Production and Purification of Silicon by Magnesiothermic Reduction of Silica Fume

by

Sarder Elius Sadique

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Materials Science and Engineering University of Toronto

© Copyright by Sarder Elius Sadique, 2010

ii

Production and Purification of Silicon by Magnesiothermic

Reduction of Silica Fume

Sarder Elius Sadique

Master of Applied Science

Department of Materials Science and Engineering

University of Toronto

2010

ABSTRACT

A new approach is discussed for the generation of high purity silicon from silica fume (SF),

which is a waste by-product from the manufacture of metallurgical grade silicon. Process steps

were developed and optimized for this purpose. These include purification of SF, reduction by

magnesium, and post-reduction acid leaching. SF is first subjected to an initial leaching with HCl

to remove soluble impurities. Reduction was carried out successfully in a sealed reaction

chamber with varying Mg/SF ratios, temperature and time. Mg/SF ratio, temperature and time

affected the production of silicon from SF with the intermediate compounds Mg2Si and Mg2SiO4

also forming. Suitable reduction conditions were found to be within the temperature range 750-

850°C and at approximately 2:1 ratio of Mg/SF. Reduction products were treated using a three

stage acid leaching. XRD, QXRD and ICP analyses of the final silicon powder product indicated

that silicon with low impurity levels particularly low boron content can be produced. Therefore,

silicon produced by magnesiothermic reduction can be an attractive source for the production of

solar grade silicon.

iii

To my beloved wife and sons, Sadid/Sami who have been giving an endless support and inspiration during my most exaggerated and jeopardized part of my life in my Canada journey which was quite

unpredictable and unprecedented

iv

ACKNOWLEDGEMENTS

I would like to acknowledge several people who contributed to this thesis and obliged for

providing me the guidance required to achieve this. First of all, I would like to express my

gratitude to my supervisors, Professors M. Barati and A. McLean, for providing me with an

opportunity to work under their supervision and without whom this project would not have been

possible. Thanks are due to Dr. Hiroshi Soda for sharing his expertise, and for showing me new

horizons.

Secondly, I would also like to thank Dr. Yang for his kind assistance to use box furnace and

carbon analysis. I would like to extend my thanks to George Kretschmann who provided

adequate support for XRD analysis of this project, Dr. Srebri Petrov who performed QXRD, Sal

Boccia for SEM, Dr. Murray Johnston and Kingsley Larbi for chemical analysis (ICP), Erfan

Sharifi and Yashashree Chaugule for invaluable support in performing experiments. I would like

to acknowledge the support given by my other research group members.

Financial support from Process Research Ortech (PRO), Ontario Center of Excellence (OCE),

and University of Toronto is also greatly appreciated. I would also like to extend my gratitude to

my friends and to the MSE administrative staff who have always been very helpful.

Finally, and definitely not least I wish to thank my family, Dalia Lucky, Sarder Sadid, Shurit

Sarder and Ruhul (immediate elder brother) for their constant love and support. I could not have

done this without you.

v

TABLE OF CONTENTS

ABSTRACT............................................................................................................................ iiiACKNOWLEDGEMENTS.................................................................................................... ivTABLE OF CONTENTS........................................................................................................ vLIST OF TABLES.................................................................................................................. viiiLIST OF FIGURES................................................................................................................. ixCHAPTER 1: INTRODUCTION........................................................................................... 1CHAPTER 2: LITERATURE REVIEW................................................................................ 3

2.1 Energy and Environment......................................................................................... 3

2.2 Solar Energy............................................................................................................ 4

2.3 Demand for Solar Grade Silicon............................................................................. 5

2.4 Specification of Solar Grade Silicon....................................................................... 7

2.5 Methods for Reduction of Silica............................................................................. 8

2.5.1 Carbothermic Reduction............................................................................. 8

2.5.2 Aluminothermic Reduction......................................................................... 9

2.5.3 Calciothermic Reduction............................................................................ 10

2.5.4 Magnesiothermic Reduction....................................................................... 10

2.6 Solar Grade Silicon Processing Methods................................................................ 11

2.6.1 Chemical Vapor Deposition (CVD): Siemens Process............................... 11

2.6.2 Direct Metallurgical Route......................................................................... 13

2.7 Silica Fume (SF)..................................................................................................... 14

2.8 Initial Leaching of Silica Based Materials and Purification of Reduction Products................................................................................................................. 18

2.8.1 Leaching Periclase (MgO).......................................................................... 18

2.8.2 Removal of Other Impurities from Silicon................................................. 18CHAPTER 3: EXPERIMENTAL PROCEDURES................................................................ 19

3.1 Overview................................................................................................................. 19

3. 2 Materials Characterization...................................................................................... 20

3.2.1 Particle Size Distribution and Morphology................................................. 20

3.2.2 Specific Surface Area.................................................................................. 20

3.2.3 X-Ray Diffraction of Silica Fume............................................................... 22

3.2.4 Chemical Analysis....................................................................................... 22

3.3 Purification of Silica Fume...................................................................................... 22

vi

3.3.1 Impurity Removal from as Received Silica Fume...................................... 22

3.3.2 Carbon Removal.......................................................................................... 23

3.4 Selection of Reducing Agent for SF........................................................................ 23

3.5 Conversion of Silica Fume to Metallic Silicon....................................................... 23

3.5.1 Reduction Procedures.................................................................................. 23

3.5.2 Experimental Variables for Reduction........................................................ 24

3.5.3 Characterization of Reduction Products...................................................... 24

3.6. Leaching and Purification of the Reduction Products............................................ 25

3.7 Characterization of Silicon Products....................................................................... 25CHAPTER 4: RESULTS AND DISCUSSION...................................................................... 26

4.1 Characteristics of Silica Fume (SF)........................................................................ 26

4.1.1 Chemical Composition...................................................................................... 26

4.1.2 Particle Size Analysis and Scanning Electron Microscopy........................ 27

4.1.3 Surface Area Analysis of Silica Fume........................................................ 28

4.1.4 X-Ray Diffraction....................................................................................... 28

4.2 Purification of Silica Fume...................................................................................... 29

4.2.1 Acid Leaching............................................................................................. 29

4.2.2 Carbon Removal from Silica Fume............................................................. 30

4.2.3 Effect of Purification Sequences on Impurity Removal.............................. 31

4.3 Reduction of Silica Fume........................................................................................ 32

4.3.1 Selection of Reducing Agents for Silica Fume........................................... 32

4.3.2 Reduction Products..................................................................................... 33

4.3.3 Effect of Mg/SF Ratio, Temperature and Time on Silica Fume Reduction 35

4.3.3.1 Effect of Mg/SF Ratio on Mg Compounds..................................... 35

4.3.3.2 Effect of Temperature on Formation of Mg Compounds............... 37

4.3.3.3 Effect of Hold Time on the Formation of Mg Compounds............ 40

4.3.4 Effect of Mg/SF Ratio, Temperature and Time on Silicon Yield............... 42

4.4 Leaching and Purification of Reduction Products................................................... 44

4.4.1 Particle Size of Reduction Products............................................................ 44

4.4.2 Leaching of MgO and Mg2Si...................................................................... 45

4.4.3 Dissolution Mechanism of MgO Particle.................................................... 47

4.4.3.1 Theoretical Consideration............................................................... 47

4.4.3.2 Correction for Area Change............................................................ 50

vii

4.4.4 Removal of Mg2SiO4............................................................................................ 53

4.4.5 Hydrofluoric Acid Purification............................................................................. 53CHAPTER 5: SUMMARY AND CONCLUSION................................................................ 60CHAPTER 6: FUTURE WORK.............................................................................................

61REFERENCES.......................................................................................................................

62

viii

LIST OF TABLES

Table 2.1 World successively increasing consumption for most types of primary energy

resources…………………………………………………………………………

3Table 2.2 Direct cost reduction potential for multi-crystalline PV modules……………… 5Table 2.3 Chemical impurities in solar grade silicon............................................................ 7Table 2.4 Target impurity concentrations in upgraded metallurgical grade (UMG) silicon

and SoG-Si. All values in ppmw............................................................................

8

Table 2.5 Obtained impurity concentrations in SoG silicon after plasma purification. All

values in ppmw, measurements by ICP and GD-MS.............................................

8Table 2.6 The chemical compositions of silica fume............................................................ 15Table 4.1 Impurities in as-received silica fume.................................................................... 26Table 4.2 Impurity levels in silica fume after initial leaching.............................................. 30Table 4.3 Comparison between impurities sequence of leaching and calcination of silica

fume.…………………………………………………………………………….

32

Table 4.4 Experimental variable (ratio of Mg/SF) for reduction and quantification of

XRD..……………………………………………………………………………

37

Table 4.5 Experimental variable (temperature) for reduction and quantification of

XRD.…………………………………………………………………………….

38

Table 4.6 Experimental variable (time) for reduction and quantification of

XRD……..………………………………………………………………………

42

Table 4.7 The apparent activation energy for different acid concentration on the

dissolution rate of Mg compound (Leaching conditions: HCl = 1M, 2M and

4M)..…………………………………………………………………………...... 52

Table 4.8 Impurity level in silicon powder........................................................................... 56Table 4.9 Comparison of impurity level in silicon powder with original silica fume and

leached and calcined silica fume...........................................................................

57

Table 4.10 Comparison of impurity level in silicon powder with MGS……………………. 58

ix

LIST OF FIGURES Fig. 2.1: Global energy consumption demand....................................................................... 3Fig. 2.2: Demand and capacity of SoG-Si with a growth rate in the PV industry…………. 6Fig. 2.3: Schematic diagram of electric arc furnace for production of silicon……….......... 9Fig. 2.4: Refining process for high purity polycrystalline silicon. ....................................... 12Fig. 2.5: Schematic process sequence of production of solar grade silicon in

SOLSILC................................................................. ...............................................

14

Fig. 2.6: Silica Fume: (a) white and (b) grey......................................................................... 15Fig. 2.7: XRD of silica fume................................................................. ................................ 16Fig. 2.8: Electron photomicrograph of silica fume: the white bar is 1 μm long…………… 16Fig. 2.9: Boiling point model of impurities in silica fume during production of silicon

metal.................................................................. .....................................................

17

Fig. 3.1: Flow chart of the scheme of experiments. .............................................................. 19Fig. 3.2: Set up for initial leaching of SF with HCl............................................................... 22Fig. 3.3: Schematic diagram of reduction setup.................................................................... 24Fig. 4.1: As received silica fume........................................................................................... 27Fig. 4.2: (a) Typical particle size distributions of as–received silica fume. Mean particle

size is 10 μm and (b) Micrograph from scanning electron microscope showing

typical as- received silica fume............................................................................... 27

Fig. 4.3: Nitrogen adsorption with respect to relative N2 pressure. PS is the sample

pressure and P0 is the saturation pressure. .............................................................

28

Fig. 4.4: Comparison of XRD patterns of as-received silica fume and crystalline

silica......................................................................................................................... 29

Fig. 4.5: Impurity ratio (final/initial) for leaching of SF with 10% HCl solution………..... 30

Fig. 4.6: Change in colors of silica fume (a) before and (b) after carbon removal

(Temperature: 700°C and Time: 2 hours)...............................................................

31

Fig. 4.7: XRD pattern of as-received silica fume after leaching and decarburization at

700°C for 4 hours....................................................................................................

31

Fig. 4.8: Ellingham diagram for various oxides. .................................................................. 33Fig. 4.9: Comparison of DTA curves for Mg and Ca with silica fume mixtures…………... 33Fig. 4.10: Example of reduced silica fume at 750°C for 2 hrs with ratio of Mg:SF =

2:1............................................................................................................................

34

Fig. 4.11: XRD pattern of reduction products of silica fume at 750°C for 2 hrs and ratio

of Mg/SF = 2:1........................................................................................................

34

x

Fig. 4.12: XRD spectrum of samples with different Mg/SF ratios. The experiments were

carried out at 750°C for 2 hours..............................................................................

35

Fig. 4.13: Effect of Mg/SF ratio on the formation of Mg2Si, MgO and Mg2SiO4

compounds in the reduction products at 750°C. Reduction time was 2 hours….....

36

Fig. 4.14: XRD pattern of SF after experiments with different reaction temperatures

(Mg/SF Ratio = 2:1 for 2 hours)..............................................................................

37

Fig. 4.15: Effect of Mg/SF ratio on the formation of MgO based on quantification of XRD

for different reduction temperatures. Reduction time was 2 hours……………….

39

Fig. 4.16: Effect of Mg/SF ratio on Mg2Si formation in the reduction products with

different reaction temperatures. Reduction time was 2 hours.......................…......

39

Fig. 4.17: Effect of Mg/SF ratio on Mg2SiO4 formation during reduction at different

reaction temperatures. Reduction time was 2 hours................................................

40

Fig. 4.18: XRD pattern of SF after reduction with different reaction times at 750°C with

Mg/SF Ratio = 2:1...................................................................................................

41

Fig. 4.19: Effect of time on Mg-compound formation during reduction at 750°C with

Mg/SF Ratio = 2:1...................................................................................................

42

Fig. 4.20: Effect of ratio of Mg:SF on silicon yield from SF with different reduction

temperatures based on quantification of XRD. Reduction time was 2 hours..........

43

Fig. 4.21: Effect of temperature on silicon yield with different Mg/SF ratios based on

quantification of XRD. Reduction time was 2 hours..............................................

43

Fig. 4.22: Effect of time on silicon yield from SF based on quantification of XRD.

Reduction temperature was 750°C and Mg:SF = 2:1..............................................

44

Fig. 4.23: Particle size distribution of reduction products used for leaching. Average

particle size is 66 μm...............................................................................................

44

Fig. 4.24: Dissolution of magnesium compounds with hydrochloric acid. Leaching

temperature was 60°C..............................................................................................

45

Fig. 4.25: Effect of leaching temperature on dissolution of magnesium compounds (Acid

concentration: 2M HCl)...........................................................................................

46

Fig. 4.26: XRD pattern after initial leaching with HCl at 60°C for 1 hr (Reduction products

of Mg/SF = 2:1 at 750°C)........................................................................................ 46

Fig. 4.27: Mass transfer model for MgO particles dissolution in HCl.................................... 47

xi

Fig. 4.28: Mass transfer model fitted by experiments to analyse the dissolution behaviour

of MgO during leaching with HCl...........................................................................

51

Fig. 4.29: Temperature dependence of the rate constant……………..................................... 52

Fig. 4.30: XRD pattern of leached product after leaching with HCl and acetic acid at 60°C

for 1 hr (Reduction products of Mg/SF = 2:1)........................................................

53

Fig. 4.31: XRD pattern of leached product after leaching with HF and acetic acid at 60°C

for 1 hr (Reduction products of Mg/SF = 2:1)........................................................

54

Fig. 4.32: XRD pattern of silicon powder after purifying with hydrofluoric acid and acetic

acid at 70°C for 1 hr (Reduction products of Mg/SF = 2:1)…………..................

54

Fig. 4.33: Silicon powder product after purifying with hydrofluoric acid and acetic acid at

70°C for 1 hr............................................................................................................

55

Fig. 4.34: Particle size distribution (PSD) of silicon powder product..................................... 55

Fig. 4.35: Scanning electron micrograph of purified silicon powder obtained by Mg

reduction and leaching of reduction products.........................................................

55

Fig. 4.36: Solid solubility in Si against segregation coefficient for various elements.

Impurity level in the silicon powder produced in this study (■), maximum

solubility at lower temperature (○) and, solubility at 1410°C (●)........................... 59

1

CHAPTER 1: INTRODUCTION

Non-renewable fossil fuels are our major sources of energy today. Due to environmental

concerns and unsustainable nature of the fossil fuels, alternative energy sources are being sought.

One of the most promising renewable energies is photovoltaic (PV). It is a safe and clean source

of electricity that can suit a wide range of applications. These include space heating and cooling

through solar architecture, house roof heating, day lighting, solar hot water, solar cooking and

high temperature process heat for industrial purposes. Although several materials are used for

manufacture of the solar cells, silicon dominates the PV industry by constituting over 90% of the

solar cells made today. Due to exponential growth of the PV industry, the demand for solar grade

silicon (SoG-Si) has increased tremendously over the past decade. At present, crude silicon

known as metallurgical grade silicon, with 98-99% purity is obtained from quartz rocks by

carbothermic reduction using electric arc furnace. This silicon is further refined into high purity

silicon through expensive and complicated purification process such as CVD process which is

used for production of electronic grade silicon. At the beginning of the PV-activities in 1980s,

the high purity scrap silicon from the microelectronic industry was used by the PV-industry

[Müller et al., 2006]. However, increased demand that surpassed the limited supply of off-

specification electronic grade silicon has created a thrust towards developing a dedicated

technology for production of solar grade silicon.

One of the approaches towards generation of SoG-Si is utilizing materials of very high purity to

produce silicon. As an industrial waste, silica fume could be one of the potential raw materials

for the production of solar grade silicon. This is based on the fact that silica fume is produced as

a byproduct of silicon and ferrosilicon process, as a rich source of SiO2. During the production of

metallurgical grade silicon (or ferrosilicon), several chemical reactions take place within the arc

furnace to produce SiO gas. The gas exits the furnace along with other gaseous species, thus

leaves a significant portion of the less volatile species inside the furnace. The SiO gas then reacts

with oxygen in air and forms silicon dioxide (SiO2) known as silica fume [Lynch, 2009].

Through the formation of silica fume distribution of impurities differ greatly between

metallurgical grade silicon and silica fume. Low content of impurities such as boron in the silica

fume offers a great advantage for production of high purity silicon from this material.

2

There is no reported work in the literature on production of silicon from silica fume. Thus the

primary objectives of this research are:

(a) To explore the possibility of converting silica fume to silicon using magnesiothermic

reduction, and

(b) To determine optimal process conditions in order to achieve a producing high purity

silicon product.

Chapter 2 of this thesis presents a background on the project and also a review of the previous

works on reduction of silica. The details of the experimental work are provided in Chapter 3.

Results are presented and discussed in Chapter 4. Finally, the significant conclusions of the

research are drawn in Chapter 5.

3

CHAPTER 2: LITERATURE SURVEY

2.1 Energy and Environment

Energy sources are divided into two categories: renewable and non-renewable. Renewable

energy sources include solar, wind, geothermal heat, biomass, hydropower, and ocean energy.

The non-renewable energy sources consist of fossil fuel as well as nuclear electricity. As shown

in Fig. 2.1 most of our energy is supplied from non-renewable energy sources, primarily because

they are cheap, easy to process and ship, and relatively non-toxic. In 2008, total global energy

consumption was 474 exajoules among which 80 to 90 percent derived from the combustion of

fossil fuels. The average world power consumption is about 15 terawatts while the usage is

successively increasing, as seen in Table 2.1.

Fig. 2.1: Global energy consumption demand [World energy resources and consumption, 2008].

Table 2.1 World successively increasing consumption for most types of primary energy

resources [World Consumption of Primary Energy, 2008].

Fuel type Average Power in TW 1980 2004 2006

Oil 4.38 5.58 5.74 Gas 1.80 3.45 3.61 Coal 2.34 3.87 4.27 Hydroelectric 0.599 0.933 0.995 Nuclear Power 0.253 0.914 0.929 Geothermal, Wind, Solar Energy, Wood 0.016 0.133 0.158 Total 9.48 15.0 15.8

4

The strong dependence on non-renewable energies poses numerous problems including emission

of greenhouse gases, limited supply resulting in price fluctuations, and depletion of such

resources. Such problems have now become major political issues and the subject of

international debates and regulations.

Supplying energy, while protecting the environment, is essential for sustainable development

[Energy and Environment, 2009] and perhaps the most important challenge of the twenty first

century. It is this need that has created a significant push towards harnessing renewable energies

that are far cleaner (in terms of greenhouse gas emissions) than fossil fuels. However,

technologies are still in the early stages to make the renewable energies a large contributor to the

energy basket. Since the focus of this work is on producing solar grade silicon (SoG-Si) for

photovoltaic applications, this form of energy is discussed in more details.

2.2 Solar Energy

Exploitation of solar energy involves the functional steps of efficient capture, conversion,

utilization and storage. A semiconductor could be used for capturing the energy of the sun’s

radiation as excited electron-hole pairs. However, this avenue is not currently competitive with

fossil fuels for a combination of cost, reliability, and performance reasons, although many routes

are available to produce electricity, fuel, and heat from solar energy.

Photovoltaics (PV) present a prime source of non-polluting energy among renewable energy

sources. The silicon-wafer based technology is the most important technology for PV today

because more than 90% of the annual solar cell production is based on crystalline silicon wafers.

The cost per Wp (Watt Peak) by the direct conversion of sunlight to electrical power by means of

wafer based solar cells has dropped and the conversion efficiency has increased as a result of

better understanding of the conversion limiting factors. The challenge of PV is to decrease the

manufacturing costs per Wp annually by 5% [Müller et al., 2006]. The cost shares per Wp of

multi-crystalline solar systems are approximately as follows:

• Solar grade silicon (SoG-Si): 20%

• Ingot and wafer production: 28%

• Solar cell processing: 13%,

5

• Solar module processing: 9%

• Installation of the PV-system including converter costs: 30%.

There are two main ways to achieve the cost reduction since approximately half of the costs are

caused by feedstock, bulk silicon production and wafer manufacturing. One way is the

development of a dedicated process for production of cheap SoG-Si feedstock material; the other

way is the development of cheap ingot manufacturing processes for multicrystalline silicon

wafers. Finally, the reduction of the silicon demand per Wp by means of the reduction of wafer

thickness and kerf loss and the increase of solar cell and module efficiency will also contribute to

the cost reduction of PV-systems.

A study shows that multi-crystalline silicon technology, currently operating at the lowest direct

production costs of 2.10 US$/Wp, still has a potential for further reductions arriving at direct

module production costs of 1.15 US$/Wp by the year 2010, and being even competitive with thin

film technologies. The cost reduction potential for the direct module of the different PV

technologies has been estimated according to four major categories: cell efficiency, yield,

production scale and material/consumables. Table 2.2 shows the estimated cost reduction

scenario for multi-crystalline silicon modules [Sarti and Einhaus, 2002].

Table 2.2 Direct cost reduction potential for multi-crystalline PV modules [Sarti and Einhaus,

2002].

Year 2000 Year 2010 Direct costs 2.10 US$/Wp 1.15 US$/Wp Cost reduction potential based on Cell efficiency (%) 14 17 Yield (%) 85 90 Production scale (MWp) 10 100 Materials ~ 20% silicon consumption

2.3 Demand for Solar Grade Silicon Fig. 2.2 shows the predicted growth in demand of silicon by the PV industry with approximately

5,000 tons in 2001 and 140,000 tons in 2020 [Müller et al., 2006]. This growth may not be

reached only by the present supply route. Securing the availability of SoG-Si at low costs would

6

be the main goal for the coming years through mass production. The shortage of SoG-Si is

leading to high prices. The PV industry experienced a deficit in silicon feedstock since 2005

resulting in prices as high as 450$/kg silicon in 2008. The need for novel technological solutions

for the processing of SoG-Si is aimed at a cost of 12-15 $/kg [Khattak and Schmid, 1987].

Therefore, it is essential to develop an independent, dedicated, energy efficient silicon feedstock

supply chain, which secures supply of low cost silicon to the PV industry [Wærnes et al., 2006].

Fig. 2.2: Demand and capacity of silicon production in tons [Müller et al., 2006].

In the metallurgical industry, metallurgical grade silicon (MG-Si) is produced by carbothermic

reduction of quartz at a capacity of 900,000 ton/yr with a purity of about 99%. The main

impurities are Fe, Al, P and B. Half of the silicon produced is used as an alloying element for

aluminum, and 45% is used in the silicone industry. The remaining 5% constitutes the raw

material for the semiconductor industry [Wærnes et al., 2006].

The silicon required for electronic application is of typical purity of 9N, thus it has to go through

a very energy consuming process in order to achieve the purity that is required by the

semiconductor industry. The quality requirements for electronic grade silicon (EG-Si) are higher

than for SoG-Si, at least for specific cases such as carbon and dopants. Silicon from the

semiconductor industry is not a very good alternative as the price is too high (30-40 times higher

than MG silicon and twice as much as the reject or non-prime material in today’s solar cells). A

new process route for feedstock to solar cells is thus of prime importance [Müller et al., 2006].

7

2.4 Specification of Solar Grade Silicon

Table 2.3 shows the specification of solar grade silicon and it is a compromise between the

necessary purity of SoG-Si and the reduction of manufacturing costs for dedicated SoG-Si

[Müller et al., 2006]. The increasing demand of SoG-Si in 2004 and 2005 led to a shortage of

SoG-Si for PV in 2005 which triggered additional discussions about the upper purity level for

SoG-Si. To achieve solar cell efficiencies of 17 or 18% on multi-crystalline silicon wafers in

particular it is crucial to minimize the level of transition metals in the raw Si material. To achieve

low enough impurity levels, usually it is essential to use a method that involves easily cleanable

silicon compounds like trichlorosilane (TCS) or monosilane. In the last five years, different

activities have been started or accelerated to purify metallurgical grade silicon by means of

directional solidification including additional steps to remove boron and phosphorus. It is a

significant challenge to achieve the mentioned impurity levels shown in Table 2.3 with low

manufacturing costs [Müller et al., 2006].

Table 2.3 Chemical impurities in solar grade silicon [Müller et al., 2006].

Element ppm

at/cm3

O 1 5x1016 C 1 5 x1016 B 0.5 2.5 x1016 P 0.025 1.25 x1016 As 0.025 1.25 x1016 Fe, Al, Cr, Ni, Ti, Mo, V, Cu, Zn Summed maximum 0.1 5 x1016 Concentration of each not mentioned metal <5.0 x1016

One purification method reported in literature is to upgrade economical metallurgical grade

silicon to higher grade of silicon using plasma purification. Target impurity concentrations are

summarized in Table 2.4 and are compared to typical impurity concentrations that are found in

MG silicon [Sarti and Einhaus, 2002]. The impurity levels of upgraded metallurgical grade

(UMG) and solar grade silicon shown in Table 2.5 have been obtained in a plasma purification

reactor [Alemany et al., 2001].

8

Table 2.4 Target impurity concentrations in upgraded metallurgical grade (UMG) silicon and

SoG-Si [Sarti and Einhaus, 2002]. All values in ppmw.

Impurities

MG-Si UMG-Si SoG-Si

B 40 <30 <1 P 20 <15 <5 O 3000 <2000 <10 C 600 <250 <10 Fe 2000 <150 <10 Al 100-200 <50 <2 Ca 500-600 <500 <2 Ti 200 <5 <1 Cr 50 <15 <1

Table 2.5 Obtained impurity concentrations in SoG silicon after plasma purification. All values in

ppmw, measurements by ICP and GD-MS [Sarti and Einhaus, 2002].

Impurities

UMG-Si SoG-Si

B 15-20 0.5-2.0 P 20-30 10-20 O 500 <10 C 100 10-15 Fe 30-90 <1.0-10 Al 80-90 <1.0-5 Ca 250-450 <1.0-10 Ti 5 <0.5

2.5 Methods for Reduction of Silica 2.5.1 Carbothermic Reduction

Silicon can be obtained from the reduction of quartz in the presence of carbon at high

temperature. At present, silicon (metallurgical grade silicon) is being produced using electric arc

furnace based on carbothermic reaction. Fig. 2.3 shows the schematic diagram of electric arc

furnace for production of metallurgical grade silicon. In this process, quartz rock and various

reducing agents such as coke, coal and wood chips are charged to the furnace and the

temperature raised up to 2400K (2127°C) by electric arc. Silica is reduced by the reaction with

9

carbon. The following intermediate reactions occur to complete the reduction process [Lynch,

2009]:

SiO2(l) + Si(l) = 2SiO(g) (2.1)

SiO(g) + 2C(s) = SiC(s) + CO(g) (2.2)

SiO(g) + SiC(s) = 2Si(l) + CO(g) (2.3)

Quartz Rock & Reducing Agents

Fume, Flames & Off-gas

Air

Quartz & Reducing Agents

Oxide RefractorySiC

Viscous Silica

Taphole

Viscous Drop of Silica formed at 2200K

Cavity SiO(g) & CO(g)

2300K

SiC & Si(l)

Furnace Centre line

SiO2

1300 - 2000K

Arc

Electrode

Fig. 2.3: Schematic diagram of electric arc furnace for production of silicon [Schei et al., 1998;

Lynch, 2009].

In this process the reductant mixture consisting of lignite, petroleum-coke, charcoal and wood

chips are the major source of various contaminants in the end product, and hence the purity level.

In Brazil, the process for obtaining metallurgical silicon uses not only high-quality quartz but

also charcoal as a reducing agent. This is reflected in the quality of Brazilian metallurgical

silicon, which, under well-controlled processing conditions and raw material, can reach a purity

of up to 99.88% [Pires et al., 2005].

2.5.2 Aluminothermic Reduction This reduction process, which uses aluminum as the reducing agent is commonly known as

aluminothermic reduction. Since aluminum is a more active metal than silicon, it can reduce

silicon dioxide to produce silicon. Such reaction is highly exothermic in nature and the liberated

heat is adequate to sustain the reaction by the rapid propagation of a combustion front without

further addition of energy. Use of aluminothermic reduction has been suggested for production

of composite materials. Examples of reactions for making composite materials are [Das et al.,

2002]:

10

Cr2O3 + 2Al = 2Cr + Al2O3 (2.4)

TiO2 + 4/3Al + C = TiC + 2/3Al2O3 (2.5)

3Fe3O4 + 8Al = 9Fe + 4Al2O3 (2.6)

SiO2 + 4/3Al + C = SiC + 2/3Al2O3 (2.7)

2MoO3 + 4Al + C = Mo2C + 2Al2O3 (2.8)

Also synthesis of alumina-aluminum-silicon composite materials has been suggested using

aluminothermic reduction with a mixture of Al and SiO2 by the following reaction [Wang and

Shi, 2002].

3SiO2 + 4Al = 2Al2O3 + 3Si (2.9)

This reaction can be applied for the production of silicon. However, once alumina is formed

through the above reaction and in presence of silica, there are other two possible reactions for the

formation of mullite and the reduction of silicon from mullite:

3Al2O3 + 2SiO2 = Al6Si2O13 (2.10)

8Al + 3Al6Si2O13 = 13Al2O3 + 6Si (2.11)

The formation of mullite and alumina as the reduction products of aluminothermic reduction

may not be suitable for acid leaching.

2.5.3 Calciothermic Reduction

Calciothermic reduction is similar to aluminothermic reduction. A research project on the

reduction of amorphous silica (obtained from rice husk) to silicon of reasonable purity by a

calciothermic process using calcium was carried out by Mishra et al. (1985). The reaction

occurred is as following:

SiO2 + 2Ca = 2CaO + Si (2.12)

Reduction of amorphous silica to silicon by calcium was carried out at 720°C. It has been

reported by Mishra et al. (1985) that final purity of silicon was 99.9% after acid leaching with

concentrated HNO3 and HF.

11

2.5.4 Magnesiothermic Reduction Silica (SiO2) can be reduced by magnesium to produce silicon through the following reaction:

SiO2 + 2Mg = 2MgO + Si (2.13)

The reaction may involve formation of Mg2Si in the early stages (Eq. 2.14), followed by

reduction of SiO2 by Mg2Si through the following chemical reactions (Eq. 2.15):

SiO2(s) + 4Mg(g) = 2MgO(s) + Mg2Si(s) ΔG°(900°C) = -308.5 kJ/mol (2.14)

Mg2Si(s) + SiO2(s) = 2MgO(s) + 2Si(s) ΔG°(900°C) = -181.8 kJ/mol (2.15)

In the presence of excess Mg in the reactants, more Mg2Si may form by consuming elemental

silicon through the reaction:

Si(s) + 2Mg(g) = Mg2Si(s) ΔG°(900°C) = -63.4 kJ/mol (2.16)

Bao et al. (2007) reported conversion of diatom frustules (SiO2) to porous nanocrystalline silicon

using Mg vapour at 650°C which is the melting point of Mg. In this case, reduction reaction

occurred from the surface to the interior of silica particles and yields the mixture of MgO and Si.

Relatively low reduction temperature at 650°C and formation of MgO phases intertwined with

the silicon product inhibited substantial coarsening and sintering of the silicon product [Bao et

al., 2007]. However, magnesiothermic method has not found wide industrial application since the

exothermic reactions cause excessive temperatures and result in forming magnesium silicide

(Mg2Si) with Si product [Zulumyan et al., 2006]. The formation of Mg2Si is affected by the

excess Mg and reduction temperature. Decreasing magnesium amount resulted in decreasing

Mg2Si and with increasing temperatures at fixed ratio of Mg to SiO2, the formation of Mg2Si

increased [Kalem, 2004]. However, in this study by Kalem (2004), Mg2SiO4 under certain

conditions was not taken into account. In addition, quantitative study of phases of the reduction

products was not performed and assessment of phases was solely based on intensity of Mg2Si,

MgO and Si peaks. For this reason, the exact effect of reduction conditions on the formation of

phases was not well established. Several intermediate steps may be involved with most of the

gas-solid reaction systems [Kalem, 2004].

12

2.6 Solar Grade Silicon Processing Methods

2.6.1 Chemical Vapor Deposition (CVD): Siemens Process

Currently, the dominant process for production of electronic and solar grade silicon uses

chemical vapor deposition technology known as the Siemens process. It is related to the

purification of silicon consisting of decomposing trichlorosilane on an inverse U-shape hot

filament (silicon seed) by the following reaction (Eq. 2.17) [Woditscha and Kochb, 2002]. High

current, up to 2 kA flows between the cathode and anode of the silicon seed where the

temperature is reached between 1050°C and 1150°C.

HSiCl3(g) + H2(g) = Si + 3HCl (2.17)

Trichlorosilane is produced using a Fluidized Bed Reactor (FBR) in which metallurgical grade

silicon and anhydrous hydrogen chloride are fed to produce trichlorosilane. Fig. 2.4 shows the

diagram of this refining process.

Fig. 2.4: Refining process for high purity polycrystalline silicon [Woditscha and Kochb, 2002].

Many process improvements have been considered in order to increase the production of silicon.

Some of the examples are:

13

i) Wacker Chemie AG Process: It is a continuous vapour deposition technique. It involves

deposition in fluid-bed reactor in which a gaseous mixture of trichlorosilane and

hydrogen flows through a bed containing silicon seed grains. A constant flow of

hydrogen is fed from the lower to the upper portion of the tube. The hydrogen flow

agitates the silicon seeds thereby creating favourable deposition of polycrystalline silicon

on the surface of the grains [Braga et al., 2008].

ii) Vapor-to-Liquid Deposition (VLD) Process: It was developed by Tokuyama Corporation

in 2005. This process is also a variation of the Siemens process. In this process,

trichlorosilane gas decomposes over a silicon liquid film instead of solid silicon seed

[Braga et al., 2008; Tokuyama Corp., 2007].

iii) MEMC process: Silane gas was produced from hexafluorosilicic acid (H2SiF6) by

reaction with sodium aluminium hydride (NaAlH4). Silane (SiH4) is decomposed in a

Fluidized Bed Reactor to silicon and hydrogen. [Woditscha and Kochb, 2002].

iv) Chisso Corporation Process: In this technology, silicon tetrachloride, produced by the

chlorination reaction of metal silicon, is reduced by zinc vapor to produce high purity

grade polysilicon. The by-product zinc chloride is separated by electrolysis into zinc and

chlorine. The latter is mixed with hydrogen to produce HCl which is recycled back to the

process [Bernreuter Research, 2010].

2.6.2 Direct Metallurgical Route Metallurgical routes involve obtaining SoG-Si directly from metallurgical grade silicon by

various processes such as slag treatment, plasma melting, vacuum melting, injection of various

gases into silicon, and acid leaching. This route for production of silicon can be more energy

efficient than the conventional Siemens process [Waernes, 2006]. In this section, the methods

that are in the development stage are discussed briefly.

i) SOLSILC Project: This proposes the development of SoG-Si by carbothermal reduction

of silicon, based on the use of high purity quartz from natural sources and high purity

carbon black. It used a two-step reduction process as shown in Fig. 2.5. First producing

SiC in a plasma furnace and the second producing Si from this SiC in a submerged arc

14

furnace. Finally, carbon removal is carried out by oxidizing dissolved carbon by Ar and

H2O [Geerligs, et al., 2002].

Fig. 2.5: Schematic process sequence of production of solar grade silicon in SOLSILC [Geerligs

et al., 2002].

ii) Elkem ASA: This process involves pyrometallurgical refining by adding calcium

containing compound in molten silicon. Purification is done by chemical treatment using

aqueous acid solutions [Ceccaroli and Friestad, 2005].

iii) Kawasaki Method: With the objective of eliminating P, Al and Ca from MG-Si, the

Japanese Kawasaki Steel Corporation uses electron beam smelting with graphite holding

vessel instead of copper vessel to achieve high heat efficiency for evaporation of

impurities [Hazanawa et al., 2001]. Argon plasma process containing water vapour over

melted silicon is used for removal of boron [Yuge et al., 1994]. Electron beam melting in

a copper vessel under high vacuum is also investigated [Pires et al., 2005]. This can

eliminate majority of impurities about 99-100% except boron.

2.7 Silica Fume (SF)

Silica fume is very fine non-crystalline silica that is produced in electric arc furnaces (EAF) as a

by-product from the production of silicon metal and also of ferrosilicon. Electric arc furnaces

(Fig. 2.4) used in the manufacture of silicon metal or ferrosilicon, release silica fume as a

byproduct. The fume has a high content of very fine spherical particles of silicon dioxide. It is a

very effective agent for strengthening cement material [Silica Fume User’s Manual, 2005]. Silica

fume is also known as condensed silica fume or microsilica. The path for formation of silica

fume is shown in Fig. 2.4 previously. Quartz rock and reducing agents are charged in the electric

15

arc furnace (EAF) and metallic silicon is produced when operating temperature rises to 2300K.

The following chemical reactions occur inside the cavity of EAF [Lynch, 2009; Wong, 1995]:

SiO2(s,l) + C(s) = SiO(g) + CO(g) (2.18)

SiO2(s,l) + Si(l) = 2SiO(g) (2.19)

SiO2(l) + C(s) = SiO(g) + CO(g) (2.20)

SiO(g) + ½O2(g) = SiO2(s) (silica fume) (2.21)

Some silicon monoxide (SiO) gas escapes from EAF as fume which reacts with oxygen in air

and forms silicon dioxide (SiO2) that is condensed as silica fume and collected in the baghouse.

The condensed fume, which has a high content of very fine spherical particles of silicon dioxide,

is collected by filtering the gases escaping from the furnaces [Malhotra and Carette, 1982]. There

are two types of silica fume commercially available: (i) white and (ii) grey which contains more

carbon (Fig. 2.6). The data of chemical analysis from literature for the silica fume are shown in

Table 2.6.

(a) (b)

Fig. 2.6: Silica Fume: (a) white and (b) grey [Silica Fume User’s Manual, 2005].

Table 2.6 The chemical compositions of silica fume. [Youssef et al., 1998; Agarwal, 2006]

Constituent wt%

wt% SiO2 93-96 90.5 Al2O3 0.20 0.90 Fe2O3 0.50 0.10 CaO - 0.40 MgO 0.50 2.0 Na2O 0.20 0.27 SO3 0-1.5 - LOI (Loss on Ignition) 0.52 2.5

16

The fume mostly is amorphous in nature according to XRD analysis shown in Fig. 2.7. Other

constituents are C, S and the oxides of Al, Fe, Ca, Mg, Na and K. The chemical composition of

the fume varies according to the type of alloy or metal being produced. For example, the fume

from a ferrosilicon furnace will generally contain more iron and magnesium oxides than that

from a furnace producing silicon metal. A typical material has a specific surface area of 28 m2/g

and 95% SiO2 content [Silica Fume User’s Manual, 2005].

Fig. 2.7: XRD of silica fume [Youssef et al., 1998].

Silica fume particles are extremely small and scanning electron micrographs of the particle size

and shape are shown in Fig. 2.8. The shape of silica fume as illustrated from the electron

micrographs is more or less spherical.

Fig. 2.8: Electron photomicrograph of silica fume: the longer white bar is 1 μm long [Silica

Fume User’s Manual, 2005]

The presence of impurities in silicon, particularly boron and phosphorus is evident from the

thermo-chemistry of the arc furnace reactions. Myrhaug and Tveit (2000) developed a boiling

point model for 30 impurity elements using mass balances at the high temperatures and the

reducing conditions in the arc furnace. The results of their study are summarized in Fig. 2.9. As

seen, some elements especially boron are expected to remain in silicon due to the fact that

boiling point of boron is higher than the operating temperature in the EAF. This means boron

17

content expected to be very low in silica fume. In contrast, elements with boiling points lower

than the operating temperature in the furnace such as sulphur and magnesium are expected to

escape into the off-gas. Elements with boiling points between two temperature limits (cavity and

top bed temperatures) were expected to distribute between the silicon, off-gas, and silica fume

(Fig. 2.9). It reveals that industrial practice is in agreement with the model, with the most serious

disagreement involving phosphorus i.e. all phosphorus should be volatilized as per prediction of

that model, whereas only approximately 25% of the phosphorus escapes from the furnace.

to Silicon

Silicon Filtered off -GasSilica Fume

Impurity Elements Expected to Report to Silicon

Expected Variation in Distribution of Impurity Element

Impurity Elements Expected to Report to Silica Fume or Off-Gas

Cavity Temp.Top Bed Temp.

Fig. 2.9: Boiling point model of impurities in silica fume during production of silicon metal

[Myrhaug and Tveit, 2000].

Up to now, there is no documented research on synthesis of silicon from silica fume. The idea of

using silica fume as the starting material for production of high purity silicon was put forward

based on the anticipation that a significant portion of the impurities do not leave to the gas phase

during the production of metallurgical grade silicon. Further, it was expected that those

impurities that volatize and appear in the gas are likely to condense at a temperature different

18

than condensation of silica fume. Thus they may not be intimately mixed with or dissolved in the

SiO2 and could potentially be removed by a purification step such as acid leaching before

reduction. It was based on this hypothesis that purification and reduction of silica fume was

undertaken to evaluate its viability for production of high purity, preferably solar grade, silicon.

2.8 Initial Leaching of Silica Based Materials and Purification of Reduction Products

2.8.1 Leaching Periclase (MgO)

Banerjee et al. (1982) and Ikram and Akhter (1988) studied magnesiothermic reduction of silica

produced from rice husk. In these reduction products one of the major phases is magnesium

oxide. Reduction products were leached with HC1 in order to remove MgO. Chemical

dissolution of periclase (MgO) has been intensively studied and it was observed by various

authors [Raschman and Fedoročková, 2004;` Segall et al., 1978; Fruhwirth et al., 1985]. For

example, Raschman and Fedoročková (2004) studied the inhibiting effect of acid concentration

on the dissolution rate of magnesium oxide during the leaching of dead-burned magnesite and

found that the dissolution of MgO was strongly affected by temperature, acid concentration and

particle size while the effect of composition of the solid was insignificant. In addition to these

detailed studies on MgO dissolution in diluted HCl, dissolution of MgO in HNO3 and H2SO4 has

also been studied [Fedoročková and Raschman, 2008]. The overall dissolution process may be

controlled by the following surface chemical reaction [Raschman and Fedoročková, 2006]:

MgO(s)+2H(aq)+ = Mg(aq)

2+ +H2O (2.22)

Also the reaction order for H+ ions seems not to be affected by the type of acid used and is

controlled by different reactions between surface anions and cations created by dissociation of

different acids [Fedoročková and Raschman, 2008].

2.8.2 Removal of Other Impurities from Silicon

Many impurities like magnesium silicate or orthosilicate were leached with HF followed by HF

and H2SO4 which were more or less completely removed [Ikram and Akhter, 1988]. On the other

hand, magnesium compounds (silicate) and untreated silica were leached with a mixture of

19

concentrated H2SO4 and HF at around 110°C in a Teflon vessel [Banerjee et al., 1982]. After

acid treatment the products were washed and dried. The final products were fine powders of

silicon.

20

CHAPTER 3: EXPERIMENTAL PROCEDURES

3.1 Overview The main purpose of this research is to characterize silica fume and to study feasibility of its use

in generation of solar grade silicon. The experiments were designed to investigate basic steps of

a complete process that consists of (i) Preliminary purification of silica fume to dissolve soluble

impurities, (ii) Reduction of silica fumes with Mg powder to produce metallic silicon and (iii)

Post-reduction leaching and refining of reduction products for the dissolution of Mg compounds.

The objective of the research is to study each of these steps and optimize the conditions to obtain

maximum yield and/or purity of Si. Fig. 3.1 shows the scope of experiments followed to achieve

the objective of this study.

Reduction by Mg

As received Silica Fume (SiO2)

XRD, QXRD, SEM

Step 1 Leaching (MgO and Mg2Si removal)

Step 2 Leaching (Mg2SiO4 removal)

Silicon

Purification

Reduction product

XRD, ICP, SEM

Characterization

Optimal Conditions

Optimal Conditions

Leaching and Decarburization

Step 3 Leaching (SiO2 removal)

Optimal Conditions

Optimal Conditions

Fig. 3.1: Flow chart of the scheme of experiments.

21

3.2 Materials Characterization

Silica fume (Country of origin: Norway) was procured from Process Research Ortech Inc.,

Mississauga, Ontario. The material was subjected to various tests to characterize its physical and

chemical characteristics.

3.2.1 Particle Size Distribution and Morphology

Malvern S Mastersizer was used to determine the particle size distribution of as-received silica

fume. It utilizes the laser diffraction method to portray particle size distribution of samples. One

gram of powder sample was suspended in water and stirred at 2100 RPM. Ultrasonic waves were

applied to break any agglomerates that might have been present in the suspended samples to

provide an accurate particle size distribution. The diffraction pattern obtained is analyzed by a

software and particle size distribution curves are plotted. The technique assumes spherical shape

of particles.

Scanning electron microscopy (SEM, Model: Hitachi S-570; URSA EDX system, Japan) was

performed on the as received silica fume to study the morphology and shape of the particles.

3.2.2 Specific Surface Area

The instrument used to measure the specific surface area was a SA3100 Surface Area and Pore

Size Analyzer, made by Coulter (Boca Raton, FL). The measurements are done in three steps:

outgassing, free space determination, and generation of an isotherm. It used helium as a backfill

gas to determine specific-surface-area (SSA) and the free space of the sample. The adsorbate gas

was nitrogen. Both of these gases were grade 5.0. Outgassing was done at 120°C for 1 hour at

less than 0.1 mmHg. The surface area was determined using the 5 point Brunauer, Emmett, and

Teller (BET) method, and the pore size distribution using the Barrett, Joyner and Halenda (BJH)

method. The purpose of outgassing is to remove volatile impurities such as moisture from the

surface of the sample before analysis. The sample and holder were then weighed, and the result

was subtracted from the mass of the holder to obtain the sample mass. At this point the analysis

was begun, the first step of which was free space calculation.

A plot of the volume of helium injected versus pressure will yield a slope that is equivalent to the

sample tube free space per unit of sample pressure, i.e., the free space correction (Coulter Corp.,

22

1996). The final step in the analysis is the creation of the adsorption isotherm. This involves a

similar measurement as is used in the calculation of the free space correction; however nitrogen

is used rather than helium. Once the injected gas volume is found, the free space determined

previously is subtracted from this value to obtain the volume of gas adsorbed by the sample. The

pressure is measured each time nitrogen is injected, as before, for a specified number of data

points. The resulting plot - called the adsorption isotherm - gives the amount of adsorbate gas

that is capable of condensing onto the sample at a given pressure and constant temperature.

In order to calculate SSA, the BET method is used. This calculation requires only the first

portion of the plot (the Coulter SA 3100 measures from near vacuum to 0.2 PS/P0, where PS is

the sample pressure and P0 is the saturation pressure), and assumes that only a uniform

monolayer of nitrogen molecules adsorbs to the surface of the specimen being analysed. The

linear form of the BET [Brunauer et al., 1938] function is expressed as follows:

0

S

MMS0A

S

PP

CV1C

CV1

PPVP

⎟⎟⎠

⎞⎜⎜⎝

⎛ −+=

− (3.1)

where VA is the volume of nitrogen adsorbed, VM is the volume of the monolayer, and C is a

constant related to the enthalpy of adsorption (Coulter Corp., 1996). The parameters VM and C

are unknown, but are determined by plotting PS/[VA(P0-PS)] versus PS/P0 and determining the

slope and y-intercept. The slope should be equal to (C-1)/(VMC) and the y-intercept equal to

1/VMC. This produces two equations with two unknowns, which may then be solved. The total

surface area of the sample, SA, is then calculated using the following equation:

g

MAM

VANVSA = (3.2)

where NA is Avogadro’s Number, AM is the cross-sectional area of a nitrogen molecule, and Vg is

the volume of one mole of an ideal gas (Coulter Corp., 1996). In this study, the adsorption

isotherm data from within the relative pressure range of 0.05 to 0.2 was used to solve Eq. 3.1.

The SSA is finally determined by dividing the result of Eq. 3.2 by the mass of the sample, which

is the only input required by the device.

23

3.2.3 X-Ray Diffraction of Silica Fume

The X-ray diffraction spectra of as-received silica fume were taken using an X-ray

diffractometer [Phillips Powder Diffractometer (Generator: PW1830); Cu X-ray tube; Control

unit: PW1310]. Copper Kα radiation with the wavelength of 1.540560 Å was used as a

monochromatic X-ray source to bombard the sample. The operating voltage and current were 40

KV and 40 mA respectively. The scans were conducted over a 2θ angular range of 10° - 100°

with a speed of 1 degree per minute. The XRD patterns obtained were used for both qualitative

and quantitative determination of the existing phases within the sample.

3.2.4 Chemical Analysis As-received silica fume was analyzed by an external laboratory (International Plasma Lab Ltd.,

British Columbia, Canada).

3.3 Purification of Silica Fume 3.3.1 Impurity Removal from as Received Silica Fume Before reducing SF, it was decided to remove the impurities that are easily leachable, to avoid

their integration into Si, due to the high temperature process of reduction. As-received silica

fume was leached initially in 0.1N HCl at 60 and 90°C for 1 and 4 hrs. The material was

subsequently washed several times with deionized water and vacuum filtered followed by drying

in an oven at 120°C for approximately 4-5 hours.

Fig. 3.2: Set up for initial leaching of SF with HCl.

24

The apparatus shown in Fig. 3.2 was used for leaching, and is composed of a Teflon beaker,

mantle heater, Teflon coated impeller and thermometer. HCl solution containing silica fume was

stirred vigorously during the leaching operation. A larger batch of SF was treated under the

optimum leaching conditions to prepare sufficient material for subsequent treatment. A sample

was analyzed by an external laboratory, as described earlier.

3.3.2 Carbon Removal

The silica fume contained about 0.5% C. It was subjected to roasting in air for 4 hours at 700°C

in a muffle furnace. The carbon content of the product was measured using the LECO

combustion technique (0.5 gm sample).

3.4 Selection of Reducing Agent for SF Two candidate reducing agents were evaluated, Mg and Ca, as both are thermodynamically

favored to reduce SiO2. To determine the exact reduction temperature of silica fume with Ca and

Mg, and to obtain an approximate idea of the reaction mechanism involved in the process, DTA

[Netzsch STA-409 ‘Luxx’ system, Netzsch, Germany; Temperature 1500°C] studies of SF-

reducing agent mixtures were conducted. 59-64mg of the powder mixture under investigation,

together with the reference sample (in this case, alumina powder), were heated at a uniform rate

of 10°C/min to 1150°C. The alumina powder used as the reference was initially fired at about

1150°C for about 4 hours to remove all moisture.

3.5 Conversion of Silica Fume to Metallic Silicon

3.5.1 Reduction Procedures A schematic diagram of the reduction setup is shown in Fig. 3.3. A mixture of purified SF and

Mg granules of 98% purity (Sigma Aldrich, Canada) was prepared by weighing appropriate

amounts of Mg and silica fume, then blending them well using a stainless steel scoop. 2 grams of

the mixture was placed within an alumina tube of 25 mm inner diameter with wall thickness of 9

mm. The tube was then inserted within another mild steel tube which was sealed tightly by two

end caps. The gap between alumina tube and mild steel tube was filled with alumina powder.

The sealed and assembled reaction tube was then inserted within a muffle furnace. The

temperature was raised at the rate of 5°C/min to the target temperature. After the desired dwell

25

time, the furnace was switched off and the sample was cooled slowly to room temperature within

the furnace. A thermocouple inserted from the front door of the furnace recorded the temperature

close to the tube. After cooling to room temperature, the reaction products were removed and

mildly hand-milled in an agate mortar.

Reaction Tube

Mg Powder and Silica Fume Mixtures

Alumina powder

Alumina Tube

Furnace

Brick Insulation

Powder Type Insulation

Thermocouple

Brick Cover

Ceramic Core

Heating Element

Steel Tube

Fig. 3.3: Schematic diagram of reduction setup.

3.5.2 Experimental Variables for Reduction

Experimental teat variables considered in this study were Mg to silica ratio, temperature and

time. Reduction tests were carried out at various Mg to silica fume ratios of 3:1, 2.5:1, 2:1, 1.5:1.

Mg to silica fume (SF) was calculated on the basis of molar ratio assuming silica content in SF is

100%. The reduction tests were conducted at three temperatures; 750, 850 and 950°C (furnace

temperatures). The dwell times were changed from 0.5 to 4 hours.

3.5.3 Characterization of Reduction Products The particle size distribution and specific surface area of the product were determined by the

methods mentioned earlier. The samples were also subjected to XRD for phase identification. In

addition, the spectra were analyzed to yield the quantity of each phase detected by XRD. The

26

method is based on the Reitveld treatment of the XRD peaks [Madsen et al., 1991]. The

computer package used for this purpose was Topas, v. 2.1, (2003).

3.6 Leaching and Purification of the Reduction Products The grounded reaction product was leached in three stages. XRD patterns show presence of

MgO, Mg2Si, Mg2SiO4, and SiO2 in addition to Si. Various leachants should be used for

effective removal of each phase. In the first stage, the powders were leached in HCl solutions of

varying concentrations (1M, 2M and 4M) to remove MgO and Mg2Si, at the temperature range

of 30 to 60°C and for 10 to 60 minutes leaching. Continuous magnetic stirring was applied for

effective leaching to take place. The HCl-leached product was washed thoroughly with deionized

water and dried after each stage of leaching. The second stage involved leaching the material

with optimized 2M HCl and 25% CH3COOH [Larbi, 2010], to dissolve Mg2SiO4. In the third

stage of leaching, the acid-purified leached products were further treated with 4.8% hydrofluoric

acid (HF) and 25% CH3COOH (60-70°C) to remove the remaining Mg2SiO4, unreacted SiO2 and

other metallic impurities. Continuous stirring was applied for effective leaching to take place.

The acid-leached product was washed thoroughly with deionized water. Finally all acid was

removed by repeated washing with warm deionized water, and the product was filtered and

dried.

3.7 Characterization of Silicon Products The final product was subjected to various characterization tests including particle size analysis,

BET surface area, XRD, SEM, and chemical assay. The chemical analysis of the Si product was

done in-house (Department of Chemistry, U of T) using ICP-OES technique and also sent to an

outside laboratory. The sample preparation involved for in-house ICP analysis was as follows:

Final silicon product (0.25 gm) was dissolved in HNO3, H2O and HF solution (ratio: 2:2:1). The

decanted solution diluted 50 times for ICP analysis. A blank sample was also prepared with the

same ratio as for digestion of silicon.

27

CHAPTER 4: RESULTS AND DISCUSSION

4.1 Characteristics of Silica Fume (SF) 4.1.1 Chemical Composition Chemical analysis of as-received silica fume shows that it contains more than 97 percent silicon

dioxide (Table 4.1). Major impurities are carbon and the oxides of phosphorus, aluminum, iron,

calcium, magnesium, sodium and potassium. However, the boron content is very low, which is a

significant advantage when solar grade silicon is the target product. As described in the literature

(Chapter 2), during the production of silicon in the electric arc furnace, boron is expected to

remain in the molten silicon due to its higher boiling temperature compared to the operating

temperature (2300K) of the furnace, whereas boiling point of phosphorus is below that of the

operating temperature. Approximately 25% of the phosphorus leaves the furnace with silica

fume [Myrhaug and Tveit, 2000]. These are reflected in the composition of silica fume given in

Table 4.1 which has low boron and relatively high phosphorus content. Silica fume contains

0.5% carbon, giving a grayish appearance to the material as shown in Fig. 4.1.

Table 4.1 Impurities in as-received silica fume.

Major Impurity Elements Percent/ppm Carbon 0.5%

P 294 ppm B <2 ppm As 12 ppm Fe 1603 ppm Al 2094 ppm Ca 2527 ppm K 7265 ppm Na 1161 ppm Mg 3004 ppm Mn 93 ppm

Silica content >97%

28

Fig. 4.1: As received silica fume.

4.1.2 Particle Size Analysis and Scanning Electron Microscopy

Result of particle size analysis is shown in Fig. 4.2(a). The particle size of as-received silica

fume varies from 0.1-100 µm and the average particle size was around 10 µm. However, it is

reported that particle size of SF is around 0.1 µm and particles tend to cluster together [Malhotra

and Carette, 1982; Silica Fume User’s Manual, 2005]. Fig. 4.2(b) shows image taken by

scanning electron microscope (SEM). It shows agglomerated particles vary in sizes confirming

the tendency to cluster.

Particle Diameter (µm.)

( )

0

10

0.01 0.1 1.0 10.0 100.0 1000.0

Volu

me(

%)

(a) (b)

Fig. 4.2: (a) Particle size distributions of as–received silica fume. Mean particle size is 10 μm

and (b) Micrograph from scanning electron microscope showing typical as- received

silica fume.

29

4.1.3 Surface Area Analysis of Silica Fume The BET method yields a specific surface area of 17.8 m2/g for as-received silica fume. This is

consistent with the values reported in the literature, showing surface area of SF of about 20 m2/g

[Phillips and Carter, 2003]. The BET isotherm allows adsorption beyond a monolayer such as

molecular adsorption of N2. It has the following shape when adsorbed volume is plotted against

pressure as shown in Fig. 4.3.

0

2

4

6

0 0.05 0.1 0.15 0.2Relative pressure Ps/P0

Volu

me A

dsor

bed

cc/g

(ST

P)

Fig. 4.3: Nitrogen adsorption with respect to relative N2 pressure. PS is the sample pressure and

P0 is the saturation pressure.

4.1.4 X-Ray Diffraction

The X-ray diffraction (XRD) of as-received silica fume is shown in Fig. 4.4. The diffraction

pattern of crystalline silica exhibits sharp peaks whereas silica fume shows a weak, broad peak

around 2θ = 22° indicating the amorphous nature of the material. This is an advantage for silica

fume, as amorphous materials are more reactive. Also, while silica is carcinogenic when

crystalline, it does not pose any health hazard in the amorphous form.

30

As received Silica Fume

Crystalline SilicaC

ount

s/s

Fig. 4.4: Comparison of XRD patterns of as-received silica fume and crystalline silica.

4.2 Purification of Silica Fume

4.2.1 Acid Leaching Due to the nature of the SF formation and collection, the associated impurities such as

phosphorus, aluminum, iron, calcium, magnesium, sodium and potassium exist as oxide within

silica fume [Agarwal, 2006]. It is anticipated that some of the oxide impurities can be removed

by acid leaching. As described in the Experimental Aspects, leaching was carried out for various

temperatures and reaction times for four conditions in a 10% HCl solution while the solid

concentration was maintained at 10%.

The impurity levels of the leached and dried silica fume were determined by inductively coupled

plasma (ICP-MS) analysis (Table 4.2). The results are also shown in Fig. 4.5 by normalizing the

concentrations in the leached product with respect to the initial concentrations. A ratio smaller

than unity shows successful removal of the impurity, the smaller the ratio, the larger is the

removal. It is clear from Fig. 4.5 that some elements like As, Pb and P were greatly reduced and

other elements such as Fe, Mg, K, Ca, Na, Mn and Sr were partially reduced; whereas B and Ti

remained unchanged.

31

Table 4.2 Impurity levels in silica fume after initial leaching (ICP analysis by International

Plasma Labs Ltd., British Columbia).

Elements B (ppm)

P (ppm)

Fe (ppm)

Al (ppm)

Ca (ppm)

K (ppm)

Na (ppm)

Mg (ppm)

Samples SF Original 2 294 1232 2153 2527 7265 1161 3004

SF_60°C_1hr 2 100 1114 2132 2053 6279 849 2678 SF_60°C _4 hr 2 100 1157 2181 2089 6330 881 2681 SF_90°C _1 hr 2 100 949 2098 2034 6149 815 2638 SF_90°C _4 hr 2 100 1059 2094 2026 6149 797 2585

0

0.2

0.4

0.6

0.8

1

1.2

As

Fe

Pb

Mg

P

K

Na

Ca

Mn

Sr

Al

B

Ti

SF Original

SF_60_1

SF_60_4

SF_90_1

SF_90_4

Original

60°C – 1 hr

60°C – 4 hrs

90°C – 1 hr

90°C – 4 hrs

Impurity Elements in SF

Impu

rity

rem

oval

effic

ienc

y

Fig. 4.5: Impurity ratio (final/initial) for leaching of SF with 10% HCl solution.

4.2.2 Carbon Removal from Silica Fume

As-received silica fume as shown in Fig. 4.6(a) contains 0.5% carbon (exhibiting grayish

appearance). This material was heated in air using a muffle furnace to remove carbon. The

material was kept in the furnace for 4 hours at 700°C. As a result, color changed from grayish to

whitish as shown in Fig. 4.6(b) suggesting carbon content was reduced. Carbon content was

analyzed and the results confirmed that carbon was reduced from 0.5% to 0.16%.

32

(a) (b)

Fig. 4.6: Change in colors of silica fume (a) before and (b) after carbon removal (Temperature:

700°C and Time: 4 hours).

XRD analysis was carried out for calcined silica fume as shown in Fig. 4.7 which confirmed

mostly amorphous silica except for a few peaks of crystalline silica. Moreover, quantification of

the XRD spectrum showed that the silica fume is still amorphous in nature.

Position, θ

Cou

nts/s

SiO

2

SiO

2

SiO

2

Fig. 4.7: XRD pattern of as-received silica fume after leaching and decarburization at 700°C for

4 hours.

4.2.3 Effect of Purification Sequences on Impurity Removal

It is possible that the carbon present within the silica fume may reduce wettability of the

particles, thereby rendering the particles unresponsive to leaching; hence removal of carbon prior

33

to leaching may be beneficial. For this reason, the effect of reversing the pre-purification

sequences from “acid leaching and carbon removal” to “carbon removal and acid leaching” was

studied. Table 4.3 shows the results of this study. It is clear that for majority of the impurities,

leaching followed by calcination provides more removal.

4.3 Reduction of Silica Fume

4.3.1 Selection of Reducing Agents for Silica Fume From the Ellingham diagram shown in Fig 4.8, Mg and Ca were considered as suitable reducing

agents for silica fume. DTA analysis was performed for silica fume mixed with Mg and other

with Ca, to determine the reduction temperature. Fig. 4.9 shows DTA records of the experiment,

exhibiting an exothermic peak for both elements. Onset temperature for reduction of silica fume

by Mg was at 643°C which is close to the melting point of Mg (650°C) whereas for Ca it was at

832°C which is again very close to the melting point of Ca (842°C). Although Ca is more

reactive compared to Mg according to the Ellingham diagram, reduction of silica fume with Mg

occurred at a substantially lower temperature and intensity of the exothermic peak for Mg

reduction was also significantly lower. Because of lower required temperature, also ease of

dissolution of Mg compounds and availability of Mg at high purity and low cost, Mg was chosen

as reducing agent.

Table 4.3 Comparison between impurities sequence of leaching and calcination of silica fume).

Elements B (ppm)

P (ppm)

Al (ppm)

Ca (ppm)

K (ppm)

Mg (ppm)

Mn (ppm)

Fe (ppm)

Conditions As received silica fume <2 294 2094 2527 7265 3004 93 1603

Leached only 14 54 1840 2213 4228 6606 184 2962 Leached & Calcined 31 125 1431 575 63 222 0 984 Calcined & Leached 13 52 1466 1640 3675 3306 604 7293

34

Fig. 4.8: Ellingham diagram for various oxides [Lynch, 2009].

0

1

2

3

4

5

6

7

200 400 600 800 1000

μV/mg

Temperature (°C)

Ca Reduction Onset Temperature: 832°C

Mg Reduction Onset Temperature: 643°C

4

μV/

mg

Fig. 4.9: Comparison of DTA curves for Mg and Ca with silica fume mixtures.

4.3.2 Reduction Products A typical product of the batch reduction experiments in closed cell reactor is shown in Fig. 4.10.

The corresponding X-ray diffractogram is shown in Fig. 4.11. The phases present in the

reduction products under the conditions of 750°C and 2 hours holding time were elemental

silicon, MgO and Mg2Si. The sharp and high intensity of MgO and Si peaks indicate that those

35

two are the major phases under the experimental conditions. The following solid/gas reactions

[Kalem, 2004] might have occurred during 2 hours reduction:

SiO2(s) + 4Mg(g) = 2MgO(s) + Mg2Si(s) (4.1)

Mg2Si(s) + SiO2(s) = 2MgO(s) + 2Si(s) (4.2)

However, the onset temperature as per DTA was 643°C which is slightly below the melting point

of Mg (650°C). It is also reported [Banerjee et al., 1982 and Kondoh et al., 2001] in DTA

analysis that onset temperature containing Mg and silica powder was approximately 493-540°C

well below the melting point of Mg. Although this suggests that the reactions take place in solid

state, it is believed that due to the very slow nature of solid-solid reactions, this is not the case.

Instead, formation of liquid Mg that is spread easily over the SiO2 particles, or Mg vapor (that

has a relatively high pressure at temperatures above 500°C) may have contributed significantly

to the progress of the reaction. This is supported by the very sharp and short heat release peak in

the DTA spectrum, indicating that the exothermic reaction occurs at very high rates.

Fig. 4.10: Example of reduced silica fume at 750°C for 2 hrs with ratio of Mg:SF = 2:1.

0100

200300400500

600700800

9001000

10 20 30 40 50 60 70 80 90 100

2 theta

Cou

nys/

s

◊ Si• Mg2Si

MgO

MgO•

◊

MgO

• ◊◊

MgO

MgO

◊ ◊•

MgO

Coun

ts/s

2 Theta (degrees) Fig. 4.11: XRD pattern of reduction products of silica fume at 750°C for 2 hrs and ratio of

Mg/SF = 2:1.

36

In addition to MgO and Mg2Si, other Mg-containing compounds such as Mg2SiO4 were formed

under several experimental conditions (Table 4.4). Formation of such compounds has been

reported by other researchers [Wynnyckuj and Rao, 1976].

4.3.3 Effect of Mg/SF Ratio, Temperature and Time on Silica Fume Reduction

4.3.3.1 Effect of Mg/SF Ratio on Mg Compounds

The X-Ray diffractograms for various ratios of Mg to silica fume (SF) were superimposed in Fig.

4.12. The results presented here are for samples heated to 750°C and held for 2 hours. Mg to

silica fume (SF) was calculated on the basis of molar ratio assuming silica content in SF is 100%.

However, the actual silica content within silica fume is 98% after carbon removal, thus the ratio

of Mg to SiO2 is slightly higher.

10 20 30 40 50 60 70 80 90 100

2 theta (degrees)

Coun

ts/s

= 2.5 : 1

= 2 : 1

= 1.5 : 1

Mg:SF Ratio     = 3 : 1

◊ Si• Mg2Si∇ SiO2

◊

••

ץ

•

◊••

◊••

∇

◊

◊

◊

◊

MgO

MgO

MgO

MgO

MgO

•

•

∇ MgO

MgO

MgO

MgO

◊

◊

◊

◊

MgO

MgO

MgO

MgO

Fig. 4.12: XRD spectrum of samples with different Mg/SF ratios. The experiments were carried

out at 750°C for 2 hours.

As seen in this figure, the formation of MgO, Mg2Si and silicon is affected by the ratio of

Mg/SF. For instance, when the Mg/SF is 3:1 Mg2Si peaks are very prominent, whereas at 2:1

ratio, they are very small. As a result, the intensity of Si peaks increases as the ratio decreases to

37

about 2.0. In order to find the quantity of phases, XRD quantification was conducted based on

spectra shown in Fig. 4.12 and the results are tabulated in Table 4.4. These results confirmed the

trend seen in XRD analysis.

It is expected that by increasing Mg content of the batch, excess Mg reacts with Si to produce

Mg2Si according to reaction (4.3) [Kalem, 2004].

Si(s) + 2Mg(g) = Mg2Si(s) (4.3)

0

5

10

15

20

25

30

35

40

1 1.5 2 2.5 3 3.5

Mg 2

Si

MG/SF Ratio

0

10

20

30

40

50

60

70

80

90

1 1.5 2 2.5 3 3.5

MgO

MG/SF Ratio

0

5

10

15

20

25

1 1.5 2 2.5 3 3.5

Mg 2

SiO

4

MG/SF Ratio Fig. 4.13: Effect of Mg/SF ratio on the formation of Mg2Si, MgO and Mg2SiO4 compounds in

the reduction products at 750°C. Reduction time was 2 hours.

38

It is known that Mg2Si causes violent reactions upon contact with HCl [Dvorina et al., 1968].

Because of this, the presence of Mg2Si is undesirable. Therefore, it is important to minimize the

formation of Mg2Si in the reduction products, to decrease the Si loss. Fig. 4.13 shows the

relationship between formations of MgO, Mg2Si and Mg2SiO4 with Mg/SF ratios, plotted from

Table 4.4. Fig. 4.13 indicates that 2:1 ratio of Mg/SF produced the least amount of Mg2Si.

Table 4.4 Experimental variable (ratio of Mg/SF) for reduction and quantification of XRD.

Reduction Conditions XRD Quantification Time (Hr) Temp (°C) Ratio of

Mg/SF Si MgO Mg2Si α-SiO2 SiO2-

cristobaliteMg2SiO4

2 750 1.5:1 16.90 67.05 6.40 0.46 - 9.16 2 750 2:1 16.08 79.07 4.86 - - - 2 750 2.5:1 10.70 64.64 21.47 - - 3.19 2 750 3:1 5.56 59.62 34.56 0.26 - -

4.3.3.2 Effect of Temperature on Formation of Mg Compounds

Fig. 4.14 shows the effect of reduction temperature on the formation of Mg compounds keeping

Mg/SF ratio and time constant. According to the intensity of XRD spectra, at higher

temperatures of 850°C and 950°C, peak of Mg2Si (•) decreased. However, at temperature of

950°C Mg2SiO4 peaks (■) appeared. In order to find the quantity of these phases, XRD

quantification was conducted.

10 20 30 40 50 60 70 80 90 100

Mg/SF Ratio

950°C 

850°C 

750°C 

2 theta (degrees)

Coun

ts/s

◊ Si•Mg2SiMg2SiO4 ◊

••

MgO

MgO

MgO

MgO

MgOMgO

◊

•

•

•

•

◊

◊

◊

◊

◊

MgO

MgO

MgO

◊

◊

◊

◊ MgO

MgO

MgO

• MgO

MgO

MgO

Fig. 4.14: XRD pattern of SF after experiments with different reaction temperatures (Mg/SF

Ratio = 2:1 for 2 hours).

39

The results are tabulated in Table 4.5 and also plotted in Figs. 4.15-17. As described previously,

the formation of Mg2Si at 750°C significantly increased at higher Mg/SF ratio. Comparing the

results of 750°C with those of 850°C and 950°C (Table 4.5), for example at Mg/SF ratio 3:1 the

amount of Mg2Si decreased from 35% at 750°C to 11% at 950°C. And at Mg/SF ratio 2.5:1 the

amount of Mg2Si decreased from 20% at 750°C to 8% at 950°C. But at 750°C with Mg/SF ratio

2:1 the amount of Mg2Si is 5% which is much lower than those of higher Mg/SF ratios (high

temperature will decrease Mg2Si content).

Table 4.5 Experimental variable (temperature) for reduction and quantification of XRD.

Reduction Conditions XRD Quantification Time (Hr)

Temp (°C)

Ratio of Mg/SF

Si MgO Mg2Si α-SiO2 SiO2-cristobalite

Mg2SiO4

2 750 1.5:1 16.90 67.05 6.40 0.46 - 9.16 2 850 1.5:1 14.97 64.57 4.18 - - 16.33 2 950 1.5:1 14.53 62.92 3.07 - 4.93 14.55 2 750 2:1 16.08 79.07 4.86 - - - 2 850 2:1 16.25 71.92 2.91 - - 8.92 2 950 2:1 13.92 67.12 3.04 - 6.01 9.90 2 750 2.5:1 10.70 64.64 21.47 - - 3.19 2 850 2.5:1 10.84 70.32 13.68 1.59 3.46 2 950 2.5:1 13.01 69.47 8.24 - 3.25 6.03 2 750 3:1 5.56 59.62 34.56 0.26 - - 2 850 3:1 9.0 68.86 20.97 - 0.82 2.35 2 950 3:1 11.74 73.15 10.88 - - -

Fig. 4.17 and Table 4.5 show that the amount of Mg2SiO4 is decreased when the reduction

temperature is lowered. In particular, amount of Mg2SiO4 was at minimum at 750°C with Mg/SF

= 2:1 ratio. At higher temperature the following reactions [Wynnyckuj and Rao, 1976], may be

favoured resulting in increased formation of Mg2SiO4.

3SiO2(s) + 2Mg(g) = Mg2SiO4(s) + 2SiO(g) (4.4)

4SiO(g) + 2Mg(g) = Mg2SiO4(s) + 3Si(s) (4.5)

40

0

10

20

30

40

50

60

70

80

90

1 1.5 2 2.5 3 3.5

MG/SF

MgO

MgO_750

MgO_850

MgO_950

Ratio

750°C

850°C

950°C

Fig. 4.15: Effect of Mg/SF ratio on the formation of MgO based on quantification of XRD for

different reduction temperatures. Reduction time was 2 hours.

0

5

10

15

20

25

30

35

40

1 1.5 2 2.5 3 3.5

MG/SF

Mg2Si

Mg2Si_750

Mg2Si_850

Mg2Si_950

Mg 2Si

750°C

850°C

950°C

Ratio

Fig. 4.16: Effect of Mg/SF ratio on Mg2Si formation in the reduction products with different

reaction temperatures. Reduction time was 2 hours.

41

0

5

10

15

20

25

1 1.5 2 2.5 3 3.5

MG/SF

Mg2

SiO

4

Mg2SiO4_750

Mg2SiO4_850

Mg2SiO4_950

750°C

850°C

950°C

Ratio

Mg 2SiO4

Fig. 4.17: Effect of Mg/SF ratio on Mg2SiO4 formation during reduction at different reaction

temperatures. Reduction time was 2 hours.

4.3.3.3 Effect of Hold Time on the Formation of Mg Compounds

The effects of hold time were also investigated for the samples with Mg/SF ratio of 2:1. Fig. 4.18

shows the summarized results of XRD diffraction patterns of reduction products at 750°C at

several reaction times. As can be seen, the formation of Mg2Si, Mg2SiO4 and MgO are affected

by the holding time. Quantification of phases was carried out from the XRD spectrum and the

results are presented in Table 4.6. In the case of Mg2Si phase, when holding time was increased

from 0.5 to 1 hr and further to 2 hrs the amount decreased respectively from 14% to 11% and

further reduced to 5%. When time increased beyond 2 hrs the formation of Mg2Si also increased.

The same trend was observed for Mg2SiO4 (Table 4.6).

On the other hand, the amount of MgO and silicon peaked at 2 hrs. Mitchell et al. (1998) used

sol-gel process to produce 2MgO.SiO2 that was then heated up to 1200°C. Crystallization started

and formed Mg2SiO4 at temperature as low as 770°C. Thus, in the present experiment at

temperature of 750°C, it may be possible that MgO and remaining SiO2 reacted to form Mg2SiO4

with time as per following equation:

2MgO + SiO2 = Mg2SiO4 (4.6)

42

10 20 30 40 50 60 70 80 90 100

2 theta (degrees)

Coun

ts/s

3 hrs

2 hrs

1 hr

4hrs

◊ Si

•Mg2Si◊

•• ◊ MgO

MgO

MgO

MgO

MgO

MgO

0.5 hr 

MgO

MgO

MgO

MgO

MgO

◊

◊

◊

◊

◊

◊

◊

◊

•

•

•

•

•

•

•

•

MgO

MgO

MgO

MgO

MgO

◊

◊

◊

◊

◊

◊

Fig. 4.18: XRD pattern of SF after reduction with different reaction times at 750°C with Mg/SF

Ratio = 2:1.

It is very likely that the sudden increase in MgO and decrease in the Mg2Si and Mg2SiO4 is due

to experimental scatter. As the trend lines show (Fig. 4.19), the formation of these phases is

plateaued at ~ 2 hours. Since the above reaction is between two solids, it may be controlled by

diffusion, which suggests time can play a critical role on the amount of the phases formed.

However, after a certain time, the reaction is stopped due to either formation of a dense shell of

the Mg2SiO4 around SiO2 particles, or depletion of one of the reactants. As seen in Fig. 4.19, this

reaction stoppage time appears to be around 2 hours.

The amount of Mg2Si is stable around 11-14%, except the single deviation at 2 hours. The

constant amount of this phase could be a result of production of Si (by Mg2Si as reductant) and

consumption of Si (by forming Mg2Si) at equal rates.

Mg2Si + SiO2 = 2MgO + 2Si (4.7)

Si + 2Mg = Mg2Si (4.8)

43

0

20

40

60

80

0 1 2 3 4 5

% M

g C

ompo

unds

Time (hrs)

Mg2SiO4

Mg2Si

MgO

Fig. 4.19: Effect of time on Mg-compound formation during reduction at 750°C with Mg/SF

Ratio = 2:1.

Table 4.6 Experimental variable (time) for reduction and quantification of XRD.

Reduction Conditions XRD Quantification Time (Hr) Temp (°C) Ratio (Mg/SF) Si Si Yield MgO Mg2Si α-SiO2 Mg2SiO4

0.5 750 2:1 11.7 26.11 60.5 13.7 7 7 1 750 2:1 13.8 30.8 66.78 11.37 - 8.05 2 750 2:1 16.08 35.89 79.07 4.86 - -

3 750 2:1 13.37 29.8 68.06 12.2 - 6.27 4 750 2:1 13.99 31.23 64.91 13.08 - 8.08

4.3.4 Effect of Mg/SF Ratio, Temperature and Time on Silicon Yield

In this section, the silicon yield was evaluated, since the main purpose of the reduction

experiments is to achieve the highest amount of Si. Figs. 4.20 and 4.21 show Si yield as a

function of Mg/SF ratio and temperature respectively. As seen, the maximum Si yield is

achieved at Mg/SF ratios of 1.5-2.0. This is primarily because of Si loss at higher Mg amounts

due to formation of Mg2Si. At this range of Mg/SF, optimum reduction temperature appears to

be 750-850°C.

44

 

0

10

20

30

40

50

60

70

80

90

1 1.5 2 2.5 3 3.5

MG/SF Ratio

Si Y

ield

(%)

Si_Y_750

Si_Y_850

Si_Y_950

750°C

850°C

950°C

Fig. 4.20: Effect of ratio of Mg:SF on silicon yield from SF with different reduction

temperatures based on quantification of XRD. Reduction time was 2 hours.

0

10

20

30

40

50

60

70

80

90

700 750 800 850 900 950 1000

Silic

on yi

eld

(%)

Temperature (°C)

Mg/SF ratio = 1.5:1

Mg/SF ratio = 2:1

Mg/SF ratio = 2.5:1

Mg/SF ratio = 3:1

Fig. 4.21: Effect of temperature on silicon yield with different Mg/SF ratios based on

quantification of XRD. Reduction time was 2 hours.

The effect of hold time on Si yield is shown in Fig. 4.22. Within the experimental conditions

investigated, the highest Si yield was obtained at 750°C with Mg:SF = 2:1 ratio for 2 hours

reduction.

45

0

10

20

30

40

50

60

70

80

90

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Time (hrs)

Si Y

ield

(%)

Fig. 4.22: Effect of time on silicon yield from SF based on quantification of XRD. Reduction

temperature was 750°C and Mg:SF = 2:1.

4.4 Leaching and Purification of Reduction Products

4.4.1 Particle Size of Reduction Products All leaching and purification experiments in this study were carried out on crushed reduction

products obtained from 750°C for 2 hrs and Mg/SF = 2:1. Since particle size has a significant

effect on dissolution properties of a material [Raschman and Fedoročková, 2006], using agate

mortar and pestle, reduction products were crushed and ground. Result of particle size analysis is

shown in Fig. 4.23. The particle size of reduction products varies from 0.25-350 µm and the

average particle size is around 66 μm.

Particle Diameter (µm.)

( )

0

10

2

3

4

5

6

7

8

9

0.01 0.1 1.0 10.0 100.0 1000.0

Volu

me(

%)

Fig. 4.23: Particle size distribution of reduction products used for leaching. Average particle size

is 66 μm.

46

4.4.2 Leaching of MgO and Mg2Si

Both MgO and Mg2Si react with HCl solution. Magnesium silicide (Mg2Si) reacts with HCl

solution and produces silane gas [Nandi et al., 1993]. Silane gas is reactive and at the presence of

O2 in air, spontaneously combusts. On the other hand, MgO is readily dissolved in dilute HCl

and remains in the solution as MgCl2 [Raschman and Fedoročková, 2004].

The effect of concentration of HCl on the dissolution of a mixture of magnesium compounds was

investigated and the results are shown in Fig. 4.24. The results showed that the highest

dissolution of magnesium compounds was observed with 2M HCl solution followed by 1M and

4M HCl. The decrease in the rate of transfer of the magnesium to the solution observed with 4M

HCl concentration could be because of two effects (a) insufficient availability of hydrolyzed

water to dissolve Mg compounds and (b) a quick reaction in the beginning that cause formation

of an inhibiting Mg-saturated layer around the particles.

0.2

0.4

0.6

0.8

1

0 20 40 60 80Time (min)

Frac

tion

of M

g C

omp.

Dis

solv

ed

1M 2M4M

Fig. 4.24: Dissolution of magnesium compounds with hydrochloric acid. Leaching temperature

was 60°C.

The dissolution of the magnesium oxide is sensitive to leaching temperature [Raschman and

Fedoročková, 2004]. For this reason, the effect of temperature on the dissolution of magnesium

compounds was also studied in the present work. The results of leaching carried out with 2M

47

HCl are shown in Fig. 4.25. These results also confirmed that temperature is an effective

parameter. For example as the temperature increased from 30 to 60°C, dissolution of magnesium

compound increased from 83% to 94%.

0.00

0.20

0.40

0.60

0.80

1.00

0 10 20 30 40 50 60 70

30°C40°C50°C60°C

Frac

tion

of M

g C

omp.

Diss

olve

d

Time (min)

Fig. 4.25: Effect of leaching temperature on dissolution  of  magnesium  compounds  (Acid

concentration: 2M HCl).

0

400

800

1200

1600

10 20 30 40 50 60 70 80 90 100

2 theta

Cou

nts/

s

◊

• Mg2SiO4◊

◊

◊

◊◊ ◊ ◊

• •

Si

degrees2θ (degrees)

Cou

nts/

s

Fig. 4.26: XRD pattern after initial leaching with HCl at 60°C for 1 hr (Reduction products of

Mg/SF = 2:1 at 750°C).

48

XRD has been carried out to find out if the dissolution of magnesium compounds especially

MgO and Mg2Si was completed. Fig. 4.26 shows XRD pattern of leached product with 2M HCl

at 60°C for 1 hr. It was found that MgO and Mg2Si peaks were not observed, suggesting the

complete dissolution of these Mg compounds. However, some minor peaks of Mg2SiO4 appeared

with major peaks of silicon.

4.4.3 Dissolution Mechanism of MgO Particle

4.4.3.1 Theoretical Consideration Chemical dissolution of MgO in hydrochloric acid is a liquid - solid reaction. A mass transfer

model could be applicable to the description of the dissolution behavior of the solid (Fig. 4.27).

In the first approximation, the particles of MgO were considered to be spheres. External mass

transfer can be applied for the overall dissolution process which involves development of a

mathematical model.

 Mg

Ci

Cb

Surf

ace

Concentration

Fig. 4.27: Mass transfer model for MgO particles dissolution in HCl.

The general equations for a mass transfer model for MgO particles dissolution can be described

as follows:

dwdt

= −k.A.(Cb −Ci) (4.9)

49

where w = weight of sample, t = time, A = surface area of sample, Cb = concentration of MgO at

bulk solution, Ci = concentration of MgO at the solid-liquid interface, k = mass transfer

coefficient. If both sides are divided by volume (v) of sample then Eq. (4.9) becomes:

)( ib CCv

kAdtv

dw

−−= (4.10)

where v

dw = bdC , Eq. (4.10) can be written as:

)( ibb CC

vkA

dtdC

−−= (4.11)

Rearranging both sides and integration generates the following equations:

tt

Ctib tv

kACCb 00,0

.)ln( −=−==

(4.12)

tv

kACCC iib .)ln()ln( −=−−−

(4.13)

ln (Ci −Cb )Ci

= −kAv

.t

(4.14)

tv

kACC

i

b .)1ln( −=−

(4.15)

)exp()1(v

kACC

i

b −=− (4.16)

Cb = Ci(1−β exp(− kAv

)) (4.17)

Under mass transfer control conditions, the chemical reaction is at equilibrium. i.e. Ci = Ceq.

Applying this boundary condition, expression of β is obtained as following:

@ t = 0 Cb = 0 β = -Ci = Ceq

50

It follows that

Cb −Ceq = −Ceq exp(− kAv

) (4.18)

From Eq. (4.18) concentration in the bulk solution can be obtained:

Cb = Ceq (1− exp(− kAv

)) (4.19)

Considering v

kAk −=* , Eq. (4.19) becomes

Cb = Ceq (1− exp(−k * .t)) (4.20)

Since the mass of Mg dissolved can be obtained as C*v, if both sides of the equation are

multiplied by the volume,

m = meq (1− exp(−k * .t)) (4.21)

Where eqm represents the amount of Mg compounds that can be dissolved at the solution, or the

mass of the dissolved species at the saturation limit.

mb = meq (1− exp(−k * .t)) (4.22)

Now, if om (the initial mass of soluble Mg compounds) is subtracted from both sides, and the

product is divided by om :

mb −mo

mo =meq − mo

mo (1− exp(−k * .t)) (4.23)

Let o

o

o mmm

mmx b −=

Δ= being the fraction of Mg compounds dissolved,

x = x*(1− exp(−k * .t)) (4.24)

where *x is a constant, equal to the maximum fraction of Mg compound that can be dissolved.

51

If the experimental results are fitted to the above expression, they should yield k*, as the

apparent mass transfer coefficient. However, in this equation, k* may not be constant, since it is

a product of k (mass transfer coefficient) and A/v. The latter term is not a constant since it

contains the surface area of the particles, which is changing as the particles are shrunk due to

dissolution. Therefore, a correction is required to account for the change in the surface area

during leaching.

4.4.3.2 Correction for Area Change Assume N particles of Mg compounds in the leaching process, all of the same radius r. If surface

area at time t is divided by original specific surface area of the sample then:

At

A0

=N.πrt

2

N.πr02 = ( rt

r0

)2 (4.25)

where A0 = original surface area of particles before leaching, At = surface area of particles after

certain leaching time (t), r0 = original radius of the solid particle, rt = radius of the solid particle

after certain leaching time (t).

Mass of the sample at time t could be related to the number of the particles as follows, with ρ

being the density of the particles.

34/3.. tt rNm πρ= (4.26)

Also the relationship in terms of radius can be obtained for mass dissolved (Δm) with respect to

original mass of the sample in terms of following equation:

3

03

0

330 )(1

rr

rrr

mm tt −=

−=

Δo

(4.27)

where Δm is the mass dissolved at time t. After simplification the following equation can be

written with respect to ratio of radius of particles:

3/1

00

]1[mm

rrt Δ

−= (4.28)

52

Then the final relationship with ratio of surface area is obtained as follows:

3/23/2

0

2

00

)1(]1[)( xmm

rr

AA tt −=

Δ−== (4.29)

As considered previously forvAkk .* = , it can be multiplied and divided by 0A to find another

relationship of k*:

0

0

0

0* ......

AA

vAk

AvAAk

vAkk === (4.30)

Where vAk 0. is a constant and may be considered as k′, the “true” mass transfer coefficient. Thus

Eq. (4.24) is modified to

)).'.exp(1( 0* t

AAkxx −−= (4.31)

2 M HCl Solution

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40t*A/A0   (min)

X, Fraction Disso

lved

60 C ‐ Calc50 C ‐ Calc40 C ‐ Calc30 C ‐ Calc60 C ‐ Meas.50 C ‐ Meas.40 C ‐ Meas.30 C ‐ Meas.

Fig. 4.28: Mass transfer model fitted by experiments to analyse the dissolution behaviour of

MgO during leaching with HCl.

One expects that if mass transfer is the rate limiting step, the above equation is followed. This

was attempted by using Excel Solver Optimizer as explained below. The fraction of solids

dissolved (x) was obtained from the mass change and the initial mass. A/A0 was calculated for

each dissolution time, using Eq. (4.26). Then, k′ was adjusted so that the sum of the square of the

53

differences (errors) between the calculated (Eq. 4.28) and the measured x values was minimized.

The results of such analysis are shown in Fig. 4.28. As seen, the model predictions fit those of

experimental measurements well, suggesting that mass transfer is likely to be the rate limiting

step under the examined experimental conditions.

A plot of ln k′ (min–1) versus 103/T (K-1) for various solutions in Fig. 4.29 indicates that an

Arrhenius type relationship holds. From the slope of the straight lines, the activation energy of

the process could be calculated.

‐2.00

‐1.75

‐1.50

‐1.25

‐1.00

‐0.75

‐0.50

2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35103/T (K‐1)

ln (K

*, m

in‐1)

1M

2M

4M

Fig. 4.29: Temperature dependence of the rate constant.

The apparent activation energy is listed in Table 4.7. As seen, the values are within the range of

13-15 kJ/mol. The relatively low activation energy is another evidence that the reaction is rate-

limited by a mass transfer process rather than a chemical reaction.

Table 4.7: The apparent activation energy for different acid concentration on the dissolution rate

of Mg compound (Leaching conditions: HCl = 1M, 2M and 4M).

HCl Concentration Activation Energy (kJ/mol) 1M 13.4 2M 15.3 4M 15.1

54

4.4.4 Removal of Mg2SiO4 As previously shown in Fig. 4.26, it was found that Mg2SiO4 remained after leaching with only

HCl at 60°C for 1 hr. An attempt was made to dissolve Mg2SiO4 leaching with a mixture of 2M

HCl and 25% acetic acid. Fig. 4.30 shows XRD spectra of leached product. However, Mg2SiO4

persisted after this treatment.

0200400600800

1000120014001600

10 20 30 40 50 60 70 80 90 100

2 theta (degrees)

Cou

nts/

s

◊

• Mg2SiO4

◊

◊

◊

◊ ◊ ◊◊• • •

Si

Fig. 4.30: XRD pattern of leached product after leaching with HCl and acetic acid at 60°C for 1

hr (Reduction products of Mg/SF = 2:1).

4.4.5 Hydrofluoric Acid Purification

For further purification of previous residue of HCl and acetic acid treatment, it was treated with a

mixture of diluted 4.8% hydrofluoric acid (HF) and 25% acetic acid (1 to 9 volume ratio) at 60°C

for 1 hr to remove silica and remaining Mg2SiO4. XRD spectrum of this leached product as

shown in Fig. 4.31 exhibited reduced minor peaks of Mg2SiO4 after this leaching treatment.

It was noted in the previous investigation [Larbi, 2010] that addition of acetic acid promoted

dissolution of MgO most likely due to the wetting effect provided by acetic acid. The hydrogen

atom in the −COOH in acetic acid (weak) can be given off as an H+ ion (proton) for acidic

nature. This portion known as hydrophilic molecule is typically charge-polarized and capable of

hydrogen bonding, enabling it to dissolve more readily in water. Hydrophilic molecules are also

55

known as polar molecules [Acetic Acid, 2010]. This nature may serve as wetting agent. For this

reason, in order to dissolve Mg2SiO4 and untreated silica, acetic acid was also combined with

hydrofluoric acid.

0

400

800

1200

1600

10 20 30 40 50 60 70 80 90 100

2 theta

Cou

nts/

s◊

• Mg2SiO4◊

◊

◊

◊◊ ◊ ◊

• •

Si

degrees2 theta (degrees)

Cou

nts/

s

Fig. 4.31: XRD pattern of leached product after leaching with HF and acetic acid at 60°C for 1

hr (Reduction products of Mg/SF = 2:1).

0

200

400

600

800

1000

1200

1400

1600

1800

2000

10 30 50 70 90

2 theta

Cou

nts/

s

Si

Si

Si

Si Si SiSi

2 theta (degrees)

Cou

nts/

s

Fig. 4.32: XRD pattern of silicon powder after purifying with hydrofluoric acid and acetic acid

at 70°C for 1 hr. (Reduction products of Mg/SF = 2:1)

For this reason, further purification has been done of the leached product with the same mixture

at 70°C for 1 hr to remove remaining Mg2SiO4. Fig. 4.32 represents XRD spectrum of the final

leached product. XRD pattern shows almost a complete removal of Mg2SiO4 and only Si peaks

56

remaining after the final leaching. This indicates that HF acid leaching has successfully removed

the remaining silicates.

The final silicon powder product after several stages of leaching is shown in Fig. 4.33. Particle

size analysis of silicon powder product can be seen in Fig. 4.34. The particle size of silicon

powder product varies from 0.2-250 µm and the average particle size is around 56 μm.

Fig. 4.33: Silicon powder product after purification with hydrofluoric acid and acetic acid at

70°C for 1 hr.

Particle Diameter (µm.)

0

10

4

0.01 0.1 1.0 10.0 100.0 1000.0

Volu

me(

%)

Fig. 4.34: Particle size distribution (PSD) of silicon powder product. Average particle size: 56

μm.

500 μm

50μm

(a) (b)

Fig. 4.35: Scanning electron micrograph of purified silicon powder obtained by Mg reduction

and leaching of reduction products.

57

Fig. 4.35 shows a representative scanning electron micrograph of the fine powder of purified

silicon. Table 4.8 presents the ICP analysis of a typical silicon sample obtained after acid

purification and indicates the impurity levels. The presence of boron of the order of 27 ppm and

phosphorus 12 ppm are in the purified silicon as per outside laboratory test. On the other hand,

in-house ICP-MS shows boron and phosphorus level below detection limit. Overall, the silicon

products were 99.1% pure (in-house). Table 4.8 Impurity level in silicon powder.

Impurity Elements

ICP Analysis at U of T (ppm)

ICP Analysis by Commercial laboratory (ppm)

B <3 27 P <100 12

Ca 1041 1018 Fe 1203 602 Al 2255 4533 K 1420 3551

Mg 3637 6125 Mn 150 43 Zn 67 14.2 Ni 40 13.8 As 1 0.4 Ba 6 13.2 Cr 9 9.6 Mo 10 1.2 V 1 3

Cd 3 0.08 Purity of Si 99.1% 98.40%

Many impurities particularly boron and phosphorus have detrimental effects on the performance

of solar grade silicon when exist in silicon at concentrations above a certain threshold level.

Therefore, it is essential to keep those impurities level as low as possible. However, several types

of impurities are intentionally or unintentionally introduced in silicon for solar cell performance.

To obtain p- and n-type doped bulk silicon materials, boron and phosphorus are commonly used

as dopant impurities [Shi et al., 1996; Pivac et al., 1996]. Doping introduces an atom of another

element (either three or five valence electrons, as opposed to silicon's four) into the silicon

crystal to alter its electrical properties.

58

A comparison of the impurity contents for SF, SoG-Si and MGS are shown in Tables 4.9 and

4.10. This shows that impurity level of P, K and Ca are reduced considerably compared to

starting SF. Impurity level of many elements are significantly lower than those of MGS. Level of

some impurities seems to be increased in the silicon powder. This may come from Mg powder

(98% purity) used in reduction.

Table 4.9 Comparison of impurity level in silicon powder with original silica fume and leached

and calcined silica fume.

Elements As received Silica Fume (ppm)

Leached and Calcined Silica Fume (ppm)

Silicon Powder (ppm)

B <2 31 <3 P 294 125 <100

Ca 2527 575 1041 Fe 1232 984 1203 Al 2153 1431 2255 K 7265 63 1420

Mg 3004 222 3637 Mn 108 0 150 Zn 65 0 67 Ni 65 - 40 As 12 - 1 Ba 18 - 6 Cr 2 - 9 Mo 1 - 10 V <1 - 1

Cd <0.2 - 3 Purity of Si 99.1%

Fig. 4.36 shows correlation of solid solubility and segregation coefficient for different impurities

in silicon [Dietl, 1983]. Open circle is maximum solubility and the closed circle shows solubility

at 1410°C. Impurity level in the silicon powder produced in the present study was superimposed

in this figure with closed square symbols. It shows that for some of the most deleterious

impurities such as As, P, and B, the concentration is well below their solubility in Si. For other

impurities such as Mg, Al, Fe, etc, the concentrations are much above the solubility limits. This

59

indicates that these impurities most likely exist along the grain boundaries or among the silicon

grains. Therefore, these impurity elements can be removed by other means of metallurgical

refining processes. Also, their content can be readily reduced by a solidification refining process

such as Floating Zone or Czochralski technique. For example, as can be seen in Fig. 4.36

majority of impurities in silicon produced in this investigation are above the maximum solubility

limits in silicon. When this silicon is melted and solidified uni-directionally impurities in the

silicon will be segregated to front of the solid-liquid interface, thereby, upgrading the purity of

bulk silicon. Other upgrading methods include electron beam, slag treatment, vacuum refining

argon refining or solvent refining [Hazanawa et al., 2001; Yuge et al., 1994; Pires et al., 2005;

Ryningen, 2010].

Table 4.10 Comparison of impurity level in silicon powder with MGS.

Elements Silicon Powder (ppm)

MGS (ppm) (Gribov and Zinov’ev, 2003)

B <3 50 P <100 40

Ca 1041 2200 Fe 1203 6000 Al 2255 4000 K 1420 -

Mg 3637 400 Mn 150 400 Zn 67 - Ni 40 110 As 1 - Ba 6 - Cr 9 300 Mo 10 - V 1 250

Cd 3 - Purity of Si 99.1% 98.63%

60

1ppm

10 ppm

0.1%

0.02%

0.1%

1%

0.5%

Ti

V

MgCr

Ag

Bi

Fe

Co

Zn

Au

Cu

Sb

Mn

Sn

AlGa

Li

Ca

PAs

B

Ag

MnFe

CoZn

Au

Cu

Bi

Al Ga

Sb

Sn

Li

As

P

V

0.01%

As

P

Cr

Zn

Mn

Al

Ca

Mg

Fe

Segregation Coefficient log K0- 6 - 5 - 4 - 3 - 2 - 1 0

1021

1020

1019

1018

1017

1016

1015

Ato

ms/

cm3

Solid Solubility in Si

•B

Fig. 4.36: Solid solubility in Si against segregation coefficient for various elements. Impurity

level in the silicon powder produced in this study (■), maximum solubility (○) and,

solubility at 1410°C (●) [Dietl, 1983].

61

CHAPTER 5: SUMMARY AND CONCLUSION

1. Silica fume was characterized using X-ray diffraction, surface area, particle size, scanning

electron microscopy and chemical analysis (ICP).

2. Magnesiothermic reduction was successfully utilized to convert silica fume to metallic

silicon, confirmed by XRD, Reitveld treatment of the XRD (QXRD), SEM and ICP studies.

3. Mg/SF ratio, temperature and time affected the conversion of Si from silica fume with the

formation of intermediate compounds Mg2Si and Mg2SiO4.

4. Maximum silicon yield was achieved at temperatures of 750-850°C at Mg/SF ratio of 1.5-

2.0.

5. Reduction products, metallic and other impurities were treated successfully with multistep

leaching using HCl, CH3COOH and hydrofluoric (HF) acid.

6. The leaching kinetics showed good agreement with a modified mass transfer model,

indicating that transport of the dissolved species to the bulk of the solution is the rate

limiting step.

7. Activation energy of the dissolution is within 13-15 kJ/mol, which is consistent with mass

transfer being the rate limiting step.

8. The chemical composition of the silicon product was determined by wet chemical analysis

(ICP) and found to be 99.1% purity with low boron, phosphorus and arsenic contents

compared to metallurgical grade silicon.

9. Silicon obtained from silica fume could be an attractive feedstock for production of solar

grade silicon.

62

CHAPTER 6: FUTURE WORK

The present work has been focused on magnesiothermic reduction of silica fume to metallic

silicon using simple laboratory facilities. The following recommendations are suggested for any

further research in this area:

(i) Further purification and refining of silicon product can be done with other metallurgical

routes like directional solidification (floating zone).

(ii) One of the major challenges in mass production due to exothermic nature of magnesium

which may break the reaction tube or vessel. Addition of heat sink such as MgO to the

reactants should be practiced and the effect on the extent of the reactions be studied.

(iii) This research tried successfully only to explore the possibility of converting silica fume

to silicon using magnesiothermic reduction. So economic evaluation of this process

maybe focused.

63

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