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 agent the
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 1st and the strongest La(OH)3 last.
• If the partial separation takes place, precipitates
can be dissolved and 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 nitrate and process is repeated.
Mixture nitrate Lu2O3 Lu2O3
+ HNO3
Lu(NO3)3
Repeated
Calcine Separate

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(NO3)3.3Mg(NO2)2.24H2O also crystallizes
well.

7. Complex formation
• The oxalates of lanthanides are insoluble but
they can be held in solution by a complexing
agent such as EDTA.
• The EDTA complex is not equally stable and
addition of acid destroys the least stable
complex, which is then precipitated as
oxalates.
(EDTA)L-complex
i) H+
ii) C2O4
2-
M2(C2O4)3
Separation is not complete, so
oxalates are re-dissolved and the
process is repeated many times

8. Solvent extraction
• The rate of partition coefficient (Kd) 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 Gd have 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. Ce4+ has a
greater charge than Ce3+, hence Ce4+ is smaller
in size and less basic and is precipitated as
Ce(OH)4 or CeO2 leaving trivalent ions in
solution

10. • Alternatively Ce4+ can be extracted with upto
99% purity from the other lanthanides in HNO3
solution using tributyl phosphate
• 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 EuSO4
• Similarly Ytterbium (Yb, 4f14, 6s2) can be reduced
to 2+ oxidation state using Na-amalgam as
reducing agent and separated as YbSO4.

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 –SO3H (sulphonic
acid) functional groups. The lanthanides
replace the functional hydrogen ions and bind
to the resin.
• M3+ + 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 Lu3+ form
stronger complex with the citrate ions than the
larger ion like La3+.
• Thus smaller 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!