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

Abstract

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.

Recovering precious metals from spent catalysts by peroxydisulfate-based advanced oxidation process

“…Several methods, including Hammett acidity 49,50 , infrared spectroscopy and etc. Electron transfer helps to generate free radicals and other oxidatively active species 54 , which can be identified using other exploration methods.…”

An acid-free process for selective REE recovery from spent NdFeB magnets by room-temperature electrolysis

“…Therefore, the [ZnCl 4 (H 2 O) 2 ] 2− coordination complex exhibited Brønsted acid behavior that can provide H + in solution. 25…”

“…The separation effect was optimal at this time, which is similar to that of previous reports. 25 In addition, the recycling performance of the electrolytes was also explored (Fig. 9b).…”

“…24 Previous studies have shown that salt solutions can achieve efficient separation of Nd 2 O 3 and Fe 2 O 3 and may have potential as electrolyte solutions. 25 Obviously, using traditional acid or organic solvent as an electrolyte is not environmentally friendly, and devices such as ion exchange membranes also have lifespan issues. Therefore, the efficient and green separation of REEs and Fe from EOL NdFeB magnets through electrochemical methods remains challenging.…”

Insights into the Selective Transformation of Ceria Sulfation and Iron/Manganese Mineralization for Enhancing the Selective Recovery of Rare Earth Elements

“…In recent years, several green solvents, such as deep eutectic solvents (DESs), 13 ionic liquid-based aqueous biphasic system (ILABS), 14 and inorganic salts, 15 were proposed to selectively recover REE from the secondary leaching solution. Among these processes, phase transformation of the components was implemented before the green solvents were employed.…”