Used method of isothermal saturation solubility of copper (II) oxide in alkali and alkaline-salt melts has been investigated at temperature range 673 -873 K. It has been determined that the solubility of CuO in the order of melts LiOH < NaOH < KOH are increased. The effect of temperature on the CuO solubility in alkali melts are described of the equation of the straight line, where C is concentration (mole fraction) of the CuO in the melts, and A and B are constants. The salt additions of lithium chloride, sodium chloride and potassium chloride to sodium hydroxide melt decreased the solubility of copper (II) oxide.

This study investigates the isothermal saturation solubility of metal oxide in alkali and alkaline-salt melts in the temperature range 673 -873 K. The effect of temperature on the metal oxide solubility in alkali melts are satisfactorily described by the equation of the straight line lnC = A + BT -1 , where C is concentration (mole fraction) of oxide in melts, and A and B are constants. The thermodynamic analysis of solutions of metal oxides in molten alkalis was carried out.

The phase equilibria of the ternary system CaCl2 – NaCl – CaO in the area which enriched of calcium and sodium chloride were investigated by the methods of differential-thermal analysis and powder X-ray phase analysis. In the systems were determined the equilibrium concentration of calcium oxide and the composition of the phases, which at the same time exist in an equilibrium state at different temperatures. The surfaces of liquidus and solidus were established, the compositions of the sections of the ternary system CaCl2–NaCl–CaO were defined, which recommended for electrochemical reduction of refractory metal oxides (titanium, zirconium and other), which allow electrolysis in the temperature range from 550 to 1000 °С. Five polythermal sections of the NaCl – CaCl2 – CaO ternary system were studied. For each polythermal section the regions of existence of the liquid and solid phases were established. For each polythermal section state diagrams were constructed. Used X-Ray phase analyses it was established the compositions of liquid and solid phases for each polythermal sections. The phases of which the system consists were determined. At a constant ratio of components [NaCl]:[CaCl2] = 1.06 (mol.) in the melts of the ternary system CaCl2 – NaCl – CaO, the equilibrium content of calcium oxide reaches 12.0 mol.%, while their crystallization temperature does not exceed 550 °C. This allows us to recommend mixtures of this composition for electrochemical reduction of refractory metal oxides in a wide range of temperatures (from 550 to 1000 °C) with a high content of both calcium and sodium chlorides (not less than 40 mol.%) and oxide. calcium (up to 12.0 mol.%). The eutectic of this ternary system has a melting point of 480 ° C and corresponds to he composition (mol.%): CaCl2 (45.8) – NaCl (47.0) – CaO (7.2).

The results of the electrochemical reduction of zirconium dioxide in molten electrolytes based on calcium or magnesium chloride on a liquid gallium cathode, bypassing the stage of granulation and sintering with carbohydrates, are presented. The liquid gallium cathode provides not only reliable contact with zirconium dioxide, but also favourable conditions for its reduction. The contact area of the gallium cathode with fine oxide powder is much larger than the contact area of the granulated and sintered zirconium dioxide with a solid conductor. This ensures a more uniform cathode polarization. Due to the lower specific mass, zirconium dioxide is located on the surface of gallium cathode, the convective movement of which provides more intense mass transfer at the interface of the phases and removal of recovery products from the zone of electrode reaction. Products of electrolysis under such conditions do not block neither the surface of zirconium dioxide nor the surface of the cathode. Zirconium, which is formed during the renewal, due to a larger specific mass, precipitates to the bottom of the electrolyser, and the layer of gallium protects it from interaction with the components of the molten electrolyte. In addition, due to the formation of alloys, the reduction of metal cations on liquid cathodes proceeds at more positive potentials than on solid indifferent cathodes, which reduces the specific energy consumption by electrolysis. The results of voltammetric studies confirm this conclusion. The reduction product is fine-grained zirconium powder with an average particle size of 1—3 microns, and purity of 99.9 %. As the density of the current increases, the value of the specific surface of the powder, the specific volume of the micropore and their average radius decrease. The degree of extraction depends on the composition of the electrolyte mixture and naturally decreases when replacing cations in the melts both on the basis of calcium chloride and on the basis of magnesium chloride in the following sequence Na+ > K+ > Li+. The melt based on compounds of calcium and sodium chloride provides the best performance. The removal degree of zirconium from such melt reaches 77 %.

The direct electrochemical reduction of titanium oxide to titanium metal in molten CaCl2 salt has been proven by the FFC Cambridge process. [1] Subsequently, the process has been applied to a number of refractory metals. However, there are limitations to the FFC Cambridge process. For example, the current efficiency of the process is quite low, 10-40%, to achieve sufficiently low oxygen content, 0.3%, in the final titanium product. [2] This could be due to a number of reasons, such as the increasing concentration of O2- ions in the melt as the reduction process proceeds. The metal oxide electrode has an inherent pore structure and sponge-type substrate electrodes can be used with a range of pore sizes, which has the advantage of high surface area and access of the melt to the oxide. However, when the metal oxide is reduced, oxide ions accumulation in the pores can significantly change the potential needed for the reduction, as shown in the Littlewood [3] predominance diagram. It can also result in the formation of other unwanted metal phases, such as those that include the salt’s metal, as it reacts with the oxide ions. Oxide ion build up close to the electrode surface can ultimately bring the reduction process to a halt, leaving the inner parts of the metal oxide unreduced. 1. Chen, G.Z., D.J. Fray, and T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. Nature, 2000. 407(6802): p. 361-364. 2. Schwandt, C., D.T.L. Alexander, and D.J. Fray, The electro-deoxidation of porous titanium dioxide precursors in molten calcium chloride under cathodic potential control. Electrochimica Acta, 2009. 54(14): p. 3819-3829. 3. Littlewood, R., Diagrammatic Representation of the Thermodynamics of Metal‐Fused Chloride Systems. Journal of the Electrochemical Society, 1962. 109(6): p. 525.