4.7 OCCURRENCE AND ISOLATION OF LANTHANIDE 

ELEMENTS

 Except promethium which is unstable and occurs only in traces, all the 

lanthanides occur in nature to a considerable extent, cerium being the most abundant 

of all the elements. There are more than hundred minerals known to contain 

lanthanides but very few are of commercial importance. Monazite sand is the best 

known and most important mineral of lanthanide elements which is essentially a 

mixture of orthophosphates, LnPO4 containing upto 12% thorium, the element of 5f-

series, small amounts of Zr, Fe and Ti as silicates, lanthanum and about 3% yttrium. 

Among lanthanides contained in monazite, the bulk is of Ce, Nd, Pr and others occur 

in minute quantities.

Extraction of lanthanide metals 

 After conventional mineral dressing which gives minerals of more than 90 

percent purity, the mineral is broken down by either acidic or alkaline attack. By 

making use of different solubilites of double salts: Ln2(SO4)3.Na2SO4.xH2O for light 

and heavy lanthanides and low solubility of hydrated oxide of thorium, the lanthanide 

fractions and thorium containing portions are separated in acidic medium. 

 Monazite is treated with hot conc. H2SO4 when thorium, lanthanum and 

lanthanons dissolve as sulphates and are separated from insoluble material 

(impurities). On partial neutralisaion by NH4OH, thorium is precipitated as ThO2. 

Then Na2SO4 is added to the solution. Lanthanum and light lanthanides are 

precipitated as sulphates leaving behind the heavy lanthanides in solution. To the 

precipitate obtained as above, is added hot conc. NaOH. The resulting hydroxides of 

light lanthanides are dried in air at 1000C to convert the hydroxides to oxides. The 

oxide mixture is treated with dil. HNO3. This brings CeO2 as precipitate and other 

lanthanides in solution. From the solutions obtained as above for heavy and light 

lanthanides, individual members of lanthanide series are isolated by the following 

methods: 

Isolation of Individual Lanthanide Elements: 

 All the lanthanides have the same size and charge (of +3 unit). The chemical 

properties of these elements which depend on the size and charge are, therefore, 

almost identical. Hence, their isolation from one another is quite difficult. However, 

the following methods have been used to separate them from one another. 

1. Fractional Crystallization Method: 

 This method is based on the difference in solubility of the salts such as 

nitrates, sulphates, oxalates, bromates, perchlorates, carbonates and double salts 

of lanthanide nitrates with magnesium nitrate which crystallize well and form 

crystals. Since, the solubility of these simple and double salts decreases from La 

to Lu, the salts of Lu will crystallize first followed by those of lighter members. 

The separation can be achieved by repeating crystallization process a number of

times. A non-aqueous solvent, viz., diethyl ether has been used to separate 

Nd(NO3)3 and Pr(NO3)3. 

2. Fractional Precipitation Method: 

This method is also based on the difference is solubility of the precipitate 

formed, which is formed on addition of the precipitant, i.e. Precipitating agent. 

If a little amount of precipitant is added, the salt with lowest solubility is 

precipitated most readily and rapidly. For example, when NaOH is added to a 

solution of Ln(NO3)3, Lu-hydroxide being the weakest base and having the 

lowest solubility product is precipitated first while La-hydroxide which is the 

strongest base and has the highest solubility product is precipitated last. By 

dissolving the precipitate in HNO3 and reprecipitating the hydroxides a number 

of times, it is possible to get the complete separation of lanthanide elements. 

3. Valency change Method: 

This method is based on the change of chemical properties by changing the 

oxidation state of the lanthanide elements. The most important application of 

this method is made in the separation of cerium and europium elements from 

mixture of lanthanides. 

(i) The mixture containing Ln3+ ions if treated with a strong oxidising agent 

such as alkaline KMnO4, only Ce3+ ion is oxidized to Ce4+ while other 

Ln3+ ions remain unaffected. To this solution alkali is added to 

precipitate Ce(OH)4 only, which can be filtered off from the solution. 

(ii) Eu2+ can be separated almost completely from Ln3+ ions from a solution 

by reducing it with zinc-amalgam and then precipitating as EuSO4 on 

adding H2SO4 which is insoluble in water and hence can be separated. 

The sulphates of other Ln3+ ions are soluble and remain in solution. 

4. Complex Formation Method: 

This method is generally employed to separate heavier lanthanide elements 

from the lighter ones by taking the advantage of stronger complexing tendency 

of smaller cations with complexing agents. When EDTA is added to Ln3+ ion 

solution, lanthanides form strong complexes. If oxalate ions are added to the 

solution containing EDTA and Ln3+ ions, no precipitate of oxalates is obtained.

However, on adding small amount of acid, the least stable complexes of lighter 

lanthanides are dissociated and precipitated as oxalates, but the heavier 

lanthanides remain in solution as EDTA complexes. 

5. Solvent Extraction Method: 

This method is based on the difference in the values of partition coefficient of 

lighter and heavier lanthanides between two solvents, e.g., water and tri-butyl 

phosphate (TBP). Heavier lanthanides are more soluble in TBP than lighter 

ones whereas reverse trend of solubility is found in water and other ionic 

solvents. La(NO3)3 and Gd(NO3)3 have been separated by this method 

because the partition coefficient of Gd-nitrate in water and TBP is different 

from that of La-nitrate. Thus, Gd-nitrate can be separated from La-nitrate by 

continuous extraction with water from a solution of these salts in TBP in 

kerosene oil or by using a continuous counter-current apparatus which gives a 

large number of partitions automatically. 

6. Modern Ion-Exchange Method:

This is the most rapid and most effective method for the isolation of individual 

lanthanide elements from the mixture. An aqueous solution of the mixture of 

lanthanide ions (Ln3+aq) is introduced into a column containing a synthetic 

cation exchange resin such as DOWAX-50 [abbreviated as HR (solid)]. The 

resin is the sulphonated polystyrene containing-SO3H as the functional group. 

As the solution of mixture moves through the column, Ln3+aq ions replace H+

ions of the resin and get themselves fixed on it: 

Ln3+aq + 3H(resin) → Ln(resin)3 + 3H+

aq 

The H+

aq ions are washed through the column. The Ln3+aq. ions are fixed at 

different positions on the column. Since, Lu3+aq. is largest (Lu3+ anhyd. is 

smallest and is hydrated to the maximum extent) and Ce3+aq. is the smallest, 

Lu3+aq. ion is attached to the column with minimum firmness remaining at the 

bottom and Ce3+aq. ion with maximum firmness remaining at the top of the 

resin column. In order to move these Ln3+aq. ions down the column and recover 

them, a solution of anionic ligand such as citrate or 2-hydroxy butyrate is 

passed slowly through the column (called elution). The anionic ligands form 

complexes with the lanthanides which possess lower positive charge than the

initial Ln3+aq ions. These ions are thus displaced from the resin and moved to 

the surrounding solutions as eluant- Ln complexes. 

For example, if the citrate solution (a mixture of citric acid and ammonium 

citrate) is used as the eluant, during elution process, NH4

+

 ions are attached to 

the resins replacing Ln3+aq. ions which form Ln-citrate complexes: 

Ln (resin)3 + 3NH4

+

 → 3NH4- resin + Ln3+aq 

Ln3+aq + citrate ions → Ln-citrate complex 

As the citrate solution (buffer) runs down the coloumn, the metal ions get 

attached alternately with the resin and citrate ions (in solution) many times and 

travel gradually down the column and finally pass out of the bottom of the 

column as the citrate complex. The Ln3+aq cations with the largest size are, 

eluted first (heavier Ln3+aq ions) because they are held with minimum firmness 

and lie at the bottom of the column. The lighter Ln3+aq ions with smaller size 

are held at the top of the column (with maximum firmness) and are eluted at 

last. The process is repeated several times by careful control of concentration of 

citrate buffer in actual practice