Separation of Gold from Other Metals in Thiosulfate Solutions by Solvent Extraction

The extraction of gold, silver, copper, zinc, and nickel present in thiosulfate solutions with or without the presence of ammonia was studied. Results show that neither a primary amine nor the mixed extractants of a primary amine with tributyl phosphate or trialkyl amine oxide can separate gold from other metals in the absence of ammonia in aqueous solutions. In the case of ammonium thiosulfate solutions, the percent extraction of gold is increased tremendously in comparison with other metals, and thus gold can be separated from them. The pH50 of gold extraction for a mixed solvent of primary amine with amine oxide is about 9.0, which is very close to the pH of the actual leaching solutions. By using the mixed solvent to extract gold in the solutions from leaching of gold-containing sulfide concentrate, the separation factors of Au to Ag and Au to Cu could reach as high as 15 and 1695, respectively, for a pH less than 8.