A review of the selective leaching of gold from oxidised copper–gold ores with ammonia–cyanide and new insights for plant control and operation

The ammonia–cyanide leach system was first patented over 100 years ago and stands out as a unique method of selectively leaching up to 90% Au and <1% Cu from oxidised copper–gold ores using <10% NaCN used in conventional cyanidation processes. However, the system has proved to be difficult to control and predict performance with different ores due to a lack of understanding of the chemistry and mechanism and proper process control. Several laboratory studies have been carried out using various empirical concentrations of ammonia and cyanide with mixed success and only a few commercial operations have been successful.This paper reviews some of the recent applied and fundamental studies on the leaching of copper–gold ores with the ammonia–cyanide system and provides insights into the mechanism to give a better appreciation of the key parameters required for the optimum leaching of gold with minimum copper dissolution. Recommended leach compositions, Eh and pH are provided to enable process control measures to be adopted for a variety of ores. The selective recovery of gold from the leach solutions by cementation or adsorption onto activated carbon or ion-exchange resins is also discussed.

Research highlights
► Ammonia-cyanide leach system reviewed with focus on past 25 years of research and commercial applications.
► System selectively leaches gold over copper with <10% NaCN consumption compared to ammonia-free system.
► Mechanism discussed and understood allowing process control to be optimised.
► Recovery of gold discussed with developments in resin functionality offering more selective RIP adsorption of gold over copper.