Inhibition of lead solubilization during the leaching of gold and silver in ammoniacal thiosulfate solutions (effect of phosphate addition)

The chemistry of precious metal ammoniacal thiosulfate leaching is very complex, especially with additives, such as EDTA, which are used to stabilize the major reactants in this system. Lead, which is co-extracted in these solutions, can be a large consumer of both EDTA and thiosulfate, decreasing the leaching capacity for gold and silver. In this research, thermodynamic analyses for different compositions of the leaching solution are performed to determine their ability to extract lead, silver and gold. These are used to explain the leaching behavior observed for a sulfide concentrate and for a mineral, both with high silver and lead contents and the latter, with an elevated concentration of gold.