Western Australian School of Mines Department of Metallurgical and Minerals Engineering A Fundamental Study of Copper and Cyanide Recovery from Gold Tailings by Sulfidisation Andrew Murray Simons This thesis is presented for the Degree of Doctor of Philosophy of Curtin University June 2015 STATEMENT OF AUTHENTICITY To the best of my knowledge and belief this thesis contains no material previously published by any other person except where due acknowledgment has been made. This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. Andrew Murray Simons i ABSTRACT Cyanide soluble copper in gold ores causes numerous problems for gold producers. This includes increased costs from high cyanide consumption and a requirement to destroy cyanide in the tailings before discharge into a tailings storage facility. Over recent years a cyanide recycling process known as SART (sulfidisation, acidification, recycle, and thickening) has gained attention as a method to remediate the increased costs and potential environmental impact caused by using cyanidation to process copper bearing gold ores. While sulfidisation processes, such as SART, have been demonstrated within the laboratory to be effective, there are several gold processing operations using sulfidisation which report high sulfide consumption compared to the expected reaction stoichiometry. Further to this problem, there is a lack of fundamental studies of sulfidisation of cyanide solutions resulting in a poor understanding of how various process variables impact the process. This thesis details work on sulfidisation of copper cyanide solutions to systematically determine the impact of process variables and impurities on the SART process. The work is divided into three distinct areas: modelling of process variables on sulfidisation performance, impact of impurities in sulfidisation, and the impact of residence time on sulfidisation. Mathematical modelling of sulfidisation of the copper, cyanide, and sulfide system was performed to determine the impact of various factors on sulfidisation. An economic model was then used to determine the overall process optimum and system sensitivity at suboptimal operation. The model showed that to maximise profit during sulfidisation, a sulfide addition of 0.56 moles per mole of copper, a pH of 4, and the lowest possible cyanide to copper ratio should be used. These optimum conditions to maximise profit ii remained the same within the range of reagent costs and product values used in the study. The model also showed that it is critical to control sulfide addition at its optimum value, and that the cyanide to copper ratio in the sulfidisation feed should be kept as low as possible, depending on upstream processing constraints. Impurities in the sulfidisation feed have numerous impacts on sulfidisation performance. It was found that silver and zinc have the largest impact on copper recovery as these two metals consume sulfide preferentially to copper. Gold, iron, nickel, thiosulfate, thiocyanate, and chloride all had no major impact on sulfidisation of copper cyanide solutions. These species did, however, have different minor effects on the process ranging from increases in acid consumption through to shifting the pH range of copper precipitation. It was found that copper can be selectively recovered from other metals, although a small amount of co-precipitation/adsorption occurs. Hence, complete separation of other metals from copper is not possible during sulfidisation. When sulfidisation is performed on mixed metal cyanide solutions, there are various complex interactions which occur. This includes the potential to precipitate gold at a higher pH than when other metals are not present, likely due to the formation of a silver gold sulfide precipitate. Selective recovery of various metals in sulfidisation is possible, although a small amount of co- precipitation/adsorption will always occur. In any case, selective precipitation would require tight control of sulfide addition and pH. The most intriguing result from the work is the discovery that long residence times during sulfidisation are the cause of high sulfide consumption experienced in some industrial sulfidisation processes. This is due to iii oxidation of the sulfidisation precipitate from a structure similar to Cu2S towards that of CuS. Ultimately, if excess sulfide is not present in solution, copper redissolution occurs reducing the recovery of both copper and cyanide in the process. Industrial surveys of an operational sulfidisation process, along with laboratory scale work using industrial solutions, confirmed the observations from the test work with synthetic copper cyanide solutions. Interestingly, the composition of the industrial solution results in a higher rate of oxidation compared to the synthetic solutions. iv ACKNOWLEDGEMENTS I would firstly like to thank my supervisors Dr Richard Browner from the Western Australian School of Mines (WASM) and Dr Paul Breuer from CSIRO. Both Richard and Paul never ceased to inspire my interest and curiosity in hydrometallurgy and science in general. I extend many thanks to all of the CSIRO Waterford site staff and fellow PhD students from WASM who have assisted me during my candidature. Of particular mention is Danielle Hewitt whom has assisted me in many aspects of my laboratory work. I’d also like to extend thanks to the Newcrest Telfer metallurgical staff for their assistance in surveying the Telfer SART plant. I would like to mention the support of my loving parents who have encouraged me over the past four years of my candidature. I am very thankful to have inherited the scientific rational of my father and the creativity of my mother. Finally, I would like to acknowledge and thank both the CSIRO Minerals Down Under Flagship and the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions for their financial support during my candidature. v CONTENTS Statement of authenticity ........................................................................................ i Abstract ...................................................................................................................... ii Acknowledgements ................................................................................................. v Contents .................................................................................................................... vi Publications ............................................................................................................. xii List of figures ......................................................................................................... xiv List of tables ...................................................................................................... xxviii 1 Introduction ....................................................................................................... 1 1.1 Importance of gold .................................................................................... 1 1.2 Processing gold ores ................................................................................. 3 1.2.1 History of gold processing ................................................................... 3 1.2.2 Gold cyanidation ................................................................................... 4 1.2.3 The impact of copper in gold cyanidation ......................................... 6 1.3 Mitigating the effect of copper in gold processing ............................... 8 1.3.1 Selective leaching of gold ..................................................................... 9 1.3.2 Selective leaching of copper................................................................. 9 1.3.3 Cyanide destruction ............................................................................ 10 1.3.4 Cyanide recycling ................................................................................ 11 1.3.5 Copper recovery and cyanide recycling by sulfidisation .............. 12 1.4 Objectives ................................................................................................. 16 1.5 Thesis structure ....................................................................................... 17 2 Minimising the impact of copper in gold cyanidation ............................ 19 2.1 Thermodynamic modelling ................................................................... 19 2.2 Comparison of technology for separating copper and cyanide ....... 19 2.2.1 Direct recycle........................................................................................ 24 2.2.2 Volatilisation ........................................................................................ 25 2.2.3 Copper cyanide digestion .................................................................. 27 vi 2.2.4 Sulfidisation ......................................................................................... 28 2.3 Factors impacting copper-cyanide-sulfide chemistry ........................ 30 2.3.1 The sulfide system............................................................................... 30 2.3.2 Effect of pH .......................................................................................... 32 2.3.3 Effect of sulfide to copper ratio ......................................................... 38 2.3.4 Effect of cyanide to copper ratio ....................................................... 43 2.3.5 Effect of volatilisation ........................................................................