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Highly crystalline faujasitic zeolites from flyash
Journal of Hazardous Materials B77 Ž2000. 123–131 www.elsevier.nlrlocaterjhazmat
Highly crystalline faujasitic zeolites from flyash S. Rayalu ) , S.U. Meshram, M.Z. Hasan National EnÕironmental Engineering Research Institute, Nehru Marg, Nagpur 440 020, India Received 5 November 1999; received in revised form 13 March 2000; accepted 15 March 2000
Abstract The process for the synthesis of flyash-based zeolites ŽFAZs. are presented, which basically includes alkaline treatment of flyash by a fusion method, followed by hydrothermal crystallization. Zeolite-Y has been identified, and conditions have been optimized for their synthesis by the fusion method. Optimal conditions for synthesis of Zeolite-Y are a NaOHrflyash ratio of 1.2:1, fusion temperature between 5008C and 6008C, crystallization time of 8–10 h and crystallization temperature between 1008C and 1108C. The cation exchange capacity ŽCEC. of FAZ-Y ranges between 400 and 450 meqr100 g. The surface area of FAZs Ž500–600 m2rg. compare well with the commercial zeolites procured from Mobil Oil. Morphological characterization of FAZ using scanning electron microscopy ŽSEM. reveals cubic structure, and XRD data reveal unit cells to be cubic system. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Flyash-based zeolite; Synthesis; Process standardisation; Waste utilization
1. Introduction In India, flyash is being generated at the rate of about 60 million tonnes per annum ŽMTPA. from about 75 thermal power plants and is expected to increase to 100 MTPA by the turn of the century. There are serious environmental health hazards associated with flyash. In addition, the land requirement envisaged for disposal of flyash is about 50,000 acre, with an annual expenditure of about Rs 500 million for transportation. These problems clearly spell out the fact that utilization of flyash is absolutely essential.
)
Corresponding author. Fax: q91-712-230673. E-mail address: [email protected] ŽS. Rayalu..
0304-3894r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 9 4 Ž 0 0 . 0 0 2 1 2 - 0
124
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
A number of technologies have been developed for gainful utilization of flyash. The utilization ranges from low to high value-added applications w1x. Utilization of flyash in India records a very low percentage of 2–3% as compared to a corresponding figure of 30–80% for developed countries w1x. This requires development of some innovative technologies to promote flyash utilization. With this in view, the possibility of synthesizing high value-added products such as zeolites from flyash was explored. Over 250 species of naturally occurring and synthetic zeolitic compositions are available. In general, crystalline zeolites are alumino-silicates that are consisted of AlO4 and SiO4 tetrahedra connected by mutual sharing of oxygen atoms and characterised by pore openings of uniform dimension. Zeolites show remarkable ion-exchange capacity, they are capable of reversibly desorbing adsorbed phases that are dispersed throughout the voids of the crystal without displacing any atoms, which make up the permanent crystal structure w2x. In general, the zeolite-synthesizing process from flyash involves alkaline treatment, using caustic soda at higher temperatures Ž80–1008C.. Most previous studies evaluated the conversion of flyash to zeolite-like materials under ambient pressure conditions. This paper addresses the synthesis of various types of zeolites from flyash by varying reaction conditions for their standardization and optimization; along with characterization of flyash-based zeolites ŽFAZs. with respect to crystallinity, elemental content, morphological observations and exchange capacity. 2. Materials and methods 2.1. Materials Flyash sample was collected from the hopper of an electrostatic precipitator at a coal-fired thermal power plant. Standard zeolites for comparison were purchased from Mobil Oil Corporation. All other chemicals were purchased from E. Merck and Loba Chemie. 2.2. Zeolite synthesis In the present investigations, FAZs were synthesized by reacting flyash with caustic soda. The methodology used was the following: 2.2.1. Fusion method The FAZ sample was synthesized by fusing flyash with sodium hydroxide. A homogenous fusion mixture was prepared by proper grinding and mixing of flyash and caustic soda in 1:1.2 ratio. This mixture was heated to at least about 5008C, preferably between 5508C and 6008C for about 1–2 h. The resultant fused mass was cooled, milled and mixed thoroughly in distilled water Žwith simultaneous addition of sodium aluminate in some cases.. The slurry was then subjected to aging for 8 h. This amorphous alumino-silicate gel was then subjected to crystallization between 908C and 1108C for about 2–24 h. The solid crystalline product was recovered by filtration and washed thoroughly until the filtrate pH was 10–11 and dried at a temperature of 50–608C. The procedure is explained in detail elsewhere w5x.
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
125
2.3. Methods of characterization The cation exchange capacity ŽCEC. of alumino-silicates was determined by the following method w6x. One liter of aqueous solution containing 0.5 g of CaCl 2 and adjusted to a pH of 9–10 with dilute NaOH was mixed with 1 g of alumino-silicate ŽFAZ-Y.. The suspension was then stirred vigorously for 15 min at room temperature Ž29–308C.. After filtration, the residual hardness of the filtrate was determined. From the difference between the hardness of the original solution and filtrate, the CBC is calculated as meqr100 g. The surface morphology of the zeolite was examined by Jeol-840-A scanning electron microscope ŽSEM.. Powder XRD analysis was employed to monitor zeolite formation process, using CuK a as source of X-rays ŽModel Philips PN-1830.. The sum ˚ . of standard zeolite as total of relative intensities of d-spacing values in angstroms ŽA compared to FAZ-Y has been used as basis for estimation of present crystallinity. The d-spacing values used are as follows: 14.15, 8.73, 7.46, 5.69, 4.78, 4.39, 3.79, 2.87, and 2.65. The chemical composition of the zeolite sample was determined using ICP-AES Žmodel YJ-24.. Morphological characterization was carried out using SEM ŽModel Jeol-64..
3. Results and discussions The elemental content of flyash collected from Koradi Thermal Power Station ŽKTPS. are as follows: SiO 2 : 62.27; Al 2 O 3 : 30.96; Fe 2 O 3 : 1.25; TiO 2 : 1.67; CaO: 3.02; Na 2 O: 0.12; K 2 O: 0.41; and LOI: 0.29. The SiO 2rAl 2 O 3 molar ratio of 3.3–3.5 for Koradi Thermal Power Station ŽKTPS. flyash is favourable for the synthesis of zeolite-Y with SiO 2rAl 2 O 3 ratio of 1.5–1.6. However, synthesis of zeolite-Y with SiO 2rAl 2 O 3 ratio of 3.0–3.5 requires the flyash to be treated so as to enhance the silica content. These modifications have been incorporated and are a subject of discussion elsewhere w7x. 3.1. Fusion method The fusion method has been employed for optimal extraction of alumino-silicates from flyash for synthesis of FAZs in pure phase. The zeolitic phases obtained by reflux method are a mixture along with traces of unreacted flyash w3,4x. The effects of variables such as fusion temperature, caustic soda concentration and crystallisation time are discussed in subsequent sections. 3.2. Effect of NaOH concentration The conversion of flyash into sodium silicate and aluminate by the fusion method depends on the amount of sodium hydroxide in the reaction system. The residual amount of sodium hydroxide in the reaction mixture influences the alkalinity of the solution undergoing hydrothermal treatments.
32 40
FAZ-Y4 FAZ-Y5
600
24 24
FAZ-Y7 FAZ-Y8
600
24 24 24 24 24
FAZ-Y10 FAZ-Y11 FAZ-Y12 FAZ-Y13 FAZ-Y14
Flyash Concentration used for samples FAZ-Y1–FAZ-Y14: 20 g.
600
600
600
600
600
Effect of crystallization time FAZ-Y9 24
800
200
Effect of fusion temperature FAZ-Y6 24
600
600
600
24
FAZ-Y3
Fusion temperature Ž8C .
600 600
NaOH concentration Žg .
Effect of NaOH concentration FAZ-Y1 8 FAZ-Y2 16
FAZ sample
8
8
8
8
8
8
8
8
8
8
8
8
8 8
Mixing time Žh .
Table 1 Reaction conditions, exchange capacity and crystallinity of FAZ-Y
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110 100 – 110
Crystalization temperature Ž8C .
24
12
10
8
4
2
10
10
10
10
10
10
10 10
Crystalization time Žh .
380
410
420
380
200
240
220
420
160
240
340
420
160 260
Calciumbinding capacity Žmeqr100 g .
98
95
90
58
50
40
50
80
25
50
60
90
30 40
Percent crystallinity ŽXRD .
Partially crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed Fully crystalline Zeolite-Y Fully crystalline Zeolite-Y Fully crystalline Zeolite-Y
Zeolite-Y formation negligible Fully crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed
Amorphous phases Partially crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed
Remarks
126 S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
127
Zeolite formation in terms of crystallinity and cation exchange capacity ŽCEC. was monitored as a function of NaOH concentration ŽFAZ-Y1–FAZ-Y5.. It is evident from the results presented in Table 1 that the level of NaOH added influences zeolitic crystallinity and CBC significantly. Formation of zeolitic phases was in traces at NaOH concentration of 8 g ŽFAZ-Y1., whereas partial conversion was observed at an increase of NaOH concentration to 16 g ŽFAZ-Y2.. Fully crystalline zeolite-Y was obtained at NaOH level of 24 g ŽFAZ-Y3.. Further increase in NaOH concentration to 32 g ŽFAZ-Y4. and 40 g ŽFAZ-Y5. had no significant influence on zeolite crystallization. This indicates that the formation of zeolite varies with the alkalinity of the system. Insufficient concentration of alkali, as observed for FAZ-Y1, leads to minimal extraction of alumino-silicates from flyash and also adversely affects the crystallization process. An optimal concentration of 22–24 g of NaOH is sufficient for extraction of alumino silicate and for maintaining the pH of reaction system so as to ensure optimal concentration. Excessive alkali-enriched FAZ systems lead to the transformation of Zeolite-Y phases to hydroxysodalite as observed in the case of FAZ-Y4 and FAZ-Y5. In terms of CBC, it can be said that it increases up to NaOH concentration of 24 g, remains almost constant at 32 g of NaOH concentration and starts decreasing with further increase in alkali content.
Fig. 1. Ža. SEM photograph of flyash. Žb. SEM photograph of FAZ-Y.
128
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
3.3. Effect of temperature on zeolite synthesis The effect of temperature on zeolite formation is quite marked ŽTable 1.. No zeolite formation was observed at temperature of 2008C ŽFAZ-Y6. indicating that conversion of flyash into silicates and aluminates was negligible. With the increase in temperature to 6008C ŽFAZ-Y7., conversion of flyash to zeolitic phases was observed with maximum
Fig. 2. X-ray diffraction pattern of commercial Zeolite-Y Ž1. and FAZ-Y Ž2..
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
129
crystallinity. At higher temperatures of 8008C ŽFAZ-Y8., the crystallinity gradually decreased probably due to formation of non-crystalline sintered glass. The trend observed for CBC as a function of fusion temperature is similar. The CBC value Ž160 meqr100 g. is quite low at 2008C, while significant increase in CBC was recorded at 6008C. Beyond a temperature of 6008C, there was a decrease in CBC value Ž220 meqr100 g at 8008C.. 3.4. Effect of crystallization time The alumino-silicate fused massrgel obtained after fusion is amorphous and changes to the crystalline state when subjected to hydrothermal crystallization. A close scrutiny of the results presented in Table 1 reveals that crystallization time influences zeolitic crystallinity significantly ŽFAZ-Y9–FAZ-Y14.. Percent crystallinity of zeolite-Y increases significantly until up to 10 h ŽFAZ-Y12. and remains almost constant beyond that crystallization time. Beyond 8 h of crystallisation time ŽFAZ-Y11., there seems to be no influence on exchange capacity. 3.5. Morphological and structural inÕestigations SEM photographs in Fig. 1 depict the transformation of flyash into FAZ-Y, under various experimental conditions. SEM photograph ŽFig. 1Žb.. shows completely grown FAZ-Y crystals of cubic morphology with approximate dimensions of 7–8 mm. X-ray diffraction patterns of fully crystalline FAZ-Y and commercial grade Zeolite-Y are presented in Fig. 2. The XRD pattern of FAZ-Y fairly matches with standard zeolite pattern confirming highly crystalline, single phase formation of FAZ-Y as also corroborated by SEM investigations. These studies confirm the optimization of reaction parameters to obtain highly crystalline and pure Zeolite-Y phase from flyash.
Table 2 Comparative characteristics of flyash-basedrcommercial Zeolite-Y Sl. no. Typical specifications
FAZ-Y3
Commercial Zeolite-Y
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
White 2–3 120–125 2.13 0.781 24–25 10–11 Y-type zeolite 95 36"0.5 20"0.5 18"0.5 1.8–2.0 500–550 yVe a
White 7–8 130 2.0 0.5 26–27 10–11 Y-type zeolite 100 38"0.5 17"0.5 17"0.5 2.1–2.2 550–600 yVe a
Appearance Average particle size Žmm. Cation exchange capacity ŽmgrCaOrg dry sample. Density Žgrcm3 . Tapped density Žgrcm3 . Moisture content Žwt.% on dry basis; ignition loss at 8008C. pH of 1% slurry Crystalline form Crystallinity Ž%. Silica content Ž%. Alumina content Ž%. Na 2 O content Ž%. SirAl molar ratio Surface area Žm2 rg. TCLP test a
Leachates do not contain any of the eight toxic elements beyond the prescribed limit.
130
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
Fig. 3. Percentage removal of ammonia and copper by FAZ-Y.
3.6. ComparatiÕe characteristics of FAZ-Y and commercial Zeolite-Y A comparative analysis of physico-chemical characteristics is presented in Table 2. The surface area of FAZ-Y is 550–600 m2rg and compares well with that reported in the literature w8x. The loss in weight due to ignition at 8008C is around 25%. The chemical composition provided for Zeolite-Y corroborates with that reported in literature. Toxicity Characteristic Leaching Procedure ŽTCLP. tests conducted for evaluating toxic or hazardous nature reveal that the level of eight toxic elements are much below the stipulated level viz. As:5.0 mgrl, Ag:5.0 mgrl, Ba:100 mgrl, Cd:1.0 mgrl, Cr:5.0 mgrl, Pb:5.0 mgrl, Se:1.0 mgrl and Hg:0.2 mgrl. FAZ-Y have been tested for their efficiency in removing certain toxic pollutants such as ammonium and copper. It shows good potential for the removal of ammonia and copper, and the results are presented in Fig. 3. It is apparent from the results that this zeolite can be used for remediation of wastewater containing ammonium and copper ions.
4. Conclusions Synthesis of FAZs is directly related to extraction of silicates and aluminates form flyash using sodium hydroxide. The SiO 2rAl 2 O 3 ratio of 3.3–3.5 in flyash favours formation of Zeolite-Y at the following optimal conditions: NaOHrflyash ratio of 1:1.2; fusion temperature of 6008C; crystallization time of 10–12 h; and crystallization temperature of 1008C. On modification of the actual SiO 2rAl 2 O 3 ratio, by increasing Al 2 O 3 content with sodium aluminate, it is possible to synthesise Zeolite-X and -A. The
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
131
synthesis of zeolites from flyash offers several advantages over the conventional process viz. raw material conservation, energy conservation and reduction in production time. Despite the low volume utilization of flyash for zeolite production technology compared to other mediumrlow value utilization, this production technology has advantages of value addition, offering an edge over other flyash utilisation technologies currently used.
Acknowledgements The help and cooperation rendered by the management of Koradi Thermal Power Station in collecting flyash for this research work is greatly acknowledged. Thanks are also due to Dr. K. Krishnan and Dr. G. Balasubramanian, Scientists, Jawaharlal Nehru Aluminium Research Development and Design Centre, Nagpur for their cooperation in characterization of FAZ product.
References w1x Proceedings of International conference on Flyash disposal and utilisation, 20–22 January 1998, New Delhi, India. w2x J. Dwyer, Chem. Ind. 7 Ž1984. 258–269. w3x C.F. Lin, H.S. Shi, Environ. Sci. Technol. 29 Ž1995. 1109. w4x A. Singer, U.B. Vadium, Environ. Sci. Technol. 29 Ž1995. 1748. w5x S.S. Rayalu, N. Labhsetwar, P. Khanna, Process for production of flyash based Zeolite-Y, Indian patent, March 1998. w6x Y. Sugahara, K. Usui, M. Ogawa, H. Kurosaki, S. Imafuku, US patent No. 4,102,977, July 25, 1978. w7x S.S. Rayalu, S.U. Meshram, M.Z. Hasan, S.N. Kaul, Flyash based zeolite technology: an illustration of waste to wealth, Proceedings of the Fifteenth International Conference on Solid Waste Technology and Management, December 12–15, 1999, Philadelphia, USA. w8x J. Weitkamp, S. Ernst, Zeolite and their use in petroleum refining, in: P.H. Ogden ŽEd.., Chemicals in the Oil Industry: Developments and Application, The Royal society of Chemistry, Cambridge, 1991, Akzo chemie UK.
Highly crystalline faujasitic zeolites from flyash S. Rayalu ) , S.U. Meshram, M.Z. Hasan National EnÕironmental Engineering Research Institute, Nehru Marg, Nagpur 440 020, India Received 5 November 1999; received in revised form 13 March 2000; accepted 15 March 2000
Abstract The process for the synthesis of flyash-based zeolites ŽFAZs. are presented, which basically includes alkaline treatment of flyash by a fusion method, followed by hydrothermal crystallization. Zeolite-Y has been identified, and conditions have been optimized for their synthesis by the fusion method. Optimal conditions for synthesis of Zeolite-Y are a NaOHrflyash ratio of 1.2:1, fusion temperature between 5008C and 6008C, crystallization time of 8–10 h and crystallization temperature between 1008C and 1108C. The cation exchange capacity ŽCEC. of FAZ-Y ranges between 400 and 450 meqr100 g. The surface area of FAZs Ž500–600 m2rg. compare well with the commercial zeolites procured from Mobil Oil. Morphological characterization of FAZ using scanning electron microscopy ŽSEM. reveals cubic structure, and XRD data reveal unit cells to be cubic system. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Flyash-based zeolite; Synthesis; Process standardisation; Waste utilization
1. Introduction In India, flyash is being generated at the rate of about 60 million tonnes per annum ŽMTPA. from about 75 thermal power plants and is expected to increase to 100 MTPA by the turn of the century. There are serious environmental health hazards associated with flyash. In addition, the land requirement envisaged for disposal of flyash is about 50,000 acre, with an annual expenditure of about Rs 500 million for transportation. These problems clearly spell out the fact that utilization of flyash is absolutely essential.
)
Corresponding author. Fax: q91-712-230673. E-mail address: [email protected] ŽS. Rayalu..
0304-3894r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 9 4 Ž 0 0 . 0 0 2 1 2 - 0
124
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
A number of technologies have been developed for gainful utilization of flyash. The utilization ranges from low to high value-added applications w1x. Utilization of flyash in India records a very low percentage of 2–3% as compared to a corresponding figure of 30–80% for developed countries w1x. This requires development of some innovative technologies to promote flyash utilization. With this in view, the possibility of synthesizing high value-added products such as zeolites from flyash was explored. Over 250 species of naturally occurring and synthetic zeolitic compositions are available. In general, crystalline zeolites are alumino-silicates that are consisted of AlO4 and SiO4 tetrahedra connected by mutual sharing of oxygen atoms and characterised by pore openings of uniform dimension. Zeolites show remarkable ion-exchange capacity, they are capable of reversibly desorbing adsorbed phases that are dispersed throughout the voids of the crystal without displacing any atoms, which make up the permanent crystal structure w2x. In general, the zeolite-synthesizing process from flyash involves alkaline treatment, using caustic soda at higher temperatures Ž80–1008C.. Most previous studies evaluated the conversion of flyash to zeolite-like materials under ambient pressure conditions. This paper addresses the synthesis of various types of zeolites from flyash by varying reaction conditions for their standardization and optimization; along with characterization of flyash-based zeolites ŽFAZs. with respect to crystallinity, elemental content, morphological observations and exchange capacity. 2. Materials and methods 2.1. Materials Flyash sample was collected from the hopper of an electrostatic precipitator at a coal-fired thermal power plant. Standard zeolites for comparison were purchased from Mobil Oil Corporation. All other chemicals were purchased from E. Merck and Loba Chemie. 2.2. Zeolite synthesis In the present investigations, FAZs were synthesized by reacting flyash with caustic soda. The methodology used was the following: 2.2.1. Fusion method The FAZ sample was synthesized by fusing flyash with sodium hydroxide. A homogenous fusion mixture was prepared by proper grinding and mixing of flyash and caustic soda in 1:1.2 ratio. This mixture was heated to at least about 5008C, preferably between 5508C and 6008C for about 1–2 h. The resultant fused mass was cooled, milled and mixed thoroughly in distilled water Žwith simultaneous addition of sodium aluminate in some cases.. The slurry was then subjected to aging for 8 h. This amorphous alumino-silicate gel was then subjected to crystallization between 908C and 1108C for about 2–24 h. The solid crystalline product was recovered by filtration and washed thoroughly until the filtrate pH was 10–11 and dried at a temperature of 50–608C. The procedure is explained in detail elsewhere w5x.
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
125
2.3. Methods of characterization The cation exchange capacity ŽCEC. of alumino-silicates was determined by the following method w6x. One liter of aqueous solution containing 0.5 g of CaCl 2 and adjusted to a pH of 9–10 with dilute NaOH was mixed with 1 g of alumino-silicate ŽFAZ-Y.. The suspension was then stirred vigorously for 15 min at room temperature Ž29–308C.. After filtration, the residual hardness of the filtrate was determined. From the difference between the hardness of the original solution and filtrate, the CBC is calculated as meqr100 g. The surface morphology of the zeolite was examined by Jeol-840-A scanning electron microscope ŽSEM.. Powder XRD analysis was employed to monitor zeolite formation process, using CuK a as source of X-rays ŽModel Philips PN-1830.. The sum ˚ . of standard zeolite as total of relative intensities of d-spacing values in angstroms ŽA compared to FAZ-Y has been used as basis for estimation of present crystallinity. The d-spacing values used are as follows: 14.15, 8.73, 7.46, 5.69, 4.78, 4.39, 3.79, 2.87, and 2.65. The chemical composition of the zeolite sample was determined using ICP-AES Žmodel YJ-24.. Morphological characterization was carried out using SEM ŽModel Jeol-64..
3. Results and discussions The elemental content of flyash collected from Koradi Thermal Power Station ŽKTPS. are as follows: SiO 2 : 62.27; Al 2 O 3 : 30.96; Fe 2 O 3 : 1.25; TiO 2 : 1.67; CaO: 3.02; Na 2 O: 0.12; K 2 O: 0.41; and LOI: 0.29. The SiO 2rAl 2 O 3 molar ratio of 3.3–3.5 for Koradi Thermal Power Station ŽKTPS. flyash is favourable for the synthesis of zeolite-Y with SiO 2rAl 2 O 3 ratio of 1.5–1.6. However, synthesis of zeolite-Y with SiO 2rAl 2 O 3 ratio of 3.0–3.5 requires the flyash to be treated so as to enhance the silica content. These modifications have been incorporated and are a subject of discussion elsewhere w7x. 3.1. Fusion method The fusion method has been employed for optimal extraction of alumino-silicates from flyash for synthesis of FAZs in pure phase. The zeolitic phases obtained by reflux method are a mixture along with traces of unreacted flyash w3,4x. The effects of variables such as fusion temperature, caustic soda concentration and crystallisation time are discussed in subsequent sections. 3.2. Effect of NaOH concentration The conversion of flyash into sodium silicate and aluminate by the fusion method depends on the amount of sodium hydroxide in the reaction system. The residual amount of sodium hydroxide in the reaction mixture influences the alkalinity of the solution undergoing hydrothermal treatments.
32 40
FAZ-Y4 FAZ-Y5
600
24 24
FAZ-Y7 FAZ-Y8
600
24 24 24 24 24
FAZ-Y10 FAZ-Y11 FAZ-Y12 FAZ-Y13 FAZ-Y14
Flyash Concentration used for samples FAZ-Y1–FAZ-Y14: 20 g.
600
600
600
600
600
Effect of crystallization time FAZ-Y9 24
800
200
Effect of fusion temperature FAZ-Y6 24
600
600
600
24
FAZ-Y3
Fusion temperature Ž8C .
600 600
NaOH concentration Žg .
Effect of NaOH concentration FAZ-Y1 8 FAZ-Y2 16
FAZ sample
8
8
8
8
8
8
8
8
8
8
8
8
8 8
Mixing time Žh .
Table 1 Reaction conditions, exchange capacity and crystallinity of FAZ-Y
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110
100 – 110 100 – 110
Crystalization temperature Ž8C .
24
12
10
8
4
2
10
10
10
10
10
10
10 10
Crystalization time Žh .
380
410
420
380
200
240
220
420
160
240
340
420
160 260
Calciumbinding capacity Žmeqr100 g .
98
95
90
58
50
40
50
80
25
50
60
90
30 40
Percent crystallinity ŽXRD .
Partially crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed Fully crystalline Zeolite-Y Fully crystalline Zeolite-Y Fully crystalline Zeolite-Y
Zeolite-Y formation negligible Fully crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed
Amorphous phases Partially crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed Fully crystalline Zeolite-Y formed Partially crystalline Zeolite-Y formed
Remarks
126 S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
127
Zeolite formation in terms of crystallinity and cation exchange capacity ŽCEC. was monitored as a function of NaOH concentration ŽFAZ-Y1–FAZ-Y5.. It is evident from the results presented in Table 1 that the level of NaOH added influences zeolitic crystallinity and CBC significantly. Formation of zeolitic phases was in traces at NaOH concentration of 8 g ŽFAZ-Y1., whereas partial conversion was observed at an increase of NaOH concentration to 16 g ŽFAZ-Y2.. Fully crystalline zeolite-Y was obtained at NaOH level of 24 g ŽFAZ-Y3.. Further increase in NaOH concentration to 32 g ŽFAZ-Y4. and 40 g ŽFAZ-Y5. had no significant influence on zeolite crystallization. This indicates that the formation of zeolite varies with the alkalinity of the system. Insufficient concentration of alkali, as observed for FAZ-Y1, leads to minimal extraction of alumino-silicates from flyash and also adversely affects the crystallization process. An optimal concentration of 22–24 g of NaOH is sufficient for extraction of alumino silicate and for maintaining the pH of reaction system so as to ensure optimal concentration. Excessive alkali-enriched FAZ systems lead to the transformation of Zeolite-Y phases to hydroxysodalite as observed in the case of FAZ-Y4 and FAZ-Y5. In terms of CBC, it can be said that it increases up to NaOH concentration of 24 g, remains almost constant at 32 g of NaOH concentration and starts decreasing with further increase in alkali content.
Fig. 1. Ža. SEM photograph of flyash. Žb. SEM photograph of FAZ-Y.
128
S. Rayalu et al.r Journal of Hazardous Materials B77 (2000) 123–131
3.3. Effect of temperature on zeolite synthesis The effect of temperature on zeolite formation is quite marked ŽTable 1.. No zeolite formation was observed at temperature of 2008C ŽFAZ-Y6. indicating that conversion of flyash into silicates and aluminates was negligible. With the increase in temperature to 6008C ŽFAZ-Y7., conversion of flyash to zeolitic phases was observed with maximum
Fig. 2. X-ray diffraction pattern of commercial Zeolite-Y Ž1. and FAZ-Y Ž2..
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crystallinity. At higher temperatures of 8008C ŽFAZ-Y8., the crystallinity gradually decreased probably due to formation of non-crystalline sintered glass. The trend observed for CBC as a function of fusion temperature is similar. The CBC value Ž160 meqr100 g. is quite low at 2008C, while significant increase in CBC was recorded at 6008C. Beyond a temperature of 6008C, there was a decrease in CBC value Ž220 meqr100 g at 8008C.. 3.4. Effect of crystallization time The alumino-silicate fused massrgel obtained after fusion is amorphous and changes to the crystalline state when subjected to hydrothermal crystallization. A close scrutiny of the results presented in Table 1 reveals that crystallization time influences zeolitic crystallinity significantly ŽFAZ-Y9–FAZ-Y14.. Percent crystallinity of zeolite-Y increases significantly until up to 10 h ŽFAZ-Y12. and remains almost constant beyond that crystallization time. Beyond 8 h of crystallisation time ŽFAZ-Y11., there seems to be no influence on exchange capacity. 3.5. Morphological and structural inÕestigations SEM photographs in Fig. 1 depict the transformation of flyash into FAZ-Y, under various experimental conditions. SEM photograph ŽFig. 1Žb.. shows completely grown FAZ-Y crystals of cubic morphology with approximate dimensions of 7–8 mm. X-ray diffraction patterns of fully crystalline FAZ-Y and commercial grade Zeolite-Y are presented in Fig. 2. The XRD pattern of FAZ-Y fairly matches with standard zeolite pattern confirming highly crystalline, single phase formation of FAZ-Y as also corroborated by SEM investigations. These studies confirm the optimization of reaction parameters to obtain highly crystalline and pure Zeolite-Y phase from flyash.
Table 2 Comparative characteristics of flyash-basedrcommercial Zeolite-Y Sl. no. Typical specifications
FAZ-Y3
Commercial Zeolite-Y
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
White 2–3 120–125 2.13 0.781 24–25 10–11 Y-type zeolite 95 36"0.5 20"0.5 18"0.5 1.8–2.0 500–550 yVe a
White 7–8 130 2.0 0.5 26–27 10–11 Y-type zeolite 100 38"0.5 17"0.5 17"0.5 2.1–2.2 550–600 yVe a
Appearance Average particle size Žmm. Cation exchange capacity ŽmgrCaOrg dry sample. Density Žgrcm3 . Tapped density Žgrcm3 . Moisture content Žwt.% on dry basis; ignition loss at 8008C. pH of 1% slurry Crystalline form Crystallinity Ž%. Silica content Ž%. Alumina content Ž%. Na 2 O content Ž%. SirAl molar ratio Surface area Žm2 rg. TCLP test a
Leachates do not contain any of the eight toxic elements beyond the prescribed limit.
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Fig. 3. Percentage removal of ammonia and copper by FAZ-Y.
3.6. ComparatiÕe characteristics of FAZ-Y and commercial Zeolite-Y A comparative analysis of physico-chemical characteristics is presented in Table 2. The surface area of FAZ-Y is 550–600 m2rg and compares well with that reported in the literature w8x. The loss in weight due to ignition at 8008C is around 25%. The chemical composition provided for Zeolite-Y corroborates with that reported in literature. Toxicity Characteristic Leaching Procedure ŽTCLP. tests conducted for evaluating toxic or hazardous nature reveal that the level of eight toxic elements are much below the stipulated level viz. As:5.0 mgrl, Ag:5.0 mgrl, Ba:100 mgrl, Cd:1.0 mgrl, Cr:5.0 mgrl, Pb:5.0 mgrl, Se:1.0 mgrl and Hg:0.2 mgrl. FAZ-Y have been tested for their efficiency in removing certain toxic pollutants such as ammonium and copper. It shows good potential for the removal of ammonia and copper, and the results are presented in Fig. 3. It is apparent from the results that this zeolite can be used for remediation of wastewater containing ammonium and copper ions.
4. Conclusions Synthesis of FAZs is directly related to extraction of silicates and aluminates form flyash using sodium hydroxide. The SiO 2rAl 2 O 3 ratio of 3.3–3.5 in flyash favours formation of Zeolite-Y at the following optimal conditions: NaOHrflyash ratio of 1:1.2; fusion temperature of 6008C; crystallization time of 10–12 h; and crystallization temperature of 1008C. On modification of the actual SiO 2rAl 2 O 3 ratio, by increasing Al 2 O 3 content with sodium aluminate, it is possible to synthesise Zeolite-X and -A. The
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synthesis of zeolites from flyash offers several advantages over the conventional process viz. raw material conservation, energy conservation and reduction in production time. Despite the low volume utilization of flyash for zeolite production technology compared to other mediumrlow value utilization, this production technology has advantages of value addition, offering an edge over other flyash utilisation technologies currently used.
Acknowledgements The help and cooperation rendered by the management of Koradi Thermal Power Station in collecting flyash for this research work is greatly acknowledged. Thanks are also due to Dr. K. Krishnan and Dr. G. Balasubramanian, Scientists, Jawaharlal Nehru Aluminium Research Development and Design Centre, Nagpur for their cooperation in characterization of FAZ product.
References w1x Proceedings of International conference on Flyash disposal and utilisation, 20–22 January 1998, New Delhi, India. w2x J. Dwyer, Chem. Ind. 7 Ž1984. 258–269. w3x C.F. Lin, H.S. Shi, Environ. Sci. Technol. 29 Ž1995. 1109. w4x A. Singer, U.B. Vadium, Environ. Sci. Technol. 29 Ž1995. 1748. w5x S.S. Rayalu, N. Labhsetwar, P. Khanna, Process for production of flyash based Zeolite-Y, Indian patent, March 1998. w6x Y. Sugahara, K. Usui, M. Ogawa, H. Kurosaki, S. Imafuku, US patent No. 4,102,977, July 25, 1978. w7x S.S. Rayalu, S.U. Meshram, M.Z. Hasan, S.N. Kaul, Flyash based zeolite technology: an illustration of waste to wealth, Proceedings of the Fifteenth International Conference on Solid Waste Technology and Management, December 12–15, 1999, Philadelphia, USA. w8x J. Weitkamp, S. Ernst, Zeolite and their use in petroleum refining, in: P.H. Ogden ŽEd.., Chemicals in the Oil Industry: Developments and Application, The Royal society of Chemistry, Cambridge, 1991, Akzo chemie UK.