25

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

26

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,

27

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.

28

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,

29

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

30

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

31

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

32

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.

33

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

34

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.

35

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:

36

(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)

37

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].

38

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.

39

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

40

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

41

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

42

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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