The leaching of silver sulfide with the thiosulfate-ammonia-cupric ion system

In recent years, then has been renewed interest in the use of thiosulfate as a substitute for cyanide in gold and silver leaching. Copper has been recognized to act as a catalyst in the process, and ammonia to stabilize the system. However, discrepancies exist in the literature concerning the oxidizing agent and, specifically in the case of silver sulfide (argentite), whether the leaching is oxidative or by substitution. The distinct mechanisms have implications concerning the feasibility of the process. The present study elucidates the chemistry of the leach through calculation of the species distribution and experiments on pure phase argentite, and also determines the role of the ammonia/thiosulfate concentration ratio. The experimental results on the pure phase show that the cupric ion reacts with thiosulfate to form the tetrathionate ion and a cuprous thiosulfate or ammonia complex, which liberates the silver by substitution in the solid phase. The results of leaching studies from a mineral concentrate indicate that, in addition to the cuprous ion concentration, the ammonia/thiosulfate ratio influences the silver extraction velocity: silver solubility and extraction are favored at low values of this quantity. A mathematical model for silver leaching in this system, which considers redox and complex equilibria, was developed. The experimental extractions were similar to those predicted by the model for low ammonia concentrations. Divergence from the simulated behavior was probably due to the slow reduction of the cupric ion at the high ammonia concentration (I M). (C) 1998 Published by Elsevier Science B.V. All rights reserved.