Silver leaching with the thiosulfate–nitrite–sulfite–copper alternative system

Abstract

In the recent years, several silver leaching technologies have been investigated in order to find out an alternative and sustainable leaching process; one of these alternatives is the thiosulfate. The main research line is focused in the use of thiosulfate with copper as a catalyst and ammonia as a cupric ion stabilizer; however, the ammonia is also another toxic reagent. For that reason, an alternative-new leaching system using thiosulfate–nitrite and copper ions was investigated in this work. This new leaching system is able to dissolve silver without the use of cyanide or ammonia due to the cupric–ammonia complexes in-situ production, which is possible through the reaction of the nitrite and the cupric ions at neutral pH and NO2:Cu molar ratio of 30:1, according to silver leaching experiments and thermodynamic predictions.On the other hand, the effect of sulfite concentration was studied in the leaching solution revealing that the reductive ability of the sulfite ions has a negative effect on the silver leaching kinetics and recovery. It was also found that an increase in the pH of the leaching solution from 7 to 9.6 promotes the precipitation of copper hydroxide species which inhibits the silver leaching kinetics.Finally, copper–ammonia complex formation was also confirmed in the silver leaching by the UV–Visible spectroscopy technique. The solid leaching residues obtained at neutral pH were analyzed by X-ray diffraction and scanning electron microscopy showing the formation of a copper hydroxysulfate species called antlerite; this result was also confirmed by species distribution diagrams.