RESEARCHING SOME PHYSICO-CHEMICAL PROPERTIES OF NITROGEN AND SULFUR-CONTAINING FERTILIZERS BASED ON MELT OF AMMONIUM NITRATE AND AMMONIUM SULPHATE

ABSTRACT

This work analyzes the methods of producing nitrogen and sulfur-containing fertilizer - ammonium sulfate-nitrate. Under laboratory conditions, samples of nitrogen and sulfur-containing fertilizers were obtained in which the mass ratios of ammonium nitrate (NH₄NO₃) to ammonium sulfate (NH₄)₂SO₄) varied from 97: 3 to 60: 40. In the samples, the pH values of a 10% solution, the beginning of temperature decomposition, strength, caking and absorbency. It is shown that the product obtained by introducing 40% ammonium sulfate into the ammonium nitrate melt contains 29.26% N (18.85% ammonia and 10.41% nitrate forms) and 9.59% S. Its granules are characterized by the following parameters: strength - 7.75 MPa, caking capacity - 1.26 kg/cm², absorption of diesel fuel - 2.29 g in relation to 100 g of product, dissolution time in water - 116 sec., pH - 3.03 and the temperature of the beginning of decomposition - 246.3℃. At the same time, for granules of pure NH₄NO₃ they are 1.32 MPa, 5.62 kg/cm², 4.82 g, 44.6 sec., 90 thermal cycles and 210℃.

АННОТАЦИЯ

В данной работе проанализированы способы получения азот и серусодержащего удобрения – сульфат-нитрата аммония. В лабораторных условиях получены образцы азот и серусодержащих удобрений, в которых массовое соотношение нитрата аммония (NH₄NO₃) к сульфату аммония (NH₄)₂SO₄) менялось от 97 : 3 до 60 : 40. В образцах определены значения рН 10 %-ного раствора, начала температуры разложения, прочность, слёживаемость и впитываемость. Показано, что продукт, полученный путем введения в расплав нитрата аммония 40% сульфата аммония содержит 29.26% N (18.85% аммиачной и 10.41% нитратной формах) и 9.59% S. Его гранулы характеризуются следующими показателями: прочность – 7.75 МПа, слёживаемость – 1.26 кг/см², впитываемость дизельного топлива – 2.29г по отношению 100г продукта, время растворения в воде – 116 сек., рН – 3.03 и температура начала разложения – 246.3°С. В то же время для гранул чистого NH₄NO₃ они составляют 1.32 МПа, 5.62 кг/см², 4,82г., 44.6 сек., 90 термоциклов и 210^оС.

Ключевые слова: плав нитрата аммония, кристаллический сульфат аммония, азот и серусодержащие удобрения, состав, начала температуры разложения, прочность, слёживаемость и впитываемость.

Keywords: ammonium nitrate melt, crystalline ammonium sulfate, nitrogen and sulfur-containing fertilizers, composition, onset of decomposition temperature, strength, caking and absorbency.

Introduction. Ammonium nitrate (AN) is an affordable, ballastless, large-capacity and concentrated nitrogen fertilizer. It is used for all types of crops and on all types of soil. Many countries of the world are engaged in the production of nitrogen fertilizers, including China, Russia, USA, India, Indonesia, Trinidad and Tobago, Canada, etc. According to IFA, in 2017 and 2018 global capacities for the production of nitrogen fertilizers reached 185.10 and 187.0 million tons of nutrient year, respectively [1].

In the CIS countries, AN manufacturers are the Russian Federation, Uzbekistan, Belarus and Kazakhstan, they have more than 28 production facilities. In this group, the leading position is occupied by the Russian Federation, where the volume of annual output exceeds 12 million tons. Here the leaders are JSC UCC Uralchem, JSC Akron, JSC MCC EuroChem, JSC PhosAgro JSC SDS Azot, producing about 8.0 million tons of nuclear power plants [1].

In Uzbekistan, AN is produced by three JSCs: “Maksam - Chirchik”, “Ferganaazot”, “Navoiazot”, the total capacity of which is 1 million 700 thousand tons.

At the same time, in recent years, both AN manufacturers and consumers have witnessed many problematic facts caused by explosions and fires that took place during the production, storage, transportation and use of ammonium nitrate [2].

The world practice of using the AN indicates that it is flammable and explosive [3]. This is due to the fact that it is prone to thermal decomposition and has oxidizing properties. Thermal decomposition of AN takes place upon slow heating, starting from 110℃ and above. At low temperatures, the products of thermal decomposition are ammonia and nitric acid, and at high temperatures, nitrous oxide, nitrogen oxides and oxygen [4]. At the same time, in the high temperature range, the formation of the latter products is explosive. Consequently, due to the thermal decomposition of the AN, a part of the product is lost. In order to completely retard the process of thermal decomposition of the AN or its deceleration, a number of methods and techniques for its stabilization are used in practice [5]. Therefore, the manufacturers have been tasked with ensuring the transition to the production of fertilizers based on AN that retain agrochemical efficiency, with significantly greater resistance to external influences and, accordingly, less explosiveness.

Currently, Uzbekistan has established the production of AFU (nitrogen-phosphorus fertilizer with a content of 22-28% N and 2-6% P₂O₅) [6] and CAN (carbonate-ammonium nitrate with a content of 22-28% N) [7] by introducing into the melt ammonium nitrate before prilling carbonate phosphorite powder (17-18% P₂O₅) or lime flour on the granite.

One of the promising approach for obtaining an explosion-proof fertilizer is the production of NS-fertilizer based on ammonium nitrate and sulfate [8]. In this case, the melt containing 80% AN and 20% ammonium sulphate can be prilled on the tower. For the purpose of physicochemical substantiation of the parameters of the ammonium sulfate-nitrate technology, the solubility of the NH₄NO₃- (NH₄)₂SO₄-Н₂О system was studied by the visual polythermal method in wide temperature and concentration ranges: from -20°С to +110 - + 150℃, close to the boiling points of solutions [9]. In this system, the existence of a crystallization field of five compounds was established: ice, NH₄NO₃, (NH₄)₂SO₄ and two double salts (NH₄)₂SO₄ • 2NH₄NO₃ and (NH₄)₂SO₄ • 3NH₄NO₃. This makes it possible to determine the rational conditions for conducting the process of obtaining ammonium sulfate-nitrate (by cooling the melt or drying the pulp), to calculate the material balances of the stages of mixing, evaporation, cooling, crystallization. It was shown in [10] that the decomposition of molten double salts (NH₄)₂SO₄ • 2NH₄NO₃ and (NH₄)₂SO₄ • 3NH₄NO₃ has no features in comparison with the decomposition of a melt of a mechanical mixture of ammonium nitrate and ammonium sulfate. The introduction of sulfate anions in the form of (NH₄)₂SO₄ into the NH₄NO₃ melt leads to a decrease in the initial rate of its thermal decomposition and the absence of an increase in the rate of decomposition during the process as compared to the kinetic regularities of the decomposition of pure NH₄NO₃. The observed effect is associated with a decrease in the concentration of molecular HNO₃ in the melt due to its ionization by the sulfate anion.

In regard to the agrochemical efficiency of sulfate nitrate [11], the new form of nitrogen fertilizer grade N: S = 32 : 5 surpasses ammonium nitrate in the collection and straw. While a dose of 0.92 ammonium sulfate nitrate is equivalent to the full dose of this fertilizer, which is associated with the positive role of sulfur in its composition. The utilization rate of nitrogen from fertilizers by plants increased by 17-22%, which has not only economic, but also environmental significance. And when growing spring rape, the introduction of sulfate-nitrate provided an increase in seed yield up to 2.48 t/ha. The increase was statistically significant and on average over two years was equal to 0.25 ± 0.04 t/ha.

Phosphogypsum - WPA waste and natural gypsum, the main component of which is calcium sulfate, can also be considered a promising sulfate additive to AN. In [12], the kinetics of the modification transformation IV®III NH₄NO₃ with the addition of CaSO₄ • 2H₂O was studied. It was found that CaSO₄ significantly increases the activation energy of the IV®III transformation and decreases the rate constant of this process by two orders of magnitude. As a result of slowing down the transformation IV®III, the properties of the AS stabilize: a decrease in caking and an increase in mechanical strength.

The aim of this work is to study the physicochemical and commodity properties of ammonium sulfate-nitrate samples obtained by introducing crystalline ammonium sulfate into the ammonium nitrate melt at a wide range of NH₄NO₃ : (NH₄)₂SO₄ mass ratios.

Experimental part. To obtain samples of ammonium sulfate nitrate, pure crystals of NH₄NO₃ and (NH₄)₂SO₄ were taken as starting materials. The latter salt was pre-ground to a particle size of less than 0.25 mm. The experiments were carried out as follows: NH₄NO₃ crystals were melted at 170℃ in a metal reactor on an electric stove. Powders of (NH₄)₂SO₄ were introduced into the melt with stirring in such an amount that the weight ratio of the NH₄NO₃ melt to the additive was equal from 97 : 3 to 60 : 40. The sulfate-nitrate melt was kept at 180-185℃ for 2-3 min, after which it was poured into a granulator, which is a metal glass with a perforated bottom, the hole diameter in which was 1.2 mm. The 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 sieved by particle size. From particles with sizes of 2-3 mm, the strength of granules was measured in accordance with GOST 21560.2-82. The products were then crushed and analyzed using known techniques. The pH value of 10% aqueous suspensions of the test samples was measured on an I-130M laboratory ionometer with an electrode system of ESL 63-07, EVL-1M3.1, and TKA-7 electrodes with an accuracy of 0.05 pH units. The accumulation of fertilizers was determined using the express method. Sagging (X - kg / cm²) was calculated by the formula: X = P / S where, P - breaking force, N (kgf); S is the cross-sectional area of the sample, cm². The absorbency of the granules in relation to liquid fuel (diesel oil) was determined according to the procedure provided for by TU 6-03-372-74 for granular porous AC grade "P". This indicator is expressed in the number of grams that 100 g of granules (g/100 g) can absorb. The temperature of the onset of decomposition of the obtained fertilizers was determined using a NETSCH STA 409 PC/PG device (Germany) in aluminum crucibles at a sample heating rate of 2 deg/min, a sample weight of 10-16 mg. The rate of dissolution of sample granules in water was determined as follows. The fertilizer granule 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. Granular samples of NH₄NO₃ (without additive) and AC (with addition of 0.28% MgO) were selected as samples for comparison with the studied samples. The results are shown in tables and in the figure.

Results and discussions. The results show that the addition of (NH₄)₂SO₄ to the NH₄NO₃ melt significantly increases the crystallization temperature of the melt (Table 1). So, at the studied ratios NH₄NO₃ : (NH₄)₂SO₄ from 97 : 3 to 60 : 40, the crystallization temperature of the nitrate melt increases from 167 (initial NH₄NO₃) to 180.4℃ (in the product at NH₄NO₃ : (NH₄)₂SO₄ = 60 : 40). The table also shows that with an increase in the amount of ammonium sulfate from 3 to 40 g in relation to NH₄NO₃ from 97: 3 to 60: 40 leads to a decrease in the total nitrogen content of nitrogen in the product from 34.53% to 29.26%, while the content of ammonia nitrogen increases slightly (from 17.57 to 18.85%), and the content of nitrate nitrogen decreases significantly (from 16.96 to 10.41). At the same time, sulfur appears in the products, the content of which ranges from 0.70 to 9.59%. This means that the composition of the AN is additionally enriched with another macroelement – sulfur. With an increase in the mass fraction of ammonium sulfate in the NH₄NO₃ melt, the amount of (NH₄)₂SO₄ from 97 : 3 to 60 : 40, the strength of the product granules increases from the initial 1.32 to 7.75 MPa in the final product (Fig. a).

Table 1.

Crystallization temperature and composition of nitrogen-containing fertilizer

Figure 1. The influence of the weight ratio of NH₄NO₃: (NH₄)₂SO₄ on strength (a), caking (b), absorbency (c), temperature onset of decomposition (d), time of complete dissolution of granules of ammonium nitrate (e) and pH of a 10% solution of the product (f)

From Fig., b it follows that the addition of any amount of (NH₄)₂SO₄ significantly reduces the caking of the AN (from 2.72 to 1.26 kg/cm²), which is 1.7-3.7 times less than the caking of standard nitrate with the addition of 0.28% MgO (4.67 kg/cm²).

Another indicator characterizing the quality of a thermostable speaker is the adsorption capacity of granules for liquid fuel. The lower the porosity of the granules, the lower their absorbency should be. As the data in Fig. c in., this situation is really linked to each other. Depending on the weight ratio of the initial components, the absorbency of the nitrogen-sulfur-containing fertilizer granules ranges from 2.29-4.18 g of fuel per 100 g of the product. It is 4.82g for granulated NH₄NO₃. With an increase in the mass fraction of ammonium sulfate in the NH₄NO₃ melt from 97 : 3 to 60 : 40, the temperature of the beginning of decomposition increases from 210℃ for pure NH₄NO₃ to 241.8-246.3℃ in the finished product (Fig. d). This means that the higher the strength and temperature of the beginning of the decomposition of the granules, the less diesel fuel gets inside the granules, and, as a consequence, the less the detonation ability of nitrate.

As the data in Fig. e, the presence of ammonium sulfate in the composition of nitrate affects the rate of dissolution of the granules of the latter. With an increase in the amount of (NH₄)₂SO₄ in the NH₄NO₃ melt, the time for complete dissolution of the finished product granules in water increases. The complete dissolution of granules of pure NH₄NO₃ and AC with a magnesian additive (0.28% MgO) in water is 44.6 sec on average. and 46.8 sec. accordingly, and with the addition of ammonium sulfate in an amount of 3 to 40 g, it slows down the dissolution rate of product granules from 70.32 to 116.24 sec.

This means that AN granules containing ammonium sulfate dissolve much more slowly in the soil solution. Therefore, the presence of ammonium sulfate in the nitrate promotes the gradual release of nitrogen from the pellet. In addition, the addition of ammonium sulfate to the NH₄NO₃ melt reduces the pH of a 10% solution of the latter from 5.17 to 4.12-3.03 (Fig. f), and this is the least effective in calcareous soils.

Conclusion. Research has been carried out to obtain a sulfur-containing nitrogen fertilizer based on melt and powdered ammonium nitrate and sulfate. It was found that the more powder-like ammonium sulfate is introduced into the ammonium nitrate melt, the lower the N content and the higher the S content in the product. It was also shown that the greater the amount of ammonium sulfate, the lower the caking and absorbency of diesel oil and the higher the strength, thermal stability, water solubility and decomposition temperature of nitrate granules. Thus, samples of ammonium sulfate nitrate have improved physicochemical, agrochemical, and lower detonation properties.

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