Mechanisms of sulfide ion oxidation during cyanidation. Part I: The effect of lead(II) ions

The mechanisms and the reaction products for the oxidation of sulfide ions in the presence lead sulfide (formed by the addition of lead(II) ions) have been established. When the leach solution contains free sulfide ions, oxidation of sulfide ions occurs via electron transfer from the sulfide ion to dissolved oxygen on the precipitated lead sulfide surface, with polysulfides being formed as an intermediate oxidation product. In the absence of cyanide, the polysulfides are further oxidised to thiosulfate and sulfite, whilst with cyanide present, thiocyanate is also formed from the reaction of polysulfides and cyanide. All of these reaction products were detected using UV following ion chromatography with an anion exchange column and perchlorate eluent.When the cyanide leach solution does not contain free sulfide ions, the oxidation of the lead sulfide to thiocyanate and lead(II) ions is the dominant sulfide oxidation mechanism, and thiosulfate and sulfite are not formed. The rate of lead sulfide oxidation is significantly slower than the lead sulfide catalysed heterogeneous oxidation of sulfide ions, although in a continuous reactor where the sulfide ion concentration is very low, the lead sulfide oxidation pathway is significant.