Physical and chemical properties of raw materials for graphite electrode production

Raw materials for producing graphite electrodes include petroleum coke, needle coke and coal tar pitch. 

Petroleum coke is a combustible solid product obtained by coking petroleum residue and petroleum pitch. It is black and porous, the main element is carbon, low ash content, generally less than 0.5%. Petroleum coke is a kind of graphitized carbon. Petroleum coke is widely used in the chemical and metallurgical industries and is the main raw material for the production of artificial graphite products and electrolytic aluminum carbon products.

Petroleum coke can be divided into raw coke and calcined coke according to heat treatment temperature. The former is obtained by delayed coking, which contains a large amount of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining raw coke. Most refineries in China only produce raw coke, and calcination is usually carried out in carbon plants.

Petroleum coke can be divided into high sulfur coke (with sulfur content above 1.5%), medium sulfur coke (with sulfur content of 0.5% - 1.5%) and low sulfur coke (with sulfur content below 0.5%) according to the sulfur content. Graphite electrode and other artificial graphite products are generally produced with low sulfur coke.

Needle coke is a kind of high-quality coke with obvious fibrous texture, especially low thermal expansion coefficient and easy graphitization. When the coke block is broken, it can split into elongated strip particles according to the texture (the aspect ratio is generally above 1.75). Anisotropic fibrous structure can be observed under the polarizing microscope, so it is called needle coke.

The anisotropy of 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 low thermal expansion coefficient. In the extrusion process, the long axes of most particles are arranged according to the extrusion direction. Therefore, needle coke is the key raw material for manufacturing high-power or ultra-high-power graphite electrodes, which have 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 normal temperature, without a fixed melting point. Soften after heating and then melt. The density is 1.25-1.35g/cm3. According to its softening point, it can be divided into low temperature, medium temperature and high temperature pitch. The yield of medium temperature pitch is 54-56% of coal tar. The composition of coal tar pitch is extremely complex, which is related to the properties and heteroatom content of coal tar, and 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 softening point, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value and fluidity of coal tar pitch.

Coal tar pitch is used as binder and impregnating agent in carbon industry. Its performance has great influence on the production process and product quality of carbon products. Binder asphalt is generally used with moderate softening point, high coking value β medium temperature or medium temperature modified asphalt with high resin, medium temperature asphalt with low softening point, low Qi and good rheological property shall be used as impregnating agent. Please consult us for the detailed explanation of graphite electrode performance.