A chemical control technique to determine the thermochemical properties of liquid metal-nonmetal systems
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t h e i r natural states, most m e t a l s e x i s t as oxides, sulfides, or as complex a g g r e g a t e s depending on the geological backgrounds of the m e t a l s . Efficient extraction of these m e t a l s from t h e i r ores can be greatly enhanced with an adequate description of b o t h the phase d i a g r a m s and the thermochemical properties of such systems, especially at the elevated process temperat u r e s . The majority of thermochemical studies perf o r m e d on these metal-nonmetal systems tend t o be s i m i l a r in n a t u r e and are essentially comprised of two stages: DEFINITION OF THE EQUILIBRIA Thermodynamic equilibrium is normally achieved by m e a n s of an appropriate CO/CO2 or H2/HzO gas m i x t u r e in the case of oxides, and H2/H2S m i x t u r e for sulfides. CO/CO2/SO2 m i x t u r e s can be used for oxidesulfide equilibria. The corresponding partial p r e s s u r e of oxygen PO2, or partial p r e s s u r e of sulfur, PS2, is thus fixed by the input m i x t u r e of these g a s e s , the temperature of the experiment, and the thermodyn a m i c s of the gaseous equilibria. Other gaseous combinations such as S J N 2 or O 2 / S O 2 are available but tend to be less popular due to the limited r a n g e of the ensuing partial p r e s s u r e s or o t h e r p r o b l e m s . In the case of oxides, the defined PO2 m a y be readily c h e c k e d b y m e a n s of a solid electrolyte o x y g e n p r o b e in the temperature r a n g e 400° to 1600°C, but for sulfides a m o n i t o r is only availabe at the lowest temperatures. S A M P L I N G AND C H E M I C A L ANALYSIS B y withdrawing a s m a l l s a m p l e of the equilibrated melt and chemically analyzing it, the composition and h e n c e activities m a y be readily calculated. H o w e v e r , p r o b l e m s m a y arise and inaccuracies develv~, since w e have I) possible loss of o x y g e n or sulfur on cooling the s a m p l e due to evolution of dissolved gas, 2) loss of o x y g e n or sulfur as H 2 0 or HES as volatile g a s e o u s species during chemical analysis, C. W. BALE, F. AJERSCH, and J. M. TOGURI are GraduateStudentsand Professor of Metallurgy and Materials Science, respectively, University ofToronto, Toronto,Ontario,Canada. Manuscript submittedMay 10, 1971. METALLURGICAL TRANSACTIONS
3) nonstandard techniques for chemical analysis of the nonmetals. S e v e r a l methods have been proposed but s i n c e many w o r k e r s ' results deviate considerably from each other, it is c l e a r that without extremely elaborate equipment and c a r e , c h e m i c a l analysis tends t o be very unreliable. Recent advances in levitation melting have reduced both container contamination and the time required for sampling the m e l t , but additional problems c r e a t e d by an inaccurate knowledge of the temperature of the l e v i tated drop and nonequilibrium conditions with the input gas introduce discrepancies in the results. Recently, the necessity of sampling and chemical analysis has been completely r e m o v e d by the developm
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