Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: The system Al2O3-H2O-KOH

Experimental solubilities of gibbsite and corundum in aqueous KOH solutions at elevated temperatures and pressures reported by Fricke and Jucaitis (1930), Wesolowski (1992), and Pascal and Anderson (1989) were used together with the Huckel (1925) and Setchenow (1892) equations for activity coefficients of aqueous species and the thermodynamic properties of the aluminate ion AlO2- to retrieve dissociation constants for the potassium aluminate ion pair KAlO20. The calculations were carried out using values of the Huckel extended-term parameter for aqueous KOH and dissociation constants for KOH0 generated in the present study. Supercritical solubilities of corundum reported in the literature were regressed with the aid of a speciation model which explicitly provides for the formation in solution of triple ion clusters (Oelkers and Helgeson, 1990). Values of the dissociation constant for KAlO20 generated from these data were subsequently regressed with the revised Helgeson-Kirkham-Flowers (HKF) equations of state (Tanger and Helgeson, 1988; Shock et al., 1989) to obtain HKF equations of state coefficients and the standard partial molal thermodynamic properties at 25 degrees C and 1 bar of KAlO20, which can be used to calculate the standard partial molal thermodynamic properties of the species at temperatures to 1000 degrees C and pressures to 5 kbar. Combining these parameters and properties with those for AlO2- (Pokrovskii and Helgeson, 1995) permits calculation to within 0.05 log units of the experimental solubilities of corundum in KOH solutions reported by Barns et al. (1963) and Anderson and Burnham (1967) at temperatures to 800 degrees C and pressures to 5 kbar. Similar calculations carried out for temperatures from 80 to 300 degrees C at P-SAT (P-SAT refers to 1 bar at temperatures < 100 degrees C and to the equilibrium pressure for coexisting liquid and vapor H2O at higher temperatures) indicate that addition of KCl to dilute KOH solutions increases the solubilities of gibbsite, boehmite, and diaspore in response to increasing formation of KAlO20 and the decrease in the activity coefficient of AlO2-. At temperatures similar to 180 degrees C, the two factors contribute comparably to the solubility of gibbsite in solutions with KCl concentrations up to 5 molal.