Solvent extraction of rare earths using a bifunctional ionic liquid. Part 1: Interaction with acidic solutions

Abstract

Bifunctional ionic liquid extractants have been examined in recent years for the separation of rare earth elements by solvent extraction. One such extractant is the quaternary ammonium phosphonate ionic liquid ‘R4N+EHEHP−’ formed from Aliquat 336 (trioctyl/decylmethylammonium chloride) and EHEHPA (2-ethylhexyl phosphonic acid 2-ethylhexyl ester, also known as P507 or PC88A).In this work, the uptake of hydrochloric acid by R4N+EHEHP− is investigated, by studying the effect of pH, chloride, diluent and extractant concentration and composition on the extraction of acid. A fundamental understanding of the interactions between R4N+EHEHP− and acid is considered crucial in order to describe the distribution of rare earths in such a system.The results show that when dissolved in low concentration in a polar phase, the acid uptake by the ionic liquid as a function of pH is indistinguishable from EHEHPA. However, when dissolved in toluene at process relevant concentrations (up to 0.5M), the protonation reaction occurs over a broader pH range and shifts to lower pH when compared with EHEHPA alone. Increasing the chloride concentration shifts the reaction to higher pH. The reaction stoichiometry, 31P{1H} NMR spectra and observed trends are consistent with protonation of the phosphonate ion and simultaneous formation of R4N+Cl−. It is suggested that ion-pairing between the quaternary ammonium and phosphonate ions is responsible for the observed change in the protonation behaviour compared with EHEHPA alone. Our results indicate that the combination of Aliquat 336 and EHEHPA behaves as a mixture of R4N+EHEHP− and R4N+Cl−+EHEHPA, with the protonation dependent on the pH, and not as a different chemical form. Hence, regardless of how the ionic liquid has been prepared, once it is exposed to acid containing solutions, the acid-base behaviour of the ionic liquid is indistinguishable from a mixture of the two reagents.