Reduction-Diffusion of Nd-Fe-B Magnet Sludge: All-Element Recovery and Submicron Powder Production

Abstract

The growing demand for electric vehicles has driven a sustained increase in the production of Nd-Fe-B magnets, with annual growth rates of approximately 10% over the past decade. Conventional Nd-Fe-B magnet manufacturing via powder metallurgy generates substantial waste, with 20-30% of the material lost as sludge during cutting and machining. This sludge, composed primarily of oxidized powder, cannot be directly reused via remelting or sintering due to its high oxygen content. However, it contains approximately 30 wt% rare earths (RE), including critical heavy rare earths (HRE) such as Dy and Tb, making its recycling highly desirable. Despite this, less than 1% of Nd-Fe-B magnets are recycled, which has remained unchanged over the past two decades. Conventional hydrometallurgical methods are limited by the use of hazardous chemicals, inefficient boron recovery, and overall process complexity. As a result, alternative approaches to recycling are being actively explored. The reduction-diffusion (RD) route using calcium (Ca) as a reducing agent has shown considerable promise as a one-step, non-hazardous process. However, its application is still limited by several drawbacks, including nonuniform mixing within the reagent mixture, incomplete conversion at higher loadings, inefficient utilization of the reductant, and relatively low overall recovery yields. To overcome these drawbacks, this study explores calcium hydride (CaH》as an alternative reductant. Owing to its brittle nature, CaH2 can be more uniformly dispersed within the sludge mixture compared to ductile metallic Ca. In this work, Nd-Fe-B sludge containing Dy and Tb was regenerated via RD using both Ca and CaH2 at varying reducing agent/sludge ratios (0.35-0.95). Both reducing agents showed the best performance at a ratio of 0.75, producing regenerated powders with high crystallinity and phase purity, primarily consisting of 2-14-1 phase. However, RD reaction behavior and final particles morphology differed considerably. Reduction with Ca resulted in coarse, agglomerated Nd-Fe-B particles, accompanied by low rare-earth recovery and overall yield. In contrast, the brittle nature of CaH2 allowed more uniform mixing with sludge particles, while the in-situ formation of CaO during RD suppressed particle growth and facilitated submicron particle formation. Thus, the powder regenerated with CaH2 consisted of particles with an average size of 0.75 pim and higher rare-earth content (35.1 wt% RE including 6.7 wt% HRE). The CaH2-regenerated powder exhibited a coercivity of 9 kOe, attributed to efficient heavy rare-earth recovery, high phase purity, and fine particle formation. This regenerated submicron powder is expected to be advantageous for fabricating high-coercivity Nd-Fe-B magnets. Therefore, these findings demonstrate that CaH2-assisted RD provides efficient all-element recovery, refined particle morphology, and a potentially scalable pathway for sustainable magnet sludge recycling.