I. Basic structure, properties and classification of graphite

Crystal structure of graphite
Graphite is a substance composed of a single carbon element. Its crystal structure belongs to the hexagonal system and presents a hexagonal layered structure. On the layer, carbon atoms are combined with σ bonds formed by sp2 hybrid orbitals and delocalized π bonds formed by Pz orbitals to form a solid hexagonal grid plane. The distance between carbon-carbon atoms is 1.42Å. The carbon atoms have a very strong bond energy (345KJ/mol), while the carbon atom planes are combined with weaker van der Waals forces (bond energy is 16.7KJ/mol), and the layer spacing is 3.354Å.

Graphite is soft and black-gray; it feels greasy and can stain paper. The hardness is 1~2 and the theoretical density is 2.26g/cm3.

There is no pure graphite in nature. Natural graphite minerals often contain impurities such as SiO2, A12O3, FeO, CaO, P2O5, and CuO. These impurities often appear in the form of minerals such as quartz, pyrite, and carbonates. In addition, it also contains gases such as water, hydrocarbons, CO2, H2, and N2. Therefore, in the analysis of graphite, in addition to determining the fixed carbon content, the volatile matter and ash content must also be determined at the same time.

 

II. Basic properties of graphite

Due to its special structure, graphite has the following excellent properties:
(1) High temperature resistance: Graphite is one of the most temperature-resistant substances. It has no melting point at normal pressure. Even if it is burned by an ultra-high temperature arc, the weight loss is very small.
(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 graphite grains and the degree of crystal development. The larger the graphite grains, the more perfect 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 higher degree of crystal development can even be ground into very thin sheets.
(6) Thermal shock resistance: Graphite has a very small thermal expansion coefficient and can withstand drastic temperature changes without being damaged during use.

 

III. Classification and characteristics of graphite

Graphite can be divided into natural graphite and artificial graphite. The two have similar structures and the same physical and chemical properties, but their uses are quite different.

01 Natural graphite
Natural graphite is a gift from nature that is formed by the transformation of carbon-rich organic matter under the long-term action of high temperature and high pressure geological environment. The process characteristics of natural graphite are mainly determined by its crystal form. Graphite minerals with different crystal forms have different industrial values ​​and uses. There are many types of natural graphite. According to different crystal forms, natural graphite is divided into three categories in industry: dense crystalline graphite, flake graphite and cryptocrystalline graphite. There are two main categories in my country: flake graphite and cryptocrystalline graphite. Dense crystalline graphite is also called block graphite. This type of graphite has obvious crystals and the crystals are visible to the naked eye. The particle diameter is greater than 0.1 mm. The crystals are arranged in a disorderly manner and present a dense block structure. The grade is very high, with a general carbon content of 60%~65%, sometimes up to 80%~98%, but its plasticity and slipperiness are not as good as flake graphite. Natural flake graphite belongs to pegmatite in crystallography. It is a single crystal named because its crystals are scaly, and there are large flakes and fine flakes. The lubricity and plasticity of this type of graphite are better than other types of graphite, so it has the greatest industrial value.
Although the grade of flake graphite ore is not high, and the carbon content is generally between 3% and 25%, it is one of the best floatable ores in nature. After multiple grinding and selection, high-grade graphite concentrate can be obtained. Cryptocrystalline graphite is also called amorphous graphite or earthy graphite. In recent years, it has begun to be called microcrystalline graphite. The crystal diameter of this graphite is generally less than 1 micron, and the crystal shape can only be seen under an electron microscope. It can be regarded as an aggregate of graphite crystals. Natural microcrystalline graphite is usually transformed from coal under high temperature and high pressure geological environment. Therefore, natural microcrystalline graphite is usually associated with coal. In natural microcrystalline graphite ore bodies, the transition zone from anthracite to natural microcrystalline graphite can often be seen. This type of graphite is characterized by an earthy surface, lack of gloss, lower lubricity than flake graphite, and poor selectivity. But the grade is higher, the carbon content is generally 60%~80%, and a few are as high as 90% or more.

02 Artificial graphite
Artificial graphite is similar to polycrystalline in crystallography. There are many types of artificial graphite, and the production processes vary greatly. In a broad sense, all graphite materials obtained by carbonizing organic matter and then graphitizing it at high temperature can be collectively referred to as artificial graphite, such as carbon (graphite) fiber, pyrolytic carbon (graphite), foamed graphite, etc. In a narrow sense, artificial graphite usually refers to block solid materials made from carbonaceous raw materials (petroleum coke, asphalt coke, etc.) with low impurity content as aggregates and coal tar as binders, such as graphite electrodes, isostatic graphite, etc.

 

VI. The difference and connection between natural graphite and artificial graphite

Since the artificial graphite prepared from natural graphite as raw material is usually in a narrow sense, this article only analyzes and discusses the difference and connection between natural graphite and artificial graphite in a narrow sense.

Crystal structure
The crystals of natural graphite are well developed. The graphitization degree of natural flake graphite is usually above 98%, while the graphitization degree of natural microcrystalline graphite is usually below 93%. The degree of crystal development of artificial graphite depends on the raw materials and heat treatment temperature. Generally speaking, the higher the heat treatment temperature, the higher the graphitization degree. The graphitization degree of artificial graphite produced industrially is usually less than 90%.

Organization structure
Natural flake graphite is a single crystal with a simple organization structure. It only has crystallographic defects (point defects, dislocations, stacking faults, etc.), and exhibits anisotropic structural characteristics on a macro scale. The grains of natural microcrystalline graphite are small, the grains are randomly arranged, and there are holes after impurities are removed, which exhibits isotropic structural characteristics on a macro scale. Artificial graphite can be regarded as a multiphase material, including graphite phases transformed from carbonaceous particles such as petroleum coke or asphalt coke, graphite phases transformed from coal tar binders wrapped around the particles, and pores formed by particle accumulation or coal tar binders after heat treatment.

Physical form
Natural graphite usually exists in the form of powder and can be used alone, but it is usually used in combination with other materials. Artificial graphite has many forms, including powder, fiber and block, while artificial graphite in a narrow sense is usually block and needs to be processed into a certain shape when used.

Physical and chemical properties
Natural graphite and artificial graphite have both similarities and differences in performance. For example, both natural graphite and artificial graphite are good conductors of heat and electricity, but for graphite powders of the same purity and particle size, natural flake graphite has the best heat transfer and electrical conductivity, followed by natural microcrystalline graphite, and artificial graphite has the lowest. Graphite has good lubricity and certain plasticity. The crystal development of natural flake graphite is more complete, the friction coefficient is smaller, the lubricity is the best, and the plasticity is the highest, while dense crystalline graphite and cryptocrystalline graphite are second, and artificial graphite is poor.

Application field
Graphite has many excellent properties, so it is widely used in metallurgy, machinery, electrical, chemical, textile, defense and other industrial sectors. The application fields of natural graphite and artificial graphite have both overlapping parts and different places. In the metallurgical industry, natural flake graphite can be used to produce refractory materials such as magnesium carbon bricks and aluminum carbon bricks due to its good oxidation resistance. Artificial graphite can be used as steelmaking electrodes, while electrodes made of natural graphite are difficult to use in steelmaking electric furnaces with harsher use conditions. In the mechanical industry, graphite materials are usually used as wear-resistant and lubricating materials. Natural flake graphite has good lubricity and is often used as an additive for lubricating oils. Equipment for conveying corrosive media widely uses piston rings, seals and bearings made of artificial graphite, and no lubricating oil is required during operation. Natural graphite and polymer resin composites can also be used in the above fields, but their wear resistance is not as good as artificial graphite. Artificial graphite has the characteristics of corrosion resistance, good thermal conductivity and low permeability. It is widely used in the chemical industry to make heat exchangers, reaction tanks, absorption towers, filters and other equipment. Natural graphite and polymer resin composites can also be used in the above fields, but their thermal conductivity and corrosion resistance are not as good as artificial graphite.

IV. Developing artificial graphite with natural graphite as raw material

In fact, it is not a new topic in the artificial graphite industry to develop new graphite products by learning from the preparation process of artificial graphite. There are many carbon graphite products prepared by artificial graphite production process with natural graphite as the main raw material or auxiliary raw material, and some have even formed a large industry.

Zinc-manganese battery carbon rod: Zinc-manganese battery (commonly known as dry battery) carbon rod produced by mixing, extrusion molding, roasting, machining, wax dipping and other processes with natural microcrystalline graphite as the main raw materials. It mainly utilizes the high conductivity and low price of natural microcrystalline graphite, and does not require high ash content, but has strict requirements on impurities such as iron and sulfur.

Natural graphite brush: Motor brush produced by mixing, rolling, grinding, molding, roasting (graphitization treatment is required if necessary), machining and other processes with natural flake graphite and coal tar as the main raw materials. Mainly utilizing the high conductivity and high orientation of natural flake graphite, requiring low impurity content such as iron and sulfur and ash content not higher than 2%, and attention should be paid to the orientation of flake graphite during machining.

Carbon graphite materials for machinery: block materials produced by mixing, rolling, grinding, molding, roasting and other processes with natural graphite and coal tar as the main raw materials, which need to be precisely machined according to the use requirements. Mainly utilizing the lubricity and high temperature and corrosion resistance of natural graphite, there are high requirements for ash and impurity content.

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 prepared according to the artificial graphite production process have the following differences in production process and product performance:
(1) The former usually needs to be graphitized at more than 2500℃ 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 organizational structure that is converted from the binder asphalt. 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 binding force with coal tar, carbon graphite products prepared from natural graphite as raw material usually have disadvantages such as large 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, when developing artificial graphite using natural graphite as raw material, the following technical issues need to be paid attention to:
Surface modification of natural graphite. Compared with carbonaceous raw materials such as petroleum coke and asphalt coke, natural graphite has fewer oxygen-containing functional groups on its surface, lower activity, and poorer binding force with coal tar. Therefore, carbon graphite products prepared by artificial graphite production process with natural graphite, especially natural flake graphite, as the main raw material inevitably have poor mechanical properties. It is necessary to properly surface treat natural graphite to increase the content of oxygen-containing functional groups on its surface.

Purification of natural graphite: Carbonaceous raw materials such as petroleum coke and asphalt coke have high purity, and the ash content is usually less than 0.5%, while the purity of natural graphite treated by flotation is low, and the carbon content is usually below 90%. Therefore, carbon graphite products prepared with natural graphite as raw material are often limited in application due to low purity and poor comprehensive performance. High purification of natural graphite is one way to solve this problem. Chemical purification has low cost, but the washing process uses a large amount of water and causes greater pollution, while high-temperature purification has the problem of high cost. Some people also believe that block graphite can be prepared according to the production process of artificial graphite, and then through high-temperature heat treatment above 2500℃, the impurities in the natural graphite phase can be removed while graphitizing the "carbon" phase. However, this will increase the production cost and the defects formed after the impurities are gasified will often cause the product performance to decline.

Particle size of natural graphite: In order to improve the process performance and product performance, most carbon graphite products, except for fine-structured carbon graphite products, need to use carbon raw materials of different particle sizes during the batching process. For some large-sized products, the particle size of carbon raw materials even reaches 16 mm, while the natural graphite treated by flotation is often in the form of fine powder, with a particle size of only tens to hundreds of microns. Therefore, the use of natural graphite as raw material is limited to the preparation of fine-structured carbon graphite products. Although natural microcrystalline graphite of different particle sizes can be obtained, due to its low purity and high cost of high-temperature purification treatment, there is no report on the preparation of coarse-structured carbon graphite products using natural microcrystalline graphite as raw material. In order to solve the problem of lack of large-particle natural graphite, it is recommended to adopt the "secondary coke" process used by the artificial graphite industry to process carbon black raw materials.

Volume shrinkage during the preparation process: During the preparation process of artificial graphite, especially during the graphitization process, the arrangement of carbon atoms gradually changes to a regular graphite structure, so the volume of the product shrinks greatly. The advantage of this volume shrinkage is that it can increase the density of the product, but when the shrinkage is uneven, it is easy to cause the product to crack. When natural graphite is used as the raw material, the density and mechanical properties of the product are low due to the small volume shrinkage during the carbonization and graphitization processes. In addition, when developing artificial graphite using natural graphite as raw material, the issue of comprehensive production costs also needs to be considered. Since the price of natural graphite after flotation is similar to that of calcined petroleum coke and asphalt coke, and after purification to a carbon content of 98%, the price of natural graphite is close to twice that of calcined petroleum coke and asphalt coke, most technical routes and technical measures will significantly increase production costs, except for the aforementioned carbon-graphite products that have formed a large industry.

Developing artificial graphite products with natural graphite as raw material is one of the important ways to expand the application of natural graphite. Natural graphite has long been used as an auxiliary raw material in the production of some artificial graphite, but there are still many problems to be solved in developing artificial graphite products with natural graphite as the main raw material.