How does solvent extraction separate actinoid elements from aqueous solutions?

Solvent extraction separates actinoid elements from aqueous solutions by utilizing the differing affinities of solutes for two immiscible liquid phases. In this process, an aqueous solution containing the target elements is mixed with an organic solvent. The solutes distribute themselves between the two phases until equilibrium is reached, based on their relative solubility in each liquid.1

For industrial applications, such as the recovery of uranium from acidic leach liquors, specific organic solvents like alkyl phosphoric acids or secondary and tertiary alkyl amines are commonly employed. The organic phase, enriched with the desired actinoid, is known as the "extract," while the remaining aqueous solution is called the "raffinate".21

When separation factors are small, a single equilibration is insufficient. In such cases, a multistage countercurrent extraction process is used, where the two phases flow in opposite directions through a series of mixer-settler tanks. This repeated equilibration allows for the effective separation of multiple elements simultaneously by exploiting their distinct migration velocities.31