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ABSTRACT
The process of producing NP-fertilizers by nitric-sulfuric acid decomposition of phosphate rock (17.20% P2O5) under
thickening slurry conditions has been studied. It was shown that the higher the recycle ratio, the lower the moisture
content of the final product. Increasing the HNO3 dosage above 30% and H2SO4 above 70% is undesirable, as the
product's moisture content exceeds 6.03% and it has increased acidity. To avoid issues during ammoniation, an optimal
rate of nitric and sulfuric acid was proposed
—
73.5-91%, which produced products containing 9.34-10.61% -P2O5total;
3.04-4.54% N; P2O5 digestive:P2O5 total = 73.4-82.3%; pH = 3.3-3.6, and granule strength of 2.1-2.3 MPa. The process of
producing NPK-fertilizers was also studied by adding nitrogen components such as K2SO4, KCl, and K2CO3 to the
products of complete nitric acid decomposition of washed and dried concentrate (24.10% P2O5) and fine fractions
(18.55% P2O5). The process includes decarbonization and decomposition of the phosphate raw material
(simultaneously in one apparatus), neutralization to a pH not lower than 3.5, evaporation of the slurry to a density of
at least 1750 kg/m3, the addition of potassium salts at N:P2O5:K2O ratios of 1:1:1, 1.5:1:1, and 1:1.5:1, product granulation,
drying in the presence of recycle, and cooling.
KEYWORDS
Research Article
IMPROVEMENT OF THE PRODUCT COMPOSITION FROM NITRIC ACID
PROCESSING OF KYZYLKUM CARBONATE PHOSPHORITES
Submission Date:
November 06, 2024,
Accepted Date:
November 11, 2024,
Published Date:
November 16, 2024
Crossref doi:
https://doi.org/10.37547/ajast/Volume04Issue11-08
Ayjamal Allamuratova
Associate Professor, Karakalpak State University, Named After Berdakh, Uzbekistan
Aktam Erkaev
Professor, Tashkent Institute Of Chemical Technology, Uzbekistan
Akhmed Reymov
Professor, Karakalpak State University. Named After Berdakh, Uzbekistan
Kazakhbaev Saparbay
Basic Doctoral Student, Karakalpak State University. Named After Berdakh, Uzbekistan
Journal
Website:
https://theusajournals.
com/index.php/ajast
Copyright:
Original
content from this work
may be used under the
terms of the creative
commons
attributes
4.0 licence.
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Phosphorites of Central Kyzylkum and Karakalpakstan, inorganic salts, activation, complex NPK fertilizers,
physicochemical analysis methods.
INTRODUCTION
Since 2005, a pilot-industrial unit for the production of
nitrocalcium phosphate fertilizer (NPF) has been
operating at OAO "Samarqandkimyo" [1]. The
production process is based on the decomposition of
unbeneficiated phosphate rock from the Kyzylkum
deposit (mass %): P2O5 17.20; CaO 46.22; CaO : P2O5 =
2.96; Al2O3 1.24; Fe2O3 1.05; MgO 1.75; F 2.00; CO2 16.0;
Cl 0.1 with 57% nitric acid (HNO3) in the range of 55-65%
of stoichiometry for binding all the CaO in the
phosphate, followed by neutralization of the free
acidity of the nitrocalcium phosphate slurry with
gaseous ammonia, evaporation, granulation, and
drying of the final product [2-5]. With incomplete HNO3
dosage, the overall reaction is as follows:
2Са5(PO4)3F + 10HNО3 = Са(Н2РО4)2 + 4СаНРО4 + 5Са(NO3)2 + 2HF
According to this reaction, the final product consists of
mono- and dicalcium phosphate salts mixed with
calcium nitrate in various degrees of hydration. The
latter depends on the crystallization conditions of
calcium nitrate from the slurry during ammoniation,
evaporation, and drying. The moisture content of the
initial slurry is 37% H2O, while the evaporated slurry has
25% H2O. The temperature of the initial slurry is 40°C,
and the evaporated slurry is at 80°C. The slurry
evaporated in contact-type apparatus contains 8-9%
P2O5, 5-6% N, and has a density of 1.7-1.8 t/m³. Due to
the combination of evaporation and neutralization of
the residual acidity in the slurry with gaseous ammonia
in a single technological unit, the pH of the slurry
during evaporation increases and fluctuates between
3.5-4.0. The product obtained from such slurry consists
of dicalcium phosphate (20-25%), monocalcium
phosphate (10-15%), and calcium nitrate (50-55%) in
terms of anhydrous salt forms. Fertilizer of this
composition is undoubtedly of great agrochemical
interest [6].
Additionally, maintaining such a ratio of water- and
citrate-soluble forms of P2O5 and CaO in the reaction
mixture throughout the entire production cycle allows
the processing of nitrophosphate slurry into finished
fertilizers without the need to separate calcium nitrate
[7]. The granulation and drying of NPF in this case can
be carried out using the well-known scheme for
producing ammonium phosphate with the use of drum
granulator dryer apparatuses [8].
It should be noted that NPF granules have very poor
physical properties: they are highly hygroscopic, sticky,
disperse poorly, and are unsuitable for mechanized soil
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application and fertilizer mixing [9]. Therefore, to
improve the physicochemical properties of NPF, we
proposed [10-12]: deep drying and instant cooling of
the product, the use of a large amount of fine product
fraction (granules smaller than 1 mm), and the
treatment and coating of granules with sulfate,
phosphate, and carbonate salts in the presence or
absence of organic substances (surfactants). These
proposed methods significantly improve the consumer
properties, particle size, and flowability. However,
during long-term storage and transportation of the
finished product, various secondary physicochemical
processes occur: moisture absorption from the air or
drying out, recrystallization due to hydration,
retrogradation,
dehydration,
polymorphic
transformations, and others.
To improve the properties of products from nitric acid
processing of carbonate phosphate raw materials,
various methods are used to remove excess calcium or
reduce the CaO:P2O5 ratio in the nitric acid slurry being
processed: 1) freezing (crystallization) of calcium
nitrate; 2) the introduction of additional amounts of
phosphoric acid; 3) precipitation of excess calcium with
sulfuric acid or ammonium sulfates; 4) precipitation of
excess calcium in the form of CaCO3 with carbon
dioxide and ammonia [13].
In the conditions of Uzbekistan, the most promising
and accessible method for improving the commercial
quality of NPF is the use of sulfuric acid to bind part of
the calcium in the nitric acid slurry into gypsum. In this
process, a target product can be obtained, containing
P2O5 partially or fully in water-soluble form. In this
case, phosphate raw material is decomposed using
nitric and sulfuric acids in a molar ratio.
In some countries, when producing nitrogen-
phosphorus fertilizer of the nitroammophosphate type
with a mass ratio of N : P2O5 = 1 : 1 and water-soluble
forms of P2O5 accounting for no less than 60% of the
total phosphorus content, the total consumption of
nitric and sulfuric acids is taken at 140% of the
stoichiometric amount to bind calcium and remove it
from the system as gypsum. In this case, the phosphate
is decomposed with nitric acid in the first two reactors,
with 60% of the total sulfuric acid being added to the
third and fourth reactors, and the remaining amount
dosed into the ammoniation reactor. The wet product
is then dried and granulated. However, to carry out this
process, high-quality phosphate raw material is
required, such as apatite concentrate from the Kola
Peninsula (39.5% P2O5, calcium modulus
—
i.e., the ratio
of CaO to P2O5
—
is 1,32) [13].
In contrast to the above method, we have developed a
process in which decarbonization and decomposition
of phosphate raw materials are carried out using low-
concentration nitric acid at various rates from the
stoichiometric amount needed to form calcium nitrate.
Both processes are conducted in a single apparatus,
with the acid being introduced into the reaction zone
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in equal portions over 2-4 minutes with active stirring.
Then, sulfuric acid is added to the resulting
nitrocalcium phosphate slurry, which facilitates further
decomposition of the phosphate raw material and
partial drying of the slurry. In this case, the liquid phase
content in the reaction slurry is 2-4 times less than that
in the developed NPF technology [2, 14]. As a result, the
decomposition of carbonate phosphate raw materials
does not result in excessive foaming or the release of
nitrogen oxides into the gas phase. The resulting thick
mass, or its mixture with neutralizing additives (such as
ammonia), is fed into a mixer where return material is
simultaneously added in the amount needed to
maintain a moisture content of 20-25%. Granulation of
the slurry is best performed in a screw or pan
granulator in the presence of return material.
METHODS
For laboratory studies, three types of phosphate raw
materials from the Kyzylkum deposit were used:
phosphate flour, washed dried concentrate, and fine
fraction. The fine fraction is produced during the
crushing and grinding of the ore to produce phosphate
flour and is currently not utilized. Given the acute
shortage of high-quality phosphate raw materials, we
believe that these materials should also be used in
agricultural production. The only requirement is to
convert the non-available P2O5 form into a form that is
accessible to plants. Kyzylkum phosphorites are
granular phosphorites, whose main mineral is
francolite. It has the unit cell parameters a₀ = 9.33A°, c₀
= 6.89A°, and contains 37% P2O5, 3.5% CO2, and up to 3%
SO3, which isomorphic enters its crystal structure.
The chemical formula of francolite (staffelite) is as
follows:
2Ca10P5.2C0.8O23.2F1.8OH
→
5.2P2O5 + 18.2CaO + 1.6CO2 + 1.8CaF2 + H2O
At the initial stage, we studied in laboratory conditions
the process of producing nitrogen-phosphorus
fertilizers through decarbonization and decomposition
of phosphate flour, the composition of which is shown
in Table 1, first with nitric acid, then sulfuric acid, with a
total acid norm ranging from 20.7% to 123% of the
stoichiometry for the formation of calcium nitrate.
Table 1
Chemical composition of various types of phosphorites from Central Kyzylkum
Name of Phosphate Raw
Materials
Content of components, mass. %
Р
2
О
5
СаО
Al
2
O
3
Fe
2
O
3
MgО
F
CO
2
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Phosphаte flour
17,20 46,30 1,23
1,04
1,70
1,98 15,5
Washed dried concentrate
24,10 46,40 0,34
0,47
0,93
2,44 9,5
Dust fraction
18,55 45,01 0,96
0,78
0,90
2,2
15,0
In this process, the amount of HNO₃ (NHNO₃) varied
from 5% to 31%, and H₂SO₄ (NH₂SO₄) varied from 7.6% to
93% of the stoichiometry required to bind the CaO in
the phosphate raw material. The concentration of the
first acid was (CHNO₃) 50% and 54%, and of t
he second
(CH₂SO₄) 65% and 92%. The interaction time between
the phosphate raw material and nitric acid was 15
minutes, and the reaction time of the nitrocalcium
phosphate slurry with sulfuric acid was also 15-20
minutes at a temperature of 75-80°C. After completing
the mixing process, the acidic reaction mixtures were
neutralized with gaseous ammonia until the pH was no
lower than 3.0. They were then dried at 90-100°C. The
mass of the product was measured, and the moisture
content of the slurry was determined. Granulation of
the wet phosphate masses was performed during the
drying process using intensive mixing and granulation
in the presence of return material. The final samples
were then cooled, crushed, and chemically analyzed
for various forms of P₂O₅ a
nd CaO, as well as nitrogen
and CO₂ content according to known methods [15]. The
degree of decarbonization of the phosphate raw
material was calculated by changes in CO₂ content. The
pH of the product was determined after shaking a 10%
aqueous suspension for one hour. The strength of the
fertilizer granules was determined using the MIP-10-1
device [15].
DISCUSSION
Experimental data showed that the higher the amount
of return material, the lower the moisture content of
the final product. For example, with an increase in the
amount of return material to 8 parts by mass in relation
to 1 part by mass of the initial product, the moisture
content decreased from the initial 22% to 1.5% in the
final product (see figure, curve-1), meaning that the
granulation process can be carried out using a pan or
screw granulator. Table 2 presents the composition of
products obtained from nitric-sulfuric acid processing
of phosphate flour, depending on the amount of nitric
and sulfuric acids used with a return ratio of 1:6.
Table 2.
Composition of products from nitric-sulfuric acid processing of ordinary phosphate flour from Central
Kyzylkum
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№
expe
rienc
e
Сoncentratio
n and norm
HNO
3
, %
Сoncentration
and norm
H
₂
SO
₄
, %
Sum
N
HNO3
+
N
H2SO4
,
%
Content
nitrogen, wt.%
Сontent
Р
2
О
5
, wt. %
Р
2
О
5
total
/
Р
2
О
5
digestiv
e
degree
of
decarbonizat
ion
of
phosphorus
raw
materials %
C
HNO
₃
N
HNO
₃
C
H
₂
SO
₄
N
H
₂
SO
₄
N
nitrate
N
ammonia
N
total
Р
2
О
5
total
Р
2
О
5
digestive
1
54
11
92
23
33
1,97 -
1,97 13,50 5,4
40,1 75,00
2
54
11
92
70
80
1,57 1,50
3,04 10,54 8,22
78,2 95,00
3
54
11
92
93
103
1,37 2,90
4,27 9,36
8,55
91,3 97,50
4
54
21
92
23
45
3,75 1,05
4,80 12,80 6,40
50,4 86,00
5
54
21
92
70
91
2,73 1,81
4,54 9,34
7,66
82,3 96,00
6
54
21
92
93
114
2,46 3,01
5,47 8,43
7,76
92,1 98,00
7
54
31
92
23
53
5,05 1,21
5,9
11,54 6,92
60,1 90,00
8
54
31
92
70
100
3,89 2,02
6,26 8,89
8,0
90,1 97,00
9
54
31
92
93
123
3,56 3,51
7,07 8,14
7,73
95,1 98,00
10
54
15
92
11,7 20,7 3,03 -
3,03 14,82 5,04
34,1 68,00
11
54
15
92
32,7 47,7 2,73 1,15
3,88 13,31 6,79
51,1 86,50
12
54
15
92
58,5 73,5 2,17 2,03
4,20 10,61 7,75
73,4 89,00
13
54
16
65
7,6
23,7 3,24 -
3,24 14,68 4,55
31,1 65,00
14
54
16
65
15,2 31,2 3,03 -
3,03 13,82 5,25
38,2 71,00
15
54
16
65
22,8 38,8 2,83 1,03
3,86 12,94 5,69
44,3 77,00
16
50
15
65
7,6
22,6 2,78 -
2,78 13,70 4,11
30,4 60,00
17
50
10
65
15,2 25,2 1,88 -
1,88 13,93 4,60
33,4 67,10
18
50
5
65
22,8 27,8 0,94 -
0,94 13,93 4,88
35,2 70,02
Return Ratio
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Figure-1. Changes in product moisture depending on the return ratio.
(1
–
powdery fraction; 2
–
phosphate flour; 3
–
washed dried concentrate)
It can be observed that with identical concentrations
of nitric and sulfuric acids (54% HNO3 and 92% H2SO4)
and a constant norm of HNO3 at 11% and 21%, increasing
the amount of H2SO4 from 23% to 93% results in a
decrease in total P2O5 content in the products from
13.50% to 9.36% and from 12.80% to 8.43%, respectively.
Conversely, the total nitrogen content (Ntotal)
increases from 1.97% to 4.27% and from 4.80% to 5.47%,
respectively. In this case, the ratio P2O5 digestive:
P2O5total increases from 40.1% to 91.3% and from 50.4%
to 92.1%, with the degree of decarbonization of
phosphate raw materials (Kdecarb.) rising from 75.0%
to 97.5% and from 86% to 98%. It is worth noting that
under these conditions, the total norms of nitric and
sulfuric acids fluctuate within 33-103% and 45-114% of
stoichiometry, respectively.
With the same changes in H2SO4 norms and a relatively
high norm of HNO3 at 31% (with a total acid norm of 53-
123%), the total P2O5 content in the products
decreases, albeit slightly (from 11.54% to 8.14%), while
the Ntotal content remains sufficiently high (5.47%-
7.07%). At this point, the phosphate raw materials are
fully decarbonized (90%-98%), and the products have a
soluble P2O5 to total P2O5 ratio ranging from 60% to
95%.
However, increasing the norm of HNO3 beyond 30%
and H2SO4 beyond 70% is undesirable, as the product
moisture exceeds 6.03% and exhibits increased acidity,
necessitating deeper ammonization of the reaction
mass. This leads to a reduction in the content of
nutrients and retrogradation of P2O5, which is the
transition of water-soluble monocalcium phosphate to
citrate-soluble dicalcium phosphate according to the
reaction:
Са(Н2РО4)2 + NH3 = CaHPO4 + NH4H2PO4
Further reduction of soluble forms of P2O5 during
ammonization can be attributed to subsequent
retrogradation of P2O5 according to the reaction:
2СаНРО4 + СаS
O4 + 2NH3 = Ca3(PO4)2 + (NH4)2SO4
To avoid additional ammonization processes that lead
to the negative phenomena mentioned above, we
selected the optimal total norms of nitric and sulfuric
acids as 73.5%, 80%, and 91%, respectively, with HNO3 at
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15%, 11%, and 21%, and H2SO4 at 58.5%, 70%, and 70%. In
these cases, the obtained nitrogen-phosphorus
fertilizers contain (weight %): in the first case - P2O5
total
–
10.61; Ntotal
–
4.20; P2O5 digestive:P2O5total =
73.4; degree of decarbonization Kdecarb
–
89; granule
strength
–
2.3 MPa; pH = 3.5; in the second case -
P2O5total
–
10.54; Ntotal
–
3.04; P2O5 digestive:
P2O5total = 78.2; Kdecarb.
–
95; granule strength
–
2.1
MPa; pH = 3.6; and in the third case - P2O5total
–
9.34;
Ntotal
–
4.54; P2O5 digestive: P2O5total = 82.3;
Kdecarb.
–
96; granule strength
–
2.2 MPa; pH = 3.3.
With this content of available phosphorus, the
products obtained are effective nitrogen-phosphorus
fertilizers.
Next, the process of obtaining complex NPK fertilizers
was studied. For this, potassium components were
added to the products of nitric acid processing of
washed dried concentrate and powdery fractions, the
compositions of which are presented in Table 1.
The
process
includes
the
simultaneous
decarbonization and decomposition of phosphate raw
materials (in one apparatus), neutralization to a pH of
no less than 3.5, evaporation of the pulp to a density of
at least 1750 kg/m³, addition of potassium salts in a
mass ratio of N:P2O5:K2O equal to 1:1:1, 1.5:1:1, and
1:1.5:1, granulation of the product, and drying in the
presence of return and cooling. The amounts of nitric
acid and potassium salts per 100 kg of phosphate rock,
given the specified mass ratio of N:P2O5:K2O, are
calculated using the following formulas:
АНNO3 = (CP2О5*63/14) *(100 / C НNO3) * (ПN / ПP2О5);
Аp/r = 100 * ПP2О5;
А к.salt= 0,5(CP2О5* Мк.salt / СK2О) *(ПK2О / ПP2О5);
Where, AHNO3, Ap/r, AK.salt, are the amounts of nitric
acid, phosphate raw material, and potassium salts,
respectively; CP2O5, CK2O are the contents of P2O5
and K2O in the phosphate raw material and potassium
salt, respectively; CHNO3 is the concentration of nitric
acid;
MK.salt is the molecular weight of potassium salts;
and ПN, ПP2O5, ПK2O
are the numerical values of the
specified mass ratios of N, P2O5, and K2O, respectively.
As potassium components, we used sulfate, chloride,
and carbonate salts, whose hygroscopicity (on a 10-
point scale) is presented below (table-3):
Table-3
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№
Components
Points (α)
Qualitative assessment of hygroscopicity
1
К
2
SО
4
>1
Almost non-hygroscopic
2
К
2
CO
3
>1
Almost non-hygroscopic
3
КCl
(3,62) 3-5
Hygroscopic
Potassium salts are added to the mixer, where the
evaporated melt is simultaneously introduced in an
amount necessary to maintain the mass ratio of
N:P2O5:K2O equal to 1:1:1, 1.5:1:1, and 1:1.5:1. Visual
observations indicate that the granulation process in
the presence of return must be conducted while
maintaining the moisture content in the mass at no
more than 7-8%. It is important to note that reducing
the density of the melt to less than 1750 kg/m³
complicates the granulation process and deteriorates
the properties of the resulting fertilizers due to
increased moisture content.
Table 3 presents the compositions of the complex
fertilizers. It is evident that based on the product of
nitric acid decomposition of washed dried concentrate
and potassium components, with the mass ratio of
N:P2O5:K2O varying from 1:1:1 to 1.5:1:1 and 1:1.5:1, the
composition of granulated NPK fertilizers appears as
follows: for K2SO4
–
7.2-10.89% N; 7.26-10.76% P2O5
with a total of nutrients
–
25.5-26.5%; 7.2-8.84% K2O; for
KCl
–
7.7-11.6% N; 7.72-11.43% P2O5; 7.7-9.53% K2O with a
total of nutrients
–
27-28.5%; for K2CO3
–
7.9-9.84% N;
7.92-11.9% P2O5; 7.9-9.84% K2O with a total of nutrients
–
27.7-29.5%.
A similar pattern is observed for NPK fertilizers based
on powdery fractions and potassium components, but
with a relatively low content of nutrients. In any case,
all samples of the products meet all the requirements
of agriculture in terms of physical-chemical and
commercial properties, making them suitable for bulk
storage, transportation, and application.
Thus, as a result of implementing the proposed option,
all the advantages of existing methods are preserved:
the drying stage is reduced, and the technology allows
for the regulation of the compositions of complex
fertilizers within a wide range of nutrient component
ratios and their physical-chemical properties. The
obtained
complex
fertilizers
possess
good
agrochemical properties and are suitable for all types
of agricultural crops.
Table 3. Composition of complex fertilizers obtained
from the products of nitric-sulfuric acid processing of
phosphorites from Central Kyzylkums and potassium
components.
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Table-4
Mass ratio.
N:Р
2
О
5
:К
2
О
Type of
phosphorites
Type of
potassium
salts
Mass ratio
original
components
А
НNO3
: А
ф/с
:
А
к.соль
Contents of components
%
Amount of
nutrients
components,%
N
P
2
O
5
К
2
О
1:1:1
1,5:1:1
1:1,5:1
Washed
dried.
concentrate
K
2
SO
4
192,9:100:77,59
298,3:100:77,59
192,9:150:77,59
8,84
10,89
7,2
8,83
7,26
10,76
8,84
7,3
7,2
26,5
25,5
25,5
1:1:1
1,5:1:1
1:1,5:1
-
KCl
192,9:100:56,86
298,3:100:56,86
192,9:150:56,86
9,53
11,6
7,7
9,53
7,72
11,43
9,53
7,7
7,7
28,5
27,0
27,0
1:1:1
1,5:1:1
1:1,5:1
-
K
2
CO
3
192,9:100:48,86
298,3:100:48,86
192,9:150:48,86
9,84
11,9
7,9
9,84
7,92
11,9
9,84
7,92
7,9
29,5
27,7
27,7
1:1:1
1,5:1:1
1:1,5:1
Dust fraction
K
2
SO
4
148,8:100:59,74
223,47:100:59,74
1489,48:150:59,74
7,62
9,76
6,33
7,62
6,51
9,49
7,62
6,51
6,33
22,86
22,78
22,78
1:1:1
1,5:1:1
1:1,5:1
KCl
148,8:100:43,78
223,47:100: 43,78
1489,48:150:43,78
8,2
10,3
6,9
8,16
6,9
10,3
8,2
6,89
6,9
24,6
24,1
24,1
Technical and economic calculations indicate the
profitability of the proposed method. Savings are
achieved through improved product quality and the
use of inexpensive highly carbonized phosphorites
(unbeneficiated
phosphorite,
washed
dried
concentrate, and powdery fraction).
CONCLUSION
Volume 04 Issue 11-2024
57
American Journal Of Applied Science And Technology
(ISSN
–
2771-2745)
VOLUME
04
ISSUE
11
Pages:
47-58
OCLC
–
1121105677
Publisher:
Oscar Publishing Services
Servi
Based on the conducted research on the improvement
of the composition of products from the nitric acid
decomposition of carbonate phosphorites: methods
for enhancing the quality of products from the nitric
acid processing of carbonate phosphate raw materials
have been analyzed; the process of obtaining NP
fertilizers
through
the
nitric-sulfuric
acid
decomposition of phosphate rock in the conditions of
thickening pulp has been studied, which excludes the
stage of ammonization of the decomposition
products; and the process of obtaining NPK fertilizers
by adding nitrogen components such as K2SO4, KCl,
and K2CO3 to the products of complete nitric acid
decomposition of washed dried concentrate and
powdery fractions has been studied. It has been
established
that
when
using
washed
dried
concentrate, products with a relatively low content of
nutrients are obtained.
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1.
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Volume 04 Issue 11-2024
58
American Journal Of Applied Science And Technology
(ISSN
–
2771-2745)
VOLUME
04
ISSUE
11
Pages:
47-58
OCLC
–
1121105677
Publisher:
Oscar Publishing Services
Servi
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