Method for extracting silicon dioxide from flyash

The invention discloses a method for extracting silicon dioxide from flyash, which comprises the following steps of: performing alkaline hydrolysis on the silicon dioxide in the flyash by using sodium hydroxide solution to prepare sodium silicate solution with the modulus between 0.4 and 0.8; directly heating to evaporate the sodium silicate solution so as to concentrate the sodium silicate solution until the solid content is 30 to 56 percent; and discharging into a cooling crystallization tank, reducing the temperature to 55-65 DEG C, adding a proper amount of seed crystals, continuously cooling to the temperature of between 30 and 35 DEG C, and performing filtering separation to obtain sodium metasilicate crystals with the modulus of 1 and NaOH solution. The NaOH solution is subjected to alkali make-up adjustment and then returns to the procedure of alkaline hydrolysis of the flyash for recycling, and the sodium metasilicate crystals are used for producing sodium metasilicate pentahydrate or anhydrous sodiu