M. Chrenková

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.

Structural and thermodynamic aspects of niobium deposition in the system LiF-KF-K2NbF7

Abstract The complex physico-chemical analysis of the system LiF-KF-K2NbF7, based on the phase diagram, density, surface tension, and viscosity measurements, was performed. The results proved the presence of the congruently melting compound K3NbF8 even in the ternary melts. The calculated co-ordinates of the two ternary eutectic points are: e1: 22.3 mole % LiF, 9.4 mole % KF; 68.3 mole % K2NbF7; te = 649 °C; e2: 45.5 mole % LiF, 51.7 mole % KF; 2.8 mole % K2NbF7; te = 486 °C. The probable inaccuracy in the calculated ternary phase diagram is 4.7 °C. The degree of thermal dissociation of the additive compound K3NbF8 α0 = 0.55 at 1100 K, calculated from the density measurement agrees well with the value determined from the analysis of the phase diagram (α0 = 0.44) as well as from the viscosity measurement (α0 = 0.45 at 1100 K) and refers to the pronounced thermal dissociation of this compound at melting. The formation of K3NbF8 was confirmed by all of the followed physico-chemical parameters and it is the general feature of this system. The deposition of Nb from the LiF-KF-K2NbF7 melts takes place via the reversible, diffusion-controlled two-step reduction mechanism [ NbF 7 ] 2− + e − = [ NbF 6 ] 2− + F − [ NbF 6 ] 2− + 4e − = Nb (0) + 6 F − In the presence of oxygen in the melt the formation of the [NbOF5]2− or/and [NbO2F]− oxyfluorocomplexes takes place, depending on the concentration of O2− anions. Good experimental conditions for niobium deposition process can be expected when nO/nNb(V) 1 inhomogeneous niobium deposits containing niobium oxide solid solution, even a non-metal deposit could be obtained. The diffusion coefficients of electroactive species were calculated from the results of electrochemical measurements.

Thermodynamic and structural aspects of electrochemical deposition of metals and binary compounds in molten salts

Abstract The influence of the ionic structure of electrolytes used in the electrodeposition of molybdenum, titanium and aluminium on the mechanism and kinetics of metal deposition is discussed. Using electrochemical methods of study and a complex thermodynamic and physico-chemical analysis it was determined that in all the electrolytes investigated the electro-deposition process is significantly facilitated by the formation of complex anions with lower symmetry of the co-ordination sphere. In the case of molybdenum deposition, complex heteropolyanions are probably created in the melt by the addition of B 2 O 3 or SiO 2 to the K 2 MoO 4 -based electrolytes. The electrodeposition of titanium from the K 2 TiF 6 -based electrolytes is enhanced by the formation of the less stable TiF 7 3− , resp. TiF 6 Cl 3− anions. In the electrolysis of aluminium from cryolite-alumina melts the creation of oxyfluoroaluminate anions facilitates the electrodeposition of aluminium.

Viscosity of the System KF–K2NbF7–Nb2O5

Abstract The viscosity of melts of the system KF–K2NbF7–Nb2O5 has been measured using computerized torsion pendulum method. An equation describing the dependency of the viscosity in logarithm values on composition at 1173 K was calculated by multiple linear regression analysis. The sum of viscosities of pure components in logarithm values multiplied by their mole fraction was used as an “additive” behaviour. In the concept of modified Redlich–Kisters type equation binary interactions of the particular components were found.

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 .

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.