Effect of rotation rate on the recovery of Nd–Fe–B sludge via rotation-reduction diffusion method

Sintered Nd–Fe–B permanent magnet materials are widely used due to their excellent magnetic properties.¹ With the rapid development of the high-tech industry, the demand and annual output of Nd–Fe–B have increased every year.^2,3 In the production process of sintered Nd–Fe–B magnets, 30–40 wt. % of waste will be produced.⁴ Among these waste products, the sludge produced during the cutting process will account for the highest proportion.^5,6 Nd–Fe–B sludge consists of about 30 wt. % rare earth element content, including Nd, Pr, Dy, and Tb.^6,7 Compared to natural rare earth ore, Nd–Fe–B sludge contains high rare earth content, as well as no radioactive elements, with an extremely high recovery value. Therefore, in recent years, research on the recycling of Nd–Fe–B sludge has become a research hotspot. However, this sludge has the characteristics of a complex phase and composition, high organic content, serious oxidation, and small particle size, which make recovery difficult. At present, Nd–Fe–B sludge has been mainly recovered by hydrometallurgy to obtain rare earth chlorides or oxides.^8,9 To obtain Nd–Fe–B magnetic powder, a series of links such as electrolysis, smelting, and hydrogen explosion are required. The entire recovery process is long, with a high energy consumption rate and the discharge of a large amount of acidic and alkaline waste liquid, which are not conducive to environmental protection. In recent years, short-process in situ regeneration technology for Nd–Fe–B sludge based on the purification and calcium thermal reduction diffusion (CTRD) method has attracted attention due to its advantages of low environmental load, low energy consumption, and a high resource recycling rate.^6,10–14 Nd–Fe–B sludge has been directly converted into regenerated Nd–Fe–B magnetic powder after the CTRD method. The high energy consumption and high pollution characteristics such as electrolysis and smelting could be avoided in the recycling process, with no harmful waste discharge; thus, reducing energy consumption and protecting the environment. Haider used the co-precipitation method combined with the CTRD method to achieve the short-process recovery of Nd–Fe–B sludge, and studied the washing and calcium removal process to improve the magnetic properties of regenerated magnetic powder.^15,16 Xu et al. used different diffusion agents to recycle Nd–Fe–B sludge, studied the influence of diffusion medium on the recovery of magnetic powder, and clarified the reaction mechanism based on the CTRD reaction of Nd–Fe–B sludge through thermodynamic calculations.^6,17 These researches on the CTRD method were comprehensive and systematic. However, in the experimental process, the high-temperature CTRD reaction was carried out after the powder was pressed. This method could shorten the reaction distance and improve the reaction uniformity; however, one of the problems was a low regeneration ability, resulting in only a few to a dozen grams of material. In this study, rotation diffusion technology was introduced to make the CTRD reaction proceed in a dynamic manner, promoting the flow of molten calcium and the effective mixing of reactants, and finally realizing the batch regeneration of Nd–Fe–B sludge. The rotation rate was the key factor in determining the performance and microstructure of regenerated magnetic powder. If the rotation rate was too slow, the powder could not be driven, with only slight shaking at the lower part of the tank due to the effect of gravity, and the rotation effect was not achieved. If the speed was too fast, the powder stuck to the wall of the tank under the action of centrifugal force, which was not conducive to reduction reaction uniformity. Only by selecting the appropriate rotation rate could the regenerated magnetic powder with uniform morphology and good performance be prepared. Therefore, in this work, the multi-wire cutting sludge was used as the raw material, and a mixture of purified sludge, diffusion agent CaCl₂, KCl, and metal calcium was reacted at 1050 °C for 3 h with different rotation rates. Then, the effects of rotation rate on the microstructure and magnetic properties of the regenerated magnetic powder prepared from Nd–Fe–B sludge recovered by the rotation calcium thermal reduction diffusion (R-CTRD) method were studied. The regenerated magnetic powder doped with rare earth-rich alloy powder and high remanence Nd–Fe–B alloy powder obtained at an optimal rotation rate were used to prepare the regenerated sintered magnets, which served as a reference for the short in situ regeneration process of the Nd–Fe–B sludge.

The Nd–Fe–B sludge used in the experiment was collected from multi-wire cutting sludge obtained from Beijing Zhongke Sanhuan High-tech Co., Ltd. First, the multi-wire cutting sludge was ultrasonically cleaned with an anhydrous ethanol solution, cleaned with a mixed solution of 0.5 g/l sodium hydroxide and 5% OP emulsifier, and then the solution and sludge powder were separated by magnetic separation. After washing, the residual mixed solution was rinsed with deionized water, and the purified sludge was obtained by vacuum drying, which could be used for the subsequent CTRD reaction.

First, 100 g of purified sludge, 20 g of CaCl₂, 20 g of KCl, and 20 g of metal calcium granules were weighed in a glove box under an argon (Ar) atmosphere and loaded into a mold. The mold was then placed into the reaction furnace and washed with Ar gas three times. Then the R-CTRD reaction was carried out at 1050 °C for 3 h using different rotation rates (2.5 rpm, 5 rpm, 10 rpm, and 15 rpm) under a high purity Ar atmosphere. After reduction diffusion, the samples were removed and ultrasonically cleaned with 75 wt. % NH₄Cl-methanol solution for 30 min, and the washing process was repeated 3 times. When no obvious gas was released and the solution was clear, the by-products were removed. Finally, the regenerated magnetic powder could be obtained by washing with absolute ethanol three times followed by vacuum drying. The regenerated magnetic powder was mixed with the self-made high remanence Nd–Fe–B alloy jet mill powder (Nd_30.5Fe_66.74A_l0.1Cu_0.2Co_0.8Ga_0.5Zr_0.2B_0.96) and the Nd₄Fe₁₄B rare earth-rich alloy jet mill powder. After orientation pressing and cold isostatic pressing, it was placed in a vacuum sintering furnace, where the temperature was increased to 400 °C for 0.5 h, heated to 600 °C for 1 h, heated to 800 °C for 1 h, heated to 1000 °C for pre-sintering for 10 min, and finally, the sintering temperature was adjusted to 1060 °C for 3 h. After sintering, air cooling was performed. Then, the second stage heat treatment was carried out, where the first stage temperature was 900 °C with a holding time of 3 h and the second stage was 500 °C for 3 h.

Qualitative phase analysis of all samples during the experiment was characterized by X-ray diffraction (XRD, MAXima XRD-7000) equipped with a Cu target (λ = 0.154 nm) at a voltage of 40 kV and current of 40 mA. In this work, the morphologies of the purified sludge, regenerated magnetic powder, and regenerated magnets were observed by a Zeiss SUPRA 55 field-emission scanning electron microscope (SEM) (Zeiss, Germany). The hysteresis loops of the Nd–Fe–B magnetic powders in this work were all measured by a vibrating sample magnetometer (VSM) produced by the Quantum Design Company, and the maximum applied magnetic field was 30 kOe. A NIM-500C B-H loop instrument was used in this work, which served as the magnetic performance measurement system for the permanent magnet materials at different temperatures and was developed by the China Institute of Metrology. The size distribution of the powder was analyzed by a HELOS-RODOS laser particle size analyzer (New Patek, Germany).

Fig. 1 shows the XRD patterns of the purified sludge and regenerated Nd–Fe–B magnetic powders obtained by the CTRD reaction at different rotation rates. After purification, large quantities of non-magnetic organic impurities and some oxide impurities were removed. The diffraction peak showed the characteristic peak of Nd₂Fe₁₄B; however, the diffraction peak widened, indicating that the crystal structure of Nd–Fe–B was destroyed, or the grain size became finer during the cutting process. No impurity phases such as the α-Fe phase and the diffraction peaks of the CaO by-products were detected after washing and removing calcium from the regenerated magnetic powder obtained at different rotation rates, where the diffraction peaks had a good match with the standard Nd₂Fe₁₄B (JCPDS no. 70-1403) pattern. This indicated that the Nd–Fe–B sludge could be reduced and diffused in the rotation rate range of 2.5–15 rpm to obtain the Nd–Fe–B magnetic powder with a single Nd₂Fe₁₄B phase, and the by-products were completely removed after washing and removing the calcium.

Fig. 2(a) shows the microscopic morphology of the purified sludge, indicating that the particles were relatively well dispersed and the surface was smooth. However, the morphology was very irregular, with debris, strips, and flakes, and the particle size distribution was uneven. When the rotation rate was 0 rpm, without rotation, the purified Nd–Fe–B sludge was subjected to the CTRD reaction. The morphology of the regenerated magnetic powder after washing and removing calcium is shown in Fig. 2(b), presenting obvious graininess, which was consistent with the previous literature. However, the particle size was polarized, with a serious sintering phenomenon between the particles. This was due to powder accumulation during the reaction process, which led to an uneven reaction. The morphologies of the magnetic powder obtained by the R-CTRD reaction at rotation rates of 5 rpm and 15 rpm are shown in the SEM image in Figs. 2(c) and 2(d). Compared to the Nd₂Fe₁₄B powder obtained without rotation, the uniformity of the particle size improved; however, there was still a large difference in particle size between the large and small particles.

According to the micro-morphology of the regenerated Nd₂Fe₁₄B magnetic powder prepared at different rotation rates, and we observed that the particle surface of the magnetic powder after washing and calcium removal was relatively smooth, with no obvious impurity residue. Different rotation rates resulted in different centrifugal forces in the spherical tank, affecting the flow speed and direction of the calcium liquid. Therefore, the dispersion and particle size distribution of the recycled Nd₂Fe₁₄B magnetic powder differed, affecting its orientation effect in the magnetic field.

Fig. 3(a) shows the morphology of the regenerated magnetic powder obtained at a rotation rate of 10 rpm. Combined with the particle size distribution in the lower left corner, we proved that the particle size of the regenerated Nd₂Fe₁₄B magnetic powder obtained at this speed was very uniform, showing a normal distribution and good dispersion, which was conducive to the magnetic field orientation. The Nd₂Fe₁₄B particles obtained after calcium removal were fully dispersed in n-hexane and mixed with the AB epoxy resin to obtain a flowing liquid. The liquid was poured into a cylindrical mold and placed in a 2.2 T static magnetic field, where the oriented Nd₂Fe₁₄B-epoxy composite was obtained after evaporation of n-hexane. When using XRD to characterize the oriented samples, the test direction was along the axis of the prepared cylinder, which was consistent with the magnetic field direction. According to the orientation and non-orientation results of the regenerated magnetic powder prepared at a rotation rate of 10 rpm in Fig. 3(b), we found that the intensity of the (006) diffraction peak after orientation significantly improved, the intensity of the (105) diffraction peak was significantly reduced, and I₍₀₀₆₎/I₍₁₀₅₎ significantly improved, with a value of 3.29. Combined with the above SEM images, we found that the particles with a uniform size and good dispersion were conducive to the magnetic field orientation.

The sludge or regenerated magnetic powder of determined quality was loaded into a VSM crucible, sealed with paraffin, placed in a 2.2 T static magnetic field, and the magnetic properties were measured after the paraffin solidified. Fig. 4(a) shows the statistical results of the saturation magnetization (M_3T) of the purified sludge where the regenerated magnetic powder was prepared at rotation rates of 2.5 rpm, 5 rpm, 10 rpm, and 15 rpm under a 3 T magnetic field. As the rotation rate increased, the magnetic properties of the regenerated magnetic powder initially increased and then decreased. When the rotation rate was 10 rpm, M_3T reached a maximum of 138.22 emu/g. The hysteresis loops of the purified sludge and the regenerated magnetic powder obtained at 10 rpm are shown in Fig. 4(b). Compared to the M_3T of the purified sludge (125.38 emu/g), the M_3T of the regenerated magnetic powder at this rotation rate increased by 10.2%.

The above research showed that when the rotation rate was 10 rpm, the regenerated Nd₂Fe₁₄B magnetic powder obtained by the R-CTRD method of the purified sludge had a uniform particle size, good dispersion, and the highest magnetic properties. Therefore, the regenerated magnetic powder obtained at this rotation rate was selected for the preparation of the regenerated sintered magnet. The initial intrinsic magnetic properties such as remanence and saturation magnetization of the multi-wire cutting sludge were low. To prepare high-performance recycled sintered magnets, a high remanence Nd–Fe–B alloy jet mill powder (referred to as high remanence alloy powder) with a composition of Nd_30.5Fe_66.74A_l0.1Cu_0.2Co_0.8Ga_0.5Zr_0.2B_0.96 was doped. The properties of the magnets prepared with this component reached B_r = 14.31 kG, H_cj = 13.31 kOe, and (BH)_max = 50.02 MGOe. The oxygen content of the regenerated Nd₂Fe₁₄B magnetic powder was 3000–4000 ppm, which was higher than the Nd–Fe–B powder prepared by the traditional powder metallurgy process. Therefore, a small amount of Nd₄Fe₁₄B rich rare earth alloy jet mill powder (referred to as rare earth-rich alloy powder) was also added.

In this work, the regenerated sintered Nd–Fe–B magnets were prepared by doping 65 wt. % high remanence alloy powder and 5 wt. % rare earth–rich alloy powder with regenerated magnetic powder. The particle size and distribution of the doped powder affected the size and uniformity of the grains of the regenerated magnets, which in turn affected the magnetic field orientation during the preparation of the sintered magnet. The high remanence alloy powder was a type of fine powder obtained by the physical method, which differed from the spherical shape of the regenerated magnetic powder, and its microscopic shape was irregular with an average particle size of 2.7 μm. The rare earth–rich alloy powder was mainly flaky, with an average particle size of about 3.5 μm, which was equivalent to the size of the regenerated Nd₂Fe₁₄B magnetic powder particles. In summary, the high remanence alloy and rare earth-rich alloy powders served as two superior dopants for preparing the recycled Nd–Fe–B sintered magnets.

Fig. 5(a) shows the backscattered SEM image of the prepared regenerated sintered magnet. The regenerated magnet was mainly composed of a gray Nd₂Fe₁₄B phase and white Nd-rich phase, where the boundary of the Nd₂Fe₁₄B main phase grain was clear and the grain size was relatively uniform. The density of the regenerated magnet was 7.51 g/cm³, which reached the normal magnet density level (≥7.5 g/cm³). The oxygen content of the regenerated magnet was slightly higher than that of the original sintered magnet, with a value of 1656 ppm. Fig. 5(b) shows the demagnetization curve of the regenerated sintered magnet. The room temperature coercivity of the magnet was 14.72 kOe, B_r was 13.28 kG, and (BH)_max was 42.87 MGOe.

In this work, the batch regeneration of Nd–Fe–B sludge was realized by the R-CTRD process. Regenerated Nd₂Fe₁₄B magnetic powder with a uniform particle size and good dispersion was successfully prepared by adjusting the rotation rate. The results showed that M_3T initially increased and then decreased with increasing rotation rate. When the rotation rate was 10 rpm, the maximum value reached 138.22 emu/g, which was 10.2% higher than the purified sludge. The orientation effect of the obtained magnetic powder in the magnetic field was obvious, and I₍₀₀₆₎/I₍₁₀₅₎ reached 3.29. The density of the regenerated sintered magnet prepared by using the regenerated magnetic powder doped with rare earth-rich alloy powder and the high remanence alloy powder obtained by this rate reached 7.51 g/cm³. For the magnetic properties, B_r was 13.28 kG, H_cj was 14.72 kOe, and (BH)_max was 42.87 MGOe. This served as a reference for the entire recovery process of the Nd–Fe–B sludge to regenerate the sintered magnet. However, there was still room for improvement in the performance of the regenerated magnet. The oxygen content of the regenerated magnet was higher than the original sintered magnet, which was 1656 ppm, causing its performance to differ from the original sintered magnet. This was mainly due to the high oxygen content of the regenerated magnetic powder. By controlling the oxygen content of the regenerated magnetic powder, the performance of the regenerated magnet could be further improved.

This work was supported by the National key R & D project (2021YFB3500801, 2020YFC1909004).

The authors have no conflicts to disclose.

Liying Cong: Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Haibo Xu: Conceptualization (equal); Methodology (equal). Qingmei Lu: Conceptualization (equal); Funding acquisition (equal); Writing – review & editing (equal). Ming Yue: Conceptualization (equal); Funding acquisition (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.