Investigation of sodium sulfate additions into cryolite-alumina melts

Abstract

In the Hall-Heroult process, sulfur impurities may not only emit harmful gases but also reduce current efficiency. To better understand this process, the behaviour of sulfur compounds in a cryolite-alumina melt at 1253 K (980 °C) was investigated in a laboratory cell. Sodium sulfate (Na2S04) was added into the molten bath as a sulfur source. Furnace off-gases were passed through a mass spectrometer for qualitative assessment. The stability of sodium sulfate in the cryolite melt was found to depend on the presence of carbon (t1/2= 116 min) and carbon- Aluminium (t1/2= 29 min). It changed dramatically during electrolysis (t1/2= 5-8 min). Detected sulfurous gases included S02, COS, CS2, and H2S.