Behavior of Sodium Sulfate in Cryolite-Alumina Melts and Formation of Sulfurous Gases
All experiments were run at 980 °C in a cryolite melt corresponding to a cryolite ratio (NaF/AlF3) equal to 2.5, and with excess amounts of AlF3, Al2O3 and CaF2 of 6.75 wt%, 4 wt% and 5 wt%, respectively. The sodium sulfate reactions and the gaseous products were investigated by adding 0.44 wt% sodium sulfate (1000 ppm sulfur) into the cryolite melt with four different conditions: 1) melt without reducing agents (graphite, aluminum), 2) melt with graphite, 3) melt with graphite and aluminum and 4) melt with graphite, aluminum and plus electrolysis. The bath samples were analysed by using ICP (Inductive coupled plasma) spectroscopy, X-Ray florescence, ion chromatography. The furnace off-gases were passed through a mass spectrometer for qualitative assessment and to support and explain the gas products formed during reactions.
The mechanisms are evaluated with respect to the sulfur concentration in the electrolyte as well as the off-gas composition. Thermochemical calculations for the formation of sulfurous gas species as well as equilibrium of the gas composition as a function of temperature have been performed.
The results indicate that the presence of reducing agents such as graphite and aluminum have an impact on the sulfur removal rate. Electrolysis has a bigger impact than the presence of graphite and aluminum alone.
The sulfurous gases that were observed are: SO2, COS, CS2 and H2S. Emissions of CS2 have not been well understood or explained in the literature. The CS2 gas has been previously observed mostly during anode effects and probably it has been rarely detected due to adsorption to alumina in the dry scrubber. The gas analysis results indicated that CS2 can be formed when there is a source of sulfur and carbon in the melt. This statement also supported by thermodynamic calculations. Measurements show that COS can be formed when there is no electrolysis. The COS was also found to follow the CO2 and cease when the signal decreased. This observed behavior may indicate that the COS formation is a chemical reaction which proceeds after electrochemical reaction.