Gold leaching by thiosulphate solutions: a critical review on copper(II)-thiosulphate-oxygen interactions

The reported reaction rates for the reduction of copper(II) by thiosulphate and oxidation of gold by copper(II) are reviewed and the results are quantified by considering the formation of NaS2O3-, NaSO4-, Cu(NH3)p(S2O3)0, Cu(NH3)p(SO4)0 and Cu(NH3)p(S2O5)0 or Cu(NH3)p(HSO3)20 (p = 2-3). The production of sulphate and trithionate during oxygenation can be rationalised according to three stoichiometric relationships corresponding to the [oxygen]/[thiosulphate] consumption ratios: 0.67/1, 1/1 and 1/2. These three ratios represent the formation of 100% trithionate, a 1:1 mixture of trithionate and sulphate, and 100% sulphate respectively. The detrimental effect of excess aeration/oxygenation on gold extraction can be related to the formation of disulphite ion and its association with copper(II)-ammonia complex ions, and subsequent disproportionation. However, controlled aeration would minimize copper(II) reduction and maintain a higher rate of gold oxidation. It would also lower thiosulphate oxidation by favoring the formation of tetrathionate and its alkaline hydrolysis. Thus, research activities in this area will be useful (i) to find a balance between maximum gold oxidation and minimum thiosulphate degradation, and (ii) to provide a deeper understanding of the mechanism of reactions responsible for the oxidation of gold and aqueous sulphur species, in the absence or presence of ammonia.