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In 2021, global alumina production reached over 135 million tons and discharged approximately 200 million tons of red mud, leading to severe environmental safety hazards. More than 70% of raw materials originated from gibbsitic bauxite. This work reviews existing and potential process from open literature to treat gibbsitic bauxite and reduce red mud discharge from the alumina production process, which are divided into two categories. The first strategy involves non-Bayer methods such as smelting reduction, reduction roasting, sub-molten salt, ammonium sulfate roasting, acid leaching, and calcification–carbonation. The current obstacle to implementing these methods is the lack of economics, making industrial application challenging. The second potential industrial application strategy is to modify the current Bayer process, with critical measures involving the enhancement digestion of inert aluminum-bearing minerals, dissociation and recovery of iron minerals, and separation of desilication products. Analyzing the reaction mechanism and process flow clarify that the modified Bayer method is more favorable to the cost-effectively and synergistic extraction of aluminum, iron, and other elements. The red mud obtained by the modified Bayer process can be efficiently utilized. Therefore, a mini-summary of the areas where red mud is expected to be substantially dissipated (e.g., the steel industry, cement building materials, and ecological restoration of red mud stockpiles) is provided. The review can advance the current technology on the comprehensive utilization of gibbsitic bauxite, especially process discharge reduction approaches and large-scale abatement ways for red mud. Our results can contribute to the future development of sustainable green alumina production. [Display omitted] • Resources distribution of gibbsitic bauxite and its red mud characteristics were investigated. • An overview of the current Non-Bayer process for treating gibbsitic bauxite was summarized. • The novel technical route to red mud discharge reduction by modified Bayer process was proposed. • Promising areas for large-scale red mud utilization and their challenges were reviewed. [ABSTRACT FROM AUTHOR]

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]

The current Bayer process focused on alumina extraction without considering the comprehensive utilization of other valuable components, such as iron, and the resulting red mud caused severe ecological and environmental safety risks. This work proposed a clean two-stage Bayer digestion technology to obtain iron-rich red mud with low Na 2 O content from high-iron gibbsitic bauxite processing. The effects of digestion conditions on alumina extraction rate and iron enrichment degree in red mud were studied systematically. The ICP, XRD, SEM, and TEM were used to investigate the chemical compositions, phase structures and microstructures of bauxite and red mud. Results show that approximately 83% Al 2 O 3 was extracted, and about 43% SiO 2 was separated simultaneously in gibbsitic bauxite in the first stage, whereas undigested aluminum-bearing minerals mainly originated from substituted aluminum in goethite. Hydrogen was produced by the reaction of glycerol with sodium aluminate solution in the second stage, which promoting the reaction of Al-goethite with titanium-bearing minerals to form Fe(Ⅱ) and Ti(Ⅳ)-substituted hematite. The increase in reductants amount, caustic alkali concentration, reaction temperature, and time favored Al-goethite transformation in digestion. The total Al 2 O 3 recovery and the Fe 2 O 3 in red mud could reach more than 99% and 86%, respectively. Moreover, the content of Al 2 O 3 , SiO 2 , and Na 2 O in red mud satisfied the iron ore concentrate standard of GB/T 36704-2018, which was attributed to avoiding the generation of desilication products (DSP) in red mud. The 93% silicon could be removed from the liquid phase by two separations of the digested slurry. This work will facilitate the large-scale reduction of red mud discharge through further the value-added use of novel red mud technology in collaboration with steel and aluminum, achieving the green and sustainable development of the alumina industry. [Display omitted] • Most of the SiO 2 was removed through the two times slurry separation. • Glycerol promoted the conversion of Al-goethite. • An iron-rich red mud with rather low Na 2 O/SiO 2 /Al 2 O 3 content could be obtained. • A new cleaner production process of high-iron gibbsitic bauxite was proposed. [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]