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Novel technology for synergetic dust suppression using surfactant-magnetized water in underground coal mines
Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 631–638
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Novel technology for synergetic dust suppression using surfactant-magnetized water in underground coal mines Qun Zhou a,b , Botao Qin a,b,∗ , Dong Ma b , Ning Jiang b a
Key Laboratory of Coal Methane and Fire Control (China University of Mining and Technology), Ministry of Education, Xuzhou, 221116, China b School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Coal dust is an increasingly serious problem in underground coal mines. This research devel-
Received 4 November 2016
oped and tested a novel dust prevention technology of surfactant-magnetized water that
Received in revised form 17 May
utilizes the synergy between magnetization and surfactants to markedly improve the wetta-
2017
bility of water, resulting in better dust suppression than water sprays alone. The technology
Accepted 24 May 2017
was systematically studied in laboratory and field conditions. A compound surfactant was
Available online 1 June 2017
developed as part of the new technology and was effective at low dosage (0.03 wt%, approxi-
Keywords:
synergetic effects with magnetization, with reduced surface tension (28.07 mN m−1 , 7.2%
Dust control
lower than that of the original solution). A new type of magnetic apparatus was designed
mately one-sixth that of conventional alternatives). The new surfactant exhibited excellent
Surfactant-magnetized water
and formed the core of the novel dust suppression technology. The magnetic device produces
Surfactant
a powerful and consistent magnetic field (300–350 mT) to achieve effective magnetization of
Dust suppression efficiency
water flow. In field tests, the new technology increased respirable dust and total dust sup-
Magnetic apparatus
pression efficiencies by 44.94% and 31.79%, respectively, compared to that of water spray. And the new technology effectively improves the atmosphere in mechanized underground coal mines, contributing to a safer and healthier working environment. © 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
tion, accidental coal dust explosions are likely to occur, leading to heavy casualties and huge economic losses (Zheng et al., 2009). An exam-
Coal dust is an urgent and constantly perplexing problem for the coal
ple occurred on 15 July 2006 in the Lin Jiazhuang coal mine in Shanxi
industry. With the continuous improvement of mechanized mining techniques in underground coal mines, dust production has increased
province, China, where a serious coal dust explosion accident killed 54 workers. At present, water spray is widely used as an economical method
markedly, causing adverse working conditions and endangering workers’ health, specifically from coal workers’ pneumoconiosis (Mo et al., 2014; Wang et al., 2015a,b). Statistical data from the China National Institute of Occupational Health and Poison Control indicates that approximately 23,000 pneumoconiosis cases were diagnosed throughout the country in 2013, of which almost 14,000 (approximately 60%) came from the coal mining industry (Han et al., 2015). Furthermore, when mixtures of floating dust and oxygen reach a certain concentra-
of dust control in underground coal mines, but the dust-prevention efficiency of water spray is usually poor owing to the hydrophobic characteristics of coal dust and the high surface tension of water (Lu et al., 2015; Zheng et al., 2012). Surfactants have been developed to solve these problems and improve dust control (Dixon-Hardy et al., 2008; Summers and Parmigiani, 2015; Wang et al., 2015a,b). These additives form a dense hydrophilic layer on the water surface to prevent contact of the water with air, allowing the surface tension of the water to
∗ Corresponding author at: School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. Fax: +86 516 83885694. E-mail address: [email protected] (B. Qin). http://dx.doi.org/10.1016/j.psep.2017.05.013 0957-5820/© 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Table 1 – Relevant parameters of Nd–Fe–B permanent magnet. Type
Br (T)
Hc (KA m−1 )
(BH)max (KJ m−3 )
N35
1.39
1000
335–395
decrease significantly, thus improving the wettability of the solution. However, the required dosage of surfactants is generally high (usually more than 0.2 wt%), rendering them a high-cost solution for dust prevention (Tessum et al., 2014; Wu et al., 2007). Thus, to safely achieve the production goals of the coal mining industry, effective and economic measures are urgently needed to reduce dust concentrations in the working environment, especially the concentration of respirable dust. In recent years, research on magnetized water has attracted increasing attention, such as for scale inhibition in pipe networks and boilers and for crop breeding (Ambashta and Sillanpaa, 2010; Moon and Chung, 2000; Silva et al., 2015). In terms of dust prevention, relevant studies on magnetized water are also continually emerging. In comparative dust prevention experiments between untreated water and magnetized water, the dust control efficiency of magnetized water was 12%–30% greater than that of untreated water (Chen et al., 2014; Zeng et al., 2014). Magnetization can change the physical–chemical properties of water, such as enhancing its wettability (Lee et al., 2013). Dust suppression using magnetized water is both environmentally and economically advantageous. Nevertheless, in its present form, this technique cannot effectively solve the dust problems of coal mines because the improvement in wettability from magnetization is limited (Pang and Deng, 2008). To overcome the individual limitations of magnetization and surfactants in dust prevention, this study developed and tested a novel dust prevention technology consisting of surfactants combined with magnetized water. This technique integrates the functions of magnetization and surfactants to significantly reduce dust control costs and improve wettability, reducing surfactant usage to only 0.03 wt%. Because most of the available magnetic systems are used in the medical and water purification fields (Dobersek and Goricanec, 2014; Mahmoud et al., 2016), the research involved first designing a surfactantmagnetized water generation system with a new type of magnetic apparatus for coal dust prevention. Field-scale experiments indicated that the new surfactant-magnetized water method had a better dust suppression efficiency than traditional water spray systems, especially in terms of controlling respirable dust.
2.
Experimental preparation
2.1.
Materials and facilities
Two surfactants were used to obtain a new low-cost surfactant mixture that would effectively improve the wettability of water under the effect of a magnetic field. These were nonionic surfactant D, which was a fatty acid methyl ester ethoxylate (FMEE), and anionic surfactant A, which was sodium dodecyl benzene sulfonate (SDBS). Both chemicals were procured from the Lin Yi Green Chemical Co. Ltd., Shandong, China. Both surfactants readily dissolve in water at room temperature and quickly decrease the surface tension of water. Permanent magnets (60 × 40 × 4 mm) made of an Nd–Fe–B alloy were used to conduct laboratory tests and were acquired from Zheng Guo Magnetic Co. Ltd., Shanghai, China. Detailed parameters of the magnets are shown in Table 1. Coal samples were acquired from the Lu Wa coal mine (Jining, Shandong, China), one-third of which were characteristically composed of coking coal. Each coal sample was crushed and sieved to produce particles finer than 325 mesh (less than 45 m). The diameter of respirable dust is less than 7 m, and the size of respirable dust throughout the
experiments was checked using a LS609-type laser particle size analyzer (Omec Instruments Co. Ltd., Zhuhai, China). In addition, the main experimental equipment included a surface tensiometer (JYW-200B, Chengde Experimental Products Co. Ltd., Hebei, China), gauss meter (TM-701, Kanetec, Nagano-ken, Japan), contact angle instrument (JGW-360B, Cheng Hui Experimental Instrument Co. Ltd., Hebei, China), high-pressure water pump (HM280, Black Cat Trade Co. Ltd., Hongkong, China), and tablet press machine (FY-24, Strong Lean Technology Development Co. Ltd., Tianjin, China).
2.2.
Experimental design
(1) Various mixtures of anionic surfactant A and nonionic surfactant D were used in synergetic combination experiments. The total mass fraction of the mixtures was 0.03 wt%, and the mixed mass ratios of A:D were 6:0, 5:1, 4:2, 3:3, 2:4, 1:5, 0:6. In addition, for the total A:D mass ratio of 5:1, the mixed mass fraction of their mixtures was 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, respectively. The effect of various compound methods on the solution surface tension was systematically measured. All samples were studied at 25 ◦ C in the laboratory. (2) To confirm the synergy between the surfactant solution and the magnetized field and to determine the optimal magnetization parameters of surfactant-magnetized water, the surface tension and contact angle of the surfactant solution were measured under various magnetic intensities and water flow velocities through the magnetic field. (3) To carry out atomization and dust suppression experiments, a laboratory scale system was constructed to simulate a spray for dust prevention in an actual coal mine (Fig. 1). The system included a dust feeder, a fan, an air diffuser device, a simulated roadway, surfactantmagnetized water generation equipment, a dust sampler device, and atomization equipment. The simulated roadway of the experimental system was an enclosed chamber (6 × 1.2 × 1.2 m) constructed of glass and steel, with an air diffuser 1.7 m long. The water flow rate for the spray system was 8 L min−1 . The air sampling method followed the China National Standard method for determination of dust in the air of workplaces (Standard, 2008), using the AKFC92A-type dust sampler (Huaqiang Mining Equipment Co. Ltd., Shandong, China) to collect dust. Generally, the dust control efficiency cannot be directly measured but must be calculated using Eq. (1) and dust concentrations measured before and after applying dust control technologies.
=
C−c × 100% C
(1)
In Eq. (1), (%) is the dust suppression efficiency, C (mg m−3 ) is the dust concentration before applying dust control technology, and c (mg m−3 ) is the concentration after applying dust control technology.
Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 631–638
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Fig. 1 – Experimental spray system for preventing coal dust (a: dust feeder; b, c: simulated roadway; d: spray nozzles; e: improvised surfactant-magnetized water generation system).
Fig. 2 – (a) Surface tension change of solution as a function of mass ratio. (b) Surface tension change of solution as a function of mass fraction.
3.
Results and discussion
3.1.
Optimal compound of surfactants
Surface tensions of the various compound surfactant solutions are shown as a function of mass ratio and mass fraction in Fig. 2. As shown in Fig. 2a, as the mass ratios between anionic surfactant A and nonionic surfactant D varied, the surface tension of the solution changed. This occurred because different types of surfactants present various modes of action with water. For example, hydrophilic groups of an anionic surfactant are arranged on the surface of water in ionic adsorption mode, generating an interface layer to isolate the contact between water and air and decrease the surface tension of
water. However, the surface tension of an aqueous solution containing only SDBS cannot reach its minimum value due to some interspaces existing in the interface layer of an SDBS aqueous solution. By contrast, a nonionic surfactant employs hydrogen bonds formed between hydrophilic groups and water molecules to produce a dense isolation layer. In the compounding process combining anionic and nonionic surfactants, the individual effects induce a synergetic interaction. Therefore, after FMEE is added to a solution containing SDBS, the interspaces caused by the coulomb repulsion among SDBS molecules will be filled by the FMEE molecules, making the isolating layer generated by surfactant molecules between air and water surfaces denser, and further reducing surface tension of the solution (Li et al., 2007; Liu et al., 2016; Zhu et al., 2016).
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Fig. 3 – The effect of different magnetic field intensities on the surface tension and contact angle of a 0.03 wt% solution.
Fig. 4 – Influence of water flow velocity through the magnetic field on the surface tension and contract angle of a 0.03 wt% solution.
Nonetheless, when the mass ratio of anionic to nonionic surfactants exceeded 5:1, the surface tension of the solution increased slightly. This occurred because the excessive content of nonionic surfactant caused the mode of action of this surfactant to gradually become the dominant factor in the solution characteristics, which weakened the synergy between the anionic and nonionic surfactants. Therefore, the optimal compound mass ratio of anionic to nonionic surfactants was 5:1. As shown in Fig. 2b, the surface tension of the solution was significantly reduced within the mass fraction range of 0.0–0.03 wt% as the surfactant content in the water increased. When the mass fraction of the surfactant solution was 0.03 wt%, the concentration of the solution reached the critical micelle concentration, the saturation concentration of surfactant at which the surface tension equalizes, indicating that the solution surface was completely covered by surfactant molecules (Bak and Podgorska, 2016). At this point, the surface tension of the solution also reached the critical micelle value (30.24 mN m−1 ), after which surface tension of the solution did not exhibit an obvious change. Therefore, for an effective economic dosage, the mass fraction of the new surfactant was confirmed to be 0.03 wt%.
As shown in Fig. 3, in the magnetic intensity range 0–350 mT, the surface tension and a contact angle of the 0.03 wt% surfactant solution had a tendency to decrease with increasing magnetic intensity, indicating that a synergy existed between the surfactant and magnetization that changed the wetting characteristics of the solution. Most aqueous solution molecules do not exist alone, but instead form large molecular clusters through hydrogen bonds. However, due to the constant motion of the solution molecules, the hydrogen bonds are always being fractured and recombining. When the surfactant solution was passed through the magnetic field, the molecular motion was accelerated, making the hydrogen bonds between solution molecules easier to fracture. Thus, the large molecular clusters were broken into small molecule groups, which weakened the solution cohesion and improved the capacity of the solution to wet dust (Ding et al., 2011; Pang and Deng, 2008; van Oss, 2003). At the magnetic intensity of 350 mT the physical-chemical characteristics of the surfactant solution were deemed to be optimal. A magnetic intensity exceeding 350 mT increased the surface tension and contact angle of the solution, exhibited in Fig. 3. This occurred mainly because hydrophilic groups of segmental anionic surfactant escaped the surface of water molecules under the effects of the magnetized field over 350 mT (ShamsiJazeyi et al., 2014), changing the combination modes between anionic and nonionic surfactants and water molecules and making the isolation layer between air and the solution surface become slightly loose. A magnetic intensity that was greater than the optimum also weakened the synergy between anionic and nonionic surfactants, further augmenting the surface tension and contact angle of the solution. Therefore, to achieve the best magnetic effect on a solution, the optimal magnetization intensity for application to the 0.03 wt% surfactant solution was 350 mT. As exhibited in Fig. 4, within the range 0–4 m s−1 of water flowing through the magnetic field, surface tension and a contact angle for the 0.03 wt% surfactant solution decreased as water velocity increased. At a water velocity of 4 m s−1 , the surface tension (28.07 mN m−1 ) and contact angle (27.02◦ ) of the surfactant solution reached their optimum values. As water velocity increased beyond 4 m s−1 , the change in solution properties was similar to that in response to different magnetization intensities. During magnetization of the solution, the Lorentz force of the magnetic field is enhanced as the water velocity increases, improving the interaction between the external magnetic field and the solution molecule
3.2. Synergetic characteristics and optimal magnetization parameters of surfactant-magnetized water Through various arrangements of hydrophilic groups on the water surface, surfactants cause differences in water wetting characteristics, which can be considered a chemical change. By reviewing relevant literature (Afshin et al., 2010; Cho and Lee, 2005; Pang and Deng, 2008) and carrying out many experiments in the laboratory, it was found that some properties of water show various degrees of change under the effects of magnetic fields; these changes include reduced surface tension and improved wetting ability. Therefore, surfactant solution wettability should be improved under the effects of a magnetized field, thereby reducing dosage of surfactants, making operation of a dust control system more convenient, and improving the prospects for long-term use. To verify this hypothesis, surface tension and contact angle experiments were conducted on the solution under various magnetic intensities and water flow velocities through the magnetic field, for which the experimental results are shown in Figs. 3 and 4, respectively.
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Table 2 – Dust concentration before and after applying different dust removal techniques. Dust control technology
C (mg m−3 )
c1
c2
c3
c = (c1 + c2 + c3 )/3 (mg m−3 )
Untreated water Magnetized water Surfactant-amended solution Surfactant-magnetized water
110.42 110.42 110.42 110.42
57.71 43.36 22.47 13.89
55.63 43.33 21.98 13.85
57.91 43.42 22.63 14.17
57.08 ± 1.26 43.37 ± 0.04 22.36 ± 0.34 13.97 ± 0.17
forces. The enhanced Lorentz force aids in breaking the hydrogen bonds or reducing the bond angles of hydrogen bonds, which decreases the free energy of cohesion among solution molecules and improves the wetting ability of the solution (van Oss, 2003). Beyond the optimal water velocity, hydrophilic groups of segmental anionic surfactants might break away from water molecules under the effects of the magnetic field, further weakening the synergetic effects of the anionic and nonionic surfactants. Consequently, the optimal flow velocity of surfactant-amended solution through the magnetic field was judged to be 4 m s−1 .
3.3. Experimental study on dust suppression effect of surfactant-magnetized water To further verify the synergy between surfactants and magnetization for dust prevention, the system described in Section 2.2 (Fig. 1) was used to measure dust suppression by untreated water, magnetized water, surfactant-amended solution, and surfactant-magnetized water. The surfactantmagnetized water (0.03 wt%) and magnetized water were produced at a magnetic field strength of 350 mT and a water flow rate of 4 m s−1 . Utilizing Eq. (1) and dust concentration data measured before and after applying the different dust removal techniques (Table 2), the dust suppression efficiency of each dust prevention technique was quantified (Fig. 5). As shown in Fig. 5, the dust control efficiency (48.3%) of untreated water was very limited. Magnetized water and surfactantamended solution had better wettability than untreated water and thus exhibited higher dust control efficiencies (60.72% and 79.75%, respectively) than untreated water. The dust suppression efficiency of surfactant-magnetized water (87.35%) was much higher than the individual efficien-
Fig. 5 – The dust suppression efficiency of untreated water, magnetized water, surfactant-amended solution and surfactant-magnetized water is shown in the bar chart as A, B, C, and D, respectively. cies of both surfactant-amended solution and magnetized water, illustrating the synergetic effect between surfactants and magnetization. Furthermore, the magnitude of the dust suppression efficiency arising from surfactant-magnetized water showed that the synergetic effect between magnetization and surfactants cannot be attributed to simple superimposition of the individual efficiencies of surfactantamended water and magnetized water i.e., magnetization could further improve the wetting ability of water while surfactant improving the dust capture ability of water. These results demonstrated that surfactant-magnetized water can efficiently capture coal dust and this represents a new technique for dust prevention in coal mines.
Fig. 6 – Schematic diagram of the novel surfactant-magnetized water generation system.
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Fig. 7 – Internal structure of an efficient magnetic apparatus (a: section of magnetic apparatus; b: external permanent magnet layout of magnetizing apparatus; c: internal helical water channel of magnetic apparatus).
Fig. 8 – Field-scale surfactant-magnetized water generation system (a: surfactant adding system; b: magnetic apparatus; c: water tank; d: pressure pump).
4. The generation system for surfactant-magnetized water To adapt to the narrow working space in an underground coal mine, and with the above experimental results in mind, a prototype field-scale surfactant-magnetized water generation system was developed and tested for dust prevention in coal mines (Fig. 6). The system included liquid handling devices, a static mixer, magnetizing apparatus, and other components. The devices for quantitatively adding liquid were composed of a metering pump, glass rotameter, solution barrel, and other hardware, the advantage of which was the accurate addition of low-concentration surfactant using dual governing methods (i.e., metering pump and glass rotameter). The static mixer utilized an internal helical structure to uniformly mix water and surfactant. Surfactant-magnetized water was generated by passing the surfactant-amended solution through the magnetizing apparatus. Finally, surfactant-magnetized water was
formed into micro-diameter spray droplets to effectively control coal dust. Through experiments investigating the optimal magnetization parameters of surfactant-magnetized water, related parameters pertaining to the magnetic apparatus were confirmed. For field application, a new type of magnetic apparatus to effectively produce surfactant-magnetized water for continuous dust control was designed (Fig. 7). As shown in Fig. 7a and b, the inner and outer permanent magnets (PM) of the magnetic apparatus were interlaced to form an alternating magnetic field environment (in contrast to a typical single magnetic field); the interlaced arrangement achieved pulsing magnetization of flowing water and a significant magnetizing effect. As depicted in Fig. 7c, the inside water channel of the magnetic apparatus utilized a spiral propulsion structure to improve the turbulence and mixing of the water. In contrast to a once-through magnetization channel for water flow, the spiral propulsion channel was more conducive to achieving uniform magnetization of the solution.
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(a) The mass ratio of anionic surfactants to nonionic surfactants was 5:1. (b) The mass fraction of the surfactant solution was 0.03 wt%. (c) The spray pressure of the surfactant-magnetized water was 3–4 MPa.
Fig. 9 – Average dust suppression efficiency for total dust and respirable dust of different dust control technologies.
The designed magnetizing apparatus has three advantages. First, the magnetic apparatus can form a powerful (i.e., 300–350 mT) and steady magnetic intensity under the effects of the inner and outer magnets, which effectively solves the problem of insufficient magnetic intensity existing in currently available magnetic devices. Second, the magnetic apparatus does not contain moving elements and does not need external power, which makes the device safer and more secure for application in coal mines. Third, as shown in Section 5, the dust capturing ability of water can be improved by more than 10% under the effect of the magnetic apparatus.
5.
Field application
Lu Wa coal mine is a modern underground mine located in Jining city, Shandong province, China. Due to implementation of mechanized mining techniques, dust hazards have been a major source of danger in Lu Wa, severely threatening the health of workers and the safe production of the mine. To provide a healthy working environment for workers and reduce the risk of dust explosion, the efficient dust control technology of surfactant-magnetized water was successfully applied in the 23012 fully mechanized face of the Lu Wa coal mine, as exhibited in Fig. 8. As a result, dust concentrations were significantly reduced. The operational parameters of the dust control technique were as follows:
Untreated water, magnetized water, surfactant-amended solution, and surfactant-magnetized water were applied in the same environment. Dust concentrations measured under the effect of the different dust control techniques confirmed the significant dust capturing efficiency of surfactant-magnetized water, which remarkably decreased the dust concentration at the working face, especially the concentration of respirable dust. Under identical spray conditions at the 23,012 fully mechanized face of the Lu Wa coal mine, the efficiencies of respirable dust control and total dust control by surfactantmagnetized water were 83.54% and 84.75%, respectively. As shown in Fig. 9, compared to that of untreated water, the respirable dust and total dust control efficiency of surfactantmagnetized water were increased by 44.94% and 31.79%, respectively. Therefore, the field application demonstrated that because of the synergy between magnetization and surfactants, the spray droplets of surfactant-magnetized water effectively captured coal dust, especially respirable dust, and markedly improved the working environment of the underground coal mine, as shown in Fig. 10.
6.
Conclusions
(a) A new surfactant was developed by mixing anionic and nonionic surfactants. The new surfactant is effective at a low dosage (0.03 wt%) in significantly improving the wettability of water. The new solution was magnetized to for use in suppressing coal mine dust and the optimal magnetization parameters were a field strength of 350 mT and a solution flow rate of 4 m s−1 . The synergy effect between the surfactant and magnetic field decreased the surface tension of the surfactant solution by approximately 7.2% (to 28.07 mN m−1 ) compared to its original value and the contact angle decreased to 27.02◦ . These changes effectively enhance the dust wetting ability of the solution. (b) A surfactant-magnetized water generation system was developed and tested. The system achieves accurate metering of low-concentration surfactant using the dual effect of a metering pump and glass rotameter.
Fig. 10 – Visual appearance of the working environment at the coal mining face after using (a) untreated water and (b) surfactant-magnetized water for dust control. The light gray color in (a) is caused by light reflecting off highly concentrated dust; by comparison, there was very little dust in (b).
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This system can efficiently and continuously produce surfactant-magnetized water. A new type of magnetic apparatus was invented for use in the system. The apparatus utilizes an interlaced arrangement of magnets to form an alternating magnetic field, achieving the effective magnetization of a solution passing through it. (c) The dust control technology consisting of a novel system to efficiently generate a new formulation of surfactantmagnetized water can significantly reduce the coal dust concentration at a mechanized working face, especially the concentration of respirable dust. Compared to that of untreated water, the respirable dust and total dust control efficiencies of surfactant-magnetized water increased by 44.94% (reaching 83.54%) and 31.79% (reaching 84.75%), respectively. The new technology effectively improves the atmospheric environment in mechanized underground coal mines, which will help guarantee the physical and mental health of workers as well as the enterprise’s safe production.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2017CXNL02), the National Key Research and Development Program of China (2017YFC0805200) and the Excellent Innovation Team of China University of Mining and Technology (2015ZY002).
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Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Novel technology for synergetic dust suppression using surfactant-magnetized water in underground coal mines Qun Zhou a,b , Botao Qin a,b,∗ , Dong Ma b , Ning Jiang b a
Key Laboratory of Coal Methane and Fire Control (China University of Mining and Technology), Ministry of Education, Xuzhou, 221116, China b School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Coal dust is an increasingly serious problem in underground coal mines. This research devel-
Received 4 November 2016
oped and tested a novel dust prevention technology of surfactant-magnetized water that
Received in revised form 17 May
utilizes the synergy between magnetization and surfactants to markedly improve the wetta-
2017
bility of water, resulting in better dust suppression than water sprays alone. The technology
Accepted 24 May 2017
was systematically studied in laboratory and field conditions. A compound surfactant was
Available online 1 June 2017
developed as part of the new technology and was effective at low dosage (0.03 wt%, approxi-
Keywords:
synergetic effects with magnetization, with reduced surface tension (28.07 mN m−1 , 7.2%
Dust control
lower than that of the original solution). A new type of magnetic apparatus was designed
mately one-sixth that of conventional alternatives). The new surfactant exhibited excellent
Surfactant-magnetized water
and formed the core of the novel dust suppression technology. The magnetic device produces
Surfactant
a powerful and consistent magnetic field (300–350 mT) to achieve effective magnetization of
Dust suppression efficiency
water flow. In field tests, the new technology increased respirable dust and total dust sup-
Magnetic apparatus
pression efficiencies by 44.94% and 31.79%, respectively, compared to that of water spray. And the new technology effectively improves the atmosphere in mechanized underground coal mines, contributing to a safer and healthier working environment. © 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
tion, accidental coal dust explosions are likely to occur, leading to heavy casualties and huge economic losses (Zheng et al., 2009). An exam-
Coal dust is an urgent and constantly perplexing problem for the coal
ple occurred on 15 July 2006 in the Lin Jiazhuang coal mine in Shanxi
industry. With the continuous improvement of mechanized mining techniques in underground coal mines, dust production has increased
province, China, where a serious coal dust explosion accident killed 54 workers. At present, water spray is widely used as an economical method
markedly, causing adverse working conditions and endangering workers’ health, specifically from coal workers’ pneumoconiosis (Mo et al., 2014; Wang et al., 2015a,b). Statistical data from the China National Institute of Occupational Health and Poison Control indicates that approximately 23,000 pneumoconiosis cases were diagnosed throughout the country in 2013, of which almost 14,000 (approximately 60%) came from the coal mining industry (Han et al., 2015). Furthermore, when mixtures of floating dust and oxygen reach a certain concentra-
of dust control in underground coal mines, but the dust-prevention efficiency of water spray is usually poor owing to the hydrophobic characteristics of coal dust and the high surface tension of water (Lu et al., 2015; Zheng et al., 2012). Surfactants have been developed to solve these problems and improve dust control (Dixon-Hardy et al., 2008; Summers and Parmigiani, 2015; Wang et al., 2015a,b). These additives form a dense hydrophilic layer on the water surface to prevent contact of the water with air, allowing the surface tension of the water to
∗ Corresponding author at: School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. Fax: +86 516 83885694. E-mail address: [email protected] (B. Qin). http://dx.doi.org/10.1016/j.psep.2017.05.013 0957-5820/© 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Table 1 – Relevant parameters of Nd–Fe–B permanent magnet. Type
Br (T)
Hc (KA m−1 )
(BH)max (KJ m−3 )
N35
1.39
1000
335–395
decrease significantly, thus improving the wettability of the solution. However, the required dosage of surfactants is generally high (usually more than 0.2 wt%), rendering them a high-cost solution for dust prevention (Tessum et al., 2014; Wu et al., 2007). Thus, to safely achieve the production goals of the coal mining industry, effective and economic measures are urgently needed to reduce dust concentrations in the working environment, especially the concentration of respirable dust. In recent years, research on magnetized water has attracted increasing attention, such as for scale inhibition in pipe networks and boilers and for crop breeding (Ambashta and Sillanpaa, 2010; Moon and Chung, 2000; Silva et al., 2015). In terms of dust prevention, relevant studies on magnetized water are also continually emerging. In comparative dust prevention experiments between untreated water and magnetized water, the dust control efficiency of magnetized water was 12%–30% greater than that of untreated water (Chen et al., 2014; Zeng et al., 2014). Magnetization can change the physical–chemical properties of water, such as enhancing its wettability (Lee et al., 2013). Dust suppression using magnetized water is both environmentally and economically advantageous. Nevertheless, in its present form, this technique cannot effectively solve the dust problems of coal mines because the improvement in wettability from magnetization is limited (Pang and Deng, 2008). To overcome the individual limitations of magnetization and surfactants in dust prevention, this study developed and tested a novel dust prevention technology consisting of surfactants combined with magnetized water. This technique integrates the functions of magnetization and surfactants to significantly reduce dust control costs and improve wettability, reducing surfactant usage to only 0.03 wt%. Because most of the available magnetic systems are used in the medical and water purification fields (Dobersek and Goricanec, 2014; Mahmoud et al., 2016), the research involved first designing a surfactantmagnetized water generation system with a new type of magnetic apparatus for coal dust prevention. Field-scale experiments indicated that the new surfactant-magnetized water method had a better dust suppression efficiency than traditional water spray systems, especially in terms of controlling respirable dust.
2.
Experimental preparation
2.1.
Materials and facilities
Two surfactants were used to obtain a new low-cost surfactant mixture that would effectively improve the wettability of water under the effect of a magnetic field. These were nonionic surfactant D, which was a fatty acid methyl ester ethoxylate (FMEE), and anionic surfactant A, which was sodium dodecyl benzene sulfonate (SDBS). Both chemicals were procured from the Lin Yi Green Chemical Co. Ltd., Shandong, China. Both surfactants readily dissolve in water at room temperature and quickly decrease the surface tension of water. Permanent magnets (60 × 40 × 4 mm) made of an Nd–Fe–B alloy were used to conduct laboratory tests and were acquired from Zheng Guo Magnetic Co. Ltd., Shanghai, China. Detailed parameters of the magnets are shown in Table 1. Coal samples were acquired from the Lu Wa coal mine (Jining, Shandong, China), one-third of which were characteristically composed of coking coal. Each coal sample was crushed and sieved to produce particles finer than 325 mesh (less than 45 m). The diameter of respirable dust is less than 7 m, and the size of respirable dust throughout the
experiments was checked using a LS609-type laser particle size analyzer (Omec Instruments Co. Ltd., Zhuhai, China). In addition, the main experimental equipment included a surface tensiometer (JYW-200B, Chengde Experimental Products Co. Ltd., Hebei, China), gauss meter (TM-701, Kanetec, Nagano-ken, Japan), contact angle instrument (JGW-360B, Cheng Hui Experimental Instrument Co. Ltd., Hebei, China), high-pressure water pump (HM280, Black Cat Trade Co. Ltd., Hongkong, China), and tablet press machine (FY-24, Strong Lean Technology Development Co. Ltd., Tianjin, China).
2.2.
Experimental design
(1) Various mixtures of anionic surfactant A and nonionic surfactant D were used in synergetic combination experiments. The total mass fraction of the mixtures was 0.03 wt%, and the mixed mass ratios of A:D were 6:0, 5:1, 4:2, 3:3, 2:4, 1:5, 0:6. In addition, for the total A:D mass ratio of 5:1, the mixed mass fraction of their mixtures was 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, respectively. The effect of various compound methods on the solution surface tension was systematically measured. All samples were studied at 25 ◦ C in the laboratory. (2) To confirm the synergy between the surfactant solution and the magnetized field and to determine the optimal magnetization parameters of surfactant-magnetized water, the surface tension and contact angle of the surfactant solution were measured under various magnetic intensities and water flow velocities through the magnetic field. (3) To carry out atomization and dust suppression experiments, a laboratory scale system was constructed to simulate a spray for dust prevention in an actual coal mine (Fig. 1). The system included a dust feeder, a fan, an air diffuser device, a simulated roadway, surfactantmagnetized water generation equipment, a dust sampler device, and atomization equipment. The simulated roadway of the experimental system was an enclosed chamber (6 × 1.2 × 1.2 m) constructed of glass and steel, with an air diffuser 1.7 m long. The water flow rate for the spray system was 8 L min−1 . The air sampling method followed the China National Standard method for determination of dust in the air of workplaces (Standard, 2008), using the AKFC92A-type dust sampler (Huaqiang Mining Equipment Co. Ltd., Shandong, China) to collect dust. Generally, the dust control efficiency cannot be directly measured but must be calculated using Eq. (1) and dust concentrations measured before and after applying dust control technologies.
=
C−c × 100% C
(1)
In Eq. (1), (%) is the dust suppression efficiency, C (mg m−3 ) is the dust concentration before applying dust control technology, and c (mg m−3 ) is the concentration after applying dust control technology.
Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 631–638
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Fig. 1 – Experimental spray system for preventing coal dust (a: dust feeder; b, c: simulated roadway; d: spray nozzles; e: improvised surfactant-magnetized water generation system).
Fig. 2 – (a) Surface tension change of solution as a function of mass ratio. (b) Surface tension change of solution as a function of mass fraction.
3.
Results and discussion
3.1.
Optimal compound of surfactants
Surface tensions of the various compound surfactant solutions are shown as a function of mass ratio and mass fraction in Fig. 2. As shown in Fig. 2a, as the mass ratios between anionic surfactant A and nonionic surfactant D varied, the surface tension of the solution changed. This occurred because different types of surfactants present various modes of action with water. For example, hydrophilic groups of an anionic surfactant are arranged on the surface of water in ionic adsorption mode, generating an interface layer to isolate the contact between water and air and decrease the surface tension of
water. However, the surface tension of an aqueous solution containing only SDBS cannot reach its minimum value due to some interspaces existing in the interface layer of an SDBS aqueous solution. By contrast, a nonionic surfactant employs hydrogen bonds formed between hydrophilic groups and water molecules to produce a dense isolation layer. In the compounding process combining anionic and nonionic surfactants, the individual effects induce a synergetic interaction. Therefore, after FMEE is added to a solution containing SDBS, the interspaces caused by the coulomb repulsion among SDBS molecules will be filled by the FMEE molecules, making the isolating layer generated by surfactant molecules between air and water surfaces denser, and further reducing surface tension of the solution (Li et al., 2007; Liu et al., 2016; Zhu et al., 2016).
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Fig. 3 – The effect of different magnetic field intensities on the surface tension and contact angle of a 0.03 wt% solution.
Fig. 4 – Influence of water flow velocity through the magnetic field on the surface tension and contract angle of a 0.03 wt% solution.
Nonetheless, when the mass ratio of anionic to nonionic surfactants exceeded 5:1, the surface tension of the solution increased slightly. This occurred because the excessive content of nonionic surfactant caused the mode of action of this surfactant to gradually become the dominant factor in the solution characteristics, which weakened the synergy between the anionic and nonionic surfactants. Therefore, the optimal compound mass ratio of anionic to nonionic surfactants was 5:1. As shown in Fig. 2b, the surface tension of the solution was significantly reduced within the mass fraction range of 0.0–0.03 wt% as the surfactant content in the water increased. When the mass fraction of the surfactant solution was 0.03 wt%, the concentration of the solution reached the critical micelle concentration, the saturation concentration of surfactant at which the surface tension equalizes, indicating that the solution surface was completely covered by surfactant molecules (Bak and Podgorska, 2016). At this point, the surface tension of the solution also reached the critical micelle value (30.24 mN m−1 ), after which surface tension of the solution did not exhibit an obvious change. Therefore, for an effective economic dosage, the mass fraction of the new surfactant was confirmed to be 0.03 wt%.
As shown in Fig. 3, in the magnetic intensity range 0–350 mT, the surface tension and a contact angle of the 0.03 wt% surfactant solution had a tendency to decrease with increasing magnetic intensity, indicating that a synergy existed between the surfactant and magnetization that changed the wetting characteristics of the solution. Most aqueous solution molecules do not exist alone, but instead form large molecular clusters through hydrogen bonds. However, due to the constant motion of the solution molecules, the hydrogen bonds are always being fractured and recombining. When the surfactant solution was passed through the magnetic field, the molecular motion was accelerated, making the hydrogen bonds between solution molecules easier to fracture. Thus, the large molecular clusters were broken into small molecule groups, which weakened the solution cohesion and improved the capacity of the solution to wet dust (Ding et al., 2011; Pang and Deng, 2008; van Oss, 2003). At the magnetic intensity of 350 mT the physical-chemical characteristics of the surfactant solution were deemed to be optimal. A magnetic intensity exceeding 350 mT increased the surface tension and contact angle of the solution, exhibited in Fig. 3. This occurred mainly because hydrophilic groups of segmental anionic surfactant escaped the surface of water molecules under the effects of the magnetized field over 350 mT (ShamsiJazeyi et al., 2014), changing the combination modes between anionic and nonionic surfactants and water molecules and making the isolation layer between air and the solution surface become slightly loose. A magnetic intensity that was greater than the optimum also weakened the synergy between anionic and nonionic surfactants, further augmenting the surface tension and contact angle of the solution. Therefore, to achieve the best magnetic effect on a solution, the optimal magnetization intensity for application to the 0.03 wt% surfactant solution was 350 mT. As exhibited in Fig. 4, within the range 0–4 m s−1 of water flowing through the magnetic field, surface tension and a contact angle for the 0.03 wt% surfactant solution decreased as water velocity increased. At a water velocity of 4 m s−1 , the surface tension (28.07 mN m−1 ) and contact angle (27.02◦ ) of the surfactant solution reached their optimum values. As water velocity increased beyond 4 m s−1 , the change in solution properties was similar to that in response to different magnetization intensities. During magnetization of the solution, the Lorentz force of the magnetic field is enhanced as the water velocity increases, improving the interaction between the external magnetic field and the solution molecule
3.2. Synergetic characteristics and optimal magnetization parameters of surfactant-magnetized water Through various arrangements of hydrophilic groups on the water surface, surfactants cause differences in water wetting characteristics, which can be considered a chemical change. By reviewing relevant literature (Afshin et al., 2010; Cho and Lee, 2005; Pang and Deng, 2008) and carrying out many experiments in the laboratory, it was found that some properties of water show various degrees of change under the effects of magnetic fields; these changes include reduced surface tension and improved wetting ability. Therefore, surfactant solution wettability should be improved under the effects of a magnetized field, thereby reducing dosage of surfactants, making operation of a dust control system more convenient, and improving the prospects for long-term use. To verify this hypothesis, surface tension and contact angle experiments were conducted on the solution under various magnetic intensities and water flow velocities through the magnetic field, for which the experimental results are shown in Figs. 3 and 4, respectively.
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Table 2 – Dust concentration before and after applying different dust removal techniques. Dust control technology
C (mg m−3 )
c1
c2
c3
c = (c1 + c2 + c3 )/3 (mg m−3 )
Untreated water Magnetized water Surfactant-amended solution Surfactant-magnetized water
110.42 110.42 110.42 110.42
57.71 43.36 22.47 13.89
55.63 43.33 21.98 13.85
57.91 43.42 22.63 14.17
57.08 ± 1.26 43.37 ± 0.04 22.36 ± 0.34 13.97 ± 0.17
forces. The enhanced Lorentz force aids in breaking the hydrogen bonds or reducing the bond angles of hydrogen bonds, which decreases the free energy of cohesion among solution molecules and improves the wetting ability of the solution (van Oss, 2003). Beyond the optimal water velocity, hydrophilic groups of segmental anionic surfactants might break away from water molecules under the effects of the magnetic field, further weakening the synergetic effects of the anionic and nonionic surfactants. Consequently, the optimal flow velocity of surfactant-amended solution through the magnetic field was judged to be 4 m s−1 .
3.3. Experimental study on dust suppression effect of surfactant-magnetized water To further verify the synergy between surfactants and magnetization for dust prevention, the system described in Section 2.2 (Fig. 1) was used to measure dust suppression by untreated water, magnetized water, surfactant-amended solution, and surfactant-magnetized water. The surfactantmagnetized water (0.03 wt%) and magnetized water were produced at a magnetic field strength of 350 mT and a water flow rate of 4 m s−1 . Utilizing Eq. (1) and dust concentration data measured before and after applying the different dust removal techniques (Table 2), the dust suppression efficiency of each dust prevention technique was quantified (Fig. 5). As shown in Fig. 5, the dust control efficiency (48.3%) of untreated water was very limited. Magnetized water and surfactantamended solution had better wettability than untreated water and thus exhibited higher dust control efficiencies (60.72% and 79.75%, respectively) than untreated water. The dust suppression efficiency of surfactant-magnetized water (87.35%) was much higher than the individual efficien-
Fig. 5 – The dust suppression efficiency of untreated water, magnetized water, surfactant-amended solution and surfactant-magnetized water is shown in the bar chart as A, B, C, and D, respectively. cies of both surfactant-amended solution and magnetized water, illustrating the synergetic effect between surfactants and magnetization. Furthermore, the magnitude of the dust suppression efficiency arising from surfactant-magnetized water showed that the synergetic effect between magnetization and surfactants cannot be attributed to simple superimposition of the individual efficiencies of surfactantamended water and magnetized water i.e., magnetization could further improve the wetting ability of water while surfactant improving the dust capture ability of water. These results demonstrated that surfactant-magnetized water can efficiently capture coal dust and this represents a new technique for dust prevention in coal mines.
Fig. 6 – Schematic diagram of the novel surfactant-magnetized water generation system.
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Fig. 7 – Internal structure of an efficient magnetic apparatus (a: section of magnetic apparatus; b: external permanent magnet layout of magnetizing apparatus; c: internal helical water channel of magnetic apparatus).
Fig. 8 – Field-scale surfactant-magnetized water generation system (a: surfactant adding system; b: magnetic apparatus; c: water tank; d: pressure pump).
4. The generation system for surfactant-magnetized water To adapt to the narrow working space in an underground coal mine, and with the above experimental results in mind, a prototype field-scale surfactant-magnetized water generation system was developed and tested for dust prevention in coal mines (Fig. 6). The system included liquid handling devices, a static mixer, magnetizing apparatus, and other components. The devices for quantitatively adding liquid were composed of a metering pump, glass rotameter, solution barrel, and other hardware, the advantage of which was the accurate addition of low-concentration surfactant using dual governing methods (i.e., metering pump and glass rotameter). The static mixer utilized an internal helical structure to uniformly mix water and surfactant. Surfactant-magnetized water was generated by passing the surfactant-amended solution through the magnetizing apparatus. Finally, surfactant-magnetized water was
formed into micro-diameter spray droplets to effectively control coal dust. Through experiments investigating the optimal magnetization parameters of surfactant-magnetized water, related parameters pertaining to the magnetic apparatus were confirmed. For field application, a new type of magnetic apparatus to effectively produce surfactant-magnetized water for continuous dust control was designed (Fig. 7). As shown in Fig. 7a and b, the inner and outer permanent magnets (PM) of the magnetic apparatus were interlaced to form an alternating magnetic field environment (in contrast to a typical single magnetic field); the interlaced arrangement achieved pulsing magnetization of flowing water and a significant magnetizing effect. As depicted in Fig. 7c, the inside water channel of the magnetic apparatus utilized a spiral propulsion structure to improve the turbulence and mixing of the water. In contrast to a once-through magnetization channel for water flow, the spiral propulsion channel was more conducive to achieving uniform magnetization of the solution.
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(a) The mass ratio of anionic surfactants to nonionic surfactants was 5:1. (b) The mass fraction of the surfactant solution was 0.03 wt%. (c) The spray pressure of the surfactant-magnetized water was 3–4 MPa.
Fig. 9 – Average dust suppression efficiency for total dust and respirable dust of different dust control technologies.
The designed magnetizing apparatus has three advantages. First, the magnetic apparatus can form a powerful (i.e., 300–350 mT) and steady magnetic intensity under the effects of the inner and outer magnets, which effectively solves the problem of insufficient magnetic intensity existing in currently available magnetic devices. Second, the magnetic apparatus does not contain moving elements and does not need external power, which makes the device safer and more secure for application in coal mines. Third, as shown in Section 5, the dust capturing ability of water can be improved by more than 10% under the effect of the magnetic apparatus.
5.
Field application
Lu Wa coal mine is a modern underground mine located in Jining city, Shandong province, China. Due to implementation of mechanized mining techniques, dust hazards have been a major source of danger in Lu Wa, severely threatening the health of workers and the safe production of the mine. To provide a healthy working environment for workers and reduce the risk of dust explosion, the efficient dust control technology of surfactant-magnetized water was successfully applied in the 23012 fully mechanized face of the Lu Wa coal mine, as exhibited in Fig. 8. As a result, dust concentrations were significantly reduced. The operational parameters of the dust control technique were as follows:
Untreated water, magnetized water, surfactant-amended solution, and surfactant-magnetized water were applied in the same environment. Dust concentrations measured under the effect of the different dust control techniques confirmed the significant dust capturing efficiency of surfactant-magnetized water, which remarkably decreased the dust concentration at the working face, especially the concentration of respirable dust. Under identical spray conditions at the 23,012 fully mechanized face of the Lu Wa coal mine, the efficiencies of respirable dust control and total dust control by surfactantmagnetized water were 83.54% and 84.75%, respectively. As shown in Fig. 9, compared to that of untreated water, the respirable dust and total dust control efficiency of surfactantmagnetized water were increased by 44.94% and 31.79%, respectively. Therefore, the field application demonstrated that because of the synergy between magnetization and surfactants, the spray droplets of surfactant-magnetized water effectively captured coal dust, especially respirable dust, and markedly improved the working environment of the underground coal mine, as shown in Fig. 10.
6.
Conclusions
(a) A new surfactant was developed by mixing anionic and nonionic surfactants. The new surfactant is effective at a low dosage (0.03 wt%) in significantly improving the wettability of water. The new solution was magnetized to for use in suppressing coal mine dust and the optimal magnetization parameters were a field strength of 350 mT and a solution flow rate of 4 m s−1 . The synergy effect between the surfactant and magnetic field decreased the surface tension of the surfactant solution by approximately 7.2% (to 28.07 mN m−1 ) compared to its original value and the contact angle decreased to 27.02◦ . These changes effectively enhance the dust wetting ability of the solution. (b) A surfactant-magnetized water generation system was developed and tested. The system achieves accurate metering of low-concentration surfactant using the dual effect of a metering pump and glass rotameter.
Fig. 10 – Visual appearance of the working environment at the coal mining face after using (a) untreated water and (b) surfactant-magnetized water for dust control. The light gray color in (a) is caused by light reflecting off highly concentrated dust; by comparison, there was very little dust in (b).
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This system can efficiently and continuously produce surfactant-magnetized water. A new type of magnetic apparatus was invented for use in the system. The apparatus utilizes an interlaced arrangement of magnets to form an alternating magnetic field, achieving the effective magnetization of a solution passing through it. (c) The dust control technology consisting of a novel system to efficiently generate a new formulation of surfactantmagnetized water can significantly reduce the coal dust concentration at a mechanized working face, especially the concentration of respirable dust. Compared to that of untreated water, the respirable dust and total dust control efficiencies of surfactant-magnetized water increased by 44.94% (reaching 83.54%) and 31.79% (reaching 84.75%), respectively. The new technology effectively improves the atmospheric environment in mechanized underground coal mines, which will help guarantee the physical and mental health of workers as well as the enterprise’s safe production.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2017CXNL02), the National Key Research and Development Program of China (2017YFC0805200) and the Excellent Innovation Team of China University of Mining and Technology (2015ZY002).
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