Chemical and electrochemical reactions of sulphur species in cryolite melts

Data on the chemical reduction of sulphate by aluminium and by graphite in cryolite melts are reported.. Chemical reduction of sulphate by aluminium or by carbon follows a similar sequence. Only SO42-, S2- and polysulphide species were detected in the solidified melts. It was found that sulphide reacts with dissolved iron(II), forming FeS, which is insoluble in cryolite melts. Because of its high density, solid FeS particles will tend to settle and react with the aluminium cathode, forming Al2S3, which is readily soluble in molten cryolite. This may be a nonelectrochemical mechanism for contaminating aluminium with iron. The cryolite-rich part of the phase diagram Na3AlF6-Al2S3 was determined. By cathodic reduction of sulphate it was found that two electrons take part in the reaction, i.e. SO42- + 2e(-) = SO32- + O2-. Since the sulphite species is not stable at high temperatures, it decomposes thermally into SO42- and S2-.