Experimental Study on Recovering Rare Earths from Separation Slag of Ni–Metal Hydride Battery Wastes by Hydrochloric Acid Leaching

The RE-bearing slag used in the experiment was obtained from Ni–metal hydride battery wastes by H2 selective reduction and melting separation method, and the slag composition mainly consisted of 46.44 wt% REO, 26.05 wt% SiO2, 17.68 wt% Al2O3 and 6.32 wt% MnO. The influence of different reacting factors on the leaching rate of rare earth was investigated, and subsequently, the leaching dynamics was analyzed at low temperature and ordinary pressure and at high pressure and high temperature separately. The results showed that the optimal leaching parameters for the low-temperature and ordinary pressure conditions were 2.69 mol L−1 hydrochloric acid, 10:1 liquid to solid ratio at 85 °C for 75 min and the rare-earth leaching rate reaching 94.94%. According to Arrhenius formula, the apparent activation energy was 57.68 kJ mol−1 and the preexponential factor was 5.02 × 104 s−1, and the related reaction orders of the hydrochloric acid concentration and particle size were 3.63 and − 1.59, respectively. For the high-pressure and high-temperature leaching process, the optimal conditions were 2.49 mol L−1 hydrochloric acid, 10:1 liquid to solid ratio at 130 °C for 25 min and the rare-earth leaching rate 96.70%. The apparent activation energy analyzed was 53.63 kJ mol−1, and the preexponential factor was 5.32 × 103 s−1. The two leaching processes were both controlled by the interfacial mass transfer and solid film diffusion.