Solubilities and Rates of Dissolution of Diaspore in NaOH Aqueous Solutions

Abstract

Abstract The solubility of diaspore (α-Al2O3·H2O or α-AlOOH) has been measured in NaOH aqueous solutions from 523.15 K to 598.15 K, and at concentrations of Na2O up to 150.7 g/l. In order to evaluate the equilibrium constant, K2°=(aAlO2−·aH2O)⁄aOH−, for the dissolution equilibrium of diaspore AlOOH(s)+OH−(l)=AlO2(l)+H2O(l), two methods have been applied to the solubility data. In the first method, analogous to the method previously applied to the dissolution equilibria of gibbsite, bayerite and beohmite, the values of K2 at relatively low concentrations of NaOH were extrapolated to zero NaOH concentrations. The temperature dependence of K2° was expressed as logK2°=−(2500⁄T)+4.45. The second method was based on the extended Debye-Hückel theory for the variation in the activity coefficients of AlO2−, OH−, and H+ ions with ionic strength at any temperature, that is, the value of the solubility product, K1°=aH+·aAlO2−, was obtained at a given temperature by extrapolating values of K1 equal to (mH+mAlO2−), to zero ionic strength. According to the second method, K2° was expressed as a function of the temperature: logK2°=−(1600⁄T)+2.52 when A=2.0 or logK2°=−(1550⁄T)+2.48 when A=1.5, where A is a parameter. From kinetic analysis of the dissolution rate of diaspore in NaOH solution, the rate constants per unit surface area of diaspore were found to be 0.31, 0.45, and 0.68 m−2 h−1 at 548.15, 573.15, and 598.15 K, respectively.