The substance of interest reacts with the reagent to form a chemical compound which is more soluble in the organic solution than in the aqueous solution. In order to obtain a high degree of concentration and a good purity the extraction and stripping processes are usually carried out in a counter-current fashion, se Figure below. Through the development of selective and inexpensive extractants it is today in principle possible to isolate any soluble substance in a concentrated and very pure form from an initially very complex mixture in a few steps at room temperature even on a large industrial scale.
Examples are the production of pure rare earths, zirconium, nickel, copper, uranium, etc.
Traditionally, solvent extraction is a much used technique in analytical chemistry, as well as a valuable research field in itself. In industry continuous operations are required and mixer-settlers, columns, centrifugal extractors, and continuous flow centrifuges are used for solvent extraction processing. The aqueous effluent from such wash stages is usually merged with the aueous feed to stage E 1.