Recovery of Gold and Silver

The high concentration of copper might interfere with the selective recovery of gold and silver. The use of ion exchange resins or activated carbon has been advocated to remove the copper ion (Cu+). Either cementation or electrolysis is adopted to recover the gold from the purified solution. The major reactions of the cementation process are the cathodic deposition of gold and anodic corrosion of zinc, which occur at the surface of zinc particles.

In practice, if the cyanide concentration is reduced too much, a passivating layer of zinc hydroxide may form on the surface of the zinc particles. It is reported that cementation of gold is practically constant over the pH range 8–11. However, impurities such as lead,copper, nickel, arsenic, antimony, and sulfur are most deleterious for gold cementation. The poisoning effect of the sulfide ion is thought to be due to the precipitation of insoluble zinc sulfide on the surface of zinc particles.

The electrolysis method has been used in concentrated solutions in electrochemical reactors with planar electrodes. However, with dilute solutions, efficiency is low. Works reported in the literature have shown that high surface area increases efficiency when used in dilute solutions.