A solid state emf study of the pyrrhotite-magnetite equilibrium in the temperature interval 850 to 1275 K
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I.
INTRODUCTION
THE
galvanic cell technique employing oxygen concentration cells with solid electrolytes has proved to be the most sensitive and accurate method for measuring differences in chemical potentials of oxygen between a sample system and a reference system. If the sample mixture contains a sulfide for which thermodynamic data are wanted, the sulfur activity must be carefully controlled. This can be achieved by allowing a gas stream containing known fractions of S02 and an inert component to pass over the solid mixture, provided intersolubility of condensed phases is negligible or is accurately known. Sulfide data thus produced should normally be the best available and can suitably be utilized, e.g., in chemical equilibrium computations for predicting and optimizing metal and energy yields in pyrometallurgical processes. The present work deals with the pyrrhotite-magnetite equilibrium, which in a general form can be written: 3 --Fel (1 -
x)
.,S(s) +
(5
2x)
(1 -
x)
O2(g)
3 Fe304(s) + - (1 -
x)
SO2(g)
available experimental information. The sulfur activity varies rapidly with composition within the pyrrhotite homogeneity range, and effects caused by changing stoichiometry during a series of measurements should therefore be considered. In the present study of equilibrium [ 1], the sublattice model will be used to calculate the pyrrhotite composition at each experimental point, to make corrections for stoichiometric variations, and to derive thermodynamic data for reaction [1] and for the formation of Fe~_xS as a function of both temperature and composition. Alternatively, the defect thermodynamic model for pyrrhotite, 6 selected by Sharma and Chang 7 in their calculation of the Fe-S system, could have been used. However, the sublattice model was thought to be simpler and contains fewer parameters and was therefore preferred. The temperature range of the measurements is restrained partly by the pyrrhotite becoming unstable together with magnetite and partly by the eutectic melting of the pyrrhotitemagnetite mixture. According to Rosenqvist, 8 these limiting temperatures are 580 ~ and 1010 ~ respectively, in a system where p (SO2) equals the atmospheric pressure.
[ 1]
Pyrrhotite is a nonstoichiometric solid with iron vacancies and iron interstitials and the composition, which ranges from FeS to at least Fe08sS, ~ is determined by the sulfur activity of the system and the temperature. Two previous experimental emf investigations of equilibrium 11] are described in the literature. Espelund and Jynge 2 reported emf values in the temperature interval 892 to 1305 K but did not specify the pyrrhotite composition. Schaefer3 assumed the pyrrhotite to be of constant stoichiometry in the studied temperature interval 839 to 1087 K and evaluated the change in standard Gibbs energy (AG ~) due to the formation of Fe09S. The emf values registered agree excellently with those of Espelund and Jynge. 2 Very recently, in an assessment of the whole Fe-S phase diagram, Fernfindez Guillermet et a l . 4 gave a thermod
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