Insight into the mechanism on co-leaching of REEs and Fe from NdFeB sludge: Elucidating phase transformation and co-leaching kinetics

The NdFeB sludge is a secondary rare earth resource abundant in both REEs and Fe elements. However, there tends to be a focus on recovering only high-value rare earths while neglecting the recycling of high-content iron, leading to a low comprehensive utilization of this secondary rare earth resource. The present study builds upon previous research on the separation and coextraction of REEs and Fe from NdFeB sludge, further investigating the phase transformation behavior during oxidation roasting and elucidating the kinetics of simultaneous leaching of REEs and Fe. The results suggest that NdFeB waste exhibited a loose morphology and demonstrated high susceptibility to dissolution in hydrochloric acid when subjected to roasting temperatures below 500℃. The morphology of NdFeB waste becomes denser and more spherical, hindering its dissolution in hydrochloric acid when the roasting temperature exceeded 500℃, resulting in a reduction in the leaching efficiencies of both roasting. The co-leaching of REEs and Fe in NdFeB sludge is governed by internal diffusion for REEs and chemical reaction for Fe, as evidenced by the kinetic results. Furthermore, it is observed that the apparent activation energy of rare earth surpasses that of iron, leading to the attainment of equilibrium for rare earth prior to that of iron during the synchronous leaching process. The findings of this study hold theoretical significance in enhancing the overall efficiency of secondary rare earth resource utilization.