Kinetics study on leaching of rare earth and aluminum from polishing powder waste using hydrochloric acid

In this study, a novel hydrometallurgical process consisting of hydrochloric acid three-stage countercurrent leaching and solvent extraction was proposed to recover rare earth oxide (REO) from the rare earth polishing powder waste (REPPW). The effects of HCl concentration, liquid–solid ratio (L/S ratio), temperature and time on the leaching yields of rare earths (in REO) and aluminum (in Al2O3) were studied. The result shows that the leaching yields of REO and Al2O3 are 90.96% and 43.89% respectively under the optimum leaching parameters of HCl concentration = 8.00 mol/L, L/S ratio = 4 mL/g, leaching temperature = 353 K and leaching time = 180 min. Meanwhile, the leaching kinetics of REO and Al2O3 were investigated in this study. The leaching behaviors of REO and Al2O3 follow a shrinking sphere/core model and the general leaching process is controlled by the surface chemical reaction. The leaching activation energies of REO and Al2O3 are 9.86 and 13.68 kJ/mol, respectively. The leaching yield of each substance in three-stage countercurrent leaching is improved substantially compared with single-stage leaching, with a change from 90.96% to 95.38% for REO and from 43.89% to 46.22% for Al2O3, respectively. Especially, the total concentration of REO in three-stage countercurrent leaching solution is greatly increased to above 300 g/L, and the acidity of which is decreased to ca. pH = 2, which is conducive to subsequent solvent extraction directly. High purity REO (99.92%) is obtained by solvent extraction separation, oxalate precipitation and calcination. The total recovery yield of REO is 85.13%.