The prediction of multicomponent ion exchange equilibria with particular reference to the system involved in the recovery of uranium.

Abstract

The problem of predicting the general ion exchange equilibria pertaining to systems of industrial significance is generated by the multicomponent nature of such systems and the nonidealities which may be present in both the solution and exchanger phases. A general framework applicable to multicomponent systems incorporating nonideal effects in both phases is presented. For the solution phase a well established procedure for calculating activity coefficients is adopted. Deviations from ideal behaviour in the exchanger phase are modelled by the Wilson equation, which expresses the excess Gibbs free energy of mixing of the resinates as a function of composition. A Significant advantage is afforded by this equation in that theoretically a multicomponent system may be predicted from the binary interaction coefficients of this equation which are determined experimentally, thereby reducing the otherwise extensive experimental program. These ideas are applied to systems of increasing complexity from simple binary characterisation experiments to the prediction of a six component system related to that encountered in the recovery of uranium from sulphuric acid leach liquors. Experimentation for the systems involving the ions S04 2-, Cl- and NO-3 and a strong base anion exchanger have provided a severe test for the procedure proposed. The agreement between the predicted and experimental resin phase composition data for this ternary system is within ± 5%. The addition of complexing agents complicates the procedure in that it becomes physically impossible to decompose the system into the desirable experimental binary systems. In this case higher order systems are characterised. Introducing a mineral acid to the ternary system discussed previously generates the HSO-4 ion which necessitates the characterisation of other ternary systems before the quaternary system may be predicted. The agreement between the predicted and experimental resin phase composition for the quaternary system is shown to be within ± 10%. The work is easily extended to include the more complex systems generated by the complexation of metal ions with the various ligands present. .Provided the stoichiometry of the complex species in the exchanger phase is well defined the complexes present no difficulties in the characterisation procedures. Experimental studies on the acidic uranyl sulphate quaternary system provide the desired ion exchange equilibrium constants and the interaction coefficients. In order that the interaction coefficients for the ion pairs such as UO2 (SO4)2-2, Cl- and U02 (SO4)2-2, NO-3 may be estimated it is necessary to characterise two quinary systems. Nevertheless the characteristics of lower order systems are employed to reduce the number of unknown parameters. Finally it is possible to predict the resin phase composition of the six component system which results from chloride and nitrate species being included in the acidic uranyl sulphate system. The quantitative effects of all the components in the solution phase on the extent of uranium loading are predicted. Although the ferric ion is an important component in the industrial situation this ion has been excluded from this work because at this stage it is not possible to identify or measure the quantity of the various ferric complexes present in the resin phase for a particular solution condition.