Graphite electrode is a high temperature resistant graphite conductive material produced by petroleum knead, needle coke as aggregate and coal bitumen as binder, which are produced through a series of processes such as kneading, molding, roasting, impregnation, graphitization and mechanical processing. material.
The graphite electrode is an important high-temperature conductive material for electric steelmaking. The graphite electrode is used to input electric energy to the electric furnace, and the high temperature generated by the arc between the electrode end and the charge is used as a heat source to melt the charge for steel making. Other ore furnaces that smelt materials such as yellow phosphorus, industrial silicon, and abrasives 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 the production of graphite electrodes are petroleum coke, needle coke and coal tar pitch.
Petroleum coke is a flammable solid product obtained by coking coal residue and petroleum pitch. The color is black and porous, the main element is carbon, and the ash content is very low, generally below 0.5%. Petroleum coke belongs to the class of easily graphitized carbon. Petroleum coke has a wide range of uses in chemical and metallurgical industries. It is the main raw material for producing artificial graphite products and carbon products for electrolytic aluminum.
The petroleum coke can be divided into two types: raw coke and calcined coke according to the heat treatment temperature. The former petroleum coke obtained by delayed coking contains a large amount of volatiles, and the mechanical strength is low. The calcined coke is obtained by calcination of raw coke. Most refineries in China produce only coke, and calcination operations are mostly carried out in carbon plants.
Petroleum coke can be divided into high sulfur coke (containing more than 1.5% sulfur), medium sulfur coke (containing 0.5%-1.5% sulfur), and low sulfur coke (containing less than 0.5% sulfur). The production of graphite electrodes and other artificial graphite products is generally produced using low sulfur coke.
Needle coke is a kind of high quality coke with obvious fibrous texture, very low thermal expansion coefficient and easy graphitization. When the coke is broken, it can be split into slender strips according to texture (the aspect ratio is generally above 1.75). An anisotropic fibrous structure can be observed under a polarizing microscope, and is therefore referred to as needle coke.
The anisotropy of physico-mechanical properties of needle coke is very obvious. It has good electrical and thermal conductivity parallel to the long axis direction of the particle, and the coefficient of thermal expansion is low. When extrusion molding, the long axis of most particles is arranged in the extrusion direction. Therefore, needle coke is the key raw material for manufacturing high-power or ultra-high-power graphite electrodes. The graphite electrode produced has low resistivity, small thermal expansion coefficient and good thermal shock resistance.
Needle coke is divided into oil-based needle coke produced from petroleum residue and coal-based needle coke produced from refined coal pitch raw materials.
Coal tar is one of the main products of coal tar deep processing. It is a mixture of various hydrocarbons, black at high temperature, semi-solid or solid at high temperature, no fixed melting point, softened after heating, and then melted, with a density of 1.25-1.35 g/cm3. According to its softening point, it is divided into low temperature, medium temperature and high temperature asphalt. The medium temperature asphalt yield is 54-56% of coal tar. The composition of coal tar is extremely complicated, which is related to the properties 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 for characterizing coal tar pitch, such as bitumen softening point, toluene insolubles (TI), quinoline insolubles (QI), coking values, and coal pitch rheology.
Coal tar is used as a binder and impregnant in the carbon industry, and its performance has a great impact on the production process and product quality of carbon products. The binder asphalt generally uses a medium-temperature or medium-temperature modified asphalt having a moderate softening point, a high coking value, and a high β resin. The impregnating agent is a medium temperature asphalt having a low softening point, a low QI, and good rheological properties.
The following picture shows the production process of graphite electrode in carbon enterprise.
Calcination: The carbonaceous raw material is heat-treated at a high temperature to discharge the moisture and volatile matter contained therein, and the production process corresponding to the improvement of the original cooking performance is called calcination. Generally, the carbonaceous raw material is calcined by using gas and its own volatiles as a heat source, and the maximum temperature is 1250-1350 °C.
Calcination makes profound changes in the structure and physicochemical properties of carbonaceous raw materials, mainly 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. .
Calcined equipment mainly includes tank calciner, rotary kiln and electric calciner. The quality control index of calcination is that 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.
Raw material crushing and ingredients
Before the batching, the bulk calcined petroleum coke and needle coke must be crushed, ground, and sieved.
The medium crushing is usually carried out by crushing equipment of about 50 mm through a jaw crusher, a hammer crusher, a roll crusher and the like to further crush the 0.5-20 mm size material required for the batching.
Milling is a process of grinding a carbonaceous material to a powdery small particle of 0.15 mm or less and a particle size of 0.075 mm or less by means of a suspension-type ring roll mill (Raymond mill), a ball mill, or the like.
Screening is a process in which a wide range of materials after a crushing is divided into several particle size ranges with a narrow range of sizes through a series of sieves with uniform openings. Current electrode production usually requires 4-5 pellets and 1-2 powder grades.
Ingredients are the production processes for calculating, weighing and focusing the various aggregates of aggregates and powders and binders according to the formulation requirements. The scientific suitability of the formulation and the stability of the batching operation are among the most important factors affecting the quality index and performance of the product.
The formula needs to determine 5 aspects:
1Select the type of raw materials;
2 determine the proportion of different types of raw materials;
3 determining the particle size composition of the solid raw material;
4 determine the amount of binder;
5 Determine the type and amount of additives.
Kneading: Mixing and quantifying various particle size carbonaceous granules and powders with a certain amount of binder at a certain temperature, and kneading the plasticity paste into a process called kneading.
Kneading process: dry mixing (20-35 min) wet mixing (40-55 min)
The role of kneading:
1 When mixing dry, the various raw materials are uniformly mixed, and the solid carbonaceous materials of different particle sizes are uniformly mixed and filled to improve the compactness of the mixture;
2 After adding coal tar pitch, the dry material and the asphalt are uniformly mixed. The liquid asphalt uniformly coats and wets the surface of the granules to form a layer of asphalt bonding layer, and all the materials are bonded to each other to form a homogeneous plastic smear. Conducive to molding;
3 parts of coal tar pitch penetrates into the inner space of the carbonaceous material, further increasing the density and cohesiveness of the paste.
Molding: The molding of carbon material refers to the process of plastically deforming the kneaded carbon paste under the external force applied by the molding equipment to finally form a green body (or raw product) having a certain shape, size, density and strength. process.
Types of molding, equipment and products produced:
Molding method
Common equipment
main products
Molding
Vertical hydraulic press
Electric carbon, low-grade fine structure graphite
Squeeze
Horizontal hydraulic extruder
Screw extruder
Graphite electrode, square electrode
Vibration molding
Vibration molding machine
Aluminum carbon brick, blast furnace carbon brick
Isostatic pressing
Isostatic molding machine
Isotropic graphite, anisotropic graphite
Squeeze operation
1 cool material: disc cooling material, cylinder cooling material, mixing and kneading cooling materials, etc.
Discharge the volatiles, reduce to a suitable temperature (90-120 ° C) to increase the adhesion, so that the blockiness of the paste is uniform for 20-30 min
2 Loading: press lift baffle —– 2-3 times cutting—-4-10MPa compaction
3 pre-pressure: pressure 20-25MPa, time 3-5min, while vacuuming
4 extrusion: press down the baffle —5-15MPa extrusion — cut — into the cooling sink
Technical parameters of extrusion: compression ratio, press chamber and nozzle temperature, cooling temperature, preload pressure time, extrusion pressure, extrusion speed, cooling water temperature
Green body inspection: bulk density, appearance tapping, analysis
Calcination: It is a process in which the carbon product green body is filled in a specially designed heating furnace under the protection of the filler to perform high-temperature heat treatment to carbonize the coal pitch in the green body. The bitumen coke formed after the carbonization of the coal bitumen solidifies the carbonaceous aggregate and the powder particles together, and the calcined carbon product has high mechanical strength, low electrical resistivity, good thermal stability and chemical stability. .
Calcination is one of the main processes in the production of carbon products, and is also an important part of the three major heat treatment processes of graphite electrode production. The calcination production cycle is long (22-30 days for baking, 5-20 days for furnaces for 2 baking), and Higher energy consumption. The quality of green roasting has an impact on the quality of the finished product and the cost of production.
The green coal pitch in the green body is coked during the roasting process, and about 10% of the volatile matter is discharged, and the volume is produced by 2-3% shrinkage, and the mass loss is 8-10%. The physical and chemical properties of the carbon billet also changed significantly. The porosity decreased from 1.70 g/cm3 to 1.60 g/cm3 and the resistivity decreased from 10000 μΩ·m to 40-50 μΩ·m due to the increase of porosity. The mechanical strength of the calcined billet was also large. For improvement.
The secondary baking is a process in which the calcined product is immersed and then calcined to carbonize the pitch immersed in the pores of the calcined product. Electrodes that require higher bulk density (all varieties except RP) and joint blanks are required to be bibaked, and the joint blanks are also subjected to three-dip four-bake or two-dip three-bake.
Main furnace type of roaster:
Continuous operation—-ring furnace (with cover, without cover), tunnel kiln
Intermittent operation—-reverse kiln, under-floor roaster, box roaster
Calcination curve and maximum temperature:
One-time roasting—-320, 360, 422, 480 hours, 1250 °C
Secondary roasting—-125, 240, 280 hours, 700-800 °C
Inspection of baked products: appearance tapping, electrical resistivity, bulk density, compressive strength, internal structure analysis
Impregnation is a process in which a carbon material is placed in a pressure vessel and the liquid impregnant pitch is immersed in the pores of the product electrode 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.
The impregnation process and related technical parameters are: roasting billet – surface cleaning – preheating (260-380 °C, 6-10 hours) – loading the impregnation tank – vacuuming (8-9KPa, 40-50min) – Injection of bitumen (180-200 °C) – Pressurization (1.2-1.5 MPa, 3-4 hours) – Return to asphalt – Cooling (inside or outside the tank)
Inspection of impregnated products: impregnation weight gain rate G=(W2-W1)/W1×100%
One dipping weight gain rate ≥14%
Secondary impregnated product weight gain rate ≥ 9%
Three dipping products weight gain rate ≥ 5%
Graphitization refers to a high-temperature heat treatment process in which a carbon product is heated to a temperature of 2300 ° C or more in a protective medium in a high-temperature electric furnace to convert an amorphous layered structure carbon into a three-dimensional ordered graphite crystal structure.
The purpose and effect of graphitization:
1 improve the conductivity and thermal conductivity of the carbon material (the resistivity is reduced by 4-5 times, and the thermal conductivity is increased by about 10 times);
2 improve the thermal shock resistance and chemical stability of the carbon material (linear expansion coefficient reduced by 50-80%);
3 to make the carbon material lubricity and abrasion resistance;
4 Exhaust impurities, improve the purity of the carbon material (the ash content of the product is reduced from 0.5-0.8% to about 0.3%).
The realization of the graphitization process:
The graphitization of carbon material is carried out at a high temperature of 2300-3000 °C, so it can only be realized by electric heating in the industry, that is, the current directly passes through the heated calcined product, and the calcined product charged into the furnace is generated by the electric current at a high temperature. The conductor is again an object that is heated to a high temperature.
Furnaces currently widely used include Acheson graphitization furnaces and internal heat cascade (LWG) furnaces. The former has a large output, a large temperature difference, and a high power consumption. The latter has a short heating time, low power consumption, uniform electrical resistivity, and is not suitable for fitting.
The control of the graphitization process is controlled by measuring the electric power curve that is suitable for the temperature rise condition. The power supply time is 50-80 hours for the Acheson furnace and 9-15 hours for the LWG furnace.
The power consumption of graphitization is very large, generally 3200-4800KWh, and the process cost accounts for about 20-35% of the total production cost.
Inspection of graphitized products: appearance tapping, resistivity test
Machining: The purpose of mechanical machining of carbon graphite materials is to achieve the required size, shape, precision, etc. by cutting to make the electrode body and joints in accordance with the requirements of use.
Graphite electrode processing is divided into two independent processing processes: electrode body and joint.
The body processing includes three steps of boring and rough flat end face, outer circle and flat end face and milling thread. The processing of conical joint can be divided into 6 processes: cutting, flat end face, car cone face, milling thread, drilling bolt And slotting.
Connection of electrode joints: conical joint connection (three buckles and one buckle), cylindrical joint connection, bump connection (male and female connection)
Control of machining accuracy: thread taper deviation, thread pitch, joint (hole) large diameter deviation, joint hole coaxiality, joint hole verticality, electrode end face flatness, joint four-point deviation. Check with special ring gauges and plate gauges.
Inspection of finished electrodes: accuracy, weight, length, diameter, bulk density, resistivity, pre-assembly tolerance, etc.
Post time: Oct-31-2019