Researchers have designed an economically feasible method to use hydrogen to melt toxic red mud produced during aluminum production to produce green steel in about 10 minutes, thereby reducing the environmental impact of the steel and aluminum industries.
The aluminum industry produces approximately 198 million tons (approximately 180 million tons) of bauxite residue - "red mud" - every year, which is extremely corrosive due to its high alkalinity and rich content of toxic heavy metals. In countries such as Australia, China and Brazil, leftover red mud is often dumped in huge landfills, where it is costly to dispose of. The steel industry also causes serious damage to the environment, accounting for 8% of the global total carbon dioxide emissions. However, demand for steel and aluminum is expected to grow by up to 60% by 2050.
But scientists at Germany's Max-Planck-Institutfür Eisenforschung may have a way to turn toxic red mud by-products from aluminum production into green steel.
"Our process can simultaneously solve the scrap problem in aluminum production and improve the carbon footprint of the steel industry," said Matic-Jovicevic-Kruger, the study's lead author. "Simultaneously solve the scrap problem in aluminum production and improve the carbon footprint of the steel industry."
Red mud contains up to 60% iron oxide. Using plasma containing 10% hydrogen to melt red mud in an electric arc furnace can reduce it to liquid iron and liquid oxide, allowing easy extraction of iron. Researchers say the plasma reduction technology only takes 10 minutes, and the iron produced is so pure that it can be directly processed into steel. The metal oxide, which is no longer corrosive, solidifies when cooled and can therefore be transformed into a glass-like material that can be used as a filler material in the construction industry.
Other researchers have used a similar method to produce iron from red mud, but using coke; however, this method results in heavy contamination of the iron and the production of large amounts of carbon dioxide. The method used in the new study avoids the emission of these greenhouse gases by using green hydrogen as a reducing agent.
Isnaldi Souza Filho, corresponding author of the study, said: "If green hydrogen is used to extract iron from the 4 billion tons of red mud produced in global aluminum production to date, the steel industry could save nearly 1.5 billion tons of carbon dioxide."
Through this process, the toxic heavy metals originally present in the red mud are "almost neutralized". Any remaining heavy metals are firmly bound in the metal oxides and will not be washed away by water like red mud left in landfills.
"After reduction, we detected chromium in the iron. It is possible that other heavy and precious metals also passed into the iron or into a separate zone. We will investigate this in further studies. Valuable metals can be separated out and reused," says Jovičević-Klug.
Researchers say using green hydrogen directly to produce iron from red mud is not only "double beneficial" to the environment, but also highly economical. According to their calculations, if the red mud contained 35% iron oxide, it would be enough to make the process economical. Calculating the cost of green hydrogen and electric arc furnace electricity at current prices, plus the cost of landfilling red mud, the proportion of iron oxide in red mud needs to reach 30% to 40%, so that the iron produced can be competitive in the market.
These are conservative estimates, as the cost of processing red mud is likely calculated to be quite low. In addition, electric arc furnaces are widely used in the metals industry, including aluminum smelters, where they can achieve greater sustainability with limited investment.
Dierk Raabe, co-author of the study, said: "In our study, it was also important to consider economic factors. It is now up to industry to decide whether to use plasma to reduce iron in red mud."
The research was published in the journal Nature.