Study of factors determining mass transfer rates in solvent extraction

Abstract

The literature relating to the mass transfer of uranium, and plutoniun, between nitric acid and tri-butyl phosphate has been reviewed. The models and theories of mass transfer and methods of measuring mass transfer coefficients, and the phenomenon of interfacial turbulence and its relationship with the extraction of uranium, were surveyed. Overall mass transfer coefficients were measured as a function of time for the transfer of uranyl nitrate and plutonium nitrate between anitric acid aqueous phase and a solvent phase containing tri-butyl phosphate. A quasi-steady state, hemispherical drop of the aqueous phase was suspended in a solvent continuous phase, and the mass transfer of solute between them observed. The uranium system was studied undervarying conditions of solute concentration and temperature. The effects of a change in organic phase diluent, aqueous phase acidity,droplet Reynolds number, and solute loading of the continuous phase were examined. In each case the overall mass transfer coefficient was found to decrease with time, levelling off to a constant value after approximately 300 seconds. Up to an order of magnitude change in the mass transfer coefficient occurred as the interface aged; most change occurred within the first hundred seconds. Both systems under investigation exhibited interfacial turbulence during extraction of solute into the organic phase. The decrease in the mass transfer coefficient was shown to correspond to a decrease in the frequency of interfacial turbulence and an explanation for this was proposed. Arrhenius plots of the variation of the mass transfer coefficient with temperature yielded an experimental activation energy of 19.5kJ/mol(4.7kcal/mol). A diffusional mechanism was concluded to control the extraction of uranium. Organic phase to aqueous phase transfer of uranium was also studied.No time dependence of the mass transfer coefficient was found,corresponding to an absence of interfacial turbulence. A solvent quasi-steady state hemispherical droplet version of the apparatus was commissioned. The extraction of 25g/1 plutonium was also examined, and demonstrated a time dependence of the mass transfer coefficient.