Gold passivation by sulfur species: A molecular picture

Abstract

Abstract The role of sulfur species in the passivation of gold surface in cyanide and thiosulfate solutions was investigated using density functional theory (DFT). The interaction energy between various sulfur complexes as well as deposited sulfur allotropes with the gold surface were computed and the contribution of each specie to the formation of a passivated layer were compared. For this purpose, a new parameter was introduced to explain and understand the tendency of each sulfur complex to interact with the gold surface from a molecular point of view. It was observed that the ratio of the hydration to the binding energy (H/B) of each sulfur specie can be well correlated with the calculated interaction energy with the gold surface. This parameter could be effectively used to predict the interaction of each specie with a substance, only based on the structural characteristics of the specie. It has been also demonstrated that the theoretically calculated energies of interactions of sulfur complexes with the gold surface are well correlated with experimentally reported data. The results showed that S, S2−, polysulfides, tetrathionate, and sulfite species affect the gold leaching process directly through the passivation of the gold surface. Sulfate, dithionate, trithionate, and thiocyanate do not contribute to the passivation effect, although they indirectly affect the gold leaching process by consuming the oxygen and cyanide required to dissolve gold in the cyanidation process. The sulfur allotropes forming on the gold surface also passivate the gold surface and limit the access of the lixiviants required for the gold leaching process. The results could shed light on the mechanism of passivation of gold surface by sulfur species in gold leaching and this effect can be controlled by inhibiting the formation of main sulfur contributors in passivation process.