Phase evolution and oxidation characteristics of the Nd–Fe–B and Ce–Fe–B magnet scrap powder during the roasting process

2.1 Materials and pretreatment

In this study, two different kinds of block magnet scrap (Nd–Fe–B and Ce–Fe–B) were used, supplied by Inner Mongolia Baotou Steel Rare Earth Magnetic Materials Co., Ltd. The chemical compositions of the magnet scrap were analyzed by inductively coupled plasma-atomic emission spectrometry, as shown in Table 1. After demagnetization under vacuum conditions, the block magnet scrap was powdered by mechanical grinding in a closed environment. A sieved powder of 80 mesh (particle size less than 180 µm) was prepared for the oxidation roasting experiment.

2.2 Oxidation roasting process

The oxidation roasting process of the Nd–Fe–B and Ce–Fe–B magnet scrap powder was conducted at a larger scale to produce a sufficient amount of oxidized samples for characterization analysis and for the subsequent leaching experiment. For the roasting test, five samples with 100 g of each scrap powder were oxidized at 800°C in an ambient air muffle furnace (without blowing extra air or oxygen into the furnace). A 100 g of scrap powder for each sample was laid in a corundum crucible with a diameter of 70 mm and a height of 80 mm.

Such a large amount of scrap powder was difficult to entirely oxidize in one cycle of roasting even for a very long time. This may be due to the formation of a protective crust, effectively sealing the bulk of the unoxidized powder from the atmosphere. Thus, the samples were roasted at 800°C for different cycles of roasting until the powder samples were completely oxidized. The repeated roasting experiments were performed with a time interval of 2 h. After each roasting, the sample was removed from the muffle furnace and cooled to room temperature. During cooling, the upper part of the crucible was covered with an alumina sheet to reduce or even avoid further oxidation of the sample. The incompletely oxidized scrap powder was again mixed and continued to roast in the muffle furnace. Correspondingly, the mass gain (%) was recorded by an analytical balance for each of the five samples and averaged. The mass gain after each roasting experiment was calculated by equation (1):

2.3 Sample analyses and characterization

The phase structures of scrap powder at different roasting times were characterized by D8 ADVANCE X-ray diffraction (XRD) with Cu-Kα X-ray radiation. The scanning 2θ range was 20–70°, and the step size increment was 0.02° with a scanning speed of 2°/min⁻¹. The treated scrap powders were mounted in epoxy resin and then polished. This operation allowed the observation of cross sections of particles contained in powders. Examinations for morphological observation, point analyses and element mapping of the oxidized phases were performed using scanning electron microscopy (SEM, JSM-6510) with energy dispersive spectroscopy (EDS) after spraying with gold.

3.1 Mass gain

According to the chemical composition listed in Table 1, Fe, REs, B and Al for Nd–Fe–B and Ce–Fe–B magnet scrap were assumed to be oxidized to their corresponding oxides, Fe₂O₃, RE₂O₃, B₂O₃ and Al₂O₃, in calculating the theoretical mass gain (%). Here, the oxidation of Nd to Nd₂O₃ was used to analyze the theoretical mass gain to simplify the calculation for the Nd–Fe–B magnet scrap. The oxidation of Ce to CeO₂, Gd to Gd₂O₃ and Nd to Nd₂O₃ were chosen due to the differences in the valence states of the stable oxides of Ce, Gd and Nd for Ce–Fe–B magnet scrap. Table 2 presents the theoretical mass gain calculated by elemental oxidation. After complete oxidation, the total theoretical mass gain for the Nd–Fe–B and Ce–Fe–B magnet powder samples should be 35.76 and 37.44%, respectively. Figure 1 shows the oxidation mass gain curve for four roasting cycles. It appears that the mass gain curve is approximately followed a parabolic law. With increasing to the fourth cycle of oxidative roasting, the mass gain for the two kinds of powder samples reached 99% of the theoretical mass gain rate. The full conversion of the components in the Nd–Fe–B and Ce–Fe–B magnet samples to their oxides was achieved after the fourth cycle of roasting.

Figure 1

Mass gain curves of the Nd–Fe–B and Ce–Fe–B magnet scrap powders oxidized at 800°C for different roasting cycles.

3.2 Phase evolution

The phase evolution of the Nd–Fe–B and Ce–Fe–B scrap powder samples during the oxidative roasting process at 800°C is illustrated in Figure 2. The major phase exhibited was the crystal structure of the Nd₂Fe₁₄B phase both for the initial Nd–Fe–B and Ce–Fe–B scrap powders. Furthermore, several small diffraction peaks were observed, but no corresponding phase was matched. Based on microstructure observations of Nd–Fe–B or Ce–Fe–B magnets [31,32], a Nd/Ce-rich grain boundary phase and a small amount of η phase were present apart from the matrix phase of the Nd₂Fe₁₄B structure.

Figure 2

Phase evolution of the Nd–Fe–B and Ce–Fe–B magnet scrap powders with different roasting cycles at 800°C. (a) Nd–Fe–B scrap powder; (b) Ce–Fe–B scrap powder.

Since the lattice constant of RE oxides is similar, the compound of Nd here represents the isomorphous compound of all RE elements for the oxidation of the Nd–Fe–B scrap powder. As shown in Figure 2(a), Fe and Fe₂O₃ were the major phases, and the peaks of Fe₃O₄ and NdFeO₃ were also observed after the first cycle of roasting. Subsequently, the intensities of the Fe peaks gradually decreased, while the intensities of the Fe₃O₄ and Fe₂O₃ peaks increased. Upon the fourth cycle of roasting, the peaks of the Fe and Fe₃O₄ phases completely disappeared, indicating that the Fe phase was fully oxidized. In the second cycle of oxidative roasting, the diffraction peaks of NdBO₃ began to appear. Then, the intensities of the NdFeO₃ and NdBO₃ peaks gradually strengthened with the increase in roasting cycles. After the fourth roasting cycle, the final oxidation phases consisted of Fe₂O₃, NdFeO₃ and NdBO₃, consistent with the oxidation products of the block Nd–Fe–B magnet reported by Firdaus et al. [29].

As presented in Figure 2(b), Fe and Fe₂O₃ were examined for the main phases, and GdFeO₃ and CeO₂ were slightly detected after the first cycle of roasting for the Ce–Fe–B scrap powder. In the second cycle of roasting, the NdBO₃ phase began to appear except for the RE-containing GdFeO₃ and CeO₂ phases. With increasing roasting cycles from the first to the fourth, the intensities of the Fe peaks were progressively reduced, while the intensities of the Fe₂O₃, GdFeO₃, CeO₂, and NdBO₃ peaks were gradually enhanced.

It should be noted that CeO₂ is barely insoluble in hydrochloric acid compared with other trivalent RE oxides [33]. Therefore, the first separation of Ce and other REs could be realized by oxidation roasting-selective leaching by hydrochloric acid for the recycling of Ce–Fe–B magnet scrap. The oxidation of Ce(iii) to Ce(iv) for the selective separation of Ce from the other RE elements by hydrochloric acid leaching is also used for bastnaesite after the calcination treatment [34,35]. Finally, an RE chloride solution containing Ce-less and Ce-rich residues can be acquired. Figure 3 shows the XRD analysis of the residue obtained by the selective leaching of the roasted Ce–Fe–B scrap powder using hydrochloric acid. Figure 3(b) and (c) shows that the individual CeO₂ phase was in the residue. Moreover, the CeO₂ peaks in the fourth cycle of roasting and the residue had very good consistency for the 2θ value, as shown in Figure 3(a) and (b). The specific results regarding the leaching of REs using hydrochloric acid for the roasted Ce–Fe–B scrap powder are reported in another article.

Figure 3

XRD patterns of completely oxidized Ce–Fe–B magnet scrap powder and the residue after leaching with hydrochloric acid. (a) XRD pattern of completely oxidized Ce–Fe–B magnet scrap powder; (b) XRD pattern of the residue; (c) standard XRD pattern of CeO₂.

Overall, the variation in the peak intensities of the oxidized phases with increasing roasting cycles was most likely similar in the Nd–Fe–B and Ce–Fe–B magnet powder samples. This implied that the oxidation processes of the elements in the two kinds of magnet scrap powders are similar in some areas. Summarily, the sustained oxidation of Fe to Fe₂O₃ and the continuous formation of the composite RE oxide were the main reaction processes for the two scrap powders as the roasting cycles increased.

According to a report [29], the oxidation process begins to occur by the dissociation of the Nd₂Fe₁₄B phase as follows:

As the oxidation process proceeds, the decomposed Nd as well as B can rapidly react with oxygen because it has a higher affinity than iron in the following form:

The XRD results in Figure 2(a) showed no evidence of Nd₂O₃, which was probably due to the low-volume fraction of Nd₂O₃ or the reaction of the formed Nd₂O₃ with Fe oxide at 800°C.

For Fe oxidation, it is initially oxidized into FeO, then Fe₃O₄ and finally to Fe₂O₃, following the principle of stepwise oxidation of Fe as follows:

With continuous oxidation roasting, the remaining Fe reacts with oxygen to form Fe oxides (FeO, Fe₃O₄ and Fe₂O₃) in succession, and thus, the intensities of the Fe peaks decreased and that of Fe₂O₃ peaks increased. The presence of Fe₂O₃ and Fe₃O₄ was confirmed by XRD, while FeO was not detected for the oxidized Nd–Fe–B scrap powder sample. It is possible to be insufficient for X-ray testing. Nonetheless, the FeO phase was apparently observed during the roasting process of the Ce–Fe–B scrap powder sample.

In the presence of Nd₂O₃, the iron oxide and boron oxide react to form NdFeO₃ and NdBO₃ by the following reaction:

The oxidation reactions of Gd, Nd, B and Fe in the Ce–Fe–B magnet scrap powder were consistent with those of the Nd–Fe–B scrap powder. The oxidation of Gd to Gd₂O₃ and finally to GdFeO₃ was represented by:

The difference was that the Ce in Ce–Fe–B magnet scrap powder can be oxidized to CeO₂ during the roasting process by the following reaction:

3.3 Microstructure variation

The backscattered electron (BSE) images of the microstructures of the Nd–Fe–B and Ce–Fe–B magnet scrap powder roasted at 800°C are shown in Figures 4 and 5, respectively. Figure 6 presents the typical microstructure with larger multiples and EDS analysis. The BSE micrographs of the unroasted Nd–Fe–B and Ce–Fe–B scrap powder samples in Figure 6(a) and (b) revealed the major matrix phase Nd₂Fe₁₄B (indicated by 2, 5), RE-rich phase at the grain boundary (indicated by 1, 4) and a small amount of η phase (indicated by 3, 6). It is obvious that the size of the RE-rich grain boundary phase in the Ce–Fe–B scrap particle was larger than that in the Nd–Fe–B particle, and its quantity was greater than that of Nd–Fe–B.

Figure 4

BSE images of the microstructure of the Nd–Fe–B magnet scrap powder roasted at 800°C with different roasting times. (a) The unroasted Nd–Fe–B scrap powder; (b) first cycle of roasting; (c) second cycle of roasting; (d) third cycle of roasting; (e) fourth cycle of roasting; (f) enlarged view of the dashed box in (b).

Figure 5

BSE images of the microstructure of the Ce–Fe–B magnet scrap powder roasted at 800°C with different roasting times. (a) The unroasted Ce–Fe–B scrap powder; (b) first cycle of roasting; (c) second cycle of roasting; (d) third cycle of roasting; (e) fourth cycle of roasting.

Figure 6

Typical BSE microstructures of the Nd–Fe–B and Ce–Fe–B magnet scrap powders roasted at 800°C and EDS analysis. (a, c, e and g) Nd–Fe–B; (b, d, f and h) Ce–Fe–B.

Generally, the increase in oxidative mass gain was dominated and limited by the oxidation process of large particles because finer particles oxidized more quickly, so the discussion of small particles was not a focus. From Figure 4, it is found that the microstructural changes for the Nd–Fe–B magnet scrap powder at different roasting cycles were obvious. For the first roasting of the Nd–Fe–B scrap powder shown in Figure 4(b) and (f), a dark gray layer (i) was observed on the surface of the big particle, and the center of the big particle (the brighter gray layer ii) was not oxidized. The dark gray layer (i) had approximately the same thickness in the large particle and was adherent to the unoxidized matrix phase. This is evidenced that the oxygen diffusion front progressed unidirectionally from the surface of the particle to its inner part. Furthermore, some white regions (iii) were observed, and their size was almost the same as the intergranular Nd-rich phase size in the initial Nd–Fe–B scrap powder, as illustrated in Figure 4(a)–(e). The positions of the white regions and the initial intergranular Nd-rich phases seemed to correspond. With increasing the roasting cycles from the second to the fourth, a black-gray layer (iv) became gradually apparent, as observed in Figure 4(c)–(e). From Figure 6(e), this layer was present as homogenous layers parallel to the particle surface. From the EDS analysis (point 7) in Figure 6(g), the outer black-gray layer only contained Fe and O, and combined with the XRD result, the layer was identified as Fe₂O₃.

When the Ce–Fe–B magnet scrap powder was subjected to the first and second roasting cycles, the interior of some large particles was not yet oxidized, as observed (viii in Figure 6(d)). The size of the white RE-rich region (such as v in Figure 5(b)) was much larger than the RE-rich grain boundary phase in the unroasted Ce–Fe–B scrap powder (Figure 5(a)). This is different from the phenomenon that the size of the white regions was almost the same before and after roasting in the large particles of Nd–Fe–B scrap. The aggregated RE-rich regions (v in Figure 5(b)) may be related to the larger RE-rich grain boundary phase in the Ce–Fe–B magnet scrap powder. The more reactive RE-rich grain boundary phase can be preferentially oxidized and easily gathered. In addition, a thin white RE-rich layer (vi in Figure 5(b) and (c)) was found on the surface of some particles, and a white RE-rich layer (vii in Figure 5(b), (c) and ix in Figure 6(d)) was also observed between the oxidized portion and the unoxidized core substrate. For the fourth cycle of roasting, a black-gray Fe₂O₃ layer was formed on the surface of some large-sized particles, similar to the fourth roasting cycle of Nd–Fe–B.

When the Nd–Fe–B and Ce–Fe–B magnet scrap powders were completely oxidized, the white areas (points 8 and 11) and the original matrix (points 9 and 12) in Figure 6(e) and (f) contained REs, Fe and O. It is worth noting that it is not known what happens to boron because it could not be detected using SEM or by XRD.

3.4 Distribution characteristics of the elements

Figure 7 shows the BSE images and the element mapping of the Nd–Fe–B magnet scrap powder at different roasting cycles. For the unroasted Nd–Fe–B, the mappings of Nd, Pr and Fe were identical and homogeneous in the whole particle (Figure 7(a)). Additionally, a small amount of O existed in the Nd-rich grain boundary phase, as observed. The occurrence of O should be due to the oxidation of the intergranular Nd-rich phase during the grinding and polishing process of sample preparation for SEM observation.

Figure 7

The mapping distribution of elements for the Nd–Fe–B magnet scrap powder oxidized at 800°C with different roasting cycles. (a) Unroasted Nd–Fe–B scrap powder; (b) first cycle of roasting; (c) second cycle of roasting; (d) third cycle of roasting; (e) fourth cycle of roasting.

With increasing the roasting cycles from 0 to fourth, the distribution of Nd and Pr did not change, and they were still distributed evenly throughout the Nd–Fe–B particle, indicating no diffusion of Nd and Pr during the whole roasting period. This phenomenon was consistent with the Nd and Pr distribution in the oxidation of block Nd–Fe–B magnet reported by Firdaus et al. [28]. Obviously, the O concentrated region gradually expanded toward the core as the roasting cycles increased. In the third cycle of oxidative roasting, the O diffused into the inner core of the particle; although at this time, the Nd–Fe–B scrap was not completely oxidized, as evidenced by the mass gain data. In the case of the fourth cycle of roasting, the O distribution in the core of the particle became more uniform. This indicates that the diffusion of O into the core of the particle always occurs during the oxidation process of the Nd–Fe–B scrap powder.

Fe was uniformly distributed in the Nd–Fe–B particle after the first cycle of roasting. For the second cycle of roasting, a Fe-poor zone was detected between the surface and core of the particle, as shown in Figure 7(c). As the roasting cycles increased to the third and fourth, an Fe-rich band with a ring shape was found in the outer layer of the particle. In this outer layer, the Fe-concentrated region was superposed with O rather than Nd and Pr (Figure 7(d) and (e)), and there was an obvious boundary. The Fe mapping for different roasting cycles suggested that the iron diffusion was out of the center of the particle to the surface, and thus, the Fe₂O₃ layer was formed on the surface of the particle.

BSE images and mapping of elements for Ce–Fe–B after different roasting cycles are revealed in Figure 8. For the initial Ce–Fe–B scrap powder, Ce, Gd and Nd were found over the cross-section of the entire particle, but the REs such as Ce seemed to be more enriched in some sporadic locations (Figure 8(a)). The scattered locations should correspond to the positions of the RE-rich intergranular phase. In the first and second cycles of roasting, the distribution of REs was no longer homogeneous in the whole Ce–Fe–B particle, different from the Nd–Fe–B scrap powder particles. The concentrations of Ce, Gd and Nd in the outer oxide layer were visibly higher than those in the internal layer, as shown in Figure 8(b) and (c). Additionally, the enriched areas of Ce, Gd and Nd increased with increasing roasting cycles from first to second. When the roasting cycle proceeded to the third and fourth, the distribution of Ce, Gd and Nd became uniform.

Figure 8

The mapping distribution of elements for the Ce–Fe–B magnet scrap powder oxidized at 800°C with different roasting cycles. (a) Unroasted Ce–Fe–B scrap powder; (b) first cycle of roasting; (c) second cycle of roasting; (d) third cycle of roasting; (e) fourth cycle of roasting.

For O, the mapping distribution with increasing roasting cycles in the Ce–Fe–B scrap powder was the same as that in the Nd–Fe–B scrap powder. It was noted that Fe was concentrated in the internal area of the Ce–Fe–B particle and seemed to be complementary with the Ce, Gd and Nd concentrated regions for the first and second cycles of roasting. The Fe-concentrated region in the core of the particle progressively decreased until it disappeared with increasing roasting cycles, and finally, Fe was evenly distributed inside the particle. Additionally, an Fe-rich layer existed on the surface of the particle, similar to that of the fully oxidized Nd–Fe–B particle presented in Figure 7(e). The distribution results of O and Fe for the Ce–Fe–B scrap powder indicated the identical inward diffusion of O and outward diffusion of Fe as that in Nd–Fe–B scrap powder during the oxidative roasting process.