Green steel from red mud through climate-neutral hydrogen plasma reduction

Abstract

Red mud is the waste of bauxite refinement into alumina, the feedstock for aluminium production¹. With about 180 million tonnes produced per year¹, red mud has amassed to one of the largest environmentally hazardous waste products, with the staggering amount of 4 billion tonnes accumulated on a global scale¹. Here we present how this red mud can be turned into valuable and sustainable feedstock for ironmaking using fossil-free hydrogen-plasma-based reduction, thus mitigating a part of the steel-related carbon dioxide emissions by making it available for the production of several hundred million tonnes of green steel. The process proceeds through rapid liquid-state reduction, chemical partitioning, as well as density-driven and viscosity-driven separation between metal and oxides. We show the underlying chemical reactions, pH-neutralization processes and phase transformations during this surprisingly simple and fast reduction method. The approach establishes a sustainable toxic-waste treatment from aluminium production through using red mud as feedstock to mitigate greenhouse gas emissions from steelmaking.

Fig. 1. The generation, storage and hazards of red muds and solution with hydrogen plasma treatment.

a, Schematic representation of the bauxite mining and subsequent Bayer process, in which the ore is chemically transformed into alumina, the feedstock material for aluminium production by means of electrolysis, and the waste red mud. b, Top, satellite image of a waste pond used to store red mud. Schematics of classical monuments are overlaid as a reference for the huge dimensions of such a reservoir. Bottom, satellite image (left) and photograph (right) showing catastrophic events when red mud dams break. c, Schematic representation of the hydrogen-plasma-based process used here to convert 15 g red mud portion into metallic iron. d, Example of red mud powder transformed into the final product after 10 min of reduction (solidified sample). The resulting sample was separated into remaining oxide-rich powder and iron nodules by mechanical crushing and magnetic separation. The oxide portions can be destined to civil construction ends and the iron to steel production. e, Diagram depicting the extracted weight change with reduction time in relation to the obtained Fe in the form of nodules, as well as the Fe and O content within the oxide portion of the samples. Credits: b (top), copyright TerraMetrics, LLC, www.terrametrics.com, imagery © 2022 Google, TerraMetrics, imagery © 2022 CNES/Airbus, Maxar Technologies, map data © 2022; b (bottom left, satellite image), © NASA Earth Observatory image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team (source https://earthobservatory.nasa.gov/images/46360/toxic-sludge-in-hungary); b, (bottom right), reprinted (adapted) with permission from ref. , copyright (2023) American Chemical Society.

Fig. 2. Phase evolution of red mud with hydrogen plasma processing and mechanism of iron recovery.

a, Normalized weight fraction of the main chemical elements within the remaining oxide portions of the partially reduced red mud samples (1–15 min) in a lean hydrogen-containing plasma (Ar-10%H₂). The black arrows indicate that the trends of iron and oxygen change with reduction time. b, Phase weight fraction in the remaining oxide portions of the partially reduced red mud samples, showing the differing complexity and evolution of oxides with reduction time. For clarity, the individual complex oxides with more than three metallic constituents are shown as individual groups. c, Chemical mechanism for iron recovery from red mud by means of hydrogen plasma reduction.

Fig. 3. Microstructure of red mud after 1 min processing with hydrogen plasma.

a, Microstructure of a red mud processed for 1 min under a lean hydrogen plasma (Ar-10%H₂) and corresponding local chemical partitioning between fresh-formed iron droplets and the remaining oxide substructures, examined by means of EDX in a scanning electron microscope. The white arrows indicate the presence of an Al-rich phase hercynite (Al₂FeO₄), mostly accompanying the Fe-rich phase. b, Microstructure and local chemical evidence for the formation of pure iron in a splatter-like morphology (arrowed) from titanomagnetite (Fe_2.5Ti_0.5O₄). Scale bars, 20 μm.

Fig. 4. Microstructure of red mud after 10 min of processing with hydrogen plasma.

Microstructural and local chemical composition of the sample partially reduced for 10 min under hydrogen plasma (Ar-10%H₂), examined by means of EDX in a scanning electron microscope. a, This region is about 1 mm below the top surface of the sample, showing a highly concentrated arrangement of oxides enriched with Al, Ca, Si and Ti and depleted of Fe. Scale bars, 1 cm (main), 10 μm (inset). b, Region examined close to the bottom of the specimen, depicting the complex resulting microstructure after reduction, showing the individual oxides and the metallic portions. The orange arrows show the chunky oxide domains enriched in Al, Fe, Mg and Cr. The white arrows show the constituents enriched in Ti that, in some cases, coincide with P enrichments. Scale bars, 1 cm (main), 20 μm (inset).

Fig. 5. Comparing the effect of non-reducing and reducing plasma gas species on red mud processing.

a, Phase weight fraction in the remaining oxide portions of the partially reduced red mud sample after 5 min reduction in pure inert Ar plasma, an experiment serving as a reference scenario with non-reducing atmosphere. b, Chemical composition of the oxide portions of the processed red mud with reducing hydrogen (for 1 min and 5 min) and inert argon (5 min) plasmas in comparison with the corresponding chemical composition of the original red mud. The composition was normalized by the mass of each corresponding specimen to visualize the weight loss of the oxide portion with the reduction process.

Extended Data Fig. 1. Graphical representation of the main oxides and their fractions within the investigated red mud.

The complete oxide composition of the investigated red mud is 38.4 ± 2.08 wt% Fe₂O₃, 15.43 ± 0.44 wt% SiO₂, 11.18 ± 3.31 wt% Al₂O₃, 10.96 ± 0.31 wt% TiO₂, 6.52 ± 0.39 wt% CaO, 4.43 ± 0.24 wt% Na₂O, 0.25 ± 0.01 wt% P₂O₅, 0.21 ± 0.00 wt% Cr₂O₃, 0.19 ± 0.05 wt% MgO and 0.16 ± 0.01 wt% ZrO₂. The composition was determined with inductively coupled plasma optical emission spectroscopy performed on a Thermo Scientific iCAP 7000. All measurements were performed in triplet, from which the uncertainty was calculated as the standard deviation. Before the measurements, the samples were prepared with microwave digestion using HClO₄, HNO₃ and HF, followed by a subsequent complexation step with H₃BO₃.

Extended Data Fig. 2. XRD data analysis of initial red mud sample with corresponding list of identified phases, both marked with peak markers and index list.

For visualization purposes, the different peak markers are exaggerated or miniaturized for easier separation of the different peak indicators. The fraction of each phase is provided in the legend in wt%. The corresponding fraction of unidentified peaks is 3.6% of the total peak signals. The numbers in brackets refer to the COD ID of the corresponding phase, that is, the identification number in the Crystallography Open Database (COD), accessed 11 July 2022.

Extended Data Fig. 3. XRD data analysis.

XRD data analysis of 1 min (a), 5 min (b), 10 min (c) and 15 min (d) reduced red mud samples with corresponding list of identified phases, both marked with peak markers and index list. For visualization purposes, the different peak markers are exaggerated or miniaturized for easier separation of the different peak indicators. The fraction of each phase is provided in the legend in wt%. The corresponding fraction of unidentified peaks is 1.45% 1.65%, 1.92% and 2.73% of the total peak signals for a, b, c and d, respectively. The numbers in brackets refer to the COD ID of the corresponding phase, that is, the identification number in the Crystallography Open Database (COD), accessed 11 July 2022.

Extended Data Fig. 4. Diffraction data for red mud samples processed under hydrogen-containing and inert Ar plasmas.

Comparative illustration of the XRD pattern of the initial state of the red mud against the oxide portion of the 1 min reduced red mud sample under hydrogen plasma (a), the oxide portion of the 1 min reduced red mud sample against the oxide portion of the 5 min reduced red mud sample (b), both under hydrogen plasma, the oxide portion of the 5 min reduced red mud sample against the oxide portion of the 10 min reduced red mud sample, both under hydrogen plasma (c), and the oxide portion of the 5 min reduced red mud sample under hydrogen plasma against the oxide portion of the 5 min reduced red mud sample under Ar plasma (d). The peaks of the main iron-holding phases are correspondingly marked with designated symbols. For the example shown in d, the additional strong oxide phase without Fe is also marked with the red star.