Process for preparing methylenediphenyl diisocyanates

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

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).

2. The process of claim 1 , wherein said alkali metal hydroxide in A) is sodium hydroxide.

3. The process of claim 1 , wherein at least part of the alkali metal hydroxide formed in C) is recirculated to A) for neutralization of said hydrochloric acid.

4. The process of claim 1 , wherein at least part of the hydrogen chloride formed in B) is converted into hydrochloric acid, at least part of which is used as said catalyst in A).

5. The process of claim 1 , wherein the concentration of alkali metal chloride in the aqueous solution comprising alkali metal chloride formed in A) is from 2 to 24% by weight.

6. The process of claim 1 , wherein the concentration of alkali metal chloride in the aqueous solution comprising alkali metal chloride formed in A) is from 5 to 15% by weight.

7. The process of claim 1 , wherein said aqueous solution comprising alkali metal chloride is adjusted to an alkali metal chloride content of >5% by weight by removal of water and/or by addition of alkali metal chlorides in solid form or in the form of a concentrated solution before being used in an electrochemical oxidation.

8. The process of claim 1 , wherein the aniline/formaldehyde reaction in A) is carried out at a degree of protonation of <30%.

9. The process of claim 1 , wherein the electrochemical oxidation in C) is carried out as an alkali metal chloride electrolysis using ion exchange membranes and/or using gas diffusion electrodes as cathode.

10. The process of claim 9 , wherein the alkali metal chloride electrolysis is carried out by the membrane electrolysis process using an ion exchange membrane, with the alkali metal chloride of the solution containing alkali metal chloride being sodium chloride and the ion exchange membrane having a water transport capacity of >3 mol of water/mol of sodium.

11. The process of claim 9 , wherein the alkali metal chloride solutions fed to the alkali metal chloride electrolysis are adjusted to a pH of <7 prior to electrolysis.

12. The process of claim 9 , wherein the concentration of alkali metal chloride in the solution comprising alkali metal chloride leaving the electrolysis cell is from 100 to 280 g/l.

13. The process of claim 9 , wherein the concentration of the alkali metal hydroxide solution obtained from the electrolysis is from 13 to 33% by weight.