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 degrees 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 C(3)A, C(12)A(7), Ca-2(Al,Fe)(2)O-5 and CA phases encapsulated the Cr(VI) by forming Cr(VI) analogue phases 3CaO center dot Al2O3 center dot CaCO4 center dot nH(2)O. 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). (C) 2015 Elsevier Ltd. All rights reserved.