Method for producing silica sol having elongated particle shape

A method for producing a silica sol containing elongated amorphous colloidal silica particles dispersed in a liquid medium and having an SiO2 concentration of 6% by mass to 30% by mass, wherein the silica particles contained in the silica sol exhibit a DL/DB ratio of 2.5 or more and a DL of 30 nm 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, and the silica particles have a particle length of 50 nm to 1,000 nm as determined by observation with an electron microscope, 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 silica sol as a raw material having an SiO2 concentration of 1% by mass to 30% by mass and a pH of 2 to 5 so that the mass ratio of the compound to SiO2 is 0.5% to 1.9%;
• and
• (b) a step of heating the raw material liquid prepared in the step (a) at 80° C. to 200° C. for 0.5 hours to 20 hours, to thereby produce a silica sol.