Preparation of phosphinate-based ionic liquids for rare earth recovery

Abstract

The rare earth elements are vital components in many modern technologies, including green energy technology such as wind turbines and electric vehicles, however the extraction and separation of these elements remains a problem. Due to the similar chemical properties of these elements, achieving pure single elemental concentrations of the REEs is difficult, often requiring multiple steps in the separation process, as well as the use of large volumes of harmful organic solvents. With the demand of these elements projected to grow in the coming decades, greener and more efficient extraction and separation methods need to be developed.

In this work, a series of hydrophobic ionic liquids were prepared through simple neutralisation and protonation reactions. The successful synthesis and characterisation of camphyl phenyl phosphinic acid is also described and used as an anion precursor in IL preparation. Five phosphinate-based ionic liquids were successfully prepared and tested for the separation of the rare earth elements, as well as for cobalt/nickel separations. Both liquid-liquid extraction methods using a neat IL, and column chromatography methods using supported ionic liquids were tested. Results showed selectivity towards the heavy rare earth elements, achieving a La/Yb separation factor as high as 55.3. For the liquid-liquid extraction experiments, La extraction efficiencies of 99% were achieved for a majority of the tested ILs. For the Yb column extractions, high extraction efficiencies were also achieved, ranging from 85.5 - 99.9%. Repeated extractions of cobalt and nickel showed the stability of the column, with minimal decrease in extraction efficiencies over 10 repeated runs, and an average separation factor of 2.79 across these runs.