An Alternative Way to Produce High-Density Graphite from Carbonaceous Raw Materials

Abstract

In this study, graphite, the most stable form of carbon, was examined for its hexagonal crystalline structure with specific dimensions (a_o = 2.46 Ǻ; c_o = 6.70 Ǻ). Its framework comprises parallel carbon atom planes, forming regular hexagons (side length 1.415 Ǻ) and 120° angles between adjacent atoms. Two structural variations exist: hexagonal symmetry (1-2-1-2-1-2 planes) and rhomboidal symmetry (1-2-3-1-2-3 planes). The aim of this research was to produce high-density graphite utilizing carbonaceous raw materials. Graphite-based materials often exhibit high porosity, necessitating additional treatment. In this study, we successfully obtained mesophase tar pitch (yield: 45%), a pivotal raw material, and high-density graphite. The resulting graphite underwent characterization for physical properties (apparent and real density, porosity, and compression strength), demonstrating conformity with the existing literature data.

1. Introduction

Graphite-based carbonaceous materials are becoming more important as their use within different industrial fields expands [1]. Depending on its state, i.e., natural or artificial, the use of graphite varies from mold fabrication [2] used in high-purity material pouring (e.g., SiO₂) [3] to the atomic energy field (cooling part of the reactors). Thus, the authors of [2] explored an innovative method for producing large-size and high-strength graphite materials without the need for conventional molding machines. This research presents a novel technique in the field of materials science and addresses the challenges associated with manufacturing graphite materials of substantial size and superior strength without the dependency on molding machines. Molding machines are traditionally used in the production of such materials, and this research seeks to eliminate this requirement.

The recent global situation has proven the necessity of semiconductor production and clearly shown the consequences of a semiconductor shortage. Therefore, the authors of [3] discussed the utilization of graphite susceptors in the Czochralski process to grow single-crystal silicon. The Czochralski process is a crucial technique in the production of high-quality silicon wafers for the semiconductor industry. This study focuses on the application of graphite as susceptors, which are materials capable of absorbing electromagnetic radiation and converting it into heat. Graphite, due to its unique thermal and electrical properties, is a suitable candidate for this role. Therefore, this paper delves into the fabrication and performance evaluation of graphite susceptors.

Different techniques are used by manufacturers to obtain graphite susceptors; the authors of [4] provided a comparative analysis of the graphitization, intercalation, and exfoliation processes of cokes and anthracites. This study investigates the transformation processes of cokes and anthracites, two different forms of carbonaceous materials, into graphite. Graphitization, the structural transformation of amorphous carbon into crystalline graphite, is a vital process for various industrial applications. These processes are essential in modifying the properties of graphite-based materials. This paper also analyzes the efficiency and characteristics of graphitization, intercalation, and exfoliation in both cokes and anthracites. Herein, we present experimental methodologies, data analyses, and conclusions regarding the comparative behavior of these materials during the mentioned processes.

The authors of [5] investigated the thermophysical properties of high-density graphite foams and their composites with paraffin. This study focuses on high-density graphite foams, a class of materials known for their unique thermal and mechanical properties. We examine the thermophysical characteristics of these foams, such as thermal conductivity and heat capacity. Additionally, we explore the development and properties of composites formed by incorporating paraffin, a phase-change material, into graphite foams.

Of great importance is a deep understanding of the manufacturing process, aspects of which are discussed within [6,7], highlighting the advancements in the synthesis and control of carbon materials, focusing on their structure and functional properties. The paper covers various forms of carbon allotropes, including, but not limited to, graphite, diamond, fullerenes, and carbon nanotubes [6]. Key topics include methods for the controlled synthesis of these materials, their structural characterization techniques, and the relationship between their structures and unique functionalities. On the other hand, ref. [7] focuses on the innovative manufacturing process for producing large-size graphite materials with high mechanical strength. Graphite, a form of carbon with unique properties, is widely used in various industrial applications, including aerospace, electronics, and metallurgy, due to its excellent thermal and electrical conductivity, as well as its mechanical strength. The study presents a novel method for the production of high-strength graphite materials on a large scale involving the incorporation of advanced processing methods, i.e., high-temperature treatments and the addition of strengthening agents, to enhance the mechanical properties of graphite. Specific parameters, procedures, and conditions optimized to achieve superior strength in the manufactured graphite materials are also detailed within the paper.

The nuclear industry is another important field for high-density graphite applications; therefore, the authors of [8,9,10] focused on the microstructural and thermal properties of nuclear graphite, which are essential for understanding its behavior in nuclear reactor environments, where the authors employed advanced analytical techniques to investigate the material’s microstructure and thermal expansion behavior [8]. The authors of [9] focused on the development of a microstructural model to predict the thermal expansion behavior of nuclear graphite. Computational simulations and detailed analysis of the graphite’s microstructure were presented in the paper. By understanding the thermal expansion at a microstructural level, the research provides valuable insights into the material’s behavior under varying temperature conditions.

The authors of [10] detailed experimental analyses to measure dimensional changes in graphite material when exposed to electron irradiation. By examining alterations in the material’s dimensions, the research provides valuable insights into the radiation-induced structural modifications of nuclear graphite, which are crucial for understanding its stability and integrity in nuclear environments. The findings enhance knowledge of graphite behavior under irradiation, aiding in the design and safety assessment of nuclear systems.

Other papers are focused on synthetic graphite-based materials used for battery electrodes, mainly anodes [11,12,13]. The authors of [11] introduced innovative methods and materials related to graphite anodes, involving modifications in the synthesis process, structural design, and electrochemical properties. The research contributes to the advancement of lithium-ion battery technology, potentially enhancing battery energy density, cycling stability, and charging/discharging efficiency, while the authors of [12] presented a study on the enhancement of the performance of spherical natural graphite anodes in lithium-ion batteries. The reported research focuses on a modified carbon-coating technique applied to spherical natural graphite particles. The modification likely involves optimizing the coating process to achieve uniform and effective carbon layers on the graphite surface. This modification is crucial as it enhances the anode’s structural integrity, prevents undesirable side reactions, and improves the electrical conductivity of the electrode material. Through systematic experimentation and analysis, the research of [12] demonstrates the superior electrochemical performance of the modified spherical natural graphite anodes. This enhanced performance could be attributed to improved lithium-ion diffusion kinetics, minimized electrode degradation during charge–discharge cycles, and enhanced overall stability of the electrode material. The synthesis process and unique properties of UCAR superfine-grain graphite are engineered to give the material exceptionally fine grains, indicating a high degree of crystallinity and structural uniformity. Such characteristics often result in improved mechanical strength, thermal conductivity, and electrical conductivity, as demonstrated by the study presented by the authors of paper [13]. The paper discusses the material’s applications across multiple industries, suggesting that UCAR superfine-grain graphite exhibits versatile properties suitable for various uses. These applications may include aerospace, electronics, manufacturing, and energy sectors, where the material’s exceptional properties make it desirable for specific purposes.

Within the manufacturing process, grain size and porosity play a crucial role in obtaining desired characteristics; thus, the authors of [14] conduct a detailed analysis of graphite samples with varying grain and pore size distributions. Experimental techniques and analytical methods are detailed to assess the mechanical properties of these samples. The research aims to understand how the arrangement and dimensions of graphite grains and pores influence the material’s strength.

The findings in this paper shed light on the critical role of microstructure in determining the mechanical strength of graphite. By exploring the impact of grain and pore size distributions, the study provides valuable insights and understanding these relationships is crucial for designing graphite-based components in various applications, including in aerospace, automotive, and manufacturing industries, where mechanical strength is a vital parameter for structural integrity and reliability. Nevertheless, other aspects related to the manufacturing process must be taken into consideration such as a detailed examination of the structural changes, crystalline rearrangements, and thermodynamic factors involved in the graphitization process, which are explored by the authors of paper [15]. Experimental techniques including X-ray diffraction, microscopy, and spectroscopy, have been used to analyze the samples at various stages of graphitization. The study elucidates the kinetics and thermodynamics governing the conversion of amorphous or non-graphitic carbon into highly ordered graphite structures and offers valuable insights into the fundamental processes of graphitization and a deeper understanding of the transformation mechanisms in carbon materials. Lastly, in paper [16], the research team explores the influence of temperature on the graphitization process of a semianthracite coal sample. The study involves subjecting the semianthracite sample to a range of temperatures and analyzing its structural and chemical changes using techniques such as X-ray diffraction, Raman spectroscopy, or electron microscopy. By varying the temperature, the study understands how different thermal conditions affect the graphitization process of the semianthracite material with critical insights into the graphitization behavior of semianthracite, a coal type with intermediate carbonization levels. Understanding the impact of temperature on the graphitization process is vital for predicting and controlling the material’s transformation into graphite-like structures. This knowledge is valuable in the production of high-quality graphitic materials for various applications, including carbon-based electrodes, composite materials, and energy storage devices. The research contributes to the fundamental understanding of graphitization kinetics and mechanisms, aiding in the optimization of industrial processes involving semianthracite and related carbonaceous materials.

Among the most important characteristics that synthetic graphite-based materials must meet are material density and porosity. Even though the research groups of scientific papers such as [17,18,19,20] have tackled the issue of material density and, on the other hand, researchers involved in papers such as [21,22,23,24,25] have tackled methods of obtaining very low material porosity, the maximum potential of such type of materials (high-density graphite-based materials) is still to be assessed. Thus, the authors of papers [17,18] outlined a novel method for manufacturing high-density graphite. Typical processes for producing high-density graphite involve the compaction of fine graphite particles under high pressure and high temperature using specialized machinery and binders. The resulting material exhibits improved mechanical, thermal, and electrical properties compared to standard graphite.

Research carried out within paper [19] is focused on an innovative graphite material designed specifically as a stripper. Strippers are essential components in various industrial applications, particularly in particle accelerators and ion implantation systems, where they remove electrons from high-energy charged particles. This research developed a highly dense and oriented graphite material optimized for this purpose. The study involved a multi-step process, including the selection of high-quality graphite precursors, advanced synthesis techniques, and possibly heat treatment processes to achieve the desired density and orientation. The researchers aimed to produce a material with superior thermal and electrical conductivity, mechanical strength, and wear resistance, characteristics crucial for efficient and long-lasting operation as a stripper.

The research carried out in paper [21] refers to a novel technique for impregnating porous materials. The method is applicable to a variety of porous substrates, including ceramics, carbon-based materials, or metal foams, which are commonly used in various industrial applications. Typical impregnation processes involve the infiltration of a material, often a resin, metal, or ceramic precursor, into the pores of a porous substrate. This infiltration process can enhance the structural integrity, mechanical strength, thermal conductivity, or other desirable properties of the porous material.

The influence of pore structure on the thermal and frictional properties of high-density graphite materials is studied by the authors of paper [22], which involves a comprehensive analysis of high-density graphite samples with varying pore structures. Characterization methods, such as scanning electron microscopy, thermal conductivity measurements, and frictional testing, were employed to assess the samples’ microstructures, thermal conductivities, and frictional behaviors, respectively. By systematically varying the pore structure, the researchers aim to understand how different pore sizes and distributions impact the thermal and mechanical properties of the graphite material. The findings provide valuable insights into the intricate relationship between pore structure and the thermophysical and frictional properties of high-density graphite. The research elucidates how variations in pore size and distribution affect the material’s heat conduction capabilities and frictional behavior.

Studies presented in papers [23,24,25] were conducted to investigate the graphitization process of mesophase, a precursor material for graphite production. The studies explored the structural transformations that occur during the graphitization of polygranular graphite derived from mesophase. The research involved subjecting mesophase-derived polygranular graphite samples to high-temperature treatments, simulating the graphitization process. Techniques such as X-ray diffraction, Raman spectroscopy, or electron microscopy were employed to analyze the structural changes in the material as it transforms into graphite. The studies aimed to understand the evolution of the mesophase structure into graphite, elucidating the mechanisms and kinetics of graphitization. The findings in these papers provide essential insights into the graphitization behavior of mesophase-derived polygranular graphite. Understanding the structural changes during graphitization is crucial for tailoring the properties of graphite materials, especially in applications requiring high crystallinity and thermal stability, such as in advanced materials, electrodes, or high-performance lubricants.

The current paper aims to investigate an innovative technique for obtaining high-density low-porosity graphite in one single stage process. Moreover, obtaining our own mesophase tar pitch and using it to decrease material porosity, instead of the current industrial practice of using molten metal, pitch, or resin impregnation, is more environmentally friendly and uses low energy since mesophase tar pitch needs to be heated just until 470 °C, unlike molten metal that often reaches ~1000 °C. Re-impregnation processes either with coal tar/petroleum pitch or resin are energy- and time-consuming since the re-impregnated material needs a second carbonization and graphitization thermal treatment, as presented by the authors of papers [26,27,28].

So, within this paper, two types of materials have been obtained: mesophase tar pitch from tar and high-density low-porosity graphite. High-density graphite was obtained by impregnating different types of graphite with mesophase tar pitch.

Natural graphite and high-density graphite share basically the same structure, with the main differences between the two of them being in terms of crystal orientation and density. Thus, both types of graphite have the same crystal structure and lattice parameters (interlayer spacing (about 3.35 Å) and bond length (about 1.42 Å)), the difference occurring after the manufacturing process. Thus, natural graphite is relatively loosely packed, allowing for easy sliding between the layers. This characteristic gives graphite its lubricating properties; also, regular graphite has a relatively lower density due to its loosely packed layers. On the other hand, high-density graphite is more tightly packed, leading to increased mechanical strength and hardness compared to regular graphite. It also shows a higher density compared to regular graphite due to its tightly packed layers.

Considering the above-mentioned aspects, the obtained high-density graphite has been compared to natural graphite only in terms of apparent density, real density, and compression strength.

2. Materials and Methods

The raw materials used for obtaining the mesophase pitch were:

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Crude tar from Donasid Calarasi, Romania; its characteristics are shown in

Table 1. Properties of crude tar pitch.

Characteristic	Value
Real density (kg/m3)	1193
Humidity (W, %)	0.1
Ash (A, %)	0.4
Benzene insoluble material (BI, %)	15.8
Quinoline insoluble material (QI, %)	6.5
ηpitch (%)	60.1

:

Raw materials for obtaining high density graphite are:

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Type E impregnation pitch from ISPAT_SIDEX Galati, Romania; its characteristics are shown in

Table 2. Properties of type E impregnation pitch.

Characteristic	Value
Humidity (W, %)	0.4
Ash (A, %)	0.2
Geiseler melting point (°C)	89.6
Benzene insoluble material (BI, %)	33.1
Quinoline insoluble material (QI, %)	14.3

.

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Colloidal natural graphite from ICSI Ramnicu Valcea, Romania; its properties are shown in

Table 3. Properties of natural graphite.

Characteristic	Value
Real density (g/cm3)	2.258
Apparent density (g/cm3)	1.558
Compression resistance, (MPa)	18
Ash (A, %)	0.8

.

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Burnt petroleum coke, 3 types from Petrobrazi refinery, Petrom, Romania; Petromidia refinery, Rompetrol, Romania; and Shell refinery, Germany. Their characteristics are shown in

Table 4. Properties of petroleum coke.

Type	Petrobrazi	Petromidia	Shell
Volatiles (Va, %)	1.0	0.9	0.8
Ash (A, %)	0.47	0.4	0.3
Real density (kg/m3)	1900	2156	2190

.

2.1. Real Density Determination

Real density represents the density of a material excluding its pores and can be calculated as the ratio between the material’s mass and its volume. The unit can be kg/m³ or g/cm³.

The real density of porous materials was determined by using the pycnometer method. Thus, empty, clean and dried-of-known-volume pycnometers with their caps and thermometers attached were weighed on an analytical balance with a precision of 0.0001 g.

Then, the pycnometer placed in a thermal bath set at 20 °C was filled in with methanol RA 99.9% purity from WWR Chemicals, Germany, and weighed again, thus resulting in the mass of the methanol. By knowing the mass of the methanol and the volume of the pycnometer, the real density of the methanol was calculated.

In order to determine the real density of the sample, 1 g of porous material was placed inside the pycnometer. After that, the pycnometer was filled in with methanol and kept at 20 °C. Then, the pycnometer was connected to a vacuum pump and kept under vacuum until no air bubbles were formed. Air bubbles formed while the air from the material’s pores was released. After that, the vacuum was released, allowing methanol to fill in the pores of the material. As expected, the level of methanol within the pycnometer decreased. The pycnometer was again filled with methanol and the volume represents the pore volume.

The real density of the material was calculated as the ratio between its mass and volume (minus the pore’s volume).

2.2. Humidity Determination

In order to determine the sample’s humidity (W_h^a), ~1 g of sample was weighed and put in an uncovered vial. Then, the vial was placed in a 105 °C heated furnace and the temperature was kept constant for 1 h. After that, the vial was covered (in order to not let air moisture reach the sample), cooled at room temperature and then weighed again.

2.3. Volatiles Determination

Volatiles (V^a) represent the total amount of eliminated substances (except humidity) during the heating of a material.

Firstly, 0.0001 precision material mass (~1 g) was placed in a quartz crucible, the lid was put on, and it was weighed. After that, the crucible was heated in a furnace at 850 °C for 7 min. Then, the crucible was taken out, left to cool to room temperature, then weighed again.

%V^a was calculated according to the equation:

$$\%V^a=\frac{\left(m_2-m_3\right)}{\left(m_2-m_1\right)\times 100-W_h^a}$$

(1)

where m₁—mass of empty crucible, lid on (g); m₂—mass of crucible containing sample, lid on before heating (g); m₃—mass of crucible containing sample, lid on after heating (g); W_h^a—sample humidity (%).

2.4. Ash Determination

Ash is the solid residue obtained after the sample was burnt at 815 °C until the remaining mass was constant. Its symbol is “A”, and represents the ratio between the residual mass and the initial mass of the sample.

In total, 1 g of sample with a particle dimension of 0.2 mm was weighed and put in a ceramic crucible. The crucible was put in a furnace at room temperature and then the heat was turned on until it reached 815 °C within 60 min. The slow heating was necessary in order to avoid material or ash particles from being eliminated along with the volatiles. After the temperature was reached, it was kept for 30 min to allow the entire sample to burn. Then, the crucible was kept in a humidity-free vessel and weighed.

The resulting ash was calculated by using Equation (2):

$$\%A^a=\frac{m_3-m_1}{m_2-m_1}\times 100$$

(2)

where m₁—mass of empty crucible; m₂—mass of crucible and sample; m₃—mass of the crucible and solid residue after burning.

2.5. Method for Obtaining Mesophase Pitch

In order to obtain the mesophase pitch, crude tar from Donasid Calarasi was used. This was mixed with toluene at a 1:1 ratio, heated up to 60 °C, and then filtered while hot. By using this method, the materials insoluble in toluene were separated from the tar. After distillation, when the toluene was removed, the resulting material was purified tar.

All the above-described analyses were performed for the obtained purified tar.

The second step was to distill the tar until 340 °C; the residue was a pitch resulting from the purified tar. Again, all the analyses were performed.

After that, the tar pitch was thermally treated as follows: from room temperature until 100 °C with a heating speed of 5 °C/min, from 100 to 250 °C with a heating speed of 3 °C/min, and from 250 to 470 °C with a heating speed of 1 °C/min. Then, the temperature was kept at 470 °C for 20 min.

Mesophase pitch was obtained at a yield of 45%; the heating treatment diagram is shown in Figure 1.

The obtained pitch was milled until its grain size was below 0.25 mm.

2.6. Method for Obtaining of High-Density Graphite

High-density graphite was obtained by mixing the pitches described above (acting as a binder) and fillers represented by the cokes and graphite described within the raw materials section.

All the obtained mixtures were thermally treated 2 times: firstly, the carbonization treatment and secondly, the graphitization treatment.

The cokes were sorted into 3 different particle dimensions: 0–40 µm, 80–100 µm, and 160–250 µm, as shown in Figure 2.

The mixtures have been configured as follows: 160–250 µm—20%; 80–100 µm—35%; 0–40 µm—45%.

As can be seen in Figure 2, there are two discontinuities within the particle size distribution, and those discontinuities have been chosen in order to ensure the maximum compaction of the material and minimum gaps.

The binder (above-described pitches) was 20% wt. compared with the filler.

The samples have been obtained by mixing different binders and fillers as follows:

• Sample 1: Natural graphite + type E impregnation pitch;
• Sample 2: Petroleum coke from Petrobrazi + type E impregnation pitch;
• Sample 3: Petroleum coke from Petromidia + type E impregnation pitch;
• Sample 4: Petroleum coke from Shell + type E impregnation pitch;
• Sample 5: Natural graphite + mesophase pitch;
• Sample 6: Petroleum coke from Petrobrazi + mesophase pitch;
• Sample 7: Petroleum coke from Petromidia + mesophase pitch;
• Sample 8: Petroleum coke from Shell + mesophase pitch.

The obtained samples were mixed, homogenized, and then pressed at 60 MPa followed by the carbonization thermal treatment in the absence of air and according to the heating diagram shown in Figure 3.

This process is commonly applied to organic materials, such as wood, coal, peat, and biomass, to produce carbonaceous materials like charcoal, activated carbon, and carbon fibers. During carbonization, the material is heated in a controlled environment to temperatures typically ranging from 600 to 1200 °C. The absence of oxygen prevents combustion and allows the carbon atoms to rearrange into a more stable form.

The carbonization process involves several stages:

• Drying (up to 100 °C): The raw material is heated to remove moisture and other volatile substances, preparing it for the main carbonization process.
• Pyrolysis (between 100 and 550 °C): At elevated temperatures, organic compounds within the material break down into smaller molecules and gases, producing volatile compounds like methane, ethylene, and tar.
• Volatilization (between 600 and 850 °C): The volatile components are expelled from the material, leaving behind a carbon-rich residue. This stage is crucial in removing impurities and creating a high-carbon content product.
• Carbonization (at 900 °C): The remaining carbon atoms undergo structural rearrangement, forming a more ordered crystalline structure. The final product, such as charcoal or activated carbon, retains the original shape of the material but with a significantly higher carbon content. The carbonization treatment was conducted within an electrically heated furnace with a volume of 675 L and a controller that allows setting the heat in 9 stages. It uses a type K thermocouple and has a thermal inertia of max. 20 °C. The furnace was produced by Nabertherm GmbH, Lilienthal, Germany.

After the carbonization treatment, the samples were subject to the graphitization process which is a high-temperature process in which carbonaceous materials, primarily amorphous carbon or carbon-rich compounds, are transformed into graphite. Graphite is a crystalline form of carbon with a unique structure that consists of layers of carbon atoms arranged in a hexagonal lattice. The graphitization process involves heating carbon materials to temperatures typically above 2500 °C in an inert atmosphere or vacuum, allowing the atoms to rearrange into a stable graphite structure.

Graphitization consists of 3 main stages:

• Heating: The carbonaceous material is heated to extremely high temperatures, causing the atoms to gain enough thermal energy to break existing bonds and rearrange into the hexagonal lattice structure characteristic of graphite.
• Crystalline transformation: The disordered carbon atoms in the original material gradually transform into ordered layers of graphite, where each carbon atom is bonded to three others in a planar arrangement.
• Formation of the graphite structure: As the material cools down after the high-temperature treatment, the carbon atoms settle into the stable graphite structure. Proper cooling rates are essential to ensure the formation of large graphite crystallites.

The heating diagram is shown in Figure 4. The graphitization process took place in an Acheson industrial graphitization furnace which uses the Joule–Lenz effect. Thus, the furnace has 2 metallic electrodes connected to the electric grid and the material subjected to the graphitization process forms the core or resistance of the furnace. By plugging in the electrodes, the electric current passes through the core formed by the carbonaceous material and heats it. Thus, the necessary temperature of 2500 °C can be reached and the graphitization process is taking place. The furnace was also produced by Nabertherm GmbH, Lilienhal, Germany, and consists of an open-brick structure with metallic electrodes at its ends. The core of the furnace was covered by a mixture of Al₂O₃ powder, coke and organic shorts in order to ensure both thermal and air insulation.

3. Results and Discussion

After all the necessary raw materials and samples were obtained and characterized as described in Section 3, these were the results:

The comparison between the characteristics of crude and purified tar is shown in

Table 5. Characteristics of crude and purified tar.

Property	Crude Tar	Purified Tar
Real density (kg/m3)	1193	1105
Humidity (W, %)	0.1	0.8
Ash (A, %)	0.4	0.0
η pitch	60.1	49

.

As can be observed in Table 5, the real density of the tar decreases after purification due to the fact that all the large molecular mass components, existing ash, and coke-like particles have been removed after filtration. As a consequence, the ash content is practically zero. Moreover, the fact that large molecular mass components have been removed decreases the yield of pitch production.

Table 6. Characteristics of obtained purified tar pitch and mesophase pitch.

Property	Purified Tar Pitch	Mesophase Pitch
Humidity (%)	0.1	0.1
Ash (%)	0.12	0.12
Volatiles (%)	62.7	15
Anisotropy (%)	-	75

shows the characteristics of the pitches obtained from crude tar: purified tar pitch and mesophase pitch.

As can be observed by comparing the data in Table 2 with the data in Table 6, type E pitch, which is commonly used for obtaining graphitic materials, has an increased humidity and ash compared with both purified tar pitch (the precursor for mesophase pitch) and obtained mesophase pitch. Also, mesophase pitch shows 75% anisotropy compared with the other two pitches (type E and purified tar), which are 100% isotropic pitches. Also, mesophase pitch has a lower volatiles content.

By analyzing the obtained samples after the graphitization process, it clearly emerges that apparent and real density are the most important parameters to be taken into account when high-density graphite is obtained. Another two important characteristics are compression resistance and porosity of the material.

Therefore, the above-mentioned characteristics are shown in

Table 7. Properties of the obtained high-density graphite.

Sample	ρapparent g/cm3	ρreal g/cm3	Porosity, %	Compression Resistance, MPa
Sample 1	1.982	2.241	11.55	24.7
Sample 2	1.851	2.210	16.24	10.6
Sample 3	1.883	2.200	14.41	14.12
Sample 4	1.906	2.204	13.52	11.27
Sample 5	2.013	2.251	10.57	28.24
Sample 6	1.892	2.219	14.73	12.35
Sample 7	1.937	2.210	12.35	17.65
Sample 8	1.954	2.215	11.78	14.12

.

By analyzing the data within Table 7, clear differences between samples 1–4 produced by using type E impregnation pitch and samples 5–8 obtained by using mesophase pitch can be assessed, such as:

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Samples 5–8 obtained by using mesophase pitch show higher density than samples 1–4 obtained by using type E impregnation pitch. The variation is consistent throughout the group, meaning that differences between samples 1–5, 2–6, 3–7 and 4–8 are exclusively due to the binding material.

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The porosity is also lower for groups 5–8 compared to groups 1–4 due to the binding material. Mesophase pitch has a lower content of volatiles (15% compared with type E 52%); thus, the pores that develop during the early stages of heating are smaller and more reduced, and, as a direct consequence, the residue obtained after the thermal treatment of the mesophase pitch is higher (apparent densities for groups 5–8 are higher than the ones for groups 1–4).

-

Real densities for groups 5–8 are also higher than the ones of groups 1–4, yet again due to the binding material. In this case, the mesophase pitch’s anisotropy is responsible for a better graphitization process than the isotropic pitches.

-

The best results from each group are obtained by samples 1 and 5, the ones that used natural graphite as the filler. For the samples that used coke as the filler, the results are poorer from a density and porosity point of view. This is due to the fact that all cokes have not been completely transformed into graphite during the graphitization heating process. Also, natural graphite has an increased tendency to self-compact, thus leading to a lower porosity of the final material.

-

While comparing the samples that used coke as the filler, samples 2 and 6 show poorer results compared to the other ones due to the fact that Petrobrazi coke is low-quality; therefore, its transformation into graphite is lower.

-

Compression resistance results are best for the materials using natural graphite as the filler due to its elasticity; therefore, its compression resistance is higher. As can be observed, materials that have not reached complete graphitization break easily. A direct link between a material’s porosity and its compression resistance can also be assessed due to the fact that materials having higher porosity display lower compression resistance.

4. Conclusions

• The nature of the used pitch as a binder has a great influence on the materials’ final characteristics. Better quality pitch (mesophase) allows lower porosities, higher densities, and higher compression resistance.
• Petroleum coke quality also influenced the materials’ final characteristics regardless of the binders used. Thus, low-quality coke results in poor characteristics such as real density, porosity, and compression resistance.
• By using discontinuous and small particle sizes as raw materials, the final characteristics of the obtained materials are improved in terms of density and porosity.
• High-density graphite can be obtained by using carbonaceous raw materials, but the binder should be specially tailored for this (mesophase pitch compared with type E impregnation pitch). Also, fillers must be of good quality (graphite and/or high-quality petroleum coke). Another important parameter that must be taken into consideration is the discontinuity of the particle sizes and particle dimensions.
• The obtained high-density graphite shows better characteristics in terms of bulk density compared with previous studies. Thus, while the density obtained for all eight samples ranges between 1.851 and 2.013 g/cm³, the reported densities from the literature are 1.85 g/cm³ [27] and 1.75–1.85 g/cm³ [28].