Measurement of electronic conduction in cryolite alumina melts and estimation of its effect on current efficiency
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I.
INTRODUCTION
THE current efficiency (CE) in the Hall-Heroult process may exceed 95 pct in modem cells. It is generally accepted that the major part of the loss in CE is due to the reaction between dissolved metal and dissolved CO2 or impurities in the electrolyte. Further development of the process will probably lead to even higher CE and lower energy consumption. However, the limit of the obtainable CE may be determined by an electronic component of the conductivity of the electrolyte. Experimental studies in binary molten alkali metalalkali halide mixtures have shown that dissolution of alkali metals in the correspon_ding salts leads to a large increase in specific conduc~ivity.t~,z~ The excess conductivity is caused by the formation of free electrons, and the electronic conductivity increases with increasing metal activity. Sodium has a certain activity in NaF-A1F3 melts in contact with aluminum due to the following equilibrium AI (1) + 3NaF (1) = 3Na (diss) + AIF3 (diss)
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Model studies t31 indicate that sodium dissolves in the electrolyte in the form of free Na while dissolved A1 is predominantly present as the monovalent species A1F2. Any electronic conductivity is most likely associated with the Na species which may form trapped electrons (analogous to F-centers in solid crystals) and electrons in the conduction band. The existence of electronic conduction in NaF-A1F3 melts was demonstrated by Borisoglebskii et a l . [41 The increase in conductivity in the presence of aluminum was determined to be about 3 pct in Na3A1F6 at 1000 ~ Morris IS1 ascribed the loss in current efficiency during AI production to electronic conduction. In a theoretical and experimental study, G.M. HAARBERG, Researcher, K.S. OSEN, Engineer, and J. THONSTAD, Professor, are with the Laboratories of Industrial Electrochemistry, University of Trondheim, Norwegian Institute of Technology, N-7034 Trondheim, Norway. R.L HEUS, formerly Chemist, and J.J. EGAN, formerly Scientist, Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973, are retired. Manuscript submitted October 15, 1992. METALLURGICAL TRANSACTIONS B
Dewing and Yoshida t61 subsequently maintained that the electronic conductivity was too low to account for the loss in CE in industrial aluminum cells. The aim of the present study was to measure the electronic conductivity of Na3A1F6-AleO3 melts and to estimate the corresponding decrease of the CE during aluminum production from this melt. Measurements were only carried out in cryolite saturated with alumina, Experiments in more acidic melts at different temperatures will be considered in the future.
II. TECHNIQUES FOR THE MEASUREMENT OF ELECTRONIC CONDUCTION AND ELECTRON MOBIL1TIES A, Electronic Conductivity The Wagner polarization technique is usually used to study electrical conduction in solids exhibiting both ionic and electronic transport. The method has been thoroughly treated in the literature especially by Wagner m and Kroger. m It has also been employed to study the movement of electrons i
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