## 4 Conclusions  
This study introduced the phase transition of REMES in high-temperature and - vacuum conditions, elucidating the obstacles to recovering REEs by vacuum distillation. And then, the investigation for the fluorination process was implemented. The main conclusions were as follows:  
1. The distillation experimental results showed that 42.04% of the rare earth fluorides and 99.99% of the lithium fluoride in the rare earth molten salt electrolytic slag could be evaporated at 1573 K and 0.1 Pa for 4 h, and the rare earth oxides as well as the rare earth oxyfluorides would be generated simultaneously. 2. The phase transition analysis elucidated that the oxide impurities could react with rare earth fluorides under high-temperature and -vacuum conditions, capturing the fluorine element and generating rare earth oxides as well as oxyfluorides. This phenomenon significantly affected the recovery efficiency of REEs. 3. The fluorination experiments indicated that the fluorination process could fluorinate both rare earth compounds and oxide impurities, and after fluorinating the slag by 20 g NH\({}_{4}\)HF\({}_{2}\) at 773 K for 6 h, the recovery efficiency of the rare earth elements could increase to 86.23% at 1573 K and 0.1 Pa for 6 h, while some problems, such as the fluorination of Fe\({}_{2}\)O\({}_{3}\), still existed. In this paper, the phase change analysis discovered the obstacle to applying vacuum distillation, and fluorination was selected as the solution for the obstacle, proving the feasibility of REE recovery by vacuum distillation. The vacuum distillation method is an \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Procedures** & **Advantage** & **Disadvantages** & **Reference** \\ \hline Fluorination–Vacuum Distillation Method & (1) Fluorination roasting; (2) Vacuum distillation. & (1) Short process; (2) Clean process; (3) High resource utilization. & (1) High energy consumption; (2) High cost of NH4 HF2. & \\ \hline Fluoride Sulfate & (1) Magnetic separation; (2) Roasting with sulfuric acid; & (1) Low energy consumption; (2) Low production cost; (3) High resource utilization. & (1) Wastewater problem; (2) The corrosion effect of HF. & [PERSON] et al. (2017) \\ Conversion Method & (3) Absorbing HF; (4) Fluorination precipitation. & (3) High resource utilization. & (1) Low energy consumption; (2) Low production cost; (3) High resource utilization; & (1) Long process of solvent extraction procedure; (2) Wastewater problem. [PERSON] et al. (2019) \\ \hline Roasting Activation Method & (1) Activation roasting; (2) Sulfuric acid leaching; (3) Hierarchical extraction. & (1) Low energy consumption; (2) Low production cost; (3) High resource utilization; & (1) Long process of solvent extraction procedure; (2) Wastewater problem. [PERSON] et al. (2019) \\ \hline Sub-molten salt Method & (1) NaOH sub-molten salt decomposition; (2) Hydrochloric acid leaching; (3) Oakate precipitation; (4) Roasting to recover RE2O3; (5) Heating to recover Li and F and recycle NaOH and Na\({}_{2}\)CO\({}_{3}\). & & \\ \hline Borax & (3) Hydrochloric acid leaching; & (1) Low energy consumption; (2) Wastewater problem; (3) Impurity enrichment. [PERSON] et al. (2015) \\ Acid Leaching Method & (4) Recycling NaOH and F; (5) Solvent extraction to recover REEs. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The comparison between the fluorination–vacuum distillation method and other techniques. environmentally friendly method, possessing the potential of recovering resources with no wastewater or gas production. Although further investigation is still necessary, this study also hopes that this attempt at vacuum distillation could inspire readers, making it possible to put an environmentally friendly treatment into practice.