With the help of hydrogen, an economical process can produce CO2-free iron from the waste of aluminium production, as proven by researchers at the Max Planck Institute for Iron Research.
The production of aluminium generates approx. 180 million tonnes of toxic red mud every year. Researchers at the Max Planck Institute for Iron Research have now demonstrated a comparatively simple method to produce green steel from aluminium production waste. Inside an electric arc furnace of a type used by the steel industry for decades, the iron oxide contained in red mud is transformed into iron with the aid of hydrogen plasma.
In this way, the four billion tonnes of red mud that currently exist worldwide could yield almost 700 million tonnes of CO2-free steel. This corresponds to over a third of annual global steel production. And, as demonstrated by the Max Planck team, this process would make sense from a business perspective, too.
According to forecasts, the demand for steel and aluminium will increase by up to 60 percent by 2050. However, conventional production of these metals comes with significant damage to the environment. For example, eight percent of global carbon emissions are generated by the steel industry, which makes it the sector with the highest greenhouse gas emissions. Every year, aluminium production generates approx. 180 million tonnes of red mud, which is highly caustic and contains traces of heavy metals like chromium. At best, this product is dried in a complex procedure and disposed of in gigantic landfills, for example in Australia, Brazil and China. Strong rains often cause red mud to be washed out of the landfill, and during dry periods, winds spread it throughout the environment in the form of dust. In addition, the highly alkaline red mud corrodes the concrete walls of the landfills and leaks out, which has resulted in several environmental disasters, for example in 2012 in China and in 2010 in Hungary. And besides, large amounts of red mud are just dumped into nature in any case.
Steel industry could reduce CO2 by 1.5 million tonnes
“Our process could at the same time solve the waste problem in the aluminium industry and improve the carbon footprint of the steel industry”, says Matic Jovičevič-Klug, a scientist who contributed significantly to the work at the Max Planck Institute for Iron Research.
In a study published in the scientific journal Nature, the team demonstrates how red mud can be used as a raw material for the steel industry. The reason for this is that up to 60 percent of this aluminium production waste product consists of iron oxide. Researchers at the Max Planck Institute melt the red mud inside an electric arc furnace while simultaneously using plasma with a hydrogen content of 10 percent to reduce the iron oxide the red mud contains to iron. This change, called plasma reduction by professionals, only takes about ten minutes, in which the liquid iron separates from the liquid oxides and can then be easily discharged. The purity of this iron is such that it can be directly worked into steel.
The remaining metallic oxides are no longer caustic and, on cooling, harden into a glass-like material which the construction industry can use as filler. Other research groups have produced iron from red mud using a similar process and coke, which, however, resulted in very impure iron and large amounts of CO2. Using green hydrogen as an agent for reduction means that these greenhouse gas emissions will no longer occur. “If you used green hydrogen to produce iron from the four billion tonnes of red mud so far accumulated in global aluminium production, the steel industry could save almost 1.5 billion tonnes of CO2”, says Isnaldi Souza Filho, head of the research team at the Max Planck Institute for Iron Research.
An economical process, even with green hydrogen and electricity
The heavy metals contained in red mud can also be rendered harmless by the same procedure. “We were able to demonstrate the presence of chromium within the iron after reduction”, says Matic Jovičevič-Klug. “Other heavy and precious metals may also be transferred to the iron or to a separate area. We will examine this in further studies. Precious metals could then be separated for further use.” According to Jovičevič-Klug, any heavy metals remaining within the metal oxides are securely bonded and can no longer be washed out by water, as is the case with red mud.
Direct production of iron from red mud, however, does not just have a double benefit for the environment. The process also makes sense from a business perspective, as the research team has demonstrated through an analysis of the costs. Using hydrogen and an electric arc furnace powered by an electricity mix from only partially renewable sources, the procedure becomes profitable if the red mud contains 50 percent of iron oxide. If the costs for disposing of red mud are considered, only 35 percent of iron oxide are enough for the procedure to turn a profit. With green hydrogen and electricity at today’s cost – and factoring in the expenses for dumping the red mud in landfills – an iron oxide of content of 30 to 40 percent is necessary for the iron to be competitive on the market. “These are cautious estimates, because the calculated costs for disposing of the red mud are probably on the low side”, says Isnaldi Souza Filho. Another advantage from a practical point of view: Electric arc furnaces are already established within the metals industry – including aluminium mills – as they are used for melting scrap metal. In many cases, the industry would therefore only need to make small investments to become more sustainable. “It was important to us to also consider the business aspects in our study”, says Dierk Raabe, Director of the Max Planck Institute for Iron Research. “Now it’s up to the industry whether they will make use of the plasma reduction of red mud to iron.”
