Investigation on the Synthesis of Zeolite NaX from Kerala Kaolin

The synthesis of zeolite NaX from a locally available kaolin has been studied. Two steps are involved in the reaction (1) thermal activation of the kaolin to get a dehydroxylated product called metakaolin (metakaolinisation) and (2) hydrothermal reaction (zeolitisation) of metakaolin in alkaline medium in presence of additional silica to crystallise the zeolite. The metakaolinisation temperature (400–1000°C) is found to have an important role in determining the type of zeolite formed during the hydrothermal reaction. Under specified conditions, the uncalcined kaolin is partially converted to hydroxy sodalite (HS), whereas the metakaolins prepared at 400, 500 and 600°C also change to HS but at a faster rate. Increasing the metakaolinisation temperature to 700°C, zeolite NaX is found to be the product (higher the temperature, faster the reaction). The quantity of additional silica which in turn determines the SiO₂/Al₂O₃ ratio of the reaction mixture and the time given for zeolitisation were also found to influence the formation of zeolite NaX. When metakaolin alone was used in the reaction (SiO₂/Al₂O₃ = 2), the product was X-ray amorphous. Higher ratios were obtained by the addition of sodium silicate. The ratios 3 and 5 gave phase pure NaX whereas 7 resulted in a mixture of NaA, NaP and HS zeolites. The optimum conditions to get phase pure zeolite NaX with good crystallinity from kaolin were found to be metakaolinisation of the clay at 900°C for 1 h, maintaining the mole ratios of SiO₂/Al₂O₃ = 3; Na₂O/SiO₂ = 1.1 and H₂O/Na₂O = 40 in the reaction mixture, an ageing of the reactants at room temperature for 24 h and heating at 87 ± 2°C for 15 h under autogenous pressure. Seeding was not essential for the zeolite crystallisation. X-ray diffraction, electron microscopy, IR and solid state NMR spectral analysis, water adsorption and calcium/magnesium exchange or binding capacity measurements and chemical analysis of the selected products substantiated these findings.