Green Recycling of Goethite and Gypsum Residues in Hydrometallurgy with α-Fe3O4 and γ-Fe2O3 Nanoparticles: Application, Characterization, and DFT Calculation

Millions of tons of hazardous iron oxide residues are produced during the iron purification process of sulfuric acid leaching solutions in the nonferrous metals hydrometallurgy industry per year. The generated iron oxide residues, which mainly contain goethite and gypsum precipitates, pose great threats to the local ecological environment and human health. We proposed a novel method, separation and recovery of goethite and gypsum by the synthetic magnetic nanoparticles (MNPs) such as α-Fe3O4 and γ-Fe2O3, to treat the residues efficiently and cost-effectively. MNPs served as the magnetic crystal nuclei of the goethite precipitates during the iron purification process, and the goethite and gypsum precipitates formed under this condition can be separated in a magnetic field for recycling purposes. The separation efficiency of the goethite and gypsum precipitates was much higher when γ-Fe2O3 was used as the crystal nuclei, indicating that the surface of γ-Fe2O3 was more favorable for the formation of goethite particles than α-Fe3O4, which has also been verified by SEM, FBRM, XRD, TEM, and XPS analysis. DFT calculations suggested that the binding energy between the MNPs and iron hydroxide plays a critical role and is responsible for the distinguished collecting efficiencies of α-Fe3O4 and γ-Fe2O3 toward goethite.