Kinetics of leaching fluoride from mixed rare earth concentrate with hydrochloric acid and aluminum chloride

A mixed rare earth concentrate from Baotou was adopted to investigate the complex leaching kinetics of fluoride with a HCl–AlCl3 solution. In addition, we explored the effects of HCl concentration, AlCl3 concentration, temperature, reaction time, liquid–solid ratio and stirring speed on leaching kinetics of fluoride. The results showed that an extraction of about 99% was achieved under the leaching conditions: 4 M HCl, 1.5 M AlCl3, 85 °C, 20 ml/g liquid–solid ratio, 300 rpm stirring speed and 90 min. SEM and XRD analysis indicated that no new reaction solid product was formed on the surface of leaching residue particle. The apparent activation energy was 40.4 kJ/mol and the reaction orders with respect to hydrochloric acid concentrations, aluminum chloride concentrations and liquid–solid ratio were determined to be 1.66, 1.54 and 1.48, respectively. A semi-empirical rate equation was obtained to describe the process. The kinetic data fitted a new variant of the shrinking core model, in which both the interfacial transfer and diffusion through the product layer affected the reaction rate.