Thiosulphate Leaching – an Alternative To

SGS MINERALS SERVICES – T3 SGS 869 10-2008

THIOSULPHATE LEACHING – AN ALTERNATIVE TO CYANIDATION IN GOLD PROCESSING ALTERNATIVES TO ADVANTAGES TECHNICAL LIMITATIONS CYANIDE IN GOLD Like cyanide, thiosulphate leaching is The chemistry involved in the thiosulphate PROCESSING an alkaline process (usually operated in leaching process is more complex and the pH 8 to 10 range) so there are no less robust than the cyanide leaching concerns with corrosion and materials process, and is therefore more difficult to THIOSULPHATE LEACHING used in construction, and the process optimize and more sensitive to operate. Thiosulphate leaching is a process that can be applied to heaps and dumps, or to The leach process involves a chemical removes gold from gold bearing ores stirred tanks. The thiosulphate process reaction between metallic gold and the without the use of cyanide. Although not offers advantages over cyanide leaching thiosulphate anion, with oxygen as the as aggressive a leaching agent as cyanide, in the following situations: oxidant, and ammonium and copper ions thiosulphate offers several technological •• Thiosulphate leaching yields as unconsumed catalysts in the reaction. advantages including its lower toxicity similar gold recovery as cyanide in One possible scheme for the overall and greater efficiency with gold deposits some cases, while in cases such reaction is presented below: associated with preg-robbing ores. The as preg-robbing ores (naturally 2– 4Au + 8S2O3 + O2 + 2H2O → 4Au thiosulphate leaching process, followed occurring carbonaceous material 3– - (S2O3)2 + 4OH by resin-in-pulp gold extraction, has been that preferentially absorbs the gold While it is relatively easy, in just a few developed by SGS to the point where it cyanide complex but not the gold tests, to establish the amenability of an is a technically and economically viable thiosulphate complex), it can yield ore to cyanide leaching and optimize alternative to cyanidation for some gold substantially better recovery than the process, the thiosulphate chemistry bearing ore bodies. cyanide. presents technical challenges and Whether the leaching is performed in •• The thiosulphate leaching of gold requires careful optimization. Thiosulphate stirred tank reactors or heaps, the size ores has great potential to reduce the leaching is a sensitive process that and type of equipment needed for a impact on the environment compared requires both dependent and independent thiosulphate leaching flowsheet is very to the cyanidation process. Unlike optimization of each of the chemical similar to a conventional cyanide leach. cyanide, which is highly toxic, the components of the leach reaction, as chemicals used in the thiosulphate SGS is a global technical leader in the well as physical parameters such as leaching process are benign, and field of thiosulphate leaching, and our pulp density and temperature, in order there is great potential to apply the laboratory and industry experts have to maximize gold recovery and minimize technology in those jurisdictions of spent years designing and developing reagent losses. the world where the use of cyanide the technique into a viable alternative to is banned outright, or is subject The sensitivity of the thiosulphate cyanide for some gold-bearing ores. SGS to intense negative publicity for leaching reaction stems from the fact that technical experts will work with you to environmental reasons. The main the reaction thermodynamics are less ensure your flowsheet development is chemical components of the leaching favorable than the cyanide leach reaction. optimized and your environmental and process (ammonium thiosulphate Consequently, higher concentrations financial risk is minimized. and ammonium sulphate) are of thiosulphate are needed to achieve common fertilizers, which opens equivalent rates of gold leaching, and a up the additional possibility of using typical thiosulphate leach solution will mine tailings solutions in agricultural have a reagent concentration of 5 to 20 applications, in regions of the world g/L versus a cyanide concentration of only where the local infrastructure and 0.25 to 1 g/L. The higher concentration environmental regulations are of thiosulphate required leads to higher favourable. reagent consumption, which is at least SGS MINERALS SERVICES – T3 SGS 869 2

partially offset by the significantly lower will evaluate the following gold recovery THE NEXT STEPS cost of thiosulphate, which could be as options for your project: Early in the planning and proposal little as one fifth the cost of cyanide. It •• Resin-in-pulp technology involving preparation stages, SGS encourages you will generally still be important either to direct recovery of gold thiosulphate to discuss your timing, anticipated project recycle the thiosulphate tailings solution from the leach pulp by adsorption on scope, environmental concerns, and back to the leach plant to minimize strong-base anion exchange resin. financial requirements. This allows SGS thiosulphate losses and new thiosulphate This process was developed by SGS, to develop the best bench or pilot plant make up requirements, or use the tailings and includes techniques for eluting testing program for you, be it a phased solution productively as a fertilizer. the gold off the resin and restoring program with a number of milestones The gold thiosulphate complex is very the resin for recycle back to the that will help you maximize your financing poorly adsorbed by activated carbon, adsorption circuit. options, or an intensive program that negating the use of the traditional •• Cementation of gold from quickly establishes the technical and carbon-in- pulp (CIP) or carbon-in-leach thiosulphate leach solution onto economic viability of the technology for (CIL) processes for the recovery of gold copper metal powder, or some other your application. directly from leach pulps. Instead, SGS reductant, following separation of solids and liquids in the leach Contact SGS for all your thiosulphate slurry by filtration or counter current leaching needs and let us help you decantation. optimize your flowsheet and reduce your operational and environmental risk and Use of the resin-in-pulp method is liability. We have the experience and the limited to those deposits requiring mild technical expertise to provide you with thiosulphate leaching conditions, since the most efficient and affordable methods strong thiosulphate leach conditions result of managing your mineral processing in competitive adsorption on the resin by requirements. the polythionate anions that are produced during leaching. SGS experts will work with you to ensure the thiosulphate leaching and recovery technology chosen is the right one for your operation.