Natural sources of raw materials for the production of mineral phosphate fertilizer.

Apatites: concentrated raw material for water-soluble fertilizers

The efficiency of phosphorus plant nutrition directly depends on the origin of the raw material from which the fertilizers are produced. The main source for obtaining water-soluble forms of phosphorus is apatite — hard crystalline minerals of magmatic origin. Due to the high strength of the crystal lattice, this raw material requires mandatory chemical or thermal processing with the removal of fluorine. Without this process, phosphorus remains in a form inaccessible to plants.

Depending on the chemical composition, four varieties of apatite are distinguished:

• Fluorapatite (Ca₅[PO₄]₃F) — the most common in nature; theoretically contains 42.24% P₂O₅ and 55.51% CaO (a portion of the calcium can be isomorphically substituted by strontium and divalent manganese).
• Chlorapatite (Ca₅[PO₄]₃Cl).
• Hydroxyapatite (Ca₅[PO₄]₃(OH)).
• Oxyapatite (Ca₁₀[PO₄]₆O).

Apatites also contain rare-earth elements of the cerium group. The largest source of this raw material in Russia is the Khibiny deposit on the Kola Peninsula, where the ore occurs in the form of apatite-nepheline rock. Deposits also exist in Buryatia (Oshurkovskoye deposit). On a global scale, large reserves of apatite are concentrated in Sweden and the carbonatites of Africa, although their industrial significance is low.

• P₂O₅ content in concentrate — about 40%
• Reserves of the Khibiny deposit — about 640 million tons of P₂O₅
• P₂O₅ content in granular phosphorites — 22–30%
• P₂O₅ content in bedded phosphorites — 28–36%

Phosphorites: porous structure for direct application

Phosphorites are sedimentary amorphous or fine-crystalline aggregates consisting of a mixture of apatite, calcite, gypsum, and other minerals. They differ from apatite in their high porosity, which significantly facilitates their dissolution. The geologically younger the phosphorite, the less pronounced its crystalline structure and the faster it is assimilated by plants.

The study of domestic phosphorites began as early as the 1860s, and in the 1880s, field trials proved the effectiveness of the first rock phosphate on sod-podzolic soil. According to the nature of the deposit, the ore is divided into nodular (concretionary), granular, and bedded types. Depending on the structure and chemical composition, the raw material is sent either for deep chemical processing or directly for grinding.

The quality of raw material and the deposition features of major Russian phosphorite sources differ significantly. Detailed characteristics of these sources are provided in the table:

Similar sandy phosphorites, cemented into a slab, also occur in the Voronezh, Tambov, Oryol, Bryansk, Kaluga, and Smolensk regions. Due to the low phosphorus content, they cannot be used to produce superphosphates. The only rational way to use such raw material remains grinding it into rock phosphate.

Domestic phosphorites have two serious drawbacks — low phosphorus concentration and high sesquioxide content. For processors, this means technological difficulties and additional raw material consumption. Sesquioxides cause retrogradation of soluble salts of phosphoric acid, converting them back into an unassimilable form. As a result, significantly more sulfuric acid is required to decompose impurities during superphosphate production.

Classification of fertilizers by solubility and availability

The solubility of a phosphorus compound directly determines its method of application and behavior in the soil. Based on this, all fertilizers are divided into three groups: water-soluble, citrate- and citrate-soluble, and poorly soluble. The main task of phosphate raw material processing technologies is to convert phosphorus into a form that plants can easily assimilate. The degree of availability determines how quickly and fully the fertilizer application will pay off on a specific soil type.

Most soils in our country have a reaction close to neutral. In such conditions, water-soluble forms of phosphorus, which are produced in the form of granules or powder, are the most effective. Citrate- and citrate-soluble fertilizers are applied as powders — they are also well absorbed by crops. Sparingly soluble phosphates require exclusively fine grinding, as they do not dissolve in water or weak acids.

Phosphorus from sparingly soluble compounds converts into an available form slowly and gradually. This process is facilitated by the natural acidity of the soil and the root exudates of the plants themselves. The release of the element is also accelerated if sparingly soluble phosphates are applied together with physiologically acidic mineral fertilizers.

• Share of water-soluble phosphates in global production — about ¾
• Share of sparingly soluble fertilizers in production — about 8 %
• Content of P₂O₅ in concentrated fertilizers — more than 25 %