Aluminum, one of the most commonly used metals in manufacturing, has a dark side. Its production generates a byproduct called bauxite residue—or “red mud”—that contains toxic minerals and has a highly basic pH. Returning red mud to the environment would mean poisoning local water supplies and contaminating ecosystems, making it a no-go. This means billions of tons of red mud are sitting in mounds or waste ponds worldwide, accumulating as demand for aluminum increases.
Scientists in Germany seem to have found a use for all that waste. In a paper published Wednesday in the journal Nature, they write that red mud might have a future as the prime ingredient in “green steel.” By turning bauxite residue into what they call a feedstock for ironmaking, fabricators could help reduce red mud buildup without unleashing its full potential for harm on the environment.
Red mud is born from bauxite ore. When alumina is separated from the ore, red mud is left behind. One of red mud’s most abundant components is iron oxide, which, when it reacts with carbon, becomes solid iron and carbon dioxide. But while solid iron is a practical material, Earth isn’t exactly short on carbon dioxide, which has its own disastrous (albeit long-term) environmental effects. To improve this process, scientists have been experimenting with combining red mud’s iron oxides with hydrogen, a more sustainably made resource that produces water instead.
A pool of “red mud.”
Credit: Ra Boe/Wikimedia Commons
This twist inspired researchers at Germany’s Max Planck Institute for Iron Research and the Federal Institute for Materials Research and Testing. Together, they heated 15 grams of red mud in an electric arc furnace (EAF), a vessel that heats material while reacting it with your atmosphere of choice. The team opted for an atmosphere of hydrogen and non-reactive argon. In under 15 minutes, they had 2.6 grams of iron and about 6 grams of water, which could be reused for hydrogen production. The process had also neutralized the pH of the leftover red mud. They’d successfully reduced their quantity of red mud, made the remainder a bit safer, and obtained two useful resources, one of which could be used to make steel.
Not all that glitters is gold (er, iron), however. Using an EAF in this way required a lot of energy, as did making the hydrogen that went into the EAF’s internal atmosphere. Additionally, while the red mud had been shrunken down and neutralized, it still contained high quantities of the very minerals that make it environmentally harmful. While some of those components, such as rare earth minerals, might someday be extracted and used in other manufacturing processes, the toxic minerals still need somewhere to go.
Nonetheless, this controlled chemical reaction offers an alternative to an inconvenient (at best) and dangerous (at worst) manufacturing method. With nearly 4 billion tons of red mud piled up around the globe, novel approaches to the byproduct’s use are in high demand. Using the sludge to create iron—and, indirectly, steel—could help mitigate those rust-hued heaps.
