Effect of diffusion medium on the regeneration of Nd-Fe-B magnet sludge by CaH2 reduction diffusion method

•The influence mechanism of the diffusion medium on the CaH2-RD reaction of Nd-Fe-B sludge was clarified.•The M3T of regenerated Nd-Fe-B powder is 38% higher than that of initial sludge.•We successfully prepared regenerated Nd-Fe-B sintered magnets with Hcj = 16.5kOe, Br = 11.7 kG and (BH)m = 31.8 MGOe. The effects of adding different diffusion medium (KCl and CaCl2) on the regeneration of Nd-Fe-B magnet sludge by CaH2 reduction diffusion (RD) method were investigated. Through thermodynamic calculations, phase-structure and microstructure analysis, we found that the liquid CaCl2 could lower the starting reaction temperature of the CaH2-RD reaction. And the gaseous KCl led to the formation of loose and porous reacted compact, which was beneficial for impurity removal and refining the grains. Besides, CaCl2 would be involved in the CaH2-RD reaction and produce the intermediate product NdOCl, while KCl would not. Therefore, the regenerated Nd-Fe-B powders with finer particle size, low impurity content and uniform particle size distribution were prepared when KCl was used as diffusion medium. The average particle size of the regenerated Nd–Fe–B powders were reduced from 3.49 μm to 1.83 μm. The contents of Ca and O for the regenerated Nd-Fe-B powders were reduced to 0.04 wt% and 0.26 wt%, respectively. The M3T of regenerated Nd-Fe-B powders increased to 152.25 emu/g, which was about 4.1% and 38% higher than CaH2-regenerated Nd-Fe-B powder and initial sludge, respectively. Finally, the regenerated Nd-Fe-B sintered magnets with properties of Hcj = 16.5 kOe, Br = 11.7 kG, and (BH)m = 31.8 MGOe were successfully prepared by adding 40 wt% Nd4Fe14B alloy powders.