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Equilibrium Phase Relations in the Al2O3–CaO–Cr2O3 System in Air

The transformation of Cr(III) to Cr(VI) during the use of refractory monolithics can cause subsequent problems with disposal of the used refractory lining. It is therefore important to study the degree of Cr(VI) formation as well as its formation mechanism from a fundamental perspective. This study investigated a simplified system of Al2O3 and CaO in the presence of Cr2O3, representative of a system commonly found in refractory castables. Equilibrium phase relations in the Al2O3–CaO–Cr2O3 system in air were investigated in this study. Samples were prepared by varying the Al2O3 contents in the range of 50–80 mass-%. Cr2O3 concentrations up to 4 mass-% and equilibrium temperatures up to 1573 K were used. After equilibrium was reached, the samples were quenched in liquid nitrogen. The phases in the quenched samples were studied using XRD analysis. The results indicated that a Cr(VI)-containing phase Ca4Al6CrO16 with a hauyne crystal structure forms in this system. The water soluble Cr(VI) in the quenched samples was quantified using the diphenylcarbazide spectrophotometric method according to the TRGS 613 standard. The water soluble Cr(VI) in all samples exceeded the allowable European limit.

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