Thermodynamic properties of hydrogen decrepitation process in Nd<sub>2</sub>Fe<sub>14</sub>B permanent magnet materials

Recycling is a sustainable strategy for the efficient utilization of rare earth resources. Hydrogenation milling has been widely adopted because of its high efficiency and environmental benefits. However, the formation of unstable phases in the hydrogenation process significantly reduces recovery efficiency, which presents new challenges for process optimization. In this study, a combination of first-principles calculations and machine learning methods is employed to systematically investigate the thermodynamic behavior of key rare earth hydrides, such as NdH<sub>2</sub>, NdH<sub>3</sub>, and Nd<sub>2</sub>H<sub>5</sub> in the hydrogenation milling process using the Debye model for lattice vibrations. The results show that a temperature centered at about 630 K at a pressure of 600 kPa may offer ideal operational conditions for the hydrogenation milling process. Under these conditions, NdH<sub>2</sub> can undergo spontaneous hydrogenation, and the formation of unstable phases can be effectively suppressed, thereby improving rare earth recovery efficiency. This study also reveals the potential adverse effects of excessively high temperatures on the stability and reactivity of NdH<sub>2</sub>, further emphasizing the importance of operating within a specific temperature range. These findings provide new insights into the thermodynamic mechanisms of the hydrogenation process in Nd<sub>2</sub>Fe<sub>14</sub>B permanent magnet material. Furthermore, they offer theoretical guidance for the optimization of industrial hydrogenation milling parameters.