Article pubs.acs.org/EF
Role of Roughness Change on Wettability of Taixi Anthracite Coal Surface before and after the Heating Process Wencheng Xia* and Yanfeng Li* Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education, School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ABSTRACT: It is well-known that the heating process can change the composition of functional groups on the coal surface and the change of the composition of functional groups can change the hydrophobicity and wettability of the coal surface. The change of coal surface topography, such as surface roughness, is usually ignored during the heating process. Therefore, the effect of the change of surface roughness on the hydrophobicity and wettability of the coal surface is also sufficiently studied. This investigation is to find out the changes of the composition of functional groups and roughness on the Taixi anthracite coal surface before and after the heating process in a quartz crucible. The effects of these changes on the wettability of the anthracite coal surface will be discussed. The composition of functional groups on the coal surface before and after the heating process was characterized by X-ray photoelectron spectroscopy (XPS), and the wettability of the coal surface was indicated by contact angle measurements. The heating process temperature was 600 °C, and the heating process time was 120 min. Throughout this paper, it was found that the surface roughness change before and after the heating process had a significant role on the wettability of the coal surface. heated coals are difficult to float. It is necessary to investigate the flotation of spontaneous combustion coal or heated coal in the future. The heating processes should change the composition of functional groups on coal, and the hydrophobicity of the coal surface is changed. In common, the increase of hydrophilic groups and the decrease of hydrophobic groups on the coal surface result in a decrease of surface hydrophobicity. However, the surface topography, such as roughness, was not considered sufficient as a result of the fine coal particles used as experimental materials. The surface roughness of the fine coal surface is difficult to obtain using the common measuring methods. In addition, the existing observations have shown the significance of surface roughness on the wettability of minerals. It was found that the wettability of quartz particles increased with the increase of the surface roughness.11−13 However, the observations about the surface roughness on the wettability and hydrophobicity of coal have not been given sufficiently. In this investigation, Taixi anthracite coal particles with a low ash content were used as the experimental materials. The purity of the coal surface has been proven to be a high level.5,6,14,15 The low-ash particles can be used to investigate the changes of the composition of functional groups and roughness on the coal surface before and after the heating process. Particularly, the role of surface roughness change on the wettability of coal before and after the heating process will be discussed. Some findings for the developments of science of the coal surface and flotation will be given in this paper.
1. INTRODUCTION Coal surface properties contain two main aspects, chemical and physical properties. Among the chemical and physical properties, the composition of functional groups and surface topography are well-known as the primary dominant factors determining the hydrophobicity and wettability of the coal surface. The composition of functional groups on the coal surface are divided into two types, hydrophobic groups (C−C, C−H, etc.) and hydrophilic groups (C−O, CO, O−CO, etc.). The surface topography of coal can also be quantized using surface roughness. Bituminous and anthracite coals usually have natural hydrophobicity and low wettability. However, the weathering/oxidation processes can destroy their natural hydrophobicity by changing their surface functional group composition and surface roughness. The number of hydrophobic groups on the coal surface decreases, whereas that of hydrophilic groups is increased after the oxidation processes.1 As a result, there are many oxidized/weathered coals around the world, which are difficult to float and sometimes refuse.2−4 Apart from the weathering/oxidation processes, the hightemperature heating caused by the coal spontaneous combustion can also change the composition of functional groups and the roughness of the coal surface. The surface hydrophobicity of anthracite coal was reduced as a result of the increasing number of hydrophilic groups and the decreasing number of hydrophobic groups on the coal surface after the high-temperature heating process.5,6 For the cleaning burning and use of coal resources, it is recommended that lowtemperature pyrolysis can be prior to the use of coal to convert the coal to the semi-coke.7−10 The burning of semi-coke can reach an environmentally friendly condition easily. The coal spontaneous combustion is considered to produce a condition of the heating process for coal. There is a great source of semicoke produced by coal spontaneous combustion, and the fine © 2015 American Chemical Society
Received: November 6, 2015 Revised: December 17, 2015 Published: December 19, 2015 281
DOI: 10.1021/acs.energyfuels.5b02621 Energy Fuels 2016, 30, 281−284
Article
Energy & Fuels
2. EXPERIMENTAL SECTION
Table 1. Fraction of C on Coal Particle Surfaces before and after the Heating Process (Relative Percentage of C 1s)
2.1. Coal Samples. Samples of Taixi ultralow-ash coal samples were selected from the Taixi Coal Preparation Plant in Ningxia province of China. The contents of elements (C, H, O, N, and S) on a dry and ash-free basis are 94.43, 3.73, 0.91, 0.79, and 0.13%, respectively. The ash content of Taixi ultralow-ash coal samples is 1.55%. As a result of its ultralow ash, we think each coal particle has similar properties. Lump coal particles are selected for the experimental materials. 2.2. Polishing Treatment. To obtain the surfaces of coal particles with different surface roughness, lump coal particles were polished using different sand papers with different meshes by hand. The sand papers were made by MATADOR in Germany. The meshes of particles on sand paper surfaces are 220, 280, 400, 800, 2000, and 5000. Using these sand papers with different meshes, the surfaces of coal particles with different surface roughnesses were obtained. The coal particles were forwarded to low-temperature heating process, contact angle, and surface roughness measurements. 2.3. Low-Temperature Heating Process. The heating process was conducted in a quartz crucible in a muffle furnace. The heating process temperature was 600 °C. The heating process time was 120 min. After the heating process, coal samples in a quartz crucible were cooled in a drying chamber. 2.4. X-ray Photoelectron Spectroscopy (XPS) Measurements. For the indication of the composition of functional group change on the coal surface before and after the heating process, the coal particle was forwarded to the XPS tests. Here, it is necessary to indicate that fine coal particles before and after the heating process were used in the XPS tests because of the testing condition of XPS. The XPS experiments were carried out at room temperature in an ultrahigh-vacuum (UHV) system with the surface analysis system (ESCALAB 250Xi, Thermo Scientific, Waltham, MA). The data processing (peak fitting) was performed with XPS peak fit software. The binding energies were corrected by setting the C 1s hydrocarbon (−CH2−CH2 bonds) peak at 284.6 eV. 2.5. Surface Roughness Measurement. The Mitutoyo SJ-210 surface roughness measurer was used to indicate the surface roughness of the coal particle surfaces owning different surface roughnesses. As a probe moved on the surface of coal particles, the surface roughness readings of coal particles were recorded on the work line and the average value was used for analysis. Three indexes (Ra, Rq, and Rz) were obtained on the basis of the results of the Mitutoyo SJ-210 surface roughness measurer. Roughness parameter Ra was used in this paper. Each coal particle was measured 3 times, and the final Ra value was obtained using the arithmetic mean values. 2.6. Contact Angle Measurement. The surfaces of coal particles of different roughnesses were measured using the water contact angle analyzer (JC2000D), such as a water droplet on the surface of coal in air. Each coal particle surface is measured 2 times, and the final contact angle is obtained using the arithmetic mean values. The contact angle changes during the heating process were obtained.
coal type before the heating process after the heating process
C−C or C−H (%)
C−O (%)
CO (%)
O−CO (%)
88.6
5.4
3.3
2.7
67.6
14.6
10.3
7.5
Figure 1. C 1s peaks for coal particle surfaces before and after the heating process.
oxygen-containing functional groups were formed during the heating process. This air oxidation process should not be very violent. Otherwise, the coal would burn during the heating process. For the coal surface, the C−C/C−H groups are the primary hydrophobic functional groups, while the C−O, CO, and O−CO groups are the primary hydrophilic functional groups.20,21 The number of hydrophobic functional groups and the number of hydrophilic functional groups on the coal surface determines the hydrophobicity and wettability of coal. The relative number of hydrophilic functional groups on the coal surface increased, whereas the relative number of hydrophobic functional groups decreased after the heating process. The wettability of the coal surface should be increased after the heating process according to the changes in the composition of functional groups on the coal surface indicated by the XPS results. 3.2. Roughness Analysis of Coal Surfaces before and after the Heating Process. Table 2 is the roughness change on coal surfaces owning different roughnesses before and after the heating process. It is obvious that the Ra value is increased after the heating process. The heating process produced many cracks and holes on the coal surface as a result of some organic components released from the coal surface.22,23 In addition, a light oxidation also occurred on the coal surface. Both the pyrolysis and oxidation processes changed the coal surface roughness. With the sand paper mesh increasing, the roughness
3. RESULTS AND DISCUSSION 3.1. XPS Analysis of Coal Surfaces before and after the Heating Process. C 1s: 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 (alcohol, phenol, or ether), CO (carbonyl or chinone) or O−C−O (in low-rank coals), and O− CO (carboxyl).16−19 The contents of C−C or C−H, C−O, CO, and O−CO groups can be calculated in Table 1 by the analysis of Figure 1. The relative content of C−C/C−H groups decreases, whereas that of C−O, CO, and O−CO groups increases after the heating process. Even though the heating process was conducted in a quartz crucible in a muffle furnace, a light oxidation process should accompany the heating process. The coal surface reacted with oxygen in the air, and some new 282
DOI: 10.1021/acs.energyfuels.5b02621 Energy Fuels 2016, 30, 281−284
Article
Energy & Fuels
decrease and the wettability of the coal surface was significantly increased. However, the coal surface with roughnesses of 0.44 and 0.18 μm changed to the surface with roughnesses of 1.25 and 0.67 μm after the heating process, while its contact angle decreased from about 76° and 75° to about 43° and 54°. The decrease in the contact angle for a relative smooth surface was not very great, and the wettability of the coal surface was not significantly increased. On the basis of the above-mentioned analysis, the role of surface roughness change on the wettability of coal before and after the heating process is also significant and even equal to the role of functional groups on the coal surface. Therefore, both the changes in the composition of functional groups and the surface roughness lead to the increase of wettability of the coal surface by the heating process. For a higher original roughness coal surface, the increase in the wettability is bigger, whereas the increase in the wettability is smaller for a lower original roughness coal surface.
Table 2. Roughness (Ra) of Coal Particle Surfaces before and after the Heating Process sand paper (mesh)
before the heating process
after the heating process
220 400 2000 2500 3000 5000
2.68 2.49 1.66 1.12 0.44 0.18
13.19 12.27 5.12 4.24 1.25 0.67
decreases quickly. The changes in roughness seem obvious, while the original surface roughness before the heating process is very high. A higher roughness might be conducive to aggravate the reactions (pyrolysis and oxidation) on the coal surface during the heating process. 3.3. Contact Angle Analysis of Coal Surfaces before and after the Heating Process. As shown in Figure 2, before
4. CONCLUSION (1) XPS results indicated that the relative number of hydrophilic functional groups on the coal surface increased, whereas the relative number of hydrophobic functional groups decreased, after the heating process. The wettability of the coal surface was, hence, increased by the heating process. (2) The roughness of the coal surface was greatly increased as a result of both the pyrolysis and oxidation processes occurring on the coal surface during the heating process. The changes in roughness seem obvious, while the original surface roughness before the heating process is very high. A higher roughness might be conducive to aggravate the reactions (pyrolysis and oxidation) on the coal surface during the heating process. (3) The role of surface roughness change on the wettability of coal before and after the heating process is significant as the changes in the composition of functional groups on the coal surface. Both the changes in the composition of functional groups and the surface roughness lead to the increase of wettability of the coal surface by the heating process.
Figure 2. Contact angles of coal surfaces with different roughnesses before and after the heating process.
the heating process, the contact angle of the coal surface increases with the increase of roughness. However, the contact angle of the coal surface decreases with the increase of the roughness after the heating process. The wettability of the coal surface before the heating process decreases, whereas that of the coal surface after the heating process increases with the increase of roughness. It is already known that the wettability of hydrophobic particles decreases with the increase of roughness in a certain roughness range.24 However, for hydrophilic particles, the experts find that the wettability increases with the increase of roughness.25−27 The coal sample used in this investigation is Taixi anthracite coal owning its natural hydrophobicity, and hence, they belong to hydrophobic particles. However, the composition of functional groups on the coal surface changed after the heating process. The coal surface became hydrophilic, and the heated coals belong to hydrophilic particles. The results of this investigation match well with the findings from the literature. In this paper, the main attempt is to make a comment on the role of surface roughness change on the wettability of coal before and after the heating process. We should compare the contact angles of coal surfaces more subtly. The coal surface with roughnesses of 2.68, 2.49, 1.66, and 1.12 μm changed to the surface with roughnesses of 13.19, 12.27, 5.12, and 4.24 μm after the heating process, while its contact angle decreased from about 88°, 87°, 86°, and 80° to only about 19°, 20°, 21°, and 32°, respectively. With the increase of roughness on the coal surface by the heating process, the contact angle had a great
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] and/or w.xia.cumt@gmail. com. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology; 2014YC08) and a Priority Academic Program Development of Jiangsu Higher Education Institutions.
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REFERENCES
(1) Kozłowski, M.; Pietrzak, R.; Wachowska, H.; Yperman, J. AP− TPR study of sulphur in coals subjected to mild oxidation. Part 1. Demineralised coals. Fuel 2002, 81 (18), 2397−2405. (2) Pietrzak, R.; Wachowska, H. Thermal analysis of oxidised coals. Thermochim. Acta 2004, 419 (1), 247−251. (3) Grzybek, T.; Pietrzak, R.; Wachowska, H. The influence of oxidation with air in comparison to oxygen in sodium carbonate solution on the surface composition of coals of different ranks. Fuel 2006, 85 (7), 1016−1023. 283
DOI: 10.1021/acs.energyfuels.5b02621 Energy Fuels 2016, 30, 281−284
Article
Energy & Fuels (4) Pietrzak, R.; Wachowska, H. Low temperature oxidation of coals of different rank and different sulphur content. Fuel 2003, 82 (6), 705−713. (5) Xia, W.; Xie, G.; Pan, D.; Yang, J. Effects of Cooling Conditions on Surface Properties of Heated Coals. Ind. Eng. Chem. Res. 2014, 53 (26), 10810−10813. (6) Xia, W.; Xie, G. Changes in the hydrophobicity of anthracite coals before and after high temperature heating process. Powder Technol. 2014, 264, 31−35. (7) Chen, X.; Zheng, D.; Guo, J.; Liu, J.; Ji, P. Energy analysis for low-rank coal based process system to co-produce semicoke, syngas and light oil. Energy 2013, 52 (2), 279−288. (8) Meng, F.; Tahmasebi, A.; Yu, J.; Zhao, H.; Han, Y.; Lucas, J.; et al. Low-Temperature Oxidation Characteristics of Lignite Chars from Low-Temperature Pyrolysis. Energy Fuels 2014, 28 (9), 5612−5622. (9) Zhang, J.; Wang, Q.; Wei, Y.; Zhang, L. Numerical Modeling and Experimental Investigation on the Use of Brown Coal and Its Beneficiated Semicoke for Coal Blending Combustion in a 600 MW e Utility Furnace. Energy Fuels 2015, 29 (2), 1196−1209. (10) Agirre, I.; Griessacher, T.; Rösler, G.; Antrekowitsch, J. Production of charcoal as an alternative reducing agent from agricultural residues using a semi-continuous semi-pilot scale pyrolysis screw reactor. Fuel Process. Technol. 2013, 106 (2), 114−121. (11) Ulusoy, U.; Hiçyılmaz, C.; Yekeler, M. Role of shape properties of calcite and Barite particles on apparent hydrophobicity. Chem. Eng. Process. 2004, 43, 1047−1053. (12) Ulusoy, U.; Yekeler, M. Variation of critical surface tension for wetting of minerals with roughness determined by Surtronic 3+ instrument. Int. J. Miner. Process. 2004, 74, 61−69. (13) Karakas, F.; Hassas, B. V. Effect of surface roughness on interaction of particles in flotation. Physicochem. Probl. Miner. Process. 2016, 52, 18−34. (14) Xia, W.; Yang, J. Changes in surface properties of anthracite coal before and after inside/outside weathering processes. Appl. Surf. Sci. 2014, 313 (18), 320−324. (15) Xia, W.; Yang, J.; Liang, C. Investigation of changes in surface properties of bituminous coal during natural weathering processes by XPS and SEM. Appl. Surf. Sci. 2014, 293 (4), 293−298. (16) Pietrzak, R.; Grzybek, T.; Wachowska, H. XPS study of pyritefree coals subjected to different oxidizing agents. Fuel 2007, 86 (16), 2616−2624. (17) Grzybek, T.; Pietrzak, R.; Wachowska, H. X-ray photoelectron spectroscopy study of oxidized coals with different sulphur content. Fuel Process. Technol. 2002, 77−78, 1−7. (18) Pilawa, B.; Więckowski, A. B.; Pietrzak, R.; Wachowska, H. Oxidation of demineralized coal and coal free of pyrite examined by EPR spectroscopy. Fuel 2002, 81 (15), 1925−1931. (19) Wang, B.; Peng, Y.; Vink, S. Diagnosis of the surface chemistry effects on fine coal flotation using saline water. Energy Fuels 2013, 27 (8), 4869−4874. (20) Cinar, M. Floatability and desulfurization of a low-rank (Turkish) coal by low-temperature heat treatment. Fuel Process. Technol. 2009, 90 (10), 1300−1304. (21) Bolat, E.; Saǧlam, S.; Pişkin, S. The effect of oxidation on the flotation properties of a Turkish bituminous coal. Fuel Process. Technol. 1998, 55 (2), 101−105. (22) Dong, C.; Jin, L.; Li, Y.; Zhou, Y.; Zou, L.; Hu, H. Integrated Process of Coal Pyrolysis with Steam Reforming of Methane for Improving the Tar Yield. Energy Fuels 2014, 28, 7377−7384. (23) Kang, L.; Zhang, J.; Lian, H.; Luo, M. Co-pyrolysis characteristics of coal and natural gas. Korean J. Chem. Eng. 2007, 24 (3), 508−511. (24) Xia, W.; Ni, C.; Xie, G. The influence of surface roughness on wettability of natural/gold-coated ultra-low ash coal particles. Powder Technol. 2016, 288, 286−290. (25) Guven, O.; Ozdemir, O.; Karaagaclioglu, I. E.; Celik, M. S. Surface morphologies and floatability of sand-blasted quartz particles. Miner. Eng. 2015, 70, 1−7.
(26) Ramiasa, M.; Ralston, J.; Fetzer, R.; Sedev, R. The influence of topography on dynamic wetting. Adv. Colloid Interface Sci. 2014, 206, 275−293. (27) Boinovich, L.; Emelyanenko, A. Wetting and surface forces. Adv. Colloid Interface Sci. 2011, 165, 60−69.
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