Influence of metakaolinization temperature on the formation of zeolite 4A from kaolin

Kaolin has been used as an alternative cheap raw material for the synthesis of zeolite 4A. Two steps are involved in the reaction: (1) dehydroxylation of kaolin at 550-900 degrees C to form an activated X-ray amorphous material called metakaolin; (2) hydrothermal treatment of metakaolin with aqueous alkali to form the zeolite. The inherent colouring impurities in kaolin, especially the Fe minerals, affect the brightness of metakaolin and the resultant zeolite. The dehydroxylation temperature is found to have a significant influence on the kinetics of zeolite formation as well as on the brightness of zeolite. The present investigation deals with the preparation of metakaolins from a good quality 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 residual Fe in the mother liquor has been estimated. The improvement in brightness and change in reactivity of the metakaolin, difference in the kinetics of its conversion to zeolites and dissolution of Fe during zeolite formation have been correlated with the calcination temperature. A calcination temperature of 900 degrees C (1 hr) is the optimum for this clay to change into a reactive metakaolin which gives detergent grade zeolite 4A of high crystallinity and maximum brightness.