Reaction mechanism and kinetics for selective recovery of REEs from Nd-Fe-B magnet sludge by low-temperature chlorination

Selective chlorination has been proven to be an energy-efficient and environmentally friendly method for recycling rare earth elements (REEs) from Nd-Fe-B waste. However, little attention has been paid to the in-depth study of Nd-Fe-B sludge chlorination until now. In this work, four isoconversion methods combined with an unreacted shrinkage core model were employed for the first time to investigate the kinetic behavior of the chlorination reaction between FeCl 3 and Nd-Fe-B sludge. Through thermodynamic calculations, TG-DSC, SEM-EDS, and XRD analysis, the chlorination process and phase transformation behavior of Nd-Fe-B sludge were elucidated. The results show that under optimal conditions (350 °C, FeCl 3 /RE=3:1, 2 h), the RE components in Nd-Fe-B sludge are selectively chlorinated to RECl 3 , avoiding the formation of REOCl, while the iron components primarily exist in the leaching residue as Fe 2 O 3 or FeOCl. And 97.32 % of REEs are selectively extracted into the leachate, while the iron leaching rate is only 4.02 %. Besides, the STA method is the most accurate, yielding E α-STA and A values of 75.01 kJ/mol and 2.6 × 10 6 min −1 , respectively. The reaction control mechanism was analyzed in three steps. In Step 1 (0.05 < α < 0.63) and Step 2 (0.63 < α < 0.75), the diffusion reaction mechanism controls the overall reaction, In Step 3 (0.75 < α < 0.95), the nucleation reaction takes over the control, which provides a good explanation for the change in E α-STA . However, the control step of the entire chlorination reaction is dominated by the internal diffusion process.