Mixed Fertilizers

Production of Mixed Fertilizers

In our country, the following methods of producing dry mixed fertilizers are currently used:

• mixing directly on farms using stationary or mobile fertilizer-mixing units;
• use of stationary high-capacity units (40–60 t/h) to meet the needs of several farms;
• mixing fertilizers at chemical plants.

Domestic and foreign practice shows the promise of establishing agrochemical centers in districts and chemicalization points on farms. Equipped with warehouses and modern equipment for preparing, mixing and applying fertilizers and chemicalization agents, chemicalization points make it possible to carry out a complex of agrochemical operations with qualified control over the receipt and use of fertilizers.

The advantage of this approach is obtaining fertilizers with high quality and good physical-mechanical and physicochemical properties. To obtain mixtures uniform in composition and reduce segregation (stratification) during application to the soil, granular fertilizers must have a particle-size distribution uniform for all forms.

During preparation and storage, mixture components may exhibit reactivity, entering into chemical interaction with each other. The quality of the resulting mixtures, their chemical composition and physical properties depend on the chemical processes that occur when fertilizers are mixed. Therefore, it is important to choose the component ingredients correctly. Basic rules for mixing fertilizers:

1. Fertilizers must not be mixed if loss of nutrients is possible or their conversion into a mass with poor physical properties that cannot be applied mechanically.

2. Because of the high hygroscopicity of the mixture, ammonium nitrate and urea must not be mixed.

3. Ammoniacal forms of nitrogen fertilizers, including compound ones, must not be mixed with fertilizers having an alkaline reaction (phosphate slags, thermophosphates, calcium cyanamide, cement dust, potash) to avoid nitrogen losses as ammonia.

4. Moisture content must not exceed the permissible value. Increased moisture reduces free-flowing properties and does not ensure uniform application. The permissible moisture content in ammonium nitrate is no more than 0.2–0.3%, in urea — no more than 0.2–0.25% (0.12%footnote:[{yagodin-1}]), in ammophos, diammophos and potassium chloride — no more than 1%, in superphosphates (simple and double) — no more than 3.5% (with free acidity no more than 1%). With elevated moisture content, granules lose strength. For ammonium nitrate this state is noted at moisture 1.7–2.0%, for urea — 1%, for potassium chloride — above 3%. Moisture content in fertilizers increases with rising storage temperature. Thus, a mixture of urea with double superphosphate and potassium chloride at initial moisture 0.2% after one month of storage at 4 °C contained 6.6% moisture, at 20 °C — 8.3%, at 40 °C — 24.9%.

5. The number of granules 1–3 mm in size must be at least 90%, including those 2–3 mm in diameter — at least 50%, particles smaller than 1 mm — no more than 1%. Granule breakdown during mixing must be no more than 3%, strength — at least 2 MPa (20 kg/cm²).

6. Acidity or alkalinity of mineral fertilizers must conform to technical specifications. Fertilizers containing free acid or having an alkaline reaction interact chemically both with each other and when mixed with other fertilizers. Content of free phosphoric acid in simple granular superphosphate — no more than 2.5%, in double — no more than 5%. Mixtures based on double superphosphate become moistened more strongly than those based on simple superphosphate. The negative effect of excess acidity of double superphosphate manifests itself when mixtures are stored under high humidity. Therefore, double superphosphate is an undesirable component of mixtures, and mixtures based on it are not prepared in advance.

Complete neutralization of superphosphate or reduction of free acid content to 1% and moisture to 4% in simple and to 3% in double superphosphate allows it to be mixed with urea and potassium chloride, producing fertilizers of 1:1:1 composition. Mixtures of granular ammophos with potassium chloride, neutralized superphosphates and ammonium sulfate have good physical properties and low hygroscopicity, which ensures the possibility of long-term storage.

7. When neutralizing materials are added, for example limestone or dolomite meal, ammonia losses are observed.

8. Good-quality mixtures are obtained on the basis of phosphorite meal. The effectiveness of mixtures based on superphosphate and phosphorite meal in a 1:1 ratio, applied to occupied fallow or under autumn plowing on acidic sod-podzolic soils and leached chernozems, is not inferior to mixtures based on pure superphosphate. For acidic soils, mixtures of potassium fertilizers with phosphorite meal are used. A mixture of ammonium nitrate and phosphorite meal can be prepared and applied under autumn plowing. It does not cake and stores for a long time. The presence of NH₄NO₃ and KCl helps increase the solubility of P₂O₅ in phosphorite meal. When 10% of a mixture of ammonium nitrate and urea is added to phosphorite meal, owing to increased hygroscopicity, the dustiness of phosphorite meal is reduced while maintaining stable operation of the spreader's metering unit.

9. Superphosphate, especially powdered, must not be mixed with ammonium nitrate, since the mixture turns into a sticky mass due to formation of hygroscopic calcium nitrate:

Free phosphoric acid in superphosphate, reacting with ammonium nitrate, leads to formation of nitric acid, which, decomposing or evaporating, leads to nitrogen losses:

Therefore, mixing these fertilizers should be carried out directly on the day of application.

10. Mixing superphosphate with urea leads to release of water of crystallization, which increases the moisture content of mixtures. For example, from interaction of components of mixtures made from standard forms of N, P and K, 12.2 to 64.7 g of water of crystallization were released per 1 kg of mixture, whereas when mixing dried products — 7.2–13.5 g per 1 kg of mixture.

11. A mixture of superphosphate and ammonium sulfate cements into a dense mass, which must be crushed and sieved before application. During mixing, the mass heats up and becomes moist as a result of water release:

then gypsum is formed:

To obtain good-quality mixtures, it is desirable to use neutral forms of phosphorus fertilizers (ammophos, ammoniated superphosphate), which make it possible to obtain dry and free-flowing mixtures with stable physical properties. Ammophos, moreover, provides a high concentration of nutrients: more than 50% NPK instead of 28–31% in superphosphate, which saves transportation costs and costs of warehouse construction, reduces the cost of loading, unloading and application of fertilizers. Among potassium fertilizers, the main component for mixing is potassium chloride; however, for chlorine-sensitive crops (potato, tobacco, grapes, citrus), it is better to replace it with chlorine-free ones, for example potassium sulfate.

Quality of fertilizer mixtures is determined by the ratio of nutrients. Mixtures with a predominance of phosphorus and potassium over nitrogen more often turn out drier and more free-flowing than mixtures of analogous composition with a balanced nutrient ratio or with a predominance of nitrogen.

In connection with the increase in production and use of urea, the possibility of using it as a nitrogen component is being studied. Mixtures with urea become moist during storage because of release of water of crystallization. The stability of the physical properties of such mixtures is increased by introducing into their composition an alkaline additive in an amount of at least 15% of the mixture mass. Urea is distinguished by high reactivity, especially with potassium chloride. When it is included in a mixture, the moisture content of the mixture increases sharply. To reduce the hygroscopicity of urea-based mixtures, it is not recommended to include chlorides in their composition, since CaCl₂ and NH₄Cl formed as a result of chemical interaction are hygroscopic and are accompanied by nitrogen losses.

In mixtures, granules 2–3 mm in size are distributed more uniformly, while particles smaller than 1 mm or larger than 3 mm are distributed unevenly. Mixtures consisting of grains of different sizes and densities are subject to segregation and become nonuniform during storage, transport, and mechanical application to the soil.

Requirements for the physicochemical properties of a mixture depend on the volumes of mixing, the timing and methods of preparation, and the scheme of transporting fertilizers to the field. The physical properties of mixed fertilizers can be improved by introducing additives: chalk, limestone, phosphorite meal.

Mixed fertilizers are produced for application immediately after mixing and in advance with subsequent storage.

Straight and compositionally unbalanced fertilizers used for dry mixing must retain free-flowing properties and particle-size distribution during transport and during storage in bulk for 6 months. The use of several components with improved physicochemical properties makes it possible to prepare mixed fertilizers suitable for long-term storage. Thus, introduction of neutralizing additives and ammoniated superphosphate eliminates the release of nitric acid and the conversion of monocalcium phosphate into dicalcium phosphate, while improving the physical properties of the fertilizer.

When preparing mixtures, the dosing of components can be promptly changed depending on the crop, fertility of a specific plot, form of fertilizers, etc. Therefore, the use of mixed fertilizers is a reserve for increasing their effectiveness. The increase in volumes of mixture preparation is associated with higher requirements for fertilizer quality, particle-size distribution and granule strength, availability of storage places, and a set of machines for mechanizing technological processes. Mechanized preparation and application give an economic effect compared with separate application of straight fertilizers.

At present, the share of mixed fertilizers produced at chemical plants is increasing. In this case, mixing fertilizers is combined with their additional chemical treatment, introduction of acids and neutralizing additives, and more advanced granulation technologies. The list of such fertilizers includes:

• Granulated complex-mixed fertilizers obtained by ammoniation of a mixture of simple superphosphate, potassium chloride and ammonium nitrate with addition, if necessary, of sulfuric and phosphoric acids.
• Complex-mixed fertilizers enriched with micronutrients or without them, obtained by ammoniation of a mixture of simple superphosphate, potassium chloride and ammonium nitrate.
• Pressed phosphorus-potassium fertilizers obtained on the basis of a mixture of simple superphosphate and potassium chloride.
• For retail trade, nutrient mixtures of grade 9-9-9 are produced, enriched with micronutrients, based on superphosphate, potassium magnesium sulfate (kalimagnesia), ammonium sulfate; with nutrient content from 22 to 56% based on superphosphate, urea, ammonium nitrate, potassium chloride or sulfate, limestone, dolomite; and a fertilizer mixture of grade 12-12-12.

References

1. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.
2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. B. A. Yagodin. Moscow: Kolos.