Oxidation of alkaline earth sulfides to sulfates: Thermodynamic aspects
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INTRODUCTION
A L K A L I N E earth oxides are generally employed for removal of sulfur or sulfur containing toxic species such as H2S from gas phase, especially in processes involving the manufacture of synthetic fuels. At low oxygen potentials the product of reaction is a sulfide, whereas at higher oxygen potentials the oxide is directly converted to a sulfate, without any intermediates. Phase relations in MO-MS-SO3 systems involving alkaline earth elements are shown in Figure 1. At a fixed temperature, the chemical potentials of certain gaseous species are unambiguously defined by condensed phase equilibria, as shown on the diagram. The thermodynamic stability of pyrosulfates of the alkaline earth elements has not been established. Although not included in Figure 1, they may form at low temperatures and high pressures of SO3. A knowledge of oxygen potentials corresponding to the transition from sulfide to sulfate is important for the evaluation of the oxygen balance in sulfur fixation reactors and methods of disposal of final products. Recently CaS has been used as an auxiliary electrode in a solid state sensor for sulfur based on CaF2. 5 Similar sensors can be designed based on SrF2 and BaF2 which are ionic conductors. Doped CaS has also been proposed 6'7 as a solid electrolyte for sulfur determination in gas, liquid, and solid phases. In atmospheres containing oxygen, the range of applicability of these sensors for chemical potential of sulfur is limited by the conversion of the sulfide to sulfate. Hence, there is a need for accurate information on phase stability limits of alkaline earth sulfides. Oxygen potentials in equilibrium with a sulfide and the corresponding sulfate can be readily measured with a high temperature oxygen concentration cell incorporating calciastabilized zirconia as the solid electrolyte. Suitability of this technique for obtaining reliable high temperature thermodynamic data on condensed phases and its range of application are well documented in the literature. J.2 Further, Larson and Elliott3 have successfully employed a similar technique to obtain the stability of sulfides from the oxygen potential of the systems M + MS + SO2 and MO + MS + SO2 where M is Mn, Zn, Mo, Ta, Nb, Pt, or Rh. Skeaff and Espelund4 have also employed a solid state cell to determine K.T. JACOB is Associate Professor, Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S IA4. G. N. K. IYENGAR is Associate Professor, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted December 22, 1981. METALLURGICALTRANSACTIONS B
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Fig. 1 - Phase relations in the ternary MO-MS-MSO4 systems involving alkaline earth metals.
the oxygen potential of a number of systems of the type MSO4 + MO + 802 (M -- Mg, Mn, Fe, Ni, Cu, and Zn), and thence derive the free energies of formation of sulfates from th
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