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simulated using a partial equilibrium model which incorporates a one-parameter mixing cell model of solution flow. Rate laws for UO2 dissolution and for CaCO 3 dissolution/precipitation were taken from the literature, as were equilibrium constants for solution phase reactions. Parameters of the model include the UO2 and C a C O 3 o r e grades, the concentrations of the H202, NH4HCO3, and (NH4)2CO 3 components, porosity, exit solution flow rate, ore and mineral densities, and mineral rate constants and surface areas. Mineral conversions, component and species concentrations, and porosity are among the time-dependent quantities calculated using the model. For the conditions simulated, calcite dissolved somewhat faster than uraninite. The resuits emphasize the importance of the coupling between the mineral reactions and solution flow. Changes in the concentrations of the CO32- and HCO3 species are particularly complicated and not predictable from the calcite kinetics alone or from a purely equilibrium model; although the simulations did not reveal any conditions under which the solution would become saturated with f a C t 3 , the pH continued to change throughout the calcite dissolution and is buffered only after calcite has been consumed.

I.

INTRODUCTION

I N S I T U leaching of uraninite is relevant to both uranium production processes and environmental remediation. Previously published mathematical models of in situ UO2 dissolution have concentrated on the transport aspects and have not fully utilized the available data on reaction rates and equilibria. In simulating the oxidative leaching of uraninite and pitchblende, Nguyen et al. m used a one-dimensional volume averaged transport model in which the reaction rate term was assumed to be first order with respect to each of the reactants (the mineral itself, dissolved O, and carbonate and bicarbonate ions); the role of solution speciation was neglected. Lake and co-workers (e.g., Walsh et al. E21)employed an equilibrium geochemical code to consider mineral precipitation and dissolution, ion exchange, and solution speciation; effects specifically caused by coupled rate processes (chemical reactions with flow in porous media) could not be described within this framework. Experimental studies have provided little information on how the solution chemistry or mineralogy changes during in situ leaching. For purposes of engineering analysis and design, mathematical models are still needed to accurately describe the essential features of the system chemistry and to explicitly treat the interactions among various rate processes, and these models need to be validated by appropriately designed experiments and field sampling programs. The present series of articles presents a new type of KNONA C. LIDDELL, Associate Professor of Chemical Engineering, is with the Department of Chemical Engineering, Washington State University, Pullman, WA 99164. RENATO G. BAUTISTA, Professor of Chemical Engineering, is with the Department of Chemical and Metallurgical Engineering, University of