Method for preparing elongated-shaped silica sol

A method of preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 8.9 to 30% by weight, and having elongated-shaped amorphous colloidal silica particles each having at least 5 as a ratio of D.sub.1 /D.sub.2 where D.sub.1 (nm) is a particle size of 40 to 300 nano meters measured by a dynamic light-scattering method and D.sub.2 (nm) is a particle size measured by a nitrogen gas-adsorbing method, having an elongated shape elongated in only one plane and having a uniform thickness along the elongation within the range of from 5 to 20 nano meters, as observed with an electron microscope, comprising the following steps (a), (b) and (c):

• (a) mixing an aqueous solution containing a water-soluble calcium salt or magnesium salt or a mixture of said calcium salt and magnesium salt with an aqueous colloidal liquid of a silicic acid containing from 1 to 6% by weight of SiO.sub.2 and having a pH in the range of from 2 to 5 in an amount of 1500 to 8500 ppm as a weight ratio of CaO or MgO or a mixture of CaO and MgO to SiO.sub.2 in the silicic acid;
• (b) mixing an alkali metal hydroxide or a water-soluble organic base or a water-soluble silicate of said alkali metal hydroxide or said water-soluble organic base with the aqueous solution obtained in the step (a) in a molar ratio of SiO.sub.2 /M.sub.2 O of from 20 to 200, where SiO.sub.2 represents the total silica content derived from the silicic acid and the silica content of the silicate and M represents the alkali metal atom or organic base molecule;
• and
• (c) heating at least a part of the mixture obtained in the step (b) to 60.degree. C. or higher to obtain a heel solution, and preparing a feed solution by using another part of the mixture obtained in the step (b) or a mixture prepared separately in accordance with the step (b) to prepare a feed solution, and adding said feed solution to said heel solution while vaporizing water from the mixture during the adding step until the concentration of SiO.sub.2 is concentrated to from 8.9 to 30% by weight.