Experimental study of the hydrothermal formation of kaolinite

Kaolinite was hydrothermally precipitated starting from amorphous aluminosilicates, with Si/Al ratio from 1.8 to 0.76, at temperatures of 150, 175, 200, 225, and 250°C, at time periods varying from 6 h to 60 days. The solutions were analysed for pH, Si, Al, and K, and their saturation state was calculated by using the computer program SOLMINEQ.88. The solids were studied by surface area, XRD and DTA–TG analyses. Kaolinite was the only crystalline phase found in the products of the runs and its formation and crystallinity depended on time, temperature, and Si/Al ratio of the starting material. The kaolinite yield increased as starting material was aged for longer times. Products synthesized at higher temperature contain more kaolinite which was more crystalline than in the experiments carried out at lower temperature. The gel with Si/Al=0.99 produced the most disordered kaolinite and better crystallinity was obtained from products richest in Al or Si. The process of kaolinite formation was the result of two stages. During the first one aggregates of domains having a kaolinite-like structure were formed. The second stage corresponded to the formation of hexagonally outlined platy crystals. The formation rates indicated that the first stage was approximately one order of magnitude faster than the second stage, with average activation energies of 82±5 kJ mol−1 and 71±5 kJ mol−1, respectively. Si/Al ratio of the starting material exerted only a slight influence on the activation energies. The precipitation rates obtained for the second stage were consistent with literature data and may be considered precipitation rates of kaolinite in hydrothermal environments.