Physical chemistry of the carbothermic reduction of alumina in the presence of a metallic solvent: Part II. Measurements
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I. I N T R O D U C T I O N A L U M I N U M is produced from bauxite ore by leaching the alumina from the ore (the Bayer process) and subsequently reducing the alumina to aluminum in an electrochemical cell (the Hall-Heroult cell). Due to cell overpotentials and ohmic losses, the electrical energy needed to produce aluminum is nearly four times the amount predicted by thermodynamics, t~l To avoid the large expenditure of energy in the production of aluminum, the aluminum industry has attempted for almost 100 years to develop a carbothermic reduction process as a replacement for the Hall-Heroult cell. The overall reaction for carbothermic reduction is A1203(s) + 3C(s) = 2Al(1) + 3CO(g)
[1]
However, efforts to develop reduction processes have met with limited success, t2-6] Some of the important physicochemical problems that have been encountered are associated with the fact that Eq. [1] will proceed only at temperatures in the region of 2200 ~ for an overall pressure of one atmosphere. These high temperatures promote the direct vaporization of aluminum and of aluminum-bearing compounds. Because of the need to employ carbon refractories and the presence of carbon in the furnace charge, there also is a high solubility of carbon in the metal product. Additionally, oxycarbide and carbide phases containing aluminum will form in competition with the formation of aluminum. The possibility of carrying out the reduction of alumina to produce aluminum by Eq. [1] at temperatures low enough to avoid the serious problems associated with
ROBERT A. FRANK, Research Assistant, and JOHN F. ELLIOTT, Professor of Metallurgy, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. CHARLES W. FINN, formerly Research Scientist, Department of Materials Science and Engineering, MIT, is Director of Research, Vacuum Industries, Inc., Somerville, MA 02143. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
the formation of vapor species, the high solubility of carbon in the metal, and the formation of intermediate compounds has been investigated theoretically, tTj The most promising strategy developed was to reduce the pressure at which the CO(g) is evolved and to reduce the activity of the aluminum produced by capturing it in a solvent metal bath. Either tin or copper appeared to be a suitable solvent. The overall reduction reaction then is A1203(s) + 3C(s) = 2A1 + 3CO(g)
[2]
where the underline indicates that the aluminum is dissolved in a solvent metal bath. Table I summarizes the thermodynamic data relevant to this work. [8,9,~~ Based on the theoretical study, an experimental investigation was undertaken to evaluate the practicality of carrying out Eq. [2] at temperatures in the range of 1700 ~ to 1850 ~ at tot
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