Abstract

The dissolution rate of uraninite crystals and of uranium roll-front ore samples has been measured in bicarbonate solutions at 25 degrees C, pH 7.0 to 8.2, and variable oxygen pressure. The rate law determined for uraninite crystals in highly undersaturated solutions is d[U]/dt = (kA/V)(O 2 ) (super 1/2) . A more complete rate law applicable over all uranium concentrations up to saturation is d[U]/dt = (kA/V)(O 2 ) (super 1/2) (U s - [U])/U s , where A is the surface area in cm 2 , V is the volume of solvent in liters, [U] is the measured molal concentration of U(VI) in solution, U s is the saturation concentration, (O 2 ) is the partial pressure of oxygen in atmospheres, t is the time in minutes, and k is the rate constant with a value of 4.9 (+ or -2.2) X 10 (super -5) (mole)(l)(m) (super -2) (atm) (super -1/2) (min) (super -1) . In this specific system, the rate-limiting step for uraninite dissolution is cleavage of the oxygen-oxygen bond in adsorbed oxygen. Parabolic rate behavior observed in some of our experiments is attributed to rapid initial dissolution of hexavalent oxides and fine-grained material. Our derived rate law for uraninite crystals applies very well to our dissolution data for uranium roll-front ores. Our rate law and experimental approach can be used to model the following processes: the formation and migration of roll-front ores, solution mining of uranium ores with oxygenated, bicarbonate solutions, and dispersion of uranium about mining sites and ore and tailings dumps.

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