Pyrogenically produced silicon dioxide powder

A process for the production of a silicon dioxide powder,

• wherein
• a mixture of silicon compounds is evaporated, separately or together, the vapours are transferred with a carrier gas to a mixing chamber, with
• SiCl4 as a first silicon compound in a proportion of 60 to 100 wt. % relative to the mixture, and
• a second silicon compound selected from the group consisting of H3SiCl, H2SiCl2, HSiCl3, CH3SiCl3, (CH3)2SiCl2, (CH3)3SiCl, (n-C3H7)SiCl3, and a mixture thereof, in a proportion of 0 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 comprising the vapour of the silicon compounds, the combustion gas and the 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 the secondary air to the primary air being in a range from 0.05 to 4,
• a resultant solid is then separated from gaseous substances and the solid is then steam-treated, thereby producing the silicon dioxide powder,
• 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 the silicon compounds, the combustion gas, the primary air and the secondary air is chosen such that an adiabatic flame temperature Tad of 1390 to 1450° C. is obtained, where
• Tad= the temperature of the feed materials + the sum of the reaction enthalpies of partial reactions/heat capacity of the substances leaving the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen, and optionally a carrier gas if it is not air or nitrogen,
• taking the specific heat capacity of these substances at 1000° C. as a basis,
• wherein the produced silicon dioxide powder is in the form of aggregates of primary particles having a BET surface area of 300±25 m2/g,
• wherein
• the aggregates display
• an average surface area of 4800 to 6000 nm2,
• an average equivalent circle diameter (ECD) of 60 to 80 nm and
• an average circumference of from 600 to 750 nm.