Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint High-temperature stability of Mg(Al,Cr) 2 O 4 spinel co-existing with calcium aluminates in air Hang Liu a , Shengqiang Song a,c,* , Andrie M. Garbers-Craig b,** , Zhengliang Xue a a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, China b Centre for Pyrometallurgy, Department of Materials Science & Metallurgical Engineering, University of Pretoria, Pretoria, 0002, South Africa c Center for Iron and Steelmaking Research, Department of Materials Science & Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213, USA ARTICLE INFO Keywords: Mg(Al,Cr) 2 O 4 spinel Refractory castables Calcium aluminates Ca 4 Al 6 CrO 16 Ca 5 Cr 3 O 12 ABSTRACT Cr 2 O 3 is a well-known corrosion resistant oxide used in refractory applications. However, it can oxidize into toxic and water-soluble Cr(VI) compounds upon reaction with calcium aluminate cement phases in the presence of oxygen, which subsequently causes disposal problems after use. This study describes the extent to which chromium in the spinel Mg(Al,Cr) 2 O 4 phase can be oxidized to Cr(VI) when it reacts with the calcium aluminate cement phases C 12 A 7 , CA, CA 2 and free CaO at 1300 °C in air, using XRD, XPS and leaching tests (TRGS 613 standard) as analytical tools. On reaction with CaO, the Mg(Al,Cr III ) 2 O 4 spinel mainly transformed into hauyne (Ca 4 Al 6 Cr VI O 16 ) and Ca 5 Cr 3 O 12 which contains both Cr(IV) and Cr(VI). The reaction of C 12 A 7 and CA with the spinel phase also resulted in the formation of Ca 4 Al 6 CrO 16 . Conversely, the reaction of Mg(Al,Cr III ) 2 O 4 spinel with CA 2 resulted in the formation of only a trace amount of Cr(VI). Water-soluble Cr(VI) leached in large quantities (> 100 mg/L) from samples where the Mg(Al,Cr III ) 2 O 4 reacted with either C 12 A 7 or CA. Almost no Cr (VI) leached from the sample when Mg(Al,Cr III ) 2 O 4 reacted with CaO, using the standard TRGS 613 leach test, but a significant amount of Cr(VI) was released into solution when leached with a HCl solution for 12 h. Both Cr (IV) and Cr(VI) present in the Ca 5 Cr 3 O 12 dissolved into acidic solution. Only a small amount of Cr(VI) leached from the sample that resulted when spinel was reacted with CA 2 , even after a prolonged HCl leach. Cr(III) in spinel Mg(Al,Cr) 2 O 4 is very stable and does not leach in either distilled water or acidic solution. 1. Introduction Refractory castables as the foremost monolithic materials have an advantage over brick linings due to their easier installation, lower en- ergy requirements and lower production cost, and are therefore in- creasingly being used in high-temperature industries [1–3]. Calcium aluminum cements consisting primarily of C 12 A 7 , CA and CA 2 are the main binders used in castables, which give these materials suitable rheological properties and sufficient green mechanical strength [4]. Cr 2 O 3 is added to certain castable formulations to prolong service life by improving their hot strengths, corrosion [5,6] and thermal shock resistances [7], especially in highly corrosive areas as found in waste melting and glass tank furnaces [8,9]. However, Cr 2 O 3 can be oxidized into hexavalent chromium at high temperatures and under oxidizing conditions as associated with refractory materials, fly ash and stainless steel plant dust [10–13]. Cr(VI) compounds being toxic, hazardous and highly water-soluble can cause disposal problems after use as it can easily contaminate the soil and groundwater [14]. Castable formulations that contain pre-formed and in-situ forming spinel (MgAl 2 O 4 , MA) are currently widely used in the steel, cement and glass industries [15,16]. The main advantages of the spinel are its ability to limit slag infiltration into the castable through the formation of a solid solution spinel phase which can absorb Fe 2+ , Fe 3+ , and Mn 2+ from the slag, and its improved thermo-mechanical properties [17]. Cr 2 O 3 has a strong tendency to go into solid solution with the MA spinel phase and can effectively promote its grain growth [18]. With the formation of the Mg(Al,Cr) 2 O 4 phase, the molten refractory exhibits higher corrosion resistance [19]. A natural further development in the Al 2 O 3 –CaO–Cr 2 O 3 -based castables would be the addition of MgO to these formulations through which the Mg(Al,Cr) 2 O 4 solid solution spinel will form in-situ. Incorporating chromium into a spinel phase can also reduce the risk of hexavalent chromium formation and subsequent leaching [20]. A previous investigation on the Al 2 O 3 –CaO–Cr 2 O 3 –MgO system confirmed that the addition of MgO to Al 2 O 3 –CaO–4%Cr 2 O 3 https://doi.org/10.1016/j.ceramint.2019.05.135 Received 18 April 2019; Received in revised form 9 May 2019; Accepted 13 May 2019 * Corresponding author. The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, China. ** Corresponding author. E-mail addresses: songs@wust.edu.cn (S. Song), Andrie.Garbers-Craig@up.ac.za (A.M. Garbers-Craig). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Hang Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.135