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

The revised Helgeson-Rirkham-Flowers (HKF) equations of state (Tanger and Helgeson, 1988; Shock and Helgeson, 1988; Shock, Helgeson, and Sverjensky, 1989) were used together with the Huckel (1925) and Setchenow (1892) equations for activity coefficients of aqueous species (Helgeson, Kirkham, and Flowers, 1981) and selected potentiometric data to interpret experimental solubilities reported in the literature for gibbsite, boehmite, diaspore, and corundum. The study resulted in an internally consistent set of thermodynamic data for Al3+, Al(OH)(2+), Al(OH)(2)(+) Al(OH)(3)(0), and Al(OH)(4)(-1) which permits calculation of the standard partial molal properties of these species at temperatures to 1000 degrees C and pressures to 5 kb. Standard partial molal Gibbs free energies of formation of boehmite and diaspore were retrieved from experimental solubilities of these minerals in NaOH solutions at P-SAT(2) by regressing solubility data reported in the literature using the Huckel extended-term parameters for aqueous NaOH, together with Setchenow coefficients and dissociation constants for NaOH0 generated in the present study. The results of these calculations indicate that appreciable concentrations of NaAl(OH)(4)(0) in NaOH solutions saturated with gibbsite, boehmite, and diaspore (see footnote 1). Dissociation constants for NaAl(OH)(4)(0) were retrieved from the solubility data for temperatures ranging from 40 degrees to 350 degrees C at P-SAT. These equilibrium constants were then used to calculate the standard partial molal thermodynamic properties and HKF equations of state coefficients for the species. Where comparisons can be made, the results of the calculations yield predicted solubilities of gibbsite in concentrated NaCl + NaOH solutions that are within +/- 0.05 log units of their experimental counterparts at temperatures as low as 6.4 degrees C and independently those of corundum in NaOH solutions within the same uncertainty limits at temperatures to 700 degrees C and pressures to 2.5 kb. The NaOH solutions saturated with corundum is comparable to, or even greater than, that of Al(OH)(4)(-). In supercritical H2O, the species Al(OH)(3)(0) and Al(OH)(4)(-) predominate in the presence of corundum. The thermodynamic properties of Al(OH)(3)(0) were calculated by taking explicit account of the formation of Al(OH)(4)(-) in these solutions. Calculated solubilities of boehmite as a function of pH generated in the present study are consistent with the bulk of the experimental data reported for alkaline solutions by Kuyunko, Malinin, and Khodakovsky (1983), Bourcier, Knauss, and Jacbon (1993), and Castet and others (1993). In neutral and acid solutions, the computed solubilities are in close agreement with their experimental counterparts at 150 degrees and 200 degrees C. However, at higher temperatures, the calculated solubilities are systematically higher than those measured by Castet and others (1993) but agree well with the experimental data reported by Kuyunko, Malinin, and Khodakovsky (1983) and Bourcier, Knauss, and Jackson (1993). It appears from the results of the present study that the discrepancy between the sets of experimental data cannot be resolved without additional experiments to better constrain the relative stabilities of Al(OH)(2+), Al(OH)(2)(+), and Al(OH)(3)(0) at temperatures >200 degrees C and pressures exceeding P-SAT. Additional experimental data are also required further to substantiate or contradict the conclusion drawn below that aluminum chloride complexing does not contribute to a substantial degree to the solubilities of aluminum-bearing minerals in concentrated alkali chloride solutions at temperatures less than or equal to 300 degrees C and pressures less than or equal to 1 kb.