Solvent extraction

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Solvent extraction is a frequently used technology in hydrometallurgy. It is a liquid-liquid extraction process used to purify and concentrate solutions obtained from the leaching of ores. Solvent extraction acts as a purification and concentration technique at the same time.

Leaching (mobilization)Solvent extractionStrippingMetal extraction
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Essentially, solvent extraction consists of two circuits of non-miscible liquids put in contact, where a valuable metal in an aqueous solution is captured by an extractant in the organic phase.

Solvent extraction (SX) is a mass transfer operation and the technique used in metallurgical industry for selective extraction of metal ions from an aqueous solution. During the process, the desired metal ion is both purified and concentrated. An extractant capable of binding metal ions is dissolved in an organic kerosene type diluent. The extractant has an active hydrophilic group bound to a long chain hydrocarbon molecule in order to reduce solubility in the organic phase. The organic phase is then intensively mixed with the metal-bearing aqueous solution (feed) allowing the metal ions to bind to the extractant. The aqueous and organic phases are then allowed to separate and the wanted metal ion is now transferred to the organic phase. The extracted metal ion is then back-extracted (stripped) into a new aqueous phase in a pure and concentrated form. Ideally, only the desired metal transfers selectively from the aqueous phase to the organic phase.


Depending on the properties of the extractant used and the chemistry of the metal-bearing solution, the extraction mechanism can be either cationic, anionic or solvating.

  • Cation extraction

Metal cations like Cu2+, Zn2+, Fe3+, etc. are extracted. Typical extractants used are carboxylic acids, phosphoric acid compounds or hydroxyoximes (Fig). Different types of hydroxyoximes are used in commercial copper extraction from heap leaching of low grade copper ores.


An example of Cu2+ extraction with a hydroxyoxime in sulphate environment is shown below:

2RH(o) + Cu2+(aq) → R2Cu(o) + 2H+(aq)

The extraction is pH-dependent and copper is usually extracted at pH values of 1.5-2 and stripping is usually achieved with ~2M sulphuric acid. In practice, spent electrolyte from the electrowinning plant is used for stripping.

  • Anion extraction

Extraction of negative metal ion complexes like CoCl42-, CuCl42-, FeCl4-, etc.. Extractants are long chain amines with primary, secondary, tertiary or quaternary amine groups. An example of Co2+ extraction with a tertiary amine in chloride environment is shown below:

2R3NH+Cl-(o) + CoCl42-(aq) → (R3NH+)2CuCl42-(o) + 2Cl-(aq)

Extraction requires high chloride concentration so that the negatively charged cobalt chloride complex can form and stripping can be done with water.

  • Solvating extraction

Non-charged metal ion complexes are extracted. A typical extractant is tributylphosphate (TBP).


The solvent extraction technique is especially important in hydrometallurgical copper production and is practiced in heap leaching operations of oxide ores with sulphuric acid and in the bioleaching of secondary copper sulphides.

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