Classification and production of specialty graphite

　　Classification of specialty graphite

　　Special graphite covers a wide range. The electric carbon products industry, natural graphite products industry and metallurgical carbon products industry have different understanding and classification methods. Regarding the customary classification of metallurgical carbon products 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 particle, fine particle and ultra-fine particle structure from the material structure. From the molding method, there are three types of special graphite: molding, extrusion and isostatic pressing. In addition, vibration molding is also Can be used to produce special graphite.

　　High-quality special graphite-isotropic graphite is produced by isostatic pressing. According to its main purposes, there are: special graphite for electric discharge machining; special graphite for making casting molds; special graphite for continuous casting of steel or copper and aluminum; high purity for Czochralski silicon furnace or for smelting precious metals and high purity materials Graphite; Graphite for synthetic artificial diamond; Special graphite for rocket and missile technology.

　　Special graphite in a broad sense also includes pyrolytic carbon (pyrolytic graphite), carbon materials for bioengineering, glassy carbon, porous carbon and porous graphite, graphite interlaminar compounds (such as flexible graphite, fluorinated graphite), graphite for lasers, etc. , Mechatronics industry also uses many special carbon and graphite materials, such as carbon brushes, carbon graphite bearings, feeder sliders for electric locomotives, graphite molds for drawing optical fibers, etc., which are not described in this article.

　　Main varieties of special graphite

　　Graphite for EDM

　　EDM is a new type of processing technology in the machinery manufacturing industry. EDM can process many hard metals, and can process parts with complex shapes and high precision requirements. It can be used as an anode tool electrode Copper materials, graphite materials can also be used.

　　Graphite materials used as tool electrodes for EDM must meet the following conditions:

　　1. Dense structure and uniform organization, should not have coarse particles and large pores;

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

　　3. Graphite tool electrodes have certain loss during the EDM process, and this loss should be as low as possible;

　　4. Stable discharge characteristics and faster processing speed.

　　Therefore, graphite for EDM generally adopts fine-grain structure graphite or ultra-fine-grain structure graphite, and the physical properties are preferably isotropic. Therefore, at present, the fine-grained isotropic graphite is mostly used in the graphite material market for EDM in China.

　　Mold graphite

　　The foundry industry in the machinery industry uses a large number of graphite materials as molds for pressure casting, centrifugal casting, and hot extrusion of super-hard alloys. Graphite molds can be used for large train wheels and small precision parts. Graphite molds can be reused many times, and the castings after demolding have a higher 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 grain size, and used for casting small precision parts The graphite must use fine-grained graphite.

　　Among the mold graphites, the quality requirements of continuous metal casting graphite are 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 precise size mold.

　　Graphite for Synthetic Diamond

　　In industry, diamond is an important cutting and grinding material, but the output of natural mining diamond is very small and expensive. Graphite and diamond belong to the same carbon element, but the crystal form is different. Graphite can be transformed into diamond crystal form under high temperature and high pressure. As early as 1954, Sweden and the United States succeeded in synthesizing synthetic diamond. China also synthesized synthetic diamond in the 1960s. Diamond, special graphite materials for synthetic synthetic diamonds were produced in the 1970s, forming a certain scale of production. The special graphite for synthetic diamonds produced in China is 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 99.99% or more. It can be divided into three types: coarse-grained structure, fine-grained structure and ultra-fine-grained 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 adopts the Czochralski method. Other methods include the magnetic field Czochralski method, the zone method and the double crucible crystal pulling method. The graphite parts are consumables, which are processed into the heating system of the Czochralski silicon furnace with high-purity graphite materials.

　　Another important use of high-purity graphite is processing into various crucibles for the production of precious metals, rare metals or high-purity metals and non-metal materials. Graphite electrode for spectral analysis is also a kind of high-purity graphite, which can be used for spectrochemical analysis of all elements except carbon. Graphite electrode for spectral analysis is formed by extrusion method. The content of impurity elements in the finished product should not exceed 6*10-5. Spectral pure carbon powder or spectral pure graphite powder is required when preparing standard samples and chemically trapping impurities in spectral analysis. The impurity content of these two high-purity materials is 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 pressing.

　　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 piles. As a structural material, graphite used in nuclear reactors is much stricter than graphite electrodes in terms of raw material selection, process control, and finished product inspection, and it is also much more expensive. Graphite used in nuclear reactors must have the following properties: low absorption of slow neutrons and good high-temperature strength , High thermal shock resistance, good deceleration performance for fast neutrons, stable size under irradiation, and minimal impurity content.

　　Nuclear graphite must have a higher bulk density, because the deceleration effect of graphite on fast neutrons is achieved by the collision of fast neutrons on carbon atoms. The more carbon atoms in the unit volume, the better the deceleration effect, so the bulk density It is one of the main indicators 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 should 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 certain physical properties of the product in the diameter direction and length direction and calculate the ratio. Some are expressed by the ratio of the thermal expansion coefficient, and the simplest is the resistance. The ratio of the rate indicates that the anisotropic ratio is within the range of 1.0-1.1, called an isotropic product, and more than 1.1 is called an anisotropic product. In addition to general petroleum coke for the production of isotropic graphite, modified pitch coke, natural pitch coke, oxidized petroleum coke, uncalcined raw petroleum coke, natural graphite, etc. are also used.

　　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 three types, as shown in Figure 1. The production of Sangao 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 select petroleum coke with few impurity elements. Although most impurity elements can be removed at a temperature of 2600-3000 degrees Celsius during the graphitization process, it is still important to select raw materials. In the link, 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%.

　　Grinding, sieving and ingredients

　　For the production of fine-grained graphite, most of the -0.075mm powder is used, and some of the powder is -0.042mm or -0.037mm fine powder; the ultra-fine structure of graphite is produced, and the particle size of the powder is less than 0.02mm or less Therefore, the calcined coke needs to use a jet mill or other grinding equipment for producing ultrafine particles. The classification of fine powder is more difficult, and special technology is used for classification. The coke particle size of the extruded product formulation is directly proportional to the cross-sectional area of ​​the product. The larger the cross-sectional area, the more large particles are needed. The largest coke particle during compression molding has little relationship with the cross-sectional area of ​​the product. The proportion of ingredients is generally a production secret, but its determination is mainly based on experience and is constantly being revised. Even if the proportion of pellets is the same in different molding methods, the proportion of binder is different. Through production practice, it is between ensuring quality indicators and improving yield Choose the best bonding point.

　　Kneading and molding

　　Kneading

　　Kneading generally uses a biaxial stirring kneading pot, and sometimes a pressure kneader is used. It is not easy to knead the paste when producing products with more micropowder. The use of pressure kneading is beneficial to improve the kneading quality.

　　forming

　　There are three molding methods for producing three high graphite, namely extrusion, molding and isostatic pressing (Figure 2). Each has its advantages and disadvantages. Extrusion molding has high production efficiency, but the finished product has a large degree of anisotropy. Compression molding has low production efficiency and is suitable for the production of fine-grained graphite. The degree of anisotropy is smaller than that of extruded products. Many compression-molded fine-grained products have to be rolled into thin sheets after being kneaded to make the binder and coke powder more compact. Good combination. The rolled sheet is cooled and crushed and then added to the mold for cold pressing or heating to a certain temperature before forming (called warm molding).

　　There are two methods for isostatic pressing. One is to heat the extruded or molded 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, and the pressure is uniformly pressurized in the high-pressure tank, and the pressure is generally 100-200MPa. Isostatic pressing has the lowest production efficiency, but it can produce finished graphite with good isotropy. Vibration molding is generally a molding method for the production of carbon products for aluminum or large-size graphite electrodes and carbon electrodes. It can also be used to form special graphite. Because the vibration molding equipment is relatively simple and the purchase price is low, it has become the mainstay of some small special graphite factories. Main molding equipment.

　　Isostatic pressing

　　Most of the molding process of isotropic graphite is changed from compression molding to isostatic pressing. The hydrostatic pressure forming equipment is mainly composed of elastic molds, high pressure vessels, frames and hydraulic systems. Elastic molds are generally made of rubber or resin synthetic materials. The size and shape of the material particles have a greater impact on the life of the elastic mold. Mold design is a key technical issue for 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 processed by machine tools after direct casting of high-strength alloy steel, which is strong enough to resist strong liquid pressure. The cylinder structure also has many forms, such as double-layer combined cylinders and prestressed steel wires. Around the reinforced cylinder, etc. The hydraulic system consists of a low-pressure pump, a high-pressure pump, a booster, and various valves. The oil is supplied by a low-pressure pump with a larger flow rate. After reaching a certain pressure, the high-pressure pump supplies oil, and the booster further increases the oil Liquid pressure.

　　Liquid isostatic pressing molding equipment is divided into two types, namely wet-bag cold isostatic press and dry-bag cold isostatic press. Figure 4 shows the schematic diagram of the two types of cold isostatic pressure tanks.

　　Wet bag cold isostatic press (Figure 4a)

　　This method hangs the mold in a high-pressure container, and can be loaded into several molds according to the size of the product. It is suitable for products with small batches, small sizes, and complex shapes. The production of carbon products mainly uses wet-bag cold isostatic press.

　　Dry bag cold isostatic press (Figure b)

　　This method is suitable for products with larger sizes and large production volumes. At this time, the cold isostatic machine equipment is also different from the cold isostatic press used in the wet bag method. The pressure punch, stopper and ejector are added. This method fixes the elastic mold in the high-pressure container and uses the stopper to position it, so it is also called the fixed mold method. During production, use a pressure punch to fill the powder into the mold and close the upper mouth. When pressurizing, the liquid medium is injected into the periphery of the elastic mold in the container to pressurize the mold. It is not necessary to take out the mold during demolding, and use the ejector mechanism to eject the formed green body. This kind of isostatic pressing equipment is often used for mass production of special refractory materials.

　　Isostatic pressing process operation procedure (production of carbon products)

　　Mold preparation

　　The mold should be made of oil-resistant and heat-resistant materials. For example, a mold made of natural rubber can only be used 1-2 times when immersed in transformer oil. Therefore, when transformer oil is used as the pressure medium, neoprene rubber with better oil resistance is generally used. You can use polyvinyl chloride plastic film to make the mold.

　　Loading

　　There are many kinds of raw materials into the mold, such as uncalcined raw petroleum coke powder (without binder); calcined petroleum coke powder mixed with asphalt and used after grinding; calcined petroleum coke is ground into 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. Vibrate at the same time when loading, so that the powdery raw material is initially dense in the mold. After the material is loaded, 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 invading the mold. In order to allow the gas in the powder to be fully discharged under pressure, an exhaust pipe is inserted in the powder in advance, and an external vacuum pump is connected to extract air. When producing certain spherical products, the powder should be pre-compressed into a sphere by compression molding, and then placed in an isostatic pressing mold of the corresponding size; the mold structure for pressing cylindrical products is shown in Figure 5. Finally, the mould filled with powder is placed in a high-pressure container, and the inlet of the high-pressure container is sealed and pressurized.

　　Boost and buck

　　Start the high-pressure pump, inject the liquid medium into the high-pressure container, and pay close attention to the pressure rise and exhaust. Pressurization is generally carried out in stages. When the pressure drops to normal pressure, open the inlet of the high-pressure container and take out the mold. It is also possible to increase the pressure by heating the high-pressure container. Because the volume of the liquid expands when heated, the pressure automatically rises after heating, but this automatic pressure rise has a certain limit.

　　Roasting and impregnation

　　For graphite products that require higher bulk density, cracks are likely to occur during baking, so a slower heating curve is used; for the baking of small-sized products, heat-resistant steel plates can be used to make square or round containers for baking. The raw products are placed in a container and filled with fillers to isolate and cover, and then loaded into the roasting furnace. Impregnation is more expensive than impregnation. High-density products need to be impregnated 2-4 times, and roasted once after each impregnation; the softening point of the impregnant should be selected correctly (related to the viscosity of the impregnant), and the pre-impregnation of the roasted product should be controlled. Heat temperature and process parameters such as the temperature, pressure, vacuum degree and pressurization time of the dipping tank to achieve the best dipping effect.

　　Graphitization

　　The larger three-high graphite is directly installed in the graphitization furnace, and the small-sized products are installed in the graphite crucible, and then the crucible is installed in the graphitization furnace; products with less strict requirements on resistivity can be less energized, and resistance Products with high rate requirements are energized to meet product quality indicators; purified gas (chlorine and freon) must be introduced in the later stage of graphitization to produce high-purity graphite, and purified gas must be introduced after the furnace temperature rises to 1800 degrees Celsius, first. Nitrogen, when it reaches about 1950 degrees Celsius, pass in chlorine, when it reaches about 2350 degrees Celsius, pass in Freon. At this time, the chlorine gas will continue to pass in. After the power supply is stopped, continue to pass chlorine and Freon 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 developed significantly and has become a major producer of carbon products. However, compared with the industrialized countries in the world, there is still a certain gap in quality, variety, energy consumption, etc. So far, many high-quality special graphites that are needed in China still rely on imports, and they must catch up with the international advanced in the field of special graphite production.