MIT engineers have developed a new nanofiltration process to curb the hazardous waste generated from aluminum production. Nanofiltration could potentially be used to process the waste from an aluminum plant and retrieve any aluminum ions that would otherwise have escaped in the effluent stream. The captured aluminum could then be upcycled and added to the bulk of the produced aluminum, increasing yield while simultaneously reducing waste.
The researchers demonstrated the membrane's performance in lab-scale experiments using a novel membrane to filter various solutions that were similar in content to the waste streams produced by aluminum plants. They found that the membrane selectively captured more than 99 percent of aluminum ions in these solutions.
If scaled up and implemented in existing production facilities, the membrane technology could reduce the amount of wasted aluminum and improve the environmental quality of the waste that plants generate.
"This membrane technology not only cuts down on hazardous waste but also enables a circular economy for aluminum by reducing the need for new mining," says John Lienhard, the Abdul Latif Jameel Professor of Water in the Department of Mechanical Engineering, and director of the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS) at MIT. "This offers a promising solution to address environmental concerns while meeting the growing demand for aluminum."
Lienhard and his colleagues report their results in a study appearing in the journal ACS Sustainable Chemistry and Engineering. The study's co-authors include MIT mechanical engineering undergraduates Trent Lee and Vinn Nguyen, and Zi Hao Foo SM '21, PhD '24, who is a postdoc at the University of California at Berkeley.
As part of aluminum's production, metal-rich ore, called bauxite, is first mined from open pits, then put through a series of chemical reactions to separate the aluminum from the rest of the mined rock. These reactions ultimately produce aluminum oxide, in a powdery form called alumina. Much of this alumina is then shipped to refineries, where the powder is poured into electrolysis vats containing a molten mineral called cryolite. When a strong electric current is applied, cryolite breaks alumina's chemical bonds, separating aluminum and oxygen atoms. The pure aluminum then settles in liquid form to the bottom of the vat, where it can be collected and cast into various forms.
Cryolite electrolyte acts as a solvent, facilitating the separation of alumina during the molten salt electrolysis process. Over time, the cryolite accumulates impurities such as sodium, lithium, and potassium ions - gradually reducing its effectiveness in dissolving alumina. At a certain point, the concentration of these impurities reaches a critical level, at which the electrolyte must be replaced with fresh cryolite to main process efficiency. The spent cryolite, a viscous sludge containing residual aluminum ions and impurities, is then transported away for disposal.
"We learned that for a traditional aluminum plant, something like 2,800 tons of aluminum are wasted per year," says lead author Trent Lee. "We were looking at ways that the industry can be more efficient, and we found cryolite waste hadn't been well-researched in terms of recycling some of its waste products."
The team reasoned that if they could selectively capture aluminum from cryolite waste, the aluminum could be poured back into the electrolysis vat without adding excessive sodium that would further slow the electrolysis process.
The researchers' new design is an adaptation of membranes used in conventional water treatment plants. These membranes are typically made from a thin sheet of polymer material that is perforated by tiny, nanometer-scale pores, the size of which is tuned to let through specific ions and molecules.
The surface of conventional membranes carries a natural, negative charge. As a result, the membranes repel any ions that carry the same negative charge, while they attract positively charged ions to flow through.
In collaboration with the Japanese membrane company Nitto Denko, the MIT team sought to examine the efficacy of commercially available membranes that could filter through most positively charged ions in cryolite wastewater while repelling and capturing aluminum ions. However, aluminum ions also carry a positive charge, of +3, where sodium and the other cations carry a lesser positive charge of +1.
Motivated by the group's recent work investigating membranes for recovering lithium from salt lakes and spent batteries, the team tested a novel Nitto Denko membrane with a thin, positively charged coating covering the membrane. The coating's charge is just positive enough to strongly repel and retain aluminum while allowing less positively charged ions to flow through.
"The aluminum is the most positively charged of the ions, so most of it is kicked away from the membrane," Foo explains.
The team tested the membrane's performance by passing through solutions with various balances of ions, similar to what can be found in cryolite waste. They observed that the membrane consistently captured 99.5 percent of aluminum ions while allowing through sodium and the other cations. They also varied the pH of the solutions, and found the membrane maintained its performance even after sitting in highly acidic solution for several weeks.
"A lot of this cryolite waste stream comes at different levels of acidity," Foo says. "And we found the membrane works really well, even within the harsh conditions that we would expect."
The new experimental membrane is about the size of a playing card. To treat cryolite waste in an industrial-scale aluminum production plant, the researchers envision a scaled-up version of the membrane, similar to what is used in many desalination plants, where a long membrane is rolled up in a spiral configuration, through which water flows.
"This paper shows the viability of membranes for innovations in circular economies," Lee says. "This membrane provides the dual benefit of upcycling aluminum while reducing hazardous waste."
Research Report:"Enhancing Resource Circularity in Aluminum Production through Nanofiltration of Waste Cryolite"
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