How does liquid-liquid solvent extraction separate rare-earth elements?

Liquid-liquid solvent extraction separates rare-earth elements by exploiting the differing affinities that individual elements have for two immiscible or partially immiscible solvents.1

In this process, a mixture containing dissolved rare earths is combined with two liquids. As the liquids are mixed, the rare-earth solutes distribute themselves between the phases until equilibrium is reached, based on their relative affinities for each solvent. The phase containing the desired solute is known as the "extract," while the remaining residue is called the "raffinate".1

To achieve effective separation, producers typically use a multistage counter-current extractor on a continuous flow basis, employing numerous mixer-settler tanks. Because rare-earth elements are separated simultaneously rather than in simple pairs, the process is complex. Producers commonly use tributylphosphate (TBP) as the organic phase to extract rare-earth ions from a highly acidic nitric acid aqueous phase. Other extractants, such as long-chained amines and di-2-ethylhexyl orthophosphoric acid, are also utilized. This method allows for the production of individual elements with purities typically ranging from 95 to 99.9 percent.12