*3.2. Phase Composition*

The main phase of the NdFeB magnet scrap is Nd2Fe14B [54,57], which accounts for 96–98%. Herbst et al. indicated the unit cell structure of Nd2Fe14B. The space group is *P*42/*mnm*, and there are four Nd2Fe14B units (68 atoms) per unit cell. All the Nd and B atoms, but only four of the 56 Fe atoms, reside in the z = 0 and z = 0.5 planes. Between these, the other Fe atoms form puckered, yet fully connected, hexagonal nets [58]. Apart from Nd2Fe14B, NdO, Nd2O3, and the minor NdFe4B4 may appear in the grain boundary [51,55].

## *3.3. Microstructural Morphology*

The coercivity of the NdFeB magnets is closely related to the interface microstructure between main phase (Nd2Fe14B) and the grain boundary phase (Nd-rich) [59]. Figure 5 shows the SEM images of the NdFeB magnet. Grain sizes of the NdFeB magnet are very small (Figure 5 left), and small agglomerates of the Nd-rich phase are also observed. The field emission gun scanning electron microscope image (Figure 5 right) shows a uniform continuous coating of Nd2Fe14B grains, where the thickness of the Nd-rich grain boundary phase is a few nanometers, separating the individual grains. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis was carried out to identify the compositional variation in the NdFeB magnets, and the results are listed in Figure 6. The results indicate that the Nd is concentrated in the grain boundaries instead of within the grains, and the Fe is concentrated within the grains.

**Figure 5.** SEM images of NdFeB permanent magnets [59]; published by Elsevier, 2004.


**Figure 6.** SEM-EDS analysis of NdFeB permanent magnets [54]; published by Elsevier, 2014.

The distribution of Nd, Dy, C, Al, Si, Fe, Ce, and Pr in the NdFeB magnet scrap was analyzed using energy dispersive X-ray spectroscopy (EDX) mapping. As shown in Figure 7, the Fe is most abundant in the matrix, while Nd and Pr are located in the grains and concentrated on the grain boundaries. Dy and Ce are nearly evenly distributed over the surface [60]. According to Önal et al., the area dominated by Fe indicates the Nd2Fe14B phase and the area highlighted by Nd and O represent the grain boundary phase. An area with a high B concentration represents the presence of Nd1Fe4B4 phase. They also concluded that all target metals are distributed over the entire microstructure of the magnet and the structure needs to be completely destroyed to recover valuable metals from the NdFeB magnet [61].

**Figure 7.** High-resolution EDX mapping of the NdFeB magnet scrap [60]; published by Elsevier, 2020.
