## 3 Results and Discussion
### The Distillation Experiments of REMES and the Phase Changes of REEs
#### 3.2.3 Further Investigation on the Phase Transformation in the Distillation Process  
To further investigate the phase transitions of REES in the distillation process, the distillation condensates harvested at 1573 K for 6 h were analyzed. The results are shown in Figure 6.  
Figure 5: The results of the experiments at different distillation temperatures: **(a)** the XRD patterns of the distillation residue; **(b)** the changes in the recovery efficiencies of the REEs and Li with temperature. Figure 6: XRD patterns and corresponding sample pictures of the condensates at 1573 K for 6 h: **(a)** the rare earth fluoride condensate; **(b)** the lithium fluoride condensate. In Figure 6a, NdF\({}_{3}\) with a small amount of PrOF was found, indicating that the evaporation of rare earth fluorides occurred at 1573 K, and PrOF also possessed the ability to evaporate in a high-vacuum environment. Interestingly, AlF\({}_{3}\) and lithium cryolite could be observed in the condensate containing LiF, which illustrated that it was difficult to obtain a high-purity LiF product. Notably, only Al\({}_{2}\)O\({}_{3}\) was discovered in the raw material, suggesting that AlF\({}_{3}\) and lithium cryolite were generated during the distillation process. These phenomena indicated that some oxide impurities might capture the fluorine from rare earth fluorides in the high-temperature and -vacuum environment, leading to the formation of REOF and RE\({}_{2}\)O\({}_{3}\). Figure 7 shows the morphology and microdistribution of the two condensates. Figure 7a shows the morphology of the condensate containing REEs, and the microdistributions of areas (b) and (c) exhibited that this condensate was mainly composed of rare earth fluorides with a small amount of Al and Ca. Figure 7e,f show the existence of aluminum and a high content of fluorine in the condensate containing Li. Although Li could not be observed by EDS, the high fluorine content suggested the possible presence of Li. These EDS results agreed with the corresponding XRD patterns. Therefore, there might be some reactions between REF\({}_{3}\) and oxide impurities, resulting in the generation of REOF during the distillation process. The potential reactions are shown as Equations (2)-(12). Since the main rare earth element was Nd, the possible reactions of Nd were used as examples. \[\text{NdF}_{3}+\text{MgO}=\text{NdOF}+\text{MgF}_{2} \tag{2}\]  
\[\text{NdF}_{3}+\text{CaO}=\text{NdOF}+\text{CaF}_{2} \tag{3}\]  
\[\text{3 NdF}_{3}+\text{Fe}_{2}\text{O}_{3}=\text{3 NdOF}+2\text{FeF}_{3}(\text{ g}) \tag{4}\]  
\[\text{3 NdF}_{3}+\text{Al}_{2}\text{O}_{3}=\text{3 NdOF}+2\text{AlF}_{3}(\text{ g}) \tag{5}\]  
\[\text{2 NdF}_{3}+\text{SiO}_{2}=\text{2 NdOF}+\text{SiF}_{4}(\text{g}) \tag{6}\]  
\[\text{NdF}_{3}+\text{Nd}_{2}\text{O}_{3}=\text{3 NdOF} \tag{7}\]  
\[\text{2 NdF}_{3}+\text{3 MgO}=\text{Nd}_{2}\text{O}_{3}+3\text{MgF}_{2}(\text{ g}) \tag{8}\]  
Figure 5:0  
Figure 5:1  
Figure 5:2  
Figure 5:3  
Equations (2)-(12) are the possible reactions in the distillation period. Due to the data of NdOF, Equations (2)-(7) were calculated according to the data of [PERSON] and [PERSON], while Equations (8)-(12) were computed by HSC 6.0 software [35; 36]. The results are shown in Figure 5:4  
Figure 8a exhibits the standard Gibbs free energy changes of Equations (2)-(7). Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\) are difficult to react with NdF\({}_{3}\), while NdOF might be generated by the reactions of MgO and CaO. Additionally, NdOF could also be produced by the reactions between NdF\({}_{3}\) and Nd\({}_{2}\)O\({}_{3}\). Figure 8b shows that the standard Gibbs free energy changes of Equations (8)-(12) will decrease with increasing temperature, while the changes are higher than 0 kJ/(mol) at 1573 K, which indicates that the reactions might have difficulty occurring at standard conditions.