A phenomenological study of the silver sulfide passivation and oxidative degradation of thiosulfate in the thiosulfate-ammonia‑copper-citrate leaching system

Abstract

The oxidative decomposition of thiosulfate is one the main problems found in the thiosulfate leaching of precious metals. This work presents a detailed analysis of the effect of different dissolved components on the silver sulfide dissolution kinetics and efficiency faced by the thiosulfate-citrate‑copper-ammonia system. The analysis comprises the quantification of thiosulfate concentration; furthermore, it is discussed the effect of copper sulfides and copper hydroxides -formed in the leaching system- on the thiosulfate decomposition and silver dissolution efficiency. The results revealed that citrate promotes the complexation of cupric ions decreasing the thiosulfate consumption in the leaching system. The individual and combined effects of copper solid species (Cu(OH)2, CuS, Cu2S and solid residues obtained from silver sulfide leaching) on the thiosulfate stability and silver extraction efficiency, showed that the thiosulfate decomposition that occurs in the system is not significantly related to these copper species, while the silver extraction (~80%) does not show any substantial change. However, the presence of Cu2S and a small amount of dissolved silver have certain effect on the the partial in-situ regeneration of thiosulfate in the leaching system. Finally, it was also found that the silver sulfide dissolution is passivated obtaining an 80% of silver extraction. Such passivation is mainly attributed to: a) the precipitation of copper-sulfide and copper-hydroxide species on the unreacted silver sulfide sites; these copper phases act as a physical barrier which inhibits the direct contact of fluid reactants with the silver sulfide, and b) the silver sulfide dissolution reaches an equilibrium state.