A simple non-conventional method to extract amorphous silica

from rice husk

C. B. Majumder a,*, Mandeep Sharma a, Gaurav Soni a

a Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttrakhand, India

Received 2 October 2013; received in revised form 20 April 2014; accepted 28 April 2014

Abstract

The paper describes the treatment of rice husk with acid and base, namely, hydrochloric acid

and sodium hydroxide to yield precipitates of silica. As rice husk contains over 80-90% silica when

converted to ash, it becomes important to extract it. A simple non-conventional method for

extracting silica in the amorphous form is based on alkaline extraction followed by acid

precipitation. Rice husk ash was prepared and washed with acid to remove the mineral impurities.

Further, it was treated with sodium hydroxide to form sodium silicate solution. The silicate

solution formed was titrated with hydrochloric acid and precipitation (in the form of gel) formation

takes place below pH 10.

Keywords: Rice husk, Rice husk ash, Silica xerogels, Minerals, Sodium silicate, Sodium hydroxide,

Hydrochloric acid.

1. Introduction

Current environmental and economic

conditions encourage us to develop and

improve technology to reduce or utilize the

agricultural waste in the best possible way.

One of these wastes is Rice husk. Asia is the

biggest zone for rice production (around 90

%) in the world.

Rice mills produce bulk amount of rice

husk as a by-product which is used as a fuel

by the industries to generate energy. Other

than its fuel importance, researcher says that

rice husk contains high amount of silica in the

form of rice husk ash (RHA).

The Table 1 and Table 2 gives

composition of rice husk and rice husk ash on

dry basis.

Table 1

Composition of Rice husk on dry basis

Element Mass Fraction

(%)

Carbon 41.44

Hydrogen 4.94

Oxygen 37.32

Nitrogen 0.57

Silicon 14.66

Potassium 0.59

Sodium 0.035

Sulphur 0.3

Phosphorous 0.07

Calcium 0.06

Iron 0.006

Magnesium 0.003

Zinc 0.006

Table 2

Composition of Rice husk ash on dry

basis

Elements Mass Fraction

(%)

Alumina 1 – 2.5

Ferric oxide 0.5

Titanium dioxide Nil

Calcium oxide 1 – 2

Magnesium oxide 0.5 – 2.0

Sodium oxide 0.2 – 0.5

Potash 0.2

Loss on Ignition 10 – 20

Silica (SiO2) 80 – 90

The above composition clearly indicates

silica as a major constituent of the rice husk

ash. As soluble silicates from silica are

widely used in ceramics glass, as adhesives

agents in detergents industries etc., thus it

becomes very important to extract it. Many

experiments have been carried out to extract

silica from rice husk ash (RHA), thus

yielding not only a valuable product but also

solving the problem of large amount of ash

disposal.

Generally, silica is present in gel,

crystalline or amorphous form. Some of its

properties are high specific surface area

(SSA), large pore volume and high reactivity.

There are two types of methods to extract

silica from rice husk. One is Conventional,

which is used in the industries where silica is

produced by smelting quartz in a high

temperature furnace, to produce ultrapure

polycrystalline silicon. Another one is the

Non-conventional method which can be

performed in the lab with the help of some

chemicals. It is based on simple low energy

chemical method for producing pure

amorphous silica by using low temperature

alkali extraction. The concept behind this

method is the low solubility of amorphous

silica at pH below 10. This unique solubility

behavior gives precipitates of silica at a lower

pH. This low energy method could be more

cost effective than the other conventional

methods.

Several authors have reported different

methods for obtaining silica from rice husk.

This paper reports a simple non-conventional

method of extracting silica by carrying out

acid-base treatment followed by the process

of burning to form precipitates of silica. The

silica obtained from this low energy method

is in the form of gel, namely Xerogel which

is mainly used to prepare dense ceramics.

2. Methods

2.1 Raw materials

Rice Husk (RH), Phosphoric Acid

(H3PO4), Hydrochloric Acid (HCl), caustic

soda (NaOH).

2.2 Rice husk ash preparation

The collected rice husk from mill was first

sieved to free it from dust and foreign

material. Then rice husk were several times

(At least 5 times) washed with distilled water

to remove more impurities of soil and dust.

Washed rice husk were dried in a hot air oven

for 24 h at 60˚C.

Dried Rice husk were carbonized with

40% H3PO4 solution in muffle furnace

maintained at 450 ˚C, for 15 minutes at

volume to weight ratio of 2:1. This leads to

ash formation.

The carbonized rice husk were washed

with distilled water and ringed. They were

dried in hot air oven for 24 h. Dried sample

was crushed to a mesh size of 300µm. Thus

the sample we get is nothing but Rice husk

ash. It’s completely ash now. The image of

Rice husk and Rice husk ash is shown in

figure 1(a) and 1(b).

2.3 Acid washing

An acid washing step was used to remove

mineral impurities. 10 g of rice husk ash

(RHA) was taken and washed with 100 ml of

2M HCl solution. The solution was shacked

in an Incubator shaker for 2 h.

Further, the solution was filtered through

Whatman No. 42 ashless filter paper and then

RHA residue were washed with 100 ml of

distilled water which dissolves remaining Na,

K, Ca, Mg, Fe and Mn contents present in the

ash. The residue were again dried in hot air

oven for 24 h at 60˚C. The dried residue were

used for alkaline solubilization.

Fig 1.Rice husk preparation, (a) Rice husk, (b) Rice

husk ash

2.4 Alkaline solubilization

The method of Kamath and Proctor (1998)

was used for extraction of silica from RHA.

100 ml of 1N NaOH solution was prepared to

wash RHA samples and was stirred in a water

bath for 2 h at 80˚C while being in a covered

250 ml flask. The water bath automatically

boils and rotates the sample at a constant

speed, see figure 2(a).

After constant stirring the solution was

filtered through Whatman No. 42 ashless

filter paper, the carbon residue were washed

with 100 ml distilled water. The filtrates and

washing were allowed to cool down to room

temperature. The filtrate formed in this

process is nothing but sodium silicate

solution, see figure 2(b). The reaction

occurred is as follows:

SiO2 + 2 NaOH ----> Na2SiO3 + H2O

(Ash) (Caustic (Sodium (Water)

soda) silicate)

Fig 2. Alkaline solubilization, (a) RHA+NaOH after

heating in water bath, (b) Sodium silicate solution

after filtration

2.5 Acid titration to form gel precipitates

The sodium silicate solution formed above

is now titrated with 2M HCl solution. As HCl

Concentration increases pH of the solution

decreases. At below 10 pH silica gel

precipitate formation started taking place

(see figure 4(a)) and no gel formation was

observed below 7 pH. The silica gel formed

were aged for 24 h.

The reaction of Sodium silicate solution

with HCl is as follows:

Na2SiO3 + HCl ---> SiO2 + NaCl + H2O (Sodium (Hydro- (Silica) (Sodium- (Water)

silicate) chloric acid) Chloride)

100 ml deionized water was added to gels and

gels were broken to make a slurry type

solution. The slurry solution was centrifuged

for 15 min at 6000 rpm. The solid gel gets

separates out (see figure 4(b)) and were kept

in a hot air oven for 12 h at 80˚C to form

xerogels (see figure 4(c)).

A flow diagram of the above procedure is

shown below in figure 3.

2.6 Moisture content of silica gels

Moisture content of the silica gels was

determined using an air oven method. About

1 g of each sample were placed in hot air oven

at 80oC for 2 h. The samples were cooled in a

dessicator and weighed. The weight loss (%)

was recorded as the moisture content of

sample.

Figure 3: (a) Silica gel precipitate formation, (b) Silica

gel after centrifuge, (c) Silica xerogel after drying

2.7 XRD Analysis of silica gel

X-ray diffraction patterns were obtained

by a dual goniometer X’pert XRD system

(Institute Instrumentation Center, IIT

Roorkee) using an acceleration voltage of 40

kV and current of 45 mA. The diffraction

angle was scanned from 10o to 90o 2h, at a

rate of 5o/ min. The X-ray diffraction patter is

shown in figure 5. The pattern shows pick at

22o which is an indication of amorphous

silica.

2.8 FTIR Analysis of silica gel

Fourier transform infrared (FTIR) data

were obtained by adding 100 interferograms

using an Impact 410 Nicolet instrument

(Chemical Engineering Department, IIT

Roorkee). See figure 6 for FTIR pattern.

Figure 4: Flow diagram of the procedure used to

extract silica from RHA.

3. Result and Discussion

Silica obtained from rice husk was white in

color, X ray diffraction analysis shows a peak

at an angle of 20-22o which shows that silica

is in amorphous form. Many times washing

with distilled water shows minimum amount

of contaminants of Ca, Mg, K, Sulphur etc.

The purity of extracted silica is above 85-

90% with some Sodium (Na) impurity

present inside. Figure 5: X-ray diffraction pattern of obtained silica.

Purified silica

Wash with water and dry at 80˚C

Xerogel

Dry at 80˚C

Centrifuge the slurry at 6000 rpm (15 min)

Add water (100 ml) and make slurry

Allow the gel to age (24 hr)

Titrate filtrate with 2M HCL (pH 7)

Collect filtrate and allowed to cool (24 hr)

Wash residue with boiling distilled water (100 ml)

Filter (Whatman 42)

Boiled with stirring (1 hr)

Disperse residue in 1 N NaOH(100 ml)

Wash residue with distilled water(100 ml) and dried (24 hr)

Filter (Whatman 42)

Stir (2 hr)

Disperse RHA (10 g) in 2M HCL(100 ml)

Crush the Rice husk ash(RHA) (300µm)

Add 40% H3PO4 at 450˚C

Wash Rice husk with distilled water(5 times) and dry (24 hr)

Figure 6: Fourier transform infrared spectra of silica

produced from RHA

4. Conclusions

This study revealed that silica xerogels

with more than 90% silica content and

minimal mineral contaminants can be

produced from RHA using a simple low

energy chemical method. The initial acid

washing of RHA resulted in a lower Ca

content in the silica xerogel. The final

washing of the xerogel led to a silica product

with lower Na, and K contents. Incorporation

of initial acid washing of RHA and final

washing of the xerogel resulted in a pure

silica with total Na, K, and Ca contents of

<0.1%.

Acknowledgements

We wish to thank shiv rice mill,

bahadrabad (Production manager) for

providing Rice husk.

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