Analysis of reaction kinetics, speciation and mechanism of gold leaching and thiosulfate oxidation by ammoniacal copper(II) solutions

Previous researchers have rationalized the thiosulfate oxidation by ammoniacal copper(II) on the basis of the rate equation −d[Cu(II)]/dt = k[Cu(II)][S2O32−]/[NH3] for the decomposition of the reaction intermediate Cu(NH3)3(S2O3)0, but no such detailed analysis has been made in the case of gold. This work revisits literature data to show that the oxidation of thiosulfate takes place via mixed complexes Cu(NH3)2(S2O3)n−(2n−2) (n = 1 or 2). The oxidation can be considered as a first-order decomposition of these complexes, with a rate constant kCu(1) = 4×10−4 s−1, to produce S2O3− which undergoes dimerization to S4O62−. It can also be treated as a second-order decomposition of Cu(NH3)3(S2O3)0 or Cu(NH3)2(S2O3)0 with rate constants kCu(2) = 0.2 and 0.1 L mol−1 s−1, respectively, which produce S4O62− in one step. terature data reported for gold dissolution in the copper(II)–ammonia–thiosulfate system show first-order dependence with respect to copper(II) and thiosulfate concentration, but it is relatively less affected by ammonia and hydroxide concentrations. At higher concentrations of copper(II) and thiosulfate, the rate becomes zero order with respect to these reagents. The leaching data obey a shrinking core model for both gold powder and carbonaceous or roasted ore with an apparent rate constant ksc≈10−5 s−1, but the ksc also shows a first-order dependence with respect to the measured copper(II) concentration in solution. The copper(II) sepeciation analysis shows Cu(NH3)2(S2O3)22−, Cu(NH3)2(S2O3)0 and Cu(NH3)3(S2O3)0 as the predominant species in ammoniacal copper(II)–thiosulfate solutions. Thus, gold oxidation can be represented by a sequence of equations representing adsorption and redox reactions.