The Raw Materials For Graphite Production

Petroleum coke is a combustible solid product obtained by coking petroleum residue and petroleum 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 belongs to the category of easily graphitized carbon. Petroleum coke is widely used in chemical, metallurgical and other industries. It is the main raw 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 heat treatment temperature. The former is the 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 oil refineries in China only produce coke, and most of the calcination operations are carried out in carbon plants.

Petroleum coke is classified according to the level of sulfur content. It 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%). The production of artificial graphite products generally uses low-sulfur coke production.

Needle coke is a kind of high-quality coke with obvious fibrous texture, low thermal expansion coefficient and easy graphitization. When the coke is broken, it can split into elongated particles according to the texture (the aspect ratio is generally above 1.75). An anisotropic fibrous structure can be observed under a polarized light microscope, so 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 parallel to the long axis of the particles, and has a low thermal expansion coefficient. During extrusion molding, most of the long axes of the particles are arranged in the extrusion direction. Therefore, needle coke is the key raw material for the manufacture of graphite. The produced graphite has low resistivity, low thermal expansion coefficient and good thermal shock resistance.

Needle coke is divided into petroleum-based needle coke produced with petroleum residue as a raw material and coal-based needle coke produced with refined coal pitch raw material.

Coal 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 after heating and then melts. 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 asphalt. 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 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 that characterize the characteristics of coal pitch, such as pitch softening point, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value, and coal pitch rheology.

Coal pitch is used as a binder and impregnant in the carbon industry, and its performance has a great impact on the production process and product quality of carbon products. Binder asphalt generally uses medium temperature or medium temperature modified asphalt with moderate softening point, high coking value, and high β resin. As impregnant, use medium temperature asphalt with low softening point, low QI, and good rheological properties.

Calcining: The carbonaceous raw material is heat-treated at high temperature to discharge the contained moisture and volatile matter, and correspondingly improve the original cooking performance of the production process is called calcination. Generally, carbonaceous raw materials are calcined using fuel gas and their own volatile matter as the heat source, and the maximum temperature is 1250-1350°C.

Calcining causes profound changes in the structure and physical and chemical properties of carbon raw materials, which are mainly reflected in increasing the density, mechanical strength and conductivity of coke, improving the chemical stability and oxidation resistance of coke, and laying the foundation for subsequent processes.

The calcination equipment mainly includes tank calciner, rotary kiln and electric calciner. The calcination quality control index is that the true density of petroleum coke is not less than 2.07g/cm3, the resistivity is not more than 550μΩ.m, the true density of needle coke is not less than 2.12g/cm3, and the resistivity is not more than 500μΩ.m.