Methods for treating waste liquid, methods for recovering and manufacturing boric acid, and methods for manufacturing polarizing plates.

Abstract

A method for treating waste liquid includes the following steps: a boric acid adsorption step, in which boric acid is selectively adsorbed onto an adsorbent from a waste liquid whose pH has been adjusted to alkaline; a boric acid removal step, in which an acidic solution is contacted with the adsorbent, thereby causing the boric acid to precipitate from the adsorbent into the acidic solution to obtain a precipitate; an NF membrane separation step, in which an alkali is added to the precipitate, and the precipitate, whose pH has been adjusted to be slightly acidic than neutral, is separated into a permeate and a concentrate by passing it through an NF membrane, wherein the permeate contains the boric acid but does not contain cations derived from anionic bases derived from acids, and the concentrate contains cations derived from anionic bases derived from acids; and a boric acid recovery step, in which boric acid is recovered from the permeate.

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