Calcination is the process of heat-treating solid carbon raw materials at high temperatures (1200-1500°C) in an airtight environment. Calcination is a pretreatment step in carbon production. Various solid carbon raw materials undergo significant changes in their elemental composition and microstructure during calcination.
Anthracite, petroleum coke, and delayed pitch coke all contain a certain amount of volatile matter and require calcination. Metallurgical coke, with its relatively high coking temperature, is roughly equivalent to the calcination temperature used in carbon plants, and thus can be dispensed with by drying out the moisture. Natural graphite can also be calcined to improve its wettability. Generally, the calcined material is harder and more brittle, making it easier to crush, grind, and screen.
The purpose of calcination is to remove moisture and volatile matter from the solid carbon raw materials, allowing them to fully shrink in volume and improve their thermal stability and physical and chemical properties. The moisture content (mass fraction) of incoming solid carbon raw materials is generally between 3% and 10%. Excessive moisture in raw materials makes crushing, grinding, and screening difficult, and also affects the raw material particles' ability to absorb binders, making molding difficult. Therefore, the moisture content (mass fraction) of the calcined material is generally required to be no more than 0.3%.
If the volatile content of the raw materials is too high, the product will undergo significant volume shrinkage during calcination due to the release of large amounts of volatiles, leading to deformation or even cracking. Therefore, volatiles must be removed from the raw materials. At calcination temperatures, as volatiles are released, high-molecular-weight aromatic hydrocarbons undergo complex decomposition and polycondensation reactions, continuously changing their molecular structure and gradually shrinking the raw material itself, thereby increasing its density and mechanical strength. Generally speaking, at the same temperature, the higher the true density of the calcined material, the easier it is to graphitize.