Process and apparatus for preparing finely-divided silicon dioxide having good thixotropic properties

A process for preparing finely divided silicon dioxide having high thickening capacity and good thixotropic properties which comprises reacting a silicon fluoride in the vapor phase with water vapor, combustible gas and free oxygen-containing gas to form silicon dioxide and hydrogen fluoride in a flame reaction zone including a burner and a flame whose base extends outwardly from the burner, the reaction zone surrounding the flame including the base of the flame, and at least the portion of the burner at the base of the flame while cooling the gaseous reaction mixture in that portion of the reaction zone surrounding the base of the flame and the portion of the burner at the base of the flame sufficiently to effect a silicon dioxide having a higher degree of agglomeration and a higher thickening number than silicon dioxide prepared as above absent said cooling, by contact with a cooling surface maintained at a temperature below 500.degree. C. but above the dew point of the reaction waste gases generated in the flame reaction.