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

The global production of aluminum generates vast quantities of red mud, a highly alkaline and environmentally hazardous waste product. Current disposal methods are costly and environmentally damaging. This research addresses the dual challenges of red mud disposal and the need for sustainable steel production by exploring the use of red mud as a feedstock for ironmaking. The study investigates the feasibility of using a hydrogen plasma-based reduction process to extract iron from red mud, eliminating the need for carbon-based reducing agents and significantly reducing CO2 emissions associated with traditional steelmaking. This approach aims to create a sustainable nexus between the aluminum and steel industries, transforming a major environmental problem into a valuable resource. The immense global accumulation of red mud (approximately 4 billion tonnes) highlights the urgency for sustainable solutions, and this work proposes a direct, efficient and climate-neutral route to address this issue.

Previous attempts to extract iron from red mud using hydrogen as a reducing agent have involved complex preprocessing steps such as roasting, milling, pelletizing, and wet magnetic separation, making the process financially unattractive and still associated with CO2 emissions. This study aims to improve upon these approaches by employing a simpler, single-step, carbon-neutral strategy.

The researchers used a lean hydrogen thermal plasma (Ar-10%H2) ignited in an electric arc furnace (EAF) to directly reduce red mud without any pretreatment. The chemical composition and crystallography of the red mud were characterized using various techniques (Extended Data Figs. 1 & 2, Extended Data Tables 1 & 2). The reduction process was monitored by tracking the weight fraction of different components (Total O in oxide, Total Fe in oxide, Fe as nodules) over time (Fig. 1e). X-ray diffraction (XRD) analysis was employed to quantify the phases present in the partially reduced samples (Fig. 2a, b). Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) was used to analyze the microstructure and local chemical partitioning in the reduced samples (Figs. 3 & 4). A control experiment using pure argon plasma was conducted to isolate the effect of hydrogen in the reduction process (Fig. 5). Thermodynamic calculations were performed to support the experimental findings (Supplementary Information). The pH of the reduced red mud was measured to assess neutralization.

The hydrogen plasma reduction process successfully extracted metallic iron from red mud in a single step, forming iron nodules within the remaining oxide matrix (Fig. 1d). The process achieved approximately 70% metallization after 10 minutes of reduction, with iron purity averaging 98 wt%. The oxygen content in the remaining oxide fraction remained relatively constant, while the iron content decreased linearly with reduction time (Fig. 1e). The XRD analysis revealed a complex evolution of oxide phases during reduction, with the formation of titanomagnetite initially, followed by hercynite, and ultimately pure iron (Fig. 2b). The reduction process directly transforms titanomagnetite into pure Fe without significant formation of wüstite, an intermediate phase that can limit the speed of reduction. Microstructural analysis confirmed the formation of micron-sized iron domains within an oxide matrix after 1 minute of exposure to hydrogen plasma (Fig. 3). After 10 minutes, a clear chemical and microstructural differentiation between the top and bottom portions of the sample was observed, reflecting the dynamics of the reduction and separation processes (Fig. 4). Comparison with an experiment using pure argon plasma showed a significant difference in iron extraction (only 7% versus 33%), demonstrating the crucial role of hydrogen as a reducing agent (Fig. 5). The pH of the residual oxide was reduced from 10.5 to near-neutral, making it suitable for various applications. The process showed potential for extracting other valuable metals like titanium, contained in red mud.

The findings demonstrate the high efficiency and simplicity of the hydrogen plasma reduction process for extracting high-purity iron from red mud. The direct transformation of titanomagnetite to iron without the formation of wüstite suggests a unique autocatalytic reaction pathway. The process's ability to achieve near-neutral pH in the residual oxide is a significant advantage, addressing both waste management and environmental concerns. The potential for extracting other valuable metals such as titanium further enhances the economic viability of this approach. The results are promising for addressing two crucial environmental challenges simultaneously – red mud disposal and reducing the carbon footprint of steel production.

This study presents a novel, sustainable, and efficient method for extracting high-purity iron from red mud using a hydrogen plasma reduction process. The method is characterized by its simplicity, speed, and ability to produce near-neutral byproducts suitable for various applications. This represents a significant step towards sustainable waste management in the aluminum industry and significantly reduces greenhouse gas emissions in steel production. Future work can focus on optimizing the process parameters, conducting large-scale experiments, and exploring the extraction of other valuable metals from the residual oxide fraction.

The current study used a relatively small amount of red mud (15 g) in the experiments. Scaling up the process to industrial levels would require further investigation to ensure maintainability of efficiency and cost-effectiveness. The influence of red mud's geographical diversity on the process's effectiveness should also be further studied.