Vladimír Daněk

Density and viscosity of the (LiFNaFKF)eutKBD4B2O3 melts

Abstract The density and viscosity of the melts of the system (LiFNaFKF)eutKBF4B2O3 have been measured. On the basis of the obtained data the values of the molar volumes, excess molar volumes, and excess viscosity were calculated to obtain information on the interaction of components and the possible ongoing chemical reactions. The composition of the melt, after cooling, was identified using X-ray diffraction and IR spectroscopy analysis. It was found that in the binary system FLINAKB2O3 the components react under the formation of potassium tetraborate, K2B4O7, and volatile boron trifluoride, BF3, or potassium tetrafluoroborate, KBF4. However, the last compound was not observed in the IR spectra. In the ternary system FLINAKKBF4B2O3 the components react under the formation of two ternary compounds K2B2OF6 and K3B3O3F6. The former compound is dominant at the molar ratio nB/nO ≥ 2, while the second one is present at the molar ratio nB/nO ≈ 1. Due to the presence of the highly polarizing Li+ cation, the BF4− anion is not stable in the melts of the FLINAKKBF4B2O3 system and decomposes partially under the formation of volatile BF3.

CALPHAD: Phase diagram of the system LiFNaFK2NbF7

Abstract The phase diagram of the ternary system LiFNaFK 2 NbF 7 , was determined using the thermal analysis method. Subsequent coupled analysis of the thermodynamic and phase diagram data were carried out to calculate the thermodynamically consistent phase diagrams. All of the three binary subsystems of our studied ternary system are simple eutectic ones. Because of the fact that all of them were already investigated and measured, this work deals with recalculation of the binary subsystems and investigation of the ternary system LiFNaFK 2 NbF 7 . The calculated coordinates of the ternary eutectic point is: 16 mole% LiF, 31 mole% NaF, 53 mole% K 2 NbF 2 ; t = 585 °C. The probable inaccuracy in the calculated ternary phase diagram is 4.3 °C.

Density and surface tension of the systems CaO-FeO-Fe2O3-MgO, CaO-FeO-Fe2O3-ZnO and CaO-Fe2O3-Cu2O

The density and surface tension of melts of the systems CaO-FeO-Fe2O3 MgO at the temperature 1623 K, CaO-FeO-Fe2O3-ZnO at 1573 K, and CaO-Fe2O3-Cu2O at 1573 K were determined using the maximum bubble pressure method. The molar volume, the excess molar volume, and the excess surface tension were calculated on the basis of the obtained data. From these properties information on the interactions of components and possible chemical reactions between them was obtained. Due to the absence of silica and the low concentration of other network-forming oxides, only isolated FeO45− tetrahedra and the CaO·FeO ionic pairs are formed in these basic melts, the donor of the oxygen atoms being either CaO, FeO, or both CaO+FeO oxides. Even the observed ternary interactions may be attributed to the formation of the anions FeO45− only.

Density of the system LiF-NaF-K2NbF7

Abstract The density of the melts in the system LiF–NaF–K2NbF7 has been measured using an Archimedean method. Based on the measured density values, the molar volumes, partial molar volumes and excess molar volumes of the melts were calculated. The measurement of density in the investigated system has shown that a significant ternary interaction exists in the melts, which can be ascribed to the formation of different complex anions.

The dissolution of FeO and FeF3 in cryolite

Abstract Theoretical derivation of relations valid for cryoscopic measurements in solvents with dystectic mode of melting (solvents, which undergo at melting a more or less extended thermal dissociation), when also chemical reaction between solvent and solute takes place, is presented. When one of the products of this chemical reaction is identical with the product of thermal dissociation of the solvent, arise the problem of defining the number of “foreign” substances, originating in the reaction. The presented approach shows that the partially dissociated solvent cannot distinguish as foreign substance between the own dissociation product and the originating one and made it easier to determine the nature of the probable chemical reaction. The presented approach is applied in the systems Na3AlF6–FeO and Na3AlF6–FeF3, which are of great importance in the fundamentals of aluminium electrolysis. In the system Na3AlF6–FeO for the dependence of the temperature of primary crystallization of cryolite on x(Na3AlF6) the equation T pc/K = 1014.4 + 267.86x(Na3AlF6) was obtained, which yields for the Stortenberker’s correction factor the value kSt = 2.09. This means that dissolving FeO in cryolite two new particles are formed. This can be interpreted by the chemical reaction 2Na3AlF6 + FeO ⇔ Na2FeF4 + Na2Al2OF6 + 2NaF. In the system Na3AlF6–FeF3 for the dependence of the temperature of primary crystallization of cryolite on x(Na3AlF6) the equation T pc/K = 1547.2 − 659.88x(Na3AlF6) + 394.99x2 (Na3AlF6) was obtained. For the Stortenbeker’s correction factor the value kSt = 1.02 was then calculated, which means that one new particle is created when FeF3 is dissolved in cryolite. The most probable reaction is 1.5Na3AlF6 + FeF3 ⇔ Na3FeF6 + 1.5NaAlF4.

Structure of the melts of the systems MF-M2SO4, where M is Li, Na, K

Abstract The densities of the melts in the systems MF-M 2 SO 4 (where M is Li, Na, K) were measured using the method of hydrostatic weighing. From the volume properties point of view the investigated systems exhibit almost ideal behaviour which indicates that the congruently melting compounds Na 3 FSO 4 and K 3 FSO 4 , which are formed in the systems NaF-Na 2 SO 4 and KF-K 2 SO 4 , respectively, exhibit a low thermal stability and at melting they undergo a considerable thermal dissociation. The degree of the thermal dissociation was calculated on the basis of both the thermodynamic analysis of the phase diagrams and the volume properties of the investigated systems. It was found that the degree of thermal dissociation of Na 3 FSO 4 at the melting point attains the value α 0 = 0.71 while that of K 3 FSO 4 is α 0 = 0.78. The dissociation enthalpies calculated on the basis of the density values are Δ dis H (Na 3 FSO 4 ) = 40.2 kJ mol −1 and Δ dis H (K 3 FSO 4 ) = 24.1 kJ mol −1 .

Phase Equilibria in the Molten System KF-K2NbF7-Nb2O5

Summary Phase equilibria in the molten system KF-K2NbF7-Nb2O5 were investigated up to 20mol% of Nb2O5 by thermal analysis and differential scanning calorimetry. In the binary and ternary systems oxofluoroniobate compounds are formed. They were evidenced by comparison of experimetal X-ray diffraction patterns with those in the international X-ray diffraction database (PDF-2). In the experimental compostion range the phase diagram of the molten KF-K2NbF7-Nb2O5 system consists of five crystallisation fields corresponding to KF, K3NbF8, K2NbF7, K2NbO3F and K3NbOF6.

CALPHAD: Phase diagram of the system KFK2MoO4SiO2

Abstract The phase diagrams of the binary systems KFK 2 MoO 4 , KFSiO 2 , and K 2 MoO 4 SiO 2 , as well as that of the ternary system KFK 2 MoO 4 SiO 2 in the range up to 50 mole % SiO 2 , were measured using the thermal analysis method. The thermodynamically consistent phase diagrams were calculated using the coupled analysis of the thermodynamic and phase diagram data. In the system KFK 2 MoO 4 the intermediate compound K 3 FMoO 4 , melting congruently at 751 °C, is formed. This compound divides this system into two simple eutectic ones. The coordinates of the individual eutectic points are: E 1 : 30.5 mole % K 2 MoO 4 , 720.4 °C, and E 2 : 58.8 mole % K 2 MoO 4 , 744.9 °C. In the binary system KFSiO 2 the liquidus curve of KF shows an inflex point, characteristic for reciprocal systems with chemical reaction taking place between components. Similar course of the liquidus curve of K 2 MoO 4 was found in the binary system K 2 MoO 4 SiO 2 , indicating the presence of the chemical reaction between components as well. The strong positive deviation from ideal behavior of the ternary system KFK 2 MoO 4 SiO 2 was ascribed to the possible formation of heteropolyanions [SiMo 12 O 40 ] 4− in the melt. In the investigated concentration range of the ternary system no eutectic point has been found. It lies most probably beyond the investigated part of the system. The standard deviation of approximation of the calculated ternary phase diagram is ± 5.9 °C, which is approximately on the same level of magnitude as the estimated experimental error of ± 4 °C.

Phase diagram of the system KFKBF4K2TiF6

Abstract The phase diagrams of the binary system KBF 4 K 2 TiF 6 and of the ternary system KFKBF 4 K 2 TiF 6 were determined using the thermal analysis method. The subsequent coupled analysis of the thermodynamic and phase diagram data was performed to obtain a thermodynamically consistent phase diagram. In the system KFKBF 4 K 2 TiF 6 , the intermediate compound K 3 TiF 7 is formed. The K 3 TiF 7 KBF 4 joint divides the ternary system into two simple eutectic ones. The calculated coordinates of the two ternary eutectic points are: e 1 : 26 mol% KF, 69 mol% KBF 4 , 5 mol% K 2 TiF 6 , t e 1 = 448°C; and e 2 : 4 mol% KF, 69 mol% KBF 4 , 27 mol% K 2 TiF 6 , t e 2 = 440°C. The system KBF 4 K 2 TiF 6 is a simple eutectic one with the calculated coordinates of the eutectic point being 28 mol% K 2 TiF 6 and 448°C. The probable inaccuracy in the calculated ternary phase diagram is 11.5°C.

Direct Methods of Investigation

This chapter discusses the proposed direct methods of investigation for the study of molten salt structures. The major direct methods used are X-ray and neutron diffraction analyses, infrared and Raman spectroscopy, NMR (nuclear magnetic resonance) measurement, and XAFS (X-ray Absorption Fine Structure) measurement in melts, are developed. However, the most frequently used direct methods discussed are X-ray and XAFS measurements, Raman spectroscopy, and NMR measurements. X-ray diffraction and XAFS measurements are powerful techniques to investigate the local atomic structure of solid and liquid phases and are successfully applied to many types of materials, such as solutions, catalysts, and amorphous solids. However, they encounter common difficulties due to high temperature, cost, and their specific properties in the experimental work. The results of Raman spectroscopy are reported for a number of inorganic melts as it provides specific information on the structure of molecules, on the chemical environment of atoms and their oxidation state. NMR examines directly the properties and behavior of a specific element. The chapter also explains Raman spectroscopy as it has proved to be a very useful tool in the study of molten salts structure.