The production process of graphite electrode is raw material calcination, extrusion, roasting, impregnation, graphitization and machining forming. Each process production process is very important.
1) Raw materials
60% of the production cost of graphite electrodes comes from raw materials, mainly petroleum coke, with needle coke and coal pitch added.
Petroleum coke is a combustible solid obtained by coking petroleum residue and petroleum pitch. Petroleum coke is a type of easily graphitized carbon and is the main raw material for the production of graphite products.
Needle coke is a high-quality coke with fibrous texture, low thermal expansion coefficient, and easy graphitization. The anisotropy of the physical and mechanical properties of needle coke is very obvious, and it has good electrical and thermal conductivity. When it is extruded, it is arranged in the extrusion direction. Therefore, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes. In addition, needle coke is divided into oil-based needle coke based on petroleum residue and coal-based needle coke produced from refined coal pitch raw materials.
In the calcination process, the raw materials remove the moisture and volatiles, which improves the physical and chemical properties of the raw materials. After the high-quality needle coke is calcined, it becomes the raw material for the production of high-performance electrodes. Generally, the maximum calcination temperature is 1250-1350℃, and the equipment adopts tank calciner, rotary kiln and electric calciner. The calcination needs to meet the indicators: the true density of petroleum coke is not less than 2.07g/cm3, the resistivity is not more than 550μΩ.m, the true density of needle coke is not less than 2.12g/cm3, and the resistivity is not more than 500μΩ.m.
The mixed raw materials are extruded and molded by an extruder. By changing the nozzle on the extrusion cylinder, an electrode with a certain diameter and length can be formed. The volatile matter is discharged by extrusion and reduced to a suitable temperature (90-120°C) to increase the bonding force. Make the viscosity of the raw material uniform and facilitate the molding for 20-30 minutes. The extrusion equipment is a horizontal hydraulic extruder and a screw extruder. The extrusion is called a green body, and its bulk density, appearance, and analysis must be checked.
Baking can greatly improve the thermal stability, mechanical strength and electrical conductivity of the material. The coal pitch in the green body is coked during the roasting process, and about 10% of volatile matter is discharged, while the volume shrinks by 2-3% and the mass loss is 8-10%. The physical and chemical properties of the electrode are also improved, the volume density is reduced from 1.70g/cm3 to 1.60g/cm3, the resistivity of 10000μΩ.m is reduced to 40-50μΩ.m, and the mechanical strength of the electrode is also improved.
The second roasting is the roasting of the roasted product after being impregnated to carbonize the pitch immersed in the pores of the roasted product. For electrodes with higher volume density requirements (except for the ordinary power graphite electrode RP) and joints, two baking is required, and the joint blanks also need to be three-immersed and four-baked or two-immersed and three-baked.
Main types of roasting furnace:
Continuous operation-ring furnace (with or without cover), tunnel kiln
Intermittent operation-inverted flame kiln, car bottom roaster, box roaster
Roasting curve and maximum temperature:
One roasting-320, 360, 422, 480 hours, 1250 ℃
Secondary roasting-125, 240, 280 hours, 700-800 ℃
Inspection of baked products: appearance knocking, resistivity, bulk density, compressive strength, internal structure analysis
The calcined product is put into a pressure vessel, and the liquid impregnant pitch is immersed in the electrode pores under certain pressure and temperature conditions. The purpose of impregnation is to reduce the porosity of the product, increase the volume density and mechanical strength of the electrode, and improve the electrical and thermal conductivity of the electrode.
The impregnation process and related technical parameters are:
Baking billet-surface cleaning-preheating (260-380 ℃, 6-10 hours)-loading into the impregnation tank-vacuuming (8-9KPa, 40-50min)-injection of asphalt (180-200 ℃) ——Pressurization (1.2-1.5MPa, 3-4 hours)——Return bitumen——Cooling (in or outside the tank)
Inspection after immersion:
Impregnation weight gain rate G=(W2-W1)/W1×100%
Weight gain rate of one-time impregnation product≥14%
Weight gain rate of secondary dipping product≥9%
Weight gain rate of three dipping products ≥5%
It refers to the high temperature heat treatment process in which the semi-finished electrode product is heated to above 2300-3000 ℃ in a high temperature electric furnace to transform the amorphous chaotic layer structure carbon into a three-dimensional ordered graphite crystalline structure. Graphitization can only be carried out by electric heating. Acheson graphitization furnace and internal heat series (LWG) furnace are used. Acheson has large output, large temperature difference and high power consumption. LWG heating time is short and power consumption is low. The resistivity is uniform but it is not easy to install the joint.
Graphitization is the process of controlling the temperature. The Acheson furnace is energized for 50-80 hours, and the LWG furnace is energized for 9-15 hours. The power consumption of graphitization is generally 3200-4800KWh. The cost of graphitization accounts for about 20% of the entire production cost. -35%.
Inspection of graphitized products: appearance knocking, resistivity test.
The purpose of electrode machining is to rely on automatic or semi-automatic CNC machining to achieve the required size, shape, precision, etc., to make the electrode body and joints that meet the requirements of use.
Graphite electrode processing is divided into two independent processing processes, the electrode body and the joint.
The electrode body is processed through three processes: boring and rough flat end face, turning outer circle and fine flat end face, and thread milling. The electrolytic joint processing passes: cutting, flat end face, turning taper surface, thread milling, drilling bolt and slotting 6 Road process.
Electrode connector connection method: conical connector connection (one inch three buckles and one inch four buckles), cylindrical connector connection, concave-convex connection (male and female buckle connection)
Processing precision control: thread taper deviation, thread pitch, joint (hole) large diameter deviation, joint hole coaxiality, joint hole perpendicularity, electrode end surface flatness, joint four-point deviation, etc. Use special ring gauges and board gauges to check.
Inspection of finished electrodes: accuracy, weight, length, diameter, volume density, resistivity, pre-installation accuracy, etc.
Graphite electrodes production process:
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