Electrochemistry of Cr(III) in the Molten System NaF–AlF₃–(Al₂O₃) at 900°C

Abstract

The diffusion coefficient of Cr(III) determined by chronopotentiometry in the molten system NaF-AlF 3 -CrF 3 was found to be D(Cr(III)) = (5.28 ± 0.14) × 10 −5 cm 2 s −1 . The molar ratio CR = n(NaF)/n(AlF 3 ) was 1.4, which allowed us to perform measurements at 900 • C. When oxygen anions were present in the melt, chromium was deposited simultaneously with aluminium at the cathode, and the apparent diffusion coefficient related to Cr(III) was too high by two orders of magnitude. This may be explained by the formation of oxofluoro complexes of chromium and aluminium. The solubility of Cr 2 O 3 was studied in the above-mentioned melt at 900 • C by two methods: chemical analysis of solidified samples and by chronopotentiometry. It was found that saturation was achieved after several hours, and the concentration of chromium was (177 ± 6) ppm, corresponding to (258 ± 9) ppm Cr 2 O 3 .

Electrolytic reduction of CrF3 and Cr2O3 in molten fluoride salt

“…However, with respect to the FLiNaK-Cr 2 O 3 system, the solubility of Cr 2 O 3 in FLiNaK is a key factor affecting chromium electroextraction. Our previous investigation and related reports [23][24][25] pointed out that the Cr 2 O 3 solubility in molten uorides was relatively low. For example, the solubility of Cr 2 O 3 in molten FLiNaK and FLiBe at 600 °C was as low as 122 ppm and 183 ppm, respectively, 23 which makes the electrolysis difficult and challenging.…”

Measurement of the changes of carbonaceous materials caused by intercalation of sodium deposited from molten salts electrolyte

“…Prior to use, the chemicals were heated for 3 h at 600 °C. Aluminium fluoride was prepared by sublimation of technical AlF 3 in a platinum crucible (Danielik et al, 2012). The Rapoport test is described in Rapoport and Samoilenko (1957).…”

Electrochemical and Raman Spectroscopic Investigations on the Speciation and Behavior of Chromium Ions in Fluoride Doped Molten LiCl-KCl

“…For the reversible Cr(III)/Cr(II) couple, the apparent standard potential can be calculated by Equation 8proposed by Matsuda and Ayabe. D Cr(II) [8] Where E eq Cr(II)/Cr(0) is the equilibrium potential, E 0 * Cr(II)/Cr(0) and E 0 * Cr(III)/Cr(II) denote the apparent standard potentials of Cr(II)/Cr(0) and Cr(III)/Cr(II), X CrF 2 is the mole fraction of Cr in the melt, E 1/2 is the half wave potential, R, T, n and F are the same as those described in Equation 1. In order to eliminate the influence of AgCl concentration on the apparent standard potentials, it is better to convert the E 0 * values in reference to the Cl 2 /Cl − couple.…”

“…7 The presence of oxide ions caused the formation of chromium oxofluoro complex at 900°C in the NaF-AlF 3 -CrF 3 system, resulting in a dramatic increase of the diffusion coefficient of Cr(III). 8 Besides the molten fluoride systems, the electrochemistry of chromium in chloride melts has also been the focus of several studies. Reduction from CrCl 3 to Cr(0) was characterized to be a twostep process mediated by Cr(II) in LiCl-KCl, and the deposition of metallic chromium could involve adsorbed Cr(II) species and initial electrocrystallization.…”