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Large-scale synthesis of artificial zeolite from coal fly ash with a small charge of alkaline solution
Fuel 84 (2005) 1455–1461 www.fuelfirst.com
Large-scale synthesis of artificial zeolite from coal fly ash with a small charge of alkaline solution Ryo Moriyamaa,*, Shohei Takedab, Masaki Onozakia, Yukuo Katayamaa, Kouji Shiotac, Tomoya Fukudac, Hiroaki Sugiharac, Yuichi Tanic a
The Institute of Applied Energy, Shinbashi SY BLDG. 14-2 Nishishinbashi 1-chome, Minato-ku, Tokyo 105-0003, Japan b The Institute of Applied Energy, 2-17, Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan c Institute of Technology, Penta-Ocean Construction Co., Ltd., 1534-1, Yonku-cho, Nishinasuno-machi, Nasu-gun, Tochigi 329-2746, Japan Received 15 December 2004; received in revised form 17 February 2005; accepted 27 February 2005 Available online 1 April 2005
Abstract A new process for converting coal fly ash to an artificial zeolite is described. The process is comprised of a high-temperature operation and water removal during the operation. Suitable operation parameters of the process were investigated using a test unit, and the optimal conditions were found to be 2.5–3.5 mol/dm3 of NaOH and 0.88–1.10 dm3/kg of liquid/solid, sodium hydroxide charge from 2.2 to 3.9 mol/kg-CFA. The zeolites obtained from a pilot plant had a higher cation exchangeable capacity than those from the test unit and were comparable to zeolites prepared using a conventional method. q 2005 Elsevier Ltd. All rights reserved. Keywords: Zeolite; Coal fly ash; High-temperature
1. Introduction A huge amount of coal fly ash (CFA) is discharged from coal-fired power plants. In Japan, 8.8 million tons of coal ash (fly ash and bottom ash) were generated, of which 1.6 million tons were disposed of without reuse in 2001. The development of reclaimed areas is restricted due to the shortage of landfill sites and tighter environmental regulations. The major method of the disposal of CFA involves mixing with cement or concrete. However, such applications are limited because of the building requirements. Thus, new ways of utilizing CFA are needed. The conversion of CFA to a zeolite has been reported by several authors [1–11], to produce a product referred to as an artificial zeolite. Zeolites are very useful materials for a wide range of applications such as ion exchangeable materials, molecular sieves, adsorbents and catalysts. Therefore, converting CFA to the zeolite not only alleviates * Corresponding author. Tel.: C81 3 4435 8284; fax: C81 3 3501 8021. E-mail addresses: [email protected] (R. Moriyama), [email protected] (R. Moriyama).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.02.026
the disposal problem but also turns an otherwise waste material into a useful one. There are several technical and economical problems associated with the commercial production of artificial zeolites. Conventionally, the long holding time, the time period for maintaining the reaction temperature [1–5], and/or large amount of alkali metal hydroxide charge [2,5,6] are needed to produce the artificial zeolite in high yield. Some processes [6,7] employ a relatively short holding time, but such processes need a large amount of alkali metal hydroxide charge. Inada et al. [6] converted CFA into zeolite at 373 K using a 5 h holding time while they charged 28 mol of sodium for each 1 kg of CFA which was several times larger than the amount required for zeolite formation from CFA. These problems are the cause of the low production rate and high material cost. As a result, the cost of an artificial zeolite is much higher than that of a natural zeolite. Recently, a new zeolite production process, licensed by the KEM Corporation [12], has been developed. The characteristics of the process are as follows; 1. The holding time is shortened by increasing the operation temperature by means of a high-pressure vessel.
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2. A reduction in the reaction rate, which is caused by decreasing the alkali concentration, is prevented by removing water during the operation. 3. Ratio of the alkaline metal hydroxide solution and the CFA is minimized, resulted in a small charge of alkali metal hydroxide. Furthermore, water removal during the operation can eliminate the need for a filtration process and wastewater treatment. The aim of this paper is to examine the following; 1. Verification of above described zeolite production process. 2. Examination of suitable experimental conditions with a test unit. 3. Development of a pilot plant for the process.
2. Experimental
Fig. 1. Particle size distributions for CFA-A and CFA-B.
2.1. Materials Four types of CFA samples were used as raw materials Their chemical compositions were determined by fluorescent X-ray spectroscopy and the data are listed in Table 1. Each CFA has a different Si/Al ratio and CaO content. Particle size distributions for CFA-A and -B are shown in Fig. 1. This figure clearly demonstrates that CFA-A is smaller than CFA-B. Particle sizes for CFA-A and -B distribute from !0.004 to 0.25 mm and from !0.004 to 4 mm, respectively. The alkali metal hydroxide employed for the zeolite synthesis was sodium hydroxide, NaOH. 2.2. Test unit The test unit was a high-pressure kneader, with a volume of 5 dm3, equipped with a hot-oil jacket around the pressure vessel, the design pressure of which was 1.1 MPa. Photographs of the kneader are shown in Fig. 2a. The kneader was equipped with two sigma-shaped blades for Table 1 Composition of raw materials
SiO2 (mass%) Al2O3 (mass%) CaO (mass%) Fe2O3 (mass%) MgO (mass%) TiO2 (mass%) SO3 (mass%) P2O5 (mass%) K2O (mass%) SrO (mass%) Ba (mass%) Si/Al (mol/mol)
CFA-A
CFA-B
CFA-C
CFA-D
63.1 31.7 1.5 2.5 0.9 1.3 0.4 0.6 0.9 !0.1 !0.1 1.69
50.7 39.5 4.2 3.9 1.0 1.5 0.7 0.6 0.7 0.1 !0.1 1.10
45.0 42.6 5.3 3.4 2.6 1.4 0.9 0.8 0.4 0.2 0.1 0.90
32.5 25.4 26.7 3.3 1.3 1.2 6.2 0.4 0.4 0.2 0.1 1.09
kneading and a steam exhaust line for removing water. 1.0 kg of CFA and known amounts of NaOH solution were heated in the closed vessel with kneading. During the heating, pressure in the vessel increased to the vapor pressure of water. The temperature of the sample was estimated from saturated vapor pressure of water. After the pressure reached a desired pressure and was held for 30 min, steam was removed via the steam exhaust line while the pressure was maintained. Fig. 3a shows typical temperature and water removal profiles for experiment using the test unit. Zero time indicates the initiation of heating. The sample was heated to 432 K for about 80 min and the temperature was maintained for about 150 min with removing water before cooling the sample. Experimental conditions used for the test unit are as follows. 1. Pressure: 0.3, 0.5 or 0.8 MPa 2. NaOH concentration: 1.5–3.5 mol/dm3 3. Ratio of NaOH solution/CFA (liquid/solid ratio): 0.7–1.23 dm3-solution/kg-CFA The operational condition for pilot plant was determined based on results obtained from the test unit. 2.3. Pilot plant A photograph and a schematic diagram of the pilot plant are shown in Figs. 2b and 4, respectively. The plant was a 0.6 m3 high-pressure kneader which had two shafts with kneading wings. Heating oil (480–490 K) was circulated around the kneader to heat the sample. The maximum pressure of operation was 0.48 MPa, which corresponds to steam vapor pressure at 426 K.
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Fig. 2. Photographs of the test unit (a) and the pilot plant (b).
Problems are typically encountered in conventional highpressure kneaders around the shaft because the sample can penetrates the sealing area of the shaft. The penetrated sample can grind the shaft or cause overload of the motor. In order to avoid such problems, this pilot plant was equipped with a sealing box around the sealing area. Sample penetration can be prevented by providing an air supply to the box and by maintaining pressure of the box slightly higher than that of the kneader. 130 kg of CFA and 156 dm3 of NaOH solution were introduced into the kneader prior to heating with the heating oil. Fig. 3b shows typical temperature and water removal profiles for experiment using the pilot plant. The sample was heated to 420 K for about 130 min and the temperature was maintained for about 150 min with water being removed before cooling the sample. The dried artificial zeolite was obtained after the cooling the sample. 2.4. Analyses Crystalline materials in CFA and artificial zeolites were identified by X-ray diffraction (XRD) measurement in the range 5–658. The cation exchangeable capacity (CEC) values for the artificial zeolites were determined using the semi-micro Schollenberger method in which 5 g of product is leached with filtering for 4–20 h with 0.1 dm3 of a 1 mol/dm3 ammonium acetate solution. The solid was then rinsed with ethanol. The ammonia absorbed by the solid was liberated by falling in drops of 0.1 dm3 of a 100 g/dm3 NaCl solution. The amount of ammonium ion in the dripped solution
was determined by steam distillation. The results are expressed as cmol per 1 kg of solids. In order to investigate inhomogeneity of CEC value in the obtained zeolite, 3 or 4 samples were selected from product obtained using both the test unit and the pilot plant. The deviations were found to be within G5 cmol/kg.
3. Results 3.1. CEC values for artificial zeolites obtained with various experimental conditions The CEC values for artificial zeolites obtained from different CFAs are shown in Fig. 5. The pressure, NaOH concentration and liquid/solid ratio used for the experiments were 0.5 MPa, 3.5 mol/dm3 and 1.10 dm3/kg, respectively. Zeolites obtained from CFA-A and -B show high CEC values while those from CFA-C and -D are lower. Based on the literature [4,8], CFA with a high CaO concentration are converted to zeolites with difficulty. This is due to a specific interaction between calcium and silicate ions that prevents the formation of a zeolite [8]. Furthermore, zeolite formation is dependent on the Si/Al ratio. Inada et al. [6] reported that SiO2-rich CFA easily formed an Na-P1 type zeolite while a SiO2-lean CFA formed a hydroxy-sodalite. The results shown in Fig. 5 are consistent with these results. CFA-A and CFA-B were employed, in order to examine suitable experimental conditions for the test unit since the CEC values for CFA-C and CFA-D were too small to
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Fig. 5. Comparison of CEC values for artificial zeolites obtained from different CFAs.
investigate the effects of operation parameters on the CEC values. Fig. 6 shows changes in CEC values for CFA-A and -B as a function of operation pressure. The CEC value for CFA-A increases with pressure while that for CFA-B decreases through maximum at 0.3 MPa. A high operation pressure means a high operation temperature and results in increasing the rate of zeolite formation. A change in the CEC value for CFA-A might be caused by a change in reaction rate while that for CFA-B can be attributed to the formation of a different crystalline structure. This structure is discussed below. Fig. 3. Temperature and water removal profiles for the test unit (a) and the pilot plant (b).
Fig. 4. Schematic diagram of the pilot plant.
Fig. 6. Changes in CEC values with operation pressure.
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Fig. 7. Changes in CEC value with liquid/solid ratio under 0.5 MPa of operation pressure.
Figs. 7 and 8 show CEC values for CFA-A at different solid/liquid ratios and NaOH concentrations, respectively. The CEC value for CFA-A increases with increasing liquid/solid ratio and NaOH concentration. Water is fundamental in the production of a zeolite since liquid acts as a transport medium of precursors of zeolite. In the case of low liquid/solid ratio, most of the water is removed in an early stage of the reaction, and, as a result, the reaction rate is decreased substantially. Therefore, the initial liquid/solid ratio should be greater than 0.88 dm3/kg.
Fig. 8. Changes in CEC value with NaOH concentration under 0.5 MPa of operation pressure.
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Alkali concentration is an important factor in solubilizing the Si and Al in the sample. Furthermore, the alkaline solution serves as a source of NaC for the zeolite. The ratio of NaC used was measured by neutralization of the residual alkalis and was greater than 0.745 mol/mol. If this ratio could be realized without removing water, the alkali concentration could be decreased from 3.5 to 0.89 mol/dm3. The concentration of NaC should be high since the rate of diffusion of NaC into the bone structure of a zeolite depends on the concentration of NaC. Regardless of a high NaC concentration as the result of water removal, the CEC value increases with initial NaOH concentration. However, the charge of NaOH should be held down to reduce the material cost of zeolite production. At an operation pressure of 0.5 MPa, the appropriate NaOH concentration and the liquid/solid ratio are 2.5–3.5 mol/dm3 and 0.88–1.10 dm3/kg, respectively. The production of artificial zeolite using this process requires an NaOH charge from 2.2 to 3.9 mol/kg-CFA. This amount is 40–70% of the maximum consumption of sodium for zeolite production and is one order less than that required for a conventional method. 3.2. Crystalline structure for artificial zeolites obtained from CFA-A and -B Figs. 9 and 10 compare the XRD profile for original CFA with that for artificial zeolites from CFA-A and -B, respectively. The XRD profiles for both of the original CFAs are comprised of peaks attributed to those of quartz
Fig. 9. XRD profiles for CFA-A (a) and artificial zeolite obtained at operation pressures of 0.5 MPa (b) and 0.8 MPa (c). Q, Quartz [SiO2]; M, mullite [3Al2O32SiO2]; G, GIS-type zeolite [Na6Al6Si10O3212H2O]; A, analcime [NaAlSi2O6H2O].
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Fig. 10. XRD profiles for CFA-B (a) and artificial zeolite obtained at operation pressures of 0.5 MPa (b) and 0.8 MPa (c). Q, Quartz [SiO2]; M, mullite [3Al2O32SiO2]; G, GIS-type zeolite [Na6Al6Si10O3212H2O]; A, analcime [NaAlSi2O6H2O].
Fig. 11. Comparison of CEC values for artificial zeolites obtained using different apparatuses.
the pilot plant are clearly smaller than those for the conventional method. and mullite. In the case of CFA-A (see Fig. 9), the intensities of these peaks decrease and peaks derived from a GIS-type zeolite appear after the operation. In the case of CFA-B (see Fig. 10), peaks attributed to GIS-type zeolite are present in the profile for the artificial zeolite operated at 0.5 MPa while peaks attributed to those of analcime are present in the profile at 0.8 MPa. The decrease in CEC value for CFA-B at high pressures might be due to the formation of analcime [1,5]. Not shown in the figure, the XRD profiles for artificial zeolites from CFA-C and -D contained peaks attributed to those of analcime and hydroxy-sodalite, respectively. According to the literature [6], the CEC value for the products from these CFA might be increased by the addition of SiO2 powder. 3.3. Results of pilot plant The CEC values for zeolite produced using the pilot plant are compared with those obtained using the test unit and by a conventional method in Fig. 11.The liquid/solid ratio, operation temperature, and holding time for the conventional method were 8 dm3/kg, 373 K, and 24 h, respectively. Furthermore, water was not removed from the sample during the operation. After the operation, the obtained slurry was filtered and dried. The alkali concentration used in these experiments was 3.5 mol/dm3. The CEC values for artificial zeolites produced using the pilot plant are higher than those for the test unit and are not inferior to those prepared by the conventional method. This process can produce artificial zeolite, with a low cost, since the holding time and NaOH charge for the operation of
4. Summary An artificial zeolite was synthesized by means of a new zeolite production process. This process can minimize the charge of NaOH and the holding time by using highpressure. Water removal during the operation eliminates the filtration process and wastewater treatment. From the results obtained with the test unit, the following were found: 1. The influence of pressure on CEC value depends on the characteristics of the original CFA. 2. The appropriate NaOH concentration and liquid/solid ratio are 2.5–3.5 mol/dm 3 and 0.88–1.10 dm 3 /kg, respectively. That is from 2.2 to 3.9 mol/kg-CFA of NaOH charge. 3. The obtained zeolite produced is a GIS-type zeolite. Based on the above results, pilot plant having a 0.6 m3 of vessel was tested. The zeolite obtained with pilot plant is not inferior to that produced using a conventional method in spite of small amount of NaOH charge.
Acknowledgements Financial support of this work by Ministry of the Environment in Japan (project J1512) is gratefully acknowledged.
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References [1] Querol X, Moreno N, Uman˜a JC, Alastuey A, Herna´ndez E, Lo´pezSoler A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [2] FerretI LS, Fernandes ID, Khahl CA, Endres JC, Maegawa A. Zeolitification of ashes obtained from the combustion of southern’s Brazil Candiota coal. International ash utilization symposium, Kentucky; 1999 [paper 89]. [3] Hollman GG, Steenbruggen G, Janssen-Jurkovicˇova´ M. A two-step process for the synthesis of zeolites from coal fly ash. Fuel 1999;78: 1225–30. [4] Mouhtaris Th, Charistos D, Kantiranis N, Filippidis A, KassoliFournaraki A, Tsirambidis A. GIS-type zeolite synthesis from Greek lignite sulphocalcic fly ash promoted by NaOH solutions. Microporous Mesoporous Mater 2003;61:57–67. [5] QuerolJ X, Uman˜a JC, Plana F, Alastuey A, Lo´pez-Soler A, Medinaceli A, et al. Synthesis of zeolites from fly ash in a pilot plant scale. Examples of potential environmental applications. International ash utilization symposium, Kentucky; 1999 [paper 12].
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[6] Inada M, Eguchi Y, Enomoto N, Hojo J. Synthesis of zeolite from coal ashes with different silica–alumina composition. Fuel 2005;84: 299–304. [7] Maruyama N, Yamamoto H, Shibata J. Mechanism of zeolite synthesis from coal fly ash by alkali hydrothermal reaction. Int J Miner Process 2002;64:1–17. [8] Catalfamo P, Di Pasquale S, Corigliano F, Mavilia L. Influence of the calcium content on the coal fly ash features in some innovative applications. Resour Conservation Recycling 1997;20:119–25. [9] Molina A, Poole C. A comparative study using two methods to produce zeolites from fly ash. Miner Eng 2004;17:167–73. [10] Chang H-L, Shih W-H. A general method for the conversion of fly ash into zeolites as ion exchangers for Cesium. Ind Eng Chem Res 1998; 37:71–8. [11] Andre´sP JM, Ferrer P, Querol X, Plana F, Uman˜a JC. Zeolitisation of coal fly ashes using microwaves. Process optimization. International ash utilization symposium, Kentucky; 1999 [paper 94]. [12] Katayama Y. Process for preparing artificial zeolite by a slurry reaction method, US Patent No. 6,599,494; 2003. AU patent No. 768253; 2003.
Large-scale synthesis of artificial zeolite from coal fly ash with a small charge of alkaline solution Ryo Moriyamaa,*, Shohei Takedab, Masaki Onozakia, Yukuo Katayamaa, Kouji Shiotac, Tomoya Fukudac, Hiroaki Sugiharac, Yuichi Tanic a
The Institute of Applied Energy, Shinbashi SY BLDG. 14-2 Nishishinbashi 1-chome, Minato-ku, Tokyo 105-0003, Japan b The Institute of Applied Energy, 2-17, Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan c Institute of Technology, Penta-Ocean Construction Co., Ltd., 1534-1, Yonku-cho, Nishinasuno-machi, Nasu-gun, Tochigi 329-2746, Japan Received 15 December 2004; received in revised form 17 February 2005; accepted 27 February 2005 Available online 1 April 2005
Abstract A new process for converting coal fly ash to an artificial zeolite is described. The process is comprised of a high-temperature operation and water removal during the operation. Suitable operation parameters of the process were investigated using a test unit, and the optimal conditions were found to be 2.5–3.5 mol/dm3 of NaOH and 0.88–1.10 dm3/kg of liquid/solid, sodium hydroxide charge from 2.2 to 3.9 mol/kg-CFA. The zeolites obtained from a pilot plant had a higher cation exchangeable capacity than those from the test unit and were comparable to zeolites prepared using a conventional method. q 2005 Elsevier Ltd. All rights reserved. Keywords: Zeolite; Coal fly ash; High-temperature
1. Introduction A huge amount of coal fly ash (CFA) is discharged from coal-fired power plants. In Japan, 8.8 million tons of coal ash (fly ash and bottom ash) were generated, of which 1.6 million tons were disposed of without reuse in 2001. The development of reclaimed areas is restricted due to the shortage of landfill sites and tighter environmental regulations. The major method of the disposal of CFA involves mixing with cement or concrete. However, such applications are limited because of the building requirements. Thus, new ways of utilizing CFA are needed. The conversion of CFA to a zeolite has been reported by several authors [1–11], to produce a product referred to as an artificial zeolite. Zeolites are very useful materials for a wide range of applications such as ion exchangeable materials, molecular sieves, adsorbents and catalysts. Therefore, converting CFA to the zeolite not only alleviates * Corresponding author. Tel.: C81 3 4435 8284; fax: C81 3 3501 8021. E-mail addresses: [email protected] (R. Moriyama), [email protected] (R. Moriyama).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.02.026
the disposal problem but also turns an otherwise waste material into a useful one. There are several technical and economical problems associated with the commercial production of artificial zeolites. Conventionally, the long holding time, the time period for maintaining the reaction temperature [1–5], and/or large amount of alkali metal hydroxide charge [2,5,6] are needed to produce the artificial zeolite in high yield. Some processes [6,7] employ a relatively short holding time, but such processes need a large amount of alkali metal hydroxide charge. Inada et al. [6] converted CFA into zeolite at 373 K using a 5 h holding time while they charged 28 mol of sodium for each 1 kg of CFA which was several times larger than the amount required for zeolite formation from CFA. These problems are the cause of the low production rate and high material cost. As a result, the cost of an artificial zeolite is much higher than that of a natural zeolite. Recently, a new zeolite production process, licensed by the KEM Corporation [12], has been developed. The characteristics of the process are as follows; 1. The holding time is shortened by increasing the operation temperature by means of a high-pressure vessel.
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2. A reduction in the reaction rate, which is caused by decreasing the alkali concentration, is prevented by removing water during the operation. 3. Ratio of the alkaline metal hydroxide solution and the CFA is minimized, resulted in a small charge of alkali metal hydroxide. Furthermore, water removal during the operation can eliminate the need for a filtration process and wastewater treatment. The aim of this paper is to examine the following; 1. Verification of above described zeolite production process. 2. Examination of suitable experimental conditions with a test unit. 3. Development of a pilot plant for the process.
2. Experimental
Fig. 1. Particle size distributions for CFA-A and CFA-B.
2.1. Materials Four types of CFA samples were used as raw materials Their chemical compositions were determined by fluorescent X-ray spectroscopy and the data are listed in Table 1. Each CFA has a different Si/Al ratio and CaO content. Particle size distributions for CFA-A and -B are shown in Fig. 1. This figure clearly demonstrates that CFA-A is smaller than CFA-B. Particle sizes for CFA-A and -B distribute from !0.004 to 0.25 mm and from !0.004 to 4 mm, respectively. The alkali metal hydroxide employed for the zeolite synthesis was sodium hydroxide, NaOH. 2.2. Test unit The test unit was a high-pressure kneader, with a volume of 5 dm3, equipped with a hot-oil jacket around the pressure vessel, the design pressure of which was 1.1 MPa. Photographs of the kneader are shown in Fig. 2a. The kneader was equipped with two sigma-shaped blades for Table 1 Composition of raw materials
SiO2 (mass%) Al2O3 (mass%) CaO (mass%) Fe2O3 (mass%) MgO (mass%) TiO2 (mass%) SO3 (mass%) P2O5 (mass%) K2O (mass%) SrO (mass%) Ba (mass%) Si/Al (mol/mol)
CFA-A
CFA-B
CFA-C
CFA-D
63.1 31.7 1.5 2.5 0.9 1.3 0.4 0.6 0.9 !0.1 !0.1 1.69
50.7 39.5 4.2 3.9 1.0 1.5 0.7 0.6 0.7 0.1 !0.1 1.10
45.0 42.6 5.3 3.4 2.6 1.4 0.9 0.8 0.4 0.2 0.1 0.90
32.5 25.4 26.7 3.3 1.3 1.2 6.2 0.4 0.4 0.2 0.1 1.09
kneading and a steam exhaust line for removing water. 1.0 kg of CFA and known amounts of NaOH solution were heated in the closed vessel with kneading. During the heating, pressure in the vessel increased to the vapor pressure of water. The temperature of the sample was estimated from saturated vapor pressure of water. After the pressure reached a desired pressure and was held for 30 min, steam was removed via the steam exhaust line while the pressure was maintained. Fig. 3a shows typical temperature and water removal profiles for experiment using the test unit. Zero time indicates the initiation of heating. The sample was heated to 432 K for about 80 min and the temperature was maintained for about 150 min with removing water before cooling the sample. Experimental conditions used for the test unit are as follows. 1. Pressure: 0.3, 0.5 or 0.8 MPa 2. NaOH concentration: 1.5–3.5 mol/dm3 3. Ratio of NaOH solution/CFA (liquid/solid ratio): 0.7–1.23 dm3-solution/kg-CFA The operational condition for pilot plant was determined based on results obtained from the test unit. 2.3. Pilot plant A photograph and a schematic diagram of the pilot plant are shown in Figs. 2b and 4, respectively. The plant was a 0.6 m3 high-pressure kneader which had two shafts with kneading wings. Heating oil (480–490 K) was circulated around the kneader to heat the sample. The maximum pressure of operation was 0.48 MPa, which corresponds to steam vapor pressure at 426 K.
R. Moriyama et al. / Fuel 84 (2005) 1455–1461
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Fig. 2. Photographs of the test unit (a) and the pilot plant (b).
Problems are typically encountered in conventional highpressure kneaders around the shaft because the sample can penetrates the sealing area of the shaft. The penetrated sample can grind the shaft or cause overload of the motor. In order to avoid such problems, this pilot plant was equipped with a sealing box around the sealing area. Sample penetration can be prevented by providing an air supply to the box and by maintaining pressure of the box slightly higher than that of the kneader. 130 kg of CFA and 156 dm3 of NaOH solution were introduced into the kneader prior to heating with the heating oil. Fig. 3b shows typical temperature and water removal profiles for experiment using the pilot plant. The sample was heated to 420 K for about 130 min and the temperature was maintained for about 150 min with water being removed before cooling the sample. The dried artificial zeolite was obtained after the cooling the sample. 2.4. Analyses Crystalline materials in CFA and artificial zeolites were identified by X-ray diffraction (XRD) measurement in the range 5–658. The cation exchangeable capacity (CEC) values for the artificial zeolites were determined using the semi-micro Schollenberger method in which 5 g of product is leached with filtering for 4–20 h with 0.1 dm3 of a 1 mol/dm3 ammonium acetate solution. The solid was then rinsed with ethanol. The ammonia absorbed by the solid was liberated by falling in drops of 0.1 dm3 of a 100 g/dm3 NaCl solution. The amount of ammonium ion in the dripped solution
was determined by steam distillation. The results are expressed as cmol per 1 kg of solids. In order to investigate inhomogeneity of CEC value in the obtained zeolite, 3 or 4 samples were selected from product obtained using both the test unit and the pilot plant. The deviations were found to be within G5 cmol/kg.
3. Results 3.1. CEC values for artificial zeolites obtained with various experimental conditions The CEC values for artificial zeolites obtained from different CFAs are shown in Fig. 5. The pressure, NaOH concentration and liquid/solid ratio used for the experiments were 0.5 MPa, 3.5 mol/dm3 and 1.10 dm3/kg, respectively. Zeolites obtained from CFA-A and -B show high CEC values while those from CFA-C and -D are lower. Based on the literature [4,8], CFA with a high CaO concentration are converted to zeolites with difficulty. This is due to a specific interaction between calcium and silicate ions that prevents the formation of a zeolite [8]. Furthermore, zeolite formation is dependent on the Si/Al ratio. Inada et al. [6] reported that SiO2-rich CFA easily formed an Na-P1 type zeolite while a SiO2-lean CFA formed a hydroxy-sodalite. The results shown in Fig. 5 are consistent with these results. CFA-A and CFA-B were employed, in order to examine suitable experimental conditions for the test unit since the CEC values for CFA-C and CFA-D were too small to
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Fig. 5. Comparison of CEC values for artificial zeolites obtained from different CFAs.
investigate the effects of operation parameters on the CEC values. Fig. 6 shows changes in CEC values for CFA-A and -B as a function of operation pressure. The CEC value for CFA-A increases with pressure while that for CFA-B decreases through maximum at 0.3 MPa. A high operation pressure means a high operation temperature and results in increasing the rate of zeolite formation. A change in the CEC value for CFA-A might be caused by a change in reaction rate while that for CFA-B can be attributed to the formation of a different crystalline structure. This structure is discussed below. Fig. 3. Temperature and water removal profiles for the test unit (a) and the pilot plant (b).
Fig. 4. Schematic diagram of the pilot plant.
Fig. 6. Changes in CEC values with operation pressure.
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Fig. 7. Changes in CEC value with liquid/solid ratio under 0.5 MPa of operation pressure.
Figs. 7 and 8 show CEC values for CFA-A at different solid/liquid ratios and NaOH concentrations, respectively. The CEC value for CFA-A increases with increasing liquid/solid ratio and NaOH concentration. Water is fundamental in the production of a zeolite since liquid acts as a transport medium of precursors of zeolite. In the case of low liquid/solid ratio, most of the water is removed in an early stage of the reaction, and, as a result, the reaction rate is decreased substantially. Therefore, the initial liquid/solid ratio should be greater than 0.88 dm3/kg.
Fig. 8. Changes in CEC value with NaOH concentration under 0.5 MPa of operation pressure.
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Alkali concentration is an important factor in solubilizing the Si and Al in the sample. Furthermore, the alkaline solution serves as a source of NaC for the zeolite. The ratio of NaC used was measured by neutralization of the residual alkalis and was greater than 0.745 mol/mol. If this ratio could be realized without removing water, the alkali concentration could be decreased from 3.5 to 0.89 mol/dm3. The concentration of NaC should be high since the rate of diffusion of NaC into the bone structure of a zeolite depends on the concentration of NaC. Regardless of a high NaC concentration as the result of water removal, the CEC value increases with initial NaOH concentration. However, the charge of NaOH should be held down to reduce the material cost of zeolite production. At an operation pressure of 0.5 MPa, the appropriate NaOH concentration and the liquid/solid ratio are 2.5–3.5 mol/dm3 and 0.88–1.10 dm3/kg, respectively. The production of artificial zeolite using this process requires an NaOH charge from 2.2 to 3.9 mol/kg-CFA. This amount is 40–70% of the maximum consumption of sodium for zeolite production and is one order less than that required for a conventional method. 3.2. Crystalline structure for artificial zeolites obtained from CFA-A and -B Figs. 9 and 10 compare the XRD profile for original CFA with that for artificial zeolites from CFA-A and -B, respectively. The XRD profiles for both of the original CFAs are comprised of peaks attributed to those of quartz
Fig. 9. XRD profiles for CFA-A (a) and artificial zeolite obtained at operation pressures of 0.5 MPa (b) and 0.8 MPa (c). Q, Quartz [SiO2]; M, mullite [3Al2O32SiO2]; G, GIS-type zeolite [Na6Al6Si10O3212H2O]; A, analcime [NaAlSi2O6H2O].
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Fig. 10. XRD profiles for CFA-B (a) and artificial zeolite obtained at operation pressures of 0.5 MPa (b) and 0.8 MPa (c). Q, Quartz [SiO2]; M, mullite [3Al2O32SiO2]; G, GIS-type zeolite [Na6Al6Si10O3212H2O]; A, analcime [NaAlSi2O6H2O].
Fig. 11. Comparison of CEC values for artificial zeolites obtained using different apparatuses.
the pilot plant are clearly smaller than those for the conventional method. and mullite. In the case of CFA-A (see Fig. 9), the intensities of these peaks decrease and peaks derived from a GIS-type zeolite appear after the operation. In the case of CFA-B (see Fig. 10), peaks attributed to GIS-type zeolite are present in the profile for the artificial zeolite operated at 0.5 MPa while peaks attributed to those of analcime are present in the profile at 0.8 MPa. The decrease in CEC value for CFA-B at high pressures might be due to the formation of analcime [1,5]. Not shown in the figure, the XRD profiles for artificial zeolites from CFA-C and -D contained peaks attributed to those of analcime and hydroxy-sodalite, respectively. According to the literature [6], the CEC value for the products from these CFA might be increased by the addition of SiO2 powder. 3.3. Results of pilot plant The CEC values for zeolite produced using the pilot plant are compared with those obtained using the test unit and by a conventional method in Fig. 11.The liquid/solid ratio, operation temperature, and holding time for the conventional method were 8 dm3/kg, 373 K, and 24 h, respectively. Furthermore, water was not removed from the sample during the operation. After the operation, the obtained slurry was filtered and dried. The alkali concentration used in these experiments was 3.5 mol/dm3. The CEC values for artificial zeolites produced using the pilot plant are higher than those for the test unit and are not inferior to those prepared by the conventional method. This process can produce artificial zeolite, with a low cost, since the holding time and NaOH charge for the operation of
4. Summary An artificial zeolite was synthesized by means of a new zeolite production process. This process can minimize the charge of NaOH and the holding time by using highpressure. Water removal during the operation eliminates the filtration process and wastewater treatment. From the results obtained with the test unit, the following were found: 1. The influence of pressure on CEC value depends on the characteristics of the original CFA. 2. The appropriate NaOH concentration and liquid/solid ratio are 2.5–3.5 mol/dm 3 and 0.88–1.10 dm 3 /kg, respectively. That is from 2.2 to 3.9 mol/kg-CFA of NaOH charge. 3. The obtained zeolite produced is a GIS-type zeolite. Based on the above results, pilot plant having a 0.6 m3 of vessel was tested. The zeolite obtained with pilot plant is not inferior to that produced using a conventional method in spite of small amount of NaOH charge.
Acknowledgements Financial support of this work by Ministry of the Environment in Japan (project J1512) is gratefully acknowledged.
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