Marián Kucharík

Density and Surface Tension of the System KF–K2TaF7–Ta2O5

The density of the system KF–K2TaF7–Ta2O5 was measured by the Archimedean method and the surface tension by the maximum bubble pressure method in the composition range up to 20 mol% Ta2O5. The density in the system KF–K2TaF7–Ta2O5 increases from KF through K2TaF7 to Ta2O5 and the surface tension increases from K2TaF7 through KF to Ta2O5. Based on the measured density values, the molar volumes of the binary and ternary melts at 860 °C, 900 °C and 950 °C were calculated as well as the partial molar volumes of particular components in different binaries. The small negative deviation from ideal behaviour in the binary system KF–K2TaF7 was identified by the calculation of the molar excess volume. The formation of complex anions [TaF8]3− is expected in this binary system. The surface tension decreases monotonically from KF to K2TaF7 in the binary system KF–K2TaF7. In the next two binaries KF–Ta2O5 and K2TaF7–Ta2O5 decrease of the surface tension was observed with the initial addition of Ta2O5 followed by the increase of the surface tension by further addition of Ta2O5. Based on measured data it was concluded that in binaries KF–Ta2O5 and K2TaF7–Ta2O5 the surface active component is Ta2O5 and in the binary KF–K2TaF7 the surface active component is K2TaF7. Mathematical description of the concentration dependence of the molar volume and surface tension of the measured ternary system was done by the polynomial function at 860°C, 900°C, and at 950°C.

Surface Tension of the System NaF –AlF3–Al2O3 and Surface Adsorption of Al2O3

Part of the molten system NaF-AlF3-Al2O3 was studied by surface tension measurements, which were performed at cryolite ratios (CR) between 1.5 and 3 [CR = n(NaF)/n(AlF3)]. The maximal bubble pressure method was applied. The surface adsorption of alumina (Al2O3) was also calculated. The obtained results were discussed in terms of the anionic composition of the melt. The addition of AlF3 to melt with CR= 3 decreases the surface tension, as AlF3 is surface-active in molten Na3AlF6. The concentration dependence of the surface tension and the surface adsorption of alumina in the title system are influenced by the formation of surface-active oxofluoroaluminates. An increase of the difference between the surface tension of NaF-AlF3 mixtures and the surface tension of pure alumina was observed with decreasing cryolite ratio.

Rapid solidification processing in molten salts chemistry: X-ray analysis of deeply undercooled cryolite-alumina melts

Rapid solidification processing (cooling rate from the interval 105–106 K s−1) was used to prepare deeply undercooled cryolite-alumina melts. Such prepared samples were analyzed by the XRD method. Besides cryolite, XRD patterns belonging to ι-Al2O3 were recorded. The influence of annealing on the XRD patterns of deeply undercooled melts was also investigated.

Notes on notation of sodium oxofluoroaluminate anions

The chemical notation of the reaction products in the system NaF—AlF3—Al2O3 was reevaluated and its modifications were suggested. Based on these modifications, the equilibrium constants of selected reactions were calculated. Some of the equilibrium constants differed by one or two orders of magnitude from the values of the corresponding original equilibrium constants.

Silver as anode in cryolite—alumina-based melts

The anodic behaviour of silver was investigated in cryolite—alumina-based melt. Silver has a lower melting point (ca. 960°C) than the other metals considered as possible inert materials for aluminium electrolysis. The working temperature used in aluminium industry is approximately 960°C, depending on the melt composition. Therefore, the stability of silver during the anodic process was tested at 870°C in an acidic electrolyte consisting of 65.5 mass % Na3AlF6 + 22.9 mass % AlF3 + 5.7 mass % CaF2 + 3.9 mass % LiF + 2 mass % Al2O3 with the melting point ca. 850°C. The electrolyte without alumina was prepared as well, with the melting point ca. 860°C. The resulting cryolite ratio (CR = n(NaF)/n(AlF3)) for both electrolytes was equal to 1.6. The behaviour of the silver anode was studied by voltammetry measurements. The electrochemical study showed that an oxidation reaction occurred at a potential below the oxygen evolution potential. Silver was not found to be stable under oxygen evolution. The degradation of the silver anode was apparent after electrolysis.