Leaching kinetics of rare earth elements and fluoride from mixed rare earth concentrate after roasting with calcium hydroxide and sodium hydroxide

The complex leaching kinetics of rare earth elements and fluoride with a HCl-AlCl3 solution was investigated for the mixed rare earth concentrate after roasting with calcium hydroxide and sodium hydroxide. The effects of HCl concentration, temperature, reaction time, liquid-solid ratio (L/S), AlCl3 concentration and stirring speed on leaching kinetics were explored. The results show that the extractions of REEs and fluoride were 86.05% and 90.23%, respectively, under the conditions: 3 mol/L HCl, 0.5 mol/L AlCl3, 70 degrees C, 10 mL/g liquid-solid ratio, 300 rpm stirring speed and 40 min. The XRD and SEM analysis of the samples before and after acid leaching show that the rare earth oxides and fluorite were completely leached. The apparent activation energies of leaching REEs and fluoride were calculated, which were 26.95 kJ/mol for REEs and 20.70 kJ/mol for fluoride. The kinetic data of extractions of REEs and fluoride all fitted a new variant of the shrinking core model, in which both the interfacial transfer and the diffusion through the product layer affected the reaction rate.