3. chemistry - ijapbcr - influence of curing
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INFLUENCE OF CURING TEMPERATURE AND TIME ON COMPLET E
CONVERSION OF FLY ASH IN TO A FRAMEWORK ALUMINOSILI CATE
UTILIZING ALKALINEHYDROTHERMAL SYNTHETIC METHODOLO GY
HEMA KUNDRA & MONIKA DATTA
Department of Chemistry, University of Delhi, Delhi, India
ABSTRACT
The need of electrical power requires adequate supply of power stations. Although there are various renewable
energy sources that do not encounter environmental problems, a number of the power stations are still in India fed by fossil
fuels. Coal is one of the major fossil fuels used in thermal power stations and is significantly increasing every year. In the
production of electricity using coal, coal fly ash is discharged from thermal power plants and globally, over 500 million tonnes of
fly ash is generated annually. About half of the discharged fly ash is used as a raw material for cement and so on and rest of it is
disposed to landfill site. Land requirement envisage for disposal of fly ash is about 50,000 acre with an annual expenditure of about
Rs500 million for transportation. Thus its disposal poses major challenges and serious environmental and economic problems.
To overcome these difficulties it is very important to promote the effective recycling of these waste materials into products of
greater value and thus mitigate the depletion of resources and environmental impacts. These objectives can be achieved
through zeolitization of fly ash.
KEYWORDS: Coal Fly Ash, Zeolite Synthesis, Hydrothermal Method, Synthetic Methodology, Hydroxysodalite Structure
Received: Sep 27, 2015; Accepted: Oct 06, 2015; Published: Dec 31, 2015; Paper Id.: IJAPBCRDEC201503
INTRODUCTION
Electricity production in India is projected to expand dramatically in the near term to energize new industrial
development, while also easing the energy shortages throughout the country. Much of the new growth in electricity
production will be fuelled by domestic coal resources; however, there is worldwide concern about increased coal use,
since coal combustion not only generates million of tonnes of coal combustion by-products as solid wastes but also
emits large amount of carbon dioxide (CO2) which will exacerbate climate change. Nearly73% of India’s total installed
power generation capacity is thermal and 90% of it is coal based [01, 02]. In the production of electricity, coal are
pulverized to powder form and blown into a furnace by high velocity hot air. The pulverized coal is burnt at a
temperature higher than melting points of most minerals which is resided within the coal, causing the transformation of
physical and chemical properties of such minerals. Some light minerals are not undergone reactions and are remained in
the exhaust gas, and this is called fly ash and its amount of fly ash depends on the mineral matter content of coal [03].
Globally, millions of tonnes of fly ash is generated annually from coal based thermal power plants. Because of the
increase in the electricity generation, there is consistent increase in the requirement of coal leading to an
increased production of fly ash [01, 04]. The disposal of waste materials poses major challenges and can cause serious
hazards to human health and the environment, if they are handled incorrectly. As a part of sustainable development,
waste management should attempt to reduce waste materials' effect on human health and the environment and to
Original A
rticle International Journal of Applied, Physical and Bio-Chemistry Research (IJAPBCR) ISSN(P): 2277-4793; ISSN(E): 2319-4448 Vol. 5, Issue 3, Dec 2015, 19-32 © TJPRC Pvt. Ltd.
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20 Hema Kundra & Monika Datta
Impact Factor (JCC): 1.9028 Index Copernicus Value(ICV) : 3.0
convert them into useful products by recycling. Fly ash is generally composed of Si and Al as major elements (in the form
of aluminosilicate) and minor amounts of Fe, Na, K, Ca, P, Ti, and S. Other crystalline minerals are also present in small
quanities such as quartz, mullite etc. There are currently several technologies developed for the disposal of fly ash
including the usage of fly ash as raw material substitution in cement production [05]. The major components (approx 80%)
in fly ash are amorphous aluminosilicate glasses, so the conversion of fly ash to zeolite has also been proposed as a viable
method [06-08]. This is not only to generate a valuable material but also to increase the value of fly ash. High cation
exchange capacity, high surface area and variable pore size are some of the special features which make them versatile materials for
targeting wide range of applications. Other applications are in agriculture, animal husbandry and construction [09]. Therefore, cost
effective production of zeolites using coal fly ash should constitute one important issue of waste management. Several research
reports have been published on synthesis of zeolites (P, A, X, Y, analcime, chabzite etc.) using fusion, refluxing, microwave assisted
methods etc [06, 10-23]. The best result reported so far, for the synthesis of zeolitic materials from fly ash using
hydrothermal method, accounts for 50% (Berkgaut and Singer, 1995) , 45% (Hollman and Steenbruggen, 1998) and 40-
55% (Querol et al, 2007) [24-26]. In present study hydrothermal synthetic methodology for complete conversion of fly ash
in to a highly crystalline single product aluminosilicate (zeolite) was explored and within view of cost effectiveness of the
process, the influence of curing temperature and time was also investigated.
EXPERIMENTAL
Materials and Methods
In the present work, fly ash sample was procured from National thermal Power Plant, India and sodium
hydroxide(AR, 98%)Pure, was procured from Qualigens, India. The zelite from fly ash was synthesized by the
modification of the reported procedure [14].
Synthesis
5g of fly ash sample was taken; fly ash sample was sieved to eliminate larger particles. The fly ash was added to sodium
hydroxide solution to make the slurry and was stirred at various temperatures followed by ageing and curing. The resultant mixture
was then cooled to room temperature and then washed several times with double distilled water to eliminate extra alkali followed by
drying in oven at 60-70°C for 6-7 hours. Keeping in mind the need for the cost effectiveness of the process, the above procedure was
optimized by ageing temperature and time by keeping other parameters constant such as concentration of alkali at 3M [27], gel
formation temperature at 80˚C for 48 hours [28] and ageing time of 48 hours [29] (“Table 1”). These samples were analyzed by
all relevant techniques to identify the structure of the synthesized product.
Table 1: Selected Parameters for Synthesis, Variation in Curing Temperature
Product Code [Alkali] Gel Formation Ageing Curing
Temp. Time (Hours) Temp. Time Temp. Time (Hours) S-02 3M 80°C 48 30°C 48 30°C 72 S-03 3M 80°C 48 30°C 48 40°C 72 S-04 3M 80°C 48 30°C 48 60°C 72
Characterization
Various analytical techniques have been employed for this purpose. To identify crystalline materials in the samples, X-ray
diffracto grams were recorded on Philips PW3710 X-ray Diffracto meter using Cu Kα (alpha) radiations with tube voltage 45
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Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 21 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology
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kV and 40 mA with a sampling step of 0.02° and a scan time of 4sec in 2θvalues ranging from 10-70°. Infrared spectroscopic (FT-
IR) studies were carried out to identify their structural features. FTIR spectra were taken in KBr on a Perkin Elmer FTIR
spectrophotometer. For each sample, spectrum has been recorded for 64 scans with 4cm-1 resolution between 4000–
400cm-1 regions. To avoid the interference from the CO2 and water, IR chamber was flushed with dry nitrogen.
Morphology of fly ash and synthesized product were investigated by using Scanning Electron Microscope (SEM), the SEM
analysis has been performed on ZEISS EVO Scanning Electron Microscope Model EVO 50. Thermal stability was
investigated by thermo gravimetric (TGA) methods. TGA was recorded on Shimadazu DTG 60 at a heating rate of 30°C/min
upto 700°C and a flow rate of 100mL/min.
RESULTS AND DISCUSSIONS
Variation in Curing Temperature
Keeping other parameters (alkali concentration, gel formation temperature and time, ageing temperature and time and
curing time) constant variation in curing temperature was investigated (“Table1”)
X-Ray Diffraction Studies
The x-ray powder diffraction patterns (“Figure 1”a, b, c and d) of fly ash (S-01) and the synthesized products (S-02, S-03
and S-04) indicates the conversion of fly ash to crystalline aluminosilicates. A relatively broad band centered at 2θ = 26.2° is the
characteristic feature of class F fly ash (glassy phase) having relatively low calcium content (“Figure 1”a) [30, 31]. The halo pattern
in the background between 2θ = 10.2° and 2θ = 40.2° indicates the presence of amorphous material [32].Several diffraction
intensities at 2θ = 13.605, 2θ = 22.56 (3.93), 30.37(2.94) which were not present in the XRD pattern of fly ash (figure 1a) has
been observed for sample S-02 (“Figure 1”b). The presence of a halo pattern at the lower diffraction angle indicates the presence
of residual fly ash. The disappearance of the diffraction peaks at 2θ = 16.34 (5.41), 30.81 (2.89), 33.11 (2.70), 35.12 (2.55), 39.13
(2.29), 40.73 (2.21), 60.50 (1.52) and at 2θ = 20.77 (4.27), 36.79 (2.44), 49.99 (1.82), predominantly of quartz and mullite (JCPDS
no. 15-0776 and 05-0490) and the increase in intensity counts of the diffraction peaks of hydroxysodalite [33-37] in case of
sample S-03 (“Figure 1”c), is indicative of conversion of fly ash in to a crystalline aluminosilicate. The presence of small hump in
the case of sample S-04 (“Figure 1”d) is indicative of the reappearance of the amorphous material [32].
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22 Hema Kundra & Monika Datta
Impact Factor (JCC): 1.9028 Index Copernicus Value(ICV) : 3.0
Figure 1: XRD Patterns of Fly Ash and the Product Obtained at Various Temperatures of Curing
Fourier Transformed Infrared Spectroscopic Studies
The FTIR spectra (“Figure 2”a, b, c and d) of fly ash (S-01) and the synthesized products (S-02, S-03 and S-04)
indicate the conversion of fly ash to crystalline aluminosilicates. The presence of quartz and mullite (as evident from the
presence of peak at 1096 cm-1 and 470 cm-1[38]. The strongest band appears at 993, 989 and 995 cm-1 in case of the
synthesized product S-02, S-03 and S-04 respectively (“Figure 2”b – d). The observed shift towards the lower wave number
(“Figure 2”b – c), indicates the presence of more number of Al+3 in to the frame work. Three well-defined medium intensity
bands at 718, 696, 661cm-1 and the bands at 463 and 434 cm-1 for the S-03 sample (“Figure 2”c) are in good agreement with
the bands reported in the literature for hydroxysodalite structure (“Table 2”) [37]. In case of sample S-04 (“Figure 2”d), the
observed shift towards higher wave number and appearance of the band because of the double 6 ring, at 555 cm-1 is indicative of
the appearance of some other zeolitic product [37].
Table 2: Reported Values for Hydroxysodalite
Reported Values for Hydroxysodalite
Assignments
3440-3520(s) O-H stretching Zeolitic Water 1635-1660(ms) O-H bending region Zeolitic Water
986(s) Si/Al-O, Asym. Stretching 729(m), 701(mw), 660(ms) Si/Al-O, Sym. Stretching
461 (ms), 432 (ms) Si/Al-O, Bending
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Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 23 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology
www.tjprc.org [email protected]
Figure 2: FT-IR Patterns of Fly Ash and the Product Obtained at Various Temperatures of Curing
Scanning Electron Microscopic Studies
Scanning electron micrographs (“Figure 3” a, b, c and d) of fly ash (S-01) and the synthesized products (S-02,
S-03 and S-04) indicates the conversion of fly ash to crystalline aluminosilicates. Spherical particles with smooth
surface have been observed for the fly ash [32]. The appearance of sharp edges on the spherical surface suggesting
the conversion of fly ash in to the crystalline material for S-02 sample (“Figure 3” b). In case of S-03 (“Figure 3”c), cubic
crystals indicate the complete conversion of fly ash in to a crystalline product. The further decrease in particle size and
deformation of cubic structure in case of sample S-04 (“Figure 3” d) has been observed which indicates the presence of
other zeolitic product. The XRD, FTIR data is also supportive of this observation.
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24 Hema Kundra & Monika Datta
Impact Factor (JCC): 1.9028 Index Copernicus Value(ICV) : 3.0
Figure 3: SEM Patterns of Fly Ash and the Product Obtained at Various Temperatures of Curing
Thermo gravimetric Studies
The thermo-grams (“Figure 4”a, b, c and d) of fly ash (S-01) and the synthesized products (S-02, S-03 and S-04) indicates
the conversion of fly ash to crystalline aluminosilicates. In case of fly ash (“Figure 4”a) first weight loss of 0.2% is observed in
the temperature range of 35-200°C (corresponding to the loss of physically adsorbed water) followed by a weight loss of
1.8% in the temperature range of 200-550°C (attributed to the decomposition of hydrated salts such as Ca
(OH)2. xH2O, CaSO3.xH2O, etc. present in fly ash). In the temperature range of 550-700°C 0.8% weight loss (attributed to
the loss due to the oxidation of unburnt carbon and decomposition of metal carbonates in fly ash) has been observed [39,
40]. Two step weight losses in each case were observed for the samples S-02, S-03 and S-04. The first step, 30°C to 100°C, amounts
to a loss of ~3%, ~3.2% and ~2.5% corresponds to physically adsorbed water and the second step, 100°C to 250°C, amounting to a
loss of ~ 4.9%, ~ 4.6% and ~ 4.5% corresponds to metal bound water and water present in zeolitic cavity for S-02, S-03 and S-04
respectively (“Figure 4” b-d) [41]. On the basis of all investigations it has been found that the best product is obtained with curing at
40°C. Therefore, further work was carried out using 40°C as the curing temperature and variation in curing time period was
investigated.
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Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 25 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology
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Figure 4: Thermo grams of Fly Ash and the Product Obtained at Various Temperatures of Curing
Variation in Curing Time
Keeping other parameters (alkali concentration, gel formation temperature and time ageing temperature and time and
curing time) constant, variation in curing time was investigated (“Table 3”).
Table 3: Selected Parameters for Synthesis, Variation in Curing Time
Product Code
[Alkali] Gel Formation Ageing Curing
Temp. Time
(Hours) Temp.
Time (Hours)
Temp. Time
(Hours) S-05 3M 80°C 48 30°C 48 40°C 24 S-06 3M 80°C 48 30°C 48 40°C 48 S-07 3M 80°C 48 30°C 48 40°C 72 S-08 3M 80°C 48 30°C 48 40°C 96 S-09 3M 80°C 48 30°C 48 40°C 120
X- Ray Diffraction Studies
The x-ray powder diffraction patterns (“Figure 5” a, b, c, d, e and f) of fly ash (S-01) and the synthesized products (S-05,
S-06, S-07, S-08 and S-09) indicate the conversion of fly ash to crystalline aluminosilicates. Appearance of increasing amount of
crystalline phase/material in case of sample S-05, S-06 and S-07with the increasing time period of curing and the disappearance
of diffraction peaks of fly ash (“Figure 5”b-d) is indicative of conversion of fly ash in to a single crystalline aluminosilicate. The
diffraction intensities in the XRD patterns were found to be in good agreement with hydroxysodalite structure (JCPDS –No. 11-
0401) [35, 36]. The presence of small hump and the decrease in the intensity counts of the diffraction peaks for sample S-08 and
S-09 (“Figure 5”e and f), is indicative of the some other zeolitic material and reappearance of the amorphous material.
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26 Hema Kundra & Monika Datta
Impact Factor (JCC): 1.9028 Index Copernicus Value(ICV) : 3.0
Figure 5: XRD Patterns of Fly Ash and the Product Obtained at Different Time Periods of Curing
Fourier Transformed Infrared Spectroscopic Studies
The FTIR spectra (“Figure 6” a, b, c, d, e and f) of fly ash (S-01) and the synthesized products (S-05, S-06, S-07, S-08
and S-09) indicate the conversion of fly ash to crystalline aluminosilicates. The strongest band appears at 999, 999, 989, 997 and
999 cm-1 in case of the synthesized product S-05, S-06, S-07, S-08 and S-09 respectively (“Figure 6”b – f). This observed shift
towards lower wave number from fly ash (“Figure 6” b–d), indicates the presence of more number of Al+3 in to the frame work.
Three well-defined medium intensity bands at 718, 696, 661cm-1 and the bands at 463 and 434 cm-1 has been observed for the S-04
sample (“Figure 6” d) and are in good agreement with the literature reported bands for hydroxysodalite structure. The observed shift
towards higher wave number and appearance of the band at 567 and 569 cm-1 because of the double 6 ring, is indicative of
the appearance of some other zeolitic product in case of sample S-08 and S-09 (“Figure 6”e and f).
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Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 27 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology
www.tjprc.org [email protected]
Figure 6: FT-IR Patterns of Fly Ash and the Product Obtained at Different Time Periods of Curing
Scanning Electron Microscopic Studies
Scanning electron micrographs (“Figure 7” a, b, c, d, e and f) of the fly ash (S-01) and the synthesized products (S-05, S-
06, S-07, S-08 and S-09) indicates the conversion of fly ash to crystalline aluminosilicates. Deformation of the smooth spherical
surface has been observed in the sampleS-05 (“Figure 7”b). The appearance of sharp edges on the spherical surface suggesting
the conversion of fly ash in to the crystalline material for S-06 sample (“Figure 7”c). In case of S-07 (“Figure 7”d), well defined
cubic crystals indicates the conversion of fly ash into a crystalline product. The further decrease in particle size and deformation
of cubic structure in case of sample S-08 and S-09 (“Figure 7” e and f) suggests the presence of the amorphous material.
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28 Hema Kundra & Monika Datta
Impact Factor (JCC): 1.9028 Index Copernicus Value(ICV) : 3.0
Figure 7: SEM Patterns of Fly Ash and the Product Obtained at Different Time Periods of Curing
Thermo gravimetric Studies
The thermo-grams (“Figure 8” b, c, d, e and f) of the synthesized products (S-05, S-06, S-07, S-08 and S-09) show two
step weight losses. The first step, 30°C to 100°C amounts to a loss of 3.8%, 2.3%, 3.2%, 3% and 6% and the second step 100°C
to 250°C, amounting to a loss of 6.7%, 6.2%, 4.6%, 6.8% and 8.9% for S-05, S-06, S-07, S-08 and S-09 respectively (“Figure 8”
b-f). It was observed that the prolonged crystallization time indicates the increase in intensity counts and crystallinity of the
synthesized product up to an optimal time of 72 hrs. Beyond this period, there is decrease in the amount of the crystalline
phase/material.
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Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 29 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology
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Figure 8: Thermo grams of Fly Ash and the Product Obtained at Different Time Periods of Curing
CONCLUSIONS
So by compiling all the studies it is concluded that complete conversion of fly ash into a crystalline, single product (zeolite)
has been achieved and it has been found to have cubic hydroxysodalite type structure [42] and the best result was found to be
curing at 40°C for 72 hours. Present process has the advantages of conservation of raw materials, cost effectiveness, technically
convenient, economical and non-tedious process, milder conditions and also partially solving the fly ash disposal problem. So, the
outcome of the proposed research work would lead to a cleaner and healthier environment.
ACKNOWLEDGEMENTS
We express our sincere thanks to the following persons for providing instrumentation and other facilities to
Department of Chemistry, University of Delhi, Dr. S. C. Datta, IARI, PUSA for extending XRD facility, AIIMS for
extending the SEM facility, UGC for the financial support, NTPC for providing the fly ash samples, Dr. Shreedhar, IICT,
Hyderabad for extending Solid State MAS NMR facility.
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