Method for producing purified active silicic acid solution and silica sol

A method for producing a purified active silicic acid solution, the method comprising:

• preparing an active silicic acid solution by subjecting an alkali silicate aqueous solution having a silica concentration of 0.5% by mass to 10.0% by mass to cation-exchange to remove alkaline components wherein the active silicic acid solution that is prepared contains an amount of plate-like fine particles having a length of one side of 0.2 to 4.0 μm and a thickness of 1 to 100 nm;
• and
• filtering the active silicic acid solution through a filter selected from the group consisting of a membrane filter, a pleated filter, a depth filter, a yarn-wound filter, a surface filter, a roll filter, a pleated depth filter, a diatomaceous-earth-containing filter, and combinations thereof, the filter having:
• a removal rate of particles having a primary particle size of 1.0 μm of 50% or more, and
• a filtering rate in a range of from 13 L/min to 400 L/min per 1 m2 of a filtration area of the filter;
• wherein:
• an amount of plate-like fine particles having a length of one side of 0.2 to 4.0 μm and a thickness of 1 to 100 nm in the purified active silicic acid solution is in a range of from 0% to 30% as measured by a measuring method A;
• the measuring method A comprises:
• observing under a scanning electron microscope at a magnification of 5,000 times a membrane filter through which 30 mL of a solution to be observed has been passed, the solution to be observed having a silica concentration of 4% by mass and a temperature of 25° C., and the membrane filter having a filtration area of 4.90 cm2 and an absolute pore size of 0.4 μm;
• measuring as one count a state in which one or more of the plate-like fine particles exist within one visual field area having a length of 15 μm and a width of 20 μm;
• and
• determining the presence or absence of the count with respect to each visual field area of 100 non-overlapping visual field areas to measure a total number of counts as the amount (%) of the plate-like fine particles;
• and
• the solution to be observed in the measuring method A is the purified active silicic acid solution.