Solubilities of RE2O3 in REF3-LiF (RE = Nd, Dy) at 1473 K

Abstract

The development of a recycling process for rare-earth-containing waste is important for improving the resource conservation and resource security of rare-earth metals. Phase equilibria between the molten fluoride and rare-earth oxide were investigated to develop a novel recycling process for neodymium magnets based on the flux-remelting method. LiF—rare-earth fluoride (REF3, RE = Nd, Dy) mixtures and rare-earth oxides (RE2O3, RE = Nd, Dy) were selected, and the phase equilibria in the Nd2O3−NdF3−Li2O−LiF and Dy2O3−DyF3−Li2O−LiF systems were investigated by differential thermal analysis and chemical equilibrating method. It was revealed that compounds NdOF, Nd4O3F6, DyOF, and Dy4O3F6 were stable as an equilibrium phase at 1473 K. In the dissolution of Nd2O3 in molten LiF–NdF3, Nd4O3F6 formed when the mixing ratio exceeds the solubility limit of the melt and the compound coexists with the melt. In the same manner, Dy4O3F6 formed when the mixing ratio exceeds the solubility limit of molten LiF-DyF3 and the compound coexists with the melt. At 1473 K, the solubilities of Nd2O3 in molten LiF—50 mol% NdF3 and Dy2O3 in molten LiF—50 mol% DyF3 were determined as 7.4 mass% and 7.6 mass%, respectively. An enhancement device for the dissolution rate of oxyfluorides is required for practical applications owing to the slow dissolution rate. The recycling process can be used to regenerate neodymium magnet waste by adding a small amount of virgin rare-earth metals.Graphical Abstract