Thermodynamic activity of magnesium in several highly-solvating liquid alloys
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I.
INTRODUCTION
CARBOTHERMIC reduction of reactive metal oxides, although often economically preferable to electrochemical reduction, has until recently appeared to be of little commercial value. The products obtained by the classical process generally are mixtures of free metal with undesired oxides and carbides, ~2 too expensive to separate. However, it is now apparent that the rational use of liquid metal solvents 3'* can give carbothermic reductions in good yield without the formation of metal carbides, oxycarbides, and suboxides. Such solvent metals are most effective when they yield very large negative deviations from thermodynamic ideality (i.e., Raoult's law). This has the effect of lowering the activity of the product metal, which not only drives the reaction, but also suppresses both vaporization and the formation of undesirable compounds, such as carbides. This work deals with the experimental evaluation of possible solvents for a carbothermic reduction of magnesium oxide. Although many other materials, such as aluminum, silicon, hafnium, zirconium, and titanium, could be reduced in this manner, 4 this study was devoted to magnesium for several reasons. Its relatively low melting point makes the temperature range for the study of the solution chemistry larger. Magnesium forms several strong intermetallic compounds with convenient solvent metals, resulting in the type of thermodynamic behavior needed. And perhaps more important, the current electrochemical production of magnesium metal is highly energy-intensive, so that a carbothermic method would be quite advantageous. The carbothermic reaction, in solvent metal M, is: (M) MgO(s) + C(M)~- Mg(M) + CO(g)
[1]
The reaction will be driven to the right by a choice of solvent where the activity of Mg is as low as possible; however, for a practical process subsequent separation of Mg from its intermetallic solution is required, so too low an activity will
interfere with such separation processes. Thus, for the rational design of such a process, precise thermodynamic data are required for a variety of solvent systems. In this paper we report emf concentration cell data for Mg in pure liquid metal Pb, Sn, Bi, and Sb solvents as well as in the mixed solvent Sn-Sb. Such data are essential to thermodynamic modeling for process design. 5'6
II.
PREVIOUS STUDIES
The magnesium-lead system was first studied by Wagner and Engelhardt 7 by the emf method. Since then a variety of thermodynamic studies on this system has been presented. 8-~7Analysis of these data shows only fair agreement for activities of magnesium at magnesium mole fractions above 0.67, and poor agreement for lower concentrations. The entropy data reported appear to be relatively poor, due primarily to some of the difficulties inherent in working with liquid magnesium. In fact, the sign of the partial molal excess entropy of magnesium is open to question. Hultgren ~8 gives a more detailed review of these data. The magnesium-tin system data are similar in that although many results are reported, ~9-2
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