PURIFICATION OF CRUDE 1,4-DIHYDROXY-2-NAPHTHOIC ACID OR ITS SALT

PURPOSE:To purify the titled crude product obtained by reacting 1,4-dihydroxynaphthalene with carbon dioxide gas under specific condition, in high yield and purity, by treating an aqueous solution of the alkali metal salt, etc. of the titled crude acid with a ketone or an ester. CONSTITUTION:Crude 1,4-dihydroxy-2-naphthoic acid (DHNA) or its salt prepared by reacting 1,4-dihydroxynaphthalene with carbon dioxide gas in the presence of an alkaline alkali metal compound under the pressure of carbon dioxide gas, is purified by treating (A) an aqueous solution of the alkali metal salt or ammonium salt of said crude DHNA, preferably an aqueous solution obtained by carrying out the above reaction in the presence of water, cooling the reaction mixture and separating the produced crystal from the solution, with (B) a ketone or an ester incompatible with water. The unreacted raw materials and by-products in the crude products can be selectively extracted by this process, and a high-purity DHNA can be obtained in high yield.