U.S. patent application number 12/275060 was filed with the patent office on 2009-03-19 for process for the manufacture of sulphur-containing ammonium phosphate fertilizers.
Invention is credited to Kenneth William KEENAN, William Patrick KENNEDY.
Application Number | 20090071213 12/275060 |
Document ID | / |
Family ID | 32309465 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090071213 |
Kind Code |
A1 |
KEENAN; Kenneth William ; et
al. |
March 19, 2009 |
PROCESS FOR THE MANUFACTURE OF SULPHUR-CONTAINING AMMONIUM
PHOSPHATE FERTILIZERS
Abstract
The invention relates to a process for the manufacture of
sulphur-containing fertilizers comprising the steps of: (a) mixing
ammonia, phosphoric acid and water in a reactor unit to obtain an
ammonium phosphate mixture; (b) introducing the mixture obtained in
step (a) into a granulator unit to obtain granules, wherein a
liquid phase comprising elemental sulphur is brought into contact
with ammonia, phosphoric acid and water in the reactor unit in step
(a) or is introduced in the granulator unit in step (b). The
invention further relates to sulphur-containing ammonium phosphate
fertilizers, to the use of these sulphur-containing fertilizers to
promote the growth of agricultural products and to the agricultural
products thus-obtained.
Inventors: |
KEENAN; Kenneth William;
(Calgary, CA) ; KENNEDY; William Patrick;
(Calgary, CA) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
32309465 |
Appl. No.: |
12/275060 |
Filed: |
November 20, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10534214 |
May 6, 2005 |
7470304 |
|
|
PCT/EP03/50821 |
Nov 12, 2003 |
|
|
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12275060 |
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Current U.S.
Class: |
71/64.03 |
Current CPC
Class: |
C05B 7/00 20130101; C05B
7/00 20130101; C05B 7/00 20130101; C05G 5/36 20200201; C05G 5/40
20200201; C05G 5/36 20200201; C05D 9/00 20130101; C05D 9/00
20130101; C05G 5/40 20200201 |
Class at
Publication: |
71/64.03 |
International
Class: |
C05D 9/00 20060101
C05D009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2002 |
EP |
02257854.6 |
Claims
1. A sulphur-containing fertilizer obtainable by a process
comprising the steps of: (a) bringing a liquid phase comprising
elemental sulphur into contact with ammonia, phosphoric acid and
water in a reactor unit to obtain an ammonium phosphate mixture,
wherein the elemental sulphur is introduced into the reactor unit
substantially at the same time as the other reactants; (b)
introducing the mixture obtained in step (a) into a granulator unit
to obtain granules.
2. A sulphur-containing fertilizer according to claim 1, wherein
the granules obtained after step (b) are dried in a drying
unit.
3. A sulphur-containing fertilizer according to claim 2, wherein
the reactor unit in step (a) is a pipe cross reactor unit or a
preneutralizer.
4. A sulphur-containing fertilizer according to claim 3, wherein
the elemental sulphur is introduced as a slurry of sulphur
particles in water.
5. A sulphur-containing fertilizer according to claim 4, wherein
the elemental sulphur is introduced as molten sulphur, the
temperature of the mixture being kept above 113.degree. C.
6. A sulphur-containing fertilizer according to claim 5, wherein
the ammonia is anhydrous gaseous ammonia or a concentrated solution
of ammonia in water.
7. A sulphur-containing fertilizer according to claim 6, wherein a
potassium salt and/or other plant nutrients has been added to the
fines.
8. A sulphur-containing fertilizer according to claim 7, wherein
the elemental sulphur is biologically produced elemental
sulphur.
9. A sulphur-containing fertilizer according to claim 1, wherein
the sulphur-containing fertilizer is a sulphur-containing
diammoniumphosphate fertilizer, a sulphur-containing
monoammoniumphosphate fertilizer, or a sulphur-containing
nitrogen-phosphorous-potassium fertilizer.
10. A sulphur-containing fertilizer according to claim 1, wherein
the sulphur content is less than 25% (w/w), based on the final
product.
11. A sulphur-containing fertilizer according to claim 1, wherein
the sulphur content is between 2% and 18% (w/w), based on the final
product.
12. A sulphur-containing fertilizer according to claim 1, wherein
the sulphur content is between 5% and 15% (w/w), based on the final
product.
13. A sulphur-containing fertilizer according to claim 3, wherein
the elemental sulphur is introduced as a slurry of sulphur
particles in water, the particle size of the sulphur particles
being between 0.5 and 150 microns.
14. A sulphur-containing fertilizer according to claim 13, wherein
the elemental sulphur is introduced as molten sulphur, the
temperature of the mixture being kept above 113.degree. C.
15. A sulphur-containing fertilizer according to claim 14, wherein
the ammonia is anhydrous gaseous ammonia or a concentrated solution
of ammonia in water.
16. A sulphur-containing fertilizer according to claim 15, wherein
a potassium salt and/or other plant nutrients has been added to the
fines.
17. A sulphur-containing fertilizer according to claim 16, wherein
the elemental sulphur is biologically produced elemental
sulphur.
18. A sulphur-containing fertilizer according to claim 3, wherein
the elemental sulphur is introduced as a slurry of sulphur
particles in water, the particle size of the sulphur particles
being between 1.0 and 100 microns.
19. A sulphur-containing fertilizer according to claim 18, wherein
the elemental sulphur is introduced as molten sulphur, the
temperature of the mixture being kept above 113.degree. C.
20. A sulphur-containing fertilizer according to claim 19, wherein
the ammonia is anhydrous gaseous ammonia or a concentrated solution
of ammonia in water.
21. A sulphur-containing fertilizer according to claim 20, wherein
a potassium salt and/or other plant nutrients has been added to the
fines.
22. A sulphur-containing fertilizer according to claim 21, wherein
the elemental sulphur is biologically produced elemental sulphur.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 10/534,214 having a 35 U.S.C. 371 date of May 6, 2005 that was
PCT filed Nov. 12, 2003 having PCT No. PCT/EP03/50821 claiming
priority from European Patent Application No. 02257854.6 filed Nov.
14, 2002, which are all incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a process for the manufacture of
sulphur-containing ammonium phosphate fertilizers.
[0003] The invention further relates to sulphur-containing
fertilizers of the ammonium phosphate type, such as
sulphur-containing diammonium phosphate (S-DAP), sulphur-containing
mono-ammonium phosphate (S-MAP) or ammonium phosphate based
sulphur-containing nitrogen-phosphorous-potassium compounds
(S-NPK).
[0004] The invention further relates to the use of these
sulphur-containing fertilizers, especially to grow agricultural
products on sulphur-deficient soil.
[0005] The invention also relates to the agricultural products
grown on soil, especially on sulphur-deficient soil which has been
treated with the sulphur-containing fertilizers of the present
invention.
BACKGROUND OF THE INVENTION
[0006] In the past a tremendous amount of work has been devoted to
the manufacture of sulphur-containing fertilizers. The growing
worldwide demand for sulphur-containing fertilizers stems from the
discovery that low crop yields in certain cases may be related to
deficiencies in sulphur in the soil. An example of a species with
high sulphur requirements is Canola. Canola is an important cash
crop in Alberta, Canada, and has high sulphur requirements at any
growth stage. A shortage of sulphur can cause serious reductions in
crop yield.
[0007] Manufacturing processes for sulphur-containing fertilizers
of the ammonium phosphate type often involve the use or
incorporation of sulphates, see e.g. U.S. Pat. No. 4,377,406, or
U.S. Pat. No. 4,762,546. A disadvantage of sulphates is that they
are very mobile in the soil and leachable. Elemental sulphur is not
leached out of the soil, as are sulphates. It is therefore more
advantageous to have the sulphur present as elemental sulphur.
Furthermore, elemental sulphur offers some additional benefits in
fertilizers: elemental sulphur acts as a fungicide against certain
micro organisms, as a pesticide against certain soil and plant
pests, it assists the decomposition of plant residues and it
improves phosphorus and nitrogen utilization and reduces the pH of
alkaline and calcareous soils.
[0008] Thus, it is advantageous to incorporate sulphur as elemental
sulphur in the sulphur-containing fertilizers.
[0009] Processes for the manufacture of sulphur-containing
fertilizers, wherein elemental sulphur is used, are known in the
art. Most of the methods involve the incorporation of molten
sulphur into the fertilizer.
[0010] In U.S. Pat. No. 5,653,782, a process for the manufacture of
sulphur-containing fertilizers has been described, wherein a
substrate containing fertilizer particles is heated to a
temperature above the melting point of sulphur and admixed with
sulphur. According to U.S. Pat. No. 5,653,782, the sulphur is
melted by the heat provided by the preheated fertilizer particles,
thereby producing a homogeneous coating on the fertilizer
particles.
[0011] U.S. Pat. No. 3,333,939, describes the coating of ammonium
phosphate granules with molten sulphur. The granules are coated in
a separate coating unit into which the sulphur is fed, by
contacting the granules with molten sulphur or with a solution of
ammonium polysulphide. Subsequently, the coated granules are dried.
Alternatively, U.S. Pat. No. 3,333,939 teaches a process for
preparing sulphur-containing fertilizer particles in which the
sulphur is interspersed throughout the particles. In this process
ammonia and phosphoric acid are allowed to react to form ammonium
phosphate. The ammonium phosphate formed is fed into a granulator
in which it is mixed with urea and dry sulphur. The granules
obtained are dried in a dryer. The disadvantage of the first
process of U.S. Pat. No. 3,333,939 is that the coating prevents a
uniform distribution of ammonium sulphate and sulphur into the
soil. The second process has the disadvantage that it requires
solid sulphur handling. The handling and grinding of solid sulphur
is highly hazardous due to the dust and risks of explosions. As
mentioned in a review by H. P. Rothbaum et al (New Zealand Journal
of Science, 1980, vol. 23, 377), explosion hazards are always due
to sulphur dust which is inflammable. Therefore, a more complex
process design is necessary to ensure the safety of the
process.
[0012] U.S. Pat. No. 5,571,303 discloses a process for the
manufacture of fertilizers in which first ammonia, water and
phosphoric acid are reacted to form ammonium phosphate.
Subsequently, the ammonium phosphate/water mixture is mixed with
molten sulphur. The mixture thus obtained is kept at temperatures
of 120-150.degree. C. until granulation. A disadvantage of this
process is that due to the preforming of ammonium phosphate either
much water is needed to keep the salt dissolved or, when relatively
small amounts of water are used, solid ammonium phosphate is
formed. The homogeneous distribution of sulphur throughout the
granule may be hampered by the existence of solid ammonium
phosphate.
[0013] Since problems with the manufacture of fertilizers of the
sulphur-containing ammonium-phosphate type, continue to be in
existence, there is a need for a manufacturing process for such
fertilizers which would diminish or even prevent the problems
experienced in the art.
[0014] It has now been found that a process for manufacturing
sulphur-containing fertilizers, wherein sulphur is introduced as a
liquid phase comprising elemental sulphur, offers advantages over
the manufacturing processes known in the art, with regard to safety
aspects as well as with respect to process control.
[0015] The process according to the invention enables the
manufacture of fertilizers with an even distribution of sulphur
throughout the fertilizer product, thereby enhancing conversion in
the soil to plant usable form, namely sulphates. The fertilizer is
thus enabled to deliver sulphates to the intended crop in a more
reliable and consistent manner.
SUMMARY OF THE INVENTION
[0016] The invention therefore provides a process for the
manufacture of sulphur-containing fertilizers comprising the steps
of:
(a) mixing ammonia, phosphoric acid and water in a reactor unit to
obtain an ammonium phosphate mixture; (b) introducing the mixture
obtained in step (a) into a granulator unit to obtain granules,
wherein a liquid phase comprising elemental sulphur is brought into
contact with ammonia, phosphoric acid and water in the reactor unit
in step (a) or is introduced in the granulator unit in step
(b).
BRIEF DESCRIPTION OF THE DRAWING
[0017] The FIGURE depicts a process scheme according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] In step (a) of the process according to the invention,
ammonia, phosphoric acid and water are contacted in a reactor unit
to obtain an ammonium phosphate mixture. The phosphoric acid is
typically manufactured by reacting sulphuric acid with phosphate or
is commercially available phosphoric acid. Suitable phosphoric
acids are for example orthophosphoric acid, or pyrophosphoric acid
or mixtures thereof. To avoid the introduction of excess process
water, the ammonia is preferably introduced as a concentrated
aqueous solution or as anhydrous gaseous ammonia. The advantage of
having a mixture with as little water as possible is that any
additional water introduced into a fertilizer process must be
handled in the process and eliminated at a later stage. Therefore,
any additional water introduced into fertilizer manufacturing
processes leads to a more complex process. Preferably, the water
content in the ammonium phosphate mixture is kept as low as
possible, preferably between approximately 10 and 20% based on the
total weight of the mixture, more preferably between 12 and 15%
based on the total weight of the mixture.
[0019] The amounts of ammonia and phosphoric acid are adjusted to
achieve the desired products. For the production of S-MAP, the
molar ratio of ammonia and phosphoric acid is typically kept
between values of about 0.5-1.0, for the production of S-DAP the
molar ratio of ammonia and phosphoric acid is typically kept
between values of about 1.2-2.0 and for the production of S-NPK the
molar ratio of ammonia and phosphoric acid is typically kept
between values of about 0.7-1.7. Preferred values for the
ammonia:phosphoric acid molar ratios are about 0.6-0.8 for the
production S-MAP, 1.3-1.8 for the production S-DAP and about
1.0-1.5 for S-NPK. More preferred values for the ammonia:phosphoric
acid molar ratios are about 0.7 for the production S-MAP, about 1.5
for the production S-DAP and about 1.3 for NPK.
[0020] Typically, the mixing takes place at atmospheric pressure
and at temperatures between about 100.degree. C. and about
130.degree. C. Preferably, water or sulphuric acid is added to the
reactor unit to control the temperature of the mixture. Typically,
water is added when a temperature reduction is needed, sulphuric
acid is added to when a temperature increase is needed.
[0021] In one embodiment of the process according to the invention,
a liquid phase comprising elemental sulphur is brought into contact
with ammonia, phosphoric acid and water in the reactor unit in step
(a). In a preferred embodiment, ammonia, phosphoric acid, water and
a sulphur slurry comprising a sulphur dispersion are mixed in a
reactor unit to obtain an ammonium phosphate mixture; this mixture
is then introduced into a granulator unit to obtain granules. In
another embodiment of the process according to the invention,
ammonia, phosphoric acid and water are mixed in a reactor unit to
obtain an ammonium phosphate mixture; this mixture is then
introduced into a granulator unit to obtain granules, while a
liquid phase comprising elemental sulphur is also introduced in the
granulator unit in step (b).
[0022] In a preferred process according to the invention, elemental
sulphur is introduced into the reactor unit in step (a)
substantially at the same time as the other reactants. It has been
found that the crushing strength of the granules can be improved if
the sulphur is added into the reactor unit in step (a).
[0023] In a preferred process, the elemental sulphur is introduced
as a slurry of water and sulphur particles. Typically, the sulphur
particles are dispersed or suspended in the slurry. Preferably, the
particles have a size ranging from between about 0.5 to about 150
microns, preferably between about 1.0 and about 100 microns. To
avoid the removal of excess water at a later stage in the process,
the water content in the sulphur slurry is typically kept as low as
possible, preferably between approximately 10 and 40% based on the
total weight of the mixture, more preferably between 15 and 30%
based on the total weight of the slurry. In the case where the
sulphur particles are suspended in the slurry, the sulphur slurry
is preferably stirred or mixed in a suitable apparatus to
homogenise the slurry prior to introducing it into the
manufacturing process. In a preferred embodiment, the sulphur
slurry contains sulphur particles which are dispersed in the water.
This type of slurry, henceforth referred to as dispersed or
emulsified sulphur slurry, comprises dispersed sulphur particles in
water, preferably dispersed micron-sized sulphur particles in
water. The sulphur particles are suitably kept in dispersion
through the addition of a suitable emulsifier. Suitable emulsifiers
are known in the art and are not critical to the invention. An
advantage of using dispersed sulphur particles is that the
precipitation of sulphur particles is kept to a minimum and the
sulphur is distributed more homogeneously throughout the water.
Thus, the need for stirring or mixing prior to introducing the
sulphur slurry into the reactor unit is reduced. Typically, the
slurry is introduced by pumping the slurry from a sulphur slurry
reservoir unit into the reactor unit.
[0024] In yet another preferred process according to the invention,
the elemental sulphur is introduced into the reactor unit in step
(a) as molten sulphur. Molten sulphur can be obtained from solid
sulphur, by melting in a suitable melting apparatus, for instance a
tube melter.
[0025] The use of molten sulphur is advantageous when sulphur is
obtained in the molten state from an industrial process. Processes
for the removal of unwanted sulphur components from natural gas
usually produce sulphur in the molten state and the use of this
molten sulphur directly in the fertilizer manufacturing process
according to the invention avoids the need for additional steps,
such as drying and grinding of the sulphur, to obtain a sulphur
slurry. An additional advantage of using molten sulphur is that no
additional water is introduced into the fertilizer manufacturing
process. When adding elemental sulphur in the molten state, the
temperature of the sulphur-containing mixture is preferably kept
above the melting point of sulphur, preferably between temperatures
of 115.degree. C. and 121.degree. C.
[0026] In an especially preferred process according to the
invention, biologically produced elemental sulphur is used.
Reference herein to biologically produced elemental sulphur is to
sulphur obtained from a process wherein sulphur-containing
components, such as sulphides or H2S, are converted to elemental
sulphur via biological conversion. Biological conversion can
suitably be effected using sulphide-oxidising bacteria. Suitable
sulphide-oxidising bacteria can be selected for instance from the
known autotropic aerobic cultures of the genera Thiobacillus and
Thiomicrospira. An example of a suitable biological conversion
process to obtain the biologically produced elemental sulphur
suitable for the process according to the invention is the process
for the removal of sulphur compounds from gases wherein the gas is
washed with an aqueous washing liquid and the washing liquid is
subjected to sulphide-oxidising bacteria, as described in WO
92/10270. Biologically produced elemental sulphur has a hydrophilic
nature, making it especially suitable for agricultural use as
fertilizer due to the relative ease with which the biologically
produced sulphur is taken up by the soil. An additional advantage
of biologically produced elemental sulphur is that fouling or
blocking of equipment is substantially reduced or even eliminated
due to the hydrophilic nature.
[0027] The reactor unit used in step (a) is any device wherein the
ammonia, phosphoric acid and water are reacted to obtain an
ammonium phosphate mixture, for example a pipe cross reactor or a
preneutralizer unit. A preneutralizer unit comprises a tank reactor
equipped with mixing equipment and appropriate inlet and outlet
devices. In a preferred embodiment, a liquid phase comprising
elemental sulphur is introduced in step (a) and a preneutralizer
unit is used. In the preneutralizer unit, the starting components
are mixed using a stirring device and ammonia is typically
introduced as gaseous ammonia. The advantage of using a
preneutralizer unit when elemental sulphur is introduced in step
(a) is that a larger amount of sulphur can be used without
experiencing operating problems such as clogging, most likely due
to a more effective mixing. Another advantage of using a
preneutralizer unit in step (a) is that the resulting
sulphur-containing granules obtained after step (b) are stronger,
reflected in their higher crush strength, even at higher amounts of
sulphur in the granule. In a pipe cross reactor, the liquid phase
comprising elemental sulphur, water and phosphoric acid are
simultaneously fed in a pipe reactor through which the reactants
are passed.
[0028] After step (a), a mixture comprising ammonium phosphate,
water and optionally elemental sulphur is obtained. In step (b) of
the process according to the invention, this mixture is introduced
into a granulator unit to obtain granules.
[0029] In a preferred embodiment, ammonia, phosphoric acid and
water are mixed in a reactor unit to obtain an ammonium phosphate
mixture; this mixture is then introduced into a granulator unit to
obtain granules, while a slurry comprising elemental sulphur is
also introduced in the granulator unit in step (b).
[0030] In a preferred process, the elemental sulphur is introduced
into the granulator unit as a slurry of water and sulphur
particles, including suspended sulphur or dispersed sulphur, as
hereinbefore described.
[0031] In an especially preferred process according to the
invention, the elemental sulphur is introduced into the granulator
unit as a slurry of water and biologically produced elemental
sulphur particles, including suspended sulphur or dispersed sulphur
as hereinbefore described.
[0032] Reference herein to a granulator is to a device for forming
granules or pellets of fertilizer product. Commonly used
granulators are described in Perry's Chemical Engineers' Handbook,
chapter 20 (1997). Preferred granulators are drum granulators or
pan granulators. Typically, the mixture is pumped and distributed
on a rolling bed of material in a drum granulator. In the
granulator, granules are formed. Reference herein to granules is to
discrete particles comprising ammonium phosphate and elemental
sulphur. Optionally, ammonia can be introduced in the granulator to
complete the ammoniation of the ammonium phosphate mixture.
Optionally, water and steam can also be fed to the granulator to
control the temperature of the granulation process as needed.
[0033] Optionally, additional ammonia and/or recycled fertilizer
particles may be added to the granulator unit. Recycled fertilizer
particles add granulation and nucleating agents. They are obtained
from the final fertilizer product. Suitably they have small
particle sizes (so-called off-spec fines). The recycle of fines is
also described in U.S. Pat. No. 3,333,939.
[0034] Other ingredients may be added during the manufacturing
process to tailor the fertilizer products to their intended
end-use. Examples include plant micro nutrients such as boron,
potassium, sodium, zinc, manganese, iron, copper, molybdenum,
cobalt, calcium, magnesium and combinations thereof. These
nutrients may be supplied in elemental form or in the form of
salts, for examples as sulphates, nitrates or halides. In this way,
granules enriched in plant nutrients are obtained. The amount of
plant micronutrients depends on the type of fertilizer needed and
is typically in the range of between 0.1 to 5%, based on the total
weight of the granules.
[0035] The sulphur-containing ammonium phosphate granules obtained
after the granulation step are optionally dried in a drying unit.
In a preferred embodiment, the granules are air-dried in the drying
unit, thereby avoiding the need for additional drying equipment.
Alternatively, drying units wherein heat transfer for drying is
accomplished by direct contact between the wet solid and hot gases
are used, thereby enabling a faster drying step. Typically, the
drying unit is a rotary dryer.
[0036] In a preferred process according to the invention, the
granules are sorted on their size in a sorting unit to achieve a
more uniform size distribution. Typically, oversized granules are
crushed and returned to the sorting unit while undersized granules
are returned to the granulator as so-called off-spec fines. A
preferred size range for the granules is between about 1.5 and 5.0
mm, more preferably between about 2 and 4 mm, expressed as the
average diameter of the granules. The use of granules which fall
within this range is more likely to enable a more even distribution
of the fertilizer ingredients in the soil after applying the
granules to the soil.
[0037] It will be appreciated that the process parameters in the
reactor unit and in the granulator unit have to be adjusted
depending on the desired products.
[0038] After a typical manufacturing process according to the
invention, sulphur-containing monoammoniumphosphate,
sulphur-containing diammoniumphosphate or sulphur-containing NPK
(nitrogen-phosphorous-potassium) fertilizer granules, optionally
enriched in plant nutrients, are obtained. The sulphur in the
sulphur-containing fertilizer granules according to the invention
may be incorporated into the fertilizer granules, or the sulphur
may be distributed on the granules or the sulphur may be both
incorporated into the granules and be distributed on the granules.
The content of elemental sulphur in these fertilizer granules is
typically up to 25%, based on the total weight of the fertilizer,
preferably between 2 and 18%, more preferably between 5 and 15%. An
elemental sulphur content higher than 25% will generally lead to a
less even distribution of sulphur on and throughout the granules,
due to clustering of elemental sulphur. In addition, the crush
strength of granules decreases with an increasing elemental sulphur
content. The most homogeneous distribution of sulphur on and
throughout the granules is achieved when the content of elemental
sulphur is between 5 and 15%, based on the total fertilizer
granule.
[0039] The invention will now be illustrated by means of the
schematic FIGURE.
[0040] The FIGURE depicts a typical process scheme of the process
according to the invention, wherein the elemental sulphur is
introduced in step (a).
[0041] Phosphoric acid is led from tank (1) via line (2) to a
reactor (3). Gaseous ammonia is led from tank (4) via line (5) to
reactor (3). Water is led from tank (6) via line (7) to reactor
(3). Sulphur is led from tank (8) via line (9) to reactor (3).
[0042] In reactor (3), the anhydrous ammonia and phosphoric acid
are reacted to form a sulphur-containing ammonium-phosphate
mixture. This mixture is pumped via line (10) to a drum granulator
(11), where it is introduced on top of a rolling bed of fertilizer
material. Gaseous ammonia is led from tank (4) via line (12) into
the drum granulator to increase the mole ratio to approximately 1.8
or 1.0 when producing S-DAP or S-MAP respectively.
[0043] In granulator (11), moist sulphur-containing
ammonium-phosphate granules are formed. The moist granules are led
via line (13) to a rotary dryer (14). In the rotary dryer (14), the
granules are dried. The dried granules are led via line (15) to a
sizing unit (16).
[0044] In the sizing unit, granules that are too large or too
small, relative to a pre-determined granules size, are removed from
the granules stream. The oversized granules are led via line (17)
to a crusher (18) where they are crushed. The crushed granules are
returned via line (19) to the sizing unit. The undersized granules
are recycled via line (20) to the granulator. The granules with a
size range of between 2.0 and 4.0 mm are led via line (21) to a
cooler (22) where they are cooled. A portion of granules with a
size range of between 2.0 and 4.0 mm is recycled via line (23) to
the drum granulator to help control the granulation process.
[0045] Ammonia and water vapours escaping from reactor (3) are led
via line (24) to a wet scrubber unit (25), where they are scrubbed
with phosphoric acid. The scrubber liquid containing ammonium
phosphate is led back via line (26) to reactor (3).
[0046] The air and dust collected from the drum granulator, dryer
discharge elevator and drum granulator surroundings are led via
lines (27) and (28) to a suitable commercially available wet
scrubber (29) where they are treated and then vented via line (30)
to the atmosphere.
[0047] The invention will now be illustrated by means of the
following non-limiting examples.
EXAMPLE 1
Comparative
[0048] DAP granules without added sulphur were prepared using the
process according to schematic FIG. 1, but without added sulphur
from tank (8). A preneutralizer reactor was used as reactor (3).
The reaction mixture in the preneutralizer reactor was maintained
at 115.degree. C., with a NH.sub.3:H.sub.3PO.sub.4 mole ratio of
1.42. Chemical analysis of the resulting granules indicated 19.0%
N, 50.5% P.sub.2O.sub.5 and 0.9% sulphate sulphur (expressed as
weight percentages based on the total weight). The average crush
strength of the granules, the minimum force required to crush an
individual granule, was 4.7 kg/granule.
EXAMPLE 2
According to the Invention
[0049] DAP granules with added sulphur were prepared using the
process according to schematic FIG. 1. The reactor used was a
preneutralizer reactor. The reaction mixture in the preneutralizer
reactor was maintained at 117.degree. C., with a
NH.sub.3:H.sub.3PO.sub.4 ratio of 1.44. Chemical analysis of the
resulting granules indicated 15.7% N, 41.8% P.sub.2O.sub.5, 0.6%
sulphate sulphur and 17.6% elemental sulphur (expressed as weight
percentages based on the total weight). Scanning electron
microscopy (SEM) analysis was performed to evaluate if the added
sulphur was evenly dispersed in the fertilizer granules. SEM
analysis of the granules and of split granules indicated that the
sulphur was distributed both on the surface of the granules and
throughout the granules. The average crush strength of the granules
was 4.3 kg/granule.
EXAMPLE 3
Comparative
[0050] MAP granules without added sulphur were prepared using the
process according to schematic FIG. 1, but without added sulphur
from tank (8). A pipe cross reactor was used as reactor (3). The
reaction mixture in the pipe cross reactor was maintained between
120 and 126.degree. C., with a NH.sub.3:H.sub.3PO.sub.4 mole ratio
of 0.67. Chemical analysis of the resulting granules indicated
11.3% N, 56.0% P.sub.2O.sub.5 and 1.0% sulphate sulphur (expressed
as weight percentages based on the total weight). The average crush
strength of the granules was 4.8 kg/granule.
EXAMPLE 4
According to the Invention
[0051] AP granules with added sulphur were prepared using the
process according to schematic FIG. 1. The reactor used was a pipe
cross reactor. Sulphur was added as emulsified sulphur. The
emulsified sulphur was agitated in a container and then transferred
directly from the container to the sulphur feed tank (8). The
reaction mixture in the pipe cross reactor was maintained at about
122.degree. C., with a NH.sub.3:H.sub.3PO.sub.4 ratio of 0.69.
Chemical analysis of the resulting granules indicated 10.3% N,
50.3% P.sub.2O.sub.5, 0.7% sulphate sulphur and 11.0% elemental
sulphur (expressed as weight percentages based on the total
weight). Scanning electron microscopy (SEM) analysis was performed
to evaluate if the added sulphur was evenly dispersed in the
fertilizer granules. SEM analysis of the granules and of split
granules indicated that the sulphur was distributed both on the
surface of the granules and throughout the granules. The average
crush strength of the granules was 4.2 kg/granule.
* * * * *