Running Title: Synthesis of silicon based compounds from rice husk
Title of Article: Synthesis of Pure Silicon and Silicon Based Compounds from Rice Husk; A
Review of Recycling the Agro-industrial By-product
Muhammad Ali, Lecturer, Institute of Advanced Materials, Bahauddin Zakariya University,
Multan-60800. Pakistan. E-mail: muhammad.ali@bzu.edu.pk, Phone: +92 332 410 9686.
Dr. Ather Ibrahim, Assistant Professor, Institute of Advanced Materials, Bahauddin
Zakariya University, Multan-60800. Pakistan. E-mail: ather.ibrahim@bzu.edu.pk, Phone:
+92 300 950 9661.
Synthesis of Pure Silicon and Silicon Based Compounds from Rice
Husk; A Review of Recycling the Agro-industrial By-product
Muhammad Ali 1*, Ather Ibrahim1
1
Institute of Advanced Materials, Bahauddin Zakariya University, Multan-60800, Pakistan
Abstract
Ultrafine silica particles are naturally embedded in the cellulosic part of rice husk which can
be exploited for the production of pure silicon as well as a variety of silicon based materials
each offering a wide spectrum of applications. Nominal cost of the raw material may
significantly reduce cost of the end products. Metallothermic reduction of rice husk leads to
the formation of solar-cell grade silicon, high temperature controlled pyrolysis to carbide,
nitride and oxynitride of silicon, oxidation to silica particles ranging from micron to nano
meter size, chlorination to silicon tetrachloride and hydrothermal process to different types
of zeolite. This work reviews these processes in order to achieve optimised yield of the
products.
Keywords: Rice husk; Carbide; Nitride; Zeolite; Silicon; Silica;
1 Introduction
Among various agro-industrial byproducts, rice husk (RH) is a potential source of sundry
technologically important metallurgical products, including carbide, nitride, oxynitride and
oxide of silicon and zeolite. Its ubiquitous availability and nominal cost are prominent
features for which manufacturers always ascertain. Otherwise, high cost is always associated
with the synthesis and close control over final properties of high-tech materials. RH is the
outermost covering of grains of rice (oryza sativa) that is separated wherefrom during rice
milling process. According to Food and Agricultural Organisation statistical data
(FAOSTAT)
[1]
, paddy rice production across the globe exceeded 670 million metric tonnes
for the year 2010, approximately 20% of which comprises husk. Besides 75 – 85% organic
*
Corresponding author: e-mail address: muhammad.ali@bzu.edu.pk
matter, RH contains 15 – 25% ash content which have up to 95% silica along with traces of
metallic oxides listed in Table 1
[2,3]
. Some traditional uses of RH include manufacturing of
some low value-added products, conversion to fertilisers or low burning fuel. Other
applications of RH need partial or complete combustion of the same. Partial combustion of
RH gives carbon black or carbonised RH (CRH) which is used as charcoal and soil
conditioner. Rice husk ash (RHA)
[3]
, obtained after complete combustion of RH, is a quality
additive in secondary steelmaking (i.e. tundish powder) and a raw material in manufac turing
insulators and refractory bricks.
Morphological studies reveal troughs and crests on the surface of RH with higher silica
content on the epidermic protuberances responsible for abrasiveness of the husk
[4,5]
as
shown in Figure 1.
Achieving different products from RH often necessitates its exposure to elevated
temperatures. Therefore, it is useful to have sound knowledge of thermal characteristics and
pyrolysis kinetics of RH which is made available through different techniques of thermal
analysis, most important being gravimetric analysis (TGA). Non-isothermal TGA is preferred
over isothermal as the former exhibits behaviour of RH against thermal exposure over a
continuous range of temperature. When RH is slowly heated to higher temperatures, it
undergoes thermal destruction through three distinct stages
[6-8]
: (i) low temperature
desiccation or loss of hygroscopic water at 50 – 150 °C, (ii) decomposition and volatilisation
of organic matter, such as lignin, cellulose, hemicelluloses etc., at intermediate temperature
range of 220 – 350 °C, (iii) combustion of carbon which continues over a range of 350 – 690
°C. Activation energy of thermal decomposition of RH can be computed by using Arrhenius
equation or Flynn and Wall expression, both entailing graphical representation of thermal
response. Thermal analyses of RH are usually carried out under inert atmosphere which
ensures that the sample subjected to thermal exposure is responding to temperature only.
However, a reactive gas can be used for some purpose, such as oxygen for measuring carbon
content of the sample by monitoring the CO2 evolution, and carbon dioxide for studying RH
conversion to char
[9]
. RH have been thermally analysed with varied conditions to study the
dependence of activation energy of thermal degradation on heating rate employed
nature of gas purged
[12-16]
[10,11]
and
. Thermogravimetric (TG) and derivative thermogravimetric
(DTG) curves under air and nitrogen atmosphere are given in Figure 4. Faster heating rate
initiates decomposition earlier
destruction
[10,11,17]
[17,18,19]
and causes increased rate of carbonisation and thermal
. Values of energy of activation computed at different heating rates for
second and third stage of thermal destruction of pulverised RH are listed in Table 2.
Moreover, value of energy of activation, when calculated over a continuous range of
temperature, increased as the degradation proceeded
[17]
.
2 Syntheses from RH
High temperature treatment of RH, assisted with suitable conditions of temperature, pressure
and atmosphere, can produce some important materials with significant applications. Yield of
the products directly depends upon various process variables involved, such as heating rate
to approach the required temperature, soaking time at that temperature, nature of atmosphere
in the reaction chamber and pressure of the purge gas to create appropriate atmosphere.
2.1 Synthesis of Silicon Carbide
High temperature pyrolysis of RH makes carbon of biomass part to reduce silica thereby
producing silicon carbide, first realised by Lee and Cutler
[20]
. The process demands less
energy and can replace other commercial processes of carbide production, such as Acheson
process which is energy intensive and eco-hazardous. Besides raw RH [4,21-23], burnt RH [21,2426]
or a mixture of RHA and carbon black [27] can also be pyrolysed to get the same product in
varying yields. Pyrolysis temperature can be attained by different heating sequences. Single
step or direct pyrolysis at 1400 °C offers highest yield as compared to two step or multistep
approach
[5,28]
. In two step process, RH is carbonised to 700 – 900 °C followed by inert
atmosphere or vacuum pyrolysis at about 1500 °C. Multistep route consists of several
soaking stages during the course of heating from ambient to pyrolysis temperature. Generally
argon or nitrogen atmosphere is employed to synthesise silicon carbide. Chemistry of
formation of carbide is discussed in various studies
[2,4,5,21,29]
. Variation in product yield with
pyrolysis temperature and effect of soaking time provided is illustrated in Figure 2 and 3.
An elaborated study on catalytic effect of sodium silicate inferred that an optimum
concentration of the same (1 g/L) gives maximum yield (about 57%) in one third times
[5]
,
i.e. one hour soaking time against three hours in Figure 3. Although sodium silicate
improved product yield by lowering the activation energy of carbide formation but it should
not be termed as catalyst because it get consumed during and cannot be recovered after the
reaction. Comparable yield in shorter soaking periods can also be obtained by using
FeCl2.4H2O or CoCl2.6H2O as catalyst
[25,30]
. Pyrolysis of RH after Fe 2O3 solution treatment
improves proportion of β-SiC and fosters the formation of whiskers rather than particulate
[21]
. Another beneficial treatment is soaking RH, prior to thermal exposure, in a mineral acid
(H2SO4, HCl, HNO3) or a boron compound (H 3BO3) resulting in the improved yield and
purity of the product [11,31].
2.2 Synthesis of Silicon Nitride and Oxynitride
Nitride and oxynitride of silicon can also be obtained through controlled pyrolysis of RH.
Presence of hydrogen in the reaction chamber ensures high yield of nitride provided that
there is no air/oxygen ingress which, if come about, results in oxynitride formation
[2,18]
.
High purity nitride is obtained by using NH 3 atmosphere or 95% N2 – 5% H2 atmosphere
[4,32]
. Although both α and β phases of silicon nitride belong to hexagonal system and have
covalent bonds in dominance, exceptionally high pressure (about 300 MNm-2) required for βnitride formation makes it a rare phase. This difficulty has been eliminated by catalytic
function of V2O5
[33]
. Pre-treatment of RH with 6% FeSO 4 solution gives maximum whiskers
at 1400 °C and further increase in catalyst concentration and/or pyrolysis temperature
converts the whiskers of nitride into globules [34].
2.3 Nano-silica from RH
Efficient removal of carbonaceous matter from RH gives silica residuum. A variety of
chemical and/or mechanical treatments can be applied to RH or RHA directly influencing the
particle size of the final product. More often RH is burnt as a low burning fuel in rice
processing mills thereby giving RHA. To synthesise submicron silica particles RH can be
used in raw form or after some preliminary treatments including acid leaching, partial
combustion and catalysis etc. Acid leaching is more efficacious in removing oily substance,
if any, and soil particles than deionised water rinsing. Furthermore, acid-treatment is
beneficial in improving the quality of silica particles than merely water rinsing as the latter
cannot remove potassium cations K + which act as catalyst to melt silica at lower
temperatures
[35,39]
. This molten silica, during solidification, can entrap carbonaceous traces
which adversely affect the purity of final product. Burning RH in a temperature range 300 –
1000 °C removes carbonaceous material
[36,37]
. Keeping the gravimetric thermogrammes in
view, it can be logically deduced that temperature as low as 500 °C is sufficient to remove
organic matter from RH and the residuum contain carbon black and RHA
[38]
. Therefore the
complete elimination of carbonaceous material requires high temperature heating up to 1000
°C [37]. Incomplete removal of carbonaceous material is the result of partial decomposition of
lignocelluloses
[39]
. On the other hand, prolonged soaking periods or excessively higher (>
800 °C) temperatures cause crystallisation of silica
[39]
and increase the operation cost. Ideal
conditions for obtaining narrow range 20 – 30 nm silica particles are heating the HCl treated
RH to 700 °C for 2 hours
[40]
. Subsequent milling operation, such as planetary ball milling,
can be carried out [40] to further increase the specific surface area of the product.
Another approach to obtain submicron silica particles is through sol-gel method. Sodium
silicate is obtained by heating NaOH solution with RHA to 100 – 150 °C
[41,42]
. Increase in
NaOH concentration effectively extracts silica provided that the latter is in amorphous form
in RHA. Sodium silicate solution is titrated and silica gel gets precipitated when pH of the
solution decreases to less than 10. Drying of silica gel gives xerogel which is subsequently
ground and purified
[42]
. Another highly efficient process route utilises silicic acid to produce
particles as fine as 5 nm. Sodium ions of sodium silicate are replaced with hydrogen ions
through ion exchange resin
[43]
giving rise to the active silicic acid. This active dilute acid is
subjected to titration [42] thereby initiating nucleation and growth of silica nanoparticles.
2.4 Silicon from RH
Silicon and silicon compounds have a variety of applications in solar-cells, semiconductor
industry, fibre optics and so on. Metallothermic reduction of RHA is an economical approach
to produce solar-cell grade silicon. Magnesium or magnesia can be employed as reducing
agent. Reduction of silica takes place at about 600 °C through gas-solid reaction
[36,44]
.
Powder silicon thus obtained can be melted and solidified to get the bulk samples which are
found to be p-type semiconductor with resistivity 0.1 – 0.3 ohm.cm within the grains
[44]
.
Aluminium is also an efficient reducing agent for this purpose giving molten silicon as the
reduction reaction proceeds above 1450 °C [45].
2.5 Silicon Chloride from RH
SiCl4 is a useful material with numerous metallurgical and electronic applications. It has
major use in semiconductor industry and is also a precursor for many other silicon based
compounds, such as ceramics, polymers and pure silicon for solar cell applications.
Pyrolysed RH is subjected to chlorination at 100 °C to get pure silicon tetrachloride
[46]
. RH,
after pyrolysis, can be pelletized and then chlorinated. Important parameters in this case
include flow rate of chlorine gas and temperature of the reaction chamber. Other factors of
secondary importance are initial grain size and compact forming pressure
TiO2 as catalyst gives best reaction yield at 1100 °C
[48]
[47]
. Use of CuO or
, however these catalysts showed no
significant effect on kinetics of reduction reaction below 1050 °C. Presence of a
diatomaceous earth as catalyst improves reaction kinetics at comparatively low temperatures,
i.e. between 700 – 900 °C
[49]
. Another route of chlorination is through HCl treatment of Si
which is extracted from metallothermic reduction of RHA with Mg
[36,44]
or Al
[45]
. This
chlorination reaction proceeds in the presence of Cu as catalyst at 300 °C which produces
trichlorosilane (SiHCl 3; boiling point 31.8 °C). In either case, chlorination proceeds in the
presence of carbon, therefore RH should be pyrolysed in inert (nitrogen or argon) atmosphere
in lieu of burning.
2.6 Zeolite from RH
Zeolites are hydrated aluminosilicates having three dimensional crystalline frameworks in
which SiO4 and AlO4 share their oxygen atoms. These compounds are conventionally
produced by subjecting an alkaline gel containing silica, alumina and cations to hydrothermal
treatment. Nature of zeolite compound produced is determined by the type of silica used.
Amorphous silica produced from RHA is a cut-price source for the production of various
types of zeolite; such as mordenite
crystalline zeolite-Y
[53]
[50]
, ZSM-48 type zeolite
and zeolite-beta
[54,55]
[51]
, ZSM-5 type zeolite
[52]
,
. NaA type zeolite, mainly being used in
detergents industry, can be produced either from raw RH or CRH, the latter gives quality
product as undesired amorphous phases appear when raw RH is used
[56]
. The process
involves hydrothermal treatment of the mixture of NaOH and RH in a sealed container.
Prolonged heating under autogenous pressure along with vigorous stirring following the
addition of sodium aluminates gives NaA zeolite. Crystallisation of zeolite-beta takes place
at 150 °C in the presence of tetraethyl ammonium hydroxide [54].
Besides these products, some novel processes have been investigated to obtain even
advanced materials from rice husk; such as silica-tin nanotubes [57] (a photocatalyst material),
lithium aluminium silicate (LAS) glass ceramics
[58]
(popular for their distinguished chemical
durability and negative coefficient of thermal expansion) and other water based materials
which are used as precursors in sol-gel techniques.
3 Conclusions and perspectives
Successful commercialisation of producing different products from RH will essentially be a
cost effective approach emerging from inexpensive starting material. Carbide and nitride of
silicon are commercially being produced through energy intensive processes which add
considerably to their cost thereby limiting their use for high-tech applications only. Large
scale production from RH will eliminate the cost constraint. Control over narrow size range
is possible when silica nanopowder is synthesised from RH. Moreover, active silica obtained
from RHA has been proved a successful precursor for preparing a number of silicon based
compounds including, but not limited to, pure single crystal and polycrystalline silicon,
silicon chloride, zeolites and silicate glass ceramics.
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Table captions and tables
Table 1 Approximate chemical analysis of RH
[2,3]
Table 2 Activation energy of pulverised RH at different heating rates
Table 1 Approximate chemical analysis of RH
CONSTITUENT (wt%)
Ash
Moisture
Carbon
Hydrogen
Oxygen
Nitrogen
Sulphur
Fe2O3
Al2O3
CaO
Na2O
MgO
K2O
MnO
[8]
[2,3]
RANGE
15 – 29
8–9
~35
4–5
31 – 37
0.23 – 0.32
0.04 – 0.08
1.34
1.06
0.86
0.66
0.22
0.18
0.06
Table 2 Activation energy of pulverised RH at different heating rates [8]
Heating Rate E
°C/min
kj/mol
225 – 350 °C
E
kj/mol
350 – 600 °C
5
10
25
50
100
14.1
21.9
21.6
21.6
11.2
Figure captions and figures
72.7
79.1
81.5
79.7
97.1
Figure 1 Outer surface of RH with Si X-ray line scan [5]
Figure 2 Pyrolysis temperatures versus product yield
Figure 3 Soaking time versus product yield
[5]
[5]
Figure 4 TG and DTG curves of rice husk: (A) Degradation in air; (B) TG in nitrogen flow;
(C) Degradation of pyrolysis product in air
[8]
Figure 1 Outer surface of RH with Si X-ray line scan [5]
Figure 2 Pyrolysis temperatures versus product yield
[5]
Figure 3 Soaking time versus product yield
[5]
Figure 4 TG and DTG curves of rice husk: (A) Degradation in air; (B) TG in nitrogen flow;
(C) Degradation of pyrolysis product in air [8]