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Why is solvent extraction widely used to produce individual rare-earth elements?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Solvent extraction is widely used in the production of individual rare-earth elements because it is an efficient, continuous process suitable for large-scale industrial separation. Unlike the ion-exchange process, which is highly effective for achieving extreme purities but is significantly slower, solvent extraction allows producers to separate complex mixtures of rare earths into individual elements with purities ranging from 95 to 99.9 percent.1

The process relies on the differing affinities of rare-earth ions for two immiscible liquids—typically an organic phase containing an extractant like tributylphosphate and an acidic aqueous phase. By utilizing a multistage counter-current extractor on a continuous flow basis, producers can effectively isolate specific elements from the mixture.2

Because of its efficiency, solvent extraction serves as the primary method for initial separation in the modern rare-earth industry. For applications requiring even higher purities (such as optical or phosphor-grade materials), the product of solvent extraction is often used as a starting material for further refinement via ion exchange.1