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

- The main factors influenced the extractions of REEs and fluoride were indicated, which were different between REEs and fluoride.
- The liquid-solid ratio of 10 ml/g was reduced compared to the data (20 ml/g) given in some literatures.
- The results of leaching experiments indicated that the leaching process was efficiently with lower acid concentrate.
- With a new variant of the shrinking core model, the apparent activation energies were 26.95 and 20.70kJ/mol for REEs and fluoride.

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 °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.

The relationship between 1/3ln(1 − x) + [(1 − x)− 1/3 − 1] and reaction time for extractions of REEs (a) and fluoride (b).A new variant of the shrinking core model was used to fit the kinetic data in this work. The rate constants of different temperature were calculated with the equation of ln(1 − x)/3 + [(1 − x)−1/3 − 1] = K3t, and the correlation coefficients were both upon 0.95. The apparent activation energies were calculated to be 26.95 kJ/mol for REEs and 20.70 kJ/mol for fluoride according to Arrhenius equation.121