Preparation of Black Amorphous Silica from Rice Husks

The formation of black (carbon containing) amorphous silica from rice husks has been studied in detail. Three parameters, viz., (i) purity, (ii) temperature of calcination and (iii) rate of heating have been found to affect the formation of black silica. The formation of black silica is higher in untreated rice husks than that in acid treated rice husks. The tendency to form black silica increases with the increase in the heating rate and the temperature of calcination of untreated rice husks. By treating with 3N HCl acid the formation of black silica can be avoided. Using a dopant/catalyst, black amorphous silica stable in air up to 1000°C was prepared from acid treated rice husks.