ZnCl₂: A Green Brønsted Acid for Selectively Recovering Rare Earth Elements from Spent NdFeB Permanent Magnets

The spent neodymium-iron-boron (NdFeB) magnet is a highly valuable secondary resource of rare earth elements (REEs). Hydrometallurgical processes are widely used in recovering REEs from spent NdFeB magnets, but they will consume large amounts of organic chemicals, leading to severe environmental pollution. This work developed an alternative green route to selectively recover REEs from spent NdFeB permanent magnets using a purely inorganic zinc salt. The Hammett acidity measurement showed that concentrated ZnCl₂ solutions could be regarded as a strong Brønsted acid. Concentrated ZnCl₂ solutions achieved a high separation factor (>1 × 10⁵) between neodymium and iron through simple dissolution of their corresponding oxide mixture. In the simulated recovery process of spent NdFeB magnets, the Nd₂O₃ product was successfully recovered with a purity close to 100% after selective leaching by ZnCl₂ solution, sulfate double-salt precipitation, and oxalic acid precipitation. The separation performance of the ZnCl₂ solution for Nd₂O₃ and Fe₂O₃ remained almost unchanged after four cycles. The energy consumption and chemical inputs of this process are about ¹/₁₀ and half of the traditional hydrometallurgy process separately. This work provides a promising approach for the green recovery of secondary REE resources.