Thermal studies of low silica zeolites and their magnesium exchanged forms

Zeolites are crystalline aluminosilicate polymers with uniform pores, channels and cavities They can be synthesised using a variety of raw materials and the history of the zeolite influences some of its properties. Thermal treatment dehydroxylates the zeolites, breaks down the structure and converts them to X-ray amorphous material. Further heating results in the crystallisation of ceramic phases depending upon the chemical composition of the system. The ion exchange capacity of the zeolite helps to introduce various cations into the structure and thereby extending the possibility of the formation of different types of ceramics. A systematic study has been carried out on the sintering behaviour of low silica zeolites ie., NaA and NaX of different origin (synthesised from kaolin/ sodium aluminosilicate gel) and their magnesium exchanged forms. The final products were albite, nepheline and jadeite for clay based zeolites. Cristobalite also is likely to form from the kaolin-based zeolite NaA and carnegeite from the corresponding NaX. Sodalite was the main product from the gel route zeolites. The magnesium exchange was partial and the remaining sodium present in the system was found to influence the sintering properties. The magnesium zeolites were found to be more heat resistant compared to their sodium forms. Mullite and magnesium spinel were the major phases formed along with small amount of albite. An important observation is that cordierite is formed at 1150 °C only from the clay based magnesium zeolites. The fired samples contain different crystalline and glassy phases and the physical properties represent an overall behaviour of this heterogeneous system.