Effect of mineralogy on Co and Ni extraction from Brazilian limonitic laterites via bioleaching and chemical leaching

Abstract

Iron-rich, limonitic laterites may contain economic grades of nickel and cobalt, but efficient extraction of these metals from such ores is still a challenge. In this study, four samples of limonitic laterites originating from the Barro Alto mine in Brazil were subjected to detailed quantitative mineralogical examination followed by bioleaching and chemical leaching tests including mineralogical leaching residue analyses. Bioleaching experiments with pure cultures and addition of elemental sulfur were conducted in 2L bioreactors under aerobic conditions with the sulfur-oxidizing acidophilic bacteria Acidithiobacillus (At.) thiooxidans and At. caldus, and under anaerobic conditions with At. ferrooxidans. Aerobic bioleaching of laterites with At. thiooxidans and At. caldus allowed for a lower pH of 1.0 and 1.1 respectively, and thus a higher Ni and Co extraction than anaerobic bioleaching with At. ferrooxidans at pH 1.5. Extraction of cobalt via bioleaching was in the range of 68–88 %, while extraction of nickel was between 17 % and 56 %. Mineralogical and geochemical analysis (XRD, SEM- MLA, DSC, LA-ICP-TOFMS, XRF) was conducted to (i) identify mineral phases, which are attacked by bioleaching and (ii) estimate the portions of Co and Ni, which are released by bioleaching of different mineral phases. Cobalt was mostly hosted by Mn-rich mineral phases, which were dissolved after reduction with ferrous iron generated by bacterial metabolism during oxidation of sulfur. Nickel was mostly hosted by goethite, the most abundant mineral phase, but nickel was also present in Mn-rich minerals (e.g. asbolane) and magnesium silicates (e.g. serpentine). Mineralogical and geochemical data revealed that goethite was almost unleached in all samples and nickel was mostly released from Mn-rich mineral phases and magnesium silicates. Results of this research were able to confirm reductive bioleaching of Mn-rich phases. Chemical leaching tests performed with different concentrations of sulfuric acid and with hydrochloric acid, and with sulfuric acid plus ferrous iron as a reducing agent supported bioleaching results. The results are not supporting published data on substantial reductive dissolution of goethite via ferric iron reduction activity of acidophilic bacteria.