Biooxidation and precipitation for iron and sulfate removal from heap bioleaching effluent streams

Effluents from bioleaching processes cause severe problems if dispersed in the environment since they typically have very low pH values and high sulfate and ferric iron concentrations. Dissolved iron may also interfere with the metal recovery. In the bioleaching circuit, partial removal of dissolved iron and sulfate is needed to alleviate process disturbances. In this study, an integrated, bench-scale process comprising a fluidized-bed reactor (FBR) and a gravity settler was developed for controlled biological oxidation of ferrous iron and precipitative removal of ferric iron and sulfate for use in waste management of heap bioleaching processes. The FBR for iron oxidation by an enrichment culture dominated by Leptospirillum ferriphilum was operated at 37 ± 2 °C. The FBR recycle liquor was partially neutralized with 10 M KOH or 50 g/L CaCO₃ slurry to promote ferric iron and sulfate precipitation. With 6 ± 1.5 g Fe²⁺/L in the feed and KOH-adjusted pH 3.5, the oxidation rate of Fe²⁺ was 3.7 g/L h and 99% precipitation of ferric iron was achieved in the process. Adjustment with CaCO₃ to pH 3.2 slightly decreased the oxidation rate to 3.3 g/L h and 98% of ferric iron precipitated. With 15 g Fe²⁺/L in the feed, the oxidation rate was 7.0 g Fe²⁺/L h coupled with 96% precipitation of ferric iron. A solid solution of jarosite was the main product of ferric iron precipitation with KOH adjustment and with minor amounts of goethite at the higher pH range. Adjustment of the pH with CaCO₃ precipitated ferric iron also as a solid solution of jarosite, and sulfate precipitated also in the form of gypsum (CaSO₄·2H₂O) especially at the higher pH values.