Method for the purification of isocyanates

A process for purifying isocyanates, wherein

• a) a stream (1) containing isocyanate, higher- and lower-boiling components and unvaporizable residue is separated, in a distillation comprising at least one theoretical plate, into a part-stream (2) which contains the unvaporizable residue and isocyanate and into a vapor stream (3) containing isocyanate and low boilers, the weight ratio of the streams being (2):(3) 20:1-1:1,
• b) the unvaporizable residue in the part-stream (2) is kept separate from the vapor stream (3) and/or from streams which at least partly contain the vapor stream (3),
• c) at least one further isocyanate-containing vapor stream (4) and a residue stream (8) having less than 2.5% by weight of desired product, and
• d) the isocyanate-containing vapor stream or streams (4) and the vapor stream (3) from a) are separated into three individual streams (5, 6, 7) having different boiling ranges by distillation, the lowest-boiling stream (5) containing a substantial part of the low boiler content of the crude isocyanate stream (1), the highest-boiling stream (7) containing a substantial part of the high boiler content of the crude isocyanate stream (1) and the medium-boiling stream (6) substantially containing desired product.