Utilization of Fly Ash Byproduct in Synthetic Zeolites

Utilization of fly ash byproduct in synthetic zeolites

WJCECT


Chintan Y. Pathak*1, Debananda Roy2 and Sumanta Das3

1*,2

Department of Environmental Science and Engineering. Faculty of Engineering, Marwadi Education Foundation, Rajkot, Gujarat. 3Department of Civil Engineering, Faculty of Engineering, Marwadi Education Foundation, Rajkot, Gujarat.

The amount of fly ash generated has been increasing at an alarming rate throughout the world. It is the finely divided mineral residue resulting from the combustion of coal in thermal power plants. It consists of inorganic, incombustible matter that has been fused during combustion into a glassy, amorphous structure. The typical thermal power plant will burn pulverized coal in a furnace with a boiler. The combustion products include fly ash, bottom ash and a boiler slag and flue gas desulfurization (FGD) materials, which creates serious danger if released into environment. This has attracted more attention within a context of progressively more stringent regulation of hazardous air pollutants (viz. fly ash) discharge from thermal power station. The disposal of such a huge quantity has become a pressing issue. Several approaches have been made for proper utilization of fly ash, either to reduce the cost of disposal or to minimize the environmental impact. This study presents the wide variety of utilization of this industrial byproduct - fly ash and its application in the field of an environment. One of the major ways out is the conversion of fly ash to zeolites, which has wide applications in the field of an environment.

Keywords: Environment, digestion, hydrothermal, microwave, fly Ash, synthesis

INTRODUCTION The increasing requirements in the sphere of environmental protection have induced search for more effective, inexpensive and ecologically safe solutions for valuable conversion of an industrial byproduct fly ash. Given that, the fly ash micro-porous solids known as molecular sieves and have absorptive and ion-exchange properties; the most important pollution of hydrocarbon (HC) (viz. petrochemical spills), may find applications in the removal of oil spill, using hydrophobic - oleophilic - high-silica content - hydro thermally stable alumino-silicates; while, simultaneously providing a solution to other environmental problems. The discussion will be applicable in wide variety of environmental pollution control applications to sensitize current trend of technocrats for some valuable conversion of these waste fly ash. As a challenging task, the authors have investigated the literature for current researches done on fly ash. Studies have been carried out in the recent past and are still being continued to understand and utilize the concept for synthesis of fly ash for better economical and technical applications. Attempt has

been made to present the researches carried out on fly ash utilization particularly for zeolite synthesis (Berkgaut and Singer (1996); Curtiss et al. (1998); Xavier et al. (1999); Abu-Sarma (1975); Kuss (1992); Criado, M.; Fernndez-Jimnez et al. (2007); Smith and Arsenault (1996); Jha and Singh (2011); Holler and Wirsching (1985)). Before a fly ash can be used for a certain application, it is necessary to characterize, to see if it has the desired properties for particular application. If not, another synthesis method should be used to meet the specifications. Fly ash fusion, alteration, classification and appliance thus are strongly related. Fly Ash Material Science has broadened and deepened by a number of interrelated and simultaneously occurring developments such as: *Corresponding author: Chintan Y. Pathak, Department of Environmental Science and Engineering. Faculty of Engineering, Marwadi Education Foundation, Rajkot,

Gujarat. E-mail: [email protected]

World Journal of Civil Engineering and Construction Technology Vol. 1(1), pp. 002-0011, March, 2014. www.premierpublishers.org ISSN: 1936-868X x

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diversification of the composition outside alumino-silicate, particularly the crystalline alumino-phosphates and Ga, Fe, B, Ti containing structures, growing number of framework structures, increasing insight in crystallization mechanisms, development of post-crystallization modification procedures, development of powerful characterization techniques. Hydrothermal Activation Method A number of technologies have been developed for gainful utilization of fly ash. Coal combustion by-products production in USA is estimated in around 115 million tons per year. A large portion of this production is accounted for the CFA. Cement and concrete manufacturing consumes most of the CFA produced. Zeolite synthesized from CFA is a minor but interesting product, with high environmental applications. Zeolite may be easily obtained from CFA by relatively cheap and fast conversion processes (Fernandez-Pereira, 2002). Synthesis of fly ash zeolite by alkali fusion followed by hydrothermal treatment was investigated by several researchers. The synthesis conditions were optimized to obtain highly crystalline zeolite. The cost of synthesized zeolite was compared with the commercial zeolite available in the market (Ojha et al. (2004)). By varying the composition of the reaction mixture at a given temperature and time; it was being achieved to control the type of zeolite and simultaneously being investigated its crystal structure, surface structure and cation exchange capacity (Vucinic et al. (2003)). It has been investigated that the synthesizing capability of zeolite from fly ash has a positive impact on utilization of wastes coming from solid fuel combustion by converting the fly ash into valuable raw material and, simultaneously, accomplishing the lithosphere protection effect (Fukui. et al. (2005); Terzano et al. (2005); Park et al. (2000); Adamczyk and Biaecka (2005)). In March 1954, the structure of zeolite A was being achieved by the (Reed et al. (1956); Breck et al. (1958)). In mid 1954, they also completed the structures of X and faujasite, with the help of single crystal X-ray data on faujasite. The iso-structural zeolite with ratios above 3 and up to 6 were named and patented as zeolite Y and evaluated as a catalyst (Beck, 1964). In February and March of 1954, Breck and Milton prepared hydrogen exchanged X with greater than 60 % of the sodium cations replaced, both by direct exchange with acid and by exchange with ammonium ions followed by heating to drive off ammonia (Milton and Breck,1956). A patent application was filed on March 13, 1956 on the catalytic cracking of hydrocarbons with hydrogen X in the names of Milton and Beck. By mid 1954, Breck and Milton had developed methods for dispersing metals on the inner surfaces of the A, X and Y zeolite and had initiated catalytic studies with them (Breck and Milton,1961); Milton (1965; 1966)). In 1953, Breck et al. (1956) had made a total of 20 crystalline zeolite including erionite,

gmelinite and 14 with no know natural counterparts. Jones (1950) co-inventor of the first major application of molecular sieves (Jones et al. (1957)). By mid 1950, Milton had discovered how to routinely make pure zeolite X (Milton (1959)). Chabazite was synthesized in late 1950 and by mid 1951, Milton made three new zeolites in the potassium-alumino-silicate system (Milton ,1961). In 1949, F.G. Straub and R.M. Barrer (Milton ,1961; Straub 1963) started synthesis using relatively insoluble forms of silica and alumina in mildly alkaline solutions (pH 8-11) at temperatures of 200-300

oC. In some reactions analcime or small pore

mordenite had formed. By end 1949, R.M. Milton (Barrer ,1948; Milton,1961) had developed a new and widely applicable method for synthesizing zeolite simultaneously with the discovery of zeolite A, zeolite B (gismondite), zeolite C (sodalite) and a crystalline impurity zeolite X. Cronstedt Christened formed a frothy mass and discovered the hydrated alumino-silicate minerals zeolite. Because of limited availability of material, research was sparese for next 200 years. Efforts to achieve hydrothermal synthesis of analogs of natural zeolite were date back to 1845, although the elevated temperatures pressures employed and the lack of proper identification techniques precluded a high degree of success. In 1985, Holler and Wirsching (Holler and Wirsching,1985) utilized fly ash as the raw material for synthesis of zeolite using hydrothermal method. After that, a number of researchers have used this process successfully, varying the hydrothermal temperature within the range of 333-573 K (Henmi,1987 a,b; Lin and His,1995; Park and Choi,1995; Park et al. (2000); Shin (1995); Chang and Shih,1998). Previous researchers have been utilizing the conventional hydrothermal method for alkali activation of fly ash (Chang and Shih,1998). A typical experimental set up for the closed reflux system is similar to an autoclave where both pressure and temperature can be varied as per the desired experimental conditions. In general, zeolite synthesis processes involve the addition of alkali agent to the fly ash slurry at higher temperatures. Miyake (Kumar et al.,2008) re-utilized the main components (SiO2 and Al2O3) and unburned carbon from the Coal Fly Ash (CFA); it was successfully converted into composite materials consisting of zeolite (NaX and/or NaA) and activated carbon using NaOH fusion treatment with a NaOH/CFA ratio of 2, followed by hydrothermal treatment. Preparation of synthetic zeolite from CFA using hydrothermal treatment with NaOH activation at low temperature was studied by Sulaiman (Sulaiman,2008). It has been found out that the crystallinity of zeolite P increases with increase in temperature and reaction time. The formation of synthetic zeolite P with a high Cation Exchange Capacity (CEC) is possible using this method. C. Belviso et al. (Belviso et al. (2007)), synthesized zeolite from coal fly ash by hydrothermal process with aid of salt and distilled water. It reveals that the zeolitization at low temperature, using industrial by-products like fly

Pathak et al. 002


ash, represents potential applications for reduction of heavy metals in contaminated areas in an economical way. In the beginning of 20

th centuary, it has been observed

that the zeolite may be easily obtained from CFA by relatively cheap and fast conversion processes (Querol et al. (2007)). The conventional alkaline conversion processes emphasized on the experimental conditions to obtain high CEC zeolite. A potential application of different zeolite products for waste water and flue gas treatment is also given with one of the main potential application of this material is the uptake of heavy metals and ammonium from polluted wastewaters. Some of the zeolite synthesized may be also used as molecular sieves to adsorb water molecules from gas streams or to trap SiO2 and NH3 from low water gaseous emissions. Fukui et al. (Fukui et al. (2003)), investigated the effects of NaOH concentration on the crystal structure and the reaction rate of the zeolite synthesized from fly ash with a hydrothermal treatment method. Fly ash and co-disposal filtrates provide a rich source of SiO2 and Al2O3 as feedstock for zeolite synthesis, which can be successfully converted into faujasite, sodalite, and zeolitic material. The type of zeolite formed was dependent on the fly ash source, because it was also contributing to the physical and chemical characteristics of the resulting zeolite. Moreover, the synthesis of zeolite P (zeolite belonging to the Gismondine series) and zeolite X (zeolite belonging to the Faujasite series) was observed for the first time directly in soil in the presence of organic matter (2.4 % C) and several mineral phases (Terzano, 2005); Park et al. (2000). Correlations between Si/Al molar ratio in solution, curing temperature and the type of synthesized zeolite were found. Si/Al ratio lower than 1 and higher curing temperatures favored the synthesis of zeolite P, while a Si/Al higher than 1 and lower curing temperatures drove the synthesis preferentially towards zeolite X. Zeolite from fly ash was synthesized and characterized by Ojha et al. (2004). Synthesis of X-type zeolite by alkali fusion followed by hydrothermal treatment was studied and characterized. Hydrothermal reaction was carried out in various alkali solutions to clarify the mechanism of zeolite synthesis from coal fly ash (Vucinic et al. (2003); Murayama et al. (2002)). All the studies by Iler and Engelhardt shows that as a general rule, the formation of more highly polymerized species is favored by decreasing pH at constant silica concentration and increasing silica concentration at constant M2O/SiO2 ratio. Mooler et al. studied the solubility of alumino-silicates in alkaline medium. This is related to the chemical behavior of aluminum. At pH larger than 11, this element exists solely as tetrahedral Al(OH)4

- ions. The host-guest relationship is described

by Barrer (Barrer,1960); Barrer et al. (1984)) is one of the most important in the chemistry of porous crystals because, without the zeolitic guest, microporous crystals could not be synthesized. The guest is required only during synthesis and is thereafter removed before the porous crystals are put to use. The study of alkaline

silicate and aluminosilicate solution is of fundamental importance for a better understanding of the mechanism of zeolite synthesis (Barrer et al. (1959); Zhdanov (1980)). Flenigen learned how to make zeolite x with silica/alumina ratio between 4-5.7. It is clearly established that the silicon is in tetrahedral coordination (Breck and Flnigen,1968; Breck and Flnigen,1968). The simplest form of silica in solution is the monosilicic acid Si(OH)4. Polymerization results from the condensation of two silanol groups with the elimination of a water molecule. A whole series of silicate anions, of various polymerization and ionization degrees are thus connected through dynamic equilibrium. The equilibrium is governed by the normal chemical parameters, namely the silica concentration, the pH and the cation type (Fortnum and Edwards,1956). In one of the sorption study, materials were measured and widespread for adsorption purification of water from petroleum and oil products by Sirotkina and Novoselova, 2005 (Sirotkina and Novoselova, 2005). Sorbent used both for elimination of oil outflow and for purification of oil-containing wastewater are considered. Natural (plant and mineral based), artificial and synthetic sorbent were studied. Querol et al. (Querol et al. (2000)) worked on the methodologies for zeolite synthesis from coal fly ash and a conventional alkaline conversion processes, with special emphasis on the experimental conditions to obtain high CEC zeolite. One of the main potential application of this material is the uptake of heavy metals from polluted waste waters and also for the uptake of ammonium from polluted waters. Some of the zeolite synthesized may be used as molecular sieves to adsorb water molecules from gas streams or to trap SO2 and NH3 from gaseous emissions. It has also been found out that the optimization of synthesis yields have to be specific for each fly ash due to differences in mineralogical and chemical compositions. Consequently, the products obtained have an important potential application in the wastewater treatment technology. Microwave Irradiation Method In recent years, increasing efforts have been directed towards the development of new crystallization methods which allow the synthesis of zeolite for practical utilization. Based on the findings of previous researchers, it can be opined that hydrothermal activation of fly ash by alkali is time consuming as it is a slow process (Breck et al. (1956); Jones and Milton,1957; Milton, 1959; 1961); Barrer,1948). Traditionally, chemical synthesis has been achieved through conductive heating with an external heat source. Heat is driven into the substance, passing first through the walls of the vessel in order to reach the solvent and reactants. This is a slow and inefficient method for transferring energy into the system because it depends upon the thermal conductivity of the various materials that must be penetrated. This process can take hours. Microwave heating, on the other hand, is a

World J. Civ. Engin. Constr. Technol. 003


very different process. The microwaves couple with the molecules those are present in the reaction mixture, leading to a rapid rise in the temperature. Because the process is not dependent upon the thermal conductivity of the vessel materials, the result is an instantaneous localized superheating of anything that will react to dipole rotation or ionic conduction, the two fundamental mechanisms for transferring energy from microwave to the substance being heated. Microwave heating also offers facile reaction control. It can be described as instant on-instant off (Das et al. (2001)). Microwave belongs to the portion of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz. Within this portion of the electromagnetic spectrum there are frequencies that are used for cellular phones, radar, and television satellite communications. For microwave heating, two frequencies, reserved by the Federal Communications Commission (FCC) for industrial, scientic and medical purposes are commonly used for microwave heating. The two most commonly used frequencies are 0.915 and 2.45 GHz (Fernandez-Pereira et al. (2002)). A microwave is a form of electromagnetic energy that falls lower frequency end of the electromagnetic spectrum and is defined in the 300 to about 300,000 MHz frequency range. Within this region of electromagnetic energy, only molecular rotation is affected, not molecular structure. Out of four available frequencies for industrial, scientific or medical applications, 2450 MHz is preferred because it has the right penetration depth to interact with laboratory scale samples and there are power sources available to generate microwaves at this frequency. Microwave energy consists of an electric field and a magnetic field, though only the electric field transfers energy to heat a substance. Magnetic interactions do not normally occur in chemical synthesis (Adamczyk and Biaecka, 2005). Microwave move at the speed of light (300,000 km/sec). The energy in microwave photons (0.037 kcal/mol) is very low relative to the typical energy required to cleave molecular bonds (80-120 kcal/mol); thus, microwave will not affect the structure of an organic molecules. In addition, repeated treatment/activation process is required to synthesis fly ash zeolite. In the last decade, the use of closed vessel microwave assisted digestion method under high temperature and pressure has become increasingly popular. Compared to the more traditional method, this procedure allows shorter digestion times and good recoveries. Furthermore, it requires smaller amount of acids, reduces risk to external contamination, which in turn, results in improved detection limits and the overall accuracy. Microwave digestion inherited some disadvantages from open air digestion; most of these problems, however, are associated with usage of HF. So far, researchers attempted to optimize the method in many different ways; some research has been done to determine the need for HF. It has been found that for digestion of coal addition of HF is not necessary and its

absence will not result in poorer recovery of trace elements However, for fly ash, addition of HF proved to be necessary. Microwave-assisted digestion method provides several advantages over traditional hot plate method and most of them come from the fact that this method is performed under higher temperatures and pressures as well as under close vessel conditions. Ashing step prior to digestion is not necessary when microwave was used. Microwave system constantly monitors the temperature of reference vessel throughout the digestion procedure and automatically increases it when necessary. For example, if at some point during the digestion of sample an exothermic reaction occurs, spike in temperature will be observed. In that case instrument will cut the energy supply in order to lower the temperature to the set value. This ensures more constant and repeatable conditions compared to conventional digestion. This condition provides the advantage of performing more complete digestion under higher temperature and pressure without loss of more volatile elements. Lastly, the time needed for digestion is significantly reduced when microwave digestion system is used (Milton,1961; Barrer,1948). The two main types of conditions used for chemical reactions, those run in the presence of solvent and those run in a solvent less environment are equally important and both can benefit from microwave heating. The Microwave synthesis technique offers the potential for convenient and often rapid sample preparation, usually affording products of high crystallinity. There is also the possibility of genuine selectivity. In the microwave synthesis of zeolite Y, crystallization of undesired phases is suppressed, even at an unusually high synthesis temperature (150C). Such selectivity is largely attributable to the high heating rates attainable in microwave syntheses and their effect upon the rival rates of nucleation and growth of competing phases (in this case, zeolites Y and P). Similarly, in colloidal silicalite synthesis, the different rates of temperature rise achievable by microwave and thermal heating made it possible to distinguish between the crystal population nucleated during the heating process and that arising from proto-nuclei generated during the room temperature aging of the precursor sols. A very rapid (3 min) microwave crystallization of ZSM-5 probably reflected a contribution from the interfacial superheating of nanocrystal seeds. However, under near-equilibrium conditions, there was no significant difference between the crystal linear growth rates in thermally and microwave heated reactions, the growing crystals acting in this case as a form of internal thermometer (Herod et al. (1999)). Savic studied the recent development of microwave systems designed for digestions under extreme acidic conditions and high temperatures and pressures, which has shifted focus toward more rapid and precise microwave assisted digestion of samples. A number of papers have been published involving research efforts toward development of quantitatively satisfying digestion methods. However, there is still no published

Pathak et al. 004


ASTM or EPA method for microwave-assisted digestion of coal and coal combustion products. The goal of this research is compare the recoveries obtained by using hot plate method with results obtained by microwave-assisted digestion, as well as to attempt to optimize method conditions for latter (Nadkarni,1974). Hu et al. have studied the microwave digestion method of fly ash using NaOH. For the silicate sample, the digestion time was 210 s in the proposed method, which was much less than that in the traditional alkali melting method and the acid soluble method. The method had signicant advantages of being simple, time-saving and energy-saving, etc. It could be applied in the pretreatment and rapid analysis of silicate samples. Compared to the microwave acid soluble method, the adding of HF was avoided (Jha and Singh, 2011). Therefore, the process to add excessive H3BO3 for eliminating HF could also be omitted. Additionally, the obtained solution from microwave digestion could be used to determine contents of SiO2, Al2O3, CaO, MgO and TiO2, etc. in the silicate sample, which solved the long-term problem the traditional microwave digestion technique whereby it was difficult to determine the contents of components directly in the silicate sample by using chemical analysis and photometric analysis. More research will be done in the near future. The ordinary microwave oven had advantages of being cheap, time-saving, etc., but safety was a potential problem. The major advantage of the closed microwave digestion technique is the high heating efficiency which can be obtained. Heating causes an increase in pressure, due to the evaporation of digestion acids and the gases evolved during the decomposition of the sample matrix. This is beneficial by increasing the boiling-point of the reagents, which aids the breakdown of the sample matrix. However, the excessive build-up of pressure, especially during the digestion of samples with a high organic content, can lead to the rupture of sealed vessels. For this reason, most digestion bombs are fitted with pressure relief valves, designed to open when the pressure becomes too great, and thus maintain safety. If venting does occur, sample losses are likely and owing to the reduction in acid vapours a less active digestion may result. Considerable research has therefore been undertaken to find ways of controlling or reducing pressure build-up during the digestion process (Walker et al. (2003)). One method of avoiding excess pressure is to pre-digest the sample, and thus enable the gases evolved from the decomposition of easily oxidized organic matter to escape before commencing the closed digestion procedure. This may be carried out by leaving the samples to predigest at room temperature. In many cases, to attain complete digestion the use of HF is necessary to decompose resistant minerals, in addition to strong oxidizing reagents such as nitric acid and sometimes perchloric acid to break down organic matter. In the literature, there is evidence to suggest that a number of elements such as Cd, Cu, Hg, Pb and Zn can be easily released after digestion with just nitric

or hydrochloric acid. This is because in many cases they are sorbed on clay minerals or are in other readily decomposed phases, rather than within the resistant framework-lattice silicates. However, other elements are more strongly bound, either as part of resistant minerals or associated with other minerals, and so in such cases the use of HF may be required (Rayalu et al. (2006)). The need for HF is very much dependent on the nature of the minerals present in the samples. Because real samples will vary in composition from that of the certified reference materials used for validation of a procedure, it would seem prudent to suggest that for the determination of all but the most weakly bound elements in sediments, digestion with HF is recommended. The choice of sample preparation method may, however, be influenced by a number of practical considerations in addition to the type of sample and elements to be determined. These may include the number of samples to be analyzed, method of analysis, safety aspects, capital and operating costs of equipment, operator skill and the degree of accuracy and precision required. The method of final analysis is an important factor, influencing the extent of the digestion required, e.g., for electro analytical techniques complete breakdown of the organic components is necessary, whereas ICP-AES can tolerate dissolved solid contents of up to 12%. Also, the addition of certain reagents during the reaction can be considered (ASTM). ICP-AES suffers from a number of interferences, particularly polyatomic ion interferences. Hence, the presence of a number of acids, including hydrochloric and sulfuric acid, in the final solution is not recommended for the determination of some elements. Therefore, adapting a digestion method for analysis by a different technique to that originally intended may not prove successful. When considering the speed of a particular procedure, it is not just the time for the actual digestion that should be considered. Other factors should also be taken into account, e.g., sample preparation before analysis, including grinding and slurry formation; pre-digestion and cooling times, including those necessary between reagent additions/heating cycles; and the washing of digestion vessels. These factors are often over looked. For batch digestions, many closed digestion procedures are developed in terms of heating at a particular power setting for a certain period of time, usually optimized to maximize energy input without causing venting of the vessels. These procedures are therefore operational, i.e., specific to the particular microwave system and bomb design used. Adaptation for use in a different laboratory may not be straightforward unless the same equipment is used, as re-optimization of the original power settings and heating times may be necessary. The optimum power and time settings may also vary considerably according to the exact nature of the sample owing to the amount of organic matter present, which influences the amount of gaseous products evolved during the reaction. It has been shown that direct temperature and pressure



measurements during the course of the digestion are possible. Such measurements can then be fed to a computer controlling the magnetron to achieve a pre-set temperature or pressure program. This technology offers the potential to produce far more reproducible and controllable procedures, reducing the possibility of venting of digestion vessels. In closed systems it enables the system to be operated to its full digestion potential. A mechanism of crystal growth is proposed; extensive heterogeneous nucleation occurs during formation of the highly supersaturated gels. Crystal growth in the solid phase then proceeds by a series of depolymerization-polymerization reactions, catalyzed by excess hydroxyl ion. There is no significant solution of the solid phase during crystallization. Growth of the crystal proceeds through a type of polymerization and de-polymerization process which involves both the solid and liquid phases. The solid phase, however, appears to play the predominant role. The hydroxyl ion behaves as a type of catalyst by breaking and remaking Si, Al-O-Si, Al bonds but does not lead to a significant dissolution of the solid phase of the gel. The formation of crystalline zeolite phases from reactive alumino-silicate gels apparently represents an unusual system for crystal growth. It differs considerably in complexity and mechanism from the more classical crystallization methods. HNO3, HCl, HF and mixtures have been tested as digestion acids. The combination of HNO3 and HF has been found to be efficient for the digestion of ash samples. In coal-fired power plants, a major part of the particles, i.e. fly ash, is precipitated by means of electrostatic precipitators or fabric filters. A small fraction is released into the atmosphere as airborne particles along with the vaporous fraction. Environmentally harmful trace metals are to a great extend associated with the particulate phase in the flue gases leaving coal combustion. During the last few years, microwave digestion has become more and more popular partly due to the reduction in digestion times. The digestion of fly ash was first carried out without any filter material, to find out which acid or acid combination is required to quantitatively dissolve the metals, and to obtain preliminary information on how trace elements are bound in the ash particles. In a second stage, the effect of commonly-used quartz fiber filter material on the determination of trace metals in fly ash materials was studied. The comparison with certified values for reference materials showed that a number of factors must be considered when choosing which one is the more suitable digestion technique: (1) the acid attack must include hydrofluoric acid to dissolve the silicate-enriched ash matrix completely since, except for Se and As, none of the other elements studied here is completely extracted from the matrix by HNO3 alone; (2) the loss of volatile elements has to be expected during the open vessel acid digestion and an alternative preparation, involving closed vessels such as in the microwave attack, is often preferable for these elements (As and Se); (3) the final solutions must be

suitable for the ICP-AES instrument (total dissolved solid kept low and possible interferences mainly due to the use of HCl or HClO4 avoided); the total solid to final solution dilution factor has to be kept minimal to allow the determination of some elements present in coal at very low level. For the small sample size of 10 mg, a dilution factor of 1000 has proved necessary to quantify several elements (Ni, Cu, Mo, Sb and to some extent Zn). There are numerous ongoing research efforts focused on optimization and improvement of sample preparation. Most of them are concerned with completeness of digestion procedure, length and time consumption, pretreatment conditions, and use of hydrofluoric acid. Das S. (Das and Chakraborty,2001), studied the microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, attempt has been made to study the heating behavior of materials in the electric and magnetic fields at microwave frequencies. The composition of fly ash was investigated by chemical sequential extraction and modified microwave digestion method. Effects of process time, container materials, aging time and salt contents were also discussed. The major elements of fly ash are Ca, Cl, Na, Si, K, Al, Mg, and Zn, and the metal species, Zn, Cr, Pb, Ca, and Cu, are mainly in the oxide phase. Under microwave processing, the fly ash was sintered into a glassceramics and the leaching concentrations of heavy metals were restrained. The stabilization efficiency increased with an increase in processing time in most of the cases. Better stabilization efficiency of fly ash was discovered by using the SiO2 or Al2O3 container than by using the Figureite plate/SiC plate. The presence of salt in the fly ash could enhance the sintering and stabilization of fly ash. During the aging time of 030 days, negligible Pb in the sintered fly ash was leached out, and the leaching concentration was lower than the criterion. Fukui et al. (Fukui. et al. (2009).) utilized NaCl for phillipsite synthesis from fly ash by hydrothermal treatment with microwave heating. The coal fly ash was treated hydrothermally with the mixture of NaOH aqueous solution and NaCl aqueous solution at 373 K, using the microwave heating and the conventional heating in order to clarify influences of the NaCl concentration in the hydrothermal solution on the growth rate and the crystalline phase of synthesized zeolite attains to the vessel by way of the wave guide. Hassan, et al. (Hassan et al. (2007)) used several different standard reference materials, namely urban particulate matter, to examine and modify US EPA method 3051 for microwave assisted digestion of sediments, sludges, soils, and oils which specifies a 10 min total digestion time and a sample mass of up to 500 mg combined with 10 mL nitric acid. They found

Pathak et al. 006


that recoveries of all elements improved by increasing the digestion time and by decreasing the sample mass to acid volume ratio recommended by US EPA method 3051 (Adamczyk and Biaecka, 2005). In a separate study, Ohki, et al. (Ohki et al. (2007)) performed microwave-acid digestion followed by ICP-AES, Figureite Furnace Atomic Absorption Spectrometry (GFAAS), and Hydride Generation Atomic Absorption Spectrometry (HGAAS) were examined the extraction of various elements in coal and CFA. The effect of microwave irradiation on the zeolite formation was investigated with emphasis on the change in yield of zeolite during the reaction (Inadaa et al. (2005)). The fly ash was mixed with 2 M NaOH solution and heated by oil bath or microwave for 2 h. Zeolite Na-P1 formed after the conventional treatment using oil bath, but no zeolitic product was obtained by microwave heating. When microwave was applied in the course of hydrothermal treatment, zeolitization was promoted by the early-stage irradiation. This is due to the stimulated dissolution of SiO2 and Al2O3 from coal fly ash. On the other hand, the microwave irradiation in the middle to later stage retarded the crystallization of zeolite. The microwave is effective to produce the zeolite from coal fly ash in a short period by control of irradiation schedule in the early stage. Because of few limitations in the earlier researches, it was not accurately explained the reaction mechanism of zeolite dissolution in hydrochloric acid solutions as a function of pH. Ojha (Ojha et al. (2004), synthesized X-type zeolite from coal fly ash by alkali fusion method followed by hydrothermal treatment. The sodium hydroxide added to the fly ash not only works as an activator, but also adjusts the sodium content in the starting material. Mullite and quartz present in the fly ash are the sources of aluminum and silicon, respectively, for zeolite formation. Cresswell studied the use of microwave ovens in research field. The relationship between microwave power applications and spectroscopic measurements in the microwave region seems also to be an exciting area for future development. Das (74) developed a microwave assisted digestion procedure for dissolution of fly ash samples prior to the inductively coupled plasma-mass spectrometric determination of their elemental composition. Gdanski concluded that the possible precipitation during the secondary reaction can adversely affect treatment success, especially in formations with high K-feldspar content or temperatures above 300 F. Herod et al. (78) performed an evaluation of two digestion methods used to extract 17 elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba and Pb) from coal and coal ash, obtained as standard reference materials. An acid digestion method in open vessels using sulfuric, hydrofluoric, perchloric and nitric acid was compared with a sealed microwave digestion method using nitric acid only. They found that the microwave method with nitric acid only cannot break down silicates, which harbor many trace elements, but can extract As and Se quantitatively.

The efficiency of different digestion methods (six microwave decomposition methods and a wet acid digestion method) in the solubilisation of six metals (Cr, Ni, Cu, Pb, Cd, Zn), for their determination in fly ashes, was compared by Morabita. The fundamentals of electromagnetic theory, dielectric response, and applications of microwave heating to materials processing, especially fiber composites, are reviewed in this article (Thostenson,1999). A major concern in laboratories performing trace element determinations is sample contamination. Sensitivity can be enhanced for trace elements by using larger samples, but this approach may increase the risk of contamination due to the need for more involved preparation techniques. With the isolation of the sample from the laboratory environment, the potential for contamination is greatly reduced during sample decomposition. In this investigation, an Environmental Evaporation Chamber (EEC) placed in a microwave digestion system was used for predigestion of biological and botanical samples prior to closed-vessel digestion. Ellin studied application of a commercially available, microwaved digestion system of hydrofluoric acid digestion of rock samples for palynological analysis allows considerable savings of time, laboratory space, chemical costs and waste disposal costs. The system offers several advantages in terms of workplace safety and environmental hazard over existing rapid silicate digestion techniques. The reaction kinetics of silicon etching in HF solution was studied experimentally. The etch rates were measured with varying HF concentrations, agitation speed, reaction temperature and time. At low HF concentration, the etch rate maintains low value and increases very slowly because of insufficient hole concentration for etching reaction. The acceptance of microwave digestion technique is based on procedures successfully carried out for most different kinds of samples (Kuss,1992). It is not only the knowledge of the mechanisms how microwaves interact with the material scientists deal with but more the experimental procedures for the most efficiently reagents attack. The goals of this paper are to gather all information concerning applications of microwave digestion methods for elemental analysis and to review the results obtained by selecting several groups of materials digested (Nadkarni NS,1974). RESULTS AND DISCUSSION It has been well thought-out by previous researchers to utilize the industrial byproduct fly ash for some valuable product to reduce the load of pollutants in our environment (Jones, R.A. and Milton, R.M.,1957; Milton, R.M.,1959, 1961; Straub, F.G.,1936; Barrer, R.M.,1948). In line with the objectives and to recycle reuse the industrial byproduct fly ash, in some valuable product (viz. zeolite), which can be applicable for oil spill clean-up; has been experimented thoroughly



Table 1. A summary of the Methods employed for Synthesis of Fly Ash Zeolites (81)

(t: activation time, Temp.: temperature)

Method L/S Reagents Temp.

(oC)

T

(h) Zeolite Remarks

Conventional hydrothermal

8 NaOH, KOH,

Na2CO3 90-150 24-96

Chabazite, Na-P1, Phillipsite, Sodalite, Zeolite KH, 4A, A, P,

Zeolite X, Y

Low yield, Low purity, Structural-

heterogeneity

Microwave

assisted

hydrothermal

10 NaOH 100 0.25-2 Na-P1 Synthesis time very

Less

Fusion

and

hydrothermal

10

NaOH/or

Sodium

Aluminate

500-650

1-2

Faujasite

Na-A,

Na-X or

zeolite X

More yield

Water 90-100 6

Molten salt -

KOH

KNO3

NaOH

NaNO3

NH4F

NH4NO3

350 3-6 Sodalite

No addition of water

Irregular

morphology

in this study. In addition, the applicability of these synthesized zeolite for oil spill clean-up has also been strategically discussed. The scope of this work was to prepare novel zeolite using both existing and novel methods. One objective of this work was to reuse and recycle industrial byproduct fly ash to synthesize zeolite by using microwave digestion technique. Another objective was to evaluate the feasibility of commercial production of zeolite. A further objective was to characterize the zeolite using ICP-AES and XRD. Moreover, the study of the properties of oil-water mixture was also focused to evaluate the possibility for pollution control. The research was to establish the applicability of fly ash synthesized zeolite in wide variety of environmental pollution control application. It is useful to work out new techniques for separation of waste oil from water using zeolite treatment. It helps in establishment of the safe method for disposal for waste (residue) and to optimize the processes for oil spill clean-up. 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 (Fernandez-Pereira et al. (2002); Nadkarni,1974). 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 temperature and time of microwave heating of aqueous matrix of different type of samples collected from various sources, needs to carried out. A critical review of the literature available on synthesis of fly ash zeolite reveals that microwave irradiation method for activation of the fly ash with acid treatment, controls the

zeolitisation. Although, the changes in the overall physical properties (viz., particle size, pore size, specific gravity, void volume and surface area), has not been studied in detail and hence proper attention is to be given on the effect of various parameters like temperature, time, HF concentration and Si/Al necessitate to be experimentally studied. With this in view, synthesis of zeolite has been tried by microwave with acid digestion method. Although, not much has been explored about the important parameter (viz. temperature, time, Si/Al and HF concentration) and their effect on the products. As step ahead, the researchers have further explained the synthesis process to maximize the consumption of acidity in silica dissolution and zeolitization in comparison with hydrothermal method to produce zeolite in less time with more yield and better properties. With this in view, the change in the mineral phase of fly ash, during its varying degree of thermal decomposition and hence acid fly ash interaction in various stages of microwave activation needs to be optimized to better understand the superiority of this method with a special focus to the mechanism of zeolitization. With all these as a challenge ahead, thorough investigations are required to establish for the physico-chemical, mineralogical and morphological characteristics of synthesized products obtained after each cycle of synthesis. In order to investigate the zeolitization potential of fly ash and its activated phase produced and the dissolution of silica and alumina in the solution, thorough characterization needs to be the matter of concern for the researchers. However, it has been opined by previous researchers that the highest release of SiO2 in the solution indicates the lowest formation of zeolites, although; the slowing down of

Pathak et al. 008


dissolution can lead to higher zeolitization (Fukui, K. et al. (2009)). It has been reported that the dissolution of Al, Fe and other trace elements are found to be below average as compared to the Si. The critical review of literature, shown in Table 1, aimed at application potential of various fly ash zeolites, their stability parameters in case of high temperature thermal exposure and their characteristics in acidic medium, remains unexplored in case of synthesis products of different methods except few. In addition, not many efforts have been made in the past to devise a simple and viable technique for large-scale conversion of fly ash to zeolitic materials for their consistent industrial applications and that too with less energy consumption. The zeolitization would recycle in small fraction of coal fly ash production, but the product would have a higher added value than current applications. Today, oil spill responders try to optimize net environmental benefit when considering how to deal with an oil spill. This simply means that the effects on the environment of whatever clean-up techniques are to be used are weighed against the damage to the site. Sometimes the decision is made not to clean up if an assessment shows that the clean-up itself will be intrusive. In the same way, the effects of the various clean-up techniques are also assessed and the least intrusive technique is chosen for a particular site. The microwave-assisted digestion method for fly ash provided comparable or better results than the conventional hydrothermal digestion method. The variations in weight of sample, temperature, and time resulted in improved recoveries of several elements. In addition, preparation and time needed for digestion was significantly reduced with this method. The microwave-assisted method for fly ash did work better than hydrothermal digestion method. There is however, scope for more improvement in future research. The use of hydrofluoric acid was absolutely necessary for digestion of fly ash because fly ash primarily consists of silicates and oxides. Most of the elements in ash were successfully extracted with hydrofluoric acid. In addition to this, the non-biodegradability of these materials is a major disadvantage since landll disposal is environmentally undesirable and incineration is very expensive. It is suggested that use of zeolites may prove very economical, technically feasible and environmentally acceptable for application in oil spill clean-up technology. Hydrophobic, oleophilic, high- SiO2 content, hydro thermally stable zeolites, particularly those prepared cheaply from fly ash, aluminium refining wastes and other solid waste materials containing silica and alumina may find application in the removal of oil spills while simultaneously providing a solution to other environmental problems. The synthesizing capability of zeolites from fly ash has a positive impact on utilization of wastes coming from

solid fuel combustion by converting the fly ash into valuable raw material, and simultaneously accomplishing the lithosphere protection effect. ACKNOWLEDGMENT I would like to thank the Director General, Pandit Deendayal Petroleum University, Gandhinagar, Gujarat and the Dean, Marwadi Education Foundations Group of Institutions, Rajkot, Gujarat; for their continuous support and motivation to complete the work. List of Abbreviations ASTM American Standards for Testing of Materials CEC Cation Exchange Capacity CFA Coal Fly Ash HF Hydrofluoric Acid ICP-AES Inductive Coupled Plasma-Atomic Emission Spectroscopy SEM Scanning Electron Microscopy XRD X-ray Diffraction XRF X-ray Fluorescence REFERENCES Abu-Sarma A (1975). Wet ashing of some biological

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Utilization of Fly Ash Byproduct in Synthetic Zeolites