Stability and kinetics of kaolinite to boehmite conversion under hydrothermal conditions

Abstract

The equilibrium constant and kinetics of the reaction: A1 2 Si 2 O 5( OH) 4+3 H 2 O⇌2 A1 OOh+2 H 4 SiO 4( aq) were experimentally studied by reacting kaolinite with boehmite in aqueous 0.1 m KCl solutions at 200° or 275°C and 0.5 kbar. The equilibrium constant ( K eq) of the reaction was determined using the quench concentration of H 4SiO 4 equilibrated with kaolinite and boehmite. The results show that log K eq is −4.98 ± 0.04 at 200°C and −4.12 ± 0.02 at 275°C. The log K eq-value increases slightly by adding trace amounts of cations in solution: −4.02 ± 0.02 at 275°C with 1.23–2.46 m m Mg 2+ and −4.00 ± 0.04 at 275°C with 0.54–1.08 m m Fe 3+. The net rates of forward and reverse reactions near equilibrium were determined by monitoring the change of quench concentration of H 4SiO 4 as a function of time at quench pH near 4.2 ± 0.2 at 200°C. The results show that the dependence of forward and reverse rates on the Gibbs free energy of reaction, ΔG r (within the range from −0.25 to 0.5 kcal mol −1) can be expressed as: ( rate)=− k( δG r / RT) where R is the gas constant; T is absolute temperature; and k (in mol m −2 s −1) is the rate constant: k = (2.2 ± 0.6)·10 −12 at 200°C and (0.85 ± 0.5)·10 −11 at 275°C. Both forward and reverse rates at 200°C can be fit to the same line within the uncertainty of the experimental data, suggesting that the near-equilibrium kinetics of the reaction may be consistent with the principle of detailed balancing. The rate constants are in good agreement with those calculated using the integrated rate method by assuming the detailed balancing principle.