Hydrothremal Synthesis Of Nano-Particle Zeolite4A Using Mineral Waste

Zeolite4A is an important class of catalyst, adsorbent and ion exchange agent, widely used in the detergent industry, petrochemical industry, fine chemical industry, environmental protection and the development of new functional materials and many other areas. Compared with normal zeolite, nano-particle zeolite exhibit more superior performance, such as it has a larger specific surface area and higher intragranular diffusion rate, and it also has higher activity and selectivity in applications such as catalysis, adsorption and environmental protections. On basis of integrated analysis of a great deal of literatures and data about the selection of raw materials, preparation methods and research progress, this paper entirely and systematically studies on the synthesis of silica sol and zeolite4A using inexpensive mineral waste as raw materials. The main contents of this paper are as follows:Silica sol was prepared using a fluorine-containing silica, and the optimum process is first to synthesize an active silicic acid solution by ion exchange method using sodium which has a solids content of8%, after the pH value was adjusted to9-10, the solution was placed into a high-pressure hydrothermal autoclave and heated to120℃for4h. The production has a uniform size distribution, it has an average particle size of20-25nm, density of1.2g/cm3and viscosity of5.85x10-3Pa-s, the SiO2content is30.2%by weight, the Na2O content is less than3%. All the indicators meet the requirements of national standards of China. After an ion exchange process, the fluoride ion from the feedstock has been substantially removed, the silica sol can be used in the synthesis of zeolite4A.By silicon hydrolysis method, silica sol was prepared using elemental silicon power as raw material. The optimum process of this method is as follows:using a feedstock ratio of water4L,NaOH5g and silicon powder500g, the reaction temperature is90℃and the reaction time is8h, a high solid content silica sol will be obtained. The SiO2content of the product by weight is30.06%, the Na2O content is less than0.11%, it also has an average particle size of25.2nm, density of1.2g/cm3 and viscosity of5.3×10-3Pa-s. Compared with the ion exchange method, this method has an easier processes and smaller energy and water consumption, at the same time the product has a smaller Na2O content and viscosity.Nano-particle zeolite4A was synthesized by hydrothermal method using homemade silica sol as feedstock. The optimum ratio is n(SiO2)/n(Al2O3)=2, n(H2O)/n(Na2O)=100, n(Na2O)/n(SiO2)=1.5, the aluminosilicate gel was prepared at room temperature and aged for6h, then it was hydrothermal crystallized at95℃for4h, the nano-particle zeolite4A was synthesized. After characterization, the product has an average size of32nm, degree of crystallinity of97%, whiteness98and the calcium ion exchange capacity is335mg CaCO3/g. It was found that low-temperature gelation and long-time aging is beneficial for the decrease of particle size. And the selection of silica sol as silicon source has an effect of eliminating the influence of impurities on the zeolite synthesis, and also conducive to the refinement of the product.Zeolite4A was also synthesized using kaolin as raw material through hydrothermal method. Kaolin was calcined at600to800℃for2h, the reactant ratio is n(H2O):n(Na2O)=50, n(Na2O):n(SiO2)-2, under alkalization temperature of55℃for3h, the aluminosilicate gel was prepared. Then after a95℃hydrothermal crystallization for6h, zeolite4A was synthesized. The product has an average particle size of less than3um, degree of crystallinity of97%, whiteness95and the calcium ion exchange capacity is317mg CaCO3/g, all the performances meet the requirements of national standard of China. This method has the advantages of low-cost, simple process and good operating flexibility.