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+ (D H+) 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