The Furnace Black Process
The furnace black process is the most modern process for the manufacture of carbon black today. It is a continuous process in a closed reactor system and therefore is flexible, economical, and less polluting. High turbulent flow prevails due to high flow velocities. Due to its importance for large-scale industrial production of carbon black for polymer fillers, the furnace black process is described in more detail in the following section.
The gaseous or liquid fuel is completely converted with air in the combustion chamber of the reactor to combustion off-gas with temperatures ranging from 1,200 °C to 1,800 °C. The reactor and throat are made of refractory linings. In the mixing zone, the preheated feedstock is injected in the hot off-gas before entering in the reaction zone of the reactor where the atomization and vaporization of the feedstock are completed. Thermal decomposition (pyrolysis) of the feedstock and the carbon black growth occur in the reaction zone. Here, the competing reactions of nucleation and particle growth are controlled by process parameters such as reaction temperature, feedstock loading rate, and residence time so that the carbon black is produced with the desired morphological property profile. High reaction (flame) temperature, low feedstock injection rate, and short residence time (short quench) favor small particle sizes (high surface areas). High feedstock/air ratios reduce the concentration of feedstock vapor and lead to lower carbon black structures. Additives can also be introduced into the flame to help with morphology control. The reactor off-gas (smoke gas) formed during conversion of the feedstock to carbon black contains CO, hydrogen, and steam which, at the temperature level of the reaction zone, react with the carbon black. These occurring water-gas and Boudouard reactions reduce the yield of carbon black. To stop these side reactions, the process gas mixture after a certain residence time is quenched by water injection. The temperature of the reactor off-gas containing the carbon black is further cooled by heat exchangers at the reactor exit. Cooled down to a temperature below 400 °C, the carbon black is separated from the tail gas by bag filters. About 50-60% of the feedstock is converted into carbon black. The balance is converted into combustible tail gas that can be used as a fuel source in the plant due to its high content of carbon monoxide, hydrogen, and methane.
The carbon black discharged from the filters is conveyed pneumatically to other sections for further processing. Grit and magnetic separators are used to purify the carbon black from contaminants such as coke particles (“grit”), abraded particles from the refractory lining of the furnace, or rust particles. The furnace process produces fluffy, low-bulk density powders that are difficult to transport and handle. One of the key steps is the conversion of these into high-bulk density, but still free-flowing, powders. This is usually carried out by a wet pelletizing process with water and must be controlled so that the pellets are strong enough to survive transport and handling, but weak enough to break down easily in their final application. The carbon black pellets are dried and then packed. The waste gas is the final off-gas after all combustibles have been burned. A schematic process flow chart is given in Fig. 1.