Surface morphology, zeolite particles

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. 

Zeolite-carbonate-disilicate system. This case is close to the previous one and gives fairly similar results. The presence of sodium silicate at Rm = 2 does not change the distribution of the calcium consumed by the carbonate and by the zeolite. The morphology of the particles in suspension is modified, however, and the size of the calcium carbonate crystals is much smaller than before. The crystals also have a different surface charge as a result of the adsorption of silicate on them as they form. 

As an alternative, the use of water in the synthesis process can be eliminated to avoid the production of liquid waste as reaction by-product. However, the complete conversion of fly ash to zeolites has not been possible due to insufficient contact of Na", in the molten stage, with the surface of fly ash particles. In addition, the yield of this molten salts reaction process has been low and this combined with the generation of irregular product morphology has led to the processes which have not been explored much. 

Abstract Fly ash is a matrix of several metal oxides, which have different molecular and stmctural properties and hence its interaction with NaOH is a complex (chemical) phenomenon. As such, synthesis of the fly ash zeolites, and their characteristics, is expected to depend on various attributes (viz., physical, chemical, mineralogical and morphological) of the fly ash. In order to realize the mechanism of the fly ash zeohtization, it would be quite prudent to picturize the fly ash particles and investigate its interaction with aUcah, and interrelate the alkali activated fly ash with zeohtes in terms of their mineralogical composition. Apart from this, the mechanism of formation of sodium aluminosihcates (the so called fly ash zeolites), after the interaction of the NaOH on the surface and the inner core of the fly ash particle, has been explained in the following. 

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... 

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