TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: Aqueous solution of (A) and concentrated sulfuric acid (H2SO4) are fed into the first reactor and silicon dioxide (SiO2) is fed into the second reactor each continuously or intermittently, and (I) formed is continuously or intermittently taken out of the first reactor. The concentrations of H2SO4 in the first and second reactors are kept at at least70 mass%. The reaction temperatures in the first and second reactors are at least60degreesC. The method further involves: contacting (I) taken out of the first reactor, with concentrated H2SO4 at at most50degreesC so that hydrogen fluoride contained in (I) is absorbed and removed; removing (I) from the first reactor and countercurrently contacting it with concentrated H2SO4 that is supplied through a channel to the first reactor; and purifying (I) with molecular sieving carbon, which is pretreated by baking in an inert gas atmosphere, so as to remove the impurities. Preferred Components: The particle size of SiO2 is at most30 mum. The removed impurities contain at least one hydrogen fluoride, hydrogen chloride, sulfur dioxide, hydrogen sulfide or carbon dioxide. The molecular sieving carbon has a smaller pore size than the molecular size of (I). EXAMPLE - A sulfuric acid solution (500 ml) was heated to 80degreesC, and aqueous hexafluorosilicic acid (20%, 4 g/minute) and sulfuric acid (98%, 16 g/minute) were added and fed into a first reactor. After work-up, tetrafluorosilane (SiF4) and hydrogen fluoride (HF) were obtained. Sulfuric acid containing HF (17.5 g/min) was reacted with silicon dioxide (3.5 g/min, 4 mass%) in a second reactor at 80degreesC to form SiF4 containing hexafluorodisiloxane, this mixture was fed into the first reactor and reacted with HF to form SiF4. After washing with sulfuric acid, the SiF4 (184 Nml/minute) was obtained, it contained impurities (vol.%): hydrogen fluoride (0.01), hydrogen chloride (1.8), carbon dioxide (0.11), carbon monoxide (0.04) and hexafluorodisiloxane (less than detection limit). The gas was introduced into an adsorption cylinder filled with an adsorbent (100 ml). The adsorbent was baked at 300degreesC with nitrogen at 400 ml/minute. The baking was continued until the outlet dew point reached at most-70degreesC, and further pretreated with SiF4. After work-up, high-purity SiF4 was obtained, which contained impurities in an amount undetectable through FT-IR i.e. less than detection limit.