Influence of Heat Transfer on Anode Reactions When Electrowinning Metal from Its Oxides Dissolved in Molten Fluorides

Abstract

Detection of CF4 coevolution during commercial electrowinning from oxide fluoride melts at anode potentials below those required for direct electrochemical formation raise doubts about published mechanisms of the cause of anode effects (AE). By linking anode-electrolyte potential gradients with anode carbon structure, cell design, and the composition of gases emitted we have obtained a better understanding of all product formation reactions. Interfacial heat transfer to satisfy the entropic energy deficit is found to be the common link between electrode potential, operating conditions and gas composition for all reactions occurring in the fluoride-oxide melts studied. Concurrent thermodynamic analysis suggests that formation of an intermediate such as a fluorinated carbon surface or COF2 would lower the required potential for the overall reactionAl2O3+2Na3AlF6l+9/2C=4Al+3COg+3/2CF4g+6NaFlThis is supported by experimental verification of the intermediate. The entropic energy deficit is linked to the buildup of the resistive or passivating intermediate fluoride film on the carbon anode surface which is capable of generating Joule heat enabling the various parallel reactions to occur with or without an anode effect.