When constructing Eh-pH diagrams, workers have previously assumed a fixed total concentration of a substance in solution, and calculations (assuming equilibrium) were made to predict mineral phases and aqueous species at various Eh-pH conditions. When dissolving a solid compound (as in in-situ leaching), it seems more plausible to assume a solid phase in solution at various Eh-pH conditions and allow the aqueous species the freedom to assume thier equilibrium concentrations.

The computer program EHPHUR was written to thermodynamically simulate the dissolution of a solid phase leading to diagrams which differ markedly from conventional Eh-pH diagrams. The name proposed for these new diagrams is dissolution diagrams. The assumptions include unit activity of uraninite, and fixed amounts of O₂, CO₂, and H₂S at 25°C and 1 atm. The parameters were chosen to simulate in-situ leaching systems currently in use.

Using these diagrams, thermodynamic arguments can be presented which are consistent with the results of kinetic experiments, for example: the first and zero order dependence of the uraninite dissolution rate on the carbonate concentration; the decrease in the dissolution rate as the pH approaches 6.0; and the dependence of the rate on the oxygen and hydrogen ion concentrations. Apparent optimum pH values for in-situ leaching are discussed.

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