Evaluation of the coal dust suppression efficiency of different surfactants: A factorial experiment

https://doi.org/10.1016/j.colsurfa.2020.124686Get rights and content

Highlights

  • A general factorial experiment has been designed.

  • Wind tunnel tests were conducted to evaluate the coal dust suppression efficiency.

  • Four surfactant solutions with seven concentrations were compared.

  • Two concentrations of initial coal dust were applied and studied.

Abstract

Coal dust is a main health hazard in underground coal mines. A recent report showed that a number of coal workers were diagnosed with pneumoconiosis in Australia. Water spray with addition of surfactant has been considered as an effective method to control the coal dust. Numbers of static studies showed surfactant could increase the coal dust wettability effectively. In fact, the contact of coal dust and surfactant droplet is dynamic and more complicated due to the short contact time. Wind tunnel tests are able to present this dynamic process. In this study, a factorial experiment was designed to evaluate the suppression effectiveness for three factors such as (i) different types of surfactants at (ii) different levels of their solutions on (iii) different initial concentrations of coal dust. Four surfactants with seven levels were investigated under two initial coal dust concentrations. The results showed that all the three-factors could affect the suppression efficiency significantly. Surfactants gave higher efficiencies when the initial coal dust concentration was low. Sodium dodecyl sulphate (SDS) and cetyltrimethylammonium bromide (CTAB) performed better than other two surfactants regardless of the initial coal dust concentration. The limitations of this study were also analysed. The results of this study could be used by researches and industry to determine an effective surfactant for coal dust suppression.

Introduction

Coal dust is generated dramatically during the mechanised mining process, and it is one of the main health hazards in coal mines. Due to its fine characteristics, it can affect the performance of mining equipment and cause adverse health effects on miners. Exposure to respirable coal dust causes various lung diseases, including coal workers’ pneumoconiosis (CWP), also known as black lung ([1,2]; G [3].). With the improvement of coal dust controls in coal mines, the prevalence of coal workers’ pneumoconiosis has decreased significantly since the last decade [4]. However, new cases still have been reported recently. Recent news reported that 20 underground coal miners in Queensland, Australia were diagnosed with black lung [5]. A resurgence of CWP has also been reported in the U.S. coal miners based on the data reported from its Coal Workers’ Health Surveillance Program [6]. Therefore, there is an urgent need for an effective dust control in coal mines to prevent coal workers’ pneumoconiosis.

Approaches commonly used for controlling coal dust include ventilation dilution, water spray suppression and stabilizing dust using foam [[7], [8], [9]]. Water spray is still the main approach due to its low-cost and easy maintenance. However, the dust suppression efficiency is relatively low due to the poor wettability of coal [10]. A number of studies have proved that the coal wettability could be increased significantly by adding surfactants in water, thus increasing the efficiency of coal dust suppression. Glanville et al. [11] evaluated the effect of surfactant solutions on the coal dust wettability. Triton X-100 (TX100) solutions with the concentration from 0 to 0.20 % were tested. The results showed that the wettability of coal dust increased with the increasing of surfactant solution concentration and temperature and large coal dust had a better wettability than the small ones. A similar study was conducted by Chander et al. [12] to determine the coal dust wetting behaviour by using noyl and octyl series of non-ionic surfactants. The surfactant concentration varied from 0 to 0.001 mol/l. Different ranks of coal were tested. This study concluded that high-rank coal had a better wettability than low-rank coal. Another research studied the effect of surfactants surface adsorption on coal dust properties [13]. The surface characteristics of three ranks of coals in the presence of three surfactants, Sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB) and Teric G12A8, were investigated. The study showed that lower rank coal have better surfactant adsorption properties. Li and others [14] compared the coal dust wetting rate of two surfactant solutions, SDS and sodium carboxymethyl cellulose (CMC). Three ranks of coals with different particle sizes were selected. Finer coal particles were observed with poor wetting abilities due to the complex micro structures of coal. Coal dust with higher volatile content also had a poor wettability. The results showed that SDS with a concentration of 0.2 % resulted in the best wetting rate. Chen et al. [10] conducted a series of laboratory tests to measure the coal particles’ wettability in the presence of two surfactants, SDS and SDBS. Four static tests were applied, including sessile drop technique, sink test, capillary rise test and drop penetration test. Surfactants with various concentrations from 0% to 0.60 % were used during the tests. Results of this study suggested that SDBS with concentration of 0.2 % could improve the coal dust wettability significantly than other solutions.

However, all of the above mentioned studies were conducted based on static tests, which do not directly reflect the actual dust suppression efficiency of the surfactant solutions. These static methods test the interactions between coal particles and surfactant bulk solution when coal particles and the solution are in firm contact for a period, thus they do not reflect the interaction of water droplet and coal particles when flowing in air. A more accurate way of evaluating the effectiveness of a water spraying system is the use of the dynamic test method with a laboratory wind tunnel. It takes consideration of airflow, contact and collision of coal particles and water droplets, and the wetting and envelopment of dust particles by water droplets. Thus, they are more reliable than those of static tests (G. [3]). The application of wind tunnel in studying coal dust suppression efficiency of different surfactants is quite recent and only a few numbers of studies have been conducted. Tessum et al. [15] conducted a wind tunnel test to evaluate the efficiency of surfactant on dust suppression in terms of different sizes of particles. Three sizes of particles with diameters of 0.6, 1.0 and 2.1 μm were injected in the wind tunnel after charging by a diffusion charger. Three surfactants, SLS, TX100 and Dodecylamine hydrochloride (DAH) were tested at either a low concentration of 1 × 10−6 mol/l or a high concentration of 1 × 10-4 mol/l. The average particle removal efficiencies of 86.6 %, 58.8 % and 21.5 % were observed for the three surfactants spayed on larger, medium and small size of particles, respectively. A similar study has been conducted by this research group later by using the same apparatus [16]. In this study, SDS, DAH and TX100 were evaluated. The results showed that the non-ionic surfactants performed best in the coal dust collection. However, it is noticed that the initial coal dust concentrations were not given in the above two studies. Zhou et al. [17] (Q [17].) developed a magnetizing apparatus to evaluate the suppression efficiency of the magnetized surfactant solution in a wind tunnel. Four solutions were compared, including untreated water, magnetized water, an unnamed surfactant solution and the magnetized surfactant solutions. After being magnetized, the suppression efficiency of water and surfactant were increased from 48.31% to 60.34% and from 79.76% to 87.35 %, respectively. The results illustrated that the magnetizing device could improve the suppression efficiency dramatically by magnetizing the surfactant solutions. A previous study was conducted by the authors [7] to evaluate the suppression efficiency of four surfactants by using wind tunnel tests. The results showed that 0.2 % SDBS solution resulted in the highest suppression efficiency of 63.09 %.

Nevertheless, it is worth noting that in the previous studies, the initial concentrations of injected coal dust were relatively high, which were more than 100 mg/m3. For example, in Zhou’s and Chang’s studies, the initial dust concentrations were 110.42 mg/m3 and around 400 mg/m3 respectively. These numbers are impractical for underground coal mines. After applying the surfactant solution, the coal dust concentrations dropped to around 14−43 mg/m3 and 150 mg/m3([7]; Q [17].). Although the suppression efficiencies were improved significantly by adding surfactant into water, the suppressed coal dust concentrations were still much higher than the limit of 2.5 mg/m3 [18]. In fact, it is challenging for underground coal mines to meet such a limit. Although the suppression efficiency has been enhanced dramatically after spraying surfactant solutions, whether the surfactant can keep the same suppression efficiency when the initial coal dust is low is still uncertain. The suppression efficiency of surfactants solution may vary from high concentration to low concentration dust. To our knowledge, no such study has been previously done to investigate the suppression efficiency under different initial coal dust concentrations.

This paper aims to evaluate the effectiveness of various levels of surfactant solutions on different coal dust initial concentrations. It will investigate if the level of initial coal dust concentration affects the suppression efficiency and whether a surfactant solution could perform a high suppression efficiency on both low and high coal dust initial concentrations. A factorial experiment was designed to achieve these objectives. In this work, a wind tunnel apparatus was used to conduct all the experiments. Two coal dust initial concentrations with around 100 mg/m3 (high concentration) and 15 mg/m3 (low concentration), were selected in this study. Previous studies have shown that SDBS, SDS, TX100, and CTAB are four most commonly used surfactants for coal dust suppression [11,[13], [14], [15], [16],19,20]. These four surfactants were also selected in this study and their suppression efficiencies were evaluated based on two concentrations of coal dust. The paper then compared the suppression efficiency of each surfactant solution. The findings in this study could be used for both research and industry equally to determine an effective water agent for coal dust suppression, thus contributing to improving the underground coal mines’ working environment.

Section snippets

Experimental procedures and apparatus

The schematic diagram of the wind tunnel used in this study is shown in Fig. 1. The wind tunnel is made up of three main parts: (1) dust generation part, which injects coal particles to the wind tunnel, (2) spray part, where water droplets are generated from a nozzle, (3) dust measurement part, where a dust monitor is placed to measure the coal dust concentration after spray, (4) dust collection part, where a cyclone dust collector is connected at the end of the wind tunnel. The dimensions of

Results

Table 1 gives the data resulted from the designed experiments. The solution level at 0% was pure water. Thus 2 runs with the pure water have been done for low and high initial coal dust concentrations, respectively. Analysis of variance (ANOVA) is an effective data analysis model to evaluate the difference among several means of the experimental data [21]. In this study, it is used to analyse the difference of mean values of different surfactant efficiencies. The ANOVA for the three-factor

Discussions

The main aim of this experimental design is to identify a surfactant that can reduce the coal dust effectively regardless of the initial coal dust concentration. From the ANOVA for the average suppression efficiency, as shown in Table 2, all of the three factors (Surfactant types, Solution levels, and Initial coal dust concentration) yielded significant main effects on average suppression efficiency. It is noticed that all the surfactants as well as water had higher suppression efficiency when

Conclusion

The aim of this study is to evaluate the coal dust suppression with different surfactants under different initial coal dust concentrations and then determine a surfactant that can reduce the coal dust effectively regardless of the initial coal dust concentration. A factorial experiment was designed to evaluate coal dust suppression efficiency of four surfactants, CTAB, SDS, SDBS and TX100 at seven different solution levels from 0.00 % to 0.30 %. Wind tunnel tests were conducted to study the

CRediT authorship contribution statement

Ping Chang: Conceptualization, Writing - original draft. Zidong Zhao: Methodology, Formal analysis, Data curation. Guang Xu: Supervision, Writing - review & editing. Apurna Ghosh: Writing - review & editing. Jinxin Huang: Investigation. Tao Yang: Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project is partially supported by the visiting scholar project from the State Key Laboratory of Coal Mine Disaster Dynamics and Control at Chongqing University in China under project number 2011DA105287-FW201902.

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