550mm graphite electrode for arc furnaces
The production process of graphite electrode
Raw materials: In carbon production, the commonly used raw materials can be divided into two categories: solid carbon raw materials, binders and impregnants. Solid carbon raw materials include petroleum coke, pitch coke, metallurgical coke, anthracite, natural graphite and graphite crushing, and binders and impregnants include coal tar pitch, coal tar, anthracene oil and synthetic resins. In addition, auxiliary materials such as quartz sand, metallurgical coke powder, and coke powder are also used in the production. Other special raw materials are used to produce some special carbon and graphite products (such as carbon fiber, activated carbon, pyrolytic carbon and pyrolytic graphite, glassy carbon).
Calcining: The process of heat-treating carbon raw materials at high temperature (1200≤1500°C) in isolated air is called calcination. Calcining is the first heat treatment process in carbon production. Calcining causes a series of changes in the structure and physical and chemical properties of various carbon-containing raw materials.
Both anthracite and petroleum coke contain a certain amount of volatiles and need to be calcined. The coking temperature of asphaltene coke and metallurgical coke is relatively high (above 1000°C), which is equivalent to the temperature of the calciner of a carbonization plant. It can no longer be calcined, it only needs to be dried with moisture. But if pitch coke and petroleum coke are mixed before calcination, they should be sent to the calciner together with petroleum coke. Natural graphite and carbon black do not need to be calcined.
Kneading: The purpose of kneading is to obtain a uniform paste with good plasticity. However, due to changes in certain factors, the kneading quality will change greatly, resulting in unstable product quality. Experiments have shown that the softening point is 80- For hard asphalt at 90°C, the temperature of the paste should be increased to 168-174°C. In this temperature range, the asphalt has the best wettability and bonding properties for dry materials. The resulting paste has good plasticity and extrusion properties. Generally, the dry mixing time is 30 minutes, and the wet mixing time is 40 minutes. If the mixing time is too long, large particles will be destroyed and the original particle composition will be destroyed, which will reduce the product’s bulk density, reduce plasticity, and increase porosity. , The mechanical strength is reduced.
Molding: Molding includes extrusion molding, vibration molding and compression molding methods. Our factory uses the extruder for molding. Before molding, the obtained paste is reduced from 168-174℃ to 116℃ in the cooler. The cooling time should not exceed 11 minutes and not less than 8 minutes. Put the heat-preserved paste into the cylinder of the extruder after 20 minutes of heat preservation on the heat preservation platform. The pre-pressing pressure is generally not less than 3-5 minutes. The product is squeezed out through the nozzle installed in front of the extruder. The machine is generally intermittent production, and the production is divided into three stages: In the first stage, when the plunger head starts to pressurize, the paste particles move quickly, and the gaps between the large paste particles are filled with small paste particles. The contact between particles tends to be close. In the second stage, when the pressure head continues to apply pressure, the electrode density gradually increases. After the density reaches a certain value, the pressure continues to increase and the density increases slowly. In the third stage, the pressure reaches the limit value and the density tends to be uniform. Then squeeze the electrode from the nozzle
Roasting: Roasting is a heat treatment process in which the pressed raw product is heated in the protective medium in the heating furnace at a certain temperature rise rate under the condition of isolating air.
The purpose of roasting is:
The main research results are as follows: (1) The volatile matter emissions of products with coal tar pitch as a binder are about 10% after roasting. Therefore, the roasting yield is generally less than 90%.
The coking raw materials are roasted according to certain process conditions, and the binder is coked, forming a coke network between the aggregate particles, so that the aggregates of different particle diameters are firmly connected together, so that the product has certain physical and chemical properties. Under the same conditions, the higher the scorch rate, the better the product quality. The residual carbon content of medium temperature asphalt is generally about 50%.
(3) During the firing process of the fixed geometric form of the raw product, the softening and the migration of the binder occur. As the temperature rises, a coking net is formed, which makes the product rigid. Therefore, as the temperature increases, its shape does not change.
(4) Reducing the resistivity During the roasting process, due to the elimination of volatiles, the pitch coking forms a coke grid, the pitch decomposes and polymerizes, and a large hexagonal carbon ring plane net is formed, and the resistivity is greatly reduced. The resistivity of the raw product is about ×10-6Ω〃m, and after firing, it drops to 40-50×10-6Ω〃m, which is called a good conductor
(5) Volume shrinkage After firing, the diameter of the product shrinks by about 1%, the length shrinks by about 2%, and the volume shrinkage is 2-3%.
Impregnation: The porosity of the raw product after compression molding is very low. However, after the raw product is roasted, part of the coal pitch is decomposed into gas and escaped during the roasting process, and the other part is coked into pitch coke. The volume of pitch coke produced is much smaller than the original volume of coal tar pitch. Although it shrinks slightly during the roasting process, many irregular and small pores with different pore sizes are still formed inside the product. The existence of a large number of pores will inevitably have a certain impact on the physical and chemical properties of the product. At a certain temperature, the oxidation speed is accelerated, the corrosion resistance is also deteriorated, and the gas and liquid are more easily penetrated.
Impregnation is a process that reduces the porosity of the product, increases the density, increases the compressive strength, reduces the resistivity of the finished product, and changes the physical and chemical properties of the product.
Graphitization: Graphitization is to heat the calcined product in the protective medium in the graphitization furnace to a high temperature, so that the hexagonal carbon atom planar grid is transformed from the disordered overlap in the two-dimensional space to the ordered overlap in the three-dimensional space, and it has a graphite structure. The high temperature heat treatment process. the purpose is:
(1) Improve the thermal and electrical conductivity of the product.
(2) Improve the thermal shock resistance and chemical stability of the product.
(3) Improve the lubricity and abrasion resistance of the product.
(4) Eliminate impurities and improve product strength.
Mechanical processing: Why do carbon products need to be mechanically processed?
(1) For the need of shaping, the pressed carbon product with a certain size and shape will undergo different degrees of deformation and damage during the roasting and graphitization process. At the same time, some fillers are adhered to the surface. It cannot be used without mechanical processing, so the product must be reshaped and processed into a specified geometric shape.
(2) According to the user’s requirements for use, processing is required. For example, the graphite electrode of electric furnace steelmaking needs to be connected and used, and threaded holes must be made at both ends of the product, and then the two electrodes must be connected with a special threaded joint.
(3) Process requirements, some products need to be processed into special shapes and specifications according to the user’s process requirements, and even require lower surface roughness