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Abstract
Selection of a leaching system for gold involves consideration of ore texture and mineralogy, chemical requirements, leaching techniques, the development of flowsheets, and environmental management. Aqueous dissolution chemistry for alkaline, neutral, and acid systems is mainly considered here. All systems require an oxidant to oxidise gold and a ligand to complex with gold in solution. Adjustment of pH is usually necessary.
Alkaline lixiviant systems (pH > 10)include cyanide, ammonia-cyanide, ammonia, sulphide, nitriles, and a few other minor possibilities. Oxygen is the main oxidant. Cyanide, which is the main ligand in these systems, forms an anionic complex, “Au(CN)2”, with Au(I). Gold dissolution rates are controlled by oxygen solubility in solution.
Neutral lixiviant systems (pH 5-9)include thiosulphate, halogens, sulphurous acid, and bacteria plus natural organic acids as the ligand. Oxygen is the normal oxidant and either Au(I) or Au(III)complexes are formed.
Acid leaching systems (pH ⩾ 3)may contain thiourea, thiocyanate, chlorine, aqua regia, or ferric chloride. Chloride is the ligand in the last three systems and the oxidants include chlorine, ferric chloride, hydrogen peroxide, and nitric acid which produce Au(III) anionic complexes, e.g. [AuClJ". Fast gold dissolution is possible but reagent consumptions are high. Thiourea is unusual in producing a cationic Au(I)complex, “Au(NH2CSNH2)2” and gold dissolution is slower.
For treating simple auriferous oxide-silicate-carbonate ores, and many otfier materials, cyanide remains the preferred lixiviant.
Most non-cyanide leaching systems appear to have little wide-spread practical application. Possible niche applications include the use of chlorine or aqua regia to dissolve coarse gold from gravity concentrates, oxidising acid chloride solutions for die treatment of auriferous base metal sulphide concentrates, thiosulphate for dissolving gold from gold-copper ores, and thiourea for auriferous hydrometallurgical intermediates.
