Process for obtaining methionine

1. A method comprising crystallizing D,L-methionine from an aqueous solution and/or suspension comprising D,L-methionine and D,L-methionine ammonium salt, having a Met content of 70-180 g/kg of solution and/or suspension, an NH 4 + content of 1-5 g/kg of solution and/or suspension, in the presence of a crystallization additive, which comprises a nonionic or anionic surfactant,

wherein a temperature of the solution and/or suspension is lowered directly or stepwise from T 1 =85-110° C. to T 2 =30-50° C., so that D,L-methionine precipitates as a solid.

2. The method of claim 1 , wherein the crystallization additive comprises at least one anionic surfactant selected from the group consisting of a compound depicted in formulae 1 to 3:

R 1 —O—SO 3 M  (formula 1)

R 2 —O—(CH 2 ) n —SO 3 M  (formula 2)

R 3 —(OC 2 H 4 ) n —O—SO 3 M  (formula 3)

wherein n is an integer from 1 to 10, M is sodium or potassium and R 1 , R 2 and R 3 are a linear, branched or cyclic, saturated or unsaturated C 8 - to C 20 -alkyl group or an aryl group.

3. The method of claim 2 , wherein n is equal to 2 and R 1 , R 2 and R 3 are linear, saturated C 8 - to C 18 -alkyl groups.

4. The method of claim 2 , wherein the anionic surfactant comprises

C n H 2n+1 —O—SO 3 Na, where n=12 to 18,

C n H 2+1 —O—C 2 H 4 —SO 3 Na, where n=8 to 18, or

C n H 2n+1 —(OC 2 H 4 ) 2 —O—SO 3 Na, where n=12.

5. The method of claim 1 , wherein the crystallization additive comprises a nonionic surfactant that is a sorbitan fatty acid ester or a mixture of sorbitan fatty acid esters.

6. The method of claim 5 , wherein the sorbitan fatty acid ester has a formula (4)

wherein w+x+y+z=20.

7. The method of claim 1 , wherein a concentration of the crystallization additive in the solution and/or suspension from which the crystallization takes place is at least 700 ppm and not more than 4000 ppm, based on the total mass of the solution and/or suspension.

8. The method of claim 1 , wherein the solution from which the crystallization takes place further comprises a defoamer.

9. The method of claim 8 , wherein the defoamer comprises silicone oil.

10. The method of claim 8 , weight ratio of defoamer:crystallization additive, based on an active ingredient, is in a range of 4:1 to 1:1, and the concentration of the crystallization additive is at least 50 ppm and not more than 1200 ppm, based on the total mass of the solution and/or suspension.

11. The method of claim 1 , wherein the crystallization takes place by introducing an aqueous solution and/or suspension comprising D,L-methionine and D,L-methionine ammonium salt heated to 85 to 110° C. into an aqueous solution and/or suspension comprising D,L-methionine and D,L-methionine ammonium salt warmed to 30 to 50° C., wherein a temperature of the resulting mixture is constantly maintained between 30 and 50° C.

12. The method of claim 11 , wherein the crystallization takes place by introducing a solution comprising D,L-methionine and D,L-methionine ammonium salt heated to 85 to 110° C. into a suspension comprising D,L-methionine and D,L-methionine ammonium salt warmed to 30 to 50° C.

13. The method of claim 1 , wherein the crystallization is carried out in two stages, wherein, in a first crystallization stage, a solution and/or suspension comprising D,L-methionine and D,L-methionine ammonium salt heated to 85 to 110° C. is introduced into a suspension comprising D,L-methionine and D,L-methionine ammonium salt warmed to 60 to 80° C. and a temperature of the resulting mixture is constantly maintained between 60 and 80° C. and wherein the suspension comprising D,L-methionine and D,L-methionine ammonium salt warmed to 60 to 80° C. obtained in the first crystallization stage is introduced in a second crystallization stage into a suspension comprising D,L-methionine and D,L-methionine ammonium salt warmed to 30 to 50° C., wherein a temperature of the resulting mixture is constantly maintained between 30 and 50° C.

14. The method of claim 1 , wherein the crystallization is carried out by vacuum crystallization, wherein a pressure in the crystallization stage is 60 to 1000 mbar and, if a two-stage crystallization is carried out, a pressure in the second crystallization stage is 35 to 200 mbar.

15. The method of claim 1 , wherein the aqueous solution and/or suspension comprising D,L-methionine and D,L-methionine ammonium salts were prepared beforehand by dissolving and/or suspending D,L-methionine in water in the presence of an appropriate amount of ammonia.

16. The method of claim 15 , wherein D,L-methionine having a methionine content of at least 90% by weight is used for the dissolution.

17. The method of claim 16 , wherein D,L-methionine is present in a form of pure methionine and/or crude methionine from a manufacturing process, having a residual moisture of 0.1 to 9.5% by weight.

18. A method for preparing D,L-methionine, comprising:

forming an aqueous solution of D,L-methionine ammonium salt by hydrolysis of D,L-methionine nitrile and/or D,L-methionine amide,

partially removing ammonia from the D,L-methionine ammonium salt, to obtain an aqueous solution and/or suspension containing comprising D,L-methionine and D,L-methionine ammonium salt, and

producing methionine by crystallization by performing the method of claim 1 .

19. The method of claim 18 , wherein the aqueous solution of D,L-methionine ammonium salt is brought to an NH 4 + content of 1-5 g/kg of solution and/or suspension by evaporating and/or stripping off of ammonia prior to the crystallization.

20. The method of claim 18 , wherein the aqueous solution and/or suspension of D,L-methionine and D,L-methionine ammonium salt is brought to a Met content of 70-180 g/kg of solution and/or suspension by addition of water and/or D,L-methionine.

21. The method of claim 1 , wherein the precipitated D,L-methionine is separated from the mother liquor and dried or is initially recrystallized and is dried after separation from the mother liquor obtained.

22. The method of claim 21 , wherein the mother liquor is fed back into a crystallization stage.