The difference between natural graphite and artificial graphite: how to use natural graphite to develop artificial graphite products

Graphite is composed of a single element of carbon. Its crystal structure belongs to the hexagonal system, forming a hexagonal layered structure. Within the layers, carbon atoms are bonded by σ bonds formed by sp2 hybrid orbitals and delocalized π bonds formed by Pz orbitals, forming a strong hexagonal grid. The carbon-carbon atomic spacing is 1.42 Å. Carbon atoms have extremely strong bond energies (345 kJ/mol), while carbon atoms are bound to each other by weaker van der Waals forces (bond energy 16.7 kJ/mol). The interlayer spacing is 3.354 Å.

Graphite is soft, dark gray, and has a greasy feel that can stain paper. Its hardness is 1-2, and its theoretical density is 2.26 g/cm³.
Pure graphite does not exist in nature. Natural graphite minerals often contain impurities such as SiO2, Al2O3, FeO, CaO, P2O5, and CuO.
These impurities often occur in minerals such as quartz, pyrite, and carbonates. In addition, graphite also contains gases such as water, hydrocarbons, CO₂, H₂, and N₂.

Therefore, in addition to determining the fixed carbon content, graphite analysis must also measure the volatile matter and ash content.

Due to its special structure, graphite has the following excellent properties:

(1) High temperature resistance: Graphite is one of the most heat-resistant materials. It has no melting point at normal pressure and loses very little weight even when burned by an ultra-high temperature arc.

(2) Electrical conductivity and thermal conductivity: Graphite has high electrical conductivity and thermal conductivity. The thermal conductivity decreases with increasing temperature. At extremely high temperatures, graphite even becomes an insulator.

(3) Lubricity: The lubricity of graphite depends on the size of the graphite grains and the degree of crystal development. The larger the graphite grains and the more complete the crystal development, the smaller the friction coefficient and the better the lubricity.

(4) Chemical stability: Graphite has good chemical stability at room temperature and is resistant to acid, alkali, and organic solvent corrosion.

(5) Plasticity: Graphite has a certain toughness and can be processed simply. Graphite with a high degree of crystal development can even be ground into very thin sheets.

(6) Thermal shock resistance: The thermal expansion coefficient of graphite is very small, and it can withstand drastic changes in temperature without being damaged during use.

3. Graphite Classification and Characteristics
Graphite can be divided into natural graphite and artificial graphite. While the two have similar structures and physical and chemical properties, their uses differ significantly.

Natural graphite is a gift from nature, formed by the long-term transformation of carbon-rich organic matter under high-temperature and high-pressure geological conditions. The processing properties of natural graphite are primarily determined by its crystal form. Graphite minerals with different crystal forms have different industrial values and applications.
Natural graphite is a wide variety of materials. Based on their crystal form, natural graphite is industrially classified into three categories: dense crystalline graphite, flake graphite, and cryptocrystalline graphite. In my country, flake graphite and cryptocrystalline graphite are the two main types.
Dense crystalline graphite, also known as massive graphite, has distinct crystals visible to the naked eye. The particle diameter is larger than 0.1 mm. The crystals are arranged in a disordered, dense, massive structure. It is of high quality, with a carbon content generally ranging from 60% to 65%, sometimes reaching 80% to 98%. However, its plasticity and lubricity are inferior to flake graphite. Natural flake graphite, a crystallographic term for pegmatites, is a single crystal named for its scaly shape. It can be either large or fine flakes. This type of graphite offers superior lubricity and plasticity compared to other types of graphite, making it the most valuable industrial material.

Although flake graphite ore is not of high grade, with a carbon content generally ranging from 3% to 25%, it is one of the most floatable ores found in nature. Through extensive grinding and selection, high-grade graphite concentrate can be obtained.

Aphanitic graphite, also known as amorphous graphite or earthy graphite, has recently begun to be referred to as microcrystalline graphite. This type of graphite typically has crystals less than 1 micron in diameter and can only be seen under an electron microscope. It can be considered an aggregate of graphite crystals.

Natural microcrystalline graphite is typically formed by the conversion of coal under high-temperature and high-pressure geological conditions. Therefore, it is often associated with coal. A transition zone from anthracite to natural microcrystalline graphite is often observed within natural microcrystalline graphite ore bodies. This type of graphite is characterized by an earthy surface, lack of luster, lower lubricity than flake graphite, and poor selectivity. However, it is of higher quality, with a carbon content generally ranging from 60% to 80%, with some exceeding 90%.

Artificial graphite is similar to polycrystals in crystallography. There are numerous types of artificial graphite, and the production processes vary widely. Broadly speaking, artificial graphite encompasses all graphite materials obtained through the carbonization of organic matter followed by graphitization and high-temperature treatment, such as carbon (graphite) fiber, pyrolytic carbon (graphite), and foamed graphite.

In a narrower sense, artificial graphite generally refers to bulk solid materials, such as graphite electrodes and isostatically pressed graphite, produced using low-impurity carbonaceous raw materials (such as petroleum coke and pitch coke) as aggregates and coal tar as binders, through a series of processes including batching, kneading, molding, carbonization (industrially known as roasting), and graphitization.

Since natural graphite is generally produced in the narrow sense of the word, artificial graphite, this article will only analyze and discuss the differences and connections between natural graphite and artificial graphite in the narrow sense.

Natural graphite has relatively well-developed crystals. The degree of graphitization of natural flake graphite is typically above 98%, while that of natural microcrystalline graphite is typically below 93%.
The degree of crystal development in artificial graphite depends on the raw material and the heat treatment temperature. Generally speaking, the higher the heat treatment temperature, the higher the degree of graphitization. Currently, the degree of graphitization of industrially produced artificial graphite is typically below 90%.

Natural flake graphite is a single crystal with a relatively simple microstructure. It contains only crystallographic defects (such as point defects, dislocations, and stacking faults), resulting in anisotropic macroscopic structural characteristics. Natural microcrystalline graphite has smaller grains, with random arrangement and pores resulting from the removal of impurities. It exhibits isotropic macroscopic structural characteristics.
Artificial graphite can be considered a multiphase material, consisting of a graphite phase derived from carbonaceous particles such as petroleum coke or pitch coke, a graphite phase derived from the coal tar binder surrounding the particles, and pores formed by particle accumulation or by heat treatment of the coal tar binder.

Natural graphite typically exists in powder form and can be used alone, but is often combined with other materials.
Artificial graphite comes in a variety of forms, including powder, fiber, and block. In the narrower sense, artificial graphite is typically in block form and needs to be processed into a specific shape before use.

Natural graphite and artificial graphite share similarities but also exhibit differences in performance. For example, both natural and artificial graphite are good conductors of heat and electricity. However, for graphite powders of the same purity and particle size, natural flake graphite has the best thermal and electrical conductivity, followed by natural microcrystalline graphite, and artificial graphite has the lowest.
Graphite exhibits good lubricity and a certain degree of plasticity. Natural flake graphite, with its well-developed crystals and low coefficient of friction, exhibits the best lubricity and highest plasticity. Dense crystalline graphite and cryptocrystalline graphite rank second, while artificial graphite performs poorly.

Graphite possesses many excellent properties, making it widely used in industries such as metallurgy, machinery, electrical engineering, chemicals, textiles, and defense. The applications of natural and synthetic graphite overlap as well as differ.
In the metallurgical industry, natural flake graphite, due to its excellent oxidation resistance, is used in the production of refractory materials such as magnesia-carbon bricks and alumina-carbon bricks.
Artificial graphite can be used as steelmaking electrodes, while electrodes made from natural graphite are difficult to use in the harsher operating conditions of electric steelmaking furnaces.
In the mechanical industry, graphite is commonly used as a wear-resistant and lubricating material. Natural flake graphite has excellent lubricity and is often used as a lubricant additive.
In equipment that transports corrosive media, piston rings, seals, and bearings made from synthetic graphite are widely used, eliminating the need for lubricant.
Natural graphite and polymer resin composites can also be used in the above-mentioned applications, but their wear resistance is inferior to that of synthetic graphite. Artificial graphite boasts corrosion resistance, good thermal conductivity, and low permeability. It is widely used in the chemical industry to manufacture equipment such as heat exchangers, reaction tanks, absorption towers, and filters.

Natural graphite and polymer resin composites can also be used in these areas, but their thermal conductivity and corrosion resistance are inferior to those of artificial graphite.

The development of new graphite products based on artificial graphite production processes is no longer a new topic in the artificial graphite industry. Numerous carbon-graphite products are produced using natural graphite as the primary or secondary raw material, following artificial graphite production processes, and some have even become large-scale industries.

Zinc-manganese battery carbon rods: Carbon rods for zinc-manganese batteries (commonly known as dry cells) are manufactured using natural microcrystalline graphite and coal tar pitch as the primary raw materials through a process of mixing, extrusion, roasting, machining, and wax impregnation.

Primarily leveraging the high conductivity and low cost of natural microcrystalline graphite, these products have low ash content requirements, but stricter requirements for impurities such as iron and sulfur.

Natural graphite brushes: Motor brushes are manufactured using natural flake graphite and coal tar pitch as the primary raw materials through a process of mixing, flaking, grinding, molding, roasting (and graphitization, if necessary), and machining.

This material primarily utilizes the high conductivity and orientation of natural flake graphite. It requires low levels of impurities such as iron and sulfur, and an ash content no higher than 2%. Care should be taken during machining to ensure the flake's orientation.

Mechanical carbon graphite material: Made primarily from natural graphite and coal tar pitch, this material is produced through a process of mixing, flaking, grinding, molding, and roasting. It requires precision machining based on application requirements.

From the above examples, it can be seen that compared with artificial graphite in a narrow sense, carbon graphite products made from natural graphite as the main raw material or auxiliary raw material and produced according to the artificial graphite production process have the following differences in production process and product performance:

(1) The former usually needs to undergo graphitization treatment at a temperature above 2500°C to obtain the required physical and chemical properties, while the latter can be graphitized or not. In order to reduce production costs, graphitization treatment is usually not performed, so there is a "carbon" phase in its structure that is converted from the binder pitch. This carbon, which is located around the graphite particles and binds the graphite particles together, has a higher hardness and a much lower conductivity than natural graphite, so it has a greater impact on the performance of the product.

(2) Since natural graphite usually exists in powder form and has poor bonding strength with coal tar, carbon graphite products made from natural graphite as raw material usually have disadvantages such as high porosity, low mechanical strength, poor oxidation resistance, and thermal shock resistance. Therefore, the product specifications cannot be too large, and the application field is also greatly limited.

Based on the above analysis and discussion, the author believes that the following technical issues require attention when developing artificial graphite using natural graphite as a raw material:

Surface modification of natural graphite. Compared to carbonaceous raw materials such as petroleum coke and pitch coke, natural graphite has fewer surface oxygen functional groups, is less active, and has poorer bonding with coal tar pitch.

Therefore, carbon-graphite products made using natural graphite, especially natural flake graphite, as the primary raw material and using artificial graphite production processes inevitably suffer from poor mechanical properties. Appropriate surface treatment of natural graphite is necessary to increase the content of surface oxygen functional groups.

Purification of natural graphite. Carbonaceous raw materials such as petroleum coke and pitch coke are relatively pure, with ash contents typically below 0.5%. However, natural graphite processed through flotation is relatively pure, with carbon contents typically below 90%. Therefore, carbon-graphite products made from natural graphite often have lower purity and poor overall performance, limiting their application areas. High-purification treatment of natural graphite is one approach to addressing this problem.

Chemical purification is less expensive, but the washing process consumes large amounts of water and causes significant pollution. High-temperature purification, on the other hand, is more expensive. Some have suggested first preparing block graphite using the production process for artificial graphite, then subjecting it to high-temperature heat treatment above 2500°C to graphitize the "carbon" phase while removing impurities from the natural graphite phase. However, this increases production costs and, secondly, the defects caused by the vaporization of impurities often lead to reduced product performance.

Natural graphite particle size. To improve process and product performance, most carbon-graphite products, except fine-structure carbon-graphite products, require carbon raw materials of varying particle sizes during the batching process. For some large-format products, the carbon raw material particle size can even reach 16 mm. However, natural graphite processed by flotation often forms a fine powder with particle sizes ranging from tens to hundreds of microns. Therefore, the use of natural graphite as a raw material is limited to the production of fine-structure carbon-graphite products.

Although natural microcrystalline graphite of varying particle sizes is available, its low purity and high-temperature purification costs make it prohibitively expensive. Therefore, there are no reports of coarse-structure carbon-graphite products produced using natural microcrystalline graphite as a raw material. To address the shortage of large-particle natural graphite, it is recommended to adopt the "secondary coking" process used in the artificial graphite industry to process carbon black raw materials.

Volume shrinkage during the production process. During the production of artificial graphite, especially during graphitization, the carbon atoms gradually shift toward a regular graphite structure, resulting in significant volume shrinkage in the finished product.
This volume shrinkage has the advantage of increasing the density of the finished product, but uneven shrinkage can easily cause cracking. Natural graphite, on the other hand, experiences less volume shrinkage during the carbonization and graphitization processes, resulting in lower density and mechanical properties.
In addition, when developing artificial graphite from natural graphite, the overall production cost must be considered.
The price of natural graphite after flotation is similar to that of calcined petroleum coke and pitch coke. After further purification to a carbon content of 98%, the price of natural graphite is nearly double that of calcined petroleum coke and pitch coke. Therefore, except for the aforementioned carbon-graphite products, which have already established significant industries, most proposed technical routes and measures will significantly increase production costs.

In summary, developing artificial graphite products using natural graphite as a raw material is an important approach to expanding the application of natural graphite.
Natural graphite has long been used as a supplementary raw material in some artificial graphite production, but developing artificial graphite products using natural graphite as the primary raw material still presents numerous challenges that need to be addressed.
The best approach to achieving this goal is to fully understand and utilize the structure and properties of natural graphite and adopt appropriate process routes and methods to produce artificial graphite products with specialized structures, properties, and applications.