Petroleum coke is a combustible solid product obtained by coking petroleum residue and petroleum pitch. Its color is black and porous, the main element is carbon, and the ash content is very low, generally below 0.5%. Petroleum coke is classified as easily graphitized carbon. Petroleum coke is widely used in the chemical industry, metallurgy, and other industries. It is the main material for the production of artificial graphite products and carbon products for electrolytic aluminum.
Petroleum coke can be divided into green coke and calcined coke according to the difference in heat treatment temperature. The former petroleum coke obtained by delayed coking contains a lot of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most refineries in China only produce coke, and most of the calcination operations are carried out in carbon plants.
According to the difference in sulfur content, petroleum coke can be divided into three types: high-sulfur coke (sulfur content above 1.5%), medium-sulfur coke (sulfur content 0.5%-1.5%), and low-sulfur coke (sulfur content below 0.5%). Graphite electrodes and other artificial graphite products are generally produced using low-sulfur coke.
Needle coke is a kind of high-quality coke with an obvious fibrous texture, particularly low thermal expansion coefficient and very simple graphitization. When the focal block breaks, it can be split into slender strip-shaped particles (the aspect ratio is generally above 1.75), and an anisotropic fibrous structure can be observed under a polarizing microscope, so it is called needle focal.
The anisotropy of physical and mechanical properties of needle coke is very obvious, and it has excellent electrical and thermal conductivity parallel to the long axis of the particles and has a low thermal expansion coefficient. During extrusion, most of the long axes of the particles are aligned in the extrusion direction. Therefore, needle coke is a key material for making high-power or ultra-high-power graphite electrodes. The graphite electrodes made have low resistivity, a 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 and then melts after being heated. The density is 1.25-1.35g/cm3. According to its softening point, it is divided into three types: low temperature, medium temperature, and high temperature. The yield of medium temperature pitch is 54-56% of that of coal tar. The composition of coal tar pitch is extremely complex, which is 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. Many characteristic indicators of coal pitch, such as pitch softening point, toluene insoluble (TI), quinoline insoluble (QI), coking value, and coal pitch rheology.
Coal tar pitch is used in the carbon industry as a binder and is impregnant. Its performance has a great influence on the production process and product quality of carbon products. The binder asphalt generally uses medium temperature or medium temperature modified asphalt with a moderate softening point, high coking value, and high β resin, and the impregnating agent should use medium temperature asphalt with a low softening point, low QI, and good rheological properties.