Classification and production of special graphite

Classification of special graphite

The coverage of special graphite is very wide. The electric carbon product industry, natural graphite product industry and metallurgical carbon product industry have different understanding and classification methods. For the customary classification of metallurgical carbon product industry, special graphite mainly refers to high strength , High density, high purity graphite products (referred to as three high graphite). Three high graphite can be divided into three types of coarse particles, fine particles and ultrafine particles in terms of material structure; from the molding method, there are three types of special graphite such as molding, extrusion and isostatic molding. In addition, vibration molding also It can be used to produce special graphite.

High-quality special graphite-isotropic graphite is produced by isostatic pressing process. According to their main uses, they are: special graphite for electrical discharge machining; special graphite for casting molds; special graphite for continuous casting of steel or copper and aluminum; high-purity for Czochralski single-crystal silicon furnace or for smelting precious metals and high-purity materials Graphite; graphite for synthetic artificial diamond; special graphite for rocket and missile technology.

Generalized special graphite also includes pyrolytic carbon (pyrolytic graphite), carbon materials for bioengineering, glassy carbon, porous carbon and porous graphite, graphite interlayer compounds (such as flexible graphite, fluorinated graphite), and graphite for lasers. The mechanical and electronic industry also uses many special carbon and graphite materials, such as carbon brushes, carbon graphite bearings, feed sliders for electric locomotives, graphite molds for drawing optical fibers, etc., which will not be described in this article.

Main varieties of special graphite

Graphite for EDM

EDM is a new processing technology in the machinery manufacturing industry. EDM can process many metals with higher hardness, and can process parts with complex shapes and high precision requirements. It can be used as a tool electrode for the anode Graphite can also be used for copper materials.

The graphite material used as the electrode for EDM tools must meet the following conditions:

1. The structure is dense and the organization is uniform. There should be no coarse particles and large pores;

2. It has high mechanical strength and good processing performance, and can process complex shapes or acute angles and flakes;

3. Graphite tool electrode has a certain loss during the EDM process, this loss should be as low as possible;

4. The discharge characteristics are stable and the processing speed is fast.

Therefore, graphite for EDM generally adopts fine-grain structure graphite or ultra-fine-grain structure graphite, and the physical properties are preferably isotropic. Therefore, the isotropic graphite with fine particle structure is currently used more in the EDM graphite material market in China.

Mold graphite

The casting industry in the machinery industry uses a large number of graphite materials as molds for processing such as pressure casting, centrifugal casting, and hot extrusion of superhard alloys, from train wheels to small precision parts. The graphite mold can be used repeatedly for many times, and the casting after demolding has a high finish, and some can be used without further processing. The graphite material used as a casting mold should be graphite with dense texture, low thermal expansion coefficient, and good oxidation resistance. The graphite used for casting large-size castings can be composed of coarser particle size, and used for casting small precision parts The graphite must use fine-grained graphite.

In the mold graphite, the quality of the continuous casting graphite is the most stringent, requiring high thermal conductivity, good thermal stability and thermal shock resistance, good lubricity, no infiltration with molten metal, no reaction with cast metal, and easy processing Into a mold with precise dimensions.

Graphite for synthetic diamond

Diamond is an important cutting and grinding material in industry, but naturally produced diamond has little output and is expensive. Graphite and diamond both belong to the carbon element, but the crystalline form is different. At high temperature and high pressure, graphite can be converted into a crystalline form of diamond. As early as 1954, Sweden and the United States successfully synthesized synthetic diamond successively. China also synthesized artificial in the 1960s Diamond, and in the 1970s produced special graphite materials for synthetic man-made diamonds, which formed a certain scale of production. The special graphites for synthetic diamonds produced in China are divided into 3 categories. The characteristics and uses are shown in the chart.

High purity graphite

High-purity graphite generally refers to graphite with a carbon content of more than 99.99%. The structure can be divided into three types: coarse particle structure, fine particle structure and ultra-fine particle structure. High-purity graphite is widely used in Czochralski single-crystal silicon furnaces . The basic material of integrated circuits is mainly silicon single crystal chips. At present, the growth process of silicon single crystals mainly uses the Czochralski method. Other methods include magnetic field Czochralski method, regional method and double crucible crystal pulling method. The graphite parts are consumables, and the high-purity graphite materials are used to process the heating system of the Czochralski single crystal silicon furnace.

Another important use of high-purity graphite is processing into various types of crucibles for the production of precious metals, rare metals or high-purity metals, and non-metallic materials. The graphite electrode for spectral analysis is also a high-purity graphite, which can be used for the spectral chemical analysis of all elements except carbon. The graphite electrode for spectral analysis is formed by extrusion. The impurity element content of the finished product should not be greater than 6 * 10-5. Spectral pure carbon powder or spectral pure graphite powder is required when preparing standard samples and collecting impurities by chemical methods in spectral analysis. The requirements for impurity content of these two high-purity materials are both 6 * 10-5; in some In terms of use, the carbon content needs to reach 99.9995%, and the total ash content is less than 5 * 10-6. There are three molding methods for high-purity graphite: extrusion molding, compression molding and isostatic molding.

Graphite materials for nuclear energy

Graphite is one of the deceleration materials and reflective materials used in the construction of nuclear reactors. The early reactors were all graphite reactors. Graphite for nuclear reactors used as structural materials is much stricter and more expensive than graphite electrodes in the selection of raw materials, process control, and product inspection. Graphite for nuclear reactors must have the following properties: low absorption of slow neutrons, good high temperature strength , High thermal shock resistance, good deceleration performance for fast neutrons, dimensionally stable under irradiation, and very little impurity content.

Nuclear graphite must have a high volume density, because the deceleration effect of graphite on fast neutrons is achieved by the collision effect of fast neutrons on carbon atoms. The more carbon atoms in a unit volume, the better the deceleration effect, so the volume density It is one of the main indexes of nuclear graphite. The volume density is also directly related to the porosity and permeability of graphite. In order to avoid the loss of nuclear fuel and heating medium, the porosity and permeability must be reduced to a certain level.

Isotropic graphite

Although the international definition of isotropic graphite needs to be further clarified, it is generally to measure some physical performance indicators of the product in the diameter direction and length direction and calculate their ratio. Some are expressed by the ratio of the coefficient of thermal expansion. The simplest is the resistance The ratio of the ratio means that the anisotropy ratio is called an isotropic product in the range of 1.0-1.1, and more than 1.1 is called anisotropic product. In addition to the use of general petroleum coke, the manufacture of isotropic graphite also uses modified asphalt coke, natural asphalt coke, oxidized petroleum coke, green petroleum coke without calcination, natural graphite, etc.

Introduction to the production of Sangao graphite

The production process of high-purity graphite depends on whether the product is isotropic graphite or anisotropic graphite, and which molding process is used. Generally, it can be divided into 3 types, see Figure 1. The production of Tri-High Graphite has similarities with the production of graphite electrodes, and the key technical issues are briefly introduced as follows:

Selected raw materials

To produce low-ash or high-purity graphite, first of all, petroleum coke with few impurity elements should be selected. Although most of the impurity elements can be removed at a temperature of 2600-3000 degrees Celsius during the graphitization process, it is still very important to select raw materials In the process, the ash content of petroleum coke is generally required to be below 0.1%, and the ash content of coal pitch is below 0.3%.

Milling, sieving and ingredients

Production of fine-grained three-high graphite, most of which uses -0.075mm powder, part of the powder uses -0.042mm or -0.037mm fine powder; produces ultra-fine structure graphite, the particle size of the powder is less than 0.02mm or less Therefore, it is necessary to use a jet mill or other milling equipment for producing ultra-fine particles after calcination. The classification of fine powder is difficult, and special techniques are used for classification. The coke particle size of the extruded product is proportional to the size of the cross-sectional area of the product. The larger the cross-sectional area, the more large particles are required. The maximum particle size of coke during compression molding is not related to the cross-sectional area of the product. The batching ratio is generally a production secret, but its determination is mainly based on experience and is constantly being revised. Different molding methods have different binder ratios even if the proportion of pellets is the same. Through production practice, between ensuring quality indicators and improving yield Choose the best combination.

Kneading and molding

Mix

Kneading generally uses a biaxial mixing kneading pot, sometimes using a pressure kneading machine, the paste is not easy to knead well when producing products with more fine powder, the use of pressure kneading is beneficial to improve the quality of kneading.

forming

There are three molding methods for producing three-high graphite, namely extrusion, molding and isostatic pressing (Figure 2), each with advantages and disadvantages. Extrusion production efficiency is high, but the anisotropy of the finished product is large. Compression molding has low production efficiency and is suitable for producing fine-grained graphite. The anisotropy is smaller than that of extruded products. Many molded fine-grained structure pastes are rolled into thin sheets after mixing and kneading, so that binder and coke Good combination. The rolled flakes are cooled and pulverized and then added to the mold for cold compression molding or heated to a certain temperature and then formed (called warm molding).

There are two methods of isostatic pressing. One is to heat the extruded or molded raw product to a certain temperature and put it into the isostatic pressing mold; the other is to put the prepared powder into the mold. The mold containing the material is placed in a sealed high-pressure tank, which is evenly pressurized in the high-pressure tank. The pressure is generally 100-200MPa. Isostatic production has the lowest efficiency, but can produce finished graphite with better isotropy. Vibration molding is generally a molding method for producing carbon products for aluminum or large-size graphite electrodes and carbon electrodes. It can also be used for molding special graphite. Due to the relatively simple vibration molding equipment and low purchase price, it has become a small special graphite factory. Main molding equipment.

Isostatic pressing

The molding process of isotropic graphite is mostly changed from compression molding to isostatic molding. Liquid isostatic pressing molding equipment is mainly composed of elastic mold, high pressure container, frame and hydraulic system. Elastic molds are generally made of rubber or resin synthetic materials. The particle size and shape of the material have a greater impact on the life of the elastic mold. The mold design is a key technical issue of liquid isostatic pressing. The elastic mold is closely related to the size and homogeneity of the product. . Most high-pressure vessels are thick-walled metal cylinders machined directly from high-strength alloy steel and machined by machine tools. They are strong enough to withstand strong liquid pressure. The cylinder structure also has many forms, such as double-layer composite cylinders and prestressed steel wires Reinforce the cylinder, etc. The hydraulic system consists of a low-pressure pump, a high-pressure pump, a booster and various valves. It starts to be supplied by a low-pressure pump with a large flow rate. After reaching a certain pressure, the high-pressure pump supplies oil, and the booster further increases the Liquid pressure.

There are two types of liquid isostatic pressing equipment, namely wet bag method cold isostatic pressing machine and dry bag method cold isostatic pressing machine. Figure 4 shows the construction principle diagram of two cold isostatic pressing tanks.

Wet bag method cold isostatic pressing machine (Figure 4a)

This method suspends the mold in a high-pressure container, and several molds can be installed according to the size of the product. It is suitable for products with small batch size, small size, and complex appearance. Wet bag method cold isostatic press is mainly used to produce carbon products.

Dry bag cold isostatic press (figure b)

This method is suitable for products with large size and large production volume. At this time, the cold isostatic machine equipment is also different from the cold isostatic press used in the wet bag method. A pressure punch, a limiter and a topping device are added. This method fixes the elastic die in the high-pressure container and uses the limiter to position it, so it is also called the fixed die method. During production, the material powder is loaded into the mold with a pressure punch and the upper mouth is closed. When pressurized, the liquid medium is injected into the periphery of the elastic mold in the container to pressurize the mold. When demoulding, it is not necessary to take out the mold, and the ejecting mechanism is used to eject the formed green body. The mass production of special refractory materials mostly uses this isostatic pressing equipment.

Operating procedure of isostatic pressing process (production of carbon products)

Mold preparation

The mold should choose oil-resistant and heat-resistant materials. For example, a mold made of natural rubber immersed in transformer oil can only be used 1-2 times. You can use PVC plastic film to make molds.

Loading

There are a variety of raw materials loaded into the mold, such as uncalcined raw petroleum coke powder (without binder); the paste of the calcined petroleum coke powder and asphalt is kneaded and then used; The powder is mixed with powdered asphalt and used. Different raw materials and ratios can obtain different molding effects and different physical and mechanical properties. Vibration should be carried out at the same time during charging, so that the powdery raw material is initially dense in the mold. After the filling is completed, the mold is properly shaped by hand, and then the other end of the mold is plugged with a rubber plug or a plastic plug, and tied with an iron wire to prevent the liquid medium from intruding into the mold. In order to allow the gas in the powder to be fully discharged when pressed, an exhaust pipe is inserted in the powder in advance, and an external vacuum pump is used for air extraction. When producing certain spherical products, the powder should be pre-pressed into spheres by molding, and then placed into the isostatic pressing mold of the corresponding size; the mold structure when pressing cylindrical products is shown in Figure 5. Finally, put the mold filled with powder in a high-pressure container, and pressurize after sealing the inlet of the high-pressure container.

Boost and buck

Start the high-pressure pump, inject the liquid medium into the high-pressure container, and pay close attention to the pressure increase and exhaust conditions. Pressurization generally takes place in stages. When the pressure drops to normal pressure, remove the mold after opening the inlet of the high-pressure container. It is also possible to increase the pressure by heating the high-pressure container. Since the volume of the heated liquid expands, the pressure automatically increases after heating, but this automatic pressure increase has a certain limit.

Roasting and impregnation

For graphite products that require a higher volume density, cracking waste products are likely to occur during firing, so a slower heating curve should be used; for the firing of small-size products, heat-resistant steel plates can be used to make square or round containers for firing. Raw products are placed in containers and filled with filler to isolate and cover, and then loaded into the roaster. The impregnation is expensive, the high-density product should be immersed 2-4 times, and roasted once after each immersion; the softening point of the impregnating agent should be correctly selected (related to the viscosity of the impregnating agent), and the pre-impregnation of the roasted product should be controlled Process parameters such as heat temperature and temperature, pressure, vacuum, pressurization time of the impregnation tank to achieve the best impregnation effect.

Graphitization

The larger three-height graphite is directly installed in the graphitization furnace, the small size products are installed in the graphite crucible, and then the crucible is installed in the graphitization furnace; products with less strict resistance requirements can be less electricity, the resistance The products with high requirements must be electrified to meet the product quality index; the production of high-purity graphite must be purged with gas (chlorine and freon) in the later stage of graphitization. Purification gas must be introduced after the furnace temperature rises to 1800 degrees Celsius. Nitrogen is introduced into chlorine when it reaches about 1950 degrees Celsius, and Freon is introduced when it reaches about 2350 degrees Celsius. At this time, chlorine gas continues to be introduced until the power supply is stopped and then chlorine and Freon are continued for several hours. This is to prevent The vaporized impurity gas diffuses toward the furnace core in the opposite direction.

Conclusion

Since the reform and opening up, China's carbon industry has made great progress and has become a major producer of carbon products. However, compared with industrial developed countries in the world, there is still a certain gap in terms of quality, variety, energy consumption, etc. Up to now, many domestic high-quality special graphites still rely on imports to catch up with the international advanced technology in the field of special graphite production Heavy duty