Kinetic study of hydroxysodalite formation from natural kaolinites by time-resolved synchrotron powder diffraction

Real-time synchrotron powder diffraction was employed to investigate the crystallization process of hydroxysodalite (SOD) from natural kaolinites, The powder diffraction experiments were carried out in isothermal mode using a transmission geometry with capillary samples, and an image plate detector. Natural kaolinites with different densities of planar disorder were thermally activated at 800 degrees C to obtain precursor materials for the syntheses. The metakaolinite precursor was also prepared from the ordered kaolinite at 600 degrees C. The hydrothermal syntheses were accomplished by leaching the activated precursor materials with a 1 M NaOH solution at temperatures in the range from 100 to 130 degrees C. The analysis of the isothermal runs permits calculation of the kinetic parameters and interpretation of the mechanism of hydroxysodalite formation in the light of the structural state of the precursor material. At 100 degrees C, prior to the crystallization of hydroxysodalite, zeolite A (LTA) is formed as an intermediate metastable phase. The mechanisms of nucleation and competing growth of the two zeolites are interpreted on the basis of the obtained kinetic parameters, and of the structural relationship between the two phases.