1.Low-temperature preheating stage (room temperature to 350℃)
When the actual heating temperature of the green body reaches 100 to 230 degrees Celsius, the green body begins to soften, the internal stress relaxes, the volume slightly expands, but not much volatile matter is discharged, and the green body is in the plastic stage. At this stage, the main function is to preheat the carbon billet. Due to the temperature and pressure differences within the green billet, some of the light components of the asphalt migrate and diffuse and flow. As the temperature continues to rise to 230-400℃, the decomposition rate of asphalt gradually accelerates. Especially within the temperature range of 350-400℃, asphalt decomposes violently and a large amount of volatile matter is discharged. At this stage, the heating rate needs to be controlled to prevent sudden temperature rise from causing internal stress concentration, and at the same time, to avoid the rapid release of volatile matter that may cause cracks in the carbon billet.
2. Medium-temperature coking stage (350℃ to 800℃)
When the actual heating temperature of the green body rises to 400-550℃, the decomposition and volatilization rate of asphalt slows down, entering a stage dominated by polycondensation reaction. At high temperatures, asphalt undergoes thermal decomposition and polycondensation to form semi-coke. At this point, the amount of volatile matter discharged decreases, and the volume of the green body changes from expansion to contraction. When the actual heating temperature of the green body reaches 500 to 700℃, the semi-coke formed by the asphalt further transforms into binder coke (asphalt coke), the volatile matter released by the decomposition of asphalt further decreases, and the carbon green body continues to shrink. At this point, the asphalt binder has transformed into binder coke, and the thermal conductivity of the carbon green body has increased. This stage is a crucial one that affects the quality of roasting. The binder undergoes a large number of complex decomposition, polymerization, cyclization and aromatization reactions. The decomposition of the binder and the re-polymerization of the decomposition products occur simultaneously, forming an intermediate phase. The growth of the intermediate phase leads to the formation of precursors. At 400℃, the product begins to show coking, but the strength is still very low, and the adhesion of asphalt decreases. At around 500℃, although there is still a small amount of volatile matter, the basic structure of the carbon has already formed. Semi-coke is formed at 500 to 550℃, and the volatile substances produced by the thermal decomposition of asphalt are basically discharged before 600 to 650℃. Coke is formed at 700 to 750℃. In order to increase the coking rate of asphalt and improve the physical and chemical properties of the products, the temperature must be raised uniformly and slowly at this stage. In addition, during this stage, a large amount of volatile matter is discharged, filling the entire furnace chamber. These gases decompose on the surface of the hot products, generating solid carbon that deposits on the pores and surface of the products, increasing the coke yield and sealing the pores of the products, thereby enhancing their strength. The most prominent feature of the reaction at this stage is the polymerization and decomposition of functional groups and the gradual increase in hydrogen content in the discharged gas.
3. High-temperature sintering stage (800℃ to 1200~1350℃)
When the product reaches above 700℃, the coking process of the binder is basically completed. During the high-temperature sintering stage, the heating rate can be increased somewhat. After reaching the maximum temperature, it is necessary to maintain the temperature for 15 to 20 hours. During the coking process, large aromatic planar molecules are formed. The peripheral dissimilar atoms and atomic groups of the planar molecules break and are excluded. As the temperature rises, the planar molecules undergo rearrangement. Above 900℃, hydrogen atoms at the edge gradually break and are eliminated. At the same time, the binder coke further shrinks and densifies. At this point, the chemical process gradually weakens, the internal and external shrinkage gradually decreases, while the true density, strength and electrical conductivity all increase.
4. Cooling stage
During cooling, the cooling rate can be slightly faster than the heating rate. However, due to the limitation of the thermal conductivity of the product, the cooling rate inside the product is less than that on the surface, thus forming temperature gradients and thermal stress gradients of different magnitudes from the center to the surface of the product. If the thermal stress is too large, it will cause uneven internal and external shrinkage and lead to cracks. Therefore, cooling should also be carried out in a controlled manner. During the cooling stage, gradient cooling is implemented. The cooling rate in areas above 800℃ does not exceed 3℃/h to avoid cracks caused by rapid cooling. The temperature at which the products come out of the furnace must be below 80℃. When using an atomized water cooling system, the water temperature should be stably maintained at 40℃±2℃ to prevent thermal shock damage.
Post time: Jun-11-2025