Thermal-reduction and -oxidation of iron oxide slags generated from rolling mill steel industry

Every year, thousands of tons of waste scales are produced during rolling mill steel processing worldwide. The scales are rich in hematite and magnetite, and different ways of reduction and oxidation have been proposed for recycling the iron oxides. Modern methods for removing and processing steel scales fall into three classes: mechanical, chemical, and electrochemical approaches. Each method has its advantages and disadvantages, and the choice of method depends on cost and users’ need for the final product. The present work reports the reduction of the mill scales by their precipitation in a mixture of HCl and HNO3 acids to form α-FeOOH nanopowder, followed by thermal oxidation in a hydrogen atmosphere at 400°C and 375°C. This approach combines chemical and physical processes and it is known as chemical metallurgical method. Sodium dodecyl sulphate is used as a surfactant to obtain Fe and Fe3O4 powders. X-ray diffraction analysis shows that thermal treatment results in the formation of γ-Fe2O3 phase. It also causes a transition from magnetite (Fe3O4) to γ-Fe2O3, with a preferential crystallite orientation of the γ-Fe2O3 phase to the (220) texture plane. Eventually, longer thermal treatment time increases the crystallite size for the γ-Fe2O3 phase and decreases it for the Fe3O4 phase.