Patent EP-0504467-A1 (EP)
Inventor: WATANABE YOSHITANE NISSAN CHEM
IPC: B01J13/00
Filed: 1991-03-22
Abstract: Disclosed is a method of preparing a stable aqueous silica sol, which comprises the following steps (a) to (g):
     (a) a step where an aqueous solution of a water-soluble alkali metal silicate as dissolved therein in a concentration of from 2 to 6 % by weight as the SiO₂ content to be derived from the silicate, the said silicate containing other polyvalent metal oxides than silica in a proportion to silica of from 500 to 10000 ppm, is passed through a column as filled with a hydrogen-type strong-acid cation-exchange resin in such an amount as being sufficient for ion-exchanging all the metal ions in the aqueous solution, at a temperature of from 0 to 60°C as the liquid temperature in the inside of the column and at a space velocity of from 1 to 10 per hour, and the aqueous solution of the active silicic acid which resulted from the passing and which has an SiO₂ concentration of from 2 to 6 % by weight and a pH value of from 2 to 4 is recovered;   (b) a step where a strong acid is added to the aqueous solution of the active silicic acid as recovered in the step (a) so that the said aqueous solution is adjusted to have a pH value of from 0 to 2.0 and then the resulting aqueous solution is kept at a temperature between 0°C and 100°C for a period of form 0.5 to 120 hours;   (c) a step where the aqueous solution as obtained in the step (b) is passed through a column as filled with a hydrogen-type strong-acid cation-exchange resin in such an amount as being sufficient for ion-exchanging all the metal ions in the aqueous solution, at a temperature of from 0 to 60°C as the liquid temperature in the inside of the column and at a space velocity of from 2 to 20 per hour, the aqueous solution which resulted from the passing is then passed through a column as filled with a hydroxyl-type strong-basic anion-exchange resin in such an amount as being sufficient for ion-exchanging all the anions in the solution, at a temperature of from 0 to 60°C as the liquid temperature in the inside of the column and at a space velocity of from 1 to 10 per hour, and thereafter the aqueous solution of the active silicic acid which resulted from the passing and which is substantially free from any other soluble substances than the active silicic acid and has an SiO₂ concentration of from 2 to 6 % by weight and a pH value of from 2 to 5 is recovered;   (d) a step where an aqueous solution of sodium or potassium hydroxide is added to the aqueous solution of the active silicic acid as recovered in the step (c), in a molar ratio of SiO₂/M₂O of being from 60 to 200, M₂O (where M means sodium atom or potassium atom) being one as derived from the hydroxide and SiO₂ being one as contained in the said aqueous solution of the active silicic acid, with keeping the liquid temperature at 0 to 60°C, thereby to form a stabilized active silicic acid-containing aqueous solution having an SiO₂ concentration of from 2 to 6 % by weight and a pH value of from 7 to 9;   (e) a step where the stabilized active silicic acid-containing aqueous solution as obtained in the step (d) is put in a container in an amount of 1 part by weight as a heel solution whereupon the liquid temperature in the inside of the container is kept at 70°C to 100°C, and from 5 to 20 parts by weight of the stabilized active silicic acid-containing aqueous solution as obtained in the step (d) is continuously fed into the said container as a feed solution for a period of from 50 to 200 hours while the said container is maintained to have a normal pressure or a reduced pressure to evaporate water in the liquid and the evaporated water is removed from the said container so that the content of the liquid in the inside of the container may be kept constant during the procedure, thereby to form a stable aqueous silica sol which has an SiO₂ concnetration of from 30 to 50 % by weight and in which the colloidal silica has a particle size of from 10 to 30 millimicrons;   (f) a step where the stable aqueous silica sol as obtained in the step (e) is passed through a column as filled with a hydrogen-type strong-acid cation-exchange resin in such an amount as being sufficient for ion-exchanging all the metal ions as contained in the sol, at a temperature of from 0 to 60°C and at a space velocity of from 2 to 20 per hour, then the solution which resulted from the passing is passed through a column as filled with a hydroxyl-type strong-basic anion-exchange resin in such an amount as being sufficient for ion-exchaning all the anions in the solution, at a temperature of from 0 to 60°C and at a space velocity of from 1 to 10 per hour, and then the acidic aqueous silica sol which resulted from the passing and which is substantially free from any other polyvalent metal oxides than silica is recovered; and   (g) a step where ammonia is added to the acidic aqueous sol as recovered in the step (f), with keeping the temperatures of the sol at 0 to 100°C to make the pH value of the sol to be form 8 to 10.5, thereby to form a stable aqueous silica sol which has an SiO₂ concentration of from 30 to 50 % by weight, which is substantially free from any other polyvalent metal oxides than silica and in which the colloidal silica has a particle size of from 10 to 30 millimicrons. The aqueous silica sol prepared has a high SiO₂ concentration of 30 to 50 wt.% and is highly pure to be free from any other polyvalent metal oxides than silica. In the sol, the colloidal silica particles have a particle size of 10 to 30 millimicrons.