The University of New South Wales Faculty of Science School of Materials Science and Engineering

Recycling of Waste Polymers in Electric Arc Furnace Steelmaking: Slag/Carbon and Steel/Carbon Interactions

A Thesis in Materials Science and Engineering

By Somyote Kongkarat

Submitted in Partial Fulfilment of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY November 2011 THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet

Surname or Family name: KONGKARAT

First name: SOMYOTE Other name/s:

Abbreviation for degree as given in the University calendar: Ph.D.

School: Materials Science and Engineering Faculty: Sciences

Title: Recycling of Waste Polymers in Electric Arc Furnace Steelmaking: Slag/Carbon and Steel/Carbon Interactions

Abstract 350 words maximum:

This project is focused on utilizing polymer/coke blends as carbon resource in EAF steelmaking process. In-depth investigations were carried out on slag/carbon and steel/carbon interactions steel at 1550ºC using the sessile drop technique. Interactions between PET/Coke and PU/Coke blends with EAF slag (30.5% FeO) were investigated. PET/Coke blend showed sustained slag foaming with the volume ratio stabilizing at 1.2 over the reaction time, while PU/Coke showed a fluctuating slag foaming behaviour with the volume ratio ranging between 0.75-1.2. These ratios are greater than that for coke where the volume ratio was ~1 initially and then decreased with time to reach approximately 0.75. The levels of CO and

CO2 generated from slag/carbon interactions for PET/Coke and PU/Coke was somewhat lower than that for coke, which allowed a better gas entrapment in the slag sample and sustained slag foaming. H2 and CH4 from the polymer/coke blends also participated in the reduction of FeO in the slag. For steel/carbon interactions, Bakelite, HDPE and PET were blended with metallurgical coke in three different ratios. Bakelite/coke blend (BK1) showed a decrease in contact angles compared to coke, but contact angles were seen to increase slightly with increasing bakelite concentration (BK2 and BK3). CaO from the bakelite caused desulphurization of steel and formed CaS at the metal/carbon interface. A small improvement in carbon transfer was observed compared to coke. HDPE/Coke and PET/Coke blends exhibited slightly better wetting behaviour with liquid steel compared to coke, and contact angles increased marginally with increasing polymers content. Volatiles from HDPE/Coke and PET/Coke slowed down the coverage of interfacial region by reaction products; oxygen from PET/Coke blends helped form FeO at the interface. A marginal improvement in carbon transfer was observed for HDPE/Coke and PET/Coke blends. Similar trends of sulphur transfer into liquid steel were observed for all polymer/coke blends.

Declaration relating to disposition of project thesis/dissertation

I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property 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 authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only).

…………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date

The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research.

FOR OFFICE USE ONLY Date of completion of requirements for Award:

THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS CERTIFICATE OF ORIGINALITY

I herby 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 acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis.

I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.

Signature……………………………... Somyote Kongkarat

ii 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 provisions 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/will apply 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 of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’

Signed ……………………………………………...... Date ……………………………………………......

iii ACKNOWLEDGEMENTS

First of all, I would like to thank my parents for their support, patience and love throughout my PhD study. I would like to express my thanks and deep appreciation to my supervisor, Prof Veena Sahajwalla for helping and guiding me in my PhD study, and for providing financial support for my studies in UNSW.

I also would like to express my special gratitude to my joint supervisor A/Prof Rita Khanna and my co-supervisor Dr. Pramod Koshy for their invaluable suggestions on the project and participation in supervision throughout my study.

I would like to thank my industrial supervisor Mr. Paul O’Kane from Onesteel, Sydney for always helping and giving me advice when needed. I am deeply indebted to his profound knowledge and support which has helped me in completing this project. My appreciation goes to Mr. Narendra Saha-Chaudhury for his assistance with carrying out the experiments, and to the other staff in School of Materials Science and Engineering, UNSW namely Dr Rahmat Kartono, Dr George Yang and Mrs. Lana Strizhevsky.

Finally, I would like to express many thanks to all my wonderful colleagues for always offering me valuable advice, suggestions and help which has given me the motivation to do my PhD.

iv ABSTRACT

Due to increasing utilization of polymeric materials, high volumes of waste plastics are generated from the municipal and industrial sectors. A majority of these waste polymers are incinerated or sent to landfill which leads to major environmental problems. Recycling of waste polymers is still quite limited. Polymers, which are carbon based materials with high volatile matter, have the potential to be used as a source of carbon in EAF steelmaking. In the EAF steelmaking process, properties of metallurgical cokes charged into the furnace are significant factors determining their ability to react with molten slag as a slag foaming agent and FeO reductant, and with molten steel as a recarburizer. The present project was focused on the possibility of using the polymer/coke blends as a source of carbon in EAF steelmaking process. The utilization of waste polymers by blending with metallurgical coke was found to modify the blend properties; thereby resulting in novel carbonaceous resources whose properties were a function of the polymers characteristics. In-depth investigations were carried out on slag/carbon and steel/carbon interactions at 1550ºC using the sessile drop technique.

Interactions between polymer/coke blends with molten slag (30.5% FeO) at 1550ºC were investigated with a focus on the foaming behaviour of molten slag reaction with the polymer/coke blends, and also the influence of chemical elements and volatiles in the polymers. Polyethylene terephthalate (PET) and Polyurethane (PU) were blended with metallurgical coke in a specific ratio. The use of PET/Coke and PU/Coke blends was found to show an improvement in slag foaming behaviour compared to the coke alone. PET/Coke blend showed sustained slag foaming with the volume ratio stabilizing at 1.2 over the reaction time. PU/Coke blend showed a fluctuating slag foaming behaviour with the volume ratio ranging between 0.75-1.2. These ratios are greater than those in the case of coke where the volume ratio was ~1 initially and then decreased with time to reach approximately 0.75. The generation of CO and CO2 gases from slag/carbon interactions in the case of polymer/coke blends was somewhat lower compared to that from coke alone, and this allowed a greater gas entrapment in the slag sample and a better foaming. H2 and CH4 released from the polymer/coke blends were also seen to participate in the reduction of FeO in the slag and these were retained over

v a longer time, thereby aiding the slag foaming. Chemical constituents of the polymer were seen to play an important role in the gasification behaviour of the carbonaceous blends, and this in turn modified the slag foaming behaviour.

Interactions between polymer/coke blends with molten iron (99.98% Fe) at 1550ºC were investigated with a focus on the wetting behaviour of the liquid steel with the carbonaceous blends, the formation of interfacial reaction products and the associated carbon and sulphur transfer into liquid steel. Bakelite, High Density Polyethylene (HDPE) and Polyethylene terephthalate (PET) were blended with metallurgical coke in three different proportions. One bakelite/coke blend (BK1) showed a decrease in contact angles compared to coke. However, the increases in bakelite concentration in the blends as BK2 and BK3 were seen to slightly increase the contact angles. Filler material, CaCO3, in the bakelite was a dominant factor affecting the formation of interfacial products. CaO formed from the decomposition of CaCO3 was found to change the chemical composition and morphology of the interfacial layer. It also participated in desulphurization reactions to form CaS at the interface and also reduced the fusion temperature of the interfacial layer. The carbon pick up values after 60 minutes of contact were found to be 0.13, 0.16 and 0.19 wt% for BK1, BK2 and BK3, respectively, while it was 0.1 wt% for coke.

HDPE/Coke and PET/Coke blends were observed to exhibit slightly better wetting behaviour with liquid steel compared to coke alone. The increase in the polymer content in the blends was found to slightly increase the contact angles. However, the wettability between carbonaceous blends and liquid steel in this study was not found to have a significant affect on the transfer of carbon. Volatiles released from HDPE/Coke and PET/Coke were found to slow down the coverage of interfacial region by the reaction products. The volatiles in HDPE/Coke did not change the chemical composition of the interfacial layer, however oxygen released from PET/Coke blends was observed to form FeO at the interface. A marginal improvement in carbon transfer was observed for HDPE/Coke and PET/Coke blends. The carbon pick-up values after 60 minutes of reaction was 0.15, 0.15 and 0.17 %wt for blends H1, H2 and H3 and 0.09, 0.15 and 0.15 wt% for blends P1, P2 and P3, respectively. Similar trends of sulphur transfer were observed for all polymer/coke blends. Blends BK1, BK2, H1, H2, P1 and P2 showed slightly higher sulphur pick up values compared to the case of coke. vi A decrease in sulphur pick up was seen for blends BK3, H3 and P3. The present study has clearly shown that waste polymers can be a potential source of carbon in EAF steelmaking.

vii TABLE OF CONTENTS

Page

CERTIFICATION OF ORIGINALITY ii COPYRIGHT & AUTHENTICITY STATEMENTS iii ACKNOWLEDGEMENTS iv ABSTRACT v TABLE OF CONTENTS viii LIST OF FIGURES xiv LIST OF TABLES xxiv LIST OF PUBLICATIONS xxviii

CHAPTER 1 INTRODUCTION 1-1 1.1 Introduction 1-2 1.2 Objectives 1-4 1.2.1 Slag/Carbon Interactions 1-5 1.2.2 Steel/Carbon Interaction 1-5

CHAPTER 2 LITERATURE REVIEW 2-1 2.1 Electric Arc Furnace Steelmaking 2-2 2.1.1 Electric Arc Furnace 2-3 2.1.2 Carbon Source in Steelmaking Process 2-4 2.1.2.1 Graphitic Materials 2-5 2.1.2.2 Non-Graphitic Materials 2-5 2.1.2.3 Alternative of Carbon Sources 2-8 2.2 Slag/Carbon Interactions 2-10 2.2.1 Fundamental of Slag Foaming 2-10 2.2.2 Measurement of Slag Foaming 2-12 2.2.3 Factors Influencing Slag Foaming 2-18 2.2.3.1 Gas Bubble Size 2-19 2.2.3.2 Gas Types and Gas Velocity 2-20 2.2.3.3 Temperature 2-21

viii 2.2.3.4 Carbonaceous Materials 2-22 2.2.3.5 Properties of Slag 2-24 2.3 Iron/Carbon Interactions 2-31 2.3.1 Fundamental of Carbon Dissolution 2-31 2.3.2 Dissolution of Carbon from Graphite 2-35 2.3.3 Influence of Sulphur on the Dissolution of Graphite 2-37 2.3.4 Dissolution of Carbon from Non-Graphitic Materials 2-42 2.3.5 Factors Influencing Carbon Dissolution 2-44 2.3.5.1 Carbon Structure 2-44 2.3.5.2 Temperature 2-45 2.3.5.3 Ash 2-46 2.4 Wettability of Iron/Carbon System 2-51 2.4.1 Influence of Melt Carbon and Sulphur Content on Wettability 2-52 2.5 Summary 2-56 2.5.1 Slag/Carbon Interactions 2-56 2.5.2 Iron/Carbon Interaction 2-57 2.6 Research Focus in This Project 2-58

CHAPTER 3 EXPERIMENTAL DETAILS 3-1 3.1 Selection and Preparation of Samples 3-2 3.1.1 Polymeric Material 3-2 3.1.2 Samples for Slag/Carbon Interactions 3-3 3.1.2.1 Slag 3-3 3.1.2.2 Carbonaceous Materials 3-4 3.1.3 Samples for Steel/Carbon Interactions 3-5 3.1.3.1 Metal 3-5 3.1.3.2 Carbonaceous Materials 3-5 3.1.4 Preparation of Samples 3-6 3.2 Experimental Instruments 3-6 3.2.1 X-ray Diffractometer (XRD) 3-7 3.2.2 Scanning Electron Microscope (SEM) 3-7 3.2.3 Gas Analyzer 3-8 3.2.4 Carbon-Sulphur Analyzer 3-8 3.2.5 High Temperature Furnaces 3-9 ix 3.2.5.1 Drop Tube Furnace 3-9 3.2.5.2 Horizontal Tube Furnace 3-10 3.3 Slag/Carbon Interaction Experiments 3-11 3.3.1 Slag Foaming 3-11 3.3.2 Observation of Reduced Iron and Entrapped Gas Bubbles 3-13 3.3.3 Analysis of Gas Generation from Slag/Carbon Interactions 3-14 3.4 Steel/Carbon Interaction Experiments 3-15 3.4.1 Carbon and Sulphur Transfer 3-15 3.4.2 Observation of Reaction Products at Steel/Carbon Interface 3-16 3.4.3 Wetting of Polymer/Coke blends by Molten Steel 3-17

CHAPTER 4 SLAG/CARBON INTERACTIONS BETWEEN POLYMER/COKE BLENDS AND MOLTEN SLAG AT 1550ºC 4-1 4.1 Characterization of Polymer/Coke Blends 4-3 4.1.1 XRD Analysis 4-4 4.1.2 Chemical Analysis 4-5 4.1.3 Morphology Studies 4-7 4.2 Dynamic Slag Foaming Behaviour 4-9 4.3 Off-gas Analysis 4-12

4.3.1 CO and CO2 Generation 4-12 4.3.2 Gas Chromatography (GC) Analysis 4-15 4.4 Optical Microscopic Investigations 4-18 4.5 Discussion 4-20 4.5.1 Influence of Gas Generation on Slag Foaming 4-20 4.5.2 Influence of Volatiles from the Polymers on Slag Foaming 4-21 4.5.3 Influence of Entrapped Gas Bubbles in the Bulk Slag on Slag Foaming 4-29 4.6 Summary 4-31

CHAPTER 5 WETTABILITY AND CARBON TRANSFER BETWEEN POLYMER/COKE BLENDS AND MOLTEN STEEL AT 1550 ºC 5-1 5.1 Bakelite/Coke Blends 5-2 5.1.1 Characterization of Bakelite/Coke Blends 5-2 5.1.1.1 XRD Analysis 5-3 x 5.1.1.2 Chemical Analysis 5-4 5.1.1.3 Morphology Studies 5-6 5.1.2 Wettability of Bakelite/Coke Blends 5-8 5.1.3 Carbon-Sulphur Transfer from Bakelite/Coke Blends 5-12 5.1.3.1 Carbon Transfer 5-12 5.1.3.2 Sulphur Transfer 5-13 5.2 HDPE/Coke Blends 5-15 5.2.1 Characterization of HDPE/Coke Blends 5-15 5.2.1.1 XRD Analysis 5-15 5.2.1.2 Chemical Analysis 5-16 5.2.1.3 Morphology Studies 5-18 5.2.2 Wettability of HDPE/Coke Blends 5-20 5.2.3 Carbon-Sulphur Transfer from HDPE/Coke Blends 5-23 5.2.3.1 Carbon Transfer 5-23 5.2.3.2 Sulphur Transfer 5-24 5.3 PET/Coke Blends 5-25 5.3.1 Characterization of PET/Coke Blends 5-25 5.3.1.1 XRD Analysis 5-25 5.3.1.2 Chemical Analysis 5-26 5.3.1.3 Morphology Studies 5-28 5.3.2 Wettability of PET/Coke Blends 5-30 5.3.3 Carbon-Sulphur Transfer from PET/Coke Blends 5-33 5.3.3.1 Carbon Transfer 5-33 5.3.3.2 Sulphur Transfer 5-34 5.4 Effect of Blending Polymers with Coke on Wettability 5-34 5.5 Effect of Blending Polymers with Coke on Carbon Transfer 5-37 5.6 Summary 5-39

CHAPTER 6 INTERFACIAL PHENOMENA BETWEEN POLYMER/COKE BLENDS AND MOLTEN STEEL AT 1550 ºC 6-1 6.1 Interfacial Phenomena between Bakelite/Coke Blends with Molten Steel 6-2 6.2 Interfacial Phenomena between HDPE/Coke Blends with Molten Steel 6-10 xi 6.3 Interfacial Phenomena between PET/Coke Blends with Molten Steel 6-15 6.4 Summary 6-24

CHAPTER 7 DISCUSSIONS ON INTERACTIONS OF POLYMER/COKE BLENDS WITH MOLTEN STEEL AT 1550ºC 7-1 7.1 Bakelite 7-2 7.1.1 Interfacial Phenomena 7-2 7.1.1.1 Estimation of Solid and Liquid Components of the Interfacial Reaction Products 7-5 7.1.2 Wettability 7-9 7.1.3 Carbon and Sulphur Transfer 7-11 7.1.3.1 Comparison of Carbon and Sulphur Pick-up from Coke and Bakelite/Coke Blends with Raw Bakelite 7-12 7.2 High Density Polyethylene (HDPE) 7-18 7.2.1 Interfacial Phenomena 7-18 7.2.2 Wettability 7-20 7.2.3 Carbon and Sulphur Transfer 7-22 7.3 Polyethylene Terephthalate (PET) 7-25 7.3.1 Interfacial Phenomena 7-25 7.3.2 Wettability 7-28 7.3.3 Carbon and Sulphur Transfer 7-29 7.3.3.1 Effect of Oxygen in PET on Carbon Pick-up 7-31 7.4 Influence of Polymers on the Carburization of Molten Steel 7-33 7.5 Summary 7-35

CHAPTER 8 CONCLUSIONS AND FUTURE WORK 8-1 8.1 Slag/Carbon Interactions 8-2 8.2 Iron/Carbon Interactions 8-4 8.2.1 Bakelite – Metallurgical Coke Blends 8-4 8.2.2 High Density Polyethylene (HDPE) – Metallurgical Coke Blends 8-6 8.2.3 Polyethylene Terephthalate (PET) – Metallurgical Coke Blends 8-6

xii 8.2.4 Role of Chemical Composition in Polymers on Carburization of Molten Iron: Studies on 100% Polymers 8-7 8.3 Future Work 8-8

CHAPTER 9 REFERENCES 9-1

APPENDIX 10-1 Appendix I Calculation of Rate of FeO Reduction 10-1 Appendix II Data Input for Thermodynamic Calculation using FactSage 6.0 10-4

xiii LIST OF FIGURES

Figure Page

2-1 Schematic showing the sectional view of an electric arc furnace (EAF) [Steeluniversity (2006)]. 2-4 2-2 Schematic of the crystal structure of graphite [Sahajwalla and Khanna (2003)1]. 2-5 2-3 Schematic representing the structure of (a) graphite and (b) coal [Krevelen (1993)]. 2-6 2-4 Schematic of the slag foaming phenomenon in EAF steelmaking [Steeluniversity (2006)]. 2-11 2-5 Images of slag droplet while reacting with metallurgical coke and HDPE/Coke blends (blend#1 and blend#3) at 1550ºC [Sahajwalla et al. (2009)]. 2-17 2-6 Volume ratios for slag reaction with HDPE/Coke blends (blend#1 through blend#4) at 1550ºC [Sahajwalla (2009)]. 2-18 2-7 The effect of the bubble size on slag foaming (Foam index) [Zhang and Fruehan (1995)1]. 2-20

2-8 Foam height of CaO-SiO2-FeO slags as a function of superficial gas velocity [Zhang and Fruehan (1995)1]. 2-21 2-9 The temperature dependence of foaming index and viscosity for

48%CaO-32%SiO2-10%FeO-10%Al2O3 [Oztuk and Fruehan (1995)]. 2-22 2-10 Slag foaming (represented through volume ration) during slag reaction with metallurgical coke and natural graphite [Rahman et al. (2009)]. 2-23

2-11 CO and CO2 generated during slag reaction with metallurgical coke and natural graphite [Rahman et al. (2009)]. 2-24

2-12 The foam index of CaO-SiO2-FeO-Al2O3 slags illustrating the effect of second phase particles [Zhang and Fruehan (1995)1]. 2-26 2-13 Effect of basicity of slag on gas hold-up index [Yi and Kim (2002)]. 2-26 -2 º 2-14 Viscosity (N.s.m ) of CaO-SiO2-FeO melts at 1400 C [Kozakevitch (1949)]. 2-28

xiv º 2-15 Isoviscosity curves for the CaO-SiO2-FeO melts at 1450 C [Bronson et al. (1985)]. 2-28 2-16 Surface tension of binary FeO melts at 1400ºC [Kozakevitch (1949)]. 2-29

2-17 Surface tension curve in FeO-CaO-SiO2 melts saturated with Fe at 1400ºC [Kozakevitch (1949]. 2-29 2-18 Effect of surface tension of molten slag on the foaminess in the bath smelting of iron ore [Hirata et al. (1985)]. 2-30

2-19 Effect of P2O5 in molten slag on the foaminess in the bath smelting of iron ore [Hirata et al. (1985)]. 2-31 2-20 Variation of carbon concentration at the iron/carbon interface. C is bulk

carbon concentration, Ci is carbon concentration at the interface, and į is the interfacial layer thickness [Gudenau et al. (1990)]. 2-32 2-21 Various experimental techniques used for carbon dissolution studies. 2-34 2-22 Influence of sulphur concentration on the overall mass transfer coefficient (k) when using graphitic and non-graphitic materials [Shigeno et al. (1985)]. 2-38 2-23 Effect of bath sulphur concentration on the dissolution of graphite [Wright and Baldock (1988)]. 2-39 2-24 Atomic distribution profile across graphite/Fe-S interface before and after simulation. Z corresponds to the number of layers normal to the interface and Z=50 is the initial contact surface [Sahajwalla and Khanna (2003)1]. 2-41 2-25 Carbon dissolution rate as a function of time for different ranges of sulphur and carbon concentrations in the melt. Time is measured in units of Monte Carlo [Sahajwalla and Khanna (2003)1]. 2-41 2-26 Carbon and sulphur transfer from coke into liquid iron at 1550ºC [McCarthy et al. (2003)]. 2-49 2-27 Contact angle (ș) of liquid iron droplet on solid carbon in a sessile

drop experimental setup. ıSL, ıSG and ıLG represent interfacial tension between solid-liquid, solid-gas and liquid-gas acting on the droplet, respectively. 2-51 2-28 The variation of contact angles as a function of time for molten Fe-S-C and graphite system [Wu and Sahajwalla (1998)]. 2-53

xv 2-29 The variation of the contact angles as a function of time for molten Fe-S-C and graphite system with varying sulphur levels [melt C level was fixed at 2% ] [Wu and Sahajwalla (1997)]. 2-54 2-30 The variation in the contact angles as a function of time for molten iron on natural graphite substrate [Wu et al. (2000)]. 2-54

3-1 Relative Proportion of plastic and coke in the carbonaceous blends (wt%) used in slag/carbon interaction experiments. 3-4 3-2 Relative proportions of plastics and metallurgical coke in the carbonaceous blends used for steel/carbon interaction experiments. 3-6 3-3 Scanning Electron Microscope (SEM Hitachi S3400). 3-7 3-4 Carbon and sulphur analyzer (LECO CS230). 3-8 3-5 Drop tube furnace (DTF). 3-9 3-6 Schematic of the Drop Tube Furnace (DTF) used in the experiments [Rahman (2010)]. 3-10 3-7 Horizontal tube furnace (HF). 3-10 3-8 Schematic of experimental set up for slag/carbon interaction experiments. 3-11 3-9 Carbonaceous substrate/slag assembly before and after experiments. 3-12 3-10 Illustration of the slag droplet during data processing indicating the radius ‘r’ and truncated height ‘h’ used in computing the slag volume ‘V’. 3-13 3-11 Cross-section of the slag droplet after reaction with the carbonaceous substrate showing the presence of reduced metal droplets and entrapped gas bubble. 3-14 3-12 Carbonaceous substrate/metal assembly before and after the experiments. 3-15 3-13 Schematic of experimental set up for steel/carbon interaction experiments. 3-16 3-14 Illustration of the contact angle measurement using ANGLE software. 3-17

4-1 Proportion of plastic and coke in the blends (wt%). 4-3 4-2 XRD patterns of a) Coke; b) Raw PET; and c) Raw PU. 4-4 4-3 XRD patterns of a) Raw PET/Coke, b) Char PET/Coke, c) Raw PU/Coke and d) Char PU/Coke. 4-5 4-4 Morphology of PET/coke and PU/coke char compared to coke [images at magnifications of 30x (left side) and 1000x (right side)]. 4-8

xvi 4-5 SEM images of PU/coke char showing: a) an agglomeration of coke and PU into a large particle and b) morphology of the agglomeration particle at magnifications of 1500x and associated EDS spectra indicating the composition of the spherical particles formed on the surface. 4-9 4-6 Changes in the volume ratio of the slag droplet during reaction with a) PET/Coke blend and b) PU/Coke blend at 1550ºC. The changes in the volume ratio for slag reactions with coke are shown as a comparison in both cases. 4-10 4-7 Images showing the dynamic changes inn the volume of the slag during reactions with coke at 1550˚C. 4-10 4-8 Images showing the dynamic changes inn the volume of the slag during reactions with PET/Coke blends at 1550˚C. 4-11 4-9 Images showing the dynamic changes inn the volume of the slag during reactions with PU/Coke blends at 1550˚C. 4-11

4-10 CO and CO2 generated during slag/carbon interactions at 1550˚C for a) PET/Coke and b) PU/Coke blends compared to coke. 4-12 4-11 Moles of a) Carbon and b) Oxygen removed as a function of time during slag/carbon interactions at 1550˚C for PET/Coke blends compared to coke. 4-13 4-12 Moles of a) Carbon and b) Oxygen removed as a function of time during slag/carbon interactions at 1550˚C for PU/Coke blends compared to coke. 4-13 4-13 Gas chromatographs showing the gases produced from the different blank substrates of a) coke, b) PET/Coke and c) PU/Coke after 1 minute at 1550˚C. 4-15 4-14 Gas chromatograph showing the gases released during slag-coke interaction at 1550˚C after a) 1 minute and b) 15 minutes of reaction for Coke, PET/Coke and PU/Coke. 4-17 4-15 Optical microscopic images of the cross sectioned slag droplets after reaction with coke at different times. 4-18 4-16 Optical microscopic images of the cross sectioned slag droplets after reaction with PET/Coke at different times. 4-19 4-17 Optical microscopic images of the cross sectioned slag droplets after reaction with PU/Coke at different times. 4-20 xvii 4-18 Rate of reaction for PET/Coke and PU/Coke blends in contact with slag at 1550ºC compared to metallurgical coke. 4-22 4-19 Percentages of FeO in the quenched slag samples after reaction with coke, PET/Coke and PU/Coke at different times. 4-27

5-1 Relative proportions between the plastics and metallurgical coke in the polymer/coke blends. 5-2 5-2 XRD patterns of a) Coke and b) Raw Bakelite. 5-3 5-3 XRD patterns of Bakelite/Coke blends: a) Raw blends and b) Chars. 5-4 5-4 Microstructural morphology of Bakelite/Coke blends compared to coke [SEM images taken at magnifications of 30X (left) and 1000X (right)]. 5-7 5-5 Morphology of blend BK3 char’s surface along with the EDS analysis indicating the deposition of CaO on the surface due to the decomposition

of CaCO3 from the bakelite. 5-8 5-6 Wetting images of steel droplet after reaction with coke for 60 minutes at 1550ºC. 5-8 5-7 Wetting images of steel droplet after reaction with blend BK1 for 60 minutes at 1550ºC. 5-9 5-8 Wetting images of steel droplet after reaction with blend BK2 for 60 minutes at 1550ºC. 5-9 5-9 Wetting images of steel droplet after reaction with blend BK3 for 60 minutes at 1550ºC. 5-10 5-10 Variation of contact angles of steel droplet with blend BK1 and coke substrates at 1550ºC. 5-10 5-11 Variation of contact angles of steel droplet with blend BK2 and coke substrates at 1550ºC. 5-11 5-12 Variation of contact angles of steel droplet with blend BK3 and coke substrates at 1550ºC. 5-11 5-13 Carbon picked up from Bakelite/Coke blends by liquid steel at 1550ºC compared to coke. 5-13 5-14 Sulphur transferred fromBakelite/Coke blends into liquid steel at 1550ºC compared to coke. 5-14

xviii 5-15 XRD patterns of a) Coke and b) Raw HDPE. 5-15 5-16 XRD patterns of HDPE/Coke blends: a) Raw blends and b) Chars. 5-16 5-17 Microstructural morphology of HDPE/Coke blends compared to coke [SEM images taken at magnifications of 50X (left) and 1000X (right)]. 5-19 5-18 Wetting images of steel droplet after reaction with blend H1 for 60 minutes at 1550ºC. 5-20 5-19 Wetting images of steel droplet after reaction with blend H2 for 60 minutes at 1550ºC. 5-20 5-20 Wetting images of steel droplet after reaction with blend H3 for 60 minutes at 1550ºC. 5-21 5-21 Variation of contact angles of steel droplet with blend H1 and coke substrates at 1550ºC. 5-21 5-22 Variation of contact angles of steel droplet with blend H2 and coke substrates at 1550ºC. 5-22 5-23 Variation of contact angles of steel droplet with blend H3 and coke substrates at 1550ºC. 5-22 5-24 Carbon picked up from HDPE/Coke blends by liquid steel at 1550ºC compared to coke. 5-23 5-25 Sulphur transferred fromHDPE/Coke blends into liquid steel at 1550ºC compared to coke. 5-24 5-26 XRD patterns of a) Coke compared to b) Raw PET. 5-25 5-27 XRD patterns of PET/Coke blends: a) Raw blends and b) Chars. 5-26 5-28 Microstructural morphology of PET/Coke blends compared to coke [SEM images taken at magnifications of 50X (left) and 1000X (right)]. 5-29 5-29 Wetting images of steel droplet after reaction with blend P1 for 60 minutes at 1550ºC. 5-30 5-30 Wetting images of steel droplet after reaction with blend P2 for 60 minutes at 1550ºC. 5-30 5-31 Wetting images of steel droplet after reaction with blend P3 for 60 minutes at 1550ºC. 5-31 5-32 Variation of contact angles of steel droplet with blend P1 and coke substrates at 1550ºC. 5-31 5-33 Variation of contact angles of steel droplet with blend P2 and coke substrates at 1550ºC. 5-32 xix 5-34 Variation of contact angles of steel droplet with blend P3 and coke substrates at 1550ºC. 5-32 5-35 Carbon picked up from PET/Coke blends by liquid steel at 1550ºC compared to coke. 5-33 5-36 Sulphur transferred fromPET/Coke blends into liquid steel at 1550ºC compared to coke. 5-34 5-37 Comparison of the initial contact angles (after1 minute) of the steel droplet with coke and its blends with polymers. 5-36 5-38 Comparison of the final contact angles (after 60 minutes) of the steel droplet with coke and its blends with polymers. 5-37 5-39 comparison of the carbon pick up by molten steel droplets after reaction with coke and polymer/coke blends for 60 minutes. 5-37

6-1 SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-2 6-2 SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-3 6-3 SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-3 6-4 SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-4 6-5 SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-5 6-6 SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-5 6-7 SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-6 6-8 SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-6 6-9 SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-7 6-10 SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-7

xx 6-11 SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-8 6-12 SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-8 6-13 SEM image (1500x) showing the steel/ BK3 interface along with the EDS analysis of points in the region after 60 minutes of reaction, A indicates

the presence of CaO.Al2O3 and B indicates the presence of CaS complex. 6-9 6-14 SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-11 6-15 SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-11 6-16 SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-12 6-17 SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-12 6-18 SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-13 6-19 SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-13 6-20 SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-14 6-21 SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-14 6-22 SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-15 6-23 SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-16 6-24 SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-16 6-25 SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-17 6-26 SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-17

xxi 6-27 SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-18 6-28 SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-18 6-29 SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions. 6-19 6-30 SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions. 6-20 6-31 SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions. 6-20 6-32 SEM image (x1500) of the steel/coke interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of

Al2O3. 6-21 6-33 SEM image (x1500) of the steel/P2 interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of

Al2O3-FeO complex. 6-22 6-34 SEM image (x1500) of the steel/P3 interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of

Al2O3-FeO complex. 6-22

7-1 SEM image and EDS analysis of the cross-section of Steel/Coke system after 60 minutes of reaction at 1550ºC. A is the metal phase, B is carbon substrate and C is the ash layer formed at the metal/carbon interface. 7-3 7-2 SEM image (1500x) comparing morphology of the interfacial layer after 60 minutes of reaction for coke and its blends with bakelite, indicating the role of CaO generated from the bakelite on reducing melting temperature of the interfacial layer. 7-4 7-3 Plots of contact angles for Bakelite/Coke blends against CaO content in the blends for a) 1 minute and b) 60 minutes of contact (the data scatter was within ± 5º). 7-10 7-4 Comparison of carbon picked up after a) 1 minute and b) 60 minutes of reaction for coke, bakelite/coke blends and raw bakelite. 7-13 7-5 Comparison of carbon content in coke, bakelite/coke blends and raw bakelite. 7-13

xxii 7-6 Comparison of sulphur picked up after a) 1 minute and b) 60 minutes of reaction for coke, bakelite/coke blends and raw bakelite. 7-17 7-7 SEM images of the reaction products formed at the interface for blends H1, H2 and H3 compared to coke. 7-19 7-8 Plots of contact angles for a) after 1 minute and b) after 60 minutes for HDPE/Coke blends against the initial volatiles content in the materials (the data scatter was within ± 5). 7-21 7-9 Plots of contact angles after 1 minute of contact for metallurgical coke, Char 1 and Char 2 against the initial volatiles content in the materials [McCarthy (2004)]. 7-22 7-10 SEM images of the reaction products formed at the interface for blends P1, P2 and P3 compared to coke. 7-26 7-11 SEM image (1500x) comparing morphology of the interfacial layer after 60 minutes of reaction for coke and its blends with PET and EDS spectra indicate the role of oxygen generated from the PET on changing

morphology and forming new phase (FeO-Al2O3) at the interfacial layer. 7-27 7-12 Plots of contact angles for a) after 1 minute and b) after 60 minutes for PET/Coke blends against the initial volatiles content in the materials (the data scatter was within ± 5). 7-29 7-13 CO generated during steel/carbon interactions at 1550ºC for coke and blend P3 compared to that generated from the blank substrates (solid substrate alone). 7-32 7-14 Comparison of carbon picked up after 60 minutes of reaction for coke, raw bakelite, HDPE and PET. 7-33

xxiii LIST OF TABLES

Table Page

3-1 Chemical composition of the raw polymeric materials 3-3 3-2 Chemical composition of raw bakelite 3-3 3-3 Slag composition used for slag/carbon interaction experiments 3-3

4-1 Chemical composition of the polymer/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 4-6 4-2 Chemical composition of the polymer/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 4-6 4-3 Ash analyses of the polymer/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 4-6 4-4 Ash analyses of the polymer/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 4-7 4-5 Calculated peak of FeO reduction rate (moles.s-1) for metallurgical coke PET/Coke and PU/Coke 4-23 4-6 Comparison of the maximum rate of FeO reduction by different carbonaceous materials obtained from literature 4-24 4-7 Chemical composition of HDPE, rubber tyre, PET and PU 4-25 4-8 The range of diameters of small gas bubbles entrapped in the slag droplet for coke and its blends with polymer between 2 – 8 minutes of reaction 4-30

5-1 Chemical composition of the bakelite/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-5

xxiv 5-2 Chemical composition of the bakelite/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-5 5-3 Ash analyses of the bakelite/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-6 5-4 Ash analyses of the bakelite/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-6 5-5 Chemical composition of the HDPE/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-17 5-6 Chemical composition of the HDPE/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-17 5-7 Ash analyses of the HDPE/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-18 5-8 Ash analyses of the HDPE/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-18 5-9 Chemical composition of the PET/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-27 5-10 Chemical composition of the PET/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-27 5-11 Ash analyses of the PET/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-28 5-12 Ash analyses of the PET/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW 5-28

xxv 5-13 Comparison of the initial and final (after 120 minutes) contact angles of pure liquid Fe with chars and metallurgical coke obtained from previous studies 5-35

6-1 Oxygen content in coke and PET/coke blends 6-21

7-1 Estimated values of percentage of solid / liquid component of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0) 7-6 7-2 Estimated values of constituents in liquid phase of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0) 7-7 7-3 Estimated values of constituents in solid phase of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0) 7-8 7-4 Comparison of contact angles and volatiles and ash chemistry of coke and bakelite/coke blends 7-9 7-5 Major ash components of coke, Bakelite/Coke blends and raw bakelite, this table shows only the main ash oxide components 7-12 7-6 Carbon dissolution rate and carbon picked up values for electrolytic pure iron (99.98% Fe) - chars and metallurgical coke systems at 1550ºC obtained from previous studies 7-15 7-7 Comparison of carbon pick up values after 60 minutes of contact for iron (99.98 %Fe) – coke system, and ash level and composition of the coke obtained from McCarthy et al. (2003) with the values from the present study 7-16 7-8 Comparison of contact angles and chemical properties of coke and HDPE/coke blends 7-20 7-9 Carbon picked up by liquid steel droplets after reaction with raw HDPE at 1550ºC for 2 and 60 minutes compared to that of coke 7-24 7-10 Comparison of contact angles and chemical properties of coke and PET/coke blends 7-28 7-11 Carbon picked up by liquid steel droplets after reaction with raw PET at 1550ºC for 2 and 60 minutes compared to that of coke 7-30 xxvi 7-12 Percentage of oxygen content in liquid steel after reactions with coke and blend P3 at 1550ºC for different times 7-31

10-1 Sample for calculation of FeO reduction rate (R0) 10-3 10-2 Data input for the thermodynamic calculation using FactSage 6.0 10-4

xxvii List of Publications

Journals

1. S. Kongkarat, P. Koshy, R. Khanna, P. O’Kane and V. Sahajwalla, Recycling Waste Polymers in EAF Steelmaking: Influence of Polymer Composition on Carbon/Slag Interactions, ISIJ Int., 2012, V. 52, No. 3.

2. S. Kongkarat, R. Khanna, P. Koshy, P. O’Kane and V. Sahajwalla, Use of Waste Bakelite as a Raw Material Resource for Recarburization in Steelmaking Processes, Steel Research International, 2011, V. 82, No.10, pp. 1228-1239.

3. V. Sahajwalla, M. Zaharia, S. Kongkarat, R. Khanna, N.S. Chaudhury and P. O’Kane, Recycling Plastics as a Resource for Electric Arc Furnace (EAF) Steelmaking: Combustion and Structural Transformations of Metallurgical Coke and Plastic Blends, Energy and Fuel, 2010, V. 24, pp. 379-391.

4. V. Sahajwalla, M. Zaharia, S. Kongkarat, R. Khanna, N.S. Chaudhury and P. O’Kane, Recycling End-of-Life Polymers in Electric Arc Furnace Steelmaking Process-Fundamentals of Polymer Reactions with Slag and Metal, Energy and Fuel, 2011 (Accepted for publication DOI: 10.1021/ef201175t)

Conferences

5. V. Sahajwalla, S. Kongkarat, M. Zaharia, P. Koshy, R. Khanna, N.S. Chaudhury, D. Knights, P. O’Kane and K. Thangaraj, Utilization of Waste Plastics in EAF Steelmaking: High Temperature Interactions between Slag and Carbonaceous Materials, AISTech 2010, USA (Presented at Conference AISTech 2010-Pittsburgh, Pa., May 3-6, 2010).

xxviii 6. V. Sahajwalla, R. Khanna, M. Zaharia, S. Kongkarat, M. Rahman, B.C. Kim, N.S. Chaudhury, P. O’Kane, J. Dicker, C. Skidmore and D. Knights, Environmentally sustainable EAF Steelmaking Through Introduction of Recycled Plastics and Tires: Laboratory and Plant Studies, AISTech 2009, USA (Presented at Conference AISTech 2009-St.Louis, Mo., May 4-7, 2009).

7. V. Sahajwalla, R. Khanna, S. Kongkarat, P. Koshy, M. Rahman, M. Zaharia, N.S. Chaudhury, D. Knights, P. O’Kane, C. Skidmore and J. Dicker, Carbon Dissolution in Steelmaking Process-Recarburization Using Waste Plastics, AISTech 2009, USA (Presented at Conference AISTech 2009-St.Louis, Mo., May 4-7, 2009).

8. V. Sahajwalla, M. Zaharia, R. Khanna, S. Kongkarat, M. Rahman, P. Koshy, N.S. Chaudhury, D. Knights, P. O’Kane, C. Skidmore and J. Dicker, Gas Phase Reaction of Coke Blends with Plastics for Chemical Energy Input into EAF Steelmaking, AISTech 2009, USA (Presented at Conference AISTech 2009-St.Louis, Mo., May 4-7, 2009).

xxix Chapter 1 Introduction

CHAPTER 1

INTRODUCTION Chapter 1 Introduction

1.1 Introduction

Plastics production and use has grown significantly in the past few decades, because of their unique and advantageous properties over traditional materials. Thus growth in the generation of plastic wastes is expected in the coming years. Post-consumer plastic wastes from both industrial and household sectors in Australia alone had reached over 1.5 million tonnes in 2008 [PACIA (2009)]; the worldwide waste generation levels are expected to be much higher. The recycling of waste plastics is still at a relatively low level, e.g., the plastics recycling rate in Australia was only 18.5% in 2008 [PACIA (2009)]. The remaining volumes end up in landfills or are incinerated. Plastics have high volume to weight ratio and are generally non-biodegradable, and thus present an environmental problem. The disposal of wastes in landfills requires an active extraction industry, the availability of land in locations relevant to waste generation, the availability of low-cost transportation and the meeting of different environmental regulatory requirements [Alter (1993)]. On the other hand, the incineration of plastics can lead to waste destruction and detoxification, which results in the emission of several environment pollutants [Brunner (1985)]. Due to the inherent limitations of these current methods of plastic waste management, alternative means which are environmentally friendly and economical are essential.

The developments in this field include the commercial utilisation of waste plastics in the power industry, in coke making processes and in blast furnaces in Japan, Korea and Germany [Asanuma et al. (1997)]. Recent developments through laboratory based research by UNSW researchers have shown that waste plastics can be used as an alternative source of carbon for inducing slag foaming in EAF steelmaking process. A partial replacement of metallurgical coke with high density polyethylene (HDPE) and rubber tyre wastes produced significant levels of slag foaming [Sahajwalla et al. (2006) and Zaharia et al. (2009)].

Electric arc furnace (EAF) steelmaking accounts for approximately 40% of the world’s production of steel [WCA (2010)]. Compared to the integrated steel production route, EAF steelmaking has the advantages of lower capital requirements and operating costs

1-2 Chapter 1 Introduction

and greater flexibility in the choice of raw materials. In recent decades, the world steel market has seen the widespread adoption of electric arc furnace steelmaking technology, since the entire pattern of the industry is changing in response to technological innovations, the energy crisis and environmental problems. To make EAF steelmaking more sustainable, new process technologies need to be developed in order to lower the consumption of coke and to reduce green house gas emissions.

Carbon is essential during steelmaking, being one of the key elements which vary the properties of different steels. Significant roles played by carbon in the steelmaking process include reducing FeO in molten slag, inducing foamy slag and carburizing molten steel. In EAF steelmaking, solid carbon is injected into the slag phase to reduce iron oxide and for slag foaming. The products of these reactions are iron and carbon monoxide. The resulting carbon monoxide is an important component of the slag foaming process. Moreover, the chemical reactions between solute carbon in molten iron and slag also give rise to the slag foaming process. Slag foaming occurs due to CO gas generation as a result of the reduction of iron oxide in the molten slag by both solid and solute carbon and also due to the oxidation of carbon. A foamy slag floats on the top of the molten iron and shields the electric arc which enables the temperature of the molten steel bath to be raised more quickly, resulting in considerable savings in energy [Schroeder (1991) and McGee and Irons (2001)]. A sustained level of slag foaming is essential for efficient operation of the EAF steelmaking process. With slag foaming also depending strongly on the concentration of carbon in molten iron (iron/slag reaction), the iron/carbon interactions, including interfacial phenomena, wettability and carbon dissolution, are important factors determining the performance of carbon in the steelmaking process.

Metallurgical coke is the conventional source of carbon in EAF steelmaking. In the steelmaking industry, consumption of coke is quite high, with about 0.6 tonnes of coke required to produce 1 tonne of steel [WCA (2010)]. The consumption of coke in steelmaking generates high levels of greenhouse gases, such as CO2. Metallurgical cokes charged into the furnace come from different sources and thus the properties are varied and difficult to control and could therefore be a problem in the steelmaking process. Properties of the coke, such as structure (Lc value) and chemical composition (fixed carbon, ash oxides, sulphur and volatiles), are significant factors in determining 1-3 Chapter 1 Introduction

its ability to react with molten slag and steel (such as FeO reduction and carburization). The interactions of metallurgical coke with both molten slag and iron have been extensively studied and a fundamental understanding related to the behaviour of metallurgical coke has been established. The interactions of coke in the steelmaking process could be improved, if the coke properties are modified. Polymers are carbon- based materials which also contain smaller amounts of hydrogen, oxygen and/or nitrogen. Four common polymeric materials, one thermoset (Bakelite) and three thermoplastics (HDPE, PET and PU), were chosen for these investigations from a wide spectrum of plastics. Experimental investigations on slag/carbon interactions had been conducted previously on HDPE and rubber tyres, which contained predominantly carbon and hydrogen [Zaharia (2010) and Rahman (2010)]. Polymers that contain oxygen and nitrogen in their molecular chains (PET and PU) could produce differences in interactions with molten slag and molten steel.

The aim of this project is to evaluate the potential of using waste polymers in EAF steelmaking. Polymers will be blended with metallurgical coke in a range of proportions and used as a source of carbon in steelmaking process. The polymer addition could help modify blend characteristics; thereby the resultant carbonaceous materials are expected to have slightly different characteristics compared to the parent coke, and also could exhibit different behaviour when reacting with molten slag and iron.

1.2 Objectives

For utilizing waste polymers as a source of carbon in steelmaking process, a fundamental understanding of the interactions of polymer/coke blends with molten slag and molten steel needs to be developed. In the present study, PET and PU were selected for the slag/carbon interaction experiments to investigate the role of additional chemical elements in the polymers (hydrogen, oxygen and nitrogen) on slag/carbon interactions. For iron/carbon interactions, bakelite, HDPE and PET were selected for in-depth investigations. Bakelite contains significant quantities of CaCO3 which is a filler material in the polymer. HDPE contains high levels of volatiles, while PET contains oxygen in its polymer chain. The differences in chemical composition in the

1-4 Chapter 1 Introduction

polymeric materials could have an impact on their interactions with iron at high temperatures and will be the focus of this project. The present project involves two parts and the objectives of this research are:

1.2.1 Slag/Carbon Interactions

¾ To investigate the high-temperature interactions between polymer/coke blends (PET/Coke and PU/Coke) with molten slag at 1550ºC with a focus on the slag reactions with the polymer/coke blends. ¾ To determine the influence of the chemical elements and volatiles in polymers on slag/carbon interactions.

1.2.2 Steel/Carbon Interactions

To investigate high-temperature interactions between polymer/coke blends (Bakelite/Coke, HDPE/Coke and PET/Coke) with molten steel at 1550ºC, with a focus on:

¾ The wetting behaviour of the liquid iron with the polymer/coke blends. ¾ The formation of interfacial reaction products at the iron/carbon interface. ¾ Associated carbon and sulphur transfer into liquid iron.

1-5 Chapter 2 Literature Review

CHAPTER 2

LITERATURE REVIEW

Chapter 2 Literature Review

Key mechanisms of carbon consumption in EAF steelmaking processes are slag/carbon and steel/carbon interactions. In the slag foaming practice, oxygen is injected into liquid steel to form FeO slag which floats on top of the liquid metal surface. Solid carbon is injected into the slag phase to reduce FeO dissolved in the slag. The reduction of FeO generates gas (CO) in the slag layer which causes slag foaming. The oxygen injected into the liquid steel oxidizes the solute carbon in the melt and this reaction also generates CO in the slag phase. Slag foaming is influenced by the melt carbon content, because the FeO in the slag can react with solute carbon in the melt and undergo reduction. Thus, solid carbon is added to the furnace to recarburize and adjust the carbon level in the liquid steel. However, slag/carbon interactions and steel/carbon interactions depend strongly on the characteristics of the carbon source. Metallurgical coke is the carbon source generally used in the EAF steelmaking process. The properties of the coke, such as carbon structure, fixed carbon and ash contents are important in controlling its interactions with the molten slag and steel. An improvement in carbon interactions with molten slag and steel could be obtained if the properties of the carbon are modified.

In this chapter, a brief overview of the electric arc furnace and its operation are presented. In addition, the fundamental understanding gained from previous studies on slag/carbon interactions and steel/carbon interactions are reviewed. In the case of slag/carbon interactions, the main focus in this investigation will be on the foaming behaviour of molten slag. For steel/carbon interactions, investigations will be carried out on the wetting behaviour of carbonaceous materials with molten steel, the interfacial phenomena occurring at the metal/carbon interface and transfer of carbon and sulphur into molten steel.

2.1 Electric Arc Furnace Steelmaking

Two key approaches used in making steel are integrated producers and the electric arc furnace. Integrated steel mills produce steel from iron ore utilizing the blast furnace and basic oxygen furnace (BOF) processes. EAF steelmakers, often referred to as minimills, produce steel by melting scrap. The use of EAF for steelmaking has grown over the past 20 years because of advances in technology, relatively low cost of production and

2-2 Chapter 2 Literature Review

high productivity compared to the blast furnace BOF based operation [Fruehan et al. (2000)]. In recent decades, a significant development in the world steel market has been the widespread adoption of EAF steelmaking technology. The entire pattern of the industry is changing in response to technological innovations, the energy crisis and environmental problems.

2.1.1 Electric Arc Furnace

The principal task of many modern EAFs is to convert a variety of raw materials (Steel scrap, DRI, Pig Iron) to liquid steel, followed by further refining in subsequent secondary steelmaking processes. Steel scrap is the most important raw materials for EAF steelmaking, contributing between 60% and 80% of the total production costs [Sandberg et al. (2007)]. The electric power for the operation of conventional EAF varies from 50 - 120 MW, with energy consumption varying from 350-700 kWh/ton of steel produced. The heat required to melt steel scrap in the EAF is generated by electric arcs, created between the graphite electrodes and scrap in the furnace. Most electric arc furnaces have one or more graphite electrodes. A direct current arc furnace has one electrode in the roof and the current returns through the conductive bottom in the base, while an alternate current furnace has three electrodes. The scrap is generally melted when the temperature reaches between 1500ºC-1550ºC, depending on the composition of the scrap [Steeluniversity (2006)]. After the scrap is melted, the temperature is generally increased, so that refining reactions can be carried out. In this process, oxygen and carbon are injected into the molten steel and slag phases, respectively. However, these reactions can create products which are detrimental to the quality of steel and thus need to be handled carefully [Steeluniversity (2006)]. A schematic of the EAF steelmaking furnace is shown in Figure 2-1.

The EAF generally consists of three sections: x A retractable roof is on the top of the furnace shell and has water cooled elements inside. x The furnace shell, consisting of sidewalls and lower steel ‘bowl’, which are lined with the refractory ceramic bricks that protect the furnace from molten steel.

2-3 Chapter 2 Literature Review

x The furnace hearth, lined with refractory bricks and hemispherical in shape.

Some components are separated from the furnace structure, such as a tilting platform, and the electrical system. An EAF furnace is normally raised off the ground to allow ladles and slag pots to be under the furnace at a taphole. A taphole located off-centre at the base of the furnace called the Electric-Bottom Tapping is used to tap molten steel without slag.

Figure: 2-1 Schematic showing the sectional view of an electric arc furnace (EAF) [Steeluniversity (2006)].

2.1.2 Carbon Sources in the Steelmaking Process

Carbonaceous materials play an important role in EAF steelmaking processes. They have a chemical role to act as the reducing agent (such as FeO reductant) and carburizing material for molten steel. Carbon sources used in EAF steelmaking generally are anthracite, metallurgical coke and coal. Synthetic graphite and calcined petroleum coke can also be used. Carbonaceous materials are characterized based on their chemical compositions (fixed carbon, ash content, volatile matter, sulphur contents etc.), physical properties (density and porosity) and crystalline structure (crystallite size). The different carbon sources used are described below.

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2.1.2.1 Graphitic Materials

Synthetic graphite has a highly ordered structure, high fixed carbon content, low level of volatile matter and ash. The structure of graphite can be described by a regular, vertical stacking of hexagonal aromatic layers with the degree of ordering characterized by the vertical dimension of the crystallite size (Lc) as shown in Figure 2-2. Each carbon atom within the aromatic layer (basal plane) is linked through covalent bonds to three other carbon atoms. The bonding between the layers is very weak and can easily be broken by external forces. Natural graphite also has a highly ordered structure (like synthetic graphite) but contains a high level of impurities [Sahajwalla and Khanna (2003)1].

Figure 2-2: Schematic of the crystal structure of graphite [Sahajwalla and Khanna (2003)1].

2.1.2.2 Non-Graphitic Materials

Non-graphitic materials used in both iron and steel making processes include char-coal, pulverised coal (PCI) and metallurgical coke. Coal is a natural mineral mainly composed of carbon, hydrogen, nitrogen, sulphur and mineral oxides. Heterogeneous in composition, it contains a relatively high level of volatile matter and mineral ash with lower density and higher porosity than graphite. Coals tend to have some carbon atoms arranged in small clusters resembling the graphite structure [Krevelen (1993), Green and Kovak (1956)]. The main difference is in their crystallite size and the degree of ordering (Lc and La). La values are difficult to quantify for coal due to small crystallite size. Inter-layer spacings are fairly similar for all carbonaceous materials ranging typically between 3-4 Å. The bonding of carbon atoms in coals is more complex than

1 Fundamental investigation of basic mechanisms of carbon dissolution in molten iron 2-5 Chapter 2 Literature Review

that in graphite. The lamelle in coal (containing up to 85% carbon) are randomly oriented and are connected by 3-D cross-links [Hirch et al. (1965)]. Structure of graphite and coal are shown in Figure 2-3. Although the crystal structure of coal is similar to that of graphite, it is partially composed of crystalline region and the 3-D arrangement is not regular. The crystallite size (Lc) is much smaller than that of graphite.

Figure 2-3: Schematic representing the structure of (a) graphite and (b) coal [Krevelen (1993)].

Metallurgical coke is the carbon source most widely used in the EAF steelmaking process to reduce iron oxide (FeO), induce slag foaming, carburize liquid steel, and to provide heat energy. It is formed from coal processing [Robert-Austen (1923)] which involves heating coals in a controlled environment in the oven (coke batteries) to produce a porous brittle mass with a cellular structure. The final product is a non- melting compound, ranging in colour from black to silvery gray. The quality and properties of cokes depend on processing conditions as well as properties of the parent coals, such as coal-rank, fluidity and inorganic matter contents.

Metallurgical coke has low volatile content, but ash oxides can be encapsulated in the resultant coke. The mineral oxides found in coke ash are silica (SiO2), iron oxide

(Fe2O3), alumina (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), potassium oxide (K2O), titanium oxide (Ti2O) and phosphorous pentoxide (P2O5). Sulphur is also found in coke, in the form of iron sulphide (FeS), calcium sulphide (CaS) and organic sulphur [Biswas (1981)]. The most significant ash components in coke which have been found to have a major impact on coke reactions with slag and steel are SiO2,

Al2O3, CaO and Fe2O3. Details of these are provided below.

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A. Silica (SiO2)

Silica is the major component of coke ash. The amount of silica present in coke makes up over 50% of the ash, although the ash yield of coke can vary appreciably from coke to coke [Kerkkonen et al. (1996)]. Silica can participate in the high temperature reactions with molten steel and slag, which will affect steel/carbon and slag/carbon interactions. For example, in steel/coke interactions, silica in the coke ash can be reduced by solute carbon in the molten steel leading to the depletion of the melt carbon level. Moreover, silica can also be reduced via a SiC intermediate as shown by Eq. 2.1.

SiO(g) + 2C(s) = SiC(s) + CO(g)……..…………….(2.1)

B. Iron Oxide (Fe2O3)

Iron oxide is also present in the coke ash but at a relatively lower amount than silica.

Iron oxide can be in the form of wustite (FeO), hematite (Fe2O3) and/or magnetite

(Fe3O4). Iron oxide has been found to participate in both steel/carbon and slag/carbon interactions. In the case of steel/carbon interactions, the presence of iron oxide at the metal/carbon interface can consume solute carbon in the molten steel through reduction reactions. In the case of slag/carbon interactions, the presence of FeO in the molten slag phase can lead to the foaming of slag due to the FeO reduction by both solid carbon (from coke) and solute carbon (from the molten steel), leading to the generation of CO gas bubbles in the molten slag layer. The reaction pathways of iron oxide reduction are shown by Eqs. 2.2 to 2.4.

3Fe2O3(l) + C(s) = 2Fe3O4(l) + CO(g)…………..……………(2.2)

Fe3O4(l) + C(s) = 3FeO(l) + CO(g)…...……..…………….(2.3) FeO(l) + C(s) = Fe(l) + CO(g)………….…………….(2.4)

C. Alumina (Al2O3)

Alumina is the other major component of coal and coke ash. The melting temperature of alumina is 2053ºC [Fischer and Hoffmann (1956)] and thus it is quite stable at

2-7 Chapter 2 Literature Review

steelmaking temperatures (1550ºC). Alumina has been found to play a role in interfacial phenomena during steel/carbon interactions. At the steel/carbon interface, the formation of an alumina layer reduces the metal/carbon contact area and thus hinders the carbon reaction with molten steel.

D. Calcium Oxide (CaO)

Calcium oxide is also present in coal and coke ash but at a relatively lower amount compared to silica and alumina. Calcium oxide plays a role in both steel/carbon and slag/carbon interactions. It is a fluxing agent that can reduce the liquidus temperature of bulk ash. In steel/carbon interactions, CaO will help desulphurize the molten steel by reacting with carbon and sulphur and transfer CaS as the reaction product to the interface (Eq. 2.5). The presence of CaO will then lead to easy removal of the product. This can help to promote the steel/carbon reactions. In the case of slag/carbon reactions, an increase of CaO in the molten slag phase changes the slag chemistry, and thus may influence the slag foaming phenomenon.

CaO(s) + C(s) + S = CaS(s) + CO(g)……………………..…(2.5)

2.1.2.3 Alternative Carbon Resources

Due to the increase in the cost of coke making and coking coal [Agarwal et al. (1996)], as well as stricter environmental regulations [Hidalgo et al. (2004)] for coke making, pulverised coal injection had been introduced to ironmaking and steelmaking processes to reduce the use of coke. Recently, innovative waste recycling technology has also been introduced in both iron and steelmaking processes and this involves utilizing waste polymers as a partial replacement of conventional carbon sources (coal and metallurgical coke) as additional fuel sources and chemical reactants in both iron and steelmaking reactions. The aim of this technology is to reduce the amount of waste polymers being landfilled/incinerated from both industrial and household waste streams, to reduce energy consumption and production costs as well as to increase productivity in the iron and steelmaking processes.

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In the iron and steel industries, plastic wastes are currently being recycled to a small extent. Several researchers have developed new routes to utilize plastic wastes, such as recycling in the cokemaking process (Japan and India) and in blast furnace ironmaking (Japan) [Asanuma et al. (2000) and Krishnan and Sharma (2006)]. Waste plastic recycling processes using coke ovens started at Nippon Steel Nagoya and Kimitsu works Japan, in 2000 [Kato et al. (2003)]. These works have focused on the utilization of waste plastics for energy saving purposes. They conducted the study on the conversion of waste plastics into chemical raw materials such as coke, tar, light oil and coke oven gas (COG) in the cokemaking process. The waste polymers used were Polyethylene (PE), Polystyrene (PS), Polypropylene (PP) and Polyethylene terephthalate (PET).

Krishnan and Sharma (2006) also recycled waste plastics in the cokemaking process. Their research work was carried out on a laboratory scale followed by plant trials at Tata Steel, India. HDPE plastic bags were used in the experiment which was conducted to study the effect of plastic addition to the top charged coal blend used for coke making. They found that it was possible to recover coke, tar, light oil and gas from waste plastics in the cokemaking process.

Asanuma et al. (2000) studied the behavior of waste plastics injected into the blast furnace at 1153ºC with the raceway hot model and a commercial blast furnace to investigate the possibility of effective waste plastic utilization in the blast furnace. The influence of particle sizes of plastics was studied. Different sizes of waste plastics ranging between 0.2-1.0 mm and 10 mm were injected into the blast furnace at the rate of 59.0 kg/min. They found that the coarse plastics gave higher combustion and gasification efficiency than fine plastics and pulverized coal. Moreover, the carbon dioxide gasification rate from the unburnt char derived from waste plastics was much higher than that of pulverized coal. This indicates that the preparation method of plastics can affect the combustion and gasification in the raceway.

These earlier studies have illustrated that waste plastics can be used in cokemaking and ironmaking processes for improving energy efficiency in a sustainable way. However, some waste plastics that contain chlorides (PVC) cannot be used in the blast furnace because hydrochloride released from the plastic will corrode the blast furnace equipment [Hotta (2003)]. 2-9 Chapter 2 Literature Review

In the EAF steelmaking process, experimental investigations have been carried out on using polymeric materials as a partial replacement for metallurgical coke used as a slag foaming agent [Sahajwalla et al. (2009), Zaharia et al. (2009) and Rahman (2010)] and FeO reductant in molten slag [Matsuda et al. (2005)] as well as for auxiliary fuels [Sahajwalla et al. (2009) and Zaharia et al. (2009)].

2.2 Slag/Carbon Interactions

In this study, the reaction between slag and carbon is focussed on the foaming behaviour of slag through slag/carbon interactions. The growing interest in slag foaming is in part due to worldwide activity in developing innovative steelmaking processes, particularly EAF steelmaking. In EAF steelmaking, slag foaming needs to be controlled in order to achieve the optimum production rate without causing sloping of the liquid slag. On the other hand, a certain amount of foamed slag is required for promoting the transfer of heat generated by post combustion of the bulk slag. Foaming slag is important for EAF steelmaking to prevent excess heat transfer to the furnace roof and sidewalls, to increase heat transfer from the arc to the metal allowing for higher rates of power input and longer arc operations and this therefore increases the thermal efficiency of the furnace.

2.2.1 Fundamentals of Slag foaming

In the steelmaking process, slag foaming phenomena originate from the expansion of molten slag by gas bubbles evolving from chemical reactions at the slag-metal interface [Hara and Ogino (1992)]. Hence, the phenomenon is caused by the rate of gas evolution by the reactions and the stability of the foam in the melt. The foam stability is dependent on the foam structure [Hara and Ogino (1992)]. Figure 2-4 describes the slag foaming phenomenon occurring in the EAF steelmaking process. In EAF slag foaming practice, oxygen is injected into liquid steel to oxidize the metal and form FeO slag which floats on the liquid metal surface, as shown by Eq. 2.6.

2Fe(l) + O2(g) = 2FeO(slag)...... (2.6)

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Then, solid carbon is injected into the slag phase to reduce the FeO in the slag (slag/solid carbon interaction), as shown by Eq. 2.7. FeO in the slag can also be reduced by the solute carbon in the liquid metal (slag/steel interactions), as shown by Eq. 2.8.

FeO(l) + C(s) = Fe(l) + CO(g)………………………...(2.7) FeO(l) + C = Fe(l) + CO(g)…...……………….…...(2.8)

Figure 2-4: Schematic of the slag foaming phenomenon in EAF steelmaking [Steeluniversity (2006)].

The reduction of FeO generates gas (CO) in the slag layer and this causes the slag to foam. The generated CO can also participate in the reduction of FeO (slag/gas interaction), as shown by Eq. 2.9, and this reaction generates CO2 in the system. The generated CO2 can react with the carbon (Boudouard reaction) and produces CO back to the system, as shown by Eq. 2.10.

FeO(l) + CO(g) = Fe(l) + CO2(g)...…………………...(2.9)

C(s) + CO2(g) = 2CO(g)………...…………...(2.10)

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2.2.2 Measurement of Slag Foaming

Slag foaming has an important role in many steelmaking processes. The foaming has both desirable and undesirable effects on the process performance. In EAF steelmaking, controlled foaming in the furnace is desirable to protect the refractories from the electric arc radiation. The presence of surface-active species is important for the formation of stable foam. Most of the oxides present in the steelmaking slag (such as

Fe2O3 and Cr2O3) are surface active, causing slag to foam [Lahiri and Seetharaman (2002)]. Several studies have been conducted to investigate the foaming behaviour of slag (CaO-SiO2 and CaO-SiO2-FeO) in a range of temperatures by blowing gases such as Ar into liquid met1al to form foam under both steady and dynamic conditions [Cooper and Kitchener (1959), Swisher and McCabe (1964), Hara et al. (1983), Ito and Fruehan (1989)1, Zhang and Fruehan (1995)2 and Kapilashrami et al. (2006)]. The aims were to measure foam life, foam index and to investigate the effects of surface-active species (such as P2O5 and Cr2O3), bubble size and chemical reactions on slag foaming behaviour.

Earlier studies carried out to investigate slag foaming behaviour in steelmaking processes have been mostly concerned with the foaming behaviour of slag at the steady state, where foam is generated by bubbling gas through a nozzle. Cooper and Kitchener (1959), Swisher and McCabe (1964) and Hara et al. (1983) described the stability of slag foams in terms of “foam life”. Cooper and Kitchener (1959) measured the time required for a certain volume of slag foam to decay from an arbitrary height after the foam reached a steady state. Their work focussed on the CaO-SiO2 slag º system, with the addition of a small amount of P2O5 at temperatures between 1350 C- º 1650 C. They did not observe a stable foaming behaviour of the CaO-SiO2 slag system, but the foam height was observed to increase with the addition of P2O5. The decrease in temperature and slag basicity was found to increase the foam height. These authors stated that the foaming was a maximum for basic and neutral slag melts, and thus the slag foam life was relatively small [Cooper and Kitchener (1959)]. Swisher and McCabe (1964) used similar experimental setup to measure the foam life of a CaO- º SiO2-Cr2O3 slag system at 1550 C. The foam life was observed to be greater for the

1 Part I: Foaming Parameters and Experimental Results 2 Effect of the bubble size and chemical reactions on slag foaming 2-12 Chapter 2 Literature Review

more acidic slags and at lower temperatures. The authors suggested that the Marangoni effect was the largest single contributor to foam stability. Hara et al. (1983) º º investigated the foaming of the CaO-SiO2-FeO slag system at 1250 C-1300 C. They observed that the high FeO containing slag showed unstable behaviour. The foam life of the slag increased rapidly with decreasing O/Si ratio in the slag. They also suggested that the increase of foam life was due to the decrease in surface tension of the slag. However, these studies [Cooper and Kitchener (1959), Swisher and McCabe (1964) and Hara et al. (1983)] could not be directly applied to predicting the height of the dynamic foam in an iron or steelmaking vessel.

The foaming index (Σ) was introduced in order to rationalize the foaming behaviour [Bikerman 1953)], which correlates the foam height (H) with the superficial gas velocity (U), as shown by Eq. 2.11. It is related to the physical properties of the slag such as surface tension (σ), density (ρ) and viscosity (μ), through Eq. 2.11.

H ¦ .....(...... 2 11) U

1 Ito and Fruehan (1989) measured the foam index of the CaO-SiO2-FeO slag system containing 30 wt% of FeO at temperatures between 1200ºC to 1400ºC. They also investigated the effects of S, CaF2 and P2O5 in the slag on the foam index. The authors found that the foam index is correlated to the physical properties of the molten slag through dimensional analysis.

Jiang and Fruehan (1991) measured the foam index or foamability (Σ) on bath º smelting slags (CaO-SiO2-Al2O3-FeO) containing less than 15 wt% of FeO at 1500 C. An improved correlation was obtained using more accurate data for the slag viscosity, surface tension and density in the dimensional analysis as shown by Eq. 2.12. They also found that the foamability decreases with increasing FeO content (FeO > 0.2 wt%), while for slags with very low FeO content (FeO < 0.2 wt%), a stable foam is not formed. P ¦ 115 ...... 2( 12) UV

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However, a more recent study by Kim and Seo (1999) showed that complex slag of the composition CaO-SiO2-FeO and CaO-SiO2-FeO-MgO-X systems (X = Al2O3, MnO,

CaF2 and P2O5) followed the relationship shown by Eq. 2.13.

CP ¦ ....(...... 2 13) UV where Σ is the foam index (sec), μ is the viscosity (Pa.s), ρ is the density (kg/m3) and σ is the surface tension (N/m); the constant C depends on the nature of the slag.

Lahiri and Seetharaman (2002) showed that the foaming index of uniform bubble size foams for several slag systems can be expressed as Eq. 2.14.

CP ¦ ....(...... 2 14) Udb

The constant C is determined by the gas fraction in the foamy slag, the bubble shape and the ratio of the bulk to surface viscosity of the slag. The authors further indicated that the foaming index is independent of the gas flow rate when the gas fraction of foam is constant, a condition that could be valid at steady state.

From Eqs. 2.12 and 2.13, it can be concluded that the slag which has high viscosity, and low density and surface tension will give good slag foaming compared to a slag with high density and surface tension. These physical properties of the slag are related to the composition of the slag and the temperatures [Jiang and Fruehan (1991)].

Interactions of molten slag with carbon, liquid steel and refractories occur during the EAF steelmaking process resulting in changes in the slag composition with time, and this could influence the physical properties of the slag. Turkdogan (1983) developed the formula to calculate the density of the slag when MnO is present in the system, and this is shown in Eq. 2.15. Surface tension was derived and shown in Eq. 2.16, while viscosity of the molten slag can be calculated using the expression in Eq. 2.17 [Mills and Sridhar (1999)].

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U 182460 (%FeO ˜ %MnO)...... (...... 2 15)

§ %SiO · § %FeO· V .0 75424 .0 5694 ˜ ¨ 2 ¸ .0 13713˜ ¨ ¸ ...(...... 2 16) © 100 ¹ © 100 ¹

§ B · P lnln A  ¨ ¸ ...... (...... 2 17) © T ¹ where T is temperature in Kelvin, A and B are constants with respect to temperature.

Kitamura and Okohira (1992) and Ozturk and Fruehan (1995) investigated slag foaming behaviour under steady state conditions at temperatures from 1250ºC to 1400ºC by blowing Ar to form slag foam. They observed that the slag foaming improved with increasing viscosity, decreasing surface tension and an increased suspension of surface-active species.

Hong et al. (1998) reported the foaming behaviour of slags originating from CO evolved from the reduction of FeO by graphite. These authors observed that no foaming occurred when the gas evolution was below a critical value. They also found that the addition of sulphur into the system suppressed slag foaming while silica promoted foaming slightly. The increase in silica level resulted in a reduction of CO bubble size through the lowering of the surface tension, while the increase in sulphur level was found to increase the CO bubble size.

Galgali et al. (2001) used a 35 kW DC extended arc plasma reactor to study FeO reduction, carbon dissolution into liquid metal and slag foaming behaviours in the bath smelting process by using coke and coal as the carbon source. They observed that the FeO reduction rate increased with increasing FeO content in the slag up to 10 wt% FeO and the rate constant increased only in the case where the reductant was added externally. The authors reported that coke used as a reductant showed a higher rate constant than coal.

Kapilashrami et al. (2006) studied slag foaming under dynamic conditions at a laboratory scale to determine the influence of properties commonly used to describe the 2-15 Chapter 2 Literature Review

foaminess and foamability of slags under steady state conditions. It was found that slag foaming systems under dynamic conditions show a more complex behaviour than systems under steady state conditions, with the observance of a fluctuating foaming behaviour.

Recently, an innovative sessile drop technique was used to study dynamic foaming behaviour of slag with carbonaceous materials [Khanna et al. (2007)]. Sahajwalla et al. (2006) used this technique to investigate the interactions between slag and carbon at high temperatures including the reduction of FeO in the slag by solid carbon, wettability of molten slag with solid carbon (contact angle measurement) and foaming behaviour of slag with solid carbon. In their studies, small amounts of slag were put on the top surface of the carbon substrate and then inserted into a high temperature furnace which was purged with Ar gas. Once the slag melted and formed a droplet, the slag/carbon reactions which occurred inside the furnace were recorded. The volume of the slag droplet was measured and the foaming behaviour of slag was represented in terms of a volume ratio of the slag droplet [the ratio between volume of slag at time t

(Vt) and initial volume of slag (V0)]. The authors reported that the slag reactions with natural graphite produced better slag foaming than in the case of chars.

The use of polymer/coke blends as a carbon resource for a slag foaming agent has been investigated previously. Zaharia et al. (2009) and Sahajwalla et al. (2009) used the sessile drop technique to investigate the foaming behaviour of molten slag with rubber/coke and HDPE/Coke blends at 1550ºC, respectively. Figure 2-5 shows an example of the variation in sizes of the slag droplet while reacting with a HDPE/Coke substrate. The volume of slag droplet was measured from the images of a slag droplet using computer software [Khanna et al. (2007)]. The slag foaming was determined through changes in volume of the molten slag droplet (volume ratio).

Sahajwalla et al. (2009) investigated carbon/slag interactions between EAF slag and HDPE/Coke blends (4 different blends). An improvement was observed in foaming behaviour of the molten slag compared to the case of metallurgical coke and it was found to depend on the concentration of the HDPE in the blends. The measured volume ratios for slag reaction with HDPE/Coke are shown in Figures 2-6. Blend#3 showed

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the best foaming behaviour with the volume ratios ranging from 1.0 to 4.0, followed by blend#1 with the volume ratios ranging from 0.5 to 2.0.

Figure 2-5: Images of slag droplet while reacting with metallurgical coke and HDPE/Coke blends (blend#1 and blend#3) at 1550ºC [Sahajwalla et al. (2009)].

The gases (CO and CO2) generated during the molten slag reaction with HDPE/Coke blends were monitored using an IR gas analyzer. It was found that the amounts of gases evolved in the case of HDPE/Coke blends were higher than that from coke alone. The authors concluded that the better slag foaming behaviour for HDPE/Coke blends compared to coke was due to the higher levels of gas generation which could arise from the higher rate of FeO reduction and associated higher amount of gas entrapped in the slag droplet [Sahajwalla et al. (2009)].

In the case of rubber/coke blends, the foaming behaviour of the molten slag was also found to be greater than that of coke alone and the foaming behaviour was found to depend on the concentration of rubber in the blends [Zaharia et al. (2009)]. However, the effect of chemical composition in the polymers (H2 and CH4) on the slag foaming, which could have an effect on the reduction of FeO, was not addressed in the two studies.

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Figure 2-6: Volume ratios for slag reaction with HDPE/Coke blends (blend#1 through blend#4) at 1550ºC [Sahajwalla et al. (2009)].

2.2.3 Factors Influencing Slag Foaming

Several researchers have conducted experimental investigations of the foaming behaviour of slags of different compositions, mostly by injecting Ar gas to produce the foams. Gas bubbles and their sizes, gas types and gas flow rate are some of the significant factors influencing the foaming of slag. The effect of the properties of the slag (basicity, viscosity and surface tension) and the temperature on foaming behaviour has also been studied previously.

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2.2.3.1 Gas Bubble Size

Foams can have different morphologies and gas bubble cells of a large size range, and hence, very different degrees of stability. Foams can be divided into two categories: foams with spherical bubbles, called kugelschaum (sphere-foam), and foams with polyhedron-shaped bubbles, called polyederschaum (polyhedron-foam) [Manegold (1953) and Ross and Morrison (1988)]. Sphere-foam consists of relatively small spherical bubbles (beer foam), while polyhedron-foam consists of large bubble cells of polyhedron shape with very thin liquid lamellae separating them (soap bubble foam). The stability of the foam is dependent not only on its physical properties, but also on the foam bubble size.

Zhang and Fruehan (1995)1 studied the effect of the bubble size on slag foaming of

CaO-SiO2-Al2O3-FeO system. The bubbles were generated by Ar injection through a nozzle with multiple small orifices and by the reaction of FeO in the slag with solute carbon in the molten iron at 1450ºC. The effect of the gas bubble size on slag foaming is shown in Figure 2-7. The authors observed that slag foams can have different structures. Foams with very fine bubbles have spherical bubble cells and are stable, while foams with large bubbles have polyhedral bubble cells and are less stable. The sulphur activity in the carbon-saturated molten iron influences the bubble size in the foam, generated by the slag/metal interaction. The average bubble diameter varied from less than 1 mm to more than 5 mm as a function of sulphur activity. Dimensional analysis was used to predict the correlation between the foam index of the molten slag as a function of its physical properties and bubble size of the slag foams, and it was found that the foam index increases with the viscosity, decreases with the density and bubble diameter, and is relatively dependent of the surface tension.

1 Effect of the bubble size and chemical reactions on slag foaming

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Figure 2-7: The effect of the bubble size on slag foaming (Foam index) [Zhang and Fruehan (1995)1].

2.2.3.2 Gas Types and Gas Velocity

The use of different types of gases was also found to affect the foamability or foaming index of slag [Ogino et al. (1988)]. The foam height of slags containing FeO was found to increase in the order of air, Ar and N2-7 pct CO as the gases used. Ogino et al. (1988) speculated that this was due to a decrease in the bubble size as a result of the Marangoni effect induced by chemical reactions. Zhang and Fruehan (1995)2 also investigated the effect of the types of gas on slag foaming. They used helium, hydrogen and argon gases to bubble through a molten slag (30% CaO-60% SiO2-10% CaF2) using alumina nozzle (orifice i.d. = 1.75 mm) at 1400ºC and 1500ºC. These authors concluded that the foam index of the molten slag was dependent on the physical properties of the gas used for bubbling. The foam index of the molten slag decreased when hydrogen and helium gases were used for bubbling instead of argon. In the case of helium, the foam index decreased by approximately 20% at 1500ºC and by 50% at 1400ºC. For hydrogen, a greater reduction of the foam index was observed (~70%) [Zhang and Fruehan (1995)2].

Zhang and Fruehan (1995)2 also investigated the effect of the gas velocity on slag foaming behaviour of two bath smelting slags (CaO-SiO2-FeO) containing different levels of FeO (%FeO = 0 and 3). The foaming index of the slag was determined by

1 Effect of the bubble size and chemical reactions on slag foaming 2 Effect of gas type and pressure on slag foaming 2-20 Chapter 2 Literature Review

measuring the foam height for different gas flow rates. The slope of a plot of foam s height (Hf) against superficial gas velocity (V g) is the foam index (Σ), as shown in Figure 2-8. It was found that the foam height increased with increasing gas flow rate and a greater foam index was observed for the higher FeO containing slag. These results were in agreement with that of the earlier work conducted by [Jiang and Fruehan (1991)].

Figure 2-8: Foam height of CaO-SiO2-FeO slags as a function of superficial gas velocity [Zhang and Fruehan (1995)1].

2.2.3.3 Temperature

Several studies have reported that a decrease in temperature leads to an increase in foam life. The influence of temperature on slag foaming of CaO-SiO2 slags was investigated by Cooper and Kitchener (1959). These authors measured the foam life as a function of CaO/SiO2 ratio, P2O5 concentration and temperature. They observed that the apparent activation energy for foaming is much higher than that for viscous flow. The foam life increased with decreasing temperature and CaO/SiO2 ratio (basicity). Similarly, Swisher and McCabe (1964) also measured the foam life of

CaO-SiO2 slag and their experimental results show that the foam life increased with decreasing basicity and temperature.

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Temperature can affect the properties of slag, such as viscosity, surface tension and the reaction rate. Increasing temperatures can lead to a decrease in the viscosity of liquid slag. The temperature dependence of the foaming index and viscosity of slag (48%

CaO-32% SiO2-10% FeO and 10% Al2O3) was reported by Ozturk and Fruehan (1995), as shown in Figure 2-9. The temperature dependence of the foaming index was obtained from the data given in Figure 2-9 and is shown by Eq. 2.18.

ª16,797º 6 .1 u 1078 5 exp ...... (...... 2 18) ¬« T ¼»

Figure 2-9: The temperature dependence of foaming index and viscosity for 48%CaO-

32%SiO2-10%FeO-10%Al2O3 [Ozturk and Fruehan (1995)].

2.2.3.4 Carbonaceous Materials

The influence of the carbonaceous materials on slag foaming behaviour during EAF steelmaking has been investigated by Sahajwalla et al. (2006). These authors used the sessile drop technique to examine the foaming behaviour of EAF slag containing ~35 wt% FeO when reacting with metallurgical coke, petroleum coke, natural and synthetic

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graphite and char at 1550ºC. The slag foaming behaviour was represented through the dynamic change in volume of the molten slag droplet (volume ratios). They concluded that the slag foaming is strongly dictated by the type of carbonaceous materials used. The volume of the slag droplet during slag/carbon interactions was found to depend on the reduction reaction occurring at the interface and wettability between the carbon and slag. These two factors depend on the type of carbon used. They also stated that an appropriate choice of carbonaceous material could play an important role in slag foaming and in enhancing the energy efficiency of EAF operations.

Rahman et al. (2009) investigated slag/carbon interactions between two carbonaceous materials [metallurgical coke (18.3 % ash) and natural graphite (2.1 % ash)] with EAF º slag (34.8 % Fe2O3) using the sessile drop technique at 1550 C. They focussed on the influence of ash impurities in the materials on the interfacial reactions of the carbon and slag. The foaming behaviour of the slag was also represented through the volume ratios. It was found that natural graphite showed better slag foaming behaviour than metallurgical coke with much greater volume ratios observed (Figure 2-10).

Figure 2-10: Slag foaming (represented through volume ration) during slag reaction with metallurgical coke and natural graphite [Rahman et al. (2009)].

However, the amount of CO and CO2 released during the slag/carbon reaction was monitored and it was found that slag/coke system showed much higher CO and CO2 generation than in the case of slag/natural graphite (Figure 2-11). The rate of gas generation from slag/coke system was also significantly faster than that from slag/natural graphite.

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Figure 2-11: CO and CO2 generated during slag reaction with metallurgical coke and natural graphite [Rahman et al. (2009)].

These results indicate a rapid iron oxide reduction, but poor foaming for metallurgical coke. Conversely, slow iron oxide reduction and good slag foaming were observed for natural graphite. These studies have brought out the role played by ash impurities in the materials on slag/carbon interactions. The authors suggested that the diffusion of ash impurities, especially SiO2, was expected to significantly influence foaming behaviour through lowering surface tension, and the large amount of gas produced from slag/coke reactions could not be contained within the slag and generally escaped the reaction assembly. In the case of natural graphite/slag, slower rates of gas generation and higher surface tension made it easier for slag to trap gases and sustain foaming [Rahman et al. (2009)].

2.2.3.5 Properties of Slag

Characteristics of slag foam can influence the foaming index (Σ) and the rate of FeO reduction. The foaming index and the reduction rate can also be correlated through these slag properties [Paramguru et al. (1997). The properties of the slag such as basicity, viscosity and surface tension are reviewed in the following sections.

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A) Basicity

The chemical composition of slags can be characterized as the basic oxides (refractory) and acidic oxides (fluxing). The basic oxides can be MgO and CaO, while the acidic oxides include FeO, MnO, SiO2, Al2O3 and CaF2. The addition of basic oxides beyond the liquidus composition leads to an increase in the effective viscosity of the molten slag [Pretorius et al. (1998)]. A balance of acidic and basic oxides is required for steelmaking so that the EAF slag is both compatible with basic refractories and is suitable for foaming. The saturation of slag with basic oxides is needed for good slag foaming [Pretorius et al. (1998)].

Various definitions for slag basicity are represented in terms of mass concentration ratios shown by Eqs. 2.19 to 2.22.

%CaO B2 ...... (...... 2 19) %SiO 2 %CaO B3 ..(...... 2 20) %SiO 2  %Al O32 %CaO  %MgO B4 ..(...... 2 21) %SiO 2  %Al O32 %CaO  %MgO B5 .(...... 2 22) %SiO 2

The relationship between modified basicity and the foaming index was analysed for different slag systems and under various conditions. Several researchers have shown that the foam height or foaming index decreases with increasing basicity up to a certain value and then increases with increasing basicity.

Zhang and Fruehan (1995) measured the foam index of CaO-SiO2-FeO-Al2O3 slags as a function of the basicity (B3) and the result is shown in Figure 2-12. They concluded that the foam index decreases initially with increasing basicity because the viscosity is decreasing. However, when the solubility limit is reached, the foam index

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increases with basicity, because the second phase particles increase the bulk viscosity of the slag.

Figure 2-12: The foam index of CaO-SiO2-FeO-Al2O3 slags illustrating the effect of second phase particles [Zhang and Fruehan (1995)1].

Yi and Kim (2002) studied the influence of basicity on slag foaming behaviour using the gas hold-up index. They concluded that the gas hold-up index as a function of basicity (B2= CaO/SiO2) showed a parabolic curve and the maximum gas hold-up index was observed at a value of CaO/SiO2 ratio of 2.5 (see Figure 2-13).

Figure 2-13: Effect of basicity of slag on gas hold-up index [Yi and Kim (2002)].

1 Effect of gas type and pressure on slag foaming 2-26 Chapter 2 Literature Review

B) Viscosity

According to Jiang and Fruehan (1991) and Zhang and Fruehan (1995), viscosity is an important physical parameter that has a significant effect on the foamability or foam index of molten slag. Good slag foaming behaviour can be observed in slag systems with high viscosity, low surface tension and low density. In the study of viscosities of mould fluxes for continuous casting, the experimental results have been represented as a function of temperature using the relation shown by Eq. 2.23.

B K AT exp( )...... (...... 2 23) T where A and B are functions of slag composition.

For the composition of slag (wt %) range 33-56% SiO2, 12-45% CaO, 0-11% Al2O3, 0-

20% Na2O and 0-20% CaF2, an interpolation formula has been derived for the parameters A and B as a function of the mole fractions of the constituents as given by Eqs. 2.24 and 2.24.

lnA = –17.51 – 35.76(Al2O3) + 1.73(CaO) + 5.82(CaF2) + 7.02(Na2O)…….....(2.24)

B = 311,140 – 68,833(Al2O3) – 23,896(CaO) – 46,351(CaF2) – 39,519(Na2O)….(2.25) where A is in units of 0.1 N.s.m-2 K-1 (poise/deg.) and B in Kelvin.

Viscosities of steelmaking slags determined experimentally and the data from Kozakevitch (1949) are shown in Figure 2-14.

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-2 º Figure 2-14: Viscosity (N.s.m ) of CaO-SiO2-FeO melts at 1400 C [Kozakevitch (1949)].

From the typical slag composition, FeO is the most important flux in the slag, since it is the only compound that is fully melted at steelmaking temperatures. The influence of FeO on the viscosity of molten slag is shown in Figure 2-15.

º Figure 2-15: Isoviscosity curves for the CaO-SiO2-FeO melts at 1450 C [Bronson et al. (1985)].

Slags that exhibit a good foaming behaviour have a fluidity that falls between “creamy and fluffy” with “watery” and “crusty” at the extreme ends of spectrum [Pretorious et al. (1998)]. This means that the optimum slags are not completely liquid (watery) but are saturated with respect to CaO (CaO.SiO2) and/or MgO (MgO-FeO solid solution).

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These second phase particles lead to a formation of a high amount of small gas bubbles in the foaming slag [Pretorious et al. (1998)].

C) Surface Tension

The surface tensions for FeO melts with different oxides at 1400ºC were measured by

Kozakevitch (1949) as shown in Figure 2-16, and that for the FeO-CaO-SiO2 system is shown in Figure 2-17.

Figure 2-16: Surface tension of binary FeO melts at 1400ºC [Kozakevitch (1949)].

Figure 2-17: Surface tension curve in FeO-CaO-SiO2 melts saturated with Fe at 1400ºC [Kozakevitch (1949].

Numerous researchers have investigated the effect of surface tension of the molten slag on its foaming behaviour. The presence of a surface active species (such as P2O5 and

Cr2O3) in slag leads to a decrease in the surface tension and thus results in good slag

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foaming [Cooper and Kitchner (1959), Swisher et al. (1964) and Kozakevitch (1969)]. Ito et al. (1981) investigated the slag foaming phenomenon during the desiliconization in a 300t torpedo ladle and discussed the method of suppression. They observed that the foaming slag level during the process was strongly dependent on the slag volume and surface tension, but the effect of viscosity was minimal [Ito et al.

(1981)]. Okuda et al. (1884) reported that the surface active substance (P2O5) in the slag promotes slag foaming. Hirata et al. (1985) observed in a laboratory test (100 kg of hot metal) that the foaming level increased with decreasing surface tension (see

Figure 2-18) and that P2O5 in the slag promotes foaminess (see Figure 2-19).

Figure 2-18: Effect of surface tension of molten slag on the foaminess in the bath smelting of iron ore [Hirata et al. (1985)].

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Figure 2-19: Effect of P2O5 in molten slag on the foaminess in the bath smelting of iron ore [Hirata et al. (1985)].

2.3 Iron/Carbon interactions

The most important phenomena that occur during iron/carbon interactions can be considered to be the wettability of carbonaceous materials by the melt, the formation of reaction products at the metal/carbon interface and the transfer of carbon and sulphur from the carbonaceous materials into molten iron. Several researchers have investigated the carbon dissolution behaviour of various carbonaceous materials into molten iron, focussing on the kinetics of carbon dissolution and the factors affecting the carbon dissolution rate. Interfacial phenomena between carbon and liquid metal have also been investigated in terms of the formation of interfacial products and their effect on carbon dissolution behaviour. This section will review previous studies on carbon dissolution into molten iron along with the formation of reaction products at the metal/carbon interface, followed by previous studies on wetting behaviour of molten iron on carbonaceous materials.

2.3.1 Fundamentals of Carbon Dissolution

When solid carbon is in contact with liquid iron, the dissolution of carbon occurs and this phenomenon takes place through two basic mechanisms.

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1. Dissociation of carbon atoms from its host lattice into the iron/carbon interface.

2. Mass transfer of carbon atoms from the interface through the liquid boundary layer into the bulk liquid iron.

The variation of carbon concentration in the interfacial layer between the molten iron and the solid carbon is shown in Figure 2-20.

Figure 2-20: Variation of carbon concentration at the iron/carbon interface. C is bulk carbon concentration, Ci is carbon concentration at the interface, and δ is the interfacial layer thickness [Gudenau et al. (1990)].

The dissolution of carbon is an endothermic reaction, where the heat of solution (ΔH) is given by Eq.2.26 [Elliot (1963)].

2 ΔH = 5400 + 5810[1- NFe ] (cal/mol)………...…………….(2.26)

where NFe is the weight fraction of iron.

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The standard Gibbs free energy for carbon dissociation is given by Eq. 2.27 [Guthrie (1992)].

ΔGº = 5400 - 10.1T (cal/mol)……………………...... (2.27) where T is the temperature (Kelvin).

At 1550ºC (1823K), the standard Gibbs free energy, ΔGº is equal to -13012.3 cal/mol (-54.5 kJ/mol). The high negative value for ΔGº at this temperature indicates a high thermodynamic potential, which allows the dissociation of carbon from its host lattice to occur spontaneously.

Several researchers [Olsson et al. (1966), Kosaka and Minowa (1968), Shigeno et al. (1985), Wright and Baldock (1988), Mourao et al. (1993), Wu and Sahajwalla (2000), Cham et al. (2004) and Khanna et al. (2005)] have used a first-order kinetic equation to describe the dissolution of carbon into liquid iron, based on the carbon concentration gradients. This equation and its integrated form are represented by Eqs. 2.28 and 2.29, respectively.

dC Ak t ( ˜ CC )...... 2( 28) dt V ts

 CC )( ln ts ˜ tK .....(...... 2 29) s  CC 0 )(

In these equations, Cs and Ct represent the saturation solubility and carbon concentration (wt%) in the liquid iron at time t, and k is the first-order dissolution rate constant (m.s-1), A and V represent the interfacial area of contact and the liquid iron bath volume, respectively, and C0 is the initial carbon concentration in the liquid metal (wt%). The overall carbon dissolution rate constant (s-1), K = Ak/V, can be calculated from the negative slope of the ln((Cs–Ct)/(Cs–C0))-vs-time plot.

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The ability of molten iron to pick up carbon is determined by the carbon saturation level (Cs) [Gudenau et al. (1990)]. In the case of pure liquid iron, the carbon saturation level (Cs) can be calculated from the empirical correlation (Eq. 2.30) proposed by Chipman et al. (1952); where T is in ºC. At 1550ºC, the carbon saturation is equal to 5.28 wt%. -3 Cs = 1.34 + (2.57 x 10 ).T…..…...... (2.30)

However, the dissolution of carbon can be affected by the chemistry of liquid iron. The elements silicon, phosphorus and sulphur are known to decrease the solubility of carbon in the melt, while manganese has been found to increase the solubility [Neumann et al. (1960)]. In the case of melts containing other chemical elements, the carbon saturation level (Cs) depends on the melt chemical composition, which can be calculated from the empirical correlation (Eq. 2.31) proposed by Neumann et al. (1960).

* Cs = C - 0.3(%Si) - 0.33(%P) - 0.45(%S) + 0.28(%Mn) ...... (2.31)

where C* = 1.34 + (2.57 x 10-3).T and T is in ºC.

Some techniques have been employed to investigate carbon dissolution from both graphitic and non-graphitic materials and these are shown in Figure 2-21. Of these only the carburizer cover and sessile drop techniques are briefly described.

Figure 2-21: Various experimental techniques used for carbon dissolution studies.

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The carburizer cover technique is widely used for carbon dissolution studies. This technique is suitable for carbonaceous materials in the form of powder and the experiments are conducted in an induction furnace. The powder is added to the top of the bath to cover the molten iron surface. Carbon dissolution from carbonaceous materials into liquid iron can be determined by the change in carbon composition of liquid iron. The advantage of carburizer cover technique is that it overcomes the problems of physical differences between the carbonaceous materials. It provides a dynamic situation (through stirring) where both solid carbon and liquid iron are present in excess.

The sessile drop technique allows for the study of interfacial reactions by creating transport-limiting conditions. The formation of reaction products at the interface as well as the dissolution of chemical elements from solid carbon into liquid iron can be investigated using this technique. This technique also allows for the study of the wettability of liquid iron with solid carbon. In this technique, carbonaceous materials are ground into powder and compacted in a die to make a flat substrate. A small piece of metal is put on the substrate and this assembly is inserted into the high temperature test chamber. The metal droplet is quenched at different times, and thus the carbon content in the iron droplet is measured. This technique has been used previously for studying carbon/iron interfacial reactions [McCarthy et al. (2003) and Zhao and Sahajwalla (2003)].

2.3.2 Dissolution of Carbon from Graphite

Several experimental studies on carbon dissolution from graphite into liquid iron have focussed on understanding the mechanisms that influence the kinetics of the process [Dahlke and Knacke (1955), Olsson and Koump (1966), Kosaka and Minowa (1968), Wright and Baldock (1988), Mourao et al. (1993) and Sahajwalla and Khanna (2003)]. It can be concluded from these previous studies that the dissolution of carbon from graphite involves the two basic mechanisms - dissociation of carbon atoms and mass transfer.

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Olsson and Koump (1966) studied the rate of dissolution of carbon from graphite into molten Fe-C alloys using the rotating cylinder technique at temperatures ranging between 1470ºC and 1640ºC. This technique involves immersing and rotating a cylindrical graphite rod in a stationary crucible containing molten Fe-C alloy. They concluded that the diffusion of the carbon from the interface into the melt was the mechanism controlling the dissolution of carbon from graphite. These authors also found that the mass transfer coefficient was linearly related to the peripheral velocity of the graphite cylinder.

Kosaka and Minowa (1968) investigated the rate of carbon dissolution from graphite into Fe-C melts at bath temperatures ranging between 1270ºC and 1550ºC. The experiments were conducted using a stationary and rotating cylindrical graphite rod immersed in the molten alloy. They found that the dissolution of graphite was controlled by the diffusion of carbon in the liquid boundary layer. These results are in agreement with the results by Olsson and Koump (1966).

Mourao et al. (1993) also investigated dissolution rates of carbonaceous materials in Fe-C melt at temperatures ranging from 1350ºC to 1660ºC. Interactions of molten Fe-C alloy with graphite, coke and low-volatile coal chars were studied. In the case of graphite, these authors found that the dissolution rate of graphite followed the correlation of natural convection under a turbulent condition. Mass transfer at the liquid boundary layer was the rate-controlling mechanism of carbon dissolution. These results are consistent with the experimental results from previous studies [Olsson and Koump (1966), Kosaka and Minowa (1968), Wright and Baldock (1988)].

Sahajwalla and Khanna (2003) used Monte Carlo simulations to model carbon dissolution from graphite into liquid iron. They found that the dissociation of carbon atoms from across the prismatic planes occurred more easily than the carbon atoms in the basal plane, due to the weak bonding between carbon atoms within the graphene layers. As dissolution proceeds, the dissociation of carbon atoms from graphite becomes faster. This simulation also shows that the rate of mass transfer of carbon atoms in the melts is slower than the corresponding dissociation rate at the Fe/C interface.

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2.3.3 Influence of Sulphur on the Dissolution of Graphite

Numerous previous studies on carbon dissolution from graphite into Fe-C alloy have indicated that the dissolution of graphite is mass-transfer controlled and can occur under kinetic or diffusion conditions. In these studies, the influence of sulphur on carbon dissolution from graphite into molten iron has also been investigated.

Grigoryan and Karshin (1972) studied the influence of adding surfactants (S and O) into molten iron on the graphite dissolution using rotating graphitic disc technique. They investigated the reactions between graphite and molten iron as a function of rotating speed, and found that when the carbon dissolution from graphite was not controlled by diffusion, the addition of sulphur into the melt decreased the dissolution rate; this can be described by an adsorption isotherm. In contrast, when the dissolution was dependent on diffusion (mass-transfer controlled), the addition of sulphur did not affect the dissolution rate.

Ericsson and Melberg (1981) also investigated the influence of sulphur on carbon dissolution from graphite using the rotating graphite rod technique. They found that the addition of sulphur into the melt decreased the rate of carbon dissolution, due to a decrease in carbon solubility. In high-sulphur containing melts, the carbon dissolution rate was controlled by mass transfer; this may be due to a decrease in carbon diffusivity. These authors suggested that the effect of sulphur on the carbon dissolution may be due to its influence on the interfacial kinetics of dissolution.

Shigeno et al. (1985) investigated the influence of sulphur and phosphorus on the dissolution rate of carbon from two types of electrode-grade graphite (non-graphitic and graphitic carbon) into the liquid iron using the rotating cylindrical graphite method at temperatures of 1300ºC-1500ºC. They also showed that the presence of sulphur decreased the carbon dissolution rate. The effect of sulphur on retarding the overall mass transfer coefficient was found to be greater than that of the phosphorus, and this suggests a more intensive surface active nature for sulphur.

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Moreover, Shigeno et al. (1985) observed that the changes in the crystalline structure of the non-graphitic electrodes due to the effect of sulphur were much greater than in the case of the graphitic ones. They explained that sulphur is selectively absorbed onto the prismatic plane of graphite. The dissolution of graphite was hindered from the plane, giving rise to the reduction of mass transfer in the presence of sulphur. The planes of non-graphitic carbon corresponding to the prismatic planes of crystalline graphite are more exposed than those of graphite, and hence the reduction of effective surface area available for carbon dissolution was higher. Figure 2-22 shows the influence of sulphur on the overall mass transfer coefficient (k) of graphitic and non- graphitic materials. Non-graphitic materials seem to be more sensitive to sulphur because the k value was observed to considerably decrease with the addition of small levels of sulphur, while the decline occurred slightly in the case of graphite.

Figure 2-22: Influence of sulphur concentration on the overall mass transfer coefficient (k) when using graphitic and non-graphitic materials [Shigeno et al. (1985)].

Orsten and Oeters (1988) also confirmed the effect of sulphur on decreasing the mass transfer coefficient because sulphur decreased the solubility and diffusion coefficient of carbon in the molten iron. These authors used the carburizer cover technique to determine the dissolution of non-graphitic ash-free activated-carbon and graphite, both with and without sulphur. The dissolution rate of the sulphur containing graphite was slower than the case of sulphur-free graphite. Similar results were observed for the activated carbon, but the effect of sulphur on reducing the dissolution rate was stronger. These authors proposed that the influence of sulphur on the dissolution rate might be

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due to the formation of heterocyclic -C-S-C bonds, since this effect was observed over the entire reaction period.

The experimental results on the dissolution of carbon from graphite in gas-stirred molten Fe-C [Wright and Baldock (1988)] also confirmed that sulphur reduced carbon solubility, thereby decreasing the driving force for carbon dissolution. This study was conducted by injecting graphite with nitrogen as a carrier gas into the bottom of the molten Fe-C alloy bath in order to investigate the influence of the gas flow rate, bath temperature, particle sizes and bath sulphur concentrations. They found that the former three parameters had a minor effect on the dissolution rate. They also concluded that under these experimental conditions, the dissolution rate of graphite increased steadily with the injection rate for up to about 85% of carbon saturation, whereas the dissolution rate was slower in the presence of sulphur in the bath. As shown in Figure 2-23, the carbon content in the bath decreased with the increase in the bath sulphur content from 0.1% to 1.0%.

Figure 2-23: Effect of bath sulphur concentration on the dissolution of graphite [Wright and Baldock (1988)].

Studies on carbon dissolution from graphite and coal by Wu and Sahajwalla (2000) also confirmed that the presence of sulphur in the melt retards the overall rate of carbon dissolution from graphite, and this effect is stronger in the case of coal. These results 2-39 Chapter 2 Literature Review

are in agreement with the findings from Orsten and Oeters (1988). However, a further increase in the sulphur level in the melt had a minor effect on decreasing the carbon dissolution rate. It can be concluded from this study that the presence of sulphur in the melt can influence the carbon dissolution through the following means:

1. Decreasing the carbon diffusion coefficient. This is in agreement with the results by Orsten and Oeters (1988), i.e. the diffusion coefficient (Dc) dropped by 20% when 1.32% of sulphur was added; and the results from Ericsson and Melberg (1981), i.e. the carbon dissolution rate was partially retarded by the reduction of carbon diffusion in the presence of sulphur. 2. Decreasing the iron/carbon interfacial area available for carbon dissolution due to formation of reaction products as a surface blockage. This is in agreement with the work by Shigeno et al. (1985), i.e. the graphite prismatic plane was blocked by sulphur leading to a decrease in the dissolution rate. 3. Decreasing wettability between the melt and carbon. This leads to a reduction in the area available for carbon transfer.

A theoretical investigation of the influence of sulphur on the carbon dissolution rate of graphite in Fe-C-S melt using Monte Carlo simulation was conducted by Sahajwalla and Khanna (2003)1. The atomic distribution profiles obtained from their study are shown in Figure 2-24. When graphite is in contact with the melt, Fe atoms penetrate the graphite block, while C atoms dissociate from the graphite lattice and dissolve into the melt. Sulphur atoms were not observed at the interface but were found to move away from the interface, deeper into the melt.

Figure 2-25 shows the dissolution rate of graphite plotted as a function of time for different ranges of C and S concentrations in the melt. In the initial stages of contact, the dissociation of carbon atoms from the host lattice is very slow due to the strong covalent bonds between carbon atoms and the basal plane; in this stage, the interfacial kinetics control the overall dissolution rate. The dissociation of carbon atoms becomes easier and faster as dissolution proceeds, and the rate-controlling mechanism changes

1 Effect of sulphur on the dissolution behaviour of graphite in Fe-C-S melts: A Monte Carlo simulation study

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over to mass transfer. The presence of sulphur in the melt reduces the overall dissolution rate by affecting both the interfacial reactions and mass transfer. Sulphur lowers the dissolution rate of graphite in the initial stages of contact and also the carbon concentration in the melt close to saturation. The decrease in the dissolution rate in the initial stages of contact due to the presence of sulphur was attributed to Fe-C, C-S and Fe-S bond energy considerations and not due to any interfacial blockage by sulphur atoms [Sahajwalla and Khanna (2003)1].

Figure 2-24: Atomic distribution profile across graphite/Fe-S interface before and after simulation. Z corresponds to the number of layers normal to the interface and Z=50 is the initial contact surface [Sahajwalla and Khanna (2003)1].

Figure 2-25: Carbon dissolution rate as a function of time for different ranges of sulphur and carbon concentrations in the melt. Time is measured in units of Monte Carlo [Sahajwalla and Khanna (2003)1].

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2.3.4 Dissolution of Carbon from Non – Graphitic Materials

Several researchers have investigated the carbon dissolution from non-graphitic carbon sources, such as coal, char and coke in order to establish a fundamental understanding of the factors affecting the dissolution of carbon [Wright and Denholm (1984), Shigeno et al. (1985), Orsten and Oeters (1988), Wright and Taylor (1993) and Khanna et al. (2005)].

Wright and Denholm (1984) investigated the kinetics of carbon dissolution from graphite, blown coal char and petroleum coke into a turbulent molten iron bath, and observed no differences in carbon dissolution rate for the three carbon types. This result is in agreement with the result by Shigeno et al. (1985) which also indicated that the dissolution rate from non-graphitic materials did not differ significantly from that of graphite. However, the carbonaceous materials used in both studies had small amounts of ash.

Orsten and Oeters (1988) investigated the carbon dissolution behaviour from coal, coke and graphite into molten iron. During the coal dissolution process, they observed that when coal particles are blown into the bath and come into contact with the hot metal, they undergo degassing (devolatilization) and are thus completely surrounded by gases. This prevents carbon particles from being directly in contact with the melt and thus hinders its reactions with the liquid metal.

Wright and Taylor (1993) developed a mathematical model to predict the dissolution of carbon particle injection into a molten iron bath. The predictions were found to be consistent with the laboratory results where in the case of high purity graphite injected into the bath mass transport controlled the dissolution. On the other hand, the laboratory results of coke deviated from these predictions. This result indicated that mass transport was not the only factor limiting the carbon dissolution rate for non- graphitic materials.

Khanna et al. (2005) studied the dissolution of carbon into liquid iron at 1550ºC from four types of PCI chars which had varying percentages of ash, using the carburizer

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cover technique. The authors focussed on the interfacial reactions occurring at the metal/carbon interface and their influence on carbon dissolution rates. They found that the formation of reaction products, such as CaS at the interface, and the consumption of solute carbon in the melt (due to the reduction of SiO2 in the ash) inhibited carbon dissolution from the chars. These authors also developed a theoretical model to estimate the contact area between the carbon and iron, which was used to calculate the first order carbon dissolution rate constant (k).

The carbon dissolution rate of coke is generally slower than that of graphite due to the more complex structure of coke, which does not allow carbon atoms to dissociate easily from their host lattices. Also, the ash content in coke is generally much higher than that in graphites and this can also influence the rate of carbon dissolution. Cham et al. (2004) studied carbon dissolution from two metallurgical cokes (Coke1 and Coke2) into liquid iron at 1550ºC using the carburizer cover technique. These authors found that the overall carbon dissolution rate (K) in the liquid iron for Coke1 was 14.7 x 10-3 s-1, while it was 1.1 x 10-3 s-1 for Coke2 and the difference was attributed to variations in the composition of their ash contents (alumina content in Coke2 was almost two times higher than that of Coke1). The authors also stated that the dissolution rates of the two cokes cannot be explained on the basis of crystallite size (LC) of the carbon peak

(002) because the LC values of the two cokes were quite comparable.

Sun (2005) also used mathematical models to analyse the reaction rates between solid carbon and molten iron. The driving force, kinetics and thermodynamic parameters for carbon dissolution when the rate is limited by mass transfer are different from that limited by the dissociation reactions. This kinetic model showed that temperature, solid structure, surface active elements, metal composition, liquid agitation, wettability between solid and liquid, solid particle size, ash and side reactions were some of the rate influencing factors. These authors also carried out experimental investigation on carbon dissolution from graphite and coke at 1300ºC-1600ºC in order to determine the rate limiting factors and to compare the results with that obtained from the kinetic model. They found that the carbon dissolution rate at high temperatures and with the induction furnace were faster than those at lower temperatures and with the resistance furnace, and the mass transfer in the melt was the major limiting mechanism for carbon dissolution. 2-43 Chapter 2 Literature Review

2.3.5 Factors Influencing Carbon Dissolution

2.3.5.1 Carbon Structure

The effect of the atomic structure on carbon dissolution has also been reported. The crystallite size (LC) indicates the degree of order of the carbon structure (as determined from the width of carbon peak (002)). Sahajwalla et al. (1993) investigated carbon dissolution from anthracites and coal char into liquid Fe-C-S bath. Their results showed that the increasing bath sulphur concentration decreased the dissolution rate, and the effect of sulphur content on the carbon dissolution behaviour from coals and anthracites were different. The differences in behaviour were due to the variation in the structure of the carbon samples. The high order (LC) materials are believed to dissolve much faster than the lower order carbonaceous materials [Sahajwalla et al. (1994)]. This may be because materials with higher crystallinity require less energy for the carbon dissociation process, and thus carbon atoms in the graphite tend to dissolve easier due to its higher crystallinity.

The carbon dissolution rates of coals and graphite in Fe-C-S melts were found to improve with increasing crystallite size (LC) [Wu and Sahajwalla (2000) and Wiblen et al. (2001)]. Wu and Sahajwalla (2000) confirmed the relationship between carbon dissolution rates and the structure of coals by quantitatively determining coal structure parameters using both X-ray diffraction (XRD) analysis and Scherrer’s equation (Eq. 2.32). .0 89O LC ....(...... 2 32) BcosTB

where LC is the crystallite size (angstroms, Å), λ is the wavelength of the incident X-ray (Å), B is the angular width at half-maximum intensity of the carbon (002) peak

(radians) and θB is the Bragg angle of the peak (002).

The carbon dissolution from coal chars into Fe-S-C melts proceeds in two steps. The first step is the dissociation of carbon atoms from their crystal lattice into the metal/carbon interface to establish a higher carbon concentration in the interfacial layer.

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The second step is diffusion (mass transfer) of carbon atoms from the interface into the bulk liquid, through the liquid boundary. If the dissociation rate of carbon atoms from coal char does not maintain the saturation level at the interface then carbon dissociation will become a significant rate-controlling mechanism for the overall carbon dissolution process. An increase in the crystallite site (LC) values results in an increase in the carbon atom dissociation and this enhances carbon dissolution [Wu and Sahajwalla (2000)].

On the other hand, an experimental investigation conducted by Ohno et al. (2010) on the use of carbonaceous materials of a lower degree of crystallinity as a carburization agent showed that the Fe-C liquid can be formed due to reactions between solid iron and these carbonaceous materials at the Fe-C eutectic point of 1154ºC. They concluded that carbon of lower crystallinity can be used a carburizer since it can enhance the initial iron carburization reaction.

2.3.5.2 Temperature

An increase in temperature changes the saturation limit of carbon in liquid iron. It has a small effect on the first order dissolution rate (k). Temperature was found not to directly affect the contact area but it could, indirectly, affect the wettability between solid carbon and liquid iron, viscosity of the melt and fusion of the ash; changes in these factors would change the contact area [Sun (2005)].

The effect of temperature on carbon dissolution was recently studied by Cham et al. (2006). These authors investigated the carbon dissolution from cokes and synthetic graphite into liquid iron at temperatures of 1450ºC-1550ºC using a carburizer cover technique. They found that the overall dissolution rate constant (K) from the cokes and graphite increased with increasing temperature, but this effect was stronger for cokes than for synthetic graphite. These authors concluded that it was due to the presence of inorganic coke ash. The inorganic matter in the coke can significantly change the viscosity and fusion temperature of the interfacial reaction products and thus affect the carbon dissolution behaviour.

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2.3.5.3 Ash

The major mineral oxides found in most metallurgical coke ash are SiO2 and Al2O3, while minor oxides include Fe2O3, CaO and others. The presence of these mineral oxides in the ash of coal and coke can affect the carbon dissolution of the material by forming a layer at the metal/carbon interface [Ericsson and Mellberg (1981), Orsten and Oeters (1986), Gudenau et al. (1990), Mourao et al. (1993) and Wu et al. (2000)] and by consuming dissolved carbon in the liquid metal via reduction reaction of some reducible oxides [Wu et al. (2000), McCarthy et al. (2003) and Cham et al. (2009)].

The mineral oxides in coke ash were found to form a viscous layer at the metal/carbon interface. This interfacial layer acts as a physical barrier at the interface reducing the contact area available for carbon dissolution, and thus retarding the dissolution of carbon into liquid iron. Orsten and Oeters (1986) also investigated the influence of ash on carbon dissolution from coke using the carburizer cover technique. The authors suggest that ash oxides in the coke affect the dissolution of coke by forming a layer which covers the metal/carbon interface, leading to the retardation of the carbon dissolution rate compared to graphite. They also examined the influence of additives like CaO and CaF2 on the carbon dissolution from coke and found that the addition of

CaO and CaF2 can enhance carbon dissolution by reducing the fusion temperature of the coke ash. These authors concluded that the dissolution of coke was limited by the fusion temperature of ash. The melting point of the ash layer could be lower than the liquid metal temperature, and thus the ash layer could be easily removed from the interface in the form of slag. This increases the contact area between the liquid metal and the solid carbon, and thus enhances carbon dissolution.

The dissolution behaviour of coke can be modified significantly by controlling the ash composition during the coking process. Gudenau et al. (1990) studied carbon dissolution from industrial and especially treated cokes using the rotating cylinder technique. They observed that the film of ash oxides on the coke surface reduces the effective area for carbon dissolution. They suggested that the carburizing ability of coke could be modified by controlling the ash composition of the coking coal. From

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their study, the presence of Fe2O3 was found to be favourable for carbon dissolution, while CaO was observed to decrease the carbon dissolution [in contrast with the results by Orsten and Oeters (1986)]. The difference in the effect of CaO on carbon dissolution found in both studies is probably due to the differences in the method used by both researchers. Gudenau et al. (1990) added pre-determined amounts of ash forming oxides before coking the coal. This technique distributes CaO throughout the coke matrix and thus the interactions between CaO, C and the impurities in the coal can occur during the coking process. Conversely, Orsten and Oeters (1986) directly mixed CaO into the coke in order to decrease the ash fusion temperature. This technique does not allow CaO and C interactions to occur prior to carbon dissolution.

Mourao et al. (1993) examined partially dissolved coke samples using electron microscopy, and observed a thin and viscous ash layer formed on the coke surface. They suggested that this layer affects carbon dissolution by decreasing iron/coke contact area, thereby retarding the carbon dissolution rate. The formation of an ash layer depends on the chemical composition of the ash in carbonaceous materials and also the dissolution time.

Other evidence of the effect of ash in carbonaceous materials on the carbon dissolution was reported by Bandyopadhyay et al. (2003). These authors investigated the effect of physical properties of a range of carbon sources on the kinetics of carbon dissolution into liquid iron at 1600ºC using the stationary rod method. They found that petroleum coke which has lowest ash yield showed highest carbon dissolution. Conversely, nut coke which has the highest ash yield shows the lowest carbon dissolution.

Mineral oxides in carbonaceous materials are a source of reducible materials (SiO2 and

Fe2O3) and can directly react with liquid iron. They can influence carbon dissolution by consuming dissolved carbon in the liquid metal via reduction reactions and by reacting with liquid iron to form reaction products at the interface [Wu et al. (2000), McCarthy et al. (2003), Chapman et al. (2007) and Cham et al. (2009)].

Silica is generally one of the largest components in coke ash. The reduction of silica involves one or more of the following reactions [Biswas (1981)].

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Slag reaction with solute carbon:

SiO2 (slag) + 2C = Si + 2CO(g)…………………….(2.33)

Silicon monoxide gas formation by silica reaction with carbon [silica can originate from either slag, or coke ash; carbon can be solid carbon (C) or solute carbon (C)]:

SiO2 + C = SiO(g) + CO(g)……...……………..(2.34)

Silicon monoxide gas reaction with solute carbon in the melt:

SiO(g) + C = Si + CO(g)……….………...….(2.35)

Overall reaction for silica transfer (from coke ash) into liquid iron:

SiO2(s) + 2C(s) = Si + 2CO(g)……...……………..(2.36)

Considering the thermodynamics of the reduction of silica [Biswas (1981)] based on Eq.2.36 (and with 1 wt% silicon in solution), the following Gibbs free energy relation for the transfer of silicon into liquid iron is seen:

ΔGº = 592,141 – 391.54T J mole-1, where T (ºC)…………………...(2.37)

In the present study, the Gibbs free energy at 1550ºC calculated from Eq.2.37 is equal to -14.7 kJ/mol.

Furthermore, the transfer of silicon from silicon monoxide to liquid iron was also reported by Ozturk and Fruehan (1995). This work determined that gas phase mass transfer controls the transfer rate of silicon. They also found that the silicon transfer into the liquid metal was not a function of melt composition or temperature, but only a function of the diffusion distance. This result confirms the transfer of silicon into the liquid iron, which can react with the solute carbon in the melt leading to the depletion of melt carbon content. 2-48 Chapter 2 Literature Review

Effect of reducible oxides, especially SiO2 on the interfacial reactions between coke and liquid iron at 1550ºC were investigated by McCarthy et al. (2003). These authors used the sessile drop technique to investigate the formation of reaction products at the iron/coke interface. It was observed that the interface after 1 minute of contact was predominantly covered by Al2O3. After 60 minutes of contact, the formation of CaS complex was observed along with Al2O3. However, SiO2 which is a major component in the coke ash was not observed. These authors concluded that SiO2 coke ash was reduced via the reduction reaction (Eq.2.36) through reactions with solute carbon in the liquid metal droplet leading to the depletion of melt carbon concentrations as shown in Figure 2-26.

In their study, the dissolution of carbon from coke was limited, with the carbon pick-up after 30 minutes of contact being 0.1 wt%, which then increased to ~1.0 wt% and 2.0 wt% after 1 and 3 hours, respectively (see Figure 2-26). The observed low carbon dissolution in their study [McCarthy et al. (2003)] was attributed to a combination of both poor dissolution rate and carbon consumption by reducible oxides in the coke ash. These authors also concluded that the production of semifused ash at the interface will reduce the interfacial area. This occurs due to the selective removal of silica at the interface initially and the subsequent removal of CaO and its conversion into CaS.

Figure 2-26: Carbon and sulphur transfer from coke into liquid iron at 1550ºC [McCarthy et al. (2003)].

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The formation of CaS and Al2O3 phases at the iron/carbon interface have been reported previously [Wu et al. (2000)]. Using the sessile drop technique, Wu et al. (2000) investigated the iron/natural graphite interface and found that although natural graphite contains 8.8 wt% ash [with the majority being SiO2 (72 wt%)], no SiO2 was observed at the interface. These authors reported that Al2O3 was observed at the interface initially, and as the reaction progressed, the proportion of CaO increased and Al2O3 decreased. Therefore, the interface was found to be replaced by Fe/Ca/S complex thereafter. They described the formation of the sulphide based complex as the effect of the desulphurization of the iron droplet. However, the mechanisms that related to the decrease in Al2O3 and the increase in CaO were not provided.

CaO present in coke ash is also known to desulphurize the liquid metal by reacting with carbon and sulphur, as shown by Eq. 2.5 [Biswas (1981)]. This reaction produces CO and transfers CaS to the metal/carbon interface. CaO can also bond with Al2O3 to form

CaO.Al2O3 at the interface [Chapman et al. (2007)], which the final composition and morphology of the interfacial layer is the result of the reactions occur at the metal/carbon interface.

Recently, Cham et al. (2009) investigated the influence of mineral matter in coke ash on the carburization rate. Carbon dissolution from two types of metallurgical cokes (Coke1 and Coke2) was carried out using a carburizer cover technique (at 1450ºC and 1550ºC) and the overall dissolution rate of coke into liquid iron was calculated (K=14.7x10-3 s-1 and K=1.1x10-3 s-1). The reaction products formed at the iron/coke interface were investigated using the sessile drop technique. They observed the marked differences in appearance and composition of the iron/coke interface between the two cokes and this was believed to cause the significant differences in the carburization rates. They found that the interface formed from Coke1 had a much greater exposed coke surface which appeared to be predominantly composed of CaS and MnS. Cham et al. (2009) concluded that the higher presence of Mn at the interface in the case of Coke1 compared to Coke2 tended to reduce the fusion temperature of the interfacial layer, which would increase its bulk fluidity, and this could cause the differences in carburization rates between Coke1 and Coke2.

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2.4 Wettability of Iron/Carbon Systems

Wetting of solid carbon (S) and liquid iron (L) is normally defined by the contact angle

(θ) between the solid-liquid interfacial tension (σSL) direction and liquid-gas interfacial tension (σLG) direction as shown in Figure 2-27 [Mukai (1992)].

Figure 2-27 Contact angle (θ) of liquid iron droplet on solid carbon in a sessile drop experimental setup. σSL, σSG and σLG represent interfacial tension between solid-liquid, solid-gas and liquid-gas acting on the droplet, respectively.

Figure 2-27 shows a drop of liquid iron on a solid carbon substrate representing on a reactive mode of wetting [Mukai (1992)]. The system is wetting when θ< 90º and non- wetting when θ> 90º [Kolasinski (2002)]. The contact angle is defined by Young’s equation [Mukai (1992), Eustathopoulos et al. (1999) and Kolasinski (2002)], as shown by Eq. 2.38. VV cosT SG SL ..(...... 2 38) V LG

The work of adhesion (Wa) of a wetting system is given by Eq. 2.39 [Mukai (1992)].

Wa LG 1(  cosTV )...... (...... 2 39)

Contact angles can be affected by reaction temperature, reaction time, morphology of the solid surface and composition of the solid, liquid and gas constituents [Jimbo et al. (1993)]. Surface active elements, such as oxygen and sulphur can affect the surface tension (γ) of the liquid metal [Jimbo et al. (1993)]. A previous study on surface tension and interfacial tensions of liquid steel by Jimbo et al. (1993) shows that the

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surface tension of liquid iron decreases with increasing oxygen and sulphur content. Also, an increase in temperature can decrease the surface tension of a pure liquid metal, such that it becomes zero at a critical temperature [Jimbo et al. (1993)]. The surface tension of the liquid iron-gas phase boundary can be determined by Eq. 2.40.

J LG 2184 .0 107 ln(1˜ akT ss )...... (...... 2 40)

ks is defined by Eq. 2.41. 9960 k  .2 75 ....(...... 2 41) s T

as is the activity of sulphur, which can be calculated from Eq. 2.42 [Sain and Belton (1978)].

s Sa (wt%)u E ..(...... 2 42) where β is the activity coefficient, defined by Eq. 2.43.

E exp[ .0 36 C({ wt%) u Cs}]...... (...... 2 43)

where Cs is the carbon saturation level in the liquid iron.

2.4.1 Influence of Melt Carbon and Sulphur Content on Wettability

Carbon is a relatively non-surface active element in molten iron [Kozakevitch and Urbain (1957), Richardson (1974), Jimbo et al. (1993)]. In a reactive wetting system, if a reaction is involved at the interface, the system free energy change will contribute to the change in interfacial tension [Cham (2007)]. Based on thermodynamic considerations, the spreading of a liquid iron droplet on a substrate can occur only if the system free energy decreases during the wetting process. The decrease in system free energy occurs as a result of interfacial reactions. The interfacial tension depends on the extent of carbon dissolution into liquid iron and iron diffusion into solid carbon, and these can change the characteristics of the interfacial layer due to the interfacial

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reactions taking place. The final interfacial tension would be determined by the resultant phases formed at the interface through the reaction.

Several previous studies have been carried out to investigate the wettability of liquid iron with graphite, coal-chars and metallurgical cokes [Wu and Sahajwalla (1998), Wu et al. (2000), Zhao and Sahajwalla (2003), McCarthy (2004) and Cham (2007)].

Wu and Sahajwalla (1998) investigated the dynamic wetting behaviour of Fe-C-S melt with graphite substrate using a sessile drop technique by focusing on the influence of melt carbon and sulphur concentration on the wettability at 1600ºC. The wettability between the Fe-C-S melt and solid graphite was presented in terms of the changes in contact angles with time (Figure 2-28).

Figure 2-28: The variation of contact angles as a function of time for molten Fe-S-C and graphite system [Wu and Sahajwalla (1998)].

From Figure 2-28, in the initial state (within ~8 seconds of contact), the system is observed to be non-wetting (~100º). Thereafter, this system becomes wetting (~60º) with a considerable decrease in the contact angles (in the time period ~8 to 90 seconds). The contact angles were observed to stabilize at ~60º for the rest of the experimental time. These authors attributed the changes in contact angles in this system to carbon and iron inter-diffusing into the solid graphite and the liquid iron.

Wu and Sahajwalla (1998) reported the influence of melt sulphur concentration on the wetting behaviour of Fe-C-S melts with graphite (see Figure 2-29). The authors found

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that sulphur, which reduced the surface tension of the melt, acted to increase the contact angles. As shown in Figure 2-29, at constant melt carbon level, the increase in the melt sulphur level from 0.07% up to 0.37% can increase the contact angles considerably, changing the system from wetting to non-wetting.

Figure 2-29: The variation of the contact angles as a function of time for molten Fe-S- C and graphite system with varying sulphur levels [melt C level was fixed at 2% ] [Wu and Sahajwalla (1998)].

Wu et al. (2000) investigated the wetting behaviour of natural graphite and liquid iron system using the sessile drop technique and the results are shown in Figure 2-30.

Figure 2-30: The variation in the contact angles as a function of time for molten iron on natural graphite substrate [Wu et al. (2000)].

They observed that the contact angles of this system decreased (from ~100º) to become wetting after the first 10 seconds (~60º). Thereafter it became non-wetting with the contact angles sharply increasing to stabilize at ~100º within 100 seconds. This

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dynamic wetting behaviour of iron and natural graphite is very different from the behaviour of molten Fe-C-S and graphite systems which were reported previously [Wu and Sahajwalla (1997)].

For chars and cokes (which are complex heterogeneous materials), several factors including the carbon structure, melt carbon and sulphur levels, contents and chemical compositions of coke ashes and interfacial reaction products can influence the wetting behaviour of these systems [Wu (1998)].

McCarthy (2004) carried out experiments on the wettability of four types of chars with both electrolytic pure iron and an iron alloy containing 2% C and 0.01% S at 1550ºC. In the case of pure iron, all samples showed a non-wetting behaviour with contact angles ranging from 106º-137º in the beginning, which decreased to 101º-119º after 60 minutes of contact. In the case of Fe-C-S alloy, all samples showed a non-wetting behaviour with contact angles ranging from 113º-127º in the beginning, followed by a decrease to 101º-125º after 60 minutes of contact. Only a marginal improvement in wettability (decrease in contact angles) was observed as a function of time. The author indentified the factors influencing the wettability as:

x Transfer of carbon and sulphur across the metal/carbon interface

x Formation of Al2O3 and CaS complex at the interface

x Reduction of reducible oxides, such as SiO2 and Fe2O3, and the transfer of Si into the liquid metal

An experimental investigation of the wetting behaviour of pure liquid iron with three types of metallurgical coke at 1550ºC was conducted by Cham (2007). This author observed that all cokes showed a non-wetting behaviour with contact angles ranging from 123º – 126º initially, which marginally decreased to 109º-114º after 60 minutes of contact. Cham (2007) concluded that the slight decrease in contact angles was due to the transfer of carbon from coke to liquid iron, which then decreased the interfacial tension. On the other hand, the effects of sulphur, ash content and composition and carbon structure on the wettability could not be observed in this study [Cham (2007)].

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2.5 Summary

Waste plastics are being used in ironmaking and steelmaking processes for improving the sustainability of the metallurgical processes. These wastes are utilised in both coke ovens and blast furnaces in order to decrease coal and coke consumption, associated environmental problems and to increase the energy efficiency. In EAF steelmaking process, waste polymers were found to be utilized as an alternative source of carbon in the slag foaming practice. Some of the key carbon consumption mechanisms in the steelmaking process are carbon dissolution into the melt, slag foaming and associated iron oxide reduction. Several researchers have conducted experimental investigations on the slag/carbon and iron/carbon interactions. The major findings are summarized below:

2.5.1 Slag/carbon Interactions

Previous studies found in the literature for slag/carbon interactions mostly focussed on slag foaming and factors that influence the foaming behaviour. Slag foaming phenomena have been studied using different techniques. It can be concluded from the literature that:

x Some researchers injected gases such as argon into molten slag and measured the foam height to investigate effects of the experimental parameters such as gas velocity, gas bubble size, temperature and surface active substance on the slag foaming behaviour. The foam index (Σ) or foamability is used to describe the slag foaming behaviour. The sessile drop technique has also been employed investigate slag foaming and slag/carbon interactions. x Small gas bubbles were shown to produce more stable slag foaming behaviour than gas bubbles of larger size. The increase in gas bubble size results in a decrease in the foaming index. Velocity of the gas injection also affects the foamability. Increasing the superficial gas velocity leads to an increase in foam height and thus foam index. x The foam life increases with decreasing temperature. Increasing temperature leads to a decrease in the viscosity of the molten slag, which decreased the foam life. Slag foaming is influenced by the type of carbonaceous materials used and

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their impurities, and this was due to the slag reactions with the different types of carbonaceous materials used. x The foam height or foaming index decreased with increasing basicity up to a certain value and then increased with increasing basicity. A high viscosity (and low surface tension) and low density slag was likely to exhibit good slag foaming. The foaming level increased with decreasing surface tension and the

presence of a surface active species (such as P2O5 and Cr2O3) in slag due to the associated decrease in surface tension. x HDPE and rubber tyres were investigated as a partial replacement of metallurgical coke for slag foaming agent in EAF steelmaking. The use of HDPE/Coke and Rubber/Coke were found to improve the foaming behaviour molten slag compared to metallurgical coke alone. The improvement was reported to be due to the greater slag reaction with the carbonaceous blends (by

solid carbon). However, the effect of volatiles from the polymers (H2, CH4, etc.) was not well investigated.

2.5.2 Iron/Carbon Interactions

Previous studies found in the literature for iron/carbon interactions were focussed on the kinetics of dissolution of carbon into molten iron along with the factors affecting the dissolution rate. The formation of reaction products at metal/carbon interface and the wetting behaviour of liquid iron on carbonaceous materials have also been investigated. The major findings of previous work on iron/carbon interactions are summarized below:

x Two key steps influence the kinetics of carbon dissolution from carbonaceous materials into molten iron: liquid side mass transfer and interactions at the metal/carbon interface. x In the case of graphite, the carbon dissolution behaviour into molten iron is controlled by the rate of mass transfer into liquid metal in most situations. Sulphur was found to retard the rate of carbon dissolution from graphite due to the decrease in the diffusion coefficient and solubility of carbon in liquid iron.

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x The carbon dissolution behaviour of non-graphitic materials (coal-char and coke) was influenced by both the mass transfer and the interfacial reactions at the metal/carbon interface. This was due to the presence of high amounts of mineral oxides in both coal and coke, which can react with the molten iron and form the interfacial products at the interface, thereby influencing molten iron from reaction with the solid carbon. For non-graphitic materials, the dissolution rate was found to be more sensitive to the presence of sulphur in the melt compared to in the case of graphite. The decrease of dissolution rate was not only affected by lowering of the carbon diffusion coefficient in the melt, but it was also affected by the decrease in interfacial contact area by either surface blockage or poor wetting behaviour of melt containing sulphur. An increase in initial melt carbon content can result in a slight decrease in the carbon dissolution rate. Moreover, the decrease in carbon dissolution rate of graphite can also be due to the decrease in temperature; however, the influence of temperature on carbon dissolution rate is insignificant. x Wettability of molten iron with solid carbon is also a factor affecting the carbon dissolution into liquid iron, due to the formation of new phases (reaction products) at the metal/carbon interface, resulting in an increase in contact angles between the liquid metal and the solid carbon, and thus leading to a reduction of the effective area available for carbon dissolution. Metallurgical coke and coal- char were reported to exhibit non-wetting behaviour with molten iron, while graphitic materials show good wetting behaviour. x The use of waste polymers as an alternative source of carbon for carburizing molten iron and the iron/carbon interactions has not been investigated in detail, and also, the role of volatiles in the polymers on the interfacial phenomena needs to be investigated.

2.6 Research Focus in This Project

The present project has been carried out to investigate the possibility of utilizing waste polymers as carbon resources in the EAF steelmaking processes. The investigation has been conducted on two key reactions that occur in the processes including slag/carbon and steel/carbon interactions. Polyethylene terephthalate (PET) and polyurethane (PU) were selected for the investigation of slag/carbon interaction, while bakelite, high

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density polyethylene (HDPE) and polyethylene terephthalate (PET) were used for steel/carbon interactions studies.

The slag/carbon interactions investigations were on the slag reactions with the polymer/coke blends. The main focus in these investigations was on the influence on chemical elements and volatiles in the polymer on slag/carbon interactions.

The steel/carbon interactions investigation were on the wettability of polymer/coke blends by molten steel, the interface and the transfer of carbon and sulphur from polymer/coke blends into molten steel and the interfacial phenomena occur at the interface.

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CHAPTER 3

EXPERIMENTAL DETAILS

Chapter 3 Experimental Details

This research investigates the possibility of using blends of recycled polymeric materials with metallurgical coke as alternative sources of carbon in EAF steelmaking for applications as slag foaming agents and as carburizers. The interactions of the carbonaceous blends with molten slag and steel will be investigated at 1550ºC. This chapter details the methodologies involved in sample preparation as well as the experimental techniques used for experiments.

3.1 Selection and Preparation of Samples

A number of plastics were selected for slag/carbon and steel/carbon interaction studies. These polymers were blended with metallurgical coke in well-defined proportions. The slag/carbon interaction study aims to understand the effects of the chemical compositions of the plastics on gas generation, volatile release, iron oxide reduction and slag foaming phenomena. The investigations of the steel/carbon interactions will help understanding of the effects of different plastics and their relative concentrations in the blends on the carbon dissolution behaviour, the formation of reaction products at the metal/carbon interface as well as the wettability between the two phases.

3.1.1 Polymeric Materials

Four types of polymeric materials were investigated in this study. Polyethylene terephthalate (PET) and Polyurethane (PU) were used for slag/carbon interaction studies. In the case of steel/carbon interaction studies, Bakelite, High Density Polyethylene (HDPE) and Polyethylene terephthalate (PET) were used. These plastics have differences in their physical and chemical properties, which are expected to lead to differences in their reactions with the molten slag and molten steel at high temperatures.

The chemical composition of the polymers was analysed in the Analytical Centre, UNSW, Australia, and is provided in Table 3-1. It can be seen from Table 3-1 that bakelite contains significant ash impurities that are present as filler materials generally found in this thermoset polymer. Further analysis of ash in the bakelite was conducted and is shown in Table 3-2.

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Table 3-1: Chemical composition of the raw polymeric materials

Chemical Composition (wt%) Impurities Polymeric Materials C H O N S (wt%) HDPE 85.6 14.4 - - - - PET 62.5 4.2 33.3 - - - PU 65.6 4.9 20.5 9.0 - - Bakelite 53.4 4.0 11.6 - 0.017 31.0

Table 3-2: Chemical composition of raw bakelite

Chemical Composition (wt%) Ash (wt%)

C H O S CaCO3 SiO2 SO3 53.4 4.0 11.6 0.017 30.03 0.91 0.06

3.1.2 Samples for Slag/Carbon Interactions

3.1.2.1 Slag

The slag used in the experiments was provided by OneSteel, Australia from their EAF steelmaking operations. The chemical composition of the slag is shown in Table 3-3.

Table 3-3: Slag composition used for slag/carbon interaction experiments

Oxides Fe2O3 CaO Al2O3 MgO SiO2 MnO (CaO/SiO2 +Al2O3)

Wt% 33.9 31.1 6.1 10.7 13.0 5.2 B3 = 1.628

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3.1.2.2 Carbonaceous Materials

Previous slag-carbon interaction studies carried out by Sahajwalla et al. (2009) and Zaharia et al. (2009) have shown that high carbon polymers such as HDPE and rubber tyres, which predominantly contain C and H, can be successfully used in EAF steelmaking as slag foaming agents. The present study will investigate the use of low carbon containing polymers (PET and PU) which contain additional elements O and N, as a slag foaming agent in EAF steelmaking process, with focus on the role played by their chemical composition.

PET ([C10H8O4]n) exists both as an amorphous and semi-crystalline polymer. Amorphous PET is transparent and mostly found in beverage bottles, whereas semi- crystalline PET is opaque or white. The PET molecule consists of C, H and O atoms.

PU ([C25H42N2O6]n) consists of C, H, O and N atoms in its polymer molecules. PU is an amorphous polymeric material and is a common name used to denote polymers consisting of a chain of organic units joined by urethane links. It is widely used in low density soft foams, rigid foams and low density elastomers, such as flexible foam seating and rigid foam insulation panels.

Carbonaceous materials used in slag/carbon interaction experiments were blends of PET and PU with metallurgical coke. Only one proportion mix was used as shown in Figure 3-1 and the blends were named PET/Coke and PU/Coke. This ratio was selected based on the best slag foaming behaviour observed from blends of coke with HDPE and rubber [Sahajwalla et al. (2009) and Zaharia et al. (2009)].

Figure 3-1: Relative Proportion of plastic and coke in the carbonaceous blends (wt%) used in slag/carbon interaction experiments.

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3.1.3 Samples for Steel/Carbon Interactions

3.1.3.1 Metal

The metal used in the experiments is an electrolytic pure iron (99.98% Fe) provided in the form of chips, by SIGMA-Aldrich Pty Ltd. It has a melting point of 1535ºC. In this study, the word “steel” refers to the electrolytic pure iron.

3.1.3.2 Carbonaceous Materials

The carbonaceous materials used in the experiments were metallurgical coke and its blends with different plastics. Coke was supplied by OneSteel1, Australia. The polymeric materials used include high density polyethylene (HDPE), polyethylene terephthalate (PET) and bakelite. HDPE ([C2H4]n) is a semi-crystalline polymer, made up of crystalline and amorphous components. The proportion of crystalline components is greater than amorphous components in the case of high density polyethylene (HDPE), in which the molecular chain in the crystalline parts is highly ordered, neatly folded, layered and densely packed. HDPE is a clear to whitish translucent thermoplastic material, consisting of long chains of C and H atoms. PET ([C10H8O4]n) which was used in the slag/carbon interaction studies was also used for the steel/carbon interaction studies. Bakelite ([C7H8O2]n) is an amorphous polymer which has a 3- dimensional cross-linked network structure, that gives it high hardness, rigidity, and strength combined with good thermal and electrical insulating properties and chemical resistance. It is a material based on the thermosetting phenol formaldehyde resin, and consists of C, H and O atoms. Calcium carbonate (CaCO3) is usually added as a filler to commercially used bakelite.

Plastic and coke were blended in three different ratios: Blend 1, Blend 2 and Blend 3. The name used are H1, H2, H3, P1, P2, P3, and BK1, BK2, BK3 for Blends 1, 2, 3 of HDPE/Coke, PET/Coke and bakelite/Coke respectively. Figure 3-2 represents the blending ratios between the plastics and coke in the carbonaceous blends used for steel/carbon interaction experiments.

1 OneSteel is an EAF steelmaking minimill located in Sydney, Australia 3-5 Chapter 3 Experimental Details

Figure 3-2: Relative proportions of plastics and metallurgical coke in the carbonaceous blends used for steel/carbon interaction experiments.

3.1.4 Preparation of Samples

Coke was ground and sieved to a particle size less than 1.0 mm. The plastics were crushed using a jaw crusher and also sieved to particle sizes less than 1.0 mm. The plastic samples were then homogeneously blended with coke in a rolling mill for 12 hours.

The blends were combusted in a drop tube furnace (DTF) at 1200ºC under an atmosphere of 80% nitrogen and 20% oxygen with a gas flow rate of 1.0 L/min. A schematic of the DTF is shown in section 3.2.5 (Figure 3-6). The samples were fed into the DTF at the rate of 0.52 g /min. Residual chars were collected from the bottom of the furnace. Produced chars were ground into a fine powder and sieved to a size <40 μm. Approximately 1.6 g of the powder was put in a die and compacted under a load of 75 kN using a hydraulic press to make a carbonaceous substrate. The cylindrical substrate (top surface area of 3.14 cm2) was used for slag/carbon and steel/carbon experiments.

3.2 Experimental Instruments

A number of experimental instruments are employed for studying slag/carbon and steel/carbon interactions. These include an X-ray diffractometer (XRD), Scanning electron microscope (SEM), Infrared (IR) gas analyzer, Gas chromatography (GC)

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analyzer, Carbon-Sulphur analyzer and high temperature furnaces. The details of these instruments are provided below.

3.2.1 X-ray Diffractometer (XRD)

XRD analysis of the carbonaceous samples was carried out using a Philips X’pert Multipurpose X-ray Diffraction System (MPD). The MPD is a theta to theta goniometer system and consists of programmable slits, a curved monochromator, and an automatic sample changer. The X-ray operating conditions used were generally 45 kV and 40 mA. The MPD is suitable for routine analysis of powder samples, i.e. phase identification and quantitative analysis. A sufficient sample amount was used to fill the holder of 25 mm diameter and 5 mm depth. The samples were ground into powder and packed into metal sample holders. X-ray scans were carried out over the range (2θ angle) 10º to 50º at a step size of 0.05º/step.

3.2.2 Scanning Electron Microscope (SEM)

A Hitachi S3400 SEM was used to investigate the reaction products formed at the metal/carbon interface and to analyse the sample surface. Hitachi S3400 SEM (Figure 3-3) is capable of imaging and X-ray microanalysis (using EDS). These instruments are fitted with secondary and backscatter electron detectors that allow for topographic and compositional (atomic number contrast) surface imaging of samples. This SEM is interfaced with an energy-dispersive X-ray spectroscopy (EDS), which is an analytical technique used for the chemical characterization of a sample.

Figure 3-3: Scanning Electron Microscope (SEM Hitachi S3400).

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3.2.3 Gas Analyzers

The gas generated from the reactions between slag and carbon were analysed using an infrared gas analyzer and gas chromatography. An infrared gas analyzer continuously measures the gas composition by determining the absorption of an emitted infrared light source through a certain air sample. In the present study, the infrared (IR) gas analyser (ABB AO2020) was used to monitor the CO and CO2 generated from slag/carbon interactions.

Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. In the present study, the gas chromatographic (GC) analyser (SRI 8610C Chromatograph Multiple Gas #3 GC configuration) was used to qualitatively identify the key constituents of gaseous species generated from the slag/carbon interactions (such as H2, O2, N2, CH4, CO and CO2).

3.2.4 Carbon-Sulphur Analyzer

A Carbon-Sulphur analyzer was used to analyse the carbon and sulphur in the steel droplet. In the present study, the carbon/sulfur analyzer (LECO's CS230) was used. Figure 3-4 show the image of the LECO's CS230. In this analyzer, the sample is combusted in an O2 atmosphere to produce CO2 and SO2, and this is used to determine the carbon and sulphur in the metal.

Figure 3-4: Carbon and sulphur analyzer (LECO CS230).

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3.2.5 High Temperature Furnaces

The furnaces used in the present study include the drop tube furnace (DTF) and horizontal tube furnace (HF).

3.2.5.1 Drop Tube Furnace (DTF)

In EAF steelmaking, carbonaceous materials injected into the furnace pass through the hot air, and thus they are devolatilized before coming into contact with the molten slag and steel. In the present study, the devolatilization of polymer/coke blend was conducted in order to simulate the actual process in EAF steelmaking. The DTF was used for devolatilization of the polymer/coke blends. This furnace is connected to a water cooling system. The operation was carried out at 1200ºC with nitrogen (80%) and oxygen (20%) purging through the furnace. An image of the DTF is shown in Figure 3- 5, and its schematic is shown in Figure 3-6.

Figure 3-5: Drop tube furnace (DTF).

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Figure 3-6: Schematic of the Drop Tube Furnace (DTF) used in the experiments [Rahman (2010)].

3.2.5.2 Horizontal Tube Furnace (HF)

A horizontal tube furnace (HF) was used as a high temperature system for slag/carbon and steel/carbon interaction experiments. This furnace has a gas inlet through which Ar gas was passed through at the rate of 1 L/min. The outlet was connected to the GC/IR. The experiments were carried out at 1550ºC (steelmaking temperature). The image of the HF is shown in Figure 3-7, and its schematic is shown in Figure 3-8.

Figure 3-7: Horizontal tube furnace (HF).

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Figure 3-8: Schematic of experimental set up for slag/carbon interaction experiments.

3.3 Slag/Carbon Interaction Experiments

3.3.1 Slag Foaming

Slag foaming experiments were carried out using the sessile drop technique in a horizontal tube furnace. Approximately 0.065 g of EAF slag was put on the carbonaceous substrate, and the assembly was then placed on an alumina sample holder. In this experiment, the gas evolving from the slag/carbon reactions was monitored along with online video recording of experimental assembly for determining volume changes in the slag droplet as a function of time. A steel rod with an alumina sample holder attached at the end was used for these experiments, and only the alumina holder was in the hot zone of the furnace. A schematic of the horizontal tube furnace used for the slag foaming experiments is presented in Figure 3-8. Figure 3-9 shows the carbonaceous substrate and slag assembly, both before and after experiments.

The experiments were carried out under an inert argon atmosphere with a gas flow rate of 1 L/min. The sample holder was put in the cold zone of the furnace where the temperature was below 1200˚C for 15 minutes, and then pushed into the hot zone where the temperature was 1550˚C. The reaction time was noted to start when the slag had completely melted and formed a droplet. The experiments were generally run for 15 minutes. During the experiments, the reactions inside the furnace were recorded using a CCD camera and DVD system. The slag droplet images were captured every 5 seconds over the entire experimental run.

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Figure 3-9: Carbonaceous substrate/slag assembly before and after experiments.

Dynamic changes in the volume of slag samples were determined from the captured images using computerized data processing software. This software is a ‘Linux’ based program used to select specific frames out of the experimental data for further processing. The second stage of data processing involved the computation of the slag droplet volume in a chosen frame. This experiment primarily focused on the quantitative measurement of gas entrapment and rapidly changing slag droplet volumes. Therefore, the slag droplet on the substrate was assumed to have a truncated spherical shape. In a semiautomatic procedure, a circular marquee was fitted manually to provide a best fit to the slag droplet, with the user playing a vital role in identifying the optimum spherical shape for the slag droplet at a given instant of time. The volume of the slag droplet and area of contact between slag and substrate were computed in terms of the radius ‘r’ of the droplet and the truncated height ‘h’, as shown in Figure 3- 10. The volume ‘V’ of the segmented sphere is given by

S 23 3(4[  hrhrV )]...... (...... )1.3 3 The area of contact ‘A’ between the slag droplet and the substrate, is given by

S 2(  hrhA )...... (...... )2.3

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Figure 3-10: Illustration of the slag droplet during data processing indicating the radius ‘r’ and truncated height ‘h’ used in computing the slag volume ‘V’.

The computed data was stored in a data file for further analysis. Minor deviations from the spherical outline were not taken into account (estimated error <5%). Further details of this software are given elsewhere [Khanna et al. (2007)]. The slag foaming behaviour was analysed in terms of volume ratio (Vt/V0) as a function of time, where Vt represents the volume of the slag droplet at time t and V0 is the initial volume. A very large number (>1000) of slag droplet images were analysed to determine changes in the slag volume as a function of time.

3.3.2 Observation of Reduced Iron and Entrapped Gas Bubbles

The slag droplets after reaction with the carbon substrates were quenched after 2, 4, 8 and 15 minutes, and these were then mounted in epoxy resin. The hardened resin was then cross sectioned and polished. The cross sectioned slag droplet images were examined using an optical microscope (Nikon 200), as seen in Figure 3-11. The diameters of the entrapped gas bubbles in the slag droplet were measured using the computer software IMAGE TOOL.

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Figure 3-11: Cross-section of the slag droplet after reaction with the carbonaceous substrate showing the presence of reduced metal droplets and entrapped gas bubble.

3.3.3 Analysis of Gas Generation from Slag/Carbon Interaction

CO and CO2 generated during slag/carbon interaction via the reduction of FeO participate in the foaming of slag. Therefore, the CO and CO2 released from the reactions were monitored using an infrared (IR) gas analyser (ABB AO2020). The gases evolved from the blank substrate alone also were monitored.

However, due to the blending of polymeric materials with coke, the gaseous species released from the decomposition of plastics is dependent on its chemical composition. These different gaseous species could also participate in the reduction reaction as well as in the foaming of slag. To determine the influence of plastic gasification on slag foaming at high temperatures, the gases released from the slag-carbon interactions at 1550˚C as well as from the blank substrate were analysed using a gas chromatographic (GC) analyser (SRI 8610C Chromatograph Multiple Gas #3 GC configuration).

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3.4 Steel/Carbon Interaction Experiments

The steel-carbon interaction studies were carried out using the sessile drop technique. These studies were focused on the dissolution of carbon from plastic/coke blends into liquid steel as well as the wettability between the liquid and the solid.

3.4.1 Carbon and Sulphur Transfer

The carbonaceous blends were ground in a ring mill to obtain a fine powder (less than 38 μm- diameter particles). Approximately 1.6 g of the powder was put in a die and then pressed with approximately 7.5 kN force using a hydraulic press. The compacted substrate had a top surface area of 3.14 cm2. It was placed on a graphite tray and then 0.5 g of electrolytic pure iron was put on top of it. Figure 3-12 shows the carbonaceous substrate and metal assembly, both before and after experiments.

Figure 3-12: Carbonaceous substrate/metal assembly before and after the experiments.

A horizontal tube furnace was employed as the high temperature test chamber as illustrated in Figure 3-13. The experiments were run under an inert argon atmosphere flowing at a rate of 1 L/min. The sample holder was first put in the cold zone of the furnace for approximately 15 minutes to protect the system from thermal shock, and then gradually pushed into the hot zone where the temperature was 1550˚C. Once the metal melts and forms the droplet, the time generator was started and the wettability of the system was observed from images captured using a CCD camera. The samples were quenched after 0.5, 1, 2, 4, 8, 15, 20, 30 and 60 minutes.

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Figure 3-13: Schematic of experimental set up for steel/carbon interaction experiments.

After carbon transfer experiments, carbon and sulphur picked up by the metal droplets were measured using a carbon-sulphur analyser (LECO CS 230). Nitrogen is used as pneumatic air to push a sample holder into the furnace. Due to combustion in the chamber, the carbon and sulphur in the samples exit as CO2 and SO2 respectively, which are detected by IR cells calibrated for carbon and sulphur. The sample weight can be in the range of 0.2-1.0 g for high carbon containing samples. Variation of the analysis was limited to ±0.006 wt% for carbon and ±0.001 wt% for sulphur.

3.4.2 Observation of Reaction Products at Steel/Carbon Interface

After carbon dissolution experiments, the metal droplets that reacted with carbon substrates at different times were examined using a Scanning Electron Microscopy (SEM Hitachi S3400-X) coupled with Energy Dispersive X-ray Spectroscopy (EDS). To mount the sample, the bottom side of the droplet which was in contact with the substrate was brushed to remove carbon powder and debris attached to it. Then the top side of the droplet was placed upside down on a carbon tape which was attached to the sample holder. EDS was used to get a semi-quantitative indication of the relative distribution of elemental species at the interface.

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3.4.3 Wetting of Polymer/Coke Blends by Molten Steel

The steel/carbon interaction experiments were carried out for 60 minutes, and the images of the wetting of the metal on the substrate were captured from the DVD of the complete process using computer software.

Figure 3-14: Illustration of the contact angle measurement using ANGLE software.

The captured images were analysed using ANGLE software that uses a non-linear regression calculation procedure to determine the left and right contact angles. The average of the contact angle values were used for the present studies. The details for the software are described elsewhere [Wu & Sahajwalla (1998)]. Figure 3-14 illustrates an example of the contact angle measurement using ANGLE software.

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CHAPTER 4

SLAG/CARBON INTERACTIONS BETWEEN POLYMER/COKE BLENDS AND EAF SLAG AT 1550ºC

Chapter 4 Slag/Carbon Interactions between Polymer/Coke Blends and EAF Slag at 1550ºC

Slag foaming is an important phenomena in EAF steelmaking as it prevents heat losses to the furnace roof and sidewalls and increases the efficiency of the process [Sahajwalla et al. (2009)]. An improvement in the slag foaming behaviour depends on the ability of slag to hold off-gases produced during high temperature reactions. The physical properties of slag (i.e. viscosity, density, surface tension and basicity) are linked to the composition of slag and the process temperatures, and these are known to affect the foamability of slag [Corbari et al. (2009) and Ozturk and Fruehan (1995)]. Generally, the reactions between slag and coke result in FeO reduction, producing CO which causes the slag to foam. By blending plastics with coke, volatiles produced during polymer decomposition along with the off-gases produced during chemical reactions with the slag would influence the slag foaming behaviour. Rahman (2010) and Zaharia (2010) have shown that blends of coke with HDPE and rubber tyres can be successfully used as a slag foaming agent in EAF steelmaking. Improved slag foaming observed from the blending of these polymeric materials was attributed to the large number of gas bubbles which were entrapped in the molten slag. If the gas generation rate is very high, the gas bubbles are likely to escape easily from the bulk liquid slag leading to poor slag foaming. If the rate of gas generation is very low, slag foaming would be poor again; an optimum gas generation is required for good foaming behaviour. Due to the success of the use of HDPE and rubber (which contain high levels of volatiles) in these previous studies, it is likely that volatiles from the polymers

(such as H2 and CH4) would also have an effect on the slag foaming behaviour. However, these studies [Rahman (2010) and Zaharia (2010)] were focussed primarily on CO and CO2 in the off-gases. The influence of volatiles like CH4 and H2 from the polymers was not investigated.

The present study investigates interfacial reactions between polymer/coke blends and molten slag at 1550ºC. Two polymers were investigated namely Polyethylene terephthalate (PET) and Polyurethane (PU). PET contains carbon, hydrogen and oxygen, while PU contains carbon, hydrogen, oxygen and nitrogen. The influence of volatiles produced from PET and PU were also investigated.

PET and PU were blended in a specific proportion with metallurgical coke (PET/Coke and PU/Coke). Herein, the word ‘coke’ used refers to metallurgical coke. The blending

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of polymers with coke is expected to modify the properties of the materials compared to the parent coke. In this chapter, the characterization of the polymer/coke chars is presented first. Then, an in-depth investigation on the interactions between slag and polymer/coke blends containing PET and PU at 1550ºC will be reported, including the slag foaming behaviour (as the dynamic change in volume of the slag droplet), off-gas analysis and observation of entrapped gases and the reduced iron in the bulk slag. The influence of chemical elements in the polymers on the slag/carbon interaction will be discussed. The slag/metallurgical coke interaction (without polymer) was also investigated for the sake of a comparison.

4.1 Characterization of Polymer/Coke Blends

PET and PU were crushed in a jaw crusher and sieved to sizes less than 1 mm, and then blended in a well defined proportion with metallurgical coke. Proportion of polymer and coke in the blends was reproduced and shown in Figure 4-1. This ratio was selected based on the optimum slag foaming behaviour observed from blends of coke with HDPE and rubber [Rahman (2010) and Zaharia (2010)].

Figure 4-1: Proportion of plastic and coke in the blends (wt%).

Raw PET/Coke and PU/Coke blends were devolatilized in a drop tube furnace (DTF) at 1200 ◦C under an atmosphere of 80% nitrogen and 20% oxygen with a gas flow rate of 1.0 L/min, and the chars obtained were used for slag/carbon interaction experiments. The devolatilization was carried out to simulate the real process in EAF steelmaking. In the EAF steelmaking process, carbonaceous materials are injected through hot air and these undergo partial combustion before coming into contact the molten slag.

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4.1.1 XRD Analysis

XRD patterns of raw PET and PU are compared with coke and shown in Figure 4-2. XRD patterns of raw PET/Coke and PU/Coke blends (prior to heating in the DTF) and chars produced from the blends are shown in Figure 4-3.

Figure 4-2: XRD patterns of a) Coke; b) Raw PET; and c) Raw PU.

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Figure 4-3: XRD patterns of a) Raw PET/Coke, b) Char PET/Coke, c) Raw PU/Coke and d) Char PU/Coke.

It can be observed from Figure 4-3 that the chars produced from PET/Coke and PU/Coke blends were not significantly different in terms of carbon structure compared to coke alone. XRD patterns of PET/Coke and PU/Coke chars are comparable to that of the parent coke. Diffraction peaks of silica, a major component of coke ash, can be observed at 2θ positions of 20.87º and 26.66º.

4.1.2 Chemical Analysis

The ultimate analyses of the raw polymer/coke blends and the polymer/coke chars used in the slag/carbon interaction studies were carried out and compared to coke and are given in Tables 4-1 and 4-2, respectively. The ash analysis results for the raw polymer/coke blends and the polymer/coke chars were compared to coke and are given in Tables 4-3 and 4-4, respectively. It was found that the blending of polymer with coke produce a significant difference in term of fixed carbon and volatiles contents compared to the parent coke; their ash and sulphur contents were comparable. PET/Coke and PU/Coke blends contain lower fixed carbon than coke, but a relatively higher volatiles level was observed in both the raw blends and the chars produced. Ash

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compositions of the raw blends and the chars for both PET/Coke and PU/Coke blends were also comparable to that of the parent coke.

Table 4-1: Chemical composition of the polymer/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Hydrogen Materials (wt%) (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 1.00 PET/Coke 60.05 23.05 16.90 0.28 2.56 PU/Coke 60.68 22.95 16.37 0.30 2.64

Table 4-2: Chemical composition of the polymer/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Hydrogen Materials (wt%) (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 1.00 PET/Coke 70.70 10.30 19.00 0.27 1.49 PU/Coke 72.90 8.30 18.30 0.27 1.53

Table 4-3: Ash analyses of the polymer/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 PET/Coke 57.10 31.80 2.30 0.92 0.74 1.10 0.32 0.34 0.56 PU/Coke 60.30 31.50 2.10 1.20 0.68 1.12 0.24 0.30 0.49 4-6 Chapter 4 Slag/Carbon Interactions between Polymer/Coke Blends and EAF Slag at 1550ºC

Table 4-4: Ash analyses of the polymer/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 PET/Coke 56.80 31.40 2.10 1.40 0.70 1.00 0.34 0.33 0.53 PU/Coke 60.20 31.60 2.00 1.10 0.69 1.10 0.26 0.31 0.47

4.1.3 Morphology Studies

Figure 4-4 shows the morphology of char particles in the blends. The PET/Coke char was observed to have a number of pores on its surface. The decomposition of the PU/Coke blend in the DTF resulted in the formation of a viscous liquid which caused an agglomeration of the carbon particles into a large mass. The surface of these large clusters was observed to have spherical particles deposited on it. These spherical particles were found to be composed of Al2O3 and SiO2, as shown in Figure 4-5. Such behaviour was not observed in the cases of PET/Coke and coke alone, where distinct carbon particles could be clearly indentified. It is interesting to note that there were only marginal differences in the ash composition of PU/Coke char compared to coke (Table 4-4).

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Figure 4-4: Morphology of PET/coke and PU/coke char compared to coke [images at magnifications of 30x (left side) and 1000x (right side)].

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Figure 4-5: SEM images of PU/coke char showing: a) an agglomeration of coke and PU into a large particle and b) morphology of the agglomeration particle at magnifications of 1500x and associated EDS spectra indicating the composition of the spherical particles formed on the surface.

4.2 Dynamic Slag Foaming Behaviour

Figure 4-6 shows the changes in the measured volume ratio (Vt/Vo) as a function of time for PET/Coke and PU/Coke blends compared to that of coke. Coke shows good slag foaming initially (volume ratio higher than 1), which then decreases and fluctuates to around 0.75 for rest of the experiment. Plastic/coke blends showed an improvement in the slag foaming behaviour compared to coke. The PET/Coke blend showed a stable slag foaming behaviour with the volume ratio fluctuating between 1.1 and 1.3 (average 1.2) throughout the experiment. In the case of the PU/Coke blend, the volume ratio increased slightly within the first minute of reaction to 1.5; afterwards, the volume ratio decreased and continued to fluctuate between 0.75-1.2. Captured video images of the slag droplets in contact with coke, PET/Coke and PU/Coke after different reaction times are shown in Figures 4-7 to 4-9.

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Figure 4-6: Changes in the volume ratio of the slag droplet during reaction with a) PET/Coke blend and b) PU/Coke blend at 1550ºC. The changes in the volume ratio for slag reactions with coke are shown as a comparison in both cases.

Figure 4-7: Images showing the dynamic changes in the volume of the slag during reactions with coke at 1550˚C.

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Figure 4-8: Images showing the dynamic changes in the volume of the slag during reactions with PET/Coke blends at 1550˚C.

Figure 4-9: Images showing the dynamic changes in the volume of the slag during reactions with PU/Coke blends at 1550˚C.

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4.3 Off-gas Analysis

4.3.1 CO and CO2 generation

CO and CO2 generated during slag/carbon interactions were measured using an online

IR gas analyser. The amounts (ppm) of CO and CO2 generated during slag/carbon interactions in the case of PET/Coke and PU/Coke blends are shown as a function of time compared to that of coke alone (Figure 4-10).

Figure 4-10: CO and CO2 generated during slag/carbon interactions at 1550˚C for a) PET/Coke and b) PU/Coke blends compared to coke.

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Reactions of the slag with the coke showed that significant amounts of CO and CO2 were generated, with the maximum gases generated reaching approximately 25000 ppm within the first 250 seconds of the reaction. The generation of CO and CO2 was found to decrease when PET/Coke and PU/Coke blends were used, with the maximum amounts of gases reaching approximately 22500 ppm and 22000 ppm within the first 150 seconds of the reaction for PET/Coke and PU/Coke, respectively.

The number of moles of carbon and oxygen removed during slag reaction with PET/Coke and PU/Coke are compared to the case of metallurgical coke alone and shown in Figures 4-11 and 4-12, respectively.

Figure 4-11: Moles of a) Carbon and b) Oxygen removed as a function of time during slag/carbon interactions at 1550˚C for PET/Coke blends compared to coke.

Figure 4-12: Moles of a) Carbon and b) Oxygen removed as a function of time during slag/carbon interactions at 1550˚C for PU/Coke blends compared to coke.

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The number of moles of carbon and oxygen removed during slag/carbon interaction were calculated by Eqs. 4.1 and 4.2, respectively.

Moles of carbon removed = Moles of CO + Moles of CO2 ……………..(4.1)

Moles of oxygen removed = Moles of CO + 2(Moles of CO2)……………...(4.2)

From Figures 4-11 and 4-12, the number of moles of carbon and oxygen removed after 10 minutes of reaction for PET/Coke were approximately 6.5x10-4 moles. In the case of PU/Coke, the number of moles of carbon and oxygen removed were approximately 6.0x10-4 moles. However, these values were observed to be lower than that in the case of coke, for which the number of moles of carbon and oxygen removed after 10 minutes of reaction was approximately 8.5x10-4 moles. The greater number of moles of carbon and oxygen removed from slag reaction with coke indicates a greater extent of reaction between slag and solid carbon in the coke substrate (reduction of FeO) compared to that in the case of polymer/coke blends.

However, despite lower amounts of gases (CO and CO2) generated and lower moles of C and O removed in the case slag reaction with PET/Coke and PU/Coke, better slag foaming behaviour was observed with the volume ratios after 10 minutes of reaction being ~1.2 for PET/Coke and ~0.75-1.2 for PU/Coke, while it was ~0.75 for coke. These results are discussed in terms of the rate of gas generation and the role of volatiles in the polymers in section 4.5.

Both polymer/coke blends contained high levels of volatiles compared to that of coke, which could be composed of H2, CH4, O2 and N2. Some of these volatiles that remained in the polymer/coke char after devolatilization in the drop tube furnace (DTF), especially CH4 and H2 can also participate in the reaction between molten slag and the polymer/coke blends. These can play a role in the slag foaming behaviour observed in the case of polymer/coke blends. Therefore, a further determination of the gaseous species devolatilised from polymer/coke blends during slag/carbon reactions was carried out using a gas chromatographic analyzer (GC), and the results are discussed in the following section.

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4.3.2 Gas Chromatography (GC) Analysis

Due to the blending of polymer with coke, the gases released from the decomposition of the blends as well as from its reactions with the slag are expected to participate in the foaming of slag. Therefore, a gas chromatographic analyzer (GC SRI 8610C) coupled with a thermal conductivity detector (TCD) was used to identify the key constituents of gases generated from the blank carbonaceous substrate (without slag) and the slag- carbonaceous substrate system at 1550ºC. The GC analysis can be used to qualitatively identify the constituents of gaseous species generated. Conversely, the GC analysis is different from the IR analysis which can be used to continuously measure the quantity of gases evolved from the reaction. The chromatograms of the gases evolved from the blank substrates of coke, PET/Coke and PU/Coke are shown in Figure 4-13.

Figure 4-13: Gas chromatographs showing the gases produced from the different blank substrates of a) coke, b) PET/Coke and c) PU/Coke after 1 minute at 1550˚C. 4-15 Chapter 4 Slag/Carbon Interactions between Polymer/Coke Blends and EAF Slag at 1550ºC

In these graphs, the Y-axis represents arbitrary units of gas yield, while the X-axis represents the retention time which is the analytical time of the analyzer to identify the gaseous species and is based on the time taken for the gases to separate in the column detector. Please note that the X-axis values do not correspond to the reaction time for slag/carbon interactions.

Figure 4-13 shows that the major gas detected from the coke substrate was CH4 which could have arisen from the decomposition of the hydrocarbons. Small amounts of CO2 were also detected. In the case of PET/Coke blend, O2 was detected along with H2,

CH4, CO and CO2. PET contains approximately 33.3% oxygen and the O2 detected could account for the oxygen remaining in the char of the PET/Coke blend after devolatilization in the drop tube furnace. A difference is seen in the case of the

PU/Coke blend with CH4, N2 and O2 were detected in the off-gas along with H2, CO and CO2. PU consists of N and O in its polymer chain, and thus the N2 and O2 detected are also expected to be generated from the N2 and O2 remaining in the char of the PU/Coke blend after being devolatilised in the drop tube furnace. These results clearly indicate that the differences in the gases generated from plastic/coke blends arise from the differences in the chemical compositions of the polymer.

Various gases evolving from the reactions of the slag with coke, PET/Coke and PU/Coke blends after 1 and 15 minutes of reaction are shown in Figures 4-14. As compared the chromatogram of slag/carbon reaction after 1 minute (Figure 4-14a) with that for the blank substrates (Figure 4-13), in the presence of slag, H2 and H2O were observed to increase in all cases, while CH4, CO and CO2 peaks were observed to be very small. The observed small peaks of CO and CO2 in the chromatograms are in agreement with the results of the gas chromatogram for the slag reaction with HDPE and metallurgical coke found in the literature [Dankwah (2011)]. On the other hand, the absence of CH4 could be attributed to its decomposition to form C and H2, which is indicated by the H2 detected. In the case of the PU/Coke blend, O2 and N2 were found to decrease. In the case of PET/Coke, the relative proportion of O2 was found to increase unexpectedly compared to the blank PET/Coke substrate. This may have occurred due to the variation in volumes of gases entering the GC analyzer.

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Figure 4-14: Gas chromatograph showing the gases released during slag-coke interaction at 1550˚C after a) 1 minute and b) 15 minutes of reaction for Coke, PET/Coke and PU/Coke.

In the presence of slag (Figure 4-14), the gases detected from slag reactions with coke after 1 minute were H2 and H2O, while the gases detected after 15 minutes of reaction were observed to be H2, CO and H2O. In the case of the PET/Coke blend, the gases detected after 1 minute of reaction were O2 along with H2 and H2O, while the gases detected after 15 minutes were H2, CO and H2O. A slight increase in the magnitude of

H2O peak was observed compared to the case of 1 minute of reaction, and this indicates the reduction of FeO in the slag by H2. For PU/Coke blend, the gases detected after 1 minute of reaction were N2, O2, H2 and H2O. After 15 minutes of reaction, CO, H2 and

H2O were clearly detected as the gaseous products arising from the reactions with slag.

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These results suggest that not only solid C reacts with slag but that the volatiles from the polymers in the form of CH4 and H2 can also participate in the reactions with slag, leading to a reduction of FeO by H2 [Katayama et al.(1982) and Nagasaka et al.

(2000)], which can be observed from the presence of a H2O peak (a reaction product of

FeO reaction with H2) in Figure 4-14. These results are discussed later in section 4.5.1.

4.4 Optical Microscopic Investigations

Optical microscopic images of the cross-sectioned slag droplets after reaction with coke, PET/Coke and PU/Coke are shown in Figures 4-15 to 4-17, respectively. The dark regions represent the entrapped gas bubbles while shiny white regions represent reduced iron.

Figure 4-15: Optical microscopic images of the cross sectioned slag droplets after reaction with coke at different times.

In the case of coke, a large number of small reduced iron droplets were observed both in the bulk slag and at the slag-gas interface after 4 minutes of reaction. After that, large iron droplets were seen to form at the slag-carbon interface. Several entrapped gas bubbles were also observed. The presence of entrapped gas bubbles in the slag region

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after 2 minutes of reaction with coke indicates rapid generation of gases which could lead to good foaming within the initial period of reactions for the slag/coke system. Thereafter, the generation of gas slowed down and the entrapped gas bubbles disappeared from the molten slag and this corresponds to a decreased volume ratio observed for coke (see Figure 4-6).

Figure 4-16: Optical microscopic images of the cross sectioned slag droplets after reaction with PET/Coke at different times.

In the case of PET/Coke blend, reduced iron droplets were seen throughout the reaction times. Entrapped gas bubbles (small and large) were also observed in the bulk slag after varying reaction times. The slag which reacted with the PU/Coke blend showed central gas bubbles of varying sizes trapped in the molten slag as well as small metal droplets after reactions for 15 minutes. These could have an impact on the improvement in slag foaming in the case of PET/Coke and PU/Coke blends compared to coke.

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Figure4-17: Optical microscopic images of the cross sectioned slag droplets after reaction with PU/Coke at different times.

4.5 Discussion

4.5.1 Influence of Gas Generation on Slag Foaming

The rate of gas generation has been reported to have a significant effect on the slag foaming behaviour and this influence was found to depend on the basic characteristics of carbonaceous materials such as ash content in the materials. Rahman et al. (2009) investigated interactions between an EAF slag containing high levels of FeO with metallurgical coke and natural graphite at 1550ºC using the sessile drop technique.

These authors found that the gas generation of both CO and CO2 from the slag/coke assembly was significantly higher than that of slag/graphite, also indicating an extensive/rapid FeO reduction by metallurgical coke. Even though the rate of gas generation for coke was very high, poor gas entrapment was observed within molten slag. Rapid gas generation can lead to the escape of gas bubbles from the slag sample. It was mentioned that high levels of gas generation resulted in a strong likelihood of convective transport of reactants and products across the slag/coke interface with oxides in coke ash partially dissolving in molten slag and modifying slag composition

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[Rahman et al. (2009)]. Natural graphite, on the other hand, showed relatively slow rates of gas generation and slow reduction of FeO; however excellent gas entrapment in the slag sample was observed leading to good slag foaming behaviour [Rahman et al. (2009)].

Slag composition can be affected in two different ways: reduction reactions and possible transfer of ash oxides from coke into molten slag. The extent of FeO reduction as a function of time can inturn lead to the differences in physical properties of the molten slag, such as viscosity, surface tension and density, and this would affect slag foaming [Paramguru et al. (1997)]. The chemical compositions of reacting slags as a function of time need to be measured for quantifying this effect. In the present study, both PET/Coke and PU/Coke blends showed a relatively slow rate of CO and CO2 generation compared to the case of coke; and a greater level of gas entrapment was observed in the slag sample. A higher rate of gas generation in the case of coke may lead to local turbulence at the interface and transfer ash oxides into the molten slag and modify slag composition. The modification of slag composition may lead to a changes in the physical properties of slag, and thus impact the ability of slag to hold the gas [Rahman et al. (2009)]. Conversely, a relatively slower rate of gas generation in the case of polymer/coke blends could have a lower impact on the modification of slag composition and this also allow for gas bubbles to be trapped in the slag sample instead of rapidly escaping from the slag. These resulted in the better slag foaming in the case of the polymer/coke blends compared to coke.

4.5.2 Influence of Volatiles from the Polymers on Slag Foaming

The foaming of slag is predominantly caused by the retention of CO and CO2 generated from the reactions between slag and solid or solute carbon. In the case of coke, FeO in the slag is reduced by the solid carbon as shown by Eq. 4.3. The CO gas generated from this reaction could further reduce FeO through gas-slag reactions, thereby generating CO2 into the system as shown by Eq. 4.4. The CO2 can be consumed via a gas-carbon reaction (Boudouard reaction) leading to CO generation back into the system as shown by Eq. 4.5 [Teasdale and Hayes (2005)].

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FeO(l) + C(s) = Fe(l) + CO(g) …………..….…(4.3)

FeO(l) + CO(g) = Fe(l) + CO2(g) …..………...…..(4.4)

C(s) + CO2(g) = 2CO(g) ….…..….….…..(4.5)

In the present study, cumulative moles of CO and CO2 generated from slag/carbon interactions after 10 minutes in the case of PET/Coke and PU/Coke blends (~6.5x10-4 moles and ~6.0x10-4 moles, respectively) were found to be less than that observed from coke alone (~8.5x10-4 moles). The rate (moles.s-1) of FeO reduction by PET/Coke,

PU/Coke and coke were calculated based on the moles of CO and CO2 monitored using similar approach taken by Min et al. (1999) and Dankwah et al. (2011) and details for the calculation are given in Appendix I.

The rates of FeO reduction by coke and polymer/coke blends have been plotted as a function of time in Figure 4-18. The maximum rate of FeO reduction for each carbonaceous sample is given in Table 4-5.

Figure 4-18: Rate of reaction for PET/Coke and PU/Coke blends in contact with slag at 1550ºC compared to metallurgical coke.

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Table 4-5: Calculated peak of FeO reduction rate (moles.s-1) for metallurgical coke PET/Coke and PU/Coke

Polymeric Materials Reaction rate (moles.s-1)

Coke 1.74 x 10-5

PET/Coke 1.55 x 10-5

PU/Coke 1.54 x 10-5

It can be observed from Figure 4-18 that the reduction of FeO in the case of PET/Coke within ~90 seconds occurred at a rate similar to that observed from coke. However, the reaction rate for PET/Coke was seen to slow down thereafter, while the reaction rate for coke was still increasing. A similar trend was observed for PU/Coke but the reaction rate was slightly lower than that for PET/Coke. The rate of FeO reduction for coke was found to reach a maximum of 1.74x10-5 mole.s-1 at ~220 seconds, while a lower reaction rate was observed in the case of polymer/coke blends with the maximum rates of 1.55x10-5 mole.s-1 (at ~90 seconds) and 1.54x10-5 mole.s-1 (at ~140 seconds) for PET/Coke and PU/Coke respectively. The rates obtained for coke and the polymer/coke blends were in the same order (x10-5) of magnitude as reported by Rahman (2010), Zaharia (2010) and Dankwah et al. (2011) for similar studies on EAF slag with other carbonaceous materials.

Interactions between FeO containing slag with HDPE/Coke and rubber/coke blends at 1550ºC using the sessile drop technique were respectively investigated by Rahman (2010) and Zaharia (2010). Using a similar proportion of polymer/coke blend used in the present study, Rahman (2010) reported that the maximum FeO reduction rate for metallurgical coke was 1.52 x 10-5 mole.cm-2.s-1, while the reaction rate was observed to increase when a HDPE/Coke blend was used with a maximum value of 1.91 x 10-5 mole.cm-2.s-1. Similarly, Zaharia (2010) reported that the maximum rate of FeO reduction for metallurgical coke was 1.89 x 10-5 mole.cm-2.s-1, while the reaction rate was also found to increase when a rubber tyre/coke blend was used with a maximum value of 2.5 x 10-5 mole.cm-2.s-1. Recently, the FeO reaction using HDPE/Coke was also conducted at 1550ºC by Dankwah et al. (2011). By using a different technique

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(slag/coke/HDPE composite pellet), this author reported that the maximum rate of FeO reduction by HDPE/Coke blend was 6.3 x 10-5 mole.s-1, which is approximately two times higher than that reported for metallurgical coke (the maximum rate was 3.3 x 10-5 mole.s-1).

The reaction rate for coke, PET/Coke and PU/Coke in the present study were compared with other values reported in literature and is shown in Table 4-6. It was found that HDPE and rubber showed a higher rate of FeO reduction than that of coke. However, the results observed in the present study indicate an opposite trend, where the rate of FeO reduction for PET/Coke and PU/Coke was found to be lower than that for coke. The differences between the values of reaction rates in the case of metallurgical coke reported in the present study and from the previous studies [Rahman (2010), Zaharia (2010) and Dankwah et al. (2011)], are possibly due to the difference in the properties of slags and cokes used. In the case of polymer/coke blends, the polymer characteristics were found to have a significant effect on their interactions with slag.

Table 4-6: Comparison of the maximum rate of FeO reduction by different carbonaceous materials obtained from literature

Polymeric Materials Reaction rate Researcher

Coke 1.52 x 10-5 mole.cm-2.s-1 Rahman (2010)

HDPE/Coke 1.91 x 10-5 mole.cm-2.s-1 Rahman (2010)

Coke 1.89 x 10-5 mole.cm-2.s-1 Zaharia (2010)

Rubber/Coke 2.5 x 10-5 mole.cm-2.s-1 Zaharia (2010)

Coke 3.30 x 10-5 mole.s-1 Dankwah et al. (2011)

HDPE/Coke 6.30 x 10-5 mole.s-1 Dankwah et al. (2011)

Coke 1.74 x 10-5 mole.s-1 Present study

PET/Coke 1.55 x 10-5 mole.s-1 Present study

PU/Coke 1.54 x 10-5 mole.s-1 Present study

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Table 4-7: Chemical composition of HDPE, rubber tyre, PET and PU

Polymeric C H O N S ash Materials (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) HDPE 85.6 14.4 - - - - Rubber tyre* 84.7 7.6 - - 2.0 5.7 PET 62.5 4.2 33.3 - - - PU 65.6 4.9 20.5 9.0 - - * Zaharia (2010)

As shown in Table 4-7, both HDPE and rubber tyre contain high levels of carbon and hydrogen, while PET and PU contain lower levels of carbon and hydrogen. However, other elements are also found in PET (O) and PU (O and N). Higher carbon and hydrogen in the form of volatile matter (CH4 and H2) in the case of HDPE and rubber tyre can aid and supplement gas generation and FeO reduction when reacted with molten slag. In the present study, although PET and PU contain oxygen in their polymer chains, lower CO and CO2 generation was observed in the case of PET/Coke and PU/Coke than for coke alone. A possible explanation could be due to the fact that PET and PU are low carbon containing polymers compared to HDPE and rubber tyre, and some of the oxygen in the polymer could have been already removed by oxidizing the carbon in the materials during the devolatilization of the samples in the drop tube º furnace (at 1200 C). However, the water vapour (H2O) produced as the reaction products of FeO reduction by H2 was detected in the case of polymer/coke blends and was observed to be higher than that observed in the case of coke, as seen from Figure 4-14.

Blending of coke with polymeric wastes results in the formation of chars that have different properties compared to the parent coke. The chemical compositions of the polymers blended with coke influence the gasification of the char at high temperatures, i.e. extent and type of gases generated due to the break down of the polymer chains at high temperatures. Volatiles released from the polymers may participate in the reduction of FeO which may in turn aid the foaming of the slag. Higher amounts of

CH4 and H2 released from the polymers compared to coke (as seen in Figure 4-13)

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would participate in FeO reduction [Ueki et al. (2008)]. CH4 can react with FeO in the slag and produce metallic Fe, CO2 and H2O into the system (Eq. 4.6). H2 also reduces FeO in slag, but at a faster rate compared to solid carbon [Katayama et al. (1982), Ban-ya et al. (1984) and Nagasaka et al. (2000)]. Moreover, the presence of freshly reduced Fe will act as a catalyst to aid the decomposition of CH4, and this would further react with FeO in the slag (Eqs. 4.7 and 4.8) [Steinfeld et al. (1995)]. The O2 from both PET/Coke and PU/Coke may participate in the combustion of CH4 thereby producing CO2 and H2O in the system as represented by Eq. 4.9 [Barrett (1972)].

CH4(g) + 4FeO(l) = 4Fe(l) + CO2(g) + 2H2O(g).………….(4.6)

CH4(g) = C(s) + 2H2(g)…………..(4.7)

FeO(l) + H2(g) = Fe(l) + 2H2O(g).……...…..(4.8)

CH4(g) + 2O2(g) = CO2(g) + 2H2O(g)…………..(4.9)

H2O(g) + CO(g) = H2(g) + CO2(g)……..…...(4.10)

H2O(g) + C(s) = H2(g) + CO(g)…….…...(4.11)

The occurrence of these reactions is confirmed by the chromatograms of slag/carbon interactions in Figure 4-14. From Figures 4-14, CH4 peaks were not detected since it could have reduced FeO (Eq. 4.6) or decomposed to C and H2 (Eq. 4.7). The H2 would reduce FeO in the slag and produce water vapour (Eq. 4.8). This could explain the higher H2O peak observed in the case of PET/Coke and PU/Coke compared to coke alone. The reaction of H2O with CO through the water gas shift reaction (Eq. 4.10) was not likely to occur at the experimental temperature (1550ºC) in the present study

[Ovesen et al. (1992)]. On the other hand, H2O could be reduced by solid carbon as carbon gasification by water vapour (Eq. 4.11). This reaction occurs above 1000ºC [Biswas (1981)] and is spontaneous at the present experimental temperature (1550ºC).

The decomposition of H2O produces H2 and CO in the system. The produced H2 can further reduce FeO in the slag and thus produce H2O back to system, while the CO produced may diffuse and be retained in the molten slag and sustain slag foaming or further react with FeO (Eq. 4.4).

These cyclic reactions would lead to sustained slag foaming for longer reaction times for the PET/Coke and slag system compared to the case of PU/Coke and coke. It can be

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seen from the chromatograms of slag/carbon interactions after 15 minutes (Figure 4-

14b) that H2O, H2 and CO peaks in the case of PET/Coke blends are higher than in the case of the PU/Coke blend and coke alone. This indicates a greater interaction of slag with PET/Coke (by CH4 and H2) and therefore a sustained slag foaming behaviour. A greater interaction of slag with PU/Coke blend was also observed because it also contains higher amount of volatiles (CH4 and H2) compared to coke. However, the absence of interaction/behaviour may be attributed to the influence of N2 from the polymer.

From the analyses of CO and CO2 gases, the reduction of FeO in the slag by coke was greater than that by PET/Coke and PU/Coke. This indicates a greater FeO reduction by the solid carbon in the coke. However, a better slag foaming behaviour was observed in the case of polymer/coke blends. Thus, it was found that volatiles (H2 and CH4) from the polymers played a role in slag/carbon interactions. To establish the role of volatiles

(H2 and CH4) on the reduction of FeO and the foaming of slag, the slag samples were quenched at different times after reaction with polymer/coke blends and coke. The quenched slag samples were ground into powder and then sieved to separate out reduced metal droplets. The FeO levels in the quenched slag powders were experimentally estimated using x-ray fluorescence (XRF). The FeO levels remaining in the quenched slag samples were plotted as a function of time and shown in Figure 4- 19.

Figure 4-19 Percentages of FeO in the quenched slag samples after reaction with coke, PET/Coke and PU/Coke at different times.

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It can be seen from Figure 4-19 that polymer/coke blends showed a greater extent of FeO reduction compared to coke. The FeO level in the slag samples after 8 minutes of reaction was ~20.4 wt% and 21.8 wt% for PET/Coke and PU/Coke, respectively, while it was ~24.1 wt% for coke. A slightly higher FeO level for PET/Coke and PU/Coke at 2 minutes compared to 1 minute was attributed to the influence of oxygen in the polymer which may reoxidize some of the reduced Fe in the slag and reform FeO into the bulk slag. These results show that FeO in the slag was not only reduced by solid carbon, but also by volatiles (H2 and CH4) in the polymers. These additional reactions can contribute to the improvement in slag foaming behaviour in the case of polymer/coke blends compared to coke. These results therefore present a modification to the results obtained from the CO and CO2 analyses using the IR gas analyzer which only take into account the reduction of FeO by solid carbon.

Several previous studies have indicated that the foamability of slag was affected by the use of different types of gases and these results were reviewed by Ogino et al. (1988). The foam height of slags containing FeO was found to increase in the order of air, Ar,

N2-7 pct CO as the gases used [Ogino et al. (1988)]. These authors speculated that this was due to a decrease in the bubble size as a result of the Marangoni effect induced by chemical reactions.

In the present study, the slag foaming behaviour (Vt/V0) in the case of PET/Coke was found to be better than the case of PU/Coke. The N2 released from the pyrolysis of PU is not expected to participate in the FeO reduction. However, it may affect the foaming of slag by acting as a gaseous solvent to dilute the concentration of other gases species in the system, especially CO and H2, and this could hinder the reactions of CO and H2 with the molten slag. Figure 4-14b shows that the CO and H2 peaks detected after 15 minutes of reaction in the case of PU/Coke blend are still high, while the H2O peak was lower compared to the case of PET/Coke, which could be due to N2 hindering various reactions. On the other hand, the higher H2O, CO and H2 peaks detected after 15 minutes of reaction in the case of PET/Coke (Figure 4-14b) indicate that the reaction between FeO in the slag with CO and H2 was still maintained in the absence of N2. N2 can also affect the foaming behaviour by forming larger sized gas bubbles in the slag droplet.

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4.5.3 Influence of Entrapped Gas Bubbles in the Bulk Slag on Slag Foaming

The foaming of slag is a result of the gas bubbles being retrained in the bulk liquid slag, and is influenced by amount and size of the entrapped gas bubbles. Zhang and Fruehan (1995)1 studied the effects of gas bubble size generated by argon gas injection into liquid slag and by the slag/metal interfacial reactions between the solute carbon in liquid metal and FeO in the slag on slag foaming. These authors concluded that foams with very fine bubbles have spherical bubble cells and are very stable, while foams with larger bubbles are less stable [Zhang and Fruehan (1995)]1.

Rahman (2010) measured the diameters of entrapped gas bubbles in the quenched droplets after reaction with coke and HDPE/Coke blends. This author reported that the minimum gas bubble diameter of the quenched slag droplets after 2, 4 and 8 minutes of reaction with coke was 90, 92 and 92 μm, respectively, while it was 29, 36, 39 μm in the case of HDPE/Coke. The smaller gas bubble diameter observed in the case of HDPE/Coke blend compared to the case of coke is in good agreement with the slag foaming (volume ratio) results from this previous study [Rahman (2010)]. The author reported the volume ratio for HDPE/Coke was in a range of 1-4, while it was <1 for coke alone.

In the present study, it can be observed from Figures 4-15 to 4-17 that the slag droplets after reaction with PET/Coke and PU/Coke blends showed a greater number of gas bubbles entrapped compared to that observed with coke. The size distribution of entrapped gas bubbles also contributes to the differences in slag foaming behaviour of the samples. The diameters of the gas bubbles entrapped in the slag droplet after reaction with coke and the carbonaceous blends were classified into 2 ranges based on their sizes. Bubbles were classified as small if their size < 100 μm and large if their size >100 μm. The ranges of diameters of the small gas bubbles entrapped in the slag are compared with the results from previous study [Rahman (2010)] in Table 4-8.

1 Effect of the bubble size and chemical reactions on slag foaming

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Table 4-8: The range of diameters of small gas bubbles entrapped in the slag droplet for coke and its blends with polymer between 2 – 8 minutes of reaction

Range of Small Bubbles Diameters Samples Researchers (μm)

Coke 44-75 Present study

PET/Coke 36-62 Present study

PU/Coke 45-68 Present study

Coke 90-92 Rahman (2010)

HDPE/Coke 29-39 Rahman (2010)

It was observed that the range of small bubbles diameters in the case of PET/Coke and PU/Coke observed between 2-8 minutes of reaction were somewhat smaller than the corresponding values for coke (44-75 μm). Among the blends, PET/Coke showed the smallest gas bubbles (36-62 μm). The small gas bubbles are generated from the reduction reactions of FeO and these bubbles could contribute to both sustaining and improving the slag foaming behaviour. These results show a trend similar to that reported by Rahman (2010) showing that the gas bubble sizes for polymer/coke blends are generally smaller than that for coke alone, which is expected to lead to a better slag foaming behaviour. The bubble diameters in the case of HDPE/Coke reported by Rahman (2010) were even smaller than that of PET/Coke and PU/Coke observed in the present study. This can explain a better slag foaming behaviour for HDPE/Coke blend (volume ratio ranged ~1-4) reported by Rahman (2010) compared to the case of PET/Coke (volume ratio ~1.2) reported in the present study.

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4.6 Summary

Our experimental results have shown that PET and PU can be utilized in the EAF steelmaking process as a slag foaming agent after blending with metallurgical coke. Slag/carbon interactions for coke and polymer blends were investigated and it was seen clearly that interaction of polymer/coke blends with slag was significantly different from that with coke. The major findings from this work are summarized below:

1. PET/Coke blend showed sustained slag foaming with the volume ratio stabilizing at 1.2 over the reaction time, while PU/Coke blend showed a fluctuating slag foaming behaviour with the volume ratio ranging between 0.75- 1.2. These ratios are greater than that observed from the case of coke where the volume ratio was ~1 initially and then decreased with time to reach approximately 0.75.

2. Despite lower levels of CO and CO2 gases generated from slag/carbon interactions in the case of PET/Coke and PU/Coke (6.5x10-4 and 6.0x10-4 moles, respectively) compared to coke (8.5x10-4 moles), the slag foaming behaviour in the case of polymer/coke blends was found to be consistently

better. The slower rate of CO and CO2 generation allowed for greater gas entrapment in the slag sample and thus better slag foaming.

3. H2 and CH4 released from the decomposition of polymer chains at high temperatures were seen to participate in the reduction of FeO in the slag and these were retained for longer times, thereby aiding the slag foaming. 4. The chemical composition of the polymeric materials was seen to play a critical role in the gasification behaviour of the blends, and this in turn modified the

slag foaming behaviour. O2 released from the plastics led to the combustion of

CH4. N2 from PU/Coke blend was found to affect the foaming behaviour, and also diluted the concentration of the other gaseous species in the system, and thereby caused a hindrance to the reactions with molten slag. 5. Present study has shown that waste polymers can be recycled in EAF steelmaking. These polymers had consistently showed a good slag foaming and iron oxide reduction behaviour.

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CHAPTER 5

WETTABILITY AND CARBON TRANSFER BETWEEN POLYMER/COKE BLENDS AND MOLTEN STEEL AT 1550ºC

Chapter 5 Wettability and carbon transfer between polymer/coke blends and molten steel at 1550ºC

Results and discussion on the high temperature interactions of polymer/coke blends with molten steel are presented in Chapters 5 through 7. These studies include investigations on wettability, the transfer of carbon and sulphur into the molten steel and interfacial phenomena occurring at the metal/carbon interface. This chapter reports in-depth characterization on polymer/coke blends, experimental results on wettability of polymer/coke blends by molten steel and carbon and sulphur transfer into the melt at 1550ºC. Three polymers including two thermoplastics (HDPE and PET) and a thermosetting plastic (Bakelite) were investigated.

5.1 Bakelite/Coke Blends

5.1.1 Characterization of Bakelite/Coke Blends

The polymers and metallurgical coke were blended in three different ratios: Blend 1, Blend 2 and Blend 3. These were labelled BK1, BK2, BK3; H1, H2, H3 and P1, P2, P3 for Blends 1, 2 and 3 of Bakelite/Coke, HDPE/Coke and PET/Coke, respectively. The proportion between the plastics and metallurgical coke in the polymer/coke blends was reproduced and shown in Figure 5-1.

Figure 5-1: Relative proportions between the plastics and metallurgical coke in the polymer/coke blends.

The raw polymer/coke blends were combusted in a drop tube furnace (DTF) at 1200ºC under an atmosphere of 80% nitrogen and 20% oxygen with a gas flow rate of 1.0 L/min, and the chars obtained were used for steel/carbon interaction investigations. The

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devolatilization was carried out to simulate the real process in EAF steelmaking. In EAF steelmaking process, carbonaceous materials are injected into the furnace and these undergo partial combustion before coming into contact the molten steel.

5.1.1.1 XRD Analysis

XRD patterns of raw Bakelite are compared with that of coke and are shown in Figure 5-2. XRD patterns of raw Bakelite/Coke blends (prior to heating in the DTF) and chars produced from the different blends are shown in Figures 5-3.

Figure 5-2: XRD patterns of a) Coke and b) Raw Bakelite.

The XRD patterns of the bakelite/coke chars show the presence of CaCO3 and CaO peaks from the bakelite (Figure 5-3 b). CaO would generate from the decomposition of

CaCO3 during heating in the DTF, while some of CaCO3 may have undergone partial decomposition when heated in the DTF. A carbon peak (002) observed from the XRD patterns of the chars produced from the different blends was found to be similar to that of coke alone.

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Figure 5-3: XRD patterns of Bakelite/Coke blends: a) Raw blends and b) Chars.

5.1.1.2 Chemical Analysis

The proximate and ash analyses of the chars of the polymer/coke blends were conducted by Amdel laboratory, Cardiff, NSW Australia. The proximate analyses results for raw bakelite/coke blends and the bakelite/coke chars produced are given in Tables 5-1 and 5-2, respectively. The ash analysis results for raw bakelite/coke blends and the bakelite/coke chars produced are given in Tables 5-3 and 5-4, respectively. The blending of bakelite with coke was found to modify the chemical composition of the blends to some extent. For raw bakelite/coke blends, fixed carbon was observed to decrease, while volatiles and ash contents were observed to increase as increasing bakelite levels in the blends; their sulphur content was comparable. In the case of bakelite/coke chars, similar trend was observed for fixed carbon and ash contents, while 5-4 Chapter 5 Wettability and carbon transfer between polymer/coke blends and molten steel at 1550ºC

volatiles and sulphur levels in the blends were comparable to that of the parent coke. The blending of bakelite with coke was found to significantly modify the ash composition of both raw blends and the chars produced. CaO content in the ash was observed to increase significantly with increasing bakelite content in the blends compared to the case of coke, while SiO2 and Al2O3 were found to decrease. This modification is attributed to the presence of CaCO3 additive in the bakelite. CaCO3 is generally found in thermoset polymer as a filler material. It is added into the polymer to modify properties of polymer properties, such as thermal conductivity, surface hardness and stiffness, and also to reduce the production costs [Murphy (2003) and Rosen (1937)].

Table 5-1: Chemical composition of the bakelite/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.80 3.00 17.20 0.29 BK1 72.95 7.75 19.60 0.26 BK2 68.56 8.87 22.57 0.25 BK3 63.10 10.25 26.65 0.28

Table 5-2: Chemical composition of the bakelite/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.80 3.00 17.20 0.29 BK1 76.30 3.20 20.50 0.27 BK2 73.40 3.10 23.50 0.25 BK3 68.40 3.30 28.30 0.27

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Table 5-3: Ash analyses of the bakelite/coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 BK1 57.10 28.80 2.40 5.10 0.61 1.10 0.60 0.38 0.21 BK2 52.40 26.70 2.50 10.10 0.54 0.95 1.10 0.32 0.22 BK3 47.50 22.60 2.30 17.90 0.53 0.81 1.68 0.34 0.20

Table 5-4: Ash analyses of the bakelite/coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 BK1 56.90 28.90 2.30 5.40 0.59 1.00 0.61 0.36 0.22 BK2 52.80 26.20 2.30 10.80 0.55 0.90 1.00 0.33 0.20 BK3 47.30 22.80 2.20 18.30 0.52 0.77 1.70 0.35 0.18

5.1.1.3 Morphology Studies

The microstructural morphology of the polymer/coke blends was analysed using Scanning Electron Microscopy (SEM, Hitachi S3400X). Microstructural morphology of chars produced from bakelite/coke blends was compared to coke and are shown in Figure 5-4. High amounts of ash were observed deposited on the char’s surface; only small numbers of pores were observed (for blend BK3). The ash deposited on the char’s surface was found to be predominantly composed of CaCO3 and CaO as evidenced by the EDS spectra of char produced from blend BK3 shown in Figure 5-5. 5-6 Chapter 5 Wettability and carbon transfer between polymer/coke blends and molten steel at 1550ºC

Figure 5-4: Microstructural morphology of Bakelite/Coke blends compared to coke [SEM images taken at magnifications of 30X (left) and 1000X (right)].

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Figure 5-5: Morphology of blend BK3 char’s surface along with the EDS analysis indicating the deposition of CaO on the surface due to the decomposition of CaCO3 from the bakelite.

5.1.2 Wettability of Bakelite/Coke Blends

The wetting images of molten steel droplet on coke, blends BK1, BK2 and BK3 after 60 minutes of reactions are shown in Figures 5-6 to 5-9 respectively.

Figure 5-6: Wetting images of steel droplet after reaction with coke for 60 minutes at 1550ºC.

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Figure 5-7: Wetting images of steel droplet after reaction with blend BK1 for 60 minutes at 1550ºC.

Figure 5-8: Wetting images of steel droplet after reaction with blend BK2 for 60 minutes at 1550ºC.

Wetting results showed variations in contact angle between the bakelite/coke blends. A general improvement in the wettability of the liquid steel with bakelite/coke blends was observed as compared to wettability with coke alone. The variations in the measured contact angles of liquid steel on bakelite/coke blends and coke for 60 minutes of reaction have been plotted in Figures 5-10 to 5-12.

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Figure 5-9: Wetting images of steel droplet after reaction with blend BK3 for 60 minutes at 1550ºC.

Figure 5-10: Variation of contact angles of steel droplet with blend BK1 and coke substrates at 1550ºC.

From Figure 5-10, it can be seen that the liquid steel droplet showed non-wetting behaviour with the coke (contact angle 129º) initially and the value increased slightly within the first few minutes. A very small change was observed throughout the experimental run with the contact angle reaching 128º after 60 mins of contact. Bakelite containing blends were also found to exhibit non-wetting behaviour with liquid steel. The liquid steel droplet exhibited better wetting with blend BK1 substrates with the measured contact angles being approximately 10º lower than that seen with coke alone. The initial contact angle was 118º which increased to 121º after 60 minutes of reaction. In the case of blend BK2 (Figure 5-11), the initial contact angle was 123º which

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decreased to 120º after 15 minutes and then increased to almost 139º after 60 minutes. In the case of BK3 (Figure 5-12), the initial contact angle was 131º which initially decreased to 120º after 25 minutes and then increased to 129º after 60 minutes.

Figure 5-11: Variation of contact angles of steel droplet with blend BK2 and coke substrates at 1550ºC.

Figure 5-12: Variation of contact angles of steel droplet with blend BK3 and coke substrates at 1550ºC.

The wetting behaviour observed in the case of blends BK2 and BK3 was similar to that found in natural graphite and liquid iron system reported by Wu et al. (2000). These authors investigated the wetting behaviour of natural graphite with liquid iron at 1600ºC using the sessile drop technique. They observed a decrease in contact angles (from ~100º to ~60º) along with a rapid increase in carbon content in the metal droplet within the first 10 seconds of reaction. Thereafter, the melt carbon content was observed to slightly

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decrease and the contact angles were observed to considerably increase from ~60º to ~100º within approximately 2 minutes of contact and then stabilize for the rest of the reaction times. Wu et al. (2000) explained that the decrease in contact angles within first the 10 seconds was due to the decrease in interfacial tension due to the dissolution of carbon into the melt. Thereafter, the contact angles increased because of the slowing down in carbon dissolution, since the phenomena related to the formation of ash layer at the metal/carbon interface became predominant (interfacial blockage and consumption of solute carbon by reducible oxides). The carbon dissolution into the metal droplet and the ash deposits in the interfacial region were also expected to influence the wetting behaviour between molten steel and bakelite/coke blends.

5.1.3 Carbon-Sulphur Transfer from Bakelite/Coke Blends

The experimental investigations on the transfer of carbon and sulphur from bakelite/coke blends into molten steel were conducted using the sessile drop technique for a range of times at 1550ºC. Quenched metal droplets, which were in contact with carbonaceous substrates for varying times, were washed using ethanol to remove any debris and carbon particles adhered to the surface. Carbon and sulphur levels in the metal droplets were then measured using a carbon-sulphur analyser (LECO CS230).

5.1.3.1 Carbon Transfer

The variation in the carbon picked up by liquid steel as a function of time for bakelite/coke blends compared to that of coke are shown in Figure 5-13. For metallurgical coke, the carbon concentration in liquid steel picked up quite slowly and reached a maximum of ~0.1 wt% after 60 minutes of contact. The level of carbon pick- up from coke was significantly below the saturation level of 5.28 wt% at 1550ºC; longer reaction times may be required for the system to reach a state of thermodynamic equilibrium. The carbon pick-up results for coke are in agreement with previous carbon pick-up studies carried out using the same technique [McCarthy et al. (2003)].

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Figure 5-13: Carbon picked up from Bakelite/Coke blends by liquid steel at 1550ºC compared to coke.

However, an improvement in carbon transfer was observed when bakelite/coke blends were used. For blend BK1, the carbon concentration in the melt picked up slowly and reached a maximum of ~0.13 wt% after 60 minutes of contact. In the case of blend BK2, faster carbon transfer was observed with the carbon concentration in the melt reaching a maximum of ~0.16 wt% after 4 minutes of contact and stabilizing for over 60 minutes. In the case of BK3, the carbon concentration in the liquid steel reached a maximum of ~0.19 wt% after 60 minutes of reaction and there was little subsequent carbon transfer.

5.1.3.2 Sulphur Transfer

Sulphur can transfer to the melt concurrently with the carbon transfer. The variation in the amount of sulphur transferred to liquid steel from coke and bakelite/coke blends is shown in Figure 5-14. Sulphur transfer from coke into liquid steel stabilized ~0.05 wt% within a few minutes. For blends BK1 and BK2, a similar trend of sulphur transfer was observed, with the level of sulphur picked up by the melt increasing from approximately ~0.08 wt% (after 2 minutes of contact) to ~0.12 wt% after 60 minutes of contact. With further increase in the bakelite content (for blend BK3) the overall sulphur pick-up was

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lower. Sulphur pick up stabilized ~0.06 wt% after 60 minutes of reaction. The decreased sulphur pick-up in the case of blend BK3 compared to the other blends could be attributed to greater desulphurization of the melt which could have occurred due to the CaO content being the highest among three bakelite/coke blends.

Figure 5-14: Sulphur transferred from Bakelite/Coke blends into liquid steel at 1550ºC compared to coke.

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5.2 HDPE/Coke Blends

5.2.1 Characterization of HDPE/Coke Blends

HDPE was blended with metallurgical coke in 3 different ratios (H1, H2 and H3). The chars produced from the combustion of the HDPE/Coke blends were analysed and used for steel/carbon interactions experiments.

5.2.1.1 XRD Analysis

XRD patterns of raw HDPE are compared with coke and shown in Figure 5-15. XRD patterns of raw HDPE/Coke blends and chars produced from the different HDPE/Coke blends are shown in Figures 5-16.

Figure 5-15: XRD patterns of a) Coke and b) Raw HDPE.

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The XRD patterns of the raw blends show two sharp peaks of HDPE at the 2θ positions of 21.5º and 23.8º, and the peaks became more prominent with increasing HDPE content. In the char samples (after DTF), HDPE peaks were detected in blend H3, but not in the other blends. This suggests that HDPE may have undergone partial decomposition when heated in the DTF.

Figure 5-16: XRD patterns of HDPE/Coke blends: a) Raw blends and b) Chars.

5.2.1.2 Chemical Analysis

The proximate analyses results for raw HDPE/Coke blends and the HDPE/Coke chars produced are given in Tables 5-5 and 5-6, respectively. The ash analyses results for raw HDPE/Coke blends and the HDPE/Coke chars produced are given in Tables 5-7 and 5- 8, respectively. The blending of HDPE with coke was found to modify the chemical

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composition of the blends compared to coke. For raw HDPE/Coke blends, a significantly increase in volatiles level was observed with increasing HDPE levels in the blends, while a decrease in fixed carbon and ash contents was observed. For HDPE/Coke chars, volatile content left after devolatilization also increased with increasing HDPE level in the blends, while a slightly decreased in fixed carbon content was seen. HDPE used in the present study was not found to contain any impurities, such as mineral oxide (as a filler material) or sulphur. The sulphur and ash content in the HDPE/Coke blends were comparable to coke, and the ash composition was also comparable.

Table 5-5: Chemical composition of the HDPE/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 H1 73.10 12.15 14.75 0.30 H2 62.70 23.90 13.40 0.29 H3 51.83 35.96 12.21 0.29

Table 5-6: Chemical composition of the HDPE/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 H1 78.90 3.50 17.60 0.29 H2 77.40 5.20 17.40 0.29 H3 75.90 7.60 16.50 0.27

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Table 5-7: Ash analyses of the HDPE/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 H1 61.30 31.60 2.10 0.81 0.65 1.20 0.26 0.33 0.25 H2 61.80 31.80 2.05 0.84 0.66 1.00 0.25 0.32 0.23 H3 61.20 31.50 2.17 0.81 0.69 1.10 0.23 0.33 0.25

Table 5-8: Ash analyses of the HDPE/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt%) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 H1 61.20 31.50 2.20 0.87 0.67 1.10 0.24 0.34 0.24 H2 61.60 31.60 2.20 0.85 0.68 1.10 0.24 0.37 0.24 H3 60.90 31.40 2.10 0.86 0.72 1.10 0.24 0.30 0.22

5.2.1.3 Morphology Studies

Microstructural morphology of chars produced from HDPE/Coke blends were compared to coke and are shown in Figure 5-17. The significant differences in morphology of chars produced from HDPE/Coke blends and coke are the presence of pores on the chars’ surface. Due to the release of gases resulting from the decomposition of the polymers, a large number of pores are seen in these materials. The pore size was seen to increase with increasing HDPE content in the blends. Such pores were clearly not present in the case of coke.

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Figure 5-17: Microstructural morphology of HDPE/Coke blends compared to coke [SEM images taken at magnifications of 50X (left) and 1000X (right)].

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5.2.2 Wettability of HDPE/Coke Blends

The wetting images of molten steel droplet on blends H1, H2 and H3 after 60 minutes of reactions are shown in Figures 5-18 to 5-20 respectively.

Figure 5-18: Wetting images of steel droplet after reaction with blend H1 for 60 minutes at 1550ºC.

Figure 5-19: Wetting images of steel droplet after reaction with blend H2 for 60 minutes at 1550ºC.

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Figure 5-20: Wetting images of steel droplet after reaction with blend H3 for 60 minutes at 1550ºC.

The measured contact angles of liquid steel droplet on HDPE/Coke blends and coke for 60 minutes of contact are plotted in Figures 5-21 to 5-23. As evident in Figure 5-21, the blend H1 showed a decrease in contact angles over the reaction time with the measured contact angles being approximately 10º lower than that observed in the case of coke.

Figure 5-21: Variation of contact angles of steel droplet with blend H1 and coke substrates at 1550ºC.

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Figure 5-22: Variation of contact angles of steel droplet with blend H2 and coke substrates at 1550ºC.

Figure 5-23: Variation of contact angles of steel droplet with blend H3 and coke substrates at 1550ºC.

A decrease in contact angles was also observed in the case of blend H2 with the contact angles decreasing from 129º to 114º within 30 minutes of contact. In the case of blend H3 (Figure 5-23), a relative smaller variation in contact angles was observed with the measured initial contact angles being 127º which decreased to 122º after 60 minutes. It can be seem from the experimental results that the contact angles for the different HDPE/Coke blends increased marginally with increasing HDPE content in the blends. Also, a general improvement in wettability has been observed when HDPE/Coke blends were used compared to the parent coke.

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5.2.3 Carbon-Sulphur Transfer from HDPE/Coke Blends

5.2.3.1 Carbon Transfer

The carbon pick-up as a function of time for HDPE/Coke blends are compared to coke and shown in Figure 5-24. An improvement in carbon transfer was not observed when blends H1 and H2 were used. However, a marginal improvement in the carbon transfer was seen in the case of blend H3.

Figure 5-24: Carbon picked up from HDPE/Coke blends by liquid steel at 1550ºC compared to coke.

The carbon transfer for blend H1 occurred slowly during initial 30 minutes of reaction with the carbon pick-up stabilizing at ~0.06 wt%. A slight increase in carbon picked was observed thereafter; the carbon content in the melts reached ~0.15 wt% after 60 minutes. Carbon transfer in the case of H2 was also observed to occur slowly, however, it was seen to pick-up slightly earlier than H1; the result from Figure 5-24 shows a slow carbon transfer to ~15 minutes of contact with melt carbon content stabilizing at ~0.06 wt%. Thereafter, the melt carbon content increased and reached 0.15 wt% after 60 minutes of reaction. A marginal difference in carbon transfer was observed in the case of blend H3; carbon pick-up occurred faster within 30 minutes compared to the case of 5-23 Chapter 5 Wettability and carbon transfer between polymer/coke blends and molten steel at 1550ºC

H2, H1 and coke. The carbon content in the metal droplet increased gradually from ~0.03 wt% within 0.5 minute and reached ~0.17 wt% within 30 minutes of reaction. The carbon transfer was observed to slow down and the melt carbon content stabilized over 60 minutes of reaction.

5.2.3.2 Sulphur Transfer

The variation in the amount of sulphur transferred to liquid steel from coke and HDPE/coke blends is shown in Figure 5-25. Sulphur transfer from blends H1 and H2 were found to be marginal higher than that observed from coke alone with sulphur pick- up by the melts ranging between 0.06-0.07 wt%. Sulphur pick-up in the case of blend H3 was observed to be slightly lower than that of the blends H1 and H2 with the overall sulphur content in the melt stabilizing at ~0.05 wt% for 60 minutes of contact. This was comparable to the corresponding results observed for metallurgical coke.

Figure 5-25: Sulphur transferred from HDPE/Coke blends into liquid steel at 1550ºC compared to coke.

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5.3 PET/Coke Blends

5.3.1 Characterization of PET/Coke Blends

PET was blended with coke in 3 proportions (P1, P2 and P3). The chars produced from the combustion of the PET/Coke blends were used for steel/carbon interactions experiments.

5.3.1.1 XRD Analysis

XRD patterns of raw PET and coke are presented in Figure 5-26. XRD patterns of raw PET/Coke blends and chars produced from the different PET/Coke blends are shown in Figures 5-27.

Figure 5-26: XRD patterns of a) Coke compared to b) Raw PET.

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The broad peak of PET can be seen in the case of raw PET/Coke blends. However the PET peaks could not be seen for the blends after combustion in the DTF. The XRD patterns for the chars appear similar to that of coke alone; the 002 carbon peak of the chars produced from the different PET/Coke blends was observed to be similar to that of coke alone.

Figure 5-27: XRD patterns of PET/Coke blends: a) Raw blends and b) Chars.

5.3.1.2 Chemical Analysis

The proximate analyses results for raw PET/Coke blends and the PET/Coke chars produced are given in Tables 5-9 and 5-10, respectively. The ash analyses results for raw PET/Coke blends and the PET/Coke chars produced are given in Tables 5-11 and 5-12, respectively. The blending of PET with coke was found to modify the chemical composition of the blends, especially volatiles levels. For raw PET/Coke blends, a

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significantly increase in volatiles level and a decrease in fixed carbon content was observed with increasing PET levels in the blends; ash and sulphur contents were comparable. A similar trend was seen for the PET/Coke chars. The chars produced (Table 5-10) were observed to have a lower fixed carbon and higher volatiles contents than coke. Unlike bakelite, the PET used in the present study was not found to contain any impurities. The PET addition was not observed to produce a significant difference in term of ash content and composition in chars of the PET/Coke blends compared to the parent coke (see Table 5-12).

Table 5-9: Chemical composition of the PET/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 P1 72.34 11.12 16.54 0.25 P2 64.15 18.65 17.20 0.27 P3 60.05 23.05 16.90 0.28

Table 5-10: Chemical composition of the PET/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Fixed Carbon Volatile Ash Sulphur Materials (wt%) (wt%) (wt%) (wt%) Coke 79.8 3.00 17.20 0.29 P1 78.30 4.50 17.20 0.26 P2 74.00 7.20 18.80 0.25 P3 70.50 10.30 19.20 0.27

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Table 5-11: Ash analyses of the PET/Coke blends before passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt% db) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 P1 60.40 31.25 2.50 1.10 0.70 1.10 0.41 0.31 0.27 P2 60.60 31.80 2.30 1.30 0.71 1.00 0.42 0.33 0.29 P3 59.90 31.60 2.40 1.20 0.69 1.10 0.36 0.32 0.50

Table 5-12: Ash analyses of the PET/Coke blends after passing through drop tube furnace (DTF) compared to raw coke. Analysis was done by Amdel, Industrial services division, NSW.

Carbonaceous Ash (wt% db) Materials SiO2 Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O Na2O Coke 61.10 32.10 1.60 0.71 0.68 1.00 0.17 0.29 0.19 P1 60.30 31.00 2.60 1.20 0.71 1.10 0.40 0.29 0.25 P2 60.50 31.60 2.40 1.40 0.73 1.10 0.39 0.34 0.30 P3 59.80 31.40 2.10 1.40 0.70 1.00 0.34 0.33 0.53

5.3.1.3 Morphology Observation

Microstructural morphology of chars produced from PET/Coke blends are compared to coke and shown in Figure 5-28. They appear quite similar to that observed in HDPE containing blends in terms of the pore size. The pore size was also seen to increase with the increasing PET content in the blends.

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Figure 5-28: Microstructural morphology of PET/Coke blends compared to coke [SEM images taken at magnifications of 50X (left) and 1000X (right)].

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5.3.2 Wettability of PET/Coke Blends

The wetting images of molten steel droplet on blends P1, P2 and P3 after 60 minutes of reactions are shown in Figures 5-29 to 5-31 respectively.

Figure 5-29: Wetting images of steel droplet after reaction with blend P1 for 60 minutes at 1550ºC.

Figure 5-30: Wetting images of steel droplet after reaction with blend P2 for 60 minutes at 1550ºC.

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Figure 5-31: Wetting images of steel droplet after reaction with blend P3 for 60 minutes at 1550ºC.

Figure 5-32: Variation of contact angles of steel droplet with blend P1 and coke substrates at 1550ºC.

The measured contact angles of molten steel droplet on PET/Coke blends after 60 minutes of contact compared to coke alone are plotted in Figures 5-32 to 5-34. A small variation in contact angles was observed in the cases of PET/coke blends compared to the case of coke. For blend P1 (Figure 5-32), the initial contact angle was 124º which decreased slightly to 121º after 60 minutes of contact. For blend P2 (Figure 5-33), a slight increase in contact angles was observed compared to blend P1, and the values were comparable to the case of coke. The initial contact angle was 128º which decreased to 127º after 60 minutes. In the case of P3 (Figure 5-34), the initial contact angle was 134º which decreased to 123º after 60 minutes. A decrease in contact angles (~10º)

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observed for blend P3 could have an impact on the carbon transfer into the melt due to the increase in contact area between the two phases.

Figure 5-33: Variation of contact angles of steel droplet with blend P2 and coke substrates at 1550ºC.

Figure 5-34: Variation of contact angles of steel droplet with blend P3 and coke substrates at 1550ºC.

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5.3.3 Carbon-Sulphur Transfer from PET/Coke Blends

5.3.3.1 Carbon Transfer

Carbon pick-up results by liquid metal from PET/coke blends are presented in Figure 5- 35. For blend P1, no improvement in carbon dissolution behaviour was observed. The carbon concentration in the melt picked up slowly and reached a maximum of ~0.09 wt% after 60 minutes of contact. A slightly improvement in carbon transfer was observed for blends P2 and P3. In the case of blend P2, the carbon concentration in the molten steel reached ~0.1 wt% after 15 minutes of contact and then increased to a maximum of ~0.15 wt% after 60 minutes of reaction. A similar trend was observed in the case of blend P3.

Figure 5-35: Carbon picked up from PET/Coke blends by liquid steel at 1550ºC compared to coke.

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5.3.3.2 Sulphur Transfer

The variation in the amount of sulphur transferred to liquid steel from coke and PET/Coke blends is shown in Figure 5-36. For blends P1 and P2, the level of sulphur picked up by the liquid steel was higher than that in the case of coke. The sulphur levels increased slightly from ~0.05 wt% after 0.5 minute to ~0.08 wt% after 4 minutes of reaction and were observed to stabilize for over 60 minutes of contact. With further increase in the PET level (for blend P3) the overall sulphur pick-up was somewhat lower compared to the other blends. Sulphur pick-up stabilized to ~0.05 wt% for 60 minutes of reaction which is comparable to that in the case of coke alone.

Figure 5-36: Sulphur transferred from PET/Coke blends into liquid steel at 1550ºC compared to coke.

5.4 Effect of Blending Polymers with Coke on Wettability

The wettability of carbonaceous materials (graphite, coal chars and coke) by liquid Fe has been investigated previously by several researchers [Wu (1998), McCarthy (2004) and Cham (2007)]. Specific characteristics of carbonaceous materials may have an influence on their wettability with liquid iron. The contact angle was reported to depend

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on characteristics of the materials, such as ash contents and compositions, and carbon structure (crystallinity) [Wu (1998)]. Graphite which is a high crystallinity material and has low ash content was reported to exhibit good wetting behaviour with liquid iron (contact angles <90º) [Wu (1998)]. On the other hand, coal-chars and metallurgical coke which are low crystallinity materials and have high ash content were found to exhibit non-wetting behaviour with liquid iron (contact angles >90º) [McCarthy (2004) and Cham (2007)]. The initial contact angle and the contact angles after 120 minutes for electrolytic pure iron and coal-chars and coke systems obtained from the previous studies are compared in Table 5-13.

Table 5-13: Comparison of the initial and final (after 120 minutes) contact angles of pure liquid Fe with chars and metallurgical coke obtained from previous studies

Contact Angle (º) Temperatures Systems º References ( C) After 120 Initial* minutes Fe/Char** 1550 106º -137º 101º -119º McCarthy (2004) Fe/Coke 1550 107º 111º McCarthy (2004) Fe/Coke** 1550 123º -129º 109º -114º Cham (2007) *The initial and final contact angles values obtained from these researchers were measured within first few minutes for initial contact angle and approximately after 120 minutes for final contact angle. **The contact angles values were measured from varieties of chars and cokes.

The contact angle of liquid iron and metallurgical coke system has been reported previously. Cham (2007) investigated the wettability of liquid iron with a variety of cokes at 1550ºC using the sessile drop technique and reported that liquid iron exhibits a non-wetting behaviour with coke with the initial contact angle ranging between 123º- 129º and slightly decreased to 109º-114º after 120 minutes of contact. Other studies on coke/electrolytic pure Fe at 1550ºC have also reported that the metal droplet exhibits non-wetting behaviour with the metallurgical coke substrate where the initial contact angle was 107º and that slightly increased to 111º after 120 minutes of contact [McCarthy (2004)]. The differences in contact angles found from the previous studies were attributed to the differences in characteristics of the materials.

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In the present study, the coke showed non-wetting behaviour with liquid steel with the measured initial contact angle was 129º and it is comparable to that reported in the literature [Cham (2007)]. The addition of polymers to metallurgical coke produced differences in the characteristics of the carbonaceous blends. HDPE/Coke and PET/Coke blends were found to have high volatile content compared to the parent coke.

On the other hand, bakelite/Coke blends contained higher level of CaO and CaCO3 which originated from fillers present in the bakelite. These differences could have been responsible for the observed for differences in wetting behaviour with liquid steel.

The polymer/coke blends also showed non-wetting behaviour with liquid steel. However, a small improvement in wettability can be observed. Figures 5-37 and 5-38 show a broad comparison of the contact angles of coke and its blends with polymers after 1 minute and 60 minutes of contact, respectively. Typical variations in the initial and final contact angles between the samples investigated were in a range of approximately 20º.

Figure 5-37: Comparison of the initial contact angles (after1 minute) of the steel droplet with coke and its blends with polymers.

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Figure 5-38: Comparison of the final contact angles (after 60 minutes) of the steel droplet with coke and its blends with polymers.

5.5 Effect of Blending Polymers with Coke on Carbon Transfer

Figure 5-39 shows a comparison of the carbon pick up by molten steel droplets after reaction with coke and polymer/coke blends for 60 minutes.

Figure 5-39: comparison of the carbon pick up by molten steel droplets after reaction with coke and polymer/coke blends for 60 minutes.

In the present study, the low carbon accumulation in liquid steel droplet was observed for coke and all polymer/coke samples investigated, and after 60 minutes of reaction time the metal droplet was far from saturation (5.28 wt%). However, a marginal

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improvement in carbon transfer was observed when plastic/coke blends were used (especially bakelite) compared to that of coke alone; there was a slight dependence on the type and concentration of the polymer in the blends.

It can be seen from Figures 5-37 to 5-39 that the modification in characteristics of carbonaceous blends in the presence of polymer had only a marginal influence on the wettability with molten steel, and these are in agreement to the corresponding carbon transfer results showing a slight improvement in carbon pick up as compared to coke. The wetting behaviour of the liquid steel with polymer/coke blends was observed to have some dependency to the carbon transfer into the melt and these would be related to the formation of reaction products at the interface. Further discussion on factors influencing wettability of liquid steel with polymer/coke blends and the transfer of carbon into molten steel is presented in chapter 7.

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5.6 Summary

The wettability of polymer/coke blends by liquid steel and carbon and sulphur transfer into the melt at 1550ºC were investigated. The experimental results show that nature of the carbonaceous blends which depends on characteristics of the polymers to some extent played a marginal role in influencing the wettability and the transfer of carbon and sulphur. Key findings from this study are:

1. Liquid steel droplets showed a small variation in contact angles with bakelite/coke blends substrates. Blend BK1 showed better wetting than the parent coke with the initial contact angle was 118º and slightly increased to 121º after 60 minutes of contact. The increase in bakelite content in the blends was found to increase contact angles, thereby resulting in poorer wettability. 2. HDPE/Coke blends showed a better wettability with the molten steel compared to that observed for coke alone. The measured initial contact angle for the coke used was 129º and decreased to 128º after 60 minutes of contact. Slightly better wettability was seen when blend H1 was used with the initial contact angle was 116º and slightly increased to 118º after 60 minutes of contact. With increasing HDPE content, blends H2 and H3 showed a slight increase in contact angles. 3. Similar trend was observed for PET/Coke blends, where the initial contact angle for blend P1 was 124º and slightly decreased to 121º after 60 minutes of contact. The contact angles at the initial and after 60 minutes of contact were observed to increase with increasing PET content in the blends. 4. The measured carbon pick-up value after 60 minutes of reaction for metallurgical coke was approximately 0.10 wt%. A marginal higher carbon pick- up was observed for bakelite/coke blends with the values after 60 minutes of reaction was 0.13, 0.16 and 0.19 wt% for blends BK1, BK2 and BK3, respectively. The addition of bakelite into coke has a moderate effect on carbon transfer behaviour; as the bakelite content increased the carbon pick-up also increased.

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5. In the case of HDPE/Coke and PET/Coke blends, only a small improvement in carbon transfer was observed with the measured carbon pick-up value after 60 minutes of reaction was 0.15, 0.15 and 0.17 %wt for blends H1, H2 and H3 and 0.09, 0.15 and 0.15 wt% for blends P1, P2 and P3, respectively. 6. The measured sulphur pick-up value after 60 minutes of reaction for metallurgical coke was approximately 0.06 wt%. A slight increase in sulphur pick-up was observed for bakelite/coke blends with the values after 60 minutes of reaction was approximately 0.12, 0.11 and 0.09 wt% for blends BK1, BK2 and BK3, respectively. The addition of bakelite into coke also has a moderate effect on sulphur transfer behaviour; however, as the bakelite content increased the sulphur pick-up decreased. 7. In the case of HDPE/Coke and PET/Coke blends, only a small change in sulphur transfer was observed with the measured sulphur pick-up value after 60 minutes of reaction was ranging between 0.07-0.08 %wt for HDPE/Coke blends and 0.05-0.08 wt% for PET/Coke blends.

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CHAPTER 6

INTERFACIAL PHENOMENA BETWEEN POLYMER/COKE BLENDS AND MOLTEN STEEL AT 1550ºC

Chapter 6 Interfacial phenomena between polymer/coke blends and molten steel at 1550ºC

This chapter reports experimental results on the formation of interfacial reaction products and associated interfacial phenomena between polymer/coke blends and molten steel at 1550ºC. The focus is on the nature of the reaction products formed at the metal/carbon interface.

6.1 Interfacial Phenomena between Bakelite/Coke Blends with Molten Steel

Three blends of bakelite and coke (BK1, BK2 and BK3) were investigated in this study. The volatile content in the bakelite/coke blends was comparable to that of coke (see Table 5-2). Therefore the influence of volatiles on the formation of interfacial reaction products was not taken into account. Blending bakelite with coke changed the chemical composition of the material, and this is expected to affect the formation and relative proportions of ash oxides layer at the metal/carbon interface.

SEM images of the metal/carbon interface as well as the EDS analysis of the interfacial region in the case of coke for 4, 15 and 60 minutes are shown in Figures 6-1 to 6-3, respectively.

Figure 6-1: SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

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The figures show that the metal/coke interface was predominantly covered by Al2O3 after 4 minutes of contact. After 15 minute of contact, the Al2O3 layer was observed to fully cover the interface and this behaviour continued over the extended reaction times

(60 minutes). The main source of this alumina was coke ash (32 wt % Al2O3). This ash layer was observed to grow with time. A small peak of SiO2 was also detected after 4 minutes of reaction. However, this SiO2 disappeared after longer reaction times possibly due to reduction reactions.

Figure 6-2: SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-3: SEM images showing the steel/coke interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

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SEM images of the metal/carbon interface along with EDS analysis of the interfacial region in the case of blend BK1 for 4, 15 and 60 minutes are shown in Figures 6-4 to

6-6, respectively. Due to the high CaCO3 content in bakelite, the blending of bakelite with coke resulted in significant differences in the ash chemistry compared to the parent coke. The thermal decomposition of CaCO3 (Eq. 6.1) can occur during the combustion of the blend in the furnace, and this increases the CaO content in the blend, thereby relatively decreasing the Al2O3 content from the coke ash (see Table 5-4). CaO formed also can help desulphurize the liquid metal through reactions with solute carbon and sulphur (Eq. 6.2) [Biswas (1981)]. This reaction transfers CaS to the metal/carbon interface as a reaction product.

CaCO3(s) = CaO(s) + CO2(g)….……..….(6.1)

CaO(g) + S + C = CaS(s) + CO(g)……..…….(6.2)

The reaction products formed at the metal/carbon interface in the case of bakelite containing blends were observed to be a combination of CaS and Al2O3. For blend BK1, bright regions representing mineral oxides were observed at the interface after 4 and 15 minutes of reaction and these were predominantly composed of CaS-Al2O3 mixture, as shown in Figures 6-4 and 6-5.

Figure 6-4: SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

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As reactions proceeded further, the composition of the ash layer was found to change and become calcium enriched due to the formation of CaS phase at the interface. A corresponding decrease in Al2O3 concentration was noted from the interfacial image after 60 minutes of reaction (see Figure 6-6).

Figure 6-5: SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-6: SEM images showing the steel/BK1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

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SEM images of the metal/carbon interface along with EDS analysis of the interfacial region in the case of blend BK2 for 4, 15 and 60 minutes are shown in Figures 6-7 to 6-9, respectively. For blend BK2, the interfacial layer formed throughout 60 minutes of reaction was predominately composed of CaS-Al2O3 phases and it morphology was similar to that observed in the case of BK1. The bright regions represent CaS-Al2O3 phases, while the dark regions indicate CaS rich phases. Denser layer composed of the similar phases was noted after 60 minutes of reaction.

Figure 6-7: SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

Figure 6-8: SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

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Figure 6-9: SEM images showing the steel/BK2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

SEM images of the metal/carbon interface along with EDS analysis of the interfacial region in the case of blend BK3 for 4, 15 and 60 minutes are shown in Figures 6-10 to 6-12, respectively. With increasing bakelite content, the change in the morphology of the interfacial layer was observed in the case of blend BK3 after 60 minutes of reaction, as shown in Figure 6-12. The ash layer appeared to be glassy due to the presence of liquid phases and was composed of CaS-Al2O3 phases.

Figure 6-10: SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

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Figure 6-11: SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-12: SEM images showing the steel/BK3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

However, it was clearly observed from a high magnification (1500x) SEM image along with the EDS analysis (Figure 6-13) that the reaction products formed at the interface after 60 minutes of reaction in the case of BK3 were a combination of CaS and

CaO.Al2O3 phases. The exact proportion of the two phases at the interface could not be accurately determined from EDS analysis. These two phases were observed only in the case of blend BK3 after 60 minutes of reaction.

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Figure 6-13: SEM image (1500x) showing the steel/ BK3 interface along with the EDS analysis of points in the region after 60 minutes of reaction, A indicates the presence of

CaO.Al2O3 and B indicates the presence of CaS complex.

The appearance of calcium aluminates (CaO.Al2O3) at the interface in the case of blend

BK3 was attributed to the reaction of CaO from the bakelite with Al2O3 from the coke.

The formation of CaO.Al2O3 at the interface was not seen in the case of coke alone, where the interface was predominately Al2O3. The mechanism leading to the formation of CaO.Al2O3 has been discussed in the literature [Chapmen et al. (2007), (2008)].

The formation of CaS and CaO.Al2O3 phases at the iron/carbon interface have been reported previously [Wu et al. (2000), McCarthy et al. (2003), McCarthy et al. (2005) and Chapman et al. (2007)]. Using the sessile drop technique, Wu et al. (2000) investigated the iron/natural graphite interface and found that although the natural graphite contained ~9 wt% ash, with the majority being SiO2 (72 wt%), no SiO2 was observed at the interface. These authors reported that Al2O3 was observed at the interface initially, and as the reaction progressed, the relative proportion of CaO increased and Al2O3 correspondingly decreased, which led to the formation of a Fe/Ca/S complex at the interface. They described the formation of the sulphide based complex as the effect of the desulphurization of the iron droplet. However, the mechanisms related to the decrease in Al2O3 and the increase in CaO was not provided.

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McCarthy et al. (2003) studied the iron/coke interface and also reported the formation of CaS complex at the interface. In the iron/coke system used in their study, the dissolution of carbon from coke was low with the carbon pick-up after 30 minutes of contact being approximately 0.1 wt%. The low carbon dissolution was attributed to the presence of ash oxides and carbon consumption by reducible oxides in the coke ash. These authors also concluded that the production of semifused ash at the interface will decrease the interfacial area, due to the selective removal of silica at the interface initially and the later removal of CaO and its conversion into CaS [McCarthy et al. (2003)]. Chapman et al. (2008) investigated the formation of a mineral layer during iron/coke reaction at the temperatures of 1450–1550ºC. They observed the formation of

Al2O3 and CaO.Al2O3 phases at the interface, with the relative proportions of phases dictating the morphology of the layer. These authors concluded that the kinetics of carbon dissolution from the coke was affected by the interfacial ash layer [Chapman et al. (2008)].

6.2 Interfacial Phenomena between HDPE/Coke Blends with Molten Steel

HDPE contains high amounts of hydrogen (~15 wt%), and therefore, HDPE/Coke blends were found to have higher volatile content compared to the parent coke (see Table 5-6). The volatiles and chemical elements in the plastics are expected to influence the formation of reaction products, which in turn could affect carbon dissolution in these systems.

The interfacial reaction products act as a physical barrier, preventing contact between the solid carbon and the liquid metal, thus limiting dissolution. The SEM micrographs of the metal/carbon interfaces as well as corresponding EDS analyses of the interfacial region in the case of blend H1 for 4, 15 and 60 minutes are shown in Figures 6-14 to 6- 16, respectively. It was observed that the interfacial ash layer formed was predominately Al2O3 which is similar to the case of coke. However, the formation of the ash layer was observed to be slower than that in the case of coke. This was expected to be due to the release of volatiles from the HDPE/Coke blends, which could induce

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the turbulence at the interface and thus not allow ash oxides to deposit at the interface. As seen in Figure 6-15 that the interface for H1 after 15 minutes was still partially covered by the ash layer and the metal surface can still be seen, while the interface in the case of coke at the same reaction time was fully covered by the ash layer (see Figure 6-2).

Figure 6-14: SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

Figure 6-15: SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

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Figure 6-16: SEM images showing the steel/H1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

Figures 6-17 to 6-19 show the SEM micrographs of the metal/carbon interfaces as well as its corresponding EDS analyses of the interfacial region in the case of blend H2 for 4, 15 and 60 minutes, respectively.

Figure 6-17: SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

Al2O3 and SiO2 are the major components in the ash of the carbonaceous blends and expected to be the major oxides presence at the metal/carbon interface. However, only

Al2O3 was observed to be a major oxide at the interface throughout the reaction times.

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SiO2 was expected to be reduced by both solid carbon in the substrate and solute carbon in the melt [McCarthy et al. (2003)] and thus was not detected at the interfacial region. After 60 minutes of reaction, the morphology of the interfacial layer in the case of H2 was similar to that of coke (see Figures 6-3 and 6-19).

Figure 6-18: SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-19: SEM images showing the steel/H2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

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SEM images of the metal/carbon interface along with EDS analysis of the interfacial region in the case of blend H3 for 4, 15 and 60 minutes are shown in Figures 6-20 to 6-

22, respectively. As seen in the cases of lower HDPE contents, Al2O3 was observed to be a major phase at the steel/H3 interface and the morphology of this phase after 60 minutes of contact was similar to that in the case of coke (see Figures 6-3 and 6-22).

Figure 6-20: SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

Figure 6-21: SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

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Figure 6-22: SEM images showing the steel/H3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

These results are different to those observed in the case of bakelite/coke blends. HDPE contains no ash impurities, and thus the blending of HDPE with coke was not found to produce any significant differences in the ash composition of the HDPE/Coke blends compared with the parent coke. It was observed that the morphology and chemical composition of the interfacial layers in the case of HDPE/Coke blends were quite similar to that of coke with the interface mainly being covered by Al2O3 which was observed to grow with time. However, this interfacial layer grew at a slower pace as compared to the case of metallurgical coke, which was attributed to the effect of volatiles released from the blends during early stages of introducing them into high temperature. An increase in the HDPE content in the blends produced no difference in terms of chemical composition of the interfacial layer.

6.3 Interfacial Phenomena between PET/Coke Blends with Molten Steel

PET contains ~5 wt% of hydrogen and has a high oxygen content (~33 wt%). PET/Coke blends were also found to have relatively higher volatile content compared to coke (Table 5-10). Oxygen in PET could play an important role in the formation of reaction products in the interfacial region.

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The SEM micrographs of the metal/carbon interfaces as well as the corresponding EDS analyses in the case of blends P1 for 4, 15 and 60 mins of reaction are shown in Figures 6-23 to 6-25 respectively.

Figure 6-23: SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

Figure 6-24: SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

The composition of the ash layer formed at the interface in the case of P1 was observed to be similar to that of coke and the morphology of the layer after 60 minutes of reaction was also similar to coke. However, the extent of formation of this layer at the

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interface was observed to be less than that in the case of coke as compared between the interfaces after 15 minutes of reaction for blend P1 and coke (see Figure 6-24 and 6- 2). The interfacial layer for blend P1 appears to be lesser densification compared to coke. This result quite similar to that observed in the case of HDPE containing blends and could be due to the presence of volatiles.

Figure 6-25: SEM images showing the steel/P1 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

Figure 6-26: SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

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With increasing PET concentration in the blends (P2 and P3), the formation of interfacial layers were observed to be different from that observed of coke. In the case of blend P2, the SEM images of the metal/carbon interfaces and the corresponding EDS analyses for 4, 15 and 60 minutes of reaction are shown in Figures 6-26 to 6-28 respectively.

Figure 6-27: SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-28: SEM images showing the steel/P2 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

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The significant different in morphology of the interfacial layer between P2 and coke was seen after 60 minutes of reaction (see Figures 6-28 and 6-3). A rough surface mainly composed of Al2O3 was seen in the case of coke after 60 minutes of contact, while a small flat shape which composed of Al2O3 and small amount of CaS and FeO was observed in the case of P2 (Figure 6-28 B).

For blend P3, the SEM images of the metal/carbon interfaces and the corresponding EDS analyses for 4, 15 and 60 mins of reaction are shown in Figures 6-29 to 6-31 respectively. After 4 minutes of reaction, only a small amount of ash was observed at the interface, and this composed of small amount of mineral oxides. With continuation of the reactions, Al2O3 and CaS were detected after 15 minutes (see Figure 6-30 A). After 60 minutes, the morphology of the interfacial layer was found to be similar to the case of P2 and this layer also composed of Al2O3 and small amount of CaS as well as some mineral oxides. The formation of small quantities of CaS at the metal/carbon interface was observed in the case of PET/Coke blends due to the desulphurization of the liquid steel. This occurred due to the reaction of CaO in the ash of the PET/Coke blends with sulphur and carbon in melt [Biswas (1981)].

Figure 6-29: SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 4 minutes, along with the EDS analysis of the regions.

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Figure 6-30: SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 15 minutes, along with the EDS analysis of the regions.

Figure 6-31: SEM images showing the steel/P3 interface after reactions at 1550ºC for, t = 60 minutes, along with the EDS analysis of the regions.

The volatiles (H2 and CH4) generated during the steel/carbon interactions in the case of HDPE/Coke blends were not found to have any significant affect on changing morphology and chemistry of the interfacial ash layer compared to the case of coke alone (see Figures 6-3 and 6-22). On the other hand, the volatiles in PET/Coke blends which consist of H2, CH4 and O2 were found to modify the morphology and chemistry of the interfacial layer (see Figure 6-32 to 6-34). Oxygen is expected to come from both PET and mineral oxides in the coke. Total oxygen content in the coke and

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PET/coke blends was analysed using an oxygen analyzer (LECO TC 436DR) and the values are given in Table 6-1. It was found that the total oxygen content of the blends increased with increasing PET concentration in the blends.

Table 6-1: Oxygen content in coke and PET/coke blends

% Total Oxygen Content* Coke P1 P2 P3 15.6 20 25.8 33.2

The metal/carbon interface after 60 minutes of reaction at higher magnification (x1500) in the cases of coke, P2 and P3 are shown in Figures 6-32 to 6-34, respectively.

Figure 6-32: SEM image (x1500) of the steel/coke interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of

Al2O3.

It is clearly seen that morphology of the interfacial layer after 60 minutes of reaction, in the cases of P2 and P3, were significantly different from that of coke. The ash layer formed at the metal/coke interface was observed to be a rough surface and rich in

Al2O3, while the ash layer in the case of P2 and P3 was observed to be composed of small platelets and these were composed of a combination of Al2O3, CaS and small amount of ash oxides. As seen in Figure 6-33, the interfacial layer in the case of P2 consists of 2 regions; point A indicates a presence of Al2O3 with small amount of FeO and CaS which the morphology of region was a small platelet, while point B indicates

Al2O3 rich region which the morphology similar to the case of the metal/coke interface. 6-21 Chapter 6 Interfacial phenomena between polymer/coke blends and molten steel at 1550ºC

Figure 6-33: SEM image (x1500) of the steel/P2 interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of Al2O3-FeO complex.

Figure 6-34: SEM image (x1500) of the steel/P3 interface after 60 minutes of reaction, coupled with the corresponding EDS analyses indicating the presence of Al2O3-FeO complex.

A further increase in PET concentration (P3) was found to increase the presence of small platelets and these were mainly composed of Al2O3-FeO mixture. The presence of FeO phase at the metal/carbon interface is demonstrated by the higher intensity of the Fe peaks (Figure 6-34). This happens through the oxidation of iron by oxygen released from PET/Coke blends (Eq.6.3). Oxygen evolved from PET/Coke blends is expected to be O2 as detected by GC analyzer for PET/Coke blend (see Figure 4-13 b). º At 1550 C, Fe2O3 formed would decompose to FeO and deposit at the metal/carbon interface (Eqs.6.4 and 6.5). The FeO observed could be reduced by solid or solute

6-22 Chapter 6 Interfacial phenomena between polymer/coke blends and molten steel at 1550ºC

carbon (Eq.6.6). However, the reduction of the FeO depends on properties of the carbonaceous materials. The formation of FeO at the interface in the case of coke was not clearly seen because the interface was predominately covered by Al2O3. The relative proportion of Al2O3 formed at the interface in the case of P3 is expected to be lower as compared to the cases of P2 and P1; however the exact proportion between the two phases could not be determined in the present study.

º º 4Fe + 3O2 = 2Fe2O3, at 1550 C, ΔG = -361 kJ……..….....(6.3)

º º 3Fe2O3 + C = 2Fe3O4 + CO, at 1550 C, ΔG = -139 kJ……..….…(6.4)

º º Fe3O4 + C = 3FeO + CO, at 1550 C, ΔG = -178 kJ…………...(6.5)

FeO + C = Fe + CO, at 1550ºC, ΔGº = -121 kJ……....…..(6.6) .

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6.4 Summary

The formation of interfacial reaction products of polymer/coke blends with liquid steel at 1550ºC was investigated. The experimental results show that the nature of the interfacial reaction products formed was a function of chemical composition of the polymers. Key findings from this study are:

1. The reaction products formed at the interface in the case of bakelite/coke blends

(BK1 and BK2) were observed to be a combination of CaS-Al2O3 phases. In the case of BK3, the interfacial reaction products were observed to be a mixture of

CaS and CaO.Al2O3 phases, and the morphology of these phases was seen to be glassy melted phases.

2. CaCO3 (present as a filler material) in bakelite was decomposed to CaO and the CaO was found to modify the chemical composition of the bakelite/coke chars compared to coke which in turn affected the composition of the interfacial ash

layer through the formation of CaO.Al2O3 and also CaS at the interface due to the desulphurization of molten steel.

3. Volatiles from HDPE (CH4 and H2) were observed to play a role in slowing down the deposition of reaction products. However, they did not affect the chemical composition of these deposits. 4. The oxygen in PET was found to participate in formation of interfacial reaction

products by oxidizing liquid steel to form FeO slag at the interface. This was altered the chemical composition and morphology of the interfacial layer compared to the case of coke alone. 5. The major component of the interfacial layer for PET/Coke blends (P2 and P3)

was a combination of FeO-Al2O3 which the morphology was a small platelet, and this could have lower melting temperature of the interfacial layer compared

to Al2O3 rich layer in the case of coke. This could affect the carbon dissolution behaviour.

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CHAPTER 7

DISCUSSIONS ON INTERACTIONS OF POLYMER/COKE BLENDS WITH MOLTEN STEEL AT 1550ºC

Chapter 7 Discussions on interactions of polymer/coke blends with molten steel at 1550ºC

Various reactions occurring between polymer/coke blends with molten steel were investigated at 1550ºC, including wettability of the carbonaceous blends by molten steel, transfer of carbon and sulphur into the melt and the formation of reaction products at the metal/carbon interfacial region. The reactions between coke and molten steel were also investigated for comparison and establishing the role of polymers. This chapter presents in-depth discussion on the effect of bakelite, which is a thermoset polymer containing high amounts of CaCO3, on the steel/carbon interactions, followed by the effect of HDPE and PET, which are thermoplastic polymers containing high levels of volatiles. The substitution of waste polymers by blending with metallurgical coke was found to modify blends characteristics; significant differences have been observed in the interactions of polymer/coke blends with molten steel as reported in chapters 5 and 6.

7.1 Bakelite

The metallurgical coke used in this study contained high levels of ash (~17 wt%) with major components being SiO2 (~61 wt%) and Al2O3 (~30 wt%). CaO was found to be a minor component present in coke ash (~0.71 wt%). While the addition of bakelite

(containing CaCO3) into coke with different ratios did not improve the carbon structure

(Lc value), it was found to affect coke chemistry, especially ash compositions.

Bakelite/coke blends were found to have relatively lower amounts of SiO2 and Al2O3, and higher CaO compared to coke. Increased levels of lime (CaO) which is a fluxing agent can significantly affect the formation of reaction products at the metal/carbon interface, and thus influence wettability and carbon transfer. These changes were found to depend on the level of bakelite in the blends.

7.1.1 Interfacial Phenomena

SEM images of the metal/coke interface as well as the EDS analysis of the interfacial region after 4, 15 and 60 minutes of contact (see Figures 6-1 to 6-3) showed the formation of ash layer (mainly Al2O3) covering the interface and growing as a function of time. This was due to a high extent of Al2O3 in the coke ash (~32%). This interfacial layer acts as a physical barrier retarding carbon transfer into liquid steel and could be

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responsible for the low carbon pick-up from the coke in the present study. In these

SEM images (Figures 6-1 to 6-3), a small peak of SiO2 was observed after 4 minutes of reaction which disappeared after longer reaction times. This is expected due to the reduction of SiO2 which can consume solute carbon and transfer Si into the melt [McCarthy et al. (2003)].

Figure 7-1: SEM image and EDS analysis of the cross-section of Steel/Coke system after 60 minutes of reaction at 1550ºC. A is the metal phase, B is carbon substrate and C is the ash layer formed at the metal/carbon interface.

Figure 7-1 shows SEM image and EDS analysis of a cross-section of steel/coke system after 60 minutes of reaction at 1550ºC. Point A indicates the metal phase where a small Si peak was detected, while point B is the coke substrate indicates the carbon and reduced Fe and Si (due to in situ reduction of Fe2O3 and SiO2 in the ash by solid carbon in the coke substrate and also through the penetration of liquid steel into the substrate).

Point C is in the ash layer formed at the interface indicating the formation of Al2O3 and other mineral oxides. This figure provides evidence for the transfer of Si from the coke ash into the molten steel which can affect carbon levels in the melt by consuming some of the solute carbon.

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The additional presence of CaO from bakelite was found to change the morphology and chemical composition of the interfacial products compared to the case of coke alone.

The interfacial products were observed to compose predominantly of CaS-Al2O3 phase mixture in the case of blends BK1 and BK2 (see Figures 6-4 to 6-9), while it was CaS-

Al2O3 and CaO.Al2O3 phases in the case of blend BK3 (see Figures 6-10 to 6-12). The morphology of the interfacial layer was also found to change significantly (see Figure 7-2) with increasing levels of bakelite.

Figure 7-2: SEM image (1500x) comparing morphology of the interfacial layer after 60 minutes of reaction for coke and its blends with bakelite, indicating the role of CaO generated from the bakelite on reducing melting temperature of the interfacial layer.

The interfacial products formed in case of coke had a mesh like structure. The interfacial layer was observed to have a crystal structure of CaS (formed from the desulphurization of molten steel) in the case of blend BK1. With increasing bakelite concentration as blend BK2, the interfacial layer was found to melt and the sharp crystalline shapes could not be as clearly as seen in the case of BK1. In the case of blend BK3, the interfacial products had a glassy or fluid like structure.

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7.1.1.1 Estimation of Solid and Liquid Components of the Interfacial Reaction Products

The relative strength of the ash layer between the solid carbon and molten iron depends on its fusion temperature which inturn depends on the chemistry of the layer [Orsten and Oeters (1986) and Gudenau et al. (1990)]. If the melting point of the ash layer is lower than the liquid metal temperature, it could be easily removed from the interface. This increases the contact area between the liquid metal and solid carbon, and thus enhances carbon transfer. Orsten and Oeters (1986) studied the influence of additives such as CaO on the carbon dissolution and found that the addition of CaO can reduce the ash melting temperature. Gudenau et al. (1990) also reported that the influence of ash on the interactions of industrial and special cokes with liquid iron was an important factor in controlling coke dissolution. They suggested that phases that have lower fusion temperature than coke ash would aid the carburizing of iron by allowing the ash to be removed from the interface.

In the present study, additional CaO from the bakelite in the blends was found to act as a fluxing agent which inturn reduced the ash melting temperature. To determine the effect of bakelite on the ash melting temperature of the carbonaceous blends, FactSage 6.0 [Bale et al. (2009)], thermodynamic software, was used to estimate the proportion of the solid and liquid components of ash oxides present in coke and bakelite/coke blends at 1550ºC. From the ash analysis of the chars produced, only the relative proportions of SiO2, Al2O3, CaO and MgO were found to vary significantly across different blends (see Table 5-4). All other ash components were low in concentration and did not show any significant differences in different blends. The data input for this software are given in Appendix II.

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Following assumptions were made for the thermodynamic calculations using FactSage:

1. Only SiO2, Al2O3, CaO and MgO were included as ash oxides in the calculation.

2. Fe2O3 was not included because it is assumed to be reduced under experimental conditions. 3. Sulphur was included in the calculations to assess its influence on the reactions at the metal/carbon interface. 4. All the components of the ash were assumed to be distributed homogeneously throughout the substrate.

Cham et al. (2006) used FactSage to estimate the solid/liquid ratio of the reaction products formed at the interface at temperatures of 1500ºC and 1550ºC to investigate the influence of the content and composition of ash oxides on the carbon dissolution behaviour from cokes. In their study, it was found that over 80% of reaction products formed at the interface were in a liquid state. These authors explained that the differences in the melting temperature and viscosity of the interfacial products resulted in the differences in the carbon dissolution behaviour of the different cokes. They also showed that the composition of ash oxides in the coke influenced the viscosity of the interfacial product and therefore the kinetics of reaction between coke and iron. In the present study, the estimated proportions of solid and liquid components of the interfacial products formed in the case of coke and bakelite/coke blends at 1550ºC are given in Table 7-1.

Table 7-1: Estimated values of percentage of solid / liquid component of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0)

Carbonaceous Samples Solid (%) Liquid (%)

Coke 40 60 BK1 23 77 BK2 7 93 BK3 3 97

7-6 Chapter 7 Discussions on interactions of polymer/coke blends with molten steel at 1550ºC

The estimated major constituents of the liquid and solid components are shown in Tables 7-2 and 7-3, respectively. Key constituents of the liquid component were found to be SiO2, Al2O3, CaO, CaS and Mg2SiO4, while major constituents of the solid component were 3Al2O3.Si2O13 (mullite) and CaS. With increasing bakelite content in the blend, the proportions of SiO2 and Al2O3 in both the liquid and solid components of the interfacial layer were found to decrease, while the levels of CaO and CaS increased relatively. The estimated results show that the presence of CaO and CaS at the interface can lower the melting temperature of the interfacial layer. As shown in Table 7-1, the interfacial reaction products in the case of bakelite/coke blends contained more liquid compared to the case of coke, and the fraction of liquid component increased with increasing bakelite content in the blend. The liquid oxide layer which is readily removed from the interface can increase metal/carbon contact area. These changes would help explain the somewhat higher carbon pick-up values in the case of bakelite/coke blends compared to coke alone. This provides the evidence that the bakelite (contains CaCO3) has a beneficial effect in decreasing the ash fusion temperature of the materials, and is in agreement with the results from literature [Orsten and Oeters (1986)].

Table 7-2: Estimated values of constituents in liquid phase of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0)

Carbonaceous Liquid Constituents (%) samples SiO2 Al2O3 CaO CaS Mg2SiO4 Coke 90.6 8.13 0.77 0.0007 0.51 BK1 71.7 20.9 5.79 0.04 1.54 BK2 59.9 27.4 10.5 0.14 2.07 BK3 52.3 25.9 18.1 0.45 3.34

7-7 Chapter 7 Discussions on interactions of polymer/coke blends with molten steel at 1550ºC

Table 7-3: Estimated values of constituents in solid phase of the interfacial products formed in the case of bakelite/coke blends compared to coke alone at 1550ºC (calculated using FactSage 6.0)

Solid Constituents (%) Carbonaceous 3Al O .Si O samples 2 3 2 13 CaS (Mullite) Coke 99.6 0.4 BK1 92.1 7.8 BK2 63.7 36.3 BK3 0 100

The differences in the percentages of the solid/liquid components estimated using the FactSage 6.0 could help explain the differences in the carbon dissolution behaviour between the bakelite/coke blends and coke alone. However, the formation of interfacial products is influenced by the kinetics of the reaction; as the dissolution reactions continue, the interfacial layer will change due to a further deposition of reaction products

Once solid carbon is in contact with liquid iron, carbon and sulphur atoms will dissociate from their host lattice into the interface and then dissolve into liquid iron. Ash oxides in the solid carbon will also form a layer at the interface. In this system, the ash layer formed is assumed to be composed of SiO2-Al2O3-CaO-MgO. Once the carbon and sulphur atoms transfer across the interface into the liquid metal, they will react with the mineral oxides in the interfacial layer, and thus transfer reaction products into the interface. The interfacial reactions that take place include desulphurization of liquid iron and reduction of silica. The CaO generated from the bakelite can react with the dissolved carbon and sulphur atoms, and form CaS at the interface as a reaction product. SiO2 can also be reduced by the dissolved carbon through both direct and indirect reduction reactions. The reduced Si is transferred into liquid iron. CaO can also bond with Al2O3 to form CaO.Al2O3 at the interface [Chapman et al. (2007)]. The final composition and morphology of the interfacial layer is the result of various reactions occurring at the metal/carbon interface.

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In the present study, the Al2O3 content in the blend samples came predominantly from the coke; therefore, by increasing the bakelite content in the blends, the relative proportion of Al2O3 present at the interface is expected to be reduced. The presence of º Al2O3 (which is a non wetting compound with high melting point ~2053 C) would hinder the interaction of liquid metal with the coke surface. On the other hand, the presence of CaS, CaO and Al2O3 mixture at the interface would aid the dissolution of carbon by reducing the melting temperature of the interfacial products. These are attributed to modifications in the interfacial reaction products formed at the metal/carbon interface. The metal droplets which reacted with blends BK2 and BK3 were found to show marginally higher carbon pick-up than coke, but the sulphur transfer was lower in the case of BK3.

7.1.2 Wettability

The nature of the interfacial layer and its composition at the metal/carbon interface are known to influence the wettability, and can be affected by the ash composition. Table 7-4 shows the factors that may have an effect on the wetting behaviour of bakelite/coke blends with molten steel.

Table 7-4: Comparison of contact angles and volatiles and ash chemistry of coke and bakelite/coke blends

Volatiles Ash SiO Al O CaO Contact Angles (º) Samples 2 2 3 (wt%) (wt%) (wt%) (wt%) (wt%) 1 minute 60 minutes Coke 3.00 17.20 61.10 32.10 0.71 129º 128º BK1 3.20 20.50 56.90 28.90 5.40 118º 121º BK2 3.10 23.50 52.80 26.20 10.80 123º 139º BK3 3.30 28.30 47.30 22.80 18.30 131º 129º

From Table 7-4, the blending of bakelite with coke produced significant differences in terms of the ash content and composition compared to the parent coke, especially CaO level in the ash. The measured contact angles after 1 and 60 minutes contact have been plotted against %CaO in the ash of bakelite/coke blends and shown in Figure 7-3.

7-9 Chapter 7 Discussions on interactions of polymer/coke blends with molten steel at 1550ºC

Figure 7-3: Plots of contact angles for Bakelite/Coke blends against CaO content in the blends for a) 1 minute and b) 60 minutes of contact (the data scatter was within ± 5º).

The increases in bakelite concentration in the blends were found to have a small effect on the wettability of the materials with molten steel. It can be observed from Figure 7- 3 that blend BK1 showed lower initial contact angle than that observed for coke. However, the initial contact angle of liquid steel droplet was observed to increase slightly with the increase in CaO content in the blends (BK2 and BK3). However this trend was not observed for the contact angle after 60 minutes of contact. This may due to the fact that the metal droplet was not directly contact with the bakelite/coke substrate, but with a layer of reaction products that more or less covered the metal/carbon interface.

The deposition of ash oxides at the interface, especially Al2O3 affects the wettability between the molten iron and solid carbon. Zhao and Sahajwalla (2003) studied the wettability of molten iron droplet on graphite/alumina substrates and found that an increase in alumina content in the substrate led to an increase in the contact angle. The

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high contact angle in the case of coke used in the present study was attributed to the high amount of Al2O3 in the coke ash (~32 wt%), which formed a layer at the interface.

However, the presence of new phase other than Al2O3 was found to decrease the contact angles. Wu et al. (2000) had observed the formation of Al2O3 and CaS phases at the interface during the reaction between natural graphite and liquid iron at 1550ºC in argon atmosphere. These phases were seen to grow with time, such that approximately 45% of interfacial coverage occurred after 60 minutes of reaction [Wu et al. (2000)]. However, the exact proportion of the two phases was not determined. They attributed º the equilibrium contact angles (102 ) to the wetting of the liquid iron with a CaS-Al2O3 layer formed at the interface because the contact angle for Fe-CaS system was 87º, º while the contact angle was 122 for the Fe-Al2O3 system [Keene (1995)].

In this study, the formation of CaS-Al2O3 complex at the interface observed in the case of BK1 and BK2 led to a slightly lower contact angles compared to the case of coke alone, where the interface was rich in Al2O3. With increasing bakelite content in the blends, the formation of new phases was observed and found to be a combination of

CaS and CaO.Al2O3 phases (see section 6.1), and these phases were observed only in the case of blend BK3. The formation of new phases (CaS and CaO.Al2O3) in the case of blend BK3 could affect contact angles compared to the case of BK1 where the interface is a combination of CaS-Al2O3 mixture. The formation of CaS-Al2O3 mixture at the interface was initially observed in the case of BK3 with a decrease in contact angles until about 15 minutes. After 15-20 minutes, the increase in contact angles was observed with the presence of CaS and CaO.Al2O3 phases mixture. The contact angle achieved a stable value of approximately 129º after about 40 minutes of contact.

7.1.3 Carbon and Sulphur Transfer

Metallurgical coke used in the present study showed poor carbon dissolution behaviour with a low carbon pick up observed after 60 minutes (0.1 wt%); and was attributed to the influence of ash oxides (interfacial blockage and consumption of solute carbon in the melt). Structure of the coke (low crystallinity) can also have an important role in the transfer of carbon. Sahajwalla et al. (1994) studied the effect of carbon structure on the carbon dissolution into molten iron, and found that the high crystallinity of the material

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resulted in a greater rate of carbon dissolution. In the present study, only a small improvement in carbon transfer into molten steel was observed when bakelite/coke blends were used compared to the case of coke alone. This can be explained as follows: bakelite addition did not change the carbon structure of the blends compared to the parent coke, but it modified the reaction products at metal/carbon interface which inturn affected the transfer of carbon and sulphur into the molten steel to some extent.

7.1.3.1 Comparison of Carbon and Sulphur Pick-up from Coke and Bakelite/Coke Blends with Raw Bakelite

In this section, carbon and sulphur picked up by molten steel from these carbonaceous materials namely, coke, bakelite/coke blends and bakelite are compared. The levels of ash of raw bakelite was significantly different from that of metallurgical coke (31.0 wt% and 17.2 wt% for the bakelite and coke, respectively), and the ash chemistry was also different. The bakelite used contained 31 wt% of ash, with the main ash component being CaCO3. The major ash components of coke, chars of bakelite/coke blends and raw bakelite are represented comparatively in Table 7-5.

Table 7-5: Major ash components of coke, Bakelite/Coke blends and raw bakelite, this table shows only the main ash oxide components

Carbonaceous Ash Composition (wt%) Ash Content Materials (wt%) SiO2 Al2O3 Fe2O3 CaO Coke 17.2 61.10 31.5 2.0 0.55 BK1 20.5 56.90 28.90 2.30 5.40 BK2 23.5 52.80 26.20 2.30 10.80 BK3 28.3 47.30 22.80 2.20 18.30 Bakelite 31.0 0.91 nd nd 55.46*

* Calculated from 99.09 %CaCO3 present in bulk ash of raw bakelite

7-12 Chapter 7 Discussions on interactions of polymer/coke blends with molten steel at 1550ºC

To understand the effect of ash chemistry of the carbonaceous materials on the transfer of carbon, the carbon pick-up from raw bakelite into liquid steel were investigated using a sessile drop technique. Raw bakelite samples were ground into powder and compacted to make a substrate and the carbon pick-up from raw bakelite was determined after 1 and 60 minutes of reaction. Results are presented in Figure 7-4.

Figure 7-4: Comparison of carbon picked up after a) 1 minute and b) 60 minutes of reaction for coke, bakelite/coke blends and raw bakelite.

The carbon pick-up from the bakelite alone was found to be 0.082 wt% after 1 minute of reaction which is comparable to that observed from blend BK1 and coke alone. The carbon pick-up for the bakelite was observed to increase thereafter with the measured carbon content in the metal droplet reaching 0.186 wt% after 60 minutes. This value was comparable to that observed from blend BK3, and higher than the case of coke and other blends.

Figure 7-5: Comparison of carbon content in coke, bakelite/coke blends and raw bakelite.

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In the case of raw bakelite, it contained relatively low levels of fixed carbon compared to the bakelite/coke blends and coke alone (see Figure 7-5) and had high ash content with a significantly different chemical composition (see Table 7-5). However its carbon pick-up was found to be somewhat higher than that from coke alone. This indicates that the fixed carbon in these materials did not play a major role in the carbon dissolution, but the role of ash chemistry was important. The content and composition of the ash in the carbonaceous materials can have a strong influence on the dissolution of carbon [Orsten and Oeters (1986), Gudenau et al. (1990), Wu et al. (2000) and McCarthy et al. (2003)]. The ash content in the bakelite used in this study was higher (31 wt%) than in the coke (17.2 wt%), and there were significant differences in their chemical composition. The coke used had high SiO2 and Al2O3 levels while the bakelite had CaCO3 as a key ash constituent which could be converted to CaO during pyrolysis in the furnace. Thus, the differences in their carbon dissolution behaviour could be attributed to the differences in the ash chemistry which played an important role in interfacial reactions.

As seen in Table 7-5 and Figure 7-4, coke which has high SiO2, Al2O3 and significantly low CaO level in its ash showed poor carbon dissolution behaviour with the carbon picked up after 60 minutes of contact was ~0.1 wt%. The addition of bakelite in coke was found to improve the carbon dissolution behaviour compared to coke alone as higher carbon pick-up were observed from the blends. This improvement can be attributed to the increase in CaO (generated from the decomposition of the

CaCO3) and decrease in SiO2 and Al2O3 present in bakelite/coke blends, which inturn influence interfacial reactions.

A number of researchers have investigated carbon dissolution from non-graphitic materials (chars and metallurgical coke) into molten iron (99.98% Fe) at 1550ºC using different techniques and a range of reaction times, and their results are compared in Table 7-6. It was found from previous studies that a variation in values of carbon pick up by liquid iron and the dissolution rates was dependent on types of materials.

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Table 7-6: Carbon dissolution rate and carbon picked up values for electrolytic pure iron (99.98% Fe) - chars and metallurgical coke systems at 1550ºC obtained from previous studies

Dissolution C Pick-up Samples Methods Rate Researchers (wt%) (x10-3 s-1)

Char 1 Carburizer cover ~3.5 atfter 40 mins 17.9 McCarthy (2004) Char 2 Carburizer cover ~2.5 atfter 40 mins 2.8 McCarthy (2004) Char 1 Sessile drop ~0.1 after 60 mins - McCarthy (2004) Coke 1 Carburizer cover >5.0 atfter 60mins 14.7 Cham et el. (2004) Coke 2 Carburizer cover >5.0 atfter 60mins 1.1 Cham et el. (2004) Coke Sessile drop ~1.0 after 60 mins - McCarthy et al.(2003)

Several factors could be limiting the carbon transfer and/or accumulation for metallurgical coke. The formation of reaction products layer would decrease the contact area available for mass transfer. The reduction of reducible oxides in the bulk ash (such as Fe2O3 and SiO2) could consume the solute carbon in the liquid steel. Carbon transfer could also be limited by the slower rates of carbon dissolution from non-graphitic materials. In the sessile drop arrangement, it was likely that the carbon accumulation in the molten steel droplet is being hindered by the consumption of solute carbon in the droplet because the formation of ash layer at the interface would be likely after longer periods of reaction time. The reductions of reducible oxides are consuming the melt carbon content as fast as it can be transferred to the molten steel droplet. It is not likely until these reactions slow down due to the decrease in levels of reactants (Fe2O3 and

SiO2), that carbon can accumulate within the molten steel droplet.

Characteristics and ash chemistry in the materials had a strong effect on the behaviour of carbon transfer into molten steel. In the present study, the experimental conditions and technique used (sessile drop) are similar to the previous study, McCarthy (2004). In order to identify the factors that may limit the carbon transfer from the coke, the measured carbon pick up values as well as the ash levels and composition are compared with the values obtained from the previous studies as shown in Table 7-7.

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Table 7-7: Comparison of carbon pick up values for iron (99.98 %Fe) – coke system, and ash level and composition of the coke obtained from McCarthy (2004) with the values from the present study

Carbon pick up (wt%) Ash SiO Al O CaO Samples 2 2 3 Researchers 1 30 60 (wt%) (wt%) (wt%) (wt%) min mins mins McCarthy Coke <0.1 ~0.1 ~1.0 11.52 55.8 31.0 1.95 (2004) Coke 0.78 0.1 0.1 17.2 61.1 32.1 0.71 Present study BK1 0.74 0.13 0.13 20.5 56.9 28.9 5.4 Present study BK2 0.13 0.16 0.16 23.5 52.8 26.2 10.8 Present study BK3 0.13 0.17 0.19 28.3 47.3 22.8 18.3 Present study

The measured carbon picked up from coke obtained from McCarthy (2004) was ~0.1 wt% after 30 minutes and then rapidly increased to ~1.0 wt% after 60 minutes of contact. In the present study, the measured carbon pick up for coke after 30 minutes was comparable to that from previous study. However, a significant difference in carbon pick up was seen after 60 minutes (~0.1 wt% and ~1.0 wt%). The difference in carbon transfer behaviour for the two cokes may be attributed to the differences in coke properties, such as ash composition and carbon structure. However, the effect of carbon structure was not reported in the previous study [McCarthy (2004)]. For the data reported in Table 7-7, the ash level of coke used in the present study was higher than that from the previous study [McCarthy (2004)] (~17 wt% and ~12 wt%, respectively), and the total SiO2 present in the cokes was calculated to be 10.5 wt% and

6.4 wt%, respectively. SiO2 was reported to have a strong effect on carbon dissolution. It can consume solute carbon in the liquid iron through the reduction reaction

[McCarthy et al. (2003)]. The difference in total SiO2 in the two cokes could be responsible for the difference in carbon transfer behaviour observed from the two studies over extended periods. This comparison could highlight the influence of ash level and its composition on the transfer of carbon. However, a slight improvement in carbon transfer behaviour was observed when bakelite/coke blends were used. Bakelite addition was found to have a marginal affect on enhancing the carbon pick up values compared to its parent coke, and this was due to the increase in CaO in the blends due to the bakelite addition.

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Figure 7-6: Comparison of sulphur picked up after a) 1 minute and b) 60 minutes of reaction for coke, bakelite/coke blends and raw bakelite.

For sulphur pick-up, as shown in Figure 7-6, a small accumulation of sulphur in the liquid metal as a function of time was seen for bakelite and bakelite/coke blends. An increase in the melt sulphur content as a function of time was not seen in the case of coke alone with the sulphur pick-up values after 1 and 60 minutes of reaction being similar (0.056 and 0.057 wt%, respectively). This could be due to the effect of interfacial blockage; high Al2O3 content in the coke ash forms a layer at the metal/carbon interface and this can significantly reduce the contact area available for mass transfer across the interface. In addition, sulphur already transferred into the melt within the first few minutes was not likely to be removed through desulphurization due to a reduction in contact area and the relatively low CaO content in the coke ash compared to that of bakelite/coke blends.

For bakelite/coke blends, blend BK1 showed higher sulphur pick-up than coke, and the sulphur pick-up increased with increasing bakelite level in the blend as seen in the case of blend BK2 (see Figure 7-6). However, a decrease in sulphur pick-up was observed with further increase in bakelite levels as blend BK3. This was expected due to the desulphurization of molten steel by CaO in the carbonaceous substrate.

The sulphur pick-up values after 1 and 60 minutes of contact in the case of bakelite alone were very low compared to the case of coke and bakelite/coke blends with the values of 0.0003 wt% and 0.002 wt%, respectively. This relatively small sulphur pick- up values by molten steel were attributed to the relatively low sulphur content in the bakelite used.

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Due to the higher carbon pick up and lower sulphur picked up for 100% bakelite compared to the case of 100% metallurgical coke, it may be possible to recycle waste bakelite as a source of carbon and lime for steelmaking process. However, further investigations are required for optimizing their utilization in steelmaking.

7.2 High Density Polyethylene (HDPE)

Thermoplastic HDPE contains significantly high level of volatiles. The volatiles in

HDPE (H2 and CH4) are expected to influence interfacial reactions with liquid steel. The addition of HDPE into coke in a range of proportions did not significantly change the chemistry of the blends; the ash content and composition of the HDPE/Coke blends were comparable to that of the parent coke. On the other hand, the volatile matter content in the blends was found to increase with increasing concentration of HDPE in the blends.

7.2.1 Interfacial Phenomena

Volatiles in HDPE (H2 and CH4) were observed to have some influence on the reaction products formed at the metal/carbon interface. In the case of HDPE/Coke blends, the volatiles in the HDPE did not change the chemical composition of the interfacial reaction products and the chemical composition of the reaction products was similar to those observed for coke alone (see Figures 6-14 to 6-22). However, their presence was found to reduce the effect of ash by slowing down the kinetics of coverage of the interface as shown in Figure 7-7. It can be clearly observed that the interface after 4 minutes of contact in the case of coke was almost completely covered by the ash layer and it was found to be completely covered after 15 and 60 minutes of contact. On the other hand, the interface in the case of all HDPE/Coke blends was observed to be partially covered by the ash layer after 15 minutes of contact. The full coverage of the interface was observed only after 60 minutes of contact. These changes can affect the carbon dissolution behaviour of the carbonaceous blends.

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Figure 7-7: SEM images of the reaction products formed at the interface for blends H1, H2 and H3 compared to coke.

Orsten and Oeters (1988) studied the degassing process of coal in liquid iron and explained that when coal particles are blown into liquid iron they are heated up and degassed, and this occurs with the release of volatiles. In the present study, the heating up of HDPE/Coke blends was expected to release volatiles which could be responsible for the formation of interfacial reaction products at the metal/carbon interface observed in this study.

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7.2.2 Wettability

Table 7-8 shows various factors that may have an effect on the wetting behaviour of HDPE/Coke blends with molten steel. It can be seen that HDPE addition only increased the volatiles level in HDPE/Coke blends compared to that of coke, while the ash content and composition were comparable. Therefore, the volatiles in the materials may have some influence on the wettability.

Table 7-8: Comparison of contact angles and chemical properties of coke and HDPE/coke blends

Volatiles Ash SiO Al O CaO Contact Angles (º) Samples 2 2 3 (wt%) (wt%) (wt%) (wt%) (wt%) 1 minute 60 minutes Coke 3.00 17.20 61.10 32.10 0.71 129º 128º H1 3.50 17.60 61.20 31.50 0.87 116º 118º H2 5.20 17.40 61.60 31.60 0.85 129º 121º H3 7.60 16.50 60.90 31.40 0.86 127º 123º

Orsten and Oeters (1988) also observed that when coal particles undergo degassing they are completely surrounded by gas and are not in contact with the liquid metal. In the present study, according to the finding from Orsten and Oeters (1988), it was likely that the devolatilization of the polymer/coke blends could affect the contact between the polymer/coke blends and molten steel and thus influence the wetting behaviour of two phases.

The measured contact angles after 1 minute and after 60 minutes of liquid steel droplet with HDPE/Coke blends were plotted as a function of volatile content in the materials, and shown in Figure 7-8. It can be observed from Figure 7-8 that the addition of HDPE into coke can slightly decrease the contact angles and this can be clearly seen in the case of blend H1 compared to coke. However, the increases in volatiles content in the blends with increasing HDPE concentration (blends H2 and H3) were found to increase the contact angles. These changes could be attributed to the devolatilization of the blends causing turbulence at the interface and thus influencing the contact with molten metal. After 60 minutes, the devolatilization of the blends would be completed.

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Figure 7-8: Plots of contact angles for a) after 1 minute and b) after 60 minutes for HDPE/Coke blends against the initial volatiles content in the materials (the data scatter was within ± 5).

The increase in contact angle with increasing volatiles content in the materials was also found from the previous study [McCarthy (2004)]. This researcher measured the initial contact angles of electrolytic pure iron droplet (99.98% Fe) when in contact with metallurgical coke, char 1 and char 2 at 1550ºC. From this study, the contact angles after 1 minute of contact for the carbonaceous materials were plotted against their initial volatiles content and shown in Figure 7-9. The contact angles from this previous study [McCarthy (2004)] were measured up to ~120 minutes and the contact angles values after 60 minutes could not obtain and thus were not compared with the present study. It was found that chars which contain higher volatile levels, showed higher contact angles compared to metallurgical coke. A small increase in contact angles was observed with increasing volatile levels.

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Figure 7-9: Plots of contact angles after 1 minute of contact for metallurgical coke, Char 1 and Char 2 against the initial volatiles content in the materials [McCarthy (2004)].

7.2.3 Carbon and Sulphur Transfer

The release of volatiles from polymer/coke blends is expected to play an important role in the transfer of carbon and sulphur. The results in Figure 5-24 show that the carbon transfer from blends H1 and H2 was slow. On the other hand, greater carbon transfer was observed from the blend H3.

Barin et al. (1987) studied coal gasification behaviour when coal particles were blown into liquid iron. By using numerical methods, they predicted that 95.95% of the coal particles will dissolve producing a melt bath with 3.5% carbon within 13 seconds at 1500ºC. They reported that the gas released from the degassing process could induce extra bath turbulence and result in intensive mixing and dispersion of coal reactants. This creates a high mass transfer which will tend to enhance carbon dissolution into liquid iron. However, such effect was not clearly seen in the present study possibly due to the limitation of experimental arrangement. Devolatilization of HDPE/Coke blends was not found to have significant effect on the carbon transfer, which at initial carbon pick up in the case of blends H1 and H2 was lower than that for coke alone. However, if the gas generation was too high (H3), they may induce extra turbulence within the metal droplet [Barin et al. (1987)] and thus produce a high mass transfer, and this could enhance the carbon transfer, as seen in the case of blend H3 compared to blends H2, H1 and coke.

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In the present study, due to the addition of HDPE into coke, the gas released during the devolatilisation of HDPE/Coke blends would consist of hydrocarbons (such as CH4) and hydrogen from the polymers, and the amount of these gases species would increase with increasing HDPE concentration in the blends [Ueki et al. (2008)]. These volatiles

(mainly H2 and CH4) could also have some effect on the carburization of liquid steel.

Sekino et al. (1995) investigated the rate of carburization of liquid iron by CH4 º º between 1400 C and 1700 C. The rate was measured for the partial pressures of CH4 in Ar in the range of 0.02 to 0.06 atm and sulphur content in the metal from 0.0006 to 0.5 wt% by injecting the gases at the position of 10 mm above the molten iron surface. The measured carbon picked up was ~0.25 wt% after 20 minutes and then increased to ~0.75 wt% after 60 minutes. The results indicate that the carburization of liquid iron occurred with the rate of carburization that may be controlled by the dissociation of

CH4 on the liquid iron surface and the sulphur was found to decrease the carburization rate. The presence of H2 was found to aid the carburization for hydrocarbons, probably through the removal of adsorbed oxygen from the steel surface.

According to the study from Barrett (1972), in the presence of iron surface, CH4 can decompose and form C and H2 into the system (Eq.7.1). The dissociated C atoms and

H2 could affect the carbon transfer into molten steel. In the present study, this could be evidenced by the carbon pick-up values by steel droplet after reaction with 100% HDPE at 1550ºC as given in Table 7-9.

º º CH4 = C + 2H2 , at 1550 C, ΔG = -110.8 kJ…………...(7.1)

Raw HDPE was used as carburizing material by compacting to form a substrate. The HDPE substrate was put on alumina tray and then 0.5 g of pure Fe chip was put on the substrate. This assembly was inserted into the furnace (1550ºC) having argon atmosphere for 2 and 60 minutes. The carbon pick-up by the liquid steel from HDPE after 2 and 60 minutes of reaction are compared with the values from coke as shown in

Table 7-9. The carbon pick up results indicate that the volatiles (CH4 and H2) released from the carbonaceous blends could affect the carburization of molten steel.

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Table 7-9: Carbon picked up by liquid steel droplets after reaction with raw HDPE at 1550 ºC for 2 and 60 minutes compared to that of coke.

Carbon content Steel droplet (wt%) Samples 2 minutes 60 minutes HDPE 2.48 2.65 Coke 0.08 0.10

The carbon pick-up values from 100% HDPE after 2 and 60 minutes were observed to comparable and the values were much higher than coke and HDPE/Coke blends (carbon pick-up values less than 0.2 wt% for all samples). The observed comparable values for 2 and 60 minutes (2.48 wt% and 2.65 wt%, respectively) could be due to the fact that there was a carburizing of molten steel (by carbon atoms released from volatiles expected to be graphitic in nature [Kim et al. (2005)]) occurred during the devolatilization of the polymers within a first few minutes of reaction. After the polymers completely burned off, there was no affect of ash and carbon consumption mechanisms involved during the polymer/steel interactions until after 60 minutes.

The associated reactions of steel with hydrogen and hydrocarbon (that still remain in HDPE/Coke blends) could explain the marginal increase in carbon pick-up observed from blends H3 (which has highest amounts of CH4 and H2 content). For H1 and H2, the influence of volatiles may not be strong enough to overcome the influence of ash oxides in the materials.

Sulphur levels in the liquid metal in the case of HDPE/coke blends were comparable to that of coke. The accumulation of sulphur in the liquid steel droplet was observed initially and the level of sulphur in the metal droplet was found to stabilize for all cases thereafter. Among the blends, a slight increase in sulphur transfer was observed in blends H1 and H2 compared to coke. A marginally lower sulphur pick-up was observed with further increase in HDPE concentration as blends H3, well within experimental errors. The decrease in sulphur transfer in the case of blend H3 was found with corresponding increase in the carbon pick-up. These may be attributed to slightly

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higher levels of solute carbon in the metal droplet in the case of blend H3, which may influence the transfer of sulphur into the metal droplet.

7.3 Polyethylene Terephthalate (PET)

PET contains significantly high amount of volatile matter and oxygen, which is expected to affect interfacial reactions with liquid steel. The addition of PET into coke with different ratios was not observed to significantly change the ash content and composition compared to that of the parent coke, but it was found to significantly enhance volatile levels in the blends with increasing concentration of PET in the blends.

7.3.1 Interfacial Phenomena

The volatiles in PET, especially oxygen were observed to have a significant influence on the formation of interfacial reaction products. The volatiles were found to slow down the coverage of the interface as shown by Figure 7-10. It can be observed that the coverage of the interface by interfacial reaction products in the case of PET/Coke blends was slower than in the case of coke; a partial coverage of the interfacial region by interfacial reaction products was observed after 15 minutes of reaction for PET/Coke blends, while the full coverage was seen in the case of coke during this period.

Moreover, the oxygen released from PET was found to change the chemical composition at the metal/carbon interface. The combination of FeO-Al2O3 phase was observed at the interface (for details see Section 6.3) and could have some effect on the carbon transfer into the melt because the presence of FeO at the interface could lower the fusion temperature of the interfacial product layer compared to that in the case of coke which was rich in Al2O3. This new phase was found to completely change the morphology of the interfacial layer compared to the case of coke (for blends P2 and P3), as reproduced again in Figure 7-11.

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Figure 7-10: SEM images of the reaction products formed at the interface for blends P1, P2 and P3 compared to coke.

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Figure 7-11: SEM image (1500x) comparing morphology of the interfacial layer after 60 minutes of reaction for coke and its blends with PET and EDS spectra indicate the role of oxygen generated from the PET on changing morphology and forming new phase (FeO-Al2O3) at the interfacial layer.

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7.3.2 Wettability

Table 7-10 shows the chemical composition of coke and PET/Coke blends and contact angles with molten steel. It can be seen that volatile levels in PET/Coke blends were significantly higher compared to that of coke, while the ash components were comparable.

Table 7-10: Comparison of contact angles and chemical properties of coke and PET/coke blends

Volatiles Ash SiO Al O CaO Contact Angles (º) Samples 2 2 3 (wt%) (wt%) (wt%) (wt%) (wt%) 1 minute 60 minutes Coke 3.00 17.20 61.10 32.10 0.71 129º 128º P1 4.50 17.20 60.30 31.00 1.20 124º 121º P2 7.20 18.80 60.50 31.60 1.40 128º 127º P3 10.30 19.20 59.80 31.40 1.40 134º 123º

The measured contact angles after 1 minute and after 60 minutes of liquid steel droplet with PET/Coke blends were plotted as a function of volatile content in the materials, and shown in Figure 7-12. Well defined trends were seen from Figure 7-12, blend P1 shows lowest contact angles compared to the other blends. Similar to that observed from Figures 5-37 and 5-38, blends containing smaller amount of the polymer (P1) showed better wettability compared to the parent coke. Higher contact angles were observed with increasing polymer content in the blends. Similar to HDPE/Coke blends, the devolatilization of PET/Coke blends could also affect the contact between the substrate and the molten steel droplet.

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Figure 7-12: Plots of contact angles for a) after 1 minute and b) after 60 minutes for PET/Coke blends against the initial volatiles content in the materials (the data scatter was within ± 5).

7.3.3 Carbon and Sulphur Transfer

The carbon transfer in the case of PET/Coke blends (Figure 5-35) was quite similar to that of coke for the first 15 minutes of reaction for all the blends. A marginal improvement in the carbon dissolution was observed after 15 minutes for blends P2 and P3 with a slightly increased carbon pick up was observed compared to coke. The devolatilization of PET/Coke blends was expected to influence the carbon and sulphur transfer. When PET/Coke blends are in contact with molten steel at high temperatures, high amount of gases are generated due to the devolatilization of these materials. The volatiles released from PET/Coke blends consist of hydrocarbons, hydrogen and oxygen from the PET, and the amount of these gases species would increase with increasing PET content in the blends.

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Hydrogen and hydrocarbon (CH4) devolatilised from the pyrolysis of PET/Coke blends could also aid the carburization of liquid steel. This could be evidenced by the carbon pick-up values of the steel droplets after reaction with 100% PET at 1550ºC as given in

Table 7-11. The carbon pick-up results also indicate that the volatiles (CH4 and H2) released from the PET/Coke blends could aid the carburization reaction and enhance the carbon pick-up [Sekino et al. (1995)].

Table 7-11: Carbon picked up by liquid steel droplets after reaction with raw PET at 1550 ºC for 2 and 60 minutes compared to that of coke.

Carbon content Steel droplet (wt%) Samples 2 minutes 60 minutes PET 2.40 2.25 Coke 0.08 0.10

The carbon pick-up values from raw PET are relatively high compared to the case of coke and PET/Coke blends. This observed result is quite similar to the one observed for

HDPE. Various carbonaceous volatiles including CH4 released from the polymer was very effective in carburizing molten metal; this was attributed to near graphitic carbons produced from the decomposition of methane. The carbon pick up value after 60 minutes was found to slight lower compared to that after 2 minutes. This may be due to the effect of oxygen which can oxidize solute carbon in the melt; however, this effect will be discussed in section 7.3.3.1.

Similar to trends observed in the case of HDPE/Coke blends, sulphur levels in the liquid metal in the case of PET/coke blends were also comparable to that of coke. The accumulation of sulphur in the liquid steel droplet was observed initially and the level of sulphur in the metal droplet was found to stabilize for all cases thereafter. Among the blends, a slight increase in sulphur transfer was observed in blends P1 and P2 compared to coke. A lower sulphur pick-up was observed with further increase in PET levels in the case of blends P3.

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7.3.3.1 Effect of Oxygen in PET on Carbon Pick up

PET/Coke blends contain high amounts of oxygen. During steel/carbon interactions, if the oxygen diffuses into the liquid metal, it can oxidize solute carbon (Eq. 7.2) and this generates CO, leading to the depletion of solute carbon in liquid steel. However, the experimental results have shown that the O2 in PET/Coke blends had insignificant effect on oxidizing solute carbon in liquid steel. Among the PET/Coke blends, P3 would contain highest amount of oxygen (due to highest level of PET in the blend) and thus, the influence of oxygen on carbon transfer was investigated in detail for blend P3 and compared to coke.

º º [C]dissolved + [O]dissolved = CO , at 1550 C, ΔG = -271.1 kJ………...(7.2)

Oxygen content diffused in the metal droplets after reaction with coke and blend P3 at different times were analysed using an oxygen analyzer (LECO TC 436DR). The oxygen content in the metal droplets for coke and blend P3 was measured and given in Table 7-12. It was found that oxygen levels in the metal droplets are far from saturation limit, which was determined to be ~22.5 wt% at 1550ºC [Sayadyaghoubi et al. (1995)].

Table 7-12: Percentage of oxygen content in liquid steel after reactions with coke and blend P3 at 1550 ºC for different times

% Oxygen in Liquid Steel Samples 2 30 60 Coke 0.0138 ± 0.0005 0.0480 ± 0.0032 0.0927 ± 0.0035 P3 0.0263 ± 0.0035 0.0550 ± 0.0037 0.0964 ± 0.0049 x The error is the standard deviation obtained for the duplicate runs of each sample

The CO generated during steel/carbon interactions at 1550ºC for coke and blend P3 was measured and compared to that generated from the blank substrates (without metal) as shown in Figure 7-13. The amounts of CO generated from the blank substrate (without

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metal) are comparable for blend P3 and coke, which the highest amount recorded, was ~22000 ppm. In the presence of molten steel (carbon substrate with steel), the CO evolved in the case of blend P3 was observed to be lower than that observed from coke, which the highest amount recorded was ~30000 ppm and ~36000 ppm, respectively.

Figure 7-13 CO generated during steel/carbon interactions at 1550ºC for coke and blend P3 compared to that generated from the blank substrates (solid substrate alone).

For blank coke substrate, the CO evolved was produced from coke gasification and in situ reduction of reducible oxides by solid carbon. In the presence of metal, additional CO evolved was produced from the reduction of silica by solute carbon in the melt. In the case of blank P3 substrate, the CO was expected to be generated from in-situ reduction of oxides by solid carbon in the substrate and also from the oxidation of carbon by oxygen from PET. In the present of metal, additional CO generated was also expected to be produced from the reduction of silica by solute carbon in the melt and oxidation of solute carbon by diffused oxygen. However, additional work is required to fully understand reaction kinetics and various mechanisms.

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Amount of CO generated from steel/carbon interaction is the difference between amount of CO generated from the blank substrate and the carbon substrate with metal. It can be seen from Figure 7-13 that the CO generated from steel/carbon interaction in the case of P3 is less than that in the case of coke, which the difference in CO amount is ~8000 ppm for P3 and ~14000 ppm for coke. Therefore, it was likely that oxygen in PET/Coke blends did not have much effect on oxidizing solute carbon in the melt.

7.4 Influence of Polymers on the Carburization of Molten Steel

In the present study, carbon pick up by liquid steel after 60 minutes of reaction with metallurgical coke was ~0.1 wt%. The addition of polymer into coke was found to have only marginal effect on improving carburization of molten steel. These were because the carburization of liquid metal was predominantly affected by coke properties, such as type of carbon in coke which has low crystallinity (small LC value) and also mineral oxides in the bulk ash. The additional of polymers in the blends was not significant to overcome the influence of coke.

Figure 7-14: Comparison of carbon picked up after 60 minutes of reaction for coke, raw bakelite, HDPE and PET.

The carbon pick up after 60 minutes of contact for 100% polymers (without coke) are compared with coke in Figure 7-14. For 100% polymers, different polymers (bakelite, HDPE and PET) showed relatively higher carbon pick up by liquid steel after 60 minutes of reaction compared to polymer/coke blends and coke alone. This indicates that polymers could play an important role on carburization of molten steel. Among the polymers, bakelite showed much lower carbon pick up compared to HDPE and PET

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with a value comparable to coke. These finding can be explained in term of the influence of volatiles (CH4 and H2) in HDPE and PET. The decomposition of CH4 produced C into the system, which is expected to be graphitic in nature [Kim et al. (2005)]. Graphitic carbon would easily dissolve into liquid metal leading to a high carbon pick up observed for HDPE and PET. On the other hand, bakelite contained relatively low volatiles and had fixed carbon. The carbon from bakelite is expected to be amorphous carbon which is similar to carbon from coke. The amorphous carbon would more difficult to dissolve into the melt compared to graphitic carbon.

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7.5 Summary

Investigations of interactions between Bakelite/Coke, HDPE/Coke and PET/Coke blends with molten steel at 1550ºC was conducted to examine their potential as a source of carbon in steelmaking. Experimental results show that the polymers had a significant effect on the interfacial phenomena by modifying the morphology and composition of the reaction products at the metal/carbon interface. However, these had a marginal influence on the wettability and the transfer of carbon and sulphur. A small reduction was observed in contact angles with small increases in carbon pick up in the case of polymer/coke blends compared to the case of coke, and these were found to be a function of chemical composition and concentrations of the polymers used in the blends. The influence of polymers on the steel/carbon interactions is summarized below:

1. Blending of bakelite with coke (blend BK1) was found to decrease the contact angles of the steel droplet with the bakelite/coke substrate. However, the increases in CaO levels in the blends due to the increasing bakelite concentration (blends BK2 and BK3) were found to slightly increase the contact angles. This was attributed to the formation of reaction products at

the interface. Filler material (CaCO3) in the bakelite was found to be a dominant factor influencing the formation of interfacial reaction products. 2. The composition of interfacial layer formed was found to play a significant role on the transfer of carbon and the wettability. CaO was found to participate in desulphurization reactions to form CaS at the interface. In addition, it also acts as a fluxing agent to lower the ash fusion temperature of the interfacial layer. Thermodynamic calculations using FactSage showed that at 1550ºC the interfacial products were predominantly liquid when the bakelite/coke blends were used compared to coke. Liquid phases at the interface could easily be removed, and this would lead to the increased exposure of carbon to the liquid metal, towards an improvement in carbon transfer.

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3. HDPE/Coke (H1) and PET/Coke (P1) blends were found to exhibit slightly better wetting behaviour with liquid steel compared to that observed from coke alone. However, the increase in polymer content in the blends was found to slightly increase the contact angle due to the release of volatile matter in the blends and associated local turbulence.

4. Volatiles (H2 and CH4) in HDPE/Coke blends were not found to change the chemical composition of the interfacial layer, however the release of volatiles was found to slow down the coverage of the interfacial region by reaction products layer. Volatiles (especially oxygen) in PET/Coke blends were found to change the morphology and composition of the interfacial layer by forming FeO at the interface. The coverage of the interfacial layer on the metal/carbon interface was found to slow down. 5. Volatiles in the carbonaceous blends were found to affect the transfer of carbon into liquid steel. When the gas generation is quite high (such as in the case of blend H3), turbulence may be induced on the metal droplet and enhance mass transfer and the transfer of carbon into molten steel. The

presence of CH4 and associated H2 due to the break-down of the polymers was also found to aid the carburization of liquid steel to some extent. 6. The trends of sulphur transfer were observed to be similar for all polymer/coke blends. Blend 1 and Blend 2 (BK1, BK2, H1, H2, P1 and P2) showed slightly higher sulphur pick up values compared to the case of coke. A decrease in sulphur pick up was seen when increasing polymer level in the blends (BK3, H3 and P3).

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CHAPTER 8

CONCLUSIONS AND FUTURE WORK Chapter 8 Conclusions and Future work

This project was focussed on determining the potential use of waste polymer as a source of carbon in EAF steelmaking. Extensive experimental investigations were carried out to investigate the interactions of polymer/coke blends with molten slag and molten steel at 1550ºC using the sessile drop technique.

8.1 Slag/Carbon Interactions

An in-depth investigation on slag/carbon interactions and the foaming behaviour of EAF slag (30.5 wt% FeO) with polymer/coke blends was carried out. The main focus was on establishing the influence of the chemical elements and volatiles present in the polymers (H2, O2, N2 and CH4) on the slag foaming behaviour and associated slag/carbon interactions. Polymers, namely Polyethylene Terephthalate (PET: 62.5% C, 4.2% H, 33.3% O) and Polyurethane (PU: 65.5% C, 4.9% H, 20.5% O, 9.0% N) were blended with metallurgical coke in a fixed proportion. Slag foaming was investigated using the sessile drop technique through dynamic changes in the volume of molten slag droplet. The formation of reduced iron and gas bubbles within the slag was investigated through microscopical investigations. Following conclusions can be drawn from this study:

1. Significant differences were observed in the foaming behaviour of molten slag with PET/Coke, PU/Coke and metallurgical coke. The chemical elements and volatiles present in the polymer were found to play a key role in slag foaming and associated slag/carbon interactions. 2. Polymer addition was found to improve the slag foaming and was a function of and polymer composition. Metallurgical coke showed a good foaming

behaviour initially with the volume ratio (Vt/V0) ~1 and then decreased with time to reach approximately 0.75 after 10 minutes of reaction. PET/Coke blend showed sustained slag foaming with the volume ratio stabilizing at 1.2, while the fluctuating slag foaming behaviour was observed for PU/Coke with the volume ratio ranging between 0.75-1.2 after 10 minutes of reaction.

3. Higher levels of CO and CO2 generation were recorded for slag-coke system compared to the corresponding gas evolution from slag-PET/Coke and slag- PU/Coke systems. Slag in contact with metallurgical coke however did not

8-2 Chapter 8 Conclusions and Future work

show much gas bubble entrapment with the size of slag droplet decreasing as a function of time. Small and large sizes of reduced iron droplets were also observed.

4. Despite lower levels of CO and CO2 generation, PET/Coke blend showed greater amount of gas bubbles entrapment with the size of the slag droplet stabilizing over the reaction times, along with a greater number of small and large sizes of reduced iron droplets. PU/Coke blend also showed greater amount of gas entrapment but it was observed to be less than that observed for PET/Coke blend, and the size of slag droplet showed fluctuations with time. A relatively larger number of reduced iron droplets in the case of PU/Coke were observed compared to that in the case of coke but their sizes were relatively smaller.

5. Higher levels of CO and CO2 detected for coke/slag system indicates significant reduction of FeO by carbon in the material. However, the presence of reduced iron droplets in the slag droplet observed in the case of PET/Coke and PU/Coke also indicate the reduction of FeO in the molten slag. 6. For polymer/coke blends, it was found that FeO in the molten slag was reduced

not only by carbon, but also by H2 and CH4 evolving from the decomposition of the polymers at high temperatures, with a reaction rate faster than the reduction

by carbon. CH4 released from the polymers can decompose to C and H2, and thus reducing FeO in the slag.

7. The reduction of FeO by H2 produces H2O vapour as the reaction product, which can react with solid C through carbon gasification by water vapour

thereby producing H2 and CO in the system. H2 can further reduce FeO in the molten slag, and CO can also react with FeO in the slag or be retained by the molten slag. These cyclic reactions are expected to lead to sustained slag foaming for longer reaction time in the slag-PET/Coke system compared to the case of coke alone. 8. The devolatilization of the polymer/coke blends is influenced by the chemical elements in the polymers, and this in turn was found to modify the slag foaming

behaviour. N2 from PU diluted the concentration of O2, H2 and CH4 in the system, and hindered reactions with molten slag. This observation is consistent with smaller number of reduced iron droplets observed for PU/Coke compared to PET/Coke. 8-3 Chapter 8 Conclusions and Future work

9. The present investigation clearly brings out the advantages of introducing plastics in blends with coke on improving of foaming behaviour in EAF steelmaking processes. This study also helps in widening the spectrum of polymers that may be suitable for recycling as a carbon resource.

8.2 Iron/Carbon Interactions

An in-depth investigation into iron/carbon interactions was carried out to assess the interactions of molten iron [electrolytic pure iron (99.98% Fe)] with polymer/coke blends. The aim was to identify the role of polymer addition on the wetting behaviour of the liquid steel with the polymer/coke blends, the formation of reaction products at the interfacial region and the associated carbon and sulphur transfer into liquid iron. Three types of polymers: Bakelite, High Density Polyethylene (HDPE) and Polyethylene Terephthalate (PET) were blended with metallurgical coke in three proportions [Bakelite/Coke (BK1, BK2, BK3), HDPE/Coke (H1, H2, H3) and

PET/Coke (P1, P2, P3)]. Bakelite contained high levels of CaCO3 as a filler material in the polymer, while HDPE and PET contained high levels of volatile matter. These differences in polymer chemistry can modify characteristics of blends and thus influence the reactions with molten iron at high temperature. Following conclusions can be made from this study:

8.2.1 Bakelite – Metallurgical Coke Blends

1. Bakelite addition was found to have a marginal effect on wetting behaviour. Liquid iron droplets showed a small variation in contact angles with bakelite/coke blends substrates. Blend BK1 showed better wetting than the parent coke with the initial contact angle being 118º then slightly increased to 121º after 60 minutes of contact. The increase in bakelite content in the blends was found to generally increase contact angles, leading to poorer wettability. This modification was attributed to the formation of interfacial reaction products.

8-4 Chapter 8 Conclusions and Future work

2. Bakelite addition was found to have a significant effect on modifying the morphology and composition of the reaction products layer formed at the metal/carbon interface, and this effect increased with increasing bakelite concentration in the blends. This was attributed to the presence of lime (CaO)

from the thermal decomposition of CaCO3 (a filler material) in the Bakelite which can in turn desulphurize the molten iron and form CaS based reaction products. 3. The interfacial reaction products in the case of BK1 and BK2 were observed to

be composed of CaS-Al2O3 phase mixture, while it was a combination of CaS

and CaO.Al2O3 phases for the blend BK3. The morphology of these interfacial layers were found to be significantly different from that observed for coke

whose interfacial layer was rich in Al2O3. The interfacial layer in the case of coke had a mesh like structure. 4. The interfacial layer was observed to be a crystal structure of CaS in the case of blend BK1. With increasing bakelite concentration as blend BK2, the interfacial layer was found to melt and sharp crystalline shapes could not be as clearly as seen in the case of BK1. The interfacial products had a glassy or fluid like structure in the case of blend BK3. CaO from the bakelite decreased the fusion temperature of interfacial deposits. The presence of Ca based materials at the interface in the case of bakelite/coke blends was found to produce more fluid phases at the interface and thus affect the transfer of carbon and sulphur. 5. Bakelite addition was found to have a moderate effect on carbon transfer behaviour compared to the case of metallurgical coke. The measured carbon pick-up value after 60 minutes of reaction for metallurgical coke was approximately 0.10 wt%. A marginally higher carbon pick-up was observed for bakelite/coke blends with the value was 0.13, 0.16 and 0.19 wt% for blends BK1, BK2 and BK3, respectively. 6. Bakelite addition was also found to have a moderate effect on the transfer of sulphur into the melt. The measured sulphur pick-up value after 60 minutes of reaction for metallurgical coke was approximately 0.06 wt%. A slight increase in sulphur pick-up was observed for bakelite/coke blends with the value was approximately 0.12, 0.11 and 0.09 wt% for blends BK1, BK2 and BK3, respectively.

8-5 Chapter 8 Conclusions and Future work

8.2.2 High Density Polyethylene (HDPE) – Metallurgical Coke Blends

1. HDPE addition was found to slightly improve the wettability with the molten iron. The measured initial contact angle for the metallurgical coke was 129º and decreased to 128º after 60 minutes of contact. Slightly better wettability was seen in the case of blend H1 with the initial contact angle was 116º that increased slightly to 118º after 60 minutes of contact. Marginal increases in contact angles were observed with increasing HDPE content (blends H2 and H3) showed a small increase in contact angles. This was attributed to the release of volatile matter from the HDPE/Coke blends.

2. Volatiles (CH4 and H2) from HDPE were not observed to play a role on changing chemical composition of the interfacial layer. The release of volatiles in the polymer was observed to however slow down the coverage of the interfacial region by reaction products layer. This had a small influence on the carbon and sulphur transfer into molten iron. 3. HDPE addition was found to also have a marginal effect on improving the carbon transfer into the melt. Local turbulence due to release of gases during devolatilization can improve rate of mass transfer. The measured carbon pick- up value after 60 minutes of reaction was 0.15, 0.15 and 0.17 wt% for blends H1, H2 and H3, while it was 0.1 wt% in the case of coke. 4. A small change in sulphur transfer was observed with the measured sulphur pick-up value after 60 minutes of reaction was ranging between 0.07-0.08 wt% for all the blends, while it was 0.05 wt% for coke.

8.2.3 Polyethylene Terephthalate (PET) – Metallurgical Coke Blends

1. PET addition was found to have a marginal effect on wettability. A slight decrease in contact angles (compared to that of coke) was seen in the case of blend P1 with the initial contact angle was 124º that slightly decreased to 121º after 60 minutes of contact. Increasing PET levels in the blends (P2 and P3) was observed to slightly increase in initial (after 1 minute) and final (after 60 minutes) contact angles.

8-6 Chapter 8 Conclusions and Future work

2. PET addition was found to produce significant differences in the formation of interfacial reaction products in term of morphology and chemical composition. Volatiles in PET (especially oxygen) oxidize liquid iron to form FeO slag at the interface. 3. The interfacial layer for PET/Coke blends (P2 and P3) was found to be

composed of a combination of FeO-Al2O3 phase mixture and the morphology was observed to be a platelet like structure. The devolatilization of PET/Coke was also observed to slow down the coverage of the interfacial region by reaction products layer. These changes were however found to have a small effect on the carbon and sulphur transfer behaviour. The formation of FeO at the metal/carbon interface can lead to an improvement in the carburization of molten steel by reducing the fusion temperature of the interfacial layer. Basic characteristic of coke were found to be a dominant factor in the carburizing behaviour. 4. PET addition was found to have a marginal effect on the carbon transfer with the measured carbon pick-up value after 60 minutes of reaction was 0.09, 0.15 and 0.15 wt% for blends P1, P2 and P3, respectively, while it was 0.1 wt% for coke. Similarly, a small change in sulphur transfer was observed with the measured sulphur pick-up value after 60 minutes of reaction was ranging between 0.05-0.08 wt% for PET/Coke blends.

8.2.4 Role of Chemical Composition in Polymers on Carburization of Molten Iron: Studies on 100% Polymers

Volatile matter (CH4 and H2) in pure HDPE and PET could play a role in the carburization of molten iron. A few experiments were carried out using 100% polymers. The carbon pick up by liquid metal after 60 minutes of reaction was found to be 2.65 wt% and 2.25 wt% for raw HDPE and PET, respectively. For metallurgical coke, a much lower value of 0.10 wt% was observed thereby indicating a relatively higher carbon pick up from pure HDPE and PET. This result was attributed to the role played by the structure of carbon. Carbon dissociated from the decomposition of CH4 due to the break-down of the polymers is graphitic in nature, and was found to carburize molten steel much faster than metallurgical coke which has poor crystallinity.

8-7 Chapter 8 Conclusions and Future work

H2 can also help clean molten metal surface and thus increase effective area available for carbon transfer. When blending polymers with coke, HDPE/Coke and PET/Coke blends were seen to have a marginal effect on enhancing the carburization of molten steel; it appears that blend behaviour was dominated by the coke characteristics and the level of polymers added resulted in minor modifications.

On the other hand, pure bakelite (without coke) which contained lower levels of volatile matter compared to HDPE and PET showed only marginal higher carbon pick up than coke. The carbon pick up by liquid metal after 60 minutes of reaction was found to be 0.19 wt% for pure bakelite. CaO formed from thermal decomposition of

CaCO3 in the bakelite was found to reduce the melting temperature of the reaction products layer at the metal/carbon interface, but this did not help much on enhancing the carburization of molten iron. Carbon in the bakelite is amorphous in nature and was not as effective for the carburization.

8.3 Future Work

These in-depth studies have shown that plastics can be used as a source of carbon in EAF steelmaking. Polymer/coke blends were found to enhance reactions with molten slag and steel compared to metallurgical coke alone. Waste plastics from industrial and household sectors are a mixture of different types of polymers, and thus a separation process is required, and this could increase the production costs. Moreover, they may contain some unwanted impurities, which could have some influence on the properties of the steel. It is recommended that future studies be focussed on the influence of mixed waste polymers on slag/carbon and iron/carbon interactions to determine the possibility of using mixed plastics as the source of carbon in EAF steelmaking. This would help to widen the possible streams of polymers that could be used in the steelmaking processes.

8-8 Chapter 9: References

CHAPTER 9 REFERENCES

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2007 Khanna R., Rahman M., Leow R. and Sahajwalla V., Novel sessile drop software for quantitative estimation of slag foaming in carbon/slag interactions, Metall. Mater. Trans. B, vol. 38B, pp. 719-723.

2007 Sandberg E., Lennox B. and Undall P., Scrap management by statistical evaluation EAF process data, Control Engineering Practice.

2008 Chapman M.W., Monaghan B.J., Nightingale S.A., Masthieson J.G. and Nightingale R. J., Formation of a mineral layer during coke dissolution into liquid iron and its influence on the kinetics of coke dissolution rate, Metall. Trans. B., vol. 39B, pp. 418-430.

9-11 Chapter 9: References

2008 Ueki Y., Mii R., Ohno K., Maeda T., Nishioka K. and Shimizu M., Reaction behaviour during heating waste plastic materials and iron oxide composites, ISIJ Int., vol. 48, No.12, pp. 1670-1675.

2009 Bale C.W., Pelton A.D., Thompson W.T., Eriksson G., Hack K., Chartrand P., Decterov S., Melancon J. and Petreson S., FactSage, 6.0 GTT Technology, Aachen, Germany.

2009 Cham T.S., Khanna R., Sahajwalla V., Sakurovs R. and French D., Influence of mineral matter on carbon dissolution from metallurgical coke into molten iron: Interfacial phenomena, ISIJ Int., vol. 49, No.12, pp. 1860-1867.

2009 Corbari R., Matsuura H., Halder S., Walker M. and Fruehan R.J., Foaming and the rate of the carbon-iron oxide reaction in slag, Metall. Mater. Trans. B, vol. 40B, pp. 940-948.

2009 PACIA, 2009 National Plastics Recycling Survey (2008 Calendar year), Australia. http://www.pacia.org.au/Content/media-21.12.2009-1.aspx

2009 Rahman M., Khanna R., Sahajwalla V. and O’Kane P., The influence of ash impurities on interfacial reactions between carbonaceous materials and EAF slag at 1550 ºC, ISIJ Int., vol. 49, No.3, pp. 329-336.

2009 Sahajwalla V., Rahman M., Khanna R., Saha-Chaudhury N., O’Kane P., Skidmore C. and Knights D., Recycling waste plastics in EAF steelmaking: Carbon/slag interactions of HDPE-Coke blends, Steel Res. Int., vol. 80, No.8, pp. 535-543.

2009 Zaharia M., Sahajwalla V., Khanna R., Koshy P. and O’Kane P., Carbon/slag interactions between coke/rubber blends and EAF slag at 1550ºC, ISIJ Int., vol. 49, No.10, pp. 1513-1521.

9-12 Chapter 9: References

2010 Ohno K., Maeda T., Nishioka K. and Shimizu M., Effect of carbon structure crystallinity on initial stage of iron carburization, vol. 50, No.1, pp. 53-58.

2010 Rahman M., Fundamental investigation of slag/carbon interactions in electric arc furnace steelmaking process, Ph.D. Thesis, The University of New South Wales, Sydney, Australia.

2010 Zaharia M., Reactions of waste rubber tyres and polypropylene plastics with gases and electric arc furnace steelmaking slags, Ph.D. Thesis, The University of New South Wales, Sydney, Australia.

2010 WCA (World Coal Association) http://www.worldcoal.org/resources/coal-statistics/coal-steel-statistics

2011 Dankwah J.R., Koshy P., Saha-Chaudhury N., O’Kane P., Skidmore C., Knights D. and Sahajwalla V., Reduction of FeO in EAF steelmaking slag by metallurgical coke and waste plastics blends, ISIJ Int., vol. 51, No.3, pp. 498- 507.

9-13 Appendix

APPENDIX

Appendix I: Calculation of Rate of FeO Reduction

Rate of FeO reduction was calculated based on the levels of carbon and oxygen removed during slag/carbon interaction using a similar approach taken by Min et al. (1999) and Dankwah et al. (2011). The oxygen level in the gases generated by the reduction reaction is accounted for the reduction of FeO in the slag. According to the mass balance for oxygen, the following equations can be derived:

−+−+=+ FeO xC Fe x CO x)1()12( CO 2 ....(...... 10 )1.

+=− 2 JJJ ...... (...... 10 )2. FeO CO CO 2

d (mol − FeO ) R −= t ×−= JA ...... (...... 10 )3. 0 dt FeO

-2 -1 where Ji = mole flux of i (mole-i.cm .s ), i can be FeO, CO and CO2. A = reaction area (cm2) -1 R0 = reaction rate (mole.s )

By assuming that CO and CO2 gases behave ideally, JCO and JCO2 can be calculated by

Eqs. 10.4 and 10.5 by using the gas compositions for CO, CO2 and Ar and the known -1 flow rate of Ar gas (FAr = 1/60 L.s ).

273 % CO 1 J F ××= ⋅ ...... (...... 10 )4. CO 298 Ar 22 × %4. r AA

273 % CO 1 J F ××= 2 ⋅ ...... (...... 10 )5. CO 298 Ar 22 × %4. r AA Appendix

The raw data of the volume of CO and CO2 gases collected from IR gas measurement was in from of part per million (ppm). The total volume of gases generated from the system is equal to

VCO + VCO2 + VAr = 1000000……..………(10.6)

Volume of Ar can be calculated from the known volume of CO and CO2. The percentage of gases composition generated from the system can be calculated by Eqs. 10.7 to 10.9.

V ppm )( %CO = CO ×100 ...... (...... 10 )7. 1000000

V ppm )( %CO = CO 2 ×100 ...... (...... 10 )8. 2 1000000

−−= % Ar 100 %% COCO 2 ...... (...... 10 )9.

The reaction area of the slag droplet in contact with the carbonaceous substrate was determined using the method expressed by Khanna et al. (2007). As consider the sessile drop assembly of the slag droplet and carbonaceous substrate in Figure 3-10 chapter 3, the reaction area (A) is represent by the contact area between slag droplet and carbonaceous substrate, which can be calculated using Eq. 10.10 [Khanna et al. (2007)].

π 2( −= hrhA )...... (...... 10.10)

The distance h and radius r can be measured using the computer software IMAGE TOOL. The sessile drop image was selected from the slag/carbon interaction experiment. By using IMAGE TOOL, the distance h was measured to be 362.86 μm and the radius r was measured to be 1391.71 μm. Therefore, the reaction area (A) can be calculated to be 0.0276 cm2.

10-2 Appendix

By assuming that the reaction area is comparable for all case (A = 0.0276 cm2), the rate of FeO reduction (R0) can be calculated. Sample of the results from the calculation is shown below.

Table 10-1: Sample for calculation of FeO reduction rate (R0)

Time %CO %CO %Ar JCO JCO2 JFeO R (sec) 2 0 0 0.13832 0.005894 99.85579 3.47935E-05 1.4826E-06 3.77587E-05 1.04214E-06 10 0.456712 0.018357 99.52493 0.000115265 4.63294E-06 0.000124531 3.43705E-06 20 1.02309 0.03223 98.94468 0.000259722 8.18191E-06 0.000276085 7.61996E-06 30 1.3645 0.04136 98.59414 0.000347623 1.0537E-05 0.000368697 1.0176E-05 40 1.73619 0.05391 98.2099 0.000444047 1.3788E-05 0.000471623 1.30168E-05 50 1.87307 0.055102 98.07183 0.000479729 1.41127E-05 0.000507955 1.40196E-05 60 1.96132 0.052274 97.98641 0.00050277 1.34001E-05 0.00052957 1.46161E-05 70 2.02285 0.048336 97.92881 0.000518848 1.23979E-05 0.000543643 1.50046E-05 80 2.06987 0.044546 97.88558 0.000531142 1.14308E-05 0.000554004 1.52905E-05 90 2.1091 0.041033 97.84987 0.000541407 1.05332E-05 0.000562473 1.55243E-05 100 2.14462 0.037357 97.81802 0.000550704 9.59268E-06 0.000569889 1.57289E-05

10-3 Appendix

Appendix II: Data Input for Thermodynamic Calculation using FactSage 6.0

To determine the effect of bakelite on the ash melting temperature of the carbonaceous blends, FactSage 6.0 [Bale et al. (2009)], thermodynamic software, was used to estimate the proportion solid and liquid components of the reaction products formed at the metal/carbon interface for coke and bakelite/coke blends at 1550ºC.

From the ash analysis of the chars of bakelite/coke blends presented in Table 5-4; only the relative proportions of SiO2, Al2O3, CaO and MgO were found to vary across different blends. Therefore, the data input for the thermodynamic calculation using

FactSage 6.0 was considered only the content of SiO2, Al2O3, CaO and MgO presented in the bulk ash. However, C and S were also taken into account in order to simulate the interfacial reactions occurred in the system. The ash oxides selected were normalized and then converted to the actual amount presented in the bulk ash of the materials. The data input for the FactSage calculation are given below.

Table 10-2: Data input for the thermodynamic calculation using FactSage 6.0

C Ash (wt%) Samples (wt%) SiO2 Al2O3 CaO MgO S* Coke 79.8 11.28 5.79 0.1 0.03 0.012 BK1 76.3 12.7 6.45 1.21 0.14 0.167 BK2 73.4 13.66 6.78 2.8 0.26 0.284 BK3 68.4 14.86 7.16 5.75 0.53 0.431

S* calculated from SO3 in the bulk ash

10-4