Removal of free carbon and crystal structure change of amorphous silica fume by calcination

Abstract

The calcination process was used to remove free carbon and to change the structure of amorphous microsilica powder. The results show that the carbon content in amorphous silica powder strongly depends on the calcination temperature. When the calcination temperature is higher than 900[Formula: see text]C, the free carbon content, SiO2 content and weight loss of microsilica powder tend to be stable, with values of 0.05%, 83.8% and 4.8%, respectively. The crystalline structure, particle size and morphology start changing at a calcination temperature of 850[Formula: see text]C. When the calcination temperature is higher than 900[Formula: see text]C, the amorphous microsilica powder is fully crystallized. The morphology of the silica fume kept its original spherical shape under 850[Formula: see text]C, while above 850[Formula: see text]C the microsilica powder recrystallizes. These results provide a basis for chemical removal of metal impurities in the preparation of high-purity silica powder.