Silicon Synthesis

Synthesis of High Purity Silicon from Rice Husks

by

Kingsley Kweku Larbi

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Materials Science and Engineering University of Toronto

© Copyright by Kingsley Kweku Larbi 2010

Synthesis of High Purity Silicon from Rice Husks Kingsley Kweku Larbi Master of Applied Science Department of Materials Science and Engineering University of Toronto

2010

Abstract Impurity optimized silicon is needed for the advancement of terrestrial photovoltaic power generation. In this study the possibility of producing solar grade silicon from rice husks has been pursued. An integrated process flowsheet was developed and practiced that included initial leaching, reduction of Rice husk ash (RHA) and post-reduction purification of silicon. Metallothermic reduction of purified RHA with magnesium was investigated within the temperature range of 500-950 oC. The reduction product was purified by two stage acid leaching sequence. Analysis of the final silicon powder product by XRD and ICP-OES showed crystalline silicon with boron content to be less than 3ppm- corresponding to reduction by a factor greater than 10, whilst the phosphorus content was reduced by a factor of over 20 and reaching less than 73ppm. The effects of temperature, magnesium amount and leaching agents optimized in this study. A one step test melting was also carried out to convert the silicon powder into silicon chunks.

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Acknowledgments With much gratitude I wish to thank several people and funding institutions who contributed in no small ways to make this research project successful. Firstly, I sincerely thank Professor Mansoor Barati and Professor Alex McLean for giving me the opportunity to work on this project and under their joint supervision throughout my study period. Secondly, I sincerely thank Dr. Hiroshi Sodha for constantly offering me his time and expert advice on this project. A sincere gratitude is extended to members of my research group – Sustainable Materials Processing Research Group, for their support and constructive criticism on this project. I wish to thank the MSE department as well for their timely technical and administrative support throughout the course of this project I wish to acknowledge the following individuals for providing me with training and technical guidance on the analytical techniques presented in this work; George Kretschmann of Geology Dept. (XRD), Dr. Srebri Petrov of Chemistry Dept.,(QXRD), Dr. Dan Mathers of Chemistry Dept.,(ICP), Sal Boccia of MSE Dept (SEM), Dr. Yindong Yang of MSE Dept. (Leco carbon analysis). I am also thankful for the financial support and technical guidance from staff and people of Process Research Ortech (PRO), especially Dr. Lakshmanan, Dr. Sridhar and Dr. Roy. I further express my gratitude to Ontario Centers of Excellence and University of Toronto Open Fellowships for their financial support. Finally, I wish to thank my friends and family, especially my wife Mrs. Josephine Larbi for her moral support and endurance of the late night hours I had to spend in the laboratory and away from her.

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Table of Contents

Abstract .......................................................................................................................................... ii  Acknowledgments ........................................................................................................................ iii  Table of Contents ......................................................................................................................... iv  List of Tables .............................................................................................................................. viii  List of Figures ............................................................................................................................... ix  List of Appendices ....................................................................................................................... xii  CHAPTER ONE: Introduction .............................................................................................. 1  CHAPTER TWO: Literature Survey .................................................................................... 3  2.1 Sources of Silicon Raw Materials ........................................................................................ 3  2.2 Characteristics of Amorphous Silica .................................................................................... 3  2.3. Properties of Silicon ............................................................................................................ 5  2.4 Impurities in Bulk Crystalline Silicon.................................................................................. 9  2.5 Existing Silicon Manufacturing Technologies ................................................................... 11  2.5.1 Production of Metallurgical-grade Silicon (MG-Si) ................................................ 12  2.5.2 Production of Electronic grade Silicon (EG-Si) ...................................................... 14  2.6 Silicon Purification Techniques ......................................................................................... 16  2.7 Summary of Literature Review .......................................................................................... 17  CHAPTER THREE: Synthesis of Silicon from Rice Husk Ash ........................................ 18  3.1 Characteristics and Availability of Rice Husks ................................................................. 18  3.2 Silicon from Rice Husk - A Review of Literature ............................................................. 19  3.3 Silicon from Rice Husks - Summary ................................................................................. 22  3.4 Thermodynamics of Metallothermic Reactions for RHA (SiO2) ....................................... 22  CHAPTER FOUR: Experimental Work ............................................................................. 24  iv

4.1 Materials and Reagents ...................................................................................................... 25  4.2 Sample Preparation – Rice Husk Ash ................................................................................ 26  4.2.1 Combustion of Rice Husks ...................................................................................... 26  4.2.2 Moisture Content Determination ............................................................................. 27  4.3 Characterization of RHA Sample....................................................................................... 28  4.3.1 Chemical Composition Analysis.............................................................................. 28  4.3.2 Particle Size Analysis .............................................................................................. 28  4.3.3 Surface Area Measurement ...................................................................................... 29  4.3.4 XRD Analysis .......................................................................................................... 30  4.3.5 Scanning Electron Microscopy (SEM) Studies ....................................................... 30  4.4. Purification Treatment of RHA ......................................................................................... 31  4.4.1 Leaching of RHA ..................................................................................................... 31  4.4.2 Roasting of Acid Leached Ash ................................................................................ 32  4.5 Selection of Reducing Agent for RHA (SiO2) ................................................................... 33  4.6 Pelletizing of Reactants ...................................................................................................... 34  4.6.1 Determination of Percent Compaction and Porosity ............................................... 36  4.7 Reduction Experiments ...................................................................................................... 36  4.7.1 FactSage™ Study ..................................................................................................... 36  4.7.2 Differential Thermal Analysis DTA ........................................................................ 37  4.7.3 Reduction in Argon Atmosphere ............................................................................. 37  4.8 Quantitative XRD Analysis of Reduction Products ........................................................... 39  4.9 Post-Reduction Purification and Analyses ......................................................................... 40  4.9.1 Leaching of Reduction Products-Kinetic Study ...................................................... 40  4.9.2 Leaching of Reduction Products .............................................................................. 41  4.9.3 Sample Digestions for ICP-OES Analysis ............................................................... 43  4.10 Test Melting of Silicon Powders ...................................................................................... 44  v

CHAPTER FIVE: Results and Discussion .......................................................................... 46  5.1 Optical Images of as-received Rice Husks Material .......................................................... 46  5.2 Characterization of As-received RHA ............................................................................... 47  5.2.1 Chemical Composition............................................................................................. 47  5.2.2 Particle Size Analysis .............................................................................................. 49  5.2.3 XRD Analysis .......................................................................................................... 50  5.2.4 SEM Analysis of as-received RHA ......................................................................... 51  5.2.5 Surface Area Analysis.............................................................................................. 52  5.3. Purification of as-received RHA ....................................................................................... 54  5.3.1 Effect of Leaching Treatment .................................................................................. 55  5.3.2 Effect of Roasting Treatment ................................................................................... 56  5.3.3 Combined Effect of Purification Treatment ............................................................ 58  5.4. Pelletizing and Reduction of Reactants............................................................................. 61  5.4.1 Pellet Properties ....................................................................................................... 61  5.4.2 Characteristics of Reduction Product....................................................................... 63  5.4.3 Effect of Composition on Reduction Product .......................................................... 66  5.4.4 Effect of Temperature on Reduction Product .......................................................... 70  5.4.5 Reduction Mechanism ............................................................................................. 76  5.5 Post Reduction Purification................................................................................................ 79  5.5.1 Effect of Reagent Type on Dissolution .................................................................... 82  5.5.2 Effect of Temperature on Rate Controlling Mechanism.......................................... 85  5.5.3 Fitting of Leaching Data to Transport Control Model ............................................. 87  5.6 Characterization of as-produced Silicon ............................................................................ 94 

vi

CHAPTER SIX: Summary, Conclusions and Future Works .......................................... 103  6.1 Summary and Conclusions ....................................................................................... 103  6.2 Future Works ............................................................................................................ 105  References .............................................................................................................................. 106 

vii

List of Tables  

Table 2.1 Thermal and mechanical properties of silicon ................................................................ 9 Table 2.2 Impurities in metallurgical grade silicon ...................................................................... 13 Table 3.1 Typical proximate analysis of rice husks ...................................................................... 19 Table 3.2 Silica-metal reaction thermodynamic data* ................................................................. 23 Table 4.1 Summary of materials and reagents .............................................................................. 25 Table 5.1 Comparison of the main oxides present in as-received RHA ..................................... 47 Table 5.2 Comparison of elemental impurities present in as-received RHA............................... 48 Table 5. 3 Pellet making conditions .............................................................................................. 61 Table 5.4 Pellets properties ........................................................................................................... 62 Table 5.5 Effect of excess magnesium on phases in reduction product at 650⁰C ....................... 68 Table 5.6 Effect of temperature on phases in reduction product .................................................. 73 Table 5.7 Physico-chemical properties of R900 reduction sample............................................... 79 Table 5.8 Leaching reagent type and effective concentrations ..................................................... 81 Table 5.9 Leaching data at different temperatures...................................................................... 85 Table 5.10 Values of km and Ceq from excel solver ..................................................................... 92 Table 5.11 Particle size analysis of RH-Si.................................................................................... 98 Table 5.12 Chemical analysis of RH-Si ....................................................................................... 99 Table 5.13 Overall impurity reduction by process..................................................................... 100

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List of Figures Figure 2.1 Typical XRD pattern of amorphous silica from RHA................................................... 4 Figure 2.2 Diamond cubic lattice of crystalline silicon .................................................................. 5 Figure 2.3 Diamond cubic lattice of silicon with interatomic sites ................................................ 6 Figure 2.4 Energy band of crystalline silicon ................................................................................. 7 Figure 2.5 Free energy of formation of oxides of elements ............................................................ 8 Figure 2.6 Effect of metal atom impurities on p-type solar cells efficiency................................. 11 Figure 2.7 Schematics of metallurgical grade silicon production process.................................... 14 Figure 2.8 Schematics of (a) Siemens reactor, (b) Czochralski crystal puller .............................. 15  Figure 3.1 XRD pattern of Magnesium reduced RHA at 620 oC ……………………….....

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Figure 4.1 Scheme of experimental work……………………………………………………..... 24 Figure 4.2 Schematic of expanded Torbed reactor ...................................................................... 27 Figure 4.3 Leaching experiment set-up ........................................................................................ 32 Figure 4.4 Set-up for roasting of acid leached RHA .................................................................... 33 Figure 4.5 Si-O-Mg phase diagram at 650°C ............................................................................... 34 Figure 4.6 Set-up for pellet making .............................................................................................. 35 Figure 4.7 Schematic of reduction experimental set-up ............................................................... 38 Figure 4.8 Leaching kinetic study set-up ...................................................................................... 41 Figure 4.9 Schematic of leaching set-up ....................................................................................... 43 Figure 4.10 Test melting of as-produced silicon powder ............................................................. 45 Figure 5.1 Photographs of rice husks and rice husk ash (RHA).................................................. 46 Figure 5. 2 Particle size analysis of as-received RHA .................................................................. 50 Figure 5.3 XRD pattern of as-received RHA............................................................................... 51 Figure 5.4 SEM micrograph of as-received RHA ........................................................................ 52 Figure 5.5 BET isotherm plot for as-received RHA .................................................................... 53 Figure 5.6 BET linear calculation model plot for as-received RHA ............................................ 54 Figure 5.7 Effect of leaching treatment on impurity reduction in as-received RHA .................... 55 Figure 5.8 Plot of % carbon removal from leached RHA............................................................. 57 Figure 5.9 Comparison of metal oxides and carbon in treated/untreated RHA ............................ 58 Figure 5.10 Comparison of silica content in treated/untreated RHA............................................ 59 Figure 5. 11 SEM micrograph (1500X) of leached and roasted RHA.......................................... 60 ix

Figure 5.12 XRD pattern of leached and roasted RHA ................................................................ 60 Figure 5.13 Photograph of as-produced pelletized reactants ........................................................ 61 Figure 5.14 Plot of percent compaction against weight of pellet ................................................. 62 Figure 5.15 Photographs of typical reduction product.................................................................. 63 Figure 5.16 SEM micrographs of reduction product at 500X ...................................................... 64 Figure 5.17 EDX spectra of reduction product SEM micrograph in Figure 5.16 ......................... 65 Figure 5.18 FactSage™ calculation of effect of magnesium content on phases in reduction product at 650 °C and inert atmosphere ................................................................................ 66 Figure 5.19 XRD pattern comparing effect of % excess magnesium on phases formed.............. 67 Figure 5.20 Experimentally measured effect of magnesium content on phases in reduction product at 650 °C and in inert atmosphere............................................................................ 69 Figure 5.21 Effect of magnesium-silica mole ratio on Si yield from reduction product .............. 70 Figure 5.22 FactSage™ calculation of effect of temperature on Mg2Si formation ...................... 71 Figure 5.23 FactSage™ calculation of effect of temperature on Si yield ..................................... 71 Figure 5.24 XRD pattern comparing effect of temperature on phases formed ............................. 72 Figure 5.25 Experimentally measured effect of temperature on Mg2Si phase

formation in

reduction product under argon atmosphere........................................................................... 74 Figure 5.26 Experimentally measured effect of temperature on Si yield in reduction product under argon atmosphere ........................................................................................................ 75 Figure 5.27 DTA result of magnesium –RHA powder mixture (48 mg)...................................... 76 Figure 5.28 Temperature-time plot for magnesium–RHA under argon atmosphere .................... 77 Figure 5.29 Pellet condition before and after on-set of reaction in pellet..................................... 78 Figure 5.30 Effect of Reagent type 1 & II on dissolution ............................................................. 83 Figure 5.31 Effect of Reagent type III & IV on dissolution ......................................................... 83 Figure 5.32 Effect of [H+] concentration on fraction of Mg dissolved ......................................... 84 Figure 5.33 Plot of fraction dissolved against time ...................................................................... 87 Figure 5.34 Effect of temperature on dissolution of R900 sample ............................................... 91 Figure 5.35 Arrhenius plot for first stage leaching of reduction product ..................................... 93 Figure 5.36 As-produced silicon from rice husks ......................................................................... 95 Figure 5.37 Comparison of XRD patterns of as-produced Si to high purity Si standard ............. 96 Figure 5.38 SEM micrograph of RH- Si (taken at 500X) ............................................................. 97 Figure 5.39 Silicon chunks from melting of RH-Si .................................................................... 101 x

Figure 5.40 SEM micrograph of polished RH-Si chunk............................................................. 101 Figure5.41 EDX spectrum of black rectangle area of Figure 5.40 ............................................. 102 Figure 5.42 EDX spectrum of white rectangle area in Figure 5.40 ............................................ 102  Figure 6.1 Suggested integrated process for producing silicon from rice husks....................... 104

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List of Appendices APPENDIX A : Effect of Sequence of Purification Treatment on Purity of RHA .................... 111 APPENDIX B : Photograph of Reduction Experimental Set-up ............................................... 112 APPENDIX C : Typical Material Recovery Factors by Process Step ........................................ 113 APPENDIX D : Quantifiable Detection Limit of Elements by ICP-OES .................................. 114 APPENDIX E: Author’s Presentations and Publications ........................................................... 116 

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CHAPTER ONE: Introduction Worldwide concerns over energy related climate change coupled with spiraling cost of fossil fuels in recent years has increased interests in renewable energy alternatives. Although solar photovoltaic power generation is a proven technology, especially in the space industry, development of this technology to meet terrestrial energy demands has been limited. One of the issues that have limited the widespread use of solar photovoltaic power has been the high cost associated with the materials used in fabricating the solar cells. Currently silicon in both monocrystalline and polycrystalline form is the dominant semiconductor material used in the fabrication of most commercially available solar cells or modules, commanding over 90% of the market share of all PV technologies [1]. Metallurgical grade and electronic or semiconductor grade silicon are produced on commercial scale by the carbothermic and Siemens process respectively. Metallurgical grade silicon is typically 98-99.5% and contains high content of electro-active impurities such as B(5-70 ppm), P(5-100ppm), Al(3000-5000 ppm) and transition metals which are recognized to reduce the conversion efficiencies of p-n junction silicon solar cells and panels. Electronic grade silicon on the other hand which has total impurities less than one part per billion (< 1ppb) is too expensive for large scale production of solar cells [2]. For fabrication of efficient silicon solar cells it is generally recognized the B and P levels have to be reduced to less than 1ppm each and with an overall silicon feedstock purity in the range of 5-7N Owing to the lack of a dedicated process for producing solar grade silicon, manufacturers of crystalline silicon solar cells have been dependent on limited rejects from crystal growth and electronic silicon production plants as well as scrap silicon from the integrated circuit industries. With an industry-wide growth rate more than double that of the electronic industry, the traditional silicon supply sources for the PV industry is in constant short supply. The use of higher-grade feedstock material for solar cells ultimately increases the cost of PV modules, a trend that does not make this clean and renewable energy technology competitive with conventional electrical power production systems. 1

Much of the current research effort to produce low cost solar grade silicon has focused either on upgrading metallurgical grade silicon or by chemical vapor phase refining [3]. The vapor phase refining processes, which are essentially modifications of the classical Siemens process succeed in making the silicon highly pure but prohibitively expensive. Moreover the processes are complex, inherently energy intensive, predominantly batch processes and involve emission of corrosive gases such as chlorine [3]. The metallurgical process route has not been efficient in reducing problematic dopant impurities such as boron (B) and phosphorus (P) to the level generally accepted for solar cells fabrication hence this approach to solar silicon remains in the research phase. An alternative approach to synthesis of low cost solar grade silicon from plant biomass such as rice husk which has a significant source of high purity silica has only been explored by relatively few research groups. However, the narrow and isolated experimental conditions of previous works and lack of an optimized process have limited the engineering application of such results. The lack of a commercially viable route for synthesis of bulk high purity silicon using amorphous silica from rice husk therefore provides a good motivation to conduct further research into the feasibility of this approach. In this study, feasibility of synthesizing high purity silicon from rice husk ash is investigated. The objective of this research is to investigate the possibility of producing solar grade silicon from Rice Husk Ash (RHA) by: ¾ Developing an integrated process for production of silicon from RHA ¾ Identifying and optimizing conditions for each step of the process ¾ Studying the fundamentals involved in the major purification steps of the process .

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CHAPTER TWO: Literature Survey 2.1 Sources of Silicon Raw Materials Dietl [4], identified two main sources of silicon raw materials that are economically useful for the synthesis of low cost silicon for photovoltaic applications. The primary source of silicon is silicon dioxide which has the chemical formula of (SiO2) and commonly known as silica. Among the known polymorphs of silica, quartz and quartzite rocks are the most stable and relatively pure form that can be found in almost all mineralogical rocks [5]. Silicon precursors or volatile compounds of silicon represent the secondary source of silicon that is widely used in technological processing industries. Notably, the halides of silicon and flousilicic acids- a waste by-product of the phosphate fertilizer industry receive continuous attention for the synthesis of low cost silicon due to their ease of purification by fractional distillation [4]. Lately, the sources of silica and silicon in biomass resources such as rice husk are being researched intensively for several industrial applications [6]. Most biomass resources are waste by-products whose disposal cause environmental nuisance hence developing uses for these waste resources are in common agreement with the global paradigm shift towards sustainable development

2.2 Characteristics of Amorphous Silica The Dictionary of Composite Materials Technology [7] defines amorphous silica as a naturally occurring or synthetically produced oxide of silicon characterized by the absence of a pronounced crystalline structure and whose X-ray diffraction patterns have no sharp peaks. This type of silica may be anhydrous or have a significant water of hydration in its structure. The classification of amorphous silica into gel or powders largely depends on the bond strengths between the ultimate colloidal particles. Silica gels are characterized by a coherent three dimensional network of contiguous particles whilst silica powders consist of small granules of silica gel or aggregates of submicron particles that are weakly linked together [8].

3

The desirable physico-chemical and mechanical properties of amorphous silica, notably their high reactivity, excellent binding properties, relatively high purity and excellent mechanical strength make this material useful in a wide range of technologies. Although hosts of synthetic silica are produced commercially, the ones produced from plant origins such as rice husks have been noted to have some significant advantages over those from mineral and synthetic origins [9]. In particular, the processing steps are relatively simple and require no elaborate infrastructure or consumption of costly reagents as in the case of the synthetic processes. In addition the final silica powder produced from plant sources contains a narrow range of metal oxide impurities [9], which makes them exceptionally desirable in applications where high purity silica at modest cost is a necessary prerequisite. A typical X-ray diffraction pattern of amorphous silica produced from rice husk combustion is shown in Figure 2.1 [10]

Figure 2.1 Typical XRD pattern of amorphous silica from RHA [10]

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2.3. Properties of Silicon Silicon is known to be exceeded in natural abundance only by oxygen in the earth crust. It however does not occur freely in nature but in combination with oxygen to form oxides mainly silica and silicates [2]. Historically, the crystalline form of silicon was first prepared by Deville in 1854[11]. The peculiar properties of silicon were not however well understood or explained until the advent of quantum mechanics in the early 1920’s, which led to the widespread use of this material in the then burgeoning solid state electronic device industry [12]. Today silicon still remains the material of choice for the microelectronic, and more so the photovoltaic industry. Silicon is the second member of the Group IVA elements from the Periodic Table of elements. It has the electronic configuration of 1s22s22p63s23p2. Silicon at atmospheric pressure crystallizes into a diamond cubic lattice structure with each silicon atom forming bonds with four nearest neighbouring atoms as shown in Figure 2.2

Figure 2.2 Diamond cubic lattice of crystalline silicon [12]

5

The diamond cubic structure is known to be more open than most crystal systems and thus leave the crystalline silicon with five large interatomic voids in which interstitial atoms or impurities can be easily accommodated along the body diagonals of the unit cell (Figure 2.3).

Figure 2.3 Diamond cubic lattice of crystalline silicon with interatomic sites [12]

Crystalline silicon (c-Si) also has an indirect energy band structure, where the highest energy valence band state Ev and the lowest energy conduction band state Ec are not directly opposite each other and therefore lie at different crystal momentum as illustrated in Figure 2.4. Since the photogenerated carriers have to reach the p-n junction of a typical solar cell device for efficient conversion into useful energy, the diffusion length that minority carriers have to travel becomes longer in indirect energy band semiconductors such as silicon [13]. This property of crystalline silicon therefore places the well known stringent purity requirements and good degree of crystal perfection of bulk silicon material used for electronic device and solar cell applications. Thus these requirements are necessary to minimize or prevent where possible 6

recombination of photogenerated carriers before they reach the conduction band for efficient device utilization.

Figure 2.4 Energy band of crystalline silicon [14, 15]

Thermodynamically, silicon forms an oxide of intermediate stability as can be seen from the Ellingham diagram (Figure 2.5) shown below. A number of metallic oxides eg. (Ca, Li Mg, Al, Ba, Zn, Ti and all the rare earth elements) are more stable than silica (SiO2). The elements of the oxides whose curve lies below that of SiO2 should thermodynamically be feasible to reduce silica to silicon. However when cost, ease of product separation, environmental impacts among other factors are considered, the candidate reductants for silica reduces to a few. At higher temperatures carbon monoxide becomes more stable than silica and hence carbon becomes a powerful reductant. Table 2.1 shows some relevant thermal and mechanical properties of silicon. 7

1

Figure 2.5 Free energy of formation of oxides of elements [11]

1

The equation for B2O3 formation is not balanced and the line has been incorrectly placed in this Ellingham diagram. 8

Table 2. 1 Thermal and mechanical properties of Silicon[2] Property

Value

Atomic weight (g/mol)

28.085

Atomic density (atoms/cm3)

5.22 x 1022

Melting point (oC)

1410

Boiling point (oC)

2355

Density (g/cm3)

2.329

Heat of fusion (kJ/g)

1.8

Heat of vaporization at melting point (kJ/g)

16

Percent contraction at melting (%)

9.5

2.4 Impurities in Bulk Crystalline Silicon Impurities may be incorporated into bulk silicon material via two modes; (i) raw materials from which bulk silicon is produced and (ii) contaminations from in-situ processing or fabrication of the bulk silicon. The atomic impurities often discussed in the literature with respect to bulk silicon can be classified into; dopants, transition metals, precipitates, non-metals and other trace elements [12]. The dopants group of impurities in silicon mainly refer to elements of group IIIA (Boron, Aluminum, and Gallium) and also elements of the group VA (such as Phosphorus, Antimony, and Arsenic). The group IIIA elements substitute silicon atoms in the crystal lattice resulting in an electron deficient bonding which tends to be supplied by electrons from neighbouring silicon atoms. Accordingly when silicon material is intentionally or otherwise contaminated with group

9

IIIA elements the resulting semiconductor material is called a p-type semiconductor and the Group IIIA elements are termed as acceptor impurities. Group VA elements substitute silicon atoms in a silicon crystal lattice with excess electrons. The resulting silicon material is termed an n-type semiconductor and donor impurity for the substitute element. Boron and phosphorus represent typical dopant impurities in silicon and are the most problematic impurities in terms of their removal from silicon. Their presence in crystalline silicon tends to modify the semiconductor properties of silicon substantially and they are therefore undesirable impurities beyond specified concentrations. Likewise, the transition metals (mainly Ti, Fe, Cr, Ni, V, Co, Mn, and Cu) impurities are known to degrade minority carrier life times and solar cell performance significantly. The minority carrier life time is the time elapsed before a free electron combines with a hole in the crystal lattice. Fortunately, these transition impurities have relatively low solid solubility and hence low segregation coefficients in silicon and are thus removed substantially by known crystal growth techniques. Non metallic impurities such as oxygen, nitrogen, carbon and hydrogen dissolve in silicon mainly as interstitial impurities. The interactive effects of non metallic impurities and single atom impurities in silicon may facilitate formation of precipitated impurities such as SiC, SiO2, silicides and others. The synergetic effects of various impurity groups have been the subject of much ongoing research The effect of some single metal impurities on the efficiency of a p-type silicon solar cell originally published by Hopkins et al [11] is shown in Figure 2.6.

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Figure 2.6 Effect of metal atom impurities on p-type solar cells efficiency [11]

2.5 Existing Silicon Manufacturing Technologies The era following the first world oil embargo in the mid 1970’s sparked a massive research effort into low cost silicon production processes for photovoltaic applications. Accordingly, several thermodynamically feasible reactions for silicon synthesis were investigated and their economic viabilities compared. Only seventeen (17) out of a possible 200 were considered both thermodynamically and economically viable at the time [11]. Currently, the most commercially established silicon synthesis methods are the carbothermic reduction of silica in submerged electric arc furnace resulting in metallurgical grade silicon (MG-Si) and the conventional Siemens or trichlorosilane process for electronic grade silicon (EG-Si). These two well established processes produce bulk silicon material that lies on the extreme end of the purity requirements for solar grade silicon. Thus the approaches to synthesis 11

of solar grade silicon are currently pursued along the improvement or modifications of these two existing technologies.

2.5.1 Production of Metallurgical-grade Silicon (MG-Si) The commercial carbothermic production of silicon is carried out in submerged arc furnaces. The furnace consists essentially of a set of pre-baked carbon electrodes, refractory lined steel crucible and the charge mix. The charge mix typically comprise of silica in the form of lumpy quartz or quartzite rocks, coke or coal, and wood chips. The coke acts as the reducing agent and the wood chips serve to improve charge porosity. After thorough blending of the charge, they are loaded into overhead mix bins from which the charge mix is fed into the open furnace through chutes. For open furnaces, a stocking machine is used to spread the charge mix after they are dropped into the furnace. This machine is also useful for adjusting the raw materials as needed to correct the charge composition as well prevent any gaseous build up in the furnace [16]. The overall reaction to produce silicon metal can be represented by the equation: SiO2(s) + 2C(s) = Si (l) + 2CO (g)

2.1

The chemistry of the process is however made complex by side reactions which yield two intermediate compounds namely gaseous silicon monoxide (SiO) and silicon carbide (SiC). In the inner hot zone of the furnace where liquid silicon is produced and where the temperature range is 1900 to 2100 oC, the key reactions may be described by the following equations [2]: 2 SiO2 (l) + SiC(s) = 3SiO (g) + CO (g)

2.2

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

2.3

In the outer zone where the temperature is typically below 1900oC, main reaction chemistry can be represented as follows: SiO (g) + 2C(s) = SiC(s) + CO (g)

2.4

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

2.5 12

The liquid silicon produced from the furnace is tapped from bottom for further refining and casting into ingots. The silica fumes are collected via a furnace hooding system and into a bag house filter which can then be sold or reused. One of the main technological advantages of the carbothermic process is that it is slag-less and thus represents a clean separation of the products. It is however disadvantaged by the fact that the elemental impurities in the raw materials and external sources tend to form alloys with the silicon due to the high regime of temperature involved in the process. Figure 2.7 shows a schematic of the entire process. A typical chemical composition of the refined metallurgical grade silicon is shown in Table 2.2

Table 2. 2 Impurities in metallurgical grade silicon [2]

Impurity Element

Concentration Impurity Range (ppm) Element

Concentration Impurity Concentration Range (ppm) Element Range (ppm)

B

5-70

Fe

300-25000

Ca

20-2000

P

5-100

Cr

5-150

Mg

5-200

Al

300-5000

Mn

10-300

O

100-5000

Mo

1-10

C

50-1500

Ni

10-100

Ti

100-1000

13

Figure 2.7 Schematics of metallurgical grade silicon production process [2]

2.5.2 Production of Electronic grade Silicon (EG-Si) Electronic grade silicon commonly known as polysilicon or semiconductor grade silicon is produced mainly by the Siemens- process. The first part of the process involves halogenation of finely ground MG-Si in a fixed or fluid bed reactor with gaseous hydrochloric acid (HCl) in the presence of copper as catalyst. Mixtures of chlorosilanes are obtained from which the lighter silanes mainly SiH3Cl, SiH2Cl2 are recycled and the heavier fractions put through a series of fractionating columns to yield purified triclorosilane (SiHCl3) [17]. The SiHCl3 is subsequently decomposed in the presence of hydrogen at temperature of 1000 -1100 oC over inverse U-shape hot filaments in a Siemens reactor Figure 2.8(a). The overall reaction process can be represented by: [3], [17] 14

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

2HSiCl3 (g) + H2 (g) = Si + SiCl4 (g) + 2HCl (g)

300 oC

2.6

900- 1100 oC

2.7

The polysilicon produced from the aSiemens reactor reaches a purity in the order of 9-11N and may be further processed into single crystal silicon by the Czochralski crystal growth technique (Figure 2.8(b)) or other techniques.

Figure 2.8 Schematics of (a) Siemens reactor and (b) Czochralski crystal puller [17] The Siemens process produces silicon of unparalleled purity however it is limited by the high cost of the final silicon products, low volume of production due to the relatively slow nature of the decomposition reactions, complex set up, energy intensive, and use of high corrosive chemicals which can potentially burden the environment in terms of emissions.[3] 15

Presently modifications have been made to the process to include decomposition of silanes (SiH4) over seed silicon granules in fluid bed reactors. However the cost reduction potential of the final silicon product by the original process and its variants still remain low.

2.6 Silicon Purification Techniques The metallurgical approaches to synthesis of solar grade silicon represent a tireless effort at upgrading the silicon metal produced by the commercial carbothermic process. A number of process techniques that have been tested and proven to remove metallic impurities other than boron and phosphorus to a reasonable extent include [18],[19] •

Solid-liquid extraction (acid leaching)

•

Al -base refining (recrystallization from Al solution)

•

Liquid-Liquid extraction (slagging)

•

Liquid gas extraction (gas blowing and evaporation)

•

High vacuum refining and plasma-arc reduction of impurities

•

Directional solidification of molten charge

Detailed descriptions of these techniques have been covered in the papers by Barthey et al [11], Dietl [18], and Gribov et al [19] along with citation of a host of research papers. The efficacy of hydrometallurgical treatments such as acid leaching on removal of dopants impurities (B, Al, and P) has often been debated in literature publications. Whilst the general consensus or perhaps perception is that the technique is ineffective in removing dopants impurities from MG-Si, it remains to be proven that the same technique can be used to purify silicon from sources other than metallurgical grade and quartz.

16

2.7 Summary of Literature Review A review of the literature suggests that the technologies to refine silicon are well advanced. The difficulty in producing solar grade silicon for photovoltaic application using the metallurgical route therefore seems to be partly an issue of silicon source material and with that comes an issue of a ‘process’. If a source of silica can be found other than mineral quartz and this alternate source of silica can be processed economically to remove the problematic boron, phosphorus and other impurity elements prior to reduction into molten liquid silicon, then perhaps the proven refining techniques can be applied to such a material to reach the goal of producing solar grade silicon. It is in view of this motivation that this research work investigates the possibility of an integrated process for synthesis of solar grade silicon from rice husks.

17

CHAPTER THREE: Synthesis of Silicon from Rice Husk Ash The metallurgical route combined with directional solidification has been successful only in removing metallic impurities with low segregation coefficients in silicon but ineffective economically, in removing high segregation impurities (B, P, As, Al), which are known to be very deleterious to the solar cells performance [20]. For these impurities, the logical approach and therefore growing consensus has been to use very high purity silica and reductant raw materials that are almost free of such impurities. It is against this background that the potential of amorphous silica obtained from rice husk ash may be explored for the production of solar grade silicon.

3.1 Characteristics and Availability of Rice Husks Rice husk is a waste by-product of the rice milling industry. It constitutes about 20% the weight of a harvested rice paddy and the mineral ash content of the rice husks ranges between 15-30% of which 87-97% is amorphous silica depending on the combustion technique and conditions employed [4], [6],[21], [22] The rice plant has high amorphous silica content because it naturally absorbs from the soil and transports silicon in the form of silicic acid to its outer surfaces. The silicic acid on reaching the outer surfaces of the rice plant becomes concentrated due to evaporation and is subsequently polymerized into silica cellulose membrane [23]. Because of this natural selectivity, the rice plant limits the uptake of heavy metallic elements that are found in large concentration in other cheap sources of silica such as quartz, bentonite and diatomaceous earth. A typical proximate analysis of rice husk is shown in Table 3.1

18

Table 3. 1 Typical proximate analysis of rice husks[21] 6%

Moisture 

16.92%

Ash 

51.98%

Volatiles  Fixed Carbon

25.10%

An estimated one hundred and twenty million (120 × 106) metric tonnes of rice husk are generated annually worldwide [24]. For this huge amount of waste, the economic importance of utilizing rice husk has attracted several research interests in recent years.

3.2 Silicon from Rice Husk - A Review of the Literature The main industrial uses of rice husk ash (RHA) are as a pozzolan in the cement and concrete industry and as an insulator in the steel industry. The synthesis of silicon from RHA has to date remained as a laboratory curiosity. Singh and Dindhaw [23] reported obtaining silicon of 6N (99.9999%) purity by reducing white rice husk ash with magnesium at temperature of 800 oC followed by several successive acid (mixtures of HF, H2SO4 and HCl) leaching treatments. The reduction was also investigated at temperatures of 850 and 900 oC. The silica in their rice husk fired beyond 800 oC was observed to have attained some degree of crystalinity. They also suggested the possibility of obtaining silicon of similar purity by directly smelting the purified amorphous silica with carbonaceous reductants in electric furnace followed by leaching with acids and repeating the smelting and leaching for about nine times. The authors however did not disclose the method used to analyze their silicon to the 6N purity.

19

Amick et al. [25] also patented a process for producing rice husk silica with adjusted silica to carbon ratio for direct reduction into high purity silicon with no addition of carbonaceous reductants. The method as described by Amick et al. comprise of leaching rice husk in semiconductor grade hydrochloric acid followed by pyrolysis of the leached husk at 900 oC in an atmosphere of 1% anhydrous HCl/Ar gas stream for a period of about one hour. The pyrolyzed rice husk which has a carbon -to -silica ratio of 4:1 is further processed in a conventional fluid bed combuster with Ar/CO2 atmosphere at a temperature of 950 oC to obtain stoichiometric carbon -to- silica ratio of 2:1. Reduction of the ash so produced at a temperature of 1900 oC reportedly yielded silicon with total impurity less than 75 ppm. The boron and phosphorus content were reported to be less than 10 ppm each. Subsequently, Hunt et al. [26] investigated the possibility of producing high purity silicon from rice husk by purifying the rice husk silica according to the method of Amick et al, followed by pelletizing and reduction in a modified electric arc furnace. The pelletizing was carried using carbon black as a reductant and sucrose as a binder. The authors concluded that purified RHA could be a potential silica source for solar grade silicon production. Bose et al. [22] subjected powdered silicon obtained by magnesium reduction of rice husk ash at a temperature of 600-650 oC to melting and directional solidification and found that boron was the active impurity in the polycrystalline silicon ingot obtained. They determined the minority carrier life time of their polycrystalline silicon material to be of the order of 1-5µs and concluded that to be promising for photovoltaic applications. A minimum carrier lifetime requirement for efficient solar cells fabricated from multicrystalline silicon wafers is however estimated as 25 µs [2]. The magnesium reduction of rice husk ash has also been investigated by Banerjee et al. [27] and by Ikram and Akther [10]. Banerjee et al reduced acid leached rice husk ash by intimately mixing the ash with magnesium powder and firing the powdered mixture in a sealed graphite crucible in a muffle furnace. The reaction product was successively leached in mineral acids (HCl, H2SO4, and HF) in a Teflon beaker. A spectrochemical analysis of the final silicon product showed a high boron content of 20-200ppm as well as high magnesium (50-1000ppm) and 20

aluminum (10-200 ppm). They attributed the contamination of the silicon to the use of laboratory grade magnesium and also from glassware. In comparison with the silica produced by Singh and Dhindaw, Barnerjee et al. reported the silica had attained some degree of crystalinity when produced from roasting of husk at temperatures between 500-600 oC. Following a similar approach but with 4N purity magnesium, Ikram and Akhter reported silicon of 99.95% purity with Boron content of approximately 2ppm.The process steps followed by Ikram and Akhter [10] , comprise boiling rice husk in 1:10 HCl and distilled water for 15 minutes, burning of the acid treated husk in air to obtain black ash, firing of the black ash in a muffle furnace to obtain white ash, leaching of the white RHA in dilute HCl , reduction of the leached white RHA with magnesium of 4N purity followed by sequential leaching with HCl, HF and a mixture of HF and H2SO4. Contrary to the report by Banerjee et al and in agreement with Singh and Dhindaw, these authors reported that no crystalinity was observed in the RHA produced at 620 oC. Only the RHA fired to 900 oC had attained significant crystallinity with reflections or sharp peaks of different phases of SiO2 in their XRD pattern. They concluded that the silicon can be upgraded to solar grade silicon by conventional refining methods. The XRD patterns of the magnesium reduced white RHA product by Ikram and Akhter is shown in Figure 3.1. Calciothermic reduction of purified rice husk ash was reported by Mishra et al.[28] They mixed a stoichiometric composition of granular calcium and purified rice husk silica and subsequently fired the powdered mixture in a sealed sillimanite crucible in a muffle furnace at temperature of about 720 oC. The reduction product was milled to fine powder and successively leached with concentrated nitric acid (HNO3) and hydrofluoric (HF) acid to obtain silicon of 99.9% purity with a boron content of 10 ppm. They suggested that the use of MgO coated crucibles and high purity reagent can lead to producing solar grade silicon by this method.

21

Figure 3. 1 XRD pattern of Magnesium reduced RHA at 620 oC [10]

3.3 Silicon from Rice Husks - Summary It can be seen from a review of the relevant literature that the main process steps involved in the efforts to synthesize silicon from rice husks are combustion of the husk, reduction of the ash with suitable reductant and purification of the reduction products with appropriate acidic reagents. The sequence of process steps and process parameters (temperature, time, type of reductants, type and concentrations of leaching reagent) have to be optimized in order for this approach to solar grade silicon to be both technically and economically feasible.

3.4 Thermodynamics of Metallothermic Reactions for RHA (SiO2) The choice of reducing reagent for silica influences the thermodynamics and kinetics of the reactions. It was shown from the Ellingham diagram in Section 2.3 that C, Mg, Ca, Al and Ti are common and readily available reductants for SiO2. The carbon reduction of SiO2 is only feasible at temperatures in excess of 1900 oC and this represents the commercially established 22

carbothermic process already described in Section 2.5. The metallic elements Mg, Ca, Al and Ti however can reduce SiO2 at comparatively lower temperature and forms mixtures of condensed phase products. The relevant overall reactions and their corresponding free energy change and adiabatic temperature rise per mole of silicon is shown in Table 3.2

Table 3. 2 Silica-Metal Reaction Thermodynamic Data* SiO2 + 2Ca = Si + 2CaO T

ΔH (650 °C )

ΔG° (650 °C )

Adiabatic Temp.

°C

kJ

kJ

°C

-363

-333

2348

-312

-261

1906

-180

1477

-33

407

650 SiO2 + 2Mg = Si + 2MgO 650

SiO2 + 4/3Al = Si + 2/3Al2O3 650

-210

SiO2 + Ti = Si + TiO2 650

-34

*HSC Chemistry 6.v12

23

CHAPTER FOUR: Experimental Work This chapter presents the experimental approach followed throughout this research work to investigate the feasibility of synthesizing silicon of high purity from rice husk ash. The scheme of experimental work is presented in Figure 4.1.

●FactSage™ Study

Figure 4.1 Scheme of experimental work

24

4.1 Materials and Reagents Rice husk and rice husk ash used in this study were of Indian origin and were provided by Process Research Ortech Incorporated (PRO). The method and apparatus used by PRO in processing the rice husk into rice husk silica or rice husk ash (RHA) will be described briefly in the section under Sample Preparation. Table 3.1 provides a summary of the samples, reagents and reducing agents used in this work. Table 4. 1 Summary of materials and reagents Material

Description

Source

Rice Husk Ash

Processed in Torbed® Reactor at Process Research Ortech 800 oC (PRO)

Magnesium Turnings

99 wt% pure

Fisher Scientific

98 wt% pure (metal basis), Magnesium Granules

-40 + 230 µm

Sigma Aldrich

Hydrochloric Acid

36.5-38 wt% Standard stock

Caledon, TMG

Nitric Acid

70 wt% Standard stock

Caledon TMG

Hydrofluoric acid

48 wt% Standard stock

Caledon TMG

Polyvinyl (PVA)

98-99 wt%

Alfa Aesar

> 6.3 grade

Lindy BIP

Argon

Alcohol

25

4.2 Sample Preparation – Rice Husk Ash 4.2.1 Combustion of Rice Husks The expanded Torbed reactor (ETR) is the apparatus used by Process Research Ortech Inc. to process the rice husks into rice husk ash (RHA). The ETR is essentially an upright cylindrical reactor with stainless steel interior walls. The base of the reactor is equipped with inlets through which a high velocity stream of air-gas mixture (process gas) is blown into the reactor to supply heat for reaction. A schematic of the ETR is shown in Figure 4.2. The reactor, after pre-heating to a wind–box temperature of about 500 oC was fed with dry rice husks from the top through a feed chute. The jet stream of air–gas mixture blown into the reactor holds the diffuse feed material (rice husk) in a cyclonic motion. The feed materials falling to the bottom of the reactor are met by the process gas stream and are forced radially outwards to the walls of the reactor by centrifugal forces incipient to the design of the reactor. The falling diffuse materials reaching the base of the reactor are re-entrained in the process gas stream for continuous solid- gas contact [29]. Products attaining sufficient combustion (800 - 830 oC for this test) exit into an adjoining cyclone where products separate into the base of the cyclone and exhaust gases through a bag house – scrubber system. The reactor is equipped with real time feedback control systems, which enable on-line close control and monitoring of the combustion process.

26

Figure 4.2 Schematic of expanded Torbed reactor [29]

4.2.2 Moisture Content Determination The moisture content of the rice husk ash was determined by the Drying Oven method. Using a Mettler HK 160 analytical balance of sensitivity 0.0001 g, a quantity of the rice husk ash was weighed into a cleaned and dried ceramic boat of predetermined tare weight. The boat and content was weighed and then placed in a Precision thelco mechanical convection oven and dried to constant weight at 105 oC over 4 to 5 hours. The ceramic boat and content was re-weighed and the moisture content determined. The moisture content was found to be below 0.5 wt% hence all weighting of RHA were considered to be on dry basis without moisture content corrections.

27

4.3 Characterization of RHA Sample 4.3.1 Chemical Composition Analysis The bulk of the as received rice husk ash (RHA) was blended and riffled using a Jones riffles to ensure homogenous and representative samples were selected for both chemical composition and other analysis. Analyses requested were multi-acid digestion Inductively Coupled Plasma (ICPMS/OES), Whole Rock Assay, and Leco total carbon content. The ANALEST laboratory of University of Toronto Chemistry Department was used for in-house chemical composition analysis. External laboratory which was involved in the chemical compositional analysis of samples from this research is International Plasma Laboratory (IPL)-Vancouver.

4.3.2 Particle Size Analysis Laser diffraction particle size analysis method was used to characterize the particle size of the as received RHA. The Malvern Master Sizer S instrument was used. The measurable particle size range of the instrument is 0.05-3000 µm and it is equipped with a small volume sample dispersion unit. A lens range of 300RF, a beam length of 2.4mm, and a presentation of 30AD with polydisperse analysis was used for this measurement. About 0.5 g of the rice husk ash was dispersed in de-ionized water in the sample dispersion unit of the instrument, vigorously mixed for about two (2) minutes at speed of 2100 rpm, and sonicated for 45 seconds. The ultrasonic waves were used to break or minimize any particle agglomerates that may be present in the suspension. Measurements were taken and the diffraction data and graphs recorded by the instrument software program.

28

4.3.3 Surface Area Measurement The Coulter SA 3100 Analyzer together with the BET (Brauner Emmet Teller) calculation model was used to determine the surface area of the as-received RHA. For sample preparation, about 2 g of the rice husk ash was weighed into the glass tube sample holder of the instrument and the free space volume (space in tube not occupied by sample) was measured using helium gas. The sample tube and contents were then outgassed with the system’s in-built vacuum pump. Analysis for the surface area began by introducing known doses of nitrogen (adsorbate gas) into the previously evacuated tube containing the sample. The glass tube was maintained at a constant temperature by immersion into a Dewar of nitrogen. When the pressure in the tube was equilibrated following each dose of the adsorbate gas, the pressure reading was recorded and then used to compute the volume of gas adsorbed on to the surface and pores of the sample. The saturation pressure was measured for every sample tube pressure data point. The isotherm data obtained were used to plot isotherm curves and a BET plot from which the surface area was calculated. The relation between the volume of adsorbed gas and the relative pressure in the sample tube is given by the BET equation in the form; 1 C-1 Ps Ps = + ( ) VA(Po-Ps) VmC VmC Po

4.1

Where: Vm = Volume of monolayer gas VA = Volume of adsorbed gas Ps = Sample pressure P0 = Saturation pressure C = Constant related to enthalpy of adsorption.

29

The BET model assumes a cross-sectional area of the adsorbate molecule (Am) to be 0.162 nm 2 for the nitrogen gas, which enables the specific BET surface area (SBET) in m2/g to be calculated from the expression: [30]

SBET =

VmNAAm

4.2

Mv

NA and Mv are respectively the Avogadro’s number and gram molar volume of an ideal gas.

4.3.4 XRD Analysis Philips Diffractometer (model PW 3710) with X’PERT graphics software package was used to analyze the structures of the rice husk ash sample. An aluminum-glass composite sample holder with a rectangular slot measuring 2 cm × 1 cm × 0.2 cm was filled with the powdered sample using the front loading method. The samples were analyzed using a CuKα radiation (λ = 1.54056Å) with a nickel filter. Bragg’s angle (2theta) range of 10 -50o and a scan speed of 0.72 degree per minute with a step-size of 0.015° was used. The Philips diffractometer was operated at 40 kV and 40 mA. The structural pattern was recorded and analyzed with X’PERT HighScore™ software.

4.3.5 Scanning Electron Microscopy (SEM) Studies The Hitachi S2500 scanning electron microscope was used to characterize the particle morphologies of the samples. Sample specimens with diameter of 12 mm each were gold coated in a gold sputter coater for 90 seconds at 15 mA current output. The gold coating was necessary to ensure a conducting surface was obtained for electron bombardment and characterization. The scanning electron microscope was operated at 20 kV and a working distance of 15 mm. Selected areas of interest were focused and micrographs were taken. For samples requiring SEM and EDX analysis, the Hitachi S570 scanning electron microscope was used.

30

4.4. Purification Treatment of RHA 4.4.1 Leaching of RHA Acid leaching of the rice husk ash was carried out to remove soluble elemental impurities and hence increase the purity of the silica content. Leaching was carried out at 10 wt% solids in 10 wt% HCl. The HCl solution was prepared from a standard HCl stock of mean concentration 37 wt% and density 1.19 g/mL. The leaching reactor was 1-2L teflon beaker with teflon lid. Holes were drilled in the lid to accommodate a separator funnel for introduction of preheated acid to the pre-heated solid to be leached; as well a thermometer and a three blade teflon coated steel impeller. The port for the separator funnel doubled as a sampling port. The experimental set-up for leaching is shown in Figure 4.3. Leaching time was varied between 1and 4 hrs and the temperature was varied between 60 and 90 o

C with continuous stirring at 250 rpm. Vacuum assisted filtration was used to separate the

residue and leach liquor. The residue was thoroughly washed with cold de-ionized water. The residue obtained was oven dried to constant weight at 105 oC. Samples of the dry residue were digested and analyzed by ICP- OES/MS to determine impurity reduction under the various leaching conditions.

31

Separator Funnel

Teflon Coated Impeller

Teflon Beaker with Lid, Thermometer

Insert

and

Sampling Port

Hotplate /Stirrer Figure 4.3 Leaching experiment set-up

4.4.2 Roasting of Acid Leached Ash Acid leached rice husk ash was heat treated to reduce the amount of carbon content before reduction and hence increase further, the purity of the silica content. The purified ash was placed in an alumina crucible and heated in air in a muffle furnace to two selected temperatures of 500 and 700 oC at a mean heating rate of 300 oC /hr and maintained at this temperature for 1- 4 hours. The set up for this experiment is shown in Figure 4.4.

32

Leached

RHA

Sample

Figure 4.4 Set-up for roasting of acid leached RHA

4.5 Selection of Reducing Agent for RHA (SiO2) It was shown in Section 3.4 that the reactive metals Mg, Ca, Al and Ti are thermodynamically favourable to reduce SiO2 at comparatively lower temperature and forming a mixture of condensed phase products. However by considering factors such as cost and availability, ease of separating the by-product of reaction from the desired product (silicon), the effect of residual levels of these elements on the useful properties of the desired silicon product and above all safety and environmental concerns, magnesium was chosen as a suitable reducing agent over the others. A look at the Si-O-Mg phase stability diagram in Figure 4.5 shows that other possible stable phases may be formed in addition to the main by-product of reaction, the MgO phase. This requires that the reduction reaction has to be controlled in order to minimise the formation of undesirable phases. 33

Figure 4.5 Si-O-Mg phase diagram at 650 °C

4.6 Pelletizing of Reactants The leached and roasted RHA having the highest silica content and lowest carbon content was selected for reduction experiments. The purified RHA and magnesium granules/turnings were wet blended in 4 wt% polyvinyl alcohol solutions and dried in argon atmosphere at a temperature of 105 oC for 10-15 minutes. The drying off of the wet alcohol improves the flowability of the reactants-mix into the compression die as well as improves the binding property of the PVA solution on the particulate reactants mix when compressed. The dried reactants-mix was poured into the compression die having a bore diameter of 16.3 mm in predetermined amounts. The 34

interior surfaces of the compression die were lubricated with a solution of stearic acid in kerosene or alternatively benzene before introduction of the reactants-mix. The quick drying action of the solution on the wall of the die leaves a thin film coating which facilitates ejection of the powder compacts. The experimental set-up for the pellet making is shown in Figure 4.6.

Plunger

Die

Figure 4.6 Set-up for pellet making

The compression die and content was placed under a hand operated hydraulic press (Carver Hydraulic Press) and then loaded to 4000 kg. This load is maintained on the die for a period of 13 minutes to ensure adequate compacting of the particulate powder mix in the die. After the elapsed time, the pressure is released and the cylindrical compact is ejected from the die with a plunger. The pellet made in this way maintains it geometrical shape up to and even beyond the critical reaction temperature as was evidenced by the reduction reaction products.

35

4.6.1 Determination of Percent Compaction and Porosity The percent compactness and porosity of the pellets may be determined by considering the geometry of the pellets along with the weight and density of the component materials in the pellets. %Compaction=

ρ ×100 ρth

4.3

ρp ρth

4.4

P

% Porosity = 100 1Where

ρp = bulk density of pellet ρth = theoretical density of pellet based on composition. The mass of silica and magnesium can be readily determined from the percent composition of the mixed reactants.

4.7 Reduction Experiments The reduction of silica (SiO2) by magnesium (Mg) was first studied theoretically using FactSage™ program and then in a small amount employing Differential Thermal Analysis. The results obtained from these preliminary studies were used to reduce the number of the bench scale experiments.

4.7.1 FactSage™ Study FactSage™ is a fully integrated database computing system in chemical thermodynamics. The FactSage™ study was undertaken to examine the effects of process variables such as temperature, and ratio of Mg: SiO2 on the relative amounts of the equilibrium phases in the reaction products.

36

The Equilibrium calculation model and the pure solids database in FactSage™ were used to investigate the effects of temperature, trace oxygen amount or partial pressure and Mg: SiO2 from the simplified reaction model equation on a gram mole basis: (gram) 60 SiO2 + Mg + O2 + Ar = (25,1,s,#1)

(25,1,s,#1)

4.5

(25,1,g,#1) (25,1,g,#1)

Where α = % fraction of reductants reagent in excess of stoichiometric requirement B = Amount of oxygen in the inert gas stream or trapped in the furnace atmosphere. The simulation was performed at 600, 650, 700, 800 and 900 oC and varying from 1 to 1.25 at 0.05 intervals with fixed at 0 and with argon as the inert gas.

4.7.2 Differential Thermal Analysis DTA Differential thermal-analysis of a stoichiometric mixture of purified RHA and magnesium was carried out using NETZSCH STA 409 PC/PG instrument. 48mg sample was loaded into DTA alumina crucible and with a certified alumina material as a reference; a heating program was initiated with a heating rate of 5 oC/min from 35 oC to 1150 oC with continuous argon flush. The specific heat absorbed or evolved (µV/mg) was recorded as function of temperature.

4.7.3 Reduction in Argon Atmosphere A schematic of the experimental set-up for the metallothermic reduction in controlled atmosphere is shown in Figure 4.7. The set-up consists of a Lindberg horizontal tube furnace with input power rating of 1300W. The work tube is recrystallized alumina with a length (L) of 760 mm, outer diameter (OD) of 60 mm and inner diameter (ID) of 54 mm. The furnace was calibrated to determine the hot zone at the desired temperature ranges for the experiments. 37

Figure 4.7 Schematic of reduction experimental set-up

The charge composition was formulated based on pure stoichiometric silica and the overall reaction equation; SiO2 + 2Mg = Si + 2MgO

4.6

The corresponding RHA equivalent calculated based on the purity of the RHA. Weight of equivalent RHA = Weight (SiO2)×

100 % Purity of RHA

4.7

The magnesium amount was then calculated based on pure silica in the RHA Weight of Mg = Weight(SiO2) g ×

48g/mol 60 g/mol

×(1+α)

Where α = % fraction in excess of stoichiometry requirement. 38

4.8

The effect of Mg/SiO2 ratio was investigated by varying the amount of magnesium in the charge. The reduction experiments in argon atmosphere were carried out at selected temperatures in the range of 500-950 °C to examine the effect of temperature. The reduction procedure in controlled atmosphere is as outlined below; •

Preheating the furnace to a temperature of 400 °C.

•

Introducing the charge into the furnace hot-zone and high argon flushing for 15 minutes.

•

Steadily heating the charge at approximately 10 °C/min under flowing argon to the selected holding temperature and holding it there for one hour.

•

Allowing furnace and charge to cool down naturally to room temperature.

4.8 Quantitative XRD Analysis of Reduction Products Phases present in the reduction product were analyzed using a Philips X-ray diffractometer with instrumental settings and software programs as previously described. Each reduction sample was prepared and scanned in the Bragg-Brentano’s 2θ range of 10 - 90° with step size of 0.015 and time per step of 1.25 s which corresponds to a scan rate of 0.72degmin-1. The relative amount of phases present in the reduction product was determined using the Reitveld method. The Reitveld method is a whole pattern fitting method which uses crystal structure data and peak profile information to generate and fit calculated XRD patterns to experimentally determined patterns by means of least squares refinements [31]. The scale factor obtained from the Reitveld method has been shown by Hill and Howard [32] to be related to the weight of each crystalline phase in a multiphase powder mixture by the following formula. Wp = Sp(ZMV)p / ∑i Si ZMV

4.9

i

Where Wp is weight of phase p in the multi-phase powder mixture,

39

S, Z, M and V are respectively the Reitveld scale factor, the number of formula units per unit cell, the mass of the formula unit and the unit cell volume. Thus the technique requires no laborious preparation of calibration standards for determining accurately relative phase amounts. The quantitative analyses on the reduction product were performed using Bruker AXS Topas v.2.1 software program. Input data for all identified phases in the XRD pattern of the reduction products were taken from Inorganic Crystal Structure Database (ICSD). The parameters that were refined included individual structural properties of the identified phases, their x-ray absorption, particle size, lattice parameters, preferred orientation and user defined peak profile function. The background of each XRD pattern was refined by Chebyshev 10th order polynomial and the modified pseudo-Voigt function was used to model the peak shape profile. Linear or microabsorption of phases in the sample as well as zero displacement was also refined [33].

4.9 Post-Reduction Purification and Analyses 4.9.1 Leaching of Reduction Products-Kinetic Study The kinetics of first stage post- reduction leaching was investigated to determine effect of leaching agent type and concentrations, as well as temperature and time on the dissolution of the magnesium by–product phases in the reduction product. The reduction product having initial charge composition of 5 wt% excess magnesium was manually ground in agate mortar and passed through US Tyler Sieve # 48 (0.297 mm) in order to ensure uniformly close sized particles. The leaching agents tested were hydrochloric acid (HCl) and mixtures of hydrochloric acid and acetic acid (CH3COOH). The leaching temperatures investigated were 20, 35, 50 and 70 °C. Leaching times were varied from few seconds to one (1) hour. All leaching kinetic studies were done using an aqua-bath (Barnstead) for close control of temperature. The set-up for leaching kinetic study is as shown in Figure 4.8.

40

Figure 4.8 Leaching kinetic study set-up

The effect of leaching agent type and concentration was tested by leaching 1 g sample of the reduction product in 500 ml of the test reagent. Aliquot samples were drawn at predetermined time intervals for ICP analysis. The HCl:CH3COOH volume ratio was varied at 4:0 and 4:1, respectively. The HCl concentrations tested were 1.25 and 4M whereas the acetic acid concentration was fixed at 4.38M (25 wt%). The effect of temperature was investigated in batches by leaching 1 g reduction product in 167 ml of the best found leaching agent for each selected time. The weight loss at each temperature was recorded as a function of time.

4.9.2 Leaching of Reduction Products The experimental set-up for the leaching is the same as previously described in Section 4.3.1 of which the schematic is shown in Figure 4.9. Preheated acid was introduced to the preheated reduction product contained in a closed and well stirred teflon beaker. Two cycle leaching sequence was adopted for purification of the reduction product. The first leach cycle was 41

designed to selectively leach out the acid soluble magnesium by-product phases and other impurities. The second leach cycle is designed to leach out silicates and other impurities that may be present. The first cycle leaching reagent comprised of hydrochloric acid (HCl) and acetic acid (CH3COOH) in volume ratio of 4:1 respectively and concentration ratio of 1.25 M HCl: 4.38M (25 wt%) CH3COOH. The leaching was carried out at 6 g/L so as to ensure effective dissolution of magnesium phases. The final leaching cycle was carried out in 4.8 wt% HF: 25 wt% CH3COOH acid mixture in a volume ratio of (1:9) respectively and at 2 wt% solids. The acids were introduced directly to the solid without the use of the glass separator funnel due to reactivity of HF acid with glass. The acetic acid was introduced first and shortly followed by the HF acid. Leaching at each cycle was carried out at 70 °C and for duration of one hour. In between cycles, the leach slurry was filtered through Whatman filter paper (Whatman # 42) using a vacuum assisted filtration assembly. The residue was either water washed on filter paper or re-pulped and leached in warm de-ionized water for another hour before final filtration and oven drying. The dried residue recovered from each cycle was used as the feed material for the subsequent leach cycle. The final residue recovered from second cycle leaching was subjected to XRD, ICP, SEM, BET surface area and laser particle size analysis as previously described.

42

Figure 4.9 Schematic of leaching set-up

4.9.3 Sample Digestions for ICP-OES Analysis The final silicon powder obtained from the second cycle leaching was prepared for ICP-OES chemical composition analysis. A 0.15 gram sample of the silicon powder was digested in a closed teflon beaker with 15 mL multi-acid mixture in the volume ratios of 1:1:1 De-ionized water, concentrated nitric acid (HNO3, 70 wt %), concentrated hydrofluoric acid (HF, 48 wt %) in the respective order. The teflon beaker and content was heated to a temperature of 50-70 °C for half an hour. The totally digested sample was transferred to HF resistant 50 ml volumetric flask or graduated cylinder. The solution was then made up to the 50 ml mark with 2 vol % nitric acid solution for ICP-OES analysis. A blank solution was prepared with the same ratios but with a volume factor of five (5) less than the prepared sample solution; so that a 3ml solution was 43

diluted with 2 vol% nitric acid to 10 mL to form the blank solution. The calculation for the impurity element (analyte) in the solid silicon sample is given by the following expression:

Analyte in (Si)solid in ppm=

mg )×prep L

1000×C′(

Vol(L)

wt of Si (g)

4.10

Where, C′ is the difference between measured ICP-OES concentration of the analyte in the sample and that in the blank solution. When the difference results in a negative concentration, the minimum quantifiable detection limit (D) of the ICP instrument for that analyte is used. The equation becomes; Analyte in (Si) solid in ppm =

mg ) L

1000 D(

prep Vol(L)

wt of Si (g)

4.11

4.10 Test Melting of Silicon Powders Samples of the silicon powders obtained after the second cycle leaching were compacted at room temperature (with no addition of binders) into 2.5 g disc pellets. A 5 g sample in the form of two disc pellets was contained in a sealed alumina crucible. The crucible and contents were then placed in the hot-zone of a vertical tube furnace and heated at 100 °C /hr to 1500 °C under flowing argon. The hold time at 1500 °C was kept at four (4) hours after which cooling to room temperature was allowed at 100 °C/hr. Figure 4.10 shows silicon compact and furnace set-up used in the test melting experiment.

44

 

Furnace set-up

Silicon compact

Figure 4.10 Test melting of as-produced silicon powder

45

CHAPTER FIVE: Results and Discussion In this chapter the results of the experimental works will be presented and discussed systematically. The discussion will be segmented into five parts namely, ( i) Characterization of as-received rice husks material, (ii) Purification of rice husk ash, (iii) Pelletizing and reduction, (iv) Post-reduction purification and leaching kinetics and finally (v) Characterization of as produced silicon material.

5.1 Optical Images of as-received Rice Husks Material Photographs of the Indian origin rice husks and rice husk ash (RHA) samples obtained from the local company (Process Research Ortech Inc) are shown in Figure 5.1 The RHA had a charcoaled-black appearance with some brown colour and a gritty feel in between fingers.

Rice husks

Rice husk ash (RHA)

Figure 5.1 Photographs of rice husks and rice husk ash (RHA)

46

5.2 Characterization of As-received RHA 5.2.1 Chemical Composition High silica content and low levels of elemental impurities are necessary pre-requisites for synthesis of high purity silicon from RHA. Accordingly chemical composition analysis of the asreceived RHA was performed as previously described. The main oxides present in the asreceived RHA together with the lost on ignition (LOI) value are summarized and compared to literature values in Table 5.1.

Table 5. 1 Comparison of the main oxides present in the as-received RHA As-received RHA

RHA [34]

wt %

wt %

2

91.5

86.9-97.3

MnO

0.04

n/a

Al O

0.62

n/a

Fe O

0.42

0.54

Na O

0.18

0.0-1.75

KO 2

1.23

0.58-2.5

CaO

0.57

0.2-1.5

MgO

0.30

0.12-1.96

P2O5

0.30

0.2-2.85

L.O.I

3.05

n/a

Oxide Impurities SiO

2 2

3 3

2

It can be seen that the silica content of 91.5 wt% is well within the range of silica content of RHA in literature publications. The “n/a” in the last column of Table 5.1 indicates that no 47

corresponding data was found from the reference source. The silica amount in the as-received RHA is however not pure enough for the intended purpose of this research work. The elemental impurity content of the as-received RHA is also summarized in Table 5.2. The third column in Table 5.2 shows the average elemental impurities in coked only rice husks as compiled by Hunts et al [26].

Table 5.2 Comparison of elemental impurities present in the as-received RHA Elements

As-received RHA

Coked Only RHA [26]

ppmw

ppmw

B

51

10

P

1600

20

Al

4000

50

Fe

2400

20

Na

1700

10

K

7900

>1000

Ca

3300

>1000

Mg

1600

1700

Mn

294

1000

Total C (wt %)

1.87

n/a

Comparing the two last columns in Table 5.2 shows that the impurity contents in the as-received RHA is generally higher than the reported values of Hunt et al. As pointed out by Genieva et al [35] compositional differences in RHA may be influenced by such factors as geographical and climatic variations, the type of rice paddy, soil chemistry as well as the type of fertilizers used in cultivating the rice. Besides these factors, the relatively high impurity contents in the as-received 48

RHA may be due to impurity pick-up from the combustion reactor. Another possibility can be due to adherent soil particles on the rice husks since no pre-cleaning treatment was done before combustion. Chakraverty et al [36] showed that pre-treating the husks in dilute hydrochloric acid before combustion significantly reduces the total impurity content of the as-produced RHA. However, the logistics and cost of pre-treating the husks has to be weighed against the purity improvement in the RHA that will be obtained when compared to the combusted husks without pre-cleaning treatment.

5.2.2 Particle Size Analysis The chemical composition of the as-received RHA has been shown to be mainly silica (SiO2). Since it is a well known fact that submicron silica particles can be a potential health hazard [6], especially when in crystalline form, particle size analysis was necessary to determine the size distribution of the RHA material for both safety and process design in terms of materials handling considerations. The cumulative particle size distribution of the as-received RHA is shown in Figure 5.2. The particle size distributions shows that approximately 80 vol% (D80) of the as-received RHA is ≤ 56 µm, 50 vol% (D50) ≤ 35.3 µm and 10 vol % (D10) is ≤ 13.3 µm. It can be seen that further grinding or milling of this material will readily bring it to submicron particle size distributions. The as- received RHA was therefore used without further screening or close sizing.

49

100 90

D80 ≤ 56 µm

Cumulative Volume %

80 70 60

D50 ≤ 35.3 µm

50 40 30 20

D10 ≤ 13.3 µm

10 0 0.1

1

10

100

1000

Particle Size (µm) Figure 5.2 Particle size analysis of as-received RHA

5.2.3 XRD Analysis The crystal structure of silica may determine its reactivity and degree of health danger. Whilst crystalline silica is less reactive and deemed carcinogenic, the amorphous silica is rather reactive and with no harmful effect. The XRD pattern of the as-received RHA is shown in Figure 5.3. The major reflections or peaks of crystalline quartz from ICSD powder diffraction files (PDF) occur at Bragg 2θ angles of 20.856˚, 26.636˚ and 36.541˚. It can be seen that no defined peaks corresponding to these Bragg 2θ angles are found in Figure 5.3. A rather broad peak spanning 2θ angle range of 18-30⁰ which is characteristic of amorphous structures is observed.

50

Figure 5.3 XRD pattern of as-received RHA

5.2.4 SEM Analysis of as-received RHA The morphological features of the as-received RHA observed by scanning electron microscopy (SEM) is shown in Figure 5.4. The micrograph was taken under the secondary electron imaging mode (SEI) at a magnification of 1000X. As can be seen from Figure 5.4 the as-received RHA shows a porous and multifaceted particle shape and size. The main components of rice husk include hydrated silica, cellulose and hemi cellulose component totaling a 55-60% and lignin component of approximately 22% [37]. The porous and honeycomb morphology seen in Figure 5.4 can be attributed to the burning out of the organic component in the rice husk during combustion. The hydrated silica subsequently polymerizes to form a skeletal silica network which may explain the flaky and honey comb-like structure in the SEM micrograph of Figure 5.4. 51

Figure 5.4 SEM micrograph of as-received RHA

5.2.5 Surface Area Analysis The BET isotherm plot of volume of nitrogen adsorbed (Vad) against relative pressure (Ps/Po) for the as-received RHA is shown in Figure 5.5. A typical BET adsorption/desorption isotherm plot for relative pressure in the range of 0-1 may comprise of three segments. An initial concave shape towards the pressure axis at low relative pressure which correspond to the uptake of nitrogen into micro pores (< 20 Å) in the sample, followed by a linear segment which represents multilayer adsorption of nitrogen on the surface of the sample. A final convex shape with respect to the pressure axis corresponding to the filling of meso pores (20-500 Å) and macro pores (>500 Å) [38]. For the determination of surface area, the linear segment of the plot having relative pressure in the range of 0.05 – 0.3 is generally used. The determination of pore size distribution 52

however requires the use of all segments in the isotherm plot together with the appropriate calculation model.

Volume Adsorbed cc/g (STP)

15

10

5

0 0

0.05

0.1

0.15

0.2

Relative Pressure (Ps/Po) Figure 5.5 BET isotherm plot for as-received RHA

The BET plot within the linear region of the adsorption isotherm plot is shown in Figure 5.6. By equating the measured slope and intercept in Figure 5.6 to the equivalent terms in the BET calculation model as previously given in equation 4.1, the monolayer volume Vm and C-value were determined to be respectively 9.0252 cc/g (STP) and 110.8. By substituting Vm, the specific surface area of the as-received RHA is calculated as:

SBET =

SBET =

Vm(cm3g-1) x NA(mol-1) xAm(m2)

5.1

Mv(cm3mol-1) Vm(cm3g-1) x 6.022 x10 23(mol-1)x16.2-20(m2) 22414(cm3mol-1) 53

= 39.3m2/g

Where NA is the Avogadro’s number, Am is the cross-sectional area of each adsorbate molecule assumed to be 16.2 x 10-20 m2 and Mv is the gram molar volume. 0.025 y = 0.1098x + 0.001 R² = 1

Ps / VA(Ps-Po) ,cc/g

0.02 0.015 0.01 0.005 0 0

0.05

0.1 0.15 Relative Pressure (Ps/Po)

0.2

0.25

Figure 5.6 BET linear calculation model plot for as-received RHA

5.3. Purification of as-received RHA The purification treatment was done in two steps. The first step was leaching to remove acid soluble impurities and the second was roasting of the leached RHA to reduce total carbon content. The expected results of these two unit process is an overall increase in the purity of the silica content.

54

5.3.1 Effect of Leaching Treatment In order to access the efficacy of the leaching step and to determine the best leaching condition, the concentration (ppm) of impurities in the leached sample is ratioed to the concentration of impurities in as-received RHA. Figure 5.7 shows the effect of the leaching treatment under various temperature and time conditions in 10 wt% hydrochloric acid for major impurity elements of interest.

60 °C/1hr

60 °C/ 4hrs

90 °C / 1hr

90 °C / 4hrs

1.2

Concentration Ratio

1.0 0.8 0.6 0.4 0.2 0.0 B

Al

P

Mn

Fe

Ti

Na

K

Ca

Mg

Impurity Elements Figure 5.7 Effect of leaching treatment on impurity reduction in as-received RHA It is seen from Figure 5.7 that the boron and phosphorus levels in the as-received RHA is removed equally well by all the leaching conditions considered in this experiment. This suggests that the boron and phosphorus in the as-received RHA is unalloyed with the silicon and thus may possibly exist as amorphous oxide. The leaching treatment at 60 °C for one hour however, was less effective in removal of titanium. The ratio of concentration of titanium appears to be greater than unity because it was unaffected by the leaching condition, whereas the entire matrix 55

elements of the sample were relatively reduced. A similar observation was made by Hunt et al [26] in their study on leached and coked RHA, and they asserted that a threefold increase in concentration can be expected for elements unaffected by a treatment if the matrix weight is reduced by one-third factor. The result in Figure 5.7 shows that the leaching condition of 90 °C for four (4) hours gave the overall best impurity removal as evidenced by the relatively lower height for all the elements of interest and thus, it was selected as the preferred leaching condition for treatment of the as-received RHA.

5.3.2 Effect of Roasting Treatment The reduction of carbon content is essential for increasing the silica content as well as improving the wettability of the surfaces of the reduction product during post-reduction leaching. The reduction of the carbon content during roasting of the leached ash may be represented by the equilibrium reactions: 2C(s) + O2(g) = 2CO (g)

ΔG˚500˚C = -360.2 kJ/mol

5.2

ΔG˚700˚C = -395.8 kJ/mol 2CO(g) + O2(g) = 2CO2 (g)

ΔG˚500˚C = - 430.8 kJ/mol

5.3

ΔG˚700˚C = -395.8 kJ/mol C(s) + CO2 = 2CO (g)

ΔG˚500˚C = +35.3 kJ/mol

5.4

ΔG˚700˚C = -0.015 kJ/mol The calculation of free energy of formation of reactions 5.2 - 5.4 using HSC Chemistry version 6.12 program shows that the reaction 5.2 and 5.3 are favorable reactions and can occur under the temperature range of the roasting tests. The reaction 5.4 however is unlikely to occur under the conditions of the roasting experiments. Whilst reaction 5.2 is seen to be more favorable at higher temperature, reaction 5.3 somewhat shows a decrease favorability with increase in temperature. Ultimately, it is desirable to have most reduction of carbon content while maintaining the structure of the silica essentially amorphous for reasons previously explained. 56

The Leco carbon analysis results of roasting leached RHA at two different temperatures and varying times is presented in Figure 5.8. It is seen that the roasting of leached RHA at 700 °C was more effective for carbon removal than roasting at 500 °C. The carbon removal was accompanied by a color change from dark to white RHA. To minimize the possibility of crystallization of silica in the leached RHA, roasting was carried out at 700 °C for time period less than two hours.

wt% Carbon Removal

100

700˚C 500˚C

80

60

40

20

0 0

1

2

3

4

Time (hr)

Figure 5.8 Plot of wt% carbon removal from leached RHA with time during roasting

57

5

5.3.3 Combined Effect of Purification Treatment The combined effect of leaching followed by roasting on the purity of the treated RHA is compared to the as-received RHA in Figures 5.9 and 5.10. Figure 5.9 compares the major metallic oxides and carbon content of the two RHA materials. It is seen that there is a general decrease in the metallic oxides and a significant decrease in the carbon content of the treated RHA. The result of the impurity reduction is evident in Figure 5.10 where the purity of the silica in the RHA has increased from 91.5 wt% to slightly over 98 wt%.

Figure 5.9 Comparison of metal oxides and carbon in treated/untreated RHA

58

Figure 5.10 Comparison of silica content in treated/untreated RHA The effect of reversing the sequence of purification treatment (by roasting followed by leaching) of the as-received RHA was also examined and a table of comparison of results is provided in Appendix A. It was observed that in general the impurity reduction was less effective than the sequence of leaching followed by roasting. It is explained that roasting of the ash causes sintering of the silica particles so that relatively less surface area and hence less of the contained impurities is exposed to the direct action of the acid. The SEM micrograph and XRD pattern for leached and roast-treated RHA is shown in Figures 5.11 and 5.12. The SEM micrograph in 59

Figure 5.11 shows that roasting has a slight sintering effect on the RHA; however the XRD pattern in Figure 5.12 remains essentially amorphous structure.

Figure 5. 11 SEM micrograph (1500X) of leached and roasted RHA

Figure 5.12 XRD pattern of leached and roasted RHA

60

5.4. Pelletizing and Reduction of Reactants The variables of interest in the reduction experiments were to determine the effect of initial charge composition and hold temperature on the yield of silicon. Pelletizing of the reactant was equally important for at least two reasons; i) enabling easy handling of compacted reactants as opposed to powdered mixtures and ii) improving reaction initiations and propagation.

5.4.1 Pellet Properties The pellets of purified RHA and magnesium granules having a fixed composition but varying weights were made using the conditions outlined in Table 5.3. Figure 5.13 shows photographs of the as-produced pellets.

Table 5. 3 Pellet making conditions Composition 54.35% SiO2(RHA)

Diameter (mm)

Axial Load (kN)

Compressive Stress ( MPa)

16.28

39.23

188.5

45.65% Mg

Figure 5.13 Photograph of as-produced pelletized reactants

61

The properties of the as-produced pellets are summarized in Table 5.4. The theoretical density of the silica in RHA was taken as 2.2 g/cm3 and that of the magnesium granules was taken as 1.74 g/cm3 in the calculations of the pellet properties.

Table 5.4 Pellets properties Pellet #

weight (g)

Height (cm)

volume (cm3)

Bulk Density kg/m3

Theoretical Density kg/m3

% Compact

1 2 3

2 2.5 3.68

0.73 0.91 1.3

1.5196 1.8943 2.7061

1316 1320 1360

1962 1962 1962

67 67.3 69.3

It is seen from Table 5.4 that the porosity levels in the as- produced pellets are typically reduced to about one third. The plot of the weight of pellets against the percent compaction is given in Figure 5.14 which suggests a slight increase in porosity reduction with weight of charge poured in the die cavity. This is due to increased packing of particles as result of increased amount of reactant materials in the fixed volume of the die. 69.5 % Compaction

69 68.5 68 67.5 67 66.5 2

2.5

3

3.5

Figure 5.14 Plot of percent compaction against weight of pellet

62

4

5.4.2 Characteristics of Reduction Product The photographs of a typical magnesium-RHA reduction product are given in Figure 5.15. Figure 5.15-A, B and C shows in their respective order, the as-produced reduction product, crushed reduction product and the reduction product ground to passing 325 mesh.

A

B

C

Figure 5.15 Photographs of typical reduction product

63

All reduction products for post-reduction leaching were prepared as shown in Figure 5.15C. A low magnification SEM micrograph of the reduction product in Figure 5.15C is given in Figure 5.16. The corresponding EDX spectra of the points identified in Figure 5.16 are shown in Figure 5.17. The analysis at points #2 and #3 in Figure 5.16 shows the crusty and fibrous surface to be magnesium-rich and could possibly be mainly MgO or MgO intermixed with Mg2Si and Mg2SiO4. The EDX spectrum of the smoothed surface at point #1 reveals a high silicon-rich solid.

#2 #3 #1

  Figure 5.16 SEM micrographs of reduction product at 500X (See Figure 5.17 for EDX Spectra of numbered points)

The un-labeled peaks in the EDX spectra in Figure 5.17 correspond to the gold–palladium (AuPd) coating applied to the samples. 64

Point #1

Point #2

Point #3 Figure 5.17 EDX spectra of reduction product SEM micrograph in Figure 5.16

65

5.4.3 Effect of Composition on Reduction Product The effect of initial composition on the reduction product formed at a fixed temperature was first investigated using the FactSage™ equilibrium calculation module and model equation 4.5. The plot of weight fraction of phases in the reduction product against initial magnesium content in the charge using FactSage calculation is shown in Figure 5.18. The conditions of calculation is given as B= 0 for purely inert atmosphere and Temperature, T = 650 °C.

Figure 5.18 FactSage™ calculation of effect of magnesium content on phases in reduction product at 650 °C and inert atmosphere The weight percent values of the phases in Figure 5.18 were calculated exclusive of gases which in this simulation is mainly argon. It observed from the Figure 5.18 that wt% Si decreases sharply after approximately 5 wt% (1.05) excess magnesium whilst the Mg2Si phases 66

progressively increases after the same initial magnesium content of 5 wt% excess. It is further observed that Mg2SiO4 reduces to nearly zero when the initial magnesium content is approximately 5 wt% excess whilst the wt% MgO shows a gentle decrease beyond approximately 5 wt% excess magnesium. Additionally it is observed that SiO2 does not appear in Figure 5.18 under the purely inert atmosphere condition of the simulation. The XRD patterns of the experimental study of the effect of initial magnesium content on the reduction product at 650 °C are compared as shown in Figure 5.19.

Figure 5.19 XRD pattern comparing effect of % excess magnesium on phases formed ● Si

○Mg2Si 67

Qualitatively, it can be seen from the XRD pattern in Figure 5.19 that the peak intensity of the Mg2Si phase increases with increases in excess magnesium whilst the peak intensity of Si appear to decrease with increasing excess magnesium. In particular the peak intensity of Mg2Si appears smaller at 5 wt% excess magnesium whilst the inverse for Si is true; the silicon intensity appear to diminish after 5 wt% excess magnesium. The quantitative phase analysis of the reduction product samples by Reitveld method is shown in Table 5.5. Table 5. 5 Effect of Excess magnesium on phases in reduction product at 650 °C Phases (wt %) % Excess Mg 0 5 11.5 25 Mean Error %

Temp. ⁰C 650 650 650 650

Si 15.98 18.38 13.00 6.60 0.5

MgO Mg2Si 63.55 12.67 61.08 10.37 67.00 13.60 50.70 30.40 1.0 0.6

Mg2SiO4 7.40 9.84 5.60 12.20 0.7

α-SiO2 0.40 0.33 0.80 0.00 0.1

The plot of weight percent of phases against % excess magnesium is given in Figure 5.20. In agreement with the observations in the XRD pattern in Figure 5.19, the weight fraction of silicon is found to be highest at 5 wt% excess magnesium in Figure 5.20. The general trends observed in the FactSage calculation model are also evident in Figure 5.20. The presence of a small amount of silica in the quantitative analysis can be attributed to the practical challenge of maintaining a perfect inert atmosphere in a furnace or reactor. The data in Table 5.5 is made clearer when the effect of magnesium-silica mole ratio on the silicon yield is plotted as shown in Figure 5.21. The yield of silicon in the reduction product goes through a maximum with increasing magnesium to silica mole ratio. As seen from Figure 5.21, the optimum silicon yield occurs at charge composition having 2.1 mole ratio of magnesium to silica or alternatively a charge with 5 wt% excess magnesium when reduction is carried out in inert atmosphere. 68

100

Weight %

80 MgO

60 40 20

Mg2Si

Si

Mg2SiO4

α-SiO2

0 0

5

10 15 % Excess Mg

20

25

Figure 5.20 Experimentally measured effect of magnesium content on phases in reduction product at 650 °C and in inert atmosphere

69

Weight % Si Yield

100 80 60 40 20 0 2.0

2.1

2.2

2.3

2.4

2.5

Mg/SiO2 Mole Ratio Figure 5.21 Effect of magnesium-silica mole ratio on Si yield from reduction product

5.4.4 Effect of Temperature on Reduction Product The FactSage simulation of the effect of temperature on the formation of Mg2Si phase in the reduction product as well as weight percent silicon yield or recovery are shown in Figures 5.22 and 5.23 respectively. The simulation was done using the same model equation 4.5 but with the composition of the charge fixed and the temperature varied from 600 to 900 °C at 25 °C increments. It is observed from Figure 5.22 that as the weight fraction of the Mg2Si reduces with increasing temperature, the silicon yield increases in slight percentages in Figure 5.23. The MgO content was found to be steady and averaged at 72 wt%.

70

2.70

Weight % Mg2Si

2.65 2.60 B=0 (1+α) = 1.05

2.55 2.50 2.45 2.40 2.35 600

650

700

750

800

850

900

950

T ⁰C Figure 5.22 FactSage™ calculation of effect of temperature on Mg2Si formation

Weight % Si Yield

97.00 96.90 96.80

B=0 (1+α) = 1.05

96.70 96.60 96.50 96.40 96.30 96.20 600

650

700

750

800

850

900

950

T ⁰C Figure 5.23 FactSage™ calculation of effect of temperature on Si yield 71

The experimentally measured effect of temperature on relative amount of phases formed is qualitatively compared in the XRD pattern in Figure 5.24.

Figure 5.24 XRD pattern comparing effect of temperature on phases formed ● Si

○Mg2Si

72

The peak intensity of silicon in Figure 5.24 is observed to increase with increasing temperature while that of the Mg2Si phase diminishes with temperature, thus confirming the trend observed in the FactSage simulation of the reduction process. The results of the quantitative phase analysis of the reduction samples whose XRD patterns is shown in Figure 5.24 is given in Table 5.6

Table 5. 6 Effect of temperature on phases in reduction product Phases (wt %) Sample ID R600_5%Mg R700_5%Mg R800_5%Mg R900_5%Mg Mean Error %

Temp. ⁰C 600 700 800 900

Si 14.80 15.00 17.60 21.50 0.55

MgO Mg2Si 65.10 14.10 65.40 13.00 71.40 4.90 72.60 0.60 0.75 0.48

Mg2SiO4 5.60 6.10 5.30 5.00 0.6

α-SiO2 0.40 0.40 0.80 0.30 0.09

It is seen from Table 5.6 that at temperatures of 800 and 900 ⁰C the MgO content in the reduction product closely matches the theoretically calculated amount using FactSage. The plot of weight percent Mg2Si formation against temperature and the plot of weight percent silicon yield against temperature are given in Figures 5.25 and 5.26 respectively.

73

16 14

Weight % Mg2Si

12 10 8 6 4 2 0 600

650

700

750

800

850

900

950

T ⁰C Figure 5.25 Experimentally measured effect of temperature on Mg2Si phase formation in reduction product under argon atmosphere

74

100

Weight % Si Yield

90

80

70

60

50 600

650

700

750

800

850

900

950

T ˚C Figure 5.26 Experimentally measured effect of temperature on Si yield in reduction product under argon atmosphere

75

5.4.5 Reduction Mechanism It has been determined in previous sections that the initial charge composition and reduction hold temperature both influence the phases that are formed in the reduction product under the inert atmosphere conditions. In particular as the yield of silicon shows significant increases with increase in temperature for a fixed composition, it was desired to understand the mechanism of the reduction process. It was also desired to determine the minimum temperature at which the reduction reaction is triggered. The result of differential thermal analysis (DTA) carried out on powdered mixture of magnesium and purified RHA is shown in Figure 5.27.

1.2 DTA (exo) µV/mg 1

C, 647.9 °C

0.8

B, 590.4°C

0.6 0.4

A, 540.8 °C

0.2 0 0

200

400

600

800

1000

1200

Temperature (°C) Figure 5.27 DTA result of magnesium –RHA powder mixture (48 mg)

The DTA result in Figure 5.27 shows the reaction of powdered magnesium- RHA mixture to have multiple heat release peaks labeled as A, B and C. The peak A has an onset temperature of 76

524.2 °C and peak temperature of 540.8 °C. The two major peaks B and C have on-set temperatures of 573.8 and 643.8 °C respectively and peak temperatures as indicated in Figure 5.27. The temperature –time plot for hollow magnesium–RHA disc pellet having a composition of 5wt % excess magnesium and weighing 1.8g is given in Figure 5.28.

Temperature ( °C)

650 630 610 590 On-set Temp(°C) 575 °C

570 550 18

19

20 Time (s)

21

22

Figure 5.28 Temperature-time plot for magnesium–RHA under argon atmosphere Compared to the DTA result in Figure 5.27, it seen in Figure 5.28 that only one major heat release peak for the pelletized reactants is recorded by the thermocouple. The on-set temperature of 575 °C is close to the on-set temperature of the heat release peak B in Figure 5.27. Figure 5.29 shows the pellet condition immediately before and after the onset of the peak in Figure 5.28

77

Pellet

Thermocouple

 

 

At 568 °C

After 575 °C

Figure 5.29 Pellet condition before and after on-set of reaction in pellet The pores appearing on the pellet surface in Figure 5.29 after the on-set reaction is indication of conversion of low density- high volume reactants to high density low volume reaction product. The energetic wave propagation due to exothermic reaction at on-set causes a fraction of the suspended pellet to be broken or chipped away. Based on the results of the effect of temperature on the phases formed in the reduction product as well as the results of the DTA and hollow- disc pellet experiment, it is thought that the reduction reaction mechanism occur as follows; As temperature increases, the vapor pressure of magnesium inside the composite pellet increases. Eventually at temperatures around 575 °C, the vapor pressure is high enough so that the reaction between SiO2 and Mg can be triggered. As the exothermic reaction proceeds, local temperatures are increased, resulting in more supply of magnesium vapor and thus an accelerated reaction. The reaction is however slowed, as the reduction products, MgO and Si limit diffusion of Mg to the SiO2 core. This creates a concentration gradient for Mg from the surface to the reaction interface. Therefore, thermodynamically, at the outer layers, conditions become favorable for formation of Mg2Si

78

Further reduction of SiO2 is made possible through diffusion of Mg through the sandwiched product layer, composed of Mg2Si, MgO and Si. Also, insufficient Mg at the SiO2/MgO interface favors formation of Mg2SiO4 rather than reduction of SiO2. The reaction then becomes diffusion controlled so that higher temperature and longer time due to heating to higher temperatures favor further reduction of SiO2, resulting in consumption of Mg2Si with resultant higher Si yield and lower Mg2SiO4 formation as observed in the QXRD results in Table 5.6.

5.5 Post Reduction Purification Table 5.7 shows the physico-chemical properties of the reduction product (hereafter referred as R900 reduction sample) used for first stage post-reduction leaching study.

Table 5.7 Physico-chemical properties of R900 reduction sample Property

Reference Value

Particle diameter d50 (µm)

28.8

Particle diameter d90 (µm)

139.5

2

BET Surface Area (m /g) QXRD Chemical Composition (wt%) MgO Si

1.464 Normalized Value 72.6 21.5

Mg2Si

0.6

α-SiO2

0.3

Mg2SiO4

5.0

The R900 reduction sample was prepared by reducing in argon atmosphere at a temperature of 900 ⁰C a charge having initial composition of 54.35 wt% SiO2 (RHA) and 45.65 wt% of Mg. 79

The composition of the R900 reduction sample as determined from quantitative XRD (QXRD) has been normalized for the major phases identified in the reduction sample on analysis by powder XRD method. The oxides of aluminum, iron, manganese and other transition group metals were not detected in the XRD but may be present as trace elemental amounts in the sample. It is seen from Table 5.7 that magnesium oxide (MgO) is the major phase whilst Mg2Si and Mg2SiO4 are the minor phases. The leaching reagent considered for dissolving the MgO phases and other elemental species were limited to hydrochloric acid (HCl) and combination of hydrochloric and acetic acid (CH3COOH). This is because in general chloride and acetate salts are soluble in aqueous medium. The generalized reaction of MgO and other metallic oxides with HCl is given by Raschman and Fedorockova [39]: MexOy(s) + 2yH+(aq.) → xMe(2y/x)+ (aq.) + yH2O(l)

5.5

Where x=y=1 for MgO, the dominant by-product for the silica-magnesium reaction. For a mixed acid of HCl and CH3COOH , the possible reactions include: HCl (aq) + H2O (aq) = H3O+(aq) + Cl- ( aq)

5.6

CH3COOH + H2O(l) = H3O+ (aq) + CH3COO-

5.7

MgO(s) + 2H3O+aq = Mg2+ (aq) + 3H2O (l)

5.8

Mg2+ (aq) + 2 Cl- = MgCl2 (aq)

5.9

Mg2+ (aq) + 2CH3COO- = (CH3COO)2Mg(aq)

5.10

The literature value for the pKa for reaction 5.7 is given as 4.756 at room temperature [40] This gives the equilibrium constant for α-molar concentration of CH3COOH with x-molar dissociation to be : Ka =

H+ [ CH3COO-] [CH3COOH]

=

x2 α-x

= 1.76x10-5

Assuming x is 40 kJ/mol) whilst purely diffusion control process with stirring or agitation the EA values are found to be 1-3kcal/mol (4-13 kJ/mol). Intermediate control processes are found to have apparent activation energy values in the range of 5-8 kcal/mol (20-32 kJ/mol) [46]. The EA value for this study appears to fall within the intermediate control category however because no stirring or agitation was employed under the condition of this experiment. The slight increases in the rate of dissolution due to temperature increase shows that the dissolution mechanism is characteristic of diffusion control. In a typical process with some agitation the apparent activation energy can be lower.

5.6 Characterization of As-produced Silicon A photograph of the as-produced powdered material after second stage leaching in 1:9 volume ratio of 5 wt% HF: 25 wt% CH3COOH reagents is shown in Figure 5.36. The material has a brownish color with a greyish tint.

94

Figure 5.36 As-produced silicon from rice husks

A comparison of XRD patterns of the as-produced silicon from RHA (hereafter referred as RHSi) and high purity crystalline silicon powder standard which was procured from Alfa Aesar is given in Figure 5.37. It is seen that the major reflections of crystalline silicon are equally matched between the two patterns. The small peak immediately before the main silicon reflection in the as-produced silicon XRD pattern was identified as residual carbon which remained after the roasting processing step.

95

Figure 5.37 Comparison of XRD patterns of as-produced Si to high purity Si standard The SEM micrograph of the RH-Si powder is given in Figure 5.38. The micrograph reveals the etching action of the HF-acetic acid reagent on the other-wise smooth surface of the silicon. It can be seen from Figure 5.38 that not all the silicon particle surfaces were etched uniformly. Some surfaces remained relatively inert to the etching action of the reagent as evident by their relative smoothness. This suggests the possibility of preferential etching along certain 96

crystallographic planes of the silicon. The pores created on the silicon resulted in a higher BET surface area value of 47 m2/g when compared to the specific area of the as-received RHA. The particle size analysis gave a distribution which is presented in Table 5.11.

Figure 5.38 SEM micrograph of RH- Si (taken at 500X)

97

Table 5.11 Particle size analysis of RH-Si

Sample ID

RH-Si

Particle Size d10

Particle Size d50

Particle Size d90

(µm)

(µm)

(µm)

4.2

18.6

45.8

The ICP-OES/MS chemical analysis of two batches of silicon produced in this research work is given Table 5.12. The analysis results in the first two columns of Table 5.12 were carried out internally using an ICP-OES instrument at the ANALEST Laboratory, University of Toronto. The results in the last column of Table 5.12 were obtained from an external laboratory (IPLCanada Inc.). The only difference between RH-Si Batch #1 and #2 is the method of post leaching rinsing or washing. With RH-Si Batch #1, the leached residue was retained on the filter paper and continuously washed with cold de-ionized water until near neutral pH. In the case of RH-Si Batch #2, the leached residue was re-pulped with warm deionized water to a volume approximately equal to the original leach solution volume. The re-pulped solution was then water leached at 40-50 °C continuously for one hour. The result in Table 5.12 shows RH-Si Batch #1 to have relatively low residual magnesium content when compared to the analysis of the RH-Si Batch #2. One possible explanation to this observation is that when the residue is re-pulped with warm de-ionized water of approximately equal volume to the initial acid solution, a neutral pH is reached quickly which results in hydrolysis of magnesium ions into insoluble Mg(OH)2 . This insoluble salt adheres to the silicon particles and is then retained during filtration. Thus it is better to maintain a slightly acidic medium during the rinsing action after leaching. 98

Table 5. 12 Chemical analysis of RH-Si RH-Si Batch#1

RH-Si Batch #2

RH-Si Batch#2 (IPL Canada Analysis)

Analyte/Element

Conc.(ppmw)

Conc.(ppmw)

Conc.(ppmw)

B P Al Fe Mg Cr Ni Mo Co Mn Ca Na K Cu

17 >22 3 4 1.5 n/a 5 2 >2 n/a 12 4 5 110