Liquid-liquid extraction
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Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their solution preferences for two different immiscible liquids, usually water and an organic solvent. It is an extraction 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 funnels, as well as a common process in chemical industry and ore processing.
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 this contact of the two liquids 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 compounds, 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 cations to be reduced at a mercury cathode to form sodium amalgam, while at an inert electrode (such as platinium) the sodium cations are not reduced, instead water is reduced to hydrogen.
It is important to note that if a detergent or fine solid can stablise an emulsion which in the solvent extraction community is known as a third phase.
[edit] 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 ΔG of the extraction process.
Sometimes the distribution ratio is referred to as the partition coefficent, 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.[1]
[edit] 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 explanined by the following example.
- Imagine that you have a litre of water containing 10 grams of a solute which has a distribution ratio of one when the solvent is nitrobenzene.
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.
- Imagine that the same original aqueous mixture was shaken with 500 ml of nitrobenzene. When the system has been brought to equilibrium, the concentration of the solute in each phase is the same. As the organic to aqueous ratio is now 1:2 the aqueous phase will now contain 6.666 g of solute while the organic layer will have 3.333 g of solute.
If the two layers are now separated and the nitrobenzene extract is kept, then the aqueous layer (raffinate) can be treated further.
- If the raffinate from this first extraction is extracted with 500 ml of new nitrobenzene, then 33.33% of the remaining solute will be extracted. This will be 2.222 g of solute in the new nitrobenzene extract. In total, 3.333 + 2.222 grams of solute for a total of 5.555 grams will have been extracted. This is a recovery of 55.5%. An additional 5% of product has been obtained.
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%.
[edit] 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 200, then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/DNi = SFAg/Ni = 20.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 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.
[edit] 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 (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]2/[HAaqueous]
[edit] Extraction with chemical change
A small review on the subject of the main classes of extraction agents (extractants) can be found at [2].
[edit] 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 [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 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 extraction.
Here in this case DU = k [TBP]2[NO3]2
[edit] 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-butylbenzene. In this case
DAm = k terpyridine1[carboxylic acid]3[H+]-3
Another example would be the extraction of zinc, cadmium or lead by a dialkyl 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.
It is important to note that 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-butylbenzene. At both high and low nitric acid concentrations the metal distribution ratio is higher than it is for an intermidate nitric acid concentration.
[edit] 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.
[edit] 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.
[edit] 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[I2.Organic]/[I2.Aqueous][I-.Aqueous]
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.
[edit] 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.
[edit] 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 contactors, spray columns, pulsed columns and mixer-settlers.
[edit] Extraction of metals
A review of the extraction methods for a range of metals is to be found here [3].
[edit] Palladium and platinium
Dialkyl sulfides, tributyl phosphate and alkyl amines have been used for extracting these metals.[4] 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.
[edit] 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.
[edit] 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}
[edit] Nickel
Nickel can be extracted using di(2-ethyl-hexyl)phosphoric acid and tributyl phosphate in a hydrocarbon diluent (Shellsol).[5]
[edit] Copper
Copper can be extracted using hydroxyoximes 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.
[edit] 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.
[edit] 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
