Reactions of sulphides in molten cryolite

The freezing point depression of cryolite (Na3AlF6) by the addition of Al2S3 and FeS was investigated. It was found that for contents of up to 10 wt.% Al2S3, it brings into the melt three new species. X-ray analysis of solidified melts of the system Na3AlF6–Al2S3 showed that it contained chiolite, Na5Al3F14 and Na2S. Chiolite originates from a reaction between Na3AlF6 and AlF3. This suggests that the system Na3AlF6–Al2S3 is a part of the reciprocal system NaF, AlF3//Na2S, Al2S3. The solubility of FeS in cryolite melt is so low that it cannot be determined by the thermal analysis. When FeO is added to the Na3AlF6–Al2S3 melt, Fe2+ cations and S2− anions react under the formation of solid FeS. A similar reaction was observed for Ni2+ and S2− ions.