Method for preparing hollow silica particles

A process for the preparation of hollow particles comprising a dense silica shell, said process comprising the steps of

• 1) precipitating active silica from an aqueous alkali metal M silicate solution, with an SiO2/M2O ratio of at least 2, on a support made of a material other than silica to form a silica slurry;
• 2) separating the formed silica particles from the silica slurry and drying the silica particles;
• and
• 3) removing the support without dissolution or destruction of the active silica shell;
• wherein, in step 1), the formation of silica slurry is carried out according to the two following stages:
• a first stage comprising forming an initial vessel heel with a pH of about 8 to about 10 by mixing:
• water
• at least one organic or inorganic support other than silica, said support is insoluble in water during step 1) and is at least partially removable without dissolution or destruction of the silica shell,
• an electrolyte alkali metal in an amount of at least about 0.4 mole of alkali metal ion per liter of vessel heel, and
• optionally a buffer or basic agent, at a temperature of about 80 to 98° C.;
• and
• a second stage comprising adding, into said vessel heel,
• the alkali metal silicate in the form of an aqueous solution comprising at least about 100 grams of SiO2/liter, and
• an acidifying agent, in order to form a reaction mixture under conditions such that the kinetics K of formation of active silica, expressed in grams of silica/hour/gram of support, corresponds to a value
• K≧3(A/200)2n
• wherein n is equal to (T-90)/10
• A is the specific surface, expressed in m2/g, of the support to be coated, and
• T is the temperature of the reaction mixture in ° C.,
• the reaction mixture exhibiting a substantially constant pH of about 8 to about 10 and being maintained at a temperature of about 80 to about 98° C., and
• wherein the particles obtained after step 2) comprise an active silica shell and a core composed of said support.