Recycling of Waste NdFeB Magnets by Supergravity-enhanced Impurity Removal

Abstract

NdFeB magnets are the most widely used rare earth permanent magnet materials at present. The increasing number of the waste NdFeB magnets and their high rare earth content motivate a search for technologies to allow their cost-effective and environmental-friendly recycling. In this study, removal of oxide inclusions from waste NdFeB magnets by supergravity technology was investigated and the separating conditions were optimized for maximum oxide removal. Under the optimized conditions of G = 800 and t = 15 min, the total oxygen of the sample decreased from 410 ppm to 28 ppm, with the oxide removal efficiency of 96.8%. The theoretical time to remove inclusions with different sizes was calculated by Stokes’ law, and the experimental phenomena were in good agreement with the calculated ones. The supergravity technology has been demonstrated highly efficient in removing oxide from waste NdFeB magnets for the recycling. A design for an industrial reactor was presented to pave the way for future commercial processing and utilization of waste NdFeB magnets.

1. Introduction

NdFeB magnets are a crucial class of rare earth functional materials that exhibit exceptional properties such as high magnetic energy and high coercivity, rendering them indispensable for various industrial applications, such as communications, aeronautics, and wind power generation.^1,2) Currently, China is the leading producer of NdFeB magnets worldwide, with an output of approximately 180000 tons in 2019, which is projected to surpass 200000 tons by 2023.³⁾ Nevertheless, the production of NdFeB magnets also generates a considerable amount of waste, approximately 30% of the total materials utilized.⁴⁾ Additionally, a considerable number of these magnets are discarded annually as they reach the end of their service lives.

Manufacture of virgin NdFeB magnets is a quite complex and costly process that involves a multitude of steps including melting pure raw materials, jet milling, aligning, pressing, sintering, and heat treatment.⁵⁾ Hence, the efficient recycling of waste NdFeB magnets is of great importance for conserving rare earth resources, reducing costs and energy usage, and ensuring environmental sustainability. In recent years, the recycling of these magnets has gained considerable attention due to depletion of valuable rare earth elements (REEs) by the long-term disorderly mining practices and the stringent environmental policies aiming at mitigating landfill issues caused by discarded magnets.

Various approaches for recycling of waste NdFeB magnets have been previously reviewed.⁶⁾ Most of the recycling methods focus on the extraction of REEs, which can be roughly divided into hydrometallurgical and pyrometallurgical processes. The hydrometallurgical technologies, such as hydrochloric acid total solution method,⁷⁾ hydrochloric acid optimal solution method,⁸⁾ and sulfate double salt precipitation method,⁹⁾ involve separating REEs from other elements by controlling pH value and obtaining single rare earth oxide through multistage extraction, precipitation, and calcination. Although hydrometallurgical methods are highly adaptable to raw materials, they have a long process duration and can be environmentally detrimental. On the other hand, pyrometallurgical technologies, including the oxidation method,¹⁰⁾ chlorinate method,¹¹⁾ and liquid alloy extraction method,¹²⁾ are based on the differences in the combining abilities of REEs and other elements with oxygen, chlorine, and alloy elements. These pyrometallurgical methods are short but energy-intensive, with the final product requiring further purification.

Another research field on the recycling of waste NdFeB magnets pertains to the direct regeneration of the waste magnets. These main methods in this regard include hydrogen detonation method,¹³⁾ doping method,¹⁴⁾ and hot pressing method.¹⁵⁾ However, the waste NdFeB magnets are often contaminated and contain a certain quantity of oxide impurities, thus leading to a decline in the properties of regenerative NdFeB magnets.^16,17) Therefore, it is necessary to develop a more process-efficient and cost-effective method for recycling of waste NdFeB magnets.

Supergravity, characterized by a gravitational acceleration significantly higher than the standard acceleration of 9.8 m/s², has seen increasing usage in recent years for the purification of metal melts. For example, based on the different density of metal melt and inclusion, Meng et al.¹⁸⁾ achieved the purification of scrap Al–Mg alloys by supergravity, leading to a concentration of nonmetallic inclusions and Fe-rich impurities at the bottom of the alloy and an increase in enrichment proportional to the gravity coefficient. Shi et al.¹⁹⁾ employed supergravity to separate Al₂O₃ and TiN inclusions from 718 superalloy, which induced the migration of most inclusions to the top of the sample and a decrease in total O and N contents from 34.4 ppm to 8.7 ppm and 133.4 ppm to 34.1 ppm, respectively. Given the successful applications of supergravity in metal purification, it is considered feasible to use supergravity to remove oxide impurities from waste NdFeB magnets. The purified material can be used to manufacture the fresh NdFeB magnets, with the potential to bring great economic and environmental benefits.

In this study, the supergravity method was used to purify the waste NdFeB magnets by removing the oxide inclusions. A thorough examination was performed to evaluate the effects of gravity coefficient (G) and separation time (t) on the distribution of oxide inclusions and total oxygen (TO) content in the sample. Additionally, calculations were performed to determine the migration velocity of oxide inclusions. The results of this study provide useful reference and guidance for future recycling of waste NdFeB magnets.

2. Experimental

2.1. Raw Material

The waste NdFeB magnets used in this study was supplied by a waste recycling station in Guangdong Province, China. The composition and TO content of raw materials were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, iCAP-RQ, Germany) and O, N, H analyzer (HORIBA, EMGR-830, Japan), and the results are shown in Table 1.

Table 1. Contents of elements in waste NdFeB magnets (wt.%).

Fe	Nd	B	Ce	Pr	TO content
65.0	19.3	0.76	9.79	6.49	0.087

2.2. Equipment

In the present study, a centrifugal device was utilized to generate a supergravity field, which has been described in detail in other papers.^20,21) The unique aspect of the current study involved the use of a silicon carbide (SiC) heating element to heat the sample, resulting in a maximum temperature of 1600°C. To ensure symmetry and stability during the experiment, the heating tank and counterweight tank were symmetrically installed on opposite sides of the rotating shaft. The temperature within the heating tank was monitored and regulated using a thermocouple in conjunction with a temperature controller. The gravity coefficient G is calculated by formula (1):   

$$G=\frac{\sqrt{g^2+{\left({\omega }^{2}r\right)}^2}}{g}=\frac{\sqrt{g^2+{\left(\frac{N^2{\pi }^{2}r}{900}\right)}^2}}{g}$$(1)

where N is the rotating speed of the centrifugal (rad/min); ω is the angular velocity (rad/s); r is the distance from the centrifugal axis to the sample (0.25 m in this work); and g is normal gravitational acceleration (9.8 m/s²).

2.3. Experimental Process and Analysis

The waste NdFeB magnets were first melted into an ingot by vacuum induction melting. The ingot was then cut into a cylinder with a diameter of 15 mm and a height of 40 mm, and placed into a corundum crucible with an inner diameter of 18 mm and a height of 80 mm. The corundum crucible was subsequently enclosed within a vacuum quartz tube. The quartz tube was first evacuated and then filled with Ar gas to 0.08 atm. To conduct the supergravity experiment, the waste NdFeB magnets was heated to 1500°C and maintained at this temperature for 20 min to ensure complete melting. Then, the centrifuge was started and the gravity coefficient was adjusted to values of G = 500, 600 and 800. After the desired gravity coefficient was achieved, the sample was kept at the constant temperature for t = 5, 10 and 15 min. After centrifugation, the sample was taken out and cooled in air. In addition, normal gravity (G = 1) was also carried out for 15 min as the control test.

The cooled sample was cut into two parts along the supergravity direction. Half was observed by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS, MLA250, USA) to investigate the composition and distribution of inclusions. The volume fraction of inclusions in the sample was analyzed by Image Pro Plus software to further understand the distribution. The other half of the sample was used for measuring TO content by O, N, H analyzer to determine the removal efficiency of inclusions, and the oxygen removal was calculated by formula (2):   

$$\eta =\frac{{\text{TO}}_{\text{n}}-{\text{TO}}_{\text{s}}}{{\text{TO}}_{\text{n}}}\times 100\%$$(2)

where η is the oxygen removal efficiency, TO_n is the TO content in the original sample, TO_s is the TO content in the supergravity treated sample.

3. Results and Discussion

3.1. Characterization of Inclusions in the Original Waste NdFeB Magnets

Figure 1 shows the composition and morphology of inclusions in the original waste NdFeB magnets as observed by SEM-EDS. It can be seen that the inclusions in the original waste NdFeB magnets were mainly comprised of rare earth oxide (RE₂O₃) and were dispersed in the matrix in a granular form. The inclusions had a diameter that ranged from approximately 2 μm to 10 μm, which presented a challenge for removal through conventional methods due to the small particle size.

Fig. 1.

SEM images and EDS spectra of inclusions in the original waste NdFeB magnets: (a) dispersed inclusions, (b) RE₂O₃ inclusions, (c) EDS spectra of RE₂O₃ inclusions. (Online version in color.)

3.2. Optimization of Supergravity Process

Figure 2 shows cross sections of the samples obtained by supergravity at G = 1 and G = 800, operating at T = 1500°C and t = 15 min. The NdFeB magnets reacted with the alumina crucible at high temperatures, forming a thin layer of neodymium oxide film (~400 μm, marked by arrows in Fig. 2) attached to the crucible wall. However, due to the lack of convection and the short duration, this neodymium oxide film would hinder further reaction of neodymium inside the melt with the corundum crucible.

Fig. 2.

Macrographs of the samples obtained by supergravity: (a) G = 1; (b) G = 800 (T = 1500°C, t = 15 min). (Online version in color.)

In order to determine the influence of gravity coefficient on the distribution of inclusions, the regions A–F in Fig. 2 were further analyzed by SEM, as shown in Fig. 3. The elemental compositions at the points presented in Fig. 3 are listed in Table 2. Point 1 represented the Nd₂Fe₁₄B matrix phase characterized by the gray color. The black phase (Point 2) was the iron phase, and the white phase (Point 3) was the Nd-rich phase. Under normal-gravity conditions (Fig. 3(a)), the inclusions exhibited a uniform distribution throughout the sample. However, as shown in Figs. 3(b)–3(f) when G = 800, a gradient distribution of inclusions was observed along the direction of supergravity, with virtually no inclusions present in the upper portion of the sample (Fig. 3(b)). Figure 4 shows the high-magnification SEM image and the corresponding elemental mappings marked by the rectangular area in Fig. 3(f), clearly revealing the compositional makeup of the inclusions as rare earth oxides.

Fig. 3.

SEM image of different regions in Fig. 2 ((a)–(f) refers to areas ‘A–F’ in Fig. 2, respectively). (Online version in color.)

Table 2. Elemental composition (wt.%) of grains marked in Fig. 3.

Points	Fe	Nd	B	Ce	Pr	Phase
1	75.5	10.5	6.5	4.6	2.9	NdFeB matrix
2	100					Iron phase
3	3.5	47.2	12.9	17.3	18.8	Nd-rich phase

Fig. 4.

SEM image and EDS mappings of the area marked by a rectangle in Fig. 3(f). (Online version in color.)

These findings illustrated the critical role that supergravity played in the removal of oxide inclusions from waste NdFeB magnets. Due to the density difference between RE₂O₃ and NdFeB melt, a large number of inclusions migrated to the bottom of the sample under the influence of supergravity. However, this phenomenon was not observed under normal-gravity conditions (G = 1). Compared with the original waste magnets (Fig. 1(a)), a significant increase in the iron phase (Point 2) appeared in the samples after the experiment, which was attributed to the precipitation of α-Fe as a result of insufficient undercooling.²²⁾ It is noteworthy that there was a higher proportion of Nd-rich phase (Point 3) in the lower part of the sample compared to the upper part, as shown in Fig. 3. One possible explanation is as follows. Due to the lower melting point of the Nd-rich phase, it precipitated at the grain boundaries of the Nd₂Fe₁₄B matrix during cooling. Under the influence of supergravity, more oxide impurities migrated to the bottom of the sample, acting as nucleation points and promoting the generation of smaller Nd₂Fe₁₄B grains with more grain boundaries at the bottom. Consequently, the Nd-rich phase preferentially precipitated at the grain boundaries, resulting in a higher proportion of Nd-rich phase at the bottom of the sample. However, further research is required to fully understand the underlying mechanisms behind this phenomenon.

To further examine the distribution of inclusions, the effects of separation time and gravity coefficient on the volume fraction of inclusions at different distances from the bottom of the sample were studied by Image Pro Plus software (Fig. 5). It is evident that after supergravity treatment, the volume fraction of inclusions in all samples showed a gradient distribution along the direction of supergravity, and the gradient exhibited a gradual steepening trend with increasing gravity coefficient and centrifugation time. The inclusion volume fraction in the middle and upper parts of the sample was close to 0, while that in the lower part of the sample was much higher. Taking the sample obtained at G = 800 and t = 15 min for instance, the volume fraction of inclusions in the upper part of the sample was only 0.102%, whereas that in the bottom was 2.912%. The volume fraction of inclusions in the upper part of the sample was only about one twenty-eighth of that in the bottom. Figure 5 also reveals a significant difference in the inclusion volume fraction at the distance of 14–21 mm from the bottom of the sample under all conditions. This phenomenon can be attributed to the migration of inclusions towards the bottom of the sample in response to supergravity. As the concentration of inclusions at the bottom increased, the viscosity of the melt increased, and the downward movement of inclusions slowed down. Thus, a distinct gradient distribution of inclusions was observed at a distance of 14–21 mm from the bottom of the sample, with the highest concentration of inclusions present at the very bottom (below 14 mm from the bottom), and the lowest concentration of inclusions in the middle and upper parts of the sample (above 21 mm from the bottom).

Fig. 5.

Volume fraction of inclusions at different distances from the bottom of samples under different conditions: (a) separation time; (b) gravity coefficient. (Online version in color.)

Figure 6 shows the effects of separation time and gravity coefficient on the TO content in the middle of the sample. Under the action of supergravity, a large number of RE₂O₃ inclusions were driven to the bottom of the sample. The results reveal that the TO content in the middle of the sample experienced a dramatic reduction within the first 5 min of separation time, followed by a slow decrease within the next 5 min, and then remained constant from 10 to 15 min. The trend in the variation of TO content with respect to the gravity coefficient follows a similar pattern, showing a significant decrease from G = 1 to 600, and then stabilization until G = 800. These findings demonstrated the efficacy of supergravity in removing the RE₂O₃ inclusions from waste NdFeB magnets.

Fig. 6.

Total oxygen content in the middle of the sample under different conditions: (a) separation time; (b) gravity coefficient.

Table 3 lists the TO content and oxygen removal efficiency for the sample at a position of 20 mm from the bottom under different conditions. It shows that when G = 800 and t = 15 min, the TO content decreased to 28 ppm and the oxygen removal efficiency (Eq. (2)) was 96.8%.

Table 3. TO content and removal efficiency for the sample at a position of 20 mm from the bottom under different conditions.

Conditions		TO (ppm)	ηO (%)
G = 1,	t = 5 min	410	–
G = 500,	t = 5 min	140	84.0
G = 500,	t = 10 min	59	93.2
G = 500,	t = 15 min	56	93.6
G = 600,	t = 15 min	38	95.7
G = 800,	t = 15 min	28	96.8

The presence of TO content exerts a deleterious effect on the magnetic properties of NdFeB materials, necessitating strict control over its content during the manufacture of NdFeB magnets.^16,23) Through supergravity purification, the levels of TO present in waste NdFeB magnets became notably low, enabling their reuse as raw materials for the production of new NdFeB magnets. This, in turn, has the potential to significantly decrease the amount of virgin materials required and mitigate the risks of environmental pollution.

3.3. Calculation of Inclusion Migration Velocity

It is known that the motion of solid particles in viscous liquid follows Stokes’ law.¹⁹⁾ Assuming that the rare earth oxide inclusion was spherical, the motion equation of the inclusion can be described by Stokes law (Eq. (3)):   

$$\frac{\text{d}r}{\text{d}t}=\frac{d^2\left|{\rho }_{\text{L}}-{\rho }_{\text{P}}\right|}{18\eta }{\omega }^{2}r$$(3)

where dr/dt, ρ_L, ρ_P, ω, r, d, η, and N are the moving velocity, liquid density, density of inclusions, angular velocity, distance between inclusion and centrifugal axis, particle diameter, viscosity of metal melt, and centrifuge rotation speed, respectively.

Assuming $A=\frac{d^2\mathrm{\Delta }\rho }{18\eta }$, $\mathrm{\Delta }\rho =\left|{\rho }_{\text{L}}-{\rho }_{\text{P}}\right|$, Eq. (3) can be written as:   

$$\frac{\text{d}r}{\text{d}t}=\frac{d^2\mathrm{\Delta }\rho }{18\eta }{\omega }^{2}r=A{\omega }^{2}r$$(4)

Assuming ω was a constant, the following equation can be obtained:   

$$t=\frac{1}{A{\omega }^2}ln\frac{r}{r\prime }=\frac{900}{A{\pi }^2{N}^2}ln\frac{r}{r\prime }$$(5)

Due to the fact that most of the Re₂O₃ in the NdFeB melt was Nd₂O₃, the density of Nd₂O₃ (7240 kg/m³) was used as ρ_P.²⁴⁾ The density of the Nd₂Fe₁₄B melt was determined by Archimedes method,²⁵⁾ which was 7000 kg/m³ at 1500°C; and the viscosity of the Nd₂Fe₁₄B melt was determined by the rotating cylinder method,²⁶⁾ which was 0.005 Pa·s at 1500°C. Substituting N = 1692 r/min, r = 0.27 m, and r′ = 0.23 m into Eq. (5):   

$$t=1.92\times {10}^{-9}\frac{1}{d^2}$$(6)

Thus, the theoretical time for inclusions with different sizes to move from the top to the bottom of the sample under the condition of G = 800 can be calculated. As shown in Table 4, under the action of supergravity, it takes 32 min for inclusions with a diameter of about 1μm reach the bottom, while only 0.08 min for inclusions with a diameter of 20 μm. The majority of inclusions present in the waste NdFeB magnets used in this study exhibited particle sizes between 2 μm and 10 μm. In addition, a portion of the inclusions underwent aggregation to diameters exceeding 20 μm (Fig. 4) as a result of collisions that occurred during the migration of the inclusions. Thus, in this experiment, most inclusions migrated to the bottom of the sample after 15 min at G = 800, aligning with the theoretical predictions.

Table 4. Theoretical time of inclusion moving from top to bottom of the sample (4 cm) under gravity coefficient G = 800.

Average size (μm)	1	2	3	4	5	10	20
Theoretical time (min)	32	8	3.6	2	1.28	0.32	0.08

3.4. Reactor Design for Future Industrialization

Figure 7 depicts the schematic design of an industrial reactor that incorporates an induction furnace and a centrifugal device within a vacuum chamber. The procedure for industrial production involves initially placing the waste NdFeB magnets into the induction furnace, followed by evacuation of the vacuum chamber. The waste NdFeB magnets are then subjected to heating and melting within the induction furnace. Upon obtaining a homogeneous melt, the induction furnace is controlled to regulate the inclination, allowing the melt to flow into the centrifugal device through the nozzle. After centrifugation, the bottom portion which is enriched with the majority of inclusions is cut off and discarded, while the residual clean portion, with a reduced oxygen content, can be employed as raw material for the production of new NdFeB materials.

Fig. 7.

Schematic design of an industrial reactor for the purification of waste NdFeB magnets by supergravity. (1: Vacuum chamber; 2: Induction furnace; 3: Nozzle; 4: Centrifugal device.) (Online version in color.)

4. Conclusion

The removal of oxide inclusions from waste NdFeB magnets by supergravity under different gravity coefficients and separation times was investigated, and the following conclusions are obtained.

(1) Supergravity separation is an effective method to remove RE₂O₃ inclusions from waste NdFeB magnets. This is attributed to the differential density between the rare earth oxide and the NdFeB melt, resulting in the migration of the rare earth oxide towards the lower portion. The migration velocities measured by experiment were in good agreement with the theoretical calculation.

(2) The volume fraction of rare earth oxides showed a gradient distribution along the direction of supergravity, with the gradient incrementally intensifying as the gravity coefficient and centrifugation time increased.

(3) At the optimized conditions of G = 800 and t = 15 min, the oxygen content in the middle of the sample decreased from 410 ppm to 28 ppm, and the oxygen removal rate reached 96.8%.

(4) After undergoing supergravity-based impurity removal, the waste NdFeB magnets can be homogenized with conventional raw materials based on their composition, thereby facilitating the manufacture of new NdFeB materials.

(5) An industrial reactor, comprising an induction furnace and a centrifugal apparatus, has been devised, which can serve as a platform for the future commercial processing of waste NdFeB magnets by supergravity-based purification.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52174275), the Fundamental Research Funds for the Central Universities (No. FRF-TP-20-018A3), and the project of State Key Laboratory of Advanced Metallurgy (No. 41621007).

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