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Liquid-liquid extraction, also known as '''solvent extraction''' and '''partitioning''', is a method to separate compounds based on their of a substance from one liquid Phase into another liquid phase. Liquid-liquid extraction is a basic technique in Chemical Laboratories , where it is done in Separating Funnel s, as well as a common process in Chemical Industry and Ore processing. This type of process is commonly performed after a chemical reaction as part of the Work-up . In an industrial application, this process is done continuously by pumping an organic and aqueous stream into a mixer. This mixes the organic (oil like solution) component with the aqueous ( water based solution) component and allows ion transfer between them. The mixing continues until equilibrium is reached. The size of the mixers are such that sufficient residence time is allowed for, to reach equilibrium. Once the ion transfer is complete (equilibrium is reached) , the mixture flows into a swimming pool type 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. In the practical use, usually one phase is a water or water-based (aqueous) solution and the other an Organic Solvent which is Immiscible with water. Solvent extraction is used in Nuclear Reprocessing , Ore processing, the production of fine Organic Compound s, the processing of Perfumes and other industries. It is interesting to note that liquid-liquid extraction is possible in non aqueous systems, for instance 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 Electrode where a metal can be reduced, the metal will often then dissolve in the mercury to form an Amalgam which modifies the 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 stablise 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. {Link without Title} One big batch of solvent or several smaller batches? When a solute is being extracted from an aqueous phase using an organic solvent, a better recovery will be obtained by using two equal volumes of solvent than the recovery that would be obtained using all the solvent in one large volume. This can be shown by experiment and can be explained by the following example.
If the aqueous solution is shaken until a Dynamic Equilibrium is reached with one litre of nitrobenzene, then five grams of solute would be transferred. The aqueous Raffinate will contain five grams of solute. 50% of the solute has been recovered.
If the two layers are now separated and the nitrobenzene extract is kept, then the aqueous layer (raffinate) can be treated further.
If the nitrobenzene was to be used in four batches of 250 ml then the outcome is theoretically better still. The first 250 ml will extract 2 grams. The second 250 ml will extract 1.6 grams. The third 250 ml will extract 1.28 grams. The fourth 250 ml will extract 1.024 grams. In total 5.904 grams is extracted; this is a recovery of 59%.
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 (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. DECONTAMINATION FACTOR This is used to express the ability of a process to remove a 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 log10(''D'') against log10( {Link without Title} ) 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 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 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 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'' = HAorganic" class="copylinks" target="_blank">{Link without Title} 2/ HAaqueous" class="copylinks" target="_blank">{Link without Title} EXTRACTION WITH CHEMICAL CHANGE A small review on the subject of the main classes of extraction agents (extractants) can be found at {Link without Title} . Solvation mechanism Using solvent extraction it is possible to extract . Here in this case DU = k {Link without Title} 2 {Link without Title} 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 ''D''Am = ''k'' Terpyridine 1 acid 3[H+]-3 Another example would be the extraction of Zinc , Cadmium or Lead by a di Alkyl phosphinic acid (R2PO2H) into a non polar 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-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 KJ Mol -1 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-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 the 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. 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 being a constant it becomes = ''k'' {Link without Title} / {Link without Title} {Link without Title} This is because the Iodine reacts with the Iodide to form I3-. The I3- 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
While solvent extraction is often done on a small scale by synthetic lab chemists using a Separating 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 {Link without Title} . Palladium and platinum Dialkyl sulfides, tributyl phosphate and alkyl amines have been used for extracting these metals. {Link without Title} An example of such chemistry can be seen in the following paper. P. Giridhar, K.A. Venkatesan, T.G. Srinivasan and P.R. Vasudeva Rao, ''Hydrometallurgy'', 2006, 81, 30-39. Neodymium This rare earth is extracted by di(2-ethyl-hexyl)phosphoric acid into Hexane by an ion exchange mechanism. J. M. Sánchez, M. Hidalgo, M. Valiente and V. Salvadó, ''Solvent Extraction and Ion Exchange'', 1999, 17, 455-474. Cobalt The extraction of cobalt from Hydrochloric Acid using alamine 336 in '' Meta ''- Xylene . M. Filiz, N.A. Sayar and A.A. Sayar, ''Hydrometallurgy'', 2006, 81, 167-173. 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). {Link without Title} 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 . Yoshinari Baba, Minako Iwakuma and Hideto Nagami, ''Ind. Eng. Chem. Res'', 2002, 41, 5835-5841. 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. K. Takeshita, K. Watanabe, Y. Nakano, M. Watanabe. Solvent extraction separation of Cd(II) and Zn(II) with the organophosphorus extractant D2EHPA and the aqueous nitrogen-donor ligand TPEN. ''Hydrometallurgy'', 2003, 70, 63-71. 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, sulphuric acid is used, at a strength of about 170g/l. TERMS
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