3.3. SEM, XRD, FT-IR Characterization and Other Test Results of Weathered Coal
(1) The SEM characterization results are shown in
Figure 3. Compared with the pre-activation ones, the particle size of weathered coal particles after mechanical activation was obviously reduced; the size specification of each particle size was more uniform and homogeneous; and the isometric aspect of the particles reduced, presenting laminated and flaky structures; the roughness of the particle surface increased; the specific surface area increased; the surface of the particles had a lot of stacked flakes; and more crevices existed.
(2) The correlation indexes of the aromaticity of weathered coal before and after activation are shown in
Table 4. It was found that the H/C value of the weathered coal after activation was reduced, and the aromaticity
and the ring number of individual C atoms
were increased, which indicated that the aromaticity increased after mechanical activation.
increased, which indicated an increase in aromatic groups and aromaticity of weathered coal after mechanical activation. With the ball milling, the temperature is increasing, according to “Hilt’s law”; the increase in coal rank with depth is due to the increase in temperature with depth, that is to say, it can be considered that the coal rank increases with the increase of temperature, and the higher the rank is, the higher the aromaticity of the coal is [
33], so it can be considered that the change in the weathered coal after the ball milling is similar to the change in the rank to a certain extent, which can be regarded as the change in the coal rank. Therefore, it can be assumed that the change in weathered coal after ball milling is similar to the change in coal rank to a certain extent and degree, which can explain why there is an increase in the aromaticity of coal after ball milling.
(3) The XRD characterization results are shown in
Figure 4. The main minerals present in the weathered coal are silica and kaolinite, and both the weathered coal before and after activation show high background intensities, suggesting that both contain a certain proportion of highly disordered material in the form of amorphous carbon [
34]. The peak with a diffraction angle of 26° corresponds to microcrystals in polycondensed aromatic rings, which are related to the stacking between aromatic rings, and the intensity of the peak at this location of the weathered coal increases after mechanical force, indicating an increase in aromaticity; except for that, mechanical force did not lead to any significant changes in the XRD patterns of the weathered coal.
(4) The results of FT-IR characterization are shown in
Figure 5. Each homing region of the infrared characteristic peaks of weathered coal is as follows: kaolinite (-Si-O-H-) vibrations at 3730–3610 cm
−1; hydrogen bonding -OH, -NH, -NH
2 telescoping vibrations at 3600–3100 cm
−1; aromatic CH telescoping vibrations at 3100–3000 cm
−1; hypromellitic CH
2 symmetric telescoping vibrations (with a small amount of methyl-CH
3) at 2970–2900 cm
−1 CH
2 asymmetric stretching vibration at 2880–2800 cm
−1 (with a small amount of methyl-CH
3); -COOH, -CO vibration at 1790–1680 cm
−1; carboxylate, aromatic C=C double bond, hydrogen-bonded carbonyl-C=O- at 1670–1530 cm
−1; aromatic nucleus, aromatic C=C double bond vibrations; methyl-CH
3 in-plane bending vibrations at 1580–1400 cm
−1; alkyl, methyl -CH
3 in-plane bending vibrations at 1390–1350 cm
−1; lipids at 1330–1100 cm
−1; hydroxyls, ketones, acetals at 1220–1070 cm
−1; clay minerals, such as kaolinite, at 1050–970 cm
−1; and Si-O- vibrations at 1330–1480 cm
−1; Si-O- vibration at 1050–970 cm
−1; quartz mineral (-Si-O-) vibration at 920–900 cm
−1; one hydrogen atom out-of-plane deformation vibration on the aromatic nucleus at 890–820 cm
−1 (class I hydrogen atom); two neighboring hydrogen atoms out-of-plane deformation vibration on the aromatic nucleus at 810–790 cm
−1 (class II hydrogen atom); four neighboring hydrogen atoms out-of-plane deformation vibration on the aromatic nucleus at 755–745 cm
−1 (class II hydrogen atom); four neighboring hydrogen atoms out-of-plane deformation vibration on the aromatic nucleus at 1220–1070 cm
−1 (class II hydrogen atom); (class IV hydrogen atom); 710–695 cm
−1 is a benzene ring folding vibration; and 595–420 cm
−1 is an inorganic mineral.
After ball milling, the peak intensities at 1600 cm
−1, 797 cm
−1, 754 cm
−1, and 694 cm
−1 were increased at four places, and the combination of the test results of
fa and
(R/
C)u values of the weathered coal and the XRD test results showed that the increase in the aromaticity of the weathered coal after activation by the optimal mechanical force activation process was due to the stacking and condensation of the various aromatic carbons in the weathered coal under the action of the compressive stress to form a new and larger aromatic layer, which was formed by the stacking and condensation of the aromatic carbons in the weathered coal under compressive stress [
33].
(5) The content of oxygen-containing functional groups of weathered coal is shown in
Table 5. In addition, as shown in
Figure 4, the intensity of the absorption peaks of each oxygen-containing functional group of the activated weathered coal increased, which, combined with the measurement results, indicates that the content of the total acidic, carboxyl, and phenolic hydroxyl groups of the activated weathered coal increased. This is because, due to the oxidation reaction accompanying the process of ball milling, the content of phenolic hydroxyl groups increased due to the fracture of some of the aryl ether bonds contained in the weathered coal, and at the same time some hydroxyl groups were oxidized to form new carboxyl groups, leading to an increase in carboxyl groups [
35]. Some of the hydroxyl groups were oxidized to form new carboxyl groups, resulting in an increase in carboxyl group content [
36].
(6) Summary: Effects of Mechanical Forces on Weathered Coal. First of all, it is physical action. In the ball milling process, under the joint action of compressive stress and shear stress, the weathered coal deformation occurs first. In this stage, the surface of the weathered coal is constantly being impacted by the steel ball, the other weathered coal, and the ball milling tank wall, resulting in the edges of the weathered coal are smoothed, appearing obtuse, which has the greatest impact of the steel ball, which is due to the highest density, the hardest, and the largest contact area with the weathered coal. When the weathered coal deformation to a certain extent and then the rupture occur, the internal material exposure, originally fixed by the minerals of the HA, can be exposed and the mechanical force so that the link between the two is broken, resulting in the release of HA and improving the free HA content. With the large particles of weathered coal broken into small particles of weathered coal, the overall particle size decreases. At this time, due to the reduction in particle size, the particles move in the gap between the steel ball, the tank wall, and the particles of the three, and the coal weathered by the steel ball impact frequency decreases, so the increase in the number of small balls is conducive to improving the efficiency of grinding. Due to the complex composition of weathered coal, the density of each region is inconsistent; in the weathered coal rupture, its particles are instantly cracked into many small particles of different sizes, and the interface formed on the rupture side will have many cracks and grooves, resulting in a surface that is relatively rough. At the same time, many smaller particles, due to the constant extrusion and collision, led to the process of movement by the particles of the interface of the roughness of the capture, and the extrusion makes the two by the combination of the two particles much closer, so it is observed that there are other small particles attached to the surface of the weathered coal particles after ball milling.
The second is chemical action. Under the action of strong mechanical force, the crystalline water inside the weathered coal is precipitated and transformed into adsorbed water, which exacerbates the agglomeration between the particles. The grain size of weathered coal decreases rapidly due to impact, extrusion, and friction by steel balls, etc. At the same time, a large number of crystal defects and lattice deformations also form, and more and more substances are transformed from crystalline to highly disordered and amorphous forms. Mechanical force breaks the organic macromolecules in weathered coal and increases the free hydroxyl groups. The mechanical force process is accompanied by an oxidation reaction; part of the benzene ring is oxidized to open the ring to produce fatty acids, and at the same time, under the action of compressive stress, the aromatic carbon will also be stacked and condensed to form a new, larger aromatic layer.
3.4. Performance Analysis of HA
(1) The results of UV-VIS characterization of HA are shown in
Figure 6. The E4/E6 value of HA is commonly used in the chemical industry to characterize the aromaticity and molecular weight of HA because of its complex structure, and the smaller the value is, the lower the aromaticity and molecular weight are. The E4/E6 value of the weathered coal HA extract before mechanical activation was 4.125, and it can be seen that the lowest point of E4/E6 in the graph was 4.004, which corresponded to a ball milling speed of 200 rpm and a ball milling time of 200 min. It is worth noting that the aroma and molecular weight of HA decreased faster with the increase in time at the speed of 300 rpm, which is presumed to be due to the high mechanical energy crushing the polycondensation of HA. The mechanical energy crushed the condensed HA, the macromolecules were broken up and decomposed, and the originally stacked aromatic layer was destroyed by the tangential force [
37].
(2) The infrared spectral results of HA [
38,
39] are shown in
Figure 7. The vibration peak at 3434 cm
−1 is a large number of free and hydrogen bonded OH groups; the peak intensity is high, indicating that the HA is rich in hydroxyl groups; and the intensity of the peaks of the two HAs is basically the same: 3000–2700 cm
−1 belongs to the fat region, the peaks at 2922 cm
−1 and 2850 cm
−1 are CH
2 asymmetric and symmetric telescopic vibration, respectively, and the peaks of the two HAs are not shifted and the intensity is basically the same; 1950–1800 cm
−1 is the carbonyl C=O vibration, presumably a carboxylic acid or ketone, and the intensity of the peaks of the activated HA is stronger than that of the activated one. The peaks at 2922 cm
−1 and 2850 cm
−1 were CH
2 asymmetric and symmetric telescopic vibration, respectively, and the two HAs did not have a large degree of peak shift, and the intensity was basically the same; the C=O vibration of the carbonyl group at 1950–1800 cm
−1 was presumed to be a carboxylic acid or ketone, and the intensity of the peaks in this area of the HA was stronger than that in the one before activation. The C=O vibration of the carboxylic acid group at 1708 cm
−1 was a stronger intensity of absorption in this area of the HA than that in the one before activation, which indicated that the content of the carboxylic acid functional groups of the HA was greater in the one extracted from the one after activation than that in the one before activation. The C=C vibration of aromatic was at 1602 cm
−1, with higher intensity after activation, which is consistent with the results of UV-VIS spectral analysis of HA in 4.5.2; the methylene deformation vibration and methyl symmetry deformation vibration was at 1422 cm
−1 and 1377 cm
−1, respectively, which are not obvious in terms of the peak intensity and the changes before and after activation. The C=O vibration was at 1231 cm
−1, which is presumed to be an Aromatic ether; 757 cm
−1 for the four adjacent hydrogen atoms on the aromatic nucleus out-of-plane deformation vibration, both of the peak intensities there were basically the same, namely 907 cm
−1 and 476 cm
−1 peaks for the minerals, both of which are very weak in these two peaks and much lower than the intensity of the peaks of the weathered coal in these two places. It can be seen that the extracted HA is very low in minerals, and the purity of the extracted HA is relatively high.
(3) The results of the ICP-OES analysis of HA are shown in
Table 6. From the analytical results, it can be seen that the content of each metal element in HA obtained from weathered coal extraction before and after activation is very low; for example, the content of the Cd element is zero, which indicates that HA extracted from this study is of high purity.