Patent ID: 8318971
Filing Date: 2012-11-27
Classification: C07C,C25B

Abstract:
1. A process for preparing methylenediphenyl diisocyanate, comprising A) reacting aniline with formaldehyde in the presence of hydrochloric acid as catalyst to form a mixture of diamines and polyamines of the diphenylmethane series (MDA) and subsequently at least partially neutralizing said hydrochloric acid with alkali metal hydroxide; wherein wherein said aqueous solution comprising alkali metal chloride is adjusted to a pH of <8 prior to purification in A4); B) reacting said mixture of diamines and polyamines of the diphenylmethane series with phosgene to form a mixture of diisocyanates and polyisocyanates of the diphenylmethane series (MDI) and hydrogen chloride; wherein C) the hydrochloric acid neutralized in A) is separated off in the form of a solution comprising alkali metal chloride and at least a portion of which is subsequently fed to an electrochemical oxidation to form chlorine, alkali metal hydroxide, and optionally hydrogen; and D) at least part of the chlorine formed in C) is used to prepare the phosgene used in B), wherein said aqueous solution comprising alkali metal chloride is adjusted to a TOC value of <200 ppm before the oxidation C).