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Microwave-assisted zeolite synthesis from coal fly ash in hydrothermal process
Fuel 84 (2005) 1482–1486 www.fuelfirst.com
Microwave-assisted zeolite synthesis from coal fly ash in hydrothermal process Miki Inadaa,*, Hidenobu Tsujimotoa, Yukari Eguchib, Naoya Enomotob, Junichi Hojob a
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 22 November 2004; received in revised form 29 January 2005; accepted 5 February 2005 Available online 19 March 2005
Abstract Coal fly ashes, which include much amount of silica and alumina, can be converted into zeolite by hydrothermal alkaline treatment. In the present work, the effect of microwave irradiation on the zeolite formation was investigated with emphasis on the change in yield of zeolite during the reaction. 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. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal fly ash; Zeolite; Microwave
1. Introduction Zeolite synthesis from coal fly ash is a well known process for the application of coal fly ash. Since coal fly ash includes a large amount of silica and alumina, they are easily converted into zeolite by hydrothermal treatment in an alkaline solution [1,2]. This process proceeds at a low temperature without much energy consumption, and the products are expected to be high-value industrial materials for ion exchanger and gas adsorbent, etc. The fly ash is derived from minerals included in coal. During coal combustion, the minerals partially melt to make fly ash particles in which crystalline phases such as quartz and mullite remain in the core, whereas a glass phase of aluminosilicate covers the surface. The glass phase plays an important role in the zeolite formation because of the high solubility into alkaline solution. The authors have studied on the synthesis condition and formation mechanism of zeolite * Corresponding author. Tel./fax: C81 92 642 3545. E-mail address: [email protected] (M. Inada).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.02.002
from coal fly ash [2], and proposed the importance of SiO2– Al2O3 composition to control the yield and type of resulting zeolite [3]. Nowadays, a microwave technique has been extended as one of hydrothermal processes for synthesis of fine oxide powders [4]. The microwave-assisted hydrothermal process was effective for zeolite synthesis from silica–alumina gel precursor [5]. The zeolitization of coal fly ash by microwave heating was also reported to be useful for shortening the reaction time [6]. Microwave is absorbed directly into water as solvent, and enables the rapid heating compared to a conventional heating process. The electromagnetic wave causes water dipole rotation, probably resulting in the activation of water molecules by break of hydrogen bonds. The short-time zeolitization is preferable for efficient industrial production because the long-time reaction consumes more energy. We have investigated the effect of microwave irradiation on the formation of zeolite from coal fly ash [7], and recently found the unexpected phenomena that the initial radiation is effective rather than the overall irradiation during the zeolitization. This paper reports the experimental results and discussion based on the formation mechanism of zeolite.
M. Inada et al. / Fuel 84 (2005) 1482–1486
2. Experimental procedure 2.1. Materials A coal fly ash was supplied from an electric power station in Japan. Table 1 summarizes the chemical and mineralogical compositions of the fly ash. The contents of metal components were determined by X-ray fluorescence analysis and the chemical composition was presented in the form of stable oxide. The contents of major crystalline phases, quartz and mullite assuming the form of 3Al2O3$2SiO2, were determined by X-ray diffraction analysis using MgO powder as standard. Commercial mullite and quartz powders were used to make their calibration curves. The contents of SiO2 and Al2O3 in amorphous glass phase, which should be included as aluminosilicate glass, were calculated by subtracting the SiO2 and Al2O3 compositions of crystal phases from the bulk compositions. About 50 wt% of glass phase mainly takes part in the zeolite formation. 2.2. Zeolite synthesis and evaluation Coal fly ash (10–15 g) was treated with 2 M NaOH aqueous solution (80–120 ml) for 2 h in the ambient atmosphere. The conventional heating was performed by using 500 ml Teflon flask equipped with a reflux condenser and an oil bath under stirring with a magnetic stirrer. The oil bath temperature was 120 8C, resulting in the solution temperature of about 100 8C. The microwave heating process was performed by using a household oven (2.45 GHz, 500 W) restructured to equip the vessel with the condenser under the ambient atmosphere. The solution was stirred with a propeller for 2 h microwave heating. The microwave was irradiated continuously to the solution, the boiling of which indicated the temperature of 100 8C or above. Conventional and microwave heating processes were combined in this study. In the combined experiment, the microwave was applied for 15 min in the course of the conventional heating, in which the vessel was moved between oil bath and microwave oven. For the short-time microwave irradiation, the solution was not stirred. After the hydrothermal treatment, the powder was centrifuged, washed with distilled water, and dried at about 100 8C. The particle morphology was observed by scanning electron microscopy (SEM). The crystalline phases were
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identified by X-ray diffraction (XRD). The change in the quantity of formed zeolite Na-P1 (PDF39-219) was represented using the intensity ratio of XRD peaks of the zeolite to MgO (PDF45-946) used as standard. The solid state NMR was used to investigate the O2K coordination state of Al3C in the product. The cation exchange capacity (CEC) was measured for NHC 4 by the ammonium acetate method. First, the NaC ions of zeolitized product were replaced by NHC 4 ions. After washing, NaCl aqueous solution was added and the NHC 4 ions were removed by exchange with NaC ions. The removed NHC 4 ions were measured by an ammonia electrode.
3. Results and discussion 3.1. Continuous microwave irradiation Zeolite Na-P1 was synthesized under the present conditions which were adopted according to the previous reports [2,3]. The NaOH concentration of 2 M was adequate for formation of zeolite Na-P1 without unfavorable product, hydroxy-sodalite. In the preliminary experiment, the liquid/ solid ratio was investigated to optimize the zeolitization. The liquid/solid ratio of 4–8 ml/g was the most effective the yield of zeolite. The present work fixed the liquid/solid ratio at 8 ml/g for safe irradiation of microwave by homogeneous absorption with water. The reaction time of 2 h was short because the fast zeolitization was expected in the present work. Fig. 1 shows the XRD patterns of original fly ash and products after treatment for 2 h by conventional and microwave heating processes. The original fly ash included quartz and mullite as the major crystalline phases. The broad hump of XRD base line was detected also, which indicated the presence of glass phase in the original ash. After the conventional heating, the broad hump became flat and the new peaks of zeolite Na-P1 and CaCO3 appeared. Quartz and mullite remained because of their low reactivity to alkaline solution compared with glass phase covering the fly ash particles. These results indicate that the glass phase of the fly ash was used for the formation of zeolite Na-P1. CaCO3 may form by reaction of CO2 in air with Ca2C ions dissolved into solution from the fly ash. On the other hand, the microwave heating yielded no zeolite product although the consumption of glass phase was observed. These results simply mean that the microwave irradiation prevented
Table 1 Chemical and mineralogical compositions of coal fly ash (wt%) Chemical composition
Mineralogical composition Crystal phase
Glass phase
SiO2
Al2O3
CaO
Fe2O3
Others
Quartz
Mullite
SiO2
Al2O3
38.3
34.8
11.0
8.1
7.8
2.2
27.2
28.4
15.3
Others: TiO2, K2O, SO3, etc.
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Fig. 1. X-ray diffraction pattern of (a) original coal fly ash and products after alkaline treatment for 2 h by (b) conventional and (c) microwave heating process; Q, quartz; M, mullite; P1, zeolite Na-P1; Ca, CaCO3.
the fly ash from forming of zeolite. However, the CEC value increased to about 70 mEq/100 g after the microwave heating although this value was lower than 150 mEq/100 g in the conventional heating (Fig. 2). This suggests that some compound forms in the microwave heating. Fig. 3 shows the SEM images of the surface of coal fly ash particles. The original fly ash particles had a smooth surface because the glass phase covers the particles. After
Fig. 2. Change in the CEC value of products after conventional and microwave heating with time.
Fig. 3. SEM photographs of (a) original coal fly ash and products after (b) conventional and (c) microwave heating.
the conventional heating, granular products, which should be zeolite Na-P1, were observed on the surface of particles. This means that the glass phase dissolves into alkaline solution and deposits as zeolite on the particles. In the microwave heating also, particulate deposits were observed on the surface of fly ash particles although no zeolite products were detected by XRD. The solid state NMR was applied to clarify the deposits. Fig. 4 shows the 27Al-NMR spectra of original fly ash and products after conventional and microwave heating processes. Aluminum has three types of coordination by oxygen. The peaks are located at 0, 35 and 65 ppm, assigned to 6-, 5- and 4-coordinate Al–O bonds, respectively. With the original fly ash, the peaks were detected at 0 and 50 ppm. The 50 ppm peak means the overlapped peak of 4- and 5-coordination. These two peaks are derived from glass phase and mullite in the fly ash. After the alkaline treatment, the 50 ppm peak shifted to 65 ppm and the 0 ppm peak became weak, indicating that the peak for 4-coordination became strong by reaction. These results suggest that Al species dissolved from the glass phase of original ash into the alkaline solution and incorporated to 4-coordinate Si–O network. In the conventional heating for 2 h,
M. Inada et al. / Fuel 84 (2005) 1482–1486
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Fig. 5. Change in XRD peak intensity of zeolite Na-P1 and CEC with timing of microwave irradiation in the course of conventional heating. The data by conventional heating for 2 h (CH2h) are indicated by lines.
Fig. 4. 27Al-NMR spectra of (a) original coal fly ash and products after (b) conventional and (c) microwave heating.
the increase in the peak intensity of 4-coordinate Al–O means the formation of zeolite because the product was identified as zeolite by XRD. On the other hand, the microwave heating also exhibited the same change in NMR peaks although the zeolite crystal was not detected. As mentioned above, the product in microwave process had a CEC to some extent. In this study, the CEC was measured C for NHC 4 . NH3 molecules, equilibrated with NH4 in the aqueous solution, can be adsorbed on an amorphous aluminosilicate having acidic points. These results suggest that the deposited product in microwave heating is an amorphous aluminosilicate gel. 3.2. Partial microwave irradiation In order to clarify the effect of microwave irradiation on the formation of zeolite, microwave was partially irradiated to the reacting suspension in the course of conventional heating. The microwave heating time was 15 min and the conventional heating time was 105 min. During the total heating time of 2 h, the timing of microwave heating was shifted at 15 min intervals. Fig. 5 shows the change in zeolite content and CEC with the timing of microwave irradiation. The XRD peak intensity ratio of zeolite Na-P1 to MgO and the CEC value of product were measured after the total heating. Compared to conventional heating only, the yield of zeolite Na-P1 and the CEC increased by the microwave heating in the early stage, significantly in term
of 0–15 min. On the other hand, the yield and the CEC decreased by the microwave heating in the middle stage. Especially, the microwave heating at 45–60 min brought the worse influence for zeolitization. After then, the yield and the CEC recovered close to those by conventional heating only, and the microwave heating exhibited little influence for zeolitization in the later stage. The formation mechanism of zeolite from coal fly ash has been proposed as follows: dissolution of SiO2 and Al2O3, especially from glass phase, into the alkaline solution, deposition of aluminosilicate gel as zeolite precursor, and crystallization of zeolite. The authors have confirmed this mechanism by spectroscopic evaluation and microscopic observation of solid product [2]. In our study, the mechanism of zeolitization is principally divided into three terms. The first step is the dissolution of SiO2 and Al2O3 into alkaline solution at 0–20 min. After 20 min, the intermediate aluminosilicate gel forms by the deposition from dissolved Si and Al species. The intermediate gel begins to change into zeolite via dissolution–reprecipitation process at 40 min, and then crystal growth occurred. According to these elemental processes, we thought that the effect of partial microwave irradiation for zeolitization as below. Zeolitization was enhanced by partial microwave irradiation in the early stage of reaction. This means that the dissolution of SiO2 and Al2O3 from coal fly ash was promoted by microwave irradiation because microwave can quickly heat up an aqueous solution. Another reason may be assigned to active water generated by break of hydrogen bonds under microwave irradiation. The active water molecules, isolated H2O molecules, should freely attack Si–O and Al–O bonds so to enhance the dissolution of the glass phase on fly ash particles. Fig. 6 shows the contents of Si and Al ions dissolved into the solution as a function of time under microwave irradiation and conventional heating for 2 h. The Si content rapidly increased in the early stage and became constant. The Al content exhibited the maximum, and after then, zeolite formation was detected
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4. Conclusion
Fig. 6. Variations of Si and Al concentrations with time in the reacting solution under microwave irradiation and conventional heating; (&, :): microwave heating, (,, 6): conventional heating. Liquid/solid ratioZ 8 ml/g.
[2]. Actually, the microwave irradiation enhanced the dissolution rate in the early stage compared to the conventional heating. Microwave irradiation in the middle stage, especially in the term of 45–60 min, retarded the zeolitization. Under the conventional heating in this stage, zeolite nuclei generated from precursor gels. These results indicate that microwave irradiation inhibited the nucleation of zeolite. The nucleation proceeds via dissolution– reprecipitation of the aluminosilicate gel. Microwave seems to accelerate the dissolution of the precursor gel, but to prevent their reprecipitation because nuclei are so unstable to be dissolved again through attacking by active water molecules. If there are many nuclei in the solution in the later stage, the crystal growth may proceed under microwave irradiation because the growing crystallites are stable. This is the reason why the microwave heating exhibited little influence for zeolitization in the later stage. The effect of ageing on the microwave synthesis of zeolite has been investigated for the formation of zeolite NaA from silica–alumina gel precursor [8], suggesting that prenucleation by ageing is needed before stimulating the zeolite formation by microwave irradiation. Otherwise, the crystalline zeolite was not obtained. These results support the present discussion on the retardation of nucleation by microwave irradiation. As described above, the formation of zeolite Na-P1 was enhanced by microwave irradiation in the early stage followed by conventional heating. In contrast with zeolite formation from the gel precursor, the zeolitization of coal fly ash requires the dissolution of glass phase. Therefore, microwave irradiation in the early stage is very important. The CEC of product including zeolite Na-P1 reached maximally about 200 mEq/100 g for 2 h of total reaction time. This CEC value was comparable to that of product after conventional heating for 5 h in the same solution. Thus, the microwave-assisted hydrothermal process is useful for shortening the reaction time of zeolitization of coal fly ash.
Zeolite Na-P1 forms by the hydrothermal alkaline treatment of coal fly ash. The microwave irradiation to the reacting solution is effective for enhancing the zeolitization and the mechanism is explained as follows. Continuous microwave irradiation retards the formation of zeolite in the crystalline form. The deposited product is presumed to be an amorphous aluminosilicate gel. This means that the microwave heating inhibits the crystallization of zeolite from the intermediate gel. Partial microwave irradiation exhibits various influences in the course of conventional heating process. The early-stage microwave heating enhances the zeolite formation. This is assigned to the stimulated dissolution of SiO2 and Al2O3 from coal fly ash by rapid heating rate and probably by strong attack of active water molecules to glass phase. On the other hand, the microwave heating in the middle stage significantly retards the zeolite formation. This is caused by the retardation of nucleation in the intermediate aluminosilicate gel. Therefore, the early-stage microwave heating followed by conventional heating is effective to enhance the zeolitization of coal fly ash. There has been a similar attempt for control of heating schedule in zeolite formation from silica–alumina gel precursor [5]. Compared to the microwave heating followed by conventional heating, the power control of microwave from high level to low level was more effective for zeolite formation. Such a power control may be also useful for the microwave-assisted zeolite formation from coal fly ash.
References [1] Querol X, Moreno N, Uman˜a JC, Alastuey A, Herna´ndez E, LopezSoler A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [2] Inada M, Eguchi Y, Uehara M, Enomoto N, Hojo J. Mechanism and morphological change in zeolite formation from coal fly ash. Trans Mater Res Soc Jpn 2004;29:2363–6. [3] Inada M, Eguchi Y, Enomoto N, Hojo J. Synthesis of zeolite from coal fly ashes with different silica–alumina composition. Fuel 2005;84: 299–304. [4] Somiya S, Roy R. Hydrothermal synthesis of fine oxide powders. Bull Mater Sci 2000;23:453–60. [5] Jansen JC, Arafat A, Barakat AK, Bekkum H. In: Occelli ML et al, editor. Molecular sieves. New York: Van Nostrand Reinhold; 1992. p. 507–21. [6] Querol X, Alastuey A, Lopez-Soler A, Plana F. A fast method for recycling fly ash: microwave-assisted zeolite synthesis. Environ Sci Technol 1997;31:2527–33. [7] Enomoto N, Inada M, Eguchi Y, Uehara M, Hojo J. Microwave synthesis of zeolite from coal fly ash and the way to scale up. Trans Mater Res Soc Jpn 2001;26:1197–200. [8] Slangen PM, Jansen JC, Bekkum H. The effect of ageing on the microwave synthesis of zeolite NaA. Microporous Mater 1997;9: 259–65.
Microwave-assisted zeolite synthesis from coal fly ash in hydrothermal process Miki Inadaa,*, Hidenobu Tsujimotoa, Yukari Eguchib, Naoya Enomotob, Junichi Hojob a
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 22 November 2004; received in revised form 29 January 2005; accepted 5 February 2005 Available online 19 March 2005
Abstract Coal fly ashes, which include much amount of silica and alumina, can be converted into zeolite by hydrothermal alkaline treatment. In the present work, the effect of microwave irradiation on the zeolite formation was investigated with emphasis on the change in yield of zeolite during the reaction. 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. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal fly ash; Zeolite; Microwave
1. Introduction Zeolite synthesis from coal fly ash is a well known process for the application of coal fly ash. Since coal fly ash includes a large amount of silica and alumina, they are easily converted into zeolite by hydrothermal treatment in an alkaline solution [1,2]. This process proceeds at a low temperature without much energy consumption, and the products are expected to be high-value industrial materials for ion exchanger and gas adsorbent, etc. The fly ash is derived from minerals included in coal. During coal combustion, the minerals partially melt to make fly ash particles in which crystalline phases such as quartz and mullite remain in the core, whereas a glass phase of aluminosilicate covers the surface. The glass phase plays an important role in the zeolite formation because of the high solubility into alkaline solution. The authors have studied on the synthesis condition and formation mechanism of zeolite * Corresponding author. Tel./fax: C81 92 642 3545. E-mail address: [email protected] (M. Inada).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.02.002
from coal fly ash [2], and proposed the importance of SiO2– Al2O3 composition to control the yield and type of resulting zeolite [3]. Nowadays, a microwave technique has been extended as one of hydrothermal processes for synthesis of fine oxide powders [4]. The microwave-assisted hydrothermal process was effective for zeolite synthesis from silica–alumina gel precursor [5]. The zeolitization of coal fly ash by microwave heating was also reported to be useful for shortening the reaction time [6]. Microwave is absorbed directly into water as solvent, and enables the rapid heating compared to a conventional heating process. The electromagnetic wave causes water dipole rotation, probably resulting in the activation of water molecules by break of hydrogen bonds. The short-time zeolitization is preferable for efficient industrial production because the long-time reaction consumes more energy. We have investigated the effect of microwave irradiation on the formation of zeolite from coal fly ash [7], and recently found the unexpected phenomena that the initial radiation is effective rather than the overall irradiation during the zeolitization. This paper reports the experimental results and discussion based on the formation mechanism of zeolite.
M. Inada et al. / Fuel 84 (2005) 1482–1486
2. Experimental procedure 2.1. Materials A coal fly ash was supplied from an electric power station in Japan. Table 1 summarizes the chemical and mineralogical compositions of the fly ash. The contents of metal components were determined by X-ray fluorescence analysis and the chemical composition was presented in the form of stable oxide. The contents of major crystalline phases, quartz and mullite assuming the form of 3Al2O3$2SiO2, were determined by X-ray diffraction analysis using MgO powder as standard. Commercial mullite and quartz powders were used to make their calibration curves. The contents of SiO2 and Al2O3 in amorphous glass phase, which should be included as aluminosilicate glass, were calculated by subtracting the SiO2 and Al2O3 compositions of crystal phases from the bulk compositions. About 50 wt% of glass phase mainly takes part in the zeolite formation. 2.2. Zeolite synthesis and evaluation Coal fly ash (10–15 g) was treated with 2 M NaOH aqueous solution (80–120 ml) for 2 h in the ambient atmosphere. The conventional heating was performed by using 500 ml Teflon flask equipped with a reflux condenser and an oil bath under stirring with a magnetic stirrer. The oil bath temperature was 120 8C, resulting in the solution temperature of about 100 8C. The microwave heating process was performed by using a household oven (2.45 GHz, 500 W) restructured to equip the vessel with the condenser under the ambient atmosphere. The solution was stirred with a propeller for 2 h microwave heating. The microwave was irradiated continuously to the solution, the boiling of which indicated the temperature of 100 8C or above. Conventional and microwave heating processes were combined in this study. In the combined experiment, the microwave was applied for 15 min in the course of the conventional heating, in which the vessel was moved between oil bath and microwave oven. For the short-time microwave irradiation, the solution was not stirred. After the hydrothermal treatment, the powder was centrifuged, washed with distilled water, and dried at about 100 8C. The particle morphology was observed by scanning electron microscopy (SEM). The crystalline phases were
1483
identified by X-ray diffraction (XRD). The change in the quantity of formed zeolite Na-P1 (PDF39-219) was represented using the intensity ratio of XRD peaks of the zeolite to MgO (PDF45-946) used as standard. The solid state NMR was used to investigate the O2K coordination state of Al3C in the product. The cation exchange capacity (CEC) was measured for NHC 4 by the ammonium acetate method. First, the NaC ions of zeolitized product were replaced by NHC 4 ions. After washing, NaCl aqueous solution was added and the NHC 4 ions were removed by exchange with NaC ions. The removed NHC 4 ions were measured by an ammonia electrode.
3. Results and discussion 3.1. Continuous microwave irradiation Zeolite Na-P1 was synthesized under the present conditions which were adopted according to the previous reports [2,3]. The NaOH concentration of 2 M was adequate for formation of zeolite Na-P1 without unfavorable product, hydroxy-sodalite. In the preliminary experiment, the liquid/ solid ratio was investigated to optimize the zeolitization. The liquid/solid ratio of 4–8 ml/g was the most effective the yield of zeolite. The present work fixed the liquid/solid ratio at 8 ml/g for safe irradiation of microwave by homogeneous absorption with water. The reaction time of 2 h was short because the fast zeolitization was expected in the present work. Fig. 1 shows the XRD patterns of original fly ash and products after treatment for 2 h by conventional and microwave heating processes. The original fly ash included quartz and mullite as the major crystalline phases. The broad hump of XRD base line was detected also, which indicated the presence of glass phase in the original ash. After the conventional heating, the broad hump became flat and the new peaks of zeolite Na-P1 and CaCO3 appeared. Quartz and mullite remained because of their low reactivity to alkaline solution compared with glass phase covering the fly ash particles. These results indicate that the glass phase of the fly ash was used for the formation of zeolite Na-P1. CaCO3 may form by reaction of CO2 in air with Ca2C ions dissolved into solution from the fly ash. On the other hand, the microwave heating yielded no zeolite product although the consumption of glass phase was observed. These results simply mean that the microwave irradiation prevented
Table 1 Chemical and mineralogical compositions of coal fly ash (wt%) Chemical composition
Mineralogical composition Crystal phase
Glass phase
SiO2
Al2O3
CaO
Fe2O3
Others
Quartz
Mullite
SiO2
Al2O3
38.3
34.8
11.0
8.1
7.8
2.2
27.2
28.4
15.3
Others: TiO2, K2O, SO3, etc.
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Fig. 1. X-ray diffraction pattern of (a) original coal fly ash and products after alkaline treatment for 2 h by (b) conventional and (c) microwave heating process; Q, quartz; M, mullite; P1, zeolite Na-P1; Ca, CaCO3.
the fly ash from forming of zeolite. However, the CEC value increased to about 70 mEq/100 g after the microwave heating although this value was lower than 150 mEq/100 g in the conventional heating (Fig. 2). This suggests that some compound forms in the microwave heating. Fig. 3 shows the SEM images of the surface of coal fly ash particles. The original fly ash particles had a smooth surface because the glass phase covers the particles. After
Fig. 2. Change in the CEC value of products after conventional and microwave heating with time.
Fig. 3. SEM photographs of (a) original coal fly ash and products after (b) conventional and (c) microwave heating.
the conventional heating, granular products, which should be zeolite Na-P1, were observed on the surface of particles. This means that the glass phase dissolves into alkaline solution and deposits as zeolite on the particles. In the microwave heating also, particulate deposits were observed on the surface of fly ash particles although no zeolite products were detected by XRD. The solid state NMR was applied to clarify the deposits. Fig. 4 shows the 27Al-NMR spectra of original fly ash and products after conventional and microwave heating processes. Aluminum has three types of coordination by oxygen. The peaks are located at 0, 35 and 65 ppm, assigned to 6-, 5- and 4-coordinate Al–O bonds, respectively. With the original fly ash, the peaks were detected at 0 and 50 ppm. The 50 ppm peak means the overlapped peak of 4- and 5-coordination. These two peaks are derived from glass phase and mullite in the fly ash. After the alkaline treatment, the 50 ppm peak shifted to 65 ppm and the 0 ppm peak became weak, indicating that the peak for 4-coordination became strong by reaction. These results suggest that Al species dissolved from the glass phase of original ash into the alkaline solution and incorporated to 4-coordinate Si–O network. In the conventional heating for 2 h,
M. Inada et al. / Fuel 84 (2005) 1482–1486
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Fig. 5. Change in XRD peak intensity of zeolite Na-P1 and CEC with timing of microwave irradiation in the course of conventional heating. The data by conventional heating for 2 h (CH2h) are indicated by lines.
Fig. 4. 27Al-NMR spectra of (a) original coal fly ash and products after (b) conventional and (c) microwave heating.
the increase in the peak intensity of 4-coordinate Al–O means the formation of zeolite because the product was identified as zeolite by XRD. On the other hand, the microwave heating also exhibited the same change in NMR peaks although the zeolite crystal was not detected. As mentioned above, the product in microwave process had a CEC to some extent. In this study, the CEC was measured C for NHC 4 . NH3 molecules, equilibrated with NH4 in the aqueous solution, can be adsorbed on an amorphous aluminosilicate having acidic points. These results suggest that the deposited product in microwave heating is an amorphous aluminosilicate gel. 3.2. Partial microwave irradiation In order to clarify the effect of microwave irradiation on the formation of zeolite, microwave was partially irradiated to the reacting suspension in the course of conventional heating. The microwave heating time was 15 min and the conventional heating time was 105 min. During the total heating time of 2 h, the timing of microwave heating was shifted at 15 min intervals. Fig. 5 shows the change in zeolite content and CEC with the timing of microwave irradiation. The XRD peak intensity ratio of zeolite Na-P1 to MgO and the CEC value of product were measured after the total heating. Compared to conventional heating only, the yield of zeolite Na-P1 and the CEC increased by the microwave heating in the early stage, significantly in term
of 0–15 min. On the other hand, the yield and the CEC decreased by the microwave heating in the middle stage. Especially, the microwave heating at 45–60 min brought the worse influence for zeolitization. After then, the yield and the CEC recovered close to those by conventional heating only, and the microwave heating exhibited little influence for zeolitization in the later stage. The formation mechanism of zeolite from coal fly ash has been proposed as follows: dissolution of SiO2 and Al2O3, especially from glass phase, into the alkaline solution, deposition of aluminosilicate gel as zeolite precursor, and crystallization of zeolite. The authors have confirmed this mechanism by spectroscopic evaluation and microscopic observation of solid product [2]. In our study, the mechanism of zeolitization is principally divided into three terms. The first step is the dissolution of SiO2 and Al2O3 into alkaline solution at 0–20 min. After 20 min, the intermediate aluminosilicate gel forms by the deposition from dissolved Si and Al species. The intermediate gel begins to change into zeolite via dissolution–reprecipitation process at 40 min, and then crystal growth occurred. According to these elemental processes, we thought that the effect of partial microwave irradiation for zeolitization as below. Zeolitization was enhanced by partial microwave irradiation in the early stage of reaction. This means that the dissolution of SiO2 and Al2O3 from coal fly ash was promoted by microwave irradiation because microwave can quickly heat up an aqueous solution. Another reason may be assigned to active water generated by break of hydrogen bonds under microwave irradiation. The active water molecules, isolated H2O molecules, should freely attack Si–O and Al–O bonds so to enhance the dissolution of the glass phase on fly ash particles. Fig. 6 shows the contents of Si and Al ions dissolved into the solution as a function of time under microwave irradiation and conventional heating for 2 h. The Si content rapidly increased in the early stage and became constant. The Al content exhibited the maximum, and after then, zeolite formation was detected
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4. Conclusion
Fig. 6. Variations of Si and Al concentrations with time in the reacting solution under microwave irradiation and conventional heating; (&, :): microwave heating, (,, 6): conventional heating. Liquid/solid ratioZ 8 ml/g.
[2]. Actually, the microwave irradiation enhanced the dissolution rate in the early stage compared to the conventional heating. Microwave irradiation in the middle stage, especially in the term of 45–60 min, retarded the zeolitization. Under the conventional heating in this stage, zeolite nuclei generated from precursor gels. These results indicate that microwave irradiation inhibited the nucleation of zeolite. The nucleation proceeds via dissolution– reprecipitation of the aluminosilicate gel. Microwave seems to accelerate the dissolution of the precursor gel, but to prevent their reprecipitation because nuclei are so unstable to be dissolved again through attacking by active water molecules. If there are many nuclei in the solution in the later stage, the crystal growth may proceed under microwave irradiation because the growing crystallites are stable. This is the reason why the microwave heating exhibited little influence for zeolitization in the later stage. The effect of ageing on the microwave synthesis of zeolite has been investigated for the formation of zeolite NaA from silica–alumina gel precursor [8], suggesting that prenucleation by ageing is needed before stimulating the zeolite formation by microwave irradiation. Otherwise, the crystalline zeolite was not obtained. These results support the present discussion on the retardation of nucleation by microwave irradiation. As described above, the formation of zeolite Na-P1 was enhanced by microwave irradiation in the early stage followed by conventional heating. In contrast with zeolite formation from the gel precursor, the zeolitization of coal fly ash requires the dissolution of glass phase. Therefore, microwave irradiation in the early stage is very important. The CEC of product including zeolite Na-P1 reached maximally about 200 mEq/100 g for 2 h of total reaction time. This CEC value was comparable to that of product after conventional heating for 5 h in the same solution. Thus, the microwave-assisted hydrothermal process is useful for shortening the reaction time of zeolitization of coal fly ash.
Zeolite Na-P1 forms by the hydrothermal alkaline treatment of coal fly ash. The microwave irradiation to the reacting solution is effective for enhancing the zeolitization and the mechanism is explained as follows. Continuous microwave irradiation retards the formation of zeolite in the crystalline form. The deposited product is presumed to be an amorphous aluminosilicate gel. This means that the microwave heating inhibits the crystallization of zeolite from the intermediate gel. Partial microwave irradiation exhibits various influences in the course of conventional heating process. The early-stage microwave heating enhances the zeolite formation. This is assigned to the stimulated dissolution of SiO2 and Al2O3 from coal fly ash by rapid heating rate and probably by strong attack of active water molecules to glass phase. On the other hand, the microwave heating in the middle stage significantly retards the zeolite formation. This is caused by the retardation of nucleation in the intermediate aluminosilicate gel. Therefore, the early-stage microwave heating followed by conventional heating is effective to enhance the zeolitization of coal fly ash. There has been a similar attempt for control of heating schedule in zeolite formation from silica–alumina gel precursor [5]. Compared to the microwave heating followed by conventional heating, the power control of microwave from high level to low level was more effective for zeolite formation. Such a power control may be also useful for the microwave-assisted zeolite formation from coal fly ash.
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