Rare earth metals occurrence and preparation

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. 

The metallothermic reduction of the oxides by La produces the metals Sm, Eu, Tm, Yb, all having high vapour pressures. The reaction goes to completion due to the removal of the rare earths by volatilization from the reaction chamber (lanthanum has a low vapour pressure). The remaining rare earth metals (Sc, La, Ce, Pr, Nd, Y, Gd, Tb, Dy, Ho, Er, Lu) can be obtained by quantitative conversion of the oxides in fluorides, followed by reduction with Ca. The metallothermic reduction of the anhydrous rare earth chlorides could be also used to obtain La, Ce, Pr and Nd. The molten electrolysis can be applied to obtain only the first four lanthanide metals, La, Ce, Pr and Nd, because of the high reactivity of the materials that limits the operating temperatures to 1100°C or lower. 

According to Abell (1989), refinement methods of the rare earth metals include vacuum re-melting (Ce, La), zone refining (Ce, La, Pr, Nd, Gd, Tb, Y), solid-state electron transport in vacuum (Ce, La, Pr, Nd, Gd, Tb, Y, Lu), solid-state electron transport under inert gas (Nd, Gd, Tb, Y, Lu, Sc, Er, Ho, Dy), distillation (Gd, Tb, Y, Lu, Sc), sublimation (Er, Ho, Dy, Eu, Sm, Yb, Tm) and distillation to remove volatile impurities from high boiling rare earth metals. 

Several aspects of the preparation of high purity rare earth metals, and of the role that impurities may play in defining their properties have been reviewed and discussed by K. Gschneidner, with a special reference to the work carried out in the highly specialized Ames laboratory in the Iowa State University (USA). 

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