ELECTROCHEMICAL STUDIES ON THE INTERACTION

ELECTROCHEMICAL STUDIES ON THE INTERACTION OF
MINERALOGY AND FERRIC OXIDANTS ON SULPHURIC ACID
LEACHING OF SPHALERITE
by
GERMINAH POLINA APHANE
Submitted in partial fulfilment of the requirements for the degree
MAGISTER TECHNOLOGIAE: METALLURGICAL ENGINEERING
In the
Department of Chemical, Metallurgical and Materials Engineering
FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT
TSHWANE UNIVERSITY OF TECHNOLOGY
Supervisor: Dr P.A. Olubambi
Co-supervisor: Dr R.K.K. Mbaya
MAY 2013
DECLARATION
I, G.P Aphane, hereby declare that the work presented for in this dissertation is
original (except where acknowledgements indicate otherwise) and that neither the
whole work, nor any part of it, is to be, has been, or is been submitted for another
degree at this or any other university.
……………………………….
……………………………
GERMINAH APHANE
DATE
Copyright© Tshwane University of Technology 2013
i
DEDICATION
I dedicate this study to Almighty God, my beloved husband, Jimmy Masombuka
and our son Patrice for their enormous support through this journey.
ii
ACKNOWLEDGEMENTS
I would like to take this opportunity to thank all people who made this Research
possible. This thesis was a long journey in obtaining my M-Tech in Metallurgical
Engineering. I have not travelled in a vacuum in this journey. There are a lot of people
who made this journey easier with words of encouragement and more intellectually
satisfying by offering different places to look to expand my theories and ideas.
First a very special thanks to my supervisor, Dr. P.A. Olubambi, the senior lecturer in
the Department of Chemical and Metallurgical Engineering, Tshwane University of
Technology, Pretoria Campus. His wide knowledge and his logical way of thinking
have been of great value for me. His understanding, encouragement, personal
guidance, and patience have provided a good basis for the present dissertation.
I am honoured to thank my Co-supervisor, Dr. R.K.K. Mbaya, senior lecturer
Department of Chemical and Metallurgical Engineering, Tshwane University of
Technology, Pretoria Campus, for his moral support and his encouragement.
I would like express my sincere appreciation to my husband, Jimmy Masombuka, for
his unflagging love, gave me the confidence and support to begin my studies, I am
also deeply grateful for his detailed and constructive comments and for his important
support throughout my study years. I learned to believe in my future, my work, and
myself. Special appreciation to my loved son Patrice for understanding during my
long distance travels in the course period and late night writing.
iii
To my family, I would like to say, “You are my feet who keep me walking. Without you
– your love and support, it would have been impossible for me to get this far.”
My sincere thanks go to Dr. A. Andrews for his detailed review, constructive criticism
and excellent advice for the improvement of this dissertation.
Tshwane University of Technology, for financial support.
National Research Foundation (NRF) for their sponsorship.
I would like to thank the research group for extensive interaction and support to
achieve this goal to date: Zanele Hazel Masuku, Linda Lethabane, Vincent
Shokane, Rasidi Sule, Tebogo Amelia Molele, Steve Ramashala, Oladayo
Olaniran, Tebogo Kale, Masego Lepule, Thato Tshepe and Senzeni Lephuthing.
I cherished the prayers and support that they have given me, and the friendships that
we have had. I treasured all precious moments we shared and would really like to
thank them for how lucky I was to have such good friends. Guys, I love you and thank
you for everything.
I would like to thank Tshwane University of Technology Management for their
permission to conduct research within the institution.
iv
ABSTRACT
The composition and mineralogical characteristic of sphalerite ores can be strongly
influenced by the mode of occurrence which often poses challenges during their
processing. This study explored the electrochemical interaction between ore
mineralogy and ferric iron oxidants on the mechanisms and kinetics of sulphuric acid
leaching of sphalerite using electrochemical techniques. The electrochemical
behavior of Import, Black mountain and Imcor (different sphalerite ores) were studied
in sulfuric acid solution with different concentrations of ferric chloride and ferric
sulphate using electrochemical
techniques such as open circuit potential,
potentiodynamic polarization and chronoamperometric measurements. An autolab
potentiostat (PGSTAT20) General Purpose Electrochemical Purpose Software
(GPES, version4.9) was used to conduct the electrochemical tests.
The working electrode was an electrode made out of powdered sample, powdered
graphite and silicon oil. The microstructures of the powered sphalerite samples were
examined using scanning electron microscopy (SEM) equipped with an energy
dispersive spectroscopy (EDS). The mineralogical phases were identified by the use
of X-Ray Diffractometry (XRD). The results showed that ferric irons enhanced the
dissolution of sphalerites. With increasing ferric ion concentration, the open circuit
potential shifted to more positive values. The potentiodynamic polarization results
indicate that increasing ferric ion concentration increased the reactivity on the
sphalerite electrode thereby increasing the recovery of zinc from sulphide ore.
v
Additionally, an increase in ferric ion concenteration reduced the passivity range of
the sphalerite electrode. The potentiodynamic polarization curves indicate that the
polarization current of these sphalerite electrode increased with both ferric chloride
and ferric sulphate concentration. It has also been shown that an increase in ferric ion
concentration resulted in the formation of passive layer on the electrode surfaces.
vi
Table of Contents
DECLARATION ......................................................................................................................... i
DEDICATION ........................................................................................................................... ii
ACKNOWLEDGEMENTS...................................................................................................... iii
ABSTRACT ............................................................................................................................... v
List of figures .......................................................................................................................... xii
List of tables ........................................................................................................................... xiv
CHAPTER ONE ....................................................................................................................... 1
1. INTRODUCTION ................................................................................................................. 1
1.1 PROBLEM STATEMENT............................................................................................ 3
1.2 AIM AND OBJECTIVES .............................................................................................. 4
1.2.1 Aim ..........................................................................Error! Bookmark not defined.
1.2.2 Objectives ..............................................................Error! Bookmark not defined.
1.3 HYPOTHESIS ............................................................................................................... 5
1.4 JUSTIFICATION FOR THE STUDY ......................................................................... 4
1.5 STRUCTURE OF THE THESIS ................................................................................ 5
CHAPTER TWO ...................................................................................................................... 6
2. LITERATURE REVIEW ...................................................................................................... 6
2.1 INTRODUCTION ............................................................................................................... 6
2.1.1 General properties for zinc ................................................................................... 7
vii
2.1.2 Electrochemical properties ................................................................................... 7
2.2 EXTRACTION OF METALS TECHNIQUES............................................................ 9
2.2.1 Extraction metallurgy ............................................................................................. 9
2.3 LEACHING OF SPHALERITE ................................................................................. 11
2.3.1 Hydrometallurgical processing of zinc .............................................................. 13
2.3.2 Pyrometallurgical extraction of zinc .................................................................. 14
2.3.3 Bioleaching ........................................................................................................... 15
2.4 REAGENTS USED DURING LEACHING OF SULPHIDE ORES...................... 15
2.4.1 Hydrochloric acid ................................................................................................. 15
2.4.2 Sulphuric acid ....................................................................................................... 16
2.4.3 Nitric acid............................................................................................................... 16
2.5 LEACHING SPHALERITE WITH FERRIC IONS .................................................. 16
2.5.1 Atmospheric ferric sulphate leaching ................................................................ 16
2.5.2 Principles of pressure leaching .......................................................................... 19
2.6 OTHER OXIDANTS USED ....................................................................................... 19
2.6.1 Oxygen .................................................................................................................. 19
2.6.2 Hydrogen peroxide .............................................................................................. 19
2.7. FACTORS AFFECTING THE LEACHING PROCESS ....................................... 21
2.7.1 Temperature ......................................................................................................... 21
2.7.2 Particle size ........................................................................................................... 21
viii
2.7.3 Agitation................................................................................................................. 22
2.7.4 Ion concentration ................................................................................................. 23
2.7.5 Solid-liquid ratio .................................................................................................... 23
2.7.6 Leaching Time ...................................................................................................... 24
2.8 INTERACTION OF SULPHIDE ORE MINERALS ................................................ 24
2.9 ELECTROCHEMICAL STUDIES ............................................................................ 29
2.9.1 Electrochemical technique ................................................................................. 34
2.9.2 The electrode cell ................................................................................................. 35
2.9.3 Role of solution potential .................................................................................... 36
2.9.4 Potentiostat ........................................................................................................... 36
2.10 ELECTROCHEMICAL MEASUREMENTS .......................................................... 38
2.10.1 The open-circuit potential ................................................................................. 38
2.10.2 Potentiodynamic polarization ........................................................................... 38
2.10.3 Chronoamperometric measurements ............................................................. 39
2.10.4 Cyclic voltammetry measurements ................................................................. 40
CHAPTER THREE ................................................................................................................ 42
3. EXPERIMENTAL PROCEDURE .................................................................................... 42
3.1 INTRODUCTION ........................................................................................................ 42
3.2 MATERIALS ................................................................................................................ 42
3.2.1 Reagents ............................................................................................................... 42
ix
3.2 MATERIAL CHARACTERIZATION ......................................................................... 43
3.2.1 Particle size ........................................................................................................... 43
3.2.2 Elemental composition determination............................................................... 43
3.2.3 Phase analysis ..................................................................................................... 45
3.2.5 Microstructural analysis ...................................................................................... 46
3.3 ELECTROCHEMICAL STUDY ................................................................................ 46
CHAPTER FOUR .................................................................................................................. 48
4. RESULTS AND DISCUSSION ........................................................................................ 48
4.1 CHARACTERIZATION OF MINERALS.................................................................. 48
4.1.2 Particle size analysis ........................................................................................... 48
4.1.3 The morphology of the samples ........................................................................ 48
4.2 ELECTROCHEMICAL BEHAVIOR ......................................................................... 50
4.2.1 The OCP behavior of sphalerites in 0.5 M H2SO4 .......................................... 50
4.2.2 The OCP behavior in 0.5 M H2SO4 with FeCl3 addition................................. 51
4.2.3 The potentiodynamic polarization of samples in 0.5 M H2SO4 ..................... 54
4.2.4 The OCP behaviour in 0.5 M H2SO4 with Fe2 (SO4)3 addition ...................... 58
4.2 EFFECT OF OXIDANT TYPE ON THE DISSOLUTION KINETICS .................. 63
4.3 THE CHRONOAMPEROMETRIC BEHAVIOUR .................................................. 65
CHAPTER FIVE ..................................................................................................................... 68
5. CONCLUSION ................................................................................................................... 68
x
5.1 CONCLUSION ............................................................ Error! Bookmark not defined.
REFERENCES ....................................................................................................................... 70
APPENDICES ........................................................................................................................ 80
APPENDIX A .......................................................................................................................... 80
APPENDIX B ..................................................................................................................... 85
APPENDIX C ..................................................................................................................... 90
APPENDIX D ..................................................................................................................... 92
APPENDIX E ..................................................................................................................... 95
APPENDIX F ..................................................................................................................... 96
APPENDIX G ..................................................................................................................... 98
xi
LIST OF FIGURES
Figure 2.1: Hydrometallurgical processing of zinc production ........................................ 13
Figure 2.2: Method of galvanic interaction amongst pyrite and sphalerite (Donati and
Sand, 2007). ........................................................................................................................... 25
Figure 2.3: The electrochemical mechanism of ZnS dissolution ( Donati and Sand,
2007) ........................................................................................................................................ 31
Figure 2.4: Role of bacteria in electrochemical mechanism of a metal sulphide (Donati
and Sand, 2007)..................................................................................................................... 33
Figure 2.5: Schematic representation of three cell electrodes ..................................... 35
Figure 2.6: The tafel plot from polarization experiment (ASTMG3-89) ......................... 38
Figure 3.1: Summarized procedure for elemental digestion (adapted from Harahsheh,
2009) ........................................................................................................................................ 44
Figure 4.2: Morphology of IMP sample showing at the magnification of 2000X. ......... 49
Figure 4.3: Morphology of BM sample showing at the magnification of 2000X.......... 49
Figure 4.4: Morphology of IMC sample showing at the magnification of 2000X. ........ 49
Figure 4.5: OCP of sphalerites (IMP, BM and IMC) in 0.5M H 2 SO4 solution for 1 hour.
.................................................................................................................................................. 51
Figure 4.6: The OCP curves showing the behavior of sphalerites (a) IMP, (b) BM and
(c) IMC dissolution in 0.5M H 2 SO4 acid and various FeCl 3 concentrations at 25◦C for
1hour. ....................................................................................................................................... 53
Figure 4.7: Potentiodynamic Polarization of IMP, BM and IMC in 0.5 M H 2 SO4
electrolyte. ............................................................................................................................... 54
xii
Figure 4.8: Polarization curve showing the behavior of sphalerites (a) IMP, (b) BM
and (c) IMC dissolution in 0.5 M H 2 SO4 and various FeCl 3 concentrations at 25◦C .. 56
Figure 4.9: Dissolution current densities of sphalerites (IMP, BM and IMC) in 0.5M
H 2 SO4 and various concentrations of FeCl 3 ...................................................................... 58
Figure 4.10: OCP curve showing the behavior of sphalerite (a) IMP, (b) BM and (c)
IMC dissolution in 0.5M H 2 SO4 and various Fe2 (SO4 ) 3 concentrations at 25◦C. .......... 59
Figure 4.11: Potentiodynamic polarization curve showing the behavior of sphalerite
(a) IMP, (b) BM and (c) IMC dissolution in 0.5M H 2 SO4 and various
Fe2 (SO4 ) 3 concentrations at 25◦C. ....................................................................................... 61
Figure 4.12: Dissolution current densities of IMP, BM and IMC in 0.5M H 2 SO4 and
various concentrations of Fe2 (SO4 ) 3 ................................................................................... 62
Figure 4.13: The dissolution current densities and concentrations of the three
sphalerites (IMP, BM and IMC) in 0.5M H 2 SO4 in both FeCl 3 and Fe2 (SO4 ) 3 . .......... 64
Figure 4.14: Chronoamperometric curves of IMP, BM and IMC in 0.5M H 2 SO4 + 0.5M
FeCl 3 at the applied potentials of 0.7, 0.9 and 1.1V. ..................................................... 66
Figure 4.15: Chronoamperometric curves of IMP, BM and IMC in 0.5M H 2 SO4 + 0.5M
Fe2 (SO4 ) 3 at the applied potentials of 0.7, 0.9 and 1.1V. ................................................ 67
xiii
LIST OF TABLES
Table 2.1: Physical properties of zinc (Choi et al., 1993) .................................................. 7
Table 2.2: The rest potential of mineral sulphide at pH = 4 (Donati and Sand, 2007) 25
Table 3.1: The chemical composition of the different concentrates .............................. 45
Table 3.2: Mineralogical compositions of the ores ........................................................... 46
Table 4.1: The particle size analysis of sphalerite minerals ........................................... 48
xiv
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND
Zinc is the fourth most used metal in the world today with a production of 7.5 million
tonnes per year (Aaltonen, 2005). It is mostly mined as sphalerite or the zinc sulphide
ore that consist of iron as the main impurity and is a metal required for various
applications in metallurgical, chemical, and textile industries (Peng Peng, 2005).
Sphalerite is the natural zinc sulphide mineral found associated with chalcopyrite
(CuFeS2), galena (PbS) and pyrite (FeS2) (Aydogan et al., 2005; Rao and
Chernyshova, 2011; Balarini et al., 2007, Peng et al., 2005; Ucar, 2009). Regardless
of the form in which sphalerite and other sulphide ores occur, they are usually very
difficult to process (Deveci et al., 2004; Rubio and Frutos, 2002). This may be due to
the close similarities in their mineralogical properties, which make them unsuitable for
conventional methods of processing. Differential flotation does not release all the
constituent phases (Majima, 1969) and so the different concentrates obtained are of
poor quality with a low rate of metal recovery. This makes further pyrometallurgical
processing of these ores difficult and costly (Rubio and Frutos, 2002) and renders
them unattractive for commercialization (Sandstrisom and Petersson, 1997).
Pyrometallurgical methods are on the decline due to high capital costs and the
emission of pollutant gases, mainly SO2 which is harmful to the environment (ALHarahsheh and Kingman, 2007, Zeng et al., 2011). As a result, the metal value is
preferably extracted from sphalerite through a hydrometallurgical process.
1
Hydrometallurgical processes are very attractive methods and are economical.
These processes suit the environment better because they can treat low grade
materials. During these processes sulphur is converted to elemental sulphur(S)
(Souza et al., 2010). Investigations reported on understanding the basic aspect of
sphalerite dissolution behaviour have however not fully produced a common
mineralogical theory for understanding of its dissolution mechanisms and kinetics.
Sulphide minerals behave differently during their hydrometallurgical processing due
to the mineralogical differences that affect their responses and behaviour in different
conventional and acid leaching which may result to different mineralogical
compositions.
In order to be able to predict and assess reactivity of the minerals and thus determine
and interpret the results of the overall metal dissolution process, it becomes
imperative that a better understanding of dissolution mechanism and kinetics be
obtained through electrochemical studies. Electrochemical study of the mineral
surface contributes basic information on mineral reactivity and the metal dissolution
mechanism, and its analysis is very important in obtaining useful information on
sulphide leaching (Antonijevic and Bogdanovic, 2004). Studies of the electrochemical
behaviour of minerals are of interest as they have provided additional insight into the
passivation phenomenon which the leaching studies will be unable to provide. The
electrochemical behavior of sulfide ores have already been investigated by several
authors (Fowler and Crundwell, 1988; Hackl et al., 1995; Fowler et al., 1999).
However, their studies focused on the methodology and sometimes on the process
2
engineering aspects, and have not always contributed to an understanding of the
intimate mechanism involved in the oxidation of the solid substrates used (Cabral and
Ignatiadis, 2001).
1.2 PROBLEM STATEMENT
In the selection of a suitable reagent for sphalerite leaching, sulphuric acid is
preferred to all other reagents in terms of cost, corrosion, wear and the ease of
regeneration during electrowinning (Biswas and Davenport, 1980). Nevertheless,
sulphuric acid is not as effective as other stronger acids like nitric acid due to its lower
oxidizing power (Çopur, 2001, 2002; Prasad and Pandey, 1998). Attention has
recently been drawn to the use of sulphuric acid for base metals recovery (Antonijevic
and Bogdanovic, 2004) and means of optimizing its dissolution kinetics. In order to
optimize sulphuric acid leaching, there is the possibility of aiding the dissolution
kinetics through the use of suitable oxidants.
It has been reported that ferric ion is the most common oxidizing agent on the
leaching of sphalerite (Santos et al., 2010). Direct oxidative leaching of sphalerite
route has been used by several investigators. It was reported that ferric chloride
leaching has an advantage over ferric sulphate leaching as the reaction kinetics are
more favorable in chloride media (Al-Harahsheh and Kingman, 2007 ; Aydogan et
al.,2005). However, ferric chloride and sulphate leaching still suffer from slow reaction
kinetics (Al-Harahsheh and Kingman, 2007). Several researchers have focused more
on sulphate media rather than chloride media due to low cost, minimal corrosion
3
problems and the ability to regenerate sulphuric acid during electrowinning (Olubambi
et al., 2006).
1.3 AIM AND OBJECTIVES
The aim of this study is to investigate the dissolution behaviour of sphalerite minerals
in sulphuric acid using ferric ions as oxidants.
The specific objectives are to study the following:
 The mineralogical characteristics of sphalerite ores.
 Mineralogical effects on dissolution kinetics in sulphuric acid.
 Effect of ferric ion concentration on the dissolution kinetics of sphalerite ores in
sulphuric acid.
1.4 JUSTIFICATION FOR THE STUDY
Mineralogy is a critical area in mineral processing and has to be considered during
process design stage, and during each processing stage. The type and concentration
of oxidizing agent depend on the mineralogical composition of the ore. Many
researchers’ investigations and test-works have been reported on leaching of
sphalerite using both ferric sulphate and ferric chloride (Al-Harahsheh and Kingman,
2007; Rath et al., 1981). However, little or no studies have been reported on the
combined oxidants.
4
1.5 HYPOTHESIS
The best ferric oxidant could improve concentration grade and recovery of minerals at
a lower cost. Sphalerite ore consist of various mineral constituents which is sulphide
minerals such as chalcopyrite, galena and pyrite. The mineralogy of ore has to be
taken in to consideration to select the suitable oxidant that can leach sulphides.
Hydrometallurgical route for zinc extraction is preferred than pyrometallurgical route
(Souza et al., 2007).
1.6 STRUCTURE OF THE THESIS
Chapter One gives an introduction to the research study, research problems, aim
and objectives of the research study. A literature review of the basic concepts and on
the previous and on-going work on the existing methods for geological mineralogy of
sphalerite ores, applied mineralogical studies, acidified ferric ions (ferric chloride and
ferric sulphate) leaching, electrochemical studies of the mechanisms of the sulphuric
acid leaching zinc from complex sulphide ores and their output is presented in
Chapter Two. Chapter Three gives a presentation of the materials and equipment
used in the research, and experimental methodology adopted for achieving the set
objectives of this research. Results and discussions of the results of the
electrochemical study of the ore are presented in Chapter Four. The conclusion is
presented in Chapter Five and lastly, a list of reference materials and appendices
are given.
5
CHAPTER TWO
LITERATURE REVIEW
2.1 INTRODUCTION
Sphalerite is an ore which has excess metal ions forming an n-type semiconductor. It
is a ZnS ores which is commonly found associated with sulphide minerals such as
chalcopyrite (CuFeS2), galena (PbS) and pyrite (FeS2) (Aydogan, 2005; Baba et al,
2010). Sulphide ore minerals are present in a large number of natural environments
and their weathering is very often an important source of pollution owing to the
production of acidity and to the release of metals to soils, surface run-off and
groundwaters (Acid Rock Drainage) (Acero et al., 2007) .
These ores are complex with complicated mineralogical associations on their
constituent minerals which makes them difficult to be processed and this could be
due to the cleanliness of the concentrates from the flotation process (Aaltonen, 2005;
Olubambi, 2006; Balaz, et al, 2003; Aydogan, 2006). Generally, they are made up of
fine intergrown minerals, in which precious metals such as gold and silver often occur
as interlocked refractory and finely disseminated metals in them. These ores
represent the major source of both non-ferrous base and precious metals.
To get optimum results in processing of these ores must start from sound and
complete at mineralogical study. The drawbacks of the sulphide ores are their lower
solubilities in many leaching reagents, low quantity of recovered metal and large
amount of ores to be processed.The knowledge of mineralogy would provide useful
6
information in understanding its interaction with ferric ions as oxidising agents on acid
leaching process. The effect of mineralogy in hydrometallurgical processing in
sulphuric acid media is not yet to fully understood.
2.1.1 General properties for zinc
Zinc is a metallic chemical element used in variety of alloys and compounds. It is
yellow-white in colour, has high electrical conductivity. The physical properties for it
are represented in Table 2.1 below.
Table 2.1: Physical properties of zinc (Choi et al., 1993).
Molar atomic weight
65.38g/mol
Melting point/ ˚C
53˚C
Boiling point/ ˚C
907˚C
Density /g.cm-3
7.14g/cm3
Electronegativity
1.65 (pauling scale)
Electrical resistivity
59.0 nΩ.m (20˚C)
Oxidation state
+2+1 0
2.1.2 Electrochemical properties
The conductivity of pure zinc is low. In the band model, the 4s orbital is associated
with the bottom of the conduction band and the 3p orbital of sulphur with the top
valence band. The d orbital of an iron impurity atom gives rise to an additional
conducting band.
7
Sphalerite dissolution in ferric chloride solution produces a variety of reaction
products such as zinc chloride, Fe (II) and Fe (III) complexes and sulphides ( H 2 S ,
HS  , S 2 ) depending on the pH solution. Below the pH of 3 it has also been found
that the principal reactions for sphalerite dissolution produces zinc chloride, ferrous
chloride and elemental sulphur. The overall chemical reaction is as follows:
ZnS + 2 Fe (III) = 2 Fe (II) + S 0
(2.1)
The standard free energy change for the reaction is -90KJ/mol. The equilibrium
constant is about 1016 indicating that the reaction lies to the right at 298K. The
leaching of sphalerite in an acidified ferric ion solution can also be expressed in terms
of the following reactions (oxidation and reduction):
Oxidation / anodic reaction are:
Zn 0 + S 2 = ZnS + 2e 
E 0 = 1.44Volts
(2.2)
Zn 2 + 2e  = Zn 0
E 0 = -0.763Volts
(2.3)
S 0 + 2e  = S 2
E 0 = -0.48Volts
(2.4)
S 0 + Zn 2 + 2e  = ZnS
E 0 = 0.197Volts
(2.5)
or ZnS = S 0 + Zn 2 + 2e 
E 0 = -0.197Volts
(2.6)
E 0 = 0.771Volts
(2.7)
The reduction/ cathode reaction is:
2Fe 3 + 2e  = 2Fe 2
The overall electrode potential between the zinc sulphide and iron (III) reaction is:
8
E 0 = E 0 anode + E 0 cathode
(2.8)
E 0 = -0.197 + 0.771 = 0.547Volts
(2.9)
2.2 TECHNIQUES OF METAL EXTRACTION
2.2.1 Extraction metallurgy
Generally, extractive metallurgy is defined as the extraction of metals from raw
materials by physical and chemical processes involving their properties in bulk as well
as at the atomic level. The first stage in this process is comminution where mined
rocks is reduced in size until most of the gangue and mineral are separated, the
reason being to release the values locked up in the ore and to reduce the particle
size which increases the surface area available for chemical reactions. The final
product, which is the metal, is produced during the extraction process.
Hydrometallurgical, pyrometallurgical and electrometallurgical processes are the
various techniques that can be used during the extraction process.
Hydrometallurgical techniques involve use of aqueous solutions as well as
inorganic solvents to extract and separate the desired product. These methods are
utilized when high purity of metallic product is required with less environmental
hazards and performed in liquid environment, involving steps like leaching of desired
metal contents in aqueous solvent followed by removal of impurities from metal
surface, Olubambi et al., 2006 and Pugaev et al, 2010.
9
Pyrometallurgical techniques work with high temperature processes and it is
applied mainly to oxide ores. In these processes ore is heated in the furnace in the
presence of heating agent resulting in the formation of molten metal and slag along
with impurities which is separated from metal at working temperature. There are
certain steps that are involved during this technique:
1) Preliminary treatment such as roasting and calcinations which changes the
physical and chemical properties of the ore to make it suitable for extraction
purposes. These pretreatments followed by the
2) Liquidification and smelting in which metals become melted and separated in
to number of liquid layers containing metals.
Electrometallurgical techniques involve various electrical operations for metals
processing. These electrical energy operations increase the efficiency of the
extraction processes but also increase the cost of the process. These include
electrical furnace processing, electrowinning and electrorefining which are explained
as follows:
Electrowinning involves the extraction of metals in purified form from their ore using
appropriate electrolytes, among various types of electrolytes, sulphuric acid is most
preferred and electrolysis is where an electric current splits a liquid (molten salts) into
its chemical parts.
Biohydrometallurgy techniques involve microbes in metals extraction e.g.
bioleaching, biosorption, biobenefication etc. Biohydrometallurgy originated about
2000 years prior to detection of microorganisms.
10
2.3 LEACHING OF SPHALERITE
Leaching is the first stage for any hydrometallurgical process which is widely used in
extractive metallurgy. It is defined as the process of solubilising, extraction or removal
of metals or minerals compound from an ore by means of a solution or lixiviant. It is
also governed by chemical reaction rates. According to Souza et al., (2007), Santos,
(2010) two similar route were proposed in the 1970s to produce zinc as a substitute
for the roast leach electrowinning process regarding chemical leaching which is direct
atmospheric leaching in which zinc sulphide concentrates are leached directly with a
ferric ion addition and pressure leaching that adopt similar method but the leaching is
performed in autoclaves.
The process involves the reaction of the reagents with metal sulphides and the
oxidising agent. Although sulphuric acid may not be as effective as stronger oxidizing
acid like nitric acid, it is however, preferred to other reagents in terms of cost,
corrosion wear and the ease of regeneration during electrowinning (Olubambi et al.,
2006). The most common oxidant used in leaching of zinc sulphide is ferric ion and
can be in either sulphate or chloride media (Santos et al., 2010). The dissolution rate
has been influenced by various factors such as temperature, concentration, stirring,
particle size, solid/liquid ratio and pH of a solution (Aydogan et al., 2005; Olubambi et
al., 2006; Santos et al., 2010).
11
Hydrometallurgical processes are easier to perform and much less harmful when
compared to pyrometallurgical processing. It is observed that the sulphide ores do not
allow the recovery of metals by direct chemical leaching in the hydrometallurgical
process because these ores are insoluble in nearly all the reagents (Olubambi, 2006).
In order for this process to take place the ore must be crushed to find valuable
mineral inclusions, grinded finer to expose minerals (mineral processing) so that the
reagent will react with metal atoms within the mineral ore.
The reagents used during this process are chemicals such as sulphuric acid, nitric
acid and hydrochloric acid which react with the pulp in combination with the one of
the above mentioned acid. The chemical pulp is further reacted with the oxidising
agent (oxygen, hydrogen peroxide, ferric ions) which makes the dissolution rate
faster. Atmospheric leaching or high pressure leaching as an alternative to the
conventional method have been proposed and the success of such methods are
evident in some applications (Santos, 2010).
During the acid leaching of sphalerite, it is selectively leached from both pyrite and
chalcopyrite due to its lower potential. The studies have shown that chalcopyrite,
sphalerite and galena are all selectively oxidised from pyrite since its potential is
higher.
12
2.3.1 Hydrometallurgical processing of zinc
During the chemical leaching for extraction of zinc, zinc is oxidised and dissolved in
an aqueous sulphuric acid solution in the presence of dissolved ferric ions as an
oxidant. The flow diagram for hydrometallurgical processing of zinc is represented in
Figure 2.1 below:
Resources
Leaching
Residue /
recycle
Front end
operation
Leach Solution
Purification
Recovery
Effluent for
disposable/recycle
Product
Metals, alloys
Figure 2.1: Hydrometallurgical processing of zinc production.
13
Back end
operation
Dreisinger, (2009) stated that the importance of hydrometallurgical process has
grown in the metallurgical industry and it has been taken into consideration after
hydrometallurgical route has become a challenge in recovery due to the secondary
reactions provoked by the galvanic interactions between minerals associated within
the concentrates, which affect the process (Madhuchhanda et al, 2000a and b).
2.3.2 Pyrometallurgical extraction of zinc
The zinc sulphide concentrate which contain about 55% zinc is roasted at a
temperature of about 800 C. The resulted roasted zinc oxide is first ground and
agglomerated and then sintered at 1200 C so as to provide feed in the form of lumps
for retort reduction and in this (retort) zinc oxide is reduced by carbon at about 1200
C. The zinc which distils off is collected in condensers and the reactions are as
follows:
ZnS  1.5O2  ZnO  SO2 (Roasting)
(2.10)
ZnO  CO  Zn( g )  CO2 (Reduction)
(2.11)
C  CO2  2CO
(2.12)
According to Harvey et al., (2002) there are approximately twenty important nonferrous metals with a combined production of 100 million tonnes per year that are
processd through pyrometallurgy.
14
2.3.3 Bioleaching
It occurs in an acidic medium that contain the concentration of Fe(III ) . The chemical
and electrochemical reactions of minerals with the leach solution and bacteria are
involved. Bioleaching of sphalerite occurs with the use of the bacteria such as
mesophilic, acidithiobacilus ferrooxidans, and thiobacillus thiooxidans. Olubambi et al,
(2006) investigated and reported the study on effect of ore mineralogy on the
microbial leaching of low grade complex sulphide ores on the recoveries of zinc and
copper. The highest bioleaching recoveries were obtained at a particle size of 75µm.
2.4 REAGENTS USED DURING LEACHING OF SULPHIDE ORES
2.4.1 Hydrochloric acid
Aydogan et al., (2006) studied the dissolution kinetics of sphalerite using hydrochloric
acid as a media and ferric ions as oxidants in hydrometallurgical processing. The
leaching of sphalerite in chloride acid solution is greater than that in sulphate
solutions. Investigations have compared leaching with the two ferric ions (sulphate
and chloride). It is also reported that ferric chloride is advantageous when compared
with ferric sulphate. It is highly corrosive when compared with hydrochloric acid. The
reaction that occurs when metal sulphide reacts with hydrochloric acid is as follows:
ZnS  2HCl(aq)  ZnCl2 (aq)  H 2  S o
(2.13)
15
2.4.2 Sulphuric acid
Most researchers have hydrometallurgical treatment done sulphide minerals leaching
with sulphuric acid as a media with various oxidizing agents because it is easy to use
and cost less for base metal recovery. This reagent was preferred in terms of cost,
corrosion wear and the ease of regeneration during electrowinning. Olubambi et al.,
(2006) investigated sulphuric acid dissolution of Nigerian complex sulphide ore in the
presence of hydrogen peroxide and it was found that mineralogy plays a major role in
the dissolution process. The reaction for it with metal sulphide:
MS  2H   M 2  H 2 S (Non-oxidative leaching)
(2.14)
2.4.3 Nitric acid
Nitric acid has also been used as a media for zinc leaching of zinc sulphide ores in
the hydrometallurgical processing. Peng et al., (2005) studied the sphalerite coupling
leaching in H 2 SO4  HNO3 solutions in the presence of C 2 Cl 4 and the zinc recovery
was 88.2% per hour. It is more effective when compared to sulphuric and hydrochloric
acid but it is expensive.
2.5 LEACHING SPHALERITE WITH FERRIC IONS
2.5.1 Atmospheric ferric sulphate leaching
Ferric ion is one of the most important oxidant in leaching processes. It was mostly
used to leach zinc sulphide (Aydogan et al., 2005; Dutrizac et al., 2003):
ZnS  2Fe 3  Zn 2  2Fe 2  S 0
(2.15)
16
Ferric ions are consumed, thus lead to the decrease of the redox potential. For this
reason it is important to carry out oxidation of the ions which was formed through
reaction 2.15. Oxygen ( O2 ) and hydrogen peroxide ( H 2 O2 ) can be used to promote
the oxidation of ferrous ions in aqueous solution.
The oxidizing media such as acidic ferric chloride solution has been used in
sphalerite leaching. The leaching reaction results in formation of zinc chloride,
sulphur and iron chloride (Aydogan et al., 2005; Santos, 2010). The dissolution of
ZnS in aqueous solution is an electrochemical process, taking place through the
coupled reactions of reduction and oxidation. In the dissolution process, aqueous
sulphuric acid is used as a solvent and ferric ions as the oxidising agent. The
leaching chemistry of ZnS with ferric sulphates (Dehghan et al., 2008)
ZnS  Fe2 (SO4 ) 3  ZnSO4  S 0
(2.16)
After the use of oxygen the following equation were observed:
2Fe 2  0.5O2  2H   2Fe 3  H 2 O
(2.17)
The total reaction according to equation 2.16 and 2.17 are then written as:
ZnS  0.5O2  2H   Zn 2  S 0  H 2 O
(2.18)
Aydogan et al., (2005) studied the dissolution kinetics of sphalerite in acidic ferric
chloride and stated that the reaction rate was increased with an increase in
temperature, ferric ion concentration while decreasing in solid liquid ratio.
17
According to Santos et al., (2010) the most important oxidants in leaching processes
are ferric ions which can be in the media of sulphate or chloride. An improvement of
sphalerite leaching occurred when using oxygen pressure leaching which has found
wide industrial application, however its oxidation is slow and higher temperature and
oxygen pressure are required to compensate for the poor kinetics (Al- Harahsheh,
2007). The kinetics of ferric ion leaching of sphalerite concentrates has been studied
by various authors (Aydogan, 2005; Souza et al., 2007). It was agreed that the
elemental sulphur is the main oxidation product and the iron content plays a key role
during leaching.
Oxidative leaching of sphalerite with ferric chloride and ferric sulphate was studied by
several investigators; ferric chloride was reported to have advantanges over ferric
sulphate as the leaching kinetics is more favourable in the chloride media (AlHarahsheh et al., 2007; Aydogan, 2005). According to Al-Harahsheh, (2007)
atmospheric ferric sulphate and ferric chloride leaching of sphalerite has been given
significant attention in recent years. Leaching of sphalerite in both ferric sulphate and
ferric chloride has been reported previously but detailed dissolution kinetics is not
available particularly for ferric chloride media (Rath et al., 1981).
Aaltonen, (2005) studied the leaching of chalcopyrite or sphalerite in the solution of
acidic ferric sulphate. The galvanic interaction between the two sulphide minerals
resulted in the selective extraction of zinc. Almost 80% of zinc conversion was
achieved; sulphur and lead sulphate solution were separated to the residue.
18
2.5.2 Principles of pressure leaching
Xie, K.Q et al., (2006) reported that at temperature below 180 degrees sphalerite was
oxidised according to the reaction 2.19;
ZnS  0.5O2  2H   Zn 2  S 0  H 2 O
(2.19)
At higher temperature sulphur was oxidised according to this equation:
2S 0  2H 2 O  3O2  2H 2 SO4
(2.20)
The sulphuric acid formed facilitates the dissolution of the base metal in high iron
sphalerite concentrate, which leads to the direct oxidation of the sphalerite according
to equation (2.21).
ZnS  2O2  ZnSO4
(2.21)
2.6 OTHER OXIDANTS USED
2.6.1 Oxygen
In the direct leaching industry oxygen has been used as an oxidizing agent for ferrous
ions.
Fe 2  1 O2  H 3O   Fe 3  3 H 2 O
4
2
(2.22)
2.6.2 Hydrogen peroxide
The dissolution of ores using sulphuric acid in conjunction with hydrogen peroxide
has been studied and successful results were obtained. Sphalerite leaching with nitric
acid and hydrogen peroxide as an oxidant was studied by Adebayo et al., (2006)
where it was stated as a strong oxidizing agent with high standard redox potential in
19
acidic medium. The oxidation of sulphide to sulphur is possible since the redox
potential of sulphur / metal sulphide is less than that of hydrogen peroxide ( H 2 O2 ) .
Moreover, the reagent hydrogen peroxide is suitable for environment. According to
Aydogan, (2006) different raw materials have been studied in conjuction with zinclead bulk concentrate, silver ore, concentrate of pyrite and sphalerite.
Hydrogen peroxide is important as an iron oxidising agent. It is also used as an
oxidising agent for iron. The reduction potential of water and hydrogen peroxide pair
oxidise the ferrous ion in agreement with the reaction in equation (2.23).
H 2 O2  2Fe 2  2H   2H 2 O  2Fe 3
(2.23)
In the process of hydrometallurgy hydrogen peroxide costs are high however its
potential is high as mentioned by current studies of sphalerite leaching. According to
Olubambi et al., (2006) hydrogen peroxide is a strong oxidizing agent and it also
promotes the oxidation and leaching potential of sulphuric acid and it is easy to
handle than gaseous ones. It can also be catalyzed into hydroxyl radicals, which are
extremely reactive. The dissolution of ores using sulphuric acid in conjunction with
hydrogen peroxide has been studied and successful results were obtained.
According to Souza et al., (2007) two similar routes were proposed in the 1970s to
produce zinc as substitutes for the roast- leach- electrowinning which are direct
atmospheric leaching in which zinc sulphide concentrates are leached directly with a
ferric ion solution and pressure leaching that adopts a similar approach except that
leaching is carried out in autoclaves.
20
2.7. FACTORS AFFECTING THE LEACHING PROCESS
2.7.1 Temperature
Temperature has a clear effect on the dissolution of sphalerite. Metal sulphide can be
leached at high temperature in the presence of oxygen or other oxidant. Many
authors agreed that an increase in temperature increases the dissolution rate (Souza
et al., 2007; Al –Harahsheh, 2007; Aydogan et al., 2005). Although temperature is
one of the main leaching parameters which influence the leaching rate of zinc
sulphide concentrate, it is difficult to prove its effects in ferric media. The chemical
reactions have high activation energy and are highly dependant on temperature
(Aaltonen, 2005). Many authors have observed that temperature increases the
leaching rate; however, the reported values of activation energy vary widely (Souza et
al., 2007).
2.7.2 Particle size
Particle size influences the extraction rate in a number of ways. The smaller the size
the greater the interfacial surface area between the solid and the liquid and therefore
the higher the transfer of the material and the smaller the distance the solute must
diffuse within the solid. Rapid dissolution rate for sphalerite dissolution is promoted
when the ore particles are finely grinded. According to Olubambi et al.,(2006) the size
of an ore plays an important role in determining the effectiveness of chemicals on
mineral dissolution. Its effects have been studied by various researchers.
21
The bigger the particle sizes, the surface area will be smaller but the smaller the
particle size the higher the surface area. Al-Harahsheh and kingman, (2007) showed
that particle size has an effect on the sphalerite leaching with hydrochloric acid
media. Souza et al., (2005) investigated sphalerite leaching on various particle sizes
and the smallest was minus 38microns of which 96.1% was recovered after 4 hours.
Many authors concluded that the dissolution rate increases with decreasing particle
size (Olubambi et al., 2006; Aydogan et al., 2005; Souza et al., 2007).
2.7.3 Agitation
According to Olubambi et al., (2006) the highest recovery was obtained at low stirring
speed (160rpm) whereby increased stirring results in faster decomposition of which
results in oxygen growth that adsorbs onto the elemental surface.
Aydogan, (2005) reported that varying agitation speed from the range of 200-600rpm
did not see changes from 200-400rpm therefore 600rpm was selected because that’s
where the oxidation reaction was chemically controlled for all the parameters. It was
reported that at 160 rpm, the oxidation reaction was completely chemically controlled.
Agitation affects the thickness of the diffusion layer surrounding the dissolving
particles. The rate of agitation on the dissolution rate depends on the ratedetermining step of the process. The rate of reaction increases with increasing
agitation until diffusion in the solution is no longer rate determining.
22
2.7.4 Ion concentration
When a metal or non-metallic element such as iron enters solution in an oxidation
state that is different from its oxidation state in the mineral, an oxidation or reduction
will occur during leaching. In such a case that the leaching state must provide an
oxidizing or reducing agent, and the solubility of the metal becomes a function of the
concentration of such agent in equilibrium with the mineral or compound leached.
Increasing the concentration of ferric ions has been the most adopted method to
increase the sphalerite dissolution rate (Souza et al., 2007, Aydogan et al., 2005).
In spite various studies found that increasing concentrations of ferric ions in both
sulphate and chloride system do not increase the leaching rate of zinc sulphide. The
presence of iron in sphalerite mineral increases the rate of the dissolution.The iron is
added as ferric sulphate Fe2 (SO4 ) 3 or ferric chloride FeCl 3 and reacts with metal
sulphides producing elemental sulphur by simple oxidation–reduction Ferric chloride
is well recognized as a leaching agent and it was reported that ferric chloride has
more advantage than ferric sulphate.
2.7.5 Solid-liquid ratio
The recovery of zinc increases with a decrease in the amount of solids (Santos,
2010). Aydogan, et al., (2005) investigated the dissolution kinetics of sphalerite in
acidic ferric chloride leaching and he stated that the effect of solid/liquid ratio on the
dissolution of sphalerite was investigated in the range of 1/5–1/50. The zinc recovery
increased with decrease in the amount of solid.
23
2.7.6 Leaching Time
Time plays an important role on leaching of sulphide minerals. The longer the contact
time between the leaching agent and the mineral the higher will be the extraction;
however there is a time when the reaction reaches equilibrium and no dissolution
occurs. The product will also in some cases diffuse back into the material when
saturated (Souza et. al., 2007). According to Santos et al., (2010) the reaction
dissolution is fast in the first 60 minutes and thereafter remained the same.
2.8 INTERACTION OF SULPHIDE ORE MINERALS
The differences in the atomic structure of minerals resulting from the type of bonding
and the distance between the atoms within different minerals, affect their
mineralogical properties and those differences in turn affect minerals’ response and
behaviour in different aqueous media (Olubambi et al., 2006). Their mineral
behaviour occurs during the interaction of two different conducting minerals with
different electrode potential and the process is called galvanic interaction. When
various sulphide minerals such as chalcopyrite, galena, sphalerite, pyrite, etc. are
present together in an acidic solution, selective dissolution occurs due to
electrochemical galvanic interactions resulting from the differences in the electrical
conductivities of the minerals.
According to Lehman et al., (2000) sulphide minerals which behave as anodic
electrode in an acidic medium can be arranged in the form of galvanic series, which is
24
shown in Table 2.2. In an electrochemical cell formed by two sulphide minerals of
different potential value, the sulphide having the low potential will dissolve first. The
rate of galvanic dissolution can be accelerated by the presence of an oxidant.
Table 2.2: The rest potential of mineral sulphide at pH = 4 (Donati and Sand, 2007)
Sulphide Minerals
Formula
Potential
Pyrite
FeS 2
0.66
Chalcopyrite
CuFeS 2
0.56
Galena
PbS
0.46
Sphalerite
ZnS
0.40
The minerals in the table above are arranged according to their potential, it is noble
from top (pyrite) and active at the bottom (sphalerite).Galvanic interaction mechanism
between pyrite and sphalerite is shown in Figure 2.2 below:
FeS 2
e
Cathode
Anode
ZnS
Figure 2.2: Method of galvanic interaction amongst pyrite and sphalerite (Donati and
Sand, 2007).
Cathode: 1 O2  H 2 O  2e  2OH 
2
(2.24)
2
Anode: ZnS  Zn  S  2e 
(2.25)
25
According to Donati and Sand, (2007) the dissimilar rest potential of the two minerals
is accountable for the galvanic corrosion formation. Factors that limit the value of
galvanic current are
1) The solution and the mineral electrical resistance.
2) The type of contact between minerals
3) The concentration of the overpotential on both electrodes which depend on the
transport of the reactants to and from the electrodes.
4) The activation of the overpotential of process electrodes on its surface.
Santos et al., (2006) and Cruz et al., (2005) showed that galvanic interactions can
increase the leaching rate and the metal recovery from various minerals. For semi
conductive minerals, such as sulphides, direct contact of different minerals with
dissimilar rest potentials initiates the galvanic effect (Cruz et al., 2005). He and others
also noted that these galvanic interactions depend on the mineralogical association
between the phases present in the ore. It was also found that these galvanic effects,
occurring between conducting and semi-conducting minerals in aqueous systems,
play an important role in the aqueous processing of ores and minerals, such as in
flotation and leaching.
According to Feng and Van Deventer, (2002); Santos et al., (2006) Galvanic
interactions play a significant role in hydrometallurgical applications whereby base-
26
metal ores such as galena, sphalerite, chalcopyrite and pyrite are semiconductors
and when any of them (minerals) contacts with the other in an electrolyte, a galvanic
couple is formed in which the mineral of lower rest potential corrodes at an enhance
rate as the anode of the couple and the other one with higher rest potential corrodes
at slower rate as cathode. An example of the galvanic coupling is the accelerated
leaching of copper from chalcopyrite in contact with pyrite. Pyrite with a higher
potential acts as a cathode: O2  4H   4e  2H 2 O O and chalcopyrite with a lower
potential becomes a dissolving anode: CuFeS 2  Cu 2  Fe 2  2S 0  4e  . Pyrites can
also corrode sphalerite (ZnS ) , which has a lower potential (Suzuki, 2001).
da Silva et al., (2003) reported that when two minerals of different conductivity
contact each other in an aqueous solution, current flows through the solution from the
mineral of the highest rest potential which forms a galvanic cell and the cell cause
the mineral of lower rest potential to be sacrificed while the mineral with higher
potential is passivated.
Olubambi et al., (2007) investigated and reported on the ore mineralogical effect on
the microbial leaching of low grade complex sulphide ores with respect to zinc and
zinc recoveries. It was observed that the mineralogical and elemental distribution
within the various particle sizes affect the mineral- microbe interaction, the galvanic
interaction as well as precipitate formation on the surfaces. He also observed that
studying the ore mineralogical effect on hydrometallurgical processing has not been
given a much attention and the influence on leaching mechanism. It was also stated
27
that research on the mineralogical effect of leaching with ferric ions should be
studied.
According to Aaltonen, (2005) a heterogenous mixture of sulphides has areas of
varying potential and galvanic coupling is known to greatly affect the rates of
dissolution. The sulphide with the higher rest potential becomes the cathode and
supports the reduction of the oxidizing agent; the one of lower potential becomes the
anode and dissolves. The oxidation potential of sphalerite is lower than that of e.g.
copper sulphide and dissolution of zinc by selective extraction is possible from a
mixed zinc-copper sulphide. Pyrite also is known to enhance the dissolution of
sphalerite by galvanic coupling.
For semi conductive minerals, such as sulfides, direct contact of different minerals
with dissimilar rest potentials initiates the galvanic effect. This effect has been
modeled with galvanic cells through the redox reactions, where the mineral with the
higher rest potential acts as the cathode, which is galvanically protected, while the
mineral with the lower rest potential acts as an anode and its dissolution is favored
through electronic interactions. These interactions occur between sulfides, involving
the flow of electrons from grains with a higher potential to grains with lower potentials,
modifying the Fermi level of both minerals.
In order to achieve high reproducibility in the electrochemical results and also to
handle particles more easily with the size distribution of the concentrates and
28
residues, carbon paste electrodes were employed as the working electrodes. For this
kind of electrodes, the presence of graphite and non-conductive binding oil does not
affect the electroactivity of mineral particles processing has not been given a much
attention and the influence on leaching mechanism. Tshilombo and Dixon, (2003)
stated that electrochemical studies in sulphide minerals should be established to
understand their interactions.
Generally, the galvanic interaction between sulfide minerals has been studied using
short-circuited galvanic cells or with attached mineral electrodes and by comparing
individual electrochemical behavior of the minerals In these type of cells, the contact
between minerals is through an electrolytic solution while, in an industrial
concentrate, the electrical contact between the mineralogical phases are in solid
state. This difference could explain the limited reliability of these galvanic cells for the
prediction of mineral reactivity (Cruz et al., 2004).
2.9 ELECTROCHEMICAL STUDIES
In the leaching mechanism of sulphides minerals the mechanism of electrochemical
was considered to be most important. The characteristics of leaching are as follows:
1) The mineral is used as the conductor or semi conductor
2) The electron is transferred from the mineral solid phase to the reactant species
and the process takes place through a redox couples in which the ions or molecules
in dissolution transport towards the solid surface where interact with electrons.
29
3)The minerals dissolves in certain localized sites, whereas the electronic transfer to
some species present in solution takes place in other different operated sites.
4) Redox reactions take place simultaneously, but each one has its own
characteristics with respect to the control kinetic mechanism (chemical or mass
transfer).
5) The dissolution rate is complex function of reactants concentration.
6) Kinetics of these reactions is affected by the own mineral crystal lattice and by the
presence of defects in it.
Therefore several aspects are important - impurities in solid solution, the presence of
different mineral phases and the presence of different ions in solution (that modify the
electrochemical behaviour when fixed on to the mineral).
According to Donati and Sand, (2007) sulphide minerals are solid semiconductors
and the oxygen can be used as an oxidant during the process of leaching. The
schematic representation for the reaction is shown in Figure 2.3.
30
Figure 2.3: The electrochemical mechanism of ZnS dissolution (Donati and Sand,
2007)
Olubambi et al., (2006) reported that the electrochemical phenomenon involved in
metal dissolution from an ore depends on the electrical properties of the solid material
and the redox characteristics of the solution. Since most sulphide minerals are
semiconductors and the kinetics of dissolution is described by an electrochemical
mechanism, the semiconducting properties influence the kinetics of dissolution. It is
noted that the principle of dissolution of semiconducting sulphide minerals in oxidizing
solutions is an electrochemical corrosion process.
31
The soluble oxidant is reduced at the mineral surface, the mineral oxidant itself
behaves as an anode. An electrochemical study would be best understood through
the minerals’ electrochemistry, which would therefore contribute basic information on
the sulphide ore dissolution mechanism, some theoretical background information on
the interaction between chemical and minerals, acid-minerals attachment and
mineralogical differences is required.
Numerous electrochemical studies on copper sulfide minerals have been conducted,
most of them aimed at understanding their behavior in different stages of mineral
processing,
particularly
in
the
leaching
and
flotation
processes.
These
electrochemical processes, which are determined by the electrochemical interactions
on dissolution at the electrode surface, are dependent on the composition and
morphology of the mineral. During these processes, the surface modification occurs
with a heterogeneous composition and morphology that depends on, among other
factors, the applied potential, pH, temperature, rate of the potential scan, type and
concentration of sulfur-containing ions and compounds, presence of different types of
chemical active solid phases, compounds influencing into the wetting properties of
surface (Arce and Gonzalez, 2002).
Olubambi et al., (2006) studied both the bioleaching and electrochemical behaviour of
sulphide minerals (galena, siderite and sphalerite) in sulphuric acid solution in the
presence of mixed cultures of Acidithiobacillus ferrooxidans, Acidithiobacillus
thiooxidans and Leptospirillum ferrooxidans, paying attention to the various particle
sizes of this ores. The role of bacteria in electrochemical leaching mechanism of
32
metal sulphide by Ballester et al., (2007) is shown in the Figure 2.4 below. Shi et al.,
(2006) also reported the electrochemical behaviors of marmatite –carbon paste
electrode in the presence and absence of bacteria strains to improve the
understanding of its mechanism. To date there is no work done on the
electrochemical behavior of the three sphalerite ores (black mountain, import and
imcor) at various concentrations of ferric chloride and ferric sulphate oxidants in
sulphuric acid media.
H
O2
H 2 SO4
H 2O
MeS
Bacteria
(Cathodic )
e
Bacteria
MS (Anodic )
S0
S0
M 2
Figure 2.4: Role of bacteria in electrochemical mechanism of a metal sulphide
(Donati and Sand, 2007).
The mechanisms of mineral sulphides dissolution have been investigated mainly for
sulphide minerals, in order to improve the efficiency of the industrial leaching
operations, including bacterial processes. The oxidation of sulphides by a chemical or
33
biological oxidant can be looked upon as an electrochemical reaction and it can be
consequently studied by electrochemical techniques (Horta et al., 2009).
2.9.1 Electrochemical technique
Urbano et al., (2007) reported that electrochemical techniques have become one of
the most powerful tools to study galvanic interactions between different mineral
associations under conditions similar to those found in sulfide mineral concentrates
these days. Electrical conductivity plays an important role when using this technique
since most sulfide minerals have good conduction due to their semi-conductivity
characteristics; however, some do not have this property (i.e. sphalerite). In this case,
the use of carbon paste electrodes (CPE) has allowed the application of
electrochemical techniques and, because of this, the knowledge of the interactions of
non-conducting sulfide minerals can be gained. The CPE is composed by a paste
made from graphite powder, binding material, and pulverized mineral. The
granulation of the graphite and the powder mineral needs to be the same to avoid
physical problems and heterogeneity. For homogenization a volatile organic
substance that does not alter the chemical and physical properties of the paste is
usually used.
Dissolution kinetics using electrochemical techniques have also been studied for
metals undergoing passivity. A kinetic model was developed to characterize zinc
oxidation incorporating a charge transfer process and the transport of anions towards
the metal surface through a non-porous passive film of zinc salt. The developed
mathematical model using mixed potential theory to describe the leaching of gold in
34
cyanide solutions, proposing reaction at the surface as the rate-controlling step. They
also accounted for gold passivity by assuming that the active surface area is
progressively covered by a passivating layer which grows at a rate proportional to the
total amount of surface area available, and which is only dominant during the later
stages of leaching.
Acero, (2007) studied the non-oxidative dissolution of sphalerite and observed that
sphalerite dissolution was first order dependent on [H +] and also studied sphalerite
dissolution by Fe (III) at 25◦C and pH around 2 and showed that sphalerite dissolution
kinetics was substantially different from pyrite dissolution kinetics.
2.9.2 The electrode cell
Figure 2.5: Schematic representation of three cell electrodes
35
2.9.3 Role of solution potential
Passivity seems to be influenced and determined by the solution chemistry wherein
species that either promote or are directly responsible for passivity can be identified.
Therefore, distribution and concentration of all chemical species determine both the
conditions for passivity to occur and the solution redox potential, which is an
important factor in the rate of atmospheric ferric-based leaching processes.
2.9.4 Potentiostat
It is the heart of electrochemistry. Depending upon the experimental technique being
used, the instrument can control the potential and measure the current, control the
current and measure the corresponding potential, or simply measure the open circuit
potential of a system. Modern instruments can be powerful, delivering stability,
sensitivity, and speed as required (Robertson et al., 2005).
It is operated in the controlled potential mode. The potential is scanned at a slow
sweep rate, typically 1 mV s-1, to simulate the reaction at steady state; in other
words, when the rate of the hydrometallurgical process is not undergoing change due
to the initial or final stages of the reaction. It is worth noting that, in some instances,
lower potential sweep rates are required, particularly when the electrochemical
reaction is hindered by the formation of a passive layer. Three typical electrodes are
used in order to control the potential while measuring the current on the potentiostat.
Those electrodes are named as the working, counter and reference. The working
electrode is made of the material of interest. The reference electrodes provide a
36
stable “reference” against which applied potential may be accurately measured. The
couter electrode is used to make a connection to the electrolyte so that a current can
be applied to the working electrode
There are a number of different electrodes, the common ones being saturated
calomel electrode (SCE), silver/ silver chloride, and mercury/mercury oxide. Each
reference electrode has a different stable potential; hence, it is very important to state
which reference electrode was used when quoting potentials. It is common to see
potentials quoted with respect to the standard hydrogen electrode (SHE) despite the
fact that it is very rarely used. Normally, the potential is measured with respect to a
more robust reference electrode such as the SCE, and then converted to the SHE
scale for publication. The counter electrode is used to provide the applied current and
as such should be composed of highly corrosion resistant material such as platinum.
According to Oubambi et al., (2009) it varies the potential at a steady rate between
two preset potentials which generates current through the cell. The current supplied
goes into the working electrode through the counter electrode. The potential of the
working electrode could thereby be measured with respect to the reference electrode.
Data logging of the potential and current could be done by the computer. The open
circuit potential (Ecorr), current density, Tafel slope and corrosion rate are usually
obtained from the graph and the graph is shown in Figure (2.6).
37
Figure 2.6: The tafel plot from polarization experiment (ASTMG3-89).
2.10 ELECTROCHEMICAL MEASUREMENTS
2.10.1 The open-circuit potential
The open circuit potential is used as a criterion for the corrosion behavior (Jimenezi et
al., 2009). This method is carried out by the use of the metal or conductive minerals
in acicdic solution with or without oxidant addition.
2.10.2 Potentiodynamic polarization
It is the characterisation of a metal specimen by its current-potential relationship.
Potentials are imposed, other than the corrosion potential, Ecorr at the metal-solution
interface by scanning slowly in the positive direction which brings about corrosion.
Polarisation characteristics are measured by plotting the current response as a
38
function of the applied potential. When the logarithm of the current function is plotted
against the potential on a semi-log graph, it is termed a potentiodynamic polarization
plot. The shape of the curve shows the corrosion behaviour of the specimen in the
electrolyte. The passivation behaviour i.e., whether the protective oxide forms or not
or no oxide, can also be determined from this curve, but one cannot identify the
passive films formed. Lu et al., (2000a and b) stated that sulphur does not cause the
passivation during the initial stage of mineral oxidation.
2.10.3 Chronoamperometric measurements
The current response as a function of time is called chronoamperometry. This
response is obtained by applying a potential which can cause electrochemical
reactions to occur on a metal surface. The response monitors the conversion of
elements into ions and the rate at which these ions diffuse out through the metal
carbide/sulphide. A plot of current against reciprocal of the square root of time should
be linear because current decays with time as indicated by Cottrell’s diffusion control
equation, reproduced here as
i = nFACD1/2 π1/2 t-1/2
(2.26)
n = number of electrons transferred/ molecule
F = Faraday’s constant
A = electrode area (cm2)
D = Diffusion coefficient (cm2s-1) and
C = Concentration (molcm-3)
39
From the plot, the slope is calculated and any one of the parameters n, A, D and C
can be determined provided any three of them are known.
2.10.4 Cyclic voltammetry measurements
Cyclic voltammetry is a method for investigating the electrochemical behaviour of a
system. In this technique current flowing between the electrode of interest and a
counter electrode is measured under the control of a potentiostat. The voltammogram
determines the potentials at which different electrochemical processes occur. The
working electrode is subjected to a triangular potential sweep, whereby the potential
rises from a start value Ei to a final value Ef then returns back to the start potential at
a constant potential sweep rate. The sweep rate applied can vary from a few millivolts
per second to a hundred volts per second.
Aaltonen, (2005) applied an electrochemical method to study the dissolution of
sulphide minerals. He found that increased temperatures and potentials accelerated
the dissolution process. The investigation was conducted with the use of three
methods which are anodic polarization, cyclic voltammetry and potentiostatic
dissolution. A graphite paste electrode was used as a sample holder for the
powdered sulphides, forming the working electrode. The graphite paste was found to
be practically inert in electrochemical 19 measurements.
The current measured during this process is often normalised to the electrode surface
area and referred to as the current density. The current density is then plotted against
40
the applied potential, and the result is referred to as a cyclic voltammogram. A peak
in the measured current is seen at a potential that is characteristic of any electrode
reaction taking place. The peak width and height for a particular process may depend
on the sweep rate, electrolyte concentration and the electrode material.
Cyclic voltammetry makes possible the elucidation of the kinetics of electrochemical
reactions taking place at electrode surfaces. In a typical voltammogram, there can be
several peaks. From the sweep-rate dependence of the peak amplitudes, widths and
potentials of the peaks observed in the voltammogram, it is possible to investigate the
role of adsorption, diffusion, and coupled homogeneous chemical reaction
mechanisms.
41
CHAPTER THREE
EXPERIMENTAL PROCEDURE
3.1 INTRODUCTION
Different experimental procedures have been performed to determine
the
electrochemical behaviour of the sphalerite minerals. These procedures are
described in detail below.
3.2 MATERIALS
The three sphalerite ores (Black mountain, imported and imcor) obtained from the
ZINCO Mine were used in this study. The ores were supplied in the form of
concentrates.
3.2.1 Reagents
Reagents used for the electrochemical studies were prepared from their analytical
grades. The 98% sulphuric acid ( H 2 SO4 ) in liquid form and the ferric oxidants
[ FeCl 3 .6 H 2 O -60% and Fe2 (SO4 ) 3 . H 2 O -70%] which were in powder form were used.
They were prepared by adding the distilled water to them. Check Appendix A for the
detailed calculations and dilution of the reagents.
42
3.2 MATERIAL CHARACTERIZATION
3.2.1 Particle size
The Malvern Mastersizer machine was used as a particle size analyzer to determine
the particle size of each sphalerite mineral (IMP, BM and IMC).
3.2.2 Elemental composition determination
The samples were digested in a microwave (CEM® MARS X) into solution form
before analyzing using inductively coupled plasma optical emission spectroscopy
(ICP-OES). The microwave digester was capable of holding up to 12 times of 80 ml of
the digestion vessels on a turntable at the same time. 11 of the 12 vessels
incorporated a pressure release valve and were capable of bearing pressures up to
about 55 bars. The 12 vessels were used to capture and monitor temperature and
pressure profiles in the vessel during heating. The summarized digestion procedure
was a modification from Al-Harahsheh et al. (2009) and has been shown in Figure
3.1.Elements such as Zn , Fe , Cu , Pb , Ni and S were analysed and their standards
were
prepared
from
salts
follows: Zn(NO3 ) 2 , Fe(NO3 ) 2 , Cu (NO3 ) 2 , Pb(NO3 ) 2
which
were
Ni(NO3 ) 2 and Na 2 SO4 .The
as
distilled
water were also added on this standard preparations and the detailed calculations
and dilution are provided in Appendix B.
The standards were prepared at five various levels of concentration for each element
and measured at suitable wavelength. The inductively coupled plasma optical
43
emission spectroscopy (ICP-OES) was used to determine the chemical composition
of the concentrates. Table 3.1 shows the chemical compositions of the concentrates
Weigh 0.1g±0.0001g of
sample in digestion
vessel.
5ml of prepared boric
acid was added to each
vessel.
5ml HNO3, 2.5ml HCl and
2.5ml HF was added to
each vessel.
Vessels were closed and
assemble them on the
turn table.
Vessels were closed and
assemble them on the
turn table.
Microwave was
heated at the
maximum power of
1200W as follows:
Ramp to 170ºC in
15min, holding for
30min in 170ºCand
cooled down to room
temperature.
Microwave was
heated at the
maximum power of
1200W as follows:
Ramp to 200ºC in
15min, holding for
30min in 200ºCadn
cooled down to room
temperature.
The content was
transferred to 50ml of
volumetric flask then
filled to the mark with
10% HNO3.
ICP- OES analysis.
Stage 1
Stage 2
Figure 3.1: Summarized procedure for elemental digestion (adapted from Harahsheh,
2009).
44
Table 3.1: The chemical composition of the different concentrates.
Minerals
Cu%
Zn%
Fe%
Ni%
Pb%
S%
IMP
0
38.01
0.27
0
0.47
18.14
BM
0.51
30.54
8.18
0.01
0.34
20.04
IMC
0.41
34.65
2.34
0
0.51
19.08
3.2.3 Phase analysis
The mineralogical composition of the ores was determined using X-ray diffractometry
(Philips PW 1830). Samples were prepared for XRD analysis using a backloading
preparation method. It was analysed with a PANalytical Empyrean diffractometer with
PIXcel detector and fixed slits with Fe filtered Co-Kα radiation. The phases were
identified using X’Pert Highscore plus software. The relative phase amounts (weights
%) were estimated using the Rietveld method. Table 3.2 shows the mineralogical
composition of each concentrate.
45
Table 3.2: Mineralogical compositions of the ores.
Minerals
Chemical Formula
IMP%
IMC%
BM%
Anglesite
PbSO4
1.25
3.02
1.83
Calcite
CaCO3
2.13
3.55
0.86
Dolomite
CaMg (CO3 ) 3
7.86
6.64
-
Gypsum
CaSO4 .2H 2 O
9.19
2.81
0.83
Sphalerite
ZnS
79.56
69.45
59.35
Polygorskite
Mg 2 Si4 O10 (OH ).4H 2 O
-
11.23
21.09
Pyrite
FeS 2
-
3.29
-
Starkeyite
MgSO4 .4H 2 O
-
-
16.03
3.2.5 Microstructural analysis
Microstructural analyses of the samples were carried out using high resolution
scanning electron microscope JSM-7600 equipped with Energy-Dispersive X-ray
analysis (EDX) for elemental analysis. Before analysis, samples were coated with
iridium to reduce charging.
3.3 ELECTROCHEMICAL STUDY
A three electrode cell was used for the electrochemical study. A glass beaker of 1000
ml with a lid having 4 holes was used as the electrochemical cell. Platinum rod was
used as the counter electrode (CE) whilst silver/silver chloride ( Ag / AgCl ) 3 M
potassium chloride ( KCl ) was used as the reference electrode (RE). The working
46
electrode consisted of a mixture of 4 g of the powdered ore, 4 g of powdered graphite
(base material conductor) to improve conductivity and 2 ml of silicon oil (as a binder).
It was compressed under pressure using a carbon paste electron holder. The
samples were prepared a day before the electrochemical test was performed for the
sample to be dry. Electrochemical tests were carried out using an Autolab
potentiostat / galvanostat instrument (PGSTAT30 computer controlled) using the
General Purpose, Electrochemical Software (GPES, version 4.9).
Electrochemical measurements were carried out in five different solutions: 0.5 M
sulfuric acid ( H 2 SO4 ) as an electrolyte and 0.5 M H 2 SO4 with four different
concentrations of ferric chloride ( FeCl 3 ) and ferric sulphate [ Fe2 (SO4 ) 3 ] as oxidants.
All the electrochemical measurements were conducted at room temperature (25 ◦C).
The
open-circuit
potential
(OCP),
potentiodynamic
polarization,
and
chronoamperometry (CA) experiments were used to study the electrochemical
behavior of the ores. The open-circuit potential was run for 1 hour, for
potentiodynamic polarization and experiments; the potential was scanned from 0 V to
1.2 V at a scan rate of 1.6 mVs−1. Chronoamperometric experiments were carried out
at different dissolution potentials which are 0.7, 0.9 and 1.1 Volts within the passivity
range.
47
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 CHARACTERIZATION OF MINERALS
4.1.2 Particle size analysis
The Malvern particle size analyzer was used to determine the particle size of each
mineral. Table 4.1 shows the results obtained from the particle size analysis. The
particle size is calculated between the diameter of 90 and 10 percentage points
relative to median diameter and D(V;50). The D50 for the the samples has been
shown in the table as 729 for IMP, 537for BM and 581 for IMC.
Table 4.1: The particle size analysis of sphalerite minerals.
Sphalerite
minerals(μm)
D10
IMP
BM
IMC
72.2
91.8
138.8
D50
729.4
537.3
581.2
D90
1303.3
1001.9
1055.1
4.1.3 The morphology of the samples
The morphology of the samples was characterized using the scanning electron
microscope (SEM). Figures 4.1 to 4.3 show the SEM micrographs of IMP, BM and
IMC
at
2000Xmagnifications.
The
SEM
and
EDX
analysis
showed
high
concentrations of Zn and S with other minor elements such as C , O and Si in all the
48
sphalerite minerals. There was also a small fraction of Fe present in the BM sample.
Thus, the light grey colour represents ZnS while white represent galena.
IMP
Figure 4.1: Morphology of IMP sample showing at the magnification of 2000X.
BM
Figure 4.2: Morphology of BM sample showing at the magnification of 2000X.
IMC
Figure 4.3: Morphology of IMC sample showing at the magnification of 2000X.
49
4.2 ELECTROCHEMICAL BEHAVIOR
The electrochemical behavior of IMP, BM and IMC in 0.5 M H 2 SO4 solution with the
addition of FeCl 3 and Fe2 (SO4 ) 3 were studied. Their electrochemical behavior in the
presence and absence of oxidant has been investigated using open- circuit potential
(OCP),
potentiodynamic
polarization
and
chronoamperometric
curves.
The
experiments were performed at room temperature. All the presented potentials are
versus silver/silver chloride ( Ag / AgCl ) reference electrode.
4.2.1 The OCP behavior of sphalerites in 0.5 M H2SO4
Figure 4.4 represents the OCP results of the three sphalerite samples (IMP, BM and
IMC) in 0.5 M H 2 SO4 solution without the addition of an oxidant. It was observed that
the behavior of OCP values were different for the different sphalerite samples
investigated. It is observed that the potential increased in the first few seconds in
samples IMC and IMP then decreased afterwards while sample BM decreased from
start of potential measurements through to the end of testing. At the end of the
exposure period, sample BM had the lowest potential followed by sample IMC and
IMP, respectively. It has been reported that the selective dissolution is due to galvanic
interaction resulting from differences in the rest potential values of the minerals in
solution (da Silver et al.,(2003); Olubambi et al., (2007). The mineralogical differences
also cause their behaviour to be different.
50
Figure 4.4: OCP of sphalerites (IMP, BM and IMC) in 0.5M H 2 SO4 solution for 1 hour.
4.2.2 The OCP behavior in 0.5 M H2SO4 with FeCl3 addition
Figure 4.5 shows the evolution of OCP values of sphalerite samples (IMC, IMP and
BM) in 0.5 M H 2 SO4 solutions and with addition of various concentrations of FeCl 3
The addition of FeCl 3 first shifted the potentials to more positive values for all the
sphalerite carbon paste electrodes as compared to potentials measured in pure
H 2 SO4 . Secondly, increasing the concentration of FeCl 3 resulted in positive shift in
the OCP values. The increase in the concentration of Fe3+ in both instances can be
due to the formation of the passive film on the surfaces of the minerals stifling the
reactivities on the surfaces of the sphalerite carbon paste electrodes. Similar
observations have been made by other researchers (Antonijević et al., 2005; Liu et
al., 2011; Shi, et al.2006). It can be seen from Figure 4.5 that the growth of the
passive film was more prevalent at higher concentrations.The increased in the
concentration of Fe3+ increased the oxide on the electrodes surface resulting in more
51
positive potential values. The results have also shown that the OCP increased with
time and eventually reached a steady value.The mineralogical compositions of the
various concentrates did not alter the rest potentials strongly.
The evolution of the OCP of sample BM was different from sample IMP and IMC. The
rest potential for sample BM after 1 hour exposure in the electrolyte was higher
compared to sample IMP and lower to sample IMC. This means that sample BM had
the less reactivity at the surface compared to sample IMP. Nevertheless, but high
surface reactivity was higher in sample IMC compared to sample BM. The differences
in the potentials can be due to the differences in their mineralogical compositions.
52
Figure 4.5: The OCP curves showing the behavior of sphalerites (a) IMP, (b) BM and
(c) IMC dissolution in 0.5M H 2 SO4 acid and various FeCl 3 concentrations at 25◦C for
1hour.
53
4.2.3 The potentiodynamic polarization of samples in 0.5 M H2SO4
Figure 4.6 shows the mineralogical dissolution behavior of three different sphalerite
samples (IMP, BM and IMC) in 0.5 M H 2 SO4 solution. The polarization behaviors
were similar and characterized by active and weak passive regions. From Figure 4.7
it can be seen that the current density increased with increasing potentials in all the
samples. Sample BM had the lowest dissolution potential as compared to samples
IMC and IMP which were consistent to the OCP values shown in Figure 4.5.This
statement revealed that the surface reactivity of sample BM was higher as compared
to samples IMP and IMC in H 2 SO4 electrolyte. The surface reactivity was slowest in
sample IMP since it has the highest rest potential.
Figure 4.6: Potentiodynamic Polarization of IMP, BM and IMC in 0.5 M H 2 SO4
electrolyte.
54
Figure 4.7 shows the potentiodynamic polarization curves of the three sphaleritecarbon paste electrodes (IMP, BM and IMC) in H 2 SO4 electrolyte with the addition of
varying concentrations of FeCl 3 (0.25 M, 0.5 M, 0.75 M and 1 M). The dissolution
behavior was different for the different samples indicating the effect of mineralogical
composition on dissolution kinetics. There were strong positive shifts in the
dissolution potentials when FeCl 3 was added. This means that the addition of ferric
ions increased the dissolution potentials. However, change in FeCl 3 concentration did
not alter the dissolution potential significantly. Thus concentration of ferric ions has
little or no effect on surface reactivity tendencies. The dissolution potential of sample
IMP was increased from 0.26 V when leached with H 2 SO4 to 0.42 V when 1 M FeCl 3
was added. For samples BM and IMC, the potentials were increased from 0.22 V to
0.46 V and from 0.26 V to 0.5 V when 1 M FeCl 3 was added, respectively.
55
Figure 4.7: Polarization curve showing the behavior of sphalerites (a) IMP, (b) BM
and (c) IMC dissolution in 0.5 M H 2 SO4 and various FeCl 3 concentrations at 25◦C.
56
The dissolution current densities were determined for the different samples and the
results are shown in Figure 4.8. It is observed that the lowest dissolution rate was
recorded in an electrolyte without any oxidant. The dissolution current densities of
samples IMP, BM and IMC at 0.5M H 2 SO4 were 1.6x10-6A/cm2, 4.8x10-7A/cm2 and
6.6x10-7A/cm2, respectively. The addition of FeCl 3 up to a concentration of 1 M
increased to the dissolution current to 1.7x10-5A/cm2, 4.5x10-5A/cm2 and 2.0x10-5 in
samples IMP, BM and IMC, respectively. The increase is observed up to the
concentration of 0.7M and the decline is observed after that.The increased dissolution
current densities with increasing FeCl 3 concentrations confirmed that FeCl 3 has an
influence on the dissolution kinetics of the sphalerite samples.
The highest current density result to high dissolution for all the samples which was
obtained at 0.75 M. It is also observed that as the concentration of FeCl 3 increased,
the passivity region also widened. The formation of the passive layer may be due to
the coating of samples with the elemental sulfur which protects the samples from
oxidation and further dissolution. The passivation is highly sensitive to FeCl 3
concentration (Olubambi et al., 2006).The dissolution current density in both samples
IMC and BM decreased at 1M FeCl 3 . The highest dissolution current density was
obtained at 0.75 M FeCl 3 in all samples.
57
Figure 4.8: Dissolution current densities of sphalerites (IMP, BM and IMC) in 0.5M
H 2 SO4 and various concentrations of FeCl 3
4.2.4 The OCP behaviour in 0.5 M H2SO4 with Fe2 (SO4)3 addition
Figure 4.9 shows the evolution of OCP values of the investigated samples in 0.5 M
H 2 SO4 and with addition of Fe2 (SO4 ) 3 . The rest potentials shifted to more positive
values with addition of Fe2 (SO4 ) 3 and the behavior was similar to the behavior
observed when FeCl 3 oxidant was added. The positive shift potentials suggest
formation of passive layer that decreased the dissolutions of the different sphalerite
electrodes. Increasing the concentration of Fe2 (SO4 ) 3 also increased the rest potential
slightly in all samples. Thus the addition of Fe2 (SO4 ) 3 influenced the surface reactivity
and dissolution kinetics of investigated samples (Olubambi, 2008). The decreased in
electrode reactivity were high in all the samples at 0.75 M Fe2 (SO4 ) 3 .
58
Figure 4.9: OCP curve showing the behavior of sphalerite (a) IMP, (b) BM and (c)
IMC dissolution in 0.5M H 2 SO4 and various Fe2 (SO4 ) 3 concentrations at 25◦C.
59
Figure 4.10 shows the potentiodynamic polarization curves of the three sphalerites
samples (IMP, BM and IMC) with and without oxidant addition. The behaviors of the
samples were also similar to that of the FeCl 3 addition with the dissolution potential
increasing with increasing concentration of Fe2 (SO4 ) 3 . The dissolution potential of
sample IMP was increased from 0.3 V to 0.46 V when leached with H 2 SO4 ; sample
BM was increased from 0.22 V to 0.5 V and sample IMC was increased from 0.28 V
to 0.5 V when 1 M Fe2 (SO4 ) 3 was added. Weak passivity features were observed in
all the samples in all the electrolytes at 0.5 V. The passivity decrease as the ferric ion
concentration increased. This could be an indication that there was continual
formation of the passive elemental sulphur layer within the potential ranges studied in
this work. Furthermore this might be due to oxidation of the elemental sulphur to
sulphates within these high potential ranges (Olubambi et al., 2009).
60
Figure 4.10: Potentiodynamic polarization curve showing the behavior of sphalerite
(a) IMP, (b) BM and (c) IMC dissolution in 0.5M H 2 SO4 and various
Fe2 (SO4 ) 3 concentrations at 25◦C.
61
Figure 4.11 shows the dissolution current densities versus concentration of IMP, BM
and IMC samples. The dissolution current densities in 0.5 M H 2 SO4 without
Fe2 (SO4 ) 3 addition were 1.3x10-6 A/cm2, 2.9x10-7A/cm2 and 5.8x10-7A/cm2 for samples
IMP, BM and IMC, respectively. When Fe2 (SO4 ) 3 concentration was increased to 1 M,
the dissolution current densities increased to 4.8x10-5A/cm2, 3x10-5A/cm2 and 7.1x105
A/cm2 in samples IMP, BM and IMC, respectively. The increased dissolution current
densities with increasing concentration of Fe2 (SO4 ) 3 suggest that Fe2 (SO4 ) 3 also has
an influence on the dissolution kinetics of the sphalerite samples.
The highest
dissolution current density was obtained at 1 M Fe2 (SO4 ) 3 in both IMP and BM
samples while the highest dissolution current density in sample IMC was obtained at
0.25M Fe2 (SO4 ) 3 . The dissolution current density for sample IMC decreased after
0.25M Fe2 (SO4 ) 3 . Sample IMP recorded the highest dissolution current density at
1M Fe2 (SO4 ) 3 .
Figure 4.11: Dissolution current densities of IMP, BM and IMC in 0.5M H 2 SO4 and
various concentrations of Fe2 (SO4 ) 3 .
62
4.2 EFFECT OF OXIDANT TYPE ON THE DISSOLUTION KINETICS
Figure 4.12 compares the effect of oxidant type on the dissolution current densities at
different concentrations of oxidants for the three sphalerite samples (IMP, BM and
IMC). The dissolution current density of sample IMP in Fe2 (SO4 ) 3 was different from
that of samples BM and IMC. The dissolution current density of sample IMP was the
highest when Fe2 (SO4 ) 3 was used. The optimum leaching conditions was achieved at
1M Fe2 (SO4 ) 3 . The dissolution current densities of samples BM and IMC were almost
similar in the presence of FeCl 3 compared to Fe2 (SO4 ) 3 . The highest dissolution rate
in both samples was achieved at 0.75 M FeCl 3 . The optimum leaching conditions for
samples BM and IMC was achieved with 0.75 M FeCl 3 .
63
Figure 4.12: The dissolution current densities and concentrations of the three
sphalerites (IMP, BM and IMC) in 0.5M H 2 SO4 in both FeCl 3 and Fe2 (SO4 ) 3 .
64
4.3 THE CHRONOAMPEROMETRIC BEHAVIOUR
Figures 4.13 and 4.14 show the chronoamperometric behaviour of the sphalerite
samples (BM, IMP and IMC) in 0.5 M H 2 SO4 and 0.5 M of FeCl 3 and Fe2 (SO4 ) 3 . The
studies were conducted under the applied potentials of 0.7 V, 0.9 V and 1.1 V. These
potentials were selected within the passive range on the polarization curves.
In the chloride solution, dissolution current density increased with an increased in
applied potentials (1.1V) in both IMP and BM while for IMC dissolution current density
was high at 0.7 V. The results suggest that leaching with H 2 SO4 + FeCl 3 in both
sphalerites could be strengthened by the applied potential (Olubambi, 2008).
In the sulphate solution the dissolution current density increased with an increase in
applied potential in BM sample. For IMP and IMC the current density increases at the
applied potential of 0.9V while for BM the maximum current density was recorded at
1.1V.
65
Figure 4.13: Chronoamperometric curves of IMP, BM and IMC in 0.5M H 2 SO4 + 0.5M
FeCl 3 at the applied potentials of 0.7, 0.9 and 1.1V.
66
Figure 4.14: Chronoamperometric curves of IMP, BM and IMC in 0.5M H 2 SO4 + 0.5M
Fe2 (SO4 ) 3 at the applied potentials of 0.7, 0.9 and 1.1V.
67
CHAPTER FIVE
CONCLUSION
The electrochemical studies for different zinc sulphide minerals (IMP, BM and IMC)
have been investigated in this work with the use of different electrochemical
techniques. The electrochemical techniques used were open-circuit potential,
potentiodynamic polarization and chronoamperometry. The tests were carried out to
explain the electrochemical or dissolution behavior of these minerals in sulphuric acid
solution with the addition of ferric chloride and ferric sulphate as oxidants. The
measured current was used to determine the dissolution rate. A mineral-carbon paste
working electrodes were used. The effects of different ferric ion oxidants at different
concentrations were studied.
The results show that the concentration of ferric ion oxidant affects the
electrochemical behavior of the sphalerite samples in sulphuric acid solution. The
OCP of the sphalerite samples increased with increasing concentration of ferric ions.
From the potentiodynamic polarization curves, increasing the ferric ion concentration
increased the dissolution rate of the sphalerite samples. Thus, at higher
concentrations of ferric ions the reaction rate increased leading to the formation of
passive layer. It was observed that the highest dissolution rate in ferric chloride was
obtained at a concentration of 0.75 M whilst in ferric sulphate it was obtained at a
concentration of 1.0 M. Additionally, the dissolution rate was higher in ferric chloride
compared to ferric sulphate for BM and IMC samples. However, sample IMP has
68
higher dissolution rate in ferric sulphate compared to ferric chloride. The reason is
due to the difference in its mineralogical composition. That is the absence of pyrite,
palygorskite and starkyite in IMP sample. Thus increasing the mass transport in the
solution does not result in an increase in the rate of dissolution process. Other
conditions such as the mineralogy of the concentrates must be considered for faster
reaction kinetics.
69
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79
APPENDICES
APPENDIX A
A.1: SOLUTION PREPARATIONS FOR ELECTROCHEMICAL TESTS
1. Sulphuric acid ( H 2 SO4 )
Molar concentration of H 2 SO4 in 2.5l
But 2.5l contains 98% of pure H 2 SO4
0.98  2500  2450 g
n
2450
 24.98 moles
98.08
Dilution of concentrated acid (1000ml)
24.98(Vconcentra tion )  0.5M  1000ml
1000
24.98
Vconcentra tion  20.016ml
Vconcentra tion  1 
2. Ferric sulphate
Ferric
sulphate hydrate Fe2 (SO4 ) 3  H 2 O
Contains 70% Fe2 (SO4 ) 3
Fe2 (SO4 ) 3 content in 500 g : 0.70  500 g  350 g
80
Preparation of 100ml of 1m
Molarity 
molesFe 2 ( SO4 ) 3
l
moles
0.100l
V  100ml
1m 
n  C V
n  0.100l  1mol / l
n  0.100mFe2 ( SO4 ) 3
MFe 2 ( SO4 ) 3  399.88
m
 m  n M
M
 0.100  399.88
n
m  39.988 g
But Fe2 (SO4 ) 3 is only 70%
Therefore 39.988 
Preparation of
100
 57.126 g of Fe2 (SO4 ) 3  H 2 O
70
100ml of 0.75m
n
V
n  C V
C
n  0.100  0.75  0.075moles
mass  0.075  399.88  29.991g
Fe 2 ( SO4 ) 3  H 2 O  29.99 
100
 42.85 g
70
81
Preparation of 100ml of 0.50M
n  C  V  0.100  0.50  0.05moles
m  0.05  399.88  19.994 Fe 2 ( SO4 ) 3
In ferric sulphate hydrate forms
19.994 
100
 28.563g
70
Preparation of 100ml of 0.25M
n  0.100  0.25  0.025
m  0.025  399.88  9.997
In ferric sulphate hydrated form
9.997 
100
 14.28 g
70
3. Ferric chloride ( FeCl 3 .6H 2 O)
MFe 2 Cl3 .6 H 2 0  270.30 g / mol
MFeCl 3  162.206 g / mol
FeCl 3 content in 500 g
82
500
 1.85moles
270.30
1moleFeCl 3 .6 H 2 O  1molFeCl 3
nFeCl 3 .6 H 2 O 
1.85mole  1.85moleFeCl 3
mFeCl 3  nFeCl 3  MFeCl 3  300.05 g
Preparation of 100ml of FeCl 3 , 1M
n  C V
n  0.100  1M  0.100moles
m  n  M  0.100  162.206  16.2206 g
% FeCl 3 
300.05  100
 60%
500
Mass FeCl 3 .6H 2 O to prepare 1M  16.2206 
Preparation of 100ml of FeCl 3 , 0.75M
nFeCl 3  0.075moles
m  0.075  162.206  12.165 g
mFeCl 3 .6 H 2 O  12.165 
100
 20.28 g
60
Preparation of 100ml of FeCl 3 , 0.50M
nFeCl 3  0.050moles
m  0.050  162.206  8.1103g
mFeCl 3 .6 H 2 O  8.1103 
100
 13.52 g
60
83
100
 27.03g
60
Preparation of 100ml of FeCl 3 , 0.25M
nFeCl 3  0.025  162.206  4.05515
mFeCl 3 .6 H 2 O  4.05515 
100
 6.76 g
60
84
APPENDIX B
B.1: SOLUTION PREPARATIONS FOR STANDARDS OF ICP-OES ANALYSIS
Copper standard
Cu 400 ppm
Cu ( NO3 ) 2 .3H 2 O
Calculation:
241.60
 3.802 g
63.546
3.802 g  1000 ppm
  400 ppm
  1.521g / l
Zinc standard
Zn 1000ppm
Zn( NO3 ) 2 .6H 2 O
Calculation:
297.48
 4.678 g
63.59
4.678 g  1000 ppm
  1000 ppm
  4.678 g
Lead standard
Pb 200ppm
Pb(NO3 ) 2
Calculation:
331.20
 1.598 g
207.2
85
1.598 g  1000 ppm
  200 ppm
  0.319 g
Iron standard
Fe 400ppm
Fe( NO3 ) 2 .9H 2 O
Calculation:
404.00
 7.234 g
55.847
7.234 g  1000 ppm
  400 ppm
  2.894 g
Nickel standard
Ni 50ppm
Ni( NO3 ) 2 .6H 2 O
Calculation:
290.80
 4.955 g
58.69
4.955  1000 ppm
  50 ppm
  0.248 g
Sulphur standard
S 1000ppm
Na2 SO4
Calculation:
142.04
 4.43g
32.066
86
4.430 g  1000 ppm
  1000 ppm
  4.430 g
87
B.2: DILUTION OF STANDARDS FOR ICP-OES ANALYSIS
Zn standard
Maximum 1000ppm
40
800
30
600
20
400
10
200
Cu standard
Maximum 400ppm
40
320
30
240
20
160
10
80
Pb standard
Maximum 200ppm
35
140
25
100
15
60
5
20
88
Fe standard
Maximum 400ppm
40
320
30
240
20
160
10
80
Ni standards
Maximum 50
40
40
30
30
20
20
10
10
S standard
Maximum 1000ppm
40
800
30
600
20
400
10
200
89
APPENDIX C
C.1: THE SCHEMATIC DIAGRAMS FOR XRD AS RECEIVED RESULTS OF IMP,
BM AND IMC MINERALS.
Counts
TUT_Germinah_Import Sphalerite
IMP
30000
20000
10000
0
10
20
30
40
50
Position [°2Theta] (Cobalt (Co))
60
70
Peak List
Sphalerite; S1 Zn1
Dolomite; C2 Ca1 Mg1 O6
Gypsum; H4 Ca1 O6 S1
Anglesite; O4 Pb1 S1
Calcite; C1 Ca1 O3
Counts
TUT_Germinah_Black Mountain Sphalerite
BM
20000
10000
0
10
20
30
40
50
Position [°2Theta] (Cobalt (Co))
Peak List
Sphalerite; S1 Zn1
Gypsum; H4 Ca1 O6 S1
Anglesite; O4 Pb1 S1
Calcite; C1 Ca1 O3
Palygorskite O; H18 Mg5 O30 Si8
Starkeyite; H8 Mg1 O8 S1
90
60
70
Counts
30000
TUT_Germinah_Imcor Sphalerite
IMC
20000
10000
0
10
20
30
40
50
Position [°2Theta] (Cobalt (Co))
Peak List
Sphalerite; S1 Zn1
Dolomite; C2 Ca1 Mg1 O6
Gypsum; H4 Ca1 O6 S1
Anglesite; O4 Pb1 S1
Calcite; C1 Ca1 O3
Pyrite; Fe1 S2
Palygorskite O; H18 Mg5 O30 Si8
91
60
70
APPENDIX D
D.1: Morphologies of IMP sample showing at different magnifications at (a) 3000X,
(b) 3500X and (c) 5000X
b
a
c
92
D.2: Morphologies of BM sample showing at different magnifications at (a) 500X, (b)
1000X and (c) 1500X
a
b
c
93
D.3: Morphologies of IMC sample showing at different magnifications at (a)
1000X,(b) 3000X and (c) 3500X
a
b
c
94
APPENDIX E
E.1: THE SEM WITH EDS RESULTS OF THE AS RECEIVED IMP, BM AND IMC
IMP
BM
IMC
95
APPENDIX F
F.1: MINERAL TABLES
Table 4.1: The electrochemical data of the sphalerites samples in 0.5M H2SO4
solution and various FeCl3 concentrations at room temperature
Minerals
Import
Black mountain
Imcor
Concentrations (M)
Ecorr (V)
Icorr (A/cm2)
0
0.26
1.6x10-6
0.25
0.42
6.0x10-6
0.5
0.42
9.0x10-6
0.75
0.42
1.7x10-5
1
0.42
1.7x10-5
0
0.22
4.8x10-7
0.25
0.42
2.1x10-5
0.5
0.46
2.1x10-5
0.75
0.46
1.4x10-4
1
0.46
4.5x10-5
0
0.26
6.6x10-7
0.25
0.42
1.3x10-5
0.5
0.46
1.5x10-5
0.75
0.46
4.1x10-5
1
0.5
2.0x10-5
96
Table 4.2: The electrochemical data of the sphalerites samples in ferric sulphate
solution at room temperature
Minerals
Import
Black mountain
Imcor
Concentrations (M)
Ecorr (V)
Icorr (A/cm2)
0
0.3
1.3x10-6
0.25
0.42
9.0x10-6
0.5
0.42
9.0x10-6
0.75
0.46
2.4x10-5
1
0.42
4.8x10-5
0
0.22
2.9x10-7
0.25
0.38
1.9x10-5
0.5
0.42
2.1x10-5
0.75
0.46
1.7x10-5
1
0.5
3x10-5
0
0.28
5.8x10-7
0.25
0.46
1.7x10-5
0.5
0.46
1.1x10-5
0.75
0.5
1.2x10-5
1
0.5
7.1x10-6
97
APPENDIX G
G.1: LIST OF SYMBOLS AND ABBREVATIONS
CuFeS2
Chalcopyrite
PbS
Galena
FeS2
Pyrite
ZnS
Sphalerite
g/mol
Gram per mole
ºC
Degrees Celsius
g/cm3
Gram per centimeter cube
n
Nano
Ω
Ohms
H2S
Hydrogen sulphide
S
Sulphur
KJ/mol
Kilojoules per mole
E0
Electrode potential
O2
Oxygen gas
SO2
Sulphur dioxide gas
CO2
Carbon dioxide gas
CO
Carbon monoxide
C
Carbon
MS
Metal sulphide
H2O2
Hydrogen peroxide
ZnSO4
Zinc sulphate
98
H
Hydrogen
V
Volts
Fe2(SO4)3
Ferric sulphate
FeCl3
Ferric chloride
e-
An electron
SCE
Saturated calomel electrode
Ag/AgCl3
Silver silver chloride
Ecorr
Corrosion rate
Icorr
Corrosion current density
BM
Black mountain
IMP
Import
IMC
Imcor
ICP-OES
Inductively coupled plasma optical emission
spectroscopy
Zn(NO3)2
Zinc nitrate
Fe(NO3)2
Iron nitrate
Cu(NO3)2
Copper nitrate
Pb(NO3)2
Lead nitrate
Ni(NO3)2
Nickel nitrate
Na2SO4
Sodium sulphate
XRD
X-Ray Diffractrometry
EDX
Energy-Dispersive X-ray
SEM
Scanning Electron Microscopy
99
KCl
Potassium Chloride
H2SO4
Sulphuric acid
OCP
Open circuit potential
CA
Chrono amperomerty
μm
Micrometer
Si
Silicon
M
Molarity
sec
Seconds
GPES
General Purpose Electrochemical Software
n
number of electrons transferred/ molecule
F
Faraday’s constant
A
Electrode area
D
Diffusion coefficient
C
Concentration
Ei
Initial potential
Ef
Final potential
CE
Counter electrode
RE
Reference electrode
100