Leaching of pure and cobalt bearing goethites in sulphurous acid: kinetics and mechanisms

The initial kinetics of sulphurous acid leaching of well characterised synthetic pure goethite (G–O) and goethites containing cobalt in sorption [G---Co(S)] and co-precipitation [G---Co(C)] modes have been studied. The interfacial reaction controlled nature of iron dissolution from different samples is indicated by the activation energy of about 65 kJ/mol. The rates of iron and cobalt dissolution from different samples showed a stronger dependence on the concentration of SO2(aq.) compared with other solution species such as HSO3− and H+. The particles were found to be non-uniformly attacked, preferentially on multi-domain grain boundaries. The results of infra-red (IR) spectroscopy on leach liquors indicated that SO42− is the major species formed during leaching. The half order of dependence of the iron dissolution rate on the concentration of SO2(aq.) was corroborated by theoretical analysis of the electrochemical leaching reaction by the Butler-Volmer equation. It is proposed that chemically controlled leaching of cobalt (Ea≈82 kJ/mol) from G---Co(S) may involve the reaction of two molecules of SO2(aq.) with one molecule of surface Co(OH)2 species. Cobalt and iron dissolve from G---Co(C) congruently; the dissolution is controlled by the structure and leachability of the host goethite phase.