Tổng hợp zeolite từ tro bay

Int. J. Miner. Process. 64 Ž2002. 1–17
www.elsevier.comrlocaterijminpro

Mechanism of zeolite synthesis from coal fly ash by
alkali hydrothermal reaction
Norihiro Murayama a,1, Hideki Yamamoto b,2 , Junji Shibata b,)
a

R&D Department, Kimura Chemical Plants Co., Ltd, Kuise Terajima 2-1-2, Amagasaki, Hyogo, Japan
b
Department of Chemical Engineering, Kansai UniÕersity, Suita, Osaka, Japan
Received in revised form 15 February 2001; accepted 3 April 2001

Abstract
To clarify the mechanism of zeolite synthesis from coal fly ash, the hydrothermal reaction was
carried out in various alkali solutions. Zeolite was synthesized in an 800-cm3 autoclave under the
condition of 393 K and 100 gr400 cm3 of solid–liquid ratio. The changes in various physical and
chemical properties, such as crystal structure, surface structure and cation exchange capacity, of
the obtained zeolites and the dissolved amount of Si 4q and Al 3q in alkali solution were
investigated during the hydrothermal reaction. The mechanism of zeolite crystallization and the
role of alkali solution on the synthesis reaction were considered.
Zeolite P and chabazite are mainly synthesized as the crystal type of zeolite from coal fly ash.
There exist three steps in alkali hydrothermal reaction of zeolite synthesis: the dissolution step of
Si 4q and Al 3q in coal fly ash, the condensation step of silicate and aluminate ions in alkali
solution to make aluminosilicate gel, and the crystallization step of aluminosilicate gel to make
zeolite crystal. The OHy in alkali solution remarkably contributes to the dissolution step of Si 4q
and Al 3q in coal fly ash, while Naq in alkali solution makes a contribution to the crystallization
step of zeolite P. This zeolite has the tendency to capture Kq selectively in the cation exchange
site. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: zeolite; coal fly ash; hydrothermal synthesis; reaction mechanism

)

Corresponding author. Tel.: q81-6-6368-0856; fax: q81-6-6388-8869.
E-mail addresses: ŽN. Murayama., ŽH. Yamamoto.,
ŽJ. Shibata..
1
Tel.: q81-6-6488-2504; fax: q81-6-6401-1143.
2
Tel.: q81-6-6368-0972; fax: q81-6-6388-8869.
0301-7516r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 7 5 1 6 Ž 0 1 . 0 0 0 4 6 - 1


2

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

1. Introduction
The recent usage of coal, which has the largest amount of deposit as an energy
source, is being reconsidered. The amount of coal usage in Japan is significantly
increasing every year in the electric power plants. The discharge of coal ash enormously
increases with an increase in coal usage, and it will be over 10 million tonsryear in
2000 ŽJapan Fly Ash Association, 1995..
Coal fly ash discharged from the coal electric power plants occupies a great part of
the total amount of coal ash. About half of discharged coal fly ash is used as the raw
material of cement and so on, but it is a practical problem that the rest of coal fly ash is
disposed in the landfill. Under these circumstances, it becomes gradually difficult to
secure the landfill for burning-up ash. The coal fly ash discharged from power plants is
designated as the specified by-product on recycling law in Japan, and the coal fly ash is
widely being tried as new recycling materials ŽHenmi, 1994; Japan Fly Ash Association,

1995..
As one of the effective usage, the conversion of coal fly ash to zeolite ŽHenmi, 1989,
1994. was investigated about 20 years ago. In the conversion method, aluminosilicate,
which is a main component of coal fly ash, is changed to zeolite crystal by alkali
hydrothermal reaction. The zeolite synthesized from coal fly ash is applied to the various
agricultural materials, which are consumed in large quantities for the purpose of water
purification, soil improvement and so on. Many researchers ŽHenmi, 1989, 1994;
Shibata et al., 1999, 2000; Poole et al., 2000; Lin and His, 1995; Shigemoto et al., 1993;
Park and Choi, 1995. report that the zeolite crystal is produced from coal fly ash by
hydrothermal reaction, but the detailed reaction mechanism of hydrothermal synthesis
has not been clarified adequately. It is important to make the reaction mechanism clear
for the purpose of designing the manufacturing equipment.
In this study, hydrothermal synthesis of zeolite from coal fly ash was investigated in
alkali solutions of a single or two components of NaOH, Na 2 CO 3 and KOH, in order to
explain the reaction mechanism and the role of alkali solution.

2. Experimentation
2.1. Coal fly ash
Coal fly ash supplied by Denpatsu Coal Tech. was used as the raw material of zeolite
synthesis. The coal fly ash was discharged from coal power plants, and it had the quality
in Japanese Industrial Standard.
2.2. Zeolite synthesis from single component alkali solutions
NaOH, Na 2 CO 3 and KOH were used as alkali sources for zeolite synthesis. The coal
fly ash was added to 1.0–4.0 molrdm3 alkali solution to prepare fly ash slurry.
Solid–liquid ratio was 100 gr400 cm3. In the 800-cm3 autoclave, zeolite was synthesized from the slurry under the agitation condition at reaction temperature of 393 K. The


N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

3

reaction time for zeolite synthesis was 3 and 24 h. According to our previous reports
ŽShibata et al., 1999, 2000., alkali hydrothermal reaction of zeolite synthesis already
starts in temperature rising process until 393 K. Therefore, the temperature rising range
until 393 K and the constant temperature range were clearly distinguished in this study.
2.3. Zeolite synthesis from two-component alkali solutions
In order to investigate the effect of cation and anion in alkali solution on zeolite
synthesis, two-component alkali solutions of NaOHrNa 2 CO 3 , NaOHrKOH and
Na 2 CO 3rKOH were used as alkali sources. Total alkali concentration in the solution
was 2.0 eqrdm3. In the case of NaOHrNa 2 CO 3 solution, the amounts of OHy and
CO 32y were changed under constant amount of Naq. On the other hand, in the case of
NaOHrKOH solution, the amounts of Naq and Kq were changed under constant
amount of OHy. Na 2 CO 3rKOH solution contained two cations and two anions in alkali
solution. The hydrothermal reaction using the above alkali solutions was carried out in
the same procedure shown in Section 2.2.
2.4. Physical properties of coal fly ash and synthesized zeolite
The physical properties of the coal fly ash and synthesized zeolites were measured as
follows. The chemical composition was analyzed by using an X-ray fluorescence
analysis equipment ŽEMAX-3770, Horiba.. The surface structure was observed by a
scanning electron microscope ŽS-2400, Hitati.. The identification of crystalline materials
in coal fly ash and synthesized zeolite was carried out by an X-ray diffraction equipment
ŽJDX-3530S, Nihon Denshi.. The amounts of Si 4q and Al 3q dissolved in alkali solution
were determined by using an inductively coupled plasma emission analysis equipment
ŽICPS-1000III, Shimadzu..
3. Results and discussion
3.1. Physical properties of coal fly ash
Table 1 shows the chemical composition of coal fly ash. The main components of
coal fly ash are the oxides of Si and Al, various metallic oxides and unburned carbon.

Table 1

Chemical composition of fly ash and the products Žwt.%.
Products
Fly ash
Zeolite synthesized in
a single component
alkali solutions

Synthesis
conditions
NaOH 2.0
molrdm3 , 3 h
Na 2 CO 3 2.0
molrdm3 , 3 h
KOH 2.0
molrdm3 , 24 h

Si

Al

Na

K

Ca

50.4
41.4

20.3

19.8

5.4
17.6

2.7
1.6

7.5
11.2

45.3

18.8

10.2

2.3

40.9

20.5

0.7

26.6

Fe

Ti

Mg

8.2
5.1

1.8
1.6

2.9
1.7

7.6

10.7

1.9

1.9

5.5

3.6

1.3

1.0


4

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

The X-ray diffraction patterns for coal fly ash are shown in Fig. 1Ža.. It is confirmed that
quartz ŽSiO 2 . and mullite Ž3Al 2 O 3 P 2SiO 2 . mainly exist as crystalline substance in coal
fly ash. The SEM photograph of fly ash surface is shown in Fig. 2Ža.. It reveals that the
coal fly ash used in this study is spherical an time 3 and 24 h.

above products are shown in Fig. 7. The crystallization of zeolite P increases with an
increase in Na 2 CO 3 concentration, but a change in the crystal structure of zeolite as seen
in NaOH solution does not occur with an increase in Na 2 CO 3 concentration.
From these results and our previous studies ŽShibata et al., 1999, 2000., it is
considered that the reason why the zeolite crystallization degree is very small in
Na 2 CO 3 solution is based on the low dissolution ability of the alkali solution. On the
other hand, the effect of Kq in alkali solution causes the slow crystallization rate in
KOH solution.


8

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

Fig. 5. X-ray diffraction patterns of the products obtained in reaction time 3 and 24 h. Qs quartz;
M s mullite; P s zeolite P; KC s potassium–chabazite; C sCaCO 3 .

3.3. Formation mechanism of zeolite crystal
In order to investigate the mechanism of zeolite crystallization, the X-ray diffraction
intensity of some crystalline substances contained in the obtained products is examined,
and Si 4q and Al 3q concentrations in alkali solution are measured through the zeolite
synthesis reaction in NaOH solution.

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

9

Fig. 6. X-ray diffraction patterns of fly ash and the products obtained in various Na 2 CO 3 concentrations.
Qs quartz; M s mullite; P s zeolite P; C sCaCO 3 .

Fig. 7. X-ray diffraction intensities of the products obtained in various Na 2 CO 3 concentrations.


10

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

The X-ray diffraction intensities of the product obtained in NaOH solution are shown
in Fig. 8 as a function of reaction time. The diffraction intensities of quartz, which is one
of the crystalline substances contained in coal fly ash, are gradually reduced through the
heating stage, while the intensities of mullite, which is a stable crystalline substance, do
not change through the hydrothermal reaction. The crystallization of zeolite P already
begins before reaching the reaction temperature, 393 K, and then the diffraction
intensities of zeolite P increase until about 2 h of reaction time.
Fig. 9Ža. – Žh. shows the SEM observation of coal fly ash and the product obtained in
2.0 molrdm3 NaOH solution as a function of reaction time. The amorphous aluminosilicates in coal fly ash are dissolved, and the particle surface changes, like unevenness,
until 373 K in a temperature rising stage ŽFig. 9Žb. – Žd... Then, the aluminosilicate gel,
which is a prematerial of zeolite, rapidly starts to deposit as a big fluke when the
reaction temperature is 393 K ŽFig. 9Že... As the reaction proceeds, the aluminosilicate
gel like a big fluke is transformed to a zeolite crystal, as the lacuna of big fluke is filled
ŽFig. 9Žf. – Žh... The SEM observation in Fig. 9 corresponds to the phenomena of the

zeolite crystallization in Fig. 8.
The changes in Al 3q and Si 4q concentrations in a liquid phase during the hydrothermal reaction with 2.0 molrdm3 NaOH solution are shown in Fig. 10. An Si ingredient
in the coal fly ash is dissolved with a linear relation as a function of time in the
temperature rising stage, while an Al ingredient rapidly increases at the beginning of the
heating stage, and then decreases after 0.5 h Ž353 K. in the temperature rising stage. The
dissolved amounts of Si 4q and Al 3q in alkali solution approach to constant value of
6300 ppm and zero after reaching 393 K, respectively.

Fig. 8. X-ray diffraction intensities of the products obtained with NaOH solution as a function of time.


N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

Fig. 9. SEM photographs of the products obtained in 2.0 molrdm3 NaOH as a function time.

11


12

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

Fig. 10. Changes of Al and Si concentrations in liquid phase during hydrothermal reaction with NaOH
solution.

It is considered that when a large quantity of silicate ion exists in the liquid phase,
aluminate ions and silicate ions are condensed to form an aluminosilicate gel with an
increase in reaction temperature, and then the aluminosilicate gel, which is a prematerial
of zeolite crystal, is created. The reason for decreasing Al 3q concentration in the alkali
solution in a temperature rising stage is explained by the consumption of aluminate ion,

based on the above condensation reaction for an aluminosilicate gel. The zeolite
synthesis reaction takes place at the interface between particle and alkali solution.
Therefore, as the condensation reaction of aluminosilicate and the formation of a zeolite
crystal proceed, aluminate ions and silicate ions are not supplied from coal fly ash
because the particle surface is covered with the deposit material, such as an aluminosilicate gel and a zeolite crystal. When aluminate ions remaining in an alkali solution are
completely consumed, Si and Al ingredients in the alkali solution are not supplied and
consumed. As a result, it is considered that Si 4q and Al 3q concentrations reach an
equilibrium state.
3.4. Zeolite synthesis from two-component alkali solutions
According to the results in Section 3.3, Naq and OHy behave independently in the
hydrothermal reaction. In order to clarify the role of cation and anion in zeolite
synthesis, the hydrothermal reaction was carried out in two-component alkali solutions
of NaOHrNa 2 CO 3 , NaOHrKOH and Na 2 CO 3rKOH. The total alkali amount was
fixed at 2.0 eqrdm3.


N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

13

The X-ray diffraction intensities of the zeolite species synthesized in various alkali
solutions are shown in Fig. 11Ža.,Žb.. The NaOHrNa 2 CO 3 solution ŽI in Fig. 11Ža.. is
the reaction system where the amount of OHy changes under constant amount of Naq.
The dissolution rate of coal fly ash is closely related to OHy concentration. If the value
of OHyrŽtotal anion. in Fig. 11Ža. is smaller, the dissolution ability for coal fly ash is
lower. When the value of OHyrŽtotal anion. becomes over 0.25, the diffraction
intensity of zeolite P is obtained to be almost the same as that of the zeolite P
synthesized in a NaOH solution. Therefore, it is considered that the hydrothermal
reaction of zeolite P can be remarkably promoted by supplying a little amount of OHy
due to the improvement of dissolution ability when Naq adequately exists in the alkali

solution.
The NaOHrKOH solution ŽI, ^ in Fig. 11Žb.. is the reaction system in which the
amount of Naq changes under a constant amount of OHy. The value of NaqrŽtotal
cation. being smaller, the reaction rate of zeolite synthesis becomes lower because of a
decrease in the crystallization speed. The diffraction intensity of zeolite P increases
almost proportionally with an increase in NaqrŽtotal cation. value. It is considered that
the amount of Naq in an alkali solution is important in the reaction rate of zeolite P.
Chabazite crystal, which is not synthesized in KOH solution in 3-h reaction time, is
produced in NaOHrKOH solutions at 3-h reaction time.
The Na 2 CO 3rKOH solution ŽB, ' in Fig. 11Ža.. is the reaction system where four
ionic species of cations and anions exist in the alkali solution. It is noted that any zeolite
species is not synthesized in a single component solution of Na 2 CO 3 and KOH in 3-h
reaction time. Zeolite P is produced in the ratio of NaqrŽtotal cation. from 0.5 to 0.75.
The diffraction intensity of zeolite P obtained in these ratios is about half compared with
that of zeolite P synthesized in the NaOH solution. The role of Naq on zeolite synthesis
is more important than that of OHy, because the production region of zeolite P in Fig.
11Žb. locates in the right side of the abscissa.
When the mixed alkali solutions containing both Naq and Kq are used, it is
important to know the ratio of Naq and Kq captured in the cation exchange site of
synthesized zeolite in order to clarify the reaction mechanism. The amount of exchangable cations in obtained zeolites is shown in Fig. 12. The exchangable cation is
calculated by the amount of Naq and Kq extracted from zeolite by an actual cation
exchange oparation. When the initial ratio of Naq and Kq in the alkali solution is 1:1, it
is found that Kq content captured in cation exchange sites of zeolite is about twice as
large as the Naq content.
In order to synthesize zeolite in a short time, it is necessary to enhance the dissolution
of coal fly ash and the crystallization of zeolite. The OHy in alkali solution contributes
to the dissolution of coal fly ash, while Naq makes a contribution to the crystallization
of zeolite P. The predominant factor for total reaction rate of zeolite formation is the
ionic species of Naq. In the presence of Naq and Kq, the crystallization degree of
zeolite P proportionally decreases with an increase in Kq ratio. It is expected that Kq is

the suppression factor for zeolite synthesis. On the contrary, zeolite P containing Kq and
potassium chabazite is synthesized in a short reaction time like 3 h, by adding a slight
amount of Naq to alkali solution, due to the ion exchange during the hydrothermal
reaction.


14

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17


N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

15

Fig. 12. Amount of exchangeable cation obtained in zeolite. Initial alkali solution; NaOH: 2.0 molrdm3
NaOH, 3 h; KOH: 2.0 molrdm3 KOH, 3 h; NaOHrKOH: 10 molrdm3 NaOHq1.0 molrdm3 KOH, 3 h;
Na 2 CO 3 rKOH: 0.5 molrdm3 Na 2 CO 3 q1.0 molrdm3 KOH, 3 h.

3.5. The role of alkali and reaction mechanism in zeolite synthesis
The role of alkali and reaction mechanism in the zeolite synthesis from coal fly ash
are investigated from the results obtained in Sections 3.1–3.4. The proposed reaction
mechanism is shown in Fig. 13. The dissolution reaction of coal fly ash occurs in a
temperature rising stage of 293–393 K as a first step at the period of Ža. in Fig. 13, and
then the particle surface changes from sphere to something like unevenness. The
dissolution rate is remarkably dependent on OHy concentration in the alkali solution. It
is possible to enhance the dissolution rate drastically by adding a slight amount of OHy.
The amount of OHy needed to promote the dissolution rate successfully is less than 2.0
eqrdm3 coal fly ash.
As the next step, the condensation or gelation reaction of silicate ions and aluminate

ions begins to take place at about 373 K, and then the concentration of Al 3q dissolved
in the alkali solution decreases by the gel formation at the period of Žb. in Fig. 13. As
the condensation reaction proceeds, the aluminosilicate gel rapidly starts to deposit on
the particle surface like a big flake. Then, the aluminosilicate gel begins to transform to

Fig. 11. Effect of cation and anion species on X-ray diffraction intensities of zeolites in two component
solutions of NaOH, Na 2 CO 3 and KOH.


16

N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

Fig. 13. Proposed reaction mechanism for zeolite synthesis from coal fly ash.

a zeolite crystal. The zeolite crystallization already occurs before the reaction temperature reaches to 393 K. In this process, the amount of Naq in the alkali solution
dominates the rate of crystallization.
In a zeolite crystallization stage, the zeolite crystal is conspicuously formed as the
last step for about 2 h after reaction temperature reaches to 393 K, at the period of Žc. in
Fig. 13. As the above gelation and zeolite crystallization progress, the surface of coal fly
ash is covered with aluminosilicate, and the Al 3q concentration in the alkali solution
substantially decreases. As a result, the zeolite crystallization becomes very slow after
all dissolved aluminate ion is consumed to form an aluminosilicate. In the presence of
Naq and Kq in an alkali solution, the zeolite containing a large amount of Kq is
synthesized. Naq type zeolite is synthesized at first, and then the Naq type zeolite
changes to Kq type zeolite in alkali solution during a series of zeolite synthesis
reactions. This is the reason why the Kq type zeolite is produced in the alkali solution
containing Naq and Kq.

4. Summary

The zeolite synthesis from coal fly ash was carried out in various alkali solutions by
using a hydrothermal reaction. The reaction mechanism and the role of alkali in the
zeolite synthesis were investigated.
Three steps, namely, dissolution, condensation and crystallization steps, exist in an
alkali hydrothermal reaction for zeolite synthesis. The dissolution of coal fly ash begins


N. Murayama et al.r Int. J. Miner. Process. 64 (2002) 1–17

17

to occur in the temperature rising stage, 298–393 K, and the amount of OHy in an
alkali solution makes a great contribution to this reaction. As the condensation of
aluminate ion and silicate ion takes place, the particle surface of coal fly ash is covered
with the deposit of the aluminosilicate gel. The amounts of Si 4q and Al 3q ions
dissolved from coal fly ash rapidly decrease according to the progress of condensation
reaction. The crystallization of zeolite P already begins to occur before the reaction
temperature attains 393 K. The crystallization rate was controlled by the amount of Naq
in the alkali solution.
The factor determining the total reaction rate of zeolite synthesis is mainly Naq
concentration in the alkali solution. When Naq and Kq coexist in the alkali solution of
the hydrothermal reaction, the crystallization rate decreases with an increase in Kq
concentration.

References
Henmi, T., 1989. A physico-chemical study of industrial solid wastes as renewable resource-zeolitization of
coal clinker ash and paper sludge incineration ash. Mem. Agric. Dept., Ehime Univ. 33 Ž2., 143–149.
Henmi, T., 1994. Sangyouhaikibutu no Zeolite-tenkanniyoru saisigenka Yuukouriyou-Gijyutukaihatu. New
Technol. Sci., 3–166.
Japan Fly Ash Association, 1995. Coal Ash Hand Book. 2nd edn., pp. II1–12.

Lin, C.-F., His, H.-C., 1995. Resource recovery of waste fly ash: synthesis of zeolite like materials. Environ.
Sci. Technol. 29, 1109–1117.
Park, M., Choi, J., 1995. Synthesis of phillipsite from fly ash. Clay Sci. 9, 219–229.
Poole, C., Prijatama, H., Rice, N.M., 2000. Synthesis of zeolite adsoebents by hydrothermal treatment of PFA
wastes: a comparative study. Miner. Eng. 13 Ž8–9., 831–842.
Shigemoto, N., Hayashi, H., Miyake, K., 1993. Selective formation of Na–X zeolite from coal fly ash by
fusion with sodium hydroxide prior to hydrothermal reaction. J. Miner. Sci. 28, 4781–4786.