Surface characteristics and wetting mechanism of respirable coal dust

oline at ww. sciencedirectcomMININGScienceDirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0365-0371ww.elsevier. com/locate/jcumtSurface characteristics and wetting mechanism ofrespirableI dustYANG Jing, wU Xiukun, GAO Jianguang, LI GalpinKey Laboratory of Mine Disaster Prevention and Control, Shandong University of Science and TechnologyQingdao 266510, chinaAbstract: In this study, FTIR and XPS spectroscopy, specific surface area and pore size analyses, wetting and other experimentalmeans have been applied to investigate respirable coal dust surface properties and wetting mechanism. The research shows that coaldust surface structures, chemical and element compositions are changed in the refining process of coal dust. Compared with largeparticle coal dust, respirable coal dust is a weak hydrophilic and strong hydrophobic substance. The reason: the smaller the particlesize of coal dust, the more unsaturated its surface. The absorption of air is strong and it is easy to form an air film on coal dust sur-faces. Coal dust contact with water is actually a compound contact of liquid in contact with a solid and gas in contact with a solidThe smaller the particle size, the larger the pore volume and the higher the degree of surface roughness, the larger the area of gas incontact with a solid and the worse the coal dust wettability. Another reason is that the changes in the composition of atoms on thedust surface and structures affect coal dust wettability. The results of this study establish a theoretical basis for the invention ofKeywords: respirable coal dust; wetting: surface characteristics1 Introductiondust size, chemical composition, chemical structures,surface pore structures and surface wettability of coalRespirable dust is any particle with a diameter less dust surfaces have been studied. Based on these re-than 5 um. When this dust enters the human alveolar, sults, the micro-mechanism of the effect of surfaceit can cause pneumoconiosis. Recent improvements wettability on particle size has been further studiedin coal mining, such as the application of high effi- This mechanism represents the microscopic nature ofciency power boring machines, as well as transporta- the change in respirable coal dust wettability, whichtion and local ventilation equipment, have not only establishes a theoretical basis for the improvement ofincreased the concentration of mineral dust in the the existing dust suppression technology and develtunnel air significantly, but also increased the propor- opment of highly effective dust suppression agents.tion of respirable dust. Although various types of dustremoval technology can effectively reduce the con- 2 Experimentalcentration of large particles, the purification efficiency of respirable dust is quite low. For example 2.1 Preparation of coal dust sampleswith spray dust suppression technology, the respirabledust removal rate is only about 28%.HowFour different amounts of coal lumps Jining,Shandong)were crushed and ground repeatedly toprove the respirable dust removal rate has become an serve as samples for coal analysis. In order to avoidimportant research focus in dust control tech-ash segregation in coal dust samples and to ensure theAlthough there are a number of reports about thetest data can accurately reflect the true characteristicswettability of coals, literature referring to the rulesof this dust, the coal samples were not screened dur-of change and mechanisms of respirable dust wet- ing the grinding process. A laser particle size instru-tability along with the change in particle size is rarement (Beckman Coulter LS 13320, USA)was thenIn our investigation, the relationship between coalused to measure the particle sizes of the coal samples.中国煤化工Received 10 September 2009: accepted II December 2009Corresponding author. Tel: 86 13730996168CNMHG NOE-mailaddresskdyangjing@163.comdoi:I0.lol6s|674-5264(09)60209XDa(um)38.151834929468Vol 20 No. 3As shown in Table 1, the average particle size of was pressed into a thin tablet and dried in a vacuumthe four coal dust samples were 38.15, 18.34, 9.29 drying oven for 12 h. The study was performed usingand 4.68 um respectively. The No 4 coal sample is an ESCALAB 250 made by ThermoFisher SCIENTIFIC2.2 Methods3 Results and discussion2.2.1 Measurement of coal dust contact angle3.1 Pore structure of respirable coal dustAn optical analytical droplet morphology me..1 Adsorption isotherm formcontact angles of coal dust. This optical instrumentAdsorption-desorption isotherms of different sizedcan accurately take images of instantaneous changes particle coal dust are shown in Figs. 1 to 4in solid-liquid contact angles. It can, at most, take 1Although the adsorption isotherm shape of eachframes/s and has various screenshot methods. In ad. coal dust sample is slightly different, with the in-dition, its intelligence data processing function makes crease in relative pressure the shapes all tend to formit an easy-to-use and high-precision instrument with an anti-s curve. This is a typical Type II adsorptionand shows an upward convex shape, indicating thatdetermine the contact angle of water and coal dust by the adsorption passes from monolayer molecular ad-the formed-coal dust method2.2.2 Measurement of specific surface area and po- sorption to multi-layer molecular adsorption. The lastrositypart of all four curves increases rapidly and does notshow the phenomenon of saturation adsorption untilrameters of different sized particles were determined the pressure is close to the saturation point. The oc-sing a SSA-4000 made by bUiLDEr (Beijing)currence of a pore cohesion phenomenon shows that2.2.3 Fourier Transform Infra Red(FTIR)spec-the coal dust contains a certain amount of mesoporesand macropores. The hysteresis loops of the coal dustModel 380 spectrometer within the range of 40t are very similar, almost a type A adsorption hysteresisThe FTIR analyses were carried out on aloops classified in the de boer classification. This fact400 cm". The coal dust samples were mixed with indicates that there are two ends opening cylindrical,KBr(without grinding)and pressed into thin trans- diamond and square pore in the coal dust For differ-ent particle sizes of coal dust, the transition point2.2.4 X-ray Photoelectron Spectroscopy (XPS) from a slow rise to a rapid increase in pressure gener.ally decreased with particle size. This shows that withX-ray photoelectron spectroscopy was the main a decrease in dust particle size, some macropores andmethod used to test surface elements of the dust(with mesopores are destroyed and the average pore di.thickness of 5-10 nm). a small amount of coal dust ameter decreasesNo IRelative pressure (PIPe)Relative pressure(P/Po)12}No.3中国煤化工CNMHG10Fig. 1 Adsorption-desorption isotherms of the different particle size coal dustYANG Jing et alSurface characteristics and wetting mechanism of respirable coal dust3.1.2 Respirable coal dust pore diameter, pore Am, the specific surface area(BJ) increases fromvolume and specific surface area( BJH)433 to 3. 093 m g. with the decrease in particle sizethe average pore radius, pore volume and surface the total pore volume( BJH) of coal dust tends to in-ea of different sized particles are shown in Table 2. crease and the average pore radis decreases. The rea-Table 2 Specific surface area and pore volume ofson for this phenomenon can be analyzed as followsdifferent sized coal dust particlesfirst, in the refining coal process some macroporesare destroyed, forming new mesopores and micro-sample particle size radius(nm)ore surface area Popores. Secondly, some fine blind pores are opduring the refining coal process, leading to an in-o.l38.15crease in the number of mesopores and micropores.No218.3415.500015565The change of surface microstructure and pore struc-13.270015876ture has an important effect on the ability of coal dustNo, 40018279to adsorb and become wetAs Table 2 shows, with a decrease in coal dust size. 3.2 Chemical structure of respirable coal dusthe specific surface area of coal dust increases. WherThe IR spectra of the different size coal dust partthe average particle size decreases from 38. 15 to 4.68 cles are shown in Fig. 2400020004000Wave number400030001000Wave number(cm)Wave number(cm)Fig 2 FTIR spectra of different sized coal dust particlesAs Fig. 2 shows, the coal dust samples have obvi- cm" increases. This fact indicates that some of theus absorption bands at 3404.9-3382.4, 2921.0- hydroxyl groups associated with the hydrogen bands2920.6, 1605. 1-1603.8 and 1444.8-1442, 4 cm. is broken and the number of unsaturated atom groupsThese are attributed to the hydroxyl group absorption increases on the coal dust surface when the size of theand associated with hydrogen bonding, the methyl coal dust particles reduces. On thee otnerhand. theabsorption band(cycloalkanes or aliphatic hydrocar- coal dust size is small with a large specific surfacebons), the aromatic rings of the C=C skeleton vibra- area, and the proportion of surface atoms is large, sotion absorption band and the methylene and methyl the atoms have a big impact on coal dust characteris-absorption band, respectively. Comparing the Ir tics. Since the surface atoms and internal atoms are inspectra of the four types of coal dust, we can see that different environments, the Ir absorption bands arewhen the coal dust size is reduced, the hydroxyl fourgroup absorption band occurring at about 3404. 9 cm中国煤化工is significantly wider and moves towards the lower 3.3CNMHGpirabie coal dust3382 cm band; in addition, the methylene andmethyl absorption band occurring at 1444.8-1442.4XPS spectra of different sized coal dust particlesMining Science and Technology20No.3are shown in Fig. 3. The relative contents of the main sized coal dust particles are shown in Table 3elements(except H)on the surface of the variously600000CIs400004000003000001200100080060040020000010008006004002000Binding energy(ev)BindiFig 3 XPS spectra of different sized coal dust particlesTable 3 Relative content of main elements on surface oelement in coal, amounting to about 74%0, which aredifferent sized coal dust particleslargely found in the coal structure unit of PAHs, inMain elements relative content (%kyl side chains and in bridge bonds. with thegradually refined coal dust, the atoms in the coal willNo I73862.76be exposed to form the surface area, so the proportion19003.15of C elements increases and the proportion of O eleNo. 3ments decreasesIn our study we pursued the statement of carbonand oxygen elements found on coal surface layersAs Fig. 3 and Table 3 show, there are four main Depending on the electronic binding energy of theelements(C, O, Si, Al)on the coal dust surfaces(ex- inner layer, different observations of carbon andcept H). with a decrease in particle size the relative oxygen(different chemical shifts)are made, furtheramount of carbon gradually increases and that of separating and fitting the carbon and oxygen spoxygen gradually decreases. That is because with the trum and obtaining the XPS spectra of Cis and OIsduction in particle size, the specific surface areThe CIs and Ois spectral peak positions, peakand the proportion of surface atoms will increase. and attribution analyses of the atoms are shown inCarbon is relatively the most frequently occurringTable 4Table 4 Corresponding assignment of Cis and O,s peak positions and areasCis photoelectronPeak position Ratio of peak Peak position orcarea(%) (evrsak area Peak position peak area position(%)(%)50452858016.1528913532.1753393284611587≤33.584.8628487159928844中国煤化工3704Carbon atoms exist in C-HCNMHAssignment C-C (including fused ringCarbon atoms exist in Oxygen atoms existC-O-H or C-o-C groups lactone or C-OOH groups C-0-groupsC=O or o-C=O groupsYANG Jing et alSurface characteristics and wetting mechanism of respirable coal dustAs Table 4 shows, with a decrease in particle size, the area of gas in contact with a solid is enlargedthe proportion of carbon atoms found in the form ofC-H or C-C bonds(including a fused ring structure ofAir filmC=C)increases, but the proportion of carbon atomsfound as C-O-H and C-o-c bonds decreases. Theproportion of carbon atoms in lactone or COOH doesnot change significantly with a change in particle size,while the oxygen atoms in these two groups do notCoal dushow any apparent regularity with changes in particleSizeFig 4 Schematic diagram of coal dust adsorption of3. 4 Respirable coal dust wettabilityair molecules and molecular formation of anThe balance contact angles of different sized coaldust particles in contact with water are shown in Ta-Contact angles are often used to measure the wetble 5 as follows. The contact angle increases withbility of solid surfaces. Youngs equation is used todecrease in particle size. Compared with large coal surface energy. But Young's equation is applicableability and stronger hydrophobicityonly to ideal surfaces and does not apply to actualsurfaces. Wenzel proposed that, when a solid surfaceTable 5 Contact angles of different coal dust particlesis rough and chemically homogeneous, the actualCoal sampleNo Icontact angle is larger than the observed contact angleContact angle() 55.9056.9857205888It has been assumed that a liquid can always fillrough surface and form a wetting contact, as shown inFig. 5a. The relationship between the apparent con-Wetting mechanism of respirable coal tact angle Bw and the actual contact angle 0 is as foldustCoS y(rsy -ysL)ysy=ycos8 (1)Compared with large coal dust particles, respirable where 0w is the apparent contact angle; and y, thecoal dust has weaker wettability and stronger hydro- roughness factor. This relationship indicates the ratiophobicity. The mechanism is analyzed as followsof an actual area of rough surface and its projected1)The more unsaturated the respirable coal dustarea.urface, the stronger the air absorption and the easierto form an air film on the coal dust surfaceThe FTIR spectra show that some of the associa-tion hydrogen bonds are broken in coal dust grindingand in the refining process. The number of unsaturated atom groups increases as well as the number ofexposed groups on the coal dust surface. The newlygenerated dangling bonds and unsaturated groupshave high surface potential energy and a very strongFig 5 Schematic diagram of rough surface wettingadsorption capacity, which easily adsorp particlesfrom the air or air molecules to reduce surface energy.As can be seen from Eq (1), the roughness factor isOn the other hand, new surfaces are constantly always greater than 1. This can increase the hydroformed in the refining process of coal dust; the phobicity of a hydrophobic surface and a hydrophilicsmaller the coal dust size, the larger the surface area surface can increase its hydrophilicityand the higher the surface energy, the stronger theHowever, for some very rough or porous surfaces,absorption of air and the easier to form an air film on Eq (1)will lose its physical meaning when ycos<-1coal dust surfaces. This air film weakens the wettabil- Cassie and Baxter proposed the Cassie theory that aity of coal dust and this will result in smaller sized droplet in contact with a rough surface is a compoundcoal dust particles with poor wettability. A schematic contact. Cassie believes that droplets on a hydropho-diagram of liquid wetting on coal dust surfaces is bic surface cannot fill the grooves of rough surfacesshown in Fig 4completely. However, air will be intercepted under2)Coal dust contact with water is actually a com- the dropletenlid-linnid contact is actually apound contact of a liquid in contact with a solid and comp中国煤化工 solid and a gas inas in contact with a solid. Compared with large coal contaldust particles, respirable coal dust particles have a apparCNMHGIg 5bThus,thecomposite contactlarger pore volume and the higher the degree of sur- surface can be calculated by the Cassie-Baxter equaface roughness, the worse the coal dust wettability, if tionMining Science and TechnologVol 20 Np.3cos 8cB=f cos 8,+f2 cos 8(2) 5 Conclusionswhere i is the proportion of the solid surface in acIn this study, the pore structure of coal dusttual contact with the liquid to the total surface within faces, chemical structures and surface compositionsthe diameter of the liquid; f2, the proportion of the have been systematically analyzed. Base on theseliquid in contact with air in the retention holes: 1, the results, the characteristics and mechanisms of respir-contact angle of a liquid with a solid and B2 is the able dust wettability have been further investigatedThe main conclusions are as follows:Because fi+f2=l and 62=180, it follows that1)In the refining process of coal dust, somecosb=fcos日+f2cos180°macropores are destroyed and new mesopores andmicropores are formed, while at the same time some=fcos 8-f2=f, cos 8+fr-1 (3) blink dust surface areas and pore volumes in-ores are opened. With a decrease in particleQ For very rough surfaces fi0 and at this point crease, the average pore size decreases and the sur-6cB→+-1,6CB→+180°face roughness increasesSubstituting the roughness factor into Eq (3), we2) FTIR spectrum analyses show that in the refin-obtain Eq (4)ing process of coal some hydroxylassociatedfwith hydrogen bands are broken and the number ofunsaturated atom groups increasesrface ofAccording to earlier experimental studies, the sur- coal dustce microstructure and pore structure changes con3)XPS spectrum analyses show that with a de-siderably when the size of dust particles changes. crease in coal dust size, carbon on the coal dustGenerally, with a decrease in size, the surface area face increases gradually and oxygen decreases graduand pore volume increases and the average pore size ally. In particular, the proportion of carbon elementsaromatIcTherefore, when water is in contact with coal dust, 2 C-C rings)decreases with an increase in the size ofincreases, leading to an increase in ]cB and the wet- coal dust particlestability of respirable coal dust becomes weak.4)Compared with large coal dust particles, respwith the reduction of coal dust size, the compoble coal dust has weaker wettability and strongersition and structure of surface atoms change. this ishydrophobicityanother important reason why the wettability of coaldust weakensAcknowledgementsThe XPS spectrum analyses show that with a decrease in coal dust size the relative amount of carbonFinancial support for this work, provided by theon coal dust surfaces increases gradually and the Science and Technology Project of the Educationoxygen decreases gradually. In particular, the propor- Department of Shandong Province(NoJO8LC12), istion of the carbon elements in the form of C-h or c-c gratefully acknowledgedbonds (including aromatic rings of C=C) decreaseswith an increase in the size of coal dust particles andReferencesthe proportion of carbon atoms in the form of C-o-Hor C-O-C groups decreases. As is well known, the [1] Lu J Z. The progress of dust control technology inC-H or C-C groups(including aromatic rings of C=C)Chinas coal mine. Coal Science and Technology, 1996.have strong hydrophobicity, so an increase of carbon24(7):1-5.( n chineselements will lead to stronger hydrophilicity of coal[2] Wang W N, Zhang H C, Wang C L. Application of dustuppression by spraying technology treat coal dust. Endust. Oxygen elements are largely found in-C-0-andergy Environment Protection, 2007, 21(3): 47-49.(Inc=O groups on coal surfaces. C-O-H is a strong polgroup and has strong hydrophilicity; the oxygen ele- 3] Ma s P, Kou z M. 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