A Novel Hydrothermal Technique

The sample, in slurry form, was prepared by mixing a solution of NaOH and the RFA in a fixed proportion. To ensure significant zeolitization by hydrothermal technique, the slurry for all the treatments was maintained with a fixed liquid to solid ratio, L/S 10 in ml/g [1—4]. In fact, the L/S as stated above has been reported to yield maximum conversion of RFA to FAZ up to 80 % [4]. 

In this context, the role of NaOH has been established by several researchers [2, 5-7]. Accordingly, the stock solution of NaOH, based on its high corrosive property, has been assumed to cause significant etching of the fly ash ingredients at the hydrothermal temperature of 100 °C. The experiment was carried out in a fabricated set up, which comprises of a 10 L reactor fitted with an open reflux condenser, thermostatically controlled heater and a magnetic stirrer. A schematic sketch of the experimental set up is presented in Fig. 5.1. 

In fact, it has been assumed that the repeated treatment of the residual ash by the ST should result in its further dissolution, decrease in IMP and increase in both 

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]. 

In this context, to differentiate each step of treatment from others, Step-1 of the activation process (the primary treatment) has been designated as PT. Subsequently, first recycled treatment (i.e., Step-2 of the TSA) has been designated as Rl, whereas, the final step i.e. second recycled treatment (i.e., Step-3 of the TSA) has been designated as R2. A typical flowchart for the step-wise synthesis of FAZ is presented in Fig. 5.2. 

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