A comparative study of leaching kinetics of limonitic laterite and synthetic iron oxides in sulfuric acid containing sulfur dioxide

Limonitic laterite ore of particle size 90–125 μm containing goethite, magnetite and hematite was leached for 6 h at a pulp density of 10% (wt/vol) in sulfuric acid in the absence or presence of sulfur dioxide at atmospheric pressure and 90 °C in a glass reactor vessel. The sulfur dioxide flow rate was kept at 0.6 L min−1 L−1 of slurry to maintain a constant SO2 concentration of ≈0.3 mol L−1 in solution, and the sulfuric acid concentration was varied between 0 and 0.72 mol L−1. The relative percentage extractions of Fe, Ni, Co and Mn indicate that the Fe and Ni extractions are inter-related at a ratio of Ni/Fe=0.7–0.9 and suggest the possibility of catalysis of manganese dissolution by solubilized iron(II). This leads to a Mn extraction of over 90% in less than 30 min compared with 20–40% Fe extraction in the same period, depending on the acid concentration. The initial rate of leaching of iron shows first-order dependence with respect to H+. Whilst the synthetic iron oxides leach according to the shrinking particle/sphere kinetic model, the results obtained in the first 4 h of laterite leaching can be described by a shrinking particle model with an insoluble product layer that retards the diffusion of H+ to the reaction sites at the interface. The heterogeneous rate constants for both models increase with the increase in H+ concentration. The effective diffusion coefficient of H+ (DH+) through the product layer (0.5×10−9 to 4×10−9 cm2 s−1), determined in the present study, is in the magnitude range of the reported data for DH+ in polycrystalline Fe3O4 and MnO2, but lower than DH+ in aqueous media, 9×10−5 cm2 s−1.