Understanding the modes of action of solvent extraction reagents

Abstract

Chapter 1 consists of a review of solvent extraction of metals, primarily focused on the fundamental chemical processes involved in the technique, and techniques previously applied to the elucidation of extraction mechanisms. Chapter 2 contains an explanation and discussion of the theoretical and experimental techniques employed in obtaining the results presented in later chapters. In Chapter 3, computational studies are presented which aim to explore the modes of action of two novel classes of reagent which have been found to function efficaciously as extractants for precious metals, and rationalise trends in the extractive strength and selectivity of these reagents observed through experiment. DFT formation energy calculations were found to reproduce experimentally-determined trends in the extractive strength of a series of amidoamine and amidoether extractants for platinum, as well as the apparent selectivity of these reagents for hexachloroplatinate (PtCl62−) over chloride, which is found to arise due to a wide array of hydrogen-bond donor groups within the extractant which leads to preferential association to larger anions. Favourability of protonation is found to be the primary determiner of extractive strength in reagents of this class. A contrasting mode of action was revealed for a class of selective amide extractants for gold, with DFT and classical molecular dynamics methods indicating the formation of large supramolecular aggregates of extractant units and tetrachloroaurate (AuCl4−) anions during the extraction process. Molecular dynamics results are consistent with, and allow the rationalisation of, results obtained from slope analysis and EXAFS experiments. Aggregate stability is found to be greater for primary amide extractants than for secondary or tertiary amide analogues, which is proposed as the origin of the superior selectivity for gold exhibited by primary amide extractants. In Chapter 4, the mode of action of commercial phosphinic acid extractant CYANEX® 272 in cobalt(II) extraction is investigated using a variety of techniques including inductively-coupled plasma optical emission spectroscopy (ICP-OES), 31P{1H} NMR spectroscopy, mass spectrometry and DFT geometry optimisation calculations. Extraction is found to proceed via differing mechanisms depending on the concentration of unassociated phosphinic acid in the system, with extended polymeric structures being formed at high levels of cobalt extraction, providing an explanation for observed increases in organic phase viscosity under such conditions. Similar techniques are applied in Chapter 5 to the case of iron(III) extraction by CYANEX® 272. A single mechanism was found to persist across all levels of Fe(III) extraction, though extraction experiments and 31P{1H} NMR spectroscopy indicate that anions other than phosphinate must be incorporated into the extracted species. Results from ICP-OES and mass spectrometry suggest that species incorporating SO42− or HSO4− anions as well as O2− or OH− anions are formed during the extraction process. Finally, in Chapter 6, a novel computational method for determination of the stability of metalate anions in aqueous solutions is discussed. Preliminary results from solution-phase DFT formation energy calculations suggest that inner-sphere co-ordination of chloride and nitrate anions to La3+ in solution is greatly disfavoured compared to formation of outer-sphere complexes, consistent with literature reports that inner-sphere co-ordination of these anions to lanthanum is not observed in solution, and experimental results suggesting that solvent extraction of lanthanum metalates is unviable. Disfavouring of inner-sphere co-ordination to La3+ is calculated to be less substantial for sulfate anions, reflective of experimental observations that inner-sphere co-ordination of sulfate to La3+ can occur to a smaller degree. Similar, disfavouring of inner-sphere chloride association to Cd2+ and Hg2+ (which is known to occur in solution) is calculated to be significantly smaller than to La3+, suggesting that such calculations could serve as qualitative predictors of the viability of formation of solution-phase inner-sphere complexes of any given metals and anions.