Effects of Si-bearing minerals on the conversion of hematite into magnetite during reductive Bayer digestion

Converting hematite to magnetite in Bayer liquor at elevated temperature plays a key role in the exploration of a cleaner technology for alumina production with high iron content bauxite. In this work, the effects of Si-bearing minerals on hematite conversion during reductive Bayer digestion were investigated by studying the reaction kinetics and mineralogy. The experimental rate data of the hematite conversion in the presence of zero-valent iron during high temperature Bayer digestion agree well with the unreacted shrinking-core model under internal diffusion control, and the activation energy for the conversion is determined to be 45.4 ± 1.8 kJ·mol−1. The interaction between the dissolved Si or the granular sodium aluminate silicate hydrate and iron powder inhibits the transformation of metallic iron into HFeO2− or magnetite. Further, the dissolved Si co-precipitates with iron-containing species to form Si–Fe hetero-aggregates, which impedes species diffusion, thus inhibits the hematite to magnetite conversion. The inhibitory effects of different Si-bearing minerals are significantly different. This work contributes to a better understanding of iron mineral conversion in Bayer liquor at elevated temperatures and to the optimization of the reductive Bayer digestion process.