Effect of reaction temperature on hydrothermal syntheses of potassium type zeolites from coal fly ash

International Journal of Mineral Processing 87 (2008) 129–133

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International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o

Effect of reaction temperature on hydrothermal syntheses of potassium type zeolites from coal fly ash Norihiro Murayama ⁎, Tomoya Takahashi, Kazuki Shuku, Hyoung-ho Lee, Junji Shibata Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, Kansai University, Japan, 3-3-35, Yamate-cho, Suita-shi, Osaka, 564-8680 Japan

A R T I C L E

I N F O

Article history: Received 13 July 2007 Received in revised form 16 February 2008 Accepted 10 March 2008 Available online 15 March 2008 Keywords: Coal ash Fly ash Zeolite Hydrothermal synthesis Recycle

A B S T R A C T Syntheses of potassium type zeolites were carried out from coal fly ash in the temperature region from 393 K to 453 K, in order to produce the zeolites containing exchangeable K+ with short reaction time. The formation behavior of zeolites and the possibility to advance the reaction time were investigated in this study. From 393 K to 453 K, the obtained zeolite species change from potassium–chabazite (K-CHA) into potassium aluminum silicate hydrate (K-H) via co-crystallization, with an increase in reaction temperature. The K-CHA and K-H are generated simultaneously in an early reaction stage at 433 K, and then the formation of K-H becomes predominant gradually with increasing reaction time in a later reaction stage. The cation exchange capacity (CEC) of zeolite obtained depends on the ratio of K-CHA formation in the potassium type zeolite. The mixture of K-CHA and K-H with high CEC of 175 meq/100 g can be synthesized as a potassium type zeolite under the reaction condition of 433 K, 3 h, 3.0 mol/dm3 KOH. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Recently, atomic power, coal, natural gases are considered as a substitution energy source of petroleum. Coal as an energy source has an advantage in terms of abundant deposits, compared with the other energy sources. In 2003, the percentage of electric power generated in coal power plants in Japan occupies about 23.8% to all amount of electric power (Nihon Furaiasshu Kyokai and Kankyogijyutsu Kyokai, 2005). By an increase in coal demand, the discharge amount of coal ash is also estimated to be increased. According to the law relating recycling in Japan, the coal ash derived from a thermal power plant is especially designated as specified by-product, and the effective usage of this coal ash is strongly required. Recently, the reuse and recycle of coal ash are tried to proceed aggressively, and the percentage and amount of effective usage in 2003 reach about 82% and 6,100,000 ton/year, respectively. However, more than 1,000,000 ton/year of coal ash must be disposed in the landfill treatment still. In the future, the lacking of landfill space is anticipated in Japan, so new recycle technologies for coal ash, by which large amount of coal ash can be consumed, are desired to develop (Nihon Furaiasshu Kyokai and Kankyogijyutsu Kyokai, 2005; Henmi, 1994). The synthesis of zeolite, which is an excellent functional material, from coal ash is noticed as one of effective usages of coal ash ⁎ Corresponding author. E-mail address: [email protected] (N. Murayama). 0301-7516/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2008.03.001

(Shigemoto et al., 1993; Lin and His, 1995; Park and Choi, 1995; Querol et al., 1997; Biserka and Boris, 1998; Sultan et al., 1998). The method to produce various zeolitic materials from coal ashes and incineration ashes (Murayama et al., 2000a, 2000b, 2002), the cation exchange property of obtained zeolite (Murayama et al., 2003a, 2003b), and the zeolite usage in the environmental protection field such as water purification and soil improvement (Murayama et al., 2003b; Shibata et al., 2002) have been reported in our previous papers. We also suggested that potassium–chabazite (K-CHA), which is one of potassium type zeolites, is synthesized from coal fly ash under the hydrothermal conditions and that the K-CHA may be used as a soil improvement agent (Murayama et al., 2003b). The effect of KOH concentration on K-CHA (Shibata et al., 2002), the physical properties of K-CHA obtained (Murayama et al., 2003c) were investigated in our reports. However, 2–4 times longer reaction time is needed to obtain K-CHA compared with sodium type zeolites, when the K-CHA is hydrothermally synthesized from coal fly ash, and this is one of the engineering problems to be improved. The effect of various reaction conditions on the potassium type zeolites or the reason why the reaction time of K-CHA needs so long is not clarified yet. In this study, we expanded the research target to potassium type zeolites except the K-CHA, and the effect of various reaction conditions on the formation of potassium type zeolites was investigated. Concretely, the syntheses of potassium type zeolites were carried out at various reaction temperatures by using KOH as an alkali source. The surface texture, kind of crystalline material and cation exchange

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Table 1 Chemical composition of coal fly ash and reaction products [wt.%] Temperature

Si

Al

Na

Mg

K

Ca

Fe

Ti

Zn

Si/Al

Coal fly ash 413 K 433 K 453 K

55.7 48.6 47.3 45.5

27.8 23.0 26.6 22.3

3.7 1.9 0 0

1.6 0.8 0.4 0.5

2.4 17.8 19.6 22.5

3.7 3.6 3.2 4.3

2.7 2.4 1.3 2.1

0.8 0.9 0.5 0.8

1.6 1.0 1.1 1.9

2.0 2.1 1.8 2.0

Experimental condition: 3 mol/dm3 KOH, reaction time: 12 h.

Generally, the CEC of zeolite indicates somewhat different value by a kind of measurement method due to ion selectivity of zeolite. In this study, the CEC value of reaction products was measured by the modified Harada–Aomine method as follows. Two grams of reaction product and 20 cm3 of 1.0 mol/dm3(CH3COO)2Ca were contacted in order to substitute K+ in the product by Ca2+ in the solution, and then this substitution operation was repeated 6 times so as to achieve a saturation substitution. The substituted product was washed 5 times with 20 cm3 of 80 wt.% ethanol solution to remove excess Ca2+ adhering on the particle surface of product. Then, NH+4 substitution operations were carried out 6 times again, with 20 cm3 of 1.0 mol/dm3 NH4Cl solution. The CEC as a unit of meq/100 g-product was determined by the amount of Ca2+ released from the product in a series of substitution operation. 3. Results and discussion

capacity (CEC) were measured for the zeolites synthesized from coal fly ash, and the formation behavior and reaction speed of zeolites were also considered from the experimental data obtained. 2. Experimental Coal fly ash (Denpatsu Coal Tech. Co., Ltd.), which was discharged in a coal power plant, was used as a raw material of zeolite, and this was satisfied with the quality of Japanese Industrial Standard (JIS). Fifty grams of coal fly ash and 200 cm3 of 3.0 mol/ dm3KOH solution were put into an autoclave made of stainless steel and a hydrothermal treatment was conducted to the mixture. The reaction temperature and agitation speed were set to be 393 K–453 K and 500 rpm, respectively. After the product obtained was washed with distilled water, the solid cake and mother liquor were separated by a vacuum filtration. The wet cake was dried at 353 K for 12 h. Chemical composition of the coal fly ash and reaction products were analyzed by an X-ray fluorescence analysis equipment (JED-2110, Nihon Denshi Co., Ltd.). An X-ray diffraction analyzer (JDX-3530, Nihon Denshi Co., Ltd.) was used to identify a crystalline material in the coal fly ash and reaction products. The surface texture was observed by using a scanning electron microscope (JSM-5410, Nihon Denshi Co., Ltd.). The concentration of various ions in aqueous solution was measured by an inductively coupled plasma atomic emission spectrometry (ICP-1000III, Shimadzu Co., Ltd.) and an atomic absorption spectrometry (AA-6800, Shimadzu Co., Ltd.). On the other hand, the concentration of NH+4 was determined by an ion chromatograph (DX-500, Nihon Dionex Co., Ltd.).

The chemical composition and X-ray diffraction pattern of coal fly ash are shown in Table 1 and Fig. 1(a). The analytical values (wt.%) of chemical composition in Table 1 are calculated by defining total metal content as 100%. About 56% of Si and 28% of Al, which are converted into zeolite structure, contained in the coal fly ash, and a slight amount of Ca, Na, K, Fe, Zn exist as another content. The ratio of Si/Al calculated from Si and Al contents in Table 1 is estimated to be about 2.0. On the other hand, the XRD pattern of coal fly ash in Fig. 1(a) shows a broad-like shape based on an amorphous material. Quartz and mullite are confirmed in the XRD pattern as a crystalline material. The potassium type zeolite with large amount of exchangeable K+ may be used as an excellent soil improvement agent, because this zeolite has not only the cationkeeping and water-keeping abilities, which many general zeolitic materials have originally, but also the ability of slow-acting fertilizer for K+ simultaneously. The CEC of zeolite may be used as a standard to estimate the applicability to a soil improve agent. As a target, the standard value of CEC was set to be over 150 meq/100 g in this study. In our previous reports (Shibata et al., 2002; Murayama et al., 2003b, 2003c), the suitable alkali condition is 2.0–3.0 mol/dm3 KOH as an alkali source of K-CHA synthesis at 393 K. In order to obtain the potassium type zeolite with high CEC efficiently in 3.0 mol/dm3 KOH, the zeolite syntheses from coal fly ash were carried out by changing hydrothermal temperature. The X-ray diffraction patterns of reaction product for 12 h at various temperatures are shown in Fig. 1(b)–(e). The peak pattern of potassium–chabazite (K-CHA) (Szostak, 1992a) is identified at 393 K and 413 K of reaction temperatures. In the case of 433 K, two peak patterns based on both K-CHA and potassium aluminum silicate hydrate (K-H) (Szostak, 1992b) are recognized in the XRD diagram. When the reaction temperature rises up to 453 K, only K-H is formed as a potassium type zeolite.

Fig. 1. XRD pattern of coal fly ash and various reaction products.

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Fig. 2. Surface texture of coal fly ash and reaction products at various temperatures.

The SEM photographs of reaction product for 12 h at various temperatures are shown in Fig. 2(a)–(d). The particles of coal fly ash have very smooth surface (Fig. 2(a)), whereas many elliptical crystals of K-CHA with 2–3 μm are deposit on the particle surface of product obtained at 413 K (Fig. 2(b)). In the case of 433 K (Fig. 2(c)), many deposits of K-H like quadratic prism are formed on the particle surface. At 453 K of reaction temperature (Fig. 2(d)), the particle surface of reaction product is perfectly covered with K-H crystals. The change in surface texture of reaction products with temperature is well corresponding to the phenomenon with the temperature change shown in Fig. 1, that is, the tendency that the obtained zeolite species change from K-CHA to K-H via co-crystallization with an increase in temperature. In order to clarify the formation behavior of potassium type zeolite from coal fly ash, time course of various physical properties was investigated for the reaction

products at various reaction temperatures. The change in XRD intensity of reaction product at 413 K is shown in Fig. 3 as a function of reaction time. The XRD peak of K-CHA near 2θ = 30.5° appears for 3 h of reaction time. The XRD intensity of K-CHA is increasing until 6 h, and the XRD intensity keeps almost constant after 6 h. The intensity of quartz decreases with the formation of K-CHA, and the intensity of mullite also declines slightly until 6 h of reaction time, and then the intensity does not change, though this intensity is not so large originally. These phenomena are mainly caused by the K-CHA formation in which K-CHA crystals deposit on the unreacted coal fly ash. As a result of K-CHA formation, the coverage percent of particle surface becomes large, and the intensities of quartz and mullite relatively decrease. Though it is not shown in a figure, in the case of hydrothermal synthesis in 3.0 mol/dm3KOH at 393 K, the K-CHA crystals are started to form at 5 h of reaction time. The similar results on the synthesis in

Fig. 3. Formation behavior of potassium type zeolite at 413 K.

Fig. 4. Formation behavior of potassium type zeolite at 433 K.

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Fig. 5. Formation behavior of potassium type zeolite at 453 K.

Fig. 7. Time course of cation exchange capacity at various reaction temperatures.

3.0 mol/dm3KOH at 393 K from other coal ash are also confirmed in our papers (Murayama et al., 2003b; Shibata et al., 2002). Therefore, it is possible to shorten the reaction time to obtain K-CHA by applying 413 K of temperature, if the reaction speed in 3.0 mol/dm3KOH at 393 K is regarded as a standard. The time course of XRD intensity at 433 K is shown in Fig. 4. At 2 h of reaction time, the peaks of both K-CHA and K-H near 2θ = 27.4° are identified in the XRD diagram. The intensity of K-CHA shows the maximum point for 3 h, and the intensity is gradually decreasing with an increase in reaction time after 3 h. On the other hand, the intensity derived from K-H is oppositely increasing after 3 h. From these results, it is considered that the K-H formation becomes predominant with reaction time in the case of 433 K. The formation rate of potassium type zeolite can be advanced remarkably at 433 K, though the zeolite species obtained are different from those at 393 K and 413 K. The change in XRD intensity at 453 K is shown in Fig. 3 as a function of time. At 453 K of reaction temperature, only K-H crystals are obtained as a potassium type zeolite, and the intensity of K-H is newly confirmed in a short time like 0.5 h. The intensity of K-H indicates almost constant values after 1.5 h. From the results in Figs. 3–5, the formation speed of potassium type zeolite can increase with an increase in temperature in the range from 393 K to 453 K. In order to consider the change from K-CHA to K-H as predominant zeolite species with reaction time shown in Fig. 4, the time course of surface texture for the reaction product at 433 K is shown in Fig. 6(a)–(d). For 0 h of reaction time at 433 K (Fig. 6(a)), very fine deposits, which can not be identified as zeolite species by a XRD method, are already found on the particle surface. These deposits change into hexagonal prism

shape for 1 h (Fig. 6(b)), compared with 0 h Fig. 6(a)). This phenomenon implies that these fine deposits can play a role on a crystal nuclear in the crystallization process of potassium type zeolite. In Fig. 6(c), many K-H crystals like quadratic prism mainly deposit on the elliptical K-CHA crystals with 5–10 μm. The crystal growth of K-H crystals like quadratic prism can be confirmed so as to fill the chinks among fine crystals for 6 h (Fig. 6(d)). According to the results in Figs. 4 and 6, both K-CHA and K-H are co-crystallized in a first reaction step by the hydrothermal treatment at 433 K, but the formation of K-H is dominant to the K-CHA formation with an increase in reaction time in the later reaction step. The reason why the intensity of K-CHA declines with reaction time is chiefly caused by the coverage of K-H on the particle surface of reaction product. It is also considered that the crystal transformation from K-CHA to K-H may take place at 433 K, but this possibility of crystal transformation is not clarified from the results in Figs. 4 and 6. The CEC value of potassium type zeolite can be regarded as a standard to evaluate the performance of soil improvement agent, and also as one physical property of reaction product, which corresponds to the formation amount of potassium type zeolite. The change in CEC of reaction products obtained at various temperatures is shown in Fig. 7. By the hydrothermal synthesis at 413 K, the CEC value is remarkably increasing until about 6 h of reaction time, and then the CEC reaches to a constant value around 210 meq/100 g after 6 h. At 433 K, the CEC of product for 3 h shows about 175 meq/100 g, and it is slightly decreasing after 3 h. The CEC is about 150 meq/100 g for 12 h. On the other hand, the reaction product at 453 K has almost constant CEC after

Fig. 6. Changes in surface texture as a function of time.

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Fig. 8. Change in XRD intensity and cation exchange capacity for 3 h of reaction time.

Fig. 9. Change in XRD intensity and cation exchange capacity for 6 h of reaction time.

1.5 h of reaction time, and its value for 12 h shows about 105 meq/100 g. Compared with the reaction products for 2–3 h, the potassium type zeolite with the maximum CEC value can be obtained at 433 K. The chemical composition of reaction products at 413 K–453 K is shown in Table 1. The contents of K in the reaction products at 413 K–453 K increase up to 17.8%–22.5% by the hydrothermal treatment, and those indicate about 7.4–9.4 times compared with that of coal fly ash. This increase in K content in the reaction products is caused by the uptake of K+ as an exchangeable cation of zeolite, or the formation of some by-product which is composed of Si, Al, O and K like potassium aluminosilicate, though the byproduct could not identified by a XRD method. The relationship between CEC value and crystal structure was investigated for various potassium type zeolites. The changes in XRD intensity and CEC of reaction product at 393–453 K are summarized in Figs. 8 and 9. For 3 h of reaction time in Fig. 8, the K-CHA and K-H are not formed at 393 K due to the lacking of reaction time. On the other hand, the peak of K-CHA is recognized in the products for 3 h at 413 K and 433 K, and the intensity at 433 K is found to be higher than that at 413 K. The peak of K-H can be confirmed for 3 h at 433 K and 453 K, and the product at 453 K has higher intensity compared with that at 433 K. In this temperature region, the main reaction product changes from K-CHA to K-H with an increase in temperature. From these results, the reaction product with 175 meq/100 g of CEC can be synthesized despite very short reaction time like 3 h. According to the results of time courses of the XRD intensity and CEC value, it is considered that the CEC of product increase with an increase in the formation ratio of K-CHA, that is, it is important to synthesize K-CHA zeolite as a potassium type zeolite to obtain high CEC. The K-CHA can be obtained at 393–433 K for 6 h of reaction time in Fig. 9, and the intensity at 413 K shows the highest value in this temperature range. The peak of K-H can be found in the products for 6 h at 433 K and 453 K, and the intensity at 453 K is higher than that at 433 K, whose tendency is almost the same shown in Fig. 8. The reaction product for 6 h at 413 K has about 205 meq/100 g of CEC, and then those at 393 K and 433 K show both about 180 meq/100 g as a CEC value. It is found from the above results that the changes in a series of CEC with reaction temperature are well corresponding to the formation behavior of potassium type zeolite. That is, the CEC of reaction product deeply depends on the kind and amount of zeolite species. From the viewpoint of reaction speed to obtain the potassium type zeolite with high CEC, it is possible to advance reaction time by an increase in reaction temperature in the range from 393 K to 453 K. As an example, the potassium type zeolite with 175 meq/100 g of CEC can be obtained under the condition of 433 K, 3 h, 3.0 mol/dm3KOH.

various reaction temperatures. The formation behavior and reaction rate of potassium type zeolite were investigated in this study. The obtained zeolite changes from K-CHA to K-H via the cocrystallization with increasing reaction temperature. The K-CHA and K-H are co-crystallized in a first reaction step at 433 K of reaction temperature. When the reaction temperature is set to be higher in the region from 393 K to 453 K, the formation speed can be increased and it is possible to synthesize the potassium type zeolite with 175 meq/ 100 g of CEC under the condition of 433 K, 3 h, 3.0 mol/dm3KOH.

4. Conclusion In order to obtain the potassium type zeolite with high CEC efficiently, the zeolite syntheses from coal fly ash were conducted at

Acknowledgments This work was supported by the Nikko Kinen Jigyodan and the MEXT, Grant-in-Aid for Scientific Research (A) (1), (17206091). References Biserka, B., Boris, S., 1998. Sep. Sci. Technol. 33, 449–466. Henmi, T., 1994. Sangyohaikibutu no Zeoraitotenkanniyoru Saisigenka Yukoriyou gijyutukaihatsu. New Technol. Sci. Tokyo 3–166. Lin, C., His, H., 1995. Environ. Sci. Technol. 29, 1109–1117. Murayama, N., Yamakawa, Y., Ogawa, K., Shibata, J., 2000a. Shigen to Sozai 116, 279–284. Murayama, N., Ogawa, K., Nishikawa, Y., Yamamoto, H., Shibata, J., 2000b. Shigen to Sozai 116, 509–514. Murayama, N., Yamamoto, H., Shibata, J., 2002. Int. J. Miner. Process. 64, 1–17. Murayama, N., Yoshida, S., Takami, Y., Yamamoto, H., Shibata, J., 2003a. Sep. Sci. Technol. 38, 113–129. Murayama, N., Tanabe, M., Yamamoto, H., Shibata, J., 2003b. Mater. Trans. 44, 2436–2440. Murayama, N., Tanabe, M., Yoshida, S., Yamamoto, H., Shibata, J., 2003c. Shigen to Sozai 119, 125–129. Nihon Furaiasshu Kyokai, Kankyogijyutsu Kyokai, 2005. Coal Ash Handbook, 4th edn. Tokyo, I-6–II-10. Park, M., Choi, J., 1995. Clay Sci. 9, 219–229. Querol, X., Plana, F., Alastuey, A., Lopez-Soler, A., 1997. Fuel 76, 793–799. Shibata, J., Yoshida, S., Murayama, N., Yamamoto, H., 2002. Shigen to Sozai 118, 419–424. Shigemoto, N., Hayashi, H., Miyake, K., 1993. J. Mater. Sci. 28, 4781–4786. Sultan, A., Shiraz, C., Keane, M., 1998. Sep. Purif. Technol. 13, 57–64. Szostak, R., 1992a. Handbook of Molecular Sieves. Van Nostrand Reinhold, New York, pp. 117–124. Szostak, R., 1992b. Handbook of Molecular Sieves. Van Nostrand Reinhold, New York, p. 240, 252.