A comparison of alkali and acid methods for the extraction of gold from low grade ores

Alternative alkali and acid methods for gold extraction from low grade copper-gold ores/tailings were studied. Several ores/tailings which originate from South and Western Australia were treated by cyanide, ammonia-cyanide, bromine and sulphuric acid, to achieve optimal reactant concentration for the selection of gold over copper. The validity of these systems was compared according to the gold leaching kinetics and other relevant criteria. It was found that different solubility of copper minerals and characteristics of predominant gangue minerals presented in the ores/tailings have a significant effect on both the concentration of copper in the solution and the consumption of the reactants. Selective copper precipitation occurs from ammonia-cyanide leach solutions, which explains the apparent selectivity of this system for gold.Leaching an ore consisting of 1.5% Cu as CuFeS 2, Cu 2S and Cu 5FeS 4 with 0.55 kg/t NaCN and 3 kg/t NH 3 allowed 89% gold extraction with only 200 ppm of copper remaining in solution. The same concentration of cyanide and a lower concentration of ammonia (2.2 kg/t NH 3) were found to be optimum for achieving 85 % extraction of gold from tailings consisting of 1.15%Cu as Cu 2S, Cu 2O and CuS. The ammonia addition in the cyanide system leads to the production of Cu(II)NH 3 species during the oxidising of gold metal to Au (CN) 2 -. Copper precipitates mostly as Cu(OH) 2, CuO, Cu 2O and CuCN. Gold is leached as Au (CN) 2 - by chemical reactions involving Cu(I) cyanides present in the system. Copper tailing containing 700-800 ppm Cu as CuFeS 2 and CuS, was leached with sulphuric acid as a pre-treatment step. This step was found to be beneficial as only 30 ppm Cu remained in solution after ammonia-cyanide leaching. Leaching of the tailing in an acid bromine solution showed much faster kinetics of gold extraction with no worse copper selection compared to the alkali systems