Burning rice husk as fuel to generate energy resulted in a waste product, namely rice husk ash.Rice husk ash is rich in silica (92-97%) and can be an economically viable raw material for production of silica gel and powders. Silica gel is the amorphous (noncrystalline) form of SiO2. It has a large surface area (~500 m2/g), which allows silica gel for showing great water adsorption ability via chemisorptions
on Si-OH groups. Silica gel from rice husk ash can be roughly prepared by 2 ways, that is to say the thermal treatment with temperatures ranging from 500-1400oC. This method requires high temperature.(1, 2) The second way of preparation is leaching by acid or basic solutions Kalapathy, et al. (2000) and(2002) and then neutralization by acid to produce silica gel. The latter consumes low energy, and is cost-effective compared to the current melting method. Besides this advantage, the process may decrease CO2 emission due to the current manufacture of sodium silicate from the reaction of
Na2CO3 and SiO2. Reaction efficiency of the reaction between NaOH and SiO2 is estimated by 2 ways.One way is by ratio of the weight of silica gel obtained from the reaction and the weight of silica content in rice husk ash. Another way is to calculate the SiO2/Na2O ratio of the system. The reaction between NaOH and SiO2 of rice husk ash can be expressed as the following reaction.