Activity coefficient of oxygen in copper-sulfur melts
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I.
INTRODUCTION
ACTIVITIES of sulfur in both sulfide-rich melts (mattes), L2, and metal-rich melts, LI, have been determined by Schuhmann and Moles, 1 Bale and Toguri, 2 S u d o , 3 and Yagihashi. 4 From these data and the phase boundaries of the metal-rich and sulfide-rich melts, the activities of copper and the integral thermodynamic properties of the melts can be calculated9 However, in mattes of practical importance, there is always a certain amount of oxygen9 Yet, relatively little work has been done in determining the activities of oxygen in mattes and the metal-rich melts9 Kuxmann and Benecke5 and Schmiedl 6 measured the solubility of oxygen in mattes saturated with the metal-rich melts from 1373 to 1823 K with partial pressures of S02 varying from 0 to 760 mm Hg. Both groups of investigators found values of the order of 1 wt pct oxygen in equilibrium with 1 atm of SO2. In contrast, Bale and Toguri, 2 who studied mattes with more than 33.5 at9 pct S using a tbermogravimetric technique, were unable to detect any appreciable oxygen solubility for these compositions. The primary objective of the present study is to measure the activity coefficients of oxygen in mattes as a function of sulfur concentration using a technique differing from those used in the literature. 2.5.6The secondary objective is to measure the activity coefficient of oxygen in the metal-rich melts in view of the discrepancies between the data of Sano and S a k a o 7 and Janke and Fischer.8 The experimental method used in the present study is a modified coulometric titration technique developed by Otsuka and co-workers. 9,1~In this technique, the electrical current contributed by electronic conduction in the electrolyte, L, is subtracted from the measured current to give the ionic current, lio,. Next, the background charge, identified primarily as the release of oxygen from the electrolyte," is measured and subtracted from the total ionic charge to yield the net ionic charge due to oxygen transfer from the liquid metal. This method was used successfully by Otsuka and co-workers in determining the activity coefficient of oxygen in many metals and binary alloys. 10.12-~5This technique was successful even for alloys with rather high vapor pressures, e.g., 5 x 10 3 atm, due to the geometrical design of the emf cell. 10,13,14In the present study, this method is extended SHINYA OTSUKA, formerly Research Associate at The University of Wisconsin, is a Research Associate with the Department of Metallurgical Engineering, Osaka University, Osaka, 565, Japan. Y. AUSTIN CHANG is Professor and Chairman of the Department of Metallurgical and Mineral Engineering, The University of Wisconsin, 1509 University Avenue, Madison, WI 53706. Manuscript submitted April 29, 1983. METALLURGICALTRANSACTIONSB
to determine the activity coefficients of oxygen in metalnonmetal melts.
II.
EXPERIMENTAL METHOD
A. The Experimental Set-Up The emf cell used in the present study is given below as, O(in Cu-S melts) I ZrO2(+CaO) lair, Pt Since the details of the experimental s
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