Thermodynamics of the oxidation of rare earth oxysulfides at high temperatures
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I.
INTRODUCTION
THE
high temperature thermodynamic properties of rare earth oxysulfides are of industrial importance in the rare earth treatment of iron and steel and the high temperature desulfurization of gaseous fuels by rare earth oxides. Rare earth oxysulfides are also used as phosphors. Despite this industrial importance, the thermodynamics of the oxidation of rare earth oxysulfides has not been studied. The oxidation behavior of oxysulfides in air has been determined qualitatively using thermogravimetric techniques, ~ and the following general conclusions can be drawn from these studies: (1) Oxysulfides of the rare earths oxidize to their respective oxysulfates when heated in air at high temperatures. Oxysulfates decompose to their respective oxides on subsequent heating at higher temperatures. (2) The decomposition of the oxysulfate may start before the oxidation of the oxysulfide is complete. The decomposition temperature of rare earth oxysulfates decreases as the atomic number of the constituent rare earth increases. In the present study, the thermodynamics of the oxidation of rare earth oxysulfides to their respective oxysulfates has been determined. The oxidation can be represented by the reaction: 89
(s) + 02 (g) = 89
(s)
taneous coexistence of RE202S, R F 4 0 2 5 0 4 , and RE203 at constant temperature, the oxygen, sulfur, and rare earth potentials are all uniquely determined. It is assumed that the oxygen partial pressure determined by the RE202S/ RE2OeSO4 equilibrium, at a constant temperature and sulfur partial pressure, is independent of sulfur partial pressure in the limited range of RE202S/RE202SO4 coexistence, i.e., the stoichiometry of RE202S and RE202SO4 is not a function of the partial pressure of sulfur, in the range of sulfur partial pressures in which RE202S and RE202SO4 coexist at constant temperature. At constant temperature, under the conditions of simultaneous coexistence of RE202S, RE202SO4, and RE203, the unique oxygen partial pressure can be measured by an oxygen concentration cell using calcia stabilized zirconia (CSZ) as the solid electrolyte. This cell may be represented as: Pt (s)IRE202S (s), RE202SO4 (s), RE203 ( s ) I c s z l Air(po2 = 0.21 atm)IPt (s) The electrode reactions are: Anode: 89 (s) + 20 = = 89 (s) + 4e Cathode: 4e + 02 (0.21 atm) = 20 = and the virtual cell reaction can then be written as: 89RE202S (s) + Odpo: = 0.21 atm) = 89RE202SO4 (s)
[1]
[2]
where RE represents a rare earth element.* Under the condi-
If the above cell is reversible and all solid phases are in their standard states, the oxygen partial pressure, Po2, corresponding to the oxysulfide/oxysulfate equilibrium is given by:
*Ce202S is a notable exception and oxidizes directly to the oxide. Cerium is not known to form an oxysulfate of the form RF-aO2SO4.
tions of coexistence of RE202S and RE202SO4, the rare earth-oxygen-sulfur system is bivariant. Thus, an additional constraint has to be imposed on the system to establish uniquely the oxygen partial pressure which can be measured using an
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