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There are many steps in solvent extraction:

  1. Extraction
  2. Scrubbing
  3. Back extraction

I understand the 1st and the 3rd. What is the principle of scrubbing? Is it just washing the organic phase? How undesirable species get removed from organic phase?

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    $\begingroup$ "Scrubbing" is (in chemical technology) a method that removes a gaseous compound. I have never heard the term used in pure chemistry. Can you please give some context? $\endgroup$ – Karl Mar 29 '19 at 7:41
  • $\begingroup$ Dictionary.com confirms that (4)dictionary.com/browse/Scrubbing I suppose it is used in transferred meaning, dealing with the emulsion of the other solvent and mainly with other co-extracted components. $\endgroup$ – Poutnik Mar 29 '19 at 12:37
  • $\begingroup$ As noted by Karl, scrubbing is usually used to describe a process in which a liquid is used to clean (scrub) a gas. $\endgroup$ – Charlie Crown Mar 30 '19 at 2:55
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"Scrubbing" in this context is the treatment of the organic phase to remove impurity metals. This is as opposed to "back extraction" which is to remove the metal of interest from the organic phase.

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I think OP's question is about solvent extraction in hydrometallurgy. In that context, solvent extraction includes three steps: (1) Extraction; (2) Scrubbing; and (3) Stripping (back extraction). This solvent extraction is a unit operation for the purification and concentration of a wide variety of metals:

  1. Extraction: The operation of transferring the metal of interest from the aqueous phase (SX Feed) to the organic phase. During the extraction, which contacts an organic phase containing an extractant with an aqueous phase containing the metal of interest (with impurities). The extractant chemically reacts with the metal to form an organic-metal complex that is soluble in the organic phase. Impurities normally do not react with the extractant and remain in the aqueous phase (known as raffinate, which is either sent for further treatment or effluent).
  2. Scrubbing: The selective removal of impurity metals from the loaded organic phase from extraction step, which contains the metal of interest (and impurity metals). Scrubbing is conducted by treatment with fresh scrub solution (aqueous phase), which contains chemicals to remove impurity metals from organic phase, selectively reversing the reaction. The spent scrub solution is normally combined with the SX Feed. The scrubbed organic containing the metal of interest is separated from aqueous phase, and is advanced to stripping.
  3. Stripping: The process of removing the metal of value from the scrubbed organic phase by reversing the extraction chemical reaction. It is normally conducted under conditions in order to produce a strip liquor containing a high concentration of the metal value (aqueous phase).

(Source)

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