# Study on the Factors Influencing on Wetting Performance of Blasting Dust in a Limestone Mine

Authors: Jinrui Nie; Shujie Yuan; Ye Li; Xiaoming Huang
Journal: Geofluids
 (2022)

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## Abstract

In order to find out the factors influencing on wetting performance of blasting dust in open-pit limestone mines, the blasting dusts in a limestone mine in Tongling, Anhui Province, were studied. The samples of hydrophobic dust (MD) and hydrophilic dust (ND) were obtained by hydrostatic separation experiment. The contact angle of water on dust samples, particle size distribution, surface oxygen-containing functional groups, surface mineral composition and content, and surface pore structure was determined. The measurement results of contact angle indicate that the wetting performance of MD is weaker than that of ND. By comparative analyzing, the surface characteristics of MD and ND factors influencing on the wetting performance of the dusts were determined. The results showed that MD has a smaller particle size, higher volume fraction of hydrophobic groups, and more complex surface morphology than that of ND, which leads to its weaker wetting performance than that of ND.


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## Body

## 1. Introduction

In the process of exploitation in open-pit limestone mines, a large amount of dust is emitted in the blasting operation [1]. The blasting dust not only increase the prevalence of pneumoconiosis among miners but also pollute atmospheric environment [2]. The gases produced by explosive explosion include CO2, CO, N2, and NOX [3]. The blasting, crushing, and other processes in the mining process have direct impact on the physicochemical characteristics of dust. The greater the energy consumed in the production process, the smaller the dust particle size, and the more complex the pore structure, which leads to the weaker wetting performance of dust [4]. The characteristics of blasting dust, such as high concentration, easy diffusion, wide pollution range, and easy to cause secondary pollution, make the dust suppression effect difficult to be improved [5]. Strengthening the research of blasting dust control technology in open-pit mines is in line with the new trend of building green mines.Lots of means have been developed to suppress blasting dust. Liu et al. and Yang et al. [6, 7] studied the influencing factors of blasting dust emission and put forward using water mist to suppress blasting dust emission. Wang et al. and Jin et al. [8, 9] developed chemical dust suppressants based on the study of the characteristics of blasting dust and suppressed dust through water-sealed blasting. Hosseini et al. [10] optimized blasting parameter to depress the blasting dust emission. In the blasting construction of tunnels and underground mine, ventilation and spraying are used to suppress dust [11]. At present, the use of wet dedusting is the most widespread, and the dust wetting performance to a large extent determines its dedusting efficiency [12]. Studies [13–15] found that the wetting performance of dust is significantly influenced by its surface physicochemical characteristics. Usually, the wetting performance of dust is directly characterized by the contact angle of water on the dust surface; the bigger the contact angle, the weaker the wetting performance of dust [16]. Fu et al. [17] analyzed the composition of the blasting dust produced in the course of blasting demolition of urban buildings and considered that the cracks and microstructure of the blasting dust were the main reasons for its hydrophobicity. Xu et al.’s research [18] shows that the wetting performance of dust is greatly affected by its particle size, and its hydrophobicity increases apparently with the decrease of particle size. The hydrophilic mineral composition of the dust surface can significantly improve its wetting performance, the common hydrophilic mineral such as calcite (CaCO3) and quartz (SiO2) [19]. The surface carbon-containing functional groups of dust apparently affect its wetting performance, which carbon-containing macromolecular structures show strong hydrophobicity of coal dust due to a large amount of surface content of carbon-containing macromolecular groups such as fatty hydrocarbons and aromatic hydrocarbons, which leads to the weak wetting performance [20–22]. Study [23] analyzed the impact of surface pore structure of dust on its wetting performance, indicating that the complex surface pore structure leads to hydrophobicity. According to Xiazhong et al. [24], the blasting dust with higher water content is more likely to produce condensation effect when it is in contact with water, thus showing hydrophilicity.As has been shown, lots of studies of dust surface characteristics influencing on its wetting performance have been conducted and some achievements have been made. However, the studies are mainly focused on coal and metal mine dust; the study of limestone mine dust needs to be further enriched. This study determined the factors affecting the wetting performance of limestone mine dust and provide theoretical support for the suppression of blasting dust.


## 2. Materials and Methods

### 2.1. The Preparation of Dust Sample

The blasting dust sample separated using a 200 mesh (74μm) was dried at 60°C in an incubator for 8 h, and then cooled at 25°C for use. As shown in Figure 1, the prepared blasting dust was poured into a beaker containing deionized water (surface tension of 73.02 mN/m). After one minute, no more dust settling to the bottom of the cup, and the blasting dust sample was separated into two different parts. The dust floating on the surface is nonwet dust, called hydrophobic blasting dust (MD), and which dust deposited on the bottom of beaker is wetted dust, called hydrophilic blasting dust (ND). Each part of the dusts was picked up and dried in an incubator at 120°C for 2 h.Figure 1
Hydrostatic separation experiment of MD and ND.


### 2.2. Wetting Performance Test

The contact angles of deionized water on the surface of MD and ND were measured using a contact angle apparatus (OCA20). The dusts were pressed into tablet-shaped samples, under the pressure of 50 MPa for 5 min. The experimental steps are as follows: the deionized water was dripped on the surface of the blasting dust samples consecutively and slowly with a syringe needle. The whole process of droplets contacting the surface of the dust samples was recorded dynamically by using SCA20 software. After adjusting the baseline, the change of the contact angle of water on the dust samples with time was obtained. The same dust sample was measured twice and the average value of the results was taken as the final result.


### 2.3. Physical Properties Test

#### 2.3.1. Particle Size Distribution Test

The laser particle size analyzer Malvern Mastersizer 2000 was used to measure the particle size distributions of MD and ND. Water was selected as the dispersive solution, and the bubbles in the dispersive solution were blown by ultrasound before the test. Dust samples were slowly added after the background value was deducted. The same dust sample was measured twice and the average value of the results was taken as the final result.


#### 2.3.2. Pore Structure Measurement

The Micromeritics ASAP 2460 fully automatic specific surface and porosity analyzer BET was used to analyze the pore structures of the MD and ND, using the nitrogen adsorption method, under condition of temperature set at 120°C and degassing time of 12 h.


### 2.4. Chemical Properties Test

#### 2.4.1. XRD Analysis

The surface mineral composition and content of MD and ND were measured by X-ray diffraction (XRD). Dust sample was placed on glass slides; the surface mineral composition of MD and ND was determined by X-ray diffractometer, using a Cu-Kα radiation source with the working voltage of 40 kV and working current of 40 mA. The diffraction angles were from 2° to 100° with angular speed of 2°/min. The data were analyzed using software MDI Jade 6.1, and the adiabatic method was used to conduct the semiquantitative analysis of the phase in the dust samples.


#### 2.4.2. XPS Analysis

The X-ray photoelectron spectroscopy (XPS) was used to measure the surface carbon-containing macromolecular groups of MD and ND. In the XPS test, the sample chamber pressure was less than2.0×10−7mbar, the spot size was 400 μm, the operating voltage was 12 kV, and the filament current was 6 mA. The full-spectrum scanning energy is 150 eV and the step length is 1 eV. The narrow-spetrum scanning energy was 50 eV and the step length is 0.1 eV. The binding energy was corrected by C1s peak with the reference value of 248.8 eV.


## 2.1. The Preparation of Dust Sample

The blasting dust sample separated using a 200 mesh (74μm) was dried at 60°C in an incubator for 8 h, and then cooled at 25°C for use. As shown in Figure 1, the prepared blasting dust was poured into a beaker containing deionized water (surface tension of 73.02 mN/m). After one minute, no more dust settling to the bottom of the cup, and the blasting dust sample was separated into two different parts. The dust floating on the surface is nonwet dust, called hydrophobic blasting dust (MD), and which dust deposited on the bottom of beaker is wetted dust, called hydrophilic blasting dust (ND). Each part of the dusts was picked up and dried in an incubator at 120°C for 2 h.Figure 1
Hydrostatic separation experiment of MD and ND.


## 2.2. Wetting Performance Test

The contact angles of deionized water on the surface of MD and ND were measured using a contact angle apparatus (OCA20). The dusts were pressed into tablet-shaped samples, under the pressure of 50 MPa for 5 min. The experimental steps are as follows: the deionized water was dripped on the surface of the blasting dust samples consecutively and slowly with a syringe needle. The whole process of droplets contacting the surface of the dust samples was recorded dynamically by using SCA20 software. After adjusting the baseline, the change of the contact angle of water on the dust samples with time was obtained. The same dust sample was measured twice and the average value of the results was taken as the final result.


## 2.3. Physical Properties Test

### 2.3.1. Particle Size Distribution Test

The laser particle size analyzer Malvern Mastersizer 2000 was used to measure the particle size distributions of MD and ND. Water was selected as the dispersive solution, and the bubbles in the dispersive solution were blown by ultrasound before the test. Dust samples were slowly added after the background value was deducted. The same dust sample was measured twice and the average value of the results was taken as the final result.


### 2.3.2. Pore Structure Measurement

The Micromeritics ASAP 2460 fully automatic specific surface and porosity analyzer BET was used to analyze the pore structures of the MD and ND, using the nitrogen adsorption method, under condition of temperature set at 120°C and degassing time of 12 h.


## 2.3.1. Particle Size Distribution Test

The laser particle size analyzer Malvern Mastersizer 2000 was used to measure the particle size distributions of MD and ND. Water was selected as the dispersive solution, and the bubbles in the dispersive solution were blown by ultrasound before the test. Dust samples were slowly added after the background value was deducted. The same dust sample was measured twice and the average value of the results was taken as the final result.


## 2.3.2. Pore Structure Measurement

The Micromeritics ASAP 2460 fully automatic specific surface and porosity analyzer BET was used to analyze the pore structures of the MD and ND, using the nitrogen adsorption method, under condition of temperature set at 120°C and degassing time of 12 h.


## 2.4. Chemical Properties Test

### 2.4.1. XRD Analysis

The surface mineral composition and content of MD and ND were measured by X-ray diffraction (XRD). Dust sample was placed on glass slides; the surface mineral composition of MD and ND was determined by X-ray diffractometer, using a Cu-Kα radiation source with the working voltage of 40 kV and working current of 40 mA. The diffraction angles were from 2° to 100° with angular speed of 2°/min. The data were analyzed using software MDI Jade 6.1, and the adiabatic method was used to conduct the semiquantitative analysis of the phase in the dust samples.


### 2.4.2. XPS Analysis

The X-ray photoelectron spectroscopy (XPS) was used to measure the surface carbon-containing macromolecular groups of MD and ND. In the XPS test, the sample chamber pressure was less than2.0×10−7mbar, the spot size was 400 μm, the operating voltage was 12 kV, and the filament current was 6 mA. The full-spectrum scanning energy is 150 eV and the step length is 1 eV. The narrow-spetrum scanning energy was 50 eV and the step length is 0.1 eV. The binding energy was corrected by C1s peak with the reference value of 248.8 eV.


## 2.4.1. XRD Analysis

The surface mineral composition and content of MD and ND were measured by X-ray diffraction (XRD). Dust sample was placed on glass slides; the surface mineral composition of MD and ND was determined by X-ray diffractometer, using a Cu-Kα radiation source with the working voltage of 40 kV and working current of 40 mA. The diffraction angles were from 2° to 100° with angular speed of 2°/min. The data were analyzed using software MDI Jade 6.1, and the adiabatic method was used to conduct the semiquantitative analysis of the phase in the dust samples.


## 2.4.2. XPS Analysis

The X-ray photoelectron spectroscopy (XPS) was used to measure the surface carbon-containing macromolecular groups of MD and ND. In the XPS test, the sample chamber pressure was less than2.0×10−7mbar, the spot size was 400 μm, the operating voltage was 12 kV, and the filament current was 6 mA. The full-spectrum scanning energy is 150 eV and the step length is 1 eV. The narrow-spetrum scanning energy was 50 eV and the step length is 0.1 eV. The binding energy was corrected by C1s peak with the reference value of 248.8 eV.


## 3. Results and Discussion

### 3.1. Wetting Performance Analysis

The contact angles of deionize water on MD and ND surface at the beginning and the ending of the test are shown in Figure2, and the changes of the contact angles of MD and ND with time are shown in Figure 3. The contact angles of MD and ND surface gradually decreased with the increase of time. The contact angles of MD and ND, respectively, decreased from 30.4° and 27.7° to 11.5° and 9.3°. The contact angles of ND decreased more significantly. The contact angle of MD is always larger than on ND. Thus, the wetting performance of MD is weaker than that of ND. This conclusion is consistent with the result of the hydrostatic separation experiment.Figure 2
Contact angles of water on the surface of MD and ND at the beginning and the ending of the test.
(a)
Contact angle of water on MD surface at 0 s(b)
Contact angle of water on MD surface at 0.8 s(c)
Contact angle of water on ND surface at 0 s(d)
Contact angle of water on ND surface at 0.8 sFigure 3
Changes of contact angle with time.


### 3.2. Particle Size Distribution Analysis

The test results of the particle size distribution of MD and ND are shown in Figure4, and the cumulative distribution percentage of MD and ND in different particle size intervals is shown in Table 1. The particle size distributions of MD and ND are quite different. Among them, the proportion of slight dust below 10 μm in MD is 10.81% higher than ND. The percentage of MD is 28.41% higher than ND in the particle size distribution range below 20 μm.Figure 4
Particle size distribution of MD and ND.Table 1
Percentage of cumulative distribution of MD and ND in different particle size intervals.
Particle size area (μm)0~1010~2020~3030~4040~5050~6060~70MD (%)31.0842.1117.166.922.270.010.00ND (%)20.2724.5122.0412.58.091.770.35The physical parameters of MD and ND are shown in Table2. D10, D50, and D90 indicate the particle size of dusts when the cumulative particle size distribution arrives 10%, 50%, and 90%, respectively. D(4,3) represents the volume-weighted average particle size [25]. As shown in Table 2, all parameters of ND are bigger than those of MD, and the mass specific surface area of MD and ND is 1.523 m2·g-1 and 0.726 m2·g-1, respectively. Study [26] showed that the smaller the particle size and the larger the mass specific surface area of dust, the stronger the surface energy and surface activity of the dust and the harder it is to be wetted. Therefore, the wetting performance of MD is weaker than that of ND.Table 2
Physical parameters of MD and ND.
ParameterD10 (μm)D50 (μm)D90 (μm)D(4,3) (μm)Mass specific area (m2∙g-1)MD3.12113.42329.93715.2921.523ND5.83416.46341.78219.8360.726


### 3.3. Analysis of Surface Pore Structure

When water touches the dust, actual contact area on the dust surface will be larger than the apparent contact area, so the surface will increase its wetting performance [27]. The difference in structure between MD and ND surface was quantitatively analyzed by N2 adsorption at low temperature.The low-temperature N2 adsorption isotherms of MD and ND are shown in Figure 5. At the liquid N2 temperature (77 K), the amount of N2 adsorption on the dust surface depends on the relative pressure of nitrogen (p/p0); “p” is the partial pressure of N2 and “p0” is the saturated vapor pressure of N2 at the liquid nitrogen temperature. Depending on the characteristics of the hysteresis loop of the low-temperature N2 adsorption isotherm, it can be divided into slit-shaped, cylindrical, wedge-shaped, and ink-bottle-shaped types [28]. The low-temperature N2 adsorption isotherms of MD and ND are more consistent with the wedge-shaped adsorption isotherm, which indicates that there are many closed or semiclosed pores on the dust surface. The maximum N2 adsorption volume of MD is 4.50 cm3·g-1, which is higher than that of ND (4.09 cm3·g-1). Therefore, the surface pore structure of MD is more developed, and its adsorption capacity to N2 is stronger.Figure 5
Adsorption-desorption isotherms of MD and ND.
(a)
MD(b)
NDThe specific surface areas (SSAs) of MD and ND were characterized by Langmuir (L) and Brunauer-Emmett-Teller (BET) according to the pore size classification of powder materials. The pore volume of the two kinds of dusts was calculated by Dubinn-Radushkevich (D-R) and Barrett-Joyner-Halenda (BJH). The pore structure parameters of the two kinds of dusts were calculated by BJH and BET methods. The pore volume and structure parameters of MD and ND are shown in Table3. The SSABET and SSAL of MD are 53.5% and 55.1% higher than those of ND, respectively. The total pore volume of MD is 86.6% higher than that of ND. The average pore size of MD is 32.1% smaller than that of ND. The results show that the pore structure of MD surface is more complex than that of ND.Table 3
Pore volume and structure parameters of MD and ND.
Dust sampleSSA (m2·g-1)Pore volume (cm3·g-1)Pore size (nm)SSABETSSALBJHa volumeD-Rb volumeTotal volumeBJH (average pore size)MD6.349.680.01160.00110.012717.5434ND4.136.240.00670.00000.006725.8414Note: a: mesopore volume, b: microhole volume, andtotalvolume=mesoporevolume+microholevolume.The distributions of pore sizes and cumulative pore volumes of MD and ND are shown in Figure6. When the pore diameter is less than 11.61 nm, the micropore volume of MD is consistently higher than that of ND, but the cumulative pore volume of MD is always higher than that of ND, which further indicates that the pore structure of MD is significantly different from that of ND. The micropore volume and cumulative pore volume on MD surface are higher than those of ND, and the pore structure of MD is more complex.Figure 6
The distributions of pore sizes and cumulative pore volumes of MD and ND.
(a)(b)In conclusion, MD has complex pore structure than ND. Because of the complex pore structure, MD has a strong adsorption ability to air, which leads to the tight air film being more likely to form on the dust surface, and hinders the contact wetting of water to dust [29]. In addition, the complex pore structure makes the specific surface area of MD larger than that of ND, which makes MD have higher surface energy and more repulsive force to water, thus hindering water from spreading and wetting on the dust surface. Therefore, the difference in the surface pore morphology between MD and ND results in the difference in their wetting performance.


### 3.4. XRD Analysis

The XRD patterns of MD and ND are shown in Figure7, and the volume fractions of mineral composition of MD and ND are shown in Table 4. The XRD patterns of MD and ND are very similar, and they are mainly composed of CaCO3 and SiO2. The volume fraction of CaCO3 in MD is 99.4%, and that of SiO2 is 0.6%. The volume fraction of CaCO3 and SiO2 in ND is 99.1% and 0.9%, respectively. The composition and content of the two kinds of dusts are basically the same. CaCO3 and SiO2 are hydrophilic minerals. Therefore, the surface mineral composition of the two dusts is not the factor causing the difference in their wetting performance.Figure 7
XRD patterns of MD and ND.Table 4
Volume fraction of mineral composition in MD and ND.
Mineral compositionCaCO3SiO2MD (%)99.40.6ND (%)99.10.9


### 3.5. XPS Analysis

The XPS test patterns of the MD and ND were charge-corrected using C(1 s) for the standard peak position (284.8 eV), and the reference peak positions of each element were aligned with the standard peak positions; the main occurrence forms of C on the dusts surfaces can be obtained [30]. The C(1 s) XPS spectra of MD and ND surface are shown in Figure 8. It can be seen in Figure 8 that there are five major groups on the surface of MD and ND, which are C-C C-OR, O-C=O, C-H, and C=O. The contents of the bound state of the elements are represented by the peak area of the spectra. The binding energy and percentage of the relative peak areas of the components on the surface of MD and ND dusts obtained from XPS analysis of C(1 s) are shown in Table 5.Figure 8
C(1 s) XPS spectra of MD and ND surface.
(a)
MD(b)
NDTable 5
The binding energy and percentage of the relative peak areas of the components on the dust surface of MD and ND obtained from XPS analysis of C(1 s).
Dust sampleC-C/C-HC-ORC=OO-C=OEB (eV)φ (%)EB (eV)φ (%)EB (eV)φ (%)EB (eV)φ (%)MD284.857.212865.822873.85289.833.12ND284.836.822866.182874.37289.852.63C-C and C-H show hydrophobic; C-OR, C=O, and O-C=O show hydrophilic [31]. According to Table 5, the volume fractions of hydrophobic groups and hydrophilic groups on MD surface are 57.21% and 42.79%, respectively. The volume fractions of hydrophobic groups and hydrophilic groups on ND surface are 36.82% and 63.18%, respectively. The volume fraction of hydrophobic groups on MD surface is 20.39% higher than that of ND, and the volume fraction of hydrophilic groups is 20.39% lower than that of ND, which is consistent with the conclusion that the wetting performance of ND is stronger than that of MD. Therefore, the surface groups of the dusts affect their wetting performance.


## 3.1. Wetting Performance Analysis

The contact angles of deionize water on MD and ND surface at the beginning and the ending of the test are shown in Figure2, and the changes of the contact angles of MD and ND with time are shown in Figure 3. The contact angles of MD and ND surface gradually decreased with the increase of time. The contact angles of MD and ND, respectively, decreased from 30.4° and 27.7° to 11.5° and 9.3°. The contact angles of ND decreased more significantly. The contact angle of MD is always larger than on ND. Thus, the wetting performance of MD is weaker than that of ND. This conclusion is consistent with the result of the hydrostatic separation experiment.Figure 2
Contact angles of water on the surface of MD and ND at the beginning and the ending of the test.
(a)
Contact angle of water on MD surface at 0 s(b)
Contact angle of water on MD surface at 0.8 s(c)
Contact angle of water on ND surface at 0 s(d)
Contact angle of water on ND surface at 0.8 sFigure 3
Changes of contact angle with time.


## 3.2. Particle Size Distribution Analysis

The test results of the particle size distribution of MD and ND are shown in Figure4, and the cumulative distribution percentage of MD and ND in different particle size intervals is shown in Table 1. The particle size distributions of MD and ND are quite different. Among them, the proportion of slight dust below 10 μm in MD is 10.81% higher than ND. The percentage of MD is 28.41% higher than ND in the particle size distribution range below 20 μm.Figure 4
Particle size distribution of MD and ND.Table 1
Percentage of cumulative distribution of MD and ND in different particle size intervals.
Particle size area (μm)0~1010~2020~3030~4040~5050~6060~70MD (%)31.0842.1117.166.922.270.010.00ND (%)20.2724.5122.0412.58.091.770.35The physical parameters of MD and ND are shown in Table2. D10, D50, and D90 indicate the particle size of dusts when the cumulative particle size distribution arrives 10%, 50%, and 90%, respectively. D(4,3) represents the volume-weighted average particle size [25]. As shown in Table 2, all parameters of ND are bigger than those of MD, and the mass specific surface area of MD and ND is 1.523 m2·g-1 and 0.726 m2·g-1, respectively. Study [26] showed that the smaller the particle size and the larger the mass specific surface area of dust, the stronger the surface energy and surface activity of the dust and the harder it is to be wetted. Therefore, the wetting performance of MD is weaker than that of ND.Table 2
Physical parameters of MD and ND.
ParameterD10 (μm)D50 (μm)D90 (μm)D(4,3) (μm)Mass specific area (m2∙g-1)MD3.12113.42329.93715.2921.523ND5.83416.46341.78219.8360.726


## 3.3. Analysis of Surface Pore Structure

When water touches the dust, actual contact area on the dust surface will be larger than the apparent contact area, so the surface will increase its wetting performance [27]. The difference in structure between MD and ND surface was quantitatively analyzed by N2 adsorption at low temperature.The low-temperature N2 adsorption isotherms of MD and ND are shown in Figure 5. At the liquid N2 temperature (77 K), the amount of N2 adsorption on the dust surface depends on the relative pressure of nitrogen (p/p0); “p” is the partial pressure of N2 and “p0” is the saturated vapor pressure of N2 at the liquid nitrogen temperature. Depending on the characteristics of the hysteresis loop of the low-temperature N2 adsorption isotherm, it can be divided into slit-shaped, cylindrical, wedge-shaped, and ink-bottle-shaped types [28]. The low-temperature N2 adsorption isotherms of MD and ND are more consistent with the wedge-shaped adsorption isotherm, which indicates that there are many closed or semiclosed pores on the dust surface. The maximum N2 adsorption volume of MD is 4.50 cm3·g-1, which is higher than that of ND (4.09 cm3·g-1). Therefore, the surface pore structure of MD is more developed, and its adsorption capacity to N2 is stronger.Figure 5
Adsorption-desorption isotherms of MD and ND.
(a)
MD(b)
NDThe specific surface areas (SSAs) of MD and ND were characterized by Langmuir (L) and Brunauer-Emmett-Teller (BET) according to the pore size classification of powder materials. The pore volume of the two kinds of dusts was calculated by Dubinn-Radushkevich (D-R) and Barrett-Joyner-Halenda (BJH). The pore structure parameters of the two kinds of dusts were calculated by BJH and BET methods. The pore volume and structure parameters of MD and ND are shown in Table3. The SSABET and SSAL of MD are 53.5% and 55.1% higher than those of ND, respectively. The total pore volume of MD is 86.6% higher than that of ND. The average pore size of MD is 32.1% smaller than that of ND. The results show that the pore structure of MD surface is more complex than that of ND.Table 3
Pore volume and structure parameters of MD and ND.
Dust sampleSSA (m2·g-1)Pore volume (cm3·g-1)Pore size (nm)SSABETSSALBJHa volumeD-Rb volumeTotal volumeBJH (average pore size)MD6.349.680.01160.00110.012717.5434ND4.136.240.00670.00000.006725.8414Note: a: mesopore volume, b: microhole volume, andtotalvolume=mesoporevolume+microholevolume.The distributions of pore sizes and cumulative pore volumes of MD and ND are shown in Figure6. When the pore diameter is less than 11.61 nm, the micropore volume of MD is consistently higher than that of ND, but the cumulative pore volume of MD is always higher than that of ND, which further indicates that the pore structure of MD is significantly different from that of ND. The micropore volume and cumulative pore volume on MD surface are higher than those of ND, and the pore structure of MD is more complex.Figure 6
The distributions of pore sizes and cumulative pore volumes of MD and ND.
(a)(b)In conclusion, MD has complex pore structure than ND. Because of the complex pore structure, MD has a strong adsorption ability to air, which leads to the tight air film being more likely to form on the dust surface, and hinders the contact wetting of water to dust [29]. In addition, the complex pore structure makes the specific surface area of MD larger than that of ND, which makes MD have higher surface energy and more repulsive force to water, thus hindering water from spreading and wetting on the dust surface. Therefore, the difference in the surface pore morphology between MD and ND results in the difference in their wetting performance.


## 3.4. XRD Analysis

The XRD patterns of MD and ND are shown in Figure7, and the volume fractions of mineral composition of MD and ND are shown in Table 4. The XRD patterns of MD and ND are very similar, and they are mainly composed of CaCO3 and SiO2. The volume fraction of CaCO3 in MD is 99.4%, and that of SiO2 is 0.6%. The volume fraction of CaCO3 and SiO2 in ND is 99.1% and 0.9%, respectively. The composition and content of the two kinds of dusts are basically the same. CaCO3 and SiO2 are hydrophilic minerals. Therefore, the surface mineral composition of the two dusts is not the factor causing the difference in their wetting performance.Figure 7
XRD patterns of MD and ND.Table 4
Volume fraction of mineral composition in MD and ND.
Mineral compositionCaCO3SiO2MD (%)99.40.6ND (%)99.10.9


## 3.5. XPS Analysis

The XPS test patterns of the MD and ND were charge-corrected using C(1 s) for the standard peak position (284.8 eV), and the reference peak positions of each element were aligned with the standard peak positions; the main occurrence forms of C on the dusts surfaces can be obtained [30]. The C(1 s) XPS spectra of MD and ND surface are shown in Figure 8. It can be seen in Figure 8 that there are five major groups on the surface of MD and ND, which are C-C C-OR, O-C=O, C-H, and C=O. The contents of the bound state of the elements are represented by the peak area of the spectra. The binding energy and percentage of the relative peak areas of the components on the surface of MD and ND dusts obtained from XPS analysis of C(1 s) are shown in Table 5.Figure 8
C(1 s) XPS spectra of MD and ND surface.
(a)
MD(b)
NDTable 5
The binding energy and percentage of the relative peak areas of the components on the dust surface of MD and ND obtained from XPS analysis of C(1 s).
Dust sampleC-C/C-HC-ORC=OO-C=OEB (eV)φ (%)EB (eV)φ (%)EB (eV)φ (%)EB (eV)φ (%)MD284.857.212865.822873.85289.833.12ND284.836.822866.182874.37289.852.63C-C and C-H show hydrophobic; C-OR, C=O, and O-C=O show hydrophilic [31]. According to Table 5, the volume fractions of hydrophobic groups and hydrophilic groups on MD surface are 57.21% and 42.79%, respectively. The volume fractions of hydrophobic groups and hydrophilic groups on ND surface are 36.82% and 63.18%, respectively. The volume fraction of hydrophobic groups on MD surface is 20.39% higher than that of ND, and the volume fraction of hydrophilic groups is 20.39% lower than that of ND, which is consistent with the conclusion that the wetting performance of ND is stronger than that of MD. Therefore, the surface groups of the dusts affect their wetting performance.


## 4. Conclusions

(1)The surface area of MD is 0.797 m2∙g-1 larger than that of ND. Due to the smaller particle size and more complex surface morphology, the specific surface area and surface energy of MD are larger, which prevents the dust from being wetted by water and makes the wetting performance of MD weaker than that of ND(2)The pore structure in MD surface is more complex than that in ND surface. The average pore size in MD surface is 8.298 nm lower than that in ND surface, and the total pore volume in MD surface is 1.90 times higher than that in ND surface. The complex surface morphology results in easier forming the air film on the surface of MD and reduces its wetting performance(3)The volume fractions of hydrophilic groups in the surface of MD and ND are 42.79% and 63.18%, respectively. The volume fractions of hydrophobic groups in the surface of MD and ND are 57.21% and 36.82%, respectively. The volume fraction of hydrophilic groups in the surface of MD is less than that in ND surface, and the volume fraction of hydrophobic groups is larger than that in ND surface, which results in weak wetting performance of MD



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*Source: 7633334-2022-12-16.xml*