Simulation of in situ uraninite leaching. Part 2: The effects of ore grade and deposit porosity

A combined partial equilibrium-mixing cell model has been used to investigate the effects of fluid flow, mineral content, porosity, and lixiviant concentrations on in situ leaching of uraninite. 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. The lixiviant considered was NH4HCO3-(NH4)2CO3-H2O. 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. 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. 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