ABSTRACT
In view of high cost of
disposal and environmental pollution, the gainful utilization of fly ash to
maximum extent is of vital importance. Therefore, development of a technique
for the treatment of coal fly ash is indispensable to the recycling and
utilization of waste matters.
In the present
project an attempt has been made to synthesize zeolite from waste coal fly ash,
and utilize this treated fly ash as catalyst for industrially important
processes like catalytic cracking of n-heptanes.
Introduction
In
the present investigation, efforts have been made to synthesize zeolite from
fly ash using classical hydrothermal treatment method preceded by alkali
fusion. The synthesis parameters were optimized to obtain X-type zeolite as the
main product. Fly ash from different Indian thermal power stations has been
used and highest conversion to X-type zeolite was observed for KHTPS (Khaparkeda
Thermal Power Station, Maharashtra India ) fly ash. Other fly ash
samples were collected from CHTPS (Chandrapur Thermal Power Station, Maharashtra , India )
and KOTPS (Korari Thermal Power Station Maharashtra India ) power stations. The original
fly ash as well as the synthesized zeolites were characterized using various
techniques such as XRD, SEM, XRF, FTIR and BET method of surface area
measurement.
The
disposal of coal ash from coal-based power plants is a problem of global
concern. In India, most of the utility thermal power stations use bituminous
and sub bituminous coal with high ash content (30-50%) resulting in the
production of a huge quantity of fly ash. Only a small portion of this huge
quantity is used as a raw material for concrete manufacturing and construction
purposes, remainder being simply dumped on the landfill sites. Currently, more
than 90 million tons of fly ash is being generated annually in India ,
with 65 000 acres of land being occupied by ash ponds. Without proper disposal
options, such a huge quantity of ash has posed a great threat to the
environment.
In
fact, high contents of reactive materials like aluminosilicate make fly ash an
interesting starting material for the synthesis of zeolite [Shigemoto et al.,
1993; Chang & Shih, 1998]. Converting fly ash into zeolites not only
alleviates the disposal problem, but also turns an otherwise waste maternal
into a marketable commodity.
THE OBJECTIVES OF PRESENT INVESTIGATION
ARE, THEREFORE, DEVOTED TO
Synthesis of Zeolites from Fly Ash
Many
academic institutions like Indian Institutes of Technology (IITs), Indian
Institute of Science (IISC), and national organizations such as National
Thermal Power Corporation (NTPC), Tata Energy Research Institute (TERI), and
National Environmental Engineering Research Institute (NEERI) are involved in
extensive research in this field.. NEERI has put some efforts in converting fly
ash into zeolite
suitable
for detergent additive.
Procedure
Before any treatment, the fly ash samples were screened through a Tyier
sieve of 80-mesh size to eliminate the larger particles. The average particle
size, chemical composition and surface area of the fly ash samples are
presented in Table.
Table Physico chemical properties of fly ash samples
obtained from different power plants
|
Components
|
Composition (wt.%) in fly
ash of
|
||
|
KHTPS
|
CHTPS
|
KOTPS
|
|
|
Na2O
Al2O3
SiO2
K2O
CaO
Fe2O3
MgO
Zn
|
2.12
30.01
55.19
1.40
0.77
4.58
1.91
0.00
|
1.14
27.86
60.03
0.00
0.54
4.08
1.876
0.00
|
0.70
22.00
76.00
0.00
0.03
0.04
0.03
1.9
|
|
Surface
Area(m2/g)
|
2.9
|
1.4
|
3.5
|
|
Mean particle size(µm)
|
24.90
|
26.08
|
13.94
|
The
unburnt carbon (4-6%) along with other volatile materials present in fly ash
was removed by calcinations at 1073K (±10°) for 2 hrs. Fly ash sample was further,
treated with hydrochloric acid to increase its activity in zeolite formation.
The acid treatment helps to dealuminate the fly ash and remove iron, thereby
increasing the activity, thermal stability and acidity of the zeolite for
catalytic applications.
Mixture of sodium
hydroxide and fly ash, in a pre-determined ratio, was milled and fused in a
stainless steel tray at different temperatures ranging from 773-923K for I hr.
The sodium hydroxide to fly ash ratio (by weight) was varied from 1.0 to 1.5.
The resultant fused mixture was then cooled to room temperature, ground further
and added to water (10 g fly ash/100 mL water). The slurry thus obtained was
agitated mechanically in a glass beaker for several hours. It was then kept at
around 363K (±5°) for 6 hrs without any disturbance unless otherwise mentioned.
The flow diagram for the synthesis process is shown in Figure. The resultant
precipitate was then repeatedly washed with distilled water to remove excess
sodium hydroxide, filtered and dried. Mullite anda-Quartz present in the fly
ash are the source of aluminum and silicon, respectively, for zeolite
formation. The synthesis conditions of the different samples are described in
Table.
|
Zeolite designation
|
Source of fly ash
|
Zeolite synthesis condition
|
||||
|
NaOH/fly ash ratio
|
Fusion temp.
(K)
|
Aging time
(hr)
|
Hydrothermal treatment
|
|||
|
Temp.(K)
|
Time (hr)
|
|||||
|
ZOP -30
ZOP -31
ZOP -21
ZOP -53
ZOP –57
|
KHTPS
KHTPS
KHTPS
CHTPS
KOTPS
|
1.3
1.3
1.2
1.3
1.3
|
823
823
823
823
823
|
24
18
12
18
18
|
363
363
363
363
363
|
6
6
6
6
6
|
Characterization of fly ash by X-ray diffraction
The
X-ray (powder) diffraction (XRD) pattern of any crystalline material is the
fingerprint of its structures. XRD patterns of different fly ash samples and
synthetic zeolitic materials were obtained using a Phillips X-ray
diffractometer (Phillips BW 1710). Operating conditions involved the use of
CoK(x radiation at4kV and 30mA. The samples were scanned from 10-50° (28, where
Q is the angle of diffraction). Various crystalline phases present in the
samples were identified with the ‘help of JCPDS (Joint Committee on Powder
Diffraction Standards, 1967) files for inorganic compounds. A quantitative
measure of the crystallinity of the synthesized zeolite was made by using the
summed heights of major peaks in the X-ray diffraction pattern [Szostak, 1976].
The major peaks were selected specifically because they are least affected by
the degree of hydration of samples and also by others. The percentage
crystallinity was taken as the sum of the peak heights.
Results and Discussions
Mineralogical Properties
The coal fly ash contains mainly
SiO2, A12O3 and some amount of Fe203
and the oxides of Mg, Ca, P, Ti, etc. The chemical compositions of fly
ash samples used in the present study are given in Table 2.1. The X-ray
diffraction patterns of original fly ash, zeolite synthesized from fly ash
(ZOP-30) and commercial 13X zeolite are shown in Figures
fig
XRD patterns of fly ash ,synthesized zeolite and commercial 13Xzeolite
The
XRD pattern of original fly ash mainly shows the presence of crystalline quartz
and mullite. Besides some crystalline phases, fly ash is primarily composed of
amorphous material. The partial hub seen in the back ground at lower
diffraction angle is responsible for the amorphous chases The full hub is not
visible as the scanning was started from the 10° and not from the origin.
Factors Influencing the Zeolite
Properties
Alkali Requirement for Fusion of Fly
Ash
Effect of Fusion Temperature
Effect of Aging Time
Effect of Hydrothermal Treatment Time
Effect of Acid (HCI) Treatment
Cost Analysis
The production
cost of the synthesized zeolite was determined by taking into account the costs
of chemicals and utilities for all the steps involved in the process. The cost
of I kg of synthesized zeolite was estimated to be Rs.253.45/-, whereas l kg of
commercial 13X zeolite (SRL Pvt. Ltd., Mumbai, India) is priced as Rs.l500/-.
The cost-analysis is given in Table.
|
Materials
|
Amount
|
Cost [Rs.]
|
|
NaOH
Fly Ash
Electricity
Miscellaneous
|
1.3 Kg.
1 Kg.
16.736 KWH
-
|
221.00
3.00
70.00
50.00
|
|
Total catalyst
|
1.2 Kg
|
344.00
|
Hence the production cost of
1 Kg of zeolite is Rs. 286.66 /-.
Applications of Treated Fly Ash
As mentioned earlier, zeolites
are very useful materials for a wide range of applications such as ion
exchange, molecular sieves, adsorbents, and catalysis [Breck, 1974]. Uniform
pore size, large surface area, high adsorption capacity and high surface
acidity make the zeolites an attractive material for a number of applications.
As catalysts, zeolite exhibits appreciable acid activity with shape- selective
features, not available in the compositionally equivalent amorphous catalysts. In addition, these materials can act
as supports for
numerous catalytically active materials. Major advances have been made
in the synthesis of molecular sieve materials since the initial discovery of
the synthetic zeolites of types A, X, and Y and a great number of techniques
have evolved for synthesis and characterization of these materials. As
described above, synthesis of various types of zeolite from waste coal fly ash
is comparatively a new addition to this field. Mainly A, X, P zeolites are
successfully synthesized from coal fly ashes using various methods. Unlike
synthetic zeolites of pure single phase, these zeolitic materials exhibit the
structural and qualitative diversities of natural zeolites. The coexistence of
several crystalline phases, the variation in T-atoms and exchangeable cations
are typical characteristics of such zeolitic materials. Furthermore, these
properties greatly affect their applications. Soil conditioner, cation
exchanger, and a broad range of sorbents have been suggested as plausible
applications of these zeolitic materials. High ion exchange capacity and
adsorptivity make these synthetic zeolites very interesting materials for
cation exchange applications.
.
Conclusions
Zeolite of X-type was
synthesized from fly ash by alkali fusion followed by hydrothermal treatment.
Quartz, the main crystalline phase of fly ash, could be converted to pure
X-type at suitable treatment conditions. The zeolite-Y was also successfully
synthesized from fly ash under certain conditions. The properties of zeolitic
material formed strongly depended upon the treatment conditions and composition
of the raw materials. Zeolites of varying surface area, silica/alumina ratio,
and crystallinity were obtained by changing the reaction parameters. The crystallinity
of the prepared zeolite was found to change with fusion temperature and a
maximum value was obtained at 823K. Fly ash from other sources (CHTPS and KOTPS)
were also tried to synthesize zeolite, but KHTPS fly ash showed the best
performance in zeolite formation among the three. The best conversion of fly
ash to Na-X zeolite was obtained at the following conditions: NaOH/Fly ash
ratio, 1.3; fusion temperature, 823K; aging time, 24 h and 6 hours of
hydrothermal treatment. Maximum yield of Na-X zeolite was obtained at the above
conditions with distorted octahedral crystal structure, confirmed by XRD
patterns and scanning electron micrographs. Subsequent experiments focused on
short reaction times for high synthesis efficiencies and monomineral synthesis
of zeolites, since these are the most important factors for possible
application of fly ash in catalysis. The cost of synthesized zeolite was
estimated to be almost one-fifth of that of commercial 13X zeolite available in
the market. The present result is, therefore, very much useful in opening up a
way to synthesize zeolite at low cost.
BIBLIOGRAPHY
Q Journal of
scientific and industrial research
Volume 63 Feb 2004 p.n. 156-162
Q Indian
journal of chemistry
volume 42 B , dec 2003 p.n. 3142-3144
Q Breck
DW,zeolites molecular sieves : structure chemistry and use ,
John Willey New
York 1974
Q Torrey,
S.1978 Coal ash utilization ,
Noyes data corporation , park ridge , New
Jersey , USA
Q TERI
Information Digest on energy and Environment
volume 1 no.4 dec 2004
Q Coal ash
utilization : environmental situation
Kamble S.K.,Patil M R1998 urja 32 (3)
p.n. 32-43
Q Producing
zeolites from fly ash
Rayalu S 1998 chemistry industry digest
11 p.n. 86-92
Q Utilization of fly ash review
Ray R.1998 chemical industry news April
P.N.335-339
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ReplyDeleteZeolite Catalyst is a good product for science technology. Thanks, I like the blog!
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