A kinetic–thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions

Abstract

In this research, an analysis of the effect of ethylenediaminetetraacetic acid (EDTA), thiosulfate and cupric ions on the silver leaching kinetics was performed. For that purpose leaching experiments with pure metallic silver were carried out at different concentrations of these reagents at room temperature. The results of this study showed that small amounts of EDTA accelerate the leaching of silver more efficiently than higher concentrations due to an increase in the oxidation potential of the leaching solution. These results were in accord with Pourbaix diagrams and redox potential measurements. A synergistic effect was found in the silver leaching kinetics when the thiosulfate and EDTA concentrations were both decreased. On the other hand, the silver leaching kinetics was reduced at low Cu(II) concentrations due to the decrease in the oxidizing ability of the leaching solution.Characterization by SEM and EDXS of the silver in the first minutes of leaching revealed that the silver particles were coated by a layer of copper sulfides and copper oxides. Furthermore, characterization of the solid residue during the precipitation of silver in the leaching experiment was performed by the same analytical techniques, showing the presence of silver sulfide on the unreacted silver surface. The mechanism by which the silver sulfide is precipitated was found to be related to the copper sulfide formation. It was also observed that an increase in the concentration of EDTA promotes the silver dissolution avoiding the formation of copper sulfides or oxides; in these cases the process is controlled by the chemical reaction. These observations were also supported with Pourbaix and species distribution diagrams.