СРАВНИТЕЛЬНЫЙ АНАЛИЗ ТЕХНОЛОГИЙ КИСЛОТНОЙ ЭКСТРАКЦИИ РЕДКОЗЕМЕЛЬНЫХ ЭЛЕМЕНТОВ В МИРОВОЙ ФОСФАТНОЙ ПРОМЫШЛЕННОСТИ

УДК 546

Abstract

This research presents a comprehensive comparative analysis of sulfuric, nitric, phosphoric, and hydrochloric acid digestion methods utilized in the processing of carbonate phosphorites into phosphorus fertilizers, specifically focusing on resources from the Central Kyzylkum region. The study investigates the distribution patterns of rare earth elements (REEs) in solution and solid phases during the digestion process. A critical finding reveals that sulfuric acid digestion, while industrially dominant, leads to 70-85% REE loss due to isomorphous substitution in the phosphogypsum crystal lattice. In contrast, nitric and hydrochloric acid methods demonstrate superior efficiency, achieving 92-98% REE recovery by ensuring a complete transition of lanthanides into the liquid phase. The influence of various acidic reagents on the selectivity of REE recovery is scientifically analyzed, considering factors such as crystal lattice isomorphism and hydrometallurgical innovations. Furthermore, the study substantiates the feasibility of nitric acid technology as a waste-free alternative for producing high-quality complex fertilizers like nitrophoska. Based on the comparative data, an optimal scheme for the complex processing of low-grade phosphate ores is proposed, balancing economic feasibility, industrial scalability, and strategic metal recovery.

Аннотация

В данном исследовании представлен всесторонний сравнительный анализ методов сернокислотного, азотнокислотного, фосфорнокислотного и солянокислотного разложения карбонатных фосфоритов, в частности Центральных Кызылкумов, при их переработке в фосфорные удобрения. Изучены закономерности распределения редкоземельных элементов (РЗЭ) между жидкой и твердой фазами. Установлено, что традиционное сернокислотное разложение приводит к потере 70-85% РЗЭ в структуре фосфогипса из-за изоморфного замещения в кристаллической решетке. В то же время азотнокислотный и солянокислотный методы обеспечивают извлечение 92-98% РЗЭ в раствор, что открывает возможности для их селективного выделения. Научно проанализировано влияние различных кислотных реагентов на селективность процесса с учетом гидрометаллургических инноваций и условий кристаллообразования. Обоснована эффективность азотнокислотной технологии как безотходного метода получения комплексных удобрений. Предложена оптимальная схема комплексной переработки низкосортного фосфатного сырья, обеспечивающая баланс между экономической эффективностью и извлечением стратегически важных лантаноидов.

Keywords: phosphorite, acid digestion, rare earth elements, selectivity, complex processing, lanthanides, hydrometallurgy.

Ключевые слова: фосфорит, кислотное разложение, редкоземельные элементы, селективность, комплексная переработка, лантаноиды, гидрометаллургия.

Introduction

 Recently rare and rare earth elements have been become crucial facility particularly increasing demand electronic devices, catalyst and optic materials.   Due to the increasing global demand for rare earth elements (REEs), their extraction from phosphate raw materials is being extensively studied at an international level. For instance, in the research conducted by V. Balaram [1], the significance of REEs in high-tech industries and their geological sources were analyzed. Dar, Sh., et al. [2] evaluated phosphate deposits as a sustainable source of REEs. Through the complex processing of phosphorites, it is possible to significantly satisfy the demand for rare earth metals in sectors such as energy, medicine, the automotive industry, and defense [3]. The issue of REE migration into phosphogypsum during sulfuric acid digestion was analyzed by Nkabinde S.  et al.  [4], who demonstrated that this phenomenon is caused by the similarity of ionic radii between calcium and rare earth ions.

The objective of this review is to provide a comprehensive comparative analysis of acidic extraction technologies for REE recovery from carbonate phosphorites, evaluating their efficiency, economic feasibility, and environmental impact.

Materials and Methods

 This review article is based on a comparative analysis of global acidic extraction technologies for rare earth elements (REEs) from phosphate resources. The data were synthesized from Scopus, Web of Science, and Google Scholar databases covering the period of 2016–2026.

Results and Discussion

 The study evaluates four primary digestion routes: sulfuric, nitric, phosphoric, and hydrochloric acid systems, focusing on REE recovery efficiency, crystal lattice isomorphism, and industrial feasibility.

The sulfuric acid digestion of phosphate raw materials constitutes the foundation of the global phosphate industry, accounting for more than 90% of the methods currently in use. The digestion of carbonate phosphorites with sulfuric acid is a complex heterogeneous process characterized by the degradation of the apatite structure and the simultaneous crystallization of calcium sulfate (CaSO₄·nH₂O). The primary digestion reaction is as follows:

Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O = 3H₃PO₄ + 5CaSO₄·2H₂O + HF

The presence of carbonate minerals (calcite, dolomite) complicates the process, as they increase acid consumption and lead to excessive foam formation:

CaCO₃ + H₂SO₄ + H₂O = CaSO₄·2H₂O + CO₂

The primary disadvantage of the sulfuric acid digestion method, in terms of rare earth element (REE) recovery, is the phenomenon of isomorphism. During the digestion process, approximately 70% to 85% of REEs are incorporated into the structure of calcium sulfate crystals. The first major cause of this is the similarity in ionic radii; the radii of REE³⁺ ions are very close to those of Ca²⁺ ions, which facilitates their substitution within the crystal lattice. The second cause is co-precipitation, where sulfate complexes of REEs (Ln(SO₄)₂) become entrapped within the crystal lattice during the formation of calcium sulfate [4-5].

Nitric acid digestion (HNO₃) differs fundamentally from the sulfuric acid method, as this process does not result in the formation of solid waste, such as phosphogypsum. All primary minerals within the carbonate phosphorite (apatite, calcite, and dolomite) are converted into soluble nitrate salts:

Ca₅(PO₄)₃F + 10HNO₃ = 3H₃PO₄ + 5Ca(NO₃)₂ + HF

CaCO₃ + 2HNO₃ = Ca(NO₃)₂ + CO₂ + H₂O

The innovative aspect of this method lies in the fact that nitric acid, being a potent oxidant and solvent, completely disintegrates the apatite crystal lattice. Consequently, 95-98% of the REEs contained in the phosphorite transition into the liquid phase (nitrate-phosphate solution). This figure is approximately five times higher than that of the sulfuric acid method (15-20%). The separation of components from the nitrate-phosphate solution is carried out stepwise. The first stage involves the crystallization of calcium nitrate, where the solution is cooled to temperatures between -5°C and -10°C. During this process, Ca(NO₃)₂·4H₂O crystallizes and precipitates, serving as a valuable raw material for the fertilizer industry. In the second stage, the selective separation of REEs is performed. In the calcium-free mother liquor, the concentration of lanthanides increases. To extract them, modern liquid-liquid extraction (e.g., using tributylphosphate - TBP) or ion-exchange sorption methods are employed. The subsequent stage is neutralization; after the REEs are separated, the remaining solution is neutralized with ammonia to produce nitrophoska, a high-quality complex fertilizer. The primary challenges of the nitric acid method include the high cost of the acid and the requirement for specialized corrosion-resistant materials (high-grade stainless steels) for the equipment. However, the high value of rare earth elements (particularly heavy lanthanides) fully offsets these additional costs [6-7].

Phosphoric acid digestion (H₃PO₄) is one of the 'cleanest' and most technologically flexible methods for processing carbonate phosphorites. This method is particularly utilized in the production of double superphosphate and for extracting rare earth elements (REEs) without introducing foreign anions into the system. In phosphoric acid digestion, external reagents such as sulfuric or nitric acids do not enter the system. The decomposition of raw materials (apatite and carbonates) using the end-product itself-phosphoric acid-enhances the selectivity of the process:

Ca₅(PO₄)₃F + 7H₃PO₄ = 5Ca(H₂PO₄)₂ + HF

Reaction with carbonate minerals:

CaCO₃ + 2H₃PO₄ = Ca(H₂PO₄)₂ + CO₂ + H₂O

The primary product obtained from this process calcium dihydrophosphate is water-soluble, with the beneficial P₂O₅ content in the fertilizer reaching 45-50%. The uniqueness of phosphoric acid digestion in REE recovery lies in the fact that the solution consists exclusively of phosphate anions. This provides a favorable environment for the formation of soluble REE complexes. Specifically, REE ions (Ln³⁺) form complex. Unlike in sulfate systems, these complexes do not co-precipitate with calcium sulfate but remain within the liquid phase. When carbonate phosphorites are digested with phosphoric acid, the REE extraction efficiency into the solution can reach 85-90%. Since phosphoric acid is a relatively weak acid, the digestion process occurs more slowly compared to the sulfate method. Consequently, the process must be conducted at temperatures of 80-95°C. Furthermore, intensive grinding or ultrasonic activation of the phosphorite meal prior to digestion increases the decomposition degree by 5-8%. Phosphoric acid digestion is considered an 'environmentally safe' approach for the complex processing of carbonate phosphorites. By studying the distribution patterns of REEs in the solution, this method can be integrated into a unified technological chain for producing high-efficiency fertilizers and strategic metals [8-9]. Hydrochloric acid digestion is a hydrometallurgical approach to processing carbonate phosphorites, representing the most promising innovative method for selective extraction of high-purity products and rare earth metals. Hydrochloric acid decomposes the apatite structure and carbonate minerals (calcite, dolomite) rapidly and completely. The primary distinction of this process from other acidic methods is the exceptionally high water solubility of all resulting chloride salts. The main reaction is as follows:

Ca₅(PO₄)₃F + 10HCl = 3H₃PO₄ + 5CaCl₂ + HF

Reaction with carbonates:

CaCO₃ + 2HCl = CaCl₂ + CO₂ + H₂O

The hydrochloric acid method does not generate solid waste (phosphogypsum), ensuring that approximately 92-96% of the REEs in the phosphorite transition into the liquid phase. In a chloride medium, rare earth elements form stable chloride complexes, which facilitates their extraction process. Extractants such as tributylphosphate (TBP) or P507 (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) are utilized to separate REEs from chloride solutions. This method allows for the concentration of lanthanides with high selectivity. By neutralizing the chloride solution with lime milk (Ca(OH)₂), high-purity dicalcium phosphate (precipitate) is obtained, which is highly valued as a feed additive in agriculture. The resulting CaCl₂ solution is used as a finished product in the national economy (e.g., for de-icing roads and in construction materials). The primary obstacles to hydrochloric acid technology are equipment corrosion and the volatility of HCl. [10-11].

 A comparative analysis of acid digestion methods is provided in the table below.

Table 1. Comparative analysis of acid digestion methods

Based on the comparative analysis presented in Table 1, it is evident that while the sulfuric acid method remains the global industrial standard due to its cost-effectiveness, it is the least efficient for REE recovery. The primary constraint is the isomorphous substitution of Ca²⁺ ions by Ln³⁺ in the calcium sulfate crystal lattice, leading to over 80% of REEs being lost in phosphogypsum waste.In contrast, the nitric acid digestion shows the highest potential for complex processing. The complete transition of REEs into the liquid phase (95-98%) without solid waste generation aligns with the principles of "Green Chemistry." However, for the Central Kyzylkum region's carbonate phosphorites, the high acid consumption due to calcite impurities remains a significant economic factor.The hydrochloric acid route offers a unique advantage for high-carbonate ores, as it ensures rapid decomposition and high selectivity for rare earth chlorides. The challenge of CaCl₂ utilization, however, requires integrated construction material production to be economically viable.Finally, phosphoric acid digestion provides a balance between fertilizer quality and REE extraction (85-90%). The absence of foreign anions simplifies the subsequent ion-exchange or solvent extraction stages for lanthanide recovery.

Conclusion

In conclusion, the analysis demonstrates that the choice of acid type in the processing of carbonate phosphorites determines the economic efficiency of the final product. Each acid variety possesses distinct advantages and limitations. For the maximum recovery of rare earth elements (REEs), nitric and hydrochloric acids are the most effective, whereas phosphoric acid digestion is most promising for the production of high-concentration fertilizers. The sulfuric acid method requires further optimization through the control of crystallization processes. The industrial implementation of the nitric acid digestion method enables the production of not only high-quality fertilizers but also concentrates of strategically important rare earth metals. Furthermore, this approach is of significant importance for environmental protection. For the sustainable development of the phosphate industry in regions like Central Kyzylkum, integrating these advanced acidic extraction technologies is essential. Further studies aimed at optimizing reagent consumption and pilot-scale testing will be crucial for the industrial transition toward zero-waste complex processing.

References:

1. Balaram V. Rare earth element deposits: sources, and exploration strategies //Journal of the Geological Society of India. – 2022. – Т. 98. – №. 9. – С. 1210-1216.
2. Dar S. A. et al. Phosphorite deposits: A promising unconventional resource for rare earth elements //Geoscience Frontiers. – 2025. – Т. 16. – №. 3. – С. 102044.
3. Dutta B. et al. A comprehensive compendium on rare earths: science, technology, sustainability, global perspective and india’s strategic approach //Coordination Chemistry Reviews. – 2026. – Т. 549. – С. 217280.
4. Nkabinde S.  et al. Pretreatment strategies for enhancing rare earth elements leaching efficiency from phosphogypsum-A review with a focus on hydrometallurgical processes //Journal of Rare Earths. – 2025.
5. Walawalkar M., Nichol C. K., Azimi G. Process investigation of the acid leaching of rare earth elements from phosphogypsum using HCl, HNO₃, and H₂SO₄ //Hydrometallurgy. – 2016. – Т. 166. – С. 195-204.
6. Xu D. et al. Recovery of rare earths from nitric acid leach solutions of phosphate ores using solvent extraction with a new amide extractant (TODGA) //Hydrometallurgy. – 2018. – Т. 180. – С. 132-138.
7. Yang X. et al. Phosphogypsum processing for rare earths recovery—a review //Nat. Resour. – 2019. – Т. 10. – №. 9. – С. 325-336.
8. Wu S. et al. Recovery of rare earth elements from phosphate rock by hydrometallurgical processes–A critical review //Chemical Engineering Journal. – 2018. – Т. 335. – С. 774-800.
9. Wu S. et al. Simultaneous recovery of rare earth elements and phosphorus from phosphate rock by phosphoric acid leaching and selective precipitation: Towards green process //Journal of Rare Earths. – 2019. – Т. 37. – №. 6. – С. 652-658.
10. Fu C. et al. A novel process for the low-temperature recovery of rare earth elements and by-products from associated rare earth phosphate ores //Separation and Purification Technology. – 2025. – С. 135728.
11. Amine M. et al. Hydrochloric acid leaching study of rare earth elements from Moroccan phosphate //Journal of Chemistry. – 2019. – Т. 2019. – №. 1. – С. 4675276.