Solubility of Cr₂O₃ in Molten Fluorides with Different ZrF₄ Contents and Fluoroacidities

Abstract

This work focused on the stability of the protective Cr 2 O 3 layer formed on alloys in molten fluorides with different fluoroacidities and ZrF 4 contents. Results showed that fluoroacidity only slightly affected Cr 2 O 3 due to its stability in basic FLiNaK and neutral FLiBe melts. When more than 1.1 mol% ZrF 4 was added to FLiNaK, the new structure [ZrF 7 ] 3− formed by the interaction of ZrF 4 and F − could be detected by Raman Spectroscopy and X-ray diffraction, and Cr 2 O 3 solubility in FLiNaK-ZrF 4 increased 19 times. In the case of FLiBe salt, the structure of [ZrF 6 ] 2− could be detected when 5 mol% ZrF 4 was added, and Cr 2 O 3 dissolution increased from 183 ppm to 320 ppm. Further structural changes from [ZrF 6 ] 2− to [ZrF 8 ] 4− occurred after adding free F − (LiF) to FLiBe-ZrF 4 (5 mol%), and solubility of Cr 2 O 3 doubled. Hence, the molar ratio of ZrF 4 additive to free F − greatly affected Cr 2 O 3 dissolution, whose mechanism can be ascribed to Cr dissolving in the form of soluble CrF 3 and the oxide ions O 2− combining with ZrF 4 and free F − to form ZrO x F y 4-2x−y complex.

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.…”

“…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. Therefore, it is necessary to nd suitable additives to increase Cr 2 O 3 solubility in molten uorides.…”

Recovery of FLiBe from ThF4–FLiBe salt using precipitation–distillation coupled method

“…LiF–BeF 2 (66.7–33.3 mol%) eutectics were provided by the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. 10 Since LiF–ThF 4 was more easily dissolved in FLiBe, the molten salt of LiF–ThF 4 was prepared first. 11 About 2.8 g of LiF was mixed with 10.2 g of ThF 4 in a nickel crucible with a diameter of 2.5 cm.…”

Inhibition effect of ZrF4 on UO2 precipitation in the LiF–BeF2 molten salt

“…2, which was described in detail in our previous studies. 7,8,16,17,20,21,24 The whole experiment was conducted in a glove box protected by dry argon atmosphere (99.999%), and the oxygen and water contents in the glove box were both strictly controlled below 0.5 ppm.…”

“…ZrF 4 can also increase the solubility of oxides because of its strong affinity toward oxygen ions. Song et al21 have studied the dissolution behavior of chromium sesquioxide (Cr 2 O 3 ) in various molten uorides and found that the ZrF 4 additive remarkably increases the solubility of Cr 2 O 3 by 19 and 2 times in FLiNaK and FLiBe molten salt, respectively, most likely yielding the dissolution product of [ZrO x F y ] 4À2xÀy complex species. Peng et al…”