EXTRACTION OF SILICAL BURNT PUDDY HURSH ASH

ABSTRACT
In present work acid resistant calcium silicate has been synthesized from silica of rice husk and calcium oxide of analytical grade. The silica from rice husk was extracted at 550°C in amorphous form and then allowed to react with calcium oxide in the presence of excess water by Sol-Gel technique to obtain calcium silicate hydrate gels. The molar ratios of Si/Ca were adjusted each time to obtain silica rich calcium silicate hydrates. The gels were dried in oven and calcined in muffle furnace at various temperatures to obtain acid resistant calcium silicate. The products were tested by analytical technique and by FTIR and XRD machines. Studies show that at higher molar ratio of Si/Ca, the heat treatment improves the acid resistivity of calcium silicate whereas at lower molar ratios the heat treatment does not make it acid resistant.

1.0 Introduction
Critical economic and environmental situations of the current days encourage companies and researchers to develop and improve technologies intended to reduce or minimize industrial wastes. As a consequence, much effort has been expended in different areas, including the agricultural production.
Rice is the second largest produced cereal in the world. Its production is geographically concentrated in Asia with more than 90 percent of world output. The United States and Brazil are the most important non-Asian producers and Italy ranks first in Europe. The rice world production was approx 400 million tons of milled rice in 2003. In most varieties rice is composed by approximately 20 % of rice hull, which contains a fibrous materials and silica; however the amount of each component depends on the climate and geographic location of rice crop. Therefore, due to its high percentage in the grain composition, the hull is considered a by-product in the mills and creates disposal and pollution problems.

Burning rice hull as a fuel substitute in order to generate energy is a useful solution which is used by many industries; however it results in a new waste, named rice husk ash (RHA). This residual ash obtained from the combustion can contain over 60 % of silica and some amount of metallic impurities. Depending on the burning process, RHA can contain silica in the amorphous form; therefore, this residue can be considered as a new economically viable raw material to produce silica or to be used as silica resource.

Silica has been used in many applications, including production of nanomaterials. Tailored materials composed of nanoparticles have potential for application in numerous technological fields. The expression nanodispersed silica covers the entire variety of silica forms including sols, gels, suspensions, and pastes. Silica sol or colloidal silica refers to a stable dispersion of colloidal silica particles in water medium. It is used in many applications, such as in refractory materials, binder for inorganic paint, and stiffener for hard coating reagents, abrasive particles, adsorbents, and catalyst. Various raw materials can be used in the manufacturing of monodispersed sols but the two main ones are tetralkyl orthosilicates and sodium silicate solution.

This latter has the advantage over the first one because it is less expensive and uses water as the solvent. Sodium silicate solutions (commercially called sodium water-glass) are complex mixtures of silicate anions and polymer silicate particles especially when silica module (SiO2: Na2O molar ratio) is >2. The manufacture process of sodium silicates is generally considered expensive due to the energy required to reach high temperatures during the calcination stages, in addition to producing considerable air pollution by emission of dust, nitrogen and sulphur oxides. Although this calcination process is widely used in industrial scale, there is another process based on the reaction of silica with aqueous sodium hydroxide (NaOH) in autoclave. This latter one has an advantage when compared with the conventional calcinations process as it requires less energy.

1.1 SCOPE OF THE STUDY
The study will cover the qualitative and quantitative analysis that is involved in the extraction of silica from rice paddy (husk ash). This analysis will determine the:
 Micrographs of silica nanoparticles obtain from CTAB
 TGA Analysis of VN3 and IISC silica
 Chemical composition of RHA analysis by AAS
 Effect of NaoH concentration and temperature on the silica conversion
 Particle size and the PH of Silica Sol

1.2 OBJECTIVES OF THE STUDY
The present study has been designed to evaluate the extraction of silica from rice paddy (husk ash). The study has also explain the production of a sodium silicate solution with silica module of approximately 3 (M ≈3; whereM = SiO2/Na2Omolar ratio) using RHA as the silica resource and then, use it to obtained silica sol via Ion-Exchange Method. In order to develop the study, the following sequence was performed: (i) characterize the RHA; (ii) investigate the influences of NaOH molar concentration and temperature on the silica conversion to sodium silicate production and (iii) utilize it to produce a silica sol.