Summary of Research Progress on Metallurgical Utilization Technology of Red Mud

Wang K., Liu Y., Dou Z., Lu G., Li X., Zhang T.

A novel method of extracting iron from high-iron red mud and preparing low-carbon cement clinker from tailings was proposed. Thermodynamics of smelting reduction were calculated. The effects of different conditions on iron recovery were investigated systematically. Properties of cement samples prepared from water quenched slag were analyzed. Results showed that increasing temperature, Na2O content and C/S and decreasing A/S appropriately were beneficial to increase the liquid content in slag. Iron recovery was 97.6% under optimal conditions, and the obtained metal met the industrial standard of pig iron for steelmaking. Vitreous content in water quenched slag was 99%, and its pozzolanic activity was comparable to that of S95 grade ore powder. Due to energy consumption and CO2 emission from decomposition of limestone in calcination process of cement clinker reducing significantly, the CO2 emission was reduced to about 400 kg per ton of cement clinker based on the 50% doping amount.

Li X., Zhang T., Wang K., Lv G., Chao X., Yang X.

In this study, the advantages of the vortex melting reduction treatment of red mud were verified. Vortex melting reduction can improve the feeding rate, promote the reaction and the directional deposition of iron, which was conducive to the separation of slag and gold. The effects of different adding methods, stirring speed and reaction time on iron recovery were investigated by using red mud, aluminum leached slag and calcified slag as raw materials. According to the experiment, the best reaction conditions were that the raw material put into the furnace by rolling pellets, stirring speed 125 RPM, and reaction time 30 min. The results provided an experimental basis for the harmless and high-value utilization of high-iron red mud treated by vortex melting reduction.

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.

Rychkov V., Botalov M., Kirillov E., Kirillov S., Semenishchev V., Bunkov G., Smyshlyaev D.

Bauxite residue (red mud) is a promising anthropogenic source of scandium because only at the aluminum plants of Ural region (Russia) nearly 150 tons of this trace metal appears in the waste dumps annually. This work describes a method of red mud carbonization by carbon dioxide that results on the one hand to scandium leaching and on the other hand decrease of toxicity of the red mud due to conversion of hydroxides to carbonates and decrease of pH value from 12 to less than 9. The ways of increase of scandium leaching degrees based on ultrasonic treatment and mechanical activation were evaluated. It was supposed that carbonated red mud had high sorption activity being the main cause of secondary loss of scandium in the carbonate treatment process.

Wang K., Liu Y., Zhang T., Li X., Chen X.

Abstract High-iron red mud presents a problem due to its alkalinity which leads to significant risks to the environment. In order to realize the harmless and large-scale utilization of high-iron red mud, the smelting reduction experiments were carried out to investigate the reaction mechanism for extraction iron from high-iron red mud. FactSage 6.4 software was used to conduct thermodynamic analysis of the carbon thermal reduction system. The results showed that the direct reduction with carbon involved a process of Fe2O3 → Fe3O4 → FeO → Fe, in which the theoretical required molar ratio of C/O (oxygen in Fe2O3) was 1:1. The maximum degree of iron extraction was 92.8% with anthracite as reductant and 88.8% without anthracite smelting at 1500 °C for 30 min in a graphite crucible. XRD was conducted to analyze the mineral phase of the samples and slags. The results showed that the minerals contained in high-iron red mud were hematite, quartz, rutile, and sodium aluminosilicate hydrate. The blank sample was consisted of hematite, nepheline quartz, and the reduced slag without quenching consisted of perovskite and gehlenite, indicating that the reaction processes occurred from sodium aluminosilicate hydrate to nepheline and then occurred from nepheline to gehlenite in slagging process. The overall smelting reduction process was described as three mass transfer steps and three chemical reaction steps. These results provide useful information for large-scale and harmless utilization of high-iron red mud.

Wei D., Jun-Hui X., Yang P., Si-Yue S., Tao C., Kai Z., Zhen W.

Red mud as the main byproduct generated during alumina extraction from bauxite. Owing to the high iron content in the red mud and the high correlation between iron and scandium during the leaching ...

Archambo M.S., Kawatra S.K.

Red mud waste from aluminum processing was utilized as a material for extraction of iron. Red mud contains a high amount of iron, comparable to feedstock to North American iron mines, and extractin...

Wei D., Jun-Hui X., Yang P., Si-Yue S., Tao C.

Red mud is the main solid waste generated by the aluminum industry for the extraction of alumina. Considering the environmental problems associated with red mud, the recovery of iron from bauxite s...

Liu G., Liu Y., Lv G., Zhang T.

Wet grinding of calcified slag was conducted to improve alumina extraction from red mud by the calcification–carbonization method. The grinding operation conditions and mechanism were investigated. The results showed that wet grinding destroyed the compact structure of spherical hydrogarnet and broke down agglomerates. The rough surface of ground hydrogarnet promoted the formation of aragonite-type calcium carbonate, and then inhibited the formation of a dense production layer on the unreacted particles, which consequently improved carbonization efficiency and alumina extraction. Alumina extraction increased with increased rotation speed within the low speed range, and decreased dramatically over a critical speed. Prolonged grinding time increased alumina extraction until large particles comminuted. Both lower and higher liquid-to-solid ratios decreased alumina extraction due to the formation of adhered layers and the decrease of ball-to-powder collision probability. A smaller ball-to-powder ratio reduced ball-to-powder collisions, whereas excess balls caused energy loss during ball-to-ball collisions, which decreased alumina extraction.

Wang Y., Li X., Zhou Q., Wang B., Qi T., Liu G., Peng Z., Pi J., Zhao Z., Wang M.

A novel design incorporating both reductive Bayer digestion and iron recovery into diasporic bauxite processing is proposed to remarkably reduce red mud discharge. This paper presents a comparative study on reductive versus typical Bayer digestion, after which an industrial validation test was carried out. During the test, relative alumina recovery of 98% and a reduction of 10.9% in the amount of red mud generated were achieved by substituting 2 wt.% iron powder for 10 wt.% lime of bauxite in the high-temperature digestion. Meanwhile, ~ 60% of the iron minerals were converted to magnetite by iron powder, meaning that processing the resulting red mud by magnetic separation could obtain iron concentrate with total iron concentration of 55.2% and iron recovery of 60.1%. The overall reduction of red mud discharge reached 50% for a ton of alumina. The proposed prototype is conducive to improve the Bayer process, aiming to achieve cleaner production of commercial alumina.

Nie Q., Hu W., Huang B., Shu X., He Q.

As an industrial waste characterized by huge volume and high alkalinity, red mud has become a serious environmental problem. The reuse of red mud has been explored in previous studies, including as building materials and for soil and waste water treatment. In this study, an innovation was made for the reuse of red mud to create a synergistic effect. Red mud was first used in flue gas desulfurization (FGD), and then the desulfurized red mud was again reused to make a geopolymer material. By using one type of original red mud and three types of fly ash, this study revealed that with high alkalinity and desulfurization capacity, the red mud could serve as an excellent FGD sorbent. After FGD, the sodium sulfate in the desulfurized red mud acted as a chemical activator for geopolymer made with class C fly ash. A 25% increase in strength was observed between the geopolymers with the red mud after FGD and with the original one. There are no significant benefits of FGD on the class F fly ash-based geopolymers and further study is required.

Wang Y., Zhang T., Zhang Y., Lv G., Zhang W.

Sodium aluminosilicate hydrate is the main equilibrium solid phase of Bayer red mud. Owing to the alkaline insoluble phase, the saltpetering of Bayer red mud tends to occur over time, even after washing procedures. This prevents the large-scale utilization of Bayer red mud for the production of cement, brick, subgrade materials, etc. In this study, a new type of red mud (C-C residue), structured through calcification–carbonization treatment, was used to produce cement clinker. The mineral transformation was studied and the phase, micromorphology, chemical composition, f-CaO content, bending strength and compressive strength of the clinker were characterized. The results show the transformation of C-C residue into the effective components (Ca3SiO5, Ca2SiO4, Ca2FexAl2–xO5 and Ca3Al2O6) of the cement clinker. The chemical composition, f-CaO content and cement strength met the Chinese national standards. Therefore, sintering C-C residue provides a promising solution to the problem of red mud stockpiling.

Rivera R.M., Xakalashe B., Ounoughene G., Binnemans K., Friedrich B., Van Gerven T.

During acid leaching of bauxite residue (red mud), the increase in dissolution of rare-earth elements (REEs) is associated with a substantial co-dissolution of iron; this poses problems in the downstream processing (i.e. solvent extraction or ion exchange). Six different slags generated by reductive smelting of the same bauxite residue sample were treated by high-pressure acid leaching (HPAL) with HCl and H2SO4 to selectively extract REEs. Thus, up to 90 wt% of scandium was extracted from the slags using H2SO4 at 150 °C, while with HCl the extraction of scandium reached up to 80 wt% at 120 °C. The extraction of yttrium, lanthanum and neodymium was above 95 wt% when HCl was used as a reagent, but it was much lower ( 90 wt%, 18 g L−1), while the concentration of the remaining iron (>60 wt%) was of 3 g L−1 in the leachate. The co-dissolution of silicon and titanium was lower than 5 wt%.

Wu H., Liao J., Zhu F., Millar G., Courtney R., Xue S.

Bauxite residue deposit area (BRDA) is a typical abandoned mining wasteland representing extreme hostile environment with increased alkalinity. Microbially-driven neutralization of bauxite residue, based on the microbial acid producing metabolisms, is a novel strategy for achieving rapid pH neutralization and thus improving its environmental outcomes. The hypothesis was that these extreme conditions promote microbial communities which are capable of novel ecologically relevant functions. Several alkaliphilic acid producing bacteria were isolated in this study. One strain was selected for its superior growth pattern and acid metabolism (termed EEEL02). Based on the phylogenetic analysis, this strain was identified as Bacillus thuringiensis. The optimized fermentation conditions were as follows: pH 10; NaCl concentration 5%; temperature 25 °C; EEEL02 preferred glucose and peptone as carbon and nitrogen sources, respectively. Based on optimal fermentation conditions, EEEL02 induced a significant pH reduction from 10.26 to 5.62 in 5-day incubation test. Acetic acid, propionic acid and CO2 (g) were the major acid metabolites of fermentation, suggesting that the pH reduction in bauxite residue may be caused by acid neutralization derived from microbial metabolism. This finding provided the basis of a novel strategy for achieving rapid pH neutralization of bauxite residue.

Khairul M.A., Zanganeh J., Moghtaderi B.

Red mud or bauxite residue is a solid waste generated from the Bayer process. The disposal of red mud leads to a serious environmental threat because of its tiny particle size and high alkaline value. The utilisation and recycling of red mud is currently a crucial issue and needs to be addressed as soon as possible. This study demonstrates the different methods and techniques recently analysed or suggested for the consumption of bulk red mud to maintain a sustainable environment. The potential utilisations of red mud include valuable metal recovery, the preparation of construction materials and as an alternative catalyst for diverse processes, including hydrodechlorination, hydrogenation and hydrocarbon oxidation. In addition, the red mud can also be used to control environmental pollution, such as in wastewater treatment, soil remediation and the purification of flue gas. However, the proper utilisation and recycling of red mud has a number of shortcomings: pyro-metallurgical recovery involves high energy costs; large volumes of acid by-product needing to be neutralised after the leaching process; the fine particle size of red mud; and the necessity of pre-treatment in some methods. Although many studies have investigated the potential utilisation of red mud, most of them have concentrated on lab-scale studies. Therefore, a large-scale investigation of the recycling and utilisation of red mud needs to be explored. Moreover, an appropriate cost-benefit analysis of the existing technologies needs to be examined in the future because of the high cost of the transportation of a large amount of red mud from alumina refineries to the point of application.

Xue H., Lv G., Zhang T.

Aktary M., Sanhoob M.A., Alzharani A.S., Alghamdi H.S., Ajeebi A.M., Aziz M.A., Shaikh M.N.

Finding environmentally acceptable and long-lasting catalysts that can convert carbon dioxide into compounds with additional value is of great interest.

Martins J.R., Novais R.M., Hotza D., Labrincha J.A., Senff L.

Abstract Coral reefs are vital to marine ecosystems, providing habitat and protection for marine life and serving as natural barriers against coastal erosion. However, coral degradation due to climate change, coastal development, and marine exploration require alternatives like artificial corals. This study explores the use of geopolymers based on industrial waste, specifically biomass fly ash and red mud, for creating artificial corals. Geopolymers offer an eco-friendly solution by repurposing waste materials and providing resistance to environmental stressors like saline water. This research investigates the mineralogical and mechanical properties of geopolymers formed using 3D printing and casting. The 3D-printed samples exhibited compressive strengths ranging from 4 to 10 MPa, sufficient to withstand marine environmental stresses, although lower than the 20 to 30 MPa observed in cast samples. The mineralogical analysis showed the presence of compatible phases including hematite and calcium carbonate, which enhance environmental compatibility. The study highlights the potential of waste-based geopolymers as sustainable materials for artificial corals, emphasizing the benefits of using 3D printing to generate complex geometries and endow marine integration. Graphical Abstract

Wang G., Chen C., Li J., Lan Y., Lin X.

Rajković M., Jelić I., Janković M., Antonijević D., Šljivić-Ivanović M.

The increasing importance of waste materials utilization with the necessary modification to remove various pollutants from industrial wastewater has been a research focus over the past few decades. Using waste material from one industry to solve pollution problems in another ultimately leads toward sustainable and circular approaches in environmental engineering, solving waste management and wastewater treatment issues simultaneously. In contemporary research and industry, there is a notable trend toward utilizing industrial wastes as precursors for adsorbent formation with a wide application range. In line with this trend, red mud, a byproduct generated during alumina production, is increasingly viewed as a material with the potential for beneficial reuse rather than strictly a waste. One of the potential uses of red mud, due to its specific composition, is in the removal of heavy metal and radionuclide ions. This study summarizes red mud’s potential as an adsorbent for wastewater treatment, emphasizing techno-economic analysis and sorption capacities. An overview of the existing research includes a critical evaluation of the adsorption performance, factors influencing efficiency rather than efficacy, and the potential for specific pollutant adsorption from aqueous solutions. This review provides a new approach to a circular economy implementation in wastewater treatment while guiding future research directions for sustainable and cost-effective solutions.

Guan S., Tang J., Sun Y., Yang Y., Li L., Wu Y., Shi F.

Red mud is rich in Fe, Al, Si and Ti elements and is the main solid waste of the alumina industry, which has great recycling value. The red mud after sodium roasting is studied in this paper, considering the influences of leaching temperature, leaching time and solid-liquid ratio on the leaching rates of Al and Si elements in red mud. The result shows that the leaching rates of Al and Si elements are 96.28% and 54.04% under the optimum condition. The dissolubility of Al and Si elements in alkali leaching solution is analyzed from the phase changes of red mud after sodium roasting. The influences of ultrasound on the synthesis conditions of Na2FeO4 are researched, and the results show that the application of ultrasound can shorten the reaction time of Na2FeO4 and increase the concentration of Na2FeO4. The mechanism of promoting Na2FeO4 formation by ultrasonic was elucidated from the effects of physics and chemistry. Moreover, the response surface method is used to optimize the preparation process of Na2FeO4 by ultrasound.

Zhao Y., Ding X., Zhou Y., Qian H., Deng X., Ran S.

Ma S., Liu X., Zhang Z., Zhu L., Wang J.

Setiawan A., Kawigraha A., Attaurrazaq B., Nahar S., Permatasari N.V., Haryanto I., Ikhwani N., Harjanto S., Rhamdhani M.A.

Yan S., Wang W., He C., Gai X., Wang S.

Shalchian H., Hajizadeh Navakh M., Birloaga I., Babakhani A., Vegliò F.

In this study, the recovery of rare earth elements (REEs) from red mud (bauxite residue) was explored through a combination of citric acid leaching and sulfation roasting–water leaching processes, introducing an innovative approach to the field. The research uniquely investigates the influence of citric acid on the leaching behavior of REEs and impurities in both untreated red mud and red mud subjected to sulfation roasting, providing a direct comparison of these methodologies. A novel aspect of this study is the evaluation of solvent extraction efficiency using DEHPA, highlighting the selective recovery of REEs over impurities from both citric acid and water-leaching solutions. Furthermore, a comprehensive phase analysis using X-ray diffraction (XRD) was conducted to track the transformations of minerals during the sulfation roasting process, an original contribution to the literature. The findings revealed that over 85% of REEs and major elements such as Fe, Al, Ca, and Ti dissolved in water after sulfation at 105 °C, while iron and titanium dissolution significantly decreased following roasting at 725 °C. Importantly, terbium, neodymium, and gadolinium extraction efficiencies were notably affected by roasting temperature. Citric acid leaching results demonstrated that the direct leaching of red mud leads to higher leaching efficiency than leaching it after the roasting process. Solvent extraction demonstrated lower terbium and neodymium recovery from citric acid solutions compared to water leaching solution. Finally, stripping experiments illustrated that 6M H2SO4 solution is capable of stripping more than 80% of rare earth elements, except terbium.

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.

Fang W., Zhou Y., Cheng M., Zhang L., Zhou T., Cen Q., Li B., Liu Z.

Red mud (RM) is a highly alkaline solid waste generated during alumina production and should be disposed of and recycled in an environmentally sound manner. Red mud is rich in metal compounds, making it a promising material for environmental remediation treatment. However, there are fewer papers on the removal of organic dyes from wastewater using modified red mud-based materials. Therefore, this paper provides a comprehensive review on the properties of dyes and reveals the adverse effects caused by dye wastewater pollutants. The application of modified red mud-based materials in dye wastewater are further reviewed. Meanwhile, the removal mechanism includes electrostatic gravitational interaction, hydrogen bonding, π-π bonding, and catalytic oxidation. In addition, this paper suggests that the activation methods for red mud materials should be optimized in future studies as well as develops red mud-based materials with absorbing many dyes. This paper aims to expand modified red mud-based materials application for removing organic pullents from wastewater and red mud utilization.

Lu J., Shen Y., Wang Y., Zhang H., Guan X., Zhu J., Liu S.

The low reactivity and high alkalinity of raw red mud (RRM) limit its utilization as supplementary cementitious materials (SCMs) in Portland cement-based grouts. This work develops an in-situ wet carbonation technique to improve the chemical reactivity of RRM using flue gas CO2. Systematic investigation of the RRM before and after carbonation revealed transformed composition and microstructure. The minerals and morphology changed significantly, with nano-CaCO3 and amorphous silica/silica-alumina gels formation. This enhanced the pozzolanic reactivity of RRM. Detailed evaluation of the effects of carbonated red mud (CRM) as SCMs on the properties of Portland cement-based grouts showed accelerated hydration kinetics and refined pore structure. Replacement of 30% cement by the CRM increased the 28 d compressive strength of the grout. The underlying mechanisms were analyzed via XRF, XRD, TGA, FT-IR, 29Si NMR, SEM, and N2 adsorption–desorption testing. The nano-CaCO3 and silica/silica-alumina gels from CRM participated in additional hydration and microstructure densification. This integrated approach enables the improved performance of grouting materials through CO2 mineralization and industrial waste recycling.

Akcil A., Swami K., Gardas R., Hazrati E., Dembele S.

Aluminum is produced from its primary bauxite ore through the Bayer process. Although Al is important nowadays in the development of humanity, its production leads to the generation of a huge amount of waste, called red mud. Globally, the estimation of the stock of red mud is about 4 billion tons, with about 10 million tons located in Turkey. The presence of rare-earth elements (REEs) in crucial materials such as red mud makes it a major source of these elements. A number of methods have been developed for treating red mud, which are employed globally to recover valuable products. The application of a suitable method for REE extraction from red mud is a way to overcome the supply risk, contributing to reducing the environmental issues linked to red mud pollution. The current review summarizes the research on red mud processing and examines the viability of recovering REEs from red mud sustainably, utilizing hydrometallurgy and biohydrometallurgy.