Extraction of High Purity Silicon From Sugarcane Bagasse Ash

Extraction of High Purity Silicon from Sugarcane Bagasse Ash

Jermaine A. Lamboso Jhonnielyn Joy T. Fidel Jireh Jan S. Villamor Blesy May G. Tolentino Wayne Laurence Bobon

Engr. Mary Ann Pandan, MS EnE, PhD EnE Adviser

University of St. La Salle

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CHAPTER 1 Introduction As the leading industry in Negros, sugar production creates waste by-products that can still be tapped for added profitability. A prime example of this is bagasse, the fibrous residue in the extraction of cane juice, which is now used as biomass for boiler operations and sometimes for the plant’s power grid (Bangcoguis, 2007). The burning of bagasse leaves an ash residue, another by-product that can be recycled as land fill and filler for building materials (Affandi, Setyawan, Winardi, Purwanto, & Balgis, 2009). This study aims to create added value for sugarcane bagasse ash when processed to produce silicon (Si), a valuable material whose applications range from aluminum and ferrous alloys for construction, to solar panels for renewable energy, to semiconductors for electronics. High purity silicon (98-99.99% Si) has been studied as an alternative to metallurgicalgrade silicon (MG-Si) for industrial uses. Metallurgical-grade silicon (MG-Si) with purity usually at 98% Si is processed from quartz sand (primarily SiO2). It serves as the raw material for the production of solar-grade (99.9999% Si) and electronic-grade silicon (99.9999999% Si) in photovoltaic and electronic industries respectively. Previous studies have produced high purity silicon from plant biomass such as rice husks as a cheap alternative source of silicon dioxide (SiO2), instead of quartz sand (Lund, Zhang, Jennings, & Singh, 2000). Quartz sand is obtained by sand mining, which has detrimental effects to the environment like land degradation, erosion, fissures, and adverse effects to water supply and quality (Saviour, 2012). The use of plant biomass as the source of silicon instead of quartz sand may address this environmental

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concern. Like rice husks, sugarcane bagasse ash is also rich in SiO2 (Abrasia, Alabado, Etang, & Taton, 2002), making it a viable source for the production of high purity silicon. The extraction of high purity silicon from sugarcane bagasse ash not only adds economic value to this waste material and increases process efficiency for the sugar industry, but also promotes environmental care by being an alternative to mining activities. 1.1 Objectives of the Study The primary objective of this study is to extract high purity silicon from sugarcane bagasse ash. Specifically, it aims to: 1.) Determine the parameters that affect the extraction of high purity silicon 2.) Devise and perform a method in extracting high purity silicon (Si) from sugarcane bagasse ash 3.) Determine the purity of silica (SiO2) obtained from sugarcane bagasse ash 4.) Characterize the final product (Si) according to its purity and impurity concentrations 1.2 Significance of the Study The study may be significant to the following: Chemists and chemical engineers. Through this study, chemists and chemical engineers may be inspired to further develop novel procedures in converting a waste material to a significant raw material. They may also have the chance to improve existing procedures so as to increase efficiency and further optimization.

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Entrepreneurs. The low-cost value of producing a significant raw material, such as silicon, can boost the profitability of a business or industry, as production costs can be lessened by using materials that are basically waste by-products, like sugarcane bagasse ash. Students. Through this study, students may be able to learn experimental methods that are a direct application of their basic classroom knowledge e.g., general chemistry. They may also be inspired to do research on their own and try to test the validity of the methods by doing further experimentation. Teachers. Through this study, teachers may be able to see the degree of learning in the student-researchers by observing the quality of the research methodologies and findings. Government officials. Through this study, the government may be able to promote policies on the use of waste from agricultural products, support cost-effective methods in the production of goods, and improve research and development funding for science and technology researchers in the country. 1.3 Scope and Limitations of the Study This study targets to perform an experimental procedure to extract silicon (Si) from sugarcane bagasse ash (SCBA) primarily at University of St. La Salle’s Chemical Engineering Research Laboratory for the first semester of academic year 2015-2016. Sugarcane bagasse ash is to be acquired by random sampling from sugar milling industries in Negros Occidental such as First Farmers Holding Corporation and Victorias Milling Corporation. Chemical reagents are to be supplied by University of St. La Salle College Science

EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH

Laboratories and Integrated Scientific and Industrial Supply. Quantitative analyses of the samples and end-product are to be done by an external institution, National Institute of Geological Sciences – UP Diliman (NIGS).

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CHAPTER 2 Review of Related Literature 2.1 Silicon and its properties, types, and uses Silicon, with the chemical symbol Si, is the second member in Group 14, formerly known as Group IV-A, of the periodic table of elements. It is characterized as a metalloid whose atomic number is 14 with an electronic configuration of [Ne] 3s2 3p2. Dating back 1824, Jöns Jakob Berzelius discovered silicon whose name was derived from silex or silicis meaning ‘flint’. It occurs in the solid state at room temperature and its melting and boiling points are 1414°C and 3265°C respectively (Royal Chemical Society, 2015). Second only to oxygen in terms of abundance on the earth’s crust (approximately 28% by mass Si), silicon is naturally found as 92%

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Si, 4.67% 29Si, and 3.1% 30Si making its average

atomic mass at 28.085 (Nave, 2015). It has a gray and lustrous appearance, and crystallizes in a diamond-cubic structure. Crystalline forms of silicon include monocrystalline, polycrystalline, and amorphous silicon. The monocrystalline type or single-crystal silicon is the purest form of silicon, characterized by its homogeneous crystal framework and lack of grain boundaries. It is commonly used in producing solar cells and is more efficient than the cheaper polycrystalline type (Heywang et al, 2004). Polycrystalline silicon, on the other hand, is not as homogeneous as single-crystal silicon as its framework is made up of multiple smaller crystals. It is more costeffective to produce commercially, thus its wide use in electronics and photovoltaics as well (Fraunhofer Institute, 2014). Amorphous silicon is a noncrystalline allotrope of silicon used in

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photovoltaics that require low power and in production of thin film transistors. A pocket calculator’s solar cell is usually amorphous silicon (New Scientist, 1985). Silicon used in the industry is classified according to their purity. There are three categories by which silicon is sorted out – metallurgical grade, solar grade and electronic grade. Safarian et al. (2012) stated that metallurgical grade silicon (MG-Si) is the initial material used for producing the other classifications. MG-Si produced through the industrial process is 98-99% pure with remnants of other elements like iron, aluminum, titanium, vanadium, boron, and phosphorus which affects the efficiency of solar grade silicon (SG-Si) and electronic grade silicon (EG-Si). SG-Si is the type applicable for the photovoltaic industry for use in manufacturing solar panel wafers. However, before they can be utilized in the solar industry they must be purified up to 99.9999% (6N). In order to qualify for EG-Si, 99.999999% (8N) or higher purity of silicon must be achieved (Fishman, 2008). Most of the world’s silicon production is used to make alloys including aluminum-silicon and ferro-silicon (iron-silicon). Silicones, or silicon-oxygen polymers, is also considered as an extensive use for silicon. Silicone oil is a lubricant added to hair products and cosmetics. Silicone rubber is also used as sealant in bathrooms, windows, pipes, and roofs. Sand (silicon dioxide or silica) and clay (aluminum silicate) are used to make concrete and cement, glass, and various ceramics. Silicon carbides have applications in abrasive and laser industries. Most common of all is the use of silicon as a semiconductor in computer, microelectronics, and photovoltaic industries (Royal Chemical Society, 2015). 2.2 Silicon economy in the Philippines

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According to The World Factbook by the United States’ Central Intelligence Agency (2015), the Philippines primarily exports semiconductors and electronic products, transport equipment, garments, copper products, petroleum products, coconut oil, fruits. A recent report by the Department of Trade and Industry showed that semiconductors and electronic products are the country’s top export, accounting for 45.96% of export goods as of April 2015. In 2014, Philippines earned $16,913,341,592.66 on integrated circuits alone (Simoes, 2014). The non-stock, non-profit organization Semiconductor and Electronics Industries in the Philippines, Inc. or SEIPI has presented that in order to maintain the growth of the electronics industry in the country, manufacturing cost control and silicon wafer fabrication for semiconductors are areas that need to be developed in the country. Presently, the country imports all materials abroad and only focuses on test and assembly of semiconductors and electronics. Manufacturing the raw materials here in the country could be an advantage for the electronics sector, especially in reducing production costs and decreasing the reliance on imports from other countries (Santiago, 2015). 2.3 Manufacture of silicon in the industry Quartz sand, basically crystalline SiO2, is the raw material for the industrial production of metallurgical-grade silicon or MG-Si whose purity ranges between 98-99%. The sand is reduced by carbon at 1900°C in an electric arc furnace. The majority of the world’s production is used as raw material for the manufacture of steel and aluminum alloys, solar cell industries, and electronics. The level of impurity in metallurgical-grade silicon is too high for photovoltaic and

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microelectronics applications, thus the need for further purification steps in producing solarready and circuit-ready silicon (Koch and Rinke, 2014). In the commercial-scale, electronic-grade silicon (EG-Si) is manufactured by the commonly used process known as the Siemens process. It is at present the standard method for purifying metallurgical-grade silicon (MG-Si) to 99.9999% pure polycrystalline silicon or use in producing semiconductor devices and solar cells (Lund, Zhang, Jennings and Singh, 2000). This is done according to the chemical reactions: Metallurgical Si(s) + 3HCl(g) SiHCl3(g) + H2(g)

(reaction 1)

SiHCl3(g) + H2(g) Si(l) + 3HCl(g)

(reaction 2)

In this process according to Lund et al. (2000), trichlorosilane (SiHCl3) is first obtained from bed of fine MG-Si particles. The metallurgical particle is fluidized and chlorinated with hydrochloric acid with copper as the catalyst in the reaction. To reduce impurities, the impure SiHCl3 then undergo succeeding fractional distillation. A chemical vapor deposition method is subsequently used to produce the EG-Si from the high purity SiHCl3. Vaporized SiHCl3 is decomposed and reduced with hydrogen at about 1000°C, resulting to silicon deposit on an inverted U-tube. The bridge is made of slim silicon rods and has been heated in a reactor by passing an electric current through it. This process can produce six polycrystalline rods of 1 m length and 12 cm diameter simultaneously. The obtained EG-Si has a purity of 99.9999999% (9N purity). Other growth techniques for photovoltaic applications are also available such as the wellknown Czochralski method and float-zone melting. The Czochralski method involves melting

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metallurgical-grade silicon in a quartz crucible at a temperature greater than 1400°C, above silicon’s melting temperature, in inert gas atmosphere commonly argon. The crucible is in a graphite container to implement homogeneous heat transfer. A monocrystalline silicon seed crystal is then introduced to the melt to facilitate nucleation and crystal growth. As the seed crystal is slowly pulled out of the melt, the crucible also counter-rotates to improve homogeneity. The pull speed here (usually just a few centimeters per hour) determines the cylindrical crystal diameter. Doping methods can also be integrated in the Czochralski method. On the other hand, float-zone melting operates by introducing also a monocrytalline seed crystal to the less pure polysilicon. A radiofrequency (RF) coil melts the polysilicon which when cooled down, solidifies to very high purity monocrystalline silicon (Koch and Rinke, 2014). The product has applications in the photovoltaic industries and is actually considered to be solar-grade silicon or SG-Si. 2.4 Production of silicon from other raw materials Although commercial industries commonly use quartz sand as raw material for the abovementioned processes, research has also been undertaken to explore other possible raw materials with high silica (SiO2) content including rice husk ash, bamboo leaf ash, and mud for the production of high purity silicon. Amick et al. (1980) patented a process for the production of solar cell-grade silicon from rice husk. The method comprised of leaching the rice husk with aqueous semiconductor-grade hydrochloric acid, followed by pyrolysis of the leached husk at 900°C in flowing argon with 1% anhydrous hydrochloric acid for 30 minutes. To adjust the carbon-to-silica ratio to 2:1, the

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sample was processed in a fluidized bed combuster with flowing argon and carbon dioxide at 950°C. Carbothermic reduction of the ash subsequently ensued at 1900°C, reportedly yielding high purity silicon with impurities less than 75 ppm. Hunt et al. (1984) investigated the feasibility of producing silicon with even higher purity by improving the purification technique of Amick et al. through pelletizing of the reactants before reduction with carbon black in a modified electric furnace. Their study claimed that their purified rice husk ash is a viable candidate as raw material for solar grade silicon synthesis. Ikram and Akther (1988) used high purity magnesium as a reductant, after preparation of the rice husk ash by acid leaching in 1:10 hydrochloric acid and pyrolysis in a muffle furnace. Additional acid purification with hydrochloric, hydrofluoric, and sulfuric acid was done after reduction. The study reported a yield of 99.95% silicon with a Boron impurity of about 2ppm. Surpassing Ikram and Akther with 99.9999% silicon purity, Singh and Dindaw (1978) also used magnesium to reduce white rice husk ash at 800°C with subsequent acid leaching treatments. The authors also suggested the possibility of smelting the obtained silica with carbonaceous reductants in a furnace, followed by acid purification, and to repeat the process nine times. The analysis method to determine the purity of silicon however was not specified in the paper. Larbi (2010) also devised a method to synthesize 99.5% pure silicon from rice husks by a pre-reduction acid treatment, reduction with magnesium, and a two-stage acid leaching process using different mixtures of hydrofluoric, acetic, and hydrochloric acid. Boron impurity was less than 3ppm. Highest silicon yield was achieved with a reduction temperature of 900°C in argon

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atmosphere and a charge with 5% excess magnesium. The obtained silicon was considered high purity: greater than metallurgical-grade purity but less than electronic-grade. Aminullah et al. (2015) extracted silicon dioxide from bamboo leaf ash by combusting the leaves in open air and ashing the obtained residue in the furnace at 400°C and then at 950°C. After acid leaching and filtration, the ash was again put in the furnace at 1000°C to obtain silicon dioxide. Magnesium was used as reducing agent, pyrolyzed at 650°C for an hour. After acid treatment with 3% HCl and drying, the resulting silicon obtained had a purity of only less than 10%. This was attributed to the type of acid used in leaching, which was insufficient to dissolve impurities. The samples were characterized by Energy Dispersive X-ray (EDX) and Scanning Electron Microscopy (SEM). Mubarok et al. (2014) employed local hot mud as raw material for preparation of silicon. Lapindo mud is an active spurt of hot mud from a drilling location in Indonesia classified as a natural disaster. It had been found to be rich in silica content, inferentially a potential source for silicon extraction. With the addition of sodium hydroxide, silica was extracted from the mud as sodium silicate. Titration with hydrochloric acid, washing and drying then produced silica xerogel. Reduction of the silica with magnesium at 650°C for 3 hours and subsequent acid leaching using hydrochloric, hydrofluoric, and acetic acid yielded silicon with 98.1% purity. Characterization was done by X-ray diffraction and X-ray fluorescence. Affandi et. al (2009) extracted silica (SiO2) xerogels with a purity of 99% from sugarcane bagasse ash by employing the sodium silicate route by extraction by adding NaOH, titration with HCl, gelation, and then drying. The study employed three methods to determine which produces

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the best purity of SiO2, namely pretreatment acid washing with 1 M HCl, cation exchange resin treatment, and post-treatment washing using demineralized water. Of the three methods, the group concluded that using demineralized water was effective in improving purity to as high as 99%. The characterization of the produced silica (SiO2) xerogels was done by X-ray fluorescence spectroscopy. All of the abovementioned methods employ the same three basic steps in the production of silicon(Si): pre-reduction treatment, reduction, and post-reduction treatment. This paper aims to devise a method that also includes these three basic steps to obtain silica (SiO2) from sugarcane bagasse ash with the appropriate and optimal parameters. 2.5 Sugarcane bagasse ash, its properties, and uses Sugarcane bagasse ash (SCBA) is obtained as a solid waste from sugar industries. After crushing of sugarcane in sugar mills and extraction of juice from processed cane by milling, the discarded fibrous matter called bagasse is used as fuel to generate power and electricity in the factory. Bagasse is burnt at to use its maximum fuel value and the residue after burning, namely bagasse ash, is collected and disposed of as landfill. In order to maximize its potential, several studies were conducted that aims to find other ways to utilize SCBA and increase its value in the industry. SCBA has benefited a number of different fields due to its remarkable properties. Studies using SCBA as cement replacement in concrete (Kawade et al, 2013), alternative pozzolanic material (Suliman et al., 2011) and supplementary cementitious material in concrete (Dhengare et al, 2015) proves its effectiveness as a construction material. SCBA is also efficient when used

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as adsorbent as shown by Kanawade et al. (2010) using bagasse ash in removing dyes from dye effluent and brilliant green dyes from aqueous solutions (Mane et al, 2007). Teixeira et al. (2010) also produced glass-ceramic materials from SCBA. SCBA as a waste material from the burning of bagasse for power generation in sugarcane industries can thus be recycled for its high silica content. A study conducted by Abrasia, Alabado, Etang and Taton (2002), characterized SCBA acquired from First Farmers’ Holding Corporation in Negros Occidental. Table 2.1 shows the different compounds that constitute SCBA. Table 2.1 Chemical composition of sugarcane bagasse ash* Component

Composition (wt. %)

SiO2

76.10

Al2O3

14.76

CaO

3.48

Na2O

0.65

other components

5.01

*(Abrasia et al., 2002) Due to its high silica content, the use of SCBA for the extraction of high purity silica (SiO2) and eventually silicon (Si) is therefore a viable route for research, coupled with the right and optimized methodology. 2.6 Methods of silica and silicon analysis

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The conversion of insoluble silicates into sodium silicate through high temperature fusion with other sodic bases is the traditional method of determining the silicon content of different materials (Silicon in Agriculture, 2001). With the continuing advance research of silicon around the world, different methods of silicon content determination have been developed including gravimetric, colorimetric and absorption/emission spectroscopy (Dai et. al, 2005). Gravimetric method is one of the classical quantitative analysis of silicon. In the analysis of silicon in soils, the method begins with oxidation to remove organic matter, acid dissolution of remaining components, filtration of silica precipitate, and finally ignition to recover silicon. Gravimetric method uses simple laboratory equipment yet time consuming and strenuous to work (Silicon in Agriculture, 2001). Colorimetric analysis is a cheaper technique of quantifying silicon content of various materials since it only uses standard analytical equipment in the laboratory. It is based on the formation of yellow silicomolybdic acid at higher silicon concentration that is further improved to blue silicomolybdic acid procedure at lower silicon concentration using a reducing solution. The latter is preferred because of its high sensitivity (Hogendorp, 2008). X-Ray diffraction (XRD) is a rapid analytical technique used to determine the crystal structure and crystalline phase of material. In the preparation of high purity silicon from raw rice hulls (RRH) the high purity silicon in the form of white ash was found out to be polycrystalline and amorphous respectively. The Raman Spectroscopy, technique that provides information also about the physical characteristics such as crystalline phase and orientation of high purity silicon, conformed to the results shown by XRD (Swatsitang et al., 2009).

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Scanning Electron Microscopy (SEM) determines the surface structure, shape, particle size, and morphology of the sample shown in the three-dimensional form (Worathanakul, 2009). Larbi (2010) revealed the morphology of high purity silicon through SEM that was observed by SEM micrograph showing the porosity of the prepared silicon due to acid leach. The combustion of the organic component contributed to the porous morphology of rice hull ash (RHA). Worathanakul et al. (2009) determined silica content in bagasse ash using x-ray fluorescence spectroscopy (XRF). Bagasse ash was heated in the furnace at 600°C, 700°C, and 800°C for three hours were analyzed through XRF and showed silica contents of 19.42%, 21.05% and 27.98% respectively. Subjecting the ash in acid treatment of 1M and 3M hydrochloric acid and oxygen feeding in the furnace at 800°C for 3 hours, silica content rose to 89.037%. From the analysis, Worathanakul et al. (2009) concluded that the increase in temperature, acid treatment, and oxygen feeding removed most of the impurities in sugarcane bagasse ash. Larbi (2010) used inductively coupled plasma optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS) to analyze the chemical composition of the final silicon powder obtained from the second cycle leaching. Fifteen (15) mL of multiple-acid mixture was prepared using the volume ratios of 1:1:1 deionized water, concentrated nitric acid (HNO3, 70 wt%), concentrated hydrofluoric acid (HF, 48 wt%) in the respective order and used this to digest a 0.15 gram sample of the silicon powder in a closed Teflon beaker. The Teflon (polytetrafluoroethylene) beaker and content was then heated to a temperature of 50-70 °C for half an hour. The totally digested sample was transferred to an HF-resistant 50-mL volumetric flask or graduated cylinder. The sample solution was then filled up to the 50-mL mark with 2

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vol% nitric acid solution for ICP-OES analysis. A blank solution was prepared with the same ratios but with a volume factor of five less than the prepared sample 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

1000

mg C ( L ) prep Vol (L) (equation 2.1) wt of Si (g)

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

1000

mg D ( L ) prep Vol (L) wt of Si (g)

(equation 2.2)

Meanwhile, Swatsitang (2009) analyzed the obtained Si from rice hulls by XRD and found to be polycrystalline Si as also confirmed by Raman spectra. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis confirmed metallic impurities such as Al, Fe, Ca, Ni, Mn, Mg, Cu, Cr and Ti in the total range of 145 – 325 wt.ppm. About 99.98 % purity of silicon was extracted from acid-treated RRH.

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CHAPTER 3 Methodology 3.1 Research Design

This chapter aims to discuss in detail the experimental procedure of synthesizing high purity silicon from sugarcane bagasse ash. Ash sample received from sugar factories in Negros Occidental is subjected to an extraction treatment to yield silica (SiO2) xerogels. Metallothermic reduction of SiO2 using magnesium (Mg) as reductant is carried out at a temperature of 650°C in a furnace. Subsequent acid leaching steps then ensue as post-reduction treatment to remove unwanted soluble phases that may have formed after reduction. The product obtained is silicon and is analyzed using energy dispersive X-ray fluorescence spectroscopy (EDXRF) A schematic flowchart for the procedure is outlined in Figure 3.1.

Sugarcane Bagasse Ash (SCBA) Extraction of SiO2 Reduction Post-Reduction Treatment High Purity Silicon Powder Test Melting High Purity Silicon Chunks

Figure 3.1 Scheme of the experimental procedure

XRF Analysis

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3.2 Materials and Reagents

The list of required materials and reagents for the experiment, their description, and their following sources are showed in Table 3.1.

Table 3.1 Materials and reagents needed for the experimental work Material/Reagent

Description

Sugarcane Bagasse Ash

Residue from combustion of bagasse

Sodium Hydroxide

2 M aqueous solution

Magnesium Turnings

99 wt% pure

Source First Farmers’ Holdings Corporation USLS College Science Laboratory USLS College Science Laboratory

1 M aqueous solution Hydrochloric Acid

33.333 vol% aqueous solution

USLS College Science Laboratory

50 vol% aqueous solution Hydrofluoric Acid

50 vol% aqueous solution

USLS College Science Laboratory

3.3 Procedure

The synthesis of silicon from sugarcane bagasse ash is accomplished in three major steps: purification treatment, reduction, and post-reduction treatment. Characterization of the raw materials and quantitative analyses of the products are done through energy dispersive X-ray fluorescence spectroscopy (EDXRF).

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3.3.1 Silica xerogel production. Silica is extracted from 100 g of SCBA using 600 mL of 2 M NaOH producing sodium silicate. The mixture is boiled for 1 hour with constant stirring. The sodium silicate is separated from the solids through vacuum filtration. The filtrate solution is the sodium silicate, which subsequently is set to room temperature. In the gelation process, the sodium silicate solution is titrated with 1 N HCl under constant stirring up to the pH of 7 to produce silica gel. The silica gel is then aged for 18 hours. After aging, the gel is gently broken by adding 1 L of de-ionized water to make slurry. The slurry is filtered and washed three times with de-ionized water. The powder is then dried in a drying oven at 80°C for 12 h. This method is adapted from Affandi et al. (2009). The flow diagram of silica xerogel production is shown in Fig. 3.2.

Bagasse Ash Extraction with 2M NaOH

Filtration

Gelation

Aging

Slurry Formation

Washing

Drying Fig. 3.2 Flow diagram of the procedure used to produce silica xerogels from SCBA

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3.3.2 Reduction treatment of SCBA (SiO2). Stoichiometric amounts of the as-produced SiO2 powder and Mg are ground by mortar and pestle to ensure homogeneity (Swatsitang & Krochai, 2009). These amounts are calculated using equations 3.1 and 3.2 as shown below. The percent purity of the produced silica (SiO2) xerogels as determined by XRF is used in equation 3.1. The mixture is then put in a muffle furnace at a temperature of 650°C for three hours (Ikram & Akther, 1988; Singh and Dindaw, 1978; Aminullah, Rohaeti & Irzaman, 2015). Weight of SiO2 = Weight SiO2 xerogels × (% purity of SiO2 xerogels)/100 (equation 3.1)

Weight of Mg = Weight of SiO2 ×

48g mol 60 g mol

(equation 3.2)

3.3.3 Post-reduction treatment.

Adapted from Swatsitang & Krochai (2009), the post-reduction treatment of the reduced product undergoes three leaching sequences. The first and second leaching sequences are basically the same process: a mixture of hydrochloric acid and water with volume ratio 1:2 is used as leaching reagent at room temperature for 10 minutes and then repeated with a different reagent which is hydrofluoric acid in water 1:2 volume ratio.

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Figure 3.3 Si-O-Mg phase diagram at 650°C (Larbi, 2010) The third leaching sequence with 1:1 volumetric ratio of acid to water is set to 95oC for 15 minutes. The leached slurry undergoes vacuum filtration through Whatman filter paper (Whatman # 42), washing with distilled water and drying in the oven at 105°C. The same leaching setup is used as in the pre-reduction acid treatment as shown in Figure 3.2. The summary for the post-reduction treatment is shown in Table 3.3.

Table 3.3 Post-reduction leaching sequences (Swatsitang & Krochai, 2009) Leaching sequence

Volume ratio

Temperature

Duration

1

1:2 HCl :H2O

Room temperature

10 minutes

Room temperature

10 minutes

1:2 HF: H2O 2

1:2 HCl :H2O 1:2 HF: H2O

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3

1:1 HCl :H2O

95°C

22

15 minutes

3.3.4 Determination of the silicon purity of the final product.

Characterization of the final product (Si) employs energy dispersive X-ray fluorescence spectroscopy (EDXRF) for elemental analysis of the impurities in the product. This is to be done by National Institute of Geological Sciences-UP Diliman (NIGS).

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CHAPTER 4 Results and Discussion This chapter presents the tabulation and analysis of data for the experimental study according to the methodology as discussed in Chapter 3. It is divided into three sections corresponding to the three main steps in the study namely preparation of SiO2, reduction of SiO2 to Si, and post-leaching treatment for the product. Findings from each of the step are presented accordingly. Appendix A presents photos for the whole process. Appendix B provides a copy of the official results from National Institute of Geological Sciences (NIGS), UP Diliman. 4.1 Preparation of Silica (SiO2) Xerogels A mixture of 100g bagasse ash and 1 liter 2N NaOH was found to boil at a temperature of 96.5°C for sample A and 97°C for B. Two hundred and ninety (290) mL of yellow to brown filtrate (sodium silicate) was obtained after filtration for A and 310 mL for B. The loss in volume can be attributed to the evaporation of water from the mixture while boiling. This volume of filtrate required 800 mL of 1 M HCl to reach a pH of 7 of A and 1015 mL for B. Silica xerogels acquired after aging of 18h, washing, and drying in the oven for 12h had the appearance of large white solid chunks with a total mass of 12g for A and 68.9g for B. The chunks were then ground to powder form. Table 4.1 presents data acquired from the experimental study. Due to financial constraints, only sample B was sent for analysis. Table 4.2 presents the composition of the product sample B. Figure 4.1 shows the X-ray diffraction profile for the silica obtained for sample B. Analysis was done using EDXRF or energy dispersive X-ray fluorescence at the National Institute of Geological Sciences (NIGS), UP Diliman.

EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH

Table 4.1 Results from Preparation of SiO2 SAMPLE

A

B

Boiling temperature of SCBA and NaOH mixture

96.5°C

97°C

Volume of filtrate (sodium silicate)

290 mL

310 mL

Volume of HCl used titrate to pH 7

800 mL

1015 mL

Mass after drying

12 g

68.9 g

Table 4.2 Relative concentrations of components in sample B Analyte SiO2

%relative concentration 54.87

Cl

40.39

K2O

3.16

SO3

1.17

Fe2O3

0.30

ZnO

0.05

CuO

0.03

Rb2O

0.02

24

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4.2 Reduction of Silica (SiO2) with Magnesium (Mg) Figure 4.1 X-ray diffraction profile of components in sample B Six grams of each powder sample A and B produced from the previous step had been subjected to reduction with a stoichiometric amount of Mg which is 5.4 grams. Data obtained from the experimental procedure is tabulated in Table 4.3. Table 4.3. Results from the reduction treatment SAMPLE

Mass, g With Mg 11.40 11.40

Without Mg 6.00 6.00

A B

After reduction 9.56 9.12

4.3 Post-Reduction Treatment through Acid Leaching The reduced samples had undergone post-treatment to leach out acid-soluble impurities. The masses of the samples used, corresponding volumes of reagents, and the masses of the final products are tabulated in Tables 4.4 to 4.6. Table 4.4 Mass determination of as-reduced product and post-leaching product

Sample

Mass Before post-leaching After post-leaching

A

8.00 g

2.19 g

B

8.00 g

3.15 g

Table 4.5 First and second acid leaching sequence

EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH

A

Total Volume of Acid 12 mL

B

12 mL

Sample

1:2 HCl : H2O HCl H2O 4 mL 8 mL

1:2 HF : H2O HF H2O 4 mL 8 mL

4 mL

4 mL

8 mL

26

8 mL

Table 4.6 Third acid leaching sequence Sample A B

Total Volume of Acid 22 mL 22 mL

1:1 HCl : H2O HCl H2O 11 mL 11 mL 11 mL 11 mL

Analyses by EDXRF of the constituents in final products A and B are presented in Table 4.8. X-ray diffraction profiles for the samples are presented in Figure 4.2 and 4.3. Table 4.8 Relative concentrations of the constituents in final products A and B

Analyte, %

Product A

Product B

Si

94.33

83.32

Cu

0.06

0.96

Al

3.65

Fe

0.22

Pb

0.05

Po

0.05

Ir

0.04

K

1.33

Rb

0.02

Os

0.02

9.76

EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH

Mn

0.64

Sc

0.18

Zn

0.06

Ti

1.48 1.17

Figure 4.2 X-ray diffraction profile of components in final product A

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Figure 4.3 X-ray diffraction profile of components in final product B CHAPTER 5 Summary, Conclusion and Recommendations This chapter presents the summary of the findings or results of the study as well as the corresponding generalizations and recommendations necessary for the development and improvement of the study. 5.1 Summary Extraction of silicon was first done starting with 100 grams of SCBA with the addition of 600mL NaOH and titrating with 1M HCl until it reaches the pH of 7. The silica gel formed was aged for 18 hours and washed to obtain the silica powder. The silica obtained is very largely amorphous. The percentage of silica content is only 54.87% which is much lower than the

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expected percent silica content of 99.16%. This silica was then reacted with Mg-ribbon through a muffle furnace at 600oC for 12 hours. The reduced product underwent post leaching with HCl and HF to leach out impurities. The obtained percent purity of the final product only reached 94.33% Si as the highest among the 2 samples as analyzed by XRF. Obtained silicon appears to be brown in powdered form. It has been shown that Sugarcane Bagasse Ash (SCBA) is a good raw material for the extraction of high purity Silicon (99% Si). 5.2 Conclusions By conducting this experimentation and analyzing the product obtained in the extraction of silicon from sugarcane bagasse ash, the following conclusions were derived: 1. The study successfully produced silicon from sugarcane bagasse ash from two experimental runs. 2. The percentage of silica content is 54.87% as analyzed by XRF which is much lower than the expected percent silica content of 99.16% based on the study of Affandi et al. 3. The obtained purity of the final product reached 94.33% Si as the highest between the 2 samples as analyzed by XRF. Obtained silicon appears to be dark brown in powdered form. 5.3 Recommendations The researchers would like to recommend the following for the improvement of the study and future works:

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1. Optimization of the different parameters such as temperature, contact time, and amounts and concentrations of reagents for a certain amount of raw material may be investigated to maximize the efficiency of the process. 2. Implementation of different methods and techniques of extracting silica with lesser time and energy requirement may also be explored. 3. A larger scale study for the process may be carried out to determine the feasibility of the process on a commercial-scale basis.

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References Abrasia, C.J., Alabado, V., Etang, M.A., & Taton, M.H. (2002). Production of glass using bagasse ash (Undergraduate Thesis). University of St. La Salle, Bacolod City. Affandi, S., Setyawan, H., Winardi, S., Purwanto, A., & Balgis, R. (2009). A facile method for production of high-purity silica xerogels from bagasse ash. Advanced Powder Technology, 20, 468–472. Amick, J. A., Milewski, J. V., & Wright, F. J. (1980). US Patent 4214920 A. Washington, DC: U.S. Patent and Trademark Office. Aminullah, Rohaeti E., & Irzaman. (2015). Reduction of high purity silicon from bamboo leaf as basic material in development of sensors manufacture in satellite technology. Procedia Environmental Sciences 24, 308 – 316. Retrieved from http://www.sciencedirect.com/science/article/pii/S1878029615001085 Bancoguis, S. R. (2007). Proceedings from 10th National Convention on Statistics: Abandoned biomass resource statistics in the Philippines. Mandaluyong, PH. Central Intelligence Agency. (2015). The World Factbook. Retrieved from https://www.cia.gov/library/publications/the-world-factbook/geos/rp.html Corathers, L.A. (2011). Silicon. 2009 Minerals Yearbook. Retrieved from http://minerals.usgs.gov/minerals/pubs/commodity/silicon/mybl-2009-simet.pdf Department of Trade and Industry. (2015). Philippine Trade Statistics [Data file]. Available from http://www.dti.gov.ph/dti/index.php/resources/statistics

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Dhengare, S., Raut, S.P., & Bandwal, N.V. (2015). Investigation into utilization of sugarcane bagasse ash as supplementary cementitious material in concrete. International Journal of Emerging Engineering Research and Technology, 3 (4), 109-116. Ganesan, K., Rajagopal, K., & Thangavel, K. (2007). Evaluation of bagasse ash as supplementary cementitious material. Cement and Concrete Research, 29, 515-524. Hogendorp, B. (2008). Effects of silicon-based fertilizer applications on the development and reproduction of insect pests associated with greenhouse-grown crops (Dissertation). Retrieved from Proquest Database. Hunt L. P., Dismukes J. P., Amick J. A., Schei A., & Larsen K. (1984). Rice hulls as a raw material for producing silicon. Journal of the Electrochemical Society, 131, 1683-1686. Ikram N. & Akhter M. (1988). X-ray diffraction analysis of silicon prepared from rice husk ash. Journal of Materials Science, 23, 2379-2381. Kalapathy, U., Proctor, A., & Shultz, J. (2002). An improved method for production of silica from rice hull ash. Bioresource Technology, 85, 285-289. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12365496 Kanawade, M., & Gaikwad, R.W. (2011). Removal of dyes from dye effluent by using sugarcane bagasse ash as an adsorbent. International Journal of Chemical Engineering and Applications, 2 (3). Kawade, U.R., Rathi, V.R., & Girge, V.D. (2013). Effect of use of bagasse ash on strength of concrete. International Journal of Innovative Research in Science, Engineering and Technology, 2 (7), 2997-3000.

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Koch, C. & Rinke, T. (2014). Silicon wafers, quartz wafers, glass wafers: Production, specifications, Si and SiO2 etching, our portfolio [Brochure]. Retrieved from http://www.microchemicals.com/brochures/silicon_quartz_glass_wafers_2014_en.pdf Larbi, K. K. (2010). Synthesis of high purity silicon from rice husks (Master’s Degree Thesis). Retrieved from TSpace Repository. Le Quéré, C., Andres, R.J., Boden, T., Conway, T., Houghton, R.A., House, J.I., …Zeng, N. (2012). The global carbon budget 1959-2011. Earth System Science Data Discussions, 5(2), 1107-1157. doi: 10.5194/essdd-5-1107-2012 Lund, Zhang, Jennings & Singh. (2000). New methods for producing low cost silicon for solar cells. Retrieved from www.solar.org.au Mane, V.S., Mall, I.D., & Srivastava, V.C. (2007). Use of bagasse fly ash as an adsorbent for the removal of brilliant green dye from aqueous solution. Dyes Pigment, 73. Mubarok, M., Setiawan, L., Utami, M., & Trisunaryanti, W. (2014). Study of acid leaching in the preparation of silicon from Lapindo mud. International Journal of Academic and Scientific Research, 2 (4). Retrieved from http://www.ijasrjournal.org/wpcontent/uploads/2014/12/DEC13-14.pdf Pacheco-Torgal, F., Lourenco, P.B., Labrincha, J.A., & Kumar, S. (2014). Eco-efficient masonry bricks and blocks: design, properties and durability. United Kingdom: Woodhead Publishing.

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Philippine Sugar Regulatory Administration. (2013). Annual Report. Retrieved from http://www.sra.gov.ph/wp-content/uploads/2014/06/Annual-Report-2013-final.pdf Santiago, E.B. (2015). About the Philippine electronics industry: a snapshot. Semiconductor and Electronics Industries, Inc. Retrieved from http://siteresources.worldbank.org/INTPHILIPPINES/Resources/ErnestoSantiago.pdf Saviour, M.N. (2012). Environmental impact of soil and sand mining: a review. International Journal of Science, Environmentcand Technology, 1(3), 125 – 134. Retrieved from http://www.ijset.net/journal/27.pdf Simoes, A. (2014). Learn more about trade in the Philippines. The Observatory of Economic Complexity. Retrieved from https://atlas.media.mit.edu/en/profile/country/phl Singh, R. & Dhindaw, B. K. (1978). Proceedings of the International Solar Energy Congress: Production of high purity silicon for use in solar cells. Suliman, M.E. & Fudl Almola, S.M. (2010). The use of sugarcane bagasse ash as an alternative local pozzolanic material: Study of chemical composition. COMSATS – Science Vision, 16-17, 65-69. Sultana, S. & Rahman, A. (2013). Proceedings from International Conference on Mechanical, Industrial and Materials Engineering: Characterization of calcined sugarcane bagasse ash and sugarcane waste ash for industrial use. Rajshahi, BD. Swanson, R.M. (2006). A vision for crystalline silicon photovoltaics. Progress in Photovoltaics: Research and Applications, 14, 443-553.

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Swatsitang, E. & Krochai, M. (2009). Preparation and characterization of silicon from rice hulls. Journal of Metals, Materials and Minerals, 19 (2), 91-94. Teixeira, S., Romero M., & Rincon, J. (2010). Crystallization of SiO2-CaO-Na2O glass using sugarcane bagasse ash as silica source. Journal of the American Ceramic Society, 93, 450-455. doi: 10.1111/j.1551-2916.2009.03431.x The Royal Chemical Society. Silicon - element information, properties and uses. Retrieved July 2015 from http://www.rsc.org/periodic-table/element/14/silicon United States Department of Agriculture. (2014). World sugar production, supply, and distribution [Data file]. Retrieved from http://ers.usda.gov/datafiles/Sugar_and_Sweeteners_Yearbook_Tables/World_Production _Supply_and_Distribution_/table01.xls USDA Foreign Agricultural Service. (2013). Philippines Sugar Annual Situation and Outlook. Retrieved from http://www.fas.usda.gov/data/philippines-sugar-annual Worathanakul, P., Payubnop, W., & Muangpet, A. (2009) Characterization for post-treatment effect of bagasse ash for silica extraction. World Academy of Science, Engineering and Technology, 3. Retrieved from http://waset.org/publications/6051/characterization-forpost-treatment-effect-of-bagasse-ash-for-silica-extraction