Solubility of Cr Species in Low Temperature Cryolite-Based Electrolyte at 900°C

Abstract

Oxidation and reduction of Cr(III) species originating from dissolution of Cr2O3 in cryolite-based melts at 900°C was studied. The molar ratio n(NaF)/n(AlF3) was 1.4, which allowed us to perform electrochemical measurements at 900°C. Cr(II) species were prepared in situ by the reduction of Cr2O3 with chromium powder. Saturation was achieved after several hours, the concentration of Cr(II) being (237 ± 11) wt ppm. Cr(III) can be oxidized to Cr(VI) both chemically by oxygen present in the air and electrochemically when using an inert anode. When a carbon anode was used, the formation of Cr(VI) species was not observed. The solubility of Cr(VI) species in the above-mentioned melt was found to be (1395 ± 22) wt ppm at a total amount of Cr species in the melt of (1574 ± 16) wt ppm.