Influence Of The Physico-Chemical Characteristics Of The Metal Oxide Cathodes On The Electro-deoxidation In The FFC Cambridge Process

Abstract:

In the FFC Cambridge process, a solid metal oxide cathode is directly reduced to the metal by molten salt electrolysis in calcium chloride medium containing small quantities of calcium oxide at ~ 900 oC. The cathodic and anodic process can be represented as MOx+ 2ne- → MOx-n+ nO2- at cathode nO2- + nC → nCO + 2ne- at anode or nO2- + n/2C → n/2CO2 + 2ne- at anodeThe cathodic process of the FFC Cambridge process, i.e the electro-generation of oxygen ions and its release to the electrolyte melt are very unique features of the process, which determine the efficiency of the ‘cathode-centric’ electrochemical process.The cathodic ionization of a solid metal oxide can take place only at those points where the three phases viz. electron (conductor), the oxide and the electrolyte meet. Also the cathodically produced oxygen ions in the bulk of the solid specimen should diffuse towards the surface of the pellet. These conditions require that the electrode be good conducting and also sufficiently porous. For the process to be efficient, the three-phase contact points must be spread across the entire volume of the solid oxide preform. When the oxide preform is poor conducting, this condition cannot be satisfied and hence it may end up with a better-reduced outer layer and a partially reduced inner bulk. Both the chemical and physical characteristics of a solid oxide preform are very important in determining the efficiency of its electro-deoxidation. We have studied some of these aspects of the FFC process with powder compacted and sintered electrodes and powder electrodes of TiO2, ZrO2 and SiO2, three metal oxide cathodes with different chemical and physical characteristics, in molten calcium chloride at 900 oC. In-house developed novel electrochemical techniques were used as part of these studies to gain insight into the electrode behaviour (electrode potentials and resistance) of the oxide cathode during the entire course of the electro-deoxidation. The studies have shown that the electro-deoxidation of a solid oxide pellet depends on many factors such as the sintering temperature-dependent morphology of the starting pellet, the intermediate compounds and the metal phase; the cathodic reactions, electrical nature of the starting pellet and the intermediate products, the IR drop of the cell, the method of application of electric potential to the electrode, the configuration of the electrodes etc. These factors influenced more prominently the electro-deoxidation of poor conducting ZrO2 than good conducting TiO2 electrode. The details of the study are presented in this paper.

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