Coke serves as the main raw material and reducing agent in the smelting of ferrosilicon. Therefore, it is important for coke to have a high fixed carbon content, ideally greater than 84%, while maintaining a low ash content. The high ash content leads to increased slag formation because the ash contains about 60% aluminum oxide (AI2O3), which makes the slag sticky and difficult to remove. In addition, the use of coke with a high ash content can lead to sintering of the surface of the material, which makes it difficult to breathe. Therefore, the ash content of coke should be kept below 13%.
Most of the carbon in coke is in the form of fixed carbon, but a small fraction exists in the form of hydrocarbons, commonly referred to as volatiles. These volatile substances evaporate at high temperatures, so their content should not exceed 2%.
In addition to the chemical composition, coke also has requirements for physical properties. Higher resistance to high temperatures allows the electrode to be inserted deeper into the charge, which increases the furnace temperature and expands the crucible. The high porosity of coke not only contributes to its resistance, but also increases the surface area for chemical reactions, thereby accelerating the reaction rate.
The particle size of the coke has a significant impact on the smelting process. If the particles are too large, the charge resistance decreases, making it difficult for the electrode to penetrate deeply, which can lead to lower furnace temperatures, smaller crucibles, and uneven melting. Conversely, if the particle size is too small or there is an excess amount of powder, it can lead to increased firing losses, causing the kiln to become sticky and air permeability. Therefore, the coke must have a specific particle size, ideally less than 8 mm for large ferrosilicon electric furnaces. Smaller ferrosilicon electric furnaces, which operate at lower temperatures, use a "smoldering" process, and the addition of some coke powder can help sinter the surface of the material, allowing the electrode to be inserted deeper. Thus, a small amount of coke powder can be used in the smelting of ferrosilicon in low-power electric furnaces.