Graphite Electrode Manufacturing Process

Graphite Electrode Manufacturing Process

Graphite electrode is a kind of high-temperature resistant graphite conductive material produced by a series of processes, such as mixing and kneading, molding, roasting, impregnation, graphitization and mechanical processing, using petroleum coke and needle coke as aggregate and coal pitch as binder.

Graphite electrode is an important high temperature conductive material for electric furnace steelmaking. Through the graphite electrode, electric energy is input to the electric furnace, and the high temperature generated by the arc between the electric extremum and the charge is used as the heat source to melt the charge for steelmaking. Other mineral furnaces for smelting yellow phosphorus, industrial silicon, abrasives and other materials also use graphite electrodes as conductive materials. Graphite electrode is also widely used in other industrial sectors for its excellent and special physical and chemical properties.

Petroleum coke is a combustible solid product of petroleum residue and petroleum pitch after coking. Black and porous, the main element is carbon, ash content is very low, generally below 0.5%. Petroleum coke belongs to a class of graphitized carbon, petroleum coke is widely used in chemical industry, metallurgy and other industries, is the main raw material for the production of man-made graphite products and carbon products with electrolytic aluminum.

Petroleum coke can be divided into raw coke and calcined coke according to heat treatment temperature. The former is petroleum coke obtained by delayed coking, which contains a large number of volatile components and low mechanical strength. Calcined coke is produced by calcination of raw coke.

Petroleum coke can be divided into high sulfur coke (sulfur 1.5%), sulfur coke (sulfur 0.5%-1.5%), and low sulfur coke (sulfur below 0.5%) three kinds, graphite electrode and other artificial graphite products generally use low sulfur coke production.

Needle coke is a kind of high-quality coke with obvious fibrous texture, especially low thermal expansion coefficient and easy graphitization. When the coke block is broken, it can be divided into slender strip particles (length to width ratio is generally above 1.75) according to the texture. The anisotropic fibrous structure can be observed under polarizing microscope, so it is called needle coke.

The physical and mechanical anisotropy of needle coke is very obvious. It has good electrical and thermal conductivity parallel to the long axis of particles and low thermal expansion coefficient. During extrusion, the long axis of most particles is arranged according to the extrusion direction. Therefore, needle coke is the key raw material for producing high or ultra high power graphite electrode, which has low resistivity, low thermal expansion coefficient and good thermal shock resistance.

Needle coke can be divided into oil series needle coke made from petroleum residue and coal series needle coke made from refined coal pitch.

Coal pitch is one of the main products of coal tar deep processing. It is a mixture of hydrocarbons, black, high viscosity, semi-solid or solid at room temperature, with no fixed melting point, softening upon heating, and then melting, with a density of 1.25-1.35g/cm3. According to its softening point is divided into low temperature, moderate and high temperature asphalt three. The yield of medium-temperature asphalt is 54- 56% of coal tar. The composition of coal tar pitch is very complex, which is related to the properties of coal tar and the content of heteroatoms, and is affected by coking process system and processing conditions of coal tar. There are many indexes to characterize the properties of coal pitch, such as softening point of pitch, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value and rheological properties of coal pitch.

Coal bitumen is used as binder and impregnating agent in carbon industry, and its performance has great influence on the production process and quality of carbon products. Binder asphalt generally uses moderate softening point, high coking value, high β resin medium temperature or medium temperature modified asphalt, impregnating agent to use low softening point, QI low, good rheological properties of medium temperature asphalt.

Calcination: carbonaceous raw materials are heat treated at high temperature to discharge the moisture and volatiles contained and improve the physical and chemical properties of raw materials. Generally, the carbonaceous raw materials are calcined by gas and their own volatiles as the heat source, and the highest temperature is 1250-1350 ℃.

Calcination causes profound changes in the microstructure and physical and chemical properties of the raw materials, mainly reflected in improving the density, mechanical strength and electrical conductivity of coke, improving the chemical stability and oxidation resistance of coke, laying a foundation for the subsequent process.

Calcination equipment mainly includes tank calcination furnace, rotary kiln and calcination furnace. The calcination quality control index is the true density of petroleum coke is not less than 2.07g/cm3, the resistivity is not more than 550μ ω.m, the true density of needle coke is not less than 2.12g/cm3, the resistivity is not more than 500μ ω.m. Raw material crushing treatment and batching

Before mixing, large calcined petroleum coke and needle coke should be crushed, ground and screened.

Medium crushing is usually the material about 50mm through jaw crusher, hammer crusher, roller crusher and other crushing equipment further broken to the size of 0.5-20mm material required for batching.

Grinding is the process of grinding carbon raw materials to small powder particles of 0.15mm or 0.075mm size by hanging stick ring roller mill (Raymond mill), ball mill and other equipment.

Screening is through a series of screens with uniform holes, will be broken after a wide range of material into a narrow size range of several particle size of the process, the current electrode production usually requires 4-5 particle size and 1-2 powder particle size.

Batching is the production process of calculating, weighing and focusing the aggregate, powder and binder of various particle sizes according to the requirements of the formula. The scientific suitability of formula and the stability of ingredient operation are one of the most important factors affecting product quality index and performance.

The formula needs to determine 5 aspects:

(1) the selection of raw materials; 

(2) Determine the proportion of different kinds of raw materials;

(3) Determine the particle size composition of solid raw materials; 

(4) determine the amount of binder;

(5) Determine the type and dosage of additives.

Mixing and kneading: the process of mixing and kneading into a plastic paste is called mixing and kneading of various carbon particles and powders with a certain amount of binder at a certain temperature.

Mixing and kneading process: dry mixing (20-35 min) wet mixing (40-55 min) mixing and kneading function:

(1) dry mixing make all kinds of raw materials mixed evenly, at the same time make different particle size of solid carbon material evenly mixed and filled, improve the density of the mixture;

(2) after adding coal asphalt, the dry material and asphalt are mixed evenly, and the liquid asphalt is evenly coated and soaked on the surface of particles, forming a layer of asphalt bonding layer, binding all materials together, and forming a homogeneous plastic paste, which is conducive to forming;

(3) Part of coal pitch penetrates into the internal void of carbonaceous material, which further improves the density and cohesiveness of paste.

Molding: the molding of carbon materials refers to the process of plastic deformation of the mixed carbon paste under the external force exerted by the molding equipment, and the final formation of green products (or raw products) with a certain shape, size, density and strength.

Molding: the molding of carbon materials refers to the process of plastic deformation of the mixed carbon paste under the external force exerted by the molding equipment, and the final formation of green products (or raw products) with a certain shape, size, density and strength.

Roasting: under the protection of filling material, the green carbon products are put into a specially designed heating furnace for high temperature heat treatment, so that the coal pitch in the green carbon is carbonized. The bituminous coke formed after the carbonization of coal pitch solidifies the carbonaceous aggregate and powder particles together. The roasted carbon products have higher mechanical strength, lower electrical resistivity, better thermal stability and chemical stability.

Roasting is one of the main processes in the production of carbon products, and also an important part of the three heat treatment processes in the production of graphite electrode. The roasting cycle is long (22-30 days for one roasting, 5-20 days for the second roasting depending on the furnace type), and the energy consumption is high. The quality of green billet roasting has certain influence on the quality of finished product and production cost.

The coal bitumen in green billet is coking in the roasting process, discharging about 10% volatiles, and producing 2-3% shrinkage in volume and 8-10% mass loss. Due to the increase of porosity, the volume density decreased from 1.70g/cm3 to 1.60g/cm3, and the resistivity decreased from about 10000μ ω.m to 40-50μ ω.m, and the mechanical strength of the roasted green was also greatly improved.

Secondary roasting is the process of charring asphalt immersed in the pores of roasted products after impregnation. The electrodes (all varieties except RP) and the joint billet with higher volume density requirements shall be double-baked, and the joint billet shall also be triple-baked or double-baked triple-baked.

Graphitization refers to the high-temperature heat treatment process in which the carbon products are heated to more than 2300 ℃ in the protective medium of high temperature electric furnace, so that the amorphous disorderly layer structure of carbon is transformed into three-dimensional ordered stone ink crystal structure.

The purpose and function of graphitization:

(1) improve the electrical and thermal conductivity of carbon materials (resistivity reduced 4-5 times, thermal conductivity increased about 10 times);

 (2) The thermal vibration resistance and chemical stability of carbon materials are improved (linear expansion coefficient is reduced by 50-80%); 

(3) the carbon material has lubricity and wear resistance; 

(4) discharge impurities, improve the purity of carbon materials (ash content of products from 0.5-0.8% to about 0.3%).

Graphitization of carbon materials is carried out at the high temperature of 2300-3000 ℃, so it can only be achieved by electric heating in industry, that is, the current directly passes through the roasted product being heated. The roasted product in the furnace is not only the conductor of high temperature produced by the current, but also the object being heated to high temperature.

Acheson graphitization furnace and LWG furnace are widely used at present. The former has large yield, large temperature difference and high power consumption, while the latter has short heating time, low power consumption and uniform resistivity, but it is not easy to install joints.

The graphitic process is controlled by measuring the temperature and determining the electric power curve corresponding to the temperature rise. The power on time is 50-80 hours for Acheson furnace and 9-15 hours for LWG furnace.

Graphitization consumes a lot of electricity, usually 3200-4800kwh, and the process cost accounts for about 20-35% of the entire production cost

Inspection of graphitized products: appearance knock, resistivity test

Mechanical processing: the purpose of mechanical processing of carbon graphite materials is to rely on cutting to reach the required size, shape, precision, etc., made to meet the requirements of the electrode body and joint.