Melting Behavior and Densities of K 2 B 2 OF 6 Melts Containing KReO 4

: Methods of simultaneous thermal analysis (differential scanning calorimetry, thermo-gravimetry) and an analysis of cooling curves were used to study the melting of K 2 B 2 OF 6 –(0–15 wt. %) KReO 4 melts. The synthesis of K 2 B 2 OF 6 was performed by alloying KF, KBF 4 , and B 2 O 3 components. The liquidus temperature dependence on the content of potassium perrhenate in the K 2 B 2 OF 6 – (0–15 wt. %) KReO 4 melts was determined. It was found that the addition of up to 6 wt. % KReO 4 caused an increase in the melt liquidus temperature to 733 K. Further increases in potassium perrhen-ate did not change the temperature of the primary crystallization (733 ± 5 K) of the K 2 B 2 OF 6 –KReO 4 melt. This fact testiﬁes to the presence of the monotectic reaction. It was found that the relative loss of mass of the K 2 B 2 OF 6 –(0–15 wt. %) KReO 4 melts did not exceed 2.1%. The delamination of the K 2 B 2 OF 6 –KReO 4 melt was revealed according to the values of the primary crystallization temperatures (liquidus temperatures) in different layers of the melt. The density of the K 2 B 2 OF 6 –KReO 4 melts as a function of potassium perrhenate content (0–15 wt. %) was investigated at 628–933 K. The temperature dependence of the K 2 B 2 OF 6 –KReO 4 melts’ densities was recorded. They are presented as linear functions. The curves of the density temperature dependence of the K 2 B 2 OF 6 –KReO 4 melts were used to determine the critical temperatures, i.e., the boundaries of the miscibility gap. The miscibility gap of the K 2 B 2 OF 6 –KReO 4 melts is limited to 1 wt. % and 15 wt. % KReO 4 content.


Introduction
Rhenium is a rare-earth metal that is widely used in heat-resistant superalloys, platinum-rhenium catalysts, and prospective structural materials for rocket and space technology [1][2][3][4][5][6][7][8][9][10][11][12][13].The trend of providing various industries with high-tech products in the next 150 years will lead to the exhaustion of the proven reserves of rhenium in the Earth's crust [14].To date, about 80% of Re is consumed in the production of superalloys, about 15% of Re is used as a catalyst in fuel production, and the remaining 5% of Re is used in alloy form, applied as electrical contacts, electromagnets, heating elements, mass spectrographs, semiconductors, thermocouples, vacuum pipes, etc. [15].The United States is the main consumer of rhenium, which is primarily used for heat-resistant alloy production; US consumption reached ~43,000 kg of Re in 2021, with a total global Re production of 59,000 kg per year.The global interest in Re is expected to grow, especially in the space industry.Rhenium is an important metal for different industrial areas, which is why having a stable rhenium production source has become a vital issue.Thus, Pratt & Whitney-an aerospace manufacturer-analyzed the market and came to the conclusion that their key operation of gas turbine engine design and production may be affected by rhenium shortages.Therefore, in 2014, the company bought 230,000 kg of Re to ensure a stable operation cycle [12].Currently, Re is produced as a by-product of pyrometallurgical molybdenum production.During this process, rhenium compounds are oxidized to volatile rhenium oxide, Re 2 O 7 .This oxide is collected, together with outgoing smoke gases, and extracted in the form of rhenium acid or ammonium perrhenate.Ammonium perrhenate is a raw product for further metallic rhenium production.Metallic rhenium is mainly produced by reduction using gaseous hydrogen at temperatures close to 1273 K [16].In this regard, a significant effort should be dedicated to identifying resource-saving technologies for the recycling of rhenium materials.The most promising secondary sources of rhenium were predicted [1] to be KReO 4 and NH 4 ReO 4 compounds or rhenium metal powder (as a result of its reduction by hydrogen).The method of hydrogen reduction has a number of disadvantages, such as complex instrumentation requirements, the usage of gaseous hydrogen at high temperatures, and the possibility of obtaining rhenium only in a powder form.
Molten salt electrolysis is a promising process for obtaining rhenium.It can enable both the production of metallic rhenium from its compounds and the compaction of metallic rhenium powders, yielding finished or semi-finished products with a density close to the theoretical one [2].The properties of molten salts are the basis of electrochemical technologies for obtaining metals [17,18].It is widely known that, by using molten salts, it is possible to obtain metallic coatings with desired properties, including roughness, mechanical strength, specific surface, density, etc. [19,20].
The fluoroborate melts' liquidus temperature and the densities' temperature dependence are required to develop the electrolytic rhenium-obtaining process.No corresponding data for fluoroborate melts were found.
The liquidus temperature determines the lowest temperature that can be used in electrolysis.Data on the melt densities are required for the technological calculations and equipment design.In addition to its practical significance, the liquidus temperature and density data are also important for the investigation of the KReO 4 and KF-KBF 4 -B 2 O 3 melts' interaction.
Nevertheless, Kataev et al. presented data on the interaction of B 2 O 3 with fluoride KF-AlF 3 and KF-NaF-AlF 3 molten salts [23].It was found that the interaction between individual components of the melts is realized according to the reaction: Oxyfluoroborate, K 2 B 2 OF 6 , can be obtained using reaction (1).The thermodynamic possibility of oxyfluoroborate formation has been verified and experimentally confirmed [24].The reaction that binds various boron complex compounds in the melts is presented as follows: The results in [24] show that the K 2 B 2 OF 6 melt is formed in the KF-KBF 4 -B 2 O 3 molten mixture in the form of oxyfluoroborate complexes according to reaction (2).The melting point of K 2 B 2 OF 6 is 628 K, and that of K 3 B 3 O 3 F 6 is 705 K [25].
Potassium perrhenate (KReO 4 ) is a rhenium chemical compound with a relatively low melting temperature of 828 K and a boiling temperature of 1643 K [26].
The purpose of this work was to obtain the densities' temperature dependence and to determine the liquidus temperature of the prospective KF-KBF 4 -B 2 O 3 -KReO 4 melts.The KF-KBF 4 -B 2 O 3 melt was chosen as a solvent because its composition corresponds stoichiometrically to the low-melting potassium oxyfluoroborate, K 2 B 2 OF 6 .The results obtained are required for determining the basics of rhenium-obtaining technology by the electrolysis of the KF-KBF 4 -B 2 O 3 -KReO 4 melts.
The potassium perrhenate powder was dried at 423 K for 3 h in the air in a glassy carbon container.Potassium tetrafluoroborate was dried at 473 K for 2 h.B 2 O 3 was remelted under vacuum and then used in the powdered form.The prepared substances were used for the melt preparation.
The substances were mixed to prepare a KF (37.28 wt.%)-KBF 4 (40.39 wt.%)-B 2 O 3 (22.33wt.%) mixture.The mixture was placed in the glassy carbon container.The container was placed inside a quartz retort.Then, it was heated to 773 K and kept in the molten state for 4 h.Thus, a stoichiometric melt corresponding to the K 2 B 2 OF 6 composition was synthesized.To confirm the phase composition of the resulting melt, an X-ray phase analysis was carried out using a Rigaku D/MAX-2200VL/PC X-ray diffractometer (Rigaku Corporation, Matsubara-cho, Akishima-shi, Tokyo, Japan).Figure 1 shows the results of the K 2 B 2 OF 6 molten salt X-ray analysis.
The potassium perrhenate powder was dried at 423 K for 3 h in the air in a glassy carbon container.Potassium tetrafluoroborate was dried at 473 K for 2 h.B2O3 was remelted under vacuum and then used in the powdered form.The prepared substances were used for the melt preparation.
The substances were mixed to prepare a KF (37.28 wt.%)-KBF4 (40.39 wt.%)-B2O3 (22.33 wt.%) mixture.The mixture was placed in the glassy carbon container.The container was placed inside a quartz retort.Then, it was heated to 773 K and kept in the molten state for 4 h.Thus, a stoichiometric melt corresponding to the K2B2OF6 composition was synthesized.To confirm the phase composition of the resulting melt, an X-ray phase analysis was carried out using a Rigaku D/MAX-2200VL/PC X-ray diffractometer (Rigaku Corporation, Matsubara-cho, Akishima-shi, Tokyo, Japan).Figure 1 shows the results of the K2B2OF6 molten salt X-ray analysis.
The required content of the potassium perrhenate powder was added to the K2B2OF6 melts to form the desired K2B2OF6-KReO4 melts.The prepared mixture was subjected to a chemical composition analysis, the results of which are presented in Table 1.The required content of the potassium perrhenate powder was added to the K 2 B 2 OF 6 melts to form the desired K 2 B 2 OF 6 -KReO 4 melts.The prepared mixture was subjected to a chemical composition analysis, the results of which are presented in Table 1.The results of the X-ray analysis show that the initial K 2 B 2 OF 6 melt is monophase.The obtained results correspond to the experimental data reported in [24].

Measuring Procedures
Density measurements are generally carried out according to the hydrostatic weighing method.The method involves measuring the change in a spherical platinum weight immersed in the melt [27][28][29].
The experimental cell was a quartz retort plugged with a vacuum rubber stopper, which was connected to a Mettler AT20 electronic balance (Mettler-Toledo GmbH, Greifensee, Switzerland).The platinum weight (diameter of 6.8 × 10 −3 m) was hung on a platinum wire, about 0.6 m long and 2 × 10 −4 m in diameter, and was connected to the electronic balance.
A special lift was used to immerse the platinum weight in and extract it from the melt.All measurements were carried out in an argon atmosphere of 99.999% purity (UralCryoGas LLC, Ekaterinburg, Russia).The spherical platinum weight was successively weighed first in the gaseous atmosphere and then in the molten salts.
The densities of the molten salts were calculated according to Equation (3), where the difference between the weight's mass in the gaseous atmosphere and in the melts is divided by the volume of the weight.
where d is the melt density, kg/m 3 ; m 1 is the mass of the weight in the air, kg; m 2 is the mass of the weight immersed in the melt, kg; and V is the volume of the Pt ball and is equal to 1.6485 × 10 −7 m 3 .The schematic of the measuring cell is presented in Figure 2.
Processes 2023, 11, x FOR PEER REVIEW 4 of 13 The results of the X-ray analysis show that the initial K2B2OF6 melt is monophase.The obtained results correspond to the experimental data reported in [24].

Measuring Procedures
Density measurements are generally carried out according to the hydrostatic weighing method.The method involves measuring the change in a spherical platinum weight immersed in the melt [27][28][29].
The experimental cell was a quartz retort plugged with a vacuum rubber stopper, which was connected to a Mettler AT20 electronic balance (Mettler-Toledo GmbH, Greifensee, Switzerland).The platinum weight (diameter of 6.8 × 10 −3 m) was hung on a platinum wire, about 0.6 m long and 2 × 10 −4 m in diameter, and was connected to the electronic balance.
A special lift was used to immerse the platinum weight in and extract it from the melt.All measurements were carried out in an argon atmosphere of 99.999% purity (UralCryoGas LLC, Ekaterinburg, Russia).The spherical platinum weight was successively weighed first in the gaseous atmosphere and then in the molten salts.
The densities of the molten salts were calculated according to Equation ( 3), where the difference between the weight's mass in the gaseous atmosphere and in the melts is divided by the volume of the weight.
where  is the melt density, kg/m 3 ;  is the mass of the weight in the air, kg;  is the mass of the weight immersed in the melt, kg; and  is the volume of the Pt ball and is equal to 1.6485 × 10 −7 m 3 .The schematic of the measuring cell is presented in Figure 2.  The density investigation of the K 2 B 2 OF 6 -KReO 4 melts was carried out in a temperature range of 929-628 K.The glass-carbon crucible SU-2000 (DONKARB GRAFITE LLC, Chelyabinsk, Russia) was used as a container for the melts.The glass-carbon crucible had a volume of 1.1 × 10 −4 m 3 (dimensions: the basic diameter was 5 × 10 −2 m; the top diameter was 7.3 × 10 −2 m; the height was 6.9 × 10 −2 m).The moving device was used to fix the platinum weight's location with an accuracy of 10 µm along the crucible axis.
The density measurements for each melt composition were repeated at least 3 times.The obtained results of parallel measurements were averaged.
The preliminary experiments elucidated that K 2 B 2 OF 6 -KReO 4 melts are layered systems.The results of the densities measurement can be represented in two positions.The top position is realized when the platinum weight is located about 5 × 10 −3 m above the melts' surfaces.The bottom position is realized when the platinum weight is located about 5 × 10 −3 m higher than the crucible bottom.Changing the platinum weight's position by 1-2 × 10 −3 m lower or higher resulted in densities' temperature dependence being irreproducible when recorded at different temperatures.This fact could be explained by the influence of two non-mixed liquids on the platinum weight.It should be mentioned here that, when using this method, the interfacial forces acting on the Pt wire at the upper-liquid-gas and lower-liquid-upper-liquid interfaces [30] cause a singularity.
The crystallization temperatures and the temperatures of phase transitions were obtained by performing a thermal analysis based on a heat release evaluation.The time dependence of the thermocouple EMF was recorded using an APPA 502 multimeter (APPA Technology Corp., New Taipei City, Taiwan) during both the heating and cooling of the melts.The temperature was automatically measured every second.The average cooling and heating rates were 4.8 and 7.1 K per minute, respectively.The differences in temperatures at the points of phase transitions obtained during heating/cooling cycles did not exceed the measurement error (~5 K).

Results and Discussion
The study of the K 2 B 2 OF 6 -KReO 4 melts was carried out in a temperature range of 307 K-873 K by STA. Figure 3 shows the data from the DSC and thermogravimetric analyses of the K 2 B 2 OF 6 melts.
Processes 2023, 11, x FOR PEER REVIEW 5 of 13 The density investigation of the K2B2OF6-KReO4 melts was carried out in a temperature range of 929-628 K.The glass-carbon crucible SU-2000 (DONKARB GRAFITE LLC, Chelyabinsk, Russia) was used as a container for the melts.The glass-carbon crucible had a volume of 1.1 × 10 −4 m 3 (dimensions: the basic diameter was 5 × 10 −2 m; the top diameter was 7.3 × 10 −2 m; the height was 6.9 × 10 −2 m).The moving device was used to fix the platinum weight's location with an accuracy of 10 µm along the crucible axis.
The density measurements for each melt composition were repeated at least 3 times.The obtained results of parallel measurements were averaged.
The preliminary experiments elucidated that K2B2OF6-KReO4 melts are layered systems.The results of the densities measurement can be represented in two positions.The top position is realized when the platinum weight is located about 5 × 10 −3 m above the melts' surfaces.The bottom position is realized when the platinum weight is located about 5 × 10 −3 m higher than the crucible bottom.Changing the platinum weight's position by 1-2 × 10 −3 m lower or higher resulted in densities' temperature dependence being irreproducible when recorded at different temperatures.This fact could be explained by the influence of two non-mixed liquids on the platinum weight.It should be mentioned here that, when using this method, the interfacial forces acting on the Pt wire at the upperliquid-gas and lower-liquid-upper-liquid interfaces [30] cause a singularity.
The crystallization temperatures and the temperatures of phase transitions were obtained by performing a thermal analysis based on a heat release evaluation.The time dependence of the thermocouple EMF was recorded using an APPA 502 multimeter (APPA Technology Corp., New Taipei City, Taiwan) during both the heating and cooling of the melts.The temperature was automatically measured every second.The average cooling and heating rates were 4.8 and 7.1 K per minute, respectively.The differences in temperatures at the points of phase transitions obtained during heating/cooling cycles did not exceed the measurement error (~5 K).

Results and Discussion
The study of the K2B2OF6-KReO4 melts was carried out in a temperature range of 307 K-873 K by STA. Figure 3 shows the data from the DSC and thermogravimetric analyses of the K2B2OF6 melts.According to Figure 3, there are a number of phase transitions in solid K 2 B 2 OF 6 -KReO 4 while heating to the melting point of K 2 B 2 OF 6 .An exothermic heat release was observed at 403 K.This can be related to the grain recrystallization caused by the compaction of the crystal structure.A significant change in the heat flux was observed at 551.4-552.2K.
The phase transition temperature coincided with the predicted one for the formation of K 10 B 38 O 62 structural groups [31].It should be noted that the samples of the K 2 B 2 OF 6 melts, including those containing KReO 4 , were X-ray amorphous.So, it is difficult to identify a specific phase transition in the solid state.It was found that the melting temperature of the K 2 B 2 OF 6 compound was 628 K.According to the differential scanning calorimetry data, the solidus temperature for the K 2 B 2 OF 6 -(0-15 wt.%) KReO 4 melts was 628 K.
It was found that the addition of KReO 4 up to 6 wt.% to the K 2 B 2 OF 6 melts resulted in the appearance of an exothermic effect at a temperature of about 773 K, which was accompanied by an increase in the mass loss of the melts (Figure 4).This fact is probably related to the crystallization area of the volatile Re 2 O 7 and the more refractory K 3 ReO 5 compound.The presence of rhenium compounds in the Re +7 oxidation state in the K 2 B 2 OF 6 -KReO 4 melts is in agreement with the results reported in [32].Presumably, the following reaction related to the release of rhenium oxide takes place in the melts: Processes 2023, 11, x FOR PEER REVIEW 6 of 13 According to Figure 3, there are a number of phase transitions in solid K2B2OF6-KReO4 while heating to the melting point of K2B2OF6.An exothermic heat release was observed at 403 K.This can be related to the grain recrystallization caused by the compaction of the crystal structure.A significant change in the heat flux was observed at 551.4-552.2K.The phase transition temperature coincided with the predicted one for the formation of K10B38O62 structural groups [31].It should be noted that the samples of the K2B2OF6 melts, including those containing KReO4, were X-ray amorphous.So, it is difficult to identify a specific phase transition in the solid state.It was found that the melting temperature of the K2B2OF6 compound was 628 K.According to the differential scanning calorimetry data, the solidus temperature for the K2B2OF6-(0-15 wt.%) KReO4 melts was 628 K.
It was found that the addition of KReO4 up to 6 wt.% to the K2B2OF6 melts resulted in the appearance of an exothermic effect at a temperature of about 773 K, which was accompanied by an increase in the mass loss of the melts (Figure 4).This fact is probably related to the crystallization area of the volatile Re2O7 and the more refractory K3ReO5 compound.The presence of rhenium compounds in the Re +7 oxidation state in the K2B2OF6-KReO4 melts is in agreement with the results reported in [32].Presumably, the following reaction related to the release of rhenium oxide takes place in the melts: The presence of the K3ReO5 compound in the samples of the K2B2OF6-KReO4 melts was detected by X-ray phase analysis (Figure 5).The data analysis showed that rhenium was present in two phases, namely, KReO4 and K3ReO5, in the solidified melts [33].The K2B2OF6 compound had a glass structure, and it did not have any individual peaks in the diffraction pattern.The presence of any other phases was not determined.The presence of the K 3 ReO 5 compound in the samples of the K 2 B 2 OF 6 -KReO 4 melts was detected by X-ray phase analysis (Figure 5).The data analysis showed that rhenium was present in two phases, namely, KReO 4 and K 3 ReO 5 , in the solidified melts [33].The K 2 B 2 OF 6 compound had a glass structure, and it did not have any individual peaks in the diffraction pattern.The presence of any other phases was not determined.An increase in the content of KReO4 from 2 to 6 wt.% resulted in a decrease in the temperature of the exothermic heat release from 793 to 773 K.We associate this with the fact that the melt composition shifts to the area of the K3ReO5 crystallization in the K2B2OF6-KReO4 phase diagram.
All thermal effects can be systemized.The majority of them are associated with phase transitions in the solid states of K2B2OF6 and K2B2OF6-KReO4, and additional investigations will be necessary in the future to determine their mechanisms (Table 2).An increase in the content of KReO 4 from 2 to 6 wt.% resulted in a decrease in the temperature of the exothermic heat release from 793 to 773 K.We associate this with the fact that the melt composition shifts to the area of the K 3 ReO 5 crystallization in the K 2 B 2 OF 6 -KReO 4 phase diagram.
All thermal effects can be systemized.The majority of them are associated with phase transitions in the solid states of K 2 B 2 OF 6 and K 2 B 2 OF 6 -KReO 4 , and additional investigations will be necessary in the future to determine their mechanisms (Table 2).The analyses of the thermogravimetric dependence indicate that the relative loss of the mass does not exceed 2.05 % for the K 2 B 2 OF 6 melts with the addition of 15 wt.% of KReO 4 at 873 K.The data obtained were used as a source for determining the liquidus temperatures of the K 2 B 2 OF 6 -KReO 4 system.Table 3 shows the liquidus temperatures of the K 2 B 2 OF 6 -KReO 4 systems depending on the KReO 4 content.It is found that an increase in the KReO 4 concentration to 6 wt.% results in the liquidus temperature rising from 628 to 733 K. Increasing the potassium perrhenate concentration above 6 wt.% does not influence the liquidus temperature.The immutability of the liquidus temperature of the K 2 B 2 OF 6 -KReO 4 melts with increasing KReO 4 content indicates the presence of a miscibility gap.Further measurements of the liquidus temperatures of the K 2 B 2 OF 6 -KReO 4 melts at various melt-level points confirmed the existence of the phase separation.The liquidus temperature stabilization at a temperature of 733 K for melt compositions of K 2 B 2 OF 6 -(4-15 wt.%) KReO 4 elucidated that the melt state was within the boundaries of the miscibility gap of the phase diagram.
The experimental data on the liquidus temperature were used to determine the temperature range of the density measurements of the K 2 B 2 OF 6 -KReO 4 melts.The temperature dependence of densities was measured at two level points of the K 2 B 2 OF 6 -KReO 4 melts: 1-immersed by 5 × 10 −3 m from the melt mirror (upper layer-"U"); 2-suspended about 5 × 10 −3 m from the bottom of the container with the melt (conditionally, the lower layer-"L").It was discovered that the temperature dependence of the melt densities could be described by linear equations (Table 2).It was found that the K 2 B 2 OF 6 and K 2 B 2 OF 6 -1 wt.% KReO 4 melts (№. 1 and №. 2 in Table 2) did not delaminate in the entire range of the temperatures studied.
Melts №. 3-№. 10 (Table 3) delaminated.The densities of the upper and lower layers differed significantly.The temperature dependence of the densities of the K 2 B 2 OF 6 -KReO 4 melts are shown in Figure 6.It was found that the addition of up to 1 wt.% KReO4 had a negligible influence on the density homogeneity of the K2B2OF6-KReO4 melts.
Phase separation was observed for the K2B2OF6-KReO4 melts with KReO4 content above 4 wt.%.It could be seen from the experimental data that the densities of the upper and lower phases started to differ, and phases did not mix with up to 15 wt.% KReO4.Such behavior is consistent with the data obtained by measuring the densities of the stratified molten salt systems [22].
The behavior of the temperature dependence of the K2B2OF6-KReO4 (4-15 wt.) melts' densities (compositions with phase separation) is in accordance with the results reported in [34].The densities' temperature dependence can be approximated by linear functions.
The density of the heavier phase of K2B2OF6-KReO4 (4-10 wt.%) decreased more rapidly.This indirectly indicates the lower solubility of the heavy component in the light layer and its better solubility in the heavy layer.The densities of the upper and lower phases approached each other as the temperature and KReO4 content increased.Figure 7 shows the density isotherms of the K2B2OF6-KReO4 melts for the upper and lower layers at T = 820 K.It was found that the addition of up to 1 wt.% KReO 4 had a negligible influence on the density homogeneity of the K 2 B 2 OF 6 -KReO 4 melts.
Phase separation was observed for the K 2 B 2 OF 6 -KReO 4 melts with KReO 4 content above 4 wt.%.It could be seen from the experimental data that the densities of the upper and lower phases started to differ, and phases did not mix with up to 15 wt.% KReO 4 .Such behavior is consistent with the data obtained by measuring the densities of the stratified molten salt systems [22].
The behavior of the temperature dependence of the K 2 B 2 OF 6 -KReO 4 (4-15 wt.) melts' densities (compositions with phase separation) is in accordance with the results reported in [34].The densities' temperature dependence can be approximated by linear functions.
The density of the heavier phase of K 2 B 2 OF 6 -KReO 4 (4-10 wt.%) decreased more rapidly.This indirectly indicates the lower solubility of the heavy component in the light layer and its better solubility in the heavy layer.The densities of the upper and lower phases approached each other as the temperature and KReO 4 content increased.Figure 7 shows the density isotherms of the K 2 B 2 OF 6 -KReO 4 melts for the upper and lower layers at T = 820 K.It was found that the melt density increased as the composition of the K2B2OF6-KReO4 (4-15 wt.%) melts changed.This is in agreement with the fact that a higher rhenium concentration (as a powerful complexing agent) increases the densities of the K2B2OF6-KReO4 melts.The densities of the upper and lower layers were found to vary.This indicates that the K2B2OF6-KReO4 (4-15 wt.%) melts are characterized by different values of critical temperature.The critical temperature is the upper boundary of the miscibility gap.At the same time, the trend changed significantly for the K2B2OF6-15 wt.% KReO4 melts (Figure 7), and it indicates the convergence of phase densities to the critical point at lower temperatures.
Tkachev et al. reported that the value of the critical temperature TCr, i.e., the upper boundary of the miscibility gap, in a molten salt corresponded to the same density of two liquids within the boundaries of the miscibility gap.Thus, the critical temperature can be predicted by solving the system of linear equations (Table 3) for one composition, provided that the densities of the upper and lower layers are equal to: where  is the temperature, K;  is the upper layer's density, g/cm 3 ;  is the lower layer's density, g/cm 3 ;  ,  ,  ,  are the linear equation coefficients of the density's temperature dependence for the upper and lower layers.We calculated the critical temperatures for the K2B2OF6-KReO4 (4-15 wt.%) molten systems.For that purpose, the system of equations of the densities' temperature dependence (data of Table 2) for the upper and lower layers was constructed.The solution to the system of equations was obtained under the condition of the equality of the upper and lower layers at the critical temperature Tcr.
The obtained values of critical temperature, as well as the liquidus and monotectic ones, can be summarized.Figure 8 shows the liquidus, monotectic, and critical temperatures of the K2B2OF6-KReO4 (0-15 wt.%) melts.It was found that the melt density increased as the composition of the K 2 B 2 OF 6 -KReO 4 (4-15 wt.%) melts changed.This is in agreement with the fact that a higher rhenium concentration (as a powerful complexing agent) increases the densities of the K 2 B 2 OF 6 -KReO 4 melts.The densities of the upper and lower layers were found to vary.This indicates that the K 2 B 2 OF 6 -KReO 4 (4-15 wt.%) melts are characterized by different values of critical temperature.The critical temperature is the upper boundary of the miscibility gap.At the same time, the trend changed significantly for the K 2 B 2 OF 6 -15 wt.% KReO 4 melts (Figure 7), and it indicates the convergence of phase densities to the critical point at lower temperatures.
Tkachev et al. reported that the value of the critical temperature T Cr , i.e., the upper boundary of the miscibility gap, in a molten salt corresponded to the same density of two liquids within the boundaries of the miscibility gap.Thus, the critical temperature can be predicted by solving the system of linear equations (Table 3) for one composition, provided that the densities of the upper and lower layers are equal to: where T is the temperature, K; ρ u is the upper layer's density, g/cm 3 ; ρ l is the lower layer's density, g/cm 3 ; a u , b l , a u , b l are the linear equation coefficients of the density's temperature dependence for the upper and lower layers.We calculated the critical temperatures for the K 2 B 2 OF 6 -KReO 4 (4-15 wt.%) molten systems.For that purpose, the system of equations of the densities' temperature dependence (data of Table 2) for the upper and lower layers was constructed.The solution to the system of equations was obtained under the condition of the equality of the upper and lower layers at the critical temperature T cr .
The obtained values of critical temperature, as well as the liquidus and monotectic ones, can be summarized.Figure 8 shows the liquidus, monotectic, and critical temperatures of the K 2 B 2 OF 6 -KReO 4 (0-15 wt.%) melts.
The results of the critical temperature calculation indicate that the peak of the miscibility gap in the K 2 B 2 OF 6 -KReO 4 melts corresponds to a temperature of about 1416 K at a potassium perrhenate concentration of 6 wt.%.The critical temperature dependence on the concentration of potassium perrhenate is extreme.An increase in the potassium perrhenate content up to 15 wt.% in the K 2 B 2 OF 6 -KReO 4 melts causes the critical temperature to decrease down to 775 K. Thus, the miscibility gap of the K 2 B 2 OF 6 -KReO 4 melts is within 1 wt.% and 15 wt.% of the KReO 4 concentration.The results of the critical temperature calculation indicate that the peak of the miscibility gap in the K2B2OF6-KReO4 melts corresponds to a temperature of about 1416 K at a potassium perrhenate concentration of 6 wt.%.The critical temperature dependence on the concentration of potassium perrhenate is extreme.An increase in the potassium perrhenate content up to 15 wt.% in the K2B2OF6-KReO4 melts causes the critical temperature to decrease down to 775 K. Thus, the miscibility gap of the K2B2OF6-KReO4 melts is within 1 wt.% and 15 wt.% of the KReO4 concentration.

Conclusions
The behavior of the K2B2OF6-KReO4 (0-15 wt.%) melts was studied by synchronous thermal analyses (differential scanning calorimetry and thermogravimetry) and the analysis of cooling curves.The liquidus and monotectic temperatures of the K2B2OF6-KReO4 melts (0-15 wt.%) were determined.It was found that the addition of up to 6 wt.% of the KReO4 compound caused the liquidus temperature to increase up to 733 K.A further increase in the potassium perrhenate concentration did not influence the primary crystallization temperature (733 ±5 K) of the K2B2OF6-KReO4 melts.This fact confirms the occurrence of the monotectic reaction.The primary crystallization temperature values of various layers of the melt reveal the presence of phase separations in the K2B2OF6-KReO4 melts.It was found that the relative mass loss of the K2B2OF6-(0-15 wt.%) KReO4 melts did not exceed 2.05%.
The densities of the K2B2OF6-KReO4 melts were studied by varying the potassium perrhenate concentration from 0 to 15 wt.% at T = 628-933 K.It was observed that the addition of 1 wt.% of the KReO4 melt to K2B2OF6 caused a slight density decrease.The densities and phase separations of the K2B2OF6-KReO4 melts increased in the KReO4 concentration range of 4-15 wt.%.This is in agreement with the conclusions on the melt phase separations obtained after the analysis of the cooling curves.
The upper boundary of the miscibility gap was determined by calculating the melts' critical temperatures.The dependence of the critical temperature on the concentration of potassium perrhenate in the K2B2OF6-KReO4 melts has an extreme character, with a maximum at about T = 416 K and 6 wt.% of the KReO4 component.

Figure 6 .
Figure 6.Experimental temperature dependence of the densities of the K 2 B 2 OF 6 -KReO 4 melts.

Figure 7 .
Figure 7. Density of the K2B2OF6-KReO4 melts depending on the content of potassium perrhenate at T = 820 K.

Figure 7 .
Figure 7. Density of the K 2 B 2 OF 6 -KReO 4 melts depending on the content of potassium perrhenate at T = 820 K.

Table 1 .
Contents of elements in the obtained deposits.

Table 1 .
Contents of elements in the obtained deposits.

Table 3 .
Coefficients of the temperature dependence of the densities of the K 2 B 2 OF 6 -KReO 4 melts.