A High-Temperature Thermal Simulation Experiment for Coal Graphitization with the Addition of SiO2

Abstract

The effect of SiO₂ on coal graphitization was investigated by adding SiO₂ as an additive to vitrinite in coal from the Gemudi mining area in Guizhou province (SW China) via a high-temperature heating treatment. The graphitization products of the samples were analyzed by X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and the influence of the SiO₂ additive on the process of coal graphitization was investigated. The results showed that, with the temperature increases, the graphitization degree of all samples was promoted, and the orderliness of the microcrystalline structure in the vitrinite increased. Compared with the samples without additives, the graphitization degree, graphite lamellae ductility, and stacking degree of the samples with SiO₂ additives were higher, and the carbon layer spacing reached 0.3379 nm at 3000 °C, entering the graphite stage. The Raman spectra showed that the peak intensities of the defect structures (D1 and D2) in the samples with SiO₂ were lower than those of the samples without additives, exhibiting fewer in-plane and interlayer defects in the samples with SiO₂. The microstructures of the experimental samples were observed by HRTEM; at the same temperature, the carbon layer stacking degree of the samples with the SiO₂ additives was higher than that of the samples without SiO₂, and large graphite lamellae with smoother and clearer edges were observed.

1. Introduction

Coal-based graphite is the main component of cryptocrystalline graphite and a type of coal-based nonmetallic mineral. Compared with crystalline graphite, coal-based graphite has enriched ore, a high grade, and easy exploitation, accounting for more than half of Chinese graphite production [1].

To explore the formation process and influencing factors of coal-based graphite, many scholars have studied coal-based graphite using laboratory simulation experiments [2,3], but the complexity of the structure and composition of coal and the diversity of controlling factors in the graphitization process make the study of the mechanism and evolution process of coal-based graphite mineralization problematic [4]. The main reason is the lack of understanding of the controlling factors in graphitization, such as the macerals, minerals, temperature, and pressure. The main controlling factors affecting the formation of coal-based graphite can be divided into two categories: internal factors (material composition) and external factors (temperature and pressure) [5]. Due to the complexity of the conditions of graphite formation and the graphitization process, the evolution rate, structural changes, and degree of graphitization vary among the different macerals; that is, there is the “differential graphitization phenomenon” [6,7].

With the gradual progress of coal graphitization research, the influence of inorganic minerals on the evolution of coal-based graphite has been emphasized. Previous studies have found that coal containing compounds of sulfur [8], manganese, iron [9], and calcium [10] was more likely to be graphitized. From the perspective of a structural analysis, some scholars believed that “shell-like carbon layers” were formed around the mineral particles, which were broken down under stress to form flat graphitic carbon layers [11,12]. Bustin [13] believed that mineral particles can transmit tectonic shear stress to the reaction site. It is very difficult to accurately study the graphitization process and influencing factors through the formed natural coal-based graphite. Therefore, some scholars have used simulation experiments to produce coal graphite to investigate the influence of minerals on coal graphitization. Noda [14,15] conducted a simulation experiment at temperatures of 600–850 °C and pressure of 0.32 GPa from the perspective of chemical reactions and proposed that calcium-containing mineral particles could directly participate in the graphitization process as reaction catalysts. Nyathi [16] found that the presence of aluminum-containing clay minerals retarded coal graphitization by simulating Pennsylvania anthracite at 3000 °C. The significant influence of minerals on graphitization has been well-demonstrated by previous simulations, but the research is not sufficiently comprehensive, and the effects of many minerals on graphitization have not yet been studied.

The common minerals in coal are quartz, clay, carbonate, and sulfide, among which SiO₂ widely exists in quartz and kaolinite. SiO₂ is chemically stable, with a melting point of about 1600 °C and a boiling point of about 2230 °C. It has a large specific surface area and high thermal stability, and is commonly used in the preparation of catalyst carriers; additionally, it can directly affect the activity, selectivity, and stability of catalytic reactions under certain conditions [17]. Gonzalez [18] and Pappan [19] conducted simulations on anthracite coal with a continuous temperature increase and discovered the promoter action of graphitization by quartz as well as an explanation of its catalytic mechanism: at temperatures up to 1800 °C, liquid quartz reacts with carbon to produce silicon carbide. Above 2600 °C, silicon carbide decomposes to produce silicon monomers and carbon monomers. The cubic crystalline silicon carbide is similar in structure to graphite and has the effect of promoting the formation of graphite.

Previous studies on the mechanism of the influence of minerals on graphitization mainly have two deficiencies. First, the effect of the differences between the different macerals in coal on the experimental results was neglected [20,21]. Each maceral should be individually tested and analyzed [22]. Second, by directly conducting simulation experiments on raw coal, there may be reactions among minerals in coal ash [23,24], which affects the coal graphitization products and obscures the real influence on the graphitization mechanism [25]. Instead, each mineral should be separately analyzed to exclude the influence of other minerals on graphitization to the greatest extent possible.

The purpose of this study was to research the effect of coal minerals on graphitization by conducting high-temperature simulation experiments. The lean coal collected from the Gemudi mining area in Guizhou province (SW China) was selected as the research object, and vitrinite-rich samples were obtained after hand selection to avoid the influence of the inertinite in the coal [5,26]. SiO₂ powder was selected as an additive to be mixed for the high-temperature simulation experiments, and the post-experimental samples were analyzed using various modern instruments to investigate the effect of SiO₂ in the coal graphitization process.

2. Experimental Samples and Methods

2.1. Selection and Processing of Experimental Materials

2.1.1. Experimental Sample

The coal samples were collected from the Gemudi mining area in Guizhou province (SW China) and had a high vitrinite content. SiO₂ powder with a particle size of 200 mesh was selected as the additive. In order to exclude the influence of the inertinite in the coal on the experiment, the coal samples were hand-selected to obtain vitrinite-enriched samples. According to the Chinese standard GB/T 15588-2013 [27] and GB/T 8899-2013 [28], the obtained samples were observed under a microscope, and the samples with 90% or higher vitrinite purity were used as raw materials for the simulation experiment. The results of the proximate analysis (GB/T 212-2008 [29]) and ultimate analysis (GB/T 476-2001 [30]) are shown in Table 1.

2.1.2. Demineralization

The purpose of this experiment was to explore the influence of SiO₂ on the graphitization of coal; however, the mineral components in raw coal are very complex, so the influence of the original minerals in the coal on the experiment should be excluded to the greatest extent possible. As a result, the raw coal required for the experiment should be acid-washed and demineralized to remove the other minerals contained in the coal so that interference factors in the experiment can be avoided [31].

The raw coal was crushed to 200 mesh (75 μm), and 15 g of coal powder was weighed into a plastic beaker and mixed with 80 mL of HCl solution (36% by mass); then, the mixture was stirred in a water bath at 60 °C for 4 h. After this, the HCl solution was filtered out, and 80 mL of the HF solution (40%) was added to the remaining coal. The water bath and acid-washing operations were repeated until the filtrate, after washing with the AgNO₃ solution test, had no precipitates. Then, the acid-washed coal was filtered out with a filter paper and put into an oven at 60 °C for 24 h to obtain demineralized coal.

2.2. High-Temperature Simulation Experiment

2.2.1. Experimental Equipment

An integrated laboratory graphitization furnace (NTG-SML-60W, NTG-SML-60W, Nuotian electric heating technology, Zhuzhou City, China) was used in this experiment. The specific experimental steps were as follows: Every dish was placed in the graphitization furnace after being loaded with 5 g of vitrinite, and the gas was replaced once with a vacuum degree of 5 Pa before heating; then, the whole process was protected by argon with a flow rate of 10 L/min. This experiment used the sectional heating method: the temperature was first raised to 1000 °C at a heating rate of 5 °C/min and kept for 60 min. Then, the temperature was raised to the target temperature point at a rate of 10 °C/min and kept for 90 min. The four experimental temperature points were set at 2100 °C, 2400 °C, 2700 °C, and 3000 °C.

2.2.2. Experimental Scheme

A group of additive-free vitrinite samples (G) and a group of SiO₂-added samples (GS) were set up for comparative analysis. The four samples in each group correspond to the four temperature points, and the mass of each sample was 5 g.

In GS sample proportioning, too much additive can affect the subsequent analysis and tests and can make it difficult to reveal regular patterns, whereas a smaller amount of additive does not have a catalytic and inhibitory effect. Therefore, the ratio of vitrinite to SiO₂ was selected as 7:3 in this experiment [32] in order to investigate the effect of SiO₂ on graphitization during the high-temperature simulation experiment. The experimental scheme adopted is shown in Table 2.

2.3. X-ray Diffraction Analysis (XRD)

SmartLab-9kW (Rigaku Corporation, Tokyo, Japan) was used as the XRD test instrument. Copper targets with a 45 kV accelerating voltage and a 200 mA current were chosen. The scan range was set as 2θ from 5° to 70°, with a scan rate of 2°/min and an X-ray wavelength of 0.15418 nm. Two diffraction peaks (2θ range 20°–30° and 40°–50°, respectively) on the XRD pattern matched the positions of the 002 and 100 peaks in the standard graphite XRD diffraction pattern. The lattice parameters (carbon layer spacing d₀₀₂, diameter La, and stack height Lc) were calculated based on Bragg’s equation and Scherre’s formula using Jade software (Materials Data, Livermore, CA, USA) [33,34].

XRD is a common method used to study microcrystalline structures in coal. d₀₀₂, Lc, and La can be obtained by the following method from the spectrum.

(1) d₀₀₂ is the average interlayer spacing of crystallites, which can be obtained according to the Bragg formula; the formula is shown as follows:

d₀₀₂ = λ/2sinθ₀₀₂(1)

In the formula, λ is the wavelength of the X-ray, λ = 0.154056 nm; θ₀₀₂ is the diffraction angle corresponding to the 002 peak, in units of degrees.

(2) Lc is the average height of crystallites in the c-axis direction; the calculation formula is shown as follows:

Lc = 1.05λ/β₀₀₂cosθ₀₀₂(2)

In the formula, β₀₀₂ is the half-width of the 002 peak.

(3) La is the average diameter of the crystallites, which can be obtained according to the Scherrer crystallite size calculation formula:

La = 1.84λ/β₁₀₀cosθ₁₀₀(3)

In the formula, β₁₀₀ is the half-width of the 100 peak, and θ₁₀₀ is the diffraction angle corresponding to the 100 peak position, in units of degrees.

2.4. Laser Raman Spectroscopy (Raman)

Raman spectroscopy is often used to analyze organic matter [35]. A Horiba LabRAM HR Evolution model high-resolution micro-Raman spectrometer (Horiba, Kyoto, Japan) was used in the Raman spectroscopic experiment. A Nd:YAG (532 nm) laser was used as the excitation light source in the experiment, with a laser power of 100 mW, scanning range of 800 cm⁻¹–3500 cm⁻¹, and exposure time of 10 s. The fitting and processing analyses were conducted with the obtained Raman spectra using the Lorentz function in Origin8.0 software (OriginLab, Northampton, US).

The measured Raman spectra were found in two regions, which were the first-order Raman (700 cm⁻¹–2000 cm⁻¹) and the second-order Raman (2000 cm⁻¹–3000 cm⁻¹). The primary Raman could be divided into four types of defect peaks (D1–D4 peaks) and one ordered graphite peak (G peak). The secondary Raman spectrum contained only two peaks (S1 and S2) at the low evolutionary stage, and as the evolution increased, the S1 peak gradually split into two peaks and the S2 peak disappeared [36].

Defect peaks caused by lattice defects and disordered carbon structures (active structures) within the graphite layers are called D peaks [37]. According to different causes and positions, it can be divided into 1350 cm⁻¹ (D1), 1620 cm⁻¹ (D2), 1500 cm⁻¹ (D3), and 1200 cm⁻¹ (D4), whereas the D3 and D4 peaks only appear in samples with high disorder [38]. The secondary Raman spectrum at 2700 cm⁻¹ (S1 peak) is related to the structural order of its three-dimensional lattice. The higher the order, the higher the splitting of its secondary peaks [39].

The quantitative evaluation parameters of Raman spectroscopy for the evaluation of carbon ordering have been studied in detail in previous research. The peak position difference between the D1 and G peaks (P) can be expressed as P(G)-P(D1); the full width at half maximum of the G peak can be expressed as FWHM (G); the intensity ratio of the D1 and G peaks can be expressed as R1 = ID1/IG; the area ratio of the D1 peak can be expressed as R₂ = AD1/A(G + D1 + D2) [40]; and the ratio of all the types of defect peaks to the total area in the first-order Raman can be expressed as R₃ = A(D1 + D2 + D3 + D4)/A(D1 + D2 + D3 + D4 + G). These parameters have been used to characterize the degree of defects in the structure or the degree of order of carbon materials [41].

These parameters showed a good positive correlation in the samples with a high degree of graphitization, but the correlation of some parameters was not obvious in the samples with a lower degree of graphitization. Therefore, the parameters R₂ and R₃ were used in this study to characterize the degree of structural defects in the samples; R₂ was called the “in-plane defect parameter” to characterize the proportion of in-plane defects (D1), which has high evaluation significance for high-graphitization-degree samples with fewer defects and mainly D1-type defects; R₃ was called the “full defect parameter” to characterize the percentage of all types of defects, which had high physical significance for low-graphitization-degree samples with more defects [42].

2.5. High-Resolution Transmission Electron Microscopy (HRTEM)

A Tecnai G2 F30 field emission transmission electron microscope host (FEI, Portland, OSU, USA) was used in the high-resolution transmission electron microscope (HRTEM) experiment with an accelerating voltage of 300 kV, point resolution of 0.20 nm, line resolution of 0.10 nm, and 0.14 nm information resolution, with a 3000× to 500,000× fold magnification. The specific method was conducted as follows: the samples were ground to 300 mesh, ultrasonically dispersed in ethanol, and then dropped on the microgrid copper mesh. Afterward, the sample was searched for overheads on the holes of the microgrid copper mesh; the particles that had the most particle characteristics were selected in the sample for multiscale observation, and high-resolution images were taken.

3. Results

3.1. X-ray Diffraction Results (XRD)

The GS and G samples after the experiment were analyzed by XRD (Figure 1). The interlayer spacing (d₀₀₂), average height of the crystallites (Lc), and average diameter of the crystallites (La) were calculated (Table 3).

By observing the XRD pattern and analyzing the lattice parameters, the following conclusions were initially obtained:

① For the same precursor, the peaks located near 2θ = 26° moved closer to the position of the standard graphite peak (2θ = 26.6°) as the experimental temperature increased, and the peak shape became sharp and symmetrical.

② The G samples showed a strong nonhomogeneous graphitization phenomenon at 2100 °C–2700 °C; the peak near 2θ = 26° exhibited asymmetry, with an ordered carbon-graphite peak (002 peak) and a disordered carbon peak inside [43], reflecting the coexistence of ordered and disordered structures in the samples. This indicated that the graphitization was nonhomogeneous inside and that a large amount of disordered carbon existed. In contrast, the GS samples exhibited nonhomogeneous graphitization only at 2100 °C, and the peaks around 2θ = 26° tended to be symmetrical from 2400 °C, with a significantly decreased internal disordered carbon content.

③ Using d₀₀₂ as a measure of the graphitization degree for evaluation, the GS samples had lower d₀₀₂ values than the G samples at the same temperature. The participation of SiO₂ obviously had a positive catalytic effect on graphitization.

④ Compared with the G samples without additives, the structure of the GS samples appeared significantly ordered at 2100 °C; after 2400 °C, the full width at half maximum (FWHM) of the peak near 2θ = 26° was significantly reduced, and an obvious sharp, symmetric graphite peak appeared. The corresponding d₀₀₂ values were all decreased to below 0.340 nm, approaching 0.338 nm; when the temperature reached 3000 °C, the d₀₀₂ of GS-4 was below 0.338 nm, indicating that the sample had entered the graphite stage [44].

⑤ At the same temperature, the GS samples had sharper 002 peaks relative to the G samples, with a smaller FWHM and lower asymmetry. At 2700 °C, the 002 peak of GS-3 was sharp and symmetrical, indicating the formation of perfect graphite crystals. In contrast, the 002 peaks of G samples were still less symmetrical at 2700 °C and only tended to be symmetrical at 3000 °C.

⑥ A sharp peak was found in the XRD pattern at 2100 °C for the GS samples at about 2θ = 35.5°, which was presumed to be the SiC diffraction peak after consulting the XRD card. The intensity of this peak significantly decreased with an increasing temperature and disappeared at 2700 °C.

3.2. Raman Results

The Raman spectra of the samples at different temperatures were fitted to the split peaks (Figure 2), and the relevant structural characterization parameters were calculated (Table 4). Through the Raman spectrograms and a comparative analysis of the data, the following conclusions were initially drawn:

① With the increasing experimental temperature, the G peak, which characterizes the ordered graphite structure, gradually increased in intensity and approached 1580 cm⁻¹, whereas the intensity of the D peak, which characterizes the disordered structure, decreased. The defect peaks of the G samples were mainly D1 peaks; the defect peaks of the GS samples were dominated by D1 peaks below 2700 °C and by D2 peaks after 2700 °C. The D2 peaks did not completely disappear even in the samples with a high graphitization degree. There were no D3 or D4 peaks representing other heterocyclic defects found in the samples.

② As the temperature increased, in the second-order Raman, the S1 peak gradually separated into two peaks, S1’ and S1’’, and the S2 peak gradually decreased. The S2 peak disappeared at 3000 °C for the G samples and at 2700 °C for the GS samples, indicating that the three-dimensional structure gradually formed and became perfected.

③ The Raman parameters (R₁/R₂/R₃) showed a decreasing trend with an increasing temperature. Compared with the G samples at same temperatures, the GS samples had lower R₁/R₂/R₃ values, which proved that the GS samples had a higher degree of graphitization and better healing of layer and interlayer defects.

④ At 3000 °C, the D1 peak disappeared, and the R₁/R₂ value decreased to 0 for GS-4, reflecting the complete healing and disappearance of defects within its graphite layers and the formation of a highly ordered structure.

3.3. High-Resolution Transmission Electron Microscopy (HRTEM)

The formation of carbon layers and the state of defects can be observed by HRTEM [45,46]. Through the HRTEM test of the experimental samples, it was observed that, with the increase in the temperature, the graphite sheets were gradually formed and perfected, whereas the length of the carbon layers and stack thickness were gradually increased.

At 2100 °C, the carbon layers in the G and GS samples started to stack with high curvature and different development directions. Compared with G-1, the carbon layer length and stack thickness were larger in GS-1. Both of them were still dominated by disordered structures (Figure 3a,b).

At 2400 °C, large-sized graphite sheets started to form in GS-2, but there were many defects overall, with a low stacking thickness at the edges and many disordered carbon layers still existing inside. The length of the carbon layers in G-2 developed with an increasing temperature, but the overall structure was still disordered (Figure 3c,d).

At 2700 °C, large graphite sheets formed in GS-3 (Figure 4a), with clear and straight graphite sheet edges, and it was observed that multiple graphite sheets were stacked together in a step-like manner (Figure 4b), reflecting good orientation, but the carbon layers in some graphite sheets were still rough. In G-3, the formation of graphite sheets was also observed, with curved and blurred edges, and there were a large number of disordered carbon layers inside. At the same time, carbon layers similar to those in G-2 were observed at some locations in G-3, indicating the existence of inhomogeneous evolution inside.

At 3000 °C, graphite sheets in GS-4 were further developed, and the stack thickness of the internal carbon layers increased (Figure 5a). Additionally, graphite sheet outcrops were observed, with straight edges and clear angles at the corners (Figure 5b). The graphitization in G-4 was also improved, which showed more homogeneous evolution and a large number of graphite sheets were produced. However, the graphite sheets in the G-4 sample still had a high defect degree; most of the graphite sheets showed curved edges and blurred corners, the internal carbon layers were poorly oriented, and the stack thickness was lower than that of GS-4 (Figure 5c,d).

Oberlin [12] found that the stages of graphitization include stacking, alignment, dewrinkling, and extension. Qin [47] studied the evolution of macromolecular structural units in coal and found that in the later stage of coalification, carbon layers will occur: splicing, stacking, and collocation of both (splicing–stacking). In the graphitization stage, stacking and splicing–stacking are the main processes. In this experiment, the stacking of carbon layers can be observed at 2100 °C in the G samples, and the number of stacked layers kept rising with the increase in the temperature, whereas simple splicing–stacking between carbon layers occurred and made the carbon layers extend; after 2700 °C, graphite sheets formed by complex splicing–stacking can be found in the G samples.

The graphitization degree of the GS samples was higher than that of the G samples, and graphite sheets had been formed at 2100 °C. During the heating process, the interior was mainly based on the process of eliminating defects. As the temperature increases, the edges of the graphite flakes became straighter and more orderly overall.

In the G samples, the change of carbon layers reflected the occurrence of splicing–stacking, which was consistent with Qin’s study. In the GS samples, the carbon layers mainly underwent the defect elimination and dewrinkling found in Oberlin’s study.

4. Discussion

The structural evolution during coal graphitization is embodied in two aspects: one is the construction of a three-dimensional graphitic structure, characterized by the decrease in carbon interlayer spacing (d₀₀₂) and the increase in crystallite sizes (La and Lc), and the other is the elimination of lattice defects, measured by Raman parameters (R₂ and R₃). The evolution process of the structural parameters (d₀₀₂, La, Lc, R₂, and R₃) of the G and GS samples with the temperature was drawn as curve diagrams, which revealed the transformation process of the three-dimensional disordered coal structure to an ordered graphite structure with the increase in the temperature, and, in this way, the role of SiO₂ in the coal graphitization process can be explained.

4.1. Carbon Interlayer Spacing Evolved with Temperature

The following rules were found using d₀₀₂ as a standard to evaluate the graphitization degree of the high-temperature experimental samples.

During the whole evolution process, at the same temperature, the d₀₀₂ values of the G samples were higher than those of the GS samples, showing a lower graphitization degree.

Although the graphitization degree of the G samples was lower than that of the GS samples, the evolution rate of the G samples was faster. The d₀₀₂ gap between the G and GS samples gradually decreased with the increase in the temperature.

During the experiment, the d₀₀₂ of the G samples uniformly decreased with the increase in the temperature, whereas the GS samples had the fastest evolution rate between 2100 °C and 2400 °C, and the evolution rate gradually decreased with the increase in the temperature; at 2400 °C–2700 °C, the decrease rate of d₀₀₂ was the lowest, and the GS samples evolved the slowest; at 2700 °C–3000 °C, the decrease rate of d₀₀₂ gradually increased, and the d₀₀₂ of the GS samples gradually advanced to 0.3380 nm, which represents the boundary between graphite and semi-graphite, and reached 0.3379 nm at 3000 °C, implying the formation of standard graphite (Figure 6).

By plotting the curve of the d₀₀₂ difference between the G and GS samples as a function of the temperature (Figure 7), it was found that d₀₀₂ gradually decreased and the reduction rate of the d₀₀₂ difference gradually increased with the increase in the temperature, which reflected that the catalytic effect of SiO₂ was gradually weakening with the increase in the temperature. It is presumed to be related to the following two points: (1) When the temperature reaches above 2200 °C, part of SiO₂ may escape out in gaseous form with argon, making the content of SiO₂ decreasing and its influence on graphitization weakening. (2) Using d₀₀₂ as a measure of the graphitization degree for evaluation, the GS samples had a higher graphitization degree than the G samples at 2100 °C, and the energy requirement to continue the graphitization process was also higher than that of the G samples; but the energy provided to G and GS in this experiment was the same, so the graphitization rate of the GS samples was lower than that of the G samples.

By comparing the XRD diffraction curves of the G and GS samples, it was observed that the 002 peak of the G samples had stronger asymmetry, reflecting the nonhomogeneous graphitization phenomenon of the G samples (the state of coexistence of graphite and nongraphitic carbon). With the increase in the temperature, the 002 peak tended to be sharp and symmetrical, and the asymmetry gradually decreased. Okabe [43] also found this phenomenon in high-temperature heat treatment of charcoal and thermosetting resin, which was explained as a result of uneven internal stress release due to the internal expansion of carbon crystals at high temperatures, thus exhibiting a nonhomogeneous spatial location of graphitization (Figure 8).

The G samples had stronger nonhomogeneous graphitization phenomena as compared with the GS samples, showing a nonhomogeneous evolutionary nature in the spatial perspective (at different positions) and a homogeneous evolutionary nature in the temporal perspective (at different temperatures); this was mainly due to the relatively consistent bridge bonds and low bond energy of vitrinite, whereas the GS samples were affected by SiO₂, resulting in a non-uniform evolution rate.

4.2. Lattice Defects Evolved with Temperature

The R₃ values for the G and GS samples showed similar evolutionary trends. The defect degree of the graphite lattice rapidly decreased with the increase in the temperature, but the reduction rate and evolutionary trajectory were not identical for the G and GS samples (Figure 9).

① Due to the high graphitization degree of the experimental samples, no D3 or D4 defect peaks existed; therefore, the R₂ values representing the in-plane defect degree and the R₃ values representing the overall defect degree showed a tendency to decrease, reflecting the disappearance of lattice defects.

② With the temperature increases, at 2100 °C–2700 °C, the reduction rate of in-plane defects (R₂) of the GS samples was always higher than that of the G samples; at 2700 °C–3000 °C, the R₂ values of the G samples decreased faster than those of the GS samples, but the D1 peak of GS-4 disappeared at 3000 °C and the R₂ value reduced to 0, reflecting the complete disappearance of in-plane defects. Throughout the experiment, the R₂ values of the GS samples were substantially lower than those of the G samples, which reflected the stronger defect healing ability of the GS samples.

③ The decreasing trend and rate of R₃ values were essentially the same for the GS and G samples, but the GS samples had a lower overall defect degree.

④ The increase in the temperature would obviously promote the reduction of R₃ values. However, there was a bottleneck in the defect reduction process. Below 2700 °C, the R₃ reduction rate gradually decreased with the increase in the temperature, but when the temperature exceeded 2700 °C, the R₃ values rapidly decreased, and the defects in the graphite lattice gradually disappeared with the increase in the temperature.

The R₃ parameter can reflect the overall defect percentage inside the sample lattice. By plotting the curve of the R₃ difference between the G and GS samples as a function of the temperature (Figure 10), it was found that the overall trend of the curve was flat, and the response of the R₃ difference was not sensitive to the temperature change, which only improved from 0.157 to 0.199. SiO₂ showed the ability to significantly promote the healing of lattice defects during the warming process, but this catalytic ability was slightly improved with the changes in the temperature.

4.3. Graphite Crystallite Structure Parameters Evolved with Temperature

By establishing regular graphs of La and Lc with the temperature (Figure 11), the influence of SiO₂ on the graphite structure evolution process of vitrinite under high temperatures was revealed, and, thus, the mechanism was also revealed:

① The evolution trends and rates of La values of GS and G samples were basically the same. The La values of the GS samples were always higher than those of G, which reflected that SiO₂ had a significant effect on increasing the diameter of graphite crystallites, and the difference between the La values of the G and GS samples was basically unchanged, indicating that the ability of SiO₂ to increase the stack height of graphite crystallites did not change with the temperature.

② At 2100 °C–2400 °C, the Lc values of the GS samples rapidly increased, and the Lc gap with the G samples reached the maximum at 2400 °C; but after 2400 °C, the Lc growth rate significantly decreased and slowly grew. The Lc gap with the G samples at 3000 °C was no longer obvious, presumably related to SiO₂.

③ Similar to the Raman parameters (R₃), there was also a bottleneck in the evolution of the graphite crystallite structure parameters (La and Lc) with the temperature located at 2700 °C. Above this temperature, the graphite crystallite structures of the G and GS samples rapidly developed, and La and Lc quickly increased.

The regular curve was drawn for the difference in the La/Lc of the G and GS samples as a function of the temperature (Figure 12). In the process of promoting graphite structure formation, SiO₂ mainly promoted the ductility of the crystallites and increased the crystal size. The higher the temperature, the better the catalytic performance.

While SiO₂ showed different effects on the stacking degree, the Lc difference gradually increased before 2400 °C and rapidly decreased after 2400 °C. After the temperature reached 3000 °C, the Lc difference decreased to 0.47, with SiO₂ basically showing no catalytic effect on carbon layer stacking.

4.4. The Mechanism by Which SiO₂ Affects Graphitization

SiO₂ has a melting point of about 1600 °C and a boiling point of about 2230 °C, so it existed in a molten–gaseous state during this experiment. The mechanism of its influence on graphitization should also be discussed in stages.

4.4.1. Catalysis of Graphitization by Molten SiO₂

Powdered SiO₂ changed into a molten state under high temperatures and mixed with the vitrinite. Pappano [19] proposed that the attachment of quartz to the surface could facilitate the growth of graphite crystals. Therefore, it is speculated that, after the vitrinite and SiO₂ were mixed in molten form, the carbon attached to the surface of SiO₂, which enlarged the specific surface area and caused the heated area to increase. At the same temperature, the GS samples obtained more energy than the G samples. According to XRD, the internal graphitization of the GS samples was more homogeneous, reflecting more uniform internal heating and more efficient energy transfer.

The XRD data also confirmed this view: the graphitization degree of the GS-1 sample (2100 °C) was even higher than that of the G-4 sample (3000 °C). The participation of molten SiO₂ enhanced the energy-harvesting ability of the GS samples, allowing a higher graphitization degree achieved under the condition of a lower energy supply. Additionally, the 002 peaks of the GS group samples were highly symmetrical, and the spatial uniformity of their graphitization was significantly higher than that of the G group samples, reflecting the fact that the internal heating was more uniform, and the overall graphitization degree was higher.

4.4.2. Catalysis of Graphitization by Gaseous SiO₂

Gonzalez [18] concluded that coal-based minerals could improve the graphitization properties of anthracite. Pappano [19] suggested that when the temperature reaches 1800 °C, molten quartz would react with carbon to form SiC, and SiC will be decomposed to form elemental silicon and elemental carbon above 2600 °C. Qiu [48] noted that the quartz contained in raw coal reacts with carbon above 2000 °C to produce the intermediate product SiC, and SiC will be decomposed to form graphite at 2600 °C.

Combined with previous research results and these experimental data, the following evolutionary mechanism is proposed (Figure 13):

① Below 2100 °C, molten SiO₂ reacted with amorphous carbon to form SiC (SiO₂ + 2C → SiC + CO₂), and a distinct SiC peak was observed at 35.5° in the XRD diffraction pattern. This was one of the reasons for the high symmetry of the 002 peak shape: part of the amorphous carbon was consumed in the reaction, causing a significant decrease in the disordered carbon peak intensity.

②Above 2230 °C, SiO₂ reached its boiling point and vaporized, and most of it escaped out of the furnace with the inert gas. A small amount of quartz remained in the furnace and continued to react with amorphous carbon in the form of a gas phase. At this time, the amount of SiC formation was greatly reduced, and the previously generated SiC started to decompose into graphite and silicon when subjected to continuous high temperatures (SiC → Si + C). As a result, the intensity of the graphite peak in the XRD pattern of GS-2 was enhanced, whereas the intensity of the SiC peak was significantly reduced. In addition, with the escape of SiO₂, its catalytic effect on graphitization gradually decreased, and the gap between the graphitization of the G and GS samples gradually decreased.

③ In the XRD pattern at 2700 °C, the SiC peak in GS-3 completely disappeared. The previously formed SiC was completely decomposed to form graphite and silicon, then the generated silicon continued to react with amorphous carbon to form SiC; the cycle continued until the amorphous carbon was exhausted or the elemental silicon was completely carried out with the inert gas.

5. Conclusions

In this study, vitrinite in coal from the Gemudi mining area in Guizhou province was used as the raw material, and, after demineralization treatment, the coal-based graphite was prepared by a high-temperature simulation experiment at 2100–3000 °C with the addition of SiO₂ additives as a variable. The experimental results showed that the SiO₂ in the high-temperature experiment had a great influence on the microstructure of coal-based graphite and played a catalytic role. After analyzing the XRD, Raman, and HRTEM results, the following conclusions can be drawn:

(1) Temperature played a crucial role in the formation and growth of graphite structures and dominated the process of high-temperature graphitization. High temperatures were conducive to the stacking of graphite sheets and the development of carbon layers, and promoted the healing of defects in the graphite lattice; thus, graphite with relatively perfect crystal structures was obtained.

(2) At the same temperature, the graphitization degree of the samples containing the SiO₂ additive (GS) was always higher than that of the samples without additives (G). The G samples contained a large amount of amorphous carbon inside, as shown by the high asymmetry of the 002 peaks, whereas the GS samples had a sharper and symmetrical 002 peak shape, exhibiting smaller carbon layer spacing (d₀₀₂), better crystallite structure (La/Lc), and a smaller lattice defect ratio (R₂/R₃).

(3) The catalytic performance of SiO₂ was evaluated according to the change in the microstructure: ① SiO₂ showed an obvious and stable catalytic effect on the healing of lattice defects, and its catalytic ability slightly increased with temperature change. ② During the crystallite structure formation, SiO₂ significantly promoted the development and growth of the crystallite plane, and its catalytic ability improved with the temperature increase.

(4) At 1600 °C–2230 °C, this stage was dominated by physical catalysis: SiO₂ created adhesion conditions for carbon in the form of a molten state, expanding the heat area of carbon and improving the energy transfer efficiency. Thus, vitrinite reached the high-temperature graphitization stage under the condition of a lower energy supply.

(5) After 2230 °C, this stage was dominated by chemical catalysis as shown in the following reaction: SiO₂ reacted with amorphous carbon and formed SiC (SiO₂ + 2C → SiC + CO₂); SiC was thermally decomposed to form elemental silicon and graphite (SiC → Si + C), and then the formed elemental silicon continually combined with amorphous carbon (Si + C → SiC). The reaction was repeated until the complete depletion of amorphous carbon or silicon, thereby improving the graphitization degree.

Author Contributions

Data curation, G.C. and M.Z.; formal analysis, G.C. and Z.L.; methodology, A.W.; resources, A.W. and Y.W.; supervision, D.C.; writing—original draft, G.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data available on request due to restrictions.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

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Figure 1. XRD pattern of experimental samples.

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Figure 2. Raman spectra of experimental samples. (red line: fitted curve; green line: original curve).

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Figure 3. HRTEM characteristics of G/GS at 2100 °C–2400 °C. (a) 2100 °C GS-1; (b) 2100 °C G-1; (c) 2400 °C GS-2; (d) 2400 °C G-2.

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Figure 4. HRTEM characteristics of G/GS at 2700 °C. (a) 2700 °C GS-3; (b) 2700 °C GS-3; (c) 2700 °C G-3; (d) 2700 °C G-3.

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Figure 5. HRTEM characteristics of G/GS at 3000 °C. (a) 3000 °C GS-4; (b) 3000 °C GS-4; (c) 3000 °C G-4; (d) 3000 °C G-4.

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Figure 6. d002 of samples with changes in temperature.

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Figure 7. d002 differences of G and GS sample evolution characteristics with temperature.

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Figure 8. Non-uniform graphitization phenomenon in XRD spectrum ((left): thermoset resin carbon [43]; (right): 2100 °C, G-1).

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Figure 9. Raman parameter (R2 and R3) evolution characteristics with temperature.

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Figure 10. Differences in R3 of G and GS sample evolution characteristics with temperature.

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Figure 11. XRD parameter (La and Lc) evolution characteristics with temperature.

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Figure 12. Difference in La/Lc of G and GS sample evolution characteristics with temperature.

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Figure 13. SiO2 mechanism diagram.

Table 1

Proximate analysis and ultimate analysis of vitrinite in Gemudi.

Sample Collection Site	Vitrinite Rmax/%	Component			Proximate Analysis				Ultimate Analysis
		Vitrinite	Inertinite	Exinite	Mad/%	Ad/%	Vdaf /%	FCd/%	St /%	Odaf/%	Cdaf/%	Hdaf/%	Ndaf/%
Gemudi	1.7	92.4	7.6	0	0.72	5.72	17.03	78.22	0.74	2.38	90.88	3.98	1.88

Table 2

High-temperature simulation experimental scheme.

	Number
Temperature	Additive-Free Vitrinite Samples	SiO2-Added Samples
2100 °C	G-1	GS-1
2400 °C	G-2	GS-2
2700 °C	G-3	GS-3
3000 °C	G-4	GS-4

Table 3

XRD parameters of experimental samples.

Temperature/°C	Number	2θ002/°	FWHM002/°	2θ100/°	FWHM100/°	Lc/nm	La/nm	d002/nm
2100	G-1	25.85	2.10	42.96	1.86	3.89	9.39	0.3443
	GS-1	26.18	1.33	42.38	1.69	6.155	10.27	0.3401
2400	G-2	25.97	1.43	42.94	1.45	5.70	12.09	0.3428
	GS-2	26.27	0.74	42.39	0.66	11.04	13.31	0.3392
2700	G-3	26.10	1.02	42.93	1.28	7.98	13.68	0.3411
	GS-3	26.3	0.71	42.37	1.15	11.49	15.17	0.3386
3000	G-4	26.23	0.69	42.75	0.99	11.85	17.60	0.3395
	GS-4	26.35	0.66	42.17	0.87	12.32	20.03	0.3379

Table 4

Raman parameter values of experimental samples.

Temperature/°C	Number	FWHM(G)	R1	R2	R3
2100	G-1	23.33	0.35	0.324	0.391
	GS-1	21.68	0.19	0.148	0.234
2400	G-2	23.17	0.27	0.296	0.332
	GS-2	20.89	0.14	0.116	0.169
2700	G-3	23.11	0.23	0.262	0.292
	GS-3	19.76	0.06	0.047	0.119
3000	G-4	20.50	0.11	0.182	0.203
	GS-4	17.60	0.00	0.000	0.040

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