THE ELECTROCHEMICAL BEHAVIOR OF SILVER SULFIDE

The electrochemical behavior of silver sulfide in sulfuric acid as well as in nitric acid was studied using electrodes made from synthetic silver sulfide. The primary techniques used were potentiostatic, potentiodynamic, galvanostatic and corrosion cell experiments. The cathodic reaction of silver sulfide produces silver and hydrogen sulfide. This reaction mechanism is a sequential two step charge transfer involving a single electron in each step. Silver ions are produced from silver sulfide upon applying an anodic potential. The dissolution rate of silver sulfide can be so high that the formation of silver sulfate occurs which partially covers the silver sulfide surface and inhibits a further rate increase. The sulfur from the silver sulfide will be oxidized at low overpotentials to elemental sulfur; at high overpotentials, the oxidation to sulfate or bisulfate is observed. The anodic dissolution of silver sulfide can be expressed by a mathematical model involving two sequential single electron charge transfer steps. The electrochemical dissolution of silver sulfide was found to be similar to the reactions determined on galena (PbS). The catalysis of silver ions in the leaching rate of chalcopyrite (CuFeS(,2)) does not continue above 10('-3) M silver ion because the formation of silver sulfate prevents a further rate increase. Predictions of the leaching rate of chalcopyrite based on the rate of anodic silver sulfide dissolution have been made. The ferrous/ferric redox couple is not affected by silver ions and galvanic effects between silver sulfide and chalcopyrite do not account for the enhanced leaching rate of chalcopyrite. The present results suggest that the catalysis of chalcopyrite by the addition of silver ions is caused by the formation and subsequent dissolution of silver sulfide leaving a porous layer behind. The understanding of the reaction mechanism of silver sulfide dissolution and its optimization will significantly improve the economic evaluation of industrial processes using the catalyzed leaching of chalcopyrite. The present knowledge of the catalysis indicates that other ions may be substituted for silver ions which would increase the feasibility of hydrometallurgical processes.