Thermodynamic Behavior and Mass Transfer Kinetics of Boron Between Ferrosilicon and CaO-SiO 2 Slag
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CONTROL of the boron content in metal is important in the production of silicon and ferrosilicon alloys alike. Commonly added in low concentrations to steels to enhance the hardness level, boron is an essential ingredient in modern steel manufacturing. In particular, the oxidation behavior of boron is important in steel and ferroalloy processes in which slags or fluxes are applied. Solar-grade silicon production is another significant application of boron oxidation, slag refining being an industrially applicable and cost-effective method for removal of boron. In terms of process control, the understanding of thermodynamic and kinetic properties of the mass transfer of boron between slag and metal phases is essential. With respect to slag refining of silicon, a decade ago the published research almost exclusively consisted of equilibrium work.[1–5] While few studies have investigated silicon refining kinetics more broadly,[6] in the past few years, the kinetic properties of slag refining of silicon have been investigated with
EGIL KRYSTAD and GABRIELLA TRANELL are with the Department of Mathematics Science and Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] LARS K. JAKOBSSON is with Elkem Technology, Fiskaaveien 100, 4675 Kristiansand, Norway. KAI TANG is with SINTEF Materials and Chemistry, P.O. Box 4760 Sluppen, 7465 Trondheim, Norway. Manuscript submitted December 26, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
special respect to boron for several slag compositions in the temperature interval 1723 K to 2073 K (1450 °C to 1800 °C). Nishimoto,[7] Krystad,[8] and Wu[9] have published studies on the mass transfer of boron between CaO-SiO2 slags and silicon. Krystad[8] also investigated CaO-SiO2-MgO slags; Safarian[10] and Fang[11] both studied boron kinetics in Si refining with Na2O-SiO2 slags. Lai[12] also investigated alkali-based slags, using Li2O-SiO2 for boron extraction. Zhang[13] and Wang[14] both quantified mass transfer between Si and slags containing halides (CaO-SiO2-Al2O3-CaF2 and CaOCaCl2-SiO2, respectively). Islam[15] passed an electrical current between molten slag and silicon for enhancement of the refining, using a CaO-SiO2-Al2O3 slag. White[16] investigated the refining kinetics of silicon with respect to boron using a CaO-SiO2 slag with the help of a graphite impeller to create stirring of the melt. While thermodynamic and kinetic data with relevance to boron transfer between silicon and slag are quickly developing, published research on the refining behavior of impurities in ferrosilicon is scarce. Jakobsson[17] investigated the thermodynamic behavior of boron and calcium in ferrosilicon-slag systems for the range 0 to 50 wt pct Fe. Here, the equilibrium partition ratio B in slag LB ¼ was calculated as 2.8 ± 0.3 for the B in metal investigated alloy range. Additional experimental results within this field are desirable, particularly for high-iron ferrosilicon alloys. Available thermodynamic databases are not detailed with r
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