How to make a graphite electrode?
The graphite electrode is made by using petroleum coke and needle coke as raw materials, coal tar pitch as a binder, and it is made by calcination, batching, kneading, pressing, roasting, impregnation, graphitization, and machining.
The following is the process flow chart of graphite electrode manufacturing
Calcining
- The production process in which carbonaceous raw materials are heat-treated at high temperatures to remove the contained water and volatile components and correspondingly improve the original cooking performance 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 1250-1350°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 batching
Before blending, the large calcined petroleum coke and needle coke must be crushed, ground, and sieved.
Medium 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 the materials with a particle size of 0.5-20mm required for batching.
Grinding is a process in which carbonaceous raw materials are pulverized to small powder-like particles 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.
Sieving is the process of dividing materials with a wide range of sizes into several granular grades with a narrower size range through a series of sieves with uniform openings. The current electrode production usually requires 4-5 granular grades and 1-2 powder granular grades.
Ingredients are 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 scientificity of the formula, suitability and stability of the batching operation are one of the most important factors affecting product quality indicators and performance.
The formula needs to determine 5 aspects:
- Choose the type of raw material;
- Determine the ratio of different types of raw materials;
- Determine the particle size composition of solid raw materials;
- Determine the amount of binder;
- Determine the type and amount of additives.
Kneading
At a certain temperature, the quantitative carbonaceous particles and powders of various particle sizes and the quantitative binder are stirred and mixed uniformly, and the process of kneading into a plastic paste is called kneading.
Kneading process: dry mixing (20-35 min) wet mixing (40-55 min)
The role of kneading:
- During dry mixing, the various raw materials are mixed uniformly, and at the same time, solid carbon materials of different particle sizes are uniformly mixed and filled to improve the density of the mixture;
- After coal tar pitch is added, 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 materials to each other to form a homogeneous plastic paste, which is beneficial forming;
- Part of the coal pitch penetrates into the internal voids of the carbonaceous material, further improving the density and cohesiveness of the paste.
Forming
The forming of carbon materials refers to the process in which the mixed and kneaded carbon paste undergoes plastic deformation under the external force exerted by the forming equipment, and finally forms a green body (or green product) with a certain shape, size, density and strength.
Types of molding, equipment and products produced:
| Molding method | Common equipment | main products |
| Molded | Vertical hydraulic press | Electric carbon, low-grade fine structure graphite |
| Extrusion | Horizontal hydraulic squeeze Press Screw extruder |
Graphite electrode, square electrode |
| Vibration forming | Vibration molding machine | Carbon bricks for aluminum, blast furnace carbon bricks |
| Isostatic pressure | Isostatic pressing machine | Isotropic graphite, anisotropic graphite |
Squeeze operation
- Cooling material: disc cooling material, cylindrical cooling material, kneading cooling material and other methods to discharge the volatiles, reduce to a suitable temperature (90-120℃) to increase the bonding force, and make the paste block uniform for 20-30 min
- Loading: press lift baffle—-discharge in 2-3 times—-4-10MPa compaction
- Pre-pressure: pressure 20-25MPa, time 3-5min, and vacuum at the same time
- Extrusion: Press down the baffle—-5-15MPa extrusion—-shear—-turn into the cooling water tank
Extrusion technical parameters: compression ratio, press chamber and nozzle temperature, cooling material temperature, pre-pressure time, extrusion pressure, extrusion speed, cooling water temperature
Inspection of green body: bulk density, appearance knocking, analysis
Roasting
Roasting: It is a process in which the green carbon products are loaded into 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 coal tar pitch carbonization 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 (22-30 days for one roasting, 5-20 days for second roasting depending on the furnace type), and Higher energy consumption. The quality of the green body firing has a certain impact 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 2-3%, and the mass loss is 8-10%. The physical and chemical properties of the carbon billet have also changed significantly. Due to the increase in porosity, the volume density is reduced from 1.70g/cm3 to 1.60g/cm3, and the resistivity is reduced to 40-50μΩ.m from about 10000μΩ.m. The mechanical strength of the calcined billet is also large. To improve.
Secondary roasting
Secondary roasting is a process in which the roasted product is impregnated and then roasted again to carbonize the pitch immersed in the pores of the roasted product. The production of electrodes with high bulk density requirements (all varieties except RP) and joint blanks need to be double-baked, and 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), tunnel kiln Intermittent operation-inverted flame kiln, car bottom roasting furnace, box roasting furnace
- Main types of roasting furnaces: Continuous operation-ring furnace (with or without cover), tunnel kiln Intermittent operation-inverted flame kiln, car bottom roasting furnace, box roasting furnace
Roasting curve and maximum temperature:
- Primary roasting-320, 360, 422, 480 hours, 1250 ℃
- Secondary roasting-125, 240, 280 hours, 700-800 ℃
Inspection of baked products: appearance knock, electrical resistivity, bulk density, compressive strength, internal structure analysis