Cyanide leaching is the dominant method of gold processing, accounting for more than half the world's gold production. It is a robust, relatively simple process that provides high gold recoveries, but poses potential safety and environmental risks in some situations.Parker Centre researchers at CSIRO Minerals and Murdoch University are investigating thiosulfate, which is considered to be the best alternative to cyanide. Thiosulfate was first proposed as a way to recover precious metals in the early 1900s, but due to technical uncertainties it has never been adopted by the gold industry. The Parker Centre is working to resolve some of those uncertainties.The chief advantage of thiosulfate - its non-toxicity - is currently outweighed by its disadvantages: the process is chemically complex, difficult to control and may result in slightly lower gold recoveries.There are no thiosulfate systems in commercial operation, but interest in the technology is growing as gold producers encounter more refractory ores.John Rumball, the Parker Centre's gold market leader, says: "Our understanding of the key parameters for thiosulfate leaching of gold is improving all the time. Although it is unlikely thiosulfate will universally replace cyanide, we can now see niche opportunities where thiosulfate offers significant advantages over cyanide. It is these niche applications where we are concentrating our research effort."Niche opportunities include the processing of sulfide and carbonaceous ores and concentrates, from which thiosulfate can unlock gold more effectively than cyanide."We are currently working with a client to examine the feasibility of leaching a gravity concentrate with thiosulfate," says Dr Rumball. "The client is attracted to this opportunity as it would mean they would no longer require cyanide on their site. Replacing cyanide with thiosulfate would significantly reduce the hazards on the site."Another niche application stems from the observation that some ores leach with unexpected ease. New work in this area indicates that ores can be conditioned to significantly improve their response in a thiosulfate leach. Dr Rumball says more details on this will emerge as the research progresses.Paleochannel gold is another opportunity for thiosulfate leaching. Some of the paleochannel gold is hosted in porous sandstones that abut impermeable beds. This creates the ideal environment for solution mining. Solution mining involves pumping a solution underground to leach the gold, and then bringing that solution back to the surface to recover the gold.This process removes the need to develop a large-scale, open-cut mine. Gold can be extracted from deep below without disturbing the surface. Thiosulfate is ideal for this application because of its low toxicity. The Parker Centre is generating a database that can be used to assess the feasibility of this process."Our focus is on understanding the chemistry that is likely to prevail in this environment," says Dr Rumball.
"Leach times, solid solution ratios and oxygen availability are all very different to those in conventional stirred tank reactors. We need to get the leach conditions right if we are to dissolve the gold."
It is possible that either an iron-oxalate or an iron-EDTA oxidant system may have application in a thiosulfate leaching process. Parker Centre researchers are developing the iron-EDTA oxidant system for thiosulfate leaching of gold from ore and concentrates as part of the AMIRA P420C Gold Processing Technology project.According to the Parker Centre, this process may be simpler to control and may provide more reproducible gold recoveries than the conventional thiosulfate system.To complete the process, the Parker Centre has been undertaking research to recover the gold once it is dissolved in thiosulfate. Precipitation, cementation and resins are the key technologies available, with resins being the method of choice because of ease of operation. Although gold thiosulfate readily adsorbs onto resins getting it back off the resin has proven problematic."Exciting new research suggests this process is about to become a whole lot simpler," says Dr Rumball.
More information can be obtained from John Rumball Phone: +61 8 9334 8000
Story courtesy of CSIRO
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