Raw materials and manufacturing process of graphite electrodes

Graphite electrode is a kind of high temperature resistant graphite conductive material produced by a series of processes such as kneading, molding, baking, impregnation, graphitization, and machining, using petroleum coke and needle coke as aggregate and coal tar as binder.

Graphite electrode is an important high-temperature conductive material for electric furnace steelmaking. Electric energy is input into the electric furnace through the graphite electrode, 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 steelmaking. Some other submerged arc furnaces for smelting yellow phosphorus, industrial silicon, abrasives and other materials also use graphite electrodes as conductive materials. Using the excellent and special physical and chemical properties of graphite electrodes, it also has a wide range of uses 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 petroleum 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 is a kind of easily graphitized carbon, which is widely used in chemical industry, metallurgy and other 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 is obtained from delayed coking, which contains a large amount of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most refineries in China only produce green coke, and calcination operations are mostly carried out in carbon plants.

According to the level of sulfur content, petroleum coke can be divided into three types: high-sulfur coke (with a sulfur content of more than 1.5%), medium-sulfur coke (with a sulfur content of 0.5%-1.5%), and low-sulfur coke (with a sulfur content of less than 0.5%). 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 block is broken, it can be split into long and thin strips according to the texture (the aspect ratio is generally above 1.75). The anisotropic fibrous structure can be observed under a polarizing microscope, so it is called needle coke.

The anisotropy of 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 thermal expansion coefficient. During extrusion, the long axis of most particles is arranged in the extrusion direction. Therefore, needle coke is a key raw material for the manufacture of 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 and coal-based needle coke produced from refined coal tar pitch.

Coal tar 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. It has no fixed melting point. It softens and then melts after being heated. The density is 1.25-1.35g/cm3. According to its softening point, it is divided into three types: Low temperature coal pitch, medium temperature coal pitch and high temperature coal pitch. The yield of medium temperature pitch is 54-56% of that of coal tar. The composition of coal tar pitch is extremely complex, 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 to characterize the characteristics of coal tar pitch, such as pitch softening point, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value and rheology of coal pitch.

Coal tar pitch is used as a binder and impregnant in the carbon industry, and its properties have a great impact on the production process and product quality of carbon products. The binder asphalt generally uses medium temperature or medium temperature modified asphalt with moderate softening point, high coking value and high β resin, and the impregnant should use medium temperature asphalt with low softening point, low QI and good rheological properties.

Calcination: The production process, in which the carbonaceous raw material is heat treated at high temperature to discharge the moisture and volatile matter contained, and the physical and chemical properties of the raw material are improved, is called calcination. Generally, carbonaceous raw materials are calcined by using gas and their own volatiles as heat sources, and the maximum temperature is 1250-1350 °C.

Calcination makes profound changes in the structure and physical and chemical properties of carbonaceous raw materials, which are mainly reflected in improving the density, mechanical strength and electrical conductivity of coke, improving the chemical stability and anti-oxidation performance of coke, and laying the foundation for subsequent procedures.

The calcining equipment mainly includes tank calciner, rotary kiln and electric calciner. The calcination quality control indicators are: 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.

Crushing and batching of raw materials

Before batching, the large pieces of calcined petroleum coke and needle coke must be crushed, ground and screened.

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

Grinding is the process of grinding carbonaceous raw materials to small powdery particles with a particle size of less than 0.15mm or 0.075mm by means of suspension roller mill (Raymond mill), ball mill and other equipment.

Screening is a process of dividing the crushed material with a wide size range into several particle grades with a narrow size range through a series of sieves with uniform openings. Current electrode production usually requires 4-5 particle sizes and 1-2 powder grades.

Batching is a production process that calculates, weighs and focuses on aggregates, powders and binders of various particle sizes according to formula requirements. The scientific suitability of the formula and the stability of the batching operation are one of the most important factors affecting the product quality index and performance.

The formula needs to determine 5 aspects: 1. Select the type of raw materials; 2. Determine the ratio of different types of raw materials; 3. Determine the particle size composition of solid raw materials; 4. Determine the amount of binder; 5. Determine the type and amount of additives.

Kneading: The process, the quantitative carbonaceous granules and powders of various sizes and the quantitative binder are stirred and mixed evenly into a plastic paste at a certain temperature, is called kneading.

The kneading process: dry mixing (20-35 min) wet mixing (40-55 min) The function of kneading: 1.During dry mixing, all kinds of raw materials are mixed evenly, and at the same time, solid carbon materials of different particle sizes are evenly mixed and filled , to improve the compactness of the mixture; 2. After adding coal tar pitch, the dry material and asphalt are mixed evenly, and the liquid asphalt evenly coats and infiltrates the surface of the particles to form a layer of asphalt bonding layer, which binds all the materials together to form a Homogeneous plastic paste to benefit forming; 3. Part of the coal pitch penetrates into the internal voids of the carbonaceous material, which further improves the density and cohesiveness of the paste.

Forming: The forming of carbon materials refers to the process in which the kneaded carbon paste is plastically deformed to a green body (or raw product) with certain shape, size, density and strength under the external force exerted by the forming equipment.

Extrusion operation:

1. Cooling material: to discharge volatile matter by disk cooling , cylindrical cooling material, kneading cooling etc., lower to suitable temperature (90-120℃) to increase the cohesive force, make the paste lump evenly beneficial Forming ,20-30 min 2. Loading: Extruder lifts baffle in 2-3 times of unloading ---- 4-10MPa compaction 3. Pre-extrusion: pressure 20-25MPa, time 3-5min, and vacuum at the same time 4. Extrusion: extruder drops baffle----5-15MPa extrusion----shear----turn into cooling water tank.

Technical parameters of extrusion: compression ratio, temperature of extruder material chamber and nozzle, material cooling temperature, pre-pressing time, extrusion pressure, extrusion speed, cooling water temperature

Inspection of green bodies: bulk density, appearance knocking, profiling.

Baking: It is a process in which the green carbon products are placed in a specially designed heating furnace for high temperature heat treatment under the protection of fillers to carbonize the coal tar pitch in the green body. The pitch coke formed after coal pitch carbonization consolidates the carbonaceous aggregates and powder particles together, and the calcined carbon products have higher mechanical strength, lower resistivity, better thermal stability and chemical stability .

Baking is one of the main processes in the production of carbon products, and it is also an important part of the three major heat treatment processes in the production of graphite electrodes. The baking production cycle is long (22-30 days for the first period baking, 5-20 days for the second period baking according to the furnace type), and the energy consumption is high. The quality of green baking has a certain influence on the quality of the finished product and the production cost.

The coal pitch in the green body is coked during the baking process, and about 10% of the volatile matter is 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 blanks have also changed significantly. Due to the increase of porosity, the bulk density is reduced from 1.70g/cm3 to 1.60g/cm3, and the resistivity is reduced from 10000μΩ.m to 40-50μΩ.m. The mechanical strength of the calcined body is also greatly improved..

Secondary baking is a process in which the baked product is impregnated and then baked again to carbonize the pitch impregnated in the pores of the baked product. Electrodes (all varieties except RP) and joint blanks with higher bulk density requirements need to be baked twice, and the joint blanks also need to be impregnated three times and four times baked or two times impregnate and three times baked.

Main types of baking furnaces: continuous operation----ring furnace (with cover, without cover), intermittent operation of tunnel kiln----down-flame kiln, car bottom type baking furnace, box type baking furnace.

Baking curve and maximum temperature: primary baking----320, 360, 422, 480 hours, 1250 ℃; secondary baking----125, 240, 280 hours, 700-800 ℃. Inspection of baked products: appearance knock, resistivity, bulk density, compressive strength, internal structure analysis.

Impregnation is a process in which the carbon material is placed in a pressure vessel, and the liquid impregnant pitch is immersed into the electrode pores 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.

The technological process and related technical parameters of the impregnation are: baking blanks - surface cleaning - preheating (260-380 ℃, 6-10 hours) - loading into the impregnation tank - vacuuming (8-9KPa, 40-50min) - Asphalt injection (180-200 ℃) - Pressurization (1.2-1.5MPa, 3-4 hours) - Asphalt return - Cooling (inside or outside the tank).

Inspection of impregnated products: weight gain rate of impregnation G=(W2-W1)/W1×100%, weight gain rate of first impregnated product≥14%, weight gain rate of secondary impregnated product≥9% weight gain rate of third impregnated product≥5%.

Graphitization refers to a high-temperature heat treatment process in which carbon products are heated to above 2300 ℃ in a protective medium in a high-temperature electric furnace to convert amorphous turbostratic carbon into a three-dimensional ordered graphite crystal structure.

The purpose and function of graphitization: 1. Improve the electrical and thermal conductivity of carbon materials (resistivity is reduced by 4-5 times, and thermal conductivity is increased by about 10 times); 2. Improve the thermal shock resistance and chemical stability of carbon materials (linear expansion coefficient 50-80% reduction); 3. make the carbon material have lubricity and wear 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%).

The realization of the graphitization process:

The graphitization of carbon materials is carried out at a high temperature of 2300-3000 °C, so it can only be realized by electric heating in industry. The electric current directly passes through the heated baked product. At this time, the baked product loaded into the furnace is not only a conductor that generates a high temperature through the current, but also an object that is heated to a high temperature.

At present, the widely used furnace types include Acheson graphitization furnace and internal heat series connection (LWG) furnace. The former has the characteristics of large output, large temperature difference and high power consumption; the latter has the characteristics of short heating time, low power consumption, uniform resistivity but not easy to install joints.

The control of the graphitization process is to determine the electric power curve suitable for the temperature rise by temperature measurement. The power-on 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 entire production cost.

Inspection of graphitized products: appearance knock, resistivity test.

Machining: The machinery of carbon graphite material relies on cutting to achieve the required size, shape, precision, etc., and makes the body and joint of the electrode that meet the requirements of use.

Graphite electrode processing is divided into two independent processing processes: electrode body and joint.

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

Electrode joint connection method: conical joint connection (one-inch three-thread and one-inch four-thread), cylindrical joint connection, concave-convex connection (male and female buckle connection).

Control of machining accuracy: thread taper deviation, thread pitch, joint (hole) major diameter deviation, joint hole coaxiality, joint hole verticality, electrode end surface flatness, joint four-point deviation, etc. Check with special ring gauge and plate gauge.

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