Oxide solubilities in melts based on alkali- and alkaline-earth metal halides

2 OXIDE SOLUBILITIES IN MELTS BASED ON ALKALI- AND ALKALINE-EARTH METAL HALIDES 

Few investigations have been performed using such halide systems as solvents for oxides, although they are of great interest for practical purposes, as media for sodium and magnesium electrodeposition. 

Castrillejo et al. studied the solubility of magnesium oxide in the molten CaCl2-NaCl equimolar mixture at 575 °C [178]. The solubility product index was found to be pPMg0 = 5.3 0.1, on the molality scale. Nevertheless, the information presented in Ref. [178] makes it possible to obtain additional data on the behaviour of MgO in the studied melt. To realize this let us consider Fig. 3.7.18. The first two points in the potentiometric curve can be established, on the basis of the treatment method in Part 6, to belong to the unsaturated 

Equilibria in solid oxide-ionic melt systems 

Strelets and Bondarenko studied the interactions between MgCl2 and CaO in carnallite melt (KMgCl3 with the melting point at 488 °C) [387]  

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Part 7. Regularities of oxide solubilities in melts based on alkali and alkaline-earth metal halides... 

From the dependence shown in Fig. 3.7.15 it can be concluded that the growth of the parameter a of the cubic crystalline lattice of the oxide increases its solubility in ionic melts based on alkali- and alkaline-earth metal halides. This may be explained by the fact that the increase in the parameter a of the crystalline lattice results in the weakening of the interaction between ions and molecules in oxide crystals, which favours dissolution of these oxide crystals in molten salts. The solubilities of ZnO and PbO are not shown in Fig. 3.7.15, since at the temperature of the experiment both oxides possess hexagonal crystalline lattices. 

All the above-said concerns alkali-metal carbonates dissolved in melts based on alkaline- and alkaline-earth metal halides. It should be mentioned that there is a lack of data on the behaviour of alkaline-earth carbonates, in high-temperature alkaline halide melts. Nevertheless, these data are necessary, in particular, for the purification of halide melts. For example, it is known that alkaline-earth oxides possess appreciable solubilities in molten alkaline halides, and that they can be used for precipitation of less soluble transition metal-oxides. After such a precipitation, the excess of alkaline-earth oxide remains in the dissolved state. The question arises, therefore, whether it can be removed from the halide melt by passing C02 according to the conventional reaction... 

For oxides of the type MeO, there exists a linear dependence of the solubility product index of the oxide against the inverse-squared radius of the metal cation. The slopes of these plots in the melts based on alkali-metal halides and alkaline-earth metal halides are stated to be approximately the same. This can give evidence that, at high temperatures in the order of 1000 K, the changes in solvation ability of the ionic melts, proceeding from one melt to another, are close for different cations with radii in the order of 0.1 nm (from 0.74 nm for Mg2+ to 1.38 nm for Ba2+). An increase in the melt temperature... 

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