PURIFICATION OF ORGANIC ISOCYANATE

PURPOSE:In removing high-boiling impurities from crude organic isocyanate by the use of a thin film evaporator, to prevent conversion of material to be treated to tar and attachment of baked polymer to the wall of the evaporator, by feeding a high-boiling inert solvent and the isocyanate to it at the same time. CONSTITUTION:The crude organic isocyanate 1 and the high-boiling inert solvent 2 having a higher boiling point than that of the isocyanate (preferably >=50 deg.C boiling point difference at normal pressure), and no compatibility with it, are fed from the pipe 11 to a thin film evaporator simultaneously, the liquid dropping along the wall of column in a wet wall state is uniformly extended by the wiper 9 of the rotator 8, the liquid is made into a thin film state, and instantly evaporated. Purified low-boiling isocyanate vapor is condensed by condenser 5, collected in the receiver 6, and taken out. The high-boiling residue dropping on the wall of column is collected in the receiver 10, and separated into the high- boiling solvent and isocyanate containing high-boiling impurities by liquid separation. Paraffin oil, triisodecyl phosphite, etc. is used as the high-boiling solvent.