The University of New South Wales Faculty of Science School of Materials Science and Engineering
Gaseous Reduction of Titanoma·gnetite Ironsand
Eungyeul Park
A Thesis submitted in Partial Fulfilment
Of the Requirements
For the Degree of
Doctor of Philosophy
September 2002 CERTIFICATE OF ORIGINALITY
I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another penon, nor material which to a substatial extent bas been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis.
I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from othen in the project's design and conception or in style, pre acknowledged.
ii To My Family
iii ACKOWLEDGEMENT
I would like to thank my supervisor, Professor Oleg Ostrovski. I have learnt from him what is an engineer as well as what is Doctor of Philosophy. I am grateful for his enthusiasm and kind guidance throughout my study. He always encouraged me with deep patience. I also wish to express my special gratitude to my supervisor for providing financial support during my study.
I wish to thank Dr. G. Zhang for the experimental set-up and his help in valuable discussions on the project, Dr. J. Zhang for his help in the former project on iron carbide process, and Dr. S. Gaal for his assistance in experimental set-up and many analysis techniques. Also, I would like to thank Mr. N. Anacleto for the discussions on the fundamentals of thermodynamics. The discussions with them always gave me new ideas and showed further directions.
The technical support provided by technician staff in the School of Materials Science and Engineering, UNSW, are also acknowledged.
I thank Mr. R. Longbottom, for conducting experiments on the reduction of ironsand by methane-containing gas mixtures.
I am thankful to the staff in Electron Microscope Unit, especially, Mr. B. Searle for his guidance in using and understanding EPMA, and Ms. V. Piegerova for SEM.
The special thanks should be given to Dr. S. Blimyukov, Dr. S. Thomson and Prof. R. Howe, for their guidance in surface analysis.
I thank to my good Aussie friends, Tim, Big Brad and Small Brad. Their warm heart made me feel home in Australia.
I appreciate Prof. Dong Jun Min in Yonsei University, Korea, for the valuable discussions and encouragement.
I wish to thank Prof. Chang Hee Rhee in POSTECH, Korea. He gave me the chance to study in UNSW and encouraged my work.
Finally, I would like to thank my family.
iv PUBLICATIONS ORIGINATED FROM TIDS PROJECT
I. Eungyeul Park, Jianqiang Zhang, Stuart Thomson, Oleg Ostrovski and Russell Howe: "Characterisation ofPhases Formed in the Iron Carbide Process by XRD, Mossbauer, XPS, and Raman Spectroscopy Analyses" Metallurgical and Materials Transaction B, 32B (5), p839-845, 2001.
2. Eungyeul Park and Oleg Ostrovski, "Reduction ofTitania-Ferrous Ore by CO COrAr Gas Mixture'\ Proceedings of I st International Conference on Advanced Materials Processing, p90-98, 19-23 Nov 2000, Rotorua, New Zealand
V ABSTRACT
This project examined the gas-solid reduction of New Zealand ironsand which contains 50-60wt°/o of iron and 7-8wt% ofTiO2• The project's aim was to develop further understanding of the reduction mechanism of ironsand by gas-solid reaction with a focus on a structure of ironsand and its transformation during reduction. The reduction of ironsand was investigated in non-isothermal and isothermal reduction experiments using carbon monoxide, hydrogen and methane-containing gas mixtures in a laboratory fixed bed reactor in the temperature range of 700-1100°C. Samples in the course of reduction were characterised using XRD, EPMA, SEM, optical microscope and BET analysis.
Ironsand 's structure and phase composition. Two types of particles were identified in raw ironsand; the major type was represented by homogeneous particles of cubic spine} titanomagnetite; and the others, non homogeneous particles, were characterised by lamellar structure of rhombohedral titanohematite, exsoluted from the titanomagnetite. The formula oftitanomagnetite was established to be (Fe3O4 ),_x(Fe2TiO4 )x, with the x value of 0.27±0.02.
The reduction sequence. Titanomagnetite was reduced to metallic iron and titanium sub-oxides via wustite, ulvospinel and ilmenite in -the following sequence:
Fe3_xTixO4 -+" FeO"+Fe + Fe3_x_6 Tix+ 6 O4 -+Fe+ Fe3_x_6 Tix+ 6 O4 -+
Fe+Fe2TiO4 -+ Fe+FeTiO3 -+ Fe+TiO2 -+ Fe+Ti3O5 -+ Fe+Ti2O3
{x= 0.27±0.2; 8, degree ofreaction, 0 ~ '5~(1-x)).
Reduction kinetics. The rate limiting stage in the ironsand reduction was the reduction of titanomagnetite to wustite, which involves the reduction 3 2 ofFe + to Fe +. The reduction of the intermediate phases, wustite and ulvospinel proceeded quickly. In the reduction of raw ironsand, the lamellar
vi titanohematite phase reduced with higher rate than titanomagnetite, enhancing the overall ironsand reduction rate.
Retarding effect oftitanium on the iron oxide reduction. Titanium, which is present in the ore mainly in the titanomagnetite, has a strong effect 4 3 on the mechanism and rate of iron oxide reduction. Ti + substitutes Fe + at octahedral sites of spinel lattice, decreasing Fe3+ activity and increasing Fe2+activity. This retards reduction ofFe3+ to Fe2+, which is a rate-limiting stage in the reduction of titanomagnetite.
Effect ofcalcination and preoxidation on the ironsand structure and reduction. The phase composition was not affected by calcination at 1100°C. Preoxidation of ironsand at l 000°C transformed titanomagnetite in ironsand to titanohematite (Fe2.yTiyO3, rhombohedral) and then partly to pseudobrookite
(Fe2 TiO5). The preoxidation of ironsand enhanced its reduction as a result of structural transformation of cubic spinel titanomagnetite to rhombohedral titanohematite in preoxidation.
The reduction experiments of the raw and the preoxidised ironsand showed that the slow reduction of New Zealand ironsand is due to two factors; 1) the spinel cubic structure oftitanomagnetite and 2) the thermodynamic stability of titanomagnetite due to the substitution of titanium. For further application of ironsand to DRI process, the project suggests the gaseous reduction of the preoxidation of ironsand is more economical than the carbothermic reduction.
The project contributes to further understanding of reduction of titanomagnetite ironsand.
vii Contents
CERTIFICATE OF ORIGINALITY 11 ACKNOWLEDGEMENTS 1v PUBLICATIONS ORIGINATED FROM THIS PROJECT v ABSTRACT vi CONTENTS viii LIST OF FIGURES xn LIST OF TABLES xvn
Chapter 1. Introduction I
Chapter 2. Literature Review 4 2.1. Ironsand deposits 4 2.1.1. Characteristics of ironsand deposits 5 2.2. Structure and thermodynamics of ironsand 6 2.2.1. Structure and thermodynamic properties of titanomagnetite 7 2.2.1.1. Structure oftitanomagnetite 7 2.2.1.2. Mixing properties of magnetite and ulvospinel in I 0 titanomagnetite 2.2.1.3. Effect of impurities on structural properties oftitanomagnetite 12 2.2.2. Phase relations in the Iron-Titanium-Oxygen system 13 2.2.3. Summary 18 2.3. Reduction of iron ore 19 2.3.1. Thermodynamics of iron ore reduction 19 2.3 .1.1. Structure of iron oxides 19 2.3.1.2. Thermodynamics of the gaseous reduction of iron oxides 21 2.3.2. Kinetics of iron ore reduction 22 2.3.3. Summary 26 2.4. Reduction of Iron-Titanium oxides 27 2.4.1. Reduction of ilmenite ore 27 2.4.2. Reduction of ironsand ore 30 2.4.2.1. The use ofironsand in steelmaking 30
viii 2.4.2.2. Reduction oftitanomagnetite ironsand 32 2.4.3. Summary 37 2.5. Objectives of the Project 38
Chapter 3. Experimental 40 3 .1. Materials 40 3.1.1. New Zealand ironsand 40 3.1.2. Iron ore 41 3.1.3. Gases 42 3.2. Experimental setup 43 3.2.1. Experimental furnace and reactor 43 3.2.2. Gas system 44 3.2.3. Monitoring of gas composition 47 3.2.3.1. Mass spectrometer 47 3.2.3.2. Dew Point Monitor 47 3.3. Analytical instruments 48 3.3.1. X-ray Diffraction analysis 48 3.3.2. Electron Probe Microanalysis 48 3.3.3. SEM analysis 50 3.3.4. Optical microscope analysis 50 3.3.5. LECO analysis 50 3.3.6. BET analysis 50 3.4. Experimental procedures 51 3.4.1. Isothermal reduction 51 3.4.2. Temperature-programmed reduction 52 3.4.3. Sample analyses 53 3.5. Data analysis 53 3.5.1. Calculation of the extent ofreduction 54
Chapter 4. Characterisation of Ironsand Ore 55 4.1. Characterisation of raw ironsand 55 4.1.1. Phase composition of the raw ironsand ore 55 4.1.2. Surface area, pore volume and size of the raw ironsand ore 61
ix 4.2. Pretreatment of ironsand 62 4.2.1. Preoxidation of ironsand 62 4.2. l. l. Non-isothermal oxidation 62 4.2.1.2. Isothermal oxidation 65 4.2.1.3. Behaviour of impurities 71 4.2.1.4. Thermodynamics of oxidation of titanomagnetite 73 4.2.2. Calcination of raw ironsand 75 4.2.2.1. Non-isothermal calcination 75 4.2.2.2. Isothermal calcination 78 4.2.3. Surface area ofpreoxidised and calcined ironsand 79 4.3. Morphology 81 4.4. Summary 85
Chapter 5. Gas-Solid Reduction of lronsand Ore 86 5.1. Temperature-programmed reduction ofironsand 86 5.1.l. Reduction of raw ironsand 86 5.1.2. Reduction of pretreated ironsand 91 5.1.2.1. Effect ofpreoxidation 93 5.1.2.2. Effect of calcination 96 5.1.3. Reduction of magnetite ore 96 5.1.4. Reduction mechanism of the ironsand ore 99 5.1.5. Summary 104 5.2. Isothermal reduction of ironsand 106 5.2.1. Gas flowrate in the reduction experiments 106 5.2.2. Reduction ofironsand by carbon monoxide 107 5.2.2.l. Effect of temperature 107 5.2.2.2. Effect of carbon dioxide 109 5.2.2.3. Progress of the reduction of ironsand by CO-Ar gas mixture 118 5.2.2.4. Morphology 123 5.2.2.5. Surface area measurement 126 5.2.2.6. Effect of pretreatments 128 5.2.3. Reduction of ironsand by hydrogen 130 5.2.3.1. Effect of temperature 130
X 5.2.3.2. Effect of hydrogen content 130 5.2.3.3. Effect of water vapour 132 5.2.3.4. Progress of the reduction of ironsand by H2-Ar gas mixture 133 5.2.3.5. Morphology 133 5.2.3.6. Surface area measurement 139 5.2.3.7. Effect of pretreatments 140 5.2.3.8. Comparison of reduction of ironsand by H2 with reduction by 142 co 5.2.4. Reduction ofironsand by CH4-H2 gas mixtures 143 5.2.4.1. Effect of temperature 143 5.2.4.2. Effect of methane content in C~-H2-Ar gas mixtures 144 5.2.4.3. Effect of hydrogen content in C~-H2-Ar gas mixtures 145 5.2.4.4. Phase transformation during the reduction of ironsand 146 by C~-H2-Ar gas mixtures 5.2.5. Comparison with previous studies 149 5.2.6. Summary 153
Chapter 6. Conclusions and Future Work 155 6.1. Characterisation of ironsand 155 6.2. Reduction of ironsand 156 6.3. Recommendations for future work 160
References 161
xi List of Figures
Figure Page 2-1-1 Location of ironsand deposits in New Zealand 4 (Wright, 1964; BHP NZ, 2000)
2-2-1 FeO-FeiO3-Ti0i ternary system (Buddington and Lindsley, 1964) 6 2-2-2 Effect of ulvospinel content on the magnetite cell lattice parameter 7 (Wechsler et al., 1984)
2-2-3 Schematic cation distribution in titanomagnetite solid solution, (Fe3O4)1_ 8 xCFe2TiO4)x (Stephenson, 1969) 3 2-2-4 Concentration Fe + in octahedral sites oftitanomagnetite lattice 9 (Wu and Mason, 1981; Trestman-Matts et al. 1983; Anderson and Lindsley, 1988) 2-2-5 Mixing properties of magnetite-ulvospinel solution at 400°C and 1200°C 10 ( dotted line: the Akimoto model; solid line: the site-mixing model; ~Gxs is excess Gibbs free energy, ~Gxs = ~Gmix -~Smix.) (Anderson and Lindsley, 1988)
2-2-6 Activities of Fe3O4 in titanomagnetite solid solution at different 11 temperatures, H&S: Hill and Sack (1987); A&L: Anderson and Lindsley (1988) (Woodland and Wood, 1994) 2-2-7 Miscibility gap in magnetite-ulvospinel solid solution (Lindsley, 1981) 12 2-2-8 Ironsand particle showing exsolution, Waikato North Head, (x650) 14 (Wright, 1964)
2-2-9 Equilibrium relation between Titanomagnetite (Mt100-x-Ulvx) and 15
Titanohematite (Hem1oo-yilmy) Numeric subscripts stand for mole percent. (Buddington and Lindsley, 1964) 2-2-10 Phase diagram of the Fe-Ti-O ternary system and isobars of oxygen at 17 1100°C [ - - - o - - - :isobars of oxygen partial pressures ] 2-3-1 Iron-oxygen phase diagram (Pelton and Bale, 1999) 19 2-3-2 Equilibrium phase diagrams of(a) Fe-C-O system and (b) Fe-H-O system 22 (Dancy, 1993) 2-3-3 Cross section of a partially reduced dense iron ore particle (Lu, 1999) 23 2-4-1 Schematic flowsheet of ironsand process 31 2-4-2 Reduction time as a function of CO/CO2 ratios and phases in Fe-C-O 33 system (McAdam, 1974)
XU 2-4-3 Reduction steps as a function of normalised time (McAdam, 1974) 34 3-2-1 Schematic experimental reactor 43 3-2-2 Schematic diagram of gas flow system (1-reactor, 2-pressure gauge, 3-six 45 way valve, 4-rotameter, 5-water bath, 6-gas flow controller, 7-gas purifier, 8-dew point monitor) 3-2-3 Calibration curves of the mass flow controllers 46 4-1-1 XRD patterns of the raw ironsand ore and magnetite 56 4-1-2 Elemental distribution in a raw ironsand particle 58 4-1-3 Mapping of Ti, Fe, Al and O in the homogeneous particle of the raw 59 ironsand ore 4-1-4 Elemental analysis of a non-homogeneous ironsand particle 60 4-2-1 XRD patterns of ironsand preoxidised by non-isothermal heating under air 63 (TTM: titanomagnetite; 1TH: titanohematite) 4-2-2 XRD patterns of the preoxidised raw ironsand ore samples. The samples 66 were oxidised for different times at 1000°C and quenched to room temperature in air. 4-2-3 The change in the elemental distributions of Fe and Ti with oxidisation time 67 at 1000°C 4-2-4 The Fe/Ti ratios of the raw and preoxidised ironsand at 1000°C 69 (Each line analysis was made on the identical particle in Figure 4-2-3.) 4-2-5 The elemental distributions in the raw and preoxidised ironsand ores 71 (Each line analysis was made on the identical particle in Figure 4-2-3.)
4-2-6 The Fe0-Fe2O3-TiO2 ternary system 74
[1] - titanomagnetite solid solution, Fe3.xTixO4, (spinel);
[2] - titanohematite solid solution, F~-y TiyO3, (rhombohedral);
[3] - ferropseudobrookite-pseudobrookite solid solution, Fe3.zTizOs, (orthorhombic) 4-2-7 XRD patterns of the raw and the calcined ironsand ore 76 4-2-8 Mapping images of the calcined ironsand 77 4-2-9 XRD patterns of the calcined ironsand ore samples. The samples were 78 calcined for 2 and 24 hours at 1100°C and quenched to room temperature in argon. 4-2-10 The specific surface area of (a) preoxidised and (b) calcined ironsand ore 79 samples 4-3-1 SEM images of particles of ironsand ore (a) Raw ore; (b) After calcination 81 at 1100°C for 24 hours
xiii 4-3-2 SEM images of particles of the ironsand ore (a) Raw ore; (b) After 82 calcination at 1100°C for 24 hours 4-3-3 Microphotographs of particles of ironsand (a) and (b) Raw ore; (c) and (d) 83 After calcination at l 100°C for 24 hours. (a) and (c) are homogeneous particles; (b) and (d) are particles with lamellar. 4-3-4 SEM images of particles of the ironsand (a) Raw ore; (b) and (c) After 84 preoxidation at 1000°C for 24 hours 5-1-1 Temperature change in the temperature-programmed reduction experiments 87 5-1-2 The progress of the temperature-programmed reduction of the raw ironsand 88 5-1-3 XRD patterns of samples of raw ironsand heated and quenched at different 89 temperatures. Reducing gas: 75vol% CO and 25vol% Ar (TTM: titanomagnetite) 5-1-4 The progress of the temperature-programmed reduction of the pretreated 92 ironsand ores 5-1-5 XRD patterns of the preoxidised ironsand samples heated and quenched at 94 different temperatures. Reducing gas: 75vol% CO and 25vol% Ar (TIM: titanomagnetite; TTH: titanohematite) 5-1-6 The progress of the temperature-programmed reduction of the magnetite 97 iron ore 5-1-7 XRD patterns of magnetite iron ore samples heated and quenched at 98 different temperatures. Reducing gas: 75vol% CO and 25vol% Ar
5-1-8 Fe-TiOrFe2O3 ternary system 99 (Arrow [1]: reduction path oftitanomagnetite; arrow [2]: reduction paths of titanohematite; arrow [3]: reduction path ofpseudobrookite) 5-1-9 The variation of the CO-COrAr gas composition equilibrium with graphite 102 vs. temperature 5-1-10 Equilibrium constant of reaction (5-16) vs. temperature 103 5-2-1 Effect of gas flowrate on extent of reduction by 75vol%CO-Ar gas mixture 106 at 1100°C 5-2-2 Reduction of ironsand by 75vol%CO-Ar gas mixture at different 108 temperatures 5-2-3 Reduction of ironsand by CO-COr25vol%Ar gas mixture with different 109 Pco l(Pco + Pco,) at 1100°C
5-2-4 XRD patterns of samples reduced by CO2-CO-Ar gas mixtures with 111 (a) Pco l(Pco + Pco,) in the range of 0.60 to 0.875 at 1100°C
xiv 5-2-4 XRD patterns of samples reduced by CO2-CO-Ar gas mixtures with 112 (b) Pco l(Pco + Pco,) in the range of0.90 to 1.0 at 1100°C
5-2-5 The Fe/Ti ratios for the raw and reduced ironsand samples by COrCO-Ar 114 gas mixtures with Pco l(Pco + Pco,) ratio 0.80 - 0.875 at 1100°C
5-2-5 (Continued.). The Fe/Ti ratios for the reduced ironsand samples by COr 115 CO-Ar gas mixtures with Pco l(Pco + Pco,) ratio 0.90- 1.0 at 1100°C
5-2-6 Mapping image of a particle in a sample reduced by 75vol%CO-Ar gas 117 mixture at 1100°C for 90 minutes 5-2-7 XRD patterns of samples reduced by 75vol%CO-Ar gas mixture at 1100°C, 119 in the progress of reduction 5-2-8 Elemental distributions in progress of reduction of ironsand by 75vol%CO- 121 Ar gas mixture at 1100°C 5-2-8 (Continued.) Elemental distributions in progress of reduction of ironsand by 122 75vol%CO-Ar gas mixture at 1100°C 5-2-9 Morphology change of homogeneous titanomagnetite particles during the 124 reduction by 75vol%CO-Ar gas mixture at 1100°C 5-2-10 Morphology change of non-homogeneous particles during the reduction by 125 75vol%CO-Ar gas mixture at 1100°C 5-2-11 Change of SSA during reduction at 1000 and 1100°C 126 5-2-12 Change in SSA during the reduction by CO-COrAr gas mixtures of 127 different compositions at 1100°C 5-2-13 Reduction of raw, preoxidised and calcined ironsand in comparison with 129 hematite and magnetite iron ore, by 75vol%CO-Ar gas mixture at 1100°C 5-2-14 Reduction of ironsand by 25vol%HrAr gas mixture at different 131 temperatures 5-2-15 Reduction of ironsand by HrAr gas mixture with different hydrogen 132 content at 900°C 5-2-16 Reduction of ironsand by 25vol%HrAr gas mixture containing different 132 water vapour content at 900°C 5-2-17 XRD patterns of samples of ironsand in progress of reduction by 134 25vol%HrAr gas mixture at 900°C 5-2-18 Morphology of a sample reduced by 25vol%HrAr gas mixture at different 135 temperatures (Reduction time: 60 minutes) 5-2-19 Morphology change of homogeneous titanomagnetite particles during the 137 reduction by 25vol%HrAr gas mixture at 900°C
xv S-2-20 Morphology change of non-homogeneous particles during the reduction by 138 25vol%H2-Ar gas mixture at 900°C S-2-21 SSA of samples after 60-minute reduction by 25vol%HrAr gas mixture at 139 different temperatures S-2-22 The change of SSA during the reduction of ironsand by 25vol%H2-Ar gas 140 mixture at 900°C S-2-23 Reduction of raw, preoxidised and calcined ironsand in comparison with 141
hematite and magnetite iron ore, by 25vol%H2-Ar gas mixture at 900°C
5-2-24 The reduction curves of ironsand, by H2 and by CO at 1100°C 142
5-2-25 Reduction of ironsand by 5vol%CH.-25vol%H2-Ar at different 143 temperatures S-2-26 Reduction of ironsand by CH.-50vol%HrAr gas mixtures containing 144 different CH. content, at 900°C 5-2-27 Reduction of ironsand by 5vol%CH.-HrAr gas mixtures containing 145
different H2 content from 0 to 70vol%, at 900°C 5-2-28 XRD patterns ofironsand samples reduced by CH.-20vol%HrAr gas 146 mixtures with different methane contents at 900°C, after 60-minute reaction 5-2-29 XRD patterns of samples of ironsand in progress of reduction by 148 20vol%CH.-20vol%HrAr gas mixture at 900°C 5-2-30 The reduction curves of the raw and the preoxidised ironsand at 1100°C 150 (Reduction gas: Present study - 70vol%CO-30col%Ar, 800ml/min; McAdam et al. - lO0Oml/min of CO) 5-2-31 The reduction curves of the raw and the preoxidised ironsand at 900°C 151
(Reduction gas: Present study- 25vo1%H2-75col%Ar, 800ml/min; McAdam et al. - lOO0ml/min ofH2; Morozov et al. - H2, Flowrate was not commented.) 5-2-32 Comparison of the gaseous reduction of ironsand with the carbothermic 152 reduction at 1100°C (Present study- 70vol%CO-30co1%Ar, 800ml/min; McAdam et al. -Pellet [1]: 200ml/min ofN2, Pellet [2]: 66.7vol%Nr 33.3vol%CO, 300ml/min)
xvi List of Tables
Table Page 2-1 Chemical composition of ironsand ores (McAdam, 1974) 5 2-2 Cell dimensions of natural New Zealand titanomagnetite ores 13 (Wright, 1964) 2-3 Reduction paths of Iron-Titanium oxide by hydrogen, at 1000°C 29 (Fihey et al., 1979) 3-1 Chemical composition of New Zealand ironsand, wt% 41 3-2 Chemical composition of the iron ore, wt«'/4 42 3-3 Gases used in the experiments 42 3-4 The crystals and standards for EPMA 49 4-1 The surface area and the pore volume and size of the raw ironsand ore 62 5-1 Calculated extent of reduction in different reduction steps 91
5-2 Phases in samples reduced by gas containing different CO2 contents 110
xvii Chapter 1. Introduction
Titanium-containing iron ores, which are found throughout the world, often in large deposits, are becoming alternative sources of iron as conventional iron ore reserves are dwindling (Ohno and Ross, 1963; Marshall, 1970; Sadykhov et al., 1993).
The mineralogy of ironsand has been studied extensively by many geologists because the ironsand is one of major Earth crust's minerals, which found in many volcanic areas around the world. lronsand is mainly composed of homogeneous titanomagnetite particles (Wright, 1964; Buddington and Lindsley, 1964; McAdam, 1974). Titanomagnetite is a solid solution of magnetite and ulvospinel, with spinel 2 3 4 cubic structure. The cation (Fe +, Fe + and Ti 1 distribution in a titanomagnetite lattice depends on the composition ofulvospinel and temperature (Ak:imoto, 1954, Neel, 1995; Chevallier et al.1955; O'Reilly and Banerjee, 1965; Woodland and Wood, 1994). There are two kinds ofpbase separation in titanomagnetite: l) phase separation caused by the miscibility gap between magnetite and ulvospinel (Anderson and Lindsley, 1998; Woodland and Wood, 1994) and 2) phase separation caused by partial oxidation oftitanomagnetite (Buddington and Lindsley, 1964; Wright, 1964, Price, 1976; Haggerty, 1991 and Krasnova and Krezer, 1995). The latter case is called exsolution, which is due to low solubility of rhombohedral phase in a cubic phase (Buddington and Lindsley, 1964). EPMA studies showed that the exsolution combines migration of cations, resulting in enrichment of titanium in rhombohedral titanohematite phase (Wright and Lovering, 1966; Akimoto et al., 1984).
BHP New Zealand Steel uses ironsand as a raw material for ironmaking at Glenbrook, New Zealand since 1970 (Marshall, 1970; McAdam, 1974). The current ironmaking technology at Glenbrook Steel works includes a carbothermic ironsand reduction using a rotary-klin. However, carbothermic reduction process has a slow reduction rate, high-energy comsumption, and relatively narrow operating temperature (Dancy, 1993; Pelton and Bale, 1999). The research into the alternative reduction of ironsand, particular the gaseous reduction has not been sufficient. It has been established that the gaseous reduction of ironsand is much slower than that of magnetite iron ore (McAdam et al., 1969a;
1 McAdam, 1974; Sadykhov et al., 1992; Morozov et al., 1998). The complete reduction of iron oxides in ironsand by carbon monoxide is possible at high temperature above 1000°C, under high reducing potential (McAdam et al., 1969a; McAdam, 1974; Morozov et al., 1998). Preoxidation enhances the reducibility of titanomagnetite ore (McAdam et al., 1969a; Morozov et al., 1998). However, understanding of the mechanism and the kinetics of the reduction is not complete and the previous studies have many contradictions with respect to the phase transformation during the reduction process, the role of cation impurities, and the reduction kinetics. Also, the most of previous studies, except McAdam' s work (1974), considered ironsand as an extension of titanium-free magnetite iron ore, lacking in the consideration of the nature oftitanomagnetite, which has been studied intensively by geologists.
The following aspects of ironsand reduction require further understanding: I) Slow reduction of ironoxides in titanomagnetite in comparison with hematite and magnetite iron ores; 2) The effect of pretreatment of ironsand on its structure and reduction; 3) The effect of titanium in titanomagnetite on the reduction ofironsand; 4) The effect of operational parameters on the rate and extent of ironsand reduction.
The aim of this study is to develop further understanding of the reduction mechanism of ironsand by gas-solid reaction with a focus on a structure of ironsand and its transformation during reduction. The aim of the project was achieved through the examination of the following aspects of the reduction process. • lronsand 's structure and phase composition, and their change in the course of reduction. • The reduction sequence. • Reduction extent and kinetics at different temperatures and gas compositions. • Retarding effect of titanium on the iron oxide reduction. • Effect of calcination and preoxidation of ironsand on its structure and reduction.
2 The structure of the thesis is as follows. The literature survey is presented in Chapter 2, which also presents the scope of the work. Experimental procedures employed in the study, experimental setup for the pretreatment and reduction tests and sample characterisation, are described in Chapter 3. Chapter 4 presents and discusses the characteristics of raw and pretreated ironsand. The mechanism of the reduction of ironsand, which is based on the characteristics ofironsand is presented in Chapter 5. Conclusions and suggestion for further work are presented in Chapter 6.
3 Chapter 2. Literature Review
2.1. Ironsand deposits
Erosion of the extensive volcanic areas in the North Island of New Zealand has produced significant coastal concentrations of titanium-containing ironsand deposits, chiefly titanomagnetite and ilmenite (Wright, 1964). The location of the ironsand deposits in New Zealand is shown in Figure 2-1-1.
N
Glanbaok S111111 Ml ._. Tauranga Bay Waikato North ·: t ··---~Head _.,• 1:-i.J" Hamilton' Gap t 0~1ly ~rm¾ I ' . 1,,:/I>:'~ (111r- '- NORTH ISLAND
Tllanomag rellle IIOrBands !II!!!!!!!!!!!! llmenlle sands
0 200km
Figure 2-1-1. Location ofironsand deposits in New Zealand (Wright, 1964; BHP NZ, 2000)
Table 2-1 summaries the compositions of some titanomagnetite ironsand ores presented in literature (McAdam, 1974). According to Table 2-1, titanium contents in
4 various ironsand ores range from 0.8 to 8.2wt%. Other impurities in ironsand are magnesium, aluminium, calcium and silicon. The content of iron varies from 51. 7wt% (Waikato North Head), to 69.8w°/o (Gillespi's Beach). The concentration of Fe3+ is also different from different ironsand deposits, from 49.3wt% Fe2O3 in Waikato Notrh Head ore to 66.5wt% Fe2O3 in Gillespie's Beach ore.
Table 2-1. Chemical composition ofironsand ores (McAdam, 1974) Ore Gillespi's Hamilton's Waikato North Tauranga Bay Concentrates Beach Gap Head Total Fe 69.8 67.6 53.8 51.7 Fe2O3 66.5 64.4 51.3 49.3 FexO 30.1 28.7 30.5 28.9 TiO2 0.8 2.0 8.2 7.6 A}iO3 1.1 0.5 3.6 3.6 MgO 0.1 0.4 2.6 2.9 SiO2 0.4 1.4 0.6 2.2 CaO 0.4 0.6 0.1 0.7 MnO 0.1 0.2 0.7 0.7 Cr2O3 - 1.2 - - V2O3 0.2 0.1 0.5 -
The ironsand used by BHP New Zealand Steel is from Waikato North Head. This ironsand ore is often called New Zealand ironsand, or simply ironsand. In this research, the term "ironsand" is the ironsand ore from Waikato North Head, which contains about 8wt% TiO2 and 2-4wt% of AhO3, MgO and SiO2.
2.1.1. Characteristics of ironsand deposits
Many attempts have been made to characterise the ironsand deposits. Wright (1964) has carried out an optical, magnetic and x-ray study ofironsand ores from various locations around New Zealand. He found that the majority of the grains in the ores were homogeneous magnetite-ulvospinel solid solution, so called titanomagnetite. McAdam (1974) also found by microstructure study using optical microscopy and
s SEM analysis that the ironsand grains were featureless titanomagnetite, although some grains were found to include apatite, pyrite and silicate inclusions.
2.2. Structure and thermodynamics of ironsand
The iron-titanium-oxygen system has been studied intensively by many geologists
1 Ti02 (Rutlle, At!otau,Brooldte) ·
Feor·~--.:Fi;:,..,-.:::-0,-.------.;m;------..;...... -~ Figure 2-2-1. FeO-Fe203-Ti02 ternary system (Buddington and Lindsley, 1964) The iron-titanium oxides of interest form a ternary system with FeO, Fe203 and Ti02 end-members (Figure 2-2-1 ). The major compounds in this system are wustite ("FeO"), magnetite (Fe304), hematite (Fe203}, maghemite (y-Fe203), pseudobrookite (Fe2 Ti05}, ferro-pseudobrookite (FeTh05), ilmenite (FeTi03) and ulvospinel (Fe2 Ti04). There are three fundamental solid-solutions in the system: magnetite-ulvospinel solid solution, which is called titanomagnetite with an inverse spinet structure, hematite-ilmenite solid solution (titanohematite) with a rhombohedral structure and orthorhombic pseudobrookite-ferro-pseudobrookite solid 6 solution. Of these, titanomagnetite and titanohematite solid solutions are the most important to this study. 2.2.1. Structure and thermodynamic properties of titanomagnetite 2.2.1.1. Structure of titanomagnetite The general formula of titanomagnetite may be written as: (Fe3O4 ) 1_x(Fe2TiO4 )x (2-1) Both magnetite and ulvospinel have the inverse spinet structure, with a space group Fd3m. This structure incorporates one tetrahedral cation and two octahedral cations 2 per four oxygen ions. In a pure magnetite lattice at room temperature, all Fe + are 3 octahedral, whereas Fe + is distributed over both tetrahedral and octahedral sites. An increase in the titanium content results in an increase in the cell lattice parameter (Figure 2-2-2). .1 j 1 I • I t , , j 4C 50 60 .10 80 90 100 .. . Molt% Usp '•i•O. Figure 2-2-2. Effect of ulvospinel content on the magnetite cell lattice parameter (Wechsler et al., 1984) 4 Substitution ofTi + for Fe ions in a magnetite spinet lattice requires the conversion of an equivalent number ofFe3+ to Fe2+, i.e. every pair ofTi4+ and Fe2+ 3 replaces two Fe + ions, conserving a total ionic charge. In a unit cell of titanomagnetite, there are twenty-four cations occupying tetrahedral and octahedral sites: 1) 8 tetrahedral sites where each cation is surrounded by four oxygen ions and 2) 16 octahedral sites where each cation is surrounded by six oxygen ions. 7 4 The distribution ofTi + in titanomagnetite was studies by X-ray and neutron diffraction, Mossbauer spectroscopy, and saturation magnetisation method. The analyses showed that the titanium ions have a preference for octahedral sites 4 (O'Reilly and Banerjee, 1965; Stephenson, 1969). Ti + takes only octahedral sites in a titanomagnetite unit cell. 2 3 The distribution ofFe + and Fe + in titanomagnetite at room temperature has been described by a number of models, based largely upon the results of magnetic measurements of natural and synthetic specimens. Akimoto (1954) suggested a simple substitution model in which the occupancy of each site varies linearly between Fe304 and Fe2Ti04, as shown in Figure 2-2-3 (a). Neel (1955) and Chevallier et al. (1955) proposed a model based on expected crystal-chemical preferences in titanomagnetite, 2 3 in which Fe + always prefers octahedral coordination while Fe + always prefers tetrahedral coordination (Figure 2-2-3 (b )). 3 Octdledral site& Tetrdledral site 0 0•2 0•4 0·6 0·8- l•O Composition X (a) Akimoto model 3 Octahedral sites Tetrahedral site Composition X (b) Neel-Chevallier model Figure 2-2-3. Schematic cation distribution in titanomagnetite solid solution, (Fe30,)1-JFe2TiO,)x (Stephenson, 1969) 8 Studies by O'Reilly and Banerjee (1965) and Wechsler et al. (1984) using the saturation magnetisation method data showed that data for titanomagnetite lie between the two models presented in Figures 2-2-3 (a) and (b).This resulted in the development of an intermediate model (O'Reilly and Banerjee, 1965) which follows the Neel-Chevallier model in the range of x from O to 0.2 and from 0.8 to 1.0, the Akimoto model for x from 0.2 to 0.8. Recent Mossbauer studies (Hamdeh et al. 1999) also showed that the cations distribution in titanomagnetite is between the two models, being close to the intermediate model proposed by O'Reilly and Banerjee (1965). 3 2 It has been reported that the distribution ofFe + and Fe + in titanomagnetite is temperature-dependent. Stephenson (1969) and Bleil (1971, 1976) proposed a 2 3 temperature-dependent ordering ofFe + and Fe + in the magnetite-ulvospinel solid 2 3 solution. The distributions ofFe + and Fe + approached the Akimoto model at high temperatures above 1100°C and the Neel-Chevallier model at low temperatures below 800°C. Wu and Mason (1981), Trestmann-Matts et al. (1983), and Anderson and Lindsley (1988) showed that the octahedral [Fe27/[Fe3+] ratio in titanomagnetite 3 varies with temperature and composition. Figure 2-2-4 shows the fraction ofFe + in octahedral sites in titanomagnetite solid solution as a function of temperature and composition. i,00 Ti• 0 0.10 T1 • O.UI _ 0.10 + f'I ~• o ... o n • o.ee. 0.10 T1 • 0.99__ N oo .. o.oo,______._ ___o-----·o··---<>·----~------~-_._ ___ ....._ _ __,;""--.__...... ,...... , "00 eoo 800 SOOD i200 l(C) 3 Figure 2-2-4. Fraction of Fe + in octahedral sites of titanomagnetite lattice (Wu and Mason, 1981; Trestman-Matts et aL 1983; Anderson and Lindsley, 1988) Literature review on the structure of titanomagnetite showed that the 3 2 distribution of Fe ions (Fe + and Fe J in octahedral and tetrahedral sites in the titanomagnetite lattice is the function of temperature and the content of titanium. 9 2.2.1.2. Mixing properties of magnetite and ulvospinel in titanomagnetite Katsura et al.(1915) derived activity composition relations from the phase equilibrium studies of Webster and Bright (1961) and Taylor (1964) at 1200°C and 1300°C. From the fact that magnetite activity was nearly equal to its mole fraction ( a Fe o = X Fe o ), they concluded that the mixing between magnetite and ulvospinel is 3 4 3 4 essentially ideal at the given temperatures. Schmahl et al. (1960) and Katsura et al. (1976) also showed that magnetite activities are close to mole fractions at 1000°C and 1200°C. Itoh et al. (1998) calculated Fe3O4 activity in the Fe3O4-Fe2TiO4 pseudo binary system in equilibrium with metallic iron at 900°C and 1100°C, using experimental data at 1100°C. Both the activities ofFe3O4 and Fe2TiO4 exhibited negative deviations from Raoult's law at lower temperature; however, with increasing the temperature, the behaviour of the solution approached to the ideal. The mixing properties of the magnetite-ulvospinel solid solution at 400°C and 1200°C calculated by the Akimoto model and the site-mixing model, based on the intermediate model (O'Reilly and Banerjee, 1965), is shown in Figure 2-2-5 (Andersen and Lindsley, 1988). s.a !i.O 12DOC ...... Gu .... , ...... ~ -1.a -; -e. ~ Ji: -sc.o ~ -10. -m. -s:,. ""2.0.CJlo------'o .OG o.zo o.c, o.eo o.oo i .oo -eo.a------o.eo D.~D O.DO o.eo 1.,0 FeaO,· Fez TIO, le:&, fe,TIO, Figure 2-2-5. Mixing properties of magnetite-ulvospinel solution at 400°C and 1200°C (dotted line: the Akimoto model; solid line: the site-mixing model; Gxs is excess Gibbs free energy, Gxs == .AGm1x -TASm1x.) (Anderson and Lindsley, 1988) Woodland and Wood (1994) determined the activity of magnetite in a magnetite-ulvospinel solid solution by measuring oxygen partial pressure at the 10 equilibrium between titanomagnetite solid solution and wustite. They showed that in the temperature range from 800°C to 1300°C, the activity of magnetite is substantially above the line represented by an equation a Fe o X ;ep , which is the ideal curve for 3 4 = 4 a disordered two-site solid solution. Under these conditions the activity ofFe304 is less than the mole fraction for X Fe o below 0.3, and higher than the mole fraction at 3 4 high concentrations (Figure 2-2-6). 1.0 1.0 1.0 9oo•c 900•c 1000°c 0.8 n..... _,. i..,.,,,. .~/.··/ .... 0.8 0.8 ~· / / nu s~ ,, / / 0.6 / 0.6 0.6 J /. /--.i/ ,I J / .· 0.4 / - 0.4 0.4 / .. 0.2 .. 0.2 0.2 ...... L-~1 A B A 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.2 0.8 1.0 0.2 0.4 0.6 0.8 1.0 x.. x.. 1.0 1.0 1200°c 0.8 0.8 0.8 0.6 J J 0.4 0.4 0.2 0.2 B 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 x.., x.. Figure 2-2-6. Activities of Fe304 in titanomagnetite solid solution at different temperatures, H&S: Hill and Sack (1987); A&L: Anderson and Lindsley (1988) (Woodland and Wood, 1994) However pure ulvospinel phase has never been separated from the host magnetite, although, there have been numerous investigations reporting the phase separation between magnetite-rich phase and ulvospinel-rich phase. Mogensen (1946) first noted the existence of a two-phase intergrowth of magnetite and ulvospinel in titanomagnetite from Sodra Ulvon. This observation subsequently led to the suggestion that the solid solution between magnetite and ulvospinel was not ideal but contained a miscibility gap at low temperatures. Early work placed the consolute point at approximately 600°C and X Fe o = 3 4 0.65 (Kawai, 1956; Vincent et al., 1957), while recent studies using homogenisation 11 and annealing experiments suggest a lower temperature of 565 ± 15°C and a composition near (Fe3O4)o.. u(Fe2TiO4O)o.<1.1 (Figure 2-2-7, Lindsley, 1981) or even below 490°C with symmetric solvus line (Price, 1981 ). The 6-9wt0/o of impurities in .the initial titanomagnetite could lower the temperature of the consolute point (Price, 1981). . . . 1 I . ' . .,,. ...., /' \ i \ I \ , \ I \ I \ l \ • . I I 11101& •• '11 Figure 2-2-7. Miscibility gap in magnetite-ulvospinel solid solution (Lindsley, 1981) Woodland and Wood (1994) derived the cation distribution in the magnetite ulvospinel solid solution at different temperatures using their activity data presented in Figure 2-2-6. From the expansion of the vibrational part of the Gibbs free energy to the Taylor series, they calculated that there is a marked departure from ideal behaviour ( aFe o X Fe o ) with the deviation being greatest at low temperatures. 3 4 = 3 4 They predicted the consolute temperature to be 600°C at (Fe3O4)0.62(Fe2TiO4O)o.Ja. 2.2.1.3. Effect of impurities on structural properties oftitanomagnetite Most of iron-titanium minerals contain magnesium, aluminium, silicon, calcium, manganese and other metals. Among them, aluminium and magnesium form spinel compounds with iron and titanium and affect the properties of titanomagnetite. Wright (1964) found using XRD that natural New Zealand titanomagnetite ironsand ores, which contain 3.3 to 5.6 wt%AhO3 and 2.7 to 3.3 wt%MgO, have nearly the same cell edge of the cubic structure, close to that of pure magnetite. The cell dimensions of various natural titanomagnetite ores are shown in Table 2-2. He 12 assumed that aluminium and magnesium dissolved mainly in titanomagnetite solid solution, and bad the effect of compensating for the tendency of ulvospinel to increase the lattice dimension of the magnetite. EPMA studies on ironsand by Wright and Lovering (1966) and Akimoto et al. (1984) showed that aluminium and magnesium are dissolved in titanomagnetite solid solution without forming separated phases. Table 2-2. Cell dimensions of natural New Zealand titanomagnetite ores (Wright, 1964) Source Fe304, Ti02, Ah03, MgO, Cubic cell edge, wt% wt°/o wt% wt% ao(A) Lake Taharoa 73 7.9 3.3 3.2 8.400±0.003 Pates 72 7.8 3.1 2.8 8.397±0.003 Raglan 70 7.0 5.6 2.7 8.401±0.003 Waikato North Head 70 8.0 3.5 3.3 8.402±0.003 Pure magnetie 100 - - - 8.396 2.2.2. Phase relations in the Iron-Titanium-Oxygen system Naturally occurring ironsand ores generally deviate from the stoichiometric composition and structure having phase inhomogenity (exsolution) and low or high temperature non-stoichiometry. Many investigators (Buddington and Lindsley, 1964; Wright, 1964; Haggerty, 1991 and Krasnova and Krezer, 1995) reported the exsolution textures in ironsand particles, which morphology is shown in Figure 2-2-8. Wright (1964) found grains of various New Zealand ironsand ores with exsolved lamellae of a rhombohedral phase. The rhombohedral phase was generally exsolved along the octahedral direction {111} in the host titanomagnetite phase, so that the two sets oflamellae cut across one another with an angle close to 60°. He also found that the lamella is a ferri-ilmenite, which is close to titanohematite. 13 .... l ·- . a.. Figure 2-2-8. lronsand particle showing exsolution, Waikato North Head, (x650) (Wright, 1964) Buddington and Lindsley ( 1964) investigated compositions of a coexisting titanomagnetite solid solution and titanohematite solid solution. They found that the solubility of rhombohedral ilmenite phase in spinet cubic magnetite phase is negligible. The low solubility of rhombohedral phases in spinel phases causes the exsolution and intergrowth of titanohematite in the host titanomagnetite. The compositions of these phases at equilibrium are dependent on oxygen partial pressure ) , as well as temperature. The relationship between oxygen fugacity ( f 02 temperature and composition in titanomagnetite-titanohematite system is shown in Figure 2-2-9 (Buddington and Lindsley, 1964). The curves in the plane are the iso-Ti content lines. For example, at 900°C, Mts0Usp20 can be formed whenlog/0 2 < 10-,o.s . 14 s·-· _____,.._._...._,__,. ______.,...._..-- ______....,.._ /' 10 o' 15 ..... 2 c,, 2 I · 20 600 700 800 . 900 1000 1100 Temperature , • C Figure 2-2-9. Equilibrium relation between Titanomagnetite (Mt100-x-Ulvx) and Titanohematite (Hem100-yllmy) Numeric subscripts stand for mole percent. (Buddington and Lindsley, 1964) As shown in Figure 2-2-9, oxygen fugacity and temperature strongly affect the exsolution. Early studies found that oxidation of titanomagnetite produced an exsolved rhombohedral phase (Vincent, 1960; Lindsley, 1961; Wright, 1964). According to Buddington and Lindsley (1964), a sequence of reactions that would take place during the progressive oxidation of titanomagnetite is: 15 6Fe2Ti04 + 0 2 = 2Fe30 4 + 6FeTi03 (2-2) 4Fe2Ti04 + 0 2 =4FeTi03 + 2Fe2 0 3 (2-3) 4Fe30 4 + 0 2 = 6Fe2 0 3 (2-4) 4FeTi03 + 0 2 = 2Fe2 0 3 + 4Ti02 (2-5a) 4FeTi03 + 0 2 = 2Fe2Ti05 + 2Ti02 (in some volcanic rocks) (2-5b) (These reactions were all simplified, in that solid solution relationships were involved in each. More accurately, for example, equation (2-2) should be read: ulvospinel-rich titanomagnetite solid solution + oxygen = magnetite-enriched titanomagnetite solid solution + ilmenite-rich titanohematite solid solution.) Literature data on oxidation of titanomagnetite are inconsistent. Wright and Lovering ( 1966) found using EPMA that in the early stage of oxidation of natural titanomagnetite there is a rapid build-up of titanium in the newly formed rhombohedral titanohematite phase. As oxidation proceeds, the ratio of titanium to iron in the rhombohedral phase increases. Akimoto et al. (1984) investigated phase compositions of partially oxidised titanomagnetite grains of volcanic rocks using EMPA. They showed that the oxidation of titanomagnetite combines migration of cations. During low-temperature oxidation below 600°C, cations with high mobility (Fe2+, Fe3+, Mg2+, Mn2+, etc.) migrate from 4 titanomagnetite lattice with enrichment of cations with low mobility (Ti +, AI3+, etc.) in the titanomagnetite lattice. As the oxidation proceeds, the cations with high mobility easily migrate out of the lattice, while the cations with low mobility remain within the lattice. When the lattice shrinks by reduction of the cell edge due to the transformation of spinet cubic to rhombohedral structure, the concentration increases further. Recently, ltoh et al. (1998) investigated phase relations in a synthetic Fe-Ti-O system by oxidation at l 100°C using X-ray diffraction. They constructed the phase diagram of the Fe-Ti-O ternary system at l 100°C shown in Figure 2-2-10. 16 1373K • : • -<>Xide (hemali!e-ilmenile U .) Fe : i""' p : pw,dobrooldlc .... r R: rulilc S: spind (m.s1neti1e-ul..,,pind u.) ' ·~... W : Wustilt 0.55 '4." I 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Xr1 Figure 2-2-10. Phase diagram of the Fe-Ti-O ternary system and isobars of oxygen at 1100°C [ - - - o - - - :isobars of oxygen partial pressures ] From the equilibrium experiments, they estimated the standard Gibbs free energy change of the following reactions in the temperature range of900 to l 100°C. Fe(s) + FeTi03( s) + 0.502 ( g) = Fe 2Ti04 (s), (2-6) tiG;er;oJ J I mol) = -271600 + 63.35T 2 Fe( s)+ Ti02 (s)+ 0.502 ( g) = FeTi03 (s), (2-7) tiG;er;o (J I mol) = -282400 + 62.93T 3 They also found that in the magnetite-ulvospinel solid solution (S in the three phase combination 'Fe+W+S' (Figure 2-3-3)), its composition shifted to ulvospinel side with increasing the temperature, that is, the molar ratio XT/(XFe + XTJ shifted from 0.235 at 900°C to 0.265 at l 100°C. 17 2.2.3. Summary Natural ironsand ores from New Zealand are mostly composed of homogeneous titanomagnetite particles. Titanomagnetite is a solid solution of magnetite and ulvospinel, with spinel cubic structure. The cation (Fe2+, Fe3+ and Ti4) distributions in a titanomagnetite lattice depend on the composition ofulvospinel and temperature. The temperature dependent cation distribution in titanomagnetite affects the mixing property of the solid solution, resulting in the miscibility gap between magnetite and ulvospinel occurred below about 600°C. Particles of titanomagnetite show a phase separation under various atmospheres. There are two kinds of phase separation in titanomagnetite: 1) phase separation caused by the miscibility gap and 2) phase separation caused by partial oxidation of titanomagnetite. The latter case is especially called exsolution, which occurs by intergrowth of rhombohedral titanohematite phase in spinet cubic titanomagnetite phase. The exsolution is due to low solubility of rhombohedral phase in cubic phase. Studies using EMP A showed that the exsolution combines migration of cations, resulting in enrichment of titanium in rhombohedral titanohematite phase. 18 2.3. Reduction of iron ore Although geologists established that titanomagnetite ironsand is different to that of pure magnetite ore, many metallurgists (McAdam et al., 1969a, 1969b, 1977; Marshall, 1970; McAdam, 1974; Sadykhov et al., 1992; Morozov et al., 1998) considered the ironsand on the basis of magnetite iron ore. Reduction of hematite and magnetite iron ores is well described in literature. This section briefly presents those aspects of iron ore reduction, which are closely related to the topic of this project. 2.3.1. Thermodynamics of iron ore reduction 2.3.1.1. Structure of iron oxides The iron-oxygen phase diagram is shown in Figure 2-3-1. Iron oxides_ exist in the form ofhematite (a-Fe2O3), magnetite (Fe3O4) and wustite (a non-stoichiometric phase Fe1-xO, further in the thesis will be referred to as "FeO"). Fe2O3 also exists in the form of y-Fe2O3, called maghemite. 2000 liq. Metal Liq. Metal 1800 + Liq. Liq. Oxide Oxide Liq.2xlda (2905°F 02 1600 1591°C 1531-C 1521-C(2712'F) (life) + Liq. Oxide 140!J, -1!1M-C(ZS41'fl 0 131M"C 1371 'C(2500'F) 0 ("fe) +Liq.Oxide 1200 f ("fe) + Wiistita Wiistite .a 1000 as 912-C(1134'F) .. lo. q,, 0 Cl) 800 ....• + Ii (afa) + Wiisllte + 0.. N E ~ J?. 600 570"C (1051°F) ....• ~ /St.38 400 (afe)+Fa304 200 Fe304 fe203 10 20 30 40 50 60 70 Atomic Percent Oxygen Figure 2-3-1. Iron-oxygen phase diagram (Pelton and Bale, 1999) 19 Hematite (a-Fe2O3) has rhombohedral corundum type lattice with a= 5.42A and a= 55°14'. The unit cell contains 18 oxygen ions arranged in a close-packed oxygen lattice and 12 ferric ions (Fe3) which occupy two-thirds of the octahedral 2 interstices. 0 - vacancies in the lattice and iron ions in additional interstitial positions lead to a small oxygen deficiency in the oxide. Maghemite (y-Fe2O3) has a cubic spinet type lattice (a = 8.34A) similar to that of magnetite which has an average of 21.!. ferric ions and 32 oxygen ions per unit cell. 3 Compared with magnetite, y- Fe2O3 is crystallographically isomorphous. It differs from magnetite in that all or most Fe ions are in the trivalent state. Cation vacancies 2 compensate for the oxidation ofFe +. Maghemite transforms to hematite at temperatures 370 to 600°C depending on its origin and the impurity content (Bernal et al., 1959; Egger and Feitknecht, 1962; Sidhu, 1988; Trone et al., 1990). However, at high enough temperatures above 500°C, magnetite changes directly to hematite. Magnetite has cubic inverse spinel type structure with a = 8.38A. The unit cell 3 2 contains 32 oxygen ions, 16 ferric ions (Fe ) and 8 ferrous ions (Fe ), and has 64 tetrahedral and 32 octahedral interstices. The oxygen ions form a close-packed cubic lattice and the smaller iron ions are distributed in the interstices. In this inverse spinet structure, 8 ferric ions are in tetrahedral and 8 ferrous ions in octahedral sites. Wustite ("FeO") has a cubic lattice, that is a close-packed oxygen lattice with the iron ions arranged in the octahedral interstices between the larger oxygen ions (Stephenson and Smailer, 1980; Edstrom, 1953; Aharoni et al., 1962). It is unstable below 568°C, decomposing eutectoidally into a- Fe and magnetite, but is easily found as a metastable phase below the temperature. Wustite has the chemical formula Fe1-xO, where the value of x changes between O and 0.1 but never goes to zero, although wustite is often written as FeO. 20 2.3.1.2. Thermodynamics of the gaseous reduction of iron oxides The major reducing agents currently used in DRI process are carbon monoxide and hydrogen. In practice, a mixture of carbon monoxide and hydrogen is used. Methane is also used in the reducing gas, mixed with hydrogen for iron carburisation. The reactions of the reduction of iron ore by carbon monoxide and hydrogen are as follows (Rao, 1985): 3Fe O (s) + CO(g) = 2Fe O.(s) + CO (g) 2 3 3 2 (2-8) AG 0 (J) = -32,907 -32.719T Fe O (s) + CO(g) = 3FeO(s)+ CO (g) 3 4 2 (2-9) AG0 (J) = 29,790-38.074T FeO(s) + CO(g) = Fe(s) + CO (g) 2 (2-10) AG 0 (J) = -22,804 + 24.476T Fe3O4 (s) + 4CO(g) = 3Fe(s) + 4CO2 (g) (T ~ 568°C) (2-11) AG 0 (J) = -38,620 + 35.356T 3Fe O (s)+H (g) = 2Fe O (s)+H O(g) 2 3 2 3 4 2 (2-12) AG 0 (J) = 3,012-85.86T Fe O.(s) + H (g) = 3FeO(s) + H O(g) 3 2 2 (2-13) AG0 (J) = 65,772-70.29T FeO(s)+H (g) = Fe(s)+H O(g) 2 2 (2-14) AG0 (J) = 13,180-7.74T Fe3O.(s)+4H2 (g) = 3Fe(s)+4H2O(g) (T ~ 568°C) (2-15) AG 0 (J) = 105,304 - 93.56T Figures 2-3-2 (a) and (b) show the equilibrium ofFe-C-O and Fe-H-O systems at 1 atmosphere of total pressure. Thermodynamically, at low temperatures below 810°C, the reduction by carbon monoxide is preferable, while at higher temperature above 810°C, the reduction by hydrogen is preferable. 21 80 .60 ~ 840 20 (b) I •-Fe J-F. 80 .eio - ...... ~~-~,-;;:::::--;---t----;i-t---;40-/, ~ 6 . :,::... 40 --+-~~_.;:~-:n:rt-~+---+----icO: Liquid oaidc 20 0 Figure 2-3-2. Equilibrium phase diagrams of (a) Fe-C-O system and (b) Fe-H-O system (Dancy, 1993) 2.3.2. Kinetics of iron ore reduction Reduction kinetics depends strongly on porosity of initial iron ore and phases formed in the course of reduction. When reduction takes place in the shrinking core mode (Figure 2-3-3), chemical reactions occur at the solid-solid interfaces and gases move across all three porous layers. 22 \ ) I Figure 2-3-3. Cross section of a partially reduced dense iron ore particle (Lu, 1999) When ore particles are dense and the reducing gas is unable to contact the magnetite layer and the hematite core, a reduction mechanism based on solid-state diffusion of ferrous ions (Edstrom, 1953) may be used. In this case, assuming that iron layer is porous enough to permit the reducing gas to diffuse to the iron-wustite interface, the wustite is reduced to iron by the reactions (2-16) and (2-18). 2 2 "Fe0"=Fe + +0 - (2-16) 02- +C0=C02 +2e- or (2-17a) 2 - 0 +H2 =H20+2e (2-17b) Fe2+ + 2e- = Fe(s) (2-18) In this way oxygen is removed from the surface of the wustite resulting in an increase in the concentration of ferrous ions (Fe2J. The solid-state diffusion of ferrous ions through the wustite crystal lattice is facilitated by the presence of lattice vacancies. The ferrous ions and electrons migrate to nucleation sites where they precipitate as metallic iron or to wustite/magnetite and magnetite/hematite interfaces, and react with magnetite and hematite to produce wustite and magnetite. The solid state diffusion of ferrous ions through wustite is faster than the gaseous diffusion of 23 either hydrogen or carbon monoxide through the ore particles at the ORI process temperatures. Changes in crystal structure take place during the stepwise reduction of hematite to magnetite, wustite and metallic iron. In the hexagonal close packed layer of the hematite structure, 2/3 octahedral interstitial sites are occupied by Fe3+ ions. The removal of oxygen from the hematite causes a change from the hexagonal to regular close packing and migration of some iron ions from octahedral to tetrahedral interstitial positions. The displacement of these ions results in the formation of magnetite, i.e. the new structure of the inverse spinel type: Fe3+[4J [Fe2+[6JFe3+C6J] 04 (at room temperature, the number in [] is a coordination number). The readjustment of oxygen and iron atoms during this reduction stage results in about 25-percent increase in volume (Edstrom, 1953). This tends to open up the structure and facilitate the subsequent reduction stages. In the transformation of magnetite to wustite the oxygen lattice remains unchanged while iron atoms diffuse in to fill the vacant sites in the iron lattice. The reduction of natural magnetite ore to iron, has been reported to be much slower than that ofhematite (Wiberg, 1948; Edstrom, 1953), and the reducibility of magnetite is greatly improved when magnetite is first oxidised to hematite before reduction (Monsen, 1994). Edstrom (1953) observed that there is no volume increase in the reduction of natural magnetite crystals to metallic iron, but there is about 4-5 percent shrinkage in the final product. The lack of expansion of magnetite during reduction causes the formation of dense layers of metallic iron surrounding remnants of wustite which cuts off access of the reducing gas to the oxide and prevents complete removal of oxygen. According to the structural change in the reduction of iron oxides, it is generally accepted that there are three different stages in the reduction process of dense iron ores (El-Geassy et al., 1977; El-Geassy and Rajakumar, 1985a; Rao, 1979);. 1) Incubation and induction: No measurable reduction occurs in incubation; the hematite structure is prepared to change iron and oxygen ions configuration which will lead to the formation of the magnetite lattice. During induction, where the rate of reduction increases, once magnetite clusters are formed, nucleation begins. This process, which begins by incubation, results in volume changes at the atomic level since these changes are related directly to the change in the 24 lattice and indirectly to changes in the microscale caused by fracture and porosity of reduced grains. 2) Acceleration: As reaction goes on, more iron is produced and transported to the grain (or phase) boundaries, forming new nuclei and increasing the growth rate of iron phase. 3) Deceleration, where the rate decreases with time up to the end of reduction. Reducing gas composition affects the rate of reduction significantly. The rate of iron ore reduction by hydrogen is much higher than that by carbon monoxide especially at the beginning of the reaction. This is because the diffusion velocity and the adsorption ability of hydrogen are much higher than those of carbon monoxide (Stephenson and Smailer, 1980; El-Geassy et al., 1977, El-Geassy and Rajakumar, 1985a; Moon et al., 1998). It has been reported that the presence of hydrogen in reducing gas improves the reducibility of iron ore by carbon monoxide and addition of small amount of carbon monoxide to hydrogen retards the rate of reduction. El-Geassy and Rajakumar (1985a) found that in the reduction ofwustite at 900 to 1100°C by hydrogen, the number of iron nuclei was much greater than in the reduction by carbon monoxide at the same extent of reduction. They did not observe incubation and induction periods in the reduction by H2 or H2-CO mixtures. On the contrary, reduction of iron ore by carbon monoxide showed distinct incubation and induction periods in the beginning of reduction. According to El-Geassy and Rajakumar (1985a), the incubation period depends on: (1) the minimum energy required to form a nucleus with a critical size; (2) the concentration of favourable sites; (3) the rate of surface reaction; and (4) the rate of atomic rearrangement in the lattice. The reduction of magnetite is also affected by gas composition. Wiberg (1948) showed that at the early stage of the reduction of magnetite, the reduction by H2 was much faster than by CO at 1000°C; however, during the final stages CO reduced magnetite faster than H2. He concluded that the slow reduction by H2 in the final stage is due to the lower pressure built up by water vapour (about 0.6 atm at 1000°C) than that by COi (about 40 atm at 1000°C), in the interface between reduced iron and wustite. CO2 pressure was sufficient to separate the reduced iron shell from the wustite and expand it to fracture, increasing permeability of the reducing gas. 25 El-Geassy and Rajakumar (1985a, b) also found the disintegration of the grains at the Fe/FeO interfaces in the reduction ofwustite by CO at 1000°C, which facilitated gas diffusion to and from the unreacted grains, while the grains reduced by H2 showed only sintering effect. Iron ore can be reduced by methane, but the reduction rate is quite low. Ghosh et al. (1986) reported that the rate of reduction ofhematite by Cli4 at 950°C was 4.5 times slower than that by H2. However, methane being mixed with CO or H2, can be used efficiently for reforming to CO and H2 by the following reactions (Barrett, 1973): CH4 (s)+C02 (s) = 2CO(s)+2H2(g) (2-19) CHis)+ H 2 0(s) = CO(s)+3Hi(g) (2-20) 2.3.3. Summary Direct Reduced Iron processes use carbon monoxide and hydrogen as reducing agents. Methane also is used in a reducing gas, usually mixed with hydrogen. It is generally accepted that there are three stages in the reduction process of dense iron ores; I) incubation and induction, 2) acceleration and 3) deceleration. The appearance of three reduction stages depends on gas composition, crystal structure of the phases of iron oxides and physical properties of ores. Generally, the rate of the ore reduction by hydrogen is higher than that by carbon monoxide, especially at the initial stage of the reduction. However, in the reduction of dense ores or magnetite iron ore, the rate of reduction by CO is higher than that by H2, in the final stage of the reduction. This is due to the pressure build-up by the formation of C02 is much higher than that of H20 at the interface between reduced and unreduced phases. The reduction ofhematite proceeds faster than that of magnetite. There is no structural change during the reduction of magnetite to metallic iron, while the reduction of hematite combines significant structural change which opens the structure for reducing gas to diffuse. The reduction rate of magnetite iron ore can be increased by pre-oxidisation. 26 2.4. Reduction of Iron-Titanium oxides 2.4.1. Reduction of ilmenite ore Composition of ilmenite ores is affected by the natural weathering. In natural 2 3 weathering, ferrous iron (Fe 1 in ilmenite is oxidised to the ferric iron (Fe 1, which is progressively leached out, resulting in the TiO2-rich natural rutile. The natural weathering of ilmenite to rutile is believed to occur through an intermediate metastable pseudorutile (Fe2TiOs), which is also termed arizonite (Palmer, 1909). Poggi et al. (1972) showed that the reduction of the iron oxide in ilmenite ore occurred in two stages. In the first stage hematite is reduced to iron according to the following reaction: (2-21) The rate of reduction of this hematite was found to be slightly lower than of pure hematite. In the second stage, the ilmenite is reduced: FeTi03 +CO= Fe+ Ti02 + C02 (2-22) They also found that synthetic ilmenite reduced at a lower rate than the natural ilmenite. The two reduction reactions (2-23 and 2-24) were shown to occur consecutively and independently to each other. At low temperatures both of these reactions were chemically controlled, while diffusion process was a controlling stage at higher temperatures. Rutile (TiO2) is reduced to lower titanium oxides in the reduction of ilmenite (Jones, 1973; Fihey et al., 1979; El-Tawil et al., 1993; Zhang, 2000). The reduction of rutile proceeds by removal of small amount of oxygen and slight crystallographic rearrangement. The general formula for reduced rutile can be written Tin02n_1, where n >2, which gives a series of mixed Ti(III)!Ti(IV) oxides with increasing content of Ti(III) as n decreases. 27 Jones (1973) investigated the phase transformation in the reduction of ilmenite by carbon monoxide. He suggested that reduction occurs in two stages. The first reduction stage in which ilmenite is formed depends upon whether the ore was preoxidised or sintered. The first stage in the reduction of non-treated ore is the reduction of ferric iron to ferrous iron: (2-23) The first stage in reduction of preoxidised ilmenite ores involves the reaction: Fe2TiO5 + TiO2 + CO= 2FeTiO3 + CO2 (2-24) In the process of reduction of sintered ilmenite ores, the first stage is: Fe3Ti3O10 +CO= 3FeTiO3 + CO2 (2-25) At temperature of 1000°C and lower, the second stage includes further reduction of ferrous iron in ilmenite to metallic iron with formation of rutile: FeTiO3 + CO= Fe+ TiO2 + CO2 (2-26) At the same time rutile is reduced to titanium suboxides: nTiO2 +CO= TinOln-l + CO2 (2-27) At temperature 1200°C and above, ilmenite is reduced first to metallic iron and ferrous pseudobrookite, then a M3Os (M represents a metal cations) phase is reduced to metallic iron and ThO3. According to Zhang (2000), in the reduction of methane-hydrogen gas, iron oxides in natural ilmenite ores were reduced to metallic iron in two stages: pseudorutile was reduced to ilmenite and rutile, and then ilmenite was reduced to metallic iron and rutile. Iron oxides in sintered and preoxidised ilmenite ores were reduced to metallic iron and rutile in a single stage. Fihey et al. (1979) studied the reduction of porous pellets of iron-titanium oxides in a hydrogen atmosphere at 1100°C. It was found that the reduction process involved five sequential reduction steps including the reduction of TiO2 to lower titanium oxide, ThO5• The reduction sequence suggested by Fihey et al. (1979) is presented in Table 2-3. 28 Table 2-3. Reduction paths of Iron-Titanium oxide by hydrogen, at 1000°C (Fihey et al., 1979) Iron-rich phase Titanium-rich phase Fe2O3 (Fe,Ti)3Os i i Step I Fe3O4 (Fe,Ti)2O3 i i FexO Fe2 TiO4+FeTiO3 i Step II i Fe Step III Fe+FeTiO3 i Step IV Fe+TiO2+FeThOs i StepV Fe+TiO2+ThOs Preoxidation was found to increase the rate of reduction of ilmenite (Jones, 1974; Merk and Pickles, 1988; Vijay et al., 1996; Zhang, 2000). According to Ostberg (1960), preoxidation of ilmenite ore converts ilmenite to pseudobrookite and rutile at temperature above 800°C by the following reaction: (2-28) He also found that single crystals of ilmenite converted into a polycrystalline array of pseudobrookite and a fine dispersion of rutile. During the reduction process ilmenite is formed by the following reaction: (2-29) In this case, iron precipitates at sub-grain boundaries within the parent ilmenite grains. In unoxidised ilmenite, shells of metallic iron form along the grain boundaries, which impairs the diffusion of reacting species and also results in the sintering of the product. Both of these effects limit the kinetics of the reduction of unoxidised ilmenite, particularly at higher temperatures. It has been reported that impurities such as magnesium oxide (MgO), manganese oxide (MnO), silica (SiO2) and lime (CaO) retard the reduction of ilmenite 29 ore. Jones (1974) found that at the temperature below 1000°C, the impurities prevented the complete reduction of the iron oxide in ilmenite by carbon monoxide. Merk and Pickles (1988) also found that these impurities not only decrease the final extent of the reduction, but reduce the rate of reduction of synthetic ilmenite by carbon monoxide. The retarding.effect of impurities was explained by so-called 'barrier effect"; unreducible impurities are enriched at the reduction front, which reduce the activity of iron and block the diffusion of CO. 2.4.2. Reduction of ironsand ore 2.4.2.1. The use of ironsand in steelmaking It has been known that the presence of titanium in iron ore has a detrimental effect on the blast furnace ironmaking increasing slag's melting point and viscocity. Currently, BHP New Zealand Steel uses ironsand ore, particular Waikato North Head irondand as a raw material for ironmaking at Glenbrook, New Zealand (Marshall, 1970; McAdam, 1974). The Glenbrook process is unique in the way of the pretreatment of the raw ore. lronsand ore is separated from other sand and_ gangue minerals by magnetic and gravity methods. Iron is reduced from the ironsand ore using coal. Up to 1.2 million tonnes of ore is used per year, along with 800,000 tonnes of coal and 1 l00GWh of electricity. Limestone is used as a flux to capture sulfur, which source is coal. 30 Coal Multi-hearth Rotary klin Furnace D Steelmaking A <- -] Electric melting Process ... Furnace Figure 2-4-1. Schematic Oowsheet of ironsand process The fl.owsheet of the process is schematically shown in Figure 2-4-1. Ironsand together with coal is dried and preheated in a multi-hearth furnace to 600°C. This converts the coal to charcoal. The mixture is then fed into a kiln, where iron oxides are reduced by char. Actually, the iron oxides in ironsand are reduced by CO releasing carbon dioxide which is converted to CO by the Boudouard reaction. The kiln products are then fed into an electric melting furnace where the charcoal reduces the remaining iron oxide to pig iron with a carbon level of about 4wt%. The pig iron is fed into an oxygen steelmaking furnace where the carbon content is lowered and iron is refined to steel. Carbothermic reduction process has slow reduction rate, high energy comsumption, and relatively narrow operating temperature (Dancy, 1993; Pelton and Bale, 1999). 31 2.4.2.2. Reduction oftitanomagnetite ironsand Reduction of ironsand was examined using hydrogen, carbon monoxide and solid carbon. It bas been established that reduction of the ironsand is slower than that of commercial iron ores, including magnetite ores (McAdam et al., 1969a, 1969b, 1977; McAdam, 1974; Sadykhov et al., 1992; Morozov et al., 1998). Effects of titanium and impurities, reduction temperatures, and pretreatment were experimentally studied, however, the reduction kinetics are not fully understood. McAdam et al. (1969a) investigated the gaseous reduction of New Zealand ironsand pellets containing 7 .6W't°/o TiO2 by hydrogen and carbon monoxide, using thermogravimetric analysis. He found that hydrogen reduction was much faster than carbon monoxide reduction, in the temperature range of 900 to 1100°C. In bis experiments, preoxidation of ironsand at temperature up to 1050°C inhanced the reduction rate, however increasing preoxidation temperature to 1100°C bad a retarding effect. In the non-isothermal carbothermic reduction using reactive coal as a reducing agent, McAdam et al. (1969b) found that the reduction ofironsand started at about 850°C. The reduction rate was close to the rate of reduction by carbon monoxide at the same temperatures. The reduction rate of the carbothermic reduction increased by an order of magnitude when a pellet was made from intimately mixed ironsand and coal. Preoxidation of ironsand accelerated reduction. McAdam (1974) also investigated the reduction of New Zealand ironsand ores by CO-CO2 gas mixtures in the temperature range of 1000 to 1200°C. He found using microprobe analysis that New Zealand ironsand occurred naturally as cation deficient solid solution of magnetite and ulvospinel. The reduction experiments showed that the stability of NZ ironsand towards reducing gases depends on the concentration ofhigh valency impurities (Ti4+, AJ3+, Si4+, etc.). By morphology analysis using SEM and optical microscope, he found that wustite grows with a parabolic rate from the magnetite-ulvospinel solid solution. He suggested that the stability of the magnetite ulvospinel solid solution was due to the existence of high-valency impurities. The stability diagram for the Fe-C-O system for New Zealand ironsand supposed by McAdam (1974) is presented in Figure 2-4-2. 32 a Gill Figure 2-4-2. Reduction time as a function of CO/C02 ratios and phases in Fe-C-0 system (McAdam, 1974) According to McAdam (1974), the reduction from titanomagnetite to iron effectively took place in consecutive stages. He calculated the time for the complete reduction by the sum of the time for the reduction of magnetite-ulvospinel solid solution to wustite and that ofwustite to metallic iron. Figure 2-4-3 shows the reduction steps as a function of normalised time. 33 1·0 1·0 magnetite 0·984 0·7 •• 0·95 CJ Gill Figure 2-4-3. Reduction steps as a function of normalised time (McAdam, 1974) In the calculation, which was based on Wen's model (1968), the rate of the reduction of magnetite-ulvospinel solid solution to wustite was appeared to be governed by diffusion (slope= 2), while the wustite to metallic iron reduction had chemical reaction control (slope= 1). McAdam et al. ( 1977) suggested that the reduction of pellets of a mixture of New Zealand ironsand concentrate and coal or char in oxidising C02-N2 atmospheres, occurred in following steps: Solid-solid reduction in the beginning of the reduction; (Fe3O4 )z(Fe2TiO4 ) 1_x + 2(1-x)C = (2 + x)Fe + (1-x)TiO2 + 2(1 + x)CO (2-30) Gas reduction; Regeneration of CO; 2(1 + x)C + 2(1 + x)CO2 = 4(1 + x)CO (2-32) 34 Also, in the particular case of Waikato North Head concentrate, x = 0.77, they calculated the standard Gibbs free energy of titanomagnetite to be, (2-33) The standard state of the equation (2-33) is for 1 mole oftitanomagnetite in ironsand, which is (Fe3O4)0.11(Fe2 TiO4)0.23, under 1 atmosphere. Sadykhov et al. (1992) studied the phase transformations in the reduction of titanomagnetite ironsand (K.uranakhsk Deposit: 7.45wt% TiO2, 54.17wt% Fe2O3, 31.76wt°/o FeO and 3.00wt°/o AhOJ), by hydrogen at the temperatures in the range of 700-1250°C, using XRD. However, although composition ofKuranakhsk ore was close to New Zealand ironsand, their results were not consistent with McAdam (1974) and McAdam et al.(1911). In their experiments, the reduction oftitanomagnetite took place in three stages. The first stage was the reduction of hematite to magnetite, wustite, and finally iron. The second stage included reduction of ulvospinel to ilmenite: (2-34) Over the temperature range 700-1000°C, the final reduction products were iron and ilmenite, and two minor phases of spinet minerals, MgAl2O4 and FeA12O4. The third stage depended on the reduction temperatures. In the temperature range of 700 to 800°C, the final products were iron and rutile. In the temperature range 900 to l 000°C, reduced rutile and other minor cation minerals in the form ofMO2-x or MnO2n-1 (M represents a metal cation) were observed. At 1100°C, the reduction of ilmenite took place in the sequence: FeTiO3 ~ FeThOs ~ FeJ-x TixOs ( 2 :S x :S 3 ), (2-35) which lead to the formation of a solid solution with a pseude-brookite structure M3O5• Morozov et al. (1998) investigated reduction oftitanomagnetite concentrate (Iturup Island deposit, in the Kuril'sk chain: 9.84wt°/o TiO2, 57.43wt°/o Fe2O3, 22.15wt°/o FeO and 3.25wt°/o MgO) thermogravimetrically by hydrogen in the temperature range of 800 to 1100°C. They examined the effect of preoxidation of the concentrate by oxidising in air at 1000°C for 2 hours. In the reduction of the titanomagnetite concentrates, the degree of metallisation reached 94% at 1000°C, however, further increase of the reduction temperature did not accelerate reduction because of primary slag formation. 35 As mentioned above, accompanying elements (Al, Mg, V, Si etc.) in a natural titanomagnetite ore affect the phase transfonnation in the reduction process. However, data reported in literature are inconsistent. According to McAdam ( 1974 ), such 3 4 impurities as Mg2+, Mn2+, ca2+, cr3+, Al +, y 3+ and Si + only influence the reduction 4 step from magnetite solid solution to wustite. He also reported that Si + appeared to 4 have a similar effect to Ti +. However, Sadykhov et al. (1992) argued that impurities in a titanomagnetite play more significant role in the whole reduction process. They showed that increase in the content of accompanying elements on the reacting interface of a titanomagnetite leaded to various reactions, taking place in reduction of the titanomagnetite, and the final products had a complex phase composition. The following reactions were supposed by Sadykhov et al. (1992), which took place during the reduction of titanomagnetite ore, by hydrogen at temperatures between 900 and 1000°C. V2 O3 + (n - 2)TiO2 = V2 Tin_ 2 O2n-t (2-36) Cr2 O3 + (n -2)TiO2 = Cr2Tin_ 2 O2n-t (2-37) (2-38) At 1100°C, trivalent cations form M3O5 crystal lattice, substitute iron in reduced ilmenite and form ulvospinel. V2 O3 + 2FeTiO3 = V2TiO5 + Fe 2TiO4 (2-40) Cr2 O3 + 2FeTiO3 = Cr2TiO5 + Fe2TiO4 (2-41) A/2 O3 + 2FeTiO3 = Al2 TiO5 + Fe2TiO4 (2-42) The newly formed ulvospinel is again reduced to ilmenite, and the V, Cr and Al titanates are dissolved in FeThO5, with the formation of a solid solution with a pseudo-brookite structure. 36 2.4.3. Summary Ilmenite ores can be reduced to metallic iron and rutile or titanium suboxides by carbon monoxjde or hydrogen at temperatures above l 000°C. The reduction oftitanomagnetite ore is much slower than that of magnetite iron ore. The complete reduction of iron oxides in titanomagnetite ore is possible at higher temperature above 1000°C, under high reducing potential. Preoxidation enhances the reducibility oftitanomagnetite ore. However, the kinetics and the mechanism of the reduction are not fully understood and the previous studies have many contradictions. While McAdam (1974) suggested that the reduction of titanomagnetite proceeded without the formation of ulvospinel phase, Sadykhov et al. (1992) claimed that ulvospinel be formed during the reduction. The role of cation impurities in titanomagnetite and the reduction kinetics are not clearly understood yet. Also, the most of previous studies, except McAdam's work (1974), lack in the consideration of the nature oftitanomagnetite, which have been studied intensively by geologists. 37 2.5. Objectives of the Project According to the literature survey, the following conclusions were drawn: 1) lronsand ores can be used as an alternative source for current ironmaking process. 2) Ironsand ore is mostly composed of homogeneous titanomagnetite particles. Titanomagnetite is a solid solution of magnetite and ulvospinel. Some particles show a phase separation which is called exsolution. The exsolution is caused by partial oxidation of spinel cubic titanomagnetite phase to rhombohedral titanohematite. 2 3) The distribution of Fe ions (Fe3+ and Fe 1 in octahedral and tetrahedral sites in the titanomagnetite lattice is the function of temperature and the content of titanium 4) The chemistry oftitanomagnetite is very different from that of magnetite even though both have the same crystal structure, spinel cubic. At high temperature above 900°C, the activity of magnetite in titanomagnetite shows a negative deviation from Raoult's law. 5) In the processing of ironsand by BHP New Zealand Steel, ironsand is reduced by carbothermic reaction. However, the carbothermic reduction has slow reduction rate, high-energy comsumption, and relatively narrow operating temperature. 6) The reduction of ironsand is slower than that of magnetite iron ore. It needs high reducing atmosphere. The stability of ironsand is due to cation imp~ties. But, there are contradictions in the role and the behaviour of the impurities, as well as in the reduction path, during the reduction process. 7) Preoxidation enhances the reduction rate significantly, but no publications are found in literature, which report the mechanism of the high-temperature oxidation of titanomagnetite. The ultimate aim of this project is to develop further understanding of the reduction mechanism of ironsand by gas-solid reaction.. The questions to be answered in this project are: • Ironsand's structure and phase composition 38 • Structure transformation during the reduction of ironsand • Phase transformation during the reduction of ironsand • The reduction sequence • Behaviour of cation impurities during the reduction • Reduction extent and kinetics at different temperatures and gas compositions • Retarding effect of titanium on the iron oxide reduction • Effect of calcination and preoxidation of ironsand on its structure and reduction 39 Chapter 3. Experimental The rate of reduction can be investigated by several methods, such as measuring the gas compositions of reduced gases or the weight loss of a sample (Thermo Gravimetric Analysis, TGA), weighing the water produced, and phase analysis using XRD or Mossbauer spectroscopy. The most popular method for iron ore reduction is TOA. However, in the reduction process using carbon monoxide, deposition of free carbon as a result of Boudouard reaction and carburisation make it difficult to distinguish the mass changes due to the oxygen removal and due to carbon deposition and carburisation. Reduction experiments in this project were based on the on-line off gas analysis. Reduction experiments were conducted in a fixed bed reactor. The design of the reactor guaranteed the uniform gas-solid reaction, under well-controlled conditions. This chapter presents the characterisation of raw materials, description of the experimental set-up and experimental procedure, methods employed for a sample characterisation and calculation of extent of reduction. 3.1. Materials This project examined reduction behaviour of New Zealand jronsand (New Zealand titanomagnetite concentrate) and conventional iron ore. The gases used in reduction experiments were carbon monoxide, carbon dioxide, hydrogen, methane and argon with addition in some experiments of water vapour. 3.1.1. New Zealand ironsand New Zealand ironsand (ironsand) was obtained from BHP New Zealand Steel. The size range of the supplied ironsand was quite narrow. Samples were screened to a size range of0.120-0.225 mm. The composition ofironsand was examined by BHP laboratory, Newcastle; the impurities in the ironsand were also analysed by XRF at UNSW. The chemical composition of the ironsand is given in Table 3-1. 40 Table 3-1. Chemical composition of New Zealand ironsand, wt% Measured by Measured at Component BHP, Newcastle lab. UNSW Fe(0) <0.01 Fe (2+) 24.0 Fe (3+) 33.1 Fe (totalt 57.1 56.4 TiO2* 7.36 7.65 Na2O 0.10 0.13 AhOJ 3.55 4.06 CaO 0.66 0.60 K2O 0.04 0.04 MgO 2.92 2.93 Mn 0.51 0.60 P2Os 0.13 0.14 SiO2 2.17 2.17 C 1.60 s 0.048 ( The total Fe content was the sum ofFe(0), Fe(2+) and Fe(3+); • All titanium was presented as TiO2.) 3.1.2. Iron ore Hematite iron ore was obtained from Mt. Whaleback, Western Australia. The iron ore was first crushed in a laboratory crusher and then screened to obtain the same size range as ironsand had 0.120-0.212 mm. The chemistry of the ore was obtained by XRF analysis conducted by BHP laboratory, Newcastle, and also at UNSW. The ore composition is given in Table 3-2. 41 Table 3-2. Chemical composition of the iron ore, wt% Measured by Measured at Component BHP, Newcastle lab. UNSW Fe, total 62.7 62.7 Fe2O3 89.56 SiOi 4.81 5.35 A}iO3 2.61 3.61 TiO2* 0.08 0.09 p 0.087 0.087 Mn 0.04 CaO 0.01 0.016 MgO 0.03 Co3O4 0.07 K2O 0.01 0.03 s 0.026 0.028 LOI 1.1 (* All titanium was presented as TiO2.) 3.1.3. Gases The reducing gases were the mixtures of carbon monoxide, carbon dioxide, hydrogen, methane and argon. The use of argon provided a reference for calculation of off-gas flow rate and composition. Water vapour was used in some experiments to examine its effect on the reduction rate. The gases were provided by BOC Gases, Australia, in gas cylinder. The specifications of the gases are listed in Table 3-3. Table 3-3. Gases used in the experiments Gas Gas Code Grade Specification(%) Carbon monoxide (CO) 156 Chemical pure 99.99 42 Carbon dioxide (CO2) 081 Industrial >99.9 Hydrogen (H2) 240 Super high purity 99.999 Methane (Cl4) 151 High purity 99.5* Argon(Ar) 062 High purity 99.997 (* From Mass Spectrometer Measurement) 3.2. Experimental setup The experimental setup for the reduction experiments in this research was based on work by Zhang (2000). The experimental setup included three parts, experimental furnace and reactor with temperature controlling system, gas system and gas analysis system. 3.2.1. Experimental furnace and reactor The furnace and reactor design provided temperature control, unifonn gas flow through the solid bed, and stable operation. Thermocouple-----, +--- Inlet Gas Thermocoupl sheath i---t---lnnerTube Outer sheath --- Figure 3-2-1. Schematic experimental reactor 43 Schematic of the reactor is depicted in Figure 3-2-1. A recrystallised alumina tube of 8.6 mm inner diameter was used as a sample holder. A porous magnesia plug was fixed at the tube bottom by an alumina pin. The temperature inside the fixed bed was measured by a B-type thermocouple and is referred to as the reduction temperature. The thermocouple was held in an alumina sheath to isolate it from contact with reducing gas, because the thermocouple wire could catalyse the carbon deposition and the reducing gas makes the thermocouple brittle. The inner tube was inserted into an outside sheath of 190mm inner diameter. The inner tube, the outside sheath and the thermocouple sheath were fixed by special metal fittings and sealed with O-rings. The gas from the top of the inner tube passed through the fixed bed and porous plug, and left the reactor through the gap between the outside sheath and the inner tube. A vertical electric high temperature furnace (Model HT 08/17, Ceramic Engineering, Sydney, Australia) was used for heating a sample to the reduction temperatures. The furnace temperature was controlled by Eurotherm 2100 temperature controller. The furnace had the maximum heating temperature of l 700°C, with the operating heating rate of up to 400°C/hour and the 9 stages of temperature-programmed heating. The furnace temperature was 23-27 degree higher than the reduction temperature in the reactor. 3.2.2. Gas system The gas system is schematically shown in Figure 3-2-2. Carbon monoxide, carbon dioxide, hydrogen, methane and argon were supplied in gas cylinders. The gases passed through the Hydropurge gas purifiers filled with 4A molecular sieve to remove moisture. The composition ofreducing gas was achieved by precisely controlling the flowrate of each gas with Brooks mass flow controllers (Model 5850E). To introduce water vapour into the reducing gas, argon in a separate line was passing over two water containers connected in series in a water bath at constant temperature of 40°C. Then the water saturated argon was mixed with the reducing gas mixture. Water vapour partial pressure in the reducing gas was controlled by the argon flow rate. The maximum capacity of 44 water addition in a gas mixture with flowrate of 800ml/min was 2.5vol%. There was another argon stream controlled by a needle valve and a rotameter, which was for purging the system before and after a reduction experiment. Either reducing gas or purging gas was directed to the reactor by switching the position of the 6-way valve. In one position, the purging gas was directed to the reactor while the reducing gas was directed to the mass spectrometer for initial gas composition analysis. In the other position, the reducing gas was introduced to the reactor. The effluent gas composition was continuously monitored by mass spectrometer and dew-point sensor. The pressure of inlet gas was measured by the pressure gauge. 8 5 to MS t t t t t 4 1 CO C02 CH4 H2 Ar Figure 3-2-2. Schematic diagram of gas flow system (I-reactor, 2-pressure gauge, 3-six way valve, 4-rotameter, 5-water bath, 6-gas flow controller, 7-gas purifier, 8-dew point monitor) The mass flow controllers supplied by manufacturer were calibrated for argon. When they were used for other gases, the relationship between the setting-point and flow rate was different from that of argon. Also, argon flowrate might have different setting- 45 value under an experimental condition. Therefore, mass flow controller had to be calibrated to use under given experimental condition. A mass flow controller used for argon was first calibrated using a bubble flowmeter, and then the mass flow controllers for other gases were calibrated with a mass spectrometer, comparing with argon flowrate. The calibration was carried out at a room temperature. The accuracy of the calibration of the mass flow controller was ±1.0% of a setting value. Figure 3-2-3 shows calibration curves of the controllers for carbon monoxide, carbon dioxide, hydrogen, methane and argon, respectively. 1000 --9-Ar 800 -a-CO C -t:r-H2 -*"""CH4 ! 600 E -¼-C02 i 400 ii:I 200 0 0 20 40 60 80 100 MFC reading, % Figure 3-2-3. Calibration curves of the mass Dow controllers Accurate control by mass flow controller requires change in the pressures of upstream and downstream gas in a small range. The change in flow rate with the pressure changes was found to be negligible. 46 3.2.3. Monitoring of gas composition 3.2.3.1. Mass spectrometer Gas analysis was primarily perfonned using a Fisons - Prima 600 scanning magnetic sector mass spectrometer (VG Gas Analysis System, UK). The precision of the mass spectrometer was ±0.1 %. Mass-spectrometer (MS} has a typical working range of 100% to 10 ppm with the Faraday detector and l000ppm to lOppb with the secondary Electron Multiplier. When a gas sample was directed to the mass spectrometer, high energy electrons ionised the gas molecules. After acceleration the ions were separated by a magnetic sector according to their mass to charge ratio (m/e). The signal intensity concentration relationships of different gases were calibrated regularly to guarantee accurate measuring of the gas composition. Practically it was found that the signal intensity changed with time. Nonnalisation of the composition to 100 % partly overcame the problem. Although the absolute error in the gas composition measurement was small, it was accumulated in calculations of the total extent of reduction. This was a main source of error in calculation of reduction. The actual error of gas analysis measured by calibration was ±1.0%. Its contribution to the final results was ±3.0% of reduction extent of a sample. The water content measurement by the mass spectrometer was not reliable. This is because water has a strong adsorption on the tube wall and there is a delay in water detection by mass spectrometry. Water content in the off-gas can be calculated from hydrogen balance, however, in practice it was not accurate. Detennination of water content by hydrogen balance calculation can only be applied at the initial stage of reduction experiments, when hydrogen consumption was relatively high. 3.2.3.2. Dew Point Monitor A dew point sensor (General Eastern Hygro M4/D-2, also called Chilled-Mirror Hygrometer) was used to measure water content in effiuent gas. The nonnal detecting range ofD-2 sensor is -35 to +25°C at nonnal ambient temperature of25°C with an 47 accuracy of 0.2°C. By mounting the D-2 sensor on a hot plate which can be heated to a maximum temperature of 85°C, the upper detecting limit is extended to 85°C. An electrical bridge circuit is used to control the temperature of a chilled mirror sensor to keep at the dew/frost temperature of the gas stream. The temperature is precisely measured and is converted to water content according to following equation (1 atm pressure, from the General Eastern Hygro Operation Manual). 17.502T ) 0 Vn (vo/%)=6.1121ex ---- for T{ C}~0 (3-1) 20 { 240.97+T Vn (vo/%)=6.1115ex ----22.452T ) for T{ 0 C}S0 (3-2) 1 0 { 272.SS+T The gas passages to the sensor were heated and insulated. The off-gas line from the reactor to the sensor was made of stainless steel with adequate insulation. The off-gas line was heated to about 60°C with a heating tape. The error of the final reduction extent from the dew point monitor was ±3.0%. 3.3. Analytical instruments 3.3.1. X-ray Diffraction analysis Phase composition of samples was analysed by powder X-ray diffraction (XRD, Simens DS0OO). The X-ray Diffractometer has a monochromator and a copper Ka X-ray source. The voltage and current in the X-ray emission tube were set at 30 KV and 30 mA, respectively. The scanning was performed from 20 to 75 °. The scanning rate of samples was set at 0.6 °/min with a step of 0.03 °. 3.3.2. Electron Probe Microanalysis Elemental composition and distribution were analysed by CAMECA SX-50 Electron Probe Microanalysis (EPMA). SXS0 has four WDS detectors interfaced to the column and, in addition, EDS detector. 48 Wavelength dispersive (WDS) detectors employ crystals diffracting characteristic x-rays emitted from the specimen according to their wavelength. These detectors allow chemical analysis of high precision. WDS is controlled by a SUN workstation that allows sophisticated analysis such as linescans and elemental area maps. Specimens can be imaged using either secondary or backscattered electrons. Characteristic X-rays generated by the elements in a sample are diffracted by suitable crystals (TAP: thallium acid phthalate; LiF: lithium fluoride; PET: pentaerythrotol; PCO, PC 1, PC2 and PC3: multiplayer crystals). The X-rays are measured in counts per second by the gas flow proportional detectors. Instrumental calibration is carried out using certified standard reference materials and employing the data reduction matrix correction procedure or a suitable comparison standard technique. The selected standard references had a similar matrix to the specimen being analysed and had been prepared under similar conditions (e.g. polishing; coating, if required, with carbon, aluminium, or copper). Primary reference standards were supplied from NIST, BCS, Brammer, Austimex, Aldrich, and Johnson Matthey. The crystals and the standard references for the analysed elements in this study are shown in Table 3-4, respectively. Table 3-4. The crystals and standards for EPMA Element Crystal Standard Fe(K.a) LiF Magnetite (Fe3O4) Ti (Ka) PET Rutile (TiO2) 0 (Ka) PC0 Magnetite (Fe3O4) Al (Ka) TAP Sanidine ((K.,Na)AlSi3Os) Mg(K.a) TAP Diopside (CaMgShO6) Si (Ka) TAP Quartz (SiO2) For quantitative analysis, the EPMA was calibrated using the standards listed in Table 3-4. The calibration process was repeated until the standard deviation of each elemental quantity was less than 3%. Imaging was available by both stage and beam image acquisition scans. 49 3.3.3. SEM analysis The morphology of samples was observed by HITACHI S-4500 Field Emission Scanning Electron Microscope (FESEM). The machine has a detection limit up to 1.5nm with tilting stage and Robinson Back-Scatter Detector and is linked with X-ray Energy Dispersive Spectrometer (EDS). Morphology of original and reduced particles and morphology of sectioned grain were examined. Samples were prepared in two ways. To observe particle morphology and size, sample powder was directly stuck on alumina stubs using doulble-sided carbon adhesive tape. To observe the sectioned profile of a particle, samples were mounted in epoxy resin, ground, polished, dried and coated with graphite. 3.3.4. Optical microscope analysis Optical microscope analysis was conducted using an optical inverted stage metallurgical microscope (Olympus PMG3, Japan). 3.3.5. LECO analysis Oxygen content in reduced samples was measured using LECO machine, TC- 436DR oxygen and nitrogen analyzer. The powder sample of0.05-0.1 gram was loaded into a nickel basket and weighed precisely. The basket was loaded into a graphite crucible which was electrically heated. The oxygen in a sample combined with carbon in a crucible to form carbon monoxide or carbon dioxide which were detected by thermal conduction detectors. The error range of the LECO 436DR was ±3.0%. 3.3.6. BET analysis The specific surface area was measured using a single point BET technique, at the boiling temperature of liquid nitrogen. The machine for the BET analysis was ASAP 2000 V2.02 manufactured by Micrometrics Instrument Corporation. The principle of the so specific surface area measurement is based on the physical adsorption of gaseous adsorbate by porous material. The BET equation was used for the calculation of specific surface area based on the adsorption isotherm. P = _1_ +-(C_-_l)_P (3-3) V(P0 -P) V.. C V.. CP 0 where P and P' are the equilibrium pressure in gas phase and saturation pressure of adsorbate, respectively. V and V,,. are the adsorption volume of adsorbate at pressure P and that adsorbed for a complete single layer of adsorbate on sample surface. C is a model parameter related to the adsorption energy. Based on the equation (3-3), the specific surface area S was calculated according to the following equation (3-4): s. =(v("} (3-4) where V0 = 22,400 mVmol, is the volume of 1 mol gas at 1 atm and 0 °C; 23 N0 = 6.02 x 10 , is the Avgardro number; 2 a= 15.8 A , is the area occupied by a single molecule of nitrogen. 3.4. Experimental procedures The reduction experiments were conducted in two modes, isothermal reduction and temperature-programmed reduction. 3.4.1. Isothermal reduction The reactor was loaded and assembled at room temperature. The porous plug was fixed at the end of the inner tube with an alumina pin. The inner tube was inserted into the outside sheath so that the end of the plug was at about 5mm from the bottom of the sheath. A sample of2.000±0.005grams was loaded into the inner tube, then the thermocouple was installed and the inlet and the outlet tubes were connected. The purging argon gas of about lUmin was introduced to the reactor from the top, and the 51 reactor was lowered so that the sample was placed in the high temperature isothermal zone of the furnace. The hot zone of the furnace was located at ±2.0cm of the sample position. The reducing gas was made-up by adjusting the mass flow controller settings. The gas mixture was directed to the mass spectrometer and its composition was measured. The experimental temperature for reduction of a sample was usually achieved in 15- 20minutes. When the reduction temperature became stable, the reducing gas was directed onto the sample by switching the 6-way valve. The effluent gas from the reactor was directed to the mass spectrometer and its composition was monitored on-line. After a sample was reduced for certain time, purging argon gas was introduced to the reactor by switching the 6-way valve to prevent the sample from reduction/oxidation during cooling. Then the sample holder was lifted from the hot zone of the furnace and the sample was quenched. When the sample was cooled to the temperature below 200°C, the reactor was disassembled and the sample was removed from the sample holder for characterisation. 3.4.l. Temperature-programmed reduction For the temperature-programmed reduction, the basic procedure was similar to that of isothermal reduction, except that the reduction experiment started at low temperature. Thus in non-isothermal experiments the initial furnace temperature was set at 200°C. The reactor was assembled as described in isothermal reduction (section 3.3.1) and inserted into the furnace for heating under argon purging. When the sample temperature reached 200°C under argon atmosphere, reducing gas was introduced, and the furnace temperature was set to increase at a fixed rate of l 00°C/hour. When the temperature reached certain value, the introduced gas to the reactor was changed from the reducing gas to purging argon. The sample was then quenched by lifting the reactor from the furnace. The sample was cooled to below 200°C and removed from the sample holder for characterisation. 52 3.4.3. Sample analyses Raw, pretreated ironsand and reduced ironsand samples were subjected to phase and elemental analysis and morphology characterisation. Powder X-ray diffraction (XRD) analysis and Electron Probe Microanalysis (EPMA) were used to determine the phase composition. The elemental distribution was analysed by EPMA. The morphology of samples was observed using Scanning Electron Microscopy (SEM) and Optical microscopy. BET method was employed for measurement of specific surface area. In some experiments, the content of oxygen in a sample was measured by LECO machine. For XRD analysis, a sample was ground to fine powder and closed packed in the cave of 10mm diameter on a plastic sample holder. For EPMA and SEM observation, particles of a sample were mounted in vacuum impregnated epoxy resin and hardened for 24 hours. The mounted sample was carefully ground and polished. After drying for 12 hours at 60°C in a muffle furnace, the mounted sample was coated with a graphite film by graphite sputtering at high vacuum to guarantee good conductivity during analysis. The accelerating voltage and the current for EPMA was 15keV and 20nA, respectively. 3.S. Data analysis The rate of the reduction of ironsand ore was calculated from the rate of oxygen evolution as carbon dioxide or water on the basis of the mass balance of inlet and outlet gases. In the reduction by carbon monoxide, the mass spectrometer data was used,. and in the reduction by hydrogen, the dew point monitor was used. It was assumed in the calculation of the gas composition from the MS or dew point data that there was no flowrate change before and after reaction. The extent of the complete reduction of ironsand was referred to oxygen combined with iron in the ore. The amount of removable oxygen in 2.00g of raw ironsand sample was approximately 0.42g, based on the chemical analysis data in Table 3-1. 53 3.5.1. Calculation of the extent of reduction The rate of oxygen evolution in the forms of CO2 in the reduction by CO, H2O in the reduction by H2 and CO, CO2 and H2O in the reduction by CH.i-H2 gas mixtures was measured on the basis of mass balance of inlet and effluent gas using mass spectrometer and dew point monitor. The rate of ironsand reduction, R, was calculated as: R(glmin)= ~[(//''' - f/n)xV; x W, ] (3-5) £..J 1::1 24.45128 Where J; is a volume percentage of CO, CO2 or H2O. v.. ~ is argon flowrate in L. The superscripts out and in represent outlet gas and inlet gas, respectively. w; is mass of oxygen in lmol of gas i, for example, 16g for CO. The value, 24.45128, is the molar volume (in L) at the temperature of the analysed gas system, which was 25°C. The extent of reduction, X, was determined by integrating of the oxygen removal rate calculated by equation (3-6), as follows: JR(g I min)dt X(%) =----~------x 100 (3-6) (Total oxygen amount in iron oxides(g)) In the reduction by H2-CH.i-Ar gas mixtures, the mass spectrometer and dew point monitor data were used together. The data obtained from each measurement were first converted to reduction rate, R, and reduction extent, X, using equations (3-5) and (3-6), and then added together in the same time scale. The final reduction rate and extent were determined by the sum of CO, CO2 and H2O evolution. 54 Chapter 4. Characterisation of lronsand Ore Titanomagnetite ironsand ores are unique in both physical and chemical aspects. Most of the particles of the ore are featureless homogeneous magnetite-ulvospinel solid solution (titanomagnetite) with low surface area (Wright, 1964; McAdam, 1974). Titanomagnetite ironsand is characterised by phase separation or exsolution under various conditions, due to the miscibility gap between magnetite and ulvospinel below 600°C (Lindsley, 1981; Price, 1981; Anderson and Lindsley, 1988) and the limited solubility between spinel phase (magnetite-ulvospinel) and rhombohedral phase (hematite-ilmenite) (Buddington and Lindsley, 1964). The cation distribution in spinel depends on the temperature (Trestman-Matts et al., 1983; Woodland and Wood, 1994). Pretreatment such as calcination and preoxidation affects the elemental and phase compositions of titanomagnetite ores, which influences the further reduction process in many ways. This chapter describes the characterisation of raw and pre-treated titanomagnetite ironsand ores. 4.1. Characterisation of raw ironsand The raw ironsand ore was examined using EPMA, XRD, SEM, optical microscope and BET analysis. 4.1.1. Phase composition of the raw ironsand ore The XRD patterns of the raw ironsand ore, hematite iron ore {Table 3-2) and magnetite obtained by partial reduction of the hematite iron ore are shown in Figure 4-1-1. The XRD pattern of the raw ironsand ore was composed of peaks for magnetite, hematite and traces of maghemite (y-Fe203). The peaks from Ti-rich phases such as ulvospinel (Fe2 Ti04), ilmenite {FeTi03) and rutile (Ti02) were not detected. The strongest peak of magnetite phase of the raw ironsand ore located at 20= 35.5886° was shifted from that of the magnetite iron ore by - 0.041 °. 55 • • Fez~ + Fe30,c oy-Fez03 • Hematite iron ore • • • • • • Magnetite iron ore • • • • • The raw ironsand ore .. • • .. 25 30 35 40 45 50 55 60 65 2 theta Figure 4-1-1 XRD patterns of the raw ironsand ore and magnetite The elemental analyses by EPMA showed that the main phase of the raw ironsand ore was homogeneous magnetite-ulvospinel solid solution (titanomagnetite) phase. Small proportion of the ironsand exhibited exsolution caused by partial oxidation (weathering) oftitanomagnetite to titanohematite, which has the identical XRD pattern to hematite. Homogeneous titanomagnetite. Particles with homogeneous magnetite-ulvospinel solid solution constituted the bulk of the raw ironsand ore. Quantitative elemental analysis across of such particle with a step of I micron is shown in Figure 4-1-2. The analysed elements were iron, titanium, aluminium, magnesium, silicon and oxygen. Iron and 56 titanium in the particle were distributed uniformly along the line of analysis with the average compositions of 32.83 at% Fe and 2.98 at% Ti. However, the concentrations of aluminium and magnesium fluctuated in the range of0.88-8.13 at°/4 for Al, and 0.64-4.26 at% for Mg. Aluminium and magnesium showed no tendency of segregation except some points. The fluctuations of the Al and Mg concentrations through the particle were independent of each other, and of the Fe and Ti concentrations. The amount of silicon in the ironsand ore particle was at most 0.26 at%(= 0.16 wt°/4) along the analysed line, although the concentration ofSiO2 in the ironsand measured by chemical analysis was 2.17 w!°/o. Quantitative point analysis of random particles showed that silicon was present predominantly in silicate inclusions. The mapping images of iron, titanium, aluminium and oxygen in a representative homogeneous particle of the raw ironsand ore are presented in Figure 4-1-3. Ttitanium in the particle was dissolved uniformly in Fe-O phase. From the EPMA and XRD data, the main phase of the raw ironsand ore was found to be homogeneous titanomagnetite solid solution, (Fe3O4)1_x(Fe2TiO4)x, with the value of x is equal to 0.27±0.02. The value of x is the average obtained by EPMA analysis for large number of homogeneous particles. Aluminium and magnesium oxides were dispersed in the titanomagnetite matrix. The average Fe/fi atomic ratio of the homogeneous particles was 10.0 with the standard deviation less than 2.6%. The ratio of the oxygen (after subtracting the oxygen in aluminium, magnesium and silicon oxides) to iron and titanium ([O]/[Fe + Ti]) was calculated to be 1.48, which is higher than the expected value of 1.33 for the stoichiometric titanomagnetite. It is thought to be due to both the existence of residual minor oxides and the low sensitivity of the EPMA for the light elements. 51 10 -<>- 0 ---Fe _._ li 0~ 1 -CU -¥-Mg --ltE-AI -o- Si 0.1 0 10 20 30 40 50 60 70 80 distance, micron Figure 4-1-2. Elemental distribution in a raw ironsand particle 58 Figure 4-1-3. Mapping of Ti, Fe, Al and O in the homogeneous particle of the raw ironsand ore The titanium in the homogeneous titanomagnetite phase increases the cell size of the cubic spine! and shifts the magnetite peaks in the XRD pattern (Wright, 1964; Wechsler et al., 1984). The cell sizes calculated from the strongest XRD peaks of the raw ironsand and the reduced magnetite ore were 8.3612A and 8.3513A, respectively. In pure magnetite-ul vospinel solid solution, the cell size of (Fe3O4) 0.73(Fe2 TiO4) 0.27 is by 0.025 A higher than in pure magnetite (Banerjee et al., 1967; Wechsler et al., 1984). The smaller increase in the titanomagnetite cell size in the raw ironsand ore, 0.0lA, can be attributed to magnesium and aluminium dispersed in the titanomagnetite phase, which reduce the cell size (Wright, 1964). 59 Non-homogeneous ironsand. While the most of the raw ironsand ore is the homogeneous titanomagnetite, some particles showed a phase separation with a titanium-rich lamellar phase with an angle close to 60°, as shown in Figure 4-1-4. The observed phase separation could be due to the miscibility gap between magnetite and ulvospinel or the exsolution between titanomagnetite and titanohematite. Figure 4-1-4. Elemental analysis of a non-homogeneous ironsand particle Composition, at% (0* is the oxygen in iron and titanium oxides.) 0 Mg Al Si Ti Fe [Fe]/[Ti] [O*]/[Fe+Ti] I 6 1.6 0.41 1.05 0.24 4.91 31.8 6.48 1.61 2 61.0 0.27 3.66 0.16 4.05 30.9 7.63 1. 57 3 63.3 0.45 2.75 0.16 6.54 26.8 4.10 1.75 4 58 .5 2.35 2.23 0.07 3.48 33.4 9.60 1.43 5 58.4 2.60 2.74 0.09 3.44 32.7 9.51 1.43 6 59.5 2.36 2.22 0.09 3.48 32.4 9.3 1 1.49 60 The origin of the phase separation was examined using quantitative point analyses, which results are shown in Figure 4-1-4. The grey phase being the majority phase must be the tjtanomagnetite as shown in Figure 4-1-1. The white phase (points I to 3) was enriched with titanium in comparison with the grey phase (points 4 to 6). Titanomagnetite has [O]/[Fe+Ti] ratio of about 1.33. However, EPMA analysis of the grey phase gave [O*]/[Fe+Ti] of 1.48, in other words, the EPMA technique tended to over-estimate the concentration of oxygen and therefore the [O*]/[Fe+Ti] ratio is higher by a factor of about I.I I. The ratio for the white phase of 1.57- 1.75 corresponds after correction to 1.41 - 1.58, which is close to the expected ratio of 1.5 for rhombohedral (M203) phase, which is titanohematite solid solution, while those ratios of the grey phase after the correction ( 1.29-1.34) were characteristic for the homogeneous titanomagnetite particle. It is known from literature (Buddington and Lindsley, 1964; Akimoto et al., 1984) that the phase separation was caused by partial oxidation of titanomagnetite to titanohematite at low temperatures and exsolution of these phases. In the white phase, the magnesium contents were also lower than those in the grey phase. In the system oftitanomagnetite ironsand in an oxidising atmosphere, thermodynamic calculation made by FACTSAGE™5.0 showed that alumina and magnesia has low solubility in rhombohedral titanohematite solid solution, resulting in the cation depletion in the titanohematite phase. The oxidation process affected by the kinetics, that is the migration of cations (Buddignton and Lindsley, 1964; Wright and Lovering, 1966; Akimoto et al., 1984). The ulvospinel-rich or magnetite-rich spinets were not found, although there is a miscibility gap between magnetite and ulvospinel with a consolute temperature between 423 and 623°C (Lindsley, 1981; Price, 1981; Woodland and Wood, 1994). 4.1.2. Surface area, pore volume and size of the raw ironsand ore The surface area of the raw ironsand ore was measured by both multi-point and single-point BET methods. The pore volume and the average pore size were measured by multi-point BET method. The 61 surface area of the hematite iron ore was also determined by the single-point BET method for comparison. Table 4-1 presents experimental data. Table 4-1. The surface area and the pore volume and size of the raw ironsand ore Multi-point [ASAP 2000] Single-point [ASAP 1200] Surface Area (m:l/g) 1.34 1.24 Pore Volume (cm3/g) 0.002 - Average Pore Size (A) 61.06 - The surface area measured by the multi-point method was consistent with data obtained by the single-point method. The surface area of the raw ironsand ore was lower than that of the hematite iron ore, which was 1.76m2/g. 4.2. Pretreatment of ironsand Pretreatment of ironsand included non-isothermal and isothermal preoxidation under air and calcination under argon. 4.2.1. Preoxidation of ironsand 4.2.1.1. Non-isothermal oxidation The raw ironsand ore was oxidised by heating under air to different temperature in the muffle furnace. The temperature was increased from the room temperature to l 100°C with the rate of200°C/hr. A sample was withdrawn from the furnace at a desired temperature and quenched. The phase composition of preoxidised ironsand ore was studied using XRD analysis. The XRD patterns of ironsand preoxidised at different temperatures are shown in Figure 4-2-1. 62 • 1TH +TIM • o y-Fe:z03 • Fe:zTiOs 1100°c •• • • •• • • • • 1000°c • • • ~ U) • C .SI .5 800°C CD 0 ~ 0 ' •• ea 0 a; • • •• 0:: • 600°C • 0 0 • • • • •• •• •• Room Temperature •• • • •• 25 30 35 40 45 50 55 60 65 2 theta Figure 4-l-1. XRD patterns of ironsand preoxidised by non-isothermal heating under air (TTM: titanomagnetite; TTH: titanohematite) 63 The oxidation of ironsand mainly included the transformation of spinet titanomagnetite to rhombohedral titanohematite phase, although the titanomagnetite phase in the raw ironsand showed high stability against the oxidising atmosphere. The main phase of the ironsand ore was titanomagnetite up to 800°C. However, oxidation of titanomagnetite phase to titanohematite phase was observed in a sample heated to 600°C. The oxidation process depended on temperature. I) Temperature below 80(/'C. At these temperatures, titanomagnetite was transformed to titanohematite phase in a two-step process. In the first step, titanomagnetite was oxidised to titanium-containing maghemite (y-Fe2O3) without a structural change. The XRD peaks of maghemite were found in the sample heated to 600°C. It has been reported (Feitknecht, 1964; Gallagher et al., 1968; Gillot et al., 1978) that pure magnetite is quickly oxidised to maghemite and further to hematite at temperatures above 300°C. At temperatures above 500°C, magnetite is oxidised to hematite directly without maghemite formation. However, the maghemite phase formed from titanomagnetite phase during the oxidation of the ironsand ore existed up to 800°C. The high stabilities of the titanomagnetite and the maghemite phases are due to high valency impurities in ironsand. The transformation of magnetite to maghemite involves an outward diffusion of Fe2+ with the creation of cation vacancies and the addition of oxygen atoms (Feitknecht, 1964; Gallagher et al., 1968; Sidhu et al., 1980). The high amount of cation impurities (Ti4+, AJ3+ and Mg2) in the ironsand ore reduced the Fe2+ thermodynamic activity, diffusion coefficient and the rate of transformation. The second step is the structural transformation of the titanium-containing maghemite phase to the rhombohedral titanohematite phase. The 37.46° peak in the XRD pattern at 600°C indicates a formation of a new rhombohedral phase, which is titanohematite. When the sample was heated to 800°C, the peak at 33.5° extended and several additional peaks of titanohematite appeared. 2) Temperatures above 100(/'C. When a sample was heated above 1000°C, the oxidation oftitanomagnetite proceeded quickly. At 1000°C, the intensity oftitanohematite peaks appeared at 800°C increased. Further increase of the temperature to 1100°C changed the 64 relative intensities of the peaks marginally. The maghemite peaks were not found in the XRD patterns at 1000°C and 1100°C, showing that titanomagnetite spine} phase was transformed to titanohematite rhombohedral phase without the formation of maghemite, or very fast transformation of titanium-containing maghemite to titanohematite. The XRD analysis and weight measurement of preoxidised samples showed that the transformation of titanomagnetite to titanohematite was not completed in non isothermal heating to 1100°C. The weight gained by the sample was approximately 2.0% 2 3 of the initial weight of the raw ore; what means that only 60% ofFe + is oxidised to Fe +. The XRD patterns did not detect Ti-rich phases such as ulvospinel (Fe2TiO 4), ilmenite (FeTiO3) or rutile (TiO2). The XRD patterns of samples heated up to 1000°C showed that titanium was present in titanomagnetite and titanohematite solid solutions. However, a trace of the formation ofpseudobrookite (Fe2TiO5) was found in the XRD pattern after heating to 1100°C as a result of the oxidation of titanohematite. Thermodynamically, Ti-rich phases can be formed when titanohematite is oxidised to pseudobrookite and hematite, and further to rutile and hematite, however it was not observed in non-isothermal oxidation. 4.2.1.2. Isothermal oxidation The raw ironsand was oxidised at 1000°C under air in the muffle furnace. A sample was held in the muffle furnace for 2, 24 and 72 hours, and quenched afterwards to the room temperature. The change in the phase composition was studied using XRD and EPMA. The XRD patterns of the preoxidised ironsand at different oxidation time are shown in Figures 4-2-2. 2 3 The complete transformation of Fe + to Fe + was not achieved in any case. The weight gained after two-hour oxidation was 2.2% while the complete oxidation of titanomagnetite would result in +3.45% weight gain. The XRD pattern of the sample oxidised for 2 hours was composed of titanomagnetite and titanohematite peaks. 65 • TIH • +TI'M o y-F~~ •• * F~Ti05 72 hours • *• • •• • • * • 24 hours •• • • I *• •• i • • I • 2 hours • • • The raw ironsand ore • • •• 25 30 35 40 45 50 55 60 65 2theta Figure 4-2-2. XRD patterns of the preoxidised raw ironsand ore samples The samples were oxidised for different times at 1000°C and quenched to room temperature in air. Oxidation for 24 hours resulted in the weight gain of2.8%, what corresponds to 81.2% of the complete oxidation oftitanomagnetite. 72 hours oxidation increased the degree of oxidation to 84.1%. The peaks oftitanomagnetite around 35.5° and 62.7° have the same position with the peaks oftitanohematite. The XRD evidence of the presence of titanomagnetite in the preoxidised ironsand was derived on the basis of analysis of peaks intensity in the hematite ore and preoxidised ironsand. The intensity ratios ofhematite peaks are (Figure 4-1-1): 66 l . 1 621• _23_l_= 1.54; - ·- = 1.10; l 35.5° l 64.2° The intensity ratios of peaks of preoxidised ironsand for 72 hours are (Figure 4-2-2): 1 33.2• = 1.41; 1 62.1• = 1.00. l 35.5° l 64.2° The lower intensity ratios of the peaks of preoxidised ironsand show that there is non-oxidised TTM phase in the ironsand, which supports the weight measurement. In the XRD pattern of a sample oxidised for 24 hours, a pseudobrookite peak was observed. Its intensity increased with increasing oxidation time to 72 hours. The atomic distributions of iron and titanium in a particle of raw ironsand and samples oxidised for 2, 24 and 72 hours were analysed by EPMA mapping analysis (Figures 4-2-3 (a) to (d)). The atomic iron to titanium (Fe/Ti) ratios along analysed lines of the raw ironsand and samples oxidised for different times are compared in Figures 4-2-4 (a) to (d). (a) Raw ore 67 (b) After 2 Hours (c) After 24 Hours ( d) After 72 Hours Figure 4-2-3. The change in the elemental distributions of Fe and Ti with oxidisation time at 1000°C 68 100 ------, (a) Raw ~ 10-loocoa:iooooo00001:l0001:xioa:lOaXIOOOCl0000000000001:l0001:xioa::OCCCIOOOCl00000CXI00000QQgg!~ u. 1 +----..-----,.------,------r-----,----,------r------, 0 10 20 30 40 50· 60 70 80 Distance, micron ~ 1o~~ioo.0!~pg..,...~Pr.t:,l:lQ--~~A..P<..JlSl9.~)0-___2!~~~~~ u. 1+----,------,.------r----,------r-----,------1 0 10 20 30 40 50 60 70 Distance, micron 100 ------, ( c) 24-hour oxidation ~ 1o~~~;tcx:l:]'2:XJXl~~~--4XltlJ:Qi~cttQ.,~~a:x:Q:i~,s:t~QD:J:CXb-.,.p;I~--,4:xx:o::.:t!} u. 1 +------,------,------r-----r-----...,....------; 0 20 40 60 80 100 120 Distance, micron 100 ~------, (d) 72-hour oxidation ~ 10$~9:S~~-re:J$.~~~-----<$1,~....,SD;sg:~~;ga:J~~~CXXXIOl:b- 0 20 40 60 80 Distance, micron Figure 4-2-4. The Fe/fi ratios of the raw and preoxidised ironsand at 1000°C (Each line analysis was made on the identical particle in Figure 4-2-3.) The mapping images (Figure 4-2-3) and the Fe/fi ratios (Figure 4-2-4) of samples oxidised at 1000°C showed the migration of titanium for the phase transformation during the oxidation. Titanohematite started to form with a preferable orientation (lamellar). Its 69 fraction increased with oxidation time (Figure 4-2-3 (b) - (d)). In the beginning of the oxidation process (Figure 4-2-4 (b)), the exsolved titanohematite phase had higher Ti content than in the titanomagnetite matrix due to the higher mobility of Fe3+ than that of Ti4+. The fluctuation ofFe/fi ratio at the early stage of the titanomagnetite to 3 titanohematite transformation (Figure 4-2-4 (b)) is mainly due to the migration ofFe +. When titanomagnetite is oxidised, the concentration ofFe3+ in a titanomagnetite lattice 2 increases due to the oxidation of Fe +, as well as the adoption of an oxygen atom. Then the structural transformation of cubic spinet to rhombohedral follows. During the 3 transformation, Fe + in the oxidising lattice is under compressive stress, resulting in 3 migration of Fe + out of the newly formed rhombohedral lattice, and accelerates further oxidation of titanomagnetite. After the titanomagnetite to titanohematite transformation, titanium also migrated, resulting in the uniform Fe/fi ratio in titanohematite phase (Figure 4-2-4 (c) and (d)). Unlike a non-homogeneous particle (Figure 4-1-4) and a sample oxidised for 2 hours, the titanohematite phase of a sample oxidised for 24 and 72 hours had uniform Fe/fi ratio along the analysed line (Figures 4-2-3 (d) and 4-2-4 (d)). In the titanohematite phase, titanium formed a new phase. In samples oxidised for 24 and 72 hours, EPMA showed the formation ofpseudobrookite phase (Fe2TiO5) in the core of the titanohematite phase. The Fe/Ti ratio of the Ti-rich phase in the sample oxidized for 24 hours was close to 2 (Figure 4-2-4 (c)), which is the stoichiometric Fe/fi ratio of pseudobrookite. The formation of pseudobrookite decreased Ti content in titanohematite phase. The average Fe/Ti ratio in titanohematite phase was 11.2, which was visibly higher than the average Fe/fi ratio for the raw ironsand ore, 10.0. After 72 hours (Figures 4-2-3 (d) and 4-2-4 (d)), titanium was partly segregated, resulting in further increase of Fe/fi ratio in titanohematite phase, 11.6 in average. The mapping image showed that titanium migrated toward the core of titanohematite phase. The formation of hematite and the further dissociation of pseudobrookite to hematite and rutile were not achieved under the given experimental condition. Formation of Ti-free hematite and Ti-rich phases (Fe2 TiOs, TiO2) depends strongly on the migration of titanium, which has lower mobility than iron. 70 4.2.1.3. Behaviour of impurities The behaviour of aluminium and magnesium in ironsand was studied using quantitative linescan EPMA technique. Figure 4-2-5 shows the elemental distributions in the raw ironsand and samples oxidised at I 000°C for different times. -o-Mg -1:r-AI --+E-Ti -.-Fe 0 10 20 30 40 50 60 70 80 distance, micron (a)Raw 10 -o-Mg ;.e. -1:r-AI -ea -*-Ti -.-Fe 1 0 10 20 30 40 50 60 70 distance, micron (b) 2-hour oxidation 71 ~Mg -lr-AI -*-Ti ~Fe 1-ti-----r------r------r------.----.------1 0 20 40 60 80 100 120 distance, micron ( c) 24-hour oxidation ~Mg -lr-AI -*-Ti ~Fe 0 10 20 30 40 50 60 70 80 90 distance, micron ( d) 72-hour oxidation Figure 4-2-5. The elemental distributions in the raw and preoxidised ironsand ores (Each line analysis was made on the identical particle in Figure 4-2-3.) 72 In the oxidation, magnesium exhibited a tendency to migrate from the Ti-rich phase and mirror the iron distribution. This is demonstrated particularly in samples oxidised for 24 and 72 hours. The migration of aluminium was observed only in a sample, oxidised for 72 hours. The aluminium content was proportional to that of titanium in some points, which was opposite to distribution of magnesium. There was no sign of the formation of Al or Mg-rich oxide phases. 4.2.1.4. Thermodynamics of oxidation of titanomagnetite The schematic oxidation oftitanomagnetite is illustrated by a FeO-Fe2O3- TiO2 ternary diagram in Figure 4-2-6. The oxidation path oftitanomagnetite phase follows the horizontal line in Figure 2 4-2-6 because only Fe + in titanomagnetite is oxidised. Ulvospinel and magnetite form a solid solution, titanomagnetite (line [l]), ilmenite and hematite also form a solid solution, titanohematite (line [2]), with a miscibility gap between them. The theoretical phase transformations during the oxidation process follow the reactions; (4-1) (4-2) (4-3) In the oxidation experiments, reaction ( 4-1) proceeded with the exsolution of rhombohedral titanohematite into the lamellar-shape phase. Reaction (4-1) included the 3 2 migration of iron (Fe + and Fe 1 and the structural transformation of spinet to rhombohedral phase. Reactions (4-2) strongly depended on the migration of titanium as shown in the mapping images of samples taken in progress of the oxidation (Figure 4-2-3). 73 Approx. initial composition of the raw ironsand ore FeO Figure 4-2-6. The FeO-Fei03-Ti02 ternary system [l] - titanomagnetite solid solution, Fe3_xTix04, (spinet); [2] - titanohematite solid solution, Fe2-y Tiy03, (rhombohedral); [3] - ferropseudobrookite-pseudobrookite solid solution, Fe3_zTizOs, (orthorhombic) Reaction (4-2) did not go to completion, and proceeded only to the extent oin accordance with the reaction: Fe T1. 0 1(2x-3o) 0 2( -- 1-x )Fe _ T1. 0 2(2x-3o) Fe T10. 2 2 3 +- --- 2 = 2 6 6 3 +- --- 2 5 2--x -x 3 3 6 2-30 2-30 3 2-30 74 Reaction (4-2) starts from the intersection of the oxidation path with the line [2] in Figure 4-2-6 with the 8value (reaction (4-4)) of2/3x and goes towards the thermodynamic equilibrium where 8= 0, which corresponds the complete oxidation of titanohematite to hematite and pseudobrookite. The formation ofhematite and rutile (reaction (4-3)) was not observed under given experimental condition. In the raw ironsand ore, the x value in reactions (4-1)-(4-4) was approximately 0.27. Isothermal preoxidation for 72 hours, resulted in complete oxidation of titanomagnetite to titanohematite. However, the oxidation did not go beyond 8= 0.16 after 72-hour oxidation. This corresponds to approximately 11 % of conversion of titanohematite to pseudobrookite. 4.2.2. Calcination of raw ironsand 4.2.2.1. Non-isothermal calcination The raw ironsand was calcined under argon in the vertical tube furnace (see Figure 3-1 ). The argon flowrate was maintained at 800ml/min with the temperature increase of 200°C/hour up to 1100°C. The XRD patterns of the raw ironsand and the calcined sample (quenched at l l00°C) are shown in Figure 4-2-7. 75 •• • TI1-I +TIM o y-F~03 Calcined ~.,, .sC • • •• •• .E a, • • • -~jg a, 0::: Raw •• • • •• • 25 30 35 40 45 50 55 60 65 2 theta Figure 4-2-7. XRD patterns of the raw and the calcined ironsand ore The XRD patterns in Figure 4-2-7 showed that there was no significant change in the phase composition during calcination except of the transformation of cubic maghemite phase in the raw ironsand to titanohematite phase. Because maghemite phase is metastable, intermediate between magnetite and hematite, it transforms easily to rhombohedral hematite at high temperatures under argon (Feitknecht, 1964; Gallagher et al., 1968; Gillot et al., 1978). The mass-spectrometer gas analysis detected no trace of oxygen evolution during the experiment. EPMA mapping of a particle of the calcined ironsand did not detect any change in elemental distributions during the calcination. Figure 4-2-8 shows that the homogeneous elemental distributions of the raw ironsand ore is retained after calcination. 76 Figure 4-2-8. Mapping images of the calcined ironsand J7 4.2.2.2. Isothermal calcination Ironsand was calcined at 1100°C under argon in the vertical furnace. A sample was introduced into the furnace at 1100°C, withdrawn from the furnace after calcination times of 2 and 24 hours, and quenched to the room temperature. The XRD patterns of the calcined samples of ironsand are shown in Figure 4-2-9 . •• 24 hours •• • • •• • 2 hours •• • •• • • • The raw ironsand ore •• • • •• 25 30 35 40 45 50 55 60 65 2 theta Figure 4-2-9. XRD patterns of the calcined ironsand ore samples. The samples were calcined for 2 and 24 hours at 1100°C and quenched to room temperature in argon. 78 In the calcination of ironsand under argon atmosphere at 1100°C, titanium containing maghemite transformed to titanohematite, as detected in the non-isothermal calcination. Further calcination up to 24 hours did not affect the phase composition. 4.2.3. Surface area of preoxidised and calcined ironsand Phase changes during preoxidation and calcination are accompanied by significant decrease in the specific surface area (SSA) of samples. The specific surface area of pre-treated samples in non-isothermal preoxidation and calcination described in previous sections measured by single-point BET method (ASAP 1200) is shown in Figure 4-2-10. 1.2 N~ E 0.8 iu, 0.4 0 200 400 600 800 1000 1200 Temperature, °C Figure 4-2-10. The specific surface area of (a) preoxidised and (b) calcined ironsand ore samples Heating in air (preoxidation) or argon (calcination) to about 600°C had only slight decrease in SSA. The major decrease in SSA took place in the range of 600 to 800°C in the calcination, and 800 to 1000°C in the preoxidation. When the samples were heated further above 1000°C, there was no measurable SSA reduction. 79 Change in SSA of ironsand in the oxidation process is caused by two processes: 1) spinel titanomagnetite to rhombohedral titanohematite transformation, and 2) sintering. The first process opens the structure of ironsand and, may be expected, increases the SSA, while sintering decreases SSA. It can be suggested that in preoxidation up to 800°C, phase transformation partly compensates the decrease in SSA as a result of sintering. In the temperature range 800 to 1000°C, effect of sintering is dominating, and SSA decreases sharply. The SSA of samples after preoxidation and calcination up to 1100°C were close to each other, approximately 0.05m2/g, which was only 4% of the SSA of the raw ironsand. The measurement of pore volume and size after the both pretreatments using the multi-point method (ASAP 2000) was inaccurate due to the small SSA. 80 4.3. Morphology The morphology of the ironsand ore was observed with SEM and optical microscope. SEM images of raw and calcined ironsand sample are shown in Figure 4-3-1. Figure 4-3-1. SEM images of particles of ironsand ore (a) Raw ore; (b) After calcination at 1100°C for 24 hours lronsand is mainly composed of spherical particles. After calcination, the pores and small cracks on the surface of the raw ironsand ore were sintered, resulting in reduction of the specific surface area (SSA). The high-resolution images of raw and calcined samples showed the change in appearance of particles more clear (Figure 4-3-2). 81 Figure 4-3-2. SEM images of particles of the ironsand ore (a) Raw ore; (b) After calcination at 1100°C for 24 hours During calcination, both homogeneous and non-homogeneous particles with exsolution maintained their morphologies. The polished sections of each particle before and after calcination are shown in Figure 4-3-3. In the particles with exsolution, the Ti rich rhombohedral phase maintained its lamellar structure. 82 Figure 4-3-3. Microphotographs of particles of ironsand (a) and (b) Raw ore; (c)and (d) After calcination at 1100°C for 24 hours (a) and (c) are homogeneous particles; (b) and (d) are particles with lamellar. During preoxidation, the change in morphology was due to phase transformation and sintering. Figure 4-3-4 shows that particles preoxidised at I 000°C for 24 hours. 83 Figure 4-3-4. SEM images of particles of the ironsand (a) Raw ore; (b) and (c) After preoxidation at 1000°C for 24 hours 84 4.4. Summary Characterisation of raw and pretreated ironsand using XRD, EPMA, SEM, optical · microscope and BET analyses showed the following. The raw ironsand is mainly composed of homogeneous titanomagnetite particles. The stoichiometric phase formula of titanomagnetite in the raw ironsand was established to be (Fe3O4 ) 1_z(Fe2TiO4 )z, with thex value of 0.27±0.2. Small portion of particles in the raw ironsand showed exsolution between titanomagnetite and titanohematite. During the exsolution, titanium is enriched in the newly formed phase of titanohematite. The phase separation due to the miscibility gap between magnetite and ulvospinel was not found. In preoxidation, titanomagnetite is transformed to titanohematite. In non isothermal preoxidation at a rate of200°C/hr, the titanomagnetite starts to oxidise above 600°C. Titanomagnetite is stable against oxidising atmosphere due to high-valency cations which retard migration of iron and titanium for the phase transformation. The oxidation oftitanohematite to hematite and rutile was not achieved in any case. During the isothermal preoxidation of the ironsand at I 000°C, pseudobrookite was formed in the core of the titanohematite phase with decrease of the Fe/Ti ratio of surrounding titanohematite. In preoxidation process, magnesium migrated from Ti-rich phase and followed iron. The migration of aluminium was negligible, compared to those of magnesium and titanium. The distribution of aluminium and magnesium depended on their mobilities. The formation of minor magnesium or aluminium oxides was not detected. Calcination of ironsand did not make a significant change in the phase composition except the structural transformation of titanium-containing maghemite phase to titanohematite. The change in the elemental distribution after calcination was negligible. The pretreatments reduced the specific surface area of the raw ironsand ore dramatically to 4% SSA of the raw ironsand. 85 Chapter 5. Gas-Solid Reduction of Ironsand Ore There were numerous studies of the reduction process of titanium-containing ironsand ores, however, most of them examined ilmenite concentrates; the research in the reduction of titanomagnetite ironsand ore was mainly focused on the carbothermal reduction and is not sufficient. This chapter reports results of study of gas-solid reduction of ironsand ore by temperature-programmed reduction and isothermal reduction of raw and pretreated ironsand by carbon monoxide, hydrogen, and methane-co~taining gas mixtures; and considers reduction mechanisms. , S.1. Temperature-programmed reduction of ironsand A temperature programmed reduction {TPR) experiments were employed to determine the temperature ranges for different reduction stages. The temperature was ramped at the rate of 100°C/hr. The difference of the furnace temperature and sample temperature due to the heat absorbed and released for reduction was negligible during the reduction experiment. The experimental temperature range was from 200 to 1100°C. When the temperature reached 1100°C, it was held at this temperature for about 2 hours. The temperature change in TPR is shown in Figure 5-1-1. S.1.1. Reduction of raw ironsand Samples of raw ironsand were reduced by CO-Ar gas mixture (75vol% CO). The total flowrate of the reducing gas mixture was 800mVmin. The reduction rate and extent obtained using mass-spectrometer data are presented in Figures 5-1-2 (a) and (b). The reduction of the raw ironsand started when the temperature reached 800°C. There are a number of reactions in the temperature range of 800 to l 100°C. The maximum rate of the reduction appeared at about 1020°C. The reduction came to the completion by the end of 2-hour exposure at 1100°C. 86 time, mln Figure 5-1-1. Temperature change in the temperature-programmed reduction experiments Phase changes during the reduction process were identified by XRD. Figure 5-1-3 presents the XRD patterns of the raw ironsand reduced and quenched at different temperatures. The phases identified during the reduction process were titanomagnetite, ilmenite and iron. The XRD pattern of a sample heated to 500°C showed that titanium-containing maghemite phase (y-Fe2O3) in the raw ironsand was reduced to titanomagnetite phase under the reducing atmosphere. However, this contributed little to the reduction, due to the small amount of the maghemite phase, and was, practically, invisible in the reduction curve. Titanomagnetite phase in the raw ironsand showed high stability in the reducing atmosphere. The formation of iron started at 800°C. Traces of ilmenite (FeTiO3) were detected in the XRD pattern of a sample heated up to 900°C. 87 0.006 1200 0.005 1000 C 800 0 ! 0.004 0 i. :Ie l! 0.003 600 C f Cl) .,0 0. u E :I 0.002 400 "C s ! 0.001 200 0 0 180 240 300 360 420 480 540 600 660 time,min (a) Rate of the reduction 100 1200 1000 80 0~ 0 .z 800 0 C M s 60 f .a =C 600 l! .2 Cl) u 40 0. -:I E "C 400 s ! 20 200 0 0 180 240 300 360 420 480 540 600 660 tlme,min (b) Extent of the reduction Figure 5-1-2. The progress of the temperature-programmed reduction of the raw ironsand 88 • TTM • FeTI03 • Fe * ® y-Fe203 1100°c, after2 hours * 1000°c • ~,,, • C .! .5 G) • ~m 'i a:: • • 500°C • • Room Temp. • • • • 25 30 35 40 45 50 55 60 65 2 theta Figure 5-1-3. XRD patterns of samples of raw ironsand heated and quenched at different temperatures. Reducing gas: 75vol% CO and 25vol% Ar (TTM: titanomagnetite) 89 The XRD pattern of a sample heated to 1100°C and kept for 2 hours at this temperature showed the peaks of metallic iron and trace of titanium oxide. The amount of oxygen removed from the sample measured by mass spectrometer (Figure 5-1-2 (b)) was 22.5wt% of the raw ironsand, which was consistent with the LECO analysis. The amount of oxygen removed was close to the amount of oxygen associated with iron oxides in the raw ironsand (see Table 3-1). During the reduction, the following reactions can be suggested on the basis of XRD analysis. , Fe3_xTi,p4 +(4-3x)C0 = (3-2x)Fe+xFeTi03 +(4-3x)C02 x = 0.27 (5-1) (5-2) Reaction (5-1) can occur via the following reactions: Fe3_xTix04 + (1-x)C0 = 3(1-x)" FeO'+xFe2Ti04 + (1- x)C02 (5-la) 3(1-x)"Fe0"+3(1-x)C0 = 3(1-x)Fe+3(1-x)C02 (5-lb) (5-lc) Wustite ("FeO") and ulvospinel (Fe2 Ti04) were observed in the course of reduction in the isothermal experiments (see section 5.2.2.3). This supports the proposal here sequence ofironsand reduction, reactions (5-la) to (5-ic). However, the XRD (Figure 5-1-3) did not detect wustite and ulvospinel. This means that once titanomagnetite reduced to wustite and ulvospinel, wustite and ulvospinel were quickly reduced to metallic iron and ilmenite. The overall reaction during the reduction process was, Fe 3_xTix04 +(4-2x)C0 = (3-x)Fe+xTi02 +(4-2x)C02 (5-3) Table 5-1 lists the contribution of different reduction steps to the extent of the reduction, based on the chemical composition of raw ironsand (Table 3-1 ). 90 Table 5-1. Calculated extent of reduction in different reduction steps Reduction steps Single step (%) Accumulative (%) Fe1-xTix04 ~ "FeO"+Fe2Ti04, x = 0.27 21.1 21.1 "FeO" ~Fe 63.3 84.4 Fe2Ti04 ~ Fe+FeTi01 7.8 92.2 FeTi03 ~Fe+Ti oxides 7.8 100 Reduction ofTiO2 to lower suboxides (Tis09, T401, ThOs, and ThOJ) could not be excluded. But very unlikely that it could go beyond TiJOs (Zhang and Ostrovski, 1999). This means that maximum extent of reduction associated with titanium oxide does not exceed about 1% with respect to iron-titanium oxides. The slowest step in the process was the reduction of titanomagnetite to wustite and ulvospinel (reaction 5-la). Further discussion of the reduction mechanism of the ironsand ore is presented in section 5.1.4. 5.1.2. Reduction of pretreated ironsand Although it has been reported by McAdam (1969a) that preoxidation accelerates reduction of ironsand, researches on the effects of preoxidation on the phase composition and the reduction of ironsand are not sufficient. Effects of preoxidation and calcination of ironsand on its reduction were investigated by comparison of reduction rates and extents of reduction of pretreated ironsand ores with that of the raw ironsand. Preoxidation and calcination of ironsand were described in Chapter 4. The preoxidised ironsand was heated for 72 hours at 1000°C in air. The calcined ironsand was heated for 24 hours at 1100°C under argon atmosphere. Samples of preoxidised and calcined ironsand were examined using temperature programmed heating up to l 100°C, in 75vol%CO-25vol%Ar gas mixture. Reduction curves obtained from the reduction experiments with preoxidised and calcined samples are shown in Figures 5-1-4 (a) and (b). 91 0.008 -,------, 0.007 +------; Calcine C 0.006 +------. I en 0.005 4------ttt-----1...... { 0.004 -+------+---.------C 0 :g 0.003 +------t-'.W-t------: ::::, ,::, Preoxldise e 0.002 +------l----'!------1------.1 o J..-.~~-""'~L.,_~~=~~~~~~!Ml...j 200 300 400 500 600 700 800 900 1000 1100 temperature, °C (a) Rate of the reduction 80 ';ft. .z C 60 ~ C 0 :p CJ 40 ,,::::, e 20 0 +---~--..,....,'---~--...... ----...... -----..--,::;:.,-~---F------l 200 300 400 500 600 700 800 900 1000 1100 temperature, °C (b) Extent of the reduction Figure 5-1-4. The progress of the temperature-programmed reduction of the pretreated ironsand ores 92 5.1.2.1. Effect of Preoxidation The reduction of the preoxidised ironsand showed very different pattern to that of the raw ironsand. In the temperature range from 300°C to l 100°C, four reduction stages were identified. The first stage includes the reduction in the temperature range of about 400-550°C, with the maximum reduction rate near 470°C. The first reduction stage contributed approximately 20% of the reduction extent. The second stage occurred .between 450 and 600°C with the maximum reduction rate at about 550°C. 40% of reduction was achieved by this stage. The third stage appeared between about 600 and 850°C, with the maximum rate at 750°C. In the fourth, the final stage which started at about 700°C, the reduction rate decreased from about 850°C, and diminished at 1100°C. Figure 5-1-5 presents the XRD patterns of samples of the preoxidised ironsand reduced and quenched at different temperatures. As reported above, at room temperature, the preoxidised ironsand was composed of titanohematite and small amount of pseudobrookite. There was no change in the phase composition up to 400°C. In the first reduction stage, titanohematite was reduced to titanomagnetite. This is shown by the XRD pattern of a sample heated up to 500°C: titanohematite peaks disappeared or decreased, with the formation of titanomagnetite phase. Some amount of titanomagnetite phase was reduced to metallic iron. The main reaction in the first reduction stage is the following reaction: Fe _YTiyO +.!.CO=~Fe Ti 0 +.!.co (5-4) 2 3 3 3 3-2y3 2y3 4 3 2 They value in the titanohematite in the preoxidised ironsand ore was approximately 0.16. The extent ofreduction after the first stage (at T = 550°C), 45%, was higher than the calculated reduction extent from reaction (5-4 ), 11.1 %. This is due to the overlap with the second reduction stage, in which titanomagnetite was reduced to metallic iron. The reduction extent after the second stage of reduction reached about 60%. In the second reduction stage, titanomagnetite was reduced to ilmenite and metallic iron. The metallic iron further transformed to iron carbide. In this stage, pseudobrookite was 93 0 + TTM VTI02 ® TTH O Fe3C 1100°c • FeTI03 * Fe o Fe2TIOs + C 800°C •• 600°C 0 ~ en sC .E Cl) 500°C ~ .!!! -Cl) • 0:: 400°C ® 0 ® Room Temp. ® 0 25 30 35 40 45 50 55 60 65 2 theta Figure 5-1-5. XRD patterns of the preoxidised ironsand samples heated and quenched at different temperatures. Reducing gas: 7Svol% CO and 2Svol% Ar. (TTM: titanomagnetite; TTH: titanohematite) 94 reduced. In the XRD pattern of a sample heated to 600°C, after the completion of the second reduction stage, the peak for pseudobrookite phase disappeared. However, the reduction path and the contribution of the reduction of pseudobrookite to the extent of reduction were not measurable; Wustite phase was not observed. The main reactions in the seco~d reduction stage were as follows: Fe 3 Ti 3 0 4 +(4-~y)C0=(3-3y)Fe+iyFeTi03 +(4-~y)C02 (5-5) 3-y -y 2 2 2 2 2 Fe2Ti05 +CO= Fe+FeTi03 +C02 (5-6) 3Fe+C0 = Fe3C+C02 (5-7) In the third reduction stage, which contributed 20% of reduction, titanomagnetite was reduced to metallic iron and rutile. However, the formations of wustite and ulvospinel were not observed, although the reduction, more likely, occurred through wustite and ulvospinel. As in the reduction of the raw ironsand, wustite and ulvospinel were reduced very quickly. The formation of rutile phase was also observed. The main reactions in the third reduction stage are as follows: (5-8) "FeO'+C0 = Fe+C02 (5-9) Fe 2 Ti04 +C0=Fe+FeTi03 +C02 (5-10) FeTi03 +CO= Fe+ Ti02 + C02 (5-11) Metallic iron was partly transformed to iron carbide by the reaction (5-7). In the fourth reduction stage which started at above 800°C, ilmenite was reduced to iron and rutile during heating a sample to 1100°C. The final sample contained iron, iron carbide, rutile and a small amount of unreduced titanomagnetite. Further reduction of rutile phase to lower titanium oxides was not detected. 95 The overall reaction during the reduction process can be described by the following reaction, Fe _ Ti,O +(3-2y)CO = (2-y)Fe+ yTiO +(3-2y)C0 (5-12) 2 1 3 2 2 3Fe+CO=Fe3C+CO2 (5-7) 5.1.2.2. Effect of calcination Reduction of calcined ironsand was similar to that of the raw ironsand, but occurred at higher temperatures. The reduction of calcined ironsand started when a sample was heated !O 970°C (Figure 5-1-4). After heating to 1100°C, the extent of reduction was only about 40%. The retardation of the reduction of the calcined ironsand is thought to be due to sintering. As shown in the previous chapter (see sections 4.3.2 and 4.3.3), calcination of ironsand did not change the phase composition of the ironsand, however, the specific surface area significantly decreased after the calcination. 5.1.3. Reduction of magnetite ore The magnetite ore, which was obtained by partial reduction of the hematite iron ore, was reduced in the temperature-programmed experiment. The extents of reduction and the reduction rates of the magnetite iron ore and the raw ironsand are presented in Figures 5-1-6 (a) and (b). The reaction rate in the temperature-programmed reduction of the magnetite iron ore was very different from the reduction of the raw ironsand in many ways. In the temperature range from 500°C to 1100°C, three reduction stages were identified. The first stage combined reduction of magnetite to metallic iron at temperature below 570°C, when wustite was unstable, and to wustite at temperature above 570°C. The second stage was reduction of magnetite to wustite. It overlapped with the third stage, which was reduction of wustite to iron. Reduced metallic iron was carburised with the formation of cementite. 96 0.005 ~------..,,-----, Raw C 0 ~ 0.002 :::, '0 ! 0.001 +------:- o •w...ww.w-W111!.Llolit,allllWIIIIIIEii-M11,W.,.....;:.....Wllollii,11,,M.....__.~!!!!::.,:__~--~___:_....::.III! 200 300 400 500 600 700 800 900 1000 1100 temperature, °C (a) Rate of the reduction 80 ~ ..C Cl) 60 =C 0 t; 40 :::, '0 f 20 0 +-----r-----r----..--c:;;__----,-----,---...-=-...------.------1 200 300 400 500 600 700 800 900 1000 1100 temperature, °C (b) Extent of the reduction Figure 5-1-6. The progress of the temperature-programmed reduction of the magnetite iron ore 97 900°C Oo • FeaO4 ®FeO 0FeaC • Fe 0 00 0 +C + 0 0 0 ~ u, • sC .E Cl) ~ea 'i Room Temp. 0:: • • • • • 25 30 35 40 45 50 55 60 65 2 theta Figure 5-1-7. XRD patterns of magnetite iron ore samples heated and quenched at different temperatures. Reducing gas: 75vol% CO and 25vol% Ar The XRD analysis of a sample, reduced in the heating up to 900°C (Figure 5-1-7) shows the presence of metallic iron, cementite, graphitic carbon and traces of wustite. The reduction was completed below 1100°C, however, XRD analysis of the final sample was not reliable due to carbon deposition. The reduction of magnetite iron ore was different to that of the raw ironsand, which started to reduce at 800°C. Magnetite and titanomagnetite have the same spinel cubic structure. The main difference between them is the presence of titanium in the lattice, and porosity and surface area are also different. The role of titanium in ironsand ore will be discussed in the following section. 98 5.1.4. Reduction mechanism of the ironsand ore Reduction oftitanomagnetite. The raw ironsand was finally reduced to metallic iron and titanium oxides. The raw ironsand was mainly composed of homogeneous titanomagnetite particles with uniform distribution of titanium (Chapter 4). The reduction path of titanomagnetite can be illustrated using Fe-Ti02-Fe203 ternary diagram (Figure 5- 1-8). Figure 5-1-8. Fe-Ti02-Fe203 ternary system (Arrow (1): reduction path oftitanomagnetite; arrow (2): reduction paths of titanohematite; arrow (3): reduction path of pseudobrookite) 99 The theoretical reduction path of titanomagnetite is described by the arrow [ 1] in Figure 5-1-8, as follows; Fe3_xTixO -+ (FeO)+(Fe2 TiO )-+ Fe+(Fe2 TiO4)-+ Fe+FeTiO3 4 4 (5-13) -+ Fe+(FeTi2O5 )-+ Fe+(TiO2 ) (The phases in parenthesis were not identified by the XRD.) During temperature-programmed reduction of raw ironsand, titanomagnetite showed higher stability against reducing atmosphere than magnetite. However, once it started to reduce, wustite and ulvospinel phases were reduced so quickly that they were not detected by XRD. Titanomagnetite is a solid solution of magnetite and ulvospinel. The phase formula oftitanomagnetite can be described as follows: 3 3 2 2 2 4 ([Fe + ],e1,JFe + ,Fe + lactaO4)1-x . ([Fe + ],e,,a [Fe + ,Ti + 1acta O4)x (5-14) magnetite ulvospine/ When the fraction ofulvospinel, x, is 0.27, at temperature above 1000°C, the titanomagnetite structure can be presented by the Akimoto model (Akimoto, 1955; Stephenson, 1969; Wu and Mason, 1981 and Trestman-Matts et al., 1983), as follows: D 3+ D 2+ ] [ D 3+ D 2+ rr.•4+ ) O [ re )-x,re X tetra' re )-x,re ,.il X acta 4 (5-14a) (tetra: tetrahedral site, and octa: octahedral sites) In a titanomagnetite lattice, one Ti4+ and one Fe2+ substitute two Fe3+, which 3 2 decreases Fe + concentration while increasing Fe + concentration in both, tetrahedral and 3 2 3 octahedral sites. The change in the concentrations ofFe + and Fe + decreases Fe + activity 2 and increases Fe + activity in the lattice in comparison with those of pure magnetite. This results in the higher stability ofFe3+ in titanomagnetite than that in pure magnetite. When the reduction of titanomagnetite starts, Fe3+ first reduces to Fe2+ with removal of one oxygen atom. It causes the further increase in Fe2+ concentration in the lattice, resulting in accelerating the reduction ofFe2+ to metallic iron and retardation of the reduction of 3 2 4 Fe + to Fe +. Also, Ti + in octahedral site surrounded by 6 oxygen atoms strengthens the lattice structure and retards the removal of oxygen in the reduction processes. These 100 factors make titanomagnetite more stable against reducing atmosphere than magnetite and accelerate reduction of wustite to metallic iron. While metallic iron migrates out of titanomagnetite lattice, the atomic Fe/fi ratio in the titanomagnetite lattice becomes close to that ofulvospinel. Ulvospinel has a low stability field in the FeO-Fe2O3-TiO2 system, and can be easily reduced to ilmenite and iron under reducing atmosphere (Krasnova and K.rezer, 1995). Therefore, in the reduction of raw ironsand, which main phase is titanomagnetite, the different reduction pattern between the raw ironsand and magnetite is due to the titanium in the titanomagnetite lattice, which decreases FeJ+ concentration and increases 2 Fe + concentration in a lattice. This also accelerates the reduction of wustite to metallic iron. Preoxidised ironsand was composed of titanohematite and pseudobrookite. The atomic Fe/fi ratio in the titanohematite phase increased to 11.6 from 10.0 for titanomagnetite in the raw ironsand with the formation of pseudobrookite, as a result of preoxidation (Chapter 4). During reduction, titanohematite and pseudobrookite in the preoxidised ironsand reduced by different reduction paths shown by dashed arrows in Figure 5-1-8. The reduction of the preoxidised ironsand ore started with the reduction of titanohematite to titanomagnetite (reaction 5-4). In this reaction, rhombohedral structure of titanohematite transforms to spinel cubic structure of titanomagnetite, which results in about 25% increase in volume (Edstrom, 1953). This opens up the structure and facilitates the subsequent reduction stages. Pseuobrookite in preoxidised ironsand was reduced in the second stage, between 450 and 600°C: Fe2 TiO 5 +CO= (Fe 2 TiO 4 )+CO2 (5-15a) (Fe 2 TiO4 )+CO=Fe+FeTiO3 +C02 (5-15b) 2FeTiO3 +CO= (FeTi2 O5 ) +Fe+ CO2 (5-15c) (FeTi 2 O5 ) +CO= Fe+ 2TiO2 + C02 (5-15d) 101 However, it was not possible to identify the intennediate phases (ulvospinel (Fe2 TiO4) and ferro-pseudobrookite (FeThOs)). These phases are unstable in reducing atmosphere. Also, they may have amorphous structure. Formation ofIron Carbide. During temperature-programmed reduction of preoxidised ironsand and magnetite iron ore, reduced iron was transformed to iron carbide. But in the reduction of raw ironsand, iron carbide was not fonned. Iron carbide fonnation strongly depends on carbon activity and temperature. Iron carbide can be formed under carbon activity higher than unity (Grabke, et al., 2001). However, it decomposes into iron and carbon due to its unstability (Zhang and Ostrovski, 2001 ). Carbon activity in CO-CO2 atmosphere can be calculated from the following reaction: 2CO(g) = C(s)+C02 (g) 2 Pco (5-16) ~G =-170,700+174.5T(J), ac = K eq.cs-, 6>-- Pco2 (Keq(5-J6J is the equilibrium constant for the reaction (5-16).) In the reducing gas in the temperature-programmed reduction experiments, the sum of partial pressures of CO and CO2 was 0.75atm (total pressure 1 atm). The variation of the gas composition with temperature at equilibrium calculated using equilibrium constant for reaction (5-16) is shown in Figure 5-1-9. Pco + Pco. = 0.75 _ 0.8 +------'------! 8 i 0.6 u ~ 0.4 +------! a.8 0.2 ______,______--! 0 +--~==---,---,---~~-~-~---! 300 400 500 600 700 800 900 1000 1100 Temperature,°C Figure S-1-9. The variation of the CO-C02-Ar gas composition equilibrium with graphite vs. temperature 102 When iron ore is reduced by carbon monoxide, iron carbide can be formed by the reaction, 3Fe+2CO=Fe C+C0 3 2 (5-17) llG =-143,904+149.64T(J) Equilibrium constant of reaction (5-17), Keq, 1000 ~~------, K = Pco2 eq 2 Pco Iron-stable zone Iron carbide-stable zone 0.001 +---...---..-----,,------"T--~----.-----t 400 500 600 700 800 900 1000 1100 Temperature, °C Figure 5-1-10. Equilibrium constant of reaction (S-16) vs. temperature When iron is formed, it catalyses carbon deposition on the surface (Hutching, et al., 1988). In the reduction of preoxidised ironsand and magnetite iron ore, iron started to form below 600°C. In the temperature range 600-800°C, this iron is transformed to iron carbide by reaction (5-17). However, in the reduction of raw ironsand, high stability of titanomagnetite against the reducing atmosphere prevented iron carbide formation by increasing reduction temperature. Metallic iron was not formed until 800°C. Above 103 800°C, even low partial pressure of carbon dioxide at gas-solid interface, Pco < 0.07, 2 can reduce carbon activity below unity (Figure 5-1-9). The iron carbide formed from preoxidised ironsand showed high stability under the reducing atmosphere. The decomposition of the iron carbide into iron and carbon was not detected even after heating up to 1100°C. However, the reason for the stability of iron carbide produced from preoxidised ironsand is not understood yet. 5.1.S. Summary Reduction of raw and pretreated ironsand was examined in temperature programmed experiments, in the temperature range of200-l 100°C. The reducing gas was 75vol%CO-25vol%Ar. In the reduction of raw ironsand, the reduction started at 800°C. Titanomagnetite was finally reduced to iron and titanium oxide. During the reduction, the intermediate products, wustite and ulvospinel were reduced very quickly. Titanium oxide was formed in amorphous form. Preoxidised ironsand was reduced in four stages. The first was the reduction of titanohematite to titanomagnetite in the temperature range of 400-550°C, which followed by the reduction of titanomagnetite to iron and ilmenite in the second reduction stage in the temperature range of 450-600°C. Metallic iron was transformed to cementite, Fe3C. Pseudobrookite phases were reduced in this second reduction stage. However, reduction path of pseudobrookite was not identified. In the third stage, in the range of 600-850°C, titanomagnetite was reduced to iron and rutile. The remaining titanomagnetite and ilmenite were reduced in the fourth reduction stage. Calcination did not change the reduction pattern of the raw ironsand, although the reduction of calcined ironsand was retarded, starting at 970°C. After heating up to 1100°C, the extent of reduction was only 40%. Reduction of raw ironsand was different from that of magnetite iron ore, although both minerals have the spinel structure; titanium stabilised the spinel structure, retarded 3 2 the reduction ofFe + to Fe +, and accelerated the reduction ofwustite to metallic iron. 104 Preoxidation of ironsand accelerated reduction of ironsand. The structural change during the reduction of titanohematite to titanomagnetite opens the structure and facilitates further reduction oftitanomagnetite. 105 S.2. Isothermal Reduction of Ironsand This section examines effects of temperature, gas composition and pretreatment of ironsand on isothermal reduction oftitanomagnetite. 5.2.1. Gas flowrate in the reduction experiments The effect of flowrate of reducing gas on the reduction of ironsand ore was investigated using raw ironsand. The gas flowrate was in the range of 500ml/min to 900ml/min. 100 80 :::ie0 C 0 :p CJ 60 ::I "'0 ....e -<>- 900ml/min ..0 40 ~~------1-0-800ml/min C -1:r-700ml/min ~ w 20 -+------;-*-600ml/min _,._5QQml/min 0 0 15 30 45 60 75 90 Time, min Figure 5-2-1. Effect of gas flowrate on extent of reduction by 75vol%CO-Ar gas mixture at 1100°C Experiments were performed at 1100°C, which is the maximum temperature in the range examined in this work. The reducing gas contained 75vol%CO and 25vol%Ar. Extents of reduction as functions of gas flowrates are presented in Figure 5-2-1. The rate of reduction increased with increasing gas flowrate from 500ml/min to 700ml/min. Further increase in gas flowrate had no significant effect. Therefore, the resistance due to the 106 external mass transfer can be neglected when the gas flowrate is above 700ml/min. This conclusion is also applicable to reduction temperatures below 1100°C, because of lower intrinsic kinetic rate of reduction at lower temperatures. The gas flowrate in isothermal reduction experiments was held at 800ml/min. 5.2.2. Reduction of ironsand by carbon monoxide 5.2.2.1. Effect of temperature Reduction of ironsand by carbon monoxide was studied in the relatively narrow temperature range from 1000 to 1100°C. At temperatures below 1000°C, carbon deposition by the reaction 2CO = C + C02 affected the reduction behaviour of ironsand. Above 1100°C, low-melting point slag formation in ironsand hinders gas-solid interface (Morozov et al., 1998); those are why these temperatures were avoided. The reducing gas contained 75vol% carbon monoxide and 25vol% argon. Reduction curves obtained at different temperatures are presented in Figure 5-2-2. In the experimental temperature range, iron oxides in the ironsand were reduced completely to metallic iron. The extent of reduction of titanium oxide was not measurable. The reduction curves were divided into three stages; the initial stage is characterised by the increasing rate in the beginning of the reduction; after achieving maximum, the rate of reduction decreases in the second stage and decays in the last stage of the reduction. In the temperature range of 1000 to 1050°C, the rate of reduction increased with increasing temperature. Decaying period started when the reduction extent reached about 90%. Further increase in temperature to 1100°C only slightly increased the reduction rate in the initial period. However, the decaying period of the reduction at 1075 and 1100°C started from the reduction extent of 80%, which was earlier than in the reduction at temperatures between 1000 and 1050°C. The retardation of reduction at the decaying period above 1075°C was due to more profound sintering effect at higher temperatures. 107 100 0~ 80 +F .sC >< 60 Cl) ~1oooc C 0 40 +---11.lll-+-<>------t-a-1025C =u :::, ~1osoc "D ! 20 +-.,_,,~------, ---*--1075C ~11ooc 0 0 15 30 45 60 75 90 time, min (a) Extent ofreduction 0.03 ....------, C 0.025 --1--,~-;------1~ 1000C l -a-1025C C_! 0.02 ~ 1050C .$ ~1075C ~ 0.015 ___,_.__ 11 ooc 0 1:; 0.01 --'--~-~-Q------1 :::, "D ! 0.005 Il----~-lJ---4)------l 0 0 15 30 45 60 75 90 time, min (b) Rate of Reduction Figure 5-2-2. Reduction of ironsand by 75vol%CO-Ar gas mixture at different temperatures 108 5.2.2.2. Effect of carbon dioxide Effect of carbon dioxide content on the reduction of ironsand was studied by reduction of raw ironsand with CO-C02-25vol%Ar gas mixture at 1100°C. The ratio of partial pressure of CO to the sum of partial pressures of CO and C02, p co l(pco + p co ) , 2 was ~hanged in the range of0.60 to 1.0 at constant (Pco + Pco ) value of0.75. The 2 reduction curves are shown in Figure 5-2-3. 100 ~ 1f 60+---H'-+-f--+-t"-~-/"---.,------:.ii~-=------t G) i Pco/(Pco+Pc<>z) C --<>-- [1.0] :u0 --o-[0.99) j 40------F--;----,,,t~------1 b. [0.98) i )( [0.965) )I( [0.95] ~ [0.90) -+--[0.875] 20 ---[0.85) • [0.83] • (0.81) - ... - .. ------•--(0.80) · - · - · - - (0.60] 15 30 45 60 75 90 time, min Figure 5-2-3. Reduction of ironsand by CO-C02-25vo1%Ar gas mixture with different Pco l(Pco + Pco2) at ll00oC 109 The extent and rate of reduction increased with increasing Pea l(Pco + Pco ). To 2 reduce wustite to metallic iron by CO at l 100°C, the Pco l(Pco + Pco ) ratio 2 (Pea + Pco = 0.75) should be above 0.731 (calculated from the equilibrium constant of 2 the reaction: + + When Pea l(Pco + Pco ) was 0.60, the Feo.947O CO = 0.947Fe CO2). 2 reaction progressed only to "FeO"; the extent of reduction after 240 minutes was about 10%. Reduction oftitanomagnetite to wustite was quite slow. Complete reduction of iron oxide in the ironsand was achieved at Pea l(Pco + Pco ) above 0.98, however, even small 2 addition of carbon dioxide decreased the reduction rate significantly. XRD patterns of samples quenched after reduction by gases containing different content of carbon dioxide are shown in Figures 5-2-4 (a) and (b). For the reductions by gases with Pea l(Pco + Pco ) below 0.85, samples were 2 exposed to a reducing gas for 180 minutes or more to approach the thermodynamic equilibrium. Phases in samples of ironsand reduced by gas containing different carbon dioxide are presented in Table 5-2. Table 5-2. Phases in samples reduced by gas containing different CO2 contents ) Pco l(Pco + Pco2 Phases present 0.6 FeO+TTM 0.80-0.81 FeO+TTM+Fe 0.83-0.85 TTM+Fe 0.875 Ulvospinel + TTM + Fe 0.90-0.95 Ilmenite + TTM + Fe 0.965-1.0 Fe + titanium oxides (TTM: titanomagnetite) 110 • Fe • 0 TTM X y-Fe20a • FeO 0.875, after 120 min. V Fe2Ti04 V 0 V 0 0 0 0 V 0 0.85, after 180 min. 0 0 0 0 0 0 0.83, after 180 min. 0 0 b 0 0 0 ·;; 0 sC .5 0.81, after 180 min. 0 CD > ~ 0 CG • 0 0 0 "i 0 0:: • • 0.80, after 240 min. 0 0 0 0 0 • 0.60, after 240 min. 0 0 0 • 0 0 Raw ironsand 0 0 0 0 0 0 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-4 (a). XRD patterns of samples reduced by CO2-CO-Ar gas mixtures with ) in the range of 0.60 to 0.875 at 1100°C Pco l(Pco + Pco2 111 • Fe o TTM ® FeTi03 A Ti02 1.0, after 90 min. * Ti30s + Ti203 0.99, after 90 min. 0.98, after 90 min. ~ en sC C ·-G) > ~ JS! G) 0.965, after 120 min. ~ 0.95, after 120 min. ® ® 0 0 0 ® ® ® • 0.90, after 120 min. 0 0 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-4 (b). XRD patterns of samples reduced by C02-CO-Ar gas mixtures with Pco ) in the range of0.90 to 1.0 at 1100°C Pco l(Pco + 2 ll2 Elemental distribution in the final samples reduced by gas of different composition was examined by EPMA. The atomic iron to titanium ratios are shown in Figures 5-2-5 (a) to (i). The change in the atomic iron to titanium ratio, Fe/Ti, with increasing CO partial pressure was consistent with the XRD analysis (Table 5-2). It showed that metallic iron migrated from titanomagnetite lattice with enrichment of the titanomagnetite with titanium. Titanium-enriched titnomagnetite then transformed to ulvospinel (Fe2 TiO4), and further to ilmenite (FeTiO3) with the formation of metallic iron phase at the shell of a particle. The average Fe/Ti ratio in a particle in the raw ironsand was about 10.0 (Figure 5-2-5 (a)). When Pea !(Pea+ Pea ) of the reducing gas was in the range of0.80 to 0.85 (Figures 5-2- 1 5 (b) to (d)), the Fe/Ti ratio in the titanomagnetite phase decreased to about 3.0, while in metallic iron phase, the Fe/Ti ratio was higher than 50. When Pea !(Pea + Pea ) was 1 0.875 (Figure 5-2-5 (e)), the formation ofulvospinel phase, where Fe/Ti ratio is 2.0, was observed in the core of the analysed particle. In the XRD pattern of the sample reduced by Pea !(Pea Pea ) the reduction gas with + 1 = 0.875 (Figure 5-2-4) also showed trace of ulvospinel. The extent of the reduction after 120 minutes was about 85%, which is consistent with the reduction oftitanomagnetite to iron and ulvospinel (see Table 5-1). The stability zone of ulvospinel phase in CO-CO2 atmosphere is narrow (Krasnova and Krezer, 1995). Ulvospinel is transformed to ilmenite by even small increase in CO partial pressure 2 to Pea !(Pea+ Pea ) of 0.90 (Figure 5-2-5 (f)). Fe + in the ulvospinel phase was reduced 1 to metallic iron and migrated from the ulvospinel resulting in the fluctuation of Fe/Ti ratio along the analysed line. When Pea /(Pea Pea ) was 0.95 (Figure 5-2-5 (g)), ilmenite + 1 phase with Fe/Ti ratio 1.0 was formed. The reduction extent was about 87%. When Pea !(Pea + Peo ) ratio was above 0.965 (Figures 5-2-5 (h) and (i)), ilmenite was reduced 2 to metallic iron and titanium oxides. 113 1000 (a) Raw 100 ~ 10 . . . .. 1 0.1 0 10 20 30 40 50 60 70 80 Distance, micron 1000 (b)0.80 100 ~ 10 1 0.1 0 30 60 90 120 150 Distance, micron 1000 100 ~ 10 1 0.1 0 20 40 60 80 100 120 Distance, micron 1000 (d) 0.85 100 E:. 10 1 0.1 0 20 40 60 80 100 Distance, micron 1000 (e) 0.875 100 E:. 10 1 0.1 0 20 40 60 80 Distance, micron Figure 5-2-5. The Fe/fi ratios for the raw and reduced ironsand samples by CO2-CO Ar gas mixtures with Pco l(Pco + Pco ) ratio 0.80 -0.875 at 1100°C 2 114 1000 ...... ------, 100 t------:no"""------=----Pob::o~rPocq--n~-~;:::---pc""qn""""""1 ~ 10 -t::-:~'--9,"'~-0r;;:;o;:-;;::d,>-O'..______---t>;:";::,::/::...... =._,;~M:t~:oe~--=---).i 1-+------1 0.1 +------r-----,-----r------r-----,----...------i 0 10 20 30 40 50 60 70 Distance, micron 1000 -.------, 100 -HF-=l:------t<.~------.lit-tJ'O----~'l'f------rO~-r'P,~f------U¥-~'-----t ~ 10 -+---+--x--vt------,,..--...,...... ,,__-+---IIJ--0----¥-+---+-----I' 1 +-4r6-~m¥1~~--~:)!.icut,cf-1---Qlu'nr(0------=::..__-1M~M'I 0.1 +-----.....------.------,------.------0 30 60 90 120 150 Distance, micron 1000 ...... ------, (h) 0.965 100 +------r..------;a;:-----.-.~------j ~ 10 .bo.~~...,d~~q.._~~~~~~&rR.~~~~~~~~~bJ~ 1-t------v------1 0.1 +-----.....------,------,------,------1 0 30 60 90 120 150 Distance, micron 1000 -.------~ (i) 1.0 100 +------==---O------Q---=------.,.------1 ~ 10 -t;:f--U-"-=--~cf--V-'--'00'----\;;:±-'U<:f'~--"tAJ..O--A:.:--i~f+ir++rf--\:.-i~---P Figure 5-2-5 (Continued.). The Fe/Ti ratios for the reduced ironsand samples by CO2- with Pco l(Pco ) ratio 0.90 -1.0 at 1100°C CO-Ar gas mixtures + Pco2 This is consistent with thennodynamics of ilmenite reduction. According to Itoh et al. (1998), pure ilmenite is reduced to metallic iron and rutile by CO-CO2 gas mixture with Pco l(Pco + Pco ) higher than 0.95 at 1100°C by the reaction: 2 115 FeTiO (s) + CO(g) = Fe(s) + TiO (s) + C0 3 2 2 (5-18) /lG 0 = 23.88T(J) When Pco l(Pco + Pco ) was 0.965, the reduction extent after 120 minutes was 2 95%. The complete reduction of iron oxides and the partial reduction of titanium oxide Pco l(Pco Pco ) phases in the ironsand were achieved when + 2 was higher than 0.98. Lower titanium oxides (Ti 0 O2n.1) appeared in the XRD pattern of a sample reduced by the Pco l(Pco Pco ) Pco l(Pco Pco ) gas with + 2 = 0.99 with very low intensity. When + 2 was 1.0, the XRD pattern was composed of a strong peak of metallic iron and weak peaks of TiO2, ThOs and ThO3. According to Jones (1973), titanium dioxide can be reduced by carbon monoxide at 1100°C by the reaction: (5-19) Under the given experimental condition, the value of n was reduced to 2. Further reduction of titanium oxide to titanium oxycarbide (TiCxOt-x) or titanium carbide (TiC) was not observed. Figure 5-2-6 shows the elemental mapping of a particle in a sample reduced by CO- 25vo1%Ar gas mixture at 1100°C for 90 minutes. A small amount of fine titanium oxide phase was uniformly dispersed in the metallic iron; segregation of titanium oxide with the size of 5 micron was also observed. 116 Figure 5-2-6. Mapping image of a particle in a sample reduced by 75vol%CO-Ar gas mixture at 1100°C for 90 minutes 11 7 5.2.2.3. Progress of the reduction ofironsand by CO-Ar gas mixture Samples taken in the progress of reduction at 1100°C by 75vol%CO-Ar gas mixture were analysed by XRD. The XRD patterns are presented in Figure 5-2-7. In the beginning of the reduction, the formation of wustite was faster than the reduction of wustite. When a sample was exposed to the reducing gas for 5 minutes, the reduction extent reached 25%; titanomagnetite was partly reduced to wustite and further to metallic iron. The 5-minute reduction curve was parabolic-shape (Figure 5-2-1). After 15 minutes, the reduction extent was 82% and the peak of metallic iron became dominant in the XRD pattern. The wustite peaks became very weak and the formation of ilmenite was detected. Wustite peaks were not observed in the XRD patterns of a sample reduced for 30 minutes. However, unreduced titanomagnetite phase remained. As discussed in section 5.1.4, the reduction ofwustite to metallic iron occurred very quickly. After 30 minutes, when 95% of reduction was achieved, ilmenite was reduced to metallic iron and rutile Rutile was reduced to titanium suboxides after 60 minutes of reduction (the reduction extent was 99.5%). The final sample, reduced for 90 minutes, was composed of metallic iron and titanium oxides, TiO2, ThOs and ThO3. The following reduction sequence can be suggested at 1100°C: Fe 3_xTix04 ---+" FeO"+Fe + ( Fe 3_x_6 Tix+6 0 4 )---+ Fe+ ( Fe 3_x_6 Tix+6 0 4 )---+ Fe + Fe 2Ti04 ---+Fe+ FeTi03 ---+Fe+ Ti02 ---+Fe+ Ti30 5 ---+Fe+ Ti 2 0 3 ' (0~8<1-x} (5-20) (The phases in parenthesis were not detected by the XRD.) In reaction (5-20), the reduction oftitanomagnetite to wustite proceeded very slowly, resulting in the parabolic shape in·the beginning of the reduction. Once wustite was formed, it reduced to metallic iron quickly (see section 5.1.4.). Titania was formed after the reduction of ilmenite, and then reduced to suboxides. 118 • Fe ® FeTi03 o TTM 11 Ti02 • x y-Fe203 • Ti30s + FeO + Ti203 90min • • • • + • 60min 30min • ·;;;~ sC 0 .5 I :i 15 min • a::CD • ® 5 min 0 • 0 • 0 Raw 0 0 0 0 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-7. XRD patterns of samples reduced by 75vol%CO-Ar gas mixture at 1100°C, in the progress of reduction 119 Impurities in the reduction ofironsand Phases containing major impurities such as aluminium and magnesium were not detected by XRD analysis of partially and completely reduced ironsand samples, due to their low contents and possibly amorphous structure. They were examined by quantitative linescan EPMA. Figures 5-2-8 (a) to (f) show the change of elemental distributions in the progress of the reduction by 75vol%CO-Ar gas mixture at 1100°C. Elemental distribution across a partially reduced particle showed that the cation 4 3 impurities {Ti +, Al + and Mg2") were expelled from metallic iron phase and formed finely dispersed oxide phases. Until 30 minutes of the reduction, the distribution of the impurities did not show a pattern of segregation or phase formation, however, the impurities segregated as the reduction proceeded. After 30 minutes of reduction, aluminium and magnesium started to migrate out of metallic iron phase. When a sample was reduced for 30 minutes, the distribution of titanium showed the formation of ilmenite where Fe/fi ratio was close to 1.0. In a sample reduced for 90 minutes, the distribution of magnesium was consistent with that of aluminium. However, the atomic ratio of magnesium to aluminium fluctuated from 0.4 to 2.0 without a tendency to the formation of Al-Mg oxides such as spinet (MgAhO4). The mapping image of a particle of sample reduced for 90 minutes (Figure 5-2-6) showed that aluminium and magnesium were finely dispersed. The concentration of the segregated of aluminium and magnesium were negligible. 120 0 10 20 30 40 50 60 70 80 distance, micron (a) Raw --o-Mg -tr--l>J _._Ti --o-Fe 0 10 20 30 40 50 60 70 80 distance, micron (b) 5 min --o-Mg -tr--l>J _._Ti --o-Fe 0 10 20 30 40 50 60 70 80 distance, micron (c) 15 min Figure 5-2-8. Elemental distributions in progress of reduction of ironsand by 75vol¾CO-Ar gas mixture at 1100°C 121 0 20 40 60 80 100 distance, micron (d) 30 min -o-Mg -t:r-AI ___._ Ti -o-Fe 0 10 20 30 40 50 60 distance, micron (e) 60 min 10 -o-Mg i -t:r-AI ftll ___._ Ti 1 -o-Fe 0 10 20 30 40 50 60 70 distance" micron (f) 9u min Figure 5-2-8 (Continued.). Elemental distributions in progress of reduction of ironsand by 75vol%CO-Ar gas mixture at 1100°C 122 5.2.2.4. Morphology Morphology of particles in the progress of the reduction ofironsand by 75vol%CO Ar gas mixture at 1100°C was examined by SEM. As shown in Chapter 4, there are two distinguished groups of particles in ironsand, one is homogeneous titanomagnetite, which is the major group and the other group includes non-homogeneous particles, which contain titanohematite and titanomagnetite. Figures 5-2-9 (a) to (h) and 5-2-10 (a) to (h) present the morphology changes of each group during the reduction process. The homogeneous particles were generally reduced in a topochemical way. The metallic iron started to form at the shell of a particle with crackling the surface of the particle (Figure 5-2-9 (c), 5 minutes of reduction), and proceeded to the core of a particle • (Figure 5-2-9 (d), 15 minutes of reduction). The homogeneous particles were about fully reduced in less than 30 minutes of reduction, which was consistent with the reduction curves, as these particles represent the majority of the ironsand. The microstructure of a particle reduced for 60 minute (Figure 5-2-9 (h)), where the reduction extent was above 99%, showed fine oxide phases (grey-coloured) dispersed in the metallic iron. The metallic iron was also very fine. The morphology change during the reduction of non-homogeneous particles was different from that of homogeneous particles. The reduction started with titanohematite phase, which has a lamellar shape (Figure 5-2-10 (a)). Metallic iron was formed even in 3 minutes of reduction of the titanohematite phase (Figure 5-2-10 (b)), while titanomagnetite phase remained unreduced. The reduction of non-homogeneous particles proceeded much faster than that of homogeneous particles. The most of the non-homogeneous particles were fully reduced to metallic iron in 15 minutes (Figure 5-2-10 (e)). The metallic iron was then coarsened and sintered (Figure 5-2-10 (f) to (h)). 123 (a) I mm (b) 3 min (c) 5 min (d)I0min (e) 15 min (f) 20 min (g) 30 min (h) 60 min Figure 5-2-9. Morphology change of homogeneous titanomagnetite particles during the reduction by 75vol%CO-Ar gas mixture at 1100°C 124 (a) 1 mm (b) 3 min (c) 5 mm (d) 10 min (e) 15 min (f) 20 min (g) 30 min (h) 60 min Figure 5-2-10. Morphology change of non-homogeneous particles during the reduction by 75vol%CO-Ar gas mixture at 1100°C 125 The fast reduction oftitanohematite phase was discussed in section 5.1.4. In the non-homogeneous particles, the structural change during the reduction of the rhombohedral titanohematite phase in the beginning of the reduction opens the structure of a particle (Figures 5-2-10 (b) to (e)), resulting in acceleration of nucleation and growth of metallic iron, and facilitates the further reduction oftitanomagnetite phase. 5.2.2.5. Surface area measurement The change in the specific surface area (SSA) of samples in the course of the reduction at 1000 and 1100°C is shown in Figure 5-2-11. The final SSA of a sample reduced at 1100°C was 75% of that of a sample reduced at 1000°C. During first 15 minutes of reduction, SSA of samples reduced at 1000 and l 100°C increased with the same rate. After this time, SSA of a sample reduced at 1000°C increased faster than that of a sample reduced at 1100°C, due to more profound sintering effect at higher temperature. 0.7 0.6 0.5 ~ E: 0.4 Figure 5-2-11. Change of SSA during reduction at 1000 and 1100°C 126 The effect of the CO-CO2-2Svol%Ar gas composition on the specific surface area (SSA) in the course ofreduction at l 100°C is shown in Figure 5-2-12. i 0.3 t---/:___~7f:::::::::===~...... ,_4=:::::::::======l -~ Pco/(Pco+Pco2) u, 0.2 -+----1-----t-+------~[1.0) -o-[0.9) -tr-[0.8) 0 +----r------,-----,----.------r------i 0 15 30 45 60 75 90 time, nin Figure 5-2-12. Change in SSA during the reduction by CO-CO2-Ar gas mixtures of different compositions at 1100°C When a sample was reduced by 7Svol%CO-Ar gas mixture, the SSA curve was similar to the reduction curve. 5-minute reduction increased SSA from 0.03m2/g to 0.16m2/g, due to the crackling of the homogeneous particles (Figure S-2-10 (d)) and the fast reduction of the non-homogeneous particles (Figure 5-2-11 (a) to (c)). The SSA increased with the formation of metallic iron until 60 minutes, and slowly decreased with further increase in reaction time as a result of sintering. The final SSA decreased with increasing CO2 content in a gas, which can be linked to the extent of reduction. 127 5.2.2.6. Effect of pretreatments As shown in section 5. l, preoxidation or calcination of ironsand affected reducibility in the temperature-programmed reduction experiments. The effects of pretreatments of ironsand on isothermal reduction process are discussed in this section. Isothermal reduction of magnetite and hematite iron ores was also investigated. The experiments were carried out at 1100°C with 75vol%CO-Ar gas mixture. Figure 5-2-13 shows the reduction curves. Preoxidisation accelerated reduction rate up to 85% of the reduction. Although the initial surface area and porosity of preoxidised ironsand were much lower than those of raw ironsand (see Table 4-1 ), it was reduced faster than magnetite iron ore and raw ironsand. Its reduction rate up to 85% of the reduction was close to that ofhematite iron ore, reflecting the structural change as a result of transformation of titanohematite to titanomagnetite. However, the decaying period in the reduction curve of preoxidised ironsand came earlier than that of hematite iron ore due to the formation of ilmenite. Calcined ironsand was reduced slower than raw ironsand, from 25% to 90% of the reduction. The retardation in the reduction of the calcined ironsand is due to the lowered surface area and porosity after calcination (see Table 4-1). 128 80 :::e0 c 0 11 60 ~ i.. ~ ..0 -o-Raw C -o-Pre-oxidised !w -tr-Calcined 20 ""'"*- Hem atite -+-Magnetite 0 0 15 30 45 60 time, min Figure 5-2-13. Reduction of raw, preoxidised and calcined ironsand in comparison with hematite and magnetite iron ore, by 75vol%CO-Ar gas mixture at 1100°C The slow reduction of raw ironsand may be explained by two factors; 1) the spinel cubic structure oftitanomagnetite and 2) the stability oftitanomagnetite. The reduction of the magnetite iron ore, which also has spinel cubic structure, was slower than the reduction of rhombohedral structure minerals, the preoxidised ironsand and the hematite iron ore. The retardation of the reduction of raw ironsand in comparison with magnetite iron ore may be explained by the stability oftitanomagnetite, which was discussed in section 5.1.4. 129 5.2.3. Reduction of ironsand by hydrogen 5.2.3.1. Effect of temperature Hydrogen is a major reducing agent in gaseous DRI processes. Reduction of ironsand by hydrogen was studied using H2-Ar gas mixtures in the temperature range of 700 to 1100°C. Reduction curves are presented in Figures 5-2-14 (a) and (b). In the experimental range, iron oxides in the ironsand were reduced completely to metallic iron at temperatures above 800°C. The reduction rate increased with temperature from 700 to 1000°C; further increase in temperature to 1100°C had a slight effect. At 1000 and 1100°C, the reduction extent increased linearly with time up 80% of reduction and slowed down afterwards until the completion of the reduction. In the temperature range of 700 to 900°C, the slope of the reduction curves slowly and steady decreased, with increasing extent of reduction. 5.2.3.2. Effect of hydrogen content Effect of hydrogen content on the reduction of ironsand was studied by reaction of raw ironsand with H2-Ar gas mixture at 900°C. The hydrogen content was varied from 5 to 50vol%. The reduction curves are presented in Figure 5-2-15. As shown in Figures 5-2-15, increase in hydrogen content from 5 to 10vol% caused a sharp increase in the reduction rate. Reduction rate increased with increasing hydrogen content up to 25vol%H2; further increase to 50vol% had a relatively slight effect. The hydrogen content in further reduction experiments was 25vol%. 130 100 0~ 80 C 0 +I CJ 60 :::, "Cl ! ~700C It- 0 40 -o--BOOC .. -A-850C C s ~900C >< 20 w ---1oooc -o-1100C 0 0 10 20 30 40 50 60 time, min (a) Extent ofreduction 0.05 C ·e o.04 0) -a C 0 0.03 +I CJ :::, "Cl ~700C ! 0.02 It- -o--B00C 0 s -A-850C c,s 0.01 ~900C 0:: ---1oooc -o-1100C 0 0 10 20 30 40 50 60 time, min (b) Rate of Reduction Figure S-2-14. Reduction of ironsand by 2Svol%H2-Ar gas mixture at different temperatures 131 100 ';ft. 80 c 0 t; 60 :I 'a ...! -<>- 501101% H2 ..0 40 --o-25wl% H2 C fJ -b-20wl% H2 w 20 -15wl%H2 -i1E-10wl% H2 -o-5wl%H2 0 0 10 20 30 40 50 60 time, mln Figure 5-2-15. Reduction of ironsand by H2-Ar gas mixture with different hydrogen content at 900°C 5.2.3.3. Effect of water vapour The influence of water vapour on the reduction ofironsand was examined by adding up to 2.5vol% of steam to the inlet gas mixture at 900°C. The hydrogen content in the inlet gas was 25vol%. As shown in Figure 5-2-16, the effect of water vapour on the reduction of ironsand was negligible in the experimental range. 100 ;I. 80 c 0 tl 60 ,,:I ! 0 40 .. -<>- No water C s -o-0.5vol% H2O )( w 20 -1.ovol%H2O -1.svol% H2O -i1E-2.5vol% H2O 0 0 10 20 30 40 50 60 time, mln Figure 5-2-16. Reduction ofironsand by 25vol%H2-Ar gas mixture containing different water vapour content at 900°C 132 5.2.3.4. Progress of the reduction of ironsand by H2-Ar gas mixture Samples in the progress of reduction at 900°C by 25vol%H2-Ar gas mixture were analysed by XRD. XRD patterns are presented in Figure 5-2-17. Metallic iron was observed after 3 minutes of reduction, when the reduction extent was about 15%. Wustite peaks were not detected except a trace in the XRD pattern of a sample reduced for 5 minutes. After 20 minutes, when the reduction extent achieved 80%, the peak for metallic iron became dominant in the XRD pattern, and weak ilmenite peaks appeared. In the XRD pattern of a sample reduced for 60 minutes, when 100% of reduction of iron oxides was achieved, the peaks of ilmenite disappeared showing that ilmenite was reduced to metallic iron and titanium oxides, however, titanium oxides were not detected by XRD. In the reduction of ironsand by hydrogen, reduction oftitanomagnetite to wustite proceeded much slower than reduction of wustite to metallic iron. Wustite was reduced very rapidly, what explains the absence ofwustite peaks in the XRD patterns. The reduction of wustite to metallic iron at 900°C by hydrogen was much faster than the reduction of ironsand by carbon monoxide at 1100°C. 5.2.3.5. Morphology Formation ofwhiskers. Figures 5-2-18 (a) to (d) present SEM images of samples reduced for 60 minutes at different temperatures, from 800 to 1100°C. A sample reduced at temperature of800°C (Figure 5-2-18 (a)), exhibited iron whiskers forming the fine feather like structure. As reduction temperature increased, the whiskers disappeared, and sintering was observed. Whisker structure is generally found in reduction of dense iron ores. The whiskers are formed when iron has a high diffusivity while the number of sites where metallic iron can nucleate and grow is not sufficient. This feather-like whisker starts to form and grow after certain time of reduction, when a grain becomes supersaturated with iron due to the energy barrier for nucleation. After nucleation of iron, iron atoms in the grain move to the nucleus for its growth and eliminate the supersaturation. 133 • • Fe o TTM • FeO ® FeTi03 60min X y-Fe203 30min 0 • 20min ® 0 ~,,, 15 min 0 C 0 s 0 0 .5 0 a, ~ a, 0 ai rx: 0 3min 0 0 0 l min 0 0 0 0 0 0 0 Raw 0 0 0 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-17. XRD patterns of samples of ironsand in progress of reduction by 2Svol%H2-Ar gas mixture at 900°C 134 (b) 900°C (c) 1000°C (d) 1100°c Figure 5-2-18. Morphology of a sample reduced by 25vol%H2-Ar gas mixture at different temperatures (Reduction time: 60 minutes) 135 Whisker-structure indicates that at a relatively low temperature of 800-900°C, the rate of reduction of wustite to iron is controlled by the rate of the chemical reaction, which is the intrinsic control. Increase in temperature increases the rate of chemical reaction faster, than the diffusion coefficient. The controlling mechanism changes from the intrinsic control to the mixed control. This is related only to the wustite to iron reduction, which itself is fast in comparison with titanomagnetite to wustite reduction. Morphology change in the course ofthe reduction. The change in morphology of particles of ironsand during the reduction by 25vol%H2-Ar gas mixture at 900°C was observed by taking SEM images (Figures 5-2-19 (a) to (h) and 5-2-20 (a) to (h)). The morphology change in the course of the reduction by hydrogen was similar to that in the reduction by carbon monoxide at 1100°C (section 5.2.2.5). The difference was that fine microstructure appeared at lower reduction temperature. The reduction of the homogeneous titanomagnetite particles started in 3 minutes (Figure 5-2-19 (b )), with crackling of the particle surface. The reduction proceeded in a topochemical way (Figures 5-2-19 (c) to (f)), however, the interface between reduced and unreduced zone in a particle was not as clear as was in the reduction by CO due to high diffusivity of hydrogen (Figure 5-2-19 (f)). The whisker structure was formed during the growth of metallic iron phase (Figure 5-2-19 (g) and (h)). The most of the whisker structure disappeared with increasing reaction time due to sintering. However, the sintering effect at 900°C was relatively low in comparison with that at 1100°C. In a particle reduced for 60 minutes, when 100% reduction of iron oxides was achieved, the metallic iron phase had a fine lamellar structure. In the reduction of non-homogeneous ironsand particles, metallic iron nucleated in the vicinity oftitanohematite phase even after 1 minute of the reduction (Figure 5-2-20 (a)). The reduction oftitanohematite to metallic iron produced the feather-like lamellar structure (Figures 5-2-20 (b) to (d)). The reduction oftitanomagnetite started ~fter titanohematite reduction (Figure 5-2-20 (e)). In a titanomagnetite grain, metallic iron grew to the core of the grain forming a whisker structure. 136 (a) 1 min (b) 3 min (c) 5 min (d) 10 min (e) 15 min (g) 30 min (h) 60 min Figure 5-2-19. Morphology change of homogeneous titanomagnetite particles during the reduction by 25vol¾Hi-Ar gas mixture at 900°C 137 (a) I min (b) 3 min (c) 5 min (d) 5 min (e) 10 min (f)15min (g) 30 min (h) 60 min Figure 5-2-20. Morphology change of non-homogeneous particles during the reduction by 25vol¾H2-Ar gas mixture at 900°C 138 Most of the non-homogeneous particles were completely reduced after 15 minutes of the reduction (Figure 5-2-20 (f)). With the increasing time, the fine whisker structure in metallic iron phase was broken. However, sintering effect at 900°C was relatively small, resulting in the final microstructure with the distinguished initial phase boundary between titanohematite and titanomagnetite (Figures 5-2-20 (g) and (h)). 5.2.3.6. Surface area measurement Effect of temperature. Reduction temperature affected the specific surface area (SSA) of reduced samples. The SSA of samples reduced for 60 minutes by 25vol%Hi-Ar at different temperatures is presented in Figure 5-2-21. 10 8 ~ "'E 6 ~ u, 4 u, 2 0 700 800 900 1000 1100 Temperature, °C Figure 5-2-21. SSA of samples after 60-minute reduction by 25vol%H2-Ar gas mixture at different temperatures The SSA decreased as reduction temperature increased, due to sintering. The SSA decreased sharply between 800 and 900°C. The SSA of a sample reduced at 1100°C was 0.35m2/g, which was only about 5% of that of a sample reduced at 800°C. The decrease of SSA with temperature reflects the breakdown and sintering of the whisker structure in reduced metallic iron phase, which was observed by SEM (Figures 5-2-18 (a) to (d)). 139 Figure 5-2-22 shows the SSA of samples of ironsand in the progress ofreduction by 25vol%H2-Ar at 900°C. The SSA of the initial sample after heating to 900°C, at t = 0, was about 0.05cm2/g. 3.5 ~ 3 2.5 ~ ~ NE 2 ~- / ~ / u, 1.5 u, 1 / 0.5 / 0 0' 15 30 45 60 Time,min Figure 5-2-22. The change of SSA during the reduction of ironsand by 2Svo1%H2-Ar gas mixture at 900°C The SSA curve is similar to the reduction curve. The SSA increased with the fonnation of metallic iron, and slightly decreased with time after 30 minutes of reduction, due to sintering effect (Figure 5-2-20 (h)). 5.2.3.7. Effect of pretreatments The effect of pretreatments of ironsand was examined in reduction of the preoxidised and calcined ironsand, in comparison with magnetite and hematite iron ores, at 900°C. The reducing gas contained 25vol% hydrogen and 75vol% argon. Figure 5-2-23 presents the reduction curves. The effect of pretreatments on the reduction rate in the reduction by hydrogen was different to what was observed in the reduction by carbon monoxide (Figure 5-2-13). The pretreated and raw ironsand ores were reduced slower than the magnetite and hematite iron ores which do not contain titanium. Hematite and magnetite iron ores were reduced with similar rate up to 85% of the reduction. The reduction rate of the preoxidised ironsand was 140 higher than those of raw and calcined ironsand, as discussed earlier (section 5 .2.2.6). However, the reduction rate was only slightly affected by calcination, which retarded the reduction rate of the raw ironsand during the reduction by carbon monoxide. Although calcination decreased the SSA, it did not affected the reduction by hydrogen, in which H2 and H2O have much higher diffusion coefficient, than CO and CO2 in the reduction by CO. In the reduction of ironsand by hydrogen, the main factor affecting the reduction rate is thought to be the chemical stability of phases in ironsand. The chemical stability of titanomagnetite in ironsand makes the reduction slow especially in the beginning of the · reduction, which resulted in the slower reduction of pretreated and raw ironsand ores than magnetite and hematite iron ores. ';/e. 80 C 0 ~ CJ 60 ,,::, ! 40 -..0 --o-Raw C -o--Pre-oxidised w~ 20 ~Calcined -*--Hematite ~Magnetite 0 0 10 20 30 40 50 60 time, min Figure 5-2-23. Reduction of raw, preoxidised and calcined ironsand in comparison with hematite and magnetite iron ore, by 25vol%H2-Ar gas mixture at 900°C 141 5.2.3.8. Comparison of reduction ofironsand by H2 with reduction by CO The reduction curves of ironsand by hydrogen and by carbon monoxide at 1100°C are compared in Figures 5-2-24 (a) and (b).The reducing gas was 25vol%H2-Ar in the reduction by hydrogen, and 75vol%CO-Ar in the reduction by carbon monoxide. 100 'ifl. c 80 0 ;j ::s 60 ! 0 40 ...C -o---25wl%H2-Ar 20 +---+--£:t------1 ~ 75wl%CO-Ar w~ 0 0 10 20 30 40 50 60 time, min (a) Extent ofreduction 0.05 C ·e -o---25wl%H2-Ar c, 0.04 +----,~:.._:_-\------I c ~ 75wl%CO-Ar 0 0.03 ;j ::s i.. 0.02 0 0.01 fJ a::: 0 0 10 20 30 40 50 60 time, min (b) Rate of reduction Figure 5-2-24. The reduction curves of ironsand, by H2 and by CO at 1100°C In the reduction of ironsand by CO, the reduction extent curve showed distinct three periods; parabolic, linear and decaying periods. The decaying period started after 15 142 minutes of the reduction, when about 80% of reduction was achieved. In the reduction by hydrogen, the curve showed only two periods; linear and decaying periods. The rate of the reduction by hydrogen was much higher than that by CO, especially in the beginning of the reduction chiefly due to the high diffusivity of hydrogen (Stepheson and Smailer, 1980; EI Geassy et al., 1977; EI-Geassy and Rajakumar, 1985a; Moon et al., 1998). The decaying period in the reduction by hydrogen started after about 10 minutes, also 80% of the reduction. However, in the decaying period, the reduction rate in the reduction by CO was slightly higher than that in the reduction by hydrogen. Conventionally this is explained by high pressure built-up by CO2 at gas-solid interface, which separates reduced iron shell and unreduced core, and supplies gaseous diffusion path for further reaction (Wiberg, 1948). S.2.4. Reduction of ironsand by CU:..-H2 gas mixtures 5.2.4.1. Effect of temperature The reducing gas was composed of25% hydrogen, 5% methane and 70% argon. The experimental temperature range was from 700 to 1100°C. The effect of the temperature on the reduction of ironsand by C!Li-Hi-Ar gas mixture is shown in Figure 5-2-25. 100 -;;. 80 C 0 1; ,,:, 60 e .... 700C 0 40 --<>- .. -o-800C C --t:r-900C ~ 20 w --M-1000C ---11ooc 0 0 10 20 30 40 50 60 time,min Figure 5-2-25. Reduction of ironsand by Svol%CU:..-25vol%H2-Ar at different temperatures 143 The temperature increase from 700 to I 000°(::: increased the reduction rate and the extent of reduction. Further increase in the temperature to 1100°C had a little effect on the rate and extent of reduction. This could be due to the increase of the rate of the decomposition of methane to carbon and hydrogen, with the subsequent deposition of carbon on the surface of particles, as well as sintering. The carbon deposited would hamper the both contact and the diffusion of the reducing gases, slowing the rate of reduction, which resulted in the extension of the decaying period of the reduction at l 100°C. 5.2.4.2. Effect of methane content in Cl-4-H2-Ar gas mixtures The effect of methane content in reducing gas on the reduction of ironsand was studied by varying methane concentration from 0 to 20 vol%, with a fixed hydrogen content of 50 vol% at 900°C. Reduction curves are shown in Figure 5-2-26. 100 0~ 80 C- 0 +i c., :::, 60 -<>-0\0I%CH4 -i:, ....! -o-1.0wl%CH4 0 40 -*-5.0Wl%CH4 C -+-7.0Wl%CH4 -.! >C -o--10wl%CH4 w 20 --+-15wl%CH4 0 -20\0I%CH4 0 10 20 30 40 50 60 time, min Figure S-2-26. Reduction of ironsand by CRt-SOvol%H2-Ar gas mixtures containing different CRt content, at 900°C 144 Although, in general, there was a slight increase in the reduction rate for the samples reduced by methane containing gas mixtures over the sample reduced by hydrogen, there was no consistent relationship between the methane content and the reduction rate. Deposited carbon and methane itself can react with water vapour enhancing the rate of reduction by hydrogen. However, it can be concluded that methane in a CH.i-H2-Ar gas mixture plays a minor role in the reduction of ironsand. 5.2.4.3. Effect of hydrogen content in CH.i-H2-Ar gas mixtures The effect of hydrogen content in reducing gas on the reduction ofironsand was investigated at 900°C, by varying hydrogen content in a CH.i-H2-Ar gas mixture from Oto 80vol%, with a fixed methane content of 5.0vol%. Reduction curves obtained are shown in Figure 5-2-27. 100 0~ 80 ~ C .,0 CJ -<>-0'.A'.>l%H2 ~ 60 "C ! ~ 1CMll%H2 .... -*-2CMll%H2 0 40 .. -¼- 30'.A'.>1% H2 C s --o-4CMll%H2 >< 20 w -+-5CMll%H2 -8CMll%H2 0 0 10 20 30 40 50 60 time,min Figure 5-2-27. Reduction of ironsand by 5vol%C}1'-H2-Ar gas mixtures containing different H2 content from O to 70vol%, at 900°C As shown in the previous section, hydrogen was the major reducing agent in a CI-Li Hi-Ar gas mixture. The reduction of ironsand by CH.i-Ar and by CI-Li-H2-Ar with with only 10% ofH2 proceeded very slowly. The reduction rate increased with increasing hydrogen 145 content to 30vol%, however, further increase in hydrogen content had no significant effect on the reduction rate. 5.2.4.4. Phase transfonnation during the reduction ofironsand by CI-LJ-H2-Ar gas mixtures Effect ofmethane content. Samples reduced by CI-LJ-H2-Ar gas mixtures with different methane contents at 900°C were analysed by XRD. The hydrogen in a gas mixture was fixed to 20vol% with varying the methane content from I to 20vol%. XRD patterns of samples after 60-minute reaction are presented in Figure 5-2-28. • Fe 0 Fe3C 20vol%CH. 0 • + Carbon 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-28. XRD patterns of ironsand samples reduced by CILt-20vol%H2-Ar gas mixtures with different methane contents at 900°C, after 60-minute reaction 146 In the XRD patterns, the major phases in the final samples were metallic iron, iron carbide (Fe3C) and graphitic carbon. Phases containing titanium were not detected because the XRD pattern was quite noisy. When methane content was lvol%, there was no effect of methane on the phase composition. The carbon activity ( ac) in the gas mixture was just over unity. The XRD pattern showed only a strong peak of metallic iron. The addition of 5.0vol% methane in a reducing gas transformed metallic iron to iron carbide. With increasing methane content in a reducing gas, the intensity of metallic iron peak decreased while the number and the intensity of iron carbide peaks increased. The peaks of graphite carbon were not detected up to 10vol% addition of methane. In the range of methane addition from 5.0 to 10vol%, the most of carbon from methane decomposition is likely to be consumed by the formation of iron carbide. When methane content was 20vol%, the carbon deposition in the form of graphite became apparent. However, there was no decrease in the intensity and the number of iron carbide peaks. This showed that the formation of graphite carbon at high methane content is from methane decomposition, not from the iron carbide decomposition. The iron carbide formation under given experimental atmosphere is favourable with 5-20vol% methane in a reducing gas. Progress ofthe reduction ofironsand Samples of ironsand in the progress of reduction at 900°C by CI-Li-H2-Ar gas mixture were analysed by XRD. The reducing gas was composed of20vol% of CJ-Li, 20vol% ofH2 and 60vol% of Ar. The XRD patterns are presented in Figure 5-2-29. Similar to the reduction by hydrogen, the weak peaks ofwustite appeared in the beginning of the reduction and totally disappeared after 8-minute reduction. Metallic iron was formed even after 2 minutes of the reduction. The intensity of metallic iron peak increased and the intensity oftitanomagnetite peaks decreased with reduction time until 10 minutes, when the most of titanomagnetite phase transformed to metallic iron and ilmenite. The formation of iron carbide was detected in the XRD pattern of a sample reduced for 15 minutes. The iron carbide peaks became more predominant with reaction time, with decreasing the intensity of the peak of metallic iron. Carbon deposition was not detected 147 • Fe x y-Fe203 oTTM 0 Fe3C 60min + carbon Raw 0 0 0 0 X X 25 30 35 40 45 50 55 60 65 2 theta Figure 5-2-29. XRD patterns of samples of ironsand in progress of reduction by 20vol%C~-20vol%H2-Ar gas mixture at 900°C 148 until 30-minute reduction. In the XRD pattern of a sample reduced for 30 minutes, the appearance of graphite carbon did not affect the intensity of peaks of iron carbide. This supports the suggestion that the deposited carbon was formed by the cracking of methane rather than by iron carbide decomposition. The final sample reduced for 60 minutes was composed of metallic iron, iron carbide, deposited carbon, and titanium oxides (which were not detected by XRD). Stability ofiron carbide. In the experimental range, the decomposition of iron carbide to metallic iron and carbon was not observed. The iron carbide synthesised in the reduction of ironsand by methane-containing gas mixtures was relatively stable. Iron carbide formed by reduction of hematite iron ore decomposes to iron and carbon with the rate increasing with increasing temperature above 725°C. At 900°C, the decomposition rate is very high (Zhang and Ostrovski, 2001). It was shown in section 5.1.2.1, that the iron carbide formed during the temperature-programmed reduction ofpreoxidised ironsand by 75vol%CO-Ar gas mixture was also stable. Iron carbide phases synthesised from the raw ironsand of spine I cubic structure and from the preoxidised ironsand of rhombohedral structure showed high stability at high temperatures in the gas with high carbon activity. This phenomenon requires further examination, which is beyond a scope of this project. 5.2.5. Comparison with previous studies In this study, the reduction of the raw and the pretreated ironsand particles by gas solid reaction was investigated. As reviewed in Chapter 2, McAdam et al. (1969a and b) investigated the reduction ofironsand pellet which made of the same New Zealand ironsand, and Morozov et al. (1998) studied the reduction of the ironsand from Itrup Island by hydrogen. The reduction of ironsand by carbon monoxide at 1100°C investigated in this study was compared with McAdam et al.'s work in Figure 5-2-30. 149 80 ~0 c 0 60 1i:s ,::s ! 'o.... 40 C ~ Pre-oxidised, present work !w --o-Raw, present work 20 ··· ·· ·· Pre-Oxidised, Mc.A.dam et al. (1969a) --Raw, Mc.A.dam et al. (1969a) 0 - 0 15 30 45 60 time, min Figure 5-2-30. The reduction curves of the raw and the preoxidised ironsand at 1100°C (Reduction gas: Present study- 70vol%CO-30col%Ar, 800ml/min; McAdam et al. - lO00ml/min of CO) In McAdam et al.' s work, pellet of 1cm diameter was used. Preoxidised pellet was prepared by sintering the pellet at 1050°C for 2 hours. In both studies, the preoxidised sample reduced faster than the raw samples. However, the reduction rates in the present study were much higher than those of McAdam et al.' s work, in the experimental condition. It is because i) the gas-solid interface is much larger in the reduction of fine particles than in the reduction of pellet, and ii) the diffusion of product gas through the reduced phase is easier in the small-size particles than in a pellet. The reduction of ironsand by hydrogen at 900°C in this study was com~ared with McAdam et al.'s and Morozov et al.'s works in Figure 5-2-31. 150 80 ';/!. ~ C 0 ~ CJ 60 ::::, "O ....! 0 40 --o---Raw, present work -C s --o-Pre-oxidised, present work >< w ______, ------Pre-oxidised, McAdam et al. (1969a} 20 --Raw, McAdam et al. (1969a} -ilE--Pre-oxidised, Morozov et al. (1998} 0 .._~------~======:;::::======:::::'...l 0 10 20 30 40 50 60 time, min Figure 5-2-31. The reduction curves of the raw and the preoxidised ironsand at 900°C (Reduction gas: Present study - 25vol%H2-75col%Ar, 800ml/min; McAdam et al. - lO00ml/min ofH2; Morozov et al. -H2, Flowrate was not commented.) In McAdam et al.' s work, samples were the ironsand pellets commented above. In Morozov et al' s work, the ironsand from Itrup Island was used, and the preoxidised particles were achieved after heating the ironsand at I 000°C for 2 hours in air. The reduction curves ofMcAdam et al.'s work showed similar pattern to the present work. Both studies showed that the preoxidation of ironsand increase the reducibility. However, the reduction pattern of the ironsand from Itrup Island was very different from New Zealand ironsand. It is thought that the high content of TiO2 (about 9.85wt%) in the ironsand from Itrup Island might decrease the reduction rate. The reduction of ironsand by gaseous reduction studied in this work was compared with the carbothermic reduction investigated by McAdam et al. ( 1969b) in Figure 5-2-32. 151 100 .. . 80 ';I!. c 0 t; 60 :I l 0.. 40 C ~ Pre-oxidised, present work !w -o-Raw, present work 20 · · · · · · · Pellet [1], fv'cAdam et al. (1969b) --Pellet [2], ~Adam et al. (1969b) 0l,J------.------,------.------1 0 15 30 45 60 time, min Figure 5-2-32. Comparison of the gaseous reduction of ironsand with the carbothermic reduction at 1100°C (Present study - 70vol¾CO-30col¾Ar, 800ml/min; McAdam et al. - Pellet (1]: 200ml/min of N2, Pellet (2): 66. 7vol¾N2- 33.3vol¾CO, 300ml/min) In McAdam et al.'s work, pellet [1] contained 20wt% char and 80wt% New Zealand ironsand. Pellet [2] contained the New Zealand ironsand and was reduced in a char bed. In the carbothermic reductions of pellet [1] and [2], the reduction temperature was achieved by rapid heating of 300°C/min. The reduction rate of the pellet [1] was the highest, and that of the pellet [2] was the lowest because the ignition of the carbothermic reduction depends on the contact of reactant and oxides. Once the reduction is ignited, it precedes fast due to the carbothermic reduction involved gas-solid reaction as well as solid-solid reaction. The reduction rate of the preoxidised ironsand was slightly lower that that of the pellet [1 ]. However, for further application of ironsand to Direct Reduced Iron processes, it can be said that the preoxidation is more economical than the production of the char containing pellet, because it takes less time and process. 152 5.2.5. Summary Reduction of ironsand was examined in isothennal experiments, using CO-Ar, H2- Ar, CO-H2-Ar and Clti-H2-Ar gas mixtures. The reduction of ironsand by carbon monoxide was studied at temperatures in the range of 1000 to 1100°C. The reduction of raw ironsand by CO completed within 60 minutes. The reduction curve exhibited three periods, parabolic, linear and decaying. The addition of CO2 retarded the reduction and affected the final phase composition of a reduced sample. At 1100°C, the complete reduction of iron oxides in the ironsand was Pco l(Pco ) achieved when + Pco2 was above 0.965. Titania was reduced to ThOs and Pco l(Pco Pco ) ThO3 when + 2 = 1.0. XRD analysis and EPMA showed that ilmenite was stable in the gas with Pco l(Pco + Pco ) between 0.90 and 0.95, and ul_vospinel had quite 2 narrow stable zone around Pco l(Pco + Pco ) of 0.875. In the reduction process by 2 75vol%CO-Ar gas mixture at l 100°C, the reduction proceeded in the following sequence: Fe 3_"Tt,,O4 -+ FeO +Fe+ Fe 3_"_6 Tin6 O4 -+Fe+ Fe3_"_6 Tin6 O4 -+ Fe + FeTiO3 -+ Fe+ TiO2 -+ Fe+ Ti 3O5 -+Fe+ Ti2O3 The reduction oftitanomagnetite to wustite was the slowest step. Wustite phase transformed to metallic iron very quickly. In a sample after the complete reduction ofiron oxides, titanium and other impurities were finely distributed in the reduced metallic iron phase. The SEM images showed that the homogeneous titanomagnetite particles reduced in a topochemical way. The non-homogeneous particles with Iamellar structure reduced faster than the homogeneous particles. The reduction of particles having a lame liar structure started with titanohematite phase and the structural change during the reduction of titanohematite to titanomagnetite accelerated further reduction. The reduction of ironsand by hydrogen was examined in the temperature range of 700 to 1100°C. The initial stage of reduction by hydrogen was much faster than that by carbon monoxide. In the reduction curves obtained using hydrogen, the parabolic shape of initial stage observed in the reduction by carbon monoxide disappeared. In the reduction at 153 900°C by 25vol%H2-Ar gas mixture, the complete reduction was achieved within 60 minutes. The fast reduction of wustite to metallic iron formed the whisker structure. When ironsand was reduced by methane-containing gas mixture at 900°C, the reduction product included iron carbide. The decomposition of iron carbide to metallic iron and carbon was not observed. Preoxidation increased the reduction rate in the both reductions by carbon monoxide and by hydrogen. Calcination retarded the reduction of ironsand by carbon monoxide, however, it had no effect on the reduction by hydrogen. In the reduction of preoxidised ironsand by CO-Ar gas mixture at 450-850°C, iron carbide was formed (section 5.1.2.1). Cementite formed by reduction ofironsand using methane-containing gas mixture or CO was stable in comparison with cementite formed in the reduction of hematite. For further application of ironsand to DRI process, the gaseous reduction of the preoxidation of ironsand is more economical than the carbothermic reduction. 154 Chapter 6. Conclusions and Future Work 6.1. Characterisation of ironsand Characterisation of raw and pretreated ironsand using XRD, EPMA, SEM, optical microscope and BET analysis showed the following. 1. The raw ironsand is mainly composed of homogeneous titanomagnetite particles. The titanomagnetite in the raw ironsand can be presented as (Fe 3O4 )._x(Fe 2TiO4t, with the x value of 0.27±0.2. 2. Small portion of particles in the raw ironsand showed exsolution between titanomagnetite and titanohematite. The exsolution is caused by the partial oxidation of titanomagnetite to titanohematite. During the exsolution, the migrations of cations 2 3 2 with high mobility (Fe +, Fe + and Mg l out oftitanomagnetite lattice resulted in the enrichment of titanium in the newly formed phase oftitanohematite. The phase separation due to the miscibility gap between magnetite and ulvospinel was not found. 3. In preoxidation, titanomagnetite is transformed to titanohematite. 4. In non-isothermal preoxidation at a rate of200°C/hr, the titanomagnetite started to oxidise above 600°C. Below 800°C, titanomagnetite was oxidised to titanium containing cubic maghemite phase, and then to titanohematite. At high temperatures above 1000°C, titanomagnetite was oxidised to titanohematite directly. 5. Oxidation oftitanohematite to pseudobrookite and hematite was not completed in non-isothermal oxidation. 6. Titanomagnetite is stable against oxidising atmosphere due to high-valency cations that retarded migration of iron and titanium. The intermediate phase in the oxidation of titanomagnetite, titanium-containing maghemite also showed high stability in oxidising atmosphere. 7. During the isothermal preoxidation of the ironsand at 1000°C, pseudobrookite was formed in the core of the titanohematite phase with decrease in the Feffi ratio of surrounding titanohematite. The formation of pseudobrookite can be described by 155 reaction; . 1 ( 2x - 38) ( 1- x ) . 2 ( 2x - 38) . Fe 2 T1 2 0 3 +- --- 0 2 = 2 -- Fe 2_6 Ti 6 0 3 +- --- Fe 2 Ti05 2--x -x 3 3 6 2-38 2-38 3 2-38 2 (0~8~-x) 3 8. During preoxidation, magnesium migrated from Ti-rich phase and followed the direction of iron. The migration of aluminium was negligible, compared to those of magnesium and titanium. The distribution of aluminium and magnesium depended on their mobilities. The formation of minor oxide phases composed of magnesium or aluminium was not detected. 9. Calcination of ironsand did not make a significant change in the phase composition except the structural transformation of titanium-containing maghemite phase to rhombohedral titanohematite. The change in the elemental distribution after calcination was negligible. 10. The pretreatments reduced the specific surface area of the raw ironsand ore dramatically to 4% of its value for the raw ironsand. 6.2. Reduction of ironsand The reduction of ironsand in temperature-programmed and isothermal experiments and XRD, EPMA, SEM, optical microscope and BET analyses showed the followings. Reduction ofironsand in temperature-programmed experiments l . The reduction of raw ironsand started at 800°C and completed at 1100°C. Titanomagnetite was reduced to iron and titanium oxide through wustite ("FeO"), ulvospinel (Fe2 TiO4) and ilmenite (FeTiO3). The intermediate wustite and ulvospinel phases were reduced very quickly. Titanium oxide was formed in the amorphous form. 2. The overall reaction of the reduction ofraw ironsand was, 156 3. Preoxidised ironsand was reduced in four stages. The first was the reduction of titanohematite (Fe2.yTiyO3) to titanomagnetite (Fe3.xTixO4) in the temperature range of 400-550°C, which followed by the reduction oftitanomagnetite to iron and ilmenite in the second reduction stage in the temperature range of 450-600°C. In this stage, pseudobrookite was reduced. In the third stage, in the range of 600-850°C, titanomagnetite was reduced to metallic iron and rutile. In the final stage, ilmenite and remaining titanomagnetite were reduced to metallic iron and rutile. Reduced iron was carburised by carbon monoxide with the formation of cementite. 4. The increase of reducibility after preoxidation of raw ironsand is due to the structural transformation of spinet cubic titanomagnetite to rhombohedral titanohematitie during the oxidation. In the reduction of preoxidised ironsand, the volume increase during the transformation of titanohematite to titanomagnetite accelerates and facilitates further reduction reactions. 5. During the reduction of the preoxidised ironsand, iron was transformed to iron carbide. Cementite formed from preoxidised ironsand showed high stability under the reducing atmosphere. The decomposition of the iron carbide into iron and carbon was not detected even after heating up to 1100°C. 6. Calcination did not change the reduction pattern of the raw ironsand. However, the reduction was retarded and started at 970°C. 7. Reduction of raw ironsand is different from that of magnetite iron ore, although both minerals have the same crystal structure. 8. Titanium in titanomagnetite stabilises the spinet structure and changes the 2 4 thermodynamics of reduction. The substitution of Fe + with Ti + decreases the activity 3 2 ofFe + and increases Fe + activity in the lattice, resulting in the retardation of the 2 reduction of Fe3+ to F e + and acceleration of the reduction of wustite to metallic iron. Isothermal reduction ofironsand I. The reduction of ironsand by carbon monoxide was examined at I 000-1100°C. The reduction ofraw ironsand by CO was completed in 60 minutes in the experimental temperature range. The reduction curve exhibited three periods, parabolic, linear and 157 decaying period. The parabolic shape in the initial stage of the reduction curves was due to the slow reduction of titanomagnetite to wustite. 2. The addition of CO2 affected the final phase composition of a reduced sample. At 1100°C, the complete reduction of iron oxides in the ironsand was achieved when Pco l(Pco + Pco ) was above 0.965, however, even small addition of carbon dioxide 2 decreased the reduction rate significantly. In the reduction by CO-Ar gas mixture, titania (TiO2) was reduced to ThOs and ThO3. Ilmenite stable zone was in the range of Pco l(Pco + Pco ) between 0.90 and 0.95. The stable zone of ulvospinel was quite 2 narrow. It was stable only when Pco l(Pco + Pco ) was near 0.875. 2 3. In the reduction process by 75vol%CO-Ar gas mixture at 1100°C, the reduction proceeded in the following sequence: Fe 3_xT(~04 --+" FeO'+Fe + Fe3_x_6 Tix+6 0 4 --+Fe+ Fe 3_x_6 Tix+ 6 0 4 --+ Fe + FeTi03 --+ Fe+ Ti02 --+ Fe+ Ti30 5 --+ Fe+ Ti20 3 (x= 0.27±0.2; o, degree ofreaction, 0 St5S(J-x)). 4. The reduction oftitanomagnetite to wustite was the slowest step. Wustite transformed to metallic iron very quickly. 5. Preoxidation increased the reducibility of ironsand in the reduction by CO. In the reduction by 75vol%CO-Ar gas mixture at 1100°C, it was reduced as fast as hematite iron ore. However, calcination of ironsand decreased the reduction rate. 6. The reduction of ironsand by hydrogen was examined in the temperature range of 700 to 1100°C. The reduction rate in the initial stage of the reduction was much higher than that by carbon monoxide. 7. The addition ofup to 2.5vol% water vapour to 25vol%H2-Ar gas mixture at 900°C did not affect the reduction rate. 8. In the reduction of ironsand by hydrogen at 700-1100°C, the whisker structure was observed. 9. In the reduction by hydrogen, hematite and magnetite iron ores were reduced with a similar reduction rate, raw and preoxidised ironsand reduced slower than the both iron ores. However, preoxidation increased the reduction rate of raw ironsand. Calcination did not affect the reduction rate of the raw ironsand. 158 I 0. In the reduction processes by carbon monoxide and hydrogen, the two different kinds of particles in raw ironsand were reduced in different ways. The homogeneous titanomagnetite particles were reduced in a topochemical way. The non-homogeneous particles were reduced faster than the homogeneous particles due to the fast reduction of titanohematite. The structural change during the reduction of titanohematite to titanomagnetite accelerated further reduction. 11. In the reduction of ironsand by methane-containing gas mixture at 900°e, metallic iron was transformed to iron carbide. The iron carbide showed high stability. 12. The slow reduction of raw ironsand in comparison with hematite and magnetite iron ores is due to two factors; 1) the spinel cubic structure oftitanomagnetite and 2) the stability of titanomagnetite. 13. For further application of ironsand to DRI process, the gaseous reduction of the preoxidation of ironsand is more economical than the carbothermic reduction. 159 6.3. Recommendations for future work The project established fundamentals of reduction of ironsand with focus on structure of ironsand and its transformation during reduction. Further research is recommended towards practical application of finding of this project, particular reduction behaviour of ironsand in shaft furnace and fluid bed reactor. Results on the reduction of ironsand by CO-H2 gas mixture were inconsistent and not included into the Thesis. The use ofCO-H2 reducing gas should be investigated further. Future work may also be recommended to study: • the effect of water vapour on the reduction of ironsand by hydrogen beyond the 2.5vol%, which was a limit in this project; • the role of titanium as well as other cation impurities, in the iron carbide formation and decomposition; • and to develop mathematical model of the reduction of ironsand. 160 References Aharoni, A.; Frei, E. H.; Schieber, M. (1962), Some Properties of y-Fe2O3 Obtained by Hydrogen Reduction of a.-Fe2O3., J. Phys. Chem. Solids, 23, p545-554. Akimoto T.; Kinoshita H.; Furuta T. (1984), Electron Probe Microanalysis Study on Processes of Low-temperature Oxidation ofTitanomagnetite., Earth & Planetary Science Letters, 71 (2), p263-278. Akimoto, S. ( 1954 ), Thermo-magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks, J. Geomag. Geoelect., 6, pl-14. Anderson, D. J.; Lindsley, D. H. (1988), Internally Consistent Solution Models for Fe-Mg-Mn-Ti Oxides: Fe-Ti Oxides, Am. Mineralogist, 73, p714-726. Barrett, D (1973), the Redction of Iron Ore by Natural Gas and the Production of Synthesis Gas. Australian Chemical Engineering (March), p 17-21. Bernal, J. D.; D~gupta, D.R.; Mackay, A. L. (1959), The oxides and Hydroxides of Iron and Their Structural Interrelationships, Clay Min. Bull., 4, p15-30. BHP Steel New Zealand (2000), www.bhpsteel.com. Bleil, U. (1971), Cation Distribution in Titanomagnetites, Z Geophys., 37, p305-319. Bleil, U. (1976), An Experimental Study of the Titanomagnetite Solid Solution Series, Pure and App. Geophys., 114, p 165-175. Buddington, A. F.; Lindsley, D. H. (1964), Iron Titanium Oxide Minerals and Synthetic Equivalents, J. Petrol., 5, p310-357. Chevallier, R.; Bolfa, J.; Mathieu, S. (1955), Titanomagnetites et Ilmenites Ferromagnetiques, Bull. Soc. Franc. Min. Crist., 78, p307-346. 161 Dancy, T. E. (1993), Development of Direct Reduction Processes, Scan. J. Metal/., 22 (3), pl00-108. Davis, C. G.; Mcfarlin, J. F.; Pratt, H. R. (1982), Direct-Reduction Technology and Economics, Ironmaking and Steelmaking, 9 (3), p93-129. Edstrom, J. 0. (1953), The Mechanism of Reduction oflron Oxides, JISI, 115, p289- 304. Egger, K.; Feitknecht, W. (1962), Uber die Oxudation von Fe3O4 zu y- and cx.-Fe2O3. Die Differenzthermoanalytische (OTA) and Thermogravimetrische (TO) Verfolgung des Reakitonsabulaufes an Kunstlichen Formen von Fe3O4. Helv. Chim. Acta, 45, p2042-2057. El-Geassy, A. A.; Rajakumar, V. (1985a), Gaseous Reduction of Wustite with H2, CO and Hi-CO Mixtures, ISIJ, 25, p449-458. El-Geassy, A. A.; Rajakumar, V. (1985b), Influence of Particle Size on the Gaseous Reduction of Wustite at 900 - 1100°C, ISIJ, 25, p 1202-1211. El-Geassy, A. A.; Shehata, K. A.; Ezz, S. Y. (1977), Mechanism of Iron Oxide Reduction with Hydrogen/Carbon Monoxide Mixtures, ISIJ, 11, p629-635. El-Tawil, S. Z.; Morsi, I. M.; Francis, A. A.(1993), Kinetics of Solid-State Reduction of Ilmenite Ore, Can. Metal/. Q., 32 (4), p281-288. Feitknecht, W. (1964), Einflu~ der Teilchengro~e auf den Mechanismus von Festk.orperreaktionen, Rev. Pure App. Chem., p423-440. Fihey, J. L.; Ajersch, F.; Dancy, E. A.(1979), Reduction Kinetics of Porous Pellets of Iron-Titanium Oxides, Can. Metal/. Q., 18, p419-434. 162 Gallagher, K. J.; Feitknecht, W.; Mannweiler, U. (1968), Mechanism of Oxidation of Magnetite to y-Fe2O3, Nature, 217, pl 118-1121. Ghosh, D.; Roy, A. K.; Ghosh, A. (1986), Reduction of Ferric Oxide Pellets with Methane, ISIJ, 26, p 186-193. Gillot, B.; Rousset, A.; Dupre, G. (1978), Influence of Crystallite Size on the Oxidation Kinetics of Magnetite, J. Solid State Chem., 25, p263-271. Grabke, H.J.; Muller-Lorenz, E.M.; Schneider, A. (2001) Carburization and metal dusting on iron, ISIJ, 41, pS 1-S8. Haggerty, S. E. (1991), Oxide Textures-A Mini-atlas, Rev. Mineral., 25, pl29-219. Hamdeh, H. H.; Barghout, K.; Ho, J.C.; Shand, P. M.; Miller, L. L. (1999), A Mossbauer Evaluation of Cation Distribution in Titanomagnetites, J. Magnetism and Ma~etic Mat., 191, p72-78. Hill, R. L.; Sack, R. 0. (1987), Thermodynamic Properties of Fe-Mg Titaniferrous Magnetite Spinels, Can. Mineralogist, 25 (3), p443-464. Hutchings, K. M.; Hawkins, R. J.; Smith, J. D. (1988), Use of Methane in a Fluidized Bed during Reduction oflron Ore, Ironmaking and Steelmaking, 15, p121-126. Itoh, S.; Inoue, O.; Azakami, T. (1998), Phase Relations and Equilibrium Oxygen Partial Pressures in Iron-Titanium-Oxygen System at 1373 K, Mat. Trans, JIM, 39 (3), p391-398. Jones, D. G. (1973), Reduction Sequences in the Reduction of Ilmenite: 2 - Gaseous Reduction by Carbon Monoxide, Trans. Inst. Min. Metall. C, 82, p 186-192. Jones, D. G. (1974), Optical Microscopy and Electron-probe Microanalysis Study of Ilmenite Reduction, Trans. Inst. Min. Metall. C, 83, pl-9. 163 Katsura, T.; Kitayama, K.; Aoyagi, R.; Sasajima, S. (1976), High Temperature Experiments Related to Fe-Ti Oxide Minerals in Volcanic Rocks, Kazan (Volcanos}, 1, p31-56. Katsura, T.; Wakihara, M.; Hara, S. I.; Sugihara, T. (1975), Some Thermodynamic Properties in Spinel Solid Solutions with the Fe304 Component, J. Solid State Chem., 13, pl07-l 13. Kawai, N. (1956), Exsolution ofTitanomagnetites and Its Effect on Rock-Magnetism III, Proceedings ofthe Japanese Academy, 32, p464-468. Lindsley, D. H. (1962), Investigations in the System FeO-Fe203-Ti02, Carnegie Inst. Wash. Yearb., 61, pl00-106. • Lindsley, D. H. ( 1981 ), Some Experiments Pertaining to the Magnetite-Ulvospinel Miscibility Gap, Am. Mineralogist, 66, p759-762. Lu, W. (1999), Direct Reduced Iron, Technology and Economics ofProduction and Use, Edited by J. Feinman and D. R. MacRae, The Iron & Steel Society, Warrendale, PA., p43-58. Marshall, T. (1970), The Development ofTitaniferous Ironsand Smelting, New ZealandJ. Sci., 13, p3-17. McAdam, G. D. (1974), Instability of Titanium-rich lronsand in Reducing Gases, Ironmaking & Steelmaking, 1 (3), p138-150. 164 McAdam, G. D.; Dall, R. E. A.; Marshall, T. (1969a), Direct Reduction of New Zealand Ironsand Concentrates, Part I-Gaseous Reduction, New Zealand J. Sci., 12, p649-668. McAdam, G. D.; Dall, R. E. A.; Marshall, T. (1969b), Direct Reduction of New Zealand Ironsand Concentrates, Part 2-Soild Reductants, New Zealand J. Sci., 12, p669-686. McAdam, G. D.; O'Brien, D. J.; Marshall, T. (1977), Rapid Reduction of New Zealand Ironsands, Ironmaking & Steelmaking, 4 (1), pl-9 Merk, R.; Pickles, C. A. (1988), Reduction of Ilmenite by Carbon Monoxide, Can. Metal!. Q., 27, pl 79-185. Mogensen, F. (1946), A Ferro-Orthotitanate Ore from Sodra Ulvon, Geologisk Forening Stockholm Forhandlinger, 68, p578-588. Monsen, B. D.; Olsen, S. E.; Kolbeinsen, L. (1994) Kinetics of magnetite oxidation, Scand. J. Metal!., 23, p74-80. Moon, I. J.; Rhee, C. H.; Min, D. J. (1998), Reduction ofHematite Compacts by H2- CO Gas Mixtures, Steel Research, 69 (8), p302-306. Moore, J. J. (1982), An Examination of the New Direct Smelting Processes for Iron and Steelmaking, JOM (June), p39-48. Morozov, A. A.; Remichenko, V. A.; Sinadskii, A. Y.; Karyazin, I. A. (1998), Metallurgical Properties of Titanomagnetite Concentrate from the Kuril' sk Islands and Its Electromelted Slags, Russ. Metal!. (Metally), pl-3. Neel, L. (1955), Some Theoretical Aspects of Rock Magnetism, Adv. Phys., 4, pl91- 243. 165 O'Reilly, W.; Banerjee, S. K. (1965), Cation Distribution in Titanomagnetites (1- x)Fe3O4-xFe2 TiO4, Phys. Let., 17, p237-238. Ohno, A.; Ross, H. U. (1963), Optimum Slag Composition for the Blast Furnace Smelting ofTitaniferous Ores, Can. Metal/. Q., 2, p259-279. Ostberg, G (1960), Solid State Reduction of Ilmenite, Jernkont. Ann., 144, p46-76. Palmer, C. (1909), Arizonite, Ferric Metatitanate. Am. J Sci., 4, p28. Pelton, A. D.; Bale, C. W. (1999), Direct Reduced Iron, Technology and Economics ofProduction and Use, Edited by J. Feinman and D.R. MacRae, The Iron & Steel Society, Warrendale, PA., p25-42. Poggi, D.; Charette, G. G.; Rigaud, M. (1972), Reduction of Ilmenite and Ilmenite Ores, Titanium Sci. Tech., 1, p247-259. Price, G. D. (1981), Subsolidus Phase Relations in the Titanomagnetite Solid Solution Series, Am. Mineralogist, 66, p75 l-758. Rao, Y. K. (1979), Mechanism and the Intrinsic Rate of Reduction of Metallic Oxides, Metal/. Trans. B, l0B, p243-255. Rao, Y. K. (1985), Stoichiometry and Thermodynamics ofMetallurgical Processes, Cambridge University Press, 1985. Sadykhov, G. B.; Naumova, L. O.; Rezinchenko, V. A. (1992), Phase Transformations in the Reduction of Titanomagnetite by Hydrogen, Russ. Metal/. (Metally), plS-24. Schmahl, N. G.; Frisch, B.; Hargarter, E. (1960), Phase relations in the Fe-Ti-O system at 1000°C, Z. Anorg. Al/gem. Chem., 305, p40-54. 166 Sidhu, P. S. (1988), Transformation of Trace Element-Substituted Magnetite to Hematite, Clays Clay Min., 36, p31-38. Sidhu, P. S.; Gilkes, R. J.; Posner, A. M. (1980), The Behaviour of Co, Ni, Zn, Cu, Mn and Cr in Magnetite during Alternation to Maghemite and Hematite, Soil Sci. Soc. Am. J., 44, p135-138. Stephenson, A. (1969), The Temperature Dependent Cation Distribution in Titanomagnetites, Geophys. J. R. Astr. Soc., 18, p199-210. Stephenson, R. L.; Smailer, R. M. (1980), Direct Reduced Iron, The Iron & Steel Society of AIME, NY. Szekely, J.; Trapaga, G. (1994), Some Perspectives on New Steelmaking Technologies, Metal/. Plant and Tech. lnt., 4, p30-94. Taylor, R. W. (1964), Phase Equilibria in the System FeO-Fe2O3-TiO2 at 1300°C, Am. Mineralogist, 49, p1016-1030. Teoh, L. L. (1985), Direct Reduction-Expanding Market, Iron and Steel lnt., 58, p16-21. Trestmann-Matts, A.; Dorris, S. E.; Kumarakrishnan, S.; Mason, T. 0. (1983), Thermoelectric Determination of Cation Distributions in Fe3O4-Fe2TiO4, J. Am. Ceram. Soc., 66, p829-834. Trone, E.; Jolivet, J.P.; Livage, J. (1990), Mossbauer Investigation of they-~ cx Fe2O3 Transformation in Small Particles, Hyperfine Interactions, 54, p737-740. Turnock, A. C.; Eugster, H. P. Fe-Al oxides: phase relationships below 1000°C, J. Petrol., 3, p533-65. 167 Vijay, P. L.; Venugopalan, R.; Sathiyamoorthy, D. (1996), Preoxidation and Hydrogen Reduction of Ilmenite in a Fluidiz.ed Bed Reactor, Metal/. Mater. Trans. B. 27B(5), p731-738. Vincent, E. A. (1960), Ulvospinel in the Skaergaard intrusion, Greenland, Neues Jahrb. Mineral., 94, p993-1016. Vincent, E. A.; Wright, J. B.; Chevallier, R.; Mathieu, S. (1957), Heating Experiments on Some Natural Titaniferous Magnetites, Mineral. Mag., 31, p624-65 5. Webster, A. H.; Bright, N. F. H. (1961), The System Iron-Titanium-Oxygen at 1200°C and Oxygen Partial Pressure Between 1 Atmosphere and 2 x 10·14 Atmospheres, J. Am. Ceram. Soc., 44, pl 10-116. Wechsler, B. A.; Lindsley, D. H.; Prewitt, C. T. (1984), Crystal Structure and Cation Distribution in Titanomagnetites (Fe3_xTixO4), Am. Mineralogist, 69, p754-770. Wen, C. Y. (1968), Noncatalytic Heterogeneous Solid-Fluid Reaction Model, Ind Eng. Chem., 60, p34-54. Wiberg, M. (1948), Discussion ofthe Faraday Society, 4, p231-233. Woodland, A. B.; Wood, B. J. (1994), Fe3O4 Activities in Fe-Ti Spinel Solid Solutions, Eur. J. Mineralogy, 6 (1), p23-37. Wright, J.B.; Lovering, J. F. (1966), Electron-Probe Microanalysis of the Iron Titanium Oxides in Some New Zealand lronsands, Mineral Mag., 35, p604-621. Wright, J.B. (1964), Iron-Titanium Oxides in Some New Zealand lronsands, New Zealand J. Geol. Geophys., 7, p424-444. Wu, C. C.; Mason, T. 0. (1981), Thermopower Measurement of Cation Distribution in Magnetite, J. Am. Ceram. Soc., 64, p520-522. 168 Zervas, T.; McMullan, J. T.; Williams, B. C. (1996a), Developments in Iron and Steel Making, lnt. J. Ener. Res., 20, p69-9 l. Zervas, T.; McMullan, J. T.; Williams, B. C. (1996b), Gas-Based Direct Reduction Processes for Iron and Steel Production, Jnt. J. Ener. Res., 20, p157-185. Zhang, G. (2000), Reduction ofRutile and Ilmenite by Methane-Hydrogen Gas Mixture, Ph.D thesis in Univ. New South Wales. Zhang, G; Ostrovski, 0. (2000), Reduction of Titania by Methane-Hydrogen-Argon Gas Mixture, Metal/. Mater. Trans. B, 31B, p129-139. Zhang, J.; Ostrovski, 0. (2001), Cementite Formation in CH4-H2-Ar Gas Mixture and Cementite Stability, ISIJ, 41, p333-339. 169