Investigation on the Recovery of Rare Earth Fluorides from Spent Rare Earth Molten Electrolytic Slag by Vacuum Distillation

Spent rare earth molten salt electrolytic slag (REMES) needs to be recovered not only because of its economic value of rare earth elements (REEs), lithium, and fluorine, but also for the environmental benefits. Vacuum distillation has many advantages, such as a short process and less wastewater. Aiming to find an environmentally friendly method to recover REEs, this research studied the challenges in recovering REMES by vacuum distillation and the solutions to handle these obstacles. Distillation experiments for the raw material were initially implemented and XRD, XPS, DSC, and SEM methods were used to investigate the phase changes of REMES, thus discovering that oxide impurities could transform REF₃ into REOF, which significantly affected the REEs recovery. Only 42.04% of the REEs could be evaporated at 1573 K and 0.1 Pa for 4 h with 99.99% of LiF. To tackle this issue, a fluorination pretreatment was proposed. NH₄HF₂ was utilized to transform oxide impurities, RE₂O₃, and REOF to fluorides with almost no waste gas released, significantly improving the recovery efficiency of the REEs, which was 86.23%. Therefore, this paper proposes this fluorination-vacuum distillation method, which has a short process to recover REF₃ from REMES efficiently with almost no wastewater or gas released.