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.     

Deuterium exchange studies of the interfacial rate of reaction of water vapor with silica-saturated iron silicate melts

Measurements have been made of the rate of dissociation of H₂O on silica-saturated iron silicate melts at 1300 °C and 1400 °C by the HDO-H₂ deuterium exchange technique. The general rate equations for the technique are developed, and the available information on the associated isotope equilibria is briefly reviewed. The rate is deduced to be first order with respect to the pressure of water vapor, and the apparent first-order rate constant is found to be essentially inversely proportional to the activity of oxygen in the melt over the range studied;i.e., pH ₂O/ pH₂ − 0.9 to 18 at 1400 °C. Comparison with the rates deduced by Sasaki and Belton from measurements of the steady-state oxygen activity of the melts in flowing H₂O-CO mixtures at 1250 °C leads to the conclusion that interfacial rates of oxidation (or reduction) of the melts in H₂O-H₂ atmospheres are given by the rate lawv = k₁(pH₂O(a′_o) −pH ₂) over the range of conditions covered by the two sets of experiments. The terma’o is the oxygen activity of the melt, defined as the equilibriumpH ₂O/pH₂ ratio, and k₁, in units of mol cm⁻²s⁻² atm⁻¹, is given by the expression: log k¹ = −6700/T − 0.08 to within a factor of about 2 over the temperature range of 1250 °C to 1400 °C. Formerly Postdoctoral Fellow, Department of Metallurgy, University of Newcastle     

Rate of interfacial reaction between liquid iron oxide and CO-CO2

The interfacial reaction rate between liquid iron oxide and CO-CO₂ was determined using a thermogravimetric technique. The measured rates were controlled by the chemical reactions at the gas-slag interface. The apparent first-order rate constant, for the oxidation of liquid iron oxide by CO₂, decreased sharply with the equilibrium CO₂/CO ratio. The rate of reduction of liquid iron oxide by CO showed a slight increase with the oxidation state of the melt. At 1773 K, the apparent first-order rate constants are given by k=4.0×10⁻⁵(CO₂/CO)^−0.8 and k=4.0 × 10⁻⁵(CO₂/CO)^0.18 mol cm⁻² s⁻¹ atm⁻¹ for the oxidation and reduction, respectively. The addition of basic oxides, such as BaO and CaO, resulted in an increased reaction rate, while the addition of acidic oxide, such as SiO₂, decreased the rate. The results are consistent with the dissociation or formation of the CO₂ molecule, involving the transfer of two charges, being the rate controlling mechanism of the reactions. 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.     

Studies of the interfacial kinetics of the reaction of CO2 with liquid iron by the14CO2-CO isotope exchange reaction

The rate of dissociation of CO₂ on liquid iron between about 1540 and 1740 °C and at CO/CO₂ ratios of 6.7 to 100 has been studied by means of the¹⁴CO₂-CO exchange reaction. It is shown that for essentially pure iron the rate constant at low oxygen potential is consistent with that for the decarburization of liquid iron by CO₂, indicating a common rate determining step. The influence of the gas composition on the rate is found to be consistent with surface blockage by adsorbed oxygen which obeys an ideal Langmuir adsorption isotherm over the experimentally accessible conditions. The adsorption coefficient for oxygen with respect to the infinitely dilute solution with 1 wt pct as standard state is deduced to be given by: logK′_o = 11270/T – 4.09 The value of K′_o at 1550 °C is found to be in good accord with the available data for the depression of the surface tension of liquid iron by oxygen. A. W. Cramb, Formerly with the Department of Materials Science and Engineering, University of Pennsylvania     

Kinetics of the carbon-oxygen reaction in molten iron

The kinetics of carbon monoxide absorption by stagnant liquid iron has been investigated over the first 10 min or so of gas-liquid metal contact. On the basis of experiments conducted at temperatures ranging between 1580° and 1700°C (PCO= 1 atm) and carbon monoxide pressures ranging between 0.1 and 1.5 atm (at 1600†C), it was concluded that the absorption kinetics of CO in liquid iron was diffusion controlled. Mass transfer equations developed to describe the process were adapted to define an “apparent diffusion coefficient” of carbon monoxide. This coefficient is a function of carbon and oxygen binary diffusion coefficients, and also depends on the initial bulk oxygen and carbon concentrations, and on the equilibrium constant for the reaction. CO = C + O Experimental D_COvalues averaged at 9.8 10−⁵cm²s−¹, while binary carbon and oxygen diffusivities were computed to be 41.2 10−⁵ and 5.2 10−T⁵cm²s−¹ respectively. Using the data obtained, the relative influence of carbon and oxygen diffusion on the kinetics of carburization and decarburization reactions is quantitatively considered. Formerly Graduate Student, McGill University, Montreal, Quebec, Canada     

Isotope exchange studies of the rate of dissociation of CO2 on liquid iron oxides and CaO-saturated calcium ferrites

Measurements of the rates of dissociation of CO₂ on liquid iron oxides and CaO-saturated liquid calcium ferrites have been made by the¹⁴CO₂-CO isotope exchange technique. For temperatures up to about 1550 °C, the apparent first order rate constants for both melts are essentially inversely proportional to the equilibrium CO₂/CO ratio over the range studied (≈0.4 to 12). Evidence is presented that the rate limiting step in the interfacial oxidation of these melts by CO₂ is the dissociation of CO₂- Rates of oxidation, in mol cm^?2 s^?1, in CO₂-CO atmospheres are deduced to be given by the equations:v =( _p CO ₂ a _o ^{? 1} ? _p CO) exp(?15,900/T ? 2.03) andv = (pCO₂ α _o ^-1 ?pCO) exp(?3800/T ? 6.93) for the iron oxides and CaO-saturated calcium ferrites, respectively, wherea ₀ is the oxygen activity of the melt expressed as the equilibrium CO₂/CO ratio and the pressures are in atmospheres. The strong dependence on the CO₂/CO ratio is shown to be consistent with the need to transfer two charges to the adsorbing or dissociating CO₂ molecule. Correlation with the existing surface tension data is inconclusive.     

Hematite single crystal reduction into magnetite with CO-CO2

In view of the striking discrepancies among previous authors as regards the transition between porous and lamellar magnetites, we have carried out a wide series of experiments with single crystals prepared by chemical vapor transport. In addition to the classical temperature and CO pct parameters, which varied over a large range (400 ≤T ≤ 1000 ≤C and 2 ≤ CO pct ≤ 50), we also investigated the influence of the crystal size and of the fractional weight change. Through observation of a great number of cross-sections of partially reduced crystals, we established that lamellar magnetite is favored by high temperature and low CO pct. This is explained by consideration of the conditions governing the competition between cation diffusion in the semi-coherent hematite-magnetite interface and chemical reaction rate. At low temperatures, the crystals are severely fractured, because hematite is not plastic enough, especially at a high CO₂ pct. The kinetic data are analyzed with the shrinking-core model, where the reaction interface is topochemical. The chemical rate constant thus obtained is ϕ = 69 exp(−8950/T), in mol(CO) · m⁻² · s⁻¹, for crystals in the range 50 to 150 μm andT varying from 500 to 900 °C. Bigger crystals yield a slightly higher preexponential term, confirming that porous diffusion does not rule the kinetics. The nucleation frequency has also been evaluated; it tends toward a kind of saturation at around 700 °C with a value of 10 to 10⁹ s⁻¹. The nuclei growth rate is in reasonable agreement with direct measurements. Formerly Graduate Student     

Hematite single crystal reduction into magnetite with CO-CO2

In view of the striking discrepancies among previous authors as regards the transition between porous and lamellar magnetites, we have carried out a wide series of experiments with single crystals prepared by chemical vapor transport. In addition to the classical temperature and CO pct parameters, which varied over a large range (400 ≤T ≤ 1000 ≤C and 2 ≤ CO pct ≤ 50), we also investigated the influence of the crystal size and of the fractional weight change. Through observation of a great number of cross-sections of partially reduced crystals, we established that lamellar magnetite is favored by high temperature and low CO pct. This is explained by consideration of the conditions governing the competition between cation diffusion in the semi-coherent hematite-magnetite interface and chemical reaction rate. At low temperatures, the crystals are severely fractured, because hematite is not plastic enough, especially at a high CO₂ pct. The kinetic data are analyzed with the shrinking-core model, where the reaction interface is topochemical. The chemical rate constant thus obtained is ϕ = 69 exp(−8950/T), in mol(CO) · m⁻² · s⁻¹, for crystals in the range 50 to 150 μm andT varying from 500 to 900 °C. Bigger crystals yield a slightly higher preexponential term, confirming that porous diffusion does not rule the kinetics. The nucleation frequency has also been evaluated; it tends toward a kind of saturation at around 700 °C with a value of 10 to 10⁹ s⁻¹. The nuclei growth rate is in reasonable agreement with direct measurements. Formerly Graduate Student     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.     

Activities of oxygen in liquid Bi,Sn, and Ge from electrochemical measurements

Modified coulometric titrations on the galvanic cell: O in liquid Bi, Sn or Ge/ZrO₂( + CaO)/Air, Pt, were performed to determine the oxygen activities in liquid bismuth and tin at 973, 1073 and 1173 and in liquid germanium at 1233 and 1373 K. The standard Gibbs energy of solution of oxygen in liquid bismuth, tin and germanium for 1/2 O₂ (1 atm) →O (1 at. pct) were determined respectively to be ΔG° (in Bi) = −24450 + 3.42T (±200), cal· g-atom⁻¹ = − 102310 + 14.29T (±900), J·g-atom⁻¹, ΔG° (in Sn) = −42140 + 4.90T (±350), cal· g-aton⁻¹ = −176300 + 20.52T (± 1500), J-g-atom⁻¹, ΔG° (inGe) = −42310 + 5.31 7 (±300), cal·g-atom⁻¹ = −177020 + 22.21T(± 1300), J· g-atom⁻¹, where the reference state for dissolved oxygen was an infinitely dilute solution. It was reconfirmed that the modified coulometric titration method proposed previously by two of the present authors produced far more reliable results than those reported by other investigators. TOYOKAZU SANO, formerly a Graduate Student, Osaka University     

The activities of sulfide and oxide components and the solubility of oxygen in copper-iron-sulfur-oxygen mattes at 1300 °C

The activities of iron and copper and the solubilities of oxygen in copper-iron-sulfur-oxygen mattes have been determined by equilibrating mattes with CO−CO₂−SO₂ gas mixtures of fixed partial pressures of oxygen and sulfur and equilibrating a small mass of platinum with the melt. Iron and copper transferred from the matte to form a platinum-iron-copper alloy in which the activities of iron and copper are the same as in the matte. The activities of iron and copper in the matte were then determined from knowledge of the activities of iron and copper in the system platinum-iron-copper. Sulfides ofW _Fe=0.1, 0.3, and 0.5 were studied, whereW _Fe=wt pct Fe/(wt pct Fe+wt pct Cu), and sulfur pressures of 0.005, 0.0158, and 0.025 atm and oxygen pressures of 3.16×10⁻¹⁰, 7.94×10⁻¹⁰, 2.00×10⁻⁹, and 3.16×10⁻⁹ were used. The activity of copper, which varied in the range 0.06 to 0.165, decreases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe and constantp _{O 2}. The activity of iron, which varied in the range 0.002 to 0.06, increases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe andp _{O 2}. The activities of the components Cu₂S, FeS, Cu₂O, FeO, and Fe₃O₄ were calculated from the activities of iron and copper, the partial pressures of oxygen and sulfur, and the approapriate equilibrium constants. The variations of the activities of these components with matte grade, oxygen pressure, and sulfur pressure are presented and discussed. Within the range of experimental conditions studied, the solubility of oxygen in the melts is given by wt pct O=2.59pO₂/0.225pS₂/−0.18 (1+9.0W _Fe)^1.86     

A comprehensive kinetic model for the CO-CO2 reaction with iron oxide-containing slags

A comprehensive kinetic model has been developed that enables the rate of the CO-CO₂ reaction with FeO _x -containing slags to be predicted. The model was developed considering a reaction mechanism involving adsorption of CO₂ at the surface, charge transfer to the adsorbed CO₂, and, finally, dissociation of CO ₂ ⁻ . The rate-determining step of the reaction was found to be the formation of the CO ₂ ⁻ ion, with the activation energy required to transfer charge to the CO₂ being dependent on slag composition. The model in its simplest form relates the apparent rate constant to temperature with an Arrhenius-type correlation, whereas both the pre-exponential and activation-energy terms are functions of slag chemistry. The model resolves the complexity previously reported in the mechanism and kinetics of the reaction and provides a self-consistent explanation for all reported observations. Comparison of calculated and measured rate constants shows that there is a good fit between the two, and the rate expression can be applied successfully for the rate-constant calculations.     

The rate of formation of intermetallic layers between aluminum and steel below 660°C

The rate of formation of intermetallic compounds between aluminum and three ferritic steels, one austenitic steel, and Inconel has been determined by an electrolytic method. The steel was held at zero potential with respect to aluminum in a NaCl-AlCl₃ melt, and the current measured. Comparison of measured thicknesses of intermetallic layers with those calculated from the integrated current gives an average deposition efficiency of 95 pct. For the Type 304 austenitic steel thickness (min), andk is given by logk= −6400/T(⁰K) +4.469. The ferritic steels show a linear rate of growth of Al₅Fe₂, with an initial higher rate such that extrapolation of the linear curve back to zero time gives an intercept of 16±7 μm. The rate constants (mm min⁻¹) may be represented by log (rate)=α/T+β, and the values of α and β are respectively −2650 and−0.788 for a plain carbon steel,−6580 and + 3.469 for a 1.3 pct Cr, 0.4 pct Mo steel, and−5950 and +2.466 for a 2.2 pct Cr, 0.9 pct Mo steel. The more highly alloyed steels are thus attacked, more slowly. Results for Inconel could not be fitted to any simple equation. With the ferritic steels growth is by aluminum diffusing inwards; with Inconel it is by nickel diffusing outward.     

The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity

The diffusivity and solubility of oxygen in liquid tin and solid silver in the temperature range of about 750° to 950°C (1023 to 1223 K) and the diffusivity of oxygen in solid nickel at 1393°C (1666 K) were determined using the electrochemical cell arrangement of cylindrical geometry: Liquid or Solid Metal + O (dissolved) | ZrO₂ + (3 to 4%)CaO | Pt, air The diffusivity and solubility of oxygen in liquid tin are given by:D _O(Sn) = 9.9 × 10⁻⁴ exp(−6300/RT) cm²/s (9.9 × 10⁻⁸ exp − 6300/RT m²/s) andN _O ^S (Sn) = 1.3 × 10⁵ exp(−30,000/RT) at. pct The diffusivity and solubility of oxygen in solid silver follow the relations:D _O(Ag) = 4.9 × 10⁻³ exp (−11,600/RT) cm²/s ( 4.9 × 10⁻⁷ exp − 11,600/RT m²/s) andN _O ^S (Ag) = 7.2 exp (−11,500/RT) at. pct The experimental value for the preexponential in the expression forD _O(Ag) is lower than the value calculated according to Zener’s theory of interstitial diffusion by a factor of 11. The diffusivity of oxygen in solid nickel at 1393°C (1666 K) was found to be 1.3 × 10⁻⁶ cm²/s (1.3 × 10⁻¹⁰ m²/s). Formerly Graduate Student, Department Formerly Graduate Student, Department Formerly Graduate Student, Department This paper is based upon a This paper is based upon a This paper is based upon a This paper is based upon a     

Direct reduction of lead sulfide with carbon and lime; Effect of catalysts:Part ii. analytical model

The experimental data presented in the Part I of this series are interpreted by means of an analytical model. The model is derived on the premise that the Boudouard reaction controls the overall rate of the reduction process in the PbS:4CaO:4C mixtures. The presence of the catalyst is considered to enhance the density of reaction sites on the carbon surface. For the uncatalyzed reduction under nitrogen, the model gives logI ₁ ^u = 4.988 (± 0.408) - 10216 (± 477)T⁻¹. A similar analysis conducted with the catalyzed reduction experiments provided the following relation: logI ₁ ^c = 4.032 (± 0.280) - 7976 (± 329)T ^-1. It is clear that the addition of the (K, Li, Na)₂CO₃ catalyst, in the amount of 2.5 wt pet, causes definite enhancement in the value of the intrinsic rate constant /,. The latter is expressed in mole · (g · C)⁻¹ · atm⁻¹ · s⁻¹. The magnitude of rate enhancement ranges from 13.6 at 800 ‡C to 6.4 at 1000 ‡C.     

The kinetics of S35 exchange between SO2/CO/CO2 gas mixtures and copper sulfide melts at 1523 K

The kinetics and mechanisms of oxidation of copper sulfide melts have been investigated using a radioisotope exchange technique. Copper sulfide melts were doped with S³⁵. The transfer of the radioisotope between the melt and SO₂/CO/CO₂ gas mixtures in chemical equilibrium with the melt was monitored by analyzing the changes in radioactivity of the gas. Analysis of the results indicates that the rate-limiting chemical reaction involves the formation of an activated complex SO, and the rate of exchange of the sulfur isotope at 1523 K is described by the relationshipR = 6.4(±2) (P _CO /P _CO2 )P _SO2 g atom S m⁻² s⁻¹. formerly Research Assistant, University of Queensland formerly Postdoctoral Fellow, University of Queensland     

On the kinetics of oxidation of liquid copper and copper-sulfur alloys by carbon dioxide

The rates of transfer of oxygen between CO₂-CO gas mixtures and liquid copper and copper-sulfur alloys have been studied by a steady-state electrochemical technique. For sulfur-free stagnant copper, and under the conditions of the experiments, the rates are shown to be controlled by the diffusion of oxygen in the metal. The resulting diffusivities are in close accord with the bulk of the previous determinations. At high sulfur concentrations, the rate is found to be controlled by an interfacial reaction which is first order with respect to the pressure of CO₂ and inversely proportional to the sulfur concentration. The rate constant, in mole (at. pct)cm⁻² s⁻¹ atm⁻¹, is approximately 8 × 10⁻⁹ at 1146°C. Formerly a Graduate Student.     

Reduction of iron-silicon-oxysulfide by CO gas injection

The reduction of liquid oxysulfide in the Fe-Si-S-O system by CO gas injection has been studied by monitoring the exit gas composition. The reduction rate of oxygen was calculated from the volume of evolved CO₂. Sulfur-bearing species such as COS were close to the detection limit of the mass spectrometer, which indicated that the reduction of sulfur was very limited. The volume of evolved CO₂ reached steady values 1 minute after CO injection. The reduction reaction was controlled by a chemical reaction. The observed maximum reduction rate of oxygen at 1250 °C was 8.3×10⁻⁶ g-O/cm² s, which was within the range of the reduction rates in other melts such as iron oxide and iron silicates.     

Oxidation of Fe (II) in sulfuric acid solutions with dissolved molecular oxygen

The oxidation of Fe(II) with dissolved molecular oxygen was studied in sulfuric acid solutions containing 0.2 mol · dm⁻³ FeSO₄ at temperatures ranging from 343 to 363 K. In solutions of sulfuric acid above 0.4 mol · dm⁻³, the oxidation of Fe (II) was found to proceed through two parallel paths. In one path the reaction rate was proportional to both [Fe⁻²⁺]² andp _{o 2} exhibiting an activation energy of 51.6 · kJ mol⁻¹. In another path the reaction rate was proportional to [Fe²⁺]², [SO ₄ ^{2 −}], andp _{o 2} with an activation energy of 144.6 kJ · mol⁻¹. A reaction mechanism in which the SO ₄ ^{2 −} ions play an important role was proposed for the oxidation of Fe(II). In dilute solutions of sulfuric acid below 0.4 mol · dm⁻³, the rate of the oxidation reaction was found to be proportional to both [Fe(II)]² andp _{o 2}, and was also affected by [H⁺] and [SO ₄ ^{2 −}]. The decrease in [H⁺] resulted in the increase of reaction rate. The discussion was further extended to the effect of Fe (III) on the oxidation reaction of Fe (II).     

Rate of decarburization of iron-carbon melts: Part II. a mixed-control model

The observed retarding effect of sulfur on the decarburization of Fe-C melts has been interpreted by means of a mixed-control mechanism involving gas-phase mass transfer and dissociative adsorption of CO₂. A mathematical model formulated on the basis of the proposed mechanism gave an excellent fit to the experimental data. The application of the model to the data provided a value of 4.42 x 10⁻³ mole · cm⁻² · s⁻¹ · atm⁻¹ for the dissociative adsorption rate constant for CO₂ on liquid iron at 1973 K; the fraction of surface sites that cannot be occupied by sulfur, even at apparent surface-saturation, was found to be 0.085. The model predicts a residual rate of decarburization at high sulfur concentrations; this prediction is borne out by the experiment. The effect of convective motion within the levitated melt on the rate of decarburization below a characteristic carbon concentration was quantified. The liquid-phase mass transfer was greatly enhanced by the stirring effect of the electromagnetic field. The effective diffusivity of carbon in Fe-C melts under levitation conditions has been found to be 3.24 x 10⁻³ cm² · s⁻¹, a value ten times as large as that under stationary conditions.