A green route for producing high-purity nano-SiO2 from silicon containing waste†

A novel, environmentally friendly synthesis method has been developed to convert silica-rich solid wastes into high-purity SiO₂. Silica fume, a by-product of industrial silicon and ferrosilicon alloy production, represents a hazardous waste with limited recycling options. In this study, sodium silicate synthesized via alkali dissolution was used as the starting material. High-purity silica powder was obtained by employing calcium oxide for impurity removal followed by a carbonation process. A single-factor experiment revealed that, under conditions of 60 °C, a reaction time of 3 h, and 6 g L⁻¹ calcium oxide, the removal rates of aluminum and iron were 50.2% and 91.4%, respectively. Optimization using response surface methodology enhanced aluminum removal to 53.1%, while iron removal remained at 91.5%. In the carbonation experiment, conducted at 75 °C, with a pH of 9.0, a concentration of 60 g L⁻¹, a CO₂ flow rate of 40 mL min⁻¹, and a stirring speed of 300 rpm, the silica recovery rate reached 92.4%, and the agglomerate particle size was 11.38 μm. Characterization techniques including XRD, FTIR, SEM, and XRF confirmed that the product was 99.79% pure amorphous silica. After further treatment with chelating agent purification and mixed acid leaching, the purity of the synthesized silica can reach 99.9914% (4N). This process offers significant environmental and economic benefits by recycling silicon-containing waste and reducing CO₂ emissions.