Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: the system Al 2O 3H 2OKOH

Abstract

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 Hückel (1925) and Setchénow (1892) equations for activity coefficients of aqueous species and the thermodynamic properties of the aluminate ion AlO 2 − to retrieve dissociation constants for the potassium aluminate ion pair KAlO 2 0. The calculations were carried out using values of the Hückel extended-term parameter for aqueous KOH and dissociation constants for KOH 0 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 KAlO 2 0 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°C and 1 bar of KAlO 2 0, which can be used to calculate the standard partial molal thermodynamic properties of the species at temperatures to 1000°C and pressures to 5 kbar. Combining these parameters and properties with those for AlO 2 − (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°C and pressures to 5 kbar. Similar calculations carried out for temperatures from 80 to 300°C at P SAT ( P SAT refers to 1 bar at temperatures < 100°C and to the equilibrium pressure for coexisting liquid and vapor H 2O 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 KAlO 2 0 and the decrease in the activity coefficient of AlO 2 −. At temperatures ∼ ≤ 80°C, the two factors contribute comparably to the solubility of gibbsite in solutions with KCl concentrations up to 5 molal.