Method of preparing high-purity aqueous silica sol

Claim 1

A method of preparing 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 mean particle size of from 10 to 30 millimicrons, the method being characterized by comprising the following steps (a), (b), (c), (d), (e), (f), (g) and (h): (a) a step where an aqueous solution of a water-soluble alkali metal silicate as dissolved therein in a concentration of from 1 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 300 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 1 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 from 0.1 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 1 to 6 % by weight and a pH value of from 2 to 5 is recovered; (d) a step where the aqueous solution of the active silicic acid recovered in the step (c) is fed over a period of time of 1 to 20 hours into an aqueous solution of an alkali metal silicate having an SiO₂ concentration of from 0.1 to 8 % by weight and a molar ratio of 1 to 4.5 as SiO₂/M₂O , wherein M represents an alkali metal atom, or into an aqueous solution of an alkali metal hydroxide having a concentration of from 0.1 to 5 % by weight of said hydroxide until a molar ratio of 30 to 300 as SiO₂/M₂O of the resulted liquid by the feeding is obtained while said resulted liquid by the feeding is maintained at a temperature of from 60 to 150° C under stirring throughout the feeding, said aqueous solution of an alkali metal silicate being formed by mixing an aqueous solution of an alkali metal hydroxide with an aqueous solution of the active silicic acid recovered in the same step as step (c) or by diluting or concentrating the resulted mixture; (e) a step where the resulted liquid in the step (d) is heated under stirring at a temperature of from 60 to 150° C for a period of time of from 0.1 to 10 hours, thereby to form a stable aqueous sol of colloidal silica having a mean particle size of from 10 to 30 millimicrons; (f) a step where the water in the formed sol in the step (e) is removed through a micro porous membrane from said sol until a stable aqueous sol of colloidal silica having an SiO₂ concentration of 30 to 50 % by weight is formed; (g) a step where the stable aqueous silica sol as obtained in the step (f) 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-exchanging 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 (h) a step where ammonia is added to the acidic aqueous sol as recovered in the step (g), with keeping the temperature of the sol at 0 to 100° C to make the pH value of the sol to be from 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 mean a particle size of from 10 to 30 millimicrons.

Claim 4

The method of preparing an aqueous silica sol as claimed in any one of claimes 1 to 3, in which the alkali metal hydroxide in the step (d) in sodium or potassium hydroxide.

Claim 5

The method of preparing an aqueous silica sol as claimed in any one of claimes 1 to 4, in which the alkali metal hydroxide solution mixed with the aqueous solution of the active silicic acid in the step (d) is an aqueos sodium or potassium hydroxide solution having a concentration of the hydroxide of from 2 to 20 % by weight.

Claim 6

The method of preparing an aqueous silica sol as claimed in any one of claimes 1 to 5, in which the micro porous membrane in the step (f) is a ultrafilter membrane having a pore size of from 1 to 7 millimicrons.

Claim 7

The method of preparing an aqueous silica sol as claimed in any one of claimes 1 to 6, in which the temperature of the sol during the removal of water from the sol in the step (f) is maintained at 20 to 80 ° C.

Claim 8

The method of preparing an aqueous silica sol as claimed in any one of claimes 1 to 7, in which the ammonia in the step (h) is an aqueous ammonia having ammonia concentration of from 5 to 30 % by weight.