GRANULATED NITROGEN-SULFUR FERTILIZERS

ABSTRACT

In this article there were obtained the samples of nitrogen-sulphur fertilizers by addition of crystalline ammonium sulphate ((NH₄)₂SO₄) and carbamide ((NH₂)₂CO) in melt ammonium nitrate (NH₄NO₃) at weight ratios of NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO from 99.5 : 0.5 : 0.5 to 95 : 5 : 5 follow up granulation of nitrogen-sulphagen-carbamid’s melt by tower prilling. There have been studied the composition and properties of the samples. Adjusting the quantities of ammonium sulfate and carbamide in the NH₄NO₃ melt appears to lower the nitrogen content in the resulting product from 34.88% to 34.35%, but on the other hand increases the sulfur content from 0.26 to 2.95%. One of the main indicators of modified ammonium nitrate (AN) is the strength of its granules. If for pure AC without any additives and nitrate with a magnesium additive (0.28% MgO) it is equal to - 1.30 and 1.58 MPa, respectively, then for fertilizer with the ratio NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO = 100 : 0.5 : 0.5 is 1.95 MPa. The time for complete dissolution of the granule AN with magnesium additive is 46.8 seconds. Increasing the amounts of (NH₄)₂SO₄ and (NH₂)₂CO in the NH₄NO₃ mixture to 5 gramm seems to extend the time required for complete dissolution of the resulting fertilizer granules to 91.25 seconds.

АННОТАЦИЯ

В работе получены образцы азотносерных удобрений путем добавления к плаву нитрата аммония (NH₄NO₃) кристаллического сульфата аммония (NH₄)₂SO₄) и карбамида (NH₂)₂CO) при массовых соотношениях NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO) от 99.5 : 0.5 : 0.5 до 95 : 5 : 5 с последующим гранулированием нитратно-сульфатно-карбамидного расплава методом приллирования на грануляционной башне. Изучены состав и свойства полученных образцов. Было показано, что увеличение количества сульфата аммония и карбамида вводимого в расплав NH₄NO₃ уменьшает в продукте содержание азота с 34.88% до 34.35%, но с другой стороны увеличивает содержание серы с 0.26 до 2.95%. Одним из основных показателей модифицированной аммиачной селитры (АС) является прочность её гранул. Если для чистой АС без всяких добавок и селитры с магнезиальной добавкой (0,28% MgO) она равна – 1.30 и 1.58 МПа соответственно, то для удобрения с соотношением NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO = 100 : 0.5 : 0.5 составляет 1.95 МПа. Время полного растворения гранулы с магнезиальной добавкой АС составляет 46.8 сек. В то время с увеличением доли (NH₄)₂SO₄ и (NH₂)₂CO в смеси с NH₄NO₃ 5г время полного растворения гранул получаемых удобрений достигает 91.25 сек.

Keywords: melt ammonium nitrate, ammonium sulphate, carbamide, nitrogen-sulphur fertilizers, composition and properties.

Ключевые слова: расплав аммиачной селитры, сульфат аммония, карбамид, азотносерные удобрение, состав и свойства.

Introduction. The chemical industry of Uzbekistan in 2015 produced 942.72 thousand tons of nitrogen, 148.42 thousand tons of phosphorus and 143.24 thousand tons of potassium fertilizers based on 100% nutrients. Of the 942.72 thousand tons of nitrogen fertilizers in the form of 100% nitrogen, 1646.66 thousand tons of ammonium nitrate (AN), 586.66 thousand tons of urea and 192.65 thousand tons of ammonium sulfate were produced in physical terms. AN is one of the most effective and widespread nitrogen fertilizers in the world [1]. However, AN has such disadvantages as caking during storage and explosion hazard. The introduction of modifying additives to saltpeter improves its consumer properties.

Currently, in the production of AN, in accordance with GOST 2-85, additives are used: ammonium sulfate, its combination with ammonium phosphate, magnesium nitrate and the product of nitrate decomposition of dolomite containing magnesium and calcium nitrates. The phosphate-sulfate additive is introduced in the form of a mixture of phosphoric and sulfuric acids into nitric acid or into an AC solution in the amount of 0.3-0.5% P₂O₅ and 0.05-0.2% ammonium sulfate in terms of the finished product, with simultaneous neutralization ammonia gas to pH 5.5-6.8. Certainly, an aqueous solution of mono- and diammonium phosphate can serve as an alternative to using phosphoric acid in certain applications. This substitution offers a viable option depending on the specific requirements of the process or product formulation. At the same time, saltpeter granules have higher strength than pure saltpeter, less caking, and higher resistance to variable temperatures. The presence of a phosphate-sulfate additive in the AN melt prevents the decomposition of the melt during deep boiling and thereby increases the safety of this process [2, 3]. The production of AN with a phosphate-sulfate additive was discontinued due to a shortage of thermal phosphoric acid.

One of the best additives to AN that eliminates caking is a magnesium additive.

Caustic magnesite is used as an additive to AN by two factories in Uzbekistan - Maksam-Chirchik JSC and Ferganaazot. The third - Navoiazot JSC also uses a magnesite additive, but in the form of the natural mineral brucite Mg(OH)₂, supplied from Russia by Vyazma-Brusit LLC and containing 61.9% MgO, 3.7 CaO, 4.52 SiO₂, 0.12% Fe₂O₃ and 0.4% H₂O.

At enterprises producing speakers with a sulfate additive, the consumption rate of ammonium sulfate is 4 kg per 1 ton of finished product. This additive practically eliminates dust in the granulation tower, ensures an optimal particle size distribution of the product, but does not increase the stability of the granules to the periodic heating-cooling cycle (20↔60ºС). The disintegration or degradation of granules usually commences after approximately 7 to 8 cycles in certain manufacturing or processing conditions [4].

Work [5] noted a significant (2-3 times) improvement in such properties of AN as the static strength of granules, caking, the yield of granules without shrinkage channels on the surface, their resistance to thermal cycles (20↔60ºС) with increasing ammonium sulfate content with 0.5 to 5%. At the same time, the number of thermal cycles of 20↔60ºС before the beginning of granule destruction increased to 110±20 times.

The goal of this work is to expand this area of application of ammonium sulfate and urea from 0.5 to 5 g in relation to 99.5 to 95 g of ammonium nitrate for the production of balanced nitrogen-sulfur fertilizers with subsequent granulation of the nitrate-sulfate-urea melt by prilling on a granulation tower.

The advantage of this fertilizer over pure AN is the presence of sulfur in it, which is part of proteins and amino acids during crop formation.

Absolutely, sulfur plays a crucial physiological role in plant nutrition, often considered third in importance after nitrogen and phosphorus. It's vital for various metabolic processes, aiding in plant growth, enzyme functions, and overall plant health [6].

In order to study the mechanism of action of the selected additive, samples of modified ammonium nitrate were obtained by mixing ammonium nitrate melt with ammonium sulfate and urea, followed by prilling the nitrate-sulfate-urea melt, that is, by simulating the granulation process in towers.

To obtain samples of modified AN, pure grade NH₄NO₃ was used as the main component.

The crystalline ammonium urea sulfate underwent pre-grinding in a porcelain mortar to achieve a particle size of 0.25 mm. This process likely aimed to standardize the particle size for specific application requirements. And as samples for comparison, granulated NH₄NO₃ and an industrial product - AN containing 34.6% N and 0.28% magnesite in terms of MgO - were selected.

Objects and methods of research. The experiments were conducted in the following manner. NH₄NO₃ was heated electrically within a metal cup to melt the sample. Then (NH₄)₂SO₄ and (NH₂)₂CO) were introduced into the melt at mass ratios NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO) from 99 : 0.5 : 0.5 to 90 : 5 : 5. Next, the nitrate-sulfate-urea melt AN kept for 3 minutes. at 170ºС, after which it was poured into a laboratory granulator, which was a metal glass with a perforated bottom, the diameter of the holes in which was 1.2 mm. A pump created pressure in the upper part of the glass and the melt was sprayed from a height of 35 m onto a plastic film lying on the ground. The resulting granules were dispersed according to particle size. Particles measuring 2-3 mm were subjected to strength testing in accordance with GOST 21560.2-82.

Subsequently, the resulting products were crushed and analyzed employing established methods [7].

The pH value of 10% aqueous suspensions of ready-made fertilizers was measured in a laboratory I-130M ion meter with an accuracy of 0.05 pH units.

To determine the rate of dissolution of granules of the studied fertilizers, a granule of the product was dipped into a glass with 100 ml of distilled water, in which its complete dissolution was visually observed and recorded. Room temperature, tests five times.

The results are shown in tables 1-2 and figures 1-2.

Results and its discussion. It is clear, from the data in Table 1, that an increase in the amount of (NH₄)₂SO₄ and (NH₂)₂CO introduced into the NH₄NO₃ melt, from 0.5 to 5 g in relation to 99 to 90 g of ammonium nitrate melt in the resulting product. The content of total nitrogen decreases from 34.88% to 34.35% and SO₃ content increases from 0.26 to 2.95%. In samples of nitrogen-sulfur fertilizers, the sum of nutritional components is already 35.14-37.30%.

It should also be noted that with the addition of (NH₄)₂SO₄ and (NH₂)₂CO, the crystallization temperature of NH₄NO₃ decreases from the initial 167°C to 158.7°C, and the pH of the 10% solution increases from 5.17 to 6.02.

These indicators are more clearly visible in figures 1 and 2.

Table 1.

Chemical composition, crystallization temperature and pH of a 10% solution of nitrogen-sulfur fertilizers

Table 2.

Strength and dissolution rate of nitrogen-sulfur fertilizer granules

Figure 1. Dependence of the change in the crystallization temperature of AN melt on the mass ratio NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO

Figure 2. Dependence of the change in pH of a 10% solution of AN melt on the mass ratio of NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO

Table 2 shows the strength and dissolution rate of granules of samples of nitrogen-sulfur fertilizers.

It shows that with the studied ratios NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO, the strength of nitrogen-sulfur fertilizer granules lies in the range of 1.95-4.16 MPa, while this indicator for pure ammonium nitrate is 1.30 MPa, AN with magnesium additive (0.28% MgO) – 1.58 MPa.

An increase in the strength of nitrate granules indicates a decrease in its porosity and internal specific surface area, which leads to a decrease in the penetration of diesel fuel into the granules, and, consequently, a decrease in the detonation ability of nitrate.

Complete dissolution of pure NH₄NO₃ granules in water averages 44.6 seconds, and the introduction of (NH₄)₂SO₄ and (NH₂)₂CO into its composition increases the dissolution rate of granules from 65.31 to 91.25 seconds. This suggests that the resulting fertilizers will be washed out of the soil much more slowly than pure ammonium nitrate.

We consider the optimal fertilizer to be obtained at the ratio NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO = 99 : 2.5 : 2.5, containing 34.64% N, 1.47 SO₃, having a granule strength of 2.75 MPa, dissolving in 76.54 seconds.

In conclusion, by adding ammonium sulfate and urea to the ammonium nitrate melt at mass ratios NH₄NO₃ : (NH₄)₂SO₄ : (NH₂)₂CO from 99 : 0.5 : 0.5 to 90 : 5 : 5, temperature 170ºС, followed by granulation of the resulting melt by prilling gives the product a new property - high strength. The nitrate composition undergoes enrichment with an additional nutrient element, sulfur.

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