Enhancing Resource Circularity in Aluminum Production through Nanofiltration of Waste Cryolite

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This study presents a novel approach to the selective separation of aluminum from waste cryolite electrolyte with two nanofiltration membranes: a conventional polyamide membrane and a membrane coated with a polyelectrolyte layer. Utilizing transmission electron microscopy and Fourier transform infrared spectroscopy, we find that the polyelectrolyte coating significantly increases the density of positively charged ammonium groups on the membrane surface, thereby enhancing the Donnan exclusion of aluminum ions. Notably, the polyelectrolyte coating enhances the sodium/aluminum separation factor by 55%. Our experimental results demonstrate that the coated membrane sustains high aluminum rejection rates, averaging 99.1%, while permitting substantial permeation of sodium, lithium, and potassium ions. This selective permeability is pronounced at lower pH levels, where the sodium/aluminum separation factor peaks at 102.02 for chloride-rich waste cryolite. Our process modeling using the Donnan steric pore model with dielectric exclusion substantiates the practical viability of Donnan-enhanced nanofiltration for processing waste cryolite. Our module-scale analysis indicates that the efficient aluminum concentration in the retentate, achieving a sodium/aluminum ratio of approximately 2.6, is viable for upcycling cryolite electrolyte and promoting a circular aluminum economy. Furthermore, the aluminum-depleted permeate, with aluminum cationic composition as low as 0.00194%, makes ample progress toward a benignly disposable effluent, reducing the aluminum industry’s environmental footprint.