In recent years, for the sake of economic development, the government in regions rich in natural graphite resources has actively promoted the development of the natural graphite industry, which has set off a wave of development of artificial graphite products using natural graphite as raw materials.

It should be said that drawing on the preparation process of artificial graphite and developing new graphite products are an important way to expand the application field of natural graphite, but because the two are both connected and different in terms of structure, performance and use, it is necessary to analyze and Discussion, so that managers and researchers can correctly understand and use natural graphite materials, so that the development of new products of natural graphite is more healthy and efficient.

The basic structure, properties and classification of graphite

Graphite crystal structure

Graphite is a substance composed of a single carbon element, and its crystal structure belongs to a hexagonal crystal system, which is a hexagonal layered structure. The carbon atoms on the plane are combined with the σ bond formed by the sp2 hybrid orbital and the delocalized π bond formed by the Pz orbital to form a solid hexagonal grid-like plane. The carbon-carbon atom spacing is 1.42Å, and the carbon atoms have extremely strong Bond energy (345KJ / mol), and the carbon atom planes are bonded with a weak van der Waals force (bond energy is 16.7KJ / mol), and the interlayer spacing is 3.354Å.

Graphite is soft, black and gray; greasy, can pollute paper. The hardness is 1 ~ 2, and the theoretical density is 2.26g / cm3.

There is no pure graphite in nature, and natural graphite minerals often contain impurities such as SiO2, A12O3, FeO, CaO, P2O5, CuO and so on.

These impurities often appear in the form of quartz, pyrite, carbonate and other minerals. In addition, it also contains water, hydrocarbons, CO2, H2, N2 and other gases.

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

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 under normal pressure. Even if it is burned by ultra-high temperature arc, the weight loss is very small.

(2) Electrical and thermal conductivity: graphite has high electrical and thermal conductivity. 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 corrosion by acids, alkalis, and organic solvents.

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

(6) Thermal shock resistance: The thermal expansion coefficient of graphite is very small, and it can withstand the drastic temperature change without damage during use.

Classification and characteristics of graphite

Graphite can be divided into natural graphite and artificial graphite. The structure of the two is similar and the physical and chemical properties are the same, but the use is quite different.

01

Natural graphite

Natural graphite is transformed from carbon-rich organic matter under the long-term effect of high-temperature and high-pressure geological environment, which is a gift from nature. The technical characteristics of natural graphite mainly depend on its crystalline form. Graphite minerals with different crystalline forms have different industrial values ​​and uses.

There are many types of natural graphite. According to different crystalline forms, industrial graphite is divided into three types: dense crystalline graphite, flake graphite and cryptocrystalline graphite. There are two main types of flake graphite and cryptocrystalline graphite in China.

Dense crystalline graphite is also called massive graphite. Such graphite crystals are obvious, and the crystals are visible to the naked eye. The particle diameter is greater than 0.1 mm. The crystal arrangement is disorderly and disorderly, showing a dense block structure. The grade is very high, the general carbon content is 60% ~ 65%, sometimes up to 80% ~ 98%, but its plasticity and greasiness are not as good as flake graphite.

Natural flake graphite is a great crystal in crystallography. It is a single crystal. It is named because its crystals are in the form of scales. There are large scales and fine scales. The lubricity and plasticity of this graphite are superior to other types of graphite, so its industrial value is the largest.

Although the grade of flake graphite ore is not high, the carbon content is generally between 3% and 25%, but it is one of the best floatable ores in nature, and high-grade graphite concentrate can be obtained through multiple grinding and multiple selection.

Cryptocrystalline graphite, also known as amorphous graphite or earthy graphite, has become called microcrystalline graphite in recent years. The crystal diameter of this graphite is generally less than 1 micron, and the crystal form can only be seen under an electron microscope, which can be regarded as an aggregate of graphite crystals.

Natural microcrystalline graphite is usually converted from coal under high-temperature and high-pressure geological environment. Therefore, natural microcrystalline graphite is usually associated with coal. The transition from anthracite to natural microcrystalline graphite can often be seen in natural microcrystalline graphite ore bodies region.

This type of graphite is characterized by a soil-like surface, lack of gloss, less lubricity than flake graphite and poor selectivity. But the grade is higher, the carbon content is generally 60% to 80%, and a few are as high as 90%.

02

Artificial graphite

Artificial graphite is similar to polycrystals in crystallography. There are many kinds of artificial graphite, and the production process is very different. Broadly speaking, all graphite materials obtained by carbonization of organic matter and graphitization and high temperature treatment can be collectively called artificial graphite, such as carbon (graphite) fiber, pyrolytic carbon (graphite), foamed graphite, etc.

In the narrow sense, artificial graphite usually refers to the carbonaceous raw materials with low impurity content (petroleum coke, asphalt coke, etc.) as aggregate, coal pitch, etc. as a binder, after batching, kneading, molding, carbonization (industry said For roasting) and graphitization, block solid materials, such as graphite electrodes, isostatic graphite, etc.

The difference and connection between natural graphite and artificial graphite

In view of the fact that natural graphite is usually produced in the narrow sense of artificial graphite, this article only analyzes and discusses the difference and connection between natural graphite and artificial graphite in the narrow sense.

Crystal structure

The crystal development of natural graphite is relatively complete. 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 degree of graphitization. At present, the degree of graphitization of artificial graphite produced in industry is usually less than 90%.

Organizational structure

Natural flake graphite is a single crystal with a simple structure and only crystallographic defects (point defects, dislocations, stacking faults, etc.), and shows anisotropic structural characteristics on a macroscopic scale. Natural microcrystalline graphite has small crystal grains, random arrangement of crystal grains, and pores after impurities are removed, which shows isotropic structural characteristics on a macro scale.

Artificial graphite can be regarded as a multi-phase material, including graphite phase converted by carbonaceous particles such as petroleum coke or pitch coke, graphite phase converted by coal pitch binder wrapped around the particles, particle accumulation or coal pitch bonding The pores formed by the agent after heat treatment.

Physical form

Natural graphite usually exists in powder form and can be used alone, but it is usually used after being compounded with other materials.

There are many forms of artificial graphite, both powdery, fibrous and massive, and artificial graphite in the narrow sense is usually massive and needs to be processed into a certain shape when used.

Physicochemical properties

Both natural graphite and artificial graphite have commonalities and performance differences. For example, natural graphite and artificial graphite are good conductors of heat and electricity, but for graphite powder of the same purity and particle size, natural flake graphite has the best heat transfer performance and electrical conductivity, followed by natural microcrystalline graphite, and artificial graphite lowest.

Graphite has better lubricity and certain plasticity. The crystals of natural flake graphite are well developed, the friction coefficient is small, the lubricity is the best, and the plasticity is the highest. The dense crystalline graphite and cryptocrystalline graphite are second, artificial graphite Poor.

Application field

Graphite has many excellent properties, so it is widely used in metallurgy, machinery, electrical, chemical, textile, national defense and other industrial sectors. The fields of application of natural graphite and artificial graphite are both overlapping and different.

In the metallurgical industry, natural flake graphite can be used to produce refractory materials such as magnesia carbon bricks and aluminum carbon bricks due to its good oxidation resistance.

Artificial graphite can be used as a steel-making electrode, and the electrode made of natural graphite is difficult to use in the steel-making electric furnace with more severe use conditions.

In the machinery industry, graphite materials are commonly used as wear-resistant and lubricating materials. Natural flake graphite has good lubricity and is often used as an additive in lubricating oil.

The equipment for transporting corrosive media widely uses piston rings, sealing rings and bearings made of artificial graphite, without the need to add lubricating oil when working.

Natural graphite and polymer resin composite materials can also be used in the above fields, but the 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 composite materials can also be used in the above fields, but thermal conductivity and corrosion resistance are not as good as artificial graphite.

Development of artificial graphite using natural graphite as raw material

In fact, drawing on the preparation process of artificial graphite to develop new graphite products is no longer a new topic in the artificial graphite industry. There are many carbon graphite products prepared with natural graphite as the main raw material or auxiliary raw materials according to the artificial graphite production process, and some have even formed a larger industry.

Zinc-manganese battery carbon rods: zinc-manganese batteries (commonly known as dry batteries) carbon rods produced by kneading, extrusion molding, roasting, machining, wax dipping and other processes using natural microcrystalline graphite and coal pitch as the main raw materials.

It mainly uses the characteristics of high conductivity and low price of natural microcrystalline graphite. It has low requirements for ash content, but stricter requirements for the content of impurities such as iron and sulfur.

Natural graphite brushes: Motor brushes made from natural flake graphite and coal pitch as the main raw materials, after mixing, kneading, flaking, grinding, compression molding, roasting (graphitization if necessary), machining and other processes.

It mainly uses the characteristics of high conductivity and high orientation of natural flake graphite, which requires low content of impurities such as iron and sulfur and ash content of not more than 2%. Pay attention to the orientation of flake graphite during machining.

Carbon graphite materials for machinery: Natural graphite and coal pitch are the main raw materials, and the bulk materials produced through kneading, flaking, grinding, compression molding, roasting and other processes need to be precision machined according to the requirements of use.

It mainly uses the lubricity, high temperature resistance and corrosion resistance of natural graphite, and has high requirements for ash and impurities.

As can be seen from the above examples, compared with artificial graphite in a narrow sense, using natural graphite as the main raw material or auxiliary raw material, the carbon graphite products 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 2500 ° C or higher to obtain the required physical and chemical properties, while the latter can be either graphitized or not graphitized.

In order to reduce production costs, graphitization is usually not carried out, so there is a “carbon” phase transformed from binder pitch in its structure.

This kind of carbon, which is located around the graphite particles and binds the graphite particles together, has greater hardness and far lower conductivity than natural graphite, so it has a great impact on the performance of the product.

(2) Since natural graphite usually exists in powder form and has poor binding power with coal pitch, carbon graphite products prepared with natural graphite as raw materials usually have large porosity, low mechanical strength, oxidation resistance and heat resistance Due to its shortcomings such as poor vibration, the product specifications cannot be too large, and the application field is also greatly restricted.

Based on the above analysis and discussion, the author believes that when developing artificial graphite using natural graphite as a raw material, it is necessary to pay attention to the following issues in terms of technology:

Surface modification of natural graphite . Compared with petroleum coke, pitch coke and other carbonaceous raw materials, natural graphite has fewer oxygen-containing functional groups on the surface, lower activity, and poorer binding power to coal pitch.

Therefore, using natural graphite, especially natural flake graphite as the main raw material, the carbon graphite products prepared according to the artificial graphite production process inevitably have the problem of poor mechanical properties. Natural graphite needs to be properly surface-treated to increase the content of oxygen-containing functional groups on its surface.

Purification of natural graphite. Petroleum coke, pitch coke and other carbonaceous materials have higher purity, and the ash content is usually less than 0.5%, while the natural graphite after flotation treatment has a lower purity, and the carbon content is usually below 90%.

Therefore, carbon graphite products prepared with natural graphite as raw materials are often limited in their application fields due to their low purity and poor overall performance. High purification of natural graphite is one of the ways to solve this problem.

The cost of chemical purification is low, but the amount of water used in the washing process is large and the pollution is large, while the high temperature purification has the problem of higher cost. Some people think that you can first prepare block graphite according to the production process of artificial graphite, and then heat treat it at a high temperature above 2500 ℃.

While the "charcoal" phase is graphitized, impurities in the natural graphite phase are removed, but first, the production cost is increased, and second, the defects formed after the gasification of the impurities often cause the performance of the product to decrease.

The particle size of natural graphite. In order to improve process performance and product performance, in addition to fine-structured carbon graphite products, most carbon graphite products require the use of different grades of carbonaceous raw materials in the batching process. Some large-scale products have a particle size of carbonaceous raw materials. 16mm, and the natural graphite after flotation treatment is often in the form of fine powder, the particle size is only tens to hundreds of microns, so the use of natural graphite as the raw material is limited to the preparation of fine-structured carbon graphite products.

Although natural microcrystalline graphite with different particle sizes can be obtained, because of its low purity and high cost of high-temperature purification treatment, there has been no report of using natural microcrystalline graphite as raw material to prepare coarse-structured carbon graphite products. In order to solve the problem of lack of large particles of natural graphite, it is recommended to use the "secondary coke" process when processing carbon black raw materials in the artificial graphite industry.

Volume shrinkage during preparation. During the preparation of artificial graphite, especially during graphitization, the arrangement of carbon atoms gradually changes to a regular graphite structure, so the volume shrinkage of the product is large.

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, due to the small volume shrinkage during the carbonization and graphitization process, the density and mechanical properties of the product are low.

In addition, when developing artificial graphite using natural graphite as a raw material, it is also necessary to consider the issue of comprehensive production cost.

Since the price of natural graphite after flotation is similar to that of calcined petroleum coke and pitch coke, and after purification treatment to a carbon content of 98%, the price of natural graphite is nearly twice that of calcined petroleum coke and pitch coke. In addition to the aforementioned carbon graphite products that have formed a larger industry, most of the technical routes and technical measures will greatly increase production costs.

Conclusion

In summary, the development of artificial graphite products using natural graphite as a raw material is one of the important ways to expand the application field of natural graphite.

Natural graphite as an auxiliary raw material has long been used in the production of some artificial graphite, but there are still many problems to be solved in the development of artificial graphite products using natural graphite as the main raw material.

Fully understanding and using the structure and characteristics of natural graphite, and adopting appropriate process routes and methods to produce artificial graphite products with special structures, properties and uses should be the best way to achieve this goal. (Source: Chenkai Petrochemical Needle Coke)