New Approach to Fly Ash Processing and Applications to Minimise

Transcription

Report: CTU/4608
New Approach to Fly Ash Processing and
Applications to Minimise Wastage to Landfill
Final Report to Defra
Project No WR0401 (formerly WRT395)
M J McCarthy, M R Jones, L Zheng and R K Dhir
November 2008
Concrete Technology Unit
Division of Civil Engineering
School of Engineering, Physics and Mathematics
University of Dundee
Dundee DD1 4HN
Scotland
ACKNOWLEDGEMENTS
The Department for Environment, Food and Rural Affairs (DEFRA), and industrial partners, Ash
Resources Ltd (Pty), British Cement Association, British Precast Concrete Federation, Castle
Cement Ltd, Center for Applied Energy Research (CAER) University of Kentucky, European
Coal Combustion Products Association (ECOBA), Quarry Products Association, ScotAsh Ltd,
and United Kingdom Quality Ash Association (UKQAA) are gratefully acknowledged for their
contributions to the project.
Acknowledgement is also given for the helpful comments made by the representatives of
DEFRA and the Industrial Partners, D Beaumont, W vom Berg, C Bennett, G Cann, C Clear, M
Clarke, R Clemmey, R Coombs, A Foster, J Gronow, T A Harrison, P Livesey, S Pepper, P
Quinn, T L Robl, L K A Sear and G W Smith throughout the duration of the project and during
the Steering Committee Meetings held in Dundee.
E.ON UK, British Energy, Scottish & Southern Energy Plc, RWE Npower Plc and ScotAsh Ltd
are thanked for their assistance with the site surveys and fly ash supply. Plean Precast Ltd and
Staffordshire University are acknowledged for their assistance with the scoping studies with the
processed materials.
CONTRIBUTORS TO THE PROJECT
UNIVERSITY OF DUNDEE
Professor R K Dhir
Dr M R Jones
Dr M J McCarthy
Dr L Zheng
Dr L J Csetenyi
Mr K Packriswamy
Miss C Scollan
Mr N Stephen
Mr D Ritchie
Mr A Anderson
DEFRA
Mr N Carrigan
Neil Carrigan Associates Ltd
INDUSTRIAL PARTNERS
Mr D Beaumont
Dr W vom Berg
Mr C Bennett
Mr G Cann
Dr C Clear
Mr M Clarke
Mr R Clemmey
Dr J Gronow
Prof T A Harrison
Mr P Livesey
Mr S Pepper
Mr P Quinn
Dr T L Robl
Dr L K A Sear
Mr G W Smith
Hargreaves (GB) Ltd
ECOBA
ScotAsh Ltd
Castle Cement Ltd
British Cement Association
British Precast Concrete Federation Limited
Hargreaves Ash Marketing Ltd
Consultant
Quarry Products Association
Castle Cement Ltd
Castle Cement Ltd
ScotAsh Ltd
CAER, University of Kentucky
UKQAA
Ash Resources Ltd (Pty)
EXECUTIVE SUMMARY
This report summarises a two year research project concerned with recovering and processing lagoon and
stockpile material (mainly fly ash) as valuable resources for a range of construction applications and
thereby to establish an integrated approach to the use of the material.
The project comprised six main work steps for achieving the above: (i) quantification of material for
recovery including site surveys, sampling and characterisation, (ii) design and fabrication of a pilot-scale
processing system, (iii) evaluation of the system to establish operation parameters, (iv) processing of
sufficient quantities of material recovered from site for scoping studies, (v) scoping studies covering a
range of applications of low, medium and high value with the processed material, and (vi) development of
guidelines for processing and use.
Based on the work of a previous survey identifying the main storage areas in the UK, material from five
power station sites covering lagoons and stockpiles was characterised. Detailed surveys were carried out
at two of the power stations (in a lagoon and stockpile). Characterisation showed that the majority of
lagoon and stockpile materials were relatively coarse and had high loss-on-ignition (LOI). This meant
that processing to sort material into a series of fraction sizes and to remove carbon would be required
towards enabling wider use. Lagoon material was found to have greater variation in properties than that
in stockpiles.
A pilot-scale processing system was developed in collaboration with research colleagues from the
University of Kentucky, which included: pre-screening, primary classification, froth flotation (column
and mechanical) and lamella hydraulic classification.
Pre-screening was used to take out very coarse particles, vegetation, etc. Primary classification removed
coarse material (i.e. > 150 m particles) and reduced carbon contents, prior to the following froth
flotation process. More than 90% of the particles > 150 m could be removed with this system.
Similarly, the LOI could be reduced by up to 2.0% (corresponding to 25 to 40% of the total carbon).
With column froth flotation, the LOI was further reduced by 2.0% compared to the feed, while the froth
had an LOI of about 40%. Mechanical flotation was also used to remove carbon. In a two-cell system,
the LOI was reduced by 6.0% compared to the feed, while the froth contained about 33% LOI. In a fourcell system, the LOI was reduced by 10.0 to 12.0% compared to the feed.
Lamella classification was used to separate material into different size fractions. Five fractions were
obtained from this, i.e. (i) coarse, U1; (ii) medium, U2; (iii) fine, U3 and U4; (iv) ultrafine, U5, and (v)
cenospheres. The results indicate that fineness and LOI of the fly ashes gradually increased and reduced
respectively from U1 to U5. There were, however, no significant changes in their chemical and
mineralogical compositions.
In total, material from three power station storage areas (a lagoon and two stockpiles) was processed. It
was found that there were some variations between these in terms of carbon removal and particle size
fraction yields. More cenospheres were collected from material which was conditioned and stockpiled,
during processing, compared to that from lagoons. Some hydration products were found on particle
surfaces (especially ultrafine) from stockpiles and lagoons, which may influence their chemical activity in
a cementitious system.
Sufficient quantities of material recovered from the test sites were processed and scoping studies for a
range of applications of low, medium and high value carried out, including (i) cement components in
standard mortars and concrete, (ii) a fine aggregate in foamed concrete, (iii) a component in cement-based
grouts, (iv) a clay replacement in bricks and a component in concrete masonry blocks, (v) a component in
road bases (mixed with lime, i.e. soil stabilisation), and (vi) a fuel or raw feed in Portland cement (PC)
manufacture.
Standard mortar test results indicate that the water-requirements of those with fine processed materials
were significantly lower than that with raw (unprocessed) material, suggesting that water savings could be
achieved when using these in concrete. The activity index of the processed material increased with
fineness.
The finer fractions, U3, U4 and U5, were used as cement components in concrete. In laboratory trials,
concrete with processed material showed satisfactory strength development and the colour was similar to
that of PC. This was also the case in the manufacture of a precast concrete element. The strength
obtained using ultrafine fractions gave little or no difference compared to raw material, which may be due
to surface reaction effects during long-term storage and/or hydraulic processing.
Medium fineness material, U2, was used as a component of cement-based grouts. The grouts prepared
with processed material demonstrated improved consistence and satisfactory strength development
compared to that with raw material.
Coarse and medium fineness materials, U1 and U2, were used in foamed concrete as a fine aggregate. A
50% replacement of sand was used and the results gave improved consistence and enhanced strength
development compared to the PC/sand reference.
Coarse material with high LOI (i.e. underflow from primary classification) was used as a component of
clay bricks. Processed fractions U2 and U4 were also examined in this application. The results indicate
that with the particular clay used, the optimum solution was low LOI/fine material with 10% to 20% clay
replacement, firing at 1100°C, in terms of achieving low water absorption and high strength. A desk
study suggests that U1 and U2 materials could also be used as a filler component in concrete masonry
units.
Processed material was used as an addition in lime soil stabilisation. This significantly reduced swelling
of high sulfate content clays stabilised by lime. At the 24% addition level, swelling was reduced to about
25% of the lime-only stabilised clay. At lower processed material addition levels, coarse fractions gave
less swelling than fine. Strength increased with addition level and there appeared to be an optimum
content for maximum strength.
Calorific values were measured in selected carbon-rich processed materials, e.g. froths from flotation, and
these were sufficiently high to have potential for use as fuel. The calorific value was also proportional to
the LOI of the materials. A desk study suggests that high LOI materials could be used as raw feed in
cement manufacture, which would be beneficial in both saving raw materials and energy.
Based on the findings of the work, estimates of the quantities of materials of different fraction sizes, and
types for recovery (at one of the power stations tested and across the UK) were made. Guidelines on the
issues that should be addressed with respect to processing, including testing of material at the site,
requirements of the processing system and operational parameters are given. Similarly, guidelines on
how the processed materials may be classified in relation to end use and the technical issues associated
with this are also covered.
With ever decreasing dependence on coal as a fuel and thereby reduced quantities of fly ash being
produced, the outcomes of this project demonstrate that long-term stored material is a valuable resource
that can be used in a wide range of applications. Furthermore, they can contribute to reducing storage
requirements around power station sites. With the substantial ‗deposits‘ of coal combustion products
across the UK, their recovery and re-use can provide significant environmental benefits.
Contents
1.
INTRODUCTION ........................................................................................................................... 1
1.1. Background............................................................................................................................ 1
1.2. Objectives .............................................................................................................................. 1
1.3. Scope of Study ....................................................................................................................... 2
2.
REVIEW OF LITERATURE ON PROCESSING TECHNIQUES ................................................... 2
2.1. Introduction ........................................................................................................................... 2
2.2. Classification ......................................................................................................................... 2
2.2.1. Air classification......................................................................................................... 3
2.2.2. Hydraulic classification .............................................................................................. 3
2.3. Screening ............................................................................................................................... 9
2.3.1. Sieving.......................................................................................................................10
2.3.2. Filtration ...................................................................................................................11
2.4. Grinding................................................................................................................................12
2.5. Flotation................................................................................................................................13
2.6. Electrostatic Separation .........................................................................................................16
2.7. Thermal Treatment ................................................................................................................19
2.8. Magnetic Separation ..............................................................................................................21
2.9. Combined Processing Technologies ......................................................................................21
2.10. Other Beneficiation Processes ...............................................................................................22
2.11. Summary ..............................................................................................................................22
3.
REVIEW OF FLY ASH USE IN VARIOUS APPLICATION ....................................................... 23
3.1. Construction Industry Applications .......................................................................................24
3.1.1. Cement and sand component in concrete and mortar .................................................24
3.1.2. Building components .................................................................................................25
3.1.3. Geotechnical applications..........................................................................................26
3.1.4. Mineral filler in asphalt paving..................................................................................27
3.1.5. Raw feed in cement manufacture ................................................................................27
3.2. Agricultural Applications ......................................................................................................28
3.3. Source to Extract Valuable Components ................................................................................28
3.3.1. Recovered carbon fuel ...............................................................................................28
3.3.2. Recovered metals .......................................................................................................29
3.4. Sorbent / Confinement Agent ................................................................................................29
3.4.1. Brownfield clean-up...................................................................................................29
3.4.2. Waste stabilization / confinement ...............................................................................29
3.4.3. Repository backfill .....................................................................................................29
3.4.4. Zeolite precursor .......................................................................................................29
3.4.5. Adsorbent ..................................................................................................................30
3.5. Constituent Material in Various Novel Products ....................................................................30
3.5.1. Filler in paints and enamels .......................................................................................30
3.5.2. Wood substitute .........................................................................................................30
3.5.3. Geopolymer mixtures .................................................................................................30
3.5.4. Metal castings / lightweight alloys .............................................................................30
3.5.5. Vitreous products / glass ceramics .............................................................................30
3.6. Summary ..............................................................................................................................31
4.
OVERALL PROGRAMME OF RESEARCH ............................................................................... 31
5.
TEST METHODS FOR MATERIAL CHARACTERISATION ..................................................... 33
5.1.
5.2.
5.3.
5.4.
5.5.
5.6.
6.
Fineness ................................................................................................................................33
Particle Size Distribution (PSD) ............................................................................................33
Loss-On-Ignition (LOI) .........................................................................................................33
Bulk Oxide Composition .......................................................................................................33
Mineralogical Composition ...................................................................................................33
Morphology ..........................................................................................................................34
MATERIAL SAMPLING AND CHARACTERISATION ............................................................. 34
6.1. Power Station 1 .....................................................................................................................34
6.1.1. Material sampling .....................................................................................................34
6.1.2. Characterisation........................................................................................................36
6.1.3. Previous survey .........................................................................................................37
6.1.4. Report from power station .........................................................................................38
6.1.5. Summary of material properties from Power Station 1 ...............................................40
6.2. Power Station 2 .....................................................................................................................40
6.3. Power Station 3 .....................................................................................................................41
6.3.1. Material sampling .....................................................................................................41
6.3.2. Characterisation........................................................................................................41
6.3.3. Previous survey .........................................................................................................45
6.3.4. Summary of material properties from Power Station 3 ...............................................47
6.4. Power Station 4 and 5............................................................................................................48
6.5. Summary ..............................................................................................................................50
7.
OVERVIEW OF PROCESSING SYSTEM ................................................................................... 51
7.1. Pre-Screening and Slurrying ..................................................................................................51
7.2. Primary Classification ...........................................................................................................53
7.3. Froth Flotation ......................................................................................................................54
7.3.1. Column flotation ........................................................................................................55
7.3.2. Mechanical flotation ..................................................................................................56
7.4. Lamella Hydraulic Classification ...........................................................................................56
7.5. Material Collection and De-watering .....................................................................................57
8.
FLY ASH PROCESSING.............................................................................................................. 58
8.1. Material from Power Station 1 ...............................................................................................60
8.2.1. Initial processing with column flotation .....................................................................66
8.2.2. Processing with mechanical flotation .........................................................................71
8.3.1. Trial 1 with column flotation......................................................................................80
8.3.2. Trials 2 and 3 with two-cell and four-cell mechanical flotation ..................................83
8.4. Summary ..............................................................................................................................95
9.
SCOPING STUDIES FOR END USE APPLICATIONS ............................................................... 95
9.1.
9.2.
9.3.
9.4.
9.5.
9.6.
9.7.
Standard Mortar Tests ...........................................................................................................95
Addition in Concrete .............................................................................................................96
Addition in Precast Concrete .................................................................................................98
Cementitious Grouts............................................................................................................ 100
Foamed Concrete ................................................................................................................ 102
Lime Soil Stabilisation ........................................................................................................ 104
Brick and Block Manufacture .............................................................................................. 106
9.7.1. Fired bricks ............................................................................................................. 106
9.7.2. Concrete masonry units ........................................................................................... 111
9.8. Other Uses .......................................................................................................................... 112
9.8.1. Fuel substitute ......................................................................................................... 112
9.8.2. Raw feed in cement manufacture .............................................................................. 113
9.8.3. Cenospheres ............................................................................................................ 113
9.9. Summary ............................................................................................................................ 113
10. SUMMARY OF RESEARCH FINDINGS .................................................................................. 114
10.1. Material Sampling and Characterisation .............................................................................. 114
10.2. Fly Ash Processing .............................................................................................................. 116
10.3. Scoping Studies for End Use Applications ........................................................................... 117
11. PRACTICAL GUIDELINES AND ACTIONS RESULTING FROM THE RESEARCH ............. 118
11.1. Guidelines for Fly Ash Processing ....................................................................................... 118
11.2. Guidelines for End Use Applications with Processed Material ............................................. 119
11.3. Quantification of Potential Fly Ash Material Available for Recovery ................................... 120
11.4. Actions Resulting from the Research ................................................................................... 121
12. REFERENCES............................................................................................................................ 122
APPENDIX A
Key Parameters and Applicable Ranges for Fly Ash Use in Various Applications .... 134
APPENDIX B
Summary of the Fly Ash Samples ............................................................................ 135
APPENDIX C
Results of Previous Study for Power Station 1 ......................................................... 137
APPENDIX D
Release Analyses of the Fly Ashes ........................................................................... 140
CTU, University of Dundee
DEFRA Project WRT 395
Page 1 of 143
1. INTRODUCTION
1.1. Background
Approximately 8 Mt of coal combustion by-products, mainly fly ash and furnace bottom ash, are
currently produced annually in the UK from the generation of electricity. Of this, about 50% is used,
mainly in construction applications, which range from highway fill, to aggregate in masonry units, to
cement components in concrete (UKQAA, 2000). Much of the remaining material is disposed of at
landfill sites, or is put into long-term storage, normally by mixing with water and transporting to
stockpiles or landfills, or pumping as slurry to lagoons or ponds. These storage areas are often at power
stations, or at nearby sites, such as disused quarries, gravel pits or low lying land (Whitbread, 1990; Sear,
2001). Recent indications are that with the development of clean coal–firing technology (Kingsley, 2006)
and the timescale required for the implementation of new electricity generating infrastructure by other
means, coal is likely to remain one of the principle fuels in this industry for the foreseeable future and,
therefore, existing practices in relation to storage look set to continue.
Estimates suggest that in the UK approximately 300 Mt of combustion residues have been deposited at or
around power stations since the 1950s and surveys indicate that approximately half of this is available for
direct recovery at operational power stations (Dhir et al., 2005). With the gradual build up of material,
storage areas around these sites have reduced and space to continue this has begun to become a problem.
Disposal of material, or extension of existing storage areas will therefore become necessary. These go
against government policy in terms of utilising by-product materials and reducing primary resource
consumption (DETR, 2004).
There is, therefore, the need to act by introducing practices which homogenise the material into specific
fractions and establish its use with confidence in a range of applications, where it provides alternatives to
other options.
1.2. Objectives
The main focus of the project was to facilitate a move towards total fly ash use as a valuable resource. In
so doing, the project also indirectly covered issues of CO2 emissions and sustainable development
associated with construction.
The overall aim of the project was to recover and process coal combustion residues, either those recently
produced, or stored at or near power stations, for use as valuable resources in a range of construction
applications and thereby establish an integrated approach to the use of the material.
Thus, the specific technical and scientific objectives of the project were to,
1. Identify, by examining international best practice, the elements of plant necessary to process coal
combustion by-products in order to produce material (mainly fly ash) with a range of qualities,
suitable for use in various construction applications. Design and develop a pilot-scale plant.
2. Carry out trials with the pilot-scale plant to evaluate the system and identify optimum controlling
parameters to obtain different materials for the various end uses.
3. Sample and test material from a lagoon at a power station to establish the quantities and qualities
available for recovery over the site. Thereafter, carry out processing with the pilot plant to produce
sufficient quantities of material for use in various scoping studies.
4. Carry out scoping studies for a range of applications of low, medium and high value with the
processed materials, including (i) a cement and sand component in concrete, (ii) fine aggregate in
concrete masonry units (iii) cement-based grouts, (iv) as a component of road bases (mixed with
lime), (v) raw feed in cement manufacture and (vi) recovered carbon as a fuel.
Page 2 of 143
5. Develop guidelines, both for (i) processing and (ii) using the recovered material in different
construction applications.
1.3. Scope of Study
The project aimed to bring a novel approach to the issue of separating the stored material by exploiting
fluid separation technologies, used in the quarrying, mining and mineral sectors, to process recovered
material.
The project involved site investigations to characterise and quantify available material in lagoons and
stockpiles across the UK, design and construction of processing plant at pilot-scale, processing of
recovered material, and use of this in various scoping studies. Details of the research programme are
described in Section 4.
2. REVIEW OF LITERATURE ON PROCESSING TECHNIQUES
2.1.
Introduction
Fly ash from coal-fired power stations has been produced for many years and a significant number of
research projects have been carried out and papers published on the subject worldwide. It has been
shown to provide significant technical benefits to concrete, particularly in the fresh state (vom Berg and
Kukko, 1991) and in terms of durability (Dhir et al., 1991, Dhir and Matthews, 1992). Other benefits in
using fly ash include reduced environment impact and cost in various applications (vom Berg and
Feuerborn, 2001).
Of the combustion residues from electricity generation annually produced in the UK only about 50% are
used, and much of the remaining material is stored in lagoons and stockpiles (in the short and long-term).
This fly ash can have different characteristics to recently produced run-of-station fly ash. The
components of lagoon and stockpile fly ash are likely to be variable in terms of their carbonaceous and
clay residue, chemical compositions and particle size distributions. Clearly these inhomogeneities reflect
differences in combustion systems, coal types and stockpile/filling procedures over the period. Therefore,
processing may be required to separate this material into specific fractions and types, prior to it being
suitable for use.
By subjecting fly ash to processing, it is envisaged that the beneficial components(i.e. pozzolanic/active
particles) of its composition can be concentrated and those negatively affecting performance (i.e.
carbonaceous and clay residue materials) reduced. In this section, the development of fly ash processing
techniques to achieve these is reviewed.
Processing technologies can generally be divided into two main categories according to the media being
used: (i) dry processing, including air classification, grinding, electrostatic separation, etc., and (ii) wet
processing including hydro-cyclone separation, sedimentation, and flotation, etc. Some techniques can
be applied to either dry or wet fly ash, for example, microwave and burnout beneficiation.
To provide more effective processing and/or to obtain several different products, some combined
processing systems have been developed, such as the Fuel-FloatTM process system (Groppo et al., 2001)
and the Minitech system (Minitech Ltd, 2003).
2.2.
Classification
Classification (by air or water) is a method of separating particle mixtures into two or more products on
the basis of the velocity with which the grains travel through a fluid medium (Wills, 1997), where,
particle density, size, and shape differences are influential.
2.2.1.
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Air classification
In air classification, air is employed as the fluid medium for classification and this is suitable for use with
dry fly ash. There are a number of air classification methods, namely centrifugal classification, aerodynamic separation, and cyclonic (cyclone separation).
Centrifugal type classifiers are used to produce fly ash of average quality (i.e. BS 3892 Part 1 PFA /
BS EN450-1 Category S fly ash) from run-of-station fly ash. Aero-dynamic separation has been used to
obtain a variety of fly ash fractions with varying particle size distributions. Cyclonic separation has been
used to remove very fine particulates, down to sub-micron levels, from bulk material.
Figure 1 shows air classifiers used in fly ash classification. Figure 1a is a large-scale air classifier with a
capacity of 40 t/hr (Bennett, 2006), and Figure 1b a commercial classifier (CAS Enterprises Ltd, 2006),
which can produce ultra fine particles down to 2 m. During processing, an opposing pneumatic
conveying air stream separates particles from the centrifugal force in the wheel. The rejected or coarse
particles are discharged and prevented from passing though the classifying wheel. This results in two size
fractions. The ―cut point‖ at which the particles are divided can be modified by a speed or air flow
change.
Currently some 500,000 tonnes per annum of classified fly ash are used in ready-mixed and precast
concrete in the UK (Barnes and Sear, 2004). A classification plant operated in South Africa produces
four types of fly ash products, i.e. (i) classified fly ash to ASTM C618 F, and EN 450; (ii) superfine fly
ash (SFFA); (iii) reactive cementitious filler and (iv) fine aggregate (Ash Resources Ltd, 1999).
(a) large-scale 40 t/hr classifier
(b) commercial classifier to produce ultrafine particles
Figure 1 Air Classifiers
2.2.2.
Hydraulic classification
In hydraulic classification, water is employed as the fluid medium and the technique is also referred to as
water classification. This methodology requires fly ash to be held in a water suspension and therefore
following this the fly ash may require to be dried.
Various classifiers have been developed, which can be free-settling or hindered-settling types. Free
settling refers to the sinking of particles in a volume of fluid, which is large with respect to the total
volume of particles, hence particle crowding is negligible. As the proportion of solids in the slurry
increase, the effect of particle crowding becomes more apparent and the falling of the particles becomes
hindered-settling.
A settling cone is the simplest form of classifier, which is often used in small-scale operations to de-slime
coarse sand products and sometimes as dewatering units. The principle of the settling cone is shown in
Figure 2 (Wills, 1997). The slurry is fed into the tank as a distributed stream at F, with the discharge S
Page 4 of 143
initially closed. When the tank is full, overflow of water and slime commences, and a bed of settled sand
builds up until it reaches the level shown. If the discharge S is opened and sand discharge maintained at a
rate equal to that of the input, classification by horizontal current action takes place radially across zone D
from the feed cylinder B to the overflow lip. Figure 3 shows two settling cones of a pilot-scale system
(Robl, 2007), one of which was used as a primary classifier for removing coarse particles (> 150 m) and
the other for dewatering.
Figure 2 Settling cone operation (Wills, 1997)
Figure 3 Primary classifier and cone thickener used in laboratory fly ash beneficiation (Robl, 2007)
For industrial processing, high efficiency, high capacity classifiers are required. Figure 4 shows a
commercial FB (fluidized bed) type classifier (Linatex, 2006a), which is widely used for removal of
deleterious materials from sand and coal/ash separations. As shown in the figure, slurry enters the
classifier through a central feed well (1) which uniformly distributes solids to a settling chamber
(2). Clean water is injected to a plenum location under the settling chamber (3). The clean water
permeates a membrane (4) and flows upward through the settling chamber, discharging over the overflow
weir (5). The interaction of the rising current and the settling solids creates a fluidized bed which inhibits
the settling of fine-size or low density material, while allowing heavier particles to pass easily to the
bottom of the chamber and then to discharge through valves (7), which is controlled by a density monitor
(6). This process is described as hindered settling. The changing dynamics of the fluidized bed enhance
stratification of materials of differing specific gravities. The fine or low specific gravity solids which
overflow the weir are typically directed to a densifying hydrocyclone, sieve bend or dewatering screen for
the next processing step.
Page 5 of 143
Figure 4 Fluidized bed classifier (Linatex, 2006a)
Figure 5 shows a density separator (Floatex Separations, 2005), which consists essentially of a hinderedsettling classifier over a dewatering cone. The density separator is a high capacity hindered settling
classifier capable of processing in excess of 600 t/hr per single machine. It is effective for cuts between
1.00 mm and 0.075 mm of a single specific gravity material.
Figure 5 Density separator (Floatex Separations, 2005)
The hydraulic classifiers shown in Figures 4 and 5 are large and sophisticated, and thus expensive to
purchase and maintain. For fly ash beneficiation, a relatively low cost high efficiency classifier, called
the Lewis Econosizer, has proved to be satisfactory (Lewis, 1990, Bradley et al., 2003, 2005). This type
of classifier is also used in field primary hydraulic classification (Hobbs et al., 1999, Groppo et al., 2001)
as shown in Figure 6.
Page 6 of 143
Figure 6 Lewis Econosizer (Groppo et al., 2001, and Lewis, 1990)
The Lewis Econosizer comprises a liquid-containment column (12) which extends vertically. The slurry
is initially fed into a hopper (18) located a predetermined distance (19) above the upper end of the column
and via a pipe (22, 23) discharged into the column onto an inclined baffle plate (27). The plate redirects
the slurry to flow upwardly in the column, as indicated by arrows (28) and (14). Finer particles according
to the desired separation, are discharged from the upper end of the column into a box (16). The movement
of material through the column is controlled by auxiliary water added at (52). The upper left edge of plate
(27) is spaced a slight distance from the adjacent column side wall to define an overflow gap (31) for
coarse particles to flow downwardly through into the hopper (25).
Hydraulic classifiers can also consist of a series of sorting columns through each of which a vertical
current of water is rising and particles are settling out (Figure 7). The rising currents are graded from a
relatively high velocity in the first sorting column, to a relatively low velocity in the last, so that a series
of products can be obtained, with the coarser, denser particles in the first discharge point and the fines in
later ones. Very fine slimes overflow the final sorting column of the classifier. This type of classifier has
been used to separate fly ash into different sizes and to obtain ultrafine fly ash as shown in Figure 8
(Robl, 2007).
Since the settling rate of finer fly ash is very slow, a large floor area may be required to obtain significant
throughput. To increase the settling rate, a lamella structure (a number of inclined plates) is used in the
classifier of Figure 8b as shown in Figure 9. This lamella structure has a number of advantages in fly ash
classification, such as an ability to operate at high solids contents, producing ultrafine fly ash and very
high recoveries (Robl, 2007). During fly ash classification, it is important that particles do not flocculate.
Therefore, dispersants are normally added during processing.
Page 7 of 143
Figure 7 Hydraulic classifiers having a series of sorting columns (Wills, 1997)
(a) Laboratory scale classifier
(b) Classifier used in the field
Figure 8 Multi-grade hydraulic classifiers used in fly ash beneficiation (Robl, 2007)
Figure 9 Lamella structure of hydraulic classifier used in fly ash beneficiation (Robl, 2007)
Page 8 of 143
While the settling force of the hydraulic classifiers described above is based on gravity, cycloning utilises
centrifugal forces to accelerate the settling rate of particles. Cycloning is a solid/liquid separation process
whose wide variety of applications include classification and thickening. This technique is essentially the
same as air-cyclonic separation, other than the medium used to carry the particles is water instead of air.
A simple procedure for the design of a hydro-cyclone is reported by Schwalbach (1988).
Figure 10 shows a hydrocyclone (Linatex, 2006b). With this system, a mixture of solids and water is fed
into the cyclone at a pressure of typically 40-150 kPa. The rotation imparted by the entry of the slurry into
the feed box causes the solid particles to be thrown outward by centrifugal forces. The higher the inlet
pressure, the greater the force, which is commonly many times that of gravity. Solids are thrown to the
wall of the cyclone, spiral down the cone and out through the apex. The bulk of the liquid spirals upwards
and leaves the cyclone through the vortex finder. The solids which exit with the bulk of the water through
the vortex finder are the particles which are so fine that the centrifugal forces are overcome by drag forces.
For a given inlet pressure or rotational speed there is a ‗cut‘ size at which the drag and centrifugal forces
are in balance. Particles finer than this cut size flow with the bulk of the liquid through the vortex finder,
and coarser particles exit through the apex.
Figure 10 Hydrocyclone (Linatex, 2006b)
General applications of the hydrocyclone include (i) classification, especially in closed circuit grinding
operations, in which coarse material is returned for further grinding, while fine material in the overflow
goes on to further processing; (ii) dewatering and desliming mineral sands, concrete sands, iron sands,
iron ore fines, phosphate rock and coal washery fines in mineral processing circuits, including back-fill
feed preparation; and (iii) fines recovery/de-gritting (e.g. removing oversize grits from cement, clay,
drilling mud, effluent and other slurries), which requires small cyclones operating at higher than normal
pressures.
Since fine separations require small cyclones, which have only small capacity, several have to be
connected in parallel if high capacity is required. Figure 11 shows some cyclone assemblies for industrial
applications.
In fly ash classification, a centrifugal classifier is applied in the RockTron fly ash beneficiation process
(Smalley et al., 2006) to produce αlpha™ and δelta™ products. The αlpha™ product is the fine fraction
of fly ash used as a cement constituent, while the δelta™ product is the coarse fraction of fly ash for other
applications.
Page 9 of 143
(a) Hydrocyclone assembly (Linatex, 2006b) (b) Units for river treatment system (NATCO Group, 2007)
Figure 11 Hydrocyclone assemblies
2.3.
Screening
The most widely used technique for separating particulate materials involves screening them into
different size fractions. Screening is generally carried out on relatively coarse materials, as the efficiency
decreases rapidly with fineness. Finer sizing normally is undertaken by classification. In addition, to
produce a series of closely sized end fractions, e.g. aggregates, where size is an important issue, screening
is also used in material processing to prevent undersize (or oversize) material entering the end product or
next stage of processing. Figure 12 shows screening using a 9.5 mm sieve adopted in the Kentucky Ash
Beneficiation System (Robl, 2007). Coarse material is removed by screening and readily marketable as a
lightweight aggregate. The material that is finer than 9.5 mm is then sent to the primary classifier for
further processing (Groppo et al., 2001).
Screening
@ 9.5mm
Figure 12 Primary screening at 9.5 mm used in fly ash beneficiation (Robl, 2007)
Page 10 of 143
The media used in screening can be (i) a sieve, or series of sieves made of woven wire cloth or (ii) a filter
(membrane) made of natural or synthetic fibres.
2.3.1.
Sieving
In this process, the bulk material is presented to a sieve, or series of sieves, of specified aperture size(s),
so the individual particles are separated according to the size and shape that permits their passage through
the apertures.
For practical purposes a lower limit aperture size of approximately 40 μm is possible. This is due to sieve
fabrication problems (Peris-Mora et al., 1991) and difficulties of encouraging particle passage through the
sieve. At these small sizes, the need for a water stream to aid passage may be required. A further
consideration is that screening will only take account of the two smallest dimensions of a particle. Thus,
large but relatively slender particles will be separated with fine particles. This shape is often found with
carbon and residue clay within fly ash. However, the use of sieves to produce fractions of fly ash has been
undertaken and has been suggested as a feasible option for large-scale fly ash classification (Sheu et al.,
1990).
In order to process a large quantity of material, it is important to allow for the removal of large particles
from the surface of the sieve. This has to be carried out so that new material can be presented. However,
the material needs to remain on the sieve long enough to ensure all particles have the chance to be
processed. The latter also requires the application of an applied force so that particles are rotated on the
surface of the sieve, such that different orientations are presented to the apertures. There are methods for
achieving this based on inclining cylindrical sieves, before discharge of the oversized particles at the base.
These are referred to as reels or trommels (Gilchrist, 1989). A typical trommel screen is shown in
Figure 13.
Figure 13 A 0.481.22m trommel screen with 75 mm screens (Gilchrist, 1989)
Flat sieves may also be employed at an inclination with the addition of applied motion, either parallel or
normal to the sieve face, therefore, resulting in particles both rolling and bouncing towards separation.
Obviously, optimal operational parameters need to be ascertained with regard to inclination angle, rate of
material feed, and frequency and amplitude of vibration, etc. Figure 14 shows a vibrating screen with a
circular motion (Shanghai Linhu Group, 2007), which is specially designed for quarries to separate
crushed stone material into different sizes.
Page 11 of 143
Figure 14 Vibrating screen with circular motion (Shanghai Linhu Group, 2007)
2.3.2.
Filtration
With this form of screening, a suspension is passed through a solid medium of natural or synthetic fibres,
which prohibit the passage of particles above a specific size. The collection of particles on the surface of
the filter is referred to as the filter cake, and as it builds up it will increase the resistance to the passage of
suspension media and smaller suspended particles and, therefore, requires frequent clearing or
replacement. Thus, filtration can handle only small quantities. Furthermore, it is time consuming due to
the resistance of passage. However, reduced pressure can be applied to accelerate the process. Industrial
membrane filtration processes are also available, whereby the suspension is fed across a membrane
surface. However, these also suffer from the build-up of particles. The application of vibration to
overcome such problems is presented as one solution (Culkin and Armando, 1992). Further developments
have been made which enable the continuous removal of filter cake using rotary drums or moving bed
filters, such that the cake is mechanically removed before the filter is returned for subjection to further
suspensions (Gilchrist, 1989).
In the fly ash beneficiation system described by Robl et al., (2005), rotary drum filtration is used to
collect ultrafine fly ashes (UFA). The UFA slurry recovered from the secondary classifier is flocculated
using a polyethylene oxide polymer and then pumped into a cone thickener. The UFA solids settle to the
base of the thickener, where they are withdrawn as thickened slurry. This thickened slurry is then
dewatered on a rotary vacuum drum filter (Figure 15) and then the dewatered UFA filter cake collected
for drying.
The rotary vacuum drum filter is one of the oldest filters applied to industrial liquid filtration (Menardi,
2003). A rotary drum filter resembles a large drum on its side. Half of the drum is submerged in slurry,
with the other half above it. The filter cloth winds around the drum and as the drum rotates, the slurry is
sucked into the cloth. As the drum rotates out of the slurry, the cake is dried. This drying is caused by a
vacuum continuously being drawn through the cake in the exposed section of the drum. At the end of the
rotation cycle, at approximately the three o'clock position, the filter cake is discharged and the process
repeats itself. The filters may incorporate a drum cloth that is caulked onto the drum itself, or they may
utilize an endless belt which tracks off and discharges away from the drum.
Page 12 of 143
Figure 15 Rotary vacuum drum filter used in fly ash beneficiation (Robl et al., 2007)
It should be noted that filtration can also be applied in dry processing. Bag filtration, which is simply
cloth barrages placed in a gas stream, is another form of filtration. In terms of fly ash, two distinct
advantages are envisaged with this method. Firstly, there is no need to suspend fly ash in liquid and
subsequently dry it after processing and, secondly, no handling of fly ash is required between production
and processing. Furthermore, hoppers can be placed below the bags so that fly ash can be shaken free
from them and thus automatically collected. By using a series of cloth barrages, it should be possible to
separate the particles into distinct groups depending on particle size. Granular bed filtration processes
have also been developed for the removal of fine particulates from flue gases, thus demonstrating the
applicability of this process (Zevenhoven et al., 1992). However, the collection of the material from the
granular medium requires further processing. Fluidised bed filtration, on the other hand, has demonstrated
the feasibility of fly ash separation in relation to size (Miyajima et al., 1985). In another study, a similar
method was also successfully employed to reduce the carbon content of fly ash: such that a high carbon
content was reduced to below 5%, measured as loss on ignition (LOI) (Mori et al., 1994).
2.4. Grinding
With grinding, the particles are reduced in size by a combination of impact and abrasion, either dry or in
suspension in water, during grinding. It is usually performed in rotating cylindrical steel vessels known
as tumbling mills. Tumbling mills are of three basic types: rod, ball and autogenous. Rod mills accept a
feed size up to 50 mm and produce a typical product size of 2-4 mm when operating in open circuit and as
fine as 500 m in closed circuit with a screen or other sizing device. Ball mills using steel balls as the
grinding medium are often used to grind cement clinker, ores and other materials to a typical product size
of 500 m or finer. Autogenous mills are usually utilised to grind run-of-mine rock, in which crushing is
achieved by the action of the ore particles themselves on each other (Metso Minerals, 2005).
Conventional grinding is an energy consuming process, especially when very fine end products are
required. In the USA, an estimated 32 billion kWh of power are consumed by size reduction equipment
(Wills, 1997). A large percentage of this power is for fine grinding applications. When fine grinding in a
tumbling mill, the production of unwanted noise and heat waste valuable energy (Metso Minerals, 2005).
Several types of mills have been developed for grinding to give a very fine end product with lower energy
consumption, such as vibratory ball mills (Russell, 1989), vertical stirred mills (e.g. Tower mill – Stief et
al., 1987 and Vertimill – Metso Minerals, 2005), and jet mills (Atritor Ltd, 2007). With this range of fine
grinding equipment, particle fractions as fine as 1-10 m can be obtained.
Page 13 of 143
Grinding provides an effective means of reducing the particle size of fly ash and fully utilises all fly ash
in comparison with air-classification which results in a ―waste‖ fraction.
Duerden (1987) found small cenospheres to be unaffected while larger spheres, agglomerates, and carbon
and clay particle species were broken down by the process. This was also found by other observers (Shen
and Zhang, 1982, Monk, 1983). In terms of fly ash improvement, there is an increase in fineness and it
may be that the vitreous glass phase of the fly ash is exposed further, thus increasing reactivity (Haertl,
1991).
A previous study (Dhir et al., 2005) indicated that grinding increased the particle density and fineness of
fly ash. However, LOI was essentially unchanged. Concrete mixes with the ground recovered fly ash
gave increased slump and strength development compared to that with original recovered material.
2.5. Flotation
This method separates particles according to density. The cut point (density at which separation occurs) is
dependent on the density of the fluid medium employed. Particles with a lower relative density than the
media will float, while those with a higher density will settle. One study has found fly ash to separate into
three distinct fractions using water (Duerden, 1987):
1) floaters of carbon and large cenospheres;
2) suspended fine cenospheres less than 10 μm;
3) sediment consisting of broken and misshapen spheres, clay residue, crystalline particles, and some
trapped cenospheres.
Flotation is in-effect already used in the processing of fly ash in power station lagoons, where floating
cenospheres are sometimes gathered for use in plastic and ceramic materials (Dower, 1984).
Another form of flotation is froth flotation. This is a widely used separation technique in the mineral
processing industry. It utilises the differences in physico-chemical surface properties of particles of
various mineral components. In this process, air bubbles are blown through a slurry and physicochemical processes take place at the interface of solid, liquid and gas phases. Separation of the minerals
is based on the surface properties of the individual mineral types. Certain minerals will attach to bubbles
and thus rise to the surface through the development of a monomolecular layer of ions on their surface,
thus giving it a high effective surface tension (Figure 16). This process can only be applied to relatively
fine particles, since if they are too large the adhesion between them and bubbles will be less than particle
weights and bubbles will therefore drop their load (Wills, 1997).
Figure 16 Principle of froth flotation (Wills, 1997)
Page 14 of 143
The use of reagents, referred to as collectors, can be utilised to define the desired separation. Collectors
are the most important reagents which adsorb on mineral surfaces, rendering them hydrophobic (or
aerophilic) and facilitating bubble attachment. This creates selective adsorption or chemisorption of
certain organic compounds on the surface of the chosen minerals. Therefore, the success of the process
depends on the control of the surface chemistry to yield selective adsorption of collectors.
Non-polar minerals, such as graphite, sulphur, talc, diamond and coal, have high natural floatability.
Although it is possible to float these minerals without the aid of chemical agents, it is possible to increase
their hydrophobicity by the addition of frothing agents. Creosote, for example, is widely used to increase
the floatability of coal (Wills, 1997).
A second reagent, known as a frother, is used to stabilise the air bubbles upon which the floatable
minerals become attached. Frothers are generally heteropolar surface-active organic reagents, capable of
being adsorbed on the air-water interface.
In addition, regulators, or modifiers, are used extensively in flotation to modify the action of the collector,
either by intensifying or reducing its water-repellent effect on the mineral surface. They thus make
collector action more selective towards certain minerals. Regulators can be classed as activators,
depressants, or pH modifiers.
A froth flotation process to separate unburnt carbon from fly ash, which was developed by Michigan
Technological University (U.S. Patents 5,047,145 and 5,227,047), claimed that a target of less than 1%
carbon in clean fly ash with 90% or more recovery, or 80% of LOI in the carbon concentrate, with 70% or
more recovery of total carbon, can be achieved with the process (Hwang et al., 1991, 1993, 1995). This
method is used to separate unburnt carbon from the fine portions (i.e. <150m) of fly ash. Therefore,
both enhanced pozzolan fly ash and marketable carbon can be obtained from this patented process.
In the fly ash beneficiation system developed at the University of Kentucky, recovery of fine carbon
product via froth flotation is also included, which is the key innovative feature of the approach with a
patented flotation reagent system (Groppo et al., 1995) that enables low cost recovery of carbon by
flotation. Figure 17 shows the conventional mechanical Denver froth flotation cells used in the Kentucky
fly ash beneficiation system (Robl, 2007).
Figure 17 Froth flotation cells (Robl, 2007)
Page 15 of 143
In addition to conventional mechanical froth flotation described above, another type of froth flotation
used in industry is pneumatic flotation.
Pneumatic machines use air blown in by means of pipes, nozzles, or perforated plates, in which case the
air must be dispersed by baffles to create adequate levels of bubbles in the slurry and to give sufficient
aeration or agitation. Figure 18 shows a pneumatic machine or Davcra cell, which has been claimed to
yield equivalent or better performance than a bank of mechanical cells (Wills, 1997). It consists of a tank
segmented by a vertical baffle. Air and feed slurry are injected into the tank through a cyclone-type
dispersion nozzle, with the energy of the jet of slurry being dissipated against the vertical baffle.
Dispersion of air and collection of particles by bubbles occurs in the highly agitated region of the tank
confined by the baffle. The slurry flows over the baffle into a quiescent region, designed for bubbleslurry disengagement.
Figure 18 Davcra froth flotation cell (Wills, 1997)
RockTron utilises pneumatic flotation cells (Figure 19) in their fly ash beneficiation process, which are
claimed to be especially suited to fine particle separations (Smalley et al., 2006). These cells produce a
low residual carbon, clean, high specification pozzolanic product and a high grade carbon concentrate.
Figure 19 Pneumatic flotation cell used in the RockTron fly ash beneficiation process
(Smalley et al., 2006)
The flotation column is another form of pneumatic equipment, and was developed in the early 1960s by
Boutin and Tremblay (Canadian patents 680,576 and 694,547). The main advantages of columns include
improved separation performance, particularly with fine materials, low capital and operational cost, less
plant space demand, and adaptability to automatic control. The typical configuration of a column is
shown in Figure 20 and consists of two distinct sections. In the section below the feed point (collection
zone), particles suspended in the descending water phase contact a rising swarm of air bubbles produced
by a sparger in the column base. Floatable particles collide with and adhere to the bubbles and are
transported to the cleaning zone above the feed point. Non-floatable material is removed from the base of
Page 16 of 143
the column as tails. Tailing particles that are loosely attached to bubbles or are entrained in bubble
slipstreams are washed back into the collection zone, hence reducing contamination of the concentrate.
The wash water also serves to suppress the flow of feed slurry up the column towards the concentrate
outlet. There is a downward liquid flow in all parts of the column preventing bulk flow of feed material
into the concentrate.
Figure 20 Froth flotation column (Finch and Dobby, 1990)
Generally speaking, column flotation is specifically designed for the recovery of very fine-sized minerals
and obtaining higher grade froth product than conventional cells, mostly due to the deeper froth zone.
The U.S. Bureau of Mines has compared column flotation with conventional flotation on a Montana
chromite ore and the results show that column flotation lead to a physical improvement in the flotation
separation process (McKay et al., 1986). However, no literature was found on the use of column flotation
in fly ash beneficiation.
2.6. Electrostatic Separation
Electrostatic separation is used to split particulate materials of different electrical conductivities. It is
mainly used as a collection method at power stations when fly ash is removed from the flue gases. Under
a high electrostatic field, the less conductive particles are polarised while the conductive particles are not,
so that a precipitator can pick up the polarised particles and hence make the separation. It is anticipated
that by extending this operation, by adjusting the operating parameters, it may be possible to define which
particles are separated from the air stream and which remain unaffected. This is based on the fact that fly
ash is a heterogeneous material and particles of different characteristics will be affected by different
electric field forces, depending on their individual conductivity properties. The lower the conductivity of
a particle, the harder it is to collect in an electrostatic precipitator (Tachibana, 1989).
Electrostatic precipitation shows the potential for removing carbon from fly ash, based on the
development of ideal operational parameters, to exploit the conductivity differences between carbon and
mineral particles. After passing the charging zone, the carbon takes a positive charge, and the mineral
particles take a negative charge. Therefore, this charged fly ash/carbon blend can be separated in a high
voltage electrostatic field. Power stations within the UK are also believed to be employing electrostatic
precipitation as a means of removing unburnt coal particles from the flue gas of a furnace, running under
low ΝOX conditions (Magee, 1996). This technique is also used by some American companies (Bittner et
al., 2003; and Boral, 2007). The unburnt carbon from fly ash can also be collected and used as an
adsorbent in removing hazardous substances, such as mercury in the flue gas (Hwang et al., 2002).
Page 17 of 143
The technique described and developed by Bittner (2003) is a tribo-electrostatic separation system as
shown schematically in Figure 21. In the separator, material is fed into the thin gap between two parallel
planar electrodes. The particles are triboelectrically charged by interparticle contact. The positively
charged carbon and the negatively charged minerals are attracted to opposite electrodes. The particles are
then swept up by a continuous moving belt and conveyed in opposite directions. The belt moves the
particles adjacent to each electrode toward opposite ends of the separator. The counter current flow of the
separating particles and continual triboelectric charging by carbon-mineral collisions, provides for a
multi-stage separation and results in excellent purity and recovery in a single-pass unit. The high belt
speed also enables very high throughputs, up to 40 tonnes per hour in a single separator. By controlling
various process parameters, such as belt speed, feed point, and feed rate, the process produces low carbon
fly ash with LOIs of 2 % ± 0.5% from feed fly ashes, ranging in LOI from 4% to over 25%. Figure 22
shows a tribo-electrostatic separation system installed at a UK power station (Bennett, 2006)
Figure 21 Tribo-electrostatic separation system (Bittner et al., 2003)
Figure 22 Tribo-electrostatic separation system installed at a UK power station (Bennett, 2006)
Page 18 of 143
Up to 2006, tribo-electrostatic separation systems were operating at eight power plants in the U.S.,
Canada, and the UK as listed in Table 1 (Gasiorowski and Bittner, 2006).
Table 1 Tribo-electrostatic separation commercial operations (Gasiorowski and Bittner, 2006)
Power Station
Location
Start
Facility Details
U.S. Generating Co. – Brayton
Point Station
Massachusetts
July 1995
2 Separators
Progress Energy – Roxboro Station
North Carolina
Sept. 1997
2 Separators
Constellation Power Source
Generation – Brandon Shores
Station
Maryland
April 1999
2 Separators
35,000 ton storage dome
Scottish Power – Longannet Station
Scotland
Oct. 2002
1 Separator,
Classification
Jacksonville Electric Authority –
St. John‘s River Power Park, FL
Florida
May 2003
2 Separators
Coal/Petcoke blends
Ammonia Removal
South Mississippi Electric Power
Cooperative R.D. Morrow Station
Mississippi
Jan, 2005
1 Separator
High Carbon Reburn
New Brunswick Power Company
Belledune Station
New Brunswick,
Canada
April 2005
1 Separator
Coal/Petcoke Blends
High Carbon Reburn
RWEnpower
Didcot Station
England
August 2005
1 Separator
In addition to the mechanical transport triboelectric method, it has been reported that a new technology
known as pneumatic transport, triboelectric separation has been developed (Lockert et al., 2003). This
approach has the potential for combining low capital and operating costs and high performance. The
system is shown in Figure 23.
Figure 23 Pneumatic transport, triboelectrostatic separation system (Lockert et al., 2003)
Page 19 of 143
2.7. Thermal Treatment
Thermal treatment represents a possibility for removing unburnt carbon from fly ash. It is also reported
that thermal processing may also increase the relative amorphous content of fly ash (Boral Lytag, 1995)
although this may be due to the reduced quantity of other materials present.
A commercial operation of Carbon Burn-Out was set up 2002 and is shown in Figure 24a (SEFA, 2003).
In the process, fly ashes are blended in a raw feed silo. A 600mm thick layer of fly ash covers the bottom
of the combustor where fly ash is burned. The flue-gas is used to pneumatically convey the product fly
ash through a shell and tube heat exchanger, where fly ash is cooled from 700 to 150ºC. Users of the
technology indicate that its selection enabled conversion of 100% fly ash to a saleable product, with the
added benefit of efficient removal of ammonia residues found on fly ash as a result of NOX treatment
operations (SEFA, 2003)
Thermal treatment generally focuses on beneficiation of fly ashes containing unburnt carbon contents
greater than 6%, in which no extra energy is required for the process.
Microwave carbon burnout (MCB) technology has been developed, pilot tested (Figure 24b) (MacLean,
2002) and marketed commercially in Canada. Carbon contents in fly ash ranging from 2 to 27% have
been successfully processed to a consistent product, with as little carbon present as deemed desirable.
(a) Carbon burn-out facility (SEFA, 2003)
(b) Pilot plant of microwave fly ash beneficiation
(MacLean, 2002)
Figure 24 Thermal treatment facilities
Table 2 gives a comparison of different carbon removal technologies, i.e. froth flotation, electrostatic
separation, conventional burnout and microwave burnout for fly ash beneficiation. The selection of the
carbon removing technologies depends on the properties of the fly ash sources and the desired products.
Froth flotation is ideal for beneficiation of lagoon and stockpiled fly ash accompanying carbon recovery,
while electrostatic separation is ideal for run-of-station dry fly ashes. Thermal treatment is suitable for
both dry and wet fly ashes with heat recovery.
In addition to the carbon removal technologies given in Table 2, a new, patented, process of dry powder
separation, which applies more than one mechanism to classify fly ash has been developed (Eiderman et
al., 2000). The equipment of the process is a dry tribo-mechanical tribo-classifier, which is based on the
interaction of different forces (centrifugal, frictional and gravitational) applied to the powder, rotating
around a vertical axis in a conical bowl. Due to the fact that the unburnt carbon is either large or with
large fly ash particles and the friction coefficient of carbon with metal is less than that of the fly ash, this
method allows separation of fly ash into two fractions according to their size and friction coefficient, with
the rotating inner surface of the bowl. Other work has been carried out to develop a fly ash beneficiation
plant based on this method (Mogilevsky, 2003), with trials carried out at lab-scale (Figure 25).
Page 20 of 143
Table 2 Comparison of different carbon removal technologies in fly ash beneficiation
Thermal Treatment
Froth Flotation
Electrostatic
Separation
conventional
burnout
microwave burnout
Principle of
Operation
Air bubbles are
blown through ash
slurry with
frothing agents.
Carbon particles
will attach to
bubbles and thus
rise to the surface
and be moved
Ash passes over
moving belt,
receives an
electrostatic charge
which causes
carbon particles to
become positive.
Carbon is removed
by passing between
charged plates
Ash is ignited to
burn using carbon
as the fuel source.
Auxiliary fuel may
be needed if
carbon is low
Microwave energy
is directly
absorbed by
carbon and
provides
combustion energy
regardless of LOI
Reactor Vessel
Froth flotation cell
Moving belt
Fluidised bed
Fluidised bed
Ash Preparation
Ash slurry
Dry ash
Dry/wet ash
Dry/wet ash
LOI Limits
No limits
15% max
6% min
No limits
Residual LOI
1 - 3%
2 - 3%
< 2%
Down to 0%
Carbon Recovery
High carbon ash
> 80% purity level
High carbon ash
> 38% purity level
None
None
Heat Recovery
No
No
Yes
Yes
Figure 25 TriboClassifier (Mogilevsky, 2003)
Page 21 of 143
2.8. Magnetic Separation
The iron oxide component in fly ash can be extracted by magnetic separation. As iron oxides are strong
paramagnetic materials, their separation efficiency is well established. Iron minerals are generally found
attached to the surface of spherical particles within fly ash and thus separation from these can be
achieved. The application of this process to fly ash is mainly for the separation of iron (Minnick, 1961,
Kerkdijk et al., 1982). However, it seems to give no improvements in terms of performance of fly ash
within cementitious systems.
Recently, a new separator (Brandner et al., 2003), taking advantage of the response of fly ash to both
magnetic forces and gas drag has been developed. The magnetic forces hold ferromagnetic and strongly
paramagnetic particles in place, while air blows away the less magnetic particles, which are
pneumatically conveyed to a cyclone that removes them from the air stream. This has successfully
separated three different products from various fly ashes: magnetite which could be sold as a substitute
for commercial grades of magnetite, cement additions with LOI < 4.0%, and lightweight aggregate with
specific gravity < 2.2.
In the RockTron process system, fly ash is subjected to magnetic separation to recover fly ash derived
magnetite following carbon recovery. This magnetite, MagAshTM typically amounts to between 3-6% of
the total feed (Smalley et al., 2006).
2.9. Combined Processing Technologies
Some combined systems have been developed for the purpose of providing: (i) a more effective
beneficiation process, and/or (ii) several different products.
An example of more effective beneficiation is the dry two-stage process to remove unburnt carbon in fly
ash, which has been developed in Korea (Lee et al., 1997). It was found that fly ash > 125 m contained
most of the unburnt carbon (33% LOI), while LOI of fly ash < 125 m was only 4-5 %. Therefore, a twostage process was designed such that the raw fly ash of 6-9.5% LOI was centrifugally classified to
remove the fraction > 125 m; then triboelectrostatic classification brought the LOI down to < 3%.
Recovery of 80% processed material was reported.
Beneficiation plants for 100% utilization of fly ash generated and collected at various thermal power
stations have been developed (Minitech Pvt. Ltd., 2003). Firstly, fly ash is classified to fine and coarse
particles, and secondly grinding of coarse particles is carried out. A drying system integrated with the
beneficiation plant for wet fly ash beneficiation has also been developed.
With regard to separation/grinding combined systems, three processes for commercial utilization of fly
ash have been developed (PMET, 2003), i.e. patented jet mill comminution/classification, mechanical
grinding/classification and mechanical separation technologies. These processes permit up to 100%
recycling of fly ash materials, while reducing or eliminating the need for land disposal.
In a number of situations, combined beneficiation systems are used to obtain several different products,
including beneficiation of fly ash, using a multi-step procedure, which yielded 0.5 %wt cenospheres,
4.5% magnetic components, and 4.0% carbon (DeBarr, 1996).
A holistic approach to fly ash beneficiation was developed by Honaker (1997). The separation and
recovery of four valuable by-products: fly ash-derived magnetite, a pozzolanic portion for cement,
cenospheres, and unburnt carbon, were achieved using a combination of dry magnetic separation, fluid
bed gravity separation, and CarefreeTM cyclone technologies.
A patented process (U.S. Patents 5,047,145, 5,227,047 and 6,027,551) was developed by the Institute of
Materials Processing at Michigan Technological University, for the beneficiation of coal fly ash (Hwang
et al., 1991, 1993, 1995).
Page 22 of 143
This wet process included the following selected steps:
1)
2)
3)
4)
Formation of a slurry mixture of fly ash material;
Gravitational removal of cenospheres from the slurry;
Separation of the unburnt carbon from the remaining slurry components by froth flotation; and
Collection of the remaining fraction of fly ash. The products were dewatered and dried for shipment
to respective markets.
As noted previously, various studies have been carried out at the University of Kentucky, Center for
Applied Energy Research (Robl et al., 2006) to examine the use of combined processing systems. This
included several patented techniques (U.S. Patent 5,456,363, 5,817,230 and 6,533,848), covering methods
for removal of carbon and production of high quality polymer filler and super-pozzolan from fly ash, and
methods for improving the pozzolanic characteristics of fly ash.
RockTron have also developed a similar fly ash beneficiation processing system, which includes
pneumatic flotation for separating carbon from fly ash and a centrifugal classifier for fly ash classification
(Smalley et al., 2006). The basic flow sheet of their processing plant consists of cenosphere recovery,
carbon recovery, magnetic ash (MagAsh™) recovery and classification to yield αlpha™ and δelta™
cement substitute products.
2.10. Other Beneficiation Processes
Some fly ashes are not suitable for use as cement additions due to their composition. In these cases,
special beneficiation processes are required to remove detrimental elements before the fly ash can be
marketed.
Some special beneficiation technologies have been developed by Maxam (2002). Preliminary laboratory
tests indicated that between 79% and 98% of several metals in commercial quantities are removable from
fly ash using this technology. Analyses of 13 of 15 electricity generating plants indicated that all
environmentally detrimental elements were reduced to acceptable levels, enabling the fly ash to meet
EPA ground water quality standards.
The University of Kentucky have also carried out studies of trace element reduction by coal and fly ash
cleaning, including the use of new advanced physical separation methods (e.g advanced column flotation
coupled with microwave treatment) to enhance removal of hazardous air pollutants (HAPs) (As, Hg, Se)
associated with pyrite, and investigation of dry triboelectrostatic methods for beneficiation of fly ash
coupled with removal of Hg (Huggins, 2003).
Power plants are increasingly using ammonia injection to mitigate NOx and SOx emissions, leaving
ammonia deposits on fly ash in typical cold-side fly ash collection systems. This ammonia should be
removed from the fly ash to prevent its release during concrete production and placement. Separation
Technologies LLC has developed a process to remove excess ammonia from fly ash and is operating a
full-scale ammonia removal system (Gasiorowski and Bittner, 2006).
2.11. Summary
There are several processing technologies available for beneficiation of fly ash for different requirements,
which can be divided into classification, screening, grinding, flotation, electrostatic separation, thermal
treatment, and magnetic separation.
The classification, screening, and grinding are mainly used to change the particle sizes of fly ash.
Classification is generally used to remove coarse particles from fly ash, or to obtain ultrafine particles as
high quality pozzolanic materials. Grinding can be integrated with classification to enable full use of fly
Page 23 of 143
ash. While sieving is used to remove extraneous materials in the primary processing stage, filtration is
usually applied in fine product collection, both in dry and wet conditions.
Flotation, electrostatic separation, and thermal treatment are mainly used to remove carbon from fly
ashes. Electrostatic techniques are mainly used to reduce the carbon content in dry fly ash and froth
flotation in wet fly ash. Thermal treatment processes, i.e. carbon burn-out or microwave treatment can be
used for both dry and wet fly ashes with heat recovery instead of carbon recovery. Magnetic Separation
is used to extract the iron oxide component from fly ash.
For more effective beneficiation and to produce several different products, combined beneficiation
systems have been developed. For some fly ashes, special beneficiation processes to remove chemical
elements have also been developed.
Combined beneficiation systems can also be divided into two main categories according to the media to
be used: (i) dry processing, and (ii) wet processing. Wet processing is more suitable for beneficiation of
lagoon and stockpile fly ashes. In addition, wet processing has the following advantages in comparison
with dry processing:
1)
2)
3)
4)
5)
6)
No problems with dust control;
More efficient in particle classification and carbon recovery;
Production of low residual carbon cementitious products;
Production of high grade carbon products;
Production of a range of value added products, with 100% utilisation of feed;
Able to cope with wide feed variations (e.g. carbon in fly ash, moisture, and throughput).
3. REVIEW OF FLY ASH USE IN VARIOUS APPLICATIONS
One of main objectives of this project is to carry out scoping studies for a range of applications of low,
medium and high value with processed fly ash. Several materials with different characteristics can be
obtained from processing as indicated in the previous section. Appropriate use of these products will
maximise the benefits of fly ash processing.
There is a long history of fly ash utilisation in various applications (UKQAA 2006; ACC-ACAA, 2007;
Iyer and Scott, 2001), mostly in the construction industry (Tyson and Blackstock, 1996). Although the
range of application areas widens (Hall and Livingston, 2002; Barnes, 2002) and uptake increases as
environmental consciousness grows (Brennan, 2003; Batra, 1996), unfavourable financial conditions,
technical issues or simply conservative thinking and lack of guidance/awareness prevent wider use.
At present, about 50% of combustion residues from electricity generation are used, with typical categories
including





Construction industry applications
Agricultural applications
Sources for extraction of valuable components
Sorbent / confinement agents
Components in various novelty products
The first two categories represent the main uses in terms of quantity, but with more accurate assessment /
monitoring of properties and more sophisticated processing of available fly ash sources, the other
application fields are developing (Bouzoubaâ and Fournier, 2005; Brennan, 2003; Ferreira et al., 2003).
Consideration is given in this section to current applications with a description of technical requirements
and coverage of benefits / limitations.
Page 24 of 143
3.1. Construction Industry Applications
3.1.1.
Cement and sand component in concrete and mortar
Fly ash has been successfully used as a addition in concrete since the 1930s. This is the largest single use
of fly ash, now covered by the ‗equivalent concrete performance concept' in BS EN 206-1 (BSI, 2000)
and BS 8500 (BSI, 2006).
Cement can also contain factory blended fly ash, for example BS EN 197-1 (BSI, 2000a) specifies
CEM II Portland-fly ash cement (6-20% and 21-35% fly ash) and Portland composite cement (up to 20
and 35% fly ash or other main constituent), CEM IV Pozzolanic cement (up to 35 and 55% fly ash or
other main constituent) and CEM V Composite cement (up to 30 and 50% fly ash or other main
constituent).
When used as an addition, fly ash must be in dry form. The most important properties of the material in
this application include fineness, LOI, chemical composition (main elements CaO, SiO2, Al2O3 and
Fe2O3), moisture content, and pozzolanic activity (activity index, strength factor), (BSI, 2005).
The principal benefits of fly ash use in concrete include enhanced consistence due to spherical fly ash
particles, reduced bleeding and water demand, increased ultimate strength, reduced permeability and
chloride ion penetration, low heat of hydration, greater resistance to sulfate attack, and alkali-aggregate
reactivity, and reduced drying shrinkage (UKQAA, 2006; Payá et al.. 2002; McCarthy and Dhir, 1999;
Sarker, 1996).
On the other hand, the use of fly ash in concrete can lead to slower early strength development, extended
initial setting time, possible difficulty in controlling air content (in air-entrained concrete), potentially
higher admixture demand due to sorption by the carbon content, seasonal limitations and quality
variations due to combustion process control (ACAA, 2003; Shi and Qian, 2003).
Fly ash can be used as a cementitious material or fine aggregate (sand) in concrete and mortar, and the
products can be classified (including its coverage in standards) as follows:
Ultra fine fly ash: Ultra fine fly ash (UFA) is a special fly ash product, classified by its particle size
which is significantly finer than normally used fly ash. Due to its high fineness and spherical shape, it
has very high pozzolanic reactivity and beneficial effects on concrete workability. Therefore, UFA has
the potential to be used in producing high-strength concrete by conventional means (Tsartsari and Byars,
2002), high performance concrete with high durability (Zhang et al., 1995) and special repair mortars
where durability and trowelability are essential (Ash Resources, 1999).
Category S fly ash: BS EN 450-1 Category S fly ash (previously called BS 3892 Part 1 PFA) is a
processed fly ash (by air classification) designed to optimise characteristics for use in concrete. This
product typically has about 8% retained on a 45µm sieve (≤ 12 % as specified in BS EN 450-1), and LOI
less than 5% (Category A), between 2.0% to 7.0% (Category B), or between 4.0% to 9.0 (Category C). It
should be noted the purpose of LOI requirement is to limit the residue of unburnt carbon in the fly ash
and only Category A and B fly ashes are allowed for use as cement additions by BS 8500.
Category N fly ash: BS EN 450-1 Category N fly ash is a product from utility boilers burning pulverized
coal without further processing. Typically, this fly ash has around 25% retained on a 45µm sieve (≤ 40 %
as specified in BS EN 450-1), and a similar LOI requirement to Category S fly ash.
The coarse portions of fly ash, including that rejected from the classifier, called grit, and that falling to the
bottom of the furnace, called furnace bottom ash are essentially inert in terms of pozzolanic activity.
They can be used as fine aggregate, or fill components in concrete and mortar.
3.1.2.
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Building components
Building bricks and blocks: Fly ash bricks and blocks can be produced in an economic and sustainable
manner in the controlled technology of a precast manufacturer. All types of fly ash can be used in this
application. While fine fly ash acts as a cement component, coarse material is effective as a fine
aggregate. Given its size and lightweight nature, furnace bottom ash has proved ideal for the manufacture
of lightweight concrete blocks (Sear, 2001).
Aerated concrete blocks: A quarter of the almost 9 million m3 of precast masonry blocks made in the UK
annually are autoclaved aerated blocks (AAC), also known as "Aircrete" blocks, due to their entrapped air
content and low densities of 400-800 kg/m3. The drying shrinkage of AAC meets the requirements of
BS 6073-2: Precast concrete masonry units (BSI, 2008), and the blocks are resistant to frost damage and
sulfate attack up to Class DS 4 soil or groundwater conditions, as defined by BRE Special Digest 1 (BRE,
2005). AAC blocks have high levels of thermal insulation, high strength/weight ratios and an ability to
meet acoustic and fire insulation requirements. The blocks contribute overall cost savings arising from a
number of secondary benefits, such as the need for lighter foundations and less insulation. They are easily
cut, worked and laid, with low maintenance and handling costs (UKQAA, 2004; Kumar, 2002). In this
application, even lagoon fly ash can be used. However, low LOI of the fly ash is required, since it will
affect the size and stability of air bubbles.
Foamed concrete: The use of unprocessed, run-of-station, low-lime fly ash in foamed concrete as a
replacement for sand has been carried out at Dundee University (Jones and McCarthy, 2005). Foamed
concrete with plastic densities ranging from between 1000 and 1400 kg/m3 and cube strengths from 1 to
10 N/mm2 have been produced. It has been shown that by using fly ash in this way, significant
enhancements to the properties of foamed concrete, including rheology and compressive strength
development and almost complete immunity to sulfate attack can be achieved. Given the high carbon
content of this type of fly ash, however, there is a need to increase greatly the amount of foam required to
achieve a specified design plastic density. Structural applications with 25 N/mm2 strength grade have also
been tested (Jones, et al., 2001).
Concrete roofing tiles: Concrete roofing tiles benefit from fly ash addition by becoming lighter and less
permeable to water (TIFAC, 2007; Karpow, 1997). This is a high performance micro concrete product
produced by precast manufacturers, with fine fly ash ideal for this application.
Cast stone: Fly ash applied in cast stone mixes, up to 60%, can result in a product very similar in
appearance and physical properties to natural granite, which can help save virgin resources (TIFAC,
2007a).
Lightweight aggregate: Sintered lightweight aggregate can contain up to 70-85% fly ash and is a good
substitute for natural aggregates in concrete. It is made by pelletising fly ash mixed with a controlled
amount of water and then heating on a sinter strand, where the temperature is between 1000 and 1250ºC.
Waste fuel oil is used to provide heat energy, with the assistance of a small amount of unburnt carbon
within the fly ash, which helps to fuse the particles together. Thus carbon in fly ash is beneficial in this
application. The water is driven off resulting in a hard, honeycombed structure of interconnecting voids
within the aggregate. The aggregate formed varies in size from 14 mm down to fines. This is then graded
into a variety of sizes (UKQAA, 2002, Verma et al., 1998). Its dry bulk densities typically range from 750
to 850 kg/m3 and particle densities from 1350 to 1650 kg/m3. It also has excellent fire resistant and good
thermal and acoustic insulation (U-value ranges from 0.08-0.56 W/mK), (TIFAC, 2007, Vilches et al.,
2002; UKQAA, 2002, Katayama, 1997). The high voids ratio, typically 40%, also gives lightweight fly
ash aggregate excellent freeze-thaw properties (UKQAA, 2002).
Lightweight fly ash aggregate is typically used in the production of structural lightweight concrete. With
this application, concrete densities range from 1550 kg/m3 to 2000 kg/m3, thermal expansion coefficients
are typically two thirds that of normal gravel concretes, and characteristic strength can range from 20 to
80 N/mm2, i.e. similar to normal density concrete. The fire resistance of this concrete is superior to most
Page 26 of 143
normal concretes, because of the reduced thermal expansion and improved insulating properties of the
lightweight aggregate (UKQAA, 2002).
In addition, lightweight fly ash aggregate is also widely used in floor and roof screeds, filler floors,
drainage media, filter media, arrestor beds, bulk fill, sports surfaces and play areas, and horticultural
growing media in the UK (UKQAA, 2002).
Fired bricks and tiles: Fly ash is also used to produce fired bricks as a clay replacement. In this
application, low quality (high LOI) lagoon fly ash can be used at high volumes (Xu, et al., 2005). Fine
fly ash may be more suited to this application, since coarse fly ash will significantly decrease the
plasticity index of the fly ash/clay mixture. However, additives can be used to improve the plasticity
index or different preparation procedures can be applied in the brick manufacture (Guo et al., 2002).
Clay tile production usually involves the addition of ground rejects (coarse particles) to make the clay
mixture leaner and thus control sintering properties. Fly ash can be successfully applied up to 35% as a
replacement and satisfy low water absorption and firing shrinkage requirements (TIFAC, 2007a, Song,
1997; Hughes, 1996; Wyszomirski and Brylska, 1996).
3.1.3.
Geotechnical applications
Soil stabilisation: Stabilized bases or sub-bases are mixtures of aggregates and binders, such as Portland
cement (PC) or lime, which increase the strength, bearing capacity, and durability of a pavement
substructure. Fly ash has been successfully used as part of the binder in stabilized base construction
applications. When fly ash is used, an activator, usually lime and PC, must be added to initiate the
pozzolanic reaction. Sometimes, combinations of lime, PC, or kiln dusts have also been used (ACAA,
2003).
Some of the properties of fly ash that are of particular interest when the material is used in stabilized base
applications include water solubility, moisture content, pozzolanic activity, fineness, and organic content.
The relevant standards are the BS EN 14227 series and ASTM C593. Other properties commonly
evaluated include compressive strength, flexural strength, modulus of elasticity, bearing strength,
autogenous healing, fatigue, freeze-thaw durability, and permeability. The chemical composition,
especially the sulfate content of fly ash is also of interest (UKQAA, 2007).
Flowable fill and grout: Fly ash is also used as a component in the production of flowable fill (also
called controlled low strength material, or CLSM), which is used as self-levelling, self-compacting
backfill material, instead of compacted earth or granular fill (Smith, 1991; Hennis and Frishette, 1993;
ACI, 1994). Flowable fill includes mixtures of PC and filler material and can contain mineral additions,
such as fly ash and/or bentonite. Filler material usually consists of fine aggregate (in most cases, sand),
but some flowable fill mixes may contain approximately equal portions of coarse and fine aggregates
(UKQAA, 2006e and 2007a).
Fly ash has been used as filler material in lower-strength applications. In higher-strength applications, the
strength of flowable fill mixes can range from 1.5 to 8 N/mm2, depending on the design requirements of
the project in question (ACAA, 2003). The quality of fly ash used in flowable fill applications need not
be as strictly controlled as in other cementitious applications. Both dry and recovered fly ash from
lagoons can be used. No special processing of fly ash is required prior to use (Mishra and Karanam, 2006;
Horiuchi et al., 2000; Mishra and Mehta, 1996).
In the UK, specification for fly ash for use in cementitious grouts is given in BS 3892, Part 3. The
fineness of the fly ash should not be more than 60 % by mass retained on a 45 m sieve and the LOI not
more than 14 %. There are two basic types of grout mixes that contain fly ash: high and low fly ash
content mixes. High fly ash content mixes typically contain nearly all fly ash, with a small percentage of
PC and enough water to make the mix flowable. Low fly ash content mixes typically contain a high
percentage of fine aggregate or filler material (usually sand), a low percentage of fly ash and PC, and
enough water to also make them flowable.
Page 27 of 143
Some of the engineering properties of grout mixes containing fly ash as a principal component that are of
particular interest include compressive strength, flowability, stability, time of set, bleeding and shrinkage,
and density (UKQAA, 2006e and 2007a).
Roller compacted concrete: Roller-compacted concrete (RCC) is a concrete mix with high cementitious
and low water content. In RCC construction, a high paste content is needed to bond successive layers of
concrete together. However, the relatively high cement content also generates heat of hydration extremes
across the section, leading to cracking and the development of internal strains. Replacement of a
proportion of the cement with fly ash reduces heat gradients. Fly ash content in RCC depends on
achieving optimum packing of all constituents in the concrete (Dunstan, 1983). A minimum paste content
is necessary to fill the voids in the fine aggregate, while as low a cement content as possible is needed to
reduce the heat of hydration as well as the cost. Therefore, fly ash content in RCC mixes could be as high
as 70 – 80% for Category S fly ash and 60 – 70% for Category N fly ash (Zheng, 1995).
Embankment or fill material: Fly ash has been used since the 1950s as an embankment or structural fill
material to substitute for natural soils. Fly ash in this application must be stockpiled and conditioned to its
optimum moisture content, and then it can be compacted to its maximum density and will perform in an
equivalent manner to well-compacted soil.
Some of the engineering properties of fly ash that are of particular interest when fly ash is used as an
embankment or fill material are its moisture-density relationship, particle size distribution (PSD), shear
strength, consolidation characteristics, bearing strength, and permeability (ACAA, 2003; Newman et al.,
1992 and 1995).
3.1.4.
Mineral filler in asphalt paving
Fly ash has been used as a substitute mineral filler in asphalt paving mixtures for many years. Gradation,
fineness, density, organic impurities, and plasticity characteristics ordinarily associated with mineral filler
specification requirements can normally be met without difficulty. Asphalt mixtures containing low
addition levels (approximately 5 percent by dry weight of aggregate) of fly ash as a mineral filler exhibit
mix design properties that are usually comparable to asphalt mixtures containing natural fillers, such as
hydrated lime or stone dust. Mineral fillers in asphalt paving mixtures consist of particles, less than
75 µm in size, that fill the voids in a paving mix and serve to improve the cohesion of the binder (asphalt
cement) and the stability of the mixture (Boehm and Suss, 1997; Zimmer, 1970).
Fly ash must be in a dry form for use as a mineral filler to avoid stripping asphalt from aggregate
particles. Fly ash that is collected dry and stored in silos requires no additional processing (Rosner et al.,
1982).
The same mix design methods that are commonly used for hot mix asphalt paving mixtures are also
applicable to mixes in which fly ash is used as a mineral filler. The percentage of fly ash filler to be
incorporated into the mix is the lowest percentage that will enable the mix to satisfy all the required
design criteria (Collins and Ciesielski, 1984).
In isolated instances, asphalt paving mixes with fly ash as the mineral filler have been observed to be
difficult to compact during hot weather. This does not appear to be a widespread problem for all sources
of fly ash during hot weather, or at other times of the year (Galloway, 1980).
3.1.5.
Raw feed in cement manufacture
Commercial-scale demonstrations of the use of high-carbon fly ash in cement manufacture have been
successfully performed (Bhatty et al., 2003). The main benefits of this application include that a new
market for high carbon fly ash was developed. As a result, costly raw materials such as shale and clay can
be saved and the high carbon content conserves energy by serving as a partial fuel substitute in the energy
intensive cement manufacturing operation.
Page 28 of 143
Prior to its use, fly ash should be analysed for its chemical composition and evaluated for compatibility
with the raw materials from the cement plant. Fly ash in this application is interground with the other raw
materials and introduced into the manufacturing process following normal procedures.
3.2. Agricultural Applications
Fly ash has the potential for use in agriculture and related applications. This is true, despite the existence
in very rare examples of heavy metal contents and radioactive levels exceeding environmental limits
(Jastrow et al., 1979). Apart from these exceptional cases, reclamation of waste land for agricultural
activity, soil amendment and uses as fertilizer are promising (Kalra et al., 1998; Lenz, 1996). Benefits of
fly ash in this application include:
Improved soil texture: The addition of appropriate quantities of fly ash can alter the soil texture, e.g.
dispersion of 70 t/ha has been reported to alter the texture of sandy and clayey soil to loamy in both
agricultural soils and strip-mined soils (TIFAC, 2007b; Page et al., 1979).
Modification of bulk density: The grain size distribution especially the fine fraction of fly ash affects the
bulk density of soil, usually causing a decrease in a variety of agricultural soils. This can contribute to
achieving an optimum bulk density, which in turn improves the porosity, workability and moisture
retention capacity of the soil, as well as improving root penetration (Hackett et al., 1999).
Increased water holding capacity of soil: Application of fly ash has been found to significantly increase
the available water content of loamy sand soil and sandy soil. Practical examples show that 8 % fly ash
addition increases the porosity of Black Cotton Soil and decreases the porosity of sandy soils, improving
their water holding capacity by about 30%. However, excessive addition of fly ash, > 20% in calcareous
soils and >10 % in acidic soils reduced the hydraulic conductivity (Phung et al., 1978).
Optimised pH value: Given that even low Calcium, fly ash is usually slightly alkaline by nature, acidic
agricultural soils can effectively be treated to increase their pH. Beyond neutralisation, fly ash addition
also has the benefit of providing essential plant nutrients to the soil, yet excessive use increases the
salinity and hence adversely affects vegetation (Sikka and Kansal, 1995; Matsi and Keramidas, 1999).
Improved soil aeration: Fly ash addition can help reduce surface encrustation, therefore enhancing the
aeration of soil and improving germination of plants grown on it (Chang et al., 1997).
Positive effect on growth and yield of crops: The positive impact of fly ash on growth and yield of crops
has been reported widely. Applied at 10 t/ha, fly ash generally results in a 15-20% increase in crop yields
(TIFAC, 2007b). The major attribute, which makes fly ash suitable for agriculture, is its texture and the
fact that it contains almost all the essential plant nutrients, except organic carbon and nitrogen (Page et
al., 1980). Although fly ash cannot substitute the need of chemical fertilizers or organic manure it can be
used in combination with these (or in some cases may part substitute their requirement) to gain additional
benefits in terms of improvement in physical characteristics of soil, increased yields etc. (Jiang et al.,
1999). As with fertilizers and other agriculture input, the quantity and method of fly ash application varies
with the type of soil, the crop to be grown, the prevailing agroclimatic conditions and also the type of fly
ash available (Gaind and Gaur, 2002; Hammermeister et al., 1998; Alva, 1995; Fail and Wochok, 1977).
3.3. Source to Extract Valuable Components
3.3.1.
Recovered carbon fuel
Dry or wet beneficiation of high LOI fly ashes can lead to selective removal of carbon particles, based on
the fact that unburnt carbon particles are usually larger and lighter than the rest of the material. The
recovery usually involves a combination of separation techniques, e.g. flotation, hydro- or air
classification, ultrasonic sieving or electrostatic separation (Walker and Wheelock, 2006; Soong et al.,
Page 29 of 143
2002; Niewiadomski et al., 1999; DeBarr, 1996). Technologies for fly ash beneficiation have been
reviewed in Section 2.
3.3.2.
Recovered metals
Volatile metals e.g. cadmium and lead in fly ash can be concentrated in a very small portion of the whole
ash flux from a plant by suitable condensors, fitted to appropriate parts of the combustion system. This
has the additional advantage of the rest of the ash mass becoming suitable for a wider range of
applications, for example, in agriculture (Narodoslawsky and Obernberger, 1996; Lebrun et al., 2002).
Supercritical-fluid extraction can also be applied to reduce certain metal contents in fly ash, again, with
concomitant improvements in the resulting ‗cleaned‘ fly ash. With suitable selection of complexing
agents, there are examples of metals such as Zn2+, Pb2+, Cu2+, Sb2+, Ni2+, and Cd2+ being recovered from
fly ash (Kersch et al., 2002).
Preventative leaching of fly ash using water, citrate, oxalate, EDTA and carbonate solutions can also
remove hazardous components, such as Mo, Se, Cr, V, and Sb. For a feasible process, however, reduction
of reagent consumption and elaboration of how to recycle process water (leachant) is required (Nugteren
et al., 2002; Shcherban, 1996).
3.4. Sorbent / Confinement Agent
3.4.1.
Brownfield clean-up
Fly ash is a pozzolana and when used in combination with lime or PC to treat contaminated land, it
hardens to form a matrix, which can confine other constituents such as heavy metals precipitated as
hydroxide and rigidly held in the structure. Reduced permeability of matrices with fly ash as an active
filler also contribute to suppression of leaching (BCA, 2004; TCC, 2005).
3.4.2.
Waste stabilization / confinement
As with the above application, dedicated matrices of fly ash with PC can successfully confine wastes by
both physical and chemical means. In order to improve the efficacy of immobilisation, no aggregate is
used and the binder paste may generate excessive heat and result in thermal crack formation. Fly ash can
mitigate the problem by reducing the autogenous heat. On the other hand, its filling effect reduces the
permeability and leachability of the matrix, contributing to effective confinement (Kostarelos et al., 2006;
Van Jaarsveld et al., 2004; Dermatas and Meng, 2003; Polat et al., 2002).
3.4.3.
Repository backfill
Hazardous wastes are usually disposed of in an engineered repository, practically as an arranged stock of
drums or other waste containers, filled with a cementitious matrix for immobilisation. In this set-up, there
are multiple barriers safeguarding against release of contaminants to the environment. The matrix, the
drum and the wall structure of the repository all contribute to this. An additional element to this system is
the backfill material which surrounds the drums hindering water ingress to and leaching of wastes from
these. Such backfill material may consist of a binder high in fly ash (Perlot et al., 2006; Coumes et al.,
2006; Trotignon et al., 2006).
3.4.4.
Zeolite precursor
Approximately two thirds of fly ash comprises a relatively reactive glassy phase. The overall composition
of fly ash typically includes 38-53% SiO2, 20-40% Al2O3, 6-16% Fe2O3, 1.8-10% CaO, 1.0-3.5% MgO,
2.3-4.5 % K2O and 0.8-2.5 % Na2O. The advantageous chemistry and reactivity make it suitable to
transform into zeolites, which are molecular sieves. They are used in filtering sub-micron particles and
various other chemical engineering processes requiring selective manipulation of components of solutions
Page 30 of 143
and suspensions. Zeolites are also used as substrate for catalysts (Kikuchi, 1999; Rayalu et al., 2006;
Moreno et al., 2002; Yang and Yang, 1998).
3.4.5.
Adsorbent
The small particle size and active carbon content (unburnt carbon) enable fly ash to be utilised to adsorb
various components from solutions and air, as well as the clean-up of spillages and soak-up of
contaminants. A special component of fly ash, cenospheres, are lighter than water and can effectively
remove oil floating on water after environmental contamination incidents (Yongqi et al., 2006; Khelifi et
al., 2002; Díaz-Somoano and Martínez-Tarazona, 2002; Hassett and Eylands, 1999).
3.5. Constituent Material in Various Novel Products
3.5.1.
Filler in paints and enamels
Fly ash exhibits extending properties and is suitable for use at 30-40 % in paints and at 18-22 % in
enamels. Cenospheres especially are valuable for this application. Other benefits of fly ash addition
include improved corrosion and abrasion resistance (Shukla et al., 2001; Vassilev et al., 2004; TIFAC,
2007a).
3.5.2.
Wood substitute
Fly ash can be applied as a filler and substitute for wood in macro defect free (MDF) panels for doors and
other furniture elements. Added benefits are increased strength (5-7 times stronger than wood) and
resistance to weather, termites, fungus and fire (Liu, 1997; TIFAC, 2007a).
3.5.3.
Geopolymer mixtures
More recently, fly ash has been used as a component in geopolymer mixtures, as it consists mostly of
silica (SiO2), alumina (Al2O3) and ferric oxide (Fe2O3), and is hence a suitable source of aluminium and
silicon (Brouwers and Van Eijk, 2002; Swanepoel and Strydom, 2002).
3.5.4.
Metal castings / lightweight alloys
The fire resistance and relatively uniform shape and size of fly ash cenospheres make them ideally suited
for use in sintered or smelted products giving them lightweight properties without adversely affecting
other aspects of performance such as toughness (UKQAA, 2002a).
3.5.5.
Vitreous products / glass ceramics
Glass ceramics are produced by controlled nucleation and transformation of a glassy melt to
microcrystalline material with improved properties compared to the equivalent glass or macrocrystalline
materials. Finely dispersed fly ash can act as extrinsic nucleation points and find use in a higher value
application (Park and Heo, 2002; Boccaccini et al., 2000).
Page 31 of 143
3.6. Summary
There are a wide range of applications in which the properties of fly ash can be exploited. Technical
requirements of fly ash for different applications vary and it can be classified into following groups:






High Performance: for use in high performance concrete and high quality precast concrete
products
Cement Component: for use in construction applications as cementitious materials
Sand/Filler: for use in construction applications as fill materials and agricultural applications
Carbon: for use as raw feed in cement manufacture, or in sintered products, such as lightweight
aggregates, and as a component in fired clay bricks and tiles, etc.
Fuel: for use as fuel in power station or as an adsorbent
Cenosphere: for special applications, e.g. filler in paints and enamels.
Appendix A gives a summary of the key parameters and applicable property ranges for fly ash use in
various applications.
4. OVERALL PROGRAMME OF RESEARCH
The project was carried out as literature review (Stage 0) and 6 main stages to recover and process coal
combustion by-products, either those recently produced, or stored in lagoons and stockpiles, at or near
power stations, for use as valuable resources in a range of construction applications and thereby establish
an integrated approach to the use of the material. A summary of the time-scale for the various stages of
the research programme is given in the Gantt chart in Table 3 and described below.
Stage 0. Literature Review
Details of the literature review on processing techniques and fly ash use in various applications are given
in Section 2 and 3 respectively.
Stage 1. Design and Fabricate Pilot-scale Processing System
The specific aim of processing recovered coal combustion by-products was to separate particles into
different fineness fractions, relevant to particular applications and to remove components which may
affect performance (i.e. carbon) to acceptable levels.
Based on the findings of a feasibility study and the literature review, it was considered that the most
appropriate route to meeting these requirements in terms of economics and least impact on the
environment would be through wet processing and following techniques used in mineral processing.
Details of the pilot-scale processing system designed and fabricated for this project are introduced in
Section 7.
Stage 2. Evaluate Pilot-scale Processing System to Establish Operational Parameters
Following manufacture of the pilot-scale plant, the components of the processing system were evaluated,
during Stage 2, in relation to its control parameters. A number of samples from different test sites were
examined to evaluate the ability of the system to handle material variations.
As part of this process, both the feed and processed material were tested for physical (fineness, PSD and
LOI), chemical and morphological properties, to examine the efficiency of processing and establish
optimum operating conditions. Material balances were established between feed and processed material
for the various stages of the plant to quantify yields. Details of this work are presented in Section 8.
Page 32 of 143
Table 3 Research plan Gantt chart
Main Work Steps
0
1
2
3
4
5
6
3
6
Duration, Months
9
12 15 18
21
24
Literature review on processing techniques and
fly ash use in various applications
Design and fabricate pilot-scale processing
system
Evaluate pilot-scale processing system to
establish operation parameters
Quantification of material for recovery at power
station sites
Processing of material recovered from sites for
scoping studies
Carry out a series of scoping studies to evaluate
material suitability for various applications
Develop guidelines for fly ash processing and use
and prepare Final Report
Stage 3. Quantification of Material for Recovery at Power Station Sites
In Stage 3, several power station sites, with coal combustion by-products stored in lagoons or stockpiles,
were considered to examine material in these areas. Surveys at selected sites were carried out and
samples obtained from these were characterised both physically and chemically across the storage areas
and with depth. Information, where available, from historical records of material produced and stored in
the areas were also collected to assist in this process. This enabled quantities available for recovery at the
sites to be estimated. Details of fly ash sampling and characterisation are given in Section 6.
Stage 4. Processing of Material Recovered from Sites for Scoping Studies
Following the findings of Stage 3 of the study in terms of available quantities and distribution of coal
combustion by-products material was excavated and processed through the pilot plant using the operating
conditions identified during Stage 2 to produce material for the scoping studies. The materials were fully
characterised, following processing, before use in these studies. Details of this work are presented in
Section 8.
Stage 5. Scoping Studies to Evaluate Material Suitability for Various Applications
In Stage 5, the use of the material and its performance were investigated in selected scoping studies. The
processed materials were divided into different grades for various applications. The fine fractions were
used as cement components in standard concrete mixes in both laboratory trials and precast concrete
production. Medium fineness material was used as a component of cement-based grouts and in foamed
concrete as fine aggregate. Soil stabilisation with lime and clay replacement in fired bricks was tested
with different processed materials including coarse/high LOI. Use in concrete masonry units was
examined in a desk study. Carbon rich materials were examined as fuel and as raw feed in cement
manufacture. The scoping study work is presented in Section 9.
Stage 6. Develop Guidelines for Fly Ash Processing and Use and Prepare Final Report
In Stage 6, guidelines on processing technology and on how the material can be used in various
applications were developed. These are given in Section 10.
Page 33 of 143
5. TEST METHODS FOR MATERIAL CHARACTERISATION
5.1. Fineness
The fineness of the samples was measured by wet sieving on a 45 μm mesh under a water pressure of
75±5 kPa in accordance with BS EN 451-2. All sieves in use were calibrated with a reference fly ash of
known fineness, to provide correction factors for use, as described in the standard.
5.2. Particle Size Distribution (PSD)
A laser particle size analyser, Malvern Mastersizer-2000, was used to determine the PSD of the samples.
Approximately 1.0 g of material was dispersed in a large (800 ml) sample vessel filled with water.
Dispersion was aided with ultra-sonic agitation. PSD curves were established (using commercial
Malvern software) from the degree of scattering of collimated, monochromatic, red and blue laser beams
passing through the sample after 1000 scans each. This method of measurement is based on the principle
that the angle of deflection increases proportionally with particle size (Malvern, 1993). The test was
generally carried out twice for each sample.
5.3. Loss-On-Ignition (LOI)
The LOI of the samples was determined following the procedure described in BS EN 450-1 and BS
EN 196-2. Two samples of material, of approximately 1 g each, were weighed into separate crucibles and
placed in a furnace at 975°C for 1 hour. The crucibles were reweighed after cooling to room temperature
in a dessicator and the LOI reported as the mean of two results, calculated in accordance with the
standards.
5.4. Bulk Oxide Composition
The bulk oxide analysis of the samples was carried out using a Philips PW1410 sequential X-ray
fluorescence spectrometer (XRFS). The samples were mixed with a few drops of organic binding agent
(2 % Moviol, i.e. polyvinyl alcohol/water solution) and then compressed in a mould under loads of 75 kN
and 150 kN for 5 minutes each, and then allowed to dry under a 70°C infra-red lamp. The samples were
then analysed by XRFS using in-house calibrations, based on international standards. Two identical
samples were tested for each variable and the results are the average of these.
5.5. Mineralogical Composition
X-ray diffraction (XRD) was carried out to identify the mineralogical composition of the samples. When
radiation hits a solid whose atoms are arranged in an ordered fashion, x-rays are diffracted by the solid at
a fixed set of angles unique to the compound. By plotting the radiation intensities against angle (in
degrees 2θ), a series of peaks are obtained known as an XRD trace.
The relative intensity of a given peak is proportional to the quantity of compound present. The crystalline
phases in each powder sample can be detected and a quantitative analysis carried out using a computer
program, Xfit (Cheary and Coelho, 1996). Since the materials involved in this study (e.g. fly ash)
comprise a number of amorphous phases, which could not be directly quantified by Xfit, internal
standards were used, which allowed the amorphous component to be estimated. The internal standard
used was spectrometer grade corundum, which was included as a 5% (by mass) component in the sample
being analysed.
The powder sample, combined with 5% corundum, was ground in a mortar and pestle. A glass slide was
placed securely on one side of the XRD slide where the sample was compacted. A metal cap was placed
on the back of the slide, allowing one face to be exposed. The slide was loaded into the diffractometer and
the diffraction runs controlled and logged by PC. Samples were analysed over the range of 5-60 degrees
2θ at a scan rate of 1 degree/minute in 0.1 degree increments, using a Hilton Brooks x-ray diffractometer
(XRD) with monochromatic CuKα source and a curved graphite single crystal chrometer (30mA, 40kV).
Page 34 of 143
The peaks on each trace were automatically established and the identities of compounds determined using
the Xfit software (Cheary and Coelho, 1996). It should be noted that the glass/amorphous phase was
derived as 100% minus the total crystalline components and LOI, and therefore is an approximate
evaluation of this.
5.6. Morphology
Observations were made to examine the morphology of the samples using a Philips XL30 Scanning
Electron Microscope (SEM), operating at an accelerating voltage of 15 kV. Samples were mounted on
stubs using carbon adhesive tape to give a thin uniform layer, and coated with 25nm of Au/Pd, using a
Cressington 208 HR sputter coater, to provide good resolution for visual analysis under the microscope.
Selected samples were analysed in the view area using Energy Dispersive X-Ray Analysis (EDAX) to
quantify elements present.
6. MATERIAL SAMPLING AND CHARACTERISATION
The main purpose of examining material in storage areas at power stations was to determine property
variations within these areas, given that the fractions/materials obtained following processing will be
influenced by the feed. Hence, this work may allow regions in storage areas to be targeted for recovery,
while at the same time assist in identifying processing requirements.
Five different power station sites were included for material sampling and evaluation, which covered
lagoons and stockpiles as indicated in Table 4 and Appendix B. This section describes the sampling
details and reports the results of the characterisation tests.
Table 4 Summary of the fly ash sampling
Site
Ash Type
Power Station 1
Power Station 2
Power Station 3
Lagoon
Stockpile
Stockpile
Dewatered
Slurry
Lagoon
Power Station 4*
Power Station 5*
Number of
Samples**
Previous Sampling Data
from Dhir et al.,2005
6
1
45
33
6
6
-
6
-
* Sampling was carried out at Ash Disposal Site
** See Appendix B, Table B1 for details.
6.1. Power Station 1
There are two main Lagoons, HL 1 and HL 2 at Power Station 1, each with a storage capacity of 400,000
tonnes (Figure 26). A further offsite storage area, Lagoons A – E, is also in use. However, access and
material recovery from this area are not generally possible. (Dhir et al., 2005).
6.1.1.
Material sampling
At Power Station 1, initial plans were made, following a visual survey of the site, to sample material by
coring into a dried lagoon area. However, it was established that tests had been recently carried out in
some of the lagoons at the power station during 2006 by the fly ash producer and data from this were
available to the project. This coupled with the fact that tests in another lagoon had been carried out by the
Page 35 of 143
CTU, University of Dundee in June 2004 (Dhir et al., 2005), meant that a significant amount of
information was available, with regard to the properties of material there. A single large sample (1 tonne)
and 5 additional samples (150 kg each) were extracted from the lagoon previously surveyed in 2004,
(Figure 26) during May and August 2007.
The single large sample was excavated by the fly ash producer from the lagoon (Figure 27) and, therefore,
the exact position was not precisely known, and it can be considered a mixed sample with material from
different parts of the storage area. In taking the 5 additional samples, surface material which was 2.0 m
below the original surface level of the lagoon was removed by the digger and the sample taken at 2-3 m in
depth from this surface, as shown in Figure 28.
1
3
2
5 4
1
Sampled in 2004
1
Sampled in 2007
Figure 26 Sampling points at Power Station 1
(a) Material excavated for characterisation
(b) Lagoon behind excavated test area
Figure 27 Lagoon storage material at Power Station 1
Page 36 of 143
(a) Excavation of test material
(b) Location of sampling pit
Figure 28 Lagoon storage material at Power Station 1 (5 additional samples)
6.1.2.
Characterisation
The physical, chemical and mineralogical characteristics of material from recent sampling at Power
Station 1 are given in Tables 5, 6 and 7 respectively. The results are discussed and compared with those
of the previous surveys in the following sections.
Table 5 Fineness and LOI of samples taken from Power Station 1
Sample No.
1
2
3
4
5
6
(Large Sample)
Fineness, 45µm
sieve retention,
% by mass
56.4
31.8
55.5
52.8
PSD, m
LOI,
% by mass
D10
D50
D90
10.7
6.0
7.1
11.7
5.8
13.0
6.4
6.1
37.4
68.5
42.9
51.2
110.6
186.4
139.0
317.0
41.3
20.6
11.4
49.2
120.6
58.6
14.7
7.3
42.9
186.6
Table 6 Bulk oxide composition of samples taken from Power Station 1
Bulk Oxide Composition, % by mass
Sample No.
1
2
3
4
5
6
(Large Sample)
CaO
SiO2
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
2.3
3.5
2.7
2.3
48.1
47.0
49.5
47.7
32.3
30.6
31.7
32.4
4.5
7.7
5.2
4.9
1.7
2.1
2.0
1.9
1.4
1.3
1.5
1.4
1.0
0.7
1.2
0.9
0.3
0.6
0.5
0.6
1.1
0.8
0.7
0.9
0.3
1.7
1.0
1.3
0.3
1.0
1.1
1.2
6.2
40.6
23.2
6.9
2.1
1.6
1.2
0.5
1.4
1.3
0.7
4.3
45.7
26.1
5.8
2.8
1.5
1.4
0.5
1.5
1.0
0.3
Page 37 of 143
Table 7 Mineralogical composition of samples taken from Power Station 1
Major Phase Composition, % by mass
Sample No.
Quartz
Mullite
Glass
6.3
17.3
60.7
6
(Large Sample)
6.1.3.
Previous survey
A site survey was carried out in 2004 (Dhir et al., 2005) when a total of 33 samples were taken from the
lagoon shown in Figure 26. The sampling procedure was as follows:

11 locations were selected at different parts of the lagoon with intervals between these of
approximately 75 metres (See Figure 26).

In general, at each location, samples were taken at three depths which were between 1-2m, 2-3m and
3-4m. However, at Locations 1 and 2, four samples were taken over the depth of the lagoon and at
Locations 6 and 7, only two samples were taken at depths of 2-3m and 3-4m.
Details of the results of the characterisation were reported by Dhir et al., (2005) and are reproduced in
Appendix C. The result ranges for fineness, LOI, and chemical and mineralogical compositions are
summarised in Tables 8 - 10 respectively.
When the results of the current study (Tables 5 - 7) are compared with those of 2004 (Tables 8 - 10), it
can be seen that the properties were generally similar between tests at different periods. The mean 45µm
sieve retention of the samples from this study was 49.4%, which was higher than that in 2004. However,
the range from the recent tests, which was from 31.8% to 58.6%, was within that obtained in 2004.
Similar effects can be seen for the other properties, except CaO%, which was higher in two of the recent
samples. These two samples also had higher LOIs. The differences in chemical composition and LOI
could have resulted from changes in fuel source and burning conditions. XRD analysis from this study
also shows a similar glass content to the mean of the 2004 samples.
Table 8 Fineness and LOI range of the 33 samples taken from Power Station 1 in 2004
Max
Min
Mean
Fineness, 45µm
sieve retention,
% by mass
61.9
12.0
39.2
PSD, m
LOI,
% by mass
D10
D50
D90
20.2
3.8
8.0
14.3
3.2
6.2
100.5
13.4
43.7
521.5
54.1
152.0
Table 9 Bulk oxide composition range of the 33 samples taken from Power Station 1 in 2004
Max
Min
Mean
CaO
SiO2
3.7
2.0
2.7
52.5
40.0
47.5
Al2O3 Fe2O3 MgO
31.4
24.3
28.6
6.8
3.3
4.9
2.1
1.1
1.6
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
1.6
1.2
1.4
1.6
0.7
1.1
3.8
0.3
1.1
1.2
0.5
0.9
6.4
0.4
1.8
1.2
0.2
0.6
Page 38 of 143
Table 10 Mineralogical composition range of the 33 samples taken from Power Station 1 in 2004
Sample
Max
Min
Mean
6.1.4.
Quartz
Mullite
Glass
13.9
3.0
32.3
13.9
72.3
47.3
8.4
22.4
59.1
Report from power station
During 2005-2006, Power Station 1 carried out an investigation of the material in its lagoons in relation to
lagoon construction (Coombs et al., 2007). Information on the properties and tests on the lagoon material,
presented in the report, are summarised in the following sections.
LOI
Figure 29 summarises LOI data for fly ash from Power Station 1 between 1997 to June 2006 (Coombs et
al., 2007). This shows increased variability in LOI with time and the trend line suggests increasing values
were obtained more recently.
Lagoon Investigation
Sampling of the lagoons (five in total, referred to as A, B, C, D, and E) located several miles east of the
power station, was carried out in 2006 during which 8 samples were taken from the top of Lagoon A (LA,
representative of new material) as well as a sample from a nearby Lagoon B (LB, representative of an
older material). Ranges of the LOI and PSD results of the samples from LA and results of LB are
summarised in Table 11 (Coombs et al., 2007).
The LOI results for the samples from LA were higher than that from LB. Visual observations showed that
the material was darker in colour than it used to be. The LOI results, indicating the recent material has a
higher carbon content, suggest that this is not unexpected. The results show general agreement with the
LOI data given in Tables 5 and 8 and the record from the power station (Figure 29).
Range of CTU survey
(Dhir, et al, 2005)
Figure 29 Increase in LOI of fly ash with time 1997 to 2006 recorded by Power Station 1
(Coombs et al., 2007)
Page 39 of 143
Table 11 Test results of lagoon samples taken during the site investigation in 2006 (Coombs et al., 2007)
Sample
PSD, m
LOI, %
D10
D50
D90
Max LA
Min LA
Mean LA
16.9
11.1
14.1
5.1
2.5
3.7
60.0
20.3
38.1
323.2
69.4
170.4
LB
7.7
4.3
45.5
152.2
The PSD of the newer material in LA indicates that it was slightly finer than the older one in LB (and
those given in Tables 5 and 8) in terms of mean D 50. This may be due to the samples being taken from the
top of the lagoons, with the coarser particles usually tending to settle downwards to lower levels of these
storage areas.
The moisture content results indicate that, in general, the material of higher LOI had a higher moisture
content. However, the moisture contents are also affected by the weather and ground conditions and
therefore are not directly comparable between surveys / investigations.
In order to provide additional samples for comparison of material produced during different periods,
3 trial pits, one in each of Lagoons C, D and E, were excavated in 2006 (Coombs et al., 2007). The
results are given in Tables 12 and 13.
Table 12 Test results of lagoon samples taken from Power Station 1 in 2006 (Coombs et al., 2007)
Sample
PSD, m
LOI, %
Lagoon C
Lagoon D
Lagoon E
13.6
14.3
16.8
D10
D50
D90
2.6
4.0
2.2
33.0
33.0
22.0
144.0
150.0
95.0
Table 13 Bulk oxide composition of lagoon samples from Power Station 1 in 2006 (Coombs et al., 2007)
Sample
Lagoon C
Lagoon D
Lagoon E
CaO
SiO2
2.0
2.8
2.9
52.6
50.7
59.4
Al2O3 Fe2O3 MgO
33.3
32.1
23.2
6.6
7.1
6.0
-
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
1.5
1.5
1.1
1.3
1.3
2.1
0.2
0.4
1.4
-
-
0.2
0.4
0.4
In comparison with the chemical analysis results given in Tables 6 and 9, SiO2 contents of the samples
from Lagoons C, D, and E were slightly higher, especially the sample of Lagoon D, the most recently
generated material. However, CaO contents were comparable to those given in Table 6. The reason for
the slightly higher CaO content in recently produced material (Nos. 5 and 6 in Table 6) is unclear. All of
the other components from Lagoons C, D, and E samples were comparable with those given in Tables 6
and 9.
6.1.5.
Page 40 of 143
Summary of material properties from Power Station 1
From the history record, the 2004 survey (Dhir et al.,2005), and sampling carried out in 2006 (Coomb et
al., 2007) and 2007 (current study), the properties of material and its variability in the storage areas of
Power Station 1 have been established and can be summarised as follows:




LOI: ranged from 4% – 21%, mean about 8% tending to increase in more recently produced
materials;
Fineness:
o 45µm sieve retention, ranged from 12% – 62%, mean about 40%,
o D10, ranged from 3 – 14 m, mean about 7 m,
o Fineness varied with material locations in the lagoons;
Chemical composition:
o CaO, ranged from 2% – 6%, mean about 3%,
o SiO2, ranged from 40% – 60%, mean about 47%,
o Al2O3, ranged from 23% – 33%, mean about 28%,
Mineralogical composition:
o Amorphous material (Glass), ranged from 47% – 72%, mean about 59%,
o Mullite, ranged from 14% – 32%, mean about 22%,
o Quartz, ranged from 3% – 14%, mean about 8%.
The properties of the large sample were generally representative of the range of samples taken.
Morphological analysis was carried out by SEM and the results are compared with processed material and
reported in Section 9.
At Power Station 2, there is a 1 Mt stockpile (Figure 30). All material at the site is conditioned and has
been produced in the last few years and the properties are relatively uniform according to the fly ash
producer. Therefore, only a single large sample of about 3.5 tonnes was taken from the stockpile.
The physical and chemical properties of the sample taken from this stockpile are given in Tables 14
and 15 respectively. The mineralogical composition of the sample is given in Table 16. All properties
were generally similar to those found for material from Power Station 1. Since it was conditioned, more
cenospheres were found during the hydraulic classification, see Section 9.
(a) Sample location
(b) Aerial photograph (Google earth)
Figure 30 Stockpile site of Power Station 2
Page 41 of 143
Table 14 Fineness and LOI of the sample taken from Power Station 2
Sample
PSD, m
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
D10
D50
D90
53.4
10.0
12.1
55.3
164.7
Ash Large
Sample
Table 15 Bulk oxide composition of the sample taken from Power Station 2
Sample
Ash Large
Sample
CaO
SiO2
4.6
41.7
Al2O3 Fe2O3 MgO
25.9
9.0
1.6
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
1.2
2.1
0.9
0.6
0.0
2.0
Table 16 Mineralogical composition of the sample taken from Power Station 2
Sample
Ash Large
Sample
Quartz
Mullite
Glass
5.3
12.9
68.9
At Power Station 3, all of the material is conditioned and then stockpiled. The stockpile area, a large
mound, was about 50 hectares, and its height around 50 meters. Estimated quantities of conditioned fly
ash at the site were over 16 Mt (Dhir et al., 2005).
6.3.1.
Material sampling
The storage site is shown in Figure 31, and 15 locations were selected for sampling. In each location, 3
samples were taken from depths 1-2m, 4-5m and 9-10m.
The site can be divided into 3 main areas: Area 1 (Locations 6, 7, 8, 9, and 10) has had material deposited
there for about 5-10 years; Area 2 (Locations 3, 4, 5, 14, and 15) in recent years; and Area 3 (Locations 1,
2, 11, 12, and 13) for about 15-20 years. Two large samples of about 1 tonne each were taken at
Locations 1 and 2 for the processing part of the study. Two visits were made during August and
September 2007 and a total of 45 samples were taken from across the site and at depth, as indicated in
Appendix B.
6.3.2.
Characterisation
The results of fineness, LOI, chemical and mineralogical characteristics of the samples from Power
Station 3 are given in Tables 17-20 respectively.
It can be seen that the 45μm sieve retention of the samples varied from a minimum of 31.1% to a
maximum of 42.3%, and the mean was 36.7%, which is lower than that of Power Station 1 (mean 40.0 %)
and less variable. D50 values ranging from about 27 to 44 m, mean 28 m, compared to Power Station 1
(13 to 100 m, mean 40 m), confirmed that the material was finer and less variable than that of the
latter. No significant fineness variations were noted from samples from different locations and depths.
Page 42 of 143
The minimum LOI of the samples from Power Station 3 was 11.2% and the maximum 25.4%, with a
mean of 15.3%, which was slightly higher but with less variation than Power Station 1 (4% to 20%, mean
8%). It was also noted that all samples with LOI higher than 20% were from Locations 3 and 4, which
was the most recently produced material. Excluding samples in Locations 3 and 4, the LOI ranged from
11.2% to 17.7% with a mean of 14.3%.
(a) Area 1
(b) Area 2
9
10
8
6
7
5
4
3
14
15
13
12
1
2
(c) Area 3
11
(d) Aerial photograph (Google earth)
Figure 31 Stockpile site of Power Station 3 and sample locations
Bulk oxide compositions compared to those from Power Station 1 were found to be slightly lower in
CaO, SiO2, and Al2O3 content, probably due to the higher LOI. When eliminating the effect of LOI, there
was no significant difference between materials from Power Stations 1 and 3.
It was noted that samples from Locations 3 and 4 contained higher levels of chloride. Since the material
of Power Station 3 is conditioned with seawater, it was anticipated that higher chloride contents would be
found in the newly placed material. However, chloride appeared to be washed away quickly at this
stockpile site, and in the majority of samples, the chloride content was very low and acceptable for the
material's use in concrete construction.
The amorphous material (glass content) of the samples from Power Station 3 ranged from a minimum of
60.7% to a maximum of 72.6%, with a mean of 67.2%, which was higher than that of Power Station 1
(47.3% to 72.3%, mean 59.1%), suggesting that material from Power Station 3 may be more reactive. It
can also be seen that the corresponding quartz and mullite contents were lower in this case than those of
Power Station 1.
Page 43 of 143
Table 17 Fineness and LOI of samples from Power Station 3
Fineness, 45µm sieve retention,
% by mass
LOI, % by mass
Depth
Depth
Sample No.
top
middle
bottom
top
middle
bottom
6
7
8
9
10
3
4
5
14
15
1
2
11
12
13
40.4
36.0
34.5
15.3
17.2
13.4
32.3
36.0
33.2
14.2
12.6
11.3
Area
32.4
31.1
36.7
13.6
11.2
11.7
1
36.6
34.3
32.9
15.8
13.4
16.3
33.5
32.1
34.1
12.9
12.4
15.8
37.5
37.2
40.1
17.2
20.5*
21.5
40.7
42.3
40.8
23.5
25.3
25.4
Area
34.0
39.6
41.2
16.3
14.7
16.1
2
35.0
34.4
33.0
12.3
11.8
11.9
39.3
36.8
34.9
13.7
17.4
13.5
33.0
37.3
34.5
14.1
14.3
17.7
37.2
38.2
38.7
13.3
15.3
13.1
Area
37.3
38.2
40.2
14.8
17.3
13.5
3
39.7
38.7
36.6
15.4
17.2
12.3
40.8
41.2
37.2
12.5
13.4
15.5
Max
40.8
42.3
41.2
23.5
25.3
25.4
Min
32.3
31.1
32.9
12.3
11.2
11.3
Mean
36.6
36.9
36.6
15.0
15.6
15.3
* Bold numbers indicated extra high values and then will not be included in the statistical analysis.
Table 18 PSD values, d10, d50, and d90 of samples from Power Station 3
PSD values, d10, d50, and d90, µm
Depth
Sample No.
top
Area 1
Area 2
Area 3
Max
Min
Mean
6
7
8
9
10
3
4
5
14
15
1
2
11
12
13
middle
bottom
d10
d50
d90
d10
d50
d90
d10
d50
d90
4.6
5.9
4.7
4.9
5.7
4.9
4.0
4.3
3.9
6.5
5.3
5.5
4.3
5.0
5.3
6.5
3.9
5.0
35.4
36.6
37.0
36.9
38.5
34.9
35.0
40.5
30.8
43.6
38.5
39.1
27.4
33.7
32.5
43.6
27.4
36.0
125.0
120.4
147.9
138.6
134.6
124.9
136.9
208.5
121.1
154.4
148.7
139.9
102.6
119.3
113.7
208.5
102.6
135.8
4.2
5.7
6.6
4.5
4.8
5.2
4.1
4.3
4.4
5.6
4.0
4.3
3.7
5.7
4.9
6.6
3.7
4.8
33.9
37.5
37.1
32.7
33.6
35.4
32.6
34.8
31.4
36.7
35.5
30.4
27.5
32.9
32.2
37.5
27.5
33.6
125.7
136.1
119.4
120.0
116.8
125.4
121.3
142.0
117.7
154.8
124.6
123.5
96.5
108.8
112.7
154.8
96.5
123.0
6.1
5.1
5.6
5.0
5.1
4.6
4.2
4.9
5.1
7.1
4.8
4.0
4.7
4.7
4.1
7.1
4.0
5.0
41.6
34.8
33.4
35.0
36.7
39.5
35.4
34.8
33.9
36.4
34.8
32.5
31.9
32.5
29.6
41.6
29.6
34.9
131.6
122.3
121.5
126.1
145.6
154.0
135.7
125.6
112.7
107.7
120.4
124.4
103.8
110.9
115.3
154.0
103.8
123.8
Page 44 of 143
Table 19 Bulk oxide composition of samples from Power Station 3
Sample No.
1
2
3
4
5
6
7
8
9
10
11
12
13
CaO
SiO2
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
Top
Middle
Bottom
2.2
2.3
2.6
1.2
2.2
2.5
2.7
2.6
2.7
2.8
2.7
2.7
2.5
2.1
2.2
1.7
2.3
1.7
1.7
1.8
3.1
1.7
1.5
1.5
1.7
1.6
1.6
1.6
1.6
1.6
1.8
2.9
3.2
3.4
3.2
39.8
39.6
39.1
38.6
39.5
40.1
38.4
38.8
38.5
38.1
37.5
38.6
39.5
41.3
42.6
41.2
35.8
40.7
41.1
40.9
38.8
40.9
42.8
41.4
39.2
41.2
40.7
40.7
40.3
40.4
45.8
41.9
42.1
41.4
42.2
21.8
22.0
21.8
21.9
21.9
22.0
19.6
19.8
19.6
19.6
19.5
19.6
20.2
21.9
19.9
22.4
19.8
22.2
22.5
22.2
21.2
22.6
23.6
23.0
21.9
22.7
22.4
22.0
21.9
21.7
21.4
21.4
21.7
21.0
21.0
8.1
7.9
7.9
7.2
8.0
7.8
6.2
6.1
6.1
6.3
6.0
6.0
6.0
6.1
5.8
5.9
8.5
5.3
7.0
7.4
7.4
7.2
6.5
6.6
6.5
6.4
6.3
6.3
6.1
6.5
6.4
6.9
6.9
6.4
6.3
1.2
1.2
1.2
0.9
1.2
1.2
1.0
1.0
1.1
1.1
1.0
1.1
1.1
1.2
1.0
1.1
1.2
1.2
1.3
1.3
1.3
1.3
1.3
1.3
1.2
1.3
1.2
1.2
1.2
1.2
1.1
1.2
1.2
1.1
1.1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.1
1.1
1.0
1.1
1.0
1.1
1.1
1.1
1.0
1.1
1.1
1.1
1.0
1.0
1.0
1.1
1.1
1.1
1.7
1.2
1.2
1.3
1.4
3.0
3.1
3.1
2.8
2.9
3.1
2.1
2.1
2.3
2.3
2.2
2.3
2.4
2.9
1.9
2.8
2.8
2.9
2.9
2.9
2.8
2.9
3.1
3.0
2.8
3.0
2.9
2.8
2.8
2.8
1.8
2.5
2.6
2.3
2.2
0.5
0.5
0.6
0.5
0.5
0.5
1.2
1.1
1.0
2.0
1.6
1.0
0.6
0.8
0.6
0.5
0.5
0.9
0.5
0.5
0.5
0.6
0.6
0.5
0.5
0.6
0.5
0.6
0.5
0.5
0.4
0.7
0.5
0.5
0.4
0.5
0.5
0.5
0.7
0.6
0.5
0.6
0.6
0.7
0.6
0.6
0.7
0.6
0.7
0.6
0.8
0.5
0.8
0.7
0.7
0.6
0.7
0.6
0.6
0.7
0.7
0.7
0.7
0.7
0.7
0.6
0.6
0.5
0.6
0.6
0.0
0.0
0.0
0.0
0.0
0.0
1.3
1.1
0.9
3.0
2.0
0.8
0.3
0.3
0.1
0.0
0.0
0.9
0.1
0.0
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.1
0.0
0.0
0.1
0.4
0.0
0.0
0.0
0.8
0.6
1.1
0.3
0.8
1.1
1.1
0.9
1.1
1.0
1.1
1.0
1.5
0.8
0.7
0.3
0.7
0.3
0.3
0.2
1.8
0.2
0.2
0.2
0.3
0.3
0.3
0.2
0.3
0.2
0.2
0.6
0.6
1.0
0.9
3.5
42.1
20.7
6.2
1.1
1.3
2.3
0.4
0.6
0.0
0.6
Top
Middle
Bottom
3.1
2.3
1.7
40.9
43.5
44.1
20.7
20.2
20.2
6.6
6.3
5.9
1.2
0.8
0.7
1.3
1.6
1.7
2.3
1.8
1.7
0.5
0.4
0.3
0.6
0.6
0.6
0.0
0.0
0.0
1.1
1.6
0.9
Page 45 of 143
Table 19 Bulk oxide composition of samples from Power Station 3 (continued)
Sample No.
14
15
CaO
SiO2
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
Top
Middle
Bottom
Top
Middle
Bottom
1.6
1.6
2.2
1.9
1.9
39.5
41.5
39.8
38.8
39.6
22.2
23.1
22.3
21.9
21.9
7.5
6.7
7.1
8.0
7.7
1.2
1.4
1.4
1.4
1.4
1.1
1.1
1.1
1.0
1.0
2.9
2.9
2.8
2.9
2.8
0.5
0.6
0.5
0.5
0.5
0.7
0.6
0.6
0.6
0.6
0.1
0.1
0.0
0.0
0.0
0.3
0.2
0.7
0.2
0.4
2.9
40.2
22.6
7.3
1.5
1.0
2.9
0.5
0.7
0.0
0.8
Max
Min
Mean
3.5
1.2
45.8
35.8
23.6
19.5
8.5
5.3
1.5
0.7
1.7
1.0
3.1
1.7
2.0
0.3
0.8
0.5
3.0
0.0
1.8
0.2
2.2
40.4
21.5
6.7
1.2
1.1
2.6
0.6
0.6
0.3
0.7
Table 20 Mineralogical composition of selected samples from Power Station 3
Sample No.
2
3
4
5
6
7
8
11
11
6.3.3.
Quartz
Mullite
Glass
Middle
Bottom
Middle
Bottom
Top
Bottom
Top
Middle
Bottom
2.8
2.5
2.5
3.3
4.4
3.1
3.1
4.7
6.8
7.7
5.0
6.1
6.3
5.3
7.9
7.7
72.6
64.1
60.7
68.5
65.6
72.1
69.0
62.5
2.4
5.6
70.1
Max
Min
Mean
4.7
2.4
7.9
5.0
72.6
60.7
3.2
6.5
67.2
Previous survey
Six samples were obtained from Power Station 3 in 2004 (Dhir et al., 2005) from different parts of the
stockpile, where they had been stored from 4 to more than 20 years, see Table 21. Results of fineness,
LOI, and chemical and mineralogical compositions are given in Tables 22-24 respectively.
The 45μm sieve retention of the samples varied from a minimum of 30.4% to a maximum of 40.5%,
which is similar to those of the recent sampling (31.1% to 42.3%). D50 values ranged from about 20 to
40 m compared to recent sampling (27 to 44 m).
The minimum LOI was 12.2% and the maximum 17.2%, with a mean of 14.8%, which was also similar
to those of the recent sampling (11.2% to 17.7%, mean 14.3%, excluding samples in Locations 3 and 4).
Page 46 of 143
There were also no significant differences in the bulk oxide and mineralogical compositions between the
two sample sets, suggesting that the samples are typical of those produced at Power Station 3.
Table 21 Samples from Power Station 3 during 2004 testing (Dhir et al.,2005)
Sample No.
1
2
3
4
5
6
Location
Sample Age
Top East Face
North End Group 5
Old Haul RD
Opposite A- Tip
Group 5 Hawl RD
Silo/East Face
7-10 years
4-5 years
15-20 years
10 years
4-5 years
20 years Plus
Table 22 Density, fineness and LOI of samples from Power Station 3 during 2004 testing
(Dhir et al., 2005)
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
1
2
3
4
5
6
30.4
33.1
40.5
39.9
33.8
39.3
Max
Min
Mean
40.5
30.4
36.2
Sample No.
PSD, m
D10
D50
D90
15.2
17.2
12.2
16.8
15.1
12.3
2.5
2.5
3.3
4.5
3.2
20.7
20.4
25.6
39.4
26.6
113.0
104.7
119.4
247.1
115.6
2.6
19.5
102.1
17.2
12.2
14.8
4.5
2.5
39.4
19.5
247.1
102.1
3.1
25.3
133.6
Table 23 Bulk oxide composition of samples from Power Station 3 during 2004 testing
(Dhir et al., 2005)
Sample
No.
CaO
SiO2
Al2O3 Fe2O3 MgO
1
2
3
4
5
6
2.2
1.9
2.0
1.8
2.4
2.1
45.0
48.0
46.9
48.3
49.8
43.8
24.4
25.2
25.5
25.4
23.9
24.2
7.9
6.5
7.5
6.5
5.7
7.4
Max
Min
Mean
2.4
1.8
2.1
49.8
43.8
46.9
25.5
23.9
24.8
7.9
5.7
6.9
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
1.3
1.2
1.4
1.2
1.2
1.6
1.1
1.2
1.1
1.2
1.4
1.1
3.3
2.8
3.3
2.8
2.1
3.4
0.6
0.5
0.7
0.5
0.5
0.8
0.6
0.6
0.7
0.7
0.8
0.6
0.0
0.0
0.0
0.0
0.0
0.0
0.6
0.7
0.5
0.3
0.6
0.4
1.6
1.2
1.3
1.4
1.1
1.2
3.4
2.1
3.0
0.8
0.5
0.6
0.8
0.6
0.7
0.0
0.0
0.0
0.7
0.3
0.5
Page 47 of 143
Table 24 Major mineralogical composition of samples from Power Station 3 in 2004 (Dhir et al.,2005)
Sample
No.
6.3.4.
Quartz
Mullite
1
2
3
4
5
6
3.2
5.2
3.6
3.2
5.6
2.4
6.3
8.2
9.5
8.5
7.9
6.6
Max
Min
Mean
5.6
2.4
3.9
9.5
6.3
7.8
Glass
72.6
67.0
72.0
68.5
69.4
76.2
76.2
67.0
71.0
Summary of material properties from Power Station 3
From the 2004 survey and recent sampling, the properties and variation of the material at Power Station 3
have been established, and can be summarised as follows:




LOI: ranged from 11% – 18%, mean about 14%, tending to increase in more recently produced
material, for which the LOI could be as high as 25 %, which is a rare value;
Fineness:
o D90, ranged from 97 – 155 m, mean about 123 m, however occasionally this was as
high as 210 m,
o Fineness variation with location was lower than the lagoon material from Power
Station 1;
Chemical composition:
o High chloride content was occasionally found in newly placed material (>1.0%).
However, in the majority of samples, the chloride content was very low (0.0% – 0.4%),
Mineralogical composition:
o Amorphous material (Glass), ranged from 61% – 76%, mean about 67%, higher than at
Power Station 1,
For the two large samples from Locations 2 and 3, Location 2 was representative of longer-term stored
material, while Location 3 represented that more recently produced.
Morphological analysis was carried out with SEM and the results are compared with processed materials
and presented in Section 9.
Page 48 of 143
6.4. Power Stations 4 and 5
Material from Power Stations 4 and 5 is deposited at the same storage site, which is slurried and pumped
to the storage site, however, that from Power Station 4 is dewatered and then stockpiled, while that from
Power Station 5 is stored in Lagoons. It is estimated that about 25 Mt of material is stored at this site.
Six samples from each power station were taken for characterisation and use in the processing work
(release analysis only). Results of fineness, LOI, chemical and mineralogical compositions of the
samples from Power Stations 4 and 5 are given in Tables 25 - 28 respectively.
It can be seen that the 45μm sieve retention of the samples from Power Station 4 varied from a minimum
of 24.4% to a maximum of 54.6%, and the mean was 41.0%. The samples from Power Station 5 varied
from a minimum of 28.0% to a maximum of 45.8%, and the mean was 33.8%, which is lower than that of
Power Station 4 and less variable.
D50 values of the samples from Power Station 4 ranged from about 29.1 to 37.5 m, mean 33.9 m, while
those from Power Station 5 ranged from about 28.2 to 34.8 m, mean 32.3 m.
The minimum LOI of the sample from Power Station 4 was 15.7% and the maximum 16.7%, with a mean
of 16.2%, while that from Power Station 5 was 13.0% and the maximum 15.5%, with a mean of 14.0%,
which is slightly lower than that of Power Station 4.
No significant difference was found in bulk oxide compositions for samples between Power Stations 4
and 5.
The amorphous material (glass content) of the samples from Power Station 4 and 5 ranged from 56.3% to
65.6%, and 58.3% to 66.0% respectively, which is within the range of Power Station 1 (47.3% to 72.3%,
mean 59.1%).
Table 25 Fineness and LOI of the samples from Power Stations 4 and 5
Power Station 4
Sample No.
1
2
3
Fineness,
45µm sieve
retention,
% by mass
42.5
54.6
32.4
Power Station 5
15.8
15.8
15.7
Fineness,
45µm sieve
retention,
% by mass
28.6
28.0
45.8
LOI,
% by mass
LOI,
% by mass
13.0
13.1
14.3
4
5
6
24.4
16.7
28.5
14.2
48.5
16.7
41.8
14.1
43.8
16.2
30.0
15.5
Max
54.6
16.7
45.8
15.5
Min
24.4
15.7
28.0
13.0
Mean
41.0
16.2
33.8
14.0
Page 49 of 143
Table 26 PSD of samples from Power Stations 4 and 5
Sample No.
Power Station 4
Power Station 5
PSD, m
PSD, m
D10
D50
D90
D10
D50
D90
1
2
3
4
5
6
2.9
4.5
2.7
3.3
3.7
32.8
33.7
34.7
35.7
37.5
158.4
118.8
273.0
143.6
147.1
2.0
3.6
3.5
4.3
4.3
31.2
33.6
31.4
34.7
34.8
122.5
135.4
121.4
132.9
133.7
4.0
29.1
147.1
3.4
28.2
115.0
Max
4.5
37.5
273.0
4.3
34.8
135.4
Min
2.7
29.1
118.8
2.0
28.2
115.0
Mean
3.5
33.9
164.6
3.5
32.3
126.8
Table 27 Bulk oxide composition of samples from Power Station 4 and 5
Sample No.
CaO
SiO2
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
5.4
5.5
6.9
5.4
5.2
44.5
42.0
38.1
41.8
43.2
19.3
19.8
20.8
23.3
20.5
5.2
5.9
6.8
4.7
6.6
1.6
1.9
1.4
1.6
1.8
1.2
1.5
1.7
1.8
1.5
2.3
1.9
1.2
1.1
1.9
0.6
0.4
0.4
0.3
0.4
1.0
1.1
1.2
1.6
1.1
0.0
0.0
0.0
0.0
0.0
1.1
1.4
2.5
0.4
0.8
6.6
39.4
21.3
6.5
1.7
1.7
1.2
0.3
1.2
0.0
1.8
Max
6.9
44.5
23.3
6.8
1.9
1.8
2.3
0.6
1.6
0.0
2.5
Min
5.2
38.1
19.3
4.7
1.4
1.2
1.1
0.3
1.0
0.0
0.4
Mean
5.8
41.5
20.8
5.9
1.7
1.6
1.6
0.4
1.2
0.0
1.3
5.2
5.7
5.2
5.6
5.5
44.6
43.8
45.2
42.9
40.7
19.4
18.9
19.8
20.9
22.5
5.2
7.3
6.9
5.7
6.0
1.5
1.8
1.9
1.7
1.9
1.2
1.3
1.3
1.6
2.2
2.3
1.9
2.0
1.8
1.9
0.6
0.7
0.7
0.4
0.4
1.0
0.8
0.9
1.0
1.1
0.0
0.0
0.1
0.0
0.1
0.9
1.4
0.7
1.1
0.9
6.1
37.1
20.2
5.4
1.4
1.8
1.1
0.3
1.3
0.0
2.6
Max
6.1
45.2
22.5
7.3
1.9
2.2
2.3
0.7
1.3
0.1
2.6
Min
5.2
37.1
18.9
5.2
1.4
1.2
1.1
0.3
0.8
0.0
0.7
Mean
5.6
42.4
20.3
6.1
1.7
1.6
1.8
0.5
1.0
0.0
1.3
Power
Station
4
Power
Station
5
1
2
3
4
5
6
1
2
3
4
5
6
Al2O3 Fe2O3 MgO
Page 50 of 143
Table 28 Mineralogical composition of samples from Power Stations 4 and 5
Power Station 4
Major Phase Composition,
% by mass
Sample No.
Power Station 5
Major Phase Composition,
% by mass
Quartz
Mullite
Glass
Quartz
Mullite
Glass
1
2
3
4
5
6
8.7
6.6
8.7
7.8
7.7
16.4
10.2
16.1
9.6
12.1
56.3
65.6
56.5
63.9
61.2
8.0
7.7
8.2
6.8
6.5
11.0
10.3
11.0
13.1
12.3
64.3
66.0
62.8
63.5
64.4
7.7
12.0
61.3
7.0
16.2
58.3
Max
8.7
16.4
65.6
8.2
16.2
66.0
Min
6.6
9.6
56.3
6.5
10.3
58.3
Mean
7.9
12.7
60.8
7.4
12.3
63.2
6.5. Summary
Five power station sites have been investigated and samples from these characterised. The sites covered
lagoons and stockpiles. Extensive surveys were carried out for Power Station 1 and 3, where material
was stored in a lagoon and stockpile. The material characteristics from the two sites are compared in
Table 29.
Table 29 Comparison of material properties between Power Stations 1 and 3
Property
Power Station 1
Power Station 3
Min
Max
Mean
Min
Max
Mean
4
21
8
11
25
14
12
62
40
31
42
37
3
13
50
14
100
500
7
44
150
4
27
100
7
44
210
5
35
120
6
60
33
3
47
28
1
36
19
3
46
24
2
40
21
47
72
59
61
76
67
14
3
32
14
22
8
5
2
10
6
7
4
Physical Properties
LOI, %
45µm sieve
retention, %
D10, m
D50, m
D90, m
Chemical compositions
CaO, %
SiO2, %
Al2O3, %
2
40
23
Mineralogical compositions
Amorphous
material, %
Mullite,
Quartz,
Page 51 of 143
The properties of the materials from the three other sites lie between these. In general, lagoon material
has a wider variation in properties than that of stockpile. The majority of lagoon and stockpile materials
have relatively high LOI and are coarse. This means that most of the material does not conform to
standards for use as a cementitious material (i.e. high value application) in concrete. Processing to
separate material into fractions suitable for the full range of possible uses is therefore necessary.
7. OVERVIEW OF PROCESSING SYSTEM
The specific aims of processing recovered coal combustion residues are to separate particles into different
fineness fractions, relevant to particular applications and to remove or reduce levels of carbon (measured
as LOI), such that the recovered material is suitable for various end uses.
Based on the findings of an earlier feasibility study (Dhir et al., 2005), and the literature review in
Section 2, the most appropriate route to meeting the requirements in terms of economics and least impact
on the environment is by wet processing. This section introduces the processing system developed for the
project.
This was based on hydraulic classification and froth flotation technology and was developed in
collaboration with research colleagues from the Centre for Applied Energy Research at the University of
Kentucky. The system includes the following components and the general process is shown in Figure 32.





Pre-screening and slurrying;
Primary Classification;
Froth flotation (column flotation (Figure 32a) or mechanical flotation (Figure 32b));
Lamella hydraulic classification; and
Product collection.
7.1. Pre-Screening and Slurrying
The equipment used for this process was a feed tank, a coarse screen and a slurry tank with mixers
(Figure 33). The material was introduced to the feed tank and mixed with water to produce a slurry with a
solids content of about 10% to 15%.
Pre-screening was applied to remove extraneous material (coarse ash, vegetation, and miscellaneous
debris) greater than 5 mm that may cause plugging problems during the following processing operations.
The material was then thoroughly mixed in the slurry supply tank.
PrePre-Screening
Page 52 of 143
Primary
Classification
Column Froth
Flotation
Lamella Hydraulic
Classification Cenospheres
Froth
Feed
Overflow
Overflow
Clean
Water
UF
(U5)
Air
U1
U2
U3
U4
Tails
4 Underflows
Slurrying
Underflows
(a) Processing system with column flotation
PrePre-Screening
Primary
Classification
Feed
Mechanical Froth
Flotation
Lamella Hydraulic
Classification Cenospheres
Froth
Overflow
Overflow
UF
(U5)
Clean
Water
Tails
Slurrying
U1
U2
U3
4 Underflows
Underflows
(b) Processing system with mechanical flotation
Figure 32 General process flow sheet
U4
Page 53 of 143
(a) Coarse screen, mixer and feed tank
Figure 33
(b) Slurry supply tank with mixers
Pre-screening and slurrying set-up
A DC Stirrer was used to initially disperse the material in water and then the slurried material was
pumped to the 800 litre slurry tank through the coarse screen using an immersible pump. Two high
torque/low speed 1 HP AC motors were used to run the agitators, each comprising two cast aluminium
propellers, one large (250 mm) at the end of the shaft and one small (100 mm) at its centre. The agitators
ran continuously during processing to keep the slurry well mixed and stable.
A digital variable speed feed pump with a capacity up to 17 l/min was used to feed the slurry into the
following processing equipment as shown in Figure 34. During processing, the feed rate from the slurry
tank was controlled in the range 1.5 to 6.0 l/min.
Figure 34
Digital variable speed feed pump (Capacity: 17 l/min)
7.2. Primary Classification
Primary classification was used to remove coarse (i.e. typically > 150 m) particles. It can also
significantly reduce the carbon content, which was beneficial for the following froth flotation process.
Page 54 of 143
The equipment used in primary classification is shown in Figure 35. Some modifications were made to
this during the evaluation of the processing system to improve its efficiency and reduce fine particle
settlement as shown in Figure 35c. During primary classification, the prepared slurry was fed into the
column at about two-thirds of its height. The finer particles were collected from the overflow while the
coarser particles settled and were collected from the underflow. Clean water was used to reduce finer
particle settlement into the underflow. The cut point of the particles (i.e. size at which particles remain in
the overflow) was controlled by adjusting the flow rates of feed, underflow and clean water.
Primary
Classification
Overflow
2.8 – 4.3 l/min
Primary
Screening
Feed
3.0 – 4.5 l/min
Clean
Water
0.0 – 0.5 l/min
Slurrying
Underflow
10 – 15 % by mass
0.5 – 0.7 l/min
(a) Primary classification;
(b) Schematic of primary classification
Overflow
2.8 – 4.3 l/min
Feed
3.0 – 4.5
l/min
Feed
Overflow
1.5 – 3.0
l/min
1.0 – 2.5 l/min
3.0 m
4.5 m
Clean
Water
0.0 – 0.5 l/min
Underflow
Underflow
0.5 l/min
0.5 – 0.7 l/min
(c) Modification during evaluation of the processing system
Figure 35 Equipment for primary classification
7.3. Froth Flotation
This process is used to remove carbon in material. Two types of froth flotation equipment were used in
the project, namely column flotation and mechanical flotation.
7.3.1.
Page 55 of 143
Column flotation
Column flotation was first developed in the early 1960s by Boutin and Tremblay (Canadian patents
680,576 and 694,547). As shown in Figure 36, bubble generation is achieved directly through an internal
sparger made from a perforated rubber pipe. Feed slurry enters about two-thirds of the column height and
descends against a rising swarm of bubbles generated by the sparger. The bubbles collect the floatable
particles (i.e. carbon) in the collection zone (Figure 36b). The collected particles are transferred into the
froth zone and removed as concentrate. The lower carbon material is collected as tails.
The prime role of wash water is to clean the froth by washing back the tail particles entrained in bubbles
from the collection zone. Hence the froth zone is also called the cleaning zone. Since the main purpose
for this project was to collect the lower carbon material (i.e. tails), wash water was not added into the
froth in the current work, but may be required when higher grade froth (carbon) is the target product.
(a) Flotation column
(b) Schematic of flotation column
Figure 36 Column flotation set-up
During trial processing, the slurry feed rate was controlled between 1.5 to 3.0 l/min and the air feed rate
between 2.5 to 5.0 l/min. An appropriate dosage of reagents (collector and frother) was added to the feed
slurry to produce a stable froth. The role of the collector is to selectively adsorb onto carbon particle
surfaces to induce hydrophobicity, while the frother is used to reduce the surface tension at the air/liquid
interface to produce significant quantities of small bubbles (i.e. large bubble surface area). The reagent
was pre-prepared with a blender and injected into the slurry feed pipe with a variable speed feed pump.
The yield ratio of the froth and tails was controlled by adjusting the hydraulic head of the column, or
using a peristaltic pump to control the outlet rate of the tails.
The LOI of the froth and tails was affected by the yield ratio (i.e. froth/tails), which can be estimated by
release analysis of the samples, as mentioned in Appendix D. During trials, the highest LOI of the froth
obtained was about 40%.
Initial work with the system was found to be effective in removing finer carbon, but did not give
significant reduction in LOI. Therefore, the focus moved to mechanical flotation, which was considered
to enable the removal of coarse carbon.
7.3.2.
Page 56 of 143
Mechanical flotation
Mechanical flotation has been commercially practiced for many years in mineral separation. The
mechanism is relatively simple; a slurry is mechanically agitated in a cell with an impeller. It was
considered that the high shearing force of this would be beneficial in separating carbon from fly ash
particles and hence in the achievement of low carbon tails. The bubbles, generated by self-aeration from
the impeller are distributed to the slurry through the diffuser. These lead to bubble-particle collisions.
When hydrophobic particles collide with bubbles, the water film around the hydrophobic particle thins
and a stable bubble-particle aggregate is formed. The buoyant bubble-particle aggregate rises to the cell
surface where coalescence occurs with other mineral-laden bubbles and a stable froth is formed (typically
50-100 mm deep). Water and some hydrophilic particles entrained in the froth drain back into the slurry
before the froth is removed from the cell by scrapers. The lower carbon content slurry is fed into the next
cell or collected as tails through a weir. Details of the system used in the current study (bank of two cells)
are shown in Figure 37.
In general, column flotation is specifically designed for the recovery of very fine-sized minerals and for
obtaining a higher grade froth product than conventional cells, mostly due to the deeper froth zone. To
achieve this, a significant amount of hydrophobic material in the raw slurry is required.
(a) Mechanical flotation equipment
(b) Schematic of mechanical flotation
Figure 37 Mechanical froth flotation equipment (Sepor Inc.)
Groppo (2007) indicates that for fly ash, there may be insufficient carbon to form a stable froth using
column flotation. Although the carbon is primarily fine, there is not enough. Where columns would be
applicable, would be to clean the froth from rougher mechanical flotation, to make a high grade froth
product. As release analyses (See Appendix D) for the materials being used in the study indicate,
flotation can be effective in reducing the LOI to the desired levels. Therefore, by using mechanical cells,
a close result to that of the release analysis can be achieved at good yield.
7.4. Lamella Hydraulic Classification
A cross flow hydraulic classifier with lamella plates was used to separate the recovered material into
different size fractions (Figures 38 and 39). There were five collected fractions from this classifier, i.e. (i)
coarse fraction, U1; (ii) medium fraction, U2; (iii) fine fraction, U3 and U4 (iv) ultrafine fraction, U5 and
(v) cenospheres.
Page 57 of 143
Figure 38 Lamella hydraulic classifier
U1
U2
U4
U3
U5
Figure 39 Lamella hydraulic classifier and product collection tanks for U1 to U5
The operation principle of the lamella hydraulic classifier is shown in Figure 40. The retention time of
the particles was controlled by the feed rate and length of the cell. The design settling distance of the
classifier, DL, is the vertical distance between lamella plates. If the particle settling distance during its
retention in the cell is larger than DL, it will settle on the lamella plate and slide down to the hopper
below. If the particle settling distance is smaller than DL, it will flow to the next cell. The ultra fine
particles will not have enough time to settle on the lamella plates and are then retained in the overflow
slurry (U5).
7.5. Material Collection and De-watering
Processed material was collected in different barrels as shown in Figure 39. For an increased scale of
production, a large tank with a capacity up to 2000 litres was used to collect ultra fine particles U5
(Figure 41). After the particles had settled in the tanks, the water was drained off and the products oven
dried, as required, before use in other stages of the study.
Page 58 of 143
er
es
If the particle is small
it goes to overflow
h
sp
o
C
en
DL
se
sh
A
r
oa
C
If the particle is
large, it goes to
underflow
Figure 40 Operation principle of the lamella hydraulic classifier (Robl, 2007)
Figure 41
Ultrafine U5 collection tank (Capacity = 2000 litres)
8. FLY ASH PROCESSING
Following the setting up of the pilot-scale plant, each component of the system was evaluated. This
included tests to examine its ability to handle material variations, using a number of samples from
different test sites.
As part of this, both feed and processed materials were tested for physical (fineness, PSD, and LOI),
chemical and morphological properties, to examine the efficiency of processing and establish optimum
running conditions.
Three raw materials were used in the processing study and their main physical and chemical properties
are given in Tables 30 and 31. As indicated, the materials from Power Stations 1, 2 and 3 did not meet
the LOI (< 7%) and fineness (<40 %) requirements of the UK standard (BS 8500) limits for use as a
cement component in concrete.
The control parameter ranges used in the processing trials are given in Table 32.
Page 59 of 143
Table 30 Properties of the three raw materials used in the processing study
PSD, m
Fineness,
45µm Sieve
Retention,
% by mass
LOI,
% by mass
D10
D50
D90
Power Station 1
58.6
14.7
7.3
42.9
186.6
Power Station 2
53.4
10.0
12.1
55.3
164.7
Power Station 3
37.9
15.0
4.5
32.7
116.5
Sample
Table 31 Bulk oxide composition of the raw materials used in the trials
Sample
CaO
SiO2 Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
Power Station 1
4.3
45.7
26.1
5.8
2.8
1.5
1.4
0.5
1.5
1.0
0.3
Power Station 2
4.4
42.4
26.3
8.8
1.6
1.2
2.2
0.9
0.6
0.0
1.3
Power Station 3
2.6
39.5
21.8
8.5
1.3
1.0
3.0
0.5
0.5
0.0
1.1
Table 32 Basic control parameters for the processing trials
Slurry Feed
Concentration,
% Solids by
mass
Primary
classification
Feed
Rate,
l/min
1.5-3.0
Additive
Feed,
g/kg solid
Output Rate, l/min
Overflow
Underflow
Clean
water
1.0-2.5
0.5
-
2.8-4.3*
0.5-0.7*
0-0.5*
-
10-15
3.0-4.5*
-
1.5-3.0
(Air:
2.5-5.0)
Collector:
2.0
Froth**
Tails**
10-30% by mass
70-90% by mass
10-30% by mass
70-90% by mass
Column froth
flotation
10-15
Mechanical
froth flotation
10-15
3.0-4.5
Frother: 1.0
Lamella
hydraulic
classification
10-15
6.0
Dispersant:
1.5-2.5
U1
U2
U3
U4
U5
0.6
0.3
0.3
0.3
4.5
* These parameters were applied after the column height was modified to 4.5 metres.
** The froth/tails ratio is determined from release analyses of the materials, See Appendix D.
Page 60 of 143
8.1. Material from Power Station 1
Two separate processing trials were carried out with Power Station 1 material: (i) primary classification +
column froth flotation and (ii) lamella hydraulic classification, as shown in Figure 42. Since the
processed fractions from classification remained at a relatively high LOI, carbon burnout at 600C was
also applied to examine this as a means of carbon removal.
The physical properties, and the chemical and mineral compositions of samples from the different
processing stages are given in Tables 33, 34 and 35 respectively.
The purpose of primary classification was to remove particles > 150 m. It can be seen that the
processing effectively removed the particles > 150 m and the overflow only contained 1% of these and
99% of the particles would pass a 150 m sieve (Table 33). The LOI of the overflow was 13.2%, reduced
by about 1.5% compared to the raw feed material. The reject product, underflow of the classification had
an increased LOI of 19.2% and contained 18% of > 150 m particles. This means significant levels of
fine material were lost during primary classification.
Initial Processing of Lagoon Material (Power Station 1)
Trial 1
Primary
Classification
Underflow
Overflow
Tails
LOI, %
Fineness, %
Raw-b
1.3
23.5
U1-b
2.2
54.7
U2-b
1.7
14.2
U3-b
0.9
14.1
U4-b
1.5
9.2
U5-b
2.0
5.5
Lamella Classification
19.2
N/A
13.2
25.5
Column Froth
Flotation
Froth
Trial 2
14.7
Raw
58.6
U1
U2
U3
19.3
9.3
U4
10.0
13.7
U5
19.9
69.1
11.5
38.9
10.4
17.8
600 C Burnout
Raw
14.7
58.6
9.9
14.8
9.1
6.4
Figure 42 Initial processing of lagoon material (Power Station 1)
To reduce the fine particle content settling into the underflow, the column was modified and clean water
introduced as shown in Figure 35. The reverse flow of the clean water reduced the settlement of fine
particles (with coarser particles) to the underflow. The additional column height increased the
productivity of the classification by about 50%. This modification was used in later processing work on
materials from Power Stations 2 and 3, and the results are covered in subsequent sections.
In column froth flotation, the LOI of the tails was 10.0%, reduced by about 3.2% compared to the feed
material, which was the overflow of the primary classification (Table 33). The LOI of the froth increased
to 19.3%, similar to the underflow of the primary classification. The difference was that the froth was
much finer than the underflow and was even finer than the tails. Figure 43 shows the PSD curves of the
froth and tails. This indicates that during froth flotation, a portion of fine particles was lost.
Page 61 of 143
Classification with the lamella hydraulic classifier was also carried out (separately from the primary
classification and froth flotation). Raw material was directly slurried and fed into the classifier (following
primary screening). Five different fineness fractions, U1 to U5, were obtained from this process.
Figure 44 shows the PSD curves of the feed and output fractions, which indicate progressively finer
material was produced from U1 to U5.
The LOI of U1 was the highest, 19.9% and was greater than that of the feed, 14.7% and similar to that of
the primary classification underflow and froth of the flotation (Table 33). The LOI of the U2 to U5
fractions gradually decreased from 11.5% to 9.1%, with all less than the feed.
Since the LOI of both froth flotation and classification products was still relatively high, with the lowest
9.1%, still above the BS 8500 UK standard limit, burnout at 600C was applied to the lamella hydraulic
classification fractions. After burnout, LOI was reduced to about 1% to 2%, and their mean particle sizes
were also reduced significantly, especially for coarser materials (Table 33).
Table 33 Fineness and LOI of samples from different processing stages
PSD, m
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
D10
D50
D90
58.6
14.7
7.3
42.9
186.6
Underflow
N/A
19.2
<150m = 82%, >150m = 18%*
Overflow
25.5
13.2
<150m = 99%, >150m = 1%*
Froth
9.3
19.3
2.7
12.7
66.5
Tails
13.7
10.0
4.2
20.0
72.9
U1
U2
U3
U4
69.1
38.9
17.8
14.8
19.9
11.5
10.4
9.9
U5
6.4
9.1
8.2
4.1
3.9
3.5
1.8
71.0
23.6
18.4
15.8
7.7
208.2
89.8
66.2
55.9
23.3
Raw-b
23.5
1.3
3.7
18.5
77.0
U1b
U2b
U3b
U4b
U5b
54.7
14.2
14.1
9.2
5.5
2.2
1.7
0.9
1.5
2.0
5.7
3.4
3.3
2.7
1.5
46.1
17.2
16.1
13.3
7.5
134.2
61.6
53.1
40.4
21.7
Sample
Feed (Raw)
Primary classification
Froth Flotation
Classification
600C Burnout
* PSD data not available, the data given here were obtained from sieve tests.
Page 62 of 143
100
Column Froth Flotation:
Power Station 1 Fly Ash
Accumulative Volume, %
80
Froth
60
Tails
40
20
0
0.1
1
10
100
1000
10000
Particle diameter, m
Figure 43 PSD curves of the froth and tails
100
Lamella Classification:
80
U4
U5
U3
U2
60
Feed
40
U1
20
0
0.1
1
10
100
1000
10000
Figure 44 PSD curves of feed and output fractions from lamella hydraulic classification
The chemical compositions of raw feed and processed fractions are given in Table 34. It can be seen that
there were no significant changes in chemical compositions after flotation and classification.
Mineralogical compositions of raw feed and processed fractions are given in Table 35. It can be seen that
after processing, the amorphous phase (glass content) of the tails and U3 to U5 fractions increased, while
that of the froth and U1 decreased. After the 600°C burnout, a slight increase in glass content was also
observed in comparison with the corresponding unburnt fractions.
SEM micrographs for raw and classified fractions are shown in Figure 45, and their corresponding 600C
burnout materials are shown in Figure 46. It can be seen that the particle size and shape changed for the
different processed materials. Further consideration of the SEM results is given in subsequent sections.
Page 63 of 143
Table 34 Bulk oxide composition of samples from different processing stages
Sample
CaO
SiO2
4.3
45.7
26.1
5.8
4.6
4.4
39.4
42.6
21.7
23.7
4.4
4.7
4.2
4.1
4.1
43.6
48.2
45.7
45.6
45.4
Raw-b
5.3
U1b
U2b
U3b
U4b
U5b
6.0
5.4
5.3
5.0
5.1
Feed (Raw)
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
2.8
1.5
1.4
0.5
1.5
1.0
0.3
5.9
4.9
2.7
2.7
1.4
1.4
1.4
1.4
0.5
0.4
1.5
1.3
0.0
0.0
0.5
0.5
27.8
22.4
26.6
26.7
27.1
4.7
8.1
5.4
5.5
5.2
2.8
2.7
2.7
2.7
2.7
1.5
1.3
1.5
1.5
1.6
1.6
1.2
1.4
1.4
1.4
0.5
0.5
0.5
0.5
0.5
1.9
1.2
1.4
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.3
0.3
0.3
0.3
0.3
44.1
25.2
5.1
3.0
1.4
1.3
0.5
1.3
0.7
0.6
43.5
43.2
43.5
43.9
43.9
23.2
24.1
24.4
24.8
25.4
6.1
5.0
5.0
4.8
4.5
3.1
3.1
3.0
3.1
3.3
1.3
1.4
1.5
1.5
1.5
1.2
1.3
1.3
1.4
1.5
0.4
0.4
0.4
0.4
0.5
1.0
1.2
1.2
1.3
1.5
0.2
0.1
0.1
0.1
0.2
0.7
0.7
0.6
0.4
0.5
Froth Flotation
Froth
Tails
Classification
U1
U2
U3
U4
U5
600C Burnout
Table 35 Mineralogical composition of samples from different processing stages
Sample
Quartz
Mullite
Glass
6.3
17.3
60.7
6.0
4.2
14.5
13.5
58.8
71.5
11.7
8.3
4.7
14.8
13.5
11.9
52.3
68.2
73.1
Raw-b
7.0
18.3
72.0
U1b
U3b
U5b
12.3
7.1
5.1
17.2
14.7
12.6
68.3
76.1
79.9
Feed (Raw)
Froth Flotation
Froth
Tails
Classification
U1
U3
U5
600C Burnout
(a) Feed (Raw)
Page 64 of 143
(b) U1
(c) U2
(d) U3
(e) U4
(f) U5
Figure 45 SEM micrographs of raw feed and output fractions from Power Station 1
(a) Raw (burnout)
Page 65 of 143
(b) U1 (burnout)
(c) U2 (burnout)
(d) U3 (burnout)
(e) U4 (burnout)
(f) U5 (burnout)
Figure 46 SEM micrographs of feed and output fractions after 600C burnout from Power Station 1
Page 66 of 143
8.2.1.
Initial processing with column flotation
During the initial processing, two separate trial processes were carried out for Power Station 2 material:
(i) primary classification + column froth flotation and (ii) Lamella hydraulic classification, as shown in
Figure 47.
With this material, each component of the processing system was evaluated by two or three runs with
different control parameters to examine the efficiency of the processing components and to establish
optimum running conditions.
The main physical properties of the samples from the different processing stages are given in Table 36.
The results of LOI and fineness of each processed product are also given in Figure 47.
Initial Processing of Stockpile Material (Power Station 2)
Trial 1
Trial 2
10.0
Raw
53.4
10.0
53.4
Raw
LOI, %
Fineness, %
Primary Classification
Run 1
Run 2
Underflow1
11.3
49.5
12.6
Underflow2
N/A
Overflow1
9.1
32.8
Overflow2
Run 1
U1
8.6
N/A
Run 2
Froth-1
10.5
12.8
Tails-1
8.7
26.5
Run 3
Froth-2
10.4
19.0
Tails-2
8.9
32.5
Froth-3
13.1
10.1
Tails-3
8.5
26.0
Run 2
U1-2
15.2
62.0
U2
9.3
31.2
U2-2
9.9
26.3
U3
8.5
20.7
U3-2
8.3
12.4
U4
7.9
18.5
U4-2
8.0
12.3
U5
7.4
3.1
Column Froth Flotation
Run 1
14.7
58.3
Figure 47 Initial processing of stockpile material (Power Station 2)
Two runs of primary classification were carried out, in which, one was set to remove particles > 150 m
and the other those > 75 m.
In Run 1, as with Power Station 1 material, the processing effectively removed the > 150 m particles and
the overflow product only contained 1% of particles > 150 m (Table 36). The LOI of the overflow was
9.1%, reduced by about 0.9% compared to the raw feed material. The reject product, underflow of the
classification, had an increased LOI of 11.3%, however, it contained more than 80% of the < 150 m
particles (Table 36). PSD curves of the raw feed, overflow and underflow materials are shown in
Figure 48. It can be seen that the overflow was finer than the underflow, and the underflow finer than the
raw feed. It therefore appears that a portion of the coarse particles was removed by the primary screening
and slurrying processes.
Page 67 of 143
In Run 2, which was aimed at removing particles > 75 m, the processing also effectively removed the
> 75 m particles and the overflow product only contained 6% of particles > 75 m. The LOI of the
overflow was 8.6%, reduced by about 1.4% compared to the raw feed material. The underflow, with the
LOI increased to 12.6%, however, contained almost 70% of particles < 75 m. The results again suggest
that significant levels of fine material were lost during the primary classification and a more effective
device was required to reduce the fine particle loss. The modification to primary classification as shown
in Figure 35 was then examined, together with mechanical flotation, as discussed in the next section.
Table 36 Fineness and LOI of samples from different processing stages
PSD, m
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
D10
D50
D90
53.4
10.0
12.1
55.3
164.7
Underflow-1
49.5
11.3
Overflow-1
35.4
9.1
Sample
Feed (Raw)
<150m = 82%, >150m = 18%
7.0
50.9
151.8
<150m = 99%, >150m = 1%
6.1
Underflow-2
Overflow-2
36.8
103.5
N/A
N/A
12.6
8.6
Froth-1
Tails-1
18.7
28.8
10.5
8.7
4.2
20.3
66.0
9.6
33.7
83.4
Froth-2
Tails-2
23.5
27.0
10.4
8.9
6.1
26.2
72.9
10.1
41.1
101.5
Froth-3
Tails-3
11.3
22.3
13.1
8.5
3.0
15.2
61.9
6.9
32.5
87.8
U1
U2
U3
U4
58.3
31.2
20.7
18.5
U5
3.1
14.7
9.3
8.5
7.9
7.4
10.6
9.6
8.3
7.0
1.7
65.9
35.7
25.0
22.5
6.6
152.8
82.9
62.0
60.0
17.0
U1-2
U2-2
U3-2
U4-2
62.0
26.3
12.4
12.3
15.2
9.9
8.3
8.0
9.8
8.6
8.0
6.2
66.2
37.3
27.5
21.9
154.0
101.2
72.1
56.6
<75m = 69%, >75m = 31%*
<75m = 94%, >75m = 6%*
Froth Flotation
* PSD data not available, the data given here were obtained from sieve tests.
Page 68 of 143
100
Primary Classification:
80
Overflow-1
60
Feed
40
20
Underflow-1
0
0.1
1
10
100
1000
10000
Figure 48 PSD curves of raw feed and outputs from primary classification
Three runs of column froth flotation were carried out. The same slurry and slurry feed rate were used
during these, however, the feed rate of air was varied as indicated in Table 37. The fineness and LOI of
the feed and outputs from the froth flotation are given in Table 36. Figure 49 shows the PSD curves of
the feed and outputs. It can be seen that during this trial, a higher air feed rate resulted in lower LOI and
coarser froth products than those at lower air feed rate. This is due to the higher air rate causing
turbulence in the column. Small particles lack sufficient mass to approach air bubbles and coarse
particles also have difficulty in settling down in turbulent conditions. Therefore, with the current
equipment, the air feed rate should be controlled between 2.5-3.0 l/min. Low air feed rates cannot
generate enough air bubbles and the effectiveness of froth flotation will then be limited. The ideal
condition is to produce as many small size bubbles as possible.
Table 37 Slurry and air feed rates used in Power Station 2 material froth flotation
Run 1
Run 2
Run 3
Slurry Feed Rate, l/min
1.6
1.6
1.6
Air Feed Rate, l/min
4.0
5.0
2.5
Page 69 of 143
100
Column Froth Flotation:
80
60
Froth-3
Froth-1
40
Tails-2
Froth-2
20
Tails-3
Feed
Tails-1
0
0.1
1
10
100
1000
10000
Figure 49 PSD test results of feed and outputs from froth flotation
Two runs of classification with the lamella hydraulic classifier were carried out. The design slurry solids
concentration, slurry feed rate and underflow rates used in these runs, together with the actual measured
values, are given in Table 38. The tests were designed to examine fraction property variations due to
changes in control parameters. The slurry solids concentrations of the two runs were controlled at two
levels, 14.7% in Run 1 and 10.2% in Run 2. The dispersant concentration was 2.5 g/(kg solids) in Run 1
compared to 3.5 g/(kg solids) in Run 2. The underflow rates for U1 to U4 for Run 2 were twice those of
Run 1.
The fineness and LOI of the feed and output fractions from the lamella hydraulic classification are given
in Table 36. Figure 50 shows the PSD curves of these materials from Run 1. As with Power Station 1
material, the fineness of the five classified fractions gradually increased from U1 to U5. U1 was the
coarsest with a mean size D 50 of 66m. U3 and U4 had similar PSD, while U5 was much finer than U4,
with a mean size D50 of 7m.
Table 38 Parameters for lamella hydraulic classification of Power Station 2 material
Slurry Feed
Run
1
Run
2
Concentration,
% Solids by
mass
Feed
Rate
Dispersant
Feed,
g/(kg solids)
Design
l/min
10-15
6.0
Actual
kg/min
14.7
Design
l/min
Actual
kg/min
Output Rate
U1
U2
U3
U4
U5
2.5
0.6
0.3
0.3
0.3
4.5
7.28
2.5
0.79
0.44
0.38
0.41
5.22
10-15
6.0
2.5
1.2
0.6
0.6
0.6
3.0
10.2
6.29
3.5
1.31
0.67
0.63
0.63
3.05
Total
7.24
6.39
Page 70 of 143
100
Powe Station 2 Fly Ash
80
U5
U2
60
U4
40
U3
Feed
U1
20
0
0.1
1
10
100
1000
10000
Figure 50 PSD test results of feed and output fractions from lamella hydraulic classification
In these five fractions, the LOI decreased from U1 to U5. The LOI of U1 was highest at 14.7% and
greater than that of the feed of 10.0%. This was also higher than that of the underflow of the primary
classification and froth of the flotation. The LOI of U2 to U5 gradually decreased from 9.3% to 7.4%,
with all lower than the feed.
In the second classification run, the fineness and LOI of the feed and output fractions were similar to that
of Run 1, although several parameters in the run were different, which suggests that the properties of the
output fractions were not very sensitive to these changes.
The chemical compositions of raw feed and processed materials are given in Table 39. As noted for
Power Station 1 material, there were no significant changes in chemical compositions after flotation and
classification.
Table 39 Bulk oxide composition of selected samples from different processing stages
Sample
Feed (Raw)
CaO
SiO2
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
4.4
42.4
26.3
8.8
1.6
1.2
2.2
0.9
0.6
0.0
1.3
4.7
4.6
37.7
41.7
23.4
25.9
8.0
9.0
1.7
1.6
1.2
1.2
2.0
2.1
0.8
0.9
0.6
0.6
0.0
0.0
1.2
2.0
4.3
4.7
4.7
4.5
4.5
41.2
42.5
41.9
41.8
40.9
27.7
23.9
24.7
26.7
26.3
6.5
11.0
10.7
8.2
8.4
1.5
1.7
1.7
1.5
1.5
1.2
1.2
1.2
1.2
1.2
2.5
1.9
2.0
2.2
2.1
1.2
0.7
0.8
0.9
0.9
0.7
0.5
0.5
0.6
0.6
0.0
0.0
0.0
0.0
0.0
1.8
2.2
1.8
2.1
2.0
Froth Flotation
Froth-1
Tails-1
Classification
U1
U2
U3
U4
U5
Page 71 of 143
The mineralogical compositions of raw feed and processed materials are given in Table 40. There were
no significant changes in mineralogical compositions after flotation, however, the coarse fraction, U1, had
a lower glass content than the feed, while that of the fine fraction, U5, was higher.
Table 40 Mineralogical composition of selected samples from different processing stages
Sample
Feed (Raw)
Quartz
Mullite
Glass
7.9
16.1
64.2
7.6
19.6
60.7
6.1
17.3
65.5
7.3
6.3
28.0
19.0
49.2
63.5
5.4
12.3
73.6
Froth Flotation
Froth-1
Tails-1
Classification
U1
U3
U5
8.2.2.
Processing with mechanical flotation
During the initial processing, it was noted that there was still a high LOI in the processed fractions,
usually ranging from 7% to 10%, which is above the UK limit (for fly ash use as an addition in concrete,
BS 8500, < 7.0%). Therefore, further work on processing technology focussed on how to effectively
reduce the carbon content of the final products. Attempts to achieve this concentrated on:
(i)
(ii)
Removing greater quantities of coarse particles during primary classification, since it was
noted that carbon is mainly present as coarse particles,
Removing carbon by mechanical froth flotation, which may be more effective for coarse
carbon particles than column froth flotation.
The process used during this trial was primary classification, mechanical flotation (two-cell or four-cell)
and lamella hydraulic classification, as shown in Figure 51.
In mechanical froth flotation, usually more than one cell is required to obtain a satisfactory result. Cells
are arranged in series forming a bank. The number of cells required for processing is dependent on the
requirements of the target material properties. More cells are normally required for achieving lower
carbon tails. During the test, two- and four-cells were used. Since the mechanical flotation equipment
used in the project was a two-cell system, the flotation process was separated into two sub-stages to
provide a four-cell process.
The processing flow chart for Power Station 2 material, together with the results of LOI and fineness of
each processed fraction, is shown in Figure 52. The main physical properties of the samples taken from
the different processing stages are also given in Table 41.
Page 72 of 143
Stage 1 Primary Classification
Overflow
PrePre-Screening
Stage 2a Mechanical Froth Flotation
2.8 – 4.3 l/min
Froth Flotation
Feed
Feed
Overflow
from Stage 1
3.0 – 4.5 l/min
(Two Cells)
3.0 – 4.5 l/min
FrothFroth-1
Flotation
Reagent Mix
20 – 50 cc/min
Clean
Water
0.0 – 0.5 l/min
Slurrying
TailsTails-1
Slurrying
8 – 12 % by mass
10 – 15 % by mass
Underflow
0.5 – 0.7 l/min
Stage 3 Lamella Hydraulic Classification
Stage 2b Mechanical Froth Flotation
Feed
TailsTails-1
Froth Flotation
Tails from
Stage 2
(Two Cells)
3.0 – 4.5 l/min
Feed
Cenospheres
6.0
l/min
Overflow
FrothFroth-2
U5
4.5
l/min
Dispersant
30 – 50 cc/min
Flotation
Reagent Mix
20 – 50 cc/min
Slurrying
TailsTails-2
8 – 12 % by mass
Slurrying
U1
U2
U3
U4
0.6
l/min
0.3
l/min
0.3
l/min
0.3
l/min
4 Underflows
8 – 12 % by mass
Figure 51 Flow sheet for processing with mechanical flotation
Processing of Stockpile Material with Mechanical Flotation (Power Station 2)
LOI, %
11.9
Fineness,
%
Raw
42.8
Underflow
14.4
63.3
U1
4.7
70.9
Overflow
9.9
33.6
U2
3.6
50.8
U3
3.2
19.4
Mechanical Froth Flotation
Run 1
Froth-1
16.3
13.8
Tails-1
7.5
39.8
Run 2
Froth-2
16.1
26.0
U4
3.2
18.1
Tails-2
4.5
47.9
U5
5.2
0.0
Figure 52 Processing of stockpile material with mechanical flotation (Power Station 2)
(Four-cell flotation, i.e two runs of two-cell mechanical flotation)
Page 73 of 143
Table 41 Fineness and LOI of samples from different processing stages with mechanical flotation
PSD, m
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
D10
D50
D90
42.8
11.9
6.3
40.0
115.1
6.3
33.6
14.4
9.9
8.3
57.4
161.3
6.6
37.4
105.3
Froth-1
13.8
16.3
3.5
20.1
76.3
Tails-1
39.8
7.5
8.4
44.6
110.6
Froth-2
26.0
16.1
4.3
27.8
90.8
Tails-2
47.9
4.5
11.8
52.8
123.8
70.9
50.8
19.4
18.1
0.0
4.7
3.6
3.2
3.2
5.2
27.2
20.0
18.2
15.2
68.0
50.2
40.2
33.1
131.6
97.8
77.6
63.6
0.9
4.5
11.8
Sample
Feed (Raw)
Underflow
Overflow
Froth Flotation
U1
U2
U3
U4
U5
The primary classification was set to remove particles >150 m. Figure 53 shows the PSD curves of the
raw feed and outputs of primary classification. The PSD of the overflow was found to be the similar to
that of the initial trial (Comparing Table 41 with Table 36, [D10, D50, D90] = [6.6, 37.4, 105.3] and [6.1,
36.8, 103.5] respectively). However, the underflow was coarser, [D10, D50, D90] = [8.3, 57.4, 161.3]
compared to [7.0, 50.9, 151.8]. Considering the feed material was finer and the feed rate was 50% higher
than that of the initial trial, the modified primary classification was more effective.
The LOI of the overflow was 9.9%, i.e. reduced by about 2.0% compared to the raw feed material. The
LOI of the underflow increased to 14.4%, 2.5% higher than that of the feed. Comparing with the initial
trial results in Table 36, the carbon removal also improved with the modified setup.
Two runs of mechanical froth flotation were carried out. The slurry fed into the second flotation was the
tails of the first run. The fineness and LOI of the feed and outputs from the froth flotation are given in
Table 41. It can be seen that the LOI was reduced to 7.5% after one run (two-cell) of mechanical
flotation, which was 2.4% lower than the feed, and better than in the initial trial (See Table 36).
However, the result was still higher than the BS 8500 limit, 7.0%. In the second run, the LOI of the tails
was reduced to 4.5%. The froth products of the two runs of flotation were similar, and the LOIs were
16.3% and 16.1% respectively. Therefore, the two froth products could be combined for applications or
further processing.
Page 74 of 143
100
Primary Classification:
80
Overflow
60
40
Feed
Underflow
20
0
0.1
1
10
100
1000
10000
Figure 53 PSD test results of raw feed and outputs from primary classification
Figure 54 shows the PSD curves of the feed and outputs. It can be seen that the tails were generally
coarser than the froths. This is similar to that in the initial column flotation trials. Therefore, for Power
Station 2 material, a certain portion of fine particles were lost during flotation processing. Material
balance calculations were carried out and the results are discussed in the next section.
The fineness and LOI of the feed and output fractions from the lamella hydraulic classification are given
in Table 41. The PSD curves are shown in Figure 55. In the five classified fractions, U1 was coarsest
with a 45 m sieve retention of 71% and mean size D50 of 68 m. From U2 to U4, fineness reduced with
45m sieve retentions of 51%, 19%, and 18%, and mean sizes D50 of 50.2m, 40.2 m and 33.1 m
respectively. U5 is an ultrafine product with D50 of 4.5 m, finer than that of the initial trial, D50 of 7 m.
100
Froth Flotation:
80
60
Froth-1
40
Froth-2
Tails-2
20
Feed
Tails-1
0
0.1
1
10
100
1000
10000
Figure 54 PSD test results of feed and outputs from froth flotation
Page 75 of 143
100
80
U2
U5
U3
U4
60
40
U1
20
Feed
0
0.1
1
10
100
1000
10000
Figure 55 PSD test results of feed and output fractions from lamella hydraulic classification
When all processed fractions were obtained, the mass balance was determined to examine the output
percentages achieved. It is also of importance to know the LOI balance and < 5m particle balance
(based on PSD data) to see the effectiveness of each process in carbon removal and related ultra fine
particle losses. The results of these calculations are given in Figure 56.
The mass balance shown gives a clear map of the material distribution. For example, it can be seen that
during the primary classification, 33.7% of the total material was removed, which took off 40.3% carbon
and 25.6% ultrafine particles. The two runs of froth flotation took 31.4% of the total material to the froth,
removed 44.8% carbon and 55.1% ultra fine particles. Therefore, before lamella hydraulic classification,
65.1% of the total material was taken off, which removed 85.1% carbon. However, 80.7% of ultra fine
particles (< 5μm) were also lost. Further work may be required to optimise the performance of the system
to reduce the loss of ultra fine particles during carbon removal processing.
Mass Balance of Stockpile Material
with Mechanical Flotation (Power Station 2)
Total Mass, %
Carbon, %
< 5m Particles, %
100
100
100
Raw
Underflow
Overflow
33.7
40.3
25.6
66.3
59.7
74.4
U1
U2
U3
Run 1
Froth-1
Tails-1
20.7
29.7
38.9
45.6
30.0
35.5
Run 2
Froth-2
Tails-2
10.7
15.1
16.2
34.9
14.9
19.3
U4
U5
16.3
8.0
5.4
6.9
2.6
3.1
5.7
1.9
1.1
4.6
1.5
1.2
0.9
0.5
7.9
Figure 56 Mass balance of stockpile material with mechanical flotation (Power Station 2)
Page 76 of 143
SEM micrographs of raw and processed materials are shown in Figures 57 to 60.
Figure 57 compares the particle shapes of feed, primary classification and flotation products. It can be
seen that the overflow was finer than the underflow from primary classification (Figure 57 b and c).
However, the tails of flotation were coarser than the froths from the two flotation processes. Therefore, a
significant portion of fine particles was lost during flotation. This is consistent with the above mass
balance analysis.
Figure 58 shows the particle size of underflows, U1 to U5, from lamella hydraulic classification. It can
be seen that the particle size and shape change for different products, from the coarsest U1 to the finest
U5. When compared with that of corresponding Power Station 1 products (Figure 45), it can be seen that
products U1 to U4 were quite ‗clean‘ with less fine particles in between. This suggests that a portion of
fine particles were lost during the four-cell flotation process. Although the particle size of U5 was similar
to that of Power Station 1, the quantity of U5 was very low in this case.
Figure 59 makes a comparison of highly magnified images of raw feed and U5 particles (10000X
compared to 250X of Figure 58), with their EDAX results. This indicates that a small quantity of
hydration products appeared on the surface of the particles in the U5 product, compared to the raw
particles. EDAX revealed slightly higher Ca and lower Al and Si concentrations on U5 particle surfaces
than those on the raw particles. This suggests that hydration processes occurred during material storage
and/or processing.
Since material from Power Station 2 is conditioned and then stockpiled, cenospheres were found during
hydraulic classification. Figure 60 shows the particle sizes and shapes of the cenospheres obtained during
processing. The particle sizes of the cenospheres ranged from around 50 to 250 μm. The large size
portion can be clearly seen under the optical microscope in Figure 60b.
Page 77 of 143
(a) Feed (Raw)
(b) Overflow
(c) Underflow
(d) Froth-1
(e) Tails-1
(f) Froth-2
Figure 57
(g) Tails-2
SEM micrographs of raw feed
and outputs from primary
classification and froth
flotation processing for Power
Station 2
(a) Feed (Tails-2)
Page 78 of 143
(b) U1
(c) U2
(d) U3
(e) U4
(f) U5
Figure 58 SEM micrographs of feed and output fractions
from lamella hydraulic classification for Power Station 2
(a) Feed (Raw)
Figure 59
Page 79 of 143
(b) U5
SEM micrographs of raw feed and U5 from lamella hydraulic classification for Power
Station 2 at high magnification with EDAX results
(a) SEM
(b) Optical Microscope
Figure 60 Images of the cenospheres obtained during Power Station 2 fly ash processing
Page 80 of 143
Three Trials were carried out for Power Station 3 material, i.e. Trial 1 with column flotation, Trial 2 with
two-cell mechanical flotation, and Trial 3 with four-cell (two runs of two-cell) mechanical flotation.
8.3.1.
Trial 1 with column flotation
The processing is shown in Figure 61. In this case, primary classification was omitted for two reasons:
(i) Power Station 3 material was relatively finer than Power Stations 1 and 2 (see Table 30), and (ii) the
initial primary classification setup lost many fine particles, as indicated in Tables 33 and 36. Indeed,
when the cut point was set to 150 m, the underflow contained about 80% of particles < 150 m, and
when the cut point was set to 75 m, the underflow contained about 70% of particles < 75 m. Primary
classification was, however, used again with mechanical flotation, after it was modified (Figure 35).
The physical properties, chemical and mineralogical compositions of the samples taken from the different
processing stages are given in Tables 42 to 44 respectively. The LOI and fineness values are also shown
in Figure 61. Figures 62 and 63 show the PSD curves of the feed and outputs from the flotation and
lamella hydraulic classification respectively. SEM micrographs for raw feed and processed materials
from the different processing stages are shown in Figure 64.
Feed
Processing of Stockpile Material
with Column Flotation
(Trial 1, Power Station 3)
Raw
15.0
46.9
Froth Flotation
12.0
15.7
U1
17.8
36.4
U2
8.8
3.2
Froth
39.4
36.7
U3
7.4
0.0
Tails
13.4
16.5
U4
6.7
0.0
U5
5.9
0.0
LOI, %
Fineness, %
Froth Flotation-2
U5-2
3.8
0.0
Figure 61 Processing of stockpile material with column flotation (Trial 1, Power Station 3)
In Trial 1, column flotation was modified mainly in terms of its height (it was increased from 3.0m to
4.5m, similar to that used for primary classification as shown in Figure 35). The modification improved
the performance of the flotation column. As can be seen (i) the froth contained nearly 40% carbon,
compared to 10% - 20% in those from Power Stations 1 and 2, and (ii) the particle size of the froth was
coarser than the tails, which meant that coarse carbon particles were also probably removed by column
flotation for Power Station 3 material.
Since in this trial only 14% of the solids were removed in the froth (i.e. 86% yield of tails), there was still
a relatively high carbon content in the tails. Fine carbon particles were found suspended in the water and
settled down more slowly than the fly ash particles in the tails. It was noted that a portion of fine carbon
was removed following a period of particle settlement and in re-slurrying the material as feed for
classification. Thus, the LOI of the feed for lamella hydraulic classification was slightly lower than that
of the froth flotation tails. Tests have also shown that a second flotation on the material produced can
further reduce the carbon content by more than 2%.
Page 81 of 143
Table 42 Fineness and LOI of the samples taken from different processing stages
Fineness,
45µm sieve
retention (%)
LOI,
% by mass
D10
D50
D90
46.9
15.0
4.6
32.7
116.5
36.7
16.5
39.4
13.4
6.4
41.7
118.4
3.5
22.9
97.5
Feed (Re-slurrying
tails)
15.7
12.0
4.1
22.8
97.9
U1
U2
U3
U4
36.4
3.2
0
0
6.6
5.2
4.8
4.4
43.7
18.5
15.2
14.1
130.7
52.3
40.1
35.0
U5
0
17.8
8.8
7.4
6.7
5.9
1.2
4.8
14.4
U5-2
0
3.8
1.0
4.8
11.2
Sample
Feed (Raw)
PSD, μm
Froth Flotation
Froth
Tails
As with materials from Power Stations 1 and 2, of the five classified fractions, U1 was coarsest and U5
finest. Since the feed of Power Station 3 material was finer than those of Power Stations 1 and 2, the
processed materials were also relatively finer, with 45m sieve retention 36% and a mean size D50 of
44m for U1. No material was retained on the 45m sieve for U3 to U5 fractions. The finest fraction U5
had a mean size D50 of 5 m.
100
Froth Flotation:
80
Tails
60
40
Feed
20
Froth
0
0.1
1
10
100
1000
Figure 62 PSD curves of raw feed and outputs from froth flotation
10000
Page 82 of 143
100
80
U4
U5
U2
U3
60
40
Feed
20
U1
0
0.1
1
10
100
1000
10000
Particle diameter, mm
Figure 63 PSD curves of feed and output fractions from lamella hydraulic classification
The chemical compositions of raw feed and processed materials are given in Table 43. Since the froth
contained more carbon, its SiO2 and Al2O3 were lower. The froth also had relatively higher CaO and SO 3
contents. These tendencies for chemical composition changes may also have occurred in the froth
flotation of materials from Power Stations 1 and 2. However, this could not clearly be seen due to
reduced effectiveness in these flotation tests. As with those from Power Stations 1 and 2, there were no
significant changes in chemical compositions of the different classification fractions. U1 contained
slightly higher CaO and SO3, and slightly lower SiO2 and Al2O3 contents, compared to those of U2 – U5.
Table 43 Bulk oxide composition of samples from different processing stages
Sample
Feed (Raw)
CaO
SiO2
Al2O3 Fe2O3 MgO
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
2.6
39.5
21.8
8.5
1.3
1.0
3.0
0.5
0.5
0.0
1.1
2.9
2.0
32.7
41.9
18.5
22.9
7.4
7.4
1.3
1.2
1.0
1.0
2.7
3.2
0.4
0.5
0.7
0.5
0.0
0.0
1.0
0.4
Froth Flotation
Froth
Tails
Feed (Reslurrying tails)
2.0
43.1
23.7
7.3
1.3
1.0
3.2
0.5
0.5
0.0
0.3
U1
U2
U3
U4
U5
2.1
1.9
1.9
1.9
1.8
40.8
42.5
42.3
43.3
43.7
22.0
23.0
23.3
23.7
24.4
7.3
7.3
7.4
7.0
6.6
1.2
1.2
1.2
1.2
1.2
1.0
1.1
1.1
1.1
1.1
3.0
3.2
3.3
3.3
3.4
0.5
0.5
0.5
0.5
0.5
0.6
0.5
0.5
0.5
0.5
0.0
0.0
0.0
0.0
0.0
0.5
0.2
0.2
0.1
0.1
U5-2
2.2
45.3
25.3
6.5
1.4
1.1
3.4
0.6
0.6
0.0
0.2
Page 83 of 143
The mineralogical compositions of raw feed and processed materials are given in Table 44. There were
no significant changes in mineralogical composition for the tails after flotation, however, the quartz,
mullite and glass contents were much reduced in the froth due to its high carbon content. There was also
no significant difference in mineralogical compositions for U1 to U4, while the finest product, U5, had a
higher glass content than the feed material.
Table 44 Mineralogical composition of samples from different processing stages
Sample
Feed (Raw)
Quartz
Mullite
Glass
4.6
8.5
70.0
3.0
4.3
51.8
4.5
7.4
73.7
7.9
7.3
6.5
7.1
10.1
10.4
10.6
11.8
62.0
71.4
74.4
71.3
3.5
9.3
80.6
Froth Flotation
Froth
Tails
U1
U2
U3
U4
U5
SEM micrographs for raw feed and processed materials are given in Figure 64. This illustrates the effect
of processing on particle sizes and shapes. Further consideration of the SEM observations is made in the
following sections in Figures 69-73.
8.3.2.
Trials 2 and 3 with two-cell and four-cell mechanical flotation
Two processing trials, Trials 2 and 3, with mechanical flotation were carried out for Power Station 3
material. The main difference between the two was in the mechanical flotation, Trial 2 was two-cell and
Trial 3 four-cell, as shown in Figures 65 and 66 respectively.
The main physical properties of the samples taken from the different processing stages are given in
Tables 45 and 46 respectively. In Trial 3, two samples, which were "1-middle" with LOI about 15% and
"3-top" with LOI about 17% (c.f. Table 17), were used in the primary classification and froth flotation
processes. Two sets of fineness and LOI data from these tests are given in Table 46. Thereafter, the tails
of Run 2, i.e. Tails-2, were then combined as feed for lamella hydraulic classification.
In Trial 2, the PSD of the overflow was finer than that of Power Station 2 material primary classification
(Comparing Table 45 with Table 41, [D10, D50, D90] = [4.0, 27.5, 98.9] and [6.6, 37.4, 105.3]
respectively). However, the underflow was coarser, [D10, D50, D90] = [7.2, 68.3, 251.1] compared to [8.3,
57.4, 161.3]. Hence the primary classification was more effective for Power Station 3 material. In
Trial 3, the fineness of the overflow was the same as that of Trial 2, however, the fineness of the
underflow was slightly greater and closer to that of Power Station 2 material following primary
classification.
The LOI reduction of the overflow ranged from 0.7 to 1.1%, while the LOI of the underflow increased by
about 0.5 to 2.4%, in comparison to the raw feeds. A better result was obtained in Trial 2 than Trial 3.
(a) Feed (Raw)
(c) Tails
Page 84 of 143
(b) Froth
(d) U1
(e) U2
(f) U3
(g) U4
(h) U5
Figure 64 SEM micrographs of feed and outputs with column flotation (Trial 1, Power Station 3)
Page 85 of 143
Processing of Stockpile Material with Two-cell Mechanical Flotation
Raw
LOI, %
Fineness, %
15.1
33.5
Underflow
17.5
69.5
U1
10.1
58.3
Overflow
14.3
25.5
U2
7.9
23.3
U3
7.4
11.7
Froth
33.1
37.2
U4
6.6
6.3
Tails
8.0
21.3
U5
6.0
0.0
Figure 65 Processing of stockpile material with two-cell mechanical flotation (Trial 2, Power Station 3)
Processing of Stockpile Material with Four-cell Mechanical Flotation
Raw
LOI, %
Fineness, %
14.9, 16.8
33.3, 34.4
Underflow
16.2, 17.3
57.1, 54.4
U1
5.8
56.5
Overflow
13.8, 16.1
24.4, 21.7
U2
3.0
19.1
U3
2.6
8.2
Run 1
Froth-1
33.0, 36.1 Run 2
31.5, 28.6
Froth-2
29.0, 31.4
32.1, 39.1
U4
2.4
4.4
Tails-1
7.2, 7.7
28.1, 20.0
Tails-2
4.5, 3.9
24.4, 19.6
U5
3.6
0.0
Figure 66 Processing of stockpile material with four-cell* mechanical flotation (Trial 3, Power Station 3)
* i.e two runs of two-cell mechanical flotation
Page 86 of 143
Table 45 Fineness and LOI of feed and outputs with two-cell mechanical flotation
Sample
Fineness,
45µm sieve retention,
% by mass
LOI,
% by mass
D10
D50
D90
33.5
15.1
4.3
33.4
123.8
69.5
17.5
14.3
7.2
68.3
251.1
4.0
27.5
98.9
33.1
8.0
5.8
38.9
114.3
3.4
24.3
94.1
10.1
7.9
7.4
6.6
6.0
10.4
6.4
5.4
5.2
55.9
26.9
21.0
19.5
138.4
80.8
61.8
55.1
0.9
4.2
10.4
Feed (Raw)
PSD, μm
Underflow
Overflow
25.5
Froth Flotation
37.2
Froth
Tails
21.3
58.3
23.3
11.7
6.3
U1
U2
U3
U4
U5
0.0
Table 46 Fineness and LOI of feed and outputs with four-cell mechanical flotation
Sample
Feed (Raw)
Fineness,
45µm sieve retention,
% by mass
33.3
LOI,
% by mass
PSD, μm
D10
D50
D90
34.4
14.9
16.8
4.2
5.7
34.9
39.5
144.6
144.7
57.1
24.4
54.4
21.7
16.2
13.8
17.3
16.1
4.9
5.1
49.9
50.5
213.0
186.6
3.4
3.3
27.2
26.8
101.9
103.7
Froth-1
31.5
28.6
33.0
36.1
4.7
4.5
40.4
36.4
125.3
120.9
Tails-1
28.1
20.0
7.2
7.7
3.6
3.8
25.6
24.6
112.0
95.1
Froth-2
32.1
39.1
29.0
31.4
3.0
3.6
29.4
32.3
109.7
131.9
Tails-2
24.4
19.6
4.5
3.9
3.0
3.9
20.2
29.3
92.8
114.7
Underflow
Overflow
Froth Flotation
U1
U2
U3
U4
U5
56.5
19.1
8.2
4.4
0.0
5.8
3.0
2.6
2.4
3.6
8.8
6.8
5.4
4.7
56.9
27.9
19.6
17.5
151.8
83.8
53.5
44.5
1.0
5.6
15.0
Page 87 of 143
Only two-cell froth flotation was used during Trial 2 processing, and the LOI of the tails was 8.0%,
higher than the UK limit for use in concrete (< 7.0%). In Trial 3, four-cell froth flotation was carried out
(i.e. two runs of two-cell) and the LOI reduced to between 3.9% - 4.5%. The results were similar to that
of Power Station 2 material flotation (See Table 41). The significant difference was that the LOI of the
froth increased to 33%-36% and the froth was coarser than the tails. This was also noticed in the initial
column flotation (See Tables 36 and 42 for Power Station 2 and 3 materials respectively). It is therefore
likely that the fineness of the froth and tails depends on the material properties, rather than on the
flotation method.
The results of lamella hydraulic classification were similar to all of the previous trials. In the five
classified fractions, U1 was coarsest and U5 finest. The fineness of the ultrafine fraction, U5 had a mean
size D50 of 5 m.
The mass balances of the two trials with mechanical flotation are shown in Figures 67 and 68. It can be
seen that during the primary classification, 20-25% of the total material was removed, which took off 2627% carbon and 14-19% ultrafine particles. Trial 3 removed slightly more carbon but took more material
and fine particles off than Trial 2.
In froth flotation, Trial 2 (two-cell flotation) took 23% of the total material to the froth, removed 46%
carbon and 20% ultrafine particles. Trial 3 (four-cell flotation) took 30% of the total material to the froth,
removed 60% carbon and 29% ultrafine particles. However, the first part of flotation in Trial 3 was
similar to Trial 2, which took 23% of the total material to the froth, removed 49% carbon and 22%
ultrafine particles.
Before the lamella hydraulic classification, Trial 2 removed 72% carbon with 44% of the total material
and 34% of the ultrafine particles lost. Trial 3 removed 88% carbon with 55% of the total material and
48% of the ultrafine particles lost. The results were better than that of Power Station 2 material processing
(See Figure 56). This also agrees with the release analyses results of the materials given in Appendix D
(Figure D7). From the release analyses results, it can also be anticipated that Power Station 1 material
processing would lose more material with carbon removal than Power Station 2.
When fewer fine particles were lost during primary classification and flotation, more ultrafine particles
could be obtained. It can be seen that 6.2% of U5 was obtained in Trial 2 and 4.1% in Trial 3. Both were
higher than that of Power Station 2 material processing, which was 0.9% (Figure 56).
Mass Balance of Stockpile Material with Two-cell Mechanical Flotation
Raw
100
100
100
Underflow
Overflow
20.3
25.9
13.6
79.7
74.1
86.4
Froth
Tails
23.4
46.4
20.0
56.3
27.7
66.4
Total Mass, %
Carbon, %
-5m Particles, %
U1
U2
U3
U4
U5
17.8
11.1
8.2
12.0
5.9
8.4
10.2
4.7
8.6
9.2
3.7
8.0
6.2
2.3
33.2
Figure 67 Mass balance of stockpile material with two-cell mechanical flotation (Trial 2, Power Station 3)
Page 88 of 143
Mass Balance of Stockpile Material with Four-cell Mechanical Flotation
Raw Ash
Total Mass, %
Carbon, %
-5m Particles, %
100
100
100
Underflow
Overflow
24.9
27.4
18.7
75.1
72.6
81.3
U2
Run 1
Froth-1
Tails-1
23.2
48.9
21.5
52.0
23.7
59.8
Run 2
14.0
5.7
8.1
10.5
2.2
7.8
8.7
1.6
8.2
8.1
1.4
9.0
4.1
1.0
19.0
U1
U3
Froth-2
Tails-2
6.6
11.7
7.8
45.4
12.0
52.1
U4
U5
Figure 68 Mass balance of stockpile material with four-cell* mechanical flotation
* i.e two runs of two-cell mechanical flotation
Mineralogical compositions of raw feed and processed materials from Trial 3 are given in Table 47. As
with Trial 1 (See Table 44), the tails had a greater glass content than the froths. There was no significant
difference in mineralogical compositions for U1 to U4, however, the finest fraction, U5, had a higher
glass content than the feed.
Table 47 Mineralogical composition of samples from different processing stages in Trial 3
Sample
Quartz
Mullite
Glass
3.8
6.0
73.6
Froth-1
Tails-1
2.3
6.1
52.4
5.5
8.4
77.1
Froth-2
Tails-2
4.3
7.5
53.0
5.6
8.9
79.7
7.5
7.2
6.3
5.8
8.7
11.6
9.4
10.2
72.4
72.9
74.3
75.6
3.4
9.8
80.0
Feed (Raw)
Froth Flotation
U1
U2
U3
U4
U5
Page 89 of 143
SEM micrographs for feed and processed materials are shown in Figures 69 to 73 to illustrate the effect of
processing on the particle sizes and shapes.
Figure 69 compares the particle shapes of feed, primary classification and flotation outputs. It can be
seen that the overflow was finer than the underflow from primary classification (Figure 69 b and c). The
tails of flotation were also finer than the froths from the two flotation processes, which was different to
Power Station 2 material processing. Indeed, there was less fine particle loss during Power Station 3
material processing, which agrees with the results of the mass balance analysis (comparing Figure 68 with
Figure 56).
Figure 70 shows the particle sizes of fractions, U1 to U5, from lamella hydraulic classification. It can be
seen that the particle size and shape changed for different fractions, from coarsest U1 to finest U5. When
compared with Trial 1 (Figure 64), it can be seen that the fractions in this case were clean with less
irregular carbon particles, but slightly coarser than the corresponding fractions. This reflects the fact that
the feed of the lamella hydraulic classification had less carbon and was coarser, due to fine particle loss
during primary classification and four-cell flotation processing.
Figure 71 makes a comparison of highly magnified images of the raw feed and different processed
materials. Since the whole shape of the large particles cannot be seen at this magnification, this
observation focused mainly on particle surfaces. An image of run-of-station dry fly ash is also shown for
comparison. It can be seen that: (i) similar small size particles can be found in all processed fractions,
therefore, at this magnification, the particle sizes and shapes of different fractions cannot be clearly
distinguished; (ii) the surface of the fly ash particles, both raw and processed, was not as smooth as that of
the reference dry fly ash. Some hydration products, particularly of needle shape, were found on particle
surfaces, especially those of small sizes.
Figure 72 shows higher magnification U5 particles, with EDAX results. Hydration products of elongated
particles (size around 0.1μm) were found surrounding fly ash particles (size around 1 to 2 μm). EDAX
revealed that these elongated particles had slightly higher Ca concentration. After acid treatment, all the
elongated particles were dissolved, as shown in Figure 72b and the Ca peak disappeared on the EDAX
spectrogram. This means: (i) the elongated particles were hydration products, calcium silicate hydrates or
calcium aluminate hydrates, and (ii) these hydrates were acid soluble.
As with that from Power Station 2, Power Station 3 material was also conditioned and stockpiled, and
cenospheres were obtained during processing. Figure 73 shows the cenospheres from Power Station 3
material processing. The particle sizes of the cenospheres ranged from around 50 to 250 μm. The large
size portion can be clearly seen under the optical microscope as shown in Figure 73b. In comparison with
cenospheres from Power Station 2, these appeared white in colour, suggesting a lower carbon content.
Page 90 of 143
(a) Feed (Raw)
(b) Underflow
(c) Overflow
(d) Froth-1
(e) Tails-1
(f) Froth-2
Figure 69
(g) Tails-2
SEM micrographs of feed and
outputs from primary
classification and froth
flotation processing for Power
Station 3, Trial 3
(a) Feed (Tails-2)
(c) U2
(e) U4
Page 91 of 143
(b) U1
(d) U3
(f) U5
Figure 70 SEM micrographs of feed and output fractions from lamella hydraulic classification
for Power Station 3, Trial 3
(a) Reference dry fly ash
(c) Overflow
(e) Froth-1
Page 92 of 143
(b) Feed (Raw)
(d) Underflow
(f) Tails-1
Figure 71 SEM micrographs of feed and output fly ashes for Power Station 3, Trial 2
(high magnification)
(g) Tails-2
Page 93 of 143
(h) U1
(i) U2
(j) U3
(i) U4
(j) U5
Figure 71 SEM micrographs of feed and output fly ashes for Power Station 3, Trial 2
(high magnification, continued)
(a) U5, without acid treatment
Page 94 of 143
(b) U5, after acid treatment
Figure 72 SEM micrographs of U5 from lamella hydraulic classification for Power Station 3 at high
magnification with EDAX results
(a) SEM
(b) Optical Microscope
Figure 73 Images of the cenospheres obtained during Power Station 3 fly ash processing
Page 95 of 143
8.4. Summary
Primary classification was used to remove coarse (i.e. > 150 m) particles and to reduce the carbon
content of material. With the final processing arrangement, more than 90% of the > 150 m particles
could be removed. The LOI of the overflow was reduced by up to 2.0% compared to the raw feed.
Around 25 to 40% of the total carbon was removed during primary classification. However, a portion of
fine particles (around 14% to 25% of the total < 5m particles) was also lost during this procedure. This
is a low cost procedure since no additives were used. The parameters of the processing, e.g. the rates of
feed, underflow and clean water can be adjusted to obtain an optimal solution in terms of coarse
particle/carbon removal and minimising fine particle loss.
Column froth flotation was used to remove carbon during trials. With the final processing arrangement,
the LOI of the tails was reduced by 2.0% compared to the raw feed, while the froth contained about 40%
LOI. This suggests that the column flotation was an effective way to obtain high carbon products.
Conventional mechanical flotation was also used to remove carbon. With two-cell, the LOI of the tails
was reduced by 6.0% compared to the raw feed, while the froth contained about 33% LOI. With fourcell, the LOI of the tails-2 was reduced by 10 to 12% compared to the raw feed. This suggests that
mechanical flotation is more effective for obtaining low carbon content fly ash. However, fine particle
loss with four-cell was also significant.
Lamella hydraulic classification was used to separate the material into different size fractions. Five
fractions were obtained from the classifier, i.e. (i) coarse fraction, U1; (ii) medium fraction, U2; (iii) fine
fraction, U3 and U4 (iv) ultrafine fraction, U5, and (v) cenospheres. It can be generally seen that the
fineness and LOI of the fractions were gradually reduced from U1 to U5. There was no significant
change in their chemical and mineralogical compositions within these fractions. The various size
fractions have potential to be used in different applications.
Some hydration products were found on the surface of ultrafine particles, especially in the U5 product,
which may affect the chemical activity of the material.
More cenospheres were found in processing materials, which were conditioned and stockpiled, compared
to those stored in lagoons.
Burnout at 600C can significantly reduce the LOI (LOI < 2%). However, this is energy intensive in
comparison with the other methods used in the study.
9. SCOPING STUDIES FOR END USE APPLICATIONS
Following the development of the processing equipment, the use of the processed materials in various
applications was investigated. These were divided into different grades corresponding to the properties
required for various applications. The fine fractions were used as cement components in standard
concrete mixes in both laboratory trials and precast concrete production. Medium fineness material was
used as a component of cement-based grouts and in foamed concrete as fine aggregate. Soil stabilisation
with lime and clay replacement in fired bricks was tested with different processed materials including
coarse/high LOI. Use in concrete masonry units was also considered. In addition, carbon rich materials
were examined as fuel and as raw feed in cement manufacture.
9.1. Standard Mortar Tests
Water requirement and strength activity index of standard mortars according to BS EN 450-1 (BSI, 2005),
which are normally carried out to assess the performance of fly ash with regard to its use in concrete,
were measured for the Power Station 3 processed materials and the results are given in Table 48.
Page 96 of 143
Table 48 Water requirement and strength activity index of processed materials
Water
Requirement
Cube Strength, N/mm²
ml
%
7d
28 d
90 d
28 d
90 d
225
225
220
205
200
200
100%
98%
91%
89%
89%
43.0
32.0
24.5
27.0
29.0
30.0
50.0
41.5
31.0
37.5
38.5
39.5
56.0
50.0
37.5
46.0
47.5
49.0
83
62
75
77
79
89
67
82
85
88
195
87%
31.5
41.5
52.0
83
93
Mix Code
PC-mortar
Raw-mortar
U1-mortar
U2-mortar
U3-mortar
U4-mortar
U5-mortar
Activity Index, %
The results show that the water requirement of the processed materials was significantly reduced.
Products U2 to U5 met the requirement of BS EN 450-1 Category S fly ash (not more than 95%), and
suggest that water savings could be achieved in using the materials in concrete.
The activity index of the processed fractions increased with fineness. Products U3 to U5 met the
requirement of BS EN 450-1 (I28≥75% and I90≥85%). The results indicate, perhaps surprisingly, that
similar activity indices were obtained between the finer processed fraction and raw material mortars.
9.2. Addition in Concrete
Fly ash generally has beneficial effects on concrete properties, which include (compared to PC concrete)
enhanced consistence due to the spherical fly ash particles, reduced bleeding and less water demand,
lower heat of hydration, increased ultimate strength, reduced permeability and chloride ion penetration,
greater resistance to sulfate attack and alkali-aggregate reactivity, and reduced drying shrinkage
(UKQAA, 2006; Payá et al., 2002; McCarthy and Dhir, 1999; Sarker, 1996).
Processed materials from Power Stations 2 and 3 were used as an addition in concrete at a level of 30% in
laboratory tests to evaluate their effect on consistence (measured as slump) and strength development.
The finer fractions of the processed materials, U3/U4 blend, U4/U5 blend and U5, were used. Table 49
gives the mix proportions for the test concrete, which had a fixed water content and represent a typical
structural concrete.
The processed material was dewatered and dried in an oven at 60°C to constant weight and then separated
by passing through a 150 m sieve. A single batch Class 42.5 N PC conforming to BS EN 197-1 (BSI,
2000a) was used throughout the laboratory tests. A commercial Category S fly ash conforming to BS EN
450-1 (BSI, 2005) was also used as a reference mix. Aggregates used were natural sand and gravel in
10mm and 20mm sizes, all conforming to BS EN 12620 (BSI, 2002). The consistence and compressive
strength test procedures were in accordance with BS EN 12350-2 (BSI, 2000b) and BS EN 12390-3 (BSI,
2002a) respectively. The results obtained are given in Tables 50 and 51 respectively.
The results indicate that the consistence of the concrete improved with the addition of fly ash, especially
with ultrafine U5 fraction. Raw material from Power Station 3 had similar results to Reference Category
S fly ash. However, raw material from Power Station 2 had lower consistence than PC concrete.
Progressive improvements in consistence compared to raw material were noted with U3/U4, U4/U5, and
U5. The results, therefore, show general agreement to those obtained from the water requirement tests on
mortar in Section 9.1 and indicate that water savings in concrete could be achieved with the materials.
Page 97 of 143
Table 49 Mix proportions for the concrete laboratory tests
Constituent Proportions, kg/m3
Mixes
W/C
Cement
Water
Aggregates
PC
Fly Ash
fine
10 mm
20 mm
PC Mix
0.5
175
350
0
575
430
860
PC/Fly Ash
Mixes
0.5
175
245
105
570
420
840
The compressive strength of the concrete mixes is given in Table 51 and indicates that those with
processed material tended to have similar or slightly lower strength than the raw material mixes, although
their 28/90 day strength development ratios were similar or higher than the raw material mixes and
reference Category S fly ash mix. These results again show general agreement with the mortar tests. The
reason why ultrafine fractions following processing did not give higher strength is unclear, but it is
possible that the material may have undergone reaction during storage and processing as detected by SEM
and EDAX (See Figures 59 and 72) and this may have influences on the subsequent reactivity within the
cementitious system. It should also be noted that advantages associated with water saving with the
processed materials were not exploited in these tests.
Table 50 Consistence (as measured slump) of concrete made
with different processed materials (30% level)
Slump,
mm
Mix Code
Mix Details
R-PC
R-Fly-Ash
PC 100%
PC 70% + Category S fly ash 30%
50
65
P3-Raw
P3-C-U3/U4*
P3-C-U5
PC 70% + Raw Power Station 3 material 30%
PC 70% + U3/U4 blended Power Station 3 material 30%
PC 70% + U5 Power Station 3 material 30%
65
75
145
P3-M-U3/U4*
P3-M-U4/U5
P3-M-U5
PC 70% + U5 Power Station 3 material 30%
70
90
125
P2-Raw
P2-M-U3/U4
P2-M-U4/U5
PC 70% + Raw Power Station 2 material 30%
45
70
85
*
C: column flotation and M: mechanical flotation. The blend ratio for the blended
processed materials was 1:1.
Page 98 of 143
Table 51 Strength development of concrete made with different processed materials
MIX CODE
*
7 days
28 days
60 days
90 days
f90/f28
R-PC
R-Fly-Ash
44.0
26.5
52.0
40.5
54.0
45.5
58.0
50.0
1.12
P3-Raw
P3-C-U3/U4*
P3-C-U5
27.0
21.5
24.5
41.0
32.0
35.5
46.5
37.0
43.0
48.0
35.5
47.0
1.18
1.12
P3-M-U3/U4*
P3-M-U4/U5
P3-M-U5
23.0
24.5
22.0
38.0
37.5
36.0
36.5
33.0
44.5
41.5
47.5
48.5
1.09
1.27
P2-Raw
P2-M-U3/U4
P2-M-U4/U5
25.5
23.5
22.5
39.0
34.0
36.5
44.5
43.0
43.5
49.0
47.5
46.0
1.26
1.36
1.24
1.32
1.34
1.26
C: column flotation and M: mechanical flotation. The blend ratio for the blended
processed materials was 1:1.
9.3. Addition in Precast Concrete
The combinations, P3-M-U3/U4 and P3-M-U4/U5 were selected for use in a precast concrete application.
Raw material was not appropriate in this application due to its dark colour. The mix proportions used in
this work are given in Table 52. The PC mix was provided by the precast plant and the concrete
compressive strength class was C32/40. Considering that the PC/fly ash mix could improve the
consistence and for the equivalent strength requirement, the w/c ratio and water content of the PC/fly ash
mix was lower as shown in the table. As shown in Figure 74, the appearances of the three mixes and their
fresh properties were similar and the concrete was used in a reinforced concrete staircase.
The concrete strength development was measured and the results are given in Table 53. The results show
that all mixes made with processed material achieved the target strength. The 28/90 day strength
developing ratios were similar to those of the laboratory results (Table 51). After 90 days, the U4/U5 mix
essentially matched the strength of the PC mix.
Table 52 Mix proportions for the precast concrete tests
Constituent Proportions, kg/m3
Mixes
Cement
W/C
Water
PC Mix
PC/Fly Ash
Mixes
Aggregates
PC
Fly
Ash
Fine
(natural sand)
Coarse
(gravel)
0.49
185
375
0
705
1115
0.42
180
300
125
630
1115
Page 99 of 143
(a) PC Mix
(b) U3/U4 Mix
(c) U4/U5 Mix
(d) Casting of element
(e) Precast element
(f) Cast cube samples
Figure 74 Photographs taken during the processed material trials in a precast concrete application
Table 53 Strength development of concrete from the precast tests
Mix Code
PC-precast
U3/U4-precast
U4/U5-precast
3 days
7 days
28 days
90 days
f90/f28
30.0
21.0
42.0
30.0
55.0
43.0
60.0
54.5
1.12
1.27
23.0
33.0
47.0
59.0
1.24
Page 100 of 143
9.4. Cementitious Grouts
Grouting is mainly applied in inaccessible voids, in particular for ground improvement, as shown in
Figure 75. It normally increases shear resistance, reduces permeability, and increases strength in this
application. Fly ash is used in cementitious grouts and offers a number of technical and economic benefits
(UKQAA, 2006e). Specification for fly ash use in this application is covered in BS 3892, Part 3. The
fineness of the fly ash should not be more than 60 % by mass retained on a 45 m test sieve and the LOI
should not be more than 14 %.
Figure 75 Grout application (UKQAA, 2006e)
There are several types of fly ash grouts: (i) fly ash only grout; (ii) fly ash/cement grout; (iii) fly ash/lime
grout; and (iv) fly ash/cement/sand grout (UKQAA, 2006e). In the current study, PC/fly ash grout was
considered. This was used in grout mixes of PC/fly ash ratio 1:1 and 1:3 and water/solids ratio 0.4 and
0.5 (without admixtures). PC only grout was also examined as a reference. It should be noted that these
grouts are at the upper end of the strength range commonly used. The properties of the grouts tested were
(i) flow using a modified Marsh cone and (ii) compressive cube strength.
In Marsh cone tests, the efflux time, i.e. that for 1.0 litre of sample to flow out of a 1.5 litre capacity
Marsh cone of 12.5 mm orifice diameter as shown in Figure 76, is taken as a measure of flow. Mixes that
flow within 60 seconds are generally satisfactory for pumping (Jones et al., 2003) while those that take
longer than 60 seconds are likely to be unsuitable.
In this application, U2 (Power Station 3, fineness = 20.0 % retained on a 45 m sieve) was used. The
grouts were mixed in a 5 litre planetary mixer. The efflux time and strength development results are
given in Table 54. Grouts with water/solids ratio = 0.4 were initially prepared, however, their efflux
times were greater than 60 seconds for all PC and fly ash mixes and, therefore, specimens were not taken
for strength tests.
It can be seen that for all mixes with water/solids ratio = 0.5, satisfactory flow was obtained with efflux
times less than 60 seconds. The PC mix exhibited greatest flow, while U2 mixes were better than the
corresponding raw material mixes. In addition, the colour of the U2 mixes was close to that of PC and
carbon floats were found in the raw material mixes (Figure 77).
The highest strength was observed in the PC mix, while U2 mixes had slightly lower strength than raw
material mixes. This may be due to U2 fraction being slightly coarser than the raw material and possibly
some reaction of fly ash during storage/processing as mentioned above.
Page 101 of 143
Figure 76 Measurement of efflux time with modified Marsh cone.
(a) PC Mix
(b) U2 /PC Mix
(c) Raw material/PC Mix
Figure 77 Grout cube samples and surface colour comparison
Page 102 of 143
Table 54 Efflux time and strength development of test grouts
Mix Code
Efflux Time,
Second
7d
28 d
90 d
14
28
39
19
33.5
12.0
6.0
10.5
45.0
21.0
9.5
20.5
47.5
26.0
11.0
25.0
23
5.5
8.5
10.0
PC-grout
Raw-grout 1:1
Raw-grout 1:3
U2-grout 1:1
U2-grout 1:3
9.5. Foamed Concrete
Foamed concrete is an increasingly popular material for diverse construction applications, from thermal/
acoustic insulation of building elements to mine reinstatement and ground stabilisation. Several research
projects have been carried out to support the technological development of the material (Dhir et al., 1999,
Brady et al., 2001, Jones et al., 2004 and Jones et al., 2007). Examples of the range of foamed concrete
applications are shown in Figure 78.
Foamed concrete block
Mine infill
Subway infill
Figure 78 Examples of foamed concrete applications (courtesy of Propump Engineering Ltd)
Page 103 of 143
In this study, U1 and U2 fractions of Power Station 3 were used to replace sand. The target density of the
foamed concrete was 1000 kg/m3, w/c ratio 0.5, and sand replacement 50%. The mix proportions were
those used in previous studies (Jones et al., 2004 and Jones et al., 2007), as given in Table 55.
The consistence of foamed concrete was measured by a modified Marsh cone as used in the grout tests.
Cube strength was measured in accordance with BS EN 12390-3 on sealed-cured (S-C) specimens at 3, 7,
28, 56, and 90 days. The efflux time and strength development results are given in Table 56.
With the addition of fly ash, consistence of the foamed concrete improved. However, as with the grout
tests, carbon floats were found in the raw material mix, as shown in Figure 79. Processed fractions, U1
and U2 were better than the raw material in improving consistence, due to their lower carbon content.
Table 55 Mix proportions for the foamed concretes
Mix Proportions, kg/m3
Mixes
Target Plastic
Density, kg/m3
W/C
PC
Sand
Fly ash
Water
Air, % by
volume
PC-foamed
1000
0.60
300
520
0
180
53
50% fly ash
1000
0.60
300
200
200
300
44
(a) PC Mix
(b) Raw material Mix
(a) U1 Mix
(c) U2 Mix
Figure 79 Fresh foamed concrete mixes
Page 104 of 143
Table 56 Efflux time and strength development of foamed concrete
Mix Code
Compressive Strength, N/mm²
Efflux Time,
Second
3d
7d
28 d
56 d
90 d
50
25
20
1.2
1.4
1.3
1.4
1.7
1.6
1.7
2.8
2.2
1.9
3.2
2.6
1.9
3.4
2.7
20
1.5
1.8
3.2
3.7
3.9
PC-foamed
Raw-foamed
U1-foamed
U2-foamed
The strength of foamed concrete also increased with the addition of fly ash. While the U2 mix achieved
highest strength, the strength of the U1 mix was lower than the raw material mix and this seems to be due
to its coarseness. However, in replacing sand, all fly ash mixes obtained higher strength than the
reference PC mix, suggesting they can be satisfactorily used in this application.
9.6. Lime Soil Stabilisation
Fly ash can be used in soil treatment on its own or in combination with a binder material (lime or cement)
to achieve enhanced properties. This is important in cohesive soils with high clay contents as these are
often of insufficient strength and stability for construction purposes, especially in wet conditions.
The binder material (e.g. 2 to 4 % lime) reacts with the components of soil and also consumes moisture.
However, depending on the sulfate content of the soil, volume stability problems may occur, resulting in
swelling of the stabilised material. These effects may only become visible after the completion of the
construction and incur significant costs to rectify. Fly ash has potential in this application to contribute to
strength properties and to mitigate swelling due to sulfate (McCarthy et al., 2009)
Two processed materials from Power Station 3, one coarse (U1) and one fine (U4), were used at 6%,
12%, 18% and 24% addition levels in this application. Kimmeridge clay, with a SO3 content of 1.0%
which will potentially give volume stability problems, was used during the test. The chemical
composition of the clay is given in Table 57. Lime conforming to BS EN 459-1 (BSI, 2001) was used for
all mixtures at a level of 3.0%.
Table 57 Bulk oxide composition of clay for soil stabilisation test
Sample
Kimmeridge
Clay
CaO
SiO2 Al2O3 Fe2O3 MgO MnO
11.8
46.0
16.0
5.3
1.8
0.0
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
0.9
3.2
0.3
0.1
-
1.0
The swelling tests were carried out following the BS EN 13286-49 (BSI, 2004), accelerated swelling test
for soil treated by lime and/or hydraulic binder. The clay/fly ash mixtures were controlled to their
optimum moisture content (around 24%) and then mixed with 3% lime and used to form a 50mm
diameter and 50mm long cylinder sample with a compressor. The samples were then immersed in a 40ºC
water bath. Volumetric swelling was measured at 7, 14, and 120 days and then unconfined compressive
strength measured according to BS 1924-2 (BSI, 1990). The swelling and strength results are shown in
Figures 80 and 81 respectively.
Page 105 of 143
15
(a) 7 days
(b) 14 days
(c) 120 days
Volumetric swelling, %
12
9
6
3
raw
U1
U4
0
0
5
10
15
20
25 0
5
10
15
20
25 0
5
10
15
20
25
Fly Ash Content, % by mass
Figure 80 Volumetric swelling of the soil stabilisation test samples (in 40ºC water bath)
Compressive Strength, N/mm2
1.5
1.2
0.9
0.6
0.3
Raw
U1
U4
0
0
5
10
15
20
25
Figure 81 Compressive strength of soil stabilisation test samples (120 days, in 40ºC water bath)
It can be seen that fly ash addition reduced swelling of the lime stabilised clay. Furthermore, greater
addition levels gave greater reductions. At the 24% addition, swelling was reduced to about 25% of the
lime only stabilised clay sample. No significant difference between raw and processed materials was
noted. However, at lower addition levels, 6% and 12%, raw material gave less swelling than processed
material and coarse fraction, U1 was less than fine one, U4. Extending the test period beyond that
specified in the standard test (14 days) may cause slight increases in volume, but these effects may not
occur in practice.
Strength at 120 days increased with addition level. The results suggest that there may be an optimum
addition level in relation to this with raw and U4 materials. The optimum addition content in terms of
strength may relate to particle packing in the mixtures. A similar phenomenon was found in fly ash
replacing bottom ash to make concrete masonry units as described in Section 9.7.2. No peak strength was
found for coarse U1 samples within the range of mixes tested.
Page 106 of 143
9.7. Brick and Block Manufacture
9.7.1.
Fired bricks
High LOI fly ash, which is unsuitable for use as cement, may have potential for use as a clay replacement
in fired clay bricks. Current specifications for clay bricks are given in BS EN 771-1 (BSI, 2003). The
property requirements for clay bricks relevant to their use in construction include compressive strength,
water absorption, dimensions/tolerances, density, thermal properties, and durability, etc.
In this application, processed material was used to partially replace clay in fired bricks. The tests were
carried out at the Ceramic Technology Group, Staffordshire University, following procedures developed
by Anderson (2002). The clay used for making test samples was Eldon Shale. The chemical composition
of the clay is given in Table 58. Three types of processed materials from Power Station 3, i.e. primary
classification underflow (LOI=17%, Fineness=54%), U2 (LOI=4%, Fineness=21%) and U4 (LOI=4.5%,
Fineness=5%) were used as a clay replacement at 10%, 20% and 30% levels.
Table 58 Bulk oxide composition of clay for brick test
Sample
CaO
SiO2 Al2O3 Fe2O3 MgO MnO TiO2
Eldon
0.41 60.44 18.21 6.50
Shale Clay
2.13
0.11
1.00
K2O Na2O P2O5
Clˉ
SO3
LOI
3.19
0.00
0.04
6.97
0.76
0.25
Small disc samples, of 38 mm diameter and 7.5 mm thickness, were prepared as shown in Figure 82, and
their bulk density, volume shrinkage, water absorption and tensile strength measured.
Three temperatures, i.e. 1000°C, 1050°C, and 1100°C were used to fire the samples. The average
temperature rise rate was controlled at 100°C/hour. The temperature was fixed for 1 hour when the target
firing temperature was achieved and then the power supply turned off to let the samples cool down
naturally. A typical firing curve is shown in Figure 83. The whole firing procedure lasted for about
48 hours.
The fired bulk density of the samples is shown in Figure 84. At 1000°C firing, the density of all clay/fly
ash mixtures decreased with increasing replacement level. Underflow and raw material samples had
similar rates of decrease. The density of U2 and U4 samples decreased more slowly than the underflow
and raw material samples. It was also found that the lower the LOI of the replacement material, the
higher the fired bulk density, which increased with firing temperature. At 1100°C firing, 10% U2
samples had the highest fired bulk density.
Fired volume shrinkage increased with firing temperature (Figure 85). However, there was no significant
difference between samples fired at the same temperature.
The variation in water absorption with replacement level was the inverse of that of the fired bulk density.
As shown in Figure 86, water absorption of all samples increased with replacement level. Underflow and
raw material samples had similar absorption rates, while that of U2 and U4 samples was lower. In all
cases, water absorption decreased with increasing firing temperature. At 1100°C firing, 10% U2 samples
had the lowest water absorption. The relationship between density and water absorption is shown in
Figure 87. The well fitted linear line indicates that the relationship was independent of the firing
temperature, and raw/processed materials and their replacement levels.
Page 107 of 143
(a) Clay and fly ash mixed with water (12%),
(b) 20g sample of mix put in cylinder mould and
then passed through a 2.36 mm sieve by brushing.
sample produced under 1 tonne load. Sample
weight and dimensions measured.
(c) Samples dried in oven at 40°C. Weight and
(d) Samples fired in an electric kiln. Weight and
dimensions measured to determine density and dimensions measured to determine density and
volume shrinkage after drying.
volume shrinkage after firing.
(e) Samples boiled in water for 2 hours. Weight
measured to determine water absorption.
(f) Indirect tensile strength measurements.
Figure 82 Test procedures for clay brick samples
Page 108 of 143
1200
Temperature, °C
1000
800
600
400
200
0
0
10
20
30
40
50
Firing Time, hr
Figure 83 1000 °C firing curve for brick sintering
2.30
(a) 1000 °C
(b) 1050 °C
(c) 1100 °C
Fired Bulk Density, g/cm3
2.20
2.10
2.00
1.90
1.80
1.70
Raw
Underflow
U2
U4
1.60
0
10
20
30
0
10
20
30
0
Figure 84 Fired bulk density of the brick samples
10
20
30
Page 109 of 143
20.0
(a) 1000 °C
(b) 1050 °C
(c) 1100 °C
18.0
Fired Volume Shrinkage, %
16.0
14.0
12.0
10.0
8.0
6.0
4.0
Raw
Underflow
2.0
U2
U4
0.0
0
10
20
30
0
10
20
30
0
10
20
30
Figure 85 Fired volume shrinkage of the brick samples
20.0
(a) 1000 °C
(b) 1050 °C
(c) 1100 °C
18.0
Water Absorption, %
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
Raw
U2
Underflow
U4
0.0
0
10
20
30
0
10
20
30
0
Figure 86 Water absorption of the brick samples
10
20
30
Page 110 of 143
25
Water Absorption, %
20
15
10
5
0
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
Fired Bulk Density, g/cm3
Figure 87 Relationship between water absorption and fired density of the brick samples
The strength of the samples (Figure 88) decreased with increasing replacement level at 1000°C and
1050°C firing. However, at 1100°C firing, higher strength was observed for U2 and U4 samples up to the
20% replacement level. The strength at the 10% level was higher than that at 20% and U4 samples also
had higher strength than U2 samples, probably due to the effects of fly ash fineness.
Therefore, for the particular clay used, the optimum solution to give low water absorption and high
strength is lower LOI and finer materials, with 10 to 20% clay replacement, firing at 1100°C.
22.0
(a) 1000 °C
(b) 1050 °C
(c) 1100 °C
20.0
Tensile Strength, N/mm2
18.0
16.0
14.0
12.0
10.0
8.0
6.0
Raw
Underflow
4.0
U2
U4
2.0
0
10
20
30
0
10
20
30
Figure 88 Tensile strength of the brick samples
0
10
20
30
9.7.2.
Page 111 of 143
Concrete masonry units
Concrete masonry units (Blocks) are usually produced by precast manufacturers and all types of fly ash
can be used in this application. While fine fly ash acts as a cement component, coarse fly ash is used as
fine aggregates. Current specifications for blocks are covered in BS EN 771-3 (BSI, 2003) and BS 60732 (BSI, 2008). The main property requirements for these include mechanical strength, bond strength,
water absorption, dimensions/ tolerances, appearance, density, thermal properties, and durability.
Bottom ash has been widely used in the manufacture of lightweight concrete blocks (Sear, 2001). This is
the coarse residue remaining after coal combustion. However, when using bottom ash instead of natural
lightweight aggregates to produce blocks, higher cement contents are generally necessary to meet strength
requirements and the products appear unusually porous compared with conventional blocks. In an effort
to increase strength without increasing cement addition, work has been carried out (Groppo et al., 2005)
to substitute processed material as fine aggregate. The results from this are shown in Figure 89.
14
Compressive Strength, N/mm 2
12
10
8
6
3 days
7 days
14 days
28 days
4
2
0
0
10
20
30
40
50
Fly Ash Substitution, %
Figure 89 Effect of fly ash substitution as fine aggregate on compressive strength for bottom ash blocks
(Cement:Aggregate = 1:4) (Groppo et al., 2005)
It can be seen that the compressive strength increased for substitution levels up to 30%. Further
increasing the level to 45% did not provide additional strength gain. At 30% substitution, strengths were
above the target 6.8 N/mm2, even after only 3 days of curing. Since pozzolanic reactions primarily occur
after longer curing times (i.e. 28 days), the early strength gains that occurred with fly ash are attributed to
decreasing the void volume.
Similar results could be achieved with material used in current study, however the optimum fly ash
substitution will depend on the fineness of processed material. Substitution with U1 is likely to be similar
to the above, while that of U2 probably slightly lower. This can be estimated with particle packing
models developed in earlier studies (Jones et al., 2003a). It is noted that similar type effects were
observed in the soil stabilisation study in Section 9.6.
Page 112 of 143
9.8. Other Uses
9.8.1.
Fuel substitute
In addition to obtaining a series of processed materials, the processing techniques used during the
investigation produced carbon rich products, such as the underflow in primary classification and the froth
in froth flotation. The reburning of these high-LOI products in utility boilers would be a relatively simple
method of utilizing this concentrated unburnt carbon.
To assess the combustion properties of carbon rich materials, calorific values of Power Station 3, highLOI processed materials were measured and the results are given in Table 59. This work was carried out
at the University of Kentucky (Robl and Groppo, 2008) according to ASTM D 5865, Standard test
method for gross calorific value of coal and coke. It can be seen that the calorific values of the froths
were high, and therefore, they could be used as fuel and worth around 0.5 of (dry ash free) bituminous
coal, in terms of its calorific contribution. It should be noted that the carbon content could be further
concentrated using column flotation. Carbon rich material with more than 70% to 90% purity level from
froth flotation was obtained and has been reported in the literature (Groppo et al., 1995, Hwang et al.,
1995, and Smalley et al., 2006). The test results also indicate that the calorific value was proportional to
the LOI as shown in Figure 90.
Table 59 Gross calorific values of Power Station 3 processed materials (Robl and Groppo, 2008)
Gross Calorific Value
Sample ID
BTU/lb
kJ/kg
Equivalent coal
Raw
1643
3822
0.13
Underflow
2125
4943
0.16
Froth-1
6508
15138
0.50
Froth-2
5981
13912
0.46
U1
609
1417
0.05
Bituminous coal
(dry ash free)
13000
30238
-
18000
Gross Calorific Value, kJ/kg
16000
14000
12000
10000
8000
6000
4000
2000
0
0
5
10
15
20
LOI, %
25
30
35
40
Figure 90 Relationship between calorific value and LOI of processed material
9.8.2.
Page 113 of 143
Raw feed in cement manufacture
With the high calorific values of the carbon rich material shown above, the material could also be used as
a portion of the raw feed in cement manufacture. The benefits are not only in saving raw clay materials
but also in reducing fuel costs.
Commercial-scale demonstrations of the use of carbon rich fly ash in cement manufacture have been
successfully performed (Bhatty et al., 2003). Fly ash with 13% LOI was used to replace the original kiln
feed at 3% level. It was noted the clinker production was increased and the fuel consumption reduced by
2.6%.
The above results could be reasonably achieved with raw material from Power Station 3 (Mean LOI
=14%). If froth materials with LOI = 31 – 36 % were used, greater fuel saving could be achieved.
9.8.3.
Cenospheres
When processing stockpile material, such as that from Power Stations 2 and 3, a considerable quantity of
cenospheres were collected. These are hollow alumosilicate microspheres (See Figures 60 and 73), which
are a valuable industrial product.
Cenospheres are mainly used as filler composites in inorganic and organic binders. With a density lower
than water (typically 0.6 – 0.8), cenospheres provide up to four times the bulking capacity of normal
weight fillers and can be used to produce a light and heat-insulating material. The microspherical-shape
dramatically improves the rheology of fillers, whether in wet or dry applications. With an alumosilicate
structure, it provides inertness and chemical stability to the material. As it is a refractory material, it can
resist high temperatures (UKQAA, 2002a, and Drozhzhin, et al., 2005).
Cenospheres can be used in plastics, GRP, light weight panels, refractory tiles and almost anywhere
traditional fillers are used. As a result of their flexibility, they are used in many high technology and
traditional industries. Aerospace, hovercraft, carpet backing, window glazing putty, concrete repair
materials, horticultural use, brake and clutch linings, intumescent coatings, insulating/refractory products
and offshore oil/gas production industries represent possible applications for these fillers (UKQAA,
2002a, and Drozhzhin et al., 2005).
9.9. Summary
After the processing work with the system developed, the materials were used in a range of scoping
studies. The main objective was to explore the range of end uses, towards enabling full use of the
recovered material.
The standard mortar test results show that the water requirement of processed material was significantly
reduced, suggesting that water savings could be achieved in using the materials in concrete. The activity
index of the processed material increased with fineness, however, no significant differences were found
between the fine processed material and raw material.
The processed materials were divided into different grades for various applications. The finer fractions,
U3, U4 and U5, were used as cement components in concrete, in both laboratory trials and precast
concrete production. The concrete made with processed material gave satisfactory strength development
and the colour was similar to that of PC. This was also the case in the manufacture of a precast concrete
element. The strength obtained with ultrafine material was not as expected, giving little or no difference
compared to raw material. This may be caused by surface hydration during the long-term storage and
processing period, which affects the pozolanic reactivity of processed materials.
Page 114 of 143
Medium fineness material, U2, was used as a component of cement-based grouts. The grouts prepared
with processed material demonstrated improved consistence, and satisfactory strength development
compared to those made with raw material.
Coarse and medium fineness materials, U1 and U2, were considered in concrete masonry units and
foamed concrete as a fine aggregate. The literature indicates that when using fly ash to replace bottom
ash lightweight aggregate, the compressive strength of masonry units increased due to the improved
aggregate grading and optimal replacement was around 20% to 30%. A 50% replacement of sand was
used in foamed concrete and the results gave improved consistence, enhanced strength development and
similar colour to the PC mix.
Coarse material with high LOI, i.e. underflow from primary classification, was used as a component of
fired clay bricks. Processed products U2 and U4 were also tried in this application. The results indicate
that with the particular clay used (Eldon shale clay), the optimal solution to achieve low water absorption
and high strength was low LOI, fine materials with 10% to 20% clay replacement, firing at 1100°C.
Processed material use as an addition in lime soil stabilisation was explored. The fly ash addition can
significantly reduce swelling of high sulfate content clays stabilised by lime. Greater addition levels gave
lower swelling. At the 24% addition level, swelling can be reduced to about 25% of the pure lime
stabilised clay sample. At lower addition level, 6% and 12%, raw material was better than processed
material and coarse fraction, U1 appeared to be better than fine one, U4 in reducing swelling. Strength
was increased with increasing fly ash addition up to an optimum level, which was approximately 18% for
raw and fine materials.
The calorific values of processed materials were measured. It was found that in high LOI products, e.g.
froths from flotation, that these were relatively high and, therefore, these could be used as fuel. The
calorific value was also proportional to the LOI. The high LOI fly ash could also be used as raw feed in
cement manufacture, beneficial both in saving raw materials and energy.
10. SUMMARY OF RESEARCH FINDINGS
The main objective of the project was to recover and process lagoon and stockpile materials for use as
valuable resources in a range of applications and thereby establish an integrated approach to its use. The
project comprised six main work steps for achieving the above: (i) quantification of material for recovery
which included site survey, material sampling and characterisation, (ii) design and fabrication of a pilotscale processing system, (iii) evaluation of the processing system to establish operation parameters,
(iv) processing of sufficient quantities of material recovered from site for scoping studies, (v) scoping
studies covering a range of applications of low, medium and high value with the processed material, and
(vi) development of guidelines for fly ash processing and use.
This section provides a summary of the main research findings.
10.1. Material Sampling and Characterisation
Five different power station sites were investigated and samples from these characterised. The
investigated sites covered lagoon and stockpile storage areas. Detailed surveys were carried out at two of
the Power Stations, covering a lagoon and stockpile.
Lagoon material
The properties of material from the lagoon and their ranges in the storage areas were as follows:


LOI: ranged from 4% – 21%, mean about 8%;
Fineness:


Page 115 of 143
o D90, ranged from 50 – 500 m, mean about 150 m;
Chemical compositions:
Mineralogical compositions:
The LOI tended to increase in more recently produced materials, suggesting that this material has a higher
carbon content. This may be due to the changes in burning conditions, e.g. lower temperature to meet
NOx emission limits.
Only a small fraction of cenospheres was found in the lagoon material, since these were probably
dispersed in the slurry water.
From the point of view of processing, the results indicate that there are a range of particle sizes and types
in the lagoon. In addition, there was some particle agglomeration. This suggests that separation of
particles into a range of fraction should be possible. However, with the appearance of some of the
particles, which were fused and contained hydration products on their surfaces, their separation may be
difficult and their reactivity may be affected.
Stockpile material
The properties of stockpile material and their ranges in the storage area were as follows:




LOI: ranged from 11% – 18%, mean about 14%;
Fineness:
o D90, ranged from 97 – 155 m, mean about 123 m;
Chemical compositions:
o High chloride content could be found in newly placed material, due to the fact it was
conditioned using sea water. However, in the majority of samples, the chloride content
was very low;
Mineralogical compositions:
The LOI tended to increase in more recently produced material, which could be as high as 25 %, and the
average was higher than that for the lagoon material.
Material fineness and its variation with location tended to be lower than that in the lagoon. No significant
changes were found in chemical and mineral properties with location and time. Significant quantities of
cenospheres were found in the stockpile material.
The limited tests at the other three power station sites gave properties within the ranges noted for the
above two sites.
Page 116 of 143
10.2. Fly Ash Processing
A pilot-scale processing system was developed in collaboration with research colleagues from the
University of Kentucky, which used primary classification, froth flotation (column and mechanical) and
lamella hydraulic classification.
Primary classification was used to remove coarse (i.e. > 150 m) particles and to reduce the coarse
carbon content, prior to the following froth flotation process. More than 90% of the > 150 m particles
could be removed with the processing arrangement. The LOI of the overflow could be reduced by up to
2.0% compared to the raw feed using this method, i.e. around 25 to 40% of the total carbon. However, a
portion of fine particles was also lost during this process. Further refinement of the system will enable
this to be improved.
Froth flotation
With column flotation, the LOI was reduced by 2.0% compared to the feed, while the froth had about
40% LOI. This suggests that column flotation is an effective way to obtain high carbon products, which
could be used as fuel.
Mechanical flotation was also used to remove carbon. With two-cell, the LOI was reduced by 6.0%
compared to the feed, while the froth had about 33% LOI. With four-cell, the LOI was reduced by 10.0
to 12.0% compared to the feed. This suggests that mechanical flotation is more effective for obtaining
low carbon materials, e.g. a cement component.
A combination of the above carbon removal methods may achieve an optimum solution, with high carbon
froth and low carbon tails, as well as low fine particle loss.
Lamella hydraulic classification
Lamella hydraulic classification was used to separate material into different size fractions. Five fractions
were obtained from the classifier, i.e. (i) coarse fraction, U1; (ii) medium fraction, U2; (iii) fine fraction,
U3 and U4 (iv) ultrafine fraction, U5, and (v) cenospheres.
The results indicate that fineness and LOI were gradually reduced from U1 to U5. LOI of U1 and U2 are
higher than the feed, while this reduced for U3, U4 and U5. When LOI of raw material was lower or just
slightly higher than that required by BS 8500, lamella hydraulic classification could be used directly in fly
ash processing without the need for froth flotation.
There were no significant changes in chemical and mineralogical compositions of the materials following
processing.
Material yields
With processing, there was a balance between the level of LOI in the separated fraction and the quantity
of material yielded. Processing was found to depend on fly ash source. The best results obtained for the
materials tested were as follows:
During primary classification, 20-25% of the total material was removed, which took off 26-27% carbon
and 14-19% ultrafine particles. If more carbon was removed, more material and fine particles tended to
be lost.
In froth flotation, two-cell flotation took 23% of the total material to the froth, removed 46% carbon and
20% ultrafine particles. Four-cell flotation took 30% of the total material to the froth, removed 60%
Page 117 of 143
carbon and 29% ultra fine particles. As above, removal of more carbon gave a greater loss of material
and fine particles.
When less fine particles were lost during primary classification and flotation, more ultrafine material
could be obtained in lamella hydraulic classification. The ultrafine fraction yield, U5 was 6.2% and 4.1%
for two-cell and four-cell flotation respectively. Further refinement to optimise could improve yields.
Burnout at high temperature (600C) gave almost no carbon content materials and avoided the loss of fine
particles.
10.3. Scoping Studies for End Use Applications
The processed materials were divided into different grades for various applications. For example: the
finer fractions, U3, U4 and U5, were used as cement components in concrete, both in laboratory trials and
precast concrete production. Medium fineness material, U2, was used as a component of cement-based
grouts. Coarse and medium fineness materials, U1 and U2, were used in masonry units and foamed
concrete as a fine aggregate. Carbon rich fractions were, for example, use as fuel and raw feed in cement
manufacture.
Concrete
Standard mortar test results show that the water requirement of processed material was significantly
reduced, suggesting that water savings could be achieved with the materials in concrete. The activity
index of the processed material increased with fineness, however, no significant differences were found
between the fine processed and raw materials.
The concrete made with processed material showed satisfactory strength development and the colour was
similar to that of PC. This was also found to be satisfactory in the manufacture of a precast concrete
element.
The strength obtained using ultrafine material was not as expected, giving little or no difference compared
to raw material. This may be caused by surface hydration during long-term storage and wet processing.
Indeed, some hydration products were found on the surface of ultrafine particles, especially in the U5
fraction, which may influence its chemical activity.
Grouts
The grouts prepared with processed material, U2, demonstrated improved consistence, and satisfactory
strength development compared to those made with raw material.
Foamed concrete
A 50% replacement of sand using U1 and U2 was tested in foamed concrete and the results gave
improved consistence, and enhanced strength development compared to the PC mix.
Fired bricks
Coarse material with high LOI, i.e. primary classification underflow, was used as a component of fired
bricks. Processed fractions U2 and U4 were also tested in this application. The results indicate that with
the particular clay used (Eldon shale clay), the optimal solution to achieve low water absorption and high
strength was low LOI and fine materials with 10% to 20% clay replacement, firing at 1100°C.
Concrete masonry blocks
Coarse fractions can be used as sand/filler components in concrete blocks to increase their packing
density and strength. There is an optimum fly ash substitution in terms of strength enhancement, which
will depend on the fineness of the processed material.
Page 118 of 143
Lime soil stabilisation
Fly ash addition can reduce swelling of high sulfate content clays stabilised by lime. This increases with
fly ash level and at 24% addition, swelling was reduced to about 25% of the lime-only stabilised clay
sample. At lower fly ash addition levels, 6% and 12%, the results indicate that raw material gave less
expansion than processed material and the coarse fraction, U1 less than the fine one, U4. Strength
increased with fly ash addition but there was an optimum level of fly ash, which was approximately 18%
both for raw and fine materials.
Fuel substitute
The calorific values of processed fractions were measured. It was found that in the carbon rich fractions,
e.g. froths from flotation, the values were relatively high and, therefore, this could be used as fuel. The
calorific value was also proportional to the LOI of the materials. The carbon rich material can also be
used as raw feed in cement manufacture, beneficial both in saving raw materials and reducing energy
requirements.
11. PRACTICAL GUIDELINES AND ACTIONS RESULTING FROM THE RESEARCH
The main outcomes drawn from the work have been presented at the end of each section and summarised
in Section 10 above. It is therefore the aim of this section to draw the various stages of the research
together, to consider (i) the practical implications arising from the study and to provide guidelines, and
(ii) issues regarding material recovery if the technology developed is used at an industrial-scale and future
research.
11.1. Guidelines for Fly Ash Processing
The processing system developed in this study at pilot scale is a combined wet processing system, which
included primary classification, froth flotation and lamella hydraulic classification to remove carbon and
separate material into different sized fractions.
The system developed is suitable for processing fly ash, which is stored in lagoons and stockpiles for
various construction applications. It should be noted that the system is only suitable for low-lime fly ash
(Class F fly ash as defined in ASTM C618), as high-lime fly ash may hydrate and harden during storage,
thereby making it potentially difficult to breakdown and handle.
A site survey and material characterisation according to BS EN 450 should be carried out to determine the
material fineness, LOI and chemical composition, which will allow estimates of the properties and
materials quantities available for recovery at the site to be established and processing requirements
determined. The use of historical records may also assist in this process.
The material to be processed should be mixed with water to produce a slurry with a solids content of
about 10% to 15%. Pre-screening should be applied if the slurry contains extraneous material (coarse ash,
vegetation, and miscellaneous debris) greater than 5 mm, which may cause plugging problems during
processing operations.
The slurry should be agitated continuously in a slurry supply tank during processing to keep it well mixed
and stable. Primary classification should be used to remove coarse (i.e. typically > 150 m) particles.
This can also significantly reduce the coarse carbon content of the material and is beneficial for the
following froth flotation process.
Froth flotation can be used to remove carbon, especially fine carbon residues in the material. There are
two options for carbon removal through froth flotation: column and mechanical flotation. Mechanical
flotation is more effective for obtaining low carbon materials. It contains a bank of flotation cells to meet
different carbon removal requirements. More cells are required when lower LOI materials are targeted.
There is, however, a balance between yields and reductions in LOI achievable. Column flotation is more
effective for obtaining high carbon concentration materials.
Page 119 of 143
When the raw feed has low LOI (i.e. lower or just slightly higher than 7%), froth flotation may not be
required since most coarse carbon can be removed by hydraulic classification.
The control parameters in the above processing may change with the specific equipment to be used and
characteristics of the material to be processed. The parameters given in the current study (Table 32) can
be used as a starting point. Trial would need to be carried out to optimise the process.
11.2. Guidelines for End Use Applications with Processed Material
There are a wide range of applications in which the use of processed material can be made. Technical
requirements vary for different applications, and the processed materials can be classified into several
grades in relation to these as given in Table 60.
When the processed material is used in a particular application, it should meet the specification
requirements for normal fly ash use in the application, such as:
Cement components in concrete – BS EN 450-1, BS EN 206-1, BS 8500;
Cement-based grouts – BS 3892-3;
Concrete masonry units – BS EN 771-3, BS 6073-2;
Foamed concrete: Specification and quality control of foamed concrete incorporating RSA
(Jones et al., 2007);
Appendix A gives a summary of the key parameters and applicable ranges for fly ash use in various
applications, which can be applied to processed materials. It should be noted that in high value
applications, i.e. when the material is used as a cement component, wet long-term storage may lead to
losses in reactivity. This may affect subsequent performance and would need to be taken account of. For
example, in concrete, the mix proportions may need to be modified.
Table 60 Classification of processed materials, their main properties and application scope
Properties
Fineness, 45µm
LOI,
sieve retention,
% by mass
% by mass
Material
Processed
Fractions
High
Performance
U5 (Lamella
classification)
near to 0.0
(D50=4.0-5.5m)
3.5-6.0
Cement
Component
U3 and U4
(Lamella
classification)
4.5-11.5
(D50=17.5-21.0m)
2.5-7.5
Sand/Filler
Component
U1 and U2
(Lamella
classification)
19.0-58.5
(D50=27.0-57.0m)
3.0-10.0
Coarse and
Carbon Rich
Underflow
(Primary
classification)
54.5-69.5
(D50=50.0-68.5m)
14.0-17.5
Carbon and
Unburnt Coal
Froth (Froth
flotation)
28.5-39.0
(D50=29.5-40.5m)
29.0-36.0
Cenosphere
Cenosphere
(Lamella
classification)
near to 100 (size
from 50 to 250 m)
< 0.5
Application Scope
High performance concrete and
precast concrete products
Majority of construction
applications, e.g. in normal
concrete as cementitious materials
Majority of construction
applications as fill materials (i.e.
aggregates)and agricultural
applications
Raw feed in cement manufacture,
or in sintered products, such as
lightweight aggregates
Potential fuel or partial coal
replacement
Very high value applications, e.g.
polymer filler
Page 120 of 143
11.3. Quantification of Potential Fly Ash Material Available for Recovery
The results of the processing work have demonstrated that a range of materials can be recovered
following processing and these are influenced by the nature of the material itself and that of the
processing techniques used. Quantification of potential material for recovery can be based on the fraction
output balance data from the processing trials. An estimation of this for Power Station 3 is given in
Table 61.
There are around 16 Mt of fly ash stockpiled at Power Station 3, for which it is assumed that around 2/3,
i.e. 10 Mt are easy to access. With the processing system developed in this study, it would be possible to
produce approximately (i) 0.5 Mt of high performance fly ash (U5); (ii) 1.8 M
ordinary quality fly
ash (U3 and U4); (iii) 2.7 Mt of sand/filler (U1 and U2); (iv) 2.3 Mt of coarse with high carbon fractions
(Underflow of primary classification); (iv) 2.7 Mt of high carbon fuel fractions (Froths of froth
flotation);and (iii) 0.05 Mt of cenospheres.
Table 61 Estimated processed fly ash materials available at Power Stations 3 and in the UK
Materials
Processed
Fractions
Total material available
High
Performance
U5 (Lamella
classification)
Cement
Component
U3 and U4
(Lamella
classification)
U1 and U2
(Lamella
classification)
Underflow
(Primary
classification)
Sand/Filler
Component
Coarse &
Carbon
Power Station 3
% by mass1
×103 tonnes
10,000 ×103 tonnes
In the UK2
×103 tonnes
60,000 ×103 tonnes3
5.2%
515
3090
18.1%
1810
10860
27.2%
2715
16290
22.6%
2260
13560
Fuel (Unburnt
Material)
Froth (Froth
flotation)
26.6%
2660
15960
Cenosphere
Cenosphere
(Lamella
classification)
0.5%
50
300
1. Values used here are the mean results of the balance calculation for Power Station 3
material processing with mechanical froth flotation (See Section 8.3.2).
2. Value is based on estimations from previous study (Dhir et al., 2005), which was around
120 Mt available in total and half easy to access.
3. Assuming similar fraction yields to Power Station 3 material processing are achievable.
According to the previous survey carried out (Dhir et al., 2005), there are around 120 Mt of lagoon and
stockpile materials available for recovery around operational UK power stations. If it is assumed that
half, i.e 60 Mt is easy to access, and that similar output rates to Power Station 3 material can be achieved
with the processing system, then the following materials would be available for recovery in the UK:
(i) 3.1 Mt of high performance fly ash (U5); (ii) 10.9 Mt of ordinary quality fly ash (U3 and U4); (iii)
16.3 Mt of sand (U1 and U2); (iv) 13.6 Mt of coarse with high carbon fractions (Underflow of primary
classification); (iv) 16.0 Mt of high carbon fuel fractions (Froths of froth flotation); and (iii) 0.3 M tonnes
of cenospheres.
Page 121 of 143
11.4. Actions Resulting from the Research
With ever decreasing dependence on coal as a fuel and thereby reduced quantities of fly ash being
produced, the outcomes of this project show valuable resources of fly ash, which are important to ensure
there is not a reversion to using PC only as cementitious construction material. Furthermore, they can
contribute to reduced storage problems around power station sites. With substantial ‗deposits‘ of fly ash
across the UK, their recovery and re-use will also contribute to greenhouse gas amelioration.
One outcome of this research project, which merits future investigation, is the potential for capturing the
energy of the separated high carbon content stream, which, given the tests carried out, has significant
calorific value and could be reburnt at the power station. This would utilise the high energy content and
improve material reactivity, which has been demonstrated previously.
The work has identified that there are substantial quantities of cenospheres, ie alumino-silicate hollow
spheres, with densities less than 1000 kg/m3, available from stockpile-stored material, which are readily
extractible using the pilot-scale processing system. Given their potentially high net worth, further work
will be carried out to characterise these both physically and compositionally and with respect to their
morphology to establish that they can be used in the same manner as cenospheres extracted after normal
production.
Regarding the processing technology developed in the project itself, further refinement to optimise the
operation parameters could improve yields for different material sources. Some improvement may be
required to reduce the fine particle loss during processing and to improve material pozzolanic reactivity
which was affected due to wet long-term storage and/or processing.
Discussions with key trade associations in terms of mortar and concrete supply have identified that ‗good
quality‘ fine aggregate is increasingly difficult to source. It is believed that recovered material used in
conjunction with modern admixtures and careful mix design can be used for this purpose. The key issue
is that very fine carbon is distributed in fine material and is uneconomic to extract. However, end use as a
high quality aggregate has significant environmental / sustainable attributes and this is an important line
of further work, which needs to be undertaken.
Scaling-up would be the next stage, but this is out with the scope of this research project, and one issue
that needs to be addressed is the perception or reality of handling recovered materials within a power
station, which would not have a waste handling licence. This is a complex area given the heterogeneous
nature of the materials in storage. Whilst this would not prevent the outcomes of the project being
exploited, the perception at least may inhibit the uptake of this type of technology. There may also need
to be national standards for such materials and this may not be suitable for British/EU standards, but may
come from organisations such as the British Board of Agrément.
It is clear that there would need to be a considerable shift in policy to significantly increase coal
combustion by-product consumption beyond its current levels. That said, there is increasing interest in
PC/fly ash combinations to reduce CO2 in cementitious construction materials. A clear metric to
recognise this needs to be agreed which would thereby enable users, specifiers and government
organisations to promote increased use.
Page 122 of 143
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Page 134 of 143
APPENDIX A Key Parameters and Applicable Ranges for Fly Ash Use in Various
Applications
Table A1 Key Parameters and Applicable Ranges for Fly Ash Use in Various Applications
Key Parameters and Applicable Ranges
Application Area
Utilisation if
known, %
Construction Industry
Cement, concrete
≤ 55
and mortar
Building blocks
20-25 typical
≤ 95 autoclave
Aerated concrete
≤ 80
blocks
Foamed concrete
≤ 50
Concrete roofing
tiles
Cast stone
≤ 60
Lightweight
≤ 85
aggregate
Fired bricks / tiles
≤35
Soil stabilisation
≤ 30
Flowable fill/grout
≤ 70
RCC
60-80
Embankment / fill
≤ 100
Filler in asphalt
≤5
Raw feed in cement
≤5
manufacture
Agricultural Application
Soil improvement
70 t/ha
Plant growth
10 t/ha
improvement
Components Extracting
Recovered Carbon
fuel
Recovered metals
Fineness, Moisture LoI, Composition, Activity Specific Bulk Colour
µm
content,
%
%
index surface, density,
%
cm2/g g/cm3
Oxide Mineral
< 40.0
< 0.5
≤ 7.0








≥ 75%














< 0.5














< 75







































Sorbent / Confinement Agent
Brownfield clean-up


Waste stabilization


Repository backfill


Zeolite precursor

Adsorbent


Constituent in Various Products
Filler in paints and
≤ 40%

enamels
Wood substitute

Geopolymer
Metal castings or

lightweight alloys
Vitreous products


or glass ceramics
 Indicates parameter may require consideration for the application.











































APPENDIX B
Page 135 of 143
Summary of the Fly Ash Samples
Table B1 Summary of the fly ash samples
Power Station 1
Sample Code
Ash large
sample
Ash-1
Ash-2
Ash-3
Ash-4
Ash-5
Date of Sampling
Amount of the sample (kg)
22/05/07
1000
09/08/07
150
09/08/07
150
09/08/07
150
09/08/07
150
09/08/07
150
Power Station 2
Sample Code
Ash large
sample
Date of Sampling
17/05/07
3000
Power Station 3
Sample Code
Ash
1-1
Ash
1-2
Ash
1-3
Ash
2-1
Ash
2-2
Ash
2-3
Date of Sampling
16/08/07
30
16/08/07
1000
16/08/07
30
16/08/07
30
16/08/07
30
16/08/07
30
Sample Code
Ash
3-1
Ash
3-2
Ash
3-3
Ash
4-1
Ash
4-2
Ash
4-3
Date of Sampling
16/08/07
30
16/08/07
1000
16/08/07
30
16/08/07
30
16/08/07
30
16/08/07
30
Sample Code
Ash
5-1
Ash
5-2
Ash
5-3
Ash
6-1
Ash
6-2
Ash
6-3
Date of Sampling
16/08/07
30
16/08/07
30
16/08/07
30
16/08/07
30
16/08/07
30
16/08/07
30
Sample Code
Ash
7-1
Ash
7-2
Ash
7-3
Ash
8-1
Ash
8-2
Ash
8-3
Date of Sampling
16/08/07
30
16/08/07
30
16/08/07
30
21/09/07
30
21/09/07
30
21/09/07
30
Sample Code
Ash
9-1
Ash
9-2
Ash
9-3
Ash
10-1
Ash
10-2
Ash
10-3
Date of Sampling
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
Sample Code
Ash
11-1
Ash
11-2
Ash
11-3
Ash
12-1
Ash
12-2
Ash
12-3
Date of Sampling
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
Sample Code
Ash
13-1
Ash
13-2
Ash
13-3
Ash
14-1
Ash
14-2
Ash
14-3
Date of Sampling
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
21/09/07
30
Page 136 of 143
Table B1 Summary of the fly ash samples (Continue)
Power Station 3
Sample Code
Ash
15-1
Ash
15-2
Ash
15-3
Date of Sampling
21/09/07
30
21/09/07
30
21/09/07
30
Sample Code
Ash-1
Ash-2
Ash-3
Ash-4
Ash-5
Ash-6
Date of Sampling
08/07
30
08/07
30
08/07
30
08/07
30
08/07
30
08/07
30
Sample Code
Ash-1
Ash-2
Ash-3
Ash-4
Ash-5
Ash-6
Date of Sampling
08/07
30
08/07
30
08/07
30
08/07
30
08/07
30
08/07
30
Power Station 4
Power Station 5
Page 137 of 143
APPENDIX C Results of Previous Study for Power Station 1
Table B1 Fineness and LOI of the fly ash samples from Power Station 1 in 2004 (Dhir et al., 2005)
Sample
Position
No.
Fineness, 45µm sieve retention, % by mass
LOI, % by mass
Position in Depth
Position in Depth
top
middle
bottom
top
middle
bottom
1
2
3
4
5
6
7
8
9
10
11
39.3
45.2
31.8
61.9
50.0
n/a
n/a
28.7
23.6
15.1
38.1
41.7
35.0
50.5
44.1
48.2
25.1
44.3
42.6
33.6
42.1
39.8
60.1
45.1
58.4
40.9
39.2
32.2
57.3
46.9
18.8
12.0
29.7
9.1
17.9
10.3
5.8
7.5
n/a
n/a
9.6
8.4
7.3
10.9
6.8
8.4
5.9
5.4
6.4
6.0
6.1
5.1
6.0
5.8
8.2
3.9
13.5
20.2
6.5
4.2
3.8
8.0
5.1
5.1
7.0
9.6
Max
Min
Mean
61.9
15.1
37.1
50.5
25.1
40.6
60.1
12.0
40.0
17.9
5.8
9.6
8.4
5.1
6.4
20.2
3.8
7.9
Table C2 PSD values, d10, d50, and d90 of the samples from Power Station 1 in 2004 (Dhir et al., 2005)
PSD values, d10, d50, and d90, µm
Sample
Position
No.
Position in Depth
top
middle
bottom
d10
d50
d90
d10
d50
d90
d10
d50
d90
1
2
3
4
5
6
7
8
9
10
11
4.80
5.32
3.62
12.4
7.01
n/a
n/a
4.33
4.88
3.20
5.10
33.1
51.6
23.4
88.9
53.0
n/a
n/a
26.6
27.7
19.2
38.0
106.2
201.6
102.2
333.6
217.6
n/a
n/a
78.5
96.4
65.7
102.1
5.52
4.87
6.39
10.0
7.49
3.61
5.64
6.02
4.80
8.64
7.04
41.2
29.6
49.7
72.9
56.2
19.5
44.9
43.2
30.5
45.6
39.4
164.2
111.1
205.6
246.1
161.0
86.9
154.0
151.0
108.3
115.6
113.3
14.3
4.45
8.79
7.15
6.50
5.80
7.42
5.97
4.15
3.29
5.09
81.0
46.6
100.5
43.0
38.2
34.4
66.6
49.6
20.5
13.4
33.5
218.9
181.8
521.4
122.2
107.6
111.7
161.9
138.9
72.3
54.1
115.0
Max
Min
Mean
12.4
3.20
5.63
88.9
19.2
40.2
333.6
65.7
144.9
10.0
3.61
6.37
72.9
19.5
43.0
246.1
86.9
147.0
14.3
3.29
6.63
100.5
13.4
47.9
521.5
54.1
164.2
Page 138 of 143
Table C3 Bulk oxide composition of the samples from Power Station 1 in 2004 (Dhir et al., 2005)
Sample
CaO
SiO2
Al2O3 Fe2O3 MgO
1top
1second
1third
1bottom
2top
2second
2third
2bottom
3top
3middle
3bottom
4top
4middle
4bottom
5top
5middle
5bottom
6middle
6bottom
7middle
7bottom
8top
8middle
8bottom
9top
9middle
9bottom
10top
10middle
10bottom
11top
11middle
11bottom
2.7
2.7
2.8
2.2
2.0
2.4
2.2
2.1
2.2
2.3
2.0
2.5
3.7
2.9
3.0
2.9
2.7
2.9
3.2
3.2
2.9
2.8
2.9
2.9
3.3
2.7
3.0
2.6
2.9
3.1
2.3
2.3
2.4
47.1
46.1
51.0
52.5
43.3
49.0
45.3
46.9
40.0
50.4
41.8
50.4
49.2
47.2
46.6
48.3
49.7
46.1
47.6
47.2
45.2
48.4
49.7
51.6
45.9
47.2
49.1
48.1
49.5
46.2
42.9
48.6
47.7
28.8
29.5
30.8
31.4
26.5
29.9
29.8
28.1
28.7
29.7
24.3
27.6
27.2
29.3
28.4
27.2
29.9
29.4
30.2
26.6
28.2
28.0
28.1
26.0
28.7
29.4
26.8
29.3
28.9
29.0
29.8
28.7
28.0
4.6
5.1
4.2
3.8
4.1
5.0
4.8
3.4
3.7
4.5
3.3
4.9
5.4
4.9
5.3
6.2
5.1
6.3
6.0
6.8
6.5
3.5
5.0
4.0
4.7
6.0
6.2
5.3
4.0
5.9
4.8
4.4
3.8
Max
Min
Mean
3.7
2.0
2.7
52.5
40.0
47.5
31.4
24.3
28.6
6.8
3.3
4.9
TiO2
K2O
Na2O
P2O5
Clˉ
SO3
1.9
1.1
1.6
1.3
1.6
2.1
1.4
1.3
1.2
1.6
1.2
2.0
2.0
2.0
1.6
1.6
1.5
1.6
1.5
1.6
1.4
1.3
1.9
1.6
1.8
1.5
1.5
2.1
1.9
1.8
1.7
1.6
1.4
1.4
1.4
1.5
1.4
1.2
1.4
1.4
1.3
1.3
1.3
1.2
1.4
1.3
1.4
1.5
1.5
1.6
1.6
1.6
1.5
1.4
1.2
1.3
1.6
1.5
1.4
1.4
1.4
1.5
1.5
1.5
1.4
1.4
1.0
0.9
1.1
1.0
1.1
1.1
1.2
0.7
0.9
0.9
0.8
1.1
1.1
1.1
1.1
1.2
1.2
1.3
1.3
1.2
1.1
0.9
1.0
1.2
1.1
1.4
0.9
1.1
0.9
1.6
0.9
0.9
1.0
0.4
0.7
0.7
1.0
0.5
0.3
0.9
0.6
3.8
0.5
1.9
1.0
0.8
0.9
0.9
1.4
0.7
1.1
0.9
1.8
1.4
0.6
2.3
1.4
1.4
0.9
2.0
0.3
0.9
0.6
1.4
1.2
1.2
1.1
1.0
1.0
0.5
0.9
0.9
0.8
0.6
1.0
1.2
0.9
0.8
0.6
1.0
0.8
0.6
0.9
0.8
0.9
0.6
0.9
1.2
1.0
0.9
1.1
0.9
1.0
0.9
1.0
0.9
1.0
1.1
1.1
1.3
0.6
0.7
0.6
0.6
0.4
1.7
1.3
6.4
1.3
2.0
1.5
1.8
2.0
2.3
2.1
1.8
2.0
2.4
2.7
2.5
1.4
1.2
2.6
1.4
2.3
2.4
0.6
2.3
1.4
2.4
1.2
1.9
0.6
0.7
0.7
0.2
0.3
0.7
0.3
0.3
0.5
0.3
0.3
0.9
1.2
0.8
0.7
0.5
0.6
0.8
0.5
0.6
0.5
0.9
0.3
1.0
0.6
0.2
0.4
0.7
0.3
0.9
0.3
0.4
0.4
2.1
1.1
1.6
1.6
1.2
1.4
1.6
0.7
1.1
3.8
0.3
1.1
1.2
0.5
0.9
6.4
0.4
1.8
1.2
0.2
0.6
Page 139 of 143
Table C4 Major Mineralogical composition of the fly ash from Power Station 1 in 2004 (Dhir et al.,
2005)
Sample
Position
No.
1
2
3
4
5
6
7
8
9
10
11
Max
Min
Mean
Position in Depth
top
middle
bottom
Quartz
7.2
3.0
4.5
13.4
9.8
n/a
n/a
4.3
4.4
7.3
Mullite
28.1
22.4
32.3
17.9
17.4
n/a
n/a
32.0
26.6
24.6
Glass
54.0
55.4
50.8
60.7
63.0
n/a
n/a
52.2
59.2
58.8
Quartz
8.0
7.6
13.0
13.4
11.4
6.2
11.0
11.0
9.0
10.7
Mullite
23.6
25.5
25.9
17.8
18.9
24.4
19.5
16.8
27.2
20.2
Glass
58.4
56.7
53.4
60.0
60.0
60.6
61.1
64.9
54.9
60.4
Quartz
13.9
4.3
11.1
8.5
11.4
7.4
9.4
7.9
8.2
6.5
Mullite
24.0
23.2
19.8
26.7
23.8
14.9
19.9
13.9
19.8
26.7
Glass
56.0
56.9
47.3
56.0
57.0
72.3
60.6
70.3
63.7
57.3
5.9
21.3
60.9
7.2
14.5
68.7
5.7
22.1
59.9
13.4
3.0
32.3
17.4
63.0
50.8
13.4
6.2
27.2
14.5
68.7
53.4
13.9
4.3
26.7
13.9
72.3
47.3
6.6
24.7
57.2
9.9
21.3
59.9
8.6
21.3
59.8
Page 140 of 143
APPENDIX D Release Analyses of the Fly Ashes
Release analyses of the fly ashes from CAER, University of Kentucky
Dr. Jack Groppo
University of Kentucky
Center for Applied Energy Research
Lexington, KY
Introduction
Froth flotation is a fine particle separation technology that is well suited for efficiently separating fine
carbon from fine ash. While this technology is used for a variety of mineral separations throughout the
world, it has yet to be commercially used for high carbon fly ash. Several research organizations and ash
marketers offer flotation-based technologies and all report efficient performance. Despite these claims, it
is often difficult to make valid comparisons of the flotation options that are available. Fortunately, there
is a laboratory technique that is frequently used in mineral processing to define the limits of separation
that can be achieved by flotation, namely the release analysis.
Test Procedure
The release analysis is a laboratory technique that uses a small quantity of representative material
(approximately 200 grams) and a batch flotation machine. The technique has been shown to be
independent of the operator, equipment, pulp density, retention time, reagent type or dosage. The release
analysis defines the limits of separation that can be achieved by flotation regardless of how it is
conducted. The limits of separation are defined by the liberation of the particles that are to be separated.
In other words, the only way to change the release analysis results is to change the liberation of the
particles to be separated by techniques such as grinding or the application of ultrasonic energy.
To conduct the procedure, the feed ash is slurried and an appropriate dosage of reagents (collector and
frother) are added to produce a stable froth. The role of the collector is to selectively adsorb on the
carbon particle to induce hydrophobicity while the frother is used to reduce the surface tension at the
air/liquid interface to produce a copious amount of small bubbles (i.e. large bubble surface area). The
froth product is removed by hand scraping until the froth is exhausted and the froth product is set aside.
Additional reagents are added to produce a second froth product which is also set aside. This procedure is
systematically repeated until all of the hydrophobic particles have been removed. The various froth
products are then each separately diluted and re-floated to reject ash particles that may have been
entrained in the froth products. Each time a single flotation stage has been completed, a separate
hydrophobic froth product and hydrophilic tails are generated, which are separately filtered and dried. At
the end of the procedure, a range of products (8 to 12) have been generated and the LOI of each product is
determined and the results are compiled.
Flotation release analyses were performed on each ash by first removing the > 150 μm solids by sieving
to simulate the effect of a primary classification step.
Power Station 1
The results obtained on the Power Station 1 fly ash sample are shown in Figure D1. The sample
contained 13% LOI and was reduce to 3% with a yield of only 35%. The carbon is much more difficult
to remove from this ash.
Power Station 2
The results obtained on the Power Station 2 fly ash samples are shown in Figure D2, and include 3
samples, two previously shipped (Fresh conditioned and Old weathered) and a recent sample (1 August
07).
Page 141 of 143
14
40
35
Froth
30
Tails
12
10
25
8
20
6
15
4
10
5
2
0
0
0
20
40
60
80
Cumulative Weight, %
Cumulative Tails Grade, % LOI
Cumulative Froth Grade, % LOI
Power Station 1 Fly Ash <150 m
45
100
Figure D1. Release analysis of Power Station 1 fly ash.
Cumulative Tails Grade , % LOI
12
Fresh Conditioned
Old Weathered
1-Aug-07
10
8
6
4
2
0
0
20
40
60
80
100
Figure D2. Release analyses of tails from Power Station 2 fly ash samples.
The fresh conditioned and old weathered samples provided essentially the same flotation behaviour. Both
contained approximately 7% LOI, which was reduced to 3% LOI with a yield of 80-85% by flotation.
Lower LOI was achieved (below 2% LOI) but with lower yield. The latest sample (1 Aug 07) contained
11% LOI and was reduced to 3% with a much lower yield of 50%. All this to say that flotation can
certainly reduce the LOI of Power Station 2 fly ash to desirable levels, but the resulting yield will depend
on how low the LOI needs to be, what the feed LOI is and how well the carbon responds to flotation.
Obviously, the steeper the initial portion of the curve is, the easier it is to remove the carbon. Figure D3
shows the behaviour of the froth phase. Since all three lines are essentially the same, the carbon is
probably quite similar, as least as far as flotation is concerned.
Power Station 3
The results obtained with the Power Station 3 fly ash are shown in Figure D4. The feed grade of 13.4%
LOI was reduced to 3% LOI with a yield of 65% and the resulting froth grade was 30% LOI. Flotation
reduced the LOI of this ash to well below 1% LOI with a yield of 50%.
Page 142 of 143
60
Fresh Conditioned
50
Old Weathered
40
1-Aug-07
30
20
10
0
0
20
40
60
80
100
Figure D3. Release analyses of froth from Power Station 2 fly ash samples.
14
12
40
10
30
8
20
6
4
10
2
0
0
0
20
40
60
80
50
100
Power Station 4
The release analysis for the Power Station 4 fly ash sample is shown in Figure D5. The < 150 μm feed
grade of 11.8% LOI was reduced to 3% LOI with a yield of 70%. The corresponding froth grade at this
yield was 33% LOI. A flotation tails grade as low as 1.1% LOI was achieved with a yield of 50%, but the
froth grade was reduced (i.e. 23% LOI).
Power Station 5
The results for the Power Station 5 fly ash sample are shown in Figure D6. The feed grade of 14.6% LOI
was reduced to 3% LOI with a yield of 45% and flotation reduced the LOI of this ash to only 2.45% LOI
at lower yield.
Summary
Results obtained with the five ash samples are compared in Figure D7. These results show that the Power
Station 4 fly ash is the most amenable to LOI reduction by flotation, followed by the Power Station 3 fly
ash. The worst of the five samples is the Power Station 1 fly ash as indicated by the shallowest slope of
the release analysis curve. The results shown in Figure D7 were all obtained by initial classification to
reject >150 μm material enriched in carbon.
Page 143 of 143
12
10
40
8
30
6
20
4
10
2
0
0
0
20
40
60
80
50
100
16
14
50
12
40
10
30
8
6
20
4
10
2
0
0
20
40
60
80
0
100
60
All 5 Fly Ashes <150 m
16
14
12
Power Station 1
10
Power Station 2
8
Power Station 3
6
Power Station 4
4
Power Station 5
2
0
0
20
40
60
80
100
Figure D7. Comparison of release analyses for tails of the 5 fly ash samples.

New Approach to Fly Ash Processing and Applications to Minimise

Transcription

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