Effect of Particle Size Refinement on the Leaching Behavior of Mixed Rare-Earth Concentrate Using Hydrochloric Acid

Abstract

The effects of particle size, temperature, and leaching time on the leaching behavior of rare-earth elements were studied. The leaching efficiency of the rare earth reached 39.24% under leaching conditions of hydrochloric acid concentration of 8.00 mol/L, particle size 95% distributed below 1.5 μm, leaching time of 120 min, and temperature of 90 °C. According to the scanning electron microscopy and X-ray diffraction analysis of the residue, the mechanism of the leaching process was also investigated. Actually, the phase of REFCO₃ transformed into that of RECl₃ and REF₃ but there was an existing intermediate transition, where the phase of REFCO₃ on reacting with hydrochloric acid generated that of REO <i>_x</i> F_3-2<i>x</i> and this process also released RE³⁺ into the solution. REO <i>_x</i> F_3-2<i>x</i> continued reacting with hydrochloric acid to release a lot of F^-, which on combining with RE³⁺ formed REF₃ precipitation. The leaching kinetics of rare earths follows a shrinking core model that can be expressed as 1 - 3(1 - <i>x</i>)^2/3 + 2(1 - <i>x</i>) = <i>k</i> ₁ <i>t</i>. The activation energies are 62.1, 54.8, 35.1, and 34.9 kJ/mol, respectively.