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Experimental investigation of coal dust wetting ability of anionic surfactants with different structures
Accepted Manuscript Title: Experimental investigation of coal dust wetting ability of anionic surfactants with different structures Authors: Chaohang Xu, Deming Wang, Hetang Wang, Liyang Ma, Xiaolong Zhu, Yunfei Zhu, Yi Zhang, Fangming Liu PII: DOI: Reference:
S0957-5820(18)30295-7 https://doi.org/10.1016/j.psep.2018.10.010 PSEP 1540
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
12-6-2018 6-9-2018 12-10-2018
Please cite this article as: Xu C, Wang D, Wang H, Ma L, Zhu X, Zhu Y, Zhang Y, Liu F, Experimental investigation of coal dust wetting ability of anionic surfactants with different structures, Process Safety and Environmental Protection (2018), https://doi.org/10.1016/j.psep.2018.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation of coal dust wetting ability of anionic surfactants with different structures
Chaohang Xua,b, Deming Wanga,b,*, Hetang Wanga,b,*, Liyang Maa,b, Xiaolong Zhua,b, Yunfei Zhua,b, Yi
Laboratory of Gas and Fire Control for Coal Mines of Ministry of Education, Xuzhou 221116, China
cXuzhou
of Marxism, Shanghai University of Finance and Economics, Shanghai 200000, China
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Corresponding author: Deming Wang, Hetang Wang
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dSchool
Anyun Mining Technology Inc., Xuzhou 221008, China
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bKey
of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China
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aSchool
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Zhangc, Fangming Liud
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E-mail address: [email protected], [email protected]
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Highlights
The wetting time doesn’t relate to the surface tension that is lower than 45 mN/m.
Hydrophobic and electrostatic interactions influence the adsorption density.
Hydrophilic-lipophilic balance (HLB) values affect the dynamic immersion process.
The surfactant structures cause different adsorption densities and HLB values.
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Both the adsorption density and HLB value determine the wetting ability.
Abstract: To better understand the coal dust wetting ability of anionic surfactants with different structures, sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDDS), and sodium
dodecyl benzene sulfonate (SDBS) were selected. The surface tension, wetting time, and infrared spectra of coal dust were tested. The hydrophilic-lipophilic balance (HLB) values of the surfactants were calculated. Results showed that the decrease in the surface tension could shorten the wetting time at low surfactant concentrations. But the decrease in the wetting time
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was no longer related to the constant surface tension that was lower than 45 mN/m. The adsorption density depends on the hydrophobic interactions and electrostatic repulsions
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between the surfactant molecules and the coal dust. During the dynamic immersion process, the
surfactants with high HLB values could bring the coal dust into the bulk solution rapidly. The wetting time of coal dust for SDBS with a high adsorption density and low HLB or SDS with
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a small adsorption density and high HLB was relatively longer. The adsorption density and
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HLB value of SDDS were intermediate between those of SDS and SDBS. Under the combined
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action of these two factors, the wetting time of SDDS was shorter than those of SDS and SDBS. Keywords: coal dust; anionic surfactant; wetting time; surface tension; adsorption density;
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hydrophilic-lipophilic balance
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1. Introduction
While coal is an important energy source worldwide, many disasters occur during the coal
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mining process (Xi et al., 2014; Cheng et al., 2017; Liang et al., 2018; Zhang et al., 2018). Coal dust is a significant hazard factor that can cause explosions and coal workers’ pneumoconiosis
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(CWP) (Fan et al., 2018), a serious occupational disease which can affect workers’ health and coal production. The patients suffer a lot of pain and cannot be cured. As the world’s largest coal producer, China generated 46.1% of the global coal in 2016 (British Petroleum, 2017). In China, due to the extensive use of the mechanized coal mining method, the amount of coal dust produced at working faces has increased dramatically (Wang et al., 2016; Zhou et al., 2017; Liu
et al., 2018). Therefore, the number of CWP patients has been increasing annually, and pneumoconiosis has become the most serious occupational disease among Chinese coal miners (Chen et al., 2013). In the USA, an upward trend in CWP has been observed since 2000. Thus, the National Institute of Occupational Safety and Health (NIOSH) recommended that the limit
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value of respirable coal mine dust should be reduced from 2 to 1 mg/m3 (Kollipara et al., 2014). However, it is difficult to achieve the statutory standards for most high production faces
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(Ralston et al., 2017). Hence, effective dust control technologies must be adopted to protect the health of coal miners.
Among various dust control technologies, water spraying technology has been widely used
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in dust suppression due to its low cost and simple implementation (Wang et al., 2018). However,
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the wetting ability and dust suppression efficiency of sprayed water is relatively poor, because
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of the hydrophobicity of coal dust and the high surface tension of water (Zhou et al., 2017a). Wetting agents composed of surfactants are usually added to water to reduce its surface tension
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to improve the dust suppression efficiency (Cybulski et al., 2015; Yao et al., 2017). The
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surfactant-magnetized water technology has also been developed to further reduce the surface tension and improve the wetting performance of water for coal dust. Studies showed that the
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suppression efficiency of surfactant-magnetized water for total and respirable dust increased by 31.79% and 44.94%, respectively, compared to that of untreated water (Zhou et al., 2017a).
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Dust suppression using foam has also been proved to be an effective technology. During this process, surfactants, water, and air are mixed in a foam generator to produce foam, which is then sprayed on the dust source (Wang et al., 2012). Compared to the water spray technology, the foam technology consumes less water, and the suppression of total and respirable dust is increased by 30–50%, due to the higher specific surface area, better wetting ability, and stronger
adhesion of the foam (Seibel, 1976; Volkwein et al., 1983; Mukherjee and Singh, 1984; Ren et al., 2012). Surfactant can improve the wetting performance of coal dust and enhance the coal suppression efficiency. The types and molecular structures of surfactants, coal rank and coal
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dust size are important factors affecting the wetting performance of coal dust (Özer et al., 2017). Various studies have obtained the same conclusion that finer coal particles are more difficult to
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be wetted (Xu et al., 2018). However, due to the large number of surfactants and different coal
ranks, it is currently difficult to choose a suitable surfactant for a specific type of coal dust based on a general methodology. Although some surfactants show good wetting ability in certain coal
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mines, they have no effect in other mines (Kissell, 2003; Zhou et al., 2015). This occurs due to
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the insufficient understanding of the interaction between different surfactants and coal dust.
selecting suitable surfactants.
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Moreover, the dynamic immersion process should also be taken into consideration when
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Coal is a complex sedimentary rock consisting of organic and inorganic components,
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which can be divided into different coal ranks, ranging from lignite to anthracite (O'Keefe et al., 2013). With the increase in coal rank, the degree of metamorphism also increases, while the
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chemical content of coal varies. Low-rank coals have high oxygen content and abundant oxygen-containing functional groups, such as carboxyl, carbonyl, and hydroxyl, while high-
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rank coals incorporate less oxygen-containing functional groups (Zhou et al., 2015; Liu et al., 2016; Gutierrez-Rodriguez et al., 1984; Arif et al., 2017). Thus, the hydrophobicity of coal increases with its rank. High-rank coals are more hydrophobic than low-rank coals, making it more difficult to wet and capture the dust of high-rank coals, therefore, can cause more harm to coal miners. Thus, the anthracite coal was selected in this work to study its wetting properties.
The structure of a surfactant consists of a hydrophilic head group and a hydrophobic tail group. According to the electrical properties of the head group, surfactants can be divided into anionic, cationic, nonionic, and zwitterionic. Numerous studies have focused on the adsorption and wetting abilities of different types of surfactants on the surface of coal. Crawford et al.
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(2001) studied the adsorption characteristics of three types of surfactants (anionic, cationic, and nonionic) on the surface of three Australian coals. Their results showed that nonionic
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surfactants caused high-rank coals to become more hydrophobic and thus demonstrated the highest adsorption density among all the types of surfactants studied. Liu et al. (2016) reported that the adsorption capacity of cationic gemini surfactants on the surface of lignite was
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significantly higher than that of ordinary cationic and anionic surfactants, due to the strong
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electrostatic attraction between the surfactants and lignite. Thus, lignite was more hydrophobic
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and its wettability was reduced after the adsorption of the cationic gemini surfactant. Singh (1999) investigated the adsorption behavior of anionic and cationic surfactants for finer coal
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dust and demonstrated that cationic surfactants achieved higher levels of adsorption than
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anionic surfactants. Although the adsorption of cationic surfactants on the surface of coal is higher than that of other types of surfactants, it usually leads to an increase in the
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hydrophobicity of coal dust. Zhou et al. (2016) studied the contact angle of four types of surfactants on coal dust. Their results indicated that anionic surfactants had the smallest contact
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angle and the best wetting ability among all the types of studied surfactants. Kilau (1990) also found that the wetting ability of anionic surfactants was superior to that of nonionic surfactants. Therefore, anionic surfactants are widely used in coal dust control technologies. Some scholars studied the effect of additives, such as polymers and inorganic salts, on the wetting ability of anionic surfactants (Kilau, 1990; Dou and Xu, 2017; Kilau and Voltz, 1991; Kilau et al., 1996;
Kilau and Pahlman, 1987). Results showed that the addition of these additives to the anionic surfactant solutions could improve the wettability of coal dust. Although the wetting abilities of anionic surfactants with different structures were tested, the results were only used to select suitable surfactants for the preparation of wetting agents (Yang et al., 2014; Zeller, 1983; Li et
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al., 2016). The effect of different molecular structures on the adsorption state of anionic surfactants on the surface of coal dust and the dynamic immersion process, both of which will
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determine the wetting performance of coal dust, have not been analyzed yet.
In this work, three anionic surfactants namely sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDDS), and sodium dodecyl benzene sulfonate (SDBS) were chosen, to
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study the effect of anionic surfactants, with different hydrophilic and hydrophobic group
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structures, on the wetting performance of coal dust. The surface tension and coal dust wetting
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time of different surfactant solutions were measured. The adsorption of surfactants on the surface of coal dust was obtained by comparing the infrared spectra of coal dust before and
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after surfactant treatment. The hydrophilic-lipophilic balance (HLB) values of the three
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surfactants were also calculated. The effect of the surface tension, surfactant adsorption density, and HLB values on the wetting time of coal dust were analyzed.
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2. Experimental 2.1. Materials
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Anthracite coal was obtained from the Wolonghu coal mine in the Anhui province, China.
After crushing it in a ball grinding mill, anthracite was screened to collect coal dust with particle sizes of 400–500 mesh. Subsequently, the coal dust samples were dried in a vacuum oven at 40 °C for 24 h. After cooling, the coal dust samples were stored in sealed plastic bags for subsequent tests. The proximate and ultimate analysis of the coal dust sample are listed in Table
1. Table 1 Proximate and ultimate analysis of the coal dust sample. Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Coal sample
Wolonghu anthracite
Mad
Aad
Vad
FCad
Cdaf
Hdaf
Odaf
Ndaf
Sdaf
1.79
11.84
9.99
76.38
92.64
4.18
1.77
1.15
0.26
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We purchased SDS (purity ≥ 93%) and SDDS (purity ≥ 98.5%) from Qingdao Yousuo Chemical Technology Co., Ltd. China. Moreover, SDBS (purity ≥ 90%) was provided by
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Tianjin Bodi Chemical Technology Co., Ltd. China. The molecular structures of the three
anionic surfactants are shown in Fig. 1. The tail group of the SDBS molecule has one more
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benzene ring than the SDS and SDDS molecules. The head group of the SDS molecule is a
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sulfate group containing one sulfur and four oxygen atoms, while the head groups of the SDDS
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and SDBS molecules are sulfonate groups consisting of one sulfur and three oxygen atoms.
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Distilled water was used for preparing all solutions.
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Fig. 1. Molecular structures of SDS, SDDS, and SDBS. The colors of the atoms and/or ions are as follows: C, gray; H, white; O, red; S, yellow; Na+, violet.
2.2. Surface tension measurements The surface tension of the surfactant solutions was measured on a JYW-200B surface tension meter (Chengde Baohui Experimental Products Co., Ltd., China) using the platinum
ring method. The average surface tension values were obtained from three measurements for each solution, and the difference between the three test results was within 0.3 mN/m. Before every measurement, the platinum ring was cleaned with distilled water and then dried over an alcohol lamp.
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2.3. Wetting time measurements Because the contact angle method is used to test the wettability between solid surfaces and
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droplets, it is not practical to evaluate the wetting characteristics of particles (Kollipara et al.,
2014; Xu et al., 2017). Thus, the Walker test method was adopted. For the experiments, 50 mL surfactant solution was placed in a beaker, and then 0.2 g coal dust was gently poured on the
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surface of the solution by passing through a glass funnel. The wetting time required for the coal
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dust to completely immerse in the solution was recorded to characterize the wetting ability of
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different surfactant solutions. The shorter the wetting time was, the better the wetting ability of
was tested four times.
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2.4. FTIR measurements
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the surfactant solution. For each surfactant solution, the wetting time of the coal dust samples
The infrared spectra of the surfactants and original coal dust were tested using a Nicolet
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6700 Fourier transform infrared (FTIR) spectrometer. Before testing, the background reference spectrum was obtained using pure KBr powder. Subsequently, the samples were placed into the
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diffuse reflector sample pool and the surface was levelled. The wave number was in the 650– 4000 cm–1 range with a resolution of 4 cm–1. Each spectrum of the testing samples was collected after 64 scans. After filtration, the coal dust samples wetted by surfactant solutions were dried in a vacuum oven at 40 °C for 24 h, and were then used for FTIR tests following the same procedures as that for the original coal dust.
3. Results and discussion
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3.1. Surface tension
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Fig. 2. Surface tension isotherms of the SDS, SDDS, and SDBS solutions.
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When surfactants are used in dust control technologies, the surface tension of the surfactant solution is one of the important factors affecting dust suppression efficiency. The
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surfactant can effectively reduce the surface tension, thereby decreasing the energy barrier for
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coal dust particles to enter the droplets and improving the capture efficiency of coal dust (Tessum et al., 2014). Therefore, the surface tension of the three anionic surfactants at different concentrations was tested, as shown in Fig. 2. The surface tension first decreased sharply and
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then had the tendency to remain constant as the concentration increased. This occurred because the increase in concentration could gradually increase the adsorption density of the surfactant at the air-water interface. After reaching saturation, the surface tension remains constant. The three types of surfactants have different abilities to reduce the surface tension. For the same concentration, except for the 1 wt.‰ concentration, the surface tension decreased from SDDS
to SDS to SDBS, indicating that SDBS had the strongest ability to reduce the surface tension, and the ability of SDDS to reduce the surface tension was the weakest of the three surfactants. 3.2. Wetting time The wetting times of coal dust for different surfactant solutions are shown in Table 2 and
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Fig. 3. When the surfactant concentration was smaller than 0.1 wt.‰, the wetting time was greater than 24 h, indicating that the coal dust could not be wetted or the wetting rate was quite
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low. When the surfactant concentration exceeded 0.5 wt.‰, the wetting time decreased
gradually with the increase in surfactant concentration. The relationship between the surface tension and wetting time was analyzed as follows. At surfactant concentrations smaller than 1
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wt.‰, the increase in concentration led to a decline in the surface tension, and the wetting time
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also decreased from over 24 h to 611.21 s, 401.66 s, and 2.3 h, for SDS, SDDS, and SDBS,
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respectively. This proved that the decrease in the surface tension could weaken the energy barrier for coal dust to enter the solution, thereby enhancing the wetting performance of coal
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dust. When the surfactant concentration was greater than 2 wt.‰, the surface tension remained
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constant with increase in the concentration, but the wetting time still decreased, which suggested that the increase in wetting ability of the surfactant solutions was no longer related
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to the surface tension. Studies showed that when the surface tension was lower than the critical surface tension of 45 mN/m, the adsorption state of the surfactant on coal dust surface began to
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affect the wetting behavior (Zhou et al., 2016; Kilau, 1990). The FTIR results in Section 3.3 showed that the increase in concentration increased the adsorption density of surfactants on the surface of coal dust, which indicated that the shortened wetting time was related to the increase in surfactant adsorption density. In addition, at surfactant concentration greater than 2 wt.‰, although the surface tension
of SDBS was the lowest, its wetting time was longer than those of SDS and SDDS. While SDDS, which had the highest surface tension of the three surfactants studied, demonstrated the shortest wetting time. This also suggested that when the surface tension was lower than the critical surface tension, it no longer determined the wetting performance of coal dust. For
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anionic surfactants with different structures, both the surfactant adsorption density and HLB values of the surfactants determined the wetting performance and caused different wetting times.
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The detailed explanation was presented in section 3.4. Table 2 Wetting time of coal dust for different surfactant solutions. Wetting time SDS
SDDS
0.01
＞24 h
＞24 h
0.1
＞24 h
0.5
2.7 h
1
611.21 s
2 3
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SDBS
＞24 h ＞24 h
2.1 h
3.8 h
401.66 s
2.3 h
139.02 s
81.47 s
1.2 h
68.59 s
54.37 s
1959.38 s
49.90 s
41.67 s
772.32 s
37.24 s
33.40 s
445.94 s
32.14 s
29.64 s
297.37 s
26.59 s
25.04 s
209.27 s
8
24.92 s
21.26 s
182.26 s
9
23.67 s
20.50 s
153.42 s
6
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7
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5
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＞24 h
4
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Surfactant concentration (wt.‰)
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Fig. 3. Wetting time of coal dust for different surfactant solutions.
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3.3. Surfactant adsorption state on the surface of coal dust
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The adsorption state of the surfactant on the surface of mineral particles can be studied by
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comparing the infrared spectra of mineral particles before and after the treatment using surfactant solutions (Liu et al., 2016; Bera et al., 2013; Zheng et al., 2018). First, we needed to
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determine the characteristic adsorption peaks of the surfactants. The infrared spectra of these
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three surfactants and original coal dust are shown in Fig. 4. The adsorption peaks of the O=S=O structure, which is contained in the head groups of all the three surfactants, were located at 1219 and 1249 cm–1, and the band separation was 30 cm–1 (Viana et al., 2012). As shown in Figs. 4(a)
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and (b), the adsorption peaks of SO2 in the infrared spectra of SDS and SDDS shifted to 1233 and 1263 cm–1, respectively, and the separation between the two peaks was also 30 cm–1. Moreover, as shown in Fig. 4(c), the adsorption peak of SO2 could not be found in the infrared spectrum of SDBS, because it was hidden by the benzene ring linked to the head group (Zheng et al., 2018). Therefore, the adsorption peaks of SO2 could not be used to characterize the
adsorption states of the surfactants on the surface of coal dust. Aliphatic chains also exist in all three surfactants. The peak at 2920 cm–1 was attributed to the aliphatic chain and could be observed in the infrared spectra of SDS, SDDS, and SDBS. The peak attributed to the aromatic C–H structure occurred at 3047 cm–1. Due to the absence
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of the benzene ring, we did not identify an adsorption peak at 3047 cm–1 in the infrared spectra of SDS and SDDS. Because of the influence of the aliphatic chain, the intensity of the
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adsorption peak at 3047 cm–1 in the spectra of SDBS was much lower than that of the 2920 cm– peak. The obvious peaks of the aliphatic chain and aromatic C–H structure could also be
observed at 2920 and 3047 cm–1 in the infrared spectrum of coal dust. If the surfactant adsorbed
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on the surface of coal dust, it would inevitably lead to an increase in the intensity of the
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adsorption peak of the aliphatic chain at 2920 cm–1, while the peak intensity at 3047 cm–1 would
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remain constant. Thus, the ratio of the peak intensities at 2920 and 3047 cm–1 was calculated to characterize the different adsorption densities of surfactants on the surface of coal dust. The
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higher the ratio was, the larger the surfactant adsorption density.
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Fig. 4. FTIR spectra of the solid powder of SDS, SDDS and SDBS, and original coal dust without surfactant treatment.
When the surfactant concentration was higher than 2 wt.‰, the surface tension remained constant, and the decrease in wetting time was related to the adsorption state of the surfactants
on the surface of coal dust. Thus, surfactant solutions with concentrations of 3, 5, 7, and 9 wt.‰ were selected, and the infrared spectra of coal dust after being wetted using surfactant solutions with the above concentrations were obtained, as shown in Fig. 5. The Kubelka–Munk (KM) absorbances of the aliphatic chains and aromatic C–H structures of the coal dust treated using
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surfactant solutions were obtained, and the ratios of the KM absorbances of the aliphatic chain to those of the aromatic C–H structures were calculated, as shown in Table 3. All the ratios for
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coal dust wetted by surfactant solutions exceed 0.87, and were higher than the 0.82 ratio for the original coal dust, which was calculated from Fig. 4(d). This demonstrated that the surfactants adsorbed on the surface of coal dust. For the same surfactant, the ratio increased gradually with
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the increase in surfactant concentration, as shown in Table 3, which suggested that the
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adsorption densities of the surfactants increased as well. Moreover, for the same surfactant
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concentration, the ratio for the coal dust wetted with SDBS was the highest, followed by the ratios for SDDS and SDS, which indicated that the adsorption density decreased from SDBS to
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SDDS to SDS.
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Fig. 5. FTIR spectra of coal dust after being wetted by surfactant solutions.
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Table 3 KM absorbances of the aliphatic chain and aromatic C–H structure of coal dust after being wetted
Aliphatic chain/aromatic C–H KM absorbances ratio
Aromatic C–H
3
0.416
0.477
0.8721
0.428
0.489
0.8753
7
0.434
0.493
0.8803
9
0.400
0.453
0.8830
3
0.449
0.497
0.9034
5
0.467
0.514
0.9086
7
0.472
0.519
0.9094
9
0.481
0.528
0.9110
3
0.477
0.497
0.9598
5
0.464
0.479
0.9687
7
0.469
0.474
0.9895
9
0.491
0.494
0.9939
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Aliphatic chain
5
SDS
KM absorbances
Concentration (wt.‰)
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Surfactant
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by surfactant solutions and their ratios.
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SDDS
SDBS
The effect of the concentration and molecular structures of surfactants on the adsorption density was explained as follows. The surface of coal dust consists of a large number of hydrophobic sites caused by hydrocarbon chains and aromatic rings. Few of the oxygencontaining functional groups are associated with hydrophilic sites with negative charges.
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Because of the strong hydrophobic interactions between the aliphatic chains of the surfactants and hydrophobic sites on the surface of coal dust, the tail groups of the surfactants adsorb onto
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the hydrophobic sites, leaving the hydrophilic head groups outward, which causes the
hydrophobic sites of coal dust to convert to hydrophilic sites. As the surfactant concentration increased, the surfactant adsorption density increased along with the number of hydrophilic
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sites on the surface of coal dust, as shown in Fig. 6. This resulted in the improvement of the
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wetting performance and the decrease of the wetting time.
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Compared to SDS and SDDS, SDBS presented stronger hydrophobic interactions on the surface of coal dust due to the longer tail group, leading to higher adsorption density. Although
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SDS and SDDS have the same tail group, their head group structures are different. The sulfate
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head group of SDS has a partial charge of –1.33, while the partial charge of the sulfonate group of SDDS is –1.00 (Wang et al., 2016a). The high negative charges of the sulfate group caused
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strong electrostatic repulsions between the SDS head groups and the negatively charged sites on the surface of coal dust, which suppressed the hydrophobic interaction. In addition, strong
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electrostatic repulsions occurred between the head groups of the SDS molecules, resulting in large distances between adjacent surfactant molecules. Thus, the adsorption density of SDS was lower than that of SDDS. The adsorption states of the three surfactants on the surface of coal dust are shown in Fig. 7.
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Fig. 6. Surfactant adsorption state on the surface of coal dust at different concentrations.
Fig. 7. Adsorption state of SDS, SDDS, and SDBS molecules on the surface of coal dust.
3.4. Dynamic immersion process
Theoretically, the wetting ability of the SDBS solution should have been the best, because its adsorption density was the highest of the three surfactants in this study, and more hydrophobic sites on the surface of coal dust were converted into hydrophilic sites in the presence of SDBS. On the other hand, SDS should have exhibited the worst wetting
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performance due to its lowest adsorption density compared to the other surfactants studied. However, SDBS rather than SDS, exhibited the longest wetting time, while the wetting time of
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SDDS was the shortest. This occurred because the coal dust needed to enter the bulk solution from the air-water interface after being adsorbed by the surfactant molecules, as shown in Fig. 8, so that the wetting process was finally completed. Most studies focused on the adsorption
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behavior of surfactants on the surface of coal dust, but the dynamic immersion process has not
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been studied yet. In this dynamic immersion process, the ability of coal dust to enter the bulk
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solution from the air-water interface plays an important role in the wetting time.
Fig. 8. Dynamic immersion process of coal dust to enter the bulk solution from the air-water interface.
After adsorption, the surface of coal dust is covered with surfactants. Thus, the ability of coal dust to enter the bulk solution depends on the ability of the surfactant to enter the bulk solution from the air-water interface, which is determined by the molecular structure of the
surfactant. The tail group of the surfactant is hydrophobic, causing the surfactant to migrate from the bulk solution to the air-water interface, while the head group is hydrophilic and tends to bring the surfactant at the air-water interface toward the bulk solution. The hydrophilicity of the head group vs. the hydrophobicity of the tail group of the surfactant molecule can be
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quantitatively evaluated using the HLB values, which can be calculated using the Davies equation (Davies, 1957):
(1)
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HLB = Σ(hydrophilic group numbers) − Σ(hydrophobic group numbers) + 7
When the hydrocarbon chain is short and the hydrophilicity of the head group is strong, HLB is high. Hence, it is easy for the surfactant to enter the bulk solution from the air-water
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interface. Therefore, the ability of coal dust to enter the bulk solution can be characterized using
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the HLB value. The higher the HLB value of the surfactant is, the better the ability of coal dust
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to enter the bulk solution, which will result in higher wetting rates and shorter wetting times. According to the Davies equation and the group numbers of different structures, the calculated
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HLB values of SDS, SDDS and SDBS were 40.0, 13.0 and 10.6, respectively (Davies, 1957;
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Li et al., 2007; Pan et al., 2012). The hydrophilicity of the sulfonate group of SDDS is poorer than that of the sulfate group of SDS, and SDDS has a shorter tail group than SDBS. Thus, the
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HLB value of SDDS is intermediate between the corresponding values of SDS and SDBS. Although SDBS had the largest adsorption density and generated the highest number of
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hydrophilic sites on the surface of coal dust compared to SDS and SDDS, its lowest HLB value resulted in the poorest ability of coal dust to enter the bulk solution and caused the longest wetting time. While SDS had the highest HLB value of the three surfactants, its lowest adsorption density led to generating the smallest number of hydrophilic sites on the surface of coal dust, which also resulted in a relatively long wetting time. The adsorption density and HLB
value of SDDS were intermediate between those of SDS and SDBS. Under the combined action of these two factors, SDDS could bring the coal dust to the bulk solution from the air-water interface the fastest. Therefore, the coal dust wetting time of SDDS was shorter than those of SDS and SDBS.
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4. Conclusions In the present work, the surface tension and the coal dust wetting time of SDS, SDDS, and
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SDBS solutions were tested. The infrared spectra of coal dust before and after being wetted by surfactant solutions were obtained, and the HLB values of the three surfactants were also
calculated. The effect of the molecular structures and surfactant concentrations on the wetting
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time of coal dust was analyzed from the surface tension, surfactant adsorption density, and HLB
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values. The main conclusions can be summarized as follows.
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At surfactant concentration lower than 1 wt.‰, the increase in concentration reduced the surface tension, thus improving the wetting ability of solutions and decreasing the wetting time.
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When the surfactant concentration was higher than 2 wt.‰, the surface tension was lower than
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the critical surface tension (45 mN/m) and remained constant as the concentration increased. The decrease in the wetting time was related to the surfactant adsorption density on the coal
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dust surface and the ability of coal dust to enter the bulk solution from the air-water interface. For the same anionic surfactant, the increase in surfactant concentration increased its
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adsorption density, which resulted in an increase in the number of hydrophilic sites on the surface of coal dust and shortened the wetting time. For anionic surfactants with different structures, the adsorption density decreased from SDBS to SDDS to SDS. The long hydrocarbon chain enhanced the hydrophobic interaction between the tail group of the surfactant and the hydrophobic sites on the surface of coal dust, while the low negative charges
of the head groups weakened the electrostatic repulsions between the surfactant molecules and negatively charged sites of coal dust. Both processes increased the surfactant adsorption density and number of hydrophilic sites on the surface of coal dust. During the dynamic immersion process, the ability of coal dust to enter the bulk solution
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from the air-water interface was characterized using the HLB value of the surfactant. The HLB value decreased from SDS to SDDS to SDBS. The higher the HLB value was, the better the
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ability of coal dust to enter the bulk solution, which resulted in higher wetting rates and shorter
wetting times. Although SDBS exhibited the largest adsorption density among all three surfactants in this study, its lowest HLB value resulted in the poorest ability of coal dust to enter
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the bulk solution and caused the longest wetting time. On the other hand, SDS exhibited the
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highest HLB value of the surfactants analyzed, but its lowest adsorption density led to
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generating the smallest number of hydrophilic sites on the surface of coal dust, which also resulted in a relatively long wetting time. The adsorption density and HLB of SDDS were
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intermediate between those of SDS and SDBS. Under the combined action of these two factors,
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SDDS brought the coal dust from the air-water interface into the bulk solution the fastest. Therefore, the coal dust wetting time of SDDS was shorter than those of SDS and SDBS.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (51474216,
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51504249 and 51874290), the Project funded by China Postdoctoral Science Foundation (2018T110574), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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S0957-5820(18)30295-7 https://doi.org/10.1016/j.psep.2018.10.010 PSEP 1540
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
12-6-2018 6-9-2018 12-10-2018
Please cite this article as: Xu C, Wang D, Wang H, Ma L, Zhu X, Zhu Y, Zhang Y, Liu F, Experimental investigation of coal dust wetting ability of anionic surfactants with different structures, Process Safety and Environmental Protection (2018), https://doi.org/10.1016/j.psep.2018.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation of coal dust wetting ability of anionic surfactants with different structures
Chaohang Xua,b, Deming Wanga,b,*, Hetang Wanga,b,*, Liyang Maa,b, Xiaolong Zhua,b, Yunfei Zhua,b, Yi
Laboratory of Gas and Fire Control for Coal Mines of Ministry of Education, Xuzhou 221116, China
cXuzhou
of Marxism, Shanghai University of Finance and Economics, Shanghai 200000, China
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Corresponding author: Deming Wang, Hetang Wang
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dSchool
Anyun Mining Technology Inc., Xuzhou 221008, China
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bKey
of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China
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aSchool
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Zhangc, Fangming Liud
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E-mail address: [email protected], [email protected]
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Highlights
The wetting time doesn’t relate to the surface tension that is lower than 45 mN/m.
Hydrophobic and electrostatic interactions influence the adsorption density.
Hydrophilic-lipophilic balance (HLB) values affect the dynamic immersion process.
The surfactant structures cause different adsorption densities and HLB values.
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Both the adsorption density and HLB value determine the wetting ability.
Abstract: To better understand the coal dust wetting ability of anionic surfactants with different structures, sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDDS), and sodium
dodecyl benzene sulfonate (SDBS) were selected. The surface tension, wetting time, and infrared spectra of coal dust were tested. The hydrophilic-lipophilic balance (HLB) values of the surfactants were calculated. Results showed that the decrease in the surface tension could shorten the wetting time at low surfactant concentrations. But the decrease in the wetting time
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was no longer related to the constant surface tension that was lower than 45 mN/m. The adsorption density depends on the hydrophobic interactions and electrostatic repulsions
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between the surfactant molecules and the coal dust. During the dynamic immersion process, the
surfactants with high HLB values could bring the coal dust into the bulk solution rapidly. The wetting time of coal dust for SDBS with a high adsorption density and low HLB or SDS with
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a small adsorption density and high HLB was relatively longer. The adsorption density and
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HLB value of SDDS were intermediate between those of SDS and SDBS. Under the combined
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action of these two factors, the wetting time of SDDS was shorter than those of SDS and SDBS. Keywords: coal dust; anionic surfactant; wetting time; surface tension; adsorption density;
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hydrophilic-lipophilic balance
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1. Introduction
While coal is an important energy source worldwide, many disasters occur during the coal
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mining process (Xi et al., 2014; Cheng et al., 2017; Liang et al., 2018; Zhang et al., 2018). Coal dust is a significant hazard factor that can cause explosions and coal workers’ pneumoconiosis
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(CWP) (Fan et al., 2018), a serious occupational disease which can affect workers’ health and coal production. The patients suffer a lot of pain and cannot be cured. As the world’s largest coal producer, China generated 46.1% of the global coal in 2016 (British Petroleum, 2017). In China, due to the extensive use of the mechanized coal mining method, the amount of coal dust produced at working faces has increased dramatically (Wang et al., 2016; Zhou et al., 2017; Liu
et al., 2018). Therefore, the number of CWP patients has been increasing annually, and pneumoconiosis has become the most serious occupational disease among Chinese coal miners (Chen et al., 2013). In the USA, an upward trend in CWP has been observed since 2000. Thus, the National Institute of Occupational Safety and Health (NIOSH) recommended that the limit
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value of respirable coal mine dust should be reduced from 2 to 1 mg/m3 (Kollipara et al., 2014). However, it is difficult to achieve the statutory standards for most high production faces
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(Ralston et al., 2017). Hence, effective dust control technologies must be adopted to protect the health of coal miners.
Among various dust control technologies, water spraying technology has been widely used
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in dust suppression due to its low cost and simple implementation (Wang et al., 2018). However,
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the wetting ability and dust suppression efficiency of sprayed water is relatively poor, because
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of the hydrophobicity of coal dust and the high surface tension of water (Zhou et al., 2017a). Wetting agents composed of surfactants are usually added to water to reduce its surface tension
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to improve the dust suppression efficiency (Cybulski et al., 2015; Yao et al., 2017). The
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surfactant-magnetized water technology has also been developed to further reduce the surface tension and improve the wetting performance of water for coal dust. Studies showed that the
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suppression efficiency of surfactant-magnetized water for total and respirable dust increased by 31.79% and 44.94%, respectively, compared to that of untreated water (Zhou et al., 2017a).
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Dust suppression using foam has also been proved to be an effective technology. During this process, surfactants, water, and air are mixed in a foam generator to produce foam, which is then sprayed on the dust source (Wang et al., 2012). Compared to the water spray technology, the foam technology consumes less water, and the suppression of total and respirable dust is increased by 30–50%, due to the higher specific surface area, better wetting ability, and stronger
adhesion of the foam (Seibel, 1976; Volkwein et al., 1983; Mukherjee and Singh, 1984; Ren et al., 2012). Surfactant can improve the wetting performance of coal dust and enhance the coal suppression efficiency. The types and molecular structures of surfactants, coal rank and coal
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dust size are important factors affecting the wetting performance of coal dust (Özer et al., 2017). Various studies have obtained the same conclusion that finer coal particles are more difficult to
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be wetted (Xu et al., 2018). However, due to the large number of surfactants and different coal
ranks, it is currently difficult to choose a suitable surfactant for a specific type of coal dust based on a general methodology. Although some surfactants show good wetting ability in certain coal
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mines, they have no effect in other mines (Kissell, 2003; Zhou et al., 2015). This occurs due to
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the insufficient understanding of the interaction between different surfactants and coal dust.
selecting suitable surfactants.
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Moreover, the dynamic immersion process should also be taken into consideration when
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Coal is a complex sedimentary rock consisting of organic and inorganic components,
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which can be divided into different coal ranks, ranging from lignite to anthracite (O'Keefe et al., 2013). With the increase in coal rank, the degree of metamorphism also increases, while the
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chemical content of coal varies. Low-rank coals have high oxygen content and abundant oxygen-containing functional groups, such as carboxyl, carbonyl, and hydroxyl, while high-
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rank coals incorporate less oxygen-containing functional groups (Zhou et al., 2015; Liu et al., 2016; Gutierrez-Rodriguez et al., 1984; Arif et al., 2017). Thus, the hydrophobicity of coal increases with its rank. High-rank coals are more hydrophobic than low-rank coals, making it more difficult to wet and capture the dust of high-rank coals, therefore, can cause more harm to coal miners. Thus, the anthracite coal was selected in this work to study its wetting properties.
The structure of a surfactant consists of a hydrophilic head group and a hydrophobic tail group. According to the electrical properties of the head group, surfactants can be divided into anionic, cationic, nonionic, and zwitterionic. Numerous studies have focused on the adsorption and wetting abilities of different types of surfactants on the surface of coal. Crawford et al.
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(2001) studied the adsorption characteristics of three types of surfactants (anionic, cationic, and nonionic) on the surface of three Australian coals. Their results showed that nonionic
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surfactants caused high-rank coals to become more hydrophobic and thus demonstrated the highest adsorption density among all the types of surfactants studied. Liu et al. (2016) reported that the adsorption capacity of cationic gemini surfactants on the surface of lignite was
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significantly higher than that of ordinary cationic and anionic surfactants, due to the strong
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electrostatic attraction between the surfactants and lignite. Thus, lignite was more hydrophobic
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and its wettability was reduced after the adsorption of the cationic gemini surfactant. Singh (1999) investigated the adsorption behavior of anionic and cationic surfactants for finer coal
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dust and demonstrated that cationic surfactants achieved higher levels of adsorption than
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anionic surfactants. Although the adsorption of cationic surfactants on the surface of coal is higher than that of other types of surfactants, it usually leads to an increase in the
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hydrophobicity of coal dust. Zhou et al. (2016) studied the contact angle of four types of surfactants on coal dust. Their results indicated that anionic surfactants had the smallest contact
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angle and the best wetting ability among all the types of studied surfactants. Kilau (1990) also found that the wetting ability of anionic surfactants was superior to that of nonionic surfactants. Therefore, anionic surfactants are widely used in coal dust control technologies. Some scholars studied the effect of additives, such as polymers and inorganic salts, on the wetting ability of anionic surfactants (Kilau, 1990; Dou and Xu, 2017; Kilau and Voltz, 1991; Kilau et al., 1996;
Kilau and Pahlman, 1987). Results showed that the addition of these additives to the anionic surfactant solutions could improve the wettability of coal dust. Although the wetting abilities of anionic surfactants with different structures were tested, the results were only used to select suitable surfactants for the preparation of wetting agents (Yang et al., 2014; Zeller, 1983; Li et
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al., 2016). The effect of different molecular structures on the adsorption state of anionic surfactants on the surface of coal dust and the dynamic immersion process, both of which will
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determine the wetting performance of coal dust, have not been analyzed yet.
In this work, three anionic surfactants namely sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDDS), and sodium dodecyl benzene sulfonate (SDBS) were chosen, to
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study the effect of anionic surfactants, with different hydrophilic and hydrophobic group
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structures, on the wetting performance of coal dust. The surface tension and coal dust wetting
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time of different surfactant solutions were measured. The adsorption of surfactants on the surface of coal dust was obtained by comparing the infrared spectra of coal dust before and
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after surfactant treatment. The hydrophilic-lipophilic balance (HLB) values of the three
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surfactants were also calculated. The effect of the surface tension, surfactant adsorption density, and HLB values on the wetting time of coal dust were analyzed.
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2. Experimental 2.1. Materials
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Anthracite coal was obtained from the Wolonghu coal mine in the Anhui province, China.
After crushing it in a ball grinding mill, anthracite was screened to collect coal dust with particle sizes of 400–500 mesh. Subsequently, the coal dust samples were dried in a vacuum oven at 40 °C for 24 h. After cooling, the coal dust samples were stored in sealed plastic bags for subsequent tests. The proximate and ultimate analysis of the coal dust sample are listed in Table
1. Table 1 Proximate and ultimate analysis of the coal dust sample. Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Coal sample
Wolonghu anthracite
Mad
Aad
Vad
FCad
Cdaf
Hdaf
Odaf
Ndaf
Sdaf
1.79
11.84
9.99
76.38
92.64
4.18
1.77
1.15
0.26
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We purchased SDS (purity ≥ 93%) and SDDS (purity ≥ 98.5%) from Qingdao Yousuo Chemical Technology Co., Ltd. China. Moreover, SDBS (purity ≥ 90%) was provided by
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Tianjin Bodi Chemical Technology Co., Ltd. China. The molecular structures of the three
anionic surfactants are shown in Fig. 1. The tail group of the SDBS molecule has one more
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benzene ring than the SDS and SDDS molecules. The head group of the SDS molecule is a
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sulfate group containing one sulfur and four oxygen atoms, while the head groups of the SDDS
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and SDBS molecules are sulfonate groups consisting of one sulfur and three oxygen atoms.
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Distilled water was used for preparing all solutions.
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Fig. 1. Molecular structures of SDS, SDDS, and SDBS. The colors of the atoms and/or ions are as follows: C, gray; H, white; O, red; S, yellow; Na+, violet.
2.2. Surface tension measurements The surface tension of the surfactant solutions was measured on a JYW-200B surface tension meter (Chengde Baohui Experimental Products Co., Ltd., China) using the platinum
ring method. The average surface tension values were obtained from three measurements for each solution, and the difference between the three test results was within 0.3 mN/m. Before every measurement, the platinum ring was cleaned with distilled water and then dried over an alcohol lamp.
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2.3. Wetting time measurements Because the contact angle method is used to test the wettability between solid surfaces and
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droplets, it is not practical to evaluate the wetting characteristics of particles (Kollipara et al.,
2014; Xu et al., 2017). Thus, the Walker test method was adopted. For the experiments, 50 mL surfactant solution was placed in a beaker, and then 0.2 g coal dust was gently poured on the
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surface of the solution by passing through a glass funnel. The wetting time required for the coal
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dust to completely immerse in the solution was recorded to characterize the wetting ability of
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different surfactant solutions. The shorter the wetting time was, the better the wetting ability of
was tested four times.
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2.4. FTIR measurements
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the surfactant solution. For each surfactant solution, the wetting time of the coal dust samples
The infrared spectra of the surfactants and original coal dust were tested using a Nicolet
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6700 Fourier transform infrared (FTIR) spectrometer. Before testing, the background reference spectrum was obtained using pure KBr powder. Subsequently, the samples were placed into the
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diffuse reflector sample pool and the surface was levelled. The wave number was in the 650– 4000 cm–1 range with a resolution of 4 cm–1. Each spectrum of the testing samples was collected after 64 scans. After filtration, the coal dust samples wetted by surfactant solutions were dried in a vacuum oven at 40 °C for 24 h, and were then used for FTIR tests following the same procedures as that for the original coal dust.
3. Results and discussion
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3.1. Surface tension
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Fig. 2. Surface tension isotherms of the SDS, SDDS, and SDBS solutions.
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When surfactants are used in dust control technologies, the surface tension of the surfactant solution is one of the important factors affecting dust suppression efficiency. The
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surfactant can effectively reduce the surface tension, thereby decreasing the energy barrier for
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coal dust particles to enter the droplets and improving the capture efficiency of coal dust (Tessum et al., 2014). Therefore, the surface tension of the three anionic surfactants at different concentrations was tested, as shown in Fig. 2. The surface tension first decreased sharply and
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then had the tendency to remain constant as the concentration increased. This occurred because the increase in concentration could gradually increase the adsorption density of the surfactant at the air-water interface. After reaching saturation, the surface tension remains constant. The three types of surfactants have different abilities to reduce the surface tension. For the same concentration, except for the 1 wt.‰ concentration, the surface tension decreased from SDDS
to SDS to SDBS, indicating that SDBS had the strongest ability to reduce the surface tension, and the ability of SDDS to reduce the surface tension was the weakest of the three surfactants. 3.2. Wetting time The wetting times of coal dust for different surfactant solutions are shown in Table 2 and
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Fig. 3. When the surfactant concentration was smaller than 0.1 wt.‰, the wetting time was greater than 24 h, indicating that the coal dust could not be wetted or the wetting rate was quite
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low. When the surfactant concentration exceeded 0.5 wt.‰, the wetting time decreased
gradually with the increase in surfactant concentration. The relationship between the surface tension and wetting time was analyzed as follows. At surfactant concentrations smaller than 1
N
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wt.‰, the increase in concentration led to a decline in the surface tension, and the wetting time
A
also decreased from over 24 h to 611.21 s, 401.66 s, and 2.3 h, for SDS, SDDS, and SDBS,
M
respectively. This proved that the decrease in the surface tension could weaken the energy barrier for coal dust to enter the solution, thereby enhancing the wetting performance of coal
ED
dust. When the surfactant concentration was greater than 2 wt.‰, the surface tension remained
PT
constant with increase in the concentration, but the wetting time still decreased, which suggested that the increase in wetting ability of the surfactant solutions was no longer related
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to the surface tension. Studies showed that when the surface tension was lower than the critical surface tension of 45 mN/m, the adsorption state of the surfactant on coal dust surface began to
A
affect the wetting behavior (Zhou et al., 2016; Kilau, 1990). The FTIR results in Section 3.3 showed that the increase in concentration increased the adsorption density of surfactants on the surface of coal dust, which indicated that the shortened wetting time was related to the increase in surfactant adsorption density. In addition, at surfactant concentration greater than 2 wt.‰, although the surface tension
of SDBS was the lowest, its wetting time was longer than those of SDS and SDDS. While SDDS, which had the highest surface tension of the three surfactants studied, demonstrated the shortest wetting time. This also suggested that when the surface tension was lower than the critical surface tension, it no longer determined the wetting performance of coal dust. For
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anionic surfactants with different structures, both the surfactant adsorption density and HLB values of the surfactants determined the wetting performance and caused different wetting times.
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The detailed explanation was presented in section 3.4. Table 2 Wetting time of coal dust for different surfactant solutions. Wetting time SDS
SDDS
0.01
＞24 h
＞24 h
0.1
＞24 h
0.5
2.7 h
1
611.21 s
2 3
N
SDBS
＞24 h ＞24 h
2.1 h
3.8 h
401.66 s
2.3 h
139.02 s
81.47 s
1.2 h
68.59 s
54.37 s
1959.38 s
49.90 s
41.67 s
772.32 s
37.24 s
33.40 s
445.94 s
32.14 s
29.64 s
297.37 s
26.59 s
25.04 s
209.27 s
8
24.92 s
21.26 s
182.26 s
9
23.67 s
20.50 s
153.42 s
6
M
ED
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7
PT
5
A
＞24 h
4
A
U
Surfactant concentration (wt.‰)
IP T SC R U
N
Fig. 3. Wetting time of coal dust for different surfactant solutions.
A
3.3. Surfactant adsorption state on the surface of coal dust
M
The adsorption state of the surfactant on the surface of mineral particles can be studied by
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comparing the infrared spectra of mineral particles before and after the treatment using surfactant solutions (Liu et al., 2016; Bera et al., 2013; Zheng et al., 2018). First, we needed to
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determine the characteristic adsorption peaks of the surfactants. The infrared spectra of these
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three surfactants and original coal dust are shown in Fig. 4. The adsorption peaks of the O=S=O structure, which is contained in the head groups of all the three surfactants, were located at 1219 and 1249 cm–1, and the band separation was 30 cm–1 (Viana et al., 2012). As shown in Figs. 4(a)
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and (b), the adsorption peaks of SO2 in the infrared spectra of SDS and SDDS shifted to 1233 and 1263 cm–1, respectively, and the separation between the two peaks was also 30 cm–1. Moreover, as shown in Fig. 4(c), the adsorption peak of SO2 could not be found in the infrared spectrum of SDBS, because it was hidden by the benzene ring linked to the head group (Zheng et al., 2018). Therefore, the adsorption peaks of SO2 could not be used to characterize the
adsorption states of the surfactants on the surface of coal dust. Aliphatic chains also exist in all three surfactants. The peak at 2920 cm–1 was attributed to the aliphatic chain and could be observed in the infrared spectra of SDS, SDDS, and SDBS. The peak attributed to the aromatic C–H structure occurred at 3047 cm–1. Due to the absence
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of the benzene ring, we did not identify an adsorption peak at 3047 cm–1 in the infrared spectra of SDS and SDDS. Because of the influence of the aliphatic chain, the intensity of the
1
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adsorption peak at 3047 cm–1 in the spectra of SDBS was much lower than that of the 2920 cm– peak. The obvious peaks of the aliphatic chain and aromatic C–H structure could also be
observed at 2920 and 3047 cm–1 in the infrared spectrum of coal dust. If the surfactant adsorbed
U
on the surface of coal dust, it would inevitably lead to an increase in the intensity of the
A
N
adsorption peak of the aliphatic chain at 2920 cm–1, while the peak intensity at 3047 cm–1 would
M
remain constant. Thus, the ratio of the peak intensities at 2920 and 3047 cm–1 was calculated to characterize the different adsorption densities of surfactants on the surface of coal dust. The
A
CC E
PT
ED
higher the ratio was, the larger the surfactant adsorption density.
A ED
PT
CC E
IP T
SC R
U
N
A
M
IP T SC R U N A M ED PT CC E A
Fig. 4. FTIR spectra of the solid powder of SDS, SDDS and SDBS, and original coal dust without surfactant treatment.
When the surfactant concentration was higher than 2 wt.‰, the surface tension remained constant, and the decrease in wetting time was related to the adsorption state of the surfactants
on the surface of coal dust. Thus, surfactant solutions with concentrations of 3, 5, 7, and 9 wt.‰ were selected, and the infrared spectra of coal dust after being wetted using surfactant solutions with the above concentrations were obtained, as shown in Fig. 5. The Kubelka–Munk (KM) absorbances of the aliphatic chains and aromatic C–H structures of the coal dust treated using
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surfactant solutions were obtained, and the ratios of the KM absorbances of the aliphatic chain to those of the aromatic C–H structures were calculated, as shown in Table 3. All the ratios for
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coal dust wetted by surfactant solutions exceed 0.87, and were higher than the 0.82 ratio for the original coal dust, which was calculated from Fig. 4(d). This demonstrated that the surfactants adsorbed on the surface of coal dust. For the same surfactant, the ratio increased gradually with
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the increase in surfactant concentration, as shown in Table 3, which suggested that the
A
adsorption densities of the surfactants increased as well. Moreover, for the same surfactant
M
concentration, the ratio for the coal dust wetted with SDBS was the highest, followed by the ratios for SDDS and SDS, which indicated that the adsorption density decreased from SDBS to
A
CC E
PT
ED
SDDS to SDS.
A ED
PT
CC E
IP T
SC R
U
N
A
M
IP T SC R U
N
Fig. 5. FTIR spectra of coal dust after being wetted by surfactant solutions.
A
Table 3 KM absorbances of the aliphatic chain and aromatic C–H structure of coal dust after being wetted
Aliphatic chain/aromatic C–H KM absorbances ratio
Aromatic C–H
3
0.416
0.477
0.8721
0.428
0.489
0.8753
7
0.434
0.493
0.8803
9
0.400
0.453
0.8830
3
0.449
0.497
0.9034
5
0.467
0.514
0.9086
7
0.472
0.519
0.9094
9
0.481
0.528
0.9110
3
0.477
0.497
0.9598
5
0.464
0.479
0.9687
7
0.469
0.474
0.9895
9
0.491
0.494
0.9939
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ED
Aliphatic chain
5
SDS
KM absorbances
Concentration (wt.‰)
PT
Surfactant
M
by surfactant solutions and their ratios.
A
SDDS
SDBS
The effect of the concentration and molecular structures of surfactants on the adsorption density was explained as follows. The surface of coal dust consists of a large number of hydrophobic sites caused by hydrocarbon chains and aromatic rings. Few of the oxygencontaining functional groups are associated with hydrophilic sites with negative charges.
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Because of the strong hydrophobic interactions between the aliphatic chains of the surfactants and hydrophobic sites on the surface of coal dust, the tail groups of the surfactants adsorb onto
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the hydrophobic sites, leaving the hydrophilic head groups outward, which causes the
hydrophobic sites of coal dust to convert to hydrophilic sites. As the surfactant concentration increased, the surfactant adsorption density increased along with the number of hydrophilic
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U
sites on the surface of coal dust, as shown in Fig. 6. This resulted in the improvement of the
A
wetting performance and the decrease of the wetting time.
M
Compared to SDS and SDDS, SDBS presented stronger hydrophobic interactions on the surface of coal dust due to the longer tail group, leading to higher adsorption density. Although
ED
SDS and SDDS have the same tail group, their head group structures are different. The sulfate
PT
head group of SDS has a partial charge of –1.33, while the partial charge of the sulfonate group of SDDS is –1.00 (Wang et al., 2016a). The high negative charges of the sulfate group caused
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strong electrostatic repulsions between the SDS head groups and the negatively charged sites on the surface of coal dust, which suppressed the hydrophobic interaction. In addition, strong
A
electrostatic repulsions occurred between the head groups of the SDS molecules, resulting in large distances between adjacent surfactant molecules. Thus, the adsorption density of SDS was lower than that of SDDS. The adsorption states of the three surfactants on the surface of coal dust are shown in Fig. 7.
IP T SC R U N
A
CC E
PT
ED
M
A
Fig. 6. Surfactant adsorption state on the surface of coal dust at different concentrations.
Fig. 7. Adsorption state of SDS, SDDS, and SDBS molecules on the surface of coal dust.
3.4. Dynamic immersion process
Theoretically, the wetting ability of the SDBS solution should have been the best, because its adsorption density was the highest of the three surfactants in this study, and more hydrophobic sites on the surface of coal dust were converted into hydrophilic sites in the presence of SDBS. On the other hand, SDS should have exhibited the worst wetting
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performance due to its lowest adsorption density compared to the other surfactants studied. However, SDBS rather than SDS, exhibited the longest wetting time, while the wetting time of
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SDDS was the shortest. This occurred because the coal dust needed to enter the bulk solution from the air-water interface after being adsorbed by the surfactant molecules, as shown in Fig. 8, so that the wetting process was finally completed. Most studies focused on the adsorption
N
U
behavior of surfactants on the surface of coal dust, but the dynamic immersion process has not
A
been studied yet. In this dynamic immersion process, the ability of coal dust to enter the bulk
A
CC E
PT
ED
M
solution from the air-water interface plays an important role in the wetting time.
Fig. 8. Dynamic immersion process of coal dust to enter the bulk solution from the air-water interface.
After adsorption, the surface of coal dust is covered with surfactants. Thus, the ability of coal dust to enter the bulk solution depends on the ability of the surfactant to enter the bulk solution from the air-water interface, which is determined by the molecular structure of the
surfactant. The tail group of the surfactant is hydrophobic, causing the surfactant to migrate from the bulk solution to the air-water interface, while the head group is hydrophilic and tends to bring the surfactant at the air-water interface toward the bulk solution. The hydrophilicity of the head group vs. the hydrophobicity of the tail group of the surfactant molecule can be
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quantitatively evaluated using the HLB values, which can be calculated using the Davies equation (Davies, 1957):
(1)
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HLB = Σ(hydrophilic group numbers) − Σ(hydrophobic group numbers) + 7
When the hydrocarbon chain is short and the hydrophilicity of the head group is strong, HLB is high. Hence, it is easy for the surfactant to enter the bulk solution from the air-water
N
U
interface. Therefore, the ability of coal dust to enter the bulk solution can be characterized using
A
the HLB value. The higher the HLB value of the surfactant is, the better the ability of coal dust
M
to enter the bulk solution, which will result in higher wetting rates and shorter wetting times. According to the Davies equation and the group numbers of different structures, the calculated
ED
HLB values of SDS, SDDS and SDBS were 40.0, 13.0 and 10.6, respectively (Davies, 1957;
PT
Li et al., 2007; Pan et al., 2012). The hydrophilicity of the sulfonate group of SDDS is poorer than that of the sulfate group of SDS, and SDDS has a shorter tail group than SDBS. Thus, the
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HLB value of SDDS is intermediate between the corresponding values of SDS and SDBS. Although SDBS had the largest adsorption density and generated the highest number of
A
hydrophilic sites on the surface of coal dust compared to SDS and SDDS, its lowest HLB value resulted in the poorest ability of coal dust to enter the bulk solution and caused the longest wetting time. While SDS had the highest HLB value of the three surfactants, its lowest adsorption density led to generating the smallest number of hydrophilic sites on the surface of coal dust, which also resulted in a relatively long wetting time. The adsorption density and HLB
value of SDDS were intermediate between those of SDS and SDBS. Under the combined action of these two factors, SDDS could bring the coal dust to the bulk solution from the air-water interface the fastest. Therefore, the coal dust wetting time of SDDS was shorter than those of SDS and SDBS.
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4. Conclusions In the present work, the surface tension and the coal dust wetting time of SDS, SDDS, and
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SDBS solutions were tested. The infrared spectra of coal dust before and after being wetted by surfactant solutions were obtained, and the HLB values of the three surfactants were also
calculated. The effect of the molecular structures and surfactant concentrations on the wetting
N
U
time of coal dust was analyzed from the surface tension, surfactant adsorption density, and HLB
A
values. The main conclusions can be summarized as follows.
M
At surfactant concentration lower than 1 wt.‰, the increase in concentration reduced the surface tension, thus improving the wetting ability of solutions and decreasing the wetting time.
ED
When the surfactant concentration was higher than 2 wt.‰, the surface tension was lower than
PT
the critical surface tension (45 mN/m) and remained constant as the concentration increased. The decrease in the wetting time was related to the surfactant adsorption density on the coal
CC E
dust surface and the ability of coal dust to enter the bulk solution from the air-water interface. For the same anionic surfactant, the increase in surfactant concentration increased its
A
adsorption density, which resulted in an increase in the number of hydrophilic sites on the surface of coal dust and shortened the wetting time. For anionic surfactants with different structures, the adsorption density decreased from SDBS to SDDS to SDS. The long hydrocarbon chain enhanced the hydrophobic interaction between the tail group of the surfactant and the hydrophobic sites on the surface of coal dust, while the low negative charges
of the head groups weakened the electrostatic repulsions between the surfactant molecules and negatively charged sites of coal dust. Both processes increased the surfactant adsorption density and number of hydrophilic sites on the surface of coal dust. During the dynamic immersion process, the ability of coal dust to enter the bulk solution
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from the air-water interface was characterized using the HLB value of the surfactant. The HLB value decreased from SDS to SDDS to SDBS. The higher the HLB value was, the better the
SC R
ability of coal dust to enter the bulk solution, which resulted in higher wetting rates and shorter
wetting times. Although SDBS exhibited the largest adsorption density among all three surfactants in this study, its lowest HLB value resulted in the poorest ability of coal dust to enter
N
U
the bulk solution and caused the longest wetting time. On the other hand, SDS exhibited the
A
highest HLB value of the surfactants analyzed, but its lowest adsorption density led to
M
generating the smallest number of hydrophilic sites on the surface of coal dust, which also resulted in a relatively long wetting time. The adsorption density and HLB of SDDS were
ED
intermediate between those of SDS and SDBS. Under the combined action of these two factors,
PT
SDDS brought the coal dust from the air-water interface into the bulk solution the fastest. Therefore, the coal dust wetting time of SDDS was shorter than those of SDS and SDBS.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (51474216,
A
51504249 and 51874290), the Project funded by China Postdoctoral Science Foundation (2018T110574), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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