Continuous flow 70 °C archaeal bioreactor for iron oxidation and jarosite precipitation

Abstract

This study is the first demonstration of a continuous culture bio-catalysed iron oxidation and jarosite precipitation reactor using thermophilic archaea, for use in hydrometallurgical process flow sheets to oxidize and partially remove iron from solution while minimizing valuable base metal (i.e. Ni and Cu) losses. A two-stage continuous stirred tank reactor (CSTR) system comprised of two CSTRs, each with its own settler, was operated to achieve biological iron oxidation and precipitation at 70°C. The two-stage reactor configuration was used to allow the growth of microorganisms that prefer different redox regimes. The bioreactors were inoculated with a mixed culture of extreme thermophilic iron oxidisers from genera Acidianus, Metallosphaera and Sulfolobus. The influent (pH1.5) contained (gL−1): 15 Fe2+, 1.5 Cu, 1.5 Ni (all as sulfates), nutrients and trace elements. The system demonstrated efficient iron oxidation and precipitation of the oxidised iron in the form of well settling jarosite with only minor losses of Cu and Ni via co-precipitation. At a total hydraulic retention time (HRT) of 13–14h in the two reactors, the overall iron oxidation rate was up to 1.0gL−1h−1 and conversion extent of >95%. During this time, the pH values were 1.3–1.6 and 1.3–1.6, and redox potentials (Ag/AgCl reference) were 440–530mV and 510–610mV, in the first and second reactor, respectively. The extent of Fe and sulfate removal, was relatively high, 52 and 25% respectively, while valuable Cu and Ni losses were low, i.e. 0.46% and 0.03%, respectively. The precipitates were exclusively jarosite, with potassium jarosite being the dominant form (38–51%), followed by hydronium (30–35%), ammonium (13–18%) and sodium jarosites (6–9%). The precipitates had good settling properties facilitating effective solid–liquid separation. Bioreactor performance was, however, found to be sensitive to instantaneous exposure to high concentrations of typical contaminants in hydrometallurgical processing streams, when added simultaneously. The addition of (gL−1) 2.0 Al, 0.05 As, 0.05F, 0.2 Co, 5.0 Mg and 0.4 Mn to the influent solution decreased iron oxidation and jarosite content in precipitates.