The use of REOS phase stability diagrams in gaseous desulfurisation and iron and steel production

The thermodynamic properties of rare earth (RE) compounds containing oxygen and/or sulfur are of industrial significance in the high temperature desulfurisation of gaseous fuels by rare earth oxides [1], the control of graphite morphology in cast irons [2] and sulfide inclusion control in steels [1]. The initial form of the phase stability diagrams was based on the thermochemical data of Gschneidner et al. [3] using cerium as ‘representative’ rare earth [4]. More recently [5], the high temperature standard free energies of formation of some REOS compounds have been determined using oxygen concentration cells with calcia stabilised zirconia (CSZ) as the solid electrolyte, and the phase stability diagrams extended to higher oxygen potentials.The LaOS and CeOS Diagrams. Updated versions of the LaOS and CeOS phase stability diagrams at 1100 K [5] are given in Fig. 1 and 2, respectively.For the LaOS diagram, oxygen concentration cells of the type:Pt/La2O3(s), La2O2S(s), La2O2SO4(s)/CSZ/Air/Pt and Pt/La2O2S(s), La2O2SO4(s), Ag(s), Ag2S(s)/CSZ/Air/Pt were used [5] to generate thermodynamic data on the equilibrium: La2O2S(s) + 2O2(g) = La2O2SO4(s) (1) These data were combined with those of Grunzweig [6] for the La2(SO4)3/La2O2SO4 equilibrium; Gschneidner et al. [3] for La2O3; Mills [7] for LaS and La2S3; and Vasileva et al. [8] for LaS2. The standard free energy of formation of La3S4, ΔG0f, La3S4, was taken to be equal to that of Ce3S4 [10].For the CeOS diagram, oxygen concentration cells of the type: Pt(s)/CeOx(s), Ce2O2S(s), Ag(s), Ag2S(s)/CSZ/Air/Pt(s) have been used [9] to generate thermodynamic data on the equilibrium: Ce2O2S(s) + (x - 1)O2(g) = 2CeOx(s) + 12S2(g) (2) The data were combined with those of Gschneidner and Kippenham [10] for the sulfides; Gschneidner et al. [3] for the oxysulfide; Bevan and Kordis [11] for the oxides; and Barin et al. [12] for Ce2(SO4)3.In both the LaOS and CeOS systems there are large uncertainties associated with the standard free energies of formation of some of the compounds and the phase stability diagrams have been constructed using internally consistent data to conform to the phase rule.Gaseous Desulfurisation. The general principles of high temperature gaseous desulfurisation have been outlined elsewhere [1, 13]. In Figs. 1 and 2, for example, the bivariant equilibria La2O3/La2O2S and CeOx/Ce2O2S represent the limits of desulfurisation which can be attained by contacting gaseous fuels containing H2S with lanthanum or cerium oxides at 1100 K, the lower the oxygen potential the greater the desulfurisation: 2REO2(s) + 12S2(g) = RE2O2S(s) + (x - 1)O2(g) (3)A gaseous fuel with a room temperature composition of 55% CO, 33% H2, 11% CO2 and 1.1% H2S by volume can be desulfurised down to about 2 ppm H2S by La2O3 and to about 72 ppm H2S by CeO2, at 1100 K; OxideTKlog pO2log pS2ppm H2SLa2O31100−19.59−13.612.3CeO21100−19.59−10.6472The stability of rare earth sulfates and oxysulfates are of importance during the high temperature regeneration of the oxides by contacting the oxysulfide with air. The decomposition temperatures of the rare earth oxysulfates decrease with increasing atomic number and La2O2SO4, with a decomposition temperature of 1943 K, is the most stable of the oxysulfates. Cerium, on the other hand, does not form an oxysulfates and the decomposition temperature of Ce2(SO4)3 is 1194 K.Graphite Morphology Control in Cast Iron. The CeOS phase stability diagram at 1500 °C, given in Fig. 3, establishes the liquid iron chemistry, in terms of the Henrian activities of oxygen and sulfur, essential to the control of nodular and compacted graphite in a cast iron melt containing 3.5% C and 2.0% Si treated with cerium [2, 14].Graphite crystal growth mechanisms are sensitive to soluble impurity concentrations at the parts per million level. Oxygen and sulfur are the principal impurities of technological interest and their activities in treated cast iron melts can be related to the activity of the treatment metal, e.g., Ce, Ca or Mg. At slow cooling rates, such as those obtained in thick sections, where the kinetic undercooling is not significant, graphite morphology can be directly related to Fig. 3.Sulfide Morphology Control in Steels. The phase stability diagram for the CeOS system at 1627 °C [1], given in Fig. 4, establishes the approximate liquid steel chemistry under rare earth deoxidation control and is based on a thermodynamic analysis [4, 15] using the data of Gschneidner and co-workers [3, 10].The sequence of precipitation of rare earth oxides, oxysulfides and sulfides during the rare earth treatment of a steel of given oxygen and sulfur contents may be determined from this diagram. The bivariant equilibrium Ce2O3/Ce2O2S determines the conditions under which Ce2O3 may precipitate, i.e., ho/hs > 4.3. This condition is easily met in the production of modern low sulfur steels.The authors should like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada, and the Department of Energy, Mines and Resources (CANMET).