Highly efficient separation of rare earths from nickel and cobalt by solvent extraction with the ionic liquid trihexyl(tetradecyl)phosphonium nitrate: process relevant to the recycling of rare earths from permanent magnets and nickel metal hydride batteries

Vander Hoogerstraete and K. Binnemans
Green Chemistry 16, 1594–1606 (2014).
DOI: 10.1039/C3GC41577E

A solvent extraction process with the ionic liquid trihexyl(tetradecyl)phosphonium nitrate has been developed to extract rare earths and separate them from nickel or cobalt. Rare earths are extracted to the ionic liquid, whereas transition metals remain in the raffinate. The process is environmentally friendlier than traditional solvent extraction processes, since no volatile and flammable diluents have to be used. Compared to conventional ionic liquid metal extraction systems, the advantage of using the new ionic liquid is that expensive and persistent fluorinated ionic liquids can be avoided. The ionic liquid can be prepared by a simple metathesis reaction from the commercially available ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101). The extraction is facilitated by an inner salting-out effect of a highly concentrated metal nitrate aqueous phase. The separation of lanthanum and samarium from nickel or cobalt, out of highly concentrated metal salt solutions by solvent extraction, is of importance for the recycling samarium–cobalt permanent magnets or nickel metal hydride (NiMH) batteries.