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Aluminium production from raw materials – primary aluminium production – requires more energy than any other industrial manufacturing method and releases large amounts of carbon dioxide (CO2) into the atmosphere. In addition, half of the ore processed ends up as waste. At the same time, aluminium and the products made from it are vitally important to economies and societies around the world.
The EU-funded ENEXAL project has developed novel ways that could make aluminium production more sustainable and competitive. One process transforms ore waste into useful products. Two others reduce the energy needed to convert the ore’s aluminium oxide – known as alumina – into aluminium, reducing greenhouse gas emissions.
The innovations show how the aluminium industry could eliminate waste and contribute to energy-saving.
“We have provided the primary production industry with green innovative technological and economical solutions,” says Thymis Balomenos, a member of the ENEXAL coordination team and currently an external associate of Mytilineos S.A., now owners of ENEXAL coordinating partner Aluminium of Greece, an aluminium-producing company).
‘Red mud’ conversion
The first part of ENEXAL investigated how to ‘close the loop’ in aluminium production by finding uses for its waste.
In the first stage of production, alumina is refined from bauxite, a naturally occurring ore containing up to 60 % aluminium oxide. The unused bauxite residue after processing – known as ‘red mud’ – represents a loss of resources.
“It amounts to throwing away half the input material, along with the energy used to process it,” says Balomenos.
To tackle the problem, ENEXAL’s researchers developed a process for turning dried red mud into pig iron, used to make wrought iron and steel. This uses up the iron oxide that makes up 45 % of the bauxite residue. They then made the remaining minerals into mineral wool felt, used for construction and insulation, reducing input losses to zero.
The project demonstrated this technique successfully for two years at Aluminium of Greece, says Balomenos.
Lower-energy refining
The second part of the project developed techniques for more sustainable energy use when converting alumina into aluminium, stage two of the production process. This step today takes place by molten salt electrolysis, a highly energy-intensive process that turns two kilos of alumina into one kilo of metallic aluminium.
ENEXAL researchers developed two conversion methods based on a technique called carbothermic reduction. Here, carbon replaces the electric current used in electrolysis, lowering the conversion’s overall environmental footprint as it uses less energy for the conversion.
M Halmann et AL.
Al2O3 + 3C = 2Al + 3CO
Al2O3 + 3CH4 = 2Al + 3CO + 6H2
Although explored for aluminium smelting for decades, carbothermic reduction has not been adopted by industry as it requires high temperatures and significant amounts of aluminium are either lost or need to be recycled in the process. In the project’s new methods, the operation temperature is lower. Meanwhile less aluminium enters side products, increasing final yields.
One technique adapts electric arc furnaces, an established technology, to produce directly aluminium-silicon alloys from alumina and silica. This saves energy as these alloys – important in construction and vehicles – are usually made by mixing separately-produced aluminium and silicon metals.
A second technique uses a novel solar furnace that refines alumina in a vacuum at 1 600 °C, which can cut overall non-renewable energy requirements by up to 68 %. In a demonstration by ENEXAL’s Israeli and Swiss research institute partners, it harnessed heat from a concentrated solar plant to produced aluminium.
“Although far from industrial application, this was a big step,” says Balomenos.
Sustainable industry
All three innovations are part of a long-term drive towards more sustainable aluminium production, needed, as recycling alone will not meet current or near future demand, Balomenos says. Aluminium companies and some of ENEXAL’s research partners are now investigating other ways to reduce waste, for example in the EU-funded SCALE and REMOVAL projects.
In terms of aluminium smelting, Balomenos says that the electric arc furnace technique has commercial potential if developed further. And although the solar furnace is still in the research realm, he predicts a snowball effect for solar metallurgy as experimental technology boosts ideas and new research opportunities.