Formation Mechanisms and Leachability of Hexavalent Chromium in Cr2O3-Containing Refractory Castables of Electric Arc Furnace Cover

Cr (III)-containing refractories are widely used as electric-arc furnace cover because of their excellent corrosion resistance properties. However, the formation of hexavalent chromium [Cr(VI)] remains a matter of concern during the service and subsequent disposal of the spent refractories. Moreover, the Cr(VI) formation mechanism and total amount of Cr(VI) generated are not clearly understood. In this study, samples from different parts of a spent electric arc furnace cover were collected from a local integrated steel plant. The phase composition, microstructure, formation and leachability of Cr(VI) were investigated using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and TRGS 613 leaching test (Technische Regeln für Gefahrstoffe-TRGS 613-October 2002)/multiple leaching tests. A Cr(VI)-containing phase namely CaCrO4 existed in the sample at a medium temperature range, while another Cr(VI)-containing phase namely hauyne (Ca4Al6CrO16) and a Cr(III)-containing phase (Al2O3–Cr2O3 solid solution) formed in the high temperature region. Concentrations of Cr(VI) in the leachates (as in the TRGS 613 procedure) exceeded the European permissible limit. During the leaching tests in an acid condition, Cr(VI) was reduced by Fe2+, which was primarily derived from the dissolution of FeO in the samples, while more Cr(VI) leached out with distilled water. Formation of an Al2O3–Cr2O3 solid solution can inhibit Cr(VI) generation.