Ammoniacal thiosulfate leaching of refractory oxide gold ore

Ammoniacal thiosulfate leaching of refractory oxide gold ore was investigated. According to X-ray fluorescence and fire assay analyses, the ore contained about 33.01 wt% Si, 8.53 wt% Al, 7.26 wt% K, 3.00 wt% Fe, and 2.80 ppm Au. Moreover, X-ray diffraction analysis and mineralogical studies using polished thin sections showed that the ore was composed of quartz, epidote, muscovite, and orthoclase as major minerals and goethite and jarosite as minor minerals. The effects of temperature, leaching time, and the concentrations of thiosulfate, copper ion, and ammonia on the gold extraction were studied. Maximum gold extraction of 55% was obtained using thiosulfate, ammonia, and copper sulfate concentrations of 0.1 M, 3 M, and 0.0125 M, respectively. This extraction value was achieved after room temperature leaching of a pulp with a density of 20% for 16 h. Stirring speed and the pH of the aqueous solution were 400 rpm and 10, respectively. Leaching rate using the preceding optimum conditions showed a transition after a leaching time of 2 h indicating a change in the process mechanism.