Separation process

PCT No. PCT/GB94/00248 Sec. 371 Date Jul. 21, 1995 Sec. 102(e) Date Jul. 21, 1995 PCT Filed Feb. 9, 1994 PCT Pub. No. WO94/19082 PCT Pub. Date Sep. 1, 1994A process for the purification of terephthalic acid is disclosed. A slurry containing crystals in an impurity-containing mother liquor under elevated pressure and temperature conditions is processed to separate the crystals from the mother liquor. The processing comprises highly efficient separation and washing of the crystals under the elevated temperature and pressure conditions in a unit (32) to produce a mass of crystals having a low level of contamination. The washed crystals are reslurried in aqueous medium (vessel 40) and transferred to a secondary separation unit (34) operating under lower pressure and temperature conditions, the transfer being effected by flashing down (in vessel 44) to the lower pressure. The filtrate (41) from unit (34), and optionally wash liquor used in unit (34), is used to establish a recirculating flow whereby the aqueous medium used for reslurrying the crystals prior to pressure let-down is repeatedly re-used.