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【Original Article】Production Process of Graphite Electrode

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Raw materials and manufacturing process of graphite electrode

 

Graphite electrode is a kind of high temperature-resistant graphite conductive produced by a series of processes such as kneading, extrusion, baking, impregnatin, graphitization, mechanical processing, etc. using petroleum coke, needle coke as aggregate, coal pitch as binder material.

 

Graphite electrode is an important high-temperature conductive material for electric furnace steelmaking. Electric energy is input to the electric furnace through the graphite electrode. The high temperature generated by the arc between the electrode end and the charge is used as a heat source to melt the charge and make steel. Some other geothermal furnaces for smelting yellow phosphorus, industrial silicon, abrasives and other materials also use graphite electrodes as conductive materials. The excellent and special physical and chemical properties of graphite electrodes are also widely used in other industrial sectors.

 

 

The raw materials for producing graphite electrodes are petroleum coke, needle coke and coal pitch

 

Petroleum coke is a combustible solid product obtained by coking of petroleum residue and petroleum pitch. The color is black and porous, the main element is carbon, the ash content is very low, generally below 0.5%. Petroleum coke belongs to the category of easy graphitizable carbon. Petroleum coke has a wide range of uses in chemical and metallurgical 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 coke obtained from delayed coking contains a large amount of volatiles and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most oil refineries in China only produce coke, and calcination operations are mostly carried out in carbon plants.

 

Petroleum coke can be divided into high sulfur coke(with sulfur content above 1.5%), medium sulfur coke(with sulfur content 0.5%-1.5%), and low sulfur coke(with sulfur content below 0.5%) according to the sulfur level. Graphite electrodes and other artificial graphite products are generally produced using low sulfur coke.

 

 

Needle coke is a kind of high-quality coke with obvious fibrous texture, particularly low thermal expansion coefficient and easy graphitization. When the coke is broken, it can be split into elongated strip-shaped particles accroding to the texture(the aspect ratio is generally above 1.75). An anisotropic fibrous structure can be ovserved under a polarizing 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 coefficient of thermal expansion. During extrusion, the long axes of most particles are arranged in the direction of extrusion. Therefore, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes. The graphite electrodes produced have low resistivity, small thermal expansion coefficient, and good thermal shock resistance.

Needle coke is divided into oil-based needle coke produced from petroleum residue oil and coal-based needle coke produced from refined coal pitch raw materials.

 

 

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. There is no fixed melting point. It softens after heating and then melts. The desity is 1.25-1.35 g/cm3. According to the level of its softening point, it can be divided into low temperature, medium temperature and high temperature asphalt. The yield of medium temperature asphalt is 54-56% of coal tar. The composition of coal pitch is extremely complicated, which is relatd to the nature 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 to characterize the properties of coal pitch, such as asphalt softening point, toluene insoluble matter(TI), quinoline insoluble matter(QI), coking value and coal pitch rheology.

 

The following pictures is the flow chart of the production process of graphite electrodes in carbon enterprises

flow chart of the production process of graphite electrodes in carbon enterprises

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  • Calcination:

The production process of carbonaceous raw materials is heat-treated at high temperature to discharge the moisture and volatile content contained, and to improve the original cooking performance accordingly is called calcination. Generally, the carbonaceous raw material is calcined by using gas and its own volatile matter as the hear source, and the maximum temperature is 1250-1350℃.

 

Calcination has profoundly changed the structure and physical and chemical properties of carbonaceous materials, mainly reflected in the improvement of coke density, mechanical strength and electrical conductivity, and improved the chemical stability and anti-oxidation performance of coke, laying a foundation for the subsequent process.

 

The main equipment for calcining is tank calciner, rotary kiln and electric calciner. The calcination quality control indicators are that the true density of petroleum coke is not less than 2.07 g/cm3, the resistivity is not greater than 550μΩ.m, the true density of needle coke is not less than 2.12g/cm3, and the resistivity is not greater than 550μΩ.m.

 

  • Crushing and batching of raw materials

Before batching, large pieces of calcined petoleum coke and needle coke must be crushed, round and sieved.

 

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

 

Grinding is the process of grinding carbonaceous raw materials to powdery small particles below 0.15mm or 0.075mm in diameter through cantilever ring roller mill(Raymond mill), ball mill and other equipment.

 

Sieving is a process of dividing a material with a wide size range after crushing into several particle sizes with a narrow size range through a series of screens with uniform openings. The current electrode production usually requires 4-5 particle sizes and 1-2 powder grades.

 

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

 

The formula should be determined in five aspects:

  1. Select the type of raw materials;
  2. Determine the proportion of different types of raw materials
  3. Determine the particle size composition of solid materials
  4. Determine the amount of binder
  5. Determine the type and amount of additives
  • Kneading:

mixing a certain amount of carbon particles and powder with a certain amount of binder at a certain temperature and mixing evenly. The process of kneading into a plastic paste is called kneading.

 

Kneading process: dry mixing(20-35min) and wet mixing(40-55min)

 

The effect of kneading:

  1. During dry mixing, various raw materials are mixed uniformly, and at the same time, solid carbon materials with different particle sizes are uniformly mixed and filled to improve the compactness of the mixture;
  2. After adding coal asphalt, the dry material and asphalt are mixed evenly, and the liquid asphalt is evenly coated and infiltrated on the surface of the particles to form a layer of asphalt bonding layer, which bonds all the materials together, thereby forming a homogeneous plastic paste, which is beneficial to forming;
  3. Part of the coal asphalt penetrates into the internal voids of the carbonaceous material, which further improves the density and adhesion of the paste

 

  • Molding

The molding of carbon materials refers to the process of mixing and kneading the carbonaceous paste to produce plastic deformation under the external force applied by the molding equipment, and finally forming a green body (or raw product) with certain shape, size, density and strength process.

 

Type of molding, equipment and products produced

Type of molding, equipment and products produced

 

 

 

 

 

 

 

 

 

 

Extrusion operation

  1. Cooling material: disc cooling material, cylindrical cooling material, kneading cooling material and other ways.
    Discharge volatile matter, reduce to a suitable temperature(90-120℃) to increase the adhesion, so that the degree of paste uniformity is favorable for forming 20-30 min
  2. Loading: The press lifts the baffle—the material is divided into 2-3 times—the compaction is 4-10Mpa
  3. Prepressure: Pressure 20-25Mpa, time35 min, and vacuum simultaneously
  4. Extrusion: Press down baffle—5-15Mpa extrusion—shear—turn into cooling water tank

 

Extrusion technical parameters:compression ratio, press material chamber and nozzle temperature, cold material temperature, precompression pressure time, extrusion pressure, extrusion speed, cooling water temperature.

 

Inspection of raw product: volume density, appearance knocking, analysis

 

  • Baking

It is a process of charging raw carbon products into high temperature heat treatment in specially designed heating furnace under the protection of fillers to carbonize coal pitch in the raw products. The pitch coke formed after the coal pitch is caronized will consolidate the carbonaceous aggregates and power particles together. The calcined carbon products have higher mechanical strength, lower resistivity, better thermal stability and chemical stability.

 

Baking is one of the main processes for the production of carbon products, and it is also an important part of the three major heat treatment processes for the production of graphite electrodes. The baking production cycle is longer (22-30 days for one baking and 5-20 days for the second baking), and higher energy consumption. The quality of raw backing has a certain impact on the quality of finished products and production costs.

 

The coal pitch in the raw product is coked during the baking process, and about 10% of the volatiles are discharged. At the same time, the volume shrinks by 2-3%, and the mass loss is 8-10%. The physical and chemical properties of the carbon billet have also undergone significant changes. Due to the increase in porosity, the bulk density has decreased from 1.70g/cm3 to 1.60g/cm3, and the resistivity has dropped from about 10000μΩ.m to 40-50μΩ.m. The mechanical strength of the calcined billet is also large to improve.

 

Secondary baking is a process in which the baked product is impregnated and then baked again to carbonize the asphalt immersed in the pores of the baked product. Electrodes(all types except RP grade) and nipple blanks that require higher bulk density are required to be sencond-baked, and nipple blanks are also required to be three-dip four-bake or two-dip three bake.

 

The main types of bake furnace:

Continuous operation—Ring furnace(with cover , without cover), tunnel kiln

Intermittent operation—Inverted flame kiln, car bottom bake furnace, box bake furnace

 

Baking curve and max. temperature:

Primary baking—320、360、422、480 hours,1250 ℃

Secondary baking—125、240 、280 hours,700-800 ℃

 

Inspection of baking products: appearance knocking, resistivity, bulk density, compressive strength, internal structure analysis

 

  • Impregnation

Impregnation is the process of placing the carbon material in a pressure vessel and impregnating the liquid impregnant pitch into the pores of the electrode of the product under certain temperature and pressure conditions. The purpose is to reduce the porosity of the product, increase the bulk density and mechanical strength of the product, and improve the electrical and thermal conductivity of the product.

 

Impregnation is the process of placing the carbon material in a pressure vessel and impregnating the liquid impregnant pitch into the pores of the electrode of the product under certain temperature and pressure conditions. The purpose is to reduce the porosity of the product, increase the bulk density and mechanical strength of the product, and improve the electrical and thermal conductivity of the product.

 

Inspection of impregnated product: weight gain rate of impregnation G=(W2-W1)/W1×100%

Weight gain rate of primary impregnated product ≥14%

Weight gain rate of secondary impregnated product ≥9%

Weight gain rate of third impregnated product ≥5%

 

 

  • Graphitization

Graphitization refers to the high-temperature heat treatment process of heating carbon products above 2300℃ in a protective medium in a high-temperaute electric furnace to convert amorphous chaotic layer structure carbon into a three-dimensional ordered graphite crystal structure.

 

The purpose and effect of graphitization:

  1. Improve the electrical and thermal conductivity of carbon materials(resistivity decreases by 4-5 times, thermal conductivity increases by about 10 times);
  2. Improve the thermal shock resistance and chemical stability of carbon materials (linear expansion coefficient reduce 50-80%);
  3. Make the carbon material lubricity and abrasion resistance;
  4. Discharge impurities and improve the purity of the carbon material(the ash content of the product is reduced from 0.5-0.8% to about 0.3%)。

 

Implementation of graphitization process:

The graphitization of the carbon material is carried out at a high temperature of 2300-3000℃, so the industry can only be achieved by electric heating, that is the current directly passes through the heated baked produt. At this time, the baked product loaded into the furnace geneerates high temperature by the current the conductor is heated to a high temperature.

 

At present, the widely used furnace types are Acheson graphitization furnace and internal thermal series(LWG) furnace. The former has large output, large temperature difference, and high power consumption, while the latter has short heating time, low power consumption, uniform resistivity, but it is n ot easy to install nipple.

 

The control of the graphitization process is to determine the electric power curve that is suitable for the temperature increase by measuring the temperature. The power-on time is 50-80 hours for the Acheson furnace and 9-15hours for the LWG furnace.

 

The power consumption of graphitization is very large, generally 3200-4800KWh, the process cost accounts for about 20-35% of the entire production cost.

 

Inspection of graphitized products: appearance knocking, resistivity rest

 

  • Mechanical processing:

The purpose of mechanical processing of carbon graphite material is to rely on cutting processing to reach the required size, shape, accuracy, etc., to make the electrode body and nipple that meet the requirements of using.

 

Graphite electrode processing is divided into two independent processes of electrode body and nipple

 

The elecrode body processing includes 3 processes: boring and rough flat end surface, turning outer and flat end surface and thread milling. The processing of conical nipple can be divided into 6 processes: cutting, flat end surface, turning cone surface, milling thread, drilling bolt and slotted.

 

The electrode nipple connection method: conical nipple conection (one inch three thread and one inch four thread). Cylindrical nipple connection, concave and convex connection( male and female thread connection)

 

Control of processing accuracy: thread taper deviation, thread pitch, large diameter deviation of nipple(hole), coaxiality of nipple hole, verticality of nipple hole, flatness of electrode end face, four-point deviation of nipple, etc. Check with special ring gauge and plate gauge.

 

Inspection of finished electrodes: accuracy, weight, length, diameter, bulk density, resistivity, precision of pre-assembled fit, etc.

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