Int J Miner Process 64 Ž2002 1–17 www.elsevier.comrlocaterijminpro Mechanism of zeolite synthesis from coal fly ash by alkali hydrothermal reaction Norihiro Murayama a,1, Hideki Yamamoto b,2 , Junji Shibata b,) a R&D Department, Kimura Chemical Plants Co., Ltd, Kuise Terajima 2-1-2, Amagasaki, Hyogo, Japan b Department of Chemical Engineering, Kansai UniÕersity, Suita, Osaka, Japan Received in revised form 15 February 2001; accepted April 2001 Abstract To clarify the mechanism of zeolite synthesis from coal fly ash, the hydrothermal reaction was carried out in various alkali solutions Zeolite was synthesized in an 800-cm3 autoclave under the condition of 393 K and 100 gr400 cm3 of solid–liquid ratio The changes in various physical and chemical properties, such as crystal structure, surface structure and cation exchange capacity, of the obtained zeolites and the dissolved amount of Si 4q and Al 3q in alkali solution were investigated during the hydrothermal reaction The mechanism of zeolite crystallization and the role of alkali solution on the synthesis reaction were considered Zeolite P and chabazite are mainly synthesized as the crystal type of zeolite from coal fly ash There exist three steps in alkali hydrothermal reaction of zeolite synthesis: the dissolution step of Si 4q and Al 3q in coal fly ash, the condensation step of silicate and aluminate ions in alkali solution to make aluminosilicate gel, and the crystallization step of aluminosilicate gel to make zeolite crystal The OHy in alkali solution remarkably contributes to the dissolution step of Si 4q and Al 3q in coal fly ash, while Naq in alkali solution makes a contribution to the crystallization step of zeolite P This zeolite has the tendency to capture Kq selectively in the cation exchange site q 2002 Elsevier Science B.V All rights reserved Keywords: zeolite; coal fly ash; hydrothermal synthesis; reaction mechanism ) Corresponding author Tel.: q81-6-6368-0856; fax: q81-6-6388-8869 E-mail addresses: kaihatsu@kcpc.co.jp ŽN Murayama., yhideki@kansai-u.ac.jp ŽH Yamamoto., shibata@kansai-u.ac.jp ŽJ Shibata Tel.: q81-6-6488-2504; fax: q81-6-6401-1143 Tel.: q81-6-6368-0972; fax: q81-6-6388-8869 0301-7516r02r$ - see front matter q 2002 Elsevier Science B.V All rights reserved PII: S - Ž 0 - N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Introduction The recent usage of coal, which has the largest amount of deposit as an energy source, is being reconsidered The amount of coal usage in Japan is significantly increasing every year in the electric power plants The discharge of coal ash enormously increases with an increase in coal usage, and it will be over 10 million tonsryear in 2000 ŽJapan Fly Ash Association, 1995 Coal fly ash discharged from the coal electric power plants occupies a great part of the total amount of coal ash About half of discharged coal fly ash is used as the raw material of cement and so on, but it is a practical problem that the rest of coal fly ash is disposed in the landfill Under these circumstances, it becomes gradually difficult to secure the landfill for burning-up ash The coal fly ash discharged from power plants is designated as the specified by-product on recycling law in Japan, and the coal fly ash is widely being tried as new recycling materials ŽHenmi, 1994; Japan Fly Ash Association, 1995 As one of the effective usage, the conversion of coal fly ash to zeolite ŽHenmi, 1989, 1994 was investigated about 20 years ago In the conversion method, aluminosilicate, which is a main component of coal fly ash, is changed to zeolite crystal by alkali hydrothermal reaction The zeolite synthesized from coal fly ash is applied to the various agricultural materials, which are consumed in large quantities for the purpose of water purification, soil improvement and so on Many researchers ŽHenmi, 1989, 1994; Shibata et al., 1999, 2000; Poole et al., 2000; Lin and His, 1995; Shigemoto et al., 1993; Park and Choi, 1995 report that the zeolite crystal is produced from coal fly ash by hydrothermal reaction, but the detailed reaction mechanism of hydrothermal synthesis has not been clarified adequately It is important to make the reaction mechanism clear for the purpose of designing the manufacturing equipment In this study, hydrothermal synthesis of zeolite from coal fly ash was investigated in alkali solutions of a single or two components of NaOH, Na CO and KOH, in order to explain the reaction mechanism and the role of alkali solution Experimentation 2.1 Coal fly ash Coal fly ash supplied by Denpatsu Coal Tech was used as the raw material of zeolite synthesis The coal fly ash was discharged from coal power plants, and it had the quality in Japanese Industrial Standard 2.2 Zeolite synthesis from single component alkali solutions NaOH, Na CO and KOH were used as alkali sources for zeolite synthesis The coal fly ash was added to 1.0–4.0 molrdm3 alkali solution to prepare fly ash slurry Solid–liquid ratio was 100 gr400 cm3 In the 800-cm3 autoclave, zeolite was synthesized from the slurry under the agitation condition at reaction temperature of 393 K The N Murayama et al.r Int J Miner Process 64 (2002) 1–17 reaction time for zeolite synthesis was and 24 h According to our previous reports ŽShibata et al., 1999, 2000., alkali hydrothermal reaction of zeolite synthesis already starts in temperature rising process until 393 K Therefore, the temperature rising range until 393 K and the constant temperature range were clearly distinguished in this study 2.3 Zeolite synthesis from two-component alkali solutions In order to investigate the effect of cation and anion in alkali solution on zeolite synthesis, two-component alkali solutions of NaOHrNa CO , NaOHrKOH and Na CO 3rKOH were used as alkali sources Total alkali concentration in the solution was 2.0 eqrdm3 In the case of NaOHrNa CO solution, the amounts of OHy and CO 32y were changed under constant amount of Naq On the other hand, in the case of NaOHrKOH solution, the amounts of Naq and Kq were changed under constant amount of OHy Na CO 3rKOH solution contained two cations and two anions in alkali solution The hydrothermal reaction using the above alkali solutions was carried out in the same procedure shown in Section 2.2 2.4 Physical properties of coal fly ash and synthesized zeolite The physical properties of the coal fly ash and synthesized zeolites were measured as follows The chemical composition was analyzed by using an X-ray fluorescence analysis equipment ŽEMAX-3770, Horiba The surface structure was observed by a scanning electron microscope ŽS-2400, Hitati The identification of crystalline materials in coal fly ash and synthesized zeolite was carried out by an X-ray diffraction equipment ŽJDX-3530S, Nihon Denshi The amounts of Si 4q and Al 3q dissolved in alkali solution were determined by using an inductively coupled plasma emission analysis equipment ŽICPS-1000III, Shimadzu Results and discussion 3.1 Physical properties of coal fly ash Table shows the chemical composition of coal fly ash The main components of coal fly ash are the oxides of Si and Al, various metallic oxides and unburned carbon Table Chemical composition of fly ash and the products Žwt.% Products Fly ash Zeolite synthesized in a single component alkali solutions Synthesis conditions NaOH 2.0 molrdm3 , h Na CO 2.0 molrdm3 , h KOH 2.0 molrdm3 , 24 h Si Al Na K Ca 50.4 41.4 20.3 19.8 5.4 17.6 2.7 1.6 7.5 11.2 45.3 18.8 10.2 2.3 40.9 20.5 0.7 26.6 Fe Ti Mg 8.2 5.1 1.8 1.6 2.9 1.7 7.6 10.7 1.9 1.9 5.5 3.6 1.3 1.0 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 The X-ray diffraction patterns for coal fly ash are shown in Fig 1Ža It is confirmed that quartz ŽSiO and mullite Ž3Al O P 2SiO mainly exist as crystalline substance in coal fly ash The SEM photograph of fly ash surface is shown in Fig 2Ža It reveals that the coal fly ash used in this study is spherical an time and 24 h above products are shown in Fig The crystallization of zeolite P increases with an increase in Na CO concentration, but a change in the crystal structure of zeolite as seen in NaOH solution does not occur with an increase in Na CO concentration From these results and our previous studies ŽShibata et al., 1999, 2000., it is considered that the reason why the zeolite crystallization degree is very small in Na CO solution is based on the low dissolution ability of the alkali solution On the other hand, the effect of Kq in alkali solution causes the slow crystallization rate in KOH solution 8 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Fig X-ray diffraction patterns of the products obtained in reaction time and 24 h Qs quartz; M s mullite; P s zeolite P; KC s potassium–chabazite; C sCaCO 3.3 Formation mechanism of zeolite crystal In order to investigate the mechanism of zeolite crystallization, the X-ray diffraction intensity of some crystalline substances contained in the obtained products is examined, and Si 4q and Al 3q concentrations in alkali solution are measured through the zeolite synthesis reaction in NaOH solution N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Fig X-ray diffraction patterns of fly ash and the products obtained in various Na CO concentrations Qs quartz; M s mullite; P s zeolite P; C sCaCO Fig X-ray diffraction intensities of the products obtained in various Na CO concentrations 10 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 The X-ray diffraction intensities of the product obtained in NaOH solution are shown in Fig as a function of reaction time The diffraction intensities of quartz, which is one of the crystalline substances contained in coal fly ash, are gradually reduced through the heating stage, while the intensities of mullite, which is a stable crystalline substance, not change through the hydrothermal reaction The crystallization of zeolite P already begins before reaching the reaction temperature, 393 K, and then the diffraction intensities of zeolite P increase until about h of reaction time Fig 9Ža – Žh shows the SEM observation of coal fly ash and the product obtained in 2.0 molrdm3 NaOH solution as a function of reaction time The amorphous aluminosilicates in coal fly ash are dissolved, and the particle surface changes, like unevenness, until 373 K in a temperature rising stage ŽFig 9Žb – Žd Then, the aluminosilicate gel, which is a prematerial of zeolite, rapidly starts to deposit as a big fluke when the reaction temperature is 393 K ŽFig 9Že As the reaction proceeds, the aluminosilicate gel like a big fluke is transformed to a zeolite crystal, as the lacuna of big fluke is filled ŽFig 9Žf – Žh The SEM observation in Fig corresponds to the phenomena of the zeolite crystallization in Fig The changes in Al 3q and Si 4q concentrations in a liquid phase during the hydrothermal reaction with 2.0 molrdm3 NaOH solution are shown in Fig 10 An Si ingredient in the coal fly ash is dissolved with a linear relation as a function of time in the temperature rising stage, while an Al ingredient rapidly increases at the beginning of the heating stage, and then decreases after 0.5 h Ž353 K in the temperature rising stage The dissolved amounts of Si 4q and Al 3q in alkali solution approach to constant value of 6300 ppm and zero after reaching 393 K, respectively Fig X-ray diffraction intensities of the products obtained with NaOH solution as a function of time N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Fig SEM photographs of the products obtained in 2.0 molrdm3 NaOH as a function time 11 12 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Fig 10 Changes of Al and Si concentrations in liquid phase during hydrothermal reaction with NaOH solution It is considered that when a large quantity of silicate ion exists in the liquid phase, aluminate ions and silicate ions are condensed to form an aluminosilicate gel with an increase in reaction temperature, and then the aluminosilicate gel, which is a prematerial of zeolite crystal, is created The reason for decreasing Al 3q concentration in the alkali solution in a temperature rising stage is explained by the consumption of aluminate ion, based on the above condensation reaction for an aluminosilicate gel The zeolite synthesis reaction takes place at the interface between particle and alkali solution Therefore, as the condensation reaction of aluminosilicate and the formation of a zeolite crystal proceed, aluminate ions and silicate ions are not supplied from coal fly ash because the particle surface is covered with the deposit material, such as an aluminosilicate gel and a zeolite crystal When aluminate ions remaining in an alkali solution are completely consumed, Si and Al ingredients in the alkali solution are not supplied and consumed As a result, it is considered that Si 4q and Al 3q concentrations reach an equilibrium state 3.4 Zeolite synthesis from two-component alkali solutions According to the results in Section 3.3, Naq and OHy behave independently in the hydrothermal reaction In order to clarify the role of cation and anion in zeolite synthesis, the hydrothermal reaction was carried out in two-component alkali solutions of NaOHrNa CO , NaOHrKOH and Na CO 3rKOH The total alkali amount was fixed at 2.0 eqrdm3 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 13 The X-ray diffraction intensities of the zeolite species synthesized in various alkali solutions are shown in Fig 11Ža.,Žb The NaOHrNa CO solution ŽI in Fig 11Ža is the reaction system where the amount of OHy changes under constant amount of Naq The dissolution rate of coal fly ash is closely related to OHy concentration If the value of OHyrŽtotal anion in Fig 11Ža is smaller, the dissolution ability for coal fly ash is lower When the value of OHyrŽtotal anion becomes over 0.25, the diffraction intensity of zeolite P is obtained to be almost the same as that of the zeolite P synthesized in a NaOH solution Therefore, it is considered that the hydrothermal reaction of zeolite P can be remarkably promoted by supplying a little amount of OHy due to the improvement of dissolution ability when Naq adequately exists in the alkali solution The NaOHrKOH solution ŽI, ^ in Fig 11Žb is the reaction system in which the amount of Naq changes under a constant amount of OHy The value of NaqrŽtotal cation being smaller, the reaction rate of zeolite synthesis becomes lower because of a decrease in the crystallization speed The diffraction intensity of zeolite P increases almost proportionally with an increase in NaqrŽtotal cation value It is considered that the amount of Naq in an alkali solution is important in the reaction rate of zeolite P Chabazite crystal, which is not synthesized in KOH solution in 3-h reaction time, is produced in NaOHrKOH solutions at 3-h reaction time The Na CO 3rKOH solution ŽB, ' in Fig 11Ža is the reaction system where four ionic species of cations and anions exist in the alkali solution It is noted that any zeolite species is not synthesized in a single component solution of Na CO and KOH in 3-h reaction time Zeolite P is produced in the ratio of NaqrŽtotal cation from 0.5 to 0.75 The diffraction intensity of zeolite P obtained in these ratios is about half compared with that of zeolite P synthesized in the NaOH solution The role of Naq on zeolite synthesis is more important than that of OHy, because the production region of zeolite P in Fig 11Žb locates in the right side of the abscissa When the mixed alkali solutions containing both Naq and Kq are used, it is important to know the ratio of Naq and Kq captured in the cation exchange site of synthesized zeolite in order to clarify the reaction mechanism The amount of exchangable cations in obtained zeolites is shown in Fig 12 The exchangable cation is calculated by the amount of Naq and Kq extracted from zeolite by an actual cation exchange oparation When the initial ratio of Naq and Kq in the alkali solution is 1:1, it is found that Kq content captured in cation exchange sites of zeolite is about twice as large as the Naq content In order to synthesize zeolite in a short time, it is necessary to enhance the dissolution of coal fly ash and the crystallization of zeolite The OHy in alkali solution contributes to the dissolution of coal fly ash, while Naq makes a contribution to the crystallization of zeolite P The predominant factor for total reaction rate of zeolite formation is the ionic species of Naq In the presence of Naq and Kq, the crystallization degree of zeolite P proportionally decreases with an increase in Kq ratio It is expected that Kq is the suppression factor for zeolite synthesis On the contrary, zeolite P containing Kq and potassium chabazite is synthesized in a short reaction time like h, by adding a slight amount of Naq to alkali solution, due to the ion exchange during the hydrothermal reaction 14 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 15 Fig 12 Amount of exchangeable cation obtained in zeolite Initial alkali solution; NaOH: 2.0 molrdm3 NaOH, h; KOH: 2.0 molrdm3 KOH, h; NaOHrKOH: 10 molrdm3 NaOHq1.0 molrdm3 KOH, h; Na CO rKOH: 0.5 molrdm3 Na CO q1.0 molrdm3 KOH, h 3.5 The role of alkali and reaction mechanism in zeolite synthesis The role of alkali and reaction mechanism in the zeolite synthesis from coal fly ash are investigated from the results obtained in Sections 3.1–3.4 The proposed reaction mechanism is shown in Fig 13 The dissolution reaction of coal fly ash occurs in a temperature rising stage of 293–393 K as a first step at the period of Ža in Fig 13, and then the particle surface changes from sphere to something like unevenness The dissolution rate is remarkably dependent on OHy concentration in the alkali solution It is possible to enhance the dissolution rate drastically by adding a slight amount of OHy The amount of OHy needed to promote the dissolution rate successfully is less than 2.0 eqrdm3 coal fly ash As the next step, the condensation or gelation reaction of silicate ions and aluminate ions begins to take place at about 373 K, and then the concentration of Al 3q dissolved in the alkali solution decreases by the gel formation at the period of Žb in Fig 13 As the condensation reaction proceeds, the aluminosilicate gel rapidly starts to deposit on the particle surface like a big flake Then, the aluminosilicate gel begins to transform to Fig 11 Effect of cation and anion species on X-ray diffraction intensities of zeolites in two component solutions of NaOH, Na CO and KOH 16 N Murayama et al.r Int J Miner Process 64 (2002) 1–17 Fig 13 Proposed reaction mechanism for zeolite synthesis from coal fly ash a zeolite crystal The zeolite crystallization already occurs before the reaction temperature reaches to 393 K In this process, the amount of Naq in the alkali solution dominates the rate of crystallization In a zeolite crystallization stage, the zeolite crystal is conspicuously formed as the last step for about h after reaction temperature reaches to 393 K, at the period of Žc in Fig 13 As the above gelation and zeolite crystallization progress, the surface of coal fly ash is covered with aluminosilicate, and the Al 3q concentration in the alkali solution substantially decreases As a result, the zeolite crystallization becomes very slow after all dissolved aluminate ion is consumed to form an aluminosilicate In the presence of Naq and Kq in an alkali solution, the zeolite containing a large amount of Kq is synthesized Naq type zeolite is synthesized at first, and then the Naq type zeolite changes to Kq type zeolite in alkali solution during a series of zeolite synthesis reactions This is the reason why the Kq type zeolite is produced in the alkali solution containing Naq and Kq Summary The zeolite synthesis from coal fly ash was carried out in various alkali solutions by using a hydrothermal reaction The reaction mechanism and the role of alkali in the zeolite synthesis were investigated Three steps, namely, dissolution, condensation and crystallization steps, exist in an alkali hydrothermal reaction for zeolite synthesis The dissolution of coal fly ash begins N Murayama et al.r Int J Miner Process 64 (2002) 1–17 17 to occur in the temperature rising stage, 298–393 K, and the amount of OHy in an alkali solution makes a great contribution to this reaction As the condensation of aluminate ion and silicate ion takes place, the particle surface of coal fly ash is covered with the deposit of the aluminosilicate gel The amounts of Si 4q and Al 3q ions dissolved from coal fly ash rapidly decrease according to the progress of condensation reaction The crystallization of zeolite P already begins to occur before the reaction temperature attains 393 K The crystallization rate was controlled by the amount of Naq in the alkali solution The factor determining the total reaction rate of zeolite synthesis is mainly Naq concentration in the alkali solution When Naq and Kq coexist in the alkali solution of the hydrothermal reaction, the crystallization rate decreases with an increase in Kq concentration References Henmi, T., 1989 A physico-chemical study of industrial solid wastes as renewable resource-zeolitization of coal clinker ash and paper sludge incineration ash Mem Agric Dept., Ehime Univ 33 Ž2., 143–149 Henmi, T., 1994 Sangyouhaikibutu no Zeolite-tenkanniyoru saisigenka Yuukouriyou-Gijyutukaihatu New Technol Sci., 3–166 Japan Fly Ash Association, 1995 Coal Ash Hand Book 2nd edn., pp II1–12 Lin, C.-F., His, H.-C., 1995 Resource recovery of waste fly ash: synthesis of zeolite like materials Environ Sci Technol 29, 1109–1117 Park, M., Choi, J., 1995 Synthesis of phillipsite from fly ash Clay Sci 9, 219–229 Poole, C., Prijatama, H., Rice, N.M., 2000 Synthesis of zeolite adsoebents by hydrothermal treatment of PFA wastes: a comparative study Miner Eng 13 Ž8–9., 831–842 Shigemoto, N., Hayashi, H., Miyake, K., 1993 Selective formation of Na–X zeolite from coal fly ash by fusion with sodium hydroxide prior to hydrothermal reaction J Miner Sci 28, 4781–4786  ... an alkali solution, the zeolite containing a large amount of Kq is synthesized Naq type zeolite is synthesized at first, and then the Naq type zeolite changes to Kq type zeolite in alkali solution... diffraction intensity of zeolite P is obtained to be almost the same as that of the zeolite P synthesized in a NaOH solution Therefore, it is considered that the hydrothermal reaction of zeolite P can be... The diffraction intensity of zeolite P obtained in these ratios is about half compared with that of zeolite P synthesized in the NaOH solution The role of Naq on zeolite synthesis is more important