A Numerical Investigation on the Electrochemical Behavior of CaO and Al 2 O 3 in the ESR Slags
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A Numerical Investigation on the Electrochemical Behavior of CaO and Al2O3 in the ESR Slags E. KARIMI-SIBAKI, A. KHARICHA, M. WU, A. LUDWIG, and J. BOHACEK Field structures including electromagnetic, concentration of ions, and flow fields in an ESR-like process composed of a graphite crucible containing a molten slag, air, and an iron electrode are computed. Both CaF2—(mass pct 2) CaO and CaF2—(mass pct 2) Al2O3 slags are examined. Tertiary current distribution is calculated. Therefore, polarization overpotential and Faradic reactions at metal–slag interface are considered using Tafel law, whereas transport of ions in the bulk of slag is determined through Nernst–Planck equations. The main goal is to shed light on the invisible phenomena such as magnetohydrodynamics caused by transport of ions, electrical conductivity of CaF2-based slag using additives (e.g., CaO or Al2O3), and the role of complexation of ions (e.g., AlO3 3 ) in the molten slag applied to the ESR. An explanation is given for the observation of higher oxygen content in the metal using Al2O3 than that using equivalent amount of CaO in the CaF2-based slag of a DC-operated ESR. https://doi.org/10.1007/s11663-020-01795-y The Author(s) 2020
I.
INTRODUCTION
MASS transfer is a major phenomenon occurring in electroslag remelting (ESR) process.[1,2] Different contributions of mass transfer are easily recognized in removal and precipitation of non-metallic inclusions in the slag and melt pool,[3,4] refinement of the alloy,[5–7] and chemical and electrochemical reactions in the slag through the process.[8,9] Numerous electrochemical (faradaic) reactions take place at the slag–metal interface aiming at obtaining an alloy ingot which is clean and chemically refined. For instance, faradaic reactions of alloying elements such as Ti, S, O, Fe, Ca, and Al were reported.[8] The CaF2-based slag, typically composed of CaF2, CaO, and Al2O3, serves as the electrolyte in ESR. Thereby, the electrochemical transport of ions such as Ca2+, Al3+, Fe2+, F-, and O2 plays a crucial role in the electrical conductivity of the slag and consequently the performance of the process.
E. KARIMI-SIBAKI and A. KHARICHA are with the ChristianDoppler Laboratory for Metallurgical Applications of Magnetohydrodynamics, Montanuniversitaet of Leoben, FranzJosef-Str. 18, 8700 Leoben, Austria. M. WU, A. LUDWIG, and J. BOHACEK are with the Chair of Simulation and Modeling of Metallurgical Processes, Montanuniversitaet of Leoben, Franz-Josef-Str. 18, 8700 Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted October 7, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Several observed phenomena during DC operation of ESR were attributed to the ionic properties of the molten slag. The melt rate of the ESR electrode with positive polarity as known as direct current reverse polarity (DCRP) was observed in situ to be higher than that for the ESR electrode with negative polarity as known as direct current straight polarity DCSP.[10–12] Mitchell et al.[13,14] reported
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