**1. Introduction**

Rare earth (RE) permanent magnets have recently been employed in high-tech industrial applications, such as electric vehicles, renewable energy, robotics, and their utilization is increasing as the world shifts towards a green economy [1,2]. Even though there are various types of magnets, such as ferrite, AlNiCo, etc., the reason for the domination of the permanent market by the RE magnets is due to their superior magnetic properties, such as high remnant magnetization and coercivity [3]. The rare-earth sublattice (4f electrons) comprises essential components to stabilize the magnetization direction for the crystal axes, i.e., high magnetic anisotropy.

Recently, the focus on carbon neutrality has led to a sharp increase in the consumption of permanent magnets to achieve net-zero carbon dioxide emission. Among the RE permanent magnets, the Nd-Dy-Fe-B magnets are widely used because of their relatively low cost and high productivity, compared to the SmCo magnets. The Nd-Dy-Fe-B magnets are generally known to be composed of about 30 wt.% REEs contents. Moreover, Dy is integrated into the magnets to enhance their thermal stability and corrosion resistance [4]. Thus, the demand for Dy has been also gradually increasing with the development of green technology. However, the production of heavy REEs (HREEs) has undergone serious balance problems related to political, geological, and technical issues as the HREEs like

**Citation:** Nam, S.-W.; Park, S.-M.; Rasheed, M.Z.; Song, M.-S.; Kim, D.-H.; Kim, T.-S. Influence of Dysprosium Compounds on the Extraction Behavior of Dy from Nd-Dy-Fe-B Magnet Using Liquid Magnesium. *Metals* **2021**, *11*, 1345. https://doi.org/10.3390/met11091345

Academic Editor: Dariush Azizi

Received: 30 June 2021 Accepted: 13 August 2021 Published: 26 August 2021

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Dy are only produced from the ion-adsorbed ores in southern China [5]. In the case of ion-absorbed ores, the REEs are absorbed on the clay, which is formed with the hydrated cations. As opposed to other rare earth ore, such as the bastnaesite, monazite, xenotime, and so on, a large number of HREEs can be effectively obtained without the emission of radioactive substances by using the chemicals. However, the usage of acids and base solutions is a cause of environmental concerns.

Therefore, diversifying the supply of HREEs is significant to improve the supply chain for sustainability. For recycling, the ways to recover end of life (EOL) magnets or magnet scraps are suggested. During the magnet fabrication process, the amount of magnet scrap can be generated to about 20% to 30% [6]. Besides, the EOL magnets can be accumulated, depending on the applications, such as small electronics, electric vehicles, and wind turbines with various shapes and sizes.

One of the recycling processes for recovering HREEs from EOL magnets or scraps, the pyrometallurgy method, is a potential alternative to collect REEs from Nd-Dy-Fe-B magnet [7–9]. Among these processes, liquid metal extraction (LME) is based on a selective reaction with target metals by using solvent metals, such as Mg, Ag, Bi, and Cu [10–13]. It has the advantage of being an environmentally friendly chemical-free process without the emission of wastes and no requirement of additional reduction processes due to the direct recovery of REEs in metal form.

The Mg is a strong candidate for being an extraction agent in the LME process. It can be selectively reacted with REEs (Nd, Dy) without Fe and B intermetallic compounds due to higher chemical affinity with REEs (Nd, Dy)) compared to with Fe [13]. Previous studies have shown that Nd is easily extracted from the Nd-Dy-Fe-B magnet and the reaction mechanism is successfully demonstrated [10,14–16]. On the other hand, the low extraction efficiency of Dy was reported because of the small amount of Dy present in the Nd-Dy-Fe-B magnet and quite a different reaction behavior with Mg. Akahori et al. thermodynamically demonstrated that the oxidation of Dy can be affected by the decreasing extraction by forming Dy2O3 and not DyNdO3. It is shown that preventing the oxidized phases is decisive in improving extraction efficiency [16]. To understand the Dy extraction mechanism, Kim et al. investigated the development of (Nd and Dy)-oxide phases in the microstructures as a result of formation during the process. They offer experimental evidence that the limited extraction of Dy is caused by the formation of Dy oxide [17]. Park et al. conducted a comparison of the extraction efficiency with increasing oxide composition. It is shown that Dy was not easily extracted in the form of Dy-oxide and Dy2Fe17, while Nd was completely extracted [18]. Even though the scrap was heavily oxidized due to the small scrap size, the Dy2Fe17 phase remained as the result of the decomposition of REFeB grain. It is supported by the experimental results of Nam et al., that the phase transformation of direct reaction between DyFeB and Mg phenomenologically shows that Dy2Fe17 is first formed in the pure DyFeB phase as a byproduct, while liquid Mg is infiltrated into the grain [19]. However, the reasoning behind the influence of the Dy2Fe17 phase in the extraction process is still unclear.

In this work, the entire extraction behavior of Dy is systematically investigated, depending on the time, activity with Mg, intermediate phase, and oxides, infiltrating Mg into the Nd-Dy-Fe-B specimen. The generation and decomposition of intermediate phases are observed in detail and the Dy2Fe17 phase is clearly identified. This demonstrates that the interplay between Dy2Fe17 and oxides affects the extraction behavior of Dy.

#### **2. Experimental**

The permanent magnets for the liquid metal extraction (LME) process were supplied by Jahwa Electronics Co. Ltd., Cheongju, Korea. Pure Mg was purchased from JC Magnesium Co., Burnaby, BC, Canada. The chemical composition of the magnet is shown in Table 1 and were determined by X-ray fluorescence (XRF; Thermo Fisher Science ARL PERFORM'X, Middlesex County, MA, USA). The preparation size of the magnet sample and pure Mg is 10 × 10 × 3 mm and 10 × 10 × 10 mm, respectively. To confirm the reaction

behavior, the magnets were placed at the bottom of a mild steel crucible with Mg on top. The crucible was then placed inside a high-frequency induction furnace and heated in an atmosphere-controlled chamber for a reaction time between 30 min and 48 h. The Mg to magnet mass ratio was 15 to 1 and LME reactions were observed at 900 ◦C. This has been known, as an ideal condition, to maximize the extraction ratio [20]. For analyzing the characteristics of the magnet obtained by furnace cooling following the change in periods at 900 ◦C, the reaction sample was cut into properly sized samples using a diamond wheel cutter with a thickness of 0.3 mm. The specimens were ground using abrasive papers, which were scaled from 200 to 4000 grit SiC. Subsequently, the samples were polished using a 0.1 μm Al2O3 suspension. After polishing, the specimens were cleaned in an ultrasonic cleaner for 5 min by steeping them in ethanol. Then, the specimens were dried using a high-pressure air spray gun. The microstructure of the samples was characterized using an FE-SEM (JSM-5310, JEOL, Tokyo, Japan) and a transmission electron microscope (TEM; JEM-F200, JEOL, Tokyo, Japan). The thickness of the diffusion layer in the magnet was measured using the BSE mode in FE-SEM and the concentrations of the REE (Nd, Dy) and Mg were investigated using XRF and EDS analyses in both the diffusion layer and Mg-zone.

**Table 1.** Chemical composition of the magnet by XRF.

