1.1. Chemistry of Rare Earths -...
Transcript of 1.1. Chemistry of Rare Earths -...
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Introduction
1.1. Chemistry of Rare Earths
Rare earth metals comprise of elements that are part of the lanthanides in the periodic
table with atomic numbers 57-71. Scandium and yttrium are grouped with the lanthanide family
because of their similar properties [1]. Rare earth elements are separated into three categories,
light rare earths, middle rare earths and heavy rare earths. The light rare earth elements include
lanthanum, cerium, praseodymium, neodymium, promethium, samarium (atomic numbers 57-62)
and they are more abundant than heavy rare earths. Middle rare earths comprise of europium,
gadolinium, terbium, dysprosium (atomic number 63- 66). The heavy rare earth elements (atomic
number 67-71) are not as predominant as light rare earths and are generally used in high
technological applications [2].
1.2. Sources and Applications
1.2.1. Sources
In nature, the rare earths do not occur in elemental state, nor do they occur as individual
rare earth compounds. The rare earths, scattered dilutely in the earth‟s crust, occur as mixtures in
many rock formations such as basalts, granites, gneisses, shales, and silicates and are present in
amount ranging from 10 to 300 ppm.
Their strong affinity for oxygen has resulted in their being found mostly as oxide
compounds even though other combinations are possible. The rare earths occur in over 160
discrete minerals. Most of these minerals are rare but the rare earths content in them, expressed
as oxides, can be as high as 60% rare earth oxide. Any rare earth mineral usually contains all the
rare earth elements of which some are enriched and others in very low concentrations.
All the world rare earth resources occur in three minerals: bastnasite, monazite, and
xenotime. These are the principal ores used for rare earth extraction. Among these, bastnasite
occurs most frequently, monazite is second, and xenotime is the third. The sources of rare earths
are the ores monazite and xenotime which are the lanthanide orthophosphates and bastnasite
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which is LnF(CO3). Other rare earth minerals those are now used as sources of rare earths
include apatite, brannerite, euxenite, gadolinite, loparite, and uraninite. The minerals, allanite and
other phosphorite sources, eudialyte, fergusonite, floreneite, parisite, perovskite, pyrochlore,
zircon, and a few other naturally occurring rare earth bearing materials are also considered
potential rare earth resources.
Monazite
The mineral monazite is a phosphate, mainly of the cerium group rare earths and thorium.
Usually monazite contains about 70% rare earth oxides, and the rare earth fraction is constituted
by 20 to 30% Ce2O3; 10 to 40% La2O3; significant amounts of neodymium, praseodymium, and
samarium and lesser amounts of dysprosium, erbium, and holmium. Yttrium content may vary
from a trace to ~5% Y2O3, and thorium content of 4 to 12% is common.
Bastnasite
The rare earth content of bastnasite is approximately 70% rare earth oxides, mostly of the
lighter elements.
Xenotime
Xenotime is an yttrium phosphate containing about 67% rare earth oxides, mostly of the
heavier elements. In addition to the three major minerals, there are several other rare earth
minerals that are of importance in the economic recovery of rare earths.
1.2.2. Applications of Rare Earths
The rare earths are becoming increasingly important in the transition to a green, low-
carbon economy. This is due to their essential role in permanent magnets, lamp phosphors,
rechargeable NiMH batteries, catalysts and other applications. The increasing popularity of
hybrid and electric cars, wind turbines and compact fluorescent lamps is causing an increase in
the demand and price of rare earths. Industrial applications of these metals are developed in
metallurgy, magnets, ceramics, electronics, chemical, optical, medical, nuclear technologies, etc.
[3]. Uses of rare earths in certain steel alloys or as cigarette lighter flints require no separation of
metal as obtained from certain ores. But in other applications such as phosphors for television
screens, medical immune assays, X-ray photography and also for the study of their chemistry,
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the individual elements must be separated [4].The first general separation procedure introduced
in 1950 was based on complexation-enhanced ion exchange process. In the mid-1960‟s liquid-
liquid extraction processes were introduced and today all large scale commercial production is
done in this way. The details of the separation techniques for lanthanide elements are discussed
in the following sections.
1.3. Techniques employed for separation of lanthanide elements
The various techniques employed to separate the lanthanides are:
(i) Precipitation
(ii) Thermal reaction
(iii) Fractional crystallisation
(iv) Complex formation
(v) Selective oxidation
(vi) Ion exchange
(vii) Solvent extraction or liquid-liquid extraction
1.3.1. Precipitation
A precipitating agent is used to solubilise the substance having the lowest solubility, most
rapidly and completely. This precipitate contains more of the elements at the right of the series
and the solution contains more of the elements at the left of the series [5].
1.3.2. Thermal reaction
After the fusion of Ln(NO3)3, at a particular temperature, the least basic nitrate changes to its
oxide. The mixture is leached with water. The nitrates dissolve and filtered off leaving the
insoluble oxides.
1.3.3. Fractional crystallisation
The technique is used to separate lanthanide salts. Non-aqueous solvents such as diethyl
ether is used to separate Pr(NO3)3 and Nd(NO3)3.
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1.3.4. Complex formation
Mixture of lanthanide ions is treated with a complexing agent like EDTA. They form
complexes with EDTA. The stability constants of the complex differ from the neighbouring rare
earth elements and so separated.
1.3.5. Selective oxidation
Some lanthanides have variable oxidation states. The properties of Ln4+
or Ln2+
are very
different from those of Ln3+
. The different properties of the various oxidation states makes
separation very easy. Cerium can be separated from Ln mixtures because it is the only one which
has Ce4+
ion stable in aqueous solution. A solution containing mixture of Ln3+
ions can be
oxidized with NaOCl under alkaline conditions to produce Ce4+
. Because of the higher charge,
Ce4+
is much smaller and less basic than Ce3+
. The Ce4+
is separated by carefully controlled
precipitation of CeO2, leaving the trivalent ions in solution.
1.3.6. Ion exchange
This is the most rapid and effective separation method for the separation and purification
of lanthanides. A solution of lanthanide ions is run down through column of synthetic ion
exchange resin such as Dowex-50 which is sulphonated polystyrene and contains the functional
group -SO3H. The Ln3+
ions are adsorbed onto the resin and replace the hydrogen atom on –
SO3H.
Ln3+
(aq) + 3H+ (resin) (s) ↔ Ln (resin) 3(s) +3H
+ (aq)
The H+ ion replaced are washed through the column. Then the metal ions are eluted and
washed off the column in a selective manner. The eluting agent is a complexing agent. But it
suffers from disadvantages like high cost of membranes and trouble to dispose the waste [6].
1.3.7. Solvent extraction
Solvent extraction is presently one of the major techniques used on the industrial scale for
the separation and recovery of metals at micro and macro level. It plays a significant role as a
separation technique because of its successful application in organic, pharmaceutical industries,
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nuclear industries and hydrometallurgy. It gained popularity due to its simplicity, rapidity and
application at micro and macro level concentrations. The separation of rare earths by solvent
extraction depends upon the preferential distribution of individual rare earths (either in the
cationic form or as complex anions or as a neutral species) between two immiscible liquid phases
that are in contact with each other. One of the liquid phases is an aqueous solution and the other
is a non-aqueous phase that is organic phase.
There are many advantages to use solvent extraction as the process for rare earth
separation. One of them is that the rare earth loading in the solvent /extractant can be very high.
These make the equipment required for the process very compact. The organic phase used in
solvent extraction usually consists of two or more substances. The extractant is responsible for
collecting rare earth species into the organic phase; however, the extractant is usually too viscous
to be used in a practical system. It is dissolved in a suitable solvent called the diluent to ensure a
good contact with the aqueous phase. The diluents are mostly aliphatic/aromatic hydrocarbons
including kerosene. A substance known as a modifier is usually added to the organic phase to
improve the hydrodynamics of the system.
1.4. Chemistry of Praseodymium and Neodymium
Praseodymium and Neodymium bearing the atomic numbers 59 and 60 are present in the
f-block of the periodic table. Some of the properties of these metals are presented in table 1.1.
Table 1.1. Properties of Praseodymium and Neodymium
Name Praseodymium Neodymium
Symbol Pr Nd
Atomic number 59 60
Atomic mass 140.90 amu 144.24 amu
Melting point 935˚C 1010˚C
Boiling point 3520˚C 3127˚C
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Density 6.77 g cm-3
7.01 g cm-3
Oxidation state +3,+4 +3
Electronic configuration 54
[Xe]4f36s
2
54[Xe]4f
46s
2
Electronegativity 1.13 1.14
1.5. Solution Chemistry of Praseodymium and Neodymium
Praseodymium, Neodymium and all other metals of this group have two electrons in the
outer most orbit (n) and one electron in the (n-1) d orbit. Therefore the +3 oxidation state
strongly dominates the chemistry of these elements. In aqueous medium both these metals
exhibit +3 oxidation state and complexes of these metal ions are formed with this valency. The
+2 and +4 oxidation state also exist, but they are not stable than +3 state. These belong to the
hard acid group according to Hard-Soft Acid Base Concept (HSAB). So, these have strong
tendency for hard ligands such as F-, SO4
2-, OH
-, NO3
- , etc.
Because of their high charge, these metals are highly electropositive in nature. In aqueous
solution, these metals show high tendency to hydrolyse. The size and charge of these ions have a
lot of implications when calculating activity coefficients. The activity coefficient of an aqueous
solution is directly proportional to the charge and inversely proportional to the ion size
parameter. Therefore, the larger ion size of these ions result lower activity in the aqueous
solutions.
1.5.1. Praseodymium and Neodymium complexes in aqueous solution
The compounds of these metals in the aqueous solutions are characterized by their high
degree of hydrolysis and complex formation. The major feature of the solution chemistry of
these metals is the strong tendency towards hydrolysis which is the most important characteristic
of the metal ions having high charge. The hydrolysis involves complicated polynuclear
equilibria. So, in order to avoid these difficulties arising from hydrolysis, all the experiments are
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carried out in low metal ion concentration. The determination of complexation equilibria and
stability constants are necessary to predict complex formation reaction in the aqueous media so
that an appropriate ligand can be selected when solvent extraction technique will be applied.
Molecules of solvent and other ions of the metal species may interact in the solution to form
various types of complexes.
(i) Hydroxide Complexes
These metals have great affinity towards hydroxide ions. These metals react with water to
form their corresponding hydroxides. The reaction is slow with cold water and becomes rapid
with hot water.
2Nd(s) + 6H2O (l) 2Nd (OH)3(aq) + 3H2(g)
2Pr(s) + 6H2O (l) 2Pr (OH)3(aq) + 3H2(g)
(ii) Halide Complexes
On reaction with the halogens, they form their corresponding halides.
2Nd (s) + 3X2 (g) 2NdX3(s)
2Pr (s) + 3X2 (g) 2PrX3(s), where X = F-, Cl
-, Br
-, I
-.
(iii) Nitrate Complexes
The metals tarnish slowly in air and burn readily to form their corresponding oxides. The
nitrate complexes of these metals are formed on reaction of their oxide with nitric acid.
2Nd2O3 + 2HNO3 2Nd (NO3)3 + 3H2O
Pr6O11 + 18HNO3 6 Pr (NO3)3 + 9H2O + O2
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A lot of literatures are available regarding the interaction of these nitrate complexes with Tri-
n-butyl phosphate (TBP), Dimethyl Sulphoxide (DMSO) to form their corresponding complexes
[7, 8].
(iv) Sulphate Complexes
These metals dissolve readily in dilute sulphuric acid to form their corresponding sulphates.
2Nd(s) + 3H2SO4 (aq) Nd2 (SO4)3 + 3H2(g)
2Pr(s) + 3H2SO4 (aq) Pr2 (SO4)3 + 3H2 (g)
1.6. Fundamentals of Solvent Extraction
1.6.1. General Introduction
This technique is one of the oldest technique used for separation and purification of
elements at micro and macro level. Over hundred and thirty years before Berthelot and
Jungfleish enunciated a law governing the distribution of metal species between two immiscible
phases [9]. Since that time, solvent extraction was in theories as an advanced knowledge of
solution chemistry and metal complexes. In the mid of 1940‟s, there was need for the separation
and recovery of radioactive metals which introduced solvent extraction for large scale operation
and hydrometallurgical separations. It gained its attention as a unit process in the
hydrometallurgical industry for the separation of non-ferrous metals. The process of solvent
extraction which may be called as liquid-liquid extraction in metallurgical industries can be
described by a simple equation.
M(aq)+E(org) ME(org) (1.6.1.1)
where M is the metal in the aqueous phase and E is the extractant in the organic phase.
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In the first step, the metal (M) is extracted from the aqueous phase to the organic phase
and forms ME complex in the organic phase. The complex formation is favoured by the shifting
of equilibrium towards right. In the second step, the metal is back extracted from the organic
phase to the aqueous phase (stripping) which means shifting of equilibrium towards left. So,
solvent extraction is the shifting of equilibrium between the extraction and stripping process.
1.6.2. Process
The entire process of solvent extraction is divided into three stages:
(i) Extraction (mixing of metal solution with the organic solvent in a contactor for a specific
time).
(ii) Scrubbing (after extraction the loaded organic phase is scrubbed with a suitable aqueous
solution to remove the impurities).
(iii)Stripping (the loaded organic phase is stripped with an aqueous solution to recover the
metal).
The general process of solvent extraction is given in the form of a schematic diagram in
figure 1.1.
Solvent
Aqueous Metal
Solvent +Impurities
Aqueous
Solvent + Metal
Metal Recovery
Solvent
Equilibration
Aqueous
feed
Scrub
Solution
Strip
solution
Solvent
Feed
Extraction
Stage
Scrubbing
Stage
Stripping
Stage
Extraction
Raffinate
Scrub
Raffinate
Strip
Liquor
Aqueous
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Figure 1.1.General process of Solvent Extraction
1.6.3. Applications
Several techniques are available for the recovery of metal, but liquid-liquid extraction or
solvent extraction is considered to be the most versatile technique for the recovery of metals.
Therefore, this technique has been efficiently used for the recovery of rare earths [10].
Hydrometallurgical techniques have fewer environmental problems as compared to the older
techniques used for recovery of metals. As in the world the high grade quality ores are
decreasing, so techniques are needed for recovery of metals from low grade ores, mixed ores and
wastes [11]. Hydrometallurgical extraction is developed as a more environmental sound and
effective technique to address all these types of challenges [12].
1.6.4. Objectives
Solvent extraction is one of the most extensively studied and most widely used
techniques for the separation and pre-concentration of trace elements [13-15].With proper choice
of extracting agents, this technique can achieve group separation or selective separation of trace
elements. The solvent extraction technique has three objectives in analytical application which
are: pre concentration of trace elements, elimination of matrix interface, differentiation of
chemical species. It works in five steps i.e. preparation of solution (aqueous and organic), mixing
of aqueous and organic phase, separation of two phases, determination of distribution ratio, data
interpretation. It involves three chemical species i.e. solute, solvent and diluent.
Solute is the metal needs to be recovered or separated.
Solvent is a mixture of an extractant and a diluent. An extractant is that which has the ability to
form complex with the metal ion (to be recovered) in the organic phase. A diluent is an organic
liquid in which the extractant is dissolved to form the solvent.
1.6.5. Basic principle
Solvent extraction is based on the principle of distribution i.e. Nernst distribution law
which states that a solute will distribute itself between two immiscible solvents in such a manner
that at equilibrium the ratio of concentration of the solute in the two phases at a particular
temperature will be constant provided the solute has the same molecular weight in each of the
phases.
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For a solute ‛S‟ distributing between two solvents 1 and 2.
S1 S2
KD = S1/S2 (1.6.5.1)
Where KD = partition coeffeicient or extraction coefficient.
S1 and S2 = concentration of solute in the two phases 1 and 2, respectively.
The above distribution law holds good for the materials which are sparingly soluble in. But,it has
certain limitations i.e:
(i) When the distributing species is involved in chemical reaction such as dissociation or
association, its distribution will be changed in the either phase, it is not valid
(ii) Activity corrections are ignored.
So, the distribution of a species in two phases is not governed by a simple equilibrium.
Therefore, to determine the overall distribution of a component in the two phases, distribution
coefficient has been replaced with distribution ratio (D) which is given as:
D = [M]org/[M]aq (1.6.5.2)
where, [M]org and [M]aq are the concentration of metal species in organic and aqueous phases,
respectively.
In the ideal condition, when there is no interaction between the two phases, D = KD.
1.6.6. Percentage Extraction (%E)
The extent of extraction depends upon the volume ratio of organic to aqueous phase. The
extent of extraction is called percentage of extraction (%E).
Let, Vorg = volume of organic phase and Vaq = volume of aqueous phase., then,
D = 100D/(D+(Vaq/Vorg)) (1.6.6.1)
If the volumes of the two phases are equal (Vorg = Vaq ), then
%E =100D/ (D+1) (1.6.6.2)
1.6.7. Separation factor (β)
The separation factor β is related to the individual distribution ratios
β = ([A]org/[A]aq)/([B]org/[B]aq)
= DA/DB, (1.6.7.1)
Where A and B represents the two solutes.
1.7. Mechanism of Extraction
The process of extraction takes place basically in the following three steps:
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(i) Formation of a metal complex.
(ii) Distribution of the extractable complex.
(iii) Interaction in the organic phase.
The nature of extracted species plays the most important role in the metal extraction
system. Basing on this concept, Ritcey and Ashbrook [16] divided all the extractants into three
classes.
(i) Compound formation.
(ii) Ion association.
(iii) Solvation.
1.7.1. Extraction by compound formation
The reagents falling on this category are called liquid cation exchangers. These
extractants operate by the exchange of H+ ion of the acidic organic compound with the cation
present in the aqueous phase.
Mn+
(aq) + nHA(org) MAn(org) + nHn+
(aq) (1.7.1.1)
Again these cation exchangers are of two types i.e. acidic extractants possessing –PO3H, -
COOH, -SO3H group and chelating extractants containing a donor group.
Acidic Extractants
The acidic extractants used for the commercial purpose are the acidic organophosphorous
and carboxylic compounds. Organophosphorous compounds include alkyl phosphoric,
phosphonic and phosphinic acids. The organophosphorous acids have pronounced tendency of
association into dimers [17]. The extraction of metal by dimers of extractants can be given as:
Mn+
(aq) + m(HA)2(org) MAn (HA)(m-n)(org) + nH+
(aq) (1.7.1.2)
Acidic organophosphorous extractants can be represented as:
P
O
OH
OH
RO
P
O
OH
OR
RO
(Mono alkyl phosphoric acid) (Dialkyl phosphoric acid)
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P
O
OH
OH
R
P
O
OH
R
R
(Mono alkyl phosphonic acid) (Dialkyl phosphinic acid)
where R is alkyl or aryl substituent.
Of all these, the dialkyl phosphonic acid i.e. di-(2-ethyl hexyl) phosphoric acid
(D2EHPA) has been proved to be the most efficient extractant for extraction and separation of
rare earths [18-20]. Among the esters of phosphonic acid, 2-ethylhexyl phosphonic acid mono-2-
ethyl hexyl ester (EHEHPA) has been widely used as extractant for the extraction and separation
of rare earths [21]. The dialkyl phosphinic acid i.e. bis (2,4,4-trimethylpentyl) phosphinic
acid(Cyanex 272) has been efficiently used for the separation of rare earths [22].
P
O
OHCH3-C-CH2-CH-CH2
CH3CH3
CH3-C-CH2-CH-CH2
CH3
CH3CH3
CH3
(Structure of Cyanex 272)
Carboxylic Acids
The metal extraction by these extractants is complicated because of their state of
aggregation in solution. Some carboxylic acids used as extractants are:
(CH2)n-COOH
R
C
COOH
CH3
R2
R1
(Naphthenic acid) Versatic 911 acid (R1,R2 = C4-C5
Versatic 10 acid (R1,R2= C6)
These extractants have also been used commercially for the extraction of many metals [23-26].
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Sulphonic Acids
Aliphatic and aromatic sulphonic acids are represented by the formula
RSO2OH(R=straight/branched chain or aromatic saturated radical). The extractants of this class
are dodecyl benzene and dinonyl naphthalene sulphonic acids. The applications of these
extractants in the commercial field are reported by several researchers [27-29].
Chelating Extractants
The chelating extractants act as weak acids and contain a donor group like oxygen,
nitrogen and sulphur to form a bidentate complex with the metal. These extractants neutralize the
charge on metal and also satisfy the co-ordination number requirement. The well known
examples of these extractants are hydroxy oximes (LIX reagents), β-diketones, dithiozone, 8-
hydroxy quinoline etc. These extractants are commercially available in the market for
hydrometallurgical operation [30-32].
CH3- (CH2)3- CH- C- CH-CH-(CH2)3- CH3
C2H5
NOH
OH C2H5
5, 8-diethyl-7-hydroxy-6-dodecane oxime (LIX 63).
O
O
CH3
C
C
CH3
Acetyl acetone
1.7.2. Extraction by ion association
It primarily involves the extraction of a species formed due to the interaction between an
anionic metal species in the aqueous phase and the cation coming from organic phase. The high
molecular weight amines are under this class of extractants. The amine salt undergoes anion
exchange with amine (B-) in the aqueous phase.
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R3N (org) +H
+(aq)+A
- (aq)→ R3NH
+A
- (org) (salt formation) (1.7.2.1)
R3NH+A
-(org) +B → R3NH
+B
-(org) +A
-(aq) (anion exchange) (1.7.2.2)
One of the important factors influencing the extraction of metal by these extractants is the
formation of emulsion, which can be effectively tackled by choosing a low surface active
extractant and a proper diluent. Another factor which influences metal extractions by amines is
the nature of the carbon chain, and the number of carbon atoms, in the amine molecule.
Normally, aliphatic amines are the best extractants since aromatic groups, especially when
attached to the amine nitrogen weaken the extractive properties, probably as a result of the
electron-withdrawing nature of the aromatic ring [33]. In many ways the problems encountered
with the use of amines as metal extractants are similar to those involving acidic extractants,
namely salt effects, aggregation of the extractant in the solvent phase, third phase formation,
solubility and so on. Salt effects generally decrease metal extraction by amines in the order: Cl->
NO3- > SO4
2-> F
-, which is the reverse order of the complexing ability of these anions. A lot of
literatures [34-36] are available on the extraction behaviour of high molecular weight amines for
the extraction of metal from mineral acids.
1.7.3. Extraction by solvation
Another important group of extraction systems is that based on the power of oxygen-
containing organic extractants to solvate inorganic molecules or complexes. By such solvation
the solubility of the inorganic species in the organic phase is greatly increased. There are two
main groups of extractants; those containing oxygen bonded to carbon, such as ethers (C-O-O),
esters (COOR), alcohols (C-OH), and ketones (C=O) and those containing oxygen bonded to
phosphorus (P=O) such as alkyl-phosphate esters. One remarkable feature of extractants
containing C-O bonds is the high degree of metal hydration that occurs in the solvent phase [37].
These systems are non-ideal in the organic phase, even at low concentrations, making a general
theoretical treatment impossible.
One distinguishing feature of the organophosphorous extractant involves the role played
by water. The strongly polar organophosphorus compounds compete with water and replace
water molecules from the first hydration sphere of a metal atom. With ethers and ketones water
is a necessary part of the complex, probably forming bridges between the organic and metal
components of the complex through hydrogen bonding. The most well known organophosphorus
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ester is undoubtedly tri- n-butyl phosphate (TBP), because of its wide use in the processing of
nuclear materials. These neutral extractants can be classified into two different types.
Extractants containing phosphorous-oxygen bond
These extractants are the derivatives of phosphoric acid. Some of the examples of these
extractants are given below:
Table 1.2. General structures and examples of esters of organophosphorous acids
Structure Ester Example
(RO)3P=O Trialkyl phosphate Tri-n-butyl phosphate (TBP)
R(RO)2P=O Dialkyl alkyl
phosphonate
Dibutyl butyl phosphonate
(DBBP)
R2(RO)P=O Alkyl dialkyl
phosphinate Butyl dibutyl phosphinate
R3P=O Trialkyl phosphine
oxide
Tri-n-octyl phosphine oxide
(TOPO)
The oxygen of the phosphoryl group of these extractants is responsible to form the co-
ordination bond with the metal. The solubility of these extractants in water is in the following
order: Phosphates < Phosphonates < Phosphinates < Phosphine oxide. The extent of extraction
by these extractants will depend on the degree of formation of the extractable species and the
solvation number of the metal. The extractability of the metal decreases in the order: Phosphine
oxide > Phosphinate > Phosphonate > Phosphate. There is a strong competition between the
extractant and water molecules to occupy the co-ordination site of the metal ion. There is the
possibility of more than one coordinate bond formation in the esters, through their other oxygen
atoms. In such cases inter-or intra-molecular bi functional complexes are formed influencing the
extraction rates for different metals as well as their stripping. These extractants have the ability to
extract acids. Considerable work has been done with an attempt to understand the mechanism of
extraction, but with little success. The fact that most metal extraction processes are carried out
from acid solution implies the possibility of co-extraction of the acids to be considered. This
ability of extractants to extract acids has been used in the production of pure phosphoric acid, by
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extraction of this acid from solutions produced by the dissolution of phosphate rock in sulfuric
acid. One of the properties of these reagents, which have been of particular interest, is the ability
to form complexes with mineral acids in ratios of acid to extractant of greater than unity e.g.
TBP.xHNO3, where x=1, 2, 3 or 4. In the extraction of metals by TBP, the general extractable
complex appears to be one in which two molecules of TBP are associated with the metal
complex. The effect of acid concentration on the extraction of metals by TBP is similar to that
with amines. As the acid concentration is increased, the extraction of metal increases until a
certain point, after which the extraction begins to decrease. The commonly used reagent of this
class are tri-n-butyl phosphate (TBP), Tributyl Phosphine oxide (TBPO), etc. Cyanex 921
extractant, better known as trioctylphosphine oxide (TOPO), was the first member of a family of
solvent extraction reagents developed by Cytec. It is 93% of trioctyl phosphine oxides.
CH3-(CH2)7-P-(CH2)7-CH3
O
(CH2)7-CH3
(Structure of Cyanex 921)
In 1980‟s, American Cyanamid industry introduced another extractant Cyanex 923,
which is a mixture of four trialkyl phosphine oxides. It gained advantages over TOPO that it is a
liquid and is completely miscible with all the types of diluents and the four major major
components of Cyanex 923 are: (i) trihexyl phosphine oxide, (ii) dihexylmonooctyl phosphine
oxide, (iii) dioctyl monohexyl phosphine oxide, (iv) Trioctyl phosphine oxide, [38].
P=OR
R
R
(Structure of Cyanex 923)
Important properties of these extractants are as follows:
Cyanex 923 is a liquid phosphine oxide while Cyanex 921 is a solid at room
temperature. The former is a potential substitute for TOPO in the conventional process for
recovering uranium from wet process phosphoric acid. It is also completely miscible with all
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commonly used diluents even at low ambient temperature. All these types of extractants have
been used by many workers [39-42] for lanthanide extraction.
Extraction involving Carbon-Oxygen bond
Extractants of this class are ethers, ketones and alcohols. Alcohol is amphoteric and
exhibits both donor and acceptor properties. These extractants gained attention in the earlier time
for metal extraction. Alcohols solvate better than ketones and ethers. Methyl isobutyl ketone
(MIBK) has been used for the extraction of metals [43-45].
Extractants involving Phosphorous-sulphur bond
This group contains alkyl thiophosphoric esters, alkyl thiophosphonic exters, alkyl
thiophosphinic esters, phosphine sulphides. Some of these are: tri-isobutyl phosphine sulphides
(TIBPS), tri isooctyl phosphine sulphide (Cyanex 471X). The extractants with oxygen donor are
considered as hard bases and those with sulphur donor are classified as soft bases. So, according
to HSAB concept, hard bases prefer hard acids like La3+
, Pr3+
etc. and soft bases prefer soft acids
like Cu2+
, Pd2+
etc. These extractants are used by many workers for the extraction of metals [46,
47].
1.8. Synergistic Extraction (Extraction with binary mixture)
Synergism is the phenomenon in which certain combinations of extractants extract a
metal ion more efficiently than does the individual ones. The synergistic enhancement occurs
due to the formation of one or several new species which are more hydrophobic than the species
involving a single extractant. The synergistic systems are generally a mixture of cation exchange
extractants and solvating extractants. The synergist should have the following properties [48]:
(i) It should be capable of displacing any residual coordinated water molecules from
the neutral chelate and rendering it less hydrophilic.
(ii) It should not itself be hydrophilic and is coordinated less strongly than the organic
chelating agent.
(iii) The maximum coordination number of the metal and the geometry of the ligands
should be favourable.
Synergistic extraction proceeds through following mechanism:
(i) Synergist becomes coordinated to the metal ion due to complete dissociation of
the ligand. In this case, the extracted complex must contain an inorganic anion in
the structure.
43
(ii) Interaction between the synergist and coordinated ligand molecule.
(iii) Coordination of the synergist ligand to the metal ion where ligand remains
bidentate.
(iv) Coordination of the synergist to the metal ion such that it occupies the empty
coordination site made available as a result of dissociation of one end of the
ligand.
The synergistic co-efficient (S.C.) and the enhancement in the distribution ratio (∆D)
determine the extent of synergism. The synergistic coefficient for the combination of any two
extractants is given by:
S.C. = log [Dmix/(D1 + D2)] (1.8.1)
Where Dmix, D1 and D2 denotes the distribution ratios using mixture of extractants (1 and 2),
extractant 1 and extractant 2, respectively. The extent of synergism is given as:
∆D = Dmix – (D1 + D2) (1.8.2)
Synergism occurs in the extraction when both S.C. and ∆D are positive. When both S.C.
and ∆D are negative, the phenomenon is called antagonism. Synergistic extraction is useful in
mutual separation, determination of metals, their recovery, determination of stability constants of
metal-ligand complexes, enhancement in the extraction, studies on the co-ordination capabilities
of the metal ion, etc. The first synergistic effect was reported for the extraction of uranium using
dialkyl phosphoric acid and neutral phosphorous esters [49]. The mixed extractant exhibits the
most desirable features of the constituent single extractant. Now, the synergistic extraction is
very common and found its wide application in the extraction of rare earths [50-57].
1.9. Factors influencing Metal Extraction
The following factors influence the extraction process.
(i) Acidity of the aqueous phase.
(ii) Presence of salting out agent in the aqueous phase.
(iii) Oxidation state of the metal ion.
(iv) Nature of extractant.
(v) Nature of diluent.
1.9.1. Acidity of the aqueous phase
The extractability of the metal complex is greatly influenced by the acidity of the aqueous
phase. In case of systems employing chelating and acidic extractants, the extraction of metal is
44
heavily dependent on the equilibrium pH of the aqueous phase. Thus, as the pH decreased, the
metal extraction will also decrease as the equilibrium will be shifted towards left. Conversely, as
the pH is increased, the metal extraction is also increased. Therefore, it is necessary to maintain
an optimum concentration of H+ ion in the aqueous phase to achieve the maximum extraction.
1.9.2. Presence of salting out agent in the aqueous phase
The addition of high concentration of inorganic salts to the aqueous phase sometimes
increases the distribution ratio of metal complexes in the organic phase. The phenomena in
which there is increase in percent of extraction because of the presence of an inorganic salt is
termed as salting out phenomena.
1.9.3. Oxidation State of the metal ion
High charge of the metal ion favours complexation of the metal with the extractant. So,
extraction will be increased with increase in the charge on the metal ion.
1.9.4. Nature of extractant
The strength of an extractant is a measure of its ability to transfer metal in loading and
stripping. This includes properties such as high metal loading capacity or selectivity to provide
pure metal free from the impurities, low aqueous solubility, high solubility in the chosen
aliphatic or aromatic diluent for both the extractant and metal complex, non-flammable, non-
aromatic and non-toxic [48].
1.9.5. Nature of diluent
The diluents are both aromatic and aliphatic. The choice of diluents may range from
simple molecule to complex mixtures. One of the important facts about the diluents is that these
are not inert as they appear to be. The most important property of the diluent is its polar nature. It
greatly affects the solvating property of the extractant and hence its extractive properties also.
The diluents should possess characteristics like (i) low viscosity and density in order to assist
phase separation (ii) should be free from objectionable components to minimize the crud
formation (iii) should be chemically stable (iv) should be insoluble in the aqueous phase (v) must
have low evaporation losses (vi) should be readily available at low cost [58].
45
1.10. Solvent Extraction of Praseodymium and Neodymium,
Literature Review
A review on the solvent extraction studies of praseodymium and neodymium with other
associated elements using various types of extractants has been presented here in a columnar
form.
1.10.1. Extraction of rare earths with Acidic organophosphorous extractants
Metals Extraction Data References
Rare earths Study of extraction of lanthanides and yttrium
using Di(2-ethyl hexyl) ortho phosphoric acid from
mineral acid solution using radiotracer technique.
[59]
Nd(III) &
Pm(III)
HCl and HNO3 medium. D2EHPA in amsco and
toluene. Extraction was 50% lower with diluent
toluene as compared to that in amsco with
commercial D2EHPA; Nd was separated as 95.35%
and Pm as 96.27%.
[60]
Rare earths Di-(2-ethylhexyl)hydrogen phosphate(HDEHP) in
toluene. Parameters like Perchlorate concentration,
HDEHP concentration were varied to obtain the
distribution ratio of the elements of the series.
[61]
Nd(III) &
Sm(III)
Extraction data was obtained using D2EHPA in
amsco hydrocarbon. Separation factors were
calculated.
[62]
Rare earth
elements
Extraction of rare earths by DEHPA, distribution
coefficient of all elements decreased with the
increasing concentration of HNO3 in the aqueous
phase. Mechanism of extraction at high HNO3
[63]
46
concentration is different from the mechanism at
low HNO3 concentration.
Rare earths Extraction of metals using mono and diacidic
organophosphates in toluene. Composition of the
extracted species was formulated.
[64]
Nd(III) &
Sm(III)
Extractant- D2EHPA, diluent-Socal 355L, a
mixture of aliphatic hydrocarbons. Addition of salt
like NH4Cl, NH4NO3, (NH4)2SO4 to the aqueous
strip solution result rapid phase disengagement.
[65]
Rare earths Di (2-ethyl-hexyl) phosphoric acid (HDEHP),
diphenyl phosphinic acid (HDPP), dibutyl
phosphoro thioic acid (HDBPT), di-n-octyl
phosphorodithioic acid (HDOPDT), or di(2-
ethylhexyl) phosphorodithioic acid (HDEHPDT) in
chloroform. Extraction constants for the lanthanides
follow the order HDPP > HDEHP > HDBPT >
HDOPDT > HDPP.
[66]
Rare earths Extraction equilibria for a series of lanthanide ions
using chloroform solutions of bis (2,4,4-trimethyl-
pentyl)-phosphinic acid (HBTMPP).
[67]
Rare earths HCl medium, DEHPA or EHEHPA in kerosene, IR
and NMR spectra were taken for the organic
extracts. Extraction mechanism followed cation
exchange mechanism at aqueous acidity and
solvating mechanism at higher acidity.
[68]
Rare earths The stoichiometry, extraction constants and
separation factors of the metals were determined.
[69]
47
Rare earths HCl and HClO4 medium. Iso propyl-3-
pentadecylphenyl phosphoric acid (IPPA = HR).
Extracted species was Ln(HR2)3.
[70]
Light rare earths A solvent extraction process was developed for the
production of magnet grade Nd2O3 from the
LREES (La, Ce, Pr, Nd and Sm) by using 20%
saponified PC88A.
[71]
Pr(III)&Nd(III) Nitric acid medium, Di-(2-ethyl hexyl) phosphoric
acid in kerosene. Impermeability of the ions
increased with the increase in the carrier
concentration up to 0.1 M DEHPA and then
independent of that. The dependence of
permeability coefficient was maximum at pH 3.
[72]
Rare earths Hydrochloric acid medium, Cyanex 302 in aliphatic
diluent. Extracted species was LnA32HA, where
HA is Cyanex 302 and A is its deprotonated form.
[73]
Nd (III) Diluents- n-heptane, toluene and benzene. Diiso
decylphosphoric acid (DIDPA, HR). The
composition of the extracted species was
formulated.
[74]
Nd (III) Distribution of Nd (III) between acidic aqueous
nitrate solutions and organic solutions of D2EHPA
in hexane. Results obtained from graphical and
numerical analysis suggested the formation of two
complexes in the organic phase: NdA3.3HA which
is the main species when the extractant/metal ratio
is higher than one and NdA3, which is the main
species when the ratio is equal to one.
[75]
48
Pr (III) & Ho
(III)
D2EHPA in kerosene. Two types of species were
formed, Ln(NO3)A2(HA)3 and LnA3(HA)3.
Formation constant and pre-dominance of each
species were calculated.
[76]
Rare earths Review on separation of rare earths using
D2EHPA, HDEHP. Suitable model was developed
to study the composition of the extracted species.
[77]
Nd (III) Study of neodymium complexes of a series of
acidic organophosphorous extractants in deuterated
toluene. Bis (2-ethyl hexyl) phosphoric acid
(HDEHP), bis(2,4,4-trimethyl pentyl) oxo
thiophosphinic acid (HC302), or bis (2,4,4-
trimethyl pentyl) dithiophos phinic acid (HC301)
were used. Composition of the complex was
formulated as Nd2(HDEHP)6 or Nd2(C302)6.
[78]
Nd (III) Hydrochloric acid medium. The equilibrium
constants for the solvent extraction of Nd with
PC88A and saponified PC88A were estimated from
the experimental data.
[79]
Th (IV)
& Pr (III)
Nitrate medium. Cyanex 301 and Cyanex 302 in
kerosene. Temperature variation studies revealed
that the extraction of these metals increased with
increase in temperature. The stoichiometry of the
extracted complex was formulated.
[80]
Rare earths Extraction by di-2-ethylhexylphosphoric acid in
kerosene from nitric–hydrochloric acid mixture.
Different acid solutions with different pH values
for stripping were studied.
[81]
49
Light rare earths Extraction of light rare earth elements were studied
in the P204(DEHPA)-HCl system and
P507(HEHEHP)-HCl system both containing acetic
acid. The separation ability of this extraction
system was better in P507-HCl system.
[82]
La (III), Ce
(IV), Pr (III) &
Nd (III)
TOPS 99 an equivalent to di-2-ethyl hexyl
phosphoric acid has been employed for the
extraction and separation of a mixture of rare earths
and seven heavy rare earths into some fractions
from phosphoric acid solutions. McCabe-Thiele
extraction isotherm was plotted to predict the
separation of rare earths.
[83]
Pr(III) & Rare
earths
Recovery of praseodymium was achieved from
mixed rare earth solution using Cyanex 272 as
extractant by membrane extraction mechanism.
[84]
Rare earths Separation of fourteen lanthanides from perchloric
acid solution by Cyanex 272. Mathematical
modeling was proposed to predict and compare the
experimental results.
[85]
La (III), Pr (III)
& Nd (III)
Separation of La from Pr and Nd was done using
Cyanex 272 in escaid 110. Cyanex 272 exhibited
best extraction affinity towards Pr and Nd than La.
Stripping of Pr and Nd from the loaded organic
phase was achieved with 1M HCl.
[86]
50
1.10.2. Extraction of rare earths with Neutral organophosphorous extractants
Metals Extraction Data References
Rare earths Extraction of lanthanides using dibutyl and tributyl
phosphate. Their individual separation was studied
using these extractants.
[87-91]
Rare earths Study of partition behaviour of lanthanides from
chloride, nitric and perchloric acid media using
methylene bis dialkyl phosphine oxide. The
equilibrium constants were calculated.
[92]
Rare earths Separation of rare earths from aqueous nitrate
solution using TOPO. The process enables the
efficient separation of yttrium from mixture of rare
earths.
[93]
Pr (III) &
Nd (III)
Extraction of metal nitrates and water from aqueous
solution by TBP in dodecane was studied. Mikulin-
Sergievskii- Dannus model was used to predict
extraction isotherm of lanthanides and water.
[94]
Ce (IV), Eu(III)
& Nd (III)
Recovery of metals from waste calcium sulphate
sludges by TBP or DBBP. Magnet grade
neodymium was obtained from light rare earth
[95]
51
fraction.
Rare earths Nitric acid medium. Lanthanides are extracted using
supercritical CO2. Metals are extracted as
Lanthanide-TBP-HNO3 complex.
[96]
Rare earths Extraction mechanism of rare earth elements with
Cyanex 923. Separation of lanthanum from rare earth
mixture was reported.
[97]
Rare earths Extraction of yttrium and some trivalent lanthanides
from thiocyanate and nitrate solutions using Cyanex
923(TRPO) in xylene. These trivalent metal ions
were extracted from nitrate solutions as M
(NO3)3.3TRPO.
[98]
Nd (III) Use of tri-n-amyl phosphate as the modifier for the
extraction of Nd(lIl) by Octyl(Phenyl)-N,N-
Diisobutyl Carbamoyl methyl phosphine oxide
(OΦDCMPO) has been studied and the results are
compared with those with TBP as modifier. . The
macro level extraction of Nd (l1I) by TAP/n-
dodecane (without OɸDCMPO) has also been
investigated
[99]
U (VI), Th(IV) &
Rare earths
Extractant Cyanex 923.The effect of different
variables like the concentration of acids, metal ion
and extractant, nature of diluent and temperature has
been studied.
[100]
Rare earths Extraction behavior of some lanthanides [La, Ce, Nd,
Eu, Gd, Ho, Yb] along with Y (III) from HNO3, HCl,
H2SO4, and H3PO4 media was studied in a toluene
solution of Cyanex 923. The stoichiometry of the
[101]
52
extracted species was formulated as
Ln (NO3)32Cyanex 923.
U (VI),Th (IV),
Sc(III), rare
earths.
The extraction of microquantities of U, Th, Sc and
rare earths from HNO3 solutions by bifunctional
neutral organophosphorous compounds in organic
diluents has been studied. The effect of HNO3
concentration in the aqueous phase and that of the
extractant concentration in the organic phase on the
extraction of metal ions has been reported.
[102]
Pr(III) & Nd (III) Supercritical carbon dioxide medium, tri-n-butyl
phosphate and tributyl phosphite (TBPO3).
Extraction equilibrium constants were calculated
from the spectral data using least-squares regression
and hard-equilibria models.
[103]
Rare earth nitrates Use of TBP in various diluents for separation of rare
earth nitrates has been reported by many workers.
The results showed that the complexes of REi(NO3)3
contained three molecules of TBP corresponding to
their oxidation states.
[104-107]
Rare earths Cyanex 923 as extractant. The effects of sorption
kinetics, extractant and nitric acid concentrations on
the uptake behaviour of metal ions were
systematically studied.
[108]
Rare earths Cyanex 925 in heptane. The effects of aqueous phase
ionic strength, Cyanex 925 concentration in the
organic phase, and temperature on Sm3+
, Nd3+
and
Y3+
extraction have been investigated.
[109]
53
Pr IIII) & Nd(III) Kinetics of extraction and back extraction of these
metal nitrates were studied with a polymer supported
tri-n-butyl phosphate (TBP). The rate of the kinetic
follow interfacial diffusion kinetics. The mass
transfer coefficient increase with the aqueous phase
containing 2M NaNO3.
[110]
Rare earths Solutions of neutral organophosphorous extractants
like TBP, TIAP, DIOMP in alkylbenzene diluent.
The major extractable complex in extraction of rare
earth nitrates is Ln(NO3)3.3NOPC,where NOPC =
TBP, TIAP, DIOMP.
[111]
Rare earths The static/dynamic extraction of rare earth elements
(Nd, Ce) from their oxides with organophosphorous
complexes with HNO3 and H2O in SC-CO2 was
investigated. The static extraction of Nd from Nd2O3
with TBP-HNO3 complex reaches 95% under
optimized conditions. The study confirmed the
feasibility of TBP-HNO3 complex.
[112]
Pr (III) &
Sm (III)
Solvent extraction of Pr (III) and Sm (III) with
Cyanex 923 in various diluents from acidic nitrate
medium was investigated. On the basis of slope
analysis, the composition of the extracted species
was found to be M(NO3)3.2Cyanex923. The results
showed that increasing dielectric constant of the
diluents decreases the percentage of extraction.
[113]
Rare earth
nitrates
TBP in kerosene. The effect of initial concentration
of TBP and rare earth nitrates were investigated in
order to find out the equilibrium constants of their
complexes. The results showed the concentration
[114]
54
change of TBP rarely affect the equilibrium
constants for formation of complexes whereas the
decrease in initial concentration of mixed rare earths
decreases the equilibrium constant for formation of
complexes.
Rare earths &
Americium
Extraction of lanthanides and americium was carried
out in thiocyanate medium using TOPO in toluene.
Slope analysis indicates that TOPO solvation
decreases from four for the light members of the
series to three (or less) for the heavy lanthanide ions.
[115]
Rare earths Solvent extraction of trivalent rare earths from
aqueous solution of hydrochloric, nitric and
perchloric acid of using lipophilic amino methyl
phosphine oxides in toluene, chloroform and
methylene chloride solvents. The effect of
concentration of mineral acids showed that the
highest extraction of lanthanides was achieved from
perchloric acid medium (80%) whereas it was only
30% with the other two acidic medium.
[116]
1.10.3. Extraction of rare earths with mixture of extractants
Metals Extraction data References
Rare earths Binary mixture of DEHPA and TOPO in kerosene.
Antagonistic effect was observed.
[117]
Rare earths Hydrochloric acid medium. Mixture of TTA and TOPO.
The composition of the extracted species was
formulated as Ln(TTA)3.TOPO and Ln(TTA)3. 2TOPO.
[118]
55
Nd (III), Ho
(III) & Er (III)
The synergistic extraction behaviour of all these
elements were studied with the mixture of thenoyl
trifluoro acetone(TTA) and tri octyl phosphine oxide
(TOPO) with various aqueous systems like ClO4-, SCN
-
, NO3-, Cl
-, etc. The extracted species was found to be
Ln (TTA)3(TOPO)2.
[119]
Rare earths Formation of mixed complexes of rare earths with
thenoyl trifluoro acetone (HTTA) and tri-n-butyl
phosphate (TBP). Organic diluent was carbon
tetrachloride.
[120]
Rare earths P350 - neutral extractant, di(1-methylheptyl)methyl
phosphonate, HTTA-chelate extractant thenoyltrifluoro
acetone. Composition of the extracted species was RE
(TTA)3 (P350)2.
[121]
Trivalent
lanthanides
Distribution ratio and equilibrium constant of trivalent
lanthanides between HTTA and TOPO in organic phase
were also determined. After interaction between the
extractants, the species like Ln(TTA)3TOPO and
Ln (TTA)3.2TOPO were formed in the organic phase.
[122]
Rare earths Mixture of TBP and HDEHP in dodecane. Extraction of
rare earths and their binary mixture from aqueous nitrate
solution was studied.
[123]
Nd (III) &
Sm (III)
Mixture of DEHPA and sec-nonyl phenoxy acetic acid.
The chemical composition of the synergistically
extracted species from chloride medium was
formulated.
[124]
Rare earths Synergistic solvent extraction of rare earths from nitrate
solution by mixture of D2EHPA and TBP in hexane and
cyclohexane. The selectivity of the extraction in a
[125]
56
synergistic system is lower for the La−Yb pair than in
the case of D2EHPA extraction under the same
conditions.
La(III), Pr (III),
Nd(III) &
Sm(III)
Extraction of these metal ions was carried out with
mixture of TBP and DEHPA in benzene.NH4NO3 was
taken as a salting out agent. Presence of NH4NO3
enhanced the extraction of Sm, Pr and La with TBP
where as inhibited the extraction of Pr and Sm with
DEHPA. The synergistic effect was observed when the
aqueous phase contained NH4NO3. Separation factors of
each rare earth ions were calculated.
[126]
Nd(III),Eu(III),
& Tm(III)
Extraction of these metals was studied with HDEHP
from various aqueous acidic media. Antagonistic effect
was observed with mixture of HDEHP and TBP or
TOPO. The extraction mechanism was studied and
composition of the extracted species was suggested.
[127]
Pr (III) &
Nd (III)
Mixture of diethylene triamine penta acetic acid
(DTPA) and a di-(2-ethylhexyl)phosphoric acid
(D2EHPA) in kerosene. The results obtained showed
that the reaction is pseudo first order and the kinetic
constants for forward and backward reaction were
calculated respectively.
[128]
Rare earths Chloride medium. 3,5-diisopropylsalicylic acid (DIPSA)
and triisobutylphosphine oxide (TIBPO) in xylene.
Mixtures of DIPSA and TIBPO give somewhat better
separation factors between the light and middle
lanthanide fractions .
[129]
Nd(III) & Synergistic effects in extraction of neodymium (III) and
erbium (III) from chloride medium with solutions of
[130]
57
Er (III) 3,5-diisopropyl salicylic acid and neutral
organophosphorous extractants containing a phosphoryl
donor group in xylene were studied. The extent of
synergistic effects for both metals follow the order of
increasing basicity caused by the replacement of alkoxy
group by alkyl group through the series:(RO)3PO <
(RO)2RPO < (RO)R2PO < R3PO. The composition of
extracted species was found to be MA3.L2, where HA is
3,5-diisopropyl salicylic acid and L is neutral
organophosphorous extractants.
Rare earths Trivalent lanthanides synergistically extracted as mixed
ligand complex with HFAA and TOPO in cyclohexane.
They are extracted as Ln (HFAA)32TOPO. Extraction
was quantitative with hydrochloric acid whereas with
sulphuric acid and perchloric acid non-reproducible
results were obtained. Of the organic solvents, non-polar
solvents found to be more efficient as compared to the
polar solvents. The percentage extraction of mixed
ligand complex increased with increase in extractant
concentration.
[131]
Rare earths Synergistic extraction and separation of lanthanides has
been investigated using mixture of Cyanex 301 and
Cyanex 923.
[132]
Uranium &
Rare earths
Phosphoric acid medium, synergistic mixture of dioctyl
phenyl phosphoric acid (DOPPA) and trioctyl phosphine
oxide (TOPO). Various parameters like H3PO4, SO42-
,
DOPPA, TOPO concentration on extraction of metals
were studied.
[133]
58
Rare earths &
actinides
Extraction of all these metals with synergistic mixture of
bis(chlorophenyl)dithiophosphinic acid and tris(2-ethyl
hexyl)phosphate as synergist was studied. High
separation factor was achieved with this synergistic
mixture. Thermodynamic parameters and composition
of the extracted species was formulated using slope
analysis.
[134]
Rare Earths Isomolar mixture of TBP and D2EHPA in kerosene is
used for extractive separation of lanthanides.
Distribution coefficients of rareearths are higher when
the concentration of extractants TBP and D2EHPA are
1.5 and 1.0M as compared to the results obtained by the
individual ones.
[135]
Pr (III) &
Nd (III)
Extraction studies of neodymium and praseodymium
with mixtures of tributyl phosphate and Aliquat-336(L)
in xylene. The synergisticc extracted species observed
was M (NO3) 4LTBP,
[136]
Lanthanides &
americium
Dilute nitric acid medium. Synergistic mixture of bis
(chlorophenyl) dithiophosphinic acid [(ClPh)2PSSH]
and tris(2‐ethylhexyl)phosphate (TEHP) . The
thermodynamical parameters like ΔH, Δ S, ΔG were
calculated.
[137]
La(III),Nd(III)
& Gd (III)
Synergistic extraction of rare earths with mixtures of
PC88A and Cyanex 923 was studied. Synergistic
enhancement decrteases with increasing ionic radius of
metals. Metals were extracted as MA3B, where HA is 2-
ethyl hexyl phosphonate and B is Cyanex 923.
[138]
Pr (III) Binary mixture of 8-hydroxy quinoline (HQ) and 2- [139]
59
&
Nd (III)
ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester
(P507, HL). For Pr (III) and Nd (III), the mixture of HQ
and P507 showed higher separation ability than the
individual ones.
Rare earths &
yttrium
Mixture of Cyanex 272 and HDEHP. Composition of
the extracted species was formulated using slope
analysis method.
[140]
Rare earths Nitric acid medium- solutions of tetraphenyl methylene
diphosphine dioxide, diphenyl (diethyl carbamoyl
methyl) phosphine oxide, and dibutyl (diethyl
carbamoyl methyl) phosphine oxide in organic solvents
in the presence of l-butyl-3-methyl imidazolium
hexafluoro phosphate (BMImPF6) was studied. The
stoichiometry of the extractable complexes was
determined.
[141]
Rare earths Mixture of di-(2-ethylhexyl) phosphoric acid (D2EHPA,
H2A2) and sec-nonyl phenoxy acetic acid
(CA100,H2B2). The separation abilities among rare
earths were determined and compared with those with
D2EHPA alone.
[142]
Rare earths Sec-nonylphenoxy acetic acid (CA100) and its mixture
with four neutral organophosphorus extractants, tri-
butyl-phosphate (TBP), 2-ethylhexyl phosphonic acid
di-2-ethyl ester (DEHEHP), Cyanex 923 and Cyanex
925. Results show that all the four mixing systems do
not have evident synergistic effects on the extraction of
rare earths.
[143]
60
Rare earths 4-(4-Fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one
in combination with the three phosphine oxide
compounds trioctylphosphine oxide (TOPO),
tributylphosphine oxide (TBPO), or triphenylphosphine
oxide (TPPO) as well as with tributylphosphate (TBP).
Composition of the extracted species was established as
LnL3·HL with HL alone and as LnL3·2S in the presence
of TOPO, TPPO, and TBP or LnL3·S with the mixture
of HL–TBPO.
[144]
La(III) &
Nd (III)
Aqueous nitric acid solution, mixture of two neutral
extractants, TOPO and TRPO in kerosene. The
composition of the extracted species was found to be
M(NO3)3(TOPO)(TRPO). Temperature variation studies
revealed that the extraction process was exothermic and
spontaneous.
[145]
Pr(III), Nd(III)
& La(III)
Separation of Pr, Nd and La from chloride solution at
aqueous pH 4.94 with Cyanex 272 and its mixture with
various cationic, solvating and anionic extractants. The
extraction percent of the LREEs decreased with increase
of cationic and solvating extractant concentration along
with 1M Cyanex 272. Extraction isotherm was
constructed for the separation of Pr and Nd over La
using mixture of saponified PC88A and TBP. The
loaded Pr and Nd were successfully stripped by 1M
HCl.
[146]
61
1.11. Aim of the present work
The aim of this study was
To study the extraction of neodymium and praseodymium using acidic and neutral
organophosphorous extractants from acidic nitrate media.
To establish the conditions for quantitative extraction.
To elucidate the mechanism of extraction process on the basis of slope analysis and there
from composition of the extracted complex.
To study extraction of these metals using binary mixture of organophosphorous
extractants.
To establish conditions for effective separation of praseodymium and neodymium with
various organophosphorous extractants.
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