The University of New South Wales
Faculty of Science
School of Materials Science and Engineering
Fundamental Investigation of Kinetics of Ferro-Silicon Reactions in Cupola Scrap Melting Processes
A Thesis in
Materials Science and Engineering
By
Pedro Javier Yunes Rubio
Submitted in Partial Fulfillment
of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
March 2013 To my family and wife 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 person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due to acknowledgment 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 others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.
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Pedro Javier Yunes Rubio COPYRIGHT STATEMENT
I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provision of the Copyright Act 1968.
I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation.
I also authorize University Microfilms to use the 350 word abstract of my thesis in
Dissertation Abstract International (this is applicable to doctoral theses only).
I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied for a partial restriction of the digital copy of my thesis or dissertation.
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AUTHENTICITY STATEMENT
‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation has occurred and if there are any minor variations in formatting, they are the result of conversion to digital formats.’
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iii ACKNOWLEDGEMENTS
I would like to express my deepest sense of gratitude and appreciation to my supervisor Professor Veena Sahajwalla for her unrelenting support and guidance throughout my research and her willingness and availability to discuss several aspects of the project. Her assertiveness and appropriate guidance as well as openness for constructive discussions were very encouraging and intellectually stimulating.
I also owe an enormous debt of gratitude to Associate Professor Rita Khanna for her constructive discussions, her guidance and continuous encouragement and support to get the work completed.
I wish to thanks the support from Russell Bush, Technology Manager from Tyco Water for his contribution and technical expertise and his valuable suggestions.
I also wish to acknowledge the financial support and interest from the Australian Research Council.
I am extremely grateful to Mr. N. Saha - Chaudhury for his great help in making things happen in our daily laboratory work. His opportune advice and support in my experimental work is highly appreciated.
I would like to sincerely thank to Professor Chris Sorrell and Dr. Haiping Sun for their valuable and constructive coaching during my research work.
And last but not least, I wish to thank to my family for their constant encouragement and inspiration throughout the course of my project. Very special thanks to my wife Marcela for her support and inspiration to achieve this result.
iv LIST OF PUBLICATIONS
Journal Papers
1- Yunes, P. J., Hong, L., Saha-Chaudhury, N., Bush, R. and Sahajwalla, V. Dynamic Wetting of Graphite and SiC by Ferrosilicon Alloys and Silicon at 1550 °C. ISIJ International, 2006. 46 (11): p. 2006.
2- Yunes, P. J., Saha-Chaudhury, N. and Sahajwalla V. Carbon Dissolution Occurring during Graphite-Ferrosilicon Interactions at 1550 °C. 2009. 49 (12) p. 1868.
3- Yunes, P. J., Khanna, R., Saha-Chaudhury, N. and Sahajwalla, V.Simultaneous Decarburisation and Oxidation Reactions occurring in Silicon and Ferrosilicon alloys at 1823 K. Steel Research International, 2013. 84 (1) p. 40.
Conference Paper
4- Yunes, P. J. and Sahajwalla, V. Kinetics of Carbon Dissolution in Ferrosilicon Alloys, International Symposium of Research Students on Materials Science and Engineering, Chennai, India, December 20-22, 2004.
v ABSTRACT
This work investigates high temperature interactions of silicon and ferrosilicon alloys with graphite as well as the reactions occurring in the presence of oxidising gases. These reactions play a key role in the scrap-melting cupola process. Using the sessile droplet method, the dynamic wetting of synthetic graphite by liquid ferrosilicon alloys containing 24.7 and 74 %
Si and silicon (98.5 % Si) at 1550 °C was investigated. Silicon 98.5% and ferrosilicon alloys containing 74 and 24.7% Si showed good wetting behaviour (θ < 90º) with synthetic graphite at 1550 °C. Full wetting was observed for silicon 98.5 and ferrosilicon 74 within the first 90 seconds. However, the final contact angle value appeared higher for the low-silicon ferroalloy and remained steady around 70 degrees during the 2 hours-run. The role of the interfacial product formed and its relationship with the dynamic wetting phenomena was also investigated. The formation of SiC at the interface appeared 30 seconds after melting for Si
98.5, while this was observed after 60 seconds for FeSi 74, and after 30 minutes for FeSi
24.7. Further wettability investigations carried out on SiC substrates showed trends similar to the ones observed on synthetic graphite. Full wetting was observed for Si 98.5 and FeSi 74 after 80 and 90 seconds, while FeSi 24.7 showed a different pattern, since the contact angle decreased rapidly during the first 10 minutes and remained steady around 40 degrees after that time. The dynamic wetting appeared to be strongly dependent on the rate of formation of
SiC at the ferrosilicon-graphite interface.
A kinetic mechanism has been developed for the carbon dissolution phenomena in ferrosilicon alloys. The overall rate constants at 1550 °C for Si 98.5, FeSi 74 and FeSi 24.7 were determined to be 3.8, 3 and 3.9 x 10-3 (s-1) respectively. These did not vary significantly across samples under investigation. A rapid increase of carbon pickup was observed during the initial few minutes and remained fairly constant later on. The faster rate observed in the
vi case of FeSi 24.7 was explained on the basis of delayed formation of SiC interfacial product which had a retarding effect on the overall process and dictated carbon transfer.
In depth and detailed investigations were carried out on the effect of the alloy composition, oxygen partial pressure and flow rate on interactions at 1550 °C . Significant differences were observed in the weight gain and carbon loss between these three alloys; both decarburisation and silicon oxidation reactions were found to occur simultaneously. There was a clear evidence for two rate regimes: the rate of decarburisation was found to be much higher during the initial 2 minutes and a much slower rate was observed in later stages for all specimens. These rate regimes were explained in terms of the extent of surface coverage with the reaction product silica. No significant effect was found on the decarburization rates when the proportion of oxidizing gas (CO2) was increased from 20 to 100%, indicating that mass transfer in the gas phase was not a dominant rate controlling step compared to chemical kinetics. The net weight gain in these alloys was found to be due to the combined influence of decarburization (weight loss due to the generation of a gaseous product) and silicon oxidation
(weight gain due to silica formation on the sample surface). The results of this investigation showed that the silicon losses in the cupola process can be better managed by using lower grade ferrosilicon alloys as well as an adequate air blowing regime.
vii TABLE OF CONTENTS
PAGE
Certificate of Originality ii
Copyright Statement iii
Acknowledgments iv
List of Publications v
Abstracts vi
Table of Contents viii
List of Figures xii
List of Tables xviii
Chapter 1 Introduction 1
1. Introduction 2
1.2 Scope of the project 4
Chapter 2 Literature Review 6
2. Literature Review 7
2.1 Outline of the Cupola Furnace 7 2.1.1 Zones of the Cupola Furnace 8 2.2 Study of the phase diagrams. 10 2.2.1 Phase diagram Fe – Si system. 10 2.2.2 Phase diagram Si – C system. 11 2.2.3 Fe-Si-C ternary diagram. 13 2.3 Reaction kinetics 15
2.4 Chemical reaction and Mass Transfer 17
2.4.1 Previous studies of carbon dissolution in iron. 18 2.4.2 Previous studies of Carbon dissolution in silicon and 21 ferrosilicon
viii 2.5 Kinetics of oxidation 24
2.5.1 Previous studies of decarburization in iron. 25
2.5.2 Previous studies of decarburization in silicon and 32 Ferrosilicon. 2.6 Wettability 38
2.6.1 The system of liquid iron and graphite. 41
2.6.2 Wettability of liquid iron on alumina 44
2.6.3 Wettability of silicon and ferrosilicon on SiC 45
2.7 Summary of the literature review. 46
Chapter 3 Experimental Details 50
3. 1 Sample selection 51
3.1.1 Silicon and ferrosilicon alloys 52
3.1.2 Synthetic graphite (SG) 54
3.1.3 Natural graphite (NG) 57
3.1.4 Silicon carbide (SiC) 57
3.1.5 Alumina 59
3.1.6 Gas composition and flow rate 60
3.2 Experimental equipment and instruments used 60
3.2.1 High Temperature Horizontal Furnace 60
3.2.2 Off-gas Analysis 63
3.2.3 Image Analysis and Contact Angle Measurements 64
3.2.4 Carbon and Sulphur Analiser 65
3.2.5 X-ray diffraction (XRD) 65
ix 3.2.6 Electro Probe Micro Analiser (EPMA) 66
3.3 Sample preparation 69
3.3.1 Sample preparation for carbon analysis 69
3.3.2 Preparation of Natural Graphite Substrate 69
3.3.3 Sample preparation for XRD 70
3.3.4 Sample preparation for EPMA 71
Chapter 4 Dynamic Wetting between molten silicon and ferrosilicon 72 alloys and different substrates. Experimental Results and Discussion.
4.1 Study of the dynamic wetting for silicon and ferrosilicon alloys on 73 synthetic graphite
77 4.2 Investigation of the product formed during the interfacial reaction
4.3 Study of the dynamic wetting in SiC substrates 82
4.4 Interdependence of dynamic wetting and SiC formation at the 88 interface
4.5 Summary 89
Chapter 5 Experimental Investigation on Carbon Dissolution in Silicon 90 and Ferrosilicon Alloys. Results and Discussion.
5.1 Carbon dissolution for silicon and ferrosilicon alloys 91
5.2 Carbon dissolution from synthetic graphite substrates 94
5.3 Carbon dissolution from SiC substrates into ferrosilicon and silicon 98 at 1550 °C
5.4 Interfacial role of SiC during carbon dissolution 100
5.5 Summary 103
x Chapter 6 Simultaneous decarburization and oxidation reactions 104 occurring in silicon and ferrosilicon alloys. Results and discussion
6.1 Investigation of weight changes for silicon and ferrosilicon alloys 105
6.2 Study of the carbon loss for silicon and ferrosilicon alloys 107
6.3 Influence of the gas composition 108
6.4 Silicon oxidation 112
6.5 Discussion 118
6.6 Summary and Conclusions 122
Chapter 7 Summary and Conclusions 124
7.1 Introduction 125
7.2 Dynamic wetting between molten silicon and ferrosilicon with 127 synthetic graphite
7.3 Carbon dissolution occurring during graphite-silicon interactions 128
7.4 Simultaneous decarburization and oxidation reactions occurring 129 in silicon and ferrosilicon alloys
7.5 Conclusions 130
7.6 Future work 132
References 133
Appendix 1 Dynamic Wetting for silicon and ferrosilicon alloys in natural 138 graphite.
xi LIST OF FIGURES
FIGURE PAGE
2-1 Outline of the Cupola Furnace 8
2-2 Schematics of the zones in the Cupola Furnace 9
2-3 Fe-Si System 12
2-4 Si – C system 12
2-5 System Fe-Si-C at 1000 °C 13
2-6 Projection on the composition plane of the Fe-C-Si 14 metastable diagram
2-7 Projection on the composition plane of the Fe-C-Si 14 stable diagram
2-8 Apparent carbon dissolution rate constant of iron in 20 synthetic graphite
2-9 Solubility of carbon in liquid silicon at different 23 temperatures
2-10 Solubility of C in Si and FeSi at different temperatures 23
2-11 The effect of Al on the solubility of carbon on pure 23 silicon and ferrosilicon at 1550 °C
2-12 Oxidation-time relationships 24
2-13 Temperature dependence of the apparent rate constant 31
2-14 Effect of temperature on the oxidation content behavior 33 of Silicon in Fe-C-Si melts by CO2 injection at 1300 °C
2-15 Variation of Carbon of Fe-C-Si melts with time by 33 CO2 injection 1300 °C
2-16 Formation of a silica layer around a rising gas bubble 36 when blowing oxygen into liquid (Ferro) silicon. The dense silica layer may result in a small mass transfer -4 coefficient kc << 10
2-17 Decarburization of 1.6 m3 FeSi 75 at 1600 °C 36
xii FIGURE PAGE
2-18 Variations in carbon content and temperature in FeSi75 38 (l) during a sequence of pouring (at time t > 0). Centric bottom Ar-stirring (0
2.19 Contact Angle θ in the solid-liquid-vapour system in 40 equilibrium
2-20 Process of solute atom transport 41
3-1 a Synthetic graphite crucibles 56
3-1 b Synthetic graphite crucibles. Dimensions 56
3.1 c Graphite crucibles placed on alumina boat. 56
3.1 d XRD scan for synthetic graphite crucibles. Peaks 56 corresponded with 2H- graphite (Clifftonite).
3-2 Graphite plate placed on alumina sample holder. 57
3-3 a Alumina crucibles, Dimensions. 59
3-3 b Alumina crucible. 59
3-4 Schematic of the experimental arrangement 61
3-5 Temperature profiles. Values decrease from the hot 62 zone to the cold zone.
3-6 Schematic of the gas cleaning system 63 3-7 Example of contact angle measurement during the 64 dynamic wetting investigations
3-8 The working flow sheet of “interface software” for 67 analysis of the contact angle of a sessile drop image
3-9a Silicon mapping after FeSi 25 – synthetic graphite 68 interaction at 1550 °C
3-9b Optical image of after FeSi 25 – synthetic graphite 68 interaction at 1550 °C
3-10 Hydraulic press 70
xiii FIGURE PAGE
3-11 Steel die 70
3-12 Schematic of the metal droplet – substrate sample used 70 for interfacial investigations
4-1 Still image of liquid silicon (Si 98.5%) droplet on 72-73 synthetic graphite substrate a) 1 second after melting , b) 13 seconds , c) 23 seconds, d) 60 seconds
4-2 Still image of liquid FeSi 74 droplet on synthetic 73 graphite substrate a) 1 second after melting b) 30 seconds, c) 60 seconds, d) 80 seconds
4-3 Still image of liquid FeSi 24.7 droplet on synthetic 74 graphite substrate a) 1 second after melting, b) 30 seconds, c) 50 seconds, d) 300 seconds, e) 570 seconds, f) 1170 seconds
4-4 Dynamic Wetting of Si 98.5 wt% on synthetic graphite 75
4-5 Dynamic Wetting of FeSi 72.6 wt% on synthetic 75 graphite.
4-6 Dynamic Wetting of FeSi 23.6 wt% on synthetic 76 graphite.
4-7 XRD spectra. Interface of Si 98.5% / synthetic graphite 77 after 0 and 30 seconds at 1550 °C.
4-8 XRD spectra. Interface of FeSi 74 % / synthetic 78 graphite after 1 second and 60 seconds at 1550 °C.
4-9 XRD spectra. Interface of FeSi 24.7 % / synthetic 79 graphite after 1 second and 1800 seconds at 1550 °C.
4-10 Back-scattered electron EPMA image at the interface 80 Si98.5/synthetic graphite (400X).
4-11 Back-scattered electron EPMA image at the interface 80 FeSi 74/ synthetic graphite (400X)
4-12 Back-scattered electron EPMA image at the interface 81 FeSi 24.7/ synthetic graphite (400X)
xiv FIGURE PAGE
4-13 Still image of liquid silicon (Si 98.5%) droplet on SiC 81-82 substrate
4-14 Still image of liquid ferrosilicon (75%) droplet on SiC 82-83 substrate
4-15 Still image of liquid ferrosilicon (25%) droplet on SiC 83-84 substrate
4-16 Dynamic wetting of Si 98.5 wt% for synthetic 85 graphite and SiC substrates
4-17 Dynamic wetting of FeSi 74 wt% for synthetic graphite 85 and SiC.
4-18 Dynamic wetting of FeSi 24.7 wt% for synthetic 86 graphite and SiC substrates.
5-1 Carbon pickup from graphite into pure iron at 1550 °C. 92
5-2 Carbon pickup from graphite into Si 98.5% at 1550 °C 92
5-2a Carbon pickup from graphite into Si 98.5% at 1550 °C 92 during the first three minutes
5-3 Carbon pickup from graphite into FeSi 74% at 1550 93 °C. 5-3 a Carbon pickup from graphite into FeSi 74% at 1550 °C 93 during the first three minutes
5-4 Carbon pickup from graphite into FeSi 24.7 at 1550 °C 93
5-4 a Carbon pickup from graphite into FeSi 24.7% at 1550 93 °C during the first three minutes
5-5 CtCs )( 95 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of Si 98.5 at 1550 °C.
5-6 CtCs )( 95 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of FeSi 74 at 1550 °C.
xv FIGURE PAGE
5-7 CtCs )( 95 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of FeSi 24.7 at 1550 °C.
5-8 Carbon pickup from SiC into Si 98.5% at 1550 °C 98
5-9 Carbon pickup from SiC into FeSi 74% at 1550 °C 98
5-10 Carbon pickup from SiC into FeSi 24.7% at 1550 °C 99
5-11 CtCs )( 100 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of Si 98.5% on graphite and SiC substrates at 1550 °C
5-12 CtCs )( 100 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of FeSi74% on graphite and SiC substrates at 1550 °C.
5-13 CtCs )( 101 Plot of ln vs. time (seconds) for carbon CoCs )( dissolution runs of FeSi24.7%on graphite and SiC substrates at 1550 °C.
6-1 a-c % Weight gain for three alloys as a function of time. 106 Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
6.-1d % Weight gain for three alloys during the initial 10 107 minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates of 2 L/min.
6-2 a-c Wt % C for three alloys as a function of time. 109 Oxidizing gas 20% CO2, 2% CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
6-2 d Change in carbon content (wt %) for different silicon 110 and ferrosilicon alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at a flow rate of 2 L/min.
xvi FIGURE PAGE
6-3 Wt% C for three alloys as a function of time. Two 111 oxidising gas 20% CO2, 2% CO, N2-balance and 100% CO2 were used at a flow rate of 1 L/min.
6-4 a-c Back-scattered electron EPMA images. Atmosphere: 113-114 20% CO2 – 2% CO. Flow rate: 2 L/min
6-6 a Changes in silicon content (wt %) and weight change 116 (%) for FeSi 74 during the first ten minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
6-6 b Changes in silicon content (wt %) and weight change 117 (%) for FeSi 74 during the first ten minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
6-6 c Changes in silicon content (wt %) and weight change 117 (%) for FeSi 24.7 during the first thirty minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
6-7 a-c Plot of rate weight change (%/min) vs. Silicon content 118 (wt. %) for the three alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates of 1 L/min
6-8 Wt % C for three alloys on a logarithmic scale as a 120 function of time. Oxidising gas 20% CO2, 2% CO, N2- balance was used at flow rate of l L/min.
A-1 Still image of liquid silicon (Si 98.5%) droplet on 140 natural graphite substrate
A-2 Still image of liquid ferrosilicon (75%) droplet on 141 natural graphite substrate
A-3 Still image of liquid ferrosilicon (25%) droplet on 143 natural graphite substrate
A-4 Dynamic wetting of Si 98.5 wt% for different 144 substrates.
A-5 Dynamic wetting of FeSi 74 wt% for different 144 substrates.
A-6 Dynamic wetting of FeSi 24.7 wt% for different 145 substrates
xvii LIST OF TABLES
TABLE PAGE
2-1 Experimental and calculated values of carbon solubility 21 in liquid silicon equilibrated with SiC
2-2 Reported Contact Angle Values of Liquid Iron on 43 Graphite.
2-3 Reported Contact Angle values of liquid Iron on 44 alumina.
2-4 Contact angle of Si (l) on SiC. 46
3-1 Typical composition of few commercially available 53 ferrosilicon alloys
3-2 a Chemical composition of the FeSi 25 54
3-2 b Chemical composition of the FeSi 75 54
3-3 a Composition of synthetic graphite. Proximate analysis 55
3-3 b Composition of synthetic graphite. Ultimate analysis 55
3-4 a Composition of natural graphite. Proximate analysis 58
3-4 b Composition of natural graphite. Ultimate analysis 58
3-5 Gas composition 60
5-1 a Constant rates and Carbon saturation limits for Si 98.5, 101 FeSi 74 and FeSi 24.7 from synthetic graphite substrate.
5-1 b Constant rates and Carbon saturation limits for Si 98.5, 101 FeSi 74 and FeSi 24.7 from SiC substrate.
5-2 Overall rates constant for ferroalloys tested on different 102 substrates (s-1)
6-1 Chemical compositions of silicon and ferrosilicon 105 alloys (wt %)
xviii TABLE PAGE
6-2 Weight gains (%) for different silicon and ferrosilicon 105 alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
6-3 Qualitative comparison of carbon dissolution and 122 oxidation reaction rates
xix CHAPTER 1
INTRODUCTION Chapter 1 Introduction
1. Introduction
The cupola furnace is considered the dominant scrap-melting facility used in scrap-iron foundries, producing approximately two-thirds of molten iron needed for casting [Katz et al
(1999)]. The cupola offers several competitive advantages relative to newer melt-furnaces, including lower energy and scrap costs, higher tolerance for harmful trace elements, and a wider allowable range for iron-production rates. One of the advantages of such a furnace is that counter flow preheating of the charge can occur. In a cupola furnace, upward flowing hot gases come into close contact with the descending burden, allowing direct and efficient heat exchange into the metallic charge (iron and steel scrap, iron returns and ferroalloys), which causes its melting and subsequent interaction with the coke lumps.
Coke that is added as part of the charge plays two important roles. In the cupola process, the major exothermic reaction involves the carbon contained in the coke and the oxygen in the blast. The combustion reactions provide the required amount of heat to melt the scrap metal and keep the liquid bath at the desired temperatures. Carbon from coke also dissolves into the liquid metal, raising the carbon content of the melt.
Silicon is another important alloying elements present in the cast irons, and is plays an important role in the effective control of iron structure and properties. Silicon is a potent graphitizer and ferritizer in cast irons, determining whether a particular iron will solidify carbidic (as in white iron) or graphitic (as in gray and ductile irons). Silicon is added to cast iron in the ladle for several reasons. It may be added as an inoculant, as an inoculant carrier for magnesium in ductile iron production, to adjust off analysis, or as an alloy to modify the properties of iron. Silicon enters in solution readily with liquid iron, thus large quantities of
2 Chapter 1 Introduction silicon can be added in the ladle, since reaction between silicon and iron is exothermic and temperature losses are minimum. In addition, silicon alters the equilibrium diagram and lowers the solubility of carbon in molten iron [Cuppola Handbook (1984)]. Because of the conditions existing in today’s cupola melt foundries, far more silicon units are purchased and charged on an annual basis than any other alloying element [Cupola Handbook (1984)].
Silicon is usually added as ferrosilicon lumps, accounting for around 2-3 % of the total metallic charge [Bush (2003)].
Globally, prices of ferrosilicon have been rising in recent years [Service R.I (2004)]. A number of factors, such as carbon steel world consumption, production cutbacks, anti- dumping duties and power prices had have a significant influence on the upward prices trend.
For some local manufacturers, around 20% of added silicon is wasted due to the oxidation during the melting process [Bush (2003)]. This oxidized silicon is transferred to the slag waste as silica (SiO2), accounting around 30-40% of the slag bulk volume, altering the slag basicity and decreasing the refractory life span. To offset some of the damage to the refractory, some limestone is added but this action tends to increase costs and slag volume.
The cost of the ferrosilicon consumption of cast iron produced was around AUD 30/ton. It also added significantly to the cost of the refractory consumption (AUD 5/ton) as well as the slag disposal (AUD 1/ton) which are likely to increase. Further, the addition of limestone costs AUD 3/ton. [Bush (2003)]
The fundamental understanding of the ferrosilicon / graphite and the ferrosilicon / gas phase interactions during the melting process will bring a positive benefit to the foundries. The operation can be conducted more effectively by reducing the ferrosilicon consumption, minimizing the generation of industrial waste and improving the control of the composition
3 Chapter 1 Introduction for the final product. In addition, the higher silicon losses, the higher the silica formed and the greater slag volume, incurring on additional cost of handling and removing it to the landfills. In basic slag cupola process, any increasing of silica content (acid oxide), it will bring decreases of the slag basicity. Managing this very important factor, would extend refractory life, and therefore, increase periods between maintenance works.
1.2 Scope of the project
Fundamental study of high temperature interaction of ferrosilicon – graphite provides key knowledge for understanding the interfacial reaction and wettability at the solid/liquid interface. Although a large body of work has investigated the wettability for silicon and ferrosilicon on SiC, the dynamic wetting and the associated interfacial phenomena of the ferrosilicon alloys – graphite system has not been investigated in depth.
Carbon dissolution phenomena also take place during the ferrosilicon – graphite interactions.
This is a key reaction for the cupola process, since the molten ferrosilicon droplets are interacting with coke lumps while descending to the furnace well. The kinetic mechanism of carbon dissolution in liquid iron had been extensively investigated, but it has been limited for silicon contents less than 10%, since the silicon content during the iron and steel making processes are usually below this limit.
As outlined on section 1.1, the interaction of ferrosilicon alloy with coke and the gaseous phase plays a key role on the silicon recovery. There are several aspects to consider during these interactions and this work aims to provide a better understanding to these key issues.
4 Chapter 1 Introduction
The three phenomena investigated in this work are
- Wettability between ferrosilicon and coke
Interfacial reaction. Formation of interfacial product and its impact on wettability
and carburization.
- Carbon pickup process during the coke-ferrosilicon interaction.
Kinetics of the carbon dissolution. Determination of the rate constant and
understanding of the carbon transfer mechanism
- Interaction of the ferrosilicon alloy with the gaseous phase.
Simultaneous silicon oxidation and decarburisation. Effect of gas flow, gas
composition and silicon content during the oxidation reactions.
This project aims to provide fundamental understanding of the ferrosilicon reactions during the melting process. Reactions involving ferrosilicon strongly affect the silicon loss and product composition in the scrap melting process. Fundamental knowledge of the ferrosilicon interaction with the gas phase and graphite during the melting would directly benefit foundries, which recycle iron and steel scrap.
5 CHAPTER 2
LITERATURE REVIEW
6 Chapter 2 Literature Review
2. Literature Review
A review of the literature relevant to this project is presented in this chapter. A brief outline of the cupola process and the main reactions involved are discussed. In addition, thermodynamic analysis of the system, wettability studies and reaction kinetics have been detailed.
2.1 Outline of the Cupola process
The cupola is a vertical, cylindrical shaft furnace similar to the blast furnace. It differs, however, with respect to the functions served and the type of charges used. Pig iron, scrap iron and scrap steel in the cupola replace the iron ore of the blast furnace, and efficient conversion melting rather than ore reduction is the principal function of the cupola. An outline of conventional cupola is shown in Fig. 2.1.
As in the case of blast furnace, the counter-flow preheating of the charge material is inherent part of the melting process in the cupola. The upward flowing hot gases come in close contact with the descending burden allowing direct and efficient heat exchange to take place. The running or charge coke is also preheated, which aids in the combustion process as it reaches the combustion zone to replenish fuel consumed from the coke bed. Alternating layers of coke, flux and metal are deposited in the cupola so that the process of combustion, heat transfer and melting can take place. In the beginning, a bed of coke is placed in the lower portion of the shaft and ignited. Metallic charge, fluxes, ferroalloys and fuel are added in alternate layers, or mixed are charged and blast air is introduced into the bed, melting the charge. Once the process starts it can be continued for a long period of time.
7 Chapter 2 Literature Review
Figure 2.1: Outline of the Cupola Furnace [Katz et al (1999)]
2.1.1 Zones of the Cupola Furnace
The five major zones, which can be easily distinguished, are: preheating, melting, reduction, oxidation and the well zone. Zone distribution and temperature profile are shown in Fig 2.2.
The zones of main interest for the present project are the melting and oxidation zones; therefore, these will be described in more details below.
8 Chapter 2 Literature Review
Metallic Charging door charge,coke and limestone and ferroalloys CO2(g) Preheat zone T = 400 – 1200 °C
Melting zone T = 1090 –1320 °C
Molten iron Oxidation (combustion) zone T = 1700 – 2100 °C O2 (Air) Well zone T = 1540 – 1570 °C
Tap hole Slag hole
Figure 2.2: Schematics of the zones in the Cupola Furnace.
The melting zone is the region from the top of the coke bed to the point where melting first occurs in the metallic charge. Hot gases coming from the upper reduction zone supply the heat necessary to melt the metal. The slag also begins to melt in this region. The liquid iron helps to liquidify more of the steel and some carbon is picked up by the steel from either the stack gases or coke in the preheat zone and serves to lower melting point of the steel.
The primary function of the oxidation zone is to supply energy for the additional superheating of the molten iron and to provide hot gases to superheat, melt and preheat charge materials in the zones above the tuyeres. These are the regions where the blast enters the cupola and reacts with the hot coke to form CO and CO2. (Eq. 2.1, 2.2)
C (s) + ½ O2 (g) = CO (g) (Eq.2.1)
9 Chapter 2 Literature Review
C (s) + O2 (g) = CO2 (g) (Eq.2.2)
The gas temperatures in this zone rise and fall rapidly and range from 1700 to 2100°C. At these temperatures, Oxygen reacts rapidly with coke and CO until it is reduced to 1-2 % level. [Cupola Handbook (1984)]
2.2 Study of phase diagrams.
The study of phase diagrams is very useful to explain the chemical behavior at the liquid / solid interface, liquid formation and solid-liquid solubilities. It is also a valuable tool for predicting the potential for a particular combination of oxides, carbides or other compound and liquid metal. In recent years, considerable amount of work has been published on the Fe-
Si, Si-C and Fe-Si-C systems, based on more accurate thermodynamic calculations and new experimental data. Due to its important role in steelmaking process and cast iron foundry, these systems have been the subject of many thermodynamic studies. A summary of some of the most recently published work on these systems is presented in the next sections.
2.2.1 Phase diagram of Fe – Si system.
The Fe-Si system has been studied by a number of authors [Kubacheski (1982)][Hultgren et al(1973)][Schurman et al (1962)][Fischer et al(1962)][Übelacker et al (1967)][Lacaze
(1991)][Lacaze and Sundman (1991)][Miettinen (1998)][Hoffman and Müller (1999)] .
Although some authors had reviewed and proposed some changes, the general shape of these diagrams has not changed substantially. Most of the changes are related with the nature of reactions occurring in the bcc-Fe-rich solution [Lacaze (1991)]. The silicon rich side has not received the same level of attention, however this region is reasonably well known.
10 Chapter 2 Literature Review
Kubachewski [Kubacheski (1982)] proposed a diagram for the Fe-Si system (Fig 2-3). In the solid state, there are only three stable phases, which are FeSi, Fe3Si7 and Si, although the second one is a low-temperature form of FeSi2. A ferrosilicon alloy with silicon content higher than 50 at % (= 33.5 wt %) will consist in a mixture of these phases, which are often found as grains. From 50 at% to 66.7% the low temperature equilibrium phases are FeSi and
FeSi2; above 66.7 at %, FeSi2 and silicon. Lacaze and Sundman [Lacaze and Sundman
(1991)] determine precisely the partial liquidus surface in the iron corner, optimizing the parameters describing the properties of various phases involved by using experimental thermo dynamical data.
Experimental work by Schürmann and Hensgen [Schürmann and Hensgen (1962)] on the Fe-
FeSi system confirms the liquid/solid equilibrium determined previously, however, they considered only the DO3 modification of the bcc phase and adopted previous views that ordering was a first order reaction [Hultgren et al (1973)]. The gamma loop has been studied by Fischer and Uberoi [Fischer and Uberoi (1962)] and Übelacker [Übelacker (1967)], whose results are in good agreement. Although there is a general agreement about the nature of reactions involving different sicilides, various temperatures are not so well established and the values proposed by Kubachewski [Kubacheski (1982)] are compromised at some extent.
2.2.2 Phase diagram of Si – C system.
Figure 2-4 shows the phase diagram for the Si-C system, summarizing the available data from literature [Hoffman et al (1999)]. The system forms a peritectic at T ~ 2800 °C. The solubility of C in liquid Si is quite low, reaching the maximum value when C (At%) is about
15%. For pure Si (< 50 At%) and temperatures above 1414 °C, there are 2 phases in the system: SiC and liquid phase. After solidification, SiC and Si appear as two independent phases.
11 Chapter 2 Literature Review
Figure 2-3: Fe-Si system [Lacaze and Sundman (1991)]
Figure. 2-4: Si – C system [Hoffman et al (1999)]
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2.2.3 Fe-Si-C ternary diagram.
Ternary phase diagram for Fe-Si-C at 1000 °C (Fig 2-5) had been derived from the published work by Sorrell [Sorrell (1995)] For Fe-rich compositions (to the left of the Fe5Si3-SiC tie line), the recrystallization of liquid will result in the formation of Fe5Si3 and α Fe. For Si-rich compositions (to the right of Fe5Si3 – SiC tie line), the recrystallization of liquid will result in the formation of Fe5Si3 and FeSi. Figures 2-6 and 2-7 provide the projection for metastable and stable diagrams for the Fe-rich corner. Despite the amount of previous work, there are still some discrepancies regarding to some lines on the Fe-Si-C system. In one of the studies,
Miettinen [ Miettinen (1998)] reassessed some thermodynamic phase data, although the Fe- rich corner was his main focus.
Figure 2-5: System Fe-Si-C at 1000 °C. [Sorrel(1995)]
13 Chapter 2 Literature Review
Figure 2-6: Projection on the composition plane of the Fe-C-Si metastable diagram. [Lacaze and Sudnman (1991)]
Figure 2-7: Projection on the composition plane of the Fe-C-Si stable diagram [Lacaze and Sundman(1991)]
14 Chapter 2 Literature Review
2.6 Reaction kinetics
Considering the heterogeneous nature of the present system, there are a number of reactions of relevance and interest:
Solid – gas: Chemical (Oxidation of carbon from the carbonaceous substrate)
Physical (Absorption of oxygen on alumina rod and tube walls)
Solid – liquid: Physical (Melting, carbon dissolution, crystallization)
Liquid – gas: Chemical (Oxidation of liquid metal)
Metallurgical transformations and reactions are generally heterogeneous in nature. They involve more than one phase and there are distinct phase boundaries. In any given system, the rate of reaction at given time depends basically on three factors, namely, the nature of the system, the time of reaction and the temperature. One of the main aims of a kinetic study is frequently the establishment of a rate law, which can be used in prediction of reaction under certain set of operating conditions.
The current system involves a number of metallurgical reactions, i.e.:
- Solid – liquid : (S- L) Physical ( melting, dissolution)
- Liquid – gas ( L-G) Chemical ( oxidation, decarburization)
The dissolution of solid into a liquid phase can occur by diffusion, convection and eddy diffusion (turbulence). Since there is absence of stirring, the fluid layer in contact with the solid surface cannot slip past the solid, and is, therefore, stagnant. The liquid film adjacent to the solid may be saturated, providing the release of atoms or molecules from the bulk solid is sufficiently rapid. The solute diffuses into the bulk liquid from the saturated layer. Most of the concentration drop takes place within a relatively finite thin layer known as the boundary layer. The boundary layer thickness (δ) is defined as the distance by which 99% drop is achieved. Since the fluid layer adjacent to the solid surface cannot slip past the solid, there must also be a velocity profiles so that flow near the boundary layer is laminar, therefore:
15 Chapter 2 Literature Review
C i J = C v = - Ci v - E D (2-3) ix ix Di C x i i x x where,
C = concentration of species i i
vix = overall velocity of the species along the x-direction
Di = diffusion coefficient of I
vx = fluid flow (convection) velocity along x-direction
ED = eddy diffusivity
But when the flow near the boundary layer is laminar, we have:
( vx )x =0 = 0
( ED )x =0 = 0
So, for diffusion across the boundary layer, we have the Fick’s law
CoCs DJ (2-4)
The mass transfer coefficient (h m) is defined as D/ δ. Consider dn moles of solid are dissolved in time dt to change the concentration by an amount of dC. If V is the total volume of the solution and A the area of dissolution, then (2-5)
dn V dC D . ( Cs – Co) (2-5) dt A dt which gives,
D A ln ( Cs – Co ) = - t + I (2-6) V
I = integration constant
At t = 0, Co = Ci, where Ci is the initial concentration in the bulk. Therefore,
16 Chapter 2 Literature Review
CoCs )( DA exp t (2-7) CiCs )( V
DA Co = Cs – ( Cs – Ci) exp t (2-8) V
DA Co = Cs – ( Cs – Ci) 1 t (2-9) V
Expressing in terms of the degree of reaction α,
CoCi )( α = , (2-10) CsCi )(
CoCs )( 1- α = (2-11) CiCs )(
And can be written as
-ln ( 1- α) = kt (2.12)
DA Where k = , corresponding with a first order equation. V
2.7 Chemical reaction and Mass Transfer
The mass transfer of carbon dissolution from graphite into liquid iron and the chemical reaction govern the interfacial phenomena to some extent. The rate of mass transfer and the rate of chemical reaction can be expressed as follow (2-13) [Gaskell (1995)], [Levenspiel
(1976)] :
C JJc D t (2-13)
Where,
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Jc = rate of chemical reaction
JD = rate of diffusion
m n And JKCc i (2-14) i1
K = rate coefficient of chemical reaction
m = the number of component i
n = the order of chemical reaction
2CCC 2 2 D JD 2 2 2 (2-15) x y z
D = diffusion coefficient
2.7.1 Previous studies of carbon dissolution in iron.
Various factors affecting the carbon dissolution from graphite into molten iron have been well studied by a number of authors [Olson et al (1966)][Kosaka and Minowa
(1968)][Oersten et al (1986)][Wright and Baldbock (1988)][Wu and Sahajwalla (2000)][Zhao
(2003)], using both experimental and theoretical approaches. The goal of those studies had been to understand the rate-controlling mechanism.
Those researches had been conducted under different experimental conditions. Dahlke et. al.
[Dahlke et al (1955)] studied the carbon dissolution by immersing graphite rods into Fe-C melts for different lengths of time. They found the change of the bath composition was controlled by mass transport within the metal. Further studies by Wright and Baldock
[Wright and Baldock (1988)] found the rate of carbon dissolution from a rotating graphite disk into different melts was limited by carbon diffusion from the graphite melt/interface to the bulk liquid.
18 Chapter 2 Literature Review
Some work had been carried out under stirring and the findings showed the mass-transfer coefficient for carbon dissolution from graphite was a function of the peripheral velocity of the rotating cylinder. It was also found the rate of carbon dissolution from graphite to the Fe-
C melts was controlled by carbon diffusion (mass transfer) from the interface (between the solid graphite and bulk liquid) when the Reynolds number ranged from 790 – 18 000 [Olson et al (1966)].
Kosaka et al [Kosaka and Minowa (1968)] measured the carbon transfer coefficient in the temperature range from 1270 – 1550 °C. They also calculated the mass transfer coefficient assuming liquid-mass transfer controlled the process of the carbon dissolution. In addition, they immersed a rotating graphite cylinder into the iron melts. The mass-transfer coefficient obtained from a first –order equation was ranging from 0.8 – 3.6 m/s at a stirring velocity of
48-52 cm/s and between 0.22 – 1.8 m/s at stirring velocity of 4.5-33 cm/s in an induction furnace.
Oersten et al [Oersten et al ( 1986) ] used the rotating cylinder method to obtain the rate limitation for carbon dissolution. In this investigation, interfacial phenomena were taken into account as phase boundary reactions. It was estimated that the rate constant of the phase- boundary reaction was ~ 0.5 x10-2 m/s, which was 16 to 33 times larger than the overall mass-transfer rate constant in the liquid side obtained from experiments. Therefore, they arrived at the conclusion that the liquid-side mass transfer rate limitation was the controlling step in carbon dissolution from graphite to Fe-C melts.
More recently, Wu et al [Wu and Sahajwalla (2000) ] investigated the carbon dissolution from graphite and coals using an induction furnace. It was confirmed mass transfer in the liquid boundary layer adjacent to the solid/liquid interface controlled the carbon dissolution from graphite. However, carbon dissolution from coals is most likely to be governed by a mixed-control mechanism that includes liquid-side mass transfer.
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Zhao [Zhao (2003)] found the apparent carbon dissolution constant rate in pure iron was
0.0419 s-1. (Fig 2-8) Carbon dissolution from graphite is thought to be a two-step process [
Wu and Sahajwalla (2000)] :
1. Dissociation of carbon atoms from its crystal site in the graphite into the carbon/melt
interface.
2. Mass transfer of carbon atoms through the adjacent boundary layer into the bulk
liquid iron.
The carbon dissolution in iron is controlled by a mass-transfer in liquid side mechanism [Wu and Sahajwalla (2000)]. It had been found the dissolution rates are governed by the diffusion of carbon from the metal-carbon interface to the bulk liquid through a boundary layer
[Zhao(2003)].
0 50 100 150 200 0
-1 y = -0.0416x R² = 0.98 -2 Co) - -3 Ct/Cs - -4 ln(Cs -5
-6
-7 Time(seconds)
Figure 2-8: Apparent carbon dissolution rate constant of iron in synthetic graphite. [Zhao
(2003)] 20 Chapter 2 Literature Review
2.4.2 Previous studies of Carbon dissolution in silicon and ferrosilicon
A summary of experimental and calculated values of carbon solubility in liquid silicon equilibrated with silicon carbide is presented in Table 2-1.
Table 2-1. Experimental and calculated values of carbon solubility in liquid silicon equilibrated with SiC.
Silicon Atmosphere Carbon Temperature Carbon solubility Ref. Melting Analysis (° C) (mass ppmw) Silica Argon Gravimetry 1520-1725 1.307x1011exp(3625 [Hall] crucible 8/T) Graphite Argon Combustion- 1720-1950 3.359X107exp(- [Scace et or alumina (3.5 MPa) IR absorption 22417/T) al (1959)] crucibles FZ with - Charged 1685 42 [Nozaki carbon particle et al painted on activation (1970)] the analysis surface Graphite Vacuum , Combustion- 1700-2150 4.217x109 exp(- [Oden et crucible Ar (135 kPa) IR adsorption 34409/T) al (1987)] Graphite Ar gas Combustion- 1435-1515 1430 °C : 60 [Suhara et crucible flowing IR adsorption 1460 °C: 70 al (1989)] 1510 °C : 90 Graphite Argon gas Combustion- 1720-1950 3.359x107 exp(- [Ottem or silica flowing IR adsorption 11417/T) (1993)] crucibles SiC Ar + CO (5%) Combustion- 1500-1600 4.266x107 exp(- [Yanaba crucible gas flowing IR adsorption 22240/T) et al Combustion (1997)] coulometric filtration
The interaction in the system Si/SiC was also studied by Naidich [Naidich (1981)]. It was found that at temperatures close to the silicon melting point, the latter dissolves small amounts of carbon. Due to current demand by stainless steel producers specifying very low carbon (< 200 ppm) in ferrosilicon (e.g. FeSi75), carbon solubility plays an important role for ferrosilicon producers,
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Ottem [Ottem (1993)] determined the solubility of carbon in silicon and FeSi alloys at different temperatures (Fig. 2-9). The carbon solubility in ferrosilicon was found to decrease when the iron content had increased in the ferroalloy. At 1550 °C, the solubility of carbon in
Si was found to be around 150 – 170 ppm, while values for FeSi75 (100 – 120 ppm) and
FeSi65 (70 – 75 ppm).Carbon solubility in FeSi 65 seems to be half of that in pure silicon.
However, at 1614° C, the solubility of C on FeSi started to increase when %Si was below 50, reaching solubility values similar to ferroalloys with 90% wt Si (Fig 2-10). Ottem [ Ottem
(1993)] also studied the effect of aluminium contained in (ferro) silicon. Aluminium also has some effect on the carbon solubility, when it is beyond certain levels. (Fig 2-11)
Klevan [Klevan (1997)] found the following equations for the solubility of carbon in silicon and ferrosilicon in equilibrium with SiC. (Eq. 2-16, 2-17)
Log [%C] Si(l) = 3.53 – 9,736 / T [mass%] ( 2-16)
Log [%C]FeSi(l) = 3.5 – 10,003 /T [mass%] (2-17)
Klevan [Klevan (1997)] found contents of 700 -1100 ppmw C when the metal was tapped from the furnace. Statistical analyses of 779 shipments over 3 years found that the carbon content dropped to an average of 300 ppmw C, after vesting, crushing and screening the material. It was found that the carbon, which precipitates as SiC particles as the temperature drops during tapping and handling, is physically removed as particles to a fairly large extent.
22 Chapter 2 Literature Review
0.04
0.035 1547 1614 0.03 1652 1674 0.025
0.02 C (wt%) C 0.015
0.01
0.005
0 30 40 50 60 70 80 90 100 Si (wt%)
Figure 2-9: Solubility of carbon in liquid silicon at different temperatures (°C).
[Ottem (1993)]
Figure 2-10: Solubility of carbon in silicon Figure 2-11: The effect of Al on the and FeSi at different temperatures. solubility of C in pure silicon and [Ottem (1993)] FeSi at 1550 °C [Ottem (1993)]
23 Chapter 2 Literature Review
2.8 Kinetics of oxidation
The oxidation of metals has been investigated by different methods, including determination of the change in thickness of the scale, in weight (mass) of the metal sample or in volume of the surrounding gas. Most quantitative data have been obtained as weight change (Δ m).
Kinetic theory has been developed to determine the relationship between oxidation and time.
Different correlations have been found empirically and relate Δ m and time t. Some representative examples are given below:
ktm linear (2-18)
)( 2 ktm parabolic (2-19)
)( 3 ktm cubic (2-20)
log()( tatkm 0 ) logarithmical (2-21)
A combination of two or more of these relationships in a single oxidation-time curve also is quite common. Fig 2- 12 represents the various relationships.
Figure 2-12: Oxidation-time relationships [Ray (1993)]
24 Chapter 2 Literature Review
The type of time relationship that can be applied to a given metal or alloy depends largely on the thickness of the film already formed, that is on time and temperature.
2.8.1 Previous studies of decarburization in iron.
Decarburization with carbon dioxide had been investigated for various reaction conditions. A review of the extensive literature had pointed to the following highlights:
- The use of the levitation technique in decarburization studies
- The importance of mass transfer considerations
- The chemical reaction mechanisms and rate parameters and
- The factors leading to changes in the rate limiting mechanism
The interfacial rate constants for the decarburization of carbon-saturated liquid iron alloys by
CO2 were first reasonably well established by the work of Sain and Belton [Sain and Belton
(1976)],[Sain and Belton (1978)]. They used jets of CO2 containing gas mixtures impinging onto the surfaces of inductively stirred iron-carbon alloys and maintained carbon saturation by having a disk cemented onto the bottom of the crucible containing liquid iron. This maintained a high driving force for liquid-phase mass transfer of carbon, and thus also eliminated it as a rate controlled step. Reaction rates were determined from the weight losses of the crucibles and contents.
Mannion and Freuhan [ Mannion and Freuhan (1989)] used a similar technique but determined the rates from the production of CO, measured by continuous mass-spectrometric analysis of the reacted gas stream. The data are consistent with the first-order rate constants, being given by the expression (2-23)
logk 5080 / T 0.21 (2-23)
25 Chapter 2 Literature Review
mol Where the first-order rate constant k is expressed in cm2 atms
For the reaction of liquid iron alloy with gases, kinetic mechanisms have been studied for gas oxidation of iron based multi-component liquid metal. For liquid iron with high concentration of carbon and silicon, the oxidation rates of carbon dioxide gases was controlled by diffusion in the gas phase. It was found the oxidation did not occur simultaneously for both elements, although the content of each individual component in the metal was far above the equilibrium level. The thermo chemical driving forces play an important role in some reactions i.e. the oxidation of each component occurs in thermodynamically favoured order at high temperatures. The oxidation of metal components other than carbon was also found to be limited by transport of oxidizing gas.
Baker [Baker et al (1967)] used the levitation technique to investigate the decarburization of iron – carbon alloys at 1600 ºC. The carbon dioxide content was varied from 1 to 100 %, using helium as the carrier gas. The results were reported for gas velocity of the order of 12.5
63 cm/s. The observed rates of decarburization were shown to be independent of the carbon concentration of the melt for carbon contents as low as 0.5 wt%. These authors developed a relationship for the gas phase flux of carbon dioxide to the surface using stagnant boundary layer theory. They found that the decarburization rate was largely limited by mass transfer in the gas phase under these conditions.
Some recent evidence suggests that their decarburization rates at higher velocity under a
100% carbon dioxide atmosphere were likely to have encountered some resistance from chemical reaction kinetics. The role of sulphur as a retarding agent during the decarburization is well known and that might have taken some part for their observed rate being lower than predicted for gas phase mass transfer control.
26 Chapter 2 Literature Review
Swisher and Turkdogan [Swisher and Turkdogan (1967)] investigated the rate of decarburization at 1580 °C using CO-CO2 gas mixtures preheated up to 1300 °C. The crucible technique was used with exit gas velocity from the lance of the order of 200 cm/s.
The authors reported that the rate of decarburization was independent of the melt carbon composition until 0.2 wt% carbon in the metal. This precludes the influence of liquid phase resistance to decarburization. These authors also found that the gas phase mass transfer coefficient at 2.6 cm/s gave the large values for the conceptual boundary layer thickness (1.33 cm). It was argued that the mass transfer coefficient and the lance exit velocities greater than
200 cm/s were not comparable. They came to the conclusion that their decarburization rates were not consistent with mass transport in the gas phase and therefore, limited by interfacial chemical kinetics. The proposed mechanism for the overall chemical reaction was considered to be
CO2(g) → CO2 (ad) adsorption (2-24)
CO2 (ad) →CO2 (activated) activation (2-25)
CO2 (activated) + C → 2CO reaction (2-26)
The dissociation and adsorption of carbon dioxide on the melt given by equation (2-25) was proposed as the rate limiting step in the decarburization process. The model proposed by
Swisher and Turkdogan [Swisher and Turkdogan (1967)] also took into consideration the retarding influence of surface active species. It also showed that the model was equally applicable to the dissolution of adsorbed species into melts. For both cases, the authors proposed that the rates of reaction were proportional to the fraction of vacant sites on the surface. The role of surface active species was to render surface area unavailable for reaction.
27 Chapter 2 Literature Review
Freuhan and Martonik [Freuhan and Martonik (1974)] using a heat transfer analogy recomputed the mass transfer coefficients for similar experimental conditions used for
Swisher et al [Swisher and Turkdogan (1967)] and obtained a comparable value with that experimentally obtained for latter authors. There was enough evidence to establish that the gas phase mass transfer offered significant resistance to the decarburization rate for this work.
Freuhan and Martonik [Freuhan and Martonik (1974)] carried out this study using carbon dioxide and hydrogen at 1527 °C. Their work was aimed at resolving discrepancies with previous work from Baker [Baker et al (1967)] and Swisher et al [Swisher and Turkdogan
(1967)] , which reported conflicting rate limiting mechanism above 1400 °C . The authors choose conditions that characterize gas phase mass transfer mechanisms for typical experimental geometries such as flow over a flat plate and gas impinging on a flat surface.
Gas velocities varied from 13 to 52 cm/s and gas composition contained 9% CO2 – 91% CO.
Decarburization rates were found to be consistent with those predicted for reactions limited by the transport of carbon dioxide in the gas phase.
Freuhan and Martonik [Freuhan and Martonik (1974)] also showed that for the crucible technique, the distance of the lance from the melt surface had a much larger influence on the decarburization rate than the exit gas velocity. The authors reported that for melt carbon less than 0.5 wt%, the decarburization rate was affected by resistance from the liquid phase transport, which becomes the dominant limiting mechanism for melt carbon contents less than 0.1 wt%. They further observed that a melt sulphur content of 0.3 wt% was found to decrease the rate of decarburization by 10%. Hence they concluded that in the presence of sulphur, slow interfacial chemical kinetics may influence the rate of decarburization due to its surface activity. In an earlier study Freuhan and Martonik [Freuhan and Martonik (1971)] investigated the decarburization of austenite using CO - CO2 gas mixtures. Strips of the iron alloy were decarburized in the temperature range 850 – 1350 °C. Gas composition were
28 Chapter 2 Literature Review chosen to avoid the formation of FeO. For gas velocities varying from 5.5 to 69 cm/s these authors showed that the observed rates did not correspond with mechanisms limited by the diffusion of carbon in the metal and concluded that:
- Above 1250 °C, the decarburization rates were limited by carbon dioxide transport in
the gas phase
- Below 1000 °C rates were limited predominantly by slow chemical reaction at the
surface
- Between 1000 – 1250 °C there was a mixed control between the above mentioned
mechanisms.
In the regime limited by slow chemical reaction the rate was shown to be first order with respect to the difference between the bulk and equilibrium partial pressures of carbon dioxide and independent of the pCO/pCO2 ratio. They concluded that the rate was limited by the dissociation of carbon monoxide on the surface. The rate constant obtained as a function of temperature was given as
10490 kj 007.3log mol cm-2 s-1 atm-1 (2-27) T
Where kj is in mol cm-2 s-1 atm-1 and T absolute temperature. These results were comparable with that obtained by Grabke [Grabke (1964)].
Two mechanisms for the adsorption of carbon dioxide on the reaction surface were considered. The intermediate chemical reaction steps were considered as
Mechanism 1: The formation of a C2O2 activated complex
CO2 (ad) + C (ad) → C2O2 (activated) (2-28)
C2O2 (activated) →2CO (g) (2-29)
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Mechanism 2: The activation of adsorbed carbon dioxide itself
CO2 (ad) → CO2 (activated) (2-30)
CO2 (activated) + C (ad) → 2CO (g) (2-31)
The authors showed that if carbon and oxygen were strongly adsorbed on the surface, the activated complex for carbon dioxide dissociation would be C2O2. If however, the fraction of vacant sites at the surface is equal to 1 i.e. carbon adsorption at the surface is weak as suggested by Grabke [Grabke (1964)], the second mechanism was indicated in their results where carbon dioxide is considered as the activated complex.
Nomura and Mori [Nomura and Mori (1973)] studied the kinetics of decarburization at 1600
°C. They used the crucible technique with gas velocities from 40 to 160 cm s-1 and found that the decarburization rate was dependent on both the gas flow rate and the size of their lance. A mixed control model was developed for gas phase mass transport and interfacial chemical kinetics. Their results showed that for the conditions employed, the rate of decarburisation was limited primarily by carbon dioxide transport in the gas phase, The chemical rate constant values was estimated to be greater than 0.001 mol cm-2 s-1 atm -1, which was comparable to an extrapolation from a later work from Freuhan et.al. [Freuhan and Martonik
(1974)]. The influence of sulphur was reported to be very low.
Lee and Rao [Lee and Rao (1982)] used the levitation technique, where liquid alloys were
-1 decarburized in CO-CO2 gas mixtures with lance exit velocities up to 65 cm s . Most of the reaction was carried out at 1700 °C.
The decarburization rates were found to be independent of the bulk carbon content of the melts down to a critical carbon concentration of about 0.17 to 0.05 wt%. The authors showed that the diffusivity of carbon in a levitated melt can be enhanced by an order of magnitude
30 Chapter 2 Literature Review with electro-magnetic stirring. Decarburisation rates were shown to be influenced by gas velocity, the presence of sulphur and the partial pressure of carbon dioxide in a manner consistent with the earlier studies reviewed. By combining the chemical kinetics rate law with the site blockage mechanism for surface active species and a relationship for the gas phase flux of carbon dioxide, a mixed control model was formulated to describe their reults. The rate constant for dissociative chemisorption of carbon dioxide on liquid iron was obtained as
4.42 x 10-3 mol cm-2 s-1 atm -1.
Further studies [Shinme and Matsuo (1987)] based on Vacuum Stirring Decarburisation
(VSD) confirmed that the carbon dropped down to ~ 10 ppm, which is considered by some authors to approach the equilibrium level. The apparent decarburization rate constant was estimated, assuming a first-order equation (2-32) and the plot is shown on Fig. (2-11).
Cd ][ A - kc C][ (2-32) dt V
It was also found the apparent decarburization rate constant decreases with increasing initial carbon content and in higher carbon content range a large amount of CO is formed and the effect of evacuation on the decarburization rate is decreased. [Shinme and Matsuo (1987)]
Figure 2-13: Temperature dependence of the apparent rate constant.[Shinme and Matsuo
(1987)]
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2.8.2 Previous studies of decarburization in silicon and ferrosilicon.
The simultaneous oxidation behaviour of silicon and carbon in molten Fe – Si – C alloys in presence of CO2 at 1300 °C has been investigated. [Ono-Nakazato et al (2001)][Taguchi et al (2003)]The desiliconization reactions of Fe-Csat-Si alloy with CO and CO2 are represented as (2-33, 2-34):
Si (mass %, in Fe- Csat alloy) + CO2 (g) = SiO2 (s) + C (s) (2-33)
Si (mass %, in Fe- Csat alloy) + 2 CO (g) = SiO2 (s) + 2 C (s) (2-34)
The sample composition of Fe-C-Si contained about 10 wt% Si. After melting, Carbon dioxide was blowed into the metal at fixed flow rates ranging 10-50 ml/min (s.t.p).
It was found that silicon would get oxidized in preference to carbon over 0.60 wt% Si under
the condition of aSiO2 a C 1 at 1300 °C. Under the condition of preference desiliconizing, namely, over 0.60 wt% Si, it was confirmed experimentally that only silicon was oxidized.
As the desiliconization reaction proceeded, the carbon content on the Fe-Si-C alloy increased slightly in the experiments by CO2 injection. The overall rate constant of the desiliconization was one order of magnitude larger than that of the decarburization. Experimental results showing variations of silicon and carbon content with time are shown in Fig 2-14 and 2-15 respectively. The overall rate constants of simultaneous carbon and silicon oxidation under
-6 -6 the condition of Pco2 = 1 were determined to be 4.0x10 m/s and 5.0x10 m/s respectively. It is inferred simultaneous oxidation of carbon and silicon causes some retarding effects on the occurrence of each reaction.
32 Chapter 2 Literature Review
Figure 2-14: Effect of temperature on the oxidation Figure 2-15:Variation of Carbon contents behavior of Silicon in Fe-C-Si melts by of Fe-C-Si melts with time by CO2 injection at 1300 °C [Ono-Nakazato(2001)] CO2 injection 1300 °C [Ono-Nakazato (2001]
These experimental studies were undertaken on melts with Si contents less than 10%. More decarburization studies for high-silicon alloys are presented as follows.
The decarburization of solar-grade silicon had been investigated by Sakaguchi et.al.
[Sakaguchi et al (1992)]. Carbon in silicon exceeding 50 ppmw was presented as SiC, and at lower concentration, as dissolved carbon.
The removal of C from silicon and ferrosilicon during the ferrosilicon manufacturing process had been investigated extensively by Klevan [Klevan (1997)]. The carbon is present in liquid silicon and ferrosilicon as SiC and dissolved carbon. The oxidation process is described using equations (2-35 to 2-39)
Si (l) + ½ O2 (g) = SiO (g) (2-35)
C + ½ O2 (g) = CO (g) (2-36)
SiO (g) = ½ Si (l) + ½ SiO2 (s) (2-37)
33 Chapter 2 Literature Review
The overall reaction can be written as:
C + ½ SiO2 (s) = ½ Si (l) + CO (g) (2-38)
With
1/ 2 aSi p CO 35069 lnK ln1/ 2 18.32 (2-39) aC a SiO 2 T
Where
aSi = activity of silicon in the metal bath
aC = activity of carbon in the metal bath
pCO = CO partial pressure in the gas phase
aSiO 2 = silica activity
From (2-35) it is seen that decarburization is favoured by low silicon activity and a high carbon activity in the metal, a high silica activity in the slag and a low CO partial pressure in the gas phase. The oxidation of carbon is enhanced by high temperatures.
Carbon monoxide has to diffuse into bubbles and then get transported by these bubbles to the surface of the metal bath. If the silica formed is a solid and dense layer around the bubbles, the diffusion through this silica layer can be a slow process. At total carbon contents above the levels given by (2-16) and (2-17) a solid –solid reaction between silicon carbide and silica is possible:
2 SiC + SiO2 (s) = 3 Si (l) + 2CO (g) (2-40)
3 2 aSi p CO 110939 lnK ln2 50.67 (2-41) aSiC a SiO 2 T
34 Chapter 2 Literature Review
With pure silicon the activity in (2-41) is all unity giving a partial pressure of CO below 0.10 atm. up to 1725 ˚C. The reduction of SiC by SiO2 is very limited and kinetically, this reaction should proceed very slowly.
Some experimental runs were conducted by Klevan [Klevan (1997)] blowing different
Argon/Air ratios at 1600 ˚C and a fixed flow rate (1.5 x 10-4 Nm3/s). Powersim® package
-4 was used for simulating, using 2 different mass transfer coefficients (kc =1x10 m/s and
5x10-6 m/s). The results showed that there was a large resistance against removal of carbon from liquid silicon when blowing air and Ar into the melt, since the simulation model gave a mass transfer coefficient 5x10-6 m/s. Bubbles tended to break up and coalescence [ Anderson et al (1987)] and bath circulation reduced the residence time [Klevan (1997)] The reaction between oxygen in the gas bubbles and liquid silicon resulted in the formation of a silica layer around the bubbles (Fig 2-15) which slowed down the decarburization.
The decarburization model in industrial case was also studied. There were two different flow rates (15 and 30 Nm3/h) at 1600 ˚C. The initial carbon level was set at 150 ppmw, which was equal to the solubility of carbon in FeSi 75. The results are presented in Fig 2-16 and resulted in a very low decarburization rate. Reduction of carbon level from 150 ppmw to 100 ppmw took 4 hours with an inert gas flow rate of 15 Nm3/h. When the flow rate was increased, the rate of decarburization improved but the removal rate was still low.
35 Chapter 2 Literature Review
Figure. 2-16: Formation of a silica layer around a rising gas bubble when blowing oxygen into liquid (ferro) silicon. The dense silica layer may result in a small mass transfer
-4 coefficient kc << 10 [Klevan (1987)]
Figure 2-17: Decarburization of 1.6 m3 FeSi 75 at 1600 °C. [Klevan (1997)]
36 Chapter 2 Literature Review
The results of these analyses combined with theoretical considerations have been summarized below (Fig 2-17): a) Carbon removal in the form of CO is caused by a decarburization reaction of the type
where the oxygen activity is controlled by the equilibrium. It may not be of much
significance in industrial handling and refining of ferrosilicon. b) With a density difference between FeSi75 and SiC of 0.055 g/cm3 at 1450 ºC, the power
of flotation of SiC particles in liquid ferrosilicon is not sufficient to give any noticeable
separation effect. c) SiC particles are effectively transferred to CaO and Al2O3 containing silicate slag being
formed when metal is exposed to air during tapping, pouring and teeming. d) In the period of inert gas stirring, a transfer of SiC particles to the ladle walls removes
carbon, where a semi-molten slag layer forms at the interface between the molten metal
and the oxide lining. This indicates that it has a limited capacity to capture SiC particles
and explains why the observed carbon removal rate decreases with time during this
period. e) Other mechanism that affect the macro and micro distribution of carbon in the cast are the
segregation of dissolved carbon along with other elements during solidification and the “
pushing” of the SiC particles ahead of the moving liquid/solid interface.
37 Chapter 2 Literature Review
Figure 2-18: Variations in carbon content and temperature in FeSi75 (l) during a sequence of pouring (at time t > 0). Centric bottom Ar-stirring (0 < t < 50 min) and casting (t >50 min) at Elkem Thamshavn. [Klevan (1997)]
2.6 Wettability
Wettability is the spontaneous response at the interface when a liquid is brought into contact with the solid surface. In metallurgical processes, the interaction between various phases is generally governed by the reactions occurring at the interface. Wetting phenomena is complex and its interpretation can sometimes be controversial. The wetting of solid surface that has a higher melting point than liquid metal occurs in many physical phenomena and technological process such as nucleation and growth, casting, soldering, welding and sintering among other. In metallurgical reactions, interfacial reactions might influence significantly the reaction rate of some process.
The analysis of wettability in the systems formed by metal and binary compounds can be, to a first approximation, carried out by considering the interactions between the liquid metal and each type of components atoms of the solid phase. The actual strength of the interaction of the metal and the compound will be weakened by the bonding between the components.
38 Chapter 2 Literature Review
Wetting refers to the macroscopic manifestation or molecular interaction between liquids and solids in direct contact at the interface between them. Such manifestations include the formation of a contact angle at the solid/liquid/gas interline, the spreading of a liquid over a solid surface and the penetration of a liquid into a porous solid medium. Various wetting phenomena may be characterized in terms of macroscopic thermodynamic properties, described as “wetting parameters”. [Andreson and Engh (1987)]
Wettability can be expressed by the value of the contact angle θ and that has been recognized as a critical parameter in bonding between two different materials. Young’s equation (2-42) relates this parameter to the surface energies (γ).
cos slsv (2-42) lv
The contact angle is a directly observable parameter describing the compatibility of a solid and liquid in equilibrium with a vapour. A liquid is said to wet a solid if θ < 90°.
If θ = 0°, the liquid completely spreads over the substrate of the solid. When θ > 90°, the system is under a non-wetting condition. There are several established methods for the measurement of contact angle. Some of the techniques that have been used are the tilting plate method [Blake (1993)], the drop weight method [Bashforth and Adams (1883)], the captive bubble method [Kemball (1946)], Neumann’s method [Rotenberg, Borucvk and
Neumann (1983)], the pendant drop method [Jimbo, Sharan and Cramb (1993)], the capillary rise technique [Sharon and Cramb (1997)] and the Wilhemy plate technique [Butler and
Bloom (1966)].
Sessile droplet method is an established method for this study and is based on (2-42). The technique uses a small liquid droplet placed in a flat, well- prepared and highly-polished surface. The contact angle is obtained by visual means and therefore, surface energy can be calculated. (Fig 2-19). The droplet must be as small as possible to render the influence of
39 Chapter 2 Literature Review gravity on deforming the droplet negligible and the shaped obtained due to the interaction liquid droplet / substrate is merely due to the balance of the interfacial force only.
Traditionally, X-ray transmission technique was used as a light source to investigate the droplet/ substrate assembly on a film negative. The contact angle was measured using the obtained photos [Kemball (1946)][Jimbo et al (1993)][Sharon and Cramb (1997)][Butler and
Bloom (1966)]. Recently, computer aided interfacial measurement systems have been developed, combining the sessile-droplet method with X-ray radiography. The Laplace equation is solved by using numerical solution.[Mehta and Sahajwalla (2000)]
The dynamic wetting can be explained as the change of the wetting of a solid / liquid system over a period of time. Chemical reaction or mass transfer is generally the main reason for those changes. Change of composition at the interface also might occur. Any factor that can cause a small change in the interfacial energy could result in significant changes in the wetting behaviour and therefore, the solid/liquid contact angle of the system. If there is no significant change in the interfacial energy during the interaction, the system is characterized by an almost steady contact angle.
Figure 2-19: Contact Angle θ in the solid-liquid-vapour system in equilibrium.
[Kozatevich et al (1955)]
40 Chapter 2 Literature Review
2.6.1 The system of liquid iron and graphite
Carbon dissolution from graphite into liquid iron occurs in this system. The chemical reaction causes solute atoms to dissociate from solid graphite to form a micro region with high solute content at the interface:
C crystal C atom (2-43)
Once the chemical reaction occurred, the mas transfer would also occur. The atom will diffuse into liquid from the interface and form a liquid boundary layer as shown in Fig. 2-20:
C atom [C] (2-44)
C atom Boundary layer Liquid
C crystal [C]
Interface
Figure 2-20: Process of solute atom transport
In the system of liquid iron and graphite, the mass transfer of carbon on the interface can be expressed as:
C 2C DKC (2-45) t x 2
Where
K – the rate coefficient of the chemical reaction
D – the diffusion coefficient
41 Chapter 2 Literature Review
Mass transfer and interfacial reaction between graphite and molten iron can allow the wetting condition (contact angle and interfacial tension) to be changed, mass transfer and interfacial reaction have a significant influence on wettability.
The wettability of pure iron on graphite has been well-investigated [Wu and Sahajwalla
(2000)][Zhao (2003)][Cham et al (2004)][Jimbo and Cramb (1992)][Espie et al
(1994)][Sharan and Cramb (1997)]. Some experimental results on graphite substrates are shown in table 2-2. These studies showed that pure iron showed good wetting at high temperatures (1300 – 1500 °C) under different atmospheres, with contact angles ranging from
0 to 66°. However, it was observed that the wetting was affected when carbon content increased in the melt. Contact angle increased from less than 90° up to around 140° [Sharan and Cramb (1992)] .
Wu and Sahajwalla [Wu and Sahajwalla (2000)] studied the influence of the sulphur on the wettability of Fe-C-S melts on graphite at 1550 °C. It was found the equilibrium contact angle in a graphite/iron wetting system increased when sulphur increased in the melt and led to a decrease in the rate of carbon dissolution in iron. The formation of an activated complex of sulphur at the interface on the graphite dissolution could be responsible for the poor wetting observed.
42 Chapter 2 Literature Review
Table 2-2. Reported Contact Angle Values of Liquid Iron on Graphite.
Metal Substrate Atmosphere T (°C) θ (°) Time Reference
(min)
Pure iron Graphite Vacuum 1550 0 [H&K]
Pure iron Graphite H2 1550 37 [H&K]
Pure iron Graphite Helium 1550 51 [H&K]
Pure iron Graphite Vacuum 1550 37-51 [Cham et al],
[N et al]
Pure iron Graphite Argon 1300 66 [Sun et al]
Pure iron Graphite Argon 1400 60 [Sun et al]
Pure iron Graphite Argon 1500 59 [Sun et al]
Pure iron Graphite Argon 1600 97-55- 0-2-30 [Wu et al]
62
Pure Iron Graphite Argon 1600 64-38 0-9 [Zhao]
H&K: [Humenik and Kingery (1954)]
N et al: [Naidich and Koleshnishencko (1961)]
Cham et al: [Cham et al (2004)]
Sun et al: [Sun et al (1998)]
Wu et al: [Wu and Sahajwalla (2000)]
Pure iron: 99.98% electrolytic iron
43 Chapter 2 Literature Review
2.6.2 Wettability of liquid iron on alumina.
Alumina is used as an important high temperature ceramic material. The wettability of alumina by liquid iron has been investigated since the 1960’s. Table 2-3 shows a summary of some experimental results.
Table 2-3: Reported Contact Angle values of liquid Iron on alumina.
Metal Atmosphere T (°C) θ (°) Time Reference
(min)
ARMCO Vacuum 1550 141 [H&K]
Electrolytic Iron H2 1550 121 [H&K]
Electrolytic Iron Helium 1550 129 [H&K]
Fe3+ 3.9%C Helium 1570 105 [A&K]
St50 Steel Helium 1560 120 Antipin
St60 Steel Helium 1560 117 Antipin
08Yu Steel Helium 1560 125 Antipin
Fe2+ 0.2%C Argon 1550 123 Mch
St20K steel Argon 1560 115 Efi
St35 steel Argon 1500 112 Pro
St35 steel Argon 1550 105 Pro
St35 steel Argon+O2 1550 74 Pro
St35L steel Argon 1560 130 [T&P]
Fe + 0.45%C 1535 105 Der
St60 steel Helium 1530 109-100 0-40 Popel
St60 steel Helium 1550 136 [V&U]
44 Chapter 2 Literature Review
H & K: [Humenik and Kingery (1954)]
ARMCO iron (ppm): 180 [C]; 100[Si]; 1600[Mn]; 60[P]; 250 [S]; 550[O]; 36[N]
A & K: [Allen and Kingery (1959)]
Antipin [Antipin et al (1978)]
Mch: [Mchedlishvili et al (1965)]
St20 steel (%): 0.19 C, 0.29 Si, 0.65 Mn, 0.03 S, 0.016 P, 0.02 Ti, 0.025 Al, 0.0075 O
Efi: [Efimov et al (1971)]
Pro: [Prokofleva (1969)]
St35 steel (%): 0.35 C, 0.3 Si, 0.7 Mn, 0.025 S, 0.026 P, 0.09 Cr, 0.29 Ni, 0.22 Cu
T & P: [Tsareskii and Popel (1963)]
St35L steel (%): 0.38C, 0.42 Si, 0.56 Mn, 0.03 S,0.04 P
Der: [Deryabin (1968)]
Popel: [Popel et al (1969)]
St60 steel (%): 0.61C, 0.67 Mn, 0.27 Si, 0.016 P, 0.2 S, 0.12 Cu, 0.03 Cr, 0.04 Ni
V & U: [Vatolin and Ukhov (1972)]
Research on the wettability of alumina by liquid iron suggests that the contact angles were in the range of 100 to 141º within the temperature interval of 1530 – 1600 ºC under different atmospheres of vacuum, helium, argon and hydrogen gases.
2.6.3 Wettability of silicon and ferrosilicon on SiC
Reported values of contact angle between Silicon and Silicon Carbide were compiled from different authors and shown on Table 2-4.
Kalogeropoulos et al [Kalogeropoulos et al (1995)] measured the wettability of -SiC by
FeSi using two different compositions (33.33 and 66.67 at% Si). The contact angle measured was 35º for both alloys at their respective melting points.
45 Chapter 2 Literature Review
Table 2-4 Contact angle of Si (l) on SiC.
Reference number (º) Temperature(° C)
[Klevan] 36 1480
[Klevan] 42 1430
[N&O] 0 1500
[N et al] 38 1410 / 1500
[N et al] 41.5 1460
[N&O]: [Nogi and Ogino (1998)]
[N et al]: [Nikolopoulos et al (1992)]
2.7 Summary of the literature review.
An overview of the cupola process and the main reactions of concern have been presented in this chapter. When the metallic charges descend towards the burden bath, the iron and steel scrap as well as the ferrosilicon melt interact with the coke bed; the hot gases (CO and CO2) ascends from below, oxidizing the metal. Some of the phenomena such as carbon dissolution, oxidation and dynamic wetting take place. These will be investigated in detail.
Phase diagrams for the Fe-Si, Si-C and Fe-Si-C systems have been reported in the literature.
Several studies had found more precision in some lines and points, although the Fe-rich corner had been the main aim in many of those researches. The formation of different phases such as SiC, FeSi and FeSi2 is satisfactorily explained by numerous previous researches and
46 Chapter 2 Literature Review thermodynamic calculations although some recent work had brought more precision to certain values.
The mechanism of carbon dissolution from graphite in pure iron had been well established by a number of studies and can be divided into two steps:
i) Dissociation of carbon atoms from its crystal site in the graphite into the
carbon/melt interface.
ii) Mass transfer of carbon atoms through the adjacent boundary layer into the bulk
liquid iron.
The carbon dissolution in iron is generally controlled by a mass-transfer in liquid side mechanism. It had been found the dissolution rates are governed by the diffusion of carbon from the metal-carbon interface to the bulk liquid through a boundary layer.
Decarburisation of iron alloys by carbon dioxide gas can be summarized as follows:
- For carbon contents higher than a critical value, the rate of decarburisation is
independent of the carbon concentration; this critical value is about 0.5 wt% carbon in
the melt. Below this critical concentration, the decarburisation rate decreased with the
carbon concentration.
- The decarburisation rate increased with increasing gas velocity, partial pressure of
CO2 and the reaction temperature.
- Several studies indicated that the CO2 dissociation was the rate limiting step for
decarburisation by CO2 in metal droplets.
Several studies regarding the carbon solubility in silicon have shown that the solubility of carbon in the system Si / SiC at equilibrium is very low. The effect of the silicon contained in ferrosilicon alloys on the carbon solubility had been also studied. It was found the carbon solubility increased with the increase of silicon in the ferroalloy. However, below certain point, carbon level started to increases when silicon content decreased.
47 Chapter 2 Literature Review
Decarburization and desiliconization in steel had been conducted by previous studies, particularly when silicon is below 10%. For liquid iron with high carbon and silicon levels, the oxidation rate was controlled by diffusion in the gas phase. Under certain conditions, silicon was oxidized in preference of carbon. The overall rate constant of the desiliconization was one order of magnitude larger than the decarburization.
When silicon contents were well above this value and in presence of Ar – air blowing showed a large resistance of carbon removal due to the reaction between oxygen in the gas bubbles and liquid silicon formed a silica layer around the bubbles, slowing down the decarburization.
There are several methods well-established for the study of wettability and contact angle measurement. Description of the sessile-drop method has been provided and some recent developments were explained. The wetting behaviour of iron on a graphite substrate and silicon and ferrosilicon alloys on SiC had been reasonably well-studied.
Pure iron showed good wetting on graphite at high temperatures under different atmospheres
(vacuum, helium, hydrogen and argon). Previous studies proved the wettability of pure iron on graphite was good and the contact angle ranged from 0 to 60°. However, when initial carbon increases the wetting behaviour changed substantially, showing poor wettability (θ >
140 °).Research on the wettability of alumina by liquid iron suggested that the contact angles were in the range of 100 to 141º under a range of different atmospheres (helium, hydrogen and argon) as well as under vacuum. It was also found that the wetting in alumina improved considerably once the metal was oxidized.
Previous work studying the wettability of silicon on SiC substrates under different temperatures (1410 – 1500 °C), agreed the wetting was good ( θ = 0 - 42°). Ferrosilicon also showed wetting and final contact angle values appeared to be independent of the % Si in the ferroalloy.
48 Chapter 2 Literature Review
However, despite the important role of the carbon pickup in the scrap-melting process, the reaction kinetics of the carbon dissolution in silicon-rich alloys is not yet well understood.
Although there is a large body of work studying the interaction of silicon and ferrosilicon alloys in SiC, those researches had been focused on bonding properties. The dynamic wetting from the metallurgical point of view had not been studied in depth, in spite the significance of the interfacial interaction in the reaction kinetics. The simultaneous decarburization and desiliconization has been studied but the information found is referred to low silicon and carbon content in the metal, which is not the situation found in the cupola process, where carbon and silicon contents are, so far, below the limits required during the steelmaking.
Present study reports the reaction kinetics of the carbon dissolution on 25, 75 and 98 wt% Si alloys, the dynamic wetting of those ferroalloys on graphite and SiC and the interaction of the gas phase with carbon-saturated ferrosilicon alloys.
49 CHAPTER 3
EXPERIMENTAL DETAILS Chapter 3 Experimental Details
This investigation aims to understand fundamental mechanisms of carbon dissolution in silicon ferroalloys during the cupola melting process, as well as interfacial phenomena during the interactions between carbon and ferrosilicon alloys. The influence of the gas phase on the metal during the melting process in the cupola has been investigated. Carbon dissolution investigations in ferrosilicon alloys were conducted at high temperatures using a horizontal furnace. Carbon transfer from the substrates to the metal was measured by the LECO CS 244
Carbon Sulphur Analyser. Dynamic wetting investigations were carried out by measuring the contact angle between the molten metal and the synthetic and natural graphite as well as 85%
SiC substrates. Interfacial studies were conducted by X-Ray Diffraction (XRD), while the
Electron Probe Micro Analyser was used for interfacial analysis and chemical characterisation of the metallic phase. Gas phase interactions of the molten alloys were determined by measuring off-gas analysis.
3.1 Sample selection
Metallurgical-grade coke added as part of the charge plays two important roles in the cupola process. The major exothermic reaction takes place between carbon contained in the coke and the oxygen in the blast. The combustion reactions provide the required amount of heat to melt the scrap metal and to keep the liquid bath at the desired temperatures. Carbon from coke also dissolves into the liquid metal, raising the carbon content up to the required levels.
Silicon is another important alloying element present in cast irons, and plays an important role in controlling iron structure and properties of cast irons. It is added predominably as ferrosilicon lumps with up to 75 wt% of silicon. During the cupola process, alternative layers of coke, iron and steel scrap, fluxes and ferrosilicon are added continuously, interacting each other and with the oxidizing gas phase.
51 Chapter 3 Experimental Details
This chapter provided details material selection and process parameters, experimental apparatus and instruments for the investigation of the carbon dissolution phenomena during the carbonaceous material – ferrosilicon alloy interaction, the interfacial reaction and the decarburisation and oxidation reactions occurring during the process.
3.1.1 Silicon and Ferrosilicon alloys
It is planned to investigate a range of compositions of ferroalloys. Ferrosilicon containing 75 and 50 wt% of silicon are used widely in steelmaking and cast iron processes, therefore their relevance to this work (Table 3.1) [Cupola Handbook (1984)].
Silicon 98.5% and FeSi 25 were chosen as reference materials in this study, covering both, high and low silicon limits. Lower silicon content up to 3.5% wt Si which are of great importance for steelmakers, have been extensively reported in the literature and are therefore not covered in the present work.
Raw materials required to prepare the high-purity ferrosilicon alloys were electrolytically - grade pure iron chips (99.98%) (CAS Number 7439-89-6 ) and high-purity silicon chunks
(98.5%) (CAS Number 7440-21-3), both were supplied by Sigma-Aldrich. The materials were carefully weighed and mixed thoroughly to achieve the desired compositions. The amount of metal sample used in the carbon dissolution experiments ranged between 0.2-0.4 grams. Samples were crushed in finer pieces and compacted into the crucibles to achieve enhance homogenization.
Mixtures were melted using a lab-scale arc smelter. Before melting, the entire system was checked for leaks in order to prevent any air/oxygen ingress. The system was under vacuum after purging inert gas several times to assure the removal of any remaining oxidizing gas.
The inert gas stream (10% Hydrogen, 90% Argon) was supplied continuously while the smelting was on. The mixture was sparked with the electric-arc for 15-20 seconds. The resultant product was a round-shape tablet. After this period of time, the process was stopped
52 Chapter 3 Experimental Details to avoid excessive temperatures (greater than 1650 °C) that could damage the equipment. The tablet was turned over and after few minutes, the above procedure was repeated few more times. Once the tablet cooled down, it was broken in smaller pieces and chemical analyses were carried out by Induced Coupled Plasma mass spectroscopy (ICP). Results are shown on
Table 3-2 a and b.
Commercial ferrosilicon alloys contain high impurity contents such as aluminium and calcium; make them unsuitable for fundamental studies. Aluminium affects the carbon solubility in the molten ferrosilicon [Ottem (1993)]. On the other hand, ferroalloy containing
50 wt% of silicon was extremely brittle, making its separation from the graphite crucibles very difficult and therefore introducing unacceptable error levels when samples were analysed using the LECO.
Table 3-1 Typical composition of few commercially available ferrosilicon alloys [Cupola
Handbook (1984)]
Composition (Weight per cent)
Si Al Ca C Fe
FeSi 50 45-50 0.50-1.25 0.60-0.90 NR* balance
FeSi 75 74-79 0.75-1.25 0.5-1.0 NR* balance
*NR – Not reported
Table 3-2 a Chemical composition of the FeSi 25
Ferroalloy Other elements composition Fe (wt%) Si (wt %) Al (wt %) (wt %) 69.8 25.8 <0.1 <0.1 68.1 26.3 <0.1 <0.1 FeSi 25 72.6 24.6 <0.1 <0.1 81 18.5 <0.1 <0.1 76.6 22.9 <0.1 <0.1
Average 73.62 23.62
53 Chapter 3 Experimental Details
Table 3-2 b Chemical composition of the FeSi 75
Ferroalloy composition Fe (wt%) Si (wt%) Al (wt %) Other elements (wt%) 26.7 72 <0.1 <0.1 24.3 73.6 <0.1 <0.1 FeSi 75 24.5 73.4 <0.1 <0.1 26 73.5 <0.1 <0.1 28 71.5 <0.1 <0.1
Average 25.9 72.8
3.1.2 Synthetic graphite (SG)
Synthetic graphite was used as the main carbon source to understand the effect of ash-free carbon on carbon transfer and to minimize the impact of other components such as the volatile matter and the ash in such reaction. Synthetic graphite was also used for the dynamic wetting investigation. The absence of impurities and negligible ash content minimized the potential interference on the wetting behavior. It is well established that even small amounts of components such as sulphur, can affect significantly the wetting behavior between the carbonaceous material and the metal. [Wu et al (1998)] An analysis of the synthetic graphite is provided on Table 3-3 and shows 0.2% of ash content. Synthetic graphite crucibles (Fig 3-1 a, b, c) were used for carbon dissolution experiments. Their structure was characterized by XRD resulting on a 2H (Clifftonite) structure (Fig 3-1d). There were also synthetic graphite plates made out the same material, which were used for wettability studies.
Typical dimensions were 24 x 24 x 2 mm. (Fig 3-2)
54 Chapter 3 Experimental Details
Table 3-3 a Composition of synthetic graphite. Proximate analysis
Proximate analysis Synthetic Graphite
Ash (%) 0.2
Volatile Matter (%) 0.4
Hydrogen NM
Nitrogen NM
IM 0.2
Carbon 99.1
Table 3-3 b Composition of synthetic graphite. Ultimate analysis
Ultimate analysis Synthetic graphite
(%)
SiO2 39
Fe2O3 22.3
Al2O3 13
TiO2 1.2
P2O5 0.28
Mn3O4 1.3 CaO 6.7
Na2O 3.0 MgO 4.8
K2O 3.6
SO3 2.4
V2O5 0.06
Cr2O3 0.60
55 Chapter 3 Experimental Details
10 mm
15 mm
Figure 3-1a: Synthetic graphite crucibles. Figure 3-1b: Synthetic graphite Crucibles. Dimensions. .
Figure 3-1c: Graphite crucibles placed on the alumina boat.
2000
1800
1600
1400
1200
1000 Intensity
800
600
400
200
0 20 25 30 35 40 45 50 55 60
2- Theta
56 Chapter 3 Experimental Details
Figure 3-1d: XRD scan for synthetic graphite crucibles. Peaks corresponded with 2H- graphite (Clifftonite)
Figure 3-2: Graphite plate placed on alumina sample holder.
3.1.3 Natural Graphite (NG)
The dynamic wetting behavior also was investigated between a natural graphite substrate and molten ferrosilicon alloys. It is well established that natural graphite shows good wetting behavior with molten steel [Timcal (2006)]. Natural graphite have greater content of ash impurities compared to synthetic graphite, but this material was especially chosen due to their relevance for industrial conditions, The pulverized natural graphite was supplied by Asbury
Graphite Mills, NJ, USA and the particle size was < 63 μm. Characterization of natural graphite is shown on Tables 3.4 and b.
3.1.4 Silicon carbide (SiC)
During the molten silicon/ferrosilicon – graphite interaction, the appearance of SiC as interfacial product is thermodynamically feasible. This was later confirmed during the preliminary interfacial studies after the carburization experiments were completed. The appearance of the SiC layer at the interface suggested it could play key role during both, carbon dissolution reaction and wettability. In view of this, Silicon Carbide was also used as a substrate for both investigations. The SiC substrates were obtained from supplied SiC tiles
57 Chapter 3 Experimental Details and were cut using a diamond saw. Selected dimension was (24 x 24 x 2 mm). The composition of the substrate was 85% SiC – 15% Si.
Table 3-4 a Composition of natural graphite. Proximate analysis
Proximate analysis Natural Graphite
Carbon (%) 96.9
Hydrogen (%) NM
Ash (%) 2.00
Mineral Matter (%) 0.2
Volatile Matter (%) 0.5
Moisture (%) 0.17
Nitrogen (%) 0.01
Table 3-4 b Composition of natural graphite. Ultimate analysis
Ultimate analysis (%) Natural graphite
SiO2 41.4
Fe2O3 16.5
Al2O3 15.2
TiO2 0.56
P2O5 0.14
Mn3O4 0.17 CaO 8.5
Na2O 1.2 MgO 9.4
K2O 2.3
SO3 2.98
V2O5 0.05
Cr2O3 0.06
58 Chapter 3 Experimental Details
3.1.5 Alumina
The crucibles for the gas phase – metal investigation were made out of 99.9% Al2O3. This material is considered ideal for the production of crucibles and other refractory parts, displaying excellent corrosion resistance and mechanical wear resistance and strength, as well as good thermal resistance. The crucibles were made by the strip casting method. A solution containing 99.9% Al2O3 was mixed thoroughly for about 2 hours and further cast in moulds.
This step was repeated twice, allowing about 5 minutes en between to settle the material. The leftovers were removed and the crucibles were left for drying during 24 hours before they were sintered at 1200 °C during 2 hours. The crucible dimensions are shown on Fig 3-3 a and b.
10 mm
12 mm
Figure 3-3a: Alumina crucibles. Dimensions. .
Figure 3-3b: Alumina crucibles.
59 Chapter 3 Experimental Details
3.1.6 Gas composition and flow rate
In order to investigate the effect of oxidizing atmosphere on carbon-saturated ferro (alloys), several experiments were undertaken under a mixed gases containing CO2 and CO.
Relative properties of gases are shown in Table 3-5. Flow rates were set at 1 liter/minute for gas 1 and 0.5, 1 and 2 liters/minute for gas 2. The gaseous mixture 2 was set up according to previous studies regarding the gas phase profile in the cupola furnace [Cupola Handbook
(1984)] for regions close to the tuyeres (oxidation zone). Argon gas was purged into the reaction chamber until samples were melted, and then, the reactive gas switched on. After keeping the samples under such atmosphere for a fixed period, the reactive gas was switched off and argon was brought back to the reactor chamber and then, the sample was quenched.
Table 3-5 Gas composition.
Composition (%) CO2 CO N2
Gas 1 100 - -
Gas 2 20 2 78
3.2 Experimental equipment and instruments used
The experimental work was carried out employing different pieces of equipment and analytical instruments. Among the most important are the horizontal tube furnace, Infrared
(IR) gas analyzer, X-Ray Diffractometer and Electro Probe Micro Analyzer (EPMA).
3.2.1 High Temperature Horizontal Furnace
The horizontal furnace used on the current project has been described thoroughly by several authors [Zhao (2003][ Wu and Sahajwalla (2000)][ Mehta and Sahajwalla (2000)] (Fig 3-4) It includes 1 meter-long and 50 mm inner-diameter high alumina reaction tube with two Super
60 Chapter 3 Experimental Details
Kanthal resistance-heating elements. The furnace is built from a double skin stainless steel with welded tubular chassis. For heat dissipation, a cooling fan was set on the top. There are two separate temperature indicators, each one with Pt-30%/Rh/Pt-6% Rh thermocouples. One of them is placed inside of the reaction chamber, near to the sample, while the others monitored the furnace temperature.
The temperature profile along the length of the furnace tube was also produced to determine the location of the hot zone. Under an established temperature, a thermocouple sample was positioned at several distances, allowing temperature measurements and obtaining a temperature profile. (Fig 3-5) The general procedure for all planned experiments was identical. The assembly of sample and substrate was placed on the cold zone for about 10 minutes. This pause was needed to allow certain temperature homogeneity, since sudden temperature variation might lead to the risk of thermal cracks in the alumina tube.
Cold zone Alumina boat Purified Argon (Inlet gas)
Thermocouple
CCD Camera
Time/Date generator
Off gas Alumina tube TV Hot zone VCR
Figure 3-4: Schematic of the experimental arrangement.
61 Chapter 3 Experimental Details
The other purpose was to flush away any remaining gas from the outside atmosphere, since the experimental conditions need to take place under an inert atmosphere. After this period of time, the sample was pushed via a sliding mechanism into the hot zone and kept there for the desired time. Once the run was over, the sample was quenched into the cold zone and kept for about 10 minutes before taking it outside of the furnace.
Due to the natural tendency of silicon to oxidize at high temperatures, a high-purity Argon gas stream (> 99%) was used during the carburization runs. Gas flow was controlled by an electronic flow controller. For removing away any remaining moisture and oxygen, an additional gas cleaning system was used based on previous successfully experimental work
[Kapilashrami (2001)] (Fig 3-6)
1600
1550
1500
1450
1400
1350
Temperature (C) 1300
1250
1200
1150 0 5 10 15 20 25 Distance ( cm)
Figure 3-5: Temperature profiles. Values decrease from the hot zone to the cold zone.
62 Chapter 3 Experimental Details
1- Silica gel 2- Ascarite 3- Magnesium per chlorate 4- Cooper turnings (823 K) 5- Magnesium chips (773 K) 6- Gas flow meter
Figure 3-6: Schematic of the gas cleaning system.
A high-resolution CCD (charged coupled device) camera was fitted with an IRIS lens. The output was channeled to a VCR-TV monitor. A time and date generator was used in the system to display the date and duration of the reaction. The recorded images were captured by using a frame grabber and copied in a computer. Due to rapid changes in the dynamic wetting, selected images were taken every 10 seconds. The wettability of the liquid ferrosilicon/substrate was investigated by measuring the interfacial contact angle using appropriate software based on Laplace equation. The software performed a curve fitting analysis using an iterative procedure.
3.2.2 Off-Gas Analysis
A BROOKS Instruments mass-flow controller, model BBC 1A11A was used to control gas flow rates. Two flow controller sensors, one for argon gas and the other for the oxidizing gas mixture, were attached to the corresponding ports. Changes in oxygen partial pressure were
63 Chapter 3 Experimental Details
tracked down by an Infra Red (IR) ABB 2020. The off-gas composition (CO2/CO) was recorded into a coupled personal computer. The data were recorded as volume (%) and
(ppm). The changes in off-gas composition usually were observed during the first 15 minutes and generally remained steady after this period. The upper detection limits of the IR were 20
% (Volume) CO2 and 5 % CO, therefore, no changes were detectable during the oxidation experiments, since the inlet gas composition was above those limits.
3.2.3 Image Analysis and Contact Angle Measurements
The wettability was characterized by measuring the contact angle at the ferroalloy/substrate interface. The experimental approach involved used the sessile- drop method as function of the type of substrate, ferroalloy composition and time. The interfacial reaction was also investigated, by studying the formation of interfacial products at the ferroalloy/substrate interface. The study correlates the dynamic wetting and the interfacial reaction. The experimental arrangement allows investigating the dynamic wetting of ferrosilicon alloys by carbonaceous materials. Obtained images were observed, recorded and captured by appropriate means for further contact angle measurements. Figure 3-11 shows an example of a screen shot and calculated parameters during the dynamic wetting experiments.
Figure 3-7: Example of contact angle measurement during the dynamic wetting investigations.
64 Chapter 3 Experimental Details
A flow diagram of the algorithm is detailed in previous work [Wu et al (1998)] as shown on
Figure 3-8. For contact angle values below 30˚, the accuracy of the above-mentioned software was insufficient. Therefore, a different methodology was followed.
Special software allowed defining the edges of the droplet, by selecting specified and varying number of points. The points were conveniently arranged by fitting a curve to the image with various degrees of polynomial equations (degrees 1 to 5) and measuring the angle between the tangent to the curve and the substrate. The contact angle could be obtained separately for the left and right sides.
3.2.4 Carbon and Sulphur Analyser
The LECO CS 244 was used to determine carbon levels during carbon dissolution experiments. The working principle for the determination of the carbon and sulphur content in the metal is by melting the sample in an induction furnace in the presence of excess oxygen. The combustion reaction takes place and all the carbon and sulphur is oxidized and the oxidised gases are detected by IR calibrated for carbon or sulphur.
3.2.5 X-Ray diffraction (XRD)
A Siemens D 5000 X-ray Diffractometer was used for the study of interface products at interface. Cooper Kα radiation (30 KV, 30 mA) was used as an X-ray source. Samples were placed on modeling compound and leveled, in order to guarantee an even exposed surface.
Samples were scanned in a step-scan mode (0.02° /step) at 1°/minute. Scanned angles started from 25 up to 100°.
65 Chapter 3 Experimental Details
3.2.6 Electro Probe Micro Analyser (EPMA)
Characterization of interfacial products, as well as mapping the element distribution throughout selected areas was carried using an Electron Probe Micro Analyzer (EPMA)
Cameca SX50. This equipment employs four wavelength dispersive spectrometers and one energy dispersive X-ray analyser for the detection and non-destructive analysis of almost all elements from Boron (4) down in the periodic table. X-ray elemental line scans were carried out across of constituents of interest. Specimens were also imaged using either secondary or backscattered electrons.
These investigations were carried out to identify the distribution of different components, not only in the bulk of the metal sample but particularly, identifying the formation of interfacial products. Scanning of selected points and areas, as well as element distribution mapping was employed. Figure 3-9 shows an example of the silicon distribution after the interaction of molten metal FeSi 25– synthetic graphite.
66 Chapter 3 Experimental Details
Select image area
Binary Image
Edge detection
y Inclination ?
n Correction
Setting top of substrate
Blanking extraneous information
Parameters selection
Profile points initialization
Curve fitting calculation
Overall sessile drop drawing graph display
Best fitting polynomial selection
Data ,image storage
End
Figure 3-8: The working flow sheet of “interface software” for analysis of the contact angle of a sessile drop image (Wu and Sahajwalla (1998)]
67 Chapter 3 Experimental Details
Metal droplet
SG substrate
Figure 3- 9a: Silicon mapping after FeSi 25 – synthetic graphite interaction at 1550°
Metal droplet
SG substrate
Figure 3- 9b: Optical image of after FeSi 25 – synthetic graphite interaction at 1550° C
68 Chapter 3 Experimental Details
3.3 Sample preparation
3.3.1 Sample preparation for Carbon analysis
The accuracy of the results depended, in a large extent, of the removal of impurities (graphite particles) from the samples. This issue was critical, since the results obtained at early stages could set distorted greatly by the presence of graphite particles adhering to the sample surface. Low-silicon samples were easily taken off the synthetic graphite crucibles, since no strong bond with the substrate was found. However, the adherence for the high-silicon samples was notably higher; therefore the separation from the substrate was carried very carefully. Graphite leftovers were removed by using sand paper (180 μm) placed in the rotating disk. After carefully visual inspection, some samples were cleaned by chemical attack. They were placed in a solution containing about 10 ml of 30% peroxide (H2O2) diluted on 200 ml of distilled water and heated at 80 – 90 ºC for about 2 hours.
The samples used for carbon dissolution on SiC substrates were cut in the boundary metal / substrate, using a very fine diamond saw at low speed. They were cleaned and sent for carbon analysis. Once the samples were cleaned, Carbon analyses were undertaken on the Carbon-
Sulphur analyser (LECO CS 244).
3.3.2 Preparation of Natural Graphite Substrates
The pulverised natural graphite and the coke samples (particle size < 63 μm) were compacted by using a steel die (Fig 3-10) without any binder. The amount of sample placed in the die was about 4 grams. The powder was compacted in a hydraulic press (Fig 3-11) under and the applied pressure was gradually increased by 1 ton steps every 2 minutes until reaching about
6 tonnes. Then, the pressure was decreased following similar pattern.
69 Chapter 3 Experimental Details
Figure 3-10: Hydraulic press Figure 3-11: Steel die
3.3.3 Sample preparation for XRD
The formation of the interfacial products was investigated using XRD. Figure 3-11 shows a schematic of the samples used for XRD analysis. The samples were removed carefully from the substrates and any remaining graphite was removed by grit paper (1200 μm) placed in the rotating polisher. The samples were molded using a thermosetting resin at 120 ˚C and then, ultrasonically cleaned.
Metal droplet Substrate
Figure 3-12: Schematic of the metal droplet – substrate sample used for interfacial
investigations
70 Chapter 3 Experimental Details
3.3.4 Sample preparation for EPMA
Interfacial studies were also carried out using EPMA and samples were similar to the ones used for XRD (Figure 3-12). Samples were mounted in epoxy resin and had a flat and well- polished surface, finished up to 0.1 micrometers. The specimen surfaces were flat with perpendicular sides. These were baked and later cooper-coated to facilitate microscopic examination.
71 CHAPTER 4
DYNAMIC WETTING BETWEEN MOLTEN SILICON AND FERROSILICON ALLOYS AND DIFFERENT SUBSTRATES
EXPERIMENTAL RESULTS - DISCUSSION Chapter 4 Dynamic Wetting
This chapter presents measurements of the dynamic wetting between silicon and ferrosilicon alloys and synthetic graphite, natural graphite and silicon carbide (SiC) at 1550 ºC.
Investigation of the interfacial product and its role in the dynamic wetting phenomena is also discussed.
4.3 Study of the dynamic wetting for silicon and ferrosilicon alloys on synthetic graphite
The influence of the ferroalloy composition on the wetting behavior was investigated for ferro (silicon) alloys ranging from 24.7 wt% up to 98.5 wt% on synthetic graphite substrate under argon atmosphere at 1550 °C. The wetting behavior of ferroalloys containing 24.7, 74 and 98.5% Si on synthetic graphite substrates is shown in Fig. 4-1 to Fig. 4-3. The samples displayed generally good wetting behavior (θ < 90º). Full wetting was observed for high- silicon ferroalloys within the first 90 seconds (Fig 4-4, 4-5). However, the final contact angle value appeared higher for the low-silicon ferroalloy (24.7 wt% Si) and remained steady around 70 degrees during the 2 hours-run (Fig 4-6)
a) 1 second b) 13 seconds
72 Chapter 4 Dynamic Wetting
c) 23 seconds d) 60 seconds
Figure 4-1: Still image of liquid silicon (Si 98.5%) droplet on synthetic graphite substrate a) 1 second after melting, b) 13 seconds, c) 23 seconds, d) 60 seconds
a) 1second b) 30 seconds
c) 60 seconds d) 80 seconds
Figure 4-2: Still image of liquid FeSi 74 droplet on synthetic graphite substrate a) 1 second after melting b) 30 seconds, c) 60 seconds, d) 80 seconds
73 Chapter 4 Dynamic Wetting
a) 1second b) 30 seconds
c) 50 seconds d) 300 seconds
e) 570 seconds f) 1170 seconds
Figure 4-3: Still image of liquid FeSi 24.7 droplet on synthetic graphite substrate a) 1 second after melting, b) 30 seconds, c) 50 seconds, d) 300 seconds, e) 570 seconds, f) 1170 seconds
74 Chapter 4 Dynamic Wetting
120
100
80
60
40
Contact Angle (degrees) Angle Contact 20
0 0 20 40 60 80 100 Time (seconds) Figure 4-4: Dynamic Wetting of Si 98.5 wt% on synthetic graphite.
160
140
120
100
80
60
40 Contact Angle (degrees) Angle Contact 20
0 0 20 40 60 80 100 Time (seconds)
Figure 4-5: Dynamic Wetting of FeSi 74 wt% on synthetic graphite.
75 Chapter 4 Dynamic Wetting
140
120
100
80
60
40 Contact Angle (degrees) Angle Contact 20
0 0 200 400 600 800 1000 1200
Time (seconds)
Figure 4-6: Dynamic Wetting of FeSi 24.7 wt% on synthetic graphite.
4.4 Investigation of the product formed during the interfacial reaction.
The occurrence of some interfacial reaction during the wetting on graphite, and therefore, the formation of interfacial product is likely to occur, according to (4.1):
Si (l) + C (s) = SiC (s) …………………….. (4.1)
At 1550 °C the Gibbs free energy for SiC = -5.58 x 104 J [Barin (1995)], so the formation of this product is thermodynamically feasible at such a temperature. Reported results from Nogi and Ogino [Nogi and Ogino (1988)] and Nikolopoulos et al [Nikolopoulos et al (1992)] showed good wetting between silicon and SiC, so this lead to further investigate the product formed during the interfacial reaction.
76 Chapter 4 Dynamic Wetting
The interface of ferrosilicon alloys/graphite was scanned by XRD in order to evaluate the formation of interfacial products. Figs. 4.7, 4.8 and 4.9 show the XRD scanning for Si 98.5%,
FeSi 74% and FeSi 24.7% interacting with synthetic graphite at 1550°C. The results showed that SiC appears 30 seconds after melting for Si 98.5%. The formation of SiC at the interface of FeSi 74%/graphite was observed 60 seconds after melting. For FeSi 24.7%, the SiC was detected after 30 minutes of the interfacial reaction. Back-scattered images using the EPMA
(400X) also showed the formation of SiC, although it was observed the interfacial layers were irregular and presented variation on the thickness. (Figs. 4.10, 4.11 and 4.12)
Silicon Carbide Silicon
700
600
500 t=30 seconds
400
Intensity 300
200
100 t=0 seconds
0 20 30 40 50 60 70 2-Theta
Figure 4-7: XRD spectra. Interface of Si 98.5% / synthetic graphite after 0 and 30 seconds
at 1550 °C
77 Chapter 4 Dynamic Wetting
SiC FeSi2 Fe5Si3
800
700
600 t=60 seconds
500
400 Intensity
300
200
100 t=1 second
0 28 33 38 43 48 53 58 63 68 73 2-Theta
Figure 4-8: XRD spectra. Interface of FeSi 74 % / synthetic graphite after 1 second and 60
seconds at 1550 °C.
78 Chapter 4 Dynamic Wetting
FeSi2
SiC 500
450
400
350 t=1800 seconds 300
250 Intensity 200
150
100 t=1 second
50
0 25 30 35 40 45 50 55 60 65 70 75 2-Theta
Figure 4-9: XRD spectra. Interface of FeSi 24.7 % / synthetic graphite after 1 second and
1800 seconds at 155 0°C.
79 Chapter 4 Dynamic Wetting
Figure 4-10: Back-scattered electron EPMA image at the interface Si98.5/synthetic graphite
(400X).
Figure 4-11: Back-scattered electron EPMA image at the interface FeSi 74/ synthetic graphite (400X)
80 Chapter 4 Dynamic Wetting
Figure 4-12: Back-scattered electron EPMA image at the interface FeSi 24.7/ synthetic graphite (400X)
4.5 Study of the dynamic wetting in SiC substrates.
Experimental runs investigating on SiC substrates on the dynamic wetting were carried out at
1550 °C. The studies were undertaken using the same ferroalloys described on section 4.1 and using SiC substrates. Still images are shown in Fig. 4-13 to Fig. 4-15.Comparative wetting behaviour of the ferrosilicon samples on synthetic and SiC are presented in Fig 4-16 to 4-18.
81 Chapter 4 Dynamic Wetting
1 second) 5 seconds)
26 seconds) 50 seconds)
71 seconds) 110 seconds)
Fig 4-13 Still image of liquid silicon (Si 98.5%) droplet on SiC substrate
82 Chapter 4 Dynamic Wetting
1 second) 10 seconds)
50 seconds) 70 seconds)
90 seconds) 110 seconds)
Figure 4-14: Still image of liquid ferrosilicon (75%) droplet on SiC substrate
83 Chapter 4 Dynamic Wetting
1 second) 10 seconds)
20 seconds) 480 seconds)
720 seconds) 1200 seconds)
Figure 4-15: Still image of liquid ferrosilicon (25%) droplet on SiC substrate
84 Chapter 4 Dynamic Wetting
120
100 Si 98.5% / synt graph
80 Si 98.5% / SiC
60
40 Contact Angle (degrees) Angle Contact 20
0 0 10 20 30 40 50 60 70 80 90
Time (seconds)
Figure 4-16: Dynamic wetting of Si 98.5 wt% for synthetic graphite and SiC substrates.
160
140 FeSi75 / synt. graphite
120 FeSi75 / SiC
100
80
60
Contact Angle (degrees) Angle Contact 40
20
0 0 20 40 60 80 100 Time (seconds)
Figure 4-17: Dynamic wetting of FeSi 74 wt% for synthetic graphite and SiC.
85 Chapter 4 Dynamic Wetting
140
120 FeSi25/synt. graphite
100 FeSi25/SiC
80
60
40
Contact Angle(degrees) Contact 20
0 0 1000 2000 3000 4000
Time (seconds)
Figure 4-18: Dynamic wetting of FeSi 24.7 wt% for synthetic graphite and SiC substrates.
The wetting of Si 98.5 wt% displayed similar trend for both, synthetic and natural graphite substrates. The initial contact angle for synthetic graphite was 105º, while this value was
52.6º for SiC. After 80 seconds, the contact angle for synthetic graphite decreased down to 0º.
Similar trend was also observed for SiC, reaching full wetting after 80 seconds.
The wettability study for FeSi 74 wt% showed similar trend to the one observed for Si 98.5 wt%. The initial contact angle value on synthetic graphite was 140º. This value was slightly lower for SiC (134 º). For both, synthetic graphite and SiC substrates, full wetting was reached after 90 seconds.
The dynamic wetting for FeSi 24.7 wt% displayed different pattern. The initial contact angle for synthetic graphite (112º) decreased down to 99º after 60 seconds. After this time, the wetting showed slight improvement up to 600 seconds, when showed a steady trend around
86 Chapter 4 Dynamic Wetting the 80º. Instead, the wetting on SiC showed an initial contact angle of 92º but this decreased rapidly after 10 minutes and remained steady around 40 degrees.
4.4 Interdependence of dynamic wetting and SiC formation at the interface
The ferroalloys tested showed good wetting on SiC. However, the dynamic wetting behaved differently for high and low silicon alloys. While the contact angle decreased down to 12° (Si
98.5%) in 70 seconds and 31° (FeSi 74%) in 80 seconds, the decrease for FeSi 24.7% was much slower, reaching 38° after 300 seconds. A similar trend was also observed when dynamic wetting was investigated on graphite substrates. It was found that the high silicon alloys reached full wetting (θ = 0°) within 90 seconds. However, the dynamic wetting changed at a slower rate for FeSi 24.7%, since the contact angle decreased after 300 seconds and remained steady at 70°.
The XRD spectra at the ferrosilicon – graphite interface provided evidence regarding the formation of SiC during their interaction, explaining the reported differences on dynamic wetting. It was clearly shown that the appearance of strong peaks of SiC occurred after 30 and 60 seconds for Si 98.5% and FeSi 74%, while they appeared at 1800 seconds for FeSi
24.7%. This suggests that the dynamic wetting is strongly related to the rate at which the interfacial reaction occurs. In addition, EPMA back-scattered electron images confirmed the appearance of the SiC as interfacial product, although that compound was distributed irregularly across the interface.
The dynamic wetting for high-silicon alloys on graphite and SiC showed similar behaviour.
Both cases exhibited a sharp decrease in the contact angle, reaching full wetting at 90 seconds on graphite, and decreasing to 12° (Si 98.5%) and 31° (FeSi 74%) before 90 seconds. The wetting for FeSi 24.7%, however, showed a different trend. The contact angle decreased slowly on both substrates, remaining steady at around 70° on graphite after 400 seconds and
38° on SiC after 180 seconds. The results obtained on SiC substrates confirmed that the
87 Chapter 4 Dynamic Wetting dynamic wetting behaviours for each ferroalloy were consistent with the ones on graphite and provide additional evidence in the role played for SiC on this phenomenon.
The experimental results indicated the strong relationship between the amount of silicon contained in the ferroalloys and the interfacial reaction rate. The early appearance of SiC as interfacial product during the interaction ferrosilicon/graphite (particularly for high-silicon alloys) is seems to be related with the changes observed in the dynamic wetting. The differences observed during the formation of SiC at the interface ferrosilicon /graphite
(occurring rapidly in Si 98.5 and FeSi 74% and slower for FeSi 24. %) might explain the differences in dynamic wetting observed in both, high and low silicon ferroalloys. The results obtained on SiC substrates confirmed that the dynamic wetting for each ferroalloy were consistent with the ones on graphite and support the role played by SiC on this phenomenon, since both have shown similar mechanisms.
4.5 Summary.
The interfacial phenomena during the interaction of ferrosilicon with silicon – graphite and
SiC were studied. This research focused on dynamic wetting and the formation of interfacial products.
The conclusions drawn from this work are that:
1) The ferrosilicon alloys showed good wetting on synthetic graphite and SiC. However,
the contact angle decreased much more quickly for high-silicon ferroalloys compared
to low-silicon ferroalloy.
2) The good wetting behaviour is due to the formation of SiC as an interfacial product.
3) The dynamic wetting seems to be strongly dependent on the rate of formation of SiC
at the ferrosilicon-graphite interface.
88 Chapter 4 Dynamic Wetting
4) The dynamic wetting for high-silicon alloys on graphite and SiC showed similar
behaviour. Both cases exhibited a sharp decrease in the contact angle, reaching full
wetting at 90 seconds on graphite. The wetting for low-silicon ferroalloy, however,
showed a different trend with the contact angle decreasing slowly on both substrates.
89 CHAPTER 5
EXPERIMENTAL INVESTIGATIONS ON CARBON DISSOLUTION
IN SILICON AND FERROSILICON ALLOYS: RESULTS AND
DISCUSSION Chapter 5 Carbon Dissolution
The carbon dissolution for silicon and ferrosilicon alloys from synthetic graphite substrate is presented in this chapter. The studies were carried out on ferrosilicon alloys containing 24.7,
74 and 98.5 wt% Si at 1550 °C. Investigation of the interfacial product at the interface was carried out, finding that SiC was formed. The role of the interfacial layer was further investigated by conducting carburization experiments in SiC substrates. Carbon dissolution kinetics and the proposed mechanism for this phenomenon are discussed.
5.1 Carbon dissolution for silicon and ferrosilicon alloys
Initially carburization investigations were undertaken with pure iron using the existing experimental set up. These results showed that the carbon saturation limit in pure iron was
5.57 wt% C, which is in good agreement with published data [Zhao (2003)]. (Figure 5-1)
Carbon pickup from synthetic graphite as function of time for ferrosilicon alloys at 1550° C has been investigated and is shown in figures 5-2 to 5-4. Carbon dissolution from synthetic graphite into ferrosilicon alloys was investigated as a function of time. Detailed experimental results are presented in Figures 5-2 to 5-4. Enlarged regions of high change are shown in figures 5-2a to 5-4a. It was observed that the carbon content increases within the initial 10 minutes but these levels remain constant after this increase. The carbon pickup trend showed no significant differences between Si 98.5% and FeSi 74%.
A different behaviour was found for FeSi 24.7%, where a sharp increase in carbon content was observed within the first 60 seconds.
91 Chapter 5 Carbon Dissolution
6
5
4
3 Wt% C Wt% 2 Current work Zhao (2003) 1
0 0 20 40 Time (minutes) Figure 5-1: Carbon pickup from graphite into pure iron at 1550 °C.
0.1600 0.1 0.1400 0.08 0.1200 0.1000 0.06 0.0800 0.04 W% C
0.0600Wt%C 0.0400 0.02 0.0200 0 0.0000 0 0.5 1 1.5 2 2.5 3 0 50 100 150 200 250 300 350 Time (minutes) Time (minutes) Figure 5-2: Carbon pickup from graphite Figure 5-2a: Carbon pickup from graphite into Si 98.5% at 1550 °C. into Si 98.5% at 1550 °C during the first three minutes.
92 Chapter 5 Carbon Dissolution
0.16 0.1 0.14 0.08 0.12 0.1 0.06 0.08 0.04
0.06 C Wt% Wt% C Wt% 0.04 0.02 0.02 0 0 0 50 100 150 200 250 300 350 0 0.5 1 1.5 2 2.5 3 Time (minutes) Time (minutes)
Figure 5-3: Carbon pickup from graphite into Figure 5-3a: Carbon pickup from graphite FeSi 74% at 1550 °C. into FeSi 74% at 1550 °C during the first three minutes
0.16 0.1 0.14 0.12 0.08
0.1 0.06 0.08 0.04 Wt%C
0.06 C Wt% 0.04 0.02
0.02 0 0 0 0.5 1 1.5 2 2.5 3 0 50 100 150 200 250 300 350 Time (minutes) Time (minutes)
Figure 5-4: Carbon pickup from graphite into Figure 5-4a: Carbon pickup from FeSi 24.7 at 1550 °C. graphite into FeSi 24.7% at 1550 °C during the first three minutes.
93 Chapter 5 Carbon Dissolution
5.2 Carbon dissolution from synthetic graphite substrates.
It is well established that there is a direct relationship between silicon and carbon content in the ferroalloy [Ottem (1993)], showing a trend for silicon content greater than 50wt%.
However, below this limit, carbon dissolved in the metal holds an inverse dependence with the silicon content in the ferroalloy. The rapid carbon pickup during the first 3 minutes suggested a similar kinetic mechanism observed for carbon dissolution in iron. In view of the similarities, a first order kinetic equation was applied for the analysis of the rate constant.
dC ( CtCsk ) …………………. (5.1) dt
After integrating (1)
dC kdt ……………….. (5.2) CtCs )(
The integrated form of (5-2) gives us,
CtCs )( ln kt …………………… (5.3) CoCs )(
Where, Cs is the melt carbon saturation; Ct is the instantaneous melt carbon content; and Co is the initial carbon content.
CtCs )( ln From plotting CoCs )( vs. time t (seconds), the constant rate k can be deduced graphically. Figure 5-5 to 5-7 shows these plots for Si 98.5%, FeSi 74 and FeSi 24.7 runs at
1550 °C on graphite substrates. 94 Chapter 5 Carbon Dissolution
Time(seconds)
0 50 100 150 200 0
-0.1 y = -0.0038x R2 = 0.9779 -0.2
-0.3 Co)} - -0.4 Ct/Cs - -0.5
ln{(Cs -0.6
-0.7
-0.8 CtCs )( Figure 5-5: Plot of ln vs. time (seconds) for carbon dissolution runs of Si 98.5 CoCs )( at 1550 °C.
Time (seconds)
0 50 100 150 200 0
-0.1
-0.2
-0.3 Co)} - -0.4 Ct/Cs - -0.5 ln{(Cs -0.6 y = -0.003x -0.7 R2 = 0.9821
-0.8
CtCs )( Figure 5-6: Plot of ln vs. time (seconds) for carbon dissolution runs of CoCs )( FeSi 74 at 1550 °C.
95 Chapter 5 Carbon Dissolution
Time(seconds)
0 50 100 150 200 0
-0.1
-0.2
Co)} -0.3 -
-0.4 Ct/Cs - -0.5
ln{(Cs y = -0.0039x -0.6 R2 = 0.9812 -0.7
-0.8
CtCs )( Figure 5-7: Plot of ln vs. time (seconds) for carbon dissolution runs of CoCs )( FeSi 24.7 at 1550 °C.
FeSi 24.7 achieved the higher carbon saturation (Cs) limit (0.149% C) compared with FeSi 74
(0.105% C) and Si 98.5 (0.116%C). Reported data from the literature [Ottem (1993)],
[Klevan (1997)] also showed lower Cs for FeSi 75 compared to pure silicon. Rate constants obtained for FeSi 24.7, FeSi 74 and Si 98.5 were 3.9 x 10-3, 3 x 10-3 and 3.8 x 10-3 respectively, showing a direct correlation with Cs values. The initial carbon content for all the ferrosilicon samples is very low (approximately 0.02 – 0.03 wt% C), but during the initial three minutes, carbon contents increased rapidly up to certain value and remained steady after this time. This indicated that at the early stages, the transfer of carbon from the graphite substrate to the metal is favoured by the carbon gradient (from high concentration of carbon in the graphite substrate to low carbon content in the ferrosilicon alloy) until carbon content in the metal approached saturation levels. This does not exclude that other process such as carbon diffusion from the bulk of the graphite plate to the surface and the interfacial reaction at the graphite-ferrosilicon interface also occurred, but the experimental results indicated that
96 Chapter 5 Carbon Dissolution the mass transfer mechanism plays a key role during the initial stages (3 minutes) of the interfacial reaction.
From previous studies [Yunes et al (2006)], it was found that an interfacial reaction occurs, producing SiC as interfacial product, and that this reaction played a significant role in dynamic wetting. XRD scanning (Figures 4-16 to 4-18) at the interface ferroalloy / synthetic graphite showed the presence of SiC as an interfacial product. Strong peaks of
SiC appeared 30 seconds after melting for Si 98.5% and the formation of SiC at the interface of FeSi 74% / graphite was observed 60 seconds after melting, while for FeSi 24.7%, the SiC was first detected after 15 minutes.
The experimental result of carbon dissolution runs on SiC substrates is discussed in the next section along with an understanding of the influence of the interfacial SiC formation on the carbon dissolution reaction.
5.3 Carbon dissolution from SiC substrates into ferrosilicon and silicon at
1550 °C
Carbon dissolution was also investigated from SiC substrates at 1550 °C. The availability of data in this case was limited by occurrence of the dynamic wetting which restricted the time frame for Si 98.5 and FeSi 74 experimental runs to a maximum of 60 seconds. Results from carburization experimental runs using SiC as substrate are shown on Fig 5-8 to 5-10. It was observed that the carbon pickup from SiC occurred at much lower levels compared to synthetic graphite.
97 Chapter 5 Carbon Dissolution
0.05 0.045 0.04 0.035 0.03 0.025
C (wt%) C 0.02 0.015 0.01 0.005 0 0 20 40 60 80 Time (seconds)
Figure 5-8: Carbon pickup from SiC into Si 98.5% at 1550 °C
0.06
0.05
0.04
0.03 C (wt%) C
0.02
0.01
0 0 20 40 60 80
Time (seconds)
Figure 5-9: Carbon pickup from SiC into FeSi 74% at 1550 °C
98 Chapter 5 Carbon Dissolution
0.07
0.06
0.05
0.04
C (wt%) 0.03
0.02
0.01
0 0 20 40 60 80 Time (seconds)
Figure 5-10: Carbon pickup from SiC into FeSi 24.7% at 1550 °C
5.4 Interfacial role of SiC during carbon dissolution
Experimental runs were carried out on 85% SiC plates using the sessile-droplet method. The experiments were performed under conditions similar to the ones described earlier on synthetic graphite crucible. Comparisons between rate constants on both substrates were plotted for each ferroalloy tested (Figures 5-11 to 5-13). As expected, the carbon dissolution rate constant from SiC substrates was an order magnitude smaller compared with the rate of carbon dissolution, as shown in Tables 5-1a and b. The formation of SiC as interfacial product creates some resistance to the carbon diffusion from the graphite substrate and therefore, has a retarding effect on the overall process. It was also observed that, despite being of similar order, the rate constants changed significantly for each composition; the rate constant for FeSi 24.7 was double that of FeSi74 and was almost three times that of Si 98.5.
(Table 5-2). This suggests that the process is favoured by lower silicon (or correspondingly higher iron) contents in the ferroalloy [Wu et al (2000)][Zhao (2004)][Cham et al (2004)].
99 Chapter 5 Carbon Dissolution
Time(seconds) 0 50 100 150 200 0 -0.1 y = -0.00017x -0.2
Co)} -0.3 - -0.4 Ct/Cs - -0.5 Si 98.5 /graphite
ln{(Cs -0.6 y = -0.0038x Si 98.5 /SiC -0.7 -0.8 CtCs )( Figure 5-11: Plot of ln vs. time (seconds) for carbon dissolution runs of Si 98.5% CoCs )( on graphite and SiC substrates at 1550 °C.
Time(seconds) 0 50 100 150 200 0
-0.1 y = -0.0004x -0.2
-0.3 Co)} - -0.4 y = -0.003x Ct/Cs - -0.5
ln{(Cs -0.6 FeSi 74 /graphite -0.7 FeSi 74 /SiC -0.8 CtCs )( Figure 5-12: Plot of ln vs. time (seconds) for carbon dissolution runs of FeSi74% CoCs )( on graphite and SiC substrates at 1550 °C.
100 Chapter 5 Carbon Dissolution
Time(seconds) 0 50 100 150 200 0 -0.1 -0.2 y = -0.0008x Co)}
- -0.3 -0.4 Ct/Cs - -0.5
ln{(Cs -0.6 FeSi24.7/graphite
-0.7 FeSi 24.7 /SiC y = -0.0039x -0.8
CtCs )( Figure 5-13: Plot of ln vs. time (seconds) for carbon dissolution runs of CoCs )( FeSi24.7%on graphite and SiC substrates at 1550 °C.
Table 5-1a Constant rates and Carbon saturation limits for Si 98.5, FeSi 74 and FeSi 24.7 from synthetic graphite substrate.
Constant Rate (s-1) Cs (wt%) Time (min) Si 98.5% 3.8 x 10-3 0.115 60
FeSi 74 3 x 10-3 0.108 60
FeSi 24.7 3.9 x 10-3 0.142 60
Table 5-1b Constant rates and Carbon saturation limits for Si 98.5, FeSi 74 and FeSi 24.7 from SiC substrate.
Constant Rate (s-1) Cs (wt%) Time (min) Si 98.5% 1.7 x 10-4 0.0453 60
FeSi 74 4 x 10-4 0.0577 60
FeSi 24.7 8 x 10-4 0.0505 60
101 Chapter 5 Carbon Dissolution
Table 5-2. Overall rates constant for ferroalloys tested on different substrates (s-1)
Substrate
Synthetic graphite SiC
Si 98.5% 3.8 x 10-3 1.7 x 10-4
FeSi 74 3 x 10-3 4 x 10-4
FeSi 24.7 3.9 x 10-3 8 x 10-4
The overall rates of carbon dissolution from graphite are quite similar for each sample. The carbon diffusion process occurs remarkably faster at initial stages but after the initial pickup, the appearance of SiC plays a retarding effect and slowed down the overall process. It can be concluded that the carbon dissolution process in silicon and ferrosilicon alloys is governed initially by diffusion and beyond this stage; the process is governed by mixed control
(including interfacial resistance by SiC), which slows down the rate of carbon dissolution.
The free diffusion of carbon occurs preferentially during the initial stages of the metal- substrate interaction. The carbon pickup occurs at similar rates across the ferroalloys and it was commonly observed that the carbon content rapidly increases at early stages. Carbon saturation levels (Cs) are consistent with the rate constant of the carbon dissolution reaction and this suggests that carbon diffusion is the primary mechanism in the initial stage. At the same time, it was observed that SiC appeared as the interfacial product, providing an interfacial resistance and slowing down the carbon dissolution from graphite.
102 Chapter 5 Carbon Dissolution
5.5 Summary
The carbon dissolution from synthetic graphite and the effect of SiC as interfacial product in ferrosilicon alloys was studied. This work investigated the kinetic mechanism of carbon dissolution reaction. The following conclusions have been drawn from the results:
- The overall rate constants do not present significant differences across the
samples, suggesting that carbon diffusion is a predominant mechanism controlling
carbon dissolution. However, the faster rate in the case of FeSi 24.7 can be
explained on the basis of delayed formation of SiC interfacial product.
- The appearance of SiC as interfacial product plays a retarding effect on the overall
process and dictates the carbon transfer. This layer appears at very early stages of
the process for Si 98.5 and FeSi 74, while it takes longer to be formed for FeSi
24.7.
- The kinetics of carbon dissolution for FeSi 24.7, FeSi 74 and Si98.5% is driven by
a mixed control mechanism, where both carbon diffusion and interfacial resistance
due to SiC formation influence the process.
103 CHAPTER 6
SIMULTANEOUS DECARBURIZATION AND OXIDATION
REACTIONS OCCURRING IN SILICON AND FERROSILICON
ALLOYS: RESULTS AND DISCUSSION Chapter 6 Silicon oxidation and Decarburisation
Dynamic weight changes during the decarburisation and silicon oxidation reactions of the molten ferrosilicon alloys during their interactions with oxidizing gases are presented in this chapter. The mechanisms governing this process are also discussed.
6.1 Investigation of weight changes for silicon and ferrosilicon alloys
Investigations were carried out on carbon-rich ferroalloys (see Table 6.1). The experimental runs were carried out on three ferrosilicon alloys respectively containing 24.7 wt%, 74 wt% and 98.5 wt% Si. The specimens were held in a (20% CO2 – 2% CO – N2
(balance) gas mixture at flow rates of 0.5, 1 and 2.0 L/minute. The % weight gain
(W*100/Wo; where W and Wo respectively represent the net weight change and initial weight) as a function of time is shown in Figure 6-1 for three specimens under investigation.
It is to be noted that y-axis scales are different for Figures 6-1(a-c) indicating significant differences in weight changes; x-axis covers a much longer time scale in Figure 6-1 c.
For a gas flow rate of 1 L/min, weight gains were quite comparable for Si 98.5 and FeSi
74. Both showed ~10% weight gain after 10 minutes. The weight gain however was very slow for FeSi 24.7, reaching a 2% weight gain after 30 minutes. For a gas flow rate of 0.5
L/min, weight gains were once again quite comparable for Si 98.5 and FeSi 74. Both showed
~3% wt. gain after 10 minutes. FeSi 24.7 showed almost no weight gain for the initial 15 minutes and then increased marginally to 0.5% after 30 minutes. Much higher weight gains were observed for gas flows of 2 L/min. The weight of Si 98.5 increased sharply reaching a massive 37% after 10 minutes. The increase was relatively slower for FeSi 74, which reached a maximum of 12%. The weight gain for FeSi 24.7 continued to be slow, reaching a maximum of 2.7% after 30 minutes. (Table 6.2). A closer comparison of the weight gain
104 Chapter 6 Silicon oxidation and Decarburisation under 2 L/min flow rate for all samples during the initial ten minutes is shown in Figure 6-1 d.
Table 6.1: Chemical compositions of silicon and ferrosilicon alloys (wt%)
Fe Si C Al Si 98.5 0.72 98.52 0.12 0.03
FeSi24.7 73.9 25.3 0.14 0.06
FeSi74 24.7 74.5 0.09 0.06
Table 6-2: Weight gains (%) for different silicon and ferrosilicon alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
Flow rate Time (minutes) (l/min) 1 2 5 10 0.5 0.64 0.7 1.38 3.34 Si 98.5 1 1.35 5.46 8.65 10.95 2 2.23 5.82 13.86 36.42 0.5 1.28 2.15 2.57 3.36 FeSi 74 1 1.19 5.6 7.38 10.18 2 3.07 6.22 8.39 11.88 0.5 0 0 0 0 FeSi 24.7 1 0 0 0 0.41 2 0 0 0.44 0.47
105 Chapter 6 Silicon oxidation and Decarburisation
Figure 6-1 a –c: % Weight gain for three alloys as a function of time. Oxidising gas 20%
CO2, 2% CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
106 Chapter 6 Silicon oxidation and Decarburisation
40
35
30
25
gain 20
15 Weight
% 10
5
0 1 2 3 4 5 6 7 8 9 10 Time Si 98.5 FeSi 74 FeSi 24.7
Figure 6-1 d: % Weight gain for three alloys during the initial 10 minutes. Oxidising gas
20% CO2, 2% CO, N2-balance was used at flow rates of 2 L/min.
6.2 Study of the carbon loss for silicon and ferrosilicon alloys
In Figure 6-2, we have plotted wt% C in three specimens for a range of gas flow rates as a function of time. Noticeable decarburization was observed across the whole range of samples during the first 2 minutes. It is to be noted that same x and y axes have been used in Figure 6-
2 (a-c). This effect was most remarkable for FeSi 24.7, where carbon levels dropped by 0.11 wt% C, followed by FeSi 74 (0.08 %C) and Si 98.5 (0.07%C) within 120 seconds. After this period, the decarburization appeared to occur at a much slower rate. For a gas flow rate of 1
L/min, the carbon content in Si 98.5 fell from 0.12%C to 0.04%C after 10 minutes of contact.
For FeSi 74 alloy, the carbon content decreased from 0.09% to 0.01%C after 10 minutes. For
FeSi 24.7, the carbon level decreased from 0.14% to 0.05% after 10 minutes. A closer comparison of carbon losses for the initial period is presented in greater detail in Figure 6-2 d. The influence of gas flow rate on decarburisation was not found to be very significant, and
107 Chapter 6 Silicon oxidation and Decarburisation similar trends were observed. For Si 98.5, while no difference was observed for 0.5 L/min and 1 L/min flow rates, increased decarburisation was observed for 2 L/min, indicating the influence of the gas phase mass transfer mechanism. Gas flow rate had a negligible influence on the decarburisation of FeSi 74 alloy. A small improvement with flow rate was observed for FeSi 24.7 alloy.
6.3 Influence of gas composition
In Figure 6-3, we have plotted %C in three specimens after exposure to two oxidising gases (20% CO2 – 2% CO – N2-balance and 100% CO2); the gas flow rate was maintained at
1 L/min for these investigations. While no discernible difference in decarburisation reaction kinetics with increasing CO2 level could be seen for FeSi 74 and FeSi 24.7 alloys, a marginal increase in decarburisation was observed for Si 98.5 specimen. This change was observed during the initial as well as in the later stages. Back-scattered images using the EPMA (400X) showed areas with substantial amounts of silica after only five minutes, particularly for high- silicon alloys (Si 98.5 and FeSi 74). However, the appearance of silica for FeSi 24.7 was found to occur at much later times. These are shown in Figures 6-4 (a-c)
108 Chapter 6 Silicon oxidation and Decarburisation
Figure 6-2 a-c: Wt % C for three alloys as a function of time. Oxidizing gas 20% CO2, 2%
CO, N2-balance was used at flow rates ranging between 0.5 to 2 L/min.
109 Chapter 6 Silicon oxidation and Decarburisation
Si 98.5 FeSi 74 FeSi 24.7
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time (Minutes)
Figure 6-2 d: Change in carbon content (wt%) for different silicon and ferrosilicon alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at a flow rate of 2 L/min.
110 Chapter 6 Silicon oxidation and Decarburisation
Figure 6-3: Wt% C for three alloys as a function of time. Two oxidising gas 20% CO2, 2%
CO, N2-balance and 100% CO2 were used at a flow rate of 1 L/min.
111 Chapter 6 Silicon oxidation and Decarburisation
6.4 Silicon oxidation
Figures 6-5 (a-c) show the changes in silicon content for the silicon and ferrosilicon samples after being under an oxidising atmosphere of 20% CO2 – 2% CO – N2-balance at flow rates between 0.5 to 2 L/min. At a flow rate of 0.5 L/min, silicon losses occurred at a low rate consistently across all samples, the effect was almost negligible for FeSi 24.7.
However, at flow rate of 1 and 2 L/min for FeSi 74 and Si 98.5 the silicon losses were significant from 2 minutes onwards; however for FeSi 24.7, higher silicon losses were observed only after 15 minutes. (Table 6.3)
The silicon depletion from the metal samples and the subsequent silica formation are closely related to the earlier weight gains results. This is clearly shown on figures 6-6 (a-c), where the weight gains and silicon content changes versus time are plotted for each alloy. For Si
98.5 and FeSi 74 alloys, the rapid weight gains corresponded clearly with significant silicon losses. After 10 minutes, weight gains for both alloys are around 10 percent, while silicon losses are also in the same order. However, for FeSi 24.7, both, silicon losses and weight gains occurred at much slower rate.
Figures 6-7 (a-c) show the relationship between silicon content changes and the rate of weight changes under an oxidising atmosphere of 20% CO2 – 2% CO – N2-balance and a flow rate of 1 liter/minute for all silicon and ferrosilicon samples. The trends showed that the rate of weight change increased very rapidly at the early stages but as the silicon content decreases in the metal, the weight gain also decreased. This is particularly noticeable for Si
98.5 and FeSi 74, whereas is less evident for FeSi 24.7.
112 Chapter 6 Silicon oxidation and Decarburisation
A1
a) FeSi 24.7 after 15 minutes -
Area A -C: 0.017 wt%, O: 0.47wt%, Si: 18.98 wt%, Fe: 79.91 wt%
B2 B1
(b) FeSi 74 after 5 minutes Area B1 - C: 0.0008 wt%, O: 53.36 wt%, Si: 46.59 wt%, Fe: 0.0371 wt% Area B2 - C: 0.004 wt%, O: 0.4815wt%, Si: 70.69 wt%, Fe: 27.79 wt%
113 Chapter 6 Silicon oxidation and Decarburisation
C2
C1
c) Si 98.5 after 5 minutes Area C1 - C: 0.0008 wt%, O: 52.00 wt%, Si: 47.98 wt%, Fe: 0.0035 wt% Area C2 - C: 0.0224 wt%, O: 0.503wt%, Si: 97.90 wt%, Fe: 0.0001 wt%
Figure 6-4 a-c. Back-scattered electron EPMA images. Atmosphere: 20% CO2 – 2% CO. Flow rate: 2 L/min
114 Chapter 6 Silicon oxidation and Decarburisation
0.5 l/min 1 l/min 2 l/min
100
90
80
70
60 0 2 4 6 8 10
75
70
65
60 0 2 4 6 8 10
25
24.5
24
23.5 0 10 20 30
Figure. 6-5 a- c: Change in silicon content (wt %) for the three alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates ranging from 0.5 to 2 L/min
115 Chapter 6 Silicon oxidation and Decarburisation
12 98 10
95.5 8 Weight gain Weight gain (%) 6
Si (wt%) Si 93
4 90.5 2
88 0 0 2 4 6 8 10 Time (minutes)
Silicon Content (wt%) weight gain
Figure 6-6a: Changes in silicon content (wt%) and weight change (%) for Si 98.5 during the first ten minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
116 Chapter 6 Silicon oxidation and Decarburisation
75 12
10 72.5 8 Weight gain Weight gain (%) Si 70 6
4 67.5 2
65 0 0 2 4 6 8 10
Silicon content weight gain
Figure 6-6 b: Changes in silicon content (wt%) and weight change (%) for FeSi 74 during the first ten minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
24.8 2
24.6 1.5
24.4 1 Weight gain (%) Silicon 24.2 0.5
24 0 0 10 20 30
Silicon content weight gain
Figure 6-6 c: Changes in silicon content (wt%) and weight change (%) for FeSi 24.7 during the first thirty minutes. Oxidising gas 20% CO2, 2% CO, N2-balance .Flow rate 1 L/min
117 Chapter 6 Silicon oxidation and Decarburisation
3
2.5
2
1.5
1
0.5
0 87 89 91 93 95 97 99
3 ) 2.5 2 1.5 1 0.5 0 66 68 70 72 74 76 Weight change rate (%/min change rate Weight
0.12
0.1
0.08
0.06
0.04
0.02
0 24.1 24.2 24.3 24.4 24.5 24.6 24.7 24.8 Silicon (wt%)
Figure. 6-7 a- c: Plot of rate weight change ( %/min) vs. Silicon content (wt%) for the three alloys during the first minutes. Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rates of 1 L/min
118 Chapter 6 Silicon oxidation and Decarburisation
6.5 Discussion
Decarburisation and silicon oxidation are the two key reactions occurring under these experimental conditions. Decarburisation reactions can lead to the loss of carbon, generating a gaseous product and an associated weight loss. Silicon oxidation reactions on the other hand produce silica resulting in weight gain. When both reactions occur simultaneously, it was seen clearly that the net weight gain is the resultant of decarburization and silicon oxidation reactions. FeSi 24.7 showed a negligible weight change during the first 10 minutes.
Although it was observed that the decarburization occurred fairly quickly during the first 120 seconds, a steady slope later on suggests that the reaction kinetics slowed down after that.
With silicon oxidation reaction also occurring during the same period, it appears that the weight loss due to decarburization was balanced by the weight gain through silica formation, thereby resulting in no weight gain during initial 120 seconds. However as decarburisation slows down after 120 seconds, the weight gain through silicon oxidation becomes higher and a net weight increase can be clearly observed. Lowest level of weight gain was observed for
FeSi 24.7.
Oxidation reactions of C and Si in Fe-C-Si melts with CO2 are represented below by
Equations (6-1) and (6-2) respectively.
C + CO2 (g) = 2CO (g) (6-1)
Si + CO2 (g) = SiO2 + C (6-2)
In Figure 6-8, we have plotted wt% C as a function of time for three alloys with two oxidising gaseous atmospheres (20% CO2, 2% CO, N2-balance; flow rate 1 L/min.) using a logarithmic scale along the y-axis. There is a clear evidence for an exponential decay and two rate regimes, labeled as I and II. The rate of decarburisation was much higher in region I,
119 Chapter 6 Silicon oxidation and Decarburisation which lasted during the initial 2 minutes. It is important to note that FeSi 74 showed the highest rate of decarburisation during initial 2 minutes as indicated by the highest slope. It was also found that decarburization rate for FeSi 74 was much higher compared to Si 98.5 in region I. Two different rate regimes can be clearly identified.
Figure 6-8: Wt % C for three alloys on a logarithmic scale as a function of time.
Oxidising gas 20% CO2, 2% CO, N2-balance was used at flow rate of l L/min.
A significant slowing down in the decarburisation rate was observed in region II, even though there were still significant amounts of carbon remaining in the alloys Si 98.5 and FeSi 24.7
(see Fig. 6-3 (a & c,). This observation suggests that, during silicon oxidation, it is thermodynamically feasible that carbon content in the melt had increased according to
Equation (6-2) and could explain the decreasing of the decarburization rate in region II.
Although the decarburization reaction occurred rapidly for FeSi 74 and Si 98.5 during early
120 Chapter 6 Silicon oxidation and Decarburisation stages, the rapid weight increases observed for both samples and the slower decarburization rates suggest that silicon oxidation prevailed over the decarburization. This is further confirmed by the significant decreases in silicon content observed during the first minutes for
Si 98.5 and FeSi 74. These results are in sharp contrast with FeSi 24.7, where the decarburization occurred in preference to silicon oxidation during the initial stages. Thus, the net “balance” between both the decarburization and silicon oxidation kinetics could explain the fact that both reactions occurred simultaneously, explaining the absence of weight change until after 10 minutes. It is not until then that the silicon oxidation reaction was seen to prevail clearly over to the decarburization. However, the weight gain is considerably lower compared to the Si 98.5 and FeSi 74, which indicate that the desiliconization rate for FeSi
24.7 is slower compared to other two alloys under investigation. (Figure 6-7 a to c)
From the flow rate data, it can be seen that the decarburization followed a trend similar with increasing gas flow rates, i.e., the reaction rate showed a sharp increase during the first
120 seconds, followed by a steady slope. This feature can be observed more clearly for the flow rate of 2 L/min. This suggests that the decarburization rate bears a direct correlation with the flow rate and indicates a key role played by mass transfer in the gas phase. Silicon oxidation rates also followed a similar trend as previous results for flow rate of 1 L/min.
Similar to decarburization reactions; desiliconization reactions were also favoured by increased flow rates, suggesting that the mass transfer of CO2 was a dominant step. A qualitative analysis for both, carburisation and oxidation reactions for Si 98.5, FeSi 74 and
FeSi 24.7 are shown on Table 6.4.
121 Chapter 6 Silicon oxidation and Decarburisation
Table 6.3: Qualitative comparison of carbon dissolution and oxidation reaction rates
Carbon Decarburization Silicon oxidation dissolution rate rate (Regime I) rate
Si 98.5 medium low high FeSi 74 low high high FeSi 24.7 high medium low
From the cupola process perspective, these findings suggest that the use of ferrosilicon alloy with lower silicon content might help to minimise silicon losses. There are of course, other considerations such as cost and availability that need to be taken into account. Our results have shown that the silicon oxidation reaction was intensified when the gas flow rate was doubled. This factor should also be weighed carefully when cupola operational parameters such as blast rate and blast volume are chosen, as these may affect the gas flow and velocity of the gases generated in the combustion zone of the cupola.
6.6 Summary and Conclusions
Key findings of this investigation have been summarized below:
1) An in-depth study has been carried out on the oxidation reactions at 1550 °C in Fe-Si
alloys with high silicon contents. Under the experimental conditions, both
decarburisation and silicon oxidation reactions were found to occur simultaneously.
2) The net weight gain in alloys was found to be the result of decarburization (weight
loss due to the generation of a gaseous product) and silicon oxidation (weight gain
due to silica formation on the surface of the sample). Key differences in weight gain
were observed between three alloys under investigation.
3) There was a clear evidence for two rate regimes, labeled as I and II. The rate of
decarburisation was much higher in region I, which lasted during initial 2 minutes; a 122 Chapter 6 Silicon oxidation and Decarburisation
much slower rate was observed in the later stages for all three alloys. No significant
effect was found on the decarburization rates when the relative proportion of the
oxidizing gas (CO2) was increased from 20 to 100%.
4) For FeSi 24.7 alloys, the decarburization reaction appeared to prevail during the first
120 seconds. The rapid drop in carbon content during the early stages suggests that
decarburization rate was higher compared to silicon oxidation. Once the
decarburization reaction slowed down, silicon oxidation was able to take over.
5) For FeSi 74 and Si 98.5 alloys, decarburization rates followed a similar trend but were
relatively slower. The silicon oxidation reaction appeared to be the dominant reaction
most of the time.
6) Decarburisation did not change significantly for FeSi 24.7 and Si 98.5 under a gas
flow rate of 0.5 and 1 L/min, but noticeable changes were observed when the gas flow
rate was increased to 2 L/min. This suggests that the mass transfer in the gas phase
played an important role in the decarburisation reaction for those alloys. This however
was not the case for FeSi 74.
123 CHAPTER 7
SUMMARY AND CONCLUSIONS Chapter 7 Summary and Conclusions
7.1 Introduction
A fundamental investigation has been carried out on the high temperature interaction between ferrosilicon and graphite; key knowledge for understanding the interfacial reaction and it relationship with the surface tension phenomenon has been developed. Using the sessile droplet method, the dynamic wetting of synthetic graphite by liquid ferrosilicon alloys containing 24.7 and 74 % Si and silicon (98.5 % Si) at 1550 °C has been reported. The study of the wetting parameters, such as contact angle, when liquid metal droplets interact with graphite substrate was found to be of significant importance during this process. The influence of the interfacial product formed and its relationship with the dynamic wetting phenomena was also investigated to develop fundamental understanding of the interfacial phenomenon and wettability.
Carburization is a key reaction for the scrap-iron process, since the metallic liquid droplets combine with the coke at high temperatures. The kinetic mechanism of carbon dissolution in liquid iron had been extensively investigated. However, there is little, if any knowledge about the kinetics of carbon dissolution when the silicon content is higher than 10%. Although the thermodynamics of Fe-Si-C system has been well studied, the kinetics of this system has not been investigated in depth. Therefore, the study of the reaction kinetics and interfacial phenomena can provide a better fundamental understanding of the carbon dissolution process occurring during ferrosilicon-carbon interaction. In this study, a kinetic mechanism has been developed for the carbon dissolution phenomena in ferrosilicon alloys. The overall-rate constants at 1550 °C were found for Si 98.5, FeSi 74% and FeSi 24.7%. The influence of some interfacial product in this process has also been investigated.
In the cupola furnace, decarburization and silicon oxidation reactions are known to occur extensively during the scrap melting process as oxidising gases react with the molten metal.
125 Chapter 7 Summary and Conclusions
From the process perspective, the oxidation of silicon needs to be controlled since excessive silicon oxidation can affect the quality of the cast iron product. Additionally, the wasted silicon forms silica, which increases the slag volume eroding more aggressively the basic refractory lining and slag capacity. Silicon oxidation and decarburization studies in steelmaking have generally been limited to Fe-based alloys containing up to 3.5 wt% Si. This thesis ,have presented high temperature investigations on ferrosilicon alloys with Si contents of 24.7 wt% and 74 wt% and a silicon alloy containing 98.5 wt% Si. The present work evaluates the effect of the alloy composition, oxygen partial pressure and flow rate on interactions at 1550 °C. Significant differences were observed in the weight gain and carbon loss between these three alloys; both decarburisation and silicon oxidation reactions were found to occur simultaneously. There was also a clear evidence for two rate regimes: the rate of decarburisation was found to be much higher during the initial 2 minutes and a much slower rate was observed in later stages for all specimens. These rate regimes have been explained in terms of the extent of surface coverage with the reaction product silica. No significant effect was found on the decarburization rates when the proportion of oxidizing gas
(CO2) was increased from 20 to 100%, indicating that mass transfer in the gas phase was not a dominant rate controlling step compared to chemical kinetics. The net weight gain in these alloys was found to be due to the combined influence of decarburization (weight loss due to the generation of a gaseous product) and silicon oxidation (weight gain due to silica formation on the sample surface). The outcomes of this investigation will assist with the development of mechanisms governing the reactions of the molten ferrosilicon and silicon alloys during their interactions with gaseous phases in the cupola process.
126 Chapter 7 Summary and Conclusions
7. 2 Dynamic Wetting between molten Silicon and Ferrosilicon with Synthetic
Graphite.
Silicon 98.5% and ferrosilicon alloys containing 74 and 24.7% Si showed good wetting behaviour (θ < 90º) on synthetic graphite at 1550 °C. The initial contact angles for Silicon
98.5, ferrosilicon 74 and 24.7 were respectively 105,140 and 112°. Full wetting was observed for silicon 98.5 and ferrosilicon 74 within the first 90 seconds (Figures 4-4, 4-5). However, the final contact angle value appeared higher for the low-silicon ferroalloy (24.7 wt% Si) and remained steady around 70 degrees during the 2 hours-run.
The presence of interfacial product was likely to occur as per equation 4.1:
Si (l) + C (s) = SiC (s)
At 1550 °C, the Gibbs free energy for SiC = -5.58 x 104 J, so the reaction is thermodynamically feasible as such temperature.
Interfacial investigations using X-ray diffraction showed the formation of SiC as an interfacial product. Reported results from Nogi and Ogino [Nogi and Ogino (1988)] and
Nikolopoulos et al [Nikolopoulos et al (1992)] showed good wetting between silicon and SiC, so this explains the good wetting behaviour observed when silicon and ferrosilicon alloys interacted with synthetic graphite. It was also found that the rate of formation of SiC at the interface between ferrosilicon and synthetic graphite was different for different ferroalloys.
The formation of SiC at the interface appeared 30 seconds after melting for si 98.5, while this was observed for FeSi 74 after 60 seconds. However, for FeSi 24.7, the SiC was detected after 30 minutes.
127 Chapter 7 Summary and Conclusions
Further wettability investigations carried out on SiC substrates showed similar trends as the ones observed on synthetic graphite. The initial contact angles for Si 98.5, FeSi 74 and FeSi 2 were 88.6, 134 and 92 degrees respectively. Full wetting was observed for Si 98.5 and FeSi
74 after 80 and 90 seconds, while FeSi 24.7 showed a different pattern, since the contact angle decreased rapidly during the first 10 minutes and remained steady around 40 degrees after that time. These results, together with the X-ray diffraction findings, explained the wettability phenomena between silicon and ferrosilicon alloys with synthetic graphite and the differences on dynamic wetting between high and low silicon alloys. The dynamic wetting seems to be strongly dependent on the rate of formation of SiC at the ferrosilicon-graphite interface.
7.3 Carbon dissolution occurring during graphite-silicon interactions
Carbon dissolution phenomenon and associated mechanisms are established for ferrosilicon alloys at 1550 °C in this investigation. The overall rate constants at 1550 °C for Si 98.5, FeSi
74 and FeSi 24.7 were 3.8,3 and 3.9 x 10-3 (s-1) respectively. Since the overall rate constants do not present significant differences across the samples, it is suggested that carbon diffusion is a predominant mechanism controlling carbon dissolution. It was also observed a rapid increased of carbon pickup during the first few minutes, remaining fairly constant after this period.
The role of SiC as interfacial product and its impact on dynamic wetting between silicon and ferrosilicon alloys with synthetic graphite have been discussed on section 7.2. Carbon dissolution on silicon and ferrosilicon alloys from SiC substrates was also investigated. It was found that the overall rate constants for Si 98.5, FeSi 74 and 24.7 were 1.7,4 and 8 x 10-4 (s-
1), being one order of magnitude smaller compared with the rate of carbon dissolution from synthetic graphite. The formation of SiC as interfacial product creates a barrier effect, thus
128 Chapter 7 Summary and Conclusions retarding the carbon diffusion process. Despite that the rate constants are in the same order of magnitude, FeSi 24.7 is double than FeSi 74.7 and almost three times compared to Si 98.5%.
This suggested that the carbon diffusion is favoured by lower silicon (or higher iron) contents in the ferroalloy.
The carbon pickup occurs at similar rates across the ferroalloys and it was found that the carbon content increased rapidly at early stages. Carbon saturation levels were consistent with the rate of carbon dissolution, suggesting that carbon diffusion is the primary mechanism in the initial stage. At the same time, the formation of SiC as interfacial product, plays a retarding effect and slowing down the carbon dissolution from graphite.
The faster rate in the case of FeSi 24.7 can be explained on the basis of delayed formation of
SiC interfacial product. The appearance of SiC as interfacial product played a retarding effect on the overall process and dictated the carbon transfer. As discussed on section 7.2, SiC layer appeared at very early stages of the process for Si 98.5 and FeSi 74, while it took longer to be formed for FeSi 24.7.
7.4 Simultaneous decarburisation and oxidation reaction occurring in Silicon and
Ferrosilicon Alloys
The interactions between ferrosilicon and silicon alloys with synthetic graphite were investigated from the perspective of the interfacial phenomenon and their implications in wettability and carburisation reactions. Both above-mentioned processes occurred inside the cupola furnace when molten ferrosilicon interacts with coke, thus the significance of this investigation. It should be noted that an oxidizing gas phase is generated inside the furnace and interacted with rich-carbon ferroalloys molten droplets that are descending to the burden.
This thesis investigates the decarburisation and silicon oxidation reactions that took place during such interactions.
129 Chapter 7 Summary and Conclusions
It was found that, both decarburisation and silicon oxidation reactions were found to occur simultaneously. There was a clear evidence for two rate regimes, labeled as I and II on Figure
6-8. The rate of decarburisation was much higher in region I, which lasted during initial 2 minutes; a much slower rate was observed in the later stages for all three alloys. No significant effect was found on the decarburization rates when the relative proportion of the oxidizing gas (CO2) was increased from 20 to 100%.
For FeSi 24.7 alloys, the decarburization reaction appeared to prevail during the first 120 seconds. The rapid drop in carbon content during the early stages suggests that decarburization rate was higher compared to silicon oxidation. Once the decarburization reaction slowed down, silicon oxidation was able to take over. For FeSi 74 and Si 98.5 alloys, decarburization rates followed a similar trend but were relatively slower. The silicon oxidation reaction appeared to be the dominant reaction most of the time.
The net weight gain in alloys was found to be the result of decarburization (weight loss due to the generation of a gaseous product) and silicon oxidation (weight gain due to silica formation on the surface of the sample). Key differences in weight gain were observed between three alloys under investigation. Decarburisation did not change significantly for
FeSi 24.7 and Si 98.5 under a gas flow rate of 0.5 and 1 L/min, but noticeable changes were observed when the gas flow rate was increased to 2 L/min. This suggests that the mass transfer in the gas phase played an important role in the decarburisation reaction for those alloys. This however was not the case for FeSi 74.
7.5 Conclusions
The main aim of this investigation is to understand the fundamentals during the interactions between ferrosilicon and silicon alloys with carbon bearing materials and impact of the oxidizing gas phase. There were three different aspects that were investigated in depth:
130 Chapter 7 Summary and Conclusions
- Wetting behavior between ferrosilicon alloys and synthetic graphite.
Interfacial reaction. Formation of interfacial product and its impact on dynamic
wetting.
- Carbon dissolution process during the interaction between ferrosilicon alloys and
synthetic graphite.
Kinetics of the carbon dissolution. Determination of the rate constant and
understanding of the carbon transfer mechanism
- Interaction of the ferrosilicon alloy with the gaseous phase.
Simultaneous silicon oxidation and decarburisation. Effect of gas flow, gas
composition and silicon content during the oxidation reactions.
The three objectives of this research were largely achieved. A mechanism for the carbon dissolution kinetics for ferrosilicon and silicon alloys was proposed. It was also found the key role played by the SiC as an interfacial product in both, carbon dissolution and dynamic wetting phenomena. These results have also provided fundamental understanding during the decarburisation and silicon oxidation reactions after the carbon-rich ferroalloys interacted with the oxidizing gas phase. The outcomes provide valuable information for cast iron manufacturers, providing practical recommendations for the adequate selection of silicon- bearing materials in the metallic charge and to achieve the optimum atmosphere in the furnace in order to achieve the best possible silicon recovery during the cupola process.
131 Chapter 7 Summary and Conclusions
7.6 Future work
This investigation established the carbon dissolution kinetics for silicon and ferrosilicon alloys. The carbon diffusion from synthetic graphite into the silicon and ferrosilicon alloys is satisfactorily explained. However, during the cupola process, the carbon-bearing material most commonly used is coke. Impurities contained in the coke might affect the carbon dissolution reaction, as well as the wettability and this also might affect the decarburisation and oxidation kinetics. Future work should be aiming to understand the impact of the impurities on these processes, as well as determining the predominant rate controlling step and effect of impurities present in coke on carbon dissolution, wettability, decarburization and oxidation kinetics. Numerical simulations and kinetic studies are also needed to study the effect of Si content on wettability on different substrates and the effect of carbon content and correlate the results with sessile drop experiments.
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137 APPENDIX I
DYNAMIC WETTING FOR SILICON AND FERROSILICON ALLOYS IN NATURAL GRAPHITE Appendix I
Dynamic wetting investigations were also carried out for ferrosilicon and silicon alloys on natural graphite (Figures A-1,A-2 and A-3). It was found that these alloys displayed very good wetting in natural graphite, comparable to the one found in synthetic graphite (Figure
A-4, A5 and A6).
These findings, although very revealing, it created further difficulties for the carburization investigations, since no reliable results for carbon analysis from the metallic sample analysis was possible. At the time that the experimental work was carried out, not natural graphite crucibles were available for the experimental work, so it was decided that the carburization study would not include natural graphite.
The wetting of Si 98.5 wt% displayed similar trend for both, synthetic and natural graphite substrates. The initial contact angle for synthetic graphite was 105º , while this value was
88.6º for natural graphite. After 80 seconds, the contact angle for synthetic graphite decreased down to 0º; while for natural graphite was only 53 seconds.
The wettability study for FeSi 74 wt% showed similar trend to the one observed for Si 98.5 wt%. The initial contact angle values on synthetic and natural graphite were both similar
(140º). Both substrates displayed full wetting and the difference was on the time needed to reach it. For synthetic graphite full wetting was reached after 90 seconds, while for natural graphite, it was observed at 65 seconds.
The dynamic wetting for FeSi 24.7 wt% displayed different pattern. The initial contact angle for synthetic graphite (112º) and natural graphite (121º) decreased down to 99º and 102º respectively after 60 seconds. After this time, the wetting showed slight improvement up to
600 seconds, when both substrates showed and steady trend around the 89º for natural graphite and 80º for synthetic graphite.
139 Appendix I
1 second) 10 seconds)
15 seconds) 18 seconds)
22 seconds) 28 seconds)
Figure A-1: Still image of liquid silicon (Si 98.5%) droplet on natural graphite substrate
140 Appendix I
1 second) 10 seconds)
30 seconds) 40 seconds)
45 seconds) 50 seconds)
Figure A-2: Still image of liquid ferrosilicon (75%) droplet on natural graphite substrate
141 Appendix I
1 second) 10 seconds)
80 seconds) 170 seconds)
900 seconds) 3000 seconds)
Figure A-3: Still image of liquid ferrosilicon (25%) droplet on natural graphite substrate
142 Appendix I
120
100 Si 98.5% / synt graph
Si 98.5 / nat graph 80
60
40 Contact Angle (degrees) Angle Contact 20
0 0 10 20 30 40 50 60 70 80 90 Time (seconds)
Figure A-4: Dynamic wetting of Si 98.5 wt% for different substrates.
160
140 FeSi75 / synt. graphite
120 FeSi75 / nat graph
100
80
60
Contact Angle (degrees) Angle Contact 40
20
0 0 20 40 60 80 100 Time (seconds)
Figure A-5: Dynamic wetting of FeSi 74 wt% for different substrates.
143 Appendix I
140 FeSi25/synt. graphite 120 FeSi25/nat. graphite 100
80
60
40 Contact Angle(degrees) Contact 20
0 0 1000 2000 3000 4000 Time (seconds) Figure A-6: Dynamic wetting of FeSi 24.7 wt% for different substrates
144