Recovery of Rare Earth Elements from Calciothermic Reduction Slag by Sulfation Roasting–Water Leaching Method

The calciothermic reduction slag (CRS) generated in heavy rare earth metal production, is rich in rare earth elements (REE) and highly amenable to recovery. In the present study, the CRS was treated with a H2SO4 roasting–water leaching method for the recovery of REEs. The feasibility of this process was confirmed by thermodynamic analysis. Key roasting and leaching factors governing the leaching efficiency of REE were identified and optimized. The maximum REE extraction efficiency reached 94.65% under the optimal conditions: roasting at 150 °C for 240 min with 15 mL of H2SO4, followed by water leaching at 20 °C for 60 min at a liquid–solid ratio of 15:1. Results of XRD, SEM, and EDS revealed that the REEs in the CRS were transformed into water-soluble rare earth sulfates after roasting. In the leaching process, the rare earth sulfate is efficiently extracted, whereas CaSO4 has low solubility in water. A CaSO4 product with a 98.10% purity was obtained with a calcium recovery of 90.79%, and the removal rate of fluorine in the CRS was 99.99%. The leaching kinetics of the REEs follow a diffusion plus interfacial transfer model with an apparent activation energy of –46.45 kJ·mol−1. This study demonstrates that sulfation roasting–water leaching is a viable route for the comprehensive utilization of CRS.