Field of the Invention
The present invention relates to a process for producing silica gel from rice husk ash.
More particularly, the present invention relates to a process for producing silica gel from
rice husk by microwave heating. Furthermore, this invention also relates to a process for
producing silica gel from rice husk performed by heating rice husk ash in sodium
hydroxide solution with various concentrations in microwave oven for 5 or 10 minutes.
Moreover, this invention also relates to evaluate the potential use of micronized rice husk
ash (RHA) for the production of xerogel silica (silica gel) and their possible use as
fertilizers in rice cultivation.
Background of the invention and related prior Art
Humidity exchange elements which are intended for use in dehumidifiers, heat
exchangers or the like, are often formed from a solid adsorbent which may adsorb
humidity and such moisture adsorbent is e.g. silica gel. Thus the adsorbent will
alternately adsorb and desorb the humidity at adsorption and regenerating of the element,
respectively.
Absorbent particulate has been produced in a small-granulated form. Silica gels have
been manufactured for many years and utilized in a plethora of different applications,
ranging from abrasives to desiccants to thickeners and the like. The typical
manufacturing process followed for silica gel production involves careful feeding of an
acid and water glass solution through a narrow tube into a large cylindrical vessel. The
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gel will form nearly instantaneously so it is imperative that the feed be undertaken in
such a fashion as to permit the resultant highly viscous gel product to form from the
bottom of the cylinder and fill up the entire structure without clogging the feed system
itself. Upon complete gel tormation, the cylinder is then tipped downward and a piston is
triggered to force he gel (in a large cylindrically shaped semi-solid form) out of the
cylinder and through a series of mcsh screens of differing gauge in order to "slice" the
large gel mass into discrete cubic (or chunks) shapes. The resultant small gel cubes are
then ammoniated and subjected to heating for a sufficient time to provide a desired pore
size level and reinforcement simultaneously. The dried particles can then be milled to a
desired size.
Moisture absorbent particles have many uses, both industrial and domestic. Silica gel has
been widely used as a drying agent and in recent years, it has been used for various
purposes as e.g. a catalyst support, a separating medium or an adsorbent. Accordingly.
requirements for pertormances of the silica gel have been diversified according to the
purposes. The perto:-mances of the silica gel are greatly influenced by physical properties
of the silica gel suef: as the surface area, the pore diameter, the pore volume. the pore size
distribution, and tbese physical properties are greatly influenced by conditions for
producing the silica gel.
As a method for producing the silica gel, the most common method is to hydrolyze an
alkali silicate such as sodium silicate with a mineral acid, and gelate the resulting silica
hydrosol, followed by drying. Many proposals have been made with respect to the details
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of the production method so as to improve the performances of the silica gel.
The patent document JP621 13713 statcs a method for producing a silica gel having a
narrow pore distribution, produced by gelling silica hydrosol formed by reaction of alkali
silicate aqueous solution and mineral acid solution, treating a silica hydrogel with acid
solution at a pH under 2.5, washing with water, adjusting the pH to '-I 9 in a buller
solution and treating hydrothermally. In Examples of the above gazelle, a silica gel
having an average pore diameter of from 6.7 to 8.5 nm and a pore volume or hom about
0.8 to about 0.9 mUg can be obtained by the above method.
The other document JP 930809 describes a method wherein a silica hydrogel is dried by
batch flow drying and then a hydrothermal treatment is carried out. Changes in
performances of the silica gel obtained by this method are also confirmed, and a silica gel
having a sharper pore distribution can be obtained. However, the pore volume, the
specific surface area and the average pore diameter can not adequately be changed. and
this method is inadequate as a method to obtain a silica gel having desired physical
properties.
The document US 6451862 discloses a process for producing a spherical silica gel, which
comprises supplying a liquid mixture resulting from mixing of an alkali metal silicate
solution and an acid solution to a spraying apparatus, spraying the liquid mixture to
obtain droplets, bringing the droplets into contact with a liquid for recovering a silica gel,
said liquid for recovering a silica gel being sprayed from an upper part of a container
accommodating the spraying apparatus and permitted to flow down along an inner wall
4
of said container, and recovering the formed spherical silica gel together with the liquid
for recovering a silica gel, as a slurry.
The patent documents US4676964 and US4401638 disclose that because of its relatively
low cost, little attention has been paid to silica gel regeneration in the past. Also, although
large volumes of silica gel are generated each year, the volume per company is generally
not sufficiently important to warrant the investment of developing advanced regeneration
technologies internally. It is well known that silica is relatively stable in strongly acidic
media or when heated at high temperatures. Because of its high temperature stability,
most regeneration processes developed in the past proposed a simple heat treatment,
alone or in combination with acid washing.
According to the other document US 5376348, the making silica gel having a large active
surface area suitable for use as an adsorbent-separator of gases and liquids in
hermetically sealed acid accumulators and as a filler in the fabrication of rubber articles.
The method produces silica gel having a large active surt:1ce area and, at the same time, a
microgranular structure. The product is in the form of aggregates having a size which
generally docs not cxcced 5 mm, so no additional grinding of the product is required.
Brazil is the most important Latin-American rice producer and the State of Rio Grande do
Sui (RS) is the country's main producer. Rice husks in India have begun to be regarded
as an agro-industrial residue of importance and as a potential raw material for thermal
and chemical processes, as well as a source of soluble silica.
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However, according to Zhang et aI., 2010, attempts to use rice husks for industrial
purposes are still facing technical difficulties, mostly due to their poor protein content,
low degradability and high mineral percentage.
For these reasons, if a suitable application could be found for residual rice husks, it could
provide a challenging business opportunity, while also bringing about benefits to the
environment and public health. At the moment, most of the rice husks remain unused
which causes environmental problems, for example, emission of CO2 and long lasting
smoke from open sky combustion and methane, when they are left to degrade on the
ground stated in Umeda and Kondoh, 2010.
Mehdinia et aI., 2011 describes that for every 4 tons of rice, 1 ton of rice husks is
produced. After combustion for power generation, 15-30% of this results in RHA
comprising around 95% silica. Thus, it is estimated that the availability of silica is around
380,000 ton year-I in the state of RS. Previous studies have shown that it is possible to
obtain silica from rice husks and Rice Husk Ash (RHA) with similar yields compared to
other processes. A simplified method of sol-gel extraction of silica at room temperature
was also reported in the literature Lima et aI., 2011.
Zaky et al. 2008 have prepared silica nanoparticles from RHA by employing statistical
design to optimise the parameters afTecting the dissolution of silica. such as
stoichiometry, time and temperature as well as achieving efficiency of 99% on the
extraction. Zhang et at. 2010 produced amorphous silica from RHA treated with HCI
6
solution, obtaining surface area of 287.86 m2 g-l, mean particle size of 50 mm and
99.87% purity. Other works have described riee husks and RHA as excellent sources for
high quality silica which has been described in Lima et af., 2011; Kalapathy et aI., 1999;
Witoon et aZ., 2008. However, these silicas have only been proposed as substitutes for
commercial sources, such as nanosilicas, ceramic and cement additives.
On the other hand, according to Nakata el af., 2008; Abdalla, 2011 silica also plays an
important role in almost all living organisms. It is absorbed by the roots of plants and
deposited on the outer walls of epidermal eells as a silica gel, where it acts as a physical
barrier against pathogenic fungi and attacks by insects. Moreover, it reduces the loss of
water through transpiration, stiffens the cell walls and keeps the leaves more erect.
According to Rodrigues et at. 2003, an increase in the Si content of rice plants by the
application of Calcium silicate to the soil, as wolastonite, explained the significant
reduction of the occurrence of sheath blight on the leaves.
The benefits of this silica gel is for the culture of rice include increased growth and
production, positive interactions with fertilizers, as well as higher resistance against
diseases, plagues, drought and salinity.
This invention solves the aforementioned problems with the conventional technologies.
Under the situation, the present inventors repeatedly and earnestly studied so as to
overcome the above-mentioned problems and, as a result, potential use of micronized
RHA for the production of xerogel silicas and their possible use as fertilizers in rice
culti vation.
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Summary of the invention
The present invention relates to a process for producing silica gel from rice husk. More
particularly, the present invention relates to a process for producing silica gel from rice
husk by microwave heating. Furthermore, this invention also relates to a process for
producing silica gel from rice husk performed by heating rice husk ash in sodium
hydroxide solution with various concentrations in microwave oven for 5 or 10 minutes.
Moreover, this invention also relates to evaluate the potential use of micronized rice husk
ash (RHA) for the production of xerogel silica and their possible use as fertilizers in rice
cultivation.
Detailed description of thc invention
It is thus a main object of the invention to overcome the disadvantages of the abovementioned
prior art. It is thcrefore one advantage of this novel process to produce a silica
gel material through a process that does not require by conventional process.
Another object of the present invention is to a process for producing silica gel from rice
husk ash.
Still other object of the present invention is to provide a process for producing silica gel
from rice husk ash by microwave heating.
Still another object of the present invention is to provide a process for producing silica
gel from rice husk ash at extremely low cost. More specifically, it concerns a process for
8
producing silica gel from rice husk performed by heating rice husk ash in sodium
hydroxide solution with various concentrations in microwave oven for 5 or 10 minutes.
Yet another object of the invention is to provide the xerogel silicas, moreover, showcd
high values for the specific surface area and favourable mean particle size for the
purpose of fertilizer.
Yet another object of the invention is to provide the xcrogel silicas for the cultivation of
rice indicated that these sources of silicon provided incremcnts in the soil Si content and
as a consequence, increasing the grain yield and dry matter production in the rice plant
leavcs.
One more objective of the present invention is that the costs of agrochemicals for the
control of plagues and diseases in the cultivation of rice can be reduced, by using a lowcost
and most abundant this product.
One more objective of the present invention is that the chemical conversion of
micronised RHA with the purpose to produce xerogel silicas was shown to be a possible
way of making use of this inconvenient agricultural residue by gcnerating products of
some added value which can feed back into the business chain.
Rice hull ash (RHA), a waste product of the rice industry is rich in silica. A simple
method based on alkaline extraction followed by acid precipitation was developed to
produce pure silica xerogels from RHA, with minimal mineral contaminants. The silica
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gels produced were heated to 80°C for 12 h to obtain xerogels. Silica and mineral
contents ofxerogels were determined by energy dispersive X-ray (EDX) and inductivclycoupled
plasma (ICP) emission spectrometers, respectively. Xerogels produced from
RHA had 93% silica and 2.6% moisture. The major impurities of silica produced from
RHA at an extraction yield of 91 % were Na, K, and Ca. Acid washing prior to extraction
resulted in silica with a lower concentration of Ca «200 ppm). However. CI{)nalwater
washing of the xerogel was more e€ective in producing silica with lower overall mineral
content (Na< 200 ppm and K< 400 ppm). X-ray diffraction patterns revealed the
amorphous nature of silica xerogel. Fourier transform infrared (FTIR) data indicated the
presence of siloxane and silanol groups. This silica xerogels with 93% silica content and
minimal mineral contaminants can be produced from RHA using a simple low energy
chemical method. The silica extraction yield from RHA was 91 %. The initial acid
washing of RHA resulted in a lower Ca content in the silica xerogel.
The process for preparing the xerogel silica from rice husk ash which compnses by
heating rice husk ash in sodium hydroxide solution with vanous concentrations in
microwave oven for 5 or 10 minutes. The obtained sodium silicate was neutralized to
give silica gel. The best condition for silica gel production was the reaction with 2.0 M
sodium hydroxide at microwave power of 800 W for 10 minutes. The ability of silica gel
as desiccant was investigated by adsorption test. The results showed that silica gel
prepared by low concentration of sodium hydroxide solution had thc highest abi lity of
water adsorption. The power of microwave and volume of reactions seemed to have only
little effect.
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109 of rice husk ash were added to various volumes and concentrations of sodium
hydroxide solution (most solutions were 2.0 M = 165 ml,
1.0 M = 330 ml, 0.5 M = 660 ml, because the above solutions provide a m1l11mUm
amount of NaOH to produce sodium silicate with Si02INa20 ratio = 1.0) in a thermoresistant
plastic container.
The mixture was then heated by microwave irradiation at 400 W, 600 W or 800 W at
100Gc for 5 min or 10 min. Thc solution was filtered through filter paper (1 a ~lm) and the
carbon residue was washed with 100 ml of de-ionized water. The filtrate and washing
were allowed to cool to room temperature. Concentrated sulfuric acid was added to the
obtained solution until pH 7.0 and incubated for 48 hours to promote silica gel formation.
The silica gel produced was separated from soluble salt solution by vacuum filtration and
washed with de-ionized water. Then silica gel was dried at 1500C for 48 hours and
ground into powder. The obtained silica gel was white rough powder. The samples for
XRD analysis were heated with 200 ml of 2 M hydrochloric acid solution by MW heating
for 10 min, then rinsed with de-ionized water dried at ISOoC for 24 hours Some
experiments were performed in a glass beaker (500 ml) using conventional heating in oil
bath (1500 W) in order to compare the eflect of heating methods. The reaction conditions
were the same as microwave heating as listed in Table 1, except for the heating method.
11 was found that under all conditions, NaOH yielded the highest amount of silica gel,
while Na2C03 which is a general reagent for production of sodium silicate produced less
than a half of silica gel from NaOH. Compared with conventional heating method in oil
bath at 1500 W at the same reaction time, MW heating gavc a higher yield of silica gel at
11
800 W by 0.5, 1.0 and 2.0 M NaOH solution. By reaction at 800 W with 1.0 M NaOH
solution, the weight of silica gel obtained from MW (6.830 g) was almost 1.6 times that
from conventional heating (3.997 g). And the reaction at 800 W with 2.0 \t1 NaOH
solution yielded 9.593 g of silica gel from 10 g of rice husk ash in 10 min. The results
showed that MW heating is capable of extraction of silica gel from rice husk ash.
Comparing concentrations of NaOH solution revealed that 0.5 M solution could give
Si02fNa20 ratio as high as 2.92 by heating at 800 W for 5 min.
Evaluation of water adsorption ability revealed that silica gel prepared by low
concentration of NaOH solution had greater ability than that prepared by high
concentration of NaOH solution. On the other hand, power of MW showed no effect on
the adsorption ability.
The micronised RHA used in this investigation was supplied by a local market developer
for RHA from the thermoelectric combustion of rice shells (Fig. 1).
After being sent to the laboratory, the rice husk ash was classified in a sieving system
using a 2.4 mm mesh sieve. The RHA samples were dried in a kiln for 24 h at 110°C:
after that, they were left to cool in a desiccator and were later stored in hermeticallysealed
glass t1asks for subsequent use. Following this, X-ray fluorescence spectrometric
analysis was conducted to determine the silica content of the micronised rice husk ash.
The silicas were prepared with the aid of NaOH and KOH extracting solutions with the
respective bicarbonates being used as catalysts, following the process (with some
modifications). Micronised RHA (20 g) was placed in a 250 mL round-bottom flask and
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mixed with the corresponding alkaline solution and catalyst, in concentrations predicted
by the Response Surface Methodology (RSM) for sodium-based silica, using as
independent variables the pH (1, 3, 5, 7 and 9), base conccntration (2.0, 3.5, 5.0, 6.5 and
8.0 mol L-'), catalyst concentration (0.6, 1.3,2.0,2.7 and 3.4 mol L-') and time (1, 2,3,4
and 5 h). The yield of the silica extraction was regarded as the dependent variable.
The mixture was then kept under reflux for 3 h in accordance with the DOE (design of
experiment). Following this, the solution was passed through a paper filter with 2.0 ~lm
porosity (Whatman, Springfield Mill, UK) and the filtered material was acidified to the
pH predicted by the RSM with a solution of 5.5 N H2S04 which formed a slightly pink
silica precipitate. It was filtered again in the same way as mentioned before, now adding
20 mL of3% H202.
The discoloured silica xerogel was dried in a kiln for 24 h; after that. it was washed with
distilled water to remove the excess acid (pH~6) and put back in the kiln for a further
drying period of 24 h. In the final stage, the silica xerogel was cooled in a desiccator,
ground in porcelain mortar and classified at 0.24 mm mesh.
The diffractrograms of the silica xerogels shown in Fig. 2. The diffractrograms of the
sodium-based and potassium-based silica samples show peaks in the range of 12.5°
which confirmed the presence of amorphous regions. In the case of the potassium-based
silica, a peak of 29° was confirmed with regard to K6Si309 (trisilicate of hexapotassium)
or tridymite, a crystalline form of silica.
Fig 1 shows extraction of silica from rice husks ash;
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Fig 2 shows diffractograms of the sodium and potassium xerogel silicas;
Fig 3 shows Isotherms of adsorption/desorption of nitrogen and
Fig 4 shows that Available silicon in soil (mg dm-]) as a function of the dosage (kg ha-').
The Specific surface area, mesopore volume and diameter and mean pariicle diameter of
silicas and micronized rice husks ash are shown in Table 1.
Propeni:~ RHA Sodium PO!1Somm Le,n t Zk.li:::;