Article pubs.acs.org/EF
Behavior of Minerals and Trace Elements during Natural Coking: A Case Study of an Intruded Bituminous Coal in the Shuoli Mine, Anhui Province, China Xibo Wang,*,†,‡ Yaofa Jiang,§ Guoqing Zhou,§ Peipei Wang,‡ Ruixue Wang,‡ Lei Zhao,† and Chen-Lin Chou∥ †
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China § Jiangsu Institute of Architectural Technology, Xuzhou 221116, China ∥ Illinois State Geological Survey (Emeritus), 615 East Peabody Drive, Champaign, Illinois 61820, United States ‡
ABSTRACT: The effects of thermal alteration by an igneous intrusion on the organic matter and inorganic constituents of a coal seam in the Shuoli mine, Anhui Province, China, have been investigated using reflected light microscopy, X-ray diffraction analysis (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma−mass spectrometry (ICP-MS), an electron microprobe, and a scanning electron microscopy system equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). A total of 11 coal benches were collected from the profile (numbered as SI5-1 to SI5-11), all of which were found to be metamorphosed from a distance of 80 cm below the sill to the sill/coal contact; benches Sl5-8 to Sl5-11, which are in direct contact with the sill, were completely converted to natural coke. The maximum paleo-temperature inferred from Ro,max of the natural coke is estimated to be 1000 °C. The minerals formed by the molten magma invasion are dominated by veins of mixedlayer illite/smectite (I/S). However, mixed-layer I/S of terrigenous origin is also present as lenses or thin layers. The mixed-layer I/S shows an increasing degree of illitization upward from the bottom. In addition to abundant mixed-layer I/S, the molten magma invasion also resulted in the precipitation of nontronite, albite, quartz, pyrite, and anatase, which mainly occur as fracture or pore fillings. Compared to the ash of the unaltered coal, the SiO2/Al2O3 ratio (average of 1.4), and the percentages of K2O (average of 4.3%), Na2O (average of 0.8%), and Fe2O3 (average of 3.2%) are higher in the ashes of altered coal and natural coke. The K2O and Na2O contents increase from the bottom to the top of the seam. Trace elements, including Be, F, Zn, As, Rb, Sr, Cs, Hg, Tl, Bi, Th, and U, exhibit a marked enrichment in the natural coke. Among these elements, the enrichment of Be, F, Rb, Sr, Cs, Th, and U is associated with the formation of molten magma-related minerals, such as mixed-layer I/S. High concentrations of Zn, As, Hg, Tl, and Bi in the natural coke are attributed to pyrite, which was related to the veins of mixed-layer I/S. The concentration of REY (rare-earth elements and yttrium) in the thermally altered coals is observed to be in the range of 267−980 μg/g (ash basis). The sill and all the coal samples are mainly characterized by a LREY-rich type (normalized to Clchondrite). The sill shows a typical europium anomaly with an Eu/Eu* value of 0.26. However, Eu/Eu* in the profile distinctly changes from weak negative to strong negative from the bottom of the seam toward the sill. The systematic fractionation change in Eu/Eu* in the profile is primarily attributed to changes in the nature of the intrusive molten magma.
1. INTRODUCTION There are many reports in the literature on thermal alteration of coal caused by igneous intrusions.1−6 Intrusion of igneous rock elevates the degree of metamorphism of the coal near the contact and results in significant physical and chemical changes in the organic matter, including increased vitrinite maximum reflectance, elevated carbon content, decreased volatile yield, nitrogen and hydrogen contents,2,7 reduced hydrogen isotopic exchange ability, and lower δ13C.8 Mineralogical compositions of coal may also be affected by igneous intrusions. Minerals in coal can be altered or destroyed because of the heat, associated fluids, and gas derived from the igneous intrusion.4,5,7 Ward et al. reported crystallographic and chemical changes in montmorillonite and kaolinite in a Permian coal of the Sydney Basin, Australia.9 In addition, new minerals can precipitate from circulating hydrothermal fluids associated with the igneous magma. As well as these influences on the organic matter and minerals, enrichment or depletion of some © 2015 American Chemical Society
trace elements may also be attributed to the igneous intrusion.1,3,9−11 Chen et al. provided a useful review of the geochemical modification of trace elements in thermally altered coal.5 Although most of these papers have discussed the effect of heat on the organic matter or inorganic constituents, little attention has been given to the effects of the intrusion composition on the development of newly formed minerals in the coal. This paper describes the effect of a felsic igneous intrusion on a low volatile bituminous coal of Permian age in the Shuoli mine, Anhui Province, China, and sheds lights on the origins of newly formed minerals and elevated abundances of some trace elements in the coal. Received: December 5, 2014 Revised: May 28, 2015 Published: May 29, 2015 4100
DOI: 10.1021/acs.energyfuels.5b00634 Energy Fuels 2015, 29, 4100−4113
Article
Energy & Fuels
Figure 1. (a) Location of the Suixiao coalfield in Anhui Province, China. (b) The Dingli stock and its distribution around the coal mines. (c) Section sampled in a tunnel face of the Shuoli mine (legend: O2, Middle Ordovician; C2, Pennsylvanian; P1, Early Permian, P2, Middle Permian, Q, Quaternary).
2. GEOLOGICAL SETTING The Shuoli mine is located in the Suixiao coalfield, Anhui Province, China (Figure 1a). The Suixiao coalfield includes the Daiweizi, Duji, Shuoli, Yuanzhuang, and Yumen mines (Figure 1b). The coal-bearing sequences are the Shanxi and Xiashihezi Formations of Permian age from the bottom up. The Xiashihezi Formation can be distinguished from the Shanxi Formation by a laterally extensive bauxite layer. The Xiashihezi Formation is composed of medium-grained sandstone, mudstone, carbonaceous mudstone, and coal beds (Nos. 3, 4, and 5 coals). The Nos. 3 and 5 coals are workable, with average thicknesses of 2.65 and 2.08 m, respectively. The Shanxi Formation mainly consists of fine sandstone, mudstone, and No. 6 coal, with an average thickness of 0.9 m. The Suixiao coalfield is characterized by Cretaceous igneous intrusions. The largest igneous body is the Dingli quartz porphyry stock (age 115.6−122.3 Ma),12 with an outcrop area of 18 km2 between the Shuoli and Yuanzhuang mines (Figure 1b). Because of the strong thermal influence of the Dingli stock, coal metamorphism in the Suixiao coalfield gradually decreases from the stock toward the surrounding area in a radial ring shape, e.g., from natural coke (anthracite) to bituminous coal.12,13 Dikes and sills are common in the coal mines, composed of quartz porphyry, diabase, or diorite porphyry (Figure 1b), and the sills usually occur as an interlayer between the roof and the coal seam. At the tunnel excavation face in the Shuoli mine, a felsic intrusion partially engulfs the No. 5 coal. The felsic intrusion is
white to white-gray in appearance and porphyritic, with the phenocrysts being mainly composed of quartz and plagioclase. Quartz occurs as rounded grains and plagioclase shows clintheriform and polysynthetic twins under cross-polarization. The groundmass is composed of quartz, K-feldspar, and biotite.
3. SAMPLING AND METHODS The No. 5 coal, which has an average thickness of 2.08 m, is the main workable seam in the Shuoli mine. However, at the tunnel excavation face, the coal seam is partially replaced by the sill, so that the coal is only ∼1.1 m thick. In this study, 11 bench samples of coal (Sl5-1−11), one floor sample (Sl5-F), and a quartz porphyry sample (SlQP) were collected from the vertical profile of the tunnel excavation face (Figure 1c). Each coal bench covers a thickness of 10 cm, a width of 10 cm, and a depth of 15 cm. Samples Sl5-11 to Sl5-8 (natural coke) are in direct contact with the sill and samples Sl5-7 to Sl5-1 (altered coal) are sequential benches under the sill (Figure 1c). In addition, an unaltered coal sample Sl5-N, which covers a thickness of 20 cm in the middle of the No. 5 coal, was collected for comparison ∼100 m away from any igneous influence. Proximate analysis, including the determination of moisture, volatile matter, and ash yield, was performed in accordance with ASTM standards D3173-03,14 D3175-02,15 and D3174-04,16 respectively. The total sulfur was determined following ASTM standard D3177-02.17 4101
DOI: 10.1021/acs.energyfuels.5b00634 Energy Fuels 2015, 29, 4100−4113
Article
Energy & Fuels Table 1. Proximate and Ultimate Analysis, Total Sulfur, and Vitrinite Maximum Reflectance of the Shuoli Coals Proximate and Ultimate Analysis, Total Sulfur
sample Sl5-11 Sl5-10 Sl5-9 Sl5-8
type natural coke natural coke natural coke natural coke
Sl5-7 Sl5-6 Sl5-5 Sl5-4 Sl5-3 Sl5-2 Sl5-1
altered altered altered altered altered altered altered
Sl5-N
unaltered coal
a
coal coal coal coal coal coal coal
maximum reflectance of vitrinite and its altered equivalents, Ro,max (%)
moisture, Mad
ash yield, Ada
volatile matter, Vdaf
total sulfur, St,da
carbon, Cdafb
nitrogen, Ndafb
hydrogen, Hdafb
Tc (°C)
11.15
3.45
25.38
4.27
0.22
97.03
1.21
2.45
1000
9.79
2.27
22.38
4.14
0.22
95.88
1.24
1.99
1000
9.83
3.79
23.49
5.29
0.26
95.83
1.39
1.49
1000
10.35
3.18
26.8
6.06
0.39
94.85
1.56
1.43
1000
8.32 6.83 6.78 5.17 4.77 2.79 2.45
2.35 2.5 1.96 1.83 1.54 1.81 1.88
30.8 26.61 27.25 25.63 24.43 25.04 28.64
9 8.29 9.03 9.37 9.87 8.87 11.57
0.41 0.31 0.37 0.38 0.39 0.26 0.61
93.77 93.12 93.02 92.36 95.57 92.95 91.14
1.76 1.66 1.65 1.62 1.70 1.69 1.59
2.40 2.16 2.50 2.93 2.98 2.90 3.54
750 650 640 565 540 435 405
2.1
1.03
11.55
19.55
0.41
90.34
1.77
4.07
200
Dry basis. bDry and ash-free basis. cCarbonization temperature estimated from Ro,max, according to Chandra’s method.33
K, Ca, Na, Mg, Fe, P, and Ti were determined by X-ray fluorescence (XRF) spectrometry (Thermofisher ARL Advant’XP+). An inductively coupled plasma−mass spectrometry (X series II ICP-MS) system, in pulse counting mode (three points per peak), was used to determine trace elements in the coal samples, except for Hg and F. The ICP-MS analysis and sample microwave digestion program, as used coal and coalrelated materials, is outlined by Dai et al.24 Arsenic and Se were determined by ICP-MS using collision cell technology (CCT) in order to avoid disturbing the polyatomic ions.25 For ICP-MS analysis, samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone). Multielement standards (Inorganic Ventures (CCS-1, CCS-4, CCS-5, and CCS-6), NIST Reference Standard 2685b, and Chinese standard reference GBW 07114) were used for the calibration of trace element concentrations. Mercury was determined using a Milestone DMA-80 analyzer. Fluorine was determined by pyrohydrolysis, in conjunction with an ion-selective electrode, following ASTM method D5987-96.26
Reflectance of vitrinite and its altered equivalents was measured on a Leica Model DM4500P microscope (at a magnification of 500×) that was equipped with a Craic Model QDI 302TM spectrophotometer. Mineral phases were identified in coal briquettes via optical microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS) (FEI, Model Quanta 650 FEG, where the working distance was 10 m, the beam voltage was 20.0 kV, the aperture was 6, and the spot size was 4.5−5.0), and electron microprobe analysis (JEOL, Model JXA-8100). The results were further confirmed by low-temperature ashing plus X-ray diffraction (LTA+XRD). Low-temperature ashing of coal was performed using an EMITECH K1050X Plasma Asher. The low-temperature ashes were ground using a agate mortar and pestle and were suspended in distilled water. The suspended particles (
