The mechanism of gold extraction and copper precipitation from low grade ores in cyanide ammonia systems

The cyanide-ammonia system is known as an alternative system for the treatment of complex copper-gold ores. Thermodynamic research on the Au-Cu(Cu2O, Cu2S)-CN--NH4+-O-2-OH--H2O O-2-OH--H2O systems resulted in producing log(mol)-mol diagrams and Delta G values for relevant reactions. In log(mol)-mol diagrams the thermodynamic possibility of the existence of both the Au(CN)(2)(-) and Cu-CN ions was found. Copper cyanidation is faster than gold cyanidation which leads to cyanide depletion and retards gold leaching. Ammonia involved in the system produces the Cu-NH3 species with a thermodynamic priority over Cu-CN species and also allows better stability of Cu (I) cyanide ions, shown in log(mol)-mol diagrams, These Cu(I) cyanide ions are responsible for gold leaching in the absence of free cyanide. Copper precipitation occurs as a parallel process, leaving sufficient Cu(I) cyanide for gold leaching, The critical value of oxygen concentration in the system was established which separates the areas of thermodynamic stability of CuCN and CuO/Cu(OH)(2). The reactions with negative Delta G values the plain the production of Cu(OH)2. The results of kinetic research showed that this component of precipitate is stable under optimal ammonia addition beyond the critical oxygen concentration, The presence of Cu-sulphides enhances gold leaching while Cu-oxides allow better copper precipitation. (C) 1997 Elsevier Science Ltd.