FORMATION OF SODALITE AND CANCRINITE FROM ILLITE UNDER BAYER DIGESTION CONDITIONS

As the transition to renewable energy gains momentum and the demand for aluminium metal increases, attention has turned to refining, via the Bayer process, complex sub-economic low-grade bauxite ores to obtain the alumina from which aluminium is produced. In this work, experiments were conducted at 90°C 150°C and 250°C with synthetic Bayer liquor and inorganic impurities in the presence of illite, a non-expandable phyllosilicate clay, with the main intent being to increase our understanding on how the presence of clay minerals affect the Bayer process and to explore the transformation pathway of illite, on the formation of desilication products (DSPs).
At 150°C, the combined presence of divalent anions (SO42- and CO32-) promoted liquor desilication compared to the experiments in the absence of these impurities as equilibrium was approached faster, with 8.0 and 34.0 % of the illite converted to DSP at 10 and 60 min, respectively, with the DSP phase formed being exclusively sodalite. At higher temperature (250°C), 91.0 % of the illite converted to DSP by 30 minutes, with both sodalite and cancrinite being the main phases formed. Using solid state-NMR and X-ray diffraction, phase transformation of sodalite to cancrinite was also investigated and discussed.
Two different sodalite morphologies were observed from microscopy analysis the usual lepispheres or “wool balls” at 150°C after 10 minutes and β-cage type “hexagon balls” of greater diameter (~ 10 μm) after 60 minutes. The latter morphology was also observed as early as 10 minutes following digestion of illite at highest temperature (250°C) but presented with rod-like cancrinite growth on the surfaces of sodalite.
Overall, the SiO2 concentration profiles showed that illite can be a source of reactive silica under Bayer digestion process conditions (150-250°C) but not under pre-desilication conditions (~90°C up to 18 hours) resulting in potential cost savings for low temperature refineries and reduced soda losses; the illite partially converts to sodalite at 150°C and at 250°C reacts readily to form sodalite and cancrinite.