Electrochemical Deoxidation of Solid Zirconium Dioxide in Molten Calcium Chloride

Abstract

The reduction of zirconium dioxide pellets by electro-deoxidation in molten calcium chloridecalcium oxide (900°C) has been studied. In this technique, the solid oxide is cathodically polarized against a graphite counter electrode under a constant applied potential. Unlike other metal oxides that have been reduced by this technique, only a small area around the cathodic current-collector wire was reduced to zirconium metal with zirconia pellets sintered at~1100°C; the rest of the sample was largely calcium zirconate. Pellets sintered above 1200°C showed better reduction near the cathode wire and the reduction extended to the entire surface of the pellet with the passage of time. However, reduction of the inner core was found to be increasingly difficult, because the surface metal layer thickened on continuous electrodeoxidation. An analysis of the experimental results showed that the poor electrical conductivity of the intermediate compound, CaZrO 3 and its blocky morphology inhibited the electrodeoxidation process. The increase in the sintering temperature of the pellet made it better conducting. However, the pores formed in the thick zirconium metal layer in such samples were too small for an ideal contact between the inner core and the molten electrolyte and hence the reduction of the inner core remained incomplete. Within the scope of this study, it is concluded that preforms with good grain growth and porosity are necessary for the electro-deoxidation of solid zirconium oxide.

Novel electrochemical measurements on direct electro-deoxidation of solid TiO2 and ZrO2 in molten calcium chloride medium

“…For example, electro-deoxidation of solid TiO 2 is reported to proceed through formation/ decomposition of different sub-oxides with decreasing valences, such as Ti 4 O 7 , Ti 3 O 5 , Ti 2 O 3 and TiO in conjunction with calcium containing ternary compounds, such as CaTiO 3 and CaTi 2 O 4 [6,7]. Our investigation on the electro-deoxidation of zirconium dioxide showed that a stable intermediate compound, CaZrO 3 , is formed during the reduction process, which undergoes electro-deoxidation in the later stages of electrolysis to yield zirconium metal [20]. Good electrical conductivity of the electrode, during the entire period of electrolysis is a pre-requisite for better efficiency of the electro-deoxidation process or for that matter any electrochemical process.…”

“…insertion reactions being Calcia-Stabilized Zirconia (CSZ), CaZrO 3 and Zr, which were transformed to a biphasic mixture of CaZrO 3 and Zr at later stages of the electrolysis [20]. The very low resistance of the cathodic reactions of TiO 2 , as seen in Fig.…”

“…The process was demonstrated for production of many metals [6][7][8][9][10][11][12], semi-metal [13,14] and alloys [15][16][17][18][19]. We have also investigated the electro-deoxidation behaviour of zirconium dioxide in the process [20].…”

Rapid reduction of titanium dioxide nano-particles by reduction with a calcium reductant

“…In addition, the electrochemical decomposition of carbon dioxide gas by an advanced OS process using a solid electrolyte anode has been reported [20,21]. Fray, Farthing, and Chen investigated the cathodic deoxidation of metal oxides in calcium chloride and calcium oxide mixture molten salts and reported the production of metallic titanium [22][23][24][25][26], iron [27], chromium [28], zirconium [29], hafnium [30], tantalum [31], and other alloys [32][33][34] through the FFC (Fray, Farthing, and Chen) Cambridge process. Several other research groups have also investigated the successive reduction of metal oxides based on electrolysis in molten calcium chloride [33][34][35][36][37][38][39][40][41].…”

Aspects of the Application of Electrochemistry to the Extraction of Titanium and Its Applications

“…Fray and C. Schwandt um oxide 28) and tantalum oxide 29) . One very important nding was that alloys and intermetallics can be prepared directly by reducing a mixture of metal oxides.…”