Process for preparing 2,6-naphthalene-dicarboxylic acid

A process for preparing 2,6-naphthalenedicarboxylic acid which comprises a) reacting 2,6-naphthalenedicarboxylic acid dipotassium salt with phthalic acid monopotassium salt in the presence of water at a temperature of 0.degree.-200.degree. C. to yield solid 2,6-naphthalenedicarboxylic acid monopotassium salt and an aqueous solution of a phthalic acid dipotassium salt;

• b) separating the solid 2,6-naphthalenedicarboxylic acid monopotassium salt from the aqueous solution containing phthalic acid dipotassium salt;
• c) reacting the 2,6-naphthalenedicarboxylic acid monopotassium salt with phthalic acid in the presence of water at 0.degree.-200.degree. C. to yield solid 2,6-naphthalenedicarboxylic acid and an aqueous solution of phthalic acid monopotassium salt;
• d) separating solid 2,6-naphthalenedicarboxylic acid from the aqueous solution containing phthalic acid monopotassium salt;
• and e) recycyling the aqueous solution containing phthalic acid monopotassium salt to step a).