The biogeochemical reactivity of phosphate during bioleaching of bornite-chalcocite ore

The interaction of acidophilic iron and sulphur-oxidising bacteria (A. ferrooxidans, L. ferrooxidans and A. thiooxidans) with a bornite-chalcocite ore at pH 4 and pH 2 was examined to determine the effect of secondary mineral coatings on bioleaching/copper solubilisation. Rock sample chips were suspended in the bacterial consortium for 24 h (providing a limited window for colonisation) to select for bacteria that could attach to mineral surfaces, and then transferred to fresh media to promote growth, i.e., colony formation and bioleaching. After 4 weeks, the ore possessed colonies on all bornite-chalcocite surfaces, evenly scattered individual cells on non-sulphidic minerals, and secondary copper phosphate at pH 4 versus iron phosphate at pH 2. Viable cell counts in both systems were two orders of magnitude higher than the original inoculum by the end of the experiment, indicating that bacteria are released from mineralised biofilms. At pH 4, the lack of iron in solution and in the precipitates formed on the ore sample indicates preferential leaching of chalcocite, with no significant solubilisation of bornite. While secondary mineral formation, i.e., copper-phosphate formed at pH 4, can limit the recovery of copper, iron phosphate mineral coatings formed at pH 2 did not impact bacterial growth or copper recovery.