Synthesis of hard magnetic NdFeB composite particles by recycling the waste using microwave assisted auto-combustion...

The present work provides a facile and energy-saving approach to recycle Nd-Fe-B wastes. Employing the severely oxidized and contaminated sintered Nd-Fe-B scraps as the raw materials, sub-micro sized Nd-Fe-B/α-Fe composite magnetic powders were successfully synthesized by microwave assisted auto-combustion method followed by a reduction diffusion process. In details, the waste magnets were dissolved in HNO₃ solution and both the rare earth oxides and iron oxide were obtained by auto-combustion approach with the help of microwave heating. These oxides were then reduced to achieve Nd-Fe-B alloys with the reducing agent CaH₂. To modify the composition of the final product, Nd(NO₃)₃·6H₂O and H₃BO₃ were added during dissolving and consequently three compositions of the final products were obtained. TEM results demonstrate that the sizes of α-Fe and Nd₂Fe₁₄B grains are below 10 nm and 20 nm, respectively. The magnetic properties of the composite powders obtained from the wastes in this study are comparable to those obtained by other methods using pure reagents. The room temperature coercivity of as-synthesized Nd₁₅Fe₇₇B₈ is higher than 400 kA/m. In addition, an appropriate mass ratio of CaH₂ to Nd-Fe-B oxides is required in order to obtain good magnetic properties.