Hydrogen Reduction of Bauxite Residue for Green Steel and Sustainable Alumina Production

Vol.:(0123456789) Journal of Sustainable Metallurgy (2025) 11:1363–1380 https://doi.org/10.1007/s40831-025-01046-x RESEARCH ARTICLE Hydrogen Reduction ofBauxite Residue forGreen Steel andSustainable Alumina Production ManishKumarKar1· MengyiZhu1· JafarSafarian1 Received: 15 November 2024 / Accepted: 17 February 2025 / Published online: 11 March 2025 © The Author(s) 2025, corrected publication 2025 Abstract This study introduces a novel approach in sustainable metallurgy for the efficient utilization and valorization of bauxite residue, aimed at producing sustainable alumina and green steel. The integrated process combines hydrogen reduction, alkaline leaching, and smelting of the leaching residue. Initially, the bauxite residue was pelletized with calcite and quicklime to create self-hardened pellets, leveraging the cementing effect of quicklime with water. These pellets underwent hydrogen reduction, achieving over 95% reduction, resulting in the formation of metallic iron and a leachable calcium aluminate phase for alumina recovery. The reduced pellets were then subjected to alkaline leaching, extracting 62% alumina. Subsequently, smelting at 1550°C facilitated the near-complete separation of iron and calcium-rich slag. The process was analyzed using various analytical techniques, including X-ray diffraction, electron probe microanalysis, and inductively coupled plasma mass spectroscopy, complemented by thermodynamic calculations using FactSage 8.1 software. Iron oxide reduction to metallic iron was achieved at 1000°C for 120min, while sodium carbonate leaching effectively extracted alumina from the calcium aluminate slag. However, residual alumina was attributed to the formation of indissoluble gehlenite and a dense calcium carbonate layer that impeded leaching kinetics. Successful iron separation during smelting required temperatures above 1500°C, though this process was challenged by the high viscosity of the oxide matrix and the purity of the iron. Graphical Abstract Keywords Bauxite residue· Hydrogen reduction· Cementing· Kinetics· Viscosity The contributing editor for this article was Anna Kaksonen. Extended author information available on the last page of the article 1364 Journal of Sustainable Metallurgy (2025) 11:1363–1380 Introduction Globally, the Bayer process produces approximately 147.04 million tons of alumina annually in 2024 [1]. This process generates a hazardous by-product, red mud, with its dewatered form known as bauxite residue (BR). The quantity of BR generated depends on the processing conditions and the mineralogical composition of bauxite ore [2], with approximately 0.7–2.0 tonnes of BR produced per ton of alumina [3]. The global annual production of BR exceeds 150 million tonnes, and over 3.4–4.6 billion tonnes of BR have been stockpiled [4–6]. The industrial utilization of the generated BR faces considerable challenges due to its high alkalinity, fine particle size, and the presence of heavy metals. To date, only less than 3% of the total BR generation has been utilized in the construction industry, leaving the majority stored in dry heaps or tailing ponds [7–9]. In fact, BR contains dominant metal resources such as iron, aluminum, silicon, titanium, sodium, and calcium, along with rare earth elements in their oxide forms [4, 8, 10, 11], where iron oxide and alumina are the major constitutes by weight of the BR. Therefore, recovering iron and alumina from the residue can significantly reduce the volume of bauxite residue. Previous research has focused on recovering iron and alumina from BR through a carbothermic reduction process [12–14]. However, the carbothermic reduction process significantly contributes to greenhouse gas emissions by generating a significant amount of CO2. An alternative method involves replacing carbon with hydrogen as a reductant in place of carbon will generate water vapor in place of CO2/CO during reduction [15, 16]; nonetheless, the generation of hydrogen should come from renewable sources. To date, only limited research has been conducted on the recovery of iron and alumina from the BR by using hydrogen as a reductant [11, 17–19]. Skibelid et.al. [18] studied the recovery of iron and alumina from BR by combined hydrogen reduction and alkaline leaching. In their study, BR pellets were sintered calcite at a high temperature of 1200°C, followed by hydrogen reduction in a temperature range from 1000 to 1250°C. They found that the reduction occurs more rapidly at 1000°C compared to 1100°C and 1200°C, attributed to the loss of porosity at higher temperatures, which shifts the reduction mechanism toward being diffusion controlled. All iron-containing compounds were converted to metallic iron, and the alumina recovery exceeded 87% during alkaline leaching [18]. Stergi etal. [19] investigated the hydrogen reduction of BR with varying sodium hydroxide at different temperatures. They observed that with mass ratio 74/26 of BR and sodium hydroxide, the alumina recovery goes above 77%, and average iron content in the solid fraction was around 38.5%. The remaining solid phase mainly consisted of Ca, Si, and Ti, with a small fraction of undissolved Na and Al that less than 6 wt.% [19]. The main difference between Skibelid etal. [18] and Stergi etal. [19] lies in the alumina recovery method. Skibelid etal. utilize alkaline leaching of calcium aluminate slag, whereas Stergi etal. recover alumina through water leaching of sodium aluminate from bauxite residue in which another process is studied. However, both approaches share a similarity in the recovery of iron and alumina from bauxite residue. Recent studies have focused on the iron recovery by magnetic separation from the hydrogen-reduced BR pellets [11, 20]. Iron separation was performed using mediumintensity magnetic separation, ranging from 1000 to 2500 gauss, and a Slon magnetic separator at 1000 gauss. It was found that as the magnetic intensity increased during magnetic separation from 1000 to 2500 gauss, the recovery improved for all particles sized in the medium-intensity magnetic separator. Particularly, the iron recovery enriched to 41% with the most appropriate particle size −106 + 74µm [21]. Hassanzadeh etal. [21] explored the iron recovery from BR-calcite reduced pellets by electrostatic separation and magnetic separation using the Davis tube and low-intensity magnetic separator. However, they discovered that while the electrostatic separation does not enhance the iron grade, there is an acceptable recoveries ranging from 22 to 37% received by Davis tube and lowintensity magnetic separator [11]. In a recent study, the recovery of iron and alumina from hydrogen-reduced BRcalcite pellets was conducted [22]. Initially, alkaline leaching by Na2CO3 solution was applied to recover alumina, followed by the magnetic separation of leaching residue to recover iron. It was found that alumina recovery goes above 75 wt.% and an enrichment of iron grade in the leaching residue to an acceptable range of 33 wt.% during magnetic separation was achieved [22]. In the physical separation studies mentioned above, the recovery is unsatisfactory, primarily attributed to the intricate structure of iron within the matrix. This study presents a novel approach for the valorization of BR, with a focus on the extraction of iron and alumina as essential products to increase economic viability and environmental sustainability. The proposed BR recycling approach integrates hydrogen reduction, alkaline leaching, and smelting of leaching residue. By prioritizing the recovery of alumina through alkaline leaching, the process ensures a more efficient subsequent iron recovery by smelting with a reduced slag viscosity. This approach not only ensures the efficient extraction of metal resources but also improves the overall process efficiency, paving the way for sustainable waste management in the industry. 1365Journal of Sustainable Metallurgy (2025) 11:1363–1380 Materials andMethods Figure1 presents the process flowsheet applied in the present work for the BR valorization. The process begins by mixing BR with quicklime and calcite for pelletization with the addition of water. Subsequently, the pellets were self-hardened through aging over a period of few days [11, 23]. These self-hardened pellets were further subjected to hydrogen reduction. The resulting hydrogen-reduced pellets undergo further milling and alkaline leaching to recover alumina. Finally, the leaching residue was melted to recover iron and a calcium-rich slag. Materials BR, limestone, and quick lime were supplied from Mytilineos S.A. (previously Aluminum of Greece), Vuglukli S.A. Greece, and NorFrakalk, respectively. These materials were received in lumpy form thus were deagglomerated and sieved to particle size of less than 500µm. The deagglomerated BR, calcite, and quicklime were then mixed using a tabular mixture with specific ratios to from these phases CaO.Al2O3, 2CaO.SiO2, and CaO.TiO2 during the heat treatment process. Methods Methods forMaterial Characterization Phase analysis of the materials was carried out by X-ray diffraction (XRD) (Bruker D8 Focus, Bruker AXS GmbH, Karlsruhe, Germany) with CuKα radiation (wavelength λ = 1.54Å). The diffractometer scans are in the range of 15° to 75° 2θ with a step size of 0.02°. The qualitative phase analysis of the raw data was analyzed by using DIFFRAC. EVA software with the database of PDF-4+ (2014, ICDD, Philadelphia, Pennsylvania, USA). To prepare the standard sample for XRD analysis, the materials were milled in WC vibratory disk mill (RS 200, RETSCH GmbH, Haan, Germany) for 45s at an 800 revolution per minute (rpm). The microstructural analysis of the materials was done by electron probe microanalyzer (EPMA). The EPMA was supported with wavelength-dispersive spectroscopy (WDS) for elemental composition measurements. Pelletization The pelletization of mixed materials (BR, calcite, and quick lime) was conducted using a disk pelletizer via the addition of approximately 10 wt.% water. The composition of bauxite residue, calcite, and quicklime is mentioned elsewhere Fig. 1 Overall flowsheet of the applied process 1366 Journal of Sustainable Metallurgy (2025) 11:1363–1380 [22]. The pelletizer was operated at a revolution around 26 ± 2 revolution per minute (rpm) with a rotation angle of 45 degree, which was determined to be optimum angle for pelletization according to our previous work [24]. The size of the green pellets was around 5–10mm. After pelletization, the green pellets were left in open atmosphere for 7days to undergo self-hardening. The main purpose to do self-hardening is to increase the pellet's strength to tolerate further handling and reduction. The self-hardened pellets were dried in an oven at 80 ± 5°C for 24h to remove the remaining excess moisture of the pellets. Reduction As illustrated in Fig.2, the dried self-hardened pellets were reduced in a vertical tube furnace. Pellets were placed onto the crucible, which was then inserted vertically from the top of the furnace. Purging gases from the bottom of the crucible pass through the pellet bed before exiting via the offgas outlet. Two thermocouples were used: one was inserted from the top into the pellet's bed, and the other one was on the furnace wall. The targeted temperature was set based on the two-thermocouple readings. The pellets were heated to the targeted temperature (1000°C) in the presence of argon gas flow. Once the targeted temperature was reached, the gas flow was changed to hydrogen for 120min with a flow rate of 4NL/min. The reduction time and temperature have been optimized based on our previous work [25]. To achieve a faster reduction rate and facilitate easier hydrogen recirculation, we used hydrogen with 100% purity. The furnace pressure was consistently maintained at 1 atmosphere. After the completion of the reduction cycle, the hydrogen-reduced pellets were cooled under the purged argon stream to avoid reoxidation. The obtained reduced pellets were then grounded in a ball mill for 30min with 25rpm to achieve a particle size below 10µm, ensuring an efficient leaching reaction. Leaching The alkaline leaching of ground-reduced pellets was carried out in an oil jacketed glass reactor as shown in the Fig.3. A mass of 100g of ground-reduced pellets was added into a 1000mL solution containing 60g/L Na2CO3 which was heated to 60°C via silicon oil circulated in a VMR thermobath. The temperature of the leaching Fig. 2 Schematic of vertical tube furnace used for hydrogen reduction 1367Journal of Sustainable Metallurgy (2025) 11:1363–1380 solution was monitored by using a thermoprobe inserted from the top of the reactor, as depicted in Fig.3. A shaft paddle impeller was also inserted from the top, the stirring speed was maintained at 400rpm, and a condenser was also connected to the reactor to prevent water loss. The alkaline leaching period was held for 120min. Upon completion of the leaching process, the slurry was filtered by using a vacuum pump with a 0.22-µm membrane. After filtration, the concentration of filtered liquor was measured by ICP-MS (inductively coupled plasma mass spectroscopy), while the leaching residue (dewatered gray mud) underwent XRD phase analysis. Smelting ofGray Mud After alumina recovery from the reduced pellets, the leaching residue predominantly composed of iron and calcite. It was melted to recover iron. The smelting of the leaching residue was conducted in a vertical tube furnace under argon purging, as illustrated and detailed in the operational description in our previous work [26]. The furnace temperature control was achieved through the use of two thermocouples: one positioned at the side of the furnace, and the other within the crucible from the top. The leaching residue was subjected to smelting at four different temperatures (1300, 1400, 1500, and 1550) °C, each held for 120min with argon gas purging. Above 1500°C, there was complete separation of metallic iron from the rest of the oxide matrix. Results XRD Analysis The phase analysis of the bauxite residue, self-hardened reduced pellets, and leaching residue is presented in Fig.4. In the bauxite residue, hematite (Fe2O3) and diaspore (AlHO2) are the major phase present. In the reduced pellets, identified phases correspond to metallic iron, mayenite (Ca12Al14O33), larnite (Ca2SiO4), lime (CaO), and perovskite (CaTiO3). Metallic iron and mayneite are the major peaks of the reduced pellets. However, in the leaching residue, iron and calcite emerge as the predominant peaks. The appearance of calcite in the leaching residue is due to its formation during the alkaline leaching of mayneite as nearly all the mayenite phase was reacted. Figure5 shows the phase analysis of the smelted leaching residue at different temperatures. The major phases identified in the smelted slags are mayenite (Ca12Al14O33), gehlenite (Ca2Al2SiO7), krotite (CaAl2O4), wüstite (FeO), perovskite (CaTiO3), grossite (CaAl4O7), dicalcium aluminate (Ca2Al2O5), and metallic iron (Fe). The slag smelted at 1300°C shows the existence of mayenite, perovskite, and wüstite. Additionally, the presence of metallic iron was found along with those phases. However, with increasing the smelting temperature, grossite (CaAl4O7) becomes more stable. In all the smelted slags, a portion of wüstite (FeO) is present, with its amount decreasing at higher temperatures. However, the majority of metallic iron separates from the oxide matrix at temperatures exceeding 1500°C. Additionally, a minor fraction of krotite is observed at temperatures above 1400°C. Microstructural Analysis The microstructure and elemental mapping of the metallic iron and slag of 1550°C are shown in Fig.6. The bright area represents the metallic iron phase, where iron is the predominant element with only very little fraction of Al and Na impurities. In the slag, two major phases are observed: the dark phase primarily composed of Al, O, and Ca, likely corresponding to calcium dealuminate (CaAl4O7), as correlated to the form XRD phase analysis, and the lighter dark slag phase is a composite mixture of various elements where Al, Fe, O, Na, Ca, and Ti are present. This region possibly comprises a composite of phases such as perovskite, gehlenite, and wüstite. In addition, Fig.6 also indicates a complete separation of iron from the slag. Figure7 shows the microstructure and elemental mapping of leaching residue smelted at 1500°C, revealing Fig. 3 Schematic drawing of leaching reactor 1368 Journal of Sustainable Metallurgy (2025) 11:1363–1380 Fig. 4 XRD spectra of bauxite residue, self-hardened reduced pellets, and leaching residue Fig. 5 Phase analysis of the smelted leaching residue at different smelting temperatures 1369Journal of Sustainable Metallurgy (2025) 11:1363–1380 Fig. 6 Microstructure and elemental mapping of smelted leaching residue at 1550°C Fig. 7 Microstructural and elemental mapping of leaching residue slag smelted at 1500°C 1370 Journal of Sustainable Metallurgy (2025) 11:1363–1380 three major phases. The bright area corresponds to metallic iron, which contains very small fraction of Al, Na, and Si. However, the metallic iron is present inside the matrix of the slag phase, forming an interconnected network. The dark slag phase mainly composed of Al, Ca, and O. The light dark area is the mixture of Al, Ca, Si, Fe, O, and Ti. Notably, the iron particle size is larger than that in residues smelted at 1400°C and 1300°C in Figs.8 and 9. Obviously, at 1550°C, almost complete separation of metallic iron from the oxide phase occurred (Fig.6), while at lower temperatures, the metal separation did not occur completely. Figure8 presents the microstructure analysis slag and metal from smelted leaching residue at 1400°C. The iron particles are uniformly distributed throughout the slag matrix. The iron particles are smaller as compared to those leaching residue smelted at 1500°C. Moreover, the metallic iron phase contains around 1 wt.% of impurities such as Al, Na, Si, and Ti. In the slag phase, there are two primaries observed. The dark phase is composed of Ca, Al, and O along with that a certain percentage of Fe also present. This Fe may be present in the form of wüstite phase within the stag structure. The light dark matrix comprises a mixture of phases similar to those observed at 1550°C and 1500°C temperatures. The elemental mapping and microstructure of leaching residue smelted at 1300°C are presented in Fig.9. Obviously, the iron particles are noticeably finer compared to those smelted at higher temperatures. The composition of the iron particle is similar to the leaching residue smelted at 1400°C. The dark slag phase contains Ca, Al, and O as the major elements along with Fe, Na, and minor fraction of Ti coexists. The light dark phase is primarily composed of Fe, Ti, and O as the major elements. Based on the weight percentage composition, it can be inferred that the dark areas represent the calcium aluminate phase, while the lighter dark areas are a mixture of the perovskite and wüstite phases. The elemental mapping clearly indicates that iron is distinct and does not overlap with other elements, suggesting that it is separated from the rest of the slag matrix. The agglomeration of metallic iron with increasing smelting temperature is presented in Fig.10. Elemental Analysis oftheLeaching Solution Table1 presents the elemental analysis of the leachate produced during the leaching of reduced pellets with a sodium carbonate solution. In addition to Al, a minor portion of Si and S was also transferred to the leaching solution. Based on our previous studies, the transfer of Si into the leaching solution during alkaline leaching can be attributed to the presence of Si in the mayenite lattice [27]. The recovery of alumina is around 62.7%. Fig. 8 Microstructural and elemental mapping of leaching residue slag smelted at 1400°C 1371Journal of Sustainable Metallurgy (2025) 11:1363–1380 Fig. 9 Microstructural and elemental mapping of leaching residue slag smelted at 1300°C Fig. 10 Micrograph of various slag with increasing smelting temperature 1378 Journal of Sustainable Metallurgy (2025) 11:1363–1380 Appendix A1 See Table2. Appendix A2 See Fig.19. Table 2 Chemical analysis of the bauxite residue, selfhardened pellets, reduced pellets, leaching residue, and slag produced at 1550°C (wt.% is normalized with respect to loss of ignition) wt.% Bauxite residue Self-hardened pellets Reduced pellets Leaching residue Slag (1550°C) SiO27.43 5.61 7.34 9.03 12.06 MnO 0.09 0.05 0.03 0.05 0.06 P2O50.12 0.07 0.11 0.12 0.16 SO31.04 0.60 1.00 0.65 0.86 Cr2O30.22 0.23 0.28 0.29 0.39 NiO 0.00 0.12 0.08 0.07 0.09 TiO25.79 4.09 5.03 5.51 7.36 V2O50.14 0.13 0.00 0.15 0.20 Al2O325.37 17.41 19.18 7.04 9.40 CaO 10.50 36.15 40.27 46.44 62.03 Fe2O3/Fe 44.72 32.98 23.08(Fe) 25.13(Fe) 0.00 ZrO20.00 0.12 0.16 0.00 0.00 MgO 1.20 0.56 0.62 0.82 1.10 Na2O 3.28 1.68 2.58 4.50 6.02 K2O 0.10 0.07 0.04 0.00 0.00 ZrO20.00 0.12 0.16 0.14 0.19 SrO 0.00 0.02 0.02 0.02 0.03 WO30.00 0.00 0.02 0.04 0.05 Fig. 19 Mass balance of the proposed process 1379Journal of Sustainable Metallurgy (2025) 11:1363–1380 Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 958307. This publication represents only the authors’ views, exempting the Community from any liability. Funding Open access funding provided by NTNU Norwegian University of Science and Technology (incl St. Olavs Hospital - Trondheim University Hospital). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 958307. Declarations Conflict of interest The authors have no conflicts of interest. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. 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In: 35th international ICSOBA conference 2017, Vol. 42, pp 243–253 1380 Journal of Sustainable Metallurgy (2025) 11:1363–1380 Authors and Affiliations ManishKumarKar1· MengyiZhu1· JafarSafarian1 * Manish Kumar Kar [email protected] 1 Department ofMaterials Science andEngineering, Norwegian University ofScience andTechnology, Alfred Getz Vei 2, 7491Trondheim, Norway 28. Kar MK, Eijk C Van Der, Safarian J (2022) Hydrogen reduction of high temperature sintered and self-hardened pellets of bauxite residue produced via the addition of limestone and quicklime. In: Hydrogen reduction of high temperature sintered and selfhardened pellets of bauxite residue produced via the addition of limestone and quicklime. p 11 29. Vance K, Falzone G, Pignatelli I etal (2015) Direct carbonation of Ca (OH)2 using liquid and supercritical CO2: implications for carbon-neutral cementation. Ind Eng Chem Res 54:8908–8918 30. Azof FI, Kolbeinsen L, Safarian J (2019) Kinetics of the leaching of alumina-containing slag for alumina recovery. In: European metallurgical conference 2019, Vol. 2. GDMB Verlag GmbH Germany Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.