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Synergistic effect of surfactant compounding on improving dust suppression in a coal mine in Erdos, China
Powder Technology 344 (2019) 561–569
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Review
Synergistic effect of surfactant compounding on improving dust suppression in a coal mine in Erdos, China Xiaonan Wang a,b, Shujie Yuan a,b,⁎, Xiang Li c, Bingyou Jiang a,b a b c
Key Laboratory of Safe and Effective Coal Mining, Ministry of Education, Anhui University of Science and Technology, Huainan, Anhui 232001, China School of Energy and Safety, Anhui University of Science and Technology, Huainan, Anhui 232001, China School of Civil and Architecture, Anhui University of Science and Technology, Huainan, Anhui 232001, China
a r t i c l e
i n f o
Article history: Received 28 September 2018 Received in revised form 5 December 2018 Accepted 12 December 2018 Available online 13 December 2018 Keywords: Coal dust control Optimal compound surfactants Wettability improvement
a b s t r a c t To effectively improve the ability of water for coal dust suppression using in a coal mine in Erdos, China, surfactant compounding was proposed because of the synergy between surfactants. First, four surfactants with good coal dust wetting ability were selected from a total of 15 surfactants through surface tension, contact angle, and coal dust sedimentation experiments. Next, a compound surfactant was produced by mixing two surfactants in equal volume. The critical micelle concentration (CMC) of surfactants was selected as the concentration of the compound solution. The four surfactants were mixed into six solutions. Then, the wettability of the compound solution was evaluated via surface tension, contact angle, and coal dust sedimentation experiments. The optimal compound solution was selected as an additive to dust suppression by sprinkling in field research in the coal mine. The results revealed that the compound surfactant solution containing 0.025 wt% FMES and 0.025 wt% CDEA was optimal for wetting coal dust, decreasing the surface tension and contact angle by 59.39% (i.e. to 29.23 mN/m) and 83.57% (i.e. to 9.23°), respectively, compared with the respective values of untreated water. Field tests proved that the dust suppression efficiency of the solution containing 0.025 wt% FMES and 0.025 wt % CDEA was N87%, considerably higher than that of water spray, and thus, effective in significantly reducing the dust concentration at the 2104 fully mechanized coal face of a coal mine in Erdos, China. © 2018 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Experimental material . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Experimental methods and facilities . . . . . . . . . . . . . . . . . . . 3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Coal dust surface chemical structure and functional groups. . . . . . . . . 3.2. Influence of monomer surfactant on wettability of coal dust . . . . . . . . 3.2.1. Effects of surfactant concentrations on the surface tension . . . . . 3.2.2. Effects of surfactant concentrations on the contact angle . . . . . . 3.2.3. Effects of surfactant concentrations on the coal dust sedimentation . 3.3. Influence of compound surfactant on wettability of coal dust . . . . . . . . 4. Field investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⁎ Corresponding author at: School of Energy and Safety, Anhui University of Science and Technology, 168 Taifeng Road, Huainan, Anhui 232001, China. E-mail: [email protected] (S. Yuan).
https://doi.org/10.1016/j.powtec.2018.12.061 0032-5910/© 2018 Elsevier B.V. All rights reserved.
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1. Introduction China is the largest producer and consumer of coal in the world, and coal production has been an important pillar industry [1]. As the level of mining mechanization continues to increase, so does the amount of coal dust produced during coal production [2–4]. In the absence of dust reduction measures, the coal dust concentration in a fully mechanized mining face can reach 2500–3000 mg/m 3 [5]. Even with measures taken, the environment of most work surfaces is still very poor, as the concentration of dust on the downstream of a coal cutter can reach higher than 1000 mg/m 3 [6], far exceeding requirement of the national standard, PC-TWA 4 mg/m 3 for the total dust and PC-TWA 2.5 mg/m3 for the respirable dust. Long-term exposure of operators to ultra-restricted coal dust can cause respiratory diseases such as CWP and chronic obstructive pulmonary disease [1]. As of 2014, 770,000 cases of pneumoconiosis have been reported in the mainland of China, with coal workers' pneumoconiosis (CWP) accounting for about 55% of the total cases and increasing by about 10,000 cases per year [7]. This causes not only great pain to pneumoconiosis patients but also great economic loss. In addition, 87.4% of China's state-owned key coal mines are at risk of explosion [6,8], which would cause heavy casualties and economic losses. The high concentration of coal dust produced in the underground coal mine production process can lead not only to the death of miners suffering from pneumoconiosis but also to coal dust explosions and even gas explosions, resulting in major personal injury and economic loss [9]. Therefore, the prevention and control of dust in coal mines is a major issue related to the occupational health and safety of coal mines. Presently, domestic and foreign researchers have conducted many theoretical and technical studies on coal dust prevention, among which the use of water to reduce dust is the most widely used. Although water does reduce dust well, due to the high hydrophobicity of coal dust, coal mine dust is still not well suppressed. Because most coal dust has a poor hydrophilic property and is mostly hydrophobic dust, adding surfactant in water to improve the wettability of coal dust is a positive and effective method of reducing dust in coal mines [10–12]. It has become an important means to prevent and control coal dust in European and American countries. China's coal dust wetting technology research began in the 1970s, and it has been developed and promoted more quickly since the 1980s [10]. In recent years, relevant research results mainly include the following: Yang et al. conducted in-depth research on the micro-mechanism of coal dust from functional groups [13]. Wu [14] conducted research on the improvement of the wetting ability of anionic, cationic, and nonionic surfactants on dust, and the effect of adding the
appropriate amount of Na 2 SO 4 on the surface activity of surfactants. The results show that the wetting property of anion and nonionic surfactants is better than that of cationic surfactants. The effect of the proper amount of Na2SO4 on the surface activity of anionic surfactant is significant. Cheng et al. [15] and Zhou et al. [16] conducted the surface tension, contact angle, and Walker settlement experiments, and mutually verified the dustreducing effect of surfactants, achieving better accuracy in their experimental results. However, there is hardly any systematic research on the optimization and compounding of surfactant in the field test of a coal mine that meets the MT 506–1996 national standard of China [17]. Therefore, 15 kinds of harmless, nontoxic and inexpensive surfactants were selected to characterize the wetting effect of each monomer and the compounding surfactant solution on coal dust based on the standard. The factors affecting the wettability of coal dust and the wetting mechanism were analyzed, and the more effective water-spray additive for coal dust suppression was selected to achieve it's better suppression effect. 2. Laboratory research 2.1. Experimental material The coal sample used in experiments was acquired from the 2104 fully mechanized coal face of a coal mine in Erdos, China, which is characteristically composed of bituminous coal. In the experiments, tap water is used to more closely mimic the actual conditions of the coal mine. The coal sample was crushed and sieved to a powder finer than 200 mesh (0.074 mm) for determining the characteristics of the dust surface. The coal powder was pressed into tablet form for the contact angle tests using the YP-2 type tablet press machine manufactured by Shanyue Science Instrument Co. Ltd. in Shanghai, which compressing pressure maintains at 20 MPa for 2 min [4]. Considering the special working environment of the coal mine, the surfactant should be non-toxic, non-irritating, non-flammable, low cost, and easy to dissolve in water [6]. In this paper, 15 commonly used surfactants (Table 1) were selected for optimization and compounding, with all chemicals purchased from Usolf Chemical Technology Co. Ltd. 2.2. Experimental methods and facilities Infrared spectroscopic analysis of the coal sample was conducted using the Nicolet380 Fourier transform infrared spectrometer machine manufactured by Thermo Co. Ltd. in USA. The infrared spectrum of the coal sample was measured by potassium bromide tabletting. The coal sample and KBr (chromatographically pure) were dried in a drying
Table 1 The surfactants used in the experiment. Reagent name
Abbreviation
Category
Molecular formula
Dodecyl dimethyl benzyl ammonium chloride Polyquaternium-7 Sodium alcohol ether sulfate Sodium alpha-olefin sulfonate Sodium fatty acid methyl esters sulfonate Fatty Acid Methyl Esters ethoxylate sulfonate Cocoamidopropyl betaine Lauroamidepropylamine oxide Disodium Lauroamphodiacetate Sodium lauroyl glutamate Cocoamidopropyl hydroxy sulfobetaine Polyethylene glycol 4000 Polyoxyethylene(20)sorbitan monooleate Coconut diethanolamide Hexyl D-glucoside
1227 M550 AES AOS MES FMES CAB-35 LAO-30 LAD-40 LG-95P CHSB PEG-4000 Tween-80 CDEA APG06
cationic cationic anionic anionic anionic anionic amphoteric amphoteric amphoteric amphoteric amphoteric nonionic nonionic nonionic nonionic
C21H38NCl C11H21ClN2O C14H29NaO5S RCH=CH(CH2)nSO3Na, R = C14–16 C27H44N2O4 C18H35CHSO3Na(OCH2CH2)7 C19H38N2O3 C15H30N2O2 C20H41N2Na2O8 C17H30NNaO5 C20H42N2O5S (C2H4O)n-H2O C24H44O6 C11H23CON(CH2CH2OH)2 C12H24O6
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Fig. 3. The relationship between contact angle and liquid-solid, vapor-solid, and vaporliquid surface tension.
Fig. 1. The experimental system used to test the dust sedimentation time, including iron support (A), metal ring (B), glass funnel (C), coal sample (D), fast qualitative filter paper (E), weighing bottle (F), locating ring (G).
oven at 100 °C for 2 h. Then they were removed, put into a dryer, and cooled to room temperature. A 2-mg sample and 200 mg of KBr were grinded together in agate mortar to a size of b2 μm. A certain amount of mixed powder is put into a grinding tool and pressed into a transparent wafer. Finally, the wafer is put into the sample chamber of the spectrometer for testing. The surface tension of the different concentration solutions were measured using the maximum bubble method and the DMPY-2C surface tension meter manufactured by Institute of Applied Physics in Nanjing. Every experimental sample was measured three times. All experiments were conducted at ambient temperature 25 °C. The contact angles of the different concentration solutions on the coal surface were detected using the sessile drop method and the SL200C contact angle instrument manufactured by Kino Co. Ltd. in USA. The photos from the testing of the contact angle of all solutions
Fig. 2. FTIR spectra of the coal dust.
were made in the first second of solution dripping for comparing the results of the experiment. Every experimental sample was measured three times. To verify the coal dust wetting ability of the different concentration solutions, coal dust sedimentation experiments were conducted using the self-built test platform according to the “Mine dust reducing agent performance measurement method” (MT 506–1996) standard. The system included an iron support, metal ring, glass funnel, fast qualitative filter paper, weighing bottle, and locating ring (Fig. 1). The fast qualitative filter paper is a kind of filter paper with good filtering property, which is loose and has a strong absorption to liquid. The experiments used the natural sedimentation method [17]. First, a 20-mg coal sample was passed through the glass funnel and directed onto the surface of fast qualitative filter paper that was supported by the metal ring, where the powder was accumulated in a conical pile. Then, the metal ring carrying the fast qualitative filter paper and powder was gradually moved downward by moving the locating ring and rotating the hook arm. At the moment that the fast qualitative filter paper contacted the solution, the filter paper quickly absorbed the solutions and separated from the coal sample. The dust sedimentation time was measured from when the fast qualitative filter paper contacted the solution until the whole coal sample was immersed into the solution. Every experimental sample was measured three times (the deviation between each measured value and the average value should be ≤7%). 3. Results and discussion 3.1. Coal dust surface chemical structure and functional groups The wettability of coal is not only related to the composition of coal but is also inseparable with the structure of coal. Fourier transform infrared spectroscopy can precisely characterize the functional groups, positions, and contents of organic macromolecular structures without destroying the structure of the coal. It is one of the important means by which the structure of organic macromolecules are examined. In this experiment, the chemical structure and functional groups of coal dust were analyzed and characterized using infrared spectroscopy, revealing the microscopic nature of the surface electrical properties and the wettability of coal dust. Fig. 2 shows the infrared spectrum of experimental coal dust. The absorption peaks of the related group are as follows. The peak at 696 cm−1 is the characteristic absorption peak of benzene ring fold vibration. The peak at 874 cm−1 is the characteristic absorption peak of hydrogen expansion vibration in an aromatic ring. The peak at 1705 cm−1 is the characteristic absorption peak of the CO stretching vibration for the carboxyl group. The peaks at 2923, 1443, and 1380 cm −1 are the characteristic absorption peaks of the stretching vibration of hydrogen for the fatty hydrocarbon and naphthene hydrocarbon groups, and the peak appearing at 3429 cm −1 is the characteristic absorption peak of the hydroxyl stretching vibration [13]. According to the infrared spectrum analysis, there are three main groups of coal dust: aliphatic hydrocarbon, aromatic hydrocarbon,
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Fig. 4. Surface tension of surfactants at different concentrations.
and oxygen-containing functional group. The large number of hydrophobic groups, such as aliphatic hydrocarbons and aromatic hydrocarbons, present on the surface of coal dust, is the fundamental reason for strong hydrophobicity on the surface of coal dust. The surface of coal dust contains oxygen-containing functional groups such as carboxyl groups, hydroxyl groups, and carbonyl groups, so that the coal dust has a certain degree of hydrophilicity. At the same time, the ionization of these groups results in the electrification of the coal dust surface. The presence of strong hydrophobic groups in the coal dust structure leads to the limitation of the water dust reduction effect. Therefore, we add surfactant to the water to enhance the dust reduction effect.
3.2. Influence of monomer surfactant on wettability of coal dust The wetting process is related to the interfacial tension of the system. A drop of liquid falls on a horizontal solid surface. When equilibrium is reached, the relationship between the contact angle and the interfacial tension is in accordance with the following equation (Young Equation) [18]: γ SV ¼ γ SL þ γ LV cosθe
ð1Þ
where γSV is the vapor-solid surface tension (mN/m), γSL is the liquid-solid interfacial tension (mN/m), γLV is the vapor-liquid
Fig. 5. Contact angle of surfactants at different concentrations.
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Fig. 6. Sedimentation velocity of surfactants at different concentrations.
surface tension (mN/m), and θe is the contact angle at the solution interface (°). For most surfactants, according to the Young Equation and Fig. 3, a smaller vapor-liquid surface tension γLV corresponds with a smaller solution contact angle θe on the coal tablet interface, which indicate their better wetting performance [18]. Therefore, to select the surfactant with good wettability, surface tension, contact angle, and coal dust sedimentation comparative experiments were conducted under conditions of various surfactant concentrations (0.01 wt%, 0.03 wt%, 0.05 wt%, 0.07 wt%, 0.1 wt%, 0.3 wt%, and 0.5 wt%).
concentration of the solution reached 0.05 wt%, as the surface tension was still large, with all of them exceeding 55 mN/m, even reaching as high as 69.42 mN/m. Meanwhile, the remaining nine solutions at 0.05% concentration, except the surface tension of the amphoteric surfactant solutions were below 50 mN/m, and the others were below 40 mN/m. Therefore, the surface tensions of the six surfactants, such as M550, were too large and were not suitable for use as a dust reducing additive, and the wettability was no longer verified by contact angle or coal dust sedimentation experiments.
3.2.1. Effects of surfactant concentrations on the surface tension The experimental results are shown in Fig. 4. The measured surface tensions of the 15 different solutions all showed a downward trend with increasing solution concentrations. After reaching a certain concentration, the surface tension did not change significantly with increasing concentration; this concentration is the CMC. The CMCs of the 15 surfactants were mainly contained in the 0.05 wt% and 0.1 wt% concentrations. The reason for this phenomenon is that at low concentrations, the surfactants in the solution exist as single molecules, because of their amphiphilic structure. As a result, these single molecules gradually enrich the surface of the solution, thus forming an adsorption layer of single molecules. When the surfactant molecules reach adsorption saturation on the surface of the solution, the stable micelles composed of several to several hundreds of ions or molecules are synthesized from a dispersed state inside the solution. At this point, the concentration of the solution has increased, the concentration of a single molecule or ion in the solution has not increased significantly, and only more micelles can be formed, so the surface tension of the solution does not change much [18]. Therefore, the smaller the critical micelle concentration is, the lower the concentration required for the surfactant to reach the saturated adsorption and micelle formation on the surface of the solution, and the higher the efficiency of the surfactant to reduce the surface tension of the solution [19], thereby it is easier to change the surface properties and produce wetting action. In addition, comparing the surface tensions of different surfactant solutions, cationic surfactant M550, anionic surfactant MES, amphoteric surfactant LG-95P, nonionic surfactants PEG-4000, Tween 80, and APG06 were less able to improved water surface tension when the
3.2.2. Effects of surfactant concentrations on the contact angle The six surfactants with weaker effects such as M550 were excluded, and the remaining nine surfactants were prepared into solutions of different mass fractions to determine the contact angle formed between them and the coal tablets. The measured contact angle between the tap water and the coal tablet in the experiment was 56.18°.The contact angle of the solutions on the coal tablets exhibited a rapid reduction (56.18°–16.74°) at low surfactant concentrations (0–0.1 wt%) and decreased at a decelerating rate when the concentration exceeded 0.1 wt % (Fig. 5). Meanwhile, the contact angle of the anionic and nonionic surfactant solutions on the coal tablets were significantly smaller than those of the cationic and amphoteric surfactants, indicating that the
Fig. 7. The adsorption process of coal dust by nonionic and anionic surfactants.
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Fig. 8. The wetting features of compound solution containing 0.025 wt% AOS and 0.025 wt% FMES (S1), compound solution containing 0.025 wt% AES and 0.025 wt% FMES (S2), compound solution containing 0.025 wt% AOS and 0.025 wt% AES (S3), compound solution containing 0.025 wt% CDEA and 0.025 wt% FMES (S4), compound solution containing 0.025 wt% AOS and 0.025 wt% CDEA (S5), and compound solution containing 0.025 wt% AES and 0.025 wt% CDEA (S6). a. Change of contact angle of nonionic-anionic compound solution. b. Change of contact angle of anionic-anionic compound solution.
wettability of anionic and nonionic surfactants to coal were superior to the cationic and amphoteric surfactants. Fig. 4 and Fig. 5 show that the surface tension and contact angle of the solution were basically consistent with the concentration change, implying that the surfactant tension and contact angle almost reached equilibrium [20,22], which is consistent with Young's formula. 3.2.3. Effects of surfactant concentrations on the coal dust sedimentation To further verify the effect of the above nine surfactants on the wetting effect of coal dust, the coal dust sedimentation experiments were conducted in accordance with the “Mine dust agent performance measurement method” (MT 506–1996) standard. If 1 g of dry coal dust was weighed according to the standard, the sedimentation time of the coal dust was too long. For example, in the solution of 0.01 wt% LAD-40, coal dust had almost no settlement over 48 h. Therefore, for the convenience of experiment timing, a unified weight of 0.02 g of dry coal dust was poured into the glass funnel, and the settling time of the coal dust was measured to evaluate the wetting capacity of the coal dust. In the experimental data, the settlement time of nine 0.01 wt% solutions was over three hours, which had no comparison significance, so the settlement time was recorded as no-sedimentation.
The experimental results are shown in Fig. 6. The velocity of the coal dust settling in different monomer solutions varies greatly, but all follow the same rule: as the concentration of the solution increased, the settling velocity of the coal dust accelerated, but the concentration that caused a significant increase in the settling velocity of the coal dust was not consistent. At concentrations below 0.1 wt%, the anionic surfactants AES, AOS, and FMES, as well as the nonionic surfactant CDEA, had a significant advantage in settling velocity. According to the theory of residual force field [23], when the anionic or nonionic surfactants are added to the water, the adsorption on the surface of the coal dust occurs (Fig. 7). In the adsorption process, the hydrophobic surface of the coal dust interacts strongly with the hydrophobic groups of the surfactant. Therefore, the tail hydrophobic group of the surfactant orientates to the surface of coal dust and the head hydrophilic group to solution. Since the hydrophilic group of the surfactant extends into the solution after adsorption, the hydrophilicity of the coal dust is enhanced, and the wetting performance of the solution to the coal dust is significantly improved. Meanwhile, the experimental results show that the sedimentation speed of anionic surfactants is faster, which is inconsistent with the conclusion of Yang et al. [13] that nonionic surfactants generally have
Fig. 9. The comparison of the monomer contact angle and the compound contact angle.
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Fig. 10. The generation system of compound solution.
better wetting of coal dust than anionic surfactants. It was also observed in the experiment that cationic surfactant 1227 and amphoteric surfactant CHSB both caused the metal ring in the device to rust in different degrees. Considering the cost of production in a coal mine, it is necessary to ensure the efficiency of dust suppression while considering economic efficiency; the selected surfactants require low costs and low concentrations. Combining the results and phenomena of the surface tension, contact angle, and coal dust sedimentation experiments, the anionic surfactants AES, AOS, and FMES as well as the nonionic surfactant CDEA with low CMC and fast settlement speed at low concentrations were selected as the compound surfactants.
3.3. Influence of compound surfactant on wettability of coal dust The purpose of surfactant compounding is to produce synergism. In other words, after mixing different types of surfactants, the performance of the mixture should be better than that of the single component, that is, the effect of “1+1 N 2” [18]. The CMCs of AOS, FMES, AES, and CDEA were all 0.05 wt%, that is, the mass fraction of the monomer solution as a factor level changed little [24]. Therefore, instead of using the orthogonal experimental optimization method, six different compounding solutions were obtained by the principle of compounding two pairs of monomers, and the CMC concentration was selected as the concentration of the mixed solution. The surface tension, contact angle, and coal dust sedimentation experiments were performed on the six solutions. A solution with significant synergistic effect was selected as an additive for dust reduction by spray in coal mines to reduce the surfactant amount, increase the efficiency, and reduce the cost. The surface tensions of the monomers were 32.96 mN/m (AOS), 32.96 mN/m (FMES), 36.94 mN/m (AES), and 33.76 mN/m (CDEA), the contact angles of the monomers were 28.12°(AOS), 19.84°(FMES), 18.06°(AES), and 27.83°(CDEA), and the coal settling times of the
monomers were 209.73 s (AOS), 764.90 s (FMES), 386.60 s (AES), and 1254.58 s (CDEA). The surface tension measurement results of the compound solution are shown in Fig. 8. The compounding of anionic-anionic surfactant had no synergistic effect in reducing the surface tension. The surface tension of the compound solution was not entirely between the surface tension of each monomer. The surface tension of the FMES and AES mixed solution increased by 9.99% and 9.04% (to 40.28 mN/m) compared to those of the single component FMES and AES, respectively. This is related to the repulsive force between the FMES and AES ion heads, in which it is difficult to form mixed micelles. The nonion-anionic surfactant blend system can produce a synergistic effect in reducing the surface tension, and the surface tension of the FMES and CDEA mixed solution decreased by 20.18% and 13.42% (to 29.23 mN/m) compared to those of the single component FMES and CDEA, respectively. This is because of the insertion of nonionic surfactant molecules into the anionic micelles to form mixed micelles, which reduces the repulsion between the anionic surfactant ionic heads. Also, the interaction between the hydrophobic chains of the two surfactants is a factor, which makes it easy to form micelles, so that the CMC of the mixed solution is reduced [18]. Moreover, the nonionic surfactants combine with H2O and H3O+ through hydrogen bonds, so that the nonionic surfactant molecules are positively charged. Therefore, the interaction between the anionic surfactant and the nonionic surfactant is similar to the electrical interaction between the anionic-cationic surfactant blend system—the surface tension decreases while the surface activity increases. The contact angle of the nonionic-anionic surfactant blend system on the coal tablet was significantly smaller than that of the single component. The contact angle of the FMES and CDEA mixed solution decreased by 59.62% and 66.83% (to only 9.23°) compared to that of the single component FMES and CDEA, respectively, which was almost instantaneously tiled (Fig. 9a). This shows that the wettability of the solution after the compounding is significantly increased. However, the contact angle of the anionic-anionic compound solution FMES and AES on the coal tablet was significantly larger than those of the respective
Table 2 The dust concentration before and after using dust control. Dust control technology
Before spraying
In spraying with untreated water
In spraying with surfactant compounding solution
Total dust concentration (mg/m3) Respirable dust concentration (mg/m3)
98.24 43.52
48.68 26.48
9.51 5.24
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in spraying with the untreated water or with surfactant compounding solution, are shown in Table 2. Comparing the dust suppression efficiencies of the three conditions, the surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) significantly decreased the dust concentration at the place operated by the shearer operator, especially regarding respirable dust (Fig. 11). The total dust and respirable dust suppression efficiencies by surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) were 90.32% and 87.96%, respectively, improving by 39.87% and 48.81% compared to that of untreated water, respectively. The field investigation results demonstrated that the compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) could effectively capture coal dust, especially respirable dust. 5. Conclusions
Fig. 11. The dust control efficiency of untreated water (A), surfactant compounding solution consisting of 0.025 wt% FMES and 0.025 wt% CDEA (B).
monomers (Fig. 9b). The contact angle was in agreement with the experimental results of surface tension. The result of the coal dust sedimentation experiments directly show the synergistic effect of the compound surfactant on the wettability of coal. The settling velocity of coal in the nonionic-anionic surfactant compounding solution was significantly faster than that of the monomer solution. In particular, the settlement velocity of the FMES and CDEA mixed solution was significantly faster; the settlement of 0.02-g coal was only 15.95 s, which was faster by 97.91% and 98.73% compared to that of the single component FMES and CDEA, respectively. The settling velocity of coal in the anionic-anionic compound solution AOS and FMES, AOS, and AES was only a compromise of the single component velocity. The surface tension of the FMES and AES compound solution increased more than that of the single component, yet the settlement velocity was significantly faster than that of the single component. However, it did not exhibit significant synergistic effects as the nonionicanionic surfactant complex system. These experimental results indicated that the wettability of the surfactant to coal dust depended not only on the surfactant vapor-liquid surface tension, but also on the solid-liquid interfacial tension between the surfactant and the coal dust. In this experiment, the anionic and nonionic surfactants were compounded, confirming that the intermolecular interaction exists and that the mixed micelles are formed in the compound system, which can produce additive and synergistic effects and can improve the wettability of the surfactant. In particular, the 0.025 wt% FMES and 0.025 wt% CDEA compound solution (anionic-nonionic surfactant compound system) showed significant synergistic effect and was determined to be the best formula for the field test of a coal mine. 4. Field investigation To further verify the dust control ability of the 0.025 wt% FMES and 0.025 wt% CDEA compound solution, a field test was conducted in the 2104 fully mechanized coalface of a coal mine in Erdos, China. The generation system of compound solution is shown in Fig. 10. Untreated water and surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) were each applied at the place operated by the shearer operator. The dust sampling followed the standard “Specifications of air sampling for hazardous substances monitoring in the workplace” (GBZ 159–2004) [25], using the AKFC-92A type dust sampler manufactured by Changshu Mining Electromechanical Equipment Co. Ltd. in Changshu to collect dust for 15 min (short-time sampling) and determine the dust concentrations before and after applying dust control. The dust concentrations in air before spraying,
(1) In low concentrations of surfactants that can wet coal, anionic and nonionic surfactant solutions are significantly better than those of the cationic and amphoteric surfactants. In the compound solution, the anionic-nonionic compounding system has the best effect on the wettability of coal. (2) The compound solution (0.025 wt% FMES and 0.025 wt% CDEA) can decrease the surface tension and the contact angle to the coal tablet by 59.39% and 83.57%, respectively, compared to that of untreated water, because of the significantly improved wettability of the solution. (3) The compound solution (0.025 wt% FMES and 0.025 wt% CDEA) can be successfully applied in the practical conditions of an operating coal mine and significantly improves the dust control efficiency of water spray. Compared to that of untreated water, the optimized compound solution can enhance the total dust and respirable dust control efficiencies by 39.87% and 48.81%, respectively, remarkably reducing the dust concentration at the coalface of an underground coal mine. Acknowledgments This work was supported by the Fundamental Research Funds for the National Natural Science Foundation of China (No. 51874009). References [1] Chineese Center for Disease Control and Prevention, Occupational Health Risk Assessment Report for Dust, http://niohp.chinacdc.cn/2015ndgzxmbg/zyjkfxpg/ Vol. 13, 2015 2018 accessed March. [2] 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) 489. [3] Q. Huang, R. Honaker, Optimized reagent dosage effect on rock dust to enhance rock dust dispersion and explosion mitigation in underground coal mines, Powder Technol. 301 (2016) 1193–1200. [4] Q. Zhou, B. Qin, J. Wang, H. Wang, F. Wang, Effects of preparation parameters on the wetting features of surfactant-magnetized water for dust control in Luwa mine, China, Powder Technol. 326 (2018) 7–15. [5] G. Zhou, Research of Theory about Dust Prevention by Water-Cloud and Relevant Techniques for Fully-Mechanized Caving Coalface, PhD Dissertation Shandong University of Science and Technology, Qingdao, China, 2009. [6] B. Qin, Q. Zhou, X. Li, J. Wang, H. Wang, Y. Ding, Synergistic technology between surfactant and magnetized water for efficient dust control in underground coal mines, J. China Coal Soc. 42 (2017) 2900–2907. [7] X. Zhang, D. Yang, J. Lou, Curative effect of large volume whole lung lavage with low concentration of poly-2-vinyl pyridine n oxide on the coal workers' pneumoconiosis after 6 years, Chin. J. Ind. Hyg. Occup. Dis. 19 (2001) 27–29. [8] Y.L. Liu, G. Fu, P. Yu, Study on diffusion rules of toxic gas from coal mine explosion, Saf. Coal Mines 39 (2008) 4–7. [9] Y.X. Zheng, Research on the Controlling Technique and Technological Process of Coal Dust with Less Water in Thin Coal Seam, PhD Dissertation Shandong University of Science and Technology, Qingdao, China, 2008. [10] W.G. Huang, F. Hu, N.Q. Liu, Study on surfactant wettability for coal dust, Mining Saf. Environ. Prot. 37 (2010) 4–6. [11] Z. Gui, R. Liu, P. Wang, S. Gou, W. Shu, X. Tan, Experimental study on surfactant effect on coal dust wettability, J. Heilongjiang Univ. Sci. Technol. 26 (2016) 513–517. [12] J.Y. Li, Y.P. Lu, C.X. Sun, Y. Shi, Experimental study on the wetting characteristics of coal dust, Chin. Energy Environ. Prot. 6 (2015) 36–38. [13] J. Yang, Y.Z. Tan, Z.H. Wang, Y.D. Shang, W.B. Zhao, Study on the coal dust surface characteristics and wetting mechanism, J. China Coal Soc. 32 (2007) 737–740.
X. Wang et al. / Powder Technology 344 (2019) 561–569 [14] C.Wu. Chemical Dust, Suppression, Central South University Press, Changsha, 2003. [15] W.M. Cheng, L.J. Zhang, G. Zhou, W. Nie, J.H. Li, Experiment of dust-settling with spraying surfactants and its application in fully mechanized caving face, J. Shandong Univ. Sci. Technol. 25 (2009) 1271–1275. [16] G. Zhou, W.M. Cheng, G. Wang, W. Nie, Dedusting technique of surfactant for fully mechanized and roof caving coal face, Ind. Saf. Environ. Prot. 35 (2009) 17–19. [17] C.N. Standard, Determination of the Performance of Mine Dust Agent, MT 506–1996, 1996. [18] S.R. Wang, X.G. Li, D.Z. Liu, Surfactant Chemistry, Chemical Industry Press, Beijing, 2010. [19] G.H. Ni, Z. Li, H.C. Xie, The mechanism and relief method of the coal seam water blocking effect (WBE) based on the surfactants, Powder Technol. 323 (2018) 60–68.
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[20] J.E. George, S. Chidangil, S.D. George, A study on air bubble wetting: Role of surface wettability surface tension, and ionic surfactants, Appl. Surf. Sci. 410 (2017) 117–125. [22] J.R. Liley, R.K. Thomas, J. Penfold, I.M. Tucker, J.T. Petkov, P. Stevenson, J.R.P. Webster, The Impact of electrolyte on adsorption at the air-water interface for ternary surfactant mixtures above the critical micelle concentration, Langmuir 33 (2017) 4301–4312. [23] Z.Q. Chen, G.X. Wang, G.Y. Xu, Colloid and Interface Chemistry, Higher Education Press, Beijing, 2001. [24] L. Zhu, G. Zhou, M.D. Sun, W. Nie, C.X. Wan, X.B. Meng, Experiment study on chemical dedusting agent for spraying dust control, Coal Eng. 46 (2014) 96–98. [25] C.N. Standard, Sampling Specification for Monitoring Harmful Substances in the Air of Workplace, GBZ 159–2004, 2004.
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Review
Synergistic effect of surfactant compounding on improving dust suppression in a coal mine in Erdos, China Xiaonan Wang a,b, Shujie Yuan a,b,⁎, Xiang Li c, Bingyou Jiang a,b a b c
Key Laboratory of Safe and Effective Coal Mining, Ministry of Education, Anhui University of Science and Technology, Huainan, Anhui 232001, China School of Energy and Safety, Anhui University of Science and Technology, Huainan, Anhui 232001, China School of Civil and Architecture, Anhui University of Science and Technology, Huainan, Anhui 232001, China
a r t i c l e
i n f o
Article history: Received 28 September 2018 Received in revised form 5 December 2018 Accepted 12 December 2018 Available online 13 December 2018 Keywords: Coal dust control Optimal compound surfactants Wettability improvement
a b s t r a c t To effectively improve the ability of water for coal dust suppression using in a coal mine in Erdos, China, surfactant compounding was proposed because of the synergy between surfactants. First, four surfactants with good coal dust wetting ability were selected from a total of 15 surfactants through surface tension, contact angle, and coal dust sedimentation experiments. Next, a compound surfactant was produced by mixing two surfactants in equal volume. The critical micelle concentration (CMC) of surfactants was selected as the concentration of the compound solution. The four surfactants were mixed into six solutions. Then, the wettability of the compound solution was evaluated via surface tension, contact angle, and coal dust sedimentation experiments. The optimal compound solution was selected as an additive to dust suppression by sprinkling in field research in the coal mine. The results revealed that the compound surfactant solution containing 0.025 wt% FMES and 0.025 wt% CDEA was optimal for wetting coal dust, decreasing the surface tension and contact angle by 59.39% (i.e. to 29.23 mN/m) and 83.57% (i.e. to 9.23°), respectively, compared with the respective values of untreated water. Field tests proved that the dust suppression efficiency of the solution containing 0.025 wt% FMES and 0.025 wt % CDEA was N87%, considerably higher than that of water spray, and thus, effective in significantly reducing the dust concentration at the 2104 fully mechanized coal face of a coal mine in Erdos, China. © 2018 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Experimental material . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Experimental methods and facilities . . . . . . . . . . . . . . . . . . . 3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Coal dust surface chemical structure and functional groups. . . . . . . . . 3.2. Influence of monomer surfactant on wettability of coal dust . . . . . . . . 3.2.1. Effects of surfactant concentrations on the surface tension . . . . . 3.2.2. Effects of surfactant concentrations on the contact angle . . . . . . 3.2.3. Effects of surfactant concentrations on the coal dust sedimentation . 3.3. Influence of compound surfactant on wettability of coal dust . . . . . . . . 4. Field investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⁎ Corresponding author at: School of Energy and Safety, Anhui University of Science and Technology, 168 Taifeng Road, Huainan, Anhui 232001, China. E-mail: [email protected] (S. Yuan).
https://doi.org/10.1016/j.powtec.2018.12.061 0032-5910/© 2018 Elsevier B.V. All rights reserved.
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1. Introduction China is the largest producer and consumer of coal in the world, and coal production has been an important pillar industry [1]. As the level of mining mechanization continues to increase, so does the amount of coal dust produced during coal production [2–4]. In the absence of dust reduction measures, the coal dust concentration in a fully mechanized mining face can reach 2500–3000 mg/m 3 [5]. Even with measures taken, the environment of most work surfaces is still very poor, as the concentration of dust on the downstream of a coal cutter can reach higher than 1000 mg/m 3 [6], far exceeding requirement of the national standard, PC-TWA 4 mg/m 3 for the total dust and PC-TWA 2.5 mg/m3 for the respirable dust. Long-term exposure of operators to ultra-restricted coal dust can cause respiratory diseases such as CWP and chronic obstructive pulmonary disease [1]. As of 2014, 770,000 cases of pneumoconiosis have been reported in the mainland of China, with coal workers' pneumoconiosis (CWP) accounting for about 55% of the total cases and increasing by about 10,000 cases per year [7]. This causes not only great pain to pneumoconiosis patients but also great economic loss. In addition, 87.4% of China's state-owned key coal mines are at risk of explosion [6,8], which would cause heavy casualties and economic losses. The high concentration of coal dust produced in the underground coal mine production process can lead not only to the death of miners suffering from pneumoconiosis but also to coal dust explosions and even gas explosions, resulting in major personal injury and economic loss [9]. Therefore, the prevention and control of dust in coal mines is a major issue related to the occupational health and safety of coal mines. Presently, domestic and foreign researchers have conducted many theoretical and technical studies on coal dust prevention, among which the use of water to reduce dust is the most widely used. Although water does reduce dust well, due to the high hydrophobicity of coal dust, coal mine dust is still not well suppressed. Because most coal dust has a poor hydrophilic property and is mostly hydrophobic dust, adding surfactant in water to improve the wettability of coal dust is a positive and effective method of reducing dust in coal mines [10–12]. It has become an important means to prevent and control coal dust in European and American countries. China's coal dust wetting technology research began in the 1970s, and it has been developed and promoted more quickly since the 1980s [10]. In recent years, relevant research results mainly include the following: Yang et al. conducted in-depth research on the micro-mechanism of coal dust from functional groups [13]. Wu [14] conducted research on the improvement of the wetting ability of anionic, cationic, and nonionic surfactants on dust, and the effect of adding the
appropriate amount of Na 2 SO 4 on the surface activity of surfactants. The results show that the wetting property of anion and nonionic surfactants is better than that of cationic surfactants. The effect of the proper amount of Na2SO4 on the surface activity of anionic surfactant is significant. Cheng et al. [15] and Zhou et al. [16] conducted the surface tension, contact angle, and Walker settlement experiments, and mutually verified the dustreducing effect of surfactants, achieving better accuracy in their experimental results. However, there is hardly any systematic research on the optimization and compounding of surfactant in the field test of a coal mine that meets the MT 506–1996 national standard of China [17]. Therefore, 15 kinds of harmless, nontoxic and inexpensive surfactants were selected to characterize the wetting effect of each monomer and the compounding surfactant solution on coal dust based on the standard. The factors affecting the wettability of coal dust and the wetting mechanism were analyzed, and the more effective water-spray additive for coal dust suppression was selected to achieve it's better suppression effect. 2. Laboratory research 2.1. Experimental material The coal sample used in experiments was acquired from the 2104 fully mechanized coal face of a coal mine in Erdos, China, which is characteristically composed of bituminous coal. In the experiments, tap water is used to more closely mimic the actual conditions of the coal mine. The coal sample was crushed and sieved to a powder finer than 200 mesh (0.074 mm) for determining the characteristics of the dust surface. The coal powder was pressed into tablet form for the contact angle tests using the YP-2 type tablet press machine manufactured by Shanyue Science Instrument Co. Ltd. in Shanghai, which compressing pressure maintains at 20 MPa for 2 min [4]. Considering the special working environment of the coal mine, the surfactant should be non-toxic, non-irritating, non-flammable, low cost, and easy to dissolve in water [6]. In this paper, 15 commonly used surfactants (Table 1) were selected for optimization and compounding, with all chemicals purchased from Usolf Chemical Technology Co. Ltd. 2.2. Experimental methods and facilities Infrared spectroscopic analysis of the coal sample was conducted using the Nicolet380 Fourier transform infrared spectrometer machine manufactured by Thermo Co. Ltd. in USA. The infrared spectrum of the coal sample was measured by potassium bromide tabletting. The coal sample and KBr (chromatographically pure) were dried in a drying
Table 1 The surfactants used in the experiment. Reagent name
Abbreviation
Category
Molecular formula
Dodecyl dimethyl benzyl ammonium chloride Polyquaternium-7 Sodium alcohol ether sulfate Sodium alpha-olefin sulfonate Sodium fatty acid methyl esters sulfonate Fatty Acid Methyl Esters ethoxylate sulfonate Cocoamidopropyl betaine Lauroamidepropylamine oxide Disodium Lauroamphodiacetate Sodium lauroyl glutamate Cocoamidopropyl hydroxy sulfobetaine Polyethylene glycol 4000 Polyoxyethylene(20)sorbitan monooleate Coconut diethanolamide Hexyl D-glucoside
1227 M550 AES AOS MES FMES CAB-35 LAO-30 LAD-40 LG-95P CHSB PEG-4000 Tween-80 CDEA APG06
cationic cationic anionic anionic anionic anionic amphoteric amphoteric amphoteric amphoteric amphoteric nonionic nonionic nonionic nonionic
C21H38NCl C11H21ClN2O C14H29NaO5S RCH=CH(CH2)nSO3Na, R = C14–16 C27H44N2O4 C18H35CHSO3Na(OCH2CH2)7 C19H38N2O3 C15H30N2O2 C20H41N2Na2O8 C17H30NNaO5 C20H42N2O5S (C2H4O)n-H2O C24H44O6 C11H23CON(CH2CH2OH)2 C12H24O6
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Fig. 3. The relationship between contact angle and liquid-solid, vapor-solid, and vaporliquid surface tension.
Fig. 1. The experimental system used to test the dust sedimentation time, including iron support (A), metal ring (B), glass funnel (C), coal sample (D), fast qualitative filter paper (E), weighing bottle (F), locating ring (G).
oven at 100 °C for 2 h. Then they were removed, put into a dryer, and cooled to room temperature. A 2-mg sample and 200 mg of KBr were grinded together in agate mortar to a size of b2 μm. A certain amount of mixed powder is put into a grinding tool and pressed into a transparent wafer. Finally, the wafer is put into the sample chamber of the spectrometer for testing. The surface tension of the different concentration solutions were measured using the maximum bubble method and the DMPY-2C surface tension meter manufactured by Institute of Applied Physics in Nanjing. Every experimental sample was measured three times. All experiments were conducted at ambient temperature 25 °C. The contact angles of the different concentration solutions on the coal surface were detected using the sessile drop method and the SL200C contact angle instrument manufactured by Kino Co. Ltd. in USA. The photos from the testing of the contact angle of all solutions
Fig. 2. FTIR spectra of the coal dust.
were made in the first second of solution dripping for comparing the results of the experiment. Every experimental sample was measured three times. To verify the coal dust wetting ability of the different concentration solutions, coal dust sedimentation experiments were conducted using the self-built test platform according to the “Mine dust reducing agent performance measurement method” (MT 506–1996) standard. The system included an iron support, metal ring, glass funnel, fast qualitative filter paper, weighing bottle, and locating ring (Fig. 1). The fast qualitative filter paper is a kind of filter paper with good filtering property, which is loose and has a strong absorption to liquid. The experiments used the natural sedimentation method [17]. First, a 20-mg coal sample was passed through the glass funnel and directed onto the surface of fast qualitative filter paper that was supported by the metal ring, where the powder was accumulated in a conical pile. Then, the metal ring carrying the fast qualitative filter paper and powder was gradually moved downward by moving the locating ring and rotating the hook arm. At the moment that the fast qualitative filter paper contacted the solution, the filter paper quickly absorbed the solutions and separated from the coal sample. The dust sedimentation time was measured from when the fast qualitative filter paper contacted the solution until the whole coal sample was immersed into the solution. Every experimental sample was measured three times (the deviation between each measured value and the average value should be ≤7%). 3. Results and discussion 3.1. Coal dust surface chemical structure and functional groups The wettability of coal is not only related to the composition of coal but is also inseparable with the structure of coal. Fourier transform infrared spectroscopy can precisely characterize the functional groups, positions, and contents of organic macromolecular structures without destroying the structure of the coal. It is one of the important means by which the structure of organic macromolecules are examined. In this experiment, the chemical structure and functional groups of coal dust were analyzed and characterized using infrared spectroscopy, revealing the microscopic nature of the surface electrical properties and the wettability of coal dust. Fig. 2 shows the infrared spectrum of experimental coal dust. The absorption peaks of the related group are as follows. The peak at 696 cm−1 is the characteristic absorption peak of benzene ring fold vibration. The peak at 874 cm−1 is the characteristic absorption peak of hydrogen expansion vibration in an aromatic ring. The peak at 1705 cm−1 is the characteristic absorption peak of the CO stretching vibration for the carboxyl group. The peaks at 2923, 1443, and 1380 cm −1 are the characteristic absorption peaks of the stretching vibration of hydrogen for the fatty hydrocarbon and naphthene hydrocarbon groups, and the peak appearing at 3429 cm −1 is the characteristic absorption peak of the hydroxyl stretching vibration [13]. According to the infrared spectrum analysis, there are three main groups of coal dust: aliphatic hydrocarbon, aromatic hydrocarbon,
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Fig. 4. Surface tension of surfactants at different concentrations.
and oxygen-containing functional group. The large number of hydrophobic groups, such as aliphatic hydrocarbons and aromatic hydrocarbons, present on the surface of coal dust, is the fundamental reason for strong hydrophobicity on the surface of coal dust. The surface of coal dust contains oxygen-containing functional groups such as carboxyl groups, hydroxyl groups, and carbonyl groups, so that the coal dust has a certain degree of hydrophilicity. At the same time, the ionization of these groups results in the electrification of the coal dust surface. The presence of strong hydrophobic groups in the coal dust structure leads to the limitation of the water dust reduction effect. Therefore, we add surfactant to the water to enhance the dust reduction effect.
3.2. Influence of monomer surfactant on wettability of coal dust The wetting process is related to the interfacial tension of the system. A drop of liquid falls on a horizontal solid surface. When equilibrium is reached, the relationship between the contact angle and the interfacial tension is in accordance with the following equation (Young Equation) [18]: γ SV ¼ γ SL þ γ LV cosθe
ð1Þ
where γSV is the vapor-solid surface tension (mN/m), γSL is the liquid-solid interfacial tension (mN/m), γLV is the vapor-liquid
Fig. 5. Contact angle of surfactants at different concentrations.
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Fig. 6. Sedimentation velocity of surfactants at different concentrations.
surface tension (mN/m), and θe is the contact angle at the solution interface (°). For most surfactants, according to the Young Equation and Fig. 3, a smaller vapor-liquid surface tension γLV corresponds with a smaller solution contact angle θe on the coal tablet interface, which indicate their better wetting performance [18]. Therefore, to select the surfactant with good wettability, surface tension, contact angle, and coal dust sedimentation comparative experiments were conducted under conditions of various surfactant concentrations (0.01 wt%, 0.03 wt%, 0.05 wt%, 0.07 wt%, 0.1 wt%, 0.3 wt%, and 0.5 wt%).
concentration of the solution reached 0.05 wt%, as the surface tension was still large, with all of them exceeding 55 mN/m, even reaching as high as 69.42 mN/m. Meanwhile, the remaining nine solutions at 0.05% concentration, except the surface tension of the amphoteric surfactant solutions were below 50 mN/m, and the others were below 40 mN/m. Therefore, the surface tensions of the six surfactants, such as M550, were too large and were not suitable for use as a dust reducing additive, and the wettability was no longer verified by contact angle or coal dust sedimentation experiments.
3.2.1. Effects of surfactant concentrations on the surface tension The experimental results are shown in Fig. 4. The measured surface tensions of the 15 different solutions all showed a downward trend with increasing solution concentrations. After reaching a certain concentration, the surface tension did not change significantly with increasing concentration; this concentration is the CMC. The CMCs of the 15 surfactants were mainly contained in the 0.05 wt% and 0.1 wt% concentrations. The reason for this phenomenon is that at low concentrations, the surfactants in the solution exist as single molecules, because of their amphiphilic structure. As a result, these single molecules gradually enrich the surface of the solution, thus forming an adsorption layer of single molecules. When the surfactant molecules reach adsorption saturation on the surface of the solution, the stable micelles composed of several to several hundreds of ions or molecules are synthesized from a dispersed state inside the solution. At this point, the concentration of the solution has increased, the concentration of a single molecule or ion in the solution has not increased significantly, and only more micelles can be formed, so the surface tension of the solution does not change much [18]. Therefore, the smaller the critical micelle concentration is, the lower the concentration required for the surfactant to reach the saturated adsorption and micelle formation on the surface of the solution, and the higher the efficiency of the surfactant to reduce the surface tension of the solution [19], thereby it is easier to change the surface properties and produce wetting action. In addition, comparing the surface tensions of different surfactant solutions, cationic surfactant M550, anionic surfactant MES, amphoteric surfactant LG-95P, nonionic surfactants PEG-4000, Tween 80, and APG06 were less able to improved water surface tension when the
3.2.2. Effects of surfactant concentrations on the contact angle The six surfactants with weaker effects such as M550 were excluded, and the remaining nine surfactants were prepared into solutions of different mass fractions to determine the contact angle formed between them and the coal tablets. The measured contact angle between the tap water and the coal tablet in the experiment was 56.18°.The contact angle of the solutions on the coal tablets exhibited a rapid reduction (56.18°–16.74°) at low surfactant concentrations (0–0.1 wt%) and decreased at a decelerating rate when the concentration exceeded 0.1 wt % (Fig. 5). Meanwhile, the contact angle of the anionic and nonionic surfactant solutions on the coal tablets were significantly smaller than those of the cationic and amphoteric surfactants, indicating that the
Fig. 7. The adsorption process of coal dust by nonionic and anionic surfactants.
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Fig. 8. The wetting features of compound solution containing 0.025 wt% AOS and 0.025 wt% FMES (S1), compound solution containing 0.025 wt% AES and 0.025 wt% FMES (S2), compound solution containing 0.025 wt% AOS and 0.025 wt% AES (S3), compound solution containing 0.025 wt% CDEA and 0.025 wt% FMES (S4), compound solution containing 0.025 wt% AOS and 0.025 wt% CDEA (S5), and compound solution containing 0.025 wt% AES and 0.025 wt% CDEA (S6). a. Change of contact angle of nonionic-anionic compound solution. b. Change of contact angle of anionic-anionic compound solution.
wettability of anionic and nonionic surfactants to coal were superior to the cationic and amphoteric surfactants. Fig. 4 and Fig. 5 show that the surface tension and contact angle of the solution were basically consistent with the concentration change, implying that the surfactant tension and contact angle almost reached equilibrium [20,22], which is consistent with Young's formula. 3.2.3. Effects of surfactant concentrations on the coal dust sedimentation To further verify the effect of the above nine surfactants on the wetting effect of coal dust, the coal dust sedimentation experiments were conducted in accordance with the “Mine dust agent performance measurement method” (MT 506–1996) standard. If 1 g of dry coal dust was weighed according to the standard, the sedimentation time of the coal dust was too long. For example, in the solution of 0.01 wt% LAD-40, coal dust had almost no settlement over 48 h. Therefore, for the convenience of experiment timing, a unified weight of 0.02 g of dry coal dust was poured into the glass funnel, and the settling time of the coal dust was measured to evaluate the wetting capacity of the coal dust. In the experimental data, the settlement time of nine 0.01 wt% solutions was over three hours, which had no comparison significance, so the settlement time was recorded as no-sedimentation.
The experimental results are shown in Fig. 6. The velocity of the coal dust settling in different monomer solutions varies greatly, but all follow the same rule: as the concentration of the solution increased, the settling velocity of the coal dust accelerated, but the concentration that caused a significant increase in the settling velocity of the coal dust was not consistent. At concentrations below 0.1 wt%, the anionic surfactants AES, AOS, and FMES, as well as the nonionic surfactant CDEA, had a significant advantage in settling velocity. According to the theory of residual force field [23], when the anionic or nonionic surfactants are added to the water, the adsorption on the surface of the coal dust occurs (Fig. 7). In the adsorption process, the hydrophobic surface of the coal dust interacts strongly with the hydrophobic groups of the surfactant. Therefore, the tail hydrophobic group of the surfactant orientates to the surface of coal dust and the head hydrophilic group to solution. Since the hydrophilic group of the surfactant extends into the solution after adsorption, the hydrophilicity of the coal dust is enhanced, and the wetting performance of the solution to the coal dust is significantly improved. Meanwhile, the experimental results show that the sedimentation speed of anionic surfactants is faster, which is inconsistent with the conclusion of Yang et al. [13] that nonionic surfactants generally have
Fig. 9. The comparison of the monomer contact angle and the compound contact angle.
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Fig. 10. The generation system of compound solution.
better wetting of coal dust than anionic surfactants. It was also observed in the experiment that cationic surfactant 1227 and amphoteric surfactant CHSB both caused the metal ring in the device to rust in different degrees. Considering the cost of production in a coal mine, it is necessary to ensure the efficiency of dust suppression while considering economic efficiency; the selected surfactants require low costs and low concentrations. Combining the results and phenomena of the surface tension, contact angle, and coal dust sedimentation experiments, the anionic surfactants AES, AOS, and FMES as well as the nonionic surfactant CDEA with low CMC and fast settlement speed at low concentrations were selected as the compound surfactants.
3.3. Influence of compound surfactant on wettability of coal dust The purpose of surfactant compounding is to produce synergism. In other words, after mixing different types of surfactants, the performance of the mixture should be better than that of the single component, that is, the effect of “1+1 N 2” [18]. The CMCs of AOS, FMES, AES, and CDEA were all 0.05 wt%, that is, the mass fraction of the monomer solution as a factor level changed little [24]. Therefore, instead of using the orthogonal experimental optimization method, six different compounding solutions were obtained by the principle of compounding two pairs of monomers, and the CMC concentration was selected as the concentration of the mixed solution. The surface tension, contact angle, and coal dust sedimentation experiments were performed on the six solutions. A solution with significant synergistic effect was selected as an additive for dust reduction by spray in coal mines to reduce the surfactant amount, increase the efficiency, and reduce the cost. The surface tensions of the monomers were 32.96 mN/m (AOS), 32.96 mN/m (FMES), 36.94 mN/m (AES), and 33.76 mN/m (CDEA), the contact angles of the monomers were 28.12°(AOS), 19.84°(FMES), 18.06°(AES), and 27.83°(CDEA), and the coal settling times of the
monomers were 209.73 s (AOS), 764.90 s (FMES), 386.60 s (AES), and 1254.58 s (CDEA). The surface tension measurement results of the compound solution are shown in Fig. 8. The compounding of anionic-anionic surfactant had no synergistic effect in reducing the surface tension. The surface tension of the compound solution was not entirely between the surface tension of each monomer. The surface tension of the FMES and AES mixed solution increased by 9.99% and 9.04% (to 40.28 mN/m) compared to those of the single component FMES and AES, respectively. This is related to the repulsive force between the FMES and AES ion heads, in which it is difficult to form mixed micelles. The nonion-anionic surfactant blend system can produce a synergistic effect in reducing the surface tension, and the surface tension of the FMES and CDEA mixed solution decreased by 20.18% and 13.42% (to 29.23 mN/m) compared to those of the single component FMES and CDEA, respectively. This is because of the insertion of nonionic surfactant molecules into the anionic micelles to form mixed micelles, which reduces the repulsion between the anionic surfactant ionic heads. Also, the interaction between the hydrophobic chains of the two surfactants is a factor, which makes it easy to form micelles, so that the CMC of the mixed solution is reduced [18]. Moreover, the nonionic surfactants combine with H2O and H3O+ through hydrogen bonds, so that the nonionic surfactant molecules are positively charged. Therefore, the interaction between the anionic surfactant and the nonionic surfactant is similar to the electrical interaction between the anionic-cationic surfactant blend system—the surface tension decreases while the surface activity increases. The contact angle of the nonionic-anionic surfactant blend system on the coal tablet was significantly smaller than that of the single component. The contact angle of the FMES and CDEA mixed solution decreased by 59.62% and 66.83% (to only 9.23°) compared to that of the single component FMES and CDEA, respectively, which was almost instantaneously tiled (Fig. 9a). This shows that the wettability of the solution after the compounding is significantly increased. However, the contact angle of the anionic-anionic compound solution FMES and AES on the coal tablet was significantly larger than those of the respective
Table 2 The dust concentration before and after using dust control. Dust control technology
Before spraying
In spraying with untreated water
In spraying with surfactant compounding solution
Total dust concentration (mg/m3) Respirable dust concentration (mg/m3)
98.24 43.52
48.68 26.48
9.51 5.24
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in spraying with the untreated water or with surfactant compounding solution, are shown in Table 2. Comparing the dust suppression efficiencies of the three conditions, the surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) significantly decreased the dust concentration at the place operated by the shearer operator, especially regarding respirable dust (Fig. 11). The total dust and respirable dust suppression efficiencies by surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) were 90.32% and 87.96%, respectively, improving by 39.87% and 48.81% compared to that of untreated water, respectively. The field investigation results demonstrated that the compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) could effectively capture coal dust, especially respirable dust. 5. Conclusions
Fig. 11. The dust control efficiency of untreated water (A), surfactant compounding solution consisting of 0.025 wt% FMES and 0.025 wt% CDEA (B).
monomers (Fig. 9b). The contact angle was in agreement with the experimental results of surface tension. The result of the coal dust sedimentation experiments directly show the synergistic effect of the compound surfactant on the wettability of coal. The settling velocity of coal in the nonionic-anionic surfactant compounding solution was significantly faster than that of the monomer solution. In particular, the settlement velocity of the FMES and CDEA mixed solution was significantly faster; the settlement of 0.02-g coal was only 15.95 s, which was faster by 97.91% and 98.73% compared to that of the single component FMES and CDEA, respectively. The settling velocity of coal in the anionic-anionic compound solution AOS and FMES, AOS, and AES was only a compromise of the single component velocity. The surface tension of the FMES and AES compound solution increased more than that of the single component, yet the settlement velocity was significantly faster than that of the single component. However, it did not exhibit significant synergistic effects as the nonionicanionic surfactant complex system. These experimental results indicated that the wettability of the surfactant to coal dust depended not only on the surfactant vapor-liquid surface tension, but also on the solid-liquid interfacial tension between the surfactant and the coal dust. In this experiment, the anionic and nonionic surfactants were compounded, confirming that the intermolecular interaction exists and that the mixed micelles are formed in the compound system, which can produce additive and synergistic effects and can improve the wettability of the surfactant. In particular, the 0.025 wt% FMES and 0.025 wt% CDEA compound solution (anionic-nonionic surfactant compound system) showed significant synergistic effect and was determined to be the best formula for the field test of a coal mine. 4. Field investigation To further verify the dust control ability of the 0.025 wt% FMES and 0.025 wt% CDEA compound solution, a field test was conducted in the 2104 fully mechanized coalface of a coal mine in Erdos, China. The generation system of compound solution is shown in Fig. 10. Untreated water and surfactant compounding solution (0.025 wt% FMES and 0.025 wt% CDEA) were each applied at the place operated by the shearer operator. The dust sampling followed the standard “Specifications of air sampling for hazardous substances monitoring in the workplace” (GBZ 159–2004) [25], using the AKFC-92A type dust sampler manufactured by Changshu Mining Electromechanical Equipment Co. Ltd. in Changshu to collect dust for 15 min (short-time sampling) and determine the dust concentrations before and after applying dust control. The dust concentrations in air before spraying,
(1) In low concentrations of surfactants that can wet coal, anionic and nonionic surfactant solutions are significantly better than those of the cationic and amphoteric surfactants. In the compound solution, the anionic-nonionic compounding system has the best effect on the wettability of coal. (2) The compound solution (0.025 wt% FMES and 0.025 wt% CDEA) can decrease the surface tension and the contact angle to the coal tablet by 59.39% and 83.57%, respectively, compared to that of untreated water, because of the significantly improved wettability of the solution. (3) The compound solution (0.025 wt% FMES and 0.025 wt% CDEA) can be successfully applied in the practical conditions of an operating coal mine and significantly improves the dust control efficiency of water spray. Compared to that of untreated water, the optimized compound solution can enhance the total dust and respirable dust control efficiencies by 39.87% and 48.81%, respectively, remarkably reducing the dust concentration at the coalface of an underground coal mine. Acknowledgments This work was supported by the Fundamental Research Funds for the National Natural Science Foundation of China (No. 51874009). References [1] Chineese Center for Disease Control and Prevention, Occupational Health Risk Assessment Report for Dust, http://niohp.chinacdc.cn/2015ndgzxmbg/zyjkfxpg/ Vol. 13, 2015 2018 accessed March. [2] 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) 489. [3] Q. Huang, R. Honaker, Optimized reagent dosage effect on rock dust to enhance rock dust dispersion and explosion mitigation in underground coal mines, Powder Technol. 301 (2016) 1193–1200. [4] Q. Zhou, B. Qin, J. Wang, H. Wang, F. Wang, Effects of preparation parameters on the wetting features of surfactant-magnetized water for dust control in Luwa mine, China, Powder Technol. 326 (2018) 7–15. [5] G. Zhou, Research of Theory about Dust Prevention by Water-Cloud and Relevant Techniques for Fully-Mechanized Caving Coalface, PhD Dissertation Shandong University of Science and Technology, Qingdao, China, 2009. [6] B. Qin, Q. Zhou, X. Li, J. Wang, H. Wang, Y. Ding, Synergistic technology between surfactant and magnetized water for efficient dust control in underground coal mines, J. China Coal Soc. 42 (2017) 2900–2907. [7] X. Zhang, D. Yang, J. Lou, Curative effect of large volume whole lung lavage with low concentration of poly-2-vinyl pyridine n oxide on the coal workers' pneumoconiosis after 6 years, Chin. J. Ind. Hyg. Occup. Dis. 19 (2001) 27–29. [8] Y.L. Liu, G. Fu, P. Yu, Study on diffusion rules of toxic gas from coal mine explosion, Saf. Coal Mines 39 (2008) 4–7. [9] Y.X. Zheng, Research on the Controlling Technique and Technological Process of Coal Dust with Less Water in Thin Coal Seam, PhD Dissertation Shandong University of Science and Technology, Qingdao, China, 2008. [10] W.G. Huang, F. Hu, N.Q. Liu, Study on surfactant wettability for coal dust, Mining Saf. Environ. Prot. 37 (2010) 4–6. [11] Z. Gui, R. Liu, P. Wang, S. Gou, W. Shu, X. Tan, Experimental study on surfactant effect on coal dust wettability, J. Heilongjiang Univ. Sci. Technol. 26 (2016) 513–517. [12] J.Y. Li, Y.P. Lu, C.X. Sun, Y. Shi, Experimental study on the wetting characteristics of coal dust, Chin. Energy Environ. Prot. 6 (2015) 36–38. [13] J. Yang, Y.Z. Tan, Z.H. Wang, Y.D. Shang, W.B. Zhao, Study on the coal dust surface characteristics and wetting mechanism, J. China Coal Soc. 32 (2007) 737–740.
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