Chemical and Biomolecular Engineering - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemSynergistic solvent extraction of rare earth elements( 2017)The separation of the rare earth elements is a difficult task owing to the chemical similarity of the lanthanides. The method most commonly utilised industrially is solvent extraction using EHEHPA (2-ethylhexyl phosphonic acid, mono-2-ethylhexyl ester). Due to the relatively small separation factors between adjacent lanthanides, a large number of equilibrium stages are required for the separation and a considerable amount of acid and base is consumed. In addition, recovery of the heavier rare earths from the solvent at the end of the process (stripping) requires quite high acid concentrations. In this work, mixed or synergistic extractants were examined to attempt to improve upon the conventional process using EHEHPA alone. Three classes of mixtures were studied using a combination of slope analysis and spectroscopy at process relevant extractant and metal concentrations. The first two systems, a mixture of EHEHPA with Cyanex 272 (bis-2,4,4-trimethylpentyl phosphinic acid) and the quaternary ammonium phosphonate ionic liquid R4N+EHEHP-, revealed antagonistic interactions that result in a lower stripping acidity, but also reduced loading capacity. 31P1H NMR spectroscopy showed that a mixed complex is formed with EHEHPA and Cyanex 272, but antagonistic interactions between the extractants are more significant for extraction results. For R4N+EHEHP-, the spectroscopic studies indicated that the extracted complex is exactly consistent with the corresponding EHEHPA-rare earth species. The quaternary ammonium ion moderates the rare earth extraction behaviour of EHEHPA by ion pairing with the phosphonate ion in a pH dependent interaction. The extraction results and spectroscopy for R4N+EHEHP- are consistent with a simple mixture of the quaternary ammonium chloride and EHEHPA once the organic phases are equilibrated at the same pH, in contrast with some reports in the literature. For the third and final class, two carboxylic acid containing mixtures were examined: CA-12 (sec-octylphenoxyacetic acid) with Cyanex 272, and a novel mixture of naphthenic acid and CA-12. The separation factors are largely determined by those of the components of the solvent mixture, and can be tailored somewhat by changing their proportion in the organic phase. The naphthenic acid/CA-12 solvent was therefore found to exhibit a promising set of separation factors for the extraction of lanthanides from yttrium, as yttrium is among the least extracted rare earth by both solvents. However, at elevated loading the separation factors revert to those of naphthenic acid alone, due to its considerably larger concentration. An analysis of the predicted capital and operating costs for several industrially relevant separations demonstrated that for most separations, the saving in stripping acid for the phosphonic/phosphinic acid mixture and R4N+EHEHP- is outweighed by an increase in equipment size due to the reduced loading capacity. However, for the separation of the heaviest elements, Lu and Yb, R4N+EHEHP- was found to be the best performing option due to the large saving in stripping acid consumption. The work shows that separation factors for the mixed extractants can largely be predicted from those of the individual components, while stripping acidity cost savings are only significant for the heaviest rare earths. New extractants are therefore required to make a significant improvement upon the conventional process, and this is suggested as the way forward for future work.