Graphite electrodes manufacture process includes calcination, forming, baking, impregnation, graphitization and machining.
60% of graphite electrode production cost comes from raw materials, using petroleum coke, needle coke as aggregate, coal asphalt as binder.
Petroleum coke is a combustible solid obtained by coking petroleum residue and petroleum pitch. Petroleum coke is a type of easily graphitized carbon and is the main raw material for the production of graphite products.
Needle coke is a high-quality coke with fibrous texture, low thermal expansion coefficient, and easy graphitization. The anisotropy of the physical and mechanical properties of needle coke is very obvious, and it has good electrical and thermal conductivity. When it is extruded, it is arranged in the direction of extrusion. Therefore, needle coke is a key raw material for manufacturing high-power or ultra-high-power graphite electrodes. In addition, needle coke is divided into oil-based needle coke with petroleum residue and coal-based needle coke produced with refined coal pitch raw materials.
In the process of calcination, the raw materials remove the moisture and volatiles, and improve the physical and chemical properties of the raw materials. After high-quality needle coke is calcined, it becomes the raw material for producing high-performance electrodes. Generally, the maximum calcination temperature is 1250-1350℃, the equipment adopts tank calciner, rotary kiln and electric calciner.
The calcination needs to meet the indicators:
Real Density of Calcined Petroleum Coke≥2.07g/cm3, Resistivity≤550μΩ.m
Real Density of Calcined needle Coke≥2.12g/cm3, Resistivity≤500μΩ.m.
Kneaded material is formed using an extrusion press. By changing the nozzle on the extrusion cylinder, an electrode of desired diameter and length can be formed.
The volatile matter is discharged through extrusion and lowered to a suitable temperature (90-120°C) to increase the bonding force, so that the paste block is uniform and facilitates molding for 20-30 minutes.
The extrusion equipment is a horizontal hydraulic extruder and a screw extruder, and the extrusion is called a green body.
Through the baking process, the thermal stability, mechanical strength and electrical conductivity of materials can be greatly improved. As the remaining volatiles are drove off by re-baking, density and electrical resistance of materials can be improved.
The coal pitch in the green body is coked during the baking 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 electrode are also improved, the bulk density is reduced from 1.70g/cm3 to 1.60g/cm3, the resistivity of 10000μΩ.m is reduced to 40-50μΩ.m, and the mechanical strength is also improved.
The rebaking is baking the baked product after being impregnated, to carbonize the pitch impregnated in the pores of the baked product. For electrodes with higher volume density (except regular power graphite electrode) and nipples, rebaking is required, and the nipple blanks also need “three-time impregnation four-time baking” or “two-time impregnation three-time baking”.
Main baking furnace types:
Continuous operation: ring furnace (with or without cover), tunnel kiln
Intermittent operation: inverted flame kiln, bogie bottom roaster, box roaster
Baking curve and maximum temperature:
Baking--320, 360, 422, 480 hours, 1250 ℃
Rebaking--125, 240, 280 hours, 700-800 ℃
The baked electrodes are impregnated with a special pitch to give them the higher density, mechanical strength, and electrical conductivity to withstand the severe operating conditions inside the furnaces.
The impregnation process and related technical parameters are:
Baking billet——Cleaning surface——Preheating (260-380℃, 6-10 hours) ——Loading into the impregnation tank——Vacuuming (8-9KPa, 40-50min) ——Impregnating pitch (180-200 ℃) ——Pressurization (1.2-1.5MPa, 3-4 hours) ——Return pitch——Cooling (in or outside the tank)
Processing the baked material at extremely high temperatures（2300~3000℃） to change its structure from amorphous carbon to graphite and obtain excellent material properties.
Graphitization can only be carried out by energized heating, and Acheson graphitization furnace and internal heat series (LWG) furnace are used.
Graphitization can only be carried out by energized heating, using Acheson graphitization furnace and lengthwise graphitization (LWG) furnace.
Graphitization is the process of controlling temperature. The Acheson furnace is energized for 50-80 hours, and the LWG furnace is energized for 9-15 hours. Graphitization consumes a lot of electricity, generally 3200~4800KWh, accounting for about 20~35% of the entire production cost.
The graphite electrodes (after cooling) are machined to exact dimensions and tolerances.
The purpose of electrode machining is to rely on automatic or semi-automatic CNC machining to achieve the required size, shape, precision, etc., to make the electrode rod and nipples that meet the requirements of use.
From Mixing to Machining the production of graphite electrodes takes about 60 days as shown in the picture here below: