Liquid-Liquid Extraction Operations and Equipment

Liquid-liquid extraction is a process for separating components in
solution by their distribution between two immiscible liquid phases.
Such a process can also be simply referred to as liquid extraction or
solvent extraction; however, the latter term may be confusing
because it also applies to the leaching of a soluble substance from a
solid.
Since liquid-liquid extraction involves the transfer of mass from one
liquid phase into a second immiscible liquid phase, the process can be
carried out in many different ways. The simplest example involves the
transfer of one component from a binary mixture into a second immiscible
liquid phase. One example is liquid-liquid extraction of an impurity
from wastewater into an organic solvent. This is analogous to
stripping or absorption in which mass is transferred from one phase to
another. Transfer of the dissolved component (solute) may be
enhanced by the addition of “salting out” agents to the feed mixture or
by adding “complexing” agents to the extraction solvent. Or in some
cases a chemical reaction can be used to enhance the transfer, an
example being the use of an aqueous caustic solution to remove phenolics
from a hydrocarbon stream. A more sophisticated concept of
liquid-liquid fractionation can be used in a process to separate two
solutes completely. A primary extraction solvent is used to extract one
of the solutes from a mixture (similar to stripping in distillation), and a
wash solvent is used to scrub the extract free from the second solute
(similar to rectification in distillation).

http://ria.servinit.cl/grabiel/libros/Perry/CHAP15.pdf