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Aspects of the reaction of CO and CO₂ with iron oxide-containing slags Chaskar, Vinay D. 1992

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ASPECTS OF THE REACTION OF CO AND CO 2 WITH IRONOXIDE-CONTAINING SLAGSbyVinay D. ChaskarB.E. ( Metallurgical Engineering ) University of Poona, India, 1973M.S. ( Metallurgical Engineering) University of Missouri, Rolla, U.S.A. 1976A THESIS SUBMITTED IN PARTIAL FULFILMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIES(Department of Metals and Materials Engineering)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIASeptember 1992© Vinay D. Chaskar, 1992In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature) Department of M^i- s^ue AA q TE RI A i- 5" EL.2 r►6^F- FT: gThe University of British ColumbiaVancouver, CanadaDate  Jo, n v 60) ^155 -?DE-6 (2/88)ABSTRACTA kinetic study of reduction and oxidation slag reactions of importance to the non-ferroussmelting industry was undertaken in the laboratory by employing a thermogravimetry technique.The reduction study included ferrous-to-iron and ferric-to-ferrous reactions whereas a limitedamount of data on ferrous-to-ferric and ferrous-to-magnetite reactions was also obtained forcomparison. The heterogeneous gas-slag reactions were investigated using unstirred slag meltsand the synthetic slags covered a wide range of melt compositions from Fe x0-Al203 toFex0-CaO-SiO2-Al203 .The results revealed that the rate of the iron formation reaction was highest during the initial10 minutes period and then dropped progressively with the passage of time. The melt silica contentinfluenced the rate values to a degree and it was observed that in silica-free melts the above trendwas reversed at high reaction driving force values. The latter trend is attributed to the melt movementinduced by the sinking iron metal. A detailed mathematical analysis showed that during the initialperiod ferrous reduction rates were limited by a mixed-control regime involving gas phase masstransport and interfacial reaction. By using intrinsic rate constant, kc, as a fitting parameter thepredicted rates were matched with the measured data and the procedure yielded an average valueof 11 x 10'5 g/cm2.s.atm at 1400 °C which is in agreement with previous work.The results of the ferric-to-ferrous reaction showed approximately constant rate during theinitial period and subsequently the rate decreased with time. Therefore, the results were analyzedusing two separate mathematical models. It is proposed that this reaction operates under a gas andinterfacial control regime during the initial period and subsequent to this the rate is controlled bycombined gas, liquid and interfacial reaction. The intrinsic rate constant value at 1400 °C isapproximately 200 times greater compared to the Fe' Fe reaction. The apparent activationiienergy value of about 44 kcal/mol is derived for the Fe 3+ Fe2+ reaction. By choosing the liquidphase mass-transfer coefficient, kL, as a fitting parameter its values were obtained for various meltsand this information was used in conjunction with the boundary layer value of 500 obtainedby MOssbauer spectroscopy to generate oxygen anion diffusivity data. The value of apparentactivation energy for diffusion, ED, for lime-containing melts was found to be 53 kcal/mole.The results obtained for both reduction reactions revealed the significance of surface activespecies in the melts and accommodation of this effect in the formulation and development of themathematical models led to accurate prediction of the rate and weight loss data. In the variousmelts studied in this investigation silica and ferric oxide were surface active species and theirindividual proportion in the melts altered the available reaction area.MOssbauer spectroscopic analyses were performed on a limited number of slag samples. Aspecial sample holder was designed to probe an entire cross-section of the quenched slag to identifythe variation in iron cations with depth. It was demonstrated for the first time that MOssabuerspectroscopy can be used to study iron distribution as a function of depth.The limited data obtained on ferrous-to-ferric and ferrous-to-magnetite reactions revealedthat the rates decreased with time in a manner similar to the reduction reactions. The weightgain-time curves for magnetite formation reaction in both simple and complex melts indicated thatsolid magnetite covered the melt surface and caused reduction in the rate values. The data on theferrous-to-ferric reaction at 1300 °C indicated that the mechanisms involved in ferric-ferrousreduction and oxidation reactions are similariiiTABLE OF CONTENTSAbstract^ iiTable of Contents^ ivList of Tables ixList of Figures^ xiiList of Symbols xviAcknowledgements^ xxChapter 1: INTRODUCTION: The Nature of Slags and Their Origin^ 11.1 Slag Treatments  41.2 Role of iron oxides in non-ferrous smelting operations^ 5Chapter 2: LITERATURE REVIEW: Role and Status of Previous Work ^ 72.1 Studies involving solid iron oxides ^  82.1.1 Equilibrium/Thermodynamic Studies  82.1.2 Kinetic Studies ^  82.2 Studies involving molten iron oxides ^  92.2.1 Equilibrium studies ^ FeOx  melts Pseudo-binary (Fex0-SiO2) melts ^ Pseudo-ternary (Fe i0-Si02-CaO) melts Pseudo-quaternary and complex melts ^  152.2.2 Kinetic studies ^ FeOx  melts Pseudo-binary Fe x0-SiO2 melts ^ Pseudo-ternary Fe x0-Si02-Ca0 melts Pseudo-quaternery melts ^ Complex melts ^  302.3 Industrial/Large scale work  30iv2.4 Critical assessment of published work ^  33Chapter 3: OBJECTIVES AND SCOPE 363.1 Objectives ^  363.2 Scope of the experimental work ^  36Chapter 4: EXPERIMENTAL WORK: Technique and Methodology ^ 394.1 Apparatus ^  394.2 Raw materials  424.3 Preparatory work ^  424.3.1 Flow meter calibration ^  434.3.2 Furnace temperature profile  454.3.3 Activity calculations ^  454.3.4 Data acquisition trials  464.4 Experimental procedure ^  484.5 Experimental variables  504.6 Laboratory methods ^  504.6.1 Chemical analysis of slag melts ^  504.6.2 SEM analysis ^  514.6.3 MOssbauer spectroscopy analysis ^  514.6.4 X-ray diffraction analysis  51Chapter 5: EXPERIMENTAL RESULTS AND PRELIMINARYANALYSIS ^ 535.1 Ferrous-to-iron reduction study between 1200 °C and 1400 °C ^ 535.1.1 Fe10 melts at 1400 °C ^  545.1.1.1 Effect of gas composition  545.1.1.2 Effect of surface area  595.1.1.3 Effect of crucible height ^  595.1.1.4 Effect of gas flow rate  615.1.1.5 Effect of excess oxygen ^  64v5.1.2 Pseudo-binary melts at 1400 'C ^  665.1.2.1 Fe„0-SiO2 melts ^  665. Effect of silica  665. Effect of excess oxygen ^  715.1.2.2 Fex0-CaO melts ^  725.1.3 Pseudo-ternary melts between 1200 °C and 1400 °C ^ 775.1.3.1 Effect of melt and gas composition ^  775.1.3.2 Effect of ferric oxide ^  775.1.3.3 Effect of excess melt oxygen ^  775.1.3.4 Effect of temperature ^  805.2 Ferric-to-ferrous reduction study between 1200 °C and 1400 °C ^ 805.2.1 Pseudo-binary Fe x0-SiO2 melts ^  815.2.2 Pseudo-ternary Fex0-SiO2-CaO melts  845.3 Ferrous-to-ferric oxidation study at 1300 °C ^  845.4 Ferrous-to-magnetite oxidation study at 1400 °C  905.4.1 Simple Fex0-Al203 melts ^  935.4.2 Complex Fex0-SiO2Ca0-Al203 melts ^  93Chapter 6: DISCUSSION^ 966.1 Ferrous-to-iron study  966.1.1 Findings of present work ^  966.1.2 Mathematical Analysis  976.1.2.1 Justification of the proposed model ^  976.1.2.2 Mixed control model formulation  1056.1.2.3 Application ^  126vi6. Calculation of kc ^  1266. Interpretation of the rate constant term ^ 1316.1.3 Reaction resistances  1336.1.5 Effet of melt composition ^  1336.1.4 Effect of excess melt oxygen  1346.1.6 Reaction mechanism  1416.2 Ferric-to-ferrous study ^  1416.2.1 Mathematical analysis  1426.2.1.1 Model formulation  1426. Constant initial rate ^  1426. Rate-time behaviour  1436.2.1.2 Application ^  1466. Calculation of lc:  1466. Calculation of apparent activation energy of chemical reaction^1516. Calculation of kL ^  1516. Estimation of diffusivity data ^  1586.2.2 Reaction mechanism  1596.3 Comparison with previous work ^  1616.4 Consequences of the results to slag fuming ^  171Chapter 7: MOSSBAUER SPECTROSCOPY  1737.1 Introduction ^  1737.1.1 Sampling of high temperature phases ^  1737.1.2 Limitations of present M6ssbauer analysis  1747.2 MOssbauer technique ^  1757.2.1 Slag preparation  1757.2.2 Generation of MOssbauer spectra ^  1757.3 Results ^  1807.3.1 Spectral parameters ^  1807.3.2 Spectra analysis  1857.3.3 MOssbauer data and wet chemical analysis ^  186vii7.3.4 Identification of crystalline phases ^  1887.4 Discussion ^  1887.5 Comments  196Chapter 8: SUMMARY AND CONCLUSIONS ^  1988.1 Recommendations for future work ^  202REFERENCES ^ 204APPENDIX I  212APPENDIX II ^  214viiiLIST OF TABLESTable 1.1^Composition of typical smelting slags ^2Table 2.1^Kinetic studies on the reduction of iron oxides and ironoxide-containing melts using carbon and carbon dissolved-in-ironas reductants ^22Table 4.1^Flow rate calibration results for argon gas. ^44Table 4.2^Comparison of aF€0, values calculated using Kellogg's model[98,99] with the other researchers. ^47Table 4.3^Slag chemical compositions obtained by wet chemical methodsand SEM-EDX. ^52Table 5.1^Reduction rate data for Fe x0-Al203 melts at 1400 °C. ^57Table 5.2^Reduction rates for FeO„ melts held in magnesia, alumina andspinel (MgO-Al203) crucibles and exposed to an Ar-CO mixture(Pm= 0.18 atm) at 1400 °C ^60Table 5.3^Rate comparison between two Fe x0-Al203 melts with varyingoxygen contents. ^65Table 5.4^Reduction rate data for Fe x0-SiO2-Al203 melts at 1400 °C(Fe3+//,Fe < 0.05)  ^69Table 5.5^Rate comparison for the reduction-oxidation-reduction runs.  ^73Table 5.6^Reduction rate data for Fe x0-CaO-Al203 melts at 1400 °C ^76Table 5.7^Reduction rate data for Fe„0-SiO2-CaO-Al203 melts between 1200and 1400 °C ^78Table 5.8^Details of Ferric-to-ferrous reaction study in Fe„0-SiO 2-Al203melts between 1200 and 1400 °C ^82ixTable 5.9^Details of Ferric-to-ferrous study at 1400 °C inFe„0-Si02-CaO-Al203 melts.  ^85Table 5.10^Details of Ferric-to-ferrous reaction study at 1300 °C inFex0-Si02-CaO-Al203 melts ^86Table 5.11^Details of Ferric-to-ferrous reaction study at 1200 °C inFex0-Si02-CaO-Al203 melts ^87Table 5.12^Details of Ferrous-to-ferric reaction study at 1300 °C inFex0-Si02-Ca0-Al203 melts ^91Table 6.1^Values of fitted liquid phase mass transfer coefficients, k L,obtianed in the ferrous reduction study at 1400 °C using gas andliquid mass transfer model ^103Table 6.2^Predicted boundary layer thicknesses using gas and liquid masstransfer model. ^104Table 6.3^Comparison of kL values obtained from two independent sources.^106Table 6.4^Expressions for the available reaction sites. ^112Table 6.5^Melt compositions of various pseudo binary and pseudo ternarymelts and the activity data for FeO, Fe 203 and Si02. ^127Table 6.6^Melt compositions of complex melts (Fe x0-CaO-Si02-Al203system) and the activity data for FeO, Fe203 and Si02 species at1200 °C, 1300 *C and 1400 °C ^128Table 6.7^Comparison of measured and predicted rates for the pseudo binaryand pseudo ternary melts at 1400 °C. (Fe 3+/EFe < 0.05)  ^129Table 6.8^Comparison of measured and predicted rates for complex meltsbetween 1200 °C and 1400 °C. (Fe3+/Y.Fe < 0.05). ^130Table 6.9^Comparison of model predicted and measured rates for ferrousreduction reaction.  ^137xTable 6.10^Comparison of overall rate constants of ferric-to-ferrous andferrous-to-iron reactions obtained using two models. ^138Table 6.11^Fitted intrinsic rate constant values for the lime-free melts.^147Table 6.12^Fitted k values for the lime-containing melts at 1400 °C.  ^148Table 6.13^Fitted lc: values for the lime-containing melts at 1300 °C.  ^149Table 6.14^Fitted lc: values for the lime-containing melts at 1200 °C.  ^150Table 6.15^Derived values of the liquid mass transfer coefficients anddiffusivity data for the melts at three reaction temperatures.^157Table 6.16^Comparison of predicted and reported ka data for the pseudo-binaryand pseudo-ternary melts at 1400 °C. ^164Table 7.1^Chemical composition and experimental parameters of slagsamples. ^176Table 7.2^MOssbauer parameters of slags quenched from 1400 °C. ^181xiLIST OF FIGURESFig. 1.1^Location of industrial slags in FeO-CaO-Si0 2 system (from ref.4).^3Fig. 2.1^The iron-oxygen system, based mainly on the data of Darken and Gurry[31,48]. ^11Fig. 2.2^State of oxidation of iron in Fe„0-Fe2O3-SiO2 melts at 1550 °C (fromref.1). ^13Fig. 2.3^Effects of temperature and melt composition on the relative stability ofFe 3+ cations (from ref.47). ^16Fig. 2.4^Some examples of reduction curves in pure liquid FeOx  (from ref.69).^21Fig. 2.5^Arrhenius plot of the apparent rate constants for pseudo-ternary(Fe.0-SiO2-CaO) melts containing equal mole fractions of CaO andSi02 at an equilibrium CO2 / CO ratio of 1. ^28Fig. 2.6^Relationship between the apparent rate constants obtained in twoseparate investigations (from ref.79) ^31Fig. 4.1^Schematic diagram of the apparatus used in this study ^40Fig. 4.2^Schematic diagram of the gas purification system ^41Fig. 5.1^Variation of the melt weight with time during ferrous-to-iron reactionstudy at 1400 °C  ^55Fig. 5.2^Relation between the initial reduction rates and P. for the Fe„0-Al203melts at 1400 °C. ^58Fig. 5.3^Relation between reduction rate and gas phase boundary layer. ^62Fig. 5.4^Weight loss-time relationships for the experiments designed to studythe effect of flow rate. ^63Fig. 5.5^Effect of silica content on the weight-time relation in threeFex0-Si02-Al203 melts ^67xiiFig. 5.6^Plots of initial reduction rate versus gas phase driving force (as perequation 5.4) for several Fex0-SiO2-Al203 melts at 1400 °C ^70Fig. 5.7^Rate versus driving force plots for pseudo-binary melts- Fe„0-S i0 2 andFex0-CaO held in alumina crucibles at 1400 °C. ^75Fig. 5.8^Plots of rate versus gas phase driving force (as per equation 5.4) forpseudo-ternary melts held in alumina crucibles at 1400 °C. ^79Fig. 5.9^Plots of initial rate versus gas phase driving force (equation 5.9) for theferric-to-ferrous reaction in Fe x0-SiO2-Al203 melts. ^83Fig. 5.10^Rate versus driving force (equation 5.9) plots for the ferric-to-ferrousreaction in Fe„0-SiO2-CaO-Al203 melts at three temperatures. ^88Fig. 5.11^Weight gain curves for the ferrous-to-ferric reaction at 1300 °C.^89Fig. 5.12^Relation between initial oxidation rate and gas phase driving force(equation 5.11) for Fe x0-SiO2-CaO-Al203 melts at 1300 °C. ^92Fig. 5.13^Weight gain curves for the ferrous-to-magnetite reaction in Fe„0-Al 203melts exposed to different Ar-CO2 mixtures at 1400 'C.^94Fig. 5.14^Weight gain curves for the ferrous-to-magnetite reaction inFex0-SiO2-CaO-Al203 melts at 1400 'C. ^95Fig. 6.1Fig. 6.2Comparison of measured ferrous reduction rates for the melts inFex0-Al203 and Fex0-SiO2-Al203 systems with the data predicted usinggas phase control model. Comparison of measured ferrous reduction rates for the melts inFex0-CaO-Al203 and Fex0-SiO2-CaO-Al203 systems with the datapredicted using gas phase control model. 99100Fig. 6.3^A plot of fraction reacted versus .NiD • tIL? obtained using unsteady statemodel ^102Fig. 6.4^Schematic representation of the (a) movement of gaseous speciesrelative to the melt surface; (b) gas and liquid phase resistances to thereduction reaction . ^ 107Fig. 6.5^Surface tension-composition relationship. (a) variations in surfacetension values of a wiistite melt due to additions of Fe 203 , Si02 and P205(ref.107) ^115Fig. 6.6^Surface tension-activity relationships for varios melts at 1400 °C.^116Fig. 6.7^Log rsi3O2 versus log asio2 plot for FeO-Si02 melts. ^119Fig. 6.8^Log F, versus log ai plots for FeO-Fe203 , FeO-Si02 and FeO-Fe203-CaOmelts at 1400 °C (based on the surface tension and activity data fromreferences 80,98,99,108 and 124). ^121Fig. 6.9^Surface tension variations in pseudo-binary Fe.0-Si0 2 melts (ref.105).^122Fig. 6.10^Surface tension of solutions of a surface active solute at differenttemperatures, t 1 , t2 and t3 (ref.107). ^125Fig. 6.11^Arrhenius plot of ln 1 versus 1/T for the ferrous reduction study.^132Fig. 6.12^Variations in apparent rate constants observed in oxidation studies usingisotope exchange technique (from ref. 92) ^140Fig. 6.13^Arrhenius plot of ln lc, versus 1/T for the ferric-to-ferrous reaction..^152Fig. 6.14^Weight loss data for the ferric-to-ferrous reaction study at 1400 °C.inlime-free melts. ^153Fig. 6.15^Weight loss data for the ferric-to-ferrous reaction study inlime-containing melts at 1400 °C ^154Fig. 6.16^Weight loss data for the ferric-to-ferrous reaction study inlime-containing melts at 1300 °C. ^155xivFig. 6.17^Weight loss data for the ferric-to-ferrous reaction study inlime-containing melts at 1200 °C (Fe 3+/IFe — 0.15)  ^156Fig. 6.18^Arrhenius plot for determination of activation energy for diffusion^160Fig. 6.19^Empirical relationship between k, and ferric content of various melts(from ref 69), Predicted lc values are shown for comparison.^165Fig. 7.1^Schematic diagram of the Miissbauer sample holder assembly. ^177Fig. 7.2^Complete set of MOssbauer spectra recorded at all depths for eight slagcompositions. ^179Fig. 7.3^Mean quadruple splitting of Fe2+ doublets versus depth for six slagcompositions. ^182Fig. 7.4^The relative area as a function of the ferrous QS values ^183Fig. 7.5^The change in Fe3+ concentration with depth. ^184Fig. 7.6^Comparison of Fe 3+11Fe values obtained by MOssbauer spectroscopyand wet chemical analysis. ^187Fig. II.1^Schematic diagram showing details of the electrical connection for thedata acquisition system. ^215xvLIST OF SYMBOLSEnglish symbolsA^reaction area, (cm2)A.^melt surface area, (cm2)activity of species iCi^concentration of species j, (g.mole/cm3)D^diffusivity or diffusion coefficient, (cm2/s)DA-B^inter-diffusivity coefficient for mixture of (A + B), (cm2/s)d^crucible diameter, (cm)ED^apparent activation energy for diffusion, (kcal/mole)f^driving force, (atm)G°^standard Gibbs free energy, (cal/mole)equilibrium constant of a reaction ik^^rate constant or mass-transfer coefficientapparent rate constant, (cm/s)chemical rate constant, (cm/s)kg^mass transfer coefficient in gas phase, (cm/s)kL^mass transfer coefficient in liquid phase, (cm/s)k,m,n,a system dependent constantsk:^temperature dependent rate constantxviL^characteristic length, (cm)1•1;^mole fraction of species iin ;^molar mass-transfer rate of species i, (g.mole/s)n•^molar quantity of species j, (g.mole)partial pressure of species i, (atm)R^gas constant, 1.987 (calf'K.mole)RT^total resistance, (RT = IK)r^reaction rate, (g-oxy/cm2.^)T^temperature, (°C or °K)t^reaction time, (s)V^melt volume, (cm3)w or W melt weight, (g)x^non-stoichiometric coefficient (of wustite)Greek symbolsa^x-ray wave lengthSiC phasesolid gamma-iron phaseactivity coefficient of species i5^gas phase boundary layer thickness, (cm)xvii8.,/^monolayer thickness, (cm)81^boundary layer thickness in the liquid phase, (cm)1-1,^viscosity, (kg/m.^) or microP^density, (g/cm3)a^surface tension, (N/m)F,^excess concentration of surface active solute i, (mole/cm2)A^change or differenceI^sum or totalxviiiSubscripts(ad)^adsorbedb^bulkg^gas(hyp)^hypothetical1^liquido original (t = 0)s^solidsl^slagt^timeSuperscriptse equilibriumo standardinterfacialOther symbols[C]^carbon dissolved in iron[n]^reference number,xixACKNOWLEDGEMENTSI would like to express my sincere gratitude to my supervisor, Dr. G.G. Richards for providingexcellent guidance, valuable help and constant encouragement throughout this work. Furthermore,I would like to extend my sincerest thanks to Dr. Catherine McCammon for her co-operation andassistance in carrying out MOssbauer spectroscopy work.The financial assistance of NSERC and B.C. Science council is greatly appreciated. Thanksare also due to the machine shop personnel and other members of the department, past and present,for their timely assistance during various stages of the project. Helpful discussions with membersof the faculty and fellow graduate students are acknowledged with sincerity.The assistance provided by Dr. Gerry Toop and the members of COMINCO assay laboratoryis greatly appreciated.A special note of thanks to my wife, Dr. Manik Chaskar for her patience, support andunderstanding. Finally, I would like to dedicate this work to all the members of my family.Chapter 1INTRODUCTION : The Nature of Slags and Their OriginMost pyrometallurgical processes generate slags. These slags perform a wide variety ofchemical and physical functions ranging from a receptacle for gangue and unreduced oxides inprimary extraction, to the reservoir of chemical reactants and absorber of extracted impurities inrefining processes. Though slag generation is considered inevitable in all the ferrous andnon-ferrous smelting operations, it is desirable to limit its quantity to the lowest possible valuefor two main reasons :(1) slag generation represents one of the "debit" items on the plant energy balance; and(2) it influences overall metal recovery values.Both the quantity of slag and its final chemical composition are dependent on raw materialassays, types of flux additions, furnace temperature, furnace atmosphere, melt oxygen potential,in addition to other prevailing operating conditions. The slags generated during smeltingoperations are sometimes referred to as " discard slags " because the bulk of their compositionis dominated by unwanted gangue materials and oxides. The chemical composition of slags forthe copper, nickel, lead and tin industries are listed in Table 1.1. For comparison the compositionof iron and steelmaking slags is mentioned in the same table. A given slag will probably consistof at least 8 or 9 different oxides. However, the composition of most industrial slags is dominatedby four oxides - " FeO ", CaO, Si02, and Al203. Minor quantities of other oxides rarely exceed5 - 10 % of the slag weight. Since representation of all oxides in a single diagram is virtuallyimpossible, the slag composition on most occasions is suitably represented in ternary diagramsby choosing the dominant oxide species. The system CaO - FeO - Si02 is reproduced in Figure1.1 to serve as an illustration of this.1Table 1.1 Composition of typical smelting slags.Slag origin ContainedmetalPredominant Oxide Species, wt% range SlagTemperatureBasicityIndexReferencewt% Ca0 Mg0 "Fe0" Al203 Si02 Other 'C ± 50 'CIron BlastFurnace- 38-44 8-10 0.2 10-12 34-38 2-3 1500 1.0 - 1.6 [1]Steelmaking Slags;BOF & Q-BOP- 45-65 2-8 5-25 1-2 10-25 10-15 1600 2.0 - 3.5 [1]CopperReverberatory Furnace1.5(Cu)5-15 0-2 30-45 8-12 35-45 5_1220 1.0 -1.5 [1-3]Nickel BlastFurnace0.2(Ni,Co)5-20 5-25 20-50 5-15 35-45 3 1230 1.3 - 1.8 [2,3]Lead BlastFurnace15-20(Zn)5-18 1-2 20-40 5-10 20-30 5 1230 1.5 - 2.0 [1,2]TinReverberatory Furnace10-15(Sn)10-15 1-2 20-30 10-15 20-30 5 1230 1.1 - 1.9 [1,2]Note: 1. "Fe0" assay includes both ferrous and ferric oxides.rot 96Cer0 4 -2wtqaMg0 + wt%Ft02. Basicity index — wivio,3. In metallurgical slags, amphoteric oxides are not included in calculating Basicity Index. (ref. 4)Se02CoO^FeOFigure 1.1 Location of industrial slags in the FeO-CaO-S10 2 system (from ref. 4 and 5)(1) Basic open hearth steel furnace.(2) Acid open hearth steel furnace.(3) Basle oxygen converter.(4) Copper reverberatory.(6) Copper oxide blast furnace.(6) Lead blast furnace.(7) Tin smelting.3Based on the information in Table 1.1, the following major differences are revealed betweenferrous and non-ferrous slags.(1) "FeO" assay: In most instances the reported assay includes both the ferrous and ferric oxidesand this is represented by notation "FeO" , FeO. or FeTO. In comparison with iron makingslags non-ferrous slags contain a much higher proportion of "FeO". In general, the ferrousslags contain very minor quantities of ferric oxide where as in non-ferrous slags the magnetiteor ferric assay may exceed 15 wt.%. The higher ferric levels in non-ferrous slags invariablylead to higher metal losses and for this reason it is important to control the ferric oxide levelsof these slags at the operating temperatures.(2) Slag forming temperature : Melting points of all non-ferrous smelting slags are about 150°C lower than iron and steelmaking slags.(3) Dollar value : This is usually related to the unit value of the contained metal. On aweight-to-weight basis each ton of either Cu, Pb, Sn or Ni produces between 2.5 - 3 tons ofslag and since this slag contains some trapped metal an attempt is almost always made torecover a portion of it. On the other hand, the loss of iron metal in ferrous slags is small.Thus the dollar value of non-ferrous slags is appreciably high in comparison with ferrousslags.1.1 Slag Treatments:The primary reasons for the growing interest in the field of slag treatments are :(1) Increasing concern over lost metal values;(2) Growing importance of issues such as the energy of metal production;(3) Stricter and more stringent environmental regulations; and(4) Gradual depletion of physically and chemically favorable raw materials.Basically there are two ways of recovering the " contained metal " in slag: firstly, the slagcan be recycled in the circuit alongside the new material and secondly, it can be processed in aseparate unit operation. The choice and justification of either is governed largely by prevailingeconomic and operational factors, but the latter practice is generally more acceptable. [6-15]4More often than not, there are additional benefits to be gained from the employment of slagtreatment practices. The "bonus" is the recovery of by-product metals - such as gold, silver andcobalt together with the main metal. For example, gold and silver are recovered with copper andcobalt is recovered with either lead, zinc, nickel or copper.Yet another area where slag treatment can be of help is the recovery of volatile impurities.In metal recovery operations based on the hot treatment of slag viz. Electric furnace slag cleaningand slag fuming, impurities such as S, As, Sb, Zn, Pb, Se, Te, Cl and F are always volatilized tosome extent. These elements are usually trapped in dust form and may require several additionaltreatments for isolation.The replacement of traditional smelting processes with new, more energy efficient andenvironmentally acceptable processes, combined with the need to maximize recoveries hasincreased the need for slag treatment. Moreover with the adoption of new smelting methods,particularly where oxygen enrichment is used, non-ferrous smelting slag compositions areexpected to change and this has wide implications on the recovery of both primary and by-productmetal values. For these reasons it is anticipated that in the near future the role played by variousslag treatments will become increasingly important.1.2 Role of iron oxides in non-ferrous smelting operations:The main iron removal reactions during the matte smelting operations of copper, nickel,tin, lead and zinc are:2FeS 0 , 1) +30 2) =2Fe0 (1) +2SO kr) (1.1)2Fe00„.,„ ) +Si020.0 = Fe2SiO 401.g) (1.2)Once ferrous oxide is formed according to the reaction (1.1) it can further react to produceeither metallic iron or ferric oxide depending on the oxygen potential. If conditions(thermodynamic and kinetic) are favorable to the formation of iron metal the situation can leadto several operating difficulties. On the other hand, if the oxygen partial pressure, Poe, is toohigh, magnetite is formed as a separate phase resulting in the changes in slag properties. Invariablythe slag viscosity is increased and this leads to higher metal losses.5The above facts clearly suggest that the control of oxygen potential is of paramountimportance in non-ferrous smelting operations to maintain the progress of desired chemicalreactions; otherwise unwanted side reactions may result. Ideally, the formation of solid magnetiteshould be avoided but often it is only practical to control its proportion to an optimum level. Theferric-to-ferrous ratio has a strong influence on both the physical and chemical properties of theslag melt and therefore on metal losses as well. For these reasons a clear understanding of theredox reactions taking place in non-ferrous slag melts is necessary. An added advantage ofstudying the iron oxide redox reaction is that the findings from such work could be extended toslags containing Mn, Cr, Sn and Ti oxides because, like iron, these metals also exhibit differentvalencies in oxide solution. Though some evidence in this regard [16] is available more data isneeded to draw firm conclusions.6Chapter 2LITERATURE REVIEW: Role and Status of Previous WorkMuch of the early research on slag-metal reactions, in the first quarter of 20th century, wasclosely associated with the development of open-hearth steelmaking. During this period it waswidely accepted that the interpretation and prediction of slag-metal distribution equilibria couldeasily be accomplished in terms of an equilibrium constant using the Law of Mass Action. Forthe successful application of this principle however, it was recognized that information on thenature of slags is essential together with the thermodynamic data. For this reason severalresearchers initiated studies to understand the nature of slag. In addition to the structure-propertyrelated research on slags, the period between 1920 and 1940 also witnessed the conception andgrowth of several slag models based on 'Molecular theory'. The concept of the "basicity index"or "v-ratio" emerged as a result of the molecular theory and this is considered to be of significantpractical importance even today.After 1940 slag molecular theories lost their original simplicity and became very complexwith a multitude of possible compounds and dissociation constants. During the same periodprogress was being made in related fields - such as glasses, fused salts, molten oxides and silicates.The new data revealed that in the molten state many species dissociated to form 'ions'. [17] Asa result of this evidence researchers began studies on slags to extend the newly developed ionicconcepts in this field. By 1950 the concept of a completely ionized slag containing a limitednumber of ionic species was well established. Commensurate with this recognition of the ionicnature of slags, the importance of measuring properties related closely to structure also grew.This trend has continued to date.Since the subject of this thesis is the reaction kinetic behavior of iron oxides it would beof interest to focus our attention primarily on past research activity in this field.72.1 Studies involving solid iron oxides:Origins of an active research interest in this area can be traced as far back as the early 1900s.The obvious reason for such an interest was an immediate utility and applicability of the researchfindings to the extraction of iron. Many developments in ironmaldng techniques came aboutprimarily as a result of the extensive research in this field. For example, between 1940 and 1980the smelting capacity of the blast furnace has about doubled without an increase in furnace size,and coke consumption has been reduced from about 1000 kg to below 500 kg per tonne of ironproduced [18]. The primary objective.of all solid oxides-related research was to understand thenature of reactions taking place within an iron blast furnace. For this purpose boththermodynamic/equilibrium and kinetic studies were initiated by the researchers.2.1.1 Equilibrium/Thermodynamic Studies:In 1945, Darken and Gurry [19] initiated equilibrium studies on the Fe - 0 system usingCO-0O2 gas mixtures and their pioneering work culminated in the development of a phasediagram. The authors calculated activities of iron and wilstite and, in addition, determined thevalues of heats of magnetite formation between 1100 *C and 1400 *C which are in excellentagreement with calorimetric measurements. Later, other researchers conducted similar work [20]and as a result our understanding of various phase fields and their interrelations has improved agreat deal.2.1.2 Kinetic Studies:Many laboratory investigations have dealt with the high temperature reduction behavior ofiron oxides - hematite (Fe203), magnetite (Fe304) and wilstite (FeO)x  using either carbon or carbonmonoxide as reducing agents [20, 21-26]. Comprehensive work on wiistite reduction wasundertaken by Watts et al. [22], John et al. [23], Steele [24] and Childs and Wagner [25] and theirresearch has offered valuable insights into gas-wiistite reactions.8In addition to offering explanations for the reactions taking place in the blast furnaceresearch on solid iron oxides has paved the way for newer technology - such as direct reductionmethods. Literature concerning solid iron oxide reactions is abundant [21,26], but any furtherreview on this topic is beyond the scope of the thesis. However, some important findings of thesolid iron oxide related kinetic work are summarized below :(1) Ferric oxide, Fe203 is reduced at a higher velocity than Fe304 . [21](2) Hydrogen at high temperatures is a better reducing agent than carbonmonoxide, but is a poorer one at lower temperatures. [18](3) The reduction of a solid oxide to solid metal by means of hydrogen or carbon monoxideinvolves phase-boundary reactions as well as diffusion processes. [22,23,26](4) The rate of the heterogeneous gas - solid reactions is influenced by the stagnant gas boundarylayer surrounding the oxide particles. [21]2.2 Studies involving molten iron oxides:In comparison with solid oxide reduction studies research on liquid oxides is very limited.One of the major difficulty in studying high temperature reactions involving iron oxide-containingmelts is containment of the melt. In almost all studies the operating temperatures are in excessof 1200 °C and this causes serious melt containment problems if an oxide crucible is used.Although iron is commonly used as a crucible material it imposes a significant thermodynamicconstraint on the experimental work. The choice of crucible material is often constrained byvariables such as melt composition, melt temperature and the duration of the experiment. Inaddition there is one inherent difficulty which is much more problematic and complex. The originof this lies in the temperature - composition relationship between iron and oxygen. S toichiometricFeO (1:1) does not exist and wiistite has a variable composition depending on temperature andoxygen partial pressure. Therefore, wiistite composition is normally represented either as Fe (,y) or Fe.O. Both x and y are positive non-zero numbers and their individual values are governed9by the proportion of ferric oxide. Within the wiistite phase, the Fe 31-/Fe2+ ratio increases withincreasing partial pressure of oxygen and this leads to decreasing x values. Turkdogan [1] hasprovided an expression for the determination of y.In the present study however, the wiistite composition is listed as FeOx  for the sake ofconvenience. Additional reasons for this choice are mentioned in section Thus thenon-stoichiometric coefficient, x, denotes the extent of deviation from stoichiometric FeO andits value is dependent on temperature and oxygen partial pressure. This fact necessitates extremecare in control of the experimental variables and as a corollary implies unreliability of the datain absence of such control.The above difficulties forced investigators into trying a variety of crucible materials otherthan the more common ones: alumina, magnesia, spinel and iron. A few researchers employedcoke and graphite crucibles. [27-31] Others resorted to using either noble metals (platinum, Pt-Rhand iridium) or expensive ceramics like zirconia, thoria and yttria. [32-36]2.2.1 Equilibrium studies:Research work prior to 1960 was mainly restricted to the determination of thermodynamicand physicochemical properties of these melts. As a result a wealth of information on severaltopics including the nature of chemical reactions, activities of compounds and phase diagramswas generated. The data was obtained for both simple and complex melt systems. FeO. melts:Darken and Gurry [19,37] studied equilibrium relations between the partial pressure ofoxygen, temperature and oxide composition at iron saturation using controlled gas mixtures. Theauthors essentially looked at the following reactions:xFe(s) + CO2w = FeOx o) + CO(g) (2.1)3Fe0 (,)+ CO24) = F e30 40)+ CO(g) (2.2)In reaction (2.1) 'x' is the nonstoichiometry coefficient. The temperature-compositionphase diagram, constructed mainly from their data, is shown in Figure 2.1.101-4^1•5Figure 2.1400 ^1^1^I^IFfs0^20 40Fa20360 F,2304 80wt-. -Fe-0 system, based on the data of Darken and Gurry [19,37].Broken lines in single Phase regions are equilibrium oxygenpartial pressures in atmospheres.11Following the classical work of Darken and Gurry several other researchers looked at the phaseequilibrium relations and the thermodynamics of the Fe-0 system between 1000 °C - 1600 °C.For a critical assessment of the available data on the above system a paper by Spencer andKubaschewski [38] is a very useful reference. Pseudo-binary (Fe.0-SiO2) melts:Because of its significance to both ferrous as well as non-ferrous operations, this systemwas a prime target for phase equilibrium and thermodynamic studies during 1940s and 1950s.[39,40] Schuhmann and Ensio [41] studied the pseudo-binary Fe x0-SiO2 system using ironcrucibles and CO-CO2 gas mixtures. The main experimental variables- temperature and silicacontent, were chosen to represent copper smelting slags and the primary objective of their workwas determination of FeO activity. One interesting feature of Schuhmann and Ensio's data isthat for a given silica percentage a does not vary significantly with the temperature.In 1955 Bodsworth [42] conducted research work similar to that of Schuhmann and Ensioand confirmed that the activity of ferrous oxide in Fe x0-SiO2 melts is essentially independent oftemperature in the range 1200 °C - 1400 °C and is only a function of melt silica content. BothBodsworth and Schuhmann and Ensio assume similar standard states for FeO and Fe namely-FeO as an ideal stoichiometric liquid and Fe as gamma-phase (f.c.c.) solid iron.The effect of temperature, melt composition and oxygen partial pressure of the gas on thestate of oxidation of iron in iron silicate melts has been measured by various investigators. [41-44]These equilibrium data for 1550 *C are collated in Figure 2.2. It is interesting to note that thedata in this figure can be used for temperatures other than 1550 °C. The temperature insensitivityof the variation of Fe3+/Fe2+ with P IPco2- - coPco/Pco ratio an increase in temperature increases the equilibrium partial pressure of oxygen,and second, for a given P02 and silica concentration the Fe/Fe2+ ratio decreases with increase intemperature. Darken and Gurry [19,37] and Larson and Chipman [45] have confirmed the abovetrend in liquid iron oxide and calcium ferrite melts a result of two opposing effects. First, for a given12Fa 3. 2.• FFa a 3.Figure 2.2 State of oxidation of iron in FeO-Fe 203-SiO2 melts at 1550 °C (from ref. 1).13In 1957 Turkdogan and Bills [46] showed that for silica saturated melts log(P co1Pco) isalmost a linear function of log(Fe 3+/Fe 2+) with a slope of 2. Such an observation is anticipatedfrom the equilibrium relation for the redox reactionCOkg) + 2Fe2+(1) = 2Fe 3+to + 0 2- + COCO^(2.3)Although the activities or activity coefficients of individual ions cannot be determinedexperimentally, the state of isothermal equilibrium for the above reaction may be represented ina hypothetical manner as follows:Pco,^Fe 3+ '443+log --mo = 2 log^+ 2 log —+ log ao_2 – log IC2.8)r - co^Fe YF.2+(2.4)A linear relationship between log(Pco/Pco) and log(Fe 3+/Fe2+) with a slope of 2 would indicatethat the sum of the terms involving the activity coefficient ratio 7F.3../YF,2+ and the oxygen ionactivity a02_ remain essentially constant in silica-saturated melts. It also follows from theseobservations that for silica saturated melts in the Fe.0-Si0 2 system the trivalent iron does notco-polymerize with the Si-0 anion group but instead remains primarily as Fe3+ cations in themelt. [1] Pseudo-ternary (Fe.0-SiO 2-CaO) melts:Early research work in this system dealt with the thermodynamic properties of these meltsat temperatures of interest to the iron and steel industry. Though the pseudo-ternary systemFex0-SiO2-CaO covers the range of compositions generated in non-ferrous smelting and refiningoperations (see Figure 1.1) the research on melts containing a higher proportion of FeOx  did notbegin until about 1955. The starting point for this research activity was determination of a n,0values and later the work diversified to include determination of physicochemical properties.In 1959 Bodsworth [42] measured the activity of ferrous oxide in iron-calcium silicate meltsbetween 1265 °C and 1365 °C. A H2-H20-Ar gas mixture was bubbled into the melt containedin an iron crucible and aFeo was calculated from the oxygen potential of the gas phase in equilibrium14with it. The author performed activity calculations using a method similar to that of Schuhmannand Ensio [41] and noted that lime additions enhanced aF00. At a fixed CaO percentage howeveraFeo decreased with increasing Si02. In other words, lower basicity ratios led to lower a n,9 values.In 1970 Timucin and Morris [36] studied phase equilibria and thermodynamic propertiesof the pseudo-ternary Fe.0-Si0 2-CaO system at 1450 °C and 1550 °C over a range of P02 from1 to about 10-11 atm. Compositions in the pseudo-binary Fe, (0-CaO were studied first and thenextended into the pseudo-ternary system by adding 5, 10, 15, 20 and 30 wt% Si0 2 to thepseudo-binary melts. In the pseudo-binary melts they observed that at constant temperature,increasing CaO contents enhanced the stability of trivalent iron, Fe3+. This tendency decreasedwith increasing temperature when CaO was held constant. Additions of Si02 to pseudo-binaryFe.0-CaO melts showed an opposite effect. These trends are revealed in the np..203/(nFeo + nF,A)versus logP02 plots in Figure Pseudo-quaternary and complex melts:There are very few equilibrium studies on complex melts and a critical review of these hasbeen published by Grieveson [47] and Grieveson and Pomfret [48]. In 1941 Taylor and Chipman[49] published the results of their equilibrium study in the CaO-MgO-Fe.0-Si02 system at 1600'C. Based on their experimental data the authors concluded that the effect of MgO is qualitativelyequivalent to CaO (in concentrations up to at least 10 wt%) and for this reason in thermodynamiccalculations magnesia can be substituted for by lime. In their subsequent study Chipman andco-workers [50] determined the activity of silica in CaO-MgO-Al 203-Si02 melts by equilibratingthe slag with Fe-Si-C melts saturated with graphite or SiC (13) at 1 atm pressure of CO andcalculated the activities of the other oxides by Gibbs-Duhem integration.Sommerville and Bell [51] measured the activity of Fe0 0) in the quaternaryFeO-CaO-Si02-TiO2 and FeO-MnO-Si02-TiO2 melts at 1450 °C at iron saturation and15•••2° 1^•••■:5-75.---,43,. I..1 -2 .3 -• .1 -• - 2 -• -1Kg •cht .. -I -3 -4 -1 -4 .7 -•1.61 oot..Figure 2.3 Effects of temperature and melt compositions on the relative stability of Fe 3+ cations(from ref. 36). (a) effect of temperature in Fex0-CaO melts at two levels of CaO; (b) effect ofsilica content in Fe x0-CaO-SiO2 melts.16represented their results as a series of iso-activity curves on ternary and pseudo-ternary diagrams.The authors initiated the work primarily to study the reduction behavior of titania in the slagsand therefore the data is not of much relevance to the present thesis.Using an emf technique Filipovska and Bell [35] measured the activities of FeO and ZnOin FeO-CaO-Si02-ZnO and FeO-CaO-Al203-Si02-ZnO systems. The iron saturated slags wereequilibrated with CO-CO2 mixtures at 1250 °C and liquid silver was used as the zinc and oxygentransfer medium. The authors calculated y,.0 and yz„0 values and found that addition of CaO tothe slag increased the activity coefficients of both FeO and ZnO. They observe that for a givenalumina content the activity ratio yp..0/7z„,0 decreases with an increase in the basicity (or molarCaO/Si02) ratio; and the ratio increases with increase in Al 203 content, but this effect is small.It is interesting to note that similar observations were made in a separate study by Richards andThome [52].2.2.2 Kinetic studies:In order to extend the utility of the equilibrium and thermodynamic data to practicalsituations in industry information on kinetics and transport properties is essential. With this inmind many researchers initiated work to cover a broad spectrum of slag compositions- ferrousas well as non-ferrous. An overview of research work relating to iron oxide-containing melts ispresented below. Fex0 melts:Grieveson and Turkdogan [33] in 1964 investigated the rates of oxidation and reduction ofthe pure molten oxide contained in iridium crucibles at 1550 °C using CO-CO2 gas mixtures. Forunstirred melts, about 3 mm deep, the reaction rates were controlled by interdiffusion of iron andoxygen within the melt. Their data fitted Fick's Second Law equation solved for unidirectionaldiffusion in a finite medium and based on these calculations the authors proposed a value ofinterdiffusion coefficients for oxygen and iron atoms as 5± 1.0 x10 -5 cm2 s"'. Additionally, theauthors observed that the rate of reduction was slightly higher (about 10-20%) than the oxidation17rate and they attributed this result to density changes and convection currents in the melts.In 1969 Mori and Suzuki [53] studied ferric oxide reduction reaction (reverse of reaction2.3) using CO-CO2 mixtures within the temperature range 1430 °C to 1530 °C and substantiatedfindings of the earlier work by Grieveson and Turkdogan [33]. In addition, they found that at aconstant temperature the diffusivity of iron and oxygen atoms decreased with an increase in theF*3 ratio and this phenomenon becomes more conspicuous at higher Fe 3+ contents (ra3+i-F7 > 0.25).Fes`The authors interpreted their results in terms of the difference in the cation-oxygen attractionbetween Fe3+ and Fe2+. They predicted that since oxygen ions are much more closely attractedto Fe3+ than to Fe2+, the melt becomes more ordered into a rigid structure at higher Fe contents.Consequently the ion mobility decreases and this leads to a decrease in diffusivity at a higherFe3+ content. Although the results obtained in these studies are in agreement, it is unfortunatethat the authors did not extend their work to study the effect of gas flow rate.Kato, Sasaki and Soma [54] in 1977 performed experiments to study the rate of reductionof molten iron oxide to liquid iron. The reactionFeO(,) + COG) = Fem + CO, ) (2.5)was studied at 1600 °C using alumina crucibles. The reductant CO was blown normally onto thesurface of the melt through an alumina tube (5 mm ID), the tip of which was held about 10 mmabove the liquid surface. For the gas flow rates investigated, 0.6-16 Nl/min, the reduction ratewas proportional to square root of gas flow rate and the rate controlling step was mass transferin the gas film (suggesting a relatively fast rate for the reaction on the surface of the melt).Evidently, mass transfer in the oxide melt did not impede the rate of reaction (2.5) in theseexperiments, presumably because of the forced convection induced by the flow of gas across thesurface.In 1981 Tsukihashi et al. [55] published the results of their work on the reduction of moltenferrous oxide to solid iron in the temperature range 1450 °C to 1600 °C. The main objective oftheir work was determination of the rate constant, icc, for the chemical reaction (2.5). They reporttwo kc values, 19.4 cm/s and 9.3 cm/s; the former for the formation of liquid Fe at 1600 °C and18the latter for the formation of solid Fe at 1450 °C. They devised a novel set up, consisting of afluidized bed transport reactor and a high temperature furnace, to overcome the effects of the gasphase boundary layer surrounding the molten iron oxide droplets. This approach to the study ofreaction kinetics is both interesting and commendable. However, the k c value obtained by themat 1600 °C (19.4 cm/s) is much lower than the value of 1350 cm/s reported earlier by Kato et al.[54]. Perhaps the main reason for this disagreement lies in the estimation of kg values by bothgroups. Furthermore Tsukihashi et al. [55] did not considered the effects of the followingimportant parameters on the rate of the reduction reaction:(1) the presence of either magnetite or Fe l+ in the FeO powder;(2) particle diameter and particle dispersion;(3) nucleation, growth and cohesion of iron in the FeO droplets; and(4) prereduction;which could have led to the observed discrepancy in the lc, value.In 1986 Nagasaka, Iguchi and Ban-ya [56] studied liquid wiistite reduction reaction usingAr-CO, CO-CO2 and Ar-CO-CO2 mixtures at 1400 °C. The reduction gas mixture was jettedonto the melt surface through a steel nozzle whose tip was about 10 mm above the melt surface.During all their experiments the melts were held in a shallow iron crucible (17 mm ID). Theauthors measured the reduction rates by monitoring the melt weight loss using a thermogravimetricset up. Assuming steady state conditions they define the reduction rate as(dw)1r.-^—dt ' Awhere^w = weight of sample (g)A = gas-liquid interfacial area (cm2)t = reduction time (s)and based on the results observe the following relationr = Ic • (Pco - Pc010.242)(2.6)(2.7)19where Ica is the apparent rate constant (g/cm 2.s.atm) and 0.242 is the equilibrium (Pc.02/Pco) ratio.Figure 2.4 is reproduced from their publication showing the variation of percent reductionwith time. The profile of reduction curves drawn for different gas mixtures reveal that equation(2.7) is not strictly valid in all the runs for the duration of the experiment. According to theauthors, the eventual decrease in the reduction rate at higher P co values results from the coverageof the melt surface by metallic iron.In 1984 Ban-Ya, Iguchi and Nagasaka [57] studied molten iron oxide reduction at 1450 °Cusing H2-inert gas mixtures. They observed that the chemical reaction:Fex0 (0 + H24) = xFe(,) + H20 4) (2.8)was extremely fast and the reduction rate was controlled by mass transfer in the gas phase.Reaction (2.8) was examined using N2-H2, Ar-H2, and He-H2 mixtures at low flow rates (4-7litres/min) whereas the experimental work at high flow rates (7-28 litres/min) utilized only He-H 2mixtures. The apparent rate constant, Ica, of the chemical reaction (2.8) at the surface was estimatedas 0.6x10-2 (g/cm2s atm) for the high flow rate data.Researchers have also studied iron oxide reduction reactions using solid carbon or carbonin iron as reductants. Table 2.1 summarizes available information on the activation energy valuesand reduction rates obtained using these reductants. Some of the important points that emergefrom the data in this table are given below.(1) The activation energy for the reduction of molten iron oxide averages about 40 kcal/mole(167 kJ/mole) using either carbon or carbon in iron as a reductant and the equivalence ofthis value with that of Boudouard reaction implies rate control via this reaction.(2) The activation energy decreases with the addition of CaO and Al 20 3 .(3) Use of carbon in iron as a reductant yields higher reduction rates.(4) With either solid carbon or carbon in the iron as a reductant, thereduction rates of ferric oxide are higher in comparison with ferrous oxide.200^20^40^60^tiutime (min)Figure 2.4 Reduction curves of pure liquid FeOx  from Nagasaka et al. (ref.56).21Table 2.1 Kinetic studies on the reduction of iron oxides and iron oxide-containing meltsusing carbon and carbon dissolved in iron as reductants.Author(Year)Reductant Iron oxide Temp. Area Reductionrate x 104Activationenergy'C cm2 mol-FeO/cm2 .s kcal/moleKondokov et al. Graphite FeO 0.3 g 1450 - 4.81 38(1960) crucible 1600 5.53Sugata et al. Graphite Si02-69% FeO 1350 31.4 0.09 40(1972) rod -78% FeO 1400 28.6 0.19-69% FeO 1450 31.4 0.23Takahashi et al. Graphite Fe203 ore 12 g 1410 =30 0.20 56(1975) crucible 1570 =30 0.79Sasaki et al. Graphite ' Fe0 10 g 1400 =25 0.11 52(1978) crucible 1500 =25 0.261600 =25 0.56CaO-Si02-80% 1400 =25 0.50 20-30Fe0 1500 =25 0.721600 =25 0.99Sato et al. Graphite Molten Fe0 in 1470 =7 0.42 31(1986) rod alumina crucible 1520 =7 0.541620 =7 0.82Dancy Carbon in FeO 0.5 g 1430 =1 31.73 43(1951) molten 1500 =1 34.80iron 1610 =1 86.99Fe304 0.5 g 1580 =1 34.80 371650 =1 48.99Mac Rae C in iron Fe203 3 g 1335 =3 4.03 27-44(1965) 1450 =3 8.80Lloyd et al. 4.15 Wt% Fe203 0.7 g 14.00 0.54 7.94 56(1975) C in iron 1600 0.44 50.12Sato et al. C in iron Molten Fe0 50 g 1470 19.6 1.19 44(1986) 1520 19.6 1.731620 19.6 3.3022Sato et al. [29] in 1987 published their work on molten iron oxide reduction by carbon.They obtained reduction rate data using solid carbon and carbon dissolved in iron as reductantsin the temperatures ranging from 1420 °C to 1620 °C. The reactionFe0(,)+ Com = Feom + COG) (2.9)was studied in alumina and steel crucibles and the reduction rates were calculated from themeasured CO gas evolution. The reduction rates of molten iron oxide by the solid carbon were0.12x10-2 and 0.46x10 -2 (g/cm2•s) at 1420 °C and 1620 °C respectively. The activation energyfor the reaction was 75 kcal/mol using a steel crucible; but a lower value of 31 kcal/mol wasobtained when an alumina crucible was used. Higher reduction rates were obtained when usingcarbon dissolved in iron as a reductant. The reported values at 1420 *C and 1620 *C were0.61x102 and 1.8x10-2 (g/cm2.^) respectively. A comparison with the data of Ban-ya et al. [57]reveals that the reduction rates for both the hydrogen and carbon dissolved in iron are comparable;however the reduction rate obtained using solid carbon is lower.Finally some remarks concerning the effect of the reaction product on oxidation andreduction rates are in order. According to Grieveson and Turkdogan [33] the reduction reactionproduct, ferrous oxide, induces convection currents because of its higher density in comparisonwith underlying ferric oxide melt and as expected the reduction rate is enhanced. The study byNagasaka et al. [56], however, claims the opposite effect i.e. a decrease in the reduction rate afterthe production of much heavier solid iron. Although these authors offer no further explanationof this phenomenon, the information is sufficient to force one to consider the role ofphysicochemical factors such as surface tension and viscosity in the overall scheme. Pseudo-binary Fe.0-SiO2 melts:Kinetic information concerning Fe.0-SiO2 melts is sparse in comparison with the equivalentthermodynamic/equilibrium data. The earliest reported work, 1966, is by Krainer, Beer andBrandl [128]. These authors used graphite and coke crucibles to study the ferrous oxide reductionrate in the 1300 °C - 1500 °C temperature range by employing a thermogravimetric technique.Based on the experimental observations it was proposed that the overall reaction23Fe001.0 + qaucible) =Fe(7,8)() + COG)^ (2.10)was the result of two consecutive reactions taking place at different sites.siag/gas: FeO(,,eso + COG) =Fe0) +CO2w^ (2.11)gaslcrucible:CO24) +C(c,,,,,ibi. ) =2C0 4) (2.12)From a consideration of the activation energy value and other relevant observations the authorsconcluded that the overall rate was controlled by reaction (2.11).Shalimov, Boronenkov and Lyamkin [30] conducted research to identify the effects of FeOconcentration, temperature and pressure on reduction reaction (2.10). They report a reductionactivation energy value of 190 kJ/mole and observe that in the 0.125 to 2 atm. pressure range therate reaches a maximum at about 1 atm. Based on the findings of the work the authors concludethat between 1300 °C and 1450 °C ferrous oxide reduction by carbon is controlled by chemicaladsorption at the gas-slag boundary.Davies, Hazeldean and Smith [31] used both coal and graphite crucibles to hold Fe x0-SiO2melts and studied ferrous oxide reduction between 1400 °C and 1500 °C, at FeO„ contents greaterthan 60%. From the data obtained using graphite crucibles they calculated an apparent activationenergy of 280 kJ/mole which is approximately 30% higher than that reported by Shalimov et al.[30]. By duplicating the experiments in coal crucibles they obtained results similar to that forgraphite crucibles and concluded that the form of the reductant material does not affect the ferrousreduction rate. However, after critical evaluation of their data, the authors failed to arrive at afirm conclusion regarding the rate limiting step(s).Instead of using carbon or graphite as a reductant material several researchers [58-60] usedcarbon dissolved in iron to study the ferrous oxide reduction reaction. This choice was popularbecause the findings of the work were of potential importance to the steelmaking industry. Manyinvestigators studied the reactionFeO(,k,g) + [CIF, = Fewag „,,,,d) + COG)^(2.13)Moreover, similar to reaction (2.10) above, reaction (2.13) can also be represented as the sum oftwo reactions taking place at slag/gas and gas/metal interfaces.24Opinions concerning the reduction mechanism and the rate limiting steps for reaction (2.13)are varied. For example, in their study Philbrook and Kirkbride [59] observed that the rate wascontrolled by an interfacial chemical reaction (2.11); but Kondakov et al. [60] concluded that thecarbon solution reaction (2.12) is rate limiting. Though the findings of various researchers donot agree on one single rate controlling step for either of reactions (2.10) and (2.13), the literaturedoes identify several other factors - such as silica reduction in melts, chemisorption of surfaceactive elements, interfacial movement and gas evolution, that influence the overall reaction rate.Grieveson [47] and Pomfret and Grieveson [48] have published excellent reviews of the importantinvestigations in the field.The use of gaseous reductants is favoured in the studies involving pseudo-binary meltsbecause of the general consensus amongst the researchers that even when solid carbonaceousreductants are used to reduce slags it is possible that in many cases the reduction occurs throughgaseous intermediates. In 1986 Nagasaka et al. [56] published the results of their research onslag reduction at 1400 °C using gas mixtures containing CO. Their rate data for FeOx  andFex0-SiO2 melts reveal a linear relationship between rate (g-oxygen/cm 2.^) and log Pco (atm.).It was observed from these plots that the a range of linear relationship for pseudo-binary meltswas much narrower (0.003-0.02 atm.) in comparison with the pure FeO melt (0.003-0.1 atm).They also noted that for each individual melt there was a critical P ro value at which the curvestarted flattening out (indicating breakdown of the linear relationship) and these critical valuesdecreased with increasing silica content in the melt. The authors propose that the deviation iscaused by the effect of mass transfer in the liquid phase. By conducting additional experimentsNagasaka et al. established conditions under which gas phase resistance was minimal and obtainedall their data by operating within this range. With this knowledge of the effects of mass transferin both the gas and liquid phases the authors derived an empirical formula to calculate the rateof the chemical reaction on the surface.25Sasaki and Belton [61] measured interfacial rate constants for the gas-slag reaction usingH2O and CO gas mixtures at 1250 °C. Silica-saturated melts were chosen for the study becausethe relatively low rates of reaction of CO2 and CO had been established and because the datarelating Fe3+/Fe2+ ratios as a function of oxygen activity was readily available from the work ofMichal and Schuhmann [62]. Based on their experimental findings the authors concluded thatthe apparent first order rate constant for the reaction of H2 exceeded that for CO by a factor ofabout 53 and similarly the apparent rate constant for the reaction of H2O exceeded that for CO2by a factor of about 20. Moreover, their data suggests that both the oxidation and reduction ratesare significantly influenced by the constitution of the melt.El-Rahaiby, Sasaki, Gaskell and Belton [34] measured the rates of dissociation of CO 2 byan isotope exchange technique on liquid slags to establish interfacial rate constants for thesereactions. Sun et al. [63] have performed similar work using calcium ferrite melts. The meltswere exposed to CO 2-CO gas mixtures enriched in 14CO2 to promote the following isotopeexchange reaction:14c02 + '2c0 = '2002 + 14C0 (2.14)by consecutive steps involving the dissociation and reforming of CO 2 molecules at the surface.For the melt temperatures ranging between 1240 *C and 1480 *C the following relationshipbetween the apparent rate constant, ka, and the gas composition was observed by the authors:ka = ka° (Pco/Pco) l (2.15)in which Ic: is a temperature dependent constant for each melt. From their data the authors derivedseveral equations relating k: to temperature. Pseudo-ternary Fe,0-Si02-CaO melts:In 1986 Nagasaka et al. [56] published results of their kinetic study at 1400 °C usingpseudo-ternary melts held in iron crucibles. Based on the weight loss data the authors derivedan empirical equation for liquid ferrous oxide reduction to solid iron:26(2.16)N2.  IF + (Pco,r = ^ki-(NF3.2+Pco^(-33000+ 2.86—=2 )exp^K^RT^)where r is the rate in g-oxy/cm2.s and IC is the equilibrium (Pc02/Pco) ratio for the backwardreaction (2.1). i.e. FeOx  + CO = xFe + CO2.El-Rahaiby et al. [34] measured the rates of dissociation of CO 2 on liquid calcium ironsilicates between 1240 °C and 1400 °C using an isotope exchange technique. Apart from thereduction study by Nagasaka et al. [56], this is the only other reported kinetic work involving thepseudo-ternary Fe x0-SiO2-CaO system. The results of a series of experiments on melts whichcontained initially 5 to 80 mol % iron oxide at gas flow rates of 240-340 ml/min are presentedin Figure 2.5. This figure reveals that the results for pseudo-ternary melts lie within those obtainedfor pseudo-binary systems, viz. Fe20-CaO and Fe20-SiO2. In general the apparent rate constantvalues for the ternary melts are closer to the Fe20-SiO2 binary throughout the temperature range.El-Rahaiby et al. proposed a redox equilibria on the surface of the form:^CO24) + 2Fe 2+ –^2CO2-(ad)^(melt)+ 2Fe 3+^ (2.17)(melt) — Based on the observed variations of the apparent first order rate constant with oxygenactivity values a charge transfer reaction was suggested. According to this hypothesis, the overallreaction of CO2 with oxidizable oxide melts involves charge transfer to produce 0 2- ions asfollows:CO:-(ad)= CO(0+ 02-^ (2.18)By assuming the weakly CO 20.1) (in equation 2.17) to be in equilibrium with CO ).2(s  the authorsobtain an expression for the surface concentration of former species:^ co22_ = M • Pco2 • (Fe2+IFe3+)2^ (2.19)where m is a temperature dependent constant. Their results suggest that the rate of dissociationis proportional to rap_ El-Rahaiby et al. [34] comment that though their data offers good27Figure 2.5 Arrhenius plot of the apparent rate constants for pseudo ternary(Fe.0-CaO-Si02) melts containing equal mole fractions of CaO and Si0 2 at anequilibrium CO2/C0 ratio of 1. (ref. 34)28evidence in support of the hypothesized charge transfer model (expressed in terms of theconcentrations of donors and acceptors), more rate data together with the appropriatethermodynamic data are required before making firm conclusions in this regard. Pseudo-quaternary melts:In 1985 Fine, Meyer, Janke and Engell [64] studied the kinetics of molten iron oxidereduction at 1873 °K in the Fez0-CaO-MgO-SiO2 system using CO as a reductant. The gas wasjetted onto the melt surface (in a manner similar to that described by Nagasaka et al. [56]) andthe product gases were analyzed with the help of an infrared gas analyzer. The authors fittedtheir results to an empirical rate equationrate =dnFedt o = — kAa; 0APcmo (2.20)where k, m and n are constants and A is gas-melt interfacial area. The rate is expressed asmo/ FeO • cm-2 • s-1 • atm-1 and the quantity APcc, equals the inlet minus the equilibrium partialpressure of CO (for the ferrous oxide reduction reaction). The authors calculated a valuesusing a 4th order polynomial equation in terms of XF,(obtained by Turkdogan and Pearson [65])for a Fez0-CaO-MgO-MnO-SiO 2-P205 system.In a recent study Kim, GrAnzdOrffer and Fine [66] report that with increasing SiO 2 contentsthe rate of reduction of ferrous oxide is decreased. Their experimental set up and data analysiswere similar to that used by Fine et al. [64] and the slag compositions were within theFez0-CaO-MgO-SiO2 system. The authors accounted for the "excess surface coverage" by silicain these melts and derived the following rate expressionr = kL1 — 0.7 (asio2 )1A^(aFgoP- co — aF,Pco2IK)0(2.21)The authors assume that the surface area available for the reduction reaction (A) is relatedto the total gas-melt area (A 0) byA(2.22). r 1L — 0.7 (asio2)15]A,29Equation (2.22) was derived by the authors based on available surface tension data for Fe 10-S i02melts. [67] The validity of the rate law represented by the equation (2.21) was determined usingthe data for all experiments with Pco values of 0.05 atm. or less. The best-fit straight line throughthe data yielded a rate constant, k, of 4.2x1 0-5 (mol/cm2 s atm). Using their own data in conjunctionwith the earlier reported data by Nagasaka et al. [56] and Belton and coworkers [34,61,68] theauthors derived an activation energy value of 135 k.I/mol for ferrous oxide reduction using COreductant. Additionally, Kim et al. [66] report the following relationship between their rateconstant k and the one reported earlier by Ban-ya and coworkers [56]:ka = kaFgo[l —0.7(asoA (2.23)This relationship is shown in Figure Complex melts:Many Russian researchers report the use of industrial slags in their kinetic investigations[28,69-72]. As a direct consequence of this choice they had to consider several different reactionstaking place in their melts and this has caused numerous difficulties in the data interpretation.Although the chemical reaction of primary importance in these studies was reduction of ZnO toZn, their results strongly suggest an important contribution by a transient iron phase and its oxidespecies in the overall reduction scheme.2.3 Industrial/Large scale work:In general there is no adequate foundation for direct extrapolation of information from thelaboratory to an industrial process and for this reason it is important to perform either large-scaleexperimental work or pilot plant studies. In spite of the potential benefits emanating from suchresearch activity the reported data concerning this topic is very limited.During 1981 and 1982 Richards, Brimacombe and Toop [9,73,74] performed industrialtrials on zinc fuming furnaces at Cominco's plant in British Columbia. Based on theirmeasurements and subsequent mathematical analysis the authors concluded that the reactionbetween coal and the slag is controlled by diffusion of the reactant species (e.g. Fe 203 and ZnO)30Ban-ya et al. (3.4)t Fe,0-SiOi -Ca01673K CaO/St0 z =1 0)• Fe,0-510,-Ca0 1673K CaO/S10.=0 5)z Fe,,U-S10 2 -Ca0 1673K CaO,'5100.0.25)o Fe,0-SiO. (1673= Fe,0-St0 2 (1593KEl-Rahaiby et al 6)• Fe,0-Si40. (1673K^o• Fe,0 -Si°. (1593K 0, 00•^°X•* . tee • 0•1 2rnEC)060E0.0o.b1 60.2^0. 4^0 6a re0[ 1 —0 7 (as,o,) 1/3 10.8Figure 2.6 Relationship between the apparent rate constants obtained in two separateinvestigations. (from ref. 56,66)31in the slag phase and/or carbon gasification (Boudouard reaction). In a separate set of trialsLehner and Lindgren [75] studied fluid flow phenomenon in Boliden's zinc fuming plant.Chemical analyses of the dust samples extracted above the turbulent slag bath, revealed about8.8% carbon in them and this observation was in good agreement with the measurements ofRichards et al. [74] Based on the results obtained in their investigation the authors conclude thatzinc fuming rates were influenced mainly by the coal and air feed rates and coal-to-air ratio.In addition to the above publications on industrial trials there are others that report on largescale laboratory work. In 1979 Floyd and Conochie [76] published results of their experimentsusing 500 gm batches of slag in aluminosilicate crucibles. The kinetics of Sn removal was studiedby injecting either a solid or gaseous reductant into the molten slag. The authors report thefollowing important conclusions:(1) Reduction rates obtained using the CO were lower than those using H2.(2) Rapid reduction of slag occurs when the injected reductants are partly burned withco-injected air; and(3) SnO reduction reactions involve the participation of both metallic iron and iron oxides.In 1981 Schmitt and Wuth [77] reported the results of top blowing experiments in complexnon-ferrous slags. The melts were held in 0.1 m diameter crucibles and the charge weights rangedbetween 0.5 and 1.5 kg. The authors analyzed their data using a "surface renewal model" andcalculated overall mass transfer coefficients assuming liquid phase control. The values of theoverall mass transfer coefficient, k. (defined by the authors as ka = koev ) ranged between 10 -4and 3 x 103 sec-1 and these were found to be in agreement with the theoretically predicted values.Malone et al. [78] in their reduction kinetic study of copper and zinc oxide bearing slagsusing carbon have considered several concurrent reactions. For example carbon, CO, Fe andFeO can all participate in the reduction of copper and zinc oxides. The authors classified thereactions via carbon as direct reduction reactions and those via either CO, Fe or FeO were termed32indirect reduction. Based on the evidence of microscopic analysis they suggest that in theFe10-Si02-Ca0 pseudo-ternary system containing either copper or zinc oxides reduction viametallic Fe may not occur.2.4 Critical assessment of published work:It is commonly stated that those who have worked in the field of high-temperature chemicalmetallurgy have devoted too much time to establishing the equilibrium properties of their phasesand too little time studying the rates at which reactions within and between phases proceed. Theinformation in the foregoing paragraphs offers some evidence in support of this opinion. Sincethe 1960s however, the interest in the field of slag reaction kinetics has grown steadily and thishas resulted in an improvement of our understanding of this area.Researchers have tried a variety of crucible materials including graphite, coal, iron, alumina,zirconia, MgO, BeO, platinum, iridium and Pt-Rh, to hold oxide melts. However, only a fewhave commented on the effects of crucible contamination on the overall reaction behavior. Afew researchers attempted to overcome the contamination problem by selecting solid carbon orgraphite as a crucible material. However, this choice introduced another problem - namelynucleation of gas bubbles. This caused discrepancies in the results between various investigatorsand as a result no unequivocal conclusion was drawn regarding the overall rate controlling step(s).Since three phases - solid, liquid and gas are involved in the reaction between iron-oxide bearingslags and solid carbon, mass transport and chemical processes may both play a part in controllingthe rate of reaction (2.9). It should be also be noted that the above reaction is strongly endothermicand hence in general terms, heat transfer might also be important in maintaining the reaction[31,79].Additionally there are conflicting opinions concerning the presence and/or participation ofiron in the reduction reactions in iron oxide-containing melts. Many Russian researchers [69-72]have speculated on the presence of an intermediate (or transient) iron phase. According to theseauthors both ferrous oxide and metallic iron can act as reductants to reduce zinc oxide but noneof the authors have provided evidence or proof to substantiate their claim. On the contrary, based33on their research Melone et al. [78] have concluded that in the reduction of copper and zinc oxidesthe reduction via metallic iron does not occur. Therefore it is important to undertake additionalstudies to identify the role played by the iron phase and to further improve our understanding ofthe complexities of slag reactions.Limited data on the ferrous reduction reaction shows that lower rates are obtained at highersilica levels in the melts and the trend is reversed with the addition of lime. [56] Further analysishas attributed such results to the changes in the apparent rate constants. The results have alsobeen interpreted in terms of changes in the surface electrochemical potential of the melts and thesame model has been proposed for the oxidation reaction. [80] Based on the work of Sun etal.[63], Sasaki et al. [68] and El-Rahaiby and co-workers [34] Belton [80] has concluded that therate of oxidation reaction is governed by the dissociation of CO 2 at the melt surface. The authorhas also shown that the results of the oxidation of liquid iron oxide, conducted by Turkdogan andGrieveson [33], can be interpreted by assuming dissociation of CO2 adsorbed on the surface tobe the rate-controlling step. In general however the fundamental reasons behind the observedvariations in the apparent rate constants in these melts are not understood with certainty andtherefore more research work is needed.The important points that emerge from the literature survey are outlined below:(1) Most reduction kinetic studies have been done at iron saturation.(2) The effects of crucible dissolution on the reaction rates, if any, are not clearly identified.(3) In many cases, the data is fitted to empirical rate expressions.(4) Bulk of the kinetic data has been obtained by jetting a gaseous mixture onto the meltsurface; but equivalent information concerning unstirred melts is very meagre.(5) The work of Kim et al. [66] highlighted the potentially important role played by surfacesilica coverage for the first time.(6) The importance of charge transfer reactions in the iron oxide-containing slag melts hasbeen suggested; but no conclusive evidence in its support is available.34(7) The role played by the physicochemical properties of the melts (e.g. density, viscosity andsurface tension) in gas-slag reactions is not understood with certainty.(8) No systematic studies are reported for comparing relative rates of oxidation and reductionreactions in non-ferrous slag systems.35Chapter 3Objectives and ScopeFrom the information provided in the two previous chapters it is clear that our understandingof slag-related reactions in general, and redox reactions involving iron oxide species in particular,is far from complete. Because gas-slag reactions are of practical importance to many non-ferrousindustries, it is important to obtain as much useful information on them as possible. Theunderstanding of gas-slag reactions is vital for either developing new pyrometallurgical operationsor improving existing ones. Since much of the previous research work (thermodynamic andkinetic) deals with gas-slag-metal reactions of importance to ironmaking and steelmakingindustries, it is also necessary to extend this research activity into the non-ferrous field. Moreover,new rate data is needed to confirm and verify certain aspects of earlier studies.3.1 ObjectivesThe primary objective of the work is to obtain quantitative data on the reduction andoxidation reactions in simple melts containing iron oxides at temperatures between 1200 °C and1400 °C by CO and CO2. The secondary objective is to investigate the nature of theferric-to-ferrous reaction in melt compositions away from both iron and magnetite saturation.The tertiary objective is to evaluate the effects of solid reaction products, namely iron metal andmagnetite, on the gas-slag reaction rates.3.2 Scope of the experimental workThe choice of synthetic slags, as opposed to industrial slags, is made for the present studyfor the following reasons:(1) By working on pure oxide melts it is hoped to restrict the number of parameters that affectreduction rates to a minimum.(2) Once the basic data on pure oxide melts is generated it will be easier to extend theinvestigation to evaluate the effects of individual oxides on overall reaction rates.36(3) Industrial slags usually contain several impurities that may strongly affect the reactionunder review. There is a possibility that these impurities e.g. S, Bi, As, Sb, and Sn, maylead to side reactions and surface effects which would pose difficulties in the analysis andinterpretation of the rate data.The temperature range of primary interest to the non-ferrous industry is chosen so that thedata could be directly applied to practical situations. Use of gaseous reductants is preferredbecause earlier research has shown that even when solid reductants are employed the reductionreactions usually involve participation of an intermediate gaseous reactant species. Unstirredmelts were exposed to gas mixtures containing different proportions of CO and CO 2. The gasmixture was not jetted onto the melt surface to avoid the possibility of melt surface movementdue to the velocity of impinging gases. Moreover, it is believed that the reaction area estimatesfor unstirred melts would be more reliable than the ones for a jetting setup.Admittedly, the stagnant system goes to the other extreme when compared to jettingarrangement and it is anticipated that the gas phase resistance would affect the reaction rates toa degree. However, the main advantage of the stagnant system is that the gas phase contributioncan be easily characterized and thereby the roles played by other interfacial and liquid phase masstransfer effects, if any, could be identified.A comprehensive experimental scheme includes a reduction reaction study at iron saturationin both simple and complex melts. A few magnetite saturation experiments are performed tocompare the effects of solid phases on reaction rates. But perhaps the most important feature ofthis work is the study of the ferric-to-ferrous reaction in lime-free and lime-containing melts awayfrom both iron and magnetite saturation. No such study using unstirred melts is previouslyreported in the literature.The experimental variables - temperature, melt composition, melt weight, ferric-to-totaliron ratio, basicity, gas composition and crucible material, are chosen to generate sufficient datafor a critical analysis of several important issues including the occurrence of charge transfer37reactions in slag melts, extent of crucible dissolution and its effect on the overall reaction rate,role of the gas-slag interface and effect of melt ferric level on rate phenomena. Further detailson the variables employed in this investigation are outlined in Chapter 4.The data generated is analyzed with the help of mathematical models to distinguish differentstages of the reduction reactions. These models are used to evaluate the roles played by gas-phasemass transfer, liquid-phase mass transfer and interfacial phenomenon. Based on the findings ofthe work several comments are made concerning reaction rate phenomena in gas-slag melts.As an aid to the interpretation of results a MOssbauer technique is used for identificationof compositional gradients in the melts. To further support these results some X-ray diffractionand scanning electron microscopic work is also done.38Chapter 4EXPERIMENTAL WORK: Techniques and MethodologyIn order to determine the rate of gas-slag reactions the change in concentration of a particularcomponent in either the gas or slag phase must be determined with time either directly or indirectly.Chromatography or infrared spectroscopy is used commonly for monitoring changes in theproduct gas composition; on the other hand the choice of thermogravimetry is often made wheninformation on the melt weight changes is required on a continuous basis. However, simultaneoususe of the two techniques, one for gas composition measurement and the other to monitor slagweight changes is very rare (only one reported use [81]) and the same was attempted in thisinvestigation. A solid electrolyte oxygen probe was installed to monitor gas composition and theslag weight changes were recorded continuously using a precision balance.4.1 Apparatus:The experimental apparatus essentially consists of three major components:(1) Gas purification system;(2) Reaction chamber; and(3) Data acquisition system.Figure 4.1 shows general arrangement of the above apparatus.Argon, carbon monoxide and carbon dioxide were used in the preparation of various gasmixtures. All three gases were purified and metered prior to mixing. A schematic of thepurification set up is shown in Figure 4.2.The reaction chamber consists of an alumina tube (length 90 cm, i.d. 3.8 cm, o.d. 4.5 cm)within a super kanthal furnace. The top of the alumina tube was closed with a metallic flangewhich had two openings - one on the side for the zirconia solid electrolyte and the other in thecentre for the alumina hanger. Through the central opening the product gases left the reactionchamber. The lower end of the alumina tube was closed using a flange and a copper chamberassembly. An inlet for the reactant gas mixture was provided in the copper chamber.39LegendF.9. Super kanthal furnaceHanger raising/loweringdevice— — — Signal connectionsGas passage1^Crucible2 Alumina reactor tube3 Alumina hanger4 Alumina pin5 Ar flow meter6 CO flow meter7 CO2 flow meterMetierBalance1.4111--- Top steel hanger-4-- Middle hangerOxygen probe.441P\ Water cooledcopper clangeDataAcquisitionHardwareDAS-8 boardDO- 1 6 boardpH meteruple AComputer Jerible capFigure 4.1 Schematic diagram of the apparatus used in this study.LegendI-1 Cooling coils- Stainless steeltubing® Denotes thatcompound A isremoved in thestainless steel columnC.P. grade Ar__A.-L.0JFlow meterC.F. grade CO 21",reactortubeGasmixingchamberC.P. grade COt.—IGas purification trainGas purification train componentsArgon1. R3-11 catalyst in reducedform to absorb oxygen.2.83-11 catalyst in oxidizedform to absorb CO and H 23. Soda lime to absorb CO 24. Magnesium perchlorate toabsorb moisture.5. Phosphorus pentoxide toabsorb moisture.Carbon monoxide1. R3-11 catalyst in reducedform to absorb oxygen.2. Soda time to absorb CO 23. Magnesium perchlorate toabsorb moisture.Carbon dioxide1. R3-11 catalyst in reducedform to absorb oxygen.2. R3-11 catalyst in oxidizedform to absorb CO and H 23. Magnesium perchlorate toabsorb moisture.Figure 4.2 Schematic diagram of the gas purification system. (a) showsarrangement of argon, carbon monoxide and carbon dioxide purificationtrains, (b) -indicates components used in individual train.41The bottom end of this chamber could be opened and closed using a screwable cap. Loading andunloading of the weighed slag crucibles was done from this end. Thermocouple A, locatedapproximately in the middle of the furnace, touched the reactor tube from outside and its emfoutput in conjunction with 'celetray' controller was used to maintain the furnace temperature atthe set level.The data acquisition system was made up of the following: balance and its bi-directionalinterface, DAS-8 and EXP-16 boards, Junction/grounding box, pH meter, chart recorder and anIBM PC-XT. Additional details are provided in Appendix I.4.2 Raw materials:Ferrous oxide, FeOx  of 99.9% purity (80 mesh pass) was obtained from Kojundo ChemicalLaboratory Ltd, (Japan). Calcium oxide and silica were of AR (Analytical Reagent) grade andferric oxide assaying a minimum of 99% purity was used in the preparation of slag melts. Boththe CaO and SiO2 powders (-200 mesh size) were thoroughly dried by heating to 1000 'C andlater an appropriate precaution was taken to store them to avoid any moisture pick up.All the gases used were of commercial/industhal quality and for the elimination ofimpurities in these the individual gases were channelled through specially designed purificationtrains prior to their entry into the reaction chamber. The slag melts were contained in aluminacrucibles on most occasions; however, magnesia (MgO) and spine! (MgO-Al 203) crucibles werealso tried in several runs to study the effect of crucible dissolution on the reaction rate.4.3 Preparatory work:To supplement the weight measurement data a scheme was planned in which changes inthe gas phase oxygen potential would be continuously monitored and recorded. For this purposea solid electrolyte probe was fabricated and installed in the reaction chamber. (See Figure 4.1)While testing and calibrating the probe it was observed that the output was very sensitive to twoparameters namely - gas velocity and carbon monoxide content of the gas mixtures. Accurateand meaningful probe readings could only be obtained using CO-CO 2 gas mixtures in which the42P colP co2 ratio was about 9 or more. Upon exposure to the argon-oxygen mixtures the proberesponse did not correspond to the expected emf output. Increase in the gas velocities of thesemixtures produced the results in the right direction however the data did not yield the reliableand exact oxygen partial pressure values. Though the observed trends are consistent with thosereported by Estell and Flengas [82] in the context of the present work these results implied thatthe probe output could not be utilized for correlating the weight changes arising from the gas-slagreactions.Prior to the commencement of actual experiments several preparatory steps were necessaryand a few important ones are briefly described below.4.3.1 Flow meter calibration:After leaving the cooling coils (see Figure 4.2) the three gases entered the rotameter tubes.Passage of the gases via cooling coils (made from 7 mm copper tubing and immersed in a waterbath) ensured gas supply to the flow meters at a constant ambient temperature. Individual flowmeters (for Ar, CO and CO2) were calibrated by adding a linear mass flow meter unit at the exitend of the rotameter. During the calibration run gas entered the purification train and then passedthrough rotameter-mass flow meter arrangement. For a constant rotameter float reading (whichwas controlled and adjusted using a precision needle valve) the corresponding flow reading inthe mass flow meter was recorded. Later the flow rate was altered by opening/closing the needlevalve at the inlet end of the rotameter tube and when the float reading stabilized the mass flowmeter rate was read. The procedure was repeated to cover an entire scale (5 to 100 mm) of therotameter tube. To supplement the above data which essentially represented high flow ratesadditional measurements were made using a dry gas meter. An average of three readings wastaken to be the rotameter flow rate. A typical set of data obtained during argon calibration islisted in Table 4.1.To calibrate low flow rate (#610 tube) rotameters a 'soap bubble' technique was employed;and similar to the high flow rate rotameter (#604 tube) calibration, further confirmation of thedata were obtained by repeating the calibration trials using a mass flow meter.43Table 4.1 Flow rate calibration results for argon gas.RotameterScaleReadingCalibratedflow rateMeasured flow rate as perStainlesssteelfloatPyrexfloat#604 tube MassFlow meterDryGas meterGasBubbletechniquemm mm cc/sec, cc/sec cc/sec cc/sec1.5 8  3.75 3.7 - 3.753  9 4.45 4.5 - 4.44 11 5.75 5.8 - 5.75 12 6.5 6.5 - 6.45^_6 13 8.0 8.0 - 7.9510 20 13.0 13.0 12.5 -15 31 22.0 21.5 22.0 -20 39  25.5 25.0 26.0 -25 49 42.0 43.0 41.0 -30 59 46.5 46.5 48.0 -35 65 53.5 54.5 52.0 -40 78 62.0 62.0 64.0 -45 87 68.0 69.0 67.5 -50 97 77.5 78.0 77.0 -55 108 87.0 87.0 86.5 -60 119 94.5 97.0 93.0 -65 127 104.0 104.0 103.0 -70 137 109.0 113.0 107.0 -75 147 118.0 124.0 115.0 -44In each individual case the calibration data was finalized when the flow rates for a particular flowmeter reading could be reproduced using two independent techniques. The accuracy of both"high flow rate" and "low flow rate" type rotameters is estimated to be ± 5%. All the flow meterswere obtained from Matheson Gas Company of Canada.4.3.2 Furnace temperature profile:The alumina reactor tube was heated by the super kanthal elements. A Pt/Pt-10%Rhthermocouple was placed approximately in the middle of the hot zone and its junction touchedthe outside wall of the alumina tube. Signal from this thermocouple was channelled to a regulatorfor controlling the furnace temperature at a set level. However, it was necessary to generateprecise information on the temperature profile within the reactor tube and for this reason anadditional thermocouple was constructed to measure the profile of the furnace maintained at 1400°C. The newly made thermocouple was traversed through the length of the reactor tube (lyingwithin the furnace) at fixed intervals of 10 mm. At each interval the emf response was read onthe chart recorder and based on this information temperature profile of the reactor tube wasobtained. Care was taken to hold the hot junction in the centre of the reactor tube and a steadyflow of argon gas was maintained during these measurements. The procedure was repeated forother furnace temperature settings at 1300 *C and 1200 'C. The data revealed the hottest zonewithin the reactor tube to be approximately 40 mm in length. This information was later utilizedto position the crucible.4.3.3 Activity calculations:In the present study the activity values were derived using the model developed by Kelloggand Goel. [83,84] A computer program was written based primarily on the data supplied byGoel and Kellogg [83] to calculate activity values of various oxide species in iron oxide-containingsilica melts (FeO-Fe203-Si02). Prior to its adoption calculations were made to check the validityof the model. For this purpose the composition of a ternary peritectic was chosen in the Fe-O-Si0 2system; and based on the wt. pct values of Si02, Fe203 and FeO species (25.0, 11.5 and 63.545respectively) and the temperature of 1163 °C the computer program was run to calculate activitiesof hypothetical species, FeO and Fe0 1.5 . These values were in turn utilized to predict the activityof solid Fe304 according to following reaction:2Fe01.5(hyp)+ Fe0(hyP) = Fe3040.a.0^ (4.1)The following relationship between the equilibrium constant, 1q4.1), and temperature for theabove reaction (4.1) was arrived at using the data in the literature [84](4.2)ln^— 17837.63 15.08 0.54•(10_3) • T + (1.28) (105) + (1.2) • ln T)^T T2Substitution of the peritectic temperature, T, value of 1436 *K yielded a K( .4)value and based onthis the aF,A was found to be 1.0338, which is in close agreement with the theoretical value of1. Similar calculations were repeated to find the correlation between the predicted and theoreticalactivity values of fayalite species. The calculated value of 0.9936 matched well with the expectedtheoretical value of unity. In addition the data obtained on silica activity were found to beconsistent with those reported by Timucin and Morris [36] and Schuhmann and Ensio [41]. Afew additional details concerning the model are presented in Appendix I.To generate activity data in the complex FeO-Fe203-Si02-CaO-Al203 system an additionalcomputer programme was written. Before using the model predicted activity values inthermodynamic calculations a comparison was made with the activity values reported by otherresearchers. Table 4.2 is prepared to illustrate the proximity of the predicted ferrous oxide activityvalues with those obtained from other sources. Though the table does not list the silica activitydata the accuracy of the predicted values was within ± 5% of those reported in the literature.[36,41] Based on these findings it was concluded that Kellogg's model provides a satisfactoryprediction of the measured activity data.46Table 4.2 Comparison of aFeo values calculated using Kellogg's model with the other researchers.Slag composition. , wt% Temp. Kellogg'smodelEstimated ape° values" based on other sourcesFeO Fe2O3 SiO2 Al 203 CaO ZnO • C aFe) ref. [41] ref. [52] ref. [42] ref. [36] ref. [85] ref. [35]63.8 0.4 35.8 - - - 1308 0.50 0.48 - 0.45 - - -71.6 1.0 27.4 - - - 1362 0.67 0.65 - 0.62 - - -77.8 3.8 18.4 - - -,1350 0.81 0.84 - 0.80 - -50.0 - 27.8 - 22.2 - 1300 0.62 0.59 0.60 0.61 0.60 -40.0 - 33.3 - 26.7 - 1300 0.44 - 0.44 0.40 0.43 0.45 -46.7 - 36.3 - 17.0 - 1600 0.68 - - - 0.67 0.69 -56.3 3.6 31.4 - 8.7 - 1200 0.75 - 0.73 - 0.69 0.78 -47.8 6.2 22.1 - 23.9 - 1260 0.74 - - 0.71 0.73 0.73 -51.9 1.0 27.3 - 19.8 - 1304 0.68 - - 0.62 0.65 0.60 -40.0 - 30.0 8.0 17.0 5.0 1250 0.43 - 0.45 - - 0.46 39.0 - 20.0 21.0 15.0 5.0 1250 0.58 - 0.59 - - - 0.5430.0 - 24.0 21.0 20.0 5.0 1250 0.48 - 0.47 - - - 0.50Note: (1) ..slag composition rounded to nearest digit.(2) Estimated values are accurate within ± 5%.4.3.4 Data acquisition trials:Three important parameters - oxygen probe output, furnace temperature and crucibleweight, were monitored during all the experiments and their permanent record was kept usingthe on-line computer. The hardware - DAS-8 and EXP-16 boards and IBM PC-XT was used inconjunction with the data acquisition software 'Labtech notebook' which allowed simultaneousdisplay of the data being stored on the computer hard drive. A brief description of the dataacquisition system is provided in Appendix II.4.4 Experimental procedure:A thoroughly dried and clean alumina crucible was weighed accurately using a precisionbalance. The crucible was reweighed after placing a known amount of ferrous oxide powder (orits mixture with silica, calcium oxide and ferric oxide). After recording the weights, the cruciblewas attached to the alumina hanger assembly using a solid alumina pin. To do this the bottomcap of the copper chamber was unscrewed. Once the crucible was suspended, the hanger assemblywas raised slightly within the copper chamber and the cap reattached. At this point, purging ofthe reactor tube with the gas mixture commenced. After about 5-10 minutes all the air was drivenout and the operation of raising the crucible within the reaction tube was started. The cruciblewas raised slowly from its lowest position in the copper chamber to the reaction site at a rate ofabout 45 °C/min. During this heating period the proportion of CO and CO 2 was controlled in theAr-CO-CO2 mixture so as to eliminate unwanted side reactions. At the end of the heating period,which averaged approximately 35 minutes, the alumina hanger was attached to the middle hanger(Figure 4.1). The middle hanger was held by the top steel hanger which in turn was suspendedfrom the balance. Prior to this the balance was calibrated by following the recommendedprocedure and other preparatory steps were taken to record the balance output. The weight ofthe crucible-hanger assembly was logged after 10 seconds and, in addition, the record of furnacetemperature (using thermocouple A) was stored on the computer hard drive. During this periodmelting of oxide mixture and its homogenization was permitted. The duration of the melting48period varied from 1-3 hours depending on the reaction temperature.At the end of the melting period the gas mixture was altered to promote the desired chemicalreaction. The flow rates employed during the reaction periods were approximately 5 times higherthan those during the heating and melting periods. The increased flow rates caused an abruptweight change at the beginning of the reaction period. The phenomenon was attributed to thedrag effect (as the reactant gases travelled from bottom to the top) and its duration was observedto be less than 2 minutes. Following this period, the flow rate related fluctuations in weightswere minimal and the melt weight changes corresponding to the chemical reaction alone wererecorded.At the end of the reaction period, which varied from 2-7 hours depending on both thetemperature and the type of reaction studied, the alumina hanger was detached from the middlehanger and was connected to a movable arm of the hanger raising/lowering device. By operatingthis device powered by a small D.C. motor the arm was lowered rapidly until the crucible reachedthe copper chamber. During this quenching procedure the crucible was brought from its reactionsite in the alumina reactor tube to the copper chamber at an ambient temperature in less than 20seconds. The arrangement enabled smooth and reliable quenching operation without inducingany melt turbulence. The reactant gas supply to the reactor chamber was maintained at least 30minutes into the quenching period to ensure complete cooling down. Later, the gas supply wasstopped and the crucible was taken out for reweighing.Though the melt weight data were collected only during the melting and reaction periods,the melt weight change in the heating period could be calculated using the following equation:Ow . g = (Initial wt. - Final wt.) - AW„.frn — AW„,wrion^(4.3)By substitution of all the known values, the weight change during heating can be calculatedreadily. This data was useful to ascertain if side reactions were taking place prior to the reactionperiod.49In all the runs slag weighed about 3 grams and the melt depth averaged between 3 to 4 mmdepending on the melt density and the crucible diameter.4.5 Experimental variables:To cover all aspects of the oxidation and reduction reactions under review the followingvariables were studied during this work.(1) Temperature; (2) Melt composition; (3) Gas composition; (4) Crucible material; and (5) Meltsurface area.4.6 Laboratory methods:In addition to the weight gain/loss data obtained using thermogravimetry, the informationon slag composition was necessary to interpret the effects of various operating variables on therate data. For this purpose it was essential to perform chemical analysis of the melts. Tosupplement the observations and fmdings of the experimental data use was made of conventionaltechniques such as electron microscopy and X-ray diffraction; but in addition to this M6ssbauerspectroscopy was employed to verify the presence of iron ion gradients with melt depth.4.6.1 Chemical analysis of slag melts:A total of 29 samples including the FeOx  starting material were analyzed for thedetermination of the following: (1) ferrous oxide; (2) total iron; (3) silica; (4) calcium oxide; (5)magnesium oxide; and (6) alumina. The ferric oxide proportion in the slags was derived fromthe total Fe and ferrous oxide assays by difference. All the assays were performed by the Comincoassay laboratory at Trail, B.C. The methods of analyses used in the determination of variouscomponents are listed below.(1) Ferrous oxide: Atomic Absorption (AA) and Volumetric - SnC1 2;(2) Total Fe: Potassium Dichromate method and Atomic Absorption;(3) Silica, (SiO2): Colorimetric - Molybdenum Blue;50(4) Calcium oxide, (CaO): Volumetric - EDTA;(5) Magnesium oxide, (MgO): Volumetric - EDTA and Atomic Absorption;(6) Alumina, (Al203): Volumetric - EDTA.4.6.2 SEM analysis:Our kinetic data indicated that silica is surface active in the melts i.e. its surface concentrationis moderately higher than the bulk composition. Though there is clear evidence in support of thisin the literature [67], measurements were made of silicon with depth using the electron probe onthe SEM. In addition, the average value obtained from such analyses was compared with the wetchemical assay for confirmation. An example of this is provided in Table 4.3 where Al 203 , MgOand Si02 assays obtained by SEM and wet chemical analyses are compared. To a limited extentthe SEM was also used to observe and identify various glassy phases in the quenched slags.4.6.3 MOssbauer spectroscopy analysis:This technique has been used primarily to study the distribution of iron cations in the slagbulk. However, in the present study an attempt is made to use this technique to trace Fe 3+ andFe2+ gradients away from the reaction interface. A special sample holder was designed for thispurpose and the Massbauer spectra were obtained in quenched slags at various depths. Detailsof this work are provided in Chapter X-ray diffraction analysis:A powder diffraction pattern was obtained on the ferrous oxide raw material to test for thepresence of iron metal and magnetite. Peaks of iron (fcc) and magnetite were identified usingtwo separate radiations i.e. Cu and FeK u. This qualitative analysis was later supplemented bythe wet chemical analysis and based on both the measurements individual proportions of Fe andFe304 were estimated to be approximately 4 wt pct.Additionally the presence of crystalline phases, if any, in the quenched slag samples wastested for using X-ray diffractometer.51Table 4.3 Slag chemical compositions obtained by wet chemical methods and SEM-EDX.Expt Slag Chemical composition, wt%# Melt type Wet chemical assay SEMFe0 MgO Al 203 SiO2 FeO Mg0 Al 203 SiO2123 _^Fe,0-Al203 93.5 - — 6.5 - 94.2 - 5.8^' -124 FeBO•Mg0 97.3 2.7 - - 97.9 2.1 - -125 Fea0-Si02•Al203 56.5 - 22.5 21.0 60.0 - 20.0 20.0126 Fe,,O-Al203-MgO 93.5 2.0 4.5 - 92.0 2.5 5.5 -Chapter 5Results and Preliminary AnalysisThe analysis of the accumulated data is based primarily on the weight change observedduring an individual run. The weight of the alumina hanger assembly (i.e. hanger rod, crucibleand pin) within the furnace and two steel hangers outside of the reactor tube (see Figure 4.1) didnot change during the experiment and therefore changes in the balance readings were attributedonly to the passage of oxygen in and out of the slag due to the gas-melt reactions. From therecorded data weight versus time curves could be plotted and slope of these curves representedreaction rates.It is felt that presentation of the results in either tabular or graphical form alone may notbe sufficient to develop an understanding of the rate phenomenon because several different factorsare responsible for the observed rates. It is therefore important to identify the contributions tothe rate from individual parameters and then study their combined behaviour. Furthermore, suchan approach is helpful in the development of a mathematical model. For these reasons preliminarydiscussion of the results is included in this chapter.5.1 Ferrous-to-iron reduction study between 1200 °C and 1400 °C:The ferrous oxide raw material, Fe,,O, used in the preparation of the melts contained — 4wt% ferric oxide and therefore the reduction reaction under review can be written asWilstite(jkig) + COG) = Fe (7)(3) + CO ,)2  (5.1)By performing wet chemical analysis of the ferrous oxide material it was possible to estimatethe value of the non-stoichiometry coefficient, x, as 0.98 ± 0.01. Additional confirmation of thiswas obtained using MOssbauer spectroscopy as discussed in chapter 7. Since the value of x isclos to unity, the standard state of wustite (ferrous oxide) is assumed to be the ideal stoichiometricliquid. In many publications [37,40-43,56,57] the authors have assumed this standard state tolist free energy change data for reaction (5.1).535.1.1 Fe„0 melts at 1400 °C:Experiments were conducted at 1400 *C to identify the effects of various parameters suchas gas composition, melt surface area, melt composition and gas flow rate on the iron formationreaction. Alumina crucibles were used and the extent of crucible dissolution was determinedfrom wet chemical analyses of the quenched slag samples. The results showed about 6-6.5 wt%Al203 in these slags which is in close agreement with its equilibrium value according to theFe.0-Al203 phase diagram. [86,87] Effect of gas composition:Three gas mixtures: Ar-CO, CO-CO2 and Ar-CO-CO2 were used In each case, variationin the melt weight with respect to time was found to be linear. In some cases however an increasein the slope could be detected after an initial period. This increase is attributed to melt movementinduced by the metallic iron. Figure 5.1 shows several weight-time curves obtained using CO-CO2and Ar-CO mixtures.It can be seen from the plots in this figure that for low values of Pc0 the slope is constantthroughout the reduction period; however at higher values of Pc0 0.6 atm) in either Ar-CO orCO-CO2 the slope increases after a certain point in the reduction period. Therefore, for comparingdata amongst different runs only the initial, linear reduction period (approximately 10 minutes)is considered for all runs. The reduction rate is derived using following equation1 dwr – —A. dt(5.2)where w is sample weight in grams, t is reaction time in seconds and A. is the gas-melt surfacearea in cm2.Alternately, the rate can also be expressed asRate = (rate constant) (Driving force)^ (5.3)(gra/ cm2 .^) (gall cm2 a Arms)^(aan)and54Time, min4 ()62IM I4031 -4 006 -CA^4 004 -▪ (1%)bA 'n3 -6)^4 NA4 040 a0.)^4 044 -4 042  a4 04 -n31 -4 036 -4 03.4 -4 032  -4 03 ^0 40min2.30 ^2.43 -2.47 -^•2.49 -2.41 -2.47-2.43 -2.41 -2.40 -2.39 -2.31 -2.37 -2.36-2.29 -^(d)2.33 -2.31 -2.33 -2.32 -2.31 -2.30 -EXPT #7Peo = 0.6Ar • CO • I Atm2.21 is^20^b^40^11',0^60^70^ICTime. min^ Time. minFigure 5.1 Variation of the melt weight with time during ferrous-to-iron reaction study at 1400'C (a) Expt #2A;(b) Expt #8; (c) Expt #1; and (d) Expt #7.f=Pcbo(5.4)where f is the reaction driving force, 40 and 402 are the bulk partial pressures of thereactant gases, IC 5`.0is the equilibrium constant for the reaction (5.1) and ap.0 is the ferrous oxideactivity in the melt. Additional details on the derivation of equation (5.4) are given in chapter 6.The reaction rate values obtained for various runs are listed in Table 5.1 along with therelevant experimental parameters. In addition a separate column is prepared to indicate the weightgained by an individual melt prior to the reduction period. This information concerning the weightgain was considered essential to verify the absence of any solid phases in the melt (either wiistiteor magnetite) prior to the reaction period. According to the Fe-0 phase diagram (Figure 2.1), at1400 °C liquid wustite can contain only 24 wt% oxygen. Any additional oxygen in the meltresults in the precipitation of a solid phase. Oxygen levels between 24 and 25.5 wt % lead tosolid wiistite formation and when the level exceeds 25.5 wt% magnetite is formed. Therefore toavoid the presence of either of the solid phases it was necessary to hold the gas phase P02. below10 atm (calculated from the thermodynamic data) during the heating and melting periods. Thedata in Table 5.1 reveals that in all the melts studied the melt oxygen content was below thisthreshold value of 24 wt% (calculated using the initial assay and equation 4.6). In the absenceof precise P02 control during the heating and melting periods the possibility of oxidation is morelikely.The reduction rate values in Table 5.1 reveal that the rate increases with an increase in P.This relationship is shown graphically in Figure 5.2. Based on this figure an equilibrium Pco/Pcoratio for reaction (5.1) is determined for comparison with its theoretical value (derived using freeenergy data and assuming unit activities of wiistite and Fe). The line OB (x-axis) represents zeroreduction rate and therefore the equilibrium point lies on this line. Along the line AB the P CO2/PCOratio changes. Since the point A is obtained by extrapolation of the rate data it represents thehighest rate at Pa) . 1 atm. Therefore, as we move away from point A the P IPco2 - co ratio increases56Table 5.1 Reduction rate data for Fe iO-Al203 melts at 1400 'C.Expt. Initial MeltWeightWeightHeatingperiodgainMpeeriodltingMelt oxygen ' prior toreduction reactionPcoGas mixturePco, PA,Reduction rateduring initial 10minuteperiodD.F.# g mg mg wt% atm_ .^atm atm g-oxy/cm2.sec atm1 4.0595 5 13 22.15  0.10 - 0.90 3.8 x 10' 0.16_ 2.5022_ 28 8 23.16 0.18 - 0.82 6.8 x 10 .6 0.187 2.0761 25 2 23.00 0.61 - 0.39 22.6 x 104 0.6116A 2.9675 2 1 21.80 0.73 - 0.27 25.0 x 104 0.732A 2.5160 19 10 22.85 0.96 0.04 - 25.5 x 10 .6  0.805 2.4380 30 5 22.71 0.88 0.12 - 13.7 x 10 .6 0.418 2.4740 34 2 23.16 0.87 0.13 - 13.0 x 10 .6 0.3619A 2.3640 6 1 22.00 0.94 0.06 - 22.2 x 10 .6 0.707A 2.5220 25 7 23.00 0.53 0.13 0.34 0.4 x 10 6 0.029 2.5450 35 3 23.19 0.65 0.13 0.22 5.0 x 10 .6 0.141911 2.3640 6^_1 22.00 0.69 0.04 0.27 16.2 x 10 .6 0.53Note:^(I) Assuming all weight gain was oxygen.(2) According to Fe-0 phase diagram (Figure 2.1) oxygen levels exceeding 24 % by wt. lead toeither solid wastite or magnetite formation.(3)During heating (-35 minutes) and melting (60 minutes) periods Ar-CO-CO 2 mixture was used.(4) Ferrous oxide raw material contained 21.7 wt% oxygen.AGas mixture (i I atm■ Ar-COo CO-0O24 Ar-CO-CO2Fez0-Al203 melts at 1400 'CPco (atm)Figure 5.2 Relation between the initial reduction rates and P. for the Fe 10-Al20,melts at 1400 'C.58and the rate decreases until at point B on the x-axis the gas is incapable of reducing the slag. Atthis point then the gas is in equilibrium with the wiistite. In Figure 5.2, the point B is located at0.80 atm and therefore assuming CO-CO 2 mixture at 1 atm the Pc02 value would be 0.20 atm.The lines OA and AB are obtained by performing linear regression analysis on the results ofAr-CO and CO-CO2 gas mixture runs respectively. Thus, the experimentally determined valueof the equilibrium Pco /Poo ratio is 0.25. Thermodynamic calculation reveals a value of 0.264for the equilibrium constant, 1q5.1), assuming unit activities of wiistite and Fe species. In ourmelts however, aFeo is less than 1 (due to the dissolution of alumina in the melt) and therefore thevalue of the equlibriumPco/Pco ratio is obtained by multiplying 0.264 by the app. For Fe x0-Al203melts at Al203 saturation an aF.0 value of 0.96 is obtained using Kellogg's model [83,84] and itsmultiplication with 0.264 yields an equilibrium Pco/Pco value of 0.253. This agrees well withboth the experimentally derived value of 0.25 and the previously reported values by Ban-ya andWatanabe (0.251) and Darken and Gurry (0.263) for the equilibrium Pco/Pco ratios. Effect of surface area:To identify the role of surface area it was necessary to conduct experiments in aluminacrucibles with different diameters using the same reaction driving force value (as per equation5.4). Accordingly, three experiments (#6, #113 and #121) were performed using a Ar-CO mixturewith Pa, = 0.18 atm. The results obtained are listed in Table 5.2. The data reveals three differentweight loss values for the initial reduction period of approximately 10 minutes. However, for allthe three experiments a rate value of 6.5 ± 0.5x10 g-oxygen/cm2s was obtained. Effect of crucible height:Experimental results showed that the rate values were dependent on the crucible height.(Table 5.2) Critical analyses of the data suggested that this effect is attributed to the increaseddiffusion length in the taller crucibles. If we consider the distance between the crucible top andthe melt surface as a gas phase boundary layer for molecular diffusion then the thickness of this59Table 5.2 Reduction rates for Fe,0 melts held in magnesia, alumina and spinel (MgO-Al 203 ) crucibles and exposedto Ar-CO mixture (Pco = 0.18 atm) at 1400 'C.Expt Crucible Gas-meltareaDiffusionlength, 8Melt oxygen prior toreduction periodWeight loss duringfirst 10 minutesReduction rate duringthe initial 10 minutesCrucibledissolution# cm2 cm wt% mg g-oxy/cm2.sec wt%6 Alumina 2.64 2.60 23.16 11.0 6.8 x 10 .6 6.5% Al20 3113 Alumina 2.08 2.63 22.00 7.5 6.0 x 10 6 6.5% Al203121  Alumina 0.39  2.60 21.90 1.6 6.8 x 10 .6 6.5% Al203114 Alumina 1.80 2.20 22.00 8.0 7.4 x 104 6.5% Al203122 Alumina 0.39 4.50 21.80 1.0 4.3 x 104 6.5% Al203120 Spinel 1.23 2.70 21.80 3.5 4.7 x 104 4.5%t1g10117 Magnesia 3.78 2.70 21.79 10.6 4.7 x 104 2.7% MgO118 Magnesia 1.84 2.40 21.70 5.6 5.1 x 10 .6 2.7% MgO119 Magnesia 3.66 1.90 21.80 14.3 6.5 x 10 .6 2.7% Mg0116 Alumina 2.09 2.60 21.95 7.5 6.0 x 10 .6 6.5% Al 203115 Alumina  2.12 2.74 21.85 7.0 5.7 x 10 .6 6.5% Al203Note: Diffusion length, 8, refers to the distance between the melt surface and the crucible top. Taller crucibles andlower melt heights lead to higher 8 values.layer is one of the rate controlling parameters. Any experimental variable that alters the gas phaseboundary, e.g. melt height and crucible height, can potentially affect the overall reduction rate.Analysis of the experimental data has revealed that the value of gas phase mass transfer coefficientDA -Bkg — 8(5.5)is in close agreement with the theoretically predicted value given by equation (5.5). In the aboveequation DA_B is the diffusivity of reactant gaseous species and 8 is the gas phase boundary layerthickness. Binary diffusivity values (i.e. diffusion coefficients for the binary system) weredetermined using the Fuller, Schettler and Giddings relation [88] and for the calculation of ternarydiffusion coefficients the procedure prescribed by Bird, Stewart and Lightfoot [89] was followed.Experiments performed using both alumina and magnesia crucibles revealed that when thevalue of gas phase boundary layer, 8 (cm), decreased an increase in the k g value (according toequation 5.5) caused a corresponding increase in the rate. To highlight this trend a plot of rateversus 8 is shown in Figure 5.3. Results obtained using alumina and magnesia crucibles areplotted separately because the extent of crucible dissolution is different in each case. Effect of gas flow rate:In the experiments #116, #115 and #113, the Ar-CO mixtures were proportioned to yielda Pco of 0.18 atm and the overall flow rates were maintained at 10.6, 26.0 and 36.5 cc/secrespectively. Apart from this difference in the gas flow rates all the other experimental parameters-melt weight, crucible material and height, duration of heating, melting and reduction periods etc.were same for these runs. Analysis of the data revealed a rate value of 6± 0.5x10 -6 g-oxygen/cm2sfor all the three runs. (Table 5.2) Additionally, the weight-time curves for three experiments areshown in Figure 5.4 and from the similarity in their slopes it is evident that within the scatter ofthe experimental data the reduction rates for these runs are equal.61S „ CMFigure 5.3 Relation between reduction rate and gas phase boundary layer, 6.6246.446.5 —Fes0-Al20, melts at 1400 .0#115Expt Flow ratecdseeSlopeVann*116#113 36.5 6.3 x 10"#115 26.0 6.0 x 10`#116 10.6 6.1^K 10.4 Ar-CO mixture; Pco = 0.18 am20^40^60^801i; 463EUX 4627.12V46.146 ^0 100^120Reduction time. (min)Figure 5.4 Weight loss-time relationships for the experiments designed to study theeffect of flow rate.635.1.1.5 Effect of excess oxygen:The data in Tables 5.1 and 5.2 shows that in all the melts the oxygen level prior to thereduction period was kept below 24 wt% using controlled Ar-CO-CO 2 mixtures to avoid theformation of solid phases: wiistite and magnetite. The weight gain data generated during a fewpreliminary experiments revealed that in absence of precise P02 control during the heating andmelting periods ferrous oxide oxidation is unavoidable. It was felt that under such circumstancethe resultant weight gain might lead to higher reduction rate (during the subsequent reactionperiod). To test this hypothesis and, in general, to better understand the role played by excessoxygen in the melt (i.e amounts exceeding 24 wt%), a special experiment was designed in whichferrous oxide was deliberately heated and melted in an argon atmosphere.Nagasaka et al. [56] employed a purified argon atmosphere during the heating and meltingperiods. Moreover they report sample heating rates of —5 °C/min It was decided to simulatethese conditions in experiment #12 and obtain the rate data for comparison. A 93 mg weight gainwas seen during the heating and melting under these conditions. In a controlled Ar-CO-CO2atmosphere employed in a total of 11 experiments (Table 5.1) the weight gain was a maximumof 38 mg. A direct consequence of this excess melt oxygen was an increase in the initial reductionrate. Table 5.3 provides a few additional details concerning the experimental parameters andobserved reduction rates for the experiments #1 and #12. Comparison of the rates for experiments#1 and #12 reveal about a 3-fold increase in the value of the latter due largely to the excess oxygenlevel of 24.95 wt.% in the melt. Because only one experiment was performed to study the effectof excess oxygen no firm conclusion is drawn on the role played by this parameter on the ratephenomenon.However, an important point that emerges from the above data concerns the validity andreliability of the rate values for reaction (5.1) obtained under these conditions. If the melt oxygenlevel exceeds 24 wt% prior to the reaction then the resultant data does not yield true rate valuesfor the reduction reaction represented by equation (5.1) due to the presence of either solid wiistite64Table 5.3 Rate comparison between two Fe,0-Al 20 3 melts with varying oxygen contents.Expt Gas-meltareaHeatingDurationperiodWt. gainMeltingDurationperiodWt. gainGas mixture usedduring heatingand meltingperiodsMelt oxygen at theend of meltingperiodReduction rateduring initialperiod# cm2 Min mg Min mg Wt% g-oxy/cm2.sec12 2.42 175 64 60 29 Ar 24.95 12.4 x 1041 2.68 35 5 60 13 Ar-CO-0O2 22.15 3.8 x 104or magnetite in the melt. Additionally it is possible that a concurrent ferric-to-ferrous side reactionis taking place due to the higher ferric levels in the melt. Using controlled Ar-CO-CO2 mixturesduring the heating and melting periods therefore significantly enhances the reliability of the data.5.1.2 Pseudo binary melts at 1400 °C:After obtaining information on the FeOx  system further experiments were performed in theFex0-SiO2 and Fex0-CaO systems to identify the contributions made by acidic and basic oxidespecies to the reaction rates. The results obtained at 1400 *C for these systems are presentedbelow. Fe10-Si02 melts:Four melt compositions were studied to identify the effects of silica on the reduction rate:5.4, 15.5, 24.6 and 29.3 mole% silica. An additional set of experiments was performed in whichferric oxide was added to give an initial ferric-to-total iron ratio of 0.18 in the resultant mixturewith silica contents of 16.6, 25.9 and 31.8 mole%. In all of the tests the slag mixture weighed 3grams and the melts were held in alumina crucibles. Effect of silica:Chemical analysis of the quenched slags revealed that the extent of crucible dissolutionincreased with the silica content of the melt and the weight loss data indicated lower rate valuescompared to the FeOx  melts. In the melts with the lowest silica content (5.4 mole%) the aluminaassayed 9.6 mole% whereas for the melts with the highest silica level (29.3 mole%) the aluminaassayed 15.2 mole%. The weight-time curves for the low silica melts were similar to thoseobtained in the Fex0 melts, i.e. the slope did not change appreciably with time. On the otherhand, in the melts with the highest silica the form of the weight-time curve altered. The slopewas highest in the beginning and as the reduction reaction proceeded the curve flattened offindicating the slowing down of reaction (5.1). To highlight these facts Figure 5.5 was prepared.66iteducam amt. (min)(c)2992 91 -191 -296 -293 -94 -2 93 -2 92 -291 -9Exrrs 20P co • Oil mmco • CO, • I Alm9^1^1^I0^20111111160^10^!CO^120^140^160Heduehon 6611.06.6133Figure 5.5 Effect of silica content on the weight-time relation in three Fep-Si02-Al203 melts (a) 5.4 mole % silica (b) 15.5mole % silica (c) and (d) 24.6 mole % silica.The weight-time data obtained for the Fe,(0-SiO2Al203 melts is shown in Figure 5.5. Theresults suggest that the onset of change in the slope is governed primarily by the silica contentof the melts. More important though is the fact that the curves in this figure clearly show thatwhen the melts of different compositions are exposed to the same reducing gas mixture differentrates are obtained. Comparison of curves (b) and (c) in Figure 5.5 reveal the above trend. Thefigure also shows that when two melts of same compositions (curves c and d) are exposed to adifferent gas mixtures the one exposed to a higher P ro yields the higher rate. These observationsindicate that both melt and gas compositions govern the rates in the Fe AO-SiO2-Al203 system.In all the silica-containing melts changes in the individual weight-time curves indicatedvariations in the rate values with time and to understand the nature of these changes it wasnecessary to compare the data obtained in all the experiments. It was felt that if the rate value isderived for an initial period ( t < 10 minutes) then the influence of physicochemical factors aswell as the reaction product would be minimal and the comparison amongst different runs wouldbe valid. Moreover such a comparison for various Fe 5O-Al203, Fer0-MgO-Al203 and FeO-MgOruns was found to yield a useful and reliable information and therefore the same procedure wasadopted for the Fex0-SiO2-Al203 melts. Table 5.4 lists the rate data for all the Fe„0-SiO 2-Al203melts. The data reveals that as the silica proportion in the melt increases the reduction ratedecreases.To further explain the qualitative effect of silica the results are plotted in Figure 5.6 withthe observed reduction rate on y-axis and the reaction driving force (equation 5.4) on x-axis.Three important features of this figure are: (1) Data points for all the Fe 7,0-SiO2-Al203 melts liebelow the best fit line drawn through Fe10-Al203 melts; (2) The rate values for a particular meltcomposition lie on the straight line; and (3) all the five lines (one for Fe„0-Al 203 and four forFe.0-SiO2-Al203) converge at the origin, indicating zero rate at zero driving force.Yet another important point emerges from Figure 5.6. According to this figure, for a fixedreaction driving force different rates result from differences in the melt composition. In general,as the silica content is increased the reaction rate decreased. Such behaviour suggests that the68Table 5.4 Reduction rate data for Fe i0-Si02-Al203 melts at 1400 C. .(Fea:Fe ratio <0.05)Expt Final melt composition,mole %Reduction gasmixture @ latmWt. changeprior toreductionperiod, mgaF.0std.stateliq.Reduction rate# FeO Fe203 SiO2 Al203 Pc0 Pc02 PA, gain loss g-oxy/cm2.sec26 83.4 1.6 5.4 9.6 0.98 0.02 - 10 - 0.92 17.6 x 10'27 0.11 - 0.89 6 - 0.92 2.0 x 10428 0.87 0.13 - 8 - 0.92 6.5 x 104130 73.2 1.4 13.6 11.8 0.18 - 0.82 2 - 0.75 2.6 x 10423 70.7 1.3 15.5 12.5 0.11 - 0.89 9 - 0.70 1.0 x 10424 0.95 0.05 -  1 - 0.70 6.7 x 10425 0.98  0.02 - 11 - 0.70 8.6 x 10420 60.1 1.5 24.6 13.8 0.98  0.02 - 4 - 0.55 4.5 x 10422 0.15 - 0.85 4 0.55 0.9 x 10418 0.95 0.05 - 8 - 0.55 1 3.2 x 10421 0.11 - 0.89 9 - 0.55 0.6 x 10429 54.2 1.3 293 15.2 0.98 0.02 - 2 - 0.40 2.8 x 10430 0.11 - 0.89 1 - 0.40 0.3 x 106^.,Note: (1) aF,0 values are derived using Kellogg's model.(2) Highest reduction rate data is obtained for the initial 10 minutesreduction period.(3) The high weight gain for experiment #14 is due to argon usageduring heating and melting periods. (In all other experimentsAr-CO-CO2 mixture was employed during the heating and meltingperiods).69■ FexO melts+ 5.4 mol% silica* 15.5 molc% silicaA 24.6 mole% silicaX 29.3 mole% silica#14 Special run■Increasingsilica■A #140.2^0.4^0.6^, 2.2▪ 21.8^- a^1.6• 1.4• 1▪^.21z^F. ^0.8▪ driving force, (atm)Figure 5.6 Plots of initial reduction rate versus gas phase driving force (as per equation 5.4) for several Fe,0-Si0 2-Al203melts at 1400 °C.values of the rate constant (in equation 5.3) are dependent on the melt silica level and can beclearly identified by this experimental procedure.. The dependence of the apparent rate constant,lc., on the melt composition has been discussed in some detail by Nagasaka et al. [56] and Beltonet al. [80] and its relationship with the overall rate constant in this study is pursued further in thenext chapter.The rate values obtained in this work in both Fe x0-Al203 and Fe10-Si02-Al203 systemsare much lower than those reported earlier by Nagasaka et al. [56] The two most important reasonsfor this difference are: (1) by jetting the reducing gas mixtures onto the melt surface Nagasakaet al. avoided restrictions imposed by the gas phase mass transport on the rate phenomenonhowever no such attempt was made in this study; and (2) type of crucibles used to contain themelts were different in two investigations. Further information on these critical issues is providedin the next chapter. Effect of excess oxygen:Experiment #14 was performed to assess the effect of excess oxygen on the reduction ratein silica-containing melts and for this purpose the procedure followed during the experiment #12(Fex0-Al203 melt) was repeated. As a result of argon usage prior to the reaction period a weightgain of about 89 mg was recorded, which is roughly 7 times more than the equivalent gain inweight using the Ar-CO-0O2 gas mixture, and a correspondent higher rate value of 2.5x10-5g-oxygen/cm2s was recorded during the subsequent reduction period. For comparison with theother melts of similar composition (experiments #18, #20, #21 and #22) this data point is shownin Figure 5.6. The location of the point clearly indicates an enhancement in the rate.For further confirmation of the above effect three additional experiments were performedin which the reaction gas mixture was deliberately altered in the middle of the reaction period topromote an oxidation reaction for a short while. The reduction reaction was then resumed usingthe same gas mixture as before. In these experiments oxidation reactions were controlled usingdifferent Pco/Pc02 ratios. In the experiment #28 (melt silica —5.4 mole%) the Pco/Pco2 ratio of1.7 was maintained to promote a reoxidation of reduced metallic iron to liquid ferrous oxide; and71the reaction was allowed to continue until the weight gain value (92 mg) exceeded that of weightloss (75 mg) prior to the oxidation period. In the experiment #29 (silica —29.3 mole%) however,the reoxidation was allowed so that the weight gain (39 mg) during the oxidation nearly equalledthe weight loss (36 mg). On the other hand in the experiment #30, an Ar-CO 2 mixture wasemployed during the oxidation period to promote magnetite formation according to reactionsFew + CO2(g) =Fe001.0 + COG) (5.6)3FeOok,o + CO24) = Fe304(,) + COG) (5.7)Comparison of the weight loss data between the two reduction periods- one prior to andthe other after the oxidation period, revealed that the rates were higher for the latter reductionperiod and the difference in rates increased with the increase in the level of melt oxygen. Table5.5 is prepared to highlight this fact.Fine et al. [64] have reported findings similar to those described above. The authors didnot attempt oxidation of the melt surface similar to the procedure described above but insteadinterrupted the reduction reaction by an introduction of argon into the reaction chamber. Theyobserved enhancement in the rate after resumption of the reduction reaction in their "stop andgo" experiments. Based on this finding Fine et al. concluded that the higher rates were causedby the reoxidation of iron species. Our observations are consistent with their reported findings.It is interesting to note that though Fine et al. used the jetting procedure yet found the same effect. F;O-CaO melts:A total of 7 experiments were conducted in the Fe x0-CaO system at 1400 °C using aluminacrucibles. Experiments #10, #11 and #93A were designed to study the effect of replacement ofsilica by lime on a weight-to-weight basis in melts containing a very small proportion of ferricoxide (Felf/IFe ratio < 0.05); whereas experiment #13 aimed to identify the effect of excess meltoxygen. In the experiments #87,#88 and #89 the lime content was progressively increased tocharacterize its effects on the reduction reaction rate at a Fe3+/EFe ratio of approximately 0.2.72Table 5.5 Rate comparison for the reduction-oxidation-reduction runs.Expt MeltsilicaReduction gasmixture @ 1 atmPco/Pcos ratios Weight gain aF,c, Reduction rate,g-oxy/cm2.secPco PCO3 PAP Equilibrium values OxidationperiodOxidationperiod _prior tooxidationafteroxidation# mole% atm atm atm Fe—,FeO FeO--►Fe 304 mg x 10' x 10"28 5.4 0.87 0.13 - 4.2 0.43 1.7 92 0.92 6.5 8.129 29.3 0.98 0.02 • 11.0 0.02 1.7 39 0.35 2.8 3.030 29.3 0.11 . 0.89 11.0 0.02 0.0 12 0.35 0.3 0.6Note: (I) During the oxidation period of #28 Few —4 FeO, ) reaction is promoted and the weight gain exceeds the loss duringprevious reduction,(2) In the experiment #29, the weight gain during oxidation was nearly same as the loss during previous reduction.(3) Magnetite formation is promoted during the oxidation period of experiment #30 and the weight gain exceeds the lossduring previous reduction.Results of chemical analysis revealed that crucible dissolution in the lime-containing meltswas much higher compared to the silica-containing melts. The alumina assay averaged about 23± 4 mole% in the Fez0-CaO-Al203 melts; whereas in the case of the Fez0-SiO2-Al203 melts theequivalent value was about 13 mole%. The weight loss data indicated that inspite of the higherlevels of alumina contamination the reduction rates for the Fe„0-CaO-Al 203 melts (Fel+aFe ratio< 0.05) are higher than the Fe„0-SiO 2-Al203 melts (Fe3-11:Fe ratio < 0.05) but lower than theFez0-Al203 melts. This is shown in Figure 5.7. The rate data for all the 7 experiments is listedin Table 5.6 together with the relevant details concerning experimental parameters. Forcomparison equivalent data for the Fe„0-SiO 2-Al203 melts is also included in this table.The rate data for the melts containing relatively higher level of ferric oxide (Fe3472:Fe ratio> 0.05) reveals that a progressive increase in the melt lime content from 16.3 to 28.2 mole% leadsonly to a moderate reduction in the rates. It can be argued that within the scatter of the experimentaldata the above trend is hardly discernible. However further scrutiny of the data reveals that apossible cause for this trend could be traced back to the alumina dissolution in the melts. It isparticularly interesting to compare the results of increasing lime content with the equivalent datain Fez0-SiO2-Al203 melts. Such a comparison reveals a distinct reduction in the rate values inthe latter due to increasing silica levels. This is a very important observation and its fullsignificance is understood only after a critical examination of the rate data using a mathematicalmodelling analysis.Moreover, when compared to the work of Nagasaka et al. [56] the observed rates are lowerby about a factor of 5 for reaction driving force values below 0.1 atm. Reasons for such discrepencyin the data are the same as those mentioned for the Fe x0-SiO2-Al203 melts.The effect of excess oxygen in the Fex0-CaO-Al203 melt was similar to the one observedin the Fez0-Al203 and Fex0-Si02-Al203 melts, namely a higher rate with higher oxygen levels.742.^ V . L^V.4^V. V^ LY . 0Reaction D. F. (as per equation 5.4), atmFigure 5.7 Rate versus gas phase driving force plots for pseudo-binary melts- Fe z0-Si02 and Fez0-CaO, held in aluminacrucibles at 1400'C.Table 5.6 Reduction rate data for Fe,0-Ca0-Al 203 melts at 1400 'C.Expt Final melt composition, mole % Reduction gas mixture@ 1 atmaF,0 Fe3* Reduction rateEFe# Fe0 Fe203 Ca0 Si02 Al203 pc0 Pm, PA , g-oxy/cm2.sec10 49.9 0,9 21.8 - 27.4 0.75 - 0.25 0.70 0.04 12.0 x 10411 0.18 -^• 0.82 0.70 0.04 2.8 x 10493A 49.6 1.2 21.8 - 27.4 0.03 - 0.97 0.65 0.05 0.4 x 10 .621 60.1 1.5 - 24.6 13.8 0.11 - 0.89 0.55 0.05 0.6 x 10 .687 57.8 6.8 16.3 - 19.1 0.03 - 0.97 0.66 0.19 1.0 x 10-688 47.1 5.5 23.9 - 23.5 0.63 0.19 0.8 x le89  39.5 5.5 28.2 - 26.8 0.57 0.22 0.7 x 10'690 63.8 7.1 - 16.6 12.5 0.03 - 0.97 0.60 0.18 1.5 x 10491 55.0 6.1 - 25.9 13.0 0.45 0.18 1.0 x 10 .692 49.6 5.5 - 31.8 13.1 0.30 0.18 0.3 x 10 .613 49.1 2.5 20.4 - 28.0 0.75 - 0.25 0.71 0.09 40.0 x 10414 58.9 2.4 - 24.7 14.0 0.75 - 0.25 0.50 0.08 25.0 x 10.6Note: (1) Effects of silica surface coverage and aho terms on the rates are revealed by comparing data for #11 and #21 .(2) Comparison of rates for #10 and #11 indicates the role of gas phase control.(3) Rate data for three experiments #90, #91 and #92, shows that as the silica content of the melt goes up the rate decreasesin a dramatic fashion however, a similar comparison for #87, #88 and #89 reveals that with the increase in the melt limecontent the corresponding rate decrease is only moderate.(4) Rate comparison between #13 and #10 reveals the effect of higher ferric level or the excess melt oxygen.5.1.3 Pseudo ternary melts between 1200 °C and 1400 °C: Effect of melt and gas composition:The experimental data revealed that for ternary Fe.0-CaO-SiO 2 melts held in aluminacrucibles at 1400 °C, as in previous cases, the initial rate is controlled by both gas and liquidcompositions. To highlight these facts the experimental rate data is listed in Table 5.7 and theplot of initial rate versus reaction driving force is presented in Figure 5.8 (a). Comparison withthe previous work by Nagasaka et al. reveals, as in previous cases, that the measured rates aremuch lower due primarily to the contributions from gas phase resistance and the surface blockageeffects by the non-reactant species of the melts. Effect of ferric oxide:The rate data showed that in the presence of ferric oxide the oxygen removal rates werehigher. Illustration of this is shown Figure 5.8(b). The location of line A representing melts withhigher ferric-to-total ratio (approximately 0.19) clearly demonstrates the rate enhancementphenomenon. The same figure shows that a point corresponding to the melt with a ferric-to-totaliron ratio of about 0.14 (Expt. #40) lies closer to the line A (and above the line representing meltswith lowest Fe 3+/EFe ratio). Nagasaka et al. [56] also report similar observations concerning therate enhancement effect of ferric oxide.5.1.33 Effect of excess melt oxygen:Experiment #15 was designed to assess the effect of excess oxygen. To induce higheroxygen levels the melt the procedure followed was similar to the one employed during experiments#12, #13, and #14. Higher rate value was obtained under these conditions (Table 5.7). Theexperimental data point is shown in Figure 5.8(a) and its location clearly shows this rateenhancement effect.7714001400Final melt composition, mole %^Reduction gasmixture @ 1 atm Reduction rate Temperatureg-oxy/cm2.sec16.2^0.95 0.0514000.7 xx 10 . 60.4 x 10"62.0 x 10'60.4 x 10 .60.6 x 10" 623.7 x 1043334140015.6^0.95 0.050.7 x 1400^0.82^0.53^0.14^0.8 x 1040.82^0.63^0.05^0.9 x 10"6^0.82^0.68^0.04^0.8 x 10 17.0^0.187.9^0.180.18 1200Table 5.7 Reduction rate data for Fei0-S10 2-Ca0-Al203 melts between 1200 and 1400 'C.aaFe's - 5 Pseudo ternary melts at 1400°CMr 1102.62.4O 2.2at^24^1.8f.4 1.6L4• 1.2O 0.8• 0.6"c74)^0.40.20 Location of pseudo ternary meltsrelative to pseudo binary melts• #15■ 5.4 mol% silica+ 15.5 mol% silicaO 24.6 mol% silica• melts with limesilica > 25 moll.Pseudo binary melts without time.--"--------- -- ..............Lime and si ica m Its(a)0.2^0.4^0.6^0.8Reaction D. F. (as per equation 5.4), atm2.62.42.21,0 0.2-c00.2^0.4^0.6^0.8(b)Reaction D. F. (as per equation 5.4) , atmFigure 5.8 Plots of initial reduction rate versus gas phase driving force for pseudo-ternarymelts held in alumina crucibles at 14(X) *C (a) shows location of pseudo-ternarymelts relative to pseudo-binary melts and (b) illustrates the effect of melt ferriccontent.795.1.3.4 Effect of temperature:Though most of the data was obtained at 1400 °C using the alumina crucibles, two additionalexperiments were performed- one at 1200 °C and the other at 1300 °C, to study the effect oftemperature on the rate phenomena. Melt composition and rate data for experiments #51 (1200°C) and #76 (1300 'C) is provided in Table 5.7 and the results show that the rates are similar.It is interesting to examine the results of experiments #51 and #76 by comparing them withthe melts of similar starting compositions but reduced at 1400 °C. Accordingly, these experiments(#51 and #76) are compared with the experiments #37 and #32 respectively. In all four runs, amelt weighing 3 grams was exposed to an Ar-00 mixture (Pm = 0.18 atm.) during the reductionperiod. Comparison between #51 and #37 reveals that in spite of the lower reaction temperatureof the former melt the rate value obtained is moderately higher than the latter. This is attributedto the lower alumina dissolution at 1200 °C and the resultant higher value of aF.0. Also at thelower reaction temperature the value of equilibrium constant for reaction (5.1), 1q5.1), is higherand this would result in a higher reaction driving force value according to equation (5.4). Fromthe comparison of the rate data between the experiments #76 (1300 'C) and #32 (1400 'C) asimilar trend is observed namely, a moderately higher rate at lower temperature.5.2 Ferric-to-ferrous reduction study between 1200 °C and 1400 °C:Fe --) Fe2+ reduction was studied using Ar-CO-CO2 mixtures. This reaction is representedas:Fe203(slag)m+ COG) = 2FeO (slag)(0 + CO24) (5.8)To ensure the forward progress of the above reaction however, it was of paramountimportance to choose the appropriate Pc02/Pco ratios and avoid either the reduction or theoxidation of the ferrous oxide species. Thermodynamic calculations were performed prior toeach run and the range of Pco2/Pco was determined where aF, and aF,304 would be less than 1.Both lime-containing and lime-free melts were used in the alumina crucibles and the reductionruns were performed at 1200 'C, 1300 °C and 1400 'C.805.2.1 Pseudo binary Fe„0-SiO2 melts:The results indicated that the rate was highest during the initial period and it decreasedprogressively with the passage of time. Table 5.8 lists the measured initial rate values for all themelts studied. Comparison of ferric-to-ferrous rates with the earlier ferrous-to-iron reductiondata (Table 5.4) reveals that the former values are lower by a factor of 10 or more for the equivalentmelt silica contents. Figure 5.9(a) shows the rate versus reaction driving force relationship forthe melts at 1400 °C. The reaction driving force expression for the ferric-to-ferrous reaction is:= Pf co(aF,203X(.5.8)) (atm)where K; . ,i) is the equilibrium constant of reaction (5.8). Details on the derivation of the aboveequation are provided in Chapter 6.The main feature of the plots in Figures 5.9(a), 5.9(b) and 5.9(c) is that they all show anincrease in the rate with an increase in the reaction driving force. This trend can also be seen inFigures 5.6 - 5.8. Comparison of plots in Figures 5.8(b) and 5.9(b) reveals higher rates at higherferric levels for both iron formation and Fe3+ Fe2+ reactions.To study the effect of temperature four additional experiments were performed at lowertemperatures- two each at 1200 and 1300'C. From the observed rate values listed in Table 5.8and the plot of initial rate versus reaction driving force in Figure 5.9(c) the trend of decreasingreduction rate with decreasing reaction temperature is evident. Yet another observation concernsthe weight-time curves. In all the runs these curves revealed reduction in the slope with time andsuch behavior implied the importance of an additional liquid phase resistance after the initialperiod. Further details on the time dependence of the rate are discussed in Chaper 6.PC.024.0^ (5.9)81Table 5.8 Ferric-to-ferrous reaction study in Fe,O-Si0 2-Al203 melts between 1200 and 1400 'C.Expt Final melt composition, mole % Reduction gasmixture @ 1 atmow ap,o, Fe'' Reduction rate TempEFe# Fe0 Fe203 Si02 Al 203 Pa, Pco2PA, g-oxy/cm2.sec 'C94 78.7 7.0 6.5 7.8 0.031 0.027 0.942 0.73 0.01 0.15 4.6 x 104 1400103 0.031 0.027 0.942 0.73 0.01 0.15 4.8 x 10' 140080A 67.4 6.0 16.6 10.0 0.022 0.025 0.943 0.65 0.012 0.15 3.8 x 10' 140096 0.022 0.025 0.943 0.65 0.012 0.15 4.6 x 10' 140086 57.9 5.1 25.7 11.3 0.017 0.041 0.942 0.53 0.014 0.15 2.4 x 10' 1400102 0.53 0.014 0.15 3.6 x 104 140081 0.014 0.038 0.945 0.53 0.014 0.15 3.0 x 10' 140082 51.3 4.6 31.3 12.8 0.008 0.042 0.950 0.44 0.016 0.15 1.3 x 104 140095 1.7 x 10' 140084 55.6 7.2 26.8 10.4 0.010 0.045 0.945 0.46 0.024 0.21 4.1 x 104 1400100 0.46 0.024 0.21 4.0 x 104 140099 0.006 0.043 0.951 0.46 0.024 0.21 1.0 x 10' 1400137 57.2 8.8 29.1 4.9 0.019 0.128 0.853 0.42 0.030 0.23 4.4 x 104 1300138 0.019 0.128 0.853 0.42 0.030 0.23 4.0 x 10' 130014158.5 9.0 29.5 3.00.018 0.037 0.951 0.40 0.040 0.21 1.5 x 10 .7 1200142 0.018 0.037 0.945 0.40 0.040 0.21 1.2 x 10' 1200Note: Activities of ferrous and ferric oxides are derived using Kellogg's model assuming liquid standardstates for both the species.0I0.40aa0.10.0130. melts at 1400 Cin alumina crucible• 6.5 mole% silica• 16.6 mole% silicao 25.7 mole% silicaA 31.3 mole% silicaMelt ferric-to-total iron ratio = 0.15Effect of term c level on the ratein melts containms 26 mole% silica•Ft3« 0.15TFT Effect of (conk-Tatumon the rate• Melts at 1400t• Melts at 1300 .c* Melts at 1200 .0Ferric-to-total Fc ratio is -0.22O 0.002 0.004 0.006 0.008 0.01 0 .013 0.014 0.016 0.018 0.07(c)(a)(b)0.90.8O 0.002 011k)4 0006 0008 001 0 .012 0 014 0016 0018 002Reaction driving force: ( airn )Figure 5.9 Plots of initial rate versus gas phase driving force (equation 5.9) forferric-to-ferrous reaction in Fe x0-SiO2Al203 melts (a) shows effect of silica at aconstant Fe3+17_,Fe ratio; (b) shows effect of ferric level and (c) shows effects oftemperature and Fe 3 +11Fe ratio.• 0.4034)0.20^0.10t.)Fr-^0.9.1=1•-■ Pseudo ternary F;O-Si02-CaO melts:The data obtained at three temperatures- 1400 'C, 1300 'C and 1200 °C is listed in Tables5.9, 5.10 and 5.11 respectively and the plots of initial rate versus reaction driving force (equation5.9) are shown in Figure 5.10. The observed rate values for 1400 'C runs suggest that the higherlime-to-silica ratio leads to higher rate values. Location of the melts with a higher molar ratioof 0.71 in Figure 5.10(a) supports this hypothesis. It is interesting to note that above trend wasnot repeated for lower temperature runs at 1200 and 1300 °C. (Figures 5.10b and 5.10c) Theseresults indicate that at lower temperatures the role played by the melt lime-to-silica ratio is perhapsmasked by the other important physicochemical properties such as surface tension, viscosity anddensity. Although the above conclusions are only tentative, as they are based on very limiteddata, the results do indicate a possible trend.A few experiments were done to study the effect of ferric level on the rate phenomenon.The results revealed a trend of higher rate values with the higher melt ferric contents. The dataobtained at 1200 °C is shown in Figure 5.10(c) and the increasing slopes of the lines with increasingFe/I:Fe ratios clearly presents a strong evidence of the expected behaviour. Also in Figure5.10(b) the location of a point corresponding to a melt Fe 3+/EFe ratio of about 0.2 (expt. #139)supports the above finding.5.3 Ferrous-to-ferric oxidation study at 1300 °C:A few oxidation experiments were performed for comparison with the ferric-ferrousreduction data. The weight gain data revealed the highest oxidation rates in the initial periodfollowed by a progressive drop with time. Such a trend suggests that once the ferric oxide speciesis formed (reverse of reaction 5.8) it preferentially occupies sites at the surface region and thebehaviour leads to the lowering of the oxidation rate. An illustration of this is provided in Figure5.11.84Table 5.9 Ferric-to-ferrous reaction study at 1400 'C in Fe,0-SiO 2-CaO-Al 203 melts. (Fe i 'lEFe = 0.15)Expt Final melt composition, mole % Reduction gas mixture@ 1 atmaho abao , Reduction rate# Fe0 Fe2O 3 Ca0 SiO2 Al203 Pcc, Pm, PA, g-oxy/cm2.sec41 29.3 2.6 20.0 32.6 153 0.017 0.014 0.969 0.45 0.010 3.0 x 10442 0.018 0.006 0.976 0.45 0.010 2.8 x 10446 37.1 3.3 15.5 29.0 15.1 0.019 0.009 0.972 0.49 0.011 2.8 x 10447 0.018 0.006 0.976 0.49 0.011 2.4 x 10443 33.1 2.9 20.3 28.4 15.3 0.019 0.009 0.972 0.48 0.009 3.4 x 10448 0.019 0.006 0.975 0.48 0.009 3.8 x 104Note: Activities of ferrous and ferric oxides are calculated using Kellogg's model (assuming liquid standard state for boththe species).Final melt composition, mole %Reduction rateTable 5.10 Ferric-to-ferrous reaction study at 1300'C in Fei 0-SiO2-CaO-Al203 melts.Table 5.11 Ferric-to-ferrous reaction study at 1200'C in Fe,0-Si0 2-Ca0-Al 203 melts.Expt Final melt composition, mole % Reduction gasmixture @ 1 atm FrO apo,Fe'' Reduction rateEFe# FeO Fe203 Ca0 Si02 Al 203 Pm Pm, PA, g-oxy/cm2.sec52A32.9 2.9 22.1 36.7 5.40.029 0.044 0.927 0.42 0.013 0.15 2.5 x 10'63 0.042 0.063 0.895 0.42 0.013 0.15 3.5 x 10453 0.024 0.044 0.932 0.42 0.013 0.15 2.2 x 10454 0.016 0.044 0.940 0.42 0.013 0.15 1.7 x 104108 0.017 0.036 0.947 0.42 0.013 0.15 2.0 x 1045537.2 3.3 22.6 32.0 4.90.029 0.044 0.927 0.53 0.011 0.15 2.1 x 10'106 0.026 0.039 0.927 0.53 0.011 0.15 2.1 x 10456 0.030 0.022 0.948 0.53 0.011 0.15 2.8 x 10.'5741.7 3.7 17.1 32.6 4.90.029 0.043 0.928 0.49 0.013 0.15 2.3 x 10'58 0.030 0.022 0.948 0.49 0.013 0.15 2.1 x 10 . '107 0.021 0.039 0.940 0.49 0.013 0.15 2.0 x 10'59 43.4 2.5 16.7 31.8 5.6 0.030 0.022 0.948 0.54 0.008 0.10 1.3 x 10460 34.1 1.9 21.4 35.7 6.9 0.030 0.022 0.948 0.51 0.010 0.10 1.1 x 10'61 0.030 0.022 0.948 0.51 ' 0.010 0.10 1.0 x 104143 35.7 4.5 22.9 32.9 4.0 0.022 0.036 0.942 0.46 0.021 0.20 4.6 x 10'0 0.0I 0.02 0.03 0.040 0.02 00-•o::)0.8c11^0.7eve0.• 0.2C.) study at 1400tPseudo ternary melts in alumina crucibles• Haar tinarorilica MAO 0314.^Meter Genera-silica 0.60• Mater lirnerarilica ratio -- 0.23Ferric-to-ferrous study at 1200tPseudo ternary melts in alumina cruciblesx FtfIX-10-4044 Fe tato • 41204- • • Ferric-arose Fe ratio a 0.13A^kft% 404041 Fe tauo^0.10• *8.^a• •••••^••• -•Ferric-to-ferrous study at 1 300tPseudo unary melts in element eructates• Higher ferric 40-40tai non rano• Mcks at same gas compositeon exposedto &fluent reaucteen stiasures* Melts of sarne,camposinan exposedto tame gas manure0^0.01^0 02^0.03^0 04RCX110tItirivtrn forcc. (win)Figure 5.10 Rate versus driving force (equation 5.9) plots for ferric-to-ferrous reaction inFex0-Si02-CaO-Al203 melts (a) 1400 'C runs; (b) 1300 'C runs; and (c) 1200 'Cruns.88go3.0113.01 -3.009 -3.001 -3.007 -3.006 -3.005 -3.004 -3.003 -3.032 -3.001 - ausguelli• ■a nagfa••■ aINasMg■•Ferrous-to-femc reaction studyPseudo ternary melts at 1300 TExpt 175(a)3 •^13.00160 40^110^120Oxidation duratioa. min160 200 240 2103.001$ • a3.0014 a3.0013 a3.0012 • ■^• a^■3.0011 ■^an a •^■•^■^a^■ a3.001 • an^• •^•■•^■1asa 3.0009 ■ a"r• 3.80033.0007 •II^■ ■3.0006 a (b)3.0003 a3.0004 • Ferrous-to-fanc reaction study3.0003 Pseudo ternary melts at 1300'C3.0002 Expt #713.0001 ■^■3 I f0^40^80^120^160^200^240^280Oxidation duration. minFigure 5.11 Weight gain curves for ferrous-to-ferric reaction at 1300 *C (a) meltF e 3 +ff-Fe ratio 0.05 (b)Fe 3 +II.Fe ratio 0.10.89The values of the highest oxidation rate at t —> 0 for the five runs are listed in Table 5.12.Some important experimental details are also provided in the same table. The limited datagenerated in this work is considered insufficient for carrying out a detailed mathematical analysis,however the available data suggests that a model similar to that for the ferric-ferrous reaction canadequately describe the overall oxidation reaction. The ferrous-to-ferric reaction2Fe001.0 + CO2(0 = Fe20302.0 + CO(g) (5.10)is essentially the reverse of reaction (5.8) for which the following equation can be derived for thereaction driving force at t -4 0,ba PF .203 COf - Pcb02 res^2(5.10)."Fe0(aim)(5.11)where 1q5.10) is the equilibrium constant for the reaction (5.10). In Figure 5.12 the initial oxidationrates (Table 5.12) are plotted against the driving force values derived using equation (5.11). Therelationship indicates that like ferric-to-ferrous reaction the ferrous-to-ferric reaction also iscontrolled by both gas and melt compositions. In addition, from the nature of theweight-versus-time curves in the Figure 5.11 it can be inferred that with the passage of timeadditional factors such as surface blockage, mass transport in the liquid phase and physicochemicalproperties of the melt such as surface tension, diffusivity and density become increasinglyimportant in dictating the progress of the reaction.5.4 Ferrous-to-magnetite oxidation study at 1400 °C:The results of all the previous runs suggest that after an initial period the rates are influencedeither by melt physicochemical properties or the diffusion of both product and reactant speciesin the melt. In the case of iron saturation runs in ferrous oxide melts the sinking metal may haveinduced melt movement and caused a moderate increase in the rates at higher Pa, values (Figure5.1). On the other hand in silica-containing melts the above trend was reversed90Table 5.12 Ferrous-to-ferric reaction study at 1300'C in Fei0-SiO 2-CaO-Al 203 melts.Expt Melt composition, mole % Reduction gas mix-ture @ 1 atm °kW ar,203 Fe'' Reduction rateEFeFeO Fe2O3 CaO SiO2 Al 202 PCO PC01 P Ar g-oxy/cm2.sec74 35.1 0.9 20.7 34.6 8.7 0.017 0.035 0.948 0.54 0.001 0.05 2.5^104x75 44.1 1.2 16.2 30.6 7.9 0.014 0.044 0.942 0.62 0.001 0.05 5.0 x 10' 272 0.018 0.035 0.947_ 0.62 0.001 0.05 3.8^104x73 39.0 1.1 20.7 29.7 9.5 0.018 0.035 0.947 0.64 0.001 0.05 4.5^10 .1x71 42.3 2.4 16.2 31.1 8.0 0.013 0.025 0.964 0.57 0.005 0.10 < 1.0^10.1x0.03 0.040^0.01^0.020.9▪• 0.5c:o• 0.4• 0.3.1=1^0 .10.10Ferrous-to-ferric reaction studyPseudo ternary melts at 1300 °CIA^Ferric-to-total Fe ratio = 0.1• *^Ferric-to-total Fe ratio = 0.05• Basicity = 0.74' Basicity = 0.6• Basicity = 0.53 *-•--*^IReaction driving force. (atm)Figure 5.12 Relation between initial oxidation rate and gas phase driving force (equation5.11) for FeO-SiO 2-CaO-Al20 3 melts at 1300 C.92possibly due to the increased melt viscosity (Figure 5.5). To further understand the role playedby the solid reaction products an additional set of experiments was done in which the magnetitewas formed according to reaction (5.7).5.4.1 Simple Fe10-Al203 melts:Results of ferrous oxidation to magnetite for two runs (#131 and #132) are illustrated inFigure 5.13. Both the curves reveal an initial linear portion representing the highest rate valuesfollowed by the progressive flattening of the curves indicating a slowing down of the oxidationreaction. It is believed that after about 10 minutes or so the surface of the melt is increasinglycovered by magnetite and this leads to a reduction in the available area for the gas-melt reaction.Microscopic as well as a visual examination of the melt surfaces after quenching support theabove hypothesis.5.4.2 Complex Fe.0-SiO 2-CaO-Al203 melts:Figure 5.14 illustrates the results obtained in experiments #44 and #45. The form ofweight-time curves for these two experiments is essentially similar to the ones shown in Figure5.13 for the ferrous oxide melts in alumina crucibles. Hence we can draw similar conclusions,i.e. the highest weight gain is observed during the initial period when the melt coverage bymagnetite is minimal; however with the passage of time the weight gain values are progressivelylowered due to the increasing magnetite coverage of the melt.It is interesting to note that the weight-time curves for both simple and complex melts havea few common features: (1) for a particular melt composition the initial slope of the curve varieswith the gas composition; (2) at a higher Pco /Pm ratio the slope is steeper and vice versa; (3)when the Pco2/Pco is low the higher initial rate is maintained for a relatively longer duration. Allthese features suggest that once the magnetite is formed at the gas-melt interface it does not sinkbelow the surface and such behavior demonstrates an important role played by the meltphysicochemical properties in this type of system.93•■■ill —1.66 —1.115 —1.84 —■1.83 —•1.88Fe.0-Al20 3 melt at 1400 °C •■ ■■ •■ ■■ ■ ■ •■■■(a)Exist. 0131Magnetite formation reaction at 1400'CFerrous oxide melt in alumina crucibleP^0.12=01.6260Oxidation time (mia)0^20^40 100^120 11541.1521.1151.8481.6461.6441.8421141.83811361.8341.132113112$1.82611241122112■■^■ ■^ ■ ■ ■■^■^■ le ^■ ▪ ■ ■ ■^■■••Fe20-Al203 melt at 1400 °C (b)ExpL 0132Magnetite formation reactioa at 1400 tFerrous oxide melt in alumina cruciblePcal 72 atm0 20^40 60Oxidation time (min)80 100 120Figure 5.13 Weight gain curves for ferrous-to-magnetite reaction in Fe xO-Al203melts exposed to different Ar-CO 2 mixtures at 1400 'C (a)Pco2 = 0.12atm and (b) Pco2 = 0.72atm.94Magnetite formation reaction at 1400 'C ■■ ■ ■Expt #45Pco2/Pco-50^■ ■■ ■■+ +++Expt #44■ ••• + +^ P co2/P co-35I•3.0213.023.0193.0183.0173.0163.0153.0143.0133.0123.0113.013.0093.0083.0073.0063.0053.0043.0033.0023.0013 11111M - 1,1111M0^20^40^60^80^100^120^140^160Oxidation time, (min)Figure 5.14 Weight gain curves for ferrous-to-magnetite reaction in Fe,0-Si02-CaO-Al 203 melts at 1400'C.Chapter 6Discussion6.1 Ferrous-to-iron study:6.1.1 Findings of present workA detailed analysis of the weight-time behaviour has revealed two trends- one showingdecreasing rate with time and the other showing an opposite pattern. A majority of the dataconformed to the former category. However the fact that the rates did increase with time in fewruns (Figure 5.1) indicated that melt movement induced by the sinking iron was a distinctpossibility. A close examination of individual weight-time curves for various melts indicatedthat the on set of the change in slopes varied with both the melt and gas compositions. (Figure5.5) On the contrary, it was observed that the slopes of weight-time plots for all the runs remainedessentially unchanged during the initial reduction period of about 10 minutes (Figures 5.1 and5.5). Therefore it was felt that a meaningful comparison amongst different runs would be morevalid for the initial period (when the influence of physicochemical factors as well as the reactionproduct would be minimal) and based on this additional useful information concerning thechemical reaction was generated.Rate data was obtained for the initial period for all the melts using equation (5.2) andcomparison amongst individual melts highlighted a few important trends, namely- decreasingrates with increasing silica (Table 5.4), decreasing rates with decreasing Pco (Table 5.1),decreasing rates with increasing distance between the crucible top and the melt surface (Table5.2 and Figure 5.3) and higher rates with higher melt oxygen contents (Tables 5.3 and 5.5). Theseobservations implied that the rate of reduction reaction is governed by both gas and meltcompositions. It is worth noting that these findings concerning the effects of silica and Pc0 onthe reduction rates are consistent with those reported earlier by Nagasaka et al. [56].96Yet another finding that was in agreement with the work of Nagasaka et al. concerned withthe rate enhancement due to higher ferric levels in the melt. Examination of the Fe-0 phasediagram (Figure 2.1) reveals that the level of oxygen in the melt also governs its constitution andat higher 0/Fe ratios the formation of solid wiistite and magnetite is favored. The role of thesefactors on the ferrous oxide reduction is not clearly understood. Nagasaka et al. addressed thisissue in their work and observed that the apparent rate constant, k., was exceedingly influencedby the melt Fe3+/Fe2+ ratio. The authors noted that k, values decreased with silica and that limehad the opposite effect. Since the overall rates obtained in the present study were influenced bythe gas phase mass transfer the results cannot be compared directly with the data reported byeither Nagasaka et al. or Kim et al. [66]. However, with the help of mathematical analysis anattempt is made to link all the three sets of data and offer an explanation on the commonobservations concerning ferric oxide and silica effects on the rate phenomena. The mathematicalscheme provided an independent means of calculating intrinsic rate data at different temperatures.It will be shown later that the intrinsic rate values obtained thus are in agreement with otherstudies.6.1.2 Mathematical AnalysisSeveral other possibilities were examined before finalizing a mixed gas phase mass transferand chemical reaction control model. However it was noted that the observed results could besatisfactorily explained in terms of the proposed model alone. Before discussing the mathematicalformulation of a mixed control model a few important reasons for the failure of other models areoutlined below. Justification of the proposed modelIt was expected that in any mathematical modelling scheme, gas phase mass transfer cannotbe overlooked due to the choice of a stagnant system. However, the main advantage of such anarrangement is that the gas phase contribution can be easily characterized and thereby the rolesplayed by other interfacial and liquid mass transfer effects, if any, could be identified. For this97reason, in the mathematical analysis a gas phase control model was used as a starting point. Arate expression was formulated on the assumptions of an instantaneous chemical reaction and noliquid phase mass transfer resistance. The predicted rate data was then compared to the measuredvalues and it was noted the model was applicable only to the simple Fe 10-Al203 melts. For allother 36 melts in the pseudo binary and ternary systems the model over-predicted the rates. Tohighlight these facts Figures 6.1 and 6.2 are prepared.Figure 6.1(a) shows an agreement between the predicted and observed rate values for 11runs in the Fex0-Al203 system however, a similar comparison for the melts in Fe x0-SiO2-Al203system in Figure 6.1(b) reveals distinct differences between the two sets of data. Rate versusdriving force plots in Figures 6.2(a) and 6.2(b) also show a clear disagreement between the modelpredicted and measured rate data for the melts in Fe 10-Ca0-Al203 and Fe10-CaO-Si02-Al203systems respectively. In a few cases in the latter system the model overpredicted the rates by ashigh as 8 to 10 times and therefore the gas phase control model was abandoned. Yet anotherreason that supported this decision was the fact that in all four special experiments in which excessoxygen was present in the melt the resultant rates were greater than the equivalent data for meltsexposed to the same reducing gas mixture but contained no excess oxygen.The liquid phase control model was written next and again no agreement between themeasured and predicted data could be reached. This model was discarded because the measuredrate data showed clearly that for any given melt composition, rate values changed with a changein gas composition. (Tables 5.1,5.4,5.6 and 5.7)An unsteady state liquid diffusion model was considered as a third possibility. In the modelformulation, quasi-steady state was assumed in the gas phase together with unsteady state in theliquid. The diffusion of reactant species in the liquid was assumed to follow Ficks Second Lawand the heterogeneous reaction was considered to be fast and not rate limiting. Essentially thesituation was considered to be equivalent to the evaporation from a liquid surface for the derivationof the governing equations.982.^^0^ model+ measureda 6(a)^0.02^0.04 0.06^0.08 4.1 0.12^0.14^0.16^2.2 ^ model2 + measured 01.81.6 ^1.4(b)1.2 ^10.80.6 a0.4a0.2 - ^^ +* 1++0 11111111 I0^0.02^0.04^0.06^0.08^0.1^0.12^0.14^0.16^0.18P co^aFeo P CoD.F., 2, (atm)Figure 6.1 Comparison of model predicted and measured rate data for the melts at1400 °C. Model predicted values are derived assuming only gas phasecontrol. (a) Fex0-Al203 Melts; (b) Fex0-SiO2-Al203 melts.1 +^aFeo990 0.02 0.04 0.06 0.08 0.1 0.12a^ model+ measured0^0^ model0measured(a)(b)▪ 0.50".^• 1.6• 1.5Cid^1.4Fr• 1.30• 1.2L• 1.1• 1it0.^0.02^0.04^0.06^0.08^0.1^0.12Pco^'aFe0 - PCOD.F., 2, (atm)1+ Ke • aF„,Figure 6.2 Comparison of model predicted and measured rate data for the melts at1400 °C. Model predicted values are derived assuming only gas phasecontrol. (a) Fe1O-Ca0-Al203 Melts; (b) Fex0-CaO-SiO2-Al203 melts.100With these equations a relationship between the fraction reacted (ratio of moles of oxygen reactedat time t to the total moles available for reaction) and .q(D it/a) was obtained. An illustration ofthis is shown in Figure 6.3. None of the data from the present work complied with the trends ofthe lines shown in this figure and therefore the model was abandoned.Finally, a gas and liquid mass transfer model was tried. For predicting rates using thismodel an expression was developed in which all parameters except k L (liquid phase mass transfercoefficient) were known. Therefore, by choosing appropriate values for kL calculated rates couldbe matched with the measured data. The predicted values of k L thus obtained could not becompared with any other data because of lack of such information in the literature. Thereforealternate methodology had to be explored to test the validity of the k L data. It was thought thatif the predicted values varied according to the melt composition in some expected manner thenperhaps the model could be validated on this ground. For example, based on the viscosity anddiffusivity data in the literature one expects an increase in lc /. with the addition of basic oxidesand an opposite trend after the addition of an acidic oxide like silica. Upon close examinationof the fitted kL for various melt systems the above trends were not evidenced. To highlight thisbehavior Table 6.1 is prepared.Further scrutiny of the fitted kL data was carried out before rejecting the gas + liquid model.Using the simple relationship,D02_- 8(6.1)the boundary layer thicknesses were calculated for various melts and an anomalous result wasobtained. It was noted that for a few lime and silica-containing melts the predicted boundarylayer values exceeded the actual melt height. Illustration of this is shown in Table 6.2. Theoxygen diffusivity values for use in the equation (6.31) were obtained assuming diffusion analogof Walden's rule [1,90] i.e. D02_ = constant. The value of the constant was derived usingSutherland equation [91].1018I^I^I3^4^5137L12 6 7Liquid phase —controlmixed controlGas phase control1.00.80Mr 0.6M.,0.41^2A=0.10.2Figure 6.3 A plot of fraction reacted versus -■ID • t/I, obtained using unsteady model. (fromref. 92) The hatched portion shows the approximate range of the measured dataand X, is the dimensionless parameter. A = DadD (Li - Lir102Table 6.1^Values of fitted liquid phase mass transfer coefficients, k L, obtained in the ferrous reduction study at 1400 *C usinggas and liquid mass transfer control model.Melt type Silica Lime Fitted k L valuemole % mole % cm/sFez° —Al203 . . 10.0 x 104FeOz  — Mg0 - - 7.0 x 104FeOz ^— Mg0 —Al203 - - 2.5 x 104Fez° —Ca0 — Al203 - —22 7.5 x 104Fez° — Si02 — Al203 —5 - 5.0 x 10'—16 - 2.5 x 10' s—25 - 1.3 x ar s—29 1.0 x 10'sFez 0 —Si0 2 —Ca0 —Al20 3 —28 —15 1.2 x 104—31 —19 0.85 x 104—27 —19 1.3 x 104Table 6.2 Predicted boundary layer thicknesses using gas and liquid mass transfer control model.Melt type Expt2ACaOmole %SiO2mole %Fitted kLcm/s1 0 x 1 0' 510xEstimatedD02 _cm2/sPredicted 8 MeltheightAvailableoxygen forthereductionOxygen inthepredicted 8Oxygenremoved in theinitial periodM^mg mg mgFe i0-Al 203 4.2 x 10 .6 200 3645^818 45 64.2 x 200 2410^508 42 38Fez0-^26^—5A 12 0 3Si025 x 104 3.5 x 10 .6 400 2910 540 74 325^—14 2.5 x 10' s 3.0 x 600 3290^I^461 84 1220 —25 1.5 x 104 2.5 x 104 665 3075^I^406 88 729^—29 1.0 x 10 -5 2 x 104 1000 2900 357 123 4.6Fe x0-^10^—17CaO.Al 20,-7.5 x 104 7 x 104 933 3860 315 76 17Fex0-^31^—15^—27Ca0-A 1 2 0 3SiO21.2 x 10 -5 4 x 104 3330 3235 274 274 3.533^—19^—31 0.9 x 104 4 x 10'6 4440 3345 218 218 1.635^—19^I^—27 1.3 x 104 4 x 104 3080 3280^I^245 230 3.2Note:^Boundary layer thicknesses, 8, are predicted using equation (6.37).In the above calculations equation (6.1) was used for estimating boundary layer thicknesseshowever the same equation can be employed for the determination of k i values by fixing theboundary layer thicknesses. The information obtained thus can then be compared with the setof fitted kL values derived independently from the gas + liquid model. The data listed in Table6.3 shows a lack of agreement between the two sets of k L values. The results implied that thepredicted kL values using the gas and liquid control model were inconsistent. On the contrary,using a mixed gas and interfacial reaction control model more meaningful data was generatedand therefore this model was finally accepted. Mixed control model formulationThe reduction of ferrous oxide in the slag melt was carried out using the carbonmonoxide-containing gaseous mixtures. Schematic representation of this arrangement isillustrated in Figure 6.4 to show the directions of movement of reactant and product gaseousspecies relative to the melt surface and the gas and liquid phase resistances to the reductionreaction (5.1). The theoretical value of the gas phase mass transfer coefficient, k g, can be calculatedfor stagnant conditions using equation (5.5) i.e. D _8,L,18 where 8 represents the distance betweencrucible top and gas-melt surface. The present model formulation is restricted to the initial 10minute period during which the role played by k L is assumed to be of little consequence.For the ferrous oxide reduction reaction (5.1) we can write the following flux equations:nco =A° • kg(40 — C;) mole^ (6.2)n CO2 = A 0 kg • (Cc'o2 — 402.) mole (6.3)where Ao is the gas-melt area in cm2, kg is gas phase mass transfer coefficient, in cm/s andconcentrations of gaseous species are in mole/cm 3. The superscripts * and b refer to interfacialand bulk concentrations respectively.105silica D02- D02.. * Expt. kL values estimated using equation (6.1) atdifferent 8 valuesmole% cm2/sx 106Nx 10"cm/s = 110µm = 500 pm = 860 pmLime-free melts — 5 3.5 1.22 #26 5 x 10 -3 18.2 x 104 4.0 x 10 .5 2.3 x 10-51.22 #28 5 x 104— 29 2 1.22 #29 1 x 9.1 x 104 2.0 x 104 1.2^104xLime-containingmelts(— 15 mole% lime)—27 4 1.02 #31 1.2 x 10' 5 36.4 x 104 8.0 x 10 -5 4.7 x 1041.02 #32 1.2 x 10'sLime-containingmelts(— 19 mole% lime)— 29 4 1.02 #35 1.3 x 10436.4 x 104 8.0 x 104 4.7^10'x4 1.02 #33 0.9 x 104MeltTable 6.3 Comparison of k L values obtained from two independent sources.Note: (1) Fitted k' values using gas and liquid mass transfer control model.(2) Boundary layer, 8, value of 110 gm as per McCarron and Belton [94].(3) Boundary layer, 8 = 500 p.m based on MOssbauer data in the present work.(4) Boundary layer thickness value of 860 gm as per M.E. Fraser and A. Mitchell [93].(5) According to Sutherland equation D 0 2...p^KeT— [4x.R 91 ]02_Crucible(a)GasCO2Melt5thCOy=0= CL, and cal- a = c;:oiMelt surfaceBoundary layerFe0(,)+COG)=F4 ) +CO24) Crucible topDco .<0ks — 6Counter diffusion ofreactant and product gaseous speciesCO,COStagnant meltFigure 6.4 Schematic representation of (a) movement of gaseous species relative tomelt surface; (b) gas and liquid phase resistances to the reductionreaction.107The rate of the chemical reaction can be written as(6.4)r =Icc • A aFeo-Ccowhere lc, is intrinsic rate constant of reduction reaction in cm/s, Ke is equilibrium constant ofwustite reduction reaction and A is the available reaction area in cm 2. Equation (6.4) can bereadily derived on the assumption of a reversible ferrous-to-iron reaction. Thus, individual rateexpressions were written for the forward and backward reactions and the overall expression wasobtained by subtraction of the latter from the former. Kim et al. [66] have followed a similarprocedure for deriving the rate expression for the melts at 1600 °C. It is worth noting here thatthe reaction driving force term used by Nagasaka et al. [56] can also be obtained using the aboveprinciple.The following assumptions are made in the model formulation:(1) steady state conditions prevail during the reduction reaction;(2) at the crucible top the composition of gaseous species (Ar, CO and CO 2) equals their bulkcomposition in the alumina reaction chamber,(3) diffusion of product and reactant gaseous species takes place under stagnant conditions;(4) Dc0 _ gas = DCO2 — gasAt steady stater =nco=nco2^ (6.5)By mathematical manipulation the two interfacial concentration terms can be eliminated to yieldr^ C b1^co2^ molesAo ^A° ^A°(C.0^2.s^+ +  A°^aFolC`^cm 2 leksaf.ok,and the same can be expressed in the practical form as16/(R • T) Pt1 ^1^1^b ^°2 2rtA° [^A, erkg.„0 r co aFeolC`^cm .s(6.6)(6.7)108The term in the first bracket represents the overall rate constant in g/cm 2.atm.s and thesecond term is the reaction driving force in atm. The area ratio, A/A 0, represents the fractionalcoverage due to the reactant species. It will be shown later that this ratio varies from melt to meltand generally lower values are obtained when a non-reactant surface active species like silica ispresent.Equation (6.7) can also be expressed as1 p b^j (6.8)W,^ . co, g 1 b+ 1 + 1 = Pco 2A0 .t^k fla^k ekosF.0 aFeo^CM .s. lC.cA Fe0^gwhere w1 is oxygen loss (g) and t is reduction time (s).In the right hand side of equations (6.7) and (6.8) all the terms except k 0 and A/A0 are knownand therefore once the fractional coverage data is obtained ko remains the only unknown. Fittingof the experimental data to the model will thus yield the value of k o. This can then be comparedto the results of other researchers. Therefore it is important to derive an expression for the A/A.term and understand its significance.Since the term "A" represents the gas-melt area available for the reaction (5.1) and A. isthe total melt area exposed to the gas their ratio corresponds to a dimensionless area parameter.In the simplest case of a pure ferrous oxide melt being held in an iron crucible the cross sectionalarea A. would equal the reaction area and therefore the A/A. ratio would have the highest valueof 1. However when molten ferrous oxide reacts with the container material (e.g. alumina) thedissolved species is expected to block some reaction sites and effectively lower the reaction areaby way of a dilution effect. Addition of an acidic oxide species like silica or phosphorus pentoxideto the melt may essentially have a similar effect however, due to the surface active nature of thesespecies an additional contribution may result via an excess surface concentration effect. It isworth mentioning here that Kim et al. [66] have employed similar approach for the determinationof the reaction area. The authors studied ferrous reduction reaction using the melts from theFe10-CaO-Si02-Mg0 system and developed the fractional coverage expression for their melts.109Alternately it is also possible to view the melt surface coverage by oxygen anions, 0 2",[80,34,68] and the reduction reaction asCO + 0 2- = (6.9)andFe2+ +CO2 - =Fe +CO2 (6.10)In this sense the area term A will account for the 0 2" coverage. When the surface is occupiedby the 02" anions alone then the fractional coverage will be unity. Because acidic oxides are glassformers they promote complex anion formation and lock up 0 2- ions which otherwise would beavailable for the reaction (6.9). This is equivalent to saying that any oxygen atom attached tosilicon atom/s is not a free 0 2' anion and this is consistent with the polymerization model proposedby C.R. Masson [129]. Addition of basic oxides to the melt may have no significant effect onthe area term "A" and one expects that in the presence of ferric oxide the area would increasebecause ferric anions can release the oxygen for continuation of reaction (6.9).A scheme was developed to quantify the fractional coverage parameter, A/A 0, in terms oftwo separate phenomena, namely - the dilution effect and the excess surface concentration effect.According to the proposed scheme, reaction (6.9) is dependent on the availablity of both the free02' anions (those associated with basic oxides) and the number of ferric anions that can provide02" (i.e. 02- associated with any complex anion that can release oxygen). Therefore the availablereaction sites or the A/A0 term can be represented asavailable reaction sites =1—net unavailable sites (6.11)where the net unavailable sites are those covered by anion species other than 0 2". Threecomplex anion species have beeen considered previously [95,96,97] - F e0; - , Al03 - andIt is appreciated that in any polymeric melt the oxygen coordination of various cations is governedby various factors - such as proportions of basic and acidic oxides and ionic radii of cations.Therefore it is very difficult to assign a particular formula to the complex anions in such melts.In the presence of amphoteric oxides (ferric oxide and alumina) in the melts further complicationsarise in the identification of the coordination states of Fe l+ and Al.+ Although from the knowledge110of the coordination states these cations should be coordinated 6-fold the facts mentioned abovesuggest that the oxygen coodination of Fe3+ and Ala* may vary from tetrahedral to octahedral inthe complex melts. Such predictions have been confirmed, to a limited extent in the case of Fe 3+cations, by the M6ssbauer spectroscopy analysis of the quenched slags. [98,99] Researchers havenoted that coordination of Al a* decreases from six to four with increasing basicity. [100] Underthe circumstance the complex A10: - anions are formed in these melts. In view of the abovedifficulties the selection of Fe03 -,A10; - as anion complexes is made for convenience only.It is important to note that whether Si0: - is polymerized or not the calculation will givethe approximate area coverage because it is based on Si0 2. Of the three anions, the latter two donot take part in the reaction (i.e. they do not release oxygen) whereas the ferric anions behavedifferently. These can release oxygen from their coordination and therefore their contributionresults in the decrease in the net (or actual) unavailable sites. Table 6.4 is prepared to highlightthis effect. The same table shows how the melt composition affects the fractional coverage term.The following assumptions were made in the derivation of various expressions listed in Table6.4.(1) alumina is not surface active;(2) alumina and ferric oxide are present in some form of cation-anion complexes but thesecan be represented by A10 1 _5 and FeOL5 species respectively; (this choice can be justifiedon the basis that in both alumina and ferric oxides the ratio of cations to anions is 1:1.5.)(3) volume fraction of the individual species equals the mole fraction; and(4) in the ferrous oxide melts both silica and ferric oxide are surface active and the excesssurface concentration of the latter is controlled to an extent by the melt silica content.111Table 6.4^Expressions for the available reaction sites.Melt type Fractional sites covered by anions otherthan 02'Net or actualunavailable sites forreactionavailable^= 1 - net unavailable sitessitesFeO; - A10: - SiO44—Pure FeO 0 0 0 0 1FeOz NF,,,,j+ N7,0 ,, 0 0 0 1FeO, ^— A10 15FeO, ^—MgO —A10, 3NF,01.3 + N7,013 Nm0 1 .50 NA,013—N,,,,1s—Npio" 1 — N^+N^+N"A1015^Fe0 1 5^Ft015FeO, — Mg0 Nriou + N7,0,3 0 0 0 1Fe,0 —Ca0 —A10, 5 Nk...0.3 + Koij N41015 0 NAK,13— Nirp Ls — N7,01j 1 —N^+N^+ N"mo 1 5^Fe015^Fe03.5Fe,0 —.110 2 —A101j Nr.00 + NZ°, j N41035 NS101+ Nrt02 N41013++N^+N"4. N:02—Nho i 4 — 40 1.11 —N,,,,,^—Nen — Mun +Ng. A^+NrgIA,..... 3 3^..,... 2^*2^. 0.., I 5^. 41..35re,0 —S00,— Ca0 —1.10tsAl peoli+ N;1,01, N.41035 N.Ss03+ no, N4101 . 5 + NS102 4. ISIZO2—Nhou —40131 — Min^—Ne1il '-'1■1;',, + N c n^+Nr"h...,15^...2^*2^, e.... I 5^. a-3 5Note: (1) Decrease in net unavailable sites is caused by the ferric anions as they can provide 0 2" for the reduction reaction.Moreover, due to their surface active they can displace some of the alumina anions.(2) The available reaction sites parameter is equivalent to the A/A t, ratioBased on the above scheme the following general expression was developedA = 1 —NA0 Nsw2 NT° + NF4,0 + 4:01.5^2^1.5^13(6.12)where N ;  the mole fraction of species i and the superscript XS is the excess concentrationof the surface active species. It was easy to calculate the mole fraction of species that are notsurface active. However for the silica and ferric oxide species additional expressions werenecessary to quantify the excess surface concentration. This task posed a special challenge becauseour melts did not qualify strictly for an approach using Gibbs adsorption isotherm equationT.. _ 1 I aa 1RT talnadmolecm 2(6.13)in which Fi is the excess quantity of solute i per unit area of interface (mole/cm2), R is thegas constant (calPK.mole), a is the surface tension (cal/cm2) and ai is the activity of i in the melt.Two important reasons for the inadequacy of above equation for the present situation are: (1) theGibbs adsorption equation was developed for a binary system whereas our melts contained atleast three or more oxide species; and (2) equation (6.13) accounts for the excess surfaceconcentration of a single surface active species whereas in the melts in this study both ferric oxideand silica are surface active species. Furthermore it is possible that other physicochemicalproperties such as density and viscosity of the melt may independently alter the excess surfaceconcentration. It is appreciated that in ionic melts the situation is further complicated due to thenature of various ionic species and therefore quantitative estimation of surface active speciesbecomes more difficult in such melts.In spite of these limitations a large number of workers have used this equation to explainlowering of surface tension values of the melts due to addition of solute to these melts and thereforean attempt has been made in the thesis to apply the concept of the Gibbs adsorption isothermequation. In this regard a reference by Kim et al. [66] is of particular interest to this work becausethese authors also studied the ferrous reduction reaction and they report the same approach for113the determination of the reaction area in the Fe z0-CaO-SiO2-MgO system. In a separate studyPal et al. [101] estimate the lowering of the reaction area due to P2O5 coverage using a similartechnique. In the present study the concept of the Gibbs adsorption isotherm is employed (to anextent, in an indirect way) to arrive at the excess surface concentration data in agreement withthe observed surface tension behavior in complex melts. The basis for this is briefly discussedbelow.P. Kozakevitch and other researchers [67,102,103] have shown that silica lowers the surfacetension of ferrous oxide melts. Therefore, the excess concentration of silica at the surface canbe determined using Gibbs adsorption isotherm. The results of such calculation are shown inFigure 6.5. The plots of excess silica versus asio2 and surface tension (a) versus log a5702 forFeO-SiO2 system highlight the relationships between the parameters in equation (6.13). A criticalpoint that emerges from Figure 6.5(b) is that the melt surface tension, a, varies in a non-linearfashion with respect to the activity of silica. To further confirm of this trend available surfacetension data was utilized in the preparation of surface tension-activity plots for other melts suchas - FeO-Fe203, Fe10-CaO, Fez0-SiO2-Al203 and Fez0-SiO2-CaO-Al203. These are shown inFigure 6.6. Comparison of curves in this figure with the curve in Figure 6.5(b) reveals someinteresting features. All the four curves in Figure 6.6 are concave-up whereas the one in the latterfigure is convex. The activity range of silica in Figures 6.5(a) and 6.5(b) is widely different andmost likely the curve for silica in the former figure is equivalent to the bottom end of a versuslog asio2 curve in the latter figure. The convex-concave pattern is typical of such plots and in factin the case of many solutes the convex curve is normally followed by a linear region, which inturn is followed by the concave region. However, without going into further details it wouldsuffice to state at this stage that the curves from Figures 6.5 and 6.6 clearly show a consistentnon-linear behavior. The important thing in the present case is to develop a method for estimatingexcess surface concentrations in comlex melts.114mole%(a)L0-o5^—40^05^0log as,02Figure 6.5 Surface tension - composition relationship. (a) shows variations insurface tension values of wiistite melt due to additions of Fe 203 , SiO2and P205 (ref. 103); (b) surface tension-activity and excess surfaceconcentration-activity relationships for Fex0-S i02 melts at 1400 °C(ref.67).115FeO,( melts (ref. 67)Fex0-CaO melts (ref. 104)6005905805705605504 — sg 540r.1-"cr.0^530►..,k,520kr;5100.—0 500L-1'-,:::^0^490o, 8480acr)^470460450440430420o Lime-free melts FeP-SiO2-Al2O3A Lime-containing melts^Fej0-CaO-SiO2-Al203I^I^1^i-6^-5.6^-5.2In ap,203Figure 6.6 Surface tension-activity relationships for various melts at 1400 'C.i^1^1^1^i^1^1^i-4.8 -4.4 -4 -3.6Due to the paucity of surface tension data in the literature, it is difficult to establish theexact relationship between the activity of the solute and the surface tension for a particular meltsystem. Under the circumstance, it can be assumed that I-1 bears some functional relationshipwith a and in turn a and a i vary together with composition. Based on this assumption it is possibleto develop an expression relating excess concentration and the solute activity. As a first step wecan writer`= a . (air^ (6.14)where a and n are constants. The above equation can be expressed aslogFi =loga+n •loga i^(6.15)Therefore, from the plots of log F i versus log ai the values of a and n can be evaluated.Based on the knowledge of these two parameters the moles of excess silica at the surface couldbe calculated by combining equation (6.14) with the following relationshipexcess moles of silica =Fsio2 • A.^ (6.16)However, in order to arrive at an overall coverage value the excess silica moles are addedto the term representing surface silica moles due to the dilution effect in the following mannermoles of silica on surface =(Fsio,- A.+6,1 • A.• C5102)^ (6.17)where 84 is the monolayer thickness (cm), C3702, is the bulk silica concentration (moles/cm 3)and A. is the melt surface area (cm 2). The above equation can be alternatively expressed in termsof area coverage due to silica asA A^= Fsio2• A. 5,/ • A. • Csio2^(6.18). —covered noface (cm2) kusl PA^Wel psi)where psi is the density of slag expressed as moles/cm 3 . After dividing both sides of equation(6.18) by A. and rearranging we getA Csio2 Fsio2= 1. PA OA • Psi (6.19)117where A/A. is the fractional area available for the reaction. The second term on the righthand side of the above equation can be expressed in terms of volume fractionCsio2 (Cs ,2) (Val/Pal)Psi^V,1(6.20)The above can be alternatively be stated as^A^rsio,A= 1 –N •0^Si°2 psi • 8,1in which substitution for Fsio2 can be made according to equation (6.14) to yield(6.21)A^ a • (awl^ (6.22)— =,2^psiFor the melt density calculations mole/cm3 units were chosen and an estimation of 861 ,(monolayer thickness of an adsorbed silica) was obtained using the information published byF.D.Richardson. [105] Values of n and a were derived from the log Fso2 versus log asio2 plotshown in Figure 6.7. Calculation based on the a, p d, 84, and n values of 6.92(10 -10) (moles/cm),3.7 (g/cm3), 5(10-8) (cm) and 0.5 respectively yields the following expression for the binaryFeO-SiO2 melts at 1400 '0A^ (6.23)= 1 –Nsio. – 0.76(aso.)"Some support for the form of equations presented above can be obtained from the work ofKim et al. [66] who have proposed the following relationship(A0 –A )(6.24)A^– 0.7(aso2)033oto evaluate the fractional surface coverage due to silica at 1600 °C. Although there are somedifferences between the set of equations proposed in this work and that by Kim et al. both clearlyhighlight the role played by the fractional coverage phenomenon on the reduction rates.1181^ 1^ 1^1^ 1^1 -0.4^-0.8 -1.2 -1.6^-2.0^-2.4log ano,Figure 6.7 Log r io, versus log awl plot for Fe0-Si0 2 melts.-10.5-10.4-10.3-10.2-10.1-10.0-9.9-9.8-9.7-9.6000 -9.5-9.4-9.3-9.2-9.1-9.0119Equation (6.23) is strictly applicable to the melts without ferric oxide. Since the melts usedin the present work contained minor proportions of ferric oxide, this equation was further modifiedto account for the excess surface concentration by ferric oxide. However the procedure for thederivation of excess surface concentrations in the melts containing two surface active solutes isnot readily available in the literature and therefore it was decided to infer some useful informationfrom the available sources.As was seen earlier, in the calculation of excess surface concentrations (using equation6.14) the log T i - log ai plots are essential because without these a and n values of the surfaceactive species cannot be determined. For the FeO-Si02, FeO-Fe203 and FeO-Fe203-CaO meltsystems such plots were prepared using the available surface tension and thermodynamic activitydata. This is shown in Figure 6.8. It is important to remember that in each of the above threesystems only one surface active species is present and therefore different trends are expected inthe presence of two surface active oxide species. It is not unreasonable to assume that for themelts containing two surafce active oxides the slopes of the lines would alter within the individuallimits set by the two solutes. Support for this hypothesis is obtained form the surfacetension-mole% solute plots shown in Figure 6.9. The slopes of the lines in this figure clearlyimply that the surface activity of Si0 2 is most marked in the melts containing < 5% Fe 203 and inaddition the effect of ferric oxide on the surface tension diminishes with increasing silica levels.(Figure 6.9b) These trends suggest that the excess surface concentration of ferric oxide woulddecrease with increase in the melt silica content and vice versa and therefore the log I .; - log aiplots for the melts containing two surface active species are expected to fall between the limitsset by the two. The behaviour also hinted that the a and n values would be largely dependenton the silica level of the melts.120-f-Fe0-Fe203■Fe0-SiO3O FeO-Fe203-CaOMelts at 1400'CLine 1/Line 37/-10.1-10.0-9.9-9.8-9.7-9.6-9.5-9.4-9.3-9.2-9.1-9.0-8.9-8.8-8.7-8.6-8.5-1^-2^-3^-4log aiFigure 6.8 Log r i versus log a; plots for FeO-Fe203, Fe0-Si02 and FeO-Fe203-CaO melts at1400 *C (where i represents ferric oxide and silica species).121O 0-5% FezOzx 5 - 10"/. Fez 03\a\^• 10-15'4 Fez 0 30(a)• \.... • x^• --.......,... ^x a -,a\O 0-...X..."'...^ k^........ ...,x^•..): X N^C2• •^ N.",...........^ N.•^r .. .. .. •^••^- ...„,i.......K• r.....^. ..... - - ...^•^.^a a x• ..„^a• •^- - - -f- a -0\ -_,- a. ^...„^0-.....,^x 0^......10^15^20^- 25^305402. mass3550004005001•15.021 .21 -23%04sol_• 154021.28%•• • •.01- •C/]35040 10Fe203.moss -Figure 6.9 Surface tension variations in pseudo-binary Fe x0-SiO2 melts (ref.102) (a) showseffect of silica at various levels of ferric oxide (b) shows effect of ferric oxide inmelts containing > 21 mass% silica.(b)20122Based primarily on these observations the following empirical relationships were developedfor the calculation of a and n coefficients for ferric oxide species in silica-containing melts:a = 0.3 +Nsio,^ (6.25)n = 0.25 + 0.6(Nso2) (6.26)For silica-free melts a and n values correspond to 0.3 and 0.25 respectively and theserepresent the intercept and slope of the line i in Figure 6.9. Due to paucity of the surface tensiondata more points could not be added to line 1. However, based on the shapes of curves in Figure6.5(a) it is assumed that the behavior of ferric oxide is similar to silica and therefore the line 1can be extrapolated. Additional calculations revealed that the coverage parameter is moresensitive to the n values. Thus for Fez0-SiO2 melts the fractional coverage expression is writtenas:A^A,^f^N0.5 m^+ (0.3 +Nsio2) faFe2,031°(0.25+0.6[Nsio,])=^vsio2, 0-76kasioi,A vFeOis(6.27)The contribution of ferric oxide is added to the R.H.S. of the above equation because thisspecies provides oxygen atoms for the reduction reaction and therefore is treated as a reactantspecies. By the extension of this logic, it is necessary to subtract the mole fraction contributionsby any species that does not release an oxygen atom. Therefore, for Fe,(0-SiO2-Al203 melts theabove equation transforms to"..A , nif, 0.5 (0.25 + (628),= — sio2 0.76(asio_Nero„ NF401.5 (0.3 +NW) faFe203}Equations (6.27) and (6.28) are applicable to melt compositions within the pseudo ternaryFe.0-SiO2-Al203 system at 1400 °C. Addition of a basic oxide (e.g CaO and MgO) to the meltdoes not alter the form of these equations and therefore the same equations are applicable to eitherFex0-SiO2-CaO-Al203 or Fe,0-Si02-Mg0-Al203 melt systems.The discussion so far has covered the effect of melt silica and ferric oxide contents on thefractional coverage parameter, A/A 0, at a constant temperature of 1400 °C. The role played bythe temperature is discussed next.123It is possible to modify the fractional coverage expression for the Fex0-SiO2-CaO-Al203system obtained at 1400 °C (i.e equation 6.28) to arrive at similar expressions for 1300 °C and1200 °C melts by substituting appropriate values of coefficient (a) and exponent (n) for the asio2and aF.203 terms. However, this task could not be undertaken due to the lack of surface tensiondata at lower temperatures and an alternate approach had to be found to complete the task. Aclue was taken from the work of P. Kozakevitch [106] who has performed a few calculations onthe temperature dependence of the surface tension in metal solutions. The author has concludedthat at lower temperatures the solute becomes more surface active. In other words, the slope,da/dC, is steeper at lower temperatures. An illustration of this is shown in Figure 6.10. Such atrend implied changes in the values of exponent (n) for both surface active species. Also it wasappreciated that for a particular activity value the decrease in n (or exponent) value led to highercoverages. Based on the above information the following equations are proposed for the complexmelts at 1300 °C and 1200 °C.A— =1+ N p.014 + (0.3 + NsiO) faF.20,1(°21)A. — Nsio2 — 0 .7602SiO2)o.a NAIOL,(6.29)A =1 + NF69 , j + (0.3 +Nsio) faF.2021(o.i 4.o.igm°21)_A. Nsio, — 0 .76(asio)"5— Nun-(6.30)The a and n values of the ferric oxide and silica species in the above equations are to someextent fitted to the experimental results. However, their individual trends are consistent with themelt behaviour expec ted at lower temperatures [106]. It is worth mentioning here that the approachchosen for the derivation of equations (629) and (6.30) was the only available option due to thepaucity of melt surface tension data at lower temperatures.124CONCENTRATIONFigure 6.10 Surface tension of solutions of a surface-active solute at different temperatures,t1 , t2 and t3. (ref.106)125Before discussing the application of the above model, it is important to clarify a few pointsconcerning the fractional coverage expressions. Firstly, it should be appreciated that theexpressions are derived by the combination of both theoretical and published data on the surfacetension of the melts. Support for this approach was obtained from the work published by Pal etal. [101] and Kim et al. [66] who have accounted for the blockage effects of P 2O5 and SiO2 speciesrespectively. Secondly, the form of the fractional coverage expressions shows that their valuesdecrease with the increasing proportions of silica or other non-reactant species. Based on thistrend, it is easier to appreciate the reasons behind the decreasing apparent rate constant valuesreported in the literature [56,34,68]. Thus, the A/A, expressions were very useful in interpretingthe nature of the apparent rate constant. Another advantage of this approach is that the rates canbe predicted for untested melts for which k, values are not available and finally the techniqueallows characterization of important reaction resistances to study their effect on the ratephenomenon. Application6. Calculation of lc,In the equation (6.7) all parameters except kc are known and therefore by choosing this asa fitting parameter the calculated rates were matched with the measured data. To illustrate thisprocedure Tables 6.5-6.8 are prepared. Tables 6.5 and 6.6 list the information on the meltcomposition, activity and A/A, for various melts studied in this work. Comparison of measuredand fitted rates for the pseudo binary and pseudo ternary melts at 1400 °C is shown in Table 6.7,whereas the data obtained for the complex melts at three reaction temperatures is listed in Table6.8. Both Tables 6.7 and 6.8 contain additional information on reaction resistances and relevantdata on the driving force and overall rate constants for the selected melt compositions.It can be seen from the information in these tables that for the melts at 1400 °C the fittedkc values range between 9 x 10-5 and 13 x 10'5 gm/cm2.s.atm, whereas for the 1300 and 1200 °Cmelts the values obtained are 7 x 10 -5 and 3.5 x 10-5 gm/cm2.s.atm. respectively.126Table 6.5 Melt compositions of various pseudo binary and pseudo ternary melts and the activity data for FeO, Fe 203 and Si02Expt Melt composition as mole fraction (N,)# Fe0 Fe0, s Si02 Ca0 Mg0 A101 5 a r d) a F, 20, a so 2 A/A0I ,6,7A,2A,19A,113,1150.87 0.05 - - - 0.08 0.96 4x104 - 1117,118 0.92 0.05 - 0.03 - 0.98 5x104 - 1120 0.87 0.04 - - 0.03 0.06 0.97 3x104 - 110,11 0.40 0.01 - 0.17 - 0.42 0.70 5x10' 5 - 0.6126,27,28 0.75 0.03 0.05 - - 0.17 0.92 4x10 -1 0.13 0.57130 0.65 0.03 0.12 - - 0.20 0.75 5x10' 0.23 0.3624,25 0.62 0.02 0.14 - .^- 0.22 0.70 7x10 -1 0.27 0.3120,21,22 0.54 0.02 0.22 - - 0.22 0.50 lx101 0.32 0.1929,30 0.47 0.02 0.25 - - 0.26 0.35 1 x10• 3 0.35 0.09Note: (1) Melt compositions at 1400'C.(2) A/A, values are calculated using equation (6.33).(3) Standard state for Fe0 and Fe 20 3 is liquid whereas that for Si02 is solid.Table 6.6 Melt compositions of complex melts (Fe.0-CaO-Si0 2-Al203 system) and theactivity data for FeO, Fe203 and Si02 species at 1200 'C, 1300 'C and 1400 'C.Expt Temp. Melt composition as mole fraction (N,)# 'C FeO Fe03 .3 Si02 CaO A101.3 are, at(, as02 A/A.31,32 1400 0.34 0.02 0.23 0.13 0.28 0.66 7x410" 0.35 0.0833,34 1400 0.27 0.01 0.26 0.16 0.30 0.41 8x410- 0.32 0.0635,36,37 1400 0.31 0.01 0.23 0.17 0.29 0.57 5x410- 0.33 0.1076 1300 0.41 0.02 0.28 0.15 0.14 0.63 1 x10- 0.30 0.19351 1200 0.37 0.01 0.28 0.20 0.14 0.92 4x410" 0.22 0.23Note: (1) A/Ao values are calculated using equation (6.28).128Table 6.7 Comparison of measured and predicted rates for the pseudo binary and pseudo ternary melts at 1400'C.(Fe 3 *IEFe < 0.05)Melt type Expt Reaction resistance (cm 2 . s • atm)Ig1/k, 11(k,• r • ar,„)• - •11(k^AA.^)Fez0-Al203 2A 5720 22570^80137A 6129 24183^1157419A 5720 22570^8013113 5363 21160^8013115 5720 22570^8013Fez0-MgO 117 5720 22110^11339118 5047 19509^10204Fez0-MgO-Al203120 5720 22338^10309Fe„0-CaO-Al20310 5720 30954 2129011 5720 30954 2129093A 5363 31251 27571Fez0-SiO2-Al20326 6600 27175 19069130 4767 24075 3367024 6600 35716 5120320 6600 50002 11111129 5363 58038 244200ReactiondrivingforcePredictedrateMeasuredrateFitted lc valueatm g/cm2 .s g/cm2.s g/cm2.s.atm0.80 22.0 x 10 .6 25.5 x 104 13 x 10'50.017 4.0 x 10' 4.0 x 10-2 9 x 10 -50.70 19.3 x 104 22.2 x 104 13 x 1040.18 5.2 x 10 -6 6.0 x 10 -6 13 x 10'50.18 5.0 x 104 5.7 x 104 13^10-5x0.18 4.6 x 104 4.7 x 104 9 x 10'50.18 5.2 x 10 .6 5.1 x 10-6 10^1 04x0.18 4.7 x 104 4.7 x 104 O x0.75 12.9 x 104 12.0 x 10' 6 11 x 10' 50.18 3.1 x 10 .6 2.8 x 104 1 1 x 1 0 - 50.03 4.7 x 10 .7 4.0 x 10' 9^10-5x0.90 17.0 x 10.6 17.6 x 10 .6 to x t o -50.18 2.9 x 104 2.6 x 10 -6 1 1 x 1 040.68 7.3 x 104 6.7 x 104 9 x 1040.83 4.9 x 104 4.5 x 10 -6 9 x 10-50.76 2.5 x 104 2.8 x 10'6 13 x 104Overall rateconstantg/cm2.s.atm2.75 x 1042.39 x 10'52.75 x 10-52.90 x 1042.75 x 1042.55 x 10' 52.88 x 1042.61 x 10'51.72 x 10'51.72 x 1041.56 x 10 -51.89 x 1041.60 x 1041.07 x 1045.96 x 1043.25 x 104Note: (1) Overall rate constant = ,+k^AIt k A^kkey Fe0(2) Equilibrium constant, ICC = 0.264.(3) Values of gas phase mass transfer coefficient, k g are calculated using equation (5.5).Table 6.8 Comparison of measured and predicted rates for complex melts between 1200 'C and 1400 'C. (Fe 3+IEFe < 0.05)1 Expt. Temp. Reaction resistance (CM 2 • S • arm)/g Overall rateconstantReactiondrivingforcePredicted rate MeasuredrateFitted lc., value# 'C 1/k, 11(k,•10 • a,,a ) i(k.. 94_ . aho ) g/cm2.s.atm atm g/cm2.s g/cm2.s g/cm2.s.atm31 1400 6600 37880 210438 3.92 x104 0.66 2.6 x 104 2.2 x 10'6 9 x 10-532 1400 6600 37880 210438 3.92 x10'6 0.18 7.1 x le 7.0 x 10'7 9 x 10 . '33 1400 6600 60978 369549 2.29 x1 0'6 0.49 1.1 x 104 1.0 x 10 -6 11 x 10'534 1400 6600 60978 369549 2.29 x104 0.18 4.1 x 104 4.0 x 10'7 11 x 10435 1400 6600 43862 216591 3.74 x104 0.62 2.3 x 10.6 2.0 x 104 9 x 10436 1400 6600 43862 216591 3.74 x104 0.18 6.7 x 10'7 4.0 x 104 9 x 10'537 1400 6600 43862 216591 3.74 x10' 0.18 6.7 x 10'7 6.0 x 104 9 x 10'576 1300 . 6723 . 32338 97847 6.31 x104 0.18 1.1 x 104 0.9 x 104 7 x 10451 1200 6868 23488 190985 4.52 x104 0.18 8.1 x 104 8.0 x 10-7 3.5 x 104Note: (1) Overall rate constant -^+ ^^g^,e gIt c FA(2) Values of Ice are 0.264, 0.33 and 0.43 at reaction temperatures of 1400 °C, 1300 °C and 1200 °C respectively.(3) Values of gas phase mass transfer coefficient, kg, are calculated using equation (5.5).Only one experiment each was performed at 1200 and 1300 °C and therefore the Ic. c values obtainedfrom these experiments have a large element of uncertainty associated with them. However, thefitted values show a right trend namely - decreasing lc, with decreasing reaction temperature.Since the kc values for the 1400 °C runs varied consistently between 9 x 10 -5 and 13 x 10-5g/cm2.s.atm an average value of 11 x 10-5 was fixed to represent all the data at this temperatureand the value was used in conjunction with those at 1200 *C and 1300 °C temperatures forconstruction of Arrehinus plot (Figure 6.11). Additional calculations were performed to arriveat the activation energy value of about 28 kcal/mole. This agrees well with the earlier reportedvalues by Nagasaka et al. (33,000 cal/mole), Kim et al. (32,000 cal/mole) and Tsukuhashi et al.(31,400 cal/mole). Interpretation of the rate constant termThe data listed in Tables 6.7 and 6.8 reveals that individual values of the overall rateconstants are dictated by three reaction resistances - 1/kg , 1/(kg • IC' • aF0) and 1/(k. • A • ape)).In a situation where gas phase mass transfer is not rate limiting the first two resistances arenegligible (because of the higher kg value) and the resultant rate constant term would then equalthe reciprocal of the remaining resistance. In the literature this term is often referred to as anapparent rate constant. Thus when the gas phase resistance is neglected the apparent rate constantcan be represented as:Ak„ = kc .—A .aF 0.(6.31)The above equation implies that the apparent rate constant values are dependent on theindividual values of A/A0 and aFio . Any changes in the melt compositions are therefore expectedto alter the apparent rate constant values. Thus with this approach it is possible to appreciate thereasons behind the reported variations in the apparent rate constants. [56,64] Further discussionon this topic is pursued in section 6.3.131Ferrous-to-iron reaction studybetween 1200 and 1400CArrhenius plot of In k, versus 1/TActvation energy - 28142 caVmoleI^I^1^I^I^I^I^I^I^I0.00059^0.00061^0.00063^0.00065^0.00067la IC -1Figure 6.11 Arrhenius plot of In Ice versus 1/T for ferrous reduction study.-11-11.2-11.4-11.6-11.8-12-12.2-12.4-12.6-12.8-13-13.2-13.4-13.6-13.8-146.1.3 Reaction resistancesThe data listed in Tables 6.7 and 6.8 shows the reaction resistances for the melts studied.It can be seen that the gas phase contribution comes from two terms containing k g parameterwhereas the remaining resistance is offered by the chemical reaction. The data shows that in thecase of simple melts, without lime and silica, the gas phase contribution to the total resistance isbetween 70 to 80% and therefore in these melts the reduction reaction is controlled by the gasphase. However, with the addition of lime and silica the gas phase contribution decreases witha commensurate increase in the chemical reaction resistance term. This result is attributed to thedecrease in the fractional coverage parameter and the lower ferrous oxide activity. The effect ismore pronounced with the addition of silica. For example, in the melt containing lowest silica(expt. #26) in the Fe.0-SiO2-Al203 system the gas phase contribution is nearly 64% whereas theequivalent value for the high silica melt (expt. #29) is about 21%. The same trend is observedin the complex melts containing both lime and silica.The above pattern of behaviour implied that the rates were not governed by the gas phasetransport alone. Although this conclusion was drawn earlier based on the discrepency betweenthe predicted rates, assuming gas phase control model, and the measured rate data (Figures 6.1and 6.2) the description in the foregoing paragraph offers further explanation for such behaviour.6.1.4 Effect of melt compositionThe measured rate data clearly showed that increasing additions of silica led to consistentlylower rate values. (Table 5.4 and Figure 5.6) Nagasaka et al. [561 report similar observation intheir study. Though these investigators employed a jetting set up their findings concerning theeffect of silica are not different. Nagasaka et al. account for the decreasing rates in terms ofreduction in the apparent rate constant values due to silica. In the present work, based on thedetailed mathematical analysis an explanation to the silica effect was sought in terms of its surfaceactive properties.133Equation (6.7) implied that the lower reduction rates could also result from the decrease inthe aF,0 values which are caused by the addition of silica to the melt. However, the observedrates were much lower than the ones predicted on the assumption of activity contribution alone.For example, the measured rate for the lowest silica-containing melt (-5 mole%) was about 6times higher than the one recorded for the highest silica (-29 mole%) melt using the same reductiongas mixture. However, the aFeo value for the former melt was only about 2 times greater thanthe latter. Such a pattern of behaviour supported the idea that in addition to the activity effectwhich acted via driving force term an additional contribution may come from the overall rateconstant term. Therefore, this idea was pursued further by quantifying the silica effect in termsof the fractional coverage parameter, A/A,,. Also the same concept was extended further to includethe role played by amphoteric oxide species like ferric oxide and alumina.With the help of a mathematical expression the role played by the melt composition canbe appreciated without much difficulty. For example, equation (6.28) clearly shows that as themelt silica and alumina levels increase the fractional coverage parameter values decrease. Asper equation (6.7), this in turn leads to lower apparent rate constant values and therefore lowerrates are predicted. Thus both the activity and surface coverage effects are considered in themathematical modelling scheme.6.1.5 Effect of excess melt oxygenSpecial experiments (#12415) were conducted to study the effect of excess oxygen in themelts and the data showed higher reduction rates. This trend was consistent in both simple andcomplex melts. According to Fe-0 phase diagram (Figure 2.1) higher oxygen contents lead tohigher ferric levels and therefore it is easier to appreciate the fact that in our study higher rateswere obtained at higher ferric levels. (Tables 5.5 and 5.7 and Figures 5.6 and 5.8) Nagasaka etal. [56] report a similar finding in their work.134The authors observed that ferrous reduction rates were exceedingly influenced by theferric-to-ferrous ratio expressed as N.30F3.2+ and proposed the following empirical relationshipbetween this ratio and the composition dependent apparent rate constantka = 8.4x 10- NF2,30F3.2,,r CM 2.s.atm (6.32)The above relation shows that the value of apparent rate constant would increasecommensurate with the melt ferric levels and this would lead to the higher rates. Due to theempirical nature of the expression however the fundamental reasons underlying the rateenhancement phenomenon were not addressed by the authors.From the expression derived by Nagasaka et al. one can see that the values of apparent rateconstant are dependent on the state of oxidation of the melt. Other researchers [68,34] have alsonoted similar trend while studying the interfacial rates of oxidation in various melt systems.Belton [80] have proposed a model involving dissociation of a doubly charged (negative) CO 2molecule according to:CO2 + 2Fe2+ = COnad)+2Fe3+ (6.33)and= CO +0 2- (6.34)to explain the trends observed in the oxidation studies. Later, using the same model andthe principle of microscopic reversibility the author has explained the results obtained in reductionstudies by both Nagasaka et al. [56] and Tsukuhashi et al. [55]. This fact suggested that in thereduction of ferrous iron the ferric-ferrous reaction is critical.To further test this possibility asditional experiments were performed where the Fe3+11Feratio of the melts was higher (between 0.14 and 0.19) than the normal melts in which this ratiowas about 0.05. (Table 6.7 and 6.8) When the measured rates were compared with the predicteddata, using the model developed earlier (i.e. gas phase mass transfer and interfacial reaction modelassuming Fe2+ ---> Fe reaction), poor agreement was noticed.135Table 6.9 shows comparison of measured and predicted rates for the high ferric oxide-containingmelts. The data highlights two important points: (1) in most cases the predicted values are lower;and (2) highest discrepency is noted for Fe x0-SiO2-Al203 melts. The same table lists the datafor the over-oxidized melts (Experiments #12415) and again these numbers reveal a pooragreement between the predicted and measured data. Lower predicted values in Table 6.9 suggesttwo things: (1) the model is wrong for these conditions or (2) a faster concurrent reaction is takingplace in these melts. The former possibility was discarded because the model could makereasonable predictions in the case of low ferric oxide-containing melts.Additional support for the concurrent ferric-ferrous reaction was obtained by comparingthe model predicted rates, assuming Fe 3 + --) Fe 2+ reaction, with the measured data. A mixed gas+ reaction control model was formulated for this purpose and the resultant rate comparisonrevealed an agreement between the two sets of data. The main reason for the success of the modelwas found to be the consideration of a simultaneous ferric-to-ferrous reaction which led to higheroverall rate constant values compared to the equivalent data obtained assuming ferrous-to-ironreaction. This comparison is shown in Table 6.10. Upon further investigation it was noted thatthe value of intrinsic rate constant for the ferric-to-ferrous reaction was nearly 200 times greaterthan the equivalent value for the ferrous-to-iron reaction. Additional details on this are providedin section 6.2.The above results were interesting because they suggested that perhaps the mechanismsinvolved in ferric and ferrous reduction reactions are different. Kinetic studies on the solid ironoxides have revealed different rates for the ferric oxide and magnetite reduction [21] and thereforethe above idea was not discarded. However, according to the model suggested by Belton [80],in the melts, the ferric-to-ferrous and ferrous-to-iron reactions are not separate.136Table 6.9 Comparison of model predicted and measured rates for ferrous-to-iron reaction.Expt. Melt composition, mole fraction (N1) FP' Measured rate Predicted rateEFeFe0 Fe013 Ca0 Si02 A1013 glcm2.s g1cm 2 .S87 0.460 0.108 0.129 - 0.303 0.19 10 x 10" 5.8 x 10"88 0.364 0.085 0.186 - 0.365 0.19 8.0 x 10" 5.4 x 10"89 0.298 0.084 0.214 - 0.404 0.22 7.0 x 10" 4.9 x 10"90 0.533 0.119 - 0.139 0.209 0.18 15.0 x 10" 4.5 x 10"91 0.460 0.104 - 0.218 0.218 0.18 10.0 x 10 .7 2.7 x 10" .92 0.464 0.115^• - 0.248 0.173 0.18 3.00 x 10" 1.5 x 10"38 0.236 0.056 0.171 0.277 0.260 0.19 7.0 x 10" 4.7 x 10"39 0.19 22.0 x 10" 23.0 x 10"40 0.244 0.040 0.164 0.268 0.284 0.14 8.0 x 10" 6.7 x 10"12' 0.672 0.243 - - 0.085 0.27 12.4 x 10" 2.3 x 10"13' 0.377 0.037 0.157 - 0.429 0.09 40.0 x 10" 14.1 x 10"14' 0.505 0.041 - 0.213 0.241 0.08 25.0 x 10" 6.6 x 10"15' 0.352 0.040 0.124 0.240 0.244 0.10 23.7 x 10" 7.1 x 10"Note: (1) gas and interfacial reaction model formulated for the ferrous-to-iron reaction.(2) Predicted rates are based on a kc value of 11 x 10's g/cm2 .s.o.tifr.(3) * Melts contain excess oxygen.Table 6.10 Comparison of overall rate constants for the melts with higher ferric oxide. (Fe 3+1£,Fe >0.05)Expt. Overall rate constant Fe 2+ --> FereactionOverall rate constant Fe --)Fe2+ reactionMelt type Fes+ --) Fe2+ reaction rate, gm/cm 2.sgm/cm2.s.atm gm/cm2.s.atm predicted measured12 2.34 x lgs 1.28 x 104 Fex0-Al203 12.8 x 104 12.4 x 10487 1.94 x lgs 3.54 x 104 Fei0-Ca0-Al20310.6 x 104 10.0 x 10488 1.63 x 104 2.44 x lg s Fei0-Ca0-Al2038.1 x 104 8.0 x 10489 1.88 x 104 4.57 x lgs Fex0-Ca0-Al2037.3 x 104 7.0 x 10490 1.49 x 104 5.39 x 104 Fex0-Si02-Al2031.6 x 10-6 1.5 x 10.691 0.91 x 104 3.84 x 104 Fe„0-Si02-Al2031.1 x 10.6 1.0 x 10.614 8.84 x 10-6 3.05 x lgs Fex0-Si02-Al20322.9 x 10-7 25.7 x 10'15 9.52 x 104 2.90 x lgs Fe,O-Ca0-Si02-Al20321.8 x 10 6 23.7 x 104Note: (1) For the Fe 2+ --) Fe reaction overall rate constant -^Its ,s ice .° ke21 aF,0(2) For the Fe3+^Fe2+ reaction overall rate constant -^(3) All the melts at 1400 'C.(4) Fitted k; value for the ferric-to-ferrous reaction averaged 0.025 (g/cm 2.s.atm)(5) Fitted Ice value for the iron formation reaction averaged 11 x 104 (g/cm2.s.atm).1 +  4 A  + ^I kt kS 'F.203^403Based on the studies by Sun et al. [63], Sasaki et al. [68] and El-Rahaiby et al. [34] Belton hasproposed the following empirical relation:ka = k' (Fe3+IFe2+)-2 (6.35)where k is a system and temperature dependent constant. Using the availableferrous-to-ferric oxidation data, Belton has shown that equation (6.35) is consistent with a rationalrate law based on the dissociation of an adsorbed CO -. By plotting log k, versus log Fe3+/Fe2+for various melts (Figure 6.12a) the value of the slope was found to be -2, as predicted by theabove equation, and this validated the proposed model. For the ferrous reduction reaction Belton'smodel would predict the dependence of apparent rate constant as follows:k.= m (Fe3+/Fe2+)2 (6.36)where m is a system and temperature dependent constant. However, Nagasaka et al. [56]report a different relation:ka=k:{(NF3:)2/(14.+)31 1/3 (6.37)where lc: is the empirical rate constant. Equation (6.37) suggests a complex dependence ofk, on the melt ferric-ferrous proportions and implies some uncertainty in the application ofBelton's model to the reduction data. To further support this point Figure 6.12(b) is shown. Thisplot of lc, versus P IPco2 co shows the results of oxidation study on the lime ferrite melt (Ca/Fe =0.3). Belton agrees that there are uncertainties in these measurements and has shown one resultfrom the reduction study by Nagasaka et al. in the same figure. Although location of the reductionpoint is in reasonable agreement with the plot it is difficult to confirm whether the same trendwill continue for other melts containing either lower or higher lime contents. Such behaviourimplies that the mechanisms of oxidation and reduction reactions may be different. It is felt thatmore data is needed to either confirm or deny such a possibility.1391^-t^is^1oFe0-Ca0 3.3^\Fe./Ca .oe \:,), Fe0 -Ca0(sat)/ .\ \\1,;\\. 15100.C-'''''\,......;\ e^A. 1550 C.1300 C:,\^L^• 1 1\8\^\\is•—x._.^0. 0^:N.\^•\• •-.S.oct,.\es*so. *\\^..\\—. • ''\\•\\-------Fe0^-\.^1400it^\\...C^. ‘^ 1500.0---........\^____F1240 .0 - e0- 5'02 (sat)'I - Fe0-Ca0-5i02equimolar 1420 Co -5rn0-61^1^111. 1111^1^1^11111t 1^1^1111111e^.Nagasaka et al 1400 C• INe: .............1is3o0toopc. e exchangeNj..=^.. .4 ._IN.N.- oxidations'.- 1362 •Ct^I 1 11 111^I^11111111^i^11 ' 1111o(7E06 10-5E-610(b)-7-20^-15^-10^-05^tl Clog Fe31Fe 2.(a)01^1 10 100pCO2 /pC0Figure 6.12 Variations in apparent rate constants observed in oxidation studies using isotopeexchange technique (ref. 80); (a) dependence on the melt Fe 34"/Fe2+ ratio; (b)variation of lc, as a function of oxygen activity for the reaction of CO 2 with acalcium ferrite melt (Ca/Fe = 0.3).6.1.6 Reaction mechanismThe ferrous reduction reaction in the present work was controlled by a mixed gas phasemass transfer and interfacial reaction. In the melts with Fe3471.Fe < 0.05 the ferrous reductionreaction can be represented asFe2+ CO2 - = Fe +CO2^ (6.38)However, in the melts with Fe3+IIFe exceeding 0.05 additional reactions involving eitherFe3+ or other ferric ion species are significant.The above scheme does not contradict the formation of CO:- as per reaction (6.9) suggestedby Belton [80] and in addition is consistent with the findings of Kim et al. [66] concerning thesilica coverage effect. The suggestion on participation of ferric anion species is essentially anextension of the idea proposed earlier by Gaskell [96] and such participation by ferric anionsdoes not exclude the presence of a charge transfer reaction (reverse of reaction 6.33) involvingferric and ferrous cations. The presence of Fe ll- ions in both the tetrahedral (anions) and octahedral(cations) coordination is justifiable on the grounds of amphoteric nature of ferric oxide species.6.2 Ferric-to-ferrous studyAlthough a certain amount of data on the ferrous-to-iron reaction can be found in theliterature very little information is available on the Fe3+ Fe2+ reaction. Since the present worksuggested the possibility of this reaction during the ferrous reduction, several additionalexperiments were performed to study the ferric-ferrous reduction reaction. These experimentswere conducted at three temperatures - 1200 *C, 1300 °C and 1400 °C and a wide range of slagcompositions was covered. Extreme care was exercised in both the selection and control of thereactant gas mixtures to avoid precipitation of either Fe or Fe 304 species during reduction. Inother words, the ferric-to-ferrous study was conducted in melts away from iron and magnetitesaturation (aF, and aF.304 < 1). It is worth mentioning here that no such work has been reportedin the past.1416.2.1 Mathematical analysisTo account for the two distinct stages observed during the reduction period two separatemodels were written. A mixed gas phase mass transfer and reaction control model described thehighest initial rates whereas in the second model a liquid phase contribution was added to theexisting reaction resistances to explain the slowing down of rate with time. Model formulation6. Constant initial rateThe procedure used in the derivation of a rate expression is essentially similar to the onedescribed earlier in the ferrous reduction study. The flux equations (6.2) and (6.3), for the COand CO2 species respectively, remain the same. However, the rate equation (6.4) is modifiedbecause the reactant and product species in the liquid phase are different. The rate per unit areafor the ferric oxide reduction (reaction 5.8) can be written as1Cc* 6,2 • teioFr = Icc. • A • a e203 . coC* ^K' (6.39)molescm 2 .swhere lc; is the intrinsic rate constant of ferric-to-ferrous reaction in cm/s and IC' is theequilibrium constant of the reaction (5.8). The available reaction area in cm2 in represented byA. Assuming steady state conditions i.e. r = rc co = iz coe the mathematical procedure given inSection can be repeated to eliminate the interfacial concentrations to arrive at the followingexpressionr^1  ,-,CO+ +^b Cto2 A. ^aka^1 ^1^L^.aFe203k;.1.-.aF„203(6.40)molescm 2 .swhere A/A. is the fraction covered by the ferric oxide species. The first term representsthe overall rate constant and the term in the bracket is reaction driving force.142Equation (6.40) can be rewritten in the practical form asr ^1Pto .al..o^g A. ^,.2••Fe0^1 ^1 ^4^ii,.^0 . 2 CMA. .aF,203^2.Sks .10.aF,203 + ke + k;.-AtA .ap.203(6.41)in which all terms except intrinsic rate constant, Ic:, are known. Therefore, by choosingappropriate values for the lc: term predicted rates could be matched to the observed data. Rate-time behaviourThe weight loss data revealed that after about 60-90 minutes into the reduction period thereaction rates slowed down with time. This fact suggested that the resistance offered by the liquidphase had to be accomodated in the rate expression during the latter stages of the reduction period.To enable this the following equations were writtenb^(6.42)4.203 = IcLA„(CF.03 -1CF*.203)andr = lc:A H *.CF A.Cco(C'.. 0.4. so2 (6.43)K.Explanation on some of the new terms in the above two equations is necessary beforeproceeding further. Superscripts * and b represent interfacial and bulk concentrations of thespecies and the activity coefficient is indicated by 7. Ferric oxide species is chosen as a convenientway to represent oxygen anion transport. The parameter f is the ratio of ferric oxide activitycoefficients for the surface and bulk regions i.e.(6.44)At equilibrium the activity of ferric oxide is same for the entire melt and the variations inconcentrations between the surface and bulk regions lead to different activity coefficient values143for these regions. The ratio is < 1 because the concentration of ferric oxide at the surface is higherthan the bulk due to its surface active nature. [102,103,107,108] Therefore, in the equation (6.43)the term in the bracket represents the activity driving force of the ferric-to-ferrous reaction.In the derivation of an earlier gas + reaction model the liquid phase contribution was ignoredand therefore equation (6.42) was excluded from the mathematical scheme. Also, it can be easilyproved that the equation (6.43) readily converts to equation (6.39) when the f.q.CF*,203 term inthe former is replaced by aF,203. The above facts suggest that the only difference between the gas+ reaction model and the gas + liquid + reaction model lies in the inclusion of liquid phaseresistance in the latter. The direct consequence of this results in the modification of equation(6.39) and the addition of equation (6.42) to the mathematical scheme.By combining equations (6.42) and (6.43) with the three previous equations (equations 6.2,6.3 and 6.5) i.e.n co = (C co — C;)n = A k (C a — Clo)co,^o• g• O2^2r = co = Am,the interfacial concentration terms were eliminated and the rate expression for the gas +liquid + reaction model was developed. The assumptions in the model formulation were sameas those mentioned for the gas + reaction model. The procedure is briefly described below.From equations 6.2, 6.3 and 6.5 we can get •CCO = + 402 —Also by equating equations (6.3) and (6.43) we getkc:A Cc* 02aLokg .A0.40 — kg .A..Cc* 0 = k:AHCF* e2o,Cco K'(6.46)Upon substitution of equation (6.45) in equation (6.46) the interfacial concentration of COcan be eliminated as per the following, Ic:AC.024,09kgA.CL,— ks,A„(40 + 402 — C.;,02)= ic:ANC;e20,(40 + 402 — C.C.02) ^(6.47)(6.2)(6.3)(6.5)(6.45)144By equating (6.3) and (6.42) after rearrangement yieldsbkL • C F...0 ki, • f' • C; oC . =^-2. 3^e2 3 + C bCO2^&^k^CO2A'S g(6.48)After combining equations (6.47) and (6.48) the interfacial concentration for the CO 2, C'02,was eliminated and a final equation with one unknown was obtained. This was a quadraticequation in terms of C;,203 . Further calculations showed that a positive root should be used forexpressing interfacial concentration of ferric oxide species (in terms of all the other parameters).The final equation by following this procedure is given below^r = iz Fjp3 =^• A° •where^k: f447(6.49)(6.50)C:..20, — [ —b +-402- 4 • a • c)])2aa=kgA^ A^2k; •T•f• IA • kz, - C t203^k; •^• aF00•f•kL (6.51)b= k - --;4i- • f • 1[ °1.•• C'.0^+ +1 kkg^kg • leandc =k: • -,1", • al•go • ICL, ' Cte203^k; • -'^• ako- ^Ct•02 b kL • C(6.52)+^F.203k^IC'^+gAcknowledging thatI.^ (6.53)Aw,. =^(rate) • dt + Am. _ 1 — (rate ),. _ 1 • (t,, — t° _ 1) + Ai% _ 1and the rate is given by the expression (6.49) a technique was developed to perform therequisite calculations. Based on the Euler method an equation was derived to predict the weightloss data for any time interval At and the same is given below= (rate ),. _ 1 • At + Aw,. _ 1^(6.54)Of the various parameters in equation (6.49), only C.',203 varies with time because the ferricoxide species is depleted due to the reduction reaction. Its highest value at t = 0 was first calculated145from a knowledge of the melt composition and the volume of the melt and in turn this was utilizedin equation (6.54) to arrive at the weight loss after any fixed time interval At. Using this weightloss value it was possible to calculate the new C;',A value assuming negligible changes in themelt volume during the time interval At. By repeating these calculations in the program theweight loss at the end of the reduction period could be determined.In the rate expression all the parameters except lc: and kL are known and therefore with theknowledge of these two paramters the rate calculations could be performed. For all the meltsstudied the intrinsic rate constant value was obtained first using gas + reaction model and laterthis value was employed in the gas + liquid + reaction model. After fixing the lc: value, k L wasused as a fitting parameter to predict the weight loss data for studying rate-time behaviour in allthe melts. A computer program was written for this purpose. Application6. Calculation of lc;By using lc: as a fitting paramter in equation (6.41) the predicted rates were matched to themeasured data. The results obtained thus for all the melt compositions are listed in Tables6.11-6.14. Additional information on reaction resistances, overall rate constants, fractionalcoverage parameters and activity data for the individual melts is also provided in these tables.Table 6.11 shows the comparison of predicted and measured rate data at three reactiontemperatures in lime-free melts (Fe.0-SiO 2-Al203 system). The information reveals that the fittedlc: values range between 0.025 and 0.035 g/cm 2 • s • atm for a reaction temperature of 1400 *Cwhereas the equivalent values for the melts at 1300 °C and 1200 °C are 0.005 and 0.001respectively. Since only limited data was obtained at lower temperatures the resultant values ofthe intrinsic rate constants may not be very accurate. However, these do show the expected trendof decreasing values with decreasing temperature.146Table 6.11 Fitted intrinsic rate constant values for the lime-free melts.Expt. R, R2 R, Overall rateconstantD.F. Predicted rate MeasuredrateaF.a F.,o, A Fitted k:# g/cm2.s.atm atm. g/cm2.s g/cm2.s g/cm2.s .atm84 0.025100 fYi 0.02599 625 5720 6720 7.65 x 104 0.001 0.8 x 10-7 1.0 x 10'' 0.46 0.024 0.248 0.025137' 402 6206 21879 3.51 x 1 0.' 0.011 3.9 x 10' 4.4 x 104 0.42 0.030 0.277 0.0055138' 402 6206 21879 3.51 x 104 0.011 3.9 x 104 4.0 x 104 0.42 0.030 0277 0.0055141— - - - - 0.017 - 1.5 x 10' 0.40 0.040 0.406 < 0.001142— - - - - 0.017 - 1.2 x 104 0.40 0.040 0.406 <0.001Note: (1) Melts * and ** are at 1300 °C and 1200 *C respectively. All other data is for 1400 C.(2) Overall rate constant — (i/R i + 1/R2 + 1/R3)R I =*^.a1•^1-'2°3R2 k1R3 — A^c • 42Fe203.  2aft°P^2C°2 Fe°(3) Reaction driving force, D.F. PCO^ ." "a fe.2031471R 1 —k..K 4 .a,.,'2-32aFt01R2 ,KtR3 = A^  a1 209Table 6.12 Fitted intrinsic rate constant values for the lime-containing melts at 1400 °C.Expt. RI R2 R3 Overall rateconstantD.F. Predicted rate MeasuredrateaF00 aF020, AA,Fitted k:# g/cm2.s.atm atm. g/cm2.s g/cm2.s g/cm2.s.atm41 1346 5363 44092 1.97 x 10' 5 0.014 2.8 x 10'7 3.0 x 104 0.45 0.009 0.126 0.02042 1435 5720 48991 1.78 x 10' 5 0.013 2.8 x 10'7 2.8 x 104 0.45 0.009 0.126 0.01846 1702 5720 47281 1.83 x 10'5 0.016 2.9 x 10'' 2.8 x 10'' 0.49 0.010 0.141 0.01547 1596 5363 47281 1.84 x 10' 5 0.016 2.9 x 104 2.4 x IC 0.49 0.010 0.141 0.01543 1914 5363 28462 2.19 x 104 0.016 3.5 x 10'7 3.4 x 10'7 0.48 0.008 0.130 0.02548 2041 5720 38462 2.16 x 104 0.017 3.7 x 10'7 3.8 x 10'7 0.48 0.008 0.130 0.025Note: (1) Overall rate constant — (1/R1+1/R2+11R3)^and KC = 80.7 at 1400 °C.Pco24,0(3) Reaction driving force, D.F. = Pco — K•Table 6.13 Fitted intrinsic rate constant values for the lime-containing melts at 1300 °C.Expt. RI R2 R3 Overall rateconstantD.F. Predicted rate Measuredrateapo aF,2o,# g/cm2.s.atm atm. g/cm2.s g/cm2.s66 1376 5763 58072 1.53 x 104 0.025 3.8 x 104 3.8 x 104 0.51 0.01270 1376 5763 50813 1.73 x 104 0.011 1.9 x 10:7 2.1 x 104 0.51 0.01267 1376 5763 45167 1.91 x 105 0.018 3.4 x 104 4.0 x 104 0.51 0.01265 1320 5763 50524 1.74 x 104 0.019 3.3 x 104 3.5 x 104 0.52 0.01368 1320 5763 50524 1.74 x 104 0.019 3.3 x IC 3.2 x 104 0.52 0.013139 731 5763 31526 2.63 x 105 0.020 5.3 x 104 5.1 x 104 0.48 0.020AA.Fitted k;g/cm2.s.atm0.205 0.0070.205 0.0080.205 0.0090.203 0.00750.203 0.00750.244 0.0065Note:^(1) Equilibrium constant, Ke = 90.8 at 1300 'C.Pc02.4,0(2) Reaction driving force, D.F. =Pco K .apd203(3) kg (cm/s) = 8067.5 x (kg in gkm2.s.atm)Table 6.14 Fitted intrinsic rate constant values for the lime-containing melts at 1200 °C.Expt. RI R2 R2 Overall rateconstantD.P. Predicted rate MeasuredrateaFe0 °Pep, AA,Fitted lc;# g/cm2.s.atm atm. g/cm2.s g/cm2.s g/cm2.s.atm54 1031 6296 68376 1.32 x 10' 5 0.010 1.3 x 10'7 1.7 x 10'7 0.42 0.013 0.225 0.005108 1031 6296 68376 1.32 x 10' 5 0.012 1.6 x 10'7 2.0 x 104 0.42 0.013 0.225 0.00552A 1031 6296 85470 1.08 x 104 0.023 2.5 x 104 2.5 x 10'7 0.42 0.013 0.225 0.00463 1031 6296 85470 1.08 x 10' 5 0.034 3.7 x 10'7 3.5 x 10'7 0.42 0.013 0.225 0.00453 1031 6296 68376 1.32 x 104 0.018 2,4 x 104 2.2 x 104 0.42 0.013 0.225 0.00555 1549 6296 78709 1.16 x 10' 5 0.018 2.1 x 10'7 2.1 x 104 0.53 0.011 0.231 0.005106 1549 6296 78709 1.16 x 10' 5 0.016 1.8 x 10' 7 2.1 x 10'7 0.53 0.011 0.231 0.00556 1549 6296 78709 1.16 x 10 -5 0.025 2.9 x 10'7 2.8 x 10'7 0.53 0.011 0.231 0.00557 1120 6296 78174 1.17 x 10' 5 0.021 2.5 x 10'7 2.3 x 104 0.49 0.013 0.246 0.00458 1120 6296 104232 0.9 x 10' 5 0.026 2.3 x 10'7 2.1 x 10'7 _ 0.49 0.013 0.246 0.003 _107 1120 6296 62539 1.43 x 104 0.014 2.0 x 10'7 2.0 x 10'7 0.49 0.013 0.246 0.00559 2211 6296 155473 0.61 x 10 -5 0.022 1.3 x 10'7 1.3 x 104 0.54 0.008 0.201 0.00460 1578 6296 170940 0.56 x la s 0.024 1.3 x 10'7 1.1 x 10'7 0.51 0.010 0.195 0.00361 1578 6296 170940 0.56 x 10' 5 0.024 1.3 x 104 1.0 x 104 0.51 0.010 0.195 0.003143 611 6296 33300 2.49 x 10' 5 0.019 4.7 x 104 4.6 x 104 0.46 0.021 0.286 0.005Note: (1) Equilibrium constant, 1(` = 103.8 at 1200'C.pco2 2F,0(2) Reaction driving force, D.F. =Pa)K .ap.203(3) kg (cm/s) = 7554.6 x (kg in g/cm2.s.atm); kg in (cm/s) is derived using equation (5.5).Tables 6.12, 6.13 and 6.14 list the data on lime-containing melts at 1400 °C, 1300 °C and1200 °C respectively. It can be seen from the information in these tables that the fitted lc: valuesrange from 0.015 to 0.02 at 1400 °C, whereas a lower value of 0.007 represents the melts at 1300°C. For a reaction temperature of 1200 °C the rate constant values range between 0.003 and 0.005g /cm 2 • s • atm . Since the ferric oxide reduction reaction is same for all the melts at three reactiontemperatures, one expects a single value for the intrinsic rate constant at each temperature. Basedon the measured data the reaction rate constant values of 0.025, 0.0075 and 0.004 are proposedfor the 1400 °C, 1300 °C and 1200 °C temperatures respectively. Calculation of apparent activation energy of chemical reactionFrom the knowledge of intrinsic rate constants at three reaction temperatures the apparentactivation energy value of about 44,500 cal/mole was derived for the ferric-to-ferrous reductionreaction. Comparison of activation energy values for the ferrous-to-iron and ferric-to-ferrousreactions reveal a relatively lower value (— 28,000 cal/mole) for the former. This differencesuggests that perhaps the reaction mechanisms involved in these two reduction reactions aredifferent. Arrhenius plot obtained for the ferric-to-ferrous reaction is shown in Figure Calculation of kLIn equation (6.49) all the parameters except kL are known and therefore by choosingappropriate values for this parameter the weight loss data were fitted. From this informationweight-time plots were prepared for all the melts studied. As an illustration, several examplesare shown in Figures 6.14 - 6.17. A trend of decreasing rate with time is evident in all these plots.The individual plots also show that for the initial reduction period (of approximately 3000 seconds)the rate does not vary much.The fitted kL values obtained for the lime-free and lime-containing melts at three reactiontemperatures are listed in Table 6.15. The data reveals the highest values of about 2 x 104 and1 x 104 cm/s for the lime-containing and lime-free melts respectively at 1400 °C. With decreasingtemperature lower values were obtained.151-6-6.2-6.4-6.6-6.8-7E -7.2:i -7.4*t .,)^-7.6g -7.8seE^-8-8.2-8.4-8.6-8.8-90.00059^0.00061 0.00063^0.000651/T IC'0.00067Figure 6.13 Arrhenius plot of In Icc versus 1/T for the ferric-to-ferrous reaction.62 107 4 6 10(d)2 1000Femc•to.ferrous study a 1400'CIn limo-rree melt12II -10 -9 -6 -4g 5 -(a)42Ferric•to-ferrous study st 1400'Cin lime-free meltEapt 0103 (6.3 mole% silica!- Model0012 ^11-10K2 -9 ,I -aE 71 6 ---2reduction time, (s)12(b)4II -10  -9 -8-2 7 -IfFerric-to-ferrous study at 1400'CIn Ilmc-free meltt Etpt 095 (31.3 mole% silica)ModelFemc•10-ferrous study it 1400 •CIn lime-free meltEttp4 080 (16.6 mole% silica)----- ModelFigure 6.14 Weight loss data for the ferric-to-ferrous reaction study at 1400 *C in lime-free melts (Fei0-Si02-Al203 system)(a) 6.5 mole% silica;(b) 16.6 mole% silica; (c) 25.7 mole% silica and (d) 31.3 mole% silica.0 62 • 10E3reduction tune (s)12•11 -10 -Fmk-so-ferrous toady at 1403 •Co lime-costaining locks+ apt 442- Model(a)(b)(c)Figure 6.15 'Weight loss data for the ferric-to-ferrous study in lime-containing melts(Fe10-Si0 2-Ca0-Al203 system) at 1400 *C (a) molar ratio 1.5, (b) molarratio 1.9 and (c) molar ratio 1.8.154(b)(c)12 1 110Fernc•o-fcrrous study at 1303 -Cin iklle-CGLII•ming Ine109$Eapt a70 fe'W.Fe .• 0.15- Model.• 7-▪ 5•2 a-(a)0 2 • 6 10Figure 6.16 Weight loss data for the ferric-to-ferrous study in lime-containing meltsat 1300 'C (a) molar ratio L8, (b) molar ratio L9 and (c) molar ratio 1.8.155(a)(b) 10 -9-Ferric-so-ferrous so.sdv at 1200m liess-ccauiaiaf meltsExfc 657--- Model1' -4-3-2 -0(c)0^•^•^12^16^20^24reduaion time, (s) s r'Figure 6.17 Weight loss data for the ferric-to-ferrous study in lime-containing melts(Fe 3+1IFe = 0.15) at 1200 'C (a) molar ratio 1.5, (b) molar ratio 1.9 and(c) molar ratio 1.8.156Table 6.15 Derived values of the liquid phase mass transfer coefficients and diffusivityfor the melts at three temperatures.Melt type Fe3+ Temp. Fitted kL DLEFe°C cm/s cm2/sLime-freemeltsFe,,O-S i02-Al2030.15-0.2 1400 — 1 x 10-4 — 5 x 10-60.23 1300 — 2 x 10-5 — 1 x 10-60.21 1200 — 2 x 10-6 — 1 x 10 7Lime-containingmeltsFe„0-SiO2-CaO-Al2030.15 1400 — 2 x 104 — 1 x 10-50.15-0.2 1300 — 7 x 10'5 — 3.5 x 10-50.1-0.2 1200 — 2 x 10-5 — 1 x 10-6Note: Diffusivity values are calculated using equation (6.1) and assuming boundary layerthichness of 500 gm.157Another important feature emerges from the data in this table. It shows that for a fixed reactiontemperature the lime-containing melts have a relatively higher k i, value when compared to thelime-free melts. Further comparison reveals that this discrepency widens at lower temperatures.The ratio of ki, values (i.e ki, for lime-containing melts to the lime-free melts) at 1200 °C is 10whereas the corresponding value for the melts at 1400 °C is about 2. The variations in ki, implythat perhaps the melt diffusivity values also change in a similar manner. Estimation of diffusivity dataDiffusivity values were determined using equation (6.1) i.e.D02-kL - 81(6.1)where 8, is the boundary layer thickness in the melt. An estimated thickness of this layeris approximately 500 gm from the MOssbauer spectroscopic analysis of the slag melts discussedin Chapter 7. It was assumed that oxygen anions move to and away from the reaction site. Theproposed diffusivity data for both the lime-free and lime-containing melts are listed in Table 6.15.A consistent trend of decreasing diffusivity values with decreasing temperatures is evident fromthis information. Furthermore the estimated diffisivity value of — 5 x 104 cm2/s at 1400 *C forthe lime-free melts agrees well with the value estimated using Sutherland equation (Table 6.3).One striking feature of the diffusivity data is that the values for lime-containing melts arehigher than those for lime-free melts. Such a comparison clearly highlights the effect of lime onenhancing the diffusivities of species in the melts. Sasabe and Asamura [109] have reportedsimilar finding based on their oxygen transport study in molten slags using electrochemicaltechnique.158With the knowledge of the melt kL values at three temperatures activation energy plot ofIn D against reciprocal temperature was constructed for lime-containing melts and this isillustrated in Figure 6.18. A similar plot was not constructed for lime-free melts because onlylimited low temperature data was available for these melts. However, rough calculations showedthat the apparent activation energy for diffusion in lime-free melts was — 96 kcal/mole. Indeed,this is not a reliable value but all the same it does show a trend in the right direction. Thecorresponding value for the lime-containing melts is about 53 kcal/mole. This value is in excellentagreement with that reported by Sasabe and Asamura [109]. For the diffusion of 0 2- anions theauthors report the apparent activation energy value of 56 kcal/mole.6.2.2 Reaction mechanismIn the proposed scheme the ionic melt consists of silicate, aluminate, ferric and oxygenanions and ferrous (Fe2+) and calcium (Ca2+) cations. Due to the amphoteric nature of both ferricoxide and alumina species however, the presence of Fe3+ and A13+ can not be ruled out. At thegas-melt interface the carbon monoxide reacts with the ferric anions to produce Fe 2+ as per thefollowing:FeOr +CO =Fe 2+ +202- +CO2 + e- (6.55)Fe03 - + e- =Fe2+ +30 - (6.56)The observed slowing down of the rate with time is attributed primarily to the decrease inthe bulk ferric oxide concentration and the accompanying structural and compositional changesin the boundary layer (top 500 gm) region in the melt. In addition to above reactions the chargetransfer reaction as proposed by Belton [80] will take place. However, due to the amphotericnature of ferric oxide species and in general due to the ionic constitution of slags it is very difficultto view a reaction mechanism in a particular context and therefore the possibilities of otherco-existing mechanisms cannot be ruled out.159Arrhenius plot of In k L versus 1/TFerric-to-ferrous reaction studyApparent activation energy of diffusion 53 kcal/mol-20-20.5-21-21.54.)6^-22-22.5-23-23.5-240.00059^0.00061^0.00063^0.00065^0.00067ha K•'Figure 6.18 Arrhenius plot for determination of the activation energy for diffusion, ED,.According to the proposed scheme the ferric-to-ferrous reaction involves breakdown (ordissociation) of ferric anions but it is likely that in addition the restructuring of anions may takeplace as per the following:2Fe2054- + CO = 2Fe03 - +2Fe2+ +302- + CO2 (6.57)Richardson [130] has suggested the following reaction in ternary silicate meltsCa2SiO4 +Fe2SiO4 =2CaSiO3+2Fe0 (6.58)to explain the positive deviation of a in these melts. This reaction indicates the occurrenceof a change in the anionic configuration of silica. Therefore, it is not unrealistic to extend thesimilar concept to the changes in ferric anion configuration.Gaskell et al. [110] have proposed 'discrete ion theory' to explain their density data insilicate melts. According to the theory, in the liquid state the extent of polymerization inorthosilicate compositions is governed by the relative energetics of M2+ - 02- and M2  - 0'associations. Since the 0210' ratio in the iron silicates is greater than the corresponding calcuimsilicate, when Fee` ions are replaced by ce the latter cations attempt to decrease the 0270' ratioin their coordination and in so doing cause a depolymerization of silicate anions in the vicinity.According to this scheme the ce ions associate preferably with the silicate anions whereas theFe2+ cations preferably associate with decreasing number of free oxygen ions. This preferredionic association introduces an element of microheterogeneity in the melt structure. It is possiblethat the behaviour of ferric anions follows that of silicate anions as per the above scehme andtherefore proposed reaction (6.57) is likely in lime-containing melts. Additional evidence insupport of this finding can be obtained from the work of Chipman and Chang [111].6.3 Comparison with previous workIn the foregoing paragraphs the data obtained in this work was critically analyzed to developan understanding of two complex reduction reactions. In order to derive additional benefits fromthe generated information the data obtained is compared with the work of Nagasaka et al. [56]161Some of the common features of the two investigations are: (1) the ferrous-to-iron reactionwas studied primarily at 1400 *C; (2) carbon monoxide-containing gas mixtures were employedfor promoting the chemical reaction and thermogravimetry technique was used; (3) a wide rangeof slag compositions were chosen to identify the effects of acidic and basic oxide species; and(4) limited low temperature work was undertaken to calculate apparent activation energy of thechemical reaction. The important points that emerged from the work of Nagasaka et al. were:(1) lime addition to the FeOx  melts led to higher rates; (2) the ferric content of the melt exceedinglyinfluenced the rate; (3) the values of composition-dependent apparent rate constant increasedwith the melt lime and ferric contents and (4) the apparent activation energy of chemical reactionis 33 kcal/mol. The present work suggests an appparent activation energy value of 28 kcal/moleand the data clearly highlights the rate enhancement due to the increased ferric proportion. Bothstudies showed that the silica additions lowered the rates. In terms of the lime effect however,the conclusion of the present work is at variance with that of Nagasaka et al.It is possible to explain an important observation made by Nagasaka et al. concerning therate enhancement due to higher ferric levels in the melts in the light of the new informationgenerated in this work. Since the intrinsic rate constant of the ferric-to-ferrous reaction is muchgreater than the one for the ferrous-to-iron reaction, the variations in the apparent rate constantsas observed by Nagasaka et al. can be explained by assuming the concurrent ferric-ferrous reaction.To demonstrate this the following procedure is employed:(1) apparent rate constant, k,, values for two melt compositions - one pseudo-binary (Fe x0-CaO)and the other pseudo-ternay (Fe x0-CaO-SiO2) are obtained from the rate versus driving forceplot provided by Nagasaka et(2) by employing gas + reaction model developed in this work and a knowledge of gas and meltcompositions rates are predicted assuming the ferric-to-ferrous reaction.(3) apparent rate constant values are predicted from the above data i.e.ka (predicted) = predicted rate ID .F .(asperNagasaka); and(4) the predicted k values are compared with those reported by Nagasaka et al.162The results obtained are listed in Table 6.16. Agreement between the two sets of apparentrate constants supports the hypothesis on the concurrent ferric-to-ferrous reaction. To furtherhighlight the above fact and the accuracy of the gas + reaction model predictions, the predictedka data for various melt compositions is superimposed on the ka versus ferric/ferrous relationgiven by Nagasaka et al. (Figure 6.19).One important feature of the gas + reaction model proposed in this work is that it clearlyidentifies the components of the gas and reaction resistances. Thus, it is possible to factor outthe gas phase contribution and look closely at the remaining term mentioned in equation 6.31 i.e.A^ (6.31)lca =lcc.z.aFiowhich essentially describes the nature of the apparent rate constant for the ferrous oxidemelts (Fe 3+11Fe < 0.05). By mathematical manipulation one can remove the activity componentfrom this term and incorporate the same in the driving force term. Thus the effect of meltcomposition on the rate can be represented by either rate constant or driving force terms. Nagasakaet a. chose the former method, however the authors account for the activity contribution by theempirical relation in terms of a complex ferric/ferrous ratio.The concept of the apparent rate constant can be extended to include the ferric-to-ferrousreaction for which the following expression can be written in absence of gas and liquid phaseresistances.. Ake =kc .—a. FA, '2°3(6.59)Confirmation on equations (6.31) and (6.59) proposed above can be obtained by comparisonof ks values reported by Nagasaka et al. and those predicted using these equations. This isillustrated in Table 6.16 and Figure 6.19.163Table 6.16 Comparison of predicted and reported lc, data for the pseudo-binary and pseudo-ternary melts at 1400 'C.Melt composition, wt% Gas composition,atmD.F.' Apparent rate constant, lc,g/cm2.s.atmReduction rate,g/cm2.sFeO Fel°, CaO Si02 pc0 pc02 atm. Nagasaka Predicted Nagasaka Predicted67.8 10.2 22.0 -0.168 0.04 0.0027 10 x 104 9.2 x 104 2.7 x 10'6 2.4 x 1040.294 0.07 0.005 8.0 x 104 8.8 x 104 4.0 x 10-6 4.2 x 1040.336 0.08 0.006 8.3 x 104 8.8 x 104 5.0 x 10.6 4.8 x 10'60.461 0.11 0.007 9.3 x 104 10 x 104 6.5 x 10'6 6.5 x 10'60.798 0.19 0.013 10 x 104 8.8 x 104 13.0 x 104 11.3 x 10458.0 20.5 14.0 7.50.0073 0.0015 0.0011 3.6 x 104 3.9 x 104 4.0 x 104 4.3 x 10''0.011 0.0022 0.002 3.5 x 104 3.4 x 104 7.0 x 10-7 6.6 x 10'70.0347 0.007 0.0058 3.4 x 104 3.6 x 104 2.0 x 10 .6 2.1 x 10 -60.05 0.01 0.01 3.3 x 104 3.5 x 104 3.3 x 104 3.0 x 10 -60.106 0.02 0.023 2.7 x 104 2.7 x 104 6.3 x 104 6.4 x 1040.257 0.05 0.05 3.0 x 104 3.1 x 104 1.5 x las 1.5 x 10'5Note:^(1) * Nagasaka et al. define reaction driving force, D.F. and ics as follows^ .r co2^observed rale D.F = P- co - 0 242 and k, -^D.F. .(2) Rate data is predicted using gas + reaction model and assuming concurrent ferric-to-ferrous reaction.(3) 1g (predicted) = predicted rate/D.F.1001 0(NE1X0.100001^00005 0001^6 .005 0.01^0 .05 01^0. 5^1(N4)2/(N, )3Figure 6.19 Empirical relationship between lc and ferric content for various melts (as perNagasaka et al. 56). Predicted lc values using the model in this work are shown inthe figure for comparison.Rahaiby et al. [34] introduced< as a temperature dependent constant to explain the observedvariations in k, and developed an expression relating the two constants. (equation 2.15) Theauthors noted that k : values decreased with increasing additions of silica and to accomodate suchbehaviour they provide 4 separate equations relating i c a° and temperature for four meltcompositions (with varying silica). Sasaki et al. [68] have extended a similar concept to the limeferrite melts and derived different relation between k ,,° and temperature. However, according tothe authors the equation is based on an overall scatter of about ± 60 pct. It would appear that sofar there has been no successful attempt made to explain the fundamental reasons for the observedvariations in the apparent rate constant. With the help of the proposed relations (equations 6.31and 6.59) between 1 s , and other parameters like intrinsic rate constant (lc and k, fractional coverageparameter (AJA,,) and the activity of the reactant component in the liquid phase it is easier toappreciate the basis for the observed trends in the k,, values.Belton [80] reproduced the ks versus ferric/ferrous plot reported by Nagasaka et al. (i.e.Figure 6.19) wherein the author has shown the location of Fex0-P205 melts relative to the line.All the points representing these melts lie considerably below the regression line and this factclearly implied that the points did not conform to the empirical relation proposed by Nagasakaet al. Belton agrees that the behaviour is explicable in terms of a surface coverage model similarto that proposed in the present work. However, the author has proposed an alternate model asan explanation for the observed phenomenon. Pal et al. [101] also noted that additions of P 2O5to lead silicate melts caused a significant decrease in the rate of reduction by H2. The depressionof the rate was attributed to surface active phosphate groups. It is an accepted fact that silica issurface active in ferrous oxide melts and therefore the surface coverage explanation can beadvanced for the results obtained by Nagasaka et al.Based on the knowledge gained in the present work it is suggested that the lower k, valuesresult from the lower activity and A/A,, values. (equations 6.31 and 6.59) In this context, it isparticularly interesting to examine critically the relative location of all the data points on the166regression line in Figure 6.19. The data points for the pseudo-binary Fe x0-CaO melts are clearlyabove the line indicating high k, values for these melts. Thus the trend is obvious - basic meltslie above the line and the acidic melts (containing surface active species) below the line. It is notunrealistic to extend the analogy to characterize the location of the melts containing higherlime-to-silica ratio with respect to those containing lower ratios. Although, the distinction is notas obvious as previous two cases (i.e CaO and P2O5 containing melts) one can place the relativelybasic melts above the line and the others below the line without much ambiguity. What thisanalysis implies is that the composition-dependent apparent rate constant is sensitive to the relativeproportions of acidic and basic oxides in the melts and this observed behaviour is explicable usingequations (6.31) and (6.59) proposed in this work.Figure 6.19 shows that lc increases with increasing Fe 3+/Fe2+ ratio. Based on the work ofNagasaka et al. alone it is very difficult to identify the role played by the individual oxide specieson the apparent rate constant. For example, from the location of pseudo-binary Fe 10-Ca0 meltsit is not possible to confirm whether the higher Ics values are due either to higher ferric level orhigher lime content or both. However, from equations (6.31) and (6.59) one can appreciate therelative significance of individual oxide species on the values of the apparent rate constant.Tsukihashi et al. [55] and Kato et al. [54] studied molten iron oxide (FeO)x  reduction at1600 °C using pure CO gas. However, both groups report substantially different values for theintrinsic rate constant, kc. The former group proposed a value of about 20 cm/s whereas the lattergroup reported the value as 1350 cm/s. Since it was observed in the present work that at 1400°C the intrinsic rate constant value of Fe+ Fe2+ reaction was approximately 200 times greaterthan the Few Fe reaction it can be inferred that perhaps Kato et al. recorded the lc, value forthe ferric-ferrous reaction (referred to in this work as k;). Such possibility cannot be overlookedespecially if, for any reason, their melts were oxidized prior to the reduction period.167Based on the data obtained in this work following expression can be derived for predicting theintrinsic rate constant of the ferric-to-ferrous reaction(6.60)= 933 exp( -44500RTUsing equation (6.60) a k value of 920 cm/s was obtained which is some what closer tothe value reported by Kato et al. This calculation implies that for the ferrous-to-iron reaction thelc, value reported by Tsukihashi et al. is more accurate than the value proposed by Kato et al.While studying the ferrous-to-iron reaction in the present work a bulk of the data wasobtained at 1400 °C and only a few experiments were performed at lower temperatures andtherefore an expression similar to the above cannot be constructed exclusively on the generateddata. However, by combining the apparent activation energy reported by Nagasaka et al. [56]and the lc value (6.875 x 10 mole/cm 2.s.atm) obtained in this work at 1400 °C it is possible toderive the value of the pre-exponential constant, a, in the following(6.61)k, = a. exp —33"RT )and develop another expression similar to equation (6.60). Simple calculation yields thevalue of 'a' as 0.14 and in turn its substitution in equation (6.61) yieldsk, = (0.14). exp( —33000^mole^(6.62) )RT^cm2 • S • aimThe kc value of about 2 x 10 -5 is predicted for 1600 °C melts using the above equation. Thisagrees reasonably well with the reported value of 4.2 x 10 -5 by Kim et al. [66] The predictedvalue converts to about 3 cm/s and this is much closer to the value of 20 cm/s proposed byTsukihashi et a. [55] It may be recalled here that the lc, value of 1350 cm/s was reported by Katoet al. [54] at the same temperature.168An important point that emerges from the above discussion concerns the accuracy of theavailable data on intrinsic rate constants. Due to the nature of slag melts it is very difficult tostudy the ferrous-to-iron reaction in isolation by eliminating the ferric-to-ferrous reaction. To adegree the latter reaction is unavoidable in the ferrous reduction studies. Therefore, based on theknowledge gained in the present work it is proposed that the extent of the Fe -4 Fe2+ reactionwould dictate the accuracy of both intrinsic rate constant and apparent rate constant values in anyinvestigation.Belton et al. [80,34,68] have proposed a charge transfer model for the redox reactioninvolving ferric and ferrous species. (equation 6.33) Upon close scrutiny of their conceptconcerning the negetively charged CO 2 molecule (COP") it appears that the authors are referringto a "gaseous anion species" at the interface. The dissociation of COI— molecule produces 02"anion and CO molecule (equation 6.34) and according to the authors this is the rate determiningstep. As an aide to the data interpretation they introduce another temperature and compositiondependent constant and propose the following empirical relationka = k .(a0)-1 (6.63)onowon2...amowhere ao is the oxygen activity expressed either as Pc02/P co or Fe3+IFe2+ ratio. It is feltthat in their analysis the authors are referring indirectly to some 'special' 0 2" anions at the surfacethat can take part in the reaction. For example, the oxygen anions surrounding Si4+ cannot takepart in the reaction and so also those surrounding the phosphorus species (i.e. phosphate ions),if present, in the melt. In general, the acidic and surface active species do not release oxygen andthis leads to lower rates. The lower ka° reported by Belton et al. are in all probability result fromthe decreased availability of 02" anions. By corollary, one expects higher rate due to the increasedavailability of reactive 0 2". Perhaps the comments made by Nagasaka et al. concerning theinfluence of melt ferric level on the rates can be viewed in this perspective. The 0/Fe ratio offerric oxide is high. Moreover, the species is surface active and therefore the number of special16902. is increased and this causes higher rates. While Belton et al. view co-existence of 02- andCO (as Con species in their model, in the present work the ferric, ferrous and other cationspecies are assumed to co-exist with the 0 2- anions.In either of the above schemes the common and perhaps the most important parameter isthe number of special 02- anions at the interface. Any variation in this number would manifesteither as a change in the surface electrochemical potential or a change in the fractional coverageparameter (A/AO which essentially is a different perspective of the same phenomenon.Alternately, the changes in either the surface electrochemical potential or the fractional coverageparameters can be viewed as different consequences of the same phenomena, namely -minimization of the surface free energy. Indeed, it is a very difficult task to visualize what isgoing on at the gas-slag interface and perhaps there are other alternate ways to view the interfacialphenomenon.However, with the technique of analysis proposed in this thesis it is easier to appreciate thereasons behind the observed variations in the apparent rate constants. For example, Belton et several different expressions for the calculation of < and additionally they note that thesimple inverse relationship between k, and k: does not hold for various melts. The authors havereported large variations in ic: with the slag basicity ratio and the melt ferric levels. Based onthe knowledge gained in this work an explanation for such behaviour can be proposed using thefollowing expressionk: =^k:^- ?I-A .(aF.49)2(Q„a„,„,2,.,,,,,,,) ri,(6.64)where k: is the apparent rate constant used by Belton et al. (for ferrous-to-ferric reaction)and k: is the intrinsic rate constant for the oxidation reaction. Due to silica additions the valuesof A/A. and a will be lowered and this would result in the lower < values. In the lime andsilica containing melts the apparent rate contant values will be lowered for the same reason. Inthe lime-saturated melts the reported k: values are about 10 times greater than the one for pure170ferrous oxide melt. [80,34,68] Explanation for this behaviour is not available in the literaturehowever, based on the proposed equation (6.64) one can speculate that the result is attributableeither to the higher lc: or the higher A/Ac, value or the combination of both. It can be postulatedthat the presence of ferric anions leads to the higher lc: or the addition of lime increases the activesites at which CO2 can be adsorbed. The latter possibility has been postulated earlier by Strachanand Grieveson [32] and this implies increase in the A/A,, ratio in equation (6.64).The model presented in this work is consistent with the accepted behaviour of varioussurface active species and it accounts for the amphoteric nature of ferric oxide species. Thus,the existence of both Few cations and ferric anions is favored in the slag melts in agreement withthe published data [96,112,102,113]. In the reactions studied in this work the presence of chargetransfer reaction cannot be denied however, the data suggests additional rate determining steps.What emerges from this study is that the behaviour of iron oxide containing melts during reductionand oxidation study is very complex and depends critically on melt composition. The interactionof surface activity effects with the transport of oxygen in the melt, as 02' and ferric anions, canlead to quite different processes. It is quite clear that a quantitative understanding of the processmust take these factors into account.6.4 Consequences of the results to slag fuming kineticsPrevious work by Richards et al. [9] has shown zinc slag fuming to be kinetically controlledby two basic processes: the rate of coal entrainment in the slag and the rate of oxidation offerrous-to-ferric iron by injected air. Based on their comprehensive in-plant measurements andsubsequent mathematical analysis the authors developed a coal particle-slag reaction model andconcluded that an increase in process efficiency would result from increasing the coal entrainmentor reducing the oxidation of ferrous iron.In the subsequent trials Cockroft et a. [114] observed that using a high-pressure pneumaticdelivery system the coal entrainment can be increased and the phenomenon led to higher fumingrates. To accomodate this change a new parameter, FHpcE, the fraction of high pressure coal171entrained in the slag, was added to the kinetic model of Richards et al. [9] Moreover, to obtainbest agreement between the measured and predicted data the 'slag circulation velocity', Vs 1, wasincreased 3-fold from the original value of 1 m/s.Based on the present thesis further modifications to the kinetic model are suggested. Sincethe experimental work has clearly identified the role of surface blockage by the non-reactantspecies, it is important to account for this effect in the model. In addition, in view of the derivedoxygen anion diffusivity data changes in the "ferric oxide" diffusivity values are proposed in theoriginal model. The effect of both these factors, especially the latter is to substantially changethe nature of the coal particle-slag reaction. As a result, the present work has promptedreassessment of three important parameters in the kinetic model, namely - fraction of oxygenoxidizing ferrous-to-ferric iron, F oc (i.e. oxygen utilization), fraction of coal combusted in thetuyere zone, Y and slag circulation velocity, Vet.172Chapter 7Miissbauer spectroscopy7.1 IntroductionMOssbauer spectroscopy is a well known technique for studying the structure of disorderedmaterials. Since 1962, chemists have made use of this technique in their study of chemicalbonding, crystal structure, electron density, ionic states and magnetic properties. Metallurgists,however, have only recently appreciated the potential importance of this technique and as a resultvery limited data is available on the Miissbauer analysis of slags. [98-99]Previous Miissbauer studies have focussed primarily on the bulk properties of the material,and questions such as how the coordination and oxidation state of the iron may vary spatiallywithin a sample have not been addressed. The present research was initiated to look at the spatialvariation of MOssbauer spectral parameters under conditions in which differences between thesurface and bulk melt compositions are expected. A technique was developed to study ferric andferrous cation distribution across the melt depth with a spatial resolution of approximately 5007.1.1 Sampling of high temperature phasesMässbauer spectroscopy works the same no matter if the phase is glassy or crystalline andtherefore it serves as a means of identifying coordination of iron cations in either cases. R.D.Jones [115] in his review paper mentions several publications that have employed this techniqueto identify various phases in the iron-carbon alloys. However because the measurements aredone on solid samples the information obtained this way is only useful if it is assumed that thehigh temperature melt structure is preserved during quenching. The validity of this assumptionis normally tested by characterizing crystallization in the individual samples using techniqueslike XRD, optical microscopy and TEM. In the present work two former methods were employedand the results obtained implied that there was minimal crystallization or segregation occuringduring quenching process.173The fact that in four out of eight samples a crystalline phase (iron or magnetite) was presentas a result of reaction under study prevented a firm conclusion from being drawn regardingcrystallization during quenching. However we believe that our assumption of minimumcrystallization during quenching is valid because of the shallow melt depths (3 - 4 mm) and ashort quench time not exceeding 15 seconds. Of the 4 remaining samples (experiments awayfrom Fe and Fe304 saturation) in 3 the XRD patterns did not reveal any crystalline phases whereasin one sample in the same series of experiments (in the Fez0-Si02-Al203 system) only a minorproportion of FeO was identified. It is believed that precipitation of FeO during quenching inthis case may not have caused substantial variation in the ferric-to-ferrous ratio of the slag andtherefore the MOssbauer data would still be of use.Bukrey et al. [116] have correlated the appearance of hfs (hyperfine splittings) with themethod of quenching. The authors report the absence of hfs at fast cooling rate. Miissbauerspectra obtained in this work showed no hfs with the exception of two samples and in both thesethe result was attributed to the presence of iron and magnetite which resulted from prevailingexperimental conditions. It is appreciated that these results provide only indirect evidence of asuccessful quenching procedure. However in this regard additional work is highly recommended.7.1.2 Limitations of present Miissbauer analysisIt is to be appreciated that the MOssbauer analysis was initiated only to supplement thefindings of the kinetic data and the mathematical modelling work. Furthermore only a few slagsamples are analysed and the data comparison with equilibrium melts is lacking. In general, thisbeing a new technique it has yet to fulfill the promise of being a reliable and reproducible analyticaltool. In spite of these impeding difficulties an attempt is made to relate the MOssbauer data tothe slag structure and develop some understanding of their interdependency.7.2 Miissbauer techniqueA typical Mässbauer spectrometer consists of a source of gamma rays, an absorber (thesample being studied) and a means of recording the amount of radiation passing through the174absorber. The source is moved relative to the absorber, shifting the energy spectrum of the sourcedue to the Doppler effect. A gamma ray emitted from the source is absorbed by a nucleus in thesample if resonance occurs, that is, if the energy of the gamma ray is identical to the differencebetween the ground and excited energy states of the nucleus. If a gamma ray is absorbed, it iseffectively removed from the flux seen by the detector and a "hole" in the energy spectrum results.The base line of the resulting spectrum therefore contains the maximum number of counts andresonance lines occur at minima. MOssbauer spectra are commonly plotted with % absorptionon the y axis and velocity (energy) on the x axis.7.2.1 Slag preparation:For the MOssbauer analysis quenched slag samples were chosen from both simple andcomplex melts in the Fex0-SiO2-CaO-Al2O3 system. Wet chemical analysis and the experimentaldetails of the slags are listed in Table 7.1.Two sections were cut from each quenched slag sample, one parallel to the slag surface(for surface analysis of the top 10 gm) and one perpendicular to the slag surface (for depthanalysis). The sections were cold mounted in a 31 mm diameter mould using a self setting resin,and ground to a thickness of approximately 10 pm using an additional resin ring to support thethin disk. The samples were numbered in pairs corresponding to the slag that they were quenchedfrom; for example #2 and #3 represent the surface and depth samples from different parts of thesame quenched slag.7.2.2 Generation of Miissbauer spectra:Schematic of the sample holder is shown in Figure 7.1. The sample was mounted on atwo-axis travelling stage that controlled the position of the sample with respect to the collimatedgamma ray beam.175Table 7.1 Chemical composition and experimental parameters of slag samples.Sample ,Slav Composition (wt470)* FeaFe* GasMixturePo,Main Reaction# FeO Fe203 Si02 CaO Al20, (atm) 17,1819,202,3 92586122200210220618160.0190.0300.0295x la"1x10-12-1x10-13Fe' +0,.,, -9 Fe 04,110Fe2+414) --) Fe 4,44At Fe saturation9,115,63429672226141424240.1370.1783x104°7x 10'9Fe' +0 ^-4 Fe2f01,43Near Fe saturation15,167,87036147322.5011.513230.1520.1492x1047x104°Fe' 4.0 ^-4 Fe2+04)Away from Fesaturation12,14 30 14 22 14 20 0.296 4x104 Fe2+060 -+Fe3+0,,,i0At magnetitesaturationNote: Assays determined by wet chemical analysis.Legend1. Lead shielding2. Aluminium body3. 500 gm hole in lead4. radioactive source5. slag sample6. aluminium body7. two-axis travelling stage8. vertical adjustment9. transducer body10.thumb screwFigure 7.1 Schematic diagram of the sample holder assembly.177The tip of the source was mounted within 2 mm of the hole in the lead shielding, resulting in avery small source-to-sample distance. The position of the hole in the lead shielding was controlledwith respect to the source using four thumb screws. To align the source directly behind the holethese screws were adjusted until the maximum count rate of gamma rays was recorded by thedetector. Additional details are provided in our previous publication [117].MOssbauer spectra were recorded for two different configurations of the quenched slag.The first, surface analysis, represents the top, approximately 10 gm, layer and the other, depthanalysis, represents two depths within the quenched slag: 500 layers centred at depths of 250pm and 1.0 mm. In the depth analysis configuration the surface of the slag was first located bymoving the y axis (depth) of the sample stage and noting the position of the drop in count rate atthe sample-resin boundary. Using this reference point the y axis of the stage was then moved tothe appropriate depth and the MOssbauer spectrum recorded. The depths could be measured to± 50 p.m, and the horizontal displacement (direction parallel to the slag surface) remained constant.The complete set of MOssbauer spectra recorded of all eight slag samples is shown in Figure7.2. The shapes of the spectral envelopes vary dramatically with composition, and in commonwith most glass spectra, they are poorly resolved with respect to individual Fe y* and Fe3+ sites.The challenge therefore is to select a fitting model that enables a comparison of MOssbauerparameters between different spectra in order to elucidate the variation of Fe and Fe2+populations. A fitting model was chosen which used three Fe y'' doublets and one Fel* doubletwhere the line widths of all doublets were constrained to 0.5 mm/s. Each doublet correspondsto a specific ion site. The spectra were fit using a nonlinear least-squares fitting method developedby the CERN Computer Centre Program Library. [118] Further details are provided in ourprevious publication. [117]178Magnetichype:finesplitting...mum.=• • 71 Tie •I^a^IFe metal^ft .:........s.....7.2 . •,:..t..A , .. i.^..1 .-:-......,. .,.4 -s^4^S x3.^2 4..111-4^-2.^...C•.14T- -.^-^..,.. ^-^-- -'•,- .17.- Africac'•• -^*.^.... ... - .. -,..^r- - •_•...••:..^..........^.•...---^ r.t,-^•^-- V -;oft-T .f"^I,-1":"-. -^'if'-f-tr .a' i -^.--- — vfaT-1r I..lamas. . 1..: .lb ..:,:i::;..^)ree7ifir . ..I^1_• ,,,.„-,-,-,fr‘1^1-^-- - -- -,f.„.-ft.lag 17.1819.20Slag 9.11Slag 5.6Slag 15.16Slag 7.8Slag 12.14Slag 2.3to (10 tun layer) surface (500^layer)^bulk2^4 y^-2^0^2^4 Velocity (mrnis)Figure 7.2 Complete set of MOssbauer spectra recorded at all depths for eight slagcompositions.1797.3 Results:7.3.1 Spectral parameters:To facilitate interpretation and comparison of the MOssbauer data for each set of quenchedslag samples, several quantities were calculated as a function of depth within the slag. Thesequantities, namely: quadrupole splitting (QS), isomer shift (IS) and relative area (area) arefrequently used in any MOssbauer spectroscopy work and additional information on these isreadily available in any textbook on this subject. The individual values of QS, IS and area obtainedin this work are listed in Table 7.2.A critical look at the data in Table 7.2 highlights distinct differences in the values of thespectral parameters with depth. This fact is evident in the reported slag spectra in the Figure 7.2as well. Since the relative differences in the spectral values are caused largely by the ferric andferrous cation gradients across the melt depth it was believed that the MOssbauer data could beused in the estimation of the liquid phase boundary layer thicknesses. On this basis a boundarylayer thickness value of roughly 500 gm is estimated. The prediction is based on the fact thatgenerally the relative difference between 'top' and 'surface' spectral parameters for an individualslag sample was smaller in comparison with an equivalent difference between 'surface' and 'bulk'parameters.The mean quadrupole splitting for Fee* in each specrum was calculated as the weightedaverage RQS 1 *Areat) + (QS2*Area2) + ....]/(Area l + Area2 + ). These values are plotted inFigure 7.3 for each set of slag samples as a function of depth. Variation of the mean quadrupolesplitting with depth indicates a change in the nature of Fe 2+ sites, probably in coordination ordegree of distortion. The distribution of quadrupole splitting values within a given slag samplecan be seen in Figure 7.4, where the quadrupole splitting of each doublet is plotted as a functionof its relative area. Figure 7.5 illustrates the variation of Fe+ relative area as a function of depthwithin the slag.180Table 7.2 MOssbauer parameters of slags quenched from 1400 C.Sample Fee t1) Fe2+09, Fe2+0.0  Fe3+QS IS area QS IS area QS IS area QS IS area#17 top 1.50 0.86 0.058 0.85 0.93 0.443 0.39 0.97 0.499 0.73 0.32 0.000#18 surface 1.73 0.95 0.239 0.86 0.93 0.471 0.25 0.90 0.263 0.98 0.30 0.027, bulk - - - - - - - - - -_ -#19 top 1.98 0.96 0.305 1.44 0.97 0.310 0.78 0.96 0.298 0.95 0.35 0.088#20 surface 1.89 0.97 0.314 1.27 0.97 0.292 0.65 0.98 0.313 0.96 0.35 0.080bulk 2.00 0.96 0.305 1.26 0.97 0.323 0.53 0.96 0.402 1.11 0.35 0.055#9 top 2.18 1.07 0.309 1.60 0.99 0.362 0.92 0.94 0.187 1.01 0.35 0.142#11 surface 2.18 1.04 0.367 1.56 0.99 0.304 0.85 0.94 0.213 1.19 0.35 0.115bulk 2.31 1.10 0.217 1.75 1.00 0.419 1.00 0.94 0.278 1.20 035 0.087#5 top 2.31 1.08 0.360 1.77 1.01 0.360 1.16 0.98 0.180 1.33 0.35 0.101#6 surface 2.29 1.06 0.324 1.85 1.02 0.355 1.26 0.97 0.207 1.27 0.35 0.114bulk 2.34 1.07 0.316 1.80 1.00 0.424 1.04, 0.94 0.155 1.25 035 0.104#15 top 2.56 1.21 0.151 1.05 0.94 0.342 0.53 0.94 0.412 1.14 0.35 0.095#16 surface 2.54 1.23 0.121 1.17 0.95 0.311 0.61 0.96 0.455 1.15 0.35 0.114bulk 2.43 1.31 0.142 1.16 0.97 0.214 0.63 0.96 0.516 1.15 035 0.129#7 top 2.19 1.12 0.340 1.53 1.01 0.272 0.75 0.91 0.200 0.94 0.35 0.187#8 surface 2.23 1.14 0.332 1.53 1.03 0.262 0.77 0.95 0.220 1.03 0.35 0.186bulk 2.22 1.13 0.365 1.50 1.04 0.235 0.64 0.96 0.295 1.17 0.35 0.105#12 top 2.57 0.73 0.170 1.73 0.83 0.396 0.98 0.85 0.271 1.50 0.35 0.163#14 surface 232 1.12 0.359 1.81 1.05 0.326 0.95 0.97 0.155 1.33 0.35 0.159bulk 2.34 1.12 0.308 1.76 1.06 0.317 0.96 0.97 0.194 1.18 0.35 0.181Fe2+(Crystall ne) Fe2+(glass)#2 top 2.85 1.17 0.257 1.61 0.97 0.743 - - - - - -#3 surface 2.76 1.19 0.337 1.58 0.95 0.663 - - - - - -bulk 2.82 1.17 0.652 1.42 1.09 0.348 - - - - - -Note: "top" refers to the top 10 grn layer."surface" refers to the top 500 p.m layer."bulk" refers to a depth of 1.0 mm within the quenched slag.181- with solid ironand no silica- - no solid ironwith silica#17,#18—0— #19,#20- - IR- - #9,#11- -12-- #5,#6--A-- #15,#16#7,#80.8^1.0^1.2Figure 7.3 Mean quadrupole splitting of Feet doublets versus depth for six slagcompositions.1820.60.54)0.4> 03# 17, # 180.00^1^2Quadrupole splitting (mm/s)0.03^0^1^2^3Quadrupole splitting (mm/s)0.6<Ts o.5#19, #207) 0.2Q.> 0.30.6cti 0.5^#15, #16 .^0.6:^0:1 0.5• a.).:.:..,^0.4tp> 0.3 - A• • ■ I■■^a^I▪ a.)C40.1 -0.0^ . .0 1^2^3^0.0 ^ '1^2Quadrupole splitting (minis) Quadrupole splitting (mm/s)0.1#7, #80.6  ^0.6#9,#1101 0.5^ a/ 0.5a, -::c4 0.4 ..:.4 0.4cu 43)> 0.3^ > 0.3'Vs ro-., 0.2 --4 02&-' 4DC40.1^ 0.10.00^1^2^3^0.0 43^1^2^3Quadrupole splitting (mm/s) Quadrupole splitting (mm/s)Figure 7.4 The relative area as a function of the ferrous QS values. The symbolsare as follows:• top 101.1m layer,o 500 pin depth; • 1.0 mm depth, andarrows indicate the direction of increasing depth. A typical set of errorbars is indicated on the sample #5,#6 dataset.1830.2Figure 7.5 The change in Fes- concentration with depth. A typical set of error barsis indicated on the sample #5,#6 dataset.184In Figure 7.5 the vertical axis is calculated as %Fe 3+(x) — %Fe34"( where x is the depthwithin the slag; therefore all samples plot initially at zero, and then show either an increase or adecrease with depth according to the variation of the relative amount of Fe..7.3.2 Spectra analysis:MOssbauer results are sensitive to the technique adopted for resolving a spectrum intodoublets and therefore even minor variations among two different techniques may causedifferences in the spectra peaks of similar slags. Because of these difficulties, conclusions onslag structure which are based on M6ssbauer spectroscopy are likely to be valid only if they arereached through consideration of relative changes in the spectra of a series of slags. All theconclusions drawn in this study have been limited to those which can be so deduced.The fitting model includes three Fe2+ doublets and the same cannot be correlated withspecific sites on a one-to-one basis because we are approximating the site-to-site variationsinherent in the disordered solid with a set of discrete Lorentzian doublets. Nevertheless, thepresence of a resolved Fe doublet at high velocity in some of the spectra (Figure 7.2) suggeststhat Fe2+ occupies at least two structurally dissimilar sites, although it is not possible to deducethe coordination of these sites. Previous investigators have concluded that Fe2+ occupies bothtetrahedral and octahedral sites in quenched Pb-Fe-O-Si glasses on the basis of similar fits [119];however these would be subject to the same uncertainties. Fe has been reported to occur inboth tetrahedral and octahedral coordination in quenched slags [98-119]. Due to the small amountsof Fe3+ in the present samples however, it is not possible to resolve different site configurations;we therefore fit only one Fe}'' doublet to each spectrum. The QS values (for iron cations) reportedby Bowker et al. [99] and the data obtained by Hollitt et al. [119], both suggest that lower QSvalues correspond to higher coordination numbers and the higher QS values to lower coordinationnumbers. In neither case however can the absolute coordination numbers can be stated. OurMOssbauer results are consistent with this observation.185Since the specific iron cationic sites are poorly resolved the following parameters of ourfitting model are of less significance: (1) the number of Fe2+ and Fe5+ doublets required to fit thespectra; and (2) the specific values of quadrupole splitting and isomer shift of each doublet.Similarly, individual values of isomer shift and quadrupole splitting should not be used todetermine the absolute coordination states (e.g. octahedral or tetrahedral) of the various sites.However, we consider the following parameters of the fitting model to be significant: (1)weighted mean values of isomer shift and quadrupole splitting averaged over all Fe 2+ doubletsfrom each sample; (2) comparison of these values with composition or depth; and (3) comparisonof relative areas of doublets between different compositions or depths.7.33 MOssbauer data and wet chemical analysis:The MOssbauer spectrum of the starting material Fe.0 was recorded and using a fittingmodel developed by McCammon and Price [120] the value of x was found to be 0.97 ± 0.01.X-ray diffraction results showed the presence of both iron and magnetite in the Fe 10, althoughdetailed quantitative information was not available. Wet chemical analysis was performed toobtain total iron and ferrous iron and the ferric iron content was determined by difference. Onthis basis a value of x = 0.98 ± 0.01 was obtained which is consistent with our MOssbauer results.A similar comparison was made between the Fell'a Fe values determined by the MOssbauermethod and those obtained by wet chemical analysis for each slag compositions. The comparisonis illustrated in Figure 7.6 using the MOssbauer data taken at 1.0 mm depth. It is to be noted thatan accurate ferric analysis by wet chemical methods is often difficult and agreement between twoindependent methods is rare.Our MOssbauer results consistently underestimate the amount of Fe+ relative to the wetchemical method, and the trend could be explained by our decision to fit only one Fe doublet.The fact that the experimental data are linearly correlated within experimental error indicates thata systematic error may be responsible for the discrepency. However, this discrepency does notaffect the conclusions in our study because only the variation of parameters with depth are usedin the analysis.1860.1^0.2^0.3Fe 3 +17_,Fe (MOssbauer)0.4Figure 7.6 Comparison of Fe3+ / IFe values obtained by MOssbauer spectroscopyand wet chemical analysis. The solid line indicates the 1:1correspondence line, while the dotted line is the linear best fit.1877.3.4 Identification of crystalline phases:According to Bukrey et al. [116] the MOssbauer technique is useful in identifying thepresence and amount of crystalline phases in the quenched slag. The authors have shown thatsuch a detection is very sensitive and moreover it can detect crystalline phases at levels whichare undetected by X-ray spectroscopy. Figure 7.2 illustrates the spectrum recorded of the top 10gm layer from sample #2,#3 where the six-line pattern corresponding to metallic iron can beclearly seen. The relative area of the iron spectrum is 7± 2%, implying a similar amount of ironpresent. The XRD patterns of this sample indicated presence of iron and fayalite species. Bycombining both the pieces of information it is estimated that in sample #2,#3 about 7 - 11% ofthe slag volume was occupied by the crystalline phases. Application of a similar criterion tosample #12, #14 revealed that nearly 15% of the slag volume was crystalline. This compositionwas run within the magnetite saturation field and the presence of magnetite was confirmed byboth XRD and MOssbauer techniques. It is likely that some magnetite may have formed duringquenching however in the interpretation of the present data this possibility is ignored.7.4 Discussion:Our analysis of the weight loss/gain versus time data suggested compositional gradients asa function of depth and therefore the M6ssbauer spectroscopy results are examined critically toconfirm this finding.(A) Effect of Si02Iron cation distribution is normally related to the nature of the surrounding anions, and itis known that 02" anions prefer coordination of smaller radius cations whereas other anions (e.g.SiO44" ) coordinate larger radius cations [121,96]. Any changes in the population density of theanions should therefore result in corresponding changes to cation coordination. According tosurface tension data silica is surface active in iron oxide melts [67,102,103,105]. Though it isappreciated that the excess concentration of silica is restricted only to the monolayer thicknessesat the surface which cannot be detected by MOssbauer technique alone as a first approximation188it is assumed that the redistribution of silica is likely to cause a change in anion distribution.Because Si + preferentially coordinates with four easily polarizable 0 2" anions, a change in ironcoordination with depth is likely to occur. The Fe.0-Si0 2-Al203 slag shows a change in thedistribution of quadrupole splitting values for Fe 2+ as a function of depth (sample #15,#16; Figure7.4), which is consistent with this conclusion, although it is difficult to isolate the effects due tothe other components. As indicated, the proportion of the lowest quadrupole splitting Fee 'decreases as the surface is approached. This implies a decrease in the 0 2" available for coordinationdue to the elevated level of Si4+ at the surface (because of its surface active nature).It is important, however, not to ignore the effect of the chemical reduction reaction takingplace at the gas-slag interface. The reduction of ferric to ferrous ions, for example2Fe033- + CO = 2F e 2+ + 502-+ CO2 (7.1a)2Fe3+ + CO + 0 2- = 2Fe 2+ + CO2 (7.1b)is expected to result in a gradient of ferric iron concentration decreasing towards the surface.Sample #15,#16 shows this trend (see Figure 7.5 and Table 7.2). It is interesting to note that inthe absence of Si02 (sample #17,#18) the coordination of ferrous ions at the surface is greaterthan the bulk, indicating an abundance of 0 2- ions. These may result from the release of 02"during the reduction of ferric anions by CO (equation 7.1) or metallic iron:2Fe033- + Fe = 3F e 2+ + 602- (7.2)With the addition of silica, even in minor amounts, it is expected that the availibility of 0 2  relativeto ferrous cations will be diminished as these anions would preferably surround Si + cations. Inthe presence of other surface active species such as ferric oxide or phosphorus pentoxide similartrends in ferrous coordination would result. Although the data is not sufficient to test thisprediction rigourously, the results obtained for sample #15,#16 support the above hypothesis (seeFigure 7.4).189In view of the limited data generated in this study, no firm comments can be made concerningthe effect of SiO2 on the Fe+ distribution in the melts. However the changes in the coordinationof Fe2+ cations (shown in Figure 7.4 as decrease in the proportion of lower QS Fel') suggest thatsilica acts as a Fe2+ stabilizer, and this behaviour is in agreement with theory [122,112].(B) Effect of CaOTo isolate the effect of CaO it is useful to examine the results for silica-free melts.Comparison of the ferric gradient with depth within the Fe„0-Al 203 system (sample #17,#18)and the Fex0-CaO-Al203 system (sample #19,#20) indicates that they are opposite to one another.While the proportion of Fe3+ in the former melt is lowest on the surface, consistent with thegradient expected due to the strong reducing atmosphere, the proportion of Fe + is highest on thesurface in the presence of CaO. Repetition of this trend is observed in other lime-containingmelts namely-samples #9,#11 and #7,#8. (Figure 7.5)Thermodynamic studies in the FeO-Fe2O3-Si02 and FeO-Fe203-CaO systems [36,110,121]indicate that the equilibrium Fe 3+/E Fe ratio of the slag is much higher in the calcium ferrite systemat a given temperature and oxygen potential. Moreover the ratio is increased by further additionsof CaO to the slag, while it is lowered by addition of SiO 2. The value of the equilibrium Fe l+aFe ratio for sample #19,#20 is calculated to be approximately 0.15 (Fe x0-CaO-Al203 melt). Theactual value, however, is 0.03 (Table 7.1). Thus there is a large difference between the equilibriumand actual Fe3+/E Fe ratios which acts as a driving force for the ferrous disproportionation reaction3Fe2+ --*2Fe3+ +Fe (7.3)The newly formed ferric cations can in turn react with the 0 2  anions to produce ferric anioncomplexes according to neutralization reactions similar to the following:2Fe3+ +50 2- ----- Fe20:- (7.4)2Fe3+ + 60 2- = 2Fe033- (7.5)The possibility of reaction (7.4) in lime containing slags has been suggested earlier byChipman and Chang [111]. The authors state that the concentration of ferrite ions (i.e. ferricanion complexes) in the slag at equilibrium with molten iron increases with increasing proportions190of either CaO or MgO with an attendent decrease in the free Fe- cation concentration. It isprobable that similar reactions are taking place in our melts. The consumption of 0 2" ions dueto above reactions (7.4) and (7.5) should lead to a depressed coordination of Fe at the surface,consistent with our data (Figure 7.4).Yet another important point is that like silica, ferric oxide can also behave as a surfaceactive species in the melts and this implies that its concentration at the surface would be higherthan the bulk level. Surface tension data provided by P. Kozakevitch [67,103] supports the surfaceactive nature of ferric oxide. Additional evidence in favor of this observation is available in theliterature [123,107,104,124,125]. Such similarities in the behaviour of silica and ferric oxidespecies imply that the latter may be present as anions. This hypothesis is also consistent with theamphoteric nature of ferric oxide.Both thermodynamic and surface tension data imply that it is anions that are surface activeand therefore ferric is only surface active when sufficient O 2  is available to complex it (reactions7.4 and 7.5). When the melt contains silica the following type of reactionsS102 + 202- = Si0:- (7.6)readily convert all silica to Si0: - or larger polymeric anions. If there is any 02" left overit is only then that reactions like 7.4 and 7.5 are possible. Such behaviour is consistent with ionicslag theory and with the amphoteric characteristics of ferric oxide. In basic slags ferric oxide isexpected to behave as an acidic oxide and as a glass former. The melt from sample #19,#20consistes of two basic oxides (FeO and CaO) that provide 02" anions and two acidic oxides (Al 203and Fe2O3) which form complexes by consuming 02" ions. Additional information concerningferric anion complexes is given by Chipman and Chang [111], Yazawa et al. [112], Gaskell [96]and Strachan [32].191(C) Effect of CaO/Si02 ratioWe have explained the effects of a single basic or acidic oxide on the melt structure andits MOssbauer parameters in the case of simple melts. We noted that in the absence of CaO theconsumption of 02" ions results in the formation of SiO4 complexes, leaving the Fe* cationswith only few 02" anions to coordinate. On the other hand when CaO is present and silica isabsent, there is an abundance of 0 2" ions and the tendency is to form ferric complexes. It is naturalto expect that when both lime and silica are present in the melt the individual cations will competefor the 02" coordination and thus slag basicity would play an important role. Moreover, in thepresence of lime and silica the changes in the overall physicochemical properties of the melt,especially the surface tension, will become more critical and complicate interpretation of theM6sssbauer data. Our studies of complex melts in the system Fe.0-CaO-Si0 2-Al203 are thereforebest examined with respect to the CaO/Si0 2 ratio.Samples #5,#6 and #9,#11 were exposed to similar reducing gas mixtures and are nearlyidentical in composition except that the molar ratio CaO/Si0 2 is approximately 16% higher insample #9,#11. The difference is more significant when the ratio of basic oxides, CaO and FeO,to silica is considered. We observe a virtual reversal in the ferric gradient with depth in sample#5,#6 consistent with the increased amount of Si0 2 (Figure 7.5) which would tend to displaceferric anions from the surface. In both samples there is a tendency towards higher values for Fe2+quadrupole splitting near the surface, with the effect being more pronounced in sample #9,#11(Figure 7.4).In order to provide an additional explanation of the observed MOssbauer results we proposethe following scheme. (It is to noted however, that similar explanation is applicable to severalother melt systems). In the ionic double layer at the melt surface the oxygen ions preferentiallyoccupy the positions in the top layer (Figure 6.12) and in turn these are coordinated by cationsto maintain the electrical neutrality. For example, in the presence of Si 4+ the tetrahedralcoordination leads to the formation of SiOt anions. In the melts studied, we suggest the presenceof both silica and ferric complexes. The ferric anions can be described by the general formula192Fex0y(2Y-3* , but the predominance of only three or four different species at most is possible [96].However, due to the amphoteric nature of the ferric oxide species (or its lower ion-oxygenattraction value) it reacts with the reducing gas mixtures according to reactions of the form:Fe20:- + CO = 2Fe 2+ + 40 2- + CO2^ (7.7)2Fe20:- + CO = 2Fe03 - + CO2 + 2Fe2+ + 302-^(7.8)using Fe20: - as an example. Both these reactions may take place simultaneously or one maypredominate over the other. In either case however, there is a net production of oxygen anions.Reaction (7.7) represents dissociation of the ferric anion species whereas reaction (7.8) leads toan increase in the Fe+ coordination by 02'. According to this scheme more 0 2- anions are releasedat the reaction site and thus there is a net increase in the availabilty of 02' ions for coordinationwith the other cations such as Si4+, Fe2+ and Ca2+. Potentially these excess 02' anios can reactwith the complex silicate anions of the type Six07 and depolymerize the silicate network. It isequally probable that these will immediately cocordinate with the remaining Fe 3+ species at thegas-slag interface. Although we cannot firmly resolve the possibility of silicate networkdepolymerization, our Mössbauer parameters for Fe- for samples #7,#8 and #9,#11 (listed inTable 7.2 under QS and area) support the second possibility i.e. increase in the ferric coordinationat the surface. Our data imply that when various cations (e.g. Fe2+, Fea÷, Si)+ are competing forcoordination with the available 0 2" ions, acidic cations will have priority for coordination and asa consequence only a small number will be available to coordinate Fe +. Such behaviour is inagreement with the ion-oxygen values derived using considerations based on radii and chargesof various ionic species. [95] The M6ssbauer data for all Fex0-CaO-SiO2-Al203 slags show atendency towards lower coordination for Fe + at the surface, consistent with the suggested scheme.The proposed reduction mechanism via reactions (7.7) and (7.8) does not exclude thepossibility of the following reaction proceeding to a degree:2Fe 3+ + 0 2- + CO = 2Fe2+ + CO2^(7.9)193The above charge transfer reaction has been suggested earlier by Turkdogan and Bills [113]and Turkdogan and Pearce [126]. Reaction (7.9) implies that once the 02' releases two electronsthese in turn can reduce any two ferric ions- even those farthest from the reaction interface,because of the unrestrained velocity of the electrons. The observed ferric gradients appear to beinconsistent with the charge transfer reaction model alone and hence the possibility of all threereactions (7.7), (7.8) and (7.9) is proposed. However the MOssbauer data suggests predomonanceof reactions (7.7) and (7.8) over (7.9).The variations in quadrupole splitting values of Fe can be viewed from another perspectiveas well. The changes in &Ay ratios surrounding the Ca2+ and Fe2+ ions could be considered theprimary cause for the variations in quadrupole splitting. According to Gaskell et al. [110] thereplacement of Fe2+ by ce in binary silicates leads to the preferential association of ce withthe silicate anions. To accomodate this new geometry the Fe2+ cations undergo changes; theymaintain the local equilibrium 0 2"/0- ratio by coordinating with decreasing numbers of 02- anions.Gaskell et al. maintain that the non-ideal behaviour of oxides in ternary silicate melts can beexplained by the occurrence of preferred ionic associations according to the above scheme.Because of the decreasing availability of 02- anions, the Fee coordination changes from spherical(higher or octahedral) to tetrahedral (lower). Hollitt et al. [119] have attributed higher values ofFe2+ quadrupole splitting to lower coordination on the assumption that higher coordination moreclosely approximates the spherical or undistorted site. Our MOssbauer data exhibits similartendency towards higher values for Fe 2+ quadrupole splitting at the surface, with the exceptionof sample #17,#18 which contains no CaO or SiO2 (Figure 7.4). The tendency is most dramaticin sample #9,#11 which has the highest CaO/Si02 ratio. Sample #12,#14 was exposed to anoxidizing gas mixture, and therefore on the basis of polymerization-depolymerization modelsuggested by Gaskell et al. [110], one expects the tendency to be reversed. From Table 7.1 it isevident that the data supports this (the proportion of lower quadrupole splitting Fe 2+ values ishighest near the surface), although as stated previously the results from this sample must beinterpreted with caution due to the presence of magnetite.194(D) Effect of Al203In the present work although the alumina level in the slags varied between 6 to 24 wt%(Table 7.2), very little attention was focussed to identify the role of alumina on the MOssbauerparameters. Alumina was added to the melt through dissolution of the crucible and therefore itis difficult to isolate the effects of the alumina component on our MOssbauer data. However, onthe basis of the consistent effects shown by other components in the presence of strong basicand/or acidic oxides, we conclude that alumina plays a relatively passive role. Our melts containpredominantly Fe2+ however, and we should emphasize that different results are likely in the caseof Fe-rich melts. Both Al3+ and Fe3+ are amphoteric, and their interactions are likely to becomplex. Yet another complication to be considered is that, in accordance with Pauling's rules,a tetrahedrally coordinated All* (or Fe) ion blocks not just a tetrahedral position but also nearbyoctahedral and tetrahedral sites [99]. We therefore feel that more data is needed to interprete therole played by Al34" on the slag structure on the basis of the MOssbauer parameters.(E) Effect of solid productsThe role of solid iron is best studied in those slags run within the Fe saturation field underhighly reducing conditions (samples #17,#18 and #19,#20). We confirmed the presence ofmetallic iron in these slags both optically and through M6ssbauer spectroscopy. MOssbauer datafor both of these slags indicates that there is a dramatic change in the mean Fe 2+ quadrupolesplitting values (Figure 7.3) and also a shift in the distribution of Fe 2+ quadrupole values withdepth (Figure 7.4).An explanation for the observed MOssbauer trends can be suggested in which the 0 2" ionssurrounding Fe are consumed according toFe2+ +0 2- +CO =Fe +CO2 (7.10)Additionally some of the newly formed iron metal can react with Fe3+ and release extra Fe 2+cations which in fact is the reverse of reaction (7.3). Under these circumstances the 021Fe2+ ratiois expected to vary and the resultant changes in the Fe2+coordination are inevitable. We believe195that the opposing trends shown by Fe 2+ quadrupole splitting values (samples #17,#18 and #19,#20;Figures 7.3 and 7.4) are due largely to the changes in the 021Fe2+ratios of two samples in the top10 pm surface region.In addition it was expected that the tendency of metallic iron to segregate would beinfluenced by the silica content of the melt via the viscosity effect. Under the circumstancemovement of solid iron away from the surface would diminish in spite of the density differential.This is indicated in the MOssbauer spectra of sample #2,#3 (Figure 7.2), which shows iron peaksonly in the top layer and not in the bulk slag.In the slag exposed to oxidizing conditions (sample #12,#14) magnetite formation reactionof the type2Fe033- + Fe 2+ = Fe304 +202- (7.11)is proposed to offer an explanation for the increased 02- coordination of Fe2+ cations in the surfaceregion. Our MOssbauer data are consistent with an increase in Fe 2+ with low quadrupole splittingat the surface of the magnetite saturated slag; this can be interpreted to indicate an increase inFe2+ coordination [119]. However, it should be noted that this is only a tentative conclusionbecause only one slag sample was analyzed.7.5 CommentsThe following important points emerge from the MOssbauer spectroscopy work:(1) Using a custom designed sample holder it is possible to probe the cross section of the quenchedslag. The technique would also be applicable to identify characteristics such as the change inproportion of iron phases, changes in iron coordination, and changes in iron oxidation state.(2) In the slag systems of importance to the non-ferrous smelting industry the MOssbauer datacould be fit with a model which included three doublets for Fe2+ and one for Felf. Although theabsolute values of the spectral parameters obtained this way lack definite physical meaning, therelative changes with both depth and composition are significant.(3) Utility and effectiveness of this technique could be further extended by repeating similar196analyses on equilibrated slag samples. For example, if such analyses were carried out in thepresent study for comparative purpose then it would have been easier to confirm the effects ofrate phenomena.197Chapter 8Summary and ConclusionsThe kinetic study of reduction and oxidation reactions of importance to the non-ferroussmelting industry was successfully carried out in the temparature range of 1200 and 1400 °Cusing unstirred melts. For the first time, a study of the ferric-to-ferrous reaction was initiated inthe lime-free and lime-containing melts by employing a thermogravimetry technique. The resultsrevealed higher rates in the lime-containing melts in comparison with the lime-free melts.The weight loss-time curves obtained during the ferrous-to-iron reaction study revealed thehighest and nearly constant slope values for the initial 10 minute period. A mathematical modelwas developed for estimating rate values and based on the agreement between the observed andfitted data it is proposed that in this experimental system the iron formation reaction operatesunder a mixed-control regime involving gas phase mass transfer and interfacial reaction. For themelts at 1400 °C an average intrinsic rate constant value of 11 x g/cm 2.s.atm was obtainedthis way which is consistent with the value reported by Nagasaka et al. [56]. Based on the limiteddata at lower temperatures an activation energy value of about 28 kcal/mole is proposed for theferrous-to-iron reaction and this agrees well with the published values by Nagasaka et al , Kimet al. [66] and Tsukihashi et al. [55].The results of experiments on the ferric-to-ferrous reaction were analyzed using two separatemathematical models. Based on the agreement between the predicted and actual weight loss datait is proposed that the ferric-to-ferrous reaction operated under a gas and interfacial control regimeduring the initial period and subsequently the rate was controlled by a combined gas and liquidmass transfer and interfacial reaction. The intrinsic rate constant value at 1400 °C is approximately200 times greater compared to the ferrous-to-iron reaction. From the knowledge of intrinsic rateconstants at three reaction temperatures the apparent activation energy value of about 44 kcal/molewas derived for the ferric-to-ferrous reaction. In the melts containing relatively higher levels of198ferric oxide the Fe3+ Fe2+ reaction occurs concurrently with the ferrous-to-iron reaction andthis offers an explanation on the important observation made by Nagasaka et al. concerning theeffect of Fe3+/Fe24- ratio on the rate.Estimates of diffusivity values were obtained for a wide range of melt compositions usingthe derived liquid phase mass transfer coefficient values and the boundary layer thickness of 500gm. The estimated D02_ values for the lime-free and lime-containing melts were about 5 x 10 -6and 1 x 10-5 cm2/s respectively at 1400 °C. These values are in reasonable agreement with thetheoretically predicted data and an additional support for these can be obtained from the researchwork of Sasabe and Asamura [109]. The predicted value of the apparent activation energy fordiffusion, ED, for lime-containing melts is about 53 kcal/mole and this is in excellent agreementwith that reported by Sasabe and Asamura.The results obtained in both the reduction reactions revealed the significance of surfaceactive species in the melts and accommodation of this effect in the development of themathematical model led to accurate prediction of rate and weight loss data. In the various meltsstudied in this investigation silica and ferric oxide were surface active species and their individualproportions altered the available reaction area. A general expression was developed to arrive atthe fractional coverage parameters for the melts at 1400 *C and the information allowed predictionof the rate data from first principles. The A/A. expressions were derived using the availablesurface tension data in the literature and they provided useful information concerning the actualreaction area. A similar approach was used by Kim et al. [66], however these authors limitedtheir observations to the ferrous-to-iron reaction in complex melts at 1600 °C and accounted onlyfor the silica coverage effect.An important aspect of the mathematical formulation proposed in this work is that it accounts(for the first time) for the differences in apparent rate constant values for FeO, FeO-S i0 2, FeO-CaOand FeO-CaO-Si0 2 melts reported by the previous researchers. Moreover, the formulation hasallowed a closer examination of the parameters, namely - intrinsic rate constants, fractional areacoverage and reactant activities, that are responsible for the variation of lc... It reasonable to expect199that in any heterogeneous reaction both the composition and the constitution of the melt surfacewould play a critical role on the rate phenomena. In this context, it makes sense to have a singlelc, value for oxygen anion surfaces and a different value for ferric anion surfaces. In the COI -dissociation model proposed by Belton et al. [34,80] however the melt surface is viewed only asoxygen anions and the explanation for the observed variations in ka is sought in terms of changesin the electrochemical surface potential. It is felt that the alternate explanation proposed in thisthesis for the variations in ka in terms of A/A,, factors and kc, k; difference may, in a sense, beequivalent to the electrochemical surface potential theory. But until this is verified, the modelproposed in this work can still be used as a means of estimating overall reduction rates of Fe 2+--+ Fe and Fe--> Fe2+ reactions in a wide range of melt compositions between 1200 °C and 1400°C.For the first time a theoretical justification is proposed for the variations in the apparentrate constant, ka, values reported in the literature. Based on the findings of the work it is suggestedthat the ferric-to-ferrous reaction is intrinsically faster than the ferrous-to-iron reaction. Moreimportant though is the fact that the information contained in the thesis has identified severalcritical aspects of the gas-slag reactions and with the help of all the available information acoherent and comprehensive picture of the phenomena is attempted at the end. In the proposedreaction mechanisms it is suggested that in both the reduction reactions the critical step involvestransfer of oxygen anions either (1) through the melt and/or (2) from or to the melt surface. Theproposed scheme is applicable to the oxidation reactions as well. The feature that differentiatesthe above scheme from the charge transfer model is that the latter does not comment on themovement of oxygen anions in the melt but merely takes care of Fe and Fe2+ balance. Moreover,the charge transfer model does not account for the presence of ferric anions, if any, in the melts.The results obtained in the present work however suggest that the role played by the ferric anionspecies should not be under-estimated especially when the available surface tension data hasshown that ferric oxide species is surface active.200The weight loss-time curves obtained for various melts during the ferric-to-ferrous reactionstudy revealed decreasing rates with time. Subsequent mathematical analysis confirmed that thistrend is caused by the additional contribution coming from the liquid mass transfer resistance.Such behaviour suggested compositional gradients across the melt depth and to test this a noveltechnique was employed. MOssbauer spectroscopic analyses were performed on a limited numberof slag samples using a special sample holder and an entire cross-section of the quenched slagwas probed to identify variations in iron cations with depth. The data revealed distinct differencesbetween the values of MOssbauer parameters obtained at the surface from those in the slag bulkfor an individual sample. Additional differences were noticed between the lime-containing andlime-free slags. In the case of lime-containing melts it was observed that in spite of the prevailingreducing conditions the surface concentration of ferric was higher than the bulk. This fmding isconsistent with the surface active nature of ferric oxide. A value of 500 pm was obtained for themelt diffusion boundary layer thickness using the MOssbauer data.The limited data on the ferrous-to-magnetite reaction revealed that rates decreased withtime in a manner similar to the reduction reactions. The weight gain-time curves for the magnetiteformation reaction in both simple and complex melts indicated that solid magnetite covered themelt surface and caused reduction in the rate values. The data on the ferrous-to-ferric reactionat 1300 °C implied that the mechanisms involved were similar to the equivalent reverse reaction.The experimental data showed that control of oxygen partial pressure during both heatingand melting periods is of primary importance in the study of iron formation reaction. The resultsrevealed that accurate and successful control is possible by using appropriate mixtures ofAr-CO-CO2 and in the absence of such control, the ferrous oxidation reaction is unavoidable.This leads to higher Fe3+/Fe2+ ratios in the melts and results in the higher reaction rates. In ageneral sense the melt oxidation prior to the reduction period complicates interpretation of resultsand hence it is should be avoided if possible.201The following important points emerge from the results of the kinetic investigation:(1) Although gas phase mass transfer limitations are present in the stagnant system amathematical approach can be employed to characterize other reaction resistances and inturn the roles played by interfacial and liquid phase mass transfer effects can be identified.(2) The values of the apparent rate constant, lc, can be evaluated from the knowledge of k c(intrinsic rate constant), A/A,, (available reaction sites) and the activities of reactant species.(3) Mixed-control models can adequately explain the observed weight changes in the slagmelts.(4) Melt physicochemical properties, namely - viscosity, diffusivity, density and surfacetension, play an important role in both the reduction and oxidation rate phenomena.(5) MOssbauer spectroscopy technique can be employed to identify thedistribution of iron cations with depth.(6) The presence of ferric anions is proposed in both lime-free and lime-containing melts andin these melts the ferric-to-ferrous reaction involves either breakdown or restructuring offerric anions.What emerges from this study is that the behaviour of iron oxide containing melts duringreduction is very complex and depends critically on melt composition. The interaction of surfaceactivity effects with the transport of oxygen in the melt, as 02- or ferric anions, can lead to quitedifferent processes. It is quite clear that a quantitative understanding of the process must takethese factors into account.8.1 Recommendations for future workThe present study has highlighted the surface active nature of ferric oxide species in thevarious melts and it is felt that additional work is necessary to support this fact. More data isneeded to obtain further confirmation on the mechanism of ferric-ferrous redox rection and theexact role played by the melt physicochemical properties on the overall rate phenomena. More202data is needed at 1200 *C and 1300 *C temperatures to confirm the proposed fractional coverageexpressions at these temperatures. Further research work is needed to confirm the oxygen aniondiffusivity data proposed in this thesis.In the present investigation, the oxygen probe data could not be used for a quantitativecomparison with the thermogravimetric data because of the flow rate dependence of its output.However, a simultaneous measurement like this could be repeated in future; perhaps by keepingthe probe in a separate furnace where the flow rate restrictions of the product gases may not hinderthe accurate and instantaneous probe measurements. Furthermore, by employing the techniquedeveloped in this work additional MOssbauer work should be undertaken to further confirmvariations of iron cations with depth.203REFERENCES[1] E.T. Turkdogan, physicochemical Properties of Molten Slags and Glasses, The MetalsSociety Publication, 1983, pp 193-404[2] N. Sevryukov, B. Kuzmin and Y. Chelishchev, General Metallurgy, MIR Publishers,Moscow, 1969, pp 143-343[3] R.E. 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Faraday Soc., Vol. 52, 1956, pp 1312-1350211Appendix IIn the present study activity values of silica and iron oxides were derived using themathematical model developed by Kellogg and Goel. [83,84] A few relevant details on this areprovided below.The original model [83] considered a mathematical correlation of binary and ternary slagsin the systems Fe-0 and Fe-O-Si0 2 that was capable of reproducing all the thermodynamicproperties of these slags as a function of composition and temperature, within the uncertainty ofthe available experimental data. In the mathematical formulation the slag phase was describedin terms of hypothetical solution species in equilibrium with each other, and behaving in anon-ideal manner according to the three-suffix Margules equations with zero ternary interactionsin the form:ln= 1/21.(ku + kx)Ni — 1/2ElkipNiNp + 1,(k — kx)Ni(Ni12 — N) + =kip — kpi)NpN,^(L1)where ku = lcz = 0.0, and the summations are carried out over all species. This form for thedependence of activity coeffients on composition is valuable because it yields values consistentwith Gibbs-Duhem relation and which obey Raoult's and Henry's laws as limiting laws.For the temperature dependence the authors employ the following equations:lnKi = Ai + BilT^ (L2)kv = cull' (I.3)The authors quote values of formation equilibria, In k; and Margules parameters forvarious species. The authors claim that reliable thermodynamic data was obtained for both Fe-0and Fe-O-Si02 systems. Additionally, they describe the miscibility gap between iron and oxidephases and the saturation boundary for Si02(c) for slags in the Fe-O-Si02 system. The range ofvalidity is for temperatures between 1150 and 1600 °C and for oxygen partial pressure, from ironsaturation to 1 atm. The authors claim that extrapolations a short distance outside of these rangeswould yield good estimates. Based on the success in estimation of the thermodynamic data inthese systems the authors have later extended the concept to the melts in the Fe-O-SiO2Ca0212system. Goel and Kellogg [84] report the same range of temperature for the applicablity oflime-containing melts. Additional restrictions on the validity of the model in these melts are: (1)mol% (CaO + SiO) < 65; and (2) molar Fe 3VT_Fe < 2/3 or molar 0/Fe < 4/3. All the melts usedin the present work satisfied the above conditions and therefore the model was considered useful.In all the melts studied by us alumina was present. However, in their mathematicalformulation Goel and Kellogg did not study the effect of this species. To resolve this it wasassumed that in silica-free melts the alumina would behave as an acidic species and therefore itwas treated as silica on a mole-to-mole basis. The resultant FeO activity data was then comparedwith the equivalent values obtained by Ban-Ya et al. [127] using thermodynamic measurements.An excellent agreement was obtained for the ferrous oxide melts containing upto 15 mol% aluminaon this basis. For example, in the melt containing about 7 wt% the model predicted aFeo valueof 0.96 which matched with that of 0.95 reported by Ban-Ya et al.To accomodate the behavior of alumina in the silica-containing melts a mole of aluminawas replaced by a mole each of lime and silica. The scheme is not considered unreasonable inview of the amphoteric nature of this species. Also similar calculations were repeated on theassumption of a mole of alumina to be equivalent to the mole each of ferrous oxide and silica.The ferrous oxide activity values obtained thus were compared with the available data in theliterature [35,52] and it was noted that the scheme involving replacement of a mole of aluminaby mole each of lime and silica yielded a more accurate data and therefore these activity datawere used in subsequent calculations. It should be noted that a similar idea has been usedpreviously to describe the viscosity behavior of the complex melts containing alumina. Turkdoganand Bills [113] have proposed a parameter, x a, which they have refered to as 'silica equivalenceof alumina'. Their findings suggest that the role of alumina, as either an acidic or basic species,is essentially governed by the lime-to-silica ratio of the melts consistent with its amphotericnature. Therefore the activity values obtained using the scheme outlined above is expected toyield reasonably accurate data.213Appendix IIThe analogue millivolt signals from the probe were shielded and ground appropriately toeliminate an electrical noise and then they were split. One was taken to a pH meter (via a junctionbox) and then stepped down with two resistors prior to its connection to a channel 3 on the EXP-16board. The other signal (thermocouple emf) was first taken to an isolation amplifier (where thelow voltage signal was smoothened and amplified) and then to a channel 2 on the EXP-16 board.In addition, both the signals were coupled to a chart recorder using parallel connections. Channel1 on the EXP-16 board received a signal from the furnace thermocouple. All the three channelssent analogue signals to an A/D converter (DAS-8 board) located in the computer. The weightsof the alumina hanger and crucible assembly were transfered through a bi-directional interfaceattached to the balance to the RS-232 port on the computer. Schematic diagram of the dataacquisition system is shown in Figure II.1.The software permitted simultaneous display of the various parameters that were stored onthe computer hard drive. For this purpose the monitor screen was divided to accomodate fourwindows and the data being recorded was displayed on x-y graphs as weight/temperature/oxygenpartial pressure versus time. The performance and accuracy of the data acquisition system-balance interface, DAS-8 and EXP-16 boards, computer, pH meter, chart recorder and isolationamplifier was tested by conducting several dummy experiments. The graphical display of theparameters was especially useful in verifying the occurrence of electrical interference, if any,over a long intervals of time and such a provision was found helpful in ensuring the properelectrical connections. Additional trials were conducted to optimize the gas flow rates. Highergas velocities (> 4 cm/sec) led to the increased pendulum-like motion of the crucible and thiscaused wide fluctuations in the weight values. Again the display mode was very helpful, as theweight changes with respect to the gas flow rates (or gas velocities) could be observed with easeand based on this information the necessary adjustments to the flow rates were made to ensurethe accurate weight measurements.214LEGENDHigh • positiveLaw • negetheR • redW • whiteB • blackP • purpleBI • blueG • greenRI a 10 k ohm resistorR2 22 1 k ohm resistorFigure II.1 Schematic diagram showing detailed electrical connections for the dataacquisition system.


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