Mechanism of interaction between zirconium dioxide and fluoride melts

Abstract

In this work, the mechanism of interaction between zirconium dioxide and fluoride melts was studied. Dissolution rates and maximum permissible oxide contents in the KF-AlF3 and NaF-AlF3 melts were determined under natural convection conditions at a temperature of 750 °C. The results of solubility measurement show that the limiting content of ZrO2 in the KF-AlF3 melt ([KF]/[AlF3] = 1.3) is 0.69 ± 0.10 mol. %, and in the NaF-AlF3 melt ([NaF]/[AlF3] = 1.3) at 800 °C it is 0.54 ± 0.04 mol. %. The calculated dissolution rate of zirconium oxide under these conditions in melts based on potassium fluoride and sodium fluoride is 0.022 mol/min and 0.017 mol/min, respectively. The influence of the cationic composition of the medium and the concentrations of additives on the liquidus temperatures of the melts was studied. It has been established that the increase in liquidus and solidus temperatures is facilitated by the addition of zirconium and aluminium oxides, as well as by the replacement of potassium fluoride with sodium fluoride while maintaining the molar ratio of the replaced cations. Analysis of thermal effects by differential scanning calorimetry (DSC) shows that, regardless of the molar ratio of the components, the onset of the thermal effect in melts based on potassium fluoride occurs at a temperature of 542 °C and in the systems with sodium fluoride additives at a temperature of 597 °C. Presumably, this dependence is associated with the melting of the KAlF4 and K2NaAl3F12 phases formed in the melts based on KF-AlF3 and KF-NaF-AlF3, respectively. It has been established that the melting of the KF-AlF3 mixture is accompanied with a mass loss of up to 1.3 wt. %, which is associated with partial evaporation of KAlF4.