# Rare Earth Magnet Recycling and Materialization for a Circular Economy--A Korean Perspective
## 4 Liquid Metal Extraction (LME)  
Mg was chosen as an extraction agent as it selectively reacts with REEs in REE magnets, leaving behind Fe-B. This was first demonstrated by [PERSON] et al. [35], whereby Mg infiltrated the magnet to pave way for the REEs to diffuse into Mg to form intermetallic compounds. Mg was used as an extraction agent in the ReMat process because: (1) it has a strong chemical affinity with Nd; (2) it hardly reacts with iron; (3) it shows high vapor pressures over 800 \({}^{\circ}\)C (0.045 atm@800 \({}^{\circ}\)C), meaning the elimination and transportation of Mg through the gas phase is easy; (4) its melting point is 922 K (649 \({}^{\circ}\)C) and it can be recovered and re-used via condensation into a pure solid [36]. Additionally, Mg is a superior metallic material for structural components in the growing aerospace and automobile industries due to its advanced physiomechanical properties [37]. Combined with the reported abundant resources of Mg in the earth's crust, this implies that Mg will be readily available for REE recycling in the near future. [PERSON] et al. proved the viability of using Mg as an extraction agent. The diffusion behavior and diffusion coefficient were investigated in detail by [PERSON] et al. [38]. It was assumed that Dy exhibits a similar behavior as Nd, although the behavior of Dy had not substantiated by research studies; therefore, [PERSON] et al. carried out research on the extraction behavior of Dy, whereby it was established that the extraction of Dy is difficult as compared to Nd because of the formation of the Dy-oxide phase. Oxygen has a greater affinity with oxygen, leading to the reaction of Dy with O, which is difficult to reduce. An experimental demonstration of the high affinity of Mg for Nd resulting in the Mg-Nd intermetallic compounds and studies on detailed diffusion behavior had previously been carried out, although the possibility of the further separation of Nd from Mg-Nd alloy remained unexplored. [PERSON] et al. [39] presented a two-step process for the direct extraction and recovery of Nd from magnet scraps. Mg-Nd alloys with varied compositions in the range of 22-63 mass% were achieved from LME reaction. These high concentrations of Nd in Mg using LME were unprecedented, which were achieved by introducing the concept of the recirculation of Mg. In the proposed apparatus, Mg reacted with magnet scrap material inside the iron crucible, taking the Nd away from it. The Nd-Mg liquid fell into the tantalum crucible, from where only Mg evaporated to condense on the top of the apparatus. The condensed Mg dropped into the iron crucible to react with the magnet scrap material. The recirculation of Mg is a novel concept, which was executed by maintaining a temperature difference between the top and bottom of the equipment. Through this recirculation process, the amount of Mg required was substantially decreased because of the recirculation principle, which was an innovative development from an environmental perspective. [PERSON] et al. [40] further optimized this process in view of the major process parameters, such as the magnet-to-scrap ratio and temperature. Figure 5: Flowchart of the zero-emission ReMAT process. Although these attempts were successful, they were only performed at the laboratory scale, meaning the feasibility at the commercial level needed to be checked. The Hitachi Corporation utilized this technology for magnet recycling [40]. They utilized a disassembly machine, which is crucial to obtain magnets from the recycling process. The machine could dissemble and remove coatings from magnets found in compressors in air conditioners and from HDDs. It takes one hour for the disassembly of 12 HDDs manually, whereas this machine can take apart 100 HDDs in the same time. Overall, a capacity to process 40 kg of magnets per day was reported. The extraction efficiencies in the semi-lab-scale process for Nd and Dy were reported to be 100% and 60%, respectively.