Reaction Mechanism of Zirconium Oxide with Fluoride Melts

The mechanism of reactions of zirconium oxide with the melts based on alkaline metal fluorides is studied. The solubilities and dissolution rates of ZrO₂ in KF–AlF₃ and NaF–AlF₃ melts under natural convection conditions are measured. According to the results obtained, the limiting content of ZrO₂ in the KF–AlF₃ ([KF]/[AlF₃] = 1.3) melt at 750°C is 0.69 ± 0.10 mol % and that in the NaF–AlF₃ ([NaF]/[AlF₃] = 1.3) melt at 800°C is 0.54 ± 0.04 mol %. The dissolution rate of weighed samples of ZrO₂ in amounts of 0.5, 1.0, and 1.5 wt % in the KF–AlF₃ melt at 750°C and in the NaF–AlF₃ melt at 800°C is 0.022 and 0.017 mol/min, respectively. The liquidus temperatures of the melts under study are measured at different contents of ZrO₂ and Al₂O₃, molar ratios, and cationic compositions of the mixture. The addition of both zirconium and aluminum oxides and the partial replacement of KF by NaF with the retention of the ([KF] + [NaF])/[AlF₃] molar ratio are found to result in an increase in the liquidus temperatures of the melts under study. The study of the thermal effects of the melts by differential scanning calorimetry (DSC) shows that regardless of the [KF]/[AlF₃] ratio the thermal effect onset in the KF–AlF₃ melts occurs at 542°C and that in the KF–NaF–AlF₃ system takes place at 597°C, which is related to the melting of the KAlF₄ and K₂NaAl₃F₁₂ phases, respectively. An insignificant (1.3%) mass loss is observed on heating the KF–AlF₃ mixture to 900°C, which is assigned, according to the mass spectroscopy data, to the partial evaporation of KAlF₄. According to the X-ray diffraction (XRD) data on the molten oxide–fluoride mixtures under study, the reaction of zirconium oxide with the fluoride melts affords fluorozirconates of the corresponding alkaline metals and more complicated complex compounds.