Ultrasound-assisted acetic acid leaching of rare earth elements from spent NdFeB permanent magnets: process optimization and kinetic analysis.

Spent NdFeB permanent magnets (SPM) represent an important secondary resource of rare earth elements (REEs), yet the simultaneous recovery of praseodymium (Pr) and dysprosium (Dy) using environmentally benign lixiviants remains underexplored. An ultrasound-assisted acetic acid leaching (UAL) process was developed for the recovery of Pr and Dy from pre-processed SPM. A four-factor Box-Behnken Design was used for process optimisation, and the developed model predicted maximum leaching efficiencies of 96.31% for Pr and 86.85% for Dy at 60.48% ultrasound amplitude, 1.42% pulp density, 2.51 M acetic acid, and 84.59 min. Analysis of variance indicated a decreasing influence of the variables on Pr leaching, whereas ultrasound amplitude exerted the greatest influence on Dy recovery, consistent with its comparatively lower accessibility within the NdFeB matrix. Although Pr consistently exhibited higher leaching efficiency than Dy, both REEs followed comparable leaching behaviour throughout the investigated operating range. Kinetic analysis showed a shrinking core model to adequately describe the leaching process, indicating diffusion through the reacted layer as the predominant rate-controlling step. The proposed mechanism explains the contribution of acoustic cavitation towards improved mass transfer and REE dissolution. The optimised process further demonstrated appreciable REE recovery at elevated pulp density, indicating its potential for operation under higher solid loading conditions. The findings demonstrate that UAL provides an effective route for the simultaneous recovery of REEs from SPM while offering mechanistic understanding of sustainable rare earth recycling processes.