C K‐edge NEXAFS study of fluorocarbon formation on carbon anodes in molten NaF–AlF₃–CaF₂

Abstract

Operational instability from processes occurring at the anode during the production of aluminum in the commercial Hall-Héroult process may lead to an increase in undesirable fluorocarbon emissions, higher energy use, and shorter plant life. One contribution to this instability may be the possible formation of a fluorocarbon film at the electrode interface. Here, the surface composition of graphite anodes after electrolysis in molten NaF-AlF 3 -CaF 2 at 990°C is investigated for evidence of fluorocarbon formation using C K-edge near edge X-ray absorption fine structure. Fluorocarbon is identified on an anode surface after prolonged anode effect (very high overpotential with increased cell resistance) and also on an anode surface after normal electrolysis without anode effect. This provides evidence that fluorocarbon formation may occur prior to anode effect lowering the surface tension of the anode and therefore resulting in dewetting to contribute to the onset of the anode effect. Confirmation that such compounds form furthers our understanding of electrochemical reactions of graphite with fluoride and of the fundamental processes that occur in an aluminum smelter cell.

Fluorination-dependent molecular orbital occupancy in ring-shaped perfluorocarbons

“…Additionally, parallels between the observed orbital change and molecule enhancement methods through fluorine treatment should be discussed. 44,45 The observations are similar with respect to the observed changes in the MO structure and the resulting changes in chemical character. [44][45][46][47] To enhance the mechanistic understanding and involvement of the orbital change in these methods and molecules, further research on chain structured and more complex perfluoro compounds is in progress and will hopefully deliver a more general understanding of these findings.…”

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

“…More recently a C-F intermediate has also been identified on the anode surface. 25 These findings are consistent with the finding that a resistive film formed on the electrode surface. 16,21,22 The thermodynamic predictions of Table I indicate that only a limited increase in anode potential from that traditionally used would be needed for this to occur.…”