Conventional Methods for Synthesis of Fly Ash Zeolites

Chapter 3 Conventional Methods for Synthesis of Fly Ash Zeolites This chapter showcases available methodologies for synthesis of the fly ash zeolites and some critical issues, associated with them, which need further attention of the researchers. 

In this context, this book has been written to present a thorough review of state-of-the-art and various innovative efforts taken in the recent past for synthesis of pure and improved grade of fly ash zeolites over those reported by previous researchers employing conventional methods. Also, attempts have been made to showcase a novel technique for synthesis of high cation exchanger (the fly ash zeolites) from fly ash and detailed characterization techniques for the products. In addition, based on previous researcher s findings, various areas of specific applications of the fly ash zeolites have been explored and compiled in a lucid way. 

It should be noted here that the one- and two- step methods mostly employ a closed reflux system [15], which facilitate hydrothermal activation of the fly ash at elevated pressure but are expensive. In addition, the two-step method employs chemicals like NaOH and NaAlOa [15], which also add to the overall cost of synthesis of the fly ash zeolites. On the contrary, the TSA being conducted in an open reflux system (refer Fig. 5.1), the cost of synthesis of zeolites is reasonably low. Also, lower energy consumption ( 72 kWh) supports the superiority of the TSA, as compared to the two-step method. In terms of purity of the zeolites, the process adopted in the TSA (refer Fig. 5.2) results in enhanced cation-exchange capacity, CEC, of the residues of the fly ah (843 meq./lOO g) as compared to the conventional methods (388 meq./lOO g for 1-step activation and 250 meq./lOO g for 2-step activation) [15]. Apart from this, the multiple recycling of the filtrates before their final disposal, in the TSA, is helpful in reducing the pH and concentration of the heavy metal ions (viz.. Si and Al) present in them, which is not the case with the conventional methods. 

In order to reduce the total time of synthesis ( 30 min), it has been reported to employ two stage synthesis processes, an initial microwave heating for around 30 min. followed by the conventional synthesis method [8, 40, 41]. Although, the purity of zeolite has been of much concern, the final yield used to contain unreacted (read residual) fly ash, which can affect the characteristics of synthesized zeolite. In fact, the presence of considerable quantity of unreacted fly ash residue in the final yield can be attributed to the factors, which are being discussed in the next chapter The mechanism of zeolite formation . 

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