Molten Salt Electrodeposition of Silicon in Cu-Si

Samira Sokhanvaran

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Departments of Materials Science and Engineering University of Toronto

Samira Sokhanvaran

Doctor of philosophy

Departments of Materials Science and Engineering University of Toronto

2014 Abstract

Widespread use of solar energy has not been realized to date because its cost is not competitive with conventional energy sources. The high price of solar grade silicon has been one of the barriers against photovoltaic industry achieving its much anticipated growth. Therefore, developing a method, which is energy efficient and will deliver inexpensive silicon feedstock material is essential. The electrodeposition of Si from a cryolite-based melt was investigated in the present work as a possible solution.

This study proposed electrowinning of Si in molten Cu-Si alloy, to decrease the working temperature and increase the efficiency. Solvent refining can be used to recover Si from Cu-Si and also as a second purification method. The physicochemical properties of the potential electrolyte, cryolite–SiO2 melts, were studied in the first step of this work. The deposition potential of Si on a graphite cathode was measured to determine the working potential and the effect of SiO2 concentration on it. In the next step, the deposition potential of Si from cryolite–

SiO2 melt on Cu and Cu-Si cathodes was determined using cyclic voltammetry. Next, the cathodic and the anodic current efficiencies of the process were measured. Continuous analysis of the evolved gas enabled the instantaneous measurement of the current efficiency and the kinetics of the deposition. Finally, the effectiveness of the process in delivering high purity Si

was investigated. Si dendrites were precipitated out of the Cu-Si cathode and recovered to determine the purity of the final product as the final step of this study. The produced Si was separated from the alloy matrix by crushing and acid leaching and the purity was reported.

The findings of this research show that the proposed method has the potential to produce high purity silicon with low B content. Further development is required to remove some metallic impurities that are remained in Si.

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Acknowledgments

This research bears the imprint of many people who shared with me their knowledge and experience. First and foremost, I wish to express my sincere gratitude to my supervisor, Dr. Mansoor Barati, for his invaluable assistance, guidance and support through completion of this research. I would like to thank him for teaching me how to solve the challenging and applied problems and for being there whenever I needed him at all stages of this research.

Deep gratitude is also due to the members of the supervisory committee, Dr. R. Ravindran, Dr. C. Jia and Dr. K. Lian whose assistance was elemental in successful completion of this study.

Also, I would like to take this opportunity to thank the Department of Materials Science and Engineering for providing me the necessary facilities and warm environment to carry out the research work. A special appreciation goes to Late Prof. T. Utigard, for the amenities and support he provided on the characterization of electrolyte. I would also like to thank Sal Boccia and Dan Grozea from the Department of Materials Science and Engineering and Mr. S. Salavati the Department of Mechanical and Industrial Engineering for their advice and assistance in sample preparation and SEM analysis. I also would like to thank RioTinto Alcan, NSERC and MSE department for providing the financial support for this research.

I really appreciate the support of all members of the Sustainable Materials Processing Research Group, specially S. Thomas, M. Li. I also thank a tireless man, K. Danaei, for his effort both in the lab and in the office. Without his wise advice, invaluable help this research would not have been possible.

I owe my deepest gratitude to my parents for their dedication and endless support and love through the years. I gained so much drive under their watchful eyes. I also appreciate my two sisters who always made me smile even on tough times when nothing worked well in the lab.

Lastly and most importantly, I would like to thank my life partner Mojtaba for standing beside me through thick and thin. He was always there cheering me up on the dull days. His unwavering love was undeniably the bedrock upon which the past twelve years of my life have been built and I dedicate this dissertation to him.

Table of Contents

Acknowledgments...... iv

Table of Contents ...... v

List of Symbols ...... xii

List of Abbreviations ...... xiv

List of Tables ...... xv

List of Figures ...... xvi

Chapter 1 Introduction ...... 1

1.1 Motivation for the thesis ...... 1

1.2 Objectives of the study...... 3

1.3 Organization of the thesis ...... 4

Chapter 2 Literature Review ...... 5

2.1 Silicon ...... 5

2.1.1 Metallurgical grade silicon ...... 6

2.1.2 Semiconductor grade silicon ...... 7

2.1.3 Solar grade silicon...... 8

2.2 SoG-Si production methods ...... 9

2.2.1 Refining of MG-Si ...... 10

2.2.1.1 Acid leaching ...... 10

2.2.1.2 Reactive gas blowing ...... 11

2.2.1.3 Slagging ...... 12

2.2.1.4 Solvent refining ...... 12

2.2.1.5 Electrorefining ...... 13

2.2.2 Silica reduction ...... 16

2.2.2.1 Carbothermal reduction of silica ...... 16

2.2.2.2 Reduction by metals and compounds ...... 17

2.2.2.3 Electrodeposition ...... 17

2.3 Electrowinning of silicon ...... 17

2.3.1 Electrowinning of solid Si ...... 18

2.3.1.1 Organic solvents...... 18

2.3.1.2 Molten salts ...... 19

2.3.1.2.1 Deposition from halide melts ...... 19

2.3.1.2.2 Deposition from mixture of oxide without halides ...... 20

2.3.1.2.3 Deposition from mixture of halides and silica ...... 20

2.3.2 Electrowinning of molten Si ...... 22

2.3.2.1 Above the melting temperature of Si ...... 22

2.3.2.2 Below the melting temperature of Si ...... 23

2.4 Cost considerations ...... 24

2.5 Physicochemical properties ...... 25

2.5.1 Density ...... 25

2.5.1.1 Density measurement ...... 25

2.5.1.2 Density of molten cathode ...... 26

2.5.1.3 Density of cryolite ...... 26

2.5.1.4 Effect of silica on density of cryolite ...... 27

2.5.2 Electrical conductivity ...... 27

2.5.2.1 Electrical conductivity measurement ...... 28

2.5.2.1.1 Cell Design...... 28

2.5.2.1.2 Measurement techniques ...... 29

2.5.2.2 Conductivity of molten cryolite ...... 31

2.5.2.3 Effect of silica on the conductivity of cryolite...... 32

2.5.3 Transference numbers ...... 32

2.5.3.1 Transference number measurement ...... 33

2.5.3.1.1 Faraday technique ...... 33

2.5.3.1.2 Stepped potential chronoamperometry ...... 34

2.5.3.2 Transference number in cryolite ...... 35

2.5.4 Phase diagram ...... 35

2.5.4.1 Determination of the phase diagram ...... 35

2.5.4.1.1 Thermal analysis (TA) method ...... 35

2.5.4.1.2 Differential thermal analysis method (DTA) ...... 36

2.5.4.1.3 Quenching method ...... 37

2.5.4.1.4 Visual observation method ...... 37

2.5.4.2 Systems containing Na3AlF6 ...... 38

2.5.4.2.1 Pure cryolite ...... 38

2.5.4.2.2 Na3AlF6- SiO2 system ...... 38

2.5.4.2.3 Reactions between cryolite and SiO2 ...... 39

2.6 Decomposition and deposition potentials ...... 40

2.6.1 Measurement techniques ...... 41

2.6.1.1 E-I method ...... 41

2.6.1.2 Cyclic voltammetry ...... 42

2.6.2 Decomposition potential of SiO2 in cryolite melts...... 43

Chapter 3 Experimental ...... 44

3.1 Phase I: Characterization of cryolite–SiO2 melts...... 44

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3.1.1 Melt preparation ...... 44

3.1.2 Density measurement ...... 45

3.1.3 Conductivity measurements ...... 46

3.1.3.1 Cell design ...... 46

3.1.3.2 Cell constant...... 47

3.1.3.3 Calculation of the melt resistance ...... 49

3.1.4 Transference number measurement ...... 50

3.1.5 Phase diagram study ...... 51

3.1.5.1 Melt Preparation...... 51

3.1.5.2 Melting and eutectic point ...... 51

3.1.5.3 Characterization of the phases ...... 52

3.2 Phase II: Determination of deposition and decomposition potential ...... 53

3.2.1 Deposition potential ...... 53

3.2.1.1 Instrumentation ...... 55

3.2.1.2 Electrodes ...... 55

3.2.1.3 Experimental procedure ...... 56

3.2.2 Decomposition potential ...... 57

3.2.2.1 Instrumentation ...... 57

3.2.2.2 Electrodes ...... 58

3.3 Phase III: Electrowinning and separation of Si...... 59

3.3.1 Determination of anodic and cathodic current efficiencies for silicon electrowinning on Cu-Si Alloy ...... 59

3.3.1.1 Master alloy preparation ...... 60

3.3.1.2 Instrumentation ...... 61

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3.3.1.3 Experimental procedure ...... 63

3.3.2 Combined electrolysis and solvent refining ...... 64

3.3.2.1 Extended electrowinning ...... 64

3.3.2.2 Solvent refining ...... 65

3.3.2.3 Acid leaching ...... 66

Chapter 4 Results and Discussion ...... 68

4.1 Characterization of cryolite–SiO2 melts ...... 68

4.1.1 Density measurements ...... 68

4.1.1.1 Effect of temperature ...... 68

4.1.1.2 Effect of silica content ...... 68

4.1.2 Conductivity measurements ...... 71

4.1.2.1 Effect of temperature ...... 71

4.1.2.2 Effect of silica concentration ...... 71

4.1.3 Transport number measurements ...... 75

4.1.3.1 Effect of temperature ...... 75

4.1.3.2 Effect of SiO2 concentration ...... 76

4.1.4 Phase diagram studies ...... 77

4.1.4.1 Phase diagram ...... 77

4.1.4.2 Characterization of the phases ...... 79

4.2 Determination of deposition and decomposition potentials...... 83

4.2.1 Deposition potential ...... 83

4.2.1.1 Cyclic voltammetry in cryolite ...... 83

4.2.1.2 Cyclic voltammetry in cryolite– SiO2 melt ...... 85

4.2.1.2.1 Theoretical potential ...... 85

4.2.1.2.2 Experimental potential ...... 86

4.2.1.2.2.1 Effect of scan rate on cyclic voltammogram ...... 91

4.2.1.2.2.2 SEM analysis ...... 94

4.2.2 Decomposition potential ...... 96

4.2.2.1 Voltammetry measurements on copper ...... 96

4.2.2.1.1 Cyclic voltammetry in cryolite ...... 97

4.2.2.1.2 Cyclic voltammetry in cryolite– 6 wt% SiO2 melts ...... 97

4.2.2.1.2.1 Effect of scan rate ...... 99

4.2.2.1.2.2 Effect of Si concentration ...... 100

4.2.2.1.2.3 SEM analysis ...... 101

4.2.2.2 Voltammetry measurements on copper−8wt% Si alloy...... 101

4.3 Electrowinning and separation of Si ...... 102

4.3.1 Characterization of master alloy ...... 102

4.3.2 Determination of anodic and cathodic current efficiencies ...... 104

4.3.2.1 Apparent cathodic current efficiency ...... 104

4.3.2.2 Actual cathodic current efficiency ...... 105

4.3.2.3 Anodic current efficiency ...... 109

4.3.2.4 Effect of cell design on the efficiency ...... 113

4.3.3 Solvent refining ...... 116

4.3.4 Acid leaching ...... 118

Chapter 5 Summary and Conclusion ...... 122

Chapter 6 Future Work ...... 125

References ...... 126

Appendix I Error analysis of density measurement ...... 138

Appendix II Molar conductivity calculation ...... 139

Appendix III Elemental mapping of cryolite- 1% SiO2 quenched at 990 °C ...... 140

Appendix IV Cu-Si phase diagram ...... 141

Appendix V Publications and presentations from this research ...... 142

List of Symbols j a SiO2 activity of SiO2 in j (j= l (liquid), s (solid)) C* bulk concentrtion

Ci concentration of charge carriers

Cl concentration of impurities in liquid

Cs concentration of impurities in solid D diffusion coefficient Eo standard potential

theoretical decomposition potential

Ej potential (j= d (decomposition), pol (polarization), conc (concentration), p (peak), p/2(with half the peak current)) E activation energy F Faraday’s constant G geometry factor (cell constant) ij conducted current (j=e (electronic), i (ionic)) ib background current ip peak current K rate constant of the reaction mj mass (j=i (initial), f (final)) M mass n no. of involved electrons

PO2 partial pressure of O2 R gas constant

Rj resistance (j=sol (ohmic resistance), pol (polarization resistance), meas (measured)) T temperature

Tm melting temperature t time tj transport no. (j=e (electronic), i (ionic)) V deposition potential v volume W weight of deposit z valence no.

Zj impedance (j= L (inductance), C (capacitance)) Z real part of impedance Z imaginary part of impedance

Epp peak separation ΔG Gibb's free energy

Hf enthalpy of fusion

m actual mass deposited

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mf Faradic mass deposited

Sf entropy of fusion ΔT change in temperature  transfer coefficient ρ density  segregation coefficient  efficiency

i mobility of carriers o  SiO2 standard potential of SiO2 j  SiO2 standard potential of SiO2 in j (j= l (liquid), s (solid))

 specific conductivity

j conductivity (j=e (electronic), i (ionic), t (total))  scan rate  frequency of AC current

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List of Abbreviations

CE current efficiency CR cryolite ratio DC direct current EDS energy-dispersive X-ray spectroscopy EIS electrochemical impedance spectroscopy EPMA electron probe microanalyzer ICP inductively coupled plasma MG-Si metallurgical grade silicon ppb parts per billion atoms ppm parts per million atoms ppt parts per trillion atoms PV photovoltaic SEM scanning electron microscope SeG-Si semiconductor grade silicon SoG-Si solar grade silicon XRD X-ray diffraction XRF X-ray fluorescence

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List of Tables

Table 2 -1. The acceptable level of impurity in MG-Si and SoG-Si [3]...... 8

Table 2 -2. Effect of temperature on the electrical conductivity of cryolite...... 31

Table 4 -1. Equation of temperature dependency of density for different silica content...... 70

Table 4 -2. Activation energy of electrical conductivity for different mixtures ...... 74

Table 4 -3. Ionic and electronic transport number for cryolite melt at different temperatures...... 75

Table 4 -4. EPMA results of white phase of 6 wt% SiO2 sample quenched from 990 and 970°C...... 82

Table 4 -5. Effect of SiO2 concentration on theoretical deposition potential of Si...... 85

Table 4 -6. Anodic and cathodic potentials in 3 and 5wt% solutions at 20 and 50 mV.s-1...... 91

Table 4 -7. Cyclic voltammetry data of silicon reduction on graphite in Na3AlF6– 5% SiO2...... 92

Table 4 -8. Diagnostic criteria for reversibility of B1 peak [58]...... 93

Table 4 -9. Electrolysis with a Cu-Si cathode from cryolite-6 wt% SiO2, T= 1040 °C...... 104

Table 4 -10. Mass balance calculation for the Cu mass loss in each experiment...... 109

Table 4 -11. Measured and calculated C consumption...... 112

Table 4 -12. Extended electrowinning experiments with different crucibles...... 113

Table 4 -13. EDS analysis of the phases presenting in Figure 4 -55-c and d...... 117

Table 4 -14. EDS analysis of the phases presenting in Figure 4 -57-b...... 119

Table 4 -15. Concentration of impurities in the alloy and final Si in ppmw ...... 121

Table 5 -1. Energy consumption and carbon footprint of different Si production methods...... 124

List of Figures

Figure 1 -1. Cost of electricity generated from different energy sources [1]...... 1

Figure 1 -2. Research plan...... 4

Figure 2 -1. Relationship between the cost and the purity of various types of Si [15]...... 6

Figure 2 -2. Schematic of the process for production of MG-Si [43]...... 6

Figure 2 -3. Schematic of the Siemens process [44]...... 7

Figure 2 -4- Effect of impurities on the performance of the p-type silicon. 1) semiconductor, 2) solar and 3) metallurgical grade silicon[48]...... 9

Figure 2 -5- Standard free energy of formation of impurity’s oxides [55]...... 11

Figure 2 -6. Comparison of segregation coefﬁcient and electronegativity of impurities in MG-Si [15, 75, 76] ...... 14

Figure 2 -7. Suggested cell design for dual refining of Si [35]...... 15

Figure 2 -8.Three layer technique for electrorefining of super pure Al [86]...... 16

Figure 2 -9. The cell design proposed for Si production [118]...... 21

Figure 2 -10. Schematic cell design for deposition of silicon above the melting temperature [13]...... 23

Figure 2 -11. Schematic electrode designs for measuring electrical conductivity of liquids [143]...... 29

Figure 2 -12.a) A typical Nyquist plot of a Randles cell, b) equivalent circuit of the Randles . ... 30

Figure 2 -13. Schematic current response to square wave potential [157]...... 34

Figure 2 -14. a) Phase diagram of a hypothetical A-B system, b) The cooling curve corresponding to phase diagram at different compositions [162]...... 36

Figure 2 -15. Differential thermal analysis method a) classical apparatus b) heat flux c) DTA curve for an endothermic reaction [163]...... 37

Figure 2 -16. Binary phase diagram of cryolite-SiO2 system at 1 atm. trd: tridymite, qz: quartz [178]...... 39

Figure 2 -17. a) Experimental setup for decomposition potential measurement, b) schematic of the response current-voltage diagram [186]...... 42

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Figure 2 -18. Potential- time wave in cyclic voltammetry, b) a typical cyclic voltammogram. .... 43

Figure 3 -1- Schematic of the experimental setup for density measurement ...... 45

Figure 3 -2. a) Dimensions and arrangement of the electrode tips, b) electrode immersion depth adjustment apparatus...... 47

Figure 3 -3. Schematic of the experimental setup for measuring charge transport properties...... 48

Figure 3 -4. Detection of the melt surface by chronomaperometry at 87 mV...... 48

Figure 3 -5. Nyquist diagram for different immersion depths in a 0.01D KCl solution...... 49

Figure 3 -6. Cell constant as a function of immersion depth recorded in 0.1 and 0.01D KCl standard solutions...... 49

Figure 3 -7. The applied polarization wave in cryolite1 wt% SiO2 melt at 1000 C...... 50

Figure 3 -8. The current response recorded during the polarization in cryolite1 wt% SiO2 melt at 1000 C...... 51

Figure 3 -9. Schematic cell design used for deposition potential measurements...... 54

Figure 3 -10. a) the complete experimental setup, b) the graphite radiation shields...... 54

Figure 3 -11. Open circuit potential of the cell containing cryolite as electrolyte...... 56

Figure 3 -12. Schematic of the experimental setup for decomposition potential measurements. . 58

Figure 3 -13. Schematic of the experimental setup for preparing the master alloy...... 61

Figure 3 -14. The master alloy quenched from 1500 C...... 61

Figure 3 -15. The Al cap used for accommodation of the electrodes and sealing the furnace tube...... 62

Figure 3 -16. Schematic of the experimental setup for current efficiency measurements...... 63

Figure 3 -17. Schematic drawing of the experimental setup for solvent refining...... 65

Figure 3 -18. Heating and cooling cycle utilized for solvent refining...... 66

Figure 4 -1. Density of cryolite as a function of temperature;  this study, Edwards [91]...... 68

Figure 4 -2. Density of cryolite- silica for different concentrations of silica at 1000 °C...... 69

Figure 4 -3. Effect of silica content on a) partial molar volume of cryolite, b) partial molar volume silica, c) molar volume of solution at 1020 °C...... 70

xvii

Figure 4 -4. Effect of temperature on conductivity of cryolite;  present study; Kalass [146];  Batashev [146];  Beljajew [195];  Vayna [194];  Edward [91];  Abramov [146];  Yim [150];  Bajcsy [148]...... 71

Figure 4 -5. Conductivity of Na3AlF6-SiO2 mixtures at 1000°C.  present study;  Grjotheim [14];  Belyaev [149]...... 72

Figure 4 -6. Comparison between the effect of SiO2 and Al2O3 [149] on conductivity at 1000°C...... 73

Figure 4 -7. Arrhenius plot of electrical conductivity for 0 and 3 wt% silica mixtures...... 74

Figure 4 -8. Effect of silica concentration and temperature on molar conductance of mixture. ... 75

Figure 4 -9. Effect of temperature on electronic, ionic and total conductivity of cryolite melts. .. 76

Figure 4 -10. Effect of SiO2 concentration on electronic, ionic and total conductivities of cryolite–SiO2 melt at 1020 °C ...... 77

Figure 4 -11. Typical cooling curve of cryolite melt with larger magnification of the freezing section...... 78

Figure 4 -12. Liquidus and eutectic line for cryolite–SiO2 system from 0 to 6 wt% SiO2. (dashed line shows the expected trend) ...... 79

Figure 4 -13. SEM images of quenched samples. a) quenched liquid, 1 wt% SiO2 b) cryolite + quenched liquid, 1 wt% SiO2 c) sodium aluminosilicate + quenched liquid, 6 wt% SiO2 d) cryolite and sodium aluminosilicate particles after solidification...... 80

Figure 4 -14. EDX spectrum of a cryolite1 wt% SiO2 sample. a) area A, b) area B in Figure 4 -13- b...... 81

Figure 4 -15. EDX spectrum of a cryolite6 wt% SiO2 sample. a) area A, b) area B in Figure 4 -13-c...... 81

Figure 4 -16. XRD pattern of the sample containing 1 wt% quenched from 970°C...... 81

Figure 4 -17. XRD pattern of the sample containing 6 wt% quenched from 970°C...... 82

Figure 4 -18. Cyclic voltammetry in molten SiO2–free cryolite; scan rate 50 mV.s-1...... 84

Figure 4 -19. Cyclic voltammetry on graphite electrode at 1040°C; scan rate 50 mV.s-1...... 86

Figure 4 -20. Cyclic voltammetry in cryolite– 3 % SiO2 melt at 1040 °C; scan rate 50 mV.s-1. . 87

Figure 4 -21. Cyclic volatmmogram showing the effect of SiO2 concentration on the deposition potential of Si; scan rate 50 mV.s-1...... 88

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Figure 4 -22. Volatmmograms showing the effect of SiO2 concentration on the deposition potential of Si; scan rate 20 mV.s-1 ...... 89

Figure 4 -23. Schematic of the expected concentration profiles of Si4+ and Si2+ ions at the electrode surface for 3 and 5wt% SiO2...... 89

Figure 4 -24. Cyclic voltammogram in cryolite–5 wt% SiO2 melt; scan rates 5, 10, 20, 50 and 100 mV.s-1...... 92

Figure 4 -25. Plot of cathodic peak current versus square root of sweep rate. (The background current was subtracted from the peak current for the corrected data)...... 94

Figure 4 -26. SEM image of electrode tip...... 95

Figure 4 -27. EDS analysis of matrix (A)...... 95

Figure 4 -28. EDS analysis of the smooth, bright phase (B)...... 96

Figure 4 -29. EDS analysis of the dark phase (C)...... 96

Figure 4 -30. a)Voltammogram in molten SiO2–free cryolite; 20 mV.s-1, b) Voltammogram in molten SiO2–free cryolite; 50 mV.s-1...... 97

Figure 4 -31. Voltammogram for deposition of Si on copper in molten cryolite– 6 wt% SiO2; 10 mV.s-1...... 98

Figure 4 -32. Effect of scan rate on the recorded voltammogram in cryolite–6 wt% SiO2 melt, scan rates 20, 50, 80, 100 mV.s-1...... 99

Figure 4 -33. Recorded voltammogram in cryolite–6 wt% SiO2 melts at scan rates of 5 and 10 mV.s-1...... 100

Figure 4 -34. Cyclic voltammogram recorded in cryolite–6 wt% SiO2 after two deposition steps of 20 min; a) scan rate 20 mV.s-1, b) scan rate 50 mV.s-1 ...... 101

Figure 4 -35. Elemental mapping of cathode after 40 min electrolysis at -1.2 V...... 101

Figure 4 -36. Voltammogram for Cu-8 wt% Si in molten cryolite–6 wt% SiO2; 50 mV.s-1. .... 102

Figure 4 -37. SEM image and the elemental mapping of the master alloy...... 103

Figure 4 -38. EDS analysis of the master alloy sample...... 103

Figure 4 -39. The XRD analysis of Cu-Si master alloy...... 103

Figure 4 -40. Dependence of the apparent cathodic CE on the current...... 105

Figure 4 -41. Cathodic CE calculated from the mass change and mass balance...... 106

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Figure 4 -42. Solidified electrolytes after electrolysis under 1.1 A...... 106

Figure 4 -43. Pictures of anode and cathode after 5 h electrolysis under different currents...... 106

Figure 4 -44. Distribution of Si across the cathode after electrolysis under a) 1.5 A, b) 1.69 A. 107

Figure 4 -45. Depletion of Si in a thin surface layer (bright area) on top section of the cathode after electrolysis under 1.1 A...... 107

Figure 4 -46. XRD pattern of solidified electrolyte after electrolysis with 1.5A...... 108

Figure 4 -47. Anodic CE calculated from gas analysis and actual CE calculated from EDS results...... 110

Figure 4 -48. Mass of Si reported to the alloy cathode and Si reduced in the cell ...... 110

Figure 4 -49. Instantaneous anodic current density...... 111

Figure 4 -50. Relationship between the cathodic current efficiency and CO2 concentration. .... 112

Figure 4 -51. Anode and cathode after electrolysis in experiment # 4...... 114

Figure 4 -52. Crucibles after Experiments a) # 4, b) # 5...... 114

Figure 4 -53. SEM observation and EDS analysis of the cathode from experiment # 4...... 115

Figure 4 -54. SEM observation and EDS analysis of the cathode from experiment # 5...... 115

Figure 4 -55. SEM images of the cathode a, b) after extended electrowinning and c, d) after solvent refining...... 117

Figure 4 -56. a) Micrograph of solvent refined alloy, b) EDS analysis across the line shown in (a)...... 118

Figure 4 -57. SEM image of the Si phase after acid leaching; a) X100, b) X998...... 119

Figure 4 -58. XRD pattern of the Si phase after acid leaching...... 120

Figure 5 -1. Overall process flow chart...... 124

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Chapter 1 Introduction 1.1 Motivation for the thesis

The global increase in energy consumption, limited reserves of fossil fuels, and the environmental pollutions associated with them are the drivers for a drastic change in our fuel– based energy towards alternative and renewable sources such as solar, wind, and biomass. Solar energy in the form of heat or photovoltaic electricity is the most abundant renewable energy form. Although it is not yet clear what portion of our energy will eventually be supplied by solar power, it is well known that its potential can far exceed the total energy demand of the globe. This potential together with the need for green energy sources have provided a unique opportunity for the photovoltaic industry to grow at a rate that is not imaginable for many other industries. Despite the high potential and well established technology, solar energy is not a main contributor to today’s energy basket. One of the current hurdles facing the widespread use of solar energy is the high cost. Figure 1 -1 compares the cost of electricity from different sources in 2013, confirming that solar power is not competitive with other sources.

Figure 1‎ -1. Cost of electricity generated from different energy sources [1].

Today, over 90% of the photovoltaic materials in commercial products are silicon based [2]. Si being the dominant photovoltaic material, accounts for 25–50% of the cost of solar arrays [3]. To reduce the cost of solar cells, it is therefore necessary either to decrease the cost of Si or shrink the Si consumption in the fabrication process. Traditionally, the majority of solar silicon was

2 from overcapacity, scraps, and rejects of the semiconductor industry, costing around $80/kg. Since 2003, demand for polysilicon has exceeded the supply due to the skyrocketing growth in the PV industry [4, 5]. As the purity requirement of silicon for solar application can be less than that of the semiconductor grade silicon (6N compared to 9N [3]), a new type of silicon known as solar grade silicon (SoG–Si) was introduced, for which the target price is $10–15 /kg. The price has in recent months been around $20–25/kg, down from over $200/kg in 2008. This price is believed to be unstable as it is dictated by the oversupply.

The existing method of SoG-Si production involves a combination of crude Si–making process, which produces metallurgical grade Si (MG–Si), and a refining process known as Siemens, through which MG–Si is upgraded to SoG–Si. This technique consumes 120–200 kWh/kg of energy and produces approximately 90 tonnes of CO2 per every tonnes of Si [3] as well as toxic gases. About 95% of required SoG–Si by the PV industry is produced by this method. However, due to the large operating cost and energy consumption, and low productivity, an alternative process is highly sought after. The feasibility of developing such a process has been studied by various researchers in the past two decades. The investigations have generally followed one of the following two approaches: direct production of SoG-Si from ultrapure feedstock or refining of MG–Si. Reduction by carbon [6, 7] or metals and compounds [8, 9] as well as electrodeposition [10-14] are the methods which directly produce high purity Si. The refining techniques such as acid leaching [15-17], reactive gas blowing [7, 18], slagging [19, 20], and solvent refining [21-25], purify MG–Si into SoG-Si. Up to now, these achievements are limited to laboratory and pilot scale and not fully implemented in commercial scale.

Among the studied methods, electrochemical approach is considered to be a promising route for silicon production due to the analogy of Si with Al that is produced by electrodeposition in the well established Hall-Heroult process. A detailed review of previous works [13, 26] reveals the great potential of the method in generating SoG–Si with low cost and energy consumption. For example, the production cost of Al is ~ 2$/kg, with the major operating cost being the pre– processing involved in isolating alumina from its ore, while the widespread availability of inexpensive high purity silica ($0.02 /kg [27]) can contribute to generating low cost Si. Although this method has been shown to be capable of delivering a very high purity Si (99.999%) [11] which can be turned into SoG–Si after one step of melting and directional solidification [28], it

3 has not been commercialized primarily due to one major problem: the high melting point of Si (1412°C).

Deposition of Si below the melting point (in the solid state) is slow [11] and results in the formation of powdery Si dispersed in the melt which is hard to recover afterward [29-31]. On the other hand, electrodeposition above the melting point enhances the production rate [31] but still suffers from high working temperature and low current efficiency. To overcome these limitations, electrodeposition of different Si alloys, molten at 1000°C, has been studied. Initial investigations on Cu–Si [12] and Al–Si [14, 32, 33] have shown that this approach is promising to produce Si with acceptable range of impurities (P, B) in the alloy. However, separation of Si from the alloy has remained a challenge. Electrorefining of the molten alloy to recover Si from the alloy has been attempted [11, 12, 34-37] but the problem of solid deposition in cahode remains a hurdle.

Improving the efficiency of the electrolytic process together with overcoming the separation challenge were the primary motivations for this study. The proposed technique combines electrowinning in a molten alloy to improve the efficiency of deposition and solvent refining to separate the Si dendrite from the alloy matrix.

1.2 Objectives of the study

The overall objective of this research was to develop an inexpensive and sustainable process for producing high purity silicon. The detailed objectives are:

1- To characterize the cryolite-SiO2 system as a possible electrolyte by measuring its physicochemical and charge transport properties.

2- To deposit Si from cryolite-SiO2 melt into a molten cathode and study the effect of applied potential and current on the purity of the final product.

3- To measure the deposition and the decomposition potential on the surface of candidate cathodes: graphite, copper and copper-silicon alloy.

4- To determine the current efficiency of the process and the electrodeposition rate.

1.3 Organization of the thesis

The thesis has been structured into 6 Chapters. Chapter 1 provides an introduction with respect to background, motivations behind the work, and objectives. Chapter 2 summarizes the knowledge in the literature pertaining to the objectives of the research. Chapter 3 presents the details of experimental work including the materials used, equipment, and the procedures for conducting the experiments and assessing the results. Chapter 4 provides the results of this research and an explanation of the findings. Chapter 5 summarizes the main achievements of each phase of this study and provides the conclusions. Finally, Chapter 6 presents the author’s suggestions for future studies that will complement this work.

The details of calculation of error analysis for density measurement are shown in Appendix I. Appendix II shows how the molar conductivity values were calculated in this thesis. Appendix

III demonstrates the elemental mapping of cryolite−SiO2 melt after quenching. Appendix IV contains the Cu-Si binary phase diagram and a list of publications from this Ph.D. work is provided in Appendix V.

A flowchart presented in Figure 1-2 provides an overview of the scope of this research.

Main Objective:

Developing an inexpensive and environmental friendly process for producing SoG-Si

Proposed Method:

Deposition of Si below its melting temperature through alloying and separation of Si from alloy matrix through solvent refining

Research Plan: Three different phases

Charactrization of cryolite- SiO2 melt Determination of the required potential Electrowinning and separation of Si 1- Density measurment 1- Deposition potential on graphite 1- Anodic and cathodic current 2- Electrical conductivity measurment 2- Decomposition potential on Cu efficiency measurement 3- Tranport number measurement 3- Decomposition potential on Cu-Si 2- Extended electrowinning 4- Phase equibliria development 3- Solvent refining 4- Reportig final purity

Figure 1‎ -2. Research plan.

Chapter 2 Literature Review

2.1 Silicon

Numerous photovoltaic materials have been investigated as possible candidates for high efficiency and low cost conversion of Sun’s energy to electricity, including silicon, cadmium telluride, copper indium deselenide and gallium arsenide. However, Si remains as the most widely used photovoltaic material due to, abundance of quartz as its source material, proven effectiveness of Si in solar cells, established low-risk technologies around Si, and minimum health and environmental issues for its use or end-of-life disposal/recycling. [38].

Silicon is the second element in group IVA of the periodic table of elements. It has a band gap of 1.12 eV at 25°C which is very close to the ideal band gap of 1.4 eV for harvesting sun’s energy [2]. Silicon is the second most available element in mass after oxygen [39]. In nature, it exists in the form of oxides and silicates, the main constituents of the Earth’s crust. Silicon reaction with oxygen is very fast and forms a thin film of silica less than 100 Å that protects the bulk of silicon from further oxidation. Silicon has a semi polymeric behavior that can cause the formation of Si-

Si, - (SiH2)p- or -(SiF2)p- chains in the structure. The characteristics of these chains are very similar to hydrocarbons and fluorocarbons chains.

Silicon has a wide range of applications in various forms such as semiconductor in microelectronic and PV devices, silicone-based polymers, ceramics (oxide, nitride, and carbide of Si), alloys, and chemicals.

There are three main categories of silicon metal available in the market depending on their level of purity: 1) Metallurgical grade silicon (MG–Si) with a purity of 98-99%, 2) Semiconductor grade silicon (SeG–Si), with the purity in the range of ppb− ppt, and 3) Solar grade silicon,

SoG–Si, with the impurity content in the range of ppm. Figure 2 -1 shows the price of these Si types as a function of their impurity content. Due to the specific purity requirement for SoG-Si, there is an opportunity to produce this material at a cost of $10- 15/kg, which enables widespread use of solar power by reaching grid parity with the traditional electricity sources. Since 2008, the

6 market price of SoG-Si has been fluctuating between $20 and $200/kg [40] depending on the supply-and-demand situation.

Figure 2‎ -1. Relationship between the cost and the purity of various types of Si [15]. 2.1.1 Metallurgical grade silicon

Metallurgical grade silicon also known as silicon metal is produced in submerged arc furnaces. The furnace is fed with quartz and carbon, where the carbothermic reduction of silica at 2000 C generates liquid Si. The metal is tapped from the bottom of the furnace (Figure 2 -2). The main impurities in this type of Si are Fe, Al, Ca, Ti, and C. Approximately, one million tons of MG-Si is produced each year that is used in making Al and steel alloys, deoxidation of steel, synthesis of silicon, and production of photovoltaic materials for the solar and electronic industries [15, 41, 42].

Figure 2‎ -2. Schematic of the process for production of MG-Si [43].

2.1.2 Semiconductor grade silicon

To achieve the required semiconductor properties, the impurity level should be as low as 1 ppb in this type of Si. SeG-Si is produced by purification of MG-Si through a vapor deposition process known as Siemens. MG-Si is first reacted with hydrogen chloride gas at 300 C to form chlorosilane or trichlorosilane:

Si (impure) + 4HCl (g) → SiCl4 (g) + 2H2(g) (‎2-1)

Si (impure) + 4HCl (g) → SiHCl3 (g) + H2(g)+1/2 Cl2(g) (‎2-2)

The impurities are separated by fractional distillation based on the difference in their boiling temperature. The vaporized chlorosilane is then decomposed to Si and HCl in the presence of hydrogen. The high purity Si grows on the surface of the seed, which is a heated Si rod at 1150°C according to Reaction ( 2-3).

SiHCl3 (pure)+ H2 (g) → Si (pure) + 3HCl (g) (‎2-3)

A schematic diagram of the Siemens process is shown in Figure 2 -3. This technique is very energy intensive (120-200 kWh/kg) and produces approximately 90 tonnes of CO2 per every tonnes of Si [3]. Currently, the bulk of Si for solar industry is produced by this method.

Figure 2‎ -3. Schematic of the Siemens process [44].

2.1.3 Solar grade silicon

Silicon used in solar cells is typically cut from the ingot of SoG-Si. A method with lower production cost and dedicated to meet the more relaxed purity requirement of SoG-Si is desired. The level of impurities in MG-Si is compared with SoG–Si in Table 2 -1. The requisite level of purity can be achieved by the conventional metallurgical purification methods for most of the impurities except P and B.

Table 2‎ -1. The acceptable level of impurity in MG-Si and SoG-Si [3].

MG-Si allowed concentration in raw impurity 98-99% pure (ppm) SoG- Si (ppm) Al 1000-4000 <0.1 Fe 1500-6000 <0.1 Ca 250-2200 <1 Mg 100-400 <1 Mn 100-400 <<1 Cr 30-300 <<1

Ti 30-300 <<1 V 50-250 <<1 Zr 20-40 <<1

Cu 20-40 <1 P 20-40 0.1-1 B 10-50 0.1-1.5

C 1000-3000 50-100

The effect of each impurity on the efficiency of p-type silicon is shown in Figure 2 -4, based on simulation results. The combined effect of several impurities is complex and not simply cumulative. Many researchers [45-47] have tried to draw a guideline for SoG–Si specifications, although a consensus has not been reached to date.

In the past two decades, many researches have been conducted to produce SoG-Si in a low-cost and environmentally friendly process. These methods are either metallurgical or chemical processes and will be discussed in the next section.

Figure 2‎ -4- Effect of impurities on the performance of the p-type silicon. 1) semiconductor, 2) solar and 3) metallurgical grade silicon[48]. 2.2 SoG-Si production methods

Generation of SoG-Si involves multiple refining processes, although repeapted directional solidification could deliver Si as pure as desired from MG-Si, but at a large cost. In directional solidification, solid Si is grown from liquid to leave the impurities behind. The distribution of impurities between liquid and solid is known as segregation coefficient, , which is defined by

 = Cs/CL (‎2-4) where Cs and CL are the equilibrium concentrations of impurities in solid and liquid, respectively. According to Equation ( 2-4), the lower the , the higher the extent of impurity removal from solid.

In practice, directional solidification is reazlised by crystal growth or zone refining. The former method, known commercially as Czogralski technique, involves pulling of a Si crystal from melt. This is achieved by mounting a seed of Si single crystal on a rod, dipping the seed in Si, and gradually pulling the rod away so that a large, single-crystal forms from the melt [49]. In zone refining, a Si rod is melted from one end using a heating element, then the heating zone moves along the rod. The melted section of the rod resolidifies, pushing the impurities into the liquid front. The rate of Si production in both of these techniques is very low and in zone refining, the rod size is limited to 10 cm in diameter. While most impurities can be removed to acceptable levels within one or a few passes, those with high segregation coefficient require numerous

10 repeats, hence prohibitively large refining cost. These elements include P and B with segregation coefficients of 0.35 and 0.8, respectively [50]. Their removal is preferentially carried out by other more effective techniques such as Siemens, while other methods with lower cost and higher productivity are desired, as explained earlier.

The researches in the recent years have been focused on low-cost purification of Si through two different approaches. The first involves the pyrometallurgical refining of MG-Si, the second uses high purity feedstock (such as carbon black, quartz or silicon fluoride compounds) and clean processing to deliver silicon with low impurity content. The following section will review the various techniques in each category.

2.2.1 Refining of MG-Si

MG-Si is an inexpensive source of silicon with an average price of $1-2 /kg and the purity about 95-99% while the required purity for solar application should be 99.99999%. The methods presented in the literature for the upgrading MG-Si are discussed here briefly.

2.2.1.1 Acid leaching

Acid leaching is a primary treatment for upgrading MG-Si due to its low cost and simplicity. The principle of acid leaching is based on low segregation coefficient of impurities in silicon. As the silicon metal is solidified, impurities with small segregation coefficient are rejected to the solid/liquid interface and eventually end up in the grain boundaries or at the interstitial positions in polycrystalline silicon. The metal is ground to small particles (50-70 m) to expose the grain boundaries to an acid solution that dissolves the impurity-rich fractions.

In a few studies on acid leaching process [16, 17, 30, 51-53], the effect of temperature, particle size and acid composition were investigated. HCl, HF, H2SO4, HNO3 and their combinations have been used for selective dissolution of impurities. Leaching MG-Si with aqua regia for long period of time has shown the most promising results [52]. These investigations were successful in removing some metallic impurities such as iron, aluminum, calcium and magnesium. However, it was believed that this process is less effective for interstitial and substitutional impurities such as boron, phosphorous, carbon and oxygen [16, 17, 51, 52]. Therefore, this process is not solely sufficient to upgrade MG-Si to SoG-Si and should be followed by a pyrometallurgical process to provide SoG-Si with acceptable range of B and P. Addition of Ca

11 to Si before acid treatment improves P removal, down to 5 ppm in concentration [53]. The maximum achieved purity by acid leaching is 99.99% [54].

2.2.1.2 Reactive gas blowing

Reactive gas blowing is one of the main pyrometallurgical techniques for upgrading MG-Si. In this process a reactive gas, which is diluted by an inert gas is bubbled through molten MG-Si or acid leached MG-Si. The gas reacts with the dissolved impurities and forms volatile compounds which are removed from the surface by the gas flow [3, 15].

Figure 2‎ -5- Standard‎free‎energy‎of‎formation‎of‎impurity’s‎oxides‎[55].

The type of gas is important in the success of this process. Cl2, O2, SiCl4, wet H2 and CO2 have been used [3, 15, 19, 56]. Cl2 is widely used for purification because it selectively reacts with impurities and forms volatile chloride compound. Among different compounds, chlorides of Al, Mg, Mn and B have very low boiling points; therefore, it is easy to transport and remove them from silicon [56]. The elements more active than silicon can also be removed as oxides, using oxygen. Figure 2 -5 shows the free energy of formation for different oxides. According to this figure, Ca, Mg, Al, and Ti oxides have lower Gibbs energy of formation, thus can be removed from Si without substantial Si loss [15]. However, Oxygen is not successful in removing P and B

12 from the silicon. The free energy of formation of these two impurity oxides are higher than that of silica, also their concentration is several orders of magnitude smaller than Si, thus significant Si oxidation occurs before their effective removal. Under oxidizing environment, different boron oxides form, among which BO is the most volatile compound [19, 57]. Lynch [58] and

Nordstrand [59] showed that H2-H2O environment can effectively remove B by formation of

HBO volatile compounds. CO, CO2 and SiF4 have been used to remove the less active elements.

CO2 is a suitable gas for P and C removal while SiF4 reacts with B, Cu, Ca and Mn and forms fluorides which are transported to the gas [3].

2.2.1.3 Slagging

This method employs molten slag containing oxides (and sometimes chloride or fluoride) of Ca, Si, Mg, Al, Na, Ba, etc. to absorb impurities from the metal. The impurities get oxidized and dissolve in the slag phase. The slag should have specific characteristics for optimal impurity pick up, including: 1) the impurity oxide should have high solubility in slag, 2) the slag should not cause severe oxidation and loss of silicon and 3) the slag and silicon should be easily separatable (i.e. different density). The amount of slag in metal refining is typically 1-5 wt% of silicon [15].

This method reduces the concentration of some elements such as Al, Mn, V and Ti by about one order of magnitude but is not successful in the case of B and P since these elements are more noble than Si [60]. CaO-SiO2 based slags are most common for boron removal [61-63]. Johnston and Barati found a linear relationship between P removal and CaO:SiO2 ratio [20]. They have reported that due to the relatively small distribution of B and P between slag and metal, this method requires large amounts of slag, making it unattractive from the operational and the cost points of view. A more recent study by the same group on slag treatment of Cu-Si alloy using

Na2O-containing slags has shown effective B removal [64], and the success has been attributed to the effect of Cu on activity coefficient of B. Phosphorus removal by slagging is however not as effective as B removal [19].

2.2.1.4 Solvent refining

Solvent refining is a controlled solidification process, during which an alloy of a metallic agent with silicon traps the impurities and leaves pure silicon dendrites behind. The process involves melting of crude Si with the impurity getter alloying element, slow cooling of the alloy to allow

13 precipitation of pure Si crystals, and separation of the Si from the alloy. Depending on the physical and chemical properties of the alloy, the Si product can be recovered by one or a combination of liquid ﬁltration, leaching or electrochemical dissolution of the solidiﬁed alloy, physical separation of the two phases based on their different density, and electromagnetically induced separation [19].

Different metals [65] have been used as the alloying elements such as Al [23-25, 66-68], Fe [21, 22], Ca [65], Cu [30, 69-71], Mg [30], Zn [42], Sn [42], Sb [42], and Ni [72]. The metal should be inexpensive and readily dissolve silicon in liquid state while posses small solubility in solid state and higher affinity for impurities than silicon.

Morita et al. [23-25, 73] performed a comprehensive study on Si-Al system. According to their findings, this method is more efficient in removing metallic impurities namely Fe, Ti, Cu, Mn and Ni rather than P and B. They also studied the effect of Ti and Ca addition to Al-Si system, and found that Ti improves B removal [66] and Ca is effective for P removal [53]. Solvent refining of Cu-Si followed by acid leaching was studied by Mukherejee [30]. According to this study, Mn, Mg, Cr, Ni, Al and Ca were partially removed by solvent refining and further treatment by aqua regia removed more Cr and Ca. A second leach in HF removed B, Fe and Mn substantially. Recently Mitrašinović and Utigard [70, 71] conducted solvent refining of Si using Cu. Based on their results, this technique is effective in removing the metallic impurities namely, Au, Co, Cr, Cu, Fe, Mn, Mo, Ni, Ti, and Zr, but not B and P. Iron was the other solvent that was used by Esfahani and Barati [21, 74]. Their findings also shows the effectiveness of iron as a getter in removing over 90% of almost all impurity elements but B and P. Khajavi [22] also studied the thermodynamics of phosphorous distribution between Si and Fe-Si alloy and found that higher P removal is expected at higher temperature, which means quenching from temperatures above the eutectic temperature is favorable for P removal.

2.2.1.5 Electrorefining

Based on the electroreﬁning principal, if Si is placed in the anode of an electrorefining cell, elements with more positive electronegativity than Si will not anodically dissolve, while those with electronegativity lower than Si will anodically dissolve but will not be cathodically deposited. Figure 2 -6 maps various elements based on their segregation coefficient in Si and electronegativity. It indicates that B and P, while particularly unresponsive to directional

14 solidification, can potentially be removed by electrorefining. These will remain in the anode during electrorefining, while other elements such as V, Ti, and Mg that migrate to the electrolyte from the anode will not deposit at the cathode.

Figure 2‎ -6.‎Comparison‎of‎segregation‎coefﬁcient‎and‎electronegativity‎of‎impurities‎in‎ MG-Si [15, 75, 76]

The idea of electrorefining of Si was first introduced by Monnier [12, 35] in 1964 when he investigated the possibility of electrorefining of either solid silicon or molten alloy (e.g. Cu-Si alloy). He proposed a dual cell design for this purpose, as seen in Figure 2 -7 [12]. In the first cell, silica is reduced to Si by electrolysis of a cryolite−silica solution and Si is deposited into a liquid Si-Cu alloy. In the second cell, the Si-Cu plays the role of an anode; Si migrates from the anode and the refined solid Si deposits onto a graphite cathode. The method has been reported to be capable of delivering 99.99% pure Si with around 75% current efficiency.

Later, Olson and Carleton [77] extended the idea of electrorefining of Cu-Si alloy to lower working temperature (750 C). This alloy was then refined in a KF-LiF-K2SiF6 melt. This technique was successful to deliver 99.999% pure silicon; this achievement is remarkable for P and B removal: up to 80 and 96% respectively.

Figure 2‎ -7. Suggested cell design for dual refining of Si [35].

In mid 1980s, Sharma and Mukherjee [78] investigated the feasibility of MG-Si purification in molten KF−LiF−K2SiF6 electrolyte, avoiding the additional alloying step. This study showed effective removal of Al, Fe and Ca as well as B (91% removal). Recently, Zou et al. [79] also tried direct electrorefining of MG-Si in KCl-NaF solution at 800 C. This technique was not effective in B removal (80%) compared to other impurities; still the total purity is higher than that in Sharma’s study.

Before 2007, all of researches on electrorefining of MG-Si had been carried out in fluoride based electrolytes. However, there are several industrial complications dealing with fluoride melts such as toxic volatiles and corrosive environment of the melt. Kongstein et al. [80] and Cai et al. [81] used a chloride based melt containing (CaCl2−NaCl−CaO), which is less corrosive and more stable at the working temperature. Cai et al. [81] conducted the electrorefining of Cu-MG Si alloy in this electrolyte at 850-950 C. Promising B and P removals (99 and 96% respectively) were achieved, but Cu and Ca were retained in the product.

In 2010, a new electrorefining technique, called three layered refining process, was proposed for upgrading MG-Si. This process is commercially used for production of super-pure aluminium [82, 83]. Electrochemical cell in this process consists of three different molten layers. The bottom layer is mostly anode, which is an alloy of impure metal with a noble metal such as Cu. The second layer is the electrolyte, which is often a molten salt, and the last layer is the deposited metal with the lowest density, which floats on top of the cell (Figure 2 -8). This principal was

16 used by Oslen et al.[84] and Lai et al. [85]. The method has been reported to be capable of removing most of the metallic impurities such as V, Ni, Mn, P, Fe, Ti, Al, Cu but is not successful in B removal.

Figure 2‎ -8.Three layer technique for electrorefining of super pure Al [86]. Despite these achievements in production of crystalline silicon, manufacturing of solar panels is accompanied with significant silicon loss during cutting and wafering. To overcome this problem, several studies have been conducted to electrodeposit Si on an appropriate substance, namely monocrystalline Si [87, 88], Ag [88] and finally graphite [28]. Osen et al. [89] investigated the electrodeposition of Si from KF-Li-K2SiF6 on different substances such as Ag, Si, W, and glassy carbon. They discussed that deposition of Si is diffusion controlled and takes place in two steps: Si (IV) Si (II) Si. The best results were achieved by electrodeposition on Ag at 800 °C without any salt inclusion and nodular surface.

2.2.2 Silica reduction

The widespread availability of highly pure silica at a relatively low cost presents an opportunity to produce high grade silicon in one step of the reduction. In the following, different routes for production of SoG-Si from SiO2 are described.

2.2.2.1 Carbothermal reduction of silica

This method is based on reducing silica by carbon, which is the main technique for MG-Si production. Principally this method can also be used to produce low cost SoG-Si if the impurity introduction into the Si is well controlled by selecting high purity silica and carbon sources and

17 operating in a clean environment. The process was first practised by Dosaj et al.[6]. The main reaction is:

SiO2+ C  Si + CO2 (‎2-5) Despite the low level of some impurities, the high concentration of carbon in the final product warrants further refining, making the process expensive [3].

2.2.2.2 Reduction by metals and compounds

This method involves reducing silicon halides by various metals or compounds. The source of silicon can be SiO2, SiF4 and SiCl4 while two different types of reductants have been used: compounds such as NH3 and CH4, and metals namely Na, K and Zn, which were used in large scale production [7, 15]. Among different possible reactions, reduction of SiF4 by Na yields the lowest concentration of impurities [90]. In this method, Both SiF4 gas and solid Na are charged to a reactor at 400 C. They react according to:

SiF4+ 4Na  Si+ 4NaF (‎2-6)

The reaction produces Si and NaF, also some Na2SiF6 is collected at the bottom of the reactor from which Si can be recovered.

2.2.2.3 Electrodeposition

Electrodeposition of Si from various electrolytes has been studied from 1950s. Since the topic of this research is on producing Si by this technique, it will be discussed in more details in the following section.

2.3 Electrowinning of silicon

This technique has been studied by different researchers trying to develop a process similar to the Hall-Héroult, for silicon [37, 91-96]. The overall reaction is decomposition of silica to silicon and oxygen, using electricity, if an inert anode is used, which presents the opportunity of a carbon-free process. It has been estimated that electrodeposition of Si requires 28.5% more energy than Al due to higher ionic charge (IV for Si compared to III for Al) [13].

Monnier [37] gives the credit of the first attempt on silicon electrodeposition to DeVille who electrodeposited silicon from a solution of KF/NaF containing SiO2 on a platinum cathode. The cathode reacted with Si and formed platinum silicide. Gore [29] also claimed to have deposited

18 silicon from an aqueous solution of potassium monosilicate. This claim has not been confirmed later. The electrodeposition of Si in elemental form was first reported by Ullik [97]. His electrolyte was KF and he used K2SiF6 as the source of silicon. Minet [98] conducted the first research on electrodeposition of Si in the form of Fe-Si and Al-Si alloy from molten cryolite,

NaCl, SiO2 and Al2O3.

In spite of the above scattered attemps, a systematic study of silicon electrodeposition was not done until 1930’s when Dodero [99, 100] electrodeposited silicon from molten alkaline or alkaline earth metal silicates, at the temperature range of 800-1250 C (benefiting from fluoride additives). In this study, alkaline and alkaline earth metals were also deposited with Si because of the large applied potential. The best result was 72% Si produced from a 5SiO2- 1Na2O- 0.2NaF melt at 1150 °C.

The electrodeposition can use MG–Si to produce a higher grade Si (i.e. electro–refining) or produce Si from its compounds (i.e. electrowinning). The latter is a more attractive process as it starts directly from Si containing species (silicates and fluorosilicates) and eliminates the reliance on the availability of MG-Si. Several studies have been performed on electrowinning of Si, trying different techniques and electrolytes to deposit highly pure SoG-Si. These works are divided into two main categories based on the operating temperature: Electrowinning below the melting point of silicon where deposited metal is solid and, electrowinning above the melting temperature where the deposited silicon is liquid.

2.3.1 Electrowinning of solid Si

Different types of electrolytes have been proposed for this process, namely organic electrolytes and molten salts.

2.3.1.1 Organic solvents

The advantage of using an organic electrolyte is low working temperature. Previous investigations reveal that it is possible to deposit amorphous Si (a-Si) at temperature close to ambient [13, 37, 101-103], resulting in low energy consumption. Several organic solvents have been used such as propylene carbonate [101, 104], dimethylformamide (DMF) [101], tetrahydrofuran [101] or its mixtures with toluene or benzene [102] and ethyl alcohol [103]. Silicon tetrachloride or triochlorosilane were used as the source of silicon.

The extracted a-Si is mostly hydrogenated which possess interesting semiconductor properties. Bucker and Amick [102] reported that the deposited a-Si is not stable against atmospheric attack and need further heat treatment to remove the excess hydrogen. In order to increase the stability, Kroger [103] tried to deposit the fluorinated a-Si. The main challenge for this type of electrolyte is low efficiency of the deposited a-Si which restricts its application in solar cells; but the technique for the deposition of a-Si is well established [13]. Deposition of Si in nano scale from ionic liquid was reported by Al-Salman [105].

2.3.1.2 Molten salts

2.3.1.2.1 Deposition from halide melts

Fluoride melts offer advantages such as high conductivity, low viscosity, high decomposition voltage and high solubility for metal oxides which is beneficial for cleaning the surface of metallic electrodes [106]. The halide electrolytes include alkaline or alkaline earth fluorides, cryolite and SiF4 [37]. The source of silicon used in combination with these electrolytes is often an alkaline fluorosilicates (Me2SiF6) which is stable below the melting temperature of Si. Therefore, the cathodic product of these electrolytes is either solid Si or molten alloy. Ullik [97] was the first to deposit silicon from KF melts. Oltowski [107] also tried to electrodeposit Si from a mixture of LiF, NaF and Na2SiF6 using Fe or Cu cathode; he produced a small quantity of silicon and silicon intermetallics. Monnier and Giacometti [92] suggested liquid Cu-Si alloy as the anode in a NaF, KF or LiF and K2SiF6 bath.

The Recent studies are mostly focused on electrodeposition of a Si layer on an appropriate substance to directly produce Si wafer for solar cells. The deposited layer can be used as n-type Si and prevent Si loss of the ingot slicing process. Cohen [108] pioneered electrodeposition of Si film from a mixture of alkaline fluoride and K2SiF6 system for direct solar application. A single crystal epitaxial layer was deposited from LiF-KF solution. A continuous film was also produced from this melt using a dissolving Si as the anode.

Rao and Elwell [26, 28, 88, 109, 110] conducted an extensive investigation on electrodeposition of Si from different electrolytes and Si sources and on a range of substrates. According to their findings, deposition of Si from binary LiF/KF eutectic and ternary LiF/NaF/KF eutectic melts and on a silver substrate have given the best results [110]. Silver is an expensive material to be

20 used in bulk deposition for solar cells so the possibility of deposition of a coherent layer of Si on the surface of graphite was also studied [28]. This research could successfully deposit a dense and continuous layer of Si at -0.75 0.05 V vs. Pt or Ag onto graphite from a K2SiF6/LiK-KF salt at 745 5 C. The deposited Si was 99.999% pure.

Boen [111] studied the effect of electrolysis parameters on the deposition of Si from Na2SiF6 in LiF/KF solution. This investigation shows that it is not possible to deposit Si on graphite or Si electrode using non-purified electrolyte. They also found that using direct current does not allow deposition of a thin and pure silicon layer; instead, pulsed current was suggested for this purpose. The level of purity obtained from two different anodes, insoluble graphite and soluble MG-Si, demonstrates that the level of most of the impurities is lower in case of graphite anode.

All of the above studies were conducted in a molten fluoride electrolyte. The research conducted by Devyatkin [112] is the first that used K2SiF6 in a chloride melt (NaCl-KCl). This study confirms the possibility of Si deposition from the chloroflouride melts. Andrikko [113] used a mixture of fluoride- chrolide melt as K2SiF6/KCl-KF solution. SiCl4 gas is another source of Si that was used for deposition of Si in LiCl-KCl melt at 450C [114].

2.3.1.2.2 Deposition from mixture of oxide without halides

The main source of Si in non-halide electrolyte systems is SiO2, which is added to a mixture of different oxides to improve the conductivity and lower the melting point. A few studies on this system have been carried out in the past noticeably the research by Andrieux [115], who deposited a-Si from a mixture of Na2O and SiO2. Jorgensen [116] studied deposition of a−Si on polycrystalline silicon by electrolysis of silica. Finally, Lyakovich [37] deposited a layer of Si on iron cathode from molten Na2O and SiO2 mixture between 1050 and 1150°C. He aimed to protect the iron by formation of intermetallic compound of Fe–Si. This electrolyte has not been used for extraction of pure Si.

2.3.1.2.3 Deposition from mixture of halides and silica

Cryolite has been used for electrodeposition of silicon because of its ability to dissolve silica and its established use in the extraction of aluminum. The process must be carried out at higher temperatures than cryolite melting point, between 970-1050°C. Minet [98] was the first to use

21 cryolite for silicon electrodeposition but he obtained an Al–Si alloy likely due to the large applied potential. Monnier et al. have reported the most number of studies on elctrowinning and electrorefinning of Si from solution of cryolite and silica [12, 35, 117]. First, they tried to electrodeposit Si from silica-cryolite mixture using different electrodes [117]. They found that when using graphite as cathode, the products are Al–Si alloy, Si and SiC, while Ni cathode results in the formation of a Ni–Si alloy without any Al [117]. Later they focused on electrorefinning of solid Si or alloy of Si–Cu using the same electrolyte bath and were successful in achieving purity between 99.7 to 99.99% [92]. Their achievements were then extended to pilot plant as the next step for commercialization but they did not succeed due to the slow rate of deposition [37]. This is the only reported pilot scale trial of Si electrodeposition and the schematic of their cell is shown in Figure 2 -9 .

Figure 2‎ -9. The cell design proposed for Si production [118].

Grjotheim and Matiasovsky [14] measured the physicochemical properties of cryolite–silica bath with and without alumina. Their studies were mostly concentrated on co–deposition of Al–Si alloy from this electrolyte. In the first step the decomposition voltage of SiO2 and Al2O3 on graphite, Al and Al–Si cathodes were determined [33]. They found that the decomposition voltage of silica on a copper cathode was a function of alumina content. The decomposition voltage of Al2O3 also changed with silica concentration [32]. Oslon [119] used cryolite-SiO2-LiF electrolyte for deposition of Si in molten tin. Si is initially deposited and dissolved in the cathode; after exceeding the solubility limit, solid Si dendrites form. These Si dendrites were separated from the alloy later. Elwell [110] investigated the electrowinning of Si from SiO2/NaF-

CaF2 or KF-CaF2 solution that resulted in fine silicon particles dispersed in the electrolyte.

Stubergh [120] examined the possibility of Si deposition from a different source of Si,

(Ca,Na)Al2-1Si2-3O8 (bytownite). A high purity Si (99.79-99.98 %) was produced on the surface of the graphite crucible. Recently, Ming [121] investigated both electrodeposition and electrorefining of Si on the surface of a graphite cathode in SiO2/Na3AlF6-LiF electrolyte. The purity of electrowon Si was lower than the starting SiO2 (99.91 vs 99.97 %) showing introduction of the impurities to Si from the electrolyte and/or anode.

Chloride electrolytes such as those based on CaCl2 are other candidates for Si deposition [122- 125] as they have been successfully utilized for Ti production in the FFC-Cambridge process.

SiO2 was the silicon source in these solutions, which does not dissolve in the electrolyte but rather forms a contacting electrode, i.e. the oxide is contacted with the cathode to be decomposed in-situ, leaving metal behind. The results of these investigations show that SiO2 can be electrochemically reduced and forms either elemental Si or Si alloys.

In all the above methods, Si is deposited in solid form. Consequently, the deposition rate is low due to the instability of liquid–solid interface at a critical current density of approximately 40 mA/cm2 which restricts the linear growth rate of Si to 45 m/hr [126]. The low deposition rate and powders dispersed in the electrolyte result in low metal yield, slow production rate, rendering the process unattractive. Further, at low current, the joules heating of the cell may not be sufficient, requiring external heating. Increasing the current density in order to increase the deposition rate would result in trapping of electrolyte and impurities during the uneven advancement of the interface. Higher current densities will eventually result in the formation of powdery silicon dispersed in the melt. As opposed to a solid cathode, there is virtually no limit to the level of current that can be applied to a cell with molten cathode. For example, in the production of Al, the current density is around 1A/cm2 (40 times larger than the current limit for solid deposition).

2.3.2 Electrowinning of molten Si

2.3.2.1 Above the melting temperature of Si

Aiming to overcome the above limitations, Mattei [127] deposited silicon above its melting point at 1450 C. The high working temperature restricts the material of electrolyte, the choices are limited to alkali or alkaline earth metal silicates. Among these SiO2/BaO-BaF2 mixture was

23 shown to deliver the most promising results. The deposited silicon has lower density than the bath and floats on the top. Downs’ cell design was proposed by Elwell and Rao [13] to easily collect Si (Figure 2 -10). Despite the deposition in liquid state, the Faradic efficiency of the system was low (15-22%), which was related to the formation of SiO due to the reaction between the deposited Si and the silicate phase. The purity of the deposited silicon was 99.97% in this method with the main impurities being Fe, Ca, Sr, Mg and Ti.

Figure 2‎ -10. Schematic cell design for deposition of silicon above the melting temperature [13].

2.3.2.2 Below the melting temperature of Si

Although electrowinning of Si in the liquid state has the advantage of high deposition rate, it still suffers from high energy consumption (due to the working temperature of +1400°C) and low current efficiency. In addition, there are few choices for electrolyte materials that are stable at that high temperature and as discussed above, special cell design and atmosphere control are required to prevent the oxidation of Si metal that floats on slag. Therefore, a new technique using liquid anode/cathode was developed by Monnier [35]. Basically, by alloying Si with another metal, liquidus temperature is lowered to below the melting point of Si. Alloys of Si have been used as both anode and cathode, for both electrowinning and electrorefining. The electrorefining was reviewed previously and the following part will discuss the electrowinning studies.

During the electrowinning, Si deposits into a molten cathode from an electrolyte containing silica or other sources of Si. Monnier [35] used this technique in combination with electrorefining process. He suggested a dual refining cell, which consists of two compartments. In the first

compartment, silica is reduced to Si by electrolysis of a cryolite–SiO2 solution. The produced Si is deposited into a liquid Si–Cu cathode. In the second compartment, the Si–Cu becomes anode, and is electro–refined to produce highly pure Si (99.99% in the study by Monnier).

Cryolite4wt% SiO2 mixture was proposed as electrolyte for both baths of the cell [35]. The current efficiency of this technique was found to be around 75%. Although the proposed technology has the advantage of liquid cathode, the final step for deposition of Si (second compartment) is still based on solid cathode, facing the same problems discussed earlier.

Grjotheim [14, 32, 33, 128] studied the electrowinning of Si alloys specially Cu-Si, Al-Si, Cu-

Al-Si, Ni-Si, Cu-Si-Fe, and Al-Si-Fe from the cryolite-SiO2 melts. Among these alloys, Al-Si showed the highest apparent current efficiency due to the supplementary effect of Al on reduction of SiO2. The ultimate goal of this alloying technique was to co-deposit both Al and Si to form the alloy, rather than recovery of pure Si.

The present research aims at electrodeposition of Si into an alloy molten at 1000°C, thus benefiting from the high current efficiency and deposition rate. The liquid cathode is then withdrawn when it is saturated with Si. On cooling, ultra–pure Si crystals precipitate from this melt that can later be recovered by a physical separation method [21]. The advantages are ease of the Si recovery, also the potential to produce very pure Si, since the refining occurs in two steps. First, a large amount of impurities is not deposited into the liquid due to their low concentration, high deposition voltage, and slow kinetics. Second, during the solidification of the cathode, a solvent refining mechanism acts so that the impurities are retained in the liquid, rather than in the growing Si dendrite. This dual refining mechanism is expected to result in a highly purified product.

2.4 Cost considerations

The analogy of a prospective Si electrodeposition process to that of Al can provide easy estimates for the cost. Al is produced at a cost of ~ $2/kg, while nearly one third of the cost is because of production of high purity alumina. In the case of Si, the raw material is quartz that is available at a much lower cost than alumina ($0.02/kg). As for the electrical energy consumption, Faraday’s law can be used for an estimate W=FVz/A. (‎2-7)

F is Faraday’s constant, V is deposition potential, z is the valence, A is the atomic weight and  is the efficiency of the process. Substituting z=4 and A=28 gives W=13.78 V/ MJ.kg-1 (‎2-8) Assuming similar applied voltages and cell efficiencies to Al (4 V and 90%), the energy consumption of the process is 60 MJ/kg Si (~ 17 kWh/kg), which is ~ 30% larger than that of Al. Therefore, combining the effects of materials and energy costs, it is expected that the price of electrodeposited Si (into the liqiuid cathode) will be comparable to Al. However, recovery of Si from the cathode requires additional steps (solidification, physical separation, and leaching) that should not add to the cost more than $5/kg based on the experiences in the metallurgical industry. If the cost of the final product is increased by a factor of even 5, i.e. $10/kg, it would still be a very attractive figure for SoG-Si, noting that today it is sold at ~ $20/kg.

2.5 Physicochemical properties

In this section, the physiochemical properties of melts pertaining to the current study and their measurement methods are reviewed briefly.

2.5.1 Density

The relative density of the electrolyte and cathode is important and it will determine whether the alloy will sink or float. Also, density and its variation with temperature and composition provide useful information about the structure of the material in liquid state.

2.5.1.1 Density measurement

Two different methods have been used for density measurement at high temperature: the Archimedes method, and the dilatometric method. The former is mostly used for higher temperature and more corrosive materials [129]. In this method, a sinker in the form of a sphere or cylinder is suspended by a wire of the same material from a balance. The volume of the sinker is predetermined using a standard melt (molten sodium chloride or potassium chloride [14, 130]). Knowing the weight of the sinker before and after immersion in the melt, the weight of the displaced fluid is equal to the mass change. The volume of the displaced fluid is equal to the volume of the sinker. Therefore the density of the melt is:

(‎2-9)

Due to the corrosive nature of cryolite the sinker should be made of a chemically resistive material like platinum or its alloys [14, 91, 131-133]. Nickel is another candidate that was used by Pascal et al. [134].

2.5.1.2 Density of molten cathode

Previous studies have shown that the density of molten Si decreases with temperature. Sato et al. [135] presented density as a function of temperature by the following equation: = 3.0052.629 10-4T (‎2-10) where T is temperature in K. This equation is valid in the range of 14251580°C. The measured densities by Rhim, et al. [136] are in good agreement with this equation while those reported by Sasaki et al. [137] show some deviations.

For the CuSi system which is used in the present research, Adachi et al. [138] have reported some data. They showed that the density of molten alloys decreases with increasing the temperature, but the decrease is negligible. The reported density at the liquidus temperature of the binary alloy is in the range of 5.77.7 gr/cm3 for the Si content of 22 2 wt%.

2.5.1.3 Density of cryolite

Several density measurements on cryolite (Na3AlF6) have been conducted by different researchers due to its importance in the aluminum industry. Edward et al. [91] reported the following correlation for temperature dependence of density. = 3.0320.937 10-3T (‎2-11) This equation is valid in a temperature range of 10001080 °C. The data reported by Matiovsky [14] is in good agreement with these results, considering the range of the experimental errors. The level of impurities has an strong effect on the calculated value of density and could be a source of error [139]. Cryolite ratio (CR), the molar ratio of NaF: AlF3 in the melt is another important factor, which can affect the density. According to Paucirova [140], the density increases with increase in the CR and reaches its maximum at 3, which is the ratio for stoichiometric cryolite. After that, the density decreases with an increase in the molar percentage of AlF3.

2.5.1.4 Effect of silica on density of cryolite

The density measurement on cryolite−silica system was conducted by Grjotheim et al. [14] at 1000 °C. Based on this study the density is a function of silica percentage: -4 = 2.10177 10  SiO2% (‎2-12)

Increase in SiO2 content decreases the melt density, however, the effect is very small. Adding silica to alumina- cryolite melt increases the density of the melt and this effect is more pronounced at higher alumina concentration [14].

2.5.2 Electrical conductivity

Electrical conductivity occurs through transport of ionic (anions and cations) and electronic (electrons and holes) charge carries in the material. The amount of current conducted by each charge carrier can be calculated from the mobility of the carrier (j), the amount of charge being carried by the carrier (zj), and its concentration (cj) using Equation ( 2-13):

j =zj . cj . j (‎2-13) Conductivity is an additive property, therefore for a material containing different charge carries the total conductivity is: (‎2-14)  

Conductivity is an important physical-chemical property from both technical and theoretical points of view. From technical standpoint, it provides information about the current and energy efficiencies of the electrolysis [141]. The energy efficiency of the process has a direct relation with the conductivity of the electrolyte. The relatively low energy efficiency is because of anodic overvoltage, anodic effect, ohmic voltage drop in the electrolyte and secondary reactions. Electrical conductivity of the electrolyte may affect the secondary reaction and as a result dictates the current efficiency of the electrolysis [142]. Fundamental studies such as the mechanism of dissolution of silica in molten cryolite, the structure of the molten fluoride, mobility of ions in the melt and the transport process[141] have been conducted based on conductivity results.

2.5.2.1 Electrical conductivity measurement

2.5.2.1.1 Cell Design

Conductivity is a material property that cannot be measured directly, but should be deducted through resistance of the cell. Equation ( 2-15) shows the relationship between the electrical conductivity and the resistance of the cell: (‎2-15)  where,  is the specific conductivity, R is the electrolyte resistance and G is the geometry factor or cell constant, which is a function of length and the cross section area of the current path

( ).

Both AC and DC signals can be used for the conductivity measurement. Measuring the conductivity by DC signal is performed using the Ohm’s law: (‎2-16)

However, the application of the DC technique is only limited to the purely electronic conductors. Applying the DC signal to ionic conductors causes the polarizations of the charge carriers. On the other hand, the AC signal can be used for all types of conductors such as purely electronic, purely ionic or mixed conductors. The main challenge of AC technique is to separate the actual resistance from the recorded impedance of the system.

Different techniques based on seven different electrode setup have been employed for measuring the electrical conductivity of the metallurgical melts [143]. Figure 2 -11 shows these cell designs in more details. The two-wire technique is the simplest cell design but it suffers from the low accuracy as the current path is not confined. In this method, the two electrodes are both driving and sensing the signal, so accumulation of opposite charge on the surface of the electrodes, known as double layer capacitance happens. This capacitance implies impedance interfering with the resistance of the melt. To overcome this limitation, the four-wire technique was introduced in which the two external electrodes carry the current and the potential is measured between the two internal electrodes. Therefore, the impedance associated with the double layer capacitance can be avoided.

Figure 2‎ -11. Schematic electrode designs for measuring electrical conductivity of liquids [143].

There are several problems involved in the conductivity measurement of fluoride salts and their mixtures with oxides, such as dependency of the cell resistance to the current frequency, corrosive environment, and high working temperature.

2.5.2.1.2 Measurement techniques

Wagner polarization technique

In this method a potential lower than the decomposition potential of the test material is applied between the two electrodes. One of the electrodes is a reversible electrode, which fixes the activity of the element corresponding to the charge carrier. The other, the blocking electrode, is an inert conducting electrode that restricts the passage of the ions. This method has been used for measuring the conductivity of the solid conductors and molten salts. The method is able to directly measure the partial electronic conductivities by electrons and electron holes.

Impedance spectroscopy technique

This method is based on the different behavior of ions and electrons at different frequencies. The sample is placed between two electron conductor but ion blocking electrodes. An AC signal is applied for a range of frequencies and the corresponding current and voltage are recorded. The impedance of the system is measured and plotted on a complex plane (imaginary vs. real part) for

30 the frequency range covered. This graph is called the Nyquist graph and is illustrated in Figure 2 -12. A simplified Randles circuit is also presented in this figure.

Figure 2‎ -12.a) A typical Nyquist plot of a Randles cell, b) equivalent circuit of the Randles .

In the more complicated situation, the measured impedance is the total impedance of the cell, which is equal to:

Zsol =Rsol+Rpol+ZL+ZC (‎2-17) where Z is the impedance of the molten salt, Rsol is the ohmic resistance of the melt, Rpol is the polarization resistance of the molten salt, ZL is the impedance due to inductance and ZC is the impedance due to the capacitance of the cell [141]. The measured impedance is a complex number and the real part, Z, indicates the resistivity:

Z=Rsol+∆Rpol (‎2-18)

The Rsol is independent of the frequency of the measured current while the Rpol has a reverse relationship with the frequency, ω, which means this resistance is almost zero in the sufficiently high frequencies. Therefore:

Z=Rsol (‎2-19)

The chemical composition of the salt and cell conductivity are two important factors which determine the critical value of the frequency [141]. Matiasovsky [141] studied the effect of frequency range on the measured values for resistivity. Based on this study, the measured resistance, Z, is plotted vs. frequency in order to find the frequency range in which the resistance is independent of frequency. The value of resistivity decreases with increasing the frequency and reaches a constant value [141]. Ribbon [144] also worked on frequency dependence of molten -1/2 fluorides. He suggested plotting measured resistance vs.  and selecting the region in which

31 resistance does not vary with frequency [144]. Both of these studies result in the conclusion that the resistance becomes independent of frequency at large frequencies.

2.5.2.2 Conductivity of molten cryolite

The electrical conductivity of molten cryolite has been the main focus of many researches [91, 95, 131, 145-148] due to its wide application in the aluminium industry. A complete investigation on the electrical conductivity of molten cryolite with and without additives was reported by Edwards et.al. [91, 145]. These measurements were conducted in a Pt cell using the Thomson bridge. The conductivity of molten cryolite without any additives was reported as 2.80 -1.cm-1 at 1000 °C. The effect of temperature on the conductivity of the melt was studied by different researchers and is shown in Table 2 -2.

Table 2‎ -2. Effect of temperature on the electrical conductivity of cryolite. Temperature °C Researcher Year 1000 1020 1040 1050 1060 1080 Arndt [146] 1924 2.23 2.30 2.37 - - - Batashev [146] 1936 3.23 - 3.48 3.59 - - Balyaev [149] 1947 2.29 - - 2.36 - - Vayna [146] 1950 2.36 - - 2.56 Edwards [145] 1952 2.80 2.85 2.90 - 2.95 3.00 Abramov [146] 1953 2.67 2.71 2.75 2.77 2.79 2.83 Yim [150] 1957 2.78 2.85 2.91 - 2.97 - Bajcsy [151] 1962 2.84 2.89 2.92 - 2.96 3.00

Matiasovsky et al. [142] presented the measured data in the range of 1010-1100 C in the form of the following equation:  = (-1.895+6.69810-3 T 2.00110-6 T2) S.cm-1 (‎2-20) where T is temperature in C. They also studied the effect of two additives, LiF and Li3AlF6, on the conductivity of cryoliteAl2O3 melts and showed that both components increase the conductivity but the effect of LiF addition is much more pronounced than Li3AlF6. The effect of

Li3AlF6 becomes noticeable only at large concentrations.

A moving electrode technique (using a boron nitride tube cell) was first invented by Kim and Sadoway [152]. Fellner et. al. [153] employed this method for measuring the electrical conductivity of molten cryolite. Later, Haarberg [154] determined the electrical conductivity of cryolite saturated with Al2O3 by the Wagner polarization technique.

2.5.2.3 Effect of silica on the conductivity of cryolite

The effect of silica on the electrical conductivity of molten cryolite was studied by Balyaev [149] in 1947. Later, Grojtheim [14] also studied this system and investigated the effect of SiO2 concentartion on the conductivity at 1000 C. Both researchers found that SiO2 supresses the conductivity of cryolite, although the measured conductivities for both cryolite and cryolite-

SiO2 mixtures by Balyaev are lower than those reported by Grojtheim. The latter provided the following dependence of conductivity to composition [130]: -2 -4 2 -1  = (5.756 8.52510 C+5.56 10 C ) S.cm (‎2-21) C is the concentration of cryolite in mol%.

Recently, Korenko [155] studied the effect of both temperature and silica concentration on the conductivity. The results show that increasing the temperature from 1000 to 1120 C, increases the conductivity. The findings on the effect of SiO2 in any isotherm are not in agreement with the previous findings. According to this study, small quantities of SiO2 (up to 10 mol%) improves the conductivity, further addition has a negative effect. Increasing the SiO2 concentration from 10 to 40 mol% dropped the conductivity by 0.7 S.cm-1. The author related the initial increase to the slight change in the cryolite ratio, and the later drop upon further addition of SiO2 to formation of a glassy network.

2.5.3 Transference numbers

Molten salts are mixed conductors, in which both electronic and ionic carriers contribute to conduction. In electrodeposition process, electronic charge carriers cause electric shunt, which is responsible for decreasing the current efficiency. Therefore, it is important to know the contribution of each type of carrier in the total conduction. This contribution is defined by the transference number. For a mixed conductor, the electronic and ionic transference numbers are:

(‎2-22)

(‎2-23)

where ie and ii are the current passed by the electronic and ionic charge carriers, respectively. The electronic and ionic portion of the conductivity can be calculated using the corresponding transference numbers and the total conductivity (t) as:

      (‎2-24) where te+ ti=1, then t= e+ i.

2.5.3.1 Transference number measurement

Two different set of methods have been used for measuring the transference numbers in the melts. Some methods that are widely used in the molten salts, i.e. Hittorf’s method, concentration cell, and radioactive isotope technique separate the cathodic and anodic portion of the ionic current and determine whether the anions or the cations are responsible for conduction. The other sets of experimental methods separate the electronic and ionic transference number without any emphasis on the ion type. The latter methods are of more practical interest and will be discussed here.

2.5.3.1.1 Faraday technique

This method is based on Farady’s law of electrolysis, which predicts the cathodic weight gain from the electrolysis parameters:

(‎2-25) A is the molecular weight of the element, I is the applied current, t is the deposition time, F is the Farday’s constant and z is the number of charge. This equation is valid when the electrolysis is 100% efficient and the current is fully used for ion transport. In the case of a mixed conductor, part of the current, which does not participate in electrolysis, is transported by the electrons, thus not resulting in reaction. Therefore, the portion of the current carried by the ions is lower than the total current. The faradic current efficiency can be calculated from:

(‎2-26) where is the actual mass deposited or evolved at the cathode. Knowing that only the inonic portion of the current is responsible for this mass change, the current efficiency can be correlated to the transference number as:

(‎2-27)

where it= ie+ii. Combining equations ( 2-23) and ( 2-27) gives:

(‎2-28) This method has been used for measuring the transference number in the oxide systems such as slags [156].

2.5.3.1.2 Stepped potential chronoamperometry

Stepped potential choronoamperometry is a relatively new method that was first utilized by Fried [157] for determination of the transference number. In this method, a corresponding response of a square potential wave is recorded and the transference number is measured from the initial and long time current. When the two electrodes are polarized initially, both electrons and ions move in the applied field (it0= ie+ii). After a sufficiently long time, the ions are fully blocked at the electrodes and the current is conducted only by the electronic carriers (it= ie). Therefore, the electronic transference number can be calculated by:

 (‎2-29)

 A schematic current response is shown in Figure 2 -13. This method has been previously used for measuring the transport properties of oxide melts [157-159].

Figure 2‎ -13. Schematic current response to square wave potential [157].

2.5.3.2 Transference number in cryolite

The initial investigations on the transference numbers in cryolite by Guskov [139] suggested that the main part of the current is transported by F- inos. However, Baimakov [139] showed later that Na+ inos are also responsible for the conduction in cryolite. Frank [160] measured the transference number of the cation as 0.99 in molten cryolite using the radioactive isothopes. Tual [161] studied the effect of acidity on the transference number in cryolite, theses results confirm that F- ions participate in conduction by increasing the acidity. However, in an exremley acidic electrolyte (7 wt% AlF3), the transference number for sodium is 0.9.

2.5.4 Phase diagram

The overall aim of this project is production of high purity silicon through electrolysis of dissolved silica in molten cryolite. In the electrolyte of interest, sodium cryolite (Na3AlF6) and

SiO2 are the main constituents, therefore this section focuses on the binary phase diagram of this system, with emphasis on the solubility of SiO2 in cryolite.

2.5.4.1 Determination of the phase diagram

Phase diagrams can be generally plotted by heating/cooling at constant composition or by monitoring the phase changes as the composition change under isothermal conditions. Often the two approaches are used together to yield the phase diagram of a system [162]. Three different methods have been used in the studies of binary systems containing cryolite.

2.5.4.1.1 Thermal analysis (TA) method

In this method, the temperature-time curve is recorded for a specific composition at a constant pressure either at a constant or varying rate of heat exchange. These curves are characterized based on breaks (discontinuities) and halts (arrest of temperature) that appear during the process. A sample of correlation between cooling curve and different regions in the corresponding phase diagram is shown in Figure 2 -14. The cooling curves which are shown in Figure 2 -14-b are the theoretical cooling curves, meaning that the cooling rate is extremely slow to allow equilibrium at any given time. Under experimental cooling rates, the system may not reach equilibrium except perhaps at the interface of the phases. Due to delayed nucleation, the liquid begins to solidify below the equilibrium liquidus. Subsequently, driven by the large temperature difference, a substantial amount of solid is formed once the nucleation starts.

Figure 2‎ -14. a) Phase diagram of a hypothetical A-B system, b) The cooling curve corresponding to phase diagram at different compositions [162].

The rapid separation of a considerable amount of solid phase during supercooling has two important effects. Firstly, the break in the cooling curve is much sharper compared to the equilibrium curve. The sudden separation of the solid phase gives rise to a substantial heat release that increases the temperature to a maximum. Secondly, the solid separation from metastable system changes the concentration of the liquid. This is the reason why the measured freezing temperature is lower than the theoretical value [162]. There are different methods to prevent large supercooling such as seeding the melt with small crystals of solid phase, stirring and cooling as slow as 0.5°C/min.

2.5.4.1.2 Differential thermal analysis method (DTA)

Figure 2 -15 shows a schematic drawing of the DTA apparatus. All the temperature measurement in this method is versus a reference material. Both the sample and reference are heated at the same heating rate, and the temperature difference between the sample and the reference material is recorded. In the case of endothermic reaction, the temperature of the sample is lower than the reference material and if ∆T (Ts−Tr) be recorded against Tr then a drop in the graph appears for an endothermic reaction such as melting.

Figure 2‎ -15. Differential thermal analysis method a) classical apparatus b) heat flux c) DTA curve for an endothermic reaction [163]. 2.5.4.1.3 Quenching method

Due to some limitation in the accuracy of the thermal analysis method, the quenching technique was introduced. The test material is melted and different samples are kept at different temperatures for a given time to reach the thermal and chemical equilibrium. The samples are then quenched in water or other media to freeze instantly. The primary phase and quenched liquid can then be characterized and quantified. The complementary analytical methods such as XRD, EPMA, and refractive index measurement are employed to identify different phases.

2.5.4.1.4 Visual observation method

This method is mostly used along the thermal analysis techniques to increase the measurement accuracy. A light beam of an arc lamp is used to observe the melt in the crucible. A telescope is used for visual observation of the melt during the cooling. The appearance of the first crystal in the melt corresponds to the break in cooling curve. Sometime these two do not correlate to each other and the break appears at a much lower temperature, in this case the visual observation is a helpful method to increase the accuracy of the measurements [164].

2.5.4.2 Systems containing Na3AlF6 2.5.4.2.1 Pure cryolite

In order to study the phase diagram of the electrolyte bath for silicon refining, the freezing point of cryolite is important. This property has been widely studied by different methods. The earliest method used to determine the melting point of cryolite was thermal analysis based on thermal arrest in the cooling curves. Due to limitation in the technique the measured freezing point was lower than the reported value by the more accurate techniques, 997 [165] and 1000 [166] compared to 1009 [94, 164, 167], 1010 [168], 1012 [169] and 1013 °C [170]. Foster et al. [169] measured the melting temperature of cryolite using the quenching technique. According to their findings, the melting point of the natural Greenland cryolite is 1012 °C ± 2 °C.

Rolin [171] studied the difference between the melting point of the natural and synthetic cryolite. The synthetic cryolite was produced by melting high purity aluminum fluoride and appropriate amount of sodium fluoride. Their measured melting point of the synthetic cryolite was 1009°C compared to 1006°C for natural Cryolite. They justified that impurities such as AlF3, Al2O3 and

CaF2 in the natural cryolite lower its melting point.

Cryolite ratio is another important factor that affects the melting point. For cryolite containing various alkalis, CR is defined as the concentration of alkali fluoride to aluminum fluoride (i.e.

CR= [NaF+LiF+KF]/[AlF3] mol/mol). In stoichiometric cryolite, Na3AlF6, this ratio is 3.0. Apisarov et al. [172] found that the cryolite with smaller ratio (1.3) has melting point as low as 780 °C. The heat of fusion of pure cryolite has been reported by different authors [169, 173-176] to be around 1142 kJ/mol.

2.5.4.2.2 Na3AlF6- SiO2 system

The previous investigations on this system were mostly focused on the ternary system of

Na3AlF6Al2O3SiO2. Batashev and Zhurin [95] studied this ternary phase diagram on the cryolite corner and reported a eutectic in the system at 3% wt SiO2 (0 wt% Al2O3) and 980 ºC. A narrow field of solid solution of SiO2 in cryolite was assumed in the cryolite corner. In SiO2 side, immiscibility of the two phases was recognized. Weill and Fyfe [177] studied this ternary diagram at two isothermal regions 800 and 1010 ºC. Based on this research the silica solubility in

39 cryolite at 1010 ºC is less than 5%, which increase rapidly up to 69% by adding 14% alumina to the system at 1010 °C. These authors reported no liquid immiscibility at these temperatures.

The investigation carried out by Dolejs and Baker [178] is the only available study for the binary cryoliteSiO2 system. They observed a eutectic point at 5% silica at 999 ºC based on DTA data. Figure 2 -16 shows the phase diagram drawn based on their limited number of measurements.

Figure 2‎ -16. Binary phase diagram of cryolite-SiO2 system at 1 atm. trd: tridymite, qz: quartz [178].

2.5.4.2.3 Reactions between cryolite and SiO2

Solubility of SiO2 in cryolite is determined from the phase diagram but the enthalpy of dissolution of silica in cryolite has not been measured so far due to the slow dissolution rate, which reported as an endothermic reaction. Different studies [11, 179] have discussed the release of SiF4 associated with silica dissolution in cryolite. Locsei [180] studied the reaction between aluminum floride and silica and concluded that mulite formation occurs above 900 °C:

12AlF3(s)+ 13SiO2(s)→ 2(3Al2O3.2SiO2)(s) (mullite) + 9SiF4(g) (‎2-30)

The formation of SiF4 starts at 600 °C and accelerates between 800 and 1000 °C when the formation of mullite starts. Between 600- 800 °C the following reaction occurs:

4AlF3(s) + 3SiO2(s) = 2Al2O3(s) + 2SiF4(g) (‎2-31)

One of the most complete studies on these reactions was carried out by Snow and Welch [181]. They analyzed the evolved gases as well as the quenched solution. Based on their observations, after 30 min the concentration of SiO2 decreased sharply and was accompanied by a rapid

40 increase in the alumina content. After 30 min, sodium oxide appeared and its concentration increased during 150 min. They concluded that this happens due to two parallel reactions; the first reaction happens in the first 30 min and is rapid. The product of this step is alumina silicates and SiF4 according to:

4Na3AlF6+ (x+3)SiO2 → 3SiF4+ 2Al2O3.xSiO2 + 12 NaF (‎2-32)

In the second period of the reaction, 30-150 min, small release of SiF4 was observed due to slow reaction between oxides and fluorides. The main reaction product of this step was jadeite

(NaAlSi2O6). According to Schairer [182], jadeite transforms to albite above 900 °C. (‎2-33) 2(Na2O.Al2O3.4SiO2)(jadeite)→ Na2O.Al2O3.6SiO2(albite)+Na2O.Al2O3.2SiO2(nepheline) Therefore, the overall reaction is:

6SiO2+ 2Na3AlF6→ (Na2O.Al2O3.4SiO2) (jadeite) + 2SiF4 + 4NaF (‎2-34)

Monnier [37] also studied this reaction and he identified only nepheline in the residue by XRD. Accordingly, he concluded that the following reaction takes place on dissolution of silica in cryolite:

2Na3AlF6 + 4SiO2 = Na2O.Al2O3.2SiO2 (nepheline) + 2Na2SiF6 (‎2-35)

In summary, these examinations cannot provide an exact image of the cryolite-silica system. It appears that in the beginning, the rate of SiF4 release is fast which reacts with NaF and forms

Na2SiF4. The first reactions also produce aluminum and sodium silicates which then react with fluoride and give a compound of three oxides such as nepheline. Therefore, the melts consist of different components: silicate anions, anions of O2-, cations of Si4+ and Na+ cations that are the main charge carriers.

2.6 Decomposition and deposition potentials

Decomposition potential is the minimum potential that has to be applied between the two electrodes for continuous electrolysis. Decomposition potential is the emf of the cell where the electrolysis products are released on the surface of the two electrodes. Deposition potential is the required potential to be applied on each electrode for continuous deposition or discharge of ions on their surfaces. The deposition potential is measured by coupling the electrode with a standard electrode and measuring the Ecell. Decomposition potential can be considered as the sum of the two deposition potentials that have to be applied on each electrode.

Theoretically, the decomposition potential of SiO2 can be calculated from the Gibbs’ free energy of the reaction, using Nernst equation:[55] o SiO2 (s) → Si (s) + O2 (g) G 1313K= 676.2 kJ (‎2-36)  (‎2-37)

In this equation, F is the Faraday’s constant (96,500 coul.mol-1) and z is the change in valence as the reaction takes place. Therefore calculated for reaction ( 2-36) is -1.75 V. In the cell design, graphite is the most suitable candidate for the anode; therefore, reaction of C with evolved O2 is possible, which results in evolution of CO or CO2 gases on the anode surface. Thus the overall reaction could be: o SiO2 (s)+C → Si (s) + CO2 (g) G 1313K= 279.9 kJ (‎2-38) o SiO2 (s)+2C → Si (s) + 2CO (g) G 1313K= 221.1 kJ (‎2-39) which decreases the decomposition potential of SiO2 to -0.73 V and -0.57 V, for CO2 and CO evolution respectively [55].

The reported experimental decomposition potential [28, 32, 33, 37, 109, 111, 117, 183-185] is considerably different from these calculated values, ~ 0.950.05 V, which is approximately 0.25 V larger than the predicted potential noting that CO is the most stable gas under the high temperature conditions. According to Boe et al. [32], this difference is mainly due to the polarization and concentration effects, which means:

(‎2-40) The interaction between electrode and electrode products such as alloying is responsible for the polarization effect; while the diffusion of species in the boundary layer near the interface is responsible for the concentration effect.

2.6.1 Measurement techniques

The decomposition potential has been measured by two different techniques as discussed below.

2.6.1.1 E-I method

In this method, a voltage is applied between the two electrodes and the current response is measured. The initial current response is very small, while it is increasing by the applied potential. The applied potential can be regulated using a variable resistance. When the potential

42 reaches the decomposition potential, the current flow in the cell increases exponentially. The experimental set up and a schematic response diagram are illustrated in Figure 2 -17.

Figure 2‎ -17. a) Experimental setup for decomposition potential measurement, b) schematic of the response current-voltage diagram [186]. 2.6.1.2 Cyclic voltammetry

This technique continuously scans over a range of potential in a symmetrical sawtooth wave and the resulting current is recorded in both the forward and reverse cycles. The current-potential response is called cyclic voltammogram. This method is typically applied as a first diagnostic test for an uncharacterized electrochemical system as it can determine the reduction/oxidation potentials of the electroactive species in a rather quick experiment. Figure 2 -18 shows the typical response in a system with a single electroactive species, O, in a single cycle. Initially, an oxidized form O is present in the system. Therefore, the scan starts in a negative direction from a potential at which no reduction occurs (Einitial). As the potential approaches the reduction potential, the cathodic current begins to increase (A) until the peak (B) is reached. After the reduction reaction is complete, the direction of the scan is reversed. In the reverse cycle, the R molecules (that are generated in the forward scan) are reoxidized back to O, causing an anodic peak.

Figure 2‎ -18. Potential- time wave in cyclic voltammetry, b) a typical cyclic voltammogram.

2.6.2 Decomposition potential of SiO2 in cryolite melts

The total experimental decomposition potential has been measured by several researchers [28, 32, 33, 111, 117, 183-185]. Monnier [117] was the first who measured the decomposition potential in cryolite by–3%SiO2 melt using E–I technique. The reported decomposition potential based on this study is 0.94 V that is in good agreement with Bϕe’s result [33, 117] in cryolite by–

5%SiO2, 1.000.5 V. Later, Rao [28, 109] measured the deposition potential of Si in a KF–LiF–

K2SiF6 electrolyte as -0.75 V vs. Pt electrode using the same technique. Boen [111] conducted the first measurement using cyclic voltammetry in 1983 in cryolite melt. According to him, the deposition potential from a fluorosilicate (Na2SiF6) is 0.8 V vs. Ag electrode. Boen also reported existence of Si2+ ions in the melt, where it was justified by the reaction between Si and Si4+ ions. In 2010, Osen [185], performed cyclic voltammetry on a glassy carbon electrode in KF–LiF

(eut)–5 mol% K2SiF6. The reduction of Si ions took place in two steps at ~-1.2 V vs. Pt. The reduction of K2SiF6 in molten LiF-KF melt was also reported happening in two electron transfer steps by Lepinay [187] and Ming [121] who determined that the reduction of K2SiF6 in

Na3AlF6LiF melt involves two reduction steps at -1.2 and -1.6 V vs. Pt.

Chapter 3 Experimental

The objective of the present study is to understand and establish the fundamental of Si electrodeposition in a CuSi cathode, and subsequent recovery of Si from the alloy. The experiments were designed in three phases to cover the existing knowledge gaps.

Data on the physicochemical properties of the cryolite–SiO2 solution are very limited, with only a few studies [14, 26, 141] on density and specific conductivity of this system. As a result, the physicochemical properties of the melt such as density, electrical conductivity as well as electronic and ionic portions of the conductivity were measured in the first phase of the study. In addition, the phase equilibria of the cryolite-silica system in the concentration/temperature range of interest was evaluated. The appropriate SiO2 concentration and working temperature that deliver the optimum properties were chosen based on these results.

Subsequently, the second part of the work was focused on determination of the deposition and decomposition potentials. Initially, the experimental deposition potential of Si was determined on a graphite cathode and the kinetics of the anodic and cathodic reactions taking place in the cell were studied. The research continued to determine the decomposition potential of SiO2 in cryolite on a Cu and Cu-Si cathode, using cyclic voltammetry.

Finally, the current efficiency of the electrodepsotion process was measured as a function of applied voltage by two different approaches: weight gain of the cathode (cathodic current efficiency) and evolved oxygen as CO/CO2 (anodic current efficiency). The continuous analysis of the evolved gas enabled instantaneous measurement of the current efficiency and the kinetics of the deposition. Besides the above fundamental investigations, the effectiveness of the process in delivering high purity Si was evaluated as the last part of the research.

3.1 Phase I: Characterization of cryolite–SiO2 melts 3.1.1 Melt preparation

Natural cryolite (supplied by Rio Tinto–Alcan), with cryolite ratio of 3.3 and total impurity content of 0.6 wt%, was ground to pass a 104 µm sieve. Silica with a purity of 99.5% and mean particle size of 2 µm, was obtained from Alfa Aesar. To investigate the effect of SiO2 in each

experiment, 500 gr of SiO2–Na3AlF6 mixtures containing different concentrations of SiO2 ( 0, 1, 3, 5, 6 wt %) were prepared. The mixture was contained in a glassy graphite crucible of 82 mm inner diameter, which fitted inside an Inconel container of 10 mm wall thickness. This setup was placed in a Thermo Scientific Blue M crucible furnace and was heated to the target temperature at the rate of 8 °C/min. A graphite lid covered the crucible to maintain a reducing environment inside and to minimize the evaporation of fluoride compounds. Through a gas nozzle, high purity argon was purged in the open space of the Inconel crucible to avoid infiltration of air into the system. Once reaching the target temperature of 1000 °C, the melt was held for 30 min to allow the dissolution of SiO2 in cryolite and homogenization of the melt. A graphite rod was used to stir the melt, to ensure the homogeneity of the melt particularly at high silica concentration. In order to investigate the effect of temperature, the experiments were conducted over a range of 1000 °C to 1100 °C in 20 °C intervals for each composition. The melt temperature was measured by a K-type thermocouple.

3.1.2 Density measurement

The density of the electrolyte is important as it determines whether the metal will sink or float to the surface of the electrolyte. This in turn affects the design of the cell and the method of metal withdrawal. The density of the cryolite-silica melt was measured based on the Archimedes principle. A spherical sinker (d20°C =19mm) of pure nickel was suspended from an analytical balance by a fine chromel−A wire (d 20°C = 0.6mm). A schematic of the experimental setup for this measurement is shown in Figure 3 -1.

Figure 3‎ -1- Schematic of the experimental setup for density measurement

The initial volume of the sinker and supporting wire was measured at room temperature using deionized water as the standard liquid. The approximate level of the sinker wire that would be immersed in the melt was determined. The sinker and wire were immersed to the same level in distilled water and the apparent mass was measured. Knowing the density of distilled water at the room temperature (0.9956 g.cm3) and the apparent mass change, the volume of the sinker was measured. According to Archimedes principle, the change in the mass of the solid is equal to the mass of the displaced water with the same volume as the sinker. This measurement was repeated four times for each recording and the average was considered as the volume of the sinker.

In the high temperature experiments, the sinker was immersed into the melt and held in for approximately 10 minutes to reach thermal equilibrium. This was noted by an initial decrease in the mass that was then stabilized. The initial decrease in the mass is believed to be because of melting (thus mass loss) of a solid shell that is formed on the cold surface of the sinker that is gradually dissolved away. The weight of the sinker was recorded for 60 seconds in 5-second intervals. The weight had small fluctuations of ±0.04 g. The average value was considered as the weight of the immersed sinker.

Based on the relation between mass, volume and density, , by measuring the mass and volume for each concentration at each temperature, the density is calculated. The mass is the difference between the mass of the sinker and wire before and after immersion. The volume was calculated from the initial volume of the sinker and considering coefficient of thermal expansion of nickel. After the sinker was withdrawn from the melt, the solidified salt on the surface of the sinker was removed by using a 13 wt% aluminum chloride solution at 80 °C.

3.1.3 Conductivity measurements

3.1.3.1 Cell design Impedance spectroscopy technique was employed to measure the conductivity of the electrolyte.

Four−terminal cell design was utilized for recording the impedance of the cell. The electrodes were made of molybdenum (d = 2 mm) which is stable in the corrosive fluoride melt under the reducing environment. To prevent shorting of the electrodes, their top parts were electrically isolated by sheathing the electrodes in alumina tubes. The immersion depth was controlled using

47 a digital indicator. The electrode tip and the apparatus for adjusting the immersion depth are shown in Figure 3 -2.

Figure 3‎ -2. a) Dimensions and arrangement of the electrode tips, b) electrode immersion depth adjustment apparatus.

These electrodes were immersed into the melt to an arbitrary depth (2.5 cm) and were left for 5 min to reach thermal equilibrium. A 1287 Solarton Potentiostat together with 1255B Solarton Frequency Response Analyzer were used to measure the cell impedance at a frequency range of 1000 Hz to 1 MHz at 100 Hz intervals. The excitation voltage was 50 mV. A personal computer and Z-Plot software were used to control the frequency analyzer and record the data. The measured impedances were analyzed subsequently. A schematic of the complete experimental setup is shown in Figure 3 -3.

3.1.3.2 Cell constant

The characteristic geometry factor of the electrodes (i.e. cell constant, G) was determined by standard 0.1 and 0.01D KCl solutions that were prepared according to the specifications of Janz and Tomkins [188]. In both calibration and high temperature experiments, the liquid surface was detected by a sudden drop in the resistance while the electrodes were being lowered gradually. A sample graph showing the change in current response is illustrated in Figure 3 -4.

Figure 3‎ -3. Schematic of the experimental setup for measuring charge transport properties.

Figure 3‎ -4. Detection of the melt surface by chronomaperometry at 87 mV.

The electrodes were first immersed 0.1 mm below the surface of the solution and impedance measurement was done at the same frequency interval and over the same range of the actual experiments. The frequency range of the scan was large enough to see the cusp in the Nyquist diagram. The electrodes were then immersed further at 1 mm intervals and the measurement was repeated. This process was repeated up to 35.5 mm immersion depth. A sample of the recorded Nyquist diagrams in 0.01D KCl at different immersion depth is shown in Figure 3 -5.

Figure 3‎ -5. Nyquist diagram for different immersion depths in a 0.01D KCl solution.

The solution resistance was derived from the Nyquist diagram as described in the previous section, and then knowing the conductivity of the standard solutions, the geometry factor was calculated from Equation ( 2-15) The measured geometry factor is plotted against the immersion depth for both solutions (Figure 3 -6). As seen, for immersion depths greater than 2 cm, the cell constant was independent of the solution concentration, therefore, 2.5 cm was considered as the immersion depth for the high temperature experiments.

Figure 3‎ -6. Cell constant as a function of immersion depth recorded in 0.1 and 0.01D KCl standard solutions. 3.1.3.3 Calculation of the melt resistance

The measured impedance Z has imaginary and real parts and has the form Z = Z′ + iZ″. Z′ was isolated from Z by electrochemical impedance spectroscopy technique. The Z′ corresponds to the

Z at minimum Z′′ and is considered as the purely resistive part of Z (Rmeas). Measured resistance

(Rmeas) has contributions from other sources i.e. electrodes, lead wires, polarization etc, therefore,

Rmeas ≠ Rliquid (Rmeas= Rliquid+Rleads+Relectrode+Rpolarization). The effect of polarization was not observed in this case. The Z´ value was read from where the cusp of the graph occurred and was considered as Rmeas. The lead wire and electrode resistances were measured with shortened electrodes by a copper foil and subtracted from the measured resistance. Using Equation ( 2-15), the specific electrical conductivity of the melt was calculated.

3.1.4 Transference number measurement

In order to quantify the contribution of each type of charge carrier in the total conductivity, the electronic and ionic transference numbers were measured and the corresponding portion of conductivity was calculated. Stepped potential chronoamperometry was employed to determine the electronic and ionic transference numbers.

The electrode set up and the cell design is exactly the same as the one used for conductivity measurements (Figure 3 -3). After melting and homogenizing the electrolyte with desired composition, the cell was polarized potentiostatically using a square wave DC signal. The melt was first polarized to an applied potential of 0 V for 100 s, followed by 20 mV (which is below the decomposition potential of SiO2) for 100 s, and ending with 0 V for another 100 s, with all potentials measured versus reference electrode. A sample of the applied polarization wave in cryolite1 wt% SiO2 melt at 1000 C is plotted in Figure 3 -7. The corresponding current response was recorded at 0.1 s intervals and was plotted against time, Figure 3 -8. The effect of temperature and SiO2 concentration were studied on both ionic and electronic conductivities.

Figure 3‎ -7. The applied polarization wave in cryolite1 wt% SiO2 melt at 1000 C.

Figure 3‎ -8. The current response recorded during the polarization in cryolite1 wt% SiO2 melt at 1000 C. 3.1.5 Phase diagram study

Knowledge about the effect of SiO2 on the melting temperature of the mixture is important to determine the appropriate working temperature for electrodeposition and the amount of SiO2 that can be dissolved in the electrolyte, which in turn affects the concentration of Si4+. The subsequent procedure was followed for this study.

3.1.5.1 Melt Preparation

Samples with different silica concentrations were prepared in graphite crucibles. The crucible was covered by a graphite cap to provide a reducing environment inside. The furnace top was covered with a ceramic cap cut from an alumina board. Two holes were drilled in the cap to accommodate the thermocouple and argon gas tube. A K-type steel thermocouple sheathed in a 2 mm protection tube was used for temperature measurement.

3.1.5.2 Melting and eutectic point

Thermal analysis method was used to record the melting and eutectic temperatures of the mixture. The sample was heated up to 1060 ºC under argon atmosphere and held at this temperature for 2 hours to obtain a homogenous melt. It was then cooled at the rate of ~5 ºC/ min and the cooling curve was recorded. The thermocouple was calibrated against melting point of NaCl (801 °C). The thermocouple was immersed in the center of the melt and Omega universal thermal connector was used to send the data to the computer every second. The melting

52 temperature and the eutectic temperature were determined from the recorded cooling curves at different compositions. To prevent supercooling, a graphite rod stirred the melt during the cooling process.

3.1.5.3 Characterization of the phases

A quartz tube attached to a rubber bulb was used to withdraw samples from the melt at three different temperatures. The 0, 1 and 6 wt% SiO2 were chosen for this study and samples were quenched in air at 1060, 990 and 970 ºC. Before quenching, the sample was held at each temperature for 15 min to reach the equilibrium. The quenched samples were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe micro analyzer (EPMA).

After grinding and polishing the mounted sample, it was coated with carbon to provide a conductive layer for SEM examination. The thickness of applied coating was about 20 nanometres. Secondary and backscattered electron imaging and EDX were used to identify the phase or phases present in the quenched samples. For this purpose, a Hitachi S570 SEM was employed. The accelerating voltage for all images was 20 kV and the working distance was 15 cm. The beryllium window was open in all investigations to allow detection of fluorine and oxygen.

XRD was used to determine the stoichiometry of the phases formed in the structure during cooling. The main aim was to investigate if the silica is segregated as SiO2 or it reacts with cryolite and forms a silicate compound. Two different samples with 1% and 6% silica were ground to powder and used in XRD analysis that was conducted in a Philips diffractometer ° PW3710 with Cu target. The applied potential was 40 kV and the K1 line (= 1.5405 A ) was used. A nickel filter was used to filter the generated X-ray to a single wavelength and single direction. The Bragg’s angle of 10 to 60° was covered with a scan rate of 2 °/min and step size of

0.02. X’ Pert High Score software was used for phase identification, pattern treatment and graphics. The XRD analysis was not able to determine the type of the aluminosilicate formed in the sample at each temperature due to the peak overlaps. Therefore, EMPA was used to quantify the distribution of elements in each phase. Cameca SX50 electron microprobe was used. The operating conditions were 40 degrees takeoff angle, and beam energy of 15 keV. The beam

53 current was 10 nA, and the beam diameter was 5 microns. The sample preparation is similar to the SEM method. The points of interest on each phase (10 points) were analyzed.

3.2 Phase II: Determination of deposition and decomposition potential

The second phase of the project involved measuring one of the fundamental electrochemical parameters: decomposition potential of SiO2. Cyclic voltammetry in cryolite–SiO2 melt, which has not been carried out before, was the main focus of this part of the research.

Initially, cyclic voltammetry was conducted on an inert cathode (graphite) to measure the required potential at which the cathodic half reaction commences on the electrode. The deposition potential was measured vs. Pt as a quasi–reference electrode. The effect of SiO2 on the deposition potential was investigated; the kinetics of oxidation and reduction reactions was also predicted.

The required cell potential to deposit Si on a cathode of interest, copper, is the fundamental information that needs to be obtained to control the co–deposition of impurities. The deposited Si on copper forms a Cu–Si alloy in which the activity of Si affects the decomposition potential. The measurements were carried out in a range of Si concentrations, allowing the prediction of potential change during the actual deposition process, where the Si content changes constantly. The results were compared with the values obtained from cyclic voltammetry on an inert graphite electrode and on a Cu−8 wt% Si alloy.

3.2.1 Deposition potential

A schematic drawing of the complete cell assembly and the actual experimental setup are presented in Figure 3 -9 and Figure 3 -10- a, respectively. A crucible furnace controlled by a digital controller was used for this set of experiments. The furnace body was made of steel, which was lined with refractory materials. The cell was held in a mulite tube to protect the wall of the furnace and the heating elements from corrosive atmosphere. The mulite tube was also protected by boron nitride and graphite paints. The tube was extended 30 cm outside the furnace chamber to allow the electrode movements, and to keep the silicon O-ring, electrical and gas connection away from the hot zone. In order to reduce the thermal loss through this part of the

54 tube, it was fully wrapped in several layers of alumina wool. The melt was contained in a graphite crucible that was positioned in the center of the tube.

Figure 3‎ -9. Schematic cell design used for deposition potential measurements.

Figure 3‎ -10. a) the complete experimental setup, b) the graphite radiation shields.

Three graphite radiation shields (Figure 3 -10-b) were used to trap the heat in the tube and to prevent heat loss through the thermal radiation. The cell was fully sealed by a water–cooled

55 stainless steel cap, which had six holes to accommodate the electrodes, the gas inlet and outlet as well as the thermocouple. This assembly enabled the vertical movement of the electrodes that is necessary for mixing the electrolyte, immersing the electrodes into electrolyte at the beginning of each experiment and removing them after each experiment.

3.2.1.1 Instrumentation

A Thermo Scientific Blue M CF56622C Helical wire coiled crucible furnace was used to heat up the cell assembly to 1040°C; the heating profile and target temperature were controlled by a Lindberg CC58114PC controller. A K-type steel thermocouple was used to measure the actual temperature inside the tube and calibrate the temperature difference between the controller and the melt. All the experiments were conducted under an inert atmosphere of Ar, which was dried using CaSO4 before entering the cell chamber. When the melt reached equilibrium, cyclic voltammetry began using a 1287 Solarton Potentiostat that has a 2A current limit. In these experiments, potential was varied in the range –2 to +0.8 V with different scan rates and the corresponding current response was recorded by a personal computer and Corrware software. Secondary and backscattered electron imaging and EDX analysis were used to identify the phase or phases deposited on the tip of the electrode.

3.2.1.2 Electrodes

Different working and counter electrode sizes and configurations were tested in some preliminary experiments and the optimum configuration which allows scanning the complete range of potential without exceeding the current limitation (< 2A) was used in this set of experiments. The graphite crucible (67.5 mm diameter and 100 mm height) served as the counter electrode; a graphite rod (6.35 mm diameter) was the working electrode, and coiled platinum wire (about 0.5 mm diameter) was chosen as a quasi–reference electrode. All the electrodes were attached to Mo rods (2 mm diameter) at the top and extended out of the cell to be connected to the potentiostat. All the lead wires were sheathed in alumina tubes to prevent short circuiting and protect them from corrosive gases. The wires were cleaned by sand paper after each experiment to remove the oxide/salt layer on them.

Cryolite is a fluoride melt, which is corrosive at high temperature; therefore, finding the appropriate material for reference electrode is challenging. The quasi–reference electrodes are

56 the most convenient type of reference electrodes, which have been widely used in fluoride melts. Among the available candidates, Pt is the most stable material with a small solubility in this melt. To ensure that Pt provides a constant potential in the melt, the open circuit potential was measured at different times during the experiment. A sample result of these measurements is shown in Figure 3 -11, confirming that the potential is relatively constant over the 500 s period.

Figure 3‎ -11. Open circuit potential of the cell containing cryolite as electrolyte. 3.2.1.3 Experimental procedure

The materials used in this experiment were the same as described in Section 3.1.1 . In order to investigate the effect of SiO2 activity (concentration) on decomposition potential, 300 g of

SiO2-Na3AlF6 mixtures with different concentrations of SiO2 (0, 3, 5, 10 wt %) were prepared. The mixture was contained in a glassy graphite crucible of 67.5 mm inner diameter, which was fitted inside the mulite tube. The furnace was first preheated to 750 °C at a rate of 8 °C/min and then heated up to 1040 °C at a rate of 2 °C/min. After reaching the final temperature, the melt was held for 30 min to allow stabilization of temperature and homogenization of the melt. The temperature was recorded by a K–type thermocouple. The electrodes were connected to the corresponding poles of the potentiostat. The working and reference electrodes were lowered down to touch the melt surface. The melt level was detected by chronoamperometry technique as described earlier. Working electrode just touched the surface (max 1mm immersion depth) while the reference electrode was immersed 3 cm in the molten salt.

Cyclic voltammetry was conducted in a potential range of –2 to +0.85 V, considering the standard potential of possible reactions responsible for silica reduction. For accurate

57 identification of reduction and corresponding oxidation peaks, the potential window was gradually opened from –0.5 to –2. Also, different scan rates (5, 10, 20, 50, 80, 100 mV/s) were employed to investigate the kinetics of oxidation and reduction reactions. After each experiment, the working and reference electrodes were lifted out of the electrolyte and the cell was cooled down. A blank test was first performed in pure cryolite melt at different scan rates to allow separating the background peaks (resulting from the electrolyte) from the SiO2 related reactions.

To confirm that the deposition peak corresponds to silicon reaction, the tip of the working electrode served in a 10 wt% SiO2 melt was analyzed by SEM and EDS. The electrode underwent several cycles between –2 to –0.5 V, which was expected to result in a complete reduction cycle and incomplete oxidation cycle. The oxidation cycle was stopped before the main anodic peak to ensure that the deposited product on the tip is due to the first cathodic peak.

The solidified salt was dissolved in AlCl3 solution and the electrode was prepared for microscopy after grinding and carbon coating.

3.2.2 Decomposition potential

Performing cyclic voltammetry on fresh copper electrode points to the deposition potential of Si on copper. Continued deposition of Si, followed by cyclic voltammetry at different times will determine the effect of Si concentration on the deposition potential. The cathode becomes liquid as soon as the silicon concentration exceeds the liquidus concentration. The measurements were carried out in a range of Si concentrations, allowing the prediction of potential change during the actual deposition process, where the Si content changes constantly. The results were compared with the values obtained from cyclic voltammetry on an inert graphite electrode and on a Cu−8 wt% Si alloy.

3.2.2.1 Instrumentation

A schematic drawing of the complete cell assembly is presented in Figure 3 -12. A vertical tube furnace with MoSi2 heating elements equipped with a proportional integral derivative (PID) controller was used to heat up the cell to 1040C. The cell was held in a gas-tight alumina tube with 70 mm OD and 63.5 ID that protects the wall of the furnace and the heating elements from corrosive atmosphere. Two water-cooled aluminum caps on both ends sealed the tube. All the experiments were conducted under an inert atmosphere using dried argon. The flow rate of the

58 inlet gas was 50 mL/min and similar flow rate was measured in the outlet to ensure that the system is fully sealed. The same potentiostat and software described in the deposition potential measurement was used in these experiments.

Figure 3‎ -12. Schematic of the experimental setup for decomposition potential measurements. 3.2.2.2 Electrodes

Copper cathode

The cell consists of two electrodes: a copper disc (25 mm diameter and 5 mm thickness) as the working electrode, and a graphite rod (6.35 mm diameter) as the counter electrode. All the electrodes were attached to Mo leads (wires of 2 mm diameter) that were extended out of the cell to be connected to the potentiostat. All the lead wires were sheathed in alumina tubes. When the melt reached equilibrium, the counter electrode was lowered to touch the melt surface and then immersed 20 mm into the molten salt.

Cyclic voltammetry was performed for a potential range of -1.25 to 0.3 V, considering the reported decomposition potential for silica. Different scan rates (5, 10, 20, 50, 80, and 100 mV/s) were applied to investigate the kinetics of oxidation and reduction reactions. To study the effect of Si concentration on the deposition potential, the melt was electrolyzed twice, each time for 20

59 min at -1.2 V; resulting in changing the concentration of Si on the surface of Cu electrode. Then, cyclic voltammetry experiments were repeated after each deposition to study the shift of the potential and the change in kinetics. A blank test was first performed in pure cryolite melt to enable recognition of background peaks resulting from the electrolyte. These results were compared with the measured deposition potential on a graphite electrode as an inert cathode.

Molten Cu-8% Si cathode

Experiments similar to above were run on the surface of a 5 g cathode of Cu−8 wt% Si alloy.

The experimental setup is similar to Figure 3 -12 except the solid copper disc is replaced with a

Cu−8% Si disc that is molten at the electrolysis temperature of 1040 C.

3.3 Phase III: Electrowinning and separation of Si

The next step of this project was focused on electrowinning of silicon in a molten cathode of Cu– 8 wt% Si. Cu was chosen as the alloying element because of its specific characteristic such as forming a low melting point alloy with Si, having a small solubility in solid Si, being inexpensive and having a higher affinity for many impurities than Si, and providing a wider range of concentration at which the alloy is liquid. The large density of Cu also facilitates settling and separation of the alloy from the electrolyte.

3.3.1 Determination of anodic and cathodic current efficiencies for silicon electrowinning on Cu-Si Alloy

The anodic and cathodic current efficiencies of the process were measured. Cathodic current efficiency was quantified by the amount of deposited Si during electrolysis and the anodic current efficiency was measured by relating the oxygen coming out of the furnace to the Si being reduced in the cell. The continuous analysis of the evolved gas enabled the instantaneous measurement of the current efficiency and the kinetics of the deposition. The effect of cell current on these efficiencies was studied.

As explained in Section 2.5.3.1.1 , the current efficiency can be determined from the theoretical mass gain (Faraday’s law) and the actual deposition of the desired element. Other than directly measuring the deposited mass, there are several related methods to measure the current efficiency, namely potential coulometry, the use of metallic tracers, and quantifying the anode

60 products (such as total oxygen evolved) [139]. In this study, the cathodic current efficiency was determined using Faraday’s law. The oxygen balance was also employed to correlate the amount of evolved gas on the anode to the amount of metal deposited on the cathode.

The primary electrochemical reaction in silicon electrolysis is one or a combination of the followings:

SiO2+2CSi+2CO (3‎ -1)

SiO2+CSi+CO2 (3‎ -2) Reaction (3 -1) is the dominant reaction under the equilibrium conditions at the anode [189], this equilibrium condition cannot be satisfied at high anodic overpotentials, resulting in an off-gas that contains both CO and CO2. In this investigation, the composition of the anode gas was analyzed for different currents (overpotentials). The current efficiency was measured using both mass and oxygen balance of the off gas. Based on these data, the primary anodic reaction as well as possible secondary reactions was determined.

3.3.1.1 Master alloy preparation

Two batches of Cu-Si master alloy were prepared to make sure that the cathode that would be used in all the experiments had a constant concentration. Each batch was a mixture of 64.4 g of copper granules (99.8% pure supplied by Sigma Aldrich) and 5.6 g silicon powder (99.9999% pure supplied by Silicor Materials). The mixture was held in an alumina crucible for melting and quenching.

A customized vertical resistance heating furnace was used for alloy making. The sample was heated up to 1500 C, above the melting temperatures of both Cu and Si, and was held at that temperature for 1 h for homogenization. Then the melt was quenched from this temperature. The entire process was under Ar gas at the purge rate of 2 L/min. Ar with flow rate of 60 mL/ min was also injected into the melt through an alumina tube for stirring the bath. The tube was above the crucible level during the heating cycle and was immersed into the melt when the temperature reached the target temperature. The flow rates of the Ar were adjusted by an Omega digital flow controller. The complete experimental set up is schematically shown in Figure 3 -13.

Figure 3‎ -13. Schematic of the experimental setup for preparing the master alloy.

The crucible was quenched in water after homogenization. A loose alumina brick was used to seal the bottom of the furnace. The brick was removed at the end of the run so the crucible was dropped in water for quenching. Figure 3 -14 shows a quenched sample. To confirm the chemical composition of the alloy and to ensure the homogeneous distribution of Si in Cu, the sample was analyzed by EDS and XRD. The same master alloy was used as a cathode for cyclic voltammetry experiments on the surface of molten alloy.

Figure 3‎ -14. The master alloy quenched from 1500 C. 3.3.1.2 Instrumentation

A vertical tube furnace equipped with a digital controller was used to heat up the cell. In order to ensure that the only source of oxygen in the furnace is from the anodic reaction, possible external

62 sources were treated or isolated. The cell was held in an alumina tube and was completely sealed by two water cooled Al caps. The caps were designed to accommodate the electrodes while maintaining the sealed environment during the electrode movement (immersion into the melt or pulling out of the melt), Figure 3 -15. An oxygen-getter setup was used to remove oxygen trances from Ar, by passing the Ar through hot titanium chips (at 300 C). Three Ti baffles were also placed inside the furnace to react with oxygen that penetrates into the tube at high temperatures through expanded ceramic pores.

Figure 3‎ -15. The Al cap used for accommodation of the electrodes and sealing the furnace tube.

The cell consists of two electrodes: a graphite rod (6.35 mm diameter) as the anode, and 5 g of Cu-8 wt% Si alloy as the starting cathode with an interpolar distance of 1.5 cm. The electrolyte

(Na3AlF6- 6 wt% SiO2) was held in a graphite crucible. The bottom of the crucible (ID: 41.3 mm) constitutes the electrical contact to the molten Cu-Si cathode. All the electrodes were attached to Mo leads (wires of 2 mm diameter) that were protected by alumina tubes. Brass Swagelok fitting and PTFE o–rings were used to seal the tubes where they go through the cap.

DC voltage was applied between the anode and cathode using a Pyramid PS-32 lab power supply. The cell current was constantly measured by a Keithly digital multimeter. The applied potential was also monitored by a separate digital voltmeter. The effect of the cell current on both anodic (from the evolved gas analysis) and cathodic (from mass change) current efficiencies was studied.

The evolved gas (including purged Ar) was directed to a gas train and then to an ABB continuous gas analyzer (EL3020). The gas train contains a ceramic wool filter to separate particulates or condensed fumes. Traces of fluorine or HF gases were absorbed in the absorption bulb holding an aqueous solution containing 0.6M AlF3, 0.03M Al2(SO4)3, and 0.06M Al(OH)3.

H2O was then removed from the stream using two Drierite columns. The flow rate of the inlet gas was adjusted such that the flow rate of the off gas right before the analyzer was 1 L/min. The flow controller was calibrated before the experiments using a soap bubble flow meter. The concentrations of the evolved CO and CO2 were determined by the gas analyzer and the measured values were transferred to a computer by a Hydra data acquisition unit. A schematic drawing of the complete experimental assembly is shown in Figure 3 -16.

Figure 3‎ -16. Schematic of the experimental setup for current efficiency measurements. 3.3.1.3 Experimental procedure

The working tube was first vacuumed to the level of 15 torr for 5 min to remove any trapped oxygen in the chamber, and then dried argon gas was purged into the furnace for 5 min. This process was repeated three times to make sure that there is no more trapped oxygen in the furnace. Next, the furnace was heated up to 1040 C at a heating rate of 5/min, and the cell was held at this temperature for 5 h.

After reaching the target temperature, the graphite anode was immersed 2 cm in the electrolyte, and the electrical wires were connected for electrodeposition. All the electrodepsotion

64 experiments were conducted for a constant time of 5 hours. Different potentials in the range from 0.75 to 1.75 V were applied between the electrodes. The response current was recorded during the electrolysis.

For instantaneous measurement of the current efficiency, the evolved gas was collected and analyzed constantly. The total volume of the evolved gas could be determined knowing the experimental time and the flow rate of the gas. The total amount of CO and CO2 gas was measured by multiplying the total volume of the off gas by the recorded concentration of each species. Oxygen mass balance for Reactions (3 -1) and (3 -2) gives the equivalent amount of Si that was reduced at the cathode. Dividing this amount by the theoretical amount of Si gives the anodic current efficiency of the process. The cathodic current efficiency was calculated from the Faraday’s law as described earlier in 2.5.3.1.1 . All the cathodes were analyzed using EDS to determine the amount of deposited Si and investigate its distribution across the cathode from top to the bottom.

3.3.2 Combined electrolysis and solvent refining

To evaluate the ability of the process in delivering high purity Si, electrowinning was done to collect sufficient alloy in the cathode so that the subsequent step (recovery of Si) can be completed. The main barrier in the previous electrowinning investigations has been removing Si from the alloy cathode. In the present study, solvent refining was proposed as a separation process for recovery of Si after electrowinning. The alloy was slowly cooled in the binary region of solid silicon and a liquid alloy to allow the formation of silicon dendrites in the melt. During the solidification, impurities are expected to be rejected into the liquid, resulting in the formation of pure silicon dendrites. The level of impurities in the liquid should be higher than solid Si due to their small segregation coefficient. The back diffusion of impurities to silicon phase is restricted by quenching the alloy at a temperature slightly lower than the eutectic temperature. The silicon dendrites were separated from the matrix through crushing and grinding followed by acid leaching. After separation, the Si particles were digested and analyzed by ICP–OES.

3.3.2.1 Extended electrowinning

The furnace set up for the electrowinning experiment is the same as the one explained in the previous section, except the off gas analysis setup, which was eliminated. The crucible wall is

65 covered by a quartz sleeve and a cavity was drilled at the bottom of the crucible to accommodate the cathode. A cathode of 6 g Cu8% Si was placed in the cavity after cleaning the surface oxides in diluted nitric acid. The anode was a graphite rod with the initial weigh of 4.70 g and the electrolyte was 100 g of cryolite−6 wt% SiO2. The cell was heated up to 1040 °C, at which the electrolysis was conducted galvanostatically at a constant current of 1.4 A for 7 h. The surface of the solidified cathode was cleaned in a 13 wt% AlCl3 solution. The cross section of the cathode was analyzed by SEM and EDS to measure the Si distribution across the specimen.

3.3.2.2 Solvent refining

The next step was solvent refining in which Si dendrites were grown and separated. The sample was held in a quartz crucible, which was covered by a graphite coated alumina lid. The lid was sealed using silica paste, and then placed in a graphite crucible. The quartz crucible was buried in Ti getter to capture any oxygen in the environment. The graphite crucible was suspended from the top of the furnace using Mo wires. The experiment was conducted under a slightly reducing atmosphere (Ar5 % H2 gas was passed through an oxygen getter chamber). The experimental setup is shown in Figure 3 -17.

Figure 3‎ -17. Schematic drawing of the experimental setup for solvent refining.

The alloy was heated up to 1250 C (100 C above the melting point) and then held for 1 h for homogenization. It was then cooled down very slowly (0.5 C/min) to provide enough time for Si dendrites to form. After reaching 780 C (20 below the eutectic temperature), the Mo wire was released to drop the crucible in water. The temperature history of the sample is presented in Figure 3 -18.

2 4 5 21 21.5

Figure 3‎ -18. Heating and cooling cycle utilized for solvent refining. 3.3.2.3 Acid leaching

The quenched alloy was ground to –160 m particles by mill mixer MM400 (30 Hz for 5 min).

The Si dendrites were separated from the alloy matrix by acid leaching. Aqua regia (HCl: HNO3 3:1) was used to dissolve the alloy matrix. Two different leaching methods were utilized and the results were compared.

Initially the acid leaching was conducted on a hot plate at 50 C. In this process, 0.28 g powder was dissolved in 50 ml of aqua regia (27 ml HCl− 9 ml HNO3−14 ml H2O) for 2.5 h. After filtration, the residue was washed by DI-water and went through the second stage of leaching in the same solution for 4 h. The residue of the second stage of leaching was digested in HF solution (5 ml HF− 5ml H2O) for ICP analysis.

The second leaching method also involved two steps, both conducted in an Ethos Microwave Digester (Milestone Inc.). In this method, 1g of each sample was leached in 20 mL of 10%

HNO3 acid. The temperature was first raised to 150 C in 10 min and then maintained for 15

67 min. The applied microwave power was 500 W. In the second stage of leaching, 5 ml of fresh aqua regia was added to the previous solution. The samples were heated in the digester to 220 C and kept at this temperature for 15 min. After filtration and several washing steps, the residue was observed and analyzed by SEM and XRD to ensure that all the copper containing phases are removed. The residue was then digested in an acid solution (10 ml HF−15 ml HNO3− 45 ml

HCl− 15 ml H2O) for ICP analysis.

The separated Si dendrites were digested in acid for ICP analysis. In addition, the cathode after electrodeposition and after solvent refining (0.2 g) were also dissolved in 56 ml HCl, 19 ml

HNO3, and 25 ml HF to quantify the level of impurities after each step.

For analysis of all solutions, a Thermo Scientific ICAP 6200 ICP-OES machine was used. All the samples were transferred to a falcon tube and diluted by a factor of 10 with DI water. Blank samples were also prepared with the same composition as the digestion solution. The standard solutions containing1, 2, 5, 10, 15, 20, 50 and 100 ppm of the desired elements were also prepared from the standards solution provided by High Purity Standards. The following equation was used to convert the concentration of elements in solution to those in the solid sample:

C(solid Si) in ppm= C*(solution) × prep volume × 1000/ wt of Si digested (3‎ -3) where C* is the difference in the concentration of element in solution and in the blank sample.

Chapter 4 Results and Discussion

4.1 Characterization of cryolite–SiO2 melts 4.1.1 Density measurements

4.1.1.1 Effect of temperature

Measurements on density of pure cryolite were conducted at different temperatures ranging from 1000 to 1100°C, matching the approximate working temperature for this electrolyte. The results are compared with the data reported by Edwards [91] and are shown in Figure 4 -1. As expected, the density decreases with increasing temperature. The small differences between the results of this study and Edwards’ are within the range of the experimental error. In addition, the difference could in part be because of the difference in composition of the cryolite used. The relation between temperature and the density for 100% cryolite melt based on these results is: = (2.798-0.810-3T) g.cm-3 (4‎ -1)

Figure 4‎ -1. Density of cryolite as a function of temperature;  this study, Edwards [91]. 4.1.1.2 Effect of silica content

The effect of silica additions to the melt on the density of cryolite was also studied. Figure 4 -2 compares the measurements from this study and the results of Grjotheim [14] at 1000 °C.

Grjotheim’s results show that the density is not a function of silica content at 1000 °C, while this work shows that the density does increase with an increase of the amount of silica up to 5 wt%. The density, then starts to decrease on further addition of silica (6 wt%- 18 mol%). The increase 3 in the first part of the diagram may be associated to the larger density of SiO2 (2.54 g/cm at

1000 °C) [190], compared to that of cryolite , 2.04 at 0 mol% SiO2. The increase is greater than the ideal behavior of the solution, implying a negative deviation, and a tendency to form compounds between the two constituents. This is supported by the fact that silica would react with cryolite to form albite at concentrations above–the saturation limit [191].

As the solubility limit of silica is reported to be 5 wt% (15 mol%) [139], the decrease in density at higher silica contents may be attributed to the saturation of the liquid for two reasons. First, on saturation silica precipitates as albite, a mineral with a specific gravity lower than silica (at 1000 °C, specific gravities are cryolite: 2.04, quartz: 2.54, albite: 2.3 [192]). Consequently, the apparent density of the melt consisting of cryolite and dispersed albite particles should be lower than the melt just at saturation. In addition, precipitation of undissolved silica particles or albite on the surface of the immersed sinker increases its apparent volume, giving rise to an underestimated density calculation. It is expected that the true density of the liquid must remain unaltered, while the apparent density (for liquid with dispersed precipitates) should slightly decrease, for the reason noted.

1000 °C

Figure 4‎ -2. Density of cryolite- silica for different concentrations of silica at 1000 °C.

The effect of silica content on cryolite density at various temperatures was also studied. It was found that the change of density at higher temperatures, up to 1100 °C, follows the same trend as

70 at 1000 °C. Besides the density relationship with temperature remains linear (i.e. = a bT) with the constants provided in Table 4 -1. Table 4‎ -1. Equation of temperature dependency of density for different silica content. silica content standard error of (wt%) a b10-4 calcualtion 0 2.72 6.78 2% 1 2.88 8.23 2% 3 2.97 8.17 1% 5 2.93 7.69 1% 6 2.94 8.12 2%

In Figure 4 -3, the molar volume of the cryolite–SiO2 solutions and the partial molar volumes of silica and cryolite at 1020 °C are plotted against SiO2 content. As seen, the effect of silica on the partial molar volume of cryolite and total molar volume is negative, and is more pronounced in higher concentration. This is because the cryolite anions can be accommodated in the free spaces of silica lattice leading to a decrease in the partial molar volume of cryolite. In spite of the rapid increase in partial molar volume of the silica, addition of silica to the cryolite melt lowers the total volume of the melt in the range up to 5 wt%. At low silica concentration the cryolite cations are able to break the silica network [193].

Figure 4‎ -3. Effect of silica content on a) partial molar volume of cryolite, b) partial molar volume silica, c) molar volume of solution at 1020 °C.

4.1.2 Conductivity measurements

4.1.2.1 Effect of temperature

The effect of temperature on the conductivity of the cryolite melt was measured, and was compared with the results published in the literature, Figure 4 -4. In ionic conductors such as molten salts, increase in the mobility of the charge carriers, i.e. ions, gives rise to higher conductivity. The measured conductivity is in good agreement with Abramov’s data [146] at temperatures higher than 1040 °C while at lower temperatures the results are more consistent with Vayna’s reported values [194]. Considering the range of error (8%), it appears that the data are in good agreement with the previous studies. The differences may be attributed to the type of the cryolite (synthetic vs. natural) and the impurity contents, as well as the technique used and the way resistance was derived from the melt impedance.

Figure 4‎ -4. Effect of temperature on conductivity of cryolite;  present study; Kalass [146];  Batashev [146];  Beljajew [195];  Vayna [194];  Edward [91];  Abramov [146];  Yim [150];  Bajcsy [148]. 4.1.2.2 Effect of silica concentration

Figure 4 -5 shows the specific conductivity data of Na3AlF6-SiO2 when silica is added at 1000 °C, in comparison with those presented by Grjotheim [14] and Belyaev [149]. The data appear to be in good agreement with Belyaev’s measurements [149] for cryolite-silica mixtures but there is about 5% difference for the silica–free cryolite melt, which is in the acceptable range of error. The trends of the variations with silica content are consistent between this work and Balyaev’s, although the absolute values deviate by about 15–20% [149]. This may be attributed

72 to the measurement method and the treatment of the results, such as elimination of the resistance of electrodes/leads and polarization effect. As observed in this work and Grjotheim’s, the conductivity should levels off as it approaches the saturation limit (6wt%).

Figure 4‎ -5. Conductivity of Na3AlF6-SiO2 mixtures at 1000°C.  present study;  Grjotheim [14];  Belyaev [149].

+ - - + In molten state, cryolite (Na3AlF6) dissociates to Na , AlF4 and F [139, 149, 176, 196, 197]. Na is the only free cation present in the melt, and because of its small size, is considered most 4+ mobile among these ions. Addition of SiO2, followed by its dissociation to Si should inherently result in a substantial increase in the conductivity, since Si4+ has a charge-to-ionic radius ten times greater than Na+ (10 A-1 for Si4+ vs. 0.98 A-1 for Na+). However, as seen above, increasing the SiO2 content decreases the melt conductivity, implying that Si in cryolite exists as a complex 2- 3- and large ion. Formation of complex Si–O–F ions such as SiO2F2 , Si2O4F3 has been reported earlier [31]. In addition to their low mobility, formation of such large anions results in increasing the melt viscosity that in turn restricts the charge transport rate of other more mobile ions.

In comparison with alumina in cryolite, silica appears to have a stronger effect on the conductivity as illustrated in Figure 4 -6. The fact that alumina has a greater solubility in cryolite (27 mol%) and it also shares the common element (Al) with the melt, implies that the dissolved species are accommodated in the liquid more readily, thus structural changes are less significant, and lower change in conductivity is expected for this system.

Figure 4‎ -6. Comparison between the effect of SiO2 and Al2O3 [149] on conductivity at 1000°C.

+ In cryolite–Al2O3 system, sodium is present in the form of Na and carries 99% of the current, - while the remaining 1% of current is carried by F ion [160]. During dissolution of Al2O3 in 1-x cryolite, O is partially substituted with F, forming AlOFx [160, 198], without a significant change in the Na+ concentration. Formation of these large complexes increases the melt viscosity, causing the conductivity to drop. In the case of cryolite–SiO2 melt, the decrease is stronger due to the combined effect of two phenomena. On one hand, the melt viscosity increases by the formation of large anions. On the other hand, formation of sodium aluminosilicates in this system reduces the concentration of free sodium cations, i.e. the charge carrying species, giving rise to lower conductivity.

Figure 4 -7 shows the effect of temperature on 0 and 3 wt% silica mixtures. Generally, both of these mixtures follow an Arrhenius–type expression,

(‎4-2)   where E is the activation energy for conductivity, T is the temperature in K and R is the gas constant. The values of activation energy for different silica contents are given in Table 4 -2. The minimum activation energy corresponded with the saturation limit of silica and after this limit, the activation energy increases, which can be because of formation of albite in liquid and reduction in the activity of free ions.

Figure 4‎ -7. Arrhenius plot of electrical conductivity for 0 and 3 wt% silica mixtures.

Table 4‎ -2. Activation energy of electrical conductivity for different mixtures

Silica content (wt %) Activation

energy (kJ/mol) 0 20.3 1 22.7 3 19.6 5 16.9 6 21.5

In various mixtures of electrolyte–SiO2, the total number of ions varies with the SiO2 concentration. Therefore, to compare the overall charge transport ability of various melts for a given number of ionic species, the molar conductivity () was calculated based on the melt densities measured earlier [199] and the total conductivity values provided above. The details of the calculations are provided in Appendix II. The results are summarized in Figure 4 -8. As shown, temperature has a positive effect on the conductivity of all SiO2–containing melts at all experimental temperatures (1020-1100°C) because of increasing the mobility of ions at higher temperature. Silica also has a consistent negative effect on conductivity within the temperature range of the experiments, which is again due to the formation of complex ions and decreasing the activity of free Na+ or F- in the melt.

Figure 4‎ -8. Effect of silica concentration and temperature on molar conductance of mixture. 4.1.3 Transport number measurements

4.1.3.1 Effect of temperature

The calculated ti and te values for cryolite melt at different temperatures are provided in Table 4 -3. The data shows that at higher temperatures the share of the electronic charge carriers in the total conductivity increases. Ionic conduction however, remains dominant in the entire temperature range studied here. For an electrodeposition process, higher electronic conduction results in reduced current efficiency. For example, Haarberg [15] found that 0.6 S.cm-1 increase in electronic conductivity decreases the current efficiency of the cell up to 3%. Table 4‎ -3. Ionic and electronic transport numbers for cryolite melt at different temperatures. Temperature ionic transport number electronic transport number

(°C) (ti) (te) 1020 0.96 0.04 1040 0.85 0.15 1060 0.83 0.17 1080 0.70 0.30

Figure 4 -9 shows the calculated ionic conductivity (i), electronic conductivity (e) together with the measured total conductivity (t) for cryolite. The values are in good agreement with those reported by Dewing [14] and Haarberg et al. [15] for cryolite–Al2O3 melt.

Figure 4‎ -9. Effect of temperature on electronic, ionic and total conductivity of cryolite melts.

As seen in Figure 4 -9, e increases with temperature, which may be related to the larger electron- hopping rate at a higher temperature. A similar trend has been observed in cryolite–Al2O3 system [15]. Haarberg justified that this is driven by dissolution of Na in cryolite, when electrolyte is in contact with liquid Al, through the Reaction ( 4-3). However, in the present system, the electronic conductivity was observed in the absence of any metallic phase. The electronic conductivity in - this electrolyte may be attributed to the presence of multivalent anions of Al–F, such as AlF4 - and Al2F7 . According to Dewing [16] who studied the thermodynamics of NaFAlF3 system, - - - AlF4 can dissociate into Al2F7 and F . Clearly, Al carries a different charge density in the two anions, creating a possibility for local charge transport, that in turn gives rise to electronic conductivity. Dewing has discussed that increasing the temperature accelerates the dissociation, which justifies why electronic conduction increased with temperature.

Al+3NaF=AlF3+3Na (‎4-3)

4.1.3.2 Effect of SiO2 concentration

The effect of SiO2 concentration on i and e at 1020 °C is presented in Figure 4 -10. It is evident that e is not dependent on the SiO2 content. The decrease in i with SiO2 content further confirms the possibility of forming complex ions between Na, Al, and Si, that lowers the effective concentration of ionic conductors namely Na+. Furthermore, according to

VogelFulcherTamman equation [35], i is a function of the ideal glass transition temperature,

T0. Therefore, addition of SiO2 to the melt increases the glass transition temperature and decreases the ionic conductivity. (‎4-4) 

Figure 4‎ -10. Effect of SiO2 concentration on electronic, ionic and total conductivities of cryolite–SiO2 melt at 1020 °C 4.1.4 Phase diagram studies

4.1.4.1 Phase diagram

To measure the melting and freezing point of cryolite–silica melt, the cooling curve was recorded for each composition. A typical cooling curve for cryolite is illustrated in Figure 4 -11, showing two breaks. The first break appears at 994 °C, which corresponds to solidification. The freezing section is magnified in which the supercooling effect is clearly visible. The second break happens at around 565 °C due to phase transformation. The freezing point of cryolite determined by cooling curve without any super cooling effect, was found to be 998.3 ºC  2 ºC which is lower than the reported melting temperature of 1012 2 ºC for pure cryolite [94, 164- 171].

Figure 4‎ -11. Typical cooling curve of cryolite melt with larger magnification of the freezing section. The liquidus curve and eutectic line are plotted and the corresponding phase diagram is illustrated in Figure 4 -12. According to this phase diagram, the eutectic point is at 5 wt% silica and 983 ºC. At the concentration lower than 5 wt%, no eutectic arrest was observed. This means either solid solubility or little heat effect caused by the small amount of eutectic. In addition, the sluggish formation of crystals from the melt can mask the expected break at lower concentrations. This effect has also been observed in cryolite–alumina system at 2.5 wt% alumina [94]. Further silica concentration at 6 wt% causes a sudden jump in liquidus curve in the silica side to 1005 C. After solidification,  transformation of cryolite was observed in all compositions at 565 ºC. This is the same transformation temperature for cryolite, which indicates a small solid solubility of silica in cryolite. Another phase transformation () was expected to affect the cooling curve, which was not observed in any of the recorded cooling curves because of its low enthalpy value, 0.4 kJ.mol-1 compared to 8.2 kJ.mol-1 for  transformation.

Both measured liquidus and eutectic temperatures are lower than those by Dolejs and Baker [178]. This difference can be because of using commercial cryolite, with a composition deviating from stoichiometric cryolite with CR of 3.0. Supercooling effect is also an important parameter, which affects the results. Stirring and seeding the crystal particle are useful methods that can increase crystal formation and reduce the supercooling.

Figure 4‎ -12. Liquidus and eutectic line for cryolite–SiO2 system from 0 to 6 wt% SiO2. (dashed line shows the expected trend) 4.1.4.2 Characterization of the phases

The SEM images of the quenched samples are presented in Figure 4-13. Figure 4-13–a shows micrographs of the quenched samples from above the liquidus line, 1060 °C. Two hypo and hypereutectic samples quenched from the twophase region of liquid+solid (990 °C) are shown in Figure 4-13–b and c, respectively. A primary phase formed in both compositions can be identified. The sample quenched from below the solidus line (6 wt% silica), 970 °C, is shown in Figure 4-13–d which has two distinct phases without any quenched liquid.

According to the phase diagram in Figure 4 -12, the sample containing 1 wt% SiO2, which was quenched at 990 °C, will have 26 wt% quenched liquid and 74 wt% cryolite as a primary phase. The presence of these phases was confirmed by SEM and EDX analysis. The areas identified as “A” in Figure 4-13–b are primary phases separated from the liquid above the eutectic temperature, which is proved to be cryolite according to the ratio between Na and Al (27/9≈3),

Figure 4 -14a. Areas marked as “B” are the quenched liquid remained between cryolite islands.

The EDX results in Figure 4 -14b confirm that silica is mostly concentrated in this phase. The elemental mapping of this sample is also presented in Appendix III.

The same calculation for a compound containing 6 wt% SiO2 and quenched from 990 °C predicts formation of 2 wt% SiO2 phase and 98 wt% quenched liquid. The SEM image in Figure 4-13c shows the two different phases in this composition. EDX analysis, Figure 4 -15, reveals that the A

80 area of Figure 4-13-c is a liquid solution of cryolite-silica. The primary phase in B area has high concentration of Si but there are elements beside Si and O, showing that the primary separated phase is not pure SiO2. Reaction in this mixture has been studied by various authors [37, 180, 181, 200, 201] and the general conclusion is that the product is a compound of three oxides (Na,

Al, Si oxides) such as nepheline, containing 75 wt% SiO2.

Figure 4‎ -13. SEM images of quenched samples. a) quenched liquid, 1 wt% SiO2 b) cryolite + quenched liquid, 1 wt% SiO2 c) sodium aluminosilicate + quenched liquid, 6 wt% SiO2 d) cryolite and sodium aluminosilicate particles after solidification.

To identify the phases in the quenched electrolyte, XRD analysis was carried out on both 1 wt% and 6 wt% silica samples quenched from below the eutectic temperature. The results are presented in Figure 4 -16 and Figure 4 -17, respectively. In the sample containing 1 wt% SiO2, the concentration of silica is low (below the detection limit of the instrument), the only detected phase is cryolite; while in the sample containing 6 wt% SiO2, some peaks other than cryolite

81 appear. The phase corresponding to these peaks could not be identified due to the overlap between the peaks of aluminosilicates or sodium aluminosilicates and cryolite.

Figure 4‎ -14. EDX spectrum of a cryolite1 wt% SiO2 sample. a) area A, b) area B in Figure 4‎ -13- b.

Figure 4‎ -15. EDX spectrum of a cryolite6 wt% SiO2 sample. a) area A, b) area B in Figure 4‎ -13-c.

Cryolite pattern

Figure 4‎ -16. XRD pattern of the sample containing 1 wt% quenched from 970°C.

Cryolite pattern Silicate phase *

* *

Figure 4‎ -17. XRD pattern of the sample containing 6 wt% quenched from 970°C.

EPMA analysis was conducted for elemental analysis on this sample quenched from 990 and 970 °C. The results confirm the presence of two different phases (matrix and white phase) with different chemical composition in both samples. Elemental analysis revealed that small amounts of Si are concentrated in the matrix of both samples. However, the concentration of Si in the white phase quenched from 990 °C is slightly lower than the concentration after quenching from 970 °C, which may be an evidence of formation of cryolitesilica eutectic at this temperature.

The measured compositions of the white phase for both temperatures are presented in Table 4 -4.

Table 4‎ -4. EPMA results of white phase of 6 wt% SiO2 sample quenched from 990 and 970 °C.

Na Al Si F O

atomic% atomic% atomic% atomic% atomic% Quenched from 990°C 4.9 13.7 16.8 16.0 48.0

Quenched from 970°C 5.0 13.8 17.0 15.0 49.0

According to previous researchers, sodium aluminosilicates do not form in a sample of this composition, while it is highly likely that Na2SiF6 and Al2O3.2SiO2 will form This may happen through the following reaction, proposed by Snow [181]: (‎4-5) 4Na3AlF6 + (x+3) SiO2 = 3SiF4 (g)+ 2Al2O3.xSiO2 + 12 NaF

He believed that this reaction happens in the first 30 min and the flouride and oxide then react to form jadite. The EPMA results confirm the presence of both alumninasilicates and sodium aluminosilicates in the sample. As it was held at liquid temperature for 2 hours, a possible

83 reaction path according to the discussion above could be formation of aluminosilicates through Reaction ( 4-5) early in the process. Then (after 30 minutes), a reaction between these compounds and NaF causes formation of jadite which then decomposes to nepheline and albite.

4.2 Determination of deposition and decomposition potentials

4.2.1 Deposition potential

Cyclic volatmmetry experiments are often conducted to study the nature and mechanism of electrode reactions. Three electrodes are employed for this type of measurements: working electrode (WE), reference electrode (RE) and counter electrode (CE). WE is the electrode under investigation for which the desired reaction and mechanism is to be studied. During cyclic voltammetry, the electrode potential increases with time at a fixed rate known as a sweep or scan rate ().The potential is applied between WE and RE, and the corresponding current is measured between CE and WE. When a complete range of potential was covered, the potential is reversed to complete a cycle. When the potential reaches a reaction potential (reduction or oxidation) the current increases until the concentration of specie next to the surface of the electrode is depleted, resulting in drop of the current. If the reaction is reversible, the corresponding (oxidation or reduction) peak appears in the reverse scan. The reduction potential as well as the reaction rate can be extracted from the obtained voltammogram. Dependency of the shape of the peak on the scan rate determines if the reaction is controlled by adsorption or diffusion of the species involved.

4.2.1.1 Cyclic voltammetry in cryolite

To study the electrochemical behaviour of the electrolyte, voltammetry was first performed in a

SiO2–free cryolite melt. The potential was swept in cathodic direction from 0.5 V and was reversed at different potentials (-1.25, -1.5, -1.75, -2 V). The resulting voltammograms are presented in Figure 4 -18.

-1 Figure 4‎ -18. Cyclic voltammetry in molten SiO2–free cryolite; scan rate 50 mV.s .

This figure shows three cathodic peaks (A, B, C) and three corresponding anodic peaks (A, B, and C), respectively. Cathodic peak B with the onset of approximately -1.2 V is associated with aluminium reduction. This potential is close to the standard deposition potential of Al on graphite cathode, -1.16 V [42]. Also, the corresponding anodic peak B shows the oxidation of the deposited aluminium. Peak C starts at approximately -1.5 V corresponding to sodium deposition. Sodium intercalation into graphite cathode has been reported before in cryolite melt during aluminium electrolysis, which results in electrode swelling [49, 50]. However, presence of the short broad wave in the reverse cycle, C, shows that intercalation of sodium did not happen, and C and C peaks are just responsible for reduction and oxidation of Na, respectively. As the working temperature is above the boiling temperature of Na (883 C), evaporation of the deposited Na occurs. Therefore, sodium reduction is an irreversible reaction and the anodic peak C of sodium oxidation is less pronounced.

Occurrence of the two anodic reactions (B and C) was observed by Ming et al. [48] in

Na3AlF6–LiF melt, but at different potentials. The nature of peak A is not clear; aluminium carbide formation on the surface of the electrode may be the possible reaction. Molten cryolite + 3- - - completely dissociates to Na and AlF6 , with the latter further dissociating to AlF4 and F [51]. When carbon is present in the system, a possible cathodic reaction is [52, 53]:

- - 4 AlF4 +3C+12e→ Al4C3+ 16 F (4‎ -6)

4.2.1.2 Cyclic voltammetry in cryolite–SiO2 melt 4.2.1.2.1 Theoretical potential

The effect of SiO2 concentration on the deposition potential of Si was calculated based on the thermodynamic principles. Knowing that cryolite–SiO2 has a eutectic point at 5 wt% SiO2 and

999 C [54], the activity of SiO2 in solution at 1040 C was calculated based on equilibrium between liquid and solid phase at the binary region of the phase diagram:   (‎4-7)

  (‎4-8) the Gibbs free energy is expressed by:

- (‎4-9)   

(4‎ -10) and    

where   are enthalpy and entropy of fusion and Tm is the melting temperature of solution in K. The activity of SiO2 can be calculated by combining equations ( 4-9) and (4 -10):

(4‎ -11) 

 was calculated at 1040 C in the binary region, assuming that SiO2 does not form any solid solution, . The melting temperature of SiO and enthalpy of fusion are 1996 K and 9.7 2 kJ/mol [42], respectively. The activity of SiO2 was determined and then the concentration potential was calculated from:

(4‎ -12)

Then the total deposition potential of Si was derived from equations (4 -12). The results of these calculations are shown in Table 4 -5.

Table 4‎ -5. Effect of SiO2 concentration on theoretical deposition potential of Si.

o  T  a SiO2 Econc E Ed Concentration (J/mol) (K) (J/mol) (V) (V) (V) Liquid +solid 9739.4 1313 3332.7 0.74 -0.01 -0.73 -0.74 SiO2 region

4.2.1.2.2 Experimental potential

Knowing the electrochemical behavior of pure cryolite, 3 wt% SiO2 was added to the melt and cyclic voltammetry was conducted. A typical voltammogram obtained on graphite electrode at 1040°C and sweep rate of 50 mV.s-1 is shown in Figure 4 -19. This voltammogram was compared with that of pure cryolite. The graph shows that addition of SiO2 to cryolite caused a new cathodic peak that forms at a potential around -0.25 V and reaches its maximum at approximately -0.690.04 V. The presence of the corresponding anodic peak at around 0.2 V indicates that silicon can be cathodically deposited and anodically oxidized. However, the shape of this graph does not resemble an ideal reversible cathodic reaction. Involvement of silicon complexes in a chemical reaction at the electrode / electrolyte interface can be the reason for this behavior.

Figure 4‎ -19. Cyclic voltammetry on graphite electrode at 1040°C; scan rate 50 mV.s-1.

The measured potential is in good agreement with the theoretical value, but has some discrepancy with the data reported by Bϕe et al. [36, 37], -1.00 V, and by Monnier et al.[40], - 0.95 V. The measurement technique for both of these studies was Le Blanc’s I-V curve method, which extrapolates the linear part of the I-V curve to zero current to obtain the deposition potential. Their method is based on two-electrode design (anode and cathode) and no reference electrode was used.

The results of cyclic voltammetry on the same electrolyte at different potential ranges are provided in Figure 4 -20. This graph reveals that three cathodic reactions take place in this potential range. The first and last peaks, A and C, originate from the background and are similar to the first and the last peak in the cryolite voltammogram (Figure 4 -18). The fluctuation observed at the most positive side of the graph is due to CO or CO2 evolution. The theoretical potential for this reaction is 1.02 V [42].

-1 Figure 4‎ -20. Cyclic voltammetry in cryolite– 3 % SiO2 melt at 1040 °C; scan rate 50 mV.s .

Peak B is the one corresponding to reduction of SiO2 on graphite. The maximum current of both anodic and cathodic peaks increases as the cathodic limit becomes more negative, which means an increased rate at the higher cathodic overpotentials. This reaction can be because of the multi– stage reduction of silicon (Si4+→Si2+→Si), which has been reported by different authors [32, 46- 48, 55, 56]. Although Si2+ compounds are not stable in free form, the presence of some compounds such as SiO and SiF2 have been reported in the fluoride melts [47]. The complexes of SinF2n+2 are also known to exist in this melt and their dissociation could be the rate controlling step, which precedes the electron transfer step on the electrode surface. A closer look to the peak region of the curve (Figure 4 -20) reveals a shallow post–peak. The presence of this peak as well as the formation of an edges on the oxidation peak started at potential of -1V confirms the multi stage oxidation of SiO2. According to Ming et al. [48], the main peak, B, is due to: Si4++2eSi2+- (4‎ -13) and the post–peak is due to the actual silicon formation:

Si2++2eSi (4‎ -14)

Furthermore, two distinguishable shoulders could be readily recognized in the more cathodic part of the graph at -1.22 and -1.55 V. The shoulder at -1.55 V can be due to deposition of Al on graphite, which is close to the measured potential by Bϕe [37] and Monnier [43], -1.45 V. The corresponding anodic edge observed in reverse cycle at -1.32 V is responsible for oxidation of Al. The other shoulder, appearing at -1.22 V, did not have any corresponding anodic shoulder, which may be due to the formation of a gaseous product which is escaping from the electrode surface. Reduction of silica by graphite according to reaction 2-38 can be responsible for this shoulder and the escaped CO2 gas did not leave any footmark in the reverse cycle.

To investigate the effect of silica concentration on the deposition potential, cyclic voltammetry was conducted in cryolite–5 wt% and 10 wt% SiO2 melts. The recorded voltammogram in a saturated melt, 5 wt%, at a scan rate of 50 mV.s-1 is shown in Figure 4 -21-a. The electrochemical behavior of the melt is compared to the behavior of an oversaturated melt in Figure 4 -21-b for the same scan rate. The same experiments were repeated at 20 mV.s-1 scan rate and the results are illustrated in Figure 4 -22.

B

Figure 4‎ -21. Cyclic volatmmogram showing the effect of SiO2 concentration on the deposition potential of Si; scan rate 50 mV.s-1.

Figure 4‎ -22. Volatmmograms showing the effect of SiO2 concentration on the deposition potential of Si; scan rate 20 mV.s-1

The voltammogram presented in Figure 4 -22-a confirmed that SiO2 is involved in the corresponding reaction in peak B. Concentration of silica affects both peak’s current and potential. Both the reduction peak B and corresponding oxidation peak B shifted towards the more negative potentials and the peaks’ current also increased. In addition, the post–peak, B2, is broader for the 5 wt% sample, showing that silica enhances both stages of Si reduction. Knowing the current is proportional to the flux towards the electrode, Figure 4 -23 illustrates schematically why both peaks have higher current intensity in the melt containing 5 wt% SiO2. Considering the similar diffusion coefficient and the diffusion thickness in both melts, higher flux in the 5 wt% melt is responsible for the larger current.

Figure 4‎ -23. Schematic of the expected concentration profiles of Si4+ and Si2+ ions at the electrode surface for 3 and 5wt% SiO2.

The over-saturated melt showed a drastically different behavior in Figure 4 -21-b. Although the deposition potential still followed the same trend and shifted to the left, the peak shrank and its height dropped. The corresponding anodic peak was also flattened. Formation of large, complex compounds or silica network in the melt at the higher SiO2 concentration can increase the viscosity of the melt and hinder the transport of Si ions towards the electrode. The same effect was observed in the conductivity measurement experiments showing that the higher SiO2 concentration declined the ionic conductivity by either fixing Na+ ions in complex ions or decreasing their diffusion coefficient by increasing the viscosity [57].

A new shoulder appeared in the 5 wt% voltammogram at -1.75 V. According to Reaction 3-36, this is the standard decomposition potential of SiO2. Therefore, it appears that direct decomposition of SiO2 is also possible, although it is not the dominant reduction mechanism. It is possible to calculate the change in concentration potential of peak B1. A 2 wt% increase in

SiO2 concentration declined the concentration potential by -0.15 V. While changing the concentrating from 5 wt% to 10 wt% shifted this potential by -0.10 V.

Generally, higher silica concentrations reduce the peak separation, which means that at lower silica concentration mass transfer rate is larger than the electron transfer rate. This is expected, since at a higher SiO2 concentration the transport of Si ions to the electrode is limited, as explained earlier. Consequently, mass transfer becomes slower than charge transfer, resulting in smaller peak separation. Increase in overpotential at a higher silica concentration, the opposite of what is expected from the thermodynamics, reveals that the extent of reactions is decided by the kinetics under the studied experimental conditions. Increase in silica concentration postponed the deposition process to larger overpotential even at 20 mV.s-1 scan rates. Comparing Figure 4 -21-a and Figure 4 -22-a reveals that potentials shifted to more oxidative potentials, and the peaks B1 and B2 merged to form one peak at a lower scan rate. Shift in potential with the scan rate indicates that the electron transfer was very slow relative to the scan rate; thus, the predicted equilibrium by the Nernst equation did not establish rapidly (relative to scan rate). Consequently, greater overpotential was required to achieve the same rate of electron transfer, leading to larger peak separation. Moreover, merging of B1 and B2 peaks is also related to the kinetics of the reaction. Considering the multi–stage reactions (4 -15) and equations (4 -16), there is a direct relationship between the current intensity and rate constant of the reaction.

K1c 4+ 2+ Si +2e⇄ Si (4‎ -15) K1a K2c Si2++2e⇄ Si K2a

(4‎ -16)

-1 B1 and B2 peaks were combined (at scan rate 20 mV.s ) when they had a similar current intensity that corresponds to a unique rate constant. Therefore, at scan rate as low as 20 mV.s-1, both reactions take place at comparable rates. Increasing scan rate to 50 mV.s-1, separated the two peaks in the way that B1 had a much higher current intensity than B2, translating into higher rate constant for the first reaction (K1c>K2c). Moreover, the presence of sharp anodic peak (B), even after B2 became prominent, confirms that the second reaction is not fast enough to consume all Si2+ that was produced by the first reaction. Then the oxidation of the accumulated Si2+ in the reverse cycle resulted in sharp anodic peak.

Table 4 -6 shows the potential separation of anodic and cathodic peaks for two different scan rates. The increase in peak separation is almost the same for both concentrations, indicating that the kinetics is the sole reason for the peak separation and ohmic potential drop has little if any effect on this.

Table 4‎ -6. Anodic and cathodic potentials in 3 and 5 wt% solutions at 20 and 50 mV.s-1. 3 wt% 5 wt% Increase in Increase in Ecathodic Eanodic Epp Ecathodic Eanodic Epp Epp Epp 20 mV.s-1 -0.730.04 0.200.04 0.93 -1.000.04 -0.200.04 0.80 0.05 0.05 50 mV.s-1 -0.750.07 0.230.04 0.98 -0.900.07 -0.05 0.85

4.2.1.2.2.1 Effect of scan rate on cyclic voltammogram

In order to investigate the mechanism of the oxidation and reduction reactions taking place on the electrode surface, cyclic voltammograms were recorded at various sweep rates, Figure 4 -24.

As seen, both the anodic (B1) and the cathodic (B) currents increased by increasing the sweep rate. This indicates that the reaction involving Si species is controlled by diffusion to some degree.

Figure 4‎ -24. Cyclic voltammogram in cryolite–5 wt% SiO2 melt; scan rates 5, 10, 20, 50 and 100 mV.s-1.

For more accurate investigation on peak B1and B, the pertinent data was derived from the

voltammogram as seen in Table 4 -7. The background current (ib) was calculated by extrapolating

relatively constant current observed before the peak to the peak potential (Ep). Epp is the

separation between anodic and cathodic peaks, Ep/2 corresponds to the potential with half the peak current and n is the number of involved electrons in the reaction. The diagnostic criteria for

reversibility of this reaction are presented in Table 4 -8. Comparing the calculated values for ,

Epp, Ep- Ep/2and with the diagnostic criteria reveals that the reaction is not truly reversible

under the experimental conditions. However, these results show that the lower sweep rate provides more time for the reaction to take place and shifts it more towards the reversibility. At the lowest scan rate, 5 mV.s-1, the calculated parameters approach those of a reversible reaction, Table 4 -8.

Table 4‎ -7. Cyclic voltammetry data of silicon reduction on graphite in Na3AlF6– 5% SiO2. B1 B

Scan rate ib ip Ep Ep- Ep/2 n ib ip Ep Ep- Ep/2 Epp -1 mV.s A A V V A A V V V 5 -0.08 -0.45 -0.86 0.17 2.5 0.13 0.60 -0.14 0.2 1.33 -0.72 -0.50 10 -0.1 -0.50 -0.86 0.18 2.3 -0.01 0.91 -0.083 0.29 1.81 -0.77 -0.47 20 -0.14 -0.52 -0.87 0.17 2.49 0.16 0.95 -0.074 0.25 1.83 -0.80 -0.47 50 -0.18 -0.56 -0.9 0.19 2.2 0.31 0.97 -0.047 0.25 1.72 -0.85 -0.47 100 -0.09 -0.71 -0.91 0.2 2.1 0.14 1.30 0.001 0.33 1.83 -0.91 -0.45 average 2.3 -0.47

Table 4‎ -8. Diagnostic criteria for reversibility of B1 peak [58].

Current Fully Reversible System

1 Ep independent of scan rate no

2  -0.72

3   0.17

4 1.33 =1

5  yes

Knowing the reaction is not reversible, the number of electrons involved in the reaction was calculated using Ep- Ep/2equation for irreversible reaction [58]:

(4‎ -17)    where R is gas constant, T is the temperature in K, F is Faraday’s constant,  is the transfer coefficient and assumed to be 0.5. The average numbers of transferred electrons in Table 4 -7 confirms that 2 electrons are involved in this reaction, which validates the hypothesis of two steps reduction of Si on graphite.

1/2 Peak current (ip) was plotted against the square root of the scan rate ( ) in Figure 4 -25. The corrected current was also calculated by subtracting the background current (ib) from the peak current (ip), ip-ib, and was plotted in this graph. The corrected current appears as a linear function of 1/2, indicating that the silicon deposition is diffusion controlled. The linear expression is: 1/2 ip=0.95  +0.39 for corrected ip (4‎ -18) Knowing the slope of the line, it is possible to calculate the diffusion coefficient of the species in an irreversible reaction using Equation [58]:

  (4‎ -19) where A is the surface area of the electrode, C* is the bulk concentration of the diffusing species and D is the diffusion coefficient. The exposed surface area of the electrode is 0.51 cm2 (considering 1 mm as the immersion depth of the electrode). C* was calculated as following:

(4‎ -20) -5 2 -1 The calculated diffusion coefficient using the corrected ip is 1.32×10 cm .s which is not significantly different from the value reported by Frazer [202], 6.410-5 cm2.s-1.

Figure 4‎ -25. Plot of cathodic peak current versus square root of sweep rate. (The background current was subtracted from the peak current for the corrected data).

Repeating the same process for peak B2 is not possible, as it did not appear at high scan rates; even at the lower scan rate, it did not appear as a sharp peak. In the reverse scan, the anodic peak emerged as a shoulder on the main B peak, which also faded by increasing the sweep rate. Therefore, the involved reaction has a very slow kinetics, which could not proceed at high sweep rates of 50 and 100 mV.s-1. Thus, this reaction is the rate determining reaction during the multi stage reduction of silicon. This is in good agreement with the observed behavior described in Section 4.2.1.2.2 and the provided explanation for the reaction (4 -15).

4.2.1.2.2.2 SEM analysis

The tip of the electrode immersed in 10 wt% electrolytes and experiencing an incomplete oxidation cycle as described earlier was analyzed by SEM and EDS to confirm the main anodic peak on the voltammogram is related to silicon oxidation. The SEM images at two different magnifications are shown in Figure 4 -26. Solidified salt on the surface of the electrode can be recognized as white islands. The presence of this salt shows that the dissolution process was not

95 effective to completely remove the salt. Penetration of the salt to the electrode bulk through the pores is noticeable. Larger magnification reveals three phases. The matrix, which is a rough and bright phase (A), is salt; a smooth, bright phase (B) that is a silicate; and dark patches (C) that are primarily Si.

Figure 4‎ -26. SEM image of electrode tip.

The EDS analysis of the matrix shown in Figure 4 -27 confirms that it is mainly cryolite. The analysis of the bright phase forming a network shows high concentrations of Si (Figure 4 -28) confirming that this is a silicate compound. Finally, analyzing the dark phase shows that this phase is mainly Si dispersed in the molten salt.

Figure 4‎ -27. EDS analysis of matrix (A).

Figure 4‎ -28. EDS analysis of the smooth, bright phase (B).

Figure 4‎ -29. EDS analysis of the dark phase (C).

This analysis confirms that the anodic peak is due to oxidation of Si and the deposited Si on the graphite was oxidized and dissolved back into the electrolyte whereas the particles away from the surface were trapped in the salt.

4.2.2 Decomposition potential

4.2.2.1 Voltammetry measurements on copper

In the following section, the effect of changing the cathode to Cu will be discussed. Cu will form a liquid cathode with Si that is expected to change the kinetics of electrolysis as well as the deposition potential by lowering the activity of Si.

4.2.2.1.1 Cyclic voltammetry in cryolite

As a blank test and to eliminate possible background peaks later, cyclic voltammogram were 4+ recorded before any Si was introduced to the system as SiO2. The potential was varied in the cathodic direction from 0.5 V and was reversed at -1.2 V to determine the electrochemical behavior of the melt. The recorded voltammograms are shown in Figure 4 -30.

-1 Figure 4‎ -30. a)Voltammogram in molten SiO2–free cryolite; 20 mV.s , b) Voltammogram -1 in molten SiO2–free cryolite; 50 mV.s .

From the figure it is seen that decomposition of cryolite starts approximately at -0.6 V. This peak is attributed to irreversible deposition of Na as no broad oxidation peak is observed in the oxidation cycle. As the working temperature of the cell was above the boiling temperature of Na (883 °C), evaporation of the deposited Na occurs and the oxidation peak is less prominent. Deposition of Al is also reported in cryolite [28, 21], and can be a reason for the serration of the graph at larger overpotentials. Oxidation of Copper was observed at around 0.4 V, which dictated the anodic range of the potential window.

4.2.2.1.2 Cyclic voltammetry in cryolite– 6 wt% SiO2 melts

Cyclic voltammograms for deposition and subsequent dissolution of Si on a copper electrode were recorded within the potential range of 0.3 to -1.4 V. Cyclic voltammetry of molten cryolite on a copper electrode shows that copper oxidation starts at 0.3 V, Figure 4 -30- a. Therefore, the anodic potential limit was set below this value to avoid the oxidation of the copper electrode. The cathodic limit was set to observe a deposition peak and at the same time to avoid exceeding the current limit of the potentiostat.

Figure 4 -31 illustrates a typical cyclic voltammogram obtained on the copper electrode for Si4+ reduction in a cryolite–6 wt% SiO2 at 1040 C. This voltammogram is compared with that for -1 pure cryolite at the same scan rate, 10 mV.s . As seen, bulk deposition of Si (C1) commences from -0.6 V with a maximum rate at -1 V. This potential is less cathodic than the equilibrium potential predicted by the Nernst equation. However, it is in good agreement with the presented potential by Grjotheim et. al. [13]. He reported -1.1 V as the cell potential for deposition of Si on

Cu. Peak identified as A1 in the reverse cycle corresponds to the stripping of electrodeposited Si. The charge consumed for the current increase at -1 V is much greater than the corresponding anodic peak. This difference in current indicates that considerable amount of current is consumed by the irreversible decomposition of solvent (as seen in Figure 4 -30).

Figure 4‎ -31. Voltammogram for deposition of Si on copper in molten cryolite– 6 wt% SiO2; 10 mV.s-1.

Two minor reversible peaks (C2 and A2) are also observed at potentials below that for bulk deposition of Si. These peaks are related to the under–potential deposition (UPD) and subsequent oxidation of Si. Underpotential deposition is mainly because of adsorption of Si ions on Cu

4+ substrate (Si solv+4eCu⇄ Siad,Cu ) [203, 204]. This phenomenon can only happen if the activity of the product in the cathode is less than unity. Beside the bulk deposition and underpotential deposition peaks, another cathodic peak (C3) also exists in this voltammogram. The most plausible explanation for this neighboring peak seems to be the interaction between Cu and Si, and the formation of Cu-Si alloy. As soon as Si deposits on the Cu electrode, a molten layer of

Cu–Si alloy forms on the surface. To form a liquid at 1313 K (1040 °C), approximately 4% Si must be present in the cathode, which can be achieved at the surface rather quickly. During the anodic scan, a very small reduction peak (A3) appears, which corresponds to de-alloying of Si from the surface before stripping of Si.

4.2.2.1.2.1 Effect of scan rate

To investigate the kinetics of the reduction reaction, the reaction was studied over a wide range of scan rates from 20 to 100 mV.s-1 in the cathodic direction from 0.3 V. The recorded voltammograms are presented in Figure 4 -32.The bulk deposition potentials independent of the scan rate indicate that the reaction is reversible. However, the potentials for underpotential deposition and alloying are influenced by this increase. The two cathodic peaks C2 and C3 merge to become one peak with larger current intensity at the scan rate of 100 mV.s-1. The merge is accompanied by a shift towards more cathodic potentials. This shift reveals that adsorption is controlled by diffusion. At low scan rates, the ions have sufficient time for diffusion, whereas this is not possible at faster scan rates.

Cyclic voltammograms recorded under the same condition as Figure 4 -32, but with a lower scan rate (5 and 10 mV.s-1) are provided in Figure 4 -33. As can be seen, the currents for both reduction and corresponding oxidation peaks increase from the first to the second cycle. In the first cycle, Si deposits on the surface of solid copper, forming a shallow melt, which enhances both adsorption and bulk deposition occurring in the second cycle.

Figure 4‎ -32. Effect of scan rate on the recorded voltammogram in cryolite–6 wt% SiO2 melt, scan rates 20, 50, 80, 100 mV.s-1.

100

Figure 4‎ -33. Recorded voltammogram in cryolite–6 wt% SiO2 melts at scan rates of 5 and 10 mV.s-1. 4.2.2.1.2.2 Effect of Si concentration As discussed earlier, the deposition potential is a function of Si activity in the cathode. According to the results of cyclic voltammetry, the deposited Si interacts with the Cu cathode and forms a Cu-Si alloy. During the deposition process, the concentration of Si in the alloy varies constantly, which results in a continuous change of the deposition potential. To investigate this effect, Si was deposited from cryolite–6 wt% SiO2 on the surface of Cu at –1.2 V in two intervals of 20 min and cyclic voltammetry was conducted on the surface of the formed alloy after each step. The recorded voltammograms at 20 and 50 mV.s-1 scan rates are presented in Figure 4 -34.

As shown in both graphs, increase in Si concentration due to longer deposition time shifts the reduction peaks to more cathodic or the predicted standard potentials. This shift should be considered during the deposition process either by adjusting the potential in a certain period or by applying a larger overpotential. Besides the change in potential, both anodic and cathodic currents increase with higher Si concentration. This change in the kinetics can be due to transformation of solid Cu to a molten Cu-Si alloy.

101

Figure 4‎ -34. Cyclic voltammogram recorded in cryolite–6 wt% SiO2 after two deposition steps of 20 min; a) scan rate 20 mV.s-1, b) scan rate 50 mV.s-1 4.2.2.1.2.3 SEM analysis

The feasibility of depositing Si was studied by analyzing the cathode after a twostep deposition at -1.2 V. The surface of the cathode was cleaned from solidified electrolyte and was studied under SEM. The image of the sample as well as the elemental mapping are presented in Figure 4 -35. The elemental mapping verifies deposition of Si, which is uniformly distributed in Cu. This distribution confirms that the deposited Si dissolves in copper and forms an alloy. The formed Cu-Si alloy melts at this temperature resulting in the formation of a liquid layer on the surface. This surface melting is responsible for the change in the shape of the cathode from cylindrical to semi hemispherical shape.

Figure 4‎ -35. Elemental mapping of cathode after 40 min electrolysis at -1.2 V.

4.2.2.2 Voltammetry measurements on copper−8wt% Si alloy

The electrochemical behavior of the melt on the surface of a Cu−8 wt% Si alloy was investigated. The recorded voltammogram is presented in Figure 4 -36. Comparing these two

102 voltammograms reveals the underpotential deposition and alloying at 0.15 and −0.04 V respectively. Bulk deposition starts at lower potential of -1.1 V compared to pure copper cathode (Figure 4 -31). One other main difference between this graph and Figure 4 -31 is the anodic peaks

(A2 and A3) corresponding to de-alloying and stripping of adsorbed Si that do not take place on the surface of Cu-Si alloy, indicating that these reactions are not reversible. Cu-Si alloy is molten at this working temperature, thus the deposited silicon dissolves in the bulk of cathode immediately, whereas in the experiments involving pure Cu cathode, the deposited Si forms a very thin solid layer that is later stripped off in the reverse cycle.

-1 Figure 4‎ -36. Voltammogram for Cu-8 wt% Si in molten cryolite–6 wt% SiO2; 50 mV.s . 4.3 Electrowinning and separation of Si

The next part of this study was concentrated on electrowinning of silicon in a molten cathode. The initial cathode was a Cu–8 wt% Si alloy that is liquid at the experimental temperature of 1040 °C. The anodic and cathodic current efficiencies of the electrodeposition process were measured and the deposited Si was further purified and separated by solvent refining.

4.3.1 Characterization of master alloy

The cathode was first analyzed by XRD and EDS to measure the concentration of Si in the alloy after solidification and verify its homogenous distribution. The SEM image and the elemental mapping of the sample are presented in Figure 4 -37. These images confirm that Si is distributed evenly in copper without any macro segregation.

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Figure 4‎ -37. SEM image and the elemental mapping of the master alloy. The BSE image of the sample as well as the EDS analysis is presented in Figure 4 -38. Two phases were observed in this image containing 8 and 9.3 wt% of Si.The overall concentration of Si in the sample is 8.2 wt%.

Figure 4‎ -38. EDS analysis of the master alloy sample. The bulk analysis of the sample by XRD is illustrated in Figure 4 -39. The detected phase is a

Cu−Si alloy (Cu0.83Si0.17), which confirms the presence of 8.2% Si in the sample.

Meas. data:CuSi-slow/Data 1 8.0e+003

6.0e+003

4.0e+003 Intensity (cps) Intensity 2.0e+003

0.0e+000 Copper Silicon, Cu0.83 Si0.17, 01-071-3786

30 40 50 60 2-theta (deg) Figure 4‎ -39. The XRD analysis of Cu-Si master alloy.

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4.3.2 Determination of anodic and cathodic current efficiencies

4.3.2.1 Apparent cathodic current efficiency

Results of electrodeposition of silicon on Cu-Si at different currents, constant temperature and fixed initial concentration of SiO2 are presented in Table 4 -9. Current density was calculated based on an estimated initial area of the cathode. The data show that the minimum required potential for deposition of Si is 1 V. At potentials below 1V the equivalent current was very low and no measurable Si was deposited on the cathode. The negative values of the actual mass show that not only weight gain did not occur, but also the cathode was partially dissolved due to the reaction of the alloy with the electrolyte and the anodic products. A blank test, in which the mass change of the cathode was measured in the absence of applied potential, confirms mass loss in the same order (0.09 g) as those experiments with potential below 1 V.

The apparent current efficiency was calculated simply based on the mass change of the cathode before and after the electrodepotion. In other words, it was assumed that the entire mass gain was because of Si deposition and the Cu remained inert and intact. Figure 4-40 illustrates the apparent cathodic current efficiency as a function of the current. The achieved current efficiency varied between 16-33% and decreased with the current. The reasons for both the low current efficiency and its dependence on the current are explained in the following section.

Table 4‎ -9. Electrolysis with a Cu-Si cathode from cryolite-6 wt% SiO2, T= 1040 °C.

Current Potential Ampere-hours Current density Mgained MTheor. CETheor. (A) (V) (A.cm-2) (gr) (gr) (%) 3.45 2 2.3 2.1 0.17 0.60 28 1.69 1.86 8.5 1.03 0.35 2.21 16 1.5 1.67 7.5 0.92 0.35 1.97 18 1.1 1.43 5.5 0.67 0.28 1.40 20 0.94 1.5 4.7 0.58 0.27 1.23 22 0.5 - 2 0.37 0.15 0.52 29 0.37 1.25 1.85 0.23 0.16 0.48 33 0.06 1 0.32 0.04 -0.03 0.08 N/A 0.01 0.75 0.06 0.01 -0.08 0.02 N/A

Considering that some of the cathode is lost in the process, the mass difference of the cathode before and after the electrodeposition is not a true measure of the amount of deposited Si into

105 cathode. The following section takes this into account to calculate the actual cathodic current efficiency.

Figure 4‎ -40. Dependence of the apparent cathodic CE on the current. 4.3.2.2 Actual cathodic current efficiency

The actual mass of Si gained in the cathode can be calculated by a simple mass balance, if the mass and concentration of Si in the cathode before and after electrodeposition are known:

(4‎ -21)

where mi and mf are the initial and final mass of cathode respectively. Accordingly, an actual cathodic current efficiency can be obtained. Figure 4 -41 presents these results along with the apparent CE values calculated based on the net mass gain. As seen, the actual CE is much greater than the apparent CE, as expected, and both decrease with the current. The decrease can be because of two reasons, one is dissolution or dispersion of Si metal in the electrolyte, and the other is the deposition of Si onto surfaces other than the alloy, e.g. crucible wall. It is expected that at higher currents, leakage of current to the wall is greater. Figure 4 -42 shows the solidified electrolytes after electrolysis under 1.1 A. A greyish layer was noticed between the electrolyte and the wall. XRD analysis of this layer confirms the presence of synthetic Si, i.e. deposition of Si on the crucible wall. Particles deposited on the wall can be dispersed in the electrolyte and are not recovered, resulting in a lower than actual current efficiency. Unlike high applied currents, in the experiments with low current this layer was not observed; also the entire solidified electrolyte was easily removed from the crucible after the experiment, showing no adhesion.

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Figure 4‎ -41. Cathodic CE calculated from the mass change and mass balance.

Figure 4‎ -42. Solidified electrolytes after electrolysis under 1.1 A.

An examination of anode and cathode after the experiments showed that the anode consumption increases at higher currents, also the colour of the cathode changes from faint gold to dark silver, indicating higher concentrations of silicon in the alloy (Figure 4 -43).

Figure 4‎ -43. Pictures of anode and cathode after 5 h electrolysis under different currents.

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The cathode from each experiment was sectioned and the concentration of Si across the section, from top to the bottom was measured using EDS. In Figure 4 -44, the results for two cathodes after 5h electrolysis with 1.5 and 1.69 A are illustrated. The homogeneous distribution of Si in the liquid cathode shows that although Si was reduced on the surface, the rate of Si diffusion was fast enough, presumably larger than the deposition rate, to allow its even distribution in the alloy. Further examination of the sections showed that a 0.5-1 mm layer of the surface has experienced Si depletion, as seen in Figure 4 -45. This indicates the Si depletion/dissolution which is probably taking place after the applied potential was withdrawn and during cooling the crucible. However, the Si concentration in this layer is not substantially lower than the rest of the specimen, also the layer thickness is small, and therefore the primary mechanism for loss of Si (low current efficiency) is believed to be the deposition on the crucible walls.

Figure 4‎ -44. Distribution of Si across the cathode after electrolysis under a) 1.5 A, b) 1.69 A.

Si-enriched area

Si-depleted layer

Figure 4‎ -45. Depletion of Si in a thin surface layer (bright area) on top section of the cathode after electrolysis under 1.1 A.

The mass balance can be performed in a different way; assuming that the mass change of cathode is only because of Si gain, the expected concentration of Si is calculated. Such calculation shows that the Si concentration in the cathode should be lower than what the analysis presents. This can

108 be explained only if some Cu is lost during the process. XRD analysis of the solidified electrolyte presented in Figure 4 -46 confirms the existence of both metallic Cu and Si in the bath.

Figure 4‎ -46. XRD pattern of solidified electrolyte after electrolysis with 1.5A. Knowing that some Cu and Si are lost to electrolyte, an overall mass balance can be written as following: Cu Si Mi M loss +M gain =Mf (4‎ -22) Cu Si where M loss is the Cu mass loss, M gain is the apparent Si mass gain, which is the balance between the actual mass of the deposited Si and the Si loss, and is calculated from the change in Si concentration before and after electrolysis as explained earlier. Therefore, Cu loss can be calculated for each experiment, (Table 4 -10). The exact mechanism for the Cu loss is not known. However, it may be postulated that the electrolyte could dissolve some Cu. In addition, as Si deposition takes place at the cathode surface, chemically and/or electrically induced flow of the interface could result in formation of small droplets of the cathode that are too fine to settle to the cathode, hence remain suspended in the electrolyte. According to Table 4 -10, the copper loss is ~ 0.45 g in average at high currents and almost negligible at low currents, which to some extent support this latter mechanism. The copper loss after electrolysis with 3.45A is 0. Due to the short electrolysis, the results of this experiment are not compared with the other experiments. However, the apparent and the actual current efficiency of this experiment are 28.4% and 28.7%,

109 respectively. The agreement between these two data verifies that Cu loss is the main source of discrepancies between the apparent and actual CEs.

Table 4‎ -10. Mass balance calculation for the Cu mass loss in each experiment. Si Cu Current Potential Mi Mf M gain M loss (A) (V) (g) (g) (g) (g) 3.45 2.0 5.12 5.29 0.17 0 1.69 1.86 5.06 5.41 0.77 0.41 1.5 1.67 5.08 5.43 0.71 0.36 1.1 1.43 5.08 5.37 1.02 0.73 0.94 1.5 5.07 5.34 0.80 0.53 0.5 - 3.85 4.0 0.06 -0.09 0.37 1.25 5.04 5.20 -0.05 -0.04 0.06 1.0 5.06 5.04 -0.04 -0.02 0.01 0.75 5.02 4.94 -0.09 -0.01

4.3.2.3 Anodic current efficiency

Measurements of transference numbers on cryolite–silica showed that at 1040 C, ti is in the range of 0.8–0.9, i.e. if an ionic portion of the current is from contribution of Si ions, 80–90% of the current should translate into silicon deposition, or CE = 80–90%. However, this cannot hold true for two reasons, one is that ions other than Si contribute to ionic conduction. Further, Si may actually be deposited, but not in the CuSi cathode. In order to quantify the actual amount of

SiO2 reduced to Si metal, the anodic current efficiency was evaluated. The fundamental assumption here is that all the oxygen evolved at the anode (as CO or CO2) is originated from the

SiO2. Continuous analysis of the off–gas enables mass balance calculation using Reactions ( 2-38) and ( 2-39) to predict the initial Si formed during the electrolysis. It also provides time- dependent data about the kinetics of the reaction.

The calculated anodic efficiency is compared with the actual cathodic current efficiency in Figure 4 -47. This graph shows that the anodic current efficiencies are larger than the cathodic efficiencies. Also, the trend with the cell current are opposite of each other, showing an increase in the anodic CE as the current increases.

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Figure 4‎ -47. Anodic CE calculated from gas analysis and actual CE calculated from EDS results. Figure 4 -48 presents the amount of Si that was recovered in the cathode and compares that with the reduced Si. As seen, at low currents the two values are close, while they become further apart as the current increases. This again confirms an increase in Si loss at higher currents due to the greater deposition on other surfaces beside the alloy. This problem is critical in an experimental cell, where the spacing between anode and cathode is small. In industrial scale however, there is a large gap between anode and cell wall; also the walls are covered with a frozen electrolyte layer (non–conductive) so that deposition onto the walls is eliminated.

Figure 4‎ -48. Mass of Si reported to the alloy cathode and Si reduced in the cell

Instantaneous current efficiencies were derived from the evolved CO/CO2. Figure 4 -49 shows that the anodic current efficiency is almost constant during the 5 hours of electrolysis and no

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anode effect takes place because of SiO2 depletion. This graph also reveals that the rate of Si deposition is not a function of the activity of Si in the alloy and remains unchanged during the electrolysis. The anodic current efficiency of this process is in the acceptable range of 60-80%, while recovery of this produced Si is still the main challenge that requires further investigations.

Figure 4‎ -49. Instantaneous anodic current density.

The oxygen balance explained above can be extended to carbon balance, noting that the reaction products are CO and CO2. Table 4 -11 presents the data for consumption of carbon, measured based on the anode mass loss, and calculated based on the evolved gas (C as CO/CO2). The carbon in the gas is greater than the anode mass change, showing that C should have partially originated from another source, i.e. crucible. This agrees with the hypothesis that during anodic reaction, both CO and CO2 are formed. CO2 reacts later with C from the crucible to form CO.

The ratio of CO/CO2 in the off-gas was in the range of 10 to 80.

The data provided in Table 4 -11support that a large part of the CO is from the post–electrolysis reaction of CO2 (from anode) with either Si or graphite crucible through one of the following reactions:

4+ 2- Si+ 2CO2 Si + 2CO+ 2O (4‎ -23) (4‎ -24) CO2+ C⇄ 2CO

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Table 4‎ -11. Measured and calculated C consumption. Current Potential C consumption form C consumption  = C from anode mass change form gas analysis crucible (A) (V) (g) (g) (g) 3.45 2 0.22 0.36 0.14 1.69 1.86 1.06 1.2 0.14 1.5 1.67 0.98 1.29 0.31 1.1 1.43 0.50 0.72 0.22 0.94 1.5 0.59 0.67 0.08 0.5 - 0.25 0.28 0.03 0.37 1.25 0.14 0.24 0.1 0.06 1 0.04 0.04 0 0.01 0.75 0.00 0.00 0

Figure 4 -50 correlates the gas composition to the actual CE (cathodic). The results show that there is a direct relationship between CO2 concentration and the current efficiency of the process, which is similar to what is reported for Al electrolysis [32, 33]. Reaction (4 -23) is responsible for metal loss in this process, therefore higher CO2 concentration translates into less metal loss or higher efficiency.

Figure 4‎ -50. Relationship between the cathodic current efficiency and CO2 concentration.

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4.3.2.4 Effect of cell design on the efficiency

As discussed earlier, the efficiency of the electrowinning was found to be low due to deposition of Si on the crucible walls and the dispersion of metallic Si in the electrolyte. Extended electrowining was conducted in three cells with different arrangements to investigate the feasibility of eliminating this problem: 1) bare crucible similar to the setup explained in Section 3.3.1.2 , 2) crucible coated with quartz paste, 3) crucible shielded from inside by a quartz sleeve. The results of these experiments are compared in Table 4 -12.

Table 4‎ -12. Extended electrowinning experiments with different crucibles. No. Crucible Current Deposition Cathode weight Anode weight Apparent CE design (A) time (hrs) gain loss (%) (g) (g) 1 Bare 1.46 6 0.27 1 11.8 2 Coated 1.5 6 0.1 0.85 4.3 3 Coated 2 2 7 0.21 - 5.7 4 Sleeved 1.41 7:20 0.27 1.39 10 5 Sleeved 2 1.40 10 0.25 1.66 6.8

The electrolytes that were used in the coated crucible experiments had a different composition

than the other 3 experiments. These electrolytes were over-saturated with SiO2 (12 wt% compared to 6 wt%) to prevent the dissolution of the thin coating, which could be the reason for

the low efficiency of these experiments compared to the others. To avoid excess SiO2 in the

electrolyte, a thicker SiO2 covering was used as a quartz sleeve with 2 mm thickness. This is assumed as a proper thickness that will still isolate the wall, even if partially dissolved. The second experiment with the quartz sleeve was conducted using high purity cryolite, but the same

concentration of SiO2. These results show that covering the wall of the crucible with quartz tube was not effective in yielding a higher amount of Si. The weight gain in experiments 1, 4, and 5 are approximately the same and independent of the crucible design. The shapes of the anode and cathode of Experiment 4 are shown in Figure 4 -51. The shapes of the crucible after Experiments 4 and 5 are compared in Figure 4 -52. The solidified electrolyte in the sleeved crucibles consists of two layers; a glassy

layer at the bottom of the crucible containing high concentration of SiO2 and a layer of solidified salt at the top. Figure 4 -52-a and b show that the Si particles deposited on the bare surface at the bottom, where the current was concentrated. The particles were also deposited in some parts of

114 the wall that was exposed to the melt due to complete dissolution of quartz in the cryolite. This indicates the insufficient thickness of the tube.

Figure 4‎ -51. Anode and cathode after electrolysis in experiment # 4.

Figure 4‎ -52. Crucibles after Experiments a) # 4, b) # 5.

One interesting result from the data presented in Table 4 -12 is the amount of mass gain in Experiment 1, 4 and 5. The data reveals that mass gain is independent of the electrolysis time, where the applied current is constant. This leads to the hypothesis that the efficiency of the electrolysis drops after deposition of a specific amount of Si in the alloy. Further electrolysis time is not effective and results in the decrease of the efficiency. This might be due to the saturation of the Cu–Si with Si, resulting in formation of Si precipitates. Such precipitates would float to the surface of the cathode and are dispersed in the melt (due to small density) or form a solid layer that promotes powdery Si in the electrolyte, hence reduced current efficiency.

The results of the SEM observation and EDS analysis of Experiment 4 are illustrated in Figure 4 -53. The analysis confirms that the sample contains 25% Si, The predicted concentration from the weight gain is approximately 12 wt%. As discussed before, the higher estimation of Si

115 concentration by EDS is due to copper reaction with the cryolite, which enriches the sample with Si.

Figure 4‎ -53. SEM observation and EDS analysis of the cathode from experiment # 4.

The same analysis of the cathode from Experiment 5 shows different results, Figure 4 -54. The concentration of Si matches well with the mass gain. Although the mass gain for this experiment is more than the one from Experiment 4, the concentration of Si in the sample is lower. This could be because of different cryolite used for this electrolysis, which reacted differently with the copper compared to the previous one.

Figure 4‎ -54. SEM observation and EDS analysis of the cathode from experiment # 5.

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A comparison between the level of impurities in the cathodes of Experiments 4 and 5 confirms that the cathode from Experiment 5 has no impurity in the weight percentage range, while the cathode from Experiment 4 contains 0.5 wt% P. This P is mainly concentrated in the phase with high concentration of Si, i.e. primary Si dendrites. The cryolite that was used for Experiment 4 was commercial grade while the one in Experiment 5 has the laboratory quality, which can be the reason for this different behavior. This can be eliminated either by using cryolite with higher purity or by pre-electrolysis of electrolyte

4.3.3 Solvent refining

The cathode recovered from Experiment 4 was chosen for the solvent refining process. It contained 24 wt% Si before solvent refining. SEM examination of the material after solvent refining, i.e. melting and controlled cooling shows that Si dendrites are present. The microstructures of the sample after solvent refining and before it (electrowon cathode) are shown in Figure 4 -55. As seen, the electrowon sample contains small Si islands whereas the Si dendrites are larger in the material after solvent refining, because of the slow cooling. In addition to these primary Si dendrites, one of which is labelled as “A” in Figure 4 -55-c, there are secondary Si particles which are formed during eutectic solidification of the alloy. The eutectic structure is clearly seen surrounding a larger Si particle in Figure 4 -55-d. As the sample was quenched just at the eutectic temperature, eutectic solidification was not complete, resulting in segregation. The EDS analyses of different regions in the sample after solvent refining are presented in Table 4 -13. The primary Si dendrites are 10–250 µm in size and contains high concentration of P and around 2 wt% Cu. The solubility of Cu in Si is 59 ppmw at room temperature [205], although Pollock [206] reports a solubility of 2 wt%, that is not reasonable, considering the phase diagram of Cu–Si. The Cu present in Si is believed to be because of trapped inclusions of Cu–Si alloy that are removed only if the Si is ground to ultrafine particles and leached properly.

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Figure 4‎ -55. SEM images of the cathode a, b) after extended electrowinning and c, d) after solvent refining.

Table 4‎ -13. EDS analysis of the phases presenting in Figure 4‎ -55-c and d (wt%). A B C D Cu 2.3 77.8 49.5 3.7 Si 95.9 22 50 94.2 P 1.8 0.2 0.5 2.1

P concentration in the Si dendrite is in the same order as that in Si islands after electrowinning. This indicates that solvent refining was not effective enough in removing P from Si, consistent with previous observations [207]. This is also validated by the P analysis across several region presented in Figure 4 -56. As seen, P concentration is higher in Si than the surrounding alloy phase. The figure also shows that a region of 20–40 µm around the Si dendrite has experienced

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Cu enrichment, or rather Si depletion, which is because of absorption of Si to the dendrite during its growth.

Figure 4‎ -56. a) Micrograph of solvent refined alloy, b) EDS analysis across the line shown in (a). 4.3.4 Acid leaching

After solvent refining, the specimen went through acid leaching to separate the Si dendrites from the alloy matrix. After crushing to ~ 160 µm, the sample was leached in a HNO3 solution followed by the second step of leaching in aqua regia. Therefore the alloy matrix was dissolved

119 in the acid and Si phase was remained as residue on the filter paper. The residue was analyzed by EDS and XRD to determine if any significant amount of alloy phase is remained in the residue. The SEM image and EDS results are provided in Figure 4 -57 and Table 4 -15, respectively.

Figure 4‎ -57. SEM image of the Si phase after acid leaching; a) X100, b) X998. Table 4‎ -14. EDS analysis of the phases presenting in Figure 4‎ -57-b (wt%). A B Cu - 0.8 Si 100 46 O - 53 P - 0.2

Based on the BSE image, the leaching process was effective in completely removing the matrix phase. The EDS analysis of the sample demonstrated that the residue is mainly Si, but Si exist in two different forms: the metallic Si and the SiO2. The oxide phase contains some P and Cu as the impurities.

To determine the phases presents in the residue, XRD analysis was conducted and the result is shown in Figure 4 -58. It confirms the EDS finding that the only Si is remained after acid leaching. The XRD pattern matches well with the synthetic Si peaks and the hump around 20 ° that represents amorphous SiO2 [208]. Oxidation of Si to SiO2 takes place during the leaching by nitric acid.

120

600

400

200 Intensity (cps) Intensity

0 Silicon, syn, Si, 01-075-0589

20 40 60 80 2-theta (deg) Figure 4‎ -58. XRD pattern of the Si phase after acid leaching.

The residue, being Si primarily, was digested in HF and analyzed by ICP-OES. The impurity level in the acid leached sample was compared with the total concentration of impurities in the solvent refined samples in Table 4 -15. This comparison reveals the impurity distribution between the alloy and the silicon phase. As seen, the sample contains high concentration of P as the main impurity (as was predicted by EDS), as well as Fe and Ca. Al and B levels in this sample were below the detection limit of the ICP–OES. B being one of the challenging elements to be removed from Si, was not detected in this sample, indicating that it was below 0.3 ppm. As discussed earlier, B removal is one of the major challenges in Si refining, so the promising results here could be an advantage of the technique. Comparing the concentration of impurities that are remaining in the Si phase with the alloy proves that P was mainly concentrated in the grain boundary, which was removed by acid leaching, but still high concentration of P exists in the recovered Si.

Cu and Fe are the two impurities which were mainly concentrated in Si phase and not much success in their removal by acid leaching. In order to investigate the effect of leaching time, the aqua regia leaching of Si residues was repeated, but for a longer time of 4 h (compared to 2.5 h the first time). The analysis showed that Cu was lowered to 187 ppm. This experiment shows that it is possible to reduce the Cu concentration by an extended leaching. Further, as Cu and Fe

121 have small segregation coefficients in Si, their removal should be easy through directional solidification which is often the last step in SoG–Si purification. Phosphorous is perhaps a more critical element that needs to be addressed. This is proposed to be done by (a) selecting more pure electrolyte and cell materials, and (b) pre–electrolysis of the electrolyte with potentials below those of Si deposition to purify the electrode. The process, with sacrificing electrodes will essentially eliminate any impurity element that is nobler than Si, including P, Fe, and B.

Table 4‎ -15. Concentration of impurities in the alloy and final Si in ppmw Si purity by Desired SoG-Si Typical MG-Si Solvent refined Silicon phase Juneja [30] [3] [3] Al <0.03 <0.03 27 <0.1 1000-4000 Ca 95 <0.03 <5 <1 250-620 Fe 917 658 60 <0.1 1500-6000 Mo 103 41 - - - Cu 79159 1426 >250 <1 15-40 P 1428 201 - 0.1-1 20-45 B <0.3 <0.3 200 0.1-1.5 40-60

Although the impurities are not in the desired level to be directly used for SoG-Si, the produced Si has a much lower concentration of impurities compared to MG-Si. Therefore, one or two directional solidification can be effective to achieve the 6N purity level. Additionally, the results presented in Figure 4 -54 confirmed that using higher purity electrolyte or pre-electrolysis of electrolyte would be the other solution for reducing the impurity concentration in the final product.

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Chapter 5 Summary and Conclusion This research was dedicated to understanding the fundamentals of a method, which is energy efficient and will deliver inexpensive high purity silicon material. This study proposed electrowinning of Si in molten Cu-Si alloy followed by Si recovery through solvent refining to decrease the working temperature and increase the productivity. The research was conducted in three separate parts.

Part I) The physiochemical properties of the potential electrolyte, cryolite–SiO2 melts, was studied in the first stage of this work. The effects of SiO2 concentration and temperature were studied on density, electrical conductivity and electronic and ionic transference numbers. The phase equilibria, which correlated the stable phases to the temperature and composition of the electrolyte mixture was also investigated. The following major conclusions were drawn from this part of the work.

1- The SiO2 addition (up to the saturation limit) to cryolite increases the density of the melt within the range of 2.04–2.15 g/cm3. -1 2- SiO2 has a negative effect on the conductivity of the melt and results in up to 0.5 S. cm reduction in conductivity when the silica content is increased to 6 wt%. y- 3- Silica forms complex (SixO2xFy ) ions, which increase its partial molar volume in the solution, also decrease the conductivity.

4- SiO2 did not appear to have an effect on the electronic conductivity, while it caused a decrease in the ionic conductivity, presumably because the otherwise mobile Na+ ions are fixed within larger complex sodium aluminosilicates.

5- Separate measurements of melting point show that there is a eutectic point at 5 wt% SiO2 and 983 C. Silica contents higher or lower than this amount increase the melting point of the electrolyte.

Part II) The deposition potential of Si on a graphite cathode was measured to determine the experimental deposition potential and to investigate the effect of SiO2 concentration on the potential. In the next step, the decomposition potential of Si from cryolite–SiO2 melt on Cu and Cu-Si cathodes was determined using cyclic voltammetry. 1- Silicon deposition on graphite is not a fully reversible reaction and takes place in two stages between -0.6 to -1 V. In the first stage Si4+ is reduced to Si2+ and in the second stage Si2+ is reduced to Si. The second stage with slow kinetics is the rate determining step.

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2- The deposition potential of Si is a function of silica concentration. Higher silica concentration reduces the peak separation, indicating that mass transfer is slower than the electron transfer rate. 3- The deposition potential of Si on graphite electrode was measured to be approximately - 1V. This is the required potential that has to be applied to graphite vs. Pt for the continuous deposition of Si. 4- Cyclic voltammetry on the surface of a Cu cathode showed that the underpotential deposition of Si occurs due to the formation of Cu-Si alloy. 5- The bulk deposition of Si on Cu occurs approximately at -1V. 6- Increase in Si concentration did not change the bulk deposition potential, while shifting the underpotential peaks towards the predicted standard potential, possibility because of increase in the Si activity.

Part III) The feasibility of deposition of Si in the molten form was investigated in this phase. The efficiency of the process was measured by two different approaches: weight gain of the cathode

(cathodic current efficiency) and evolved oxygen (as CO/CO2) in the anode (anodic current efficiency). Finally, the effectiveness of the process in delivering high purity Si was investigated. Si dendrites were precipitated out of the Cu-Si cathode and recovered to determine the purity of the final product. 1- The cathodic CE based on mass balance was ~ 40-65%. 2- An increase in the applied potential (and current) decreases the current efficiency due to deposition on the crucible walls. 3- The anodic CE based on evolved gas was ~ 60-80%. 4- The results of the off gas analysis show that the gas mainly contains CO, while the

detailed analysis reveals that CO2 was initially formed on the anode and later reacted with carbon to form CO. 5- Instantaneous current efficiencies were almost constant during the 5 hours of electrolysis and did not show a sign of decline due to the anodic effect. 6- The extended electrowinning increases the Si concentration in the alloy from 8 wt% to 24 wt%. 7- The impurity level in the final Si after the acid leaching is lower than metallurgical grade and higher than the desired concentrations for solar grade Si. Boron levels appeared to be extremely low in the product.

Estimates of energy consumption and carbon footprint for the proposed method are compared with the Siemens and carbothermic processes in Table 5 -1. The total energy consumption of this method was calculated by the sum of the required energy for electrowinning of Si (17 kWh/kg Si

124 as described in section 3-4) and the required energy for solvent refining and leaching (6 kWh/kg

Si [21]). The carbon footprint was calculated from the CO2 of the required electricity

(considering natural gas as the primary source) to produce Si and CO2 emission from the electrolysis process. It must be noted that the estimates here are for the Si with the reported purity. Additional steps to further purify Si would increase the consumption that need to be considered. However, it is expected that the total energy consumption be far less than the Siemens process.

Table 5‎ -1. Energy consumption and carbon footprint of different Si production methods. Energy Consumption Carbon footprint

kWh/kg Si kg/kg Si Siemens process [3] 120-200 90 Carbothermic process [43] 13 6 Electrowinning + Solvent Refining 23 11

From energy consumption and environmental pollution perspectives, the method looks promising, although numerous challenges need to be addressed to make commercial implementation viable. This fundamental study provided the basic knowledge with respect to design and use of the electrolyte, cell operation, and the overall processing scheme that would enable such technology. The overall overview of the proposed process is illustrated in Figure 5 -1. Si Purification Purified Si

Filtration

Cryolite-SiO2 Cu-Si Si Cu-Si

Electrowinning cell Crushing Separation Solvent refining Cu-Si

Recycling Cu- Si alloy

Slag cleaning agent

Figure 5‎ -1. Overall process flowsheet.

125

Chapter 6 Future Work

Although the combined electrowinning and solvent refining was successful in delivering high purity Si directly from SiO2, the reported purity of the final product is not in the acceptable range for solar applications. For further improvement more work needs to be carried out: 1- Improving the purity of the deposited Si: Using high purity materials, i.e. cryolite and

SiO2, is recommended. As replacing high purity electrolyte is not possible in large scale, pre-elctrolysis of the melt is suggested for this purpose. 2- Improving the purity by acid leaching: An initial investigation on the effect of multi stage leaching in fresh acid showed that this process decreased the Cu content in Si. Further investigation of this effect on all the impurities may be bifacial towards improving the overall the purity of the Si.

Besides the work dedicated to improving the purity of Si, more work can be dedicated to improving the efficiency of the process: 1- Study alternative alloys that can be used as cathode. Al- Si is one of the most appropriate options. It may be possible to produce this alloy directly by co–deposition of the two metals because of their close deposition potential. In addition, Al–Si system has shown to be the most effective impurity getter during solvent refining of Si. 2- Improving the cell design so that deposition on surfaces other than cathode is eliminated. One solution for example would be to use a larger crucible and cathode mass, so that the cathode covers the entire bottom, rather than forming a button. Greater spacing between a node and the wall in a larger crucible would reduce deposition on the side walls. 3- Decreasing the environmental impact of the process by replacing the graphite anode with a non consumable anode.

126

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Appendix I Error analysis of density measurement

Based on principle of error analysis:

   (‎I-1)

Mass error: For the calculation the mass error three types of errors, random, reading and systematic, were calculated and the maximum value was considered as the error of the mass. The random error was calculated by the standard deviation of the recorded value for each point which is 0.04 for most of the points and the systematic value was recorded based on the information of repeatability of data reported by the manufacturer of the balance which is 0.02. The reading error was the amount calculated from the accuracy of the device. For the digital devices such as balance this number is half of the accuracy of the device, 0.005 for balance. Therefore, finally 0.04 was considered as the error of mass for most of the points.

Volume error: The same types of errors in mass were also calculated for volume, but the error calculation for volume is more complicated due to different parameters which were measured for calculating the volume at high temperature:

(‎I-2)

Three types of errors for each of these parameters were calculated separately and the maximum value for each of them was considered for error calculation based on the following formula.

If :

(‎I-3)

Then,

(‎I-4)

It should be noticed that the error of volume measurement at room temperature is also the combination of error of mass measurement of sinker after immersion in graduated cylinder and density measurement of water by weighting a specific volume of water.

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Appendix II Molar conductivity calculation

Molar conductivity is defined as the conductivity of an electrolyte divided by the molar volume of the electrolyte. It shows the effectiveness of one mole of species in the electrolyte in conducting charge, and can be calculated as following

  (‎II-1) 2 -1 -1 where molar conductivity  is in S.m .mol ,  is conductivity in S.cm , and is the molar 3 volume in m /mol. of the electrolyte was calculated from the weight percentage of each component and density of the electrolyte at the given temperature (Table 4-1): (‎II-2)

Where Wi is the weight percentage of component i, Mi is the molecular weight of component i and is the density of the melt. For SiO2- cryolite electrolytes, the equation become:   (‎II-3)

i= A, B, C,....

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Appendix III Elemental mapping of cryolite- 1% SiO2 quenched at 990 °C

141

Appendix IV Cu-Si phase diagram

142

Appendix V Publications and presentations from this research

Journal Articles

 Sokhanvaran S., Abdolkarim Danaei and Barati M. Determination of Cell Potential for Silicon Electrodeposition. Metallurgical and Materials Transaction E, 1A (2014), 187-193.

 Sokhanvaran S. and Barati M., Electrochemical Behavior of Silicon Species in Cryolite Melt. Journal of Electrochemical Society, 161 (2014), E6- E11.

 Johnston M., Tafaghodi Khajavi L., Li M., Sokhanvaran S. and Barati M., High temperature refining of metallurgical-grade silicon: A review. JOM, 64 (2012), 935-945.

 Sokhanvaran S., Barati M. and Thomas S., Charge transport properties of cryolite–silica melts. Electro–Chimica Acta, 66 (2012), 239-244.

Conference Proceedings

 S. Sokhanvaran and M. Barati, Determination of Anodic and Cathodic Current Efficiencies for Silicon Electrowinning on Cu-Si Alloy in Proc. Of Silicon for the Chemical and Solar Industry XII Ed., Norway, 229–239, 2014 (invited paper).

 Sokhanvaran S. and Barati M., Cyclic voltammetry study of anodic and cathodic reactions on graphite in cryolite- silica melt. ECS Transactions, 53 (19), 33-45, 2013.

 Sokhanvaran S., Barati M. and Thomas S. Physicochemical properties of cryolite- silica melts. Proceeding of the 50th Annual Conference of Metallurgists, 25-34, 2011.