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Mixed fertilizers

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Mixed fertilizers – complex fertilizers, obtained by mechanical mixing of fertilizers, contain several nutrients. They are used when it is necessary to apply several nutrients at the same time.

Dry mixing of fertilizers is an available, simple and economical method of obtaining complex fertilizers.

In terms of their agrochemical qualities are practically the same as complicated fertilizers. The advantage is the ability to produce a wide range of fertilizers with any ratios of nutrients that meet the requirements of agriculture. For example, in Western European countries the range of mixed fertilizers includes about 100 brands.

Mixtures of fertilizers can be easily adjusted to the requirements of crops, soil and climatic conditions both in terms of concentration and ratio of nutrients. This distinguishes them from complex fertilizers with a constant composition.

Depending on the type of mixed fertilizers the content of nutrients can vary from 25-30%, as with simple superphosphate, ammonium sulfate or ammonium nitrate, to 40% or more in mixtures based on concentrated fertilizers.

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Fertilizers

Mixed fertilizer production

In Russia, the current methods of obtaining dry mixed fertilizers are:

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

Domestic and foreign practice shows the prospects of creating agrochemical centers in districts and chemicalization points in farms. Equipped with warehouses and modern equipment for preparation, mixing and application of fertilizers and chemical means chemicalization points allow to carry out a complex of agrochemical works with qualified control over the receipt and use of fertilizers.

The advantage of this approach is to obtain fertilizers with high quality, good physical and mechanical and physical-chemical properties. To obtain homogeneous mixtures and reduce segregation (stratification) when applying to the soil, it is necessary that granular fertilizers have a homogeneous granulometric composition for all forms.

In the process of preparation and storage components of mixtures may be reactive, entering into chemical interaction with each other. The quality of mixtures obtained, their chemical composition and physical properties depend on the chemical processes that take place when mixing fertilizers. Therefore, it is important to choose the right components. Basic rules for mixing fertilizers:

  1. Do not mix fertilizers if it is possible to lose nutrients or their transformation into a poor in physical properties mass, not amenable to mechanized application.
  2. Due to the high hygroscopic nature of the mixture, ammonium nitrate and urea must not be mixed.
  3. Do not mix ammonia forms of nitrogen fertilizers, including complex fertilizers, with fertilizers with an alkaline reaction (phosphate slag, thermophosphate, calcium cyanamide, cement dust, potash) to avoid loss of nitrogen in the form of ammonia.
  4. Moisture content should not exceed permissible value. Higher moisture content reduces flowability and does not ensure uniform application. Acceptable moisture content in ammonium nitrate – no more than 0,2-0,3%, in urea – no more than 0,2-0,25% (0,12%[1]Yagodin BA, Zhukov JP, Kobzarenko VI Agrochemistry / Edited by BA Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.), ammophos, diammophos, and potassium chloride no more than 1 percent, in superphosphates (simple and double) no more than 3.5 percent (with free acidity not more than 1 percent). If the moisture content is increased, granules lose their strength. For ammonium nitrate this condition is noted at moisture content of 1.7-2.0%, urea – 1%, potassium chloride – over 3%. Moisture content in fertilizers increases with increasing storage temperature. Thus, a mixture of urea with double superphosphate and potassium chloride at initial moisture content of 0.2% after a month of storage at 4 ° C contained 6.6% moisture, at 20 ° C – 8.3%, at 40 °C – 24.9%.
Fertilizer mixing scheme
Fertilizer mixing scheme
  1. The number of granules 1-3 mm in size must be at least 90%, including a diameter of 2-3 mm – at least 50%, particles less than 1 mm – no more than 1%. Destruction of granules during mixing should be not more than 3%, strength – not less than 2 MPa (20 kg/cm2).
  2. The acidity or alkalinity of mineral fertilizers must comply with technical specifications. Fertilizers containing free acid or having an alkaline reaction, chemically interact with each other, and when mixed with other fertilizers. The content of free phosphoric acid in simple granulated superphosphate – no more than 2.5%, in double – no more than 5%. Mixtures based on double superphosphate are wetter than those based on simple superphosphate. The negative effect of excess acidity of double superphosphate is manifested when mixtures are stored in conditions of high humidity. Therefore, double superphosphate is an undesirable component of mixtures, and mixtures based on it are not prepared in advance.

Full neutralization of superphosphate or reducing the free acid content to 1% and moisture content to 4% in a simple one and to 3% in a double one, allows to mix it with urea and potassium chloride, receiving fertilizer of 1:1:1 composition. Mixtures of granulated ammophos with potassium chloride, neutralized superphosphates and ammonium sulfate have good physical properties, low hygroscopicity which provides the possibility of long-term storage.

  1. When neutralizing materials are added, such as limestone, dolomite flour, ammonia losses are noted.
  2. Mixtures of good quality are obtained on the basis of phosphate meal. The effectiveness of mixtures on the basis of superphosphate and phosphate flour in a ratio of 1:1, made in the occupied fallow or under plowing on acidic sod-podzolic soils and leached chernozems are not inferior to mixtures on pure superphosphate. For acidic soils, mixtures of potash fertilizer with phosphate meal are used. A mixture of ammonium nitrate and phosphate flour can be prepared and made under autumn plowing. It does not caked, stored for a long time. The presence of NH4NO3 and KCl increases the solubility of P2O5 phosphate meal. Adding 10% of ammonium nitrate and urea mixture to phosphate flour due to increased hygroscopicity reduces the dispersibility of phosphate flour while maintaining the stability of the seeding apparatus of the spreader.
  3. Superphosphate, especially in powder form, should not be mixed with ammonium nitrate, as the mixture turns into a sticky mass due to the formation of hygroscopic calcium nitrate:

Ca(H2PO4)2 + 2NH4NO3 = Ca(NO3)2 + 2NH4H2PO4.

Free phosphoric acid of superphosphate, interacting with ammonium nitrate, leads to the formation of nitric acid, which, decomposed or evaporating, leads to a loss of nitrogen:

Н3РО4 + NH4NO3 = HNO3 + NH4H2PO4;

4HNO3 = 4NO2 + 2H2O +O2;

Therefore, the mixing of these fertilizers should be done right on the day of application.

  1. Mixing superphosphate with urea leads to the release of crystallization water, which increases the moisture content of mixtures. For example, 12.2 to 64.7 g of crystallization water per 1 kg of mixture was released from the interaction of the components of mixtures from standard N, P, and K forms, whereas 7.2 to 13.5 g per 1 kg of mixture was released when dried products were mixed.
  2. The mixture of superphosphate and ammonium sulfate is cemented into a dense mass, which must be crushed and sifted before application. During mixing, the mass is heated and made wet by the release of water:

Са(Н2РO4)2⋅Н2О + (NH4)2SO4 → 2NH4H2PO4 + CaSO4 + H2O,

then the gypsum is formed:

CaSO4 + 2H2O = CaSO4⋅2Н2O.

To obtain good quality mixtures it is desirable to use neutral forms of phosphate fertilizers (ammophos, ammoniated superphosphate), which allow to obtain dry and loose mixtures with stable physical properties. Ammophos, moreover, provides a high concentration of nutrients: more than 50% of NPK instead of 28-31% in superphosphate, which saves transportation costs, warehouse construction costs, reduces the cost of loading, unloading and application of fertilizers. Of potash fertilizers the main component for mixing is potassium chloride, but for chlorophobic crops (potatoes, tobacco, grapes, citrus) it is better to replace it with chloride-free, such as potassium sulfate.

The quality of fertilizer mixtures is determined by the ratio of nutrients. Mixtures with a predominance of phosphorus and potassium over nitrogen are more likely to be drier and drier than mixtures of similar composition with an equal ratio of nutrients or with a predominance of nitrogen.

Due to increased production and use of urea, its use as a nitrogen component is being investigated. Mixtures with urea are moistened during storage due to the release of crystallization water. Stability of physical properties of such mixtures is increased by introduction of alkaline additive in an amount not less than 15 % of mixture weight. Urea is highly reactive, especially with potassium chloride. When it is included in the mixture moisture content increases sharply. To reduce the hygroscopicity of mixtures based on urea it is not recommended to include chlorides, as formed by the chemical interaction of CaCl2 and NH4Cl, are hygroscopic and accompanied by nitrogen loss.

In the mixtures, granules of 2-3 mm in size are more evenly distributed and particles less than 1 or more than 3 mm are unevenly distributed. Mixtures consisting of grains of different sizes and densities are subject to segregation, become heterogeneous during storage, transportation, mechanical incorporation into the soil.

Requirements for physical and chemical properties of the mixture depend on the amount of mixing, timing and methods of preparation, the scheme of transportation of fertilizers to the field. Physical properties of mixed fertilizers can be improved by introducing additives: chalk, limestone, phosphate meal.

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

Used for dry mixing one-sided and unbalanced composition fertilizers must retain loose and granulometric composition during transportation and storage in bulk for 6 months. The use of several components with improved physical and chemical properties allows to prepare mixed fertilizers, suitable for long-term storage. For example, the introduction of neutralizing additives and ammoniated superphosphate eliminates the release of nitric acid, the conversion of monocalcium phosphate in dicalcium phosphate, while improving the physical properties of the fertilizer.

When preparing mixtures you can quickly change the dosage of components depending on the crop, the fertility of the particular site, the form of fertilizer, etc. Therefore, the use of mixed fertilizers is a reserve for increasing their effectiveness. Increasing the volume of mixtures preparation is associated with higher requirements for the quality of fertilizers, granulometric composition and strength of granules, the availability of storage sites, a set of machines to mechanize technological processes. Mechanized preparation and application of the economic effect compared with the separate application of one-sided fertilizers.

Currently, the share of mixed fertilizers produced at chemical plants is increasing. This combines the mixing of fertilizers with their additional chemical treatment, the introduction of acids and neutralizing additives, more advanced granulation technology. The list of such fertilizers includes:

  1. Granulated complex mixed fertilizers obtained by ammonization of a mixture of simple superphosphate, potassium chloride and ammonium nitrate with the addition, if necessary, of sulfuric and phosphoric acids.
  2. Complex mixed fertilizers, enriched with or without trace elements, obtained by ammonization of a mixture of simple superphosphate, potassium chloride and ammonium nitrate.
  3. Pressed phosphate-potassium fertilizers, produced on the basis of a mixture of simple superphosphate and potassium chloride.
  4. Nutrient mixtures of 9-9-9 grade, enriched with microelements, on the basis of superphosphate, potassium magnesia, ammonium sulfate; with 22 to 56% of nutrients on the basis of superphosphate, urea, ammonium nitrate, potassium chloride or sulfate, limestone, dolomite, and fertilizer mixture of 12-12-12 grade are produced for retail sale.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry/Under ed. B.A. Yagodin. – M.: Kolos, 2002. – 584 p.: ill.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.