The Contents of Alkali and Earth Alkaline Metals in the Aluminium Cathode in Aluminium Electrolysis

摘要

The contents of sodium, lithium, calcium and magnesium in molten aluminium in contact with cryolite-based-melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950 - 1030℃. A thermodynamic model describing the equilibrium experimental data on the contents of sodium, lithium, calcium and/or magnesium in aluminium was developed. The experimental data were compared with the thermodynamic model. It was found that the addition of alumina and/or CaF_2 to the NaF -AlF_3 system had only a minor effect on the content of sodium in aluminum, while the addition of LiF lowered it. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a minor effect on the concentrations of lithium, calcium or magnesium in aluminium in industrial cells. The increase of sodium with increasing current density, caused by a concentration gradient at the cathode, gives rise to concentration overvoltage. This cathodic overvoltage was measured as a function of composition, temperature, and current density in a laboratory cell.