Gold extraction and recovery by using ammonium thiosulfate solution

Cyanide, which is a highly toxic chemical, has been used for gold leaching since the late 19th century. Thiosulfate has been proposed as a cyanide substitute due to its low toxicity. However, it is still not widely applied in the gold mining industry because of high reagent consumption and the uncertainty in the gold recovery from the leach solution. To investigate these problems, ammonium thiosulfate leaching test was conducted on a 16 g/t Au silicate type ore followed by cementation. The favourable gold leaching condition found above study was compared to that of various copper-bearing sulfide ores to investigate the effect of impurity. The efficiency of an ion exchange process was also determined.; On the gold extraction from a 60 wt%-75 μm-sized particles of ore, the preferable reagent combination was 1 mol/dm3 NH₄OH, 0.01 mol/dm3 CuSO₄ and 0.4 mol/dm3 (NH₄)₂S₂O₃ at pH 9.5, which produced 93% of gold extraction and 23 kg/t-ore of ammonium thiosulfate consumption. Upon further reduction of the ore size to 100 wt%-75 μm-sized particles, the reagent concentration was successfully reduced to 0.3 mol/dm3 NH₄OH, 0.0001 mol/dm3 CuSO₄ and 0.05 mol/dm3 (NH₄)₂S₂O₃ at pH 9.5 with improving the gold extraction efficiency to 94% and only 3 kg/t-ore of (NH₄)₂S₂O₃ consumption. From this pregnant solution, zinc powder could precipitate the gold with nearly 100% gold recovery. Additional experiments using copper and aluminium as the precipitant resulted in less than 40% gold recovery.; Despite the importance of copper as a catalyst for gold leaching, the increase of copper concentration caused a detrimental effect on the gold extraction efficiency. For example, as the copper concentration was increased from 0.0001 mol/dm3 to 0.001 mol/dm3, the ammonium thiosulfate consumption was greatly increased from 3 kg/t-ore to 21 kg/t-ore. This problem was solved by the replacement of copper by nickel as the catalyst. In the range of 0.0001∼0.005 mol/dm3 Ni2+, the ammonium thiosulfate consumption was only 1∼5 kg/t-ore with a gold extraction of 95%. The feasibility of recycling the barren solution was positive after gold cementation by zinc. Copper bearing sulfide ore required higher reagent concentrations on gold leaching because the dissolution of copper and sulfur decomposes thiosulfate.; In the ion exchange process for gold recovery, the use of copper-catalyzed ammonium thiosulfate solution caused a significant loss of catalyst (copper) during the ion exchange process. When considering of the possibility for recycling the barren solution, 70∼80% of the copper was co-adsorbed along with gold. (Abstract shortened by UMI.)...