Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.10, pp.919-928, 2010 Printed in the USA. All rights reserved
Chemical Solubilization of Coal using HF a nd C ha ra ct er ization of Products
by FTIR, FT Raman, SEM and Elemental Analysis
B. Manoj* and A.G. Kunjomana
Department of Physics, Christ University, Bangalore, Karnataka, India 560029
Research and development Cell, Bharathiar University, Coimbatore, Tamil Nadu, India
*Corresponding Author:
An examination of the structural features of sub-bituminous coal samples from Godavari coal
field, India were carried out by Scanning Electron Microscopy (SEM), FTIR, FT Raman, and
elemental analysis. SEM micrograph analysis of virgin coal revealed the features of lithophiles
like aluminium, silicates and calcium. To remove minerals and enhance the carbon content, the
sample was leached with HF of different concentrations. FT Raman spectra in the range 100 to
3500 cm-1 region were obtained for coal samples. The same sample was recorded with FTIR for
comparison purpose. The spectrums were found to have similar broad characteristics. Certain
bands were found to be both Raman and IR active. Variation in band intensity and position was
found to be sample dependent. Graphite and defect band was observed on the Raman spectrum.
The absence of the features corresponding to inorganic elements in HF leached residual samples
was an indication of demineralization. The CHNS analysis showed that the oxygen content
decreased by 63% when treated with HF (20%) where as carbon content increased by about
29.63%. The calorific value a remarkable increase of 37% with HF (10%) leaching. In the SEM
micrograph of leached sample cracks and devolatization holes were observed. Proximate
analysis showed a systematic decrease in ash content from 12.87 wt% to 3.06 wt% with leaching.
It was evident from the results that Hydrofluoric acid had significant effect in removing the
mineral matter and oxygenated functional groups from the coal.
Key words: FT Raman; SEM; FTIR; chemical leaching; elemental analysis.
In the study of hydrocarbons, the characterization and leaching of coal is of prime importance.
The heavy minerals present in coal liberate to environment leading to pollution. This main draw
back and low calorific value prevent the utilization of huge reserve of coal in an efficient way.
920 B. Manoj and A.G. Kunjomana Vol.9, No.10
Present study investigates the efficiency of hydrofluoric acid on leaching and characterization of
the product by FT Raman, FTIR, SEM, ultimate and proximate analysis.
Vibrational spectroscopy in particular has been widely used for the study of clay mineral
structure for a very long time [1-2]. It was found that definitive analysis of the IR spectra enabled
structural analysis to take place. The use of FT Raman spectroscopy offers the advantages of
reduced fluorescence, improved signal to noise by co adding of scans and the longer wavelength
of light reduces sample degradation. It also helps us to study about the IR inactive functional
group. The use of Scanning Electron Microscopy (SEM) and FTIR has many current and
potential applications for coal and mineral processing industries. It is used to investigate the
formation and deposition of unwanted ash by- products and in analyzing substances of
environmental concern.
2.1 Extraction Procedure
Five grams portion of the coal sample under study was extracted separately with HF of
concentrations 30%, 20% and 10%. The specified amount of coal was slurried in 50 ml of
extracting solution in a beaker and stirred for 60 minutes at 30oC. After being treated for the
specified time, the slurry was allowed to settle down and then precipitate was removed. The
slurry was filtered using filter paper to remove the capture solution and later washed in a column
of distilled water for a day and filtered again. The filtrates were dried at of 80oC and allowed to
cool slowly in a dessicator. Ahead of Raman and infrared spectroscopic analysis, samples were
analysed using SEM techniques. Ultimate and proximate analysis of the sample was conducted
and recorded in table.1
2.2 Elemental & Proximate Analysis
Elemental analysis was carried out using Vario EL III CHNS analyzer. The analysis was able to
determine CHNS on dried samples. Oxygen content was obtained subtracting the total weight of
Carbon, Hydrogen, Nitrogen and Sulphur from the total. Proximate analysis was conducted using
a muffle furnace by standard procedure.
2.3 Infrared and Raman Spectroscopy
The FTIR spectrum was recorded by using Shimadzu FTIR -8400 spectrometer in the region
3500 - 500 cm-1. This spectrometer had the resolution of 4 cm-1. To obtain consistent records the
FT-IR spectra was recorded in 20 scan mode. The Raman spectra were obtained using a Bruker
RFS 100/S spectrometer equipped with a Raman accessory. This comprised a Spectron Laser
Vol.9, No.10 Chemical Solubilization of Coal 921
systems SL301 Nd-YAG laser operating at a wavelength of 1064 nm, and a Raman sampling
compartment incorporating 180 degree optics. The Raman detector was a highly sensitive
standard Ge detector and was operated at 22oC. Under these conditions Raman shifts would be
observed in the spectral range 3600- 100 cm-1. A laser power of 200 mW was used. This power
is low enough to prevent damage to the minerals, but was sufficient to produce quality spectra in
reasonable time. No heating as may be evidenced by the lack of thermo luminescent background
was observed.
2.4 SEM Analysis
The SEM micrographs of the virgin and residual coal samples obtained by Scanning Electron
Microscope (SEM) model JSM 6390 from JEOL Company in Japan.
3.1 CHNS & Proximate Analysis
The CHNS analysis of the sample (Table 1) showed an increase in carbon content of 29.62% and
corresponding decrease in oxygen content of about 63% for HF(20%) leaching. With HF (10%)
leaching the carbon and hydrogen content is increased by 26% and 46% respectively with 61%
reduction in oxygen content. The ash content in the sample decreased from 12.87 wt% to 3.06
wt% as the concentration of HF increased from 10% to 30% (Figure.1). Percentage of volatile
matter and moisture also decreased with leaching with maximum of 50.5% and 82.5% removal
respectively with HF (10%). The gross calorific value of the sample is also increased by 37%
and 31% respectively with HF leaching (10% and 20%). Total sulphur content is decreased
during leaching with maximum removal for HF (30%).
0510 15 20 25 30
Ash content wt%
Concentration of HF
Figure 1. Variation of ash content with concentration of HF
922 B. Manoj and A.G. Kunjomana Vol.9, No.10
Table 1. Proximate and CHNS analysis of the sample
3.2 Infrared and Raman Spectroscopy
The FT-Raman spectrum of virgin sub-bituminous coal is shown in Figure.2 and HF leached
sample in Figure.3. The corresponding FTIR spectrum is shown in Figure.4. (Spectrum GX is for
Virgin sample and HF 10%, 20%, 30% are leached samples). FT Raman bands were found at
122 cm-1 and 137 cm-1 in the respective samples. Similar bands have been identified using
conventional dispersive Raman spectroscopy. Earlier studies reported bands corresponding to
kaolinite at 129 cm-1 and 120 cm-1 [3]. The spectra indicate that the disordered kaolinite
frequency occurs at a higher frequency than that of ordered kaolinite. Farmer [2] predicts the
vibrations of an ideal hexagonal (Si2O5) layer and Ishii et al [4] predicts the frequencies of these
vibrations of which one is the Raman active symmetric bend with a predicted frequency of 127
cm-1. The possible assignment is that of the O-Si-O symmetric bend. The very large change in
intensity of this band with chemical leaching can occur only as a result of a vibrational mode
which induces a very large change in polarizability. It is likely that the variation in the peak
position of the particular clay mineral is related to the stress on the kaolinite crystal structure and
a consequence is sample dependent. The shift in intensity of this band with HF leaching may be
due to change in layer stacking. Leaching changed the chemical environment and bond strength
of kaolinite mineral structure.
The intense band at 602 cm-1 and 570 cm-1 may be due to halloysite. There are two peaks which
are due to the symmetry reduction. The band is infrared active but is weak. The band has been
attributed to a Si-O-Si stretch [1, 3]. With HF leaching these bands shifted to lower wavelength
of 596 and 561cm-1 respectively. The characteristic IR bands in the 680 cm-1 – 600 cm-1 region
are attributed to the C-H bending modes. The intensity is very strong in IR, while it is weak in
Vol.9, No.10 Chemical Solubilization of Coal 923
The intense infrared bands at 1014 cm-1, 1036 cm-1 and 1108 cm-1 are attributed to silicate
minerals are observable as small three peaks [5-6]. These peaks disappeared or decreased its
intensity when treated with HF. This confirms demineralization of silicate minerals with HF. The
Raman spectra give less information in the 900 cm-1 to 1200 cm-1 of the spectrum and the peaks
are of very low intensity. This is due to the fact that Raman depends on the changes in the
polarizability of the bonds and consequently is applicable to symmetric vibrations and vibrations
that involve atoms of similar size; infrared on the other hand is a function of the change in bond
dipole and thus is more useful for looking at asymmetric vibrations and vibrations that involve
atoms of different sizes [7].
The C-C stretching bands (1161-758 cm-1) are weak in IR and not simple characteristic
frequencies in Raman. The identification of these vibrational bands is more difficult because
other types of C-C stretching vibrations also occur in the same region and can interact with each
other. The weak band at 3243 cm-1 and 3213 cm-1 observed in sub bituminous coal sample is due
to the -OH stretching of coordinated water. Further with leaching the band is shifted to lower
wavelength of 3136cm-1. This band is not active in the IR spectrum.
Manoj et al (8) assigned the bands at 1580 cm-1 to graphite structure and band at 1350 cm-1 to
defect band on earlier studies. Presence of D-band was an indication of condensed benzene rings
in amorphous carbon and is stronger with HF leaching. There are many small Raman absorptions
in the region 1500 cm-1 to 1800 cm-1. The strong, highly polarized nature of this vibrational
mode makes it a very useful Raman band. This may be due to C=O ketone group and carboxylic
group. Much change in Raman absorption is not observed in leached sample. This region of the
spectrum is sample independent. In the IR spectrum strong absorption at 1600 cm-1 is observed.
With HF leaching IR absorption becomes stronger in all the leached samples.
Nishide et al [9] observed strong Raman bands in the region 2000-2200 cm-1 in single wall
carbon nano tubes and identified as due to axial- stretching fundamentals of the linear carbon
molecules. The band at 2100-2300 cm-1 is moderate in virgin and leached samples. This is
attributed to CC bond as well as combination of band arising from 1400 cm-1 and 800 cm-1.
With HF leaching there is no major change in intensity is observed for this band. This band is
absent in the infrared spectra.
The asymmetric and symmetric CH3 bending occurs at 1470-1430 cm-1. The intensity of
asymmetric bend near the 1460 cm-1 is medium in IR and weak in Raman. The asymmetric and
symmetric CH2 stretching appears strongly at about 2926 cm-1 and 2853 cm-1 in IR but is weak
in Raman. The four bending vibrations of CH2 groups are scissoring, rocking, wagging, and
twisting. The scissoring mode of the CH2 group gives rise to a characteristic weak band near
1465 cm-1 in IR and Raman. This band overlaps with CH3 asymmetrical bending. This band
924 B. Manoj and A.G. Kunjomana Vol.9, No.10
normally appears in low rank coals. Manoj et al earlier reported this in the case of sub-
bituminous coal [10-11]. With HF leaching this band disappears in both IR and Raman spectrum.
This confirms that demineralization with HF increased the quality of coal by removing lignin
Figure 2. FT Raman spectrum of virgin sample
Figure 3. FT Raman spectrum of HF leached sample
Vol.9, No.10 Chemical Solubilization of Coal 925
3500 3000 2500 2000 1500 1000500
HF 20%
HF 10%
HF 30%
Transmittance %
wavenmber cm-1
Figure 4. FTIR spectrum of virgin and HF leached sample.
3.3 SEM Analysis of Sample
The SEM micrographs of the virgin and HF leached sample were provided in Figures.5-6.
Figure.5 represents the SEM image of the virgin coal sample. A bulk microstructure composed
of homogeneously distributed network of small crystallites showed the presence of minerals. In
the matrix, luminous as well as non luminous features could be seen. These features indicate the
presence of minerals distributed in the organic matrix. Many fissures, cleats, cracks and veins
were also observed. The bright luminosity was due to the presence of aluminium,potassium or
sodium. The dark luminosity was mainly due to the presence of chalcophiles [6, 10-11]. Etiched
pits, layers, some islands and hills & valleys could also be seen randomly distributed throughoutt
the micrograph.These might haveresulted from the calcinations of dolomite and calcites or their
assemblages due to thermal shock during metamorphism. It was inferred that the coal under
study contains large proportins of silica, calcium carbonates ,dolomite and some proportions of
elements such as aluminium, potassium and sulphur.
With HF leaching (Figure.6) the surface underwent a drastic change. Cracks and devolatization
holes observed on the surface confirms demineralization. The size of the mineral grains is
reduced. With HF(10%) leaching all the minerals were removed and only trace amount of
sulphur content was present in the demineralized coal. These findings are already reported by
manoj et al. [10] in the sub-bituminous coal.
926 B. Manoj and A.G. Kunjomana Vol.9, No.10
Figure 5. SEM micrograph of Virgin coal sample.
Figure 6. SEM micrograph of HF leached coal sample.
Vol.9, No.10 Chemical Solubilization of Coal 927
It is found that the kaolinite mineral band at 122 cm-1 is sample dependent. This band is shifted
to higher wave number with HF leaching. It is found that the silicate mineral bands are IR active
but Raman inactive. With HF leaching these bands decreased its intensity. The C=C and C-C
stretching bands are active in IR but weak in Raman spectra. Presence of graphite and defect
bands are observed in the coal. The CC band is IR inactive but present in Raman spectrum. The
asymmetric and symmetric bending bands of methylene groups disappeared with HF leaching
which in turn increases the quality of coal. The ultimate analysis of the sample showed an
increase of 29.63% in carbon content 37% in calorific value with HF leaching. Correspondingly
the sample showed a decrease in oxygen content of about 63%. Proximate analysis revealed a
systematic reduction in ash content with HF leaching with least value of 3.06% for HF (30%).
The SEM micrograph showed the presence of mineral grains on the surface of virgin sample and
the leached sample shows devolatization holes and cracks which is an indication of
demineralization. It is inferred from the study that leaching with hydrofluoric acid especially
10% and 20% could increase the calorific value of the sub bituminous sample along with
decrease of mineral and ash content.
The author wishes to thank the research and development centre, Bharathiar University for
allowing registering for PhD work. The author is highly indebted to Christ University, Bangalore
for providing financial assistance to carry out this work. The help rendered by the school of
chemical science, M.G. University, Kottayam is also gratefully acknowledged in connection with
SEM, CHNS and FTIR analysis.
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