Needle coke performance index 

At present, needle coke is mainly used to manufacture graphite electrode for ultra-high power electric arc furnace, which has high conductivity, high mechanical strength, excellent thermal shock resistance and oxidation resistance. Graphite electrode is made of needle coke as main material and medium temperature pitch as binder through molding, baking, impregnation and graphitization. The performance of graphite electrode largely depends on the performance of aggregate needle coke, which must have high anisotropy, easy graphitization and high purity. Otherwise, the graphite electrode is easy to break during EAF steelmaking, resulting in loss.

The quality of needle coke varies greatly with different raw materials and processes. Generally, density, coefficient of thermal expansion, strength, resistivity, ash and sulfur content are taken as the main performance indexes 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. True density reflects the density and regularity of the particle arrangement of needle coke. High true density indicates that the material has dense structure, orderly arrangement of microcrystals, high graphitization degree and low content of heteroatoms. The true density of needle coke is generally not less than 2.1g/cm³, mostly in 2.13g/cm³.

Bulk density is related to true density, porosity and pore structure. When the true density is the same, the larger the porosity, the smaller the bulk density. Generally, the more developed the needle structure, the smaller the porosity and the greater the bulk density.

② Coefficient of thermal expansion (CTE)

Thermal expansion coefficient is an important performance index of needle coke. The small needle coke CTE value indicates that when the temperature changes, the needle coke volume change is small and the thermal resistance is good. The coefficient of thermal expansion is closely related to the microstructure of needle coke. The decisive factors are the preferred orientation degree and pore structure of microcrystals in coke particles, which are reflected in the arrangement degree of fibrous structure of needle coke, coke particle size, length width ratio of needle structure and so on. The higher the content of fibrous structure and the larger the length-width ratio, the smaller the thermal expansion coefficient. It is generally believed that the thermal expansion coefficient of coal series needle coke is smaller than that of oil series needle coke. See the table below：

③ Strength

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 texture direction. With the decrease of thermal expansion coefficient, the anti-crushing and anti-wear properties of needle coke decrease. The strength of coal series needle coke is generally lower than that of oil series needle coke, the reason is that coal measure needle coke is porous, well arranged, easy to break. In addition, the hydrophilicity between coal series needle coke and binder pitch is not good, which also leads to the low strength of coal series needle coke.

④ Resistivity

The resistivity is mainly affected by the true density and porosity, and also related to the particle shape. When the electrode is formed, if the pressure method is used, the coke particles are arranged horizontally and the resistivity is large. If extrusion molding is adopted, the needle coke particles will be arranged along the horizontal extrusion direction along the long direction, and the resistivity will be reduced.

⑤ Sulfur and ash

Sulfur is a harmful component, and irreversible volume expansion (crystal expansion) occurs during graphitization of needle coke, reduces the bulk density, increases the resistivity and reduces the strength of the produced graphite material. Crystal expansion is related to the sulfur and nitrogen content and needle coke structure. When needle coke is graphitized, sulfur will escape sharply in the heating stage, causing the material to expand or even burst. Nickel oxide and cobalt oxide are generally added in industry to inhibit crystal expansion. In addition, calcination of needle coke at higher temperature can reduce the degree of crystal expansion during graphitization.

⑥ Moisture and volatiles

Moisture and volatile matter are related to raw materials and process, reducing  moisture and volatile matter and improving calcination yield are also an important index to evaluate the performance of needle coke, cosulting us with the advanced technical news of needle coke. 

Physical and chemical indexes of several kinds of needle coke

National performance index standard for coal series needle coke