Process for Producing Elongated-Shaped Silica Sol

A method for producing an elongated-shaped silica sol comprising the following (a), (b), (c), (d), and (e);

• wherein a particle diameter (DB2 nm) of colloidal silica particles obtained through (e) determined by a nitrogen adsorption method is 5 to 20 nm, a ratio of particle diameters (DL2/DB2) of the particle diameter (DB2 nm) and a particle diameter (DL2 nm) of the colloidal silica particles determined by dynamic light scattering is 4 to 20, and a particle diameter (DB1 nm) of colloidal silica particles obtained through (c) determined by the nitrogen adsorption method and a particle diameter (DL1 nm) of colloidal silica particles obtained through (c) determined by the dynamic light scattering as well as the particle diameter (DB2 nm) of colloidal silica particles obtained through (e) determined by the nitrogen adsorption method, and the particle diameter (DL2 nm) of colloidal silica particles obtained through (e) determined by the dynamic light scattering satisfy the relationship represented by the following formula (I):
• (DL2/DB2)/(DL1/DB1)≧1.2 (I);
• (a) adding an aqueous solution containing a water soluble calcium salt, magnesium salt, or a mixture of the water soluble calcium salt and magnesium salt to an aqueous colloidal solution of active silicic acid with an SiO2 concentration of 1 to 6% by mass and a pH of 2 to 5 in a mass ratio of CaO, MgO, or both of CaO and MgO to SiO2 in the active silicic acid of 1500 to 15,000 ppm, and mixing;
• (b) adding alkali metal hydroxide, a water soluble organic base, or water soluble silicate of the alkali metal hydroxide or the water soluble organic base to an aqueous solution obtained through (a) in a molar ratio converted by a formula represented by SiO2/M2O (where SiO2 represents a total content of a silica content derived from the active silicic acid and a silica content in the water soluble silicate, and M represents the alkali metal atom or the organic base molecule) of 20 to 200, and mixing;
• (c) heating a mixture obtained through (b) at 85 to 200° C. for 0.5 to 20 hours so as to obtain a colloidal solution;
• (d) removing, from the colloidal solution obtained through (c), part of water and at least part of anions derived from the aqueous solution containing the water soluble calcium salt, magnesium salt, or the mixture of the water soluble calcium salt and magnesium salt;
• and
• (e) heating a colloidal solution obtained through (d) at a temperature of 80 to 195° C. that is lower than a heating temperature in (c) for 0.5 to 20 hours.