Direct determination of the NaF/AlF3 molar ratio by Raman spectroscopy in NaF-AlF3-CaF2 melts at 1000 °C.
For the last 40 years, Raman spectroscopy has been very useful in investigating the structure of corrosive molten salts, such as the cryolite-based melts widely used as electrolyte in the Hall-Heroult process. Even if this process remains the most economically efficient for metallic aluminum electro-production, it suffers from a high energy loss, which is dependent on the melt composition. Therefore, controlling the chemical composition of the electrolyte is essential. The present paper proposes to apply Raman spectroscopy for the direct determination of the NaF-AlF3 molar ratio in NaF-AlF3-CaF2-based melts. Despite the experimental difficulties, a calibration curve based on equilibria taking place in the melt has been developed and the procedure has been successfully compared to industrial samples of known compositions. The possible exportation of the laboratory scale procedure to an industrial environment application for the control of the Hall-Heroult process is finally discussed.
A viscosity model for the (NaF + AlF3 + CaF2 + Al2O3) electrolyte
Abstract A model has been developed for the viscosity of multicomponent inorganic molten salts or liquid alloys. Over the past several years, through critical evaluation of all available thermodynamic and phase equilibrium data, we have developed a quantitative thermodynamic description of multicomponent molten salt systems and liquid alloys using the Modified Quasichemical Model in the Quadruplet (or Pair) Approximation for short-range ordering. The viscosity is modelled using an Eyring equation with molar activation energy for viscous flow expanded as a first-order polynomial in the quadruplet (or pair) mole fractions. The quadruplets (or pairs) may be regarded as structural units and can be calculated from the thermodynamic model. In the present article, the viscosity model is applied to the (NaF + AlF 3  + CaF 2  + Al 2 O 3 ) electrolyte. All available viscosity data were collected and critically evaluated, and optimized model parameters have been found. This new model can be used with Gibbs free energy minimization software, to calculate the viscosity of cryolite-based melts used for the electro-reduction of alumina in Hall–Heroult cells. The viscosity model described in the present paper has not yet been tested on molten salts whose viscosity can increase by more than one order of magnitude.
Raman spectra of aluminum fluoride containing melts and the ionic equilibrium in molten cryolite type mixtures
The structure of molten aluminum fluoride−alkali fluoride mixtures has been investigated by Raman spectroscopy at 700−900°C using a captive liquid windowless cell. Changes in spectra with temperature and melt composition reflect an equilibrium between the species AlF4− and AlF63−. Complete spectrum of the AlF63− was obtained confirming an octahedral coordination; the frequencies of the ν1, ν2, and ν5 vibrations were found to be 555, 390, and 345 cm−1, respectively. A strong cation effect on the bandwidth of the AlF63− ν1 band was observed when the cation is changed from Li+ to Cs+. This effect is believed to be due to a restriction of the rotational mobility of the anion in the presence of a smaller cation. Quantitative analysis of the intensity ratios of the AlF4− and AlF63− ν1 bands as a function of the composition of the melt permits the calculation of the stoichiometric dissociation constant K = XAlF4−XF−2/XAlF63−. An average value of 3±1×10−2 (concentration scale in mole fractions) was found at 780°C...