Different mining processes, e.g., coal mining and transportation, generate varying amounts of dust. Airborne dust poses a serious threat to the health and safety of mine workers (Ahmed et al. 2017; Zhou et al. 2018a; Xu et al. 2019a; Wang et al. 2019a; Hua et al. 2020). In recent years, according to the National Occupational Diseases Statistics Report by the National Health Commission of China, most new pneumoconiosis cases were reported to occur in the coal and non-ferrous metal mining industries. Specifically, 40% of new pneumoconiosis cases were reported to occur in the coal mining industry (Vedal 1997; Li et al. 2002; Yin et al. 2019; Bao et al. 2020; Zhang et al. 2020). Thus, adoption of effective dust suppression technologies to reduce the concentration of dust in the coal mining field is imperative (Nie et al. 2017a, b; Peng et al. 2019).

Currently, spray-based dust reduction is the main dust control measure in underground mines (Dey 2012; Wang et al. 2019b, e; Zhou et al. 2019a, b; Han et al. 2020a). However, sprayed water has a large surface tension and coal dust is extremely hydrophobic, which limits the efficiency of dust suppression sprays in coal mines (Wang et al. 2020a; Xu et al. 2018; Han et al. 2020b, a). Some studies report the addition of surfactants to water, which provide the resulting solution with a lower surface tension and improve the wetting performance of the spraying process (Yang et al. 2007; Tessum and Raynor 2017). Other studies have shown that surfactants significantly reduce the surface tension of water, and that the reduction effect is dependent on surfactant type and concentration (Omane et al. 2018; Wang et al. 2019d). Tessum et al. (2014) studied the ability of various surfactant droplets to trap dust particles with a surface charge. The analysis indicated that different types of surfactant performed differently in trapping dust particles with a surface charge; in particular, it was shown that non-ionic surfactants had the highest dust trapping efficiency. Yang et al. (2014) reported that surface active ions in anionic surfactants generally carry a negative charge, and thus repelled coal dust particles with a negatively charged surface. The anionic surfactant particles exhibited low adsorption on the surface of coal dust particles. Therefore, anionic surfactants have a weaker wetting ability compared to that of non-ionic surfactants.

In addition, some researchers have developed and studied novel dust suppressants (Fan et al. 2018; Hu et al. 2019; Zhou et al. 2019a, b; Yan et al. 2020). Tong (2013) and Jiang et al. (2013) conducted theoretical and experimental analyses and concluded that the application of suitable compounding significantly improved the wetting performance of a solution during coal dust trapping. Li et al. (2010) prepared a new dust suppressant using a surfactant compounding method and obtained the optimal concentration through orthogonal experimental design. Based on the experimental results, compared to clean water, the dust suppressant improved the dust reduction efficiency by 20%. Kilaua and Pahlman (1987) found that the wetting performance of the surfactant solution is further enhanced via the addition of metal inorganic salts. Subsequently, Du and Zeng (2002) and He et al. (2008) investigated the variation in surface tension of surfactant solutions under the action of metal salts. Wu and Gu (2001) investigated the impact of inorganic salt additives on the wetting performance of anionic surfactant solutions through experimental and theoretical analyses. The results showed that the wetting ability of anionic surfactant solutions is significantly enhanced following the addition of Na₂SO₄. Li et al. (2016) compared the wetting properties of a sodium dodecyl sulfate (SDS) solution with five different additives and assessed its suitability for coal dust trapping using surface tension, contact angle, and reverse osmosis experiments.

Coking coal is a type of bituminous coal and is more hydrophobic than other types of coal dust, meaning its combination with water droplets is challenging. Thus, the dust capture rate of traditional spray technologies is extremely poor. The development of dust suppressants specifically for coking coal is rarely reported in the literature. In addition, inorganic salts are rarely added as water-retention agents and additives during the formulation of dust suppressants. In this study, two surfactant monomers were used as surfactant components to prepare a dust suppressant for coking coal dust via monomer optimization and compounding analysis. At the same time, through compounding of the surfactant monomer and inorganic salt, the reverse osmosis moisture absorption of each solution was measured. Using the reverse osmosis water retention results, a specific inorganic salt was chosen as the dust suppressant additive. Finally, the optimal combination of concentrations of the three components was obtained via orthogonal experimental design to obtain the spray dust suppressant formulation for coke coal dust.

2.1 Materials

Coking coal samples were obtained from the Shanxi Wanfeng coal mine. The coking coal from the Shanxi Wanfeng coal mine had poor wetting properties (making wetting using water difficult), and a large contact angle (88.76°) (Liu 2006; Luo et al. 2016; Zhou et al. 2018b; Wang et al. 2019c). First, a pulverizer was used to crush the coal samples for one minute, and then the sample was passed through a sieve using a 150-mesh standard industrial sieve. Next, the obtained coal dust sample was dried in a vacuum dryer at 80 °C for 480 min. The coal dust sample was stored in a sealed pouch prior to analysis. The industrial evaluation indexes and characteristic particle diameters of the coal dust samples are shown in Table 1. Figure 1 shows the contact angle and size distribution of the coal dust samples. As the mining area uses tap water, tap water was used during the experiments.

a Contact angle and b particle size distribution of the coal dust particles

If the suppressant spray is applied directly to the coal mine operation site, it will come into direct contact with coal mine workers. Thus, there are strict requirements for the selection of surfactants. When selecting dust suppressant sprays, they should be: non-toxic, harmless, non-corrosive, and easily soluble in water. In addition, the selected suppressants should be economical and easy to transport. Based on the above principles, through market research and field investigation, ten types of surfactant underwent preliminarily screening, as shown in Table 2.

During dust suppressant formulation as well as using multiple surfactants for compounding, the addition of hygroscopic inorganic salt additives to further improve the wetting and water retention properties of the solution should be considered (Tang et al. 2016; Kumar and Mandal 2016). The effect of hygroscopic inorganic salts and surfactants differ; some have synergistic effects and some antagonistic effects. In the experiments reported here, four types of hygroscopic inorganic salts (NaCl, CaCl₂, Na₂SO₄, and Na₂SiO₃) were selected. The effect of each inorganic salt and surfactant was analyzed, and the inorganic salt with optimal performance was selected.

2.2 Experimental scheme

2.2.1 Preparation scheme

1. (1)

   Surface tension experiments were used to investigate the ability of ten surfactant monomers to decrease the solution surface tension. At the same time, we investigated the performance of ten types of surfactant monomer for wetting coal dust using contact angle and reverse osmosis analyses. A total of six surfactant concentrations were tested during the experiments i.e., 0%, 0.00005%, 0.0005%, 0.005%, 0.05% and 0.5%. Based on the surface tension and wetting performance experimental results, six suitable surfactant monomers were selected.

2. (2)

   The six surfactant monomers were tested using binary compounding analysis, and the performance of the compounded solutions was evaluated using reverse osmosis analysis. Based on the results of the performance evaluation, an optimal compounding scheme was selected. The concentration of the six monomer surfactants was set at 0.05% during the binary compounding analysis.

3. (3)

   Four inorganic salts were used to prepare solutions with mass fractions of 1%. The preferred surfactant monomer selected during the compounding scheme in step (2) was used to prepare a 0.05% solution. Then, binary compounding of the surfactant monomer and inorganic salt solutions was performed, and the wetting performance of the compounded solution was investigated using reverse osmosis analysis. Solutions of the four different types of inorganic salt at equal mass concentrations were added to the coal dust samples and their water retention properties were investigated. One of the four inorganic salts was selected as the optimal additive for the dust suppressant, based on the water retention and reverse osmosis results.

4. (4)

   A three-factor, three-level reverse osmosis orthogonal experimental design was performed using the two surfactant monomers and the inorganic salt selected in the binary compounding analysis described above. In addition, based on the ranges of the three factors and the comprehensive average of each level, the dust suppressant configuration scheme with optimal the dust suppression effect for coking coal was obtained. The manufacturing process for the dust suppressant is shown in Fig. 2.

Flow chart of the dust suppressant

2.2.2 Parameter measurement method

1. (1)

   Surface tension analysis. A German Kruss K20 surface tension meter was used to detect the surface tension of the surfactant monomer and the compound solution. The surface tension of all solutions was measured at 25 °C using the hanging piece method. First, the platinum piece was placed and maintained in a vertical position on the experiment table, and then the instrument was rotated to allow the platinum piece to interact with the surface of the water. Then, the minimum tensile force required for the platinum piece to exit the aqueous solution was measured.

2. (2)

   Contact angle analysis. In order to obtain the cylinder test piece of the coal samples, a mold containing 400 mg of pulverized coal was placed in a bench-top powder grinding machine for one minute at a pressure of 50 MPa. The obtained test pieces were 2 mm thick and had a smooth surface. A CA100B contact angle measurement instrument was used to measure the contact angle of the solution on a coal dust sample. Three test pieces were measured in triplicate for each solution to obtain the average value.

3. (3)

   Reverse osmosis analysis. A custom-designed reverse osmosis device was used to evaluate the moisture absorption of the coal dust. First, a 10 mm-diameter glass tube containing 3 g of coal sample sealed with filter paper was weighed. Then, the glass test tube was submerged in a water tank for two hours. Next, the glass test tube was removed from the water tank and reweighed it. The difference between the two weights was used to calculate the moisture absorption of the coal dust samples.

4. (4)

   Water retention analysis. The four inorganic salts i.e., NaCl, CaCl₂, Na₂SO₄, and Na₂SiO₃, were used to prepare 1% solutions. Then, 10 mL of each solution was mixed with 10 g of dry coking coal dust in a petri dish and the petri dish was dried at room temperature. The petri dish was weighed at 12-h intervals to calculate the coking coal moisture content. The method used to calculate the moisture content is shown in Eq. (1).

   $$\eta = \frac{{m_{2} - m_{1} }}{{m_{1} }} \times 100\%$$

   (1)

   where, η is the moisture content, m₁ is the mass of the coal sample before wetting, and m₂ is the mass of the coal sample after wetting.

3.1 Surface tension analysis

The experimental results in Table 3 show that the surface tension was reduced by adding a surfactant into water. Due to the amphiphilic structure, the surfactant molecules were positively adsorbed onto the surface of the solution, causing a significant reduction in solution surface tension. The results showed that for most coal dust samples the critical surface tension was approximately 45 mN/m (Wu and Gu 2001). Table 3 also shows that the critical surface tension of some surfactants exceeded 45 mN/m at the highest concentration (0.5%). For example, the critical surface tension of SLS was 48.9 mN/m and that of 1227 was 45.6 mN/m. Therefore, coal dust wetting using these two surfactants was not optimal and they were eliminated. c_m refers to the mass fraction of surfactant solution.

Table 3 also shows that as the surfactant mass concentration increased within the low range, the surface tension was significantly reduced. In addition, the surface tension remained stable after the mass concentration exceeded a certain value, which is defined as the critical micelle concentration (CMC) (Wang et al. 2020b; Jin et al. 2019). At high concentrations, the interface adsorption between amphiphilic molecules became saturated leading to the formation of colloidal aggregates of the amphiphilic molecules in solution and resulting in stable surface tension (Ma et al. 2004). In this study, the change in surface tension slope for the ten types of surfactant were measured in different concentration ranges to obtain the CMC of each surfactant (Fig. 3).

The surface tension rate of change for the ten surfactants in different concentration ranges: a a-SAA, b n-SAA, c z-SAA, d c-SAA. Note: ‘1–2’ refers to the surfactant concentration is between 0% and 0.00005%, ‘2–3’ refers to the surfactant concentration is between 0.00005% and 0.0005%, ‘3–4’ refers to the surfactant concentration is between 0.0005% and 0.005%. The rest can be deduced by analogy

Figure 3 indicates that when the concentration is between 0.05% and 0.5%, the surface tension rate of change for nine of the surfactants was below 5%; however, Tween-80 was an exception. The results showed that the CMC of Tween-80 was significantly higher than that of the other surfactants. At the highest concentration, the corresponding surface tension of Tween-80 was approximately 40 mN/m, which is close to the critical surface tension of coal dust. Thus, Tween-80 should not be selected as the dust suppressant. Based on the results in Fig. 3, the six surfactants i.e., AES, SDBS, Tween-80, AEO-9, Tween-20, CAT-97, and OB-2, have CMC values of approximately 0.005%, while SLS, CTAB, and 1227 have CMC values of approximately 0.05%. Based on these surface tension experimental results, three types of surfactant, SLS, 1227 and Tween-80, were eliminated.

3.2 Wetting performance analysis

The wetting performance of a surfactant solution is highly dependent on the type of surfactant and the mass concentration. The contact angle measurements and reverse osmosis moisture absorption values for the ten types of surfactant at various mass concentrations are shown in Fig. 4.

a Contact angle measurements and b moisture absorption values for the ten types of surfactant

From Fig. 4a, it can be seen that the solution contact angle on the surface of the coal dust tablet continuously decreases with the increasing mass concentration of the surfactant. According to Young's equation, the relationship between the contact angle of a liquid on a solid surface and the interfacial tension is as follows:

where, θ is the contact angle (°); γ_sg, γ_sl, γ_lg are the tension (mN/m) at the solid–gas, solid–liquid, and liquid–gas interfaces, respectively. When a surfactant is present in the solution, γ_sl and γ_lg significantly decrease due to the adsorption of the surfactant at the solid–liquid and liquid–gas interfaces, which leads to a smaller contact angle value θ, as shown in Fig. 5.

Contact angle projections for coking coal in different SDBS mass fractions: a 0%, b 0.00005%; c 0.0005%, d 0.005%, e 0.05% and f 0.5%

The results of the reverse osmosis analysis in Fig. 4b show that for all ten surfactant solutions, the reverse osmosis moisture absorption increases with increasing mass concentration. By comparing Fig. 4a, b, we can see that both the reverse osmosis hygroscopicity and the contact angle essentially follow the same trend. Combining the contact angle and reverse osmosis results allowed us to select six surfactants based on wetting performance: anionic AES and SDBS, non-ionic AEO-9 and Tween-20, zwitterionic OB-2, and cationic CTAB.

Using Origin 9.0 software (Edwards 2002), curve fitting of the data for the reverse osmosis moisture absorption and contact angle analyses for the above six surfactant monomers was performed. The fitting results shown in Fig. 6 indicate that the amount of moisture absorption decreases as the contact angle increases. In addition, the moisture absorption and contact angle were negatively correlated, with a correlation coefficient of approximately 0.85. This indicates that it is feasible to utilize contact angle and reverse osmosis moisture absorption to assess the wetting performance of surfactant solutions. The relatively high error in the contact angle analysis and the difficulties encountered in producing the coke coal test piece meant the analysis was challenging (Huang et al. 2010). Therefore, in subsequent compounding analyses, only reverse osmosis absorption analysis was utilized to assess the wetting performance of the solution.

Relationship between contact angle and moisture absorption

4.1 Surfactant binary compounding

A binary compound reverse osmosis experiment was conducted using the six selected monomer surfactants and the results are listed in Table 4.

From Table 4, it can be seen that most of the compound solutions have stronger wetting effects and a higher moisture absorption than the monomer solution (Zhang 2019; Xu et al. 2019b). By considering the moisture absorption growth rate, four compounding schemes i.e., Scheme 1 (AES + SDBS), Scheme 2 (AES + AEO-9), Scheme 10 (AEO-9 + 20), and Scheme 13 (AEO-9 + CTAB), were considered most effective. The wettability of the mixed solution was more than 20% higher than the monomer. Based on further comparisons, we found that the final moisture absorption value for Scheme 1 was the highest. In view of the properties of each surfactant and their usage habits in coal mines, the two surfactant monomers in Scheme 1 were chosen as the key components for the dust suppressant.

4.2 Inorganic salt additive analysis

Following the surfactant binary compounding analysis above, we selected AES and SDBS as the two basic components for the dust suppressant. Figure 7 shows the results of the reverse osmosis analysis using the two surfactant monomers (AES and SDBS) and inorganic salt additives. From Fig. 7, it can be seen that compounding of the surfactant AES with the four inorganic salts enhanced the moisture absorption of the coal dust to various degrees. This enhancement is because the surface of coal dust consists of hydrophilic and hydrophobic crystal lattices. The addition of a suitable surfactant creates an adsorption effect on the hydrophobic lattice, thus improving the wettability of the coal dust (Hapgood and Khanmohammadi 2009; Zhao et al. 2011; Hu 2014). However, adsorption of surfactant onto the coal dust surface was limited by electrostatic forces between the surfactant and the layers on the surface of the coal dust and the addition of inorganic salts resulted in the presence of ions in solution, which decreased the electrostatic force distance in the two-layer arrangement. As a result, the adsorption density of the surfactant ions was improved, and the wetting ability of the surfactant for coal dust was further enhanced. Therefore, the reverse osmosis moisture absorption capacity of the coal dust particles was enhanced by the addition of the salt additives (Zhan and Guo 2013; Wang et al. 2020b).

The moisture absorption capacity of a solution of surfactants and inorganic salts

Figure 7 shows that for anionic SDBS, compounding with salts does not lead to significant improvement in the wetting performance of the solutions. Compared to the monomer solution, the addition of CaCl₂ and NaCl increased the reverse osmosis moisture absorption up to a maximum of 10%. Further, Na₂SiO₃ and Na₂SO₄ produced an antagonistic effect on SDBS and resulted in a lower compound solution moisture absorption compared to that of the monomer; this indicates that these two inorganic salts are not suitable for use as dust suppressant additives to enhance the solution wettability.

Figure 8 shows the results of the moisture retention analysis for four inorganic salt solutions. It can be seen that the coal dust sample without inorganic salt additives has the fastest moisture evaporation rate; after 132 h, the moisture content was less than 5%. In contrast, the addition of inorganic salts increased the moisture content of the coal dust samples by more than 15%. This is because after the coal dusts meet water, lots of water molecules are adsorbed on the coal dust surface. The addition of an inorganic salt into this solution results in the formation of ions, which penetrate the crystal layers of the coal dust samples. As a result, the coal dust surface charge and the ions adsorbed on the crystal layers adsorb water molecules and a water film is formed. This water film absorbs and stores water, thereby enhancing evaporation resistance (Yao et al. 2017; Wang et al. 2020b).

Time course of the change in moisture content in the coal dust samples

Prior to 168 h, the moisture content of the coal dust samples containing NaCl was the highest. However, between 168 and 192 h, the moisture content of the coal dust samples containing NaCl decreases while evaporation for the other three coal dust samples reached equilibrium between 156 and 168 h. At 192 h, the moisture content of the CaCl₂ coal dust samples was the highest (8.40%) and the moisture content of the coal dust with added NaCl, Na₂SiO₃, and Na₂SO₄ were 4.88%, 4.55%, and 3.38%, respectively. Based on the water retention analysis results, we found that the addition of CaCl₂ led to optimal water retention. The addition of CaCl₂ slows water evaporation in coking coal dust. Therefore, CaCl₂ is recommended as a surfactant solution additive to further enhance the wettability of the surfactant solution and provide excellent water retention. Therefore, we recommended this inorganic salt is used as the additive for dust suppressants applied in engineering applications.

4.3 Orthogonal experimental design

Through the previous compounding experiments, we identified AES, SDBS and CaCl₂ as the dust suppressant components. However, the optimal concentration of each component has not been determined. Orthogonal experimental design was used to determine the optimal concentration of the three compounds. A three-factor, three-level orthogonal experimental design was performed using the orthogonal table L9 (3³). The results of the orthogonal experimental design are shown in Table 5.

Table 5 shows that, of the nine schemes analyzed, Scheme 1 is optimal. Under Scheme 1, the reverse osmosis hygroscopic mass was 120 mg. In order to determine the influence of the optimal concentration levels of the three compounds, the comprehensive average and reverse osmosis moisture absorption range for each level are listed in Table 6.

Range is a key indicator in evaluating the importance of each influencing factor. A larger range indicates that the factor has a greater influence on the result (Sun et al. 2019; Li et al. 2019). In contrast, a smaller range indicates that the factor has a smaller influence. Using the range analysis results in Table 6, the three factors were ordered based on influence of reverse osmosis moisture absorption as follows: AES > CaCl₂ > SDBS. The comprehensive average calculation results in Table 6 show that K1 is the largest factor for AES, K2 is the largest factor for SDBS, and K1 is the largest factor for CaCl₂. Therefore, the concentrations of AES, SDBS, and CaCl₂ in the recommended formulation are 0.03%, 0.05%, and 0.4%, respectively. Using this formulation, the reverse osmosis hygroscopic mass of the solution was 128 mg, which indicates that the solution has improved wetting performance.

In this study, six surfactants were selected from a total of ten surfactant monomers as suitable dust suppressants for coking coal dust, using surface tension, contact angle, and reverse osmosis analyses. Next, the six selected surfactant monomers underwent binary compounding and the wettability of the solution was investigated by determining the reverse osmosis moisture absorption capacity. Based on the results, SDBS and AES were determined to be the optimal components of the dust suppressant. The wetting properties of SDBS and AES compounded with four inorganic salts were then investigated. By combining the wetting properties and the water retention properties of the four inorganic salt solutions, CaCl₂ was selected as the optimal inorganic salt additive for the dust suppressant. Finally, three-factor, three-level orthogonal experimental design was performed to obtain the optimal combination of concentrations for the three components i.e., AES (0.03%), SBS (0.05%), and CaCl₂ (0.4%). The dust suppressant solution prepared using the above scheme has a moisture absorption of up to 128 mg and excellent wetting performance. Using the methods described above, an improved surfactant formulation for the treatment of coking coal dust was obtained, which will provide direction for future research.