Application of sol–gel technique for preparation of nanosilica from coal powered thermal power plant fly ash

Fly ash, a thermal power plant by-product is increasing and it is expected that this by-product will increase with increase in energy demand, especially in developing countries. Considering this, current study presents a novel process for the utilization of thermal power plant waste i.e., (fly ash) by fractionating it into silica particles and aluminum enriched residual ash. Fractionation of fly ash helped to extract industrial grade pure form of nanosilica. As a first step fly ash was added into 2N Sodium hydroxide solution, and resulting suspension was autoclaved for 120 min duration at 100 °C. After filtration, supernatant was collected and 1N HCl was added drop-wise, allowing precipitation of silica in the solution. A white cloudy gel-like precipitate of silica was formed which was filtered and washed several times to remove the impurities of Na, K, and chloride salts. This procedure yielded the nanosilica a final product, which was characterized for crystalline index and composition by using X-ray diffraction and fluorescence technique. The result of X-ray diffraction shows a broad hump in the region of 20°–22° indicating amorphous nature of the product was studied using field emission scanning electron microscopic and transmission electron microscopic analytical technique was used to study about internal structure, morphological features and size of the molecules which depicts spherical globular silica particles in 10–40 nm range. Further, chemical bonds were studied by infrared analysis (fourier transform infrared) and results indicated the presence of silica oxygen bonding (–Si–O–Si–) at 785 cm⁻¹ and Si–OH bonds 3000–3500 cm⁻¹. Energy dispersive X-ray analysis shows the presence of silicon and oxygen elements, which forms 99.9% of prepared nanosilica particles by weight.