C K-edge NEXAFS study of fluorocarbon formation on carbon anodes in molten NaF-AlF_3-CaF_2

摘要

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.