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

Shengqiang Song, Andrie M. Garbers-Craig

Centre for Pyrometallurgy, Department of Materials Science & Metallurgical Engineering, University of Pretoria, Pretoria/South Africa

Revision

Volume 8, Issue 1, Pages 70 - 74

Abstract

Cr2O3 can be present in refractory monolithic castables either as a specific addition or as a residual element present in certain raw materials. 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.

Keywords

Al2O3–CaO–Cr2O3, phase relations, calcium alumina cement, Cr(VI), Ca4Al6CrO16

References

[1] Sanchez-Ramos, S.; et al.: Analytical and mineralogical studies of ore and impurities from a chromite mineral using X-ray analysis, electrochemical and microscopy techniques. Talanta 74 (2008) [5] 1592–1597 [2] Richard, F.C.; Bourg, A.C.M.: Aqueous geochemistry of chromium – a review. Water Res. 25 (1991) [7] 807–816 [3] Dayan, A.D.; Paine, A.J.: Mechanisms of chromium toxicity, carcinogenicity and allergenicity: Rev. of the literature from 1985 to 2000. Hum. Exp. Toxicol. 20 (2001) [9] 439–451 [4] Frias, M.; de Rojas, M.I.S.: Total and soluble chromium, nickel and cobalt content in the main materials used in the manufacturing of Spanish commercial cements. Cem. Concr. Res. 32 (2002) [3] 435–440 [5] Tandon, R.; Aarts, B.: Chromium, nickel and cobalt contents of some Australian cements. Contact Dermatitis 28 (1993) [4] 201– 205 [6] Wahlberg, J.E.; Lindstedt, G.; Einarsson, O.: Chromium, cobalt and nickel in Swedish cement, detergents, mold and cutting oils. Berufs- Dermatosen 25 (1977) [6] 220–228 [7] Frias, M.; et al.: Contribution of toxic elements – hexavalent chromium in materials used in the manufacture of cement. Cem. Concr. Res. 24 (1994) [3] 533–541 [8] Frias, M; Rojas, M.I.S.: Determination and quantification of total chromium and water-soluble chromium contents in commercial cements. Cem. Concr. Res. 25 (1995) [2] 433–439 [9] Stephan, D.; et al.: High intakes of Cr, Ni, and Zn in clinker Part I. Influence on burning process and formation of phases. Cem. Concr. Res. 29 (1999) [12] 1949–1957 [10] Sinyoung, S.; et al.: Chromium behavior during cement-production processes: A clinkerization, hydration, and leaching study. J. Hazard. Mater. 191 (2011) [1–3] 296–305 [11] TRGS 613: The Technical Rules for Hazardous Substance, 2002 [12] N.N.: Slag Atlas. 2nd ed., Düsseldorf 1995 [13] Antao, S.M.; Hassan, I.; Parise, J.B.: Chromate aluminate sodalite, Ca8Al12O24(CrO4)2: Phase transitions and high-temperature structural evolution of the cubic phase. Can. Mineral. 42 (2004) 1047–1056 [14] EU directive 2003/53/EC: Restrictions on the marketing and use of cements and cementcontaining preparations, 2003 [15] Taylor, H.F.W.: Cement Chemistry. 2nd ed., London 1997 [16] He, H.Y.; Suito, H.: Immobilization of hexavalent chromium in aqueous solution through the formation of 3CaO·(Al,Fe)2O3·Ca(OH)2· xH2O phase, ettringite and C–S–H gel. ISIJ Int. 42 (2002) [2] 139–145

Copyright

Göller Verlag GmbH