INFLUENCE OF METAKAOLINIZATION TEMPERATURE ON THE FORMATION OF ZEOLITE 4A FROM KAOLIN

Kaolin has been used as an alternative cheap raw material for the synt hesis of zeolite 4A. Two steps are involved in the reaction: (1) dehyd roxylation of kaolin at 550-900 degrees C to form an activated X-ray a morphous material called metakaolin; (2) hydrothermal treatment of met akaolin with aqueous alkali to form the zeolite. The inherent colourin g impurities in kaolin, especially the Fe minerals, affect the brightn ess of metakaolin and the resultant zeolite. The dehydroxylation tempe rature is found to have a significant influence on the kinetics of zeo lite formation as well as on the brightness of zeolite. The present in vestigation deals with the preparation of metakaolins from a good qual ity kaolin at different temperatures and their characterization by XRD , IR, TGA, MAS NMR and brightness measurements. Hydrothermal reactions of these metakaolins with aqueous alkali have been conducted. The res idual Fe in the mother liquor has been estimated. The improvement in b rightness and change in reactivity of the metakaolin, difference in th e kinetics of its conversion to zeolites and dissolution of Fe during zeolite formation have been correlated with the calcination temperatur e. A calcination temperature of 900 degrees C (1 hr) is the optimum fo r this clay to change into a reactive metakaolin which gives detergent grade zeolite 4A of high crystallinity and maximum brightness.