Preparation of silica from rice husks

Preliminary leaching of rice husks with a solution of hydrochloric acid before their combustion at 600 degrees C is shown to be required to obtain relatively pure silica (similar to 99,5%) with a high specific surface area (similar to 260 m(2)/g) that is maintained even after heating at 800 degrees C, Transmission electron microscopy observations indicate that this material has a homogeneous size distribution of nanometric particles. However, if the leaching with HCl is performed on the white ashes obtained by combustion of the rice husks at 600 degrees C, an amorphous silica with the same purity also is obtained, but its specific surface area decreases to 1 m(2)/g. This behavior is due to a strong interaction between the silica and the potassium contained in the rice husks, which leads to a dramatic decrease of the specific surface area if KC cations are not removed prior to the heat treatment at 600 degrees C. This finding leads to a better understanding of the effect of potassium on the morphology of silica.