Kinetics of leaching of a Chinese laterite containing maghemite and magnetite in sulfuric acid solutions

The kinetics of leaching of a Chinese laterite containing maghemite and magnetite in sulfuric acid solutions was studied in a batch autoclave at temperatures ranging from 230 to 270 °C. Temperature, acid concentration and particle size had significant effects on the leaching rate while the influences of solid/liquid ratio and agitation speed greater than 450 rpm were negligible. The leaching kinetics followed the shrinking core model, with surface chemical reaction as the rate-controlling step in the early and middle stages, changing to acid diffusion control in the later stage. The leaching of magnetite consumed sulfuric acid, causing the decrease of acid concentration in the early stage. After magnetite had completely dissolved, the leaching of maghemite did not consume acid and the acid concentration kept constant. This resulted in the change of kinetic model in the early and middle stages. The activation energy was 61.710 kJ/mol for the early stage, 57.258 kJ/mol for the middle stage and 11.174 kJ/mol for the later stage. The variation of rate-controlling step was mainly due to the sheet hematite being deposited randomly in the early and middle stages and the formation of a dense hematite layer around the unreacted grains of maghemite in the later stage, as observed by scanning electron microscopy.