Two-zone ligand-assisted displacement chromatography for producing high-purity praseodymium, neodymium, and dysprosium with high yield and high productivity from crude mixtures derived from waste magnets†
Three rare earth elements (REEs), neodymium (Nd), praseodymium (Pr), and dysprosium (Dy), are essential ingredients of permanent magnets, used widely in electronics, motors, hybrid cars, generators, televisions, sensors, and windmills. Conventional methods for producing high-purity REEs employ two-phase liquid–liquid extraction methods, which require thousands of mixer-settler units in series or in parallel and generate large amounts of toxic waste. In this study, a two-zone ligand-assisted displacement chromatography (LAD) system with a new zone-splitting method is developed for producing high-purity (>99%) Nd, Pr, and Dy with high yields (>99%) and high sorbent productivity from crude REE mixtures derived from waste magnets. The zone-splitting method based on selectivity-weighted composition factors enables a two-zone design to achieve two orders of magnitude higher productivity than that of a single column design. The design and simulation methods are based on first principles and intrinsic (or scale-independent) engineering parameters. They can be used to design processes for a wide range of feed compositions or production scales. The overall productivity of the two-zone LAD can exceed 100 kg REEs m−3 day−1, which is 100 times higher than those of the conventional extraction methods. The LAD for the purification of the ternary mixture requires only three chromatography columns, a safe extractant, EDTA, and other environmentally friendly chemicals. Most of the chemicals can be recycled, generating little waste. This method has the potential for efficient and environmentally friendly purification of the REEs from waste magnets. The method may also help transform the current linear REE economy (from ores to pure REEs, to products, to landfills) to a circular and sustainable REE economy.