Process for the purification of high-melting organic products

Abstract

The purification of high-melting organic products, such an anthraquinone and substituted anthraquinones, which is substantially free from lower-boiling impurities, by melting the product to be purified by heating, while mechanically conveying the same along a path with a screw conveyer, as for example, in a melting screw; degassing the melt at a reduced pressure in a column, as for example, at a pressure of 50 to 400 mm Hg; passing the degassed melt through a thin film evaporator at a reduced pressure of, for example, 5 to 250 mm Hg to evaporate the high-melting organic product and discharging the remaining high-boiling impurities and such impurities which are not capable of being distilled from the evaporator through a conically tapering sump, while mechanically scraping residue from the sump wall, preferably with the use of a rotating helical screw in the sump; the discharge from the sump being effected into and through an enclosed path at a rate sufficient to prevent liquid accumulating in the sump, while maintaining the central portion of the enclosed discharge path at a lower temperature than its end portion to form a crystal sludge in the central portion, which acts as a pressure seal; and solidifying and recovering the product discharging from the enclosed path. The enclosed path is preferably established through a screw-type pump which feeds into a crystallizing screw where the solidification takes place, the crystallizing screw being preferably maintained above the screw-type pump and is maintained filled with an amount of sump product which approximately corresponds to the amount of product in the sump take-off screw. The vapors produced in the thin film evaporator are passed via a heated rectifying column into a condenser, and a portion of the condensate is passed to the column as a reflux and another portion taken off and solidified in a crystallizing screw.