A model for the electrochemical reduction of metal oxides in molten salt electrolytes

Abstract

The reduction of numerous metal oxides is being investigated by electro-deoxidation in molten salts due to the low-oxygen content advantage these processes offer. One of these processes is the Fray–Farthing–Chen (FFC) process that involves the direct reduction of titanium dioxide to titanium. A model for the multi-stage reduction of titanium dioxide to titanium is reported herein. The modeling approach adopted is based on the porous electrode theory used for studying lithium-ion batteries. [J.S. Newman, C.W. Tobias, J. Electrochem. Soc. 109 (12) (1962) 1183; K.E. Thomas, J.S. Newman, T.M. Darling, in: B. Scrosati, W. van Schalkwijk (Eds.), Advances in Lithium-Ion Batteries, Kluwer Academic Publishers, New York, 2002; J.S. Newman, K.E. Thomas, Electrochemical Systems, 3rd ed., Wiley-Interscience, 2004; V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 151 (10) (2004) A1517; V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 151 (10) (2004) A1530; J. Christensen, V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 153 (3) (2006) A560; J.S. Newman, K.E. Thomas, H. Hafezi, D.R. Wheeler, J. Power Sources 119 (SI) (2003) 838; C.R. Pals, J.S. Newman, J. Electrochem. Soc. 142 (10) (1995) A3274]. The reduction of an individual sintered pellet of TiO 2 as it undergoes electro-deoxidation in a molten salt bath of CaCl 2 is modeled. This model has been applied to study the effect of physical variables such as the porosity, radius of pellet, radius of grains (particle size) in the pellet and the effect of starting with a partially reduced oxide. The effect of calcium titanate formation on the reduction is also incorporated and it is shown that if the reduction is started with a pellet of partially reduced titanium dioxide, one can avoid titanate formation. Though the FFC process is not completely successful in complete reduction due to titanate formation and needs to be developed further it has succeeded in the reduction of metals that are not as complex as the titanium oxygen system. This model could also be used for studying the reduction of other metal and mixed oxides such as SiO 2, NiO, Cr 2O 3, Nb 2O 5 and ZrO 2 that are simpler to reduce than TiO 2.