Simulation ofin situ uraninite leaching-part II: The effects of ore grade and deposit porosity

Abstract

A combined partial equilibrium-mixing cell model has been used to investigate the effects of fluid flow, mineral content, porosity, and lixiviant concentrations onin situ leaching of uraninite. The model couples the rate processes of reactive transport (uraninite and calcite dissolution kinetics and leach solution flow) with solution phase equilibria (acid-base and complexation equilibria). Solution circulation and porosity changes have been explicitly treated in the following way: reacted solution was assumed to be pumped from the system at a constant rate and replaced by fresh lixiviant; the additional void volume resulting from CaCO3 or UO2 dissolution was immediately filled with lixiviant. A solution volume of 1 cm3 was taken for the base, and it was assumed that on each 1200-second increment, loaded solution was removed at the rate of 1.67 × 10-5 cm s-1, equivalent to removal of 2.0 pct of the base volume. The lixiviant considered was NH4HCO3 (NH4)2CO3-H2O2 with reference case concentrations of 1.0 × 10-4, 1.0 × 10-4, and 2.2 × 10-5 mol cm-1. The parameters that were varied in this investigation were the mass fractions of UO2 (0.000 to 0.015) and CaCO3 (0.00 to 0.40) and the initial porosity of the deposit (0.20 and 0.30). Major factors found to affect the uranium content of the solution were UO2 content and initial porosity. Higher UO2 grades were associated with higher U(VI) concentrations, and these were maintained for much longer periods; the consumption of the peroxide oxidant was under mass transfer control. As the leaching reaction slowed, solution replacement began to control the component concentrations, causing decreasing U(VI) concentrations. Higher porosity caused reduced maximum U concentrations and a faster decline. The calcite content had a slight effect on the rate of U leaching; this occurred because high CaCO3 mass fractions led to increased HCO3- concentrations. Early in the leaching process, a lower initial porosity or a higher calcite content led to a higher (less negative) value of the CaCO3 saturation index; however, for the conditions simulated, the solution did not actually become saturated. Also, decreases in the saturation index occurred sooner for higher initial porosities or lower calcite grades. The final porosity was effectively determined by the initial calcite content; dissolution of calcite continued until it had completely reacted, and the uraninite content was too low for it to contribute significantly. Changes in concentrations of the various solution species occurred more rapidly if the ore was more porous, but there were no other significant differences attributable to initial porosity. The H+ concentration was virtually constant throughout leaching if the ore did not contain any calcite; with high calcite contents (40 pct), it remained constant for an extended period following an initial sharp decrease. Changes in the OH-, NH4/+ and NH3 concentrations could be readily predicted from those of H+, and changes in the Ca species concentrations were closely related to those of the Ca and CO3 components. Total U and total H2O2 concentrations behaved oppositely (as required by the reaction stoichiometry), but changes in the concentrations of the minor U(VI) and peroxo species were more complicated. The concentrations of the CO32- and HCO3- species could not readily be predicted from the reaction kinetics, and variations in their concentrations did not reliably indicate pH.