THIOCYANATE LEACHING OF GOLD

by

Roselyn Sarpomah Yeboah

B.Sc., University of Mines and Technology-Tarkwa Ghana, 2015

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE

in

THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES

(Materials Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA

(Vancouver)

September 2019

© Roselyn Sarpomah Yeboah, 2019

The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled:

Thiocyanate Leaching of Gold

submitted by Roselyn Sarpomah Yeboah in partial fulfillment of the requirements for the degree of Master of Applied Science in Materials Engineering

Examining Committee:

Dr. David Dreisinger, Materials Engineering Supervisor

Dr. David Dixon, Materials Engineering Supervisory Committee Member

Dr Liu Wenying, Materials Engineering Supervisory Committee Member

ii Abstract

This study focused on evaluating sodium thiocyanate as an alternative reagent to the conventional cyanidation process for leaching of gold ores. Goldcorp’s Coffee project in Yukon-Canada supplied three mineral samples namely, Supremo Oxide 68151, Supremo oxide 68151, Supremo Composite 72142 for this study.

As a baseline for comparison with thiocyanate extraction results, cyanidation tests performed on all the three samples showed that the samples are amenable to the conventional cyanidation leaching, yielding gold extractions as high as 97% for Supremo composite 72142.

A series of leaching tests were performed on the 72142 B sample with SCN- solutions to determine the feasible regions of gold dissolution and to maximize gold dissolution. The leaching tests were conducted in the acidic regime (pH 1.5 -2) for these samples.

Notable results with SCN-, ferric sulphate and potassium iodide variation showed gold extractions of 91 % in solutions containing 0.15 M thiocyanate; 92% with 0.15 M SCN- and 0.10 M Fe(III); 94 % with 0.10 M SCN- and 0.05 M KI; 94 % with 0.10 M SCN-, 0.05 M Fe(III) and 0.02 M KI - and 95 % with 0.15 M SCN and 10 g/L H2O2.

The kinetic leach data were well fitted by the CIP/CIL leach model developed by Nicol et al, giving estimates of the leach rate parameter of 72.1 hr-1 and leach tails grade after infinite leach time of 0.17 g/t, confirming fast leaching of the 72142 B sample.

The tests ended with gold adsorption from thiocyanate solution onto carbon. The average gold loading onto carbon was 2100 g/t and 48 g/t for carbon concentrations of 0.25 g/L and 20 g/L, respectively. Results obtained were excellent with greater than 98 % gold adsorbed in less than 0.5 hr when carbon concentration was above 5 g/L.

The results show that the gold ore from the Goldcorp’s Coffee project in Yukon-Canada is amenable to extraction with acidified thiocyanate based lixiviant and subsequent adsorption of the gold-thiocyanate complex onto activated carbon, giving gold extraction results that are comparable

iii to cyanide-based gold extraction. The thiocyanate system is therefore a competitive and alternative leaching reagent to conventional cyanidation.

iv Lay Summary

Sodium cyanide is a reagent used by the gold mining industry to dissolve gold into solution. It is inexpensive and highly efficient. However, its toxicity has raised environmental concerns, leading to strict regulatory scrutiny. Consequently, alternative reagents are being sought. Thiocyanate is one the promising reagents that can replace cyanide because of its high efficiency, low toxicity and fast leaching kinetics.

This work examined the possibility of using sodium thiocyanate as an alternative reagent to dissolve gold into solution. Conditions that improved gold extraction such as reagent dosage, increased addition of potassium iodide as a catalyst, use of different oxidants besides the conventional iron salt oxidant, were tested to determine the amenability of gold dissolution in thiocyanate solution. These tests were performed on oxidized mineral samples taken from the Goldcorp Coffee Project.

Results showed that thiocyanate is a viable reagent and can dissolve gold in the acidic regime.

v Preface

This thesis is original, unpublished, and independent work by the author, Roselyn Sarpomah Yeboah.

This thesis originated from the consultation of Dr. Marcus Tomlinson of Newmont Goldcorp with the help of my thesis advisor Prof. David Dreisinger. The total supervision, guidance and editing was done by Professor David Dreisinger.

All the experimental work reported were conducted by the author at the Materials Engineering Laboratory, University of British Columbia (Vancouver, B.C.) with the help of Dr Be` Wassink. Chemical analyses of samples were conducted by either SGS Canada, or the Department of Earth and Ocean Sciences (EOS) University of British Columbia (Vancouver Campus).

This work was sponsored by Mitacs through the Mitacs-Accelerate Program and by Goldcorp Incorporated now Newmont Goldcorp Incorporated, which also provided the mineral ore samples for this project.

vi Table of Contents

Abstract ...... iii

Lay Summary ...... v

Preface...... vi

Table of Contents ...... vii

List of Tables ...... xiii

List of Figures ...... xv

List of Symbols and Abbreviations...... xviii

Acknowledgements ...... xix

Dedication ...... xx

Chapter 1: Introduction ...... 1

1.1 Background and Thesis Objective ...... 1

Chapter 2: Background and Literature Review ...... 3

2.1 Gold Production and Research ...... 3

2.1.1 Gold in History ...... 3

2.2 Gold Mineralogy ...... 3

2.2.1 Classification of Gold Ores ...... 3

2.3 Gold Cyanidation ...... 5

2.4 Why the Need for an Alternative Reagent for Gold Leaching? ...... 5 vii 2.5 Gold Leaching using Alternative Reagents ...... 6

2.5.1 Thiosulphate ...... 8

2.5.2 Thiourea ...... 8

2.5.3 The Halide (Chlorine, Bromine & Iodine) ...... 9

2.6 The Challenges to Developing Alternatives for Cyanide ...... 11

2.7 Thiocyanate Leaching of Gold (Chemistry and Thermodynamics) ...... 11

2.7.1 Chemical Properties of Thiocyanate ...... 11

2.7.2 Gold Extraction with Thiocyanate ...... 12

2.7.3 Stability of Thiocyanate ...... 13

2.7.4 Leaching of Gold in Thiocyanate Solutions ...... 16

2.8 The Use of Different Oxidizing Agent ...... 21

2.8.1 Hydrogen Peroxide as an Alternative Oxidant ...... 22

2.8.2 Potassium Iodide as an Additive to Thiocyanate Leaching ...... 23

2.9 Toxicity and Environmental Concerns with Thiocyanate ...... 26

2.10 Recovery of Gold from Thiocyanate Solutions ...... 27

Chapter 3: Experimental Design and Methodology...... 29

3.1 Experimental Design ...... 29

3.1.1 Goldcorp Coffee Sample...... 29

3.1.2 Sample Preparation ...... 29

viii 3.1.3 Dry Grinding ...... 30

3.1.4 Sampling for Testwork and Analyses ...... 31

3.1.5 Solid SG Determination ...... 31

3.2 Mineralogical and Chemical analyses – Head grade, XRD and ICP Analysis ...... 32

3.2.1 Head Grade Analysis ...... 32

3.2.2 Mineralogical Analysis ...... 33

3.2.3 ICP Analysis ...... 34

3.3 Experimental Setup ...... 34

3.4 Analysis of Results and Analytical Methods ...... 35

3.5 Reagents used in Gold Leaching Tests ...... 36

Chapter 4: Results and Discussion ...... 37

4.1 Introduction ...... 37

4.2 Cyanide Leaching ...... 37

4.2.1 Cyanidation Test ...... 38

4.3 Thiocyanate and Ferric Sulphate Variation ...... 40

4.3.1 Effect of Thiocyanate Concentration on Gold Extraction ...... 41

4.3.2 Effect of the Concentration of Fe(III) on Gold Extraction ...... 44

4.3.3 Effect of Fe(III) and SCN Concentration on Thiocyanate Consumption ...... 46

4.4 Effect of the Concentration of Low and No Fe(III) Iron Addition on Gold Extraction ... 48

ix 4.4.1 Effect of Low Iron Concentration ...... 49

4.4.2 Effect of No Fe(III) Iron Addition on Gold Dissolution...... 50

4.5 The pH Variation Test ...... 56

4.5.1 Effect of pH on Gold dissolution ...... 58

4.5.2 Effect of pH on the Leaching Potential ...... 60

4.6 Potassium Iodide Leaching Test ...... 62

4.6.1 Gold Leaching with Potassium Iodide ...... 63

4.6.2 Gold Leaching by Thiocyanate with Addition of Iodide ...... 64

4.6.3 Gold Leaching by Iron(III)-Thiocyanate with Addition of Potassium Iodide ...... 66

4.6.4 Reagent Consumption ...... 70

4.7 Hydrogen Peroxide Test ...... 72

4.7.1 Gold Leaching with SCN Only ...... 73

4.7.2 Effect of Hydrogen Peroxide concentration on Gold Dissolution ...... 73

4.7.3 Effect of Peroxide Concentration on Leaching Potential ...... 74

4.7.4 Effect of Hydrogen Peroxide Concentration on NaSCN Consumption...... 75

4.8 Kinetic Leach Test ...... 77

4.8.1 Effect of Leaching Time on Gold Dissolution ...... 77

4.8.2 Effect of Leaching Time on Thiocyanate Consumption ...... 80

4.9 Mixture of Reagents ...... 81

x 4.9.1 Effect of Oxygen, Lead Nitrate and Hydrogen Peroxide on the Gold Extraction .... 83

4.10 Adsorption Test using Activated Carbon ...... 85

4.10.1 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 1 86

4.10.2 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 2 87

4.10.3 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 3 88

4.10.4 Comparison of the Three Conditions ...... 90

Chapter 5: Summary, Conclusions and Recommendations Future Work ...... 93

5.1 Summary and Conclusions ...... 93

5.2 Recommendations for Future Work...... 95

References ...... 97

Appendices ...... 100

Appendix A: Analytical Methods ...... 100

A1: Preparation of Au-Standards for AAS ...... 100

A2: Free Cyanide Titration Procedure ...... 102

A3: Residual Thiocyanate Titration Procedure...... 103

A4: Solids Specific Gravity ...... 104

Appendix B: Mineralogical Analysis Results ...... 105

B1: XRD Imaging Results for Untreated Samples ...... 105

B2: Results of ICP Analysis ...... 108

xi Appendix C: Gold Extraction Calculations ...... 111

C1: Gold extraction...... 111

C2: Gold extraction from thiocyanate solutions ...... 113

Appendix D: Iron Leaching ...... 119

D1: Effect of pH on iron concentration ...... 119

D2: Effect of pH on Oxidation Potential ...... 119

Appendix E: Leaching Model ...... 120

Appendix F: Grind Characterisation ...... 121

F1: Sieve analysis Results ...... 121

xii List of Tables

Table 1 – Classification of gold ores [7] ...... 4

Table 2 - Typical leaching conditions used in leaching gold with halides (Aylmore 2005) ...... 10

Table 3: Stability constants of thiocyanate complexes with iron and gold at 25oC [32] ...... 21

Table 4- Standard Potentials for Half-Reactions of Iodide and Thiocyanate Redox Couples in Aqueous Solution at 25°C...... 26

Table 5- Sample Identification ...... 29

Table 6 – Solid SG result ...... 32

Table 7 – Head grade analysis results ...... 32

Table 8 – Results of XRD analysis for the three samples ...... 33

Table 9 – Chemical analysis of three Coffee sample ...... 34

Table 10 : Chemical Reagents Used for Leaching Tests ...... 36

Table 11- Leaching conditions for cyanidation tests ...... 37

Table 12 – Baseline cyanidation test results with their reagent consumptions ...... 39

Table 13 – Leaching condition for the SCN and Fe(III) optimization ...... 40

Table 14 - Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant Fe(III) concentration ...... 47

Table 15- Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant SCN concentration ...... 47

Table 16 – Test condition for Low and No iron addition ...... 48

xiii Table 17 Comparison between the three conditions with 0.1 M SCN...... 54

Table 18 - Thiocyanate Consumption for the No iron addition varied SCN concentration ...... 55

Table 19 – Leaching Condition for pH Variation Test ...... 57

Table 20 - Leaching Conditions for potassium iodide tests ...... 63

Table 21 – Results of the gold leaching with KI only ...... 63

Table 25 – Leaching test result for thiocyanate leaching only at 0.1 M NaSCN ...... 73

Table 26 – Leaching condition for kinetics test ...... 77

Table 27 – Leaching condition for mixture of reagents ...... 81

Table 28 – Results of gold leaching experiments conducted under different leaching conditions82

Table 29 Initial composition for the three solution samples ...... 86

xiv List of Figures

Figure 1 - Eh-pH diagram showing typical operating regions for alternative gold lixiviants [2] .. 7

Figure 2 - Eh–pH diagram for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C. Short dashed lines show water stability limits [27]...... 13

Figure 3 - Eh–pH diagrams for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C. Short dashed lines show water stability limits ...... 15

Figure 4 - Eh–pH diagrams for the Au–H2O system at Au concentration of 0.0001 M at 25 °C. Short dashed lines show water stability limits...... 17

Figure 5 - Eh–pH diagrams for the AuSCN–H2O system at SCN concentrations of 1.0 M, and Au concentration of 0.00001 M at 25 °C...... 18

3+ o 3+ Figure 6 - Species distribution diagram for the Fe - SCN-H2O system at 25 C, pH 2 and Fe activity of 10-1 [31] ...... 20

Figure 7 - Product of crushed sample ...... 29

Figure 8 - Grinding mills used for the grinding process at UBC Mining Department ...... 30

Figure 9 – Particle size distribution of the 72142 B gold ore samples ...... 31

Figure 10 – A pictorial view of the experimental set-up ...... 35

Figure 11 – Cyanidation of the three samples ...... 38

Figure 12 - Effect of the NaSCN concentration on the gold extraction, [Fe(III)] 0.10 M ...... 41

Figure 13 - Effect of the NaSCN concentration on the oxidation potential, [Fe(III)] 0.10 M ..... 43

Figure 14: Leach solution with ferric addition ...... 43

Figure 15 - Effect of the Fe(III) concentration on the gold extraction, [SCN] 0.1 M ...... 45 xv Figure 16 - Effect of the Fe(III) concentration on the oxidation potential, [SCN] 0.1 M...... 46

Figure 17 - Effect of thiocyanate concentration on gold extraction [Fe(III)] 0.01 M ...... 49

Figure 18 - Effect of Fe(III) (0.01 M and 0.1 M) concentration on gold SCN 0.1 M ...... 50

Figure 19 - Effect of the thiocyanate concentration on gold (No Iron addition) ...... 51

Figure 20: Leach solution with and without ferric ...... 52

Figure 21 - Effect of the thiocyanate concentration on leaching potential ...... 53

Figure 22– Effect of pH on Iron concentration at (a) 10 mins (b) 60 mins (c) 120 mins and (d) 180 mins ...... 58

Figure 23 - Effect of pH and the SCN concentration (0.025 M and 0.2 M) on gold extraction ... 59

Figure 24 - Effect of pH on the leaching oxidation potential @ SCN 0.025 M ...... 60

Figure 25 - Effect of pH on leaching oxidation potential (ORP) @ SCN 0.2 M ...... 61

Figure 26 - Effect of KI concentration on gold extraction, SCN 0.15 M ...... 64

Figure 27 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M ...... 65

Figure 28 - Effect of the KI concentration on gold extraction, SCN 0.15 M ...... 66

Figure 29 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M ...... 67

Figure 30 - Effect of the KI concentration SCN concentration: 0.15 ...... 69

Figure 31 - Effect of the H2O2 concentration on gold extraction, SCN 0.15 M ...... 73

Figure 32 - Effect of the H2O2 concentration on the potential readings, SCN concentration: 0.10 M ...... 75

Figure 33 - Effect of the H2O2 concentration on thiocyanate consumption, SCN 0.10 M ...... 76 xvi Figure 34 - Kinetics of Gold dissolution in thiocyanate solution, (SCN 0.15 M and Fe(III) 0.1 M) ...... 78

Figure 35 – Kinetic leaching model fitting ...... 79

Figure 36 - Effect of leaching time on thiocyanate consumption ...... 80

Figure 37 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold extraction ...... 83

Figure 38 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold dissolution ...... 84

Figure 39 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.1 M, Fe (III) 0.05 M, pH 1.98, Temp 25 oC ...... 87

Figure 40 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.1 M, Fe (III) 0.05 M, KI 0.02 M, pH 2, Temp 25 oC ...... 88

Figure 41 - Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.2 M, pH 2.06, Temp 25 oC ...... 89

Figure 42 – Comparison of the carbon adsorption between the 3 conditions study. (a) runs at 0.25 g/L carbon at the 3 conditions, (b) runs at 0.5 g/L carbon at the 3 conditions, (c) runs at 5 g/L carbon at the 3 conditions, (d) runs at 10 g/L carbon at the 3 conditions, (e) runs at 20 g/L carbon at the 3 conditions ...... 91

xvii List of Symbols and Abbreviations

AAS Atomic Adsorption Spectroscopy DI De-Ionized water DO Dissolved Oxygen in solution| Eh Electrochemical potential, in Volts (V) E° Standard electrochemical potential at 25°C, in Volts (V) ΔGf°298K Standard Gibbs Free Energy (of formation at 298K or 25°C), in kJ/mol Hr Hour

ICP-AES/OES LECO - Combustion analysis of total sulphur content using LECO® instruments ICP-MS - Inductively coupled plasma mass spectrometry LPM Litres Per Minute Mins Minute denoting time M Molarity (mol/L) ORP Oxidation/Reduction Potential, in Volts (V) pH Measure of acidity/alkalinity in solution ppm Parts Per Million, also equivalent to grams per tonne PLS Pregnant Leach Solution, the aqueous leachate solution at the end of leaching P80 the diameter of screen hole that allows 80% of ore particles to pass through RPM Rotations Per Minute XRD X-Ray Diffraction analysis

xviii Acknowledgements

I would like to express my deep and sincere gratitude to the following people without whom this project could not have been accomplished.

My uttermost appreciation goes to the Almighty God for His steadfast love and mercies which have taken me through all the storms of academic life. My mouth will continually be filled with your praise, declaring your splendor all day long.

I would like to express my most heartfelt appreciation to my supervisor, Professor David Dreisinger for granting me the opportunity to be part of the hydrometallurgy team (UBC) and not only that being a father; providing invaluable guidance throughout my study.

I am highly indebted to Dr Kodjo Afewu for the day to day guidance, encouragement and support which helped me to successfully complete this program but also his mentoring in my life that went far beyond helping me in my research work. I say Ayekoo and God bless you for all that you do.

Thank you to Dr, Marcus Tomlinson and Goldcorp Incorporated, for funding this project, providing the samples for my analysis and most especially the technical advice and feedback given at the start and during my research. My sincere thanks also go to the Mitacs Accelerate program and their representative, Dr. Sherry Zhao, for the financial contribution for this work.

To Dr. Wassink Berend, thank you for your close supervision, guidance, technical support in the set-up and analysis of the experiment which enabled me to successfully complete this research on schedule. God bless you.

I would also like to extend my appreciation to the Materials Engineering group at University of British Columbia (UBC) for the love. I also thank all the professors for the knowledge they imparted to me throughout the period I spent on the program.

To my friends and colleagues, Chih Wei, Clara Asamoah, Patrick Aboagye, Prince Adu and Richard Osae, I appreciate you.

xix Dedication

I dedicate this work to God, who is the pillar of my life, without Him I am nothing. Thank you for your grace that has kept me.

Though the fig tree does not bud and there are no grapes on the vines, though the olive crop fails and the fields produce no food, though there are no sheep in the pen and no cattle in the stalls, yet I will rejoice in the Lord, I will be joyful in God my Savior. The Sovereign Lord is my strength; he makes my feet like the feet of a deer; He enables me to tread on higher heights.

Habakkuk 3:17-19

I also dedicate it to my family, Benjamin Asante and Dr Kodjo Afewu. God bless you for being there for me. I love you all.

xx Chapter 1: Introduction

1.1 Background and Thesis Objective

The most common leaching process for gold dissolution involves cyanidation, a process that has been practiced for over a century and still remains dominant. Cyanide has been used successfully in the processing of gold and silver ores, forming very stable cyano complexes in solution. It has been the leach reagent of choice because of its high efficiency and relatively low cost. As a result, over 85% of all gold extracted worldwide produced by hydrometallurgical extraction use cyanide. In Canada, more than 90% of the gold mines use cyanide [1].

In recent years, one area of research activity for gold recovery has focused on alternative processes involving non-cyanide lixiviants. Non-cyanide lixiviants for gold extraction have been considered because of safety, environmental and permitting issues associated with the use of cyanide. Also, while cyanide is very effective in leaching free milling ores, there are certain classes of ores that are termed refractory or difficult-to–treat ores, that are mostly not amenable to the conventional cyanidation process [2]. The search for an alternative, apart from the reasons mentioned above, is also geared towards finding a highly economic reagent to replace cyanide since the processing cost, (including CN destruction, waste water treatment, etc.) of cyanidation is increasingly becoming expensive [3]. These reasons have fuelled the consideration of an alternative to cyanide with effort devoted to finding an alternative that might compete with the conventional cyanidation.

Thiocyanate (SCN-) is amongst the many alternatives that has received much attention over the years and stands as a competitor to replace cyanide. Goldcorp Incorporated (now known as Newmont Goldcorp), has many mines around the Americas and a general interest in non-cyanide lixiviants for gold. One deposit that is in the development pipeline is the Coffee deposit, a potential high-grade open pit heap leach gold mine, located approximately 130 km of Dawson City, Canada. This deposit is still in the exploration and development stage and it is an ideal opportunity to examine new, improved and safer methods to process the ore. In this thesis, a series of fundamental leaching tests with ferric thiocyanate solution were conducted on selected mineral samples from the Coffee project. The primary objective was to ascertain the amenability of extracting gold in thiocyanate solutions. This will help determine: a) the feasible conditions of

1 leaching gold in thiocyanate solutions b) the methods of improving the leach performance with addition of leach enhancers/additives (such as potassium iodide) to increase the dissolution of gold in thiocyanate systems c) the effect of alternative oxidants to ferric sulphate on the gold dissolution rate. Finally, the adsorption of gold from thiocyanate solutions on carbon has also been studied. This contributes to the development of an overall competitive process in the gold mining industry.

The thesis is divided into 5 chapters. Chapter 2 covers the relevant literature background information about thiocyanate leaching; Chapter 3 discusses ore characterization and sample preparation including the process of ore particle size reduction and methodology for reactor leaching and carbon adsorption; Chapter 4 focuses on the results and discussions of the leaching and adsorption tests conducted; Chapter 5 provides conclusions and recommendations for future work.

2 Chapter 2: Background and Literature Review

2.1 Gold Production and Research 2.1.1 Gold in History

Gold has been known since pre-historic times and is one of the first noble metals to be recovered because of its high dissemination in the earth crust [4]. It appeared as nuggets in streams and flowing waters and was identified by virtue of its bright yellow color. Humans have valued gold for its physical and chemical properties (lustrous color, ductility and durability) which make it useful to the electronic, technological and jewellery industries. It is also used in design of religious artefacts and is a form of wealth for individuals and countries serving as revenue when mined and sold in the international market [5].

The high demand requires more gold to be sought and mined. Primitive methods (including gravity separation and amalgamation) have been employed to recover and process gold. However, these processes are only suitable for placer or free gold and are unsuitable for fine gold or gold associated with sulphides. Many lixiviants were tested before the invention of the cyanidation process; for example chlorination was used in the year 1887 and 1889 before cyanidation was patented which really revolutionized the extractive metallurgy of gold and increased production [6].

2.2 Gold Mineralogy 2.2.1 Classification of Gold Ores

Generally gold ores can be classified as “free milling”, “complex” or “refractory”. Free milling ores are mostly oxide and low sulphide containing gold ores. They are easy to treat and give good gold recoveries > 90 % by direct cyanidation. Complex gold ores are those that contain cyanide and oxygen consuming materials like tellurides, cyanicides, etc. These ores are mildly refractory. Refractory gold ores are those that do not respond to simple cyanidation process and give gold recoveries of less than 80 %, in some cases much less than 50 % [7]. The degree of refractoriness is expressed in Table 1.

3

Table 1 – Classification of gold ores [7]

90%-100% recovery Free milling

80%-90% recovery Mildly refractory

80%-50% recovery Moderately refractory

< 50 % recovery Highly refractory

Generally, gold bearing materials can be grouped into 15 mineral-based processing categories [5]. This includes placers, oxidized, iron sulphides gold bearing materials, etc. Oxidized ores are normally weathered materials mostly in a shallow zone that is typical of the primary sulfide deposit. During oxidation and other hydrothermal alteration processes, the rock structure breaks down due to exposure to the atmosphere and weather conditions resulting in increased permeability and leachability. These ores are mostly free milling ores and are ideal for cyanidation processes as high leaching extractions can be achieved. The treatment processes for these ores are simple and are cost effective. However, the disadvantage of working with these kinds of ores is that the weathering can produce hydrated, amorphous clay minerals that sometimes adversely affect downstream processes (like slurry viscosity and such mass transfer processes as leaching and carbon adsorption [5]).

Sulphide containing ores (refractory ores) are common throughout the world. Pyrite is a common mineral associated with sulphide gold ores. Sulphide ores are mostly resistant to the conventional cyanidation process. Leaching of fine gold grains contained within pyrite is a major difficulty in gold ore treatment. Sulphide ores may require pre-treatment in order for cyanidation to be effective in the recovery of gold. Fine gold inclusions in pyrite may require fine grinding and/or strongly oxidizing conditions to liberate the gold for leaching. Moreover, for carbonaceous gold ores or concentrates, gold extraction by direct cyanidation is usually low. A few alternatives like thiosulphate has been successful in treating these ores. However, not much work has been reported on the leaching of gold from these ores using thiocyanate solutions. 4 2.3 Gold Cyanidation

Cyanidation has been used as the typical gold leaching method for over 100 years. Cyanide is known as a powerful lixiviant for gold and silver. Cyanide has simple reaction chemistry, lower dosage requirements and better metallurgical performance than many other reagents. Cyanide, - complexes with gold after oxidation to form an anionic complex as Au(CN)2 [1, 5, 8]. The reaction of gold in an aerated solution of sodium cyanide was demonstrated by Elsner in 1846 as:

Reaction 1: 4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH

The most common and effective oxidant used in the cyanide leaching system is oxygen, which can be supplied from air making the process cheaper. The cyanidation process has been successfully used in agitation leaching, heap leaching, and intensive cyanidation.

2.4 Why the Need for an Alternative Reagent for Gold Leaching?

The use of cyanide presents significant safety, environmental and production challenges. These are summed up as follows:

1. Environmental concerns and high toxicity of cyanide compounds:

The high toxicity of cyanide is mainly due to its production of the toxic gas (HCN) when pH is low. The release of cyanide into the environment represents potential environmental and health hazards. Cyanide spills from mining wastes into the environment can cause damage to aquatic life, loss of wildlife and plant life. For example, the cyanide spill in Summitville, Colorado, in the year 1992, resulted in the loss of aquatic species in the 22-mile Alamosa river in Colorado. Birds were found dead in the immediate vicinity which was believed to have resulted from the birds drinking the cyanide contaminated waters [1, 9].

5 2. Increasing Regulatory Scrutiny:

The environmental accidents that have occurred around the world have raised concerns about the use of cyanide in many parts of the world. Some gold mining companies have left scars in some communities due to improper handling and uncontrolled disposal of cyanide in mining wastes. This has increased the regulatory scrutiny for new miners wanting to set up plants that use cyanide.

3. Nonadaptability of “stubborn” ores and concentrates:

Aside from the environmental concerns, one major reason for the search for the alternative is the difficulty of cyanide to leach highly refractory gold ores. Cyanide is effective to leach free milling ores. However, there are certain classes of gold and silver ores (i.e., carbonaceous, pyritic, arsenical, manganiferous, cupriferous) that are considered refractory to conventional cyanidation and record low gold dissolution with cyanide.

These together with the long leaching period with cyanidation have been the critical driver for the industry to evaluate new and improved reagents with effort devoted to finding an alternative that might compete with conventional cyanidation.

2.5 Gold Leaching using Alternative Reagents

More than 25 new alternative reagents have been seriously studied in order to find a reagent to replace cyanide for gold leaching. These reagents include the halides (chlorine, bromine and iodine), the thiosystem (thiosulfate, thiocyanate and thiourea), polysulfide, the ammonia system and novel reagents and technologies. Researchers have made comparative studies on the chemistry, thermodynamics and application of these reagents. Quite a few pilot plants and review papers based on these research results have been reported [10]. A consideration of environmental effects using these reagents have also been discussed [11],[ [6, 12, 13].

The operating regime, oxidation potential and pH of some of the alternatives are presented in Figure 1. The Eh-pH diagram classifies these reagents into acidic, basic and neutral regime and shows their corresponding potentials. 6

Figure 1 - Eh-pH diagram showing typical operating regions for alternative gold lixiviants [2]

Various oxidants are used in these kinds of system. In the alkaline systems including cyanide, ammonia-cyanide and ammonia, oxygen, air or peroxide are the main oxidants used. Gold dissolution rates are controlled by oxygen solubility in solution. The neutral lixiviant systems which includes the thiosulphates, halogens, etc., oxygen and iron sulphate are the main oxidants used. In the acid leaching systems which includes, thiourea, thiocyanate, chlorine, etc., various oxidants such as chlorine, ferric chloride, hydrogen peroxide, ferric sulphate, nitric acid are reported. Gold dissolution in acidic solutions has been reported to be fast. However, major disadvantages reported for some of these reagents which operate in the acidic regime include high reagent consumptions and corrosion of materials of construction. Among all the lixiviants examined, (thiosulfate [14, 15], thiourea, halogens, thiocyanate [16],) thiocyanate is one of the few promising alternatives that can replace cyanide. Some of the alternatives have been discussed briefly below.

7 2.5.1 Thiosulphate

Thiosulphate as an alternative to cyanide for the leaching of gold and silver ores has been extensively studied [17, 18]. The dissolution of gold in thiosulphate solution is accomplished by the formation of the gold-thiosulphate complex. Acceptable gold leaching rates using thiosulphate have been achieved in the presence of ammonia with cupric ion acting as the oxidant [15]. However, reports show that the chemistry is quite complex (mainly due to the presence of thiosulphate, ammonia and copper), and recovery of gold from thiosulphate solution has been the main limitation to the process [15]. The relatively simple oxidation of metallic gold to the aurous Au+ ion in ammoniacal thiosulphate in the presence of Cu(II) can be simply represented as follows:

2- 2+ 3- 5- Reaction 1: Au + 5 S2O3 + Cu(NH3)4 = Au(S2O3)2 + 4 NH3 + Cu(S2O3)3

Senanayake [19] showed that the leaching of gold in thiosulphate occurs at a potential between 0.1- 0.36 V and a slightly alkaline pH between 9-10. Li et al [6] reported that maintaining suitable concentrations of thiosulphate, ammonia, copper and oxygen in the leach solution, and consequently, suitable Eh and pH conditions, thiosulphate leaching can be made practical. However, high thiosulphate consumption has been one of the limitations for the wide industrialization of thiosulphate leaching. Barrick Gold Corporation [20] after recording low recoveries, applied thiosulphate to the treatment of pre-oxidized carbonaceous gold ores that exhibited preg-robbing characteristics and a gold recovery of 95% was recorded. Thiosulphate is generally regarded as the most popular alternative to cyanide and more work has been completed to ascertain the process.

2.5.2 Thiourea

Thiourea (H2NCSNH2) dissolves in water to yield an aqueous form which is also stable in acidic solutions and forms strong gold complexes [21]. It has been known to have many advantages over cyanide due to its high leaching rates. In gold dissolution, thiourea operates in the acidic regime. The main oxidants used for gold dissolution are sodium peroxide, hydrogen peroxide, ferric iron, oxygen (pure oxygen or air), ozone, manganese dioxide, manganate, dichromate, and others. However, ferric ion is known to be the common and the most effective [21, 22]. The process can be described by the following chemical equation: 8 3+ + 2+ Reaction 2: Au + 2CS(NH2)2 + Fe = [Au(CS(NH2)2)2] + Fe

Thermodynamically, the potential for leaching gold with thiourea in the presence of ferric is between 0.4 and 0.45 V (vs SHE) and at a pH between 1 - 3. It has been reported that thiourea is intrinsically unstable and at a higher pH and potential, oxidizes rapidly to form formamidine disulfide. Formamidine disulfide decomposes to a number of intermediate species where elemental sulfur forms as the final product which may passivate the gold surface and stop the leaching process [22]. Most successful applications with thiourea have been carried out on ores that have high contents of cyanicides such as antimony, or sulfide ores that have undergone pretreatment such as bacteria oxidation or pressure leaching. This makes the process more advantageous since the pretreated ore can be directly leached with thiourea without a neutralisation step as would be required for leaching with cyanide[2]. Thiourea has been demonstrated in a pilot- scale agitation leaching, percolation leaching, in-stope leaching experiments at a Witwatersrand mine, underground leaching at a Canadian mine.

However, in recent years, interest in this leaching approach has decreased due to the potential carcinogenic properties of thiourea, higher reagent consumption during gold dissolution, serious corrosion of equipment due to the acidic media in which it is operated, no selectivity of gold during leaching and its complicated regeneration procedures [6].

2.5.3 The Halide (Chlorine, Bromine & Iodine)

The halogens are well known for leaching gold and silver. Chlorine dissolves gold in aqueous solutions by forming soluble Au(I) and the more stable Au(III) chloride complexes. A big advantage of chlorination is the high dissolution rate compared to alkaline cyanide leaching which is due to higher solubility of chlorine and chlorides in water. The typical operating region lies at pH values lower than 1.5 and temperature between 50oC - 70oC yielding a recovery of 99% in small-medium scales [23].

The chlorination is capable for higher scale processing and has already been applied industrially for pre-treating refractory and carbonaceous ores in several plants in the USA in the 1980’s [2] [23]. Renewed interest in halides as a lixiviant for leaching gold occurred in the 1990’s after

9 several patents based on the bromine/bromide systems were lodged. A general equation for the reaction with gold and chlorine or bromine is:

- - Reaction 3: 2Au + 3X2 + 2X = 2(AuX4) where X is Cl, Br

These reagents are strongly oxidizing and show much higher dissolution rates than cyanide leaching. Table 2 gives a summary of the leaching conditions used in leaching gold by the halogens.

Table 2 - Typical leaching conditions used in leaching gold with halides (Aylmore 2005)

Reagent Ligand Oxidant Gold Complex Typical Conditions pH

- - Chlorine Cl Cl2 or HClO [AuCl4] 5-10 g/L Cl2 or NaCl <3

- - Bromine Br Br2 [AuBr4] 2-5 g/L Br2 5-8

- - Iodine I I2 [AuI2] 1g/L I2, 9 g/L NaI 5-9

The disadvantage of these halogens is the difficult handling of the strongly corrosive chlorine solution and the need for robust materials of construction and closed reaction conditions because of the formation of chlorine gas [2]. Bromine and iodine have not been used industrially because of difficult handling, high reagent costs and associated health issues.

10 2.6 The Challenges to Developing Alternatives for Cyanide

For the past 30 years, there has been considerable effort to find an alternative to cyanide. However, the main challenge lies in between finding an alternative that would match the characteristics of the ore giving gold recoveries better or comparable to that in the cyanidation process. The general challenges of alternatives appears to be high reagent consumption and difficult process control and recovery of gold from the leach solution. For example, thiosulphate has received much attention over the years and has been reported to have fast leaching kinetics, but it has complex solution chemistry and is difficult to removing gold from leach solution. Thiourea has been reported also to have a high reagent consumption and difficult management due to the potential carcinogenic nature of thiourea. A suitable alternative should be selected to offset all these challenges. [2].

2.7 Thiocyanate Leaching of Gold (Chemistry and Thermodynamics) 2.7.1 Chemical Properties of Thiocyanate

Thiocyanate SCN- previously known as rhodanide (from a Greek word for rose) is a free ion of thiocyanogen (SCN)2. It is a pseudo halide forming thiocyanogen, (SCN)2, and thiocyanic acid, HSCN. This property is due to its similarities with that of the halogens [24]. Thiocyanate forms insoluble salts or complexes with silver, mercury, lead and copper ions under certain conditions. Thiocyanate forms stable soluble complexes with gold. It has a linear structure and shares its negative charge almost equally between sulfur and nitrogen; thus, it is able to form covalent compounds and complexes. SCN- is synthesized or manufactured as a salt of ammonium, mercury, sodium, potassium and transition metals like copper. It is an ambidentate ligand (meaning its unidentate ligand can bond through different atoms to form different coordination compounds) and has a donor function either via the S or via the N atom. The SCN− species can also bridge two (M−SCN−M) or even three metals (>SCN− or −SCN<). Experimentally it has been reported that hard metals such as (Fe, Mn, Ni, Cu and Zn) tend to form N-bonded thiocyanate complexes, whereas soft metals/acids such as second- and third-row transition elements, including Au and Ag tend to form S-bonded thiocyanate complexes [25].

11 2.7.2 Gold Extraction with Thiocyanate

Thiocyanate as an alternative to cyanide for the leaching of gold ores was first discovered by White in 1905. He demonstrated the effectiveness of thiocyanate by dissolving a gold leaf in a ferric thiocyanate solution and also tested the effectiveness of various oxidizing agents [14].

In 1986, when safety concerns on the use of cyanide started becoming public, Fleming [20] revisited the subject of thiocyanate leaching of gold. He investigated a South African pyritic ore, extracting gold and uranium simultaneously by using the acid thiocyanate system with ferric as oxidant. This was done to reduce production cost of uranium-gold ore. The uranium market was in economic decline and thus the company wanted to cut down on the production cost by extracting gold and uranium simultaneously. He reported after his findings that increasing thiocyanate - concentration does not affect uranium extraction and favours gold extraction, forming Au(SCN)2 - and Au(SCN)4 complexes.

Later in the 1990’s, Broadhurst and Du Preez [26] postulated the thermodynamics of the thiocyanate leaching of gold in acidic ferric sulphate medium. Eh-pH diagrams and speciation diagrams were constructed for all the gold/thiocyanate/iron/sulphate/water systems in concentrations and at pH values that would be feasible in a typical leaching system. These diagrams were used to predict the feasibility and the optimum leaching parameters of the thiocyanate leaching of gold in the ferric medium. It was successful to also predict the factors affecting the oxidative dissolution of gold.

More recent work has been done by Barbosa and Monhemius [16] on the thiocyanate leaching of gold. This includes; detailed fundamental and thermodynamics study, kinetic and electrochemical study and finally recovery of gold in thiocyanate solutions. As part of their findings they reported that the mechanism of gold dissolution by iron (III) thiocyanate complex is linked to an autoreduction process, in which SCN- is oxidized by the spontaneous reduction of ferric to ferrous.

12 2.7.3 Stability of Thiocyanate

Thiocyanate ions are metastable and can be oxidized in water to form carbonates, sulphates and ammonia according to the following reactions 4, 5 and 6. Depending on the pH, reaction 4 produces cyanide or hydrogen cyanide which is considerably less stable and oxidizes quickly to cyanate ion. This reaction is irreversible as cyanide and sulphate cannot be reduced back in any form (chemically or electrochemically) to thiocyanate. Reaction 5 behaves like reaction 4 and is irreversible. Reaction 6 is reported to have more stable products.

- -2 - + - Reaction 4: SCN + 4H2O = SO4 + CN + 8 H + 6e

- -2 - + - Reaction 5: SCN + 5H20 = SO4 + CNO + 10 H + 8e

- -2 -2 + - Reaction 6: SCN + 7H2O = NH3 + SO4 + CO3 + 11 H . + 8e

Figure 2 - Eh–pH diagram for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C. Short dashed lines show water stability limits [27].

13 The Eh-pH diagram by Li et al [28] for two levels of thiocyanate concentrations (0.5 and 0.005 M) is shown in Figure 2. The concentration of all other species is 0.1 M. The Eh-pH diagram suggests that thiocyanate oxidation to ammonia, sulfate and carbonic acid (Reaction 6) occur over a pH range of 2 - 6.4. The Eh-pH diagram also suggests that the SCN-H2O system is not sensitive to SCN- concentration in the absence of other ions.

Thiocyanic acid will form at low pH with a pKa of 0.9 according to Equation 7:

Reaction 7: SCN- + H+ = HSCN

As mentioned in the introduction, thiocyanate is a pseudo halide and its related pseudohalogen is thiocyanogen and related acid is thiocyanic acid. Thiocyanic acid exists in two tautomeric forms, H-S-C=N and H-N-C=S. The formation of thiocyanic acid is known to lower the dissolution of gold in a typical leaching process. This is due to the reduction in the activity of SCN- as a result of its protonation to thiocyanic acid [27]. Also, the thiocyanic acid is less reactive and does not participate directly in the dissolution gold.

It should be noted that several intermediate species are formed in the SCN- oxidative process. The - most important species are thiocyanogen (SCN)2 and trithiocyanate (SCN)3 , as suggested by Itabashi [29] and later confirmed by Barbosa-Filho [16]. The formation of these metastable intermediate products is indicated in Figure 3. The Eh-pH diagram corresponding to these metastable species, thiocyanogen and trithiocyanate was constructed with data collected from other sources [16, 27]. This provides insight of the mechanism of thiocyanate oxidation. The diagram also confirms the predominance of trithiocyanate. The formation of HOSCN on the diagram (one of the by-products of the H2O2/SCN mixture) occurs at a very high potential and does not form in a Fe(III)/SCN complex solution.

14

Figure 3 - Eh–pH diagrams for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C. Short dashed lines show water stability limits

The standard potentials for these two intermediate products are as follows:

- - o Reaction 8: (SCN)2 + 2e = 2SCN E = 0.770 V

- - - o Reaction 9: (SCN)3 + 2e = 3SCN E = 0.650 V

− It is believed that under certain conditions trithiocyanate (SCN)3 is the most pre- dominant species with respect to thiocyanogen (SCN)2. It decomposes to form thiocyanogen and thiocyanate as shown in Reaction 10.

- - Reaction 10: (SCN)3 = (SCN)2 + SCN

- 2- Thiocyanogen can also rapidly hydrolyse in acid to form SCN , HCN and SO4 according to the following reaction:

− 2− + Reaction 11: 3(SCN)2 + 4H2O = 5SCN + HCN + SO4 + 7H

15 Li et al [27] reported that the formation of these metastable species enhances gold dissolution as these species act as oxidant and complexant at the same time under certain oxidative leaching conditions and is mostly catalysed by an auto reduction process of ferric to ferrous. Also, under an actual leaching condition with ferric as the oxidant, thiocyanate can be oxidized by ferric thermodynamically into carbonates, sulphates and ammonium as expressed in the reaction below:

- 3+ 2- 2- + + 2+ Reaction 12: SCN + 7H2O + 8Fe = SO4 +CO3 + NH4 + 10H 8Fe

△Go = -378.5 kJ/mol.

It should be noted that the formation of these metastable intermediate products depends on leaching conditions such as concentrations of the oxidants, thiocyanate concentration, pH and solution potential.

2.7.4 Leaching of Gold in Thiocyanate Solutions

In the Au-SCN-H2O system, gold can be dissolved in the acidic region, which allows the use of several oxidizing agents that are suitable for acidic media. The Eh-pH diagram for the stability of gold in water is presented in Figure 4. The short-dashed line indicates the stability region of water.

Activities of all dissolved species are regarded as 1 M. The predominant area is Au(OH)3(s). The diagram was constructed with data collected from relevant sources [16, 30]. This diagram indicates that metallic gold is more stable than gold oxide compounds and gold ions under the conditions for stability of water. The gold metal has no coexistence boundaries with the gold oxides within the water boundaries [10]. Therefore, the oxides would not be produced directly from the metal by oxidation in water. The dissolution of gold by oxidation only can be achieved at a higher oxidation potential and other complexing agent, such as cyanide ion, thiocyanate, etc. to produce a gold complex which remains in solution.

16

Figure 4 - Eh–pH diagrams for the Au–H2O system at Au concentration of 0.0001 M at 25 °C. Short dashed lines show water stability limits.

Most of the earlier research focused on thiocyanate leaching of ‘metallic’ gold and silver with very few studies on gold ores. There are no current research works on the leaching of an oxide gold ore in thiocyanate solution. The chemical reactions for thiocyanate leaching of gold were described by Barbosa [16]. In his postulation, he described that the two most important complexes - formed from the dissolution process are aurothiocyanate, Au(SCN)2 , and/or aurithiocyanate,

- Au(SCN)4 . The formation of these complexes can be achieved over a wide range of pH and potential according to Reaction 13 and 14.

- - o Reaction 13: Au + 2 SCN = Au(SCN)2 + e- E = 0.691 V

- - o Reaction 14: Au+4SCN =Au(SCN)4 +3e- E = 0.659 V

17

Figure 5 - Eh–pH diagrams for the AuSCN–H2O system at SCN concentrations of 1.0 M, and Au concentration of 0.00001 M at 25 °C.

The Eh-pH diagram of the Au-SCN-H2O system presented in Figure 5 shows the predominance of the two gold thiocyanate complexes. Gold may be dissolved at acidic pH values, which enables the use of different oxidants such as iron(III), hydrogen peroxide and manganese dioxide. The − − species are Au(SCN)2 which occurs at potentials around 0.630 V or higher and as Au(SCN)4 at potentials above 0.680 V. In this regard, the potential for an actual gold leaching system as reported by Li et al [27] should be controlled in a range of 0.4 to 0.8 V versus SHE.

- Barbosa [16] reported that the dissolution of gold as the auric complex Au(SCN)4 occurs at the - same potentials in which the predominant thiocyanate species is being oxidized to (SCN)3 whereas - - the aurous complex, Au(SCN)2 , is formed at potentials at which the SCN predominates. The - formation of the auric complex in the presence of (SCN)3 is shown in Reaction 15 as:

- - Reaction 15: AuSCN + (SCN)3 = Au(SCN)4

18 The oxidation potential is very important in this work as it determines the kind of complexes formed. In a typical acid leach condition (pH 1 to 2), this potential can be controlled by such factors as choice of oxidant, concentration of thiocyanate, pH, etc. As mentioned above, higher oxidation − potential is needed to make Au(SCN)4 predominant whereas lower potential signifies the − predominance of the Au(SCN)2 complex. The overall calculations also show that the redox potentials required in a typical leaching process should be around 600-700 mV (vs. SHE).

Almost all the reports on thiocyanate leaching of gold refer to ferric ion as the most suitable oxidizing agent. It should be noted that using ferric ion as an oxidant is very important in a typical leaching system for most gold ores and also for gold leaching following oxidative pre-treatment of sulfidic ores. Most oxide and refractory gold ores have a high amount of iron. Soluble iron as an oxidant can be generated in-situ. Refractory ores are also associated with pyrite and other sulfide minerals. High concentrations of ferric ion in an acidic media can be generated from these ores after oxidative pre-treatment using roasting, bio-oxidation or pressure oxidation.

The overall gold dissolution reaction by thiocyanate in the presence of ferric can be written as:

- 3+ - 2+ Reaction 16: Au + 2SCN + Fe = Au(SCN)2 + Fe

It is well understood that SCN- forms stable complexes with ferric and that the presence of ferric increases gold dissolution rates. The presence of ferric in the SCN- leaching system is also known to release SCN- upon the reduction of ferric to ferrous which thereby increasing the thiocyanate concentration [27]. The reduction of ferric to ferrous is shown in Reaction 17 as:

Reaction 17: Fe3+ + e- = Fe2+ Eo = 0.771V

Since the oxidation potential of ferric is higher than that of SCN- it should be noted that the formation of stable compounds could reduce the oxidizing potential of ferric ions and the concentration of free thiocyanate required for gold dissolution.

The speciation diagram in Figure 6 gives an idea of complexes formed from the SCN/Fe(III) mixture at different SCN- concentration and ferric fractions. One characteristic property of the iron(lll)- thiocyanate complex is the formation of the red blood colour which occurs when solutions 19 containing ferric ions and thiocyanate are mixed together. The existence of any of these species depend on the thiocyanate concentration and the molar ratio of the SCN/Fe3+ complex. For example, at a molar ratio of 1 the predominant species Fe(SCN)2+.

3+ o Figure 6 - Species distribution diagram for the Fe - SCN-H2O system at 25 C, pH 2 and Fe3+ activity of 10-1 [31]

As the formation of these complexes depend on the thiocyanate concentration, it should be noted that lower SCN concentrations lead to the formation of cationic complexes such as Fe(SCN)2+ etc., whereas higher thiocyanate concentrations yield anionic complexes. However, these complexes will coexist in a typical leaching process with SCN- concentration above 10-3 M [26].

The only complex reported so far for the complexation between ferrous and thiocyanate is the FeSCN+. This is due to the weak stability constant between thiocyanate and ferrous ion. This indicates that the ability of thiocyanate to complex with ferrous ions is much weaker than that with ferric ions [16]. The complex formation between thiocyanate complexes with iron and gold is presented in Table 3. The stability constants presented in Table 3 signifies that the iron-thiocyanate complexes are less stable than gold- thiocyanate complexes.

20 Table 3: Stability constants of thiocyanate complexes with iron and gold at 25oC [32]

Metal Ion Complex Stability Constant

Fe2+ FeSCN+ 2.04 x101

Fe3+ FeSCN2+ 1.05 x103

3+ + 5 Fe Fe(SCN)2 2.00 x10

3+ - 5 Fe Fe(SCN)4 3.31 x10

3+ 2- 6 Fe Fe(SCN)5 1.58 x10

3+ 3- 6 Fe Fe(SCN)6 1.26 x10

+ - 19 Au Au(SCN)2 1.45 x10

3+ - 43 Au Au(SCN)4 4.57 x10

3+ 2- 43 Au Au(SCN)5 4.17 x10

2.8 The Use of Different Oxidizing Agent

Ferric sulphate is the most researched oxidant in the thiocyanate system and almost all the reports on thiocyanate leaching of gold refer to ferric ion as the most suitable oxidizing agent. The use of ferric as an oxygen surrogate is to help overcome the slowness of oxygen reduction at room temperature. The leaching of gold in thiocyanate solutions with ferric allows gold to be leached at a pH of 1-3. The low pH of operation is due to the solubility of ferric sulphate as a function of pH. Ferric is highly soluble in the acidic regime, and above pH > 3, the solubility of ferric decreases with the formation of iron (III) hydroxide which precipitates at higher pH values [26].

Other potential oxidizing agents for the leaching of gold in acidic thiocyanate solution are hydrogen peroxide, oxygen, air and MnO2. However as mentioned before, the major drawback of the use of oxygen and air as an oxidizing agent is its low solubility in acidic solutions. Though it is very effective for cyanide systems in the basic medium, it is expected not to be effective at any pH above 3 in the thiocyanate system. This is due to the slow rates of oxygen reduction on gold 21 surfaces in non-cyanide solutions [2]. This explains why SCN- generated in cyanidation plants does not participate in the gold dissolution process and simply reports as a cyanide loss.

Hydrogen peroxide and MnO2 have not received much attention. However, Barbosa [16] reported that hydrogen peroxide can be used in both gold and silver leaching, the former requiring a weakly acidic pH (pH < 2.5). Potentials for this kind of process are a bit higher as compared to that in the Fe(III)/SCN systems. A few of these alternative oxidants have been discussed below:

2.8.1 Hydrogen Peroxide as an Alternative Oxidant

Hydrogen peroxide is known to be a good oxygen supplier. It has been used successfully to increase dissolution of gold in alkaline cyanide solutions. Hydrogen peroxide decomposes to form water and oxygen. Even though it has been used in most cases as an auxiliary oxidant, its decomposition reaction enables it to serve as the main oxidant by increasing the oxygen to catalyse a reaction [33].

Early studies with hydrogen peroxide in cyanidation showed that the dissolution rate of gold with small amount of hydrogen peroxide is very slow under oxygen-free solutions. Other studies have shown that an increase in the hydrogen peroxide concentration increases the dissolution rate of gold significantly [34]. However, hydrogen peroxide has been used many times as an oxygen supplement in many gold mining process plants to increase the concentration of dissolved oxygen above that attainable with simple air sparging systems.

- Wilson and Harris [35] investigated the reaction of H2O2 with SCN . According to their investigation, it was found that the oxidation reaction between thiocyanate and hydrogen peroxide is pH dependent and catalysed by the production of H+ ion at pH below 2. The oxidative reaction between the thiocyanate and H2O2 proceeds through an initial step of the reduction of H2O2 to H2O and the production of an intermediate species; hypothiocyanite, (according to Reaction 18) which undergoes a series of fast reactions leading to end products, which are dependent on the pH of the medium. They observed that at a higher acid concentration (pH below 2), the reaction is acid- catalyzed. This is due to the fast production of H+ ions and the formation of hypothiocyanite (as seen in Reaction 19). Hypothiocyanite also undergoes fast reactions leading to end products of cyanide and sulphate (according to Reaction 22). 22 Cyanide reacts further with HOSCN and forms sulphur dicyanide, S(CN)2. Also, at a lower pH of 4 -12, the reaction between thiocyanate and hydrogen peroxide was found to be pH independent − and the hypothiocyanite produced further oxidizes to sulphate, ammonia and bicarbonate (HCO3 ).

- - Reaction 18: SCN + H2O2 = HOSCN + OH

+ - Reaction 19: H3O + SCN + H2O2 = HOSCN + 2H2O

Reaction 20: HOSCN + H2O2 = HOOSCN + H2O

Reaction 21: HOOSCN + H2O2 = H2SO3 + HOCN

+ + Reaction 22: HOOSCN + H3O = H2SO3 + HCN + H

- Reaction 23: HOSCN + HCN = (SCN)2 + H2O

- + Reaction 24: HOCN + 2H2O = HCO3 + NH4

The rate determining reactions would be Reactions 18 and 19 for the pH-independent (pH>4) and the acid catalyzed (pH<2) reactions, respectively.

2.8.2 Potassium Iodide as an Additive to Thiocyanate Leaching

Iodide/Iodine are halides that have been reported to leach gold, have high oxidizing potentials and forms stable gold complexes in the aurous and auric states [36]. Chlorine, bromine and iodine are halides and have been successfully used to dissolve gold. However, iodide solutions are known to be the most stable amongst the halides [11]. It has been suggested for the extraction of precious minerals such as platinum, gold and silver ores. Previous results have shown that gold leaching rate in iodide solution is much higher than that of cyanides and other lixiviants such as, thiosulphate, thiourea etc. Furthermore, iodide leaching can be carried out over a wide pH range [30].

23 Gold dissolution in iodide solution takes place in the presence of a suitable oxidizing reagent (ferric ions, hydrogen peroxide, iodine, hypochlorite, etc.) [36]. Among these, iodine seems to be the most widely researched and suitable reagent due to its ability to perform at a wide range of pH (usually below 11) yielding high gold dissolution rates.

It is understood that under general conditions, iodine dissolves in the presence of iodide to form triiodide ion, which acts as the oxidant for the oxidation of elemental gold to gold(I)-iodide complex. The oxidant reacts with the iodide which forms the tri-iodide and subsequently oxidizes - - gold to form the stable complexes AuI2 and AuI4 . The solubility products of the gold iodide (AuI) -23 -46 and gold tri-iodide (AuI3) in water are 1.6×10 and 1×10 , respectively [11, 36, 37].

Early studies on the use of the Iodine/Iodide system was first proposed by McGrew and Murphy [38]. In their study, an electrolyte containing iodide ions was used for leaching sulphidic gold ores (gold associated with marcasite, pyrite, galena, etc.). This electrolyte contained a mixture of iodide ions and elemental iodine which served as the oxidant while the former acting as a complexant. Gold dissolution was dependent on the concentrations of iodide ion and molecular iodine, which recorded a redox potential sufficient for extraction [39]. This was made possible by the continued addition of iodide which increased the iodide ion to achieve the desired concentration for leaching gold. McGrew and Murphy [38] emphasized that due to the sparingly soluble nature of iodine in low concentrations, to ensure sufficient iodide formation, the iodine lixiviant should be added to an ore containing iodine reducing components. Gold was recovered on activated carbon and the excess iodide formed during the process was re-oxidized electrochemically to iodine and reused and thus requiring no addition of iodine. It should be noted that gold did not start to dissolve until sufficient concentration of iodine species remained in solution.

A leaching test conducted by Morteza [30] on two different gold ores in the iodine/iodide solutions; oxide and carbonaceous gold ores for 24 hr showed only 20 % extraction for the carbonaceous and 89 % for the oxide. The same test was repeated with iodate as the oxidant and no gold was leached in 48 hr. He reported that the presence of sulphides and ferrous minerals slowly consumed iodine and increased reagent consumption.

24 Thermodynamically, the dissolution of gold in iodide/iodine solutions is an electrochemical process. Gold in its natural state is stable in water and can only go into solution when a mixed potential is created by adding a complexant and an oxidant. The latter prepares the gold surface by + 3+ - oxidizing it to Au and Au and the former bonds with it to form a complex in the form, AuI2 or - AuI4 which brings the gold into solution. The gold dissolution chemical reactions in the iodine/iodide solution are shown as follows [31]:

Reaction 25: Au+ + e- = Au Eo = 1.68 V

- - - Reaction 26: Anodic Au + 2I = AuI2 + e

- - - Reaction 27: Au + 4I = AuI4 + 3e

- - - o Reaction 28: Cathodic I3 + 2e = 3I E = 0.537 V

The oxidation of iodide is achieved by the cathodic half-cell reaction according to Reaction 28. The overall reaction is written as:

- - - Reaction 29: 2Au + I3 + I = 2AuI2 E = -0.042 V - - - Reaction 30: 2Au + 3I3 = 2AuI4 + I E = -0.024 V

- It should be noted that I3 is the main oxidant of the iodine /iodide solution. The standard potential - - + of the I3 /I is 0.536 V which is far lower than the standard potential of the gold couple Au /Au + (1.68 V) or Au3 /Au (1.52 V).

Also, since the standard potentials for the overall reaction are negatives -0.042 V according to Reaction (30) and -0.024 V for Reaction (31), it signifies that gold dissolution in iodine solutions is not spontaneous under standard conditions. However, it has been reported that when triodide and iodide concentrations are varied, it is possible to bring about a net spontaneous reaction [37].

Recently, experimental work done by Barbosa [39] on the effect of iodine/iodide additions on the dissolution of gold in iron (III) thiocyanate solutions showed that small amounts of iodine/iodide - - addition was successful in increasing the extraction rates. He stated that species such as I2 and I3 that form from the iodine/iodide system are very stable and form stable gold complexes as - compared to the intermediate species, (SCN)2 and (SCN)3 formed from the thiocyanate system.

25 - It is believed that the production of these intermediate oxidation species ((SCN)2 and (SCN)3 ) caused by the autoreduction of ferric is the mechanism for gold dissolution in the thiocyanate system. However, these species are not stable as their production is not continuous due to their fast decomposition by hydrolysis. The instability of these intermediate species can be overcome by additions of small amounts of iodide or iodine ions. It is revealed that the rate of extraction increased with increase in thiocyanate and iodine or iodide concentrations. However, the addition of iodide ions lowers the oxidation potential making the formation of the most active intermediate - species (SCN)3 thermodynamically not feasible. However, Barbosa [39] explained that ions such - as I2 and I3 are very stable in solution and complex with gold and thiocyanate forming stable - - - - species I2SCN and I(SCN)2 to cause gold dissolution. Thus, I2SCN and I(SCN)2 are believed to - cause faster gold dissolution by increasing leaching kinetics than (SCN)2 and (SCN)3 in ordinary thiocyanate systems. The comparison of their half reactions is shown in Table 4.

Table 4- Standard Potentials for Half-Reactions of Iodide and Thiocyanate Redox Couples in Aqueous Solution at 25°C.

Half Reaction Eo (V)

- - I2(aq) + 2e + 2I aq 0.621 - - - I3 (aq) + 2e = 3I aq 0.536 - - (SCN)2(aq) +2e = 2SCN aq 0.77 - - - (SCN)3 (aq) + 2e = 3SCN aq 0.68

2.9 Toxicity and Environmental Concerns with Thiocyanate

The use of sodium thiocyanate has been reported to be less harmful than cyanide. The reagent shows some promise and will require further investigations on its toxicity. Li et al [27] highlighted that thiocyanate is a naturally occurring compound which is present in many food products such as cabbage, beets, cauliflower with its source being Cruciferae.

In animals, sudden death may occur from SCN- toxicity but usually the effects develop gradually. Toxicity symptoms in animals may include vomiting, diarrhea, emaciation, general weakness, loss 26 of equilibrium and buoyancy, tremors, convulsion, coma and death. A person weighing 150 pounds will have to ingest 29 g of potassium thiocyanate as a potential single lethal dose [24].

In the gold leaching process, the stability of the thiocyanate species is strongly dependent on oxidation potential and leaching environments. The oxidation of thiocyanate produces species such as sulfate, ammonia, metastable cyanide, hydrocyanic acid, etc. Hydrocyanic acid is not harmful and needs very high potential to form. However, its formation is unlikely and it is known not to be toxic even if it does form [40]. However, further investigation is needed to determine the correct methods for waste disposal.

2.10 Recovery of Gold from Thiocyanate Solutions

In gold processing, once gold is dissolved, the recovery of gold from the pregnant solution can be carried out by different procedures e.g. adsorption, solvent extraction, etc. Adsorption of gold from cyanide solutions using activated carbon is currently the most commonly used process in the gold mining industry. The adsorption of gold with activated carbon in the cyanide system and some of the alternatives to cyanide has been researched. However, not much has been done on the carbon adsorption of gold from thiocyanate solution.

The strength/ability of activated carbon to adsorb gold in other solutions as discussed by Marsden and House [5] has the following order:

AuCl2 > Au(CN)2 > Au(SCN)2 > AuSC(NH2)2)2 > Au((S2O)3)2

Activated carbon adsorption in thiocyanate solutions as reported by Li et al [40] gave a very high gold recovery of about 98% and stands to be one of the ideal adsorbents for the adsorption of gold in thiocyanate solutions. Carbon was observed to serve as a catalyst for the auto reduction process of ferric to ferrous whiles oxidizing thiocyanate causing the formation of the two intermediate - species, namely (SCN)2 and (SCN)3 . This will become an advantage in CIL/CIP process systems with SCN as the lixiviant, as the presence of these intermediate species will cause extra gold to dissolve. The equilibrium of gold loading onto activated carbon were reported not to be affected

27 by temperature and pH. According to other studies [40], activated carbon has strong adsorption properties for Au-SCN complexes for high gold recovery, and rapid kinetics, but lacks selectivity, and needs stripping at high temperatures (about 150 °C).

Among the first alternative adsorbent tested was iron cementation for adsorbing gold in thiocyanate solutions. The author concluded that the process was very effective, however, the gold reaction was affected by solution pH, stirring speed, initial gold concentration, iron/gold mass ratio, dissolved oxygen and presence of iron (III), while the reaction temperature and thiocyanate concentration only slightly affected the cementation reaction. Also, the gold cementation reaction was diffusion controlled and obeyed first-order kinetics with an activation energy of 9.3 kJ/mol. It was observed that the de-aeration process of the solution enhanced the gold recovery and the presence of ferric ions resulted in the low efficiency of gold precipitation as well as the low grade of the cement [32].

In summary, thiocyanate SCN- is one of the most promising reagents among the several alternatives to replace cyanide. With a careful control of leach conditions, the leaching of gold in thiocyanate solutions can be controlled to attain maximum gold extraction. Thiocyanate was evaluated for the extraction of gold samples (Supremo Oxide 68151 A, Supremo oxide 68151 B, Supremo Composite 72142 B) from the Goldcorp Coffee project in Yukon, Canada with the following objectives to:

1. Investigate the optimal conditions for dissolution of gold using thiocyanate 2. Improve gold extraction by the addition of additives (potassium iodide, hydrogen peroxide and lead nitrate) 3. Recover gold from pregnant thiocyanate solution using activated carbon

28 Chapter 3: Experimental Design and Methodology

3.1 Experimental Design 3.1.1 Goldcorp Coffee Sample

Three different samples were received from Goldcorp’s Coffee Project, Yukon, Canada, for this study. The samples were drawn from different zones of their mineralisation with the following identity designations: 68151 A, 68151 B and 72142 Composite. The details of the as-received samples are as shown in Table 5.

Table 5- Sample identification

KCA Sample Client ID Mass (Kg) 68151 A Supremo Oxide - 3.5 mm 5.76 68151 B Supremo Oxide - 1.70 mm 10.04 72142 B Supremo T2-T4 Comp - 62.5 mm 31.88

3.1.2 Sample Preparation

The purpose of this sample preparation was to have an evenly distributed representative sample for each test.

Sample as received (72142 B) 1st Stage Crushing of the 2nd Stage crushing of the same same sample sample

Figure 7 - Product of crushed sample

29 The initial samples were received as rough rocks as seen in Figure 7. Upon receipt, the samples were physically examined and weighed. Each of the three as received ore samples was crushed in a two-stage crushing i.e. Jaw crusher following a cone crusher to reduce the particle size and expose the gold surface for the subsequent processes. After the two-crushing stages, the samples were dried at 60 oC to remove moisture, and dry-milled in a laboratory rod mill to P80 of 150 µm.

3.1.3 Dry Grinding

Grind characterisation was conducted under dry milling conditions and in batches (approximately 1 kg per sample) with the aim of attaining P80 of 150 µm. Each batch was ground (in Figure 8) with 20 rods and subsequently sieved. The sieving times were varied at 5, 10, 15, and 18 minutes to determine the time at which P80 of 150 µm was attained. The grind curve showed that P80 of 150 µm was attained after 15 minutes (see Appendix F for grind characterisation curve). The rest of the samples were subsequently ground for 15 mins.

Figure 8 - Grinding mills used for the grinding process at UBC Mining Department

30 The product of the 72142 B material from the grinding stage was split using a riffle splitter to obtain a representative sample for particle size analysis. The result of the particle size analysis conducted 72142 B composite is shown in Figure 9.

Figure 9 – Particle size distribution of the 72142 B gold ore samples

3.1.4 Sampling for Testwork and Analyses

The samples were riffle split and sub-sampled into aliquots for the leaching testwork and mineralogical and chemical analyses.

3.1.5 Solid SG Determination

Solid SG was determined for all the three gold ore samples. This was necessary to help in metallurgical accounting and calculation purposes. Approximately 50 g of each of the sample was placed in a known mass of 250 volumetric flask which was half filled with distilled water. The flask containing the slurry was gently swirled and deaerated using a vacuum chamber to remove 31 any entrained air in the water. This was topped up with deaerated water and the new mass was recorded. The solid SG is calculated as shown in equation in Appendix A5.

Table 6 – Solid SG result

Sample ID Supremo 68151 A Supremo 68151 B Supremo 72142 B

SG 2.60 2.62 2.66

3.2 Mineralogical and Chemical analyses – Head grade, XRD and ICP Analysis

Samples were submitted for mineralogical analysis by XRD, elemental chemical analysis by ICP and head gold grade analysis by Fire Assay.

3.2.1 Head Grade Analysis

Samples were submitted to SGS Vancouver for head gold grade analysis by fire assay followed by acid digestion. The results of this is presented in Table 7:

Table 7 – Head grade analysis results

Sample ID Supremo 68151 A Supremo 68151 B Supremo 72142 B

Head Grade (g/t) 1.40 1.42 1.98

32 3.2.2 Mineralogical Analysis

Table 8 is the results of an XRD analysis conducted on the three samples received from Goldcorp. The samples were prepared and analysed according to the International Centre for Diffraction Database PDF-4 using Search-Match software by Bruker. The analysis was conducted at the UBC Earth and Ocean Sciences.

Table 8 – Results of XRD analysis for the three samples

Percentages of Supremo Ores #1 #2 #3 Mineral Ideal Formula 68151 - 68151 - 72142 - A B B

Quartz SiO2 56.0 57.4 55.3

Illite- Muscovite 2M1 K0.65Al2.0Al0.65Si3.35O10(OH)2/KAl2AlSi3O10 15.9 15.4 15.3

Illite-Muscovite 1M K0.65Al2.0Al0.65Si3.35O10(OH)2/KAl2AlSi3O10 15.2 15.6 10.6

2+ Biotite K(Mg,Fe )3AlSi3O10(OH)2 1.2

Kaolinite Al2Si2O5(OH)4 4.4 4.3 8.5 K-feldspar KAlSi3O8 5.3 4.7 4.2 (microcline) Plagioclase NaAlSi3O8 – CaAlSi2O8 2.1 1.5 4.9 (oligoclase)

Rutile TiO2 0.5 0.5

Chalcopyrite CuFeS2 0.2 0.3

2+ Dolomite-Ankerite CaMg(CO3)2,Ca(Fe ,Mg,Mn)(CO3)2 0.4 0.4 Total 100.0 100.0 100.0

Results of the XRD analysis on all the three samples showed high amount of quartz. The results of quantitative phase analysis by Rietveld refinements are given in Table 8 and with the scans shown in Appendix B1. The major phases of minerals for all three samples were quartz, clay minerals (illite), kaolinite, plagioclase and feldspar. These amounts represent the relative amounts of crystalline phases normalized to 100%.

33 3.2.3 ICP Analysis

SGS (Vancouver) conducted the chemical analysis of the samples by using the 56-element ICP, and Carbon and Sulphur species by the Leco method. The results of the chemical analysis are partly shown in Table 9, with the rest provided in Appendix B2. The elements with the highest

Table 9 – Chemical analysis of three Coffee sample

Cu (ppm) Fe (%) Ag (ppm) As (ppm) C (%) S (%) Al (%)

Supremo Oxide 68151 A 20 2.95 <1 1430 0.154 0.034 6.97 Supremo Oxide 68151 B 20 2.83 <1 1540 0.193 0.034 6.93 Supremo 72142 B 10 4.1 <1 1500 0.033 0.018 7.57

3.3 Experimental Setup

The gold dissolution experiments were carried out in a one-liter cylindrical baffled reactor obtained from CANSCI. The presence of the baffles was to prevent vortex effect and to ensure that slurry was well mixed. The reactor was then immersed in a thermostatically controlled water bath with a suspended mixer over the reactor to mix the slurry, as shown in Figure 10.

The reactor was covered with a four-holed lid to prevent evaporation. Three of these holes were covered and one was left open throughout the test for sampling and reading of ORP and pH’s. The pH and oxidation-reduction potential (ORP) of the solution were also measured using a hand-held Oakton pH pH/mV Meter.

Slurry samples were taken at time 2, 4, 8, 12 and 24 hr. After the leaching test was completed, the final slurry was filtered using a vacuum pump attached to a buchner funnel/bottle and the gold pregnant solution (filtrate) collected for gold assay and residual thiocyanate analyses.

The residue retained on the filter, was generously washed to remove any residual soluble gold using deionized (DI) water to strip away all gold pregnant solution remaining in the cake. The cake was placed in an oven and dried at 60 °C. The dried cake was crushed using a roller and a sample

34 was taken to SGS for tails grade analysis by fire assay followed by acid digestion. Solution grade was analysed with AAS and/or ICP MS.

Figure 10 – A pictorial view of the experimental set-up

3.4 Analysis of Results and Analytical Methods

Gold in solution was analysed using Atomic Absorption Spectroscopy (on a Varian 240 AAS instrument) with an air-acetylene flame and ICP MS (this was done in the UBC Geological laboratory to confirm values from the AAS). Gold in solution was extracted into the organic phase using DIBK containing 10g/L aliquat 336. Gold standards were prepared in the concentration of 0.1 – 5 ppm (see procedure in appendix) from a 1000 ppm gold standard stock solution. All the solution gold grade analysis was done in the organic phase.

Residual thiocyanate was determined argentimetrically by the Volhard method titration [41]. This was performed by pipetting a known volume of the titrand into a volumetric flask, acidified with

2-3 drops of concentrated HNO3, and titrated directly with AgNO3 in a glass burette. Ferric sulphate solution was used as the indicator. The reaction of ferric and thiocyanate gives a deep red colour. Silver nitrate precipitates thiocyanate quantitatively as the white solid AgSCN. Silver thiocyanate is a stronger complex than ferric thiocyanate and the solution becomes colourless when 35 all of the thiocyanate is complexed with silver. (The Ksp of AgSCN is 1.0 x 10-12). The end point was indicated by colour change of the precipitate from dark red to white/ colourless. Residual thiocyanate was calculated as:

mg SCN/litre = ((Vol. titrant, ml) x (0.1mmoles Ag+/ml) x (58.08 mg SCN/mole))/litres

Residual cyanide was also determined argentimetrically. Samples were titrated directly with 0.1M

AgNO3. Rhodamine was used as the indicator. The reaction between the AgNO3 and the NaCN is as: Cyanide reacts with Silver Nitrate (AgNO3) according to the following equation below:

AgNO3 +2NaCN NaAg(CN)2 +NaNO3

The end point was indicated by colour change of the precipitate from colourless to pink.

3.5 Reagents used in Gold Leaching Tests

All the reagents used in the leaching test are summarised in Table 10. These were used without further purification or analysis to confirm their purity values. Deionised water was used for all the gold leaching tests.

Table 10 : Chemical reagents used for leaching tests

Reagent Grade/Purity Source Form CaO (ACS Reagent Grade) Alfa Aesar/Thermo Solid (Pellets) Fisher Scientific NaOH (ACS Reagent Grade) BDH Chemicals/VWR Solution Analytical NaCN 95% (ACS Reagent Grade) Anachemia (VWR) Solid (Powder)

AgNO3 Certified Analytical Grade Fisher Scientific Solution KI (ACS Reagent Grade) ACROS Solid (Powder) NaSCN 95% (ACS Reagent Grade) Anachemia (VWR) Solid (Powder)

H2O2 30% w/w (ACS Reagent Grade) Fisher Scientific Solution

36 Chapter 4: Results and Discussion

4.1 Introduction

This chapter discusses the results of the leaching and adsorption tests conducted in this study. Preliminary leaching tests were conducted on all the three samples (68151 A, 68151 B and 72142 B composite material) to determine the responsiveness/amenability of the ores to gold extraction by cyanide and thiocyanate solutions. This was followed by a series of leaching experiments aimed at optimizing the thiocyanate leaching of gold. All the subsequent leaching tests excluding the preliminary tests were conducted on the 72142 B composite material using the experimental setup in Section 3.3 so as to eliminate sample variability from the interpretation of the results.

4.2 Cyanide Leaching

A cyanidation test was conducted in duplicate for each of the ores sample, the first one at a default leaching time of 24 hr and a sodium cyanide concentration of 500 ppm and the duplicate at a leaching time of 32 hr with the same cyanide concentration at 500 ppm. Two of the openings of the reactor lid were left open to allow air into the reactor and thus, no air sparging equipment was used. The purpose of the cyanidation test was to determine the maximum extraction attainable by cyanide leaching and also to serve as a reference/baseline for the thiocyanate tests.

All the samples were subject to the same test procedure and sample preparation as detailed in Section 3.3 and Section 3.1.2 respectively, with the leaching conditions summarized in Table 11.

Table 11- Leaching conditions for cyanidation tests

Volume of Solution ~ 0.6 L Pulp density 40 % solid Solution pH 10.5 – 10.8 Agitation Speed 700 rpm Atmospheric condition Air /no air sparging Temperature 20 -25 oC Residence Time 24 - 32 hr NaCN Concentration 500 ppm

37

4.2.1 Cyanidation Test

The result of the 32-hr cyanidation test performed on the three Coffee samples is shown in Figure 11.

Figure 11 – Cyanidation of the three samples

The result presented in Figure 11 indicates that the ore is free milling and amenable to conventional cyanidation process. All three ores showed a recovery of ~90% after 2 hr of leaching indicative of fast leaching kinetics. Supremo 72142 B exhibited the fastest kinetics and the highest gold extraction rate after 2 hr relative to 68151 A and 68151 B.

The slight fluctuations seen in the extraction curves can be associated with the continuous washing of measuring probes and the method of sampling employed in this test (i.e. by stopping the reaction completely) as well as the gradual rise after time 8 hr could also be related to evaporation due to continuous stirring resulting in an increase of the gold concentration. Overall gold extractions are

38 good and were calculated based on the final solids and solution grades at the end of the tests (Appendix C1).

A slight increase in gold extraction of less than 1 % for two of the ore types was observed when the leaching time was extended from 24 hr to 32 hr, as shown in Table 12.

Table 12 – Baseline cyanidation test results with their reagent consumptions

Au Reagent Sample ID Residue Extraction Consumption & Au Grade Head Au Grade (g/t) (%) (kg/t) Composition (g/t) Calculated Measured Measured NaCN CaO Cyanide Leaching Tests ([NaCN] 500 ppm)

68151 A (24 hr) 0.070 1.29 1.28 94.6 0.62 1.87

68151 A (32 hr) 0.060 1.20 1.28 95.0 0.68 1.54

68151 B (24 hr) 0.080 1.47 1.45 94.6 0.60 1.95

68151 B (32 hr) 0.080 1.44 1.45 94.4 0.73 1.19

72142 C (24 hr) 0.075 2.00 1.98 96.3 0.77 1.33

72142 C (32 hr) 0.060 2.02 1.98 97.0 0.54 1.92

The sodium cyanide consumptions (as seen in Table 12) for all the ore samples were low and within a similar range (0.5-0.7) kg/t ore and also indicating that a lower sodium cyanide concentration can be used to achieve similar gold extraction results. The lower consumptions could be attributed to the high pH range in which the test was conducted – as there was little or no cyanide loss in the form HCN gas. Furthermore, due to the low content of base metals and sulphides in the samples (as shown from the chemical and mineralogical analyses), the CN- losses from reactions with the gangue matter would be minimal.

39 The calculated and measured head grade showed a very good correlation in terms of accountability (within +/- 5%). Overall, the cyanidation test conducted provided much information on reaction and leaching kinetics.

4.3 Thiocyanate and Ferric Sulphate Variation

In this section, sodium thiocyanate and ferric sulphate concentrations were varied over a wide range of sodium thiocyanate and ferric sulphate concentration ratios. This was aimed at attaining a high gold extraction for the SCN/Fe(III) system. The Fe(III)/SCN ratio was important to cause higher gold dissolution and also increase leaching potential [28]. Thus, sodium thiocyanate was varied in the range of 0.005 M - 0.2 M at a ferric sulphate concentration of 0.1 M while ferric sulphate was varied from 0.05 M - 0.5 M at a thiocyanate concentration of 0.1 M in an initial series of experiments.

The optimization test for the thiocyanate and ferric sulphate were conducted as per the experimental procedure outlined in Section 3.3 with test condition outlined in Table 13.

Table 13 – Leaching condition for the SCN and Fe(III) optimization

Volume of Solution 0.6 L

Solid grade ~ 2 g/t gold ore Pulp density 40 % solid

Solution pH 1.5 -2.0

Agitation Speed 700 rpm

Atmospheric condition Air

Temperature 20 -25 oC

Residence Time 24 hr

40 4.3.1 Effect of Thiocyanate Concentration on Gold Extraction

The thiocyanate optimization test was conducted by maintaining all the leaching parameters outlined in Table 13 and varying the thiocyanate concentration.

Freshly prepared 0.1 M ferric sulphate solution acidified with sulphuric acid to a pH of ~1.4 was used. The initial pH and ORP’s were recorded for the acidified ferric sulphate solution. These were in the range of 1.3 -1.4 and 650 - 700 mV (Ag/AgCl) respectively. The final pH and ORP’s after adding ore sample and calculated mass of thiocyanate were also in the range of 1.6 -1.7 and 500 -560 mV respectively, and these became the initial pH’s and ORP’s for the leaching test. The increase in pH from ~1.4 to ~ 1.7 was due to the basicity of the ore whereas the drop in ORP was due to the addition of thiocyanate solid to the leaching solution as well as the reaction chemistry between thiocyanate and gold and possibly other minerals.

Figure 12 - Effect of the NaSCN concentration on the gold extraction, [Fe(III)] 0.10 M

41 It is apparent from Figure 12 that increasing the thiocyanate concentration from 0.005 M to 0.15 M increases the gold extraction at a faster rate up to 0.05 M SCN and a further increase in thiocyanate concentration insignificantly affects gold extraction. The increase in gold extraction when the concentration of sodium thiocyanate was increased from 0.05 M to 0.15 M was marginal. Barbosa [16] explained that the dissolution of gold in thiocyanate solution is dependent on the Fe(III)/SCN- ratio and as the thiocyanate concentration is increased at a constant Fe(III) concentration, there is the formation of various complexes that changes the speciation of the system and affects the gold dissolution rate. Broadhurst [26] mentioned that thiocyanate ions have the tendency of complexing with gold more quickly than iron; forming gold thiocyanate complexes that are more stable than iron complexes and therefore increasing gold dissolution with an increase in thiocyanate concentration.

Therefore, it can be concluded from Figure 12 that increasing thiocyanate concentration increases the gold dissolution up to a point (around 0.05 to 0.1 M in this test) beyond which a further increase becomes insignificant. Thus, for the subsequent leaching tests, the thiocyanate concentration was maintained at 0.1 M. These results substantiate the other results [16, 26, 28]. Gold extraction results obtained from these tests were reproducible (as some of the runs were replicated) as seen in Table 14.

The potentials measured during the leaching tests are reported in Figure 13. It is observed that an increase in the thiocyanate concentration decreases the leaching potential. This is due to the increasing complexation of ferric by thiocyanate. The leaching potential also decreases with leaching time signifying the reduction of ferric to ferrous.

42

Figure 13 - Effect of the NaSCN concentration on the oxidation potential, [Fe(III)] 0.10 M

It is believed that the drop in the leaching potential can be attributed to the reduction of Fe(III) and the various thiocyanate species and ferric complexes that are formed when thiocyanate is mixed with ferric sulphate [16]. A typical example is the formation of the two intermediates species - 2+ ((SCN)2 and (SCN)3 ) and possibly iron complexes such as Fe(II) and FeSCN . The formation of FeSCN2+ complex was confirmed from the change in colour of the leach solution from colourless to red-blood when the thiocyanate solid was immediately added to the ferric solution during the leaching tests. The change in colour of the leaching solution after the leaching test is shown in Figure 14.

Figure 14: Leach solution with ferric addition

43 During the leaching process, it was observed that the initial solution potentials were higher at higher Fe(III) concentration and lower SCN- concentration (usually around 600 mV as seen in Figure 13) and lower when the thiocyanate concentration was increased. The latter could be mainly due to the Fe(III)/Fe(II) redox couple and also the complex formation between thiocyanate and ferric ligand. Also, since SCN- has a stronger affinity to complex with Fe(III) than with Fe(II), the potentials decreased with increasing free SCN- concentrations resulting in an increase in the concentration of Fe(II). However, in all the tests, the oxidation potentials were high enough to oxidize both gold and thiocyanate.

The above mentioned corroborates the work of Barbosa and Li [16, 28] that the addition of sodium thiocyanate to ferric sulphate solution, drops the leaching potential and turns leaching solution to deep red blood colour (as shown in Figure 13).

4.3.2 Effect of the Concentration of Fe(III) on Gold Extraction

The leaching condition of the thiocyanate test was replicated for the Fe(III) iron variation test. The only notable difference was the variable ferric sulphate concentration. The thiocyanate concentration was maintained at 0.1 M. Broadhurst [26] mentioned that maintaining a high ferric concentration relative to thiocyanate concentration improves the kinetics. This is due to the - production of the intermediate species (SCN)2 and SCN3 . Also, high concentration of Fe (III) is needed to keep thiocyanate stable increasing the concentration of free thiocyanates in solution; thus high concentration range (0.05M – 0.5 M) of the ferric sulphate concentration was chosen to evaluate its effect on gold extraction and oxidation potential.

44

Figure 15 - Effect of the Fe(III) concentration on the gold extraction, [SCN] 0.1 M

There was a discernible increase in gold dissolution with increase in Fe (III) iron concentration from 0.05 M to 0.2 M and thereafter a very slight fall (relatively flat across the range) in gold dissolution for Fe (III) concentration between 0.2 and 0.5 M as shown in Figure 15. One important thing to note is that maintaining a high concentration of Fe(III) iron relative to thiocyanate significantly improves the gold dissolution. The improved gold extraction are believed to be what is termed the auto reduction process in which SCN- ion is oxidized by the spontaneous reduction of Fe(III) to Fe(II) releasing the two intermediate species which oxidize and complex gold to cause dissolution to occur. Broadhurst [26] also reported that high ferric iron concentration is favorable for thiocyanate stability, increasing concentration of free thiocyanate. However, very high Fe(III) has been reported to insignificantly affect the dissolution of gold. Li et al [28], found that above a certain limit of ferric iron concentration (0.022 M), the gold dissolution in their test, was controlled by surface reaction making gold dissolution reaction independent of ferric concentration. Thus, the concentration of Fe (III) may at least be 0.1 M or 0.2 M and further increase beyond this has little or no effect on gold dissolution. Ferric ion was fixed at 0.05 M and 0.1 M for subsequent tests conducted in this research. The slight downtrend of gold dissolution for

45 Fe(III) iron concentrations at 0.5 M experienced in this work is not well understood and would need further investigation.

Figure 16 - Effect of the Fe(III) concentration on the oxidation potential, [SCN] 0.1 M.

The potentials were continuously monitored throughout the leaching test to ascertain the potential - - range in which the AuSCN2 and AuSCN4 form. However, in all the tests, the initial potentials were sufficiently high to oxidize both gold and thiocyanate. Solution potentials were also observed to decrease with time and Fe (III) concentration as seen in Figure 16. This was mainly due to the reduction of Fe (III) to Fe (II) and formation of SCN- complexes with Fe (III) and gold.

4.3.3 Effect of Fe(III) and SCN Concentration on Thiocyanate Consumption

The effect of Fe(III) and NaSCN concentration on thiocyanate consumption are shown in Tables 14 and 15. From Table 14, it can be seen that increasing SCN- concentration increases the SCN- consumption. On the other hand (Table 15), increase in Fe(III) ion concentration decreases the consumption of thiocyanate. This is due to the stability of SCN- that Fe(III) provides as explained above.

46 Table 14 - Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant Fe(III) concentration

Residue Au Reagent Sample ID & Au Extraction Consumption Composition Grade Head Au Grade (g/t) (%) (kg/t)

(g/t) Calculated Measured Measured NaSCN H2SO4 SCN (0.005 M) 1.67 1.89 1.98 11.7 0.39 14.30

SCN (0.01 M) 0.28 1.86 1.98 84.77 0.59 16.31

SCN (0.02 M) 0.22 1.82 1.98 87.90 0.63 14.30

SCN (0.02 M) 0.23 1.81 1.98 87.55 0.65 12.27

SCN (0.05 M) 0.17 1.86 1.98 90.86 1.25 14.21 SCN (0.05 M) 0.19 1.79 1.98 89.58 1.74 14.17 SCN (0.1 M) 0.17 1.86 1.98 91.09 1.94 12.19

Table 15- Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant SCN concentration

Sample ID Residue Au Reagent Au Extraction Consumption & Grade Head Au Grade (g/t) (%) (kg/t)

Composition (g/t) Calculated Measured Measured NaSCN H2SO4 Fe3+ (0.05 M) 0.208 1.85 1.98 88.8 1.92 9.34

Fe3+ (0.1 M) 0.206 1.86 1.98 88.9 1.85 11.03

Fe3+ (0.2 M) 0.177 1.99 1.98 91.1 1.45 9.68

Fe3+ (0.5 M) 0.206 2.06 1.98 90.0 1.46 9.41

47 4.4 Effect of the Concentration of Low and No Fe(III) Iron Addition on Gold Extraction

Fe(III) iron has been reported and shown in this research to be necessary to keep thiocyanate stable, thus increasing leaching potential which subsequently increases the gold dissolution and concentration of free thiocyanate. However, beyond a certain concentration point, gold dissolution is marginally enhanced by further increase in Fe(III) iron concentration in solution (as seen in Section 4.3.2). This motivated testing of the following two conditions:

1. A lower iron concentration of 0.01 M 2. Leaching with no iron addition

Thiocyanate solution was prepared with 0.1 M sulphuric acid to a pH of 1.5. The concentrations of thiocyanate in these tests were varied over a wide range to determine the effect iron addition on gold dissolution.

Test condition 1 had 0.01 M Fe(III) with varying SCN- concentration (0.005 M – 0.2 M) while condition 2 had no Fe(III) added with varying thiocyanate concentration (0.1 M – 0.6 M). The general leaching test condition for both tests is summarised in Table 16.

Table 16 – Test condition for Low and No iron addition

Volume of Solution 0.6 L

Solid grade ~ 2 g/t gold ore

Pulp density 40 % solid

Solution pH 1.5 -1.9

Agitation Speed 700 rpm

Atmospheric condition Air

Temperature 20 -25 oC

Residence Time 24 hrs

48 4.4.1 Effect of Low Iron Concentration

Figure 17 shows that; gold dissolution increases with increasing thiocyanate concentration at low Fe(III) iron concentration (0.01 M). The increase in gold extraction with change in thiocyanate concentration was sharp from a thiocyanate concentration of 0.005 M to 0. 1 M and a gradual increase in gold extraction was seen thereafter. However, the gold extraction did not achieve a plateau which signifies that a further increase in thiocyanate concentration could possibly increase gold extraction.

Figure 17 - Effect of thiocyanate concentration on gold extraction [Fe(III)] 0.01 M

Comparatively, from Figure 18, the gold dissolution when the ferric concentration was increased tenfold; that is from 0.01 M to 0.1 M showed a significant difference in gold extraction. The increase in gold extraction is huge especially for low values of SCN- as seen in Figure 18. Gold extraction with 0.1 M Fe(III) achieved a plateau at a lower SCN- concentration (0.05 M) whereas Fe(III) at 0.01 M did not achieve any plateau. This confirms the importance of high Fe(III) to the leach solution, however very high Fe(III) iron above certain concentration point (> 0.2 M) plays insignificant role as seen in Section 4.3.2. It can be concluded from Figure 18 that; high gold

49 extractions can be obtained with either high Fe (III) (~ 0.1 M) and low thiocyanate concentration ( ~0.05 M) or low Fe (III) (~0.02 M or 0.05 M) and high SCN- concentration (>0.1 M).

Figure 18 - Effect of Fe(III) (0.01 M and 0.1 M) concentration on gold SCN 0.1 M

4.4.2 Effect of No Fe(III) Iron Addition on Gold Dissolution

In order to operate the process economically, the study was extended to determine the effects of reducing reagent consumption for gold dissolution in the thiocyanate system. One such test was studying the effect of gold dissolution in thiocyanate systems without addition of Fe(III). The main aim of this was to compare this to the traditional cyanidation process where air is normally used as the main oxidant.

The test was conducted on the 72142 B composite material. Thiocyanate solution acidified with sulphuric acid to a pH of about 1.5 was used. One important thing to note here is that the leach solution pH ranged from 1.5 -1.9; meaning there was no sequential addition of acid to maintain

50 the pH of the leaching solution to a particular pH value. The leaching test was conducted according to the test procedure outlined in Section 3.3.1 and conditions in Table 16.

Figure 19 - Effect of the thiocyanate concentration on gold (No Iron addition)

Surprisingly, gold dissolution increased with increasing thiocyanate concentration without the addition of Fe (III) as shown in Figure 19. A thiocyanate concentration of 0.1 M gave 81.5 % gold extraction. When this was increased 6-fold with the same leaching condition; i.e. 0.6 M thiocyanate, the gold extraction increased to 90.9 %.

Even though ferric was not added to the leach solution, it was visually observed that ferric was generated in situ (leached from the ore sample as shown in Figure 20 b) mainly because the leached solution turned red immediately when thiocyanate was added to the leach solution. The dissolution of iron was due to the acidification of the leached solution which possibly extracted some soluble iron into solution. The color intensity of the solution increased (red blood colour) within the first

51 30 minutes signifying the formation of the FeSCN2+ complex. A comparison of the leach solution with and without iron is shown in Figure 20.

a. Leach solution with Ferric addition b. Leach solution without ferric addition

Figure 20: Leach solution with and without ferric

Though concentrations of dissolved oxygen were not measured during the test (without Fe addition), the dissolution of gold may be attributed to the presence of dissolved oxygen acting as oxidant thus possibly driving the iron in the ore to proceed through the auto reduction process (Fe(III) to Fe(II)). Moreover, the low pH (1.5 - 2) of the leached solution could possibly have dissolved part of the 4.1 % (from mineralogical analysis) iron from the ore. The dissolution of iron into the leach solution could have caused the auto reduction process to proceed; producing the two intermediate species which cause gold dissolution.

52

Figure 21 - Effect of the thiocyanate concentration on leaching potential

A quick look at the solution potentials in these tests showed a decrease with time and plateaued after time 8 hr as seen in Figure 21. The reduction in the leaching potential signifies the auto reduction of Fe(III) to Fe(II). The auto reduction process of Fe(III) to Fe(II), producing the two intermediate species, is known as the mechanism that causes gold to dissolve in the thiocyanate system. This supports the assertion that the gold extraction observed in this test could be due to the oxidative dissolution of iron leached from the ore.

Moreover, higher concentrations of thiocyanate were seen to produce lower leaching potentials. The trend seen here is similar to that in the Fe(III)/SCN system. Barbosa postulated that, “In the runs with high free thiocyanate concentrations (0.05-0.5) M, the overall autoreduction process is much faster in the initial moments and then slows down drastically, due to the marked Fe2+ and FeNCS+ build up in solution. For lower free thiocyanate concentrations (<0.05 M), on the other hand, the initial autoreduction rate is lower and, as the Fe2+ and FeNCS+ production is also slower, the overall rate decrease is more gradual’’. Solution potentials were found to be in the acceptable range (550 – 600 mV) for gold dissolution.

53 Table 17 compares the extraction of gold and NaSCN consumption among the three test conditions outlined. The thiocyanate concentration used in these tests is 0.1 M.

Table 17 Comparison between the three conditions with 0.1 M SCN

Residual Gold Calculated Measured NaSCN Gold Extraction Head Head Grade Consumption Test Condition (g/t) (%) Grade (g/t) (g/t) (Kg/t)

Fe (0.1 M) 0.206 88.9 1.86 1.98 1.85

Fe (0.01 M) 0.391 79.9 1.95 1.98 1.13

Fe (0.00 M) 0.357 81.5 1.93 1.98 0.78

Surprisingly, gold extraction obtained with 0.01 M Fe (III) is somewhat lower than that with no Fe(III) as seen in Table 17. This could be due to unstable/inactive complexes formed between the thiocyanate and the added Fe (III). Moreover, because a smaller amount of Fe(III) was used, the slowness of the Fe (III) to auto reduce itself to Fe(II) causing the production of the two thiocyanate intermediate species which are known to act as oxidant and complexant to cause the gold dissolution [16].

It is not surprising that 0.1 M Fe(III) achieved an 88.9 % gold extraction. As previously mentioned high Fe (III) is needed to keep thiocyanate stable. Comparing this to the low iron concentration (0.01 M), the low gold extractions can be attributed to the Fe(III) concentration being too low to keep thiocyanate stable exacerbating the formation of other complexes that affected the concentration of thiocyanate. The increase seen in the test without Fe(III)/oxidant would need further investigation. However, the red blood colour of the leaching solution (which is lighter than tests with added Fe(III) iron) suggests that iron was leached from the ore sample which could serve as the oxidant for the leaching process. Moreover, since the test was partly opened to allow air into the slurry, the extraction could be attributed to the presence of dissolved oxygen which could have aided in the production of the two intermediate species needed for gold dissolution in the

54 thiocyanate system by oxidizing Fe dissolved from the ore to Fe(III). Thus, for an optimal performance of the acidic thiocyanate leach, Fe(III) concentration should be either 0.1 M or preferably 0.2 M at 0.15 M or 0.1 M SCN-. Lower Fe(III) concentration like 0.05 M would need higher amount of SCN 0.2 M.

Table 18 - Thiocyanate consumption for the no iron addition varied SCN concentration

Description SCN (0.1 M) SCN (0.2 M) SCN (0.6 M) NaSCN Consumption (Kg/t) 0.78 1.87 2.84

The thiocyanate consumption increased with increasing thiocyanate concentration as seen in Table 18. The lowest consumption occurred at 0.1 M and this increased when SCN concentration was increased to 0.6 M.

55 4.5 The pH Variation Test

The traditional cyanidation process for most ores makes use of blower/compressed air/atmospheric air as the main source of oxidizing agent. This process was tested with the leaching of gold in thiocyanate solution.

The results obtained from Section 4.4.2 motivated the leaching of gold at varying pH in thiocyanate solution without the addition of Fe(III) to be conducted. This is due to the red colour formation of the leach solution observed in Section 4.4.2 which signified the presence of FeSCN2+ signifying that iron was leached from the ore.

Thus, the main objective was to leach gold ore at low pH in thiocyanate solution in order to leach iron from the ore. Moreover, since the solubility of iron in solution increases with reducing pH, the pH was varied with the aim of ascertaining the pH which will cause sufficient iron in solution for highest gold dissolution. Thus, the gold ore was leached with thiocyanate only; without the addition of Fe(III) iron.

Prior to this, iron leaching from the ore using sulphuric acid was also conducted at varying pH to determine the amount of iron that could be leached from the ore. The 72142 composite gold ore which showed the highest percentage of iron in the ore (4.1%) was leached with sulphuric acid at varying pH. The main aim was to determine the amount of iron that could be leached from the ore at a varying pH. This if successful would mean that iron would be self-generated from the ore for the leaching process. The parameters of the iron leaching test are summarised in Table 19 with four variables pH, leaching time, oxidation potential and iron in concentration considered and measured. The pH was kept constant and varied at 1.0, 1.5, 2.0, 2.5. The effect of pH on iron concentration for times 10, 60, 120 and 180 minutes is shown in Figure 22 (and in Appendix D). ORP’S were also monitored and recorded at these times.

56 Table 19 – Leaching condition for pH variation test

Volume of Solution 0.6 L Pulp density 40 % solid Agitation Speed 700 rpm Atmospheric condition Air Temperature 20 -25 oC

(a) (b)

57

(c) (d)

Figure 22– Effect of pH on Iron concentration at (a) 10 mins (b) 60 mins (c) 120 mins and (d) 180 mins

From Figure 22, the iron concentration increased with time and decreased with pH. The highest iron concentration of 3135 ppm was recorded at time 180 mins and a pH of 1. From the mass of solid used (~200 g), this signifies approximately 11 % of iron leached from the 4.1% iron reported from the ICP analysis. Above pH of 2, the concentration of iron decreased approaching zero which is due to Fe(III) iron hydrolysis and/or lack of extraction.

4.5.1 Effect of pH on Gold dissolution

The effect of pH on gold dissolution in 0.025 M and 0.2 M sodium thiocyanate solution is shown in Figure 23. The results indicate an increase in gold extraction with increasing thiocyanate concentration. Gold extractions recorded were low at lower thiocyanate concentrations. The gold extractions at 0.025 M SCN were low and increased slowly with increasing pH. However, a thiocyanate concentration of 0.2 M showed a higher gold extraction relative to a concentration of 58 0.025 M. The highest gold extraction recorded for 0.2 M thiocyanate concentration was 87 % at a pH of 1.5 with the lowest being 72.6% at a pH of 2.5.

Figure 23 - Effect of pH and the SCN concentration (0.025 M and 0.2 M) on gold extraction

However, a slightly lower gold extraction was recorded at pH of 1 which is supposed to have the highest dissolved iron in solution. The reason for the low gold extractions recorded at pH of 1 even though it has the highest dissolved iron (as seen in Section 4.4 above) has been discussed. Barbosa mentioned that in a very acidic solution, i.e. pH values below 1, gold dissolution is expected to decrease due to the reduction in the activity of SCN-, which is caused by the protonation of thiocyanate to thiocyanic acid (pKa for HSCN = 0.9). The slightly lower gold extraction obtained at pH of 1 attest to this point. Also, another drop-in gold extraction was seen at pH of 2.5. At pH of 2.5, iron in solution is expected to decrease due to hydrolyzation of the iron which could probably slow down the autoreduction process (due to insufficient iron in solution) causing lower gold extraction.

59 4.5.2 Effect of pH on the Leaching Potential

The leach solution potentials recorded for tests at 0.025 M and 0.2 M are shown in Figure 24 and Figure 25 respectively. The solution potentials increased to a large extent with time, a reverse of the observation SCN/Fe(III) systems. The leaching oxidation potentials measured by Ag/AgCl electrode ranged from 450 mV to 500 mV and from 390 mV to 470 mV for tests with 0.025 M and 0.2 M thiocyanate, respectively. This also confirms that an increase in thiocyanate concentration decreases the solution leaching potential.

For the 0.025 M thiocyanate, the initial potential at pH of 1 was low, however, it increased with time. The XRD analysis on the gold ore used in this test showed high amount of iron in the form of ferrous. This increase could be associated with the oxidation dissolution of iron from the ore to ferric in the solution. Although the solution potentials were higher and fell within the range needed for gold dissolution, the dissolution rates were still low for this concentration (0.025M).

Figure 24 - Effect of pH on the leaching oxidation potential @ SCN 0.025 M

60 At 0.2 M thiocyanate, though the potentials were relatively lower as compared to the 0.025 M, they were still within the range for gold dissolution in thiocyanate solution. As seen in the lower concentration 0.025 M SCN, initial solution potential at pH of 1 was lower. A similar pattern was seen with the 0.2 M SCN, however, the potentials in this test did not take any particular trend probably due to the sequential addition of sulphuric acid to keep the pH constant. The addition of sulphuric acid to the leach solution was seen to increase the leaching potentials of the leach solution.

Figure 25 - Effect of pH on leaching oxidation potential (ORP) @ SCN 0.2 M

61 4.6 Potassium Iodide Leaching Test

In the quest to increase the dissolution rate of gold in thiocyanate system, the effect of small amounts of additives potassium iodide were explored. Potassium iodide (KI) is a halogen salt and an alternate lixiviant to cyanide.

Barbosa postulated that adding small amounts of potassium iodide to the SCN/Fe(III) complex would tend to stabilize the thiocyanate in solution and improve leaching performance. KI was therefore used as an additive to improve gold leaching performance.

To ascertain the effects of potassium iodide on the gold dissolution in SCN systems, it was expedient to know how it works separately, thus the leaching tests were conducted in 3 different phases at SCN concentration of 0.1 M and Fe (III) concentration of 0.05 M;

(1) A baseline test with 0.1 M KI without SCN and Fe (III)

(2) Addition of potassium iodide to;

a) SCN, designated as SCN/KI b) SCN and Fe (III) solution designated as Fe (III)/SCN/ KI

The leaching tests were carried out at KI concentration of 0.1 M for the phase 1 (leaching with potassium iodide only). The test procedure same as described in Section 3.3. KI was varied over a wide range in the subsequent tests with SCN only and SCN/Fe(III) mixture, to know its effect on gold dissolution. The pH ranged from 1.5 - 1.9 and was continuously maintained by the addition of sulphuric acid. The residence time was 24 hr and solution samples (of volume 7 mL) were taken at selected time intervals (2, 4, 8 and 24) hrs and analyzed for gold by organic method by AAS.

The leaching condition for all the tests is summarized in Table 20 for the phase 1 and phase 2 (i) and 2(ii).

62 Table 20 - Leaching conditions for potassium iodide tests

Volume of Solution 0.5 L Pulp density 40 % solid Solution pH 10.5 – 10.8 Agitation Speed 700 rpm Atmospheric condition Air Temperature 20 -25 oC Residence Time 24 hr NaSCN Concentration 0.1 M

4.6.1 Gold Leaching with Potassium Iodide

Leaching with 0.1 M KI was conducted to serve as a reference/baseline for the SCN- tests. The reaction chemistry of gold iodide system has been discussed in the literature review. Gold dissolution requires the use of an oxidant and a complexant. However, in this test, gold leaching experiment was conducted with KI only without the addition of Iodine (oxidant). However, two of the openings on the lid of the leach vessel was left opened to allow some air into the slurry. A high concentration of KI (0.1 M) was chosen. This was necessary to know the operating window in which KI would give comparable gold dissolution rate to that in Fe(III)/SCN mixture.

Table 21 – Results of the gold leaching with KI only

Sample ID Au Reagent Residue Au Extraction Consumption & Grade Head Au Grade (g/t) (%) (kg/t)

Composition (g/t) Calculated Measured Measured H2SO4 Iodide (0.1 M) Only 0.1075 1.94 1.98 94.46 13.23

63 A 94.46 % gold extraction was obtained with the 0.1 M KI obtained under the conditions studied (as seen Table 21). Although, iodine was not added as oxidant in this test, the high dissolution - rate signified the production of I3 which is both an oxidant and complexant for gold dissolution [10]. Thus, for the subsequent leaching process for phase 2 i.e. SCN/KI and Fe(III)/SCN/KI, the KI was varied in a lower range of (0.001 M – 0.1 M) and (0.001 M – 0.02) respectively

4.6.2 Gold Leaching by Thiocyanate with Addition of Iodide

The experimental results with 0.1 M SCN concentration achieved a 79.9% gold extraction. Six experiments based on various KI concentrations were conducted. The KI concentration used were 0.001, 0.002, 0.005, 0.01, 0.05 and 0.1 M. SCN concentration was constant at 0.1 M with a residence time for each test of 24 hrs. The result of the gold extraction for phase 2(i) is illustrated in Figure 26 and its corresponding potentials in Figure 27.

97% Au extraction attained for NaCN at 32 hr

Figure 26 - Effect of KI concentration on gold extraction, SCN 0.15 M

64 From Figure 26, the addition of KI to SCN solution improved the gold extractions. The gold dissolution rate obtained at a given iodide concentration above 0.01 M were higher than those obtained in the Fe/SCN mixture. Gold extraction is seen to increase gradually with increasing KI concentration and plateaued at a KI concentration of 0.05 M. This could possibly be the optimum gold dissolution rate for this set of conditions. The addition of 0.1 M KI to the 0.1 M SCN yielded the same gold extraction of 94 %, similar to that with 0.05 M KI. The gold extraction with the 0.1 M KI and 0.1 M SCN can be said to be an iodide leaching other than a thiocyanate leaching since the same gold extraction was recorded when iodide was used alone.

Figure 27 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M

The leaching potentials as shown in Figure 27 in these tests with the addition of KI ions were lower compared to Fe(III)/SCN tests conducted in Section 4.3. This making the formation of the most - stable intermediate species; SCN3 (which causes gold dissolution to occur) thermodynamically - not feasible. However, iodide species such as l2 and l3 , formed from the oxidation of iodide by - ferric are reported to be very stable species which complex with gold to form stable Aul2 complex and thus causing high gold dissolution rate in this system.

65 4.6.3 Gold Leaching by Iron(III)-Thiocyanate with Addition of Potassium Iodide

The effect of KI to the ferric-thiocyanate mixture on gold dissolution rate is presented in Figure 28 and solution potential in Figure 29. The leaching tests were conducted with the condition stated in Table 20. The concentration of SCN and Fe(III) used in this section are 0.1 M and 0.05 M, respectively. KI concentration was varied at 0.001 M, 0.002 M, 0.005 M, 0.01 M and 0.02 M.

97% Au extraction attained for NaCN at 32 hr

Figure 28 - Effect of the KI concentration on gold extraction, SCN 0.15 M

When the three reagents SCN/KI/Fe(III) were added together, the effect was significant (showing an increase in gold dissolution with small amount of KI) as compared with the previous tests using SCN/KI without ferric iron, suggesting a synergistic effect. The synergistic effect of iodide on the gold dissolution rate can be attributed to the formation of relatively stable mixed iodide- - - thiocyanate complexes which are I2SCN and ISCN2 . These are the main intermediates species formed and are known to participate in the gold dissolution process, as in the reactions 32 and 33 below [39] :

66 - - - Reaction 31 I + 2SCN = lSCN2 + 2e

- - Reaction 32 2I + SCN- = l2SCN + 2e

Gold extraction results obtained for the three mixed reagents were much higher than those obtained in the KI/SCN without Fe(III) and the SCN/Fe(III) mixture. Gold extraction of 92.7% was obtained at iodide concentration of 0.001 M, Fe(III) concentration of 0.05 M and SCN concentration of 0.1 M. This increased gradually with increasing iodide concentration up to about 94 % at iodide concentration of 0.02 M.

Figure 29 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M

67 The leaching potential of the KI/SCN/Fe(III) dropped drastically from 555 mV to ~ 480 mV. The drastic drop is associated to the presence of ferric being reduced to ferrous. Though the interaction between ferric and iodide is known to be slow, the reduction of ferric to ferrous is also known to catalyse the formation of the two intermediates species produced during thiocyanate oxidation - (namely, SCN2 and SCN3 ) which aid again in the dissolution rate of gold and probably causing the higher reduction in the leaching potential [27] as seen in Figure 29. The reduction in the leaching potential followed basically a similar pattern already described for KI/SCN in Section 4.6.3.

Comparatively, the potentials for the KI/SCN pair (in Section 4.6.3) were generally lower than that of the KI/SCN/Fe(III) system. One interesting thing to note is the increase in the potentials in both cases with leaching time. While there was a decrease in the potential in the ferric thiocyanate solutions due to the autoreduction process, the reverse took place in the iodide system. The initial autoreduction of iron in both cases were initially faster and decreased sharply and plateaued after time 4 hr. A slight increase was seen after time 8 hr as shown in Figure 27 and Figure 29. The - potential rise as explained by Barbosa [16] is due to the production of the species l2 and l3 , which are generated by the oxidation of I- by Fe(III). The behaviour observed in the experiments reported in this section is similar to that reported both by Qi and Hiskey [10].

Moreover, the leaching solution (slurry) in the iodide tests did not produce a red blood colour as seen in other leaching tests with and without the addition of Fe(III). This implies the absence or slow formation of FeSCN2+ complex. A rather close to light orange colour was observed. Although in other tests which were without the addition of ferric sulphate, it was evident that iron was being leached from the ore forming FeSCN2+ complex and making the solution turn red the reverse was seen in these tests. This might be due to the low concentration of iron in the form of Fe(III) present in the solution and also the high affinity of iodide to complex with SCN more quickly than with Fe(III) and Fe(II); making SCN unavailable for iron complexation.

68

Figure 30 - Effect of the KI concentration SCN concentration: 0.15

A comparison of the two results of iodide leaching gives a clear distinction between the Au dissolution rate. A linear gold dissolution rate was seen with the SCN/KI/Fe(III) mixture with increasing thiocyanate concentration (as shown in Figure 30). This means that increasing iodide concentration from 0.002 M in the KI/SCN/Fe(III) mixture increased the gold dissolution gradually (from 85% to 90%). This figure also indicates the need for ferric to be used in the thiocyanate leaching process with respect to the intermediate species produced.

69 4.6.4 Reagent Consumption

Table 22 Reagent composition and the result of gold extraction and sodium thiocyanate consumption on KI/SCN mixture

Sample ID Residue Au Au Extraction Reagent & Grade Head Au Grade (g/t) (%) Consumption (kg/t)

Composition (g/t) Calculated Measured Measured NaSCN H2SO4

KI (0.1 M) Only 0.107 1.94 1.98 94.46 13.23

KI 0.1 M + 0.05 M SCN 0.114 1.95 1.98 94.15 3.69 12.04

KI (0.1 M) + 0.1 M SCN 0.108 1.91 1.98 94.36 8.66 15.39

0.001 KI + SCN (0.1 M) 0.500 2.23 1.98 77.61 2.55 27.32

0.002 KI + SCN (0.1 M) 0.285 1.96 1.98 85.45 5.46 24.17

0.005 KI + SCN (0.1 M) 0.241 1.92 1.98 87.43 5.89 25.15

0.01 KI + SCN (0.1 M) 0.181 1.93 1.98 90.62 4.88 29.77

70 Table 23 Reagent composition and the result of gold extraction and sodium thiocyanate consumption on KI/Fe(III)/SCN mixture

Residue Au Reagent

Sample ID & Au Extraction Consumption Composition Grade Head Au Grade (g/t) (%) (kg/t)

(g/t) Calculated Measured Measured NaSCN H2SO4 0.001 KI + SCN + Fe3+ 0.141 1.93 1.98 92.71 2.83 22.09 0.002 KI + SCN + Fe3+ 0.136 1.78 1.98 92.35 3.63 25.80 0.005 KI + SCN + Fe3+ 0.125 1.88 1.98 93.34 3.74 25.15

0.01 KI + SCN + Fe3+ 0.121 2.06 1.98 94.11 3.82 28.16

0.02 KI + SCN + Fe3+ 0.115 2.05 1.98 94.40 4.60 28.84

In these two tests (SCN/KI and SCN/KI/Fe(III)), the consumption of NaSCN and H2SO4 were almost similar for both test results as shown in Table 22 and Table 23. The presence of ferric in the test with KI/SCN/Fe(III) tests did not affect the consumption of SCN. However, increasing the KI concentration to 0.1 M increased the SCN consumption. The average consumption of thiocyanate was about 6 times higher than cyanide. The average NaCN consumption was 0.5 kg/t, while NaSCN consumption was 3 kg/t for the SCN/KI and 3.5 kg/t for the SCN/KI/Fe(III). However, SCN consumptions in these tests were higher than those seen in the SCN/Fe(III) mixture. Thiocyanate consumption could be due to the formation of various complexes other than the gold complex.

71 4.7 Hydrogen Peroxide Test

Barbosa[16] postulated that the presence of Hydrogen Peroxide increases the leaching potential and maintains it within the limits for the formation of the Au-SCN complexes. Therefore, hydrogen peroxide was employed as an alternative oxidant to ferric sulphate to increase the leaching potential and also serve as an alternative oxidant to ferric due to the constraint in the use of iron. This is because iron hydrolyzes at higher pH’s (> 2.5) and thus will require robust equipment in the acidic regime.

Gold leaching tests were conducted in the acidic (1.5-1.8) medium to ascertain its effectiveness.

Freshly prepared sodium thiocyanate solutions acidified with 0.1 M H2SO4 to pH of 1.5 was used for all the tests. Sodium hydroxide was also used to adjust pH in the basic regime.

In order to determine the effect of hydrogen peroxide on the gold dissolution rate, the gold leaching tests were carried out at a thiocyanate concentration of 0.1 M with varying amounts of hydrogen peroxide ( 0, 4, 8, 10 and 14 g/L). All the experiments were conducted under atmospheric condition (two of the holes on the lid were open to atmosphere). Gold leaching tests were conducted on the Composite 72142 material. Sample was prepared as stated in Section 3.1.2. The test condition is summarized in Table 24.

Table 24 - Leaching conditions of peroxide tests

Volume of Solution 0.6 L Solid ~ 2 g/t Pulp density 40 % solid Solution pH 1.5 -2 Agitation Speed 700 rpm Atmospheric condition Air Temperature 20 -25 oC Residence Time 24 hr NaSCN Concentration 0.1 M

H2O2 Concentration 0, 4, 8, 10, 14 (g/L)

72 4.7.1 Gold Leaching with SCN Only

The test conducted without peroxide was to serve as reference point for the H2O2/SCN system. The SCN concentration used was 0.1 M. Result is shown in Table 25.

Table 22 – Leaching test result for thiocyanate leaching only at 0.1 M NaSCN

Residual Gold Calculated Measured NaSCN Gold Extraction Head Head Grade Consumption Test Condition (g/t) (%) Grade (g/t) (g/t) (Kg/t)

SCN Only 0.357 81.5 1.93 1.98 0.78

4.7.2 Effect of Hydrogen Peroxide concentration on Gold Dissolution

Figure 31 - Effect of the H2O2 concentration on gold extraction, SCN 0.15 M

73 From Figure 31, it can be seen that gold dissolution rate increased appreciably with an increase in peroxide concentration. The initial gold extraction without peroxide was 81.5% (as seen in Table 25). This increased to 90% with 4 g/L hydrogen peroxide and later peaked to 94% with 10 g/L hydrogen peroxide. A fall in gold dissolution was observed when the hydrogen peroxide concentration was increased further to 14 g/L.

The reason for the drop is not well understood, however, Barbosa [16] in their paper on using hydrogen peroxide to increase the leaching potential by re-oxidizing ferrous back to ferric to cause - the production of the intermediate species SCN2 and SCN3 and also maintain the leaching potential within the region of the formation of the gold thiocyanate complexes mentioned that new - - - intermediates species such as HOSCN, OSCN, HOSCN , OSCN2 , HO2SCN, O2SCN , are formed when concentration of H2O2 is varied. These complexes are less active and may decrease the dissolution of gold. Wilson also [35] reported HOSCN to be the main intermediate species formed from the complexation of SCN and H2O2 and mentioned that the formation of this is dependent on pH of the solution. Thus, the 14 g/L H2O2 used was too high and could possibly have a stronger effect on the Au/SCN systems, producing more inactive intermediate species which possibly slowed down the leaching rate (at 14g/L H2O2). On a more practical note, the leaching of gold in thiocyanate solution with the addition of peroxide can be done sequentially aiming at keeping the leaching potential within a limit where the two gold thiocyanate complexes form. In that way, a lower amount of hydrogen peroxide may be used. More work should be done to expediate this process.

4.7.3 Effect of Peroxide Concentration on Leaching Potential

The leaching potentials were continuously monitored and measured with Ag/AgCl electrode. Higher concentrations of hydrogen peroxide recorded higher leaching potentials. From Figure 32, it can be seen that the highest leaching potential was recorded for hydrogen peroxide concentration of 14 g/L. This potential increased gradually with time. The potential recorded for the 14g/L was higher than the other potentials and the potential region (500 mV-650mV) for the formation of the two gold thiocyanate complexes. At such a high potential, the dominant species becomes HOSCN

74 which might have inhibited the dissolution process. This could have contributed to the lower gold dissolution recorded for this test. The potentials of the other tests with H2O2 concentrations below 14 g/L decreased slowly with time. However, they were found to be within the formation region (500 mV - 650 mV) of gold dissolution.

Figure 32 - Effect of the H2O2 concentration on the potential readings, SCN concentration: 0.10 M

4.7.4 Effect of Hydrogen Peroxide Concentration on NaSCN Consumption

The residual sodium thiocyanate was analysed after the gold leaching tests. The leaching solutions were orange signifying the presence of little amount of Fe(III). This colour was enhanced with the addition of few drops of 1 M ferric sulphate solution which served as an indicator for the sodium thiocyanate titration. The sodium thiocyanate was determined argentimetrically with 2-3 drops of

HNO3 acid to increase intensity of the solution colour and titrated with 0.1 M AgNO3 solution. A colour change from deep red to white precipitate was observed. Thiocyanate consumption was calculated according to equation in Appendix A3.

75 The consumption of SCN increased with an increase in hydrogen peroxide concentration evident in Figure 33. The consumption of SCN from the test without peroxide was pretty low, at 0.78 Kg/t.

The decrease in the peroxide consumption for the 14g/L H2O2 could possibly be the formation of inactive complexes as mentioned above. The experimental results discussed in this section point towards the need for further investigation.

Figure 33 - Effect of the H2O2 concentration on thiocyanate consumption, SCN 0.10 M

76 4.8 Kinetic Leach Test

A kinetic test was conducted to determine the leaching kinetics of the SCN/Fe(III) system. Gold leaching experiments were conducted at selected times 1, 2, 4, 8, 12 and 24 hr in a fresh SCN/Fe(III) solution. The leaching test was completely stopped after each of the times (1, 2, 4, 8, 12 and 24 hr) and analysed for solution and solid (leached residue) grade by AAS and fire assay, respectively. The concentration of NaSCN and Fe(III) used were 0.15 M and 0.1 M respectively. Test was conducted on the 72142 B composite sample. The leaching condition is summarized in Table 26.

Table 23 – Leaching condition for kinetics test

Volume of Solution 0.6 L Pulp density 40 % solid Solution pH 1.5-1.8 Agitation Speed 700 rpm Atmospheric condition Air Temperature 20 -25 oC Fe(III) Concentration 0.1 M NaSCN Concentration 0.15 M

4.8.1 Effect of Leaching Time on Gold Dissolution

Results of the leaching kinetics is plotted in Figure 34. In the kinetics plot, there was a sharp increase in gold dissolution for the first 2 hr leading to a high gold extraction of close to 90%. The kinetics tends to decrease after 2 hr and the plot starts to plateau toward 24 hr with minimum increase in gold dissolution (91.6% at 24hr). The kinetics exhibited by the interrupted leaching test indicated that sodium thiocyanate has the ability to dissolve gold quickly and requires just about 2 hr – 4 hr to dissolve most of the available gold. This agrees with work done by Li et al [28].

77

Figure 34 - Kinetics of Gold dissolution in thiocyanate solution, (SCN 0.15 M and Fe(III) 0.1 M)

Nicol et al (1984) said though the rate of gold leaching from ores by cyanidation was a complex kinetic problem encompassing chemical, particle size distribution, mass-transport, and mineralogical factors, it could be expressed by a relatively simple expression as given by

Equation 1:

푑[퐴푢] 2 − 푝,푡 = 푘 ([퐴푢] – [퐴푢] ) 푑푡 푝 푝,푡 푝,푒 where

[Au]p,t is the concentration of gold in the ore at time t

[Au]p,e is the corresponding quantity at infinite time of leaching (the minimum achievable residue grade)

kp is a rate constant.

The kinetic data obtained for this test was fitted to the model.

78 [퐴푢0 + 푘푝푡 (퐴푢표 − 퐴푢푝푒)] 퐴푢푡 = [푘푝푡(퐴푢0 − 퐴푢푝푒) + 1]

Where Auo is the head grade of the ore (prior to leaching)

The curve was calculated from the integrated form of Equation 1, and best-fit values for the parameters Aupe and kp were derived from a non-linear least squares treatment of the data as provided in Appendix G. The agreement between the raw tails data (dots) and the predicted tails -1 data (curve) as expressed in Figure 35 can be seen to be good, giving kp and Aupe to be 72.1 hr and 0.17 g/t, respectively. The profile shows that 89.7 % of the contained gold was leached within 1-2 hr and slowed down with little or no further leaching (no decrease in tails gold grade) between 4 and 24 hr.

Figure 35 – Kinetic leaching model fitting

79 4.8.2 Effect of Leaching Time on Thiocyanate Consumption

The thiocyanate consumption increased gradually with increased in leaching time and slowed down after 12 hr as seen in Figure 36. The thiocyanate consumption increased from 0.71 kg/t at time 1 hr to 1.58 kg/t at time 24 hr. The increase in thiocyanate consumption at time 24 hr is about twice that at time 1 hr.

Figure 36 - Effect of leaching time on thiocyanate consumption

80 4.9 Mixture of Reagents

Finally, a combination of oxidants and reagents were tested. Different oxidants, complexants and additives were combined in different proportions to determine their effects on gold dissolution. Oxygen, hydrogen peroxide, lead nitrate and ferric sulphate were used together with thiocyanate.

The gold leaching experiments were conducted in the acidic regime to determine the behaviour of thiocyanate in those conditions. Oxygen was sparged into the slurry using an oxygen sparger. This was inserted into the slurry throughout the leaching test.

- Pb(NO3)2 is known to increase gold dissolution in CN systems by releasing its divalent cations Pb2+ to speed up the leaching reaction. It was added in very small quantity to determine the effect on the gold dissolution rate.

All other leaching parameters were maintained in the concentration outlined in Table 27. The tests were conducted on the 72142 Composite oxide material. The gold extractions and reagent consumptions recorded for these series of tests are shown in Table 28.

Table 24 – Leaching condition for mixture of reagents

Volume of Solution 0.6 L Pulp density 40 % solid Solution pH 1.5-1.8 Agitation Speed 700 rpm Atmospheric condition Air Temperature 20 -25 oC Fe(III) Concentration 0.2 M & 0.5 M NaSCN Concentration 0.05 M & 0.1 M

PbNO3 50 g/t

Oxygen 3 LPM

H2O2 3.5 g/L

81 Table 25 – Results of gold leaching experiments conducted under different leaching conditions

Sample ID Residue Au Reagent Au Extraction Consumption & Grade Head Au Grade (g/t) (%) (kg/t)

Composition (g/t) Calculated Measured Measured NaSCN H2SO4 3+ Fe 0.2 M + SCN 0.05 M + O2 0.21 1.94 1.98 89.24 1.88 18.60 3+ Fe 0.2 M + SCN 0.05 M + O2 +

H2O2 0.19 1.95 1.98 90.27 3.22 12.32

3+ Fe 0.5 M + SCN 0.1 M + O2 + H2O2 0.18 1.93 1.98 90.88 2.60 7.04 3+ Fe 0.2 M+ SCN 0.05 M+ H2O2 +

Pb(NO3)2 0.17 1.89 1.98 90.93 3.99 13.92 3+ Fe (0.5 M), SCN(0.1 M), H2O2,

Pb(NO3)2 0.15 1.90 1.98 92.20 6.91 13.66 3+ Fe (0.2 M), SCN(0.05 M), O2,

Pb(NO3)2 0.23 1.81 1.98 87.33 2.60 11.95 3+ Fe (0.2 M), SCN(0.05 M), O2,

H2O2, Pb(NO3)2 0.22 1.89 1.98 88.60 3.39 6.50 3+ Fe (0.5 M), SCN(0.1 M), O2, H2O2,

Pb(NO3)2 0.18 1.86 1.98 90.28 6.56 6.77

The results obtained in this table have been graphed in Figure 37 and Figure 38. The thiocyanate and ferric concentrations were varied in each of them. The effect of the addition of the different reagents to these have been grouped in the

82 4.9.1 Effect of Oxygen, Lead Nitrate and Hydrogen Peroxide on the Gold Extraction

SCN 0.05 M Fe(III) 0.2 M 92 91.1 90.9 91 90.3 90 89.2 89 88.6

88 87.3

Gold Extraction (%) Extraction Gold 87

86

85 - O2 O2 +H2O2 H2O2 + O2 + O2 + H2O2 + Pb(NO3)2 Pb(NO3)2 Pb(NO3)2 Additives

Figure 37 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold extraction

The gold extraction for SCN and Fe(III) at 0.05 M and 0.20 M, respectively, without any additive was 91.1% (as seen Figure 37). In the Figure 37, the addition of oxygen, hydrogen peroxide and lead nitrate to the SCN/Fe(III) solution insignificantly affected the gold extraction reaction.

Even though there was insignificant increase in the gold extraction in each of these tests, it was observed that leaching potentials were a little higher (ORP ~50 mV higher) with tests with the addition of 3.5 g/L hydrogen peroxide than those without peroxide addition. However, the little increase in the leaching potential had no impact on the gold extraction. The highest gold extraction being 92.2% was recorded for a combination of 0.5 M Fe(III), 0.1 M SCN, 3.5 g/L H2O2 and 50 g/t Pb(NO3)2 as seen in Figure 38.

83 SCN 0.10 M, Fe(III) 0.50 M 93.0 92.2 92.0 90.9 91.0 90.3 90.0

89.0

88.0

Gold Gold Extraction(%) 87.0

86.0

85.0 O2 + H2O2 H2O2 + Pb(NO3)2 O2 + H2O2 + Pb(NO3)2

Additives

Figure 38 Effect of oxygen, lead nitrate and hydrogen peroxide on the Gold dissolution

Moreover, the addition of Pb(NO3)2 to enhance the gold extraction did not seem to have any effect on the gold extraction. This could be due to the mineralogy of the ore, since the treated ore is not refractory, and the quantities of sulphide recorded from the analysis in Section 3.2.3 was not high.

Leaching potentials were observed not to be affected by the addition of Pb(NO3)2 and oxygen. The leach solution in these tests were still in its deep red colour signifying the presence of the FeSCN2+.

The addition of H2O2 affected the colour of the leaching solution by changing it to light orange. This shows that peroxide affected the thiocyanate and the ferric ions complexation by unknown means.

84 4.10 Adsorption Test using Activated Carbon

The adsorption test was carried out on filtered gold solution. Approximately, 1 L of pregnant gold leach solution was generated using the 72142 composite gold ore sample with the leaching conditions as listed below:

Condition 1

• NaSCN concentration – 0.1 M • Ferric Sulphate concentration – 0.1 M

Condition 2

• NaSCN concentration 0.1 • Ferric Sulphate concentration 0.05 • Potassium Iodide 0.01

Condition 3

• NaSCN concentration – 0.2 M

The initial gold concentration, pH and thiocyanate concentration of the filtered solution were measured for each of the tests (values recorded in Table 29). The effect of mass of carbon added was investigated. These were conducted to determine the amenability of gold adsorption onto carbon from thiocyanate liquors. Prior to the adsorption test, activated carbon was sieved through a 1.19 mm sieve to remove all fines and soaked/washed with 0.1 M hydrochloric acid. This was rinsed very well with distilled water and dried in an oven.

Five different tests with respect to carbon concentration (0.25, 0.5, 5, 10, 20) g/L were conducted. For each test, 100 ml of solution were measured out into 250 mls plastic bottles and specified masses of the treated virgin carbon, calculated from the carbon concentration was added to each bottle. The contents of the flask were agitated at 500 rev per minute on a mechanical shaker for 4 hr. Samples (of volume of 5 mL) were taken at selected times 10, 30, 60, 120, 240, 360 and 420 mins for gold analysis. DIBK was immediately added to these samples to prevent further reaction. 85 The initial solution composition of the three samples before the carbon adsorption are summarised in Table 29. NB: Because the initial gold concentrations were close to each other the gold in concentration data was not normalised.

Table 26 Initial composition for the three solution samples

Condition 1 Condition 2 Condition 3

Gold in solution (ppm) 0.97 1.001 0.98

pH 1.98 2.0 2.06

NaSCN concentration (Kg/t) 9.16 9.4 18.6

4.10.1 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 1

The solution chemistry of test condition 1 was 0.1 M SCN, and 0.05 M Fe(III). The initial concentration of gold before carbon addition for test condition 1 was 0.97 ppm.

Adsorption of gold in this solution was measured and the rate of adsorption measured with variation in carbon concentration. The recovery of gold as seen in Figure 39 shows that increasing carbon concentration increases the rate of adsorption. The adsorption profile for carbon concentration at 0.25 g/L and 0.5 g/L showed very low adsorption rate. The highest recovery obtained after 4 hrs for the 0.25 g/L and 0.5 g/L was about 43.3 % and 53 % respectively. However, carbon concentrations of 5 g/L, 10 g/L and 20 g/L all gave greater than 98 % recovery at time 4 hr. The adsorption was very fast for 10 g/l and 20 g/L and almost close to completion within 0.5 hr.

86 1.20

1.00

0.25 g/L Carbon 0.80

0.5 g/L Carbon 0.60 5 g/L Carbon

0.40

10 g/L Carbon Au Concentration Au (mg/L)

0.20 20 g/L Carbon

0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Time (h)

Figure 39 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.1 M, Fe (III) 0.05 M, pH 1.98, Temp 25 oC

4.10.2 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 2

The solution chemistry of test condition 2 was 0.1 M SCN, 0.02 M K1 and 0.05 M Fe(III). The initial concentration of gold before carbon addition was 1.001 ppm.

Condition 2 behaviour was similar to that in condition 1. Low carbon addition gave lower adsorption rate. The recovery of gold as seen in Figure 40 showed that increasing carbon concentration increases the rate of adsorption. The recovery for carbon concentration at 0.25 g/l and 0.5 g/L after 4 hrs gave 44 % and 50 %, respectively, which was slightly higher than condition 1. However, gold adsorptions with carbon concentrations of 5 g/L, 10 g/L and 20 g/L approached completion after 0.5 hr, yielding gold recovery of between 99 % and 100% within 4 hr.

87 1.20

1.00

0.25 g/L Carbon 0.80 0.5 g/L Carbon

0.60 5 g/L Carbon

10 g/L Carbon

0.40 Au Concentration Au (mg/L) 20 g/L Carbon

0.20

0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Time (h)

Figure 40 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.1 M, Fe (III) 0.05 M, KI 0.02 M, pH 2, Temp 25 oC

4.10.3 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 3

The solution chemistry of test condition 3 was 0.2 M SCN. The initial concentration of gold before carbon addition was 0.98 ppm.

According to the results presented in Figure 41 for Condition 3, the adsorption of gold onto carbon increased with an increase in carbon concentration. This behaviour is similar to that in conditions 1 and 2. Low carbon addition gave lower adsorption rate. The recovery for carbon concentration at 0.25 g/l and 0.5 g/L after 4 hrs gave 53.67 % and 54.38 %, respectively, which was slightly higher than from conditions 1 and 2. However, carbon concentrations of 5 g/l, 10 g/l and 20 g/L were fast and approached completion after 0.5 hr yielding gold adsorption of between 99 % and 100% within 4 hr. 88

Figure 41 - Adsorption of gold with variation in activated carbon in acidic thiocyanate solution. SCN 0.2 M, pH 2.06, Temp 25 oC

89 4.10.4 Comparison of the Three Conditions

A comparison between the three conditions is presented in Figure 42. Apart from carbon concentration at 0.25 and 0.5 g/L, the rest are overlapping , signifying that the adsorption of gold from thiocyanate solutions may not be impacted by the presence of potassium iodide (at 0.02 M) and Fe(III) (at 0.05 M).

(a) Carbon concentration 0.25 g/L (b) Carbon concentration 0.5 g/L

(c.) Carbon concentration 5 g/L (d) Carbon concentration 10 g/L

90

(e) Carbon concentration 20 g/L

Figure 42 – Comparison of the carbon adsorption between the 3 conditions study. (a) runs at 0.25 g/L carbon at the 3 conditions, (b) runs at 0.5 g/L carbon at the 3 conditions, (c) runs at 5 g/L carbon at the 3 conditions, (d) runs at 10 g/L carbon at the 3 conditions, (e) runs at 20 g/L carbon at the 3 conditions

From Figure 42, the carbon concentrations of 0.25 g/l and 0.5 g/L loaded gold slowly and incompletely. Increasing contacting time could probably increase the adsorption extent. However, carbon concentrations above 5 g/L is very effective and showed very fast rate of adsorption.

One interesting thing to note is the slowness of condition 1 which had external Fe(III) added to the solution. However, the test without external iron showed the best adsorption at lower carbon concentration. The reason for the slight slowness of condition 1 has been discussed by Li et al.[40] It is believed that activated carbon promotes the reduction of dissolved ferric ion in the thiocyanate leach solution to ferrous iron, however this process in adsorption is not desirable as it has been seen to slightly slow down adsorption rate. This assumption will need further investigation.

91 Moreover, an average gold loading onto carbon with all three conditions showed 2100 g/t and 48 g/t of gold adsorbed at time 4 hrs (results are shown in Appendix C2). The gold loading did not reach equilibrium

Overall, the results obtained in these tests are similar to that reported by Li et al[40]. They reported that activated carbon is very effective in adsorbing gold from thiocyanate solutions yielding a recovery above 98% in 1 h for a carbon concentration of 10g/L. Also, from their studies they reported that the recovery of gold is not dependent on the ratio of solution volume (mL)/amount of AC (g), indicating that the system is suitable to a wide range of ratios under the experimental conditions they studied (Au 10 mg/L, Fe (III) 1 g/L, SCN 0.05 M, 23 °C, activated carbon content 10 g/L, at pH 1.8).

92 Chapter 5: Summary, Conclusions and Recommendations Future Work

5.1 Summary and Conclusions

a. Gold dissolution in thiocyanate solutions has been evaluated. The study focused on leaching an oxide gold ore from Goldcorp Coffee Project Yukon- Canada, in thiocyanate solution. Three samples namely Supremo 68151 A, Supremo 68151 B and 72142 composites received from Goldcorp Coffee Project, Yukon, Canada were used for the study. The solution chemistry of the gold-ferric-thiocyanate system was reviewed to understand reaction chemistry and possible mechanisms for gold leaching in thiocyanate solutions. b. The study began with ore preparation of the rock samples that were received. This was crushed in a laboratory jaw and cone crusher and further milled in a laboratory rod mill to reduce the particle size and to expose the gold surface for the leaching process. Representative samples were taken for chemical and mineralogical analysis. The head grade analysis for the three samples; Supremo 68151 A, Supremo 68151 B and 72142 composite material were 1.28 g/t, 1.45 g/t and 1.98 g/t, respectively. Sub samples were taken for the leaching tests. c. A baseline cyanide leaching of the three samples were conducted with 500 ppm cyanide with the leach vessel left half opened to allow air into the slurry. Calcium hydroxide was used to control the pH of the solution to be above 10.5. The leaching test lasted for 24 hr with periodical sampling at various times during the leaching process. Gold extraction obtained for Supremo 68151 A, Supremo 68151 B and 72142 composites were 94.6 %, 94.6 % and 96.3 %, respectively for time 24 hr and 95%, 94.4 % and 97 % for 32 hr. Results obtained showed the amenability of the ore to cyanidation and also indicated that the oxide ore is free milling. d. Ferric ion concentration was varied at 0.05 M – 0.5 M. The highest gold extraction obtained at this concentration range was 91.1 % at a SCN concentration of 0.1 M and Fe(III) of 0.2 M. Fe(III) concentration above 0.2 M was found to insignificantly improve gold dissolution. However, from all the leaching tests conducted with Fe(III), it can be concluded that high Fe(III) concentration ( between 0.1 M and 0.2 M) helps in keeping

93 thiocyanate stable and in the auto reduction of Fe(III) and thus reduces thiocyanate consumption. e. Increase in thiocyanate concentration was found to increase gold extraction. Gold extraction increased from 11.7 % to 91 % as SCN concentration was increased from 0.005 M to 0.10 M. However above 0.05 M SCN, the increase of gold dissolution was found to be marginal. f. The addition of potassium iodide to the SCN and SCN/Fe(III) solution increased gold extraction. When the two reagents (KI and SCN) were put together, the highest gold extraction of 94.1 % was obtained at KI concentration of 0.05 M. On the other hand, when the three reagents were put together, a synergistic effect was achieved yielding a 94.4 % gold extraction with lower KI concentration of 0.02 M. Overall results obtained for KI/SCN/Fe(III) is encouraging and would need further investigation. g. The use of hydrogen peroxide was found to be effective in the dissolution of gold in the thiocyanate system. Approximately 94 % gold was extracted at 0.1 M SCN and 10 g/L

H2O2. A slight drop of gold extraction was seen when H2O2 was further increased to 14 g/L. h. Leaching with SCN only in an atmospheric condition without the addition of any oxidant was found to be possible. However, it was observed that the gold dissolution was aided by internal iron which was leached from the ore by acid added. When SCN concentration was increased from 0.1 M to 0.6 M, gold extraction increased from 81.5 to 90.9 % gold extraction. i. The leaching kinetics of the SCN/Fe(III) system proved to be very fast yielding a rate constant of 72.1 hr-1 when fitted to the leach model proposed for CIL/CIP systems by Nicol et al. The gold extraction was very fast, obtaining 90 % after 2 hr of leaching and 91.6 % at time 12 hr, at which it reached a plateau for the condition studied. j. Carbon adsorption test performed on the three “best” leaching conditions (that obtained the highest gold extraction) showed that adsorption of gold onto carbon in thiocyanate solution is possible yielding > 98 % gold adsorption in less than 1 hr.

94 5.2 Recommendations for Future Work

The leaching of gold in thiocyanate is still in the development stage and further research is recommended in the following areas:

a. It is important to investigate the leaching of gold in the KI/SCN/Fe(III) mixture. The results obtained from this study show promise and further work should be continued to optimize gold extraction. b. Another area of research would be finding a suitable oxidant to leach gold in thiocyanate solution over a wider range of pH which will make the process more convenient. Hydrogen peroxide was very efficient in the acidic regime but less efficient above pH of 4. Further investigation can be done to ascertain its effectiveness. c. The safety issues pertaining to the use of hydrogen peroxide as an oxidant in the leaching of gold in acidic thiocyanate should be investigated. By-products such as HCN, formed from thiocyanate degradation in the acidic regime is a progenitor of sodium cyanide. This HCN is a highly toxic and poisonous substance that the gold mining industry is finding ways to get rid of. The formation of HCN as a by-product of SCN degradation occurs at a very high oxidation potential making its formation impossible, however, this should be thoroughly investigated. d. The leaching of the different gold ore types (carbonaceous, sulphidic, etc.) in thiocyanate solutions should be investigated. For example, most ore types are not amenable to cyanidation and that has been one of the drivers for the pursuit of alternative reagents. Most of these ores would need either an initial pretreatment option, addition of additives or leaching at higher temperatures to achieve satisfactory results. e. The current testwork on adsorption was done on filtered gold solutions. The testwork should be extended to encompass in-pulp gold adsorption. Gold desorption should be studied as well. f. The current testwork as performed for only one leach feed particle size of P80 of 150 microns. Gold extraction and reagents consumptions should be studied with respect to leach feed particle size. g. This SCN leaching study should be extended to leaching gold from pre-treated refractory ores and concentrates, for example from bacteria leaching, roasting and pressure oxidation. 95 The products of these pre-treatments are acidic and will sync with SCN gold leaching in acidic media. Current industry-practice is to wash off the acid in counter-current decantation (CCD) thickeners from the pre-treated material and then adjust the pH with lime prior to cyanidation. Elimination of the acid washing and lime dosage will reduce capital and operating costs. Furthermore, lime dosage to raise the pH of the acid solution to slurry pH of 10.5 can result in high slurry viscosity that can impair all mass transfer reactions like gold leaching and gold adsorption from the slurry. h. No elution experiments have been undertaken on the loaded carbon. That should be investigated along with any effects (if any) that leach additives may have on gold loading on carbon and gold desorption from carbon.

96 References

1. Laitos, J.G., The current status of cyanide regulations. Engineering and Mining Journal, 2012. 213(2): p. 34-40.

2. Aylmore, M., Alternative lixiviants to cyanide for leaching gold ores. Vol. 15. 2005.

3. Eisler, R. and S.N. Wiemeyer, Cyanide hazards to plants and animals from gold mining and related water issues, in Reviews of environmental contamination and toxicology. 2004, Springer. p. 21-54.

4. Eugene, W.W.L. and A.S. Mujumdar, Gold extraction and recovery processes. Minerals, Metals, and Materials Technology Centre, National University of Singapore, 2009.

5. Marsden, J. and I. House, The chemistry of gold extraction. 2006: SME.

6. Li, J. and J.D. Miller, A REVIEW OF GOLD LEACHING IN ACID THIOUREA SOLUTIONS. Mineral Processing and Extractive Metallurgy Review, 2006. 27(3): p. 177- 214.

7. Vaughan, J., The process mineralogy of gold: the classification of ore types. Jom, 2004. 56(7): p. 46-48.

8. Yang, X., et al., Thiourea–thiocyanate leaching system for gold. Hydrometallurgy, 2011. 106(1): p. 58-63.

9. Akcil, A., A new global approach of cyanide management: international cyanide management code for the manufacture, transport, and use of cyanide in the production of gold. Mineral Processing & Extractive Metallurgy Review, 2010. 31(3): p. 135-149.

10. Qi, P.H. and J.B. Hiskey, Dissolution kinetics of gold in iodide solutions. Hydrometallurgy, 1991. 27(1): p. 47-62.

11. Angelidis, T.N., K.A. Kydros, and K.A. Matis, A fundamental rotating disk study of gold dissolution in iodine-iodide solutions. Hydrometallurgy, 1993. 34(1): p. 49-64.

12. Sharma, P. and T.S. Bhatti, A review on electrochemical double-layer capacitors. Energy Conversion and Management, 2010. 51(12): p. 2901-2912.

13. Xu, B., et al., A review of thiosulfate leaching of gold: Focus on thiosulfate consumption and gold recovery from pregnant solution. Metals, 2017. 7(6): p. 222.

14. White, H., The solubility of gold in thiosulphates and thiocyanates. South African Journal of Science, 1905. 1(1): p. 211-215.

97 15. Muir, D.M. and M.G. Aylmore, Thiosulphate as an alternative to cyanide for gold processing – issues and impediments. Mineral Processing and Extractive Metallurgy, 2004. 113(1): p. 2-12.

16. Barbosa-Filho, O. and A. Monhemius, Leaching of gold in thiocyanate solutions: Part 1: chemistry and thermodynamics. Transactions of the Institution of Mining and Metallurgy- Section C-Mineral Processing, 1994. 103: p. C105.

17. Zhang, H. and D.B. Dreisinger, The kinetics for the decomposition of tetrathionate in alkaline solutions. Hydrometallurgy, 2002. 66(1-3): p. 59-65.

18. Marchbank, A.R., et al., Gold recovery from refractory carbonaceous ores by pressure oxidation and thiosulfate leaching. 1996, Google Patents.

19. Senanayake, G., Gold leaching in non-cyanide lixiviant systems: critical issues on fundamentals and applications. Minerals Engineering, 2004. 17(6): p. 785-801.

20. Fleming, C., et al., Recent advances in the development of an alternative to the cyanidation process: Thiosulfate leaching and resin in pulp. Minerals and metallurgical processing, 2003. 20(1): p. 1-9.

21. Örgül, S. and Ü. Atalay, Reaction chemistry of gold leaching in thiourea solution for a Turkish gold ore. Hydrometallurgy, 2002. 67(1): p. 71-77.

22. Zheng, S., Y.-y. Wang, and L.-y. Chai, Research status and prospect of gold leaching in alkaline thiourea solution. Minerals Engineering, 2006. 19(13): p. 1301-1306.

23. Ximing, L., et al., Chlorine leaching of gold-bearing sulphide concentrate and its calcine. Hydrometallurgy, 1992. 29(1-3): p. 205-215.

24. Gure, A., Anaerobic dissociation of thiocyanate under controlled pH and temperature. 2016.

25. Fortmann, S.P., et al., Indirect measures of cigarette use: Expired-air carbon monoxide versus plasma thiocyanate. Preventive medicine, 1984. 13(1): p. 127-135.

26. Broadhurst, J. and J. Du Perez, A thermodynamic study of the dissolution of gold in an acidic aqueous thiocyanate medium using iron (III) sulphate as an oxidant. Hydrometallurgy, 1993. 32(3): p. 317-344.

27. Li, J., et al., Thiocyanate hydrometallurgy for the recovery of gold. Part I: Chemical and thermodynamic considerations. Hydrometallurgy, 2012. 113-114: p. 1-9.

28. Li, J., et al., Thiocyanate hydrometallurgy for the recovery of gold.: Part II: The leaching kinetics. Hydrometallurgy, 2012. 113-114: p. 10-18.

98 29. Itabashi, E., Spectroelectrochemical characterization of iron (III)-thiocyanate complexes in acidic thiocyanate solutions at an optically transparent thin-layer-electrode cell. Inorganic Chemistry, 1985. 24(24): p. 4024-4027.

30. Baghalha, M., The leaching kinetics of an oxide gold ore with iodide/iodine solutions. Hydrometallurgy, 2012. 113-114: p. 42-50.

31. Wang, H.-x., et al., Study on gold concentrate leaching by iodine-iodide. International Journal of Minerals, Metallurgy, and Materials, 2013. 20(4): p. 323-328.

32. Li, J., et al., Thiocyanate hydrometallurgy for the recovery of goldPart III: Thiocyanate stability. Hydrometallurgy, 2012. 113-114: p. 19-24.

33. Van Asbeck, B., Oxygen toxicity: Role of hydrogen peroxide and iron, in Antioxidants in therapy and preventive medicine. 1990, Springer. p. 235-246.

34. Nicol, M., R. Paul, and C. Fleming. The chemistry of the extraction of gold. 1987. Mintek.

35. Wilson, I. and G. Harris, The oxidation of thiocyanate ion by hydrogen peroxide. II. The acid-catalyzed reaction. Journal of the American Chemical Society, 1961. 83(2): p. 286- 289.

36. Davis, A. and T. Tran, Gold dissolution in iodide electrolytes. Hydrometallurgy, 1991. 26(2): p. 163-177.

37. Konyratbekova, S.S., et al., Thermodynamic and kinetic of iodine–iodide leaching in gold hydrometallurgy. Transactions of Nonferrous Metals Society of China, 2015. 25(11): p. 3774-3783.

38. McGrew, K.J. and J.W. Murphy, Iodine leach for the dissolution of gold. 1985, Google Patents.

39. Barbosa-Filho, O. and A.J. Monhemius, Iodide—thiocyanate leaching system for gold, in Hydrometallurgy ’94: Papers presented at the international symposium ‘Hydrometallurgy ’94’ organized by the Institution of Mining and Metallurgy and the Society of Chemical Industry, and held in Cambridge, England, from 11 to 15 July, 1994. 1994, Springer Netherlands: Dordrecht. p. 425-440.

40. Li, J., et al., Thiocyanate hydrometallurgy for the recovery of gold. Part IV: Solvent extraction of gold with Alamine 336. Hydrometallurgy, 2012. 113-114: p. 25-30.

41. Fortune, W. and M. Mellon, Determination of iron with o-phenanthroline: a spectrophotometric study. Industrial & Engineering Chemistry Analytical Edition, 1938. 10(2): p. 60-64.

99 Appendices

Appendix A: Analytical Methods

A1: Preparation of Au-Standards for AAS

Organic Method from Stock solutions

a. Gold Stock Solution (100 ppm) (g/ml)

Pipette 25 ml of 1000 ppm standard gold solution into a 250 ml volumetric flask containing about 150 ml of distilled water and 50 ml of concentrated hydrochloric acid which has been allowed to cool. Make up to the mark and mix well.

b. Gold Stock Solution (50 ppm in DIBK)

Pipette 50 ml of 100 ppm gold stock solution into a 250 ml flask. With a pipette, add 100 ml DIBK, containing 10g/L Aliquat 336, and shake vigorously for 60 seconds.

c. Gold Working Standard (3, 2, 1, 0.5, 0.1 etc. ppm Au)

Pipette the respectively amount of the 50 ppm Stock solution into a clean, dry 100 ml volumetric flask. Dilute to the mark with DIBK/Aliquot 336 and shake well to mix.

Aqueous Method from stock solution

a. Gold Stock Solution

0.100 g/l (100 mg/L) Au - Into a 100 ml volumetric flask, pipette 30 ml of 1N NaOH and 10 ml of 10 g/l NaCN solution. Mix thoroughly, then pipet 10 ml of the g/l Au stock solution into the flask and dilute to the mark with distilled water.

100 a. Working Solution

0.05 g/l (50 mg/l) Au - Into a 100 ml volumetric flask, pipet 15 ml of 1N NaOH and 10 ml of 10 g/l NaCN solution. Mix thoroughly, then pipette 5ml of the 1 g/l Au stock solution into the flask and dilute to the mark with distilled water b. Gold Standard

g/l (10 mg/l) Au - Into a 100 ml volumetric flask, pipette 10 ml of the 0.100 g/l Au (Solution I) stock solution. Bring to volume with NaOH (0.1 N) -NaCN (1 g/l) solution. c. Gold Standard

g/l (1 mg/l) Au - Into a 100 ml volumetric flask, pipette 10 ml of the g/l Au (Solution III) stock solution. Bring to volume with NaOH (0.1 N)- NaCN (1 g/L) solution.

101 A2: Free Cyanide Titration Procedure

Reagents:

• 0.2% (in Acetone) rhodamine as indicator

• Weigh out 3.2647 g of dried AgNO3, dissolve in distilled water, and dilute to 1000 mL (1 mL = 1 3 mg CN).

Titration Procedure:

a. Filter slurry sample to a clear solution and pipette 10ml into a 100ml conical flask. b. Add about 2 to 3 drops of rhodamine indicator. This gives a pale-yellow coloration. c. Fill the 50ml burette with Silver Nitrate solution to above the zero mark. d. Titrate slowly with the Silver Nitrate solution, swirling at the same time until a pale pink colour is seen. (This is best seen under a white background) + e. Note the volume of the Silver nitrate used: V(Ag ) and calculate concentration as below:

Cyanide reacts with Silver Nitrate (AgNO3) according to the following equation:

AgNO3 +2NaCN Na[Ag(CN)2 ]+ NaNO3

+ + - - Calculation: C(Ag ) x V(Ag ) = 2 x C(CN ) x V(CN )

+ Note: C(Ag ) = Volume of AgNO3

+ V(Ag ) = Concentration of AgNO3

- C(CN ) = Concentration of NaCN

- V(CN ) = Volume of NaCN sample taken

- + + C(CN ) = 2 x C(Ag ) x V(Ag )

- V(CN )

102 A3: Residual Thiocyanate Titration Procedure

Principle:

Thiocyanate is precipitated from the red ferric thiocyanate complex by adding silver ion (as silver nitrate solution). Silver thiocyanate is a stronger complex than ferric thiocyanate and the solution becomes colourless when all of the thiocyanate is complexed with silver. (The Ksp of AgSCN is 1.0 x 10-12).

Reagents:

• 0.1 N Silver Nitrate solution (keep in amber bottle, away from direct sunlight)

• HNO3, concentrated

• 0.1 M Ferric Sulphate solution: Dissolve 47.1 grams Fe2(SO4)3-5H2O in DI water, and dilute to 1 litre in a volumetric flask. Transfer to a plastic container.

Titration procedure:

a. Pipet an aliquot of sample solution into an erlenmeyer flask. Add a stir bar. b. For aliquots less than 100 ml, dilute to 125-150 ml with DI water. c. Add 3 ml concentrated nitric acid. d. For samples not already containing iron, add 1 ml 0.1 M Ferric Sulphate solution. If there is thiocyanate in the sample, the solution will turn dark red. e. Titrate with 0.1 N silver nitrate solution, just to the disappearance of the red colour. In samples containing abundant thiocyanate, periodic settling of precipitate might become necessary to see the red colour remaining in solution. f. Calculation: mg SCN/litre = ((Vol. titrant, ml) x (0.1 moles Ag+/ml) x (58.08 mg SCN/mole))/litres sample

103 A4: Solids Specific Gravity

1. Obtain a dry representative sample, minimum of 50g dry is required for testing. Use a rolling pin to break up any larger clumps that may have developed from drying

2. If sample has any coarse particles it will need to be sieved at 2000-microns.

5. Weigh out 25 grams (+/- 0.5) of sample into a tarred calibrated volumetric flask. Record the mass and repeat for second flask

6. Fill the flasks with distilled water halfway (leaving room in the flask for air to escape), gently swirl in the palm of your hand until entire sample is wet.

7. Place both flasks into the vacuum chamber along with a beaker of distilled water covered with a watch glass (to be used to top of the flasks after de-airing) and turn vacuum pump on.

10. Check for the production of air bubbles within the flask, this indicates that the vacuum seal is good and working. Allow the samples to remain under vacuum for 2-4 hours, periodically giving the chamber a “knock” to release the air bubbles forming.

11. When the sample is de-aired turn off the pump and allow the vacuum built up in the chamber to gradually release

12. Using the water that was placed in the vacuum chamber with the sample, top the volumetric flasks up to their 250mL line.

13. Weigh and record the mass of the volumetric flask and record in template.

14. Check that SG results are matching, if not double check volumes and re-weigh. Retest if unable to get repeatable results.

15. The solids SG can be calculated with the following formula:

Solids SG = Solids / [(Flask Mass&Water + Solids) – Mass of Flask Water & Solids]

104 Appendix B: Mineralogical Analysis Results

B1: XRD Imaging Results for Untreated Samples

a. 68151 A Sample

1RY_68151-A.raw Quartz low 56.00 % Illite/Muscovite 2M1 15.92 % Kaolinite 1A 4.40 % 150 Microcline (ordered) 5.32 % Illite/Muscovite 1M 15.16 % Rutile 0.50 % Dolomite ? 0.42 % Chalcopyrite ? 0.21 % Albite low, calcian 2.07 %

100 Sqrt(Counts)

50

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2Th Degrees

Figure 1. Rietveld refinement plot of sample 68151 - A (blue line - observed intensity at each step; red line - calculated pattern; solid grey line below - difference between observed and calculated intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual diffraction patterns of all phases.

105 b. 68151 B Sample

2RY_68151-B.raw_1 200 Quartz low 57.36 % Illite/Muscovite 2M1 15.44 % Kaolinite 1A 4.35 % Microcline (ordered) 4.69 % Illite/Muscovite 1M 15.57 % Rutile 0.46 % Dolomite ? 0.40 % Chalcopyrite ? 0.28 % 150 Albite low, calcian 1.45 %

100 Sqrt(Counts)

50

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2Th Degrees

Figure 2. Rietveld refinement plot of sample 68151 - B (blue line - observed intensity at each step; red line - calculated pattern; solid grey line below - difference between observed and calculated intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual diffraction patterns of all phases.

106 c. 72142 C Composite Sample

3RY_72142-B.raw_1 200 Quartz low 55.26 % Illite/Muscovite 2M1 15.30 % Kaolinite 1A 8.54 % Microcline (ordered) 4.24 % Illite/Muscovite 1M 10.55 % Albite low, calcian 4.93 % Biotite 1M 1.18 %

150

100 Sqrt(Counts)

50

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2Th Degrees

Figure 3. Rietveld refinement plot of sample 72142 - B (blue line - observed intensity at each step; red line - calculated pattern; solid grey line below - difference between observed and calculated intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual diffraction patterns of all phases.

107 B2: Results of ICP Analysis

Supremo Oxide Supremo Oxide Supremo T2-T4 ANALYTE DETECTION UNITS 68151 - A 68151 - B Composite 72142 - B Al 0.01 % 6.97 6.93 7.57 Ba 10 ppm 760 640 960 Be 5 ppm <5 <5 <5 Ca 0.1 % 0.4 0.5 0.3 Cr 10 ppm 200 170 250 Cu 10 ppm 20 20 10 Fe 0.01 % 2.95 2.83 4.1 K 0.1 % 2.9 2.8 2.6 Li 10 ppm 20 20 20 Mg 0.01 % 0.31 0.31 0.53 Mn 10 ppm 540 520 900 Ni 5 ppm 20 18 91 P 0.01 % 0.04 0.04 0.05 Sc 5 ppm 11 10 17 Si 0.1 % >30 >30 >30 Sr 10 ppm 230 230 150 Ti 0.01 % 0.25 0.25 0.36 V 5 ppm 85 80 107 Zn 5 ppm 36 38 46 Ag 1 ppm <1 <1 <1 As 5 ppm 1430 1540 1500 Bi 0.1 ppm 0.2 0.3 0.4 Cd 0.2 ppm <0.2 <0.2 <0.2 Ce 0.1 ppm 61.6 62.4 61.8 Co 0.5 ppm 10.4 10.7 18.3

108 Supremo Oxide Supremo Oxide Supremo T2-T4 ANALYTE DETECTION UNITS 68151 - A 68151 - B Composite 72142 - B Dy 0.05 ppm 2.96 3.16 3.72 Er 0.05 ppm 1.65 1.89 2.23 Eu 0.05 ppm 0.87 0.93 1.16 Ga 1 ppm 15 15 18 Gd 0.05 ppm 3.25 3.35 4.14 Ge 1 ppm 1 1 1 Hf 1 ppm 3 4 4 Ho 0.05 ppm 0.62 0.63 0.73 In 0.2 ppm <0.2 <0.2 <0.2 La 0.1 ppm 35.9 35.8 33.4 Lu 0.05 ppm 0.31 0.32 0.35 Mo 2 ppm 7 6 5 Nb 1 ppm 11 10 11 Nd 0.1 ppm 22.6 23 25.9 Pb 5 ppm 18 19 19 Pr 0.05 ppm 7.08 7.02 7.51 Rb 0.2 ppm 142 143 125 Sb 0.1 ppm 82.9 67.5 71 Sm 0.1 ppm 3.8 3.9 4.9 Sn 1 ppm 4 4 4 Ta 0.5 ppm 1 1 0.7 Tb 0.05 ppm 0.47 0.5 0.63 Th 0.1 ppm 22.4 22.8 15.6

109 Supremo Oxide Supremo Oxide Supremo T2-T4 ANALYTE DETECTION UNITS 68151 - A 68151 - B Composite 72142 - B Tl 0.5 ppm 1.5 1.6 2.9 Tm 0.05 ppm 0.26 0.28 0.34 U 0.05 ppm 15.3 15.4 20.3 W 1 ppm 6 6 10 Y 0.5 ppm 17.5 18.1 21.1 Yb 0.1 ppm 1.9 2 2.3 Zr 0.5 ppm 118 133 168 C 0.005 % 0.154 0.193 0.033 S 0.005 % 0.034 0.034 0.018 Cs 0.1 ppm 4.8 4.6 6.2

110 Appendix C: Gold Extraction Calculations

C1: Gold extraction

The gold extraction calculations are explained below:

Example of the analytical spreadsheet used for the gold extraction is shown in the Table below:

Analtyical Data and Calculated Results

Time Amount Assays Au Distribution Overall Soln Sample Soln Sample Residue Assay (g/t) Product (h) (g or mL) (mg/L or g/t) (%) (mg Au) (mg Au) (mL) Sample A Sample B Pregnant solution 0 365.2 0.000 0.0 0.000 5 0.285 0.285 2 365.2 0.939 75.6 0.343 0.0047 4 365.2 0.957 78.1 0.354 0.0048 8 337.3 0.964 73.7 0.335 0.0048 Extraction (%) 24 337.3 1.107 85.5 0.388 0.0055 Residue 231.6 0.285 14.5 0.066 Leach 85.5 Leach Head (Calc.) 231.6 1.959 14.5 0.454

Pregnant Solution 85.5 0.388 Residue 14.5 0.066 Test Head (Calc.) 1.96 100.0 0.454 Test Head (Assayed) 231.6 1.98 Accountability 99

The gold extraction was calculated

푂푣푒푟푎푙푙 푔표푙푑 푖푛 푠표푙푢푡푖표푛 % 푇표푡푎푙 퐺표푙푑 퐸푥푡푟푎푐푡푖표푛 = ∗ 100% 푂푣푒푟푎푙푙 푔표푙푑 푖푛 푠푎푚푝푙푒

Where

Overall gold in solution = ∑퐴푢𝑖푛푡푒푟푚푒푑𝑖푎푡푒 푠푎푚푝푙푒푠 + 퐴푢푝푟푒𝑔푛푎푛푡 푙푒푎푐ℎ 푠표푙푢푡𝑖표푛

Overall gold in Sample =

푇표푡푎푙 퐴푢푠표푙푢푡𝑖표푛 + 퐴푢푟푒푠𝑖푑푢푒

111 Calculated Head grade

The calculated head grade was calculated based on the overall gold in the mass of solid tested by the mass of the residue (which is the mass of solid after leaching) expressed in g/t. The calculation is as follows :

푂푣푒푟푎푙푙 푔표푙푑 푖푛 푆푎푚푝푙푒 퐶푎푙푐푢푙푎푡푒푑 퐻푒푎푑 푔푟푎푑푒 = 푀푎푠푠 표푓 푟푒푠푖푑푢푒

Where,

푂푣푒푟푎푙푙 푔표푙푑 푖푛 푆푎푚푝푙푒 = 퐺표푙푑 푖푛 푟푒푠푖푑푢푒 (푡푎푖푙푠) + ∑퐺표푙푑 푖푛 푠표푙푢푡푖표푛

Accountability

The accountability was calculated to determine the

퐶푎푙푐푢푙푎푡푒푑 퐻푒푎푑 퐺푟푎푑푒 퐴푐푐표푢푛푡푎푏푖푙푡푦 = ∗ 100 퐴푠푠푎푦푒푑 퐻푒푎푑 퐺푟푎푑푒

112 C2: Gold extraction from thiocyanate solutions

The extraction of gold from pregnant thiocyanate were calculated as follows:

푀푒푎푠푢푟푒푑 푔표푙푑 푔푟푎푑푒 ∗ 푉표푙푢푚푒 표푓 푠푎푚푝푙푒 퐴푢 푖푛 푠푎푚푝푙푒푑 푠표푙푢푡푖표푛 = (푚푔) 1000

(푎 − 푏) ∗ 푐 + ∑푑 퐶표푟푟푒푐푡푒푑 퐴푢 푐표푛푐푒푛푡푟푎푡푖표푛 푖푛 푠표푙푢푡푖표푛 = 푎

Where

a. = Total Volume of Solution (L)

b. = Total Volume of sample taken at before the specific time (mL)

c.= Measured gold grade at specific time (mg/L)

d. = Sum of Au in sample before the specific time (mg)

The results of the carbon adsorption are tabulated in Tables I – VI

113 Table I: The raw data for Condition 1 (SCN = 0.1 M Fe(III)= 0.05 M)

Carbon Mass (g) 0.05 0.05 0.5 1 2 Solution Vol (mL) 100 200 100 100 100 Carbon Conc (g/L) 0.5 0.25 5 10 20 Solution Head Grade (mg/L Au) Initial Gold Concentration = 0.97 Time (h) 0 0.970 0.970 0.970 0.970 0.970 0.17 0.840 0.890 0.289 0.107 0.091 0.5 0.700 0.821 0.138 0.031 0.003 1 0.595 0.812 0.014 0.000 0.000 2 0.541 0.799 0.003 0.000 0.000 3 0.527 0.750 0.000 0.000 0.000 4 0.508 0.710 0.000 0.000 0.000

Table II: The corrected data of Condition 1 based on the equation above

Carbon Conc (g/L) 0.25 0.5 5 10 20

0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 0.970 0.970 0.970 0.970 0.970 0.17 0.890 0.840 0.289 0.107 0.091 0.5 0.823 0.707 0.146 0.035 0.007 1 0.814 0.613 0.034 0.007 0.005 2 0.802 0.567 0.025 0.007 0.005 3 0.756 0.555 0.022 0.007 0.005 4 0.721 0.541 0.022 0.007 0.005

114 Table III: The gold loading onto carbon in g/t of Condition 1

Carbon Conc (g/L) 0.25 0.5 5 10 20 Condition 1 0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 0 0 0 0 0 0.17 320 260 136 86 44 0.5 589 526 165 94 48 1 624 715 187 96 48 2 674 807 189 96 48 3 855 829 190 96 48 4 995 858 190 96 48

Table IV: Raw data for Condition 2 SCN = 0.1 M KI = 0.01 M Fe(III) = 0.05 M

Carbon Mass (g) 0.05 0.05 0.5 1 2 Solution Vol (mL) 100 200 100 100 100 Carbon Conc (g/L) 0.5 0.25 5 10 20 Solution Head Grade (mg/L Au) Initial Gold Concentration = 1.001 Time (h) 0 1.001 1.001 1.001 1.001 1.001 0.17 0.920 0.964 0.095 0.073 0.041 0.5 0.789 0.862 0.039 0.013 0.006 1 0.696 0.799 0.014 0 0 2 0.604 0.745 0 0 0 3 0.556 0.712 0 0 0 4 0.508 0.693 0 0 0

115 Table V: The corrected data for Condition 2

Carbon Conc (g/L) 0.25 0.5 5 10 20

0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 1.001 1.001 1.001 1.001 1.001 0.17 0.964 0.920 0.095 0.073 0.041 0.5 0.865 0.796 0.042 0.016 0.008 1 0.803 0.712 0.019 0.004 0.002 2 0.752 0.634 0.007 0.004 0.002 3 0.721 0.595 0.007 0.004 0.002 4 0.705 0.559 0.007 0.004 0.002

Table VI: The gold loading onto carbon in g/t of Condition 2

Carbon Conc (g/L) 0.25 0.5 5 10 20 Condition 2 0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 0 0 0 0 0 0.17 148 162 181 93 48 0.5 546 411 192 99 50 1 792 578 196 100 50 2 997 735 199 100 50 3 1119 812 199 100 50 4 1185 884 199 100 50

116 Table VII: Raw data for Condition 3, SCN = 0.2 M

Carbon Mass (g) 0.05 0.05 0.5 1 2 Solution Vol (mL) 100 200 100 100 100 Carbon Conc (g/L) 0.5 0.25 5 10 20 Solution Head Grade (mg/L Au) Initial Gold Concentration = 0.98 Time (h) 0 0.980 0.980 0.980 0.980 0.980 0.17 0.890 0.897 0.082 0.059 0.023 0.5 0.680 0.701 0.024 0.008 0.006 1 0.577 0.598 0.000 0.000 0.000 2 0.501 0.534 0.000 0.000 0.000 3 0.444 0.489 0.000 0.000 0.000 4 0.390 0.425 0.000 0.000 0.000

Table VIII: The corrected data for Condition 3

Carbon Conc (g/L) 0.25 0.5 5 10 20

0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 0.980 0.980 0.980 0.980 0.980 0.17 0.897 0.890 0.082 0.059 0.023 0.5 0.706 0.691 0.027 0.011 0.007 1 0.605 0.598 0.005 0.003 0.001 2 0.545 0.533 0.005 0.003 0.001 3 0.503 0.488 0.005 0.003 0.001 4 0.447 0.447 0.005 0.003 0.001

117 Table VI: The gold loading onto carbon in g/t of Condition 3

Carbon Conc (g/L) 0.25 0.5 5 10 20 Condition 3 0.25 g/L 0.5 g/L 5 g/L 10 g/L 20 g/L Time (h) Carbon Carbon Carbon Carbon Carbon 0 0 0 0 0 0 0.17 332 180 180 92 48 0.5 1096 579 191 97 49 1 1498 764 195 98 49 2 1741 894 195 98 49 3 1908 985 195 98 49 4 2132 1066 195 98 49

118 Appendix D: Iron Leaching

The results of the iron leaching of the ore in sulphuric acid are presented in Tables D1 and D2:

D1: Effect of pH on iron concentration

Fe Concentration (ppm)

Time pH 1 pH 1.5 pH 2 pH 2.5 pH 3 pH 3.5 pH 4

0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 10 1547.0 702.8 370.1 60.6 20.4 9.8 0.3

30 2104.2 1277.2 420.2 71.8 11.8 4.4 2.8 60 2306.9 1252.4 432.9 61.2 8.6 2.9 4.8 90 2282.5 1268.7 518.8 91.7 7.2 2.8 10.2

120 3033.9 1284.7 534.3 89.4 5.5 1.4 11.0 150 3061.3 1438.4 560.6 92.2 6.2 1.3 8.1

180 3137.5 1486.1 599.9 80.1 5.2 1.4 9.2

D2: Effect of pH on Oxidation Potential

ORP vs. Ag/AgCl

Time pH 1 pH 1.5 pH 2 pH 2.5 pH 3 pH 3.4 pH 4 0 0 0 0 0 0 0 0 10 512.0 467.80 468.0 494.0 489.9 444.0 444 30 504.8 471.80 470.3 497.0 492.1 445.0 444.9 60 506.3 478.40 476.8 508.9 493.2 446.0 444.0 90 474.4 475.80 479.1 519.7 499.9 448.0 446 120 475.9 479.30 497.5 539.9 509.5 454.1 446.4 150 478.8 482.80 496.3 543.8 514.7 461.7 449 180 480.7 486.80 494.8 544.7 514.7 465.0 449

119 Appendix E: Leaching Model

The results of the leaching model as presented by Nicol et al is presented below:

kp 72.13 Solid Tails Grade (g/t) Aupe 0.183236 Head Grade (g/t) Aui 1.980 Data Model 2 Time (h) Au (ppm) Aut Diff 0 1.98 1.980 1 0.195 0.197 0.0000 2 0.197 0.190 0.0000 4 0.185 0.187 0.0000 8 0.181 0.185 0.0000 12 0.171 0.184 0.0002 24 0.198 0.184 0.0002

Sum 0.0005

120 Appendix F: Grind Characterisation

The sieve analysis result for Supremo 72142 B material is presented in Table F1 and Figure F1

F1: Sieve analysis Results

Sieve Mass Retained (g) % Retained % Cum Passing (um) Time 5 10 15 18 5 10 15 18 5 10 15 18 (mins) 212 490.7 109.2 2.9 1.1 45.6 10.2 0.3 0.1 54.4 89.8 99.7 99.7 150 126.3 294.2 194.1 176 11.7 27.5 18.3 16.6 42.6 62.3 81.4 83.0 106 115 183.1 296.2 279.8 10.7 17.1 27.9 26.4 31.9 45.1 53.5 56.6 75 98.2 189.9 293.2 298 9.1 17.8 27.7 28.1 22.8 27.4 25.8 28.5 53 143.6 207.4 189.5 213.2 13.4 19.4 17.9 20.1 9.4 8.0 7.9 8.4 -53 101.3 85.3 83.9 89.1 9.4 8.0 7.9 8.4

Sum 1075.1 1069.1 1059.8 1057.2

Figure F1: Results of the grind characterisation to determine optimum time for grinding the Supremo 72142 B composite material 121