Solvent extraction of rare earth elements using a bifunctional ionic liquid. Part 2: Separation of rare earth elements.

Abstract

The extraction and separation of rare earths with bifunctional ionic liquid extractants has been the subject of a number of recent studies. 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), for which several claims have been made concerning separation for rare earths and likely extraction mechanisms.In Part 1 of this paper, the reaction of the extractant with HCl/NaCl media was studied, and found to be consistent with conversion to phosphonic acid and quaternary ammonium chloride forms over a broad pH range.In this work, the extraction and separation of rare earth elements by R4N+EHEHP− is explored, at process relevant extractant concentrations (up to 0.5M) and acidities (pH5 and lower). An analysis of the effect of acid, chloride, extractant and metal concentrations on the distribution of rare earths is presented, supplemented by visible absorption and 31P{1H} NMR spectroscopy to examine the extracted complex directly. This was combined with our previous study of the protonation reaction to suggest an extraction mechanism for rare earth elements. Other process relevant considerations such as the loading capacity, stripping acidity and separation factors were also determined.The results suggest that the reaction mechanism is one of cation exchange, with antagonism compared with EHEHPA. This is thought to be caused by the decrease in free EHEHPA dimer concentration due to ion pairing between the quaternary ammonium and phosphonate ions. The extractant is not recommended for light rare earth circuits due to the low loading capacity at high pH, while the antagonistic effect would be beneficial in heavy rare earth separations due to reduction in the stripping acidity.