Liquid-liquid extraction studies of the rare earth elements.
Creators: name: Otu, Emmanuel Ogbonna.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1990
Subjects: subject: Chemistry, Analytical.

description: This thesis is directed towards the study of the solvent extraction behaviour of the lanthanides and aluminium, bismuth, calcium and zinc whose radii and/or charges are similar to those of the REE. A brief review of the fundamentals and classification of extraction systems and of extractants is presented in Chapter 1. Chapters 2 and 3 are reviews of the properties and uses of the lanthanides and solvent extraction using phosphonate and sulphonate extractants, respectively. Chapter 4 deals with experimental procedures, results, discussions and conclusions. The extraction of zinc (II), calcium (II), aluminium (III), bismuth (III) and some lanthanide ions from aqueous perchlorate solutions into hexane solutions of 2-ethylhexyl phenylphosphonic acid, HEH$\Phi$P, was studied. The mechanisms of extraction are discussed on the basis of the results obtained by slope analysis. Depending upon the size and charge of the ion, the extracted species contain varying numbers of extractant molecules and phosphonate groups as ligands. Monomeric complexes are formed in the presence of excess extractant. High loadings of the extractant phase with the metal ion resulted in suppression of the extraction. Alkali ions were not extracted but alkali perchlorate suppressed the extraction through the effect of ionic strength on the metal ion activity. To further investigate the mechanism of extraction, $\sp{31}$P NMR of the organic phase following extraction of lanthanum was studied. A polymeric lanthanum-HEH$\Phi$P complex which precipitates out in the organic phase is formed at high (saturation) loading of this phase. A structure is proposed for this complex. The thermodynamics of extraction of these ions from perchlorate solutions into petroleum ether solutions of dinonylnaphthalene sulphonic acid, DNNSA, and HEH$\Phi$P were studied. In the case of DNNSA, extraction of the trivalent ions is dominated by the enthalpy of complexing. Electrostriction of large complex micelles by the complexed ion is postulated in order to account for the entropy effect. For the divalent ions, the enthalpy of dehydration of the ion is more important. A strategy for improving the separation factors is proposed. In the case of HEH$\Phi$P, charge density of the cation has a major influence upon the mechanism of the reaction and in turn upon the thermodynamic parameters. The ionic strength of the aqueous phase influences the thermodynamic parameters in the HEH$\Phi$P and DNNSA systems. Amongst the REE, lanthanum shows a singular behaviour. The extractions have been compared with those that employ dinonylnaphthalene sulphonic acid and factors that are responsible for the greater selectivity of the phosphonate have been elucidated. Development of an extraction chromatographic separation procedure for the lanthanides with a view to separating them from other matrix elements and fractionating them among themselves was studied. This was to be achieved by employing an organic polar solvent in the later stages of column elution. However, low recoveries were observed upon ashing the organic eluent and DCP determinations. The presence of phosphate (as KH$\sb2$PO$\sb4$ or H$\sb3$PO$\sb4)$ was found to lead to depression of analyte signal (concentration) in the DCP.