3.1 Analysis of the Influence of Single Factors on the Adhesion and Compression Performance of Dust Suppressants
Under the condition when other factors unchanged, the influence of each single factor on both the viscosity and the compressive strength of the dust suppressant were measured, and the experimental data was fitted. The results of the data fitting are shown in Fig. 3.
Figure 3a shows the effect of the mass ratio of the MAA and SPI monomers on the viscosity and compressive strength of the synthesized dust suppressant. It can be seen that the monomer mass ratio has a great influence on the viscosity and the compressive strength. As the volume ratio increases, the viscosity and compressive strength values first increase rapidly and then their rate of increase slows. Because acrylic monomers agglomerate very easily, the use of too much MMA will affect the stability of the solution. The inflection point of the curve where the growth rate slows is at an MAA/SPI mass ratio of 3:4. The viscosity value at this mass ration is 69 mPa⋅s, and the compressive strength is 0.88 MPa. Therefore, when the MAA/SPI mass ratio is 3:4, the dust suppressant has good adhesion and compression performance.
The effect of the amount of cross-linking agent on the viscosity and compressive strength of the dust suppressant is shown in Fig. 3b. As the amount of cross-linking agent increases, the viscosity first increases, and then at 0.8 g, the viscosity begins a gradual decrease. At 0.8 g the increase in the compressive strength slows. As the amount of the cross-linking agent increases beyond 0.8 g, the data point where the cross-linking occurs increases. During the cross-linking reaction, the gaps between the dust suppressant networks become smaller. As a result, it is difficult for water to enter these spaces, the degree of cross-linking decreases, and the viscosity decreases. Therefore, the selected amount of cross-linking agent is 0.8g.
Figure 3c shows the influence of the amount of initiator on the viscosity and compressive strength of the dust suppressant. As the amount of initiator increases, both the compressive strength and the viscosity first increase and then decrease. Based on the analysis, as the amount of initiator increases, the concentration of free radicals in the solution increases, and the grafting points of SPI chains increase. This increases the entanglement between molecular chains, causing the viscosity and compressive strength to increase sharply. When the amount of KPS is > 0.2 g, the molecular chain entanglement decreases. As a result, the increase in viscosity diminishes, and a downward trend appears. The compressive strength value decreases first and then increases slowly. Therefore, 0.2 g KPS is appropriate for the synthesis of the dust suppressant.
The effects of the grafting reaction temperature and time on the viscosity and compressive strength of the dust suppressant are presented in Fig. 3d, e. As the reaction time and temperature increase, the viscosity and compressive strength first increase and then become stable. When the reaction temperature is 60 ℃ and the reaction time is 30 min, the grafting reaction reaches completion. As the reaction time and temperature continuously increase beyond these levels, the viscosity and compressive strength of the dust suppressant do not increase appreciably. Therefore, the best grafting reaction condition is at 60°C for 30 min.
By testing the viscosity and compressive strength of each single factor, the best process conditions for the synthesis of dust suppressants have been determined to be the combination of an MAA/SPI mass ratio of 3:4, 0.8 g of SHMP, and 0.2 g of KPS, reacted for 30 min at 60 ℃. The dust suppressant prepared under the determined optimal process conditions exhibits good adhesion and compression performance. It can effectively prevent dust from becoming airborne and creating air pollution.
3.2 SEM and EDS Analysis
To clearly observe the microscopic state of the interaction between the dust suppressant and the sprayed coal dust, SEM observations and EDS elemental analysis were conducted, as shown in Fig. 4–6. Figure 4a shows an SEM image of the morphology of the dust suppressant at 300⋅ magnification. It can be seen that the droplets of the dust suppressant look like spherical particles with depressions on the surface, similar in shape but with different sizes. Figure 4b shows the morphology of the solidified shell formed by the dust suppressant on the surface of the coal sample at 300⋅ magnification. It can be seen that the dust suppressant droplets are fused together, and the dust suppressant droplets combine with the coal dust to effectively wrap the coal dust particles in the coal. A dense cured film is formed on the dust surface. Figure 4c shows the morphology of the combination of dust suppressant and coal dust at 10,000⋅ magnification. It can be seen that the coal dust and dust suppressant are bonded and cross-linked together, and the dust suppressant droplets can effectively bond to coal dust particles, showing strong adhesion. Figure 4d shows that the dust suppressant forms a dense dust suppression film on the surface of coal dust, and the coal dust particles are covered underneath. The clear outline of the coal dust particles can be seen from the circled position. This dust suppression film has notable compressive strength and is able to resist wind erosion and prevent the coal dust from becoming airborne.
Figure 6 EDS and elemental surface distribution map of solidified layer of dust suppressant
3.3 FTIR Spectra Analysis
Figure 7 shows the FTIR spectra of SPI, the SPI-MAA intermediate product, and the dust suppressant synthesized by SPI-MAA-SHMP. In the FTIR spectrum of SPI, the peaks at 3435 and 1654 cm− 1 correspond to the O-H and N-H stretching vibration peaks of the hydroxyl and amide groups, respectively. The characteristic peaks of the amide I band at 1032 cm− 1 belong to the C = O stretching vibration of the carbonyl group in the SPI. The FTIR spectra of the modified forms of SPI, SPI-MAA, and SPI-MAA-SHMP can be compared with the SPI spectrum. Regardless of the peak position or shape, the FTIR spectra of all three materials are similar. Therefore, it can be preliminarily judged that the synthesis of the dust suppressant is successful. The peak position at 3400 cm− 1 is roughly the same for all three materials, but the peak position gradually shifts to the right. The shift is due to the graft copolymerization between the SPI and MAA monomers. The intensity of the absorption peak near 1650 cm− 1 in the SPI-MAA and SPI-MAA-SHMP spectra gradually decreases. The decrease suggests that after SPI participates in the reaction, its active groups are consumed. The characteristic absorption peak of the cross-linker SHMP at 987 cm− 1 shows that SHMP successfully participated in the reaction to form a dust suppressant with a network structure [28–30].
3.4 DSC Analysis
SPI is a protein with a naturally formed structure. Chemical polymerization causes the protein to undergo a transformation. DSC is used to measure the thermal denaturation temperature of the protein and to evaluate whether the graft copolymerization modification of SPI is successful.
Figure 8 shows the DSC curves of SPI, SPI-MAA, and SPI-MAA-SHMP in the liquid and solid states. To clearly see the peak changes, a section of Fig. 8a is enlarged to make Fig. 8b. It can be seen in Fig. 8b that with the continuous modification of SPI, the melting peak shifts toward higher temperatures, and the melting peak temperature of the same substance in the liquid state is slightly greater than the solid melting peak, i.e. TSPI−L > TSPI−S, TSPI−MAA−L > TSPI−MAA−S, and TSPI−MAA−SHMP−L> TSPI. The shift in melting temperature occurs because the liquid substance contains water, which causes the melting peak to shift to the right during the heating and evaporation process. The melting peak of SPI-S is 101.85 oC. After adding MAA, the melting peak of SPI-MAA-S is 111.36 ℃, and the melting peak shifts to the right. The shift may be due to the graft copolymerization of SPI and MAA. More energy is required to melt the connected SPI-MAA than the SPI, which is manifested as a shift of the melting peak to higher temperatures. Similarly, after cross-linking with SHMP, the melting peak of the product SPI-MAA-SHMP continues to shift to higher temperatures, and the melting peak of SPI-MAA-SHMP-S is 115.57°C. The change in melting temperature is caused by the increase in molecular weight after cross-linking, which can lead to the increase of denaturation temperature of cross-linked proteins [31, 32].
3.5 Analysis of TSP Results
Table 1
Weather report during the monitoring periods
Date | Temperature /℃ | Wind /degree | Direction of wind |
2020/11/2 | 4–18 | 5–6 | North |
2020/11/3 | 2–12 | 4–5 | North |
202011/4 | 7–14 | 3–4 | South |
2020/11/5 | 11–16 | 3–4 | West-south |
Figure 9 shows the TSP monitoring data of the test group and the control group at the monitoring points over 4 days, and Table 1 shows the weather conditions during the monitoring period. On the second and third days, the wind was strong, and the TSP value of monitoring point 1 in the control group was higher. The maximum TSP value of 9.1 µg/m3 occurred at 18:00 on the second day. According to the data analysis, the average TSP of monitoring point 1 in the control group over 4 days was 5.18 µg/m3. The average TSPs at monitoring point 2 and the downwind monitoring point 3 in the experimental group over 4 days were 1.80µg/m3 and 1.85µg/m3, respectively. Compared with the control group, the experimental group had an average TSP reduction of 79.95%. This finding showed that spraying of the dust suppressant can effectively fix the coal dust on the surface of the coal pile. The consolidation layer formed by spraying the dust suppressant can effectively prevent the coal powder from being blown aloft by the wind and can therefore reduce air pollution.