I SCHOOL of MATERIALS and MINERAL RESOURCES

SCHOOL OF MATERIALS AND MINERAL RESOURCES ENGINEERING

UNIVERSITI SAINS MALAYSIA

ELECTROWINNING OF GOLD FROM CYANIDE LEACH LIQUOR BY KAVIPRIYA ARUMUGAM SUPERVISOR: DR. NORLIA BAHARUN

Dissertation submitted in partial fulfilment of the requirements for degree in Bachelor of Engineering with Honours (Mineral Resources Engineering)

Universiti Sains Malaysia

May 2019

DECLARATION

I hereby declare that I have conducted, completed the research work and written the disertation entitled ‘Electrowinning of Gold from Cyanide Leach Liquor’. I also declare that it has not been submitted previously for the award of any degree or diploma or other similar title of this for any other examining body or University.

Name of the student: KAVIPRIYA ARUMUGAM Signature:

Date: 9 July 2019

Witness by:

Supervisor: DR. NORLIA BAHARUN Signature:

Date: 9 July 2019

ACKNOWLEDGEMENT

This thesis becomes a reality with the kind support and help of many individuals, I would like to extend my sincere thanks to all of them.

Foremost, I want to offer this endeavour to our GOD Almighty for the wisdom he bestowed upon me, the strength, peace of mind and good health in order to finish this research.

I would like to express my sincere gratitude towards my family for the encouragement which helped me in completion of this thesis.

I am highly indebted to my supervisor Dr. Norlia Baharun for the continuous support, for her patience, motivation, enthusiasm, and immense knowledge. Her guidance helped me in all the time of research and writing of this thesis. I could not have imagined having a better advisor for my final year project.

My thanks and appreciation also goes to my coursemates and people who have willingly helped me out with their abilities. Not forgetting Mr. Zulkarnain Hasbolah, Madam Haslina

Zulkifli and other technical staffs of the School of Materials and Mineral Resources

Engineering, Universiti Sains Malaysia who helped in completing my laboratory work.

Last but not least, I would like to acknowledge my parents for their continuous support throughout my academic years in the university.

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TABLE OF CONTENTS

DECLARATION ...... ii

ACKNOWLEDGEMENT ...... iii

TABLE OF CONTENTS ...... iv

LIST OF FIGURES ...... vii

LIST OF TABLES ...... ix

ABSTRAK...... x

ABSTRACT ...... xi

CHAPTER 1 INTRODUCTION ...... 1

1.1 Research Background...... 1

1.2 Problem Statement ...... 3

1.3 Objectives ...... 4

1.4 Scope of Research ...... 4

1.5 Thesis Outline ...... 5

CHAPTER 2 LITERATURE REVIEW ...... 6

2.1 Introduction ...... 6

2.2 Gold Ore Characterization ...... 8

2.2.1 X-Ray Diffraction (XRF) Analysis ...... 8

2.2.2 X-Ray Diffraction (XRD) Analysis ...... 9

2.2.3 Scanning Electron Microscope (SEM/EDX) Analysis ...... 9

2.3 Liquid-Solid Reaction – Leaching ...... 10

2.4 Chemistry of Gold Recovery from Cyanide Solution ...... 11

2.4.1 Advantages of Cyanidation ...... 12

2.4.2 Disadvantages of Cyanidation ...... 13

2.5 Solvent Extraction ...... 13

2.6 Electrochemistry ...... 14

2.7 Electrowinning ...... 15

2.7.1 Advantages of Electowinning ...... 17

2.8 The Thermodynamics of Reaction at an Electrode ...... 17

2.9 The Deposition Process ...... 18

2.10 Morphology of Gold Deposited ...... 20

2.11 Effect of pH ...... 21

2.12 Effect of Temperature ...... 23

CHAPTER 3 METHODOLOGY ...... 24

3.1 Introduction ...... 24

3.2 Sample Preparation ...... 26

3.3 Characterization of Gold Sample ...... 26

3.3.1 Phase Identification by X-Ray Diffraction (XRD) Analysis...... 27

3.3.2 Determination of Chemical Composition by X-Ray Fluorescence (XRF) Analysis ...... 27

3.3.3 Loss on Ignition Test to Eliminate Volatile Substance ...... 27

3.3.4 Identification of Minerals using Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) ...... 28

3.3.5 Preparation of Polished Section ...... 28

3.4 Detemination of Gold Content by Fire Assay Method ...... 29

3.4.1 Cupellation Process ...... 30

3.4.2 Acid Digestion ...... 30

3.5 Determination of Free Gold Content by Bulk Leach Extractable Gold (BLEG) Test 32

3.6 Cyanide Leaching Experiment ...... 33

3.7 Solvent Extraction ...... 34

3.8 Setting Up of Electrowinning Cell ...... 35

3.9 Electrowinning Cell Design ...... 36

3.10 Electrode Preparation ...... 38

3.11 Electrowinning Process ...... 38

3.11.1 Determination of Decomposition Potential ...... 39

3.11.2 The Effect of Temperature on Recovery of Gold ...... 39

3.11.3 The Effect of pH on the Recovery of Gold ...... 40

3.11.4 Surface Morphology of Gold Deposited ...... 41

CHAPTER 4 RESULTS AND DISCUSSION ...... 42

4.1 Introduction ...... 42

4.2 Raw Sample Characterization of Gold Ore ...... 42

4.2.1 Optical Microscopy Study ...... 43

4.2.2 Chemical Composition by XRF ...... 44

4.2.3 Phase Identification by XRD Analysis ...... 46

4.2.4 Identification of Element Present by SEM/EDX ...... 47

4.3 Fire Assay ...... 50

4.4 Bulk Leach Extractable Gold (BLEG) Test ...... 52

4.5 Cyanide Leaching ...... 53

4.6 Electrowinning ...... 54

4.6.1 Determination of Decomposition Potential ...... 55

4.6.2 Effect of Temperature on the Recovery of Gold ...... 57

4.6.3 The Effect of pH on the Recovery of Gold ...... 60

4.6.4 Surface Morphology of Gold Deposited ...... 63

4.6.5 Experimental Observation during Electrowinning Process ...... 69

CHAPTER 5 CONCLUSION AND RECCOMENDATION ...... 71

5.1 Conclusion ...... 71

5.2 Recommendation ...... 72

REFERENCES ...... 73

APPENDICES………………………………………………………………………………..77

LIST OF FIGURES

Figure 3.1: Summary of the gold extraction process ...... 25 Figure 3.2: Riffler sample divider ...... 26 Figure 3.3: Process flowchart for fire assay ...... 31 Figure 3.4: Poly-ethylene bottle used for BLEG test on a milling machine ...... 33 Figure 3.5: Organic and aqueous phases in the separating funnel after solvent extraction (top layer – organic phase and bottom layer – aqueous phase) ...... 35 Figure 3.6: Schematic diagram of an elecrowinning cell ...... 36 Figure 3.7: Setup of electrowinning cell design used throughout the experiment ...... 37 Figure 4.1: Optical microscope image of polished section under 100× magnification showing the presence of quartz and arsenopyrite ...... 43 Figure 4.2: XRD diffractogram showing the major phase of the gold sample ...... 46 Figure 4.3: SEM photomicrograph of gold ore polished section with 1000× magnification .. 47 Figure 4.4: EDX diffractogram showing elements associated with gold ore at spot 1 ...... 48 Figure 4.5: EDX diffractogram showing elements associated with gold ore at spot 2 ...... 48 Figure 4.6: EDX diffractogram showing elements associated with gold ore at spot 3 ...... 49 Figure 4.7: EDX diffractogram showing elements associated with gold ore at spot 3 ...... 49 Figure 4.8: Formation of borosilicate slag in the silica crucible after fusion process ...... 50 Figure 4.9: Lead button formed after fusion process ...... 51 Figure 4.10: The silver prill formed after cupellation process ...... 51 Figure 4.11: Plot showing the highest percentage of gold leached at 97.11% after 24 hours . 54 Figure 4.12: Plot of current vs voltage showing the decomposition potential for gold standard solution ...... 55 Figure 4.13: Plot of current vs voltage showing the decomposition potential ...... 56 Figure 4.14: The effect of temperature on the recovery of gold...... 59 Figure 4.15: The effect of pH on the recovery of gold ...... 61 Figure 4.16: A part of Au-Cl pourbaixdiagram in the [퐴푢퐶푙4]¯ domain at25 °C (Marsden and House, 2005) ...... 62 Figure 4.17: Homogeneous equilibrium diagram of potential versus pH for the 퐴푢-퐻2푂-퐶푁 system at 25°C (after Finkelstein)...... 63

vii

Figure 4.18: Photomicrograph of gold deposited on steel wool cathode at a magnification of 10000× ...... 67 Figure 4.19: Photomicrograph of gold deposited on stainless steel plate cathode at a magnification of 10000× ...... 67 Figure 4.20: SEM diffractogram of gold deposited on steel wool cathode ...... 68 Figure 4.21: SEM diffractogram of gold deposited on stainless steel plate cathode ...... 68 Figure 4.22: Colour change at the end of electrowinning process using stailess steel plate as cathode...... 69 Figure 4.23: Colour change at the end of electrowinning process using steel wool as cathode...... 70

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LIST OF TABLES

Table 3.1: The composition of flux material used in fire assay ...... 29 Table 4.1: Optical properties of quartz, arsenopyrite and gold ...... 44 Table 4.2: Mineral composition of ground ore sample from XRF analysis ...... 45 Table 4.3: Weight of lead button, silver prill and its gold content obtained after acid digestion process ...... 52 Table 4.4: The concentration of free milling gold in g/t in the sample ...... 52 Table 4.5: The gold concentration, amount of gold leached, and percentage of gold recovery ...... 53 Table 4.6: Electrowinning parameters used to the study effect of temperature on gold recovery ...... 57 Table 4.7: The percentage recovery of gold for four different temperatures ...... 58 Table 4.8: Electrowinning parameters used to study the effect of pH on the recovery of gold ...... 60 Table 4.9: Parameters used in the electrowinning of gold standard solution using steel wool as cathode...... 64 Table 4.10: Initial and final concentration of gold standard solution in the electrowinning process ...... 64 Table 4.11: Parameters used in the electrowinning of gold standard solution using stainless steel plate as cathode ...... 65 Table 4.12: Initial and final concentration of gold standard solution in the electrowinning process ...... 65

PROSES ELEKTROLEHAN EMAS DARI LIKOR PELARUTLESAPAN SIANIDA

ABSTRAK

Proses elektrolehan emas dari likor pelarutlesapan sianida telah dijalankan pada pelbagai kondisi eksperimen. Kajian eksperimen ini terdiri daripada merekabentuk satu sel elektrolehan dan pemilihan pelbagai jenis bahan elektrod dalam elektrolehan emas. Sel elektrolehan telah digunakan untuk mengkaji kesan suhu, pH, dan morfologi pemendapan emas pada dua jenis elektrod iaitu keluli tahan karat dan dawai keluli. Perolehan emas dalam masa sejam meningkat apabila suhu meningkat pada sesetengah julat suhu. Perolehan emas daripada elektroelahn pelarutlesapan sianida meningkat dari 36.3% pada 25 °C kepada 41.3% pada 40

°C. Hal ini kerana, peningkatan suhu meningkatkan pekali resapan AuCN yang juga meningkatkan kadar pemendapan emas pada elektrod. Walaubagaimanapun, pemendapan emas pada 60 °C menurun kepada 11.9% kerana pemusnahan sianaida akan berlaku pada suhu yang sangat tinggi. Kesan pH pada perolehan emas bergantung pada komposisi elektrolit.

Perolehan emas dari aurochloride adalah tinggi pada pH 2 sepeti yang ditunjukkan dalam rajah pourbaix, di mana [AuCl4]¯ adalah stabil dalam julat pH 0-8. Manakala, perolehan emas dari aurosianida adalah tinggi pada pH 11 kerana CN mendominasi pada pH > 9 dan mengelakkan pembebasan gas HCN. Pada akhir proses elektrolelah terdapat sedikit peningkatan pH untuk setiap eksperimen yang dilakukan. Peningkatan pH ini berlaku disebabkan oleh tindakbalas sampingan seperti pengoksidaan sianida dan penurunan air. Pemendapan emas pada elektrod dawai keluli mempunyai berat peratus lebih tinggi sebanyak 43.8% berbanding dengan elektrod keluli tahan karat sebanyak 14.2%. Elektrod dawai keluli mempunyai sifat semulajadi; nisbah permukaan bagi elektrod dawai keluli kepada isipadu adalah lebih besar dan keliangan pada dawai keluli membolehkan taburan sekata ion-ion logam pada permukaan elektrod semasa proses elektrolehan.

ELECTOWINNING OF GOLD FROM CYANIDE LEACH LIQUOR

ABSTRACT

The electrowinning of gold from cyanide leach liquor was investigated at various experimental conditions. The experimental study consisted of designing an electrowinning cell and selectivity of different types of electrode material for the electrowinning of gold. The electrowinning cell was used to investigate the effect of pH, temperature and the morphology of gold deposited on two different types of electrode which are stainless steel plate and steel wool. The recovery of gold from the electrowinning of cyanide leach liquor increased from

36.3% at 25 °C to 41.3% at 40 °C. The increase in temperature, increases the diffusion coefficient of AuCN thus increasing the deposition rate. However, the recovery decreased to

11.85% at 60 °C because cyanide destruction occurs at an elevated temperature. The effect of pH on the recovery of gold depends on the composition of the electrolyte. For aurochloride the recovery is high at pH 2 as shown in Au-Cl pourbaix diagram where the [AuCl4]¯ is stable at the pH range of 0-8. Whereas, for aurocyanide the recovery is high at pH 11 because CN¯ predominates at pH >9 and prevents the evolution of HCN gas. There is a slight increase in the pH of the electrolyte solution at the end of each electrowinning experiment. This increase in pH in the electrolyte solution was due to the side reactions such as cyanide oxidation and water reduction. The deposition of gold on steel wool electrode has a higher weight percent at 43.48% compared to weight percent deposition on stainless steel plate, which is at 14.26%. Steel wool electrode has its inherent properties; its high surface area to volume ratio and porosity allow the metal ions to be distributed throughout the electrode during electrowinning process.

CHAPTER 1

INTRODUCTION

1.1 Research Background

Hydrometallurgical processes are used to extract, purify and recover mineral and metal in aqueous systems. The majority of gold extraction flow sheets use hydrometallurgical techniques. The most important of which are leaching, solution purification and gold recovery.

Hydrometallurgical pre-treatment methods like chlorination, pressure oxidation and bacterial oxidation can also increase gold extraction in the subsequent leaching stage by liberating contained gold and by converting interfering constituents into less reactive forms.

The methods used to recover metals from ore depends on the physical and chemical properties, mineralization and mode of occurrence in ore deposit. Gold is usually found as native metal or alloyed with silver and other metal like Tellurides. It is commonly associated with sulphides of iron, silver, arsenic, copper, and in compounds of selenium and antimony.

The recovery process is defined as removal of gold from solution into a concentrated solid. The gold can be recovered from solution by electrolysis; a process called electrowinning in the industry. Electrowinning process involve the deposition of gold at the electrode (cathode) immersed in the solution. This method uses direct current to drive an otherwise non- spontaneous chemical reaction.

Electrowinning refers to the process of extraction of metal using electrolysis. The anode is an insoluble conductor and electrolyte in either case is solution with sufficiently high electrical conductivity and solute concentration. The cathode may be pure metal built upon a pure ‘starting sheet’ or sometimes, on a blank of another metal. The voltage applied across

1 cathode and anode results in cathodic reaction which in turn results in deposition of metal from solution onto cathode. In the recovery of most metal, oxygen is evolved from water at positive electrode.

The aim of this work was to study the effect of pH, temperature, and morphology of gold deposited on steel wool cathode and stainless steel cathode on the direct electrowinning of gold from cyanide leach liquor.

During gold electrowinning, the following cathodic reactions should be considered:

− − − 퐴푢(퐶푁)2 + 푒 → Au + 2 퐶푁 ; E ° = -0.600V (1.1)

− − 푂2 + 2퐻2푂 + 4 푒 → 4 푂퐻 ; E ° = 0.401V (1.2)

− − − 푂2 + 퐻2푂 + 2 푒 → 푂퐻 + 퐻푂2 ; E ° = -0.065V (1.3)

− − 2퐻2푂 + 2 푒 → 퐻2 + 2 푂퐻 ; E ° = -0.828V (1.4)

− The complex 퐴푢(퐶푁)2 is reduced to metallic gold, under diffusion control, according to Reaction (1.1). Reactions (1.2) and (1.3), representing oxygen reduction in alkaline solutions, are the main cathodic reactions competing with gold deposition and consuming a great deal of the available current on the cathode, as the electrolyte is supposed to be saturated with oxygen. Reaction (1.4) represents the hydrogen evolution in alkaline solutions, which can also occur, along with gold deposition, at more cathodic potentials.

1.2 Problem Statement

Ore from mines contains base metals like copper, nickel and zinc. Although the concentrations of these base metals are very low, they effect electrowinning process in terms of decreased current efficiencies, passivation of the cathode surface and by influencing the morphology of the plated gold. Depending on their concentration in gold cyanide solution, cathodic deposition occurs. A major disadvantage of the gold electrowinning process is its inability to produce discardable tailings. This is mainly due to the cathodic shift in the reduction

− potential of the 퐴푢(퐶푁)2 /Au half reaction. When the gold in solution decreases, resulting in preferential deposition of base metals, also a decrease in the mass transfer rate of aurodicyanide to the cathode surface, which may cause a significant decrease in the efficiency of the electrowinning process (Steyn and Sandenbergh). Variations in conditions or factors like pH

(hydrogen potential), temperature, current density, cell voltage, duration, eluate concentration

(gold content in the cyanide solution), caustic strength and flow rate can have an effect on electrowinning efficiency.

1.3 Objectives

The following are the objectives of this research work:

i) To investigate the effect of pH on the electrowinning of gold.

ii) The study the effect of temperature on the deposition of gold during electrowinning

process.

iii) To study the morphology of gold deposited on stainless steel plate and steel wool

cathode.

1.4 Scope of Research

The research work of this project is to extract gold from cyanide leach liquor through electrowinning process. Breaking down large chunks of rock into smaller pieces was the first step in this process. Crusher was used to reduce the ore to pieces no larger than road gravel.

Mineralogical study was carried out on the crushed ore in order to study the distribution of mineral present in the ore.

The mineralogical characteristic of sample was investigated using the optical microscope study. Mineral samples are prepared as grain mounts for study in the laboratory with a petrographic microscope. Then, the presence of other minerals was identified by X-ray diffraction (XRD) and scanning electron microscope / energy dispersive

X-ray (SEM/EDX).

This process continues with bulk leach extractable gold (BLEG) test to determine the free gold content in sample. Cyanidation leaching was then carried out to extract the gold, followed by electrowinning as a process of gold recovery from solution.

Different types of electrode materials was used, to study the morphology of gold deposited on the cathode in the electrowinning process. The electrode materials used in this experiments were stainless steel plate and steel wool.

Two different type of electrolyte solution were studied; gold standard solution, cyanide leach liquor. The electrowinning process was studied at room temperature, 30 °C, and 40 °C to investigate the influence of temperature on the deposition of gold.

At the end of electrowinning, the solutions were sent for AAS analysis to determine the final concentration of gold and the electrode plate is sent for scanning electron microscope / energy dispersive X-ray (SEM/EDX) analysis to study the surface morphology and to check for the deposition of gold on the elctrode.

1.5 Thesis Outline

The thesis is divided into five chapters. Chapter one is the introductory chapter and it consists of background of study, statement of problem, objectives of the study, scope of the study, and organization of the study. Chapter two deals with literature review on recovery of gold. It begins with an overview of hydrometallurgy process. The chapter gives the description of the leaching and electrowinning process of the mining company. It also brings into fore previous work in the field and throws more light on the subject matter. The theoretical concepts are well explored here. Chapter three explains the methodology of the study which includes the procedures of experiments carried out.

Chapter four is the analysis and findings. Results obtained from the study, analysis is evaluated and possible scientific meanings behind the observed results are discussed. Chapter five is the conclusion of research work, recommendation and suggestions for future work on this field of processing gold ore.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The extraction of a metal from its ore will always include a reduction step from whichever oxidation state is most stable in our oxygen-rich natural environment to the zero oxidation state. In principle, this oxidation can almost always be accomplished by electrolysis and many metals, are at least on a small scale, isolated by an electrolytic route. Commonly, however, routes based on carbon or sodium reduction are preferred for economic reasons. On a large scale, electrolysis is only used for the manufacture of the very electropositive elements, where the electrolysis medium is a molten salt and for elements where the chemical route has environmental problems or the electrolytic route has advantages in terms of the purity of the metal this latter group includes particularly copper, zinc and nickel. Cobalt, chromium, manganese, gallium, rare earths, tantalum and niobium are other examples where electrolysis has been used, at least on a small scale, for their extraction. Large electrolytic plants for metal extraction are heavy consumers of electrical power and it is therefore common for them to be sited close to cheap sources of electricity (Derek and Frank, 1990).

Cyanidation is the predominant process by which gold is recovered from its ore in metallurgical operations, and it is recognized that the Carbon in Pulp, the Merrill-Crowe, the

Ion Exchange and Solvent Extraction processes are used for concentration and purification of gold from cyanide solutions. The common process for recovery of gold solution is electrowinning.

Electrowinning is the oldest industrial electrolytic process. For the first time in 1807, an English chemist Humphry Davy obtained sodium metal in elemental form by the electrolysis of molten sodium hydroxide.

Metals such as lead, copper, gold, silver, zinc, aluminium, chromium, cobalt, manganese, and the rare-earth and alkali metals commonly undergo electrowinning process.

This is the only production process employed for aluminium. Several industrially important active metals (which react strongly with water) are produced commercially today by electrolysis of molten salts. The key parameters in developing an effective electrowinning unit are metal deposition rates. Units are most effective when installed in rinse tanks immediately following drag-out tanks. Electrowinning is not a viable recovery technology for all metals, while the process works satisfactorily for metals with high electro-potential, such as gold.

However, the process is very pH sensitive and must be rigorously maintained for any deposition to occur.

Gold is found in ores made up of rock with tiny or microscopic particles of gold. This gold ore is often found together with quartz or sulphide minerals; pyrite or also known as the

‘fool’s gold’. Native gold is found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits. Such free gold is always abundant at surface of gold-bearing veins owing to the oxidation of associate minerals followed by weathering and washing of the dust into streams and rivers, where it collects and can be welded by action of water to form nuggets (Wiberg, 2001).

The recovery of gold is greatly affected by the process parameters considered in hydrometallurgical route. Cyanide is used as lixiviant and it targets the gold that is interlocked with the gangue minerals. The increase in cyanide concentration, increases the dissolution of gold up to an extent where-by the concentration will only be competing internally.

2.2 Gold Ore Characterization

Gold characterisation is the study of minerals in terms of their size, habit, chemical composition, morphology, textural position, association and other attributes. There is an established and increasingly important need for this type of study applied to gold deposits. This is due to the realisation that mineral characterisation is important to decide the suitable hydrometallurgical method for the extraction of gold.

2.2.1 X-Ray Diffraction (XRF) Analysis

The basis of X-ray fluorescence spectrometry is the interaction of X-ray photons from a separate excitation source with atoms of the elements of interest found in the sample (filter deposit). When these excitation photons interact with the atoms in the sample, the photons cause the ejection of inner shell electrons. Outer shell electrons then fall into these vacancies.

These transitions result in emission of X-rays characteristic of the element. The energy of the characteristic X-ray is equal to the difference in the electron binding energies of the two electron shells involved in the transition. Because the electron binding energies are a function of the atomic number, the energy of the X-ray is characteristic of the element. The number or intensity of X-rays produced at a given energy provide a measure of the amount of the element present by comparisons with standards.

2.2.2 X-Ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) analysis is a mineralogical identification method that permits semi to full-quantitative assessment of the minerals present in a given sample and in what relative proportions they occur. The material to be analysed is finely ground, homogenized, and average bulk composition is determined.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. The cathode ray tube generates these x-rays, filtered to produce monochromatic radiation, collimated to concentrate, and directed towards the sample.

Constructive interference (and a diffracted ray) is produced when the incident ray interacts with the sample and this must satisfy Bragg's Law (nλ=2d sin θ). The wavelength of electromagnetic radiation is related to the diffraction angle and the lattice spacing in a crystalline sample in this law. These X-rays are collimated and directed onto the sample.

The intensity of the reflected X-rays is recorded, as the sample and detector are rotated.

When the geometry of the incident X-rays hitting the sample satisfies the Bragg Equation, constructive interference and a peak in intensity occurs. Processed X-ray signal is converted to a count rate which is then output to a device such as a printer or computer monitor.

2.2.3 Scanning Electron Microscope (SEM/EDX) Analysis

The generation of the X-rays in a SEM is a two-step process. In the first step, the electron beam hits the sample and transfers part of its energy to the atoms of the sample. This energy can be used by the electrons of the atoms to “jump” to an energy shell with higher energy or be knocked-off from the atom. If such a transition occurs, the electron leaves behind a hole. Holes have a positive charge and, in the second step of the process, attract the negatively-charged electrons from higher-energy shells. When an electron from such a higher-

9 energy shell fills the hole of the lower-energy shell, the energy difference of this transition can be released in the form of an X-ray. X-rays are electromagnetic radiation, and consist of photons. Detectors are placed under an angle, very close to the sample, and have the ability to measure the energy of the incoming photons that belong to the X-rays. As the solid angle between the detector and the sample increases, the detection probability of the X-ray increases, thus the likelihood of acquiring the best results. The data that is generated by EDX analysis consists of spectra with peaks corresponding to all the different elements that are present in the sample. The EDX spot gives a semi-quantitative analysis (wt%) of gold and other minerals associated.

2.3 Liquid-Solid Reaction – Leaching

Leaching of ore is rather similar to gas-solid reactions but is conducted at a uniform temperature. The ore lumps contain tiny particle of valuable mineral embedded in gangue and the exact mechanism of leaching by solution and diffusion depends on spatial distribution of the two phases and on the permeability of the gangue. If the gangue is impermeable the mineral particle must be exposed and solution will proceed on a narrow front, solvent ions diffusing inwards and dissolved ions diffusing out to the liquor outside by the narrow path etched out by the solution reaction. If the gangue is fissured or permeable, solvent may reach the surfaces of totally enclosed mineral particles. Solution may then be easy but the escape of the dissolved ions can only be by diffusion and must be slow. As a result, the concentration of metallic ion within the particle must be high and near to saturation during most of the process. If the process involves a reaction such as the oxidation of mineral prior to its solution the oxidizing agent must also reach the reaction site by diffusion. The process is almost certain to be rate controlled

10 by the diffusion of one or other of the ions involved – whichever is the slowest having regard to both their size and the concentration gradient determining their motion (Gilchrist, 1989).

2.4 Chemistry of Gold Recovery from Cyanide Solution

The sodium cyanide salt is readily soluble in water producing cyanide ions and metal cations. The cyanide ions will hydrolyze in water to produce molecular hydrogen cyanide according to equation 2.1 below.

− − 퐶푁 + 퐻2푂 ↔ HCN + 푂퐻 (2.1)

Hydrogen cyanide is a weak acid (Ka = 4.89 ×10−10) whichdissociates in aqueous solutions according to equation 2.2

HCN ↔ 퐻− + 퐶푁− (2.2)

All experiments were carried out at a pH greater than 10.5 to prevent dangerous levels of HCN gas being produced. Three main cathodic reactions are likely to occur in the catholyte compartment which contains the alkaline gold cyanide solutions. The reactions are gold deposition, oxygen reduction eq.(1.2) and water reduction eq. (1.4). From a thermodynamic point of view, oxygen reduction is the most favorable reaction followed by gold deposition.

Thus, in order to increase the current efficiency, the catholyte compartment was deaerated with nitrogen gas before and during experiments to remove the dissolved oxygen. According to eq.

(2.1), the equilibrium potential for the gold deposition reaction is strongly dependent on both cyanide ion and gold cyanide complex concentrations. The gold deposition will occur at less negative potentials with a decrease in cyanide ion concentration. Lower cyanide concentrations will limit gold deposition because of the lowered conductivity of the solution. Thus, the experiments were carried out in 0.041 M of NaCN solution. As expected, with the decrease in

11 gold concentration, the gold deposition will shift to more negative potentials resulting in greater difficulty for the reduction process. In the galvanic recovery of gold, the driving potential is provided by the cell electromotive force itself. When operated in a short circuit condition, zinc ions will be released at the anode and form zinc cyanide in the anolyte compartment, while electrons are transferred through the external circuit to the cathode where the electrons are used up by the gold cyanide complex. Deposition of gold occurs at the cathode substrate. Zinc cyanide which is generated is well contained and can be disposed of by other methods of cyanide remediation. It can also be used as secondary raw material in the zinc plating industries.

The reactions that occur in the anolyte and catholyte compartments are spontaneous and self- driven. The negative free Gibbs energy is used to drive an electrochemical cell to produce a relatively pure form of gold metal on the cathode together with the by-product of direct current.

By using this electrogenerative technique, the potential control is achieved by choosing the right anode rather than regulating the voltage as in the electrowinning technique (Marsden and

House, 1992).

2.4.1 Advantages of Cyanidation

Since the 1890's, cyanide has been used to recover gold from gold bearing ores. And today, over 115 years later, most of the worlds gold is recovered with cyanide playing a large part in the beneficiation of the yellow precious metal. This cost-effective, proven method of ore extraction provides maximum recovery for many gold ores, including low grade and some refractory ores.

2.4.2 Disadvantages of Cyanidation

Both the use and disposal of cyanide present significant safety and environmental risks. Cyanide and cyanide gas are both extremely toxic and great care has to be taken during ore processing to avoid exposure for workers. Solutions containing cyanide have to be carefully managed to prevent the formation of cyanide gas. Cyanide gas is a colourless, extremely poisonous gas, which may cause gasping, irregular heartbeats, seizures, fainting, and even rapid death. In addition, there are significant problems with the disposal of cyanide- containing waste. Pure cyanide in open air breaks down into other compounds relatively quickly, however the exact composition and toxicity of these products is not well understood. One of the common breakdown products is nitrate, which itself can cause both environmental and human health problems. Lastly, cyanide can form complexes with certain metals, such as cobalt, and these can persists for many years in groundwater (Charles, 2012).

2.5 Solvent Extraction

During precious metals refining, solvent extraction has been applied industrially for the separation of gold, platinum group metals and base metals. The use of solvents for purification and concentration of gold in dilute leach solution has been proposed but has not been applied commercially. Liquid extractants offer some potential advantages over activated carbon and ion exchange resin; namely, rapid extraction kinetics and high gold loadings. These factor have the potential to reduce process equipment requirements, reduce gold inventory and possibly simplify refining requirements of the product. However, unlike carbon and resin material, liquid extractants cannot easily be applied directly in-pulp, and their potential use is likely to be restricted to treatment of clarified solution. Also, liquid extractants have some solubility in water, which results in solvent (and gold) losses to aqueous phase.

Solvent extraction uses suitable organic extractants to selectively remove gold species from aqueous solution. The extractant is dissolved in a supporting matrix (or diluent), such as kerosene, to distribute the functional groups in an optimal concentration for metal extraction.

Typically, solvent extrcation of 10% to 20% in the diluent are used, although this may vary depending on the type of solvent. Gold is recovered from the loaded extractant either directly, by precipitation or electrolysis, or indirectly by stripping the solute back into aqueous phase to allow recovery by electrowinning.

The kinetics of solvent extraction are typically much faster than carbon adsorption and ion exchange resin processes, and high level of extraction can usually be achieved within a few minutes. This is attributed to superior mass transport properties of the liquid – liquid system.

However, the kinetics of stripping are much slower, and 2 to 4 hour may be required to achieve satisfactory metal recovery.

2.6 Electrochemistry

In an electronic conductor, an electric current is the result of a net movement of electrons in the structure of the conductor when an electrical potential is applied. Electrons have negligible mass compared with the remainder of the structure, and the flow of electricity is not accompanied by a significant movement of matter. Electrolytic conductors contain mobile ions, which carries electrical charge. When electrical potential is applied, these ions will move in the direction appropriate to their charge. As in electronic conductance, the electric current is a movement of electrical charge, but this time the charge is carried by ions of significant mass and electrolytic conductance is accompanied by mass transfer. Conductors may exhibit a combination of electronic and electrolytic conduction, but frequently the

14 proportion of one type is so dominating that it is permissible to assume that the conduction is entirely by the dominating mechanism (Derek and Frank., 1990).

This property of electrolytic conduction plays a major role in some of the processes and phenomena important to the metallurgist - corrosion and oxidation of metal, electroplating, electropolishing, electrolytic extraction, and refining of metals for example. The nature of electrolytes and electrolytic conductance is considered first, and then their function in electrolysis (Derek and Frank., 1990).

2.7 Electrowinning

In contrast to pyro and hydro metallurgy, which have been used in ancient times, electrometallurgy is a very recent technology that was born after the discovery of electric current (Seyed, 2009).

Electrowinning operations forms a critical part of Gold recovery; one of the earliest uses was in 1807, when the english chemist Humphry Davy obtained sodium metal in elemental form by the electrolysis of molten Sodium Hydroxide and in 1842, Henry Fox plated gold on electrodes of a galvanic cell (Shreir, 1963).

The process of electrowinning involves passing an electric current through an electrolyte (eluate). Electricity is passed from the cathode; the negative electrode through the solution into the anode; the positive electrode completing the electrical circuit. The current causes the gold to plate out onto the steel wool cathodes. The electrolyte in gold electrowinning is the cyanide solution. High throughput of the electrowinning cells provides low cost option for silver producers in particular and therefore leads to increased profit.

Electrowinning is particularly applicable for removing metal from solutions containing a moderate to high concentration of metal ions. The conventional electrowinning process becomes inefficient for concentration below 1000 mg/l of metal. Therefore, it is not a compliance technology, i.e. a technology that will meet waste water discharge standard. Rather its benefit is in recovering valuable metals that would otherwise be converted to metal hydroxide sludge by the wastewater treatment system.

Electrowinning is applied to a wide variety of chemical solution found in extractive electrometallurgy industry. Most common metals recovered by electrowinning are copper, gold and silver. For practical purposes, the degree to which a metal can be recovered by electrowinning depends on its position in the electromotive series. In general, metals with higher electrode potential plate easier compared to metals with lower electrode potential. For example, noble metals like gold and silver can be removed from solution to less than 1 mg/l using flat plate cathodes whereas with copper and tin, concentration in the range of 0.5 to 1 g/L or more is required for a homogenous metal deposit (Defour and Jim, 2006).

The electrowinning of metal, or the production of metals from their ores one put in solution or liquefied, is the oldest industrial electrolyte process. Metal deposition rates are key parameters in developing an effective electrowinning unit. Electrowinning is not a viable recovery technology for all metals. While the process works well for metals with high electropotential, such as gold, silver, copper, cadmium and zinc, it does not work as well on others, such as chromium. Nickel can be recaptured also. However, the process is pH sensitive and thus must be rigorously maintained for any deposition to occur.

2.7.1 Advantages of Electowinning

High throughput, state-of-the-art electrowinning cells provide a low-cost option for gold producers, in particular, and leads to increased profits. Electrowinning offers a significantly higher efficiency of gold recovery with low operating costs, compared to zinc precipitation methods. In addition, electrowinning produces a clean gold that is straightforward to smelt.

2.8 The Thermodynamics of Reaction at an Electrode

The chemical reaction at electrode can define an electrode as an electronic conductor in contact with electrolytic conductor. The reaction at metal electrode in which ions of the same metal discharge at the electrode from the electrolyte, the reaction can be represented by

푧+ 푀 + 푍∈− = M. (2.3)

Depending on the electrolyte, whether it is aqueous, solid or fused salt, the metal ions will form part of the structure of that electrolyte. They may be present as aquo-ions surrounded by a solvation sheath of water molecules complex ions such as complex formed between cyanide ions or surrounded by silicate network in liquid slag. The metal electrode will consist of metal ions in a crystal structure bound together by the attraction of their free electrons, which give the metal its electronic conductivity. The metal ions in the electrolyte possess a free energy, 퐺푒 and metal ions in the electrode possess a free energy, 퐺푚. Then, if the metal ion is to leave its place in the electrolyte structure and take up a position in the structure of the metal electrode, the free energy change accompanying this process will be

∆퐺 = 퐺푚 − 퐺푒 (2.4) and the reaction (2.1) can be written

푧+ 푧+ 푀푒 = 푀푚 (2.5)

푧+ 푧+ Where 푀푒 is the metal ion in the electrolyte, 푀푚 the metal ion in the metallic electrode

(Elges, 1984).

2.9 The Deposition Process

The cathode attracts predominantly positive ions to a region near its surface which is known as the Helmholtz double layer. Moreover, negatively charged complex ions, such as the

− 퐴푢(퐶푁)2 , ions in the gold(I) cyanide plating electrolyte, that approach this layer become polarised in the electric field of the cathode. The distribution of ions around the metal is thereby distorted and the diffusion of the complex ion into the Helmholtz layer is assisted. Finally, within the Helmholtz layer the complex cracks. Its component ligand ions or molecules are disengaged and the metal released in the form of the positively charged metal cation which is deposited as the metal atom on the cathode.

It will be appreciated from the figure of events at the cathode, that the ease of gold

− deposition from gold(I) cyanide baths, which contain gold in the form of 퐴푢(퐶푁)2 ions, will depend on the dissociation of ion into 퐴푢+ and 퐶푁− ions, thus:

− + − 퐴푢(퐶푁)2 ↔ 퐴푢 + 2퐶푁 (2.6)

The mass action law dictates that for a reversible reaction:

[퐴푢(퐶푁)−] 2 = a constant (2.7) [퐴푢+][퐶푁−]

− + − where 퐴푢(퐶푁)2 , 퐴푢 and 퐶푁 are the concentrations of the respective ions in solution. This

− constant provides a measure of the stability of the 퐴푢(퐶푁)2 complex and is referred to as its stability constant ꞵ2(the subscript 2 indicates that the reaction is two stage). The value of ꞵ2(for

− 383 the 퐴푢(퐶푁)2 ion has been determined as 10 . This is exceedingly large and implies that the

− equilibrium in reaction (1) is far to the left and that the 퐴푢(퐶푁)2 ion is a very stable one. The concentration [ 퐴푢+] of 퐴푢+ ions is given by equation:

− [ 퐴푢+] = 퐴푢(퐶푁)2 . 10383 (2.8) [퐶푁−2]

This is an unusually low concentration and it would seem as if sensible rates of deposition of gold from gold(I) cyanide solutions are possible only because of the polarisation

− of 퐴푢(퐶푁)2 ions which approach the cathode surfaces and their decomposition in accord with equation (1) within the Helmholtz layer. The electrodeposition of gold from its other complexes occur in a similar manner. Gold(I) Cyanide Baths these may be operated under alkaline, neutral or acid conditions. Under acid conditions, the value of [ 퐴푢+] is increased as a result of the equilibrium reaction:

퐻++ 퐶푁− ↔ HCN (2.9) in which the formation of undissociated HCN is highly preffered. Acid gold(I) plating baths are therefore designate by low free cyanide ion concentrations. However, under alkaline conditions and especially if potassium cyanide is present, the value of [퐶푁−] is greatly increased. Following equations 1 to 3 this means that in acid baths the value of [ 퐴푢+] is increased, whereas in alkaline baths it is decreased. Despite the fact that the stability constant

− of the 퐴푢(퐶푁)2 ion is unchanged, this complex ion therefore appears to be unstable in acid compared to alkaline media. The range of usefulness of gold(I) cyanide baths in acid media is

− limited, however, by the tendency of the 퐴푢(퐶푁)2 complex to decompose at pH's below about

3 with precipitation of AuCN thus: 19

− − 퐴푢(퐶푁)2 → AuCN + 퐶푁 (2.10)

Both polarographic and potentiometric studies have shown that the gold(III) cyanide

− complex 퐴푢(퐶푁)4 is formed in acid gold(I) cyanide plating baths during operation, presumably by oxidation at the anode. It is important to realise that any gold oxidised to the trivalent state in this manner is not available for deposition at the cathode because gold(I) and hydrogen are preferentially deposited. The efficiency of the bath therefore drops, not because some gold is being deposited from gold(III) cyanide, but because the effective concentration of gold(I) cyanide in the bath has been decreased. Also gold(III) is just as susceptible to attenuate as the gold(I) and the metal's valency does not affect its price. Modern acid gold(I) cyanide baths are therefore formulated with the addition of reducing agent so as to reduce the formation of gold(III) cyanide, but baths formulated several years ago can give rise to gold(III) cyanide formation especially under atypical plating conditions, such as wire plating. Since the gold(III) complex is also very stable, it is better to avert its formation than to reduce it back to gold(I) cyanide (Marsden and House, 1992).

2.10 Morphology of Gold Deposited

From an industrial point of view, with conventional EW it is essential to obtain loosely adherent precipitates that can be easily removed with high pressure water. If this cannot be achieved, some gold will get locked up inside the conventional electrowinning cells, which can result in electrowinning being uneconomical. The electrolyte temperature, current density, and conductivity of the electrolyte have typically proved to be the main parameters that influence the adhesion of gold precipitates. Co-deposition of base metals, such as copper, may also have an effect on the morphology and adhesion of the precipitate.

The electrodeposition of gold in industrial electrowinning cells is typically operated under mass transfer control. Various testing protocols have been developed over the decades to allow important operational parameters to be investigated and allow more effective cells to be investigated and compared.

Synthetic solutions are prepared to simulate plant conditions. For example, the electrolyte may be prepared by adding 2 % caustic soda together with 0.1 % CN, to simulate conditions normally encountered in plant practice. Chemical compositions typically ranged from low to high copper and gold concentrations in order to determine at which concentrations the influence of copper became problematic. The influence of gold and copper at various concentrations is typically studied, as is the morphology of the deposited metal.

A decrease in the rate of gold electrowinning occurs when too-high overpotentials are applied. This is likely to be due to a decrease in the effective cathode area available for electrowinning because of the enhanced hydrogen evolution, as well as dislodging of the gold from the cathode surface due to the increased surface turbulence caused by the evolution of hydrogen gas. The optimum applied potential in conventional EW proved to be -1.2 VSSE, since the fastest electrowinning rate was obtained at this potential and a more loosely adherent precipitate can be expected than for gold plated at an applied potential of -1.0 VSSE.

2.11 Effect of pH

The value of pH is depending upon the composition of electrolyte. The pH value should be maintained for good result. The pH of the electrolyte influences the hydrogen evolution voltage, the precipitation of basic inclusion, the decomposition of the complex or hydrate from which the metal is deposited, and the extent of adsorption of additives. In a complex bath, pH

21 may influence equilibrium between various processes. When the anode is insoluble, oxygen evolution takes place at the anode

+ − 2퐻2푂 →2푂2 + 4퐻 + 푒 (2.11)

On the other hand, hydrogen evolution at the cathode is accompanied by the production of hydroxide ion:

− − 2퐻2푂 + 2푒 →2푂퐻 + 퐻2 (2.12)

In a neutral bath, if the current efficiency is greater at the anode than at the cathode, the bath becomes more alkaline. If the electrode efficiencies are similar, the pH of the bath remains unchanged. Hence change in pH of a plating bath is a good indication of electrode efficiencies.

In certain conditions precipitation of metal hydroxides may occur locally within the cathodic double layer, which get co-deposited with the plated metal and give defective deposit while increasing the pH due to hydrogen evolution. Thus, buffers are necessary to minimize these pH changes.

During Electro deposition of aqueous solution, hydrogen ions are discharged together with the ions of the metal being deposited. The hydrogen evolved not only has a detrimental influence on the plating rate and on the cathodic current efficiency, but it often also unfavorably affects the structure and properties of the metal being deposited by causing spongy or powdery deposits, pitting or other defects. In the Electro deposition, the metal of iron group or the-metal with ‘low hydrogen overvoltage are very sensitive to the concentration of hydrogen ions in the electrolyte, a change in the pH value considerably affects both the cathodic current efficiency and the structure of the electrodeposits.

The pH value of the cathodic film is not always the same as that of the bulk of the electrolyte. The Hydrogen ions take part in the electricity transport and also affect the changes

22 taking place in the electrode film. The pH value of the cathodic film will in principle be higher than the bulk of the Electrolyte, if the number of hydrogen ions transported by the current is smaller than the number of the hydrogen ions discharged in unit time and vice versa. A change in the pH value of the cathodic film causes diffusion, which tends to equalize the activity of the hydrogen ionic discharged in the bulk of the solution and in the cathodic film.

The difference between the pH values of the cathodic film and the bulk Solution, which tends to increase with the current density, either become stabilized or continues to increase, depending on the composition of the solution. [Lainer, 1970]

2.12 Effect of Temperature

In general, an increase in bath temperature causes an increase in the crystal size.

Increase in bath temperature increases solubility and thereby the transport number, which in turn leads to increased conductivity of the solution. It also decreases the viscosity of the solution, thereby replenishing the double layer relatively faster. High bath temperature usually decreases less adsorption of hydrogen on the deposits and thereby reduces stress and tendency toward cracking.

By increasing the bath temperature from 45°C to 55°C, the grain size of deposit partial decreased, whereas further increase of bath temperature resulted in a contrary effect (Kumar,

Pande and Verma, 2015)

CHAPTER 3

METHODOLOGY

3.1 Introduction

Processes on gold extraction are being proposed on a regular basis; however, there have in fact been no dramatic changes in the metallurgical techniques since the introduction of the cyanide process (cyanide leaching) by McArthur and Forrester in 1887. It is the most common used process for gold extraction. This process involves the dissolution of gold from the ground ore in a dilute cyanide solution. The solubilized gold is then electrowon.

As the first stage of experimental work, mineral characterization of the sample was done. Characterization is the fundamental step as it gives information in order to understand the gold ore better and the information obtained from mineral characterization is useful in designing a suitable hydrometallurgical process in order to extract gold, Characterization and mineralogical study of the gold sample were done using X-ray Diffraction (XRD), X-ray

Fluorescence (XRF), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray

(EDX) analysis.

Gold content of the gold sample was then identified using fire assay technique. It is important to determine the gold content as it will be used to compare the recovery of the gold after the electrowinning process. Bottle roll test or BLEG test was also done as to determine the free gold content in the sample. The concentration of gold from the BLEG test will then be identified by using Atomic Absorption Spectroscopy (AAS).

This research was conducted to study the effect of temperature and pH on the electrowinning of gold and the morphology of gold deposited on the surface of cathode.

Sample

Gold ore sample

Representative Sample

Jones Riffle Splitter

Batch A Batch B

(Kept as reference)

Characterisation of Gold Determination of Gold Cyanide Leaching Ore Content

XRD Bulk Leach Solvent Extraction

XRF Extractable Gold SEM (BLEG) AAS analysis

Fire Assay Electrowinning AAS analysis

AAS analysis

Figure 3.1: Summary of the gold extraction process

3.2 Sample Preparation

John riffling was used as a method of sampling. A riffler as shown in Figure 3.2 was used to divide the sample into approximately two equal parts. The sample was poured into the top of the box (feeder) and divided logitudinally. The procedure of dividing is repeated, discarding the portions from alternate slots, until a portion of suitable size is obtained for analysis.

Figure 3.2: Riffler sample divider

3.3 Characterization of Gold Sample

Characterization of gold ore is important as it helps in deciding the suitable hydrometallurgical method for the extraction of gold. Mineralogy of ore, phase and chemical composition can be identified through XRD, XRF, and SEM/EDX analysis.

3.3.1 Phase Identification by X-Ray Diffraction (XRD) Analysis

X-ray diffraction analysis was used to identify the crystal structure of the gold sample as well as the phase of the sample. This analysis was conducted to identify the major mineral phases present in the sample.

This analysis was carried out at the scanning range (2θ) from 10˚ to 90˚ .Approximately

1g of sample with size less than 75μm was sent for this analysis. The diffraction peak pattern were produced from the incident X-ray; Cu-Kα radiation with the wavelength about 1.5404퐴̇ to the sample and prescribed through the Bragg’s Law.

3.3.2 Determination of Chemical Composition by X-Ray Fluorescence (XRF) Analysis

Approximately 25g of sample is required to be used in this analysis. Significantly, the size of the sample sent for XRF analysis is passing -75μm. Loss on ignition (LOI) was conducted prior to the XRF analysis.

3.3.3 Loss on Ignition Test to Eliminate Volatile Substance

Loss on ignition is used in inorganic analytical chemistry, particularly in the analysis of minerals. It consists of strongly heating ("igniting") a sample of the material at a specified temperature, allowing volatile substances to escape, until its mass ceases to change. Beside it is used to estimate organic matter and carbonate content in the sample. Mass of the sample before and after ignition was recorded and the LOI is determined by using the Equation 3.1 below.

Approximately 3g of sample was used to carry out this test. The samples was place in the crucible. Replication of sample was done to obtain a result that are more precise and the average value will be taken as the value of LOI test. The crucible with a sample in it was placed inside a furnace and ignited at temperature of 950˚C. Retention time for the ignition was set for 1 hour as to allow the volatile substance such as carbon to be eliminated.

𝑖푛𝑖푡𝑖푎푙 푤푒𝑖푔ℎ푡 표푓 푠푎푚푝푙푒 (푏푒푓표푟푒 𝑖푔푛𝑖푡𝑖표푛)−푤푒𝑖푔ℎ푡 표푓 푠푎푚푝푙푒 푎푓푡푒푟 𝑖푔푛𝑖푡𝑖표푛 퐿푂퐼 = (3.1) 𝑖푛𝑖푡𝑖푎푙 푤푒𝑖푔ℎ푡 표푓 푠푎푚푝푙푒 (푏푒푓표푟푒 𝑖푔푛𝑖푡𝑖표푛)

3.3.4 Identification of Minerals using Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX)

SEM is used to study the surface morphology of a sample while EDX EDX spot gives a semi-quantitative analysis (wt %) of gold and other minerals associated. A polished section which was prepared beforehand was used in this analysis.

3.3.5 Preparation of Polished Section

The preparation of polished surfaces free from scratches is essential for the interpretation of ore minerals using the reflected-light microscope. Ground sample with a size

75μm was mixed with epoxy resin and hardener at a ratio of 1:1. The mixture was poured into a mould and the impregnated sample was left overnight to harden. The hardened sample was then polished with sandpaper sized of various sizes; 200 mm, 300 mm, 400mm, 600 mm, 1200 mm. An oil based lubricant was added to assist the polishing process.

3.4 Detemination of Gold Content by Fire Assay Method

Fire assay is an ancient method that has been used for centuries. It is indeed one of the best methods to determine the metal content of the precious metal in ores. Fire assaying is a quantitative determination in which metals are separated from its impurities through fusion.

A finely ground sample (-75μm) weighing 50.0 g was dried in the oven at 110 °C for 2 hours (Ong, 2016). The sample was then mixed with 150.0 g flux consisting of sodium carbonate, litharge, borax, starch and silver nitrate. The composition of the flux is shown in

Table 3.1. The mixture of sample and flux were shaken in a plastic bag to make it homogenous which was later transferred into a silica crucible. The silica crucible was placed inside a maffle furnace at 1100 °C for 1 hour to complete the fusion process.

Upon the completion of fusion process, two products will be obtained, which is the glassy slag and a metal button known as lead button. The composition of glassy slag is silica and other oxide metal that crystallizes at the top of the crucible. Whereas the lead-gold alloy that solidifies, settled at the bottom of the crucible. The crucible is hammered to crack the slag and separate the lead button. The lead button is then weighed before proceeding to cupellation.

Table 3.1: The composition of flux material used in fire assay

Chemical composition Chemical Formula Weight (%) Individual weight (g)

Sodium carbonate 푁푎2퐶푂3 49.95 74.29 Lead (II) Oxide PbO 25.50 38.25

Silver nitrate 퐴푔푁푂2 0.53 0.80

Borax 푁푎2퐵4푂7 21.25 31.88 Flour C source 2.76 4.14 Total 100 150

3.4.1 Cupellation Process

Cupellation is a process that uses small clay crucible known as the ‘Cupel’ designed to absorb the lead from the lead button. The lead button was placed into the cupel and the cupel inside a furnace for 75 minutes. The temperature of the furnace was 975 °C (Ong, 2016) with a heating rate of 10 °C per minute. At the end of cupellation, the lead was absorbed leaving a shiny silver prill. The prill consists of silver and gold. The prill was weighed and then digested by aqua regia or also known as the acid digestion process.

3.4.2 Acid Digestion

Acid digestion is a process where the silver prill is digested with use of hydrochloric acid (HCl) and nitric acid (퐻푁푂3). This process eliminates the silver from gold. The ratio of hydrochloric acid to nitric acid is 1:3. The silver prill was placed in a beaker filled with 5 ml of nitric acid. The beaker was then placed on to a hot plate and heated at 100 °C for 1 hour.

Once the prill had been completely digested, 15 ml of hydrochloric acid was added from time to time until there was no more yellow fume. The remaining solution was cooled before being filtered. The filtrate was diluted in a 50 ml volumetric flask and was analysed by AAS. Figure

3.3 shows the overall process flowchart of fire assay.

Gold ore sample

Flux preparation

Chemical composition Chemical Formula Weight (%) Individual weight (g)

Sodium carbonate 푁푎2퐶푂3 49.95 74.29 Lead (II) Oxide PbO 25.50 38.25

Silver nitrate 퐴푔푁푂2 0.53 0.80

Borax 푁푎2퐵4푂7 21.25 31.88 Flour C source 2.76 4.14

Total 100 150

Fusion

 1100 °C  1 hour

Cupellation Product Product

 975 °C  Lead button  Silver prill  75 minutes  Slag

Acid digestion

 5 ml 퐻푁푂3 AAS analysis  15 ml HCl  100 °C  Gold  Filter and make concentration up to 50 ml in ppm

Figure 3.3: Process flowchart for fire assay

3.5 Determination of Free Gold Content by Bulk Leach Extractable Gold (BLEG) Test

BLEG is a geochemical sampling/analysis tool used during exploration for gold. It was developed in the early 1980s to address concerns relating to the accurately measuring fine ground gold, and dealing with problems associated with sample heterogeneity.

The sample was ground to - 75μm, so that it can be leached by cyanide. This procedure is required to liberate the gold particle interlocked with other minerals associated with it such as silica, hematite and etc.

Approximately 50 g of sample was used for each cyanidation test. The sample was leached in 1000 ppm of sodium cyanide solution with pH 11 at room temperature for 24 hours with solid: liquid (S:L) ratio of 1:3. After 24 hours of cyanidation, the sample solution was filtered, producing solid residue and pregnant leached solution.

Mixture of sodium cyanide and sample was sealed in a poly-ethylene bottle to prevent leakage. The poly-ethylene bottle was put into another bigger bottle and sealed. The bottle was then placed on the rolling bar for agitation movement aided for leaching process. Figure 3.4 shows the bottle roll test of cyanided sample.

Figure 3.4: Poly-ethylene bottle used for BLEG test on a milling machine

The gold pregnant solution is filtered after 24 hours using a vacuum pump. Filtration process is carried out in a fume cupboard to avoid the contact of hazardous gases.

Then, solvent extraction process using DIBK, aliquot 336 and 25% is conducted on the filtered solution to convert it into organic phase. The solution is then analysed using AAS to determine the free gold concentration.

3.6 Cyanide Leaching Experiment

Gold cyanidation a hydrometallurgical technique for extracting gold from its ore by converting the gold to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction. The leaching process was done in a fume cupboard to avoid inhalation of harmful gaseous.

The procedure began with weighing 0.5654g cyanide using an electronic balance. The sodium cyanide was put into a 1000ml volumetric flask. The flask was marked up with

33 deionised water. 100g sample was weighed and put in 1000ml beaker. The beaker was filled with 300ppm sodium cyanide prepared from 1000ml volumetric flask. A mechanical stirrer was switched on to run the sample. Seven to eight drops of sodium hydroxide was added to obtain pH 11. The experiment is conducted for 24 hours at an average speed of 450rpm. The solution was then filtered using vacuum pump.

Finally, carry out solvent extraction using DIBK, KCl and Aliquot 336 and the sample was ready for AAS analysis.

3.7 Solvent Extraction

The gold leached in the pregnant solution has to be extracted into the organic phase because the AAS instrument can only accept acid and organic phase solution. In this solvent extraction, the gold was extracted into the organic phase by contacting the pregnant leached solution with diisobutyl ketone (DIBK), potassium chloride (KCl) and aliquat 336. About 50 ml volume of the pregnant leached solution was placed in a separating funnel and mixed with

1 % aliquat 336 and 25% KCl solution. Figure 3.5 shows the separating funnel after solvent extraction creating two layers of organic and aqueous phases. The funnel containing the mixture above was shaken for 60 seconds and the stop cork was released at an intermediate time of 30 seconds to ensure mixing well of the solution. After several mixing stages, gold was extracted into the organic phase. The separating funnel was clamped onto a retort stand until two layers were observed in the funnel. The bottom layer which contained the aqueous phase was removed from the funnel. The gold species extracted into the organic phase were then analysed using AAS.

Organic Phase

Aqueous

Phase

Figure 3.5: Organic and aqueous phases in the separating funnel after solvent extraction (top layer – organic phase and bottom layer – aqueous phase)

3.8 Setting Up of Electrowinning Cell

Figure 3.6 below shows the schematic diagram of the complete circuit in the electrowinning cell. The negative terminal of the power supply is connected to the cathode and the positive terminal is connected to the anode. A crocodile clip is used to connect the voltmeter to the negative terminal of the power supply and connected the voltmeter to the cathode via an ammeter. The cable also connected the voltmeter directly to the positive terminal of the power supply and to the anode. The clamps were used to hold both electrodes in a fixed position in the electrowinning cell. The complete circuit was tested before the experimental work is conducted.

Voltmeter

+ − Rectifier Ammeter

Cathode: stainless Cathode: stainless steel steel

Cyanide leach

liquor

Figure 3.6: Schematic diagram of an elecrowinning cell

3.9 Electrowinning Cell Design

Figure 3.7 below shows the electrowinning cell design. A 250 ml beaker is used to hold

the electrolyte and clothespins are used to clamp the electrodes at the right position when it is

immersed into the electrolyte. The beaker is used because it allows a clear observation of the

reaction during electrowinning process. For instance, the colour change of electrolyte solution

and the virtual deposition of gold.

The electrodes used are stainless steel plate and steel wool. The area exposed to the

electrolyte is 6cm2, with 0.1cm thickness. The distance between the electrodes is fixed at 3cm.

because shorter distance leads to lower resistivity according to the equation below.

푅 = 푙 휌 (3.2) 퐴

Lowering resistivity increases electric current defined by Ohm’s Law

푉 퐼 = (3.3) 푅

Electron transfer occurs at closer distance.

Two multimeters were used as ammeter and voltmeter; ammeter is used to measure the current

passing through the circuit whereas the voltmeter is used to measure the voltage passing

through the circuit.

Connecting wires Power supply

Ammeter

Voltmeter

Beaker

Figure 3.7: Setup of electrowinning cell design used throughout the experiment

3.10 Electrode Preparation

The initial stage of experimental work begins with the preparation of electrodes. The materials used as electrodes are stainless steel plate and steel wool. The surface of stainless steel wool plate need to be polished before use. It removes the impurities on the surface which may prevent the deposition of gold during electrowinning. The dimension of the given stainless steel plate is (10 × 2)푐푚2. Whereas for stainless steel wool, it was measured according to its weight which was 2.8145g.

Polishing of electrodes is done with various sizes of sand paper starting from coarse to fine size; 120 mm, 240 mm, 300 mm, 400 mm, 600 mm, and 800 mm. The electrodes were then washed in distilled water and dried. In order to remove organic matter or compound that might still be on the electrode surface, it is soaked in ethanol for a few minute.

3.11 Electrowinning Process

Electrowinning was conducted on two different types electrolyte, gold standard solution and cyanide leach liquor The electrowinning cell was used to investigate the effect of pH, temperature and the morphology of gold deposited on two different types of electrode which are stainless steel plate and steel wool. Do not pour the electrolyte solution before the electric circuit of the cell is prepared.

3.11.1 Determination of Decomposition Potential

Decomposition potential is the minimum voltage required for the electrowinning process to take place. Firstly, 200 ml of gold standard solution was poured into the electrowinning cell. The initial temperature and pH was measured using thermometer and pH meter respectively. The electric circuit was prepared based on Figure 3.5. The direct current power supply was switched on and the potential was gradually increased up to 4V.

Cell potential at which anodic and cathodic processes began to occur (indicated by oxygen and hydrogen formations at the anode and cathode respectively) was observed.

Relationship between the current and cell potential (every 0.25V increment of cell potential and it was terminated when the cell potential reaches 4 volt) was recorded. The observation results was plot in a A vs. V diagram and the value of decomposition potential was indicated.

The similar steps were repeated for cyanide leach liquor.

3.11.2 The Effect of Temperature on Recovery of Gold

To study the effect of temperature on the recovery of gold four different temperatures were chosen: 25 °C, 30 °C, 40 °C and 60 °C. The electrowinning cell was filled with 200 ml of gold standard solution. The temperature and pH of solution were measured and tabulated.

The electrodes were then immersed into the electrolyte and all the wire connections was checked before the power supply was switched on. The electrowinning cell was let to run for

1 hour. The colour change in the electrolyte solution and any changes of the electrode was observed and recorded. After an hour the power supply was switched off and both the cathode and anode was removed. The cathode and anode was dried in room condition. The final pH of the solution was recorded. The electrowinning cell was placed in a hot plate for the experiment

39 of 30 °C, 40 °C and 60 °C. Approximately 10 ml of electrolyte was pipette and stored in a brown bottle for AAS analysis. The steps above were repeated using cyanide leach liquor.

3.11.3 The Effect of pH on the Recovery of Gold

The electrowinning process was carried out in two different pH condition, acidic and alkaline. The pH is known to be dependant upon the composition of the electrolyte. Thus, this experiment was carried out on gold standard solution and cyanide leach liquor. The electrowinning cell was filled with 200 ml of gold standard solution. The pH of the solution was measured and tabulated. The electrodes were then immersed into the electrolyte and all the wire connections was checked before the power supply was switched on. The electrowinning process was carried out for 1 hour. The colour change in the electrolyte solution and any changes in the electrode was observed and recorded. After an hour the power supply was switched off and both the cathode and anode was removed. Both the anode and cathode was dried in room condition. The final pH of the solution was recorded. Approximately 10 ml of electrolyte was pipette and stored in a brown bottle for AAS analysis. All the steps above were repeated for cyanide leach liquor.

3.11.4 Surface Morphology of Gold Deposited

Electrowinning test were carried out with two different types of cathode materials, stainless steel plate and steel wool cathode with gold standard solution as the electrolyte. The differences between the two electrodes are further discussed in chapter 4. 200 ml of gold standard solution was used in this test. Both the steel wool and the stainless steel plate were weighed before and after the electrowinning process. The electrodes were then immersed into the electrolyte and all the wire connections was checked before the power supply was switched on. The electrowinning cell was let to run for 1 hour. The colour change in the electrolyte solution and any changes in the electrode was observed and recorded. After an hour the power supply was switched off and both the cathode and anode was removed. Both the electrodes were dried in room condition. The final pH of the solution was recorded. Approximately 10 ml of electrolyte was pipette and stored in a brown bottle for AAS analysis.

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Introduction

The present study was an attempt to know about the electrowinning of gold from cyanide leach liquor. As stated in previous chapter, this process involves the dissolution of gold from the ground ore in cyanide solution. On the representative sample, mineralogical characteristics was investigated using the optical microscope. Then, the presence of other minerals was identified by X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscope / energy dispersive X-ray (SEM/EDX).

This process continues with bulk leach extractable gold (BLEG) test to determine the free gold content in sample. Cyanidation leaching is then carried out to extract the gold, followed by electrowinning as a process of gold recovery from solution.

4.2 Raw Sample Characterization of Gold Ore

Characterization study provides the information regarding the chemical composition, mineral phases and associated minerals present in the sample. Information obtained from XRD, XRF, and SEM/EDX analysis is discussed further.

4.2.1 Optical Microscopy Study

A mineralogical study on the sample was carried out to identify the presence minerals in the gold sample. A polished section was prepared to be observed under the optical microscope.Identification of minerals associated in a sample helps design the most effective and economical method of gold extraction. Figure 4.1 shows the image of polished section observed under microscope.

From the image it is observed that there are major presence of quartz and the metallic mineral is identified as arsenopyrite. Arsenopyrite is always mistaken with gold. Gold can be differentiated from arsenopyrite based on its distinctive shape and colour as shown in Table

4.1 below.

Figure 4.1: Optical microscope image of polished section under 100× magnification showing the presence of quartz and arsenopyrite

Table 4.1: Optical properties of quartz, arsenopyrite and gold

Mineral Chemical Crystal Colour Bi-reflectance Anisotropy Internal

Formula System Reflection

Quartz 푆푖푂2 Trigonal Colourless/ None Anisotropy Milky white

Arsenopyrite FeAsS Monoclinic Steel grey – Very weak Blue, reddish None silver white brown, green

Gold Au Cubic Bright None None None yellow/ golden

4.2.2 Chemical Composition by XRF

XRF provides detailed information on the bulk chemical properties of various fractions.

It gives important information on the presence of valuable elements and contaminants in the

samples.

The gold ore samples were sent for XRF analysis to determine the elemental

composition of major and trace elements. The result of XRF analysis is shown in Table 4.2.

Table 4.2: Mineral composition of ground ore sample from XRF analysis

Compound wt% Compound wt% Compound wt%

SiO2 81.14 MgO 0.21 ZnO 0.01

Al2O3 11.18 Na2O 0.03 Ga2O3 0.01

Fe2O3 2.17 P2O5 0.02 Rb2O3 0.01

K2O 2.10 CaO 0.01 SrO 0.01

SO3 0.78 MnO 0.01 ZrO2 0.01

As2O3 0.59 NiO 0.01 Au2O 0.01

TiO2 0.21 CuO 0.01 PbO 0.01 Loss on Ignition (LOI) = 1.53 %

The result show the chemical composition (%wt/wt) in ore. All the elements exist in the ore presented as phase of oxide. Major oxides present in the ore sample were SiO2 and

Al2O3 with 81.14% and 11.18% respectively. Fe2O3 and K2O also exist in a sigificant amount with 2.17% and 2.10% respectively. Although Au present as trace element but the presence of

As element indicate the existance of gold in the form of arsenopyrite where the both elements are path indicator for gold ore. Indicator minerals are mineral species that, when appearing as transported grains in clastic sediments, indicate the presence in bedrock of a specific mineralization.

4.2.3 Phase Identification by XRD Analysis

XRD helps in the identification of phases in an ore sample. The identification is obtained by comparing the x-ray diffraction pattern. Based on the analysis, minerals that present in the gold ore sample is silicon, oxide, potassium and iron. Figure 4.2 shows the XRD result of the sample.

Figure 4.2: XRD diffractogram showing the major phase of the gold sample

4.2.4 Identification of Element Present by SEM/EDX

The polished section was analysed by SEM/EDX. Figure 4.3 presents the data obtained.

From the analysis, the mineral associated with gold can be identified.

Figure 4.3 below has four semi-quantitative spot analysis on the sample as shown. From the EDX diffractogram shown Figure 4.4 Spot 1 has high weight percent of As which is

43.22%. This may indicate that the the gold is interlocked in the form of arsenopyrite. In Figure

4.5 and 4.6 shows high Fe weight percent which is 92.10% wt and 71.60% wt respectively.

Figure 4.7 shows 54.74% wt O. This may be due to the presence of Fe2O3 from XRF analysis and Fe3O4 from XRD analysis.

Spot 1

Spot 2

Spot 4 Spot 3

50 μm

Figure 4.3: SEM photomicrograph of gold ore polished section with 1000× magnification

Figure 4.4: EDX diffractogram showing elements associated with gold ore at spot 1

Figure 4.5: EDX diffractogram showing elements associated with gold ore at spot 2

Figure 4.6: EDX diffractogram showing elements associated with gold ore at spot 3

Figure 4.7: EDX diffractogram showing elements associated with gold ore at spot 3

4.3 Fire Assay

In this test, a lead button was produced when the mixture of sample and flux was calcined in a furnace. The lead button contained silver and gold. Other elements were separated into a form of crystal slag. Figure 4.8 shows the image of a borosilicate slag formed in the crucible after the fusion process. The crucible was hammered to remove the slag and get the lead button. The molten lead from fusion process collects gold in the form of lead button as shown in Figure 4.9. The lead button was the weighed and recorded in Table 4.3.

Figure 4.8: Formation of borosilicate slag in the silica crucible after fusion process

Figure 4.9: Lead button formed after fusion process

The process continued with cupellation in the muffle furnace at a high temperature. A shining silver prill as shown in Figure 4.10 was produced. The silver prilled was weighed and recorded in Table 4.3. The prill was then digested in aqua regia before it analysed using AAS.

The gold concentration from AAS analysis is recorded in Table 4.3.

Figure 4.10: The silver prill formed after cupellation process

Table 4.3: Weight of lead button, silver prill and its gold content obtained after acid digestion process

Sample Weight of lead Weight of silver Gold concentration Gold content button prill (mg/L) (g/t) Raw sample 25.60 0.68 31.38 31.38

The total gold content obtained from fire assay analysis is 31.38 g/t.

4.4 Bulk Leach Extractable Gold (BLEG) Test

This test was conducted to determine the free milling gold. The concentration of cyanide used was 1000ppm. The samples were bottle rolled for 24 hours. The sample was then filtered and undergone solvent extraction. Solvent extraction pre-concentrates the solution into high concentration using DIBK, Aliquot 336, and potassium chloride (KCl). The results obtained is tabulated in Table 4.4.

Table 4.4: The concentration of free milling gold in g/t in the sample

Sample Time (hr) Gold concentration Gold content (g/T) (mg/L)

SHG 1 24 20.11 20.11

The free gold content in the sample is 20.11 g/t. From the results obtained in fire assay the total gold content is 31.38 g/t. From these results the amount of interlocking gold is calculated to be 11.27 g/t. Cyanide leaching is only effective when it comes in contact with gold particles.

4.5 Cyanide Leaching

Ore is crushed to a size at which the surface of the gold is exposed. Cyanide leaching will only be effective if the cyanide can come in contact with the gold particle. Cyanidation process was conducted using sodium cyanide (NaCN) concentration of 300 ppm. The recovery of gold from cyanide leaching is tabulated in table 4.5.

Table 4.5: The gold concentration, amount of gold leached, and percentage of gold recovery

Percentage Cumulative

Time Concentration Leached Au Cumulative of Au percentage of

(ppm) (mg) leached Au rcovery per Au rcovery

(mg) hour (%) (%)

15 min 0.981 0.0196 0.0196 3.310 3.310

30 min 1.145 0.0229 0.0425 3.867 7.177

45 min 1.576 0.0315 0.074 5.319 12.496

1 hr 1.748 0.03550 0.1090 5.910 18.406

2 hr 2.591 0.0518 0.1608 8.747 27.153

3 hr 2.798 0.0560 0.2168 9.546 36.609

4 hr 3.134 0.0627 0.2795 10.588 47.197

5 hr 3.358 0.0672 0.3467 11.348 58.545

6 hr 3.990 0.0798 0.4265 13.475 72.020

24 hr 5.942 0.1486 0.5751 25.093 97.113

From the table, the percentage of gold leached is 97.11%. Graph of total amount of gold leached vs time and graph of cyanidation gold recovery is shown below in Figure 4.11.

Plot of Au Recovery against Time 120

100

0 15 min 30 min 45 min 1 hr 2 hr 3 hr 4 hr 5 hr 6 hr 24 hr CUMULATIVE PERCENTAGE OF AU RECOVERY (%) RECOVERY AU OF PERCENTAGE CUMULATIVE TIME

Figure 4.11: Plot showing the highest percentage of gold leached at 97.11% after 24 hours

4.6 Electrowinning

Electrowinning test was conducted using two different electrolytes, gold standard solution and cyanide leach liquor. This test was conducted to study effect of temperature, pH, and morpholygy of gold deposition on two different type of cathode materials. The results of the experiment work are discussed below.

4.6.1 Determination of Decomposition Potential

Decomposition potential refers to the minimum voltage (difference in electrode potential) between anode and cathode of an electrolytic cell that is needed for electrolysis to occur.

a) Determination of Decomposition Potential for Gold Standard Solution

The relationship between the cell potential and current in the circuit is shown in

Figure 4.12. The test was conducted to indicate the decomposition potential of gold

standard solution at the concentration of 10 ppm. The extrapolated line in the graph

below indicates the decomposition potential. From the plot of current vs voltage the

value is known be 1 V.

0.14 0.12 0.1 0.08 0.06 0.04

Current (A) Current 0.02 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Voltage (V)

Figure 4.12: Plot of current vs voltage showing the decomposition potential for gold standard solution

a) Determination of Decomposition Potential for Cyanide Leach Liquor

The relationship between the cell potential and current in the circuit is shown in

Figure 4.13. The test was conducted to indicate the decomposition potential of cyanide

leach liquor at 16 ppm. The extrapolated line in the graph below indicates the

decomposition potential. From the graph, the value is read as 1.9 V.

0.02 0.018 0.016 0.014 0.012 0.01

Current (A) Current 0.008 0.006 0.004 0.002 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 voltage (V)

Figure 4.13: Plot of current vs voltage showing the decomposition potential

The decomposition potential for cyanide leach liquor is higher than gold standard solution. This is because there is presence of impurities in cyanide leach liquor. The impurities will cause an increase in the resistance. Thus, the increase in resistance will increase the voltage as per the equation V =IR.

4.6.2 Effect of Temperature on the Recovery of Gold

This test was carried out at 4 different range of temperature, 25 °C, 30 °C, 40 °C and

60 °C. It was carried out on gold standard solution and cyanide leach liquor. The results are

tabulated below.

Table 4.6: Electrowinning parameters used to the study effect of temperature on gold recovery

Parameter Specification

Electrolyte Gold standard solution Cyanide leach liquor

Temperature (°C) 25 30 40 60 25 30 40 60

Initial concentration (ppm) 10.10 10.45 10.38 10.23 16.96 15.26 18.46 15.36

Final concentration (ppm) 3.82 2.209 1.271 3.667 10.78 8.96 9.054 13.54

Volume of solution used (ml) 200 200 200 200 200 200 200 200

Initial pH 2 2 2 2 11 11 11 11

Final pH 3 3 3 3 11.5 12 12 12

Voltage (V) 1.0 1.0 1.0 1.0 1.9 1.9 1.9 1.9

Current (A) 0.05 0.08 0.16 0.16 0.18 0.18 0.21 0.23

Current density (A/푐푚2) 0.008 0.013 0.027 0.027 0.030 0.030 0.035 0.038

Duration (hr) 1 1 1 1 1 1 1 1

Exposed surface of electrodes (푐푚2) 6 6 6 6 6 6 6 6

Distance between electrodes (cm) 3 3 3 3 3 3 3 3

From the table above, it is observed that the increase in temperature, increase current density.

Resistance of the electrolyte solution is reduced with the rise in temperature leading to

57 increased rate of electrodeposition. The percentage of recovery was calculated from the concentration of gold. The result is tabulated in Table 4.7.

Table 4.7: The percentage recovery of gold for four different temperatures

Recovery (%)

Temperature (°C) Gold standard solution Cyanide leach liquor

25 62.16 36.32

30 78.86 41.28

40 87.76 50.95

60 64.15 11.84

In Figure 4.14, it is observed that when increasing the temperature of the solution the recovery of gold increases too. For example, for a temperature of 40 ºC the gold recovery was

50.95% for a time of 1 hour. Elevated temperature increases the diffusion coefficient of Au cyanide thus increasing the deposition rate. Besides, solution conductivity increases and solubility of oxygen decreases, reducing the amount of oxygen available for reduction at cathode. However, further increase resulted in a contrary effect. At temperature of 60 ºC, the recovery reduces to 11.84%. This is due to the cyanide destruction that occurs at high temperature. The instability of formate at higher temperatures results in the following overall reaction:

− − 퐶푁 + 3퐻2푂→푁퐻3 + 퐶푂2 + 퐻2+푂퐻 . (4.2)

Nonetheless, according to John O. Marsden and C. Lain House (1993), effect of temperature is quite small, particularly when compared with the effects of gold concentration, the degree of mixing of electrolyte and the cathode surface area.

100 90 80 70 60 50 40 30

Recovery (%) Recovery 20 10 0 0 10 20 30 40 50 60 70 Temperature (°C)

Recovery (%) Gold standard solution Recovery (%) Cyanide leach liquor

Figure 4.14: The effect of temperature on the recovery of gold.

4.6.3 The Effect of pH on the Recovery of Gold

The effect of pH is dependant upon the composition of electrolyte. Therefore, two solution with different nature was chosen to carry out the test, gold standard solution which is aurochloride and cyanide leach liquor, which is aurocyanide. The results are tabulated below.

Table 4.8: Electrowinning parameters used to study the effect of pH on the recovery of gold

Parameter Specification

Electrolyte Gold standard solution Cyanide leach liquor pH 2 11 2 11

Initial concentration (ppm) 10.10 10.10 7.479 16.96

Final concentration (ppm) 3.82 6.030 7.258 10.78

Volume of solution used (ml) 200 200 200 200

Initial pH 2 11 2 11

Final pH 3 11.5 3 11.5

Voltage (V) 1.0 1.0 1.9 1.9

Current (A) 1 1 1 1

Duration (hr) 6 6 6 6

Exposed surface of electrodes (푐푚2) 3 3 3 3

Whereas, for cyanide leach liquor the recovery increases as the pH increases.

70.00% 60.00% 50.00% 40.00% 30.00% 20.00%

10.00% Recovery (%) Recovery 0.00% 0 2 4 6 8 10 12 pH

Gold standard solution Cyanide leach liquor

Figure 4.15: The effect of pH on the recovery of gold

This is due to the nature of the solution. As for gold standard solution it exists in the form of aurochloride. Under highly acidic condition (pH < 3.5) and in the presence of chloride ion, aqueous chlorine is formed. All of the chlorine species are powerful oxidants but HOCl is the most effective. Thus, pH should be maintained in the range of HOCl stability. A part of the

Pourbaix diagram of Au–Cl in the [퐴푢퐶푙4]¯ domain is shown in Figure 4.16. The [퐴푢퐶푙4]¯ is stable in the 0–8 pH range and in potentials greater than 0.9 volts, depending on the concentration of gold and chloride ion.

Figure 4.16: A part of Au-Cl pourbaixdiagram in the [퐴푢퐶푙4]¯ domain at25 °C (Marsden and House, 2005)

Cyanide leach liquor exists in the form of aurocyanide. High pH allows higher concentration of free cyanide and the solubility of AuCN increases. The lower the pH, the higher the concentration of hydrogen ions; this favours deposition of hydrogen instead of gold.

From Figure 4.17, it can be seen that at pH < 9.21, the main complexing agent is HCN, while at pH > 9.21, CN predominates. Operating in the range pH > 9.21 prevents the evolution of

HCN gas. A main characteristic of the Eh-pH diagrams for the gold system is that the stability region of the aurocyanide complex, 퐴푢(퐶푁)2¯, grows with increase in cyanide concentration and decreases with increase in dissolved metal concentration.

Figure 4.17: Homogeneous equilibrium diagram of potential versus pH for the 퐴푢-퐻2푂-퐶푁 system at 25°C (after Finkelstein)

4.6.4 Surface Morphology of Gold Deposited

a) Electrowinning of gold standard solution using steel wool as the cathode

The experimental data from the electrowining of gold standard solution using

steel wool as the cathode is tabulated in Table 4.9 below. From the result of AAS

analysis in Table 4.10, the concentration of the electrolyte has decreased from 10.38

ppm to 0.678 ppm.

Table 4.9: Parameters used in the electrowinning of gold standard solution using steel wool as cathode

Parameter Specification Electrolyte Gold standard solution Initial concentration 10.38 ppm Final concentration 0.678 ppm Volume of solution used 200 ml Initial pH 2 Final pH 3 Voltage 1.0 V Current 0.08 A Duration 1 hour Anode Stainless steel Cathode Steel wool Distance between electrodes 3 cm

Table 4.10: Initial and final concentration of gold standard solution in the electrowinning process

No Initial concentration (mg/l) Final concentration (mg/l) 1 10.30 0.792 2 10.48 0.636 3 10.37 0.607 Mean 10.38 0.678

The efficiency of electrowinning process is:

% recovery of Au = 100% − 0.678 × 100 10.38

= 93.47%

b) Electrowinning of gold standard solution using stainless steel plate as the cathode

The experimental data from the electrowining of gold standard solution using

steel wool as the cathode is tabulated in Table 4.11 below. From the result of AAS

analysis in Table 4.12, the concentration of the electrolyte has decreased from 10.10

ppm to 3.82 ppm.

Table 4.11: Parameters used in the electrowinning of gold standard solution using stainless steel plate as cathode

Parameter Specification Electrolyte Gold standard solution Initial concentration 10.10 ppm Final concentration 3.82 ppm Volume of solution used 200 ml Initial pH 2 Final pH 3 Voltage 1.0 V Current 0.05 A Duration 1 hour Anode Stainless steel Cathode Stainless steel Distance between electrodes 3 cm

Table 4.12: Initial and final concentration of gold standard solution in the electrowinning process

No Initial concentration (mg/l) Final concentration (mg/l) 1 10.12 3.723 2 10.08 3.834 3 10.09 3.910 Mean 10.10 3.822

The efficiency of electrowinning process is:

% recovery of Au = 100% − 3.822 × 100 10.10

= 62.16%

After gold deposition from gold cyanide solution was carried out for 1 h on each electrode, were analysed using SEM. The morphologies of the deposits are shown in Figure

4.18 and Figure 4.19. Different substrates produce gold deposits with different morphology and sizes. Gold deposits on stainless steel plates are in crystalline form and are not evenly distributed . The gold is deposited in granular form on steel wool cathode. Figure 4.20 and

Figure 4.21 shows that the deposition on steel wool has an enhanced value compared to stainless steel. This is because it has higher surface area. Steel wool electrodes have their inherent properties; its high surface area to volume ratio allow the metal ions to be distributed throughout the electrode. This arrangement can overcome the limitation of mass transport and small specific surface area.

2 μm

Figure 4.18: Photomicrograph of gold deposited on steel wool cathode at a magnification of 10000×

2 μm

Figure 4.19: Photomicrograph of gold deposited on stainless steel plate cathode at a magnification of 10000×

Figure 4.20: SEM diffractogram of gold deposited on steel wool cathode

Figure 4.21: SEM diffractogram of gold deposited on stainless steel plate cathode

4.6.5 Experimental Observation during Electrowinning Process

At the beginning of the electrowinning process the electrolyte was very light yellow in colour. As the electrowinning time increases the solution turned brownish as shown in Figure

4.22 indicating the presence of iron. There were also slight pH change at the end of each experiment. This increase in pH in the electrolyte solution was due to the side reactions such as cyanide oxidation and water reduction (Adams, 1990).

Stainless steel plate cathode

Gold standard solution containing Fe

Figure 4.22: Colour change at the end of electrowinning process using stailess steel plate as cathode.

Steel wool cathode

Gold standard solution containing Fe

Figure 4.23: Colour change at the end of electrowinning process using steel wool as cathode.

CHAPTER 5

CONCLUSION AND RECCOMENDATION

5.1 Conclusion

From the characterisation study conducted on gold ore sample, the major mineral associated with gold is silica, iron, potassium and oxide. The major phase presence according to XRD analysis is silica and oxide. XRF analysis detected high amount of SiO2 and Al2O3 with 81.14% and 11.18% respectively. Determination of free gold content through BLEG test resulted with 20.11 mg/L. the amount gold content through fire assay was 31.38 mg/L.

From the experimental work, gold deposition was found to depend on the type of electrode material, temperature and pH. The electrode material used in this experiment is stainless steel and steel wool. Steel wool is a better choice as it gave the highest recovery in both gold standard solution and cyanide leach liquor, which is 93.47% and 67.95% respectively.

As the temperature increases, the percentage of recovery also increase but further increment, above 40 °C shows decrease. That was due to the cyanide destruction at high temperature.

The effect of pH is dependent on the composition of electrolyte. As for gold standard solution the recovery is best at pH 2 because cyanide is stable within the pH range of 0-8.

Whereas, cyanide leach liquor has a better recovery at pH 11.

Gold is found to have better deposition on the steel wool because it has higher surface area. Steel wool electrodes have their inherent properties; its high surface area to volume ratio allow the metal ions to be distributed throughout the electrode.

5.2 Recommendation

In future, for electrowinning of gold, zinc anode should be tried instead of stainless steel plate to prevent the corrosion of electrode. Zinc is a metal with low galvanic potential and has low tendency to corrode.

On top of that, several other factors such as the effects of gold concentration, the degree of mixing of electrolyte and the cathode surface area should be studied because these factors in its optimum conditions are believed to have high gold recovery.

Rotating electrode also has the possibility if increasing the recovery percentage since it can provide a balanced surface to distribute the gold deposition on the cathode surface. The gold will not have the tendency to accumulate at a particular point.

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