Preparation And Characterization Of Binderless 4A Zeolite

LTA zeolite can be widely used as adsorbent, catalyst, ion-exchanger and so on, in petrochemical industry, environmental protection, high-tech and agriculture. However, powder zeolite is inconvenient to use because of difficult operation and recovery, and easy deactivation and reunion, so it is shaped by adding a binder. However, the binder can influence performances of shaped body by reducing the effective content and blocking channels of zeolite. Sometimes, it also may cause some secondary reactions. Therefore, binderless zeolites have attracted intensive research.Aimed at the key issues such as the thermal phase transition law of kaolin, the function and additive of micropore generating material, activation of the green bodies, aging conditions of shaped bodies, crystallization of zeolite precursor, the transformation mechanism of zeolite and so on, the technical approach of preparing binderless 4A zeolite from kaolin directly was discussed intensively.Kaolin as the raw material was mixed with micropore generating material (4wt%-12wt% of kaolin) and was shaped. Green bodies were obtained. After drying, the green bodies were activated at the temperature of 650～730℃for 1.5-3h. Then the molding bodies were gained. Zeolite was prepared in the in solution of NaOH with the alkalinity of 3～6mol/L by thermally treating molding bodies. Aging and crystallization conditions were 60～75℃for 4～7h and 80-95℃for 3～5h respectively. After washing and drying, the product was obtained.The content of SiO2 and Al2O3 was enriched to 99.1wt% from 78.28wt% by roasting kaolin, which facilitates much more the formation of LTA zeolite. The relationships between micropore generating material and precursor pore, the rate of zeolite formation were investigated by means of SEM, mercury porosimetry and measure of static water adsorption. The optimal micropore generating material is carbon powder, and the additive amount is 10wt%. Experimental conditions for preparing binderless zeolite from kaolin were obtained. The optimal alkalinity of NaOH solution is 5mol/L. The aging condition is 70℃for 6h., and crystallization condition is 95℃for 4h. The static water adsorption of product synthesized under the optimal conditions is 21.11%. Meanwhile, the rate of zeolite formation is 76.8%.The influences of aging and crystallization conditions on the adsorption of product and the rate of zeolite formation were studied. It’s shown that the effect of aging conditions on the zeolite transformation is greater than crystallization conditions. The elements influencing zeolite transformation are ordered from larger to little as aging temperature, aging time, crystallization time, crystallization temperature. Thus, the aging conditions should be stringently controlled.On the basis of experiments and characterization, the mechanism was discussed. It’s realized that there are three stages in the process of preparing binderless zeolite from kaolin, namely, calcinations of the green bodies, aging of the molding bodies, and crystallization of precursor. The major is solid hydro-gel transformation mechanism, and liquid transformation is the auxiliary mechanism.