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Difference between calcination and roasting in graphite electrode production process

Aug 29, 2020 Source: DanCarbon Writer: Yvonne
Difference between calcination and roasting in graphite electrode production process
Calcination temperature is controlled at (1200-1500°C), Roasting requires real-time control of temperature changes.
► Graphite electrode production process
1) Calcination
During the calcinating process, the physical and chemical properties of raw materials are increased.
2) Forming
Kneaded material is formed using an extrusion press.  
3) Baking
Through the baking process, the thermal stability, mechanical strength and electrical conductivity of materials can be greatly improved.
4) Impregnation
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.
5) Graphitization
Processing the baked material at extremely high temperatures to change its structure from amorphous carbon to graphite and obtain excellent material properties.
6) Machining
Machining the graphitized material to achieve the required size, shape and accuracy, to make the electrode body and pin according to the specification of customers.
graphite electrode production process
Graphite electrodes production process:
https://youtu.be/rDigaz27Txo

► Difference between calcination and roasting
• Calcination
The process of heat treatment of carbon raw materials at high temperature (1200≤1500°C) under isolated air conditions is called calcination. Calcination is the first heat treatment process in carbon production. Calcination causes a series of changes in the structure and physical and chemical properties of various carbonaceous raw materials.
Both anthracite and petroleum coke contain a certain amount of volatilization and need to be calcined. The coking temperature of bituminous coke and metallurgical coke is relatively high (above 1000℃), which is equivalent to the temperature of calcining furnace in carbonization plant. It can't be calcined anymore. It only needs to be dried. However, if bituminous coke is mixed with petroleum coke before calcining, it shall be sent to the calciner together with petroleum coke. Natural graphite and carbon black do not require calcination.
• Roasting
Roasting is a heat treatment process in which compressed raw products are heated at a certain rate under the condition of isolating air in the protective medium in the furnace.
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 purpose of roasting is:
1) Exclude volatiles
In general, 10% volatiles are discharged from the products which use coal asphalt as binder after roasting. Therefore, the rate of roasted products is generally below 90%.
2) Binder coking
The raw product is roasted according to certain technological conditions to make the binder coking and form a coke network among the aggregate particles, so that all the aggregate of different particle sizes are firmly connected together and the product has certain physical and chemical properties. Under the same conditions, the higher the coking rate, the better the quality. The coking rate of medium - temperature asphalt is about 50%.
3) Fixed geometry
In the roasting process of raw products, the phenomenon of softening and binder migration occurred. With the increase of temperature, the coking network is formed, making the products rigid. Therefore, its shape does not change as the temperature rises.
4) Reduce the resistivity
In the roasting process, due to the elimination of volatiles, the coking of asphalt forms a coke grid, the decomposition and polymerization of asphalt, and the formation of a large hexagonal carbon ring plane network, etc., the resistivity decreased significantly. About 10000 x 10-6 raw products resistivity Ω m., after roasting reduced to 40-50 x 10-6 Ω m., called good conductors
5) Further shrinkage
After roasting, the product shrinks by about 1% in diameter, 2% in length and 2-3% in volume.

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