Kinetics of mass transfer among silicates and aqueous solutions

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Scientific paper

Theoretical and empirical considerations suggest that the kinetics of mass transfer among aluminosilicates and aqueous solutions can be described by a simple parabolic rate law, which is consistent with diffusional mass transfer and applicable to incongruent as well as congruent reactions. The rate limiting step at both low and high temperatures consists of diffusion of material from the reactant mineral through a surface layer (or layers) of intermediate reaction products out into the bulk solution, which means that precipitation of the reaction products can be described by reversible chemical reactions and equilibrium thermodynamics. The sequential appearance of these products is manifested by changes in the parabolic rate constant, which ranges from 10 -10 to > 10 -8 moles (cm 2 of surface area) -1 day -1/2 for the incongruent hydrolysis of feldspar at 25°, 100°, 150° and 200°C, depending on the reaction and temperature. Extrapolation to higher temperatures yields rate constants of the order of 10 -5.5 moles (cm 2 of surface area) -1 day -1/2 at 700°C. The activation energy for the reaction of feldspar with an aqueous phase to produce boehmite and/or kaolinite is ~8700 cal. mole -1 . Phase equilibria, mass transfer and reaction paths predicted from theoretical equations and thermodynamic data are nearly identical to those observed experimentally. The thermodynamic calculations afford detailed interpretation of the results of experimental kinetic studies, evaluation of complicated chemical reactions in multicomponent systems and prediction of mass transfer in geochemical processes in terms of space/time coordinates.

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