# Rare Earth Magnet Recycling and Materialization for a Circular Economy--A Korean Perspective
## 4 Liquid Metal Extraction (LME)
RE magnets available for recycling are present in different shapes and sizes in EoL products; however, no study had been performed to assess the effect of scrap size on the extraction efficiency. [PERSON] et al. [41] compared the extraction efficiencies of three different scrap sizes with varying shapes i.e., 0.5, 1-5, and 5 mm. Oxidation and extraction were reported to have a direct relation. The smaller scrap size particles were observed to have high amounts of Nd\({}_{2}\)O\({}_{3}\) due to the high surface-to-volume ratio compared to larger particle sizes. The maximum LME reaction time was 50 min, which is insufficient to reduce Nd\({}_{2}\)O\({}_{3}\). The incomplete decomposition of Nd\({}_{2}\)O\({}_{3}\) under the conditions employed in this experiment was explained thermodynamically using a binary phase diagram of Mg-Nd\({}_{2}\)O\({}_{3}\) obtained from Factsage software. Dy had not been the focus of most of the prior research studies, even though Dy is a critical component in rare earth element magnets, especially in high-temperature applications. [PERSON] et al. [42] investigated the optimum conditions of extraction by LME reaction for both Nd and Dy in terms of the temperature, time, and Mg-to-magnet ratio, which are shown in Figure 6a-c, respectively. A maximum extraction efficiency of 67% was reported for Dy after an extraction time of 6 h at 1000 \({}^{\circ}\)C, with a Mg/magnet mass ratio of 10:1. Dy\({}_{2}\)O\({}_{3}\) was reported as the major hurdle for incomplete decomposition. The extraction of Nd is favorable compared to Dy, as 100% extraction of Nd was observed after 3 h, with a Mg/magnet mass ratio of 2:1, even at 800 \({}^{\circ}\)C. The reason why the extraction rate of Nd is relatively much faster than Dy is the reduction behaviors of Nd\({}_{2}\)O\({}_{3}\) and Dy\({}_{2}\)O\({}_{3}\), which affect the extraction efficiency. Nd\({}_{2}\)O\({}_{3}\) and Dy\({}_{2}\)O\({}_{3}\) are obstacles to extraction in their metallic form; hence, Gibbs energy changes were calculated to observe the possibility of each reduction behavior. It was shown thermodynamically that Mg can reduce Nd, irrespective of whether in the form of Nd\({}_{2}\)O\({}_{3}\) or Nd, under the aforementioned conditions. Additionally, Dy\({}_{2}\)O\({}_{3}\) can be thermodynamically reduced by Mg under the aforementioned conditions, although the activity of Mg is more required to reduce Dy\({}_{2}\)O\({}_{3}\) than Nd\({}_{2}\)O\({}_{3}\); therefore, the relatively incomplete reduction of Dy\({}_{2}\)O\({}_{3}\) causes the slower extraction rate of Dy than Nd. In order to describe the diffusion behavior of rare earth elements to Mg, Figure 7 shows an SEM image and corresponding XRD patterns of the experimental results for a reaction time of 3 h at a temperature of 900 \({}^{\circ}\)C, with a magnet-to-Mg ratio of 1:10. The upper part of Figure 7 shows the overall microstructure of the specimen, which can be divided into three distinct zones. These three distinct zones can be seen in the figure, i.e., the magnet  
Figure 6: Effects of different parameters on the REE extraction efficiency: (**a**) temperature; (**b**) time; (**c**) weight ratio of Mg to magnet material. Reprinted with permission from [42]. area (unreacted raw material--magnet only), reaction area (reaction between magnet and Mg), and Mg area, where rare earth elements are diffused from the magnet. The NdFeB magnet was composed of RE\({}_{2}\)Fe\({}_{14}\)B (gray matrix), RE-oxide (white circular phases in grain boundary), and RE-rich (white rectangular phases in grain boundary) phases, as shown in the magnet area. Mg penetrated the magnet along the grain boundaries to decompose the RE-rich phase and RE2 Fe14B matrix phase to Fe (gray matrix) and RE\({}_{2}\)Fe\({}_{17}\) (white circular phases on the Fe matrix) in the reaction zone; however, RE-oxide in the magnet cannot be reduced easily due to its affinity with oxygen, meaning it remained in grain boundary, as shown in the reaction zone.