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Performance indicators of needle coke

Performance indicators of needle coke

(Summary description)At present, needle coke is mainly used to manufacture graphite electrodes for ultra-high power electric furnace steelmaking. This kind of electrode has high electrical conductivity, high mechanical strength, excellent thermal shock resistance and oxidation resistance.

Performance indicators of needle coke

(Summary description)At present, needle coke is mainly used to manufacture graphite electrodes for ultra-high power electric furnace steelmaking. This kind of electrode has high electrical conductivity, high mechanical strength, excellent thermal shock resistance and oxidation resistance.

Information

At present, needle coke is mainly used to manufacture graphite electrodes for ultra-high power electric furnace steelmaking. This kind of electrode has high electrical conductivity, high mechanical strength, excellent thermal shock resistance and oxidation resistance. Graphite electrodes are made of needle coke as the main material and medium-temperature pitch as the binder, after forming, roasting, impregnating and graphitizing processes. The performance of graphite electrodes largely depends on the performance of the aggregate needle coke, which requires high anisotropy, easy graphitization, and high purity. Otherwise, the graphite electrode will easily break during steelmaking in the electric arc furnace, which will cause losses.

The quality of needle coke varies greatly because of different raw materials and processes. Generally, density, thermal expansion coefficient, strength, resistivity, ash content and sulfur content are used as the main performance indicators of needle coke to measure the quality of needle coke.

①Density

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 has a dense structure, neatly arranged microcrystals, a high degree of graphitization, and a small amount of heteroatoms. The true density of needle coke is generally not less than 2.1g/cm3, mostly 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 lower the volume density. Generally, the more developed the needle-like structure, the smaller the porosity and the greater the volume density.

②Coefficient of thermal expansion

The thermal expansion coefficient is an important performance index of needle coke. The small CTE value of needle coke indicates that the volume change of needle coke is small when the temperature changes, and the heat resistance performance is good. The coefficient of thermal expansion is closely related to the microstructure of the needle coke. The decisive factor is the degree of preferred orientation of the crystallites in the coke particles and the pore structure, which is reflected in the degree of arrangement of the needle coke fibrous structure, the size of the coke particles, and the needle structure. The aspect ratio and so on. The higher the fibrous structure content and the larger the particle aspect ratio, the smaller the thermal expansion coefficient. It is generally believed that the thermal expansion coefficient of coal-based needle coke is smaller than that of oil-based needle coke. The following table

 

sample

Total fiber content%

Thermal expansion coefficient/10-6K-1

Mizushima Coke

47.4

2.54

Anshan Jiao

57.6

2.15

Nitten

71.7

1.55

 

③Strength

The mildness of needle coke depends on the total number of absolute pores and is related to the thermal expansion coefficient along the grain direction. As the coefficient of thermal expansion decreases, the anti-cracking 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. The reason is that the coal-based needle coke has many pores, is well arranged, and is easy to break. In addition, the coal-based needle coke has poor wettability with the binder pitch, which also leads to the lower strength of the coal-based needle coke.

④Resistivity

The resistivity is mainly affected by the true density and porosity, in addition to the particle shape. When the electrode is formed, if the sample is prepared by the pressure method, the coke particles are arranged horizontally, and the resistivity is relatively large. If extrusion molding is used, the needle coke particles will be aligned along the longitudinal direction along the horizontal extrusion direction, and the resistivity will decrease at this time.

⑤Sulfur and Ash

Sulfur is a harmful component. Needle coke will undergo irreversible volume expansion (crystal swelling) during graphitization, which will reduce the volume density, resistivity, and strength of the graphite material produced. Crystal swelling is related to the sulfur and nitrogen content and structure of needle coke. When needle coke is graphitized, sulfur will escape sharply during the heating stage, causing the material to expand or even burst. Industrially, nickel oxide, cobalt oxide, etc. are generally added to suppress crystal swelling. In addition, calcining needle coke at a higher temperature can reduce the degree of crystal swelling during graphitization.

⑥Moisture and volatile matter

Moisture and volatile matter are related to the production materials and processes. Reducing moisture and volatile matter and improving the calcination yield are also an important indicator for evaluating the performance of needle coke.

Several physical and chemical indexes of needle coke

 

Variety

Moisture%

Volatile matter%

True density g/cm3

Ash content%

Sulfur%

Xin Nihua Coke (Coal)

0.13

0.28

2.12

0.01

0.28

Mitsubishi coke (coal series)

0.14

0.68

2.13

0.01

0.28

Anshan Jiao (Coal Measures)

0.55

0.81

2.12

0.03

0.28

Jinzhou tar (oil series)

0.24

0.48

2.12

0.08

0.48

 

National Performance Index Standard for Coal Series Needle Coke

Performance

Metrics

Superior

First level

Level 2

True density g/cm3

2.13

2.13

2.12

sulfur %

0.4

0.4

0.5

nitrogen %

0.5

0.6

0.7

Volatile %

0.3

0.4

0.4

Ash %

0.2

0.3

0.3

Moisture %

0.15

0.15

0.15

Thermal expansion coefficient(100-600)10-6K-1

1.0

1.3

1.5

Resistivity um.m

600

600

600

Tap density g/cm3

0.9

0.88

0.85

 Excerpt from: Pitch-based carbon materials

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