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Experimental study on synergistic wetting of a coal dust with dust suppressant compounded with noncationic surfactants and its mechanism analysis
Powder Technology 356 (2019) 1077e1086
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental study on synergistic wetting of a coal dust with dust suppressant compounded with noncationic surfactants and its mechanism analysis Guo-Qing Shi a, b, *, Cong Han a, Yan-ming Wang a, He-Tang Wang a, * a b
College of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2019 Received in revised form 10 September 2019 Accepted 14 September 2019 Available online 16 September 2019
To improve the ability of water spray additives to wet coal dust, the compounding wetting characteristics of several nonionic and anionic surfactants on bituminous coal dust were studied, the surface tension, contact angle and the settling time of coal dust were measured. Results showed that the compounding of sodium dodecyl sulfate (SDS) and two other anionic surfactants exhibits an antagonism effect and reduces the wettability of water. The compounding of primary alcobol ethoxylate (JFC), ethylene glycol polyoxyethylene ether (PEG800) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) achieves the strongest synergistic effect at a mass ratio of 3:1:1. Furthermore, the infrared spectra of raw coal and coal treated with dust suppressant showed that the hydrophilicity of coal dust is improved after the treatment by dust suppressant. Finally, the synergistic effect mechanism of JFC, PEG800 and SDBS was explained from the perspective of molecular and hydrogen bond. © 2019 Published by Elsevier B.V.
Keywords: Coal dust Surfactant Wetting ability FTIR Functional group
1. Introduction In the coal mining industry, coal dust is an important source of danger for coal worker’s pneumoconiosis (CWP). Compared with production accidents, CWP caused by coal dust is more common in major coal-producing countries in the world [1e3]. From 2005 to 2015, the incidence of CWP in the eastern United States was 2.4% [4]; 7.3% of a sample of 248 coal workers in Australia had CWP [5]; the incidence of CWP in China was high and on the rise [6]. CWP is an irreversible illness of high incidence [7e9]. With the continuous improvement of mechanized mining techniques, coal dust is produced rather intensely in underground working face and is becoming an increasingly serious problem [10]. Therefore, dust control is of great significance to the coal industry. Hydraulic methods, such as water spray, coal seam water injection and chemical dust suppression [11e16], are the main way to reduce dust during coal production at present [17,18]. Compared with coal seam water injection and chemical dust suppression, water spray is a dust-reducing method which boasts simple layout,
* Corresponding authors at: College of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail addresses: E-mail address: [email protected] (G.-Q. Shi). [email protected] (H.-T. Wang). https://doi.org/10.1016/j.powtec.2019.09.040 0032-5910/© 2019 Published by Elsevier B.V.
low cost and wide scope of use. However, the wetting degree of coal dust is low due to the excessive surface tension of water and the hydrophobic nature of coal surface. In recent years, the method of adding surfactant in water has been adopted to reduce the surface tension of water and improve the atomization quality and wettability of water [19e23]. While surfactant can significantly reduce the surface tension of water, the effect of surface tension reduction is related to the concentration of surfactant. The surface tension of solution will not be lower than the critical surface tension of coal wetting until the concentration reaches a certain value. Hence, the cost of use is relatively high [24,25]. Theoretical analysis and experimental research on improving the dust-wetting ability of surfactants have been extensively carried out at home and abroad. Li et al. [26] studied the correlation between the fractal dimension and contact angle of coal dust and selected 0.2% sodium dodecyl sulfate (SDS) as the surfactant to improve the water spray effect. However, the cost of dust reduction by a single surfactant is high. Kilau et al. [24,27] investigated the effects of a variety of sodium and potassium salts of multivalent anions on the dust-wetting performance of sodium di (2ethylhexyl) sulfosuccinate anionic surfactant. Meanwhile, they analyzed the mechanism of electrolyte of multivalent anions in enhancing the dust-wetting ability of anionic surfactant. Kilau and Voltz [24] investigated the dust-wetting model of anionic
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surfactant and polyethylene oxide system, revealing that the double-layer structure of adsorption layer system improved the hydrophilicity of coal surface. By performing surface tension, contact angle and capillary rise tests, Li et al. [28] compared the effects of the compounding of SDS with five additives on the dust-wetting performance and selected 0.10 wt% SDS and 0.05 wt% NaAc as the wetting agents. Nevertheless, the three experimental methods yielded inconsistent results and the study lacked theoretical analysis. The addition of electrolytes and polymers is the primary research direction to enhance the dust-wetting ability of surfactants, but the compounding effects of different types of surfactants on coal dust wetting are rarely reported. In this study, the compounding characteristics of noncationic surfactants for wetting coal dust were experimentally investigated. An efficient dust suppressant compounding scheme was selected based on the parameters of surface tension, sedimentation time and contact angle. Besides, the surface wettability variation of coal sample treated by the surfactant was explored. Finally, according to the experimental results, the mechanism of synergistic wetting of coal dust by nonionic and anionic surfactants was explained from a microscopic perspective. 2. Experiment 2.1. Experimental materials and equipment 2.1.1. Surfactants Surfactants can be classified into ionic and nonionic surfactants according to whether they can be ionized in water, and ionic surfactants can be further classified into anionic and cationic surfactants according to the charge of groups. After the positively charged cationic surfactant group adsorbs to coal surface, its hydrophobic tail faces the solution, because the surface of coal dust is generally negatively charged. As a result, cationic surfactants generally result in poor wettability [29]. Therefore, in this study, several noncationic surfactants with relatively good surface activity were selected for the test of coal dust wetting, as listed in Table 1. The reagents used all reached the grade of analytical reagent. Tap water was adopted as the experimental water to simulate the actual situation of water spray in coal mines. 2.1.2. Coal sample The experimental coal sample was bituminous coal from Shicao Village, Ningxia, China. The results of industrial analysis are given in Table 2. According to Chinese Classification of Coals (GB/T 57512009) [30], the coal sample belongs to non-caking coal with a low metamorphic degree. After the coal sample was ground by a ball mill, the experimental coal powder (particle size: smaller than 74 mm) was screened out by 200 mesh sieve. Coal tablets for contact angle test were prepared by a powder tablet compressing machine (FY-24, Strong Lean Technology Development Co. Ltd., Tianjin, China). Specifically, first, 300 mg of experimental coal powder was placed in a mold and compressed by the machine with 20 MPa of pressure for 2 min. Next, coal powder on the tablets was blown off with a rubber suction bulb to make the coal surface smooth and flat. The coal sample required for the infrared spectroscopy experiment
Table 2 Proximate analysis of coal sample. Mad (%)
FCad (%)
Vdaf (%)
Ad (%)
11.6
54.66
35.88
4.05
Note: Mad ¼ moisture (air-dry basis); FCad ¼ fixed carbon Vdaf ¼ volatile matter (air-dry basis); Ad ¼ ash (dry basis).
(air-dry
basis);
was first immersed in the dust suppressant solution for 12 h and then washed with water for several times. The washed coal sample was dried in a vacuum drying oven at 40 for 24 h.
2.1.3. Equipment An interfacial shear rheometer (TECLIS Mechanics, Lyon, France) was used to measure surface tensions of liquids. A contact angle measurement instrument (JGW-360B, Chenghui Experimental Instrument Co. Ltd., Hebei, China) was used to measure the contact angle between liquid and coal. The Fourier transform infrared spectroscopy (FTIR) Vertex 80 V (Bruker, Germany) was used to determine the infrared spectra of coal sample. The infrared spectrum analysis adopted a high-precision dynamic calibration Myerson interferometer whose parameters are as follows: The spectrum range was 4000e400 cm1; the resolution was 4 cm1; and each coal sample was scanned 32 times.
2.2. Experimental process and method The selected noncationic surfactants were compounded at the mass ratios of 4:0, 3:1, 2:2, 1:3 and 0:4, respectively. The solution concentration was 0.2 wt%. The surface tension measurement and sedimentation experiment were performed on the compound solutions, and the compound scheme of dust suppressant was put forward based on the results of surface tension and sedimentation time measurements. The surface tensions and contact angles of dust suppressant at different concentrations (0.01 wt%, 0.02 wt%, 0.05 wt%, 0.1 wt% and 0.2 wt%) were measured. Surface tension is a typical parameter to characterize the ability of liquid to wet coal dust. The lower the surface tension is, the stronger the ability of liquid to wet coal dust is [31e34]. However, surface tension cannot reflect the permeation and wetting effect of liquid on coal dust [28]. In fact, the permeation and wetting effect are usually compared through sedimentation experiments [35]. Therefore, in this study, a test tube sedimentation method was designed to compare the wetting effect of surfactant solution on coal dust. The wetting rate of coal dust in solution is compared by measuring the time of coal dust settling to the bottom of solution after entering into solution. The test tube is a standard glass test tube with a diameter of 15 mm and a height of 150 mm. In the experiment, the height of liquid column in the test tube was 10 cm, and the weight of coal powder was 0.2 g; and each group of experiment was performed three times to take the averaged value.
Table 1 Surfactants used in the experiment. Type
Reagent name
Abbreviation
Molecular formula
Anionic surfactant
Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Sodium dodecyl sulfonate Primary alcobol ethoxylate Ethylene glycol polyoxyethylene ether
SDS SDBS SDDS Penetrating agent JFC PEG800
C12H25SO4Na C18H29NaO3S C12H25SO3Na RO(CH2CH2O)5H, R ¼ C7~9 HO(CH2CH2O)nH
Nonionic surfactant
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3. Results and discussion 3.1. Compounding wetting characteristics of surfactants 3.1.1. Surface tension of solution The premise of coal wetting is that the surface tension of liquid is lower than the critical surface tension of coal [31]. The surface tension of pure water (about 72.8 mN/m [36]) is much higher than the critical surface tension of coal wetting (about 45 mN/m [37]), leading to its poor coal dust wetting effect. The synergistic wetting of the surfactants is reflected in the lower surface tension of the compound solution than that of each monomer surfactant solution, while the antagonistic wetting is opposite. Figs. 1e3 display the surface tensions of surfactant compound solutions with different mass ratios. Fig. 1 shows the surface tensions of anionic surfactant compound solutions. The surface tensions of the compound solutions of SDS and other two kinds of anionic surfactants are higher than that of monomer surfactant solutions, indicating an antagonistic effect of the compounding. While the surface tension values of the compound solutions of SDBS and SDDS all fall between the surface tension values of monomer SDBS and SDDS, which indicates that there is neither antagonism nor synergism between SDBS and SDDS. Fig. 2 exhibits the surface tensions of nonionic surfactants JFC and PEG800 compound solutions. Among the JFC and PEG800 compound solutions, the one with JFC: PEG800 ¼ 3: 1 achieves the strongest synergistic effect. In this case, the surface tension of the compound solution is 25.045 mN/m which is 2.36% and 3.91% lower than those of monomer solutions of JFC (25.636 mN/m) and PEG800 (26.025 mN/m) with the same concentration, respectively. The synergistic effect of the compounding is limited. Fig. 3 is the surface tensions of nonionic and anionic surfactant compound solutions. In Fig. 3(a), the synergistic effect is produced when nonionic surfactant JFC is compounded with anionic surfactants SDBS and SDDS at a certain mass ratio. Among the compound solutions, the one with JFC: SDBS ¼ 3: 1 boasts the lowest surface tension 25.209 mN/m which is 1.69% and 33.68% lower than those of monomer solutions of JFC (25.636 mN/m) and SDBS (33.70 mN/ m) with the same concentration, respectively. The effect of compounding is significantly improved compared with that of anionic surfactant SDBS. In Fig. 3(b), the compounding of PEG800 and the three anionic surfactants shows neither antagonism nor synergism.
3.1.2. Speed of coal dust wetting The shorter the sedimentation time of coal dust is, the faster the permeation and the stronger the wettability of surfactant solution
Fig. 1. Surface tensions of anionic surfactant compound solutions.
Fig. 2. Surface tensions of nonionic surfactant compound solutions.
on coal dust are. Figs. 4e6 present the sedimentation time of coal dust in surfactant solutions of different mass ratios. Fig. 4 shows the sedimentation time of coal dust in anionic surfactant compound solutions. The sedimentation time of coal dust in the compound solutions of anionic surfactants are longer than that in the solutions of each monomer surfactant. In Fig. 5, the compounding of two nonionic surfactants shows a weak synergistic effect between mass ratio 4:0 and 2:2, which is similar to the variation trend of surface tension in Fig. 2. In Fig. 6, the sedimentation time of coal dust in the compound solutions of JFC and three anionic surfactants are shorter than that of coal dust in the compound solutions of PEG800 and three anionic surfactants, which is consistent with the determination results of surface tension in Fig. 3, indicating that the compound solutions of JFC and anionic surfactants have stronger wetting ability.
3.2. Scheme of dust suppressant compounding By comparing the surface tension and sedimentation time, the wetting characteristics of the compound solutions of nonionic surfactants and anionic surfactants were studied. The compounding of anionic surfactant SDS and two other kinds of anionic surfactants show antagonistic effect. The compounding of nonionic surfactants exerts weak synergistic effects; the compounding of nonionic surfactant JFC and the selected anionic surfactants produce significant synergistic wetting effects; the compounding of PEG800 and selected anionic surfactants have no effect on the wettability of coal dust. The compounding characteristics of surfactants provide a basis for the preparation of water spray dust suppressants. According to the determination of surface tension and sedimentation time, the dust suppressant was compounded with only one anionic surfactant, namely SDBS, in this study. The compound scheme is given in Table 3. Fig. 7 exhibits fitting curves of the relationship between wetting parameters and concentrations of three dust suppressant solutions. Fig. 7(a) illustrates fitting curves of the relationship between surface tensions and concentrations of solutions. The surface tension of the experimental tap water is 69.853 mN/m, and those of the three dust suppressant solutions with the concentration of 0.01 wt % are all lower than 30.6 mN/m which is about 128.28% lower than that of tap water and 14 mN/m lower than the critical surface tension of coal wetting. This suggests that dust suppressant solutions with this concentration can effectively wet coal dust. In the low-concentration range (0.01 wt%-0.05 wt%), the surface tensions of dust suppressants A, B and C decrease rapidly. In the high-
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Fig. 3. Surface tensions of nonionic and anionic surfactant compound solutions.
Fig. 4. Sedimentation time of coal dust in anionic surfactant compound solutions.
solution interface increases with the rise of surfactant concentration, and the surface tension decreases rapidly accordingly. After the adsorption layer is saturated, the surface tension of solution reaches an equilibrium state and hardly decreases any more. Among the three dust suppressants, the dust suppressant C is the first to reach the state of surface tension equilibrium, and its CMC is about 0.05 wt%. Fig. 7(b) presents fitting curves of the relationship between contact angles and solution concentrations of solutions, and Fig. 8 is comparison of the measured contact angles. As can be seen from Figs. 7(b) and 8, the contact angle between water and coal is 60.45 , while contact angles between the three dust suppressant solutions with the concentration of 0.01 wt% are all smaller than 33 which is 83.18% lower than that between water and coal. This demonstrates that the compounding obviously improve the wetting effect. In the low-concentration range (0.01 wt%-0.05 wt%), the contact angles between the three dust suppressant solutions and coal decrease rapidly. Among them, the contact angle between the dust suppressant C and coal drops the fastest from 32.6 to 19.23 by 69.53%. In the high-concentration range (0.05 wt%-0.2 wt%), the contact angles change little. The dust suppressant C was selected as the water spray additive according to the two wetting parameters, namely surface tension and contact angle, and the concentration was determined as 0.05 wt% in consideration of use cost. In order to further study the effect of dust suppressant treatment on the hydrophilic and hydrophobic properties of coal surface, the changes of functional groups of raw coal and coal treated by dust suppressant C (hereafter referred to as treated coal) were analyzed by infrared spectroscopy.
3.3. Effect of dust suppressant treatment on the structure of groups on coal surface
Fig. 5. Sedimentation time of coal dust in nonionic surfactant compound solutions.
concentration range (0.05 wt%e0.2 wt%), the surface tensions change little, almost no longer falling. The phenomenon is related to the critical micelle concentration (CMC) of surfactants. That is, in the low-concentration range, the density of adsorption layer at the
The wettability of coal mainly depends on its hydrophobic and hydrophilic functional groups, of which the former includes aliphatic groups such as CeC and CeH and aromatic groups such as C¼C and ¼CeH while the latter includes oxygen-containing functional groups such as CeO, C]O, OeCeO and OeH [38]. The information on functional groups of raw and treated coal samples was analyzed by FTIR, and their wettability can be compared based on the changes of carbon and oxygen groups on coal surface [39e41] by using the peak area method. Fig. 9 displays the infrared spectra of raw coal and treated coal samples. In infrared spectra, the absorption peak shape and absorbance of raw coal and treated coal
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Fig. 6. Sedimentation time of coal dust in anionic and nonionic surfactant compound solutions.
Table 3 Compounding scheme of dust suppressants. Scheme
Dust suppressant A
Dust suppressant B
Dust suppressant C
Mass ratio
JFC:SDBS ¼ 3:1
JFC:PEG800 ¼ 3:1
JFC:SDBS:PEG800 ¼ 3:1:1
Fig. 7. Relationship between wetting parameters and concentrations of dust suppressants.
samples are related to their basic structures. For example, 3000e2850 cm1 is the stretching vibration region of eCH3/eCH2 where 2870 cm1 is the characteristic absorption peak of symmetrical stretching vibration of eCH3; 1067 cm1 and 1062 cm1 are the characteristic absorption peaks of vibration of ether bonds of raw coal and treated coal samples, respectively. The infrared spectrum of raw coal is divided into four regions according to the main absorption vibration regions of hydrophilic functional groups and hydrophobic functional groups. Among the four regions, the regions of hydrophilic functional groups mainly include 3800e2960 cm1 (Region I) and 1800e1650 cm1 (Region II), while the regions of hydrophobic functional groups mainly include 1600e1300 cm1 (Region III) and 900e650 cm1 (Region IV). For the treated coal sample, the regions of hydrophilic functional groups are 3800e2900 cm1 (Region I0 ) and
1800e1700 cm1 (Region II’), while the regions of hydrophobic functional groups are 1600e1300 cm1 (Region III’) and 974e646 cm1 (Region IV’). The area of absorption peak in each region was calculated by using the peak fitting method, while the curve fitting was conducted with the aid of Origin 9.0 software. Figs. 10 and 11 show the peak fitting results of absorption peaks of carbon and oxygen groups in the infrared spectra of two samples. Table 4 lists the percentage of group structure in the fitting regions. The fitting regions I and I0 belong to the stretching vibration of OeH, of which the vicinity of 3100e2900 cm1 is a stretching vibration region of OeH containing hydrogen bonds. The fitting regions II and II’ are mainly stretching vibration of carbonyl groups (C¼O), of which 1650 cm1 is the characteristic absorption peak of stretching vibration of carbonyl (C¼O). The groups in the above four regions are all hydrophilic. The fitting regions III and III’ are mainly the
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Fig. 8. Comparison of contact angles (a) Tap water; (b) 0.01 wt% solutions; (c) 0.05 wt% solutions.
bending vibration regions of aliphatic groups CeC and CeH, and the fitting regions IV and IV’ are mainly the bending vibration regions of aliphatic hydrocarbons and aromatic hydrocarbons. The groups in these four regions are all hydrophobic. Compared with raw coal sample, the treated coal sample experienced the following changes: The proportion of hydroxyl (OeH) drops by about 0.41%; the proportion of carbonyl (C¼O) grows by about 0.52%; the total proportion of oxygen-containing functional groups hydroxyl (OeH) and carbonyl (C¼O) increases by about 0.11%. The increased proportion of oxygen-containing groups indicates that the dust suppressant treatment promotes the hydrophilicity of coal dust. The change of proportion of oxygencontaining functional groups in the treated coal sample may be related to the ether‑oxygen units in the surfactant. In fact, both JFC and PEG800 contain ether‑oxygen units which can form hydrogen
bonds with hydroxyl groups (OeH). As a result, the content of hydroxyl (OeH) in the coal is reduced while the proportion of oxygencontaining functional groups is raised. After the dust suppressant treatment, the total aliphatic and aromatic proportion of coal decreases from (17.48 þ 10.54)% to (16.54 þ 11.37)% by 0.11%. In short, the dust suppressant treatment promotes the wettability of coal dust by raising the proportion of hydrophilic functional groups and reducing that of hydrophobic functional groups.
3.4. Mechanism of synergistic effect produced by noncationic surfactant compounding Some objective facts can be taken as the theoretical basis for the mechanism of synergistic wetting of coal dust by noncationic surfactants.
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Fig. 9. Infrared spectra of raw coal and treated coal.
Fig. 10. Peak Fitting of infrared spectrum of raw coal.
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Fig. 11. Peak Fitting of infrared spectrum of treated coal.
Table 4 Proportions of main functional groups of raw coal and treated coal. Fitting method
Gaussian
Raw coal
Treated coal
Spectrum region
Area
Proportion (%)
Spectrum region
Area
Proportion (%)
I II III IV
10.14 0.526 2.59 1.56
68.43 3.55 17.48 10.54
I0 II0 III0 IV0
9.99 0.598 2.43 1.67
68.02 4.07 16.54 11.37
(1) Due to the hydrophobic effect of surface surfactants, their hydrophilic heads are inserted into water while the hydrophobic tails face air. In this way, an oriented adsorption layer which isolates the contact between air and water and reduces the surface tension of water is formed at the air-water interface. (2) Anionic surfactants can be dissociated in water, and their active groups are negatively charged. Nonionic surfactants cannot be dissociated in water, and their molecular groups are electrically neutral.
3.4.1. Adsorption layer at the water-air interface Electrostatic repulsion exists between ionic groups in the
adsorption layer of anionic surfactant SDBS, so that the distance between groups is relatively large. Accordingly, the low density of adsorption layer leads to the high surface tension of water, as shown in Fig. 12(a). The nonionic surfactants JFC and PEG800 are both polyethylene oxides. Since polyethylene oxides have a strong hydrogen-bonding affinity with water and their hydrogen-bonding force is greater than the ion adsorption, the molecular groups of JFC and PEG800 can fill the gap in the adsorption layer of SDBS ionic groups [42]. Besides, the electrically neutral molecules of JFC and PEG800 weaken the electrostatic repulsion between SDBS ions. Thus, the adsorption layer at the water-air interface becomes denser, and the surface tension of solution is further reduced, as illustrated in Fig. 12(b).
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Fig. 12. Synergistic effect of noncationic surfactant compounding at the water-air interface.
Fig. 13. Synergistic effect of noncationic surfactant compounding at the solid-liquid interface.
3.4.2. Adsorption layer at the solid-liquid interface Coal is a kind of hydrophobic substance with a large number of hydrophobic sites and a small number of hydrophilic sites on the surface. The adsorption of hydrophobic tails of surfactants at the hydrophobic sites of coal can make the coal surface more hydrophilic, but the adsorption of hydrophilic heads on the hydrophilic sites of coal can also cause wettability loss [12], as presented in Fig. 13(a). In the nonionic and anionic surfactant compound solution, the molecular groups of nonionic surfactants, depending on the hydrophobic interaction of their hydrophobic tails, gather on the hydrophobic tails of anionic surfactants, which multiplies the area of hydrophilic layer on coal surface. Meanwhile, the hydrophobic tails of nonionic groups bind to the hydrophobic tails of anionic groups at the hydrophilic sites, which reduces the wettability loss of anionic surfactant, as shown in Fig. 13(b). The compounding of JFC, PEG800 and SDBS improves the hydrophilicity of coal surface, so that coal dust is wetted more easily.
4. Conclusions (1) The results of the surfactants compounding experiment show that the anionic surfactant SDS and two other anionic surfactants have antagonistic effect on the wetting of Shicao Village bituminous coal. The compounding of nonionic surfactants JFC and PEG800 exert weak synergistic effects; the compounding of nonionic surfactant JFC and anionic surfactant SDBS produces a significant synergistic effect, and the compound solutions achieve notably promoted wetting effects compared with the anionic monomer SDBS solution;
the compounding of PEG800 and anionic surfactants has no effect on the wetting of coal dust. (2) According to the compounding characteristics of noncationic surfactants, three schemes of dust suppressant compounding were proposed. The results of surface tension and contact angle measurements reveal that the dust suppressant C boasts the best wetting effect on coal dust. Compared with tap water, the surface tension of dust suppressant C solution with the concentration of 0.05 wt% decreases from 69.853 mN/m to 25.679 mN/m by about 172% (44.174 mN/m), and its contact angle drops from 60.45 to 19.23 by about 214.35% (41.22), indicating its enhanced ability to wet coal dust. The treatment by the dust suppressant C promotes the hydrophilicity of coal by increasing the proportion of hydrophilic functional groups and decreasing that of hydrophobic functional groups. (3) The wetting synergistic effect produced by the compounding of nonionic and anionic surfactants is related to hydrogen bonds and hydrophobic interaction. At the water-air interface, the ether‑oxygen units in nonionic surfactants JFC and PEG800 form hydrogen bonds with water molecules and fill the gaps in the SDBS ion adsorption layer by hydrogenbonding force, which reduces the surface tension of water. At the solid-liquid interface, the hydrophobic tails of JFC and PEG800 bind with the hydrophobic tails of SDBS, which improves the hydrophilicity of coal surface. Acknowledgements This work was supported by the China National Key R&D
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Program (Grant No. 2018YFC0808100), National Natural Science Foundation of China (Grand No. 51774274, 51874290), National Science Foundation for Distinguished Young Scholars of China (Grand NO. 51825402) and the Fundamental Research Funds for the Central Universities (2017XKQY026). References [1] G. Zhou, Y. Ma, T. Fan, G. Wang, Preparation and characteristics of a multifunctional dust suppressant with agglomeration and wettability performance used in coal mine, Chem. Eng. Res. Des. 132 (2018) 729e742, https://doi.org/ 10.1016/j.cherd.2018.02.021. [2] H. Chen, Q. Feng, R. Long, H. Qi, Focusing on coal miners’ occupational disease issues: a comparative analysis between China and the United States, Saf. Sci. 51 (2013) 217e222, https://doi.org/10.1016/J.SSCI.2012.06.025. [3] J. Mo, L. Wang, W. Au, M. Su, Prevalence of coal workers’ pneumoconiosis in China: a systematic analysis of 2001-2011 studies, Int. J. Hyg. Environ. Health 217 (2014) 46e51, https://doi.org/10.1016/j.ijheh.2013.03.006. [4] L.E. Reynolds, D.J. Blackley, A.S. Laney, C.N. Halldin, Respiratory morbidity among U.S. coal miners in states outside of central Appalachia, Am. J. Ind. Med. 60 (2017) 513e517, https://doi.org/10.1002/ajim.22727. [5] D.C. Glass, R. Cohen, M. Roberts, K. Almberg, R. Hoy, L. Go, M.R. Sim, 1497 Review of the Respiratory Component of the Queensland Coal Mine Workers’ Health Scheme, 2018, https://doi.org/10.1136/oemed-2018ICOHabstracts.1249. [6] L. Han, Q. Gao, J. Yang, Q. Wu, B. Zhu, H. Zhang, B. Ding, C. Ni, Survival analysis of coal workers’ pneumoconiosis (CWP) patients in a state-owned mine in the east of China from 1963 to 2014, Int. J. Environ. Res. Public Health 14 (2017), https://doi.org/10.3390/ijerph14050489. [7] M.H. Ross, J. Murray, Occupational respiratory disease in mining, Occup. Med. (Chic. Ill.) 54 (2004) 304e310, https://doi.org/10.1093/occmed/kqh073. [8] D.E. Pollock, J.D. Potts, G.J. Joy, Investigation into dust exposures and mining practices in mines in the southern Appalachian region, Min. Eng. 62 (2010) 44e49. [9] F.N. Ayoglu, B. Acikgoz, E. Tutkun, S. Gebedek, Descriptive characteristics of coal workers’ pneumoconiosis cases in Turkey, Iran. J. Public Health 43 (2014) 389e390. [10] N.B. Hall, D.J. Blackley, C.N. Halldin, A.S. Laney, Continued increase in prevalence of r-type opacities among underground coal miners in the USA, Occup. Environ. Med. 76 (2019) 479e481, https://doi.org/10.1136/oemed-2019105691. [11] J. Kim, J.C. Tien, Effect of operating parameters of a liquid spray system on coal dust suppression, CIM Bull. 93 (2000) 72e75. [12] P. Cai, W. Nie, D. Chen, S. Yang, Z. Liu, Effect of air flowrate on pollutant dispersion pattern of coal dust particles at fully mechanized mining face based on numerical simulation, Fuel 239 (2019) 623e635, https://doi.org/10.1016/ j.fuel.2018.11.030. [13] Z. Xi, L. Jin, J.Y. Richard Liew, D. Li, Characteristics of foam sol clay for controlling coal dust, Powder Technol. 335 (2018) 401e408, https://doi.org/ 10.1016/j.powtec.2018.05.037. [14] L. Jin, W. Ni, Q. Wang, W. Wei, A penetrative addicitve for water infusion in coia seams, J. Univ. Sci. Technol. Beijing 10 (2003) 16. [15] L. Jin, Z. Jiang, R. Jin, Characteristics of water absorption and the loss of duststicking agent, J. Univ. Sci. Technol. Beijing Miner. Metall. Mater. (Eng. Ed.) 6 (1999) 1e3. [16] S. Dey, Enhancement in hydrophobicity of low rank coal by surfactants d a critical overview, Fuel Process. Technol. 94 (2012) 151e158, https://doi.org/ 10.1016/j.fuproc.2011.10.021. [17] T.F. Tomb, J.E. Emmerling, R.H. Kellner, Collection of airborne coal dust by water spray in a horizontal duct, Am. Ind. Hyg. Assoc. J. 33 (1972) 715e721, https://doi.org/10.1080/0002889728506736. [18] W. Cheng, H. Yu, G. Zhou, W. Nie, The diffusion and pollution mechanisms of airborne dusts in fully-mechanized excavation face at mesoscopic scale based on CFD-DEM, Process. Saf. Environ. Prot. 104 (2016), https://doi.org/10.1016/ j.psep.2016.09.004. [19] K. Cybulski, B. Malich, A. Wieczorek, Evaluation of the effectiveness of coal and mine dust wetting, J. Sustain. Min. 14 (2015) 83e92, https://doi.org/10.1016/
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Experimental study on synergistic wetting of a coal dust with dust suppressant compounded with noncationic surfactants and its mechanism analysis Guo-Qing Shi a, b, *, Cong Han a, Yan-ming Wang a, He-Tang Wang a, * a b
College of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2019 Received in revised form 10 September 2019 Accepted 14 September 2019 Available online 16 September 2019
To improve the ability of water spray additives to wet coal dust, the compounding wetting characteristics of several nonionic and anionic surfactants on bituminous coal dust were studied, the surface tension, contact angle and the settling time of coal dust were measured. Results showed that the compounding of sodium dodecyl sulfate (SDS) and two other anionic surfactants exhibits an antagonism effect and reduces the wettability of water. The compounding of primary alcobol ethoxylate (JFC), ethylene glycol polyoxyethylene ether (PEG800) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) achieves the strongest synergistic effect at a mass ratio of 3:1:1. Furthermore, the infrared spectra of raw coal and coal treated with dust suppressant showed that the hydrophilicity of coal dust is improved after the treatment by dust suppressant. Finally, the synergistic effect mechanism of JFC, PEG800 and SDBS was explained from the perspective of molecular and hydrogen bond. © 2019 Published by Elsevier B.V.
Keywords: Coal dust Surfactant Wetting ability FTIR Functional group
1. Introduction In the coal mining industry, coal dust is an important source of danger for coal worker’s pneumoconiosis (CWP). Compared with production accidents, CWP caused by coal dust is more common in major coal-producing countries in the world [1e3]. From 2005 to 2015, the incidence of CWP in the eastern United States was 2.4% [4]; 7.3% of a sample of 248 coal workers in Australia had CWP [5]; the incidence of CWP in China was high and on the rise [6]. CWP is an irreversible illness of high incidence [7e9]. With the continuous improvement of mechanized mining techniques, coal dust is produced rather intensely in underground working face and is becoming an increasingly serious problem [10]. Therefore, dust control is of great significance to the coal industry. Hydraulic methods, such as water spray, coal seam water injection and chemical dust suppression [11e16], are the main way to reduce dust during coal production at present [17,18]. Compared with coal seam water injection and chemical dust suppression, water spray is a dust-reducing method which boasts simple layout,
* Corresponding authors at: College of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail addresses: E-mail address: [email protected] (G.-Q. Shi). [email protected] (H.-T. Wang). https://doi.org/10.1016/j.powtec.2019.09.040 0032-5910/© 2019 Published by Elsevier B.V.
low cost and wide scope of use. However, the wetting degree of coal dust is low due to the excessive surface tension of water and the hydrophobic nature of coal surface. In recent years, the method of adding surfactant in water has been adopted to reduce the surface tension of water and improve the atomization quality and wettability of water [19e23]. While surfactant can significantly reduce the surface tension of water, the effect of surface tension reduction is related to the concentration of surfactant. The surface tension of solution will not be lower than the critical surface tension of coal wetting until the concentration reaches a certain value. Hence, the cost of use is relatively high [24,25]. Theoretical analysis and experimental research on improving the dust-wetting ability of surfactants have been extensively carried out at home and abroad. Li et al. [26] studied the correlation between the fractal dimension and contact angle of coal dust and selected 0.2% sodium dodecyl sulfate (SDS) as the surfactant to improve the water spray effect. However, the cost of dust reduction by a single surfactant is high. Kilau et al. [24,27] investigated the effects of a variety of sodium and potassium salts of multivalent anions on the dust-wetting performance of sodium di (2ethylhexyl) sulfosuccinate anionic surfactant. Meanwhile, they analyzed the mechanism of electrolyte of multivalent anions in enhancing the dust-wetting ability of anionic surfactant. Kilau and Voltz [24] investigated the dust-wetting model of anionic
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surfactant and polyethylene oxide system, revealing that the double-layer structure of adsorption layer system improved the hydrophilicity of coal surface. By performing surface tension, contact angle and capillary rise tests, Li et al. [28] compared the effects of the compounding of SDS with five additives on the dust-wetting performance and selected 0.10 wt% SDS and 0.05 wt% NaAc as the wetting agents. Nevertheless, the three experimental methods yielded inconsistent results and the study lacked theoretical analysis. The addition of electrolytes and polymers is the primary research direction to enhance the dust-wetting ability of surfactants, but the compounding effects of different types of surfactants on coal dust wetting are rarely reported. In this study, the compounding characteristics of noncationic surfactants for wetting coal dust were experimentally investigated. An efficient dust suppressant compounding scheme was selected based on the parameters of surface tension, sedimentation time and contact angle. Besides, the surface wettability variation of coal sample treated by the surfactant was explored. Finally, according to the experimental results, the mechanism of synergistic wetting of coal dust by nonionic and anionic surfactants was explained from a microscopic perspective. 2. Experiment 2.1. Experimental materials and equipment 2.1.1. Surfactants Surfactants can be classified into ionic and nonionic surfactants according to whether they can be ionized in water, and ionic surfactants can be further classified into anionic and cationic surfactants according to the charge of groups. After the positively charged cationic surfactant group adsorbs to coal surface, its hydrophobic tail faces the solution, because the surface of coal dust is generally negatively charged. As a result, cationic surfactants generally result in poor wettability [29]. Therefore, in this study, several noncationic surfactants with relatively good surface activity were selected for the test of coal dust wetting, as listed in Table 1. The reagents used all reached the grade of analytical reagent. Tap water was adopted as the experimental water to simulate the actual situation of water spray in coal mines. 2.1.2. Coal sample The experimental coal sample was bituminous coal from Shicao Village, Ningxia, China. The results of industrial analysis are given in Table 2. According to Chinese Classification of Coals (GB/T 57512009) [30], the coal sample belongs to non-caking coal with a low metamorphic degree. After the coal sample was ground by a ball mill, the experimental coal powder (particle size: smaller than 74 mm) was screened out by 200 mesh sieve. Coal tablets for contact angle test were prepared by a powder tablet compressing machine (FY-24, Strong Lean Technology Development Co. Ltd., Tianjin, China). Specifically, first, 300 mg of experimental coal powder was placed in a mold and compressed by the machine with 20 MPa of pressure for 2 min. Next, coal powder on the tablets was blown off with a rubber suction bulb to make the coal surface smooth and flat. The coal sample required for the infrared spectroscopy experiment
Table 2 Proximate analysis of coal sample. Mad (%)
FCad (%)
Vdaf (%)
Ad (%)
11.6
54.66
35.88
4.05
Note: Mad ¼ moisture (air-dry basis); FCad ¼ fixed carbon Vdaf ¼ volatile matter (air-dry basis); Ad ¼ ash (dry basis).
(air-dry
basis);
was first immersed in the dust suppressant solution for 12 h and then washed with water for several times. The washed coal sample was dried in a vacuum drying oven at 40 for 24 h.
2.1.3. Equipment An interfacial shear rheometer (TECLIS Mechanics, Lyon, France) was used to measure surface tensions of liquids. A contact angle measurement instrument (JGW-360B, Chenghui Experimental Instrument Co. Ltd., Hebei, China) was used to measure the contact angle between liquid and coal. The Fourier transform infrared spectroscopy (FTIR) Vertex 80 V (Bruker, Germany) was used to determine the infrared spectra of coal sample. The infrared spectrum analysis adopted a high-precision dynamic calibration Myerson interferometer whose parameters are as follows: The spectrum range was 4000e400 cm1; the resolution was 4 cm1; and each coal sample was scanned 32 times.
2.2. Experimental process and method The selected noncationic surfactants were compounded at the mass ratios of 4:0, 3:1, 2:2, 1:3 and 0:4, respectively. The solution concentration was 0.2 wt%. The surface tension measurement and sedimentation experiment were performed on the compound solutions, and the compound scheme of dust suppressant was put forward based on the results of surface tension and sedimentation time measurements. The surface tensions and contact angles of dust suppressant at different concentrations (0.01 wt%, 0.02 wt%, 0.05 wt%, 0.1 wt% and 0.2 wt%) were measured. Surface tension is a typical parameter to characterize the ability of liquid to wet coal dust. The lower the surface tension is, the stronger the ability of liquid to wet coal dust is [31e34]. However, surface tension cannot reflect the permeation and wetting effect of liquid on coal dust [28]. In fact, the permeation and wetting effect are usually compared through sedimentation experiments [35]. Therefore, in this study, a test tube sedimentation method was designed to compare the wetting effect of surfactant solution on coal dust. The wetting rate of coal dust in solution is compared by measuring the time of coal dust settling to the bottom of solution after entering into solution. The test tube is a standard glass test tube with a diameter of 15 mm and a height of 150 mm. In the experiment, the height of liquid column in the test tube was 10 cm, and the weight of coal powder was 0.2 g; and each group of experiment was performed three times to take the averaged value.
Table 1 Surfactants used in the experiment. Type
Reagent name
Abbreviation
Molecular formula
Anionic surfactant
Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Sodium dodecyl sulfonate Primary alcobol ethoxylate Ethylene glycol polyoxyethylene ether
SDS SDBS SDDS Penetrating agent JFC PEG800
C12H25SO4Na C18H29NaO3S C12H25SO3Na RO(CH2CH2O)5H, R ¼ C7~9 HO(CH2CH2O)nH
Nonionic surfactant
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3. Results and discussion 3.1. Compounding wetting characteristics of surfactants 3.1.1. Surface tension of solution The premise of coal wetting is that the surface tension of liquid is lower than the critical surface tension of coal [31]. The surface tension of pure water (about 72.8 mN/m [36]) is much higher than the critical surface tension of coal wetting (about 45 mN/m [37]), leading to its poor coal dust wetting effect. The synergistic wetting of the surfactants is reflected in the lower surface tension of the compound solution than that of each monomer surfactant solution, while the antagonistic wetting is opposite. Figs. 1e3 display the surface tensions of surfactant compound solutions with different mass ratios. Fig. 1 shows the surface tensions of anionic surfactant compound solutions. The surface tensions of the compound solutions of SDS and other two kinds of anionic surfactants are higher than that of monomer surfactant solutions, indicating an antagonistic effect of the compounding. While the surface tension values of the compound solutions of SDBS and SDDS all fall between the surface tension values of monomer SDBS and SDDS, which indicates that there is neither antagonism nor synergism between SDBS and SDDS. Fig. 2 exhibits the surface tensions of nonionic surfactants JFC and PEG800 compound solutions. Among the JFC and PEG800 compound solutions, the one with JFC: PEG800 ¼ 3: 1 achieves the strongest synergistic effect. In this case, the surface tension of the compound solution is 25.045 mN/m which is 2.36% and 3.91% lower than those of monomer solutions of JFC (25.636 mN/m) and PEG800 (26.025 mN/m) with the same concentration, respectively. The synergistic effect of the compounding is limited. Fig. 3 is the surface tensions of nonionic and anionic surfactant compound solutions. In Fig. 3(a), the synergistic effect is produced when nonionic surfactant JFC is compounded with anionic surfactants SDBS and SDDS at a certain mass ratio. Among the compound solutions, the one with JFC: SDBS ¼ 3: 1 boasts the lowest surface tension 25.209 mN/m which is 1.69% and 33.68% lower than those of monomer solutions of JFC (25.636 mN/m) and SDBS (33.70 mN/ m) with the same concentration, respectively. The effect of compounding is significantly improved compared with that of anionic surfactant SDBS. In Fig. 3(b), the compounding of PEG800 and the three anionic surfactants shows neither antagonism nor synergism.
3.1.2. Speed of coal dust wetting The shorter the sedimentation time of coal dust is, the faster the permeation and the stronger the wettability of surfactant solution
Fig. 1. Surface tensions of anionic surfactant compound solutions.
Fig. 2. Surface tensions of nonionic surfactant compound solutions.
on coal dust are. Figs. 4e6 present the sedimentation time of coal dust in surfactant solutions of different mass ratios. Fig. 4 shows the sedimentation time of coal dust in anionic surfactant compound solutions. The sedimentation time of coal dust in the compound solutions of anionic surfactants are longer than that in the solutions of each monomer surfactant. In Fig. 5, the compounding of two nonionic surfactants shows a weak synergistic effect between mass ratio 4:0 and 2:2, which is similar to the variation trend of surface tension in Fig. 2. In Fig. 6, the sedimentation time of coal dust in the compound solutions of JFC and three anionic surfactants are shorter than that of coal dust in the compound solutions of PEG800 and three anionic surfactants, which is consistent with the determination results of surface tension in Fig. 3, indicating that the compound solutions of JFC and anionic surfactants have stronger wetting ability.
3.2. Scheme of dust suppressant compounding By comparing the surface tension and sedimentation time, the wetting characteristics of the compound solutions of nonionic surfactants and anionic surfactants were studied. The compounding of anionic surfactant SDS and two other kinds of anionic surfactants show antagonistic effect. The compounding of nonionic surfactants exerts weak synergistic effects; the compounding of nonionic surfactant JFC and the selected anionic surfactants produce significant synergistic wetting effects; the compounding of PEG800 and selected anionic surfactants have no effect on the wettability of coal dust. The compounding characteristics of surfactants provide a basis for the preparation of water spray dust suppressants. According to the determination of surface tension and sedimentation time, the dust suppressant was compounded with only one anionic surfactant, namely SDBS, in this study. The compound scheme is given in Table 3. Fig. 7 exhibits fitting curves of the relationship between wetting parameters and concentrations of three dust suppressant solutions. Fig. 7(a) illustrates fitting curves of the relationship between surface tensions and concentrations of solutions. The surface tension of the experimental tap water is 69.853 mN/m, and those of the three dust suppressant solutions with the concentration of 0.01 wt % are all lower than 30.6 mN/m which is about 128.28% lower than that of tap water and 14 mN/m lower than the critical surface tension of coal wetting. This suggests that dust suppressant solutions with this concentration can effectively wet coal dust. In the low-concentration range (0.01 wt%-0.05 wt%), the surface tensions of dust suppressants A, B and C decrease rapidly. In the high-
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Fig. 3. Surface tensions of nonionic and anionic surfactant compound solutions.
Fig. 4. Sedimentation time of coal dust in anionic surfactant compound solutions.
solution interface increases with the rise of surfactant concentration, and the surface tension decreases rapidly accordingly. After the adsorption layer is saturated, the surface tension of solution reaches an equilibrium state and hardly decreases any more. Among the three dust suppressants, the dust suppressant C is the first to reach the state of surface tension equilibrium, and its CMC is about 0.05 wt%. Fig. 7(b) presents fitting curves of the relationship between contact angles and solution concentrations of solutions, and Fig. 8 is comparison of the measured contact angles. As can be seen from Figs. 7(b) and 8, the contact angle between water and coal is 60.45 , while contact angles between the three dust suppressant solutions with the concentration of 0.01 wt% are all smaller than 33 which is 83.18% lower than that between water and coal. This demonstrates that the compounding obviously improve the wetting effect. In the low-concentration range (0.01 wt%-0.05 wt%), the contact angles between the three dust suppressant solutions and coal decrease rapidly. Among them, the contact angle between the dust suppressant C and coal drops the fastest from 32.6 to 19.23 by 69.53%. In the high-concentration range (0.05 wt%-0.2 wt%), the contact angles change little. The dust suppressant C was selected as the water spray additive according to the two wetting parameters, namely surface tension and contact angle, and the concentration was determined as 0.05 wt% in consideration of use cost. In order to further study the effect of dust suppressant treatment on the hydrophilic and hydrophobic properties of coal surface, the changes of functional groups of raw coal and coal treated by dust suppressant C (hereafter referred to as treated coal) were analyzed by infrared spectroscopy.
3.3. Effect of dust suppressant treatment on the structure of groups on coal surface
Fig. 5. Sedimentation time of coal dust in nonionic surfactant compound solutions.
concentration range (0.05 wt%e0.2 wt%), the surface tensions change little, almost no longer falling. The phenomenon is related to the critical micelle concentration (CMC) of surfactants. That is, in the low-concentration range, the density of adsorption layer at the
The wettability of coal mainly depends on its hydrophobic and hydrophilic functional groups, of which the former includes aliphatic groups such as CeC and CeH and aromatic groups such as C¼C and ¼CeH while the latter includes oxygen-containing functional groups such as CeO, C]O, OeCeO and OeH [38]. The information on functional groups of raw and treated coal samples was analyzed by FTIR, and their wettability can be compared based on the changes of carbon and oxygen groups on coal surface [39e41] by using the peak area method. Fig. 9 displays the infrared spectra of raw coal and treated coal samples. In infrared spectra, the absorption peak shape and absorbance of raw coal and treated coal
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Fig. 6. Sedimentation time of coal dust in anionic and nonionic surfactant compound solutions.
Table 3 Compounding scheme of dust suppressants. Scheme
Dust suppressant A
Dust suppressant B
Dust suppressant C
Mass ratio
JFC:SDBS ¼ 3:1
JFC:PEG800 ¼ 3:1
JFC:SDBS:PEG800 ¼ 3:1:1
Fig. 7. Relationship between wetting parameters and concentrations of dust suppressants.
samples are related to their basic structures. For example, 3000e2850 cm1 is the stretching vibration region of eCH3/eCH2 where 2870 cm1 is the characteristic absorption peak of symmetrical stretching vibration of eCH3; 1067 cm1 and 1062 cm1 are the characteristic absorption peaks of vibration of ether bonds of raw coal and treated coal samples, respectively. The infrared spectrum of raw coal is divided into four regions according to the main absorption vibration regions of hydrophilic functional groups and hydrophobic functional groups. Among the four regions, the regions of hydrophilic functional groups mainly include 3800e2960 cm1 (Region I) and 1800e1650 cm1 (Region II), while the regions of hydrophobic functional groups mainly include 1600e1300 cm1 (Region III) and 900e650 cm1 (Region IV). For the treated coal sample, the regions of hydrophilic functional groups are 3800e2900 cm1 (Region I0 ) and
1800e1700 cm1 (Region II’), while the regions of hydrophobic functional groups are 1600e1300 cm1 (Region III’) and 974e646 cm1 (Region IV’). The area of absorption peak in each region was calculated by using the peak fitting method, while the curve fitting was conducted with the aid of Origin 9.0 software. Figs. 10 and 11 show the peak fitting results of absorption peaks of carbon and oxygen groups in the infrared spectra of two samples. Table 4 lists the percentage of group structure in the fitting regions. The fitting regions I and I0 belong to the stretching vibration of OeH, of which the vicinity of 3100e2900 cm1 is a stretching vibration region of OeH containing hydrogen bonds. The fitting regions II and II’ are mainly stretching vibration of carbonyl groups (C¼O), of which 1650 cm1 is the characteristic absorption peak of stretching vibration of carbonyl (C¼O). The groups in the above four regions are all hydrophilic. The fitting regions III and III’ are mainly the
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Fig. 8. Comparison of contact angles (a) Tap water; (b) 0.01 wt% solutions; (c) 0.05 wt% solutions.
bending vibration regions of aliphatic groups CeC and CeH, and the fitting regions IV and IV’ are mainly the bending vibration regions of aliphatic hydrocarbons and aromatic hydrocarbons. The groups in these four regions are all hydrophobic. Compared with raw coal sample, the treated coal sample experienced the following changes: The proportion of hydroxyl (OeH) drops by about 0.41%; the proportion of carbonyl (C¼O) grows by about 0.52%; the total proportion of oxygen-containing functional groups hydroxyl (OeH) and carbonyl (C¼O) increases by about 0.11%. The increased proportion of oxygen-containing groups indicates that the dust suppressant treatment promotes the hydrophilicity of coal dust. The change of proportion of oxygencontaining functional groups in the treated coal sample may be related to the ether‑oxygen units in the surfactant. In fact, both JFC and PEG800 contain ether‑oxygen units which can form hydrogen
bonds with hydroxyl groups (OeH). As a result, the content of hydroxyl (OeH) in the coal is reduced while the proportion of oxygencontaining functional groups is raised. After the dust suppressant treatment, the total aliphatic and aromatic proportion of coal decreases from (17.48 þ 10.54)% to (16.54 þ 11.37)% by 0.11%. In short, the dust suppressant treatment promotes the wettability of coal dust by raising the proportion of hydrophilic functional groups and reducing that of hydrophobic functional groups.
3.4. Mechanism of synergistic effect produced by noncationic surfactant compounding Some objective facts can be taken as the theoretical basis for the mechanism of synergistic wetting of coal dust by noncationic surfactants.
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Fig. 9. Infrared spectra of raw coal and treated coal.
Fig. 10. Peak Fitting of infrared spectrum of raw coal.
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Fig. 11. Peak Fitting of infrared spectrum of treated coal.
Table 4 Proportions of main functional groups of raw coal and treated coal. Fitting method
Gaussian
Raw coal
Treated coal
Spectrum region
Area
Proportion (%)
Spectrum region
Area
Proportion (%)
I II III IV
10.14 0.526 2.59 1.56
68.43 3.55 17.48 10.54
I0 II0 III0 IV0
9.99 0.598 2.43 1.67
68.02 4.07 16.54 11.37
(1) Due to the hydrophobic effect of surface surfactants, their hydrophilic heads are inserted into water while the hydrophobic tails face air. In this way, an oriented adsorption layer which isolates the contact between air and water and reduces the surface tension of water is formed at the air-water interface. (2) Anionic surfactants can be dissociated in water, and their active groups are negatively charged. Nonionic surfactants cannot be dissociated in water, and their molecular groups are electrically neutral.
3.4.1. Adsorption layer at the water-air interface Electrostatic repulsion exists between ionic groups in the
adsorption layer of anionic surfactant SDBS, so that the distance between groups is relatively large. Accordingly, the low density of adsorption layer leads to the high surface tension of water, as shown in Fig. 12(a). The nonionic surfactants JFC and PEG800 are both polyethylene oxides. Since polyethylene oxides have a strong hydrogen-bonding affinity with water and their hydrogen-bonding force is greater than the ion adsorption, the molecular groups of JFC and PEG800 can fill the gap in the adsorption layer of SDBS ionic groups [42]. Besides, the electrically neutral molecules of JFC and PEG800 weaken the electrostatic repulsion between SDBS ions. Thus, the adsorption layer at the water-air interface becomes denser, and the surface tension of solution is further reduced, as illustrated in Fig. 12(b).
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Fig. 12. Synergistic effect of noncationic surfactant compounding at the water-air interface.
Fig. 13. Synergistic effect of noncationic surfactant compounding at the solid-liquid interface.
3.4.2. Adsorption layer at the solid-liquid interface Coal is a kind of hydrophobic substance with a large number of hydrophobic sites and a small number of hydrophilic sites on the surface. The adsorption of hydrophobic tails of surfactants at the hydrophobic sites of coal can make the coal surface more hydrophilic, but the adsorption of hydrophilic heads on the hydrophilic sites of coal can also cause wettability loss [12], as presented in Fig. 13(a). In the nonionic and anionic surfactant compound solution, the molecular groups of nonionic surfactants, depending on the hydrophobic interaction of their hydrophobic tails, gather on the hydrophobic tails of anionic surfactants, which multiplies the area of hydrophilic layer on coal surface. Meanwhile, the hydrophobic tails of nonionic groups bind to the hydrophobic tails of anionic groups at the hydrophilic sites, which reduces the wettability loss of anionic surfactant, as shown in Fig. 13(b). The compounding of JFC, PEG800 and SDBS improves the hydrophilicity of coal surface, so that coal dust is wetted more easily.
4. Conclusions (1) The results of the surfactants compounding experiment show that the anionic surfactant SDS and two other anionic surfactants have antagonistic effect on the wetting of Shicao Village bituminous coal. The compounding of nonionic surfactants JFC and PEG800 exert weak synergistic effects; the compounding of nonionic surfactant JFC and anionic surfactant SDBS produces a significant synergistic effect, and the compound solutions achieve notably promoted wetting effects compared with the anionic monomer SDBS solution;
the compounding of PEG800 and anionic surfactants has no effect on the wetting of coal dust. (2) According to the compounding characteristics of noncationic surfactants, three schemes of dust suppressant compounding were proposed. The results of surface tension and contact angle measurements reveal that the dust suppressant C boasts the best wetting effect on coal dust. Compared with tap water, the surface tension of dust suppressant C solution with the concentration of 0.05 wt% decreases from 69.853 mN/m to 25.679 mN/m by about 172% (44.174 mN/m), and its contact angle drops from 60.45 to 19.23 by about 214.35% (41.22), indicating its enhanced ability to wet coal dust. The treatment by the dust suppressant C promotes the hydrophilicity of coal by increasing the proportion of hydrophilic functional groups and decreasing that of hydrophobic functional groups. (3) The wetting synergistic effect produced by the compounding of nonionic and anionic surfactants is related to hydrogen bonds and hydrophobic interaction. At the water-air interface, the ether‑oxygen units in nonionic surfactants JFC and PEG800 form hydrogen bonds with water molecules and fill the gaps in the SDBS ion adsorption layer by hydrogenbonding force, which reduces the surface tension of water. At the solid-liquid interface, the hydrophobic tails of JFC and PEG800 bind with the hydrophobic tails of SDBS, which improves the hydrophilicity of coal surface. Acknowledgements This work was supported by the China National Key R&D
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