Researchers have devised an economically viable way of reducing the environmental impact of both the steel and aluminum industries by using hydrogen to melt down the toxic red mud left over from aluminum production to produce green steel in around 10 minutes.
The aluminum industry produces around 198 million tons (180 million tonnes) of bauxite residue – ‘red mud’ – yearly, which is extremely corrosive because it has high alkalinity and is rich in toxic heavy metals. In countries such as Australia, China and Brazil, the leftover red mud is usually disposed of in gigantic landfills, with high processing costs. The steel industry is equally environmentally damaging, responsible for 8% of global carbon dioxide emissions. Yet, the demand for steel and aluminum is forecast to increase by up to 60% by 2050.
However, scientists from the Max-Planck-Institut für Eisenforschung, Germany, a center for iron research, may have a solution to turn the toxic red mud by-product left over from aluminum production into green steel.
“Our process could simultaneously solve the waste problem of aluminum production and improve the steel industry’s carbon footprint,” said Matic Jovičević-Klug, the study’s lead author.
Red mud consists of up to 60% iron oxide. Melting the mud in an electric arc furnace using a plasma containing 10% hydrogen reduces it to liquid iron and liquid oxides, allowing the iron to be easily extracted. The plasma reduction technique takes 10 minutes and produces iron so pure, say the researchers, it can be processed directly into steel. And the no-longer-corrosive metal oxides solidify on cooling, so they can be transformed into glass-like material that could be used as a filling material in the construction industry.
Other researchers have produced iron from red mud using a similar approach, but with coke; however, it results in highly contaminated iron and large quantities of carbon dioxide. The approach taken in the new study, 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," said Isnaldi Souza Filho, corresponding author of the study.
The toxic heavy metals originally found in the red mud are “virtually neutralized” using this process. Any heavy metals that remain are firmly bound within the metal oxides and can’t be washed away with water, as can happen with red mud left in a landfill site.
“After reduction, we detected chromium in the iron,” Jovičević-Klug said. “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.”
The researchers say that producing iron from red mud directly using green hydrogen benefits the environment “twice over " and is economically beneficial. Using their calculations, if the red mud contains 35% iron oxide, this is enough to make the process economical. Taking the cost of green hydrogen and electricity to power the arc furnace at today’s prices and including the cost of landfilling the red mud, a proportion of 30% to 40% iron oxide in the mud would be needed for the resulting iron to be competitive in the market.
“These are conservative estimates because the costs for the disposal of the red mud are probably calculated rather low,” said Souza Filho.
In addition, electric arc furnaces are widely used in the metal industry – including in aluminum smelters – which would require industries to make only limited investments to become more sustainable.
“It was important for us to also consider economic aspects in our study,” said Dierk Raabe, a study co-author. “Now it’s up to the industry to decide whether it will utilize the plasma reduction of red mud to iron.”
The study was published in the journal Nature.
Source: Max-Planck-Gesellschaft
