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Synthesis of zeolite NaA from sugarcane bagasse ash
Materials Letters 108 (2013) 243–246
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of zeolite NaA from sugarcane bagasse ash Murilo Pereira Moisés a, Cleiser Thiago Pereira da Silva a, Joziane Gimenes Meneguin b, Emerson Marcelo Girotto a, Eduardo Radovanovic a,n a b
Department of Chemistry, State University of Maringá, Av. Colombo 5790, CEP 87020-900 Maringá, Paraná, Brazil Department of Chemical Engineering, State University of Maringá, Av. Colombo 5790, CEP 87020-900 Maringá, Paraná, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 29 January 2013 Accepted 23 June 2013 Available online 28 June 2013
Zeolite NaA was synthesized using sugarcane bagasse as silica source under hydrothermal condition at 80 1C for 72–160 h. The silicon was extracted by alkaline fusion for 40 min, at 550 1C with an alkali:ash ratio of 1. Zeolite A was obtained without phase contamination. The ash and synthesized zeolite NaA were analyzed by granulometry, XRD, SEM, FTIR, XRF and TG/DTA. In XRD results, all signals were perfectly indexed to zeolite A. The vibration bands at ca. 557 cm−1 suggested the presence of double-fourring (D4R) zeolite A structure. The synthesized material has a potential application as a catalyst, as adsorbent, and as an ion exchanger. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zeolite A Alkaline fusion Sugarcane bagasse ash Crystal growth Phase transformation
1. Introduction Sugarcane bagasse is a hazardous solid waste generated in large amounts in sugar mills. Combustion of sugarcane bagasse in boilers, used for steam and electricity generation, produces a great amount of another solid waste, denominated sugarcane bagasse ash (SCBA) [1]. Therefore, the development of new procedures for its productive reuse is relevant. Actually, the accumulation of this waste, which is quartz-abundant, can be avoided if employed as a silicon source. By means of an alkali fusion extraction method, quartz particles can be dissolved and used as silicon source for synthesizing silica-based materials such as zeolites. Most researchers have used coal fly ash as a low cost silicon and aluminum source to produce zeolites. Different types of zeolites such as X [2,3], ZSM-5 [4], hydroxysodalite[5], Na–P1 [6–9] and zeolite A [10,11] were synthesized by applying many synthesis methods. Therefore, the application of this process using SCBA is an important procedure to increase the value of SCBA and to avoid environmental pollution caused by this waste. Due to the thermal stability of quartz crystal, the alkaline fusion is indicated for the extraction of silicon from SCBA, making it available for zeolite synthesis [12]. The purpose of this study was to synthesize zeolite from SCBA in two steps: silicon extraction method by alkaline fusion and hydrothermal treatment for the zeolite A crystallization process.
n
Corresponding author. Tel.: +55 44 3011 3653; fax: +55 44 3011 4125. E-mail addresses: [email protected], [email protected] (E. Radovanovic). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.06.086
This research demonstrates the potential of SCBA extract to be used as a reliable silica source for preparing pure zeolite A.
2. Experimental procedure Sugarcane bagasse ash (SCBA) was collected from sugarcane industry located in the region of Maringá City, Paraná, Brazil. This quartz material was placed in a horizontal furnace and heated in air atmosphere at 20 1C min−1 from room temperature to 600 1C and kept for 4 h (SCBA600). For the zeolite synthesis, 30 g of SCBA600 was homogeneously mixed with NaOH in a 1.5 ratio (45 g of NaOH). Then, the mixture was heated in a nickel crucible in air atmosphere at 550 1C for 40 min. The resultant fused mixture was dissolved in 1.0 L of distilled water. Soon after, an amount of 1.0 L of sodium aluminate solution 0.48 mol L−1 (39 g of sodium aluminate-Sigma-Aldrich-in 1.0 L of distilled water) was added to the silicate solution. The molar ratio of this solution was 2SiO2:1Al2O3:4Na2O:480H2O. The mixture (2.0 L of result solution) was transferred to ten polypropylene reactors (0.2 L each) and kept at 80 1C for different crystallization periods of time (1, 3, 6, 16, 25, 44, 72, 96, 136, and 160 h). Then, the solid was separated by filtration, washed with distilled water and dried overnight at 100 1C. Characterization: The raw SCBA and SCBA600 were characterized by X-ray fluorescence spectroscopy (XRF) (Rigakum model ZSX mini II, palladium tube) and granulometric analysis. The thermal behavior was observed through TGA-DTA (Netzsch, model STA 409 PG/4/G) in air from room temperature to 900 1C at a fixed heating rate of 10 1C.min−1. The synthesized zeolites were
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Fig. 1. Granulometric analysis and scanning electron micrograph of SCBA (1A), TGA curves and first derivative of the thermal analysis of sample SCBA (1B), and XRD analysis of the SCBA and SCBA600 samples (1C).
Fig. 2. XRD diffraction patterns of zeolites synthesized in different periods of time in hydrothermal process.
Fig. 3. FTIR spectra showing the evolution of zeolite synthesis for different periods of time in hydrothermal process.
M.P. Moisés et al. / Materials Letters 108 (2013) 243–246
characterized by the Fourier transform infrared spectrometry (Bomem-Michelson MB-100 with a resolution of 4 cm−1 using a KBr disc method), XRD analysis (Shimadzu, model XRD-6000 X-ray operated at 40 kV and 40 mA, with Cu Kα as the radiation source, diffraction angle – 2θ – in the range 101–601), and scanning electron microscopy (SEM) (Shimadzu SSX-550 Superscan).
3. Results and discussion Fig. 1A shows the scanning electron micrograph and granulometric analysis of SCBA. SCBA is composed of 10 mm–1 mm particles, with a mean value of ca. 200 mm. Fig. 1B displays the thermogravimetric and differential thermal analyses (TGA/DTA). The SCBA thermogram presented a weight loss of 16% which was assigned to water and the exothermic peak at ca. 500 1C (DTA) evinces the existence of organic residues. Thus, the treatment up to 600 1C removes all undesired organic compounds (OC) to the zeolite synthesis. Fig. 1C shows the XRD patterns of SCBA and SCBA600, indicating a higher crystallinity of SBCA600 caused by the calcination process, due to decreasing in the amount of OC and volatile materials, leading to the crystallization of amorphous phases in the raw material. These results are in good agreement
245
with the thermal analysis. The crystalline phase presented by the ash is quartz (standard pattern number 02-0458 – ICDD database). The chemical composition (weight percent) of SCBA found by XRF was 86.2% SiO2, 2.8% Al2O3, 1.6% P2O5, 2.4% K2O, 1.9% TiO2, 1.5% CaO, 2.9% Fe2O3 and 0.7% of trace elements, and of SCBA600 was 92.0% SiO2, 1.6% Al2O3, 1.1% P2O5, 1.0% K2O, 0.6% TiO2, 0.8% CaO, 1.5% Fe2O3, and 1.4% of trace elements. This composition indicates the potential application of this waste in zeolitization process. Fig. 2 displays the X-ray diffraction patterns of zeolite synthesis for each period of time. At 25 h zeolite A was detected and this transformation coincided with Ostwald's rule of successive transformations [13]. Following the crystallization time, an increasing zeolitization is observed. After 72 h all diffraction peaks were perfectly indexed to zeolite A belonging to cubic crystal system and Pm-3m space group (standard pattern number 71–0784 – ICDD database and standard pattern of International Zeolite Association – IZA). The peaks in 171 and 381 found before 44 h could appear from the impurities of sugarcane bagasse ash. Fig. 3 presents the FTIR spectra of the zeolites as a function of hydrothermal process period of time. The existence of zeolite A is suggested by peaks in the lattice region of 1200–400 cm−1. The band at 870 cm−1, which can be assigned to T–OH bond (T ¼Si or Al), was found in the amorphous material precursor of zeolite A.
Fig. 4. SEM micrograph of zeolites synthesized in different periods of time. Inset of Z-160 h (5 μm): EDS spectrum of Z-160 h.
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Decottignies [13] noticed that this band disappears as the sample becomes more crystalline. The broad band centered at 1001 cm−1 is assigned to the internal vibration of (Si, Al)–O asymmetric stretching (tetrahedrally coordinated). This band is slightly shifted and become sharper as the amorphous material was transformed to crystalline zeolite A. The signal centered at 671 cm−1 is assigned to internal vibration of (Si, Al)–O symmetric stretching, and the signal at 467 cm−1 is assigned to the internal vibration of (Si, Al)–O bending. The OH band, related to deformational vibrations of adsorbed water molecules in zeolite channels also appears at ca. 1655 cm−1 [12]. These results show that FTIR is suitable for monitoring crystallinity of zeolite A during the hydrothermal synthesis. The peak intensity changes of the characteristic signals corresponding to the external double-four-ring (D4R) vibration for the zeolite framework at 557 cm−1 can be used to study the progress of zeolite crystallization [14]. After 44 h both signals level up and then remain constant. These results are in good agreement with the XRD and microscopy results. Fig. 4 shows the SEM images of the zeolites synthesized in different periods of time. A well-defined morphology is observed from a hydrothermal synthesis of 72 h (Z-72 h), which corroborates the XRD and FTIR results. For the sample Z-160 the zeolite showed a particle size of 2.44 70.28 mm and a multifaceted shape. The chemical composition analysis through EDS indicated a silicon:aluminum ratio of 1:1, which corresponds to the zeolite A. 4. Conclusions This work showed that the Brazilian sugarcane bagasse ash can be successfully used as raw material for the hydrothermal synthesis of zeolite A, which have potential applications in adsorption, catalysis, and ion exchangers. This research contributes to materials and environmental science, suggesting the recycling of a contaminant solid waste generated in large amount in sugar mills.
Acknowledgements The authors thank both the COMCAP – UEM for SEM analyses and Laboratório de Adsorção e Troca Iônica - UEM for XRD analyses realized in the study. References [1] Balakrishnan M, Batra VS. Valorization of solid waste in sugar factories with possible applications in India: A review. J Environ Manage 2011;92 (11):2886–91. [2] Shigemoto N, Hayashi H, Miyaura K. Selective formation of Na–X zeolite from coal fly-ash by fusion with sodium-hydroxide prior to hydrothermal reaction. J Mater Sci 1993;28(17):4781–6. [3] Jha VK, et al. Zeolite formation from coal fly ash and heavy metal ion removal characteristics of thus-obtained Zeolite X in multi-metal systems. J Environ Manage 2009;90(8):2507–14. [4] Chareonpanich M, et al. Synthesis of ZSM-5 zeolite from lignite fly ash and rice husk ash. Fuel Process Technol 2004;85(15):1623–34. [5] Naskar MK, Kundu D, Chatterjee M. Coral-like hydroxy sodalite particles from rice husk ash as silica source. Mater Lett 2011;65(23–24):3508–10. [6] Inada M, et al. Synthesis of zeolite from coal fly ashes with different silica– alumina composition. Fuel 2005;84(2–3):299–304. [7] Hollman GG, Steenbruggen G, Janssen-Jurkovicova M. A two-step process for the synthesis of zeolites from coal fly ash. Fuel 1999;78(10):1225–30. [8] Querol X, et al. Synthesis of zeolites from fly ash at pilot plant scale. Examples of potential applications. Fuel 2001;80(6):857–65. [9] Shih WH, Chang HL. Conversion of fly ash into zeolites for ion-exchange applications. Mater Lett 1996;28(4–6):263–8. [10] Wang C-F, et al. Influence of NaOH concentrations on synthesis of pure-form zeolite A from fly ash using two-stage method. J Hazardous Mater 2008;155 (1–2):58–64. [11] Tanaka H, Fujii A. Effect of stirring on the dissolution of coal fly ash and synthesis of pure-form Na–A and -X zeolites by two-step process. Adv Powder Technol 2009;20(5):473–9. [12] Yao ZT, et al. Synthesis of zeolite Li-ABW from fly ash by fusion method. J Hazardous Mater 2009;170(2–3):639–44. [13] Decottignies M, Phalippou J, Zarzycki J. Synthesis of glasses by hot-pressing of gels. J Mater Sci 1978;13(12):2605–18. [14] Loiola AR, et al. Structural analysis of zeolite NaA synthesized by a costeffective hydrothermal method using kaolin and its use as water softener. J Colloid Interface Sci 2012;367(1):35–9.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of zeolite NaA from sugarcane bagasse ash Murilo Pereira Moisés a, Cleiser Thiago Pereira da Silva a, Joziane Gimenes Meneguin b, Emerson Marcelo Girotto a, Eduardo Radovanovic a,n a b
Department of Chemistry, State University of Maringá, Av. Colombo 5790, CEP 87020-900 Maringá, Paraná, Brazil Department of Chemical Engineering, State University of Maringá, Av. Colombo 5790, CEP 87020-900 Maringá, Paraná, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 29 January 2013 Accepted 23 June 2013 Available online 28 June 2013
Zeolite NaA was synthesized using sugarcane bagasse as silica source under hydrothermal condition at 80 1C for 72–160 h. The silicon was extracted by alkaline fusion for 40 min, at 550 1C with an alkali:ash ratio of 1. Zeolite A was obtained without phase contamination. The ash and synthesized zeolite NaA were analyzed by granulometry, XRD, SEM, FTIR, XRF and TG/DTA. In XRD results, all signals were perfectly indexed to zeolite A. The vibration bands at ca. 557 cm−1 suggested the presence of double-fourring (D4R) zeolite A structure. The synthesized material has a potential application as a catalyst, as adsorbent, and as an ion exchanger. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zeolite A Alkaline fusion Sugarcane bagasse ash Crystal growth Phase transformation
1. Introduction Sugarcane bagasse is a hazardous solid waste generated in large amounts in sugar mills. Combustion of sugarcane bagasse in boilers, used for steam and electricity generation, produces a great amount of another solid waste, denominated sugarcane bagasse ash (SCBA) [1]. Therefore, the development of new procedures for its productive reuse is relevant. Actually, the accumulation of this waste, which is quartz-abundant, can be avoided if employed as a silicon source. By means of an alkali fusion extraction method, quartz particles can be dissolved and used as silicon source for synthesizing silica-based materials such as zeolites. Most researchers have used coal fly ash as a low cost silicon and aluminum source to produce zeolites. Different types of zeolites such as X [2,3], ZSM-5 [4], hydroxysodalite[5], Na–P1 [6–9] and zeolite A [10,11] were synthesized by applying many synthesis methods. Therefore, the application of this process using SCBA is an important procedure to increase the value of SCBA and to avoid environmental pollution caused by this waste. Due to the thermal stability of quartz crystal, the alkaline fusion is indicated for the extraction of silicon from SCBA, making it available for zeolite synthesis [12]. The purpose of this study was to synthesize zeolite from SCBA in two steps: silicon extraction method by alkaline fusion and hydrothermal treatment for the zeolite A crystallization process.
n
Corresponding author. Tel.: +55 44 3011 3653; fax: +55 44 3011 4125. E-mail addresses: [email protected], [email protected] (E. Radovanovic). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.06.086
This research demonstrates the potential of SCBA extract to be used as a reliable silica source for preparing pure zeolite A.
2. Experimental procedure Sugarcane bagasse ash (SCBA) was collected from sugarcane industry located in the region of Maringá City, Paraná, Brazil. This quartz material was placed in a horizontal furnace and heated in air atmosphere at 20 1C min−1 from room temperature to 600 1C and kept for 4 h (SCBA600). For the zeolite synthesis, 30 g of SCBA600 was homogeneously mixed with NaOH in a 1.5 ratio (45 g of NaOH). Then, the mixture was heated in a nickel crucible in air atmosphere at 550 1C for 40 min. The resultant fused mixture was dissolved in 1.0 L of distilled water. Soon after, an amount of 1.0 L of sodium aluminate solution 0.48 mol L−1 (39 g of sodium aluminate-Sigma-Aldrich-in 1.0 L of distilled water) was added to the silicate solution. The molar ratio of this solution was 2SiO2:1Al2O3:4Na2O:480H2O. The mixture (2.0 L of result solution) was transferred to ten polypropylene reactors (0.2 L each) and kept at 80 1C for different crystallization periods of time (1, 3, 6, 16, 25, 44, 72, 96, 136, and 160 h). Then, the solid was separated by filtration, washed with distilled water and dried overnight at 100 1C. Characterization: The raw SCBA and SCBA600 were characterized by X-ray fluorescence spectroscopy (XRF) (Rigakum model ZSX mini II, palladium tube) and granulometric analysis. The thermal behavior was observed through TGA-DTA (Netzsch, model STA 409 PG/4/G) in air from room temperature to 900 1C at a fixed heating rate of 10 1C.min−1. The synthesized zeolites were
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Fig. 1. Granulometric analysis and scanning electron micrograph of SCBA (1A), TGA curves and first derivative of the thermal analysis of sample SCBA (1B), and XRD analysis of the SCBA and SCBA600 samples (1C).
Fig. 2. XRD diffraction patterns of zeolites synthesized in different periods of time in hydrothermal process.
Fig. 3. FTIR spectra showing the evolution of zeolite synthesis for different periods of time in hydrothermal process.
M.P. Moisés et al. / Materials Letters 108 (2013) 243–246
characterized by the Fourier transform infrared spectrometry (Bomem-Michelson MB-100 with a resolution of 4 cm−1 using a KBr disc method), XRD analysis (Shimadzu, model XRD-6000 X-ray operated at 40 kV and 40 mA, with Cu Kα as the radiation source, diffraction angle – 2θ – in the range 101–601), and scanning electron microscopy (SEM) (Shimadzu SSX-550 Superscan).
3. Results and discussion Fig. 1A shows the scanning electron micrograph and granulometric analysis of SCBA. SCBA is composed of 10 mm–1 mm particles, with a mean value of ca. 200 mm. Fig. 1B displays the thermogravimetric and differential thermal analyses (TGA/DTA). The SCBA thermogram presented a weight loss of 16% which was assigned to water and the exothermic peak at ca. 500 1C (DTA) evinces the existence of organic residues. Thus, the treatment up to 600 1C removes all undesired organic compounds (OC) to the zeolite synthesis. Fig. 1C shows the XRD patterns of SCBA and SCBA600, indicating a higher crystallinity of SBCA600 caused by the calcination process, due to decreasing in the amount of OC and volatile materials, leading to the crystallization of amorphous phases in the raw material. These results are in good agreement
245
with the thermal analysis. The crystalline phase presented by the ash is quartz (standard pattern number 02-0458 – ICDD database). The chemical composition (weight percent) of SCBA found by XRF was 86.2% SiO2, 2.8% Al2O3, 1.6% P2O5, 2.4% K2O, 1.9% TiO2, 1.5% CaO, 2.9% Fe2O3 and 0.7% of trace elements, and of SCBA600 was 92.0% SiO2, 1.6% Al2O3, 1.1% P2O5, 1.0% K2O, 0.6% TiO2, 0.8% CaO, 1.5% Fe2O3, and 1.4% of trace elements. This composition indicates the potential application of this waste in zeolitization process. Fig. 2 displays the X-ray diffraction patterns of zeolite synthesis for each period of time. At 25 h zeolite A was detected and this transformation coincided with Ostwald's rule of successive transformations [13]. Following the crystallization time, an increasing zeolitization is observed. After 72 h all diffraction peaks were perfectly indexed to zeolite A belonging to cubic crystal system and Pm-3m space group (standard pattern number 71–0784 – ICDD database and standard pattern of International Zeolite Association – IZA). The peaks in 171 and 381 found before 44 h could appear from the impurities of sugarcane bagasse ash. Fig. 3 presents the FTIR spectra of the zeolites as a function of hydrothermal process period of time. The existence of zeolite A is suggested by peaks in the lattice region of 1200–400 cm−1. The band at 870 cm−1, which can be assigned to T–OH bond (T ¼Si or Al), was found in the amorphous material precursor of zeolite A.
Fig. 4. SEM micrograph of zeolites synthesized in different periods of time. Inset of Z-160 h (5 μm): EDS spectrum of Z-160 h.
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M.P. Moisés et al. / Materials Letters 108 (2013) 243–246
Decottignies [13] noticed that this band disappears as the sample becomes more crystalline. The broad band centered at 1001 cm−1 is assigned to the internal vibration of (Si, Al)–O asymmetric stretching (tetrahedrally coordinated). This band is slightly shifted and become sharper as the amorphous material was transformed to crystalline zeolite A. The signal centered at 671 cm−1 is assigned to internal vibration of (Si, Al)–O symmetric stretching, and the signal at 467 cm−1 is assigned to the internal vibration of (Si, Al)–O bending. The OH band, related to deformational vibrations of adsorbed water molecules in zeolite channels also appears at ca. 1655 cm−1 [12]. These results show that FTIR is suitable for monitoring crystallinity of zeolite A during the hydrothermal synthesis. The peak intensity changes of the characteristic signals corresponding to the external double-four-ring (D4R) vibration for the zeolite framework at 557 cm−1 can be used to study the progress of zeolite crystallization [14]. After 44 h both signals level up and then remain constant. These results are in good agreement with the XRD and microscopy results. Fig. 4 shows the SEM images of the zeolites synthesized in different periods of time. A well-defined morphology is observed from a hydrothermal synthesis of 72 h (Z-72 h), which corroborates the XRD and FTIR results. For the sample Z-160 the zeolite showed a particle size of 2.44 70.28 mm and a multifaceted shape. The chemical composition analysis through EDS indicated a silicon:aluminum ratio of 1:1, which corresponds to the zeolite A. 4. Conclusions This work showed that the Brazilian sugarcane bagasse ash can be successfully used as raw material for the hydrothermal synthesis of zeolite A, which have potential applications in adsorption, catalysis, and ion exchangers. This research contributes to materials and environmental science, suggesting the recycling of a contaminant solid waste generated in large amount in sugar mills.
Acknowledgements The authors thank both the COMCAP – UEM for SEM analyses and Laboratório de Adsorção e Troca Iônica - UEM for XRD analyses realized in the study. References [1] Balakrishnan M, Batra VS. Valorization of solid waste in sugar factories with possible applications in India: A review. J Environ Manage 2011;92 (11):2886–91. [2] Shigemoto N, Hayashi H, Miyaura K. Selective formation of Na–X zeolite from coal fly-ash by fusion with sodium-hydroxide prior to hydrothermal reaction. J Mater Sci 1993;28(17):4781–6. [3] Jha VK, et al. Zeolite formation from coal fly ash and heavy metal ion removal characteristics of thus-obtained Zeolite X in multi-metal systems. J Environ Manage 2009;90(8):2507–14. [4] Chareonpanich M, et al. Synthesis of ZSM-5 zeolite from lignite fly ash and rice husk ash. Fuel Process Technol 2004;85(15):1623–34. [5] Naskar MK, Kundu D, Chatterjee M. Coral-like hydroxy sodalite particles from rice husk ash as silica source. Mater Lett 2011;65(23–24):3508–10. [6] Inada M, et al. Synthesis of zeolite from coal fly ashes with different silica– alumina composition. Fuel 2005;84(2–3):299–304. [7] Hollman GG, Steenbruggen G, Janssen-Jurkovicova M. A two-step process for the synthesis of zeolites from coal fly ash. Fuel 1999;78(10):1225–30. [8] Querol X, et al. Synthesis of zeolites from fly ash at pilot plant scale. Examples of potential applications. Fuel 2001;80(6):857–65. [9] Shih WH, Chang HL. Conversion of fly ash into zeolites for ion-exchange applications. Mater Lett 1996;28(4–6):263–8. [10] Wang C-F, et al. Influence of NaOH concentrations on synthesis of pure-form zeolite A from fly ash using two-stage method. J Hazardous Mater 2008;155 (1–2):58–64. [11] Tanaka H, Fujii A. Effect of stirring on the dissolution of coal fly ash and synthesis of pure-form Na–A and -X zeolites by two-step process. Adv Powder Technol 2009;20(5):473–9. [12] Yao ZT, et al. Synthesis of zeolite Li-ABW from fly ash by fusion method. J Hazardous Mater 2009;170(2–3):639–44. [13] Decottignies M, Phalippou J, Zarzycki J. Synthesis of glasses by hot-pressing of gels. J Mater Sci 1978;13(12):2605–18. [14] Loiola AR, et al. Structural analysis of zeolite NaA synthesized by a costeffective hydrothermal method using kaolin and its use as water softener. J Colloid Interface Sci 2012;367(1):35–9.