Chemical Reaction Potential of the Fly Ash

Si extracted is 120 g/kg i.e., slightly lesser than the maximum value (126 g/kg). This indicates that the fly ash sample with Si/Al ratio equal to 2.8 can be graded as an optimum sample from the point of view of synthesis of ash zeolites by its alkali activation, where the amount of extracted Si plays an important role in nucleation and crystallization of zeolite crystals. Furthermore, the amount of silica extracted from the fly ash into the alkali solvent will depend upon the chemical composition of particles and their reactivity with the alkali. 

The variations in products of the above reactions can be controlled by opting desired variations in the molarity of alkali (e.g., NaOH), Si02/Al203 ratio of the fly ash, reaction time and temperature. It has been demonstrated that an increase in pH (i.e., 10) of the solution can increase the solubility of the amorphous silica in the alkaline solution. This can contribute to an increase in dissolution of more monomeric silica and aluminum complexes leading to the formation of negatively charged ion complexes as presented by the product side of Eqs. 4.1 and 4.2. 

In fact, replacement of silica by aluminum in the tetrahedron can cause differentiation in the redistribution of the electric charge between the Al-O and Si-0 bonds, which result in polarization of the chemical bonds and the enhancement of the chemically active centers (of positive and negative charge) in the crystal lattice of ash zeolites. This can be attributed to the development of terminal groups (viz., =Si-OH, =Si-0-Na, =Si-0-, (=Si-0)3 and A1-0-) in the alkaline solvent, which can lead to the synthesis of more complex products (i.e. zeolites). 

For example, to understand the mechanism of development of morphology of the synthesis product in the hydrothermal method, the t) pe of alkati source employed and its reactivity with the fly ash, can influence the input and output streams of each as depicted in Fig. 4.4. In fact, alkali activation of the fly ash particles, with NaOH, causes etching of the outer surface of its particles and increases its surface roughness because of the large scale dissolution of Si and Al in the alkali solution. This can also be attributed to the precipitation reaction products, their nucleation and subsequent crystallization, as fine crystals of zeolite P, mostly seen as surface deposits in the activated fly ash residue particles (refer Fig. 4.4a) [12]. The increase in the rate of dissolution can be directly correlated with the increase in alkali concentration and/or temperature, which can finally result in the rapid nucleation and crystallization of big spherical crystals of Sodahte, which has been shown as projecting out of the surface as seen in Fig. 4.4a. 

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 

---