Occurrence
All the lanthanides, except promethium, occur in nature. Elements with even atomic numbers (with an even number of protons in the nucleus) are more abundant in nature than their neighbours with odd atomic numbers. This is called Harkin’s rule.
Elements with even atomic numbers have a more significant number of stable isotopes. Elements with odd atomic numbers never have more than two stable isotopes.
Lanthanides are found mainly in three minerals.
- Monazite: A mixture of lanthanum phosphate and trivalent phosphates of Ce, Pr, and Nd. In addition, it contains a smaller amount of Y, the heavier lanthanides, and thorium phosphate.
- Bastnaesite: A mixture of flurocarbonates and LnCO3
- Xenotime: A complex mixture of lanthanide compounds, mainly yttrium phosphate.
Separation of the Lanthanide Elements
The lanthanides have almost identical chemical properties because (i) all of them are typically trivalent (same charge) and (ii) all the trivalent ions (Ln³+) are almost identical in size (its charge and size mainly determine the properties of a metal ion). Therefore, the separation of the lanthanides is very difficult. Only two methods are currently used to separate the lanthanides- the ion exchange method and the valency change method.
Ion Exchange Method
This method can be carried out either based on the basicity differences or by complex formation.
- A procedure based on basicity differences: The basicity of the tripositive Ln³+ decreases with increasing atomic number. As the pH of an aqueous solution of the mixture of metals increases (the basicity of the medium increases), the least basic Ln³+ gets precipitated first. Yttrium is removed effectively from the mixture by this method.
- Complex formation: The stabilities of complex ions of the tripositive lanthanides increase with increasing Z. Differences in the stabilities of the complex species of a given type can be the basis for separation. This method involves adsorption of the mixed Ln³+ ions on a column of cation exchanger, followed by elution at a controlled pH with a complexing agent such as ammonium citrate. The smallest ion with the maximum complexing ability gets eluted first, followed successively by the other ions.
(a) A solution of lanthanide ions is run down a column of synthetic ion exchange resin bearing -SO3H groups. The Ln³+ ions replace the H+ ions in the -SO3H groups.
Thus, the Ln3+ ions get fixed to the resin, and the H+ ions get washed down the column.
(b) Then, a buffered solution of citric acid/ ammonium citrate (eluent) is passed through the resin. This solution elutes (removes) the Ln³+ ions from the resin one by one. The Ln+ ions form soluble complexes with the citrate ion and flow down gradually. The smaller Ln³+ ions are complexed first as their citrate complexes. These are more stable than the other citrate complexes. Therefore, these smaller ions come out of the column first, one by one. Separation is possible because the ions move out of the column (desorbed) at different rates. The solution flowing out of the column is collected as different portions in different containers; each container receives a solution portion (eluate) rich in a particular metal. On adding ammonium oxalate solution to each solution portion, the metal ion is precipitated as its oxalate, which on heating, yields the oxide.
Valency Change Method
The ability of some of the lanthanides to exist in variable valencies is employed in separating these elements.
- For example, Ce4+ is stable and has properties different from Ce³+ and other Ln³+ This fact is used for separating Ce from the other lanthanides. To a mixture of Ln³+ ions, NaOCl is added; Ce³+ gets oxidised to Ce4+. Then Ce4+ is precipitated from the mixture as Ce(OH)4, which can be heated to give CeO₂. Alternatively, Ce4+ alone can be extracted with tributyl phosphate.
- Eu²+ is quite different in properties compared to the other Ln³+ The Eu³+ in the first is reduced electrolytically to Eu2+ and then precipitated as EuSO4; it is filtered off from the other (unaffected) Ln3+ ions in the solution.
- A similar procedure separates Sm and Yb through their divalent species.
This method of changing the valency and separating the lanthanides is called the valency change method. It helps separate some of the Ln³+ ions from the mixture; the remaining mixture can be separated using the ion exchange method.
Lanthanides can be separated by two other methods-fractional crystallization and liquid-liquid extraction.
Fractional Crystallisation
This method is based on the formation of isomorphous compounds with a significant difference in their solubilities and marked changes in solubilities with temperature. For example, the double nitrate of Mg and Ce can be separated from the double nitrates of Mg and other lanthanides.
Liquid-liquid Extraction Method
Ln³+ ions have significantly different solubilities in organic solvents. The metal ions in the mixture may first be complex, and then one or more of the products are transferred to an organic solvent layer. This method can separate Ce(IV) and Th(IV) from the other Ln³+ ions. They react with dilute mineral acids to give H₂.
None of the methods can be used to separate all the metals from a mixture; each may be used to separate a group of three or four metals.
