A nanofiltration technique has the potential to minimize the harmful waste produced during aluminum manufacturing.
Aluminum, widely used in products such as soda cans, foil, circuit boards, and rocket boosters, ranks as the world’s second-most-produced metal, trailing only steel. By the decade’s end, a marked increase in demand is anticipated, which could boost aluminum production by 40 percent globally. Such a significant escalation could amplify environmental effects, particularly the pollutants linked to its production waste.
Engineers from MIT have introduced a novel nanofiltration technique aimed at reducing hazardous waste from aluminum production. This method could effectively process aluminum plant waste and reclaim aluminum ions that would otherwise be lost in waste water. The retrieved aluminum can potentially be repurposed and reintroduced into the production stream, boosting output while simultaneously lessening waste.
The researchers showcased the membrane’s capabilities through laboratory-scale tests, employing an innovative membrane to filter solutions that mimic the waste produced by aluminum manufacturing. Their findings revealed that the membrane effectively captured over 99 percent of aluminum ions from these solutions.
If expanded and implemented in current production facilities, this membrane technology could dramatically cut down on aluminum waste and enhance the environmental quality of the waste generated by these plants.
“This membrane technology not only reduces hazardous waste but also fosters a circular economy for aluminum by minimizing the need for new mining,” explains John Lienhard, the Abdul Latif Jameel Professor of Water in the Department of Mechanical Engineering and the director of the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS) at MIT. “This presents a promising way to tackle environmental issues while satisfying the growing aluminum demand.”
In a study published in ACS Sustainable Chemistry and Engineering, Lienhard and his colleagues, including MIT mechanical engineering undergraduates Trent Lee and Vinn Nguyen, along with postdoc Zi Hao Foo SM ’21, PhD ’24 from the University of California, Berkeley, report their findings.
A Recycling Opportunity
The group led by Lienhard at MIT is focused on developing filtration and membrane technologies for seawater desalination and wastewater treatment. While searching for new applications, they identified an untapped area in aluminum production and the associated wastewater.
Aluminum’s production process begins with the mining of bauxite, a metal-rich ore, followed by chemical processes aimed at separating aluminum from other materials. This process yields aluminum oxide, commonly known as alumina, which is then delivered to refineries. Here, alumina powder is combined with a molten mineral called cryolite in electrolysis vats. A strong electric current is applied, which breaks the chemical bonds in alumina to derive aluminum and oxygen atoms. The resulting pure aluminum settles to the bottom of the vat for collection and casting.
“Our research revealed that a traditional aluminum plant wastes around 2,800 tons of aluminum annually,” states lead author Trent Lee. “We sought out ways for the industry to become more efficient and discovered that recycling options for cryolite waste hadn’t been sufficiently explored.”
A Charged Innovation
The researchers aimed to create a membrane process capable of filtering cryolite waste to recover aluminum ions that inevitably enter the waste stream. They specifically focused on retaining aluminum while permitting other ions, notably sodium, to pass through, as sodium builds up in cryolite over time.
The objective was to capture aluminum from cryolite waste so that it could be returned to the electrolysis vat without introducing too much sodium, which would slow down the electrolytic process.
The newly designed membrane builds upon existing water treatment membrane technologies. Traditional membranes comprise a thin polymer sheet with tiny pores calibrated to allow specific ions and molecules to pass.
Typical membrane surfaces carry a natural negative charge, which repels negatively charged ions while attracting positively charged ones to move through.
Team members collaborated with the Japanese membrane manufacturer Nitto Denko to investigate the performance of commercially available membranes that could filter most positively charged ions in cryolite wastewater while capturing aluminum ions. However, aluminum ions possess a positive charge of +3, whereas sodium and other cations only carry a +1 charge.
Building on their previous research into membranes for extracting lithium from salt lakes and used batteries, the team tested a new Nitto Denko membrane that features a thin, positively charged coating. This coating is designed to repel more positively charged aluminum while allowing less positively charged ions to flow through.
“Aluminum is the most positively charged ion present, which is why it tends to be repelled by the membrane,” Foo clarifies.
In their experiments, the team tested the membrane using solutions with varying ion compositions similar to those in cryolite waste. They discovered that the membrane consistently captured 99.5 percent of aluminum ions while allowing sodium and other cations to pass through. Additionally, varying the solutions’ pH revealed that the membrane maintained its efficiency even in highly acidic conditions for several weeks.
“Cryolite waste streams can vary in acidity,” Foo explains. “We found that the membrane performs exceptionally well, even under the harsh conditions we anticipated.”
The experimental membrane is about the size of a playing card. To extract aluminum ions from cryolite waste at an industrial scale, the researchers envision a larger version of the membrane reminiscent of those used in many desalination facilities, where it is configured in spirals that allow water to flow through.
“This research highlights the potential of membranes for advancing circular economies,” Lee asserts. “This membrane technology not only recycles aluminum but also mitigates the problem of hazardous waste.”
