Spontaneous Emulsification of a Metal Drop Immersed in Slag Due to Dephosphorization: Surface Area Quantification
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AN emulsion is a mixture of two liquids that are usually immiscible. If two immiscible liquids are at thermodynamic equilibrium, the interfacial area and the system free energy are at a minimum. In this case, in order to promote emulsification, energy has to be supplied to the system and the emulsified state is not thermodynamically stable. However, if two immiscible liquids that are not initially at equilibrium are brought into contact, dynamic interfacial phenomena caused by temporary interfacial instabilities can promote the spontaneous emulsification of the system without external stirring. The first observation of this phenomenon dates back to 1879 and has been usually observed in organic and biological ternary systems.[1] This phenomenon, however, has also important kinetic impacts in steelmaking, where the refining of steel relies on good mixing between gas-metal-slag phases. In oxygen steelmaking (OSM), hot metal and scrap are rapidly refined by the use of oxygen.[2] The blow of oxygen creates a three phase gas-slag-metal emulsion, ANDRE N. ASSIS, PhD Candidate, and RICHARD J. FRUEHAN, U.S. Steel Professor, are with the Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA. Contact e-mails: anogueira.assis@gmail. com; [email protected] JASON WARNETT, Research Fellow, STEPHEN SPOONER, PhD Candidate, MARK A. WILLIAMS and SEETHARAMAN SRIDHAR, Professors, are with the Warwick Manufacturing Group, University of Warwick, Gibbet Hill Road, Coventry, U.K. Manuscript submitted April 21, 2014. Article published online December 9, 2014. 568—VOLUME 46B, APRIL 2015
which drastically increases the total interfacial area available for several chemical reactions to occur between the phases. Metal droplets are continuously ejected from the bulk metal bath into the slag-gas mixture where they can react for a given amount of time. The generation and behavior of these metal droplets are important for refining processes that are kinetically limited, and several authors have studied different aspects of this topic.[3–11] Various researchers have shown that, during periods of intense mass transfer of surface-active elements, such as oxygen, spontaneous emulsification can occur due to a very pronounced reduction in the interfacial tension between metal-slag systems.[12–17] This behavior is kinetically important as it temporarily increases the interfacial area between metal slag, thus increasing the rate of a given chemical reaction. However, actual quantification of dynamic surface area changes in such systems remains scarce. The work done by Rhamdhani et al.[15–17] is an example of the few studies that have actually succeeded in documenting how the interfacial area of a metal-slag system changes as a given chemical reaction is allowed to occur between the two phases. They studied the behavior of Fe-Al drops immersed in CaO-SiO2-Al2O3 slags. The interfacial area of the recovered metal droplets was measured using different sizes of papers to cover the droplets and then calcul
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