Leaching and electrochemistry of gold, silver and gold–silver alloys in cyanide solutions: Effect of oxidant and lead(II) ions

The leaching and electrochemistry of gold, silver and gold–silver alloys in cyanide solutions has been investigated using air, oxygen and/or hydrogen peroxide as oxidant in the presence or absence of lead(II) ions. It has been found that both the gold–silver oxidation half reaction and the oxygen reduction half reaction occur more readily with increasing silver content in the alloy, and both half reactions are significantly enhanced by the under-potential deposition of lead onto the metal surface. However, the under-potential deposition of lead can be inhibited by high leach potentials, resulting in no beneficial effect for the cyanidation under these conditions (e.g. cyanidation in oxygen saturated solutions). A selective dissolution of gold over silver was found for low silver content alloys, which diminished with increasing silver content. The leach rates were less than the oxygen diffusion limited rate other than for gold alloyed with silver in air saturated solutions with lead(II) ions present. A low concentration of hydrogen peroxide had no positive effect on the leach rate as oxygen reduction occurs preferential to hydrogen peroxide reduction; enhanced hydrogen peroxide reduction could occur at leached surfaces resulting in enhanced leach rates. It has also been found that the gold leach rate predicted by an Evans' diagram could be misleading and a combination of the measured leach point (rate and potential) and Evans' diagram revealed some important information occurring during the leaching.

► Both metal oxidation and oxygen reduction were enhanced by silver and lead.
► High leach potential may inhibit the deposition of lead and render it ineffective.
► Selective dissolution of gold over silver was found for low silver content alloys.
► Low concentration of hydrogen peroxide had no positive effect on the leach rate.
► Evans’ diagram can be more informative if interpreted with measured leach point.