P. Sun, J.L. Anderson
Ames National Laboratory,
United States
Keywords: wastewater remediation, rare earth element recovery, electronic waste, precipitation
Summary:
The efficient separation of critical elements from ores and secondary sources, such as electronic waste, is essential for achieving circularity in metal supply chains. Liquid–liquid solvent extraction employing organic organophosphorus extractants has been widely used to separate and recover critical elements on an industrial scale. However, tremendous amounts of wastewater are produced in the separation process which contains slightly water-soluble extractants from the organic phase. Removing the contaminants poses to be a challenge due to their relative low concentration and large volumes. We have found that the same contaminants can be a potential source of precipitation agents that are known to selectively bind metal ions, thus recovering them from the leaching solution. The precipitation experiments have demonstrated that the precipitation efficiency of metal ions is highly dependent on the coordination chemistry of the dissolved extractants and metal ions. Trivalent rare earth ions easily form precipitates, while divalent transition metal ions can form stable homogenous cloudy solution. By optimizing the experimental conditions, efficient removal of extractant (>99%) and recovery of rare earth (>99%) can be achieved simultaneously. The precipitate is characterized by SEM, EDS, XPS, FTIR as well as EXAFS to demonstrate the interaction mechanism and coordination structure between rare earth ions and extractants in the precipitates. Selective precipitation of rare earth ions (La3+, Ce3+, Nd3+, Sm3+, Pr3+) from transition metal ions (Co2+, Ni2+, Mn2+, Zn2+, Fe2+) is achieved through mixing ligand-containing wastewater and the simulated leachate from the nickel metal hydride (NiMH) batteries or real leachate from Sm-Co magnets.