Thermodynamics of liquid Al-Na alloys determined by using CaF 2 solid electrolyte
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y electrolysis from cryolite melts inevitably contains some sodium. Because of its tendency to form Na films, which may lead to formation of cracks during hot-rolling processes, the sodium needs to be removed from aluminum melts by refining. Commonly applied processes involve reactions between dissolved sodium and refining agents such as Cl2 (g), MgCl2 (l), and AlF3 (s).[1] The present work was initiated in order to determine the activity coefficient of sodium in liquid aluminum, so that the aforementioned type reactions can be analyzed on the basis of reliable thermodynamic data. Such data are also of interest for the industrial production of aluminum by electrolysis. Previous authors[9–18] have discussed whether sodium in liquid aluminum obeys Henry’s law[2,3] given by Eq. [1]. ai ⫽ ␥ oi xi
[1]
Investigators studying liquid Al-Na alloys in the period 1906 to 1950 tried to determine the solubility of Na by quenching and subsequent chemical analysis.[4–8] Results by Ransley and Neufeld[8] were used in drawing the Al-Na phase diagram[14] and also for estimation of the activity coefficient ␥Na[10–13] using the assumption that Henry’s law S.G. HANSEN, Research Scientist, formerly with the Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, is with Sintef Materials Technology, N-7465 Trondheim, Norway, Contact e-mail: [email protected] J.K. TUSET and G.M. HAARBERG, Professors, are with the Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Manuscript submitted February 16, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
is obeyed from infinite dilution to saturation. Mitchell and Samis[9] determined the distribution of Na between liquid Al-Na and Pb-Na alloys and found that ␥Na was a strongly increasing function of xNa between 0 and 85 ppm Na. Dewing,[10,11,12] who used a vapor pressure method, assumed that Henry’s law is obeyed in the liquid solubility region and derived equations for estimation of the activity coefficient ␥Na on the basis of his own experimental data for very dilute solutions and those of Ransley and Neufeld for saturated solutions. Brisley and Fray,[15] who used sodium beta alumina sensors, stated that 1 to 50 ppm Na in pure liquid Al at 725 ⬚C obeys Henry’s law with ␥ oNa ⬇ 350. Brisley and Fray and Yao and Fray,[16] found that 1 to 100 ppm Na in Al with 0.5 pct impurities obeys Henry’s law with ␥ o ⬇ 260 at 795 ⬚C and 825 ⬚C, whereas ␥Na was drastically reduced and increased from 17 to 110 with increasing sodium concentration at 725 ⬚C and 750 ⬚C. Sun and Yang,[17] who also used sodium beta alumina sensors measured, ␥Na values increasing from 0.16 to 22 with increasing sodium concentration from 1 to 150 ppm Na in liquid Al with 0.11 pct Si at 725 ⬚C. Dubreuil and Pelton[18] measured potentials of a sodium beta alumina sensor immersed in liquid Al with 1 to 100 ppm Na at 710 ⬚C to 750 ⬚C. They normalized their measurements to 735 ⬚C and plotted the normal
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