Kinetics of Hydragylite to Boehmite Transformation in Water and Water Vapor

The kinetics of the isothermal transformation of hydrargillite with an average particle size of 3.5-μm into boehmite during autoclave treatment at 160–180°C in a liquid water and steam media has been studied. Samples of hydrargillite with and without the addition of boehmite previously obtained at 200°C have been used. It has been found that when boehmite is added to hydrargillite, the induction period decreases, while the rate of boehmite formation does not change. In water vapor, the induction period is sharply reduced, while the rate of boehmite formation increases compared to the rate in liquid water. A study of the morphology of the transformation products has shown that the formation and growth of boehmite crystals occurs on the surface of hydrargillite particles. The values of the activation energy of 89.3 ± 11.2 and 147.6 ± 11.4 kJ/mol have been determined from the temperature dependence of the rate constants of the isothermal boehmite formation in liquid water and steam, respectively. The mechanism of transformation of hydrargillite into boehmite has been discussed.