The engineers claim that the nanofiltration could be used to process the waste from an aluminium plant and retrieve aluminium ions that would have escaped in the effluent stream.
The filtered aluminium is then upcycled and added to the bulk of the produced aluminium - reducing waste and increasing yield. Pouring the captured aluminium into the electrolysis vat without the excessive by-products would not slow down the electrolysis process.
The researchers used a novel membrane to filter various solutions that were similar in content to the waste streams produced in aluminium plants. They found that their membrane captured more that 99% of aluminium ions in these solutions.
The MIT engineers membrane process filters cryolite waste, a by-product that is formed during the aluminium production process that reduces efficiency. The membrane also recovers the aluminium ions lost in the cryolite waste while letting the other by-products and ions, like sodium through.
The new experimental membrane is about the size of a playing card.
“This membrane technology not only cuts down on hazardous waste but also enables a circular economy for aluminium 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 aluminium.”
The filters design has been adapted by membranes used in conventional water treatment plants - made from a thin sheet of polymer materials that is perforated by nanometre pores.
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 aluminium ions.
However, aluminium ions also carry a positive charge, of +3, where sodium and the other cations carry a lesser positive charge of +1.
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 aluminium while allowing less positively charged ions to flow through.
“A lot of this cryolite waste stream comes at different levels of acidity,” Zi Hao Foo, co-author of the project said. “And we found the membrane works really well, even within the harsh conditions that we would expect.”
