OBTAINING THERMOPHOSPHATES BY CALCINATION OF PRODUCTS OF PHOSPHORIC ACID DECOMPOSITION OF HIGH-CARBONATE PHOSPHATE FLOUR

АBSTRACT

The article considers the process of acid-thermal treatment of phosphorite powder to obtain thermal phosphates with improved characteristics. The study involved two-stage treatment: decomposition with phosphoric acid and thermal calcination at temperatures of 800–1200°C. It was found that a decrease in the calcium modulus and an increase in the calcination temperature significantly reduces the fluorine content and increases the solubility of phosphates. The resulting products meet the standards for feed phosphates, providing high nutritional value for farm animals. The work offers an optimized approach to the production of safe and effective mineral supplements from phosphorites of the Central Kyzylkum Desert.

AННОТАЦИЯ

Статья рассматривает процесс кислотно-термической обработки фосфоритного порошка для получения термофосфатов с улучшенными характеристиками. В ходе исследования выполнена двухстадийная обработка: разложение фосфорной кислотой и термическая прокалка при температурах 800–1200°C. Установлено, что снижение кальциевого модуля и повышение температуры прокалки значительно уменьшает содержание фтора и увеличивает растворимость фосфатов. Полученные продукты соответствуют стандартам кормовых фосфатов, обеспечивая высокую питательную ценность для сельскохозяйственных животных. Работа предлагает оптимизированный подход к производству безопасных и эффективных минеральных добавок из фосфоритов Центрального Кызылкума.

Keywords: phosphorite flour, extraction phosphoric acid, calcium modulus, acid-thermal treatment, feed phosphate, fluorine.

Ключевые слова: фосфоритная мука. экстракционная фосфорная кислота, кальциевый модуль, кислотно-термическая обработка, кормовой фосфат, фтор.

Calcium feed phosphates must comply with the specifications outlined in GOST 23999-80. According to this standard, the phosphorus content should be no less than 18-20%. while calcium should constitute at least 24-34% of the composition. The permissible levels of harmful substances, including fluorine, arsenic, and heavy metals, are strictly limited as follows (by mass percentage): fluorine – a maximum of 0.2%; arsenic – no more than 0.005%; lead – 0.002%; cadmium – 0.001%; and mercury – 0.0001%.

These regulations are essential for ensuring the safe and effective use of calcium phosphates as a nutritional additive in animal feeds. Phosphorus and calcium are vital for skeletal development and metabolic functions in animals, but the presence of contaminants such as fluorine and heavy metals poses significant health risks. Therefore, adherence to GOST 23999-80 standards not only safeguards animal health but also aligns with environmental safety and food quality standards in agricultural practices.

In the context of Uzbekistan. Central Kyzylkum (CK) phosphorites can be considered a viable raw material for producing calcium feed phosphates. According to data sources [2]. the concentrations of heavy metals, such as arsenic (As), lead (Pb), cadmium (Cd), and mercury (Hg), in CK phosphorites are below 0.1 mg/kg. a level that is acceptable for use in animal feed phosphates. This low concentration of heavy metals makes CK phosphorites a promising feedstock. However, the primary challenge remains the removal of fluorine from these phosphorites.

It is well-established that hydrothermal treatment of fluorapatite at high temperatures (1400-1450°C) initiates a process of isomorphic substitution, where fluoride ions are replaced by hydroxyl ions. This transformation can be represented by the following chemical reaction:

Ca10​(PO₄​)₆​F₂​+2H₂​O→Ca₁₀​(PO₄​)₆​(OH)₂​+2HF

Through this reaction, fluorine is released in the form of hydrofluoric acid (HF). while the resulting product, hydroxyapatite, remains fluorine-free and suitable for use as a feed phosphate. This hydrothermal treatment thus provides a practical approach for fluorine removal, making CK phosphorites a feasible resource for producing high-quality calcium phosphates that meet safety standards.

The ability of fluoride and hydroxyl ions to undergo isomorphic substitution is due to the similarity in their ionic radii, which allows them to replace each other within the crystal lattice of the mineral. This substitution mechanism is fundamental to processes that aim to reduce fluoride content in fluorapatite, thus enhancing the mineral's suitability for feed phosphates.

Another effective approach involves introducing silica into the feedstock mixture. Silica actively reacts with fluorapatite in the following reaction:

Ca10​(PO₄​)₆​F₂​+2H₂​O +0.5SiO₂​→3Ca₃​(PO₄​)₂​+0.5Ca₂​SiO₄​+2HF

In this reaction, silica interacts with fluorapatite in the presence of water. facilitating the release of fluorine in the form of hydrofluoric acid (HF). This process not only reduces fluoride levels but also results in the formation of tricalcium phosphate and calcium silicate, both of which are valuable byproducts. The inclusion of silica. Therefore, provides an efficient method for defluorination while simultaneously yielding additional compounds that can be beneficial in various industrial applications.

Under these conditions, the primary phase of the resulting product is α-tricalcium phosphate. However, these processes are energy-intensive and require meticulous quality control. Additionally, reactive silica is a scarce material, making it difficult to implement this approach widely. Given these constraints, an acid-thermal method is more suitable for Central Kyzylkum granular phosphorites.

In the acid-thermal method, the phosphorite is initially decomposed with extraction phosphoric acid (EPA). This step breaks down the mineral matrix, facilitating the release of phosphorus while minimizing fluoride content. The decomposition product is then subjected to drying and calcination. processes that help stabilize the desired phases and ensure the final product meets quality standards for feed phosphates.

This method is advantageous as it reduces dependency on high-purity silica and enables better control over the fluoride removal process, ultimately producing a phosphate product with enhanced bioavailability and safety for animal consumption.

Phosphorus compounds derived from acid decomposition products not only support the growth and development of crops but also play a crucial role in enhancing livestock productivity. In this context, ensuring that the population has access to foodstuffs with a diverse range of nutrients - such as proteins, carbohydrates, and fats - is of paramount importance in social life. For livestock, poultry, and fish, the diet must include not only protein- and fat-rich feed but also valuable mineral supplements. These mineral additives significantly contribute to improving meat quality, increasing milk fat content, enhancing egg production, and more. Among these additives, feed phosphates with high feed value are particularly important. They provide essential phosphorus, a vital element for animal growth and productivity, thus playing a central role in agricultural and animal husbandry practices aimed at achieving sustainable and efficient food production. [4]

In light of these considerations, we selected phosphorite flour and extraction phosphoric acid (EPA) as the research objects in laboratory conditions. The composition of the phosphorite flour is as follows (by mass %): 16.47% P₂O₅​; 48.67% CaO; 0.73% Fe₂O₃​; 0.95% Al₂O₃​; 12.97% CO₂​; 3.24% SO₃​; 2.21% F; and 6.89% of other substances. The EPA. used as a decomposing agent, has the following composition (by mass %): 16.62% P₂O₅; 0.051% CaO; 1.09% MgO; 0.269% Fe₂O₃​; 0.409% Al₂O₃​; 2.94% SO₃​; and 0.96% F.

Table 1

Composition of Products from Phosphoric Acid Decomposition of Phosphorite Flour with Varying CaO/P₂O₅ Ratios

By analyzing these specific compositions, we aim to optimize the acid-thermal treatment process to achieve effective defluorination while preserving the desirable phosphorus and calcium content in the final feed phosphate product. This experimental approach allows for a controlled examination of decomposition reactions and the effectiveness of EPA in breaking down the mineral structure of phosphorites, paving the way for the development of efficient and cost-effective production methods tailored for Central Kyzylkum resources.

Laboratory experiments on the acid decomposition were conducted in a thermostatically controlled glass reactor with a screw mixer, which was connected to a variable autotransformer (LATR). The decomposition process lasted for 30 minutes at 65°C. During the experiments, the amount of extraction phosphoric acid (EPA) used was based on different CaO:P₂O₅​ molar ratios: 1.67. 1.45. 1.31. 1.18. 1.00. and 0.79. After the decomposition was completed, the reaction mass was dried at 90°C until it reached a constant weight. Following this, a chemical analysis was carried out to determine the composition of the phosphoric acid decomposition products.

It was shown that during the acid treatment of phosphorite flour, depending on the CaO:P₂O₅ ​ ratio, the total P₂O₅ ​ content in the products increased from 25.89% to 38.31%. The lower the CaO: P₂O₅ ​ ratio, the higher the P₂O₅ content and the lower the CaO content. The relative form of P₂O₅​. determined using 2% citric acid and Trilon B (EDTA), ranged from 46.54% to 91.59% and from 40.59% to 81.86%. respectively. However, the fluorine content remained quite high, ranging from 2.28% to 2.86%. which does not meet the standards of GOST for feed phosphates. (table. 1).

In order to remove fluorine from the decomposition products, these dried masses were ground in a porcelain mortar to a particle size of 0.16 mm and placed in a muffle furnace (SNOL. Russia) for calcination. The calcination temperature varied from 800°C to 1200°C. It should be noted that during the calcination process, the calcium phosphate samples melted and agglomerated into large clumps. Therefore, they were re-ground in the mortar to a particle size of 0.16 mm. Subsequently, the ground thermophosphate samples were subjected to chemical analysis [5]

The results of the studies show that as the calcium modulus decreases from 1.67 to 0.79 and the calcination temperature increases from 800°C to 1200°C, the fluorine content decreases from 2.12% to 0.01%. This represents an average reduction factor of 212. Meanwhile, the total P₂O₅ content in the products increases from 26.09% to 47.89%.

For feed phosphates, a key quality indicator is the soluble form of P₂O₅. measured in a 0.4% hydrochloric acid solution. The results indicate that the relative amount of soluble in hydrochloric acid ranged from 24.01% to 43.58%. The observed trends indicate that a lower calcium modulus (i.e.. a higher phosphoric acid ratio) and a higher calcination temperature lead to a greater amount of P₂O₅ and less residual fluorine. [5-8]

Considering the consumption of phosphoric acid, the most economical sample, in our opinion, is the one obtained with a CaO:P₂O₅ ratio of 1.00 at 1200°C. The product from this condition exhibited the following composition (by mass %): total P₂O₅ – 42.39%; soluble P₂O₅ in 0.4% HCl – 38.98%; total  CaO – 45.45%; soluble  CaO in 0.4% HCl – 41.81%; fluorine – 0.20%.

These results suggest that acid-thermal processing is an acceptable method for processing easily decomposable phosphorites from the Central Kyzylkum region and producing calcium feed phosphates that meet the GOST 23999-80 standards [1]

Thus, the pathways for improvement and reduction of energy costs remain an area for further research. Continued investigation is required to optimize the acid-thermal treatment process, enhance the efficiency of fluorine removal, and explore methods to reduce the overall energy consumption, all while maintaining the desired quality of the feed phosphate products. These efforts could lead to more cost-effective and sustainable production methods for calcium feed phosphates, particularly when utilizing local phosphorite resources such as those from the Central Kyzylkum region. [6.7.8]

Phosphorus compounds derived from acid decomposition products not only support the growth and development of crops but also play a crucial role in enhancing livestock productivity. In this context, ensuring that the population has access to foodstuffs with a diverse range of nutrients - such as proteins, carbohydrates, and fats- is of paramount importance in social life. For livestock, poultry, and fish, the diet must include not only protein - and fat-rich feed but also valuable mineral supplements. These mineral additives significantly contribute to improving meat quality, increasing milk fat content, enhancing egg production, and more. Among these additives, feed phosphates with high feed value are particularly important. They provide essential phosphorus, a vital element for animal growth and productivity, thus playing a central role in agricultural and animal husbandry practices aimed at achieving sustainable and efficient food production.

Conclusion

The study demonstrated that acid-thermal treatment of phosphorite flour is an effective method for producing thermophosphates with reduced fluorine content and enhanced phosphorus availability. Through a two-stage process, first involving phosphoric acid decomposition at controlled CaO:P₂O₅ ​ ratios and then thermal calcination at temperatures between 800°C and 1200°C, it was possible to achieve significant fluorine reduction. Results showed that a lower calcium modulus and higher calcination temperature led to optimal results, with the fluorine content decreasing by an average of 212 times. The final product, particularly at a CaO:P₂O₅ ratio of 0.79 and calcination at 1200°C, met feed phosphate standards, with high total and soluble P₂O₅ content and acceptable fluorine levels. These findings suggest that acid-thermal processing is a viable method for producing high-quality feed phosphates from Central Kyzylkum phosphorites, offering a promising solution to address both agricultural and livestock nutrition needs. Further research should focus on optimizing process conditions to enhance efficiency and reduce energy consumption.

Список литературы:

1. Feed calcium phosphate. GOST 23999-80. - 9 p.
2. Kist A.A., Danilova E.A., Osinskaya N.V., KhusniddinovaS.Kh., Beglov B.M., Volynskova N.V. Impurity elements in phosphorites of the central Kyzylkum and their transition to ammophos, phosphogypsum, soil and plants // Chemical industry. - 2014.- v. 91. No. 8. - P. 418-428.
3. Pozin M.E. Technology of mineral fertilizers. - L.: 1983. - 336 p.
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