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One-step high efficiency crystallization of zeolite A from ultra-fine circulating fluidized bed fly ash by hydrothermal synthesis method
Fuel 257 (2019) 116043
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Full Length Article
One-step high efficiency crystallization of zeolite A from ultra-fine circulating fluidized bed fly ash by hydrothermal synthesis method
T
Ze Liu , Siqi Li, Li Li, Jixiang Wang, Yu Zhou, Dongmin Wang ⁎
School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, PR China
ARTICLE INFO
ABSTRACT
Keywords: Circulating fluidized bed fly ash Zeolite A Hydrothermal synthesis Box-Behnken design Crystallinity
The present work focused on the crystallization of zeolite A directly from ultra-fine circulating fluidized bed fly ash (UF-CFA) by alkali-activated hydrothermal synthesis method without high temperature calcining and longterm crystallizing. Response surface methodology, namely Box-Behnken design was employed to optimize the synthesis conditions by varying the crystallization temperature, crystallization time, and NaOHaq concentration. The obtained zeolite A was subjected to mineralogical (X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and chemical (Fourier transform infrared spectrometer) characterization. The results showed that zeolite A begins to form at 90 °C – 2.5 h with NaOHaq concentration of 2.6 mol/L. The crystallinity at 3 h was approximately 90.1%, and pure zeolite A was formed at 6 h with maximum crystallinity of 97.9%. The solid-liquid separation of the synthesized mixture was carried out under the conditions of the appearance of zeolite A at 2.5 h. Crystal structure and solid-liquid composition of the product during the hydrothermal reaction were analyzed. It is explored the growth mechanism of zeolite A by clarifying the evolution of substances in solid-liquid phases and composition of products in the process of zeolite formation.
1. Introduction In the past decades, a variety of technologies have been developed for the effective utilization and safe management of fly ash. At present, Japan and the United States have the most outstanding research results in the preparation of zeolite molecular sieves using fly ash, and the comprehensive utilization rate of fly ash in Japan is as high as 100% [1] Some researchers have also studied the process, mechanism and application of zeolite synthesis from fly ash in recent years [2–5]. The physical activity of fly ash is mainly reflected in that it can promote the gelling activity of products and improve the performance of coal ash products to some extent [6]. The chemical activity of fly ash mainly comes from the amorphous SiO2 and Al2O3. In the presence of alkali, hydration reaction can occur [7]. Due to the high content of silica and alumina, fly ash can be converted into valuable zeolite products [8–10]. In fact, fly ash is a valuable resource for the production of building materials, whether for cement or concrete admixtures [11]. In terms of the purification of waste gas, fly ash can well absorb SO2 and SO3 in the waste gas, and can also activate and adsorb NOx and mercury vapor in the waste gas [12–14]. Other applications include soil improvement, waste water treatment, the extraction of alumina from fly ash, and so on [5,15]. Fly ash in coal burned boiler is mostly spherical, and surface is ⁎
dense and smooth, mainly containing glass phase, quartz and mullite [16]. Unlike ordinary pulverized coal furnace fly ash (PFA), circulating fluidized bed fly ash (CFA) is produced by burning coal at 800–900 °C in a circulating fluidized bed. It is a kind of fly ash with high SO3 content, porous and large specific surface area [17]. In recent years, as a new type of coal-fired technology, circulating fluidized bed combustion technology has been widely used because of its low emission concentration of nitrogen oxides, low pollution emission and high thermal efficiency [18,19]. With the increase of circulating fluidized bed power plants in China, CFA is also produced in large quantities [20–22]. Therefore, making full use of CFA to turn waste into treasure is of great significance for developing a friendly environment and promoting the economic development [23,24]. Holler and Wirsching [25] firstly used fly ash to synthetic zeolite molecular sieves. Since then, the use of fly ash to prepare zeolite molecular sieves has become one of the research hotspots of fly ash resource utilization. The SiO2/Al2O3 ratio of fly ash used in this paper is 1.76, which is an ideal molar ratio for the preparation of zeolite A [26,27]. Therefore, the use of such fly ash to make zeolite A does not require the addition of any Si source and Al source. It is focused on the synthesis of zeolite A from UF-CFA in this paper. According to the previous literatures, it has been found that there have been many studies on the preparation of zeolite A by fly ash. Gong et al. [28]
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.fuel.2019.116043 Received 11 April 2019; Received in revised form 10 August 2019; Accepted 17 August 2019 Available online 30 August 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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Table 1 Chemical composition of fly ash (wt %).
Table 2 Experiment designs with levels in the BBD.
Raw material
SiO2
Al2O3
CaO
SO3
Fe2O3
TiO2
K2O
MgO
P2O5
ZrO2
UF-CFA
43.62
42.22
3.89
2.76
2.54
1.86
1.18
1.11
0.24
0.123
synthesized zeolite A with well crystal shape and large specific surface area from fly ash by hydrothermal synthesis method, the result indicated that: optimum synthesis conditions were 600 °C of alkaline fusion temperature, mass ratio 1–2 of fly ash/NaOH, 90 °C of hydrothermal temperature, 24 h of hydrothermal time. Li et al. [29] determined the optimum conditions for synthesizing Zeolite A by hydrothermal synthesis were 750 °C of calcining temperature, mass ratio 1.3 of fly ash/NaOH, 100 °C of crystallizing temperature, 10 h of crystallizing time. Rentsenorov et al. [30] synthesized zeolite A from fly ash performed with an acid-washing process by hydrothermal treatment and the highest content of zeolite A was formed in the mixture treated at 80 °C for 8 h. Ge [31] used alkali melt decomposition sample of coal fly ash as raw material, added aluminum hydroxide and sodium hydroxide solution to adjust the ratio of each substance in the reaction solution. It indicates that the conditions for obtaining zeolite A were 98 °C of crystallizing temperature, 5 h of crystallizing time. Meng et al. [32] proposed that alkaline assisted calcination activation or alkaline calcination combined with hydrothermal processes can be utilized to achieve pure zeolites, which is superior to the traditional thermal decomposition method, as discussed by Feng [33], and Zhao [34]. In this paper, it is firstly synthesized zeolite A directly from UF-CFA by alkali-activated hydrothermal synthesis method at 90 °C for 3 h without high temperature calcining and long-term crystallizing. The response surface method, namely box-behnken design, is used to obtain the optimal synthesis parameters to maximize the percentage of crystallinity. Zeolite A was characterized by X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersive (SEM/EDX), and Fourier transform infrared spectrometer (FT-IR). It was also explored the growth mechanism of zeolite A from UF-CFA with low reaction temperature and short synthesis time [14].
Run
NaOHaq concentration
Crystallization temperature (°C)
Crystallization Time (h)
Percent crystallinity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2.6 2.6 1.6 1.6 0.6 0.6 1.6 2.6 1.6 1.6 0.6 1.6 1.6 0.6 1.6 2.6 1.6
120 90 60 90 90 120 90 60 60 90 90 90 120 60 90 90 120
4 2 6 4 6 4 4 4 2 4 2 4 6 4 4 6 2
88.7 73.4 78.3 81.2 68.3 53.2 81.2 78.4 69.8 81.2 57.3 81.2 83.2 73.5 81.2 97.9 63.5
by X-ray fluorescence spectrometer (XRF). The results are shown in Table 1. X-ray analysis of UF-CFA (Fig. 1a) has shown the presence of quartz, anhydrite and hematite. The raised background at 20° to 30° indicates the presence of the amorphous phase [35]. Fig. 1b has shown that the particle size distribution of the raw material is between 1 and 6 μm, with a relatively high content of 1–2 μm and 3–5 μm. Compared with PFA, particle size of UF-CFA is very small, which contributes to the sufficient reaction of the raw materials. Different from the spherical morphology of PFA, the morphology of UF-CFA is irregular granular or flocculent as shown in Fig. 6a, which is mainly caused by the amorphous structure of UF-CFA. Therefore, UF-CFA has higher activity than PFA [17]. According to the previous studies [36,37], the optimum siliconaluminum ratio of fly ash to form zeolite A is generally between 0.75 and 1. The initial Si/Al ratio of raw material used in this study is 0.88. Therefore, the study was to prepare zeolite A using UF-CFA without adjusting silicon-aluminum ratio. In addition, sodium hydroxide (NaOH, A.R., > 99% pure, Beijing Chemical Works, China) were employed as alkaline activator (AA) in the experiment.
2. Experimental 2.1. Materials The chemical composition of UF-CFA (Shanxi, China) was measured
Fig. 1. (a) XRD pattern of UF-CFA, (b) Particle distribution of UF-CFA. 2
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2.2. Design of experimentation
Table 3 Experiment designs with levels.
Response Surface Method (RSM), a statistical test design for optimizing biological processes [38], is used to establish a continuous variable surface model and evaluate the factors of affecting biological processes and their interactions. In this study, RSM-box Behnken method was firstly used to design the experiment to reduce the blindness of the experiment. Based on earlier studies, we have chosen three important parameters with the ranges as NaOHaq concentration 0.6–2.6 M, crystallization temperature 60–120 °C, and crystallization time 2–6 h. The specific experimental scheme is shown in Table 2. The results showed that, the highest crystallinity percentage reached 97.9% at NaOHaq concentration of 2.6 M, crystallization temperature of 90 °C, and crystallization time of 6 h. In order to verify and optimize these three parameters, the single-factor control variable method is adopted to design the experiment by taking points in the upper and lower range of the optimal parameters and is listed in Table 3.
NaOHaq concentration
2.3. Synthesis of zeolite A UF-CFA, sodium hydroxide, and distillated water were used as raw materials. The sodium hydroxide solution was prepared by dissolving sodium hydroxide in deionized water by stirring at room temperature for about 15 min. Then this solution was transferred to a 100 mL capacity, Teflon-lined hydrothermal reaction vessel, autoclave, and then UF-CFA was added. According to the conditions of designed experimental scheme, the autoclaves were then heated in an oven at certain conditions as shown in Tables 2 and 3. After crystallization periods, the autoclaves were removed from the oven and cooling to stop the crystallization process. The reacting solution was filtrated and the obtained products was washed several times with deionized water to a pH of 8–9. Finally, the hydrothermal products were dried at 80 °C for 12 h for characterization.
Crystallization temperature (°C)
Crystallization Time (h)
Percent crystallinity (XRD)
Remarks
Effect of NaOHaq concentration 0.6 90
6
68.3%
1.0
90
6
71.5%
1.8 2.6 3.0
90 90 90
6 6 6
83.2% 97.9% 89.8%
Amorphous phases Amorphous phases Zeolite A Zeolite A Zeolite A
Effect of Crystallization temperature 2.6 60 6
80.4%
2.6 2.6 2.6
6 6 6
97.9% 89.6% 77.6%
Effect of Crystallization Time 2.6 90
0.5
56.2%
2.6
90
2.0
78.4%
2.6
90
2.5
89.8%
2.6 2.6 2.6 2.6 2.6 2.6
90 90 90 90 90 90
3.0 4.0 6 12 24 48
90.1% 93.6% 97.9% 81.9% 70.5% 72.1%
90 120 150
Amorphous phases Zeolite A Zeolite A Sodalite Amorphous phases Amorphous phases Partially crystalline Zeolite A Zeolite A Zeolite A Zeolite A Zeolite A Sodalite
2.4. Characterization Both the raw minerals and the synthesized zeolite A were characterized by X Ray Fluorescence (XRF), X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The particle size of UF-CFA was analyzed by using a LS-C (IIA) particle size analyzer. The PANalytical XRF detector was used for chemical composition analysis. XRD patterns were obtained by using a X'Pert PRO MPD X-ray diffractometer with Cu Kα radiation operating at 40 kV and 40 mA over the 2θ range of 5–90°. FT-IR was recorded on a Nicolet iS10 spectrometer in the range 500–4000 cm−1. SEM and energy dispersive X-ray (EDX) spectrum were obtained with a emission scanning electron microscope energy spectrum analyzer (Model JSM-7001F + INCA X-MAX), operating at an accelerating voltage of 15 kV.
Fig. 2. XRD pattern of the effect of different NaOHaq concentrations on the synthesis of zeolite A.
3. Results and discussion 3.1. Optimize experimental parameters
concentration largely affects the extent of hydrothermal reaction. No other type of zeolite form in lower alkalinity. According to Murayama et al. [39], the concentration of Na+ ions in the alkaline solution is the main factor determining the total reaction rate of zeolite synthesis. It's probably that higher alkalinity can promote the dissolution of the silicaalumina phase. In this study, the silica-alumina ratio of the dissolved phase was favorable for forming zeolite A when the alkalinity was higher than 1.8 M. However, excessive alkalinity will also inhibit the formation and growth of zeolite A.
3.1.1. Effect of NaOHaq concentration on zeolite synthesis The effect of NaOHaq concentration on zeolite synthesis plays an important role in the zeolite synthesis process. Fig. 2 shows the XRD pattern of the effect of different concentrations of NaOHaq (0.6, 1.0, 1.8, 2.0, 2.6, and 3.0 M) on the synthesis of zeolite A at 90 °C–6 h. XRD patterns clearly reveal that zeolite A presents when the NaOHaq concentration is 1.8 M or more, and a large amount of zeolite A is formed when the NaOHaq concentration is 2.6 M. Therefore, NaOHaq
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intensities of the sodalite gradually increase with the increasing hydrothermal time. These results indicate that zeolite A as metastabile phase is decomposed gradually and results in the supersaturation of liquid phase with aluminosilicate anions, which promotes the formation of sodalite nuclei. Therefore, reasonable control of time can obtain zeolite A in a short time. 3.2. Characterization of products Comprehensive analysis of the above experimental data, diffraction peaks of analyzed phases at 90 °C–2.6 M–6 h are sharp and characterized by high intensity, which indicates a full formation of the crystal structure. The XRD data under this condition were analyzed, and the structural parameters (90 °C–2.6 M–6 h) of the zeolite were calculated (Table 4), which were compared to the standards taken from the PDF-2 diffraction database (2009 release) formalized by the JCPDS-ICDD. The obtained zeolite materials are characterized by very similar unit cell parameters in relation to the standards. Neither the length of the unit cell parameters a, b, c, the α, β, γ angles, nor the volume showed any significant difference with respect to the reference materials (standards). That is, almost pure zeolite A is formed at 90 °C–2.6 M–6 h. In addition, a certain amount of zeolite A has been synthesized at 2.5 h, and the main product is still zeolite A after 12 h of reaction. Fig. 5 is an FTIR pattern of the effect of different reaction times on the synthesis of zeolites A at 90 °C–2.6 M. A ν (O–H) asymmetric and symmetric stretching vibration at 3430–3460 cm−1 can be assigned as caused by H2O molecules in silicate gel and hydrated aluminosilicates. With the reaction time increasing, the position of the absorption peak changes to a low wave number. This may be due to the variation of concentration of hydration products and acidic character of the terminal O–H groups [40]. 1640–1660 cm−1 is the bending vibration peak of H2O molecules and also represents the vibration of H–O or H–OH in the product hydration aluminosilicates [41]. The strongest peak appeared at 990–1100 cm−1 demonstrates the phase change of amorphous phases into high crystalline zeolite A. The wave number of the absorption peak at 990–1100 cm−1 gradually decreases with the increase of time, which is clearly shown in Fig. 5. This is because, in the process of forming zeolite A, the silicon ions and aluminum ions released into the solution gradually form silicon tetrahedron (SiO4) and alumina tetrahedron (AlO4) under strong base conditions [42]. Moreover, there are asymmetric stretching vibrations of Si–O, Al–O and Si–O–Al in the tetrahedron, which are susceptible to the composition of Si and Al in the structure. The position of the absorption peak shifts to a low wave number as the number of Al atoms in the tetrahedron
Fig. 3. XRD pattern of the effect of different temperature on the synthesis of zeolite A.
3.1.2. Effect of crystallization temperature on zeolite synthesis Fig. 3 shows the XRD pattern of the effect of different reaction temperatures (60 °C, 90 °C, 120 °C, and 150 °C) on the synthesis of zeolite A at 6 h–2.6 M. It can be seen that most of UF-CFA didn't react at 60 °C; UF-CFA started to produce a relatively pure zeolite A until 90 °C; sodalite appears and zeolite A is no longer pure when the temperature is raised to 120 °C; zeolite A becomes less and the product is mainly Na-P1 zeolite and sodalite at 150 °C. It is considered that the type of zeolite formed is largely affected by reaction temperature. Thus, it can be concluded that low crystallization temperatures result in low crystallinity percentage and crystal size. The type and purity of the finally obtained crystal can be changed by increasing the reaction temperature. And excessive temperatures decompose the synthesized zeolite to reduce crystallinity. 3.1.3. Effect of crystallization time on zeolite synthesis The crystallization time also has an important influence on the hydrothermal synthesis of zeolite A. Fig. 4 shows the XRD pattern of the effect of different reaction times (0.5–48 h) on the synthesis of zeolite A at 90 °C–2.6 M. The increase of time contributes to the formation of zeolite A, but the quite long term will produce other crystal phases and resulting in the synthesis of zeolite A is impure. Fig. 4 indicates that
Fig. 4. XRD pattern of the effect of different reaction time on the synthesis of zeolite A.
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Table 4 Calculated structural parameters of the received zeolite A compared to the patterns. Parameter
Zeolite A (90 °C –2.6 M–2.5 h)
Zeolite A (90 °C–2.6 M–6 h)
Zeolite A (90 °C–2.6 M–12 h)
Standard Zeolite A with HS no ref.: 00-039-0222
a,b,c [Å] α,β,γ [°] V [Å3]
24.580 90 14850.66
24.590 90 14878.79
24.630 90 14941.47
24.610 90 14905.10
Fig. 5. FTIR spectra of zeolite A at 90 °C–2.6 M under different reaction time, (a) 30 min, 2 h, 2.5 h, 3 h, and 4 h, (b) 6 h, 12 h, 24 h, and 48 h.
increases. Similarly, the absorption peak at 1640–3450 cm−1 may also be related to water molecules located in the zeolite channels or exchangeable cations. The amorphous phase gradually dissolves with the reaction time increasing. This peak demonstrates the components in the raw material gradually react into new structure and the structure of zeolite A becomes more obvious. The SEM pattern of synthesized zeolite A at different time is in good agreements with the XRD and FTIR as shown in Figs. 4 and 5. Fig. 6c shown that there are 2 μm cubes appeared in the product after 2.5 h, which is growing zeolite A. However, these cubes had unclear edges and were likely to be covered with unconverted amorphous materials. Most UF-CFA was converted to zeolite A at 3 h, and only a few crystals may have some defects. Fig. 6g clearly reveals that zeolite A formed at 6 h is characteristic cubic crystal. This morphological form is the smallest feature of traditional crystallographic systems. The sizes of obtained zeolite A are about 4 μm. However, SEM patterns indicate that the sodalite gradually increase with the increasing hydrothermal time. Not only zeolite A but also sodalite appeared at 24 h. Thus, the formation of zeolite A requires shorter hydrothermal time and it is considered that the stages of transformation are from UF-CFA, zeolite A to sodalite.
ANOVA results is shown in Table 5, where the closeness of the R2 value (adjusted sum) further validates the applicability of the polynomial model. Experimental run results indicate a maximum percent crystallinity of 97.9%. The final equation of the actual factor is as follows:
Crystallinity (%) = + 56.71450 7.37750×NaOHaq concentration +0.18533 × Temperature + 3.80625×Time +0.25500 × NaOHaq concentration×Temperature +0.68750 × NaOHaq concentration×Time +0.046667 × Temperature×Time 3.61250 × NaOHaq concentration2 4.59722 E 3 × Temperature 2 0.84062 × Time 2 The above formula expresses the percentage of crystallinity by actual factors. The coefficient of NaOH/UF-CFA ratio describes the most critical effect of percent crystallinity. A 3D plot of the percentage of crystallinity is shown in Fig. 9. Response surface methodology (RSM) is the optimization method [45]. It takes the response (crystallinity) of the system as a function of one or more factors (such as NaOH concentration, temperature, and time), and uses graphical techniques to display this functional relationship. The highest point in Fig. 9 is to present the optimal conditions in a more intuitive way.
3.3. Fitness of proposed model for percent crystallinity of zeolite A
3.4. The formation mechanism of zeolite A
The normal probability map and actual versus predicted plots are shown in Fig. 7. Fig. 7a shows the probability distribution of the simulation results. The data exhibit obvious concentricity and symmetry, and normally distribution. The closeness of actual and predicted value of crystallinity percentage is clearly described in the Fig. 7b. Fig. 8 indicates a bar graph of the comparison chart for experimentation (as shown in Table 2) and prediction of crystallinity percentage [43,44]. The results shown in Figs. 7 and 8 verify the reliability of the polynomial model. The percentage of the factorial model selected by the
To explore the growth process of zeolite, in the presence of zeolite A at 2.5 h, the mixture of synthesized zeolite product was subjected to solid-liquid separation by means of suction filtration. And the solid is first washed to about pH = 8 with deionized water, and the solid product is washed with absolute ethanol, and the liquid is allowed to stand for a short period of time. Separated gels and dried solids were characterized [46]. Fig. 10 is SEM image of the solid-liquid separation product at 90 °C−2.6 M–2.5 h. As can be seen from the Fig. 10, there
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Fig. 6. SEM images of the effect of different reaction time on the synthesis of zeolite A at 6 h–2.6 M (a) UF-CFA, (b) 0.5 h, (c) 2 h, (d) 2.5 h, (e) 3 h, (f) 4 h, (g) 6 h, (h) 12 h, (i) 24 h, and (j) 48 h.
Fig. 7. (a) Normal probability plot, (b) predicted vs. actual plot of crystallinity percentage.
are some cubes that are about 2 μm in size and have not yet clear edges and corners in the solid phase product. EDX spectrum analysis was performed on the red frame in the figure, and its element composition is
shown in Table 6. According to the data of XRD, these cubes can be judged to be growing zeolites A [47]. The product in the liquid phase is the gel phase that reacts for 2.5 h during the synthesis of the zeolite. It is
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Fig. 8. Comparison bar chart for experimental and predicted crystallinity percentage. Table 5 Results of ANOVA for the crystallinity percentage of factorial model. Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
Model A-NaOHaq concentration B-Temperature C-Time AB AC BC A^2 B^2 C^2 Residual Lack of Fit Pure Error Cor Total Summary of quadratic model Response %CV
1956.08 926.65 16.25 507.21 234.09 45.56 31.36 54.95 72.08 47.61 19.42 19.42 0 1975.5
9 1 1 1 1 1 1 1 1 1 7 3 4 16
217.34 926.65 16.25 507.21 234.09 45.56 31.36 54.95 72.08 47.61 2.77 6.47 0
78.35 334.06 5.86 182.85 84.39 16.43 11.31 19.81 25.98 17.16
< 0.0001 < 0.0001 0.0461 < 0.0001 < 0.0001 0.0049 0.012 0.003 0.0014 0.0043
Significant
S.D.* 1.67
Mean 75.97
R2 0.9902
A-R2 0.9775
P-R2 0.8427
C.V.% 2.19
AP 33.437
* S.D.: standard deviation, C.V. %: coefficient of variation, R2: R-squared, A-R2: adjusted R-squared, P-R2: predicted R-squared, AP: adequate precision.
an essential component of the synthetic zeolite. During the later reaction, the gel phase will participate in the formation of zeolite crystals. Combined with XRD and FTIR, it can be seen that the first step of the formation of zeolite A is to dissolve Si4+ and Al3+ from fly ash in the alkali solution, and then the silicate ions and aluminate ions dissolved in the alkali solution to form silicate aluminate gel. With the increase of reaction time, zeolite A is formed. Fig. 11 displays the growth process of zeolite A [48,49].
NaOHaq concentration. However, reaction temperature directly affects the type of zeolite. In this study, not only zeolite A, but also sodalite is formed at 120 °C, and Na-P1 zeolite appeared at 150 °C. Similarly, the increase of time contributes to the formation of Zeolite A, but the quite long term will produce other crystal phases and result in the impurity of synthesized zeolite A. Response surface methodology employed in this study, namely Box-Behnken design was accounted to optimize the synthesis conditions and finally obtained the optimal parameter NaOHaq Concentration of 2.6 M, the crystallization temperature of 90 °C. Zeolite A had been formed by the reaction at 2.5 h, the crystallinity at 3 h was approximately 90.1%, and pure zeolite A was formed at 6 h with maximum crystallinity of 97.9%. In addition, the mechanism of synthesis of zeolite A from UF-CFA by alkali-activated hydrothermal method was explored. It mainly divided into the following three steps: Silicate ions and aluminate ions are dissolved from the raw material; Silicate ions and aluminate ions dissolved into alkali solution form silicate aluminate gel; and crystallizes under certain reaction conditions to form zeolite A. Based on these results, it can be concluded that the
4. Conclusion The focus of this paper is to one-step high efficiently synthesize zeolite A from UF-CFA by alkali-activated hydrothermal synthesis method. Experimental results show that the effect of NaOHaq concentration on zeolite synthesis plays an important role in the zeolite synthesis process. NaOHaq concentration not only affects the reaction degree of UF-CFA, but also affects the content of zeolite A. The content of zeolite A increases first and then decreases with the increase of
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100 100
90
C ry s ta llin ity (% )
90
C ry s ta llin ity (% )
80 70 60 50
120.00 114.00 108.00 102.00 96.00 90.00 84.00 78.00 72.00 B: Crys.Temp.(Deg C) 66.00 60.00 0.60
2.60
80 70 60 50
6.00
2.10
A: NaOHaq concentration
1.10
2.60
5.00
1.60
2.10
4.00
C: Crys.Time(hr)
1.60 3.00
1.10 2.00 0.60
A: NaOHaq concentration
100
C ry s ta llin ity (% )
90 80 70 60 50
120.00 114.00 108.00 102.00 96.00 90.00 84.00 78.00 72.00 B: Crys.Temp.(Deg C) 66.00 2.00 60.00
6.00 5.00 4.00 3.00
C: Crys.Time(hr)
Fig. 9. 3D plots obtained from the response of crystallinity percentage.
Fig. 10. SEM images of solid-liquid separation synthesized at 90 °C–2.6 M–2.5 h. (a) solid products, (b) liquid product. Table 6 Elemental composition of the solid-liquid separation product at 90 °C–2.6 M–2.5 h. Solid product (at. %)
Liquid product (at. %)
Elements Atomic (%)
1 2 3 4 5
O
Na
Al
Si
Elements
64.4 64.32 64.41 64.35 64.4
8.38 8.39 8.35 8.33 8.35
13.06 13.15 13.02 13.00 13.12
14.16 14.14 14.22 14.32 14.13
Atomic (%)
8
1 2 3 4 5
O
Na
Al
Si
60.98 60.98 60.96 61.01 60.99
16.74 16.72 16.68 16.70 16.74
11.13 11.12 11.15 11.15 11.10
11.15 11.18 11.21 11.14 11.17
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Experimental study on synthesis of zeolite A from coal fly ash. China Resourc Comprehens Utilizat 2011;29:43–5. [30] Rentsenorov U, Davaabal B, Temuujin J. Synthesis of zeolite A from mongolian coal fly ash by hydrothermal treatment. Solid State Phenomena 2018;271:1–8. [31] Ge Y. Synthesis of zeolite 4A from alkali melt decomposition sample of coal fly ash. China Resourc Comprehens Utilizat 2003;01:25–7. [32] Meng QP, Chen H, Lin JZ, Lin Z, Sun JL. Zeolite A synthesized from alkaline assisted pre-activated halloysite for efficient heavy metal removal in polluted river water and industrial wastewater. J Environ Sci (China). 2017;56:254–62. [33] Feng H, Li CY, Shan HH. In-situ synthesis and catalytic activity of ZSM-5 zeolite. Appl Clay Sci 2009;42:439–45. [34] Zhao YF, Zhang B, Zhang X, et al. Preparation of highly ordered cubic NaA zeolite from halloysite mineral for adsorption of ammonium ions. J Hazard Mater 2010;178:658–64. [35] Tauanov Z, Shah D, Inglezakis V, Jamwal PK. Hydrothermal synthesis of zeolite production from coal fly ash: A heuristic approach and its optimization for system identification of conversion. J Cleaner Prod 2018;182:616–23. [36] Tümsek F, İnel O. Evaluation of the thermodynamic parameters for the adsorption of some n-alkanes on a type zeolite crystals by inverse gas chromatography. Chem Eng J 2003;94:57–66. [37] Tanaka H, Fujii A, Fujimoto S, Tanaka Y. Microwave-assisted two-step process for the synthesis of a single-phase Na-A zeolite from coal fly ash. Adv Powder Technol 2008;19:83–94. [38] Tan XH, Wang JG, Liu XR. Improved response surface method and its application to reliability analysis. Chinese J Rock Mech Eng 2005;24:5875–9. [39] Murayama 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. [40] Mouhtaris T, Charistos D, Kantiranis N, Filippidis A, Kassoli-Fournaraki A, Tsirambidis A. GIS-type zeolite synthesis from Greek lignite sulphocalcic fly ashes promoted by NaOH solutions. Microporous Mesoporous Mater 2003;61:57–67. [41] Visa M. Synthesis and characterization of new zeolite materials obtained from fly ash for heavy metals removal in advanced wastewater treatment. Powder Technol 2016;294:338–47. [42] Zhang KY, Jiang H, Qin G. Utilization of zeolite as a potential multi-functional proppant for CO2 enhanced shale gas recovery and CO2 sequestration: A molecular simulation study on the competitive adsorption of CH4 and CO2 in zeolite and organic matter. Fuel 2019;249:119–29. [43] Torkittikul P, Chaipanich A. Utilization of ceramic waste as fine aggregate within Portland cement and fly ash concretes. Cem Concr Compos 2010;32:440–9. [44] Sakai E, Miyahara S, Ohsawa S, Lee S-H, Daimon M. Hydration of fly ash cement. Cem Concr Res 2005;35:1135–40. [45] Chen YX, Wang YX, Liu J, Zhang MM. Experimental study on strength of solidified mud based on response surface method. Shanxi Architect 2019;45:89–91. [46] Bosnar S, Antonić T, Bronić J, Subotić B. Mechanism and kinetics of the growthof zeolite microcrystals. Part 2: Influence of sodium ions concentration in the liquid phase on the growth kinetics of zeolite A microcrystals. Microporous Mesoporous Mater 2004;76:157–65. [47] Nešić AR, Veličković SJ, Antonović DG. Modification of chitosan by zeolite A and adsorption of Bezactive Orange 16 from aqueous solution. Compos Part B: Eng 2013;53:145–51. [48] Fu KM, Liu TX, Zhang QS, Yuan DL. Hydrothermal synthesis of zeolite A from fly ash and its growth mechanism. J Wuhan Univ Technol 2008;30:25–7. [49] Abdullahi T, Harun Z, Othman MHD. A review on sustainable synthesis of zeolite from kaolinite resources via hydrothermal process. Adv Powder Technol 2017;28:1827–40.
Fig. 11. The growth process of zeolite A.
synthesis of zeolite A from UF-CFA by alkali-activated hydrothermal synthesis has several advantages, such as easy operation, low reaction temperature, and short reaction time. The view is to propose a process for the production of the well-formed zeolite A in a short time if a large full-scale synthesis would be considered. Acknowledgement This work was financially supported by the National Key Research and Development Program of China (2017YFC0703200) and Beijing Municipal Natural Science Foundation (2192047). References [1] Bukhari SS, Behin J, Kazemian H, Rohani S. Conversion of coal fly ash to zeolite utilizing microwave and ultrasound energies: A review. Fuel 2015;140:250–66. [2] Valle-Zermeno R, Giro-Paloma J, Formosa J, Chimenos JM. APC fly ash recycling: Development of a granular material from laboratory to a pilot scale. Waste Biomass Valorizat 2017;8:1409–19. [3] Paramitha T, Wulandari W, Rizkiana J, Sasongko D. Performance evaluation of coal fly ash based zeolite A for heavy metal ions adsorption of wastewater. IOP Conf Ser: Mater Sci Eng 2019;543:012095. [4] Ojumu TV, Du Plessis PW, Petrik LF. Synthesis of zeolite A from coal fly ash using ultrasonic treatment – A replacement for fusion step. Ultrason Sonochem 2016;31:342–9. [5] Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. A comprehensive review on the applications of coal fly ash. Earth Sci Rev 2015;141:105–21. [6] Hollman GG, Steenbruggen G, Janssen-Jurkovičová M. A two-step process for the synthesis of zeolites from coal fly ash. Fuel 1999;78:1225–30. [7] Lin CF, Hsi H-C. Resource recovery of waste fly ash: synthesis of zeolite-like materials. Environ Sci Technol 1995;29:1109–17. [8] Shih W-H, Chang HL. Conversion of fly ash into zeolites for ion-exchange applications. Mater Lett 1996;28:263–8. [9] Querol X, Moreno N, Umaña JC, Alastuey A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [10] Jha VK, Nagae M, Matsuda M, Miyake M. Zeolite formation from coal fly ash and heavy metal ion removal characteristics of thus-obtained Zeolite X in multi-metal systems. J Environ Manage 2009;90:2507–14. [11] Lieberman RN, Knop Y, Querol X, Moreno N, Muñoz-Quirós C, et al. Environmental impact and potential use of coal fly ash and sub-economical quarry fine aggregates in concrete. J Hazard Mater 2018;344:1043–56. [12] Pan Z, Luo JJ, Xue SS, Luo JY. Retrospect and outlook on utilization of fly ash. Environ Sanit Eng 2008;16:19–22. [13] Deng H, Yi HH, Tang XL, Yu QF, Ning P, Yang LP. Adsorption equilibrium for sulfur dioxide, nitric oxide, carbon dioxide, nitrogen on 13X and 5A zeolites. Chem Eng J 2012;188:77–85. [14] Cardoso AM, Horn MB, Ferret LS, Azevedo CM, Pires M. Integrated synthesis of zeolites 4A and Na-P1 using coal fly ash for application in the formulation of detergents and swine wastewater treatment. J Hazard Mater 2015;287:69–77. [15] Bukhari SS, Rohani S, Kazemian H. Effect of ultrasound energy on the zeolitization of chemical extracts from fused coal fly ash. Ultrason Sonochem 2016;28:47–53. [16] Ren CF, Wang DM, Zheng DP, Li DL. Progress of characteristics and utilization study of circulating fluidized bed fly ash. Ready-mixed Concrete 2016;1:26–9. [17] Huang Y, Qian JS, Wang Z, Zhang ZW. Comparative study of CFB ashes and PC ashes. Fly Ash Comprehen Utilizat 2009;3:7–13. [18] Sivalingam S, Sen S. Optimization of synthesis parameters and characterization of coal fly ash derived microporous zeolite X. Appl Surf Sci 2018;455:903–10.
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Full Length Article
One-step high efficiency crystallization of zeolite A from ultra-fine circulating fluidized bed fly ash by hydrothermal synthesis method
T
Ze Liu , Siqi Li, Li Li, Jixiang Wang, Yu Zhou, Dongmin Wang ⁎
School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, PR China
ARTICLE INFO
ABSTRACT
Keywords: Circulating fluidized bed fly ash Zeolite A Hydrothermal synthesis Box-Behnken design Crystallinity
The present work focused on the crystallization of zeolite A directly from ultra-fine circulating fluidized bed fly ash (UF-CFA) by alkali-activated hydrothermal synthesis method without high temperature calcining and longterm crystallizing. Response surface methodology, namely Box-Behnken design was employed to optimize the synthesis conditions by varying the crystallization temperature, crystallization time, and NaOHaq concentration. The obtained zeolite A was subjected to mineralogical (X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and chemical (Fourier transform infrared spectrometer) characterization. The results showed that zeolite A begins to form at 90 °C – 2.5 h with NaOHaq concentration of 2.6 mol/L. The crystallinity at 3 h was approximately 90.1%, and pure zeolite A was formed at 6 h with maximum crystallinity of 97.9%. The solid-liquid separation of the synthesized mixture was carried out under the conditions of the appearance of zeolite A at 2.5 h. Crystal structure and solid-liquid composition of the product during the hydrothermal reaction were analyzed. It is explored the growth mechanism of zeolite A by clarifying the evolution of substances in solid-liquid phases and composition of products in the process of zeolite formation.
1. Introduction In the past decades, a variety of technologies have been developed for the effective utilization and safe management of fly ash. At present, Japan and the United States have the most outstanding research results in the preparation of zeolite molecular sieves using fly ash, and the comprehensive utilization rate of fly ash in Japan is as high as 100% [1] Some researchers have also studied the process, mechanism and application of zeolite synthesis from fly ash in recent years [2–5]. The physical activity of fly ash is mainly reflected in that it can promote the gelling activity of products and improve the performance of coal ash products to some extent [6]. The chemical activity of fly ash mainly comes from the amorphous SiO2 and Al2O3. In the presence of alkali, hydration reaction can occur [7]. Due to the high content of silica and alumina, fly ash can be converted into valuable zeolite products [8–10]. In fact, fly ash is a valuable resource for the production of building materials, whether for cement or concrete admixtures [11]. In terms of the purification of waste gas, fly ash can well absorb SO2 and SO3 in the waste gas, and can also activate and adsorb NOx and mercury vapor in the waste gas [12–14]. Other applications include soil improvement, waste water treatment, the extraction of alumina from fly ash, and so on [5,15]. Fly ash in coal burned boiler is mostly spherical, and surface is ⁎
dense and smooth, mainly containing glass phase, quartz and mullite [16]. Unlike ordinary pulverized coal furnace fly ash (PFA), circulating fluidized bed fly ash (CFA) is produced by burning coal at 800–900 °C in a circulating fluidized bed. It is a kind of fly ash with high SO3 content, porous and large specific surface area [17]. In recent years, as a new type of coal-fired technology, circulating fluidized bed combustion technology has been widely used because of its low emission concentration of nitrogen oxides, low pollution emission and high thermal efficiency [18,19]. With the increase of circulating fluidized bed power plants in China, CFA is also produced in large quantities [20–22]. Therefore, making full use of CFA to turn waste into treasure is of great significance for developing a friendly environment and promoting the economic development [23,24]. Holler and Wirsching [25] firstly used fly ash to synthetic zeolite molecular sieves. Since then, the use of fly ash to prepare zeolite molecular sieves has become one of the research hotspots of fly ash resource utilization. The SiO2/Al2O3 ratio of fly ash used in this paper is 1.76, which is an ideal molar ratio for the preparation of zeolite A [26,27]. Therefore, the use of such fly ash to make zeolite A does not require the addition of any Si source and Al source. It is focused on the synthesis of zeolite A from UF-CFA in this paper. According to the previous literatures, it has been found that there have been many studies on the preparation of zeolite A by fly ash. Gong et al. [28]
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.fuel.2019.116043 Received 11 April 2019; Received in revised form 10 August 2019; Accepted 17 August 2019 Available online 30 August 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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Table 1 Chemical composition of fly ash (wt %).
Table 2 Experiment designs with levels in the BBD.
Raw material
SiO2
Al2O3
CaO
SO3
Fe2O3
TiO2
K2O
MgO
P2O5
ZrO2
UF-CFA
43.62
42.22
3.89
2.76
2.54
1.86
1.18
1.11
0.24
0.123
synthesized zeolite A with well crystal shape and large specific surface area from fly ash by hydrothermal synthesis method, the result indicated that: optimum synthesis conditions were 600 °C of alkaline fusion temperature, mass ratio 1–2 of fly ash/NaOH, 90 °C of hydrothermal temperature, 24 h of hydrothermal time. Li et al. [29] determined the optimum conditions for synthesizing Zeolite A by hydrothermal synthesis were 750 °C of calcining temperature, mass ratio 1.3 of fly ash/NaOH, 100 °C of crystallizing temperature, 10 h of crystallizing time. Rentsenorov et al. [30] synthesized zeolite A from fly ash performed with an acid-washing process by hydrothermal treatment and the highest content of zeolite A was formed in the mixture treated at 80 °C for 8 h. Ge [31] used alkali melt decomposition sample of coal fly ash as raw material, added aluminum hydroxide and sodium hydroxide solution to adjust the ratio of each substance in the reaction solution. It indicates that the conditions for obtaining zeolite A were 98 °C of crystallizing temperature, 5 h of crystallizing time. Meng et al. [32] proposed that alkaline assisted calcination activation or alkaline calcination combined with hydrothermal processes can be utilized to achieve pure zeolites, which is superior to the traditional thermal decomposition method, as discussed by Feng [33], and Zhao [34]. In this paper, it is firstly synthesized zeolite A directly from UF-CFA by alkali-activated hydrothermal synthesis method at 90 °C for 3 h without high temperature calcining and long-term crystallizing. The response surface method, namely box-behnken design, is used to obtain the optimal synthesis parameters to maximize the percentage of crystallinity. Zeolite A was characterized by X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersive (SEM/EDX), and Fourier transform infrared spectrometer (FT-IR). It was also explored the growth mechanism of zeolite A from UF-CFA with low reaction temperature and short synthesis time [14].
Run
NaOHaq concentration
Crystallization temperature (°C)
Crystallization Time (h)
Percent crystallinity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2.6 2.6 1.6 1.6 0.6 0.6 1.6 2.6 1.6 1.6 0.6 1.6 1.6 0.6 1.6 2.6 1.6
120 90 60 90 90 120 90 60 60 90 90 90 120 60 90 90 120
4 2 6 4 6 4 4 4 2 4 2 4 6 4 4 6 2
88.7 73.4 78.3 81.2 68.3 53.2 81.2 78.4 69.8 81.2 57.3 81.2 83.2 73.5 81.2 97.9 63.5
by X-ray fluorescence spectrometer (XRF). The results are shown in Table 1. X-ray analysis of UF-CFA (Fig. 1a) has shown the presence of quartz, anhydrite and hematite. The raised background at 20° to 30° indicates the presence of the amorphous phase [35]. Fig. 1b has shown that the particle size distribution of the raw material is between 1 and 6 μm, with a relatively high content of 1–2 μm and 3–5 μm. Compared with PFA, particle size of UF-CFA is very small, which contributes to the sufficient reaction of the raw materials. Different from the spherical morphology of PFA, the morphology of UF-CFA is irregular granular or flocculent as shown in Fig. 6a, which is mainly caused by the amorphous structure of UF-CFA. Therefore, UF-CFA has higher activity than PFA [17]. According to the previous studies [36,37], the optimum siliconaluminum ratio of fly ash to form zeolite A is generally between 0.75 and 1. The initial Si/Al ratio of raw material used in this study is 0.88. Therefore, the study was to prepare zeolite A using UF-CFA without adjusting silicon-aluminum ratio. In addition, sodium hydroxide (NaOH, A.R., > 99% pure, Beijing Chemical Works, China) were employed as alkaline activator (AA) in the experiment.
2. Experimental 2.1. Materials The chemical composition of UF-CFA (Shanxi, China) was measured
Fig. 1. (a) XRD pattern of UF-CFA, (b) Particle distribution of UF-CFA. 2
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2.2. Design of experimentation
Table 3 Experiment designs with levels.
Response Surface Method (RSM), a statistical test design for optimizing biological processes [38], is used to establish a continuous variable surface model and evaluate the factors of affecting biological processes and their interactions. In this study, RSM-box Behnken method was firstly used to design the experiment to reduce the blindness of the experiment. Based on earlier studies, we have chosen three important parameters with the ranges as NaOHaq concentration 0.6–2.6 M, crystallization temperature 60–120 °C, and crystallization time 2–6 h. The specific experimental scheme is shown in Table 2. The results showed that, the highest crystallinity percentage reached 97.9% at NaOHaq concentration of 2.6 M, crystallization temperature of 90 °C, and crystallization time of 6 h. In order to verify and optimize these three parameters, the single-factor control variable method is adopted to design the experiment by taking points in the upper and lower range of the optimal parameters and is listed in Table 3.
NaOHaq concentration
2.3. Synthesis of zeolite A UF-CFA, sodium hydroxide, and distillated water were used as raw materials. The sodium hydroxide solution was prepared by dissolving sodium hydroxide in deionized water by stirring at room temperature for about 15 min. Then this solution was transferred to a 100 mL capacity, Teflon-lined hydrothermal reaction vessel, autoclave, and then UF-CFA was added. According to the conditions of designed experimental scheme, the autoclaves were then heated in an oven at certain conditions as shown in Tables 2 and 3. After crystallization periods, the autoclaves were removed from the oven and cooling to stop the crystallization process. The reacting solution was filtrated and the obtained products was washed several times with deionized water to a pH of 8–9. Finally, the hydrothermal products were dried at 80 °C for 12 h for characterization.
Crystallization temperature (°C)
Crystallization Time (h)
Percent crystallinity (XRD)
Remarks
Effect of NaOHaq concentration 0.6 90
6
68.3%
1.0
90
6
71.5%
1.8 2.6 3.0
90 90 90
6 6 6
83.2% 97.9% 89.8%
Amorphous phases Amorphous phases Zeolite A Zeolite A Zeolite A
Effect of Crystallization temperature 2.6 60 6
80.4%
2.6 2.6 2.6
6 6 6
97.9% 89.6% 77.6%
Effect of Crystallization Time 2.6 90
0.5
56.2%
2.6
90
2.0
78.4%
2.6
90
2.5
89.8%
2.6 2.6 2.6 2.6 2.6 2.6
90 90 90 90 90 90
3.0 4.0 6 12 24 48
90.1% 93.6% 97.9% 81.9% 70.5% 72.1%
90 120 150
Amorphous phases Zeolite A Zeolite A Sodalite Amorphous phases Amorphous phases Partially crystalline Zeolite A Zeolite A Zeolite A Zeolite A Zeolite A Sodalite
2.4. Characterization Both the raw minerals and the synthesized zeolite A were characterized by X Ray Fluorescence (XRF), X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The particle size of UF-CFA was analyzed by using a LS-C (IIA) particle size analyzer. The PANalytical XRF detector was used for chemical composition analysis. XRD patterns were obtained by using a X'Pert PRO MPD X-ray diffractometer with Cu Kα radiation operating at 40 kV and 40 mA over the 2θ range of 5–90°. FT-IR was recorded on a Nicolet iS10 spectrometer in the range 500–4000 cm−1. SEM and energy dispersive X-ray (EDX) spectrum were obtained with a emission scanning electron microscope energy spectrum analyzer (Model JSM-7001F + INCA X-MAX), operating at an accelerating voltage of 15 kV.
Fig. 2. XRD pattern of the effect of different NaOHaq concentrations on the synthesis of zeolite A.
3. Results and discussion 3.1. Optimize experimental parameters
concentration largely affects the extent of hydrothermal reaction. No other type of zeolite form in lower alkalinity. According to Murayama et al. [39], the concentration of Na+ ions in the alkaline solution is the main factor determining the total reaction rate of zeolite synthesis. It's probably that higher alkalinity can promote the dissolution of the silicaalumina phase. In this study, the silica-alumina ratio of the dissolved phase was favorable for forming zeolite A when the alkalinity was higher than 1.8 M. However, excessive alkalinity will also inhibit the formation and growth of zeolite A.
3.1.1. Effect of NaOHaq concentration on zeolite synthesis The effect of NaOHaq concentration on zeolite synthesis plays an important role in the zeolite synthesis process. Fig. 2 shows the XRD pattern of the effect of different concentrations of NaOHaq (0.6, 1.0, 1.8, 2.0, 2.6, and 3.0 M) on the synthesis of zeolite A at 90 °C–6 h. XRD patterns clearly reveal that zeolite A presents when the NaOHaq concentration is 1.8 M or more, and a large amount of zeolite A is formed when the NaOHaq concentration is 2.6 M. Therefore, NaOHaq
3
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intensities of the sodalite gradually increase with the increasing hydrothermal time. These results indicate that zeolite A as metastabile phase is decomposed gradually and results in the supersaturation of liquid phase with aluminosilicate anions, which promotes the formation of sodalite nuclei. Therefore, reasonable control of time can obtain zeolite A in a short time. 3.2. Characterization of products Comprehensive analysis of the above experimental data, diffraction peaks of analyzed phases at 90 °C–2.6 M–6 h are sharp and characterized by high intensity, which indicates a full formation of the crystal structure. The XRD data under this condition were analyzed, and the structural parameters (90 °C–2.6 M–6 h) of the zeolite were calculated (Table 4), which were compared to the standards taken from the PDF-2 diffraction database (2009 release) formalized by the JCPDS-ICDD. The obtained zeolite materials are characterized by very similar unit cell parameters in relation to the standards. Neither the length of the unit cell parameters a, b, c, the α, β, γ angles, nor the volume showed any significant difference with respect to the reference materials (standards). That is, almost pure zeolite A is formed at 90 °C–2.6 M–6 h. In addition, a certain amount of zeolite A has been synthesized at 2.5 h, and the main product is still zeolite A after 12 h of reaction. Fig. 5 is an FTIR pattern of the effect of different reaction times on the synthesis of zeolites A at 90 °C–2.6 M. A ν (O–H) asymmetric and symmetric stretching vibration at 3430–3460 cm−1 can be assigned as caused by H2O molecules in silicate gel and hydrated aluminosilicates. With the reaction time increasing, the position of the absorption peak changes to a low wave number. This may be due to the variation of concentration of hydration products and acidic character of the terminal O–H groups [40]. 1640–1660 cm−1 is the bending vibration peak of H2O molecules and also represents the vibration of H–O or H–OH in the product hydration aluminosilicates [41]. The strongest peak appeared at 990–1100 cm−1 demonstrates the phase change of amorphous phases into high crystalline zeolite A. The wave number of the absorption peak at 990–1100 cm−1 gradually decreases with the increase of time, which is clearly shown in Fig. 5. This is because, in the process of forming zeolite A, the silicon ions and aluminum ions released into the solution gradually form silicon tetrahedron (SiO4) and alumina tetrahedron (AlO4) under strong base conditions [42]. Moreover, there are asymmetric stretching vibrations of Si–O, Al–O and Si–O–Al in the tetrahedron, which are susceptible to the composition of Si and Al in the structure. The position of the absorption peak shifts to a low wave number as the number of Al atoms in the tetrahedron
Fig. 3. XRD pattern of the effect of different temperature on the synthesis of zeolite A.
3.1.2. Effect of crystallization temperature on zeolite synthesis Fig. 3 shows the XRD pattern of the effect of different reaction temperatures (60 °C, 90 °C, 120 °C, and 150 °C) on the synthesis of zeolite A at 6 h–2.6 M. It can be seen that most of UF-CFA didn't react at 60 °C; UF-CFA started to produce a relatively pure zeolite A until 90 °C; sodalite appears and zeolite A is no longer pure when the temperature is raised to 120 °C; zeolite A becomes less and the product is mainly Na-P1 zeolite and sodalite at 150 °C. It is considered that the type of zeolite formed is largely affected by reaction temperature. Thus, it can be concluded that low crystallization temperatures result in low crystallinity percentage and crystal size. The type and purity of the finally obtained crystal can be changed by increasing the reaction temperature. And excessive temperatures decompose the synthesized zeolite to reduce crystallinity. 3.1.3. Effect of crystallization time on zeolite synthesis The crystallization time also has an important influence on the hydrothermal synthesis of zeolite A. Fig. 4 shows the XRD pattern of the effect of different reaction times (0.5–48 h) on the synthesis of zeolite A at 90 °C–2.6 M. The increase of time contributes to the formation of zeolite A, but the quite long term will produce other crystal phases and resulting in the synthesis of zeolite A is impure. Fig. 4 indicates that
Fig. 4. XRD pattern of the effect of different reaction time on the synthesis of zeolite A.
4
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Table 4 Calculated structural parameters of the received zeolite A compared to the patterns. Parameter
Zeolite A (90 °C –2.6 M–2.5 h)
Zeolite A (90 °C–2.6 M–6 h)
Zeolite A (90 °C–2.6 M–12 h)
Standard Zeolite A with HS no ref.: 00-039-0222
a,b,c [Å] α,β,γ [°] V [Å3]
24.580 90 14850.66
24.590 90 14878.79
24.630 90 14941.47
24.610 90 14905.10
Fig. 5. FTIR spectra of zeolite A at 90 °C–2.6 M under different reaction time, (a) 30 min, 2 h, 2.5 h, 3 h, and 4 h, (b) 6 h, 12 h, 24 h, and 48 h.
increases. Similarly, the absorption peak at 1640–3450 cm−1 may also be related to water molecules located in the zeolite channels or exchangeable cations. The amorphous phase gradually dissolves with the reaction time increasing. This peak demonstrates the components in the raw material gradually react into new structure and the structure of zeolite A becomes more obvious. The SEM pattern of synthesized zeolite A at different time is in good agreements with the XRD and FTIR as shown in Figs. 4 and 5. Fig. 6c shown that there are 2 μm cubes appeared in the product after 2.5 h, which is growing zeolite A. However, these cubes had unclear edges and were likely to be covered with unconverted amorphous materials. Most UF-CFA was converted to zeolite A at 3 h, and only a few crystals may have some defects. Fig. 6g clearly reveals that zeolite A formed at 6 h is characteristic cubic crystal. This morphological form is the smallest feature of traditional crystallographic systems. The sizes of obtained zeolite A are about 4 μm. However, SEM patterns indicate that the sodalite gradually increase with the increasing hydrothermal time. Not only zeolite A but also sodalite appeared at 24 h. Thus, the formation of zeolite A requires shorter hydrothermal time and it is considered that the stages of transformation are from UF-CFA, zeolite A to sodalite.
ANOVA results is shown in Table 5, where the closeness of the R2 value (adjusted sum) further validates the applicability of the polynomial model. Experimental run results indicate a maximum percent crystallinity of 97.9%. The final equation of the actual factor is as follows:
Crystallinity (%) = + 56.71450 7.37750×NaOHaq concentration +0.18533 × Temperature + 3.80625×Time +0.25500 × NaOHaq concentration×Temperature +0.68750 × NaOHaq concentration×Time +0.046667 × Temperature×Time 3.61250 × NaOHaq concentration2 4.59722 E 3 × Temperature 2 0.84062 × Time 2 The above formula expresses the percentage of crystallinity by actual factors. The coefficient of NaOH/UF-CFA ratio describes the most critical effect of percent crystallinity. A 3D plot of the percentage of crystallinity is shown in Fig. 9. Response surface methodology (RSM) is the optimization method [45]. It takes the response (crystallinity) of the system as a function of one or more factors (such as NaOH concentration, temperature, and time), and uses graphical techniques to display this functional relationship. The highest point in Fig. 9 is to present the optimal conditions in a more intuitive way.
3.3. Fitness of proposed model for percent crystallinity of zeolite A
3.4. The formation mechanism of zeolite A
The normal probability map and actual versus predicted plots are shown in Fig. 7. Fig. 7a shows the probability distribution of the simulation results. The data exhibit obvious concentricity and symmetry, and normally distribution. The closeness of actual and predicted value of crystallinity percentage is clearly described in the Fig. 7b. Fig. 8 indicates a bar graph of the comparison chart for experimentation (as shown in Table 2) and prediction of crystallinity percentage [43,44]. The results shown in Figs. 7 and 8 verify the reliability of the polynomial model. The percentage of the factorial model selected by the
To explore the growth process of zeolite, in the presence of zeolite A at 2.5 h, the mixture of synthesized zeolite product was subjected to solid-liquid separation by means of suction filtration. And the solid is first washed to about pH = 8 with deionized water, and the solid product is washed with absolute ethanol, and the liquid is allowed to stand for a short period of time. Separated gels and dried solids were characterized [46]. Fig. 10 is SEM image of the solid-liquid separation product at 90 °C−2.6 M–2.5 h. As can be seen from the Fig. 10, there
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Fig. 6. SEM images of the effect of different reaction time on the synthesis of zeolite A at 6 h–2.6 M (a) UF-CFA, (b) 0.5 h, (c) 2 h, (d) 2.5 h, (e) 3 h, (f) 4 h, (g) 6 h, (h) 12 h, (i) 24 h, and (j) 48 h.
Fig. 7. (a) Normal probability plot, (b) predicted vs. actual plot of crystallinity percentage.
are some cubes that are about 2 μm in size and have not yet clear edges and corners in the solid phase product. EDX spectrum analysis was performed on the red frame in the figure, and its element composition is
shown in Table 6. According to the data of XRD, these cubes can be judged to be growing zeolites A [47]. The product in the liquid phase is the gel phase that reacts for 2.5 h during the synthesis of the zeolite. It is
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Fig. 8. Comparison bar chart for experimental and predicted crystallinity percentage. Table 5 Results of ANOVA for the crystallinity percentage of factorial model. Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
Model A-NaOHaq concentration B-Temperature C-Time AB AC BC A^2 B^2 C^2 Residual Lack of Fit Pure Error Cor Total Summary of quadratic model Response %CV
1956.08 926.65 16.25 507.21 234.09 45.56 31.36 54.95 72.08 47.61 19.42 19.42 0 1975.5
9 1 1 1 1 1 1 1 1 1 7 3 4 16
217.34 926.65 16.25 507.21 234.09 45.56 31.36 54.95 72.08 47.61 2.77 6.47 0
78.35 334.06 5.86 182.85 84.39 16.43 11.31 19.81 25.98 17.16
< 0.0001 < 0.0001 0.0461 < 0.0001 < 0.0001 0.0049 0.012 0.003 0.0014 0.0043
Significant
S.D.* 1.67
Mean 75.97
R2 0.9902
A-R2 0.9775
P-R2 0.8427
C.V.% 2.19
AP 33.437
* S.D.: standard deviation, C.V. %: coefficient of variation, R2: R-squared, A-R2: adjusted R-squared, P-R2: predicted R-squared, AP: adequate precision.
an essential component of the synthetic zeolite. During the later reaction, the gel phase will participate in the formation of zeolite crystals. Combined with XRD and FTIR, it can be seen that the first step of the formation of zeolite A is to dissolve Si4+ and Al3+ from fly ash in the alkali solution, and then the silicate ions and aluminate ions dissolved in the alkali solution to form silicate aluminate gel. With the increase of reaction time, zeolite A is formed. Fig. 11 displays the growth process of zeolite A [48,49].
NaOHaq concentration. However, reaction temperature directly affects the type of zeolite. In this study, not only zeolite A, but also sodalite is formed at 120 °C, and Na-P1 zeolite appeared at 150 °C. Similarly, the increase of time contributes to the formation of Zeolite A, but the quite long term will produce other crystal phases and result in the impurity of synthesized zeolite A. Response surface methodology employed in this study, namely Box-Behnken design was accounted to optimize the synthesis conditions and finally obtained the optimal parameter NaOHaq Concentration of 2.6 M, the crystallization temperature of 90 °C. Zeolite A had been formed by the reaction at 2.5 h, the crystallinity at 3 h was approximately 90.1%, and pure zeolite A was formed at 6 h with maximum crystallinity of 97.9%. In addition, the mechanism of synthesis of zeolite A from UF-CFA by alkali-activated hydrothermal method was explored. It mainly divided into the following three steps: Silicate ions and aluminate ions are dissolved from the raw material; Silicate ions and aluminate ions dissolved into alkali solution form silicate aluminate gel; and crystallizes under certain reaction conditions to form zeolite A. Based on these results, it can be concluded that the
4. Conclusion The focus of this paper is to one-step high efficiently synthesize zeolite A from UF-CFA by alkali-activated hydrothermal synthesis method. Experimental results show that the effect of NaOHaq concentration on zeolite synthesis plays an important role in the zeolite synthesis process. NaOHaq concentration not only affects the reaction degree of UF-CFA, but also affects the content of zeolite A. The content of zeolite A increases first and then decreases with the increase of
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100 100
90
C ry s ta llin ity (% )
90
C ry s ta llin ity (% )
80 70 60 50
120.00 114.00 108.00 102.00 96.00 90.00 84.00 78.00 72.00 B: Crys.Temp.(Deg C) 66.00 60.00 0.60
2.60
80 70 60 50
6.00
2.10
A: NaOHaq concentration
1.10
2.60
5.00
1.60
2.10
4.00
C: Crys.Time(hr)
1.60 3.00
1.10 2.00 0.60
A: NaOHaq concentration
100
C ry s ta llin ity (% )
90 80 70 60 50
120.00 114.00 108.00 102.00 96.00 90.00 84.00 78.00 72.00 B: Crys.Temp.(Deg C) 66.00 2.00 60.00
6.00 5.00 4.00 3.00
C: Crys.Time(hr)
Fig. 9. 3D plots obtained from the response of crystallinity percentage.
Fig. 10. SEM images of solid-liquid separation synthesized at 90 °C–2.6 M–2.5 h. (a) solid products, (b) liquid product. Table 6 Elemental composition of the solid-liquid separation product at 90 °C–2.6 M–2.5 h. Solid product (at. %)
Liquid product (at. %)
Elements Atomic (%)
1 2 3 4 5
O
Na
Al
Si
Elements
64.4 64.32 64.41 64.35 64.4
8.38 8.39 8.35 8.33 8.35
13.06 13.15 13.02 13.00 13.12
14.16 14.14 14.22 14.32 14.13
Atomic (%)
8
1 2 3 4 5
O
Na
Al
Si
60.98 60.98 60.96 61.01 60.99
16.74 16.72 16.68 16.70 16.74
11.13 11.12 11.15 11.15 11.10
11.15 11.18 11.21 11.14 11.17
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Fig. 11. The growth process of zeolite A.
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