electrically enhanced slag-metal reactions...electrically enhanced slag-metal reactions md saiful...

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

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.

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

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.

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.

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.

To my parents

This Page Intentionally Left Blank

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

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

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

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

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

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

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

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

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


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


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

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

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

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

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


SiO2 -10 wt.% Al2O3 slag system (at temperature 1550°C and basicity 1.21) ............. 111


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


SiO2 -15 wt.% MgO slag system (at temperature 1550°C and basicity 1.21) .............. 113


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


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


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


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


Figure 4-39: Boron partition ratio as a function of slag/silicon ratio for CaO-SiO2-15.9


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




1550°C assuming nth order kinetic with n = 1.8 ............................................................ 146



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


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


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


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


Figure 5-17: A plot between ln k and 1000/T from the experimental data (Si-370 ppm B


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


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


CaO-SiO2-9.59%Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ...................... 173


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


CaO-SiO2-15.9 wt.% Al2O3 slag, with slag to silicon ratio of 2.0 at 1550°C ............... 175


CaO-SiO2-9.59 wt.%Al2O3 slag at 1550°C with applied potential of 3.0V and 5.0V. The



CaO-SiO2-15.9 wt.%Al2O3 slag at 1550°C with applied potential of 3.5V and 5.0V. The


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


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

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 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


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


temperature for CaO-SiO2-10 wt.% Al2O3 slag system (basicity 1.21 and slag/silicon ratio

2.0) ................................................................................................................................ 227


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


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


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


1600°C assuming nth order kinetic with n = 0.8. ......................................................... 232


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

Figure F-9: A plot between ln k and 1000/T from the experimental data (Si-370 ppm B

and CaO-SiO2 slag). ...................................................................................................... 234

1 | P a g e

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.

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

3 | P a g e

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).

4 | P a g e

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

5 | P a g e

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

6 | P a g e


7 | P a g e

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

8 | P a g e

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

9 | P a g e

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

11 | P a g e

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

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