Mechanism and rate of reaction of Al 2 O, Al, and CO vapors with carbon
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Mechanism and Rate of Reaction of Al2O, Al, and CO Vapors with Carbon R.J. FRUEHAN, Y. LI, and GERALD CARKIN 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.
I. INTRODUCTION
DURING the past 50 years, there have been numerous attempts to produce aluminum by carbothermic reduction, eliminating the use of the traditional Hall cell. The major driving forces for the development of carbothermic reduction are a decrease in energy consumption and cost and a significant increase in smelting intensity. Of particular note, the processes that reached a significant level of development are the Pechiney and Reynolds processes. Recently Alcoa, with technical support from Elkem Research and Carnegie Mellon University, has been carrying out research and developmental work on unique technologies to produce aluminum by carbothermic reduction.[1] The smelter consists of two stages. In stage 1, (Al4C3-Al2O3) slag (hereafter, parentheses indicate that the specie is in solution in slag) is formed by the reduction of Al2O3 to Al4C3 by carbon at about 2000 °C. In stage 2, the Al metal is formed by reaction of the Al4C3 with Al2O3 at higher temperatures. In any carbothermic reduction process for aluminum, extremely high temperatures (2000 °C) are required. At these temperatures, a large quantity of Al and Al2O gaseous species are associated with the CO off-gas. Depending on the temperature and melt (Al2O3-Al4C3) co
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