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Volume# 1 (2009)
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Suggested Citation format for this article:
Belviso, C., Cavalcante, F., Lettino, A., Fiore, S., 2009, Zeolite Synthesised from Fused Coal Fly Ash at Low Temperature Using Seawater for Crystallization. Coal Combustion and Gasification Products 1, 8-13, doi: 10.4177/CCGP-D-09-00004.1
I S S N 1 9 4 6 - 0 1 9 8
j o u r n a l h o m e p a g e : w w w . c o a l c g p - j o u r n a l . o r g
Zeolite Synthesised from Fused Coal Fly Ash at Low Temperature Using Seawater forCrystallization
Claudia Belviso*, Francesco Cavalcante, Antonio Lettino, Saverio Fiore
Laboratory of Environmental and Medical Geology, IMAA–CNR, Tito Scalo (PZ), 85050, Italy
A B S T R A C T
A sample of coal fly ash from an Italian thermoelectric power plant was used in order to synthesize zeolite by hydrothermal
activation after a pre-treatment fusion with NaOH. The experiments involved were performed at different temperatures of
crystallization, ranging from 35 up to 60uC, with seawater and distilled water, separately, during hydrothermal process. A
comparison between the results obtained from the use of the different kinds of water showed that at low temperature (35–40 uC)
the synthesis yield of zeolite X is higher using seawater as crystallizing agent than using distilled water. This implies a possible
application for seawater in the solution to the problem of high water volume involved in the zeolite synthesis on a pilot plant
scale.
f 2009 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association
All rights reserved.
A R T I C L E I N F O
Article history: Received 24 June 2009; Received in revised form 24 September 2009; Accepted 3 December 2009
Keywords: fly ash; zeolite; distilled and seawater
1. Introduction
Fly ash is a by-product of thermal power plants partly used in
concrete and cement manufacturing. More than half of it is
disposed of in landfills because it finds no other application. Fly
ash is composed of minerals such as quartz, mullite, subordinately
hematite and magnetite, carbon, and a prevalent phase of
amorphous aluminosilicate (Hower et al., 1996, 1999; Vassilev
and Vassileva, 1996; Bayat, 1998; Scheetz and Earle, 1998; Mollah
et al., 1999; Sokol et al., 2000; Hall and Livingston, 2002; Kukier et
al., 2003; Mishra et al., 2003; Koukouzas et al., 2006). The
abundance of amorphous aluminosilicate glass, which is the
prevalent reactive phase, is what makes fly ash an important
source material in zeolite synthesis.
Zeolites are hydrated aluminosilicate minerals with a three-
dimensional open structure making them very useful for solving
the mobility of toxic elements in a number of environmental
applications (Pansini and Colella, 1990; Kesraoui-Ouki et al., 1994;
Curkovic et al., 1997; Torracca et al., 1998; Ouki and Kavannagh,
1999; Querol et al., 1999, 2001, 2002; 2006; Singh et al., 2000;
Langella et al. 2000; Woolard et al., 2000; Moreno et al., 2001a,b;
Babel and Kurniawan, 2003; Inglezakis et al., 2002, 2003;
de’Gennaro et al., 2003; Rayalu et al., 2006; Kocaoba et al.,
2007; Stefanovic et al., 2007; Wu et al., 2008). All this is strictly
connected with their ability to exchange cations, their large surface
area, and their typical structural characteristics (such as porosity)
which facilitate pollutant absorption and encapsulation. Over the
last few decades, the synthesis of zeolite from fly ash has been
gaining ground and numerous methods have been suggested.
These differ from one another in alkaline solution, the molarity of
alkaline agents, the solution/solid ratio, temperature, time of
reaction, pressure and the type of incubation.* Corresponding author. Tel.: +390971427224. E-mail: [email protected]
doi: 10.4177/CCGP-D-09-00004.1
f 2009 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.
Two well-known processes are generally employed: i) the
hydrothermal process (Holler et al., 1985; Querol et al., 1995,
1997, 2001; Lin and His, 1995; Singer and Berkgaut, 1995;
Amrhein et al., 1996; Shih and Chang, 1996; Steenbruggen and
Hollman, 1998; Lee et al. 2001; Tanaka et al., 2003; Murayama et
al., 2002; Inada et al., 2005a; Somerset et al., 2005) and ii) the
hydrothermal process with a fusion pre-treatment at high
temperatures (Shigemoto et al., 1993, 1994; Berkgaut and Singer,
1996; Chang and Shih, 2000; Rayalu et al., 2000, 2001; Molina and
Poole, 2004; Somerset et al., 2004; Mishra and Tiwari, 2006). The
studies of zeolites synthesis by microwave reported high amount of
zeolite and, most importantly, highly shortened synthesis times
when compared to conventional heating methods (Querol et al.,
1997; Slangen et al., 1997; Andres et al., 1999; Katsuki et al., 2001;
Inada et al., 2005b). X-type, A-type, and P-type zeolite can be
synthesized with these heating methods. The X-type zeolite is
particularly interesting for its features and possible uses, as it is
able to exchange cations due to its large-pored structure.
In this study, a sample of coal fly ash from an Italian
thermoelectric power plant was used. X-type and ZK-5-type
zeolites were synthesized by direct conversion method after a
pre-treatment fusion with NaOH. The experiments involved were
performed at different temperatures – from 35 up to 60uC – using
seawater during hydrothermal process. The results were compared
with the products obtained from hydrothermal process carried out
with distilled water. The advantages of using seawater consist in
cutting down the costs for a large-scale use of distilled water and
also cutting down or reducing drastically the costs to attain the
incubation temperature when using warm seawater from powder
plants located near the sea.
2. Materials and Methods
2.1. Materials
Coal fly ash was supplied by the ENEL thermoelectric power
plant in Cerano (Brindisi, Italy). The chemical characterization for
major chemical constituents and trace elements was carried out by
X-ray fluorescence (XRF) and inductively-coupled plasma spec-
trometry (ICP-MS) analysis, respectively. The latter after acid
sample dissolution. Table 1 summarizes fly ash chemical compo-
sition.
The mineralogical composition was determined by X-ray
diffraction (XRD), using a Rigaku Rint 2200 powder diffractometer
with Cu-Ka radiation and graphite monochromator. The X-ray
pattern (Fig. 1) shows quartz and mullite as major crystalline
phases together with the amorphous component based on an
alluminosilicate glass. A scanning electron microscope (SEM, Zeiss
Supra 40) was used to characterize the morphology of fly ash
which consists of cenospheres (Fig. 2a) and plenospheres (Fig. 2b)
of different sizes (ranging from very few mm to dozens of mm). The
size of fly ash particles was determined by laser granulometry
(Malvern Mastersizer/E), too. Figure 3 shows the particle size
distribution.
Table 1
Chemical composition of fly ash
Major Oxide wt. % Minor element ppm
SiO2 48.47 Ni 15
Al2O3 28.03 Cu 101
Fe2O3 4.38 Cd 0.2
TiO2 1.45 As 20
MnO 0.08 Co 69
MgO 1.36 Cr 157
CaO 6.38 Pb 71
Na2O 0.49 Zn 75
K2O 0.94 V 211
P2O5 0.44 Se 4
Fig. 1. X-ray diffraction pattern of the fly ash. Mul 5 Mullite; Qtz 5 Quartz.
Fig. 2. SEM images of the fly ash. a) cenospheres; b) plenospheres.
8 Belviso et al. / Coal Combustion and Gasification Products 1 (2009)
2.2. Procedure for zeolite synthesis
A 1:1.2 weight ratio of fly ash and NaOH was ground in a
mechanical mortar for a few minutes, and then the powder, which
was well mixed, was fused at 550uC for 1 h in accordance with
previous studies (Chang and Shih, 1998). The resultant fused
mixture was cooled and milled again. The powder thus obtained
was mixed with 43 ml (Chang and Shih, 1998) of distilled water
and seawater, separately, and kept in a stirring condition for one
night at room temperature. After being stirred, the solution was
incubated for 4 days at a temperature between 35 and 60uC. The
choice of such a long time of incubation was taken considering the
low temperature of crystallization and the literature data (Chang
and Shih, 1998; Terzano et al., 2005). At the end of this process, the
solid part separated by centrifugation, was dried for 12 h at 80uCafter being rinsed with distilled water (Chang and Shih, 1998).
2.3. Product characteristics
The zeolitic phases in the products were characterized by XRD,
spectroscopy transform infrared (FT-IR), and SEM coupled with an
energy dispersive X-ray (EDX) analyzer.
Fig. 3. Particle size distribution of the fly ash.
Fig. 4. X-ray diffraction patterns of treated fly ash with a) seawater; b) distilled water; c) incubation temperature of 35uC; d) incubation temperature of 60 uC. X 5 X
zeolite, ZK 5 5 ZK5 zeolite.
Fig. 5. Graphical representation of the amount of zeolites synthesised using
seawater and distilled water at different temperature.
Belviso et al. / Coal Combustion and Gasification Products 1 (2009) 9
The relative amount of zeolite synthesized was determined by
applying the Rietveld refinement, using the EXPGUI software
(Toby, 2001) after X-ray powder collection with internal standard.
In addition, FT-IR spectroscopy was used for monitoring the
process of zeolite formation at different temperatures. Measure-
ments were made with a Thermo Nicolet infrared spectrophotom-
eter using KBr pellets.
Morphological and chemical analyses were performed by SEM-
EDS.
3. Results and discussion
In order to investigate the differences between zeolite synthe-
sized with seawater and distilled water at low temperatures, our
experiments were carried out at temperatures of crystallization
ranging from 35 up to 60uC.
As shown in Fig. 4a, X-ray diffraction patterns of fused fly ash
kept at 35, 40, 45, and 60 uC reveal that the synthesis of X-type and
ZK-5-type zeolite with seawater takes place readily at 35 and 45uC,
respectively. Zeolite synthesis with distilled water was also done
(Fig. 4b). It is evident that the amount of zeolite synthesised with
seawater is higher at low temperatures (35 uC) (Fig. 4c) and that the
synthesis of ZK-5-type zeolite does not take place at 45 uC when
using distilled water. Minimal differences were detected in the
experiments carried out at 60 uC. However, only X-type zeolite is
synthesized at this temperature (Fig. 4d).
Figure 5 reports the total amount of X-type zeolite synthesized
at these different temperatures; Zk5-type zeolite was not worth
considering because of its low abundance. This figure shows that
there is a yield of about 17% at 35uC when utilizing seawater. This
value is expressed as a weight percentage. When increasing the
incubation temperature to 40uC the yield of X-type zeolite
increases to about 23% and goes on increasing at 45uC and
60uC, being about 27% and 30%, respectively.
The same process with distilled water gave lower yields in the
range of 35–40uC, and, in any case, lower values in the range of
45–60uC. In fact, at 35uC the yield is approximately 2–3%, at 40uCapproximately 5–7%, and in the range of 45–60uC it is about 24%
and 27%, respectively.
Besides the surprising amount of X-type zeolite formed below
60uC through a hydrothermal process with a pre-treatment fusion
within the incubation temperature range of 35–40uC, the use of
seawater facilitates the formation of X-type zeolite with higher
yields, when compared to the same process with distilled water.
This difference disappears at 60 uC, here the XRD patters are quite
comparable.
The scanning electron micrographs of the original fly ash and
treated fly ash are shown in Figure 6. SEM observations reveal the
typical octahedral crystals of X-type zeolite and provide evidence
of the crystalline growth of zeolites under increasing temperature
conditions using both seawater and distilled water.
FT-IR spectra show the transformations of the alluminosilicate
solid phase after undergoing the different incubation temperatures
(Fig. 7). At 35 uC spectra show the typical X-type zeolite bands.
The region between 1250 and 950 cm21 is attributed to interior
bonds of the tetrahedral asymmetric stretching zone and the
asymmetric stretching of external bonds between tetrahedral zones
(Flanigen, 1980). The main band is placed at 980 cm21 and is
asymmetric due to its overlapping another peak at 1065 cm21. The
data obtained are consistent with those available on Na X zeolite. In
the spectral zone 750–650 cm21 the three bands each at 750, 660,
and 692 cm21 are in good agreement with the values reported for
Na-X zeolite (746, 668, 690 cm21) (Flanigen, 1980). In the spectral
region 650–500 cm21, relative to the double-ring bond vibration
zone, there is a band at 564 cm21. This is consistent with the values
known for Na-X zeolite (560 cm21). In the sector within 500–
420 cm21, related to the deformations of the O-T-O bond, there is
evidence of one band at 461 cm21 characterized by a moderate
intensity and good symmetry; the band is in good agreement with
the one Na-X zeolite (458 cm21) (Flanigen, 1980). The comparison
between the spectra obtained from the use of seawater and distilled
water confirms the XRD data and shows the X-type zeolite formation
at lower temperatures when using seawater.
Our results are not completely in accordance with the data
available in literature. The synthesis of zeolite with seawater is
described in a previous article (Lee et al., 2001), although the
process involved is the hydrothermal one (without a pre-treatment
fusion) used in order to form Na-P1-type zeolites with incubation
temperatures above 100uC. When comparing the patterns of the
products, the authors of this article report that the use of seawater
neither disturbs nor accelerates the zeolite formation, and the
amount of zeolite synthesized with seawater is comparable with
that synthesized with distilled water.
Our data indicate that synthesis of zeolite from fused coal fly ash
at 550 uC, using seawater as crystallization media takes place at
lower temperatures and the amount of zeolite synthesized is higher
comparing with results obtained with distilled water at the same
temperature. The role played by seawater in the crystallization
process of zeolite X at low temperatures is probably connected
with the action of some cations or/and impurities present in
seawater. Research experiments are ongoing.
In view of interesting results, more experiments are now being
carried out in order to improve the synthesis yield of zeolites as
well as their purity by modifying the temperature and/or reducing
the time of incubation.
4. Conclusion
Our experiments demonstrate that zeolites can be synthesized at
very low temperatures from fused fly ash using seawater as
crystallization media. The results indicate that a higher amount of
zeolite occurs using seawater instead of distilled water during the
hydrothermal crystallization at temperature , 60 uC. Any
differences are detectable at temperatures higher than 60 uC with
sea or distilled water.
The advantages of using seawater lie in an increase in the yield
of X-type zeolites, the elimination of costs for large-scale use of
distilled water and the elimination of or a drastic reduction in the
costs to attain the incubation temperature. As the X-type zeolite is
formed at low temperatures with seawater, the out flowing
seawater used to cool the turbine in thermal power plants can be
employed directly in this process. Actually most thermal power
plants are situated near the coast and a lot of seawater containing
waste heat is eliminated. The use of hot waste seawater in artificial
zeolite synthesis could reduce the heating energy and the cost of
water. Our results show that seawater used to cool the turbines of
electric power plants can be recycled for the X-type zeolite
synthesis without employing any other type of energy, or with a
reduced energy input to further increase in incubation temperature
in the zeolite production process.
10 Belviso et al. / Coal Combustion and Gasification Products 1 (2009)
Fig. 6. SEM micrograph of fly ash (a) and zeolite X synthesised at 35uC (b), 40uC (c), 45uC (d) and 60uC (e).
Belviso et al. / Coal Combustion and Gasification Products 1 (2009) 11
Acknowledgments
Our thanks go to the management of ENEL Thermal Power
Station for their cooperation and to Mr Garofalo for collecting fly
ash for this research. We also wish to thank the reviewers for their
useful suggestions.
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