Scientists at the Max-Planck-Institut für Eisenforschung, a center for iron research, have developed a process to convert the toxic red mud waste resulting from the refinement of bauxite into alumina into valuable and sustainable feedstock for iron-making using green hydrogen-plasma-based reduction. An open-access paper on their work is published in the journal Nature.

About 180 million tonnes of red mud is produced each year, making it one the the largest environmentally hazardous waste products, with an aggregate of some 4 billion tonnes accumulated on a global scale.

Using an electric arc furnace similar to those used in the steel industry for decades, the Max-Planck researchers convert the iron oxide contained in the red mud into iron using hydrogen plasma. With this process, almost 700 million tonnes of CO₂-free steel could be produced from the four billion tonnes of red mud that have accumulated to date—corresponding to a good third of annual steel production worldwide. The process would also be economically viable.

Schematic representation of the hydrogen-plasma-based process used to convert 15 g red mud portion into metallic iron. Jovičević-Klug et al.

According to forecasts, demand for steel and aluminum will increase by up to 60% by 2050. The conventional production of these metals has a considerable impact on the environment. Eight percent of global CO₂ emissions come from the steel industry, making it the sector with the highest greenhouse gas emissions.

Red mud is highly alkaline and contains traces of heavy metals such as chromium. In Australia, Brazil and China, among others, this waste is at best dried and disposed of in gigantic landfill sites, resulting in high processing costs. When it rains heavily, the red mud is often washed out of the landfill, and when it dries, the wind can blow it into the environment as dust. In addition, the highly alkaline red mud corrodes the concrete walls of the landfills, resulting in red mud leaks that have already triggered environmental disasters on several occasions—for example in China in 2012 and in Hungary in 2010. In addition, large quantities of red mud are also simply disposed of in nature.

The red mud, however, also consists of up to 60% iron oxide. The Max-Planck scientists melt the red mud in the electric arc furnace and simultaneously reduce the contained iron oxide to iron using a plasma that contains 10% hydrogen. The plasma reduction takes just ten minutes, during which the liquid iron separates from the liquid oxides and can then be extracted easily. The iron is so pure that it can be processed directly into steel.

The remaining metal oxides are no longer corrosive and solidify on cooling to form a glass-like material that can be used as a filling material in the construction industry, for example. Other research groups have produced iron from red mud using a similar approach with coke, but this produces highly contaminated iron and large quantities of CO₂. Using green hydrogen as a reducing agent avoids these greenhouse gas emissions.

If green hydrogen would be used to produce iron from the four billion tonnes of red mud that have been generated in global aluminum production to date, the steel industry could save almost 1.5 billion tonnes of CO₂.

—Isnaldi Souza Filho, Research Group Leader at the Max-Planck-Institut für Eisenforschung

The heavy metals in the red mud can also be virtually neutralized using the process.

After reduction, we detected chromium in the iron. Other heavy and precious metals are also likely to go into the iron or into a separate area. That's something we’ll investigate in further studies. Valuable metals could then be separated and reused.

—Matic Jovičevič-Klug, lead author

Heavy metals that remain in the metal oxides are firmly bound within them and can no longer be washed out with water, as can happen with red mud.

With hydrogen and an electricity mix for the electric arc furnace from only partially renewable sources, the process is worthwhile, if the red mud contains 50% iron oxide or more. If the costs for the disposal of the red mud are also considered, only 35% iron oxide is sufficient to make the process economical.

With green hydrogen and electricity, at today's costs—also taking into account the cost of landfilling the red mud—a proportion of 30 to 40% iron oxide is required for the resulting iron to be competitive on the market.

Resources

• Jovičević-Klug, M., Souza Filho, I.R., Springer, H. et al. (2024) “Green steel from red mud through climate-neutral hydrogen plasma reduction.” Nature 625, 703–709 doi: 10.1038/s41586-023-06901-z