Separation of Lanthanides/ Lanthanides and Actinides

Methods of separation of Lanthanides

1. Separation of Lanthanides

2. •The lanthanides are trivalent and are almost identical
in size, their chemical properties are almost identical.
•The separation of one metal from another is as
difficult as the separation of isotopes.
•Different method used are
1.Precipitation
2.Thermal Reaction
3.Fractional Crystallization
4.Complex Formation
5.Solvent Extraction
6.Valence Charge
Separation Methods

3. Precipitation
•Precipitating agent is used for separation
•With a limited amount of precipitating agentthe
substance with the lowest solubility is precipitated
most rapidly and most completely.
•If ions are added to the solution of
lanthanide nitrates, the weakest base Lu(OH)
3
is
precipitated 1
st
and the strongest La(OH)
3
last.
•If the partial separation takes place, precipitates
can be dissolvedand the process is repeated

4. Thermal reaction
•If the nitrates are fused (treated at high
temperature, calcined) a temperature will be
reached when least basic lanthanoid forms the
oxides. The mixture is then washed with water.
•Oxides being insoluble remains. It is reconverted
to nitrateand process is repeated.
Mixture nitrate Lu
2
O
3
Lu
2
O
3
+ HNO
3
Lu(NO
3
)
3
Repeated
CalcineSeparate

5. Fractional Crystallization
•Fractional crystallization of simple salts such
as nitrates, sulphates, bromates, perchlorates
etc. has been frequently used for the separation
of lanthanides.
Fraction crystallization is a process in which
mixed compounds are dissolved in solvent,
heated and then gradually cooled, so that each
fraction crystallizes and is removed from the
solution in pure form

6. •The solubility decreases from La to Lu. The
process is repeated many times due to close
solubility product of next elements.
•Moreover, the double salts
L(NO
3
)
3
.3Mg(NO
2
)
2
.24H
2
O also crystallizes
well.

7. Complex formation
•The oxalates of lanthanides are insolublebut
they can be held in solution by a complexing
agent such as EDTA.
•The EDTA complex is not equally stableand
addition of acid destroys the least stable
complex, which is then precipitated as
oxalates.
(EDTA)L-complex
i) H
+
ii) C
2
O
4
2-
M
2
(C
2
O
4
)
3
Separation is not complete, so
oxalates are re-dissolvedand the
process is repeated many times

8. Solvent extraction
•The rate of partition coefficient (K
d
) is 1:1.06, though
the difference is quite small, a very large number of
partitions can be performed using a continuous
counter current apparatus.
•Kg quantities of 95% pure Gdhave been obtained by
this method

9. Valency Change
•Valency change is a very important method for
separation of Ce and Eu.
•Ce can be separated from lanthanides by
oxidizing the solution with permanganate or
bromate under alkaline condition. Ce
4+
has a
greater charge than Ce
3+
, hence Ce
4+
is smaller
in size and less basic and is precipitated as
Ce(OH)
4
or CeO
2
leaving trivalent ions in
solution

10. •AlternativelyCe
4+
can be extracted with upto
99% purity from the other lanthanides in HNO
3
solution using tributylphosphate
•Europium (Eu) can be obtained in 2+ oxidation
state by either electrolytic reduction (at cathode)
or using Zn/Hg (zinc amalgam) as reducing agent,
followed by precipitation of EuSO
4
•SimilarlyYtterbium (Yb, 4f
14
, 6s
2
) can be reduced
to 2+ oxidation state using Na-amalgam as
reducing agent and separated as YbSO
4
.

11. Ion Exchange
•It is the most important, most rapid and
effective method for the separation and
purification of lanthanons.
•The solution of lanthanide ions is run down in
a column of ion exchange resin which has –
COOH (carboxylic acid) or –SO
3
H (sulphonic
acid) functional groups. The lanthanides
replace the functional hydrogen ions and bind
to the resin.
•M
3+
+3H(resin)M(resin)+3H
+

12. •The H
+
ions produced are washed through the column.
•Then a buffer solution of citric acid and ammonium
citrate is used to elute the metal ions in a selective
manner.
M(resins) + 3H(citrate) 3H(resin) + M(citrate)
2
•As the citrate solution flows down the column,
lanthanide ions come off the resin and form citrate
complex.

13. •The smaller lanthanides such as Lu
3+
form
stronger complex with the citrate ions than the
larger ion like La
3+
.
•Thussmaller ions spend more time in solution
and less time on the column and are eluted out
first.
•The process of many separation or many
crystallization performed prior to ion exchange
can even help to obtain 80% pure elements by one
pass through on an ion exchange column

14. Happy Learning!