Sponsored by: LMD Aluminum Committee
Program Organizer: Ms Fiona J Stevens, Comalco Research and Technology, Comalco Research Centre, PO Box 316, Thomastown, Victoria 3074, Australia
Monday, AM Room: A9
February 5, 1996 Location: Anaheim Convention Center
Session Chairperson: Jomar Thonstad, Department of Electrochemistry, Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim, Norway
8:30 am
THE TRANSPORTED ENTROPIES OF IONS IN SOLID STATE FLUORIDES AND B"-ALUMINA: V.S. Sharivker, Institute of Structural Macrokinetics, Russian Acadamy of Sciences, Chernogolovka, Moscow Region, Russia 142432; S.K. Ratkje, Department of Physical Chemistry, Norwegian Institute of Technology, University of Trondheim, N-7034, Trondheim, Norway
Transported entropies of Na+ in solid state mixtures of NaF and Na3AlF6 are determined from thermocell experiments. The experiments were favourably described by the electric work method. The variation observed in the thermocell emf with composition is explained by the probable path of charge transfer in the electrolyte. The transported entropy of sodium ions in cryolite is S[[sigma]]cryNa+ = 140 +/- 7 J K-1 mol-1 between 380deg.C and 500deg.C. The Thomson coefficient in this temperature range is [[alpha]]cry Na+ = 33 +/- 10 JK-1 mol-1. It is predicted that the transported entropy for Na+ in the liquid electrolyte probably is larger than 140 J K-1 mol-1. This makes us predict that the transported entropy for Na+ in the molten electrolyte mixture for aluminium production is substantial, and that the reversible heat effects in the aluminium electrolysis cell are the same. It implies that the reversible heat balances calculated for the electrodes in the aluminium electrolysis need to be revised.
9:00 am
PELTIER HEATS IN CRYOLITE MELTS WITH ALUMINA: Belinda E. Flem, Signe Kjelstrup Ratkje, Division of Physical Chemistry, Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim, Norway; Asmund Sterten, Division of Industrial Electrochemistry, Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim, Norway
The Seebeck coefficient was measured for cells with molten mixtures of NaF and AlF3 saturated with Al203. The electrodes were either a pair of oxygen electrodes or a pair of aluminium electrodes. For the molar ratio NaF/AlF3 equal to 1.8, 1.2 and 1.0, we obtained the Seebeck coefficients -1.80 mV K-1 at 971[[ring]]C, -1.63 mV K-1 at 813.6[[ring]]C and -0.583 mV K-1 at 758[[ring]]C, respectively, for the oxygen electrodes. For the aluminium electrodes, we obtained the Seebeck coefficient - 1.23 mV K-1 at 962[[ordmasculine]]C, for NaF/AlF3 of 1.8. These suggest that there is a substantial reversible heat consumption at the anode during aluminium electrolysis and a large reversible heat production at the cathode. The highest temperature in the Hall-Heroult cell is then closer to the cathode than the anode. The transported entropies of Al3 and O2- were calculated to 77 J mole-1 K-1 and 10 J mole-1 K-1, respectively, when NaF/AlF3 was 1.0.
9:30 am
APPLICATION OF NONEQUILIBRIUM THERMODYNAMICS TO ALUMINUM ELECTROLYSIS CELLS: Ellen Marie Hansen, Signe Kjelstrup Ratkje, Belinda Flem, Dick Bedeaux
The theoretical expression for the dissipated energy, Ts[[sigma]]s, of the electrode surfaces in the aluminium electrolysis cell is derived from nonequilibrium thermodynamics, considering polarizable fluids at an interface. Here [[sigma]]s is the excess entropy production rate at the surface and Ts is the surface temperature. It is shown that the energy dissipated at the electrode surfaces is a significant part of the total energy dissipated in the cell. Both vectorial and scalar contributions to the dissipated energy are presented. The vectorial contributions describe processes parallel to the surface, while the scalar contributions describe transport processes normal to the surface. Coupled transport of heat, mass, and charge is discussed in particular. The premises used in the derivation of the theoretical expressions are local thermodynamic equilibrium and electroneutrality at the surface. Discontinuities in the intensive variables at the surfaces (e.g. temperature and chemical potential), are predicted and estimated.
10:00 am
DEFINING BATH RATIO IN THE PRESENCE OF LiF AND MgF2: Warren Haupin, 2820 Seventh Street Road, Lower Burrell, PA 15068
When Lewis acids other than AlF3 and Lewis bases other than NaF are present, the ratio of NaF to AlF3 is no longer a good measure of Lewis acidity. The formula Req = 1/2{[(NaF)/42 + 0.32 (LiF)/25.4]/ [AlF3) /84 + 0.41 (MgF2)/62.3]} produces nearly constant acidity of AlF4- and nearly constant vapor pressures. However, at constant Req, LiF lowers: liquidus temperatures, metal solubility, alumina solubility and bath density while raising electrical conductivity. At constant Req, MgF2 lowers: liquidus temperature, alumina solubility and electrical conductivity but raises metal solubility, and bath density.
10:25 am BREAK
10:35 am
ELECTRICAL CONDUCTIVITY MEASUREMENTS IN CRYOLITE ALUMINA MELTS IN THE PRESENCE OF ALUMINUM: G.M. Haarberg, J. Thonstad, Department of Electrochemistry, The Norwegian Institute of Technology, N-7034 Trondheim, Norway; J.J. Egan, Brookhaven National Laboratory, Upton, NY 11973; R. Oblakowski, S. Pietrzyk, Academy of Mining and Metallurgy, Krakow, Poland
Ac resistance measurements were performed in cryolite alumina melts to study the effect of dissolved metal on electrical conductivity. The specific electrical conductivity was found to increase substantially in the presence of Al. A certain activity of sodium is established due to the interaction between Al and the melt. Dissolution of Na gives rise to the formation of mobile electrons, which are responsible for the increase of the conductivity. The electronic conductivity was taken as the difference between the conductivity in melts equilibrated with Al and the ionic conductivity. In the pure cryolite alumina melt saturated with Al at 1000C the electronic conductivity contributes to more than 20% of the total conductivity. Results are in good agreement with previously reported data using a polarization technique. Effects of temperature CaF2 and xs AlF3 were studied. Model calculations showing the effect of electronic conduction on the current efficiency for producing Al were made.
11:05 am
DENSITY, ELECTRICAL CONDUCTIVITY AND VISCOSITY OF LOW MELTING BATHS FOR ALUMINIUM ELECTROLYSIS: M. Chrenkova, V. Danek, A. Silny, Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava, Slovakia, Department of Metallurgy and Materials Science, T.A. Utigard, University of Toronto, M5S 1A4, Toronto, Canada
Density, electrical conductivity and viscosity of melts of the Na3AlF6 - AlF3 - LiF - Al203 system have been measured. Empirical equations for the concentration and temperature dependencies of these parameters have been developed. The reliability of these equations is compared with those published by other authors. An equation for density, was derived; which is valid from 850-1050C, AlF3 up to 30% and Al203 up to the solubility limit. An equation for electrical conductivity was obtained; which is valid in the same ranges. An equation for viscosity (within the standards deviation of 1.83.10 -2 mPa.s) is given which is valid from 850 - 1050C, AlF3 from 9-26% and LiF up to 7%.
11:35 am
METAL SOLUBILITY IN LOW RATIO BATH: Qiu Zhuxian, Tie Jun, Yu Yaxin, Northeast University, Shenyang 110006, China
For the molten salt system of cryolite - excess AlF3 - CaF2-Al203, the metal
solubility was measured by gravimetric method. The metal solubility in the
molten salt was obtained by plotting the relationship curve of "Metal wt loss
versus time" in a time duration of 6 hours. An increase in the excess AlF3 -
content from 10% to 28% (mass), will cause the metal solubility to decrease
significantly, for two super-heat temperatures of 15[[ring]]C and 50[[ring]]C
above the corresponding liquidus temperatures. In a graphite crucible with
BN-lining the metal solubility may be decreased to 0.1% (mass). A comparison
between the gravimetric method and volumetric method is discussed.
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