Iron oxidation and jarosite precipitation in a two-stage airlift bioreactor

Continuous iron oxidation and removal of jarosite precipitates from solution at low pH, ambient temperature and pressure was successfully demonstrated with a two-stage airlift bioreactor (ALBR) system. The ALBR system, comprising 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 optimization of the overall reactor kinetics by facilitating the growth of two distinct groups of iron oxidizers at low (430 mV vs. Ag/AgCl) and high (517 mV) redox potential in the two reactors, respectively. The influent with 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 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. The process demonstrated performance that is promising for a variety of hydrometallurgical process flow sheets, where it allows iron removal from ferrous solutions without chemical addition and with negligible base metal co-precipitation losses.