Formation, leachability and encapsulation of hexavalent chromium in the Al2O3-CaO-Fe2O3-Cr2O3 system

Refractory castables can contain Cr2O3, either as a specific addition or as a impurity element in the raw materials. When calcium aluminate cement is used as a binder in such castables, Cr(III) can transform to Cr(VI) during use, which can cause subsequent problems with the disposal of the used refractory lining. The formation of Cr(VI) in Al2O3-CaO-Fe2O3-4wt% Cr2O3 mixtures, representing a simplified refractory castable system, was investigated as well as its leachability. Samples were prepared from reagent grade chemicals, equilibrated at 1300 °C in air and quenched in liquid nitrogen. A Cr(VI)-containing phase Ca4Al6CrO16 (hauyne) formed in all the examined samples. During the ASTM D3987-85 leach test the co-existing C3A, C12A7, Ca2(Al,Fe)2O5 and CA phases encapsulated the Cr(VI) by forming Cr(VI) analogue phases 3CaO·Al2O3·CaCO4·nH2O. These phases decreased the leachability of Cr(VI) to below the EPA limit of 5 mg/l. Decreasing the pH and increasing the temperature can however remobilise the Cr(VI).