Method for preparing particles comprising a core and a silica shell

A process for the preparation of particles presenting a dense silica shell comprising the steps of:

• 1) precipitating an active silica from an aqueous alkali metal M silicate solution, with an SiO.sub.2 /Na.sub.2 O ratio of at least 2 with adjustment of the pH using an acidifying agent, on a support made of a material other than silica in order to obtain a silica slurry;
• 2) separating the silica slurry in the form of a silica suspension;
• and
• 3) drying said silica suspension, wherein in step 1) the formation of silica slurry by precipitation comprises the following stages:
• a first stage comprising employing an initial vessel heel with a pH of about 8 to about 10 comprising water, at least one organic or inorganic support other than silica which is insoluble in water under the conditions of the slurry formation, an electrolyte salt of an alkali metal, the amount of the electrolyte present being at least about 0.4 mol, of alkali metal ion per liter of vessel heel, and optionally a buffer or basic agent, at a temperature of the order of about 80 to about 98.degree. C.;
• and
• a second stage comprising introducing, into said vessel heel, the alkali metal silicate in the form of an aqueous solution containing at least about 100 grams of SiO.sub.2 /liter, and the acidifying agent, 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.gtoreq.3'(A/200)2.sup.n,
• n being equal to (T-90)/10,
• A representing the specific surface, expressed in m.sup.2 /g, of the support to be coated, and
• T the temperature in .degree.C. of said reaction mixture, the reaction mixture exhibiting a constant pH of about 8 to 10 and being maintained at a temperature T of about 80 to 98.degree. C., until the desired amount of active silica has been formed.