EXTRACTION OF SILICA FROM RICE HUSK ASH

Background of Study
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 approximately 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.