Evaluation of the mechanisms of rare earth elements extraction from citrate solutions in the recycling of NdFeB magnets

Recycling rare earth elements (REEs) from secondary resources has gained significant attention due to their classification as critical metals by the European Union and their essential role in sustainable development. This study investigates the extraction of light and heavy REEs from citrate leachate solutions derived from end-of-life NdFeB magnets and examines the underlying mechanisms. Key parameters influencing REEs extraction efficiency are evaluated, including extractant and diluent types, extractant concentration, organic-to-aqueous phase volume ratio, contact time, and equilibrium pH. Results revealed that D2EHPA was the most efficient extractant, achieving approximately 100 % recovery for heavy REEs and 88 % for light REEs, with negligible iron extraction within 10 min. The extraction mechanism was found to differ significantly between light and heavy REEs. The equilibrium constant for heavy REEs (Dy, Y, and Tb) was approximately 50 times higher than for light REEs (Nd and Pr), indicating stronger interactions with the extractant. This difference is further supported by the smaller ionic radii of heavy REEs compared to light REEs, which result in higher charge density and stronger coordination preferences for heavy REEs. Moreover, it was observed that the extracted forms vary, with LREEs forming LREE H1.6 Cit.R1.6 (RH)2 complexes and HREEs forming HREE H2.4 Cit.R2.4 (RH)0.4 complexes. These differences contribute to the overall efficiency of the extraction process, with the final oxide product containing approximately 82 % REEs and only 0.1 % iron, demonstrating the high selectivity of the process.