The industrial production of aluminum involves electrolysis of alumina (Al2O3) in molten cryolite, a mineral used as a solvent for the aluminum oxide raw material. Over time, the cryolite accumulates impurities, such as sodium, lithium and potassium ions, which degrade the cryolite’s ability to dissolve aluminum oxide. When the spent cryolite is replaced, a significant amount of aluminum is typically lost.
A team of engineers from the Massachusetts Institute of Technology (MIT; Cambridge, Mass.; www.mit.edu) and the Japanese membrane maker Nitto Denko Corp. (Osaka, Japan; www.nitto.com) developed a nanofiltration process designed to capture aluminum ions that would otherwise be wasted, and return them to the aluminum production process. At the same time, the process reduces the requirements for handling hazardous waste from the production of aluminum.
“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 MIT professor John Lienhard, who led the study.
The research team developed a novel membrane that can selectively capture more than 99% of aluminum ions from solutions similar to aluminum plant waste streams. The membrane design is an adaptation of the polyamide membranes used in water treatment. Collaborating with Nitto Denko, the MIT researchers started with thin films of polymer material with nanometer-scale pores whose size is engineered to allow specific ions through, while blocking others. They then applied a thin, positively charged coating over the nanopore membrane. The coating’s charge is tuned such that it strongly repels the Al ions (with +3 charge), while allowing ions with lesser charge (+1; including Li+, Na+ and K+) to pass through. Experiments revealed that the membrane (photo) consistently captured 99.5% of aluminum ions, while allowing other cations through. The team also found the membrane maintained its performance in highly acidic solution.
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.
The research was recently published in the journal ACS Sustainable Chemistry & Engineering.