# Rare Earth Magnet Recycling and Materialization for a Circular Economy--A Korean Perspective
## 5 Vacuum Distillation  
The relationship between the magnesium vapor pressure and temperature can be calculated from Equations (1) and (2), where P is the magnesium vapor pressure in Pa and T is the temperature in K [46]:  
\[ \begin{split}\text{log P}=(3.27-2.95\times 10^{-3}\text{T}-7.64\times 1 0^{-3}\text{T}+8.74\times 10^{-7}\text{T}^{2}+2.5\text{logT})\times 133.3\\ 570<\text{T}<924\text{K}\end{split}  \tag{1}\]  
\[ \begin{split}\text{log P}=(11.61-7.61\times 10^{3}\text{T}-1-1.02 \text{logT})\times 1.33\\ 924<\text{T}<1380\text{ K}\end{split}  \tag{2}\]  
Only one research study has so far reported the use of vacuum distillation for the separation of REEs from Mg alloy in recycling processes [36]. [PERSON] et al. performed VD on several 8 g samples of Mg-Nd alloy. The vacuum distillation experiments were performed in two steps to enhance the Mg removal efficiency; Mg-Nd alloy was heated to a temperature of 1123 K for two to six hours, then subsequently it was heated at 1313 K in the second step. Nd of 97.7 mass% was achieved as a result of this two-step vacuum distillation process. As mentioned earlier, the Mg-Nd master alloy obtained after the LME experiment was cut into several pieces of 10 g each for the next step in the VD process. The distillation temperature must be above the melting point and below the boiling point of the metal with the higher vapor pressure (in this case Mg) so that it can melt and subsequently evaporate. The vapor pressure of Nd is extremely low compared to Mg, even at 1000 \({}^{\circ}\)C, i.e., approximately 0.1 Pa compared to 73,967.3 Pa for 0.73 Mg at the same temperature. This implies that liquid Mg has weak intermolecular forces and evaporates easily as compared to Nd [47]. It was not necessary for the temperature to be much greater than the melting point of magnesium (650 \({}^{\circ}\)C); therefore, a temperature of 800 \({}^{\circ}\)C was set as the temperature for the first batch of VD experiments and distillation experiments were carried out for periods of 10 min to 6 h.  
Although Mg evaporation did take place even after 10 min of distillation, there was no change in the morphology as compared to that of the LME sample, and Mg remained the dominant phase. Mg evaporation increased from 5 min onwards, although no change in the  
Figure 8: Extraction efficiency of rare earth elements from the magnet to Mg with time. microstructure was observed until 20 min. The microstructure of the resultant samples after distillation times of 20, 40, 60, 90, 120, and 180 min are shown in Figure 9a-f, respectively. During the vaporization of Mg, Mg-Nd phases can be evolved into other phases. Because Mg\({}_{12}\)Nd is the most stable phase in this system, Mg-Nd phases exist only as Mg\({}_{12}\)Nd, as shown in Figure 9. For the 20 min specimen, only dark Mg and light gray Mg\({}_{12}\)Nd coexisted, as shown Figure 9. As mentioned above, Mg is easily vaporized under pressure, hence Mg-related phases are converted to other phases very quickly. In addition, Mg\({}_{12}\)Nd is the most stable phase in this system, meaning Mg-Nd phases exist only as Mg\({}_{12}\)Nd; therefore, after only 40 min of distillation time, a white region containing Nd metal started to appear in the resultant Mg-Nd alloy, which has been highlighted. The dark region that was previously identified as \(\alpha\)-Mg phase and the light grey region that was previously identified as Mg\({}_{12}\)Nd decreased with increasing time. This was because the phase transformation of Mg to Mg\({}_{12}\)Nd and then to Nd occurred. Finally, the white region increased with increasing distillation time until it became the dominant phase at distillation times of 120 min and above. In order to confirm these findings, XRF measurements of the samples at the six mentioned distillation times were carried out to check the Nd purity levels in wt.% with increasing distillation time. The results are shown in Figure 10.  
As expected, the Nd concentration increased with increasing distillation time and the Mg concentration decreased.