Addition of Fe3O4 as electron mediator for enhanced cementation of Cd2+ and Zn2+ on aluminum powder from sulfate solutions and magnetic separation to concentrate cemented metals from cementation products

Abstract The zinc leach residues (ZLRs) generated by Zn hydrometallurgical process are disposed of in landfills due to a lack of proper treatment methods although they contain toxic heavy metals like Cd and Zn. To remove these heavy metals, this study investigated cementation using aluminum (Al) together with magnetite (Fe3O4) from sulfate solutions. Cementation using only Al could not remove Cd2+ and Zn2+ because the insulating Al oxide (Al2O3) layer on Al surface was not removed at high pH (5.0) from sulfate solution, and inhibiting electron transfer from Al to metal ions. The addition of Fe3O4, however, enhanced the cementation of Cd2+ and Zn2+ from 0% to 83% and 92%, respectively. The results of SEM-EDS analysis for the cementation products confirmed that Cd and Zn were deposited on Fe3O4 attached on Al surface. Electrochemical impedance spectra for Al electrode in sulfate solutions confirmed that charge transfer resistance decreased when Fe3O4 powder was attached on the electrode surface, suggesting that Fe3O4 acts as a mediator to enhance electron transfer from Al to metal ions in solutions. By applying magnetic separation for the cementation products, the majority of cemented Cd and Zn were recovered together with Fe3O4 in magnetic fraction, and they were separated from Al in non-magnetic fraction.