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Experimental study on improving performance of dust-suppression foam by magnetization
Colloids and Surfaces A 577 (2019) 370–377
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Experimental study on improving performance of dust-suppression foam by magnetization
T
⁎
Hetang Wanga,b,c, , Xinyi Chenc, Ying Xiec, Xiaobin Weib,c, Wei Victor Liud a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, Jiangsu, China c School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China d Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Dust suppression Foam Magnetization Foaming capacity Foam stability Foam size
Foam is an effective material for controlling industrial dust. To optimize and upgrade the performance of dustsuppression foam, this paper proposes a method of exposing the foaming agent solution to a magnetic field of specific intensity prior to foaming. The effects of magnetization on foaming capacity, foam stability and foam size were investigated. Two types of foaming agent, anionic and non-ionic, were selected to test the foam expansion (FE) and foaming time (FT), both reflecting the foaming capacity, and foam stability (FS) at different concentrations before and after magnetization. The size of the bubbles, which directly affects the dust suppression performance, was also examined. The experimental results indicated that the foaming capacity and foam stability of the foaming agent solution after magnetization were higher than that of the original solution, and the size distribution of the bubbles has a trend to concentrate towards smaller size ranges. It proves that magnetization can enhance the performance of dust-suppression foam and reduce the quantity of foaming agent. Then we explored the mechanism of the observed experimental phenomena. This study is of significant importance in promoting foam as a more efficient material in dust control.
⁎
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.colsurfa.2019.05.069 Received 21 April 2019; Received in revised form 21 May 2019; Accepted 26 May 2019 Available online 27 May 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 577 (2019) 370–377
H. Wang, et al.
1. Introduction
supply to generate a steady current. The device as a whole is symmetrical to offset the horizontal magnetic induction component in the space between the injector, so as to generate a uniform and easy-tocontrol magnetic field. In the experiment, the injector and the FoamScan™ short hose are connected before conducting magnetization, which makes injection and foaming possible immediately after electromagnetization. To guarantee that each solution sample be exposed to magnetic field of the same intensity for comparison, we controlled the current at 8A. The EMF Tester indicates that the magnetic field intensity was approximately 700 Gs.
Dust is a ubiquitous occupational hazard that occurs in industrial production processes and poses a severe threat to the health and safety of workers [1–3]. It is the primary cause of pneumoconiosis, a currently incurable serious occupational disease [4–6]. In China, 22,701 cases of occupational pneumoconiosis were reported nationwide in 2017 [7]. Furthermore, dust can be considered a significant combustion and explosion hazard, causing tremendous casualties [8,9]. Examples of this include two recent coal dust explosions the (Jim Walter No. 5 Min. and the Upper Big Branch Mine disasters) which resulted in the fatality of 42 miners in the USA [10]. Among a multitude of dust control technologies, foam dust suppression is an efficient one. Past studies have shown that the efficiency of foam dust suppression can be increased by more than 30% and water consumption reduced by over 70% as compared to that of the conventional spray [11,12]. More recently, foam dust suppression has received much attention in dust control, including the successful development of novel foaming agent adding devices [13–15], foam generating devices [16–18], spray devices (foam nozzle) [19], which enhance the safety and reliability of foam preparation and the refinement of foam utilization. Foaming agents are crucial in foam formation, which directly affects the characteristics of dust-suppression foam. Xu et al. studied coal dust wetting ability of anionic surfactants with different structures [20], aiming to optimize the composition and concentration of foaming agents. Wang et al. investigated the effect of foaming agent concentration and temperature on the foaming capacity and foam stability [21,22]. Such studies have allowed for significant progress to be made in foam dust suppression technologies. However, this promising technology still faces relatively high long-term operating costs. Thus, improving the performance of dust-suppression foam can allow for the deployment of the clean production technology more feasible. In this study, the authors propose the concept of magnetization foam and its implement method. This particular approach uses a magnetic field of a certain intensity to magnetize the foaming agent solution to enhance the performance of the dust-suppression foam, thereby reducing the concentration of foaming agent used. This is based on the concept that the performance of foam is closely related to the mechanics and motion behavior of foaming agent (surfactant) molecules, and the magnetic field has a significant effect on the interaction between water molecules and foaming agent molecules. In order to explore the effect of magnetization on foaming capacity of foaming agent, foam stability and foam size, the foaming expansion (FE), foaming rate, indicated as foaming time (FT), foam stability (FS) and bubble radius distribution of foam before and after magnetization were tested. This study advances beyond the state of the art by improving the performance of dust-suppression foam, promoting its efficient and economical application.
2.2. Experimental materials Considering factors such as cost, environmental friendliness and industry application circumstances, five types of surfactants were selected in this experiment. Information of the purity and origin of each foaming agent is shown in Table 1. Solutions with concentrations of 0.03%, 0.05%, 0.08%, 0.10%, 0.15% and 0.20% were prepared for each foaming agent respectively. 2.3. Experiment method FE and FT, which characterize the foaming capacity of surfactant solution, were measured by optical devices attached to the FoamScan™ system during the foaming process. FE was calculated by Eq. 1:
FE =
Vfoam Vili − Vfli
(1)
where Vfoam represents the volume of foam (the experiment is set to stop foaming at the foam volume of 200 ml), and Vili and Vfli represent the volume of foaming solution at the start and the end of foaming, respectively. In addition, the FT to reach the pre-set foam volume and half-life of the liquid, indicating the foaming rate and FS respectively were also measured by the FoamScan™system. The foam size analysis was completed by a Cell Size Analyzer™. The image at the end of foaming was chosen for the Cell Size Analyzer™ to measure the radius and compile statistics of the bubble number of a specific radius range. The inlet compressed air velocity was selected at 120 mL/min for this experiment with a system temperature of 21 ± 1℃. 3. Results and discussion 3.1. Effect of magnetization on foaming capacity of the foaming solution 3.1.1. Foaming expansion (FE) The experimental results show that the foaming expansion of the magnetized surfactant solution increased in varied degrees. The experimental data is shown in Fig. 2. The FE-mass concentration curve before and after magnetization tended to be parallel, but the effect of magnetization on different surfactants types and different mass concentrations is different. Taking FMEE solution with 0.1% mass concentration as an example, the FE before magnetization was 8.1, whilst after magnetization it reached 15.2, an increase of 46.7%. When the concentration of the foaming agent solution approached or exceeded the critical micelle concentration (CMC), the surfactant molecules began to form micelles, and the FE decreased with the increase of concentration. This is discussed further in Section 3.4.
2. Experiment 2.1. Experimental setup The main instrument used for analysis in this study is the FoamScan™ experimental system developed by Teclis Instruments, France, for foam generation and foaming performance parameters measurement. Owing to the special injector of fixed size that the main instrument adapts, in order to weaken the magnetization effect attenuation caused by unavoidable transfer procedures of external magnetization, we devised a magnetization device matching the FoamScan™ experiment system, as shown in Fig. 1. The magnetization device consists of a cylinder core made of silicon steel with high permeability that matches the size of the special injector with layers of close-wound pure copper coils, and a DC regulated power
3.1.2. Foaming rate The foaming rate of the magnetized AOS surfactant solutions increased the most significantly, reflected by the decrease in FT. The foaming time of 200 ml varied with the mass concentration curve as shown in Fig. 3. Similarly, based on micelle theory, the effect of magnetization on the foaming rate is inferior to that of the micelle effect 371
Colloids and Surfaces A 577 (2019) 370–377
H. Wang, et al.
Fig. 1. Structure diagram of magnetizing device matched with FoamScan™.
solution present a similar alternating result owing to the two effects. In short, controlling the concentration of the foaming agent solution, the improved magnetized dust suppressing foam stability can extend dust capture times in underground mining operations, and reduce efficiency losses by other disturbances caused by non-dust particles related impaction.
Table 1 Experimental sample information. Surfactant
AOS
SDBS
K12
FMEE
AES
Source
Zhejiang Sinolight 92
Tianjin Dingshengxin ≥90
Linyi Lusen 93
Mexico Pemex 70
Linyi Lusen 70
Purity (%)
3.3. Effect of magnetization on foam size when the concentration of the solution reaches its CMC (such as a mass concentration of 0.1% AOS). When the mass concentration of the solution continues to increase, the magnetization effect again gains an upper hand.
The experimental results show that the bubble radius of the foam generated from the magnetized surfactant solution typically reduces and tends to be homogeneous, as shown in Figs. 5 and 6. Taking K12 solution with a mass concentration of 0.5% as an example, the radius of foam before magnetization is distributed in seven equal intervals between 0.016-0.506 mm. After magnetization, the foam radius is concentrated in the smaller intervals, 0.016 - 0.086 mm and 0.086 - 0.15 mm, accounting for 86.5% of the total bubble amount. The uniformity of the size of bubbles is also a key factor affecting the stability of foam. One of the main reasons for defoaming is the absorption between bubbles of various size. Therefore, the increase of uniformity and decrease of overall size is favorable to improve stability, thus prolonging foam life and attaining higher efficiencies. Moreover, the diminishing of bubble size helps to improve dust capturing performance. The dust capturing mechanism can be explicated by the aerodynamic model of inertial impaction between dust flow and the bubbles. The process diagram is shown in Fig. 7. When the dust flow approaches the bubbles, the streamlines flow around the bubble and the dust particles near the bubble axis are captured due to the inertia effect, which keeps the original direction of movement, or changes the direction of movement but still deviates from the
3.2. Magnetization foam stability As illustrated in the experimental results, the stability of the surfactant solution after magnetization was considerably enhanced, as shown in Fig. 4. Taking AES solution with 0.15% mass concentration as an example, the FS before and after magnetization was 62 s and 80 s respectively, an increase of 29%, and tends to be steady when the concentration continues to increase. At low concentrations (e.g. 0.03%), magnetization increases FS more significantly, and the FS value between these high or low mass concentrations appears abnormal. This occurred when the mass concentration of solution reaches CMC, the effect of magnetization on FS was weakened by the micelle effect (similar to FE and FT). When the mass concentration exceeds a certain value, the formation of micelles reaches saturation state, and the effect of magnetization is dominant again and tends to be stable. As indicated above, the changes of FT before and after magnetization of each 372
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H. Wang, et al.
Fig. 2. Change of FE with mass concentration before and after magnetization.
capturing will be improved. 3.4. Mechanism exploration 3.4.1. Improvement of foaming capacity of the surfactant solution after magnetization 3.4.1.1. Qualitative discussion. From the microscopic point of view, the foaming principle of surfactant can be elaborated as follows. A surfactant particle has a hydrophilic group and a hydrophobic group. Due to the hydrophobic group, the particle moves spontaneously to the gas-liquid interface when gas is inflated into the solution. This forms a layer of surfactant molecular membranes, which reduces the surface tension of the gas-liquid interface and weakens the shrinkage accompanied and defoaming of bubbles along with it due to surface tension. The process is shown in Fig. 8. Water as a solute has strong polarity, of which the molecules are bound in hydrogen bonds and is arranged around the surfactant molecules in a cage structure as shown in Fig. 9. Nuclear magnetic resonance (NMR) experiments of water suggest that the proton resonance absorption peak of magnetized water shifts to a variable field compared with distilled water. This indicates that the density of the proton electron cloud increases and the shielding increases in magnetized water, which is not conducive to the formation of hydrogen bonds [23], that is, the hydrogen bond between the pion electrons on the benzene ring in the hydrophobic group of the surfactant molecule and its surrounding H2O molecules. By analogy, the hydrogen bond formed between the surfactant molecule and the water molecule is also partially destroyed after magnetization. Due to the destruction of two types of hydrogen bonds, the cage structure formed by the surfactant and its surrounding water molecules is destroyed. This is conducive to the movement of surfactant molecules to the gas-liquid interface, and the solution viscosity decreases. As such, this explains the increased foaming rate and foam expansion of magnetized solution. When the bubbles leave the solution carrying more surfactant molecules on the liquid membrane, the surface tension is reduced and the shrinkage and drainage of bubbles are slowed down. Their stability is also significantly improved as a result. Evidently, under the experimental conditions, FE of the foaming solution with higher concentration than the critical micelle concentration decreases with the increase in mass concentration. This is due to the surfactant molecules in the foaming solution are easy to form
Fig. 3. Change of FT with mass concentration before and after magnetization.
streamline, and collides with the bubble. The ratio between the cross sectional area of the cylinder, formed by curves of the maximum distance from the axis where dust particles can inertially impact with the bubble (limit curves), and that of the bubble in the direction of the flow indicate the probability of the impaction between the particles and the bubble. The dust catching capacity of the foam is characterized and expressed by Eq. 2.
Pc =
D 2cylinder 2 Dbubble
(2)
According to the inertial impaction theory, when the diameter of the foam bubble is large, the dust flow starts to flow around the bubble at quite a distance away from it. Under such conditions, the dust particles have sufficient time for acceleration to change their direction of movement, so the diameter of the cylinder formed by limit trajectories is relatively small, and the probability of inertia impaction is consequently lower. On the contrary, if the diameters of the bubbles are smaller, the dust flow is bound to flow around the bubble with little distance between them. Theoretically, if the diameter of the limit trajectory cylinder can reach the bubble diameter, the probability of inertial impaction will be greater, and thus the efficiency of dust 373
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Fig. 4. Change of FS with mass concentration before and after magnetization.
3.4.1.2. Modeling. The gas influx mode of FoamScan™ foaming device is simplified to n continuous spherical bubbles with surface area As , as shown in Fig. 10. Considering the absence of gas escaping and dissolution, the volume of gas in the foam is n initial bubbles’ volume after completion of foaming. Surface expansion modulus [24] is introduced from the Gibbs-Marangoni effect shown in Eq. 3:
micelles. For the experimental results that the foaming capacity of high concentration solutions is still enhanced after magnetization, the following assumptions are made: 1) the magnetic field has the same effect on the free surfactant molecules in solutions where micelles are already formed; 2) the effect of magnetic field on surfactant molecules in solution weakens the repulsion of hydrophilic groups and the attraction of hydrophobic groups in the micelles, thereby partially destroying them. The deformation of some micelles increases the number of free molecules. From the macro point of view of thermodynamics, the solubility of surfactants decreases due to the directional arrangement of molecules in magnetized solutions and the decrease of solution entropy, of which the macro phenomenon is that hydrophobic groups are more hydrophobic.
(3)
ε = dσ / d (ln As )
where σ is the change of surface tension with the change of an area infinitesimal. And the volume of air in the foam Vair , can be expressed as Eq. 4: n
Vair =
σi
4 Ce ε 3 )2 4π
∑ 3( i=1
Fig. 5. Change of bubble radius distribution before and after magnetization (1). 374
(4)
Colloids and Surfaces A 577 (2019) 370–377
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Fig. 6. Change of bubble radius distribution before and after magnetization (2).
Fig. 7. Inertial impaction between dust flow and bubbles of different size.
Fig. 8. The schematic diagram of foaming mechanism of surfactant.
It is evident that FE increases with the increase in surface tension and the decrease of surface expansion modulus, and that both surface elasticity modulus and solution viscosity come into effect due to intermolecular forces, thus giving a positive correlation. This mechanism quantitatively explains the reason for the increase of FE through magnetization.
Therefore the relationship between FE and surface tension and surface elastic modulus is given by Eq. 5:
FE =
=
Vfoam Vili − Vfli
=
Vfoam Vfoam − Vair
Vfoam n
σi
4 Ce ε 4π
Vfoam − ∑i = 1 3 (
3
)2
, i = 1, 2, 3, ..., n , (5)
3.4.2. Shrinkage of bubble size after magnetization Inspired by the Laplacian equation [25] as per equation:
where C is the integral constant and σi is the surface tension of the ith initial bubble. 375
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H. Wang, et al.
Fig. 9. The influence of magnetization on the water-surfactant cage structure.
4. Conclusions This paper proposes magnetization to improve the performance of dust suppression foam through experimental investigation. The following conclusions can be drawn. 1) By magnetization, the foam expansion and foaming rate of the dust suppression foam increase. Verified by mathematical deduction, magnetization increases the number of surfactant molecules at the gas-liquid interface through two mechanisms, reducing the surface tension and increasing the surface elastic modulus. 2) Post magnetization, the liquid half-life increased significantly. This can be interpreted as a positive effect for foam stability. The dust suppression foam is in a relatively stable state, thus the efficiency of its use is increased. 3) The size of foam bubbles decreases and tends to be more homogeneous after magnetization. Through magnetization, the surface tension of the liquid membrane is reduced, so that the size distribution of foam has a tendency to concentrate into a smaller radius range. The probability of inertial impaction between dust suppression foam and dust is believed to increase remarkably. Fig. 10. Diagram of the simplified air influx model.
ΔP = γ (
1 1 + ) R1 R2
To summarize, magnetization is helpful in providing more efficient dust suppression foam. Based on the experimental results, low concentration foaming agent solution combined with magnetization is suggested to achieve high expansion ratio, high stability and high foaming rate, reducing the use of foaming agent. In addition, foaming stability and bubble size distribution were prominently optimized. Ultimately, this could bring about the improvement of foam dust suppression effect and reduce the costs associated with the application of dust-suppression foam.
(6)
where ΔP is the pressure difference at the gas-liquid interface and γ is the surface tension coefficient of the liquid. A line is made perpendicular to the surface at a point on the surface, through which a plane is made. The intersection of the plane and the surface is defined as a curve. R1 is the radius of the circle tangent to the curve at this point, or the radius of curvature. R2 is the radius of curvature of a second intersection of another plane that is perpendicular to the first plane, through a line perpendicular to the surface and the surface. The bending of the liquid surface is expressed by R1 and R2. The surface tension coefficient γ is defined by Eq. 7:
γ=
ΔE ΔS
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2018QNA04) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education (PAPD) .
(7) References
illustrating that γ numerically equates to the increase of free energy per unit surface area. In other words, the work done by external forces or in the context of this study, the difficulty of foaming in the system and it is clear that this decreases as a consequence of the increased number of surfactant molecules. Eq. 6 shows that when the pressure difference is a constant, bubble radius is proportional to the surface tension coefficient, the decrease of which after magnetization results in the decrease of the bubble radius.
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377
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Experimental study on improving performance of dust-suppression foam by magnetization
T
⁎
Hetang Wanga,b,c, , Xinyi Chenc, Ying Xiec, Xiaobin Weib,c, Wei Victor Liud a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, Jiangsu, China c School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China d Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Dust suppression Foam Magnetization Foaming capacity Foam stability Foam size
Foam is an effective material for controlling industrial dust. To optimize and upgrade the performance of dustsuppression foam, this paper proposes a method of exposing the foaming agent solution to a magnetic field of specific intensity prior to foaming. The effects of magnetization on foaming capacity, foam stability and foam size were investigated. Two types of foaming agent, anionic and non-ionic, were selected to test the foam expansion (FE) and foaming time (FT), both reflecting the foaming capacity, and foam stability (FS) at different concentrations before and after magnetization. The size of the bubbles, which directly affects the dust suppression performance, was also examined. The experimental results indicated that the foaming capacity and foam stability of the foaming agent solution after magnetization were higher than that of the original solution, and the size distribution of the bubbles has a trend to concentrate towards smaller size ranges. It proves that magnetization can enhance the performance of dust-suppression foam and reduce the quantity of foaming agent. Then we explored the mechanism of the observed experimental phenomena. This study is of significant importance in promoting foam as a more efficient material in dust control.
⁎
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.colsurfa.2019.05.069 Received 21 April 2019; Received in revised form 21 May 2019; Accepted 26 May 2019 Available online 27 May 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 577 (2019) 370–377
H. Wang, et al.
1. Introduction
supply to generate a steady current. The device as a whole is symmetrical to offset the horizontal magnetic induction component in the space between the injector, so as to generate a uniform and easy-tocontrol magnetic field. In the experiment, the injector and the FoamScan™ short hose are connected before conducting magnetization, which makes injection and foaming possible immediately after electromagnetization. To guarantee that each solution sample be exposed to magnetic field of the same intensity for comparison, we controlled the current at 8A. The EMF Tester indicates that the magnetic field intensity was approximately 700 Gs.
Dust is a ubiquitous occupational hazard that occurs in industrial production processes and poses a severe threat to the health and safety of workers [1–3]. It is the primary cause of pneumoconiosis, a currently incurable serious occupational disease [4–6]. In China, 22,701 cases of occupational pneumoconiosis were reported nationwide in 2017 [7]. Furthermore, dust can be considered a significant combustion and explosion hazard, causing tremendous casualties [8,9]. Examples of this include two recent coal dust explosions the (Jim Walter No. 5 Min. and the Upper Big Branch Mine disasters) which resulted in the fatality of 42 miners in the USA [10]. Among a multitude of dust control technologies, foam dust suppression is an efficient one. Past studies have shown that the efficiency of foam dust suppression can be increased by more than 30% and water consumption reduced by over 70% as compared to that of the conventional spray [11,12]. More recently, foam dust suppression has received much attention in dust control, including the successful development of novel foaming agent adding devices [13–15], foam generating devices [16–18], spray devices (foam nozzle) [19], which enhance the safety and reliability of foam preparation and the refinement of foam utilization. Foaming agents are crucial in foam formation, which directly affects the characteristics of dust-suppression foam. Xu et al. studied coal dust wetting ability of anionic surfactants with different structures [20], aiming to optimize the composition and concentration of foaming agents. Wang et al. investigated the effect of foaming agent concentration and temperature on the foaming capacity and foam stability [21,22]. Such studies have allowed for significant progress to be made in foam dust suppression technologies. However, this promising technology still faces relatively high long-term operating costs. Thus, improving the performance of dust-suppression foam can allow for the deployment of the clean production technology more feasible. In this study, the authors propose the concept of magnetization foam and its implement method. This particular approach uses a magnetic field of a certain intensity to magnetize the foaming agent solution to enhance the performance of the dust-suppression foam, thereby reducing the concentration of foaming agent used. This is based on the concept that the performance of foam is closely related to the mechanics and motion behavior of foaming agent (surfactant) molecules, and the magnetic field has a significant effect on the interaction between water molecules and foaming agent molecules. In order to explore the effect of magnetization on foaming capacity of foaming agent, foam stability and foam size, the foaming expansion (FE), foaming rate, indicated as foaming time (FT), foam stability (FS) and bubble radius distribution of foam before and after magnetization were tested. This study advances beyond the state of the art by improving the performance of dust-suppression foam, promoting its efficient and economical application.
2.2. Experimental materials Considering factors such as cost, environmental friendliness and industry application circumstances, five types of surfactants were selected in this experiment. Information of the purity and origin of each foaming agent is shown in Table 1. Solutions with concentrations of 0.03%, 0.05%, 0.08%, 0.10%, 0.15% and 0.20% were prepared for each foaming agent respectively. 2.3. Experiment method FE and FT, which characterize the foaming capacity of surfactant solution, were measured by optical devices attached to the FoamScan™ system during the foaming process. FE was calculated by Eq. 1:
FE =
Vfoam Vili − Vfli
(1)
where Vfoam represents the volume of foam (the experiment is set to stop foaming at the foam volume of 200 ml), and Vili and Vfli represent the volume of foaming solution at the start and the end of foaming, respectively. In addition, the FT to reach the pre-set foam volume and half-life of the liquid, indicating the foaming rate and FS respectively were also measured by the FoamScan™system. The foam size analysis was completed by a Cell Size Analyzer™. The image at the end of foaming was chosen for the Cell Size Analyzer™ to measure the radius and compile statistics of the bubble number of a specific radius range. The inlet compressed air velocity was selected at 120 mL/min for this experiment with a system temperature of 21 ± 1℃. 3. Results and discussion 3.1. Effect of magnetization on foaming capacity of the foaming solution 3.1.1. Foaming expansion (FE) The experimental results show that the foaming expansion of the magnetized surfactant solution increased in varied degrees. The experimental data is shown in Fig. 2. The FE-mass concentration curve before and after magnetization tended to be parallel, but the effect of magnetization on different surfactants types and different mass concentrations is different. Taking FMEE solution with 0.1% mass concentration as an example, the FE before magnetization was 8.1, whilst after magnetization it reached 15.2, an increase of 46.7%. When the concentration of the foaming agent solution approached or exceeded the critical micelle concentration (CMC), the surfactant molecules began to form micelles, and the FE decreased with the increase of concentration. This is discussed further in Section 3.4.
2. Experiment 2.1. Experimental setup The main instrument used for analysis in this study is the FoamScan™ experimental system developed by Teclis Instruments, France, for foam generation and foaming performance parameters measurement. Owing to the special injector of fixed size that the main instrument adapts, in order to weaken the magnetization effect attenuation caused by unavoidable transfer procedures of external magnetization, we devised a magnetization device matching the FoamScan™ experiment system, as shown in Fig. 1. The magnetization device consists of a cylinder core made of silicon steel with high permeability that matches the size of the special injector with layers of close-wound pure copper coils, and a DC regulated power
3.1.2. Foaming rate The foaming rate of the magnetized AOS surfactant solutions increased the most significantly, reflected by the decrease in FT. The foaming time of 200 ml varied with the mass concentration curve as shown in Fig. 3. Similarly, based on micelle theory, the effect of magnetization on the foaming rate is inferior to that of the micelle effect 371
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Fig. 1. Structure diagram of magnetizing device matched with FoamScan™.
solution present a similar alternating result owing to the two effects. In short, controlling the concentration of the foaming agent solution, the improved magnetized dust suppressing foam stability can extend dust capture times in underground mining operations, and reduce efficiency losses by other disturbances caused by non-dust particles related impaction.
Table 1 Experimental sample information. Surfactant
AOS
SDBS
K12
FMEE
AES
Source
Zhejiang Sinolight 92
Tianjin Dingshengxin ≥90
Linyi Lusen 93
Mexico Pemex 70
Linyi Lusen 70
Purity (%)
3.3. Effect of magnetization on foam size when the concentration of the solution reaches its CMC (such as a mass concentration of 0.1% AOS). When the mass concentration of the solution continues to increase, the magnetization effect again gains an upper hand.
The experimental results show that the bubble radius of the foam generated from the magnetized surfactant solution typically reduces and tends to be homogeneous, as shown in Figs. 5 and 6. Taking K12 solution with a mass concentration of 0.5% as an example, the radius of foam before magnetization is distributed in seven equal intervals between 0.016-0.506 mm. After magnetization, the foam radius is concentrated in the smaller intervals, 0.016 - 0.086 mm and 0.086 - 0.15 mm, accounting for 86.5% of the total bubble amount. The uniformity of the size of bubbles is also a key factor affecting the stability of foam. One of the main reasons for defoaming is the absorption between bubbles of various size. Therefore, the increase of uniformity and decrease of overall size is favorable to improve stability, thus prolonging foam life and attaining higher efficiencies. Moreover, the diminishing of bubble size helps to improve dust capturing performance. The dust capturing mechanism can be explicated by the aerodynamic model of inertial impaction between dust flow and the bubbles. The process diagram is shown in Fig. 7. When the dust flow approaches the bubbles, the streamlines flow around the bubble and the dust particles near the bubble axis are captured due to the inertia effect, which keeps the original direction of movement, or changes the direction of movement but still deviates from the
3.2. Magnetization foam stability As illustrated in the experimental results, the stability of the surfactant solution after magnetization was considerably enhanced, as shown in Fig. 4. Taking AES solution with 0.15% mass concentration as an example, the FS before and after magnetization was 62 s and 80 s respectively, an increase of 29%, and tends to be steady when the concentration continues to increase. At low concentrations (e.g. 0.03%), magnetization increases FS more significantly, and the FS value between these high or low mass concentrations appears abnormal. This occurred when the mass concentration of solution reaches CMC, the effect of magnetization on FS was weakened by the micelle effect (similar to FE and FT). When the mass concentration exceeds a certain value, the formation of micelles reaches saturation state, and the effect of magnetization is dominant again and tends to be stable. As indicated above, the changes of FT before and after magnetization of each 372
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Fig. 2. Change of FE with mass concentration before and after magnetization.
capturing will be improved. 3.4. Mechanism exploration 3.4.1. Improvement of foaming capacity of the surfactant solution after magnetization 3.4.1.1. Qualitative discussion. From the microscopic point of view, the foaming principle of surfactant can be elaborated as follows. A surfactant particle has a hydrophilic group and a hydrophobic group. Due to the hydrophobic group, the particle moves spontaneously to the gas-liquid interface when gas is inflated into the solution. This forms a layer of surfactant molecular membranes, which reduces the surface tension of the gas-liquid interface and weakens the shrinkage accompanied and defoaming of bubbles along with it due to surface tension. The process is shown in Fig. 8. Water as a solute has strong polarity, of which the molecules are bound in hydrogen bonds and is arranged around the surfactant molecules in a cage structure as shown in Fig. 9. Nuclear magnetic resonance (NMR) experiments of water suggest that the proton resonance absorption peak of magnetized water shifts to a variable field compared with distilled water. This indicates that the density of the proton electron cloud increases and the shielding increases in magnetized water, which is not conducive to the formation of hydrogen bonds [23], that is, the hydrogen bond between the pion electrons on the benzene ring in the hydrophobic group of the surfactant molecule and its surrounding H2O molecules. By analogy, the hydrogen bond formed between the surfactant molecule and the water molecule is also partially destroyed after magnetization. Due to the destruction of two types of hydrogen bonds, the cage structure formed by the surfactant and its surrounding water molecules is destroyed. This is conducive to the movement of surfactant molecules to the gas-liquid interface, and the solution viscosity decreases. As such, this explains the increased foaming rate and foam expansion of magnetized solution. When the bubbles leave the solution carrying more surfactant molecules on the liquid membrane, the surface tension is reduced and the shrinkage and drainage of bubbles are slowed down. Their stability is also significantly improved as a result. Evidently, under the experimental conditions, FE of the foaming solution with higher concentration than the critical micelle concentration decreases with the increase in mass concentration. This is due to the surfactant molecules in the foaming solution are easy to form
Fig. 3. Change of FT with mass concentration before and after magnetization.
streamline, and collides with the bubble. The ratio between the cross sectional area of the cylinder, formed by curves of the maximum distance from the axis where dust particles can inertially impact with the bubble (limit curves), and that of the bubble in the direction of the flow indicate the probability of the impaction between the particles and the bubble. The dust catching capacity of the foam is characterized and expressed by Eq. 2.
Pc =
D 2cylinder 2 Dbubble
(2)
According to the inertial impaction theory, when the diameter of the foam bubble is large, the dust flow starts to flow around the bubble at quite a distance away from it. Under such conditions, the dust particles have sufficient time for acceleration to change their direction of movement, so the diameter of the cylinder formed by limit trajectories is relatively small, and the probability of inertia impaction is consequently lower. On the contrary, if the diameters of the bubbles are smaller, the dust flow is bound to flow around the bubble with little distance between them. Theoretically, if the diameter of the limit trajectory cylinder can reach the bubble diameter, the probability of inertial impaction will be greater, and thus the efficiency of dust 373
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Fig. 4. Change of FS with mass concentration before and after magnetization.
3.4.1.2. Modeling. The gas influx mode of FoamScan™ foaming device is simplified to n continuous spherical bubbles with surface area As , as shown in Fig. 10. Considering the absence of gas escaping and dissolution, the volume of gas in the foam is n initial bubbles’ volume after completion of foaming. Surface expansion modulus [24] is introduced from the Gibbs-Marangoni effect shown in Eq. 3:
micelles. For the experimental results that the foaming capacity of high concentration solutions is still enhanced after magnetization, the following assumptions are made: 1) the magnetic field has the same effect on the free surfactant molecules in solutions where micelles are already formed; 2) the effect of magnetic field on surfactant molecules in solution weakens the repulsion of hydrophilic groups and the attraction of hydrophobic groups in the micelles, thereby partially destroying them. The deformation of some micelles increases the number of free molecules. From the macro point of view of thermodynamics, the solubility of surfactants decreases due to the directional arrangement of molecules in magnetized solutions and the decrease of solution entropy, of which the macro phenomenon is that hydrophobic groups are more hydrophobic.
(3)
ε = dσ / d (ln As )
where σ is the change of surface tension with the change of an area infinitesimal. And the volume of air in the foam Vair , can be expressed as Eq. 4: n
Vair =
σi
4 Ce ε 3 )2 4π
∑ 3( i=1
Fig. 5. Change of bubble radius distribution before and after magnetization (1). 374
(4)
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Fig. 6. Change of bubble radius distribution before and after magnetization (2).
Fig. 7. Inertial impaction between dust flow and bubbles of different size.
Fig. 8. The schematic diagram of foaming mechanism of surfactant.
It is evident that FE increases with the increase in surface tension and the decrease of surface expansion modulus, and that both surface elasticity modulus and solution viscosity come into effect due to intermolecular forces, thus giving a positive correlation. This mechanism quantitatively explains the reason for the increase of FE through magnetization.
Therefore the relationship between FE and surface tension and surface elastic modulus is given by Eq. 5:
FE =
=
Vfoam Vili − Vfli
=
Vfoam Vfoam − Vair
Vfoam n
σi
4 Ce ε 4π
Vfoam − ∑i = 1 3 (
3
)2
, i = 1, 2, 3, ..., n , (5)
3.4.2. Shrinkage of bubble size after magnetization Inspired by the Laplacian equation [25] as per equation:
where C is the integral constant and σi is the surface tension of the ith initial bubble. 375
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Fig. 9. The influence of magnetization on the water-surfactant cage structure.
4. Conclusions This paper proposes magnetization to improve the performance of dust suppression foam through experimental investigation. The following conclusions can be drawn. 1) By magnetization, the foam expansion and foaming rate of the dust suppression foam increase. Verified by mathematical deduction, magnetization increases the number of surfactant molecules at the gas-liquid interface through two mechanisms, reducing the surface tension and increasing the surface elastic modulus. 2) Post magnetization, the liquid half-life increased significantly. This can be interpreted as a positive effect for foam stability. The dust suppression foam is in a relatively stable state, thus the efficiency of its use is increased. 3) The size of foam bubbles decreases and tends to be more homogeneous after magnetization. Through magnetization, the surface tension of the liquid membrane is reduced, so that the size distribution of foam has a tendency to concentrate into a smaller radius range. The probability of inertial impaction between dust suppression foam and dust is believed to increase remarkably. Fig. 10. Diagram of the simplified air influx model.
ΔP = γ (
1 1 + ) R1 R2
To summarize, magnetization is helpful in providing more efficient dust suppression foam. Based on the experimental results, low concentration foaming agent solution combined with magnetization is suggested to achieve high expansion ratio, high stability and high foaming rate, reducing the use of foaming agent. In addition, foaming stability and bubble size distribution were prominently optimized. Ultimately, this could bring about the improvement of foam dust suppression effect and reduce the costs associated with the application of dust-suppression foam.
(6)
where ΔP is the pressure difference at the gas-liquid interface and γ is the surface tension coefficient of the liquid. A line is made perpendicular to the surface at a point on the surface, through which a plane is made. The intersection of the plane and the surface is defined as a curve. R1 is the radius of the circle tangent to the curve at this point, or the radius of curvature. R2 is the radius of curvature of a second intersection of another plane that is perpendicular to the first plane, through a line perpendicular to the surface and the surface. The bending of the liquid surface is expressed by R1 and R2. The surface tension coefficient γ is defined by Eq. 7:
γ=
ΔE ΔS
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2018QNA04) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education (PAPD) .
(7) References
illustrating that γ numerically equates to the increase of free energy per unit surface area. In other words, the work done by external forces or in the context of this study, the difficulty of foaming in the system and it is clear that this decreases as a consequence of the increased number of surfactant molecules. Eq. 6 shows that when the pressure difference is a constant, bubble radius is proportional to the surface tension coefficient, the decrease of which after magnetization results in the decrease of the bubble radius.
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