Fractional rare earth metals

A1 Zn Zr Th Rare earth metals Fractionated rare earth metals Be Mn Ag... 

Gr. neos, new, and didymos, twin) In 1841, Mosander, extracted from cerite a new rose-colored oxide, which he believed contained a new element. He named the element didymium, as it was an inseparable twin brother of lanthanum. In 1885 von Welsbach separated didymium into two new elemental components, neodymia and praseodymia, by repeated fractionation of ammonium didymium nitrate. While the free metal is in misch metal, long known and used as a pyrophoric alloy for light flints, the element was not isolated in relatively pure form until 1925. Neodymium is present in misch metal to the extent of about 18%. It is present in the minerals monazite and bastnasite, which are principal sources of rare-earth metals. 

Due to the great similarity of the chemical properties of the rare earth elements, their separation represented, especially in the past, one of the most difficult problems in metallic chemistry. Two principal types of process are available for the extraction of rare earth elements (i) solid-liquid systems using fractional precipitation, crystallization or ion exchange (ii) liquid-liquid systems using solvent extraction. The rare earth metals are produced by metallothermic reduction (high purity metals are obtained) and by molten electrolysis. 

Praesodymium may be recovered from its minerals monazite and bastana-site. The didymia extract of rare earth minerals is a mixture of praesodymia and neodymia, primarily oxides of praesodymium and neodymium. Several methods are known for isolation of rare earths. These are applicable to all rare earths including praesodymium. They include solvent extractions, ion-exchange, and fractional crystallization. While the first two methods form easy and rapid separation of rare earth metals, fractional crystaUization is more tedious. Extractions and separations of rare earths have been discussed in detail earlier (see Neodymium and Cerium). 

Thorium sulfate, being less soluble than rare earth metals sulfates, can be separated by fractional crystallization. Usually, solvent extraction methods are applied to obtain high purity thorium and for separation from rare earths. In many solvent extraction processes, an aqueous solution of tributyl phosphate is the extraction solvent of choice. 

Eecent work by L. M. Dermis2 and his co-workers has shown that electrolysis may be of considerable value in effecting a complete or partial separation of the oxides of the rare earth metals. Prom a neutral solution of the nitrates of neodymium, praseodymium, lanthanum, and samarium, nearly all the lanthanum is deposited as hydroxide in the last fractions discharged on the cathode. The hydroxides are deposited fractionally in order of their basicity, and the deposition is not dependent upon the... 

Fractional crystallization and precipitation are classical methods of separation of rare earth metal ions. Complex forming agents may be used to give better separations than simple or double salts. Some of the complexing agents used in fractionation separation are given below. 

For example, in the determination of the atomic weight of lanthanum the salt lanthanum ammonium nitrate was recrystallized from water 126 times, eadi crystallization involving the separation of die material into as many as 12 fractions. Analysis of the final fraction showed constancy of composition to 1 part in 20,000. Lanthanum compounds, like other rare-earth compounds, are hard to purify because corresponding compounds of the rare-earth metals differ only slightly from one another in properties. 

Table 10.4. Fractions of free (o m) and complexed (o(mi..O rare earth metal ions and average number of ligands (n) in 4 mM HIBA electrolyte solution at pH 4.1...

Neodymium, being one of the more abundant rare earth metals, is more easily obtained in pure cum pounds, Thu last impurity to be removed is usually praseodymium, which is separated by the methods already given. The following methods are also valuable Fractionation of the meta-nitrobenznnte, of the simple nitrates in strong nitric acid, or the fractional precipitation of the chloride by HC1 gas. 

Basic carbonates and double carbonates are formed by some of the rare earth metals. The stability and, solubility of the latter class of compounds increases with increasing atomic weight of the rare earth metal. Both the sodium and ammonium double carbonates are less soluble than the corresponding potassium compounds. These salts are likewise useful in fractionations. 

Separation. — The separation of thorium from the rare earth metals with which it is still mixed may be accomplished by three methods (1) the carbonate separation depends on the fact that thorium carbonate is much more soluble in sodium carbonate than the carbonates of the rare earth metals (2) by the fractional crystallization of the mixed sulfates at 15°-20°, crystals of Th(S04)2 8 H20 are obtained at the insoluble end of the series (3) thorium oxalate forms a soluble double salt with ammonium oxalate, while the rare earth oxalates are almost insoluble in this reagent. Some other methods which have been suggested are fractionation of the chromates,4 of the hydrogen alkyl sulfates,5 of the acetates, by the use of sebacic add 6 and hydrogen peroxide. 

In the rare-earth metal overlayers, the introduction of Eu or Yb onto the Co and Ni surface leads to a continuous decrease in hydrogenolysis activity, whereas the activity of hydrogenation, H2-D2 exchange, C6Hi2-D2 exchange and dehydrogenation shows a tendency to increase. A marked decrease in suitably arranged sites with the fractions of... 

The removal of impurities which are more volatile than the rare earth metals is usually called vacuum melting as described above and since distillation of the rare earth metals is done after the vacuum melting operation, the two processes together could be called fractional distillation. About half of the rare earth metals are purified by fractional distillation (sections 1.1.2 and 1.1.3). The more volatile impurities Ca, Cap2 and H are removed by vacuum melting, while... 

From a solution containing iron and some rare earth metals, Debierne precipitated a mixture of hydroxides. It was radioactive, an activity that could not have its origin in uranium, radium or polonium. A new element could be isolated by fractional crystallization of magnesium lanthanum nitrate. The element was named actinium after the Greek word aktinos, meaning ray . Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300°C. 

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