Melting Behavior and Densities of K2B2OF6 Melts Containing KReO4

Abstract

Methods of simultaneous thermal analysis (differential scanning calorimetry, thermogravimetry) and an analysis of cooling curves were used to study the melting of K2B2OF6–(0–15 wt. %) KReO4 melts. The synthesis of K2B2OF6 was performed by alloying KF, KBF4, and B2O3 components. The liquidus temperature dependence on the content of potassium perrhenate in the K2B2OF6–(0–15 wt. %) KReO4 melts was determined. It was found that the addition of up to 6 wt. % KReO4 caused an increase in the melt liquidus temperature to 733 K. Further increases in potassium perrhenate did not change the temperature of the primary crystallization (733 ± 5 K) of the K2B2OF6–KReO4 melt. This fact testifies to the presence of the monotectic reaction. It was found that the relative loss of mass of the K2B2OF6–(0–15 wt. %) KReO4 melts did not exceed 2.1%. The delamination of the K2B2OF6–KReO4 melt was revealed according to the values of the primary crystallization temperatures (liquidus temperatures) in different layers of the melt. The density of the K2B2OF6–KReO4 melts as a function of potassium perrhenate content (0–15 wt. %) was investigated at 628–933 K. The temperature dependence of the K2B2OF6–KReO4 melts’ densities was recorded. They are presented as linear functions. The curves of the density temperature dependence of the K2B2OF6–KReO4 melts were used to determine the critical temperatures, i.e., the boundaries of the miscibility gap. The miscibility gap of the K2B2OF6–KReO4 melts is limited to 1 wt. % and 15 wt. % KReO4 content.