Gas transfer kinetics and redox potential considerations in oxidative precipitation of manganese from an industrial zinc sulphate solution with SO2/O2

In this work the oxidative precipitation of manganese from a neutral leach zinc solution ([Zn] = 150 g/L) at 80 °C using the gas mixture of sulphur dioxide (SO2) and oxygen (O2) is described from the stand point of gas transfer kinetics, effect of Oxidation–Reduction Potential (ORP) and precipitate characteristics. The study involved semi-batch experiments, where the effects of impeller type and configuration, SO2 flowrate and open vs. closed reactor on manganese removal kinetics and consumption of SO2 and base were determined. Moreover the complex relationship between SO2/O2 ratio and ORP in terms of residual manganese left in solution was established. The study showed that SO2/O2 is a fast-acting and effective oxidant for removing manganese down to ppm level provided that the appropriate reactor design, agitation and SO2/O2 ratio are employed. The precipitate was found to be in the form of rounded particles of about 10 μm diameter consisting of poorly crystalline complex manganese oxide hydrate considered to be of birnessite type.