Reactions of Iron Minerals in Sodium Aluminate Solutions

Abstract

Settling properties of bauxite residue are affected by the form of iron mineral present in the system. The two forms of iron minerals generally found in bauxites are aluminous goethite, α-(Fe,Al)OOH and hematite, α-Fe₂O₃. Model systems comprising of synthetic goethite and hematite in pure sodium aluminate solutions were used to study reactions representative of those occurring in the Bayer Process. Hematite exhibits considerably faster settling rates as compared to those of goethite, owing to the surface properties unique to each mineral. Transformation of α-FeOOH → α-Fe₂O₃ at high temperatures leads to improved settling. The kinetics and mechanism of this transformation reaction have been established. The transformation proceeds via a dissolution, reprecipitation mechanism. Factors that affect each step in the mechanism influence the overall reaction and, hence, clarification. These factors include: temperature, liquor composition, and presence of hematite seed. Also reported is the effect of temperature and liquor composition on the dissolution of goethite.