ELECTROCHEMICAL INTERACTIONS BETWEEN GOLD AND ITS ASSOCIATED MINERALS DURING CYANIDATION

Gold occurs in the Witwatersrand gold mines in association with sulphide, oxide and gangue minerals. The leaching behaviour of gold in contact or in association with various minerals depends largely on the galvanic interaction between gold and the mineral, and partially on the formation of a passivating film on the gold surface. Gold in contact with conducting minerals will passivate as a result of the enhanced magnitude of the cathodic current, resulting in decreased gold dissolution rates. Chalcopyrite, pyrite and pyffhotite cause the largest decrease in the rate of leaching when in contact with gold. Galena strongly enhances the dissolution rate owing to the action of dissolved Pb2+ ions on the surface of the gold. When these Pb2+ ions are prevented from coming into contact with the gold, galena inhibits the leaching rate. In all experiments the rotating disc of gold passivated so that the rate of dissolution was much slower than that predicted by a mass-transport limiting model. The various films that occur on the surface of the gold and associated minerals, as well as the galvanic interaction (related to electrical conductivity), depend largely on the pretreatment of the ore. Pre-elimination of host minerals from the gold-bearing ore has a marked positive effect on the dissolution rate of gold, and explains the kinetics of reaction on the gold surface to a large extent. From the results presented in this paper on passivation owing to both film formation and galvanic interaction, it can be seen that passivation is not merely a laboratory curiosity, but it can reduce the efficiency of industrial operations and is thus most relevant to practical leaching processes.