Fabrication and properties of amorphous silica particles by fluorination of zircon using ammonium bifluoride

Abstract

A method to prepare amorphous silica particles from zircon has been developed using the fluorination method through a three-step process: the fluorination of zircon using ammonium bifluoride at 230 ℃, the sublimation of (NH4)2SiF6 at 300 ℃ and the hydrolysis of (NH4)2SiF6. During above progress, the composition and phases of materials produced during fluorination of zircon were analyzed; the chemical reaction of silica with ammonium bifluoride, the sublimation behavior of (NH4)2SiF6 and its composition were determined; the effect of hydrolysis temperature (30 ℃, 45 ℃ and 60 ℃) on the hydrolysis ratio of Si and the size of silica particles were investigated. The results indicated that (NH4)3ZrF7 and (NH4)2SiF6 formed after the fluorination of zircon at 230 ℃ for 5 h. (NH4)2SiF6 started to sublimate at 175 ℃, but the sublimation dominated at 280 ℃ and ended at 300 ℃. The hydrolysis results indicated that uni-modal silica particles in micrometer size range were produced at 45 ℃, while bi-modal particles in a variety of sizes ranging from nanometer to micrometer were produced at 30 ℃ and 60 ℃. The amorphous silica particles obtained at 45 ℃ exhibited high purity above 99.96 wt.%, narrow size distribution (average: 254 ± 17 nm), high specific surface (136.8 m2/g), high dibutyl phthalate (DBP)-absorption (2.7 cm3/g), low density (0.532 g/cm2) and pH (5.7). Therefore, the amorphous silica obtained has potential application as filler in plastic and rubber compounding. After the hydrolysis process, ammonium bifluoride can be recycled through the evaporation of NH4F solution. Thus, this work provides a novel strategy for processing zircon by ammonium bifluoride to produce amorphous silica particles and recycle of ammonium bifluoride.