Process for biological oxidation and control of dissolved iron in bioleach liquors

Iron has a central role in bioleaching and biooxidation processes. Fe2+ produced in the dissolution of sulfidic minerals is re-oxidized to Fe3+ mostly by biological action in acid bioleaching processes. To control the concentration of iron in solution, it is important to precipitate the excess as part of the process circuit. In this study, a bioprocess was developed based on a fluidized-bed reactor (FBR) for Fe2+ oxidation coupled with a gravity settler for precipitative removal of ferric iron. Biological iron oxidation and partial removal of iron by precipitation from a barren heap leaching solution was optimized in relation to the performance and retention time (τFBR) of the FBR. The biofilm in the FBR was dominated by Leptospirillum ferriphilum and “Ferromicrobium acidiphilum.” The FBR was operated at pH 2.0 ± 0.2 and at 37 °C. The feed was a barren leach solution following metal recovery, with all iron in the ferrous form. 98–99% of the Fe2+ in the barren heap leaching solution was oxidized in the FBR at loading rates below 10 g Fe2+/L h (τFBR of 1 h). The optimal performance with the oxidation rate of 8.2 g Fe2+/L h was achieved at τFBR of 1 h. Below the τFBR of 1 h the oxygen mass transfer from air to liquid limited the iron oxidation rate. The precipitation of ferric iron ranged from 5% to 40%. The concurrent Fe2+ oxidation and partial precipitative iron removal was maximized at τFBR of 1.5 h, with Fe2+ oxidation rate of 5.1 g Fe2+/L h and Fe3+ precipitation rate of 25 mg Fe3+/L h, which corresponded to 37% iron removal. The precipitates had good settling properties as indicated by the sludge volume indices of 3–15 mL/g but this step needs additional characterization of the properties of the solids and optimization to maximize the precipitation and to manage sludge disposal.