A kinetic model of UO 2 dissolution in acid, H 2 O 2 solutions that includes uranium peroxide hydrate precipitation
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I.
INTRODUCTION
S A N D S T O N E uranium deposits constitute the largest source of low-cost uranium reserves in the United States. The richest and most accessible deposits have been, or are, being mined by conventional open pit or underground methods. However, direct recovery of uranium from low grade sandstone deposits by in situ leaching is a viable alternative to conventional methods of uranium mining that is presently being used to recover uranium from roll front deposits in Wyoming and south Texas. Knowledge of the chemistry and kinetics of UO2 dissolution is necessary if in situ leaching processes are to be understood and optimized, and could provide a basis for design and optimization of acid vat leaching of uraninite concentrate. This paper reports the results of initial rate experiments designed to determine the dissolution rate of UO2 in acid solutions containing H,_O2 and conditions similar to those expected in an acid in situ leach operation (Table I). An expression is derived that gives the rate of UO2 leaching as a function of H202 and total sulfate concentrations and pH. The mechanism of UO2 dissolution is described. UO2 dissolution by H202 is a particularly interesting process because, even at low pH conditions, a uranium peroxide hydrate (UO4" XH20) forms on the uraninite grains, and this precipitation affects the dissolution kinetics. The mathematical model derived from the experiments takes into account the effects of uranium peroxidehydrate precipitation on UO2 dissolution kinetics, and is consistent with the experiments and theories of heterogeneous reaction kinetics. The mechanisms of reactions in heterogeneous systems, including the UO2 dissolution reaction considered in this paper, often involve a similar series of steps: "(1) adsorption of fluid species onto surface, (2) reaction of adsorbed species among themselves or with the surface atoms, and L. E. EARY is Graduate Student, Department of Geosciences, Pennsylvania State University, University Park, PA 16802. L.M. CATHLES, formerly Associate Professor of Geosciences, Department of Geosciences, Pennsylvania State University, University Park, PA, is now Senior Research Geophysicist with Chevron Oil Field Research Company, La Habra, CA 9063 I. Manuscript submitted March 19, 1982. METALLURGICALTRANSACTIONS B
(3) desorption of product species." (Lasaga-') Interactions at the oxide-water interface have been described in general by Parks 3 and Davis. 4 First a hydroxylatcd surface can be expected to form on any metal oxide in an aqueous environment and can be represented as shown in reaction [1] for the UO2 surface. The symbols 0,-05 represent the fraction of the oxide surface present as the indicated surface species. S indicates the surface. S--UO2 + H20 = S - - - O U / O H 01
02
[1]
\OH
The hydroxylated surface may develop a charge due to amphoteric protonation or dissociation of the adsorbed hydroxyl groups. Protonation, reaction [2], is much more likely at the low pH values used in the dissolution experiments (Table I). The zero point
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