Effect of Experimental Variables on the Extraction of Silica from the Rice Husk Ash

Silica was extracted from the locally obtained rice husk ash (RHA) by heating its dispersion in sodium hydroxide solution for various periods of time. Following this treatment, silica content of the dispersed RHA particles leached out into the liquid phase in the form of sodium silicate. On separation from the solid, the alkaline solution of sodium silicate was made acidified with sulfuric acid, which transformed the dissolved silicate into precipitated gelatinous silica. The yield of silica was dependent on the strength of the alkaline solution, aging time, etc. Optimum conditions were established under which maximum extraction could be made from the rice husk ash. The as-extracted silica maintained its gelatinous nature after washing with water; however, on washing with ethanol, it transformed into powder of discrete particles. On calcination at 750 degrees C, the water washed silica converted into powder form, comprised of ellipsoids, whereas the ethanol washed silica particles sintered together. Selected samples of the extracted materials were then subjected to characterization by various physical methods, such as SEM, EDX, XRD, and FT-IR.