OVERVIEW

Extraction is a process for the separation of one or more components through intimate contact with a second immiscible liquid called a solvent. If the components in the original solution distribute themselves differently between the two phases, separation will occur. Separation by extraction is based on this principle. When some of the original substances are solids, the process is called leaching. In a sense, the role of solvent in extraction is analogous to the role of enthalpy in distillation. The solvent- rich phase is called the extract, and the carrier- rich phase is called the raffinate. A high degree of separation may be achieved with several extraction stages in series, particularly in countercurrent flow.

Processes of separation by extraction, distillation, crystallization, or adsorption sometimes are equally possible. Differences in solubility, and hence of separability by extraction, are associated with differences in chemical structure, whereas differences in vapor pressure are the basis of separation by distillation. Extraction often is effective at near- ambient temperatures, a valuable feature in the separation of thermally unstable natural mixtures or pharmaceutical substances such as penicillin.

The simplest separation by extraction involves two immiscible liquids. One liquid is composed of the carrier and solute to be extracted. The second liquid is solvent. Equilibria in such cases are represented conveniently on triangular diagrams, either equilateral or right