Abstract Many 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.

中文翻译：

---

Nd-Fe-B和Ce-Fe-B磁粉在焙烧过程中的相演变和氧化特性

摘要 许多已开发的从磁铁废料中回收稀土 (REs) 的技术对废料在高温下的氧化焙烧过程高度敏感。本研究侧重于广泛使用的 Nd-Fe-B 和高电位 Ce-Fe-B 废粉在 800°C 下焙烧过程中的相演变、微观结构变化和元素分布。Fe 持续氧化成 Fe2O3 和复合稀土氧化物的不断形成是两种废粉随着焙烧周期增加的主要反应过程。对于 Nd-Fe-B 废粉，完全氧化相包括 NdBO3、NdFeO3 和 Fe2O3，而对于 Ce-Fe-B 废粉，最终产品是 NdBO3、GdFeO3 和 Fe2O3 以及单独的 CeO2。观察到氧扩散前沿，在大颗粒表面形成几乎相同厚度的深灰色氧化层。此外，当两种废粉氧化完成时，Fe2O3 层覆盖在颗粒表面。在氧化的 Nd-Fe-B 颗粒中，观察到的白色区域对应于氧化的富含 Nd 的晶间相，如焙烧前后尺寸和位置几乎相同。在 Ce-Fe-B 颗粒中，氧化的晶间相似乎聚集和生长，并且在氧化物/未氧化层之间出现了富含 RE 的层。总之，铁向外扩散和氧向内扩散主要由 Nd-Fe-B 和 Ce-Fe-B 颗粒的氧化决定。在氧化的 Nd-Fe-B 颗粒中，观察到的白色区域对应于氧化的晶间富 Nd 相，如焙烧前后尺寸和位置几乎相同。在 Ce-Fe-B 颗粒中，氧化的晶间相似乎聚集和生长，并且在氧化物/未氧化层之间出现了富含 RE 的层。总之，铁向外扩散和氧向内扩散主要由 Nd-Fe-B 和 Ce-Fe-B 颗粒的氧化决定。在氧化的 Nd-Fe-B 颗粒中，观察到的白色区域对应于氧化的富含 Nd 的晶间相，如焙烧前后尺寸和位置几乎相同。在 Ce-Fe-B 颗粒中，氧化的晶间相似乎聚集和生长，并且在氧化物/未氧化层之间出现了富含 RE 的层。总之，铁向外扩散和氧向内扩散主要由 Nd-Fe-B 和 Ce-Fe-B 颗粒的氧化决定。