Monitoring of Low Cell Voltage PFC Formation in 200 to 300 kA Aluminium Electrolysis Cells

The existence of perfluorocarbons (PFCs) formed at low cell voltage has been documented for larger size alumimium electrolysis cells with relatively few point feeders and intermediate current density. It has been proposed that less uniformity in dissolved alumina for larger cells may elevate individual anode overvoltage sufficiently to produce PFC. Low cell voltage PFC formation was monitored after the dry scrubber on a train of 28 cells at a Norwegian smelter. For this work a fourier-transform infrared spectrometer was used. Equipped with a mercury cadmium telluride (MCT) detector and retrofitted with a 35 m / 11 l measurement cell it was possible to get the detection limit down in the low ppb range needed for this study. It was discovered that also small cell could emit PFC that was not directly related to the full anode effect. Low cell voltage PFC was found to be discrete emissions mostly in the form of CF4, however, the results indicate that C2F6 may also form outside the main anode effect. No numerical data for the contribution of low cell voltage emissions to the overall PFC was calculated, nonetheless, judging by the difference in intensity of the events it will be small for this particular smelter.