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Electrically enhanced slag-metal reactions...Electrically Enhanced Slag-Metal Reactions Md Saiful...
Transcript of Electrically enhanced slag-metal reactions...Electrically Enhanced Slag-Metal Reactions Md Saiful...
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Electrically Enhanced Slag-Metal
Reactions
Md Saiful Islam
A Thesis Presented for the Degree of Doctor of
Philosophy
Faculty of Science, Engineering, and
Technology
Swinburne University of Technology
Melbourne, Australia
2015
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Some of the results presented in this thesis have been published in journals and
conferences as listed below:
Journal Publications: 1. Islam, M. S., M. A. Rhamdhani, and G. A. Brooks. "Electrically Enhanced Boron
Removal from Silicon Using Slag." Metallurgical and Materials Transactions B
45.1 (2014): 1-5 2. Rhamdhani, M.A., Khaliq, A., Brooks, G.A., Masood, S., Ahmad, S., Islam, M.S.,
"More from Less, Generating Wealth from Lower Grade and Urban Metal/Ore
Sources", Advanced Materials Research, vol.112, 2015, pp.481-484.
International Conference Proceedings 1. Islam, Md S., Rhamdhani, M.A., Brooks, G.A., “Solar-grade silicon: current and
alternative production routes”, Chemeca 2011 Conference, Chemeca 2011, 18-
21 Sept. 2011, Sydney, NSW, Australia.
2. Islam, Md S., Rhamdhani, M.A., Brooks, G.A., "Electrochemical slag-metal
reaction for silicon production", Proceedings of 4th HTP Symposium 2012, pp:
24-25; Swinburne University of Technology, Victoria, Australia, 2012.
3. Islam, Md S., Rhamdhani, M.A., Brooks, G.A., " Kinetics of Silicon Refining
using Slag Treatment ", 4th & 5th February, 2013; Proceedings of 5th HTP
Symposium 2013, pp: 26-28; Swinburne University of Technology, Victoria,
Australia, 2013. 4. Islam, Md S., Rhamdhani, M.A., Brooks, G.A., “Electrically enhanced metal
purification using slag”, TMS (Celebrating the Megascale: Proceedings of the
Extraction and Processing Division Symposium on Pyrometallurgy in Honor of
David G.C. Robertson) (2014), pages 587 – 595.
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Abstract
Boron removal from molten silicon using slag has been studied in this study. A
system of Si-B alloys reacting with CaO-SiO2-Al2O3 slag was chosen in the present study.
This system is important as it is relevant to metallurgical grade silicon refining process to
produce solar grade silicon. This study comprised of a combination of thermodynamic
modelling and experimental study to study the distribution coefficient of boron for
selected slag systems. The thermodynamic modelling was carried out using FactSage 6.4
to predict the equilibrium concentration of elements in molten silicon and slag. The
thermodynamic modelling and experimental results on the distribution coefficient of
boron were assessed and compared with previous data. The thermodynamic modelling
predicted the ranges of boron partition ratio from 2.8 to 9.9 at different basicity for
selected slag systems. It was also predicted that higher values of boron partition ratio can
get with increasing slag-silicon ratio. The experimental results for CaO-SiO2 and CaO-
SiO2-Al2O3 bearing slags show that boron partition ratio change with basicity following
a negative parabola relationship with a maximum value of 2.8 for CaO/SiO2 ratio 1.20.
Moreover, the experimental results demonstrate that the boron partition ratio increases
with slag/silicon ratio and temperature in the system.
Kinetic analyses were also carried out in this study. The change in the
concentration of boron in silicon was tracked at different intervals of time and the rate at
which the refining takes place was determined. Reaction mechanism of boron transfer
from molten silicon with slag system was determined in this study. Specifically, the
author determined the mass transfer coefficient, the rate controlling step, and the
activation energy associated with boron removal in slag refining processes. Moreover,
the kinetic data were validated with kinetic models to understand the reaction mechanism
and to quantify the values. From the kinetics plots for used slag systems, the rate of boron
removal follows first order with respect to boron in molten silicon. It was found that
kinetics of slag-silicon reaction was controlled by mass transport in the slag phase and
the mass transfer coefficient increases with increasing the reaction temperature. It was
also calculated that the activation energy and mass transfer coefficient were 98.85 kJ/mol
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and of 1.86 X 10-6 m/sec respectively for boron removal from silicon using CaO-SiO2-
9.59 wt.% Al2O3 slag system .
The author has demonstrated in selected experiments the effect of application of
external potential on the reaction rate, and the equilibrium end point for the system
studied. The apparent equilibrium and the kinetic of the boron removal from silicon using
slag were altered by applying electrical potential across the slag-silicon phase. The effect
of applied potential was more pronounced on enhancing the kinetics rather than altering
the equilibrium. It was found that that by applying external potential difference
(maximum 5V) between the Si and CaO-SiO2 and CaO-SiO2-Al2O3 slag at 1823K, the
apparent boron partition ratio and the mass transfer coefficient were increased by
maximum of a factor of 1.77 and 1.79, respectively, in the condition studied.
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Acknowledgements
I would like to express my gratitude to Allah, the Almighty, who guides me throughout
my PhD study and beyond.
I am truly indebted and obliged to my supervisor, Associate Professor M. Akbar
Rhamdhani, for his guidance and support, and countless reading revisions of this thesis.
He has been supporting me throughout this challenging journey while I have been
juggling through my PhD journey. I really admire his enthusiasm on research and his
contribution to the society, which also motivates me to do this research. I would like to
thank him for introducing me to this field of research and the high temperature processing
research community.
I also would like to express my gratitude my second supervisor, Professor Geoffrey A.
Brooks, for his valuable discussions. To my external supervisor, Associate Professor
Mansoor Barati, I would like to thank for his valuable inputs on my research. I always
gain more insight on this project from my corresponding supervisors, especially about the
analytical skills as well as experimental skills.
I would like to thanks Prof. George Kaptay for the discussion that provides idea for
treatment of equilibrium at electrified interface.
I owe sincere thankfulness to technical staff at the Faculty of Science, Engineering and
Technology: Mr Phil Watson, Mr Alec Papanicolaou, Mr David Vass, Mr Walter
Chetcuti, Mr Krys Stachowicz, and Mr Andrew Moore for assisting me to construct
experimental set-up and help me technically. Without their assistances, it was very
challenging to construct and develop a novel laboratory and experimental rigs. I also
would like to thanks Dr. James Wang for assisting me conducting SEM/EDS analysis,
and XRD analysis.
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Thanks to all my colleagues at the High Temperature Processing group (in no special
order): Neslihan Dogan, Mohammad Dewan, Nazmul Huda, Morshed Alam, Bernard Xu,
Reiza Mukhlis, Abdul Khaliq, Behrooz Fateh, Jaifar Younus, Mehdi bin Muhammad and,
Sazzad Ahmed, Shabnam Sabah, and Muhammad Al Hossaini Shuva. In particular, I
would like to thank Reiza Mukhlis, Sazzad Ahmed, and Hasnat Jamil for all wonderful
discussions and sharing of knowledge and experiences during the duration of my PhD
journey.
I am honestly thankful to my wife, Rabia Sultana, who always supports me in any way to
finish my study. And also to my parents and sisters (Lovely Akter and Fatema Akter) in
my home country whose support and encourage me to finish this study, I would like to
thank them.
This thesis is dedicated to my beloved parents, Nazma Khatun and Suruj Mollah. They
have raised me with a love for science and always support me in any conditional way to
help me achieve my success since my childhood until now.
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To my parents
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Table of Contents
Declaration ....................................................................................................................... iii
Abstract ............................................................................................................................. v
Acknowledgements ......................................................................................................... vii
Table of Contents ............................................................................................................. xi
List of Tables ............................................................................................................... xviii
List of Figures ................................................................................................................ xxi
........................................................................................................................... 1
Introduction ................................................................................................................... 1
1.1 Background of the Study ......................................................................................... 1
1.2 Objectives of Present Study .................................................................................... 2
1.3 Outline of the Thesis ............................................................................................... 4
........................................................................................................................... 7
Literature Review .......................................................................................................... 7
2.1 Introduction ............................................................................................................. 7
2.1.2 Silicon Impurities ............................................................................................ 11
2.1.2.1 Sources of Impurities ............................................................................... 11
2.1.2.2 Effect of Impurities on Solar Cell Performance ....................................... 12
2.1.2.3 Effect of Boron and Phosphorus .............................................................. 12
2.1.2.4 Effect of Carbon, Oxygen and Nitrogen .................................................. 13
2.1.2.5 Effect of Transition Metals ...................................................................... 13
2.2 Industrial Production of Silicon............................................................................. 15
2.2.1 Metallurgical Grade Silicon ............................................................................ 15
2.2.2 High–Purity Silicon Production ...................................................................... 16
2.2.3 Chemical/Metallurgical Productions of Solar-Grade Silicon ......................... 19
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2.2.3.1 Trichlorosilane Routes ............................................................................. 19
2.2.3.2 Silane Processes ....................................................................................... 20
2.2.3.3 Fluoride Processes ................................................................................... 21
2.2.3.4 Zincothermic Reduction ........................................................................... 22
2.2.3.5 Aluminothermic Reduction ...................................................................... 23
2.2.3.6 Reduction by Alkali/Alkaline Earth Metals ............................................. 23
2.2.3.7 Silicon Halides Reduction by Hydrogen .................................................. 24
2.2.3.8 Halidothermic Reduction ......................................................................... 24
2.2.3.9 Carbothermic Reduction of Silica ............................................................ 24
2.2.3.10 Gas Phase Reduction of Pure SiO2 ........................................................ 25
2.2.4 Electrochemical Productions of Solar-Grade Silicon ..................................... 25
2.2.4.1 Three Layer Electrorefining ..................................................................... 25
2.2.4.2 Direct Reduction of SiO2 ......................................................................... 26
2.2.5 Other Refinement Techniques of Metallurgical Grade Silicon ...................... 27
2.2.5.1 Etching/Acid Leaching Process ............................................................... 27
2.2.5.2 Slag Process ............................................................................................. 28
2.2.5.3 Electron Beam Melting and Gas Blowing (Plasma) ................................ 29
2.2.5.4 Solidification from Si-Al Alloy ............................................................... 30
2.2.6 Summary of Industrial Production of Silicon ................................................. 30
2.3 Background Theory of Thermodynamics Modelling ............................................ 31
2.3.1 Basic Thermodynamics .................................................................................. 31
2.3.2 Solution Models .............................................................................................. 33
2.3.2.1 Ideal Solution ........................................................................................... 34
2.3.2.2 Regular Solution ...................................................................................... 34
2.3.2.3 Dilute Solution ......................................................................................... 35
2.3.2.4 Substitutional Solution Model ................................................................. 36
2.3.2.5 Modified Quasi-Chemical Model ............................................................ 38
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2.4 Background Theory on Kinetics of Metallurgical Reactions ................................ 39
2.4.1 Introduction ..................................................................................................... 40
2.4.2 Kinetics Theory on Liquid-Liquid reactions .................................................. 41
2.4.2.1 Chemical Reaction Controlled Kinetics ................................................... 41
2.4.2.2 Diffusion and Mass Transfer Controlled Kinetics ................................... 43
2.4.2.3 Mixed Control Kinetics ............................................................................ 44
2.4.3 Mass Transfer Models .................................................................................... 45
2.4.3.1 Two Film Model ...................................................................................... 45
2.4.3.2 Boundary Layer Model ............................................................................ 45
2.4.3.3 The Penetration Theory ............................................................................ 46
2.4.3.4 Surface Renewal Theory .......................................................................... 46
2.5 Thermodynamics and Kinetics of Boron Removal from Silicon Using Slag Process
..................................................................................................................................... 47
2.5.1 Basics of Slag Process .................................................................................... 47
2.5.2 Thermodynamics of Boron Removal .............................................................. 49
2.5.2.1 Effect of Slag/Silicon Ratio on Boron Removal Efficiency .................... 50
2.5.3 Previous Works on the Partition Ratio of Boron during Slag Process ........... 52
2.5.3.1 The Binary CaO–SiO2 Slag System ......................................................... 52
2.5.3.2 The CaO–SiO2-CaF2 Slag System ........................................................... 58
2.5.3.3 The CaO-SiO2- Al2O3 Slag System ......................................................... 59
2.5.3.4 The CaO-SiO2-MgO Slag System ............................................................ 61
2.5.3.5 Other Slag System .................................................................................... 62
2.6 Electrochemically Enhanced Slag-Metal Reaction ............................................... 65
2.6.1 Introduction ..................................................................................................... 65
2.6.2 Rate of Electrochemical Reaction................................................................... 66
2.6.3 Literature Survey on Electrochemically Enhanced Slag-Metal Reaction ...... 67
2.6.3.1 Electrochemical Sulphur Transfer............................................................ 67
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2.6.3.2 Electrochemical Iron Oxide Reduction/Decarburization Reaction .......... 69
2.6.3.3 Other Electrochemical Systems ............................................................... 71
2.7 Summary ............................................................................................................... 72
......................................................................................................................... 75
Experimental Methodology ......................................................................................... 75
3.1 Introduction ........................................................................................................... 75
3.2 Thermodynamic Assessment Approach ................................................................ 75
3.3 Experimental Study ............................................................................................... 77
3.3.1 Sample Preparation ......................................................................................... 77
3.4 Temperature Profile Measurement ........................................................................ 81
3.5 Main Experimental Program ................................................................................. 82
3.5.1 Thermodynamical Experiments ...................................................................... 82
3.5.2 Kinetic Experiments ....................................................................................... 84
3.5.3 Electrically Enhanced Experiments ................................................................ 86
3.6 Material Characterisation ...................................................................................... 88
3.6.1 Sample Preparation Microscopy (OM, SEM, EDX) ...................................... 90
3.6.2 Scanning Electron Microscope ....................................................................... 90
3.6.3 Energy Dispersive Spectroscopy .................................................................... 91
3.6.4 ICP-AES ......................................................................................................... 91
3.7 Error Analysis........................................................................................................ 91
......................................................................................................................... 93
Thermodynamic Study on Boron Removal ................................................................. 93
4.1 Introduction ........................................................................................................... 93
4.2 Thermodynamic Assessment for Silicon and Slag System ................................... 93
4.2.1 Modelling Methodology ................................................................................. 94
4.2.1.1 Development of Thermodynamic Assessment ........................................ 94
4.2.2 Equilibrium Calculation.................................................................................. 98
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4.2.2.1 Effect of Basicity on Boron Partition Ratio ............................................. 99
4.2.2.2 Effect of Slag/Silicon Ratio on Boron Partition Ratio ........................... 108
4.2.2.3 Effect of Alumina Content on Boron Partition Ratio............................. 116
4.2.2.4 Effect of MgO Content on Boron Partition Ratio .................................. 118
4.2.2.5 Effect of CaF2 Content on Boron Partition Ratio ................................... 120
4.2.3 Remarks on Thermodynamic Assessment .............................................. 122
4.3 Analysis on Distribution of Boron between Slag and Silicon from Experimental
Study .......................................................................................................................... 124
4.3.1 Introduction ................................................................................................... 124
4.3.2 Effect of Basicity for Different Slag Systems .............................................. 125
4.3.3 Effect of Slag/Silicon Ratio on Boron Partition Ratio .................................. 130
4.3.4 Remarks on Experimental Results ................................................................ 136
....................................................................................................................... 139
Kinetics Study ........................................................................................................... 139
5.1 Introduction ......................................................................................................... 139
5.2 Kinetic Study of B Removal Using CaO-SiO2 and CaO-SiO2-Al2O3 ................. 140
5.3 Analysis of Experimental Data ............................................................................ 144
5.3.1 Order of the Reaction .................................................................................... 144
5.3.2 Final Kinetic Equation .................................................................................. 148
5.3.3 Interfacial Area Measurement ...................................................................... 149
5.3.4 Effect of Slag/Silicon Ratio .......................................................................... 152
5.3.5 Effect of Temperature ................................................................................... 155
5.3.6 Mass Transfer Models .................................................................................. 158
5.3.7 Effect of Alumina on Mass Transfer Coefficient (ks) ................................... 160
5.4 Summary ............................................................................................................. 161
....................................................................................................................... 163
Electrically Enhanced Boron Removal ..................................................................... 163
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6.1 Introduction ......................................................................................................... 163
6.2 Preliminary Experiments for Slag-Silicon System .............................................. 164
6.3 Kinetic Results with Applying Voltage............................................................... 167
6.3.1 CaO-SiO2 System .......................................................................................... 168
6.3.2 CaO-SiO2-9.59% Al2O3 System ................................................................... 171
6.3.3 CaO-SiO2-15.9% Al2O3 System ................................................................... 173
6.3.4 Kinetic Analyses with 5 Volt Potential ........................................................ 175
6.3.5 Summary ....................................................................................................... 180
6.4 Equilibrium Shifting with Potential .................................................................... 181
6.4.1 Introduction .................................................................................................. 181
6.4.2 Equilibrium Results ...................................................................................... 183
6.6 Summary ............................................................................................................. 187
....................................................................................................................... 189
Conclusions and Recommendations .......................................................................... 189
References ..................................................................................................................... 193
Appendix A ........................................................................................................ 204
Compositions of Si-alloys and Master Slags ............................................................. 204
Appendix B ........................................................................................................ 209
Temperature profile in Vertical Tube Furnace .......................................................... 209
Appendix C ........................................................................................................ 211
Melting Furnaces ....................................................................................................... 211
Appendix D ........................................................................................................ 213
Examples of Equilibrium Calculation ....................................................................... 213
Appendix E ........................................................................................................ 221
Other Results of Thermodynamic Assessment ......................................................... 221
A. Thermodynamic data for CaO-SiO2-15 wt.% Al2O3 slag system ............... 221
B. Effect of Temperature on Impurities Contents in Slag and Silicon ............ 226
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Appendix F ......................................................................................................... 230
Kinetics Data during Slag-Silicon Reaction .............................................................. 230
Appendix G ........................................................................................................ 235
Mass Transfer Control Kinetic Equations ................................................................. 235
G-1 Silicon mass transfer controlled equation ....................................................... 235
G-2 Slag mass transfer equation ............................................................................ 236
Appendix H ................................................................................................................... 239
Error Analysis ............................................................................................................ 239
H-1 Master silicon and slag preparation ................................................................ 240
H-2 Temperature measurement inside the furnace ................................................ 241
H-3 Starting and ending time of the slag-silicon reaction ..................................... 241
H-4 Sample positioning inside the furnace ............................................................ 242
H-5 Sample preparation after the experiments ...................................................... 242
H-6 Sample characterisation techniques error ....................................................... 242
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List of Tables Table 2-1: Typical impurity levels for MG-Si, SOG-Si and EG-Si (Wang and Ciszek,
2001) ................................................................................................................................. 8
Table 2-2: Some physical properties of silicon (Zulhane, 1993) ...................................... 9
Table 2-3: Global polycrystalline silicon market data, (in tons) (Source: QY Research,
2007) ............................................................................................................................... 11
Table 2-4: Selected solar grade polycrystalline silicon production and refinement
processes ......................................................................................................................... 17
Table 2-5: Previous works on slag processes for boron removal ................................... 54
Table 2-6: Previous studies on the kinetics experiments for boron removal .................. 65
Table 3-1: Purity and Composition of raw materials ...................................................... 78
Table 3-2: Composition analysis of the Master Silicon alloy used in the experiments .. 79
Table 3-3: Composition Analysis of the Master Slag#1 used in the experiments .......... 81
Table 3-4: Experiments performed for determination of the distribution coefficient of
boron ............................................................................................................................... 83
Table 3-5: Experimental parameters for kinetics experiments ....................................... 84
Table 3-6: Experiments performed with applying external potential across slag-silicon
interface ........................................................................................................................... 86
Table 4-1: FactSage built-in database used in this study ................................................ 96
Table 4-2: Parameter conditions investigated for equilibrium calculations. .................. 97
Table 4-3: FactSage built-in solution database used in this study .................................. 98
Table 4-4: General features observed from thermodynamic assessment using FactSage
....................................................................................................................................... 123
Table 4-5: Experimental plan for the thermodynamic analyses of boron removal from
molten silicon. ............................................................................................................... 125
Table 4-6: Initial and final slag-silicon composition for CaO-SiO2 slag system for
different basicity ranges ................................................................................................ 127
Table 4-7: Initial and final slag-silicon composition for CaO-SiO2-10% Al2O3 slag system
for different basicity ranges .......................................................................................... 129
Table 4-8: Initial and final slag-silicon composition for CaO-SiO2-15% Al2O3 slag system
for different basicity ranges .......................................................................................... 130
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Table 4-9: Initial and final slag-silicon composition for CaO-SiO2 slag system for
different slag/silicon ratio ............................................................................................. 131
Table 4-10: Initial and final slag-silicon composition for CaO-SiO2-10 wt.% Al2O3 slag
system for different slag/silicon ratio ............................................................................ 132
Table 4-11: Initial and final slag-silicon composition for CaO-SiO2-15%Al2O3 slag
system for different slag/silicon ratio ............................................................................ 134
Table 4-12: Initial and final slag-silicon composition for CaO-SiO2-10%CaF2 slag system
for different slag/silicon ratio ........................................................................................ 135
Table 4-13: Initial and final slag-silicon composition for CaO-SiO2-10 wt.% MgO slag
system for different slag/silicon ratio ............................................................................ 136
Table 4-14: General features observed from experimental results ............................... 137
Table 5-1: Experimental plan for the kinetic analysis of boron removal from molten
silicon. ........................................................................................................................... 140
Table 5-2: Reaction interfacial area calculation after the kinetic experiments at time 180
minutes for CaO-SiO2-9.59 wt.% Al2O3 at 1550°C ...................................................... 151
Table 5-3: Reaction interfacial area calculation after the kinetic experiments for
slag/silicon ratio 2.0, at different time intervals for CaO-SiO2-9.59 wt.% Al2O3 at 1550°C
....................................................................................................................................... 152
Table 5-4: The calculated mass transfer coefficient and the activation energy data for
different slag systems .................................................................................................... 156
Table 5-5: Slag compositions and the viscosities (Using FactSage 6.4)....................... 158
Table 6-1: Experimental plan for the kinetic analysis with applied potential of boron
removal from molten silicon. ........................................................................................ 168
Table 6-2: Experimental plan for the thermodynamic analysis with applied potential of
boron removal from molten silicon. .............................................................................. 184
Table 6-3: Initial and final slag-silicon phase compositions for CaO-SiO2 slag system for
different applied voltage ............................................................................................... 185
Table 6-4: Initial and final slag-silicon composition for CaO-SiO2-9.59 wt.% Al2O3 slag
system for different applied voltage .............................................................................. 186
Table 6-5: Initial and final slag-silicon composition for CaO-SiO2-15.9 wt.% Al2O3 slag
system for different applied voltage .............................................................................. 187
Table A-1: Composition of raw MG-Si ........................................................................ 204
Table A-2: Composition analysis of the master slag#2 used in the experiments .......... 205
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Table A-3: Composition analysis of the master slag#3 used in the experiments ......... 206
Table A-4: Composition analysis of the master Slag#4 used in the experiments ......... 207
Table A-5: Composition analysis of the master slag#5 used in the experiments ......... 208
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List of Figures
Figure 2-1: Installed capacity by country reported to International Energy Agency in
2012(Source: IEA-PVPS Trends in photovoltaic Applications 2012) ............................ 10
Figure 2-2: Solar power generation as percentage of world electricity consumption
(2011). (Source: IEA-PVPS Trends in photovoltaic Applications 2011) ....................... 10
Figure 2-3: Effect of metal impurities on P-type solar cell efficiency (Hopkins et al., 1986)
......................................................................................................................................... 14
Figure 2-4: Schematic diagram of Siemens process (Zadde et al., 2002) ....................... 20
Figure 2-5: Schematic diagram of Silane process (Zadde et al., 2002) .......................... 22
Figure 2-6: A schematic diagram of three-layer electrorefining of Si (Olsen and Rolseth,
2010) ............................................................................................................................... 26
Figure 2-7: Schematic diagram of slag process .............................................................. 28
Figure 2-8: A schematic flow diagram of SOG-Si production (Khattak et al., 2002) .... 29
Figure 2-9: The five possible rate-controlling mechanisms in the slag-metal reaction:
metal phase control (a); slag phase control (b); mixed mass transfer control (c); chemical
control (d); and (e) mixed control (Richardson, 1974) ................................................... 41
Figure 2-10: Elingham diagram for oxides (Lynch, 2009) ............................................. 48
Figure 2-11: Boron distribution versus basicity for CaO-SiO2 binary system (Teixeira et
al., 2009) ......................................................................................................................... 57
Figure 2-12: Boron distribution versus basicity as a function of slag composition for CaO-
SiO2 binary system (Jakobsson and Tagstad, 2014) ........................................................ 57
Figure 2-13: Boron distribution versus basicity for CaO-SiO2-CaF2 slag system .......... 59
Figure 2-14: Boron distribution versus basicity for CaO-SiO2- Al2O3 slag system ....... 60
Figure 2-15: Boron distribution for CaO-SiO2- Al2O3 slag system (Jakobsson and
Tangstad, 2014) ............................................................................................................... 61
Figure 2-16: Boron distribution versus basicity for CaO-SiO2- MgO slag system ........ 62
Figure 3-1: Summary of experimental methodology ...................................................... 76
Figure 3-2: Slag Preparation Technique ......................................................................... 80
Figure 3-3: Induction melting of slag sample ................................................................. 80
Figure 3-4: Schematic representation of the vertical tube furnace for kinetic experiments
......................................................................................................................................... 85
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Figure 3-5: Schematic representation of the vertical tube furnace for electrochemical
experiments; Legend: 1-Silicone O-ring, 2- MoSi2 heating element, 3-Al2O3 crucible, 4-
Silicon melt, 5-Slag melt, 6-Al2O3 pedestal, 7-Al2O3 tube, 8-Water cooled flanges, 9-
Copper ............................................................................................................................. 88
Figure 3-6: LabView coding to record the voltage during experiments ......................... 89
Figure 4-1: Boron partition ratio as a function of basicity for CaO-SiO2 slag system
(slag/silicon ratio 2.0) ................................................................................................... 100
Figure 4-2: Equilibrium calcium and boron content as a function of basicity for CaO-SiO2
slag system (at temperature 1550°C and slag/silicon ratio 2.0) .................................... 100
Figure 4-3: Equilibrium slag composition as a function of basicity for CaO-SiO2 slag
system (at temperature 1550°C and slag/silicon ratio 2.0) ........................................... 101
Figure 4-4: Boron partition ratio as a function of basicity for CaO-SiO2-10 wt.% Al2O3
slag system (slag/silicon ratio 2.0) ................................................................................ 101
Figure 4-5: Equilibrium calcium, aluminium and boron content as a function of basicity
for CaO-SiO2-10 wt.% Al2O3 slag system (at temperature 1550°C and slag/silicon ratio
2.0) ................................................................................................................................ 103
Figure 4-6: Equilibrium slag composition as a function of basicity for CaO-SiO2-10 wt.%
Al2O3 slag system (at temperature 1550°C and slag/silicon ratio 2.0) .......................... 103
Figure 4-7: Boron partition ratio as a function of basicity for CaO-SiO2-10 wt.% MgO
slag system .................................................................................................................... 104
Figure 4-8: Equilibrium calcium, magnesium and boron content as a function of basicity
for CaO-SiO2-10wt.% MgO slag system (at temperature 1550°C and slag/silicon ratio
2.0) ................................................................................................................................ 105
Figure 4-9: Equilibrium slag composition as a function of basicity for CaO-SiO2-10 wt.%
MgO slag system (at temperature 1550°C and slag/silicon ratio 2.0) .......................... 105
Figure 4-10: Boron partition ratio as a function of basicity for CaO-SiO2-10 wt.% CaF2
slag system .................................................................................................................... 106
Figure 4-11: Equilibrium calcium and boron content as a function of basicity for CaO-
SiO2-10wt.% CaF2 slag system (at temperature 1500°C and slag/silicon ratio 2.0) ..... 107
Figure 4-12: Equilibrium slag composition as a function of basicity for CaO-SiO2-10
wt.% CaF2 slag system (at temperature 1500°C and slag/silicon ratio 2.0) .................. 107
Figure 4-13: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2 slag
system (CaO/SiO2 ratio = 1.21) .................................................................................... 108
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Figure 4-14: Equilibrium calcium and boron content as a function of slag/silicon ratio for
CaO-SiO2 slag system (at temperature 1550°C and basicity 1.21) ............................... 109
Figure 4-15: Equilibrium slag composition as a function of slag/silicon ratio for CaO-
SiO2 slag system (at temperature 1550°C and basicity 1.21) ....................................... 109
Figure 4-16: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2- 10
wt.% Al2O3 slag system ................................................................................................ 110
Figure 4-17: Equilibrium calcium, aluminium and boron content as a function of
slag/silicon ratio for CaO-SiO2- 10 wt.% Al2O3 slag system (at temperature 1550°C and
basicity 1.21) ................................................................................................................. 111
Figure 4-18: Equilibrium slag composition as a function of slag/silicon ratio for CaO-
SiO2 -10 wt.% Al2O3 slag system (at temperature 1550°C and basicity 1.21) ............. 111
Figure 4-19: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2- 10
wt.% MgO slag system (at temperature 1550°C and basicity 1.21) ............................. 112
Figure 4-20: Equilibrium calcium, aluminium and boron content as a function of
slag/silicon ratio for CaO-SiO2- 10 wt.% MgO slag system (at temperature 1550°C and
basicity 1.21) ................................................................................................................. 113
Figure 4-21: Equilibrium slag composition as a function of slag/silicon ratio for CaO-
SiO2 -15 wt.% MgO slag system (at temperature 1550°C and basicity 1.21) .............. 113
Figure 4-22: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2- 10
wt.% CaF2 slag system .................................................................................................. 114
Figure 4-23: Equilibrium calcium and boron content as a function of slag/silicon ratio for
CaO-SiO2- 10 wt.% CaF2 slag system (at temperature 1500°C and basicity 1.21) ...... 115
Figure 4-24: Equilibrium slag composition as a function of slag/silicon ratio for CaO-
SiO2 -10 wt.% CaF2 slag system (at temperature 1500°C and basicity 1.21) ............... 115
Figure 4-25: Effect of initial alumina content in slag on boron partition ratio (slag/silicon
ratio 2.0) ........................................................................................................................ 117
Figure 4-26: Effect of initial alumina content in slag on equilibrium calcium, aluminium
and boron content in silicon (at temperature 1550°C and basicity 1.21) ...................... 117
Figure 4-27: Effect of initial alumina content in slag on equilibrium slag compositions (at
temperature 1550°C and basicity 1.21) ......................................................................... 118
Figure 4-28: Effect of initial MgO content in slag on boron partition ratio ................. 119
Figure 4-29: Effect of initial magnesium oxide content in slag on equilibrium calcium,
magnesium and boron content in silicon (at temperature 1550°C and basicity 1.21) .. 119
-
Figure 4-30: Effect of initial MgO content in slag on equilibrium slag compositions (at
temperature 1550°C and basicity 1.21) ......................................................................... 120
Figure 4-31: Effect of initial CaF2 content in slag on boron partition ratio .................. 121
Figure 4-32: Effect of initial CaF2 content in slag on equilibrium calcium and boron
content in silicon (at temperature 1500°C and basicity 1.21) ....................................... 121
Figure 4-33: Effect of initial CaF2 content in slag on equilibrium slag compositions (at
temperature 1500°C and basicity 1.21) ......................................................................... 122
Figure 4-34: Boron partition ratio as a function of basicity for CaO-SiO2 slag system
(slag-silicon mass ratio 2.0) .......................................................................................... 126
Figure 4-35: Boron distribution versus basicity for CaO-SiO2 binary system (Teixeira et
al., 2009) ....................................................................................................................... 127
Figure 4-36: Boron partition ratio as a function of basicity for CaO-SiO2-Al2O3 slag
system ........................................................................................................................... 129
Figure 4-37: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2 slag
system ........................................................................................................................... 131
Figure 4-38: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2-9.59
wt.% Al2O3 slag system ................................................................................................ 132
Figure 4-39: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2-15.9
wt.% Al2O3 slag system ................................................................................................ 133
Figure 4-40: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2-10 wt.%
CaF2 slag system ........................................................................................................... 134
Figure 4-41: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2-10 wt.%
MgO slag system ........................................................................................................... 136
Figure 4-42: Boron partition ratio as a function of temperature for different slag system
(slag-silicon ratio 2.0) ................................................................................................... 138
Figure 5-1: The change of Boron content of different slag/silicon ratio during reactions
(Si-370ppm B alloy with CaO-SiO2-9.59 wt.% Al2O3 slag) at 1550°C. ...................... 142
Figure 5-2: The change of Boron content at 1500°C, 1550°C and 1600°C during reactions
(Si-370ppm B alloy with CaO-SiO2-9.59 wt.% Al2O3 slag). ....................................... 142
Figure 5-3: The change of Boron content of different slag/silicon ratio during reactions
(Si-350ppm B alloy with CaO-SiO2-15.9 wt.% Al2O3 slag) at 1550°C. ...................... 143
Figure 5-4: The change of Boron content at 1500°C, 1550°C and 1600°C during reactions
(Si-370ppm B alloy with CaO-SiO2-15.9 wt.% Al2O3 slag) ........................................ 143
-
Figure 5-5: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-9.59 wt.% Al2O3 at
1550°C assuming nth order kinetic with n = 0.1 ........................................................... 145
Figure 5-6: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-9.59 wt.% Al2O3 at
1550°C assuming nth order kinetic with n = 0.8 ........................................................... 146
Figure 5-7: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-9.59 wt.% Al2O3 at
1550°C assuming nth order kinetic with n = 1.8 ............................................................ 146
Figure 5-8: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-9.59 wt.% Al2O3 at
1550°C assuming nth order kinetic with n = 1.0 ........................................................... 148
Figure 5-9: Crosssectional view of the crucible after the experiment. ......................... 151
Figure 5-10: Integrated rate plots of Si-370 ppm B + CaO-SiO2-9.59 wt.% Al2O3 at
1550°C, using kinetic equation for silicon mass transfer control ................................. 153
Figure 5-11: Integrated rate plots of Si-370 ppm B + CaO-SiO2-9.59 wt.% Al2O3 at
1550°C, using kinetic equation for slag mass transfer control ..................................... 153
Figure 5-12: Integrated rate plots of Si-370 ppm B + CaO-SiO2-15.9 wt.% A2O3 at
1550°C, using kinetic equation for silicon mass transfer control ................................. 154
Figure 5-13: Integrated rate plots of Si-370 ppm B + CaO-SiO2-15.9 wt.% Al2O3 at
1550°C, using kinetic equation for slag mass transfer control ..................................... 154
Figure 5-14: Integrated rate plot assuming mass transport in the slag, with respect to B in
the Silicon bath, showing the effect of temperature on the reaction rate. (Si-370 ppm B
and CaO-SiO2-9.59 wt.% Al2O3 slag) ........................................................................... 156
Figure 5-15: A plot between ln k and 1000/T from the experimental data (Si-370 ppm B
and CaO-SiO2-9.59 wt.% Al2O3 slag) ........................................................................... 157
Figure 5-16: Integrated rate plot assuming mass transport in the slag, with respect to B in
the Silicon bath, showing the effect of temperature on the reaction rate (Si-370 ppm B
and CaO-SiO2-15.9 wt.% Al2O3 slag) ........................................................................... 157
Figure 5-17: A plot between ln k and 1000/T from the experimental data (Si-370 ppm B
and CaO-SiO2-15.9 wt.% Al2O3 slag) ........................................................................... 158
Figure 5-18: A plot of ks versus T/ƞs of the experimental data .................................... 160
Figure 5-19: A plot between ks and wt.% alumina ....................................................... 161
Figure 6-1: Open-circuit voltage measurement set-up of slag melt .............................. 165
Figure 6-2: Open-circuit voltage measurement set-up between slag and silicon melt . 166
Figure 6-3: Potential difference between slag and silicon with time for reaction between
CaO-SiO2-9.59 wt.% Al2O3 slag and Si-B bath ............................................................ 166
-
Figure 6-4: Experimental set-up for applying external voltage .................................... 167
Figure 6-5: The change in boron concentration (in ppm) in silicon with time using the
CaO-SiO2 slag at 1823 K with applied potential of 0V, 2V and 3V. The weight ratio of
slag to silicon was 2.0. .................................................................................................. 169
Figure 6-6: The integrated rate plot for boron removal for different applied potential for
CaO-SiO2 slag, with slag to silicon ratio of 2.0 at 1550°C ........................................... 170
Figure 6-7: The change in boron concentration (in ppm) in silicon with time using the
CaO-SiO2-9.59 wt.% Al2O3 slag at 1550°C with applied potential of 0V, 2V and 3V. The
weight ratio of slag to silicon was 2.0 ........................................................................... 172
Figure 6-8: The integrated rate plot for boron removal for different applied potential for
CaO-SiO2-9.59%Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ...................... 173
Figure 6-9: The change in boron concentration (in ppm) in silicon with time using the
CaO-SiO2-15.9%Al2O3 slag at 1550°C with applied potential of 0V, 2.5V and 3.5V. The
weight ratio of slag to silicon was 2.0. .......................................................................... 174
Figure 6-10: The integrated rate plot for boron removal for different applied potential for
CaO-SiO2-15.9 wt.% Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ............... 175
Figure 6-11: The change in boron concentration (in ppm) in silicon with time using the
CaO-SiO2-9.59 wt.%Al2O3 slag at 1550°C with applied potential of 3.0V and 5.0V. The
weight ratio of slag to silicon was 2.0 ........................................................................... 176
Figure 6-12: The change in boron concentration (in ppm) in silicon with time using the
CaO-SiO2-15.9 wt.%Al2O3 slag at 1550°C with applied potential of 3.5V and 5.0V. The
weight ratio of slag to silicon was 2.0 ........................................................................... 176
Figure 6-13: The integrated rate plot for boron removal for 3V and 5V applied potential
for CaO-SiO2-9.59% Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ............... 177
Figure 6-14: The integrated rate plot for boron removal for different applied potential for
CaO-SiO2-15.9 wt.% Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ............... 178
Figure 6-15: Current change during Si-370 ppm B + CaO-SiO2-9.59 wt.% Al2O3 at
1550°C when voltage was set constant of 5V ............................................................... 179
Figure 6-16: Current change during Si-370 ppm B + CaO-SiO2-15.9 wt.% Al2O3 at
1550°C when voltage was set constant of 5V ............................................................... 179
Figure 6-17: Effect of voltage on the mass transfer coefficient .................................... 180
Figure 6-18: The dependence of the partition ratio of boron on the applied anodic potential
calculated by Equation 6.6 using 3oBL , T = 1823 K .................................................. 183
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Figure 6-19: Effect of voltage on the boron partition ratio ........................................... 185
Figure 6-20: Reaction mechanism involving with applying DC potential across the slag
and silicon bath ............................................................................................................. 188
Figure B-1: Temperature profile in vertical tube furnace at 1500°C ............................ 209
Figure B-2: Temperature profile in vertical tube furnace at 1550°C ............................ 210
Figure B-3: Temperature profile in vertical tube furnace at 1600°C ............................ 210
Figure C-1: Induction melting facilities (a) melting unit (b) control panel .................. 211
Figure C-2: Slag melting facilities (a) melted slag (b) control panel ............................ 211
Figure C-3: Vertical tube resistant furnace used for experiments................................. 212
Figure D-1: Input species for the equilibrium reaction ................................................. 213
Figure D-2: Details of databases considered in the equilibrium calculation ................ 214
Figure D-3: Solution models and equilibrium conditions ............................................. 214
Figure E-1: Boron partition ratio as a function of basicity for CaO-SiO2-15 wt.% Al2O3
slag system .................................................................................................................... 221
Figure E-2: Equilibrium calcium, aluminium and boron content as a function of basicity
for CaO-SiO2-15 wt.% Al2O3 slag system (at temperature 1550°C and slag/silicon ratio
2.0) ................................................................................................................................ 222
Figure E-3: Equilibrium slag composition as a function of basicity for CaO-SiO2-15 wt.%
Al2O3 slag system (at temperature 1550°C and slag/silicon ratio 2.0) .......................... 222
Figure E-4: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2- 15 wt.%
Al2O3 slag system .......................................................................................................... 223
Figure E-5: Equilibrium calcium, aluminium and boron content as a function of
slag/silicon ratio for CaO-SiO2- 15 wt. % Al2O3 slag system (at temperature 1550°C and
basicity 1.21) ................................................................................................................. 223
Figure E-6: Equilibrium slag composition as a function of slag/silicon ratio for CaO-SiO2
-15 wt.% Al2O3 slag system (at temperature 1550°C and basicity 1.21) ...................... 224
Figure E-7: Equilibrium calcium, aluminium and boron content as a function of
temperature for CaO-SiO2-15wt.% Al2O3 slag system (basicity 1.21 and slag/silicon ratio
2.0) ................................................................................................................................ 224
Figure E-8: Equilibrium slag composition as a function of temperature for CaO-SiO2-15
wt.% Al2O3 slag system (and basicity 1.21 and slag/silicon ratio 2.0) ......................... 225
Figure E-9: Equilibrium slag composition as a function of temperature for CaO-SiO2 slag
system (basicity 1.21 and slag/silicon ratio 2.0) ........................................................... 226
-
Figure E-10: Equilibrium calcium and boron content as a function of temperature for
CaO-SiO2 slag system (basicity 1.21 and slag/silicon ratio 2.0) .................................. 226
Figure E-11: Equilibrium calcium, aluminium and boron content as a function of
temperature for CaO-SiO2-10 wt.% Al2O3 slag system (basicity 1.21 and slag/silicon ratio
2.0) ................................................................................................................................ 227
Figure E-12: Equilibrium slag composition as a function of temperature for CaO-SiO2-10
wt.% Al2O3 slag system (and basicity 1.21 and slag/silicon ratio 2.0) ......................... 227
Figure E-13: Equilibrium calcium, magnesium and boron content as a function of
temperature for CaO-SiO2-10 wt.% MgO slag system (basicity 1.21 and slag/silicon ratio
2.0) ................................................................................................................................ 228
Figure E-14: Equilibrium slag composition as a function of temperature for CaO-SiO2-10
wt.% MgO slag system (and basicity 1.21 and slag/silicon ratio 2.0) .......................... 228
Figure E-15: Equilibrium calcium and boron content as a function of temperature for
CaO-SiO2-10 wt.% CaF2 slag system (basicity 1.21 and slag/silicon ratio 2.0) ........... 229
Figure E-16: Equilibrium slag composition as a function of temperature for CaO-SiO2-10
wt.% CaF2 slag system (and basicity 1.21 and slag/silicon ratio 2.0) ........................... 229
Figure F-1: The Change of Boron contents of different slag/silicon ratio (Si-370 ppm B
alloy during reactions with CaO-SiO2 slag) at 1550°C. ............................................... 230
Figure F-2: The Change of Boron contents at 1500°C, 1550°C and 1600°C during
reactions (Si-370 ppm B alloy with CaO-SiO2 slag). ................................................... 231
Figure F-3: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-15.9 wt.% Al2O3 at
1600°C assuming nth order kinetic with n = 0.1. .......................................................... 231
Figure F-4: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-15.9 wt.% Al2O3 at
1600°C assuming nth order kinetic with n = 0.8. ......................................................... 232
Figure F-5: Kinetics plot of Si-370 ppm B reacting with CaO-SiO2-15.9 wt.% Al2O3 at
1600°C assuming nth order kinetic with n = 1.8. ......................................................... 232
Figure F-6: Integrated rate plots of Si-370 ppm B + CaO-SiO2 at 1550°C, using kinetic
equation for silicon phase mass transfer control. .......................................................... 233
Figure F-7: Integrated rate plots of Si-370 ppm B + CaO-SiO2 at 1550°C, using kinetic
equation for slag phase mass transfer control. .............................................................. 233
Figure F-8: Integrated rate plot assuming mass transport in the slag, with respect to B in
the Silicon bath, showing the effect of temperature on the reaction rate (Si-370 ppm B
and CaO-SiO2 slag). ...................................................................................................... 234
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Figure F-9: A plot between ln k and 1000/T from the experimental data (Si-370 ppm B
and CaO-SiO2 slag). ...................................................................................................... 234
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Introduction
1.1 Background of the Study
Silicon is an important semiconducting and photovoltaic material. It is also widely used
for chemical and metallurgical applications. The rapid growth in the demand of solar
photovoltaic (PV) cell results in the shortage of solar-grade (SOG) silicon feedstock.
Expensive scrap electronic grade (EG) silicon (99.9999999% Si) is commonly used as
the raw material to produce SOG-Si (99.9999% Si). The Siemens process, which is based
on the hydrogenous reduction of trichlorosilane (SiHCl3), is the current dominating
production method of SOG-Si. Many researchers have reported that relatively
inexpensive metallurgical grade (MG) silicon (98-99% Si) can be used as an alternative
raw material for the production of SOG-Si using alternative production processes. The
fundamental approach of the production of obtainment of solar silicon from metallurgical
silicon is to remove the existing impurities. All the impurities under consideration can be
divided into two groups. The first group includes the metallic impurities that can be
effectively removed by directional solidification such as Fe, Co, Ni, and Cu. The second
group of impurities has sufficiently large segregation coefficients (i.e. B and P) and
cannot be removed by directional solidification technique. Fortunately, these impurities
can be removed from molten silicon by oxidation.
Boron is one of the most detrimental impurities in metallurgical grade silicon and need to
be removed to produce SOG-Si because boron is used as a dopant to produce controlled
conductivity of a solar cell. Slag treatment is one of the promising methods for removal
from MG-Si. It has been shown that the removal of boron by slag is determined by the
slag properties, compositions and the amount of slag (Weiss and Schwerdtfeger, 1994).
Large amount of slags is not desirable in high temperature processes from a cost and
environmental standpoint. Thus, the amount of slag can be lowered by selecting proper
slag compositions and constituents where boron absorption will be maximized.
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2 | P a g e
Although boron removal from molten silicon using slag has been studied by a number of
investigators, the details of the mechanism of boron removal are not fully understood. By
using thermodynamics, kinetics and electrochemical modelling with high temperature
experimentation, this study will address the following questions:
- What is the effect of reaction parameters such as slag/silicon ratio, basicity,
reaction temperature, and slag composition to the distribution coefficient of boron
between Si-B alloy and CaO-SiO2-Al2O3 Slag?
- What is the kinetics mechanism of B removal from molten silicon, and what is
the overall rate-controlling step during slag-silicon reaction?
- Is slag-silicon reaction electrochemical in nature?
- Can the rate of boron removal be enhanced by applying electrical voltage across
the slag-silicon interface?
- Is it possible to shift the thermodynamic end point toward higher B removal in
the slag-silicon system by applying external driving force?
These fundamental questions will be addressed in this current study to obtain a complete
understanding of boron removal in order to enhance, optimize and improve the current
slag-silicon refining system.
1.2 Objectives of Present Study
The aim of the current study is to study the enhancement of boron removal from molten
silicon by oxidation at the slag-silicon interface. This study comprises of three parts;
Thermodynamic modelling and experimental investigation of equilibrium
between slags and Si-B alloy.
Kinetic investigation of B removal from molten silicon to elucidate the reaction
kinetics mechanism.
Electrochemical investigation of B removal by applying electrical potential across
the slag-silicon interface to improve the process.
In the first part, this research was comprised of a combination of thermodynamic
modelling and experimental study to study the distribution coefficient of boron for
selected slag systems. The thermodynamic modelling was carried out using FactSage 6.4
to predict the equilibrium boron and boron oxide concentration in molten silicon and slag
respectively at temperatures of 1500°C, 1550°C and 1600°C. These thermodynamic data
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provided the basis for the experiments performed in the current study. The equilibrium
results obtained in this current study will be compared with previous studies (Johnston et
al., 2012; Teixeira and Morita., 2009)
In the second part, the changing concentration of boron was tracked for different intervals
of time and the rate at which the refining takes place was determined. There are only
limited studies on the kinetics of the boron removal from silicon to the slag in the
literature (Nishimoto et al., 2011; Krystad et al., 2012). Moreover, reaction mechanism
of boron transfer from molten silicon with slag system has not been clearly identified in
literature. Specifically, the author wanted to determine the mass transfer coefficient, the
rate controlling step, and the activation energy associated with boron removal in slag
refining processes. In the current study, the kinetic data has been analysed and validated
with model to understand the reaction mechanism and to quantify the values.
In the third part of the study, the author sought to increase the rate of boron removal by
applying electrical potential across the slag-silicon interface. Some slag-metal reactions
in the refining of common metals (such as iron and steel) are electrochemical in nature
because the components in a slag are predominantly in the ionic state, e.g. when the slag
has high basicity. Moreover, the electrochemical nature of slag-metal reaction has been
demonstrated by a number of investigators, for example in the case of reaction between
Fe-C alloys and CaO-SiO2-Al2O3 slags, it has been shown that the rate of reaction can be
increased by a applying a voltage between slag and metal bath (Krishna Murthy et al.,
1993; Woolley and Pal., 1999). In the study, the author sought to test the following
hypothesis,
the slag-silicon reaction can be electrochemical in nature,
the rate of boron removal can be increased by applying a voltage across the slag
layer,
the equilibrium boron partition ratio can be shifted by applying electrical potential
across the slag layer.
The author has demonstrated with preliminary experiments that the application of
external potential has an effect on the reaction rate and the equilibrium end point for this
present system (Islam et al., 2014).
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1.3 Outline of the Thesis
This thesis consists of seven chapters. In order to briefly introduce the thesis, Chapter 1
explains the background of the research as well as the objectives of the present study.
In order to explain the experimental and modelling works in this current study, a literature
review is given in Chapter 2 to provide the basic knowledge and understanding of this
particular research area. This includes general information of industrial production of
silicon and the alternative production routes, common impurities and their impact on the
properties of solar grade silicon. The literature review has been focused on boron, one of
the common impurities, and its removal with slag process. In this section, a description
of the basic thermodynamic modelling theories, the basic kinetics of liquid-liquid
reactions and mass transfer theories is provided. It is followed by the basic
thermodynamics and kinetics of the boron removal using slag, slag and silicon alloy
physio-chemical properties and a survey of earlier studies on slag process are provided.
In the final section, Chapter 2 also includes the basics of electrochemical kinetic and
thermodynamics theories and the literature survey of electrically enhanced slag-metal
reactions.
Chapter 3 describes the thermodynamic modelling approach, the preparation of master
silicon and slags and the experimental techniques used in the current study including the
experimental procedures and apparatus. It also includes the description of sample
characterization techniques used in this current research.
Thermodynamic modelling of Si-370 ppm B and slag (CaO-SiO2, CaO-SiO2 with CaF2,
Al2O3 and MgO) systems is given in Chapter 4. This chapter also explains the equilibrium
predictions of boron removal with some selected slag systems at temperatures 1500°C,
1550°C and 1600°C. The effect of the slag/silicon mass ratio, basicity and alumina
content is explained in Chapter 4. Chapter 4 also outlines the experimental results of
distribution coefficient of boron in molten silicon at temperatures 1500°C, 1550°C and
1600°C.
Chapter 5 describes the kinetic experiments and the analysis of experimental data
obtained for different slag/silicon mass ratio and reaction temperature. The kinetic study
focuses on understanding of the reaction mechanism of boron removal for selected slag
refining processes by changing reaction parameters. The mechanism is elucidated from
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the kinetics data obtained from reactions between Si-370 ppm B alloy and, CaO-SiO2
with 0, 9.59 and 15.9 wt.% Al2O3, at different time 0 min to 240 min; at temperatures
1500°C, 1550°C and 1600°C.
In Chapter 6, a technique to investigate the effect of electrical potential on slag/silicon
reaction is established by performing short circuit and applied-voltage experiments in
vertical tube furnace, in which the kinetic data, electrochemical data and the equilibrium
shifting data were measured. The results of the study are discussed and a model of the
reaction rate was developed. Finally, in Chapter 7, includes a summary of the work,
conclusions and the suggestions for further work. To enhance the flow of the thesis, most
of the raw data and other supplementary information are given in several appendices that
include;
Appendix-A: Compositions of Si-alloys and Master Slags
Appendix-B: Temperature profile in Vertical Tube Furnace
Appendix-C: Melting Furnaces
Appendix-D: Examples of Equilibrium Calculation
Appendix-E:
A. Other Results of Thermodynamic Assessment B. Other Equilibrium Experiments Results
Appendix-F: Kinetics Data during Slag-Silicon Reaction
Appendix-G: Mass Transfer control Kinetic Equations
Appendix-H: Error Analysis
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Literature Review
2.1 Introduction
Silicon is a non-metallic semiconducting element, which makes up 25.7 mass% of the
earth’s crust and is the second most abundant element on earth, exceeding only by
oxygen. It is not found free in nature, but it does occur as silicon dioxide and silicates in
minerals (sand, quartz, rock crystal etc.). Silicon can be prepared commercially by heating
silica and carbon in an electric furnace, using carbon electrodes (Fishman, 2008).
Silicon is widely used as an alloying element in the aluminium industry and as a reducing
element in the steel industry. The purity of silicon used directly in metal industry is 98%
and commonly called metallurgical-grade silicon (MG-Si). A small portion of silicon is
used in the electronic/semiconductor industry as electronic chips such as transistors,
liquid crystal displays, diodes, etc. The purity of the silicon used in this industry is
99.99999999% (eight nines) or higher, and referred to as electronic-grade silicon (EG-
Si). Another application of silicon is for solar photovoltaic (PV) panel wafers. For this
application silicon must be purified to 99.9999% (six nines) purity, and is usually called
solar-grade silicon (SOG-Si).
Table 2-1 shows the typical impurity levels for metallurgical grade silicon, solar grade
silicon and the electronic grade silicon respectively.
Table 2-1: Typical impurity levels for MG-Si, SOG-Si and EG-Si (Wang and Ciszek,
2001)
Impurity MG-Si
(ppma)
SOG-Si
(ppma)
EG-Si
(ppma)
Al 1200-4000 0.08-0.5 0.0008
B 10-50 0.1-3 0.0002
C 700 60 0.5
Ca 590 0.1 0.003
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Cr 50-140 0.006-0.05 0.003
Cu 24-90 0.3 0.003
Fe 1600-3000 0.02-0.3 0.010
Mn 70-80 0.015-0.05 0.003
Mo ≤ 10 15 x 10-5 0.003
Ni 40-80 0.1-0.2 0.010
P 15-50 0.1-1.0 0.0008
Ti 140-200 0.1 0.003
V 100-200 5 x 10-5 0.003
Group IIIA element like boron is usually used as dopant for pure silicon, where one
silicon atom substituted with boron in the crystal structure. Therefore, it provides one less
valance electron than silicon and one valence electron of silicon can shift to that hole to
become extrinsic conductor. Solar grade silicon with this type of doping are referred to
as p-type semiconductors. In the same way, if silicon atom substituted with a group VA
element, such as P, there is one extra electron in the bonding. Solar grade silicon with this
type of doping are referred to as n-type semiconductors. Therefore, by doping pure silicon
with electrically active elements, such as Al, B or P, decrease the electrical resistivity of
the pure silicon. In addition to that, electrical resistivity of silicon decreases with
increasing temperature. Some physical properties of silicon are given in
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Table 2-2: Some physical properties of silicon (Zulhane, 1993)
Atomic mass 28.086
Atomic density 5.00 × 1022 atoms/cm3
Density at 300 K 2.329 g/cm3
Density at high temperature
(Sato and Nishizuka et al., 2000) 𝜌 = 3.005 − 2.629 × 10−4𝑇(T in K)
Volume increase at
transformation from liquid to
solid
+ 9.1 %
Melting point 1687 K
Boiling point 3504 K
Latent heat of fusion 50.66 kJ/mol
Heat of evaporation 385 kJ/mol
Band gap (300 K) 1.126 eV
Electron mobility 1440 cm2 V-1 s-1
Hole mobility 484 cm2 V-1 s-1
In recent years, PV power generation has increased significantly. In 2012, the countries
under the European Photovoltaic Industry Association Programme have installed 25.3
GW of PV, with a minimum worldwide installed capacity totalling 28.6 GW. Germany
(26%), and Italy (13%), China (12%) and USA (11%) were dominating the installed solar
capacity in 2012 as shown in Figure 2-1 (Source: IEA-PVPS Trends in photovoltaic
Applications, 2012). While the current global solar capacity has yet to have significant
impact on the world’s electricity consumption, it does represent the tremendous growth
opportunities for solar power generation in the future. According to the latest estimates
from Jeffries and Energy Information Administration (Source: IEA-PVPS Trends in
photovoltaic Applications, 2011), solar power generation is projected to be 11% of the
total world capacity demand by 2030, as shown in Figure 2-2.
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Figure 2-1: Installed capacity by country reported to International Energy Agency in
2012(Source: IEA-PVPS Trends in photovoltaic Applications 2012)
Figure 2-2: Solar power generation as percentage of world electricity consumption
(2011). (Source: IEA-PVPS Trends in photovoltaic Applications 2011)
About 95% of the current solar PV cell module market is for silicon based solar cells, i.e.
using silicon as raw material, of which 60% is polycrystalline silicon and 30% is single
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crystal silicon (Kawamoto and Okuwada, 2007). Table 2-3 shows the global
polycrystalline silicon production and demand data (Source: QY Research, 2007). It can
be seen from Table 2-3 that there is a shortage of stock of polycrystalline silicon. The
production of polycrystalline Si for photovoltaic solar wafer is mainly relying on the off-
spec high-purity scrap of electronic-grade silicon from the semiconductor industry. With
the increase demand of polycrystalline silicon (and depletion of world reserves of EG-Si
scrap), it is imperative to develop a new process that is more sustainable with lower
environmental impact.
Table 2-3: Global polycrystalline silicon market data, (in tons) (Source: QY Research,
2007)
Year Available
polycrystalline-Si
Demand of
polycrystalline-Si
Stock polycrystalline-
Si
2005 30,680 33,850 -3,170
2006 33,390 39,520 -6,130
2007 37,500 46,900 -9,400
2008 51,000 62,940 -11,940
2009 73,500 81,340 -7,840
2010 96,500 103,440 -6,940
2011 115,200 121,560 -6,360
2012 142,000 148,150 -6,150
2013 168,000 173,200 -5,200
2.1.2 Silicon Impurities
2.1.2.1 Sources of Impurities
Solar grade silicon production use low-grade silicon feedstock comes from off-spec
semiconductor silicon or rejected material from the microelectronic industry. Hence, the
starting raw materials have higher level of impurities and it is well known that impurity
atoms have a strong effect on the efficiency of photovoltaic silicon. Silicon impurities
may be incorporated into bulk silicon material via two modes; (i) raw materials from
which bulk silicon is produced and (ii) contaminations from manufacturing processing or
fabrication of the metallurgical grade silicon.
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The photovoltaic grade solar silicon requires the use of a very pure silicon (99.9999 %
Si). To obtain solar grade silicon specification, the process starts with carbo-thermal
reduction of quartz with carbon in an electric furnace, using carbon electrodes. The
impurity levels in quartz depends on the geographical location, and the most detrimental
elements in the use of the quartz for the manufacturing of the photovoltaic cells are first
of all the boron and the phosphorus because of their difficulty of removal (Istratov et al,
2006). Other impurities (calcium, aluminium and other metal oxides are also undesirable
impurities in quartz. Quartz usually contains iron and aluminium, whose level is higher
than the required minimum content in the photovoltaic application. For that reason, higher
grade quartz is most important requisite for subsequent operation to produce solar grade
silicon.
2.1.2.2 Effect of Impurities on Solar Cell Performance
Impurities with higher concentration can precipitate at preferred sites, such as extended
defects, grain boundaries, and the defect clusters. Impurities can affect direct impact on
device performance and also can degrade crystal structure. Moreover, crystal structure
breakdown (constitutional supercooling) or precipitation effects largely affect junction
behaviour. Impurities depreciate cell performance by reducing diffusion length by trap
formation and degrade junctions via precipitates/inclusions.
2.1.2.3 Effect of Boron and Phosphorus
Group IIIA elements, such as boron, substitute silicon atoms in the crystal lattice resulting
in an electron deficient bonding to satisfy the four covalent neighbour bonds. These
impurities act as substitutional impurities in silicon, and this gives rise to holes weakly
tied to the Group IIIA atoms. Accordingly, boron creates energy levels for electrons in
the band gap, and it is termed as acceptors.
Group VA elements, such as phosphorus, have intentionally replaced a silicon atom with
excess electrons. When phosphorus replaces a silicon atom, four d-electrons are bound to
the silicon with covalent bonds, while the fifth electron activates to the conduction band.
And, silicon material doped with group VA element is termed an n-type semiconductor
and donor impurity for the substitute element.
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The presence of boron and phosphorus in crystalline silicon must be maintained in
specified concentration. Unfortunately, these impurities are the most problematic
impurities to remove during silicon refining. The presence of these impurities above the
concentration of the doping requirement will modify the semiconductor properties of
silicon substantially.
2.1.2.4 Effect of Carbon, Oxygen and Nitrogen
Oxygen, nitrogen, carbon and hydrogen are non-metallic impurities dissolve in silicon
mainly as interstitial impurities. These impurities react with silicon to form SiC, SiO2,
silicides or silicon nitride, etc.
Carbon has four valence electrons like silicon and is therefore electrically neutral. The
carbon atom is smaller than the silicon atom and may expand the lattice like silicon di
oxide. Carbon is present at levels above the solid solubility limit in metallurgical grade
silicon and there has SiC precipitate.
Oxygen atoms are electrically inactive as carbon in solid solution. Since oxygen has a
high diffusion coefficient, the distribution of oxygen particles may change by heat
treatment. During heating, oxygen particles can dissolve and can grow during cooling.
Moreover, oxygen can react with other impurities, which is called internal gettering. The
oxygen can introduce during melting in high purity quartz crucible if there has any crack
in Si3N4 coating onto the surface of the crucible.
2.1.2.5 Effect of Transition Metals
The transition metals impurities found in silicon are mainly 3d transition metals (Sc, Ti,
V, Cr, Mn, Fe, Co, Ni, and Cu). These elements are presented by the symbols 3d, 4d and
5d, which mean the outer electron configuration of a neutral atom. Most of the transition
metal impurities forming deep energy levels between the conduction band and the valence
band in silicon, and have therefore a large influence on the solar cell properties of silicon.
The most detrimental effect is transition metals impurities are known to degrade the
minority carrier life time. The time elapsed before a free electron combines with a hole
in the crystal lattice is called minority carrier life time. The minority carrier life time, τ0,
is inversely proportional to the impurity concentration, N (1/cm3), which is related to
(Graff, 2000)
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τ0 = (σ ʋ N) −1 (2.1)
Where, σ (cm2) represents the impurity atoms effective cross-section for the capture of a
minority carrier. σe and σh represent the carrier capture cross-section for electron in p-
type silicon and hole in n-type silicon respectively. And, ʋ is the thermal velocity, which
is the average speed of the electrons as they randomly collide with atoms, impurities or
other defects.
The capture cross-sections for different transition metals can differ by several orders of
magnitude. As a consequence, the carrier lifetime of a silicon sample can even be
determined by an impurity of minor concentration if this is a “lifetime killer” with a high
minority-carrier capture cross-section. Therefore, the tolerable impurity concentration for
acceptable lifetime values depends upon the chemical nature of the respective impurity,
and its carrier capture cross-section for electrons in p-type silicon and for holes in n-type
silicon. Copper and nickel have high diffusivities and low capture cross-sections. These
elements will rapidly enter a low solid solution level af