Low pH cyanidation of gold

Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Hydrogen cyanide gas can be evol ved directly from the pulp surface or purged from the leach tank in oxygen or air that is sparged into the tanks to provide oxygen for the dissolution of gold. Savings in lime consumption can be achieved by leaching at a reduced pH; es pecially in aqueous solutions high in magnesium such as to be found in the Kalgoorlie Goldfields in Western Australia but a greater percentage of cyan ide is then present as aqueous hydrogen cyanide. Another proposed procedure to reduce lime consumption is closed tank leaching where a reduced pH or t he natural pH of the ore is wed and hydrogen cyanide loss is prevented by t he closed vessel. The dissolution rate of pure gold as a function of pH for constant total cy anide concentration has been determined. Dissolution was directly proportio nal to the concentration of the cyanide ion present with the contribution t o dissolution from aqueous hydrogen cyanide too low to be measured. Ore leached in high magnesium, hyper-saline, process water showed increased gold dissolution at low pH compared to the pure gold. Reducing pH caused m ove cyanide to be consumed by the pulp but did not significantly increase t he amount lost to the atmosphere. (C) 1999 Elsevier Science Ltd. All rights reserved.