Mechanochemical processing of gold-bearing sulphides

Samples of pure pyrite and arsenopyrite and a gold-bearing pyrite/arsenopyrite concentrate have been mechanically milled in inert and oxidizing atmospheres. Pyrite was found to be essentially inert with only slight oxidation to FeSO₄ evident only after 100 h milling in oxygen. Arsenopyrite completely oxidized to As₂O₃ and FeSO₄. The milled concentrate was more complex and showed quartz, pyrite and an unidentified phase as the products of milling in oxygen. Leaching of the milled concentrate in 3% HCl showed 93% dissolution leaving quartz and pyrite as the only phases present. The gold, which was present only in the arsenopyrite, remained in the solid residue which would be expected to be readily treated by alkaline cyanidation. Thus, it appears that milling in oxygen allows selective oxidation of arsenopyrite over pyrite providing an alternative, low temperature process for extracting gold from these types of ore.