Leaching:All hydrometallurgical gold extraction routes use a leaching step to produce a gold solution as an intermediate product. Currently diluted alkaline cyanide solutions are use exclusively for gold dissolution, although chlorine media has been use in the past. Others lixiviants such as thiourea, thiosulphate, bromide and iodide solutions are also potential alternatives to cyanide leaching, but none has yet been used commercially.
Cyanide leaching can be applied in several forms: agitated leaching, heap or dump leaching, vat leaching, and intensive leaching. Agitated leaching systems are use for the treatment of ground slurries or reclaimed tailings. The product from agitation leaching must either be subjected to one or more stages of solid-liquid separation to allow gold recovery from the solution or may be treated in pulp with carbon or resin for gold recovery. These in pulp processes can be also incorporated into the leaching circuit for treatment of mildly carbonaceous ores and are referred as carbon in leach and resin in leach.
Heap or dump leaching can be applied to ores where gold occurs in a form that can be al least partially liberated without grinding. The process is performed on run-of-mine or crushed ore, and is most suitable for treatment of permeable ore types, although agglomeration processes have been developed to improve the performance with less permeable ores.
Vat leaching is essentially a flooded heap leach with the solution and the ore contained within a vessel or other suitable impermeable impoundment. Its application is limited to the leaching of unusual materials that no respond well to heap or dump leaching but do not require grinding for gold liberation, for example low grade oxide/free milling ores with most the gold present as coarse particles. This is a rarely used process option because of the generally superior economics of heap and agitated leaching systems.
Intensive cyanidation leaching has been used commercially for the treatment of gravity concentrates containing coarse gold. The leaching kinetics are increased by increasing cyanide and oxygen concentrations and, where necessary, by elevating temperature and pressure. Fig. 4.14 Carbon in Pulp Process.