Identification of surface species that inhibit and passivate thiosulfate leaching of gold

The anodic dissolution of gold in thiosulfate solution is accelerated by the presence of ammonia and significantly enhanced by the addition of copper. It is inhibited by the presence of adsorbed surface species and passivation is observed under certain conditions. Sulfur, polythionate and copper sulfide species have been identified on gold surfaces in thiosulfate solutions using surface enhanced Raman scattering spectroscopy (SERS) under potential control and open-circuit leaching conditions. Correlations are made between the gold leaching rate and the extent and composition of surface layers, determined quantitatively by anodic stripping voltammetry. In thiosulfate alone, the inhibiting surface layer on gold is sulfur with the coverage of one sulfur atom per three surface gold atoms (13ML) in the potential region of leaching, increasing to close to 1 ML at higher potentials. When ammonia is added, the sulfur coverage under the same conditions remains at 13ML, but co-adsorption of a polythionate occurs at higher potentials and causes passivation. Ammonia copper polysulfide is formed on the gold surface when copper is present as well as ammonia, with the surface layer under conventional leaching conditions developing to a composition of 23ML S and 13ML Cu.