PURPOSE: To provide a purification method capable of not only efficiently removing impurities in crude 2,6-naphthalene dicarboxylic acid (NDCA) but also obtaining pure NDCA uniform and large in the diameters of the crystal granules.
CONSTITUTION: A dialkylnaphthalene or its oxidation intermediate is oxidized with molecular oxygen in the presence of a cobalt-manganese catalyst in acetic acid solvent, and the produced crude NDCA dialkali salt is crystallized with a mineral acid or an acidic gas in the presence of water and a water-soluble organic solvent (e.g. an alcohol, a ketone or acetonitrile). The produced NDCA crystals are filtered out, sufficiently washed with water, and subsequently dried to recover the pure NDCA free from a coloring substance, a heavy metal, and 2-formyl-naphthoic acid which is a reaction intermediate. When the NDCA is treated with active carbon before the acidic crystallization, the NDCA crystals having a higher purity are obtained. On the acidic crystallization, the concentration of the NDCA dialkali salt is 0.2-15wt.% in the aqueous solution, and an organic solvent is used in an amount of 50-200g per 100g of the water.
KUJIRA KATSUFUMI
SUZUKI NAOKI
SHIRASAKI YOSHIKAZU
SAKATA TOMOYA
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