Energy Fuels 2009, 23, 5536–5543 Published on Web 10/27/2009
: DOI:10.1021/ef900589d
Investigation of Bituminous Coal Hydrophobicity and its Influence on Flotation Li Ping Ding*,† Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia. † Current address: School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Received June 9, 2009. Revised Manuscript Received October 5, 2009
The hydrophobicity of bituminous coal surface has a significant effect on the separation of coal and mineral matters by flotation process. In this work, water contact angles on bituminous coals displaying different brightness in terms of specular reflectance were measured to characterize their surface hydrophobicity. It is found that water contact angle on coal increases with specular reflectance for both bright and dull coals, and the bright coals have higher contact angle and specular reflectance than the dull coals. The oxidation of coal surface will render it less hydrophobic, but has little effect on the specular reflectance. Results from petrographic analysis show remarkable differences in volume percentage of maceral groups for bright and dull coals. Bright coals contain more vitrinite, less inertinite and mineral mater, than the dull coals. X-ray photoelectron spectroscopy (XPS) analysis indicates that the dull surface has more hydrophilic, oxygen-bearing functional groups. These results imply that vitrinite is more hydrophobic than inertinite, and should float better than the latter. The combustible recoveries of bench scale flotation tests increase with the corresponding contact angles and their relationship can be accurately described by a sigmoidal equation. The maximum combustible recovery for vitrinite is 96.76% under the current operation conditions. The results also indicates that in order to reach the targeted combustible recovery in flotation, the contact angle of coal particles needs to reach certain value. slime coating of coal particle surfaces by hydrophilic fine clay particles.6,7 Therefore the characterization and description of coal hydrophobicity is crucial for the design, control, and optimization of coal flotation operation. Coal is composed of a number of distinct organic entities called macerals. Different maceral groups with different physical and chemical properties control the overall behavior of coal, including its hydrophobicities.8,9 The maceral groups in coal can be identified by petrographic analysis, which is conducted by observing the reflectance of a polished coal surface under a microscope. Holuszko and Laskowski10 investigated the effect of macerals on the wettability and floatability of coal particles by studying their critical surface tension. They found that vitrinite is more hydrophobic than inertinite and mineral matter. The prediction of water contact angle on coal is very difficult due to its heterogeneity. Based on the Cassie equation11 for contact angle on heterogeneous surfaces, similar expressions considering the effects of coal rank, carbon content, and oxidation were proposed in the literature.12,13
1. Introduction Flotation is a selective separation process of solid from aqueous solutions using gas bubbles. In this process, the fine solid particles contact and attach to gas bubbles of much larger diameters and float to the surface of the pulp. Some wastewater treatment plants and paper mills used this method for the removal of particulate matters and deinking. Froth flotation has been widely applied in mineral and coal processing to separate the valuables from the gangue material. In a coal processing plant, flotation is commonly used to clean fine coals with particle size less than 150 μm.1 With the development of new technology and reagents for coal flotation, coal particles of larger sizes can also be effectively processed in the flotation circuit. Flotation is a surface-based separation method, and the hydrophobicity of the particle surface is one of the most important factors in determining the separation efficiency by flotation. Coal is intrinsically hydrophobic2 because of its chemical composition (surface aromatic and aliphatic groups). In practice poor coal flotation can happen due to the decrease of coal surface hydrophobicity. The coal surface may be less hydrophobic because of oxidation resulting in the formation of hydrophilic carbonyl, carboxyl, and ester groups.3-5 The decrease of coal hydrophobicity can also be attributed to the
(6) Arnold, B. J.; Aplan, F. F. Int. J. Miner. Process. 1986, 17, 225– 242. (7) Xu, Z.; Liu, J.; Choung, J. W.; Zhou, Z. Int. J. Miner. Process. 2003, 68, 183–196. (8) Ting, F. T. C. Coal macerals. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 7-49. (9) Arnold, B. J.; Aplan, F. F. Fuel 1989, 68, 651–8. (10) Holuszko, M. E.; Laskowski, J. S. In Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the 1st UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, Vancouver, B.C., 1995; Laskowski, J. S.; Poling, G. W., Eds.; pp 145-63. (11) Neumann, A. W.; Good, R. J. Techniques of Measuring Contact Angles. In Surface and Colloid Science Vol. III Experimental Methods; Good, R. J.; Stromberg, R. R., Eds.; Plenum Press: New York, 1979; pp 31-91. (12) Rosenbaum, J. M.; Fuerstenau, D. W. Int. J. Miner. Process. 1984, 12, 313–16. (13) Keller, D. V., Jr. Colloids Surf. 1987, 22, 21–35.
*Author to whom correspondence should be addressed. Phone: þ617-3365 8835. Fax: þ61-7-3365 4199. E-mail:
[email protected]. (1) Laskowski, J. S. Coal Flotation and Fine Coal Utilization; Elsevier: Amsterdam, 2001. (2) Brady, G. A.; Gauger, A. W. J. Ind. Eng. Chem. (Washington, D. C.) 1940, 32, 1599–1604. (3) Xiao, L.; Somasundaran, P.; Vasudevan, T. V. Colloids Surf. 1990, 50, 231–40. (4) Buckley, A. N.; Lamb, R. N. Int. J. Coal Geol. 1996, 32, 87–106. (5) Somasundaran, P.; Zhang, L.; Fuerstenau, D. W. Int. J. Miner. Process. 2000, 58, 85–97. r 2009 American Chemical Society
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Previous study also demonstrated that the water contact angles obtained from the sessile-drop method were larger than those from the captive bubble method. Gosiewska and coworkers15 investigated the distribution and inclusion of mineral matters on coal surfaces and their effect on coal wettability. They applied the captive-bubble method for contact angle measurement on polished coal surfaces. Their results showed great variation between the advancing and receding contact angles. Despite of previous efforts on coal hydrophobicity study and contact angle measurement, the relationship between coal surface composition, contact angle and flotation has not been fully established. This is important in process design and control, as it provides valuable information for the prediction of flotation performance. The aim of this work is to provide a reliable method to characterize the hydrophobicity of bituminous coal in terms of water contact angle, and also to investigate the origin of the different hydrophobicities in terms of the chemical composition and physical properties of the coal surface. Because contact angle measurement can only be applied to the analyzed samples, it is crucial to adopt a suitable sampling method so that a representative sample can be selected. In this study, different analytical techniques complementing each other were utilized in order to identify the hydrophobic and hydrophilic compositions on coal surface. The effect of these species on contact angle, and ultimately the effect on flotation will be investigated. These results will form the basis for a flotation model incorporating coal hydrophobicity (contact angle), particle size, and cell hydrodynamics. Particle size also has a significant effect on coal flotation. Flotation rate and recovery tend to reach maximum at intermediate particle sizes due to the decrease of collision efficiency for small particles and attachment efficiency for large particles.16,17 In the bench scale flotation tests conducted in this work, different coal samples of the same size range were prepared by the same method, so the effect of particle size can be eliminated. The hydrodynamic conditions in the flotation machine, such as air flow rate, agitation speed, were essentially the same for all the tests. The contact angle on coal particles seems more relevant to flotation. However, the available techniques for measuring particle contact angle, such as the Washburn method,18 skin flotation19,20, and induction time measurement,21 were difficult and not reliable when applied to coal due to its porosity and somewhat high hydrophobicity. The latter two methods are indirect methods to extract the contact angle values. Coal particles have a short residence time in flotation cells (5-20 min),22 so the porosity should have little effect on flotation as pore diffusion is slow. Therefore the contact angle measured on flat surface of appropriately selected coal
samples should be able to represent the hydrophobicity of the particles. 2. Experimental Section 2.1. Sample Selection and Preparation. A new approach on sample selection for contact angle measurement was adopted in this work. Because coal is a heterogeneous material, sampling is very important in order to obtain results representative of the bulk. All the coal samples used in this study were provided by BHP Billiton Mitsubishi Alliance (BMA) from their Queensland coal fields. The coals received in our laboratory were large lump samples with size greater than 10 cm, and were crushed through a jaw crusher, gyratory crusher and rolls crusher, before dry screened to obtain the desirable size range. During the crushing process, coal specimens of about 3 3 cm were handpicked based on their appearance, such as roughness and brightness. The surface of selected samples was as flat and homogeneous as possible, so that there were no cracks or visible mineral inclusions on the surface. More importantly, coal samples of either bright or dull appearances were selected from each coal types, respectively. Each coal sample was stored in a sealed plastic bag in a cool place before use. During sample preparation for contact angle measurement, the chosen sample was cut to a desired size (1-2 cm). The sample was then put in a beaker with Milli-Q water and washed in a ultrasonic bath for 15 min to remove loose particles from the surface. The Milli-Q water with a conductivity of 18.2 mΩ 3 cm was obtained by sequential treatment using reverse osmosis, two-stage ion exchange and activated carbon treatments prior to final filtration. The washing procedure was repeated twice for 5 and 2 min respectively, before the water contact angle on coal samples was measured. Following contact angle measurement on unpolished surfaces, coal samples were ground and polished. The specimens were first ground by different grades of silicon carbide abrasive papers, followed by washing in the ultrasonic bath of Milli-Q water for 10 min. Then the specimen was polished by alumina powders (of 5 and 0.05 μm diameters, respectively). Following the polishing steps, the samples were cleaned in a ultrasonic bath for 10 min, to remove any loose particles. More details of the polishing process can be found in Drelich’s work.23 The cleaned specimens were stored in a desiccator. 2.2. Measurement of Contact Angle. Contact angle measurements were performed using the captive bubble method. The instrument consists of a sample stand, a dosing unit and a microscope connected to a video capture apparatus. The sample was immersed in Milli-Q water in a rectangular glass cell (Starna). Experiments were performed at 25 C. The volume of the initial air bubble was about 1.6 μL, which gave a bubble diameter of about 1.45 mm. The volume of the air bubble was increased and decreased to obtain the receding and advancing contact angles respectively. The image of the air bubble was captured and analyzed using ImageJ software.24 The angles on both side of the bubble were calculated and the average was taken. The measurement was carefully conducted in a clean room environment with a reproducibility of (2. Figure 1 shows the images collected by the captive bubble method on coal samples having 60 and 38 contact angle. Although hydrophobic surfaces are normally assumed to have water contact angles between 90 and 180, contact angles in solid-liquid systems rarely exceed 150.25 In fact, the largest contact angle value observed for water on a smooth solid surface is about 120.26
(14) Gutierrez-Rodriguez, J. A.; Purcell, R. J., Jr.; Aplan, F. F. Colloids Surf. 1984, 12, 1–25. (15) Gosiewska, A.; Drelich, J.; Laskowski, J. S.; Pawlik, M. J. Colloid Interface Sci. 2002, 247, 107–116. (16) Al Taweel, A. M.; Delory, B.; Wozniczek, J.; Stefanski, M.; Andersen, N.; Hamza, H. A. Colloids Surf. 1986, 18, 9–18. (17) Polat, M.; Polat, H.; Chander, S. Int. J. Miner. Process. 2003, 72, 199–213. (18) Washburn, E. W. Phys. Rev. 1921, 17, 273. (19) Fuerstenau, D. W.; Williams, M. C.; Narayanan, K. S.; Diao, J. L.; Urbina, R. H. Energy Fuels 1988, 2, 237–241. (20) Fuerstenau, D. W.; Diao, J.; Hanson, J. S. Energy Fuels 1990, 4, 34–7. (21) Hanning, R. N.; Rutter, P. R. Int. J. Miner. Process. 1989, 27, 133–46. (22) Yianatos, J. B.; Bergh, L. G.; Dı´ az, F.; Rodrı´ guez, J. Chem. Eng. Sci. 2005, 60, 2273–2282.
(23) Drelich, J.; Laskowski, J. S.; Pawlik, M.; Veeramasuneni, S. J. Adhes. Sci. Technol. 1997, 11, 1399–1431. (24) Rasband, W. S. ImageJ. http://rsb.info.nih.gov/ij/ (1997-2009). (25) Shchukin, E. D.; Pertsov, A. V.; Amelina, E. A.; Zelenev, A. S. Colloid and surface chemistry; Elsevier: Amsterdam, 2001. (26) Barnes, G.; Gentle, I. Interfacial Science: An Introduction; Oxford University Press: Oxford, 2005.
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angles, therefore it is necessary to obtain the morphology information. Surface roughness was measured using a portable surface finish measuring instrument Handysurf E-35A (Zeiss). The surface profile was collected and the root mean squared (rms) roughness Rq and peak-to-valley roughness Rpv were calculated. 2.4. Petrography Analysis of Coal Samples. Two bright coal samples and two dull samples were selected for petrography analysis. The sample was set in resin, polished and then analyzed using incident light microscopy. The details of sample preparation and maceral analysis can be found in Australian standards AS2856.131 and AS2856.2.32 The analyses of block samples were conducted by counting points over the whole surface in a grid pattern, similar to the procedure utilized for analysis on particles. This work was conducted by CSIRO Mining and Exploration in Queensland. 2.5. Surface Analysis by X-ray Photoelectron Spectroscopy. The selected bright and dull coal samples were analyzed using X-ray photoelectron spectrometer (XPS). This method can provide quantitative information about the chemical compositions at the uttermost layers of the surface. The X-ray photoelectron spectrometer (XPS), Perkin-Elmer Physical Electronics Division PHI5100, with a MgK R X-ray source operated at 300W was used. A pass energy of 18 eV was applied for all elemental spectral regions. The scan area is 300 700 μm. The analyzer chamber was evacuated overnight to obtain ultrahigh vacuum of 10-7 Pa. The size of the samples was about 5 5 3 mm. The sample was obtained by manually breaking a large piece of coal to avoid contamination with metals, and it was stored in a container inside a vacuumed desiccator before use. Both elemental and high resolution C1s spectra were obtained and analyzed. 2.6. Bench Scale Flotation Test.33 The flotation tests were conducted in a Denver D12 flotation machine fitted with a 4 L cell. This machine was connected with a regulated air supply where the air flow rate was monitored. The flotation test was conducted according to the procedure similar to the one described by the Australian Standard AS4156.2.1.34 No collector was used in the flotation tests, and a small amount of methyl isobutyl carbinol (MIBC) was applied as frother. The flotation conditions were kept the same for all the tests. The concentrates and tailings were weighed, filtered, dried, sampled and analyzed for their ash content determined using the methods in the Australian Standard AS1038.3.35 Flotation performances of different coals were compared according to their combustible recoveries. Combustible recovery was calculated by the weight percentage of the combustible material in the concentrate in the combustible material in the feed.
Figure 1. Images collected on coal samples having contact angle of 60 (left) and 38 (right) using captive bubble method.
The water contact angle on graphite, generally accepted as hydrophobic, is 86.27 Therefore for coal surfaces exhibiting both chemical and geometrical heterogeneity, coal surfaces having contact angles in the range of 60-80 will be regarded as hydrophobic. This assumption is consistent with previous studies.9 On each coal sample, the contact angle was measured on three different spots of the surface and an average value was calculated. During the analysis, the tangent line at the three phase contact point was always toward the water phase, so the value obtained was the water contact angle on the coal surface. For both unpolished and polished samples, coal surface was chemically oxidized to study the effect of chemical composition on contact angle and brightness. Following contact angle measurements, the surface of the sample was immersed in 0.5% potassium permanganate aqueous solution for 1 h. The sample was then rinsed in Milli-Q water in a ultrasonic bath repeatedly until the pH of the solution became constant. Finally, the contact angles of oxidized surfaces were measured using the method described above. 2.3. Measurement of Reflectance and Morphology. The reflectance was measured for each sample in order to quantify the brightness of the surface. For opaque materials like coal, most incident light is reflected while color is seen in the diffuse reflectance and gloss is seen in the specular reflectance. The glossiness is related to the texture and roughness of the surface, therefore it was expected that the specular reflectance might be related to coal surface composition, such as maceral groups, and roughness, which also affects contact angle. It has been well established that different coal maceral groups have different optical properties.28,29 Following contact angle measurements, the samples were dried in a desiccator before the reflectance of each sample was measured using a Color Quest XE spectrophotometer (HunterLab). The spectra were collected for the visible light wavelength range of 400-700 nm with a resolution of 10 nm. The reflectances with specular included and excluded were measured, therefore surface brightness represented by the specular reflectance can be calculated from: Rsp ¼ Rin - Rex ð1Þ
3. Results and Discussion 3.1. Hydrophobicity of Coal Surfaces. The relationship between contact angle and specular reflectance for bright and dull coals is displayed in Figures 2 and 3, respectively. These results were obtained on coal samples from five different coal types. In both figures, the left and right figures show contact angle vs specular reflectance of the coal before and after polishing, respectively. Due to the variance of the origin and rank of the coal samples, the values of contact
where Rin is the total reflectance including both specular and diffuse reflectance, Rex is only diffuse reflectance. The difference between them accounts for the contribution from the specular reflectance Rsp. The reflectance was measured for both unpolished and polished dry samples after contact angle measurement. The effect of surface roughness on brightness can be evaluated by comparing the reflectance of unpolished and polished surfaces. Surface roughness also has a significant effect on contact
(31) Standard Australia, Coal Petrography - Part 1: Preparation of coal samples for incident light microscopy, AS 2856.1, 2000. (32) Standard Australia, Coal Petrography - Part 2: Maceral analysis, AS 2856.2, 1998. (33) Quast, K.; Ding, L. P.; Fornasiero, D.; Ralston, J., Effects of particle size on coal flotation, Proceedings of the CHEMECA 2007, Melbourne, Australia, September 23-26, 2007. (34) Standard Australia, Coal Preparation - Part 2.1: Higher rank coal - froth flotation - basic test, AS 4156.2, 2004. (35) Standard Australia, Coal and Coke - Analysis and testing - Part 3: proximate analysis of higher rank coal, AS 1038.3, 2000.
(27) Adamson, A. W. Physical Chemistry of Surfaces; 5th ed.; Wiley: New York, 1990. (28) Mastalerz, M.; Drobniak, A. Int. J. Coal Geol. 2005, 62, 250–258. (29) Tang, L. G.; Gupta, R. P.; Sheng, C. D.; Wall, T. F.; O’Brien, G. Energy Fuels 2004, 19, 130–137. (30) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface Sci. 1996, 179, 37–50.
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Figure 2. The relationship between contact angle and specular reflectance for five different bright coals. The left figure shows the result on unpolished surfaces, and the right figure shows the result on polished surfaces.
Figure 3. The relationship between contact angle and specular reflectance for five different dull coals. The left figure shows the result on unpolished surfaces, and the right figure shows the result on polished surfaces. Table 1. Surface Roughness of Different Coals unpolished
polished
roughness
bright
dull
bright
dull
Rq (μm) Rpv (μm)
0.6 3.1
2.6 9.9
0.4 1.6
0.3 1.5
angle spanned a wide range, which reflected the heterogeneity of coal surfaces. Polishing increased contact angles on bright coals and rendered most of the bright surfaces hydrophobic, as shown in Figure 2. Although polishing also increased contact angles on dull surfaces, they remained to be hydrophilic in terms of contact angle values as displayed in Figure 3. For unpolished surfaces, contact angle increases with brightness (specular reflectance) for both bright and dull coals. The reflectance of bright samples is mostly in the range of 2-9%, compared with that for the dull sample of less than 2%. This validates our sampling approach and indicates that the bright samples have much higher specular reflectance than the dull samples. After polishing, the specular reflectance was reduced significantly for bright sample, but increased slightly for the dull sample, therefore the same specular reflectance for bright and dull surfaces in the right of Figures 2 and 3 is observed. This is because polishing can remove the features of the original surfaces and make the bright and dull surfaces optically similar. For the bright coal surfaces,
Figure 4. The relationship between advancing contact angle and specular reflectance for fresh and oxidized coals.
the measurement results gave a higher diffuse reflectance for the polished sample than unpolished samples, whereas the total reflectances on both surfaces were almost the same. Therefore the specular reflectance of the polished bright surface was decreased. Nonetheless, the differences between polished bright and dull surfaces can be observed through petrographic analysis under a microscope as shown in the next section. 5539
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Figure 5. Representative micrographs showing maceral groups on bright coal samples from petrography analysis.
Figure 6. Representative micrographs showing maceral groups on dull coal samples from petrography analysis.
Figures 2 and 3. This is caused by the roughness and chemical heterogeneity on coal surfaces. Therefore, in order to simulate coal hydrophobicity in terms of contact angle, both Cassie equation11 and Wenzel equation39 should be considered. For bright coals, the receding angle barely changed after polishing, while the range of advancing angle changed from 30 to 75 to 40-75 after polishing. This is because the surface roughness has slightly decreased after polishing, especially the removal of asperities from the unpolished surface as illustrated by the decrease of Rpv in Table 1, which would increase the contact angles on the lower end. For dull coals, both advancing and receding angles are increased after polishing, so is the specular reflectance. For example, the advancing contact angle increased from 20 to 50 to 30-65 after polishing, as a result of the significant decrease of surface roughness as shown in Table 1. The advancing contact angles of oxidized surfaces are compared with those of fresh bright coal surfaces in Figure 4. Oxidation made coal surfaces completely hydrophilic as indicated by the contact angle values in Figure 4. It clearly shows that the contact angle was decreased dramatically by oxidation using KMnO4, but the specular reflectance was only slightly decreased after oxidation. Some researchers had found increased reflectance of vitrinite by oxidation at increased temperatures.40,41 Oxidation by KMnO4 resulted in the formation of hydrophilic functional groups, such as ;C;O; and ;COO;, and caused the decrease of contact angles. However, the change of chemical composition on the surface did not affect its appearance as shown by the similar specular reflectance values in
Table 2. Maceral Composition of Different Coals maceral group (%)
vitrinite inertinite mineral
bright samples
dull samples
1
2
1
2
81.2 16.3 2.5
78.9 16.5 4.6
2.6 65.3 32.1
6.9 90.6 2.5
The surface roughness for bright and dull samples before and after polishing is listed in Table 1. Both rms roughness and peak-to-valley roughness were calculated from the profile data. For the bright coal, polishing slightly decreased rms roughness from 0.6 to 0.4 μm. For dull coals, the surface became much smoother after polishing, with rms roughness decreased from 2.6 to 0.3 μm. The polishing process also reduced the peak-to-valley roughness of dull samples significantly. These results indicate that dull coals are rougher than bright coals. The geometrical differences observed for bright and dull coals can be related to the differences in their maceral compositions. Various maceral groups may have different mechanical properties due to their different geological origin and formation.36,37 Therefore different maceral groups exhibit different breakage structures under mechanical stress during crushing process. The effect of surface roughness on particle-particle and particle-bubble interactions, as well as the rheology of the slurry system and flotation,38 has yet to be explored. There is a large hysteresis between the advancing and receding angles for both bright and dull coals as shown in (36) Man, C. K.; Jacobs, J.; Gibbins, J. R. Fuel Process. Technol. 1998, 56, 215–227. (37) Wang, G. X.; Wang, Z. T.; Rudolph, V.; Massarotto, P.; Finley, R. J. Fuel 2007, 86, 1873–1884. (38) Davis, R. H.; Zhao, Y.; Galvin, K. P.; Wilson, H. J. Philos. Trans. R. Soc. A 2003, 361, 871–894.
(39) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (40) Bend, S. L.; Kosloski, D. M. Int. J. Coal Geol. 1993, 24, 233–243. (41) Calemma, V.; Del Piero, G.; Rausa, R.; Girardi, E. Fuel 1995, 74, 383–388.
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Figure 7. XPS analysis of bright coal. The top figure shows the survey spectrum and the corresponding elemental compositions, and the bottom figure shows the high resolution C1s spectrum and its compositions.
Figure 4. This suggests that it is difficult to determine whether a coal is oxidized or not by the observation of brightness. Alternatively, the contact angle measurement is able to differentiate between fresh and oxidized surfaces. 3.2. Maceral Compositions and Coal Hydrophobicity. Petrography analysis was conducted to determine the maceral compositions of coal samples with different appearances. The representative photomicrographs for bright and dull coals are displayed in Figures 5 and 6, respectively. The size of each image is 400 320 μm. These images clearly illustrate the different appearance between bright and dull coal
surfaces. In the left of Figure 5 it shows the thick band of telovitrinite (the whole picture is part of the band), and in the right we can see the detrovitrinite in association with fusinite of open cell structure and bright reflectance. In the right of Figure 6, a large macrinite particle was associated with inertodetrinite and clays, while the fusinite and semifusinite were displayed in the left photo. The volume percentage of each maceral component for two bright coals and two dull coals are listed in Table 2. It can be found that the bright coals predominantly consist of vitrinite (greater than 78% for both samples). The dull coals mainly consist of inertinite, as well as some mineral matters. By comparing the results for 5541
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Figure 8. XPS analysis of dull coal. The top figure shows the survey spectrum and the corresponding elemental compositions, and the bottom figure shows the high resolution C1s spectrum and its compositions.
bright and dull coals, it can be concluded that the dull coals are more heterogeneous than the bright coals in terms of maceral compositions, as well as the size and distribution of different components. Contact angle results have already shown that bright surfaces are more hydrophobic than dull surfaces. The maceral analysis can explain the reason as it contains more vitrinite. The dull surface is abundant of inertinite and mineral matters which makes the surface more hydrophilic. The reason is that inertinite contains more hydrophilic oxygen-bearing functional groups as shown by Shu et al.42
This conclusion can also be validated by our observation from flotation tests, in which concentrates were found to be mainly comprised of bright coal particles. Previous study had shown that vitrinite is more hydrophobic than inertinite, and the contact angle range of 60-70 for vitrinite and 25-40 for inertinite.9 These values are generally within the range of the results obtained from the current measurement. The discrepancy may be attributed to that samples studied in this work are not of a pure maceral group; therefore the results obtained give a wider range than (43) Ofori, P.; O’Brien, G.; Firth, B.; McNally, C.; Nguyen, A. Improved Flotation Recovery via Hydrophobicity Adjustment, ACARP project C16039 final report; ACARP; Brisbane, January 2009.
(42) Shu, X.; Wang, Z.; Xu, J. Fuel 2002, 81, 495–501.
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of the fitting parameters was obtained by the least-squares regression of the experiment data using eq 2, and the results are R¥ = 96.76, θ0 = 39.85 and b = 10.66. These values are related to the operation conditions in the bench scale flotation tests. It can be concluded from Figure 9 that when the coal is hydrophobic with average contact angle over 70, the majority of combustible contents can be floated with recovery greater than 90%. For poor floating coal, the combustible recovery can only reach around 50% at contact angle less than 40. For intermediate coal, the combustible recovery increases monotonously with coal hydrophobicity (contact angle). Although the amount of mineral matter in coal also has a significant effect on flotation result, similar relationship for the recovery versus ash content was not observed for the coals studied as shown in Figure 9. For example, it can be found that the coal with lower contact angle and lower flotation recovery contained less mineral matters in the feed than the coal with higher contact angle and higher recovery. Therefore, flotation recovery is better correlated with coal hydrophobicity than the ash content. The reason is that ash content is a bulk property, whereas contact angle represents the surface characteristic. In practice, coal blending is used to obtain the product with targeted specifications, therefore, further work on the quantitative representation of coal particle brightness, hydrophobicity and flotation performance is needed for the improved prediction of coal flotation performance.
Figure 9. Relationship between advancing angle and combustible recovery for eight different coals. b experimental data (the error bar showing the standard deviation of the contact angle measurement), 0 ash content, - fitting result.
the previous study. A recent study measuring the contact angle on different maceral groups produced advancing contact angles on vitrinite, inertinite, and liptinite in the range of 80-90.43 The high value of contact angle may be due to the sessile drop method used. It has been found that sessile drop method produced higher contact angle values than captive bubble method.9,14 3.3. Chemical Compositions from XPS Analysis. The XPS analysis of bright and dull coals was conducted and the results are displayed in Figures 7 and 8, respectively. The survey spectra show that bright coal has higher carbon and lower oxygen content than the dull coal. Although both coals contain some mineral matter in the form of aluminosilicate clays, the dull sample has much higher mineral content than the bright one as shown by the higher Al and Si contents. The high resolution C1s spectra show that bright coal has higher aliphatic groups than the dull coal, while the latter should be more hydrophilic due to more oxygen-bearing functional groups, such as carbonyl and carboxyl groups on the surface. The quantitative XPS results further verifies the conclusion that bright coals are more hydrophobic than dull coals, and the reason is that bright coal surfaces have a higher percentage of hydrophobic functional groups, but less hydrophilic groups and mineral matter, than the dull coal surfaces. This is consistent with the result from the petrography analysis. 3.4. Effect of Hydrophobicity on Flotation. The flotation results of eight different coals are presented in Figure 9, showing the combustible recovery versus the average of advancing contact angles of bright coal samples selected from each coal category. The ash content of the feed material is also displayed for each coal. For all the coals studied, the majority of the samples were bright and hydrophobic, they were expected to float well. Because the dull coals are hydrophilic, they were expected to remain in the tailings after flotation. The figure shows that the recovery increased with contact angle (hydrophobicity) and their relationship can be adequately described by a sigmoidal equation given by ð2Þ R ¼ R¥ =f1 þ exp½ -ðθ -θ0 Þ=bg
4. Conclusions Coal hydrophobicity has a significant effect on its flotation. By selecting both bright and dull samples from the bulk and measuring their water contact angle, we would be able to compare the hydrophobicity of different coals, and further to predict their performance in bench scale flotation test. The results obtained from this study suggest that when performing contact angle measurements on bituminous coal surfaces, it may not be necessary to prepare the surfaces by grinding and polishing, to simplify the sample preparation process and avoid surface contamination. The brightness of coal surface can be measured by specular reflectance, and it is related to the morphology and the maceral compositions of the surface. The difference in coal surface chemistry is caused by oxidation, different maceral groups, and the inclusion of mineral matter. It is found that dull coals are more heterogeneous than bright coals in terms of surface composition, and containing more hydrophilic functional groups on their surfaces. The existence of different maceral groups resulted in the differences on both brightness and hydrophobicity. Vitrinite is brighter than inertinite, and more hydrophobic than the latter. Acknowledgment. Financial support from BHP Billiton Mitsubishi Alliance (BMA) and Australia Research Council (ARC) through the ARC Linkage scheme is gratefully acknowledged. The author also acknowledges the discussion with and help on sample crushing and flotation test from Dr. Daniel Fornasiero and Mr. Keith Quast.
where R and θ is the combustible recovery and contact angle, respectively. The fitting parameters are R¥ of the maximum combustible recovery, θ0 of the contact angle at the inflection point and b standing for the slope of the function. The value
Note Added after ASAP Publication. Reference 39 was modified in the version of this paper published ASAP October 27, 2009; the correct version published ASAP November 19, 2009.
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