Occurrence and genesis of amorphous graphite in ultra anthracite

Abstract

Amorphous graphite is widely developed in super anthracite (also called metamorphic anthracite). To analyze the characteristics and causes of the amorphous graphite in coal, taking typical samples of Yongan coalfield in Fujian Province as an example, the optical properties, material composition, crystal structure, and other characteristics of the microcrystalline in coal are identified and analyzed by optical microscopy, scanning electron microscope and transmission electron microscope. The results show that the amorphous graphite is mainly produced in the coal micro-cracks and cavities, most of which have fiber-like microstructure similar to petroleum coke, and bubble film structure can be seen; The maximum reflectivity of amorphous graphite is 9.29% – 10.83%, which is much higher than the original micro component in coal; Under the condition of orthogonal polarized light and gypsum test board, the amorphous graphite shows the first-order yellow, second-order blue interference color, mosaic, regional and fiber-like microstructure, and the local orientation is obvious.

SEM and EPMA showed that amorphous graphite was mostly scaly and showed flow structure and pore structure, and showed the characteristics of pure carbon. The high-resolution transmission electron microscopy (TEM) observed that the lattice fringes of amorphous graphite are in a straight and directional arrangement, and the selected electron diffraction shows a typical pattern of graphite lattice ring pattern. The results show that the carbon of the amorphous graphite in coal comes from a highly molten (variable) shell group and hydrogen-rich (variable) vitrinite.

The heat source is Yanshanian granite that intrudes around the coalfield and the bottom of the basin. The molten carbon-bearing materials converge and flow in the pore space and remove impurities at high temperature, and then crystallize the amorphous graphite with significant rank under the strong magma invasion pressure; At the same time, it is not ruled out that the possibility of the reactivation of the oozing asphaltene to form amorphous graphite in the coal of the study area is not excluded.

Ultra anthracite (also known as metamorphic anthracite) often can be observed under the optical microscope with a clear outline, strong optical anisotropy, various occurrence and different morphology of microscopic components, and the number varies with the area and coal seam [1-5]. Recently, the author found that the optical structure and microscopic composition of coal from Yongan and other places in Fujian are relatively rich, and most of them are microcrystalline graphite with complete crystal shape [5]. High-quality amorphous graphite is usually the main component of earthy graphite, most of which are graphite carbon. They have stable chemical properties and are not affected by strong acid or alkali. They have the characteristics of high-temperature resistance, heat transfer, conductivity, lubrication, and plasticity, and are widely used.

With the discovery and application of graphene materials, graphite has become an important raw material for the preparation of graphene [6-7]. At the same time, about half of the aphanitic graphite produced in China every year is used as coal fuel, which makes graphite resources a great waste [8]. In this paper, the occurrence observation, optical properties, material composition, and crystal structure characterization were used to study the microcrystalline graphite in Yongan coalfield, Fujian Province, to deepen the understanding of the occurrence characteristics and genesis of microcrystalline graphite in coal and provide the basis for the evaluation of coal-based materials.

1. Geological background

Yong’an coalfield is located in the northern margin of Yong’an basin in Fujian Province [9], adjacent to the southeast coastal volcanic belt. It is one of the six major coal mining areas in Fujian Province (Fig. 1). The coal-bearing stratum is the Middle Permian Tongziyan Formation, with many coal seams and a thin single layer, which can be divided into three lithologic sub-sections from bottom to top. The lower section is composed of barrier island lagoon facies siltstone and carbonaceous mudstone, intercalated with thin coal seams; The middle section is composed of thick siltstone with siderite nodule of shallow marine facies, without coal; The upper member is composed of thick siltstone of sea-land interaction facies, intercalated with carbonaceous mudstone and coal seam, and develops main coal seams in the mining area [10].

After the formation of the coal seam, it experienced multiple structural changes, developed NE and NW trending deep faults, and derived a series of gently dipping faults and folds, including the nappe structure under extension and the nappe structure under compression. The coal-bearing strata are strongly affected by structural compression deformation, and the coal seams are locally repeated or missing. The Mesozoic Cenozoic magmatic activity is extremely frequent, especially in the early Cretaceous. The distribution of magmatic rocks in this period is obviously controlled by NE trending faults, showing zonation and directionality in the region [11], and exposed in the large areas in the study area and its surrounding area (Fig. 1). Magmatic rock intrudes into coal-bearing strata along the fault zone, and the accompanying thermal and huge emplacement pressure makes the coal metamorphism in the study area generally very high, and the graphitization characteristics of coal macerals are remarkable.

2. Sampling and experimental methods

Six coal samples are taken from the upper member of Tongziyan Formation of Middle Permian in Yong’an coalfield, and the plane position of sampling points is shown in Figure 1. Macroscopically, the luster of the samples is strong, and the color is steel gray, black streaks, and easy to change

It is greasy with hands. Except for the yasm-6 sample, the other five samples are mylonitic structure, wrinkle structure, and scale structure are developed, and smooth friction mirror is common on the surface.

According to GB / T 212-2008 method for industrial analysis of coal, GB / T 19143-2003 method for analysis of carbon, hydrogen, and oxygen in rock organic matter, GB / T 214-2007 method for determination of total sulfur in coal and GB / T 6948-2008 method for microscopic determination of vitrinite reflectance of coal, the samples were analyzed for coal quality and tested for coal petrography (Rmax).

QinYong et al. [1-2] research indicates that when the vitrinite reflectance of coal is more than 8.0%, the ductility of the basic structural unit (BSU) of coal increases rapidly, indicating the end of coal gasification and the beginning of graphitization. According to the classification index of coal graphitization degree proposed by caddying et al. [12], the sample in this paper has reached the stage of semi graphite. According to MT / T 1158-2011 classification of vitrinite reflectance and ISO 11760-2018 international coal classification, the degree of sample metamorphism in this paper reaches stage III or anthracite a.

The experimental methods are as follows:

(1) The coal and rock micro components and reflectance test samples were prepared into powder coal flakes. The instrument was a Leica 4500p microscope equipped with a craic micro photometer. The microscopic components were identified by using cross-polarized light, orthogonally polarized light plus gypsum test board, and rotating loading platform, and the occurrence was observed and reflectance was tested [2,13-15].

(2) The SEM of the samples was observed by Zeiss sigma field emission scanning electron microscope (SEM) with the accessories of Oxford x-max20. The working voltage was 20 kV, the resolution of the energy spectrum was better than 127 EV and the peak shift was less than 1 eV.

(3) The samples were purified before the observation and analysis of TEM. The coal samples ground to 200 mesh (0.074 mm) were reacted in HF and HCl mixed solution for 24 hours, then washed and filtered repeatedly with deionized water. After drying, the micromorphology and selective electron diffraction (SAD) analysis were carried out on a gem-2100f field emission transmission electron microscope (TEM), and the acceleration voltage was 200 kV, The resolution of the point is 0.19 nm and the line resolution is 0.10 nm.

3. Results and discussion

3.1 Identification of amorphous  graphite in coal

The amorphous graphite in coal is difficult to distinguish from coal by the naked eye because of its small grain size and often coexisting with coal. Different observation methods are needed to determine the morphology characteristics [1-2,5]. The microscopic composition of coal is mainly (variable) vitrinite, and the contents of (variable) inertinite, (variable) EXINITE, and mineral components are different (Table 2). amorphous graphite exists in the pore space of coal with high reflective light (Fig. 2a-fig. 2C), which is one of the main characteristics identified. In addition to the strong reflective property, the optical anisotropy and inhomogeneity are strong under the reflected monopolar light, and there are locally oriented lamellae. Under the purplish-red background of orthogonally polarized light plus gypsum test board, the amorphous graphite shows the interference color of flake or fiber-like petroleum coke. When the insertion direction of the gypsum test board is perpendicular to the lamellar direction, it is the first-order yellow, and when it is parallel to the lamellar direction, it is second-order blue (Fig. 2d-fig. 2f), It is significantly different from macerals of vitrinite, inertinite and EXINITE in coal.

The oil-immersion reflectance (R) of the samples yasm-1 and yasm-3 with a large amount of amorphous graphite is tested. The results are shown in Table 3. The Rmax of amorphous graphite in the two samples ranged from 9.29% to 10.22% and from 9.67% to 10.83%, with an average of 9.64% and 10.16%, respectively, which was significantly higher than the average maximum reflectance (Rmax) of vitrinite in the two samples. In addition, the minimum reflectance (Rmin) of amorphous graphite from the above two coal samples is 0.54% ~ 1.35% and 0.47% ~ 1.26% respectively, with an average of 1.13% and 0.85% respectively. In the two samples, the average difference between the maximum reflectivity and the minimum reflectivity of amorphous graphite is 8.51% and 9.31%, respectively, which reveals the strong optical anisotropy of amorphous graphite, which is consistent with the optical characteristics of natural graphite.

Under the scanning electron microscope, the natural cross-section of amorphous graphite is scaly, irregular, and stacked along a certain direction (Fig. 3a, FIG. 3b). The size of microcrystals is mostly at the micron level. The amorphous graphite filled in the fracture has a flowing shape and pore structure, and the bubble film structure can be seen (Fig. 3C, FIG. 3D), which is a sign of high temperature melting and cooling crystallization. Irregular aggregate at the same time, there are more disseminated minerals on the surface of amorphous graphitetal, and more free granular minerals can be seen around the fracture. The results of EDS semi-quantitative analysis show that the main component of lamellar amorphous graphite is C, and no organic elements such as h, N, s are found. The existence of trace o element may be related to the oxygen-containing functional groups in the edge or defect of amorphous graphitetal structure; In addition, there are a small number of aluminosilicate minerals on the amorphous graphite, which are characterized by rich Si and Al elements and are clay minerals adhered to the surface of the amorphous graphite (Fig. 3e-fig. 3G).

In the process of graphitization of carbonaceous materials, aromatic layer, microcolumn, crumpled, and straight graphite structure are formed successively, and BSU gradually transforms from disordered vortex layer to regular and orderly graphite layer [16]. Under the transmission electron microscope, the surface of the amorphous graphitetal is flat and the edge is irregular (FIG. 4A). After magnification, the graphite lattice stripe clusters extending along with different directions and partially overlapping (Fig. 4b) can be seen, showing obvious local orientation. The selected area electron diffraction pattern is dominated by diffuse annular spots, and the scattering rings 002 and 10 are clearly visible (Fig. 4C). The locally concentrated speckle diffraction on the scattering ring reveals that BSU has a preferred orientation and a strong rank physicochemical degree. With the increase of the observation times, the straight and continuous lattice fringes of the layered graphite can be seen clearly (Fig. 4d-fig. 4F), and the length is generally more than tens of nanometers, up to 500 nm.

3.2 Occurrence and quantity of amorphous  graphite in coal

As a kind of mineral, amorphous graphite was detected in all six samples, and the content of amorphous graphite varies greatly among different samples (Table 2). The volume fraction of amorphous graphite in the sample yasm-1 is as high as 6.68%, and that in the other five samples is 0.99% ~ 2.77%. Because amorphous graphite is mainly produced in pores and fissures, the content of amorphous graphite is related to the development degree and scale of pores and fissures in coal. The results show that there are two types of amorphous graphite: one is attached to the inner wall of cracks or pores in coal, forming large microcrystal layers, which are abundant in yasm-1, yasm-3, yasm-4, and yasm-5 samples; The other is to fill the original maceral cavity in coal and form dense granular microcrystals, which are typical of yasm-2 and yasm-6.

Under the optical microscope, according to the size of the isochromatic zone (primary yellow or secondary blue) under the orthogonally polarized light and gypsum test board, the amorphous graphite was divided into three kinds of microstructure: mosaic, regional and fibrous. The linear diameter of the isochromatic zone of mosaic amorphous graphitetal is less than 10 μ m. It mainly occurs in the relatively small space of pores in the coal, mainly in the cellular pores of macerals, and the microcrystals in different directions are in close contact; The linear diameter of the isochromatic zone of regional amorphous graphitetal is between 10 and 50 μ m. It occurs in the fracture space and is the main type of microstructure; The long axis diameter of the isochromatic zone of fibrous amorphous graphite is more than 50 to hundreds of microns, and the number is relatively small, which is found in large fracture space.

At present, anthracite and coal formed graphite are divided as follows: first, according to the change of element composition and the intensity of reflected light, such as H / C and the maximum reflectance r max of vitrinite; The second is the defect degree of carbon structure, such as the graphite characteristic peak and defect peak area ratio R2; The third is the lattice spacing D 002 and the graphitization degree based on this calculation [8]. The above research and analysis show that the identification of amorphous graphite in coal employing microscope and scanning electron microscope and the determination of the reflectivity of amorphous graphite is another important means to identify the amorphous graphite in coal measures and the degree of graphitization of coal.

The graphitization degree is based on this calculation [8]. The above research and analysis show that the identification of amorphous graphite in coal employing microscope and scanning electron microscope and the determination of the reflectivity of amorphous graphite is another important means to identify the amorphous graphite in coal measures and the degree of graphitization of coal.

3.3 Discussion on the genesis of amorphous  graphite in coal

The amorphous graphite in the pores and fissures of ultra anthracite coal has the microstructure and interference color of flake petroleum coke, which is the display of the thermal melting of organic matter in coal. Different types of microstructures are formed in the cooking process of vitrinite and EXINITE in bituminous coal, including petroleum coke or pitch coke with fibrous structure [14,17-18]. The thermal activity of different macerals is different, the thermal evolution process is significantly different, and the carbon source contribution to the formation of amorphous graphite in coal is different（ The softening and decomposition temperature of the (variable) EXINITE is lower than that of the (variable) vitrinite, and the flowing structure can be formed after hot melting, cooling, and solidification [14]. Some hydrogen-rich vitrinite also have high thermal activity and graphitization potential（ The thermal activity of the (variable) inertinite is the lowest, and its optical anisotropy is still relatively weak in the ultra anthracite stage, and the graphitization process lags far behind the (variable) EXINITE and (variable) vitrinite [1-2,5]. Therefore, it can be reasonably inferred that the carbon of amorphous graphite in coal mainly comes from (variable) EXINITE and part of hydrogen-rich (variable) vitrinite, which is the internal reason for the formation of amorphous graphite in coal in the study area.

The melting of some macerals in coal to form microcrystalline graphite requires a strong heat source and enough tectonic stress. Similar thermal and tectonic stress sources in coal measures are related to the emplacement of large magmatic bodies. The study area is located on the southeast coast of China, which is one of the eight Mesozoic volcanic intrusive belts in China. The magmatic activity occurred during the Yanshan movement, and the intrusive body is mainly large granite [19]. The large intrusive rocks in this period are exposed in a large area in the study area and its surrounding areas (Fig. 1). At the same time, some macerals in the coal were melted and the impurity organic elements were removed by volatilization. The strong magmatic emplacement pressure created the tectonic coal in the study area on the macro level and promoted the orientation and rank physicochemical of carbon network slices in the coal on the micro-level. The combined action of magmatic heat and emplacement pressure is the external factor for the formation of microcrystalline graphite in coal in the study area.

In addition, in the process of coal evolution, different amounts of petroleum substances will be generated in the oil generation window stage, and some of them will be discharged from the coal block and flow into the coal seam pores and fissures. After solidification, the secondary macerals in the coal will exude asphaltite [20]. Exudate asphaltenes have strong thermal activity, and they are often cracked in the subsequent thermal evolution process, further forming gaseous hydrocarbons and leaving behind “microsomes” and other residues of secondary macerals. Under the action of high temperature and emplacement pressure brought by the large-scale magmatic intrusion, it is not ruled out that the oozing asphaltite filled in the pore space of the coal seam can be activated again, and then microcrystalline graphite in coal can be formed. The formation mechanism similar to that of pitch coke may also be an important reason for the formation of amorphous graphite in coal.

4. Conclusion

(1) Amorphous graphite is commonly found in ultra anthracite. Under the optical microscope, the reflected light property and optical anisotropy of the microcrystals are significantly stronger than those of the original macerals. Under the orthogonally polarized light plus gypsum test board, the first-order yellow and second-order blue interference colors are shown. Under the scanning electron microscope, the microcrystals are irregular flake-like and the molten bubble film can be seen, The results of high-resolution transmission electron microscopy (HRTEM) show that the lattice stripes are straight and the selected area electron diffraction (SAED) ring porphyritic lattice is obvious.

(2) In the ultra anthracite coal of Yongan mining area, Fujian Province, the amorphous graphite is mainly produced in the pore space of the coal, and the content of different samples varies greatly. According to the line diameter of the interference color isochromatic zone, the amorphous graphite are is divided into mosaic, regional and fibrous microstructure, and the regional type is the main one, reflecting the significantly preferred orientation. The special morphology, high reflectivity, and significant optical anisotropy of amorphous graphite in coal are different from the original macerals in coal, which is one of the indexes to evaluate the degree of graphitization.

(3) The internal reason for the formation of amorphous graphite in the coal of the study area is the existence of melting softening (variable) EXINITE and hydrogen-rich (variable) vitrinite in the coal, and the external reason is the strong thermal power and emplacement pressure brought by the large-scale Yanshanian magmatic activity, so the possibility of the formation of amorphous graphite in the coal of the study area can not be ruled out.