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Computational chemistry was used to make predictions regarding the stepwise hydrolysis and fluoride dissociation reactions of ZrF 4 and HfF 4 in aqueous solutions. Specifically, density functional theory was used to predict Gibbs formation energies for ZrF 4 and HfF 4 in aqueous solutions, which were compared to experimental results reported in the literature. Following this, Gibbs reaction energies for the stepwise hydrolysis and fluoride dissociation reactions of ZrF 4 and HfF 4 were predicted, also with density functional theory, and used to predict stability constants that describe the distribution of the various zirconium and hafnium species in an aqueous solution. Finally, given these stability constants, distribution of species diagrams were constructed, which display which ZrF 4 and HfF 4 species are present at a given pH. Speciation, ZrF4, HfF4, density functional theory, stability constant.

The suitability of Liquid-liquid extraction (LLE) for the selective extraction of chemically similar tantalum (Ta) and niobium (Nb) mixtures was investigated by determining the influence of the acid concentration, extractant to metal mole ratio (E:M) as well as the ageing of the feed on extraction using the extractants, bis (2-ethylhexyl) phosphate (D2EHPA) and di-iso-octylphosphinic acid (PA). The system consisted of a solvent in varying E:M ratio’s, diluted in cyclohexane with 3% (v\v) 1-octanol added as modifier and a feed solution containing sulphuric acid and 100 ppm of the NH4TaF6and NH4NbF6complexes. Depending on the acid concentration, extraction percentages (%E) of up to 100% for Ta and 10-20% for Nb were attained. An initial lack of repeatability in experimental results was shown to be caused by variations in the age of the feed solution. The change in extraction trends expressed as a normalized %E for the aged feed solutions were nearly identical for both extractants when using 3, 6, 9 and 14mol/dm3H2SO4. For the 3 and 6mol/dm3solutions, the %E decreased significantly within the first 4-5 hours of ageing. At 9mol/dm3the %E remained stable for feed ages up to 3.5 hours before declining, while the %E remained near constant at 14mol/dm3even after ageing for 24 hours.