Note: Descriptions are shown in the official language in which they were submitted.
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Abrasion resistant, tropicalised potash fertiliser granulate, method of its
production and device for carrying out the method
Description
This invention relates to an abrasion resistant, tropicalised potash
fertiliser granulate, a
method of its production and a device for carrying out the method.
Brief description of the state of the art
Potash fertiliser granulates are used throughout the world and are valuable
substances
which are employed as nutrients in agriculture. Problems which occur again and
again
with this type of fertiliser granulate are on one hand the low mechanical
strength of the
granulates which leads to the unwanted formation of fine-grained dusts and the
low
resistance to moisture or the high moisture absorption of normal fertiliser
granulates,
which leads both to increases in weight and to impairment of the product
integrity, which
can cause problems particularly with the use of fertiliser granulates.
To overcome these problems, various methods are known in the state of the art.
Potash
fertiliser granulates are normally pressed by compacting fine-grained salts on
roller
presses to form so-called scabs with a thickness of about 5 to 12 mm. Then the
flat
scabs are further processed in a crushing and classifying process. The coarse-
grained
crushing product in the range of grain size from about 1 to 5 mm represents
the
required finished product (granulate) and the undersized particles are fed
back into the
granulating process (compaction followed by crushing) (Ulimanns Enzyklopadie
der
technischen Chemie (Ullmann's Encyclopaedia of technical chemistry), 4th
edition, Vol.
13, page 471).
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This sort of granulate consists of edged to tapered grains with a surface
which is
comparatively rough due to the crushing process. With loading and unloading
and
transport stages it is inevitable with this type of granulate that there will
be abrasion due
to mechanical stressing, leading to the formation of unwanted undersized
particles (fine
dusts). On further processing or with the application of the granulates, these
materials
lead to the release of dust, which is undesirable with regard to industrial
safety and
product efficiency.
Furthermore, potash fertilisers can be basically classified as hygroscopic.
They absorb
moisture from the air, which however at temperate latitudes, affects the
strength and
storage capabilities of the granulates in manageable orders of magnitude. But
in zones
with tropical climates with relatively high air humidity, a significantly
higher water
absorption of the granulates occurs, leading to lowering of the grain
strength, increase
of the fine-grained proportion and to hardening of the product during storage.
With regard to the problems described above, there are rework methods for
potash
fertiliser granulates. For example, for the reduction of the formation of fine
grains due to
mechanical abrasion and the breakdown and hardening of the granules due to
moisture
absorption, there is the suggestion in DE-OS-1 36 956 for minimising the
abrasion
involving a combined application of the process steps of dust extraction in a
fluidised
bed and wetting the granulate surface with water. With this combination of
process
steps the objective is an improvement in the abrasion properties of the
granulates and
to facilitate the production of non-dusty potash fertilisers. The quoted dust
extraction in
the fluidised bed occurs at air speeds of 1 to 4 m/s. The water added to the
granulate
heated to 400 to 80 C amounts to 0.5 to 2% weight referred to the total mass,
preferably
1% weight. Drying of the granulate then takes place with a residual moisture
content of
0.1 to 0.2% weight by exploiting the thermal content of the granulate or by
introducing
preheated air currents. The granulates with this residual moisture content are
cooled to
room temperature and then processed further, for example, packaged. Also,
addition of
CA 02448820 2003-11-10
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organic substances after cooling is possible, preferably mineral oil in
amounts of 0.2 to
1 g/kg of granulate, whereby evaporation of the residual water is delayed and
additional
binding of dust is ensured during application later.
DE-OS-210 650 explains as a supplement to the above quoted German disclosure
document that to achieve an optimum gain in strength, the potash fertiliser
granulates
are brought to a temperature of preferably 800 to 100 C in a mixer before
being wetted
with water. Furthermore, a minimum dwell period of 10 s in an atmosphere of
high
moisture content should be maintained.
Although the methods outlined above are, where applicable, able to rectify
strength
problems of the potash fertiliser granulates, the problem of moisture
absorption during
transport or loading and unloading processes and during periods of storage is
not
solved by such methods.
The problem of the absorption of moisture is however addressed in EP-A-
1048634.
Here it is disclosed that an increase in the strength of the granulate and a
reduction in
the moisture absorption of the granulate can be achieved if 0.1 to 2% weight
of an
additive is added to the fine salt of potash fertiliser used for granulation
before the
compacting process. This additive, which is preferably added in a quantity of
1 to 1.5%
weight, is preferably dry magnesium oxide or dry calcium oxide. Alternatively,
burnt
chalk and bumt dolomite is mentioned in the above quoted European application.
The addition of these additives, which is also described in the corresponding
American
application, US-A-6,379,414, results in granulates which exhibit a moisture
absorption
of 1.3 to 1.5% weight during storage at 20 C in 70% relative humidity after
eight days. A
disadvantage to this approach however is that the added additive is also
sensitive to
moisture. It is therefore to be expected that with high relative moisture
contents, as with
the above described test conditions, hydration of the additive will occur,
which is
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associated with swelling. This sort of swelling of the granulates would
however severely
damage the grain structure of the granulates or lead to the breakdown of the
granulates
containing the additive. This sort of swelling effect is, for example,
described in
"Feuerfestkunde" (Refractory theory) (Harder und Kienow), Springer-Verlag
1960, and
can amount up to 53% by volume. It is therefore thoroughly conceivable that
the
addition of the additives in fact causes precisely an opposite effect, in
particular with
very high relative humidity, as in tropical zones.
It is therefore the object of this invention to provide an improved potash
fertiliser
granulate which with high relative humidity remains capable of storage without
the
above quoted problems occurring. Furthermore, the improved potash fertiliser
granulate
should exhibit an adequate strength, so that the formation of undersized
grains is
largely prevented. In addition, this invention assumes the task of providing a
method
for the production of this sort of potash fertiliser granulate and of
providing a device for
carrying out the method.
Brief description of the invention
In another aspect, the present invention provides a method for producing
improved
potash fertiliser granulates, comprising the treatment of a potash fertiliser
raw
granulate with a solution containing silicate or carbonate, followed by the
application of
kinetic energy in the form of vibrations.
In another aspect, the present invention provides a potash fertilizer
granulate obtained
using the method of the present invention.
In another aspect, the present invention provides a device for producing an
improved
potash fertiliser granulate, including an appliance for applying a solution to
a raw
granulate and an appliance for transferring kinetic energy in the form of
vibrations to
the raw granulate which has been treated with the solution.
The term potash fertiliser as used in this application includes the usual
known fertilisers
containing potassium, such as for example potassium nitrate, potassium
chloride,
potassium sulphate, potassium carbonate and the mixed salts derived from
potassium,
such as potassium magnesium chloride, potassium magnesium chloride sulphate,
potassium magnesium sulphate, etc. Those potash fertilisers are particularly
preferred
CA 02448820 2003-11-10
which preferably contain potassium chloride or potassium sulphate and mixtures
of
these components as the principal constituents.
Brief description of the figures
Figure 1 shows a flow chart of a normal method for producing potash fertiliser
granulate.
Figure 2 shows a flow chart of a method according to the invention for the
production of
a potash fertiliser granulate.
Figure 3 shows preferred devices for coating the granulates. Figure 3a shows a
schematic view of a vibrating fluidised bed dryer/cooler and Figure 3b shows a
schematic side view of a weir, adjustable in height with removable operating
appliance.
Figure 4 shows a further flow chart of a method corresponding to this
invention.
Detailed description of the invention
Starting from the known moisture sensitivity of potash fertiliser granulates,
examinations
were carried out to establish the reason for the moisture absorption of the
potash
fertiliser granulates. Here, it was found that the alkaline earth chlorides
present in
potash fertiliser granulates, which are the usual components accompanying the
natural
salts, influence very significantly the moisture absorption of the potash
fertiliser
granulates even in slight amounts. Magnesium chloride and calcium chloride
have a
very strong influence on the moisture absorption capacity of potash fertiliser
granulates.
This influence is shown in the following table.
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Content of alkaline earth chlorides Moisture absorption of the potash
(MgC12+CaCI2 content) fertiliser granulate*
None (< 0.05% mass) 0.45% mass
0.1 % mass 1.40% mass
0.2% mass 2.55% mass
0.5% mass 5.60% mass
*Standard conditions: 20 C, 70% RH, 8d
Even a content of only 0.1 % mass of alkaline earth chloride results in a very
noticeable
moisture absorption of the potash fertiliser granulate.
In the following, this invention, which has been based on the above quoted
findings, is
described in detail. Firstly, the method according to the invention is
discussed. Details,
which are provided in conjunction with the method according to the invention,
also apply
correspondingly to the product according to the invention and the device
according to
the invention (equivalent in each case, where applicable) and vice versa.
According to the invention, a normal raw granulate of a potash fertiliser
granulate is
used. Preferably, this raw granulate includes the fraction of the grain size
of 1 to 5 mm
quoted at the start. This is first treated with a solution containing silicate
or carbonate.
Such preferred solutions are aqueous solutions, in particular aqueous
solutions
containing silicate of potassium, soda, silicate of sodium or potash.
Preferred solutions
contain the components containing silicate or carbonate in an amount of 5 to
25%
weight, preferabiy 10 to 20% weight and especially 14 to 20% weight. This
solution is
applied to the raw granulate using methods known in technology, for example by
spray
application or by mixing in an intensifier-type mixer. The amount of the
solution to be
used is preferably 1 to 2.5% weight referred to the total mass of raw
granulate and
applied solution, preferably 1.3 to 2.0% weight and particularly preferably
1.5 to 1.8%
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weight. The treatment of the raw granulate with the solution described above
leads to a
transformation of the alkaline earth chlorides contained in the raw granulate
to silicate
compounds or compounds containing carbonate which are not hygroscopic. Due to
this
process stage a substantial reduction of the moisture absorption capabiiity of
the raw
granulate is already obtained. A surprising feature is that a positive effect
can be
obtained due to this type of simple wet chemical reaction, without additional
additives
such as magnesium oxide, calcium oxide, burnt chalk or burnt dolomite having
to be
added to the granulate as suggested in the European patent application
mentioned
above. At the same time the application of the amount of liquid leads to
further binding
of any included fine-grain content so that the application of the liquid as
such
furthermore has a positive effect on the raw granulate.
After application of the solution described above, according to the invention
kinetic
energy is then applied by treatment with vibration. This vibration treatment,
in contrast
to compaction methods which are carried out in presses with high pressures,
involves
relative simple equipment and leads to the required grain deformation. Due to
the rather
point-type energy application by the vibration method, smoothing of the
surface of the
granulate grains is achieved, resulting in rounding of the grains and a
reduction of the
specific surface area. This produces a clearly improved abrasion resistance
with a
simultaneous further reduction of the moisture absorption capacity.
Overall, the positive effect of the stage with treatment of the solution
containing
carbonate or silicate solution is again reinforced by the second stage of the
method
according to the invention and furthermore also the mechanical properties of
the
granulates are improved. Consequently, the abrasion of the granulates during
transport,
loading and unloading is again clearly reduced, the flow properties of the
granulates
under handling are improved and the tendency to hardening during storage is
reduced
by reducing the flat contact points.
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In contrast to compaction where crystal deformation is forced due to high
compression
pressures on the rollers (180 to 300 bar), surprisingly in the method
according to the
invention, the grain deformation of the moistened granulates can be achieved
by
vibrations in the frequency range from 10 to 70 Hz, preferably from 15 to 30
Hz, with
amplitudes from 2 to 20 mm, preferably from 2 to 10 mm and particularly
preferably
from 5 to 8 mm.
It has been found that the moistened granulate should be subjected to a dwell
period in
the vibration treatment zone of 10 to 180 s in order to ensure adequate
compaction.
Longer dwell periods are basically however not detrimental, but rather only
disadvantageous with regard to the economics of the method.
The adequate relatively short dwell periods open up however additional
possibilities for
the integration of the process step of vibration treatment into other process
stages,
which can lead to further simplification of the process and to a reduction in
the
investment and operating costs. For example, it is possible to carry out the
vibration
treatment in a vibration zone which is not made available specially for this
purpose, but
instead to position it in a feed section of a vibration-aided fluidised-bed
dryer.
Taken overall, the method according to the invention comprises the two main
stages of
application of a solution containing carbonate or silicate and the vibration
treatment for
grain deformation and grain compaction as described above.
Following the process steps, essential to the invention and quoted above, the
usual
post-treatments can be carried out which are briefly described below.
Following the vibration treatment, drying of the moistened granulates can take
place.
This drying can be carried out in normal equipment, such as for example,
fluidised-bed
apparatus or in vibration-aided fluidised-bed dryers. Normally the next
process stage
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following drying is cooling, whereby the granulate at a temperature of over
100 C is
cooled to about 40 C and therefore the vapour is extensively extracted.
The following treatment stage of the granulate is normally formed by a coating
with
organic substances, such as for example oils, amines, oil/amine mixtures and
waxes, to
ensure free-flowing properties and the storage capability of the granulates
over still
longer time periods by the partial coating of the granulate surface. The
process controls
and equipment needed for this are known to the skilled person. Coating is
normally
carried out in mixers, as shown in Figure 1 for example. Altematively, the
coating can
be located in a vibration-aided fluidised-bed dryer, as indicated in Figure 2.
To achieve
this, a weir, as shown in Figure 3 and which is adjustable in height, is
provided in the
transition area of drying/cooling with an overhang strip of at least 10 cm
width mounted
in the application direction, under which an airless nozzle device can be
installed which
can also be replaced during the process. The nozzles can be individually
pressure
controlled depending on throughput and adjusted such that the height at about
10 to 15
cm of the granulate fluidised bed is not penetrated. Depending on the
consistency of the
coating agent, melting on may be necessary, e.g. with waxes or tempering with
oil/amine mixtures.
If a coating is provided in the method according to the invention, then this
coating takes
place using the usual organic substances, whereby, for example, palmitin or
stearin
waxes are preferred. Normally, fatty amines with a chain length of 16 to 18 C
atoms,
mineral oils, fatty acids (e.g. palmitin acid) or waxes (e.g. stearin) are
used for an
organic coating. The coated amount is generally I to 3 kg of coating substance
per t of
granulate, preferably about 2 kg of coating substance per t of granulate.
The product according to the invention is explained in more detail in the
following.
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The potash fertiliser granulate according to the invention is a product which
can be
obtained by the method of this invention described above. This product is
characterised
in that in comparison to the state of the art, the abrasion resistance is
increased and the
moisture absorption reduced, as shown in the following examples.
Simultaneously, the
product according to the invention contains no additionally introduced
additives, as
mentioned at the start, in particular no MgO and no CaO, which are added as
moisture-
absorption inhibiting additives in the European application discussed above.
The
product according to the invention, obtained according to the main process
stages as
described above, exhibits preferably an abrasion value of < 4%, particularly
preferred
< 3.8% and especially preferred < 3%. The abrasion value is determined
according to
the potash standard (works standard KALI 97 - 147, 08/87). This abrasion value
is the
proportion < 0.5 mm in % mass of the amount used, which is determined after
exposure
of the sample material in a vibration device (frequency 240 min'', amplitude
30 mm) in a
pan with 36 steel balls by sieving. The abrasion values according to the Busch
method,
which is also used, are normally on average 10 to 15% below those of the
method
according to the potash standard.
The moisture absorption, measured under standard conditions of a temperature
of
C, a relative humidity of 70%, with a storage period of 8 days, amounts to
preferably
less than 2% with the product according to the invention, particularly
preferably less
than 1.5% and especially preferably less than 1%. By additional application of
the
coating layer describe above, i.e. by partial coating with the organic
substances
described above, the moisture absorption can be reduced still further,
preferably to a
value less than 1.8%, particularly preferably less than 1% and especially
preferably still
less than 0.8%.
These values apply to potash fertiliser granulate which is not partially
coated with a
water repellent organic substance.
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Here, it is surprising that according to the invention the potash fertiliser
granulate does
not need to contain any oxidic additives, such as calcium oxide or magnesium
oxide,
which are described in the European application mentioned above as essential
components for the reduction of the moisture absorption. Since however the
addition of
such materials is associated with the problems described at the start, this
invention
makes an overall improved potash fertiliser granulate available.
The method according to the invention furthermore has the result that, as
described at
the start, the specific surface area of the granulate grains is reduced and
the grain
shape slightly modified. Whereas the usual granulates are angular shapes
running to a
taper (due to the fracture surfaces arising with the usual treatment), the
product of this
invention exhibits a rather rounded grain shape due to the modification of the
granulate
grain area due to the application of kinetic energy by the vibration treatment
process.
Overall, a product results with an improved storage stability, in particular
because the
moisture absorption is reduced which prevents unwanted hardening of the
granulate
(baking). Simultaneously, the production of dust when handling the granulate
is reduced
due to the increased granulate strength, improving the application conditions
and
contributing to improved handling safety.
Overall, the product according to the invention is characterised by a hitherto
unattained
combination of mechanical strength and low moisture absorption, whereby it is
ensured
that no oxidic additives need to be employed.
In the following the device for carrying out the method of this invention is
more precisely
described.
The device according to the invention comprises as the essentiai eiements
first an
apparatus with which the liquid described in conjunction with the method
according to
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the invention (containing carbonate or silicate) is applied to the raw
fertiliser granulate.
This apparatus is preferabiy a mixer in which the raw fertiliser granulate is
moved while
the liquid is added. The addition of the liquid can either occur through an
inlet opening in
the mixer or preferably by spray nozzles provided, whereby an improved
distribution of
the liquid to be applied is achieved. This type of mixer is basically not
restricted, but
intensifier-type mixers are preferred.
The second element of the device according to the invention is a vibration
appliance in
which the raw granulate grains, which are exposed to the liquid, are treated
to apply the
kinetic energy by vibration oscillations. This element of the device according
to the
invention is preferably a vibrating conveyor, equipped such that the raw
fertiliser
granulate grains treated with the liquid are subjected to a frequency of 15 to
70 Hz at an
amplitude of 2 to 20 mm, with a dwell time of 10 to 180 s. The ranges for
these
parameters described above in conjunction with the method according to the
invention
also apply to the device according to the invention. Alternatively, the
apparatus for
applying the kinetic energy is arranged as an entry zone on a vibration-aided
dryer. This
simplifies especially the equipment needed, because normally drying in a dryer
follows
the application of kinetic energy through vibration, for example, a fluidised-
bed dryer or
a vibration-aided dryer.
Optionally, the device of this invention includes furthermore a device for
applying the
above described coating layer of an amine, oil, wax or a mixture of these.
This device,
which can be fitted for example below the dryer unit, is preferably also a
spray nozzle
appliance, provided for example in a weir, adjustable in height, in the
transition
drying/cooling section of a fluidised-bed dryer. The application preferably
occurs by
replaceable airless jetting appliances, whereby the nozzles preferabiy can be
pressure
controlled individually in relation to the throughput. Normally, adjustment is
made such
that the granulate fluidised bed, about 10 to 15 cm high, is not penetrated.
Where
applicable, this unit also includes an apparatus for heating the organic
coating agent to
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be applied, because, for example with waxes, melting on is needed and with
oils or
oil/amine mixtures tempering is required.
This invention is illustrated further based on the following embodiments.
Example 1
A dry potash fertiliser salt raw granulate, produced on a roller press, with a
magnesium
chloride/calcium chloride content of 0.16% is sprayed according to Figure 2 in
a mixer
with a 15% weight soda solution for a feed of 1.5% weight, mixed thoroughly
and
thereby the alkaline earth chlorides are subjected to a transformation
reaction, resulting
in the carbonates. The moistened raw granulate is surface deformed, smoothed
and
compacted in the following vibrating conveyor and also in the entry section of
the
vibrating fluidised-bed dryer. The vibrating conveyor is run at a frequency of
30 Hz and
an amplitude of 5 mm. The granulate is dried in the vibrating fluidised-bed
dryer/cooler
and the dust extracted in the airflow. In the transition section of the
dryer/cooler there is
the jetting appliance with which 2 kg of amine/oil mixture (ratio of amine :
oil = 1:1) is
applied to one tonne of granulate. In technical testing the granulate produced
in this way
exhibits a single grain strength of 36.2 N, an abrasion of 2.8% and a moisture
absorption (standard conditions, as described above) of 0.75%. As a comparison
raw
granulate was treated in accordance with Figure 1. Here, a single grain
strength of 34.1
N, an abrasion of 4.5% and a moisture absorption of 1.90% resulted. The raw
granulate
exhibited a single grain strength of 34.4 N, an abrasion of 6.8% and a
moisture
absorption of 2.05%. It should be pointed out that in this connection, the
values
described above were measured in each case on a granulate on which no
amine/oil
mixture had been applied.
It is therefore apparent that the method according to the invention is able to
provide a
superior potash fertiliser granulate in accordance with this invention which
exhibits an
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increased strength and a reduced moisture absorption compared to the granulate
produced by the usual method, whereby at the same time no additional oxidic
additives
need to be employed.
After application of the amine/oil mixture a moisture absorption in the
product according
to the invention of 0.6% resulted and with the comparison product 1.05%
(whereby the
abrasion values were approximately the same and were about 0.2%).
ExamAle 2
A dry potash fertiliser salt raw granulate, produced on a roller press, with a
magnesium
chloride/calcium chloride content of 0.6% is treated in accordance with Figure
4 in an
intensifier-type mixer or equivalently on a vibrating conveyor with a 20%
weight
potassium metasilicate solution and intensively and thoroughly mixed. The
potassium
metasilicate solution was produced from a potassium metasilicate with a
density of 1.24
to 1.26 g/cm3. The applied amount of solution containing silicate was 1.7%
weight. The
different facilities used in this example, were used alternatively and can be
selected in
practical operation according to constructional prerequisites.
Then the moistened raw granulate was surface deformed and then dried in the
following
vibrating fluidised-bed dryer with a granulate deformation section of at least
1 m and
reduced air feed at a frequency of 18 to 25 Hz at an amplitude of 7 to 8 mm.
The
coating was then applied, in this example with a palmitin melt heated to 80 C
with an
additive amount of 2 kg of paimitin per t of granulate.
Before the application of the paimitin coating, the granulate produced in this
way
exhibits in the technical tests a single grain strength of 32.9 N, an abrasion
of 3.6% and
a moisture absorption of 1.9% (measured in each case in accordance with the
standard
CA 02448820 2003-11-10
method mentioned at the beginning). The comparison granulate, produced in
accordance with Figure 1, exhibited a single grain strength of 28.2 N, an
abrasion of
4.8% and a moisture absorption of 6.2%. The raw granulate showed a single
grain
strength of 31.0 N, an abrasion of 7.9% and a moisture absorption of 6.5%. It
was
therefore found again that the product according to the invention particularly
exhibited a
very substantially reduced moisture absorption.
After application of the paimitin coating the values for the abrasion on the
product
according to the invention and on the comparison product were in each case
approximately equal (0.2%). The moisture absorption was reduced in the
comparison
product to about 4.2%, but in the product according to the invention to 1.6%.
Overall it has been shown that the method according to the invention provides
an
improved potash fertiliser granulate. Since particularly the addition of
oxidic additives
can be omitted, the product is improved with regard to the long-term storage
capability,
because the swelling effects mentioned at the start cannot occur in the
granulate
according to the invention. The device according to the invention is
characterised by a
relatively simple structure and therefore facilitates economical production of
the product
according to the invention and economical implementation of the method
according to
the invention.
