Yuxiang Yao,† Nina F. Farac,† and Gisele Azimi*,†,‡
†Laboratory for Strategic Materials, Department of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto,
Ontario M5S 3E5, Canada
‡Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S 3E4, Canada
Today’s world relies upon critical green technologies that are made of elements with unique properties irreplaceable by other materials. Such elements are classified under strategic materials; examples include rare earth elements that are in increasingly high demand but facing supply uncertainty and near zero recycling. For tackling the sustainability challenges associated with rare earth elements supply, new strategies have been initiated to mine these elements from secondary sources. Waste electrical and electronic equipment contain considerable amounts of rare earth elements; however, the current level of their recycling is less than 1%. Current recycling practices use either pyrometallurgy, which is energy intensive, or hydrometallurgy that rely on large volumes of acids and organic solvents, generating large volumes of environmentally unsafe residues. This study put emphasis on developing an innovative and sustainable process for the urban mining of rare earth elements from waste electrical and electronic equipment, in particular, a nickel metal hydride battery. The developed process relies on supercritical fluid extraction utilizing CO2 as the solvent, which is inert, safe, and abundant. This process is very efficient in the sense that it is safe, runs at low temperature, and does not produce hazardous waste while recovering ∼90% of rare earth elements. Furthermore, we propose a mechanism for the supercritical fluid extraction of rare earth elements, where we considered a trivalent rare earth element state bonded with three tri-n-butyl phosphate molecules and three nitrates model for the extracted rare earth tri-n-butyl phosphate complex. The supercritical fluid extraction process has the double advantage of waste valorization without utilizing hazardous reagents, thus minimizing the negative impacts of process tailings.
KEYWORDS: Rare earth elements, Supercritical fluid extraction, Recycling, Urban mining, Waste electrical and electronic equipment, Nickel metal hydride battery