Graphite electrodes, or artificial graphite electrodes, are an essential high-temperature conductive material for electric furnace steelmaking. Electrical energy is transferred to the furnace through graphite electrodes, and the high temperatures generated by the arcs between the electrode tips and the furnace charge are used as a heat source to melt the materials. This process underpins the principle of electric arc steelmaking. The upstream sector of the graphite electrode supply chain primarily consists of raw material suppliers such as petroleum coke and needle coke, along with production equipment vendors. Raw materials account for a significant portion of the production costs of graphite electrodes, exceeding 65%. Midstream comprises graphite electrode manufacturing companies, while downstream involves sectors like electric furnace steelmaking, silicon smelting, and yellow phosphorus production in electric furnaces.
Graphite Electrode Raw Materials
Solid Carbonaceous Materials
Solid carbonaceous materials include petroleum coke, needle coke, pitch coke, natural graphite, and crushed graphite, among others.
Petroleum coke is a solid combustible product obtained from the coking of petroleum residual oil and asphalt. It is black and porous, primarily composed of carbon with very low ash content, typically below 0.5%. Petroleum coke belongs to the category of carbon that is easily graphitized and can be differentiated into green coke and calcined coke based on the heat treatment temperature. Green coke is produced through delayed coking and contains a significant amount of volatile matter with low mechanical strength. Calcined coke is obtained by calcining green coke. Most refineries in China only produce green coke, with the calcining process often carried out within carbon factories. Depending on sulfur content, petroleum coke can be categorized into high-sulfur coke (with more than 1.5% sulfur), medium-sulfur coke (with 0.5%-1.5% sulfur), and low-sulfur coke (with less than 0.5% sulfur). Low-sulfur coke is generally used in the production of graphite electrodes and other artificial graphite products.
Needle coke is a premium coke with a distinct fibrous texture, exceptionally low coefficient of thermal expansion, and a high propensity for graphitization. The physical and mechanical properties of needle coke exhibit significant anisotropy; it demonstrates excellent electrical and thermal conductivity parallel to the long axis of the particles and has a low coefficient of thermal expansion. During extrusion, most particles align their long axes with the direction of extrusion. Consequently, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes. The resulting graphite electrodes have lower resistivity, a smaller coefficient of thermal expansion, and better thermal shock resistance.
Binder And Impregnating Agents
Binders and impregnating agents include coal tar pitch, coal tar, anthracene oil, and synthetic resins.
Coal tar pitch is one of the main products derived from the further processing of coal tar, consisting of various hydrocarbon compounds. At room temperature, it is a black, highly viscous semi-solid or solid. Coal tar pitch is used as a binder and impregnating agent in the carbon industry, where its properties greatly influence the production process and quality of carbon products. As a binder, a moderate softening point pitch with high coking value and high β-resin content is generally used, while as an impregnating agent, a medium-temperature pitch with a lower softening point, low QI (quinoline insoluble), and good rheological properties are employed.
Additionally, in the production of graphite electrodes, auxiliary materials such as silica sand, metallurgical coke powder, and coke powder are also utilized.
Graphite Electrode Manufacturing Process
The manufacturing process of graphite electrodes primarily includes the steps of raw material calcination, crushing and screening, batching, mixing and kneading, molding, baking, impregnation, (secondary baking), graphitization, machining, and final product inspection. The main differences in the processes between manufacturers are reflected in the refined management of raw material ratios, innovations in molding technology, and temperature control during the baking and graphitization stages. AGRM employs an automated control system to optimize these critical steps, thereby obtaining higher-quality products.
1. Calcination
The thermal treatment of carbonaceous materials at high temperatures (1250-1350°C) to expel moisture and volatile components and correspondingly enhance the physicochemical properties of the raw materials is known as calcination.
Calcination induces profound changes in the structural organization and physicochemical properties of the carbonaceous materials. Primarily, it increases the density, mechanical strength, and electrical conductivity of the coke, as well as its chemical stability and resistance to oxidation, laying the foundation for subsequent operations. The primary equipment used for calcination includes tank calciners, rotary kilns, and electric calciners. The quality control indicators for calcination are that the true density of petroleum coke should not be less than 2.07g/cm³, with an electrical resistivity not greater than 550μΩ.m; for needle coke, the true density should not be less than 2.12g/cm³, with an electrical resistivity not greater than 500μΩ.m.
2. Crushing And Screening
Before batching, large pieces of calcined petroleum coke and needle coke must undergo processes of medium crushing, grinding, and screening.
Medium crushing typically involves further breaking down materials of about 50mm in size to the granularity required for batching, which is 0.5-20mm. Grinding is the process of reducing the carbonaceous materials to a fine powder with particles smaller than 0.15mm or 0.075mm in diameter. Screening is the process of classifying crushed materials with a wide range of sizes into several narrower size fractions using a series of sieves with uniform openings. Current electrode production typically requires 4-5 granular size fractions and 1-2 powder size fractions.
3. Batching
Batching is the production process where various particle sizes of aggregates, powders, and binders are calculated, weighed, and gathered according to formula requirements. The scientific appropriateness of the formula and the stability of the batching operation are among the most important factors affecting product quality indicators and performance.
4. Mixing And Kneading
The process of mixing and kneading involves blending quantitative amounts of various particle sizes of carbonaceous materials and powders with a fixed amount of binder at a certain temperature to form a plastic paste. The quality of mixing and kneading directly affects the stability of product quality. The process includes dry mixing (20-35 min) and wet mixing (40-55 min). The quality of the mixing and kneading can be influenced by the binder pitch, mixing temperature, and duration of the process.
Functions:
- Ensure uniform distribution and mixing of different materials
- Increase the compactness of the materials
- Unify the structural organization of dry materials with the binder, imparting plasticity to the paste to facilitate molding
5. Molding
The molding of carbon materials refers to the process where the plastically deformed carbon paste, obtained from mixing and kneading, is shaped into a green body (or green product) with certain shapes, dimensions, density, and strength under the external forces applied by the molding equipment. Molding methods include extrusion molding, vibration molding, and compression molding, among which extrusion molding is commonly adopted in factories. Extrusion molding employs horizontal extruders and is suitable for producing rods, tubes, or other profiled products with relatively large length-to-diameter ratios.
Advantages of Extrusion Molding:
- Continuous production
- High production efficiency
- Simple operation
- The high degree of mechanization
6. Baking
Baking is the heat treatment process where the pressed green products are heated in a protective medium within a heating furnace, in an airtight condition, at a certain rate of temperature rise. Baking is one of the main procedures in the production of carbon products and an important part of the three major heat treatment processes in the production of graphite electrodes. Baking has a long production cycle (first bake 22-30 days, second bake 5-20 days depending on the furnace type) and high energy consumption. The quality of green body baking affects both the quality of the finished product and the production cost.
Purpose:
- Volatile removal: Products using coal tar pitch as a binder emit about 10% of their volatile content after baking.
- Binder coking: The coke raw material is baked under certain process conditions, leading to the coking of the binder and forming a coke network between the aggregate particles, firmly connecting different particle sizes of aggregates, thus endowing the product with certain physicochemical properties. Under the same conditions, the higher the coke yield, the better the product quality.
- Fixing geometric shape: During baking, the green product softens, and the binder migrates. As the temperature rises, a coke network forms, rigidifying the product. Therefore, even if the temperature rises further, its shape does not change.
- Reducing resistivity: During baking, due to the removal of volatiles, the formation of a coke grid from the coking of pitch, and the decomposition and polymerization reactions of pitch resulting in large hexagonal carbon ring plane networks, the resistivity decreases significantly. The resistivity of green products is about 10000×10^-6Ω·m, which after baking reduces to 40-50×10^-6Ω·m, making it a good conductor.
- Adequate shrinkage in volume: After baking, the diameter of the product shrinks by about 1%, and the length shrinks by about 2%, with a volume reduction of 2-3%.
7. Impregnation
Impregnation is the process where carbon materials are placed in a pressure vessel, and liquid impregnating agent pitch is forced into the pores of the electrode products under certain temperature and pressure conditions. The purpose is to reduce the porosity of the products, increase the bulk density and mechanical strength, and improve the electrical and thermal conductivity of the products.
Purpose:
- Reduce product porosity
- Increase density
- Enhance compressive strength
- Lower finished product resistivity
- Alter the physicochemical properties of the products
8. Secondary Baking
Secondary baking is the process where the impregnated baked products are baked again to carbonize the pitch that has penetrated the pores of the baked products. Electrodes requiring a higher volume density (all varieties except RP) and joint blanks need to undergo a second bake, and joint blanks also require a third impregnation and fourth bake, or a second impregnation and third bake.
9. Graphitization
Graphitization is the high-temperature heat treatment process where baked products are heated in a graphitization furnace within a protective medium to high temperatures, causing the two-dimensional disordered overlap of hexagonal carbon atom plane grids to transform into an ordered three-dimensional overlap with a graphite structure.
Purpose:
- Improve the thermal and electrical conductivity of the products.
- Enhance the thermal shock resistance and chemical stability of the products.
- Increase the lubricity and wear resistance of the products.
- Eliminate impurities and enhance product strength.
10. Machining
The machining of graphite electrodes is divided into two independent processes: the electrode body and the joint. Body machining includes three operations: boring and rough flatting of the end face, turning the outer circle fine flatting of the end face, and milling threads. The machining of conical joints can be divided into six operations: cutting, flatting the end face, turning the conical surface, milling threads, drilling for pin installation, and slotting.
Purpose:
- Need for shaping: After pressing, green carbon products with certain sizes and shapes will undergo varying degrees of deformation and damage during baking and graphitization, and their surfaces will also be attached with some filling materials. If not machined, they cannot be used, so it is necessary to shape the products into the specified geometric shapes through machining.
- Need for use: Machining is performed according to the user’s requirements. For example, graphite electrodes used in electric furnace steelmaking need to be connected for use; threaded holes must be machined at both ends of the product, and then special threaded joints are used to connect two electrodes for use.
- Technological needs: Some products need to be machined into special shapes and specifications according to the technological needs of users, and even require a lower surface roughness.
11. Finished Product Inspection
Quality control runs through the entire production process. Starting with the selection of raw materials, quality must be ensured through chemical analysis and physical testing. During production, non-destructive testing methods such as X-ray and ultrasonic testing are used to monitor the internal structure of the products. In addition, finished products undergo multiple performance tests including precision, weight, length, diameter, bulk density, resistivity, and pre-assembly fit precision to ensure they meet the stringent requirements of industrial applications.