Rheological Transitions of the Solid-Bearing Slag During Cooling Process
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INTRODUCTION
IN industrial practice, slag is constantly exposed to complicated kinetic conditions. For example, the basic oxygen furnace (BOF) slag in the converter is stirred by a high velocity stream of oxygen which is injected into the converter via a top-blowing oxygen lance for the decarburization purpose. The kinetic conditions (e.g., shear rate) can significantly influence the viscosity of slag, particularly the viscosity of solid-bearing slag. Different aspects of non-Newtonian behavior (a viscosity that varies with applied shear rate) have been reported in solid-bearing slags. Wright et al.[1] found the CaOMgO-Al2O3-SiO2 slag saturated with MgAl2O4 particles displays a yield stress (a finite stress to be exceeded for a material to flow[2]) up to 3 Pa at a solid fraction larger than 8 wt pct. Their later study on CaO-FeOx slag containing Fe3O4 particles[3] suggested a shear thinning behavior in the solid-bearing slag, but the mechanism was not discussed. Wu et al.[4] found the steelmaking slag (CaO-MgO-Al2O3-SiO2) containing solid 2CaOÆSiO2 or MgO particles shows Newtonian behavior at particle content less than 10 vol pct. Zhen et al.[5] observed that the CaO-MgO-Al2O3-SiO2 slag containing TiN particles exhibits shear thinning behavior once the particle fraction
ZHUANGZHUANG LIU, ANNELIES MALFLIET, BART BLANPAIN, and MUXING GUO are with the Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium. Contact e-mail: [email protected] Manuscript submitted March 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
is above 3 vol pct. Although various flow behavior such as the Newtonian,[4] shear thinning[3,5] and yield stress[1,3] have been reported in the sub-liquidus slag, the threshold and mechanism of the rheological transitions have not been clarified. This leads to difficulties in accurately estimating the slag viscosity in industrial processes and a sudden rheological transition can bring about operational difficulties such as blocking of tapping holes, splashing, and excessive slag foaming during production. An interesting slag system to study the rheological transitions is BOF slag as it has a liquid–solid coexisting region in a wide temperature range, which makes it easy to correlate the crystal fraction to the flow regime.[6] Moreover, in order to recycle BOF slag, a hot stage slag engineering (processing the slag during molten state before, during or after slag tapping, e.g., addition of solids in the slag or selection of appropriate cooling path to deliver the desirable product) has been proposed to inject materials such as SiO2 into the BOF slag to achieve desired chemistry for slag valorization. According to the microstructure analysis and taking into account the cost of the materials, adding 20 wt pct SiO2 or 10 wt pct Al2O3 into the BOF slag has potential for the BOF slag valorization.[7,8] Furthermore, adding Na2O in conjunction with SiO2 or Al2O3 into the BOF slag is a probable way of reducing the slag viscosity to prevent potential operational difficultie
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