STUDY ON REACTION KINETICS BETWEEN SILICA GLASSES AND HYDROFLUORIC ACID

The kinetic mechanism of the reaction between three kinds of silica glasses and HF acid solution was investigated systematically. The results indicate that the reaction rate is controlled by the chemical reaction, and depends on the breaking of the siloxane bonds in ≡Si-O-Si≡ network structure. The main factors influencing reaction rate include temperature, composition and concentration of reactive species in HF acid, and also the structure of silica glasses. Due to the existence of a large number of defects and metal ions in the structure of type I silica glass, its reaction rate is the fastest in three kinds of silica glasses. In addition, because of the high content of hydroxyl groups in type III silica glass, its reaction rate is higher than that of type II silica glass. During the reaction, the nucleophilic attack of (HF)_2 or HF~-_2 on silicon atom results in the breaking of siloxane bond, while H~+ ions adsorbed on the surface of silica glass can catalyze the reaction. At high HF contents (above (7 mol/L)) the reaction rate increases rapidly because more reactive polymeric (HF)_n F~- ions are present in HF acid.