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
Jan 29, 2016
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 2
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.
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 3
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 4
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).
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 5
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 6
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 8
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 9
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 10
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 11
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 12
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 13
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 14
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).
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 15
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 16
(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)
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 17
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 18
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).
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 19
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 20
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.
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 21
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)
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 22
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).
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 23
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 24
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 25
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 26
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 27
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 28
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
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 29
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:
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 30
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.
EXTRACTION OF HIGH PURITY SILICON FROM SUGARCANE BAGASSE ASH 31
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