Petrographic and geochemical contrasts and environmentally significant trace elements in marine-influenced coal seams, Yanzhou mining area, China

Journal of Asian Earth Sciences 23 (2004) 491–506 www.elsevier.com/locate/jseaes

Petrographic and geochemical contrasts and environmentally significant trace elements in marine-influenced coal seams, Yanzhou mining area, China Guijian Liua,*, Pingyue Yanga, Zicheng Penga,b, Chen-Lin Chouc a School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China Key Laboratory of Loess and Quaternary Geology, Institute of Earth and Environment, CAS, Xi’an, Shanxi 710075, China c Illinois State Geological Survey, 615 East Peabody Drive, Champaign, IL 61820, USA

b

Received 4 June 2002; revised 7 July 2003; accepted 24 July 2003

Abstract The Yanzhou mining area in west Shandong Province, China contains coals of Permian and Carboniferous age. The 31 and 32 seams of the Permian Shanxi Formation and seams 6, 15 – 17 of the Carboniferous Taiyuan Formation were analyzed for coal petrology, mineralogy and geochemical parameters. The parameters indicate that the coal is high volatile bituminous in rank. The coal is characterized by high vitrinite and low to medium inertinite and liptinite contents. These properties may be related to evolution of the coal forming environment from more reducing conditions in a marine influenced lower delta plain environment for the early Taiyuan coals to more oxidizing paleoenvironments in an upper delta plain for the upper Shanxi coal seams. The major mineral phases present in the coal are quartz, kaolinite, pyrite and calcite. Sulfur is one of the hazardous elements in coal. The major forms of sulfur in coal are pyritic, organic and sulfate sulfur. Pyritic and organic sulfur generally account for the bulk of the sulfur in coal. Elemental sulfur also occurs in coal, but only in trace to minor amounts. In this paper, the distribution and concentration of sulfur in the Yanzhou mining district are analyzed, and the forms of sulfur are studied. The sulfur content of the Taiyuan coal seams is considerably higher than that of the Shanxi coals. Organic sulfur content is positively correlated to total and pyritic sulfur. The vertical variation of Cu, Zn, Pb, As, Th, U and sulfur contents in coal seam 3 of the Shanxi Formation in the Xinglongzhuang mine show that all these trace elements, with the exception of Th, are enriched in the top and bottom plies of the seam, and that their concentrations are also relatively high in the dirt bands within the seam. The pyritic sulfur is positively correlated with total sulfur, and both are enriched in the top, bottom and parting plies of the seam. The concentrations of the trace elements are closely related to sulfur and ash contents. Most of the trace elements are correlated with the ash content, and may be associated with the mineral matter in the coal. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal geochemistry; Sulfur in coal; Environmentally significant trace elements, Yanzhou mining area, Chinese coal

1. Introduction A large coal resource occurs in the Yanzhou mining area in Shandong Province, China (Fig. 1). The coal resource is concentrated in two coalfields, the Yanzhou coalfield and the Jining coalfield. Annual coal production for both coalfields, estimated to be at about 40 Mt for 1998, is used mainly for power generation with a minor proportion for coke production. * Corresponding author. Tel.: þ 86-551-3603714; fax: þ 86-5513621485. E-mail address: [email protected] (G. Liu). 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2003.07.003

Currently there are 15 working underground coal mines in the Yanzhou coalfield. Their total annual production is 26 Mt, of which about 5 Mt are exported. The Jining coalfield has six working underground mines with a total production estimated at about 13.5 Mt in 1998, and five exploration areas for which additional mines are being designed and built. All twenty-one mines and exploration areas have been studied (Liu and Wang, 1999) in the mining district. The larger mines in the district are the Xinglongzhuang mine, the Baodian mine, the Dongtan mine, the Jining-2 mine, the Jining-3 mine, and the Tangkou exploration area where a mine is being built (Fig. 2).

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Fig. 1. Location of the Yanzhou mining district in Shandong Province, China.

Samples were collected from the Xinglongzhuang mine, which had a production of 4.0 Mt in 1999. The coal seams are numbered in ascending sequence from 17 to 1, with seams 17– 4 in the Taiyuan Formation (Upper Carboniferous) and seams 3– 1 in the Shanxi Formation (Lower Permian). Fig. 3 shows the stratigraphy of the coal-bearing sequences in the Yanzhou – Jining area. Coal rank in western Shandong is mainly high volatile bituminous, although medium volatile bituminous coal occurs in the deeper areas of the coalfield. Coal rank in the Laiwu field (Fig. 1) has been locally increased to low volatile bituminous coal and anthracite by Jurassic igneous intrusions. Some trace elements may be released to the atmosphere through coal burning (Swaine, 2000; Liu et al., 1999; Spears and Zheng, 1999; Spears et al., 1999; Yan et al., 1999; Foner et al., 1999; Helble, 1994; Wang and Ren, 1996; Qiu et al., 1999; Rong and Wang, 1990). Coal combustion, along with coal mining operations, agricultural operations and industrial processes, is a major source of potentially hazardous air pollutants, which include As, Be, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb, Se, and radionucleides, such as Th and U (Finkelman, 1994; Helble, 1994; Wang and Ren, 1996; Qiu et al., 1999). Systematic studies of the potentially hazardous elements in coal have therefore been made to determine the possible environmental impact of coal utilization (Swaine, 2000; Liu et al., 1999; Yan et al., 1999; Finkelman, 1994; Finkelman and Gross, 1999; Clarke,1993; Block and Dams, 1975; Querol et al., 1995;

Kizishtein and Kholodkov, 1999; Zheng et al., 1999; Gutta, 1999; Fyfe, 1999). Geochemical studies of toxic trace elements in coal have intensified in recent years, due to a growing awareness of the potential effects of these elements on the environment, and also as advanced analytical techniques have been developed. Despite increased research activities, knowledge of the distribution of most trace elements within the various host phases of the coal is limited. This is partly due to the paucity of trace element data for many coals, and partly due to the fact that the trace elements in coal are derived from a variety of host phases. A proportion of the trace elements in coal originated from the parent plant materials (Chen et al., 1987), and these organically associated trace elements may be redistributed during coalification. Another portion of the trace elements is hosted in various accessory minerals in coal, these minerals having a variety of syngenetic to epigenetic origins (Spears, 1987). It is normally assumed that chalcophile trace elements are associated with pyrite and other sulfides in coal, and lithophile elements with carbonates or clay minerals. Building on previous studies (Chen and Wang, 1993; Foner et al., 1999; Li and Wei, 1998; Liu and Yang, 1999a; Liu et al., 1995; Wu, 1990; Wu et al., 1995; Han and Yang, 1980; Yang and Liu, 1994) a study has been made of the coal geology, mineralogy, petrography and sedimentary environments of the Yanzhou mining area. This study

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Fig. 2. Location of the coal mines and the exploration areas in Yanzhou mining district.

concentrated on the geochemistry of the coals and aimed to provide a good understanding of the abundances and geochemistry of sulfur and potentially hazardous trace elements (As, Cu, Pb, Zn, Th, U) in the coals of the study district. The results are also relevant to environmental protection during coal mining and utilization.

2. Geological setting and coal geology 2.1. Regional geological setting The Yanzhou mining area is part of the West Shandong mining district located in the west and middle parts of

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The Permo-Carboniferous strata of the West Shandong coal mining district are located in the east of the North China Platform, which was a large down-warped basin in the Late Paleozoic. This basin is divided into northern, central, and southern zones. The West Shandong coal deposits are located in the eastern part of the basin (Chen et al., 1993; Li and Wei, 1998; Wu et al., 1995). Based on sedimentological studies, Li and Wei (1998) and Han and Yang (1980) suggest that the North China Platform subsided and formed a broad down-warped basin in the Middle Carboniferous. A transgression from the east formed a broad shallow epicontinental sea and littoral – alluvial to littoral plains in the basin (Li and Wei, 1998; Liu and Yang, 1999a; Yang and Liu, 1994). The sedimentary facies consist mainly of shallow marine limestones and mudstones and littoral sandstones. The maximum transgression occurred in the Late Carboniferous, and in the Western Shangdong mining district this transgression resulted in the deposition of marine carbonate and mudstone, littoral clastic sediments, and marsh peat. In Late Carboniferous time, uplift of the Yinshang, in the northern margin of the basin, resulted in an increased supply of terrigenous clastic sediment. Coals of the Western Shangdong mining district were formed in environments that ranged from marine to terrestrial (Han and Yang, 1980; Li and Wei, 1998). Minor limestone deposits occur in the upper Taiyuan Formation in the southern areas, generally consisting of one to two beds with a cumulative thickness of 2 – 5 m. 2.2. Geological setting of the Yanzhou mining area

Fig. 3. Stratigraphic section of the Upper Carboniferious Taiyuan Formation and the Lower Permian Shanxi Formation in the Yanzhou mining district.

Shandong Province, China (Fig. 1). The Tan –Lu Fault marks the eastern boundary of the West Shandong district and the Yi– Du and the Liao – Kao Faults are the northern and western boundaries, respectively (Fig. 1). To the south, the coal mining district extends into Jiangsu Province, where it is bounded by the Fengqiu and Shangqiu Faults. The coal basin covers a total area of about 90,000 km2.

The geology of the Yanzhou mining district has been reported by Chen et al. (1993), Li and Wei (1998), Han and Yang (1980), Liu et al. (1995, 1999), and Yang and Liu (1994). The district is bounded by the Yishan Fault on the east, and the Jiaxiang Fault on the west and coal outcrops to the south and north (Fig. 2). The coal basin covers an area of about 1300 km2 and contains a coal resource of approximately 8– 9 Gt (Liu et al., 1999). The major coal beds of the Yanzhou mining district occur in the Upper Carboniferous Taiyuan Formation and the Lower Permian Shanxi Formation (Fig. 3).The coal-bearing formations are underlain by the Middle Carboniferous Benxi Formation and overlain by the Upper Permian Xiashihezi Formation. Both the overlying and underlying formations contain minor coal seams. The Taiyuan Formation ranges from 130 to 190 m in thickness and is composed primarily of sandstone, limestone, mudstone, and coal (Fig. 3). Out of a total of 20 coal seams, 2 –4 are mineable throughout the area and five to seven are mineable locally. The Shanxi Formation consists of sandstone, mudstone and coal (Fig. 3). Its total thickness ranges from 50 to 120 m. The formation contains one to three coal seams, with a cumulative workable coal thickness of 3 –8 m. Of these

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seams, only coal seam 3 of the Shanxi Formation is mined in the Yanzhou mining district. Coal 3 is usually divided into two sub-seams, referred to in places as coal seams (31) and 32.

3. Sampling Coal seams 3 (31), 32, 6, 10, 15, 16 and 17 were sampled in the Yanzhou mining area. The samples of coal seams 3 (31) and 32 were taken from the Xinglongzhuang mine and the Tangkou exploration area, and those of coal seams 6, 10 and 15– 17 from the Baodian mine and the Tangkou exploration area. Samples from the Tangkou exploration area were collected from borehole cores in the processing of exploration, and the samples from the borehole cores were collected from full seam channels. In addition, 21 samples of coal seam 3 and associated clay partings were collected from an underground mine (Xinlongzhuang coal mine). The samples were analyzed to identify the vertical distribution of sulfur and selected trace elements. These samples are identified as M1 – M21 from bottom to top of the coal seam. Samples M14 and M15 represent clay partings in the coal seam, and M21 is roof of the seam (also coaly claystone). All samples were collected as full channels from the fresh coal seam. The individual plies were approximately 10 –12 cm thick, and the 10 cm thick clay parting in the coal seam was sampled as a separate ply.

4. Methods The bulk coal samples were air-dried, milled and split until a representative 0.5 kg sample milled to pass a 0.250 mm sieve was obtained for mineralogical, proximate, ultimate, and chemical analyses. Other splits were made at different size fractions (, 0.20 mm) for petrological studies. Proximate and ultimate analyses were performed following ASTM (1992) standard procedures. Ashes and coals were subjected to mineralogical, chemical and petrographic analysis using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The trace elements in the coals were determined by atomic adsorption spectrometry (AAS) and inductively coupled plasma-optical emission spectrometry (ICP-OES) after the samples were first ashed at about 815 8C. Cu and Zn were determined by flame AAS and Pb by graphite-furnace AAS. Arsenic was determined by a spectrophotometric method. Maceral composition was determined and random vitrinite reflectance was measured on telovitrinite using Leitz MPV-compact and MPV-3 microphotometers. The petrographic and chemical analysis results are listed in Tables 1– 5.

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5. Results and discussion 5.1. Coal characterization The data in Table 1 were obtained from coal samples from the Dongtan coal mine in the Yanzhou coal field and the Jining-3 coal mine in the Jining coal field. The data in Table 2 were obtained from the Jining-2 coal mine in the Jining coal field. Table 1 shows the results of the proximate and ultimate analyses of the coal seams from the Yanzhou and Jining coalfields. Results of proximate analyses, sulfur, and vitrinite reflectance for the coal seams in the Jining coal field are listed in Table 2. The variations are graphically shown in Fig. 4. The moisture content of the coals in the Shanxi Formation (Seam 3) and Taiyuan Formation (Seams 6– 17) is less than 3% (Tables 1 and 2). The moisture in the Shanxi Formation coal seams is slightly higher than that in the Taiyuan Formation coals. The upper coal seams in the Shanxi Formation have an ash yield that ranges from 8.74% (M6) to 24.7% (M13). The Taiyuan Formation coals have a lower ash yield, ranging from 4.2% (Table 2) to 21.3% (Table 1). Disregarding clay partings, ash yields are generally higher at the top and bottom of the individual coal seams (Tables 2 and 5). Table 3 shows ash composition. The coal ashes consist mainly of SiO2 and Al2O3, with secondary CaO, Fe2O3 and SO3, and minor proportions of MgO, TiO2 and other oxides. SiO2 and Al2O3 make up 42.5– 82.8% of the total ash. SiO2, Al2O3 and Fe2O3 are more abundant in the top and bottom of seam 3 than in the middle, while MgO, MnO2, CaO and SO3 are higher in the middle of seam 3. SiO2, Al2O3, Fe2O3, CaO and SO3 are more abundant in the top than in the bottom of the Taiyuan coal seam 16. The coals are of high volatile bituminous rank (Tables 1 and 2), with vitrinite reflectance ranging from 0.6 to 0.8%. Vitrinite reflectance shows a downward decrease from the Shanxi coals to the Taiyuan coals. This may be due to impregnation of the vitrinite by hydrocarbons (Goodarzi et al., 1987; Gentzis and Goodarzi, 1994; Suarez-Ruiz et al., 1994; Liu et al., 1999). However, it may also reflect marine influence on the coals in the lower part of the section (Gurba and Ward, 1998, 2000). The reverse trend is apparent for volatile matter (Tables 1 and 2), which is higher (. 43% on a dry ash-free basis) in the lower seams (6 – 17) than in the upper seam (3) (, 40% daf). Such an inverse trend is impressive because coal samples from the Shanxi and Taiyuan Formations were taken from the same vertical succession (Table 1). However, the benches within seam 3 (Table 2) show significant variation, and the lowest volatile matter content is in samples 3 –9 (lowest part of seam 3). Two samples from the top and bottom of seam 16 in the Tangkou exploration area show little variation in their volatile matter content and vitrinite reflectance (Table 2).

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Table 1 Proximate and ultimate analyses of representative coal seam samples from the Yanzhou and Jining coal fields Coal seam

Moisture (wt%)

Ash (wt%)

Volatile matter (%daf)

Gross calorific value (MJ/kg maf)

Vitrinite reflectance values, Ro (%)

C (%daf)

H (%daf)

N (%db)

S (total, %db)

Yanzhou coal field 3 2.5 6 2.4 10 2.2 15 2.3 16 1.9 17 2.0

14.5 13.9 17.4 14.4 12.0 12.9

38.4 43.3 43.7 43.6 43.6 44.2

27.79 28.48 27.15 27.42 29.09 29.21

0.75 0.67 0.68 0.73 0.70 0.69

83.8 83.7 82.6 83.3 82.5 82.7

5.4 5.6 5.8 5.7 5.6 5.7

1.4 1.5 1.6 1.5 1.3 1.3

0.6 2.9 3.4 3.4 3.2 3.5

Jining coal field 3(31) 2.4 32 2.5 6 2.2 10 2.0 15 2.1 16 2.2 17 2.1

14.6 16.6 13.8 21.3 14.6 12.5 13.9

38.5 39.7 43.2 46.0 45.2 43.9 44.6

27.61 26.71 29.00 25.91 28.47 29.43 29.03

0.72 0.76 0.67 0.67 0.64 0.70 0.66

83.0 82.7 82.3 81.5 81.1 82.0 82.4

5.4 5.4 5.8 5.5 5.6 5.5 5.6

1.5 1.6 1.5 1.5 1.5 1.3 1.4

0.5 0.6 2.9 4.7 3.8 4.0 4.2

daf: dry ash-free basis, db: dry basis.

The vitrinite content of the Taiyuan Formation coals is higher than that of the Shanxi Formation coal seams, but the contents of intertinite and liptinite in the Shanxi Formation coal seams are higher than in the Taiyuan Formation coals. Based on the results of this work and previous studies (Liu et al., 1999; Chen and Wang, 1993; Querol et al., 1999), the telinite content is higher at the bottom than at the top of the coal seams. The top of coal seams 16 and 17 is characterized by a lower telinite content and higher telocollinite and desmocollinite concentrations compared to the bottom parts. The proportions of vitrinite group macerals in coal seam 16 do not show any significant variation (Chen and Wang, 1993). As with desmocollinite, inertinite macerals are enriched at the top of coal seam 3, compared to the bottom of the seam. Both coal seams 3 and 16 from the Tangkou exploration area show a high inertinite content.

There is no clear difference in gross calorific value (25.91 –29.43 MJ/kg) and total carbon content between the Shanxi and Taiyuan coals. Based on the vitrinite reflectance and the volatile matter values, the coal rank is high volatile bituminous for the coals studied. 5.2. Macerals The coal in the Tankou exploration area and in the Xinglongzhuang mine is characterized by high vitrinite and low to medium inertinite and liptinite contents (Table 4). The major macerals of the vitrinite group are telinite, telocollinite and desmocollinite. The coal seams of the Shanxi Formation contain mainly desmocollinite, with secondary telinite and minor telocollinite, while the coals of the Taiyuan Formation have telocollinite . desmocollinite . telinite.

Table 2 Moisture, ash, total sulfur and volatile matter contents and virtrinite reflectance values for profile samples from the Jining coalfield used for petrographic analysis Coal seam sample numbers

Thickness (m)

Moisture (%)

Ash (%db)

Volatile matter (%daf)

Sulfur (total, %db)

Ro (%)

3-1 (top) 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 (bottom) 16-1 (top) 16-2 (bottom)

0.42 0.34 0.37 0.32 0.41 0.38 0.31 0.27 0.29 0.41 0.46

2.8 2.9 2.5 3.3 2.8 2.7 2.7 2.7 1.5 1.8 1.9

13.4 11.4 17.7 20.1 10.8 10.1 9.5 18.4 36.8 4.2 14.9

38.6 38.8 36.9 36.1 37.0 35.9 37.3 39.1 31.4 47.5 44.2

0.72 0.69 0.51 0.49 0.55 0.62 0.62 0.66 0.61 2.44 4.95

0.76 0.75 0.76 0.77 0.73 0.75 0.73 0.70 0.72 0.72 0.74

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Table 3 Ash composition (%) of coal seam 3 in the Shanxi Formation and coal seam 16 in the Taiyuan Formation Samples

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

TiO2

K2 O

Na2O

MnO2

P2O5

3-1 (top) 3-2 3-3 3-4 3-5 3-6 3-7 3-8 16 (top) 16 (bottom)

44.05 46.60 32.13 21.84 26.94 28.40 39.14 45.33 18.49 17.33

35.18 35.32 30.36 20.68 23.25 24.85 34.09 37.50 41.00 51.10

4.52 3.94 3.01 2.53 4.86 5.98 6.17 4.13 19.49 14.88

8.39 7.89 24.31 46.30 34.67 31.71 13.53 8.32 9.38 7.99

0.68 1.11 1.56 0.92 1.31 1.11 1.35 0.87 0.61 0.81

1.60 1.58 1.55 2.95 4.14 3.60 2.92 1.60 6.11 4.65

1.58 1.24 1.00 1.03 0.93 1.24 1.40 1.25 0.80 0.78

0.35 0.56 0.23 0.05 0.23 0.28 0.33 0.36 0.71 0.74

0.59 0.41 0.45 0.40 0.81 0.80 0.77 0.34 0.14 0.17

0.071 0.086 0.142 0.295 0.182 0.237 0.104 0.060 0.064 0.056

0.18 0.19 1.33 1.06 0.75 0.49 0.44 0.54 0.07 0.07

The proportion of liptinite macerals is relatively low. The major liptinite macerals are sporinite, cutinite, and suberinite. The progressive increase in the inertinite content from the lower to the upper coal seams could be attributed to the presence of more reducing conditions of the sedimentary environments for the Taiyuan Formation coals and more oxidizing paleoenvironments for the Shanxi seams. A similar evolution could account for the vertical increase in inertinite abundance from bottom to top of coal seam 3. The stratigraphic trends are very consistent, with evidence of a lower delta plain in the basal Taiyuan Formation, with marine influence, high subsidence, and probably more reducing environmental conditions, and a progressive upwards gradation to a more proximal upper delta plain environment in the Shanxi Formation. 5.3. Mineral matter The major mineral phases in the coals of the study area, identified by XRD and SEM, are quartz, kaolinite, pyrite

and calcite. There are traces of other minerals (dolomite, ankerite, illite, opal, feldspar and marcasite), and weathering products such as gypsum, melanterite and hematite are also observed in several coals using electron and optical microscopy. The top, middle, and bottom benches of seam 16 of the Taiyuan Formation were sampled for this study at the Tangchun mine (Table 6). The major mineral phases in this coal, indicated by chemical analysis are calcite . kaolinite . pyrite . quartz. Calcite is highest (12.3%) in the top of the seam 16; the lowest calcite content (1.5%) is in the middle of the seam. The variations in kaolinite, pyrite and quartz contents are similar to the variation in calcite content, top . bottom . middle. The Permian coal seam 3 has a slightly higher ash yield than the Carboniferous coal seams (Table 1). The Permian seam also has a lower quartz content than the Carboniferous coal seams. The lower proportion of quartz may be attributed to marine influence on the lowest coal seams, since recrystallized quartz bands (centimeter size) are often

Table 4 Maceral and mineral matter contents of profile samples as determined by coal petrology analysis in vol% Samples

31-1 (top) 31-2 31-3 31-4 (bottom) 16 (top) 16 (bottom) 17 (top) 17 (bottom)

Vitrinite

60.62 54.69 48.39 51.50 75.34 71.72 72.48 70.54

Average for Jining coal field 31 52.3 32 53.4 6 55.0 15 75.2 16 74.3 17 74.6

Intertinite

Semi-vitrinite

Semi-intertinite

12.51 12.62 13.53 13.81 1.30 2.05 1.61 5.06

1.03 1.07 2.97 2.05 1.95 2.23 2.06 4.57

10.65 13.78 20.02 14.79 5.48 5.26 8.70 11.97

10.0 6.5 6.4 3.4 4.0 3.2

4.2 7.4 5.6 3.1 2.8 3.7

16.5 12.3 11.9 8.9 7.3 6.2

Liptinite

8.59 7.34 7.79 6.95 6.60 7.84 3.95 4.87 8.8 13.7 15.3 4.5 4.0 2.4

Mineral matter% Pyrite

Clay

Carbonate

0.80 0.41 0.75 1.30 1.70 4.41 2.60 0.71

0.31 0.44 0.73 1.61 2.90 1.80 0.60 0.30

5.50 9.32 5.21 7.70 1.10 2.90 3.90 1.70

0.3 0.1 2.0 1.7 1.7 2.3

6.4 5.6 2.4 0.8 2.4 2.8

1.0 0.6 1.1 1.9 2.0 1.9

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Table 5 Ash yield (%), total sulfur (St, %), pyritic sulfur (Sp, %), sulfide sulfur (Ss, %), organic sulfur (So, %) and selected environmentally significant trace element concentrations (ppm), all presented on a dry whole coal basis, Shanxi coal seam 3 Sample

Ash%

St%

Sp%

Ss%

So%

Cu

Zn

Pb

U

Th

As

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 Seam World meansa China meansb Seam/China Seam/World

19.81 12.30 10.70 13.70 9.65 8.73 11.25 10.05 9.45 12.31 11.72 14.70 24.70 56.67 59.96 20.11 17.20 11.50 12.30 11.72 60.81 19.97

2.31 1.02 0.92 0.88 0.72 0.95 0.86 0.93 1.02 1.32 1.15 1.42 2.87 3.96 4.32 3.26 4.35 0.86 0.91 1.02 8.42 2.07

1.69 0.71 0.64 0.55 0.48 0.65 0.62 0.63 0.74 0.95 0.91 1.03 2.11 2.98 3.41 2.71 3.02 0.51 0.59 0.71 6.96 1.55

0.09 0.07 0.05 0.02 0.01 0.02 0.02 0.04 0.07 0.04 0.05 0.03 0.05 0.10 0.12 0.08 0.06 0.06 0.08 0.07 0.26

0.53 0.24 0.23 0.31 0.23 0.28 0.22 0.26 0.21 0.33 0.19 0.36 0.71 0.88 0.79 0.47 1.27 0.29 0.24 0.24 1.20 0.45

41.76 34.12 30.10 31.62 30.11 29.71 28.96 29.65 36.71 29.76 32.17 40.55 48.97 70.81 68.95 45.21 32.12 31.11 30.13 28.96 50.70 38.20 15 28 1.36 2.54

20.18 18.72 12.10 13.42 11.93 10.16 11.37 10.15 13.21 15.41 16.32 17.56 23.15 28.79 30.13 22.55 18.71 7.32 10.16 9.73 28.10 16.63 50 94 0.18 0.33

25.91 17.34 15.41 15.10 13.20 14.72 11.34 10.47 14.32 13.20 11.70 12.56 20.71 30.56 34.76 17.95 18.40 15.91 14.32 14.96 45.76 18.50 40 24 0.77 0.46

9.31 9.47 8.35 7.42 9.66 8.71 9.32 8.90 8.88 9.10 11.37 10.45 13.78 14.79 14.01 13.25 12.78 9.62 9.73 9.46 15.85 10.68 2 7 1.53 5.34

5.75 6.32 5.61 5.38 6.67 6.72 6.91 5.21 5.72 6.91 6.87 6.69 8.96 8.74 9.01 8.56 7.32 7.56 6.38 6.09 6.10 6.83 4 7 0.98 1.71

3.78 3.24 1.25 1.32 1.05 2.12 1.96 1.38 1.62 2.01 2.17 2.32 3.21 4.76 4.54 3.86 2.97 3.01 2.31 2.05 3.63 2.60 10

a b

Swaine (1990). Wang et al. (1997).

Fig. 4. Vertical variation in proximate and ultimate analysis properties of the workable coal seams from the Tangkou exploration area.

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Table 6 Correlation coefficients ðrÞ among the selected trace elements, ash yield, total sulfur (St) and pyritic sulfur (Sp) contents

Ash St Sp Cu Zn Pb U Th As

Ash

St

Sp

Cu

Zn

Pb

U

Th

As

1.00

0.85 1.00

0.86 1.00 1.00

0.91 0.66 0.66 1.00

0.87 0.81 0.80 0.92 1.00

0.93 0.90 0.92 0.77 0.81 1.00

0.83 0.87 0.85 0.79 0.85 0.76 1.00

0.46 0.36 0.32 0.66 0.58 0.32 0.70 1.00

0.76 0.67 0.65 0.82 0.82 0.74 0.77 0.69 1.00

detected in the marine-influenced coal beds at the top of the Taiyuan Formation (Li and Wei, 1998; Liu et al., 1999; Querol et al., 1999). Kaolinite is uniformly distributed in the coal seams studied. Kaolinite is present in the coal seam in two species with different crystallinity, a low crystallinity detrital kaolinite and a high crystallinity neomorphic kaolinite (Ward, 1989; Ward et al., 1999; Querol et al., 1999). Pyrite is more abundant in the coals of the lower Taiyuan Formation than in those of the Shanxi, reflecting the marine influence on the depostional environment. Pyrite is the major sulfide mineral, but marcasite is also detected. Pyrite in the coals occurs as typical syngenetic framboidal, euhedral and massive cell-filling forms. Carbonate minerals are common, with calcite, dolomite and ankerite as the dominant carbonate species in the coal seams. Because the roof of most of the Taiyuan coal seams is limestone, especially seam 16, some Ca ions are thought to have migrated into the coal seams, where they reacted with other ions in the coals to form calcite. Hence, the calcite content of the Taiyuan coal seams is higher than that of the Shanxi coals. Major weathering and oxidation products identified in the seams are gypsum, melanterite, and hematite, which are present at trace levels in the coals studied. The occurrence and distribution of the different minerals in the Shandong coal suggests the following paragenetic sequence (Querol et al., 1999), based on the diagenetic evolution stages recognized for the mineral matter. The early syngenetic stage is characterized by detrital minerals supplied to the original peatland, as well as framboidal, euhedral and massive cell-filling pyrite. Syngenetic precipitation of ankerite also occurred in this stage of diagenesis, but only in the non-marine coal seams of the upper Shanxi Formation. Syngenetic dissolution of quartz and reprecipitation around quartz grains or as opaline phases replacing the cellular tissue took place in this stage. This gave rise to the silicified microstructures found in the marine-influenced coal beds of the Taiyuan Formation. The middle and late syngenetic stages are characterized by the precipitation of radial and fibrous marcasite

aggregates, subsequently cemented by massive pyrite. This last process accounted for an intensive pyritization of the typical marcasite structures. Relict marcasite structures are commonly found in the late-syngenetic massive pyrite. The sulfide precipitation stage occurred only in the marineinfluenced coals of the Taiyuan Formation, since seawater sulfate is a major source of the sulfur in high-sulfur coals (Chou, 1990, 1999). The epigenetic stage is characterized by the precipitation of carbonates as cleat fillings in all the coal seams studied, and of pyrite in cleats and fractures in the high-sulfur coal seams. 5.4. Sulfur Sulfur contents in the Taiyuan coals are much higher than in the Shanxi coals (Tables 1 and 2; Figs. 5 and 6). The difference could be due to marine influence on the depositional environment of the coal seams, as was pointed out by Chou (1990, 1999), Wu (1990), Wu et al. (1995) and Yuan (1999). Chou (1990, 1999) outlined the origin and formation of pyritic sulfur and organic sulfur in high-sulfur coals. Nearly, all the iron in many coals in the study district is contained in pyrite and siderite. Table 5 shows the ash yield, total sulfur, pyritic sulfur, sulfate sulfur, organic sulfur, and selected environmentally significant trace element (ESTE) concentrations, all presented on a dry whole coal basis, in a channel sample from the Shanxi coal seam 3 from the Xinglongzhuang mine. Within the section of the Shanxi coal seam 3 analyzed in detail (Table 5), the total sulfur content of the coal mine samples varied from 0.72 wt% (M5) to 4.35 wt% (M17). Dirt bands were excluded from the average calculation. Therefore, the average total sulfur content of the coal itself is 1.49 wt%, excluding dirt bands M14, M15 and M21. nThere is an increase in the total sulfur content for this seam section in both of the basal plies and the upper ply. However, in the middle of the seam, the total sulfur contents of samples M13 –M17 are unusually high because they are associated with banded pyrite lamellae. Pyritic sulfur is the predominant form sulfur of the in the coal samples. SEM and XRD examination, however, also shows the presence of other sulfur bearing minerals

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Fig. 5. Variation in sulfur and selected trace elements concentrations in section of seams studied from the Xinglongzhuang coal mine. So: organic sulfur, Ss: sulfide sulfur, Sp: pyritic sulfur, St: total sulfur.

including marcasite, sphalerite, galena and chalcopyrite. Pyrite occurs in several forms, most commonly as nodules and partings (e.g. M15 and M17). Disseminated pyrite, ranging in size from a few millimeters to less than one micrometer, is also present, as well as platy pyrite in cleats,

pyrite veins, and pyrite in permineralized peat or coal balls (Liu et al., 1999; Swaine, 1994; Chou, 1990, 1999). The pyritic sulfur contents of the plies in the Xinglongzhuang coal mine seam section varied from 0.48 to 6.96 wt%, with an average of 1.55 wt% (Table 5). With the exception

G. Liu et al. / Journal of Asian Earth Sciences 23 (2004) 491–506 501

Fig. 6. Variation in ash, total sulfur, pyritic sulfur and selected trace element concentrations in sections of the seams studied from Xinglongzhuang coal mine.

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of samples M14, M15 and M21 (coaly claystone), the pyritic sulfur content of the seam varies from 0.48 to 3.02 wt%, with an average of 1.07 wt%. Pyritic sulfur is correlated closely with total sulfur with respect to its variation. There are several distinct forms of pyrite in M21. An early, syngenetic phase is developed as framboids and isolated euhedral crystals. The second phase of pyrite occurs as an anastomosing array of veins, sub-parallel to the coal banding, which cross-cut the framboidal clusters. These veins are themselves cut by cleat-fill pyrite of a third phase. From Table 5, it can be seen that the coal with a marine roof in the underground coal mine presents a more complex situation. Not only are the total sulfur and pyritic sulfur concentrated in the lower and upper plies of the seam, but also plies with higher sulfur content occur in the middle of the seam (M13 – M17). The high levels of total sulfur in the basal and top plies could be explained by the development of peat mire in a transgressive marine setting and inundation of the peat surface by sulfate-rich sea water. The concomitant high pyritic sulfur levels would indicate the availability of ferrous iron in peat. According to previous data from this district (Liu et al., 1999), pyrite becomes more abundant in the lower coal seams with intensifying marine influence of the peat depositional environment (Gayer et al., 1999; Helbel, 1994). Pyrite is present at very low concentrations in the Lower Permian coal. However, pyrite concentrations in the Carboniferous coal seams are much higher. The major sulfide mineral is pyrite, but marcasite is also detected. Sulfide aggregates occur as typical syngenetic framboidal, euhedral and massive cell-filling pyrite in all coals in the study district. Radial and fibrous marcasites with external pyrite rings can also be identified in the high-sulfur marine-influenced coal seams of the Carboniferous Taiyuan Formation. Petrographic analysis indicates the presence of typical fibrous morphologies, which were then transformed into pyrite aggregates. Consequently, this implies a late syngenetic transformation stage to account for the conversion of marcasite to pyrite. Finally, typical epigenetic sulfides were also formed in the coal as cleat fillings. In this study, the organic sulfur contents are given in Table 5, which shows that organic sulfur ranges from 0.19 to 1.27 wt%, with a average of 0.45 wt%. The organic sulfur levels show the correlation with the total sulfur. The organic sulfur of the study district has been reported and studied by Liu et al. (1999), Steenari et al. (1997) and Chen et al. (1993). Macerals were separated from the coal seam by means of isopycnic density gradient centrifugation. Direct measurement of the organic sulfur content and its distribution in the coal macerals was made by transmission electron microscope (Chen et al., 1993). According to the data obtained, the highest concentration of organic sulfur in the macerals of the coal seam is 1.27 wt%. The range of organic sulfur contents in liptinite is much wider than that of other macerals. The liptinite maceral group is composed

of diverse individual macerals including sporinite, cutinite, resinite, backinite, bituminite and alginite, etc. Those individual macerals have various proportions of organic sulfur (Ward and Gurba, 1998, 1999). Inertinite in the seam has the lowest organic sulfur concentration. The organic sulfur concentration of vitrinite lies between the liptinite and the inertinite. Organic sulfur in whole-coal samples is calculated to be the difference between total sulfur and the sum of pyritic plus sulfate sulfur. Pyritic sulfur is determined on the basis of the amount of iron extracted by nitric acid and a significant error can be caused by extraction of non-pyritic iron or incomplete dissolution of pyrite. As a result, the errors made in measurement of total sulfur, pyritic sulfur and sulfate sulfur are reflected in the organic sulfur values. In order to avoid these errors, researchers have tried to determine organic sulfur directly (Ward and Gurba, 1998, 1999). The coals contain minor amounts of sulfate sulfur. Sulfate minerals such as gypsum and barite are found in some study samples. The following iron sulfate minerals may be weathering products of pyrite: szomolnokite, rosenite, melanterite and coquimbite. The sodium sulfate minerals, mirabilite and thenardite, found in coal refuse, are reaction products of iron sulfate with sodium associated with coal. We observed that the content of sulfate sulfur in the samples is less than 0.1 wt%, with the exception of samples M14, M15 and M21. With an average content of 0.07 wt%, sulfate sulfur is much lower than the other sulfur forms. 5.5. Trace elements Trace elements are present in coals at different concentrations, depending on the various processes by which they have entered the coal at the different stages of coalification. Most elements are associated with the mineral matter in coal; however, certain elements may have an organic affinity (Finkelman, 1981, 1989, 1993, 1994; Swaine, 1990). Element associations with coal ash have been widely reported, with the information obtained from the degree of organic/inorganic association. The elements associated with mineral matter are variably affected by combustion, but are mostly concentrated in ash. (Spears and Zheng,1999; Yan et al., 1999; Finkelman, 1994; Nathan et al., 1999; Liu and Yang, 2000; Lu, 1995; Lu and Ali, 1996; Sun, 1986; Swaine, 1994; Xu, 1997, Yang et al., 1983). From Figs. 5 and 6, it can be seen that the trace elements Cu, Pb, Zn and As show similar variations from floor to roof of the coal seam in the Yanzhou coalfield. Liu and Wang, 1999 also noted high Pearson correlation coefficients for these elements when present in coals. The trace elements also delineate the boundaries between coal and dirt bands by responding to changes in mineral matter content (Gentzis and Goodarzi, 1997). They have higher concentrations at the top and bottom of the seam and in the sedimentary

G. Liu et al. / Journal of Asian Earth Sciences 23 (2004) 491–506

parting (Fig. 5), which suggests an affinity with the mineral matter in the coal. However, Th behaves differently by showing a lower concentration in the top and bottom of the seam. It has a higher concentration in the upper plies (samples M13 –M17) of the interburden parting, in which some of the other elements (Cu, Pb, Zn and U) also have relatively high concentrations. Previous work in the Yanzhou mine area (Liu et al., 1999), shows that most of the trace elements (e.g. Cl, F, Ge, Ga and U) have higher concentrations in the top and bottom of the coal seams, although the concentrations of trace elements in the bottom is generally higher than in the top of the coals. Analyses of selected ESTEs for the coal seam are presented on a whole coal basis in Table 5. Table 5 also gives the ratios of the mean concentration of the elements in the Yanzhou coalfield compared with the average concentration in world and Chinese coals, as listed in Swaine (1990) and Wang et al. (1997). This ratio gives an indication of any unusually high or low concentrations in the study area. In comparison to world coals, the seam has a slightly higher concentration of Th, U and Cu, whereas in comparison to Chinese coals, the seam has a slightly higher concentration of U and Cu. Compared with average concentrations in world coals, Zn, As and Pb are depleted in the coal mine by factors of between 0.1 and 0.83. However, Th, U and Cu are enriched by factors of 1.71, 5.34 and 5.46, respectively. Compared with the average concentration in Chinese coals, Zn, Th and Pb are reduced by factors of between 0.18 and 0.98, but Cu and U are enriched by factors of 1.36 and 1.53, respectively. As mentioned above, concentrations of Cu and U in this coalfield are higher than the global and Chinese average. In general, the ESTE occur in two different relationships. Some of the ESTE concentrations correlate with the total sulfur concentration, while the others appear to correlate with the variation in ash content. These correlation are most clearly seen in plots of variations in ash, sulfur and ESTE concentrations vertically through the coal seam from the underground coal mine in study district (Figs. 5 –7). In order to clarify the relationships among ESTE, ash and sulfur contents, the correlation coefficients are listed in Table 7. Table 5, Figs. 5 and 6 show the variation in sulfur content and ash yield throughout the bulk of the seam. With the exception of ply M17, the correlation between ash and sulfur content is very strong. The exception is ply M17, in which ash yield correlates with high total and pyritic sulfur. In the top and bottom of the seam, the relatively higher ash yield is matched by high sulfur content. The majority of the selected the ESTEs (e.g. As, Pb, Zn and Cu) correlate strongly with the sulfur content. The correlation coefficients of Cu, Zn, U and As to total sulfur and to pyritic sulfur range from 0.65 to 1.00. However, the correlation coefficients between Th and total sulfur and

503

Fig. 7. Mean concentrations of selected trace elements in low ash, medium ash and high ash coal.

pyritic sulfur are 0.36 and 0.32, respectively, showing that Th is not associated with sulfur. These data indicate that As, Cu, Pb and Zn are probably concentrated in the pyrite in the coal samples, and hence are associated with sulfur (Liu et al., 2000; Swaine, 1994; Helble, 1994; Steenari et al., 1997). For example, the toxic trace element As, may be associated with sulfur in the forms of FeAsS, AsS and As2S3 (Rong and Wang, 1990; Yuan, 1999). Other studies have showed that most of As in coal is associated with pyrite (Finkelman, 1994; Ward et al., 1999; Karayigit et al., 2000). The relatively strong correlation between sulfur and most of the ESTE analyzed suggests that the origin of most the selected ESTE in the analyzed coal is linked with the origin of the sulfur. However, a lower correlation coefficient in some cases (e.g. Th, r , 0:4) may imply that some ESTE have more than one mode of occurrence in the coal. Most trace elements in coal are associated with the mineral portion of the coal. As a result, they are considered as having an inorganic association (Liu and Yang, 1999b; Gentzis and Goddarzi,1997; Vassilev et al., 1997). A number of the trace elements in the seam show a positive correlation with ash yield (Table 5 and Fig. 7). Pb, Cu, As, U and Zn Table 7 Selected trace element concentrations in low ash, medium ash and high ash coals in the seam Low ash (n ¼ 9) Range (ppm) Cu Pb Zn U Th As

Medium ash (n ¼ 7) Mean Range (ppm) (ppm)

28.96–36.71 30.83 7.32–13.21 11.37 10.47–15.91 13.56 8.35–9.66 9.36 5.21–7.56 6.37 1.05–3.01 1.89

High ash (n ¼ 5) Mean Range (ppm) (ppm)

27.76–41.76 34.29 10.15–20.88 16.31 12.56–25.91 16.12 7.42–12.78 9.76 5.38–7.32 6.39 1.32–3.78 2.56

45.21– 70.81 22.55– 30.13 17.95– 45.76 13.25– 15.85 6.10– 9.01 3.21– 4.76

Mean (ppm) 56.93 26.54 29.95 14.34 8.27 4.00

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Table 8 Mineral matter contents in coal seam 16 from Tangchun mine (wt%) Samples

Thickness (m)

Calcite

Kaolinite

Pyrite

Quartz

16 (top) 16 (middle) 16 (bottom)

0.30 0.28 0.31

12.3 1.5 4.2

9.1 1.0 3.1

4.7 2.1 4.0

1.2 0.4 1.0

6.0

4.4

3.6

0.9

Average

appear to be not only correlated with ash yield, but with sulfur content also. The trace element concentrations in the coal samples have been grouped in terms of the coal ash yield (Table 8). Low ash, medium ash and high ash coals are defined as having less than 12%, 12 –20% and more than 20% total ash, respectively. Fig. 7 indicates the concentrations of selected trace elements in the coal samples. It can clearly be observed that the concentration of the elements is highest in the high-ash coal plies. The concentration order of the trace elements is generally high ash coal . medium ash coal . low ash coal. However, there is not much significant difference between the As concentration in the low ash and the medium ash coals. Table 8 shows that the mean concentration values of selected ESTEs are enriched in the high ash coal samples. They also correlate positively with dry ash yield, demonstrating an association with the inorganic constituents in the coal seams. Similar significant positive correlations with ash were recorded for As, Cd, Cr, Ni, Pb, Sb and U in the Al lignite bed, Turkey (Karayigit and Kholodkov, 2000). Table 7 indicates that with the exception of Th ðr , 0:40Þ; all elements have correlation coefficients between ash and selected trace elements more than 0.70. Table 7 also shows strong correlations between Cu, Zn, Pb and As. For example, r ¼ 0:82 between As and Pb, indicating that As is related to the occurrence of Pb. Th has the weakest correlation with the other elements in this table. Perhaps, the most significant samples are the middle plies, M13 – M17 of the seam studied. Not only do these have the highest sulfur levels, but they also have the highest Cu, Zn and As concentrations. More than 70 percent of the sulfur is present as pyritic sulfur, which is consistent with the majority of the high ash yield originating as pyrite.

6. Conclusions 1. The Taiyuan Formation is made up of sandstone, mudstone, limestone and coal, which are related to a stratigraphic trend characterized by the evolution of relatively more reducing conditions. The unit was formed in a marine-influenced lower delta plain environment. The Shanxi Formation was formed in an oxidizing paleoenvironment, dominated by an upper delta plain setting.

2. The coal of the study area is of high volatile bituminous rank. Petrographic studies indicate a downward decrease in vitrinite reflectance values from the Shanxi coals to the Taiyuan coals, and an inverse trend for volatile matter content, which is higher in the Taiyuan coal than in the Shanxi coal seams. This reflects a greater aboundance of vitrinite and a suppression of vitrinite reflectance associated with the marine influence. 3. The Shanxi coal seams have slightly higher ash yield than the Taiyuan coal seams. The ash consists mainly of SiO2, Al2O3, Fe2O3, CaO and SO3.The minerals in the coal seams are chiefly quartz, kaolinite, pyrite and calcite. 4. The sulfur content of the Taiyuan coal seams is considerably higher than that of the Shanxi coals. In the Shanxi coal seam 3, the total sulfur and pyritic sulfur contents are both relatively high in the basal and top plies and in the upper parting ply of the coal seam. Organic sulfur content is positively correlated to total and pyritic sulfur. 5. Cu, Pb, Zn and As are more highly concentrated in the top and bottom of the seam. There are general similarities in the vertical variations of Cu, Pb, Zn and As. However, in the upper dirt band ply of the seam, all of the selected trace elements have an even higher concentration. 6. The concentrations of Th, Cu and U in the seam studied are between 1.71 and 5.34 times greater than the world mean of these elements in coals. Compared with the average concentration in Chinese coals, Cu and U are enriched by factors of 1.36 and 1.53, respectively. 7. The origin of the sulfur enrichment in the basal and top plies, and in the dirt band of the seam, is thought to be from sulfate ions carried into the palaeoenvironments during the peat forming process. 8. The trace elements investigated in the study are enriched in the high ash samples. Cu, Pb, Zn, and As appear to be correlated with ash content and sulfur content, the trace elements being more closely correlated with ash yield than with sulfur content.

Acknowledgements This work was supported by National Key Natural Science Found of China (40133010). We wish to thank Zhongxin Hu for assistance during the sampling. We thank Professor C.R. Ward, Dr R.B. Finkeman, Ms. Leslie Ruppert, Dr Harvey E. Belkin and Editors for reviewing the manuscript and giving us many constructive comments.

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