Process for the preparation of urea granules

1. Process for the preparation of urea granules in a fluid-bed granulator comprising the steps of:

(a) feeding a film of a urea melt from at least one feeding device to a fluidized bed of solid urea nuclei, and

(b) bringing the solid urea nuclei into contact with the film of the urea melt so as to cause the nuclei to grow by solidification of the urea melt thereon to form the urea granules, wherein

the urea melt and the urea granules contain amounts of biuret and water which fulfill the following relation

b

m

·

b

g

w

m

·

(

w

m

-

w

g

)

=

0.1

-

20

wherein

b m =the % by weight of biuret in the urea melt

b g =the % by weight of biuret in the urea granules

w m =the % by weight of water in the urea melt

w g =the % by weight of water in the urea granules.

2. Process according to claim 1 , wherein the value of the quotient in the relation is 0.1-10.

3. Process according to claim 1 , wherein value of the quotient in the relation is 0.2-5.

4. Process according to claim 1 , wherein the urea melt is obtained in and/or downstream of an evaporator.

5. Process according to claim 1 , wherein the urea melt is obtained by using two evaporators arranged in series, with part of the urea melt leaving the first evaporator being fed to the second evaporator and part of the urea melt leaving the first evaporator being combined with the urea melt leaving the second evaporator.

6. Process according to claim 1 , which comprises adding water to the urea melt between the evaporator and the feeding device to obtain the urea melt.

7. Process according to claim 1 , which comprises adding a biuret-rich stream between the evaporator and the feeding device to obtain the urea melt.

8. Process according to claim 1 , wherein a vessel is present between the evaporator and the feeding device in which the urea melt is obtained.

9. Process according to claim 1 , wherein the temperature of the urea melt is raised between the evaporator and the feeding device to obtain the urea melt.

10. Process according to claim 1 , wherein step (a) is practiced by feeding the urea melt to the fluidized bed in the form of a virtually closed conical film.

11. Process according to claim 1 , wherein the at least one feeding device comprises a central conduit for feeding the urea melt to the fluid-bed granulator and a conduit concentric with the central conduit through which a gas stream is delivered.

12. Process according to claim 1 , wherein the urea melt has a velocity when exiting a central channel of the at least one feeding device of between 10 and 25 m/s.

13. Process according to claim 1 , wherein the fluidized bed is fluidized by a gas stream, and wherein the urea melt is fed to the fluid-bed in a higher place than the gas stream.

14. Process according to claim 1 , wherein the fluidized bed is fluidized by a gas stream, and wherein the gas stream has a velocity of 50-400 m/s and a feed pressure of 0.11-0.74 MPa.

15. Process according to claim 1 , wherein the fluidized bed is fluidized by a gas stream, and wherein a weight ratio of the gas stream to the urea melt is 0.2-0.6.

16. Process according to claim 1 , wherein the fluid-bed granulator is fluidized by a gas stream which causes the nuclei to penetrate the film and to thereby be moistened with the urea melt.

17. Process according to claim 16 , wherein step (a) includes forming a substantially closed conical film of the urea melt from the at least one feeding device.

18. Process according to claim 17 , wherein step (a) includes imparting rotation to the urea melt to obtain the substantially closed conical film thereof.