Pure RE Metals

The rare earth metals are produced by molten salt electrolysis and by metaUothermic reduction methods. Some details are given below about the applications of the latter. 

1 Metals with High Boiling Point and Low Vapor Pressure 

The metal oxide is treated with hydrofluoric acid. The fluoride formed is reduced with calcium. The advantage of using the fluoride instead of the oxide is that liquid calcium fluoride and not solid calcium oxide is formed. This can easily be separated from the pure RE metal. In a subsequent high-temperature process calcium residues are vaporized in vacuum. A very pure RE metal remains. This technique is used for production of the following metals [boiling point ( C) in parentheses] La (3457), Ce (3427), Pr (3520), Nd (3074), Gd (3273), Tb (3230), Dy (2567), Ho (2700), Er (2868), Lu (3402). 

2 Metals with Low Boiling Point and High Vapor Pressure 

The final step, vaporization of calcium, does not work for metals with high vapor pressure, as the metal itself would vaporize with the calcium. Instead the oxide is reduced with metallic lanthanum, which has a very low vapor pressure. Lanthanum oxide and a melt of the actual RE metal are formed. The reduction reaction occurs in a tantalum container. The reduced RE metals e.g. samarium, are vaporized and deposit on the walls of the tantalum container. This method is used for [boiling point (°C) in parenthesis] Sm (1794), Eu (1529), Tm (1950), Yb (1196). 

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Auer von Weis bach in Austria developed a new separation method. If a mixture of RE oxides is attacked by nitric acid the most basic oxides dissolve, while the less basic will be enriched in the insoluble residue. By using this method he was able, in 1884, to separate lanthanum (the more basic) from didymium. He continued to apply the same method to the didymium fraction and carried out more than 100 fractional crystallizations, each taking two days. In this way didymium was separated into two main fractions. In 1885, spectroscopic studies confirmed that two different elements were present. Auer called the green-colored one praseodymium, the green twin", while the other got the name neodymium, the new twin". This naming has been accepted, although it involved a deviation from the earlier rule that when a supposedly pure RE metal is divided in two, one of the elements shall keep the older name, while the other is given a new name by the discoverer. 

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