František Šimko

Determination of the enthalpy of fusion of Na3FSO4

The areas of the fusion and crystallization peaks of Na3FeF6 and of four calibration substances (KCl, NaCl, Na2SO4, and K2SO4) were measured using the DSC mode of a high-temperature calorimeter. Using the measured quantities and known values of the enthalpy of fusion of the calibration substances, the enthalpy of fusion of Na3FeF6 was determined. Its value at the temperature of fusion 1224 K was 70 ± 4 kJ mol−1.

Density of the systems (NaF/AlF3)—AlPO4 and (NaF/AlF3)—NaVO3

The densities of two systems of molten mixtures (NaF/AlF3)—NaVO3 and (NaF/AlF3)—AlPO4 were measured using an Archimedean method. Each molten mixture contained varying amounts of “impurity” constituents. The measurements were performed at various NaF/AlF3 mole ratios (CR, cryolite ratio), equal to 3, 2.5, 2, and 1.5.

Electrical conductivity of systems based on Na3AlF6-SiO2 melt

The electrical conductivity of molten binary and ternary mixtures based on the NaF-AlF3-SiO2 system was investigated by means of a tube-cell (composed of pyrolytic boron nitride) with stationary electrodes. An impedance/gain-phase analyser (National Instruments; a high-performance modular chassis controlled by Labview™ software) was used for the cell impedance measurement. The conductivity was found to vary linearly with temperature in all the mixtures investigated. The concentration dependence of electrical conductivity (isotherms) thus obtained was divided into two parts. The first represents the concentration region of up to 10 mole % of SiO2, the second the region with a higher concentration of SiO2 (from 10 mole % to 40 mole %). While the conductivity decreased considerably with the concentration of SiO2 in the second part, it increased surprisingly in the low concentration range. From these results, the influence of electrolyte composition and temperature on the electrical conductivity was examined.

Density, Viscosity and Electrical Conductivity of the Molten Cryolite Electrolytes (Na 3 AlF 6 –SiO 2 ) for Solar Grade Silicon (Si–SoG) Electrowinning

Density, viscosity and electrical conductivity of the molten system Na 3AlF 6–SiO 2 have been investigated in the concentration range up to 50 mole % of SiO 2. Density was measured by means of a computerized Archimedean method, viscosity of the melt by computerized torsion pendulum method and the electrical conductivity by means of the tube–cell (pyrolytic boron nitride) with stationary electrodes. It was found that the density, viscosity and conductivity vary linearly with the temperature in all investigated mixtures. On the basis of the density data, the molar volume of the melts and the partial molar volume have been calculated.

On the structure of crystalline and molten cryolite: Insights from the ab initio molecular dynamics in NpT ensemble

Ab initio molecular dynamics simulations in isobaric-isothermal ensemble have been performed to study the low- and the high-temperature crystalline and liquid phases of cryolite. The temperature induced transitions from the low-temperature solid (α) to the high-temperature solid phase (β) and from the phase β to the liquid phase have been simulated using a series of MD runs performed at gradually increasing temperature. The structure of crystalline and liquid phases is analysed in detail and our computational approach is shown to reliably reproduce the available experimental data for a wide range of temperatures. Relatively frequent reorientations of the AlF6 octahedra observed in our simulation of the phase β explain the thermal disorder in positions of the F(-) ions observed in X-ray diffraction experiments. The isolated AlF6(3-), AlF5(2-), AlF4(-), as well as the bridged Al2Fm(6-m) ionic entities have been identified as the main constituents of cryolite melt. In accord with the previous high-temperature NMR and Raman spectroscopic experiments, the compound AlF5(2-) has been shown to be the most abundant Al-containing species formed in the melt. The characteristic vibrational frequencies for the AlFn(3-n) species in realistic environment have been determined and the computed values have been found to be in a good agreement with experiment.

Thermal analysis of (NaF/AlF3)-FeF3 and (NaF/AlF3)-FeO systems

Thermal analysis of the two systems, (NaF/AlF3)-FeF3 and (NaF/AlF3)-FeO, was carried out with three different cryolite ratios. In these systems, “impurity compounds” decreased the temperature of primary crystallisation with decreasing cryolite ratios. From the slope of the dependencies, it can be assumed that the excess of AlF3 plays a more significant role in changes in the “Fe(III)” systems than in “Fe(II)” systems.

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.

Surface tension of the system LiF-NaF-K2NbF7

Abstract The surface tension of the molten system LiF-NaF-K2NbF7 has been determined using the maximum bubble pressure method. The error in the surface tension measurement was estimated to be ±1%. Based on the obtained data the composition dependence of the surface tension and of the surface tension excess of the investigated system was calculated. The results are discussed in term of the anionic composition. From the surface tension excess results the negative deviation from the ideal behavior of the studied system.

(Oxo)(Fluoro)–Aluminates in KF–Al2O3 System: Thermal Stability and Structural Correlation

Precise investigation of part of the phase diagram of KF-Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.