Baking is a heat treatment process in which the pressed raw product is heated at a certain heating rate in the protective medium in the heating furnace under the condition of isolating air.
The purpose of baking is:
(1) Eliminate volatile matter. Products using coal tar pitch as a binder generally emit about 10% of volatile matter after baking. Therefore, the firing yield is generally below 90%.
(2) Binder coking. The raw product is baked according to certain technological conditions to coke the binder and form a coke network between the aggregate particles, which firmly connects all aggregates of different sizes together, so that the product has certain physical and chemical properties. Under the same conditions, the higher the coking rate, the better the quality. Generally, the coking residue rate of medium temperature pitch is about 50%.
(3) Fixed geometric form. During the baking process, the raw product softens and the binder migrates. As the temperature rises, a coking net is formed, making the product rigid. Therefore, if the temperature increases, its shape does not change.
(4) Reducing resistivity. During the baking process, due to the elimination of volatiles, the pitch coking forms a coke grid, and the pitch decomposes and polymerizes, resulting in a large hexagonal carbon ring plane net. The resistivity drops significantly. The resistivity of the raw product is about 10000×10-6Ω·m, and after baking, it drops to 40-50×10-6Ω·m, which is called a good conductor.
(5) Further volume shrinkage. After baking, the diameter of the product shrinks by about 1%, the length shrinks by about 2%, and the volume shrinkage is 2-3%.