Synthesis of fly ash zeolites

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. 

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... 

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. 

It can be realized that there is a huge scope of conducting research in the fields of synthesis, characterization and adequate utilization of fly ash zeolite. Attempts in the direction of zero effluent zeolite synthesis technique, application of the zeolites from the TSA, quantification of pore network in such zeolites would be prudent for the future researchers. In addition, for large scale synthesis of fly ash zeolites, details of the plant level synthesis should also be worked out. 

Chapter 3 Conventional Methods for Synthesis of Fly Ash Zeolites This chapter showcases available methodologies for synthesis of the fly ash zeolites and some critical issues, associated with them, which need further attention of the researchers. 

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. 

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. 

Rios, C.A.R., Williams, C.D., Roberts, C.L. A comparative study of two methods for the synthesis of fly ash-based sodium and potassirrm type zeolites. Fuel 88, 1403-1416 (2009)... 

Chapter 5 Novel Technique for Synthesis and Characterization of Fly Ash Zeolites This chapter presents innovative approach adopted by the authors for synthesis and characterization of fly ash zeolites. Also, attempts have been made to explain methods of characterization of the fly ash and the end products obtained from it. This chapter also presents a discussion on the suitability of the fly ash for its effective zeolitization. 

Zeolite Synthesis The synthesis of Na-Y zeolites is carried out in five steps. Fly ash is initially screened through a 355 pm mesh and the particles retained on the screen are put... 

Fukui K., Nishimoto T, Takiguchi M., Yoshida H. Effects of NaOH concentration on zeolite synthesis from fly ash with a hydrothermal treatment method. J. Soc. Powder Technology, Japan. 

In context to the characterization of the S3uithesized zeolites, it has been reported that the quantity of zeoUtic minerals present in the end product has been found to vary widely ( 20-65 % by weight of fly ash), mainly depending on the various parameters (viz., type of fly ash, alkali concentration, temperature, time and liquid to solid ratio) involved in the synthesis process. A t3q>ical flow chart is depicted in Fig. 3.1 to highlight the different procedural steps required for this method. 

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]. 

Park et al. [15, 16] have established the molten-salt method for the synthesis of zeolitic materials without any addition of water. The zeohtization of fly ash has been demonstrated by employing various mixtures of NaOH, KOH, or NH4F as mineralizers and NaNOs, KNO3, or NH4NO3 as solvent instead of demineralized water in hydrothermal method. It has been reported that zeolites like Hydroxy-sodalite and Cancrinite can get crystallized from mixture of NaOH and... 

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. 

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