Environ. Sci. Technol. 2001, 35, 2812-2816
Salt-Thermal Zeolitization of Fly Ash CHOONG LYEAL CHOI,† MAN PARK,† DONG HOON LEE,† JANG-EOK KIM,† BYOUNG-YOON PARK,‡ AND J Y U N G C H O I * ,† Department of Agricultural Chemistry, Kyungpook National University, Taegu, 702-701, Korea, and Department of Environment Science, Catholic University of Taegu, Kyungpook, 712-702, Korea
The molten-salt method has been recently proposed as a new approach to zeolitization of fly ash. Unlike the hydrothermal method, this method employs salt mixtures as the reaction medium without any addition of water. In this study, systematic investigation has been conducted on zeolitization of fly ash in a NaOH-NaNO3 system in order to elucidate the mechanism of zeolite formation and to achieve its optimization. Zeolitization of fly ash was conducted by thermally treating a powder mixture of fly ash, NaOH, and NaNO3. Zeolitization of fly ash took place above 200 °C, a temperature lower than the melting points of salt and base in the NaOH-NaNO3 system. However, it was uncertain whether the reactions took place in a local molten state or in a solid state. Therefore, the proposed method is renamed the “salt-thermal” method rather than the “molten-salt” method. Mainly because of difficulty in mobility of components in salt mixtures, zeolitization seems to occur within a local reaction system. In situ rearrangement of activated components seems to lead to zeolite formation. Particle growth, rather than crystal growth through agglomeration, resulted in no distinct morphologies of zeolite phases. Following are the optimal zeolitization conditions of the saltthermal method: temperature, 250-350 °C; time, 3-12 h; weight ratio of NaOH/NaNO3, 0.3-0.5; weight ratio of NaNO3/ fly ash, 0.7-1.4. Therefore, it is clear from this work that the salt-thermal method could be applied to massive zeolitization of fly ash as a new alternative method for recycling this waste.
Introduction Efficient disposal of fly ash has been a worldwide issue because of its massive production and harmful effects on the environment (1-3). As a technique for recycling fly ash, as well as other mineral wastes, zeolitization has attracted a great deal of attention (4-12). Zeolitization could lead to massive recycling of mineral wastes because zeolitic materials have been used for a wide range of purposes, including agricultural, environmental, and industrial applications (1315). However, zeolitization has been achieved exclusively by the hydrothermal method. The hydrothermal method not only requires excess water for the reaction, but it also results in production of an alkaline waste solution. Consequently, this method has certain limitations that discourage largescale industrial applications. To overcome these problems, * Corresponding author phone: +82-53-950-5717; fax: +82-53953-7233; e-mail:
[email protected]. † Kyungpook National University. ‡ Catholic University of Taegu. 2812
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we have recently proposed the molten-salt method as a new approach to the synthesis of zeolitic materials from mineral wastes (16, 17). Unlike the hydrothermal method, the proposed moltensalt method employs a salt mixture instead of an aqueous solution as the reaction medium (16). Various mineral wastes, including fly ash, were easily converted into zeolitic materials by a simple thermal treatment in molten salt mixtures. Salt mixtures, such as NaOH-NaNO3, NaOH-KNO3, KOHNaNO3, and NH4F-NH4NO3, led to formation of zeolites, specifically cancrinite and sodalite. Compared to the hydrothermal method, this method exhibited much higher product yield (twice higher based on total reactant weight) and lower elemental losses (17). Although specific zeolite phases and somewhat low cation exchange capacity resulted from this molten-salt method (17), it is expected to have relatively few restrictions on application of its product. Not only do the cancrinite-sodalite mixtures retain their potential for application as soil conditioners, fertilizer additives, and broad cation exchangers, but this method also has technological importance as an environment-friendly method for large-scale disposal of harmful mineral wastes. Despite these advantages of the proposed molten-salt method, there are no systematic investigations that are required to establish this method as a new zeolitization method. No evaluations were made about the effects of various treatment factors such as temperature, time, and ratio of components. Furthermore, the mechanism of zeolite formation in the molten-salt state remains unexplored. In this study, a systematic investigation has been conducted on zeolitization of fly ash in a NaOH-NaNO3 system without any addition of water. Our focus is on establishment of the proposed molten-salt method as a new zeolitization technique by elucidating the mechanism of zeolite formation in the NaOH-NaNO3 system and by achieving its optimization.
Experimental Section Fly ash donated from Yeongwoel power station in Korea was used. Originated from an anthracite coal, the fly ash contained some crystalline phases such as quartz and mullite, along with amorphous glass materials (16). Its major chemical composition (dry basis at 105 °C) was SiO2, 44.4%; Al2O3, 29.5%; Fe2O3, 3.7%; TiO2, 1.4%; CaO, 0.9%; MgO, 0.7%; K2O, 3.6%; and Na2O, 0.2%; and its ignition loss was 15.1%. Zeolitization of fly ash was conducted without any addition of water in the following manner. Typically, mixtures containing 0.7 g of fly ash, 0.3 g of NaOH, and 1 g of NaNO3 were ground into a fine powder in a Pt crucible, and were thermally treated at 350 °C (( 5 °C) for 12 h. Treatment times, temperatures, and weight ratios of NaOH/NaNO3 and NaNO3/fly ash were varied to examine zeolitization kinetics and their effects. After the resultant lump was cooled to room temperature, it was crushed, and then washed with 50 mL of deionized water 7 times to remove excess bases and salts. X-ray diffraction (XRD) patterns, differential thermal analysis (DTA) curves, 27Al CP/MAS NMR spectra, and scanning electron micrographs (SEM) were examined to characterize the materials synthesized from fly ash. XRD patterns were obtained by the powder method using Nifiltered CuKR radiation at 40 KV and 100 mA at a scanning speed of 5°/min (Rigaku D/Max 2500). DTA curves were recorded at a heating rate of 3°/min with Al2O3 as a reference (TA-50 WSI, Shimadzu). 27Al NMR spectra (Varian, Unity 300 Inova) were obtained at rotation speed of 5 kHz with 1 µs excitation pulse in conjunction with recycle delays of 1 s to enable quantitative estimates of aluminum species. The 10.1021/es0017817 CCC: $20.00
2001 American Chemical Society Published on Web 05/25/2001
FIGURE 1. Effect of temperature on zeolitization of fly ash thermally treated for 12 h in NaOH-NaNO3 mixture. solution of 1 M Al(H2O)6+3 was taken as a reference (chemical shift, 0.0 ppm). A SEM image was obtained with gold coating by using SEM (S-2300, Hitachi).
Results and Discussion Effect of Reaction Temperature. Reaction temperature in hydrothermal treatment influences cystallization kinetics and formed phases of zeolites. In general, an increase in temperature accelerates crystallization of zeolite (18, 19), and extremely high temperature leads to dense zeolite phases such as analcime, sodalite, and cancrinite (18). On the other hand, temperature governs the physical states of salt mixtures in the proposed molten-salt method. Depending on temperature, zeolitization may occur in either the solid or liquid (molten) state. In a previous report (17) it was assumed, based mainly on mobility of framework components, that zeolitization in salt mixtures took place in a molten state. Figure 1, however, shows that zeolitization of fly ash in a salt mixture occurred at a temperature even lower than the melting points of the salt and the base (310 °C for NaOH and 335 °C for NaNO3). Although no zeolite phases were detected in XRD patterns below 150 °C, significant zeolitization was observed above 200 °C. Mixed zeolite phases of cancrinite and sodalite resulted as major crystalline phases in the product, as reported previously (16). As the temperature was increased to 250 °C, zeolitization was remarkably enhanced. However, negligible effects were observed by increases in temperature between 250 and 350 °C. Activation of components for zeolite framework seemed to be initiated around 200 °C. Enhanced zeolitization between 200 °C and 250 °C seemed to result from increased mobility of the components, especially silicates and aluminates. In this temperature range, increased mobility could not be expected from overall melting of NaOH or NaNO3 because of their high melting points. Therefore, it is suggested that zeolitization would occur mainly in a local reaction system. Increased mobility could be expanded by progressive liquefactions of NaOH and NaNO3 within the local reaction system. A molten state was probably established within the local reaction system around 250 °C, which led to negligible effects of further increases in reaction temperature up to 350 °C. Above 350 °C, all NaOH and NaNO3 were melted into a liquid state that allowed the NaOH molecules remaining out of the local reaction system to diffuse and attack the newly formed zeolite framework. Consequently, destruction of zeolites was found above 350 °C, indicated by a decrease in XRD intensity of zeolite peaks.
FIGURE 2. Differential thermal curves with the combination of various mixtures: A, fly ash; B, mixture of 0.3 g of NaOH - 0.7 g of fly ash ; C, mixture of 0.3 g of NaOH - 1.0 g of NaNO3 - 0.7 g of fly ash. However, the increase in temperature did not result in the transformation of the zeolite phases that frequently occurred in hydrothermal treatment (18). This is probably due to the fact that cancrinite and sodalite are thermally very stable phases among zeolites (18). Zeolitization in the local reaction system was further supported by thermal behaviors of the NaOH-NaNO3-fly ash mixture. A DTA curve of fly ash showed no distinct thermal responses below 500 °C, as known (7). However, a clear endothermic response was observed around 170 °C in NaOH-fly ash mixture. This partially resulted from the reaction of NaOH with labile components such as amorphous silicates and aluminates, as reported by Berkgaut et al. (7). On the other hand, the NaOH-NaNO3-fly ash mixture exhibited two clear endothermic responses around 150 and 240 °C (C in Figure 2). The endothermic response around 150 °C seems to be due to the reaction of NaOH. A lower temperature compared to that in the NaOH-fly ash mixture probably resulted from coexistence of NaOH and NaNO3. In fact, the DTA curve of the NaOH (0.3 g)-NaNO3 (1.0 g) mixture exhibited a distinct endothermic response around 300 °C that is much lower than the melting point of each salt (this result was not shown). Consequently, the other endothermic response around 240 °C seemed to indicate localized melting of the NaOH-NaNO3 mixture in the presence of fly ash. Overall melting of NaOH and NaNO3 was indicated by a broad endothermic response above 300 °C. These thermal behaviors were well matched with zeolitization of fly ash at different temperatures (Figure 1), indicating that zeolitization in a NaOH-NaNO3 mixture would occur mainly in a local reaction system. These results indicate that effective temperatures for zeolitization in salt mixture are within the range from 250 to 350 °C. However, it is still not clear whether zeolitization in a salt mixture occurs in a solid or liquid (molten) state reaction. Therefore, the proposed molten-salt method is renamed “salt-thermal method” and will be referred to as such hereafter. Effect of Component Weight Ratios. The hydrothermal method leads to a variety of zeolite phases by changing ratios of (Si+Al)/OH- and/or (Si+Al)/H2O (18). However, the saltthermal method resulted only in the mixed zeolite phases of cancrinite and sodalite. Despite this restriction on zeolite phases, zeolitization by the salt-thermal method was greatly affected by ratios of NaOH/NaNO3 and NaNO3/fly ash. The effect of NaOH/NaNO3 ratio on zeolitization of fly ash is shown in Figure 3. Increase in the NaOH/NaNO3 ratio VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Effect of NaOH/NaNO3 ratios on zeolitization of fly ash treated at 350 °C for 12 h (mixtures consisted of 0.7 g of fly ash and 1.0 g of NaNO3. up to 0.3 resulted in considerable enhancement of zeolitization. Between the ratios of 0.3 and 0.5, there was no observable change in XRD intensities of zeolite phases. Destruction of all crystalline phases began when the ratio exceeded 0.5. XRD intensities of quartz and mullite steadily decreased with increasing NaOH level. Because mobility of the NaOH molecules could be very restricted even in the molten state, zeolitization may take place only by NaOH molecules within the local reaction system. Consequently, zeolitization could be enhanced by increasing the NaOH concentration up to a level (ratio of 0.3) that is enough for zeolite formation within the local reaction system. Negligible effects could be expected by further increase because of the difficulty in diffusion of NaOH molecules and the stability of zeolite against NaOH. However, a high concentration level (more than 0.5 ratio) could lead to decomposition of newly formed zeolites, like fusion treatment with NaOH. This result clearly shows that NaOH not only acts as a mineralizer in a low level for zeolitization but also decomposes all crystalline silicates and aluminosilicates into reactive amorphous phases when present at a high level. Therefore, the effective ratio of NaOH/NaNO3 seems to be in the range of 0.3 to 0.5. Difficulty in diffusion of components was also clearly shown by effect of the NaNO3/fly ash ratio (Figure 4). Introduction of NaNO3 greatly enhanced zeolitization, whereas poor zeolitization was observed in the absence of NaNO3. However, increasing the ratio of NaNO3/fly ash to more than 0.7 did not affect zeolitization. Because zeolitization occurred in the local reaction system and diffusion of NaNO3 was also restricted just like that of NaOH, excess NaNO3 molecules out of the local reaction system could not be involved in the reaction. Therefore, it was difficult to observe dilution effect by further increase in NaNO3 in this study. In addition, this result clearly showed that NaNO3 molecules serve as solvent as well as stabilizer. As reported previously (16, 17), the presence of nitrates in cancrinite and sodalite cages indicated their role as a stabilizer. Meanwhile, remarkable enhancement of zeolitization confirms their role 2814
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FIGURE 4. Effect of NaNO3/fly ash ratios on zeolitization of fly ash treated at 350 °C for 12 h (mixtures consisted of 0.7 g of fly ash and 0.3 g of NaOH).
FIGURE 5. Changes in 27Al NMR spectra of fly ash treated at 350 °C in NaOH-NaNO3 for different times. The spinning sidebands of tetrahedral Al atom are indicated by asterisks (*). as reaction medium. Figure 4 suggests that a plausible ratio of NaNO3/fly ash is in the range of 0.7-1.4. Zeolitization Kinetics of Fly Ash. Crystallization of zeolites in the hydrothermal method takes place through several distinct stages such as supersaturation, nucleation, and crystal growth (18). These stages are usually reflected in kinetics of crystallization (9, 19). Zeolitization kinetics of fly ash in the NaOH-NaNO3 mixture was examined by the change in 27Al NMR spectra of the products because zeolitization of fly ash in a NaOH-NaNO3 mixture could be clearly detected by the formation of tetrahedral Al species characteristic of zeolite frameworks (17).
FIGURE 7. XRD pattern of NaOH-fused fly ash thermally treated in NaNO3. (1 g of fly ash was fused by 1 g of NaOH at 550 °C for 2 h. Fused fly ash (0.2 g) was thermally treated with 1 g of NaNO3 at 350 °C for 4 h). 2 hr of treatment, no further changes in morphologies of the product were observed. Unlike the hydrothermal method, a supersaturation stage is short-lived and/or absent in the salt-thermal method. Consequently, an induction period may be hardly observed in either the molten or solid states. Simultaneous dissolution and transformation (A in Figure 6) suggest that the components available to zeolitization are activated and transformed into zeolite frameworks instantaneously. Crystal growth seemed to be very restricted because of local zeolitization and difficulty in diffusion. Consequently, particle growth instead of crystal growth took place through agglomeration to result in no distinct morphologies of zeolite phases (B and C in Figure 6). Therefore, it is strongly suggested that zeolitization in the salt-thermal method occurs by in situ rearrangement of activated components. Zeolitization by the salt-thermal method seems to be complete within 3 h.
FIGURE 6. SEM images of fly ash treated in NaOH-NaNO3 mixture at 350 °C for different times: A, 0.5 h; B, 1 h; C, 2 h. A new peak at 59 ppm (value of chemical shift) in Figure 5 was assigned to tetrahedral Al species that resulted from zeolitization of fly ash. Relative intensity of this peak dramatically increased within 0.5 h. The increment became less with extended treatment up to 3 h. After the 3-hr treatment, there was no noticeable change in relative intensity of this peak. This result showed quite different kinetics of crystallization from those typical of the hydrothermal method in which induction periods were usually found for formation of nuclei (9, 18, 19). No clear induction period was observed in kinetics of zeolitization by the salt-thermal method. This was confirmed by SEM images of fly ash treated for different periods (Figure 6). SEM images of fly ash treated less than 0.5 h showed simultaneous fracture and transformation of amorphous glassy materials into fine particles. As reactions proceeded, the fine particles agglomerated into bigger spherelike particles and original shapes in fly ash disappeared. After
Complete Conversion of Fly Ash into Zeolite Phase. Complete zeolitization was not accomplished by the simple salt-thermal method despite a relatively high reaction temperature. Quartz and mullite were clearly detected by XRD pattern in the final product. In addition, it was not possible to synthesize a single-phase zeolite. As well established in the hydrothermal method, mullite, as well as quartz, could be completely converted into a zeolite phase by NaOHfusion treatment (7, 13). Complete conversion of fly ash into a zeolite phase was attempted with pretreatment of fly ash by fused NaOH. Pure sodalite based on XRD pattern was obtained by the salt-thermal method from NaOH-fused fly ash (Figure 7). NaOH-fused fly ash led to a single zeolite phase. Only sodalite was synthesized even with replacement of NaNO3 by KNO3 (result not shown). Previously, the use of KNO3 as a salt resulted in somewhat preferred formation of cancrinite from fly ash (16). In fact, cancrinite was not formed at all when commercial Si and Al sources such as sodium silicate, silica, Al metal, sodium aluminate, etc., were employed as raw materials (results not shown). It is still unknown and interesting why cancrinite did not form the above-mentioned raw materials but formed from mineral wastes.
Acknowledgments The work was partially supported by the Korea Research Foundation Grant (KRF-2000-GA0005). VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review October 17, 2000. Revised manuscript received March 20, 2001. Accepted April 3, 2001. ES0017817
