Thiosulfate Leaching
Thiosulfate leaching is an alkaline process which removes gold from carbonaceous ores instead of using cyanide. It is less destructive and toxic, and has a higher efficiency with preg-robbing ores as a leaching agent. The major chemical components of thiosulfate are common fertilisers which include ammonium thiosulphate and ammonium sulphate. A two-part method was developed, resulting in it being an economically viable replacement to cyanidation for various types of ores which contained gold; the process of thiosulfate leaching is followed by resin-in-pulp gold extraction.
Thiosulfate leaching can produce comparable gold recovery to cyanidation and in some cases, it can produce a better recovery than cyanide leaching. This only occurs in naturally occurring carbonaceous substances such as preg-robbing ores which absorb more of the gold cyanide than the thiosulfate cyanide. By using thiosulfate instead of cyanide, a highly toxic chemical, the impact on the environment can be reduced. The process of thiosulfate leaching uses non-threatening chemicals which increases the potential to operate the technology in regions of the world where using cyanide is subject to intense negative publicity or is prohibited for environmental reasons. Since the main chemical components are common fertilisers, it allows for the possibility of using the mine tailings in agriculture applications in regions where the residential infrastructure and environmental parameters are advantageous.
Even though the process of thiosulfate leaching is more environmentally friendly, it involves more complex chemistry than cyanidation process, thus it is more difficult to optimize and more complicated to operate. It involves a chemical reaction between the thiosulphate anion, metallic gold, oxygen (oxidant), ammonium and copper ions (unconsumed catalysts) in the reaction:
4Au + 8S2O2-3 + O2 + 2H2O ? 4Au(S2O3)3-2 + 4OH
The process of thiosulphate leaching is complicated since the reaction thermodynamics are less favourable than the cyanidation reaction and it requires the optimization of each of the chemical components as well as physical parameters (i.e. pulp density and temperature) to maximize gold turnover and minimize substance losses. Therefore, more concentrated solutions are required to obtain comparable rates of gold leaching. A standard thiosulphate solution has a concentration of 5 to 20 g/L compared to a cyanide concentration of 0.25 to 1 g/L. This slightly counterbalances the cost of thiosulfate which is one fifth the cost of cyanide; significantly lower cost.

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