Article pubs.acs.org/EF
Effect of Natural Weathering Processes on Size and Density Compositions of Bituminous Coal Wencheng Xia,*,† Guangyuan Xie,† Chuancheng Ren,‡ Zelin Zhang,† Chuan Liang,§ and Xiaodong Ge∥ †
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ‡ Dezhou University, Dezhou, Shandong 250323, People’s Republic of China § Shandong Energy Linyi Mining Group Company, Limited, Linyi, Shandong 276017, People’s Republic of China ∥ Dadi Engineering Development (Group) Company, Limited, Tianjing 300381, People’s Republic of China ABSTRACT: It is well-known that natural weathering processes can change the surface properties of coal. The carbon content on the coal surface can be reduced, while the oxygen content can be increased, by natural weathering processes. However, there is little direct and deep research into the effect of natural weathering processes on the size and density compositions of coal. In this investigation, the changes in the size and density compositions of bituminous coal before and after natural weathering processes were discussed. X-ray photoelectron spectroscopy and scanning electron microscopy measurements were used to explain these changes in micro aspects. Throughout this paper, both the size and density compositions of bituminous coal could be affected by natural weathering processes. Natural weathering processes not only reduce the coal particle size but also increase the coal particle density. the fine coal particle is difficult to separate using the gravity separation technology. Meanwhile, the coal density difference determines the gravity separation efficiency.14−16 In this investigation, the changes in the size and density compositions of bituminous coal before and after natural weathering processes were discussed. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements were also used to explain these changes in micro aspects. Throughout this paper, some interesting findings may be obtained.
1. INTRODUCTION Coal weathering/oxidation processes can change the surface properties of coal. It is well-known that the carbon content on the coal surface can be reduced, while the oxygen content can be increased, by natural weathering processes.1−4 The surface O/C atomic ratio has been found to be in good correlation with the mass loss of coal samples after weathering/oxidation processes.5,6 The surface roughness of the coal surface could also be increased by natural weathering processes, and some scraps would be produced on the bituminous coal surface after weathering/oxidation processes.2 In most cases, natural weathering/oxidation processes make C−C and C−H groups oxidize to C−O groups. Then, the C− O group is oxidized to the CO group, and the CO group is oxidized to the OC−O group. At last, the OC−O group can be oxidized to a further extent, such as some gas components (CO2 and CO) being released.6−10 During the coal oxidation/weathering processes, some organic materials are oxidized and a part of them can release as oxidation products, such as gas components (CO and CO2) and water. Meanwhile, the content of oxygen-containing functional groups on the coal surface can be increased, while the content of hydrophobic functional groups is decreased, after the coal oxidation/weathering processes. As a result, the coal oxidation/ weathering processes usually reduce the natural hydrophobicity of the coal surface.11−13 The research into the changes in surface properties of coal before and after oxidation/weathering processes has been widely investigated. However, there is little direct and deep research into the effect of natural weathering processes on the size and density compositions of coal. The size and density compositions of coal should be also affected by natural weathering processes. The size and density compositions of coal play a very important role in the coal preparation. Usually, © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials and Experiment Design. Bituminous coal samples were obtained from the Shanxi province of China. Coal samples were dry-ground in a laboratory mill to pass a 0.5 mm sieve. Natural weathering processes of bituminous coal were conducted on the roof. Bituminous coal underwent the breakdowns from the sun, wind, and water. The oxidation processes occurred under a natural environment. The weathering times were 3 and 6 months. The coal samples in this paper were three types: coal 1 (fresh coal), coal 2 (coal was oxidized for 3 months), and coal 3 (coal was oxidized for 6 months). The proximate analyses of three coal samples are shown in Table 1, where Mad is the moisture content, Vad is the volatile matter content, FCad is the fixed carbon content, and Aad is the ash content, all on an air-dry basis. Table 1 shows that the moisture and ash contents were increased after natural weathering processes, while the volatile matter and fixed carbon contents were reduced. It indicates that natural weathering processes should change both the chemical and physical properties of bituminous coal.17 2.2. Size and Density Composition Measurements. The three coal samples were screened into five size fractions using an Received: April 20, 2014 Revised: June 17, 2014 Published: June 18, 2014 4496
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Table 1. Proximate Analysis of Fresh and Oxidized Coals (Air Dried, wt %) coal type
Mad
Vad
FCad
Aad
coal 1 coal 2 coal 3
2.91 3.60 5.14
16.54 16.27 15.79
58.32 56.55 52.03
22.23 23.58 27.04
experimental vibrating sieving machine. These five size fractions were 0.5−0.25, 0.25−0.125, 0.125−0.074, 0.074−0.045, and −0.045 mm. On the basis of the gravity concentration (using organic heavy liquids obtained by the different ratios of benzol and bromoform) in an experimental centrifuge (a GL-21 M high-speed centrifuge with a centrifuge speed of 3000 revolutions/min), the coal samples were classified into seven density fractions: −1.2, 1.2−1.3, 1.3−1.4, 1.4−1.5, 1.5−1.6, 1.6−1.8, and +1.8 g/cm3. The ash contents of each density and size fraction could be obtained by a series of electric muffle furnace burning tests. 2.3. XPS and SEM Measurements. The XPS experiments were carried out at room temperature in an ultra-high vacuum (UHV) system with the surface analysis system (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA). The base pressure of the analysis chamber during the measurements was lower than 1.0 × 10−9 mbar. Al Ka radiation (hν = 1486.6 eV) from a monochromatized X-ray source was used for XPS. For all analyses, the takeoff angle of the photoelectrons was 90° and the spot size was 900 μm. The spectra of the survey scan were recorded with the pass energy of 100 eV; the energy step size was 1.00 eV. High-resolution spectra were recorded with the pass energy of 20 eV, and the energy step size was 0.05 eV. The data processing (peak fitting) was performed with XPS Peakfit software, using a Smart type background subtraction and Gaussian/ Lorentzian peak shapes. The binding energies were corrected by setting the C 1s hydrocarbon (−CH2−CH2− bonds) peak at 284.6 eV. The FEI Quanta 250 SEM was used to analyze the surface morphology of fresh and oxidized coals. The magnification times were fixed at 4000 and 10 000. The coal samples were prepared by surface cleaning using absolute ethyl alcohol. After surface cleaning, the coal samples were dried in air. Before SEM tests, the coal samples were sputter-coated with a layer of gold.
Figure 1. XPS wide energy spectrum of bituminous coals surface before and after natural weathering processes.
3. RESULTS AND DISCUSSION 3.1. XPS and SEM Analyses. The C 1s peak is near 285 eV; the O 1s peak is near 533 eV; the Si 2p peak is near 103 eV; and the Al 2p peak is near 75 eV.18,19 Figure 1 is an XPS wide energy spectra of the bituminous coal surface before and after natural weathering processes. Figure 1a is the XPS wide energy spectrum of coal 1 (fresh coal). Figure 1b is the XPS wide energy spectrum of coal2 (coal oxidized after 3 months). Figure 1c is the XPS wide energy spectrum of coal 3 (coal oxidized after 6 months). Figure 1 indicates that the strength of C 1s is reduced from coal 1 to coal 3, while the strength of O 1s is increased. Therefore, the content of atomic carbon should be decreased, while the content of atomic oxygen is increased, from coal 1 to coal 3. Furthermore, the atomic carbon, atomic oxygen, atomic aluminum, and atomic silicium contents are calculated on the basis of Figure 1 using XPS analysis software. Table 2 is the contents of atomic carbon, atomic oxygen, atomic aluminum, and atomic silicium. The content of atomic carbon decreases from 63.86 to 37.65%, and the content of atomic oxygen increases from 24.30 to 42.81%, from coal 1 to coal 3. Meanwhile, the content of atomic silicium increases from 6.66 to 11.05%, and the content of atomic aluminum increases from 5.18 to 8.49%, from coal 1 to coal 3. It indicates that the content of organic materials (carbon) decreases but the content of inorganic materials (aluminum and silicium) increases after natural weathering processes. It seems that a
Table 2. Contents of C 1s, O 1s, Si 2p, and Al 2p on the Bituminous Coal Surface before and after Natural Weathering Processes coal type
C 1s (%)
O 1s (%)
Si 2p (%)
Al 2p (%)
coal 1 coal 2 coal 3
63.86 50.99 37.65
24.30 33.44 42.81
6.66 8.76 11.05
5.18 6.81 8.49
part of organic materials may shed from the coal surface after natural weathering processes. In other words, a part of organic materials may be oxidized to gas fractions (CO and CO2) and H2O.2,4,10 Figure 2 is C 1s peaks for the bituminous coal surface before and after natural weathering processes. For C 1s peaks, peaks at binding energies of 284.6, 285.6, 286.6, and 289.1 eV correspond to the following groups: C−C or C−H, C−O, CO, and OC−O.18−20 The contents of these groups can be calculated in Table 3. The content of C−C and C−H groups is decreased from 55.31 to 8.79%, while the content of the OC−O group is increased from 0 to 8.77%, from coal 1 to coal 3. Meanwhile, the content of C−O increases from 38.74 to 50.99%, and the content of CO increases from 5.95 to 31.45%, from coal 1 to coal 3. C− C and C−H groups can be oxidized to C−O groups. Then, the C−O group is oxidized to the CO group, and the CO 4497
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Figure 3. SEM results for the bituminous coal surface before and after natural weathering processes.
Figure 2. C 1s peaks for the bituminous coal surface before and after natural weathering processes.
Table 4. Size Compositions of Coal 1, Coal 2, and Coal 3 Table 3. Fraction of C on the Bituminous Coal Surface before and after Natural Weathering Processes (Relative Percentage of C 1s) coal type
C−C and C−H (%)
C−O (%)
CO (%)
OC−O (%)
coal 1 coal 2 coal 3
55.31 27.80 8.79
38.74 50.94 50.99
5.95 18.83 31.45
0.00 2.42 8.77
coal 1
group is oxidized to the COOH group. At last, the COOH group can be oxidized further and releases some gas components (CO and CO2) and water.2,5−7,9,10 As shown in Figure 3, the surface morphology of bituminous coal is also changed by natural weathering processes. The surface roughness of bituminous coal is increased, and some scraps are produced by natural weathering processes. Some holes are also produced on the bituminous coal surface after natural weathering processes. During natural weathering processes, the organic materials can be oxidized, while the inorganic materials were difficult to be oxidized by air or water.21−24 The organic materials can be oxidized into some oxidation products. As a result, a part of organic materials will release some gas components (CO and CO2) and water. 3.2. Size Composition Analysis. As shown in Table 4, the weight of the 0.5−0.25 mm size fraction in coal 1 is higher than that in coal 2 and/or coal 3. However, the weights of other size
coal 2
coal 3
size fraction (mm)
weight (%)
ash (%)
weight (%)
ash (%)
weight (%)
ash (%)
0.5−0.25 0.25−0.125 0.125−0.074 0.074−0.045 −0.045
45.95 20.22 14.82 14.85 4.16
24.45 24.06 22.55 14.98 13.49
44.85 20.52 14.98 15.29 4.36
24.11 24.98 25.76 18.80 20.73
42.29 20.91 15.46 16.52 4.82
24.45 29.92 30.70 26.41 27.62
fractions (0.25−0.125, 0.125−0.074, 0.074−0.045, and −0.045 mm) in coal 1 are lower than those in coal 2 and/or coal 3. It indicates that natural weathering processes can change the size composition of coal particles, such as reduce the weight of coarse coals (size bigger than 0.25 mm) and increase the weight of fine coals (size smaller than 0.25 mm). Apart from that, the ash content of each size fraction is also changed. The ash content of the 0.5−0.25 mm size fraction in coal 1 is similar to that in coal 2 and/or coal 3. However, the ash contents of other size fractions (0.25−0.125, 0.125−0.074, 0.074−0.045, and −0.045 mm) in coal 1 are higher than those in coal 2 and/or coal 3. A specific coarse coal particle can be oxidized, and its surface may be disintegrated during natural weathering processes. After the weathering and denudation processes, some small fragments may shed from this specific coarse coal particle, and 4498
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During natural weathering processes, a part of organic materials can be released as gas components (CO and CO2) and water from the coal surface, and hence, the density of a specific coal particle may be increased. Low-density fractions primarily consist of the organic materials, while high-density fractions primarily consist of the inorganic materials. If a part of the organic materials releases from the coal surface, the inorganic materials would like to stay on the coal surface. It shows that the ash content of the coal surface is increased by natural weathering processes. It is well-known that the density of inorganic materials is much higher than that of organic materials. Therefore, a specific coal particle density may be increased by natural weathering processes because the relative content of inorganic materials is increased. The coal particles with the lower density fractions can be weathered into the coal particles with the higher density fractions. For example, a specific coal particle with the density of 1.295 g/cm3 may become another coal particle with the density of 1.302 g/cm3. Before natural weathering processes, this specific coal particle belongs to the 1.2−1.3 g/cm3 density fraction. However, this specific coal particle belongs to the 1.3−1.4 g/ cm3 density fraction after natural weathering processes. Furthermore, the coal particles belonging to the 1.3−1.4 g/ cm3 density fraction may also become the coal particles in the 1.4−1.5 g/cm3 density fraction, etc.
hence, the size of this specific coarse coal may be reduced, while some small coal fines (fragments from this specific coarse coal particle) may be newly produced. From Table 4, the weight of the 0.5−0.25 mm size fraction is decreased, while the weights of other size fractions are increased, by natural weathering processes. Coal particles with the 0.5−0.25 mm size fraction may be weathered into the 0.25−0.125 mm size fraction. Furthermore, the 0.25−0.125 mm size fraction may be weathered into the 0.125−0.0.074 mm size fraction, etc. In here, it cannot be ignored that the ash contents of size fractions (0.25−0.125, 0.125−0.074, 0.074−0.045, and −0.045 mm) are increased by natural weathering processes. Coarser size fractions can be weathered into finer size fractions. During the processes of size decreasing, the ash content of the same coal particle may also be increased. From Figure 1 and Table 2, it can be known that the ash content on the coal surface is increased by natural weathering processes. It can be concluded that the ash content of a specific coal particle may be increased by natural weathering processes. If a specific coal particle is oxidized and becomes a new particle (the size has been relatively reduced), this new particle may stay in the original size fraction or in another size fraction. For example, for a specific coal particle with the size of 0.27 mm, its size may be reduced to 0.26 mm, and hence, it stays in the original size fraction of 0.5−0.25 mm. If its size is reduced to 0.24 mm, it should be in the size fraction of 0.25−0.125 mm. Because a part of organic materials can be oxidized into some gas components (CO and CO2) and water, the total ash content is increased by natural weathering processes, such as 22.31% in coal 1, 23.86% in coal 2, and 27.01% in coal 3, in Table 1. 3.3. Density Composition Analysis. As shown in Table 5, the weights of −1.2 and 1.2−1.3 g/cm3 density fractions in coal
4. CONCLUSION (1) Natural weathering processes can reduce the content of organic materials on the bituminous coal surface but increase the relative content of inorganic materials. The surface roughness of bituminous coal can also be increased by natural weathering processes. (2) The size composition of bituminous coal can be changed by natural weathering processes. The coal particle size becomes finer, and the ash content of the coal sample can also be increased, after natural weathering processes. (3) The density composition of bituminous coal can be changed by natural weathering processes. The density of a specific coal particle may be increased by natural weathering processes. The coal particles with the lower density fractions can be weathered into the coal particles with the higher density fractions. (4) The changes in the size and density compositions of bituminous coal may be attributed to a part of organic materials that is weathered into gas components (CO and CO2) and water. These oxidation products can be released from the coal surface.
Table 5. Density Compositions of Coal 1, Coal 2, and Coal 3 coal 1
coal 2
coal 3
density fraction (g/cm3)
weight (%)
ash (%)
weight (%)
ash (%)
weight (%)
ash (%)
−1.2 1.2−1.3 1.3−1.4 1.4−1.5 1.5−1.6 1.6−1.8 +1.8
9.54 39.76 20.67 9.16 2.85 3.84 14.18
4.27 5.49 11.06 20.03 33.13 62.47 85.86
2.30 22.64 37.13 13.11 4.81 3.98 16.02
4.02 5.32 8.05 15.33 27.44 51.46 86.86
1.25 15.35 23.90 19.72 12.33 8.67 18.79
3.77 3.65 7.24 12.71 20.88 42.14 84.99
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] and/or xiawencheng@cumt. edu.cn.
1 are higher than those in coal2 and/or coal 3. However, the weights of 1.4−1.5, 1.5−1.6, 1.6−1.8, and +1.8 g/cm3 density fractions in coal 1 are lower than those in coal 2 and/or coal 3. Furthermore, the weight of the 1.3−1.4 g/cm3 density fraction increases from coal 1 to coal 2 and has a decrease from coal 2 to coal 3. It can also be found that the weight of the 1.3−1.4 g/ cm3 density fraction in coal 3 is still higher than that in coal 1. It indicates that natural weathering processes can reduce the weights of low-density fractions but increase the weights of high-density fractions. It is also found that the ash contents of −1.2, 1.2−1.3, 1.3−1.4, 1.4−1.5, 1.5−1.6, and 1.6−1.8 g/cm3 density fractions are reduced after natural weathering processes. However, the ash content of the +1.8 g/cm3 density fraction in coal 1 is similar to that in coal 2 and/or coal 3.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Advanced Analysis and Computation Center of China University of Mining and Technology.
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REFERENCES
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