Liquid-liquid extraction 2393984 224233361 2008-07-07T22:34:51Z Freestyle-69 6502796 /* See also */ c/e {{Cleanup|date=August 2006}} {{Chemical equilibria}} '''Liquid-liquid extraction''', also known as '''solvent extraction''' and '''partitioning''', is a method to separate compounds based on their relative [[solubility|solubilities]] in two different [[miscible|immiscible]] [[liquid]]s, usually [[Water (molecule)|water]] and an [[solvent|organic solvent]]. It is an [[wiktionary:Extraction|extraction]] of a substance from one liquid [[phase (matter)|phase]] into another liquid phase. Liquid-liquid extraction is a basic technique in [[chemistry|chemical]] [[laboratory|laboratories]], where it is performed using a [[separatory funnel]]. This type of process is commonly performed after a chemical reaction as part of the [[work-up]]. Solvent extraction is used in [[nuclear reprocessing]], [[ore]] processing, the production of fine [[organic compound]]s, the processing of [[perfumes]] and other industries. In an industrial application, this process is done continuously by pumping an organic and aqueous stream into a mixer. This mixes the organic component with the aqueous component and allows ion transfer between them. The mixing continues until equilibrium is reached. Once the ion transfer is complete (equilibrium is reached), the mixture flows into a vessel, where the organic and aqueous are allowed to separate, similar to the way oil and water would separate after mixing them. Fresh material is continuously fed into the mixer, and a two continuous streams is removed from the settler (one organic, and one aqueous). The process is commonly used to process copper and uranium, but has recently been adapted for zinc, at [[Skorpion Zinc]] mine in Namibia. Liquid-liquid extraction is possible in non-aqueous systems: in a system consisting of a [[molten metal]] in contact with [[molten]] salt, metals can be extracted from one phase to the other. This is related to a [[mercury (element)|mercury]] [[electrode]] where a metal can be reduced, the metal will often then dissolve in the mercury to form an [[amalgam]] which modifies its electrochemistry greatly. For example it is possible for [[sodium]] [[cation]]s 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. If a [[detergent]] or fine [[solid]] can stabilise an [[emulsion]] which in the solvent extraction community is known as a [[third phase]]. ==Distribution ratio== In solvent extraction, a distribution ratio is often quoted as a measure of how well-extracted a species is. The distribution [[ratio]] (''D'') 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 Δ''G'' of the extraction process. Sometimes the distribution ratio is referred to as the partition coefficient, which is often expressed as the [[logarithm]]. See [[partition coefficient]] for more details. Note that a distribution ratio for [[uranium]] and [[neptunium]] between two inorganic solids ([[zirconolite]] and [[perovskite]]) has been reported.[http://www-ssrl.slac.stanford.edu/pubs/activity_rep/ar98/2370-vance.pdf] ==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 [[solute]]s. For instance if the distribution ratio for [[nickel]] (D<sub>Ni</sub>) is 10 and the distribution ratio for [[silver]] (D<sub>Ag</sub>) is 100, then the silver/nickel separation factor (SF<sub>Ag/Ni</sub>) is equal to D<sub>Ag</sub>/D<sub>Ni</sub> = SF<sub>Ag/Ni</sub> = 10. ==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 0.1 / 0.01 = 10. ==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 log<sub>10</sub>(''D'') against log<sub>10</sub>([[''Z'']]) will be two. ==Batchwise single stage extractions== This is commonly used on the small scale in chemical labs, it is normal to use a [[separating funnel]] For instance if a chemist was to extract [[anisole]] from a [[mixture]] of [[water]] and 5% [[acetic acid]] using [[diethyl ether|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]]. ==Multistage [[Countercurrent exchange|countercurrent]] continuous processes== These are commonly used in [[industry]] for the processing of [[metals]] such as the [[lanthanides]], because the [[separation factor]]s between the lanthanides are so small many extraction stages are needed. In the multistage processes the aqueous [[raffinate]] from one extraction unit is feed as 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 (chemistry)|stripping]] section to win back the metal from the organic phase. ==Extraction without chemical change== Some solutes such as [[noble gas]]es can be extracted from one phase to another without the need for a chemical reaction (See [[Absorption (chemistry)]]). This is the most simple type of solvent extraction. Some solutes which do not at first sight appear to undergo a reaction during the extraction process do not have distribution ratio which is independent of concentration, a classic example is the extraction of [[carboxylic acids]] ('''HA''') into non polar 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>]]<sup>2</sup>/[['''HA'''<sub>aqueous</sub>]] ==Extraction with chemical change== A small review on the subject of the main classes of extraction agents (extractants) can be found at [http://www.cognis.com/mining/mid/services/pdfs/solvente.pdf]. ===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]] [[tri-n-butyl phosphate]]. The PUREX process 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 (chemistry)|complex]] [UO<sub>2</sub>(TBP)<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>] is formed, then the organic layer bearing the uranium is brought into contact with a [[dilution|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]] can be added which converts the [[plutonium]] to the trivalent [[oxidation state]]. 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]] [[wiktionary:Extraction|extraction]]. Here in this case D<sub>U</sub> = k [[TBP]]<sup>2</sup>[[NO<sub>3</sub>]]<sup>2</sup> ===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 ''D''<sub>Am</sub> = ''k'' [[terpyridine]]<sup>1</sup>[[carboxylic acid]]<sup>3</sup>[[H+]]<sup>-3</sup> Another example would be the extraction of [[zinc]], [[cadmium]] or [[lead]] by a di[[alkyl]] phosphinic acid (R<sub>2</sub>PO<sub>2</sub>H) into a non polar diluent such as an [[alkane]]. A non-[[Polar molecule|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-di[[pentyl]]-1,2,4-triazin-3-yl)-[[2,2'-bipyridine]] and 2-bromo[[hexanoic acid]] in ''tert''-[[butyl]] [[benzene]]. At both high and low nitric acid concentrations the metal distribution ratio is higher than it is for an intermidate 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 which is more likely to be encountered by the '' 'average' '' chemist is the use of a [[phase transfer catalyst]], these are charged species which transfer 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 according to F. Scholz, S. Komorsky-Lovric, M. Lovric, ''Electrochem. Comm.'', 2000, '''2''', 112-118 the 31.1 [[kilojoule|kJ]] [[mole (unit)|mol]]<sup>-1</sup> is required to transfer an [[acetate]] anion into nitrobenzene, while according to A.F.Danil de Namor and T.Hill, ''J.Chem. Soc Fraraday Trans.'', 1983, 2713 the energy required to transfer a chloride anion from an aqueous phase to nitrobenzene is 43.8 kJ mol<sup>-1</sup>. 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 the given out by the reaction. A 43.8 to 31.1 kJ mol<sup>-1</sup> = 12.7 kJ mol<sup>-1</sup> 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. ==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 due to the fact that the rate of ligand exchange at these metal centres 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 <math>D_{\mathrm{I}^{+2}}</math> being a constant it becomes <math>D_{\mathrm{I}^{+2}}</math> = ''k''[[I<sub>2</sub>.<sub>Organic</sub>]]/[I<sub>2</sub>.<sub>Aqueous</sub>] [[I<sup>-</sup>.<sub>Aqueous</sub>]] This is because the [[iodine]] reacts with the [[iodide]] to form I<sub>3</sub><sup>-</sup>. The I<sub>3</sub><sup>-</sup> anion is an example of a [[polyhalide]] [[anion]] which is quite common. ==Industrial process design== Typically 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 which might be present. ==Equipment== {{video float | align = right | filename = Separation02.ogg | title = Two layers separating during a liquid-liquid extraction | description = An organic [[MTBE]] solution is extracted with [[aqueous]] sodium bicarbonate solution. This base removes [[benzoic acid]] as [[benzoate]] but leaves non-acidic [[benzil]] (yellow) behind in the upper organic phase. }} While solvent extraction is often done on a small scale by synthetic lab chemists using a [[separatory funnel]], it is normally done on the industrial scale using machines which bring the two liquid phases into contact with each other. Such machines include [[centrifugal contactor]]s, [[spray column]]s, [[pulsed column]]s and [[mixer-settler]]s. ==Extraction of metals== A review of the extraction methods for a range of metals is to be found here [http://www.cognis.com/mining/mid/services/pdfs/thesolve.pdf]. ===Palladium and platinum=== Dialkyl sulfides, tributyl phosphate and alkyl amines have been used for extracting these metals.[http://www.halwachs.de/solvent-extraction.htm]<ref>P. Giridhar, K.A. Venkatesan, T.G. Srinivasan and P.R. Vasudeva Rao, ''Hydrometallurgy'', 2006, '''81''', 30-39.</ref> ===Neodymium=== This rare earth is extracted by di(2-ethyl-hexyl)phosphoric acid into [[hexane]] by an ion exchange mechanism.<ref>J. M. Sánchez, M. Hidalgo, M. Valiente and V. Salvadó, ''Solvent Extraction and Ion Exchange'', 1999, '''17''', 455-474.</ref> ===Cobalt=== The extraction of cobalt from [[hydrochloric acid]] using alamine 336 in ''[[meta]]''-[[xylene]].<ref>M. Filiz, N.A. Sayar and A.A. Sayar, ''Hydrometallurgy'', 2006, '''81''', 167-173.</ref> Cobalt can be extracted also using Cyanex 272 {''bis''-(2,4,4-trimethylpentyl) phosphinic acid}. ===Nickel=== Nickel can be extracted using di(2-ethyl-hexyl)phosphoric acid and [[tributyl phosphate]] in a hydrocarbon diluent (Shellsol).<ref>{{cite web | url = http://www.biomet.com.au/Extract/NiCoFS.htm | publisher = BioMetallurgical Pty Ltd | author = Lee W. John | title = A Potential Nickel / Cobalt Recovery Process}}</ref> ===Copper=== Copper can be extracted using hydroxy[[oxime]]s as extractants, a recent paper describes an extractant which has a good selectivity for copper over [[cobalt]] and [[nickel]].<ref>Yoshinari Baba, Minako Iwakuma and Hideto Nagami, ''Ind. Eng. Chem. Res'', 2002, '''41''', 5835-5841.</ref> ===Zinc and cadmium=== The 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.<ref>{{cite journal | author = K. Takeshita, K. Watanabe, Y. Nakano, M. Watanabe | title = Solvent extraction separation of Cd(II) and Zn(II) with the organophosphorus extractant D2EHPA and the aqueous nitrogen-donor ligand TPEN | journal = [[Hydrometallurgy]] | year = 2003 | volume = 70 | pages = 63–71}}</ref> 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 at a high pH (around pH 4-5 Zinc is selective). To strip the Zinc from the D2EHPA, sulfuric acid is used, at a strength of about 170g/l. ==Terms== * [[Solvent]] is the term for the organic layer * [[Diluent]] is the term for an inert liquid used to dissolve an extractant, and to dilute the system. * [[Extractant]] is the term for a metal extraction agent * [[Raffinate]] is the term for the aqueous layer after a solute has been extracted from it * [[Scrubbing]] is the term for the back extraction of an unwanted solute from the organic phase * [[Stripping]] is the term for the back extraction from the organic phase ==See also== * [[wiktionary:Extraction|Extraction]] on Wictionary * [[Acid-base extraction]] * [[Separatory funnel]] * [[Multiphasic liquid]] * [[Work-up]] ==References== <references/> {{Separation processes}} [[Category:Unit operations]] [[Category:Laboratory techniques]] [[Category:Separation processes]] [[Category:Flavor technology]] [[cs:Extrakce]] [[de:Extraktion (Verfahrenstechnik)]] [[es:Extracción líquido-líquido]] [[eo:Ekstraktado]] [[ko:추출]] [[it:Estrazione liquido-liquido]] [[nl:Extractie (scheikunde)]] [[ja:抽出]] [[pl:Ekstrakcja]] [[pt:Extração líquido-líquido]] [[ru:Экстракция]] [[uk:Екстракція]] [[zh:萃取]]