Thiosulphate as an alternative to cyanide for gold processing - issues and impediments

Thiosulphate is an attractive alternative reagent to cyanide for processing gold ores. It is relatively cheap and non-toxic, forms relatively strong gold and silver complexes, and readily leaches gold ores in ammoniacal solutions, catalysed by Cu (II). It is particularly suited to carbonaceous ores where the recovery of gold by cyanide is poor. However, research over the past 20 years has identified a number of issues and impediments that must be understood or overcome before an overall process can be demonstrated as practical and economic for typical oxidised gold ores. This paper reviews the current understanding of thiosulphate chemistry and mechanism of gold dissolution, and discusses the stability of thiosulphate and polythionates. It then reviews the results (of leaching various gold ores that have been reported in the literature, discusses gold recovery options, and considers environmental and toxicity issues. Some of 'the issues' that require further research include decreasing the oxidation of thiosulphate by Cu(II) or other oxidants, increasing the extraction (of gold compared to cyanide, and understanding the variation in gold recovery between ores. More work is also required to minimise the effect of polythionates on gold recovery using resins or alternative absorbents, to improve the elution of copper and gold from I resins, and to recycle or minimise the concentration of reagents. (C) 2004 IoM Communications Ltd.