How to produce green steel from aluminum’s red mud

Red mud near Stade, Germany.(Reference image by Ra Boe, Wikimedia Commons.)

Scientists at the Max-Planck-Institut für Eisenforschung, a centre for iron research, have shown how green steel can be manufactured from aluminium production waste in a relatively simple way.

In a paper published in the journal Nature, the researchers point out that the feasibility of their technique relies on the fact that the production of aluminium generates millions of tonnes of toxic red mud every year.

In an electric arc furnace similar to those used in the steel industry for decades, they were able to convert the iron oxide contained in 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 worldwide to date – which corresponds to a third of annual steel production worldwide.

Dealing with an environmental passive

According to forecasts, demand for steel and aluminium will increase by up to 60% by 2050. However, 8% of global CO2 emissions come from the steel industry, making it the sector with the highest greenhouse gas emissions. Meanwhile, the aluminium industry produces around 180 million tonnes of red mud every year, which 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.

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 Hungary in 2010 and in China in 2012. In addition, large quantities of red mud are also simply disposed of in nature.

“Our process could simultaneously solve the waste problem of aluminium production and improve the steel industry’s carbon footprint,” Matic Jovičevič-Klug, co-author of the study, said in a media statement.

The reason why the new process works is because red mud from aluminium production consists of up to 60% iron oxide and the transformation it goes through, known in technical jargon as plasma reduction, takes just 10 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 filling in the construction industry, for example. Other research groups have produced iron from red mud using a similar approach and employing 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 aluminium production to date, the steel industry could save almost 1.5 billion tonnes of CO2,” head researcher Isnaldi Souza Filho said.

Green hydrogen in the mix

In addition to the prior, the research team discovered that the heavy metals in the red mud can also be virtually neutralized using the process.

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

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.

The scientists note that producing iron from red mud directly using hydrogen not only benefits the environment twice over, but it pays off economically too. 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.

“These are conservative estimates because the costs for the disposal of the red mud are probably calculated rather low,” Souza Filho said.

And there is another advantage from a practical point of view: electric arc furnaces are widely used in the metal industry – including in aluminium smelters – as they are used to melt down scrap metal. In many cases, the industry would need to invest only a little to become more sustainable.

“Now it’s up to the industry to decide whether it will utilize the plasma reduction of red mud to iron,” Dierk Raabe, director at the Max-Planck-Institut für Eisenforschung, said.