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Volume# 1 (2009)

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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: belviso@imaa.cnr.it

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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