Material transfer - Liquid-liquid extraction

Liquid-liquid extraction involves the transfer of solute between two liquid phases that are not totally miscible.

Unlike distillation, the solute does not change state during transfer, so thermal phenomena usually play only a negligible role.

Let's consider a binary solution (A–B) that needs to be separated by liquid-liquid extraction: this operation requires the introduction of a third body C called solvent, which preferentially extracts one of the two constituents of the solution in question, A for example. After contact and separation, two phases are obtained: the extract, a solvent-rich phase enriched in transferred solute, and the raffinate, the starting solution depleted in solute.

This method of separation has two main areas of application: the large-scale organic chemical industry on the one hand, and hydrometallurgy and the nuclear industry on the other. Liquid-liquid extraction enables the separation of compounds with similar boiling temperatures (aliphatic or benzene hydrocarbons), thanks to their different physico-chemical properties (distillation, due to very similar vapour pressures, would require much larger, and therefore more costly, equipment).

In the case of unstable or heat-sensitive products such as penicillin, liquid-liquid extraction enables rapid, non-degrading separations. Finally, hydrometallurgy is its preferred field: separation of metals in solution, treatment of irradiated nuclear fuels, concentration of solutions without excessive energy input (unlike distillation or evaporation).