graphite electrode HP400

The raw materials and manufacturing process of graphite electrodes

Graphite electrodes are a kind of high-temperature resistant graphite conductive material produced through a series of processes such as kneading, molding, calcination, impregnation, graphitization and mechanical processing, using petroleum coke and needle coke as aggregates and coal tar pitch as binders.

 

Graphite electrodes are important high-temperature conductive materials for electric furnace steelmaking. Electrical energy is input into the electric furnace through graphite electrodes, and the high temperature generated by the electric arc between the electrode tip and the charge is used as the heat source to melt the charge for steelmaking. Some other electric furnaces for smelting yellow phosphorus, industrial silicon, abrasives and other materials also use graphite electrodes as conductive materials. Taking advantage of the excellent and special physical and chemical properties of graphite electrodes, they also have extensive applications in other industrial sectors. The commonly used grades of graphite electrodes include graphite electrode RP, graphite electrode HP, and graphite electrode UHP, which are used for electric furnaces of different powers.

 

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

 

Petroleum coke is a combustible solid product obtained after coking of petroleum residue oil and petroleum asphalt. It is black and porous, with carbon as the main element and a very low ash content, generally below 0.5%. Petroleum coke belongs to the category of easily graphitized carbon. It has extensive applications in the chemical, metallurgical and other industries and is the main raw material for the production of artificial graphite products and carbon products for electrolytic aluminum.

 

Petroleum coke can be classified into two types based on heat treatment temperature: green coke and calcined coke. The former is obtained from delayed coking, containing a large amount of volatile matter and having low mechanical strength. Calcined coke is produced by calcining green coke. Most oil refineries in China only produce raw coke, and calcination operations are mostly carried out within carbon plants.

 

Petroleum coke can be classified into three types based on sulfur content: high-sulfur coke (with sulfur content above 1.5%), medium-sulfur coke (with sulfur content ranging from 0.5% to 1.5%), and low-sulfur coke (with sulfur content below 0.5%). Low-sulfur coke is generally used in the production of graphite electrodes and other artificial graphite products.

 

Needle coke is a high-quality coke with obvious fibrous texture in appearance, particularly low thermal expansion coefficient and very easy graphitization. When the coke block breaks, it can split into slender strip-like particles according to the texture (the aspect ratio is generally above 1.75). Under a polarizing microscope, anisotropic fibrous structures can be observed, thus it is called needle coke.

 

The anisotropy of the physical and mechanical properties of needle coke is very obvious. It has good electrical and thermal conductivity in the direction parallel to the long axis of the particles, and the coefficient of thermal expansion is relatively low. During extrusion molding, the long axes of most particles are arranged in the extrusion direction. Therefore, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes. The graphite electrodes made from it have a low resistivity, a small coefficient of thermal expansion, and good thermal shock resistance.

 

Needle coke is divided into oil-based needle coke produced from petroleum residue oil as raw material and coal-based needle coke produced from refined coal tar pitch raw material.

 

Coal tar pitch is one of the main products of the deep processing of coal tar. It is a mixture of various hydrocarbons. At room temperature, it is a black, high-viscosity semi-solid or solid with no fixed melting point. It softens and then melts when heated, with a density of 1.25-1.35g/cm ³. It is classified into three types according to its softening point: low-temperature, medium-temperature and high-temperature asphalt. The yield of medium-temperature asphalt 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 the processing conditions of coal tar. There are many indicators characterizing the properties of coal tar pitch, such as the softening point of pitch, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value and rheological properties 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 significant impact on the production process and product quality of carbon products. Binder asphalt generally uses medium-temperature or medium-temperature modified asphalt with a moderate softening point, high coking value and high β resin. Impregnating agents should use medium-temperature asphalt with a lower softening point, low QI and good rheological properties.