Electrochemical Reduction of Zirconia in Melts Based on Mixture of Calcium Chloride and Calcium Oxide
Authors: Gritsai Larisa V. , Omel'chuk Anatolii A 
Published: February 2020

The results of studies of the electrochemical reduction of zirconium dioxide on liquid gallium cathode in a molten mixture of CaO-CaCl2-MCl (M = Li, Na) are presented. The fine zirconia powder were used for reduction.

As a result of a complex electrochemical and chemical reactions during electrolysis (Fray-Farthing-Chen (FFC) -Cambridge-Process), zirconia is reduced on liquid gallium cathode to zirconium forming wherein a fine powder that does not contain impurities both of oxygen and gallium. Oxygen ions associated with zirconium in zirconia, are moved into the melt under the influence of the electric field, are transported to the anode and discharged thereon. The components of the melt and the gallium are practically not spent on the recovery of zirconia. One of the necessary conditions of the realization of this process is the presence of a suitable melt capable of dissolving a sufficient amount of metal oxides (Ca, Mg, etc.). These oxides are necessary for the electrochemical reduction of ZrO2 and for discharge of oxygen anions, which are formed herewith. The mixture of CaO(5,0) - CaCl2(39,9) - LiCl(55,1 mol.%) tmp. = 4950C, and CaO(5,0) -CaCl2(48,0) -NaCl(, 0 mol.%) tmp. = 5000C were investigated as such melts.

The powder of zirconium, which is forming during electrolysis, is easily removed from the cell due to the difference in the specific weights and crystallization temperatures of the molten oxide-chloride mixture and the gallium. The extraction degree of zirconium from zirconia depends on the cation composition of the melt and is the higher, the lower the its acid strength is. For example, the extraction degree of zirconium from zirconia is not lower than 76% and essentially independent of temperature in the temperature range 650-8500C in the melt CaO-CaCl2-NaCl, which acid strength is less than that of melt CaO-CaCl2-LiCl. At lower temperatures, the degree of extraction of zirconium in this melt decreases. The intensive reduction of zirconia begins at lower temperatures (580-6000C) in the molten CaO-CaCl2-LiCl mixture. The extraction degree of zirconium from zirconia also substantially independent of temperature in this melt and is not lower than 65% in the temperature range of 600-8000C. The extraction degree of zirconium from zirconia reaches a maximum after reaching of certain values of the current density and subsequently is practically independent from it. This may be due to the fact that reducing agent of the zirconia in this range of current density become preferably intermetallic compounds of calcium with gallium.

The maximum extraction degree of zirconium in the melt CaO-CaCl2-NaCl is achieved at current densities exceeding 0.3 A/cm2, and in the melt CaO-CaCl2-LiCl - at current densities above 0.7 A/cm2.

The molten mixture of CaO-CaCl2-NaCl provides more favorable conditions for the recovery of zirconia. The average size of the particles of obtained zirconium powder is of 0.001-0.003 microns, specific surface area and the average of micropore size decreases with increasing current density.