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

The spent rare earth molten salt electrolytic slag (REMES) needs to be recovered not only its economic value of rare earth elements, lithium and fluorine, but also the environmental benefits. Vacuum distillation has many advantages, such as a short process and less wastewater. In this work, the structural characterization of REMES and recovery of REMES by vacuum distillation was analyzed. The structural characterization showed that the slag was mainly composed of rare earth fluorides with some forms of rare earth oxyfluorides, carbon and some impurities oxide. The directly distillation experiments showed that 42.04% of rare earth and 99.99% of lithium fluoride could be evaporated at 1573 K and 0.1 Pa for 4 h, and discovered the phase transition from REF3 to NdOF in the function of SiO2、Al2O3、Fe2O3 of in the distillation process, which terribly affected the recovery efficiency of REEs. Therefore, the fluorination pretreatment was firstly proposed to transfer oxide impurities, rare earth oxides and rare earth oxyfluoride to fluorides. Fluorinating the slag by 20 g NH4HF2 significantly improved the recovery of rare earth elements, which was 86.23%. This research aimed to find a new method based on vacuum distillation to recover rare earth elements with no waste water, promoting the advance of environmentally friendly secondary resources recovery methodologies.