Co-evolution of Carbon Oxides and Fluorides During the Electrowinning of Aluminium with Molten NaF–AlF₃–CaF₂–Al₂O₃ Electrolytes

The recent finding of continuous background PFC co-evolution during apparently normal operations of large operating smelting cells has raised the question as to whether or not these arise from a different reaction mechanism since they occur at similar cell voltages to what is considered to be below the threshold when calculating greenhouse gas emissions emanating from cells. The analysis presented from this combination of studies suggest that all PFC emissions are triggered by the anode potential approaching that necessary for COF₂(g) formation, with the intermediate formed on the electrode surface decomposing to release an increased amount of CO(g) as well as CF₄(g) according to the overall reaction Because of the difference in kinetics of discharge of the two anions, and the extra thermal energy necessary to satisfy the overall enthalpy, partial passivation of the electrode surface can occur when it is under cell voltage control. The situation also applies to individual anode in large cells and therefore gives rise to PFC co-evolution through spatial variations. As the spatial changes propagate the same process leads to a full anode effect in the operating cell.