A method for producing a silica sol containing a small amount of metal impurities and wherein colloidal silica having an elongated particle shape is dispersed in a solvent, by addition of a compound as an anion source and ammonia as an alkali source and heating of the resultant mixture at a predetermined temperature, includes the following steps: (a) preparing a raw material liquid by adding at least one compound as an anion source selected from the group of inorganic acids, organic acids, and ammonium salts of these acids, and ammonia to a silica sol as a raw material having SiO.sub.2 of 1% by mass to 30% by mass and a pH of 2 to 5 so the mass ratio of the compound to SiO.sub.2 is 0.5% to 1.9%; and (b) heating the raw material liquid prepared in step (a) at 80° C. to 200° C. for 0.5 hours to 20 hours.

A preparation method for a silica aerogel, comprising the following steps: A) raw material containing a solid silicon source and an alkaline solution is used to produce an aerogel precursor after mixing; and B) the aerogel precursor is dried to obtain a silica aerogel. An improved silica aerogel preparation method, comprising the following steps: A) a cation exchange resin and a silicate solution are used as raw materials and mixed; B) the mixed material is allowed to stand to obtain an aerogel precursor; and C) the aerogel precursor is dried to obtain a silica aerogel.

A colloidal silica solution which includes two or more colloidal silica compositions or suspensions having differing particle sizes and specific surface areas, the compositions or suspensions resulting in a multimodal particle size distribution in which the solution or suspension can be bimodal in nature and composed of, but not limited to, particles with a mode of 4 nm and 20 nm or composed of particles 7 nm and 12 nm. The solution or suspension can also be trimodal and composed of, but not limited to, 4 nm, 7 nm and 15 nm or 3 nm, 5 nm, and 20 nm. The solution or suspension can also include other multimodal systems which would give superior water drainage and fiber and ash retention on paper machines. The colloidal silica solution is a drainage and retention aid in the making of paper.

A process for producing a charge-reversed aqueous silica sol includes: (a) providing a slurry of an acidic cation-exchange solid in an aqueous liquid; (b) providing a starting aqueous silica sol with an alkaline pH and including a monovalent cation(s); (c) contacting the slurry with the starting sol; (d) separating the acidic cation-exchange solid from the mixture (c) to leave a decationised aqueous silica sol with an acidic pH and a reduced monovalent cation(s) content compared to the starting sol; and (e) contacting the decationised sol with a compound(s) including a modifying element(s) that can adopt a +3 or +4 oxidation state to produce a charge-reversed aqueous silica sol whose silica particles include the modifying element(s) on their surface. The S-value of the starting sol is from about 10 to about 50%, and the surface area of colloidal silica particles in the starting sol is at least about 500 m.sup.2 g.sup.1.

A method for producing a silica sol containing a small amount of metal impurities and in which colloidal silica having an elongated particle shape dispersed in a solvent, includes: preparing a raw material liquid by adding a compound as an anion source selected from inorganic acids, organic acids, and their ammonium salts, and ammonia to a colloidal aqueous solution of activated silica having an SiO.sub.2 concentration of 1 to 6% by mass and a pH of 2 to 5 so that the mass ratio of the compound to SiO.sub.2 is 3.0 to 7.0%; and heating the raw material liquid at 80 to 200 C. for 0.5 to 20 hours. The elongated colloidal silica particles exhibit a D.sub.L/D.sub.B ratio of 2.5 or more. D.sub.L is an average particle diameter as measured by the dynamic light scattering method. D.sub.B is a primary particle diameter as measured by the nitrogen gas adsorption method.

A method for producing a silica sol containing a small amount of metal impurities and wherein colloidal silica having an elongated particle shape is dispersed in a solvent, by addition of a compound as an anion source and ammonia as an alkali source and heating of the resultant mixture at a predetermined temperature, includes the following steps: (a) preparing a raw material liquid by adding at least one compound as an anion source selected from the group of inorganic acids, organic acids, and ammonium salts of these acids, and ammonia to a silica sol as a raw material having SiO.sub.2 of 1% by mass to 30% by mass and a pH of 2 to 5 so the mass ratio of the compound to SiO.sub.2 is 0.5% to 1.9%; and (b) heating the raw material liquid prepared in step (a) at 80 C. to 200 C. for 0.5 hours to 20 hours.

There is provided a method for obtaining a purified aqueous solution of silicic acid containing less metal impurities such as Cu and Ni using water glass as a raw material with less number of purification steps than that in conventional methods without using any unnecessary additives. The method for producing a purified aqueous solution of silicic acid, the method comprising the steps of: (a) passing an aqueous solution of alkaline silicate having a silica concentration of 0.5% by mass or more and 10% by mass or less through a column filled with a polyamine-, iminodiacetic acid-, or aminophosphoric acid-type chelating resin, and (b) passing the aqueous solution passed in the step (a) through a column filled with a hydrogen-type cation exchange resin.

The invention relates to a method for producing aerogels, in particular silica aerogels, by way of a sol-gel process.

An insulating composition containing silica particles, a resin, and a curing agent, wherein: when an aqueous solution of the silica particles having an SiO.sub.2 concentration of 3.8% by mass is heated at 121 C. for 20 hours, the amount of Na ions eluted from the silica particles is 40 ppm/SiO.sub.2 or less, especially, wherein the amount of Na ions eluted from the silica particles after the heating may be 5-38 ppm/SiO.sub.2, and, the silica particles may contain a polyvalent metal oxide so the ratio by mole of a polyvalent metal M to Si is 0.001-0.02; the mass ratio of Na.sub.2O/SiO.sub.2 in the silica particles may be 700-1,300 ppm; and the silica particles may have, on the surfaces thereof, a layer having a thickness of 0.1-1.5 nm and an Na.sub.2O/SiO.sub.2 mass ratio of 10-400 ppm, and may have an average particle diameter of 5-40 nm.

The invention relates to a method for producing aerogels, in particular silica aerogels, by way of a sol-gel process.