Investigation of Sodium Sulfate Additions into Cryolite-Alumina Melts

In the Hall-Héroult 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 (Na2SO4) 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 SO2, COS, CS2, and H2S.