Graphite electrodes are high-temperature, conductive graphite materials produced through a series of processes—including mixing, forming, roasting, impregnation, graphitization, and mechanical machining—using petroleum coke and needle coke as aggregates and coal tar pitch as a binder. Graphite electrodes serve as crucial high-temperature conductive materials in electric arc furnaces for steelmaking. By feeding electrical energy into the furnace via graphite electrodes, a high-temperature arc is generated between the electrode tips and the charge material, providing the heat necessary to melt the charge and carry out steelmaking. Other types of submerged arc furnaces used in the smelting of yellow phosphorus, industrial silicon, abrasives, and other materials also rely on graphite electrodes as conductive materials. Due to their excellent and unique physicochemical properties, graphite electrodes find extensive applications in various other industrial sectors as well.

The raw materials for producing graphite electrodes include petroleum coke, needle coke, and coal pitch.

Petroleum coke is a combustible solid product obtained by coking petroleum residuum and petroleum asphalt. It is dark in color and highly porous, with carbon as its primary element and a very low ash content—typically below 0.5%. Petroleum coke belongs to the class of easily graphitizable carbons. It finds extensive applications in industries such as chemical engineering and metallurgy and serves as a key raw material for the production of synthetic graphite products and carbon-based materials used in electrolytic aluminum production.

Petroleum coke can be classified into two types based on its heat-treatment temperature: raw coke and calcined coke. The former, obtained through delayed coking, contains a high volatile content and has low mechanical strength. Calcined coke is produced by calcining raw coke. Most refineries in China produce only raw coke, and the calcination process is typically carried out at carbon plants.

Petroleum coke can be classified according to its sulfur content into three types: high-sulfur coke (with a sulfur content of 1.5% or higher), medium-sulfur coke (with a sulfur content between 0.5% and 1.5%), and low-sulfur coke (with a sulfur content below 0.5%). Generally, low-sulfur coke is used in the production of graphite electrodes and other synthetic graphite products.

Needle coke is a high-quality coke characterized by a distinct fibrous texture, an exceptionally low thermal expansion coefficient, and remarkable ease of graphitization. When the coke blocks are fractured, they split along their texture into slender, elongated particles (with an aspect ratio typically exceeding 1.75). Under polarized light microscopy, these particles exhibit anisotropic fibrous structures, which is why they are referred to as needle coke.

The anisotropy of the physical and mechanical properties of needle coke is highly pronounced: it exhibits excellent electrical and thermal conductivity along the direction parallel to the long axis of the particles, and has a relatively low coefficient of thermal expansion. During extrusion molding, the long axes of most particles align along the extrusion direction. Therefore, needle coke is a crucial raw material for manufacturing high-power or ultra-high-power graphite electrodes. Graphite electrodes made from needle coke have low resistivity, a small coefficient of thermal expansion, and excellent resistance to thermal shock.

Needle coke is divided into oil-based needle coke, which is produced from petroleum residue, and coal-based needle coke, which is produced from refined coal tar.

Coal tar pitch is one of the primary products obtained from the deep processing of coal tar. It is a mixture of various hydrocarbons, appearing as a black, highly viscous semi-solid or solid substance at room temperature. It has no fixed melting point; when heated, it softens first and then melts. Its density ranges from 1.25 to 1.35 g/cm³. Based on their softening points, coal tar pitches are classified into three types: low-temperature, medium-temperature, and high-temperature pitches. The yield of medium-temperature pitch typically accounts for 54–56% of the coal tar. The composition of coal tar pitch is extremely complex, closely related to the properties of the original coal tar and the content of heteroatoms, and also influenced by the coking process conditions and the processing parameters of the coal tar. Numerous indicators are used to characterize the properties of coal tar pitch, including its softening point, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value, and rheological behavior of the pitch.

Coal tar pitch is used as a binder and impregnant in the carbon industry, and its properties significantly influence both the production process and the quality of carbon products. As a binder, coal tar pitch typically consists of medium-temperature or medium-temperature modified pitches with moderate softening points, high coking values, and high β-resin content. For impregnation purposes, medium-temperature pitches with lower softening points, low QI values, and excellent rheological properties are preferred.

The following diagram shows the production process flow chart for graphite electrodes used by carbon enterprises.

Calcination is a production process in which carbonaceous raw materials are subjected to high-temperature thermal treatment to remove their moisture and volatile components, thereby enhancing their physicochemical properties. Typically, carbonaceous raw materials are calcined using gas and their own volatile components as heat sources, with the maximum temperature ranging from 1250 to 1350°C.

Calcination induces profound changes in the microstructure and physicochemical properties of carbonaceous raw materials, primarily by enhancing the density, mechanical strength, and electrical conductivity of coke, as well as improving its chemical stability and antioxidant performance, thereby laying a solid foundation for subsequent processing steps.

The main equipment used for calcination includes retort kilns, rotary kilns, and electric calcination furnaces. The quality control indicators for calcination are as follows: the true density of petroleum coke shall be no less than 2.07 g/cm³, and its resistivity shall be no greater than 550 μΩ·m; the true density of needle coke shall be no less than 2.12 g/cm³, and its resistivity shall be no greater than 500 μΩ·m.

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