Fly ash zeolite

In this context, this book has been written to present a thorough review of state-of-the-art and various innovative efforts taken in the recent past for synthesis of pure and improved grade of fly ash zeolites over those reported by previous researchers employing conventional methods. Also, attempts have been made to showcase a novel technique for synthesis of high cation exchanger (the fly ash zeolites) from fly ash and detailed characterization techniques for the products. In addition, based on previous researcher s findings, various areas of specific applications of the fly ash zeolites have been explored and compiled in a lucid way. 

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. 

Si/Al molar ratio in the activated fly ash, zeolites can be classified/graded as low silica zeolites , intermediate silica zeolites and high silica zeolites , as listed in Table 2.2. In general, for zeolites, an increase in this parameter (i.e., Si/Al from 0.5 to infinity) [5] can significantly result in the increase in various other parameters (viz., acid resistivity, thermal stability and hydrophobicity) except few parameters (viz., hydrophilicity, acid site density and cation concentration) which get decreased [5, 8, 10,40, 41]. In general, synthetic zeolites hold some key advantages over their counterparts i.e. natural zeolites. Zeolites type A, X, Y, P and Na-Pl are well known synthetic zeolites synthesized from fly ash which have a wider range of industrial applications than the natural zeolites [1, 8, 20, 22, 36, 42, 43]. 

The most common physical properly of the ash zeolites is its specific surface area, which is dependent on the extent of dissolution of fly ash particles in alkaline solvents [19, 22, 31]. In line with this, another important physical property of zeolites is their void volume which can directly be correlated with the CEC of the synthesized product (e.g., fly ash zeolites, Na-Pl) and which in turn depends upon the specific area as depicted in Fig. 2.3a, b [1, 22]. Moreover, both CEC and the surface area of the ash zeolites are found to undergo significant variations with increase in molarity and the reaction time, as depicted in Fig. 2.3c, d [22, 29]. From the trends depicted in Fig. 2.3c, it can be observed that the CEC increases, marginally, with an increase in concentration, however, the same is noticed to be fluctuating, randomly, with an increase in reaction time. This can be attributed to the variations in the pore size and volume, as depicted in Fig. 2.3a. On the contrary, the surface area maintains an increasing trend with increase in concentration and the reaction time, as depicted in Fig. 2.3d, which can be attributed to increase in dissolution of fly ash ingredients (viz., glass. Quartz and Mullite). 

Zeolites consist of aluminium oxide, calcium oxide, iron oxide, magnesium oxide, potassium oxide, silicon oxide and sodium oxide within their strucmre with water molecules and/or cations in the pores and the cages [10, 20, 27, 46-48]. A certain fraction of the mass of the zeolites is lost on ignition because of loss of water. Researchers have suggested that, for a material to get zeolited, the ratio of (Si + Al)/0 in it should be equal to 0.5 [16, 46-48]. The cation exchange capacity (CEQ, adsorption properties, pH, and loss on acid immersion of zeolites are some of the chemical properties which are reported to depend on the chemical composition of the synthesized products. Table 2.4 presents typical chemical composition of a fly ash, its crystalline constituents (viz.. Quartz and Mullite), one commercial grade synthetic zeolite, a fly ash zeolite and their comparison with a natural zeolite [47, 48]. 

It can be noticed from the data presented in Table 2.4 that the chemical composition of the fly ash zeolites (i.e., synthesized by Ojha et al. [48] and Park et al. [47]) is very close to the commercial grade synthetic zeolite 13X with Si/Al ratio equal to 1.5 [12,47,48], whereas, natural zeolite is comparatively rich in silica with Si/Al ratio equal to 4 [12, 21]. Hence, it can be opined that a wide range of chemical... 

Table 2.11 Properties of some commonly available fly ash zeolites [8]... 

Abstract Though, naturally occurring and chemically synthesized (pure grade) zeolites have been used for various industrial applications in the past, their increasing demand for several novel applications (viz., as adsorbent or absorbent for waste water decontamination, soil remediation as fertilizers, aqua-culture purification, etc.) warrants their enhanced production. With this in view, several researchers have attempted to synthesize zeolites from the fly ash, an abundantly available industrial by-product, as described in this chapter. Furthermore, different methods employed for synthesis of fly ash zeolites, the mechanism of zeolites formation and potential fields of their appUcations have also been included herein. 

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]. 

Sulaiman [38] has synthesized fly ash zeolites (FAZ) from coal fly ash. It has been reported that the hydrothermal reaction of magnetically pre-treated fly ash can produce stable zeolites P by initiating its activation in aqueous NaOH solution at 70 °C for 48 h. The crystallinity of the zeolites has been found to increase with the increase in temperature and reaction time. It has been concluded that the mineral transformation takes place due to dissolution of aluminosilicate glass and formation of zeolites P. 

Rayalu et al. [12] have estimated the crystallinity of fly ash zeolites-A using XRD and infrared (IR) spectroscopy for identification, quantification and their framework structure. Zeolite A has been synthesized by fusion of mixtures of fly ash and sodium hydroxide in 1 1.2 ratios at temperatures from 550 to 600 °C for 1-1.5 h of heating time. It has been reported that powder XRD analysis was employed to monitor the formation of zeolite-A. The infrared, IR, technique has been proposed for monitoring crystaUinityof end product after synthesis. Based on the characterization results, it has been opined that the unreacted fly ash associated with the zeohte-A as impurities in the final yield, have negligible influence on its application as an adsorbent or cation exchanger. As such, the crystallinity of end product as per interpretation of XRD peaks of commercial zeolite-A have been reported to match closely with the crystallinity interpreted from IR spectrum of the mineral. 

The state-of-the-art for synthesis of fly ash zeolites from fly ash employs various chemicals like NaOH, KOH, NaaCOa, KNO3, NaNOa, NH4F and NH4NOa as weU as different techniques like hydrothermal, fusion, combination of both fusion and hydrothermal, molten salt method, microwave irradiation method. However, these methods yield final end products blended with zeolites, which comprise of some... 

Czurda, K.A., Haus, R. Reactive barriers with fly ash zeolites for in situ ground water remediation. App. Clay. Sci. 21, 13-20 (2002)... 

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. 

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. 

Based on the variation in the reaction conditions, the quantification of Si and A1 concentrations in the supernatant of each step of the TSA by hydrothermal techniques has been given significance. Also, the final product obtained from each of the experiments, may contain varying proportions of fly ash zeolites. As such, it is of utmost significance to completely characterize the final product for various parameters [19, 22-25]. 

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