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

EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 1

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 metallurgical-

grade 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


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.


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


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).


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 3p

2. 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% 28

Si, 4.67% 29

Si, and 3.1% 30

Si 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 cost-

effective 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


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


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


microelectronics applications, thus the need for further purification steps in producing solar-

ready 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 well-

known Czochralski method and float-zone melting. The Czochralski method involves melting


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


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


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


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


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


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).


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


(equation 2.2)

vol% nitric acid solution for ICP-OES analysis. A blank solution was prepared with the same

ratios but with a volume factor of five 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 C

(mgL) prep Vol (L)

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 D (

mgL) prep Vol (L)

wt of Si (g)

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.

(equation 2.1)


Sugarcane Bagasse Ash (SCBA)

Extraction of SiO2

Reduction

Post-Reduction Treatment

High Purity Silicon Powder

Test Melting

High Purity Silicon Chunks

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.

Figure 3.1 Scheme of the experimental procedure

XRF

Analysis


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 Source

Sugarcane Bagasse Ash Residue from combustion of bagasse First Farmers’ Holdings

Corporation

Sodium Hydroxide 2 M aqueous solution USLS College Science

Laboratory

Magnesium Turnings 99 wt% pure USLS College Science

Laboratory

Hydrochloric Acid

1 M aqueous solution

33.333 vol% aqueous solution

50 vol% aqueous solution

USLS College Science

Laboratory

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).


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


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.


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.

Leaching sequence Volume ratio Temperature Duration

1 1:2 HCl :H2O

1:2 HF: H2O

Room temperature 10 minutes

2 1:2 HCl :H2O

1:2 HF: H2O

Room temperature 10 minutes

Figure 3.3 Si-O-Mg phase diagram at 650°C (Larbi, 2010)

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


3 1:1 HCl :H2O 95°C 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).


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.


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

Analyte %relative

concentration

SiO2 54.87

Cl 40.39

K2O 3.16

SO3 1.17

Fe2O3 0.30

ZnO 0.05

CuO 0.03

Rb2O 0.02

Table 4.1 Results from Preparation of SiO2

Table 4.2 Relative concentrations of components in sample B


4.2 Reduction of Silica (SiO2) with Magnesium (Mg)

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

Without Mg With Mg After reduction

A 6.00 11.40 9.56

B 6.00 11.40 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

Table 4.5 First and second acid leaching sequence

Mass

Sample Before post-leaching After post-leaching

A 8.00 g 2.19 g

B 8.00 g 3.15 g

Figure 4.1 X-ray diffraction profile of components in sample B


Sample Total Volume of

Acid

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

HCl H2O HF H2O

A 12 mL 4 mL 8 mL 4 mL 8 mL

B 12 mL 4 mL 8 mL 4 mL 8 mL

Table 4.6 Third acid leaching sequence

Sample Total Volume of

Acid 1:1 HCl : H2O

HCl H2O

A 22 mL 11 mL 11 mL

B 22 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 9.76

Rb 0.02

Os 0.02


Mn 0.64

Sc 0.18

Zn 0.06 1.48

Ti 1.17

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


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


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:


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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Extraction of High Purity Silicon From Sugarcane Bagasse Ash