Studies of the interfacial kinetics of the reaction of nitrogen with liquid iron by the15N-14N isotope exchange reaction

The rate of dissociation of N₂ on high purity liquid iron and iron-sulfur alloys between 1550 and 1650 °C has been studied by means of the¹⁵N-¹⁴N exchange reaction. It is shown that the rate constants at given sulfur concentrations are consistent with those for the absorption of nitrogen into iron-sulfur alloys, indicating a common rate determining step. The rate constant for high purity liquid iron, in units of mol cm^?2 s^?1 aim^?1, is given by: logk _f = ?340/T ? 1.38. The rate constant is found to be independent of carbon concentration up to about 4.3 wt pct and to be closely consistent with ideal chemisorption kinetics. The results are combined with those of previously published studies to give rational equations for the apparent rate constants for Fe-S and Fe-O alloys. Consistent values for the adsorption coefficients at 1600 °C for sulfur and oxygen are deduced to be about 130 and 220, respectively, for a standard state of the 1 wt pct ideal solution.     

The influence of sulfur on interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of continuously carbon-saturated liquid iron by CO₂ have been studied between 1280 and 1600?C at sulfur concentrations between 0.01 and 1 wt pct. The results are consistent with a surface blockage mechanism by chemisorbed sulfur which shows an essentially ideal adsorption isotherm. The adsorption coefficient of sulfur, in (wt pct)^-1, is given by the equation logK = 3600/T + 0.57 for carbon-saturated alloys. A small residual rate at apparent surface saturation is observed. This leaves about 1.4 pct of the active surface sites available for reaction, essentially independent of temperature. Studies with varying carbon concentration suggest that to a first approximation, and above about 3 wt pct C, the adsorption equilibrium for sulfur depends only on the thermodynamic activity of sulfur.     

The influence of sulfur on interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of continuously carbon-saturated liquid iron by CO₂ have been studied between 1280 and 1600‡C at sulfur concentrations between 0.01 and 1 wt pct. The results are consistent with a surface blockage mechanism by chemisorbed sulfur which shows an essentially ideal adsorption isotherm. The adsorption coefficient of sulfur, in (wt pct)^-1, is given by the equation logK = 3600/T + 0.57 for carbon-saturated alloys. A small residual rate at apparent surface saturation is observed. This leaves about 1.4 pct of the active surface sites available for reaction, essentially independent of temperature. Studies with varying carbon concentration suggest that to a first approximation, and above about 3 wt pct C, the adsorption equilibrium for sulfur depends only on the thermodynamic activity of sulfur. DR. SAIN was formerly a Graduate Student.     

The diffusion of carbon in iron-carbon alloys at 1560°C

Measurements have been made of the rate of absorption of carbon from CO-CO₂ mixtures into stagnant liquid iron and of the rate of unidirectional dissolution of graphite into stag-nant liquid iron and iron-carbon alloys at 1560°C. Total absorptions and concentration pro-files were found to be consistent with a composition dependent chemical diffusivity given byD = 1.1 (1 + wt pct CJ5.3) X 10∼⁴ cm²Js with an uncertainty of about ±0.2 x 10^-4. It is shown that many of the previous determina-tions of the chemical and self-diffusivity are consistent with this suggested dependence. Formerly graduate student at the University of Pennsylvania     

Steady-state studies of the reactions of H2O-CO and CO2-H2 mixtures with liquid iron

Studies have been made of the steady-stata composition of liquid iron exposed to high flow rates of H₂O-CO mixtures at 1550 °C to 1700 °C and CO₂-H₂ mixtures at 1600 °C. Values of the steady-state activity of oxygen have been established by measurement of either the carbon concentration or the silicon concentration when the iron was held in a silica crucible. Additions of sulfur or selenium to the iron have been found to result in steady-state oxygen activities, which differ significantly from those expected from water-gas equilibrium. The results are interpreted to show that the ratio of the apparent first-order rate constants for the reactions of H₂O and CO₂ with liquid iron is about 3 at 1600 °C. It is shown that the dependencies of the rate constants on the activities of sulfur, oxygen, and selenium must, even if complex, be similar for the H₂O and CO₂ reactions with liquid iron, to a good approximation.     

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.     

Sulfur Removal by Adding Iron During the Digestion Process of High-sulfur Bauxite

This paper proposes a novel approach to sulfur removal by adding iron during the digestion process. Iron can react with high-valence sulfur (S₂O₃^2?, SO₃^2?, SO₄^2?) to generate S^2? at digestion temperature, and then S^2? enter red mud in the form of Na₃FeS₃ to be removed. As iron dosage increases, high-valence sulfur concentration decreases, but the concentration of S^2? increases; sulfur digestion rate decreases while sulfur content in red mud markedly increases; the alumina digestion rate, conversely, remains fairly stable. So sulfur can be removed completely by adding iron in digestion process, which provide a theoretical basis for the effective removal of sulfur in alumina production process.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

The effect of carbon content on the rate of reduction of FeO in slag relevant to iron smelting

In the iron smelting, or bath smelting, process the tapped metal contains high amounts of sulfur and the slag contains high amounts of FeO, relative to blast furnace slag. After tapping, the FeO can be further reduced by carbon in the metal, which will also lead to better desulfurization. Although there have been many studies of the reaction of carbon in iron with FeO in slag, discrepancies exist with regards to the effect of carbon in iron on the rate of FeO reduction in slag, which is the subject of this study. Experiments were conducted at 1723 K, using a slag with basicity close to one with an FeO mass content of 5 %. The rate of reduction was measured using a pressure increase technique. For moderate and high sulfur contents, as in the case of iron smelting, the rate is primarily controlled by the dissociation of CO₂ on the surface of the molten iron. Furthermore, if the effect of carbon on sulfur is taken into account, for the range of carbon mass contents of 2 to 4.5 %, there is no effect of the carbon level on the rate of FeO reduction. At low sulfur contents it was found that there is considerable slag foaming, which inhibits mass transfer of FeO in the slag, and significantly reduces the rate. Even when there is no slag foaming at low sulfur contents, mass transfer of FeO in the slag can influence the rate of FeO reduction.     

The rates of desulfurization of liquid iron by solid CaO and CaO-saturated liquid iron oxide at 1600‡c

The rates of desulfurization of Fe-O-S melts by CaO crucibles and by CaO-saturated liquid iron oxide have been measured at 1600 ‡C. It was found that irons containing 1.62 wt pct and 0.64 wt pct sulfur and 0.070 wt pct oxygen are desulfurized by a reaction with the containing CaO crucible which does not involve the formation of a CaS product layer. The rate of desulfurization reaction is controlled by diffusion of sulfur in the iron melt, and a value of 6.7 ±1.7 × 10^-5 cm² per second was obtained for the diffusion coefficient of sulfur in liquid iron. Iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen is not desulfurized by solid CaO. The rate of desulfurization of liquid iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen by CaO-saturated liquid iron oxide is significantly greater than that calculated on the assumption of diffusion control in the metal phase, and evidence is presented in support of speculation that the reaction rate is enhanced by Marangoni turbulence at the slag-metal interface. The addition of 4 wt pct CaF₂ to the CaO-saturated liquid iron oxide has no influence on the rate of desulfurization of the melt. A. Saelim formerly Lecturer, Faculty of Engineering, Prince of Songkla University, Thailand     

Interfacial rates of reaction of CO2 with liquid iron silicates,silica-saturated manganese silicates,and some calcium iron silicates

Measurements of the rates of dissociation of CO₂ have been made by the¹⁴CO₂-CO isotope exchange technique on liquid iron silicates, calcium iron silicates, and silica-saturated manganese silicates as functions of temperature and imposed equilibrium CO₂/CO ratio. It is shown that the rates of reduction of the liquid iron silicates and an iron oxide-rich slag in CO-CO₂ atmospheres are consistent with the rates of isotope exchange, indicating a common rate determining step. The dependences of the apparent first order rate constant on the oxygen activity for the dissociation of CO₂ on silica-saturated iron silicates and an equimolar “FeO”-CaO-SiO₂ melt are found to be closely consistent with the ability to transfer two charges to the adsorbing or dissociating CO₂ molecule, as was previously found for liquid iron oxide and lime-saturated calcium ferrites. Apparent rate constants at fixed oxygen activity are found to increase generally with the basicities of the melts. Formerly Postdoctoral Fellow, Department of Metallurgy, University of Newcastle, New South Wales, Australia     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15?_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal.     

Dissolution kinetics of spent petroleum catalyst using two different acidophiles

Leaching studies using spent petroleum catalyst containing Ni, V and Mo were carried out using two different acidophiles, iron oxidizing (IOB) and sulfur oxidizing (SOB) bacteria. XRD analysis proved the existence of V in oxide form, Ni in sulfide form, Mo both in oxide and sulfide forms, and sulfur in elemental state. Both bacteria showed similar leaching kinetics at the same leaching parameters, such as pH, nutrient concentration, pulp density, particle size and temperature. The dissolution kinetics for Ni and V was much higher than Mo. Bioleaching kinetics was observed to follow dual rates, initially faster followed by a slower rate. So, dissolution mechanism was based on surface- and pore-diffusion rate. The dissolution process was found to follow 1st order kinetics. Unified dissolution rate kinetics equations were developed using 1st order rate kinetics. Various thermodynamic parameters were also calculated. Rate determining step for both the bacteria were evaluated and the average D₁ (surface) and D₂ (pore) values were found to be around 7 × 10^− 9 and 1 × 10^− 10 cm² respectively. The lower value of D₂ suggested that the pore diffusion is the rate determining step during bioleaching process.     

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.     

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     

The rate of decarburization of liquid iron by CO2 and H2

The rate of decarburization of liquid iron in CO-CO₂ mixtures and hydrogen at 1800 K has been investigated. The effect of sulfur on the rate in CO-CO₂ was also determined. Two experimental techniques were employed, one with the gas flow parallel to the surface of the melt, the other with gas flow perpendicular to it. The rate of decarburization in both CO-CO₂ mixtures and hydrogen at high carbon contents is controlled primarily by diffusionsion in the gas film boundary layer near the surface of the liquid. The presence of 0.3 wt pct sulfur reduced the rate of decarburization in CO-CO₂ by about 10 pct indicating that a slow chemical reaction on the surface is effecting the rate slightly when the surface is covered with sulfur atoms. The rate of decarburization at low carbon contents in CO-CO₂ is controlled primarily by carbon diffusion in the metal. The mass transfer relationships for the experimental geometries employed were investigated by measuring the rate of oxidation of graphite in CO-CO₂ mixtures. Previous work in which it was concluded that a chemical reaction was controlling the rate were re-examined and it was concluded that gas phase mass transfer was in fact controlling the rate of the reaction.     

The Interfacial Kinetics of the Reaction of CO2 with Nickel. Part I: The 14CO2-CO Exchange Reaction and the Influence of Sulfur

The rate of dissociation of CO₂ on nickel has been studied by means of the ¹⁴CO₂-CO exchange reaction. For pure polycrystalline nickel between 500 and 1240°C the forward rate constant, in mole cm^?2 s^?1 atm^?1, is given by: $$\ln k_f = - 7300/T - 2.57.$$ This is shown to be closely consistent with ideal chemisorption kinetics. Lower apparent rate constants when Ni₃S₂ is added to the surface or at bulk saturation with sulfur are consistent with a surface blockage mechanism by chemisorbed sulfur which shows a Langmuir adsorption isotherm. The adsorption coefficient of sulfur, in at. pet^?1, is deduced to be given by the equation: $$\ln K_s = 29,100/T - 15.82.$$ Results from associated Auger electron spectroscopic studies are in good accord with an ideal Langmuir adsorption isotherm for sulfur on polycrystalline nickel.     

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.     

Effect of grain boundary chemistry on the intergranular fracture of iron at cathodic potentials

The percent intergranular fracture and tensile ductility of iron tested at cathodic potentials in IN H₂SO₄ was found to depend primarily on the grain boundary sulfur concentration. Zone refined and vacuum melted iron with different bulk sulfur, oxygen, and nitrogen but similar carbon concentrations were evaluated. The grain boundary chemistry was measured by Auger Electron Spectroscopy and the fracture mode and ductility by uniaxial straining electrode tests at potentials of -0.60 to -2.0 V (SCE) in IN H₂SO₄. The tensile ductility, as measured by the total strain and reduction of area, of both irons decreased with increasing cathodic potentials. The fracture mode and ductility at a potential of - 0.6 V (SCE) was related to the grain boundary sulfur concentration with increasing sulfur resulting in an increasing percent intergranular fracture and a decreasing ductility. The fracture mode and ductility was not related to the grain boundary oxygen, nitrogen or carbon concentrations but large bulk nitrogen concentrations did promote cleavage and quasicleavage fracture.     

Reaction mechanism of FeO reduction by solid and dissolved carbon

The reduction reactions of FeO by carbon have been studied in order to be able to understand the fundamental phenomena occurring in smelting reduction process. The reduction of pure FeO by solid carbon proceeds mostly according to the same reaction mechanism as that by dissolved carbon in iron, the rate of which was experimentally determined to be controlled by the interfacial chemical reaction between Fe-C melt and intermediate CO₂ gas. Hence, the reduction rate of pure FeO by solid carbon is also chemically controlled by the Boudouard reaction between the dissolved carbon and CO₂ at the interface of by-product Fe droplet/gas phase, the activation energy of which was found to be about 193.2 kJ/mol. In addition, the reduction reaction of FeO in CaO-SiO₂-Al₂O₃-FeO slags by the dissolved carbon in Fe melt was also investigated over the FeO mass content less than 20 %. The reduction rate shows first order dependence with respect to FeO concentration. The surface active sulphur content in iron does not affect the reduction rate, and the temperature dependence of reduction rate gives the activation energy of 24.78 kJ/mol. Therefore, the reduction rate of FeO in slags by the dissolved carbon can be safely mentioned to be controlled by the liquid phase mass transfer of FeO through the slag phase diffusion-resistant boundary layer over the limited FeO concentration range. The empirical expression for the mass transfer controlled reactioe, deren Aktivierungsenergie ca. 193.2 kJ/mol beträgt. Außerdem wurde die Reduktion von FeO in CaO-SiO₂-Al₂O₃-FeO-Schlacken mit dem in der Eisenschmelze gelöstem Kohlenstoff fär FeO-Massengehalte von weniger als 20% untersucht. Die Reduktionsgeschwindigkeit weist hinsichtlich der FeO-Konzentration eine Abhängigkeit 1. Ordnung auf. Der Anteil an oberflächenaktivemn rate was determined as r = 5.94(±0.07).10^?6.exp(-24780/RT).(%FeOP)^0.96 over the reaction conditions employed.