The interfacial kinetics of the reaction of CO2 with nickel: Part II. the decarburization of liquid nickel

The kinetics of decarburization of liquid nickel in CO₂-CO mixtures have been studied at 1400 and 1500°C, using the experimental arrangement of the impinging jet. At carbon concentrations above about 1 wt pct, pressures of CO₂ ⪯ 0.1 atm, and for total gas flow-rates above about 40 l/min (STP) impinging on a metal surface of 2.08 cm², it is concluded that the interfacial reaction step controls the rate. Comparison with isotope exchange studies indicates that dissociative chemisorption of CO₂ is the rate determining step. Rate constants, based on the nominal surface area, are 1.2 ×10⁻³ and 1.4 × 10⁻³ mol/cm² · s · atm at 1400 and 1500°C, respectively. on leave of absence from the Homer Research Laboratories of the Bethlehem Steel Co.     

The interfacial kinetics of the reaction of CO2 with liquid nickel

Measurements of the rate of dissociation of CO2 on liquid nickel have been made by the¹⁴CO2-CO isotope exchange technique between 1490 and 1670 °C at CO2/CO ratios between 0.01 and 7. Apparent first order rate constants are given by the expression:ka = (1 + 2pCO₂/pCO)⁻¹exp(−12700/T - 0.65) mol cm⁻² s⁻¹ atm⁻¹. It is shown that the results are consistent with blockage of the surface by oxygen which exhibits ideal Langmuirian adsorption over the conditions of the experiments. The adsorption coefficient of oxygen with respect to the infinitely dilute solution with 1 wt pct as the standard state is deduced to be given by the equation: logK′_o = 11880/T - 4.6. It is deduced that the interfacial rate of oxidation of nickel by CO₂ is given by the rate of dissociative chemisorption of CO₂. Measurements of the rate of decarburization of liquid nickel are reexamined in the light of the present results.     

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.     

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     

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.     

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,     

Kinetics of reduction of hematite with hydrogen gas at modest temperatures

The kinetics of hydrogen reduction of thin, dense strips of hematite were investigated in the range 245 °C to 482 °C. Pure hydrogen gas at 1 atm was used as the reducing agent. Because of the relative thinness (only 136 /μm thick) of the specimens used, the pore-diffusion of gases offered no significant resistance to the reduction process. The interfacial-reaction-rate constantk _s ^* , which has been corrected for film-mass-transfer effects, is found to be given by logk _s ^* = −1.032 (±0.138) -[7860 (±200)]/2.303r where k _s ^* is in g · atom O · cm⁻² · s⁻¹ · atm⁻¹. The activation energy for the reduction process is found to be 65,325 (±1650) J · mol⁻¹; the rate-controlling step appears to be the Fe₃O₄ → Fe conversion step.     

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.     

Decarburization kinetics of liquid Fe−Csat alloys by CO2

The kinetics of the decarburization reaction of Fe−C_sat liquid alloys by CO₂ were investigated at 1600°C under conditions where the rate is not significantly affected by liquid- or gas-phase mass transfer. Rates of decarburization were measured by monitoring the change in gas composition with an in-line mass spectrometer. Small amounts of S, P, Sn, and Pb were alloyed with the Fe−C_sat melt to determine their effect on the interfacial rate constant. The rate constant measured for the Fe−C_sat−S alloys is in agreement with previous results for Fe−S alloys and was found to be inversely proportional to sulfur concentration at low sulfur levels. For high S-containing alloys where the surface is saturated with sulfur, a residual rate was observed. The effects of phosphorus and lead on the rate constant are negligible. Tin decreases the rate constant, but the effect is small, even when the tin content is as high as 2.6 wt pct. F.J. MANNION, formerly Graduate Student, Carnegie Mellon University     

Interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of liquid iron have been studied between 1160 and 1600°C under conditions where mass transport of reactants is not rate determining. Studies with continuously carbon-saturated iron and of iron with varying carbon concentration have been used to show that the slow step at high concentrations of carbon is independent of carbon concentration and is first order with respect to the pressure of CO₂. For high purity iron, the forward rate constant, in mole cm² s^-1 atm^-1, is given by the equation ln k_f = -11,700/T-0.48. It is concluded that the data are consistent with the chemisorption process as the rate limiting step. A marked sensitivity of the rate to trace amounts of sulfur has been found and it is shown that this is consistent with ideal adsorption of sulfur and is in fair accord with the existing measurements of the depression of the surface tension of iron-carbon alloys by sulfur. D. R. Sain was formerly a Graduate Student.     

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     

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 effect of chromium on the activity coefficient of sulfur in liquid Fe−Cr−S alloys

The effect of chromium on the activity coefficient of sulfur in the ternary system Fe−Cr−S has been determined in the temperature range 1525° to 1755°C for chromium concentrations of up to 40 wt pct, using a levitation melting technique in H₂−H₂S atmospheres. The first order free energy interaction coefficient,e _S ^Cr , which is derived on the assumption that the thermal diffusion error is constant for both binary Fe−S and ternary Fe−Cr−S melts under controlled levitation conditions, is given by the relationship:e _S ^Cr =−94.2/T+0.040 The first order enthalpy and entropy interaction coefficients are found to beh _S ^Cr =−430±70 ands _S ^Cr =−0.183±0.007 respectively. These results are in good agreement with recently published data.     

Ionic conductivity of solid calcium sulfide at 650 to 1000° C

The electrical conductivity of solid calcium sulfide has been measured at various temperatures and sulfur pressures with the aid of an alternating current bridge. At partial pressures less than about 10⁻⁶ atm at 770°C to 1000°C, the conductivity is independent of sulfur pressure and can be expressed by log₁₀ σ (Ω⁻¹ ·cm⁻¹) = −5.185/T(K) − 0.0901 The apparent activation energy for the conduction is 23.7 (± 1.04) kilocalories per mole. Both the relatively large activation energy and the lack of dependence of the specific conductivity on sulfur pressure suggest that the conduction is predominantly ionic for sulfur pressures less than about 10⁻⁶ atm. However, for pressures greater than 10⁻⁶ atm, the specific conductivity increases with an increase of the sulfur pressure, suggesting positive hole conduction. Some comments are included on the possibility of the application of calcium sulfide as a solid electrolyte of a sulfur concentration cell.     

Dissolution of solid magnetite in Fe-S-O melt

The dissolution rate of solid magnetite in binary Fe-S and ternary Fe-S-O melts was measured at 1493 K using a rotating magnetite rod of 5 mm in diameter. At rotating speed higher than 52 rpm, the flow of sulfide melt was turbulent, and the effect of natural convection on the dissolution rate was negligible. Magnetite dissolution took place without the evolution of SO₂ gas at the rod surface immersed in the interior of the melt. On the other hand, SO₂ gas was evolved from a portion of magnetite rod which was in contact with the melt surface. The overall rate of dissolution was controlled by mass transfer through liquid boundary layer on the rod surface, and the dissolution rate of magnetite decreased with increasing oxygen concentration of the sulfide melt. Mass transfer coefficient was between 1.8 × 10⁻³ and 4.6 × 10⁻³ cm · s⁻¹ at the rotating speed of 156 rpm, and it decreased with increasing oxygen concentration of the melt. Formerly Graduate Student Formerly Graduate Student Formerly Graduate Student     

The rate of reaction of solid iron with oxidized “FeO”-CaO-SiO2-Al2O3 slags at 1360 °C—the chemical diffusivity of iron oxide

Measurements have been made of the rate of reduction of oxidized iron oxide-containing 41CaO-38SiO₂-21Al₂O₃ (wt pct) slags at 1360 °C by a rotating disc of solid iron. For initial total iron concentrations of between 1.8 and 13.4 wt pct and rotation speeds up to 1000 rpm, the rate is shown to be determined by mass transfer in the liquid phase. The chemical diffusivity of iron oxide (in cm² s⁻¹) is found to be given by the empirical expression log D = −6.11 + 0.08 (wt pct Fe). It is concluded that the values of the diffusivity are for melts at close to iron saturation. It is shown that the available measurements of the diffusivity of iron oxide in liquid slags are consistent with increasing diffusivity with increasing state of oxidation, with about a tenfold increase between melts in equilibrium with iron and those in equilibrium with oxygen at 1 atm.     

Enthalpies of formation of chromium carbides

Adiabatic oxygen combustion calorimetry has been used to determine the enthalpies of combustion of the chromium carbides Cr₂₃C₆, Cr₇C₃ and Cr₃C₂ to be—15,057.6±12.4 kJ ·mole⁻¹,—4985.3±3.8 kJ ·mole⁻¹ and—2400.5±0.9 kJ ·mole⁻¹ respectively. The products of combustion in all cases were Cr₂O₃ and CO₂. Using standard data for Cr₂O₃ and CO₂, the enthalpies of formation of the carbides have been calculated to be:fΔH ₂₉₈ ^o Cr₂₃C₆=−290.0±27.6 kJ·mole⁻¹ fΔH ₂₉₈ ^o Cr₇C₃=−149.2±8.5 kJ·mole⁻¹ fΔH ₂₉₈ ^o Cr₃C₂=−81.1±2.9 kJ·mole⁻¹     

Trapping of hydrogen by sulfur-associated defects in steel

Reversible and irreversible trapping behaviors of sulfur-associated defects in steel were studied through electrochemical hydrogen permeation experiments and the results were analyzed to obtain the follow-ing information: an apparent diffusion constant of hydrogen influenced by reversible and irreversible trapping by sulfur-associated defects,D *, was shown to be given as a function of sulfur content, S, in wt ppm.byD * = 1.12·10⁻⁶(1 + 0.0933S^0.548)⁻¹cm² s⁻¹. Associated parameters of reversible and irreversible trapping, λ/μ and k, were also expressed as a function of sulfur content, S. Both parameters, λ/μ, and k, are shown to increase with S, suggesting an increase of both reversible and irreversible trap sites with increase in sulfur content in steel.     

Rate of decarburization of iron-carbon melts: Part I. experimental determination of the effect of sulfur

The kinetics of decarburization of iron-carbon melts with CO-CO₂ gas mixtures were investigated at 1700 ° using the levitation technique. The influences of different experimental variables on the decarburization kinetics were determined. It was found that sulfur has a clear and reproducible retarding effect on the decarburization of iron-carbon melts; and this effect is most pronounced at sulfur concentrations in the range of 0 to 0.05 wt pct. The initial carbon concentration has no discernible effect on the decarburization kinetics. Melts containing 2.48 wt pct C and 0.92 wt pct C initially were found to decarburize at virtually identical rates until a substantial portion of the carbon was removed. The decarburization rate of a melt with a specified initial carbon content was found to remain essentially constant until the carbon content fell to a characteristic level below which the rate tended to level off. The partial pressure of CO₂ of the gas mixture has a marked effect on the decarburization kinetics. The flow-rate of the gas mixture has a small but finite effect on the rate of decarburization.     

Influence of chromium and nickel on the dissociation of CO2 on carbon-saturated liquid iron

At 1600 °C, under conditions where the rate was not significantly affected by liquid-phase or gasphase mass transfer, the rate of dissociation of CO₂ was determined from the rate of decarburization of iron-based carbon-saturated melts containing varying amounts of chromium and nickel. The rate was determined by monitoring the change in reacted gas composition with an in-line spectrometer. The results indicate that neither chromium nor nickel had a strong effect on the kinetics of dissociation of CO₂ on the surface of the melt. Sulfur was found to significantly decrease the rate, as is the case for alloys without chromium or nickel, and the rate constant is given by $$k = \frac{{k^0 }}{{1 + K_s a_s }} + k_r $$ where k ⁰ denotes the chemical rate on pure iron, K _s is the adsorption coefficient of sulfur, a _s is the activity of sulfur corrected for Cr, and k _r represents the residual rate at a high sulfur level. The rate constants and adsorption coefficient were determined to be: $$\begin{array}{*{20}c} {k^0 = 1.8 \times 10^{ - 3} mol/cm^2 s atm} \\ {k_r = 6.1 \times 10^{ - 5} mol/cm^2 s atm} \\ {K_s = 330 \pm 20} \\ \end{array} $$ Experiments run at lower carbon contents showed that only a very small quantity of chromium was oxidized, immediately forming a protective layer. However, this oxidation occurred at a higher carbon content (2 pct) than what was expected from the thermodynamics.