CHAPTER IV COAL QUALITY, MINERALOGY, CHEMISTRY AND DEPOSITIONAL ENVIRONMENT

Transcription

1 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. 100

2 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 101

3 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 Mt, out of which 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 ( 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 102

4 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: ) 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, 103

5 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. 104

6 105 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.

7 4.5 METHODS OF STUDY 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 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 106

8 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) 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 (R 0max ) 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) Mineral analysis Representative split of coal samples were grained to -200 mesh (75 µm) and ovendried 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 107

9 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. 108

10 4.6 RESULTS AND DISCUSSION 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 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 ( %; 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. 109

11 Sample No Table 4.1: Proximate Analysis of Coal- (Air dried basis) Moisture (%) Ash (%) Volatile matter (%) Fixed carbon (%) d d d d Average Ultimate analysis The Pabedana coal contains high C ( %, 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). Sample No Table 4.2: Elemental composition of Coal (D.m.m.f: basis). C (%) H (%) N (%) Sulfur organic (%) O (%) O/C H/C d d d d Average D.m.m.f basis = Dry, Mineral-Matter Free Basis 110

12 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 Gassy Gassy Fat Fat Coking Fat Coking Cokunable Lean Cokable Bituminous Semi- Anthracite Anthracite

13 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). 112

14 Table 4.4. Different types of sulfur present in coals of the Pabedana area. Sample No Total sulfur (%) Pyritic sulfur (%) Sulfate sulfur (%) Organic sulfur (%) d d d d Average 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 113

15 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 114

16 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 Coal rank The Pabedana coals have been classified as a high volatile, bituminous coal in accordance with the vitrinite reflectance values ( %) 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 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. 115

17 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 (%) d d d d Average 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 116

18 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. 117

19 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 ( %, av.= 65.73%), medium inertinite ( %, av.= 23.15%), and low liptinite ( %, 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 ( %) and low ash ( %) 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 118

20 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) 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. 119

21 Fig. 4.8A Fig. 4.8B 120

22 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. 121

23 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. 122

24 Table 4.6: Major elements analytical data of Pabedana coal. Seam SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O MnO P 2 O 5 TiO 2 Cl 2 O d d d d Table 4.7: Values of Pearson s coefficient of correlation of major elements of coals. Element SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O MnO P 2 O 5 TiO 2 Cl 2 O SiO Al 2 O Fe 2 O MgO CaO Na 2 O K 2 O MnO P 2 O TiO Cl 2 O Table 4.6 shows the major element content of the Pabedana coal. The SiO 2 content varies narrowly from to The Al 2 O 3 content ranges from to 16.54% and Fe 2 O 3, from 8.44 to 12.56%. MgO and CaO are the dominant components of the inorganic constituents and vary between and 24.72%, and and 36.04% respectively. The 123

25 Na 2 O content varies from 0.87 to 2.63%, K 2 O, from 0.1 to 0.35%, MnO, from 0.48 to 0.61, P 2 O 5, from 0.40 to 0.78 and Cl 2 O, from 0.96 to 1.12%. The content of TiO 2 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 K 2 O and Al 2 O 3 (r = 0.94), the positive correlation between SiO 2 and Al 2 O 3 (r = 0.20) and between TiO 2 and Al 2 O 3 (r = 0.62), no correlation between K 2 O and SiO 2, 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 ( %). The Ti/Al ratio is close to , 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. 124

26 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 P 2 O 5 and Cl 2 O may be contributed by apatite. The strong positive correlation between P 2 O 5 and ash content (r = 0.86) further shows that P is mainly present in the form of phosphate minerals. The positive correlation between Na 2 O and ash content, (r = 0.46) and the negative correlation between Na and Cl (r = ) 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 Fe 2 O 3 content in coals ranges from 8.44 to 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. 125

27 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). 126

28 Table 4.8: Trace elements analytical data of Pabedana coal (mg/l). Elements d2 d4 d5 d6 Elements d2 d4 d5 d6 Hg Mo Be Nb Co Pb Cr Rb Cu Sb Li Sn Mn Te Ni Tl Sc U Sr W Th Ti V Ta Y Se Zn S B Re Ba P Zr Na Ag <1 <1 <1 <1 Ca As Mg Au La Bi K Cd Al Cs Ce Ga Fe Ge 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 physicochemical environment of peat formation, (5) precipitation from groundwater by reaction with 127

29 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 (r ash >0.7): These elements have high inorganic affinity. Most of these elements have a high positive correlation coefficient with SiO 2 and Al 2 O 3 (r Si+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= ), 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 128

30 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 Mo Be 0.89 Nb Co 0.98 Pb Cr Rb 0.56 Cu 0.66 Sb Li Sn 0.25 Mn 0.88 Te 0.21 Ni Tl Sc U Sr 0.60 W Th 0.77 Ti 0.40 V 0.95 Ta 0.99 Y 0.57 Se Zn S 0.44 B 0.76 Re Ba P 0.90 Zr Na 0.61 As Ca 0.74 Au 0.92 Mg 0.64 Bi La 0.61 Cd 0.75 K 0.58 Cs 0.19 Al 0.92 Ga 0.77 Ce Ge 0.66 Fe 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 129

31 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+Fe 2 O 3 )/(SiO 2 +Al 2 O 3 )], 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+Fe 2 O 3 )/(SiO 2 +Al 2 O 3 )] ratios. This data indicates deltaic environmental depositional condition. 130