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.

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

Influence of Curing Temperature and Time on Complete Conversion of Fly Ash in to 21 a Framework Aluminosilicate Utilizing Alkalinehydrothermal Synthetic Methodology


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].



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



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.



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.



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.



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).



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.



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.



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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