Process for preparing amorphous silicon dioxide exhibiting hydrophobic properties

Zp sob spo v v tom, e se roztok vodn ho skla s obsahem 3 a 10 % hmotn. oxidu k°emi it ho sr sm s kyseliny chlorovod kov a roztoku chloridu v penat ho tak, aby pH sr ec sm si bylo 2 a 8 a hmotnostn pom r chloridu v penat ho k oxidu k°emi it mu byl 1,2 a 2,5. Vysr en² produkt se uprav organickou slou eninou k°em ku v mno stv 0,5 a 5 % hmotn., vzta eno na obsah oxidu k°emi it ho.\ The proposed preparation process of amorphous silicon dioxide exhibiting hydrophobic properties is characterized in that aqueous solution of water glass containing 3 to 10 percent by weight of silica is subjected to precipitation with a mixture of hydrochloric acid and calcium chloride solution to achieve pH value of the precipitation mixture within 2 to 8 and the weight ratio of calcium chloride to silica is 1.2 to 2.5. The precipitated product is then treated with an organic silicon compound amounting to 0.5 to 5 percent by weight, based on the silicon dioxide amount. The method is based on the fact that an aqueous solution of sodium silicate with a volume of 3 to 10 % by weight of silicon dioxide is precipitated with a mixture of hydrochloric acid and solution of calcium chloride so that the pH of the precipitation mixture is 2 to 8 and the mass ratio of calcium chloride to silicon dioxide is 1.2 to 2.5. The precipitation product is modified with an organic compound of silicon at 0.5 to 5 % by weight, relative to the content of silica.