Comprehensive Evaluation on Reduction Behavior of Rare Earths (Neodymium/Dysprosium) Oxide with Magnesium Masses

The recycling process for rare earths (RE) is regarded as necessary process that can alter unstable and unsustainable RE production. One of the promising methods is the liquid metal extraction (LME) using magnesium (Mg). The mechanism of LME for RE has been elucidated with various assessments. However, there is lack of understanding on the practical reduction behavior of RE-oxides in the permanent magnets. In this study, we investigated the unrevealed mechanism of reaction between RE-oxide and Mg during LME process. The thermodynamic calculation and practical experiments are carried out simultaneously for observing theoretically reversed reduction of RE-oxide from Mg masses. After the reaction between RE-oxide and Mg, the microstructural studies, crystallography and quantitative analysis are examined. The reaction yields 100% and 41.94% of reduction efficiency for Nd-oxide and Dy oxide, respectively, which have good consistency with Gibbs free energy calculation results. Finally, RE-oxide reduction behavior during LME can be explained by observation on the radical mechanism and kinetics in this study. Furthermore, it is suggested that the Mg mass can control the reduction of RE-oxide for complete recovery of RE with metallic form based on collected understanding.