kinetics of oxidation of divalent iron to trivalent state in liquid feo-cao-sio2 slags

Kinetics of Oxidation of Divalent Iron to Trivalent Statein Liquid FeO-CaO-SiO2 Slags

ANNA SEMYKINA, VOLODYMYR SHATOKHA, MASANORI IWASE,and SESHADRI SEETHARAMAN

This work was devoted to the kinetics studies of the oxidation of divalent iron in liquid FeO-CaO-SiO2 slags to the trivalent state. The experiments were carried out using a thermogravi-metric technique (TGA) in the temperature range of 1623 K to 1773 K (1350 �C to 1500 �C) inan oxidizing atmosphere. The reaction products after oxidation were analyzed by X-ray dif-fraction and optical and scanning electron microscopy. The results obtained show that duringthe first 10 to 15 minutes of oxidation, 70 to 90 pct of the Fe2+ in the slag was oxidized. Kineticanalysis of the TGA results indicates that the oxidation process may consist of three distinctsteps, viz an initial incubation period, followed by a chemical-reaction-controlled stage, andlater, a diffusion-control stage. Appropriate mathematical relationships were set up for the firsttwo consecutive steps. After combining these equations suitably as the mechanism of oxidationshifts from one form to another, the experimental results for the first two parts could bereproduced. A linear correlation was found between the thermodynamic activity of FeO in theslag and the degree of oxidation.

DOI: 10.1007/s11663-010-9425-x� The Minerals, Metals & Materials Society and ASM International 2010

I. INTRODUCTION

IN iron- and steelmaking, nearly 12 Mt of steelmak-ing slag is produced every year in Europe (around 50 Mtin the world). Although less than 65 pct is used inrecovering the valuable elements in the slag, the rest ofthe slag is used for land filling.[1,2] Potentially usefulcomponents in the latter, such as metallic iron inclu-sions and iron-bearing compounds, are discarded alongwith rest.

The most widespread approach to recover the ironvalues existing as inclusions in the slag matrix insteelmaking slags is to crush the slag at ambienttemperature, separate the metallic iron by magneticseparation, and recycle the same material by introducingit to a steelmaking process. The nonmagnetic iron-bearing compounds from the slag (up to 30 pct of FeOdepending on the steel grade and technology) are notrecovered. Apart from the loss of Fe, the iron oxide leftin the slag limits its applications in civil engineering.Wustite present in these slags lowers the value forcement applications as the mechanical propertiesare seriously affected. Along with other problems

(e.g., disintegration of calcium silicates[3,4]), this is amajor reason for the insufficient recycling rate.The reduction of iron-bearing oxide compounds

followed by a magnetic separation of metallic iron hasbeen investigated by a few scientists.[5,6] The reductionroute using fossil fuels cannot be considered as anenvironmentally friendly option for the selective extrac-tion of iron values from the slag.A more sustainable approach to use the steelmaking

slag components is based on the transformation ofnonmagnetic iron monooxide to magnetite by oxidation.This process allows selective recovery of iron-bearingand non iron-bearing slag constituents for specificpurposes.In our previous work,[7] preliminary studies on the

feasibility of oxidizing FeO in the liquid slag werecarried out. The present work aims at investigating themechanism of oxidation of Fe2+ from molten slagscontaining FeO. The slags investigated were syntheticternaries in the FeO-CaO-SiO2 system.

II. THERMODYNAMIC CONSIDERATIONS

In the present work, the oxidation of Fe2+ to Fe3+ inCaO-FeO-SiO2 slags of five different compositions wasconsidered. The slag compositions examined are pre-sented in Table I.The oxidation studies were carried out in the single-

phase liquid region with the lowest temperature being1623 K (1350 �C). The reaction considered in theseexperiments can be written as follows:

2Fe2þ þ 1

2O2 ¼ 2Fe3þ þO2� ½1�

ANNA SEMYKINA, Ph.D. Student, is with the Royal Instituteof Technology, SE-100 44 Stockholm, Sweden, and also with theNational Metallurgical Academy of Ukraine, Dnipropetrovsk,49600 Ukraine. Contact e-mail: [email protected] VOLODYMYRSHATOKHA, Professor, is with the National Metallurgical Academyof Ukraine. MASANORI IWASE, Ph.D. Professor, is with the Uni-versity of Kyoto, Sakyo-Ku, Kyoto 606-8501, Japan. SESHADRISEETHARAMAN, Professor, is with the Royal Institute ofTechnology.

Manuscript submitted May 13, 2010.Article published online August 26, 2010.

1230—VOLUME 41B, DECEMBER 2010 METALLURGICAL AND MATERIALS TRANSACTIONS B

2FeO(slag)þ 1

2O2 ¼ Fe2O3ðslag) ½1a�

In considering the standard Gibbs energy change forReaction [1a], the activities of ‘‘FeO’’ as well as forFe2O3 dissolved in the molten slag should be considered.Both activity values corresponding to the experimentalslags could be retrieved from FactSage 6.1 (ThermfactLtd., Montreal, Canada) using the Fact53 and FToxidedatabases (GTT-Technologies, Aachen, Germany). Thestandard states for FeO and Fe2O3 were pure liquids atthe experimental temperature. The Gibbs energychanges for Reaction [1a] DG1a for the five slagcompositions studied were computed and are presentedin Figure 1. It is shown that the DG1a values for all slagsare similar.

The system under consideration consists of liquid andgas phases; the former contains four components, vizCaO, SiO2, FeO, and Fe2O3, and the latter involves O2.Thus, the degrees of freedom F will be given by Gibbs

phase rule as follows: F = (C¢� r)+2� P = (5� 1)+2� 2 = 4 (whereC¢ is the number of components, r is thenumber of equations relating them, and P is the numberof phases. While the oxidation Reaction [1a] proceeds onthe right-hand side, the atomic ratios of Ca/Si as well asFe/Ca within the liquid slag are kept constant. Thus, thevalues (pct CaO)/(pct SiO2) and {(pct FeO+2(pctFe2O3)}/(pct CaO) are fixed throughout an experimentalrun. At a given experimental temperature, only onedegree of freedom remains, which means that changes inPO2

would result in a variation of (pct FeO)/(pct Fe2O3)only.To follow the oxidation reaction, it is necessary to

examine the phase stability diagram for the system CaO-FeO-Fe2O3-SiO2 in the temperature and oxygen partialpressure ranges investigated in the present work. Such adiagram was constructed using FactSage 6.1 and repro-duced, in the case of slag 5 (Table I), in Figure 2.Figure 2 shows that above approximately 1650 K

(1377 �C), the slag occurs as a homogeneous liquidphase. As the system is cooled to a temperature below1550 K (1277 �C) with an approximate log10P(O2) of 2,magnetite precipitates along with CaSiO3, whereas at

Table I. Chemical Composition of the Slags Studied and the Temperature Range of the Experiments

No.

Slag Composition,Weight Pct Liquidus Temperature

Calculated UsingFactSage6.1, K (�C)**

Liquidus Temperatureby Slag Atlas, K (�C)�

Temperature Range Usedin the Experiments, K (�C)CaO FeO* SiO2

1 26.0 30.0 44.0 1505 (1232) <1523 (1250) 1623 (1350)–1773 (1500)2 31.0 30.0 39.0 1516 (1243) �1523 (1250) 1623 (1350)–1773 (1500)3 35.0 30.0 35.0 1501 (1228) �1500 (1227) 1623 (1350)–1773 (1500)4 40.0 20.0 40.0 1573 (1300) �1623 (1350) 1623 (1350)–1773 (1500)5 37.5 25.0 37.5 1550 (1277) �1548 (1275) 1623 (1350)–1773 (1500)

*Assumed stoichiometric; the actual composition corresponds to Fe–wustite phase boundary at 1273 K (1000 �C).**The liquidus temperatures in FactSage 6.1 calculations were carried out at Log10PO2

(atm) = �10.�For the slag atlas, the values were read off from the CaO-FeO-SiO2 ternary phase diagram, which corresponds to low oxygen partial pressures.

Fig. 1—DG1a for Reaction [1a] for the five slag compositions studiedin the present work as a function of temperature. The standardstates for ‘‘FeO’’ and Fe2O3 are pure liquids in the experimentaltemperature range. Temperature in the figures is presented in Kelvin.The following conversion was done: 1000 �C+273 = 1273 K.

Fig. 2—The phase stability diagram corresponding to slag 5. Linemarked (A) corresponds to the partial pressure of oxygen in air. P(O2)presented in Pa. Temperature in the figures is presented in Kelvin. Thefollowing conversion was done: 1000 �C+273 = 1273 K.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 41B, DECEMBER 2010—1231

higher oxygen pressures, hematite is formed along withCaSiO3. The calculations for other slags are somewhatsimilar and are not reproduced here for the sake ofbrevity.

Two minor points to note are as follows: (1) therelative stabilities of hematite and magnetite in air. Onceagain, FactSage 6.1 calculations (conducted at P(O2)equal to the partial pressure of oxygen in air) show thatpure hematite decomposes to magnetite at a temperatureslightly above 1673 K (1400 �C). (2) The melting pointof magnetite is ca 1869 K (1596 �C) [slag atlas].

III. EXPERIMENTAL

A. Materials and Sample Preparation

To produce wustite ‘‘FeO,’’ required amounts of iron(<10 lm, purity of 99.9+ pct) and Fe2O3 (<5 lm, purityof 99+ pct) powders supplied by Sigma Aldrich Chemie(Steinheim, Germany) were mixed carefully in the appro-priate ratio so that the final composition corresponded tothat of ‘‘FeO’’ in equilibrium with iron at 1273 K(1000 �C). The mixture then was compacted and placedin an iron crucible (purity of 99.9 pct) and kept in argonatmosphere at 1273 K (1000 �C) in a vertical furnace for24 hours, after which the crucible was quenched. The‘‘FeO’’ produced was examined by X-ray diffraction(XRD), and the absence of both metallic iron andmagnetite were confirmed. From the diffraction pattern,the lattice parameter of the ‘‘FeO’’ produced was com-puted to be 4.30 A, which is in agreement with theliterature value of 4.3088 A.[8] Other materials used in theslag preparation—CaO powder with a purity of 99.9 pctand SiO2 powder with a purity of 99.5 pct—were suppliedby Sigma Aldrich Chemie. SiO2 powder was dried for24 hours at 1073 K (800 �C) and CaO powder was driedat 1273 K (1000 �C) for 2 hours in Ar atmosphere priorto mixing. The total mass of the sample for each TGAexperiment was 1 g. The powder mixtures were com-pacted into tablets under a pressure of 5 t. Platinumcrucibles for holding the slags were hand-fashioned fromplatinum foil (Alfa Aesar, Karlsruhe, Germany) with0.1 mm thickness and 99.99 pct purity.

B. Apparatus and Procedure

1. TGA experiments with synthetic slagsThe oxidation of FeO in the synthetic slags was

investigated by thermogravimetric analysis (TGA) usinga SETARAM TAG 24 (Setaram instrumentation,Caluire, France) unit. The experimental assembly con-sists of an electronic microbalance, a graphite furnacewith two chambers (Figure 3), and a gas cleaning system(Figure 4). During the experiments, a platinum cruciblecontaining pure, dehydrated alumina powder was hungfrom one of the balance arms, whereas in the other, theslag sample with nearly the same mass was kept in asimilar Pt crucible. The Al2O3 powder in the firstchamber served as a reference sample for TGA. Themicrobalance has a detection limit of 0.001 mg. Thesystem is operated fully by a computer.

Platinum crucibles with 10-mm inner diameter and aheight of 8 mm were used as the containers for the slagsamples. In all experiments, the same level of slag in thecrucible was maintained so that the results are compat-ible. To ensure homogeneity, the slags were premelted.The crucible was placed in a Pt basket suspended from thebalance with a Pt wire and placed in the alumina reactiontube with 21-mm inner diameter kept inside a graphitefurnace. The sample was positioned in the uniformtemperature zone of the furnace and carefully centeredto ensure that neither the sample crucible nor thesuspension wire came into contact with the walls of thealumina tube. The furnace temperature was controlledby a programmable regulator Eurotherm 2408 with aPt-30 pct Rh/Pt-6 pct Rh thermocouple as the sensorproviding a control accuracy of less than 1 K. Thethermocouples in the dual furnace systemwere positionedas closely as possible to the bottom of the crucibles.Before the experiments, the chambers with the sample

and the reference were purged with purified argon for24 hours. The furnace then was heated at 20 deg/min to

Fig. 3—Experimental setup for TGA: 1, electronic microbalance; 2,carrier gas inlet; 3, gas controller; 4, thermocouple; 5, gas outlet; 6,Pt-basket for the Pt-crucible; 7, Pt-crucible; 8, vacuum pump; 9,graphite furnace.

Fig. 4—Gas cleaning system.


the desired temperature, with Ar gas flowing at the rateof 0.15 l/min. The sample was kept at the targettemperature for 3400 seconds to ensure the attainmentof thermal equilibrium. No mass change was noticedduring the heating or the soaking period, thereby ensuringthat the argon gas neither was oxidizing nor reducing theslag sample.Ar thenwas replaced by the oxidant gas air asthe oxidation experiments were started.

The starvation rate for the flow of the oxidant gas, vizair, was determined in the preliminary experiments.These experiments showed that at air flow rates below0.15 l/min, the slag oxidation rate was proportional tothe air flow rate, whereas the flow rate had no influenceon the rate of oxidation beyond 0.15 l/min (Figure 5).All experiments were conducted in isothermal mode for1200 seconds. The mass and the temperature of thesample were registered every 1 second. After the exper-iments, the samples were cooled in the furnace inargon atmosphere at the maximum possible rate, viz25 deg/min.

The experimental target temperatures were chosen sothat they were above the liquidus temperatures calculatedusing FactSage 6.1 software (the calculation results andthe experimental temperatures are presented in Table I).

The basicities of the synthetic slags were chosen as0.6, 0.8, and 1.0 so that for a given FeO content, theliquidus temperatures of the slags studied were belowthe maximum temperature limit of the furnace, viz1873 K (1600 �C). In the present work, basicity, B isdefined as follows:

B ¼ CaO

SiO2½2�

After the experiments, the slag samples were takenout and analyzed by optical microscopy, XRD, andscanning electron microscopy (SEM).

For the XRD analysis, a Siemens D5000 X-ray unit,with a copper Ka X-ray source was used. SEM analysiswas carried out in a JEOL SEM unit.

IV. RESULTS

Typical experimental curves for the isothermal weightgain during oxidation at different temperatures are

presented in Figure 6. In this figure, the horizontal linesindicate the theoretical levels corresponding to thecomplete oxidation of Fe2+ in the slag to Fe3+

(theoretically corresponding to hematite) and evenpartial oxidation corresponding to magnetite stage.The characteristics of the curves demonstrate thatduring the first 10 to 15 minutes of the experiment, a70-to-90 pct oxidation level was reached for the oxida-tion to hematite stage.Figure 6 shows that an increase of temperature

between 1623 K and 1723 K (1350 �C and 1450 �C) isfollowed by the progressive mass gain caused byoxidation. The maximum oxidation level achieved alsoincreases for all slags studied with a rise in temperature.However, at 1773 K (1500 �C), the oxidation behavior issomewhat different; oxidation proceeds rapidly in theearly stage but slows down at the later stage. The finalmass gain (i.e., oxidation level achieved) is below thecorresponding levels at 1673 K and 1723 K (1400 �Cand 1450 �C). This pattern is observed more distinctly inFigure 7, showing the first derivative of the mass gainwith respect to time represented as follows:

dDWdt¼ d Wt �Wið Þ

Dt½3�

Fig. 5—The isothermal mass change curves for the following compo-sition: 25 pct FeO, 37.5 pct CaO, and 37.5 pct SiO2 for differentflow rates.

Fig. 6—The isothermal mass changes curves for the following com-position: 25 pct FeO, 37.5 pct CaO, and 37.5 pct SiO2 for differenttemperatures. Temperature in the figures is presented in Kelvin. Thefollowing conversion was done: 1000 �C+273 = 1273 K.

Fig. 7—Differential oxidation rate with respect to time as a functionof reaction time for the following sample: 25 pct FeO, 37.5 pct CaO,an 37.5 pct SiO2, with temperatures of 1: 1673 K (1400 �C), 2:1723 K (1450 �C), 3: 1773 K (1500 �C). Temperature in the figures ispresented in Kelvin. The following conversion was done:1000 �C+273 = 1273 K.


where Wt is mass recorded at time t, Wi is the initialmass of the slag in mg, and Dt is the time interval inseconds.

To explain the divergence of the oxidation behavior at1773 K (1500 �C) from the lower temperature experi-ments, the samples, cooled at the maximum possiblecooling rate in the thermoanalyzer after TGA experi-ments (viz 25 deg/min) were studied by optical micros-copy and SEM. Optical images are presented inFigure 8, and the SEM image corresponding to1773 K (1500 �C) is shown in Figure 9.

Different phases (magnetite, hematite, calcium sili-cates, and even calcium ferrites) were observed in thesamples that were oxidized below 1773 K (1500 �C).Pores and even cracks were observed in the productlayer. On the other hand, in the sample oxidized at1773 K (1500 �C), optical microscopy images show amonolite structure without cracks. The SEM analyses ofthis sample (Figure 8) shows that the layer formed onthe sample surface has a nonporous dense crystalloidstructure. Furthermore, the surface layer containedmostly magnetite (and some hematite), as shown in

XRD patterns in Figure 10(a). On the other hand, XRDanalysis of the sample, oxidized at 1673 K (1400 �C),shows a multiphase structure in the surface layer(Figure 10(b)). The phases present are in agreementwith the FactSage 6.1 calculations shown in Figure 2.With an increasing basicity of the sample, the degree

of oxidation increased (Figure 11). This is likely causedby the increase in the thermodynamic activity of FeOwith an increasing basicity of the slag, facilitating theoxidation of FeO. Figure 12 shows the oxidation curvesfor different FeO contents in the slag. For a slag withunit basicity, the oxidation level at 1673 K (1400 �C)increased with the increase in the FeO content in theslag. Although the oxidation rates and the final oxida-tion degree were somewhat similar for slags containing25 and 30 wt pct FeO, the slag containing 20 wt pctFeO showed a lower degree of oxidation, as thethermodynamic activity will be low. FactSage 6.1calculations show that the activity of FeO in this slagis about 0.03 as compared with 0.06 for other slagsaround 1700 K (1427 �C).

V. DISCUSSION

The thermograms show differences in slopes atdifferent stages of oxidation that indicate the changeof mechanism as the reaction progressed. An initialincubation time (about 100 seconds) also was necessary,and in this period, little mass increase occurred. It wouldbe interesting to understand the mechanism of oxidationat the various stages considering the oxidation of theliquid slag in air.The oxidation process can be controlled by the

following steps: (1) an initial oxygen dissolution period,(2) a chemical reaction, and (3) oxidation resultingfrom the diffusion of oxygen through the stagnant topslag layer with high viscosity because of high amountsof Fe3+.The three steps of the oxidation reaction would

correspond broadly to the three slope changes in a

Fig. 8—Images of the sample surface (25 pct FeO; 37.5 pct CaO,and 37.5 pct SiO2), observed by optical microscopy for the tempera-tures (a) 1673 K (1400 �C); and (b) 1773 K (1500 �C). Magnification9250.

Fig. 9—Image obtained by SEM (96000) for the sample treated at1773 K (1500 �C).


typical thermogram as presented in Figure 13. The regionwhere the transition from chemical control to diffusioncontrol is marked in the figure as ‘‘mixed control.’’

A. Initial Oxygen Dissolution as Rate-ControllingStep

With the onset of the oxidation process, the oxygenmolecules impinge on the slag surface leading to theoxidation of FeO in the slag. Reaction [1] would occurat the gas–slag interface.As the oxidant gas is let in, the thermograms show an

incubation period, which is likely to correspond to theinitial dissolution of oxygen in the slag. At 1560 K(1287 �C), confocal microscopic studies of the oxida-tion phenomenon, carried out in collaboration withProfessor S. Sridhar at Carnegie Mellon University(Pittsburgh, PA), confirmed the formation of the par-ticles in agreement with the FactSage 6.1 calculationspresented in Figure 2. A typical confocal image fromthis work is presented in Figure 14.In the present work, an attempt was made to describe

this incubation period with the following Avrami-typeof equation[9]:

1� X ¼ exp �KNtnð Þ ½4�

Fig. 10—XRD pattern of the top layer of the sample (25 pct FeO, 37.5 pct CaO, and 37.5 pct SiO2) at temperatures (a) 1773 K (1500 �C) and(b) 1673 K (1400 �C). o = Fe2O3 (rhombohedra); ¤ = Fe3O4 (FCC); d = CaSiO3; . = Ca2Fe2O5. Temperature in the figures is presented inKelvin. The following conversion was done: 1000 �C+273 = 1273 K.

Fig. 11—The isothermal mass-change curves at 1673 K (1400 �C) fordifferent basicity (FeO) = 30 pct.

Fig. 12—The isothermal mass-change curves at 1673 K (1400 �C),basicity = 1.0 for different (FeO).

Fig. 13—Illustration of the plausible steps in the oxidation mecha-nism marked on a typical thermogram.


This equation can be rearranged to a linear form byexpressing it in the following logarithmic terms:

ln ln1

1� X

� �¼ lnKN þ n ln t ½5�

where X is the reacted fraction, KN is a rate constant(can be determined graphically), t is the time in seconds,and n is the growth exponent. Parameter n may repre-sent either the interface-controlled growth of crystalsor the diffusion limited growth of a product (e.g., forthe chemical composition no. 5 from Table I, a plot ofthe left-hand side in Eq. [5] as a function of ln t,corresponding to 1623 K (1350 �C) is presented inFigure 15). The slope of the line that corresponds ton in Eq. [5] has a value of 1.94, and the intercept on they-axis ln KN is �11.68.

B. Chemical Reaction Rate-Controlling Step

In the early stages of oxidation, the effect of theproduct layer formed is less significant, and the chemicalreaction would be the rate-controlling step.

The main equation corresponding to the chemicalreaction control step can be expressed as follows:

DmA¼ X exp

�DEa

RT

� �t ½6�

where X is a preexponential term, DEa is Arrheniusactivation energy, and t is time in seconds.

Fig. 14—Typical confocal image for the oxidation of a sample of athree-component system CaO-FeO-SiO2, basicity = 1 in air (nucle-ation step).

Fig. 15—Evaluation of the n and KN coefficients in Eq. [5] for thenucleation step.

Fig. 16—Arrhenius plot for evaluating the activation energy for thechemical reaction step.

Table II. Chemical Compositions of the Slag

Samples and Activation Energies for the Chemical

Reaction-Controlling Step

No.

Slag Composition,Weight Pct

Activation Energy, kJCaO FeO* SiO2

1 26.0 30.0 44.0 64.022 31.0 30.0 39.0 118.893 35.0 30.0 35.0 171.274 40.0 20.0 40.0 107.875 37.5 25.0 37.5 170.84

*Assumed stoichiometric; the actual composition corresponds toFe–wustite phase boundary at 1273 K (1000 �C).

Fig. 17—The isothermal mass-change curves for the following com-position: 25 pct FeO, 37.5 pct CaO, and 37.5 pct SiO2 for differenttemperatures, = calculated by kinetic model; o experimental data.Temperature in the figures is presented in Kelvin. The following con-version was done: 1000 �C+273 = 1273 K.


By plotting the term on the left-hand side of Eq. [6],viz (Dm/A), at different temperatures and as a functionof time, the coefficient g (which is defined asg ¼ X exp �DEa

RT

� �can be estimated from the slope. From

the Arrhenius plot, where lng is function of 1/T, theactivation energy for the chemical reaction rate-control-ling step could be evaluated. For example, the activationenergy for the oxidation of sample #5 (Table I) can befound from the slope of the line in Figure 16 to be170.84 kJ (Table II).

The simulations of the initial incubation period andthe chemical reaction steps are compared with theexperimental results in Figure 17.

C. Diffusion as Rate-Controlling Step

The oxidation reaction would lead to a build up of astagnant slag layer at the top of the slag melt with a highcontent of Fe3+. This layer would be expected to behighly viscous because of the higher amounts oftrivalent iron, depicted here as a product slag layer.Thus, downward diffusion of oxide ions through thislayer is likely to be slow. The oxidation process at laterstages is likely to be controlled by the diffusion ofoxygen through this slag layer. A schematic illustrationof the oxidation phenomenon is presented in Figure 18.The oxygen potential at the top and the bottom of thisproduct layer would be different, with the former closerto the oxygen potential of the gas phase, depicted inthe figure as Cs

O; and the latter closer to that of theequilibrium oxygen potential prevailing between theFeO dissolved in the slag and the product layer, Ci

O; inthe figure.

The figure portrays, for the sake of simplicity, the slaglayer as having a uniform concentration. In reality, aconcentration gradient is likely to be present dependingon the experimental conditions and the transfer of heatcaused by the exothermic oxidation reaction, which cancause local convection.

The possible transfer steps that may occur during thisstage of the oxidation process are (1) gas phase transferof oxygen to the slag–gas interface, and (2) transfer ofreacting species through the product layer to thereaction front.

The rate of oxidation may be controlled by one or bya combination of these steps. The gas phase diffusionmay not have an impact on the reaction rate as theoxidation is carried out with the flow of the oxidant

above the starvation rate for the reaction. Thus, step 1may be ruled out.Considering step 2, an increased growth of the layer by

oxidation requires diffusion ofFe2+ ions and/orO2� ions.These two diffusion processes can be described as follows:

(a) Fe2+ ions can diffuse from the liquid slag–magnetiteinterface to the outer surface where they will reactwith the oxygen in the air. This reaction would beaccompanied by the transfer of electrons or the‘‘counter-current’’ diffusion of O2�, maintainingcharge neutrality.

(b) Alternatively, because of the exothermicity of theoxidation reaction, it is likely that a slight surfaceturbulence would exist, which would cause the O2

molecules or O2� ions on the surface to come intocontact with Fe2+ in the liquid slag, causing oxi-dation to Fe3+.

With the progress of the oxidation reaction, thethickness of the product layer would increase andsteady-state conditions may prevail.The mass transport could be explained based on the

application of Fick’s law for the diffusion to this stage ofoxidation by the present authors. Such an exercise wascarried out using empirical coefficients for the variousterms in Fick’s equation with reasonable success. But, inview of the empiricism involved because of a lack ofactual experimental data, this part is left out of thepresent article.

D. Error Analyses

For all controlling steps the total error was calculatedas follows:

r ¼ rT1þ rT2

þ . . .

N½7�

where N is the number of the experiments, andrT1; rT2

. . . are the error at each experiment and are de-fined as follows:

rTi¼ 1

b

Xbi¼1

Xir � Xic

Xic

�� ½8�

where, b is the number of points and Xic and Xir are thevalues calculated and experimentally determined,respectively.For the initial incubation step, r was estimated to be

0.034, and for the chemical reaction rate-controllingstep, the value of r was estimated to be 0.033.

E. Degree of Oxidation

The degree of oxidation for the synthetic slags wascalculated by the following equation:

n ¼ mr=moð Þ � 100 ½9�

where n is the degree of oxidation in pct, mo is theestimated mass gain that occurs when all iron in the slagis oxidized to the hematite, mg, and mr is the real massgain, mg.

Fig. 18—A schematic illustration of the oxidation phenomenon.


The degree of oxidationwas correlatedwith the activityof FeO (Figure 19). The activity of FeO was estimatedusing FactSage 6.1. The standard state for FeO waschosen as liquid at the experimental temperature. Thedegree of oxidation was a linear function of the thermo-dynamic activity of FeO in the liquid slag (standard stateis pure FeO liquid) in the entire experimental temperaturerange. The equation corresponding to the line inFigure 19 can be written as follows:

n ¼ 1513:9aðFeOÞ þ 4:032

R2 ¼ 0:8½10�

Equation [10] allows an approximate estimation ofthe activity of FeO in the slag system studied. Alter-nately, it is possible, from a knowledge of the thermo-dynamic activity of FeO in the slag, to estimate the finaldegree of oxidation of Fe2+ in the slag by using Eq. [10].This approach currently is being verified for multicom-ponent slag systems containing FeO.

VI. CONCLUSIONS

In the current work, the oxidation of FeO insteelmaking slags (i.e., the oxidation of Fe2+ to Fe3+)has been investigated by TGA in synthetic slags in theternary system CaO-FeO-SiO2.

The TGA experiments showed that, during the first 10to 15 minutes, 70 to 90 pct of oxidation was achieved.An increase of the temperature in the range 1623 K to1723 K (1350 �C to 1450 �C) caused an increase in therate of the reaction. The oxidation rate increased withincreasing basicity and pct FeO in the system. Thethermograms for oxidation showed an initial incubationfollowed by a chemical-reaction-controlled stage. Atlater stages, the reaction rate most likely was controlledby diffusion.

An attempt was made to describe the reaction kineticsby means of appropriate equations. A linear correlationwas found between the thermodynamic activity of FeOand the degree of oxidation.

ACKNOWLEDGMENTS

The authors are thankful to the Swedish Founda-tion for Strategic Environmental Research (MISTRA)for the financial support through the project Eco-SteelProduction (Sub project no.: 88035), administered bythe Swedish Steel Producers Association (Jernkontoret).The financial support for Anna Semykina from theSwedish Institute is gratefully acknowledged. Theauthors are extremely thankful to Dr. Lidong Teng forvery useful discussions.

NOMENCLATURE

A area, cm2

aFeO Activity of FeO in the slagB BasicityCO Oxygen concentration, mol/m3

C¢ Number of components in the Gibbs phaserule

DO Diffusion coefficient of oxygen, cm2/sF Degree of freedom in the Gibbs phase ruleJ Flux of oxygen, mol/(cm2 s)K Constant of the reactionKN, n Constantsn Degree of oxidation, pctmO2

Mass of oxygen, mgm3þ

Fe Mass of the trivalent oxide of iron, mgMO2

Molar mass of oxygen, g/mol, mg/molM3þ

Fe Molar mass of the trivalent oxide of iron,g/mol, mg/mol

mo Estimated mass gain, occurred when all theiron in the slag is oxidized to the hematite, mg

mr Real mass gain, mgn Number of molqFe3O4

Magnetite density mg/cm3

R 8.314 J/(mole K)r Number of the reactions in the Gibbs phase

ruler Error in the experimentsP Number of phases in the Gibbs phase rulePO2

Partial pressure of oxygen, atm, PaT Temperature, Kt Time, sx Distance along the direction of oxygen flux, cmX Reacted fractionDEa Activation energy, JX Preexponential term in Eq. [6]

REFERENCES1. A. Altun and I. Yılmaz: Cem. Concr. Res., 2002, vol. 32 (8),

pp. 1247–49.2. H. Motz and J. Geiseler: Waste Manag., 2001, vol. 21 (3), pp. 285–

93.3. S.A. Mikhail and A.M. Turcotte: Thermochim. Acta, 1995, vol. 263,

pp. 87–94.4. D. Fabbro, M. Stefanutti, and C. Ceschia: Quarry Construction,

2001, pp. 81–91.5. D. Podorska and J. Wypartowicz: Arch. Metall. Mater., 2008,

vol. 53, pp. 595–600.

Fig. 19—Relationship between the activity of FeO in the slag andthe degree of oxidation (pct).


6. R.K. Paramguru, R.K. Galgali, and H.S. Ray: Metall. Mater.Trans. B, 1997, vol. 28B, pp. 805–10.

7. A. Semykina, V. Shatokha, and S. Seetharaman: Molten2009,Santiago, Chile, 2009, p. 65.

8. D.R. Lide and H.P.R. Frederikse: Handbook of Chemistryand Physics, 78th ed., CRC Press, Cleveland, OH, 1997, pp. 4–149.

9. M. Avrami: J. Chem. Phys., 1939, vol. 7, pp. 1103–09.


kinetics of oxidation of divalent iron to trivalent state in liquid feo-cao-sio2 slags

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