The main application of needle coke is to manufacture graphite electrodes for ultra-high power electric arc furnaces, which have high electrical conductivity and mechanical strength, excellent heat resistance, and oxidation resistance. Graphite electrodes are made of needle coke as the main material and coal tar pitch as the binder and are made through the processes of molding, roasting, impregnation, and graphitization. The performance of graphite electrodes largely depends on the performance of aggregate needle coke, which requires that needle coke must have high anisotropy, easy graphitization, and high purity. Otherwise, the graphite electrode is easy to break during steelmaking, resulting in a loss.

Depending on the raw material and process of needle coke, the quality varies greatly. Generally, density, thermal expansion coefficient, strength, resistivity, ash, and sulfur are used as the leading performance indicators of needle coke to measure the quality of needle coke.

① Density

The density index of needle coke is divided into true density and bulk density. The true density reflects the density and regularity of the arrangement of the particles that make up the needle coke. The high true density indicates that the material structure is compact, the microcrystals are neatly arranged, the degree of graphitization is high, and the content of heteroatoms is small. The true density of needle coke is generally not lower than 2.1g/cm3, mostly 2.13g/cm3.

Bulk density is related to true density, porosity, and pore structure. In the case of the same true density, the larger the porosity, the smaller the bulk density. Generally, the more developed the needle-like structure, the smaller the porosity and the larger the bulk density.

②Thermal expansion coefficient

       The thermal expansion coefficient is an important performance index of needle coke. The CTE value of needle coke is small, indicating that when the temperature changes, the volume change of needle coke is small, and the heat resistance performance is good. The coefficient of thermal expansion has a great relationship with the microstructure of needle coke, and the decisive factors are the preferred orientation degree and pore structure of the crystallites in the coke particles, which are manifested in the arrangement degree of the fibrous structure of the needle coke, the size of the coke particles, and the needle structure. aspect ratio etc. The higher the content of the fibrous structure and the larger the aspect ratio of particles, the smaller the coefficient of thermal expansion. It is generally believed that the thermal expansion coefficient of coal-based needle coke is smaller than that of oil-based needle coke. As follows：

③Strength

The strength of needle coke depends on the total number of its absolute pores and is related to the coefficient of thermal expansion along the grain direction, as the coefficient of thermal expansion decreases, the anti-breakage and anti-wear properties of needle coke decrease. The strength of coal-based needle coke is generally lower than that of oil-based needle coke, because coal-based needle coke has many pores, is well arranged, and is easy to break. In addition, coal-based needle coke has poor wettability with binder pitch, which also leads to the low strength of coal-based needle coke.

④Resistivity

Resistivity is mainly affected by true density and porosity and is also related to particle shape. When the electrode is formed, if the pressure method is used to prepare the sample, the coke particles are arranged horizontally and the resistivity is relatively large. If extrusion molding is used, the needle coke particles will be arranged along the horizontal extrusion direction along the long direction, and the resistivity will decrease at this time.

⑤ Sulfur and Ash

Sulfur is a harmful component. When the needle coke is graphitized, it will experience irreversible volume expansion (crystal expansion), which will reduce the volume density, increase the resistivity and decrease the strength of the produced graphite material. Crystal expansion is related to the sulfur and nitrogen content and structure of needle coke. When needle coke is graphitized, sulfur will escape rapidly during the heating stage, causing the material to expand and burst. In the industry, nickel oxide, cobalt oxide, etc. are generally added to inhibit crystal expansion. In addition, calcining needle coke at a higher temperature can reduce the degree of crystal expansion during graphitization.

⑥Moisture and volatile matter

Moisture and volatile matter are related to the production of raw materials and processes. Reducing water and volatile matter and increasing calcination yield are also important indicators for evaluating the performance of needle coke. For more information on needle coke technical indicators, please contact us.