Bhagwanjee Jha

Establishing two-stage interaction between fly ash and NaOH by X-ray and infrared analyses

The interaction of the fly ash and NaOH, in an open reflux hydrothermal system at 100°C, has been explored by several researchers and formation of fly ash zeolites has been confirmed based on the X-ray diffraction analysis of the residues. However, this method does not reveal much about the characteristic transitions (viz. elemental, electro-negativity and cation exchange capacity) of the residues. In this situation, resorting to Fourier transform-infrared radiation (FT-IR) spectroscopy on the residues obtained from two-stage hydrothermal treatment process, described in this manuscript, appears to be a novel idea to establish transitions in chemical bonds (viz., -Si-OH-Al-, OH-Na, OH-Al-), crystallinity and cation exchange capacity of these residues. Based on extensive studies, it has been demonstrated that FT-IR spectroscopy is extremely useful for 1) detection of chemical bonds in the residues, 2) evaluation of zeolites in the residues and 3) also establishing the superiority of the two-stage interaction of the fly ash with NaOH for synthesizing better fly ash zeolites (viz., Na-P1 and Hydroxysodalte) as compared to those obtained from the conventional single-stage treatment of the fly ash.

Determination of Crystallinity of Alkali Activated Flyash by XRD and FTIR Studies

Alkali activation of a flyash causes dissolution of its mineral constituents and also, significant transition in the characteristics of the residue. As a result, the residue attains the crystallinity, which is different from the ash. In order to quantify the final crystallinity, peak intensity of a mineral present in the X-ray diffractograms (XRD) of these samples has been demonstrated to be a useful parameter. In this context this paper presents a methodology to determine crystallinity of the residue by employing XRD and the Fourier transform infrared (FTIR) analysis of these samples. Also, this study establishes that such activation causes significant changes in the structural framework, which confirms formation of flyash zeolites (viz., Na-P1, hydroxysodalite and cancrinite), as well.

Synthesis of Higher Grade Fly Ash Zeolite X From Fly Ash via Three-Step Fusion

Several researchers have established the fusion of fly ash–NaOH mixtures at 450°C to 900°C. The fly ash gets partially converted to zeolite X when the fused ash is further treated in a hydrothermal system. However, such a solid residue (the end product) exhibits a low cation-exchange capacity. A culprit of such a deficiency in zeolite is improper contact between the NaOH and the fly ash particles during their fusion. To enhance the degree of fusion, in the present study we attempted to demonstrate a three-step fusion technique that does not favor any further treatment in a hydrothermal system. Observed changes in the characteristics (cation exchange capacity, pore volume, pore area, mineralogy, and thermal stability) of the end product of three-step fusion corresponding to an NaOH/fly ash ratio equal to 1.0 proves the suitability of this fusion technique for the synthesis of a better grade of zeolite X (viz., Na-X and Na-Y).

Basics of Zeolites

Tectosilicates have been commonly established as zeolites, which are found in nature as well as synthesized artificially. Depending upon the type of source (read raw) materials used and the method(s) of synthesis adopted, properties of the zeolites would vary and hence their application as adsorbents could be different. Keeping this in view, an in-depth description of the zeolites, their types and properties are presented in the following.

Synthesis and Characterization of Ca and Na Zeolites (Non-Pozzolanic Materials) Obtained From Fly Ash–Ca(OH)2Interaction

The conventional method for alkali activation of fly ash utilizes Ca(OH)2 and NaOH for the formation of pozzolanic material and fly ash zeolites, respectively. Sodium-based fly ash zeolites (say, Na-zeolite) mostly employ NaOH (the high-grade mineralizer and alkali) for activation of fly ash before its application as an absorbent. However, the Na-zeolites as absorbents (in agro- and aqua-culture) result in sodicity (i.e., excess of Na, present as impurity in the zeolite), which in turn reduces their holding capacity of moisture, nutrient [i.e., nitrogen, phosphorous, and potassium (NPK fertilizers)], microorganisms (viz., microbial spores), or heavy metals and negatively affect the growth of plant and aquatic life. To resolve such problems, the present study is focused on synthesis of agro-grade blend (dominated by Ca-zeolite) of zeolites by using Ca(OH)2 as major alkali and two well-established mineralizers, NaOH and NaCl, used in the trace quantity. To monitor activation of the fly ash in two different conditions, the synthesis of zeolites could be carried out by employing (1) conventional (the open hydrothermal system), and (2) autoclaving (the closed hydrothermal system) methods. The main attributes that control the entire study include temperature and reaction times for both methods. In addition, the present study demonstrates (1) effectiveness of Ca(OH)2 in creation of blend of zeolites with considerable cation exchange capacity when an optimum chemical composition (comparable to a pure agro- and aqua-grade zeolite 4A), and (2) suitability of the conventional hydrothermal method over autoclave method in synthesizing the blend of Ca- and Na-zeolites possessing a cation exchange capacity up to 394 meq/100 g. The formation of needle/star/spherule/small cube-shaped crystals (i.e., Na-P1, the Na-zeolite) and prismatic/cuboidal shaped crystals (i.e., heulandite, the Ca-zeolite), confirms suitability of the end product as a good sorbent and manure.

Characterization of Na and Ca Zeolites Synthesized by Various Hydrothermal Treatments of Fly Ash

For the past several decades, researchers have studied the zeolitization of coal fly ash (class-F) by following different methods (viz., open and closed hydrothermal, and fusion followed by hydrothermal). In fact, these methods involve sequential processes like (i) dissolution of silica and alumina from the fly ash, (ii) nucleation of zeolite, and (iii) crystallization (growth of zeolite) in the reactant solution. Also, performance of these processes has been reported to vary with the type of alkali used as reactant and often, NaOH has been preferred for high cation exchange capacity, resulting in sodium zeolites. However, large scale applications of Na-based zeolites in soil and water are questionable due to the presence of high sodium, thereby increasing the sodicity and salinity of the soil/water. In addition, performance of the zeolites, as adsorbent, synthesized by different methods is expected to depend on various characteristics (viz., mineralogy, structural bonding, specific surface area, pore volume, and morphology), of the zeolites. In order to address the above issues, the present study is focused to investigate the various characteristics of the synthesized zeolites by (i) the above mentioned three methods, (ii) using Ca(OH)2 as reactant, and (iii) considering Na and Ca present in the fly ash. Thus, the aim of the study was to ascertain (i) a suitable method out of the three and (ii) characteristics of the blend of Na- and Ca-zeolites from the fly ash, which can be used as a controlled release fertilizer, as sorbent for water and soil decontamination.

Conventional Methods for Synthesis of Fly Ash Zeolites

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 applications have also been included herein.

Major Findings of the Three-Step Activation Technique

The present chapter deals with the inferences derived from the “three-step activation” of the hopper ash, which has been ascertained to be the superior ash over lagoon ash, as described in Chap. 5. This technique has been found to be superior over conventional hydrothermal technique for obtaining the residues with high cation exchange values (viz., the higher grade zeolites, Na-P1, Hydroxy-sodalite, Faujasite, Cancrinite, Na-A). This technique has been established to be highly effective for purification of fly ash zeolites as well. Attempts have been made in the following to discuss about the quantification of transitions occurring into the filtrate and the macro to micro pores of the residues obtained from the three-step activation. Furthermore, efforts have been made to monitor the three-step activation of the fly ash by fusion method to synthesize high grade zeolite X. The final inference of this study is that the fly ash zeolites synthesized by following the hydrothermal technique is superior, as compared to the products of fusion technique.

Mechanism of Zeolitization of Fly Ash

Fly ash is a matrix of several metal oxides, which have different molecular and structural 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 zeolitization, it would be quite prudent to picturize the fly ash particles and investigate its interaction with alkali, and interrelate the alkali activated fly ash with zeolites in terms of their mineralogical composition. Apart from this, the mechanism of formation of sodium aluminosilicates (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.

Novel Techniques for Synthesis and Characterization of Fly Ash Zeolites

As discussed in the previous chapters, conventional techniques of synthesis of fly ash zeolites have not been found successful in synthesizing zeolites of higher grades (i.e., zeolites possessing high cation exchange capacity), mainly due to incomplete zeolitization of the fly ash, the complexities associated with the liquid by-product and the impurities present in the activated residues. The degree of activation of the type fly ash may also depend on its zeolitization potential and hence identification of the most suitable fly ash which would yield products of improved grade becomes a prime focus. In this context, out of the two types of the fly ashes (viz., hopper ash and lagoon ash, available as dry powder at the electrostatic precipitator and as wet powder at lagoons, respectively), which have different characteristics (viz., physical, chemical, mineralogical and morphological), depending upon their disposal site conditions (dry and wet) were used in this study. Based on detailed experimentation, hopper ash has been ascertained to have faster reaction with NaOH and thus yields superior residue with higher cation exchange capacity than the lagoon ash. This could be observed from the X-ray fluorescence results, X-ray diffractograms and micrographs of the two ashes and their products, after hydrothermal treatment. Finally, it has been demonstrated that the hopper ash exhibits better zeolitization potential than the lagoon ash. Furthermore, to synthesize higher grade fly ash zeolites from the hopper ash, a technique which involves a very innovative synthesis process, has been developed and its details are presented in this chapter. Contrary to the conventional hydrothermal technique, this novel technique is based on ‘three-step activation’ of the hopper fly ash by employing hydrothermal as well as fusion activation and hence results in synthesis of zeolites of very high cation-exchange capacity.