Immobilization of Chromium in Stainless Steel Slag Using Low Zinc Electric Arc Furnace Dusts
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SIGNIFICANT amounts of stainless steel slag that contain harmful chromium are generated during stainless steel production.[1] For a long time, the massive amounts of stainless steel slag were land-filled or stored in slag yards as hazardous solid waste (waste category in China: HW21), which not only occupied a large amount of land but also polluted the natural environment. Previous studies[2] have shown that trivalent chromium in unstable phases among the stainless steel slag could be converted into highly toxic hexavalent chromium in
YONG LIN and BAIJUN YAN are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, P.R. China. Contact e-mail: [email protected] TIMO FABRITIUS is with the Process Metallurgy Research Unit, University of Oulu, 90014 Oulu, Finland. QIFENG SHU is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing and also with the Process Metallurgy Research Unit, University of Oulu. Contact e-mails: [email protected], qifeng.shu@oulu.fi Manuscript submitted September 30, 2019. Article published online January 24, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B
air and aggravated in acidic oxygen-rich condition. Continuous leaching of hexavalent chromium can contaminate the surrounding environment, such as soil and underground water.[2] Therefore, stainless steel slag must be treated harmlessly before being reused or released to the environment. The utilization of stainless steel slag must take its disintegration behavior, volume stability and especially the leachability into account. The disintegration behavior and volume stability are mainly caused by the transition from b-Ca2SiO4 (monoclinic) to c-Ca2SiO4 (orthorhombic) accompanied by a volume expansion (approximately 12 pct)[3] or the hydration process of f-CaO and f-MgO.[4] The leachability[5] refers to the leaching ability of hazardous elements such as chromium and nickel during the storage or recycling as construction materials. Immobilization treatment can effectively reduce the leachability of heavy metals by converting them into chemically stable mineralogic phases, such as the spinel and glass phases, which are resistant to dissolution.[6,7] Hence, the immobilization of chromium in stainless steel slag has been investigated by many researchers.[8–15] Romero-Serrano et al.[8] reported that chromium leaching was suppressed by adding MgO
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to the CaO-SiO2-Cr2O3 system, which was attributed to stable binding of chromium in the MgCr2O4 phase. They also found that the addition of FeSO4 and FeS2 had positive effects on the immobilization of chromium.[9] Albertsson et al.[10,11] indicated that slow cooling, low oxygen partial pressure and low slag basicity (1.0 to 1.4) can improve the spinel phase precipitation. Shu et al.[12] found that the size of the spinel phase increased with increasing MnO content and decreasing CaO/SiO2 ratio. However, disintegration was found in samples with a high MnO
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