Production technologies and agrochemical properties of potassium chloride

Potassium chloride. Chemically pure KCl contains 63.1% K2O, while the fertilizer, which has a certain amount of impurities, contains from 50 to 60% K2O.

In global production of potash fertilizers, potassium chloride accounts for 80–90%. Potassium chloride is a fine-crystalline fertilizer, white or cream-colored, with an admixture of orange or red crystals. It is obtained by separating sylvinite into potassium and sodium chlorides using the hydrocyclone method, as well as by halurgic and flotation enrichment of potash ores. In the hydrocyclone method, coarse-crystalline potassium chloride is obtained by separating potassium and sodium chlorides based on specific gravity in special "Hydrocyclone" apparatuses. The halurgic production method is based on the different solubility of sylvinite components—potassium chloride and sodium chloride—when the temperature is raised to 90–100 °C. In this process, in solutions saturated with both salts, the content of potassium chloride increases approximately twofold, while that of sodium chloride decreases. Upon subsequent cooling of the solution to 20–25 °C, potassium chloride crystallizes, while sodium chloride remains in the solution. After drying the resulting crystals, fine-crystalline potassium chloride is obtained, which tends to cake during storage.

The flotation method of producing potassium chloride differs from the previous one in that amines are added to the sylvinite as a surfactant to separate KCl from NaCl; these adsorb only on the surface of KCl crystals. During intense aeration, these crystals float, while NaCl crystals settle. Flotation potassium chloride has larger natural crystals, and the reagents on the surface of the KCl crystals drastically reduce the caking of the fertilizer.

Potassium chloride obtained by the halurgic method is a white crystalline substance; that obtained by the flotation method is in the form of reddish particles. Crystalline potassium chloride (grade "K") contains 62–62.5% K2O, while flotation grade (grade "F"), depending on the cultivar, contains from 54 to 60% K2O.

When applied to the soil, potassium chloride dissolves easily in the soil solution and then interacts with the soil adsorption complex through exchange-type and, partially, non-exchangeable adsorption.

Exchangeable, i.e., physicochemical, adsorption of potassium ions by the soil accounts for a significant portion of the exchange capacity. The reaction of physicochemical (exchangeable) adsorption of the K+ cation by the soil is reversible. Exchangeable potassium adsorption proceeds differently depending on soil properties. For soils saturated with calcium, the exchange process can be represented as follows:

[SAC]Ca + 2KCl ⇄ [SAC]2 2

As a result, calcium passes into the soil solution. Soil properties do not change significantly in this process.

In acidic soils, in exchange for potassium ions, the soil solution becomes enriched with hydrogen and aluminum cations:

[SAC] [SAC]

[SAC]Al + 3 KCl ⇄ [SAC]3 3

AlCl3 + 3 Н2О 3 + 3 HCl

The appearance of H+ and Al3+ ions in the soil solution leads to its acidification. Furthermore, additional acidification of the soil solution occurs due to the physiological acidity of potassium chloride: plants consume the potassium cation (K+) significantly more intensely than the accompanying chlorine anion. As a result of exchange reactions, hydrochloric acid is formed in the soil solution. Additionally, further acidification of the soil solution occurs due to hydrochloric acid generated by the hydrolysis of aluminum chloride. Therefore, on acidic sod-podzolic soils, the effectiveness of potash fertilizers decreases. However, the acidifying effect of potash fertilizers on the soil solution is much lower than the similar influence of ammonium nitrate and ammonium fertilizers, and, as a rule, it manifests only after prolonged use of these fertilizers for potassium-loving crops that consume large amounts of potassium.

As a result of potassium transitioning to an exchange-adsorbed state, its mobility in the soil is limited, and leaching beyond the plough layer is prevented, except in soils of light particle-size distribution with low exchange capacity. Potassium from fertilizers that is exchange-adsorbed by the soil is highly accessible to plants.

Partially, potassium is adsorbed by soils in a non-exchangeable manner. Fixation, i.e., non-exchangeable adsorption of potassium by soils, can occur in two ways: through the illitization of a portion of the swelling mineral phase and through the direct entry of potassium cations into existing vacant positions in the crystal lattice of minerals of the hydromica and montmorillonite groups. Potassium cations are especially strongly adsorbed non-exchangeably by vermiculite. Fixed potassium cations are less accessible to plants than exchange-adsorbed ones.

Potassium chloride is the most common potash fertilizer and can be used for all crops and on any soil. It is suitable for basal, at-sowing fertilization, and top dressing. Its application is not recommended for chlorine-sensitive crops. It must be said that the chlorine ion in potassium chloride, in reasonable doses, is not only not dangerous but also has a beneficial effect on plant life. Cells of virtually all plants contain a large amount of chlorine in their vacuolar sap. Plants possess a specific Cl-transport system that ensures rapid absorption of the chlorine ion. Unlike other anions, chlorine does not undergo chemical transformations within the cell and, at a concentration of 2–20 mg/kg of dry matter, plays a positive role in physiological processes:

– participates in photolysis reactions during photosynthesis;

– enhances the uptake of K+, Ca2+, and Mg2+ cations into cells and maintains electroneutrality;

– influences plant hydration by increasing osmotic pressure in cells and their water-holding capacity, controlling the operation of the stomatal apparatus – all this contributes to plant resilience under adverse conditions;

– participates in creating energy reserves in the cell.

With a chlorine content of 15% in fertilizers, it has no negative effect on plants, even those sensitive to this element. The "chlorine-free fertilizer" label serves only for advertising purposes and to justify the high cost of the fertilizer.

Potassium salt (KCl) is a crystalline salt of gray color with inclusions of pink crystals. It contains 41–44% K2O, 20% Na2O, and 50% Cl; it is a physiologically acidic fertilizer. It is obtained by mechanical mixing of potassium chloride with finely ground sylvinite or kainite. Compared to potassium chloride, potassium salt has lower hygroscopicity, but it cakes during storage; in a dry state, it spreads satisfactorily. It is most effective when applied as a sugar beet fertilizer and for forage root crops, which respond positively to sodium and are slightly sensitive to chlorine. It is a good fertilizer for hayfields and pastures. When applying high rates of nitrogen fertilizers, the combination of potassium with sodium in potassium salt reduces the accumulation of nitrate nitrogen in plants and enhances carbohydrate metabolism. For crops sensitive to excess chlorine, it is less suitable than potassium chloride. It is recommended to apply potassium salt as a basal fertilizer. According to technical requirements, it must contain at least 40% K2O.

Potassium sulfate (sulfate of potash). Chemically pure K2SO4 contains 54.1% K2O, while in the technical product used as fertilizer, the K2O content ranges from 45 to 52%. It is produced by methods of exchange decomposition of KCl and MgSO4 (I) and thermal reduction (II):

I. 2 KCl + 2 MgSO4 = K2SO4 · MgSO4 + MgCl2;

K2SO4 · MgSO4 + 2 KCl = 2 K2SO4 + MgCl2

Potassium sulfate precipitates due to low solubility, while highly soluble MgCl2 remains in the solution. The potassium sulfate precipitate is filtered and dried.

II. 2 K2SO4 · 2 MgSO4 + C = K2SO4 + 2 MgO + 2 SO2 + CO2.

Potassium sulfate is leached with water at 100°C, and magnesium oxide remains in the residue; sulfur dioxide is reduced with methane to elemental sulfur.

Potassium sulfate is a fine crystalline salt of white or cream color. The fertilizer has good physical properties: it is non-hygroscopic, spreads well, and does not cake; it should be transported in containers or in bulk in covered railcars. It can be used on any soil and for all crops. Usually, this fertilizer contains 1–2.5% chlorine. Potassium sulfate is a valuable fertilizer for potatoes, vegetables, flax, tobacco, pome/stone fruits, grain crops, and perennial grasses. The presence of the sulfate ion in the fertilizer can have a positive effect on the harvest of cruciferous and legume crops, which consume a significant amount of sulfur from the soil.

Potassium chloride-electrolyte. A product obtained during the production of magnesium from Solikamsk carnallite, it contains 32–45% K2O in the form of KCl; in addition, it contains about 30% NaCl and 2–3% MgCl2 (16% Na2O and 0.2%). Potassium chloride-electrolyte is a highly dusty, fine crystalline powder with a yellow tint; it does not cake and is transported in paper bags or in bulk. The Berezniki plant has mastered the production of granulated potassium electrolyte with a content of 42% K2O and 6–7% MgO. As a basal fertilizer, potassium chloride-electrolyte can be applied in the autumn for all crops. In terms of efficiency, it approaches KCl; on magnesium-deficient light soils, it is more effective than potassium chloride.

Potash (potassium carbonate) K2CO3 is a by-product of nepheline ore processing. It contains 52–55% K2O. This is a concentrated, powdery, easily soluble, highly hygroscopic fertilizer with an alkaline reaction. Potash is recommended for application under potatoes and fruit/berry crops on acidic soils. Its disadvantage is high hygroscopicity. Its alkalinity and lack of chlorine make this fertilizer highly effective on acidic sod-podzolic soils.