Team describes how to produce ‘green’ steel from toxic red sludge

Aluminum production generates around 180 million tonnes of toxic red sludge each year. Scientists from the Max-Planck-Institut für Eisenforschung, an iron research center, have now shown how green steel can be produced relatively simply from aluminum production waste. In an electric arc furnace similar to those used in the steel industry for decades, they convert iron oxide in the red mud into iron using hydrogen plasma.

With this process, nearly 700 million tonnes of CO₂Carbon-free steel could be produced from the 4 billion tonnes of red mud accumulated to date worldwide, which corresponds to a good third of the world’s annual steel production. As Max Planck’s team shows, the process would also be economically viable.

Demand for steel and aluminum is predicted to increase by up to 60% by 2050. Yet conventional production of these metals has a significant 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. At the same time, the aluminum industry produces around 180 million tonnes of red sludge each year, which is very 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 landfills, leading to 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.

Additionally, the highly alkaline red sludge corrodes the concrete walls of landfills, causing red sludge leaks that have already triggered environmental disasters on several occasions, for example in China in 2012 and Hungary in 2010. Additionally, large quantities of red mud mud are also simply disposed of in nature.

Saving potential of 1.5 billion tonnes of CO₂ in the steel industry

“Our process could simultaneously solve the problem of waste from aluminum production and improve the carbon footprint of the steel industry,” says Matic Jovičevič-Klug, who played a key role in the work as a scientist at Max-Planck-Institut für Eisenforschung. In a study published in the journal Nature, the team shows how red mud can be used as a raw material in the steel industry. In fact, waste from aluminum production contains up to 60% iron oxide.

Max Planck scientists melt the red mud in an electric arc furnace and simultaneously reduce the iron oxide content to iron using plasma containing 10% hydrogen. The transformation, known in technical jargon as plasma reduction, takes only ten minutes, during which the liquid iron separates from the liquid oxides and can then be easily extracted. Iron is so pure that it can be made directly into steel.

The remaining metal oxides are no longer corrosive and solidify on cooling to form a glass-like material which can be used as a filler 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 amounts of CO.₂. Using green hydrogen as a reducing agent helps avoid these greenhouse gas emissions.

“If green hydrogen were used to produce iron from the 4 billion tonnes of red sludge generated to date in global aluminum production, the steel industry could save almost 1.5 billion tonnes of CO .₂“, says Isnaldi Souza Filho, head of the research group at the Max-Planck-Institut für Eisenforschung.

An economical process, particularly with green hydrogen and electricity

The heavy metals contained in the red mud can also be practically neutralized using this process. “After reduction, we detected chromium in the iron,” explains Jovičevič-Klug. “Other heavy and precious metals are also likely to be embedded in the iron or in a separate area. This is something we will investigate in future studies. The precious metals could then be separated and reused. “

In addition, the heavy metals that remain in the metal oxides are firmly bound there and can no longer be removed with water, as can happen with red mud.

However, producing iron from red mud directly using hydrogen not only benefits the environment twice; it is also economically profitable, as the research team demonstrated in a cost analysis. With hydrogen and an electric mix for the electric arc furnace coming only from partially renewable sources, the process is worth it if the red mud contains 50% iron oxide or more.

If we also consider the costs of red mud disposal, only 35% iron oxide is enough to make the process economical. With green hydrogen and electricity, at current costs – also taking into account the cost of landfilling red sludge – a proportion of 30 to 40% iron oxide is required for the iron obtained to be competitive on the market.

“These are conservative estimates because the costs of red sludge disposal are probably quite low,” explains Isnaldi Souza Filho. And there is another advantage from a practical point of view: electric arc furnaces are widely used in the metallurgical industry, including in aluminum smelters, as they are used to melt scrap metal. In many cases, the industry would therefore only need to invest little to become more sustainable.

“It was important for us to also take economic aspects into account in our study,” explains Dierk Raabe, director of the Max-Planck-Institut für Eisenforschung. “It is now up to the industry to decide whether it will use plasma reduction of red mud to iron.”