Visualization of Anode Effect in Aluminum Electrolysis

The aluminum smelting industry is a significant emitter of greenhouse gases. In addition to the emission of CO 2 during normal operation, perfluorocarbons (PFCs) are generated when the Al 2 O 3 concentration in the electrolyte falls below a certain critical level, a phenomenon referred to as anode effect . Understanding the mechanism of anode effect formation has become essential to mitigating or preventing its occurrence. Here, the anode effect occurring at an industrial carbon anode with an underside surface area of 50 cm 2 (10 × 5 cm) was observed in a high-temperature see-through Hall-Héroult electrolytic cell operating at 940 °C. The visual observation of the phenomenon confirmed that the anode effect is caused by the formation of an intermediate, insulating CF x film. No gas layer or large bubbles were observed during the anode effect. The sharp increase of the cell voltage is due to the insulation of the continuous intermediate unbroken film, which has relatively much higher electrical resistance than the carbon anode and electrolyte.