Recovery of Amorphous Silica in Pure Form from Rice Husk

Rice husk has been used for preparation of chemically pure and amorphous silica. When husk is heated in air, the resulting silica ash varies in colour from grey to black and contains inorganic impurities along with unburnt carbon. The ash, if further heated, resulted in formation of crystalline forms of silica. However, by treating the husk with nitric acid followed by heating in air, pure white amorphous silica was obtained. A possible explanation for the ready removal of carbonaceous matter through this route has been advanced.