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Abstract Clarifying the interactions among iron and titanium-bearing minerals in the treatment of diasporic bauxite with Bayer liquor at elevated temperature is crucial to understanding reductive Bayer digestion, in which the inhibition of sodium titanate on diaspore digestion is eliminated by the use of non-lime additive. In this work, we carried out a thermodynamic analysis, which predicted that ilmenite (FeTiO 3), ulvite (Fe 2 TiO 4), and sodium iron titanium oxide (NaFeTiO 4) will result from the interactions among iron and titanium bearing minerals in Bayer digestion, whereas pseudobrookite (Fe 2 TiO 5) cannot be generated. In addition, the perovskite (CaTiO 3) is more stable than the produced Ti-Fe compounds, thus, the Ti-Fe compounds could be converted into this perovskite by the addition of extra lime. In order to confirm the thermodynamic analysis results, we then carried out both typical and reductive Bayer digestion experiments. Unlike the dense sodium titanate layer formed on the other minerals present in typical Bayer digestion, the fibrous porous sodium iron titanium oxide layer produced in reductive Bayer digestion allows the passage of the alkaline solution through the precipitated layer to the mineral particles beneath, and thus ensures the efficient digestion of the diaspore. The reaction of HFeO 2 − and titanium-bearing mineral to form Ti-Fe compounds occurs prior to the reaction of HFeO 2 − and hematite to form magnetite, meanwhile some of the insoluble Ti-Fe compounds have low iron content, therefore only a small amount of iron powder is needed to eliminate the inhibition. These results are important for the efficient digestion of diasporic bauxite through the addition of iron powder, which avoids the loss of alumina through the hydro-garnet produced by adding excess lime. Highlights • FeTiO 3 , Fe 2 TiO 4 and NaFeTiO 4 formed in the reductive Bayer digestion. • A fibrous porous structure mainly comprised of NaFeTiO 4 was observed. • The elimination of sodium titanate occurs prior to iron minerals transformation. • Achieving 98.91% relative alumina recovery by adding 1% iron powder of bauxite. [ABSTRACT FROM AUTHOR]

The simultaneous transformation and dissociation of minerals containing iron in reductive Bayer digestion of bauxite are important to separate iron minerals from red mud and thus reduce the waste solid disposal in alumina production. In order to better understand the reaction mechanism of iron and hematite in sodium aluminate solution, the phase composition, valence variation and micro-morphology of transformation products were studied by XRD, XPS, FT-IR and SEM respectively. The results show that the intermediate alumogoethite, α-Fe (1 − x) Al x OOH, preferentially forms by the reaction of HFeO 2 − and AlO 2 − and then converts to magnetite in Fe-Na 2 O-Al 2 O 3 -H 2 O system at digestion temperature of 533 K. Meanwhile, the transformation of hematite to magnetite in Fe-Fe 2 O 3 -Na 2 O-Al 2 O 3 -H 2 O system sequentially involves HFeO 2 − generation by dissolution of iron powder, HFeO 2 − diffusion to hematite particles, and magnetite formation on hematite particles by reaction of HFeO 2 − and hematite or dissolved FeO 2 − . Fe addition can prevent hematite particle from becoming much smaller by surface magnetization in reductive Bayer digestion compared with current industrial Bayer digestion. This work is conducive to develop a novel technology for efficiently processing high-iron diasporic bauxite. [ABSTRACT FROM AUTHOR]

During the high-temperature Bayer process, the inhibitory effect of titanium-containing minerals on diaspore digestion is believed to be resulted from the formation of a dense sodium titanate layer covering the diaspore particles, however, there is a lack of direct observational evidence to date. This work adopted X-ray photoelectron spectroscopy (XPS), cross-section SEM and in-situ analysis to clarify the inhibitory mechanism by investigating the interactions between Al-/Ti-/Si-bearing minerals in the simulated high-temperature Bayer digestion process. The observation of a sodium titanate layer provided direct evidence for the solid layer theory. XPS result further indicates that the Ti-O-Al bond is a significant factor in promoting the covering of sodium titanate on alumina-bearing mineral particles. Cross-section SEM and in-situ analyses illustrate that the dissolution of diaspore is carried out not only on the surface of diaspore particles but also on the inner surface such as pores and gaps of particles. Meanwhile, as similar to Ti-bearing minerals, the Si-bearing minerals also inhibit the diaspore digestion by forming a sodium aluminate silicate hydrate layer at an elevated temperature. • Direct observation of a sodium titanate layer on Al-bearing minerals. • Ti-O-Al bond promotes the covering of sodium titanate on disapore. • Sodium aluminate silicate hydrate layer also inhibits the diaspore digestion. • A better method to observe the changes at mineral-solution interface. [ABSTRACT FROM AUTHOR]

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 HFeO 2 − 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. • Magnetite forms primarily through the hematite reacting with HFeO 2 −. • Fixation of DSP inhibits the direct adsorption of Fe (II) ions onto the iron mineral surface. • The contributions of Si-bearing minerals to the inhibitory effect are significantly different. [ABSTRACT FROM AUTHOR]