a novel process for the purification of crude aromatic carboxylic acids , such as , for example , terephthalic acid , orthophthalic acid , trimesic acid , isophthalic acid , 2 , 6 - naphthalenedicarboxylic acid and pyromellitic acid , which are obtained by oxidising the corresponding aromatic precursors is described ; the process is based on the subsequent oxidation in heterogeneous phase of the crude product in aqueous solution .