Used in everything from soda cans and foil to circuit boards and rocket boosters, aluminum is the second most produced metal in the world after steel. In the next decade, global aluminum production is expected to grow by 40% due to increasing demand. This dramatic increase will increase aluminum’s environmental impact, including the pollutants released with manufacturing waste.
MIT engineers have developed a new nanofiltration process to limit hazardous waste generated from aluminum production. Nanofiltration could be used to treat waste from aluminum plants and capture aluminum ions that leak into wastewater streams. The resulting aluminum could be recycled and added to the majority of aluminum produced, thus increasing production and reducing waste.
The researchers demonstrated the membrane’s performance in laboratory-scale experiments by filtering a variety of solutions with compositions similar to waste streams generated by aluminum plants through the new membrane. They found that the membrane selectively captured more than 99 percent of the aluminum ions in these solutions.
Scaling up and implementing the membrane technology in existing manufacturing facilities has the potential to reduce aluminum emissions and improve the environmental quality of waste generated by the plants.
“Not only does this membrane technology reduce hazardous waste, it also creates a circular economy for aluminum by reducing the need for new mining,” said John Lienhard, the Abdul Latif Jameel Professor of Water Science in MIT’s Department of Mechanical Engineering and director of the Abdul Latif Jameel Institute for Water and Food Systems (J-WAFS). “This is a promising solution to address environmental challenges while meeting growing demand for aluminum.”
Lienhardt and his colleagues report their findings in a study published today in the journal ACS Sustainable Chemistry and Engineering . Co-authors of the study include MIT mechanical engineering students Trent Lee and Vinn Nguyen, and Zi Hao Foo (SM ’21, PhD ’24), now a postdoctoral researcher at the University of California, Berkeley.
Recycling Niche
Lienhard’s group at MIT develops membrane and filtration technologies to desalinate seawater and treat a variety of wastewater sources. Looking for new areas to apply their research, the team found an untapped opportunity in aluminum, and specifically in the wastewater generated from the production of this metal.
Aluminum production begins with the extraction of the metal-rich ore called bauxite in an open-cut mine, then a series of chemical reactions separate the aluminum from the rest of the mined rock. These reactions ultimately produce a powdered form of aluminum oxide called alumina. Much of this alumina is then transported to a smelter, where the powder is poured into electrolytic tanks containing a molten mineral called cryolite. When a strong electric current is applied, the cryolite breaks the alumina’s chemical bonds, separating the aluminum and oxygen atoms. Pure aluminum precipitates as a liquid to the bottom of barrels, where it is collected and cast into a variety of shapes.
The cryolite electrolyte acts as a solvent, facilitating the separation of alumina during molten salt electrolysis. Over time, cryolite accumulates impurities such as sodium, lithium and potassium ions, gradually reducing its effectiveness at dissolving alumina. At some point, the concentration of these impurities reaches a critical level, at which point the electrolyte must be replaced with fresh cryolite to maintain the efficiency of key processes. Spent cryolite is a viscous sludge containing residual aluminum ions and impurities, which is then shipped off for disposal.
“We know that traditional aluminum plants waste about 2,800 tonnes of aluminum each year,” said lead author Trent Lee, who conducted the new study as part of the MITEI Energy UROP program. “We were looking for ways to help the industry become more efficient, and we found that recycling some of the cryolite waste hadn’t been well-studied.”
Strong Kick
In their new study, the researchers aimed to develop a membrane process to filter cryolite waste and capture the aluminum ions that inevitably find their way into the waste stream. Specifically, the team aimed to capture aluminum while allowing all other ions, especially sodium, to significantly accumulate in the cryolite over time.
The team reasoned that if they could selectively recover aluminium from the cryolite waste, they could return it to the electrolysis tank without having to add too much sodium, which would slow down the electrolysis process.
The researchers’ new design is an improvement over membranes used in conventional water treatment plants. These membranes are typically made from thin sheets of polymer materials perforated with nanometer-sized holes, the size of which is tailored to allow certain ions or molecules to pass through.
Conventional membranes have a naturally negatively charged surface, so they repel similarly negatively charged ions and attract positively charged ions, allowing them to pass through.
The MIT team worked with Japanese membrane company Nitto Denko to test the effectiveness of a commercially available membrane that could filter out most of the positively charged ions in cryolite wastewater while repelling and capturing aluminum ions, although aluminum ions also carry a positive charge of +3, while sodium and other cations carry a smaller positive charge of +1.
Inspired by recent work on membranes for recovering lithium from saltwater lakes and used batteries, the team tested Nitto Denko’s new membrane, which has a thin, positively charged coating that is positive enough to repel and strongly hold on to aluminum, while still allowing weakly positive ions to pass through.
“Aluminum is the most positively charged ion, so most of it gets pushed out of the membrane,” Hu explains.
The team tested the membrane’s performance by passing it through solutions with different ion balances, similar to those found in cryolite waste. The researchers found that the membrane consistently captured 99.5 percent of the aluminum ions while allowing sodium and other cations to pass through. They also varied the pH value of the solutions and found that the membrane maintained its performance even after being immersed in highly acidic solutions for several weeks.
“Many of these cryolite wastes have a wide range of acidity,” Hu says, “and we found that the membrane performed very well even under the harsh conditions that we expected.”
The new test membrane is about the size of a playing card. The researchers envision a scaled-up version of a membrane similar to the type used in many desalination plants to treat cryolite waste from industrial-scale aluminum plants, in which a long membrane would be wound into a spiral shape through which water would flow.
“This paper demonstrates the feasibility of the membrane for innovation in the circular economy,” said Li. “The membrane offers the dual benefit of reducing hazardous waste while recycling aluminum.”
