Behavior of Sodium Sulfate in Cryolite-Alumina Melts and Formation of Sulfurous Gases

Sulfurous gases are known for their detrimental effect on local air pollution (particulate matter), acid rain, health and global climate change. Sulfur emissions from an aluminum smelter with an annual production of 200000 tonnes can be as high as 1000 tonnes sulfur per year. The most important reported sulfurous gases from aluminum reduction cells are SO_2, COS, H₂S and CS₂, first two being the predominant. The source of sulfurous gases from aluminum smelters is mainly from the sulfur in carbon anodes. Small amounts of sulfur are also introduced by the feed materials, mostly in the form of sulfates. The study of the behaviour of sulfur compounds has particular interest not only due to the emitted harmful gases, but also its effect on the process of aluminum production. Recent study shows that sulfur has an impact on current efficiency of the aluminum smelting due to several oxidations states available.

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: SO_2, 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.