Light Metals 2007 Edited by Morten Ssrlie TMS (The Minerals, Metals & Materials Society),2007 Kinetic Study of the Magnesium Removal from Molten Aluminum Using Ar-SFs-O1 Gaseous Mixtures Alfredo Flores*, David Villegas*, Zygmunt Kolenda**, and Janusz Donizaka* *CINVESTAV-IPN Unidad Saltillo, Saltillo-MonterreyHighway Km. 13.5, Ramos Arizpe, Coahuila, Mkxico. ** St Staszic University of Mining and Metallurgy, Faculty of Non-ferrous Metals, 30 Mickiewicz Ave., 30059, Cracow, Poland. Abstract In this paper it is shown that pneumatically injected gaseous mixtures containing SF6(g) are useful to oxidize the magnesium dissolved in molten aluminum alloys. Although the oxidation of magnesium with SF6@) is thermodynamically favorable, the addition of q(g) causes that AG is additionally reduced by -249.16 KJlmol Mg. After chemical analysis of the final contents of magnesium in the treated alloys, the identification of compounds in the slag by x - ray diffraction, and chemical analysis of gaseous emissions produced, a scheme of reaction is proposed considering no emission of volatile fluorides or sulfides. Kinetic equations describing the magnesium removal rate are presented, taking into account that kinetics of the process is controlled by chemical reaction. Introduction Gas injection from porous diffusers has been widely used in molten metal refining. The pneumatic injection of binary gaseous mixtures composed of C121Ar or C12/N2 allows not only enhancing mixing conditions but also the oxidation and removal of alkali &d alkaline-eartb metals from molten aluminum. Moreover, the elimination of hydrogen implicitly occurs [I]. However. due to the highly corrosive effects of chlorine bubbling, the use of argon based gaseous mixtures containing low amounts of 02(g) and SFs(g) are suitable. Recently, the reactivity of SFdg) has been studied in removing arsenic, antimony and bismuth from molten copper alloys [2]. Other investigations demonstrate that gaseous mixtures composed of SF6, 02, and Ar, are useful to reduce the magnesium content fiom molten aluminum alloys [3]. Table I contains the set of chemical reactions that are believed to take place when using these gaseous mixtures for molten aluminum refining. The theoretical thermodynamic results were calculated using the software from Outokumpu, HSC Chemistry for Windows. This software allows the obtention of the values of the Gibbs free energy for all reactions proposed as well as the values of the equilibrium constants [4]. A result that justifies the use of SF6-O2 gaseous mixtures for rdnement of aluminum scrap is obtained analyzing the values of equilibrium constants and Gibbs free cnergies of the reactions R4 and R6 from Table I at 1000 K, taking the activity coefficient of magnesium equal to 1. From the values reported, it could be stated that reaction R6 is favored with respect to reaction R4 due to a difference in the Gibbs free energy equal to - 249.16 KJ per mole of Mg. Therefore, the oxidation of pure magnesium would be high when the composition of the gaseous mixture rcaches a molar ratio 1:1 for O2@) and SF&). According to this, the objective of this paper is to present the results of a kinetic study performed to refine magnesium fiom a molten aluminum alloy processed from scrap, using Ar/SFA2 gaseous mixtures. TABLE L- Reactions suggested, the values of Gibbs free energy and of the equilibrium constants calculated with HSC, corresponding to the oxidation of pure A1(I) and MgO) at 1000 K by Ar/SF& gaseous mixtures. Metal oxidation AGIOOOK reactions (KJ/mole) KB~ 413 Al(l) + O2(g) + -906.91 2.37x 1~47 213 A120,