Phase evolution and oxidation characteristics of the Nd–Fe–B and Ce–Fe–B magnet scrap powder during the roasting process

Abstract

AbstractMany developed techniques for rare-earths’ (REs) recovery from magnet scraps are highly sensitive to the oxidative roasting process of scraps under high temperature. This study focused on phase evolution, microstructural changes and element distribution during the roasting of the widely used Nd–Fe–B and high-potential Ce–Fe–B scrap powders at 800°C. The sustained oxidation of Fe to Fe2O3 and the constant formation of composite RE oxides were the main reaction processes with increasing roasting cycles for the two scrap powders. The complete oxidation phases consisted of NdBO3, NdFeO3 and Fe2O3 for the Nd–Fe–B scrap powder, while the final products were NdBO3, GdFeO3 and Fe2O3 as well as individual CeO2 for the Ce–Fe–B scrap powder. An oxygen diffusion front was observed, forming a dark gray oxidized layer with almost the same thickness on the large particle surface. Additionally, a Fe2O3 layer covered the particle surface when the oxidation of the two scrap powders was complete. In oxidized Nd–Fe–B particles, the observed white regions corresponded to the oxidized intergranular Nd-rich phase as indicated by the almost same size and position before and after roasting. In Ce–Fe–B particles, the oxidized intergranular phase appeared to gather and grow, and a RE-rich layer appeared between the oxide/unoxidized layer. Conclusively, the iron-outward diffusion and the oxygen-inward diffusion were dominated by the oxidation of both Nd–Fe–B and Ce–Fe–B particles.