Solubility and dissolution behavior of ZrO2 in KF–AlF3 molten salts

Abstract The fluorine salt system is a potential electrolyte for the preparation of metals and alloys from metal oxides via molten salt electrolysis. The solubility of ZrO2 in KF–AlF3–Al2O3 molten salts (KF/AlF3 molar ratio = 2.1–2.8, Al2O3 content = 0–9 wt.%) was measured using the isothermal saturation method in this study. The influence of temperature and electrolyte composition on ZrO2 solubility was analyzed. The dissolution mechanism of ZrO2 was then determined based on the analysis of X–ray diffraction and Raman spectroscopy results. The experimental results show that the solubility of ZrO2 in the 2.4KF–AlF3 molten salt at 1233 K could reach up to 6.62 wt.%. Besides, the solubility of ZrO2 increase with the increase of temperature, the decrease of KF/AlF3 molar ratio and Al2O3 content. X-ray diffraction and Raman spectroscopy analyses show that the dissolved ZrO2 in KF–AlF3 molten salts existed in the form of [Zr2O2F6]2- and [ZrF6]2- complex ions under melting and cooling conditions, respectively. And the dissolution process of ZrO2 in KF–AlF3 molten salts can be described using a mass transfer control model.