Review of Extraction of Silica from Agricultural Wastes using Acid

Leaching Treatment.

C.P. Faizul, C. Abdullah & B. Fazlul

School of Materials Engineering, Universiti Malaysia Perlis,

P.O Box 77, D/A Pejabat Pos Besar, 01007 Kangar, Perlis, Malaysia

e-mail: faizul@unimap.edu.my

Keywords: silica, rice husk, palm ash, agricultural wastes, organic acid

Abstract. Large quantities of agricultural wastes such as palm ash and rice husk are found in

Malaysia, have a large possibility to be employed as usefully renewable to produce energy and

silica (SiO2). Extensive researches have been carried out to extract silica from agricultural wastes

such rice husk, because silica is useful raw material for industrial application. In the previous

studies, the strong acid leaching treatment was carried out on rice husk to remove metallic

impurities and organics contained in them. Leaching treatment is a proper route to extract the silica.

Sulphuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3) solutions are

conventionally used in leaching treatment to prepare silica materials [1]. A strong acid leaching

treatment, however, is significantly hazardous to the environment and humans. This paper reviews

the common extraction method used and the latest research trends in extraction of silica.

Introduction

This review paper discusses the common extraction method used and the latest research trends in

extraction of silica from agricultural wastes using organic acid. Silicon dioxide (SiO2) or commonly

known as silica is one of the basic materials and the valuable inorganic multipurpose chemical

compounds. Silica is occurring naturally as quartz, sand or flint. It can exist in gel, crystalline and

amorphous forms. It is the most abundant material on the earth’s crust. Nowadays, most silica was

produced from quartz or sand by the extraction process. Sodium silicate, the precursor for silica

production is currently manufactured by smelting quartz sand with sodium carbonate at 13000C [2].

From this production of silica, it will be used to fulfil the requirement in its major applications such

as for ceramic product, electronic component and additive in concrete. However, manufacturing of

pure silica is energy intensive.

Silica also can be found in agricultural wastes such as palm ash and rice. These waste are needed

to be disposed properly, otherwise it may cause a major environmental sustainable issue. The large

amount of this waste can be a new source of silica production for this country. For example,

According to the Malaysian Palm Oil Board, the amount of palm ash produce in Malaysia in 2007

approximately 3 million tons [3] and based on the annual grain production of 2.2 million tonnes, the

output of rice husks is about 0.44 million tonnes in Malaysia [4]. Even though the production of

silica from the agricultural wastes are not much compared to the production from quartz or sand, it

is still can be used to fulfil the industrial demands.

Leaching treatment is a proper route to extract the silica. Sulphuric acid (H2SO4), hydrochloric

acid (HCl) and nitric acid (HNO3) solutions are conventionally used in leaching treatment to

prepare silica materials [1]. Beside hazardous to the environment and humans, the strong acid

leaching treatment also has an economical problem due to a necessary use of expansive materials

with corrosion resistance to strong acid and a special disposal treatment of used strong acids.

Advanced Materials Research Vol. 626 (2013) pp 997-1000Online available since 2012/Dec/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.626.997

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Palm Ash. Palm oil is an important export commodity in tropical countries and especially in

Malaysia due to its wide spectrum of an acknowledged usability such as precursors of food products

and biofuel. Extraction of the oil from fresh oil palm fruitlets requires separation of the fruitlets

from empty fruit bunches prior to further processing. These empty fruit bunches, which consists of

fibres and shells, are often used as boiler fuel by palm oil mill to produce steam for electricity

generation and palm oil extraction [5]. Combustion produces approximately 5% of palm as.

Because of limited uses for palm ash, it is currently disposed of as landfill; this could lead to

environmental problems in the future.

Table 1: Chemical Composition of rice husk ash

Compound Concentration (%)

SiO2 46.0

MgO 3.7

P2O5 6.1

K2O 23.9

Fe2O3 3.5

CaO 15.0

Others 1.8

Table 1 shows that palm ash has about 46.0% silicon element. It is possible to extract silica from

palm ash.

Rice Husk Ash. Rice husk is an agricultural residue abundantly available in rice producing

countries. Rice husk are the coating part of the seed or grain of the rice husk. The beneficiation of

rice generates as by-product rice husk that corresponds about 23% of its initial weights [6]. This

husk can be used as a fertilizer in agricultural or as an additive for cement and concrete fabrication.

Rice husk as is one of the most silica rich raw materials containing about 90-98% silica (after

complete combustion) among the family of other agro – wastes [7]. Silica is the major constituent

of rice husk ash and Table 2 give typical composition of rice husk ash. With such high silica content

in the rice husk ash it becomes economical to extract silica from the ash, which has wide market

and also takes care of ash disposal.

Table 2: Chemical Composition of rice husk ash

Compound Concentration (%)

SiO2 80 – 90

Al2O3 1 – 2.5

Fe2O3 0.5

CaO 1 – 2

MgO 0.5 – 2.0

Na2O 0.2 – 0.5

K2O 0.2

Leaching Process. Leaching generally refers to the removal of a substance from a solid via a liquid

extraction media. The desired component diffuses into the solvent from its natural solid form. There

are three important parameters in leaching which are temperature, contact time per area and solvent

selection. The temperature can be adjusted to optimized solubility and mass transfer. Leaching can

be divided into two categories which is percolation and dispersed solid. For percolation, the solvent

998 Advanced Materials Engineering and Technology

is contacted with solid in a continuous or batch method and widely used for extreme amount of

solids. In dispersed solids, the solid are usually crushed into small pieces before being contacted

with solvent. In simple words, percolation is for liquid added into solid while dispersed solid is for

solid added into liquid [1, 8-10].

Leaching treatment is a proper route to extract the silica. Through conventional method, silica is

digested from ash using caustic soda (NaOH) as sodium silicate (Na2SiO3). As example, to obtain

silica; rice husk ash is treated with sodium hydroxide forming sodium silicate. Sodium silicate is

then treated with sulphuric acid to precipitate silica [11]. The reaction is:

SiO2 + 2NaOH Na2SiO3 + H20 (1)

(ash) (caustic soda) (sodium silicate) (water)

Then, Rreaction of sodium silicate with sulphuric acid precipitates silica.

Na2SiO3 + H2SO4 SiO2 + N2SO4 + H20 (2)

(sodium silicate) (sulphuric acid) (silica) (sodium sulphate) (water)

The purification and drying produce silica in white amorphous powder form [7, 12]. This process

will remove any impurities or organic matter in the agricultural wastes.

In the previous study done by other researchers, the strong acid leaching treatment was carried

out on rice husk to remove metallic impurities and organics contained in them [8-10]. In some

research, acid leaching performed by reflux boiling in 3% (v/v) HCl and 10% (v/v) H2SO4 [13].

Riveros et al. also used 3% HCl in their leaching process14]. Conradt et al. was performing acid

leaching by reflux boiling in 2.4 molar HCl or 3.6 molar H2SO4 [15]. The silica content will be

extract from the agricultural wastes by removing the organic matter in the agricultural wastes using

acidic solution.

Many researchers have concluded that preliminary leaching of rice husk with a solution of HCl,

HNO3, H2SO4, NaOH and NH4OH, boiled before thermal treatment with temperatures ranging from

500 to 14000C for various time intervals, proved to be effective in substantially removing most of

the metallic impurities and producing ash-silica completely white in colour with a high specific area

[6].

The problem with conventional acid leaching treatment was used strong acid as the extraction

media. Strong acid is classified as very dangerous chemical due to the level of hazardous which is

high. The strong acid leaching treatment also has an economical problem due to a necessary use of

expensive materials with corrosion resistance to strong acids, water rinsing of husks and a special

disposal treatment of used strong acid.

Recent Trends

Organic acid is an alternative chemical to replace the strong acid in the leaching process. This is

because the organic acid have low level of hazardous compared to the strong acid. Researchers all

over the world are focusing on ways to establish an environmentally benign process to produce

silica from agricultural wastes by using organic acid rather than using strong acid

Several studies have been carried out by researcher from Joining and Welding Research Institute,

Osaka University, high purity of amorphous SiO2 materials from rice husk has been established by

using carboxylic acid leaching treatment [1,8-10]. In their studies, the concentration and

temperature of the carboxylic acid solution, and stirring time in the solution were selected as the

operating parameters. This due to the chelate reaction between carboxyl groups (-COOH) and

metallic impurities contained in husk strongly depends on the mentioned parameters.

Advanced Materials Research Vol. 626 999

References

[1] Junko Umeda & Katsuyoshi Kondoh (2008). “High-purity Amorphous Silica Originated in Rice

Husks via Carboxylic Acid Leaching Process”, Journal of Materials Science, 43, 7084-7090.

[2] U. Kalapathy, A. Proctor & J. Shultz (2000), “A Simple Method for Production of Pure Silica

from Rice Hull Ash”, Boiresources Technology, 73, 257-262.

[3] Chea Chandra, Khairun Azizi Mohd Azizli & Zainal Arifin Ahmad (2010). “Mineralogical

Component of Palm Ash Oil Fuel Ash with or without Unburned Carbon”, Proceedings of

International Conference on X-Rays & Related Techniques in Research & Industry 2010, 103-

106.

[4] Vadivelooa, J., Nurfarizaa, B. and Fadel, J.G. (2009). “Nutritional Improvement of Rice

Husks”, Animal Feed Science and Technology 151, 299–305.

[5] Chun Yang Yin, Sharifah Aishah Syed Abdul Kadir, Ying Pei Lim, Sharifah Nawirah Syed-

Ariffin, Zurinawati Zamzuri. (2008). “An Investigation into Physicochemical Characteristics of

Ash Produced from Combustion of Oil Palm Biomass Waste in A Boiler”, Fuel Processing

Technology, 89(7), 693-696, 2008

[6] Della, V.P., Kühn, I. And Hotza, D. (2002). “Rice Husk Ash as an Alternate Source for Active

Silica Production”, Materials Letters 57, 818-821.

[7] Davinder Mittal (1997). “Silica from Ash”, Resonance 7, 64-66.

[8] Junko Umeda & Katsuyoshi Kondoh (2010). “High-Purification of Amorphous Silica

Originated from Rice Husks by Combination of Polysaccharide Hydrolysis and Metallic

Impurities Removal”, Industrial Crops and Products 32, 539-544.

[9] Umeda Junko & Kondoh Katsuyoshi (2008). “Process Optimization to Prepare High-Purity

Amorphous Silica from Rice Husk via Citric Acid Leaching Treatment”, Transaction of JWRI

30(1), 13-17.

[10] Umeda Junko, Imai Hissashi & Kondoh Katsuyoshi (2009). “Polysaccharide Hydrolysis and

Metallic Impurities Removal Behavior of Rice Husks in Citric Acid Leaching Treatment”,

Transaction of JWRI 38(2), 13-18.

[11] Shelke, V.R. et al. (2010). ”Mesoporous Silica from Rice Husk Ash”, Bulletin of Chemical

Reaction Engineering & Catalysis 5(2), 63-67.

[12] Amutha, K. et al. (2010). ”Extraction, Synthesis and Characterization of Nanosilica from

Rice Husk”, International Journal of Nanotechnology and Application, 4, 61-66.

[13] Yalcin, N. and Sevinc, V. (2001). “Studies on Silica Obtained From Rice Husk”, Ceramics

International 27, 219-224.

[14] Riveros, H. and Garza, C. (1986). “Rice Husks as a Source of High Purity Silica”, Journal of

Crystal Growth 75, 126-131.

[15] Conradt, R., Pimkhaokham, P. and Leela-Adisorn, U. (1992). “Nano Structured Silica from

Rice Husk”, Journal of Non-Crystalline Solids 145 (75-79).

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