The separation of mixtures of substances is among the central problems of thermal process engineering. For difficult separation problems such as, for example, in the case of substances which have virtually the same boiling point, form an azeotropic mixture or are perhaps temperature-sensitive, extraction offers economical problem solutions. Extraction involves dissolving a particular substance (C) out of, or extracting it from, a carrier liquid (A) with the aid of a liquid extraction agent (B). The liquid-liquid extraction process is based on the unequal distribution of a substance between two liquids which are not soluble in each other. The substance (C) to be produced is in general transferred to the extraction agent by a purely physical exchange process. However, the extraction agent can also act as a reactant as for example in metal salt extraction.

Capacity, selectivity, insolubility in the carrier phase, economic regeneration and mechanical separation of the liquid/liquid dispersion are criteria affecting the choice of a suitable extraction agent.