Journal of Minerals & Materials Characterization & Engineering, Vol. 5, No.2, pp 131-142, 2006 Printed in the USA. All rights reserved
Leaching of Minerals in Degari Coal
Mohammad Shakirullah, Imtiaz Ahmad, Mohammad Arsala Khan,
Mohammad Ishaq, Habib ur Rehman, and Uzma Khan
Department of Chemistry, University of Peshawar, N.W.F.P, 25120, Pakistan
Contact: Phone: +92-091-9216652, E-mail:
This paper reports the demineralization of coal with EDTA, citric acid, HCl, HNO3,
acid mixture and CH3COONH4. The residual coal from each treatment was analyzed
using Scanning Electron Microscopy. All the micrographs are bright field and reveal
several features correspond to the mineral grains comprised of lithophiles like
aluminum, potassium, and sodium; sidrophiles like iron and chalcophiles like copper
and germanium. The absence of some morphological features correspond to inorganic
elements in residual samples confirms demineralization. It is evident from the results
that amongst the leachants used; acid mixture and EDTA have caused significant
removal of mineral phases.
Keywords: Coal; Minerals; Leaching; Particle Morphology
Fossil energy, particularly derived from coal, has been investigated from many
directions. Many environmentalists see coal as inherently dirty. Coal minerals are
objectionable due to process as well as environmental problems. Coal minerals may be
epigenetic and syngentic. Both constitute the inorganic part of the coal and if their
concentration exceeds certain levels, they are considered unwanted because of no role in
combustion of coal. Minerals are desired to certain levels due to catalytic effects in
gasification and liquefaction [1-3]. Concerted efforts are needed to reduce the ash
forming inorganic elements and to develop clean methods of using coal.
Demineralization prior to utilization is an effective way to ensure environmentally
friendly combustion of coal and to compel the user to use coal for domestic as well as
commercial power generation.
Many techniques are in line for demineralization of coal [4-6].However, all these
treatments are tedious, obnoxious and costly. Introduction of less time consuming and
cost effective techniques is the need of the day. New techniques are being viewed for
removal of ash forming inorganic elements from coal [7-8]. EDTA [9], citric acid [10],
132 Shakirullah et al Vol.5, No.2
some lixiviants [11] ,HCl, HNO3, NaOH & H2O2 [12] and forced leaching using water,
citrate, oxalate, EDTA & carbonate solutions are in use [9].
Various analytical protocols have been used in the past for mineral mater
identification [13-17]. Atomic absorption spectroscopy, x-ray diffraction, energy
dispersive scattering and inductively coupled plasma mass spectrometry (ICP-MS), FT-
IR Imaging Direct-Current/Arc Optical Emission Spectrometric Analysis, LTA, XRD,
FT-IR, and SEM-EDX , Fourier transform infrared reflection-absorption spectroscopy
(IRRAS), Mid-Infrared Evanescent Wave Sensors, FT-Raman Spectroscopy techniques,
X-ray photoelectron spectroscopy (ESCA) are enjoying popularity. Scanning electron
microscopy is gaining importance during recent years for minerals identification [18-
The present paper demonstrates the use of scanning electron microscopic technique
to evaluate the ability of some leachants in demineralizing the coal understudy.
Preparation of Coal Sample
The run of mine coal sample was obtained from Degari coal mines through
Pakistan Mineral Development Corporation, crushed and ground in a pestle and mortar,
screened through 250 µm sieves using a sieve shaker. The definite sized coal sample was
dried in a vacuum oven at 70 oC for one hour and cooled in a dessiccator. The proximate
and ultimate analysis of the coal understudy is provided in Table 1.
Table 1. Proximate analysis of coal used.
Parameter Level
Moisture 7.2
Volatile Matte
Ash 11.18
Fixed Carbon 46.85
Total Sulfu
Sulfatic Sulfu
retic Sulfu
anic Sulfu
Chlorine 0.55
Vol.5, No.2 Leaching of Minerals in Degari Coal 133
Extraction Procedure
Five grams portion of coal sample understudy was extracted separately with
EDTA, buffered ETDA, citric acid, HCl, HNO3 , acid mixture made of H2O, HNO3, HCl
and HF (molar ratio of 10:5:1:1) and ammonium acetate. The specified amount of coal
was slurried in 50 cm3 of extracting solution in a beaker. A Teflon coated magnetic
stirring bar was also immersed. The beaker was placed on a water bath and the whole
assembly was placed on a magnetic stirrer. The contents were stirred for time duration of
5 hours at 50 0C. The temperature was maintained throughout the extraction process by
addition of hot water into the water bath. After being contacted for the specified duration
of time, the slurry was filtered using What man filter paper (No. 542) to remove the
capture solution. The residual coal was washed exhaustively with copious amount of hot
distilled water, dried in vaccum oven at 70 o C till constant mass.
SEM Analysis
SEM micrographs of the virgin and residual coal samples obtained by Scanning
Electron Microscope (SEM) Model JSM 5910 from JEOL Company in Japan. Each of
the samples was mounted on disc and coated with gold.
The mineralogy of coal is dependent upon the processes that have occurred in the
source areas, during transport and deposition and during post-depositional early
diagenesis. The variation in mineral contents of different coals is due to the different
extents of some or all of these factors. Coal understudy has been found to have high ash
content and in turn highly contaminated by inorganic elements [23].An attempt was made
to demineralize it with some leachants under different extraction conditions.
SEM micrographs of the virgin and variously leached samples are provided in
Figs. 1-9. Fig.1 represents the SEM image of the virgin coal. It can be seen a bulk
microstructure which in turn is composed of a homogeneously distributed network
comprised of small fistulous and filamentous crystallites showing minerals. In the matrix,
luminous as well as non luminous features can be seen. These features indicate the
presence of minerals distributed in the organic matrix and as surface coverage. Many
fissures, cleats, cracks and veins can also be seen. The bright luminosity is due to the
presence of lithophiles like aluminum, potassium, sodium & sidrophiles like iron and the
dark luminosity is mainly due to the presence of chalcophiles [24]. Etiched pits, layers,
some islands and hills & valleys can also be seen randomly distributed though out the
micrograph. These might be resulted from the calcinations of dolomite and calcites or
their assemblages due to the thermal shock during metamorphism. Some discrete and
coherent crystals (framboids, and euhedral) of irregular shapes represent the presence of
iron [20-22]. Veins correspond to iron oxide can also be seen. It is inferred that the coal
under study contains large proportions of silica, calcium carbonate, and dolomite, as well
134 Shakirullah et al Vol.5, No.2
as some proportions of elements such as aluminum, iron, and potassium that proved to be
in agreement with the work of previous researchers [23].
In order to enhance channeling for effective access of the leachant, the same coal
was carbonized. The SEM micrograph is provided in Fig. 2. It can be seen a very large
channel almost drilled in to the matrix due to catastrophic effect of carbonization
temperature. Some rounded vesicles along the channel are dominant. These bright
rounded formation might be nucleated minerals resulted from high temperature treatment.
An attempt was made to remove the mineral inclusions and to enrich the coal in
useable carbon. Leaching was performed with a chelating agent (EDTA) and the
mineralogical study of the residual coal was done. The SEM picture is provided in Fig. 3.
Numerous aggregated particles can be seen. The porosity has been increased and
provides strong evidence that significant amounts of inorganic elements are being
removed. However, the surface coverage is still bright and luminous indicating the
presence of mineral phases. An attempt was made to exacerbate and stimulate the
leaching with EDTA and buffered EDTA. The micrograph of the residue from this
treatment is provided in Fig. 4. It can be seen that this leachant did little harm to the
surface and the surface is as intact as in the case of virgin coal. Some minute fissures and
cracks, however, are evident. The surface is bright and mostly protruded. Some islands
can also be seen. The micrograph reveals that the leaching has undergone poorly. The
reason might be some of the dolomitic mineral phases have gotten dissolved at the
selected pH and have redeposited on the surface instead of their extraction.
40.00 µm
Fig. 1- SEM micrograph of virgin coal
Vol.5, No.2 Leaching of Minerals in Degari Coal 135
40.00 µm
Fig. 2- SEM micrograph of carbonized coal
40.00 µm
Fig. 3- SEM micrograph of EDTA leached residual coal
136 Shakirullah et al Vol.5, No.2
Leaching was also performed with citric acid. The SEM image or the residual coal
from this treatment is provided in Fig. 5. It can be seen that this leachant caused
morphological changes in the particle and did enormous harm to the surface by leaching
some of the inorganic elements. By comparing the micrograph with the one displayed in
Fig. 3, one can suggest that EDTA has caused effective removal of inorganic phases than
this leachant. However, this leachant seems to be more effective than the buffered EDTA
(Fig. 4) in demineralizing the coal under study.
SEM micrograph of the residual coal leached with acid mixture is displayed in
Fig. 6. It can be seen enormous disintegration of the mineral part of the coal as envisaged
from the dominant dark organic part. However, some catastrophes are numerous. These
can be removed if the coal under study is kept in contact with the leachant for longer
contact time followed by washing. The micrograph upon comparing with the
aforementioned images, established its importance for the removal of inorganic phases.
An attempt was also made to study the effectiveness of HCl as leachant. The
micrograph of the residual coal from this treatment is provided in Fig. 7. The surface
seems to be intact as in case of original coal. The surface coverage with grains is also
evident which confirms poor demineralization with this leachant.
40.00 µm
Fig. 4- SEM micrograph of buffered EDTA leached residual coal
Vol.5, No.2 Leaching of Minerals in Degari Coal 137
Fig. 6- SEM micrograph of acid mixture leached residual coal
Leaching was also performed with HNO3 as leachant. The SEM micrograph
displaying several features like generation of some fissures, kerfs, and vesicles (Fig. 8).
40.00 µm
Fig. 5- SEM micrograph of citric acid leached residual coal
40.00 µm
138 Shakirullah et al Vol.5, No.2
This is evident from demineralization of some of the inorganic elements like pyrites.
Mineral acids have been proved as effective leachant for pyrite removal [25-26].
Fig. 7- SEM micrograph of HCl leached residual coal
Fig.8- SEM micrograph of HNO3 leached residual coal
40.00 µm
40.00 µm
Vol.5, No.2 Leaching of Minerals in Degari Coal 139
Some coal mineral matter may be discrete, organically bound and trace. An
attempt was made to remove organically bound elements as well. The sample understudy
was leached with ammonium acetate as leachant. The residual coal was studied by SEM.
The corresponding micrograph is displayed in Fig. 8. One can see surface disintegration
in the form of generation of micro fissures, cracks, crevices, kerfs, protrusions and
valleys which indicate the possibility of the removal of organically bound elements.
It can be concluded from the results that amongst the lea chants employed, EDTA
and acid mixture proved very effective for the removal of inorganic mineral phases.
However, in the case of EDTA, prolonged or extended leaching time is suggested.
Acknowledgements: The authors are grateful to Mr Mohammad Umar, and Khalid Shah
, Centralized Resource Laboratory, Department of Physics, University of Peshawar,
N.W.F.P Pakistan .
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40.00 µm
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