Raw Materials and Manufacturing Process of the Graphite Electrode

Graphite electrode is a kind of high temperature resistant graphite conductive material produced by a series of processes such as kneading, molding, roasting, impregnation, graphitization, and mechanical processing using petroleum coke and needle coke as aggregates and coal pitch as the binder.

Electrode de graphite 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 the heat source to melt the furnace 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. The excellent and special physical and chemical properties of graphite electrodes are utilized, and it also has a wide range of uses in other industrial sectors.

1. The raw materials for the production of the graphite electrode include petroleum coke, needle coke and coal pitch

Petroleum coke is a combustible solid product obtained by coking petroleum residue and petroleum pitch. Its color is black and it is porous, the main element is carbon, and the ash content is very low, generally below 0.5%. Petroleum coke belongs to the category of easily graphitized carbon. Petroleum coke is widely used in chemical, metallurgical 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 petroleum coke obtained from delayed coking contains a large amount of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most oil refineries in China only produce coke, and most of the calcination operations are carried out in carbon plants.

Petroleum coke is classified according to the level of sulfur. It can be divided into three types: high-sulfur coke (sulfur content above 1.5%), medium-sulfur coke (sulfur content 0.5%-1.5%), and low-sulfur coke (sulfur content below 0.5%). Low-sulfur coke is generally used in the production of graphite electrodes and other artificial graphite products.

Needle coke is a kind of high-quality coke with obvious fibrous texture, low thermal expansion coefficient and easy graphitization. When the coke is broken, it can split into elongated particles according to the texture (the aspect ratio is generally above 1.75). An anisotropic fibrous structure can be observed under a polarized light 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 in the direction parallel to the long axis of the particles, and has a low thermal expansion coefficient. During extrusion molding, most of the long axes of the particles are arranged in the extrusion direction. Therefore, needle coke is the key raw material for the manufacture of high-power or ultra-high-power graphite electrodes. The resulting graphite electrodes have low resistivity, low thermal expansion coefficient, and good thermal shock resistance.

Needle coke is divided into oil-based needle coke produced with petroleum residue as raw material and coal-based needle coke produced with refined coal pitch raw material.

Coal pitch is one of the main products of deep processing of coal tar. 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 after heating and then melts. The density is 1.25 to 1.35g/cm3. 

According to its softening point, it can be divided into low temperature, medium temperature and high temperature asphalt. The yield of medium temperature pitch is 54 to 56% 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 that characterize the characteristics of coal pitch, such as pitch softening point, toluene insoluble matter (TI), quinoline insoluble matter (QI), coking value, and coal pitch rheology.

Coal pitch 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. Binder asphalt generally uses medium temperature or medium temperature modified asphalt with moderate softening point, high coking value, and high β resin. The impregnant uses medium temperature asphalt with low softening point, low QI, and good rheological properties.

Calcining: The carbonaceous raw material is heat-treated at high temperature to discharge the contained moisture and volatile matter, and correspondingly improve the original cooking performance of the production process, which is called calcination. Generally, carbonaceous raw materials are calcined using fuel gas and their own volatile matter as the heat source, and the maximum temperature is 1250 to 1350°C.

Calcining causes profound changes in the structure and physical and chemical properties of carbon raw materials, which are mainly reflected in increasing the density, mechanical strength and conductivity of coke, improving the chemical stability and oxidation resistance of coke, and laying the foundation for subsequent processes.

The calcination equipment mainly includes tank calciner, rotary kiln and electric calciner. The calcination quality control index 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 treatment and burdening.

Before batching, the large calcined petroleum coke and needle coke must go through processes of intermediate crushing, grinding powder, and screening.

Intermediate crushing usually involves crushing materials with a size of about 50mm through jaw crushers, hammer crushers, counter-roll crushers and other crushing equipment to further crush materials with a particle size of 0.5 to 20mm required for burdening.

Grinding is a process in which carbonaceous raw materials are pulverized to small powder graphite particle with a particle size of 0.15mm or 0.075mm by means of a suspended-rod ring roller mill (Raymond mill), ball mill and other equipment.

Screening is the process of dividing materials with a wide range of sizes into several granular grades with a narrow size range through a series of sieves with uniform openings. The current electrode production usually requires 4 to 5 granular grades and 1 to 2 powder granular grades.

Burdening is a production process in which aggregates, powders, and binders of various sizes are calculated, weighed, and focused according to the requirements of the formula. The scientific suitability of the formula and the stability of the batching operation are most important factors affecting product quality indicators and using performance.

The formula needs to determine 5 aspects: 

①Select the type of raw materials; 

②Determine the ratio of different types of raw materials; 

③Determine the particle size composition of the solid raw materials; 

④Determine the amount of binder; 

⑤Determine the type and amount of additives.

Kneading: Stir and mix quantitatively various carbonaceous particles and powders with a quantitative binder at a certain temperature, and knead it into a plastic paste, which is called kneading.

The process of kneading: dry mixing (20 to 35 min) and wet mixing (40 to 55 min). 

The function of kneading: 

①Dry mixing makes various raw materials uniformly mixed, and at the same time makes solid carbon materials of different particle sizes uniformly mixed and filled, so as to improve the density of the mixture; 

②After adding coal tar pitch, the dry material and asphalt will be evenly mixed, and the liquid asphalt will evenly coat and infiltrate the surface of the particles to form a layer of asphalt bonding layer, which will bond all materials together to form homogeneous plastic paste, which is conducive to molding; 

③Part of the coal pitch penetrates into the internal voids of the carbonaceous material, further improving the density and cohesiveness of the paste.

Molding: The molding of carbon materials refers to the process by which the mixed and kneaded carbon paste produces plastic deformation under the external force applied by the molding equipment, and finally forms a green body (or green product) with a certain shape, size, density and strength process.

Extrusion operation: 

①Cooling material: disc cooling material, cylindrical cooling material, kneading type cooling material and other methods to discharge the volatile matter, reduce to the appropriate temperature (90 to 120 ℃) to increase the bonding force, make the lumpiness of paste uniform, which is good for forming into 20 to 30 min;

②Loading: press lifts the baffle----discharge in 2 to 3 times----4 to 10MPa for compaction;

③Pre-compaction: pressure 20 to 25MPa, time 3 to 5min, and vacuumize at the same time;

④Extrusion: The press lowers the baffle----5 to 15MPa extrusion----shear----turn into the cooling water tank.

Technical parameters for extrusion: compression ratio, press chamber and nozzle temperature, cooling material temperature, pre-compaction pressure time, extrusion pressure, extrusion speed, cooling water temperature.

Inspection of green body: bulk density, appearance knocking, and analysis

Roasting: It is a process in which the green carbon product is placed in a specially designed heating furnace for high-temperature heat treatment under the protection of the filler, so that the coal pitch in the green body is carbonized. The pitch coke formed after carbonization of coal tar pitch consolidates the carbonaceous aggregate and powder particles together, and the roasted carbon product has higher mechanical strength, lower electrical resistivity, better thermal stability and chemical stability.

Roasting 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 roasting production cycle is longer (22 to 30 days for one baking, 5 to 20 days for second baking), and energy consumption is high. The quality of the green body roasting 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 roasting process, and about 10% of the volatile matter is discharged, while the volume shrinks by 2 to 3%, and the mass loss is 8 to 10%. The physical and chemical properties of the carbon billet have also changed significantly. Due to the increase in porosity, the volume density has decreased from 1.70g/cm3 to 1.60g/cm3, and the resistivity has dropped to 40 to 50μΩ.m from about 10,000μΩ.m. The mechanical strength of the calcined billet is also largely improved.

Secondary roasting is a process in which the roasted product is immersed and then roasted again to carbonize the pitch immersed in the pores of the roasted product. The production of electrodes with higher bulk density requirements (all varieties except RP) and joint blanks need to be subjected to two baking, and the joint blanks also need to be subjected to three immersion and four baking or two immersion and three baking.

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

Firing curve and maximum temperature: primary firing-320, 360, 422, 480 hours, 1250 ℃ secondary firing-125, 240, 280 hours, 700 to 800 ℃ inspection of roasted products: appearance knocking, 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 volume 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: baking billet-surface cleaning-preheating (260 to 380 ℃, 6 to 10 hours)-loading into the impregnation tank-vacuuming (8 to 9KPa, 40 to 50min) -Inject pitch (180 to 200 ℃)——pressurization(1.2 to 1.5MPa, 3 to 4 hours)——Return pitch——Cooling (in or outside the tank).

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

Graphitization refers to a high-temperature heat treatment process in which carbon products are heated to above 2300 ℃ in the protective medium of a high-temperature electric furnace to transform the amorphous chaotic layer structure carbon into a three-dimensional ordered graphite crystalline structure.

2. The purpose and function of graphitization

①Improve the electrical and thermal conductivity of carbon materials (resistivity is reduced by 4 to 5 times, and thermal conductivity is increased by about 10 times); 

②Improve the thermal vibration resistance and chemical stability of carbon materials (coefficient of linear expansion is reduced by 50 to 80%); 

③make the carbon material have lubricity and abrasion resistance; 

④discharge impurities and improve the purity of the carbon material (the ash content of the product is reduced from 0.5 to 0.8% to about 0.3%).

3. Realization of graphitization process

The graphitization of carbon materials is carried out at a high temperature of 2300 to 3000 ℃, so in the industry, it can only be realized by electric heating, that is, the electric current directly passes through the heated roasted product. At this time, the roasted product in the furnace is both a conductor which generates high temperature through electric current and an object which is heated to high temperature.

Currently widely used furnace types include Acheson graphitization furnace and 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 not easy to install joints.

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

Graphitization consumes a lot of electricity, generally 3200 to 4800KWh, and the process cost accounts for about 20 to 35% of the entire production cost.

Inspection of graphitized products: appearance knocking, resistivity test.

Mechanical processing: The purpose of mechanical processing of carbon graphite material is to rely on cutting 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 processes: the electrode body and the joint.

The body processing includes 3 processes of boring and rough flat end face, cylindrical turning and fine flat end face, and thread milling. The processing of conical joints can be divided into 6 processes: cutting, flat end face, turning conical surface, milling thread, drilling and anchoring and slotting.

The connection method of electrode connector: conical connector for connection (one inch three buckles and one inch four buckles), cylindrical connector for connection, concave-convex connection (male and female buckle for connection).

Processing precision control: thread taper deviation, thread and 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, bulk density, resistivity, pre-installed matching accuracy, etc.

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