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 review of primary aluminium tapping models
The removal of molten aluminum from Hall–Heroult cells is known as the tapping process. Metal produced in the aluminum electrolytic cell is removed by vacuum using a crucible through a tapping tube. The purpose of metal tapping is to keep the metal volume at the optimum production level. It is possible that a certain amount of bath is entrained with metal when the metal is siphoned into the crucible. One of the challenges faced by aluminum smelters is the high volume of dross processed in the cast house plant as a result of bath tapped with metal from the reduction cells. The aim of this review is to discuss previous aluminum tapping models and trends in technology developments with the aim of improving the tapping operation. The advantages and limitations of existing model studies are also discussed in the context of electrolytic bath carried over with liquid metal during tapping from the reduction cell. A fundamental approach of the quality of tapping flows is also proposed for further investigation.
Structural entities in NaF?AlF3 melts containing alumina
Abstract An ionic structure model is developed for NaFAlF3 melts containing alumina. The model can be described by the following dissociation equilibria, Na+ is proposed to be the only cation present in the system. The model is derived from activity data for NaF and AlF3 in melts saturated with alumina at 1285 K and the assumption of an ideal ionic mixture, with randomly distributed anions on the anion positions in the melt. The available data for the system seem to fit the model reasonably well. The entity Al2F−7 will only be present in AlF3 rich melts. Oxygen atoms in the complexes are most probably involved in bridging bonds of the type AlOAl and . It appears that aluminium atoms are involved in 4 bonds in AlF3-rich melts. The number of bonds changes partly towards 5 and/or 6 with increasing NaF content of the melt.