Novel techniques for synthesis and characterization of fly ash zeolites

Abstract

As discussed in the previous chapters, conventional techniques of synthesis of fly ash zeolites have not been found successful in synthesizing zeolites of higher grades (i.e., zeolites possessing high cation exchange capacity), mainly due to incomplete zeolitization of the fly ash, the complexities associated with the liquid by-product and the impurities present in the activated residues. The degree of activation of the type fly ash may also depend on its zeolitization potential and hence identification of the most suitable fly ash which would yield products of improved grade becomes a prime focus. In this context, out of the two types of the fly ashes (viz., hopper ash and lagoon ash, available as dry powder at the electrostatic precipitator and as wet powder at lagoons, respectively), which have different characteristics (viz., physical, chemical, mineralogical and morphological), depending upon their disposal site conditions (dry and wet) were used in this study. Based on detailed experimentation, hopper ash has been ascertained to have faster reaction with NaOH and thus yields superior residue with higher cation exchange capacity than the lagoon ash. This could be observed from the X-ray fluorescence results, X-ray diffractograms and micrographs of the two ashes and their products, after hydrothermal treatment. Finally, it has been demonstrated that the hopper ash exhibits better zeolitization potential than the lagoon ash. Furthermore, to synthesize higher grade fly ash zeolites from the hopper ash, a technique which involves a very innovative synthesis process, has been developed and its details are presented in this chapter. Contrary to the conventional hydrothermal technique, this novel technique is based on ‘three-step activation’ of the hopper fly ash by employing hydrothermal as well as fusion activation and hence results in synthesis of zeolites of very high cation-exchange capacity.