Systematic study of critical parameters on magnesium saturated thiosulfate gold leaching process; part A: Effect of ammonia and lime for pH adjustments and concentration of magnesium and copper(II)

Abstract

Thiosulfate gold leaching is an alternative gold leaching method, especially for leaching of preg-robbing gold ores. Here we report a thiosulfate process that has been developed for gold leaching, with emphasis on lowering thiosulfate consumption in the process. Ammonium thiosulfate has been used as the primary lixiviant along with magnesium hydroxide to saturate the solution with magnesium in the presence of copper(II) as an oxidizing agent. The resulting magnesium thiosulfate solution (Mg-TS) can potentially leach gold from the ore at considerably high efficiency. The potential for substantial decrease in the thiosulfate consumption, while achieving high gold recovery, gives the Mg-TS process a cutting-edge advantage over other processes. The study demonstrates the effect of pH, magnesium, and initial Cu(II) concentration on gold leaching kinetics in the Mg-TS process. As a comparison, pH of the tests was controlled with Ca(OH)2 and NH4OH; it was concluded that high pH, if controlled by NH4OH, is beneficial for gold leaching due to increase in ligand (NH3) availability and favorable pH range. Increasing pH using Ca(OH)2 was only effective up to a pH threshold of 9.8, after which the efficiency of the process decreases due to the precipitation of Cu(II) catalyst/oxidant. Increasing copper(II) concentration to 8 mM leached 87.8% of gold which was very close to the gold recovery with cyanidation (87.3%). The Mg-TS process identified is an operationally green and sustainable, efficient and low-cost thiosulfate process, which is feasible for in-situ/heap leaching of low-grade gold ores and is believed to present a competitive advantage over cyanidation process.