Oxidation Kinetics of Hydrogenated NdFeB Magnet

Pretreatments such as grinding and oxidation play an important role in recovering rare earth elements (REEs) from spent NdFeB magnets in terms of selectivity and efficiency. As high content of iron (Fe) usually results in poor oxidation rate, improving oxidation rate is a critical aspect for recovery of REEs. The hydrogenation process, especially when the temperature was over 750 ℃, which induced both the dissociation of Nd₂Fe₁₄B matrix and hydrogenation of the Nd element, would be a good way in increasing oxidation rate with the growth of Nd₂O₃ particle because the hydrogenated magnet was known to be unstable and easily oxidized. When hydrogenated, the oxidation rate was increased and only 3 h. was needed to obtain full oxidation when hydrogenated at 650 ℃. TG/DSC analysis showed the linear weight gain for the hydrogenated magnet with the lower oxidation temperature. Combined with EMPA results, it can be inferred that the hydrogenated magnet possessed the increased oxygen diffusion paths due to the numerous Nd hydride dispersed in the α-Fe, which allowed the facile diffusion of oxygen and the reduced activation energy. Also, the hydrogenation process provided the improved resistance to the NdFeO₃ formation. The oxidation at 650 ℃ did not induce the formation of NdFeO₃, while NdFeO₃ was formed when oxidized over 700 ℃.