Note: Descriptions are shown in the official language in which they were submitted.
~L~18049~
This invention relates to the production of slurries
for mixed fertilizer manufacture, particularly fertilizers con-
taining nitrogen plus phosphorus.
The invention is more especially concerned with a pro- '
cess for producing a slurry suitable for the manufacture of a
mixed fertilizer containing nitrogen plus phosphorus and
possibly potassiuml by leaching crude phosphate with nitric
acid to produce an acid slurry, which is thereafter neutralized
with ammonia and to which some other acid is added.
The source of phosphorus commonly used in the manu-
facture of mixed fertilizers containing nitrogen, phosphorus,
and potassium, i.e., so called ~PK fertilizers, lS a crude
phosphate, e.g., apatite. The aim is to convert the phosphorus
present in apatite to such a form that it is available to plants.
This occurs when nitric acid is caused to react with crude
phosphate, whereby phosphorus is released as phosphoric acid
In the successive stages the produced acid mixture and the fed
additional acids, phosphoric acid and'possibly sulfuric acid, ~ ;
are neutralized to the desired degree by means of ammonia. '
Apatite contains per one P205 molecule an average of
3.35 CaO molecules, of which 3 are combined with P205 and 0.35 '
with fluorine and other negative anions. Such a compou'nd forms
an apatite molecule, in which the P205 is not available' to
plants. In the process the apa~ite molecule is dissociated by
means of nitric acid, and thereby the P205'is obtained in a
soluble or ionizable form.
A slight excess of nitric acid is used in the reactions
between apatite and nitric acid, and thereby a short reaction
period and a high efficiency are achieved. With apatite, 6.8
mol nitric acid is used per one apatite molecule
On ammonization of the mixture, ammonium nitrate is
thus obtained, the P205 changes into dicalcium phosphate
gg
(P20$, 2 CaO) through monocalcium phosphate (P205, CaO), and
part of the calcium remains in calcium nitrate Ca(NO3)2,
Reaction between apatite and nitric acid:
P205 3-35 CaO + 6-8 HNO3 -~ P205 + 3-35 Ca(N3)2 + 0-1 HNO3
~ eutralization reaction:
P205 ~ 3.35 Ca(~03)2 + 0.1 H~O3 + 4.1 NH3 -~ P205 . 2CaO +
1.35 Ca(~03)2 + 4.1 ~H4MO
The calcium nitrate, Ca(~03)2, must be removed since
in a fertilizer it would cause insurmountable difficulties owing
to its highly hygroscopic nature.
In the Odda process (Farm Chemicals, May 1967, p. 29)
part of the calcium derived from the leaching of crude phosphate
i9 removed by crystallizing it as calcium nitrate tetrahydrate.
This process has, however, drawbacks, i.e., the expensive
crystallization and the great number of by-products which are
difficult to market.
In another process, the PEC process (Farm Chemicals,
May 1967, pa 28), calcium is precipitated by means of some acid,
e~g./ phosphoric acid, in numerous ammonization tanks. A draw-
back of this process i9 the great number of ammonization tanks r
needed for raising the pH of the nitrophosphate slurry by
degrees to a value suitable for the manufacture of ~ixed fertilizers.
Also known are different processes in which phosphoric
acid is added together with nitric acid to the crude phosphate
leaching stage, whereby phosphoric acid is used instead of part
of the phosphorus in the crude phosphate. When nitric acïd
reacts with crude phosphate, phosphoric acid and calcium nitrate,
among other things, are formed which, together-with the phos-
phoric acid added, are thereafter neutralized with ammonia~ ~
These processes usually comprise several neturalization stages, ;
or part of the neutralization is performed in a granulator,
for th~ slurry greatly tends to thicken.
iC913~499
Now, surprisingly, it has been found that the a~ove
drawbacks can be eliminated and ~he number of neutralization
stages simultaneously reduced to two by feeding at least appro~.
6~/o of the phosphoric acid to the second neutralization stage, at
which the pH is already so high (5.6-6.0) that the slurry cannot
thicken. In addition, a good stability is obtained in the re-
actors.
By the process according to the present invention the
phosphorus present in a crude phosphate, e.g., apatite, can be
converted to a form available to plants. The invention,relates
to a process for producing a slurry suitable for the manufacture
of a mixed fertilizer containing nitrogen plus phosphorus from a
crude phosphate which comprises: leaching the crude phosphate with
nitric acid to produce an acid slurry and neutralizing the acid
slurry with ammonia under simultaneous cooling, in two stages
in a first and a second neutralization zone. Phosphoric acid is
added to the acid slurry in the two stages, to reduce the amount
of crude phosphate required to sstisfy the phosphorus requirement
of the mixed fertilizer.
In the first neutralization stage the phosphoric acid
is added in an amount of at most 4~/0 of the total added phos-
phoric acid, and the acidity o~ the slurry is maintained at a pH
from 2.5 to 4.5. The remainder of the phosphoric acid is added
in the secon~ neutralization stage at a pH of 5 to 6. The slurry
is cooled during the neturalization stages, preferably to the
temperature range of 110 to 130C.
The addition of nitric acid to the crude phosphorus,
for example, apitite, releases the phosphorus as phosphoric
acid. Part of the phosphorus can also be added directly as phos-
phoric acid in a manner known ~ se, It has now been realized
that the thickening of the slurry caused by the amount of
phosphorus entering the neutralization is eliminated by replacing
part of the crude phosphate by phosphoric acid, at least about
6~o by weight, of which is not added until the second
neutralization stage, where the pH is already so high that the
slurry no longer thickens.
In order to obtain water-soluble phosphorus, phosphoric
acid and possibly sulfuric acid are added in excess to the
neutralization stages, and the advantageous concentration of
the phosphoric acid is approx. 30-50% by weight P S~
During the ammonization, phosphoric acid and possibly
.
sulfuric acid are thus added, whereby calcium phosphate or
gypsum is produced at the expense of calcium nitrate~
The total reactions can be expressed as follows:
P2O5 . 3.35 CaO + 6.8 HNO3 * 6.8 NH3 + 1.35 H
1.675 ~P2O5 . 2CaO) * 6.8 ~H4~O
P2O5 . 3.35 CaO + 6.8 HNO3 + 6.8 NH3 + 1-35 H2S04 ~ r
P2O5 . 2CaO f 1.35 CaSO4 + 6.8 ~H4~O3
The phosphorus obtained in this manner is not solùble
in water but in ammonium citrate, in which case it is already
available to plants. To obtain water soluble phosphorus, the
- phosphorus should be in the fertilizer in the form of ammonium
phosphate. This is achieved by using phosphoric acid and
sulfuric acid in excessO The ammonization of phosphorus is
usually performed to a point between mono- and diammonium ~
phosphate.
When H3PO4 with a concentration of 30-50% P 05 is used -
in excess
P205 . 3.35 CaO ~ 6.8 H~O3 + 11.825 ~lH3 + 4.7 ~ PO ~ ~`
1.675 (P205 . 2CaO) + 1.675 (P205 . 3~3) + 6-8 ~H4~O3
is obtained.-
~-
When H2S04 is used in excess
P2O5 3.35 CaO + 6.8 HNO3 + 8.3 NH3 + 2-35 H2S4 --t
2 5 ) .5 (P205 3NH3) + 2.35 CaS04 ~ 6.8 ~H ~O
is obtained.
When apatite is treated with nitric acid, the fluorine
present in the apatite is released in the form of hydrofluoride
HF. On ammonization the part which has remained in solution
is converted to calcium fluoride and remains in the fertiliz~r.
CaF2 + 2~03 ~ Ca(NO3)
Ca(N3)2 ~ 2HF + 2~H3 ~~~ Ca~2 ~ 2NH4~O3
The invention is described below in more detail with
reference to the enclosed drawing which diagrammatically
depicts the apparatus intended for the application of the
process according to the invention.
In the figure, A indicates the leaching reactor, B the
first neutrali~ation reactor, C the second neutralization
reactor, and D and E the washers.
Crude pho~phate is fed into the leaching reactor A,
provided with a mixer, through the pipe 1 and nitric acid
through the pipe 20 When the nitric acid reacts with the
crude phosphate, an acid phosphoric-acid-bearing slurry is
produced, the temperature of which is adjusted to approx.
50-80C, preferably 70C. From the leaching reactor A the
acid slurry is caused to flow through the pipe 9 into the
first neutralization reactor B, into which part of the
phosphoric acid i9 simultaneously fed through the pipe 3 and
part or all of the sulfuric acid along the pipe 12. The
acid slurry and the added acids are neutralized to pH 2.5-405
with ammonia, which is fed into the reactor B through several
nozzles 4. The reactor B is cooled to 110-130C.
From the reactor B the slurry is fed along the pipe 10
into the second neutralization reactor C, into which the rest
~8~9~
of the phosphoric acid and sulfuric acid is fed along the
pipes 3 and 12, and the slurry and the acids are neutralized
to pH 5.0-6.0 by feeding ammonia through several nozzles 7
into the reactor. The temperature in this second neutraliza-
tion reactor C is adjusted to the same range as in the first
neutralization reactor B. Finally the slurry is removed from
the reactor C along the overflow pipe 11. At this time the
water content in the slurry is approx. 16-26~, usually 18%,
and the slurry is directed to a system for further processing,
e.g., granulation and drying.
The potassium salt and the trace elements are usually
added to the second neutralization stage C along the pipe 14.
The gases generated in the leaching reactor A are directed
along the pipe 6 to the washer D and the washing water emerging
from the washer D is directed along the pipe 6' to the first
neutralization stage B, and the gases emerging from this
neutralization reactor B are fed along the pipe 13 into the
same cycle. The gases generated in the neutralization reactor
C are directed along the pipe 5 to the washer E and the
washing water from the washer E is returned to the reactor
C along the pipe 5'.
At maximun 40~, but usually approx. 10~, of the total
phosphoric acid and some sulfuric acid are fed into the
first neutralization reactor B to be mixed with the slurry
arriving there as an overflow from the leachinq reactor. The
obtained mixture is neutralized with ammonia to pH 3.3-3.8,
preferably 3.5. The ammonia is fed into the reactor through
four pipes.
The ammonization reactors ~enerate a great amount o
heat and therefore the reactor temperature rises considerably.
The temperature is adjusted by means of coolin~ water which
is fed to the mantle-cooling coil and by means of a sufficient
feed of the gas-washing water. The aim is to maintain the
reactor temperature at approx. 110C. A raise in the tempera-
ture results in losses of ammonia and fluorine and causes
a decrease in the soluble phosphorus amount; it also affects
the corrosion resistance of the reactor.
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The reactor is provided with a powerful mixer which
maintains the slurry in swift motion~ In general the slurry
does not tend to thicken in this reactor but in the next one. ~
It has been noted, however, that it is the operation conditions, ~;
the pH and phosphoric acid batching, in the first neutralization
reactor that have an effect on the conditions in the second
. . .~,
neutralization reactor. Especially the pH range is very import-
ant, and deviations must not be made from the pH range 3.3~3~8.
Potassium, dust, and trace elements can be fed into
the first neutralization reactor if all of them cannot be
fed into the second one.
In the second neutralization reactor C the rest of raw
materials, such as the rest of the phosphoric acid (60-95%),
possibly part of the sulfuric acid, depending on the formula,
potassium, trace elements, dust, and gas-washing waters, are
added, and the mixture is neutralized with ammoniaD A great
deal of heat is generated by the reaction, and the temperature
is adjusted, as in the first neutralization reaction, by
means of cooling water and gas-washing water. The temperature
in the reactor is maintained at approx. 110C, the pH within the
ranga 5.5-5.8, and the water content at 16-26%. The reactor has
been provided with a similar powerful mixer as the first
neutralization reactor. The volume of the reactor is 25 m and
the delay period 1 hour.
The invention is described below in more detail with
the aid of examples.
Example 1
Raw materials
- apatite 39% P205
- nitric acid 60-65% HNG3
- phosphoric acid 40-50~0 P205
- ammonia (gas)
- potassium salt 6~/o K20
~ race element fertilizer borate
~o~9~
When manufacturing a 15-20-15 = N:P205: K20 fertilizer ~;
at the rate of 42 tons/hour, 118 kg of apatite was fed into
the leaching reactor along the pipe 1. 253 kg of nitric acid
was fed into the same reactor along the pipe 2, The tempera-
ture in the leaching reactor was 50C.
The acid slurry was directed thereafter to the first
ammonization reactor B, into which 10% of the phosphoric acid
was fed along the pi~e 3, and gaseous ammonia was fed along '~
the pipe 4 through several nozzles to the bottom of the
reactor so that the pH of the slurry rose to 3.5. .
By extraneous cooling and by the evaporation of water
the temperature in the reactor B was maintained at 125C.
The gas-washing waters from the washer D were also fed into
it.
From the reactor B the slurry flowed into the second
ammonization reactor C. The rest of the phosphoric acid was
fed into it along the pipe 3. The total consumption of
phosphoric acid was 160 kg~ Sulfuric acid was not used.
Ammonia was fed through several nozzles (pipe 7) to the bottom
of the reactor so that the pH of the slurry in the reactor ;
C rose to 5.5. Gas-washing waters from the washer E were fed -
into the reactor.The total ammonia consumption of the ammoniza-
tion reactors B and C was 114 kg.
The reactor temperature was 125C. 260 kg of a potassium
salt was also fed into it along the pipe 8 and 2.2 kg of ~ ;
fertilizer borate.
The raw material quantities have been based on one ton.
The product was an NPK slurry which had a water content
of 18~ and was suitable for further processing by conventional
granulation methods.
Example 2
In the example the crude phosphate was morocco phosphate
and ~rothing was checked by derothers.
9 ~ )499
Raw materials
- morocco phosphate 34% 2 5
- nitric acid 40-65% HN03
- phosphoric acid 40-50% P205
- sulfuric acid 93% H2S04
- ammonia gas "
- potassium ~alt 60% K20
- magnesium sulfate and fertilizer borate as trace elements
The product was of the type ~ : P205 : K20 = 20:10:10
Morocco phosphate was fed into the reactor 1, i.e., the
leaching reactor; at the rate of 62 kg/h, nitric acid at
608 kg~h, ~nd additional water at 0.6 kg/h.
The reactor temperature was 67C, and thereafter the
acid slurry was fed into the first neutralization reactor,
into which ammonia and 10% of the phosphoric acid were fed,
so that the p~ of the slurry rose to 3.3. The temperature
in the reactor was 110C.
From the first neutralization reactor the slurry was
directed to the second neutralization reactor, into which the
rest of the ammonia and 90% of the phosphoric acid were fed.
The pH of the siurry was thereby raised to 5.6. The tempera-
ture was maintained at 110C by means of a control system
(water jacket). The water content in the slurry was 18%.
Sulfuric acid at a rate of 78 kg~h, potassium salt at
167 kg~h, and the trace elements (magnesium sulfate at
43 kgfh and fertilizer borate at 2.2 kg/h) were also fed
into the second neutralization reactor.
Gàs-washing waters were fed into both neutralization
reactors.
The product with a water content of 18% was fed into a
granulator and a high-~uality 20-10-10 granulated fertilizer
was there~y obtained.
. ~ . : :.i -, ,. ,~ . : -- -
499
Example 3
This is a reference example in which the conditions
were not within the range delimited by the invention.
Raw materials:
- apatite 39% P205
- nitric acid 60-65% HN0
- phosphoric acid 40-50% P205
- sulfuric acid 93~ 2 4
- ammonia gas
- potassium salt 60% K20
Apatite at the rate of 12 kg/h and nitric acid at
45.8 kg/h were fed into the reactor 1, i.e., the leaching
reactor. The reactor temperature was 66C. The acid slurry
from the reactor 1 was fed thereafter into the first
neutralization reactor, into which 50% of the phosphoric acid
and some ammonia were fed so that the pH of the slurry rose
to approx. four. The reactor temperature was 104C.
Thereafter the slurry was fed into the second neutraliza-
tion reactor, into which the rest, lOe., 50%, of the phosphoric
acid, sulfuric acid at 2.2 k~/h, potassium salt at 25.1 kg/h,
and ammonia were added so that the pH rose to 5.6.
In this case water had to be added in lar~e quantities
into the neutralization reactors since the slurry tended
to solidify drastically, especially in the second neutraliza-
tion reactor. Although the water content was as hiqh as
30%, the flow of the slurry was very poor or nil, and the
process could not be applied to the manufacture of industrial
fertilizers.
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