Selective leaching behavior of Nd from spent NdFeB magnets treated with combination of selective oxidation and roasting processes

Abstract

Selective leaching behavior of the spent NdFeB magnets was investigated to find out how controlled phase system through the roasting conditions gave an impact on the performance of the selective leaching. The pre-formed Nd₂O₃ by the selective oxidation made it possible to prevent the formation of NdFeO₃ at high roasting temperature. This resulted in the ideal Nd₂O₃ and Fe₂O₃ phase system, which was obtained by roasting at 660 °C, suitable for the selective leaching of Nd. While the roasting at 600 °C was still not enough to get full oxidation, the roasting at 720 °C made the NdFeO₃ formed. These characteristics with the roasting conditions affected leaching behavior. The NdFeB powder sample of 32 μm particle diameter, roasted at 600 °C showed the highest Nd leaching efficiency as well as the highest Fe leaching efficiency, which seemed to be ascribed to the existence of the metallic Fe. The poor leaching efficiency for the large powder size after roasting was ascribed to the poor diffusion characteristic of the H₂SO₄ solution. With the controlled phase system, the 100% leaching efficiency of Nd could be obtained by reducing the particle size to 16 μm. The formation of NdFeO₃ as a co-product, which had resistance to the reaction with the leaching agent, could prevent the dissolution of Nd₂O₃ around the NdFeO₃ layer. Thus, the 720 °C roasting condition showed the lowest leaching efficiency. The co-product Fe₂O₃ also showed good resistance to leaching so its leaching efficiency was just 5–6% after the leaching time of 3–4 h.