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Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration (lower free energy). The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. 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 called as mixer settlers. 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. From a hydrometallurgical perspective, solvent extraction is exclusively used in separation and purification of uranium and plutonium, zirconium and hafnium, separation of cobalt and nickel, separation and purification of rare earth elements etc., its greatest advantage being its ability to selectively separate out even very similar metals. One obtains high-purity single metal streams on 'stripping' out the metal value from the 'loaded' organic wherein one can precipitate or deposit the metal value. Stripping is the opposite of extraction: Transfer of mass from organic to aqueous phase. Liquid–liquid extraction is also widely used in the production of fine
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s, 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 can be substantially accelerated in microfluidic devices, reducing extraction and separation times from minutes/hours to mere seconds compared to conventional extractors. 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 An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
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 Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
cations to be reduced at a mercury
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...
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 Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
can be used to stabilize an emulsion, or third phase.


Measures of effectiveness


Distribution ratio

In solvent extraction, a distribution ratio (D) is often quoted as a measure of how well-extracted a species is. The distribution ratio is a measure of the total concentration of a solute in the organic phase divided by its concentration in the aqueous phase. The partition or distribution coefficient (Kd) is the ration of solute concentration in each layer upon reaching equilibrium. This distinction between D and ''Kd'' is important. The partition coefficient is a thermodynamic equilibrium constant and has a fixed value for the solute’s partitioning between the two phases. The distribution ratio’s value, however, changes with solution conditions if the relative amounts of ''A'' and ''B'' change. If we know the solute’s equilibrium reactions within each phase and between the two phases, we can derive an algebraic relationship between Kd and ''D''. The partition coefficient and the distribution ratio are identical if the solute has only one chemical form in each phase; however, if the solute exists in more than one chemical form in either phase, then ''Kd'' and ''D'' usually have different values. 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 Gibbs Free Energy (Δ''G)'' of the extraction process. 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 non-polar mineral oil.


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 Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
(DAg) is 100, then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/DNi = SFAg/Ni = 10.


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.


Techniques


Batch wise 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. After equilibration, the extract phase containing the desired solute is separated out for further processing.


Dispersive liquid–liquid microextraction (DLLME)

A process used to extract small amounts of organic compounds from water samples. 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.


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. Another example of this application is extracting anisole from a
mixture In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...
of
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
and 5%
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...
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 Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, and water.
Caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class and is the most commonly consumed Psychoactive drug, psychoactive substance globally. It is mainly used for its eugeroic (wakefulness pr ...
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.


Multistage countercurrent continuous processes

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. 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.


Mixer–settlers

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. A novel settling device, Sudhin BioSettler, can separate an oil-water emulsion continuously at a much faster rate than simple gravity settlers. In this photo, an oil-water emulsion, stirred by an impeller in an external reservoir and pumped continuously into the two bottom side ports of BioSettler, is separated very quickly into a clear organic (mineral oil) layer exiting via the top of BioSettler and an aqueous (coloured with a red food dye) layer being pumped out continuously from the bottom of BioSettler. 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)Liquid–Liquid Extraction Equipment
Jack D. Law and Terry A. Todd, Idaho National Laboratory.


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.


Extraction without chemical change

Some solutes such as
noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
es 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 In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
(HA) into nonpolar media such as
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
. 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'' = ''HAorganicsup>2/ ''HAaqueous


Solvation mechanism

Using solvent extraction it is possible to extract
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
, plutonium, thorium and many rare earth elements from acid solutions in a selective way by using the right choice of organic extracting solvent and diluent. 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 In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
( kerosene), the uranium(VI) are extracted from strong
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
and are back-extracted (stripped) using weak nitric acid. An organic soluble uranium complex O2(TBP)2(NO3)2is 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 In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...
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 Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
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. In this case, DU = k BPsup>2 O3sup>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 A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...
; 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 Americium is a synthetic element, synthetic chemical element; it has Chemical symbol, symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element e ...
by a combination of terpyridine and a
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
in ''tert''- butyl
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
. In this case :''D''Am = ''k'' erpyridinesup>1 arboxylic acidsup>3 +sup>−3 Another example is the extraction of
zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
, cadmium, or
lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
by a di
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...
phosphinic acid (R2PO2H) into a nonpolar diluent such as an alkane. A non- polar diluent favours the formation of uncharged non-polar
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
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 Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
by a combination of 6,6'-''bis''-(5,6-di pentyl-1,2,4-triazin-3-yl)- 2,2'-bipyridine and 2-bromo hexanoic acid in ''tert''- butyl
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
. 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 Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
concentration is high, it is possible to extract
americium Americium is a synthetic element, synthetic chemical element; it has Chemical symbol, symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element e ...
as an anionic nitrate complex if the mixture contains a
lipophilic Lipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly") is the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such compounds are c ...
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 An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
anion into nitrobenzene, while the energy required to transfer a chloride anion from an aqueous phase to nitrobenzene is 43.8 kJ mol−1. 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.


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 a
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...
, and the light phase is generally
Polyethylene glycol Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular wei ...
(PEG). Traditionally, the polysaccharide used is dextran. However, dextran is relatively expensive, and research has been exploring using less expensive polysaccharides to generate the heavy 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 An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
. Polymer–salt systems. Aqueous two-phase systems can also be generated by generating the heavy phase with a concentrated salt 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.


DNA purification

The ability to purify DNA from a sample is important for many modern biotechnology processes. However, samples often contain nucleases that degrade the target DNA before it can be purified. It has been shown that DNA fragments will partition into the light phase of a polymer–salt separation system. If ligands known to bind and deactivate nucleases are incorporated into the polymer phase, the nucleases will then partition into the heavy phase and be deactivated. Thus, this polymer–salt system is a useful tool for purifying DNA from a sample while simultaneously protecting it from nucleases.


Food industry

The PEG–NaCl system has been shown to be effective at partitioning small molecules, such as peptides and nucleic acids. These compounds are often flavorants or odorants. The system could then be used by the food industry to isolate or eliminate particular flavors.
Caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class and is the most commonly consumed Psychoactive drug, psychoactive substance globally. It is mainly used for its eugeroic (wakefulness pr ...
extraction used to be done using liquid–liquid extraction, specifically direct and indirect liquid–liquid extraction (Swiss Water Method), but has since moved towards super-critical CO2 as it is cheaper and can be done on a commercial scale.


Analytical chemistry

Often there are chemical species present or necessary at one stage of sample processing that will interfere with the analysis. For example, some air monitoring is performed by drawing air through a small glass tube filled with sorbent particles that have been coated with a chemical to stabilize or derivatize the analyte of interest. The coating may be of such a concentration or characteristics that it would damage the instrumentation or interfere with the analysis. If the sample can be extracted from the sorbent using a nonpolar solvent (such as toluene or carbon disulfide), and the coating is polar (such as HBr or phosphoric acid) the dissolved coating will partition into the aqueous phase. Clearly the reverse is true as well, using polar extraction solvent and a nonpolar solvent to partition a nonpolar interferent. A small aliquot of the organic phase (or in the latter case, polar phase) can then be injected into the instrument for analysis.


Purification of amines

Amines (analogously to ammonia) have a lone pair of electrons on the nitrogen atom that can form a relatively weak bond to a hydrogen atom. It is therefore the case that under acidic conditions amines are typically protonated, carrying a positive charge and under basic conditions they are typically deprotonated and neutral. Amines of sufficiently low molecular weight are rather polar and can form hydrogen bonds with water and therefore will readily dissolve in aqueous solutions. Deprotonated amines on the other hand, are neutral and have ''greasy'', nonpolar organic substituents, and therefore have a higher affinity for nonpolar inorganic solvents. As such purification steps can be carried out where an aqueous solution of an amine is neutralized with a base such as sodium hydroxide, then shaken in a separatory funnel with a nonpolar solvent that is immiscible with water. The organic phase is then drained off. Subsequent processing can recover the amine by techniques such as recrystallization, evaporation or distillation; subsequent extraction back to a polar phase can be performed by adding HCl and shaking again in a separatory funnel (at which point the ammonium ion could be recovered by adding an insoluble counterion), or in either phase, reactions could be performed as part of a chemical synthesis.


Temperature swing solvent extraction

Temperature swing solvent extraction is an experimental technique for the desalination of drinking water. It has been used to remove up to 98.5% of the salt content in water, and is able to process hypersaline brines that cannot be desalinated using reverse osmosis.


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 Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
or
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
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 D_ being a constant it becomes :D_ = ''k'' 2 (organic) 2 (aq)I (aq)] 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.


Liquid-liquid equilibrium calculations

In order to calculate the phase equilibrium, it is necessary to use a thermodynamic model such as NRTL, UNIQUAC, etc. The corresponding parameters of these models can be obtained from literature (e.g. Dechema Chemistry Data Series, Dortmund Data Bank, etc.) or by a correlation process of experimental data.


Equipment

While solvent extraction is often done on a small scale by synthetic lab chemists using a separatory funnel, Craig apparatus or membrane-based techniques, 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.


Extraction of metals

The extraction methods for a range of metals include:


Cerium

Ce(IV) can separated from other rare earth (III) ions by using sulfuric acid and HDEHP to produce high-purity cerium oxide.


Cobalt

The extraction of cobalt from hydrochloric acid using Alamine 336 (tri-octyl/decyl amine) in '' meta''- xylene. Cobalt can be extracted also using Ionquest 290 or Cyanex 272 .


Copper

Copper can be extracted using hydroxy oximes as extractants, a recent paper describes an extractant that has a good selectivity for copper over
cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
and nickel.


Gadolinium

The rare earth element Gadolinium can be extracted with ''n-''tributyl phosphate and nitric acid to yield over a kilogram of gadolinium oxide.


Neodymium

The rare earth element Neodymium is extracted by di(2-ethyl-hexyl)phosphoric acid into
hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...
by an ion exchange mechanism. Neodymium can also be separated from dysprosium through selective precipitation of dysprosium with Cyanex 272 when the HNO3 concentration was 0.001 mol/L.


Nickel

Nickel can be extracted using di(2-ethyl-hexyl)phosphoric acid and tributyl phosphate in a hydrocarbon diluent (Shellsol).


Palladium and platinum

Dialkyl sulfides, tributyl phosphate and alkyl amines have been used for extracting palladium and platinum.


Polonium

Polonium is produced in reactors from natural 209Bi, bombarded with
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s, creating 210Bi, which then decays to 210Po via beta-minus decay. The final purification is done pyrochemically with sodium hydroxide at . This is then followed by liquid-liquid extraction, with dibutyl Carbitol as the extractant.


Thorium

Thorium can be extracted from other rare earths by using sulfuric acid and the extractant, N1923, to produce thorium oxide with 99.5% purity and 99% recovery.


Zinc and cadmium

Zinc and cadmium are both extracted by an ion exchange process, the ''N,N,N′,N′''-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) acts as a masking agent for the zinc and an extractant for the cadmium. In the modified Zincex process, zinc is separated from most divalent ions by solvent extraction. D2EHPA (Di (2) ethyl hexyl phosphoric acid) is used for this. A zinc ion replaces the proton from two D2EHPA molecules. To strip the zinc from the D2EHPA,
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
is used, at a concentration of above 170g/L (typically 240-265g/L).


Lithium

Lithium extraction is more popular due to the high demand of lithium-ion batteries. TBP (Tri-butyl phosphate) and are mostly used to extract lithium from brine (with high Li/Mg ratio). Alternatively, Cyanex 272 was also used to extract lithium. The mechanism of lithium extraction was found differently from other metals, such as cobalt, due to the weak coordinating bonding between lithium ions and extractants.


See also

* Fragrance extraction * Dortmund Data Bank * Non-random two-liquid model - (NRTL model) LL Phase Equilibrium Calculation * UNIQUAC - LL Phase Equilibrium Calculation


References


Further reading

*B.L. Karger, 2014, "Separation and Purification: Single-stage versus multistage processes" and "Separation and Purification: Separations Based on Equilibrium", Encyclopædia Britannica, se

an

accessed 12 May 2014. *Gunt Hamburg, 2014, "Thermal Process Engineering: liquid–liquid extraction and solid-liquid extraction", se

accessed 12 May 2014. *G.W. Stevens, T.C., Lo, & M. H. I. Baird, 2007, "Extraction, liquid–liquid", in Kirk-Othmer Encyclopedia of Chemical Technology, , accessed 12 May 2014. *Colin Poole & Michael Cooke, 2000, "Extraction", in Encyclopedia of Separation Science, 10 Vols., , se

accessed 12 May 2014. *Sikdar, Cole, et al. Aqueous Two-Phase Extractions in Bioseparations: An Assessment. Biotechnology 9:254. 1991 *Szlag, Giuliano. A Low-Cost Aqueous Two Phase System for Enzyme Extraction. Biotechnology Techniques 2:4:277. 1988 *Dreyer, Kragl. Ionic Liquids for Aqueous Two-Phase Extraction and Stabilization of Enzymes. Biotechnology and Bioengineering. 99:6:1416. 2008 *Boland. Aqueous Two-Phase Systems: Methods and Protocols. Pg 259-269 *https://web.archive.org/web/20100702074135/http://ull.chemistry.uakron.edu/chemsep/extraction/
Topological Analysis of the Gibbs Energy Function (Liquid-Liquid Equilibrium Correlation Data). Including a Thermodynamic Review and a Graphical User Interface (GUI) for Surfaces/Tie-lines/Hessian matrix analysis
- University of Alicante (Reyes-Labarta et al. 2015-18) {{DEFAULTSORT:Liquid-liquid extraction Extraction (chemistry) Laboratory techniques Flavor technology Articles containing video clips