Interrelationship between lixiviants and galvanic interaction during dissolution of gold

Abstract

Galvanic interactions between gold and associated substances are amongst the many factors that could influence the rate of Au dissolution in a cyanide, bromine or an acidic thiourea solution. Some of these galvanic interactions were investigated by short circuiting two-rotating disc electrodes either in one reactor or in two separate reactors linked by a salt bridge. In cyanide solution it was established that the greatest decrease in the rate of Au dissolution was obtained when Au was in contact with copper, chalcopyrite and pyrite in the same reactor. On the other hand, in acidic thiourea solution, chalcopyrite and pyrite increased the rate of Au dissolution when both electrodes were in the same reactor. This was ascribed to sulphur species in solution that inhibited the degradation of thiourea to formamidine disulphide, one of the by-products of the dissolution of thiourea. Gold in contact with pyrite in Br solution also enhances the Au dissolution rate. Galena produced a very marked increase in the rate of dissolution of Au in cyanide solutions when both electrodes were in the same reactor. This could be ascribed to the action of Pb(II) ions on the surface of the Au. Galena causes the largest decrease in the rate of leaching when in contact with Au in Br solution in the same reactor. This can be attributed mainly to the galvanic interaction between the two substances. However, galena had little or no effect on the rate of Au dissolution in an acidic thiourea medium. As could be expected, the specific dissolution rate of Au on its own in acidic thiourea solution was higher than that in acidic Br solution, which in turn was significantly higher than that in cyanide.