Process for the production of polyisocyanates

A process for preparing polyisocyanates by reaction of at least one polyamine with urea and at least one secondary amine to form the corresponding polyurea and subsequent dissociation of the resulting polyureas into the corresponding polyisocyanates, which comprises a multistage process for continuously preparing organic isocyanates by reaction of the corresponding organic polyamines with urea and at least one secondary amine to form the corresponding polyureas in at least one mixing apparatus with downstream reactor and the dissociation of said polyureas, said process

• a) mixing together at least one organic polyamine with urea and with at least one secondary amine,
• b) reacting the mixture obtained from a) in at least one following residence reactor or two or more residence reactors,
• c) removing a resultant ammonia at the same time as the reaction course in stage b) or subsequently,
• d) removing an excess secondary amine, and other by-components which boil at temperatures lower than said polyureas, from the discharge from b),
• f) in a continuous dissociation apparatus, dissociating said polyureas into a corresponding isocyanate and secondary amine, to give at least one isocyanate-containing stream and at least one secondary amine-containing stream,
• g) purifying the isocyanate-containing stream obtained from the dissociation f) by
• g1) removing solvent, if solvent was used beforehand,
• g2a) in the case of distillable isocyanates, subjecting them to distillative purification, or
• g2b) in the case of undistillable isocyanates, subjecting them to optional purification by nondistillative means,
• and
• i) purifying the secondary amine-containing stream obtained from the dissociation f) and optionally recycling it.