Направления оптимизации переработки экстракционной фосфорной кислоты

DOI:

Phosphorus is one of the essential nutrients for plants and plays a crucial role in achieving high yields in agriculture. Therefore, extractive phosphoric acid (EPA), which is used as the main raw material in the production of phosphate fertilizers, is considered a strategically important product in the chemical industry [1-4]. However, EPA obtained from natural phosphorites and apatites contains numerous impurities, such as iron, aluminum, magnesium, organic substances, as well as fluorine and sulfate-containing compounds, which reduce the quality of the product and limit its use in the production of high-grade phosphate fertilizers and other chemicals.

In recent years, special attention has been given to improving the purification and concentration processes of EPA. In particular, new technological solutions have been developed based on crystallization, extraction, ion exchange, membrane separation, precipitation, and thermal methods. Such approaches make it possible to improve product quality, reduce waste, and save energy resources.

From this perspective, an in-depth study of purification and concentration technologies for EPA, as well as the identification of their promising directions, is a relevant scientific and practical task. This paper analyzes existing technologies, examines their advantages and disadvantages, and highlights the prospects of innovative solutions that may be applied in industry.

Recent developments in wet-process phosphoric acid (WPA) and EPA processing have also been driven by the need to meet stricter environmental regulations and sustainability requirements. Industrial production generates significant amounts of phosphogypsum and other by-products, which pose challenges for storage and disposal. Consequently, modern purification strategies are designed not only to enhance product quality but also to minimize waste volumes and enable recycling of process residues into commercially valuable materials.

Another important factor influencing the choice of technological solutions is the variability in the composition of phosphorus-containing raw materials from different deposits. For example, Karatau, Central Kyzylkum, and other phosphorites differ in their impurity profiles, requiring tailored processing methods to achieve optimal purification and concentration [3]. This makes the development of flexible, adaptive technological schemes a priority for ensuring stable production under varying supply conditions.

Furthermore, economic considerations play a decisive role in the implementation of new purification technologies. While some methods offer high removal efficiency, they may involve significant reagent costs or energy consumption. Therefore, research must aim at balancing technological performance with cost-effectiveness, enabling large-scale application in existing industrial systems without excessive capital investments.

MATERIALS AND METHODS

To date, many methods for purifying phosphate solutions have been studied, mainly including extraction with organic solvents, ion exchange, recrystallization, precipitation, and electrochemical techniques (see Table 1). The ion exchange method is completely unsuitable for purifying phosphoric acid due to the high content of impurities in the solution. Recrystallization is also ineffective because of the low concentration of phosphoric acid in the solution. The basis of extractive phosphoric acid purification lies in precipitation and extraction methods using certain organic solvents. The use of other purification methods is mainly advisable for obtaining highly concentrated phosphoric acids.

Table 1. Advantages and disadvantages of phosphoric acid purification methods.

Method	Advantages	Disadvantages	Field of Practical Application
Ion exchange	Theoretically allows for a high degree of purification	Unsuitable due to the high amount of impurities in the solution	Not applied in practice
Recrystallization	Can be applied under simple laboratory conditions	Very low efficiency due to the low concentration of H3PO4 in the solution	Limited to theoretical or restricted laboratory work
Precipitation	Performed using inexpensive reagents, widely used in industry	Very low efficiency due to the low concentration of H3PO4 in the solution	Limited to theoretical or restricted laboratory work
Extraction with organic solvents	High selectivity, ability to remove many impurities	High solvent consumption, regeneration issues are relevant	Effectively applied in industry

Method	Advantages	Disadvantages	Field of Practical Application
Electrochemical method	Environmentally friendly, does not require additional reagents	High energy consumption, rarely used in industry	Mainly at the level of scientific research
Other methods (membrane, ion-selective technologies)	Innovative approach, potentially high efficiency	Not widely tested on a large scale, expensive	Promising for production of highly concentrated WPA (wet-process phosphoric acid)

In studies related to the purification of wet-process phosphoric acid, the main challenge is the removal of various impurities, which requires the use of multistage and labor-intensive methods. In this context, different organic compounds are needed to separate impurities of both cationic and anionic nature. Nevertheless, complete purification of WPA remains practically impossible, and it cannot be achieved without the combined application of various chemical, physical, and physico-chemical methods.

One of the key issues in WPA purification is the separation of sulfate ions. These are formed as a result of processing apatite with sulfuric acid and become part of the phosphogypsum precipitate, which in turn causes serious difficulties in filtering and separating phosphoric acid from the precipitate. One of the promising methods for clarifying WPA is the separation of solid precipitates in high-rate settlers with thin layers, where the process is accelerated in the presence of coagulants.

Table 2 summarizes the key technological issues associated with phosphogypsum formation during the wet-process production of phosphoric acid. It outlines the main problems, their underlying causes, and potential solution pathways, while also highlighting the respective advantages and limitations of each approach. The comparison provides insight into optimizing sulfate ion removal and improving wet-process phosphoric acid quality.

Table 2. Problems and solutions in phosphogypsum formation.

Problem	Cause	Solution Pathways	Advantages	Limitations
Formation of phosphogypsum	Sulfate ions transfer into the precipitate during the processing of apatite with H2SO4	Separation in thin-layer settlers	High-rate clarification, improved filtration efficiency	Increased consumption of coagulants
Filtration difficulties	Phosphogypsum crystals are fine and sticky	Use of coagulants (accelerated sedimentation)	Process is accelerated, precipitate becomes denser	Requires optimal selection of coagulants
Incomplete removal of sulfate ions	Conventional precipitation is insufficient	Precipitation with strontium salts	Enables complete removal of sulfate ions	High reagent cost, requires economic justification
Barriers to obtaining high-quality WPA	Residual SOi’ ions remain in solution	Combination of chemical and physico-chemical methods	Improves WPA quality, makes it suitable for subsequent stages	Increases technological complexity of the process

It has been established that the use of precipitation reagents such as strontium salts has a positive effect in the deep purification of wet-process phosphoric acid, making it possible to completely remove sulfate ions. A developed method for obtaining WPA through the utilization of phosphorus-containing wastes from phosphorus production provides for the significant neutralization of the negative effects of MgO in technological processes. To achieve this, a number of innovations with a positive influence on the process were applied, whereby MgO was precipitated from the phosphoric acid solution and removed together with phosphogypsum.

RESULTS AND DISCUSSION

The developed impurity removal methods are interlinked and function as inseparable components of a unified technological complex. This complex plays a central role in the production of WPA from Karatau phosphorites or other types of phosphorus raw materials.

The interdependence of the purification stages lies in the fact that their effective implementation ensures the stability of the overall process and determines the quality of the final product. At the same time, the developed methods not only serve to remove impurities from phosphoric acid, but also contribute to the efficient utilization of phosphorus-containing wastes generated in the phosphorus production process.

As a result, this approach allows for:

• •    improvement of phosphoric acid quality,

• •    ensuring the energetic and economic efficiency of the technological process,

• •    reduction of environmental burden through waste minimization.

Thus, applying impurity removal methods as a unified system forms an integral part of WPA production technology and creates a scientific and practical basis for its future improvement.

In the literature, various methods for the purification of technical WPA are widely covered, including evaporation and precipitation, solvent extraction, ion exchange, adsorption, and recrystallization [5-7]. These studies compare the scientific basis of purification technologies and their efficiency.

Particular importance is attached to experiments conducted on washed and calcined phosphate concentrate from the Central Kyzylkum region [6]. In these experiments, phosphoric acid was produced by the dihydrate method with the additional provision of a mass ratio of phosphoric acid to acetic acid (H3PO4 : CH3COOH = 1 : 4). The mixing time was set at 30 minutes, and the temperature at 25 °C. Under these conditions, the following impurities were effectively removed from WPA:

• •   CaO – 69.29%;

• • MgO – 79.44%;

• •   Al₂O₃ - 81.41%;

• •   Fe2O3 - 82.9%;

• •   SO3 - 85.66%.

As a result, the amount of major impurities in the acid was significantly reduced, thereby improving the quality of WPA.

In addition, the physicochemical properties of phosphoric acid obtained from apatite were studied. It was found to have the following parameters:

• •    density: 1650-1750 kg/m³;

• •    composition, %:

• •   P2O5 — 52—54;

• •    SO3 - 3.4-4.2;

• •   (Fe,Al)2O3 - 1.2-1.3;

• •   SiO2 - 0.1-0.4;

• •    F – 0.5–0.8.

For the efficient organization of the evaporation process, special barbotage concentrators are used. These devices, made of heat-resistant materials, operate by bubbling hot gases through the acid layer during the evaporation process. This method makes it possible to achieve a high degree of WPA concentration and is widely applied in industrial practice.

During the concentration of WPA to 52–55% in vacuum-evaporation units, the degree of fluorine release into the gas phase reaches 80–90%. Under these conditions, the residual fluorine content in the concentrated acid decreases to 0.5–0.8%. However, when superphosphoric acid (70-76% P2O5) is obtained in drum-type concentrators, the residual fluorine content is further reduced to 0.05–0.15%.

3SiF 4 + (n +2)H 2 O = 2H 2 SiF 6 + SiO 2 + nH 2 O.

These data show that in evaporation technology, the type of equipment used and the process conditions directly affect not only the concentration of phosphoric acid, but also the content of harmful impurities, particularly fluorine. Therefore, the concentration stage plays a crucial role in obtaining high-quality wet-process phosphoric acid.

Thus, several methods can be employed for the purification of WPA. One of the most important stages in the purification of WPA is the removal of fluorine. In phosphoric acid, fluorine is mainly present in the form of hydrofluoric acid (HF) and fluosilicic acid (^SiFg). One of the practically applied methods for removing fluorine from the solution is its precipitation in the form of fluosilicate salts.

In this process, soluble sodium salts such as sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), or sodium chloride (NaCl) are used as precipitating agents. The following reactions occur in this case:

H 2 SiF 6 + Na 2 CO 3 = Na 2 SiF 6 + CO 2 + H 2 O;

H 2 SiF 6 + Na 2 SO 4 = Na 2 SiF 6 + H 2 SO 4 ;

H 2 SiF 6 + 2NaCl = Na 2 SiF 6 + 2HCl.

As a result, the sodium fluosilicate (Na2SiFg) formed precipitates as a solid phase and can be easily separated from the solution. This ensures significant purification of WPA from fluorine-containing impurities.

The advantage of this method is that the precipitating agents are inexpensive and readily available, while the resulting precipitate is stable and significantly improves the quality of phosphoric acid.

The sodium fluosilicate (Na2SiFe) obtained from fluorine precipitation in WPA is a poorly soluble salt. Since its solubility in water at 20 °C is only 0.78%, it precipitates from the solution as a stable solid phase.

In practice, soda (Na2CO3) or its mixture with water glass (Na2SiO2) is most often used as a precipitating agent. Potash (K2CO3), however, has not been widely applied on an industrial scale due to its high cost.

The resulting Na2SiFg usually forms a fine-dispersed precipitate, which is separated from the solution either by sedimentation or filtration. During filtration, special filter presses are often employed. To accelerate the process and improve the agglomeration of the precipitate, flocculants are added to the solution. The use of flocculants leads to the separation of the precipitate in denser aggregates, thereby increasing the efficiency of the filtration process.

Let us consider the purification of wet-process phosphoric acid by organic solvents. The purification of WPA using organic solvents is based on the principle of extraction from its aqueous solutions into an organic phase [8, 9]. In this process, unlike the “impure acid” which retains fluorine and other impurities, phosphoric acid is transferred into the organic solvent. Subsequently, the organic extract is separated, and purified acid is recovered from it by re-extraction (back-extraction with water) or by distillation.

A wide range of organic compounds are used in industry and experimental practice for the extraction of phosphoric acid, including: methyl isobutyl ketone (MIBK), isobutanol, tributyl phosphate (TBP), isopropyl alcohol, acetone, trialkylamines, and other solvents.

This approach enables a high degree of WPA purification; however, the economic efficiency of the process depends on the price of the solvent, its regeneration capability, and the efficiency of the extraction itself.

Some studies have focused on removing impurities from WPA obtained from Central Kyzylkum phosphorites. At the initial stage of the process, desulfation (neutralization of sulfate ions) is carried out. Then, the gypsum-containing solid suspension formed is separated. According to the studies, the initial WPA composition was as follows (wt.%):

• •   P2O5 - 22.6;

• •    CaO – 0.52;

• •    F – 1.94;

• •    solid particles in suspension – 1.5.

In the neutralization process, the norm of NMSK (neutralizing material – calcium sulfate) was calculated relative to the amount of CaO and determined on the basis of SO3 ions bound in CaSO4. This norm varied within 20–100% of stoichiometry.

The process was carried out at a temperature of 60 °C under continuous stirring for 20 minutes. As a result, the sulfate ion content in WPA was significantly reduced, and the separation of gypsum precipitates made it possible to further improve the acid quality at subsequent stages.

CONCLUSION

The conducted analysis confirms that the optimization of extractive phosphoric acid and wetprocess phosphoric acid production is a complex yet achievable goal when based on a systematic combination of chemical and physico-chemical purification methods. The study highlights that effective removal of major impurities such as fluorine, sulfate, iron, magnesium, and aluminum compounds significantly enhances the quality of the resulting acid, making it suitable for the production of high-grade phosphate fertilizers and various chemical products.

Among the examined methods, precipitation with strontium salts, separation in thin-layer settlers using coagulants, and the application of precipitation agents such as sodium carbonate and sodium sulfate have shown promising results. These approaches ensure the efficient removal of sulfate and fluorine ions while maintaining acceptable economic parameters. At the same time, extraction with organic solvents remains one of the most selective and industrially viable techniques, especially for obtaining highly purified WPA.

The integrated implementation of these technological measures not only improves the purification efficiency and concentration stability of phosphoric acid but also enables the rational utilization of phosphorus-containing waste, thereby reducing the environmental impact. Future research should focus on developing low-cost, energyefficient reagents and hybrid purification systems that combine chemical, extraction, and membrane processes to achieve deeper removal of impurities and higher process sustainability.