Rare metals, often called “industrial vitamins,” are irreplaceable in high-tech fields such as catalysis, lasers, agriculture, biology, and electromagnetics. They are also a core component of national strategic resources.Due to their highly similar chemical properties, rare earth elements are often found in ores in a symbiotic form. Their efficient separation and high-purity single-element preparation have always been technological bottlenecks hindering the development of related fields.Currently, approximately 90% of rare earth metal separation processes rely primarily on solvent extraction, a core technology.
The basic principle of solvent extraction is to contact organic phase containing a specific extractant (kerosene or mineral spirits) with an aqueous phase containing metal ions, selectively transferring the target metal into the organic phase.Then, the organic phase is recovered through a stripping step, achieving separation and purification.
Extractant molecules must first possess a fundamental structural characteristic: they must contain both active groups capable of coordinating with metal ions (such as -OH, -NH, -SH, or groups containing O, N, or S) and hydrophobic chains (such as alkyl or aryl groups) that enhance oil solubility. This structural design allows the extractant to “grasp” the metal ions like a dragon playing with a pearl: the active groups bind to the metal ions on the inside, while the hydrophobic structure dissolves in the organic phase on the outside, forming a transferable “extractant,” thereby enabling the interphase migration and separation of the metal ions.
After having this basic structure, the extractant must also have the following 9 properties:
1. Separation Factor
An excellent extractant must possess the ability to accurately identify, particularly when achieving a one-in-a-million separation between rare earth elements with very similar chemical properties. The prerequisite for achieving this separation is that it must efficiently capture the target metal within a complex ionic system.
2. Speed
Extraction rate directly impacts process efficiency. This covers both the forward extraction (aqueous phase → organic phase) and the reverse extraction (organic phase → aqueous phase) processes of metals. The value of faster mass transfer rates is reflected in three aspects: shortening the contact time between the two phases, reducing equipment size and energy consumption, and achieving time-difference separation by utilizing differences in metal extraction kinetics.
3. Stripping Ability
The extractant must be able to release metal ions under mild conditions (such as low-concentration acid or alkali) to achieve enrichment and recovery, and to regenerate the extractant itself. Excessively high stripping conditions not only increase costs but may also create environmental pressures.
4. Stability
Extractants must maintain structural stability, resist degradation and ineffectiveness in the presence of strong acids, strong bases, temperature fluctuations, and repeated cycles. This is the foundation for their continuous industrial application.
5. Saturation Capacity
Represents the amount of metal that can be loaded per unit volume of organic phase. High capacity means less extractant is required, which helps achieve equipment miniaturization, reducing energy consumption and waste emissions.
6. Solubility
The ideal extractant should have extremely low solubility in water to prevent loss during use. The hydrophobic structure of the long carbon chain can effectively inhibit water solubility and maintain the stability of the extraction system.
7. Safety
The extraction agent should have the characteristics of low toxicity, non-flammability, non-volatility, and non-corrosiveness to ensure that the entire process of production, transportation, use and waste liquid treatment is harmless to the environment and personnel.
8. Synthesis
An excellent extractant should be easy to synthesize, with common raw materials and simple steps, and suitable for large-scale production to ensure supply and control costs.
9. Source
The raw materials required for preparation should be sufficient and inexpensive, which is the key support for the commercial application of the extractant.
Among the millions of organic compounds, only a few extractants can meet these nine requirements simultaneously.Currently, the extractants widely used in industry include P507, P204, TBP, TIBP, extractant 272 and extractant 235.
