[P1] RE-Fe-B Magnets

[P1-28] Recycling of Waste Nd-Fe-B Sintered Magnets via Ca-reduction and two-step washing process

*Seol-mi Lee¹, Tae-Hoon Kim¹, Ki-Suk Lee², Sang-hyub Lee³, Dong-Hwan Kim³, Jung-Goo Lee¹ (1. Korea Institute of Materials Science (KIMS) (Korea), 2. Ulsan National Institute of Science and Technology (UNIST) (Korea), 3. Star Group Ind. CO., Ltd. (Korea))

Keywords:

Recycling process,Waste Nd-Fe-B magnet,Ca-reduction,Two-step washing process

Nd-Fe-B magnets, which account for about two-thirds of the permanent magnet market, are superior to other types such as ferrite and SmCo due to their excellent magnetic H_c, B_r, and (BH)_max [1]. The growth of the eco-friendly energy sector, especially in electric and hybrid vehicles, has significantly increased the demand for Nd-Fe-B sintered magnets [1]. Given the lifespan of these components (typically two to three years for household electronics and around 20 to 30 years for wind turbines), a substantial amount of waste Nd-Fe-B magnets is anticipated in near future [2]. The most critical problem of the waste magnets is their high oxygen concentration, which degrades the magnetic properties of the magnets [3]. Due to this, the waste magnets cannot be re-used directly without an oxygen reduction process [3]. In other words, developing the techniques to reduce the oxygen concentration of waste magnets is crucial for producing high-performance recycled magnet using the waste magnets as a precursor. From this perspective, the Ca-reduction process for the wasted magnet particles, seems to be an effective way to reduce the oxygen concentration of waste magnets [4]. In this approach, the bottle-neck point is the washing process that is required for eliminating the CaO byproduct formed after the Ca-reduction process [5]. During the washing process, the CaO should be selectively eliminated without any re-oxidation of reduced Nd-rich and Nd₂Fe₁₄B particles, which is very difficult to realize under the conventional washing process using the H₂O [5]. A novel re-oxidation-prohibited washing process should be developed for recycling the waste Nd-Fe-B magnets successfully. Thus, out study aims to develop a washing process that can remove CaO and residual Ca while preventing re-oxidation of the reduced powders to recover the magnetic properties of waste magnets. The waste Nd-Fe-B magnets (originally 50M grade) with the oxygen concentration of 0.54 wt.% were prepared from the end-of-life motors. The waste magnets were pulverized to approximately 3.3 µm via HD treatment and jet-milling. To reduce the waste Nd-Fe-B particles, 4.5 wt.% of Ca was mixed with the waste particles, then the mixed powders were heat-treated at 900 °C for 30 min under the Ar atmosphere. The two-step washing process were employed to eliminate the CaO (and residual Ca) without re-oxidation of Nd-Fe-B particles. In the first step, the powders were coated with the ethylenediamine [C₂H₄(NH₂)₂] to prevent the particle surface from the oxidation, then were washed using the solution of distilled water, methanol, and ammonium nitrate (NH₄NO₃). In the second step, the 1st washed particles were washed again with the dilute acetic acid to chemically etch the oxide layer formed on the particle surface. As shown in Fig. 1(a), in the recycled Nd-Fe-B powders, the diffraction peaks from the of Nd₂O₃ were disappeared, and also no peaks from Ca and CaO were observed. This indicates that the Nd₂O₃ in the waste magnets was effectively reduced by Ca, and the CaO and residual Ca remained after the reduction process are successfully eliminated without re-oxidation of reduced particles. As a result, as shown in Fig.1(b), the 4 µm sized recycled powders with the oxygen concentration of 0.15 wt.% was obtained. In this presentation, we will introduce a detailed recycling process, specifically focused on the two-step washing process. The advantages of our recycling process will also be discussed based on the magnetic and microstructural character of the recycled bulk magnets

Reference
[1] Kaya, Muammer. "An overview of NdFeB magnets recycling technologies." Current Opinion in Green and Sustainable Chemistry 46 (2024): 100884
[2] van Nielen, Sander S., et al. "Towards neodymium recycling: Analysis of the availability and recyclability of European waste flows." Journal of Cleaner Production 394 (2023): 136252.
[3] Firdaus, Muhamad, et al. "Review of high-temperature recovery of rare earth (Nd/Dy) from magnet waste." Journal of Sustainable Metallurgy 2 (2016): 276-295.
[4] Xu, Haibo, et al. "Reaction mechanism of Ca-reduction diffusion process used for sustainable recycling Nd-Fe-B sludge." Journal of Alloys and Compounds 909 (2022): 164744
[5] Wang, Y., et al. "Effect of washing process on the magnetic properties of Nd-Fe-B nanoparticles prepared by reduction-diffusion method." Journal of Magnetism and Magnetic Materials 439 (2017): 91-94.

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