Corrosion mechanisms of magnesia-chrome refractories in copper slag and concurrent formation of hexavalent chromium

Magnesia-chrome refractories have been essential materials for application as wear lining in copper production furnaces for many years. Formation of hexavalent chromium (Cr(VI)) has been a major concern despite its admirable corrosion resistance in copper slag. Magnesia-chrome refractory specimens (9, 12, 15 and 18 wt% Cr₂O₃) with addition of Al₂O₃ and TiO₂ were fabricated and subsequent corrosion tests using copper slag was conducted in air. The corrosion mechanism between magnesia-chrome refractories fabricated and synthetic copper slag were investigated by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), and concurrent formations of Cr(VI) were assessed effectively by leaching test. All the magnesia-chrome specimens prepared are composed of periclase and composite spinel Mg(Al,Cr,Ti)₂O₄ phases after firing at 1700 °C. Densification of the specimens decreases with the increase in Cr₂O₃ content due to the increased volume expansion, while TiO₂ addition can improve their densification as mass transfer is enhanced during the sintering process. The corrosion resistance of the specimens decreases slightly with the Cr₂O₃ content due to the increased apparent porosity, which would result in penetration of more corrosive slag at the initial stage of the corrosion test while a newly formed (Mg,Cu)(Cr,Fe)₂O₄ spinel dense layer between the slag and penetration layer effectively hinders further penetration of slag and well protects the specimens. The Cr(VI) in the specimens prepared increases obviously with the Cr₂O₃ added content, while the TiO₂ addition can suppress the formation of Cr(VI) effectively and the copper slag can cause the formed Cr(VI) to be reduced further during the corrosion process. For all the specimens, concentrations of Cr(VI) in the leachates is much lower than the US-EPA limit of 100 mg/kg but exceeds the European limit of 2 mg/kg after the corrosion test.