Red Mud as a Secondary Resource of Low-Grade Iron: A Global Perspective

Shoppert A., Loginova I., Napol’skikh J., Kyrchikov A., Chaikin L., Rogozhnikov D., Valeev D.

Bauxite residue, known as “red mud,” is a potential raw material for extracting rare-earth elements (REEs). The main REEs (Sc, Y, La, Ce, Nd, Nb, and Sm) from the raw bauxite are concentrated in RM after the Bayer leaching process. The earlier worldwide studies were focused on the scandium (Sc) extraction from RM by concentrated acids to enhance the extraction degree. This leads to the dissolution of major oxides (Fe2O3 and Al2O3) from RM. This article studies the possibility of selective Sc extraction from alkali fusion red mud (RMF) by diluted nitric acid (HNO3) leaching at pH ≥ 2 to prevent co-dissolution of Fe2O3. RMF samples were analyzed by X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and inductively coupled plasma mass spectrometry (ICP-MS). It was revealed that Sc concentration in RMF can reach up to 140–150 mg kg−1. Sc extraction was 71.2% at RMF leaching by HNO3 at pH 2 and 80 °C during 90 min. The leaching solution contained 8 mg L−1 Sc and a high amount of other REEs in the presence of relatively low concentrations of impurity elements such as Fe, Al, Ti, Ca, etc. The kinetic analysis of experimental data by the shrinking core model showed that Sc leaching process is limited by the interfacial diffusion and the diffusion through the product layer. The apparent activation energy (Ea) was 19.5 kJ/mol. The linear dependence of Sc extraction on magnesium (Mg) extraction was revealed. According to EPMA of RMF, Sc is associated with iron minerals rather than Mg. This allows us to conclude that Mg acts as a leaching agent for the extraction of Sc presented in the RMF in an ion-exchangeable phase.

Kannan P., Banat F., Hasan S.W., Abu Haija M.

Bauxite residues from the alumina industry are highly alkaline and should be neutralized before using them for industrial or agricultural purposes. This review evaluates the effectiveness of seawater and other brines as neutralizing agents, which have advantages over alternatives such as mineral acids. Brines not only lower pH but also prevent reversion by immobilizing alkalinity in the form of Ca and Mg minerals such as hydrotalcite, which can also bind to potential contaminants. The analysis of solid residues, leachates, and supernatant liquors from brine neutralization demonstrated that Ca- and Mg-rich brines (natural and synthetic) can outperform seawater by lowering the pH and total alkalinity of the bauxite residue. The use of industrial waste brines to lower the alkalinity of bauxite residue can be a cost-effective and environmentally friendly “waste management by waste” technique. • Bauxite residue neutralization by various brines was reviewed. • Reduction of residue pH and alkalinity using brines was well demonstrated. • Ca- and Mg- rich brines enhance the long-term stabilization of product residue. • Large-scale use of industrial brines for residue neutralization appears promising.

Cao Y., Sun Y., Gao P., Han Y., Li Y.

As an alternative reductant for fossil fuel in the future, straw-type biomass contributes to emission reduction and green utilization in the suspension roasting process. In this study, the influences of the roasting time, roasting temperature and dose of straw-type biomass after suspension magnetization roasting (SMR) and separation were investigated. The optimal conditions were determined to be a roasting time of 7.5 min with a straw-type biomass dose of 20 wt% and a roasting temperature of 800 ℃, in which an iron grade of 71.07% and recovery of 94.17% were obtained for the iron concentrate. The maximum saturation magnetization under optimal conditions was 35.05 A·m 2 ·g −1 , and the gaseous regulation of the biomass revealed that cumulative reducing gas volume was 293.93 mL at the optimal roasting time of 450 s. The transformation of hematite to magnetite was detected by X-ray diffraction (XRD). During microstructure evolution, the outer layer consisting of fissures and tiny holes continuously deepened toward the core.

Liu X., Han Y., He F., Gao P., Yuan S.

Red mud (RM) is the major waste material with strong alkaline discharged which is during the alumina extraction process. The global stock of RM has exceeded 4 billion tons and its disposal as a solid waste has always been a thorny environmental problem. However, RM is widely considered to be a potential resource due to its high content of valuable metal components such as iron. High-iron RM is rich in iron and can potentially become a valuable resource if the iron can be extracted effectively. It is of great research value and profound significance to recover iron from high-iron RM. This paper systematically reviews the iron recovery methods for resource utilization of high-iron RM, and divides the technology of iron recovery from high-iron RM into three aspects: physical separation method, pyrometallurgy method (reduction smelting and reduction roasting) and hydrometallurgy method (acid leaching). The basic principles and effect of iron extraction of the above technologies are summarized respectively, and the advantages and disadvantages of different technologies are compared. It is pointed out that the feasibility and economic cost are the main factor restricting the industrial application of these technologies. Therefore, it is of great significance to overcome various problems and difficulties, and develop innovative processes and technologies, which can realize the recycling and utilization of iron in high-iron RM and realize the reduction of RM emission at the same time. • The hazards caused by red mud and trends in red mud research were presented. • The research status of iron recovery methods from high-iron red mud is reviewed. • The advantages and disadvantages of several iron recovery methods are compared. • Technologies suitable for industrial production needs to be studied in the future.

Agrawal S., Dhawan N.

• Red mud is evaluated as a polymetallic source for Fe, Ti , Al, Sc. • Scandium associated with Ti > Fe > Al and REEs occur as discrete phases or adsorbed on mineral surfaces. • Metallurgical application of red mud is important for the sustainable supply of critical metals. • This exhaustive review reports metal extraction by pyro- and hydro-metallurgical processes, from laboratory to pilot scale. • Proposed route for recovery of metallic values and valorization of red mud. Red mud is an industrial waste generated during alumina production containing residual minerals of bauxite ore. Significant metallic values with appreciable concentration make it a potential polymetallic source. The current red mud management involves storage in artificial ponds/dams or dry stacked in open areas, which poses an environmental risk, and disposal processes are costly. Recently, red mud is utilized for cement production; however, it results in the loss of valuable minerals, which could be strategically advantageous for resource-deficient nations. Red mud utilization for producing concentrate can significantly improve process efficiency of alumina production, reduce industrial liability and environmental impact. The globally generated red muds are compared according to the mineralogy, composition, and associated values. Hematite is the chief constituent in the red mud, with anatase, gibbsite, goethite, quartz, and desilication phases such as cancrinite, sodalite, and olivine in a significant amount. Red mud originated from karst bauxites containing higher rare-earth content as adsorbed ions on the mineral surfaces, isomorphous substitution, and discrete rare-earth minerals such as bastnaesite, xenotime, chuchite, and allanite. Scandium holds 95% of the rare-earth economic value in the red mud and is associated with hematite, goethite, and anatase. The paper presents a critical overview of the laboratory, pilot, and commercial processes employed to recover iron, aluminum, titanium, scandium, and REEs from various red muds. The response parameters such as alumina-to-silica ratio, iron grade, total rare-earth elements were used to determine the appropriate processing route for recovery of metals from red mud. An integrated process is proposed to recover metallic values from the red mud while ensuring minimal waste generation.

Panda S., Costa R.B., Shah S.S., Mishra S., Bevilaqua D., Akcil A.

The growing demand for rare earth elements (REEs) confronted with a parallel supply risk, draws major interest to utilize secondary resources bearing higher REE content than the primary resources. The European Commission has recently identified bauxite as a Critical Raw Material (CRM). In particular, unexploited bauxite residues have invited due attention owing to their abundance (worldwide generation at 120 – 150 million tons/yr) and presence of REEs (0.5 - 1.7 kg/ton) and scandium (Sc) in particular, with Fe: 14–45%, Al: 5–14%, Si: 1–9%, Na: 1–6% and Ti: 2–12%. Nevertheless, it has also to be taken into consideration that higher amassing of this waste is turning into a global concern due to its hazardous impacts and disposal issues owing to its high alkalinity, fine particle size and metal content. Industrial valorization of REEs from stockpiled bauxite residues could possibly unlock approximately a 4.3 trillion-dollar economy globally. This review foresees bauxite as a potential resource for REEs and identifies the problems associated with disposal of bauxite residues. Considering the recycling potential of bauxite residues for supplying valuable metals for technology, biotechnology is seen as a promising alternative to the conventional methods. Comprehensive details including role and challenges of biotechnology in green recovery of REEs from bauxite residues, their scale-up and environmental issues are critically discussed. Furthermore, w.r.t. the bauxite residues, the REE market potential is presented with discussions into future prospects, following the current impact of COVID-19 pandemic on the demand and supply of REE to industrial sectors.

Jin J., Liu X., Yuan S., Gao P., Li Y., Zhang H., Meng X.

• A novel co-roasting of red mud and coal gangue is presented. • Fe and Al minerals could be effectively separated and enriched by co-roasting. • Concentrate of the iron content of 57.25% and recovery of 65.22% is obtained. • Fe content in magnetite is raised to 67.30% in product from 0.26% in raw material. • Resource utilization of red mud and coal gangue is realized. Red mud and coal gangue are industrial solid wastes discharged during alumina extraction and coal mining, respectively. As these are hazardous materials, their disposal leads to serious environmental issues. In this study, an innovative utilization of red mud through co-roasting with coal gangue for separation and recycling of iron and aluminum minerals is presented. Under optimum co-roasting conditions (550 °C for 50 min), an iron concentrate containing 57.25% TFe (total iron content) with the recovery of 65.22%, and an aluminum-rich product containing 27.26% Al 2 O 3 with the recovery of 71.37% were obtained after magnetic separation. The characteristics of mixed raw material and products all indicated that the goethite and hematite phases in the mixed raw material were transformed into a magnetite phase after co-roasting, although some of the magnetite generated during co-roasting was oxidized to hematite again. This study demonstrates that co-roasting of coal gangue and red mud is a promising technology for the reduction of iron and activation of aluminum to realize resource recycling without additional materials.

Pasechnik L.A., Skachkov V.M., Chufarov A.Y., Suntsov A.Y., Yatsenko S.P.

Storage of red mud – bauxite processing waste – leads to serious environmental problems due to its high alkalinity and particle dispersity. Full or partial utilization or recycling of red mud could reduce the harmful effect on the environment. Scandium is the most valuable ingredient of red mud, yet it's extraction is poorly commercialized due to its high cost. The new efficient extraction technologies promise an ensured supply of scandium and a significant drop in cost. Here, scandium concentrate, extracted from leachate after carbonate treatment of red mud, was subjected to sulfatisation by H 2 SO 4 to separate silica from water-soluble sulfates. To recover and selectively separate scandium from other impurity metals, the crystallization of two complex scandium and ammonium sulfates – NH 4 Sc(SO 4 ) 2 and (NH 4 ) 3 Sc(SO 4 ) 3 – is proposed. The solubilities of these sulfatoscandiates in water, established by isothermal method, are 33.4 and 72.4 g/L, respectively. For the less soluble NH 4 Sc(SO 4 ) 2 a further considerable reduction of solubility has been observed in H 2 SO 4 solutions of concentration above 3.5 М in the presence of 0.5 М NH 4 Cl at 20 ± 1 °C. More than 99% of scandium in the form of micron-sized NH 4 Sc(SO 4 ) 2 crystals has been recovered from a multicomponent liquid at 5–6 М H 2 SO 4 and 0.5 М NH 4 Cl. The product contains extremely low levels of impurities. The precipitation of NH 4 Sc(SO 4 ) 2 offers a much higher selectivity in separation of Sc from the other main constituents, as demonstrated by the large separation coefficients between scandium and other metals β Sc/M (e.g., for the couple with aluminum β Sc/Al = 4280). The recrystallization product after calcination at 1000°С contains 99% Sc 2 O 3 . • Sulfuric dry digestion is applied in Sc extraction without silicon dissolution. • NH 4 Sc(SO 4 ) 2 and (NH 4 ) 3 Sc(SO 4 ) 3 are crystallization products from sulfate media. • A decline of NH 4 Sc(SO 4 ) 2 solubility is observed with increasing acidity. • Efficiency of selective precipitation of Sc(III) in solid form, NH 4 Sc(SO 4 ) 2 , reaches 99%. • Attractive separation coefficients for Sc among Al, Fe Ti, Zr and Ca are obtained.

Li S., Pan J., Zhu D., Guo Z., Shi Y., Dong T., Lu S., Tian H.

Red mud is one kind of by-products rejected from industrial production of alumina plant and characterized by high content of alkaline and containing many valuable metals such as iron, aluminum, titanium, etc. It is urgent to effectively utilize red mud for the sustainable development of aluminum metallurgy industry. In this paper, a new route was proposed to fully recover Fe, Al 2 O 3 and TiO 2 from the red mud, which includes three procedures, i.e. pre-reducing-smelting, alkaline leaching and acid leaching. The pre-reducing-smelting step was employed to recover iron from the mud, producing pig iron at an iron recovery of some 98.15% and slag containing 43.17%Al 2 O 3 and 15.71%TiO 2 , respectively. Then, based on the thermal dynamic calculations, a modification of the slag-alkaline leaching and hydrochloric acid leaching process were conducted to obtain the stepwise recovery of Al 2 O 3 and TiO 2 , respectively, from the smelting slag. Aluminum and titanium were recovered in the form of sodium aluminum solution and perovskite at a recovery of 85.85% and 95.53%, respectively. The innovative route is probably a potential way to achieve comprehensive and clean utilization of red mud.

Cardenia C., Balomenos E., Wai Yin Tam P., Panias D.

In this study an integrated process is presented as a suitable method to transform Fe3+ oxides present in bauxite residue into magnetic oxides and metallic iron through a microwave roasting reduction, avoiding the formation of hercynite (FeAl2O4). In the first step, all the alumina phases were transformed into sodium aluminates by adding sodium carbonate as a flux to BR and then leached out through alkali-leaching to recover alumina. Subsequently, the leaching residue was mixed with carbon and roasted by using a microwave furnace at the optimum conditions. The iron oxide present in the sinter was converted into metallic iron (98%). In addition, hercynite was not detected. The produced cinder was subjected to a wet high intensity magnetic separation process to separate iron from the other elements.

Zhang J., Yao Z., Wang K., Wang F., Jiang H., Liang M., Wei J., Airey G.

• Red mud is a huge stock of industrial waste and causing serious environmental problems. • The physical–chemical behaviors of red mud were reviewed and its potential properties were analyzed. • The utilization of red mud in road bases and asphalt mixtures were reviewed and future research priorities were pointed out. Sustainable utilization of the bauxite residue (red mud) generated from the alumina refining has recently increased due to increased environmental concerns because of its high alkalinity and problematic pollutants when placed in landfills. This paper attempts to review recent research findings of utilizing red mud as a road material in pavement structures in literature, including road bases and asphalt mixtures. Previous laboratory investigations indicated that red mud is feasible to be used as a raw material in road bases with satisfactory unconfined compressive strength (UCS), frost resistance and durability performance, but their durability during the service life of the pavement is still unclear. The incorporation of red mud in asphalt mastics was found to be able to improve its stiffness, such as softening point, complex modulus, and viscosity. In terms of asphalt mixtures, replacing limestone filler in mixtures by red mud resulted in some positive effects on mechanical behaviors, high bulk density and good rutting resistance. However, the moisture susceptibility and raveling resistance of asphalt mixtures became relatively worse because of the addition of red mud. Referring to this overview, it is necessary to qualify the long-term service performance of road base materials prepared with red mud by using laboratory accelerating evaluation or road trials. For asphalt mixtures prepared with red mud, how to improve durability, in particular moisture resistance, will be a future research focus. In addition, the influence of red mud on cracking resistance at low temperature, fatigue properties and long-term durability of asphalt mixtures is still needed to be further explored.

Li S., Kang Z., Liu W., Lian Y., Yang H.

A large amount of red mud is discharged in the aluminum oxide production process, which contains a variety of valuable metals and is considered as a secondary resource. In order to reveal the mechanism of red mud carbon thermal reduction process, isothermal reduction experiments on carbon-bearing pellets of red mud were investigated with biomass carbon as a reducing agent. In this study, the reduction temperatures were conducted using a microwave stove in the temperature range from 850 to 1250 ℃, and the C/O molar ratio was 1.1. The results show that the reduction process of red mud is governed by a carbon gasification reaction, and the apparent activation energy is 88.44 kJ/mol. The optimum reduction process conditions were established to be 1150 ℃ for 13 min.

Bao Q., Guo L., Guo Z.

Phosphorus (P) is one of the most deleterious elements in iron ore, it is easy to form iron phosphides that make steel brittle during reduction processes. A new process (direct reduction and flash smelting separation (FSS)) of treating high phosphorus iron ore (HPIO) is introduced in this paper. By limiting the smelting separation between slag and iron to the level of about 1 s, the reduction and migration process of P element was restrained, and the content of P element in the metallic iron obtained after separation was limited to about 0.3 wt%. The effects of flash smelting temperature and ore particle size on the flash smelting process were studied. The migration process of phosphorus was analyzed, and a mathematical model of this smelting process was established to facilitate the visualized understanding and regulation of this process. • The flash smelting separation method is introduced in this study. • The metallic iron will melt into a spherical ball wrapped by slag phase. • The rapid smelting separation hinders the reduction and diffusion of P. • Proper particle size is suggested when using this method. • The dephosphorization behavior can be interpreted with the mathematical model.

Agrawal S., Dhawan N.

The red mud is an abundantly generated by-product of the aluminum industry rich in iron values mainly in the form of hematite and goethite phase. The favorable dielectric properties of iron oxide enable efficient microwave heating of red mud, and ~750 °C is achieved in 10 min with 10.7% C at 800 W. The heating rate in the mixture depends on the charcoal dosage and phase composition. The theoretical thermal decomposition kinetics suggest that the hematite to magnetite conversion in the red mud is chemically controlled and requires activation energy of 20.95 kJ/mol. The carbon gasification findings suggest that the CO formation is initiated below the bulk temperature of 700 °C during microwave heating, possibly because of accelerated microwave-induced heating in carbon having a significant effect on the reduction reaction mechanism. Volumetric heating enables efficient reduction and iron enrichment in magnetic concentrate with iron grade 52% and ~98% recovery constituting the magnetite phase majorly in 10 min, consuming 478.8 kJ energy. The iron metallization is dependent on the charcoal dosage, and ~15% metallic iron is formed in 30 min with 1435.4 kJ energy expenditure. The non-magnetic fraction is leached in 0.5 M HCl to recover alumina values in the mullite phase with an Al purity of 48% suitable for refractory applications. The non-magnetic residue containing iron values can be mixed with magnetic fraction to increase the Fe-product yield to ~84%.

Harini K., Karthiyaini S., Shanmugasundaram M.

Khanna R., Konyukhov Y., Zinoveev D., Li K., Maslennikov N., Burmistrov I., Kargin J., Kravchenko M., Mukherjee P.S.

The present study developed a novel approach for transforming red mud (RM) into soft magnetic materials (SMMs) for applications in advanced electrical devices in the form of Fe-Si and Fe-Si-Al alloys. A total of ten blends were prepared based on two RMs, three iron oxide additives (Fe2O3, black and red mill scales), alumina and carbonaceous reductants in a range of proportions. Carbothermic reduction of the blends was carried out in a vertical Tamman resistance furnace at 1600–1650 °C for 30 min in an argon atmosphere; synthetic graphite was used as a reductant. Reaction products were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray fluorescence (XRF) and X-ray diffraction (XRD). Significant amounts of Fe-rich metallic droplets/regions of different grain sizes (0.5 to 500 μm) were produced in these studies. The formation of Fe-Si alloys with Si contents from 3.9 to 6.7 wt.% was achieved in 8 out of 10 blends; the optimal levels of Si for SMMs ranged from 3.2 to 6.5 wt.%. There was clear evidence for the formation of Fe-Si-Al (up to 1.8 wt.% Al) alloys in 4 out of 10 blends. In addition to lowering operating challenges associated with RM processing, blending of RMs with iron oxide additives and alumina presents a novel recycling approach for converting RMs into valuable SMMs for possible emerging applications in renewable energy, storage, electrical vehicles and other fields. Along with reducing RM stockpiles across the globe, this approach is expected to improve resource efficiency, mitigating environmental impacts while generating economic benefits.

Huang Z., Li A., Zhou W., Li J., Zhang J.

The effects of red mud on the microstructures and high-temperature tensile properties of the ZL109 aluminum alloy have been investigated. Red mud/ZL109-based composite materials with added red mud (a major byproduct of the aluminum industry), which has been coated with nickel by chemical deposition, have been prepared through gravity casting. The results show that the addition of red mud promotes the alloy’s microstructure and helps to uniformly distribute the eutectic silicon. It also increases the content of heat-resistant phases, such as the Q-Al5Cu2Mg8Si6 and γ-Al7Cu4Ni phases. These changes significantly enhance the alloy’s high-temperature tensile properties. The alloy with 1% (wt.%) red mud exhibits the best tensile strength at both room temperature and 350 °C, reaching 295.4 MPa and 143.3 MPa, respectively. The alloy with 1.5% (wt.%) red mud demonstrates excellent performance at 400 °C, achieving a tensile strength of 86.2 MPa through the cut-through method and Orowan mechanism. As a reinforcing material, red mud not only improves the high-temperature resistance of the aluminum alloy but also provides a way to recycle industrial waste. This study offers a new way to address the red mud waste problem and helps develop high-performance, heat-resistant aluminum alloys. It shows the potential of these alloys in high-temperature applications.

Shoppert A., Loginova I., Diallo M.M., Valeev D.

The use of reduction leaching in the production of alumina from bauxite by the Bayer process in order to decrease the amount of waste (bauxite residue) by adding elemental iron or aluminum, as well as Fe2+ salts and organic compounds in the stage of high-pressure leaching, requires the purchase of relatively expensive reagents in large quantities. The aim of this study was to investigate the possibility of the use of electrolytically reduced bauxite residue (BR) as a substitute for these reagents. Reduced BR was obtained from Al-goethite containing BR using a bulk cathode in alkaline suspension. The degree of deoxidation of Fe3+ compounds was 55% after 2 h of electrolysis with a current yield of more than 73%. The addition of reduced BR according to the shrinking core model leads to a change in the limiting stage of the high-pressure boehmitic bauxite leaching from a surface chemical reaction to internal diffusion. The activation energy decreased from 32.9 to 17.2 kJ/mol by adding reduced red mud. It was also shown that the addition of reduced BR increased the rate of thickening of the slurry after leaching by a factor of 1.5 and decreased the Na2O losses by 15% without the addition of lime. The solid residue was examined by means of X-ray diffraction analysis and scanning electron microscopy to confirm the presence of magnetite and elemental iron. A preliminary techno-economic analysis was carried out to assess the applicability of the proposed process.

Cheng F., Pang J., Berggren S., Tanvar H., Mishra B., Arlos M.J.

Mousavi S., Damizia M., Hamidi R., De Filippis P., de Caprariis B.

Angelopoulos P.M., Oustadakis P., Anastassakis G., Pissas M., Taxiarchou M.

Akhtar M.S., Ali S., Zaman W.

The growing presence of diverse pollutants, including heavy metals, organic compounds, pharmaceuticals, and emerging contaminants, poses significant environmental and health risks. Traditional methods for pollutant removal often face limitations in efficiency, selectivity, and sustainability. This review provides a comprehensive analysis of recent advancements in innovative adsorbents designed to address these challenges. It explores a wide array of non-conventional adsorbent materials, such as nanocellulose, metal–organic frameworks (MOFs), graphene-based composites, and biochar, emphasizing their sources, structural characteristics, and unique adsorption mechanisms. The review discusses adsorption processes, including the basic principles, kinetics, isotherms, and the factors influencing adsorption efficiency. It highlights the superior performance of these materials in removing specific pollutants across various environmental settings. The practical applications of these adsorbents are further explored through case studies in industrial settings, pilot studies, and field trials, showcasing their real-world effectiveness. Additionally, the review critically examines the economic considerations, technical challenges, and environmental impacts associated with these adsorbents, offering a balanced perspective on their viability and sustainability. The conclusion emphasizes future research directions, focusing on the development of scalable production methods, enhanced material stability, and sustainable regeneration techniques. This comprehensive assessment underscores the transformative potential of innovative adsorbents in pollutant remediation and their critical role in advancing environmental protection.

Vasyunina N.V., Dubova I.V., Druzhinin K.E., Gilmanshina T.R.

High-iron Bayer red mud, containing over 30% of iron, is considered low-grade iron ore. Due to the global iron deficiency in recent decades, the effective utilization of the iron contained in high-iron red mud has received increasing attention. In this work, a technological scheme was developed for the extraction of iron into cast iron from red mud by smelting reduction, followed by rapid cooling to separate the metal from the slag. The influence of various experimental parameters, including temperature, basicity, and reduction time, on the recovery of iron from red mud was studied in detail. The results demonstrated that the separation of metal from slag was complete. The maximum extraction of iron into cast iron was obtained at a temperature of 1450 °C, with approximately 88.5% achieved in the absence of sodium carbonate and 91.5% with sodium carbonate. The optimal experimental result is of great importance for the large-scale and highly efficient recycling of red mud.

Yu H., Liu L., Liu M., Zhang H., Mao R.

Red mud (RM) and phosphogypsum (PG) are both large-scale industrial solid wastes, which cause waste of resources reserved in them and pose a huge threat to the environment if not treated appropriately. Guide by the concept of "waste control by waste", the individual and cooperated effect of PG and sodium sulfate (Na2SO4) on iron recovering from RM through reductive roasting-magnetic separation process was comparably investigated. Systematical factorial tests verified the advantage of binary additive. An iron concentrate with iron grade and recovery of 89.02 % and 87.46 % was obtained under the optimized conditions. Mechanistic studies demonstrated that iron oxides were converted to metallic iron through reductive roasting, along with which PG was beneficial for the releasing of iron from complex iron-hosting minerals to a certain extent while binary additive was even more efficient for the reducing, decomposing and grain growing processes. This work realizes the simultaneous utilization of both waste resources.

Kannan R., Stevens A.G., Nandwana P.

This study highlights the potential of red mud/bauxite residue, an industrial waste product generated during alumina extraction as a flux for iron and steelmaking. The thermodynamic feasibility of using red mud as a flux in the presence of different reducing agents is evaluated. Potential approaches to decarbonize iron and steel production while utilizing red mud as a flux, as well as the benefits of red mud remnant after iron extraction, are discussed.

Zhen Z., He C., Wang Y., Ma H.

A synthetic flocculant of aluminum (Al) and iron (Fe) extracted from red mud (RM) has been widely used in sewage treatment, while the remaining RM residue has been ignored. This study aimed to synthesize polymeric aluminum ferric sulfate (PAFS) flocculant from RM by acid leaching and then use the acidified RM residue to produce an acid RM-based ceramsite (ARMC) by mixing bentonite, hydroxypropyl methylcellulose, and starch. Our results showed that sintering, reaction temperature, H2SO4 concentration, reaction time, and liquid-to-solid ratio had an obvious effect on the leaching of Al and Fe in RM, which was a necessary prerequisite for the efficient PAFS flocculants. At a PAFS dosage of 60 mg/L, turbidity and phosphate removal rates were 95.21 ± 0.64% and 89.17 ± 0.52%, respectively. When the pH value was 8.0, the turbidity and phosphate removal efficiency were 99.22 ± 0.66% and 95.98 ± 1.63%, respectively. Considering the adsorption capacity and mechanical properties, the best conditions for ARMC production included using 60% ARM and ceramsite calcination at 600 °C, with the BET surface area 56.16 m2/g and a pore volume of 0.167 cm3/g. Thermogravimetric analysis indicated that 400 °C was a reasonable preheating temperature to enhance the ARMC mechanical strength, as this temperature allows the removal of surface-adsorbed and constituent water. Under a scanning electron microscope, the ARMC appeared rough before adsorption, while relatively uniform pores occupied it after adsorption. Our conclusion will help to improve the zero-waste strategy of RM and speed up the industrial production of RM in flocculants as well as utilizing ARMC as a new type of adsorbent for phosphorus adsorption in sewage treatment.

Xu X., Guo Z., Zhu D., Pan J., Yang C., Li S.

Non-Ferrous Metallurgical Wastes, including copper slag, red mud, jarosite residue, zinc-bearing dust, pyrite cinder, and nickel slag, are hazardous waste products released during the metal extraction process. Metallurgical solid wastes pose significant risks to the environment due to their content of hazardous elements, which may impact the quality of surface water and groundwater. However, the iron content in non-ferrous metallurgical solid wastes such as copper slag, red mud and zinc-containing dust can often reach 30–50%, which is higher than the average iron grade (27%) of the iron ores found in China and has a high utility value. In this manuscript, we summarize the primary optimization process of the coal-based direct reduction-magnetic separation method for metallurgical recovery and further purification of metallurgical solid waste. We discuss technological advancements, including rotary kiln, rotary bottom furnace, and tunnel kiln methods, and address environmental and economic aspects, especially in coal-rich regions with limited natural gas resources. Additionally, we have highlighted the technical challenges and developmental bottlenecks of the direct reduction-magnetic separation process; our findings reveal that the improved processes, characterized by high recycling efficiency and low energy consumption and secondary environmental pollution, continue to be the focus of research and development for the sustainable utilization of metallurgical solid waste.

Kong H., Zhou T., Wang Z., Li C., Zhang M., Yang H.

AbstractPyrometallurgical method of iron recovery from red mud (RM) has advantages of simple procedures, high recovery efficiency and significant waste minimization. The fluidization reduction process using CO as reductant addresses the issues of high energy consumption and long reaction time of pyrometallurgical method. In order to optimize the operational conditions of the fluidization reduction process, it is necessary to study the reaction characteristics of RM and CO under fluidization condition. In response to the problems of the current kinetic study including the unsatisfied fluidization condition and possible errors introduced by the estimation method, we carried out improvements in both experiment and data processing. In the experiment aspect, thermo‐gravimetric analyzer (TGA) test rig with large sample capacity and gas flow was established, and approximate fluidization condition was achieved by intensifying diffusion by increasing the gas flow rate and decreasing the sample mass. In the data processing aspect, we developed a program with data cleaning and kinetic function fitting capabilities, and the goodness of fit was evaluated by Akaike information criterion (AIC). The results indicated that within the temperature range of 500–600°C and CO concentrations of 5%–15%, the reaction between RM and CO can be divided into two steps based on the criterion of complete formation of Fe3O4. The first step reaction has a relatively fast reaction rate, conforming to the F1 kinetic function with rate equation given as . The second step reaction displays a more complex pattern and the fitted rate equation is . The obtained results could provide a reliable reference for the operational design of the fluidization reduction of RM.

Pilla G., Hertel T., Douvalis A.P., Kapelari S., Blanpain B., Pontikes Y.

Bauxite residue (BR) is a valuable polymetallic resource, but its utilization is hindered by challenges such as high alkalinity, fine particle size, and complex mineral composition. Hydrogen reduction is a promising approach for recovering metals from BR, fostering a sustainable circular economy. Successful metal recovery necessitates comprehensive material characterization. This study investigates BR reduction from 400 to 700 °C, varying time (30–120 min), and NaOH addition (10–25 wt %) under different H2 environments to maximize magnetite (Fe3O4) and water-soluble aluminate (NaAlO2) formation in reduced pellets to extract valuables (such as Fe, Al, Ti, and REEs including Sc). The reduced products are analyzed using different analytical techniques. H2 reduction conditions and parameter interactions are explored using response surface methodology (RSM). The findings reveal that NaAlO2 formation increases until 700 °C, and complete hematite to magnetite conversion occurs at 500 °C. Metal recovery can be hindered by the formation of sodium iron oxide, wüstite, sodium aluminum silicate with the higher H2 flow rate (≥45 L/h (or H2 amount ≥450 L/(h.kg)) under 5 vol % H2 + 95 vol % N2 environment or with 20 L/h (or H2 amount 200 L/(h.kg)) under 100 vol % H2 concentration), >600 °C, and ≥25 wt % NaOH. These trends are aligned with thermodynamic calculations. The study advances the understanding of H2 reduction of BR for sustainable processes and polymetallic resource recovery.