Systems containing halides and oxides

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

The physico-chemical properties such as density, conductivity, and viscosity of the magnesium chloride electrolyte are of substantial importance for the current efficiency of the magnesium electrolysis. The solubility of the reaction products, Mg and CI2, in the electrolyte is also important to attain the high current efficiency. Interfacial properties between the Mg electrolyte and the metal may depend significantly on the oxide concentration of the bath. 

Combes et al. (1980) used calcia-stabilized zirconia electrodes to determine the concentration in the MgCl2-NaCl-KCl mixtures at 1000 K with different BaO additions. They suggested that the following reactions take place in the melt  

The solubility of MgO in the equimolar NaCl-KCl mixture at 1000 K was found to be very low. The solubility of other alkaline-earth oxides in the equimolar NaCl-KCl melts are low, but are considerably higher than that of MgO and do increase in the sequence MgO CaO SrO BaO. However, in earth alkali metal chloride-rich melts. 

Boghosian et al. (1991) studied the solubility of earth alkali metal oxides in alkali metal-earth alkali metal chlorides and NaCl-MeCl2 melts. They found that the oxide solubility is in general very low and increases markedly with the MeCl2 concentration and with increasing atomic number of the earth alkali metal atom. The very low oxide solubility in earth alkali metal chlorides can be explained by the reaction (e.g. for the MgCl2 melts) 

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