Stabilization of Free Lime in BOF Slag by Melting and Solidification in Air
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(Basic Oxygen Furnace) slag is a main by-product during steelmaking. Currently, about 20 million tonnes of steel slag are produced annually in Europe,[1] of which almost half is BOF slag.[2] Therefore, the recycling of BOF slag is key for the sustainability of the steelmaking industry and for environmental protection in general. BOF slag valorization is, however, limited by the volume expansion during natural aging, attributed to the presence of free lime. Hydration and carbonation of free lime can induce up to 10 pct swelling,[3] resulting in the ‘‘dusting’’ of the slag. Different methods to stabilize free lime have been investigated. Chemically bonding the free lime by adding SiO2 or carbonation in the molten state has been intensively studied in recent years.[2,4–7] Gautier et al.[8] studied the effect of cooling rate on the BOF slag microstructure after solidification in oxidizing atmosphere and found that free lime content is decreased. The stabilization of free lime was attributed to the suppression of the decomposition of tricalcium silicate by rapid cooling. However, there is no CHUNWEI LIU, Ph.D. Student, MUXING GUO, Senior Expert Researcher, LIEVEN PANDELAERS, Research Manager, BART BLANPAIN, Full Professor, and SHUIGEN HUANG, Senior Researcher, are with the Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 box 2450, 3001 Leuven, Belgium. Contact e-mail: [email protected] Manuscript submitted July 19, 2016. Article published online September 6, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
dedicated experimental study of the influence of different oxygen partial pressures on the BOF slag mineralogy after solidification. In this work, free lime containing industrial BOF slag was remelted and solidified under Ar and air atmospheres, followed by controlled slow cooling. Quantitative X-ray diffraction (QXRD) and electron microprobe analysis (EPMA) were used to characterize the solidified microstructure and mineralogy. The effect of the oxygen partial pressure on the solidification behavior was discussed by comparison with thermodynamic calculation using FactSage. The results reveal the mechanism of free lime stabilization by air and provide a novel method to stabilize BOF slag. Industrial BOF slag samples were used in this study. The slag was ground and milled to powder below 200 lm. The typical ranges of chemical and mineral composition of the original slags are given in Tables I and II, respectively. Basicity is defined as the mass ratio of CaO/SiO2. For the tests under Ar, 20 g slag was loaded in an MgO crucible (ID: 32 mm, OD: 70 mm), which was placed in a vertical tube furnace (HTRV 100-250/18, GERO, Germany) under Mg-purified Ar atmosphere with a flow rate of 0.4 L/min. The sample was melted at 1673 K (1400 °C) for 1 hour and then cooled to room temperature in the furnace at 5 K/min. The oxygen partial pressure was measured by an oxygen sensor (Rapidox 2100, Cambridge Sensotec) and was maintained below 1 9 1018 atm. For the tests in air, 20 g slag was heated to 1673 K (1400 °C) in an Mg
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