Patent ID: 7491375

Claim:
A process for the production of a composition comprising a pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles, wherein the composition has a BET surface area of 200±25 m 2 /g and the aggregates display an average surface area of 7000 to 12000 nm 2 , an average equivalent circle diameter (ECD) of 80 to 100 nm and an average circumference of 850 to 1050 nm; and wherein in the process a mixture of silicon compounds is evaporated, separately or together, the vapours are transferred by means of a carrier gas to a mixing chamber, with SiCl 4 as the first component in a proportion of 60 to 95 wt. % relative to the mixture, and a second component chosen from the group comprising H 3 SiCl, H 2 SiCl 2 , HSiCl 3 , CH 3 SiCl 3 , (CH 3 ) 2 SiCl 2 , (CH 3 ) 3 SiCl, (n—C 3 H 7 )SiCl 3 , in a proportion of 5 to 40 wt. % , relative to the mixture, and a combustion gas and primary air, which can optionally be enriched with oxygen and/or be preheated, are transferred separately to the mixing chamber, the mixture of the vapour of silicon chlorides, combustion gas and primary air is ignited in a burner and the flame burns into a reaction chamber, secondary air, which surrounds the flame, is introduced into the reaction chamber, the ratio of secondary air to primary air being in a range from 0.05 to 3, the solid is then separated from gaseous substances and the solid is then steam-treated at 250° C. to 750° C., wherein the total amount of oxygen is at least sufficient for the complete combustion of the combustion gas and the silicon compounds and the amount of feed materials consisting of silicon compounds, combustion gas, primary air and secondary air is chosen such that an adiabatic flame temperature T ad of 1570 to 1630° C. is obtained, where T ad =the temperature of the feed materials+the sum of the reaction enthalpies of the partial reactions/heat capacity of the substances leaving the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen, and optionally the carrier gas if it is not air or nitrogen, taking the specific heat capacity of these substances at 1000° C. as a basis.