Abstract: A process for producing an aromatic dicarboxylic acid by simple operations using a simple apparatus is disclosed, which enables to effect separation and washing of the product crystals efficiently without suffering from clogging of the apparatus, while permitting recovery of the reaction solvent and catalyst with permission of an efficient replacement of the solvent.

Abstract: A method for recovering crystals from a crystal-containing slurry, which comprises supplying the slurry to a rotary filter having a rotating cylindrical filter medium, filtering the supplied slurry in a filtering region to retain the crystals on the filter medium and washing the filter cake formed on the rotating filter medium repeatedly in a plurality of washing regions by spraying a washing liquid onto the cake in each region by supplying any one of washing regions on the aft side in the rotating direction of the filter medium with the spent washing liquid of the washing region adjacent to said one washing region on the fore side in the rotating direction.

Abstract: It is contemplated by the present invention to produce a high quality aromatic carboxylic acid by a liquid phase oxidation of an alkylbenzene by contacting it with a molecular oxygen-containing gas in a solvent containing a lower aliphatic carboxylic acid, under exclusion of any acceleration of decomposition of the solvent molecules and danger of explosion accident.