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I.

INTRODUCTION

THE search for materials with which nonconsumable anodes for aluminum production might be constructed has led to renewed interest in the solubilities of metal oxides in cryolite melts. Zinc oxide is well known as an electronic conductor, at least when it is in a slightly reduced form, and its solubility, together with the solubility of the associated zinc aluminate, ZnAl2O4, is the subject of this work. Existing literature data are unsatisfactory. Chasanov[1] reported a solubility of 0.75 wt pct and Belyaev et al.[2] one of 0.51 wt pct, both at 1000 7C. It must be noted, however, that pure cryolite melts at 1011 7C,[3] and those quantities of ZnO are quite insufficient to lower the melting point to 1000 7C. One therefore suspects that the cryolite was impure, and, if one of the impurities was alumina, it is known to have a drastic effect since Belyaev et al.[2] also reported a solubility of 0.004 wt pct in the presence of 5 wt pct Al2O3. Hayakawa and Kido[4] determined the phase diagram and reported a eutectic solubility of about 2.5 wt pct at 975 7C, but they too gave the melting point of cryolite as 1000 7C, and the slope of their liquidus is much steeper than can be accounted for by a normal cryoscopic calculation. Rolin and Bernard[5] used a visual technique to determine solubility and reported 0.53 wt pct at 1030 7C. None of these authors made any mention of the existence of the zinc aluminate spinel, ZnAl2O4. The Gibbs free energy of the reaction ZnO (s) 1 Al2O3 (a) 5 ZnAl2O4 (s)

[1]

has been determined by Jacobs[6] in the region of 700 7C to 900 7C as DG7 5 210,750 1 1.57 T 5 150 cal

[2]

The enthalpy term is in virtually perfect accord with the

ERNEST W. DEWING, 648, Pimlico Pl., Kingston, ON, Canada K7M 5T8, is retired. S. ROLSETH, Scientist, and L. STØEN, Technician, The Foundation for Scientific and Industrial Research, and J. THONSTAD, Professor of Electrochemistry, Institute for Industrial Electrochemistry, are with the Norwegian University of Science and Technology, 7034 Trondheim, Norway. Manuscript submitted June 14, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B

calorimetric value of 210,560 5 300 cal from Navrotsky and Kleppa.[7] Thus, at 1020 7C (1293 K), aZnO z aAl2O3 5 0.0336

[3]

in equilibrium with the aluminate. On the basis of an existing[8] relation between alumina activity and concentration, it was predicted that an activity of 0.0336 would occur at 4.0 wt pct Al2O3, so that ZnO was expected to be stable at Al2O3 contents below that, and ZnAl2O4 above. The prediction has proved impossible to reconcile with the results subsequently described, and that has led to a reassessment of activities in the binary Al2O3-Na3AlF6 system.[9] Information on the chemical state of the dissolved material in dilute solutions can be obtained from cryoscopic measurements, but the reassessment of the Al2O3-Na3AlF6 system[9] has shown the necessity to allow for the fact that solid cryolite melts incongruently;[10] that has been done here. For more concentrated solutions, the variation o