From red mud to green steel

Toxic red mud is set to turn green. Brazilian metallurgist Isnaldi R. Souza Filho and his colleagues at the Max Planck Institute for Iron Research (MPIE) in Düsseldorf, Germany, may have found an answer to environmental concerns associated with aluminium and steel, the world's most produced metals.

While the process of making aluminium — an abundant, cheap, and lightweight metal with applications ranging from kitchen foils to aircraft parts — is relatively clean, it leaves behind a nasty by-product called red mud. This brownish-red slurry, a cocktail of metal- and silicon-rich oxides, is one of the most abundant industrial waste materials on the planet. Its red hue stems mainly from the presence of iron oxides.

About 4 billion tonnes of red mud have been dumped in gigantic landfills around the world so far. For every tonne of alumina (a hydrated form of aluminium oxide) produced, 1.3-1.5 tonnes of red mud are generated. According to an industry expert who earlier worked with the National Aluminium Company Limited (NALCO), an audit carried out in 2019 showed that the tailing pond of the NALCO plant, located at Damanjodi in Odisha, carried some 55 million tonnes of red mud.

Now, Souza and his colleagues are turning red mud into an eco-friendly source of iron for making steel. The work, reported in the journal Nature in January 2024 (bit.ly/gsteel), has the potential to solve two knotty environmental issues at once: safely disposing of toxic red mud, and reducing the carbon footprint associated with steel production.

Red mud is highly alkaline and contains traces of many heavy metals. When it leaches into water bodies and fields, it damages crops and impacts human health. The storage of red mud also often holds up vast tracts of land. Every year, aluminium plants worldwide add 180 million tonnes of red mud to the existing stockpile, which will only increase in the coming years with an increase in aluminium production.