Method for producing silica sol having elongated particle shape

A method for producing a silica sol containing elongated colloidal silica particles dispersed in a liquid medium and having an SiO2 concentration of 6 to 30% by mass, wherein the elongated colloidal silica particles have a particle length of 50 to 1,000 nm as determined by observation with an electron microscope and exhibit a DL/DB ratio of 2.5 or more and a DL of 30 to 300 nm wherein DL is an average particle diameter (nm) as measured by the dynamic light scattering method, and DB is a primary particle diameter (nm) as measured by the nitrogen gas adsorption method, the production method comprising the following steps (a) and (b):

• (a) a step of preparing a raw material liquid by adding at least one compound as an anion source selected from the group consisting of inorganic acids, organic acids, and ammonium salts of these acids, and ammonia to a colloidal aqueous solution of activated silica having an SiO2 concentration of 1 to 6% by mass and a pH of 2 to 5 so that the mass ratio of the compound to SiO2 is 3.0 to 7.0%;
• and
• (b) a step of heating the raw material liquid prepared in the step (a) at 80 to 200° C. for 0.5 to 20 hours, to thereby produce a silica sol.