Corundum solubility and aluminum speciation in KOH aqueous solutions at 400°C from 0.5 to 2.0 kbar

Abstract

Corundum solubility has been measured in KOH aqueous solutions under supercritical conditions (400°C, from 0.5 to 2 kbar). The investigated concentration range (0.001 < MKOH < 0.1 mol/kg H2O) allowed to vary the species balance between diluted solutions dominated by aluminate ion, Al(OH)4− , and more concentrated ones in which association of aluminate ion with K+ ion is appreciable. The dissociation constant of the complex KAl(OH)40 and the equilibrium constant for hydrolysis of corundum have been simultaneously fitted to the measured solubilities through a speciation calculation.The corundum hydrolysis (1/2 Al2O3 + 5/2 H2O ⇔ Al(OH)4− + H+) equilibrium constant is in close agreement with the values calculated from thermodynamical data for corundum (Berman, 1988) and aqueous species (Johnson et al., 1992). Using the Density Model (Anderson et al., 1991) for extrapolating thermodynamical properties of aqueous species results in appreciably higher values for this equilibrium constant, not consistent with experimental data, due to the inadequacy of the Density Model for extrapolating water properties in the supercritical P-T range.For the KA1 (OH)40 dissociation reaction, the following values of log K: −1.94 ± 0.17, −1.73 ± 0.14, and −1.56 ± 0.11, are obtained at 0.5, 1.0, and 2.0 kbar, respectively. These constants correspond to equal contributions of the aluminate ion and of the KAl(OH)40 complex to the total dissolved A1 in 0.1 m KOH solutions. At 600 and 700°C, 2 kbar, the same equilibrium constants calculated from literature experimental corundum solubilities are −2.43 and −3.16, respectively.From the calculated aluminate ion concentration in the system Al2O3H2O and the literature values of corundum solubility in water, the neutral hydroxyde Al(OH)3 is inferred to be probably the dominant A1 aqueous species in the neutral pH range in this system.