Low-cost and Fast Production of Nano-silica From Rice Husk Ash

Weerachon Phoohinkong1,a* and Udomsak Kitthawee 1,b

1Faculty of Science and Technology, Suan Dusit Rajabhat University, Bangkok, 10700, Thailand

ap_weerachon@hotmail.com, budomsak_kit@dusit.ac.th

Keywords: Rice husk ash; Silica; Sodium silicate

Abstract. Silica with nanostructure are the high quality silica that are used in many industry areas.

The applications of silica nanostructure frequently depend on physical properties such as

morphology and size of structure. Rice husk ash is the waste from biomass power plants and is a

high quality, raw material as a silica source. The conventional methods for synthesis of nano-silica

from rice husk ash are energy consumption or time consumption. The objective of this work was to

investigate the synthesized of nano-silica from rice husk ash via sodium silicate solution.

Nano-silica particles were obtained via alkaline extraction and a fast acid precipitation method at

room temperature by adding inorganic salts and without surfactant or template. The flow synthesis

was investigated at ambient temperature, varying the concentration of hydrochloric acid, sodium

chloride, and flow-rate while fixing the concentration of sodium silicate. The samples were

characterized by field emission scanning electron microscopy (FESEM), transmission electron

microscopy (TEM), and X-ray diffraction (XRD). The results revealed that the sodium chloride is

significantly inorganic salt for generated nano-silica, with uniform spherical morphology (80-150

nm), without curing or aging time. In the flow synthesis method, the silica nanoparticles, of

diameter around 10 nm and aggregate particles of around 50 to 200 nm, were obtained. This method

may be applicable to control different grade of silica and can easily scaling up of silica production

for different industries.

INTRODUCTION

Silica with nanostructure have unique properties and usage for numerous applications such

as lubricants, drug delivery, active food packaging, nanoelectronics, transistors, diode, solar cells,

sensors, polymer filler, thermal insulation, and catalysts [1-5]. Rice husk is an abundant agricultural

by-product. In Thailand, rice is the top export in the world and rice farmers are praised as being the

national’s backbone. Most of the rice husk is useful for biomass fuel in industry. Rice husk ash

(RHA) is the waste from power plants that is used to generate heat for electricity power production.

RHA is composed of 87% to 97% silica content by mass for complete combustion. Hence, RHA

from power plants is a high quality grade of amorphous silica sourced raw material, easy to use

without further combustion, and is very cheap compared with common raw materials like alkoxide

organometallic compounds, or chlorosilane gas.

There are two conventional methods for preparing silica from rice husk: the combustion

method and the chemical method. The combustion method is the burning of the rice husk at a high

temperature to remove carbon and organic content. The chemical method, by a reaction of rice husk

with alkaline to extract silica via sodium silicate, is a lower energy consumption than the

combustion method and is able to produce silica nanostructure [6]. The aim of this work is to create

a simple, low-cost, suitable for scale-up, and fast preparation of nano-silica using RHA waste from

rice husk power plants.

Advanced Materials Research Vol. 979 (2014) pp 216-219Online available since 2014/Jun/24 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.979.216

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EXPERIMENTAL

All chemicals were reagent grade and used without further purification. RHA was the waste of

rice husk agricultural by-product from power plants from the Bua Sommhay Roi Et Thailand.

The RHA were washed with water and soaked in 1M hydrochloric acid for 24 h at room

temperature. The RHA were filtered and washed with distilled water several times until the washed

water was neutral, and then the residual was dried in an air oven at 110 °C for 12 h. Determination

of the silica content in RHA was by combustion of 1g of the RHA sample at 900 °C for 1 h, and the

residual solid was then weighed. Silica was extracted from RHA as sodium silicate solution, in the

typical synthesis method, 2M NaOH 3,000 ml were add to 450 g washed RHA and boiled at 80 °C

for 1 h while stirring, and was cooled to room temperature for 24 h. The solution was heated to

80 °C and the filtrate solution was a 2M sodium silicate solution. Silicas were prepared by batch

reaction and flow-synthesis reaction. For the typical batch reaction, the equal molar amounts of

hydrochloric acid solution was added dropwise into 2M sodium silicate at different salt

concentration, for the other acid as well as for hydrochloric acid. The resultant silica particles were

obtained by centrifugation and were washed 5 times with distilled water followed by ethanol 3

times and then dried in an air oven at 60 °C for 2 h. For the flow-synthesis reaction, we used a

t-shaped tube reactor with two inlet of a diameter of 2 mm and one outlet of a diameter of 2 mm and

a length of 10 mm. In the typical flow-synthesis method, the solution of 2M sodium silicate and

sodium chloride and solution of hydrochloric acid were pumps in to the t-shaped tube reactor. The

effects of sodium chloride concentrations were study at 5% and 10%, and the effects of

hydrochloric acid concentration were study at 2M and 4M. The effects of flow-rates were

investigated at 0.1, 0.5, 1.0, and 2.0 ml/min for both inlet reactants. Morphology and particle sizes

were characterized by scanning electron microscope (HITACHI-S4700) and transmission electron

microscope (FEI-TECNAI T20 G2).

Results and Discussion

After burning the RHA sample to remove carbon and organic compounds, the results showed

that the silica content in RHA samples were around 87% by mass fraction. The addition of NaCl

showed that NaCl is a significant inorganic salt for fast synthesis with reaction at the moment. The

optimal concentration of reactant to minimum percentage of NaCl for fast reaction and high

productivity in various acids are 5%, 15%, and 15% sodium chloride for hydrochloric acid, nitric

acid, and sulfuric acid, respectively. The morphology of silica nanoparticles obtained from batch

reactions in optimal conditions of all three acids were of a similar size and uniform spherical-like

morphology. The FESEM image in Fig. 1. shows the silica prepared from hydrochloric acid and

sodium chloride condition (Fig.1(a) and (b)). High magnitude TEM images indicate that the particle

size is around 100 nm (Fig. 1(c) and (d)). These obtained silica particles are similar to the silica in

Dawai Li and Xifeng Zhu [7] from the TEM image, which was synthesize a high specific surface

area silica from rice husk char via sodium silicate solution, and used polyethylene glycol as a

template, the acidification process lasts about 25 min in total. In this work, the similar high quality

silica have lower production costs due to lower raw material costs, without using any template, and

took few steps.

Fast reaction at a high concentration of NaCl can be due to high ionic strength, a high acidity

solution that can lead to high rate interaction and growth of particles because the thickness of

electrical double layers and electrostatic repulsion decrease [9-10]. The FESEM images of silica

prepared from flow-synthesis are shown in Fig. 2. The reaction in t-shaped tube reactors occurs, and

the solution became white and milky.

Advanced Materials Research Vol. 979 217

The non-uniform morphology silica were obtained. The sodium chloride content had a

significant effect to morphology of the silica particles product. The flow rate was not a significant

effect to morphology but was a significant effect to productivity, for at low flow rate the

productivity was high. At flow rate 0.1 ml/min 5% sodium chloride, the granular silica particles of

diameter of around 10 nm were obtained (Fig. 2(a)) and the small amount of aggregate particles of

diameter size around 50 nm (Fig. 2(b)) and 200 to 300 nm (Fig. 2(c)) were obtained. At flow rate

0.1 ml/min 10% sodium chloride, the granular silica particles of diameter size around 10 nm were

obtained (Fig. 2(d) and (f)) and the small amount of aggregate spherical like particles of diameter

size around 200 nm (Fig. 2(e)) were obtained. The non-uniform morphology from flow synthesis

assumed due to dead volume at the outlet tube. The result from XRD showed broaden peaks around

2-Theta 21-25° (Fig. 3.) that the obtained silica samples were amorphous. XRD patterns showed

sodium chloride (Fig. 3(d)) and sodium nitrate (Fig. 3(a)) in product samples prepared by batch

reaction indicated that the flow synthesis (Fig. 3(b) and (c)).

Fig. 1. FESEM image of silica from batch synthesis with 2M Na2SiO3, 10% NaCl and 2M HCl

(a, b), TEM image of silica from 2M Na2SiO3, 10% NaCl and 2M HCl (c, d).

Fig. 2. FESEM image of silica particles prepared by flow-synthesis with 5% sodium chloride and

2M hydrochloric acid at flow rate 0.1 ml/min (a, b, and c), 10% sodium chloride and 2M

hydrochloric acid at flow rate 0.1 ml/min for typical (d, e, and f).

(a) (b) (c)

(d) (e) (f)

218 Applied Physics and Material Science

Fig. 3. X-ray diffraction patterns of silica samples obtained at different synthesis condition, batch

reaction sodium chloride with hydrochloric acid (d), nitric acid (a), and flow synthesis at 0.1 ml/min

with 10% sodium chloride (b), and 5% sodium chloride (c).

Conclusions

RHA waste from rice husk power plants is a high-quality grade raw material silica source and

can be extracted to prepare nanosilicas with simple methods and at low cost. Spherical silica

particles with diameter of around 100 nm can be successfully prepared from RHA waste of power

plants with fast precipitation and smaller particles around 10 nm by flow synthesis. This work

indicated that silica nanoparticles can be easily prepared from sodium silicate solution by salt

assistant without surfactant or template by using a flow-synthesis method. Flow synthesis is a

continuous process that easily scales up production and controllable product quality.

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Advanced Materials Research Vol. 979 219

Applied Physics and Material Science 10.4028/www.scientific.net/AMR.979 Low-Cost and Fast Production of Nano-Silica from Rice Husk Ash 10.4028/www.scientific.net/AMR.979.216

DOI References

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