SIMULATION OF IN-SITU URANINITE LEACHING .3. THE EFFECTS OF SOLUTION CONCENTRATION

The effects of variations in the concentrations of leaching reagents h ave been simulated for in situ leaching of UO2 by H2O-(NH4)(2)CO3-NH4H CO3. The model used in the simulations incorporates rate laws for the mineral reactions, equilibrium reactions among the solution species, a nd a mixing cell representation of solution flow. Of the component con centrations, the major factor affecting the rate of uraninite dissolut ion is the oxidant concentration. High peroxide concentrations lead to more rapid reaction with an early maximum in the U(VI) concentration. If lower oxidant concentrations are used, the reaction is under mixed kinetic and mass transfer control and the U(VI) concentration is lowe r but approximately constant for an extended period. Because they incr ease the concentration of the HCO3- anion, high ammonium carbonate and ammonium bicarbonate concentrations also result in some enhancement i n the rate of U leaching; the reaction is known to be half-order in bo th HCO3- and H2O2. A 10:1 ratio of (NH4)(2)CO3 to NH4HCO3 concentratio ns was found to result in a nearly constant pH during most of the leac hing process. Calcite-containing gangue causes an immediate pH increas e from about 8.9 to 9.3. The rate of the calcite reaction, calcite sat uration index, and porosity are all independent of the lixiviant conce ntrations. Detailed calculations of solution speciation are necessary to predict the concentrations of individual species from those of comp onents.