Hydrometallurgy in the Processing of REE
Once a REE-containing concentrate has been produced by physical ore beneficiation the next step is dissolution, separation and purification of rare earth elements. For this, hydrometallurgy is one of the main procedures. Three unit operations have to be considered:
- Dissolution of the rare earth content in acid, sometimes at elevated pressure and temperature.
- Separation of the different REE into pure and concentrated solutions, by solvent extraction or ionic liquid extraction and ion exchange.
- Generation of individual and pure rare earth elements.
There are many alternative ways to carry out a complete metal recovery process, and to develop such a process it is necessary to combine different operations in a unique way.
Different dissolution and separation methods will be studied by partners in EURARE including MEAB, RWTH, NTUA and KUL. All partners have documented experience and access to advanced laboratory facilities for the development of hydrometallurgical processes.
Solvent extraction is a selective separation procedure for isolating and concentrating substances from aqueous solutions with the aid of an immiscible organic solvent. The procedure has gained rapidly growing industrial importance in hydrometallurgy and has been widely adopted for the recovery and separation of the different REE into pure and concentrated solutions.
In a solvent extraction procedure, an aqueous feed solution containing REE is mixed with an organic solvent containing an organic reagent. The REE react with the reagent to form compounds that are more soluble in the organic solvent and, consequently, are extracted into the solvent.
In order to clean the organic solvent from co-extracted substances, a scrubbing function may be included. The REE are subsequently re-extracted (stripped) from the organic solvent by mixing it with a "new" aqueous solution in which the REE are again more soluble. The REE are concentrated into this solution, at concentrations often 10-100 times that of the original feed solution, achieved by adjustment of the flow rates.
General Separation Procedures for the REE
Various processes for separation and purification of rare earth elements, in groups or individually, utilize the small differences in basicity resulting from decrease in the ionic radius from the LREE to the HREE. The basicity differences influence the solubility of the salts, the hydrolysis of ions, and the formation of complex species. These properties also form the basis of the separation techniques.
In addition to a tri-valent oxidation state, cerium, praseodymium and terbium can also occur in the tetra-valent state. Europium, samarium and ytterbium exhibit a di-valent state. Selective oxidation and reduction of these rare earth elements is useful in an effective separation procedure because in the di-valent and tetra-valent states the rare earth elements have a different chemical and physical behaviour compared to that in the tri-valent state.
Organo-phosphorus acids are typical cation exchange reagents used in separation of the REE, and involve the displacement of a hydrogen ion from the reagents by the extracted rare earth element. The distribution coefficient in an organo-phosphorus rare earth system increases with an increase in rare earth atomic number, allowing separation to be achieved. The separation of co-extracted rare earth elements can be enhanced by the use of a scrubbing function. The exchange reaction is also pH dependent.
In addition to the organo-phosphorus acids, solvation reagents have been studied and used for REE separations. Tri butyl phosphate (TBP) appears to be very effective under certain conditions.
In an operation using an organo-phosphorus acid or a solvating reagent, yttrium is anomalous, acting as a HREE in chloride media, and as a LREE in thiocyanate media. This behaviour has been utilized in a proposed process for preparing high purity yttrium oxide.
Of the various other basic organic reagents, only quaternary ammonium salts, marketed as Aliquat 336, has turned out to be promising for separation and purification of REE.
The EURARE project includes laboratory batch investigations to evaluate the solvent extraction process conditions, which will be used to prepare pilot plant activities, and also be used for verification and determination of equipment and investment costs.
Studies of Rare Earths Separation based on Norra Kärr Middlings - Extraction of rare earth elements in chloride solution with di(ethylhexyl) phosphonic acid (Ionquest 801 or P507) followed by Yttrium separation using tri butyl phosphate (TBP).
Information provided by EURARE partner MEAB