Sulfur and fluorine containing anode gases produced during normal electrolysis and approaching an anode effect

Abstract

The gases evolved from a laboratory scale alumina reduction cell were monitored with a mass spectrometer to determine the species present and qualitatively how they varied. In conjunction with the gas analysis, thermodynamic predictions were made of the range and composition of the gases produced under different conditions. Carbonyl sulfide was evolved at a steady rate during normal electrolysis, but no SO 2 was detected. Thermodynamic predictions indicate that when the crust is broken or the cell gas is burned SO 2 is dominant. The production of CF 4 and C 2F 6 did not start until anode effect, and while small amounts of C 2F 6 could not be differentiated from CF 4 in the technique used, it was evident that CF 4 dominated. Carbonyl fluoride was detected in the period shortly before anode effect, this period ranging up to 5 minutes. Thermodynamic analysis supports the formation and subsequent decomposition of COF 2, suggesting this leads to the initiation of an anode effect.