Patent GB-797431-A (GB)
Title: Process for the refining or beneficiation of the isomers of benzene hexachloride
Inventor: CHUFFART ROBERT CHARLES
IPC: C07C17/38
Filed: 1955-04-04
Abstract: Substantially pure gamma isomer or a gamma-enriched fraction of benzene hexachloride is separated from a mixture of alpha and of gamma isomers, optionally containing other isomers and/or impurities, formed during the production of benzene hexachloride, which comprises causing a solution of the mixture which is in a state of metastable supersaturation in respect of at least the gamma isomer to flow upwards through a crystallization vessel wherein it crystallizes substantially isothermally in the presence of a large surface of gamma crystals which are maintained distributed throughout the crystallizing solution in a fluidized state by the upward flow of the solution, the rate and manner of the flow being so adjusted that the crystallization vessel acts as a crystal classifier so that the fine crystals consisting mainly of alpha and beta isomers, but including some fine gamma crystals, are carried to the upper part of the vessel and withdrawn with the stream of spent solution, while relatively coarse crystals of purified gamma isomer collect in the lower portion of the vessel whence they are withdrawn.  The process is preferably carried out in a continuous manner whereby the stream of spent solution carrying the fine crystals withdrawn from the upper portion of the crystallization vessel is divided, crystals of the alpha isomer or the alpha and beta isomers being separated from one portion of the stream which is then reunited with the other portion, the stream being fortified by a controlled quantity of the original material, suitably added as a solution, and subsequently brought to the requisite state of metastable supersaturation by cooling, or concentrating and cooling, before being reintroduced into the lower portion of the crystallization vessel.  The fortification of the spent solution is suitably effected by passing through a heating zone and bringing the stream into contact with a further quantity of the original material, for example by passing the stream into the lower portion of a classifying extraction vessel operating on the fluidized bed principle and which is fed with crude chlorination product or primary concentrate.  The delta isomer, which tends to accumulate in the system, is removed by withdrawing from the stream from the upper portion of the crystallization vessel a controlled fraction of the spent solution, sufficient to remove the same amounts of delta isomer and other impurities as are introduced in a given period of time. The delta purge from the spent solution is suitably passed to a recovery unit which comprises a second crystal classification process in a fluid medium, the solvent being changed or debased to give a higher delta/gamma solubility ratio.  Suitably the recovery unit employs a higher degree of supersaturation than the main stage of the process so that crystal agglomerates containing alpha, beta and gamma isomers but no delta isomer form and are recovered.  Suitably the solvent used in the main process is methanol which is diluted with water in the delta purge recovery process.  The invention is described with reference to drawings illustrating suitable apparatus and flow sheets. Specifications 392,829, 418,349, 457,301, [all are in Group III], 573,693, [Group IV], 616,351, [Group III], 727,477 and 758,006 are referred to.