An Electrochemical And Chemical Study Of The Leaching Of Copper Sulfides In Acidified Ferrous Sulfate Solutions Sparged With An O2 - SO2 Mixture

Electrochemical and chemical leaching tests were carried out on chalcocite, covellite and chalcopyrite in acidified iron sulfate solutions sparged with a mixture of O2 - SO2. The gas mixture catalyzed the ferrous to ferric oxidation reaction in situ and, therefore, increased the redox potential and oxidizing strength of the solution. Anodic polarization curves in dilute sulfuric acid on copper sulfide electrodes showed that chalcopyrite passivates and dissolves at current densities orders of magnitude less than chalcocite and covellite. The addition of a depassivating agent (Ag+) increased the passive current density of chalcopyrite and caused breakdown near 0.625 V (SCE). The depassivating effect was also observed during the leaching of chalcopyrite particulate samples as small amounts of Ag+ added to acidified iron sulfate solutions in the presence of O2 - SO2 mixtures greatly increased the leaching rate and the total amount of Cu oxidized (> 90%). Passivation was prevented in the absence of Ag+ by decreasing the initial oxidizing power of the solution and by pretreating the sample in a reductive leach. Both of these methods resulted in transformation of the chalcopyrite surface from its characteristic greenish-gold into deep black. Increased concentrations of Fe+++ due to 02 - S02 sparging were then able to oxidize the transformation product.