COMPLEX GRANULAR NITROGEN-CONTAINING FERTILIZERS BASED ON MELTED AMMONIUM NITRATE AND KARAKALPAKSTAN VERMICULITE

ABSTRACT

In this scientific study, the process of producing granulated complex nitrogen-containing fertilizers was investigated by incorporating vermiculite mineral (VM) from the Tebinbulak deposit (Karakalpakstan) into molten ammonium nitrate (AN) at mass ratios of AN : VM ranging from 100 : 5 to 100 : 45. The resulting fertilizer melts were granulated using the prilling method in a tower-type granulation unit. The composition and physicochemical properties of the AN-VM samples were subsequently characterized. As the quantity of vermiculite introduced into 100 g of AN melt increased from 2 g to 45 g, the nitrogen content of the product decreased from 33.91% to 23.46%. However, this was accompanied by a notable increase in the concentrations of potassium (from 0.15% to 1.02%), calcium (from 0.22% to 1.46%), and magnesium (from 0.94% to 6.20%). Within the tested AN : VM ratios of 100 : (5-45), the mechanical strength of the granules increased from 5.02 MPa to 12.14 MPa. Concurrently, the caking tendency of the product decreased significantly-from 5.62 kg/cm² for pure ammonium nitrate to 1.36 kg/cm², representing a reduction by a factor of approximately 4.1. Furthermore, AN granules containing VM exhibited a significantly slower dissolution rate in water compared to those modified with magnesite.

АННОТАЦИЯ

В данной научной работе исследован процесс получения образцов гранулированного комплексного азотного удобрения путем добавления вермикулитного минерала (ВМ) месторождения “Тебинбулак” (Каракалпакстан) к плаву аммиачной селитры (АС) при массовом соотношении АС : ВМ от 100 : 5 до 100 : 45. Расплав образцов удобрений гранулировались методом приллирования в грануляционном устройстве башенного типа. Определены состав и свойства образцов АС с добавкой ВМ, полученных таким способом. При увеличении количества вермикулитного минерала, входящего в 100г плава АС с 2 до 45г снижается количество азота в продукте с 33,91% до 23,46%, но с другой стороны показано, что количество калия, кальция и магния увеличивается с 0,15 до 1,02%, с 0,22 до 1,46% и с 0,94% до 6,20% соответственно. При исследованных соотношениях АС : ВМ = 100 : (5÷45) прочность гранул продукта увеличивается от 5,02 до 12,14 МПа. При этом слёживаемость гранул продукта снижается с 5,62 кг/см² исходной «чистой» марки NH₄NO₃ до 1,36 кг/см², то есть почти в 4,1 раза. Гранулы АС с добавкой ВМ растворяются в воде значительно медленнее, чем АС с добавкой магнезита.

Ключевые слова: аммиачная селитра, вермикулит, состав, прочность, слёживаемость, пористость, впитываемость и скорость растворения гранул.

Keywords: ammonium nitrate, vermiculite, composition, strength, caking, porosity, absorbency and dissolution rate of granules.

Introduction. The majority of agricultural crops in Uzbekistan are cultivated on approximately 3.73 million hectares of land [1]. To achieve high-quality and high-yield harvests, the rational use of both simple and complex mineral fertilizers is essential. When applied correctly and in appropriate amounts, mineral fertilizers can increase crop yields by 40-50% [2].

It is important to note that the primary macronutrients required for plant growth are nitrogen, phosphorus, and potassium, followed by secondary nutrients such as sulfur, calcium, and magnesium. Among these, nitrogen is supplied through nitrogen-based fertilizers, which are critical for vegetative growth. Key nitrogen fertilizers include urea, ammonium nitrate (AN), urea-ammonium nitrate (UAN), and ammonium sulfate.

Ammonium nitrate is the second most widely used nitrogen fertilizer globally, following urea. According to the International Fertilizer Association, global production of AN in 2007 reached approximately 43 million tons for international trade purposes [3].

The largest production capacities for ammonium nitrate are located in the United States and Russia, which together account for approximately 13% of global output [4]. Currently, Uzbekistan produces about 2 million tons of ammonium nitrate annually to meet both domestic and export demands [5].

Ammonium nitrate is highly effective for agricultural use due to its excellent water solubility, which facilitates nutrient uptake by crops. However, it has two notable drawbacks: first, its granules tend to agglomerate or cake because of this solubility, and second, it possesses a high explosive potential [6-8].

Technical and scientific literature indicates that the caking tendency of ammonium nitrate granules can be mitigated by incorporating additives such as magnesium or ammonium sulfate, magnesite or brucite, ammonium sulfate-phosphate, and borates into the molten AN during production [5].

Therefore, inorganic compounds with high magnesium content serve as effective modifiers for ammonium nitrate, particularly when processed into agro minerals. Soil fertility plays a crucial role in the year-round cultivation of agricultural crops, especially melons and vegetables grown in greenhouses, significantly impacting yield quality and quantity. Moreover, the use of greenhouse-grown seedlings is increasingly essential for achieving high-quality vegetable crop production [9, 10].

Approximately 20% of vegetable crops in Russia are cultivated using seedlings. Although this method of growing melons and vegetables in greenhouses is somewhat costlier, it offers considerable advantages such as reduced seed consumption, earlier harvests, and other related benefits [11]. A key factor contributing to this success is the use of calcined vermiculite mixed with humus-rich soil, which significantly enhances soil fertility, especially in greenhouse environments. Vermiculite is also a valuable source of essential nutrients such as potassium, calcium, and magnesium, all critical for plant nutrition.

In Uzbekistan, calcined vermiculite is widely applied in gardening, vegetable production, and floriculture as a soil conditioner and magnesium fertilizer. It is also utilized as a raw material to improve seed germination and seedling growth.

A substantial deposit of vermiculite has been identified at the Tebinbulok site in the Karauzyak district of Karakalpakstan, Uzbekistan, with estimated reserves totaling approximately 1,332,620 tons. Consequently, extensive scientific investigations are underway to characterize this mineral. The primary chemical composition of the vermiculite from this deposit is detailed in Table 1 and illustrated in Figure 1.

Table 1

Chemical composition of vermiculite "Tebinbulak"

X-ray structural analysis revealed that the vermiculite contains significant amounts of macronutrients - 3.28% K₂O, 4.70% CaO, and 19.92% MgO - as well as essential micronutrients including 2.26% Na₂O, 5.74% Al₂O₃, 8.03% Fe₂O₃, and 1.32% TiO₂, all of which are vital for plant growth and development.

• SiO₂ (quartz): represents the silicate phase in the composition of vermiculite, increasing the resistance of the raw material to high temperatures;

Figure 1. X-ray structural analysis of a sample of vermiculite Tebinbulak

• MgO: these peaks, corresponding to magnesium oxide, are considered a nutrient element for plants and further increase the strength of the materials;

• Fe₂O₄ (magnetite): comes from iron-containing minerals and contributes to the overall thermal stability of the structure;

• Al₂O₃ (aluminum oxide): indicates that the mineral is highly crystalline and likely comes from aluminum silicates;

• K₂O (potassium oxide): potassium, found in small amounts in vermiculite, is an important macronutrient for agricultural crops.

Using the obtained data, we decided to add Tebinbulak vermiculite mineral (VM) to the saltpeter melt in order to reduce both the caking of granules and the explosiveness of AN.

Objects and methods of research. Prior to incorporation into the ammonium nitrate (AN) melt, the vermiculite mineral (VM) was thoroughly dried in a drying oven at 90–105°C. It was then finely ground in a porcelain mortar to a particle size of approximately 0.25 microns and subsequently passed through a sorting sieve.

Laboratory experiments were conducted using a 0.5-liter stainless steel reactor fabricated from corrosion-resistant grade 12X18N10T steel. A measured 100 g portion of ammonium nitrate, with the addition of magnesite (grade B), was placed into the reactor and heated to 170-175°C on an electric stove until it melted. The temperature of the molten saltpeter was continuously monitored using a precision thermometer to ensure it did not exceed 175°C.

Vermiculite mineral from Tebinbulak was added to the AN melt in mass ratios ranging from 100:5 to 100:45 (AN:VM). The resulting vermiculite–nitrate melt was stirred continuously with a glass rod for 10 minutes to ensure homogeneity. Subsequently, the mixture was poured into a granulator constructed from 12X18N10T stainless steel, equipped with holes measuring 1.2 microns in diameter. The vermiculite-modified nitrate melt was then sprayed from the 10th floor of the building using a high-pressure hand pump. Upon exposure to cold air during this process-analogous to tower prilling-the sprayed droplets solidified into granules measuring between 1 and 5 microns. Granulated fertilizer samples were collected on polyethylene film spread on the ground.

The granulated fertilizer samples obtained were first dried and ground prior to analysis. The chemical composition and physical properties of the granules were then determined using established analytical methods as follows:

- Elemental nitrogen content was measured by the Kjeldahl method using a K9840 Automatic Distillation Unit [12];

- Concentrations of potassium, calcium, and magnesium oxides were quantified via flame photometry [13];

- The pH of a 10% aqueous suspension of the samples was measured using a Mettler Toledo pH meter;

Physical properties including granule strength, caking tendency, porosity, diesel fuel absorbency, and complete water solubility of 2-3 micron-sized granules were also evaluated [5].

Results and their discussion. The results are presented in Tables 2-4 and Figure 2. As shown in Table 2, for the studied ammonium nitrate to vermiculite ratios (AN:VM = 100:(5-45)), an increase in the mass fraction of vermiculite corresponds to a decrease in nitrogen content - from 32.58% to 23.72% - in the nitrogen complex fertilizer composition.

Table 2.

Composition of samples of complex nitrogen fertilizers obtained on the basis of melt AN and VM

Table 3.

Assimilable forms of calcium and magnesium elements in samples of complex nitrogen fertilizers obtained on the basis of AN melt and VM

Conversely, the concentrations of potassium, calcium, and magnesium increase from 0.15% to 1.01%, 0.22% to 1.44%, and 0.93% to 6.21%, respectively.

It is important to note that at a temperature of 170-175°C, the ammonium nitrate melt activates the vermiculite by converting CaO and MgO from forms that are not bioavailable to plants into digestible forms. Consequently, within the studied weight ratios of AN:VM = 100:(5-45), the relative proportions of plant-available CaO and MgO in the complex nitrogen fertilizer, as measured by 2% citric acid extraction, range from 89.25% to 68.13% for CaO and from 50.37% to 26.10% for MgO, respectively.

Figure 2 (a) shows that the modification of AN with the addition of VM has a positive effect on the rate of dissolution of the resulting fertilizer granules in water.

Figure 2. Dependence of the change in the rate of complete dissolution of water (a) and the strength (b) of AN granules with the addition of VM on the AN : VM ratio

AN granules with the addition of magnesite (containing 0.28% MgO) exhibit an average complete dissolution time in water of 46.8 seconds. In contrast, for complex nitrogen fertilizer samples produced by adding vermiculite (VM) to ammonium nitrate at ratios of 100:(5–45), the dissolution time increases significantly, ranging from 84.83 to 117.55 seconds. This indicates that AN granules modified with VM dissolve more slowly than conventional AN granules with magnesite. Therefore, the incorporation of VM into the nitrate matrix facilitates a more gradual release of nitrogen from the granules.

The addition of 5 to 45 g of VM to 100 g of AN melt increases the granule strength from 5.02 MPa to 12.14 MPa. This represents a 3- to 7.6-fold improvement compared to AN granules with magnesite addition, which have a strength of 1.6 MPa. The inclusion of VM not only enhances the mechanical strength of nitrate granules but also reduces their caking tendency, porosity, and diesel fuel absorption. This improvement is attributed to VM particles filling the pores within the AN granules, acting as crystallization nuclei and thereby decreasing granule volume. Moreover, the high hydrophilicity of VM particles leads to increased moisture absorption from the nitrate, further contributing to these effects.

Table 4 shows the main properties of the product of AN granules with the addition of VM. If the caking of granules for “pure” NH₄NO₃ is 5.62 kg/cm², then for AC granules with the addition of VM it is 2.23-1.36 kg/cm². These values are 2.5 and 4.1 times lower than the caking of pure NH₄NO₃.

In AN samples with VM added at mass ratios of AN:VM = 100:(5–45), the porosity ranges from 8.51% to 3.44%. In contrast, the porosity of pure AN granules without additives is 22.0%, which is approximately 2.6 to 6.4 times higher than that of the VM-modified samples.

Table 4.

Caking, porosity and absorption of granules of complex nitrogen fertilizers obtained on the basis of AN melt and VM

This significant reduction in porosity correlates with a decreased capacity for diesel fuel absorption in the fertilizer granules, which in turn contributes to a lower risk of explosiveness.

Table 4 demonstrates that the properties of caking, porosity, and diesel fuel absorption are closely interrelated. The addition of 5 to 45 g of VM to 100 g of AN melt reduces the diesel fuel absorption of the granules from 4.06 g to 1.14 g, compared to 4.82 g for pure granulated NH₄NO₃. These findings explain the observed increase in granule strength in complex nitrogen fertilizer samples. The reduced adsorption of liquid fuel by these complex fertilizers contributes to lower detonation sensitivity and, consequently, improved thermal stability of ammonium nitrate.

Furthermore, incorporating VM into the AN melt raises its pH from an initial value of 6.17 to a range of 6.42-7.69. This increase in pH helps mitigate soil acidification when the fertilizer is applied to crops. Additionally, it is well-established that thermal decomposition of NH₄NO₃ is exacerbated by increased acidity of its melt. The buffering capacity of VM additives stabilizes the acidity of the NH₄NO₃ melt by neutralizing nitric acid (HNO₃) as it forms, thereby preventing strong acidification and enhancing the thermal stability of the fertilizer.

Conclusion. Integrating powdered Tebinbulak VM into molten ammonium nitrate at ratios ranging from 100:5 to 100:45, followed by granulation in a tower system, facilitates the synthesis of advanced complex nitrogen fertilizers exhibiting superior physical and chemical properties.

Notably, as granule strength increases, their porosity and internal surface area decline, effectively limiting the infiltration of diesel fuel. This structural refinement significantly reduces the explosive potential of the vermiculite-enhanced ammonium nitrate, offering a safer and more stable fertilizer product.

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