Superior fly ash zeolites

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. 

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. 

With this in view, the change in the mineral phase of fly ash, during its varying degree of thermal decomposition and hence alkali-fly ash interaction in the dry stage of fusion and wet stage of hydrothermal activation needs to be optimized to better understand the superiority of both the methods with a special focus to the mechanism of zeolitization of the fly ash, discussed in detail, in the next chapter. 

Further, alkali activation of the fly ash with Na2C03 can result in a coating of the thin film of zeolite P around the fly ash particles (refer to Fig. 4.4b), whereas, that with KOH can develop a surface deposit of egg shaped, ellipsoidal crystals of Chabazite (refer to Fig. 4.4c). Based on the variation in the developed morphologies, it can be confirmed that different types of alkalis can display their superiority with reference to... 

Chapter 6 Major Findings of the Three-Step Activation Technique This chapter deals with the inferences derived from the novel method three step activation of the hopper ash, which has been ascertained to be the superior ash over the lagoon ash, as described in Chapter-5, by following hydrothermal activation method. Furthermore, this chapter also showcases the outcome of the three-step activation of the fly ash by fusion method to synthesize high grade zeolite-X. 

---