Vitrification and Crystallization Behavior of CaO-SiO 2 -MgO-Al 2 O 3 -Fe 2 O 3 -Cr 2 O 3 System
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TRODUCTION
CHROMIUM-CONTAINING slag, a byproduct mainly from stainless-steel production, contains various amounts of chromium oxide, typically approximately 3.5 wt pct Cr2O3.[1] Currently, the vast majority of the slag is discarded and dumped. It has been shown that chromium in the slag, even as Cr3+, is gradually oxidized to Cr6+ when free lime and oxygen are available.[2,3] Therefore, it is necessary to restrict its use as landfill and prioritize adequate treatment to reduce environmental risk. Generally, leaching of chromium from slag depends on the structure of the chromium-bearing phase. Several studies have shown that the formation of spinel[4–7] and/or glass[4,8–12] can be expected to immobilize Cr ions and limit their leaching. However, because of the complex composition of the slag (CaO, MgO, SiO2 Al2O3, Fe2O3, Cr2O3, etc.), it is not surprising that molten slag will crystallize. Mostafaee et al.[13] observed that the magnesiochromite spinel content significantly increased with chromium oxide
YONGLI XIAO and HUALIN WANG are with the Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Meilong Road 130, Shanghai, 200237, China. Contact email: [email protected] XINGMEI SHEN and XINGRONG WU are with the Key Laboratory of Metallurgical Emission Reduction & Resources Recycling of Ministry of Education, Anhui University of Technology, Ma’anshan, 243002, China. Manuscript submitted January 25, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
content in the slag and that the effect of basicity (CaO/ SiO2) on the precipitation of spinel was limited. Albertsson et al.[14] showed that the formation of spinel at 1873 K (1600 C) was favored in the basicity range of 1.0 to 1.4 in the CaO-MgO-SiO2-Cr2O3 system. Li et al.[15] noted that rapid cooling was favorable for enrichment of chromium in the spinel phase in high basicity (B > 2.0) stainless-steel slag. A previous study[16] showed that not all chromium in the slag can be immobilized by the spinel structure during the cooling process. In another approach, vitrification has been applied to convert slag into glass, and this approach has received increasing attention. However, glass formation from molten slag is mainly affected by its composition and the cooling method. Liao et al.[4] prepared glass-ceramics based on the CaO-MgO-SiO2-Al2O3-Cr2O3 system (B = 0.47) and observed that the spinel structure and residual glass successfully immobilized chromium. Daugherty et al.[17] noted that acid slag with lower basicity (B < 1) more readily form glassy material than basic slag when cooled rapidly. Richet et al.[18] observed that no glass was formed in the CaO-SiO2-FeOx system at the cooling rate of 10 C/s. Nevertheless, exploration of the effects of basicity and Cr2O3 content on the glass formation in chromium-containing slag has been limited. In this work, the relationships between basicity, Cr2O3 content, and vitrification were investigated in the melt-quenched CaO-MgO-SiO2-
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