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THIOCYANATE LEACHING of GOLD by Roselyn Sarpomah Yeboah B.Sc

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THIOCYANATE LEACHING of GOLD by Roselyn Sarpomah Yeboah B.Sc 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..........................................................................................
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