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

Abstract

The cyanide-ammonia system is known as an alternative system for the treatment of complex copper-gold ores. Thermodynamic research on the AuCu(Cu 2O, Cu 2S)CN −NH 4 +O 2 OH −H 2O systems resulted in producing log(mol)-mol diagrams and δG 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NH 3 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 δG values explain 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. The established process mechanism was proved during leaching of complex ores with 0.5 g/l NaCN, an initial concentration of less than 300 ppm of Cu in solution is achieved. The only cyanide determined was Cu(CN) 2 − and greater than 85% of the gold was extracted over 24 hours with subsequent copper precipitation leaving about 100 ppm Cu remaining in solution.