Rare earth elements are an important strategic resource, known as the "industrial vitamin" and the "mother of new materials". They are widely used in traditional fields such as metal smelting, petrochemicals, ceramics, glass, health care, agriculture, etc. They also play an important role in modern functional materials such as magnetic materials, luminescent materials, hydrogen storage materials, superconducting materials, etc. They are even more indispensable in high-tech fields such as national defense, military industry, cutting-edge electronics, aerospace, and communications.

Rare earth is a general term for 17 elements in the periodic table, including lanthanide elements, scandium, yttrium, etc. According to the mineral paragenesis and the different properties produced by different ion radii, they can be divided into light rare earth (from lanthanum to samarium) and heavy rare earth (from gadolinium to lutetium). Their outer electron configurations in the periodic table are highly similar, which makes them indispensable in various high-tech and green technologies.

Mechanism of extraction separation

Among rare earth elements, different groups of elements have different solubility in sulfuric acid. The solubility of sulfate complex salts of cerium group elements, such as lanthanum, cerium, praseodymium, neodymium, etc., is low and belongs to the category of insoluble; the solubility of sulfate complex salts of terbium group elements, such as samarium, europium, gadolinium, terbium, dysprosium, etc., is moderate and belongs to slightly soluble; the solubility of sulfate complex salts of yttrium group elements, such as yttrium, erbium, thulium, ytterbium, lutetium and scandium, is high in sulfuric acid and belongs to easily soluble. This difference provides the basis for the extraction and separation of rare earth elements.


One of the core issues: the choice of extraction agent

Solvent extraction, also known as liquid-liquid extraction, utilizes the difference in solubility or distribution coefficient between two immiscible or slightly soluble solvents to transfer the solute from one solvent to another. After repeated extractions, the purpose of enriching the solute can be achieved. This method has many advantages, such as large processing capacity, good separation effect, high recovery rate, fast equilibrium speed, simple equipment, and high automation. It is currently the most widely used rare earth separation and purification technology.

The extractant is a key factor affecting the effect of solvent extraction. According to its chemical properties, the extractant can be divided into acidic, neutral, amine and ionic liquid systems.


1. Acidic extraction system
Commonly used are di(2-ethylhexyl) phosphate (HDEHP or P204), 2-ethylhexyl phosphate and 2-ethylhexyl ester (P507) and Cyanex272, carboxylic acid extractants CA12, CA100 and cyclohexane acid, etc., which have shown excellent extraction and separation performance in industry.

2. Neutral extraction system
Including neutral phosphorus extractants, neutral oxygen-containing extractants, neutral sulfur-containing extractants and substituted amide extractants, among which tributyl phosphate (TBP), Cyanex921 and di(1-methylheptyl) methylphosphonate (P350) are the most commonly used extractants.

3. Amine and amide extraction systems
This type of extractant is a general term for rare earth extractants with nitrogen atoms as the extraction functional group. Commonly used amines include primary amines (N1923), secondary amines, tertiary amines and quaternary ammonium salts, etc. They have high extraction capacity, selectivity and stability. Amide extractants are mainly divided into monoamides, diamides, imides, amide pods, etc.

4. Ionic liquid system
Ionic liquids can be used as extractants or diluents in rare earth separation. According to the chemical structure of the cations in them, they can be divided into quaternary ammonium salts, quaternary phosphonium salts, imidazoles, pyridines, etc.

In addition to the above extraction systems, a synergistic extraction system is sometimes used in the rare earth extraction process, that is, two or more extractants are added at the same time, and the synergistic effect between them is used to enhance the extraction capacity of rare earths.

This system usually includes a binary synergistic extraction system and a ternary synergistic extraction system. Currently, the binary synergistic extraction system is more commonly used in industry, such as P204+P507, CA12+Cyanex272, P204+TBP, P507+Cyanex272, P507+N923, P507+N235, etc.

In the 1980s, domestic scholars began to apply cylindrical centrifugal extractors to the extraction of rare earth elements, using five-stage series extraction for experiments, with extraction efficiency of each stage above 90%. The results showed that centrifugal solvent extraction technology is feasible for the extraction and separation of rare earth elements. Since the cylindrical centrifugal extractor has a small liquid storage volume and a short residence time of the material in the equipment, it has significant advantages over the mixer settler.



LC centrifugal solvent extraction technology is a new type of efficient extraction and separation technology that combines liquid-liquid extraction and two-phase separation. It takes the centrifugal extractor as the core equipment and has the characteristics of large processing capacity, high extraction efficiency, good separation effect, small stage retention, wide operating range, continuous operation, and automation. It is currently widely used in many fields such as hydrometallurgy, chemical industry, nonferrous metals, rare earths, medicine, and environmental protection.

A multi-stage series system is formed by LC centrifugal extraction equipment. The number of series stages is determined according to the process parameters such as the type, concentration, type of extractant, and phase ratio of rare earth elements. Generally, extraction, washing, and stripping sections are set up to achieve efficient extraction and separation of rare earth elements, thereby improving the purity of rare earth elements. At present, centrifugal solvent extraction has achieved industrial applications of more than 200 series.

When using organic phosphorus extractants (such as P507, P204, etc.) to extract rare earth elements, it is usually necessary to add a saponification section to pre-treat the extractant, that is, use NaOH or NH3·H2O to neutralize and remove H+ to ensure the extraction performance of the extractant for rare earths.

In the future, rare earth element solvent extraction technology will be applied in collaborative extraction systems, complex extraction systems, etc. to improve extraction efficiency and product purity. At the same time, breakthroughs will be made in two-phase aqueous extraction systems, pitting extraction technology, non-aqueous solvent extraction systems, etc., and green and efficient extraction agents will be developed to achieve clean production of the entire rare earth separation process and realize green and high-quality development.