Specialties graphite covers a wide range. There are different understandings and classification methods in the electro carbon products industry, natural graphite products industry and metallurgical carbon products industry. According to the customary classification and interpretation of metallurgical carbon products industry, specialties graphite mainly refers to high strength, high density and high purity graphite products (referred to as "three high graphite").
The production process of high purity graphite depends on whether the product is isotropic graphite or anisotropic graphite. The process can be divided into three types. The similarities and key technical problems between the production of three-high graphite and the production of graphite electrodes are briefly introduced as follows:
Selecting raw materials
In the production of low ash or high purity graphite, petroleum coke with few impurities should be selected first. Although most impurities can be removed at 2600-3000 degrees Celsius during the graphitization process, the selection of raw materials is still an important link. The ash content of petroleum coke is generally required to be less than 0.1% and that of coal tar asphalt to be less than 0.3%.
Grinding, Screening and Ingredients
For the production of three-high graphite with fine grain structure, most of them use -0.075mm powder, some of them use -0.042mm or -0.037mm powder; for the production of ultra-fine graphite, the particle size of the powder is less than 0.02mm or smaller, so it is difficult for calcined coke to use airflow mill or other grinding equipment to produce ultra-fine particles. The classification of fine particles is difficult, and special technology should be used for classification. The coke particle size of extruded products is directly proportional to the cross-section area of products. The larger the cross-section area, the more large particles are needed. The maximum coke particle size has little relationship with the cross-section area of products during molding. The proportioning ratio is generally classified as a production secret, but its determination is mainly based on experience and constantly revised. Different forming methods have different proportion of binder even if the proportion of granular material is the same. Through production practice, the best combination point is selected between ensuring quality index and improving yield.
Kneading and Forming
Kneading
In general, the mixing pot with two axes is used for mixing and kneading. Sometimes the pressure kneading machine is used. When producing products with more micro-powder, the paste is not easy to mix well. Pressure kneading is beneficial to improve the quality of mixing.
Forming
There are three forming methods for producing three-high graphite, namely extrusion, die pressing and isostatic pressing (Fig. 2), each of which has its advantages and disadvantages. The production efficiency of extrusion forming is high, but the anisotropy of finished products is high. Molding production efficiency is low, suitable for the production of fine-grained graphite; anisotropy is smaller than extrusion products, many of the fine-grained structure products after mixing paste to roll into thin sheets, so that binder and coke powder better combination. The rolled sheet is crushed after cooling and then cold pressed into the die or heated to a certain temperature for forming (called warm molding).
There are two ways of isostatic pressing: one is to heat the raw products after extrusion or die pressing to a certain temperature and put them into the isostatic pressing die; the other is to load the prepared powder into the die. Dies with materials are placed in sealed high-pressure tanks, which are uniformly pressurized. The pressure is generally 100-200MPa. Isostatic pressing has the lowest production efficiency, but it can produce finished graphite with better isotropy. Vibration molding is generally used to produce carbon products for aluminum or large size graphite electrodes and carbon electrodes. At present, it can also be used to molding special graphite. Vibration molding equipment is relatively simple and the purchase price is low. It has become the main molding equipment of some small special graphite factories.
Isostatic pressing
The molding process of isotropic graphite is mostly changed from molding to isostatic molding. The hydrostatic forming equipment is mainly composed of elastic die, high pressure vessel, frame and hydraulic system. Elastic molds are usually made of rubber or resin synthetic materials. The size and shape of material particles have a great influence on the life of elastic molds. Die design is a key technical problem in hydrostatic pressing. Elastic molds are closely related to the size and homogeneity of products. Most of the high pressure vessels are thick-walled metal cylinders made of high strength alloy steel directly cast and processed by machine tools. Their strength is strong enough to resist strong liquid pressure. The cylinder structure also has many forms, such as double-layer composite cylinders, pre-stressed steel wire wound reinforced cylinders, etc. Hydraulic system is composed of low pressure pump, high pressure pump, supercharger and various valves. It starts with a high flow rate low pressure pump. After a certain pressure, it is supplied by a high pressure pump, and the supercharger further increases the liquid pressure in the high pressure vessel.
There are two types of liquid isostatic pressing equipment, i. e. wet bag cold isostatic pressing machine and dry bag cold isostatic pressing machine. Fig. 4 is the structure schematic diagram of two kinds of cold isostatic pressing tanks.
Wet Bag Cold Isostatic Pressing Machine
This method hangs the die in the high pressure vessel and can be loaded into several dies according to the size of the product. It is suitable for small batch, small size and complex shape products. Carbon products are mainly produced by wet bag cold isostatic press.
Dry Bag Cold Isostatic Pressing Machine
This method is suitable for products with large size and large production capacity. At this time, the cold isostatic press equipment is also different from the cold isostatic press used in wet bag process. Pressure punch, limiter and ejector are added. This method fixes elastic die in high pressure vessel and locates it with limiter, so it is also called fixed die method. In production, the powder is loaded into the die with a pressure punch and the upper opening is closed. When pressurized, the liquid medium is injected into the periphery of the elastic die in the container to pressurize the die. When demolding, it is not necessary to take out the mold, and the green billet after molding is ejected by ejector mechanism. This kind of isostatic pressing equipment is widely used in mass production of special refractories.
Isostatic Pressing Procedure (Production of Carbon Products)
Mold preparation
Dies should be made of oil-resistant and heat-resistant materials, such as natural rubber molds immersed in transformer oil can only be used 1-2 times, so when transformer oil is used as pressure medium, chloroprene rubber with good oil-resistance is generally used, and polyvinyl chloride plastic film can also be used to make molds.
Filling the material
There are many kinds of raw materials to be loaded into the mold, such as raw petroleum coke powder without binder, paste made by mixing calcined petroleum coke powder with asphalt for grinding, and petroleum coke powder for grinding and mixing with powdered asphalt for use. Different raw materials and proportions can obtain different forming effects and different physical and mechanical properties. Feeding should be vibrated at the same time to make the powder material initially dense in the die. After filling, the mold is properly shaped by hand, and then the other end of the mold is plugged with rubber plug or plastic plug, and tightened with iron wire to prevent the liquid medium from intruding into the mold. In order to make the gas in the powder fully discharged under pressure, the exhaust pipe is inserted in the powder beforehand, and the vacuum pump is connected to suction. In the production of some spherical products, the powder should be pre-pressed into spheres by molding and then placed into the corresponding size of isostatic pressing molds. The die structure for pressing cylindrical products is shown in Fig. 5. Finally, the powder mold is placed in the high pressure vessel, and the pressure is applied after sealing the entrance of the high pressure vessel.
Boosting and Decreasing Pressure
Start high-pressure pump, inject liquid medium into high-pressure vessel, and pay close attention to boost and exhaust. Pressure is generally carried out step by step. When the pressure drops to normal pressure, open the inlet of the high pressure vessel and take out the die. The method of heating high pressure vessel can also be used to boost pressure. Because the volume of liquid heated expands, the pressure rises automatically after heating, but the pressure rises automatically to a certain extent.
Roasting and impregnation
For graphite products requiring higher bulk density, crack waste is easy to occur during roasting, so a slower heating curve should be used. For small-sized products, heat-resistant steel plates can be used to make square or circular containers for roasting. Raw products can be placed in containers and filled with fillers to isolate and cover, and then loaded into the roasting furnace. Impregnation is important. High density products should be impregnated 2-4 times and roasted once after each impregnation. The softening point of the impregnating agent should be selected correctly (related to the viscosity of the impregnating agent), and the preheating temperature before impregnation and the technological parameters such as temperature, pressure, vacuum and pressure time of the impregnating pot should be controlled to achieve the best impregnation effect.
Graphitization
The larger three-high graphite is directly packed in graphitization furnace, the smaller products are packed in graphite crucible, and then the crucible is packed in graphitization furnace; the products with less resistance requirement can be less charged, and the products with higher resistance requirement can be electrified to meet the product quality index; the production of high-purity graphite needs to be injected into purified gas (Chlorine and Freon) at the later stage of graphitization and into the graphitization furnace. Purified gases need to be carried out after the furnace temperature rises to 1800 degrees Celsius. Nitrogen is introduced first, chlorine is introduced when the furnace temperature reaches about 1950 degrees Celsius, and Freon is introduced when the furnace temperature reaches about 2350 degrees Celsius. At this time, chlorine continues to enter, and chlorine and Freon are continued for several hours until the power supply is stopped. This is to prevent the vaporized impurity gases from diffusing in the opposite direction to the furnace core.
Epilogue
Since the reform and opening up, China's carbon industry has developed greatly and has become a major producer of carbon products. However, compared with the developed industrial countries in the world, there are still some gaps in quality, variety and energy consumption. Up to now, much high-quality special graphite needed in China still rely on imports. It is a long way to go to catch up with the advanced international technology in the field of special graphite production.
