Liquid–liquid extraction

Liquid–liquid extraction (LLE) consists in transferring one (or more) solute(s) contained in a feed solution to another immiscible liquid (solvent). The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called raffinate.

Solvent extraction.

Liquid–liquid extraction also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid into another liquid phase. Liquid–liquid extraction is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work up.

The term partitioning is commonly used to refer to the underlying chemical and physical processes involved in liquid–liquid extraction, but on another reading may be fully synonymous with it. The term solvent extraction can also refer to the separation of a substance from a mixture by preferentially dissolving that substance in a suitable solvent. In that case, a soluble compound is separated from an insoluble compound or a complex matrix.

Solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, the processing of perfumes, the production of vegetable oils and biodiesel, and other industries. It is among the most common initial separation techniques, though some difficulties result in extracting out closely related functional groups.

Liquid–liquid extraction is possible in non-aqueous systems: In a system consisting of a molten metal in contact with molten salts, metals can be extracted from one phase to the other. This is related to a mercury electrode where a metal can be reduced, the metal will often then dissolve in the mercury to form an amalgam that modifies its electrochemistry greatly. For example, it is possible for sodium cations to be reduced at a mercury cathode to form sodium amalgam, while at an inert electrode (such as platinum) the sodium cations are not reduced. Instead, water is reduced to hydrogen. A detergent or fine solid can be used to stabilize an emulsion, or third phase.

Measures of effectiveness

Distribution ratio

In solvent extraction, a distribution ratio is often quoted as a measure of how well-extracted a species is. The distribution ratio (Kd) is equal to the concentration of a solute in the organic phase divided by its concentration in the aqueous phase. Depending on the system, the distribution ratio can be a function of temperature, the concentration of chemical species in the system, and a large number of other parameters. Note that D is related to the ΔG of the extraction process.

Sometimes, the distribution ratio is referred to as the partition coefficient, which is often expressed as the logarithm. Note that a distribution ratio for uranium and neptunium between two inorganic solids (zirconolite and perovskite) has been reported.[1] In solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. In this experiment, the nonpolar halogens preferentially dissolve in the nonpolar mineral oil.[2]

After performing liquid–liquid extraction, a quantitative measure must be taken to determine the ratio of the solution’s total concentration in each phase of the extraction. This quantitative measure is known as the distribution ratio or distribution coefficient.[3]

Separation factors

The separation factor is one distribution ratio divided by another; it is a measure of the ability of the system to separate two solutes. For instance, if the distribution ratio for nickel (DNi) is 10 and the distribution ratio for silver (DAg) is 100, then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/DNi = SFAg/Ni = 10.[4]

Decontamination factor

This is used to express the ability of a process to remove a contaminant from a product. For instance, if a process is fed with a mixture of 1:9 cadmium to indium, and the product is a 1:99 mixture of cadmium and indium, then the decontamination factor (for the removal of cadmium) of the process is 0.11 / 0.01 = 11.

Slopes of graphs

The easy way to work out the extraction mechanism is to draw graphs and measure the slopes. If for an extraction system the D value is proportional to the square of the concentration of a reagent (Z) then the slope of the graph of log10(D) against log10([[Z]]) will be two.

Measures of success

Success of liquid–liquid extraction is measured through separation factors and decontamination factors. The best way to understand the success of an extraction column is through the liquid–liquid equilibrium (LLE) data set. The data set can then be converted into a curve to determine the steady state partitioning behavior of the solute between the two phases. The y-axis is the concentration of solute in the extract (solvent) phase, and the x-axis is the concentration of the solute in the raffinate phase. From here, one can determine steps for optimization of the process.[5]


Batchwise single stage extractions

This is commonly used on the small scale in chemical labs. It is normal to use a separating funnel. Processes include DLLME and direct organic extraction.

Dispersive liquid–liquid microextraction (DLLME)

A process used to extract small amounts of organic compounds from water samples.[6] This process is done by injecting small amounts of an appropriate extraction solvent (C2Cl4) and a disperser solvent (acetone) into the aqueous solution. The resulting solution is then centrifuged to separate the organic and aqueous layers. This process is useful in extraction organic compounds such as organochloride and organophsophorus pesticides, as well as substituted benzene compounds from water samples.[6]

Direct organic extraction

By mixing partially organic soluble samples in organic solvent (toluene, benzene, xylene), the organic soluble compounds will dissolve into the solvent and can be separated using a separatory funnel. This process is valuable in the extraction of proteins and specifically phosphoprotein and phosphopeptide phosphatases.[7]

Another example of this application is extracting anisole from a mixture of water and 5% acetic acid using ether, then the anisole will enter the organic phase. The two phases would then be separated. The acetic acid can then be scrubbed (removed) from the organic phase by shaking the organic extract with sodium bicarbonate. The acetic acid reacts with the sodium bicarbonate to form sodium acetate, carbon dioxide, and water.

Caffeine can also be extracted from coffee beans and tea leaves using a direct organic extraction. The beans or leaves can be soaked in ethyl acetate which favorably dissolves the caffeine, leaving a majority of the coffee or tea flavor remaining in the initial sample.[8]

Multistage countercurrent continuous processes

Coflore continuous countercurrent extractor.

These are commonly used in industry for the processing of metals such as the lanthanides; because the separation factors between the lanthanides are so small many extraction stages are needed.[9] In the multistage processes, the aqueous raffinate from one extraction unit is fed to the next unit as the aqueous feed, while the organic phase is moved in the opposite direction. Hence, in this way, even if the separation between two metals in each stage is small, the overall system can have a higher decontamination factor.

Multistage countercurrent arrays have been used for the separation of lanthanides. For the design of a good process, the distribution ratio should be not too high (>100) or too low (<0.1) in the extraction portion of the process. It is often the case that the process will have a section for scrubbing unwanted metals from the organic phase, and finally a stripping section to obtain the metal back from the organic phase.


Battery of mixer-settlers counter currently interconnected. Each mixer-settler unit provides a single stage of extraction. A mixer settler consists of a first stage that mixes the phases together followed by a quiescent settling stage that allows the phases to separate by gravity.

In the multistage countercurrent process, multiple mixer settlers are installed with mixing and settling chambers located at alternating ends for each stage (since the outlet of the settling sections feed the inlets of the adjacent stage’s mixing sections). Mixer-settlers are used when a process requires longer residence times and when the solutions are easily separated by gravity. They require a large facility footprint, but do not require much headspace, and need limited remote maintenance capability for occasional replacement of mixing motors. (Colven, 1956; Davidson, 1957)[10]

4 stage battery of mixer-settlers for counter-current extraction.

Centrifugal extractors

Centrifugal extractors mix and separate in one unit. Two liquids will be intensively mixed between the spinning rotor and the stationary housing at speeds up to 6000 RPM. This develops great surfaces for an ideal mass transfer from the aqueous phase into the organic phase. At 200–2000 g, both phases will be separated again. Centrifugal extractors minimize the solvent in the process, optimize the product load in the solvent and extract the aqueous phase completely. Counter current and cross current extractions are easily established.[11]

Extraction without chemical change

Some solutes such as noble gases can be extracted from one phase to another without the need for a chemical reaction (see absorption). This is the simplest type of solvent extraction. When a solvent is extracted, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. Some solutes that do not at first sight appear to undergo a reaction during the extraction process do not have distribution ratio that is independent of concentration. A classic example is the extraction of carboxylic acids (HA) into nonpolar media such as benzene. Here, it is often the case that the carboxylic acid will form a dimer in the organic layer so the distribution ratio will change as a function of the acid concentration (measured in either phase).

For this case, the extraction constant k is described by k = [['''HA'''<sub>organic</sub>]]2/[['''HA'''<sub>aqueous</sub>]]

Solvation mechanism

Using solvent extraction it is possible to extract uranium, plutonium, or thorium from acid solutions. One solvent used for this purpose is the organophosphate tributyl phosphate (TBP). The PUREX process that is commonly used in nuclear reprocessing uses a mixture of tri-n-butyl phosphate and an inert hydrocarbon (kerosene), the uranium(VI) are extracted from strong nitric acid and are back-extracted (stripped) using weak nitric acid. An organic soluble uranium complex [UO2(TBP)2(NO3)2] is formed, then the organic layer bearing the uranium is brought into contact with a dilute nitric acid solution; the equilibrium is shifted away from the organic soluble uranium complex and towards the free TBP and uranyl nitrate in dilute nitric acid. The plutonium(IV) forms a similar complex to the uranium(VI), but it is possible to strip the plutonium in more than one way; a reducing agent that converts the plutonium to the trivalent oxidation state can be added. This oxidation state does not form a stable complex with TBP and nitrate unless the nitrate concentration is very high (circa 10 mol/L nitrate is required in the aqueous phase). Another method is to simply use dilute nitric acid as a stripping agent for the plutonium. This PUREX chemistry is a classic example of a solvation extraction.

Here in this case DU = k TBP2[[NO<sub>3</sub>]]2

Ion exchange mechanism

Another extraction mechanism is known as the ion exchange mechanism. Here, when an ion is transferred from the aqueous phase to the organic phase, another ion is transferred in the other direction to maintain the charge balance. This additional ion is often a hydrogen ion; for ion exchange mechanisms, the distribution ratio is often a function of pH. An example of an ion exchange extraction would be the extraction of americium by a combination of terpyridine and a carboxylic acid in tert-butyl benzene. In this case

DAm = k terpyridine1carboxylic acid3H+−3

Another example is the extraction of zinc, cadmium, or lead by a dialkyl phosphinic acid (R2PO2H) into a nonpolar diluent such as an alkane. A non-polar diluent favours the formation of uncharged non-polar metal complexes.

Some extraction systems are able to extract metals by both the solvation and ion exchange mechanisms; an example of such a system is the americium (and lanthanide) extraction from nitric acid by a combination of 6,6'-bis-(5,6-dipentyl-1,2,4-triazin-3-yl)-2,2'-bipyridine and 2-bromohexanoic acid in tert-butyl benzene. At both high- and low-nitric acid concentrations, the metal distribution ratio is higher than it is for an intermediate nitric acid concentration.

Ion pair extraction

It is possible by careful choice of counterion to extract a metal. For instance, if the nitrate concentration is high, it is possible to extract americium as an anionic nitrate complex if the mixture contains a lipophilic quaternary ammonium salt.

An example that is more likely to be encountered by the 'average' chemist is the use of a phase transfer catalyst. This is a charged species that transfers another ion to the organic phase. The ion reacts and then forms another ion, which is then transferred back to the aqueous phase.

For instance, the 31.1 kJ mol−1 is required to transfer an acetate anion into nitrobenzene,[12] while the energy required to transfer a chloride anion from an aqueous phase to nitrobenzene is 43.8 kJ mol−1.[13] Hence, if the aqueous phase in a reaction is a solution of sodium acetate while the organic phase is a nitrobenzene solution of benzyl chloride, then, when a phase transfer catalyst, the acetate anions can be transferred from the aqueous layer where they react with the benzyl chloride to form benzyl acetate and a chloride anion. The chloride anion is then transferred to the aqueous phase. The transfer energies of the anions contribute to that given out by the reaction.

A 43.8 to 31.1 kJ mol−1 = 12.7 kJ mol−1 of additional energy is given out by the reaction when compared with energy if the reaction had been done in nitrobenzene using one equivalent weight of a tetraalkylammonium acetate.[14]

Types of aqueous two-phase extractions

Polymer–polymer systems. In a Polymer–polymer system, both phases are generated by a dissolved polymer. The heavy phase will generally be Polyethylene glycol (PEG), and the light phase is generally a polysaccharide. Traditionally, the polymer used is dextran. However, dextran is relatively expensive, and research has been exploring using less expensive polysaccharides to generate the light phase. If the target compound being separated is a protein or enzyme, it is possible to incorporate a ligand to the target into one of the polymer phases. This improves the target's affinity to that phase, and improves its ability to partition from one phase into the other. This, as well as the absence of solvents or other denaturing agents, makes polymer–polymer extractions an attractive option for purifying proteins. The two phases of a polymer–polymer system often have very similar densities, and very low surface tension between them. Because of this, demixing a polymer–polymer system is often much more difficult than demixing a solvent extraction. Methods to improve the demixing include centrifugation, and application of an electric field.

Polymer–salt systems. Aqueous two-phase systems can also be generated by introducing a high concentration of salt to a polymer solution. The polymer phase used is generally still PEG. Generally, a kosmotropic salt, such as Na3PO4 is used, however PEG–NaCl systems have been documented when the salt concentration is high enough. Since polymer–salt systems demix readily they are easier to use. However, at high salt concentrations, proteins generally either denature, or precipitate from solution. Thus, polymer–salt systems are not as useful for purifying proteins.

Ionic liquids systems. Ionic liquids are ionic compounds with low melting points. While they are not technically aqueous, recent research has experimented with using them in an extraction that does not use organic solvents.


Kinetics of extraction

It is important to investigate the rate at which the solute is transferred between the two phases, in some cases by an alteration of the contact time it is possible to alter the selectivity of the extraction. For instance, the extraction of palladium or nickel can be very slow because the rate of ligand exchange at these metal centers is much lower than the rates for iron or silver complexes.

Aqueous complexing agents

If a complexing agent is present in the aqueous phase then it can lower the distribution ratio. For instance, in the case of iodine being distributed between water and an inert organic solvent such as carbon tetrachloride then the presence of iodide in the aqueous phase can alter the extraction chemistry.

Instead of being a constant it becomes = k[[I<sub>2</sub>.<sub>Organic</sub>]]/[I2.Aqueous] [[I<sup>−</sup>.<sub>Aqueous</sub>]]

This is because the iodine reacts with the iodide to form I3. The I3 anion is an example of a polyhalide anion that is quite common.

Industrial process design

In a typical scenario, an industrial process will use an extraction step in which solutes are transferred from the aqueous phase to the organic phase; this is often followed by a scrubbing stage in which unwanted solutes are removed from the organic phase, then a stripping stage in which the wanted solutes are removed from the organic phase. The organic phase may then be treated to make it ready for use again.

After use, the organic phase may be subjected to a cleaning step to remove any degradation products; for instance, in PUREX plants, the used organic phase is washed with sodium carbonate solution to remove any dibutyl hydrogen phosphate or butyl dihydrogen phosphate that might be present.


While solvent extraction is often done on a small scale by synthetic lab chemists using a separatory funnel, Craig apparatus or membrane-based techniques,[17] it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, Thin Layer Extraction, spray columns, pulsed columns, and mixer-settlers.

A typical modular process system for liquid–liquid extraction includes processing equipment such as columns, reactors, heat exchangers and pumps, mounted within a strong structural steel frame. Additional features of large-scale modular process systems include thermal insulation, lighting, safety showers, control systems, and fire protection systems.[18]

Extraction of metals

The extraction methods for a range of metals include:[19]

See also


  1. (PDF) Archived from the original (PDF) on March 9, 2008. Retrieved January 21, 2006. Missing or empty |title= (help)
  2. pnjjrose. "Solvent Extraction Notes".
  4. "Basic Technology and Tools in Chemical Engineering Field - S. Wesley - Documents".
  6. 1 2 Rezaee, Mohammad; Assadi, Yaghoub; Milani Hosseini, Mohammad-Reza; Aghaee, Elham; Ahmadi, Fardin; Berijani, Sana (2006). "Determination of organic compounds in water using dispersive liquid–liquid microextraction". Journal of Chromatography A. 1116 (1-2): 1–9. doi:10.1016/j.chroma.2006.03.007. ISSN 0021-9673.
  7. Shacter, Emily (1984). "Organic extraction of Pi with isobutanol/toluene". Analytical Biochemistry. 138 (2): 416–420. doi:10.1016/0003-2697(84)90831-5. ISSN 0003-2697.
  8. Senese F (20 September 2005). "How is coffee decaffeinated?". General Chemistry Online. Retrieved 3 August 2009.
  9. Binnemans, Koen (2007). "Lanthanides and Actinides in Ionic Liquids". Chemical Reviews. 107 (6): 2592–2614. doi:10.1021/cr050979c. ISSN 0009-2665.
  10. Liquid–Liquid Extraction Equipment, Jack D. Law and Terry A. Todd, Idaho National Laboratory.
  11. "Riegel's Handbook of Industrial Chemistry".
  12. Scholz, F.; S. Komorsky-Lovric; M. Lovric (February 2000). "A new access to Gibbs energies of transfer of ions across liquid|liquid interfaces and a new method to study electrochemical processes at well-defined three-phase junctions". Electrochemistry Communications. Elsevier. 2 (2): 112–118. doi:10.1016/S1388-2481(99)00156-3.
  13. Danil de Namor, A.F.; T. Hill (1983). Journal of the Chemical Society, Faraday Transactions: 2713. Missing or empty |title= (help)
  14. zamani, Dariush. "Extraction Operation".
  15. Peker, Hulya; Srinivasan, M. P.; Smith, J. M.; McCoy, Ben J. (1992). "Caffeine extraction rates from coffee beans with supercritical carbon dioxide". AIChE Journal. 38 (5): 761–770. doi:10.1002/aic.690380513. ISSN 0001-1541.
  16. Emden, Lorenzo. "Decaffeination 101: Four Ways to Decaffeinate Coffee". Coffee Confidential. Retrieved 29 October 2014.
  17. Adamo, Andrea; Heider, Patrick L.; Weeranoppanant, Nopphon; Jensen, Klavs F. (2013). "Membrane-Based, Liquid–Liquid Separator with Integrated Pressure Control". Industrial & Engineering Chemistry Research. 52 (31): 10802–10808. doi:10.1021/ie401180t. ISSN 0888-5885.
  19. Mackenzie, Murdoch. "The Solvent Extraction of Some Major Metals" (PDF). Cognis GmbH. Retrieved 2008-11-18.
  20. Filiz, M.; Sayar, N.A.; Sayar, A.A. (2006). "Extraction of cobalt(II) from aqueous hydrochloric acid solutions into alamine 336–m-xylene mixtures". Hydrometallurgy. 81 (3-4): 167–173. doi:10.1016/j.hydromet.2005.12.007. ISSN 0304-386X.
  21. Baba, Yoshinari; Iwakuma, Minako; Nagami, Hideto (2002). "Extraction Mechanism for Copper(II) with 2-Hydroxy-4-n-octyloxybenzophenone Oxime". Industrial & Engineering Chemistry Research. 41 (23): 5835–5841. doi:10.1021/ie0106736. ISSN 0888-5885.
  22. Sanchez, J.M.; Hidalgo, M.; Salvadó, V.; Valiente, M. (1999). "EXTRACTION OF NEODYMIUM(III) AT TRACE LEVEL WITH DI(2-ETHYL-HEXYL)PHOSPHORIC ACID IN HEXANE". Solvent Extraction and Ion Exchange. 17 (3): 455–474. doi:10.1080/07366299908934623. ISSN 0736-6299.
  23. Lee W. John. "A Potential Nickel / Cobalt Recovery Process". BioMetallurgical Pty Ltd.
  24. "Precious Metals Refining By Solvent Extraction". Halwachs Edelmetallchemie und Verfahrenstechnik. Retrieved 2008-11-18.
  25. Giridhar, P.; Venkatesan, K.A.; Srinivasan, T.G.; Vasudeva Rao, P.R. (2006). "Extraction of fission palladium by Aliquat 336 and electrochemical studies on direct recovery from ionic liquid phase". Hydrometallurgy. 81 (1): 30–39. doi:10.1016/j.hydromet.2005.10.001. ISSN 0304-386X.
  26. Schulz, Wallace W.; Schiefelbein, Gary F.; Bruns, Lester E. (1969). "Pyrochemical Extraction of Polonium from Irradiated Bismuth Metal". Ind. Eng. Chem. Process Des. Dev. 8 (4): 508–515. doi:10.1021/i260032a013.
  27. K. Takeshita; K. Watanabe; Y. Nakano; M. Watanabe (2003). "Solvent extraction separation of Cd(II) and Zn(II) with the organophosphorus extractant D2EHPA and the aqueous nitrogen-donor ligand TPEN". Hydrometallurgy. 70: 63–71. doi:10.1016/s0304-386x(03)00046-x.

Further reading

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