Lanthanides

The lanthanides occur naturally in many minerals but are most concentrated in monazite, a heavy dark sand, found in Brazil, India, Australia, South Africa, and the United States. The composition of monazite varies depending on its location, but generally contains about 50% of lanthanide compounds by weight. Like any group of elements that have similar properties and that occur in nature together, the separation and purification of the lanthanides requires considerable effort. Consequently, commercial production of the lanthanides tends to be expensive.

To separate the lanthanides from other elements occurring with them, they are chemically combined with specific substances to form lanthanide compounds with low solubility (oxalates and fluorides, for example). A process known as ion exchange is then used to separate the lanthanides from each other. In this process, a solution of the lanthanides in ionic, soluble form is passed down a long column containing a resin. The lanthanide ions "stick" to the resin with various strengths based on their ion size. The lanthanum ion, being smallest, binds most tightly to the resin, whereas the largest ion, lutetium, binds the weakest. The lanthanides are then washed out of the ion exchange column with various solutions, emerging one at a time, and so are separated. Each is then mixed with acid, precipitated as the oxalate compound, and then heated to form the oxide. A number of methods have been used to obtain the lanthanides in metallic form. For example, the oxides can be converted to fluorides or chlorides which are then reduced with calcium to metallic form.