Kinetics of manganous oxide sulfidization in carbonyl sulfide (COS) contaminated atmospheres

To contribute to the mitigation of man-made emission of sulfurous compounds, the susceptibility of manganous oxide for carbonyl sulfide under reducing atmospheres (C-O-S system) has been investigated over the temperature range 700 to 1010 °C (973 to 1283 K). The kinetic investigation employed thermogravimetry and anin situ solid electrolyte oxygen probe to follow the topochemical reaction of spherical MnO pellets under various experimental conditions. The apparent activation energy of sulfidization of manganous oxide, under measured oxygen potentials in the C-O-S system, was determined to be 12.06 (±1.5) kcal/mol (52.33 (±6.28) kJ/mol). Overall sulfidization appeared to proceed by mixed control involving convective mass transfer of COS across the boundary layer and diffusion through the product layer. Formerly with the Department of Theoretical Metallurgy, Royal Institute of Technology, Stockholm     

Desulfurization of CO:SO2 contaminated gases by manganous oxide

To contribute to the curtailment of anthropogenic emission of sulfurous compounds, the susceptibility of manganous oxide for sulfur under reducing atmospheres (C−O−S system) has been investigated over the temperature range 700 to 900°C (973 to 1173 K). A thermodynamic evaluation of the Mn−S−O system and the C−O−S system was undertaken and the basic, bivariant, and univariant equilibria in the Mn−S−O system over the temperature range 700 to 1100°C are listed. The kinetic investigation employed thermogravimetry and anin situ solid electrolyte oxygen probe to follow the sulfidization reaction of spherical MnO pellets under various experimental conditions. The kinetic evaluation was complemented by X-ray diffraction and krypton B.E.T. surface area analyses. Apparent activation energy of sulfidization of manganous oxide, under measured oxygen potentials in the C−O−S system, was determined to be 6.26 (±2.0) kcal/gmol (26.2 (±8.4) kJ/gmol), and a comparison with some alternative desulfurization agents in the H−O−S system is included.     

Interdiffusion in the TiO2 oxidation product of Ti3Al

Oxidation experiments have been conducted on Ti₃Al in the temperature range 1023 to 1273 K in a one-atmosphere pure oxygen environment. The oxidation products were analyzed using the scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques and found to be predominantly TiO₂ (rutile). The oxidation rate was observed to obey the parabolic rate law. Diffusivity data were obtained using the parabolic rate constant for interdiffusion of Ti and O in the oxide layer. Parabolic rate constants were calculated from oxidation rate data, and Valenci equations for flat sheets were used to calculate diffusion coefficients. The activation energy, Q, was found to be 295.43±5.90 kJ/mol, and the frequency factor, D ₀, was calculated to be 0.68±0.01 m²/s for oxygen in the TiO₂. The activation energy obtained in this study matches closely with that of oxygen diffusion in TiO₂ reported in the literature.     

Kinetics of decarburization of iron-chromium melts in highly oxidizing atmosphere

The kinetics of decarburization in Fe-Cr-C melts were studied to determine the rate con-trolling step for the process. The experiments were carried out under nitrogen-oxygen at-mosphere in a resistance-heated vertical-tube furnace. The liquid melt was held in a freshly prepared magnesite crucible. Sampling and chemical analysis of the metal phase led to time-carbon concentration curves for the system. An iron oxide layer just below the impinging are a and a general boil were observed. Results obtained by varying param-eters such as temperature, partial pressure of oxygen, flowrate of the oxidizing gas and amount of melt determined the limiting reaction mechanism. The rate has been found to be almost independent of flow rate and partial pressure of oxygen (between 1.0 to 2.0 l/min. and 0.5 to 1.0 atm of oxygen). The amount of melt and temperature have a marked effect on the reaction rate. The apparent activation energy has been found to be 48.0 ± 5.4 K cal/mol. The carbon oxidation reaction has been proposed to occur predominantly at CO bubble/metal interface. On the basis of the experimental results and discussions reaction involving reduction of oxides by carbon has been proposed to be the rate controlling step. Formerly a graduate student of IIT, Kharagpur     

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO₂-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10⁻³ cm²/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Reduction of cobalt oxide with methane

The present work deals with reduction of cobalt oxide, CoO, with methane. Thermogravimetry was used for finding kinetic parameters of the reduction reaction. The reaction of cobalt oxide with methane was carried out in the temperature range 800 °C to 950 °C at atmospheric pressures with porous pellets prepared from cobalt oxide powder with a mean particle size of 10.6 μm. Assuming that the reaction is first order with respect to methane concentration, the activation energy is f6ound to be 155 kJ/mol (±20).     

Gibbs energies of formation of intermetallic phases in the systems Pt-Mg,Pt-Ca,and Pt-Ba and some applications

The standard Gibbs energies of formation of platinum-rich intermetallic compounds in the systems Pt-Mg, Pt-Ca, and Pt-Ba have been measured in the temperature range of 950 to 1200 K using solid-state galvanic cells based on MgF2, CaF2, and BaF2 as solid electrolytes. The results are summarized by the following equations: ΔG° (MgPt₇) = −256,100 + 16.5T (±2000) J/mol ΔG° (MgPt₃) = −217,400 + 10.7T (±2000) J/mol ΔG° (CaPt₅) = −297,500 + 13.0T (±5000) J/mol ΔG° (Ca₂Pt₇) = −551,800 + 22.3T (±5000) J/mol ΔG° (CaPt₂) = −245,400 + 9.3T (±5000) J/mol ΔG° (BaPt₅) = −238,700 + 8.1T (±4000) J/mol ΔG° (BaPt₂) = −197,300 + 4.0T (±4000) J/mol where solid platinum and liquid alkaline earth metals are selected as the standard states. The relatively large error estimates reflect the uncertainties in the auxiliary thermodynamic data used in the calculation. Because of the strong interaction between platinum and alkaline earth metals, it is possible to reduce oxides of Group ILA metals by hydrogen at high temperature in the presence of platinum. The alkaline earth metals can be recovered from the resulting intermetallic compounds by distillation, regenerating platinum for recycling. The platinum-slag-gas equilibration technique for the study of the activities of FeO, MnO, or Cr₂O₃ in slags containing MgO, CaO, or BaO is feasible provided oxygen partial pressure in the gas is maintained above that corresponding to the coexistence of Fe and “FeO.” Formerly Professor and Chairman, Department of Metallurgy, Indian Institute of Science Formerly Visiting Scientist, Department of Metallurgy, Indian Institute of Science     

Kinetics of the solid-state carbothermic reduction of wessel manganese ores

Reduction of manganese ores from the Wessel mine of South Africa has been investigated in the temperature range 1100 °C to 1350 °C with pure graphite as the reductant under argon atmosphere. The rate and degree of reduction were found to increase with increasing temperature and decreasing particle sizes of both the ore and the graphite. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron to MnO and FeO. The rate control appears to be mixed, both inward diffusion of CO and outward diffusion of CO₂ across the porous product layer, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO₂ diffusivities generated by the mathematical model are in the range from 2.15 x 10⁻⁵ to 6.17 X 10⁻⁵ cm².s⁻¹ for different ores at 1300 °C. Apparent activation energies range from 81. 3 to 94.6 kJ/kg/mol. (2) The second stage is slower during which MnO and FeO are reduced to mixed carbide of iron and manganese. The chemical reaction between the manganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process The rate constant of chemical reaction between MnO and carbide on the surface of the impervious core was found to lie in the range from 1.53 x 10⁻⁸ to 1.32 x 10⁻⁷ mol . s⁻¹ . cm⁻². Apparent activation energies calculated are in the range from 102.1 to 141.7 kJ/kg/mol. Formerly Doctoral Student, Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Johannesburg,     

Oxidation of Ti3SiC2 composites in air

The oxidation of Ti₃SiC₂ composites (75 at. pct Ti₃SiC₂ and 25 at. pct TiC _x ), prepared via self-propagating high-temperature synthesis (SHS) and subsequent shock consolidation, has been studied in the range of 1073 to 1573 K in this research. The oxidation kinetics are parabolic with an activation energy of approximately 240±20 kJ/mol. As shown by transmission electron microscopy (TEM) during the very early stages of oxidation, the oxide layer formed contains amorphous SiO₂ and crystalline rutile (TiO₂). As oxidation proceeds, a two-layered oxide scale is observed with the outer oxide layer consisting of columnar TiO₂ with trace amounts of SiO₂ and the inner oxide layer being comprised of a mixture of amorphous SiO₂ and fine crystalline TiO₂. The grain size of the outermost oxide (TiO₂) increases with increasing oxidation temperature. The oxidation resistance of the Ti₃SiC₂ composites prepared by SHS and subsequent shock consolidation is similar to the oxidation of Ti₃SiC₂ prepared by other means with comparable parabolic constants.     

Oxidation Kinetics of a Pb-30 At. Pct In Alloy

The solid-state oxidation kinetics of a Pb-30 at. pct In alloy were studied from 22 °C to 175 °C using Auger electron spectroscopy (AES) combined with argon ion sputtering. At 22 °C, oxidation followed a direct logarithmic relationship. The data were interpreted in terms of a model presented previously. [2] The predicted value of the model parameter using the structural data of In₂O₃ is within a factor of 2 from the experimentally determined value. At temperatures higher than 100 °C, the oxidation kinetics changed to a different mechanism. Rapid oxidation occurred initially, followed by slower parabolic oxidation. The oxide formed was found to be In₂O₃ with possible existence of a metastable indium suboxide near the oxide/alloy interface. These data were described quantitatively by the model proposed by Smeltzeret al. [5] The model assumes the existence of short-circuit diffusion in addition to lattice diffusion. The degree of short-circuit diffusion decreases exponentially with time. The activation energy for the parabolic oxidation was found to be 30.8 ± 4 kJ/mol (0.32 eV). The rather low activation energy was rationalized by the fact that the diffusion of oxygen vacancies in In₂O₃ responsible for the parabolic oxidation occurred in the extrinsic region. This rationalization was made by analyzing the defect structure of In₂O₃ using oxygen diffusion data and the enthalpy of forming oxygen vacancies in In₂O₃ reported in the literature. [10,11,12]     

Effect of Annealing on the Interfacial and Structural Properties of Amorphous Silicon-Hafnia Films

Chemical and physical properties of the interface formed between amorphous silicon (a-Si) and hafnium oxide (HfO₂) were investigated using X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS). XRR showed that the interface layer formed between the a-Si and HfO₂ layer had a thickness of 0.7 ± 0.3 nm and a density of 4.4 ± 0.2 g/cm³. Based on the XPS results, the interface comprised HfO₂, silicon oxide, and hafnium silicate phases. High-temperature annealing led to the conversion of elemental silicon and nonstoichiometric silicate into silicon dioxide phases. This reaction was also accompanied by the chemical reduction of the HfO₂ phase. Annealed films crystallized into the monoclinic phase of HfO₂ with a preferred orientation along the (111) axis.     

Standard molar enthalpies of formation of MeAl (Me = Ru,Rh, Os,Ir)

The standard molar enthalpies of formation of RuAl, RhAl, and IrAl have been determined by the direct combination method using a high-temperature calorimeter operated at (1473 ±2) K. The following values are reported: ΔH _f ^o (RuAl) = −(124.1 ± 3.3) kJ/mol; ΔH _f ^o (RhAl) =-(212.6 ± 3.2) kJ/mol; and ΔH _f ^o (IrAl) = -(185.5 ± 3.5) kJ/mol. For OsAl, an approximate value is −77 kJ/mol. The results are compared with available data for related alloys and with predicted values.     

Thermodynamics of Cobalt Aluminate Formation in Molten Cobalt

The oxygen concentration of liquid cobalt in equilibrium with cobalt aluminate and alumina was measured by suction sampling at temperatures in the range of 1783 to 1873 K. Experiments were made with cobalt of high and low initial oxygen content and with the addition of cobalt aluminate. The Gibbs free energy of the formation of cobalt aluminate in equilibrium with cobalt containing dissolved oxygen and alumina is: Co (l) + (1+x) Al₂O₃ + [O]_{1 wt% in Co} = CoO·(1+x)Al₂O₃ with ΔG° = ‐(226400 ± 8730) + (93.04 ± 4.74) T [J/mol]. The oxygen partial pressure in equilibrium with the dissolved oxygen in molten cobalt was determined by zirconia stabilized with magnesia oxygen sensors. The Gibbs standard free energy of dissolution of oxygen in molten cobalt in the range of 1783 to 1873 K is: 1/2 O₂ = [O]_{1 wt% in Co} with ΔG° = ‐76410 – 2.16 T [J/g·atom].     

Equilibrium phase relations and thermodynamics of the Cr-0 system in the temperature range of 1500 °C to 1825 °C

Phase relations and thermodynamic properties of the Cr-O system were studied at temperatures from 1500 °C to 1825 °C. In addition to Cr and Cr₂O₂, a third crystalline phase was found to be stable in the temperature range from 1650 °C to 1705 °C. The atomic ratio of oxygen to chromium of this phase, which decomposes upon cooling to form Cr and Cr₂O₃, was determined as 1.33 + 0.02, in good agreement with the formula Cr₃O₄. Temperatures and phase assem blages for invariant equilibria of the Cr-O system were determined as follows: Cr₂O₃ + Cr + Cr₃O₄, 1650 °C ± 2 °C; Cr₃O₄ + Cr + liquid oxide, 1665 °C ± 2 °C; and Cr₃O₄ + Cr₂O₃ + liquid oxide, 1705 °C ± 3 °C. The composition of the liquid oxide phase at the eutectic temperature of 1665 °C was found to be close to CrO. Relations between oxygen pressure and temperature for the univariant equilibria of the Cr-O system were established by equilibrating Cr and/or Cr₂O₃ starting materials in H₂-CO₂ mixtures of known oxygen potentials at temper atures from 1500 ΔC to 1825 °C. From this information, the standard free-energy changes (ΔGΔ) for various reactions were calculated as follows: 2Cr (s) + 3/2O₂ = Cr₂O₃ (s): ΔG ° = -1,092,442 + 237.94T Joules, 1773 to 1923 K; 3Cr (s) + 2O₂ = Cr₂O₄ (s): ΔG ° =-1,355,198 + 264.64T Joules, 1923 to 1938 K; and Cr (s) + l/2O₂ = CrO (1): ΔG ° =-334,218 + 63.81T Joules, 1938 to 2023 K. Formerly Graduate Research Assistant, The Pennsylvania State University Formerly Professor     

Dissolution Behavior of Rhodium in the Na2O-SiO2 and CaO-SiO2 Slags

To understand the behavior of rhodium during its recovery process, the dissolution behaviors of rhodium in Na₂O-SiO₂ and in CaO-SiO₂ slags at temperatures ranging from 1423 K to 1623 K (from 1150 °C to 1350 °C) and from 1773 K to 1873 K (from 1500 °C to 1600 °C), respectively, in an oxidizing atmosphere were investigated. The solubility of rhodium in the slags was found to increase with increasing oxygen partial pressure, temperature, and the basic oxide content. The correlation between the solubility of rhodium and the oxygen partial pressure suggested that rhodium dissolved into the slags as RhO_1.5. The dissolution of rhodium was slightly endothermic: the enthalpy change of the dissolution of solid rhodium was determined to be 50 ± 10 kJ/mol for the 50(mass pct)Na₂O-50SiO₂; and 188 ± 94 kJ/mol for the 56(mass pct)CaO-44SiO₂ slag systems. The increase in the solubility of rhodium with the basic oxide content indicated that rhodium exhibits acidic behavior in slags. The correlation between the solubility of rhodium and the sulfide capacity of the slags suggested that the ionic species of rhodium in slags is the rhodate ion, RhO ₂ ^? . The rhodate capacity of the slags was defined, and its application to estimate the possible rhodium content in various slag systems was proposed.     

Galvanic cell measurements on the sodium-oxygen system

Abstract

The sodium - disodium oxide equilibrium was studied by means of galvanic cells using thoria-based solid electrolytes of various compositions and the accurately known nickel- nickel oxide system as the reference electrode. The free energy of formation of disodium oxide, per gram-mole of oxygen, was found to be given by

?G° =? 193,890 +61.66 T ±800 cal (714°–934°K)

Unsaturated solutions of oxygen in sodium, with oxygen concentrations ranging from 0.56 to 6.85 ppm, were studied in similar cells where the tin - tin oxide system was used as the reference electrode. The measurements yielded the following expression for the solubility of oxygen in sodium equilibrated with disodium oxide:

log (ppm) = (7.076 ± 0.004) ? 2865 ± 180/T

Evidence is incontrovertible that free disodium oxide corrodes heavily the thoria-based electrolytes and pure thoria and pure yttria to a much smaller extent As these tests were carried out on specimens of very high purity, this corrosion cannot be attributed to impurities.

Résumé

L'équilibre sodium-oxyde de sodium a été étudié par des mesures de piles utilisant des électrolytes solides à base de thorine et Ie système de référence Ni/NiO. L'enthalpie libre de formation de Na₂0, par mole d'oxygène s'écrit:

?G° =?193,890 + 61.66T ± 800 cal (714° ? 934°K)

Des solutions non saturées d'oxygéne dans Na, avec des concentrations en O₂ variant de 0.56 à 6.85 ppm, ont été étudiées dans des cellules semblables mais avec le système étain/oxyde d' étain comme référence. Ces mesures ont conduit à l'expression suivante pour la solubilité de l'oxygéne dans le sodium en équilibre avec Na₂O.

log (ppm) = (7.076 ± 0.004) ? 2865 ± 180/T

Il est incontestable que le Na₂O corrode fortement l'électrolyte à base de thorine et beaucoup moins la ThO₂ pure et Y₂O₃ pure. Comme ces tests ont été effectués sur des spécimens de très haute pureté, cette corrosion ne peu t être attribuée aux impuretés.     

Raman spectroscopy of glasses and melts of the Na<Subscript>2</Subscript>O-GeO<Subscript>2</Subscript> system

Glasses and melts of the system X mol % Na₂O · GeO₂ (X = 0, 10, 20, 30) are investigated using high-temperature Raman spectroscopy. Addition of an oxide modifier is shown to bring about an increase in the coordination number of central germanium atoms along with the depolarization of the three-dimensional GeO₂ network. The glass-melt transition in germanate systems with a low oxide-modifier content is accompanied by the transition of six-coordinated germanium atoms into four-coordinated (with oxygen) ones and a simultaneous formation of nonbridge oxygen atoms.     

Determination of the standard gibbs energies of formation of Ca3As2, Ca3Sb2, and Ca3Bi2

The standard Gibbs energies of formation of Ca₃As₂, Ca₃Sb₂, and Ca₃Bi₂ were determined by a chemical equilibration technique yielding the following results: 3Ca(1)+2As(1)=Ca₃As₂ (s) ΔG°=−723,800+172.8T (±23,700)(J/mol) 1273 to 1573 K 3Ca(1)+2Sb(1)=Ca₃Sb₂(s) ΔG°=−726,300+159.3T(±24,600) (J/mol) 1273 to 1573K 3Ca(1)+2Bi(1)=Ca₃Bi₂(s) ΔG°=−696,400+195.6T(±23,200) (J/mol) 1148 to 1323 K The thermodynamic data for removal of arsenic, antimony, and bismuth by other experimental investigations were discussed in terms of the activity coefficients of these compounds in slags. The stabilities of these compounds were also discussed by using the critical oxygen partial pressures calculated from the above equations. D.J. MIN, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan     

Study of the mechanism of fast diffusion of cobalt in thorium by diffusion and internal friction measurements

The activation energy for diffusion,Q _d, of cobalt in fcc thorium was determined to be 83.7±4 kJ/mol from chemical diffusion measurements between 900 °C and 1332 °C. An internal friction peak associated with cobalt was identified in thorium at 45°C, and the activation energy for relaxation,Q _R, was found to be 76.0±1.5 kJ/mol by a frequency variation technique. The nearly equal values forQ _D andQ _R eliminate the interstitial-vacancy pair mechanism for fast diffusion in this system. The host-solute diplon and substitutional-interstitial (S-I) pair models for fast diffusion are considered, however, and preference is given to the latter mechanism for cobalt in thorium. S.C. AXTELL, formerly Graduate Student, Ames Laboratory, Iowa State University