A review of factors affecting gold leaching in non-ammoniacal thiosulfate solutions including degradation and in-situ generation of thiosulfate

Cyanide has been used for more than a century as a preferred lixiviant for extracting gold from ores and concentrates. While cyanide is very effective in treating certain ores and concentrates, it is not suitable for carbonaceous and high copper-bearing ores. Research activities over many decades have indicated that thiosulfate has potential as an alternative lixiviant for extracting gold from problematic ores/concentrates which are not amenable for cyanidation. A number of studies have shown faster gold leaching kinetics with thiosulfate in the presence of ammonia and copper(II) in the form of a cupric tetraamine complex that acts as an oxidant for gold. However, ammonia is toxic and the lixiviant system is highly complex due to the presence of several ligands, which particularly affects speciation of the copper ions and their reactions in the system. For this reason, recent studies have been focusing on alternative oxidants that do not use ammonia. In this article, a comprehensive review of fundamental studies of leaching of gold from different ores in ammonia-free thiosulfate solutions is presented with special emphasis on factors that affect gold leaching kinetics including mineralogical composition of ores and concentrates. Important aspects of thiosulfate gold leaching such as thiosulfate stability, in-situ generation and consumption of thiosulfate and passivation of gold are reviewed.