SIMULATION OF IN-SITU URANINITE LEACHING .2. THE EFFECTS OF ORE GRADEAND DEPOSIT POROSITY

A combined partial equilibrium-mixing cell model has been used to inve stigate the effects of fluid flow, mineral content, porosity, and lixi viant concentrations on in situ leaching of uraninite. The model coupl es the rate processes of reactive transport (uraninite and calcite dis solution kinetics and leach solution flow) with solution phase equilib ria (acid-base and complexation equilibria). Solution circulation and porosity changes have been explicitly treated in the following way: re acted solution was assumed to be pumped from the system at a constant rate and replaced by fresh lixiviant; the additional void volume resul ting from CaCO3 or UO2 dissolution was immediately filled with lixivia nt. A solution volume of 1 cm(3) was taken for the base, and it was as sumed that on each 1200-second increment, loaded solution was removed at the rate of 1.67 x 10(-5) cm s(-1), equivalent to removal of 2.0 pe t of the base volume. The lixiviant considered was NH4HCO3-(NH4)(2)CO3 -H2O2 with reference case concentrations of 1.0 x 10(-4), 1.0 x 10(-4) , and 2.2 X 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 CaC O3 (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 associa ted with higher U(VI) concentrations, and these were maintained for mu ch longer periods; the consumption of the peroxide oxidant was under m ass transfer control. As the leaching reaction slowed, solution replac ement began to control the component concentrations, causing decreasin g U(VI) concentrations. Higher porosity caused reduced maximum U conce ntrations and a faster decline. The calcite content had a slight effec t on the rate of U leaching; this occurred because high CaCO3 mass fra ctions led to Increased HCO3- concentrations. Early in the leaching pr ocess, a lower initial porosity or a higher calcite content led to a h igher (less negative) value of the CaCO3 saturation index; however, fo r the conditions simulated, the solution did not actually become satur ated. Also, decreases in the saturation index occurred sooner for high er 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 c ontent was too low for it to contribute significantly. Changes in conc entrations of the various solution species occurred more rapidly if th e 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; w ith high calcite contents (40 pct), it remained constant for an extend ed 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 concentrati ons behaved oppositely (as required by the reaction stoichiometry), bu t changes in the concentrations of the minor U(VI) and peroxo species were more complicated. The concentrations of the CO32- and HCO3- speci es could not readily be predicted from the reaction kinetics, and vari ations in their concentrations did not reliably indicate pH.