An analysis of the efficiency and sustainability of the thiosulfate-copper-ammonia-monoethanolamine system for the recovery of silver as an alternative to cyanidation

Abstract

In this research, silver leaching experiments from pure phases of Ag and Ag2S were done using the S2O32−-Cu2+-MEA-NH4 alternative system and varying the S2O32− and MEA concentrations. The results revealed that MEA can stabilize the cupric ions forming several Cu-MEA complexes. It was also found that MEA can form Ag-MEA complexes, and thus it can work as a co-leaching agent for silver, mainly when metallic silver is used.Furthermore the leaching experiments with metallic silver showed a continuous oscillation process (silver dissolution-precipitation phenomenon), which was related to the formation of the redox couples Ag-MEA/Ag and Cu-NH3/Cu-MEA, as well as with the oxidative decomposition of thiosulfate which could lead to the formation of copper sulfide species. The leaching experiments with silver sulfide revealed that the presence of MEA in the leaching system is advantageous in two ways: a) it decreases the precipitation of copper oxides and b) it assists the stabilization of silver in solution avoiding its precipitation.Furthermore, the pregnant leaching solutions were treated by cementation using a metallic copper wire and the chemical and microstructural characterizations revealed that silver was deposited on the copper wire surface. When the leaching solution is re-utilized after the cementation step in a second leaching with Ag2S, it is possible to dissolve 85% of silver. This paper presents an alternative-sustainable thiosulfate leaching system which permits the recovery of silver from metallic silver or silver sulfide at short processing times.