Note: Descriptions are shown in the official language in which they were submitted.
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A process for bindincL dust in fertilizer granules
The invention relates to a process for preventing the formation
of dust when loading or transporting granulated materials or
pellets, in particular fertilizer granules.
To an increasing extent, fertiiizers.are being produced in
granulated farm and transported in bulk. The granules are
produced mainly by the granulation method which forms the fine-
grained salt into flakes fxom which, subsequently, granules
having preferentially a grain size range of 1.0 to 4.0 mm, are
formed by crushing and screening.
It is a known fact that when fine-grained materials are
granulated, adjuvants are added to the starting mixture in
order to facilitate the granulation process.
For example, German Patent Application (Auslegeschrift) DE-AS
21 01 585 describes the addition of 2 to 3% organic or
inorganic binding agents such as molasses, starch, calcium
chloride and alkali and alkaline-earth sulfates arid nitrates
when compacting Thomas meal (phosphate fertilizer). With this
method it is essential to stay within certain temperature
ranges and to follow certain time sequences during the actual
compacting and secondary treatment stages.
German Patent DE-PS 36 18 058 describes a process for
granulating water-soluble fertilizers containing a high
proportion of kieserite, characterized by the fact that soluble
and/or slightly colloidal soluble substances of the mono-, di-
and polysaccharide class and/or simple hydrophilic derivatives
thereof are added in solid form or as a solution in amounts
between 0.1 and 5%; in addition, saccharose in solid form or as
sugar-rich molasses is added.
The granules formed in this way are characterized above a11 by
sharp edges and corners, which are formed during the subsequent
crushing stage and which are then abraded during transportation
or handling of the material, thereby giving rise to the
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undesired generation of dust during transportati4n and further
use. This dust causes unacceptable environmental pollution. It
is a known fact that attempts are made to counter this
phenomenon by screening out the extremely fine grain sizes
prior to loading at the manufacturer's plant. However, it has
been discovered in practice that this secondary treatment is
not enough by itself to remove the adhering residue of fine
dust or to improve the abrasion resistance of the edges,
corners and also the sides of each and every granule.
German Patent DD-PS 136 956 describes a procedure for improving
the abrasion resistance of potash fertilizer granules wherein,
following the granulation process, the potash fertilizer
undergoes secondary treatment consisting of the following
steps: dedusting in a fluidized bed, followed by surface
treatment of the granules with water or aqueous additives,
then drying and cooling of.the granules treated in this manner.
The intention of this process is to remove any existing
unstable tips or edges by dissolving and recrystallizing the
material, thereby strengthening the surface of the granules.
German Patent Application (Offenlegungsschrift) DE-OS 30 03 883
supplements the information provided in the aforementioned
patent application by prescribing a temperature range of 80 -
100° at which an optimum strength gain is achieved, if at the
same time a minimum dwell time of 10 seconds in a high-humidity
atmosphere is guaranteed.
The treatment method described requires a great deal of
investment for the equipment installed downstream of the actual
granulating and screening stages, and it can only be used at
the manufacturer's premises because the heat required for the
process comes from the production of the granules. Thus, quite
apart from the investment costs and the resulting equipment
maintenance costs, this method cannot be used, for example,
for granulated potash products which are stored for long
periods of time in intermediate storage piles from where they
must again be loaded onto means of transport. It is a known
fact that long storage times and the associated physical
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effects, such as compression and alternating atmospheric
humidity levels, can have negative effects on granules treated
in this manner (caking tendency).
It is therefore the purpose of the invention to create a
process which prevents the formation of dust during the loading
and transporting of granules or pellets, in particular
fertilizer granules, and which at the same time optimally
satisfies a11 the requirements which might be made as regards
environmental compatibility, long storage life of the treated
granules and simplicity of application.
These goals are achieved by the process according to the
invention wherein a mixture of at least two organic substances,
which are completely soluble one with the other, are added to
the granules; the second organic substance is a water-soluble,
oxygen-containing hydrocarbon and the first organic substance
possesses an adhesive effect, and the second oxygen-containing
organic substance controls the moisture content of the mixture.
The dust-binding mixture used must therefore possess an
adhesive effect on the surface of the granules. It may not be
diluted by absorbing an excessive amount of moisture nor may so
much moisture be driven off by drying processes that it loses
its adhesive and bonding effect.
Generally speaking, the substances used for this purpose
possess relatively high viscosity at room temperature as well
as water-repellent properties. Such viscous substances should
possess the highest possible flash point because as a rule they
have to be spray-applied at approx. 80° C in order to achieve
good distribution.
The long storage life of the material treated in this manner is
due to the long-lasting dust-binding effect of the dust-binding
mixture according to the invention.
A preferred mixture (dust-binding mixture) consists of molasses
and at least one further water-soluble, oxygen-containing
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hydrocarbon) The mixture is added in an extremely finely
distributed form to the granules.
In the process, the molasses provides the adhesive effect, thus
binding the dust, while by controlling the water vapour
pressure of the solution, the second organic substance ensures
that the film of dust-binding mixture adhering to the granules
remains moist and thus active.
The molasses, which is a particularly suitable substance for
the purpose, accumulates as the second raw sugar stage in the
process of beet sugar refining. The second organic substance
consists of water-soluble, oxygen-containing hydrocarbons from
the group comprising glycerine, polyethylene glycol and
triethanolamine, used either singly or in a mixture containing
two of these compounds. These substances satisfy particularly
well the requirements which a dust-binding agent must meet.
Even just one of these substances is effective together with
molasses, but it is particularly advantageous to use a mixture
of two or three substances in a ratio of 1 : 1. In each case,
the substances glycerine, polyethylene glycol and
triethanolamine should be used alone or mixed together in a
proportion equal to at least 10% of the total amount of dust-
binding mixture (including molasses) used.
The subject of the invention is in particular a homogeneous
mixture of molasses and of the second organic substance in the
mixture ratio 1 : 0.3 to 1 : 2.
It was found, for example, that in the case of potash
fertilizer granules even a small addition, namely 0.6 to 1.2
wt.%, and preferentially 0.7 to 0.9 wt.% of the dust-binding
mixture relative to the weight of the bulk product, gives the
desired optimal effect. When the mixture is applied in this way
to potash fertilizer granules, the useful substance content
(Kz0) does not drop below the prescribed tolerance range.
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The homogeneous mixture can be prepared without any difficulty
at room temperature in a container fitted with an agitator
device.
The granules which are to be loaded and from which the very
fine dust has already been removed by screening, can be coated
with the mixture by applying it at an appropriate pressure and
at room temperature up to a maximum temperature of 50° C, using
an appropriately selected nozzle. In contrast to other, purely
organic, non-aqueous dust-binding agents, it is not absolutely
necessary to heat the mixture.
The desired dust-binding effect is determined according to the
method described below:
Samples of the granules to be conditioned are screened to
remove any particles that may be adhering to them. Then, the
binding agent to be tested is uniformly distributed and spray-
applied to the samples, each of which weighs 200 g. After the
mixture has been applied, dust (<0.2 mm) from the bulk product
is added in the amount of~exactly 1% relative to the weight of
the granules and a11 the samples are then mixed by rotating
them for 5 minutes in sample bottles.
The samples obtained in this way are then examined, for example
after they have been stored for a certain amount of time. The
examination is carried out by screening the entire contents of
the sample bottle on an Alpine air-jet machine fitted with a
0.063 mm screen.
The screening must last exactly three minutes. Then the filter
paper loaded with dust must be removed from the machine and it
is either weighed or, in the case of soluble dusts, e.g. potash
salts, the dust is washed off the filter and the amount is
determined by titration. The dust-binding effect in per cent is
calculated from this value.
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In the comparative tests listed in the following Table, in each
case 9 kg of binding agent were spray-applied to one tonne of
potassium sulfate pressed granules at a temperature of 45° C.
The individual chemicals were used in the following form:
- Molasses with a solids content of 47%
- Glycerine (trihydroxypropane), single distilled
86-88%
Density at 20° C = 1.23 g/cm3; flash point 180° C
- Polyethylene glycol
Density at 20° C = 1.13 g/cm3; with a mean molecular weight
of 400 g/mole; flash point: > 100° C
- Triethanolamine
N(CHZCHZOH)3;
Density at 25° C = 1.11 g/cm3; content 85%; max. 0.5% HZO;
i5 approx. 15% iminobisethanol.
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TABLE
Dust-binding in relation to the null sample in %
after 1 after 6
week weeks
1. Null sample 0 0
2. Molasses (100%) 62 66
3. Glycerine (100%) 61 40
4. Polyethylene glycol (100%) 56 49
5. Molasses/glycerine (1:1) 87 86
6. Molasses/polyethylene glycol 87 82
(1:1)
7. Molasses/polyethylene glycol 89 87
(2:1)
8, Molasses/triethanolamine (1:1).96 86
9. Molasses/glycerine/polyethylene86 80
glycol (1:0.5:0.5)
10. Molasses/glycerine/ 99 90
triethanolamine (1:0.5:0.5)
11. Molasses/polyethylene/glycol/ 96 87
triethanolamine (1:0.5:0.5)
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It can be seen that compared with an untreated sample, very
good dust binding can be achieved.
When used by themselves, the individual substances are nowhere
nearly as effective as the mixtures.
On the one hand, the mixtures have a much more intensive and
complete effect, and on the other the effect lasts longer. High
dust-binding capability is still retained after 6 weeks.
The examples of use listed below describe the invention without
in any way limiting it.
Example 1
Potassium sulfate granules from the press granulation process
are screened and then stored in a storage shed without being
conditioned. At a product temperature of 40° G the granules are
screened to remove the extremely fine dust fraction prior to
being bulk loaded, and then they are treated with the dust-
binding agent before reaching the discharge point of the
conveyor belt and before entering the transportation container;
at the treatment point, fan jet atomizing nozzles are arranged
in such a way that the falling mass of material is covered on
a11 sides by the spray cone. Downstream of this point are
arranged deflection devices to ensure that the wetted product
is thoroughly mixed prior to loading. The atomizing pressure is
approx. 3 to 5 bar)
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The media are on average at room temperature. The mixture used
consists of 50% molasses (content 47%) and 50% glycerine
(content 86% and density 1.23 g/cm3) and it is applied in an
amount of 0.8%, relative to the KZS04 granules which are to be
conditioned.
Example 2
The granules coming from the press granulation stage are
screened and then fed directly to the loading process. After
the very fine dust has also been screened out, the conditioning
agent is applied - in the manner described under Example 1 - in
an amount of 4 kg molasses and 4 kg polyethylene glycol
(density = 1.13, and mean molecular weight = 400 g/mol) per
tonne of granules, but because of the temperature of the
freshly produced granules, it is not necessary to heat up the
applied conditioning agent if an atomizing pressure of at least
3 to 5 bar is available.
