Catalysis of the leaching of gold in cyanide solutions by lead, bismuth and thallium

The anodic activity of gold in aqueous cyanide may be inhibited at potentials just positive of the reversible potential by the formation of passivating species. In this work the mechanisms proposed for the passivation of the gold, as well as its activation by metals such as lead, bismuth and thallium are reviewed. It is proposed that the activation by these metals are due to underpotential deposition of sub-monolayer quantities of the metals onto the surface of the gold; these then prevent the formation of passivating species from forming. By using potentiostatic experiments it was shown that bulk plating of the metals is not possible at the potentials where activation of the gold occurs. The presence of the activating metals on the surface of the gold was identified using XPS.