Coupling phenomena in the transfer of elements between a gas phase and iron melts

The simultaneous transfer of Si and C from a gas phase containing SiO and CO to liquid Fe-C alloys has been investigated. It was found that, although the silicon content of the melt increased with time as expected, the carbon content initially decreased, in spite of the fact that the carbon potential of the gas phase was above that of the liquid alloy. These phenomena are interpreted in terms of irreversible thermodynamics which shows that the overall transfer reactions are comprised of coupled reactions: SiO(g) + CO(g) →Si + CO₂(g) andC + CO₂(g) → 2CO(g). It is also shown that the simultaneous transfer of carbon and oxygen from gas mixtures of CO and CO₂ to liquid iron occurs via the coupled reactions CO₂(g) →O + CO(g) and 2CO(g) →C + CO₂. In each case there is a predominant, or driving reaction which promotes the other.     

Simultaneous desulfurization and dephosphorization reactions of molten iron by soda ash treatment

Desulfurization and dephosphorization reactions of molten iron by soda ash has been studied on laboratory heats of Fe-C, Fe-C-S, Fe-C-P, and Fe-C-S-P alloys at 1573 and 1623 K. The alloys were melted in helium gas flow and preheated soda ash was added; metal samples were taken at certain time intervals and analyzed for sulfur, phosphorus, and carbon. Evolved gas samples were also taken at certain time intervals and analyzed. The phosphorus and sulfur contents in metals decreased rapidly, reaching the lowest values two to four minutes after the soda ash addition. The degree of desulfurization was generally greater than that of dephosphorization, and both degrees were higher at lower reaction temperature. The major component of evolved gas was CO with small amounts of CO₂. Phosphorus appeared to form a stable phosphate compound with Na₂O, possibly 3Na₂O-P₂O₅, in the slag phase. Soda ash reacts with carbon resulting in decarburization of molten iron and vaporization of sodium; this reaction may cause the fading of soda ash and can be expressed as: Na₂CO₃(1) + (1 +x)C = (1 -xNa₂O(1) + 2xNa(g_ + (2 +xCO(g). For the phosphorus containing melt, the reaction can be expressed as: Na₂CO₃(l) +yC + 2x/3P =x(Na₂O · 1/3P₂O₅)(1) + (2 −y − 8x/3)Na₂O(l) + 2(−l + y + 5x/3)Na(g) + (1 +y)CO(g) and for the sulfur containing melt: Na₂O(l) +C +S = Na₂S(l) + CO(g). Katsumi Mori, Formerly Visiting Associate Research Scientist, University of Michigan, Ann Arbor, MI     

The diffusivity of oxygen in liquid copper-lead alloys

The oxygen diffusivity in liquid copper-lead alloys at 1403 K (1130° C) was measured using the electrochemical cell: Ni−NiO/ZrO₂(+CaO)/O in liquid Cu−Pb alloy(I)/ZrO₂(+CaO)/O in liquid Cu−Pb alloy (II). Oxygen in liquid Cu−Pb alloy (I) was transferred to the right by applying a preselected voltage between the two liquid Cu−Pb alloys. The oxygen diffusivity in liquid Cu−Pb alloy(I) was calculated from the emf change with time between the Ni−NiO and liquid Cu−Pb alloy (I) electrodes. The results were: It was found that the oxygen diffusivity in liquid copper-lead alloys did not change drastically over the entire composition range, in contrast with that reported by other investigators for liquid copper-nickel alloys. The oxygen diffusivity in pure liquid lead agreed with the results of our previous work using an FeO−Fe₃O₄ mixture as a sink for oxygen.     

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     

Measurement of oxygen potentials in Ag-Pb system employing oxygen sensor

Accurate and instantaneous analysis of dissolved nonmetallic or metallic species in molten metals at elevated temperatures using an online electrochemical sensor is important for continuous process control during metal refining and alloying operations in metallurgical industries. In this article, the application of long-life, solid-state electrochemical sensors for oxygen has been demonstrated to measure the oxygen potential as a function of the lead concentration in a molten Ag-Pb system at 1323 K. Yttria-stabilized zirconia (YSZ) in the form of one end-closed tube 20 mm long, 3 mm inner diameter, and 1 mm wall thickness has been used as a solid electrolyte in the oxygen sensor. Electromotive force (EMF) of the solid-state electrochemical cell (−)Pt, Ni-NiO//YSZ//O _Ag-Pb, Mo(+) has been measured at 1323 K. The reference electrode consisted of Ni + NiO biphasic mixture; the working electrode was composed of a molten Ag-Pb alloy of varying concentrations of lead. The concentrations of lead in silver ranged from 0.02 to 10.20 wt pct. Samples of the molten alloy were drawn after each addition of Pb to Ag and analyzed by induction coupled plasma (ICP).     

Activities of oxygen in liquid copper and silver from electrochemical measurements

Modified coulometric titrations on the galvanic cell:O in liquid Cu or Ag / ZrO₂( + CaO) / Air, Pt, were performed to determine precisely the oxygen activities in liquid copper and silver in the range of relatively low oxygen concentration. The present experimental results were remarkably reproducible in comparison with the published data. The standard Gibbs energies of solution of oxygen in liquid copper and liquid silver for 1/2 O₂(l atm) → O(l at. pct) were determined respectively to be ΔG° (in Cu) = −18040 −0.03 T(K) (± 120) cal · g-atom−1 = −75500 −0.12 T(K)(± 500) J · g-atom−1, ΔG°(inAg)= -3860+ 1.56 T(K) (±90) cal · g-atom−1 = −16140 + 6.52 T(K)(±380) J · g-atom−1 where the reference state for dissolved oxygen was an infinitely dilute solution. The present value of the partial entropy of oxygen dissolved in liquid copper differs significantly from that suggested by many investigators. Further, the present equation for liquid copper has been found to be consistent with a correlation proposed previously by the present authors. The equation for liquid silver is in good agreement with the published ones.     

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     

Equilibria between cerium or neodymium and oxygen in molten iron

Autoradiographs show that there is an obvious reaction between Ce and Nd in liquid iron and the MgO/CaO crucible wall. For reaching the Ce-O, Nd-O equilibrium, a long melting period and the addition of rare earth elements (RE’s) in several batches were needed to ensure the full reaction between the RE’s in the melt and crucible wall. The dissolved Ce or Nd content in iron was measured by means of radioactive isotopes¹⁴¹Ce or¹⁴⁷Nd and electrolysis in the organic electrolyte. The oxygen activity was measured by solid electrolyte sensors made of ZrO₂(MgO) tube. The relationships between the equilibrium constants and the temperatures for reactions Ce₂O₃ (s) = 2[Ce] + 3[O], CeO₂ (s) = [Ce] + 2[O], and Nd₂O₃ (s) = 2[Nd] + 3[O], as well as the corresponding thermodynamic parameters, have been determined. Formerly Graduate Student at the University of Science and Technology Beijing     

Calcium deoxidation equilibrium in liquid iron

Calcium-oxygen equilibrium was studied at 1873 K under normal pressure by the method of immersing a pure iron capsule containing calcium metal into liquid iron, which was equilibrated with CaO-Al₂O₃ slags in Al₂O₃ and CaO crucibles. On the basis of these and previous results obtained in the equilibrium experiments between liquid iron and CaO-containing slags, the equilibrium constant,K _Ca, for the reaction, CaO (s) =Ca +O, and the first-order interaction parameter,e ₀ ^Ca , were estimated. The measured value forK _Ca reported in previous experiments, which was found to be significantly different from that calculated from the reliable thermodynamic data, was discussed. Nitrogen distribution ratios between CaO-Al₂O₃ slags and liquid iron were also measured. Formerly Graduate Student, Department of Metallurgical Engineering, Tohoku University     

Kinetics of chromium oxide reduction from a basic steelmaking slag by silicon dissolved in liquid iron

The reduction of chromium oxide from a basic steelmaking slag (45 wt pct CaO, 35 wt pct SiO₂, 10 wt pct MgO, 10 wt pct A1₂O₃) by silicon dissolved in liquid iron at steelmaking temperatures was studied to determine the rate-limiting steps. The reduction is described by the reactions: (Cr₂O₃) + Si = (SiO₂) + (CrO) + Cr [1] and 2 (CrO) +Si = (SiO₂) + 2 Cr [2] The experiments were carried out under an argon atmosphere in a vertical resistance-heated tube furnace. The slag and metal phases were held in zirconia crucibles. The course of the reactions was followed by periodically sampling the slag phase and analyzing for total chromium, divalent chromium, and iron. Results obtained by varying stirring rate, temperature, and composition defined the rate-limiting mechanism for each reaction. The rate of reduction of trivalent chromium (reaction [1] above) increases with moderate increases in stirring of the slag, and increases markedly with increases in temperature. The effects of changes in composition identified the rate-limiting step for Cr⁺³ reduction as diffusion of Cr⁺³ from the bulk slag to the slag-metal interface. The rate of reduction of divalent chromium does not vary with changes in stirring of the slag, but increases in temperature markedly increase the reaction rate. Thus, this reaction is limited by the rate of an interfacial chemical reaction. The reduction of divalent chromium is linearly dependent on concentration of divalent chromium, but is independent of silicon content of the metal phase.     

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.     

Activities of oxygen in liquid thallium and indium from electrochemical measurements

Modified coulometric titrations on the galvanic cell;O in liquid Tl or In/ZrO₂(+CaO)/Air, Pt, were performed at 973, 1073, and 1173 K to determine the oxygen activities in liquid thallium and liquid indium. The standard Gibbs energies of solution of oxygen in liquid thallium and liquid indium for l/2 O₂ →O (1 at. pct) were determined respectively to be δG‡(in Tl) = -22000 + 0.74T (±300) cal/g-atom = -92000 + 3.10T (±1300) J/g-atom, δG‡(in In) = -42450+ 3.30T (±350) cal/g-atom = -177600 + 13.8T (±1500) J/g-atom, where the reference state for dissolved oxygen was an infinitely dilute solution. It was reconfirmed that the apparent initial oxygen concentration observed in the range of very low oxygen concentration in liquid metal was attributed to the oxygen released from the solid electrolyte.     

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     

Absorption of gaseous oxygen by liquid cobalt,copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts’ method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600°C, and for copper was 1250°C. For initial oxygen pressures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) O₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and proportional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissociative adsorption of oxygen molecules at the gas/oxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative adsorption of oxygen molecules at the gas/oxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen absorption rate compared to that of liquid cobalt. R. H. RADZILOWSKI, formerly a Graduate Studient at The University of Michigan     

Mullite as an electrochemical probe for the determination of low oxygen activity in liquid iron

Oxygen activities in liquid iron deoxidized with aluminium were measured at 1873 K, using a mullite electrolyte having a Cr-Cr₂O₃ reference mixture. These results were compared with those obtained using commercial tube-type ZrO₂-8 mol% MgO and plug-type ZrO₂-9 mol% MgO probes, along with oxygen activities calculated from analysis. Oxygen activities from EMF measurements using mullite and plug-type probes were found to be in agreement with those calculated from the Al /AI₂O₃ equilibrium and those estimated from analyzed oxygen contents. Polarization effect due to electrochemical oxygen transport was observed in a commercial tube-type ZrO₂-based probe. Supersaturation in liquid iron deoxidized with aluminium was also measured at 1873 K.     

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.     

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.     

Towards coal based continuous steelmaking Part 1 – Iron ore fines and scrap to low carbon steel via melt circulation

Abstract

Integrated iron- and steelmaking is effected by depositing a mixture of powdered coal and iron ore fines onto a moving melt surface without prior agglomeration. The resulting metallised solid raft is propelled out of the ironmaking loop onto the melt surface of the first of two steelmaking loops. By progressively adding oxygen to the gas first produced in ironmaking, decarburisation is conducted not with oxygen directly but rather by CO₂ and H₂O so that subsurface formation of CO is never permitted, via careful manipulation of the rates of gas phase mass transfer, interfacial chemical kinetics and liquid phase mass transfer. During ironmaking, infiltration of the melt via capillary rise greatly enhances the rate of metallisation. All the endothermic heat is supplied from beneath the circulating melt, which picks up its heat when post-combustion of CO and H₂ is completed after gases have first passed through the steelmaking loops.     

Solubility and activity of oxygen in liquid manganese

The oxygen concentration of liquid manganese in equilibrium with MnAl_2+2xO_4+3x and α−Al₂O₃ has been determined in the temperature range 1520 to 1875 K. The oxygen content of quenched samples, wrapped in oxygen-free nickel foil, was determined by an inert gas fusion technique. The results are combined with accurate data now available on the Gibbs energies of formation of MnO and Al₂O₃−saturated MnAl_2+2xO_4+3x to derive the oxygen content of liquid manganese in equilibrium with MnO and the Gibbs energy of solution of diatomic oxygen gas in liquid manganese. The enthalpy and entropy of solution of oxygen in manganese are compared with similar data on other metal-oxygen systems.     

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