Two-Stage Airlift Bioreactor System for Efficient Iron Oxidation and Jarosite Precipitation

Abstract

Continuous high-rate iron oxidation and removal of jarosite precipitates from solution at low pH and ambient temperature and pressure was successfully demonstrated. The bio-catalysed iron oxidation and jarosite precipitation is promising as a unit process for a variety of hydrometallurgical process flow sheets, where it allows for iron removal from ferrous solutions without the requirement for chemical addition and with negligible base metal co-precipitation losses. The process demonstrated performance that could be used in a large scale industry unit. A two-stage airlift bioreactor (ALBR) system comprised of two ALBRs, each with its own settler, was operated for iron oxidation and precipitation at room temperature with a mixed culture of mesophilic iron oxidisers. The two-stage reactor design allowed for optimization of overall reactor kinetics by facilitating the growth of low (430 mV vs Ag/AgCl) and high (517 mV) redox potential iron oxidizers in the respective reactors. The influent (pH 1.5) contained (g L-1) 15 Fe2+, 1.5 Cu, 1.5 Ni, nutrients and trace elements. The hydraulic retention time (HRT) was decreased stepwise to evaluate process performance. With the lowest HRTs (8 h in ALBR1 and 10 h in ALBR2), the overall iron oxidation and precipitation rates of the two-stage system were 0.75 ± 0.02 g L-1 h-1 and 0.15 ± 0.01 g L-1 h-1, respectively and overall iron oxidation and precipitation efficiencies of 94 ± 3% and 18 ± 1 %, respectively. The percent of influent Fe, S, Cu and Ni removed as precipitates from settlers were 30.9%, 16.7%, 1.1% and 0.2%, respectively. The precipitates were predominately comprised of (>95%) jarosite with potassium jarosite being the dominant form, followed by hydronium, ammonium and sodium jarosites. In conclusion, the two-stage ALBR system allowed efficient iron oxidation and precipitation of the oxidised iron as well settling jarosite with only minor loss of Cu and Ni via co-precipitation.