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Advances in Molten Salts

ISSN:
1-56700-142-4 (Print)

The influence of CaF2 and AIF3 upon mechanism and kinetics of the anodic reaction in cryolite-alumina melts

Adolf Kisza
Faculty of Chemistry, University of Wroclaw, 50383 Wroclaw, Poland.

Jomar Thonstad
Department of Electrochemistry, NTNU, 7034 Trondheim, Norway

Jan Hives
Department of Inorganic Technology, SUT, 81237 Bratislava, Slovakia

Abstract

The anodic reaction on spectral pure graphite (SPG) was studied in the potential range from 1.2 V to 2.1 V versus an aluminium reference electrode in cryolite-alumina melts containing also CaF2 and AIF3 at 1000°C by means of Electrochemical Impedance Spectroscopy (EIS). The experimentally recorded impedance spectra were interpreted in terms of an equivalent circuit approach and the kinetic theory of Bai and Conway. Using the values of the effective rate constants of the two charge transfer steps and the values of the Tafel constants, the role of the two additives: CaF2 and AlF3 is clarified. The addition of CaF2 and AIF3 to cryolite -alumina saturated melt, increases the overpotential of the anodic reaction by decreasing the value of the effective rate constant of the first step, and this decrease is much greater in case of AIF3. In the industrial type melt, the influence of the CaF2 seems to be prevailing above the influence of AIF3.