The reduction of copper(II) and the oxidation of thiosulfate and oxysulfur anions in gold leaching solutions

Due to the increasing environmental and public concerns associated with cyanide leaching of gold, much of the current research in gold hydrometallurgy is focussed on alternative systems, such as thiosulfate. One of the major problems with the thiosulfate leaching system is the undesirable reaction between thiosulfate and copper(II). The current paper presents a study on the effect of various anions on the rate of this reaction. It will be shown that the presence of anions such as sulfate, chloride and phosphate significantly reduce the rate at which copper(II) is reduced by thiosulfate. The presence of other oxysulfur species such as sulfite, tetrathionate and trithionate were also studied. These species were found to be readily oxidised by copper(II) and hence greatly increased the rate of copper(II) reduction. The rate of copper(II) reduction by tetrathionate is relatively unaffected by the addition of anions; however, anion addition was found to significantly influence the rate of copper(II) reduction by sulfite and trithionate. Based on these results, inner sphere mechanisms were proposed for the reaction of copper(II) with thiosulfate, sulfite and trithionate. The kinetics of copper(II) reduction is thus very complex and can be represented by a number of parallel-series reaction sequences.