Thermodynamic analysis of Na-S-Fe-H2O system for Bayer process

Abstract

Thermodynamic diagrams of Na-S-Fe-H2O system were constructed to analyze the behavior of sulfur and iron in the Bayer process. After digestion, iron mainly exists as Fe3O4 and Fe2O3 in red mud, and partial iron transfers into solution as Fe(OH)3-, HFeO2-, Fe(OH)4- and Fe(OH)42-. The dominant species of sulfur is S2-, followed by SO42-, and then SO32- and S2O32-. The thermodynamic analysis is consistent with the iron and sulfur species distribution in the solution obtained by experiments. When the temperature decreases, sulfur and iron can combine and precipitate. Controlling low potential and reducing temperature are beneficial to removing them from the solution. XRD patterns show that NaFeS2·2H2O, FeS and FeS2 widely appear in red mud and precipitates of pyrite and high-sulfur bauxite digestion solution. Thermodynamic analysis can be utilized to guide the simultaneous removal of sulfur and iron in the Bayer process.