Recycling the industrial waste ZnFe2O4 from hot-dip galvanization sludge

Abstract

In this study, the goal was to find lower temperature for separating the Zn and Fe content of ZnFe2O4 by sulfation reaction than previously achieved and to study the various reaction steps of sulfation. Hence, the reaction of ZnFe2O4 with Mohr’s salt containing iron(II), i.e., (NH4)2Fe(SO4)2·6H2O, and ammonium iron alum containing Fe(III), i.e., NH4Fe(SO4)2·12H2O, was studied. At first, the thermal decomposition of precursor salts in air was studied by TG/DTA-MS to find the proper temperature for sulfation. Then ZnFe2SO4/precursor salt mixtures with ratios 1:2 and 1:5 were prepared and annealed at 400, 425 and 450 °C. The solubility of the products obtained at different annealing temperatures in water (e.g., ZnSO4, FeSO4, Fe2(SO4)3) and in HCl (Fe2O3, ZnO, Fex(OH)ySO4, Znv(OH)wSO4 basic sulfates) was studied. The morphology and structure of the starting materials was investigated by SEM, XRD and FTIR, the crystalline phases after each annealing and washing step were studied by XRD. The Fe in the starting materials and the products obtained at 425 °C was measured by Mössbauer. Based on the obtained results, it was demonstrated that the sulfation reaction with ammonium iron sulfates could be performed at lower temperatures than with iron sulfates. It was possible to detect the reaction intermediates and to obtain information about the reaction intermediates. With our sulfation reaction, depending on the reaction conditions, it is possible to obtain Fe2O3 as final product, but the Zn and Fe metals can be obtained also as sulfates. Our results open up further possibilities to recycle the ZnFe2O4 waste material.