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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.

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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

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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

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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,

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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.

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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.

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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

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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

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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.

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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.

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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

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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

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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).

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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

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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

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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.

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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

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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.

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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

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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.

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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.

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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.