A way to turn red mud into green steel

Aluminum is vital for making lightweight cars, beverage cans, and kitchen utensils. But its production creates a by-product called red mud, a caustic slurry containing iron and other metals. The large global aluminum market has led to almost 4 billion metric tons of red mud sitting in waste ponds or dumped into landfills or the environment.

Now researchers report a relatively simple way to extract iron from red mud: by blasting it with ahydrogen plasma in an electric-arc furnace (Nature 2024, DOI: 10.1038/s41586-023-06901-z) . Renewable electricity could power the process, says Isnaldi R. Souza Filho, a materials engineer at the Max Planck Institute for Iron Research. If the team can scale up the process, it would be a sustainable route to produce iron for steelmaking. Most of steel’s carbon dioxide emissions come from smelting iron ore.

Red mud is a growing environmental hazard. It forms when bauxite ore is treated with sodium hydroxide to extract aluminum oxide. Each metric ton of aluminum manufactured creates up to 4 metric tons of red mud, which has high water content and a composition that varies widely bygeography. Only about 3% of red mud is recycled to make bricks and pigments. Most is discarded and can leach corrosive materials and toxic metals into groundwater.

Researchers are exploring red mud as a source of iron and rare earth metals such as scandium. But the caustic, complex slurry can be challenging to work with. Extracting iron typically begins with roasting and pretreating the sludge and reducing iron oxides using carbon, all of which emits CO₂.

Souza Filho and his colleagues put untreated 15 g samples of red mud on a copper plate in an electric-arc furnace fed with a gas mixture containing inert argon and 10% hydrogen. The high furnace temperature melted the ore, while electrons from the arc collided with hydrogen atoms, creating a plasma of hydrogen ions.

The ions stripped oxygen from iron oxides in the viscous melt, forming pockets of pure liquid iron, Souza Filho explains. “Removing oxygen precipitates iron because it is the element in red mud with the lowest affinity to oxygen.” When the researchers switched off the furnace after 10 min, the water-cooled copper plate quickly solidified the entire melt into nodules, and the density difference between liquid iron and the remaining red mud caused iron to form spherical particles within the nodules.

The process extracted 70% of the red mud’s iron, says postdoctoral researcher Matic Jovičević-Klug, who used a hammer to smash the nodules and collect the iron spheres. In a larger furnace, the samples could be cooled more slowly, he says, “so all the iron would coalesce and sink to the bottom, making one large piece that is easier to separate.”

The use of hydrogen plasma to reduce iron oxides is novel and important for reducing carbon use, says Corby Anderson, a metallurgical engineer at the Colorado School of Mines. But the cost and energy use of the furnace could be a hurdle for large-scale adoption of this technique, and the complex nature of various red muds could pose a challenge.

Jovičević-Klug says the Max Planck team is now optimizing the process by tweaking parameters such as temperature, with plans to scale up and “look beyond iron” to extract other valuable metals from red mud.