Transactions of the Indian Institute of Metals

, Volume 65, Issue 1, pp 63–70

Determination of Silica Activity Index and XRD, SEM and EDS Studies of Amorphous SiO2 Extracted from Rice Husk Ash

Authors

    • Department of Metallurgical and Materials EngineeringVisvesvaraya National Institute of Technology
  • Jatin Bhatt
    • Department of Metallurgical and Materials EngineeringVisvesvaraya National Institute of Technology
  • Dilip Peshwe
    • Department of Metallurgical and Materials EngineeringVisvesvaraya National Institute of Technology
  • Shailkumar Pathak
    • Department of Metallurgical and Materials EngineeringVisvesvaraya National Institute of Technology
Original Paper

DOI: 10.1007/s12666-011-0071-z

Cite this article as:
Deshmukh, P., Bhatt, J., Peshwe, D. et al. Trans Indian Inst Met (2012) 65: 63. doi:10.1007/s12666-011-0071-z

Abstract

Rice husk ash (RHA) contains 20% SiO2 in hydrated amorphous form (Si–OH). On thermal treatment, the SiO2 converts to cristobalite, the crystalline form which is not reactive. However under controlled conditions, amorphous SiO2 with high reactivity is produced. Therefore rice husk has been one of the useful bio-mass. The silica activity index was determined to be equal to 97.73 which was used to determine the percentage of amorphous SiO2 in RHA. The values of the soluble fraction and silica activity index were also stated. Comparative study of amorphous and crystalline SiO2 done by X-ray diffraction revealed the total amorphous nature of SiO2. The scanning electron microscopy (SEM) images displayed the comparative morphological features of the rice husk and RHA. The energy dispersive spectroscopy analysis of rice husk was done to determine the presence of SiO2 on the upper portion of rice husk and to determine the percentage of SiO2 in RHA. The SiO2 particles in an agglomerated form was found to be of micron size when observed under SEM.

Keywords

EDSSEMSoluble fractionSilica activity indexXRD analysis

Introduction

Many plants during their growth take up SiO2 from the earth. When plants residues are burned, organic materials are broken down as carbon dioxide, water vapors etc. The remaining ash contains inorganic residues, notably the SiO2. Examples are rice husk ash (RHA), rice straw ash, bagasse ash etc. Of all plant residues, the ash of rice husks contains the highest percentage of SiO2. Rice husk, a form of agricultural biomass, is generated in large quantities as a major by-product in the rice milling industry. The estimated world-wide rice husk production is about 100 million tons, of which about 90% is generated in developing countries. Disposal of these vast amounts of rice husk has been one of the major problems facing the rice milling industry. By burning rice husk, 20% ash is obtained from it [1, 2]. The SiO2 present in ash has a purity of 94–96% with major impurities like K2O, Na2O and Fe2O3. It has been shown that purity of SiO2 obtained by incineration of rice husk can be further extended to 98–99%, if rice husk is subjected to pre treatments like washing with distilled water and boiling with acid such as HCl which helps in removing above mentioned impurities. The SiO2 obtained by burning rice husk is amorphous and can be transformed to quartz, tridymite and cristobalite by heating it at high temperature over 900°C. The crystal phase, to which form the amorphous SiO2 transform depends on the purity of the SiO2 [3, 4].

The “SiO2” powder is widely used as raw material for cement, glass, porcelain, refractory, the filler for plastic, rubber and tire and so on. Comparison of natural “SiO2” and rice husk SiO2 exhibits two main differences. The first one is that use of natural “SiO2”, destroys nature more or less. On the other hand, rice husk SiO2 is one of the biological resources. Secondly, the crystal phase of “SiO2” is quartz while that of rice husk SiO2 is amorphous. Transforming quartz to tridymite and cristobalite needs high temperature of about 1,400°C. On the other hand rice husk SiO2 can transform to cristobalite at low temperature of 1,000°C. Rice husk SiO2 can be crushed to very fine powder easily and it is a kind of natural resource, which can be applied as the filler for cosmetics and foods [5].

SiO2 has been widely used in vegetable oil refining, pharmaceutical products, detergents, adhesives, chromatograph column packing, and ceramics because of its amorphous nature as the amorphous SiO2 is more reactive in nature as compared to crystalline. SiO2. Sodium silicate, the precursor for SiO2 production is currently manufactured by smelting quartz sand with sodium carbonate at 1,300°C contains over 60% SiO2 and can be an economically viable raw material for the production of silicates and SiO2. The amorphous nature of RHA SiO2 makes it extractable at lower temperature and hence provides a low energy method as an alternative to the current high energy method [6].

The main objective of the present work is to extract amorphous silica from RHA and study its properties. This process has the combined benefit of not only producing valuable silica powder at lower cost but also of reducing disposal as well as pollution problems. In this paper the comparative analysis was done on the SiO2 obtained by burning the rice husk for 12 h at the various temperatures such as 400, 450, 500 and 900°C. The morphological and elemental analysis of these samples was done by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The conversion from amorphous to crystalline phases with the increase in the temperature was done by X-ray diffraction (XRD) technique. The values of silica activity index for each variety of RHA were found out first time in this paper.

Materials and Methods

Sample Preparation

The rice husks used in this study was supplied by The Bajarang rice mill (Bhandara road, Nagpur). This was washed with water to remove dirt and other contaminants present in them and then dried in an oven at 100°C for 24 h. An adequate process of acid leaching was carried out with 3% HCl by boiling for 2 h, at a ratio of 50 g husk/l. The solution was filtered and the husk was washed with distilled water several times until the filtrate was free from acid. The acid leached husk was dried at 100°C for 12 h to remove moisture. These samples were burnt at temperatures such as 400, 450, 500 and 900°C for 12 h with the heating rate of 10°C per minute.

Soluble Fraction of Silica

The RHA may contain the SiO2 in an amorphous as well as in an crystalline form. So to separate amorphous silica from the crystalline SiO2 the procedure of soluble fraction of silica was carried out. The ash obtained from above four samples were taken, weighed in an analytical balance and boiled in 100 ml of 2.5 N (10%) NaOH. The solution was filtered through a filter paper and washed with mineralized water. This residue was dried in an oven at 800°C, cooled and weighed to get the weight of the insoluble silica. Soluble silica remained as dissolved in the solution [7]. This experiment was repeated for 5–6 times and then the average was found out. There was the +/− error of 0.0011 for each case approximately.

Silica Activity Index

Thus, the reactivity of RHA SiO2 depends on the crystalline/amorphous ratio. Therefore, for RHA characterization, the evaluation of the amount of amorphous silica becomes very important. For this purpose, there are some specific methods in the literature. One of them, proposed by Paya [8] established that the degree of amorphousness of silica is estimated by the “silica activity index,’’ which is determined by calculating the percentage of available silica that is dissolved in an excess of boiling 0.5 M sodium hydroxide in a 3-min extraction period. Thus the reactive silica was estimated indirectly as acid-soluble silica. The reactive silica was estimated conventionally as the difference between total silica and free silica present in the RHA before and after washing with 1:1 hydrochloric acid. This experiment was repeated for 5–6 times and then the average was found out. There was the +/− error of 0.0014 for each case approximately.

XRD, SEM and EDS Analysis

The amorphous and the crystalline nature of rice husk silica were examined by XRD analysis (X’Pert PRO PANalytical)) using Cu-Kα radiation at a scan speed of 2.5°/min. The morphological features of the rice husk, RHA and silica were studied with a SEM (JEOL JSM-6380A Analytical SEM). SEM coupled with microanalysis for the identification of amorphous silica. The EDS analysis of rice husk silica was also performed to determine the % of silica in RHA.

Results and Discussions

Soluble Fraction of Silica

The corresponding reaction for the determination of soluble fraction of silica is as follows
$$ \mathop {{\text{SiO}}_{2} }\limits_{{ ( {\text{Ash)}}}} + \mathop {2{\text{NaOH}}}\limits_{{ ( {\text{Caustic \;soda)}}}} \to \mathop {{\text{Na}}_{2} {\text{SiO}}_{3} }\limits_{{ ( {\text{Sodium\; Silicate)}}}} + \mathop {{\text{H}}_{2} {\text{O}}}\limits_{{ ( {\text{water)}}}} $$
(1)
$$ \mathop {{\text{Na}}_{2} {\text{SiO}}_{3} }\limits_{{ ( {\text{Sodium \;Silicate)}}}} + \mathop {2{\text{HCl}}}\limits_{{ ( {\text{Hydrochloric\; acid)}}}} \to \mathop {{\text{SiO}}_{2} }\limits_{{ ( {\text{silica)}}}} + \mathop {2{\text{NaCl}}}\limits_{{ ( {\text{Sodium \;chloride)}}}} + \mathop {{\text{H}}_{2} {\text{O}}}\limits_{{ ( {\text{Water)}}}} $$
(2)
Soluble silica remained in the solution.
As we go on increasing the temperature from 400 to 900°C we observed the decreasing trend in the values of soluble fraction of silica. From Table 1 it is reveled that the soluble fraction of silica is highest for the rice husk sample burnt at 500°C (12 h). It is because, as the temperature and duration of burning increases the RHA becomes more amorphous and reactive in nature so soluble fraction was also more as compared to the other samples. At 900°C the silica become crystalline and is not soluble in the above reaction (1) and (2). These results are in accordance with that reported by Tzong-Horng Liou [9].
Table 1

Determination of soluble fraction of silica at different temperatures

Burning temperature (°C)

Soluble fraction of silica

400°C (12 h)

72

450°C (12 h)

98

500°C (12 h)

99

900°C (12 h)

69

Silica Activity Index

Silica activity index was found out by calculating the percentage of available silica that is dissolved in an excess of boiling 0.5 M NaOH in a 3 min extraction period.

From the above graph (Fig. 1) it is observed that the silica activity index for the sample burnt at 500°C for 12 h has highest value as compared to the other samples. The reason behind this is as the temperature of burning increases beyond 500°C the amorphous silica converts into crystalline silica (Fig. 2) and in a 3 min extraction period only amorphous silica is soluble. If the extraction period is increased beyond 3 min then crystalline silica is also soluble.
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Fig. 1

Comparative study of silica activity index of rice husk burnt at different temperatures

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Fig. 2

XRD diffractogram displaying the crystalline nature of silica when rice husk was burnt at the higher temperature above 500°C

Highly reactive silica can be produced by maintaining the combustion temperature below 500°C under oxidizing conditions for relatively prolonged period or up to 680°C provided the high temperature exposure was less than 1 min. Prolonged heating above this temperature may cause the material to convert (at least in part) to crystalline silica; first to cristobalite and then to tridymite. Silica in RHA can remain in amorphous form at combustion temperatures up to 900°C if the combustion time is less than one hour, whereas crystalline silica is produced at 1,000°C with combustion time greater than 5 min. Other reports claim that crystallization of silica can take place at temperatures as low as 600, 500, or even at 350°C with 15 h of exposure [10]. X-ray diffraction analysis revealed the presence of quartz whose origin may be attributed to contamination or re-crystallization of silica during calcinations.

SEM and EDS Analysis

Figures 3 and 4 shows SEM micrographs of rice husk sample. Figure 3a, b shows the outer epidermis of rice husk, which is well organized and has a corrugated structure. Figure 4a, b shows the inner epidermis of rice husk, which has a lamella structure. The morphology is different for the outer and inner surfaces of rice husk. The silica is mainly localized in the tough interlayer (epidermis) of the rice husk and also filling in the spaces between the epidermal cells [11, 12]. The concentration of silica was high on the external surface of the husk (Fig. 3a, b) and much weaker on the internal face and practically non-existent within the rice husk (Fig. 4a, b). Figure 3 shows the regular spherical platelets of almost equal size (40–50 μm) appearing in parallel rows. However Fig. 4a, b we note that there is no occurrence of Silica in the lower portion of rice husk. According to the study done by [13], most of the silica was present in the outer epidermal cells, being particularly concentrated in the dome-shaped protrusions.
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Fig. 3

Scanning electron micrographs (a) along with EDS (b) of outer epidermis of rice husk (500×)

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Fig. 4

Scanning electron micrographs (a) along with EDS (b) of inner epidermis of rice husk (500×)

When rice husk was burnt in air, Fig. 5 shows that many residual pores are distributed within the ash sample, indicating that the silica is a highly porous material with a large internal surface area. The rice husk have broken up during thermal decomposition of organic matter, thus leaving a highly porous structure. The silica obtained after burning of rice husk in air has a pure white colour (Fig. 10). By comparison of these micrographs (Figs. 4, 5), it was observed that the surface of unreacted samples was relatively nonporous, whereas a burnt sample exhibits a porous surface, as indicated by the pore structure analysis (Fig. 5). As the temperature of heating increases the loss of volatile matter also increases. This gives the more porous structure of silica, which is active in nature. SEM images of samples are shown in Fig. 5. Figure 5a, b shows the aggregates of clearly defined layers of loose flakes. The flaky morphology of rice husk in Fig. 5c suggests that surface is more loosely bound than in Fig. 5a, b, which makes it highly amorphous and reactive.
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Fig. 5

Transformations in the structure of rice husk with the increase in the temperature. a RHA burnt at 400°C. b RHA burnt at 450°C. c RHA burnt at 500°C. d RHA burnt at 900°C

When the rice husk burnt beyond 500°C i.e., at 900°C there is start of crystallization in the rice husk, the structure becomes more defined or ordered (Fig. 5d). As found from the Table 1 also the value of soluble fraction of silica of rice husk burnt at 500°C for 12 is 99 which is highest among all the temperatures. The structure of the rice husk burnt at this temperature was found to be highly porous (Fig. 5c). The SEM image and EDS analysis was done at the magnification of 1,000× (Fig. 6a, b).
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Fig. 6

a and b SEM image and EDS of silica obtained by burning rice husk at 500°C for 12 h

The SEM of amorphous silica (after precipitation) in an agglomerated form displays the particle size in the μm (Fig. 7) and with the magnification at 1,000× displayed porous structure of the silica (Fig. 7a).
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Fig. 7

Scanning electron microscopy of amorphous silica in the precipitated form at 500× (a) displaying the particle in an agglomerated form. The magnified SEM image at 1,000× (b) displaying the fine silica particles forming the porous agglomerate

XRD Analysis

The rice husk samples were burnt in a furnace for the temperatures such as 400, 450, 500, 900°C and the XRD plot were observed (Fig. 8). XRD analysis was performed for selected samples to identify differences in the formation of amorphous or crystalline silica for different combustion temperatures. A qualitative assessment of the crystallinity of the samples can be obtained from the intensity of the narrow reflections as compared to the broad band around 22° (2θ) for RHA burned at 500°C. The intense broad peak observed for the RHA at 500°C samples indicates the amorphous nature of silica (Fig. 8). The start of crystallization that is, α-quartz starts from the temperature 500°C onwards. All the RHA samples burnt below 500°C shows the amorphous nature.
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Fig. 8

Comparative XRD plot of RHA burnt at various temperatures

For the RHA at 900°C some reasonably sharp and intense reflections start to show up on top of the broad amorphous background indicating that cristobalite starts to form at this temperature. Only after incineration above 900°C the material becomes highly crystalline as evident from the XRD pattern of a RHA at 1,000°C sample consisting of sharp reflections which can be observed from the following pattern (Fig. 8).

The XRD pattern of a RHA 1,000°C sample shows sharp reflections and that can be assigned to cristobalite and tridymite Fig. 9.
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Fig. 9

XRD pattern of an RHA 1,000°C sample showing sharp lines and without broad background indicating the crystalline nature of the sample. Two crystalline phases could be identified being cristobalite and tridymite

The transition from amorphous to crystalline silica of rice husk takes place with the increase in the temperature and also at lower temperature but with prolonged heating [7]. α Quartz converts to β quartz at 573°C. Quartz converts to cristobalite at 870°C and then converts to tridymite at 1,470°C.

The color of the ash generally reflects the completeness of the combustion process as well as the structural composition of the ash (Fig. 10).
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Fig. 10

Transition in the color of RHA with the increase in temperature. a 400°C, b 500°C, and c 900°C

Generally, dark ash exhibit higher carbon content (with the exception of those that may be darker due to soil chemistry or region (Fig. 10a). Lighter ash have achieved higher carbon burnout (Fig. 10b) while those showing a pinkish tinge have higher crystalline (tridymite or cristobalite) content (Fig. 10c) [12]. The complete white colour of the ash shows the total amorphous structured silica. From Fig. 10 it was observed that at 400°C RHA is black in color due to high carbon content. Whereas at 500°C bright white color ash is seen indicating total transformation of RHA to amorphous silica. However with further increasing the incineration temperature to 900°C the ash attains the pinkish tinge which shows the transformation of amorphous silica to crystalline form.

Conclusions

Rice husk is a valuable natural resource and also an excellent source of high quality silica. The SEM and EDS techniques employed provided complementary information on husk surface topography. It is apparent that rice husk has unique surface features with a highly irregular outer surface rich in silica. The values of the silica activity index have been found first time in this paper. The optimum burning temperature to obtain the desired properties was set at 500°C (12 h of burning) from the various characterization tests carried out in the paper. The comparative XRD plot of the RHA obtained by burning rice husk at various temperatures showed the difference between the amorphous and crystalline silica. The structure of the rice husk becomes more porous as the temperature of burning increases. The porous structure leads to the formation of active and amorphous silica. The flaky morphology of the amorphous silica sample as seen in the SEM micrograph suggests that the surface is more loosely bound, which makes it highly reactive. Samples containing crystalline phases cristobalite and tridymite are not active.

Acknowledgment

The authors expresses thanks to the Institution of Engineers (India) for supporting the financial aid during this project under the grant SCK/T-R&D/96/2009-2010.

Copyright information

© Indian Institute of Metals 2011