Simulation of in situ uraninite leaching-part III: The effects of solution concentration
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I.
INTRODUCTION
D U R I N G in situ leaching for uranium or other metals, there are very few operating conditions that can be set a priori. Leaching reagents and their concentrations can be chosen, the number and location of injection and production wells selected, and pumping rates specified. Often these decisions have been taken with little idea of the consequences they would have on ultimate recovery. For optimum values of operating parameters to be identified, the ore body must be characterized thoroughly; however, in the open literature, mineralogical and hydrological data are sparse. During leaching, fluid flow, advective and diffusive mass transport, mineral dissolution and precipitation reactions, and solution phase reactions are tightly coupled, and their inter-relationships determine the technical success and profitability of the operation. The partial equilibrium-mixing cell model developed for a peroxide carbonate-bicarbonate lixiviant in Part ItU incorporates rate equations for uraninite dissolution and for dissolution (or precipitation) of calcite equilibrium equations for important solution phase reactions, a one-parameter equation involving the rate of solution flow and an equation relating porosity changes to the mineral reactions. The model assumes that mineral dissolution and solution flow are relatively slow rate processes and that the solution phase reactions rapidly re-equilibrate in response to changes in component concentrations; this is the concept of partial equilibrium applied to an open system in a geologic medium. Within a mixing cell (the control volume), conditions are assumed to be time varying but homogeneous
KNONA C. LIDDELL is Associate Professor of Chemical Engineering, Washington State University, Pullman, Washington 99164. RENATO G. BAUTISTA is Professor of Chemical Engineering, University of Nevada, Reno, Reno, Nevada 89557. Manuscript submitted April 12, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS B
and isotropic; a mixing cell is analogous to an unsteadystate stirred tank reactor and may be modeled in a similar way. Effects of fluid flow on mineral conversion (fraction reacted), calcite saturation index, porosity, and the concentrations of five solution components and 15 individual species were described in Part I,t~] those of UO2 and C a C O 3 mass fraction and initial deposit porosity are discussed in Part II.t21 This article is concerned with the effects of the concentrations of (NH4)2CO3, NHaHCO3, and H202 in the lixiviant. Unless otherwise specified, input parameter values are the same as those in Part I. Calculations are based on a solution volume of 1 cm 3. Fresh lixiviant is assumed to be supplied at a rate sufficient to completely fill the void volume, which becomes larger as the minerals dissolve.[ 2j Input parameters that were kept constant during the following simulations are UO2 mass fraction (0.5 wt pct), CaCO3 mass fraction (20 wt pct), initial porosity (30 pct), and output solution flow rate (1.67 • 105 cm 3 S-l). Further details on the assumptions and
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