3.1 Characterization of raw coal gangue
The coal gangue used in the experiment was analyzed via X-ray fluorescence, and these results are shown in Table 1. The primary components of coal gangue are SiO2, Al2O3, Fe2O3 and TiO2. XRD results show that (Fig. 1) the primary mineral phases of coal gangue selected in the test are quartz, kaolin, calcite and kaolinite-montmorillonite layers, of which the kaolin and kaolinite-montmorillonite layers have rich layered structures.
Table 1
Chemical composition of coal gangue (%)
Project
|
SiO2
|
Al2O3
|
Fe2O3
|
CaO
|
TiO2
|
MgO
|
Relative quantity
|
43.97
|
18.43
|
14.75
|
12.11
|
4.45
|
2.18
|
3.2 Structural succession of thermally activated coal gangue
3.2.1 TGA-DSC analysis
The TGA-DCS curve of gangue is shown in Fig. 2. There are three troughs at 108.04℃, 503.92℃ and 748.7℃ and two peaks at 453.00℃ and 676.44℃. The trough at 108.04 °C is primarily caused by the desorption of water adsorbed on the surface of gangue. The trough at 503.92 °C is caused by the endothermic conversion of kaolinite into metakaolinite due to the loss of crystalline water. The trough at 748.7 °C is caused by the heat absorption of calcite decomposition. The wave peak at 453.00 °C may be the exothermic peak generated by the combustion of organic matter in gangue, while the wave peak at 676.44 °C may be caused by the combustion of carbon in gangue. The DSC curves from 0 to 1200 ℃ show that the gangue warming process experienced 3 phase changes. The TGA curve shows that the process of gangue warming experienced 4 stages of weight loss. The first stage is the slow weight loss stage at 50~200 °C. The weight loss in this stage is primarily caused by the desorption of adsorbed water from coal gangue, and the weight loss rate was 1.775%. The second stage is primarily the stage of accelerated weight loss at 200~600 °C. The weight loss in this stage is primarily caused by the conversion of organic matter in the gangue into carbon dioxide and water, and the loss of crystalline water in kaolin, with a weight loss rate of 4.986% (Hao et al., 2022). The third stage is the rapid weight loss stage at 600~800 °C. The weight loss in this stage is primarily caused by the decomposition of calcite to overflow carbon dioxide and the combustion of carbon to form carbon dioxide overflow, and the weight loss rate is 5.943% (Li et al., 2016a). The fourth stage is the stable weight loss stage at 800~1200℃. The weight loss in this stage is primarily caused by the desiccation of internal structure water. Thus, the temperature of gangue thermal activation should be selected after the decomposition of kaolin and calcite but before the formation of a new stable mineral phase (i.e., above 748.7 °C).
3.2.2 Succession law of ore facies of thermally activated coal gangue
The X-ray diffraction patterns of raw coal gangue and coal gangue calcined at 200~900 °C are shown in Fig. 3a. With calcination at 200~500 °C, the mineral phases of quartz and calcite remained unchanged, while the peaks of kaolinite-montmorillonite and kaolin gradually decreased with increasing calcination temperature. The peaks of kaolinite and kaolinite-montmorillonite were not found in the coal gangue calcined at 600 °C, indicating that the kaolin has been completely transformed into metakaolin at 600 °C. The calcite characteristic peak began to weaken at 700 °C, which is consistent with the results of differential thermal analysis. The characteristic peaks of calcite disappeared at 800 °C; thus, the reaction activity of thermally activated coal gangue was the strongest at 800 °C. To describe the effect of holding time on the phase structure of coal gangue, the coal gangue was calcined at 800 ℃ for 0.5, 1.0, 2.0, 3.0 and 5 h of holding time. After natural cooling, the XRD patterns of the calcined samples under different holding times were analyzed (Fig. 3b). Results show that the XRD pattern waveforms of thermally activated coal gangue with different holding times are similar.
3.3 Succession law of the physical properties of thermally activated coal gangue
3.3.1 FTIR analysis of thermally activated coal gangue
The FTIR spectrum of coal gangue shown in Fig. 4a indicates that the absorption peak at 3696 cm-1 is formed by the vibration of the outer kaolinite hydroxyl (Al-O-H, structural water) in the coal gangue; the absorption peak at 3623 cm-1 is formed by the vibration of hydroxyl groups (interlayer water between silicon oxygen tetrahedron and aluminum oxygen octahedron) in kaolin(Torres-Luna and Carriazo, 2019), Madejová, 2003); the absorption peak at 796 cm-1 is formed by the vibration of Si-O-Si in kaolin; and the absorption peak at 694 cm-1 is formed by the strong bending vibration of Si-O in kaolin (Li et al., 2019). These results are consistent with the result of kaolin contained in coal gangue analyzed by XRD in Fig. 3. The absorption peak at 3431 cm-1 should be formed by the telescopic vibration of adsorbed water in coal gangue. The absorption peak at 1426 cm-1 can be attributed to the absorption frequency of Ca in calcite, which is formed by the reverse asymmetric stretching vibration of the vertical C-axis of the CO32-group. The absorption peak at 877.26 cm-1 can be attributed to the reverse bending vibration of the CO32-group of calcite parallel to the C axis (Sruthi and Reddy P, 2017), Ferone et al., 2015), which indicates that the coal gangue contains calcite. There is also a weak absorption peak at 1088 cm-1, which is formed by the stretching vibration of the Si-O bond in montmorillonite and should be formed by the expansion and contraction vibration of the Si-O bond in the high mask mixed layer in the coal gangue. The absorption peak at 1031 cm-1 should be formed by the asymmetric stretching vibration of the Si(Al)-O-Si bond (Li et al., 2015). The infrared spectrum shows few changes below 500 ℃. The absorption peaks near 3696, 3623 and 694 cm-1 began to weaken at 500 °C. At 600 °C, the absorption peaks near 3696 cm-1 and 3623 cm-1 disappeared, while the absorption peak at 694 cm-1 shifted to 690 cm-1 and nearly disappeared, indicating that at 600 °C, the kaolinite in the coal gangue was almost completely converted to metakaolinite. The absorption peak at 1426 cm-1 moved in the direction of the wavenumber decrease with increasing temperature and started to weaken at 700 ℃. The absorption peak at 877.26 cm-1 moved in the direction of increasing wavenumber with increasing temperature and began to weaken at 700 °C. The absorption peaks near 1426 cm-1 and 877 cm-1 disappeared at 800 ℃, indicating that calcite completely decomposed at 800 ℃. At 700℃, a faint absorption peak appears at 3642 cm-1, which may be formed by hydroxyl stretching vibrations of Al-OH and Ca-OH (张吉秀, 2010), where the absorption peak is enhanced at 800℃ and the absorption peak is weakened at 900℃, indicating that a new substance may form at 700℃, and this new material is most abundant at 800℃. At 900 ℃, in addition to the adsorption of water, only the absorption peak of quartz is left, and the adsorption of water in this study may be formed by incomplete drying of the material. The absorption peak at 1031 cm-1 gradually moved toward the direction of increasing wavenumber with increasing temperature, and widening occurred, which indicates that the spacing between layered silica tetrahedron structures is reduced due to the release of interlayer structural water from clay minerals during calcination. Also, widening begins after 600 ℃ because the crystallinity of silica tetrahedron decreases with increasing temperature (Zhang, 2010). The characteristic peak of quartz at 796 cm-1 is weak below 400 °C, and the characteristic peak begins to strengthen at 400~600 °C, which is due to the increase in the relative concentration of quartz caused by the combustion of carbon in gangue and the removal of hydroxyl groups in kaolin. The FTIR spectrum waveform of thermally activated gangue with different holding times were similar, and the absorption peak near 3642 cm-1 first increased and then decreased with increasing holding time. The peak was the highest at 2 h (Fig. 4b), which showed that the activity of thermally activated gangue was the strongest at 2 h.
3.3.2 Effect of activation temperature on the morphological characteristics of coal gangue
Fig. 5a shows an SEM image of the unburned coal gangue. The surface structure of the unburned coal gangue is relatively compact, and the structure of the thermally activated coal gangue at 200~500 ℃ is similar (Fig. 5b, c, d, e), maintaining the original structural state of the coal gangue. At 600 ℃, the layered structure of thermally activated coal gangue loosens, which may occur because the hydroxyl group of kaolin is removed to reduce the binding of the layered structure, which makes the interlayer structure loose. The relaxation of the crystal structure is conducive to improving the porosity of coal gangue (Fig. 5f). The layered structure of activated gangue at 700~800℃ did not change markedly, but the relaxation of the crystal structure increased due to the loss of interlayer water (Fig. 5g, h). The layered structure of activated coal gangue at 900 ℃ markedly changed: the layered structure collapses, which makes the porosity and surface area drop markedly (Fig. 5i, j).
3.3.3 Absorption and desorption curve of thermally activated coal gangue
The pore structure of the thermally activated coal gangue sample at different temperatures was characterized by an adsorption-desorption isotherm of N2 at 77 K, and the results are shown in Fig. 6. The N2 suction-desorption isotherm of the coal gangue sample with a thermal activation temperature of less than 800℃ is similar. According to the classification basis proposed by the International Union of Pure and Applied Chemistry (IUPAC), the heat activation temperature of less than 800℃ of the coal gangue suction and desorption isotherm is in line with the characteristics of the V-type isothermal and H3 hysteresis loop. These results show that the structure of the pores in the gangue soil, and that the sample heated to 800℃ is primarily mesoporous, which can be attributed to the slit hole formed by the accumulation of layered particles (Thommes et al., 2015). At low relative pressure (p/p0˂0.4), the adsorption amount of N2 slowly increases with increasing relative pressure, at which time the adsorption branch line and the desorption branch line completely coincide, primarily mesoporous monolayer adsorption, and no microporous adsorption is present (Kresge et al., 1992). As the relative pressure rises again, the adsorption of N2 increases markedly and forms a hysteresis loop with the desorption branch, which is primarily due to the phenomenon of capillary condensation of the mesoporous material during nitrogen suction and desorption. Adsorption is caused by two factors: multilayered adsorption of pore walls and agglomeration in the pores, while desorption is only caused by capillary agglomeration. Thus, multimolecular layer adsorption first occurs during adsorption, and condensation can only occur when the adsorption layer on the pore wall reaches a sufficient thickness. When desorption occurs under the pressure of the same p/p0 ratio, the steam that occurs only on the liquid surface in the capillary cannot desorb the molecules adsorbed under p/p0, and to desorb, a smaller p/p0 is required; thus, the lag phenomenon of desorption occurs, which is actually caused by the irreversibility of adsorption under the same p/p0. The shape of the hysteresis loop reflects a certain pore structure, reflecting a slit mesoporous produced by a flaky granular material in the pores of thermally activated coal gangue below 800 ℃(Jin et al., 2021), Thommes et al., 2015).
According to the Kelvin equation and capillary coagulation theory, the early and late closure of the upper and lower ends of the hysteresis loop indicates the width and narrowness of the mesoporous pore size distribution, respectively; the farther the upper and lower ends of the closure point are, the wider the mesoporous aperture distribution. Also, the closer the upper and lower end closure points are, the narrower the pore size distribution of the mesopore. Fig.6 shows that the gangue and activation products below 800℃ are primarily mesoporous, and the distribution of mesoporous pore size does not change much. The absorption and desorption isotherm of the thermally activated gangue product at 900℃ is nearly straight, the adsorption amount is small, and the hysteresis loop is also almost gone, indicating that the pores in the activated gangue samples gradually disappear at this temperature.
3.3.4 Succession law of the specific surface area of thermally activated coal gangue
The multipoint BET method was used to analyze the succession law of the specific surface area of thermally activated coal gangue. The specific surface area of coal gangue first decreases, then increases, and finally decreases with increasing calcination temperature (Fig. 7a). Calcined coal gangue at 200℃ shows a large decrease in the specific surface area of the original soil due to the decrease in the specific surface area caused by the shrinkage of the clay minerals in the coal gangue, which is caused by the shrinkage of the crystals due to the adsorption of water. The specific surface area of coal gangue calcined at 200~300 ℃ continues to decrease, which is primarily due to the continuous stripping of interlayer water of layered silicate minerals with increasing temperature, which causes interlayer hydrogen bond fracture, resulting in the reduction of part of the interlayer spacing and the reduction of the specific surface area caused by overlap. The specific surface area of coal gangue calcined at 300~600 ℃ increases slowly with increasing temperature, which is primarily caused by two aspects. First, the increase in mesopores caused by the combustion of coal gangue carbon increases the specific surface area. The other is that the specific surface area increases due to the increase in mesopores and micropores caused by the dehydration of layered silicate mineral structure water. The specific surface area of coal gangue calcined at 600~800 ℃ decreases with increasing temperature, which is due to the layered structure sintering of silicate minerals caused by the increase in temperature and the pore blockage caused by the decomposition of calcite caused by the increase in temperature. The specific surface area of calcined coal gangue at 900 ℃ dropped to below 4.00 m2/g, indicating that the layered structure began to collapse at 900 ℃.
3.3.5 Pore size and pore volume succession of thermally activated coal gangue
The size, shape and number of pores have a great influence on the measurement results of the surface area. Concurrently, the pore volume and pore diameter of materials strongly affect the adsorption, catalysis, stability, subsequent modification and loading of the materials themselves. The change in average aperture shows a rising-decreasing-rising-decreasing-rising trend (Fig. 8a). The increase in the pore diameter of coal gangue calcined at 200 ℃ may be due to the removal of adsorbed water and the shrinkage of its structure. The decrease in the pore diameter of coal gangue calcined at 300 ℃ may be caused by the desorption of interlayer water. The increase in the calcined coal gangue pore diameter at 300~500 ℃ may be caused by the combustion of carbon in coal gangue. The decrease in the average pore diameter of coal gangue calcined at 500~600 ℃ is primarily caused by the increase in micropores due to the loss of kaolin crystal water. The increase in the average pore diameter of coal gangue calcined at 600~800 ℃ is primarily caused by the increase in mesopores caused by the deformation of Si-O bonds and Al-O bonds. The change rule of pore volume is the same as that of specific surface area (Fig. 8b), showing a trend of first decreasing, then increasing, and then decreasing. The specific reasons are basically the same as those that cause the change in specific surface area.
3.3.6 Pore size distribution of thermally activated coal gangue
The pore size distribution of porous materials affects the adsorption, permeation, filtration and loadability of the material. In this study, the pore size of coal gangue and its thermal activated products is primarily mesoporous, and micropores can be ignored. Therefore, the BJH (Barrett Joyner Halenda) model is used to analyze the pore size distribution of coal gangue and its thermally activated products. When using the BJH method to analyze the pore size distribution, it is also necessary to consider the use of adsorption branch or desorption branch data. The desorption process generally considered to be more similar to the physical process; thus, many scholars choose to use desorption branch data to analyze the pore size distribution of mesoporous materials. Determination of pore size distribution and porosity of solid materials by mercury intrusion method and gas adsorption method - Part 2: Analysis of mesopores and macropores by gas adsorption method (GBT21650.2-2008) provides suggestions about the selection of mesoporous analysis data and notes that careful analysis should be performed. For the narrow H1 hysteresis curve produced by the relatively simple hole structure of a relatively uniform cylindrical hole, the desorption branch of the curve is often used for aperture analysis. When the connecting hole is plugged, producing seepage, it is unsafe to use the adsorption branch or desorption branch because there may be a mixing effect with both delayed agglutination and network penetration. If a certain method is used to consider the influence of pore size on delayed agglutination, particularly in the metastable range of pore fluid, the adsorption branch can be used for pore size analysis. If the so-called "tension strength effect" occurs during evaporation, the desorption branch of the curve will drop markedly at a certain P/P0, which varies with the type and temperature of the adsorbed gas. For the adsorbed gas of 77 K N2, the p/p0 value is 0.42. A more realistic pore size distribution can be obtained according to the adsorption branch of the curve. The H3 hysteresis ring appears in the N2 adsorption and desorption curve of coal gangue and thermal activation products (Fig. 6), which indicates that the pores in the material are irregular, and the so-called "tensile strength effect" occurs during evaporation; thus, the pore size distribution curve of coal gangue and heat-activated coal gangue in this paper is calculated by adsorption branches. The radius corresponding to the highest peak in the differential curve is the most likely pore size.
As shown in Fig. 9, the pore size distribution curve of the BJH adsorption branch of raw coal gangue and thermally activated coal gangue at different temperatures shows that there is only one peak of raw coal gangue and thermally activated products, which indicates that the pore size distribution is relatively concentrated. The position and intensity of the peak fluctuate with increasing thermal activation temperature (Table 2). The peak position of calcined coal gangue (including 300 ℃ calcined coal gangue) remains consistent below 300 ℃, which indicates that the pore structure of thermally activated coal gangue undergoes few changes within this temperature range. The peak position of coal gangue calcined at 400 ℃ was pushed back to 2.8192 nm, which may be caused by the combustion of C in coal gangue. The peak position of coal gangue calcined at 500 ℃ returns to 2.4903 nm, the peak position of coal gangue calcined at 600~700 ℃ returns to 2.8 nm, and the peak position of coal gangue calcined at 800 ℃ or above returns to 2.5 nm, which may be caused by the decomposition of calcite. The intensity change of the peak is thus dependent on the change in pore volume, and the peak effectively disappears at 900 °C, which indicates that the structure has effectively collapsed at 900 °C.
Table 2
Most probable pore size and strength of activated coal gangue at different temperatures
Sample
|
Most probable pore size(nm)
|
dV/D(cc/nm/g)
|
raw
|
2.5174
|
0.00878
|
200℃
|
2.4784
|
0.00374
|
300℃
|
2.4881
|
0.00253
|
400℃
|
2.8192
|
0.00313
|
500℃
|
2.4903
|
0.00441
|
600℃
|
2.7867
|
0.00628
|
700℃
|
2.8303
|
0.00447
|
800℃
|
2.4629
|
0.00332
|
900℃
|
2.4845
|
0.00139
|