Partial-equilibrium model based on the kinetics of dissolution and equilibrium in solution of the UO2-FeS2-Fe2(SO4)3-H2SO4 system

A mathematical model for determining the concentration changes in solution during the dissolution of a uranium dioxide-pyrite mixture has been developed using a Fe2(SO4)3-H2SO4 lixiviant. When modeling such a leaching process, two different types of reactions must be considered: heterogeneous dissolution reactions and homogeneous solution reactions. The dissolution reactions occur at kinetically limited reaction rates which are slow enough so that the rapid solution reaction can be assumed to be at continual equilibrium. This situation allows for the partial equilibrium approach to be utilized to model the overall leaching process. In setting up the model, the major solution reactions and aqueous species present during the dissolution process were identified. The initial equilibrium concentration for each of these aqueous species is then determined for a given lixiviant. The modeling process then determines the equilibrium changes for each of these aqueous species for every increment of solid dissolution, with the relative amount of uranium dioxide to pyrite reacting based on the experimentally determined rate expressions. The calculated concentration changes are obtained from simultaneously solving a series of linear equations arising from each equilibrium reaction, a charge balance, and material balances for Fe(III), Fe(II), UO22+, and SO42-. A computer model was written to perform this iterative process. Results from the model for the dissolution of a uranium dioxide and pyrite mixture were found to agree with the experimental results obtained for similar leach conditions. The Fe2(SO4)3-H2SO4 solution was found to be an effective lixiviant provided leaching conditions were adjusted for the selective leaching of uranium dioxide