Tian Qiu, Jian-guo Yang, and Xue-jie Bai, Insight into the change in carbon structure and thermodynamics during anthracite transformation into graphite, Int. J. Miner. Metall. Mater., 27(2020), No. 2, pp.162-172. https://dx.doi.org/10.1007/s12613-019-1859-9
Cite this article as:
Tian Qiu, Jian-guo Yang, and Xue-jie Bai, Insight into the change in carbon structure and thermodynamics during anthracite transformation into graphite, Int. J. Miner. Metall. Mater., 27(2020), No. 2, pp.162-172. https://dx.doi.org/10.1007/s12613-019-1859-9
Tian Qiu, Jian-guo Yang, and Xue-jie Bai, Insight into the change in carbon structure and thermodynamics during anthracite transformation into graphite, Int. J. Miner. Metall. Mater., 27(2020), No. 2, pp.162-172. https://dx.doi.org/10.1007/s12613-019-1859-9
Cite this article as:
Tian Qiu, Jian-guo Yang, and Xue-jie Bai, Insight into the change in carbon structure and thermodynamics during anthracite transformation into graphite, Int. J. Miner. Metall. Mater., 27(2020), No. 2, pp.162-172. https://dx.doi.org/10.1007/s12613-019-1859-9
National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, China
2)
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
Funds: This work was financially supported by the“China Postdoctoral Science Foundation”and China National “Twelfth Five-Year” Plan for Science & Technology (No. 2014BAB01B02). The authors would like to thank Shenhua Ningxia Coal Industry Group for financial support and providing Taixi anthracite samples. We also want to thank the support of Advanced Analysis & Computation Center of China University of Mining and Technology
The thermodynamic and kinetic mechanisms of Taixi anthracite during its graphitization process were explored. To understand the variation trends of carbon arrangement order, microcrystal size, and graphitization degree against temperature during the graphitization process, a series of experiments were performed using Raman spectroscopy and X-ray diffraction (XRD). Subsequently, the influencing factors of the dominant reaction at different temperatures were analyzed using thermodynamics and kinetics. The results showed that the graphitization process of Taixi anthracite can be divided into three stages from the perspective of reaction thermodynamics and kinetics. Temperature played a crucial role in the formation and growth of a graphitic structure. Meanwhile, multivariate mechanisms coexisted in the graphitization process. At ultrahigh temperatures, the defects of synthetic graphite could not be completely eliminated and perfect graphite crystals could not be produced. At low temperatures, the reaction is mainly controlled by dynamics, while at high temperatures, thermodynamics dominates the direction of the reaction.
In recent years, carbonaceous substances have caused extensive concerns and have become one of the most widespread materials worldwide. Graphite, as one of the most popular carbonaceous materials, has excellent characteristics of corrosion resistance, heat resistance, self-lubricity, thermal and electrical conductivity, and porosity. Therefore, it has many practical applications in various fields, such as aerospace engineering, nuclear and military industries, and many civilian industries [1–8]. However, with the increasing depletion of natural graphite [9–10], synthetic graphite has emerged, and great progress has been made in its production and application [11–14]. Recently, many attempts have been made to prepare graphite materials from a variety of precursors, including mesocarbon microbead [15], petroleum coke and needle coke [16], sponge coke [17], pitch [18], unburned carbon from fly ash [19], and anthracite [20]. These research efforts have shown that carbonaceous materials can be transformed into graphite under specific conditions (e.g., high temperature, high pressure, or under the action of catalysts) [21–28]. At present, needle coke from petroleum is still the main raw material in the industrial production of graphite. However, as the price of petroleum has been rising in recent years, the cost of synthetic graphite has increased [29]. Therefore, considering the increasing demand for synthetic graphite, a search for inexpensive, abundant, and environmental friendly precursors for synthetic graphite materials is urgently needed.
Coal, a conventional fossil fuel, is known as the cheapest and most abundant natural carbon source in the world, and it has been exploited as an important resource for the production of a wide variety of carbon materials. Coal could be an important raw material to make up for the shortage of high-quality natural graphite resources and the high cost of other raw materials, making it one of the most prospective materials for producing graphite. Most recently, several researchers have intensively investigated the possibility and feasibility of preparing synthetic graphite or graphite-like products, focusing on using anthracite as starting materials. For instance, Gao et al. [30] prepared coal-derived graphite from Chinese anthracite as a carbon source to synthesize graphene sheets. Cabielles et al. [31] prepared graphite materials from anthracite combustion fly ashes through high-temperature treatment. García and co-workers [20] reported the successful production of synthetic graphite from anthracite via high-temperature treatment under inert atmosphere.
The study of graphitization mechanism for petroleum coke, needle coke, and some model compounds (e.g., acenaphthylene, fluorene, and anthracene) is now well developed [32–34]. Regarding anthracite, however, its structural complexity makes the graphitization process more sophisticated than that of petroleum [35]; therefore, little research has been conducted on the graphitization mechanism of anthracite, especially in terms of the graphite crystal nucleation growth mechanism, structural changes, and influencing factors from the perspective of molecular thermodynamics and kinetics. The lack of basic research has also constrained the large-scale production of anthracite-based synthetic graphite.
Recent developments of coal-based graphite have shown that its graphitization can be regarded as a self-organized process of amorphous carbon atoms in coal macromolecules. In this process, the basic aromatic structural units in coal macromolecules are condensed, and the number of aromatic rings increases, making the accumulation structure to gradually change to a graphitic structure [33,36]. However, from the perspective of physical chemistry, the graphitization process of coal is a solid-phase reaction process in which coal is transformed from an amorphous state to a crystalline state. The key problem involved in this process is how to reduce the activation energy in the crystalline transformation process and how to avoid the formation of a metastable state. This would further improve the graphitization degree of the product. It is necessary to study and analyze the thermodynamics and dynamics in the graphitization process of anthracite and to understand the graphitization mechanism, which will lay the foundation for the industrial production of anthracite-based graphite. Therefore, the thermodynamics and kinetics involved in the graphitization are the basic theories for studying the energy relationship and growth orientation of crystal lattices.
In this study, Taixi anthracite was employed as the raw material for producing synthetic graphite, and a series of anthracite-based graphite were obtained at different temperatures. X-ray diffraction (XRD) was used to characterize the graphitization degree variation with temperature. XRD can also reflect the graphitization rate of anthracite during the heat-treatment process, by which we can study the kinetics mechanism of graphitization. Subsequently, the combustion heat and the heat capacity of anthracite at different temperatures were measured, which were used to calculate the graphitization thermodynamic function. Finally, the graphitization mechanism of anthracite was obtained based on the combination of the kinetics and thermodynamics results.
2.
Experimental
2.1
Materials
The anthracite samples were provided by Taixi Coal Preparation Plant (Ningxia, China). The proximate analysis results and ash compositions of the sample are shown in Tables 1 and 2, respectively.
Table
1.
Proximate analysis and ultimate analysis results of the Taixi anthracite sample wt%
Proximate analysis
Ultimate analysis (daf)
Mad
Ad
Vdaf
C
H
O
N
S
1.33
2.85
8.63
96.81
1.17
1.35
0.59
0.08
Note: Mad—moisture (air dry basis); Ad—ash (dry basis); Vdaf—volatile (dry and ash-free basis); daf—dry and ash-free basis.
In this work, anthracite bricks approximately 2 cm in length and 1 cm in height were used as the samples. The anthracite samples were placed into a graphite pot, which was subsequently covered and placed in a vertical ultrahigh temperature graphitization furnace (model: IGBT). In the graphitization process, the pressure in the furnace cavity was 20–30 kPa higher than that of normal atmosphere, with high-purity argon used as the protective gas. The furnace was heated at a rising rate of 10 K/min up to a specified temperature and was heat-preserved for 3 h. Afterward, the furnace was naturally cooled to room temperature to obtain the coal-based graphite products.
2.3
Analysis and testing
Raman spectra were obtained on a French JY LabRam HR800 under 632.8 nm (He–Ne laser, 1.96 eV) laser excitation. XRD spectra were obtained on a PANalytical B.V. X'Pert3 Powder diffractometer, using the following parameters: copper target and potassium radiation, λ = 0.154056 nm; tube voltage 40 kV; tube current 100 mA; scan temperature 50–60°C; scanning range 10°–80°, and high-purity silicon as the internal standard. To prepare the XRD samples, blocky coal-based graphite products were ground to around 48 μm prior to measurements. The combustion heats of the coal-based graphite samples were obtained with a bomb calorimeter. The heat capacity was determined using a differential scanning calorimeter.
2.4
Calculation of graphitization degree
In this paper, the Maire and Mering equation [37] was adopted to determine the structural parameters of carbon materials, as shown in Eq. (1):
g=0.3440−d0020.3440−0.3354×100%
(1)
where g is the graphitization degree (%), 0.3440 is the carbon layer spacing (nm) of original carbonaceous materials, 0.3354 is the carbon layer spacing (nm) of an ideal graphite crystal, and d002 denotes the carbon layer spacing (nm) of the sample.
The diffraction angle (θ) obtained by XRD is substituted into the Bragg equation:
d002=λ2sinθ
(2)
Then, the d002 and g of the sample can be calculated.
3.
Results and discussion
3.1
Optimization process of graphitization conditions
3.1.1
Study on heating rate and heat preservation time
To determine the optimum heating rate for the graphitization process, we placed Taixi anthracite samples in a graphitization furnace heated to 3273 K, employing different heating rates of 5, 10, and 20 K/min. After 1 h of heat preservation, the product was naturally cooled to room temperature and taken out. The products were ground to powder for proximate analysis and XRD characterization, and the graphitization degrees (g) of the products were calculated according to Eqs. (1) and (2), as presented in Table 3.
Table
3.
Proximate analysis and the graphitization degree of products with different heating rates
As can be seen from Table 3, the heating rate had no significant influence on the moisture, ash, volatile matter, fixed carbon content, and graphitization degree of the final product. Considering that high heating rate leads to large loss and energy consumption of equipment and low heating rate greatly increases the time cost, we selected 10 K/min as the best heating rate for the anthracite graphitization.
To determine the best holding time for the graphitization process, we placed Taixi anthracite samples in a graphitization furnace heated to 3273 K at the heating rate of 10 K/min but with different holding times. After 1, 1.5, 2, 2.5, 3, 3.5, and 4 h of heat preservation, the products were naturally cooled to room temperature and were then taken out and ground to powder. Proximate analysis and XRD characterization were performed on them, and the graphitization degrees (g) of the products were calculated according to Eqs. (1) and (2). The results are presented in Table 4.
Table
4.
Proximate analysis results and the graphitization degree of products with different heat preservation times
As can be seen from Table 4, in the initial stage, with the continuous increase of the holding time, the ash content of the obtained products exhibited a declining trend, and the fixed carbon content and graphitization degree also increased. However, after increasing the holding time to more than 3 h, the ash content, fixed carbon content, and graphitization degree of the products all tended to be stable, without any change, and reached an equilibrium state. Therefore, we finally selected 3 h as the best heat preservation time for the graphitization of anthracite.
Through the above experiment, we can believe that in anthracite, the amounts of overflow of the inorganic mineral component (i.e., the ash content) and the aliphatic hydrocarbon on the side chain of a carbon hexagonal network (i.e., the volatile) are not related to the holding time at a certain temperature and the rate of temperature rise. It is the final temperature of heat treatment that determines the breaking and spilling of minerals with different decomposition temperatures and side chain molecules with different bond energies.
Therefore, we finally determined the relatively suitable heating rate and holding time for anthracite graphitization. Later in the exploration experiment, we adopted a heating rate of 10 K/min and a holding time of 3 h as the process conditions for the preparation of anthracite-based graphite.
3.1.2
Effect of the heat treatment final temperature on the product
To explore the effects of the heat treatment final temperature on the physical and chemical properties of coal-based graphite, we placed a sample of Taixi anthracite in a graphitization furnace and heated to 1773, 2073, 2473, 2873, and 3273 K at a heating rate of 10 K/min. After 3 h of heat preservation, the product was naturally cooled to room temperature and taken out, and its pictures were taken with a digital camera. The result is shown in Fig. 1. The product was then ground to powder, and proximate analysis was performed on it. The results are presented in Table 5.
Fig.
1.
Photos of the product with different heat treatment temperatures: (a) anthracite; (b) 1773 K; (c) 2073 K; (d) 2473 K; (e) 2873 K; (f) 3273 K.
As can be seen from Fig. 1, when the temperature reached 2473 K, macroscopic cracks began to appear on the product and the color also transformed from the original bright black to matte gray. The number of cracks increased with the graphitization heat treatment temperature. The main macromolecular network structure in anthracite coal is a stable “stationary phase” at high temperatures, while there is a certain number of “free-phase” low-molecular substances in the macromolecular network. Low-molecular matter and macromolecular networks exist in the form of physical association. At a high temperature, the physical association is destroyed, and the aliphatic hydrocarbon in the free phase generates hydrocarbon gas through a free radical cracking mechanism. As the heat treatment temperature continues to rise, the alkyl side chains on the aromatic ring in the macromolecular network also break off, resulting in the volatilization of small molecules. Thus, the graphitized product has many macroscopic cracks. Moreover, the change in the molecular structure of the substance will inevitably lead to the change of its absorption wavelength; therefore, on the macro level, the product of different heat treatment temperatures exhibited different colors.
From the data in Table 5, it can be concluded that the moisture, ash content, and volatile matter in the anthracite were mainly concentrated under low temperature (1773 K), i.e., the initial reaction stage. With the continuous increase of the graphitization heat treatment temperature, the moisture content, ash content, and volatile content of the product as a whole decreased gradually, while the fixed carbon content increased gradually. This is because the forms of moisture, ash, and volatiles in coal are different, complex, and changeable, and the binding forces between coal macromolecules are different; therefore, the temperatures that lead to the overflow of the abovementioned constituents are also different. The decomposition is not instantaneous at a certain temperature; rather, it is a dynamic change process that stretches across a broad temperature range. As the temperature increases, the moisture in coal changes from a liquid phase to a gas phase, the low fusibility ash changes from a solid phase to a gas phase, the high fusibility ash remains unchanged or changes from a solid phase to a liquid or gas phase, and the volatile matter changes from solid or liquid phase to gas phase. The rapid diffusion and escape of gas-phase materials at high temperatures reduces the overall content of water, ash, and volatile matter and increases the fixed carbon content. This is consistent with the conclusion in Fig. 1.
3.2
XRD study and calculation of graphitization degree
XRD was performed on the products obtained from the heat-treated anthracite. Fig. 2 and Table 6 respectively show the XRD spectra of coal-based graphites heated at different temperatures and the trend of the graphitization degree (g) of coal-based graphites increasing with the heat-treatment temperature. The graphitization degree was calculated using Eqs. (1) and (2).
Fig.
2.
XRD curves of the product under different heat treatment temperatures and the graphitization degree changes with temperature.
As depicted in Fig. 2 and Table 6, before 3273 K, the diffraction peaks constantly shifted to higher angles as the temperature rose; meanwhile, the peaks grew narrower and their intensity became higher. Hence, it can be calculated that the graphite microcrystalline layer spacing was getting smaller [20,38]. At 3273 K, the d002 of the anthracite decreased to 0.3361 nm, approaching the ideal layer spacing of graphite, 0.3354 nm. This was because with the temperature continuing to rise, the random layer structure of the anthracite absorbed more energies, leading to an increase in the vibration frequency and amplitude of carbon atoms. This was a transition period between a two-dimensional random layer structure and the ideal three-dimensional structure. During this period, the hexagonal ring layer gradually increased along the c-axis direction, and the layer spacing d002 decreased. Meanwhile, defects in the disordered layer structure were gradually eliminated, and the degree of ring condensation increased, causing the two-dimensional microcrystalline layer to grow gradually along the a and c axes [39].
3.3
Study of reaction kinetics
Fig. 3 shows the Raman spectra of the product obtained from Taixi anthracite at different heat treatment temperatures. The calculated results based on the intensities of peak D and peak G in the spectrum are listed in Table 7.
Fig.
3.
Raman spectra of anthracite at different heat treatment temperatures.
The intensity ratio of peak D to peak G（ID/IG）in the Raman spectra showed the mole number ratio of sp3 hybrid carbon atom to sp2 hybrid carbon atom (XD/XG). Where, XD represents the number of sp3 hybrid carbon atoms, while XG stands for the number of sp2 hybrid carbon atoms. The ID/IG constantly decreased as the temperature rose, indicating that the mole fraction of the sp2 hybrid carbon atom, which formed a graphite layer in the products, grew continuously [40–41]. Therefore, XD and XG were expected to be approximate to the proportion coefficients of ID and IG, respectively, which can be estimated by XG = IG/(ID + IG). Meanwhile, XG can also be approximately regarded as the reaction rate constant k in the graphitization process [42].
The optimization process of the graphitization conditions shows that at a certain temperature, the carbon layer spacing in carbon materials, which was transforming into the spacing of a graphite structure, decreased with the extension of the heat preservation time. However, the spacing stopped decreasing after a certain period of time. This phenomenon indicates that the transformation of disordered structure into ordered structure was a highly dynamic process when reaching a certain temperature. More specifically, equilibrium was achieved when the ordering rate was equal to the disordering rate at this temperature, and the carbon layer spacing no longer changed. This kinetic relationship can be explained by the famous Arrhenius empirical equation:
lnXR=lnA−EaRT
(3)
where XR is the reaction rate constant, A is the pre-exponential factor, Ea is the apparent activation energy, R is the molar gas constant, and T is the thermodynamic temperature. According to Eq. (3), the scatter figure of lnXR–1/T was plotted based on the obtained experimental data and was subsequently fitted into a straight line. The apparent activation energy (Ea) could be obtained from the slope of the plot, and the pre-exponential factor (A) could be determined by the intercept.
The Arrhenius empirical equation is only applicable to a narrow range of temperatures, where the apparent activation energy (Ea) can only be considered as a constant. However, wide ranges of temperatures were presented in the graphitization process in this work; therefore, the fitting lines of low temperatures (1773–2473 K) and high temperatures (2473–3273 K) were plotted separately based on the Arrhenius kinetic equation, as shown in Fig. 4.
Fig.
4.
Graphite mole fraction as a function of different heated temperature ranges using the Arrhenius kinetic equation.
It can be calculated from Fig. 3 that at low temperatures (1773–2473 K), the activation energy (ΔE1) was 2483.33 J/mol, and the pre-exponential factor (A1) was 0.512, while at high temperatures (2473–3273 K), ΔE2 = 4173.91 J/mol and A2 = 1.16. Compared with the activation energy value of the polyacrylonitrile(PAN)-based graphite (7360 ± 110 J/mol) [42], the Taixi anthracite in this work should be categorized into “soft carbon,” which can be easily graphitized. Moreover, Ea showed a lower value under high temperatures than under low temperatures, which may be caused by the decomposition of silicon carbide. The presence of silicon made it possible to promote the graphitization process of carbon and reduced the apparent activation energy of the system.
It can be seen from the experimental results of XRD that the graphitization degree of products increased rapidly in the temperature range of 1773 to 2473 K. The growth rate, however, became moderate at higher temperatures, until it reached saturation state, at around 3000 K. This was consistent with the calculation results of the pre-exponential factor calculated in the kinetics study. According to the Arrhenius empirical equation, the magnitude of the pre-exponential factor (A) was found to proportionally increase the reaction rate constant, which in turn affected the reaction rate. Therefore, it could be calculated that the pre-exponential factor value decreased at high temperatures, leading to reductions in the reaction rate.
3.4
Study of reaction thermodynamics
The graphitization process of anthracite involves many components, including solid, liquid, and gas reaction systems. Besides the main reaction, many side reactions exist. Therefore, the type and amount of final substances in the system were determined by the interaction of the main and side reactions. Theoretically, the thermodynamic analysis requires that all the components in the reaction system and all of the possible reactions between substances should be determined. However, the thermodynamic data of many intermediate products cannot be calculated due to limitations of the characterization methods. Therefore, to simplify the calculation, we studied the thermodynamics of the main reaction of the anthracite in the temperature range of 1773–3273 K. That is, C(amorphous carbon) → C(graphite carbon).
According to the first law of thermodynamics, enthalpy is one of the basic concepts of thermodynamics, and it is a state function for the energy of a system. The enthalpy change of a chemical reaction (ΔH) represents the difference in enthalpy between the products and reactants. Under constant pressure, ΔH is equal to the numerical value of the reaction heat and is thus an important factor for evaluating whether a chemical reaction can occur or not. Therefore, the combustion heats (Q) of coal-based graphite heat-treated with different temperatures were measured using an oxygen bomb. The combustion heat (Q) was used as a thermodynamic basis for calculating the enthalpy change of the graphitization process. The results are presented in Table 8.
Table
8.
Combustion heat of the product generated from anthracite treated at different high temperatures
It can be seen from Table 8 that the combustion heat of the product at different temperatures showed a significant decrease compared with the raw anthracite. This is because the enthalpy change of solids was approximately equal to their internal energy. The enthalpy of a substance is equivalent to the internal energy stored within it at a certain temperature. Given that the internal energy of a substance can be measured from its combustion heat, it can easily be concluded that combustion heat of a substance is proportional to its internal energy. With the presence of C–H, –C≡CH and =C=CH2 groups at the end of the anthracite macromolecules, the combustion heat of the product was reduced because the average bond potential energy of these groups was higher than the C–C conjugated bond.
According to thermodynamics, the enthalpy change of a combustible substance is equal to the opposite number of its internal energy and is also equal to the formation enthalpy of the product.
The thermochemical equation of transforming anthracite into coal-based graphite can be obtained from the difference in the combustion reaction of product and raw material:
It can be derived from Table 8 that ΔH was below zero, demonstrating that the transformation of anthracite from amorphous carbon to ordered crystalline graphite was an exothermic process. Less perfect structure of carbon material led to higher internal energy, which further resulted in a greater combustion heat. As we know, the coking and carbonization processes of coal before 773 K are endothermic reactions. Moreover, the heat carbon absorbed was stored in the form of internal energy within the imperfect disordered carbon structure. At a higher temperature, the anthracite underwent a graphitization process, where the internal energy was released in the form of heat.
To investigate the effect of the ash content of anthracite on enthalpy change during the graphitization process, anthracites with different ash contents (i.e., anthracite, demineralized anthracite and anthracite-silica mixture) were heated at different temperatures to measure the combustion heat values (Q) of their products. Using the data in Table 8, their ΔH could be obtained. Thus, the variation trend of the enthalpy change of anthracite with different ash contents during the graphitization process against temperature is illustrated in Fig. 5.
Fig.
5.
Relationship between enthalpy and temperature during graphitization.
It can be seen from Fig. 5 that the enthalpy change of the product did not decrease monotonically as the temperature rose. Instead, a significant increase in the enthalpy change was observed in the temperature range of 2073–2473 K for anthracite and anthracite-silica mixture. This indicates that the reaction in that temperature range was endothermic, and the structure of the carbon material tended to be disordered. However, for demineralized anthracite, its enthalpy change at that temperature stage decreased steadily. For anthracite-silica mixture, which was considered as high-ash-content coal, the enthalpy change at that temperature became more intense than that of low-ash-content anthracite. Hence, we strongly believe that during the anthracite graphitization process, the silica in the anthracite reacted with amorphous carbon to form silicon carbide in the temperature range of 2073–2473 K. As a result, the enthalpy of the system was increased.
Fig. 5 also illustrates that in the temperature range of 2473–3273 K, the enthalpy change of each anthracite declined slowly. Moreover, the decline rate was much moderate than that of 1773–2073 K. This indicates that at ultrahigh temperatures, the endothermic process (the thermal motion of gas-phase carbon atoms) was often accompanied by the exothermic process (the orderly arrangement of carbon atoms) in the system, and these two processes were mutually restricted.
Apart from enthalpy, entropy is also an important state function in thermodynamics. During the anthracite graphitization process, the entropy of the anthracite changed as the amorphous carbon transformed into polycrystalline graphite. The entropy of a substance can be determined by measuring its constant pressure heat capacity (Cp) at different temperatures, and then the entropy of the substance at a certain temperature can be calculated using Eq. (8).
ST=S298+∫T298CpTdT
(8)
where ST denotes the absolute value of the entropy of a substance at a certain temperature T (e.g., S298 represents the absolute value of entropy of a substance at 298 K). Therefore, the entropy change ΔST of a substance at temperature T can be obtained by
ΔST=ST−S298=∫T298CpTdT
(9)
The heat capacity of anthracite at different temperatures could be determined using differential scanning calorimetry. The entropy change of anthracite during the graphitization process as a function of temperature is illustrated in Fig. 6.
Fig.
6.
Entropy changes with temperature during graphitization: (a) entropy at different temperatures; (b) change in entropy over different temperature ranges.
As shown in Fig. 6, the entropy change value was above zero, indicating that the entropy value of the system in the temperature range of 1773–3273 K increased continuously compared with that of 298 K. This confirmed that the graphitization could be carried out spontaneously under this condition.
The entropy value of the system decreased in the temperature range of 1773–2073 K; thus, the entropy change of the reaction was below zero. Combined with the abovementioned analysis of enthalpy change, it can be concluded that the reaction was exothermic. This indicates that the graphitization process was not an isolated system and that heat was exchanged between the system and the environment. Thus, the entropy change reduction caused the exothermic process of the system, and as a result, the entropy value of the environment increased. In addition, the increase of the environmental entropy was greater than the decrease of the system entropy. Therefore, the sum of the entropy change of the system and the environment is still greater than zero; therefore, the graphitization reaction was consistent with the second law of thermodynamics in this temperature range.
In the temperature range of 2073–2473 K, the system entropy gradually increased and the entropy change of reaction was above zero. This indicates that side reactions took place at this temperature to form disordered products; that is, silicon carbide was formed in this stage, and high-melting-point impurities in coal were further removed. This agrees well with the results of the enthalpy change discussed above.
In the temperature range of 2473–3273 K, the entropy change of reaction was below zero again, and the system released very little heat in this temperature range. This demonstrates that although changes in the carbon structure and the formation of orderly arranged carbon atoms took place during the graphitization process, the thermal motion of carbon atoms also led to the local disordered carbon structure in the ultrahigh temperature range of 2473–3273 K. Therefore, when graphite with a certain perfect structure continued to transform into a much orderly structure, its entropy value decreased rather than increased.
To sum up, we conclude with the following:
(1) The graphitization of anthracite occurred in an open system and was characterized by both energy transfer and material transfer between anthracite and the environment. Below 1773 K was the initial heating stage, where most of the moisture, ash, and volatile matter in anthracite gradually turned into a gas phase, which was greater in entropy, and finally overflowed the reactor. This was an endothermic process that promoted the pyrolysis and condensation reaction of the anthracite macromolecules. Meanwhile, it was also accompanied by a physical change: some two-dimensional microcrystalline boundaries began to close, and the original interface energy could be released in the form of heat, which subsequently acted as a driving force to promote the growth of a two-dimensional microcrystalline layer.
(2) In the temperature range of 1773–2473 K, the graphitization system could obtain greater energy; thus, the vibration frequency and amplitude of carbon atoms were increased. Unstable low-molecular-number aliphatic hydrocarbons and impurity groups on side chains of the aromatic ring of anthracite obtained higher reactivity and began to break or gasify. Some discharged out of the reactor with argon, while the rest of them were deposited on the carbon layer, forming an ordered crystalline state with lower energy. Dislocation lines and boundaries on the two-dimensional microcrystalline surface disappeared, and the carbon layer spacing shrunk, leading to a further release of latent heat in the system. Meanwhile, some impurities started to form carbides and then decomposed at a higher temperature, resulting in thermal defects. This was an endothermic reaction, where amorphous carbon in the anthracite was transformed into amorphous graphite with a three-dimensional ordered structure. However, the solid-phase reaction process that continued to transform highly ordered crystalline graphite with a laminated structure needed to be carried out at a higher temperature.
(3) When the temperature reached the ultrahigh temperature range of 2473–3273 K, carbon atoms in the anthracite gained more energy, and the gas-phase carbon atoms in the carbon plane molecules or between molecules began to undergo an intense thermal movement. The active material exchange between the solid phase and the gas phase promoted the arrangement of carbon atoms. This recrystallization process gradually eliminated lattice defects in the amorphous graphite structure, making the graphite crystal grow to a perfect structure.
Therefore, temperature plays a crucial role in the formation and growth of a graphitic structure during the graphitization process of anthracite, especially in the temperature range of 2473–3273 K. Ultrahigh temperature is beneficial to the stacking of laminated graphite and the growth of a carbon net, causing a decrease in d002 and increase in La and Lc (La is the diameter of the aromatic ring laminates, Lc is the stacking height of the aromatic ring laminates). Thus, graphite with a relatively perfect crystal structure could be obtained. Meanwhile, multivariate mechanisms coexisted in the graphitization process; this feature was related to the chemical atmosphere of many carbon atoms. In these stages, not only the formation and decomposition of carbides occurred, but also the growth and recrystallization of microcrystals as well as the elimination of defects. At ultrahigh temperatures, due to the restriction by the local disordered structure caused by the thermal motion of carbon atoms, the defects of synthetic graphite could not be completely eliminated and perfect graphite crystals could not be produced. Hence, additional catalysts should be added to reduce the reaction activation energy.
4.
Conclusions
In summary, we introduced Raman spectroscopy and XRD to study the kinetics and thermodynamics mechanisms during the graphitization process of anthracite. Based on the results, we proposed that the graphitization process of anthracite can be divided into three stages from the perspective of reaction thermodynamics and kinetics.
(1) In the temperature range of 298–1773 K, the anthracite macromolecular structure underwent cracking and polycondensation reactions, and these processes were mainly endothermic.
(2) From 1773 to 2473 K, the random layer structure gradually became ordered, and the carbon layer spacing decreased, leading to the release of the latent heat in the system. Meanwhile, the formation and decomposition reactions of carbide, which were endothermic, caused thermal defects of the system. Therefore, exothermic reaction coexisted with endothermic reaction in this temperature range.
(3) In the ultrahigh temperature range of 2473–3273 K, graphite with lattice defects absorbed heat and recrystallized, resulting in the elimination of the lattice defects and further improved the ordered arrangement of the carbon atoms.
A high temperature is a necessary and important condition for graphitization. However, the ordered structural transformation in the graphitization process is always accompanied by the locally disordered thermal motion of atoms. Hence, when the graphite with a certain perfection is converted into a higher stage, its entropy value decreases, making it impossible to produce a defect-free graphite. In conclusion, the graphitization process of anthracite is complex and features multiple conversion mechanisms. In the low temperature stage, the reaction is mainly controlled by dynamics, while in the high temperature stage, thermodynamics dominates the direction of the reaction.
Acknowledgements
This work was financially supported by the China Postdoctoral Science Foundation and China National “Twelfth Five-Year” Plan for Science & Technology (No. 2014BAB01B02). The authors would like to thank Shenhua Ningxia Coal Industry Group for financial support and providing Taixi anthracite samples. We also want to thank the support of Advanced Analysis & Computation Center of China University of Mining and Technology.
D.D.L. Chung, Review graphite, J. Mater. Sci., 37(2002), No. 8, p. 1475. DOI: 10.1023/A:1014915307738
D.D.L. Chung, Carbon materials for structural self-sensing, electromagnetic shielding and thermal interfacing, Carbon, 50(2012), No. 9, p. 3342. DOI: 10.1016/j.carbon.2012.01.031
S.W. Sharshir, G.L. Peng, L.R. Wu, F.A. Essa, A.E. Kabeel, and N. Yang, The effects of flake graphite nanoparticles, phase change material, and film cooling on the solar still performance, Appl. Energy, 191(2017), p. 358. DOI: 10.1016/j.apenergy.2017.01.067
Y.N. Yang, Y. Pang, Y. Liu, and H.W. Guo, Preparation and thermal properties of polyethylene glycol/expanded graphite as novel form-stable phase change material for indoor energy saving, Mater. Lett., 216(2018), p. 220. DOI: 10.1016/j.matlet.2018.01.025
L.F. Castañeda, F.C. Walsh, J.L. Nava, and C.P. de León, Graphite felt as a versatile electrode material: Properties, reaction environment, performance and applications, Electrochim. Acta., 258(2017), p. 1115. DOI: 10.1016/j.electacta.2017.11.165
G.M. Butyrin, Density, porous structure, and gas-dynamic characteristics of finely grained graphites (a review), Solid Fuel Chem., 49(2015), No. 5, p. 304. DOI: 10.3103/S0361521915050055
A.P. Yu, P. Ramesh, M.E. Itkis, E. Bekyarova, and R.C. Haddon, Graphite nanoplatelet−epoxy composite thermal interface materials, J. Phys. Chem. C, 111(2007), No. 21, p. 7565. DOI: 10.1021/jp071761s
Y. Shibayama, H. Sato, T. Enoki, and M. Endo, Disordered magnetism at the metal-insulator threshold in nano-graphite-based carbon materials, Phys. Rev. Lett., 84(2000), No. 8, p. 1744. DOI: 10.1103/PhysRevLett.84.1744
S.C. Chelgani, M. Rudolph, R. Kratzsch, D. Sandmann, and J. Gutzmer, A review of graphite beneficiation techniques, Miner. Process. Extr. Metall. Rev., 37(2016), No. 1, p. 58. DOI: 10.1080/08827508.2015.1115992
H.H. Cai, Y.S. Li, and X.L. Luo, The characteristics and prospect of reserves of graphite resources in China, China Min. Mag., 25(2016), No. S2, p. 5.
C.L. Ma, Y. Zhao, J. Li, Y. Song, J.L. Shi, Q.G. Guo, and L. Liu, Synthesis and electrochemical properties of artificial graphite as an anode for high-performance lithium-ion batteries, Carbon, 64(2013), p. 553. DOI: 10.1016/j.carbon.2013.07.089
D. Marchand, C. Fretigny, M. Lagues, A.P. Legrand, E. McRae, J.F. Mareche, and M. Lelaurain, Surface structure and electrical conductivity of natural and artificial graphites, Carbon, 22(1984), No. 6, p. 497. DOI: 10.1016/0008-6223(84)90082-4
X.B. Fu, X.L. Song, and Y.M. Zhang, Facile preparation of graphene sheets from synthetic graphite, Mater. Lett., 70(2012), p. 181. DOI: 10.1016/j.matlet.2011.12.002
M. Mundszinger, S. Farsi, M. Rapp, U. Golla-Schindler, U. Kaiser, and M. Wachtler, Morphology and texture of spheroidized natural and synthetic graphites, Carbon, 111(2017), p. 764. DOI: 10.1016/j.carbon.2016.10.060
F.M. Courtel, S. Niketic, D. Duguay, Y. Abu-Lebdeh, and I.J. Davidson, Water-soluble binders for MCMB carbon anodes for lithium-ion batteries, J. Power Sources, 196(2011), No. 4, p. 2128. DOI: 10.1016/j.jpowsour.2010.10.025
C.L. Fan, H. He, K.H. Zhang and S.C. Han, Structural developments of artificial graphite scraps in further graphitization and its relationships with discharge capacity, Electrochim. Acta, 75(2012), p. 311. DOI: 10.1016/j.electacta.2012.05.010
T. Liu, R.Y. Luo, S.H. Yoon, and I. Mochida, Anode performance of boron-doped graphites prepared from shot and sponge cokes, J. Power Sources, 195(2010), No. 6, p. 1714. DOI: 10.1016/j.jpowsour.2009.08.104
S.L. Huang, H.J. Guo, X.H. Li, Z.X. Wang, L. Gan, J.X. Wang, and W. Xiao, Carbonization and graphitization of pitch applied for anode materials of high power lithium ion batteries, J. Solid State Electrochem, 17(2013), No. 5, p. 1401. DOI: 10.1007/s10008-013-2003-9
T.S. Yeh, Y.S. Wu, and Y.H. Lee, Graphitization of unburned carbon from oil-fired fly ash applied for anode materials of high power lithium ion batteries, Mater. Chem. Phys., 130(2011), No. 1-2, p. 309. DOI: 10.1016/j.matchemphys.2011.06.045
I. Cameán, P. Lavela, J.L. Tirado, and A.B. García, On the electrochemical performance of anthracite-based graphite materials as anodes in lithium-ion batteries, Fuel, 89(2010), No. 5, p. 986. DOI: 10.1016/j.fuel.2009.06.034
S. Yang, I.J. Kim, I.S. Choi, M.K. Bae, and H.S. Kim, Influence of electrolytes (TEABF4 and TEMABF4) on electrochemical performance of graphite oxide derived from needle coke, J. Nanosci. Nanotechnol., 13(2013), No. 5, p. 3747. DOI: 10.1166/jnn.2013.7331
C. Pantea, J. Qian, G.A. Voronin, and T.W. Zerda, High pressure study of graphitization of diamond crystals, J. Appl. Phys., 91(2002), No. 4, p. 1957. DOI: 10.1063/1.1433181
J.Y. Zhu, S. Zhang, L.X. Wang, D.Z. Jia, M.J. Xu, Z.B. Zhao, J.S. Qiu, and L.X. Jia, Engineering cross-linking by coal-based graphene quantum dots toward tough, flexible, and hydrophobic electrospun carbon nanofiber fabrics, Carbon, 129(2018), p. 54. DOI: 10.1016/j.carbon.2017.11.071
Y.T. Zhang, K.K. Li, S.Z. Ren, D.Y. Wang, A.N. Zhou, and J.S. Qiu, Metal-modified coal-based graphene composites and the photocatalytic performance, Imaging Sci. Photochem., 34(2016), No. 5, p. 475.
Q. Zhou, Z.B. Zhao, Y.T. Zhang, B. Meng, A.N. Zhou, and J.S. Qiu, Graphene sheets from graphitized anthracite coal: preparation, decoration, and application, Energy Fuels, 26(2012), No. 8, p. 5186. DOI: 10.1021/ef300919d
D. González, M.A. Montes-Morán, I. Suárez-Ruiz, and A.B. Garcia, Structural characterization of graphite materials prepared from anthracites of different characteristics: a comparative analysis, Energy Fuels, 18(2004), No. 2, p. 365. DOI: 10.1021/ef030144+
D. González, M.A. Montes-Morán and A.B. Garcia, Graphite materials prepared from an anthracite: a structural characterization, Energy Fuels, 17(2003), No. 5, p. 1324. DOI: 10.1021/ef0300491
J.B. Geng and Q. Ji, Multi-perspective analysis of China's energy supply security, Energy, 64(2014), p. 541. DOI: 10.1016/j.energy.2013.11.036
F. Gao, J.Y. Qu, Z.B. Zhao, Q. Zhou, B.B. Li, and J.S. Qiu, A green strategy for the synthesis of graphene supported Mn3O4 nanocomposites from graphitized coal and their supercapacitor application, Carbon, 80(2014), p. 640. DOI: 10.1016/j.carbon.2014.09.008
M. Cabielles, M.A. Montes-Morán, and A.B. Garcia, Structural study of graphite materials prepared by HTT of unburned carbon concentrates from coal combustion fly ashes, Energy Fuels, 22(2008), No. 2, p. 1239. DOI: 10.1021/ef700603t
D.B. Fischbach, Kinetics of graphitization of a petroleum coke, Nature, 200(1963), No. 4913, p. 1281. DOI: 10.1038/2001281a0
J.N. Rouzaud and A. Oberlin, Structure, microtexture, and optical properties of anthracene and saccharose-based carbons, Carbon, 27(1989), No. 4, p. 517. DOI: 10.1016/0008-6223(89)90002-X
K. Bratek, W. Bratek, I. Gerus-Piasecka, S. Jasieńko, and P. Wilk, Properties and structure of different rank anthracites, Fuel, 81(2002), No. 1, p. 97. DOI: 10.1016/S0016-2361(01)00120-X
A. Oberlin and G. Terriere, Graphitization studies of anthracites by high resolution electron microscopy, Carbon, 13(1975), No. 5, p. 367. DOI: 10.1016/0008-6223(75)90004-4
W. Ruland, X-ray studies on the structure of graphitic carbons, Acta Crystallogr., 18(1965), No. 6, p. 992. DOI: 10.1107/S0365110X65002414
W. Li and Y.M. Zhu, Structural characteristics of coal vitrinite during pyrolysis, Energy Fuels, 28(2014), No. 6, p. 3645. DOI: 10.1021/ef500300r
H. Badenhorst, Microstructure of natural graphite flakes revealed by oxidation: Limitations of XRD and Raman techniques for crystallinity estimates, Carbon, 66(2014), p. 674. DOI: 10.1016/j.carbon.2013.09.065
L. Bokobza, J.L. Bruneel, and M. Couzi, Raman spectroscopic investigation of carbon-based materials and their composites. Comparison between carbon nanotubes and carbon black, Chem. Phys. Lett., 590(2013), p. 153. DOI: 10.1016/j.cplett.2013.10.071
M.J. Matthews, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, and M. Endo, Origin of dispersive effects of the Raman D band in carbon materials, Phys. Rev. B, 59(1999), No. 10, p. R6585. DOI: 10.1103/PhysRevB.59.R6585
Y. Wang, S. Serrano, and J.J. Santiago-Avilés, Raman characterization of carbon nanofibers prepared using electrospinning, Synth. Met., 138(2003), No. 3, p. 423. DOI: 10.1016/S0379-6779(02)00472-1
Jisong Yi, Ping Wang, Jie Fu, Baohong Tu, Jin Li, Jianxin Liu, Wu Wen, Chenggang Huang.
Thermodynamics of Slag-desulphuration in VD Process
[J]. International Journal of Minerals, Metallurgy and Materials, 2000, 7(1): 14-17.
Wen-quan Niu, Jing-song Wang, Guang Wang, et al. Analysis of existence state and deterioration mechanism of coke in a blast furnace hearth. Journal of Iron and Steel Research International, 2025, 32(4): 883.
DOI:10.1007/s42243-024-01327-x
2.
Jiguo Tu, Xiaoyun Wang, Lili Jiang, et al. Efficient graphitization conversion strategies of low-value carbonaceous resources into advanced graphitic carbons. Chemical Engineering Journal, 2025, 505: 159472.
DOI:10.1016/j.cej.2025.159472
3.
Xiaofei Li, Yinao Jiao, Yupeng Li, et al. Dual-drive mica-based magnetic composite phase-change materials for photothermal and magnetothermal conversion. RSC Advances, 2025, 15(16): 12713.
DOI:10.1039/D4RA06902A
4.
Chenggang Tao, Zhengjie Chen, Wenhui Ma, et al. Comparative Study on the Graphitization Index of Carbonaceous Reducing Agents Used in Silicon Smelting. Silicon, 2024, 16(7): 3121.
DOI:10.1007/s12633-024-02910-3
5.
Tian Qiu, Weining Xie, Xuejie Bai, et al. Study on the Formation Mechanism of a Core–Shell Structure during Graphitization of Anthracite. ACS Omega, 2024, 9(21): 22581.
DOI:10.1021/acsomega.3c09596
6.
Milad Ahmadi Khoshooei, Xijun Wang, Gerardo Vitale, et al. An active, stable cubic molybdenum carbide catalyst for the high-temperature reverse water-gas shift reaction. Science, 2024, 384(6695): 540.
DOI:10.1126/science.adl1260
7.
Shanxin Xiong, Fengyan Lv, Chenxu Wang, et al. Coal-based activated carbon prepared by H2O activation process for supercapacitors using response surface optimization method. Frontiers of Chemical Science and Engineering, 2024, 18(6)
DOI:10.1007/s11705-024-2422-4
8.
Lipeng Wang, Zhiang Li, Chenxian Du, et al. High-temperature graphitization of coke and lithium storage properties of coke-based graphite. International Journal of Coal Preparation and Utilization, 2024, 44(1): 19.
DOI:10.1080/19392699.2023.2168656
9.
Hao Zhang, Xiaoyu Ding, Yongjie Yang, et al. Temperature-dependent evolution of synthetic coal-derived graphite fillers and their reinforcement in styrene butadiene rubber composites. Composites Science and Technology, 2024, 258: 110883.
DOI:10.1016/j.compscitech.2024.110883
Yude Zhang, Yan Li, Qian Zhang, et al. Graphitization of anthracite catalyzed by single metal oxides and its enhanced electrical conductivity. International Journal of Coal Preparation and Utilization, 2024, 44(9): 1227.
DOI:10.1080/19392699.2023.2270925
12.
Lei Zhang, Chunjiang Liu, Ruikang Song, et al. Low-temperature graphitization of carbon foam using nickel-based catalysts for enhanced thermal conductivity: application in the pyrolysis of oil-rich coal. Journal of Porous Materials, 2024, 31(3): 1087.
DOI:10.1007/s10934-024-01562-3
13.
Sara Ramandi, Mohammad H. Entezari. Study of structural and electrochemical properties of graphene/polyaniline nanotube/ZIF-67 nanocages as a 3D novel ternary composite hydrogel for all-solid-state supercapacitors. Ionics, 2024, 30(5): 3003.
DOI:10.1007/s11581-024-05476-7
14.
Hao Zhang, Haiyue Cao, Liang Yuan, et al. Temperature-Dependent Structural Evolution of Coal-Based Graphite During Synthetic Graphitization Process. Energy & Fuels, 2024, 38(18): 17370.
DOI:10.1021/acs.energyfuels.4c02240
15.
Zongwei Wang, Tuo Zhou, Jing Dai, et al. A biochar electrode based on conductive straw for Cd removal and recovery from wastewater by DC electrochemical deposition. Biomass and Bioenergy, 2024, 189: 107347.
DOI:10.1016/j.biombioe.2024.107347
16.
Chenggang Tao, Zhengjie Chen, Wenhui Ma. Analysis of Microstructural Change Mechanism During High-Temperature Pyrolysis of Carbonaceous Reducing Agents for Silicon Smelting. Silicon, 2024, 16(2): 647.
DOI:10.1007/s12633-023-02701-2
17.
A.V. Sybir, S.M. Hubynskyi, S.S. Fedorov, et al. Peculiarities of high-temperature refining of carbon materials. Voprosy Khimii i Khimicheskoi Tekhnologii, 2023.
DOI:10.32434/0321-4095-2023-151-6-177-186
18.
Jinmei Xu, Pengfei Gao, Xiangyun Qiu, et al. Kinetic and transport characteristics of LiNi0.8Co0.1Mn0.1O2 lithium-ion batteries. Solid State Ionics, 2023, 395: 116216.
DOI:10.1016/j.ssi.2023.116216
19.
Buxin Su, Guangwei Wang, Renguo Li, et al. Co-Combustion Behavior of Paper Sludge Hydrochar and Pulverized Coal: Low Rank Coal and Its Product by Hydrothermal Carbonization. Energies, 2022, 15(15): 5619.
DOI:10.3390/en15155619
20.
Dongyao Zhang, Chuanchang Li, Niangzhi Lin, et al. Mica-stabilized polyethylene glycol composite phase change materials for thermal energy storage. International Journal of Minerals, Metallurgy and Materials, 2022, 29(1): 168.
DOI:10.1007/s12613-021-2357-4
21.
Xiaojun Ning, Han Dang, Runsheng Xu, et al. Co-hydrothermal carbonization of biomass and PVC for clean blast furnace injection fuel production: Experiment and DFT calculation. Renewable Energy, 2022, 187: 156.
DOI:10.1016/j.renene.2022.01.082
22.
Sen Yang, Shuaiqing Zhang, Wei Dong, et al. Purification mechanism of microcrystalline graphite and lithium storage properties of purified graphite. Materials Research Express, 2022, 9(2): 025505.
DOI:10.1088/2053-1591/ac513f
23.
Huihui Zeng, Baolin Xing, Yijun Cao, et al. Insight into the microstructural evolution of anthracite during carbonization-graphitization process from the perspective of materialization. International Journal of Mining Science and Technology, 2022, 32(6): 1397.
DOI:10.1016/j.ijmst.2022.06.009
24.
A.V. Sybir, M.V. Hubynskyi, О.K. Balalaiev , et al. Effect of parameters of the anthracite heat treatment on the properties of carbon materials during shock heating. Voprosy Khimii i Khimicheskoi Tekhnologii, 2022.
DOI:10.32434/0321-4095-2022-144-5-94-101
25.
Nan Zhang, Guangwei Wang, Chunmei Yu, et al. Physicochemical structure characteristics and combustion kinetics of low-rank coal by hydrothermal carbonization. Energy, 2022, 238: 121682.
DOI:10.1016/j.energy.2021.121682
26.
Tian Qiu, Zhaoyi Yu, Weining Xie, et al. Preparation of Onion-like Synthetic Graphite with a Hierarchical Pore Structure from Anthracite and Its Electrochemical Properties as the Anode Material of Lithium-Ion Batteries. Energy & Fuels, 2022, 36(15): 8256.
DOI:10.1021/acs.energyfuels.2c01892
27.
Rui Li, Long Qing, Wilfred Emori, et al. Ultrafine SnO2 Nanoparticles Decorated on N-Doped Highly Structurally Connected Carbon Nanospheres as Anode Materials for High-Performance Sodium-Ion Batteries. Energy & Fuels, 2022, 36(9): 4957.
DOI:10.1021/acs.energyfuels.2c00115
28.
Fredina Destyorini, Rike Yudianti, Yuyun Irmawati, et al. Temperature driven structural transition in the nickel-based catalytic graphitization of coconut coir. Diamond and Related Materials, 2021, 117: 108443.
DOI:10.1016/j.diamond.2021.108443
29.
Yingke Wu, Kuo Li, Zhaoguo Wang, et al. Fluctuations in Graphitization of Coal Seam-Derived Natural Graphite upon Approaching the Qitianling Granite Intrusion, Hunan, China. Minerals, 2021, 11(10): 1147.
DOI:10.3390/min11101147
30.
Xiao-man Tian, Shen-xu Bao, Yi-min Zhang. Adsorption properties of V(IV) on resin-activated carbon composite electrodes in capacitive deionization. International Journal of Minerals, Metallurgy and Materials, 2021, 28(11): 1777.
DOI:10.1007/s12613-020-2100-6
31.
Artem Sybir, Mykhailo Hubynskyi, Serhii Fedorov, et al. Effect of heat shock on graphitization of Donbass anthracite. Mining of Mineral Deposits, 2020, 14(3): 43.
DOI:10.33271/mining14.03.043
32.
Hao-bin Zhu, Wen-long Zhan, Zhi-jun He, et al. Pore structure evolution during the coke graphitization process in a blast furnace. International Journal of Minerals, Metallurgy and Materials, 2020, 27(9): 1226.
DOI:10.1007/s12613-019-1927-1
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