Solvent extraction

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Leaching (mobilization)Solvent extractionStrippingMetal extraction
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Definition and application

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

Process

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.

More detailed solvent extraction is a mass transfer operation in which the feed is contacted with an immiscible liquid (solvent) that exhibits preferential affinity or selectivity towards one or more of the components in the feed. Two streams result from this contact: the extract, which is the solvent rich solution containing the desired component, and the raffinate, the residual feed solution containing small amounts of the desired component.


Solvent extraction (SX) is a 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 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.

Depending on 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+, …. 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).


Ideally only the desired metal transfers selectively from the aqueous phase to the organic phase. The solvent extraction technique is especially important in hydrometallurgical copper production and is practiced in heap bioleaching of secondary copper sulphides.

See also

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