The solubility equilibrium and the rate of solution of quartz in water at high temperatures and high pressures

Abstract

The reasonable chemical equilibrium for solution of quartz in water under supercritical conditions seems to be represented by the following equation: SiO_2(quartz)+2H_20(gas)⇄Si(OH)_4(gas). The constants K_f and K_e for this equilibrium were calculated using the data already published by the author. From a comparison of K_e as a function of temperature, the heat of solution ΔE was estimated to be 7.80 kcal/mol. The changes in the amount of dissolved quartz were measured by the quenching method at temperatures 400, 440 and 480℃ and under specific volumes of solvents, 3.0, 2.0 and 1.6ml/g. The following rate equation was found to be applied to the solution of quartz under supercritical conditions: -dC/dt=kS(Ce-C)/Ce, where k is the rate constant, C, the concentration of monosilicic acid (Ce at equilibrium) and S, the constant for the surface of quartz. The rate of solution was accelerated by sodium chloride added to water.