HIGH TEMPERATURE NMR IN  MOLTEN SALTS

 

Catherine BESSADA

 

CNRS-CRMHT 1D Av. de la Recherche Scientifique, 45071 Orléans cedex 2 France

 

 

 In situ experimental study of molten salts structure is extremely difficult because of the volatility and reactivity of these liquids at high temperature (up to 1200°C for fluorides). The different techniques currently used for the structural characterisation of materials : Raman, EPR, NMR, EXAFS.. have to be specially adapted. This has been done successfully in the case of Raman spectroscopy for systems as difficult as fluorides, giving an experimental description of such melts in terms of ionic complexes distributions.

Nuclear Magnetic Resonance has proved to be a powerful tool for the investigation of local structure around a selected nucleus -cation or anion-,  without limitations due to disorder in liquids or glasses. Recent developments of NMR spectroscopy at high temperatures allow now to investigate a great number of  molten systems and to propose a more precise description of the  microscopic structure of the melt in terms of different species, average coordinations, or nature of first neighbours. The heating system developed in Orleans is based on the direct irradiation of the sample by a CO₂ laser. This system insures the heating of the sample in the NMR probe with a minimal thermal power and without perturbation of the RF coil. The design used up to 1500°C is associated with a closed boron nitride crucible that can be filled in a gloves box under dried argon. This system has been successfully used for high temperature experiments in molten fluorides or chlorides, very sensitive to moisture or oxygen and known for their volatility and corrosiveness towards a number of materials.

       It is the case for cryolite-based melts used in the Hall-Heroult electrolytic process for the production of Aluminium. In situ NMR experiments have been performed in the ternary NaF-AlF₃-Al₂O₃ system up to 1030°C with the observation of the different nuclei present in the system, ²⁷Al, ²³Na, ¹⁹F and ¹⁷O. We can propose now a structural description of these liquids in terms of different aluminium bearing species that coincides rather well with the Raman data given by B.Gilbert et al. supporting the existence of AlF₅^- species. Moreover, the ¹⁷O NMR spectra give a selective and quantitative description of alumina dissolution in molten Cryolithe. From the evolution of ¹⁷O and ²⁷Al  chemical shifts with Al₂O₃ additions in the melt, we can confirm the existence of different Al-O-F species in the melt by mean of their direct experimental evidence.

This study have been extended to the characterisation of metallic aluminium dissolution in the cryolitic melts. In the industrial cells, the metal produced is heavier than the bath and falls down on the bottom of the cell. The metal is then in contact with the electrolyte and the graphite container and can induce secondary reactions that will influence strongly the electrical rate of the process. The aluminium solubility in molten cryolite is always a subject of discussion. Compared to the other metals-molten salts systems, the system Al-NaF-AlF3 is considerably more complex. In addition of the Al dissolution in the salt, one must take into account the exchange reactions as Al + 3NaF D AlF3 + Na, and the appearance of new species. This is expressed through the NMR spectra, and their complex evolution with metal contents and temperature.