Thermodynamic study of zinc-rich zinc-sodium alloys
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[1]
The activity of sodium at saturated concentrations is given by aNa(l) 5 gNaXNa(l) 5
1 K14a13 Zn(l)
[2]
where aNa(l) and aZn(l) are the activities of sodium and zinc in zinc-rich Zn-Na melts at solid NaZn13 saturated concentrations, respectively; gNa and XZn(l) are the corresponding activity coefficient and atom fraction of sodium; and K is the equilibrium constant for Reaction [1]. The activity of solid
X.Y. YAN, Lecturer, is with the Research Centre for Metallurgical Process Engineering, Institute for Advanced Materials Processing, Tohoku University, Sendai, Japan 980-77. D.E. LANGBERG, Project Leader, and W.J. RANKIN, Research and Development Manager, are with CSIRO Minerals, Clayton, Victoria, Australia 3169 Manuscript submitted May 25, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
NaZn13 is equal to unity when the pure substance is taken as the standard state. It can be seen from Eq. [2] that information about the activity and activity coefficient of sodium in zincrich Zn-Na melts saturated with sodium is required in order to determine the saturation concentrations of sodium in molten zinc. The thermodynamic data of the Na-Zn system were assessed by Pelton[11] and more recently by Cetin and Ross.[12] However, experimental thermodynamic data for molten zinc-rich Zn-Na alloys are scarce;[13,14] and, especially, thermodynamic properties of the Zn-Na alloys in the range relevant to the zinc refining process have not been investigated experimentally hitherto. The aims of this work were to determine experimentally the thermodynamic properties of zinc-rich Zn-Na alloys over the range relevant to zinc refining and to study the behavior of the dilute solutions. Reversible solid-state galvanic cells with Na+b-alumina electrolytes were used and electromotive force (emf) measurements were made on zinc-rich Zn-Na melts in the composition range 3.32 3 1024 to 6.35 3 1023 atom fraction of sodium and for the two-phase Zn (l)1 NaZn13 (s) alloys. The various solid-state cells used have the following configurations. Cell I: Ta, Na (l).Na+b-alumina. Bi-Na.Na+b-alumina.Zn-Na (l), Ta (703 K) Cell II: Ta, Bi(l) 1 Na3Bi (s).Na+b-alumina. Bi-Na.Na+b-alumina.Zn-Na (l), Ta (738 K) Cell III: Ta, Na (l).Na+b-alumina.Bi-Na.Na+b-alumina. Zn (l) 1 NaZn13 (s), Ta (703 to 773K) Zinc (99.9999 pct) was added in small fragments into the electrolyte tube through the open end, and then a tantalum wire of 1 mm in diameter was placed in contact with the zinc. Finally, a stainless steel Swagelok fitting with graphite ferrules was used to make a gas-tight seal between the tantalum wire and the alumina tube. The technique for sealing between the alumina disc and Na+b-alumina tube was based on that used by Petric et al.[15] Bismuth (99.995 pct) was added in small pieces into the electrolyte tube through the hole in the disc and an alumina-sheathed tantalum wire was then passed through the hole and placed in contact with the bismuth. In all the cases, loading of metals into the electrolyte tubes and final sealing of each half-cell were performed
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