Red mud is piling up. Can scientists figure out what to do with it? | Science

A worker inspects ponds holding 30 million tons of red mud at an aluminum plant in Hungary. A 2010 spill from the ponds killed 10 people.

BELA SZANDELSZKY/AP PHOTO

Practical and glamorous, aluminium is prized for making products from kitchen foil and beverage cans to Tesla Roadsters and aircraft. But the silvery metal—abundant, cheap, lightweight, and corrosion resistant—has a dark side: red mud. This brownish red slurry, a caustic mishmash of metal- and silicon-rich oxides, often with a dash of radioactive and rare earth elements, is what’s left after aluminum is extracted from ore. And it is piling up. Globally, some 3 billion tons of red mud are now stored in massive waste ponds or dried mounds, making it one of the most abundant industrial wastes on the planet. Aluminum plants generate an additional 150 million tons each year.

Red mud has become trouble looking for a place to happen. In 2010, an earthen dam at one waste pond in Hungary gave way, unleashing a 2-meter-high wall of red mud that buried the town of Ajka, killing 10 people and giving 150 severe chemical burns. (See more on the dangers posed by waste dams.) Even when red mud remains contained, its extreme alkalinity can leach out, poison groundwater, and contaminate nearby rivers and ecosystems. Such liabilities, as well as growing regulatory pressure on industry to develop sustainable practices, have catalyzed global efforts to find ways to recycle and reuse red mud. Some researchers are developing ways to extract the valuable rare earth metals, whereas others turn the mud into cement or bricks.

“There is hope here,” says Yiannis Pontikes, a mechanical engineer at KU Leuven. But economic and marketing hurdles remain, and “the clock is ticking” as regulators consider new controls, says Efthymios Balomenos, a metallurgical engineer at the National Technical University of Athens. “At some point we will not be able to produce waste. So, there is an urgent need to make changes.”

Aluminum is one of the most commonly recycled materials, with 75% of all aluminum ever produced still in use. But there is an ever-burgeoning demand. Aluminum production starts with mining bauxite, a rock rich in aluminum oxide that also contains a wealth of other elements, including silicon, iron, and titanium. Workers extract the aluminum with a combination of treatments, including caustic chemicals, heat, and electricity. What remains is usually red, because of the iron, but its exact makeup can vary from region to region, depending on the ore, making it still harder to contend with. “The composition of [red mud] varies so much it means one [type of solution] will not work,” says Brajendra Mishra, a materials scientist at the Worcester Polytechnic Institute.

One approach that does seem to be working is tapping red mud as a source of scandium, a rare earth metal used to strengthen metal alloys. Researchers have recently shown that scandium-aluminum alloys are as much as 40% stronger than pure aluminum. That has manufacturers eagerly eyeing the alloy; aircraft manufacturers, for instance, could use it to build planes that have lighter aluminum framing and burn less fuel. But scandium currently costs $3500 per kilogram, so there’s plenty of incentive to find new, cheaper sources.

Scientists have come up with several ways to purify scandium from red mud. Balomenos’s group, for example, has shown it can use both sulfuric acid and compounds called ionic liquids to extract the rare earth. Ultimately, red mud could meet 10% of Europe’s demand for scandium, Balomenos says. Rusal, one of the largest aluminum producers in the world, is already building a pilot plant that uses related methods to extract scandium from red mud at one of its facilities in the Ural Mountains of Russia. But scandium makes up only about 140 parts per million of red mud, Pontikes notes, so “99.99% of the residue” still remains.

Other approaches aim to use more of the waste. One idea is to harness red mud, which is typically 40% to 70% iron oxide, to make iron-rich cements. The world uses more than 4 billion tons of cement per year, mostly as the binder in concrete. The most common version is Portland cement, made from calcium silicates that react with water to make create a tough, hard matrix.

But in 2015, researchers in New Zealand reported that by adding a common cement additive called silica fume to red mud, together with a modest amount of iron, they could create a cement with roughly the same hardness as Portland cement. Pontikes and his colleagues are working to extend these findings, by developing recipes that would enable manufacturers to make cement from a wide range of red muds with varying iron concentrations. The team hopes red mud could become a source of both the extra iron added to their cements and the alkaline compounds needed to catalyze the hardening reactions.

In the meantime, Pontikes’s lab is already producing about 1000 kilograms of iron-rich cements per day. They’ve even used their product for demonstration projects, such as a 2-ton staircase made with ultra–high-strength concrete. “This is no longer a lab-scale endeavor,” Pontikes says. He’s begun to talk with companies about making the cement on an industrial scale.

Red mud could form the basis for other construction materials. Pontikes and his team have found that if they add about 10% clay and silicate minerals to red mud and bake the mixture in a furnace, they can make bricks able to withstand 80 megapascals of compressive force, 40 times more than conventional bricks. They’re now looking to scale up the technique, which could be used to make everything from roofing tiles to sidewalk pavers.

Because of its chemistry, red mud can also capture and lock away carbon dioxide (CO2), the major climate warming gas. In Australia, aluminum producer Alcoa bubbles CO2 into red mud, creating a mild acid that reacts with the alkaline waste, forming carbonate minerals that turn the red mud into red sand that can be used to level road beds. The company estimates that the red mud from a single aluminum refinery can lock up 70,000 tons of CO2 per year, equivalent to taking more than 15,000 cars off the road.

Yet these glimmers of progress could fade, Balomenos says, just as earlier hopes have. Since 1964, he notes, researchers have patented some 700 uses for red mud, including tapping it to make decorative ceramics, dyes, and even fertilizer. Yet just 3% of red mud is currently recycled.

One major reason is that many schemes envision using red mud to make commodities that are already cheap and produced with methods that have been optimized over a century or more. In addition, red mud isn’t easy to handle. The iron industry has shied away from extracting the metal from it, for example, because the caustic waste destroys key components in their smelters. “The industry has iron ore available with much better quality,” Mishra says.

Balomenos argues that countries could push progress by establishing zero waste mandates for aluminum makers, or other incentives that force companies to recycle red mud instead of letting it pile up. The European Union has considered instituting a tax on waste deposited in landfills, for example. But it hasn’t done so, and there appears to be little appetite elsewhere for similar ideas.

Another obstacle, Balomenos says, is international opposition to allowing hazardous materials to cross borders. As a result, it can be cumbersome and costly to move red mud that contains even trace amounts of heavy metals or radioactivity. For now, he says, simply putting the waste in a landfill is both cheaper and far simpler.

Finally, there is the question of consumer acceptance. Even if scientists and engineers manage to come up with a suite of practical uses for red mud, consumers still have the final say in whether they will buy products with such a noxious starting point. “Will you use roofing tiles made with red mud?” Pontikes asks. “It’s up to the market to say ‘yes.’”

Kent

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