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CHAPTER IV
COAL QUALITY, MINERALOGY, CHEMISTRY AND DEPOSITIONAL
ENVIRONMENT
4.1 INTRODUCTION
Energy plays most important role in ensuring industrial progress, which depends on the
creation of wealth and establishment of high standard of living for the people. Amongst fossil
fuels viz., coal, petroleum and natural gas are note worthy. Among these, the coal play a vital
role needs no emphasis. Coal is a mixture of organic and inorganic compounds and the
proportion of these compounds varies in different types of coals. Coal originates from plant
remains. The ultimate constituents of pure coals are the same as those found in plants viz.,
carbon, hydrogen, oxygen, nitrogen and minor amounts of sulfur and other elements. In order
to explain the various characteristics and properties of coal, which are prerequisite for its
utilization in industries, chemical analysis in addition to ultimate and proximate analyses are
to be carried out. The popular method of proximate analysis of coal have been carried to
determine the moisture content, volatile matter, fixed carbon, etc., is the first simple step in the
attempt to get an idea of the constituent compounds. Investigations on coal petrology also
have a bearing on the utilization of coal.
Coals are classified according to:
i) type i.e., depending upon the character of the original vegetal matter into humic,
cannel and boghead coal
ii) rank i.e., based on the degree of coalification with anthracite ranking highest
followed in descending order by bituminious, subbituminous, lignite and peat.
iii) grade i.e., depending on the proportion of impurities contained in coal.
101
This chapter deals with a brief review on the coal resources of Iran followed by quality
characterization of coal of the study area which include the major and trace element
geochemistry, mineralogy and petrography of representative samples of coals currently mined
from Pabedana area. Depositional environment evaluated by analyzing the coal composition
and trace element contents is provided at the end of the chapter.
4.2 PREVIOUS WORK
Iranian coal reserve are estimated to be about 7 – 10 Gt. Most of the coal deposits
occur in two main basins, one in northern Alborz basin and another in Central Kerman basin.
Geological studies on the coal resources of the Alborz basin are limited (Yazdi and Shiravani,
2004), but exploration so far has resulted in the discovery of numerous coal occurrences
during past 4 decades 50 coal mines have been developed in the Alborz region which are
mostly underground mines. Coking coals are the primary target of development.
Comprehensive literature reviews of the geological character of Iranian coals have
been carried out by Zadeh Kabir (1991) and Razavi Armagani and Moinsadat (1994). In the
early geological work, the emphasis was on the determination of the minability of the coals in
the Alborz region. Little is known about the geochemistry and mineralogy of Iranian coals
except a brief reference to the petrology of a coal sample from Zerab by Teichmuller (1982)
and a study carried out by Stasiuk et al., (2003) on the petrology, rank and liquid petroleum
potential of the Zarab coals. Yazdi and Shiravani (2004) have recently reported major oxide
and some minor element concentrations from the Lushan coal field of northern Iran. The
Iranian Steel Corporation carried out studies on coking properties of Iranian coals (Razavi
Armagani and Moinsadat ,1994; Shariat Nia, 1994). Goodarzi et al., (2006) carried out a
102
preliminary study of mineralogy and geochemistry of coals from the Central Alborz region of
northern Iran.
4.3 CHARACTERISTICS OF IRANIAN COALS
In Iran, coal reserves are confined to upper Triasic and lower middle Jurassic
sediments and are associated with (i) Shemshak Formation in Alborz region of the northern
part of Iran and (ii) Nayband Formation in Kerman region of the central Iran. In these regions,
coal was formed in active tectonic basins. Alborz coal bearing zone confined to Shemshak
Formation is divided into three parts namely northern Khorasan, eastern Alborz and western
Alborz (Shariat Nia, 1993). All the major mines of Alborz zone are distributed along the
Alborz mountain belt. Alborz coals are mostly of thermal type and less cokable. The thickness
of coal beds vary from 0.2 to 2m. Total reserves of the identified coal deposits in Alborz is
800 – 850 Mt, out of which 200 – 250 Mt are cokable and remaining ones are thermal type.
A brief review on the characteristics of coal of the Alborz basin has made so as to
know the depositional environment of coal in the northern part of Iran. It is followed by the
characterization of coal of Pabedana region. Although the coal characteristics in different coal
fields of the Alborz basin are similar, there are some differences in their macerals constitution
(Yazdi and Shirvani, 2004). In the coal seams of Lushan coal field of northern Alborz region,
vitrinite (50-80%) which is followed by inertinite (10-30%), liptinite (2-8%) and mineral
matter. The Lushan coals are low to medium in ash content (3-22%) but relatively high in
heating value (15.6-19.1 Mj/kg) (Razavi Armagani and moinosadat, 1994). The dominant
mineral phases are pyrite, detrital quartz, siderite, calcite, gypsum, barite, phosphate and illite.
Pyrite is present either in epigenetic or syngenetic form Syngenetic pyrite forms are fine
grained and deeply embedded in the fabric of the coal as framboids. Epigenetic pyrite is
103
normally present as coarse grains. Chemical analysis show that the carbon content in the coal
samples from the Lushan coal field is generally high; it ranges from 80% to 95% but the more
common values are in the range of 86 to 88% (Yazdi and Esmaeilnia, 2003).
The coals of the western Alborz coal fields are characterized by variable sulfur content
(0.5-4%), low vitrinite matter (3-22%), high C (88%) and relatively low H (5%) contents
(Razavi Armagani and Moinosadat, 1994). In general, coals from the central Alborz region
have an ash content of between 10 and 50% and their sulfur and phosphorous contents are 1.1
to 5.0% and less than 0.01 to 0.1%, (Razavi Armagaini and Moinosadat, 1994). As indicated
by their low barium content (9-33mg/kg) (Goodarzi, 1995), these coals were deposited in a
fresh water environment (Razavi Armagaini and Moinosadat, 1994).
In Alborz basin coal contain minerals represented by syngenetic pyrite, marcasite,
detrital quartz, siderite, calcite, illite and kaolinite (Zadeh Kabir, 1991; Yazdi and Shiravani,
2004). According to Goodarzi et al., (2006), Alborz coals are high volatile bituminous (%
Romax: 0.61-1.04) and have variable ash (1.36 to 20.97wt% db), volatile matter (31.03 to
37.70wt% db) and fixed carbon (41.33 to 64.41wt% db) contents. These coals are low in
sulfur and consist of kaolinite, halloysite and carbonates in the eastern part of the Alborz
basin. Deposition of coal has taken place in lacustrine environment, where as in the central
part of the basin coal was deposited in freshwater environment.
4.4 COAL RESERVES OF THE STUDY AREA
Nayband Formation in Kerman region of central Iran consists of large reserves of coal,
which is estimated to be around 1.3 billion tons. 350 Mt of the reserve is cokable, 330 MT is
thermal and the remaining coal has intermediate properties. Among the several coal seams of
the Kerman basin, coal seams at Pabedana which is situated 65 Km north of Zarand city,
104
Kerman province is significant and have been selected for the present study, since not much of
work have been carried out related to processing and environmental issues.
In the Pabedana coal field, which is underground mine contain coal that is cokable type
and Mining was started in the year 1977 and in 2009 as much as 1,62,000 tons coal have been
extracted. Absolute reserve of the mine is estimated to be around 315 MT. In the Pabedana
coal field, 13 coal bearing layers (seams) with thickness varying from 0.1 to 2.5 m have been
identified (Fig 4.1). Among these coal seams, coal seam numbered as d2, d4, d5 and d6 are
more productive and economical. Nayband formation mainly consists of sandstone, shale,
siltstone and clay stone. Coal is predominantly confined to shale and argillite. These sediments
with coal intercalations were happen to be deposited under humid climatic conditions. The
result of the detailed investigations carried out on the coal characteristics of the Pabedana
region including coal petrography and major and trace element studies are discussed in the
following sections.
105
Fig 4.1: Stratigraphic column of upper Triassic to lower Jurassic coal-bearing strata in Pabedana mine. (1) Sandstone, (2) Coal, (3) Siltstone, (4) Carbonaceous argillite Coal, (5)Argillite, (6) Colluvium.
106
4.5 METHODS OF STUDY
4.5.1 Sampling
A total of sixteen samples were collected from four coal seams viz., d2, d4, d5 and d6
of the Pabedana underground mines and were analyzed for their mineralogical and
geochemical compositions. Samples were collected adopting channel sampling technique. The
sampling channel was 0.12 cm wide, 20 cm long and 5 cm thickness. These samples were
taken from fresh surface of the mine by driving a channel across the beds and digging inside
the coal bed (~ 0.5 m thickness) to avoid weathered surface. The coal samples are black in
color, light weight and massive without visible sedimentary structures such as lamination. The
collected samples were numbered and placed in plastic storage bags to prevent contamination
and to minimize oxidation. In the laboratory, these sixteen samples were reduced to four
samples through composite sampling of each seam. Three types of analyses were performed
namely chemical, maceral and mineral. The methodology adopted is shown in the form of a
flow chart (Fig. 4.2) and is as follows.
4.5.2 Chemical analysis
The bulk samples from the field were air dried and reduced to 0.5 kg by coning and
quartering method. The coal samples were analysed (proximate, ultimate, major and trace
element analyses) at the Organization of Geology and Exploration of Minerals in Tehran, Iran.
The samples for proximate, ultimate and chemical analyses were pulverized to less than -200
mesh size and dried for 12 hour in a dessicator. These powdered samples were subjected to
major and trace element determinations using inductively coupled plasma-atomic emission
spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). The
procedures used for ICP-AES involve two different dissolution methods. A sinter digest was
107
used to determine the concentration of major elements (Si, Al, Ca, Mg, K, Fe, Ti, P) and trace
elements (B, Ba and Zr). An acid digest was used to determine the concentrations of Na, Be,
Co, Cr, Cu. Li, Mn, Ni, Sc, Sr, Th, V, Y and Zn. Acid digest solution similar to the above was
used to carry out ICP-MS analyses. Concentrations of As, Au, Cd, Cs, Ga, Ge, Mo, Nb, Pb,
Rb, Sb, Sn, Tl and U were determined adopting the procedure described by Meier (1997). Hg
and Se were determined directly on the coal samples by cold-vapor atomic absorption analysis
and hydride generation atomic absorption, respectively as described by O’Leary (1997).
4.5.3 Maceral analysis
Representative splits of coal samples were ground, cast in epoxy and polished for
spectrographic analyses following the procedure outlined in Pontolillo and Stanton (1994).
Two sample mounts were made from each sample. Measurements of maximum vitrinite
reflectance in immersion oil (R0max) were performed according to ASTYM D2798 methods
and procedures (ASTM, 2002). Identification of liptinite was carried out on each sample
mount. Vitrinite and inertinite macerals were identified under oil immersion with a standard
white light source and the adopted maceral nomenclature is according to the International
Committee for Coal and Organic Petrology (ICCP) (ICCP, 1998, 2001).
4.5.4 Mineral analysis
Representative split of coal samples were grained to -200 mesh (75 µm) and oven-
dried overnight before ashing at low-temperature. Low temperature ash residues were cast into
pressed pellets and analysed on an automated powder diffractometer (D/Max-1200 from
Japan). X-ray diffraction (XRD) patterns (Fig. 4.8 A-D) were analysed with commercial
reference pattern library (International Centre for Diffraction Data, 1997) and also using a
108
USGS program with a common coal mineral reference pattern library (Hosterman and Dulong,
1989).
Fig. 4.2: Flow chart of the analytical methods used by the Organization of Geology and Exploration of Minerals in Tehran, Iran, for the analysis of coal samples of Pabedana region.
109
4.6 RESULTS AND DISCUSSION
4.6.1 Coal quality
Coal quality of the study area has been evaluated through proximate, ultimate, calorific
and forms of sulphur values and the obtained data are tabulated (Tables 4.1, 2, 3 and 4) and
described in the following sections.
4.6.1.1 Proximate analysis
Results of the proximate analysis of the Pabedana coals are presented in Table 4.1. As
seen from the table, there is no major variations in the moisture, ash, volatile matter and fixed
carbon contents in the coals of different seams of the study area. The coals of the study region
are characterized by low moisture content (1.05-1.23%; mean 1.15%). The content of volatile
matter ranges from 29% to 31% db, with an average of 30.47%. Fixed carbon content ranges
from 56.27% to 58.16%, with an average of 57.19%. Ash content in Pabedana coal samples
varies from 9.82% to 12.95% with an average of 11.19%. In coals of Central Alborz region,
the ash content ranges from 10 to 50 %. Low ash contents in the Pabedana coal indicate
relatively quick burial of vegetative matter. Further, moderately low ash content indicates
short distance transportation. Volatile matter (31.03 to 37.70wt% db) and fixed carbon (41.33
to 64.41wt% db) contents of Alborz region are comparable to those of Pabedana coals. The
slight variation in the volatile matter contents is probably due to the compounds released from
organic and mineral matter in coals.
110
Table 4.1: Proximate Analysis of Coal- (Air dried basis)
Sample
No
Moisture
(%)
Ash
(%)
Volatile
matter (%)
Fixed
carbon (%)
d2 1.16 9.82 30.86 58.16
d4 1.15 11.55 30.17 57.13
d5 1.05 12.95 29.73 56.27
d6 1.23 10.43 31.12 57.22
Average 1.15 11.19 30.47 57.20
4.6.1.2 Ultimate analysis
The Pabedana coal contains high C (81.30-83.32%, mean 82.61%), relatively low H (
av. 5.04%) and O+N combined comprise 7.98%. Atomic H/C and O/C ratios determined for
the Pabedana coals are indicative of humic nature for coal, which are in agreement with the
nature and origin (Table 4.2).
Table 4.2: Elemental composition of Coal (D.m.m.f: basis).
Sample
No C (%) H (%) N (%)
Sulfur
organic
(%)
O (%) O/C H/C
d2 83.32 4.70 1.35 0.61 6.13 0.07 0.06
d4 81.30 5.10 1.20 0.68 7.75 0.10 0.06
d5 81.65 5.65 1.42 0.58 6.44 0.08 0.06
d6 82.87 4.70 1.22 0.68 6.38 0.08 0.06
Average 82.61 5.04 1.30 0.64 6.68 0.08 0.06
D.m.m.f basis = Dry, Mineral-Matter Free Basis
111
4.6.1.3 Calorific value
Thermal value is amount of heat which produces by burning one kilogram of coal. This
parameter measured by calorimeter analyzes with kilocalorie per kilogram unit. In study area
flamy coal yields minimum Calorific value (7430 Kcal/Kg) and coking coal yields maximum
calorific value (8900 Kcal/Kg) (Table 4.3).
Table 4.3: Combustion parameters of the coals of the study area.
Different type
of coal
Plastometry
Coefficient
Volatile
material percent
Reflection
coefficient
(R*10)
Calorific
value
(Kcal/Kg)
Carbon
percentage
Peat _ _ _ _ _
Brown Coal _ _ _ _ _
Lignite _ _ _ _ _
Flamy coal 0 37-51 70-80 7430-7710 76-80
Gassy 6-25 37-46 80-85 8033-8485 83-86
Gassy Fat 6-25 31-37 83-89 8200-8700 86
Fat 26 33 86-97 8400-8800 86-89
Coking Fat 6-26 25-31 89-95 8300-8800 89
Coking 6-25 17-25 94-102 8500-8900 89-91
Cokunable 0-6 17-31 89-102 5000-8000 91
Lean Cokable 6-10 13-17 103-106 8500-8750 91
Bituminous 4-7 27-35 114-120 8600-8700 91-92
Semi-
Anthracite 0 7-10 114-120
8200-8500 92-98
Anthracite 0 4-7 120
112
4.6.1.4 Sulfur forms
Based on the level of sulfur content, coals have been classified into three types viz.,
low-sulfur coal that contains less than 1% total sulfur, medium sulfur coal that contains 1-3%
total sulfur and high sulfur-coal that contains more than 3% total sulfur (Chou, 1990). The
total sulfur content in Pabedana coals varies from 0.73 to 0.85% (db) with an average of
0.79% (db) (Table 4.4). Based on sulphur content the Pabedana coals may be classified as
low-sulfur coals.
Analyses of different sulfur present in Pabedana coal have been determined. The
sulfate sulfur content among the different coal seams do not show noticeable variation and
ranges from 0.01 to 0.02% db with an average of 0.015% db. Pyritic sulfur varies from 0.09%
db to 0.18% db with an average of 0.14% db and organic sulfur ranges from 0.58% db to
0.68% db with an average of 0.64% db (Table 4.4). These values indicate that a significant
proportion of the sulfur occurs as organic sulfur and pyrite is a minor constituent of the total
sulfur. The same is represented in the ternary plot (Fig. 4.3). X-ray diffraction and
petrographic analyses indicate the presence of pyrite in coal samples. Coal samples with lower
S contents are characterized by the predominance of organics over pyritic sulphur (3-4:1),
whereas in the samples with higher sulfur levels (> 1%) the proportion of pyritic sulphur is
higher (1:2).
113
Table 4.4. Different types of sulfur present in coals of the Pabedana area.
Sample
No
Total sulfur
(%)
Pyritic sulfur
(%)
Sulfate sulfur
(%)
Organic sulfur
(%)
d2 0.81 0.18 0.02 0.61
d4 0.85 0.16 0.01 0.68
d5 0.73 0.13 0.02 0.58
d6 0.78 0.09 0.01 0.68
Average 0.79 0.14 0.02 0.64
Fig. 4.3: Ternary diagram depicting forms of sulfur values (dry basis) for Pabedana coals.
In Pabedana coals, sulphate sulfur is usually found in very low levels (Table 4.4). The
highest content of sulfate sulfur is 0.02%., Sulfate sulfur in coals mainly originates from the
114
oxidizing products of pyrite (Lin et al., 2001). The content of sulfate sulfur in coals exposed to
air increases with time due to weathering (Goodarzi, 1987). Occurrence of sulfate sulfur in
trace amounts (Table 4.4) in the Pabedana coals could be the result of the partial oxidation of
pyrite during weathering.
The abundance of sulfur in coals is related to the depositional environment of coal
seams (Chou, 1990, 1997; Liu et al., 2001, 2004, 2007; Zheng et al., 2008). Sulfur content is
thought to originate within the precursor peat environment of the coal (Chou, 1990) and the
high sulfur content of the coal immediately overlain by a marine roof is well documented
(Chou,1997). It is known that the sulfur content of marine water, where sulfur bacteria had a
special role, is much higher than that of fresh water and so the peats which are formed under
marine influenced condition posses more sulfur content. Coals of the Nayband Formation in
d2, d4, d5 and d6 seams do not show any significant variations in their sulfur contents and as
mentioned earlier are considered as low-sulfur coals. The low sulfur contents in the Pabedana
coal and relatively low proportion of pyritic sulfur suggest a possible fresh water environment
during the deposition of the peat of the Pabedana coal. From the moderate amount of organic
sulfur present in the Pabedana coal, it can be inferred that the parent plant debris contained
moderate amount of sulfur.
Carbon against moisture shows no distinct relation between them. Similarly correlation
of carbon and volatile matter do not show any relation between them. A linear correlation is
seen between oxygen and carbon. There is no genetic relation between carbon and sulfur. The
sulfur content might have been controlled by the depositional conditions prevalent during the
period of formation of coal. Moreover, it is observed that the organic sulfur played a vital role
115
in the environment. Positive correlation of O + Sorg against carbon content indicates that
some of the oxygen of the coals might have been replaced by organic sulfur.
4.6.2 Coal rank
The Pabedana coals have been classified as a high volatile, bituminous coal in
accordance with the vitrinite reflectance values (58.75-74.32%) and other rank parameters
(carbon, calorific value and other volatile matter content). Calorific values of the coal of the
study region vary from 7430 to 8900 Kcal/Kg. The calorific values indicate a high volatile
bituminous rank for the Pabedana coal, but much of the volatile maybe possibly due to the
high suberine (waxy) content of the coal. In the Pabedana coals the vitrinite maceral group
predominates (> 58% vol.mmf) followed by macerals of the inertinite (from 16.10% to
28.42% vol.mmf) and liptinite groups (from 1.29% to 3.33% vol.mmf) (Table 4.5). The
variations in the preservation of initial organic material have been attributed to changes in the
physico-chemical sedimentary conditions of the water table.
According to the proximate and ultimate analyses data and also based on maceral
compositions, the samples are high volatile bituminous coal in rank with ash content ranging
from 9.82% to 12.95% and moisture content varying from 1.05 to 1.23%. Furthermore, the
macerals are dominantly composed of vitrinite.
4.6.3 Coal petrography
Lithotypes of the Pabedana coals are mainly consist of alternate layers of dull and
semi-dull coal, thin bands of semi-dull coal and semi-bright coal with a few medium and thick
bands of semi bright and bright coal. Fusain (type of charcoal) is present in high proportions in
the dull and semi-dull coals. Pabedana coals are dominated by dull and semi-dull coals with a
few semi-bright and bright coals.
116
coal is determined with a reflected light microscope (light is reflected from the sample
towards the analyst) at magnifications of about 500x, using tungsten filament and gas arc light
sources. The macroscopic study of Pabedana coals shows a banded aspect, typical of humic
coals. The aspect of each sample varies with the predominant lithotype from glassy (mainly
vitrain) to dull (mainly durain). Clarain is the most abundant lithotype followed by vitrain
layers, which are smaller in thickness (less than 0.5 cm). Carbonates and pyrites are
commonly found in cracks in the coal samples.
Table 4.5: Maceral analysis (vol%) of Pabedana Coal.
Sample
No
Vitrinite
(%)
Inertinite
(%)
Liptinite
(%) Ash (%)
Mineral matter
(%)
d2 62.17 25.73 2.72 9.82 9.38
d4 58.75 28.42 3.33 11.55 9.50
d5 74.32 16.10 1.29 12.95 8.30
d6 67.68 22.35 1.47 10.43 8.70
Average 65.73 23.15 2.20 11.19 8.97
The present analysis of Pabedana coal indicate that these are as homogeneous maceral
proportions in all samples dominated by vitrinite. Vitrinites are the coalified remains of humic
plant substances, primarily lignin and cellulose. As mentioned earlier, Pabedana mines
produce primary coking coals and contain high vitrinite (58%-67%), moderate inertinite
(16.10%-28.42%) and low liptinite contents (1.29%-3.33%). The dominant macerals of the
vitrinite group appears to be telinite and collinite. The vitrinite macerals are set in the matrix
of argillaceous mineral matter (Fig. 4.4 a,b and c). Spherical and oval shapes sporinite in are
found embedded in collotellnitic ground mass (Fig. 4.5). The maceral sporinite is thought to
117
be derived from spores and pollen. Fractures in vitrnite bands developed due to escape of
gases during coalification process are commonly seen in the macerals (Fig. 4.6a and b).
Fig. 4.4 a,b,c: Photomicrograph of vitrinite macerals in the matrix of argillaceous mineral matter.
Fig. 4.5: Photomicrograph showing Sporinite which is spherical and oval shape.
118
Fig. 4.6 a,b: Photomicrograph of fractures in vitrinite bands developed during escape of gases during coalification process.
Table 4.5 shows quantitative data on maceral content of selected samples from coal
seams d2, d4, d5 and d6. The results shows relatively a high vitrinite (58.75-74.32%, av.=
65.73%), medium inertinite (16.10-28.42%, av.= 23.15%), and low liptinite (1.29-3.33%, av.=
2.20%) contents. It is clearly evidenced that from bottom to top, the vitrinite, inertinite and
liptinite contents of the samples of the coal seams do not show much variation suggesting an
uniform depositional environment of vegetative matter. Pyrite is present mainly as massive
cell-filling mineralization thus suggesting its formation mainly during the diagenetic stage.
According to the maceral composition of the studied coal samples, the evolution of the type of
coal facies in the studied coal seam viz., moderate inertinite (16.10-28.42%) and low ash
(9.82-12.95%) suggests a low lying marsh with relatively oxidizing open water body and
higher detrital influence.
The ternary maceral and mineral matter data plotting (Singh and Singh, 1996) has
revealed the existence of vitric and mixed coal types (Fig. 4.7) in Pabedana area. The maceral
analysis and reflectance study suggest that the coals in all the four seams are of good quality
with low maceral matter association. Petrographic investigations indicate that the Pabedana
119
coal is dominated by terrestrially derived organic debris (vitrinite and liptinite) with low
amounts of inertinite.
Fig. 4.7: Depositional conditions based on the maceral and mineral matter content (after Singh and Singh, 1996).
4.6.4 Mineral analysis
Mineralogical investigations using optical microscope and XRD (Fig. 4.8 A – D)
indicate that the inorganic fraction in the Pabedana coal samples is dominated by carbonates
thus constituting the major inorganic fraction of the coal samples. Illite, kaolinite, muscovite,
quartz, feldspar, apatite and hematite occur as minor or trace phases. Carbonates, mainly
represented by ankerite, are commonly found as crack fillings in the coals. At places, pyrite is
found associated with ankerite. The high content of epigenetic ankerite mineralization is
responsible for the higher Ca, Mg, Fe and Mn contents in coals.
120
Fig. 4.8A
Fig. 4.8B
121
Fig. 4.8C
Fig. 4.8D
Fig 4.8 (A-D): XRD patterns of d2, d4, d5 and d6 coal samples. A-ankerite; Q - quartz; I-
illite, K- kaolinite; H- hematite; M- muscovite; F- feldspar; P- pyrite; Ap- apatite; C- calcite.
122
4.6.5 Major elements
The mode of occurrence of elements in coal can be determined using indirect or direct
methods (Finkelman, 1983, 1994, 1995). The indirect method is statistical which was first
used by Nicholls (1968) and followed by many researchers (Glauskoter, et al., 1977; Kamar et
al., 1986). Generally elements in coal occur associated either with inorganic constituents
(minerals) or with organic constituents (Zhang et al., 2002). According to Nicholls (1968), the
concentrations of organically bound elements in coal decrease or remain almost constant with
increasing ash content in coal. Further, the concentration of inorganically bound elements in
coal increase with increasing ash content in coal. According to Shao et al., (2004), the mode of
occurrence of an element in coal can be identified from its association with particular mineral
(s) or major element (s), based on pearson’s correlation coefficients between elements.
Elements exhibiting positive correlation with the ash yield indicate an inorganic
association and suggest that these elements are the components of minerals in coal. Elements
with positive correlation with the organic carbon contents (TOC) indicate their organic
association in coal (Baioumy, 2009). The direct method for determining the occurrence of
elements in coal is sequential leaching, which was adopted by Finkelman (1983) and Wang
(1994). The indirect method (Nicholls, 1968; Shao et al., 2004) was used in this study to
determine the organic / inorganic affinity of elements in coal. The major elements in coal
generally occur in minerals (Liu et al., 2001) rather than in organic matter (Pike et al., 1989).
Therefore, major elemental analyses can be used as a tool for discriminating element-mineral
associations.
123
Table 4.6: Major elements analytical data of Pabedana coal.
Seam SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O MnO P2O5 TiO2 Cl2O
d2 10.38 12.24 8.65 24.72 36.04 1.02 0.10 0.51 0.40 2.84 0.99
d4 10.57 16.54 12.56 21.48 31.53 0.87 0.35 0.61 0.43 3.98 1.12
d5 19.93 15.07 9.65 19.70 28.00 2.63 0.23 0.54 0.78 2.55 0.96
d6 13.53 14.59 8.44 21.11 34.75 2.40 0.29 0.48 0.48 2.97 1.05
Table 4.7: Values of Pearson’s coefficient of correlation of major elements of coals.
Element SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O MnO P2O5 TiO2 Cl2O
SiO2 1.00
Al2O3 0.20 1.00
Fe2O3 -0.22 0.79 1.00
MgO -0.76 -0.75 -0.25 1.00
CaO -0.77 -0.64 -0.45 0.81 1.00
Na2O 0.86 0.00 -0.50 -0.71 -0.43 1.00
K2O 0.00 0.94 0.64 -0.67 -0.38 0.10 1.00
MnO -0.15 0.69 0.97 -0.19 -0.51 -0.52 0.50 1.00
P2O5 0.98 0.25 0.00 -0.75 -0.85 0.77 0.00 0.00 1.00
TiO2 -0.64 0.62 0.84 0.00 0.05 -0.67 0.66 0.74 -0.56 1.00
Cl2O -0.62 0.62 0.69 0.00 0.17 -0.51 0.76 0.53 -0.60 0.95 1.00
Table 4.6 shows the major element content of the Pabedana coal. The SiO2 content
varies narrowly from 10.38 to 19.93. The Al2O3 content ranges from 12.24 to 16.54% and
Fe2O3, from 8.44 to 12.56%. MgO and CaO are the dominant components of the inorganic
constituents and vary between 19.70 and 24.72%, and 28.00 and 36.04% respectively. The
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Na2O content varies from 0.87 to 2.63%, K2O, from 0.1 to 0.35%, MnO, from 0.48 to 0.61,
P2O5, from 0.40 to 0.78 and Cl2O, from 0.96 to 1.12%. The content of TiO2 ranges from 2.55
to 3.98%. The variation in major elements content is relatively narrow between different coal
seams.
The elements Si, Al, Ti and K are mainly associated with quartz and clay minerals. The
significantly positive correlation between K2O and Al2O3 (r = 0.94), the positive correlation
between SiO2 and Al2O3 (r = 0.20) and between TiO2 and Al2O3 (r = 0.62), no correlation
between K2O and SiO2, demonstrate that Si, Al, K and Ti mainly originate from illite and not
from kaolinite. Illite has been reported as one of the major clay minerals in the coal deposits of
Iran. By assuming that Al in the coal is exclusively derived from detrital alumina-silicate
sources (Murray et al., 1992), the positive correlation between Al and Si, K, Ti and P (r =
0.20, 0.94, 0.62 and 0.25 respectively) indicate the detrital origin of these elements, which
may occur as detrital clay minerals (Table 4.7).
Ti is present in concentration close to 3% in pabedana coals. This range of Ti contents
is high when compared with the usual Ti content in coal elsewhere (0.05-0.2%). The Ti/Al
ratio is close to 0.04-0.05, but when high Ti levels are present, this ratio increases up to 0.2.
The small variation in the Al/Ti ratios in the Pabedana coal implies that the detrital material
supplied to the site of deposition had near equal values of Al/Ti ratio. The constant Ti/Al ratio
supports an association of Ti with the aluminium fraction, but the presence of significant
amounts of anatase or rutile may be deduced when high Ti/Al ratios are sporadically
measured. In sediments rutile is known to form during the reconstitution processes in clays
and shales and also known to occur as a common detrital mineral.
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The Pabedana coals are characterized by high contents of Ca, Mg, Mn, Ba and Sr
which in turn reflect the high carbonate and phosphate contents. The MgO wt.% (which
ranges from 19.70% to 24.72%) and CaO wt.% (which varies from 28% to 36.04%), are
positively correlated (r = 0.81). Optical and X-ray studies and the ratio of Mn:Fe ≤ 4:1 confirm
the undoubted presence of ankerite as the most dominant constituent among the carbonate
minerals in the coal samples. Megascopic studies of coal reveal the occurrence ankerite along
cleavages and joints. Ankerite is low temperature metasomatic origin. However, some amount
of CaO in coals may be present as minor amounts of calcite. This is in contrast to the coal
fields of Alborz region of northern Iran, wherein it is reported that carbonates are largely made
up of dolomite and calcite (Zadeh Kabir, 1991; Razavi Armagani and Moinosadat, 1994;
Yazdi and Shiravani, 2004; Goodarzi et al., 2006). Further, minor amounts of Ca, along with
P2O5 and Cl2O may be contributed by apatite. The strong positive correlation between P2O5
and ash content (r = 0.86) further shows that P is mainly present in the form of phosphate
minerals.
The positive correlation between Na2O and ash content, (r = 0.46) and the negative
correlation between Na and Cl (r = - 0.51) indicate that Na mainly occurs in minerals rather
than in pore water, the latter is generally considered as a source of Na. The presence of
feldspar group of minerals account for Na in coals. The Fe2O3 content in coals ranges from
8.44 to 12.56 wt.% and indicates the presence of variable amounts of pyrite. Low sulfur
content indicates low contents of sulphates (barite and gypsum). There is a positive correlation
between Fe and S, showing association of these elements with sulfide minerals, pyrite in
particular. Some exceptionally high content of Fe suggests the presence of iron oxides
(hematite) and Fe-bearing clay minerals.
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4.6.7 Trace elements
The study of trace elements (TE) in coals is a complex issue. This makes it possible to
detect elevated contents of valuable elements and thus upgrade the feasibility of coal mining,
on the one hand, and to elucidate hazardous contents of toxic elements, pose problems related
to environmental contamination by coal combustion, and find ways of salvaging the
environment. Trace elements in coals have been studied by many workers. Comprehensive
literature reviews are to be found in Raask (1985), Swaine (1990) and Clarke and Sloss
(1992). In the early geochemical work, the emphasis was on the determination of elemental
concentrations in coals and other earth materials in order to define the laws governing element
distributions. Due to the special geochemical environment involved during peatification and
coalification processes, many trace elements, especially potentially toxic trace elements
(PTTE), can be enriched in coal. Organic matter and diagenetic minerals can act as enrichment
traps for these trace elements (Swaine 1990). Concentrations of trace element in coal samples
shown in Table 4.8.
The relationships between trace element concentration and ash yield have been widely
reported (Finkelman, 1983; Goodarzi, 1988; Spears and Zheng, 1999; Spears et Al., 1999; Dai
et al., 2005). The ash content of coal and its geochemical character depends on the
environment of deposition and subsequent geological history. It is generally considered that
most trace elements in coal are associated with the mineral matter (Gentzis and Goodarzi,
1997).
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Table 4.8: Trace elements analytical data of Pabedana coal (mg/L).
Elements d2 d4 d5 d6 Elements d2 d4 d5 d6
Hg 0.34 0.1 0.23 0.18 Mo 1.3 0.85 1.44 1.63
Be 1.67 2.31 2.86 1.22 Nb 10.1 4.3 6.6 5.4
Co 4.15 6.9 8.18 5.77 Pb 44.2 53.3 40.3 64.1
Cr 98 66 33 54 Rb 96.1 83.4 110.2 71.1
Cu 42 110 80 50 Sb 1.3 0.9 0.94 0.8
Li 160 215 76 90 Sn 1.35 2.4 1.6 1.8
Mn 704 1016 1100 590 Te 0.06 0.08 0.17 0.23
Ni 124 83 101 69 Tl 0.44 0.6 0.3 0.45
Sc 21.3 15.6 8.9 13.6 U 2.8 2.3 3.4 2.4
Sr 1222 962 1640 1118 W 1.2 0.95 1.1 1.3
Th 10.3 9.7 13.2 9.48 Ti 1556 2174 1720 1620
V 58 87 93 61 Ta 0.45 0.93 1.2 0.68
Y 17.1 38.8 26.2 12.7 Se 1.5 1.8 1.41 2.4
Zn 52.5 63.3 58.4 79 S 9654 8200 11000 9271
B 7.6 8.1 9.2 6 Re 0.002 0.003 0.002 0.004
Ba 296 405 260 304 P 393 468 560 481
Zr 271 295 188 314 Na 1450 1460 1600 1120
Ag <1 <1 <1 <1 Ca 6375 9233 8330 6140
As 20.1 8.4 12.3 37.2 Mg 3880 4010 3980 3730
Au 0.04 0.06 0.08 0.06 La 26.4 18.5 22.3 30.3
Bi 0.31 0.54 0.22 0.6 K 8720 11340 9880 7940
Cd 0.16 0.15 0.38 0.23 Al 67900 72650 93440 68800
Cs 6.4 14.4 7.3 8.5 Ce 30.5 38.2 24.5 42.3
Ga 24.9 18.6 12.1 15.2 Fe 35400 48300 46450 43870
Ge 0.55 0.46 0.27 1.03
Elements exhibiting positive correlation with ash yield indicate inorganic affinity
(Nicholls, 1968). The inorganic affinity may be explained as a result of the causes such as : (1)
presence of the element in the inorganic detritus accumulating together with the peat from
which the coal is formed, (2) sorption from circulating waters by this inorganic detritus during
original peat accumulation, (3) sorption from groundwater by the inorganic fraction during
diagenesis, (4) precipitation from circulating waters of compounds stable under physico-
chemical environment of peat formation, (5) precipitation from groundwater by reaction with
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compounds already present in the formation during diagenesis and (6) introduction of mineral
matter into coals at a late stage in their formation or even after their formation operating in
isolation or in union.
Table 4.9 provides correlation coefficients between element contents and ash yields.
Based on the values of correlation coefficients between elements and ash yield, the elements
are classified into four group. The first group of elements (Be, Co, Mn, Th, B, V, Au, Cd, Ga,
Ta, P, Ca, Al, Fe) has a very high positive correlations with ash yield (rash>0.7): These
elements have high inorganic affinity. Most of these elements have a high positive correlation
coefficient with SiO2 and Al2O3 (rSi+Al>0.7). The second group includes eight elements (Cu,
Sr, Y, Rb, Na, Mg, La, K) and shows medium positive correlations with ash yields (r = 0.51 to
0.69). This group of elements exhibits inorganic affinity. The third group of elements (Sn, Te,
Ti, S) exhibits weak correlation with ash yield (r = 0.21 to 0.50). Only one element, namely
Cs, belongs to fourth group which shows the lowest correlation with ash yields (r<0.20).
Many researchers have reported that some elements including As, Hg, Sb, Co and Se
are associated with pyrite (Finkelman et al., 1992; Ward et al., 1999; Ding et al., 2001). In the
coals of the study area, these elements are not clearly related with pyrite except As. Elements
like As, Ni, Be, Mo and Fe show relatively high positive correlation coefficients with pyritic
sulphur (r = 0.53 to 0.80).
Elements Sc (r= -0.85), Cr (r= -0.83), Zr (r= -0.74), Ga (r= -0.77), Ge (r= -0.66), La
(r= -0.62), As (r= -0.59), W (r= -0.55), Ce (r= -0.51), Sb (r= -0.44), Nb (r= -0.45), Th (r= -
0.44), Pb (r= -0.42), Se (r= -0.40), Tl (r= -0.39), Bi (r= -0.39), Hg (r= -0.38), Re (r= -0.29), Li
(r= -0.28), Zn (r= -0.12), Mo (r= -0.12) and Ba (r= -0.11) show varying negative correlation
with ash yield. These elements possibly have an organic affinity. These elements may be
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present as primary biological concentrations either with tissues in living condition and/or
through sorption and formation of organometallic compounds.
Table 4.9: Correlation coefficients of trace elements and ash yields.
Element Correlation Element Correlation
Hg -0.38 Mo -0.11
Be 0.89 Nb -0.45
Co 0.98 Pb -0.42
Cr -0.83 Rb 0.56
Cu 0.66 Sb -0.44
Li -0.28 Sn 0.25
Mn 0.88 Te 0.21
Ni -0.13 Tl -0.38
Sc -0.85 U -0.48
Sr 0.60 W -0.55
Th 0.77 Ti 0.40
V 0.95 Ta 0.99
Y 0.57 Se -0.39
Zn -0.12 S 0.44
B 0.76 Re -0.29
Ba -0.11 P 0.90
Zr -0.74 Na 0.61
As -0.58 Ca 0.74
Au 0.92 Mg 0.64
Bi -0.39 La 0.61
Cd 0.75 K 0.58
Cs 0.19 Al 0.92
Ga 0.77 Ce -0.50
Ge 0.66 Fe 0.75
4.7 Depositional environment
Boron (B) is a palaeosalinity indicator of coal forming environments (Goodarzi, 1987;
Dominik and Stanley, 1993; Goodarzi and Swaine, 1994; Hower et al., 2002). Goodarzi
(1987) and Goodarzi and Swaine, (1994) showed that there is a good relationship between the
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B content of coal and palaeo-environmental settings. The boundaries between mildly brackish
and marine environments are defined at 50 and 110 mg/kg B, respectively. The B content in 4
samples from coal seam shows a narrow range from 6.00 to 9. 2 ppm with average value of
7.7 ppm, which indicates the depositional environment was fresh water condition oriented
(Goodarzi and Swaine, 1994).
The elemental ratios Th/U, Sr/Ba, B/Ga, [(CaO+MgO+Fe2O3)/(SiO2+Al2O3)], imply a
reductive littoral to brackish swamp environment during deposition. A perusal of literature
reveals that there are some inconsistencies in the interpretation (Chao et al., 1994). The
inconsistency of elemental ratio can be attributed to differences in plant species, geologic time
and local tectonic activities. In addition the Pabedana coals contain low B, Mo and U and low
B/Ga and [(CaO+MgO+Fe2O3)/(SiO2+Al2O3)] ratios. This data indicates deltaic
environmental depositional condition.