Recycling of rare earths from NdFeB magnets using a combined leaching/extraction system based on the acidity and thermomorphism of the ionic liquid [Hbet][Tf2N]

Abstract

The continuous miniaturization of electric motors, hard disks and wind turbines is causing an increasing demand for high-performance neodymium–iron–boron magnets (NdFeB). The supply risk for the rare-earth elements (REEs) used in these magnets is a growing concern and has sparked the development of recycling schemes for these end-of-life products. In this paper a new recycling process for (microwave) roasted NdFeB magnets is proposed, based on the carboxyl-functionalized ionic liquid: betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf₂N]. Using the thermomorphic properties of the [Hbet][Tf₂N]–H₂O system, a combined leaching/extraction step was designed. The change from a homogeneous system during leaching (80 °C) to a biphasic system at room temperature causes the dissolved metal ions to distribute themselves amongst the two phases. The valuable elements (Nd, Dy, Co) are thus separated from the iron with high separation factors. The stripping was done very efficiently using oxalic acid to precipitate the REE(III) and cobalt(II) ions while transferring the iron(III) from the ionic liquid to the water phase as a soluble oxalate complex. The cobalt (present in certain magnets) was removed by treating the mixed (REE/Co) oxalate precipitate with aqueous ammonia. The remaining REE oxalate was then calcined to form the REE oxides (99.9% pure). The ionic liquid is regenerated during the stripping step and contamination of the water phase was avoided by salting-out the ionic liquid with Na₂SO₄. This innovative recycling process features a combined leaching/extraction in mild conditions using a reusable acidic ionic liquid and an energy-efficient microwave roasting of the magnets. These aspects all contribute towards the green character of this process which can be considered as a sustainable and efficient alternative to mineral acid leaching and solvent extraction.