SYSTEMATIC STUDY OF MAGNESIUM THIOSULFATE GOLD LEACHING PROCESS
Thiosulfate gold leaching is an attractive alternative gold leaching method, especially when it comes to preg-robbing gold ores. The Goldstrike operation of Barrick Gold Corp, Nevada, USA is currently the only calcium-thiosulphate (Ca-TS) leaching operation in the world to leach pregrobbing gold ores. However, recovery with this thiosulfate leaching process is lower than expected. A new thiosulfate technology has been identified for the recovery of various gold ores. Ammonium thiosulfate has been used as the lixiviant along with magnesium hydroxide in the presence of copper as an oxidizing agent. The potential for substantial reduction in thiosulfate consumption while achieving high gold recovery, gives it a cutting-edge advantage over Ca-TS and other thiosulfate gold leaching processes mentioned in the literature. The aim of the study is to evaluate the effect of selected process parameters on gold extraction from the magnesium thiosulfate gold leaching process. These parameters included pH, temperature, concentration of lixiviant (S2O32-), oxidizing agents (Cu), magnesium as magnesium hydroxide as well as air (oxygen source). When pH is increased from 9.00 to the optimum 9.80 using calcium hydroxide, gold recovery drastically improves from 28.4% to 54.3%, showing significantly beneficial impact of pH on MgTS process. Further increasing the copper concentration from 1 mM to 2 mM at constant pH 10 spiked the final recovery (24 h) to be 74.4%, due to increase in the availability of the primary oxidant of copper(II)tetraamine complex while making up for any precipitated copper in the presence of Ca(OH)2. Further, response surface methodology has been deployed to the critical factors to statistically model, analysis, and optimize the Mg-TS system. Maximum gold recovery obtained under the range of conditions studied in DOE study gave 74.7% recovery. Given the optimization guidelines, optimal conditions were determined as: thiosulfate concentration of 0.14 M, cupric concentration of 2.00 mM, magnesium of 0.04 g/g and pH 11.00 giving 70.4% gold recovery. Under these conditions, the percentage of gold leaching was slightly compromised (about 4%) in an effort to reduce thiosulfate concentration from 0.19 M to 0.14 M, which amounts to a substantial reduction in reagent cost in practical application of the process.