Liquid–liquid extraction, also known as
solvent extraction and
partitioning, is a method to separate compounds based on their relative
solubilitiesSolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
in two different
immiscibleMiscibility is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases , but the main focus is usually on the solubility of one liquid in another...
liquids, usually water and an
organic solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
. 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 performed using a
separatory funnelA separatory funnel, also known as separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture between two immiscible solvent phases of different densities Typically, one of the...
. This type of process is commonly performed after a chemical reaction as part of the work-up.
The term
partitioning is commonly used to refer to the underlying chemical and physical processes involved in
liquid–liquid extraction but may be fully synonymous. 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 reprocessingNuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...
, ore processing, the production of fine
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s, the processing of
perfumePerfume is a mixture of fragrant essential oils and/or aroma compounds, fixatives, and solvents used to give the human body, animals, objects, and living spaces "a pleasant scent"...
s, the production of
vegetable oilsVegetable fats and oils are lipid materials derived from plants. Physically, oils are liquid at room temperature, and fats are solid. Chemically, both fats and oils are composed of triglycerides, as contrasted with waxes which lack glycerin in their structure...
and
biodieselBiodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....
, and other industries.
Liquid–liquid extraction is possible in non-aqueous systems: In a system consisting of a molten metal in contact with
molten saltMolten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...
s, metals can be extracted from one phase to the other. This is related to a
mercuryMercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
where a metal can be reduced, the metal will often then dissolve in the mercury to form an
amalgamAn amalgam is a substance formed by the reaction of mercury with another metal. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.The...
that modifies its electrochemistry greatly. For example, it is possible for
sodiumSodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
cations to be reduced at a mercury
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
to form
sodium amalgamSodium amalgam, commonly denoted Na, is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions involving mercury as a major component. Sodium amalgam is often used in reactions as strong reducing agents with better handling properties compared...
, while at an inert electrode (such as platinum) the sodium cations are not reduced. Instead, water is reduced to hydrogen. A
detergentA detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
or fine
solidSolid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
can be used to stabilize an
emulsionAn emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
, or
third phaseThird phase is the term for a stable emulsion which forms in a solvent extraction system when the original two phases are mixed.The third phase can be caused by a detergent or a fine solid...
.
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
concentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of a
soluteIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
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 coefficientIn chemistry and the pharmaceutical sciences, a partition- or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium. The terms "gas/liquid partition coefficient" and "air/water partition coefficient" are...
, which is often expressed as the
logarithmThe logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
. Note that a distribution ratio for
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and
neptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
between two inorganic solids (
zirconoliteZirconolite is a mineral, calcium zirconium titanate; formula CaZrTi2O7. Some examples of the mineral may also contain thorium, uranium, cerium, niobium and iron; the presence of thorium and/or uranium would make the mineral radioactive....
and
perovskiteA perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
) has been reported.
In solvent extraction, two immiscible liquids are shaken together. The more
polarIn chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
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.
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
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
(D
Ni) is 10 and the distribution ratio for
silverSilver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
(D
Ag) is 100, then the silver/nickel separation factor (SF
Ag/Ni) is equal to D
Ag/D
Ni = SF
Ag/Ni = 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
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
to
indiumIndium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
, and the product is a 1:99 mixture of
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
and
indiumIndium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
, 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
10(
D) against log
10(
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 were to extract
anisoleAnisole, or methoxybenzene, is the organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances...
from a
mixtureIn chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...
of
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and 5%
acetic acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
using
etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, 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 bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...
. The acetic acid reacts with the
sodium bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...
to form
sodium acetateSodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...
,
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
.
Multistage countercurrent continuous processes
These are commonly used in
industryIndustry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
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
raffinateRaffinating something is a technique used in metallurgy to remove impurities from liquid material. There are many different kinds of raffination, for example you can use vacuum to extract hydrogen from metals....
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
countercurrentCountercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
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
strippingStripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have co-current or countercurrent flows. Stripping is usually carried out in either a packed or trayed column.-...
section to obtain the metal back from the organic phase.
Multistage Podbielniak contactor centrifuges produce three to five stages of theoretical extraction in a single countercurrent pass, and are used in fermentation-based pharmaceutical and food additive production facilities.
Extraction without chemical change
Some solutes such as
noble gasThe noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
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 (
HA) into nonpolar media such as
benzeneBenzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. 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 =
HAorganic2/
HAaqueous
Solvation mechanism
Using solvent extraction it is possible to extract
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
,
plutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, or
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
from acid solutions. One solvent used for this purpose is the
organophosphateAn organophosphate is the general name for esters of phosphoric acid. Phosphates are probably the most pervasive organophosphorus compounds. Many of the most important biochemicals are organophosphates, including DNA and RNA as well as many cofactors that are essential for life...
tri-n-butyl phosphate. The
PUREXPUREX is an acronym standing for Plutonium - URanium EXtraction — de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid-liquid extraction ion-exchange.The PUREX process was invented by Herbert H. Anderson and...
process that is commonly used in
nuclear reprocessingNuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...
uses a mixture of tri-n-butyl phosphate and an
inert-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
(
keroseneKerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
), the uranium(VI) are extracted from strong nitric acid and are back-extracted (stripped) using weak nitric acid. An organic soluble uranium
complexIn chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
[UO
2(TBP)
2(NO
3)
2] is formed, then the organic layer bearing the uranium is brought into contact with a
diluteIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
nitric acid solution; the equilibrium is shifted away from the organic soluble uranium complex and towards the free TBP and
uranyl nitrateUranyl nitrate is a water soluble yellow uranium salt. The yellow-green crystals of uranium nitrate hexahydrate are triboluminescent.Uranyl nitrate can be prepared by reaction of uranium salts with nitric acid...
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 agentA reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
that converts the
plutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
to the trivalent
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
can be added. This
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
does not form a stable complex with TBP and
nitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
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
solvationSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
extraction.
Here in this case D
U = k
TBPTributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
2NO
32
Ion exchange mechanism
Another extraction mechanism is known as the
ion exchangeIon exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
mechanism. Here, when an ion is transferred from the aqueous phase to the organic phase, another
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is transferred in the other direction to maintain the charge balance. This additional ion is often a
hydrogen ionHydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
; for ion exchange mechanisms, the distribution ratio is often a function of
pHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. An example of an ion exchange extraction would be the extraction of
americiumAmericium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...
by a combination of
terpyridineTerpyridine is a heterocyclic compound derived from pyridine. This colourless solid is used as a ligand in coordination chemistry.-Synthesis:...
and a
carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
in
tert-
butylIn organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula -C4H9, derived from either of the two isomers of butane....
benzeneBenzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. In this case
DAm =
k terpyridineTerpyridine is a heterocyclic compound derived from pyridine. This colourless solid is used as a ligand in coordination chemistry.-Synthesis:...
1carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
3H+In chemistry, a hydron is the general name for a cationic form of atomic hydrogen : most commonly a "proton". However, hydron includes cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons , deuterons , and tritons...
−3
Another example is the extraction of
zincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
,
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, or
leadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
by a dialkyl phosphinic acid (R
2PO
2H) into a nonpolar
diluentA diluent is a diluting agent.Certain fluids are too viscous to be pumped easily or too dense to flow from one particular point to the other. This can be problematic, because it might not be economically feasible to transport such fluids in this state.To ease this restricted movement, diluents...
such as an
alkaneAlkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
. A non-polar diluent favours the formation of uncharged non-polar
metalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
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
lanthanideThe lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
) extraction from
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
by a combination of 6,6'-
bis-(5,6-di
pentylIn organic chemistry, pentyl is a five-carbon alkyl substituent with chemical formula -C5H11. It is the substituent form of the alkane pentane. In older literature, the common non-systematic name "amyl" was often used for the pentyl group....
-1,2,4-triazin-3-yl)-
2,2'-bipyridine2,2'-Bipyridine is a organic compound with the formula . This colorless solid, commonly abbreviated bipy or bpy , is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals...
and 2-bromo
hexanoic acidHexanoic acid , is the carboxylic acid derived from hexane with the general formula C5H11COOH. It is a colorless oily liquid with an odor that is fatty, cheesy, waxy, and like that of goats or other barnyard animals...
in
tert-
butylIn organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula -C4H9, derived from either of the two isomers of butane....
benzeneBenzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. 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
nitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
concentration is high, it is possible to extract
americiumAmericium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...
as an anionic nitrate complex if the mixture contains a
lipophilicLipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...
quaternary ammonium salt.
An example that is more likely to be encountered by the
'average' chemist is the use of a
phase transfer catalystIn chemistry, a phase transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase transfer catalysis is a special form of heterogeneous catalysis. Ionic reactants are often soluble in an aqueous phase but...
. This is a charged species that transfers another
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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
molThe mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
−1 is required to transfer an
acetateAn acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
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 acetateSodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...
while the organic phase is a nitrobenzene solution of
benzyl chlorideBenzyl chloride, or α-chlorotoluene, is an organic compound with the formula C6H5CH2Cl. This colourless liquid is a reactive organochlorine compound that is a widely used chemical building block.-Preparation:...
, then, when a phase transfer catalyst, the acetate anions can be transferred from the aqueous layer where they react with the
benzylIn organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C6H5CH2-. Benzyl features a benzene ring attached to a CH2 group.-Nomenclature:...
chlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
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
nitrobenzeneNitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...
using one
equivalent weightEquivalent weight is a term which has been used in several contexts in chemistry. In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will:...
of a tetraalkylammonium acetate.
Aqueous two-phase extraction
Using an aqueous two-phase system, it is possible to generate two immiscible water phases. This can then be used to extract proteins, which would denature if exposed to 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
palladiumPalladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
or
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
can be very slow because the rate of ligand exchange at these metal centers is much lower than the rates for
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
or
silverSilver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity 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 tetrachlorideCarbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
then the presence of
iodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
in the aqueous phase can alter the extraction chemistry.
Instead of
being a constant it becomes
=
kI
2.
Organic/[I
2.
Aqueous] I
-.
Aqueous
This is because the
iodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
reacts with the
iodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
to form I
3-. The I
3- 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 carbonateSodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
solution to remove any dibutyl hydrogen phosphate or butyl dihydrogen phosphate that might be present.
Equipment
Two layers separating during a liquid–liquid extraction.
While solvent extraction is often done on a small scale by synthetic lab chemists using a
separatory funnelA separatory funnel, also known as separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture between two immiscible solvent phases of different densities Typically, one of the...
or Craig apparatus, 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 extractors, spray columns,
pulsed columnsPulsed columns are a type of liquid-liquid extraction equipment; examples of this class of extraction equipment is used at the BNFL plant THORP....
, and mixer-settlers.
Extraction of metals
The extraction methods for a range of metals include:
- Cobalt – The extraction of cobalt from hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
using alamine 336 in metaMeta- , is a prefix used in English to indicate a concept which is an abstraction from another concept, used to complete or add to the latter....
-xyleneXylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...
. Cobalt can be extracted also using Cyanex 272 {bis-(2,4,4-trimethylpentyl) phosphinic acid}.
- Copper – Copper can be extracted using hydroxyoxime
An oxime is a chemical compound belonging to the imines, with the general formula R1R2C=NOH, where R1 is an organic side chain and R2 may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds...
s as extractants, a recent paper describes an extractant that has a good selectivity for copper over cobaltCobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
and nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
.
- Neodymium – This rare earth is extracted by di(2-ethyl-hexyl)phosphoric acid into hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
by an ion exchange mechanism.
- Nickel – Nickel can be extracted using di(2-ethyl-hexyl)phosphoric acid and tributyl phosphate
Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
in a hydrocarbon diluent (Shellsol).
- Palladium and platinum – Dialkyl sulfides, tributyl phosphate and alkyl amines have been used for extracting these metals.
- 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. 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 is used, at a concentration of above 170g/l (typically 240-265g/l).
See also
- centrifugal extractor
A centrifugal extractor uses the rotation of the rotor inside a centrifuge to mix two immiscible liquids outside the rotor and to separate the liquids in the field of gravity inside the rotor...
- Acid-base extraction
Acid-base extraction is a procedure using sequential liquid–liquid extractions to purify acids and bases from mixtures based on their chemical properties....
- Ionic transfer
Ionic transfer is a term which can be used to describe the transfer of ions from one liquid phase to another.This is related to the phase transfer catalysts which are a special type of liquid-liquid extraction which is used in synthetic chemistry....
- Multiphasic liquid
A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases. Biphasic mixtures consisting of two immiscible phases are very common and usually consist of an organic solvent and an aqueous phase...
- Separatory funnel
A separatory funnel, also known as separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture between two immiscible solvent phases of different densities Typically, one of the...
- Work-up (chemistry)
- ITIES
ITIES is an acronym used in electrochemistry for the "Interface between Two Immiscible Electrolyte Solutions". Usually, one electrolyte is an aqueous electrolyte composed of hydrophilic ions such as NaCl dissolved in water and the other electrolyte is a lipophilic salt such as tetrabutylammonium...