METHODE DE PRODUCTION D'UREE EN GRAINS AUX PROPRIETES AMELIOREES

PROCESS FOR THE PRODUCTION OF UREA GRAINS WITH IMPROVED PROPERTIES

Abrégé anglais

ABSTRACT


Urea grains having a content of from 0.1 to 10% by weight of dicyandiamide,
based on the total amount of area and dicyandiamide concentrated urea
solutions or molten urea is converted into granulated urea with the di-
cyandiamide being added to the urea solutions or molten urea before conversion
of the urea into grains. The urea grains are free flowing and thus have
improved storage properties. Abrasion during transport and storage is
substantially reduced.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Urea grains having a content of from 0.1 to 3.5% by weight of
dicyandiamide, based on the total amount of urea and of dicyandiamide.
2. Process for the production of urea grains by the conversion of
concentrated urea solutions or molten urea into granulated urea, characterized
in that 0.1 to 3.5% by weight of dicyandiamide, based on the total amount of
urea and of dicyandiamide, is added to the urea solution or to the molten
urea, before the conversion of the urea into grains.
3. Process according to claim 2, characterized in that molten urea,
which contains 0.1 to 3.5% of dicyandiamide, is prilled.

Description

5a73

This invention relates to urea grains having improved
properties.

Urea is used either per se or mixed with other nit-
rogenous compounds, e.g. dian~onium phosphate or di-
cyandiamide or its salts(DE-PS 1 592 768,DE-OS 2 531
962) on a large scale for fertilizers.~owever the urea
has the disadvantage that the grains which are made
from it, e.g. prills t pOSsess poor storage properties
due to their relatively low hardness and to the
resultant abrasion during transport and loading or un-
loading, and above all the prills tend to cake together in a
humid atmosphere.

Numerous proposals have been made to improve the storage
properties of urea prills. Thus apart from additives, which
are applied to the urea grains,it is also recommended
that such additives be applied to the molten ureas or
to aqueous urea solutions. A known process is the im-
provement of the urea grains by the addition of aqueous
formaldehyc1e solution or of aqueous formaldehyde~urea
condensates such as di- and trimethylol urea(DE-OS
2 139 27S, DE-OS 2 ~25 039).But it has been shown that
the resulting improvement of the grain hardness is only
minor.In addition, processes based on formaldehyde are
unsatisfactory; for the small amounts of methanol which
are al~a,vs present in formaldehyde solutions are con-
centrated during the urea process, so that after a rel-
atively short time the operation comes to a standstill.
~1oreover the addition of aqueous solutions is generally
regarded as unfavorable, since to produce the urea
grains, water has to be removed completely.For the same
reason, the other proposed additives such as polyvinyl
alcohol, dissolved in water or aqueous glue, starch and
cellulose solutions should be regarded as disadvantageous.



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Surprisingly it has now been found that urea grains withimproved properties can be produced i~ 0.1 to3 5% by
weight of dicyandiamide i5 added to the urea smelt or to
a concentr~ted urea solution. Owing to the stated relat-
ively small amounts of dicyandiamide, already at the lower limit
o~ the cited ranye a big increase in the hardness of the
urea yrains extracted from such a smelt is attained.

This also substantially reduces abrasion during transport
and storage processes.

Whereas during the fracture of pure urea grains, break up
into numerous little fragments takes place, increasing the
hygroscopy o the ùrea and thereby the danger of caking,
with the urea grains obtained from the invention, which for
example fra~ment during transportati^on, fracture results in
only a few large fragments. The urea ~rains thus remain free
flowing even if stored for long periods. In yeneral, addit-
ions of 0.1 to 1.5~ by weight are adertuate to ensure a major
increase of the stability of the urea grains. ~dditions of
more than 3.5~ by weight o dicyandiamide do not in fact
lead to any ~urther substantial increase in hardness of the
urea grains, but are beneficial for many uses, e.g. for app-
lications in agriculture. Dicyandiamide also has the advant-
age that it is a substance bene~iting plants,and its nitro-
gen content has fertil~ing effects. Dicyandiamide can,with
advantage,be added directly to molten urea, and simultaneous-
ly other conventional additives, such as e.g. small am-
ounts o~ acids, can be supplied. But dicyandiamide can be
added, advantageously in aqueous solution, also to a
concentrated aqueous urea solution, though this has the
dra~lback that the water then has to be removed a~ain. The
dicyandiamide additive is specially suitable for improving
the abrasion resistance and breaking strength of urea
prills. The urea grains according to the inVentiQn
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can have the same sizes as were previously known for
urea grainsO An average diameter between 0.1 and 15 mm
is preferred, while special preference is given to be~
tween 0.5 and 6 mm. The invention will he more closely
described in the following examples:
Example 1

0.5~ by weight o dicyandiamide was continuousl~ addecl
by a dosing device to a urea smelt such as is convent-
ionally used in the prill device at the -top oE a prill-
tower. From the prills extracted from the smelt in the
usual manner, a screen fraction of from 1.6 - 1.8 mm
was screened out and was tested for breaking strength
(proportion of frac~ure on collision with a wall at a
s~eed of 13 m/sec.) and also for compression strength
~loading capac:ity to fracture in g).

Result:
~ by weight Worktng
Fracture Lo d in g
-
Urea without additive 4.4 620
20Urea with 0.5% by weight of
dicyandiamide 0.7 890

Example 2
In analogy with example 1l urea prills with an additive o~
3% by weight of dicyandiamide were prepared and tested.

Result: % by weight Working Load
Eracture in g _
Urea without additive 4.3 605
Urea with 3% by weight of
dicyandiamide 0.4 950

Example 3
An aqueous solution of dicyandiamide was continuously
added to a 75~ aqueous urea solution in such an amount
that the percentage share by weight o; dicyandiamide was
~, l% by weight.

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The urea solution was processed in the conventional manner,
- forming crystals. The latter showed the following proper-
ties:
~ by weight Working
Fracture Load in g

Urea without additive 4.0 580
Urea with 1 ~ by weight
o dicyandiamide 0.5 905


Example 4
To cristalline urea 0,2 % by weight o dicyandiamide
are added and thorou~hly mixed. The mixture so ob-tained
was prilled in the conventional manner by melting and
cooling of the melted droplets. The prills were tested for
strength as follows:
20 grs. prills were filled into a vertical drop tube and
loaded 10 times with a drop weight of 500 grs. and a drop
distance of 25 cm (10 inches). The prills are then sieved
over a 0,2 mm sieve. The percent mechanical abrasion under
dynamic stress is calculated by multiplication of the
weight of the C0,2 mm raction by the actor 5. The mea-
sured mechanical abrasion was 7,5 ~ corresponding to an
increase in strength of 8 ~ compared with pure urea.




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