Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 817-824
Published Online August 2012 (
Coal Chemistry and Morphology of Thar Reserves,
Anila Sarwar1, M. Nasiruddin Khan2*, Kaniz Fizza Azhar3
1Fuel Research Centre, Pakistan Council of Scientific & Industrial Research, Karachi, Pakistan
2Department of Chemistry, University of Karachi, Karachi, Pakistan
3Scientific Information Centre, Pakistan Council of Scientific & Industrial Research, Karachi, Pakistan
Email:, *,
Received May 5, 2012; revised June 21, 2012; accepted July 9, 2012
The surface of Thar coal has been characterized by spectroscopic, microscopic and chemical methods using atomic ab-
sorption spectroscopy, fourier transform infrared analysis, X-ray diffraction, scanned electron microscopy and pH titra-
tion. The samples contained high moisture, low volatile and low to moderate sulfur content and ranked as lignite (heat-
ing value 2541 - 4289 kcal/kg on moist, mineral-matter-free basis). Scanned electron micrographs show porous matrix
with calcium, potassium or sodium minerals. Fourier transform infrared analysis also confirmed the presence of alumi-
num, silica and hydrate mineral constituents. The spectra showed C=C aromatic groups at 1604 - 1609 cm–1. Phenolic
ester and carboxylic acid are identified by C=O stretching vibration peaks at 1702 cm–1. The peaks of quartz and kaolin-
ite were observed at 900 - 1100 cm–1. Point of zero charge of Thar coal has been estimated as 6.00 to 6.27 through ad-
sorption of H+ and OH ions by suspending coal particles in aqueous electrolyte solution. Oxygen containing functional
groups, mineral matter, and metal oxides are found to have a remarkable impact on point of zero charge. The surface
characterization study will be helpful in the separation of hydrophilic impurities during coal preparation processes con-
sidering pzc as the controlling factor.
Keywords: Coal Chemistry; Lignite; Surface Characterization; Scanned Electron Microscopy; Fourier Transform
Infrared Analysis; X-Ray Diffraction Analysis; Point of Zero Charge
1. Introduction
Run-of-mine coal consists of coal, minerals and contami-
nants of large particle size. It needs a series of coal prepara-
tion steps after pulverization. During mining, crushing
and other mechanical operations a significant amount of
coal fines (particle size < 0.5 - 0.6 mm) is produced. The
handling of fine particles is difficult, expensive and
needs special attention [1]. There are several physical
methods for the separation of minerals and other impuri-
ties from coal [2,3]. Physical cleaning through gravity
separation has been recommended for lumps of coal but
it is considered inefficient for fine particles [4]. Of the
existing fine coal cleaning techniques, froth flotation
method is the most common and effective. The process
consists of bubbling air through coal/water slurry. The
separation occurs by preferential physical attachment of
air bubbles to the coal. The coal particles floated at the
surface are removed, while the unwanted particles com-
pletely wetted and stay in the water phase. The ability of
air bubbles to selectively adhere to coal is concerned
with the surface chemistry. Therefore, surface charac-
terization of coal plays a vital role in coal preparation
processes (such as floatation, dispersion, wettability, and
coal-water slurry) prior to its utilization.
Generally fourier transform infrared (FT-IR) analysis,
X-ray diffraction (XRD), and scanned electron micros-
copy (SEM) techniques are used for surface characteriza-
tion of coal and coal-derived products [5,6]. Various in-
dustrial operations depend on the pH where electrical
charge density on coal surface becomes zero (pzc); but
limited data is reported in which pzc of coal has been
considered as an important surface parameter [7,8].
When coal is immersed in a liquid environment, a charge
is developed on its surface by dissociation of functional
groups or by the adsorption of H+ or OH ions [9]. The
surface charge together with counter ions constitutes the
electrical double layer and responsible to control the sta-
bility of coal suspensions (Figure 1). During adsorption
phenomenon, if the pH is equal to pzc, the surface of coal
acts as neutral specie. Below pzc, the surface of adsorb-
ent becomes positively charged and attracts anions.
Conversely, above pzc the surface becomes negatively
*Corresponding author.
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Flocculation Sedime
pH ntation
Figure 1. Schematic representation showing role of pH on surface dependent proce sse s of coal.
charged and attracts cations. Therefore, the charge on the
surface of coal plays a significant role in the removal of
unwanted species.
Thar coalfield has been recently discovered in Thar-
parker District of Sindh Province. It is the largest coal-
field of the country (covering an area of 9100 km2) with
estimated reserves of about 175.5 billion tones. Unfortu-
nately the huge reserves are not exploited yet. However,
now serious efforts are going on at government level to
utilize these reserves. Our present understanding about
the nature of Thar coal is surprisingly limited, which
restricted from its efficient utilization [10]. The main
objective of this study is surface characterization of Thar
coal. Direct examination of surface of Thar coal by SEM,
FT-IR, and XRD has been performed to obtain some
useful qualitative information related to morphology,
functional groups and mineral matters identification.
These information are useful in the utilization of the re-
serve for various applications. SEM describes the suit-
ability of coal surface for gasification process because
diffusion of gases is easily permitted from porous surface.
FT-IR analysis is helpful for the determination of func-
tional groups which are the most reactive components in
conversion processes. XRD tells about the mineral con-
stituents of coal which are converted into ash during
combustion. Mineral composition determines the mode
Copyright © 2012 SciRes. JMMCE
of ash removal. An important outcome of the present
study is surface quantification by measuring pzc. With
the help of this important parameter one can identify the
pH where the coal surface behaves as the least stable
state. It is a major controlling factor in the separation of
oxides, silicates, etc. Proper choice of pH is a require-
ment for selective flotation of one mineral from another.
2. Experimental
Eight samples of coal were obtained from Thar coalfield
of Pakistan. The samples were air dried (ASTM D-3302-
00a) and ground into fine powder of particle size 60 mesh
(ASTM D2013-00a). Thermogravimetric analyzer (TGA-
2000 A, Las Navas Instruments) was used to measure
proximate analysis of coal according to ASTM D7582.
Isoperibol bomb calorimeter (Parr 6300, USA) was used
to measure the heating value in accordance with ASTM
D5865-00. The elemental composition of the samples (C,
H and N) was calculated using empirical formula derived
by Carpenter and Diederichs [11]. Total sulfur was esti-
mated using SC-32 LECO sulfur determinator (ASTM D
4239-00). The percentage of oxygen was taken by dif-
ference. Each analysis was performed in three replicates
to check the reproducibility. Experimental data was con-
verted into different basis using ASTM D388-99.
Fourier Transform Infrared (FT-IR) spectra of coal
samples were recorded using NicoletTM 380, Fourier
Transform Infrared Spectrophotometer (Thermo Electron
Corporation, USA), with spectral resolution of 4 cm–1.
Major and minor oxides in the combustion residue of
coal were analyzed by Perkin Elmer-2380 model atomic
absorption spectrophotometer (ASTM D-3286). National
Bureau Standards 1633a and 1635, Washington DC
were used as the standard reference materials. Minera-
logical characterization was performed by X-ray diffrac-
tion using monochromatic Cu K radiation at 40 kV and
30 mA (
= 1.5406 Å). Microscopic observations were
made using a JEOL JSM-6380A type scanning electron
microscope at the accelerating voltage of 10 - 15 kV.
The surface charge at coal/water interface was deter-
mined by pH titration [12]. Two experiments were con-
ducted for the purpose, each with 10 g of coal suspended
in 35 ml of 0.1 M NaNO3. After waiting 15 min to attain
the equilibrium, one suspension was titrated with 0.1 M
HNO3 and the other with 0.1 M NaOH. The surface charge
qH and qOH for HCl and NaOH additions respectively was
estimated using Equations (1) and (2).
where Ca and Cb are the concentration (mol·L–1) of added
acid and base, respectively, H+ and OH are the molar
proton and hydroxide ion concentration, and m is the
mass of solid in g. The surface charge as a function of pH
is calculated and plotted using Equations (1) and (2). The
pzc was estimated as the pH where the surface charge
crosses the x-axis (q = 0).
3. Results and Discussion
Physico-chemical properties of coal samples are shown
in Tab le 1. The quality of coal was assessed on as-received
basis [13].
Table 1. Classification of Thar coal samples (basis are shown in superscript).
Sample No:
Parameters 1 2 3 4 5 6 7 8
Proximate Compositiona (%)
Moisture 41.19 37.62 42.09 48.10 45.80 48.80 46.38 45.98
Volatile matter 16.47 14.38 13.45 13.69 18.03 11.95 12.37 17.64
Ash yield 5.06 19.17 0.63 2.49 1.90 3.98 6.78 3.40
Fixed carbon 37.28 28.83 43.83 35.72 34.27 35.27 34.47 32.98
Ultimate Compositio n (%)
Carbonb 27.49 21.28 17.88 29.16 32.03 26.85 25.41 29.88
Hydrogenb 6.06 7.49 5.01 5.45 5.65 5.52 5.73 5.76
Nitrogenb 2.22 2.95 1.91 1.99 1.88 2.13 2.21 1.93
Sulfura 1.42 0.19 0.22 0.40 0.21 0.23 1.20 1.24
Oxygenc 58.17 48.27 74.39 60.26 58.58 60.81 56.94 56.96
GCVd (kcal/kg) 4289.46 3194.16 2541.36 3279.75 3850.16 2729.25 2980.58 2978.76
ASTM Rank Lignite
aar basis = as-received basis; bdaf basis = dry, ash-free basis; cdb basis = dry basis; 3m, mmf basis = moist, mineral-matter-free basis.
Copyright © 2012 SciRes. JMMCE
The samples show high moisture, low volatile matter
and low ash content (except sample 2 of high ash). High
moisture content is attributed to the water aquifers which
are present at Thar at an average depth of 50 m, 120 m
and more than 200 m. The sulfur content in the samples
is low to moderate. Ultimate analysis shows total carbon
17.88% - 32.03%, hydrogen 5.01% - 7.49%, nitrogen
1.88% - 2.95% and oxygen 48.27% - 74.39% on dry-ash-
free (daf) basis. The samples are classified as lignite on
the basis of gross calorific value (GCV) on moist, mineral-
matter-free basis (ASTM D388-99).
3.1. SEM Micrographs
SEM images of Thar samples are shown in Figure 2 (a to
h). The samples show interconnected and open micro-
pores (<4 - 12 Å), mesopores (12 - 300 Å), and macro-
pores (>300 Å) [14]. The SEM micrograph shows that
coal matrix is covered with bright and dark luminous
materials indicating the presence of minerals. The bright
glow is due to the presence of calcium, aluminum, potas-
sium or sodium. The dark luminosity is mainly due to the
presence of chalcophiles [15]. The minerals are in the
form of irregular shaped aggregates. Non luminous por-
tion on the surface is mainly made up of carbon content.
Randomly distributed fissures, cracks and etched pits
could also be seen on the micrograph. These might be
produced from the calcinations of dolomite and calcites
as a result of thermal shock during metamorphism [16].
It is evident from the images that Thar coal contains
large proportions of silica, calcium carbonates and dolo-
mite, as well as some proportions of elements such as
aluminum, potassium and sulfur.
3.2. FT-IR Analysis
Figure 3 shows FT-IR spectra of the samples. It is evi-
dent that Thar coal contains aliphatic CH, CH2, and CH3
groups, as well as aromatic ring systems. It also contains
C-O-, C-O-C and associated -OH or NH bonds, and few
C=O bonds. The band at 3350 - 3385 cm–1 is assigned to
associated OH and NH groups (hydrogen bonded). All
samples exhibit band at 2916 cm-1 with a shoulder peak
at 2850 cm–1 showing the presence of aliphatic C-H
stretching vibration. The strong band at 1604 - 1609 cm–1
is attributed to aromatic ring vibrations, enhanced by
oxygen groups [14]. The shoulder peak at 1702 cm–1
(C=O stretching vibration) is represented phenolic ester
and carboxylic acid [15,16]. All samples of Thar show
lower intensity of the aliphatic methylene band at 1460
cm–1 as compared to the methyl band at 1370 cm–1 indi-
cates that coal is composed of fewer aliphatic methylene
groups than methyl groups [14]. Weak bands at 1560
cm–1 was identified showing condensed aromatic ring
C=C at their surfaces. The prominent peak at 1030 - 1035
cm–1 with a shoulder peak at 1005 - 1010 cm–1 is repre-
sented to Si-O bending vibration. Quartz and kaolinite
were identified by the presence of bands in the region of
912 - 917 cm–1 [5]. The aromatic character of coal due to
C=C stretching vibration is found to be more pro-
nounced than the aliphatic character as the band intensity
at 1620 cm–1 is higher than the bands at 2960 cm–1
(stretching vibration of methyl’s group). The same find-
ing has been observed in FT-IR spectrum of Pittsburgh
No. 8 coal [5].
3.3. Point of Zero Charge (pzc) Determination
The surface of coal is generally considered as negatively
charged due to the presence of polar functionalities, do-
minantly carboxylic and phenolic groups. pzc of coal
depends on the relative affinity of the coal surface for H+
and OH. Figure 4 describes that Thar coal behave at the
least stable state at pH 6.00 - 6.27. At this pH range non-
charged coal particles are supposed to be unable to bond
with water. The maximum hydrophobicity of coal at this
pH range supports the maximum floatability [17, 18]. pzc
of Thar coal is attributed to the presence of weakly acidic
oxygen groups, and ash forming minerals such as kaolin-
ite and quartz etc. pzc of pure carbon is 6.5 - 7.0 while
pzc of silicate minerals is approximately 2.0.
The presence of silicate minerals has been confirmed
by X-ray diffraction analysis (Figure 5). pzc of Thar coal
also depends on the minor oxides present in significant
amount in the inorganic part of coal such as Fe2O3 (pzc
8.5) and CaO (pzc 8.1) [19]. The pzc of Thar coal will be
of great importance for process engineers to control wet
washing processes which depends on stability and co-
agulation of colloidal dispersion.
3.4. XRD Analysis
A typical XRD spectrum of Thar coal has been shown in
Figure 5. The samples were composed of hexagonal
shaped quartz low, dauphine-twinned SiO2, monoclinic
carbon oxide hydrate (COOH)2·2H2O and triclinic Kao-
linite 1 A Al2(Si2O5)(OH)4. The mineralogical analysis
was supplemented with the metal oxide quantification by
atomic absorption spectrophotometer (Table 2). Higher
amounts of SiO2 and Al2O3 verified XRD observations.
3.5. Comparison of Thar Coal with Other
Lignite Coalfields
In addition to characterization of the samples, chemical
composition of Thar coal was compared with other lignite
coals of the world. The samples of Thar are seems to closely
resembles with the lignite coal mines of North Dakota
with ultimate composition (C 30.46% - 34.25%, H 6.49% -
7.12% and N 0.50% - 0.56%) and oxide composition of
Copyright © 2012 SciRes. JMMCE
Figure 2. SEM images of Thar coal: (a) to (h) represents sample 1 to 8 respectively.
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Figure 3. FT-IR spectra of Thar coal.
Figure 4. Point of zero charge of Thar samples, Sample No: 1(), 2 (), 3 (-), 4(×), 5 (), 6(), 7 (), 8 ().
Copyright © 2012 SciRes. JMMCE
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Figure 5. A typical XRD spectrum of Thar coal.
Table 2. Major and minor elements in Thar coal.
Sample No:
Oxides (%) 1 2 3 4 5 6 7 8
SiO2 44.45 36.2 27.45 32.34 29.95 31.48 31.59 25.35
Al2O3 10.52 18.49 19.94 24.44 24.45 19.32 20.36 17.77
Fe2O3 21.22 17.38 20.25 15.44 17.85 20.34 18.34 5.53
Na2O 1.48 1.67 1.04 1.54 1.27 1.11 1.05 3.49
K2O 0.2 0.19 0.14 0.02 0.24 0.31 0.34 1.04
MgO 4.31 4.27 5.55 4.3 4.99 2.56 5.36 8.73
CaO 14.81 18.24 22 17.45 18.44 23.26 20.34 31.85
MnO2 0.01 0.01 0.03 0.01 0.01 0.02 0.02 0.03
ash (23.16% - 39.13% SiO2, 7.66% - 13.14% Al2O3,
1.94% - 14.87% Fe2O3, 13.14% - 18.39% CaO, 4.77% -
6.44% MgO, 4.53% - 6.61% Na2O, and 0.40% - 0.52%
K2O) [20]. Chemical composition of Thar coal is also
similar to the coal mine of Velenje, Slovenia with total
moisture 38%, ash content 20%, heating value 3956
btu/lb, S 1.4%, C 27.1%, H 2.1%, O 11.0% and N 0.4%
[21]. The heating value of Thar coal is higher to a large
extent than the lignite coal of Greece mined from the
basin of Megalopolis (1620 - 1980 btu/lb) and Ptolemais
Basin (2070 - 2520 btu/lb) [22].
Thar coal has an advantage of low to moderate sulfur
compare to the world’s average sulfur (2.42%) in lignite
coal [6]. Oxides of sulfur are the most concerned emis-
sion pollutants from the regulatory stand point. Low sul-
fur content in Thar coal makes it acceptable for power
generation without exceeding the emission standard for
sulfur oxides (1%).
4. Conclusion
Thar coal has been ranked as lignite. SiO2 and Al2O3 were
identified as the major and Fe2O3 and CaO as the minor
metal oxides in the ash residue. The surface of coal has
been characterized by macro, meso and micropores with
the irregular aggregates of minerals of calcium, sodium,
potassium and aluminum. pzc of Thar coal ranges from
pH 6.00 - 6.27. At this pH fines of coal float relatively
easily, so the coal washing of low-rank coal of hydrophilic
nature may work fast.
5. Acknowledgements
The authors are grateful to Dean of Science, University
of Karachi, for the financial support.
[1] A. Das, B. Sarkar and S. P. Mehrotra, “Prediction of Se-
paration Performance of Floatex Density Separator for
Processing of Fine Coal Particles,” International Journal
of Mineral Processing, Vol. 91, No. 1-2, 2009, pp. 41-49.
[2] A. U. Kurniawan, O. Ozdemir, A. V. Nguyen, P. Ofori
and B. Firth, “Flotation of Coal Particles in MgCl2, NaCl,
and NaClO3 Solutions in the Absence and Presence of
Dowfroth 250,” International Journal of Mineral Proc-
essing, Vol. 98, No. 3-4, 2011, pp. 137-144.
[3] X. Li, R. Shaw and P. Stevenson, “Effect of Humidity on
Dynamic Foam Stability,” International Journal of Min-
eral Processing, Vol. 94, No. 1-2, 2010, pp. 14-19.
[4] K. S. Birdi, “Handbook of Surface and Colloid Chemis-
try,” 3rd Edition, CRC Press, Taylor & Francis Group,
Boca Raton, London, New York, 2009, pp. 655-679.
[5] Y. D. Abreu, P. Patil, A. I. Marquez and G. G. Botte,
“Characterization of Electroxidized Pittsburgh No. 8
Coal,” Fuel, Vol. 86, No. 4, 2007, pp. 573-584.
[6] V. Bouska and J. Pesek, “Quality Parameters of Lignite
of the North Bohemian Basin in the Czech Republic in
Comparison with the World Average Lignite,” Interna-
tional Journal of Coal Geology, Vol. 40, No. 2-3, 1999,
pp. 211-235. doi:10.1016/S0166-5162(98)00070-6
[7] S. E. Kuh and D. S. Kim, “Effects of Surface Chemical
and Electrochemical Factors on the Dewatering Charac-
teristics of Fine Particle Slurry,” Journal of Environ-
mental Science and Health. Part A: Toxic/Hazardous
Substances & Environmental Engineering, Vol. 39, No. 8,
2004, pp. 2157-2182. doi:10.1081/ESE-120039382
[8] M. Kosmulski, “pH-Dependent Surface Charging and
Points of Zero Charge. IV. Update and New Approach,”
Journal of Colloid and Interface Science, Vol. 337, No. 2,
2009, pp. 439-448. doi:10.1016/j.jcis.2009.04.072
[9] M. N. Khan and A. Sarwar, “Determination of Points of
Zero Charge of Natural and Treated Adsorbents,” Surface
Review and Letters, Vol. 14, No. 3, 2007, pp. 461-469.
[10] A. Sarwar, M. N. Khan and K. F. Azhar, “Kinetic Studies
of Pyrolysis and Combustion of Thar Coal by Thermo-
gravimetry and Chemometric Data Analysis,” Journal of
Thermal Analysis and Calorimetry, Vol. 109, No. 1, 2012,
pp. 97-103. doi:10.1007/s10973-011-1725-0
[11] R. C. Carpenter and H. Diederichs, “Experimental Engi-
neering,” 8th Edition, Wiley, New York, 1913.
[12] M. Davranche, S. Lacour, F. Bordas and J. C. Bollinger,
“An Easy Determination of the Surface Chemical Proper-
ties of Simple and Natural Solids,” Journal of Chemical
Education, Vol. 80, No. 1, 2003, pp. 76-78.
[13] The International Coal Encyclopedia, “Coal Services In-
ternational,” Vol. 1, Time off set Pte Ltd., 1990.
[14] S. C. Tsai, “Coal Science and Technology Series 2: Fun-
damentals of Coal Beneficiation and Utilization,” El-
sevier Scientific Publishing Company, Amsterdam, 1982.
[15] M. Shakirullah, I. Ahmad, M. A. Khan, M. Ishaq, H.
Rehman and U. Khan, “Leaching of Minerals in Degari
Coal,” Journal of Minerals & Material Characterization
and Engineering, Vol. 5, No. 2, 2006, pp. 131-142.
[16] B. Manoj, A. G. Kunjomana and K. A. Chandrasekharan,
“Chemical Leaching of Low Rank Coal and Its Charac-
terization Using SEM/EDAX and FTIR,” Journal of Min-
erals & Materials Characterization & Engineering, Vol.
8, No. 10, 2009, pp. 821-832.
[17] K. H. Nimerick and B. E. Scolt, “New Method of Oxi-
dised Coal Flotation,” Mining Congress Journal, Vol. 66,
1980, pp. 21-22.
[18] A. J. Rubin and R. J. Kramer, “Recovery of Fine-Particle
Coal by Colloid Flotation,” Separation Science and
Technology, Vol. 17, No. 4, 1982, pp. 535-560.
[19] K. Y. Zhang, H. P. Hu, L. J. Zhang and Q. Y. Chen,
“Surface Charge Properties of Red Mud Particles Gener-
ated from Chinese Diaspore Bauxite,” Transactions of
Nonferrous Metals Society of China, Vol. 18, No. 5, 2008,
pp. 1285-1289. doi:10.1016/S1003-6326(08)60218-6
[20] B. C. Folkedahl and C. J. Zygarlicke, “Sulfur Retention in
North Dakota Lignite Coal Ash,” Preprints Papers—
American Chemistry Society, Division of Fuel Chemistry,
Vol. 49, No. 1, 2004, pp. 167-168.
[21] J. Oman, A. Senegacnik and B. Dejanovic, “Influence of
Lignite Composition on Thermal Power Plant Perform-
ance: Part 2: Results of Tests,” Energy Conversion and
Management, Vol. 42, No. 3, 2001, pp. 265-277.
[22] M. J. Galetakis and F. F. Pavloudakis, “The Effect of
Lignite Quality Variation on the Efficiency of On-Line
Ash Analyzers,” International Journal of Coal Geology,
Vol. 80, No. 3-4, 2009, pp. 145-156.
Copyright © 2012 SciRes. JMMCE