Thermodynamics of the Ca-S-O, Mg-S-O, and La-S-O systems at high temperatures
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I.
INTRODUCTION
HIGHtemperature thermodynamic data and phase stability relationships for the Ca-S-O, Mg-S-O, and La-S-O systems are of industrial importance in processes such as gaseous desulfurization, the refining of liquid iron and steel by external deoxidation and desulfurization processes, inclusion morphology control in steel, and graphite morphology control in cast iron. In recent years, galvanic cell techniques based on calcia stabilized zirconia solid electrolytes have been successfully used for the determination of various reaction equilibria in metal-sulfur-oxygen systems. Oxygen potentials associated with various metal sulfide/metal sulfate equilibria ~3 and various rare earth oxysulfide/oxysulfate equilibria~ have been measured by employing CSZ as a solid electrolyte in oxygen concentration cells. For a two phase equilibria in a metal-sulfur-oxygen system involving the metal oxide as one of the phases, a fixed oxygen potential can be obtained R V. KUMAR, Research Fellow, and D. A R KAY, Professor, are with the Department of Metallurgy and Materials Science, McMaster Universtty, Hamilton, ON L85 4L7, Canada. Manuscript submitted June 12. 1984.
METALLURGICALTRANSACTIONS B
for a given temperature and total pressure only by introducing an additional thermodynamic constraint. Larson and Elliott 5 determined the thermodynamics of several metal oxide/metal sulfide equilibria, providing the additional constraint by fixing the value of pSO2 at 1 arm. Later Skeaff and Espelund 6 used a similar technique to investigate metal oxide/metal sulfate equilibria for Mg, Mn, Fe, Ni, Cu, and Zn, by maintaining pSO2 + pSO3 = 1 atm for each system. Dwivedi 4 used Cu/Cu2S or Ag/Ag2S couples to fix the sulfur potentials in the investigation of Re203/ Re202SO4 (Re = La, Pr, Nd, Sm, Eu, Gd, Tb, and Dy), CezO3/CezOaS, and Y203/Y202S equilibria. In the present study, the following electrochemical cells were used in order to investigate the thermodynamics of the Ca-S-O, Mg-S-O, and La-S-O systems: Cell I PtlCu(s), Cu2S(s), CaS(s), CaO(s)lCSZlair]Pt Cell IX PtlCaS(s), CaSO4(s)lCSZlairlPt Cell I1! PtlCu(s), Cu2S(s), CaSO4(s), CaO(s)]CSZlairlPt Cell IV PtlAg(s), Ag2S(s), MgS(s), MgO(s)]CSZlair]Pt Cell V PtlAg(s), Ag2S(s), MgS(s), MgO(s)lNa/3- and ~-A12031Ni(s), NiO(s)[Pt Cell VI Pt[Cu(s), Cu2S(s), La202S(s), La203(s)lCSZlair[Pt
VOLUME 16B. JUNE 1985--287
Cell VII PtlAg(s),Ag2S(s), La2S3(s), La202S(s)]CSZlairlPt Cell VIII Pt[Ag(s), Ag2S(s), La2S3(s), La2OzS(s)lNa/3- and oe-Al2031Ni(s), NiO(s)tPt The emf data from cells I, III, and VI were combined with the values of AG~ for Cu2S 7 in order to calculate the standard free energy changes for the following reactions: [1] CaO(s) + ~S2(g) ~ CaS(s) + ~_Oz(g) Cell I [3]
CaO(s) + ~O2(g) + ~_S2(s)~
CaSO4(s)
TO
STEEL TUBE COUPLING
THERMOCO~
'RING
Cell IIl
[5] La203(s) + ~S2(g) ~ La202S(s) + 89 Cell VI The results from cells IV and VII were each combined with the values of AG~ for Ag2S s to determine the standard free energy changes of the following reactions: [4] MgO(s) + "S2(g) ~
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