The cyanidation of gold. III. Mechanistic details of the role of lead and sulfide ions

The role of lead ions in the cyanidation of gold has been extensively studied in this investigation. In addition to a speciation analysis using accepted thermodynamic data, the equilibrium potential for the deposition of lead under cyanidation conditions has been estimated. This has been used in conjunction with other electrochemical measurements to confirm that bulk lead metal is not deposited on a gold surface during cyanidation. The underpotential deposition of lead has been confirmed and it has been shown that an incomplete layer of lead is responsible for the increased rates in the presence of lead. The detrimental effect of high lead concentrations appears to be the result of the formation of a complete lead layer on the gold surface. A less extensive study of the effect of sulfide ions on the rate of cyanidation has confirmed that low (micro-molar) concentrations of sulfide ions enhance the dissolution of gold in cyanide solutions but inhibit the rate of dissolution at high concentrations. The detrimental effect is associated with the anodic oxidation of gold and the formation of a passivating layer of Au 2 S is possibly responsible. There does not appear to be a significant effect of sulfide on the kinetics of the cathodic reduction of oxygen. A gold alloy containing 10 % silver is less susceptible to the inhibitory effect of sulfide ions. • Speciation of lead in cyanide solutions • Bulk lead not deposited on gold in cyanidation • Lead deposited as underpotential layer • Detrimental effect of lead at high concentrations • Low concentrations of sulfide enhance gold leaching • High sulfide results in passivation.