Mechanism and rate of reaction of Al2O, Al, and CO vapors with carbon

During the production of aluminum by carbothermic reduction, large quantities of Al2O and Al vapor are generated. For the process to be economical, the aluminum and energy associated with these species must be captured and used in the process. This is accomplished by reacting them with carbon to form Al4C3. The mechanism and rate of the reactions of gas containing Al and Al2O with various forms of carbon was studied. The Al2O-Al-CO gas was generated by reacting an Al4C3-Al2O3 melt with carbon at high temperatures (2000 °C to 2050 °C). The gas then reacted with carbon at lower temperatures (1900 °C to 1950 °C). The only form of carbon that reacted extensively, forming Al4C3, was wood charcoal; with other forms of carbon, such as metallurgical coke and petroleum coke, primarily only Al2O3 condensed on the surface formed. The rate of formation of Al4C3 on wood charcoal was found to be controlled by the diffusion of Al2O and Al through the Al4C3 product layer, and their effective diffusivities were estimated to be 0.82 and 1.31 cm2/s, respectively. Over 90 pct of the carbide is formed by Al2O and only 10 pct by Al vapor. When an Al4C3-Al2O3 dense slag was formed on the charcoal at lower temperatures (1920 °C to 1930 °C) and then reacted at a higher temperature, it appears that the slag and carbon reacted to form Al4C3 relatively fast. The volume of Al4C3 formed is much greater than that of the original carbon. It is believed that this is the reason the other forms of carbon with lower porosity (25 pct vs 60 pct) did not react significantly. Any amount of Al4C3 formed would quickly fill the pores of the more dense carbon, stopping any further reaction.