Three-step hydrothermal process

In order to synthesize zeolites from fly ash by its activation with NaOH, attempts have been made to identify a suitable fly ash out of its two disposal sites (viz., dry site at the electrostatic precipitator and wet site at the lagoons in the thermal power plants) for conventional (i.e., one step) hydrothermal activation technique [1-10]. Subsequently, the fly ash ascertained to exhibit improved zeolitization potential has been prefered to undergo novel hydrothermal treatment processes (viz., three step activation by hydrothermal technique and three step fusions) to activate the fly ash significantly for synthesis of fly ash zeolites with high cation exchange cqjadfy [11-15]. The details of both the types of alkali activations (viz., conventional with the two ashes and three step activations with the superior ash) are presented in the following. 

In view of the above, this technique focuses to recycle/resuse initial products (i.e., the residue, AAF and the supernatant, ST) up to three steps of treatment, which has been termed as three step activation (TSA) by hydrothermal technique [12-15]. Moreover, uniform conditions of various parameters (viz., L/S, duration of treatment, temperature and pressure) were maintained during each step of the treatment. As such, with an intention to investigate the relative variation in the characteristics (viz., chemical, mineralogical and morphological) of the products of several recyclings, TSA techniques for advanced characterization of the end products have also been devised. This is a major modification in the process of zeolitization of fly ash, by the hydrothermal technique in this study, over conventional techniques demonstrated by the earlier researchers [2, 16]. 

Based on the hndings presented in this chapter, it can be concluded that the three-step activation of the hopper ash with NaOH results in minor variation in pH and reduction in electrical conductivity of the supernatant. Such activation is also responsible for reduction in Si and A1 contents of the supernatant, obtained after recycled treatments. Hence, the ftnal grade of the fly ash zeohtes gets improved (with high CEC and specific surface area, enhanced specific gravity, nano-sized fine particles of zeohtes and micro-sized new pores). The three-step activation of the fly ash by adopting the fusion technique has also been found to be effective for zeolitization of the fly ash. However, the presence of impurities in the ash residues obtained from the fusion process makes them inferior as compared to those obtained from the hydrothermal treatment. 

The objective of this chapter is to review the open literature on molecular sieve zeohte synthesis, highlighting information regarding the fundamental mechanisms of zeolite crystallization in hydrothermal systems. The text, therefore, focuses on the three primary mechanistic steps in the crystallization process nucleation of new populations of zeolite crystals, growth of existing populations of crystals, and the role played by existing zeolite crystal mass in the subsequent nucleation of new crystals or the growth of zeolite crystals in the system. 

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