Home » Hydrogen innovation unlocks green steel from toxic red mud

Hydrogen innovation unlocks green steel from toxic red mud

by Mark Cantrell
Scientists claim to have developed an economical way to make green steel from the toxic red mud generated by the production of aluminium

Now this is some recycling ‘alchemy’; scientists claim to have developed an economical way to make green steel from the toxic red mud generated by the production of aluminium.

The ‘magic’ ingredient is hydrogen. Scientists at the Max-Planck Institute’s centre for iron research in Germany have shown how it can be used to extract the steel from aluminium production waste in what they say is a relatively simple way.

The process uses an electric arc furnace similar to those that have been used in the steel industry for decades, however the team converted the iron oxide in the mud into raw iron using a hydrogen plasma.

With this process, the scientists claim, almost 700m tonnes of CO2-free steel could be produced from the four billion tonnes of red mud that have accumulated around the world to date. This equates to a “good third” of annual steel production globally. Furthermore, the Max Planck team says its process is economically viable.

It’s a critical issue, given that both steel and aluminium are such ubiquitous materials, with widespread use across all kinds of applications in modern industry and society. Demand for steel and aluminium is forecast to increase by up to 60% by 2050. Yet, both metals exert a massive detrimental hit on the environment, one way or another.

Heavy metal

Eight percent 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 180m tonnes of the red mud every year. The sludge is highly alkaline and contains traces of heavy metals such as chromium, but up to 60% of its content is iron oxide.

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.

In addition, the mud’s high alkali content corrodes the concrete walls of the landfills, resulting in red mud leaks that have already triggered environmental disasters on several occasions, for example in China in 2012, and in Hungary in 2010. In addition, large quantities of red mud are also simply disposed of in nature.

So, clearly, turning the aluminium’s waste product into a raw material to generate green steel goes a long way to addressing those negative impacts.

Matic Jovičevič-Klug, one of the Max Planck scientists involved in the project, said: “Our process could simultaneously solve the waste problem of aluminium production and improve the steel industry’s carbon footprint.”

Electric arc

The scientists melt the red mud in an electric arc furnace and simultaneously reduce the contained iron oxide to iron using a plasma that contains 10% hydrogen. The transformation, known in technical jargon as plasma reduction, takes just ten minutes, during which the liquid iron separates from the liquid oxides and can then be extracted easily. The iron is said to be 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 – for example – be used as a filling material in the construction industry.

Other research groups have produced iron from red mud using a similar approach but with coke. However this produces highly contaminated iron and large quantities of CO2, says the Max Planck team. By using green hydrogen as a reducing agent, these greenhouse gas emissions are avoided. avoids these greenhouse gas emissions.

Isnaldi Souza Filho, research group leader at the Max Planck Institute, said: “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 1.5bn tonnes of CO2.”

According to the team, producing iron from red mud directly using hydrogen not only benefits the environment twice over; it pays off economically too, as the research team demonstrated in a cost analysis.

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.

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

There’s another advantage from a practical point of view, the scientists say: 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 therefore need to invest only a little to become more sustainable.

Dierk Raabe, director at the Max Planck Institute’s centre for iron research, said: “It was important for us to also consider economic aspects in our study. Now it’s up to the industry to decide whether it will utilize the plasma reduction of red mud to iron.”

Main image: A dump for red mud close to Stade, Germany. Wikimedia CC-by-3.0

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