Note: Descriptions are shown in the official language in which they were submitted.
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1333548
It is known that modern fertilizer technology req-
uires granulated nutrient-carriers. In contrast to other sin-
gle and multiple nutrient fertilizers, granulating processes
for potassium-sulphate became known only relatively recently.
It has been found that growing primary particles together to
form an adequate grain structure is difficult to achieve.
As a possible solution to the problem of producing
granular potassium-sulphate, German Patent 28 10 640 des-
cribes a compacting process in which the material to be gran-
ulated is compacted in a special way between the rolls of a
high-pressure roller-press to form so-called shells. The
shells are crushed and the satisfactory grain is screened
out. The product is obtained in the desired grain-size and,
with after-treatment, it is of good quality.
The compacting process has been found very satis-
factory for producing granulated potassium-sulphate. In spite
of this, it would be desirable to provide for a roller-granu-
lating process for this fertilizer, since the compacting pro-
cess delivers angular grains which do not mix very well with
the rounded grains of other fertizilers. Furthermore there is
an increase in abrasion in the mixture with repeated tran-
shipment.
International mixing of individual granulated fert-
ilizers in dealer warehouses is becoming increasingly popular
in Europe, since mixing close to the point of use makes it
possible to adjust the nutrient ratio to the respective de-
mand. This procedure, however, means that all fertilizers
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13335Q8
which are to be mixed must be largely of the same grain-size
and also, as far as possible, of the same (granule) density,
in order to facilitate mixing and to reduce the danger of
segregation during transhipment and spreading.
Nearly spherical grains are also preferred because
bulk-material of this nature flows better into and out of
storage than chip-like materials.
Various substances are suggested as aids to granul-
ation. German OS 26 03 917 makes use of calcium compounds
- which can be combined with potassium-sulphate to form syngen-
itic double-salts.
German OS 27 48 220 discloses the use of mono- and
dibasic ammonium-, sodium- or potassium-phosphate in the
granulation of potassium-chloride, potassium-nitrate, potas-
sium-sulphate, potassium-carbonate, and langbeinite.
German Patent 20 21 963 attempts to solve the prob-
lem of rollergranulating potassium-sulphate by using starch
as an aid to granulation. In this connection, it is found
that starch alone acts far too weakly. In order to moisten
the granulating mixture, therefore, it is necessary to use a
salt-solution containing magnesium-sulphate and possibly also
potassium-sulphate. During drying of the green granules,
double salts are supposed to form and these, upon crystal-
lizing, cement the primary potassium-sulphate particles.
It has now been found, surprisingly enough, that
the action of starch as an aid to granulation may be sharply
improved if, instead of a salt-solution, practically salt-
~ 1333~8
free drinking water is used to moisten the potassium-sulph-
ate, some of which has been subjected to additional dry-
grinding. Highly abrasion-resistant granules were obtained by
using 2 to 3 kg of paste starch to 1 t of satisfactory grain.
By crushing a large part of the potassium-sulphate particles
to less than 50 ~m, this additional grinding increases the
particle-surface per unit of mass and activates the stressed
particles.
Recrystallizing is presumably accelerated by the
immediately preceding mechanical action. It may also be imag-
ined that activated potassium-sulphate dissolves very quickly
in the water used for moistening and the starch-paste, col-
loid-dissolved starch being partly expelled from the solution
and being fixed upon the surfaces of the potassium-sulphate
particles. This film may possibly promote the growth of
potassium-sulphate which has passed into solution upon the
surfaces of undissolved potassium-sulphate particles, whereas
the starch which has remained in solution suppresses nuclea-
tion in the film of solution between the particles.
The method according to the invention is character-
ized in that approximately 20 to approximately 40% of the
potassium-sulphate, in relation to the amount of product (satisfactory
gra~) d~charged, is subjected, prior to the production of
the moist granulating mixture, to dry activation-grlnding in
a mill which grinds more than half of the material to less
than 50 pm. The amount activated is combined, in the mixer,
with the main amount of material to be granulated. The
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mixture is moistened with water to which l to 5 kg of starch
per t of satisfactory grain have been added in the form of a
paste and the mixture is rolled. The green granules are
dried, at a temperature of between 105 and 120C, to a resid-
ual moisture of 0.1 to 0.5% and the product (satisfactory grain) is
finally screened out of the dried granular material and dis-
charged, whereas the rejected granules (unsatisfactory grain), after being
crushed, is returned to the mixer.
The method may be further characterized in that
either some of the potassium-sulphate which is fed to the
process freshly as the satisfactory grain is discharged, or
some of the unsatisfactory potassium-sulphate grain screened
out of the granulated product, is subjected, after crushing,
to dry activation-grinding.
The method according to the invention may also be
characterized in that a water-soluble salt of a lignin-sulph-
onic acid is added, in an amount of between 0.1 and l kg per
t of discharged satisfactory material, to the potassium-sul-
phate mixture to be granulated, together with the pasted
starch.
As in the case of any granulating process, the
method according to the invention delivers over-size and
under-size dried granulated material which must be screened
out and returned, whereas the satisfactory grain is dis-
charged as the product. In the stationary case, the unsatis-
factory grain forms a part-circuit with a partly circular
flow. Fresh potassium-sulphate is added to the process as the
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1333~48
satisfactory grain is discharged.
It has been found, in principle, immaterial whether
some of the fresh potassium-sulphate or an equal amount of
potassium-sulphate, recirculating as unsatisfactory grain, is
subjected to activation-grinding. In order to regain its
granulatability, the returned material must be moistened by
rolling with the fresh material. In order to compensate for
the disadvantage of drying and remoistening in the returned-
material part-circuit, it is desirable to use a filter-wet
crystalline material as fresh potassium-sulphate. This is
possible if the dry activation-grinding is applied to the un-
satisfactory grain. The result is scarcely any different from
that obtained by using dry and partly activated, fresh
potassium-sulphate.
Although the fact that lignin-sulphonate saves
water is known from the preparation of concrete in construct-
ion work and other fields, the addition of lignin-sulphonate
to the granulating mixture cannot be generally recommended,
since these salts often have a structure-loosening effect.
In granulating potassium-sulphate, not only was the
expected saving in water achieved, but also, surprisingly
enough, a noticeable increase in resistance of the granules
to abrasion. The moisture needed for rolling build-up was re-
duced, by the addition of O.S kg of sodiumlignin-sulphonate
per t of satisfactory grain, by about 20% as compared with no
addition.
If it is considered that even such a fine potas-
1333548
sium-sulphate as is obtained with the industrial potassium-
sulphate process, with a main grain-size of between 90 and
250 ~m (58%), cannot be compacted to green-strength granules
by rolling, and that addition of starch ln the form of a
paste to the granulating material moistened with water
provides little improvement, then the progress achleved with
the method according to the invention becomes clear.
It is not the individual measures, but the combina-
tion of activation-grinding and the addition of starch that
produces structural compacting during rolling and a mechanic-
ally strong bond between the primary particles within the
granules upon drying. In contrast to this, green granules
from untreated potassium-sulphate have such a loose structure
that most of them cannot withstand drying. Although the ad-
dition of 1% of starch in the form of a paste does improve
the structure in green granules, they lose most of their res-
istance to abrasion upon drying. Testing reveals an abrasion
coefficient of 32%.
If 30% of the potassium-sulphate used is subjected
to activation-grinding, if the activated material is added to
the untreated sulphate, and this is followed by moistening!
rolling and drying, this produces granules which, when test-
ed, show 18% abrasion.
Although activation-grinding alone improves the
strength of the granules almost as much as the proposal con-
tained in German Patent 20 21 963 (without after-treatment),
total success is achieved only by the addition of 2 to 3 kg
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of pasted starch per t of product. Under test, granules thus
produced exhibit only 4 to 8% abrasion. It is also worthy of
note that, when thrown into water, these granules do not dis-
integrate; instead they dissolve like large individual cryst-
als. This indicates that the primary particles which build up
the granules grow together particularly firmly. Such cohesion
between primary particles can be obtained in granules pro-
duced by other method only by complex after-treatment of the
material, for example by spraying with water, solutions,
vapours or the like and renewed drying.
In contrast to the method according to German
Patent 21 21 963, the method according to the invention also
makes it possible to save energy by using filter-wet potas-
sium-sulphate, the optimal moisture-content of the material
being adjusted by matching the additional amount of water to
the ratio between the dry returned material and the moist
fresh material.
The following examples are intended to explain the
method according to the invention, without restricting its
principle to the numerical data provided.
Example 1
In this case, the substance to be granulated is a
non-crystalline potassium-sulphate. 3% by mass of the sulph-
ate has a grain-size in excess of 500 ~m, 70% by mass is be-
tween 200 and 500 ~m, 19% by mass is between 100 and 200 ~m,
and 3~ by mass has a grain-size of less than 100 ~m.
40 parts by weight of this potassium-sulphate are
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ground in a ball-mill for activation until 2/3 thereof has
assumed a grain-size of less than 45 ~m. The activated potas-
sium-sulphate is co~bined, in the mixer, with 60 parts by
weight of potassium-sulphate of the original grain-size and
is mixed with 0.3 parts by weight of starch which has been
converted into a paste by boiling in 4 parts by weight of
water. During the mixing process, drinking water is added
until preliminary granulation is visible. This requires about
6 parts by weight of water.
The granules are formed and compacted by rolling on
a granulating pan. They are then dried at 100C to a residual
moisture of 0.2%. The test-material screened out had a burst-
ing strength of 24 N/grain after 1 day and of 35 N/grain
after 30 days. Abrasion measured 7% after 1 day and 6% after
30 days.
Example 2
Based upon the same substance and the same dry
preparation as in Example 1, the starch-paste is mixed with
the moistening water before being placed in the mixer. The
rolled green granular material is dried at 105C to a resid-
ual moisture of 0.4%. Bursting strength after 1 day rose to
26 N/grain; abrasion was 6%.
Example 3
The basic substance in this example is a dry com-
mercial potassium-sulphate containing the following grain-
sizes:
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> 500 ~um 2%
200 - 500 ~m 25~
90 - 200 ~m 41%
~ 90~um 32%
500 parts by weight of returned unsatisfactory
grain are crushed to a grain-size of less than 500 ~m. 150
parts by weight of the commercial potassium-sulphate are sub-
jected to activation-grinding in a rod-mill. 73% of the acti-
vated potassium-sulphate had a grain-size of less than 45 ~m.
The returned material, and the activated potassium-sulphate,
are passed, together with 350 parts by weight of untreated
potassium-sulphate, to a ploughbar-mixer. The dry material is
mixed with 105 parts by weight of water in which 1.5 parts by
weight of paste starch are dissolved colloidally, followed by
thorough mixing. The moist mixture is passed to a rotating
drum, 5 more parts by weight of water being sprayed onto the
inlet-end thereof. As the material rolls through the drum,
almost spherical green granules, with smooth surfaces, are
produced. The material emerging from the rolling drum is
dried to a residuaL moisture of 0.2%.
After the satisfactory material to be discharged
has been separated, the unsatisfactory material is returned
to the start of the process. The product shows a bursting
strength of 31 N/grain and 6% abrasion.
Example 4
The procedure in this case is the same as in
Example 3, as far as the moistening process.
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1333548
In this case, 0,3 parts by weight of sodiumlignin-
sulphonate are dissolved in the moistening water, in addition
to the 1.5 parts by weight of starch. Using the otherwise
similar procedure of the preceding example, this recipe pro-
duces granules having a bursting strength of 34 N/grain after
1 day and of 41 N/grain after 30 days, together with 6
abrasion after 1 day and 3% after 30 days.
Example 5
Commercial potassium-sulphate, of the grain-size
given in Example 3, has, in the undried condition, a moist-
ure-content of 11%, i.e. 100 parts by weight of the dry mat-
erial contains, in the filter-wet condition, 12.5 parts of
water. The total addition of potassium-sulphate may be ef-
fected in the moist condition if the unsatisfactory : satis-
factory-grain ratio is not less than 2 : 3.
The flow of returned material in the process
amounts to 400 parts by weight per hour. A flow of 180 parts
by weight per hour is branched off and is passed through the
activation-mill. After the air-separating ball-mill, the two
dry partial flows are reunited and are passed to a continuous
mixer together with 600 parts by weight of moist potassium-
sulphate. Also fed into this mixer continuously is a solution
of 1.8 parts by weight of starch and 0~.12 parts by weight of
megnesiumlignin-sulphonate in 21.6 parts by weight of water.
Upon entering the rolling drum, the material thus
produced has a water-content of 96 parts by weight to 1000
parts by weight of dry material, corresponding to a maisture
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content of 8.9%. Five more parts by weight of water per hour
are sprayed into the rolling drum. The water-content of the
green granules is about 9.2% and they are dried at 100C to a
residual moisture of 0.2%. After screening , 600 parts by
weight of satisfactory grain are discharged as the product.
The 400 parts by weight of unsatisfactory grain, after being
crushed to less than 250 pm, are returned to the start of the
process. With this method of operation, 0.165 t of H2O/t of
product are to be vapourized as compared with a saving of
0.124 t of vapourized H2O/t of dry material in the moist
potassium-sulphate used. Thus the granulating process need
only deliver 0.041 t of vapourized H2O/t of product more than
potassium-sulphate drying without granulation. Tests showed a
bursting strength after 1 day of 7% and, after 30 days, 4%.
Under test, granules thrown into water do not dis-
integrate. They dissolve within about two hours, like cryst-
als of the same size.
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