Graphite electrodes are high-temperature-resistant graphite conductive electrodes produced by a series of processes such as kneading, molding, roasting, impregnating, graphitization, and mechanical processing, using petroleum coke and needle coke as aggregates and coal tar pitch as raw materials. binder. Material.
Graphite electrode is an important high-temperature conductive material for electric furnace steelmaking. The electric energy is input into the electric furnace through the graphite electrode, and the high temperature generated by the arc generated between the electrode end and the charge is used as the heat source to melt the charge for steelmaking. Some other submerged arc furnaces for smelting yellow phosphorus, industrial silicon, abrasives, and other materials also use graphite electrodes as conductive materials. Utilizing the excellent and special physical and chemical properties of graphite electrodes, it is also widely used in other industrial sectors.
The raw materials for producing graphite electrodes include petroleum coke, needle coke, and coal tar pitch
graphite electrode
Petroleum coke is a combustible solid product obtained by coking petroleum residue and pitch. The color is black and porous, the main element is carbon, and the ash content is very low, generally below 0.5%. Petroleum coke is an easily graphitizable carbon. Petroleum coke is widely used in the chemical industry, metallurgy, and other industries. It is the main raw material for the production of artificial graphite products and electrolytic aluminum carbon products.
According to the heat treatment temperature, petroleum coke can be divided into green and calcined coke. The former is petroleum coke obtained by delayed coking, which contains a large amount of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most refineries in China only produce green Coke, and most calcination operations are carried out at carbon plants.
According to the level of sulfur content, petroleum coke can be divided into high sulfur coke (sulfur content greater than 1.5%), medium sulfur coke (sulfur content is 0.5%-1.5%), and low sulfur coke (sulfur content is less than 0.5%). Artificial graphite products such as graphite electrodes are generally produced with low sulfur coke.
Needle Coke is a high-quality Coke with an obvious fibrous structure, extremely low thermal expansion coefficient, and easy graphitization. When the Coke block is broken, it can be split into long and thin strip particles according to the texture (the aspect ratio is generally above 1.75). The anisotropic fiber structure can be observed under a polarizing microscope, so it is called needle focusing.
The anisotropy of the physical and mechanical properties of needle coke is very obvious. It has good electrical and thermal conductivity parallel to the long axis of the particles, and a low thermal expansion coefficient. During extrusion molding, the major axes of most particles are aligned along the direction of extrusion. Therefore, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes, which has the characteristics of low resistivity, small thermal expansion coefficient, and good thermal shock resistance.
Needle coke is divided into oil-based needle coke produced from petroleum residue and coal-based needle coke produced from refined coal tar pitch.
Coal tar pitch is one of the main products of coal tar deep processing. It is a mixture of various hydrocarbons. It is a black high-viscosity semi-solid or solid at room temperature. It has no fixed melting point. It softens when heated and then melts. The density is 1.25-1.35g/cm3. According to its softening point, it can be divided into three types: low-temperature, medium-temperature, and high-temperature asphalt. The yield of medium-temperature pitch is 54-56% of that of coal tar. The composition of coal tar pitch is extremely complex, related to the properties of coal tar and the content of heteroatoms, and is also affected by the coking process system and coal tar processing conditions. There are many indicators to characterize the characteristics of coal tar pitch, such as pitch softening point, toluene insoluble (TI), quinoline insoluble (QI), coking value, rheology of coal tar pitch, etc.
Coal tar pitch is used as a binder and impregnating agent in the carbon industry, and its performance has a great influence on the production process and product quality of carbon products. Generally, medium-temperature or medium-temperature modified asphalt with moderate softening point, high coking value, and high β resin is used as an impregnating agent for bonding asphalt, as well as medium-temperature asphalt with low softening point, low QI, and good rheological properties.
The figure below shows the production process flow chart of graphite electrodes in carbon enterprises
Calcination: The production process in which carbon-containing raw materials are heat-treated at high temperatures to discharge the moisture and volatiles contained in them, and correspondingly improve the physical and chemical properties of raw materials is called calcination. Carbonaceous raw materials are generally calcined with gas and its volatiles as the heat source, and the maximum temperature is 1250-1350 °C.
Calcination causes profound changes in the organizational structure and physical and chemical properties of carbonaceous raw materials, mainly reflected in the improvement of coke density, mechanical strength, and electrical conductivity, as well as the improvement of coke chemical stability and oxidation resistance, which is the follow-up process.
graphite electrode