Note: Descriptions are shown in the official language in which they were submitted.
1~463~
.
The present invention provides for an improved
process of manufacture of ammonium nitrate prills and
granules.
There are two types of ammonium nitrate prill which
may be distinguished by their free flowing bulk densities.
Those with a high density (free flowing bulk density > o.85
g/ml) are generally used as fertilisers. Those with low
densities (free flowing bulk density ~o.8S g/ml) are
generally used in explosive compositions. The two types
of ammonium nitrate prills may be made by variations of
a process involving the reaction of nitric acid and
ammonia to form a hot liquid comprising ammonium nitrate,
the spraying of this hot liquid down a tower so that solid
prills are formed and the cooling of the prills.
Low density ammonium nitrate prills are manufactured
by reacting nitric acid with ammonia in a neutraliser,
evaporating most of the water from the resulting ammonium
nîtrate solution in an evaporator to form a concentrate,
causing the hot concentrate, containing typically between
1% and 6% water, from the evaporator to fall from the top
of a prilling tower, and collecting, drying and cooling
the prills 80 formed.
High density ammonium nitrate prills are manufactured
by reacting nitric acid with ammonia in a neutraliser,
evaporating the resulting ammonium nitrate solution in an
evaporator to reduce the water content of the ammonium
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nitrate solution to less than 1~ water to ~orm a con-
centrate in the form of a melt, causing the hot melt from
the e~aporator to fall from the top of a prilling tower,
and collecting and cooling the dry prills so formed. By
5 dry we mean less than 0.2~ H20 content.
Ammonium nitrate granules are made by preparing
a hot concentrate of ammonium nitrate in the manner des-
cribed for the manufacture of low density am~onium nitrate
prills. This hot concentrate is added to recycled solid
10 ammonium nitrate in a granulator in which granules are
produced by agglomeration of the recycled solid ammonium r
nitrate and solidifcation of the concentrate. The
~ranules are dried and screened. The granules of a size
out~ide the desired product size are crushed and recycled
15 to the granulator to provide the solid ammonium nitrate re-
cycle.
In UOS. Patent Specification 3,639,643 it has been
disclosed that the mechanical strength of ammonium nitrate
prills and granules may be enhanced by preparing them so
20 that they contain colloidal aluminium hydroxide; the
aluminium hydroxide being formed by the addition of
aluminium sulphate to the ammonium nitrat~ concentrate
after the evaporation stage. High mechanical strength
of the prills and granules mani~est~ itself in high re-
25 sistance to prill andgranule breakdown during handling,
to compaction and cakin~ on storage, and to degradation
during temperature changes about the crystal phase
transition temperature.
We have now found that, in the manufacture of
30 ammonium nitrate prills and granules which contain alumin-
ium compounds to impart high strength, it is advantageous
to add an aluminium salt as a source of aluminium com-
`pounds to the reactants in the neutraliser or, at the
latest, before the concentrate is formed.
35We have observed that the drying characteristic~
oP ammonium nitrate prills and granules made by a process
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in which an aluminium salt was added in the neutraliser
were superior to those made without aluminium salt
addition and to those made by a process in which the
aluminium salt was added after the evaporation stage.
Much less rigorous conditions of drying are required.
Although this benefit is more significant in the manufacture
of low density prills and granules~ it also helps in the
drying o high density prills during the prilling process.
Less rigorous drying conditions mean that le~s prill
attrition is likely to occur and less prill tower fume
will form. Prill tower fume is the phenomenon of the carry
over of very fine solid particles of ammonium nitrate en-
traine~ in the gases passed through the prill tower.
Another advantage lies in the better control of pH
which may be achieved throughout the whole process.
Ideally the neutraliser is operated by titrating the ammonia
and nitric acid to form the composition which is obtained by
the reacting of stoichiometric amounts of these reactants,
that is ammonia (NH3) and nitric acid (HN03~ in a malecular
ratio oP 1:1; the pH value of this composition is 3.9 and
it is referred to as the equivalence point. (Throughout
this specification and claims, unless otherwise stated, pH
value8 are those of an aqueous solution derived from the
composition in question diluted or dissolved in water so
that the solution so formed contains approximately 10% w/w
o~ ammonium nitrate, the pH measurement being made with
a conventional pH meter at room temperature.) However,
because of the steep nature of the ammonia - nitric acid
neutralisation curve it is impossible to control the
addition of the reactants to the neutraliser and maintain
the contents at the equivalence point. For safety
reasons most neutralisers are operated on an excess of
ammonia with a consequently higher pH. Typically a
neutraliser is run so that its contents have a pH value
of 5.2 to 5.5, below that pH range it is difficult to
control the reaction in a stable manner. However, we have
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now found that if an aluminium salt is added to the re-
actants in the neutraliser, stable control can be achieved
at a lower pH, for example as low as 4~6. One advantage
of this is that the amount of ammonia slip at the sub-
sequent evaporation stage is reduced. During the conr
centration of the hot ammonium nitrate solution ammonia
is also removed with the water from the solution until the
stoichiometric equivalence point is reached and the pH
value of the concentrate drops t~ 3.9. Therefore the lower
the pH value at which the neturaliser may be operated, the
less the ammonia slip from the evaporator and the less
ammonia which has to be scrubbed from the effluent and
recycled. However the process of the present invention
may be carri:ed out using any desired pH profile throughout
the process including those conventionally used. Con-
veniently the pH value of the solution in the neutraliser
may be in the range of 4.2 to 5.5 inclusive without
significantly affecting the aforesaid advantages which
accrue in the drying operations ~hen the process of the
invention is used.
Another advantage accruing by adding aluminium
salts prior to the evaporation stage rather than later in
the proce~ that the pH value of the concentrate from
the evaporator i9 not affected. ~or example, when
aluminium sulphate is added to the conaentrate from the
evaporator, the pH of that concentrate may be reduced to
below 3Ø ~his means extra ammonia has to be added to
raise the pH to the desirable level ~or prilling or
granulating, which i~ typically 4.0 to 5.0 for low density
prills and granules, and 5.0 to 6.0 for high density
prill~
Accordingly there is provided an improvement in
the proces~ for the manufacture o~ prilled or granulated
material comprising ammonium nitrate which process comp-
rise~ manufacture of a solution of ammonium nitrate bythe reaction of ammonia and nitric acid in a neutraliser,
concentration of said solution by evaporation to the
concentration required for prilling or granulating, and
subjecting the thus formed concentrate to a prilling or
granulating process to form prills or granules which are
optionally dried and cooled, the said improvement comp-
rising the addition of at least one water soluble aluminium
salt to the said solution of ammonium nitrate prior to
the completion of the concentration stage.
The said process may be carried out batchwise,
continuously or semicontinuously.
J The actual aluminium compound present in the
ammonium nitrate solution, concentrate, prill or granule,
depending on the stage of the process, is difficult to
identi~y. However, it has been found that the benefits
already described as accruing from the present invention
may be achieved by adding a water-soluble aluminium salt
to the said solution of ammonium nitrate. However, as it
is undesirable to have halides present, even in small
amounts, in ammonium nitrate manu~acturing processes,
aluminium halides are specifically excludea ~rom our
invention. Suitable water soluble aluminium salts may be
selected from the sulphates such as aluminium sulphate,
and soaium aluminium sulphate - preferably aluminium
sulphate is used. Aluminium nitrate is also suitable.
It is preferred to add the water soluble aluminium
salt as an aqueous solution. For example, a convenient
form of solution to use is th~t commonly employed in water
treatment, i e~ an aqueous solution of aluminium sulphate
containing 7.5% o~ aluminium sulphate expressed as Al203
and a free sulphuric acid content of 0.5% H2S04 both
expressed on a w/w basis.
~he process o~ our invention involves the addition
of water soluble aluminium salts so that the final product
prills contain at least 100 parts by weight o~ aluminium
compounds (expressed as Al203) per million parts by weight
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of ammonium nitrate. Preferably the level of addition is
between 800 and 2000 parts by waight per million parts
by weight o~ ammonium nitrate.
It is preferred in the present invention to derive
only part of the desired content of aluminium compounds
in the prilled or granulated material by addition of an
aluminium salt to the solution of ammonium nitrate prior
to evaporation. The remainder required to give the desired
content of aluminium compounds in the prilled or granulated
products may be added at any convenient stage in the
process stream after the evaporation stage. It is pre-
ferred that at least 20% by weight of the total weight of
aluminium added is added to the process reactant stream
prior to the concentrate being formed.
Accordingly there is provided in a preferred
embodiment of the present invention a process for the
manufacture of prilled or granulated ammonium nitrate,
as hereinbefore described, wherein, in a first addition,
at least one water soluble aluminium salt is added to the
said solution of ammonium nitrate prior to the said
concentration being formed and wherein, in a second
addition, at least one water soluble aluminium salt i9
added to the said process stream after said concentrate
has been formed.
Conveniently the water soluble aluminium salt added
in the second addition may be selected from the following
group:
Aluminium sulphate
Aluminium nitrate
Double Aluminium salts, such a~:
Ammonium aluminium sulphate
Sodium aluminium sulphate
Potassium aluminium sulphate
Aluminates, such as:
Sodium aluminate
Potassium aluminate
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Preferably aluminates are used because they do not
depress the pH value of the concentrate when they are
addea to it and thus the amount of ammonia which has to
be added to the concentrate to adjust its pH value to the
5 desired value for the prilling or granulating process is
reduced.
- It is generally more convenient to add the aluminium
salt in the second addition as an aqueous sQlution but in
some circumstances, such as in the manufacture of high
; 10 densi~y prills, it is better to add the ~alts as solids
because less additional water is introduced thereby.
~he process of the invention may be carried out
such that the pH value in the solution of ammonium nitrate
formed by the reaction of ammonia and nitric acid is in a
15 range from 4.2 to 5.5 inclusive. Preferably it is in a
range from 4.6 to 5.0 inclusive.
In another preferred embodiment of the invention
ammonium nitrate prills or granules are manufactured by
a continuous process wherein the solution of ammonium
20 nitrate is made continously by the reaction of ammonia
and nitric acid in a preformed solution of ammonium nitrate
and wherein a water soluble aluminium salt is added
continuously to the ammonium nitrate solution before it is
concentrated to the degree th~t it is suitable for prilling
25 or granulating.
In this preferred embodiment involving a continuous
process the rates of addition of ammonia and nitric acid
are controlled so that the pH value of the solution of
ammonium nitrate is maintained-in the range of 4.2 to 5.5
30 inclusive, preferably in the range of 4~6 to 5.0 inclusive.
The term "continuous" and its derivatives are used
in this specification to identify the mode of operation
a~ being distinct from "batchwise".
Although the subsequent benefits in respect to
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drying will be obtained if the aluminium is added at any
stage before the concentrate,i.e. a solution or melt
containing not mo~e than 6~o w/w Of water,is formed, the
benefits, hereinbefore described, will only be enjoyed to
the full if the addition is made during the reaction stage.
Therefore it is a preferred feature of this invention
that the pre-evaporator addition of the aluminium compound
is made to the neutraliser in which the nitric acid is
being re-acted with ammonia. It is preferred that the
addition is made to the neutraliser so that the aluminium
~ compounds are introduced into a reactant medium of pH
;~ between 4.2 and 5.5, more preferably between 4~6 and 5Ø
When the process of the invention is carried out in
the preferred continuous mode the water soluble aluminium
salt may be added continuously to the neutraliser con-
currently with the addition of the nitric acid and ammonia.
Prèferably the aluminium salt is added direct to the
preformed solution of ammonium nitrate in an amount which
will cause the solution of ammonium nitrate tc have a
content of aluminium compounds-in an amount in the range
from 100 to 2000 parts by weight, expressed as Al203, per
million parts of ammonium nitrate, expressed as NH4N03,
contained in the said solution.
In some circumstances, particularly in manufacturing
complexes in which a number of different products are
being manufactured from a common stream of ammonium nitrate
solution, it is more convenient to add the aluminium salt
to a holding tank of ammonium nitrate solution which is
destined to be used in the process of this invention.
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g
our invention is now illustrated by, but is in no
- way limited to, the following examples 1,2,5,8, and 9
w'nerein all parts and percentage~ expres~ed ~n a weight
basis unless otherwise specified,
Example 1
This illustrates the process of the present invention
as it was used for the manufacture of low density prills~
Ammonium nitrate solution containing about 85% ammonium
nitrate was made in a neutraliser by continuously adding
10 nitric acid and gaseous ammonia to a preformed ammonium
nitrate solution, Simultaneously aluminium sulphate,
in the form of an aqueous solution prepared by dis-
solving 50 parts of Al2 ~S04) 3,18HzO in 50 parts water,
was added to the ammonium nitrate solution in the neutral-
15 iser in the proportion of 280 parts of aluminium sulphate(expressed as Al203) per million parts of ammonium nitrate
formed (expressed as ~H4~03), The level of ammonia
addition was controlled so that the pH of the solution in
the neutraliser was maintained at a value between 5,2 and
20 5,4.
The solution from the neutraliser was concentrated
in a falling film evaporatar to form a concentrate con-
taining 95% ammonium nitrate, The pH value of the solu-
tion fell during this evaporation process to 3,9 Gaseous
25 ammonia was added to adjust the pH value of the concentrate
in a range from 4,2 to 4,6, The concentrate was then
prilled in a conventional prilling tower,
.The prills were then dried to give a product having
a moisture content in the range from 0,10 to O,lS% H2),
30 as measured by the conventional Karl Fischer Test, The
drying proce9s wa9 carried out in a conventional drying
system comprising three rotating drying drums in series,
The residence time in the drying system was 58 minutes
and the amount of drying air used was 4,1 kg air per
35 1,0 kg of product in the form of prilled ammonium nitrate,
,~
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Example 2
This example illustrates a further process of the
present invention which was used to manufacture low
density ammonium nitrate prills. A solu~ion containing
85~ ammonium nitrate was prepared in a similar manner to
that used in example 1 except that aluminium sulphate in
the form of an aqueous solution of aluminium sulphate~
containing 7.5% of aluminium sulphate expressed as A1203
and a free sulphuric acid content of 0.5~ H2S04 both ex-
pressed on a w/w basis, was added to the ammonium nitratesolution in the neutraliser in the proportion of 600 par~s
of aluminium sulphate (expressed as A1203) per million
parts of ammonium nitrate formed (expressed as ~H4N03).
Ammonia was added so that the pH value of the ammonium
nitrate solution in the neutraliser was in the range from
4.7 to 4.9.
The solution was evaporated to form a concentrate
containing 95% ammonium nitrate as in example 1, and a
further 600 parts of aluminium sulphate (expressed as
A1203) per million parts of ammonium nitrate present were
added to the concentrate as a solution of the same con-
centration of aluminium sulphate as that added to the
neutraliser. Gaseous ammonia was added to the concentrate
prior to the addition of the aluminium sulphate so that
the pH value o~ the concentrate after the addition of the
aluminium salt was at lea~t 4.2.
The concentrate was then prilled in a conventional
prilling tower and the product prills were dried to a
moisture content between 0.09 and 0.13% H20 as determined
by the Karl Fischer test. The drying system used was the
same a~ that used in Example 1. The residence time in the
drying system was 58 minutes and the amount of drying
air used was 3.9 kg air per 1.0 kg of product.
Example 3
By way of comparison this example illustrates the
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process of manufacturing low density ammoni~m nitrate
prills containing aluminium compounds by a process in
which an aluminium salt was added only to the concentrate.
A solution containing 83% ammonium nitrate was prepared in
S a similar manner to that used in Example 1, but no aluminium
sulphate was added. The minimum level of gaseous ammonia
addition to give stable pH control was found to be that
which produced a solution having a pH value in the range
5.2 to 5.4.
0 J The solution was concentrated to give a concentrate
containin~ 95% ammonium nitrate and having a pH value of
3.9. On the addition of 1000 parts of aluminium sulphate
(expre~sed as A1203) per million parts of ammonium nitrate
(expressed as NH4N03) in the concentrate, the pH value
dropped to below 2Ø Gaseous ammonia was added to in-
crease the pH value of the concentrate to a suitable value
before it was prilled under conventional prilling conditions.
The difficulty in drying the prills made in the
manner just described, which is not according to the present
20 invention, is shown by the fact that the product prills
could not be dried below a moisture content, as determined
by the Karl Fischer method, in the range 0.16~ to 0.30%
H20 using the same drying system as was used in examplès
1 and 2 e~en with the maximum amount of drying air
available, namely 4.5 kg air per 1.0 kg product.
ExamPle 4
As a comparison this example illustrates the process
of manufacturing low density ammonium nitrate prills with-
out the addition of aluminium at any stage of the process.
The steps of neutralising, evaporating, prilling and drying
described in example 1 were followed, but at no stage was
any aluminium compound added. The pH values at various
stages of the process were
stable pH in neutraliser = 5.1 - 5.3
pH after evaporation = 3.9 - 4.1
pH at prilling = 4.5 _ 4 7
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Product prills containing 0.11 to 0.13% H20 as
determined by the KarlFischer method were produced using
the sa~e drying system as was used in examples 1 to 3 but
with operating conditions altered so that the residence
5 time of the prills in the drying system was 84 minutes and
the amount of drying air used was 6.2 kg air per 1.0 kg
of product.
Samples of the prills made according to the
processes described in examples 1 to 4 were tested to
lO measure their quality using the following tests:
The hardness of the prills was compared by the
following friability test:
It will be appreciated that hardness varies
inversely with the friability, i.e. a highly friable
material has a low hardness.
An air stream from 11 psig air supply was expanded
through a 2.5 mm orifice into a tube of 13 mm internal
diameter, which opened into a standard cast iron water
pipe elbow of 25 mm internal diameter. Prills which were
20 retained by a No 16BSS sieve were dropped into the air
stream 184 mm upstream from the elbow. After they had
impinged against the elbow, the prills were collected
and the percentage by weight which passed through a No 16
BSS sieve was determined.
The temperature cycling tests were conducted as
follows;
Polyethylene sample bags partly fitted with prill~
which were retained on a No 16 BSS sieve were temperature
cycled 15C to 45C in six hours by placing them in an
oven having a suitable heating and cooling mechanism
to achieve such a temperature pattern. This cycle was re-
peated four times, so that the prills passed 8 times
through this 32C transition temperature.
Bulk density is the mass of prills per unit volume
of a con~ainer which has been filled and levelled without
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_ 13 -
pressure or vibration.
The moisture in the prills was measured by the
conventional Karl Fischer test.
The results of these tests on products from
examples 1 to 4 are given in Table 1.
TABLE 1
PRODUCT OUALITY TESTS ON LOW DENSITY AMMONIUM NITRAT,E PRILLS
' .............................. . . . . I ,
Sample from example 1 2 3 4
, . _, ' .... .... ~_,
Moisture content % H20 0.10 0.09 0.160.12
Bulk density (kg/l) 0.75 0.72 0.72o.76
pH 5.4 5.0 4'9 5.2
No 16 BSS mesh) 2.0 o.6 5.415.0
.... . _
After temPerature cyclin~
Bulk density (kg/l) o.66 0.68 o.66 0.62
Friab 1 ty (% passing 30 19 31 50
ExamPle S
This example illustrates a process according to the
invention as it was used to manufacture high density prills.
A solution containing between 83% and 85% ammonium nitrate
was prepared in a similar manner to that described in
example a except that the level of addition of aluminium
sulphate was 1000 parts (expressed as A1203) per million
parts of ammonium nitrate (expressed as NH4N03). Ammonia
was added to give stable pH control in the range 4.6 -
5Ø
The solution was converted to a melt by evaporation
so that the water content was less than o.o8% H20. 200
parts of aluminium sulphate (expressed as A1203) per
million parts of ammonium nitrate were added to the melt.
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The pH value of a lO~o solution of the melt was
adjusted to a value in the range of 4.4 to 4.8 by addition
of gaseous ammonia to the melt. The melt was then prilled
in a conventional prilling tower and cooled by a con-
ventional cooling process. The bulk density of the'productwas 0.95 kgjl, its pH value was 5.0, the friability was
0.4%, and moisture content between 0.12% and 0.18% H20,
as determined by the tests previously described.
Example 6
-~ By way of comparison a process not according to
our invention by which high density prills were made is
now described. A solution containing between 83~ and 85%
ammonium ni~rate was prepared in a similar manner to that
described in Example 3. Again the minimum stable pH value
obtained in the neutraliser was in the range 5.2 to 5.4.
Th,e solution was converted to a melt, prilled and
cooled, in a similar manner to that described in Example
5 except that 500 parts of aluminium sulphate ~expressed
as the A1203) per million parts of ammonium nitrate were
added to the melt.
The bulk density of the product was 0.95 kg/l, its
pH value was 5.0, the friability was 0.9~ and moisture
content was 0.22% to 0.26% H20, as determined by the
, tests previously described.
ExamPle 7
By way of comparison a process for making high
density ammonium ni$rate prills without any aluminium
addltion is described. High density prills were made
according to Example 6 except that no aluminium sulphate
was added. The bulk density of the product was 0.95 kg/l,
lts pH value was 5.6, the friability 3.5% and the moisture
content between 0. o8% and 0.12% H20, as determined by
the tests previously described.
Samples from the products of examples 5, 6 and
7 were ~ubjected to the temperature cycling test
described be~ore. The results are recorded in Table 2.
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TABLE 2
TE~oeER~TURE CYCLING TESTS ON HIGH DENSITY AMMiONIUM l!iiITRATE
PRILLS
. . . . _
Sample from example 5 6 7
. _
Moisture content % H~0 0.15 0,24 0.10
Bulk density after cycling kg/l 0.87 0.84 0.80 ' :~
Friability after cycling
(% passing No. 16 BSS mesh) 12 28 4S .
ExamPle 8
In order to exemplify an emboaiment of th~ process
of the invention in which sodium aluminate is added in
the second addition an ammonium nitrate concantrate was,
prepared in a similar manner to that described in
Example 2, Sodium aluminate was added to the concentrate
in an amount sufficient to provide 1200 parts by weight '~
of aluminium compounds (expressed as Al203) per million
10 parts by weight of ammonium nitrate, expressed as NH4~03,
contained in the concentrate. The pH value of the con-
centrate on the addition of the sodium aluminate rose
to a value in the range from 4.0 to ~.0 thereby eliminating
the need for any further ammonia addition to the con-
15 centrate before prilling,ExamPle 9
This example illustrates the application of the process
of the invention on a manufacturing complex in which severa~
ammonium nitrate containing products are made from one
20 common process ~tream made by the reaction of ammonia
and nitric acid, In the manufacture of low density prills
ofammonium nitrate containing 1200 parts by weight of
aluminium compounds (expressed as Al203) per million parts
by weight of ammonium nitrate product an ammonium nitrate
25 solution is made according to the process of Bxample 3.
. : . ,.. : ,.. . ..
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A portion of this solution is subjected to a preliminary
concentration process involving evaporation so that its
concentration is increased to 92~o w/w ammonium nitrate.
The partially concentrated solution is stored at elevated
temperature to prevent crystallisation. It is sparged
with ammonia to prevent its pH value falling below 4.5.
Its pH value varies within the range of 4.5 to 7Ø
~he partially concentrated solution is transferred to a
dosing tank. During this transfer its pH value is adjusted
to 4.5 by addition of nitric acid, if necessary. Aqueous
aluminium sulphate solution, as described in Example 1,
is added to the solution in the dosing tank to provide
720 parts by weight of aluminium compounds (expressed
as Al203) per million parts by weight of ammonium nitrate,
(expressed as NH4N03) in the partially concentrated solut-
ion. The pH value of the solution in the dosing tank
is maintained between 5.2 and 5.7 by sparging in ammonia.
Solution from the dosing tank is then concentrated
by evaporation to yield a concentrate having a fudge
point of 118C, equivalent to a concentration of 94.8%
ammonium nitrate.
Aluminium sulphate solution, as described in Example
1, is added to the concentrate to bring the total content
of aluminium compounds to the desired level of 1200ppm
on the basis already defined. The concentrate is then
prilled to give the desired product.
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