The Effect of Silicon-Containing Minerals on Coal Evolution at High-Temperature Pre-Graphitization Stage

Abstract

Coal is a carrier of carbon enrichment, so it has the potential for the preparation of coal-based carbon materials. In this paper, LT anthracite and TSG bituminous coal were selected, and the corresponding graphitized samples were prepared from high-temperature treatment. The effects of silicon-containing minerals on coal evolution during the high-temperature pre-graphitization stage were investigated by XRD, Raman spectroscopy, and SEM. The results showed that with increasing temperature, the silicon-containing samples showed smaller d₀₀₂ and ID₁/IG, and higher L_c, while L_a presented a slight increase. It was found by SEM that the micromorphology of all samples was mainly massive structures. Meanwhile, irregular polyhedral structures also were observed in silicon-containing samples at 1300 °C, which were related to the formation and deposition of SiC. The carbothermal reactions of silicon-containing minerals continued to generate SiC and precipitate with increasing temperature, resulting in the gradual transformation of the needle-like structures into polyhedral structures. However, SiC was completely decomposed at 2800 °C. These changes indicated that during the pre-graphitization stage, silicon-containing minerals form SiC to advance the reduction of the interlayer spacing and the increase of longitudinal layer stacking height, thereby enhancing structural ordering and graphitization degree, while it had less effect on the lateral size. This will help to further understand the role of silicon-containing minerals in the coal pre-graphitization stage and also provide useful information about synthetic coal-based graphite.

1. Introduction

Graphite has been applied in the electrode, chemical, biomedical, refractory, and other traditional industries because of its special structures and excellent properties [1,2,3,4]. With further research on graphite, it will be used for new energy, precision electronic components, nuclear and aerospace, and has become an important strategic resource to support the development of high technology fields [3,4,5,6,7]. Graphite can be divided into natural graphite and artificial graphite. Natural graphite has more intergrowth minerals, and the disseminated grain size is very small [8]; therefore, it is difficult for natural graphite to be sorted and purified, which greatly limits the utilization of natural graphite. Furthermore, with the increase in the price of natural graphite, the utilization of large-scale graphite resources is affected by the cost [3]. Thus, it is necessary to make reasonable use of natural graphite and explore suitable materials to develop artificial graphite. Compared with natural graphite, artificial graphite is usually made of organic carbon materials through high-temperature carbonization and graphitization, which has better performance and adjustability due to the influence of raw materials and temperature [2,4,7].

Coal is a carbon-rich material, and the heteroatoms and small molecular compounds are continuously removed, and the final product of evolution is graphite under the combined effect of stratigraphic temperature and pressure [9,10,11]. Thus, it is a good precursor for preparing artificial graphite and has great potential in the future. A number of researchers have paid more attention to the graphitization of coal and obtained great results [12,13,14,15,16,17,18,19,20]. Franklin experimentally confirmed that anthracite was graphitizable above 2500 °C [21]. Atria et al. [13] studied the structural ordering of three Pennsylvania anthracites at different heat-treated temperatures and reported that the temperature of 2700 °C may be a “graphitization jump”, indicating that temperature played an important role during coal graphitization. By studying anthracite heated at high ambient pressure at temperatures up to 2800 °C, Bustin et al. [22] reported that graphite began to appear at temperatures as low as 600 °C, suggesting that the pressure compensated for the partial strain energy of coal graphitization. Cao et al. [23] concluded that temperature was the fundamental driving force for coal graphitization and the tectonic stress shortened the time of graphitization. Zhang et al. [24] proposed the formation of graphite-like structures was a successive process during high-temperature graphitization of coal, followed by carbonization (<1000 °C), secondary carbonization and pre-graphitization (1000~2000 °C), and graphitization (>2000 °C). The reflectance of vitrinite (R_o) is one of the important parameters to characterize the degree of coalification. The extension of basic structural units in coal sharply increases, which marks the end of coalification and the beginning of graphitization [9,25]. The turbostratic structures begin to evolve into ordered structures when d₀₀₂ = 0.334 nm, which is also regarded as the starting point of graphitization [26]. In addition, d₀₀₂ is usually used to divide graphite (<0.338 nm), semi-graphite (0.338~0.340 nm), semi-anthracite (0.340~0.348 nm), and anthracite (>0.344 nm) [9,24].

Besides the treatment temperature and pressure, some characteristics of anthracite have been explored to influence its graphitization process. Among them, minerals have been considered graphitization catalysts [2,6,7,27,28,29,30,31]. Baraniecki et al. [32] found that the addition of iron or ferrosilicon particles considerably reduced the temperature at which coal graphitization occurs. Pappano et al. [28,33] found that the minerals that played a key role during coal graphitization were similar to those contained in high-quality graphite deposits. Lin et al. [34] systematically studied the graphitization process of a graphite block doped with Si and Ti by HRTEM at the atomic scale and proposed the related catalytic mechanisms. Liu et al. [35] reported that the transformation of creating silicon carbide above 1300 °C in carbothermal reduction played a significant catalytic role in the graphite structure of the coal. Li et al. [36] discussed the catalytic graphitization process of coke carbon with iron and pointed out that the carbon dissolution–graphite precipitation mechanism could explain this process very well.

Although many studies have been conducted on coal graphitization, there is still a lack of systematic research on the influence of silica-containing minerals at the coal pre-graphitization stage. In this paper, quartz has been added into demineralized coal as a typical silicon-containing mineral for preparing silicon-containing samples. Longtan No. 6 (LT) anthracite and Tangshangou No. 12 (TSG) bituminous coal were selected, and the related raw coal, demineralized coal, and quartz-added demineralized coal were prepared for high-temperature treatment. XRD, Raman spectroscopy, and SEM were applied to analyze the structural evolution of samples. Based on the above analysis, we also tried to further understand the role of silicon-containing minerals on coal pre-graphitization. The findings of this work provide new ideas for the research of coal-based graphite and the clean efficient utilization of coal. It will also be of great practical significance to improve the added value and resource utilization of coal resources.

2. Samples and Methods

2.1. Sample Selection and Preparation

The experimental coal samples were anthracite collected from the Longtan No. 6 coal seam (LT) and bituminous coal from the Tangshangou No. 12 coal seam (TSG) according to the Chinese National Standard GB/T 482-2008. The samples were crushed into 18 meshes to make polished grain mounts for the mean maximum vitrinite reflectance (R_o) measurement, 80 meshes for proximate and ultimate analysis, and 200 meshes for demineralization and high-temperature experiments.

Table 1. Basic parameters of coal sample.

Sample	Ro	Proximate Analysis (%)			Ultimate Analysis (wt.%, daf)
		Mad	Ad	Vdaf	C	H	O *	N	S
LT	5.67	3.64	19.76	4.96	94.90	1.18	2.79	0.56	0.56
TSG	1.02	2.37	18.23	32.08	83.65	3.91	10.93	0.90	0.60

M: moisture; A: ash; V: volatile; ad: air dry; d: dry; daf: dry ash free; C: carbon; H: hydrogen; O: oxygen; N: nitrogen; St: total sulfur. * By difference.

lists the results of R_o proximate and ultimate analyses of the samples. For LT raw coal, the minerals were mainly quartz, chlorite-serpentine, kaolinite, calcite, and a small amount of granular pyrite, while the minerals in TSG raw coal were dominated by kaolinite, quartz, and pyrite. The raw coal samples were called TL6 and TSG12, respectively. The demineralization of raw coal was treated via a series of acid treatments (HF and HCl solutions) and was marked as LT6T and TSG12T, respectively. The detailed procedure for demineralization was described in previous work [5]. After demineralized treatment, the silicon-containing minerals were almost removed from raw coal.

Quartz, a typical silicon-containing mineral, was selected to explore the effect of the silicon-containing minerals on the coal pre-graphitization stage. To better observe its role, the quartz was ground into 200 mesh and then thoroughly mixed with the demineralized coal at the mineral mixing ratio of 10%, which were individually called LT6T-Q and TSG12T-Q.

2.2. High-Temperature Treatment

The final temperature of the high-temperature treatment was set as 1300 °C, 1450 °C, 1600 °C, 1900 °C, and 2800 °C, respectively. Every 5 g of the sample within a graphite crucible was placed in a medium-frequency induction graphitization furnace and heated to the final temperature. The heating rate was controlled at 10 °C/min and the temperature was maintained for 3 h after reaching the final temperature. To avoid interference with atmospheric factors such as air, argon was used to preserve the sample during the whole heating process. Then, treated samples were sealed and stored for further analysis and testing.

2.3. X-ray Diffraction

The XRD data collection was recorded by a D8 ADVANCE diffractometer with CuKα radiation. The operating conditions of the X-ray tube: I = 10 mA, U = 30 kV. Powdered samples were scanned from 10° to 80° in the 2θ range with a 0.02° step interval and a 0.012θ step width. The average lateral size (L_a), stacking height (L_c), and interlayer spacing (d₀₀₂) of the average crystallite structures have been established as XRD structural parameters. The d₀₀₂ was determined by Bragg’s Equation (1), and L_a and L_c were calculated by Scherrer Equations (2) and (3), respectively. The relevant equations are as follows:

$${\mathrm{d}}_{002}=\mathsf{\lambda }/2{\sin \mathsf{\theta }}_{002}$$

(1)

$${\mathrm{L}}_{\mathrm{c}}=0.89\mathsf{\lambda }/{\mathsf{\beta }}_{002}{\cos \mathsf{\theta }}_{002}$$

(2)

$${\mathrm{L}}_{\mathrm{a}}=1.84\mathsf{\lambda }/{\mathsf{\beta }}_{100}{\cos \mathsf{\theta }}_{100}$$

(3)

The λ is the wavelength of the radiation used, which is 0.154056 nm in this experiment. θ₀₀₂ and θ₁₀₀ are the positions of the peaks of 002 and 100, respectively, and β₀₀₂ and β₁₀₀ are the peak width at half height of the peaks of 002 and 100, respectively.

2.4. Raman Spectroscopy

Raman spectra were measured using a Renishaw Invia Raman spectrometer with microscopy, equipped with 10× and 50× objectives. Raman spectra were excited by an argon ion laser (532 nm) with the experiment power of 50 mW. The sample was scanned in the range of 100~3500 cm⁻¹. Each measurement was accumulated 10 times to reduce noise in average spectra, and the data acquisition time for each spectrum was 20 s. To obtain further data, peak separation Raman spectroscopy analysis was carried out using the Origin software.

2.5. SEM/EDS

SEM examination was used to observe the microstructure characteristics of samples under different heating temperatures by using a SU8220 field emission scanning electron microscope with a magnification of up to 800,000 times and an acceleration voltage of 3~5 kV. Then, the samples treated with 1300 °C, 1900 °C, and 2800 °C were observed by SEM.

3. Results and Discussion

3.1. Mineral Features at the Pre-Graphitization Stage

The XRD profiles of the selected samples are illustrated in Figure 1. Similar to the results reported by other authors [37,38,39], each XRD profile has two obvious peaks in the range of 10/15~30° and 40~50° (2θ), corresponding to the 002 and 100 peaks, respectively. The 002 and 100 peaks individually correlated to the stacking of aromatic layers and the extension of the aromatic molecules in the plane of the layers [40]. The 002 peak of the graphite is symmetrical and sharp [41], but for coal, the shape of the 002 peak was broad and asymmetric, and the intensity of the 002 peak was higher than that of the 100 peak, which was attributed to the γ peak on its left-hand side. The γ peak was associated with the aliphatic side chains or also assigned to the irregular packing of buckled aromatic layers [38,42]. As seen in Figure 1, all samples showed broad 002 and 001 peaks at 1300 °C. From 1300 °C to 1900 °C, the position of the 002 peak shifted to higher angles, the shape became sharper and the intensity increased with increasing temperature. When the temperature reached 2800 °C, only the characteristic graphite peaks—such as the 002, 100, 101, and 004 peaks—were observed in the XRD profile. The clear appearance of the (004) peak, a nearly three-dimensional crystalline order structure in coal-based graphite samples is supposed to exist [41]; however, the difference between the samples was the mineral peaks formed.

For LT6 (Figure 1a), the β-SiC and Fe₃Si peaks were found in the XRD profile at 1300 °C and existed almost in the range of 1300~1900 °C, while they disappeared at 2800 °C. However, there was only Fe₃Si in TL6T at 1300 °C and presented at 1300~1900 °C (Figure 1c). Up to 2800 °C, the XRD spectrum of LT6T also exclusively observed the characteristic peaks of graphite, which was similar to LT6. The XRD patterns of TSG12 showed significantly similar features to LT6 as the temperature increased (Figure 1b), whereas Fe was observed in TSG12T at 1300 °C and decomposed higher than 1900 °C (Figure 1d). For two quartz-added demineralized samples, their XRD patterns presented similar evolution trends with increasing temperature, shown in Figure 1e,f. It can be seen that β-SiC and Fe₂Si were characterized in both quartz-added samples at 1300 °C and were continuously observed at the pre-graphitization stage. Similarly, only graphite characteristic peaks were found at 2800 °C. Therefore, it can be inferred that the original minerals were transformed to form new silicon-containing minerals (such as SiC and Fe₃Si) during the pre-graphitization process, promoting the development of graphitization [11,32]. After demineralization treatment, the silicon was removed, and some Fe reacted with carbon to produce austenitic-like structures in TSG12T. As the temperature gradually increased, austenitic-like structures were removed and only the coal-based graphite was left in the reaction system. However, the Fe₃Si observed in LT6T may be attributed to incomplete demineralization. For quartz-added demineralized coal, SiC was generated by the reduction of quartz, and Fe₃Si was converted into Fe₂Si due to the excess of silicon at 1300 °C. This process also kept occurring within the pre-graphitization stage. When the temperature reached 2800 °C, the SiC peaks disappeared and the graphite peaks were clearly observed.

3.2. Chemical Structural Evolution during the Pre-Graphitization Stage

3.2.1. XRD Analysis

XRD is a non-destructive technique to determine the stacking structure in carbon materials for reflecting the graphitization degree of carbonaceous materials [39,43]. To obtain accurate structural parameters, the 002 and 100 peaks were peak-fitted and the calculated structural parameters (d₀₀₂, L_c, L_a) of different samples are shown in Figure 2.

The parameter d₀₀₂ is a measure of the perfection in the stacking structure periodicity [38]. The d₀₀₂ values of all samples showed a decreasing trend with the increase in temperature (Figure 2a,b). Compared with raw coal and demineralized coal, the d₀₀₂ of LT6 and TSG12 at the pre-graphitization stage was always correspondingly smaller than that of LT6T and TSG12T, respectively (Figure 2a). At 1900 °C, the d₀₀₂ of LT6 was closer to 0.3440 nm, making the beginning of the graphitization of coal [44]. Up to 2800 °C, the d₀₀₂ of LT6 and LT6 reached almost the same value, while the d₀₀₂ of TSG12T was slightly larger than that of TSG12. In Figure 3b, the d₀₀₂ of demineralized coals was also larger than that of the corresponding quartz-added coals at the pre-graphitization stage and then tended to be the same at 2800 °C. This indicated that silicon-containing minerals showed a positive effect on the reduction of interlayer spacing of the coal crystallite structure at the pre-graphitization stage. In addition, the d₀₀₂ of LT6 coal was relatively smaller at the same preparation conditions, suggesting that anthracite had a better promotion effect.

Figure 2c,d showed the change in L_c for different samples with increasing temperature. All coal samples had similar trends, in which the L_c increased slightly below 1900 °C, and then significantly increased above 1900 °C. For LT coal, the L_c of both LT6 and LT6T-Q was larger than that of LT6T at 1300~1900 °C, while their L_c was closed at 2800 °C. Similarly, the evolution of L_c for three TSG samples also showed the same trend. Therefore, the changes of L_c suggested that silicon-containing minerals were beneficial to the growth of stacking height at the pre-graphitization stage.

As seen in Figure 2e,f, at the pre-graphitization stage, the L_a of LT6T was larger than that of LT6T and closer to LT6T-Q, while the L_a of TSG12T was slightly higher than that of TSG12 and TSG12T-Q. This change indicated that silicon-containing minerals presented less effect on the lateral size of coal crystallites or slightly inhibited its extension at the pre-graphitization process.

3.2.2. Raman Spectroscopy Analysis

Raman spectroscopy has been widely used to provide important information for the degree of structural order of carbonaceous materials, and its characteristic spectra generally exist in the first-order region and second-order region, which corresponded to 800~2000 cm⁻¹ and 2400~3400 cm⁻¹, respectively [45,46,47,48,49,50,51]. The Raman spectra of all samples exhibited a similar evolution trend, as shown in Figure 3. In the first-order Raman spectrum, just one band at about 1580 cm⁻¹ (G band) for the single crystals of graphite was found and defined as the E_2g2 mode of graphite [52,53]. On the other hand, the spectrum of the graphite bar exhibits additional bands (D or “defect” bands), which are known to be characteristic of disordered graphite [54]. Compared with the G peak, the D band becomes more intense with an increasing degree of disorder in the graphitic structure [54]. As seen in Figure 3, the D₁ peak was clear, the shape of the D₁ and G peaks of samples were wider, and the intensity of the D₁ peak was also greater than that of the G peak at the pre-graphitization stage, indicating that the coal structure was disordered in this stage. Up to 2800 °C, the D₁ and G peaks of samples became sharp and the intensity of the D₁ peak significantly decreased, which was much lower than that of the previous stage, indicating that the coal-based graphite crystals with smaller particles possibly begin to form in the sample and the defects structures still existed [36]. The graphite mainly has three non-overlapping bands located at about 2480, 2670, and 3240 cm⁻¹ in the second-order Raman spectrum, indicating the second-order Raman spectrum of graphite is simple [50]. However, for the samples, it can be seen in Figure 3 that the second-order Raman spectra were complex and significantly different from that of the graphite, which separately were the 2D₁ peak (the overtone of the D₁ peak), the D₁+G peak (the combination of the D₁ peak and G peak), and the 2D₂ peak (the overtone of the D₂ peak). As the temperature increased, the 2D₁ peak shape gradually became narrower and more intense, which also indicated that the coal structure tended to be ordered with the increase in temperature. A similar result was reported by Xu et al. that the intensity of 2D₁ increased with the enhancement of coalification because the high-temperature treatment of coal was equivalent to the process of coalification [50]. At the same time, a new peak around 2450 cm⁻¹ was observed that is hard to interpret because its origin is still controversial. Li et al. [55,56] considered this a sign of the formation of graphitized structure in the structural evolution of carbon materials, while it was not present during the process of carbonization. As shown in Figure 2, it can be inferred that the existence of silicon-containing minerals in coal strongly promoted the degree of structural order. Raman spectra were peak-fitted to more accurately describe the differences in crystallite structure, and detailed assignments for various functional groups of coals are listed in

Table 2. Chemical shifts and assignment of Raman spectrum of coal [45,46,48,49,57].

Type	Shift/cm−1	Assignment
D1	1350	The ring breathing vibration in the graphite subunit or polycyclic aromatic hydrocarbon compounds (PAHs) or to aromatics with 6 or more rings
D2	1620	Disordered graphite lattice
D3	1500	Caromatic-Calkyl aromatic (aliphatic) ethers, C-C on hydro-aromatic rings, C-C on aliphatic structures or olefin-like structures
D4	1200	Caromatic-Calkyl; aromatic (aliphatic) ethers; C-C on hydro-aromatic rings; hexagonal diamond carbon sp3; C-H on aromatic rings
G	1580	The aromatic ring breathing in the graphene sheets, aromatic ring quadrant breathing, alkene C=C
2D1	2560	Overtone of the D1 band, C-C between aromatic rings, large aromatic rings system
D1+G	2860	Combination of D1 band and G band, large aromatic rings system
2G	3180	Overtone of the G band, the ring breathing vibration

[47,48,49,50]. The ID₁/IG (the peak area or integrated intensities) has been extensively used to evaluate crystalline or graphite-like carbon structures, and the ID₁/IG ratio normally decreases with an increasing extent of graphitization [57,58]. The band peak is a function of the peak intensity and the FWHM and is virtually a combined parameter of those two parameters [52,57]. Therefore, ID₁/IG characterized the peak area ratio between the D₁ peak and G peak in this paper, and the variation of ID₁/IG for different samples was calculated and shown in Figure 4.

For LT coal (Figure 4a,c), the ID₁/IG of LT6 significantly increased at the pre-graphitization stage and then rapidly decreased above 1900°C, whilst ID₁/IG of LT6T and LT6T-Q showed a decreasing trend at the pre-graphitization stage and also sharply decreased above 1900 °C. It was obvious that the ID₁/IG of LT6T-Q was always relatively smaller than that of LT6T. These changes indicated that the mixed minerals in raw coal would hinder the ordered arrangement of structures at pre-graphitization, but the existence of only silicon-containing minerals in coal would promote structural alignment and become more gradually ordered in this stage. In TSG coal (Figure 4b,d), the turning point of ID₁/IG with temperature occurred at 1600 °C for three samples, which showed that the ID₁/IG increased at 1300~1600 °C and then significantly decreased above 1600 °C. This indicated that the structural evolution of bituminous coal must first be spliced with disordered microcrystalline layers, which reduced the degree of order, and then the layers gradually became parallel and ordered again as the temperature increased [59]. Similarly, the ID₁/IG of TSG12T-Q was always relatively smaller than that of TSG12T. Therefore, the evolution of ID₁/IG could prove that silicon-containing minerals promoted structural rearrangement and improved the order of coal at the pre-graphitization stage.

3.3. Characteristics of Minerals and Morphology

The SEM images of raw coal and demineralized coal at different heating temperatures are presented in Figure 5. At 1300 °C (Figure 5a–h), the micromorphology of the sample was clear, mostly angular and dense massive structures. In addition, some irregular polyhedron structures with a few tiny particles were also found, whose EDS showed that C was the main element. In addition, there were more pore structures on the surface of demineralized coal, which may be caused by the minerals being dissolved during the demineralization. At 1900 °C (Figure 5i–p), the micromorphology of the sample was similar to that at 1300 °C, it was still dominated by clearly distinguishable massive structures. However, the difference was that the surface of the massive structures clearly showed layers at this stage. Up to 2800 °C (Figure 5q–x), similar to the SEM characteristics at other temperatures, it was still dominated by massive structures with porous or layered surfaces, and a small amount of polyhedral structure. Combined with XRD, d₀₀₂ at this temperature was extremely close to 0.3354 nm, which was the interlayer spacing of the ideal single-crystal graphite. It was inferred that most of the substances in the sample, except graphite, had evaporated at this stage, so it was not conducted for EDS analysis. Therefore, with or without demineralization, the whole micromorphology of the samples was dominated by massive structures, indicating that they did not change with temperature. Based on EDS results, it can be inferred that the formation of the few polyhedral structures observed may be related to the minerals. Meanwhile, based on the samples and test results, pyrite was present in both raw coals, and iron-containing minerals were detected in XRD and SEM of raw coal and demineralized coal, as well as in XRD of quartz-added demineralized coal. Therefore, it was inferred that the polyhedron structures may also be associated with the unremoved pyrite due to the insufficient dissolution of pyrite by the acid used for demineralization [11].

Figure 6 illustrates the SEM images of quartz-added samples. At 1300 °C (Figure 6a–d), the micromorphology of the samples was mainly massive structures, with sizes ranging from 10 to 40μm. In addition, some fine needle-like structures with different lengths were also observed in Figure 6b,d, in which the main element was C with less O and Si. With the increase in temperature, the massive structures were still the main micromorphology in coal with smooth or layered surfaces (Figure 6e–l). Some long columnar structures were also found at 1900 °C, and the constituent elements were unchanged, but the content of Si had a slight increase, as shown in Figure 6f,h. Up to 2800 °C, the long columnar structures were transformed into polyhedral structures (Figure 6i–l). Combined with XRD analysis, the needle-like structures can be attributed to the formation of the needle-like multilayer structures of quartz-SiC-carbon, in which the recrystallized quartz reacted with excess carbon to produce SiC growth based on quartz crystals and the surface of carbonaceous elements [19]. As the temperature increased, the reaction continued to produce SiC attached to the needle-like structures, increasing its length and diameter, and eventually producing the long columnar structure. At 2800 °C, SiC in samples had completely decomposed and the C elements in it were redeposited on the columnar structures to form the polyhedral structure.

3.4. Effect of Silicon-Containing Minerals on the Pre-Graphitization Stage

The comparison of raw and demineralized coals showed that the raw coals exhibited smaller d₀₀₂ and larger L_c values than the corresponding demineralized coal (Figure 2). Combining Raman and SEM analysis, it can be inferred that the existence of mixed mineral matter in raw coal gave rise to a decrease in interlayer spacing and an increase in stacking height and made it easier for the structure to rearrange into a more ordered structure. In order to further explore the influence of silicon-containing minerals, demineralized samples containing 10wt% quartz were conducted, and significant changes in its chemical structural characteristics and micromorphology were observed. It showed smaller d₀₀₂ and ID₁/IG, and higher L_c, while L_a presented a slight increase. The reduction of the quartz occurred at 1300 °C, forming needle-like SiC. With increasing temperature, the needle-like structures gradually grew in length and diameter, eventually producing the quartz-SiC-carbon multilayer long columnar structure. Above 1900 °C, SiC was decomposed by heat and the carbon-rich polyhedral structures became the main microstructure. These changes indicated that the formation and dissolution of SiC at the pre-graphitization stage was conducive to the movement and rearrangement of the coal molecular structure, resulting in the reduction of interlayer spacing, the development of longitudinal layer stacking height, and the enhancement of structural ordering. Therefore, the height of the aromatic planes grew during the pre-graphitization stage, and the width increased slightly [34,60].

4. Conclusions

In order to further explain the effect of silica-containing minerals on coal during the pre-graphitization stage, LT anthracite and TSG bituminous coal were selected as the subjects in this paper. Graphitized samples of raw coal, demineralized coal, and quartz-added demineralized coal were prepared via high-temperature treatments. XRD and Raman spectroscopy were applied to reveal structural change, and the micro-morphological features were observed by SEM. The effect of silicon-containing minerals on coal at the pre-graphitization stage was discussed by comparing the characteristics of different samples. The results showed that silicon-containing minerals played a positive role in structural evolution at this stage, as evidenced by the relatively smaller d₀₀₂ and ID₁/IG, and the relatively larger L_c and L_a of the samples with silicon-containing minerals. Meanwhile, SEM observation found that the main micromorphology of the samples at different temperatures were massive structures, and the increase in temperature did not change the basic morphology but only decreased its mineral content. A small amount of irregular polyhedral structures was also found at the pre-graphitization stage, which was formed by the reaction of silicon-containing minerals with carbon in samples and the continuous accumulation of quartz-SiC-carbon multilayer polyhedral structures. At higher temperatures, SiC was completely decomposed and the carbon element was redeposited to produce graphite. Therefore, it could be inferred that silica-containing minerals promoted structural rearrangement by forming SiC during the pre-graphitization stage, which was beneficial to improve the structural order, reducing the interlayer spacing and promoting the development of longitudinal layer stacking height, thereby accelerating the graphitization process. In addition, it was found that LT anthracite showed a relatively better graphitization effect.