Abstract: A filter cake of crude isophthalic acid is dispersed as a slurry in acetic acid, and thereafter heated to a temperature bringing about the dissolving of the isophthalic acid. Such hot solution is subjected to very rapid evaporative crystallization in a single crystallization zone maintained at a pressure at which volatilization of the acetic acid occurs and at a temperature within the range of 180.degree.-250.degree. F., the heat input being controlled to promote such very rapid volatilization that more than half of the solvent quickly is volatilized. Moreover, more than half of the isophthalic acid content of the effluent is in the solids suspended in the slurry of such effluent. Because the volatilization is so rapid, only a relatively short residence time in the single crystallization zone is required for obtaining an effluent suitable for transfer to a separation zone. Isophthalic acid crystals are separated from the mother liquor in such separation zone.

Abstract: The severity of oxidation conditions in each of two stages is regulated so that the concentration of metatoluic acid in the liquid does not exceed 2 weight per cent in the first stage and does not exceed 0.4 weight per cent in the second stage. Total impurities and intermediate aromatic oxidation products are thus sufficiently low that the particles of isophthalic acid which grow in the slurry of reaction mixture in acetic acid do not contain an excessive amount of impurities. The flow rates of feed, interstage slurry, and product slurry as expressed in mols of aromatic components per hour are approximately equalized and regulated as a part of the monitoring to assure compliance with such oxidation severity standards, both stages being in the 100.degree.-130.degree.C. range.