MODELS TO OPTIMIZE THE EVALUATION AND EXTRACTION OF URANIUM FROM SEDIMENTARY ORE DEPOSITS WITH APPLICATIONS TO IN-SITU LEACHING
ODDO, JOHN EDWARD
Doctor of Philosophy
With increasing demands for nuclear power supplies, more efficient means of evaluation and extraction of sedimentary uranium ore deposits, which form 96% of the U.S. reserves, are required. The fluvial, deltaic, and near-shore marine sands and associated lignites of the Texas coastal plain uranium deposits can assist in the need for increased nuclear power supplies. Tetravalent uranium is essentially immobile, while hexavalent uranium is easily complexed and very mobile in sedimentary aqueous systems. Fixation of uranium in the sediments seemed to be controlled by sorption and/or reduction by organic matter, H(,2)S, clays, zeolites, and carbonates. Texas uranium ore typically occurs in roll-front type deposits and these are discussed in conjunction with fixation and mobilization mechanisms. The basic strategies of the carbonate and acid leach systems are discussed. By monitoring effluent uranium and Rn-222 and cumulative uranium and Rn-222 extracted, it is shown that predictions can be made concerning mining efficiency, degree of secular equilibrium, future profitability, and mining duration. Dissolution Eh-pH diagrams constructed by assuming an infinite source of uraninite in water with various complexing agents are shown to agree more accurately with kinetic data of uraninite dissolution than conventional stability Eh-pH diagrams. At pH values between 6.0 and 9.0, the reduced reaction rate in this pH range, the first order dependence of the oxygen partial pressure and the hydrogen ion concentration, and the first order dependence of the carbonate at low concentration and zero dependence at higher concentration can be explained by the formation of (UO(,2))(,3)(OH)(,5)('+) predicted by the dissolution diagram. It is proposed that carbonate leach systems be operated at pH values between 9.0 and 10.0 and sulfate acid leach systems may be operated at a pH as high as 3.0. Utilizing evidence that uraninite dissolution is not diffusion limited and that in-situ leach solutions are quite undersaturated with respect to uranium, it is shown that uraninite dissolution is independent of hydrological parameters with the exception of the flow rate which regulates oxidant introduction to the ore body. The La Place source and sink equation is utilized to gain insight into parameters which can be adjusted to maximize mining efficiency while constraining the leach system. The optimum well spacing in an in-situ leach system is found to be a complex function of flow rate, the mobilization inhibiting factor (MIF), the rate of total oxidation of the aquifer, and the ability of the well system and the aquifer to confine the system. The results of the research are used to generate models to optimize parameters in the in-situ leach. The models are found to predict values in good agreement with literature values for uranium in-situ leach operations.