Patent ID: 8318971

Claim:
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 A1) water introduced with the starting materials or formed during the reaction of aniline with formaldehyde in the presence of hydrochloric acid as catalyst to form a mixture of diamines and polyamines of the diphenylmethane series (MDA) is at least partly removed from the reaction mixture; A2) after neutralization of the reaction mixture formed in A1), an aqueous solution comprising alkali metal chloride and an organic phase comprising said mixture of diamines and polyamines is obtained; A3) the organic phase from A2) is separated off from the aqueous phase and worked up to give said mixture of diamines and polyamines; and A4) the aqueous solution comprising alkali metal chloride remaining after A3) is optionally at least partly combined with any water removed in A1) and/or with aqueous phases obtained in the work-up of said organic phase in step A3); purified; and at least partly fed to the electrochemical oxidation C); and 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).