Chloride-promoted leaching of chalcopyrite concentrate by biologically-produced ferric sulfate

The effects of chloride ions on chalcopyrite leaching by biologically-produced ferric sulfate solution and on the iron-oxidizing culture were determined. Chloride ions significantly increased chalcopyrite leaching by ferric sulfate at 67°C and 87°C, but slowed down the leaching at 50°C. At 90°C, chloride at 5gdm^-3 (0.25 g Cl^- g^-1 concentrate) increased the copper yield from 60 to 100% in approximately 2 weeks. Further increase in Cl^- concentration did not affect the leaching. Addition of chloride increased both leaching yields and iron precipitation, which shows that the passivation was not due to iron precipitation. A decreased Ag-potential of 60 mV against an Ag/AgCl reference electrode in the presence of Cl^- indicates the accumulation of partially oxidized forms of dissolved sulfur compounds such as thiosulfate and polythionate instead of elemental sulfur and, thus, a decrease in sulfur passivation. A chloride concentration of 5 g dm^-3 did not affect the iron oxidation rate of the iron-oxidizing culture dominated by Leptospirillum ferriphilum.