Micro-structural characterization of EAF black slag and aspects associated to the release of polluting chemical species

Black slag produced during the melting process in Electric Arc Furnace can represent a good opportunity in the operation of environmental recovery, in the building operation of road grounds and in the production of concrete. On the other hand, the chemical species contained in the scraps and the dirty material on their surfaces constitute a pollution source that causes the release of chemical species regarded as dangerous by the technical standards and by the limits imposed by some national rules, i.e. Cr, Ba, V, Mo, etc.. Several experimental trials have been performed on the slag produced by two Italian steelmaking plants producing reinforcing bars, whose production is usually interested by a significant heterogeneity and poor selection of the scraps. The chemical compositions of the final obtained slag belong to a range in which the slag has shown a releasing behaviour that is significantly different as a function of the different ratios among the fundamental constituting chemical species, i.e. CaO, SiO2, Al2O3, MgO etc.. The chemical composition has been characterized through the optical basicity and the slag has been investigated from a microstructural point of view through SEM-EDS analysis that has permitted to identify the main phases contained in the slag and to characterize their chemical composition and the dangerous polluting elements solved in each phase. The combination of the data coming from chemical analysis, micro-structural examination and data coming from the releasing tests have permitted to identify a chemical range that allows to avoid dangerous chemical releasing and this statement has provided the possibility to identify a correct procedure for treating and modifying the slag.