- Red mud, a byproduct of aluminium production, contains up to 60% iron and poses significant environmental risks when disposed of improperly.
- Researchers propose a method to transform red mud into low-carbon iron using green hydrogen plasma, potentially saving 1.5 billion tonnes of CO2 in the steel industry.
- Economic analysis suggests the process is viable with certain iron oxide content levels in red mud, offering both environmental and economic benefits for sustainable steel production.
Aluminium production remains a major contributor of global carbon emissions, with far higher emissions per ton than steel (though with lower overall volumes). The biggest contribution comes from the smelting process, converting alumina to aluminium, with over 15MWh of electricity per tonne of aluminium. As ever, where this electricity comes from (i.e., from low-quality coal-burning power plants up to hydroelectric) has the biggest impact on the actual CO2e.
Refining of bauxite, the red-coloured rock with ~15-20% aluminium content, is by comparison relatively low impact. The Bayer process that turns bauxite into alumina operates at relatively low temperatures, and subsequently energy requirements are a fraction of the smelting process. What the Bayer process does produce however — and produces prodigiously — is ‘red mud’.
Red mud is a slurry of waste products with a pH level similar to bleach, around pH 10-13, and containing various heavy metals, oxides and other unpleasantness. Or rather it’s unpleasant because currently the stockpile of red mud from bauxite refining is either pumped into huge waste lakes, piled as it dries or landfilled. That excludes the unknown proportion that is simply ‘returned to nature’. The potential for human and environmental disaster is therefore ‘non-neglible’, through leaching, dust distribution and spillage. There’s currently estimated to be 4 billion tonnes of red mud, growing at about 180 million tonnes per year.
One person’s waste…
What red mud does contain is up to 60% iron in various oxides, though this percentage changes by location. Researchers from the Max-Planck-Institut für Eisenforschung, Institut für Bildsame Formgebung, RWTH Aachen University, Aachen, and Bundesanstalt für Materialforschung und -prüfung, Berlin, have published a paper on the creation of a sustainable iron feedstock for the steel industry using the abundant waste. In what the researchers claim is a “surprisingly simple and fast reduction method”, useful low-carbon iron for green steel is created from the red mud using green hydrogen as a reducing agent.
The red mud is melted in an electric arc furnace and simultaneously the contained iron oxide to iron is reduced using a plasma that contains ten percent 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 extracted iron is pure enough to go directly into steelmaking.
“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.”Isnaldi Souza Filho, Research Group Leader at the Max-Planck-Institut für Eisenforschung.
Once the iron has been removed, the red mud could be re-processed to liberate the traces of valuable and/or harmful metals remaining including cadmium, yttrium, vanadium and scandium.
The direct production of iron from red mud using hydrogen presents dual benefits: environmental and economic. A cost analysis conducted by the research team indicates that using hydrogen alongside an electricity mix for the electric arc furnace—derived from partially renewable sources—renders the process economically viable when the red mud comprises at least 50 percent iron oxide.
“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.”Dierk Raabe, Director at the Max-Planck-Institut für Eisenforschung.
Incorporating the expenses associated with red mud disposal lowers the requisite iron oxide content to 35 percent for the process to be considered economically feasible. Employing green hydrogen and electricity, taking into account current costs and including landfilling expenses for the red mud, necessitates an iron oxide content ranging from 30 to 40 percent for the iron produced to be competitive in the market.
According to Isnaldi Souza Filho, these figures are based on conservative estimates, with the disposal costs of the red mud likely underestimated. Additionally, from a practical perspective, the widespread use of electric arc furnaces in the metal industry, including aluminium smelters for melting scrap metal, implies minimal investment is required for industry players to enhance sustainability.