NITROGEN-PHOSPHORUS-SULFUR FERTILIZERS OBTAINED BY INTRODUCING KYZYLKUM PHOSPHORITE FLOUR AND PHOSPHOGYPSUM INTO THE AMMONIUM NITRATE MELT

ABSTRACT

In this scientific study, the process of obtaining samples of granular complex nitrogen-phosphorus-sulfur fertilizers was investigated by introducing phosphorite flour (PhF) from the “Kyzylkum” deposit (Navoi) and the technogenic waste phosphogypsum (PG) into the melt of ammonium nitrate (AN). The mass ratio of AN : PhF ranged from 80 : 15 to 98 : 1. Phosphogypsum (PG) was added in amounts of 1, 3, and 5% of the total mass of the AN and PhF mixture. The melt of the fertilizer samples was granulated using the prilling method in a tower-type granulation unit. The chemical composition and granule strength of ammonium nitrate samples containing PhF and PG additives obtained by this method were determined. It was found that with an increase in the amount of PhF and PG introduced into the ammonium nitrate melt, the total nitrogen content (Nₜₒₜ_.) in the product decreased from 34.1% to 28.0%. However, at the same time, the contents of phosphorus, sulfur, and calcium increased from 0.192% to 2.739%, from 0.52% to 2.67%, and from 0.82% to 8.97%, respectively. At the studied mass ratios of  AN : PhF : PG, the strength of fertilizer granules increased from 4.11 to 10.28 MPa. The high strength of ammonium nitrate granules containing PhF and PG indicates their improved thermal stability.

АННОТАЦИЯ

В данной научной работе исследован процесс получения образцов гранулированного комплексного азотнофосфорносерного удобрения путем добавления фосфоритного мука (ФМ) месторождения “Кызылкум” (Навои) и техногенного отхода фосфогипса (ФГ) к плаву аммиачной селитры (АС) при массовом соотношении АС : ФМ от 80 : 15 до 98 : 1. А фосфогипс добавляли в количестве 1, 3 и 5% от общей массы смеси АС и ФМ. Расплав образцов удобрений гранулировались методом приллирования в грануляционном устройстве башенного типа. Определены состав и прочность гранул образцов АС с добавкой ФИ и ФГ, полученных таким способом. При увеличении количества ФМ и ФГ, входящего в плава АС снижается количество N_общ. в продукте с 34.1% до 28.0%, но с другой стороны показано, что количество фосфора, сера и кальция увеличивается с 0,192 до 2.739%, с 0,52 до 2.67% и с 0,82% до 8.97% соответственно. При исследованных соотношениях АС : ФМ : ФГ прочность гранул продукта увеличивается от 4.11 до 10.28 МПа. Высокая прочность гранул ФМ и ФГ содержащей аммиачной селитры свидетельствует о её термической стабильности.

Ключевые слова: аммиачная селитра, фосфоритовая мука, фосфогипс, декарбонизация, химический состав и прочность гранул.

Keywords: ammonium nitrate, phosphate rock, phosphogypsum, decarbonization, chemical composition and strength of granules.

Introduction. Ammonium nitrate (AN – NH₄NO₃) is one of the most widely produced nitrogen fertilizers in the world, and its agrochemical effectiveness becomes evident very rapidly when applied to various soil types and agricultural crops. Therefore, ammonium nitrate is also referred to as a universal and highly concentrated nitrogen fertilizer. According to data from the International Fertilizer Association (2007), its global production capacity reached 43 million tons per year. In Uzbekistan, approximately 2 million tons of ammonium nitrate (AN) are produced annually at the joint-stock companies Maxam-Chirchiq, Navoiyazot, and Farg‘onaazot. However, ammonium nitrate has two significant drawbacks. The first is the caking tendency of ammonium nitrate granules packed in polyethylene bags during storage, which leads to the formation of a monolithic mass. The second drawback is its high explosion hazard [1].

The problem of caking of ammonium nitrate granules has been partially solved by introducing various inorganic modifying additives into its composition, such as ammonium or magnesium sulfate, sulfate-phosphate, sulfate-phosphate-borate, caustic magnesite, or brucite [2].

However, the issue of its explosive hazard still remains unresolved. This is because NH₄NO₃ can exhibit reducing properties in the presence of strong oxidizing agents, and oxidizing properties in the presence of strong reducing agents.

Currently, ammonium nitrate producers are facing an urgent challenge: the need to develop nitrogen fertilizers based on ammonium nitrate that retain high agrochemical efficiency, increase the mechanical strength of granules, and exhibit greater resistance to external influences, thereby reducing the risk of explosion. According to technical and scientific literature as well as patent materials, the following compounds can be cited as inorganic additives capable of reducing the explosive properties of ammonium nitrate:

♦ Carbonate-containing compounds obtained from natural sources or processing (limestone, dolomite mineral, calcium carbonate, chalk);

♦ Potassium-containing compounds (potassium chloride, potassium sulfate);

♦ Inorganic compounds containing the ammonium cation, such as ammonium sulfate, ammonium ortho- and polyphosphates;

♦ Ballast materials that improve the rheological properties and mechanical dilution of ammonium nitrate, including natural gypsum, phosphogypsum, gypsum, and other inorganic substances [3].

At the Russian enterprises Kemerovo “Azot”, Kirovo-Chepetsk Chemical Combine, Rossosh “Minudobreniya”, and Novgorod “Acron” Joint-Stock Companies, the production of thermally stabilized ammonium nitrate has been established by introducing phosphorus-containing inorganic additives (phosphoric acid or mono- and polyammonium phosphates) into the composition of ammonium nitrate [4, 5].

The main disadvantages of this method include corrosion caused by fluorine contained in phosphoric acid, as well as blockage of equipment pipelines due to the precipitation of iron, aluminum, magnesium, and calcium salts.

In 2002, at OJSC “Cherepovets Azot”, the industrial production of a granulated complex nitrogen-phosphate fertilizer (NPF) containing 32.3% nitrogen and 5.2% phosphorus was introduced. This product was obtained by adding a liquid complex fertilizer based on superphosphoric acid containing mono- and polyammonium phosphates to the ammonium nitrate melt [6, 7].

Although this method reduces the explosive hazard of ammonium nitrate, it has certain disadvantages. During the mixing of the liquid complex fertilizer with ammonium nitrate, precipitates are formed, which causes difficulties during the evaporation and granulation stages of the process. In addition, phosphoric acid is not produced in Uzbekistan. Therefore, the search for suitable phosphorus-containing raw materials for the thermal stabilization of ammonium nitrate is ongoing.

Therefore, in 2007, at JSC “Navoiyazot” located in the Navoi region of Uzbekistan, the production of nitrogen-phosphate fertilizer (NPF) was introduced. This fertilizer was obtained by adding Kyzylkum phosphorite flour to the ammonium nitrate melt and granulating the resulting phosphate–nitrate melt using the prilling (spraying) method from a tower. According to the approved technical specification (TS 6.1-00203849-111:2007) for the production of NPF, the obtained complex nitrogen-phosphorus fertilizer contains 22-28% nitrogen and 1-6% phosphorus. In 2011, 110.6 thousand tons of NPF were produced [8-10].

Compounds containing ballast materials (natural gypsum and phosphogypsum) are also considered promising for improving the consumer properties of ammonium nitrate (AN) [11-14]. In these studies, a technology for producing thermally stabilized nitrogen fertilizers based on ammonium nitrate was developed by introducing phosphogypsum dihydrate, hemihydrate, and natural gypsum into the ammonium nitrate melt. Modified ammonium nitrate with gypsum additives exhibits significantly higher thermal stability compared with “pure” ammonium nitrate without additives. In particular, the activation energy for pure ammonium nitrate was determined to be 160 kJ/mol, whereas for ammonium nitrate containing phosphogypsum additives the maximum value reached 240 kJ/mol.

Objects and methods of research. In laboratory conditions, we decided to investigate the process of producing nitrogen-phosphorus-sulfur-containing fertilizers by simultaneously introducing two promising additives into the ammonium nitrate melt: Central Kyzylkum phosphorite flour (PhF) and phosphogypsum (PG), which is a technogenic waste generated during the production of extraction phosphoric acid at JSC “Ammofos-Maksam.” Approximately 80 million tons of phosphogypsum waste have accumulated within the territory of JSC “Ammofos-Maksam.” This PG exists mainly in the form of calcium sulfate dihydrate (CaSO₄·2H₂O) and contains 18-20% moisture, therefore it was first thoroughly dried. After that, its chemical composition was determined.

The results showed that PG contains 1.59% P₂O_5tot., including 1.48% P₂O_5ava. and 1.12% P₂O_5ws.; 37.47% CaO_tot., including 19.08% CaO_ava. and 11.26% CaO_ws.; and 54.49% SO_3tot., including 27.40% SO_3ava. and 16.88% SO_3ws.

This PG exists in the form of calcium sulfate dihydrate (CaSO₄·2H₂O) and contains 18-20% moisture, therefore it was first thoroughly dried. Subsequently, its chemical composition was determined and found to be as follows: P₂O_5tot. – 1.59%; P₂O_5ava. – 1.48%; P₂O_5ws. – 1.12%; CaO_tot. – 37.47%; CaO_ava. – 19.08%; CaO_ws. – 11.26%; SO_3tot. – 54.49%; SO_3ava. – 27.40%; SO_3ws. – 16.88%.

The PhF had the following chemical composition (wt.%): P₂O_5tot.  - 17.76; CaO_tot. – 47.51; MgO – 1.79; CO₂ – 17.02; Fe₂O₃ – 0.73; Al₂O₃ – 0.95; insoluble residue – 5.27; P₂O_5ava. – 3.15; P₂O_5ava./P₂O_5tot.= 17.74. The particle size distribution was as follows: (+1 mm) - 1.8%; (−1+0.63 mm) - 3.4%; (−0.63+0.4 mm) - 7.4%; (−0.4+0.315 mm) - 11.3%; (−0.315+0.25 mm) - 4.4%; (−0.25+0.16 mm) - 2.4%; (−0.16+0.1 mm) - 18.4%; (−0.1+0.05 mm) - 8.8%; (−0.05 mm) - 2.5%. In order to ensure effective filling of pores and microcracks in the granules of “pure” grade NH₄NO₃, both PF and PG were preliminarily ground to a particle size of 0.25 mm. The laboratory experimental studies were carried out as follows. First, a pre-weighed amount of “pure” grade NH₄NO₃ was placed into a metal cup reactor and melted at a temperature of 170°C using an electric heater PhF was added to the ammonium nitrate melt in such an amount that the AN : PhF ratio in the mixture varied from 98 : 1 to 80 : 15. PG was then introduced in amounts of 1, 3, and 5% of the total mass of the AN and PhF mixture. At a temperature range of 165-170⁰C, the resulting phosphate-gypsum-nitrate melt was thoroughly mixed for 10 minutes, after which it was poured into the granulator. The granulator consisted of a stainless-steel cylindrical vessel with a perforated bottom plate having holes with a diameter of 1.2 mm. Pressure was created in its upper part using a manual pump, and the melt was sprayed from the 11th floor of the building onto a polyethylene film spread on the ground. The granules obtained by the prilling method were cooled, and the commercial fraction of 2-3 mm was separated using a sieve. The mechanical strength of the fertilizer granules was determined according to GOST 21560.2-82 [15]. After that, the nitrogen-phosphorus-sulfur fertilizers were ground and their chemical composition was analyzed. The total nitrogen content in the products was determined by the Kjeldahl method [16], the SO₃^2- ion content by the gravimetric method, the various forms of P₂O₅ by the differential method, and the various forms of CaO by the complexometric method [17]. The CO₂ content was determined volumetrically by the decomposition of carbonates with hydrochloric acid. The degree of decarbonization of the raw materials was calculated based on the change in the CO₂ content.

The results are presented in Tables 1 and 2 and in Figure 1.

Results and their discussion. As can be seen from Table 1, when the ammonium nitrate melt (AN) is mixed with phosphorite flour from the Kyzylkum deposit, activation of the latter occurs, i.e., the conversion of non-available P₂O₅ into forms available for plant uptake. If the relative content of citric acid-soluble P₂O_5ava. in the initial phosphorite flour was 17.74%, then in the obtained products this value ranges from 68.28 to 89.96%, depending on the ratio of the components used. With an increase in the amounts of phosphorite flour and phosphogypsum introduced into the AN melt from 1 to 15 g and from 1 to 5 g, respectively, per 80-98 g of AN, the total nitrogen content in the fertilizer decreases from 34.1 to 28.0%.

Table 1.

Chemical composition of NPS-fertilizers obtained based on ammonium nitrate melt, phosphorite flour and phosphogypsum.

At the same time, the contents of P₂O_5tot. and SO_3tot. increase from 0.195 to 2.739% and from 0.52 to 2.67%, respectively. The relative contents of citric acid-soluble forms of P₂O₅ and CaO decrease from 89.96 to 68.28% and from 92.79 to 71.85%, respectively. The values of the relative contents of available forms of P₂O₅, SO₃, and CaO indicate that the gypsum-phosphate component in ammonium nitrate can be considered a highly effective nitrogen-phosphorus-sulfur fertilizer. Such NPS-fertilizers can be applied on various soil types and for all agricultural crops.

The appearance of the water-soluble form of CaO in the products indicates that, due to its acidic nature, ammonium nitrate at elevated temperatures partially decomposes calcium carbonate present in the phosphorite flour, resulting in the formation of calcium nitrate:

2NH₄NO₃ + CaСО₃ ® Ca(NO₃)₂ + 2NН₃ + СО₂ + H₂O.

Fig. shows the dependence of the degree of decarbonization of PhF from the Kyzylkum deposit and PG on the mass ratio of AN : PhF : PG in the melt.

Figure 1. Degree of decarbonization of phosphate raw materials as a function of the AN : PhF : PG ratio

The essence of the activation of PhF and PG by the AN melt lies in the destruction of the structure of the gypsum-phosphate-containing mineral, namely the deformation of its crystal lattice without decomposition. As a result, the relative content of plant-available forms of P₂O₅ and CaO in the fertilizer increases. The partial decomposition of CaCO₃ with the release of gaseous products also contributes to the deformation of the crystal lattice of the phosphate mineral.

Table 2 presents the values of the mechanical strength of nitrogen-phosphorus-sulfur fertilizer granules depending on the mass ratio of ammonium nitrate melt, phosphorite flour from the Kyzylkum deposit, and phosphogypsum. It can be seen that with an increase in the amount of phosphorite flour from 1 to 15 g and phosphogypsum from 1 to 5 g in 80-98 g of ammonium nitrate melt, the granule strength increases from 4.11 to 10.28 MPa, which is significantly higher than the strength of granules of pure ammonium nitrate.

Table 2.

Granule strength and dissolution rate in water of NPS-fertilizer samples obtained from AN melt, PhF and PG

We consider the most optimal option for obtaining stabilized ammonium nitrate in the form of a nitrogen-phosphorus-sulfur fertilizer to be the introduction of 15 g of phosphorite flour and 5 g of phosphogypsum into 80 g of ammonium nitrate melt. Under these conditions, a fertilizer is obtained containing 28.0% nitrogen, 2.739% P₂O_5tot. and 2.67% SO_3tot. The relative content of the citric acid-soluble form of P₂O_5ava. is 68.28%, while the relative content of the available form of CaO₍_ava.₎ reaches 71.85%. The granule strength of this fertilizer is 10.28 MPa.

Conclusion. Thus, phosphorite flour from the Kyzylkum deposit and phosphogypsum can be considered highly promising additives both for ensuring the thermal stability of ammonium nitrate and for producing nitrogen-phosphorus-sulfur fertilizers with a high content of available forms of P₂O₅, SO₃, and CaO. This will significantly expand the raw material base for the production of highly efficient nitrogen-phosphorus-sulfur fertilizers with improved chemical and physicochemical properties.

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