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Effects of step change of heating source on synthesis of zeolite 4A from coal fly ash
Journal of Industrial and Engineering Chemistry 15 (2009) 736–742
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of step change of heating source on synthesis of zeolite 4A from coal fly ash Jae Kwan Kim *, Hyun Dong Lee Power Generation Laboratory, Korea Electric Power Research Institute, KEPCO, Daejon 103-16, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 February 2009 Accepted 31 May 2009
Effects of step change of heating sources on the crystallization of zeolite 4A from coal fly ash by hydrothermal reaction were investigated with emphasis on the change in the crystallinity of the synthesized zeolite 4A. Most of the Si and Al components were effectively transformed into zeolite 4A by step change of the first conventional heating and then the second microwave heating of synthesis mixture dissolved from coal fly ash, and maximum crystallinity of zeolite 4A obtained was 91%. The first conventional heating also plays an important role in enhancing the nuclei formation that Si and Al in synthesis mixture reacted to form ring-like structures for combining sodalites, and further to small zeolite 4A seeds. The second microwave heating increases the crystallization rates from small zeolite 4A nuclei to more zeolite 4A crystals. The cation exchange capacity (CEC) of the zeolite 4A crystallized by step change of heating source from the conventional to the microwave was 5.5 meq/g compared to 5.7 meq/g for commercial zeolite 4A. Test results showed that removal efficiency of heavy metals by zeolite 4A synthesized from fly ash was more than 98% and similar to commercial zeolite 4A. ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Fly ash Zeolite 4A Microwave synthesis Conventional heating CEC
1. Introduction About 6.0 million tons of coal fly ash was produced in Korea last year, of which only 70% were utilized with remaining 30% being disposed as a waste. Due to the construction of new coal power plants, the fly ash production in Korea will increase for next 5 years and is expected to reach up to 8.3 million tons in 2013. This rapid increase of the fly ash production causes serious concerns over fly ash disposal amid growing awareness of environmental issues and depletion of landfill sites [1]. Many efforts have been made at finding alternative and meaningful applications for this waste. Since fly ash contains mainly amorphous aluminosilicate (glassy phase) and some crystalline minerals (quartz, mullite, hematite, etc.), it can be used as a raw material for the synthesis of zeolite-like materials. The glassy phase plays an important role in the zeolite formation because of the high solubility into alkaline solution. However, in many of these studies [2–5], the total conversion time was generally long (72 h or more), synthesis temperature was very high (90–225 8C), and a little SiO2 and Al2O3 contained in fly ash was utilized due to the low dissolution rate of conventional heating such as autoclave, heating mantle and oil (water) bath. Nowadays, microwave heating at hydrothermal process was effective for
* Corresponding author. E-mail address: [email protected] (J.K. Kim).
zeolite synthesis from silica–alumina gel [6,7]. The zeolite synthesis from coal fly ash by microwave heating also reported to be useful for shortening the reaction time [8]. Microwave was absorbed directly into water as a solvent, and enabled the rapid heating compared to a conventional heating. We have investigated the effect of microwave irradiation on the dissolution of Si and Al from coal fly ashes, and the crystallization of zeolite 4A from the dissolved solution [1]. It is recently found the unexpected phenomena that dissolution rate of silicon and aluminum from coal fly ash by microwave irradiation improved dramatically compared to conventional heating, but nuclei of zeolite 4A was not generated because Si and Al dissolved from fly ash less reactive and microwave have too energy to disturb the combination of among sodalites. Therefore, the conventional heating at initial step on crystallization of zeolite 4A was introduced to produce many small zeolite 4A nuclei and seeds from mixture solution. 2. Experimental 2.1. Coal fly ash sample A fly ash sample was collected from the Tean thermal power plant owned by the Korea Western Electric Power Corporation. Table 1 shows the chemical and mineralogical composition of fly ash sample. The content of metal components was determined by X-ray fluorescence analysis and the chemical composition was presented in the form of stable oxide. The contents of major
1226-086X/$ – see front matter ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2009.09.055
J.K. Kim, H.D. Lee / Journal of Industrial and Engineering Chemistry 15 (2009) 736–742
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Table 1 Chemical and mineralogical composition of coal fly ash. Chemical composition (wt.%)
Mineralogical composition (wt.%) Crystal phase
Glass phase
SiO2
Al2O3
CaO
Fe2O3
Others
Quartz
Mullite
SiO2
Al2O3
54.5
26.3
6.0
3.2
10.0
30.6
16.8
19.1
14.2
Others: MgO, Na2O, TiO2, K2O, SO3, P2O5, etc.
crystalline phases such as quartz and mullite presented in the form SiO2 and 3Al2O32SiO2 were determined by a quantitative X-ray diffraction analysis. Commercial mullite and quartz powders were used to make their calibration curves. The contents of SiO2 and Al2O3 in amorphous glass phase, which included as aluminosilicate were calculated by subtracting the SiO2 and Al2O3 compositions of crystal phases from the bulk compositions. 2.2. Experimental apparatus and procedures Microwave equipment used in this study consisted of an industrial type micro-oven with a working frequency of 2.45 GHz (Korea High Frequency Inc., KMIC-2 KW) and an output power that can be varied from 0 W to a maximum of 2 kW. The microwave synthetic equipment was modified by the introduction of a thermocouple inside the cavity in order to monitor reaction temperature, and the water condenser to conduct atmospheric pressure experiments in Fig. 1. Atmospheric pressure has been chosen because atmospheric hydrothermal synthesis was the common method for manufacturing zeolite NaA on a commercial scale and allows an easy change of reaction parameters, like agitating. The agitation shaft was equipped with a SUS 316 blade and stirring rate was varied from 0 to 500 rpm. The impeller was also operated to reflect transparent microwave through Teflon lining Pyrex reactor, which has the volume of 2000 mL and the high alkali resistant materials. Appendix equipments were tuned to minimize microwave for reflecting from the reactor, the coupler for transmitting the signal of microwave strength to be reflected at reactor into both magnetron and power supply, the circulator to
prevent the damage of magnetron by microwave reflected from reactor, and the dummy load which eliminates microwave inducted through circulator. 2.3. Preparation of mixed solution A mixture of 250 g of fly ash and 1000 mL of 5 M NaOH solution (Yakuri Inc.) in a 2000-mL sealed teflon bottle was heated using a conventional insulating mantle [1] or a microwave reactor at 100 8C for 1–8 h. The solution was filtrated with filter paper lined funnels. The volume of filtrate obtained from the solution heated at 100 8C for 5 h was roughly 940 mL, the Si, Al and Na concentrations were 56.8, 0.75 and 134.2 mg/L, respectively. Next, the filtrate solution was mixed with 130 g of sodium aluminate (Daejung Chemistry Inc., Na2O 0.31 mg/mL, Al2O3 0.34 mg/mL), 132 g of NaOH pellets, and 790 mL of distilled water to adjust the molar ratio of Si:Al:Na:H2O as 1.0:1.0:2.5:100. The mixed solution was stirred with 250 rpm for 24 h at room temperature. 2.4. Synthesis of zeolite 4A In the synthesis of zeolite 4A, four kinds of experimental methods were compared as shown in Table 2. The first method (KZ-1) was carried out only by the microwave heating without the conventional heating as insulating mantle. That is, 1750 mL of the mixed solution prepared at above section was injected into the microwave reactor and was crystallized by a microwave irradiation under agitation of 250 rpm at 100 8C for 2 h. The second and third methods were conducted only by the conventional heating
Fig. 1. The apparatus of microwave reactor for dissolution and crystallization process.
J.K. Kim, H.D. Lee / Journal of Industrial and Engineering Chemistry 15 (2009) 736–742
738 Table 2 Synthesis conditions of zeolite 4A. Symbol
KZ-1 KZ-2 KZ-3 KZ-4
Crystallization conditions First conventional heating
Second microwave heating
N 100 8C for 1 h 100 8C for 2 h 100 8C for 1 h
100 8C for 2 h N N 100 8C for 1 h
N, not conducted.
process without the microwave heating process as shown as KZ-2 and KZ-3 in Table 2. 1750 mL of the mixed solution was crystallized at the same condition for 1 and 2 h, respectively. The fourth method carried out both process of the conventional heating and the microwave heating. The mixed solution was hydrothermally heated in a conventional heating mantle at the temperature of 100 8C for 1 h and then the heated mixture was injected to the microwave reactor. Zeolite 4A was finally crystallized with an agitator under the microwave heating of 100 8C for 1 h. Microwave heating system for the extraction of Si and Al source from fly ash and for the crystallization of zeolite 4A from mixed solution were conducted in Fig. 1. After a given period of synthesis, the precipitate was extracted from the solution, and then washed with a deionized water until the pH reached around 10. The samples were kept in an oven and dried at 100 8C for 12 h. Hereafter, labeling of the synthesized products was denoted as KZ-1–KZ-4 in Table 2. 2.5. Analysis of physicochemical properties The physical properties of coal fly ash and synthesized products were measured as follows. The chemical composition was analyzed using an X-ray fluorescence analysis equipment
Fig. 2. The concentration of silicon and aluminum in solution eluted from fly ash by the microwave irradiation and the conventional heating.
(EMAX-3770, Horiba). The phases were characterized by X-ray diffraction (XRD) using a PhilipsPW 1830 diffractometer with CuKa radiation (1.5496 A˚). XRD was used to estimate the degree of crystallinity of zeolite 4A and sodalite employing a quantitative analysis based on the ‘‘peak summation procedure [9] by using the following equation: Sx 100 crystallinity ð%Þ=4A ¼ Sr where Sx is the sum of integral peak intensities for the sample and Sr is the sum of integral peak intensities for the reference zeolite 4A. Particle morphology was observed by scanning electron microscopy (SEM, JEOL 5200-2AE). The concentrations of Si, Al and Na dissolved from fly ash in alkali solution were determined by
Fig. 3. SEM morphology of fly ash and the residues of fly ash obtained in 5 M NaOH by the microwave irradiation and the conventional heating. (a) Original fly ash, (b) conventional heating, (c) conventional heating and (d) microwave heating.
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an inductively coupled plasma emission analysis equipment (ICPS1000III, Shimadzu). In addition, CEC of the zeolite products was measured by means of the calcium chloride method [10]. The BET surface area has been determined with the help of air permeability apparatus (ASAP2010, Micromertics). Batch experiments on the removal of heavy metal of the synthesized products were carried out in order to determine the adsorption isotherms for Cd2+, Pb2+ and Zn2+. Three single component solutions were prepared by adding each 1 g of synthesized zeolite in three 500 mL sealed polypropylene bottles containing 100 mL of 100 mg/L Cd2+, Pb2+ and Zn2+ solution, respectively. The pH of solutions was each adjusted to pH 7.0 to prevent agglomeration of heavy metal. Then, the solutions were set in water bath, agitated for 10 min and then filtrated with filter paper lined funnels. The concentration of heavy metals in filtered solutions was analyzed by an inductively coupled plasma emission analysis equipment (ICPS-1000III, Shimadzu). These batch experiments were carried out in duplicate to check the reproducibility of their results. 3. Results and discussion 3.1. Microwave dissolution of Si and Al from fly ash Fig. 2 shows that the dissolution percentage of Si and Al in the filtrate solution as a function of time under microwave irradiation or conventional heating at 100 8C for 1–8 h in 5 M NaOH. Dissolution percentage of Si at the conventional heating, constantly increased initially (2 h) and later remained constant. The dissolution percentage of Al reached the maximum and after then constantly decreased with heating time. Therefore, SEM morphology of residue as fly ash dissolved in 5 M NaOH for 5 h showed needle-like precipitate in Fig. 3(b). Zeolite P and sodalite formation at the surface of dissolving fly ash was ascertained in Fig. 4(a). Under the conventional heating in this stage, zeolite P (Na6Al6Si10O3212H2O) and sodalite (Na4Al3Si3O12(OH)) crystals formed from Si and Al was deposited on the surface of the fly ash particles in Figs. 3(c) and 4(a). Actually, the microwave irradiation slowly enhanced the dissolution rate of Si in the early stage (4 h) compared to the conventional heating, and after then rapidly increased into heating time of 5 h, but from 6 h, it slowly decreased with increasing heating times. Al dissolution was with similar trends to Si dissolution rate. Microwave irradiation in the middle stage, especially in the term of 5–6 h, retarded the zeolite formation of dissolved Si and Al. These results indicate that the microwave irradiation for Si and Al dissolved from fly ash in 5 M NaOH inhibited the nucleation to form zeolite P and sodalite on the surface of dissolving fly ash in Figs. 3 and 4 Figs. 3(d) and 4(b). In Fig. 3(a), the original fly ash had a smooth surface because the surface was covered by an aluminosilicate glass phase. After reaction with alkali (5 M NaOH) solution by the microwave irradiation, the surface on the fly ash generate crevice and became rough in Fig. 3(d). Fig. 4 shows the XRD main peak intensities of residues after dissolved in various NaOH concentrations at 100 8C for 5 h by the conventional (a) or microwave irradiation (b) heating. The diffraction angles (2u) of main peaks of zeolite P, sodalite, quartz, and mullite are 28.08, 14.08, 26.68, and 26.18, respectively. Fig. 4 shows that main peak intensities of the zeolite P at all NaOH solutions were larger in the conventional heating than the microwave irradiation. This tendency reversely consists with the concentration of Si and Al dissolved in 5 M NaOH for 5 h in Fig. 2. When NaOH concentrations ranged from 3.0 to 4.0 M, the zeolite P peaks showed the maximum in the microwave irradiation (b) but did not reach to the zeolite P intensities at the conventional heating (a). On the other hand, the sodalite peaks at NaOH concentration from 1.0 to 3.0 M by the conventional heating were
Fig. 4. XRD intensities of the residues of fly ash obtained at various NaOH concentrations by (a) conventional heating and (b) microwave irradiation.
not generated because Si and Al dissolved from fly ash are consumed to generate zeolite P. This is the reason for the formation of zeolite P even at the low concentration with less than 4 M NaOH. The present results suggest that heating method by the microwave irradiation in 5 M NaOH solution for 5 h was important for preventing the formation of zeolite P and sodalites deposited on the surface of dissolving fly ash during the dissolution process of Si and Al from coal fly ash. Therefore, for obtaining a maximum dissolution rate of Si and Al from fly ash, it was desirable to react at 5 M NaOH solution for 5 h in the microwave irradiation. 3.2. Effect of step change of heating source on zeolite 4A formation Conventional heating before the microwave crystallization of mixed solution has been introduced for the pre-nucleation and seed formation of zeolite. Fig. 5 shows the XRD patterns of the synthesized products formed at various conditions. As the microwave heating on the crystallization process was conducted, zeolite 4A crystals were not formed whereas sodalites were crystallized. Microwave irradiation seems to prevent the formation of zeolite 4A seeds through the combination of their sodalite because zeolite 4A nuclei are so unstable to be dissolved again through attacking by active water molecules in microwave irradiation sources. If there are many zeolite 4A nuclei and seeds in the solution at the microwave crystallization stage, the crystal growth may proceed under microwave irradiation because the growing zeolite 4A crystals are stable. This is the reason why the microwave heating exhibited little influence for zeolite formation in the crystallization stage [11]. But, both zeolite 4A and sodalites
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J.K. Kim, H.D. Lee / Journal of Industrial and Engineering Chemistry 15 (2009) 736–742
Fig. 5. XRD patterns of the product obtained at various crystallization conditions. (a) KZ-1, (b) KZ-2, (c) KZ-3 and (d) KZ-4.
Fig. 6. SEM morphology of the products obtained at various synthesis conditions. (1) KZ-1, (2) KZ-2, (3) KZ-3 and (4) KZ-4.
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Fig. 7. Crystallinity of the products obtained at various conventional heating times without the microwave heating.
Fig. 8. Crystallinity of the products obtained at various microwave heating times after the conventional heating of 1 h.
in products obtained at the conventional heating of 1 h without the microwave heating were coexisted. Sodalites contribute to the main crystalline phase, zeolite 4A with zeolite impurities included faujasite-Na (F, 12-0228:Na2Al2Si3.3O10.67H2O, 12-0246:Na2Al2Si2.4O8.86.7H2O), sodium silicate (SS, 19-1239:Na2SiO39H2O), silicon oxide (SO, 12-0708:SiO2) as shown in Fig. 5(c). It was shown that step change of both the conventional heating and the microwave heating on the crystallization of zeolite 4A be simultaneously conducted to enhance the crystallinity of zeolite 4A into more than 91% and selectively synthesize the zeolite 4A with high purity. The morphology of the products obtained under different synthesis conditions is shown in Fig. 6. Morphology of KZ-1, sodalite obtained without the conventional heating as shown in Fig. 6(a) was very irregular and its particle size was larger than zeolites 4A of cubic form. Morphologies of KZ-2 and KZ-3 synthesized at the conventional heating of 1 and 2 h without the microwave heating were presented as mixture of both the zeolite 4A with cubic form and the sodalite with rare crystallized form while zeolite 4A (KZ-4) obtained through the step change from the conventional heating to the microwave heating were in the form of cubes in Fig. 6(d). It was believed that the cubics morphology was due to the initial Si/Al concentration used in this study, rather than the step change of heating sources [13]. Particle size of zeolite 4A crystals obtained by the conventional heating for 1 h and then the microwave heating of 1 h was in the range of less than 0.4 mm in Fig. 6(d) while that of zeolite 4A synthesized with heating time for 2 h was more than 1 mm in Fig. 6(c). The particle size of zeolite 4A by the microwave heating in this study was very fine than most of the commercial zeolite 4A crystals in the range of 2–3 mm. It was well known that the microwave heating of aluminosilicate gel markedly influences the lowering of particle size [12,14]. Fig. 7 depicts the crystallinity of the products obtained at various conventional heating of 0–4 h without the microwave
heating. Zeolite 4A crystals at the conventional heating were easily formatted. As the heating times from 1 to 4 h was increased, crystallinity of zeolite 4A linearly increased. It showed that crystallinity of zeolite 4A obtained at heating time of 4 h was about less than 76%, and it was unsuitable for use as a commercial utilization. But, previous zeolite studies [1–6] reported that the conventional heating can synthesize the zeolite 4A from Si and Al mixtures which dissolved from fly ash. On the hand, crystallinity of sodalite was inversely proportional to that of zeolite 4A because sodalites were consumed as the medium (frame) materials of zeolite 4A crystals. Also, the products obtained at various heating times by the microwave irradiation after the conventional heating for 1 h were easily formed by the zeolite 4A crystals as shown in Fig. 8. Even with the microwave heating of 30 min compared to only conventional heating, the crystallinity of zeolite 4A was drastically increased, while the crystallinity of sodalite was constantly decreased with increasing the crystallinity of zeolite 4A. These data reveal that the microwave heating was essential for increasing the crystallization rates of a zeolite 4A in Si and Al mixture dissolved from fly ash, and in the presence of conventional heating, small zeolite 4A crystals were selectively synthesized. 3.3. Characteristic of the synthesized zeolite The physicochemical properties of products synthesized at various conditions are compared in Table 3. Zeolite 4A(KZ-4) synthesized at the microwave heating of 1 h after the conventional heating of 1 h has a crystallinity of 91%, CEC of 5.5 meq/g, BET surface area of 980 m2/g, pore size of 4.82 A˚, and pore volume of 0.28 cc/g. The relatively high BET surface area compared to commercial zeolite 4A may be due to the presence of high pore volume and small pore size. CEC have 5.7 meq/g for commercial zeolite 4A and 5.5 meq/g for the zeolite 4A crystallized by heating source change of microwave heating after conventional heating.
Table 3 Physical and chemical properties of synthesized products. Symbol
Crystallinity (%)
CEC (meq/g)
BET surface area (m2/g)
Pore size (A˚)
Pore volume (cc/g)
KZ-1 KZ-2 KZ-3 KZ-4 Commercial zeolite A
0 22 45 91 100
1.1 1.6 2.5 5.5 5.7
209 221 250 980 702
5.46 5.35 5.12 4.82 5.24
0.12 0.18 0.24 0.28 0.26
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Fig. 9. Removal efficiency of heavy metals from wastewaters with synthesized products from coal fly ash.
These results showed that crystallinity and CEC of zeolite 4A synthesized from fly ash compared to a commercial zeolite 4A was little low, but the characteristics of particle size of KZ-4 were excellent due to lowering of particle size by the microwave crystallization. As a result of zeolite synthesis, CEC, increased from 1.1 meq/g for the sodalite (KZ-1) to circa 5.5 meq/g for the zeolite product (KZ-4). The adsorption test results show that the removal efficiency of heavy metals such as Pb, Cd, and Zn by zeolite 4A (KZ4) with high crystallinity was more than 98% while sodalite materials (KZ-1) with low crystallinity was less than 42% in Fig. 9. A drastic removal of the heavy metals in 10 min was obtained by zeolite 4A (KZ-4) with high purity synthesized from fly ash. The test results showed that zeolite 4A crystallized by step change from the conventional heating to the microwave heating like a commercial zeolite 4A was suitable for the removal of heavy metals ions such as Pb, Cd, and Zn from wastewater. 4. Conclusion The results obtained in this study were summarized as follows. Effects of the microwave irradiation to increase the
crystallization rate of zeolite 4A and dissolution percentage of Si and Al from coal fly ash were demonstrated. However, limiting effect of microwave irradiation was identified as a lack of nuclei formation in the synthesis mixture whereas in addition to the conventional heating, nucleation of zeolite 4A seeds occurs even during the short time of less than 1 h. Therefore, the conventional heating was required to format the nuclei of zeolite 4A seeds and the microwave irradiation was used to increase the crystallization rate of zeolite 4A. Zeolite 4A(KZ-4) synthesized at the microwave heating of 1 h after the conventional heating of 1 h has a crystallinity of 91%, CEC of 5.5 meq/g, BET surface area of 980 m2/g, pore size of 4.82 A˚, and pore volume of 0.28 cc/g. The relatively high BET surface area compared to commercial zeolite 4A may be due to the presence of high pore volume and small pore size. The adsorption test results show that the removal efficiency of heavy metals such as Pb, Cd, and Zn by zeolite 4A (KZ-4) with high crystallinity synthesized from fly ash was more than 98% while sodalite materials (KZ-1) with low crystallinity was less than 42%. References [1] J.K. Kim, Synthesis technology of zeolite from coal fly ash using microwave heating, Ministry of Commerce, Industry and Energy, Electric Power Industry Developmental Fundation, 2006 R&D Report, R-2004-0-275, pp.123–198 (2006). [2] G.G. Hollman, G. Steenbruggen, M. Janssen-Jurkovicova, Fuel 78 (1999) 1225. [3] Y.H. Oh, J.H. Lee, D.H. Lee, J. Kor. Waste Eng. Soc. 17 (1) (2000) 36. [4] S.Y. Ju, I.J. Yeon, K.Y. Kim, J. Kor. Waste Eng. Soc. 16 (6) (1999) 632. [5] T.G. Ryu, J.C. Ryu, C.H. Choi, C.G. Kim, S.J. Yoo, H.S. Yang, Y.H. Kim, J. Ind. Eng. Chem. 12 (3) (2006) 401. [6] (a) S. Somiya, R. Roy, Bull. Mater. Sci. 23 (2000) 2363; (b) G. Talebi, M. Sohrabi, S.J. Rayaee, R.L. Keisyi, M. Huuhtanen, H. Imamverdizadeh, J. Ind. Eng. Chem. 14 (2008) 614. [7] J.C. Jansen, A. Arafat, A.K. Barakat, H. Bekkum, Molecular Sieves, Van Nostrand Reinhold, New York, 1992. [8] X. Querol, X.A. Alastuey, A. Lopez-Solar, F. Plana, Environ. Sci. Technol. 31 (1997) 2527. [9] D. Caputo, B.D. Gennaro, B. Liguori, F. Testa, L. Carotenuto, C. Piccolo, Mater. Chem. Phys. 22 (2000) 120. [10] P.K. Kolay, D.N. Singh, M.V.R. Murti, Fuel 80 (2001) 739. [11] P.M. Slangen, J.C. Jansen, J.C. Bekkum, Microporous Mater. 9 (1997) 259. [12] M.A. Scott, C.A. Kathleen, K.D. Prabir, Handbook of Science and Technology, Sasel, New York, 2003, pp. 36–38. [13] E.I. Basaldella, A. Kikot, J.C. Tara, Mater. Lett. 31 (1997) 83. [14] H.J. Koroglu, A. Sarioglan, M. Tatlier, A.E. Senatalar, O.T. Savasci, J. Cryst. Growth 241 (2002) 481.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of step change of heating source on synthesis of zeolite 4A from coal fly ash Jae Kwan Kim *, Hyun Dong Lee Power Generation Laboratory, Korea Electric Power Research Institute, KEPCO, Daejon 103-16, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 February 2009 Accepted 31 May 2009
Effects of step change of heating sources on the crystallization of zeolite 4A from coal fly ash by hydrothermal reaction were investigated with emphasis on the change in the crystallinity of the synthesized zeolite 4A. Most of the Si and Al components were effectively transformed into zeolite 4A by step change of the first conventional heating and then the second microwave heating of synthesis mixture dissolved from coal fly ash, and maximum crystallinity of zeolite 4A obtained was 91%. The first conventional heating also plays an important role in enhancing the nuclei formation that Si and Al in synthesis mixture reacted to form ring-like structures for combining sodalites, and further to small zeolite 4A seeds. The second microwave heating increases the crystallization rates from small zeolite 4A nuclei to more zeolite 4A crystals. The cation exchange capacity (CEC) of the zeolite 4A crystallized by step change of heating source from the conventional to the microwave was 5.5 meq/g compared to 5.7 meq/g for commercial zeolite 4A. Test results showed that removal efficiency of heavy metals by zeolite 4A synthesized from fly ash was more than 98% and similar to commercial zeolite 4A. ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Fly ash Zeolite 4A Microwave synthesis Conventional heating CEC
1. Introduction About 6.0 million tons of coal fly ash was produced in Korea last year, of which only 70% were utilized with remaining 30% being disposed as a waste. Due to the construction of new coal power plants, the fly ash production in Korea will increase for next 5 years and is expected to reach up to 8.3 million tons in 2013. This rapid increase of the fly ash production causes serious concerns over fly ash disposal amid growing awareness of environmental issues and depletion of landfill sites [1]. Many efforts have been made at finding alternative and meaningful applications for this waste. Since fly ash contains mainly amorphous aluminosilicate (glassy phase) and some crystalline minerals (quartz, mullite, hematite, etc.), it can be used as a raw material for the synthesis of zeolite-like materials. The glassy phase plays an important role in the zeolite formation because of the high solubility into alkaline solution. However, in many of these studies [2–5], the total conversion time was generally long (72 h or more), synthesis temperature was very high (90–225 8C), and a little SiO2 and Al2O3 contained in fly ash was utilized due to the low dissolution rate of conventional heating such as autoclave, heating mantle and oil (water) bath. Nowadays, microwave heating at hydrothermal process was effective for
* Corresponding author. E-mail address: [email protected] (J.K. Kim).
zeolite synthesis from silica–alumina gel [6,7]. The zeolite synthesis from coal fly ash by microwave heating also reported to be useful for shortening the reaction time [8]. Microwave was absorbed directly into water as a solvent, and enabled the rapid heating compared to a conventional heating. We have investigated the effect of microwave irradiation on the dissolution of Si and Al from coal fly ashes, and the crystallization of zeolite 4A from the dissolved solution [1]. It is recently found the unexpected phenomena that dissolution rate of silicon and aluminum from coal fly ash by microwave irradiation improved dramatically compared to conventional heating, but nuclei of zeolite 4A was not generated because Si and Al dissolved from fly ash less reactive and microwave have too energy to disturb the combination of among sodalites. Therefore, the conventional heating at initial step on crystallization of zeolite 4A was introduced to produce many small zeolite 4A nuclei and seeds from mixture solution. 2. Experimental 2.1. Coal fly ash sample A fly ash sample was collected from the Tean thermal power plant owned by the Korea Western Electric Power Corporation. Table 1 shows the chemical and mineralogical composition of fly ash sample. The content of metal components was determined by X-ray fluorescence analysis and the chemical composition was presented in the form of stable oxide. The contents of major
1226-086X/$ – see front matter ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2009.09.055
J.K. Kim, H.D. Lee / Journal of Industrial and Engineering Chemistry 15 (2009) 736–742
737
Table 1 Chemical and mineralogical composition of coal fly ash. Chemical composition (wt.%)
Mineralogical composition (wt.%) Crystal phase
Glass phase
SiO2
Al2O3
CaO
Fe2O3
Others
Quartz
Mullite
SiO2
Al2O3
54.5
26.3
6.0
3.2
10.0
30.6
16.8
19.1
14.2
Others: MgO, Na2O, TiO2, K2O, SO3, P2O5, etc.
crystalline phases such as quartz and mullite presented in the form SiO2 and 3Al2O32SiO2 were determined by a quantitative X-ray diffraction analysis. Commercial mullite and quartz powders were used to make their calibration curves. The contents of SiO2 and Al2O3 in amorphous glass phase, which included as aluminosilicate were calculated by subtracting the SiO2 and Al2O3 compositions of crystal phases from the bulk compositions. 2.2. Experimental apparatus and procedures Microwave equipment used in this study consisted of an industrial type micro-oven with a working frequency of 2.45 GHz (Korea High Frequency Inc., KMIC-2 KW) and an output power that can be varied from 0 W to a maximum of 2 kW. The microwave synthetic equipment was modified by the introduction of a thermocouple inside the cavity in order to monitor reaction temperature, and the water condenser to conduct atmospheric pressure experiments in Fig. 1. Atmospheric pressure has been chosen because atmospheric hydrothermal synthesis was the common method for manufacturing zeolite NaA on a commercial scale and allows an easy change of reaction parameters, like agitating. The agitation shaft was equipped with a SUS 316 blade and stirring rate was varied from 0 to 500 rpm. The impeller was also operated to reflect transparent microwave through Teflon lining Pyrex reactor, which has the volume of 2000 mL and the high alkali resistant materials. Appendix equipments were tuned to minimize microwave for reflecting from the reactor, the coupler for transmitting the signal of microwave strength to be reflected at reactor into both magnetron and power supply, the circulator to
prevent the damage of magnetron by microwave reflected from reactor, and the dummy load which eliminates microwave inducted through circulator. 2.3. Preparation of mixed solution A mixture of 250 g of fly ash and 1000 mL of 5 M NaOH solution (Yakuri Inc.) in a 2000-mL sealed teflon bottle was heated using a conventional insulating mantle [1] or a microwave reactor at 100 8C for 1–8 h. The solution was filtrated with filter paper lined funnels. The volume of filtrate obtained from the solution heated at 100 8C for 5 h was roughly 940 mL, the Si, Al and Na concentrations were 56.8, 0.75 and 134.2 mg/L, respectively. Next, the filtrate solution was mixed with 130 g of sodium aluminate (Daejung Chemistry Inc., Na2O 0.31 mg/mL, Al2O3 0.34 mg/mL), 132 g of NaOH pellets, and 790 mL of distilled water to adjust the molar ratio of Si:Al:Na:H2O as 1.0:1.0:2.5:100. The mixed solution was stirred with 250 rpm for 24 h at room temperature. 2.4. Synthesis of zeolite 4A In the synthesis of zeolite 4A, four kinds of experimental methods were compared as shown in Table 2. The first method (KZ-1) was carried out only by the microwave heating without the conventional heating as insulating mantle. That is, 1750 mL of the mixed solution prepared at above section was injected into the microwave reactor and was crystallized by a microwave irradiation under agitation of 250 rpm at 100 8C for 2 h. The second and third methods were conducted only by the conventional heating
Fig. 1. The apparatus of microwave reactor for dissolution and crystallization process.
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738 Table 2 Synthesis conditions of zeolite 4A. Symbol
KZ-1 KZ-2 KZ-3 KZ-4
Crystallization conditions First conventional heating
Second microwave heating
N 100 8C for 1 h 100 8C for 2 h 100 8C for 1 h
100 8C for 2 h N N 100 8C for 1 h
N, not conducted.
process without the microwave heating process as shown as KZ-2 and KZ-3 in Table 2. 1750 mL of the mixed solution was crystallized at the same condition for 1 and 2 h, respectively. The fourth method carried out both process of the conventional heating and the microwave heating. The mixed solution was hydrothermally heated in a conventional heating mantle at the temperature of 100 8C for 1 h and then the heated mixture was injected to the microwave reactor. Zeolite 4A was finally crystallized with an agitator under the microwave heating of 100 8C for 1 h. Microwave heating system for the extraction of Si and Al source from fly ash and for the crystallization of zeolite 4A from mixed solution were conducted in Fig. 1. After a given period of synthesis, the precipitate was extracted from the solution, and then washed with a deionized water until the pH reached around 10. The samples were kept in an oven and dried at 100 8C for 12 h. Hereafter, labeling of the synthesized products was denoted as KZ-1–KZ-4 in Table 2. 2.5. Analysis of physicochemical properties The physical properties of coal fly ash and synthesized products were measured as follows. The chemical composition was analyzed using an X-ray fluorescence analysis equipment
Fig. 2. The concentration of silicon and aluminum in solution eluted from fly ash by the microwave irradiation and the conventional heating.
(EMAX-3770, Horiba). The phases were characterized by X-ray diffraction (XRD) using a PhilipsPW 1830 diffractometer with CuKa radiation (1.5496 A˚). XRD was used to estimate the degree of crystallinity of zeolite 4A and sodalite employing a quantitative analysis based on the ‘‘peak summation procedure [9] by using the following equation: Sx 100 crystallinity ð%Þ=4A ¼ Sr where Sx is the sum of integral peak intensities for the sample and Sr is the sum of integral peak intensities for the reference zeolite 4A. Particle morphology was observed by scanning electron microscopy (SEM, JEOL 5200-2AE). The concentrations of Si, Al and Na dissolved from fly ash in alkali solution were determined by
Fig. 3. SEM morphology of fly ash and the residues of fly ash obtained in 5 M NaOH by the microwave irradiation and the conventional heating. (a) Original fly ash, (b) conventional heating, (c) conventional heating and (d) microwave heating.
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an inductively coupled plasma emission analysis equipment (ICPS1000III, Shimadzu). In addition, CEC of the zeolite products was measured by means of the calcium chloride method [10]. The BET surface area has been determined with the help of air permeability apparatus (ASAP2010, Micromertics). Batch experiments on the removal of heavy metal of the synthesized products were carried out in order to determine the adsorption isotherms for Cd2+, Pb2+ and Zn2+. Three single component solutions were prepared by adding each 1 g of synthesized zeolite in three 500 mL sealed polypropylene bottles containing 100 mL of 100 mg/L Cd2+, Pb2+ and Zn2+ solution, respectively. The pH of solutions was each adjusted to pH 7.0 to prevent agglomeration of heavy metal. Then, the solutions were set in water bath, agitated for 10 min and then filtrated with filter paper lined funnels. The concentration of heavy metals in filtered solutions was analyzed by an inductively coupled plasma emission analysis equipment (ICPS-1000III, Shimadzu). These batch experiments were carried out in duplicate to check the reproducibility of their results. 3. Results and discussion 3.1. Microwave dissolution of Si and Al from fly ash Fig. 2 shows that the dissolution percentage of Si and Al in the filtrate solution as a function of time under microwave irradiation or conventional heating at 100 8C for 1–8 h in 5 M NaOH. Dissolution percentage of Si at the conventional heating, constantly increased initially (2 h) and later remained constant. The dissolution percentage of Al reached the maximum and after then constantly decreased with heating time. Therefore, SEM morphology of residue as fly ash dissolved in 5 M NaOH for 5 h showed needle-like precipitate in Fig. 3(b). Zeolite P and sodalite formation at the surface of dissolving fly ash was ascertained in Fig. 4(a). Under the conventional heating in this stage, zeolite P (Na6Al6Si10O3212H2O) and sodalite (Na4Al3Si3O12(OH)) crystals formed from Si and Al was deposited on the surface of the fly ash particles in Figs. 3(c) and 4(a). Actually, the microwave irradiation slowly enhanced the dissolution rate of Si in the early stage (4 h) compared to the conventional heating, and after then rapidly increased into heating time of 5 h, but from 6 h, it slowly decreased with increasing heating times. Al dissolution was with similar trends to Si dissolution rate. Microwave irradiation in the middle stage, especially in the term of 5–6 h, retarded the zeolite formation of dissolved Si and Al. These results indicate that the microwave irradiation for Si and Al dissolved from fly ash in 5 M NaOH inhibited the nucleation to form zeolite P and sodalite on the surface of dissolving fly ash in Figs. 3 and 4 Figs. 3(d) and 4(b). In Fig. 3(a), the original fly ash had a smooth surface because the surface was covered by an aluminosilicate glass phase. After reaction with alkali (5 M NaOH) solution by the microwave irradiation, the surface on the fly ash generate crevice and became rough in Fig. 3(d). Fig. 4 shows the XRD main peak intensities of residues after dissolved in various NaOH concentrations at 100 8C for 5 h by the conventional (a) or microwave irradiation (b) heating. The diffraction angles (2u) of main peaks of zeolite P, sodalite, quartz, and mullite are 28.08, 14.08, 26.68, and 26.18, respectively. Fig. 4 shows that main peak intensities of the zeolite P at all NaOH solutions were larger in the conventional heating than the microwave irradiation. This tendency reversely consists with the concentration of Si and Al dissolved in 5 M NaOH for 5 h in Fig. 2. When NaOH concentrations ranged from 3.0 to 4.0 M, the zeolite P peaks showed the maximum in the microwave irradiation (b) but did not reach to the zeolite P intensities at the conventional heating (a). On the other hand, the sodalite peaks at NaOH concentration from 1.0 to 3.0 M by the conventional heating were
Fig. 4. XRD intensities of the residues of fly ash obtained at various NaOH concentrations by (a) conventional heating and (b) microwave irradiation.
not generated because Si and Al dissolved from fly ash are consumed to generate zeolite P. This is the reason for the formation of zeolite P even at the low concentration with less than 4 M NaOH. The present results suggest that heating method by the microwave irradiation in 5 M NaOH solution for 5 h was important for preventing the formation of zeolite P and sodalites deposited on the surface of dissolving fly ash during the dissolution process of Si and Al from coal fly ash. Therefore, for obtaining a maximum dissolution rate of Si and Al from fly ash, it was desirable to react at 5 M NaOH solution for 5 h in the microwave irradiation. 3.2. Effect of step change of heating source on zeolite 4A formation Conventional heating before the microwave crystallization of mixed solution has been introduced for the pre-nucleation and seed formation of zeolite. Fig. 5 shows the XRD patterns of the synthesized products formed at various conditions. As the microwave heating on the crystallization process was conducted, zeolite 4A crystals were not formed whereas sodalites were crystallized. Microwave irradiation seems to prevent the formation of zeolite 4A seeds through the combination of their sodalite because zeolite 4A nuclei are so unstable to be dissolved again through attacking by active water molecules in microwave irradiation sources. If there are many zeolite 4A nuclei and seeds in the solution at the microwave crystallization stage, the crystal growth may proceed under microwave irradiation because the growing zeolite 4A crystals are stable. This is the reason why the microwave heating exhibited little influence for zeolite formation in the crystallization stage [11]. But, both zeolite 4A and sodalites
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Fig. 5. XRD patterns of the product obtained at various crystallization conditions. (a) KZ-1, (b) KZ-2, (c) KZ-3 and (d) KZ-4.
Fig. 6. SEM morphology of the products obtained at various synthesis conditions. (1) KZ-1, (2) KZ-2, (3) KZ-3 and (4) KZ-4.
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Fig. 7. Crystallinity of the products obtained at various conventional heating times without the microwave heating.
Fig. 8. Crystallinity of the products obtained at various microwave heating times after the conventional heating of 1 h.
in products obtained at the conventional heating of 1 h without the microwave heating were coexisted. Sodalites contribute to the main crystalline phase, zeolite 4A with zeolite impurities included faujasite-Na (F, 12-0228:Na2Al2Si3.3O10.67H2O, 12-0246:Na2Al2Si2.4O8.86.7H2O), sodium silicate (SS, 19-1239:Na2SiO39H2O), silicon oxide (SO, 12-0708:SiO2) as shown in Fig. 5(c). It was shown that step change of both the conventional heating and the microwave heating on the crystallization of zeolite 4A be simultaneously conducted to enhance the crystallinity of zeolite 4A into more than 91% and selectively synthesize the zeolite 4A with high purity. The morphology of the products obtained under different synthesis conditions is shown in Fig. 6. Morphology of KZ-1, sodalite obtained without the conventional heating as shown in Fig. 6(a) was very irregular and its particle size was larger than zeolites 4A of cubic form. Morphologies of KZ-2 and KZ-3 synthesized at the conventional heating of 1 and 2 h without the microwave heating were presented as mixture of both the zeolite 4A with cubic form and the sodalite with rare crystallized form while zeolite 4A (KZ-4) obtained through the step change from the conventional heating to the microwave heating were in the form of cubes in Fig. 6(d). It was believed that the cubics morphology was due to the initial Si/Al concentration used in this study, rather than the step change of heating sources [13]. Particle size of zeolite 4A crystals obtained by the conventional heating for 1 h and then the microwave heating of 1 h was in the range of less than 0.4 mm in Fig. 6(d) while that of zeolite 4A synthesized with heating time for 2 h was more than 1 mm in Fig. 6(c). The particle size of zeolite 4A by the microwave heating in this study was very fine than most of the commercial zeolite 4A crystals in the range of 2–3 mm. It was well known that the microwave heating of aluminosilicate gel markedly influences the lowering of particle size [12,14]. Fig. 7 depicts the crystallinity of the products obtained at various conventional heating of 0–4 h without the microwave
heating. Zeolite 4A crystals at the conventional heating were easily formatted. As the heating times from 1 to 4 h was increased, crystallinity of zeolite 4A linearly increased. It showed that crystallinity of zeolite 4A obtained at heating time of 4 h was about less than 76%, and it was unsuitable for use as a commercial utilization. But, previous zeolite studies [1–6] reported that the conventional heating can synthesize the zeolite 4A from Si and Al mixtures which dissolved from fly ash. On the hand, crystallinity of sodalite was inversely proportional to that of zeolite 4A because sodalites were consumed as the medium (frame) materials of zeolite 4A crystals. Also, the products obtained at various heating times by the microwave irradiation after the conventional heating for 1 h were easily formed by the zeolite 4A crystals as shown in Fig. 8. Even with the microwave heating of 30 min compared to only conventional heating, the crystallinity of zeolite 4A was drastically increased, while the crystallinity of sodalite was constantly decreased with increasing the crystallinity of zeolite 4A. These data reveal that the microwave heating was essential for increasing the crystallization rates of a zeolite 4A in Si and Al mixture dissolved from fly ash, and in the presence of conventional heating, small zeolite 4A crystals were selectively synthesized. 3.3. Characteristic of the synthesized zeolite The physicochemical properties of products synthesized at various conditions are compared in Table 3. Zeolite 4A(KZ-4) synthesized at the microwave heating of 1 h after the conventional heating of 1 h has a crystallinity of 91%, CEC of 5.5 meq/g, BET surface area of 980 m2/g, pore size of 4.82 A˚, and pore volume of 0.28 cc/g. The relatively high BET surface area compared to commercial zeolite 4A may be due to the presence of high pore volume and small pore size. CEC have 5.7 meq/g for commercial zeolite 4A and 5.5 meq/g for the zeolite 4A crystallized by heating source change of microwave heating after conventional heating.
Table 3 Physical and chemical properties of synthesized products. Symbol
Crystallinity (%)
CEC (meq/g)
BET surface area (m2/g)
Pore size (A˚)
Pore volume (cc/g)
KZ-1 KZ-2 KZ-3 KZ-4 Commercial zeolite A
0 22 45 91 100
1.1 1.6 2.5 5.5 5.7
209 221 250 980 702
5.46 5.35 5.12 4.82 5.24
0.12 0.18 0.24 0.28 0.26
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Fig. 9. Removal efficiency of heavy metals from wastewaters with synthesized products from coal fly ash.
These results showed that crystallinity and CEC of zeolite 4A synthesized from fly ash compared to a commercial zeolite 4A was little low, but the characteristics of particle size of KZ-4 were excellent due to lowering of particle size by the microwave crystallization. As a result of zeolite synthesis, CEC, increased from 1.1 meq/g for the sodalite (KZ-1) to circa 5.5 meq/g for the zeolite product (KZ-4). The adsorption test results show that the removal efficiency of heavy metals such as Pb, Cd, and Zn by zeolite 4A (KZ4) with high crystallinity was more than 98% while sodalite materials (KZ-1) with low crystallinity was less than 42% in Fig. 9. A drastic removal of the heavy metals in 10 min was obtained by zeolite 4A (KZ-4) with high purity synthesized from fly ash. The test results showed that zeolite 4A crystallized by step change from the conventional heating to the microwave heating like a commercial zeolite 4A was suitable for the removal of heavy metals ions such as Pb, Cd, and Zn from wastewater. 4. Conclusion The results obtained in this study were summarized as follows. Effects of the microwave irradiation to increase the
crystallization rate of zeolite 4A and dissolution percentage of Si and Al from coal fly ash were demonstrated. However, limiting effect of microwave irradiation was identified as a lack of nuclei formation in the synthesis mixture whereas in addition to the conventional heating, nucleation of zeolite 4A seeds occurs even during the short time of less than 1 h. Therefore, the conventional heating was required to format the nuclei of zeolite 4A seeds and the microwave irradiation was used to increase the crystallization rate of zeolite 4A. Zeolite 4A(KZ-4) synthesized at the microwave heating of 1 h after the conventional heating of 1 h has a crystallinity of 91%, CEC of 5.5 meq/g, BET surface area of 980 m2/g, pore size of 4.82 A˚, and pore volume of 0.28 cc/g. The relatively high BET surface area compared to commercial zeolite 4A may be due to the presence of high pore volume and small pore size. The adsorption test results show that the removal efficiency of heavy metals such as Pb, Cd, and Zn by zeolite 4A (KZ-4) with high crystallinity synthesized from fly ash was more than 98% while sodalite materials (KZ-1) with low crystallinity was less than 42%. References [1] J.K. Kim, Synthesis technology of zeolite from coal fly ash using microwave heating, Ministry of Commerce, Industry and Energy, Electric Power Industry Developmental Fundation, 2006 R&D Report, R-2004-0-275, pp.123–198 (2006). [2] G.G. Hollman, G. Steenbruggen, M. Janssen-Jurkovicova, Fuel 78 (1999) 1225. [3] Y.H. Oh, J.H. Lee, D.H. Lee, J. Kor. Waste Eng. Soc. 17 (1) (2000) 36. [4] S.Y. Ju, I.J. Yeon, K.Y. Kim, J. Kor. Waste Eng. Soc. 16 (6) (1999) 632. [5] T.G. Ryu, J.C. Ryu, C.H. Choi, C.G. Kim, S.J. Yoo, H.S. Yang, Y.H. Kim, J. Ind. Eng. Chem. 12 (3) (2006) 401. [6] (a) S. Somiya, R. Roy, Bull. Mater. Sci. 23 (2000) 2363; (b) G. Talebi, M. Sohrabi, S.J. Rayaee, R.L. Keisyi, M. Huuhtanen, H. Imamverdizadeh, J. Ind. Eng. Chem. 14 (2008) 614. [7] J.C. Jansen, A. Arafat, A.K. Barakat, H. Bekkum, Molecular Sieves, Van Nostrand Reinhold, New York, 1992. [8] X. Querol, X.A. Alastuey, A. Lopez-Solar, F. Plana, Environ. Sci. Technol. 31 (1997) 2527. [9] D. Caputo, B.D. Gennaro, B. Liguori, F. Testa, L. Carotenuto, C. Piccolo, Mater. Chem. Phys. 22 (2000) 120. [10] P.K. Kolay, D.N. Singh, M.V.R. Murti, Fuel 80 (2001) 739. [11] P.M. Slangen, J.C. Jansen, J.C. Bekkum, Microporous Mater. 9 (1997) 259. [12] M.A. Scott, C.A. Kathleen, K.D. Prabir, Handbook of Science and Technology, Sasel, New York, 2003, pp. 36–38. [13] E.I. Basaldella, A. Kikot, J.C. Tara, Mater. Lett. 31 (1997) 83. [14] H.J. Koroglu, A. Sarioglan, M. Tatlier, A.E. Senatalar, O.T. Savasci, J. Cryst. Growth 241 (2002) 481.