Alkali Activation of the Fly Ash

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. 

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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... 

In order to investigate the effectiveness of different types of alkalis, for activation of the fly ash, researchers have, mosdy, adopted NaOH or KOH aqueous solutions by varying the temperature between 80 and 200 °C and activation period between 3 and 48 h. 13 different types of fly ash zeolites have been synthesized by employing this method [3, 5, 6, 19-28]. 

It can be noticed from Fig. 4.5 that the variation of concentration of soluble silica in the solution sharply increases up to 10,000 ppm with only 6 h of alkali activation and thereafter it decreases up to 6400 ppm, corresponding to 20 h of treatment. Further, an equilibrium concentration of silica in the solution is achieved corresponding to wide variations in activation time (from 20 to 96 h). Moreover, the trend of variation in the concentration of aluminium ion in the solution is found to increase initially up to 460 ppm, corresponding to 4 h of activation, which later decreases sharply up to 20 ppm, corresponding to 12 h of activation and finally gets stabilized and equilibrated. It is interesting to observe that there is no increase in the concentration of silica and aluminium ions in the solution despite continuous activation of the fly ash particles (for 20 to 96 h). This can be attributed to... 

Keeping in view the all above fact, this book is intended to provide present and future generation researchers a single compiled reading material on fly ash and its recycled product, the zeolites. The under graduate, post graduate and doctoral students would find this very lucid to understand the potential of the fly ash, an industrial by-product. Above all, the students and the researchers would get state-of-the art methodology and mechanism of alkali activation of fly ash, the quality of the zeolitic products, and techniques to further purify them by chemical activation method. 

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. 

Furthermore, the time of activation has been reduced significantly by employing microwave assisted heating in hydrothermal method. Although, the alkali dissolution of fly ash can be accelerated by fast heating and attack of active water at molecular level, however, it has been found that the end product is of low yield, which has not been explored further. In addition, no efforts have been made for understanding the effects of the physical variation of the fly ash particles on the quality of final products. With this in view, an optimization of the time of micro-wave heating of aqueous matrix of different L/S ratio, alkali concentration as well as the type of fly ash collected from various sources (viz., bottom ash, hopper ash, ash from electrostatic precipitator), needs to be carried out. 

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. 

Fig. 4.4 Pichnial re nesentation of the cross section of the fly ash particle undergoing alkali activation with a NaOH, b Na2C03 and c KOH and development of the surface morphology by hydrothermal method...

The Si/Al ratio of the solution can only be influenced by the synthesis conditions (viz., temperature, liquid to solid (L/S) ratio, molarity of the alkali, types of the alkali and fly ash) employed, which can significantly affect the chemical and mineralogical compositions of the synthesized zeolites [3, 5, 13, 22-24]. A summary of the detailed study of the available literature is presented in Table 4.2 to exhibit the effect of variation in L/S ratio, temperature, activation time and molarity of the alkali solution on the final yield (i.e., in terms of quantity of the product as weight % of fly ash) of zeolitization of the fly ash. 

It can be observed from Tables 5.20, 5.21, 5.22, 5.23 and 5.36 that the sum total of major oxides (AI2O3 + SiOa + Fe203) present in the raw fly ash, RFA (i.e., the original hopper ash) is about 95 % and hence it can be characterized as Class-F fly ash (ASTM C618-08) [3]. It can be noted from Fig. 6.3 and Tables 5.20, 5.21, 5.22 and 5.23 that activation of this fly ash with NaOH results in loss of SiOa (in majority) and AI2O3 (in traces) in the residues (the alkali activated fly ash, AAF), which is represented as the second data point, from the origin, on the duration axis, for all the durations. Incidentally, reduction in the value of oxides increases with an... 

This chapter logically presents the mechanism of chemical transformation of the fly ash into zeolites. Attempts have been made to highlight the effects of alkali activation on the overall characteristics of the fly ash. Also, a need to innovate novel methods of synthesis of the fly ash zeolites, and to improve their overall characteristics, has been the main focus of this chapter. In addition, the chapter presents details of die applications of various advanced characterization tools (viz., physico-chemical, mineralogical and morphological) for exploring the overall properties of the zeolites. 

The geopolymer samples were made from fly ash and FGD gypsum G-II thennally-treated at 150- 800 °C for 1-3 h, separately. And the fly ash was replaced by the thermally-treated G-II at ratios of 10 to 50 wt%. For all the samples, the water to the blend of fly ash and G-II were 0.4. The water includes the water in the mixed alkali activator and the distilled water added. The mix ratios of the geopolymer pastes are provided in Table 5. The pastes were poured into molds of 20 x 20 x 20 mm to make cubic samples. 

M. Criado, A. Polomo and A. Femandez-Jimenez, Alkali Activation of Fly AsheS, Part I Effect of Curing Conditions on the Carbonation of the Reaction Products, Fuel, 84,2048-2054, (2005). 

The fly ash zeolites (FAZ) are available in hydrated alumino-silicate mineral forms and are synthesized from fly ash by its alkali activation. Zeolites can be broadly classified as ion-exchangers, catalysts and molecular filters based on their characteristics and performance. As such, the principal advantage of using the zeolites in industries are their high cation exchange capacity CEC), adsorption and catalytic capacities for various environmental clean-up projects such as adsorbents for removal of heavy metal ions and other wastes. 

The conventional hydrothermal method (either open or closed reflux systems) for alkali activation of fly ash has been extensively utilized by previous researchers [1, 5]. A typical experimental set up employed for the closed reflux system is similar to an autoclave [18], where both pressure and temperature can be varied as per the desired experimental conditions. 

Murayama et al. [3] have explained the mechanism of zeolite synthesis from coal fly ash by its hydrothermal reaction with alkali. They have observed that alkaline medium type (viz., NaOH, Na2C03 and KOH) affects the mechanism of crystallization of zeolites. The authors have employed synthesis matrix bearing a specific solid/slurry ratio (i.e., 100 g/400 cm ), with the slurry being an aqueous mixmre of two different alkalis (viz., NaOH and Na2C03, NaOH and KOH and Na2C03 and KOH) to investigate the effect of the presence of different cations and/or anions on the alkali activation of fly ash. It has been reported that zeohtes, P and Chabazite, are the main crystals present in the synthesized product. The OH in the alkali solution remarkably contributes to the dissolution of Si" and Al " from coal fly ash, whereas Na" makes a contribution to the crystaUization of zeolite P, which has the tendency to capture K" in the cation exchange process. 

In order to augment the quantity and quality of final synthesis yield as obtained from conventional hydrothermal activation, another modified method has been introduced which utihzes two different steps, an initial high temperature fusion of fly ash-alkah mixture, prior to employing the final stage of hydrothermal activation of the fused product. The main variables have been fusion temperature and time, alkali type and its concentration and crystallization time in hydrothermal synthesis process, which can affect the quahty and yield of final product. As such, it has been confirmed that the final yield can be quantified to exhibit zeolitic conversion up to 62 % together with by production of alkaline waste solution which can become a threat to the environment after disposal. A flowchart of the synthesis process is depicted in Fig. 3.3 [1, 2, 9, 10, 12, 43, 44]. 

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. 

Criado, M., Femandez-Jimenez, A., Palomo, A. Alkali activation of fly ash Effect of the Si02-Na20 ratio. Part I FT-IR study. Micropor. Mesopor. Mater. 10, 180-191 (2007)... 

Thompson Argent 2002) predicts the formation of an alkali sulphate-based melt, with the majority of the other elements also forming sulphates, implying a complex melt and solid solution(s) based on sulphates. At the present, information on the activity of cations in mixed sulphate melts is lacking and identification of actual phases is difficult because of the problem of concentrating the sulphate fraction. It is therefore difficult to predict with certainty exactly how a fly ash sample will behave in water and hence the need for standardized leaching tests. 

In alkali-activated fly ash binders a fly ash is eombined with an alkali metal eompound, preferably sodium silieate, which will secure a high alka-linity (pH=13-14) of the liquid phase. The alkaline activator may be blended with the ash prior to mixing with water, or— preferably—dissolved in the mixing water before mixing with the ash. 

The binder may be suitable for applications in which high chemical corrosion, including acid attack, is likely. It has also been suggested that alkali-activated fly ash binders should be used for fixation of hazardous waste, including radioactive wastes (Brough ei a/., 1995 Roy efo/., 1995). 

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