Pre-weakening of mineral ores by high voltage pulses

Minerals Engineering 24 (2011) 455–462
Contents lists available at ScienceDirect
Minerals Engineering
journal homepage: www.elsevier.com/locate/mineng
Pre-weakening of mineral ores by high voltage pulses
Eric Wang, Fengnian Shi ⇑, Emmy Manlapig
The University of Queensland, Sustainable Minerals Institute, Julius Kruttschnitt Mineral Research Centre, Qld 4068, Australia
a r t i c l e
i n f o
Article history:
Received 25 October 2010
Accepted 15 December 2010
Available online 12 January 2011
Keywords:
Comminution
High voltage pulse breakage
Energy efficiency
a b s t r a c t
A new comminution method has been developed by applying high voltage pulses at specific energy 1–
3 kWh/t to pre-weaken mineral particles, leading to reduction in energy consumption in the downstream
grinding process. Four ore samples were tested using high voltage pulses and conventional crushing in
parallel for comparison. Evidence of cracks and microcracks measured with X-ray tomography and mercury porosimetry supported the principle of high voltage pulses induced damage on rocks in the electrocomminution process, which resulted in energy saving up to 24% found in this study. Ore surface texture
and mineral properties affected the efficiency of high voltage pulse breakage. The feasibility of the electro-comminution and its benefits need to be investigated case by case.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Comminution remains by far the largest energy consumer on
most mine sites, with reported specific energy consumption varying from a few kWh/t for crushing, to 10–60 kWh/t for AG/SAG
milling and ball milling, and to over 100 kWh/t for ultra-fine grinding. The high energy consumption implies both high operational
cost and greenhouse footprint.
The sustainable development and use of raw resources require new, better and more efficient processes. Novel methods
of comminution are continually being sought which offer the
prospect of achieving the required outcomes (size reduction
and mineral liberation) at lower energy consumption. These
methods have included optimisation of comminution circuit
and the development of novel comminution devices such as high
pressure grinding rolls (Schonert, 1988), microwave treatment
(Kingman et al., 2000), electrohydraulic fragmentation (Delius,
1994; Zhong and Preminger, 1994) and electrodynamic fragmentation (Andres et al., 1999).
The work using high voltage pulses to break rocks appears
interesting. Andres et al. (2001a) reported comparative liberation
tests on oxide ore containing hematite and PGM and sulphide ores
containing complex Cu sulphides and pentlandite. The results
indicated that disintegration of ore aggregates by electric pulses
generated a higher percentage of liberated particles and lower
percentage of fine material than that obtained by mechanical
comminution. Lastra and Carbri (2003) showed in a comparative
liberation study of Merensky reef sample comminuted by electric
pulse disaggregation and by conventional crusher that liberation
of gangue was similar using either method, but liberation of
⇑ Corresponding author.
E-mail address: [email protected] (F. Shi).
0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.mineng.2010.12.011
chromite, pentlandite, pyrrhotite and PGM was higher than that
obtained by conventional jaw crusher. Ito et al. (2009) found that
electric disintegration resulted in preferential breakage of coal
substances and mineral particles along their boundaries.
Literature shows that better liberation can be achieved with
high voltage pulse breakage. However, very scarce data were published regarding the energy consumption in the high voltage pulse
breakage process. Andres et al. (2001b) reported that the consumption of energy per unit volume of the tested ores varied widely, but
on average was 2–3 times higher than in conventional mechanical
comminution. They quoted 50 kWh/t for the mechanical comminution and 90 kWh/t for the electric disintegration. Although it
can be argued that the high voltage pulse breakage can generate
overall savings of energy because the higher grade of the concentrate produced from high voltage pulse breakage decreases the
mass of material fed to the smelting process, which consumes
2.5 MW h/t (Andres et al., 2001b), the energy consumption of
90 kWh/t is significant comparing to the conventional comminution operation treating the similar size of feed ore.
In the last two years, a new approach to exploiting high voltage
pulse breakage has been developed at the Julius Kruttschnitt Mineral Research Centre (JKMRC). This approach is distinguished by:
(1) the application of only a very small specific energy (e.g. 1–
3 kWh/t) in the high voltage pulse breakage process, aiming
at damaging the rock particles and reducing rock strength,
and
(2) the consideration of the effect of this process on the energy
consumption across the whole comminution circuit. The
approach does not attempt to fracture the feed rocks into
micron size particles to liberate the valuables, but pre-weakens the ore particles so that downstream comminution process requires reduced energy.
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E. Wang et al. / Minerals Engineering 24 (2011) 455–462
This manuscript summarises the major findings of pre-weakening of various types of ore by high voltage pulses.
2. Experimental work
2.1. High voltage pulses treatment (selFrag Lab)
The high voltage pulse breakage experiment was conducted
with a selFrag Lab machine, manufactured by selFrag AG based
in Switzerland. The name of selFrag refers to selective fragmentation. The equipment consists of a high voltage (HV) power supply,
HV pulse generator, portable process vessels and a lifting table for
easy loading and unloading of the process vessel (see Fig. 1). The
process chamber is enveloped by an outer shell specially designed
to guard against sound and electromagnetic emissions. The HV
components are insulated with oil and gas, and walled with a steel
shielding. To ensure safe operation the HV parts of the equipment
are protected by safety interlocks.
The machine selFrag Lab is designed to treat mineralogical and
geological samples in the 1 kg range. The operation occurs in batch
mode. The samples are loaded in a portable process vessel filled
with water. Then the process vessel is placed onto the lifting table
in the loading section. The desired process parameters can be entered on a touch panel (Fig. 1). Settings to choose are: number of
pulses; discharge voltage (90–200 kV) that controls the energy
per pulse; electrode gap (10–40 mm); frequency of discharge (1–
5 Hz). After the interlocked safety door is closed the process can
be started. The HV pulse generator is continuously charged by
the HV power supply. When the predetermined voltage is reached,
the energy of the pulse generator is discharged from the electrode
through the solid sample to the grounded bottom (counter electrode) of the process vessel. This charging and discharging cycle repeats itself at a given frequency until the selected number of pulses
has been reached. To avoid over-breaking the target minerals a
process vessel with interchangeable sieve bottoms is in use. Already liberated minerals in the desired size range escape through
the sieve from the process chamber to a collection tray below. Typically sieve apertures from 2 mm to 0.3 mm are used.
2.2. Materials tested
Four different mineralised ores from three major mining companies were selected for the pre-weakening study, which consisted
of one copper ore, two copper/gold ores and one lead/zinc ore.
Mine site A is located in the Central West of New South Wales,
Australia. Copper mineralization occurs with K-feldspar alteration
and is mostly hosted by stockwork and sheeted quartz veins. Bornite is the dominant Cu sulphide with chalcopyrite and minor
chalcocite.
Mine site B is in the centre of Queensland, Australia. The ore
samples collected were from sedimentary exhalative Pb–Zn deposits, with finely banded galena, sphalerite and pyrite.
Mine site C is also in the centre of Queensland. The copper–gold
deposit lies within a breccia system developed in a sequence of altered, porphyritic, intermediate volcanic rocks. Valuables mainly
consist of magnetite, chalcopyrite, pyrite, gold, cobalt molybdenum, rare earth elements and less uranium.
Mine site D is located in New South Wales, Australia. The gold–
copper mineralization occurs as quartz veins, sheeted quartz sulphide veins and stockworks. The gold occurs mainly as free grains
in quartz or on the margins of sulphide grains. The principal copper
sulphide minerals are chalcopyrite and bornite.
A total of 3600 kg ore samples were collected from the four
mine sites for the study. Particles in the size range of 12–45 mm
were used. Each sample was split into two parts:
(a) one half of each sample was pre-weakened by high voltage
pulses using selFrag Lab in Kerzers, Switzerland; and
(b) the other half of the sample was conventionally crushed in a
jaw crusher at JKMRC, University of Queensland.
Both products (a and b) were processed and measured in the
same way to determine fragments residual hardness by JKRBT (JK
Rotary Breakage Tester), Bond Work indices using a Bond rod mill,
and cracks/microcracks measurements using X-ray tomography
and mercury porosimetry. More than 1000 kg of the sized ore samples were transported to Switzerland for high voltage pulse treatment and sent back to Australia for product processing and
measurement.
2.3. Product residual hardness measurement
In order to quantitatively assess the effect of pre-weakening,
residual hardness of products from high voltage pulse breakage
and conventional mechanical breakage was measured using a
JKRBT (Fig. 2). The recently developed JKRBT was used for rapid
particle breakage characterisation tests. The JKRBT uses a rotor–
stator impacting system, in which particles gain a controlled kinetic energy while they are spun in the rotor and are then ejected
Fig. 1. selFrag Lab with main components (dimensions 2 2 0.8 m) and process vessel with interchangeable sieve bottom.
E. Wang et al. / Minerals Engineering 24 (2011) 455–462
and impacted against the stator, causing particle breakage (Shi
et al., 2009).
In the standard JKRBT test, four particle size fractions are
tested (45 + 37.5 mm, 31.5 + 26.5 mm, 22.4 + 19 mm and
16 + 13.2 mm). Each particle size is tested at three different
impact energy levels (0.25, 1.0 and 2.5 kWh/t). In the current design,
the unit can achieve a wide range of specific impact energy range
varying from 0.001 kWh/t to 3.8 kWh/t. About 30 particles per size
are tested at each energy level.
Due to the availability of the amount of product, revised JKRBT
tests were implemented with four sizes (37.5 + 26.5 mm,
26.5 + 22.4 mm, 19 + 16 mm, and 12.5 + 9.5 mm), each size
being subjected to three specific energy levels (0.15, 0.5 and
1.0 kWh/t). After breakage, the product was collected and sieved
to give a product size distribution. The measured size distribution
together with the impact specific energy was then used to determine breakage characteristic parameters for the material.
457
Fig. 3. Skyscan 1172 Cone Beam Tomography installed at JKMRC.
2.4. Bond Work index measurement
Standard Bond rod mill tests were carried out on both high voltage pulses and conventional crusher products. As the top feed size
for Bond ball mill test is 3.35 mm while for Bond rod mill it is
12.7 mm, Bond rod mill is preferred in this study as more particles
in coarse size can be tested. The Bond rod mill tests were conducted consistently with a closing screen of 1.18 mm aperture.
The Bond Work indices were used to assess the pre-weakening effect and the energy saving benefits by high voltage pulses, if any.
2.5. Cracks/microcracks measurement
For qualitative and quantitative assessment of cracks/microcracks within particles of the high voltage pulses product and
mechanical crusher product, X-ray Cone Beam Tomography (CBT,
see Fig. 3) and mercury intrusion porosimeter (Fig. 4) were used.
Mercury porosimeter characterizes a material’s porosity by applying various levels of pressure to a sample immersed in mercury.
The pressure required to intrude mercury into the pores of the
sample is inversely proportional to the size of the pores. This is
called mercury porosimetry, or often, ‘‘mercury intrusion’’. In this
Fig. 4. Micrometrics 9320 mercury porosimeter for measurement of pores from
0.04 to 240 lm.
study, CBT was used to provide evidence of cracks/microcracks,
and the mercury porosimeter was used to quantitatively determine
the total amount of cracks/microcracks generated.
3. Comparison of product residual hardness
3.1. Product hardness index
The product residual hardness was tested with JKRBT. From the
size distribution of fragments produced by JKRBT, a single parameter t10 was calculated. The parameter t10 is defined as cumulative
percentage passing 1/10th of the initial size. For example, for an
initial feed size 19 + 16 mm, t10 = 20 means
20%
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi of the product
1
material is smaller than 1.74 mm (¼ 10
19 16 mm). The larger
t10 value indicates the finer breakage product.
The product ‘fineness’ index t10 is often plotted against the associated specific impact energy (Fig. 5a). The graph shows that t10 increases as the impact specific energy increases, as expected. The
data also depicture that subjecting to the same specific energy, larger particles tend to produce larger t10. This is a well known size
effect on particle breakage, as larger particles may have higher
crack density (Krajcinovic, 1996; Tavares and King, 1998).
An equation (Eq. (1)) is used to describe the relationship (Napier-Munn et al., 1996):
t10 ¼ Að1 ebEcs Þ
Fig. 2. JKRBT used to measure the product residual hardness.
ð1Þ
where Ecs is the specific comminution energy (kWh/t), and A and b
are the ore impact breakage parameters that can be fitted to the
E. Wang et al. / Minerals Engineering 24 (2011) 455–462
40
40
35
35
30
30
25
25
t 10 (%)
t 10 (%)
458
20
15
37.5 x 26.5 mm
26.5 x 22.4 mm
19x16 mm
12.5 x 9.5 mm
Ab fit
10
5
0
0.0
0.5
1.0
20
15
37.5 x 26.5 mm
26.5 x 22.4 mm
19x16 mm
12.5 x 9.5 mm
Fit
10
5
0
1.5
Ecs (kWh/t)
0.0
0.1
0.2
0.3
0.4
0.5
fmat.x.k.Ecs
(a) The prior-art JKMRC model fitting
(b) The new breakage model fitting
Fig. 5. Relationship between size reduction t10 and specific energy of JKRBT breakage product from Mine A ore sample.
experimental data. The product A b has been used as an indicator
of ore softness, larger A b indicating softer ore, or less resistance to
breakage. The typical A b values in the JKMRC database that consists of more than 2000 standard breakage testing data for ore particles are between 20 and 300, A b values less than 40 indicating
very hard ore, and larger than 100 very soft. It is popular nowadays
for many mineral processing engineers and researchers to use the A
and b parameters in their comminution circuit design, simulation,
optimisation and operation.
The use of ‘average’ set of A and b parameters assumes that particles of different sizes would be broken in the same way when
subjected to the same impact energy, which is questionable. A
new breakage model incorporating particle size effect has been
developed (Shi and Kojovic, 2007), which is modified from Vogel
and Peukert’s work (2004), and takes the following form:
t 10 ¼ Mf1 exp ½fmat: x kðEcs Emin Þg
ð2Þ
where M (%) represents the maximum t10 for a material subject to
breakage, fmat. (kg J1 m1) is modelled as a function of the material
breakage property, x (m) the initial particle size, k the successive
number of impacts with the single impact energy, Ecs (J kg1) the
mass-specific impact energy, and Emin (J kg1) the threshold energy.
In this application, two variables were set constant k = 1 and
Emin = 0 respectively.
Eqs. (1) and (2) take the similar exponential form, and the
parameters of the two equations are convertible. The ore softness
indicator A b in Eq. (1) can be calculated from the parameters
in Eq. (2) using the following relationship:
A b ¼ 3600 M fmat: x
ð3Þ
where constant 3600 is used for unit conversion. Eq. (3) gives the
size-specific A b values. The overall A b value can be taken as
an average of all particle sizes tested.
Eq. (2) has been applied to many datasets and proved to be valid. Fig. 5b shows the result of Eq. (2) fitted to the Mine A JKRBT
breakage data (same data as shown in Fig. 5a). Fig. 5b demonstrates that most data points of various sizes fall on one trend line,
indicating that the particle size effect on breakage has been well
described and corrected in Eq. (2).
mines. The legends in each graph indicate the breakage type, the
parent particle size before breakage, and the specific energy recorded during the breakage process.
The error bars shown in Fig. 6 were estimated from repeat tests.
Three repeat JKRBT tests on mechanical breakage products and
four repeats on two selFrag products tested at two energy levels
for Mine D sample were conducted. Standard Deviations (SD) were
calculated from the repeat tests. Coefficient of Variation (CoV) defined as a ratio of the standard deviation to the mean A b value
was determined. The averaged CoV from all the repeat JKRBT tests
was taken, which was used to calculate the SD for individual JKRBT
test. The 95% confidence intervals are determined as ±z SD, where
z = 1.96 is the 2-sided normal ordinate for a probability P = 0.05.
The averaged CoV determined from the repeat JKRBT tests was
0.042, which was consistent with that estimated for Drop Weight
tests (CoV = 0.040) (Stark et al., 2008).
Fig. 6 demonstrates that for majority of the ore samples collected from the four mines the selFrag products are softer than
the mechanical breakage products, across all fragment size ranges
tested. This difference in product ore softness is real, as it exceeds
95% significance. Fig. 6 provides strong evidence that products
treated with high voltage pulse breakage become weaker, even at
low specific energy levels of 1–3 kWh/t.
The only exception is Mine C selFrag product generated at
3.2 kWh/t, which shows almost identical softness as the mechanical breakage product subjected to 2 kWh/t. This was probably due
to the fact that different parent particle sizes were tested with selFrag comparing to the mechanical breakage.
Another interesting observation is that most fragments appear
softer at larger product size (ascending trend lines), regardless of
the pre-treatment by selFrag or mechanical breakage, except the
ore from Mine C. This is the well known size effect as larger particles may have higher cracks/microcracks density. The opposite
trend as exhibited in Mine C product is often observed in chromite
ore and some of iron ores. As described in the Experimental work
section, the sample from Mine C contains large amount of magnetite that may be responsible for the observed negative size effect
on breakage.
3.3. Product softness in relation to selFrag input specific energy
3.2. Product softness in relation to fragment size
Eqs. (2) and (3) were applied in this study to derive the residual
ore hardness parameters. The fitted model parameters A and b are
presented in Fig. 6 in a form of A b in relation to breakage fragment size produced by selFrag in comparison with the conventional crushing for ore samples collected from the four different
Experiment was conducted to investigate the influence of input
energy from the high voltage pulses on the residual hardness
of fragments. Sample from Mine C was used (19 16 mm size
fraction). The specific energy varied between 3.2 kWh/t and
7.7 kWh/t. The residual hardness as a function of selFrag input
specific energy is presented in Fig. 7.
E. Wang et al. / Minerals Engineering 24 (2011) 455–462
459
Fig. 6. Product ore softness in relation to size of fragments produced by selFrag in comparison with the conventional crushing for samples collected from the four different
mines, error bars indicating 95% confidence intervals.
The result suggests that as energy increases, the product becomes softer. This trend is as expected, attributing to the fact that
more cracks or microcracks may be generated at higher pulse
breakage specific energy. However, achieving more benefits in
pre-weakening would be at a cost of more energy consumption.
Economical balance would help to design a circuit to optimise
the energy usage in the high voltage pulse breakage stage.
3.4. Comparison of product residual hardness
The A b values for each ore sample treated with high voltage
pulsed breakage and mechanical breakage were calculated and
summarised in Table 1. Percent change in hardness as listed in
the last row of Table 1 is calculated based on the A b value of
mechanical breakage.
The results suggest that for all four ores, the high voltage pulses
products generated at an energy range 1–3 kWh/t have higher
A b values, with 9–52% increase in A b values comparing to
the conventional crusher product generated at similar specific energy. The data confirm the pre-weakening effect induced by high
voltage pulses.
Fig. 7. The effect of specific energy of high voltage pulse breakage on the residual
hardness of fragments, Mine C (Cu/Au ore) sample, 19 16 mm feed particles.
4. Comparison of product grindability
The objective of the grindability study was to investigate the
energy saving benefits in the downstream grinding process treating the pre-weakened ore by high voltage pulse breakage. Compar-
ison of Bond rod mill Work indices between the high voltage pulse
breakage and the mechanical breakage products is given in Table 2.
Percent energy saving is calculated by taking the mechanical Work
index as a baseline.
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E. Wang et al. / Minerals Engineering 24 (2011) 455–462
Table 1
Ore residual hardness of breakage products.
Ore source
Mine A
(Cu)
Mine B
(Pb/Zn)
Mine C
(Cu)
Mine D
(Au/Cu)
Energy applied by high
voltage process
(kWh/t)
Energy applied by
crusher (kWh/t)
Initial feed size (mm)
A b for high voltage
process
A b for crusher
Change in hardness (%)
2.3
1.5
3.2
1.1
2
2
2
2
45 + 37.5
46.3
45 + 37.5
55.4
19 + 16
73.9
45 + 37.5
55.6
35
32
50.7
9
61.1
21
36.6
52
Data in Table 2 provide evidence of significant reductions in
Bond Work indices after high voltage pulses treatment. For Mines
A and D the energy saving of 18% and 24% can be realised with the
pre-treatment by high voltage pulses discharge. It is worth mentioning that the high voltage pulse pre-treatment was conducted
at a specific energy range 1–3 kWh/t that was similar to the conventional crushing. Thus there is a potential to employ the high
voltage pulses mechanism to replace the conventional crushing
and to achieve total energy saving in the comminution circuit for
certain ores.
Mine C is an exception. The ore appears soft subjected to
mechanical breakage. This is evidenced by the Bond Work index
(9.4 kWh/t). However, it appears ‘‘hard’’ subjected to electro-comminution. It was noticed that when the ore sample from Mine C
was treated by selFrag at a specific energy 2 kWh/t, there were little fragments generated. In order to provide sufficient material of
12.7 mm for the Bond rod mill tests, a specific energy of
7.7 kWh/t from high voltage pulses was used. As shown in Table 2,
the Bond Work index of Mine C ore product from high voltage
breakage was almost identical as that from mechanical breakage
(9.7 kWh/t vs. 9.4 kWh/t), though higher specific energy in the
electro-comminution stage was applied. Similarly, Mine B ore sample exhibits marginally reduced Bond Work index from the selFrag
product comparing to the mechanical product (17.9 kWh/t for selFrag product vs. 19.1 kWh/t for crushed product). It was believed
that this phenomenon was caused by the surface texture of the
ores. This influence will be further discussed in the next section.
The relative reduction in fragment strength shown by JKRBT impact tests (Table 1) was greater than those shown by the Bond
Work index tests (Table 2). For example, Sample D exhibited 52%
reduction in the residual hardness, but only 24% reduction in Bond
Work index. Sample C was another extreme example, as 21%
reduction in hardness (A b) was completely lost in the Bond Work
index comparison.
These differences were elucidated through the influences of
particle size and the testing environment. The decrease in product
strength after high voltage processing was hypothesized due to the
Table 2
Comparison of Bond rod mill Work index of product ores treated with high voltage
pulses and mechanical breakage.
Ore sources
Mine
A
(Cu)
Mine
B
(Pb/
Zn)
Mine
C
(Cu/
Au)
Mine
D
(Au/
Cu)
Energy applied by high voltage process
(kWh/t)
Energy applied by crusher (kWh/t)
Bond Work index (kWh/t) – high voltage
process product
Bond Work index (kWh/t) – conventional
crusher product
Energy Saving by high voltage process (%)
2.3
1.5
7.7
1.1
2
16.5
2
17.9
2
9.7
2
17.5
enhancement of cracks and microcracks in the electro-comminution products. In the JKRBT tests, particles in the size range of
37.5–9.5 mm were treated, except Sample C in which 19–6.7 mm
particles were tested. While in the Bond rod mill tests, material
in the size 12.7 + 0 mm was used. Large particles normally have
higher crack/microcrack density, hence appear weaker.
Another explanation was that only one impact breakage event
happened in the JKRBT test, while in the Bond test, particles were
subjected to incremental breakage. The existing cracks/microcracks generated from the high voltage pulses may be destroyed
after a few tumbles inside the mill. The survivors of particle become harder to break with larger energy threshold (Emin as shown
in Eq. (2)). Hence in the tumbling environment, the incremental
breakage had much lower energy efficiency than the single impact
breakage (Larbi-Bram, 2010).
5. Cracks and microcracks generation
The data in Tables 1 and 2 consistently demonstrated that the
high voltage pulse breakage products generated at 1–3 kWh/t specific energy were significantly weaker than the mechanical breakage product generated at the similar energy for ore samples from
Mines A and D. As a result, energy saving can be expected in the
downstream comminution process. However, the pre-weakening
effect by the electro-comminution on the other two ore samples
was not as pronounced as the samples from Mines A and D. To elucidate the phenomenon, X-ray tomography was performed on the
product particles generated by the two comminution routes. Fig. 8
presents the tomography images of the four mine samples treated
with high voltage pulses discharge.
It was observed from the images that many cracks and microcracks existed within the high voltage pulse products, particularly
Mines A and D samples having the highest crack density, while the
crushed products were hardly found any cracks. This is in good
agreement with the JKRBT hardness tests and the Bond Work index
tests, in which these two ores have the highest strength reduction.
Fig. 9 shows CBT image of internal microcracks of a fragment
from Mine D treated with high voltage pulses discharge at
1.1 kWh/t specific breakage energy. The width of the internal
microcracks varies from 2.7 lm to 230 lm.
The porosimetry measurement indicated that for Mine D ore,
the total voids volume in the mechanical breakage product was
3.2%, while the total voids volume in the electro-comminution
product was 13.7%, more than four times of cracks/microcracks
generated by electro-comminution.
The CBT images and the porosimetry data all supported the
mechanism that the high voltage pulses created cracks/microcracks on ore particles, even at 1–3 kWh/t specific energy. The formation of cracks/microcracks led to reduction in particle strength.
As a result, the electro-comminution product appeared weaker,
and required less energy consumption to be ground to a desired
product size.
A question may be raised regarding the reasons why samples
from Mines B and C treated with high voltage pulses could not
achieve the same degree of reduction in ore residual hardness
and energy saving in the downstream comminution as the Mines
A and D samples did. Andres and Timoshkin (1998) suggested that
when assuming the energy of electric field is equal to the mechanical energy required for generation of crack, the order of magnitude
of the breakdown voltage can be found from equation
U
20.1
19.1
9.4
23
18
6
–
24
YG
e20 e2
14
r 3=4
ð4Þ
where G is the energy of creating of unit area of new crack surface, Y
is Young’s modulus of solids, r is the radius of protrusions, e0 is
E. Wang et al. / Minerals Engineering 24 (2011) 455–462
461
Fig. 8. CBT images of particles (13 + 9.5 mm) treated with high voltage pulses for the four ore samples.
Fig. 9. CBT scanned image of internal microcracks (in white hairlines) in the high
voltage pulses product, Mine D sample.
vacuum permittivity constant, and e is dielectric permittivity. This
expression is based on the Griffith criterion of initiation and propagation of cracks which suggested that a harder dielectric would
have much higher breakdown strength (voltage). The results from
Mines B and C samples cannot be explained by Eq. (4), as the ores
from Mines B and C are softer (higher A b as shown in Table 1),
which infers smaller Young’s modulus, one would expect smaller
breakdown strength from Eq. (4), and hence easier to be pre-weakened by electro-comminution.
This was believed due to the difference in particle surface texture, mineralogy and grain size among the four samples investigated. It was observed that many particles from Mine B were
covered with galena mineral on surface, and particles in Sample
C were rich of pyrite minerals on surface, while the samples from
the other two mines did not exhibit any particular minerals concentrated on surface. During the experiment, it was noticed that
streamer propagation and the development of breakdown channel
within the solids were not possible for Mines B and C ore samples
at low voltages of 90 kV, while for the other two samples it could
do.
Andres et al. (1999) suggested that the area of maximum electric field determines the location of the breakdown channel. When
the maximum electric field is concentrated inside solid dielectrics,
the path of the breakdown goes through this material. In the case
of Mines B and C ores, the maximum field can occur on the particle
surface. This leads streamer to propagate across the sample surface, chipping and shaving away the conductive minerals (e.g. galena or pyrite). This type of streamer propagation on particle
surface prevents the disintegration to take place by internal electric explosion inside the solid. This problem can be overcome by
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E. Wang et al. / Minerals Engineering 24 (2011) 455–462
increasing the voltage applied. The higher voltage causes higher
field to the system, thus smaller energy is required for development of the pre-breakdown cracks by the strictional tension.
Numerical simulations of the electric field under various particle
conditions were conducted to verify the explanations, and will be
published in a separate paper.
Apparently, ore texture, particle mineralogy and other electrical/mechanical properties all affect the efficiency of high voltage
pulse breakage. It is emphasized that the feasibility of electro-comminution and the benefits can be achieved need to be investigated
case by case.
6. Conclusion
A new comminution method has been developed by applying
high voltage pulses to pre-weaken mineral particles, leading to
reduction in energy consumption in the downstream grinding process. Four ore samples of over 3000 kg collected by major mining
companies were tested in parallel using high voltage pulses at specific energy 1–3 kWh/t and using conventional crushing at the similar energy. Product residual hardness of the ores treated with the
two routes was measured by JKRBT. Energy requirements in downstream grinding were determined by the Bond rod mill Work index
tests. The data provided strong evidence that the ores treated with
high voltage pulses appeared weaker (9%–52% change in the softness indicator A b value) than the crushed product, which resulted in energy saving up to 24% in the subsequent grinding
process.
The mechanisms of pre-weakening ores by high voltage pulses
were investigated by using X-ray Cone Beam Tomography and
mercury porosimeter to qualitatively and quantitatively assess
the induced cracks and microcracks during the process. Evidence
of cracks and microcracks was found in the electro-comminution
products – four times more crack/microcracks were generated in
one sample.
There were pronounced variations in the pre-weakening effect
on the four ores treated with high voltage pulses. Ore surface texture, mineralogy, electric property of minerals and particle
mechanical properties all affect the efficiency of electro-comminution. The feasibility of electro-comminution and its benefits need
to be investigated case by case.
Acknowledgements
The authors would like to acknowledge financial support from
the Australian Research Council Linkage Scheme (AMSRI –
LP0667828), AMIRA International, the State Governments of South
Australia and Victoria, and the sponsors of AMIRA International
Project P924: BHP/Billiton, Rio Tinto, Orica Explosives, Anglo Platinum, Xstrata Technology, Freeport McMoran and AREVA NC. selFrag AG kindly provided the high voltage pulse testing facility for
the experiment. Assistance provided by Dr. Alexander Weh of selFrag is gratefully appreciated. Support from Rio Tinto, Xstrata and
Newcrest mining companies in sample collection is gratefully
acknowledged.
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