The Pyrite Catalysed Oxidation of Sulfide Ions

Abstract

The oxidation of sulfide ions in cyanide solutions by oxygen is catalysed by pyrite. Polysulfides are detected as an intermediate product in the oxidation reaction for both cyanide and non-cyanide alkaline solutions. The major product for oxidation in the absence of cyanide is thiosulfate, whilst thiocyanate is dominant for the cyanide containing system. Electrochemical studies of sulfide oxidation are consistent with the formation of polysulfides being the initial oxidation step for both the cyanide and non-cyanide systems. However, the reduction of oxygen is significantly slower on pyrite in the presence of cyanide, leading to reduced rates of oxidation of sulfide by oxygen. Another important observation was that some pyrite oxidation by oxygen occurred concurrently with sulfide oxidation, despite the mixed potential being in the region where pyrite oxidation is very slow in the absence of sulfide.