Rare Metal

a conventional name for any one of the more than 50 metals shown in Table 1. These metals are either relatively new to technology or else have so far received only limited application. The scale of production and fields of application of these metals are continuing to expand. The term “rare metals” came into use in the USSR in the 1920’s; in other countries, these elements are sometimes referred to as less common metals. Most rare metals are not widely distributed and are often dispersed in the earth’s crust. The considerable technological difficulties encountered in the extraction and purification from source material explain the relatively late discovery, study, and technological application of rare metals.

The production of rare metals has been developing at an especially high rate since World War II. The metals are essential for such new fields of technology as high-speed aviation, rocket construction, electronics, and atomic power engineering. Of course, it is true that the term “rare metals” loses its original meaning as the production of and demand for these metals increase.

Table 1. Technical classification of rare metals
Group of the periodicsystem	Elements	Group of rare metals
‘Germanium, selenium, and tellurium are arbitrarily classed as metals, although they are actually semiconductors
I II		Light
IV V VI		Refractory
III IV VI VIII		Dispersed
III		Rare-earth
I II VI VII		Radioactive

A technical classification system for rare metals has been developed (see Table 1) based on similarities of physicochemical properties, production technology, and certain other indicators. This classification is arbitrary, since many elements can belong to more than one group. For example, Rb and Cs can be classed as either light or dispersed elements; the typical dispersed element Re is also a refractory metal; the typical refractory metals V and Hf are also dispersed elements; and Ti can be classed as either a refractory or a light metal.

Light rare metals have a low density (from 0.54 g/cm³ for Li to 1.87 g/cm³ for Cs) and are highly active chemically. They resemble light nonferrous metals (Al, Mg, Ca, Na) in both properties and methods of extraction.

Refractory rare metals are included among the transition metals of groups IV, V, VI, and VII of the periodic system; in the atoms of refractory metals the completion of electron d sub-shells takes place. Refractory metals are noted for their high melting points (from 1670°C for Ti to 3410°C for W) and their ability to form refractory metal-like compounds with a number of nonmetals. Examples are carbides, nitrides, suicides, bor-ides, and beryllides.

Dispersed rare metals generally exist in the form of isomorphic admixtures to the minerals of other elements and are extracted as by-products from the tailings of metallurgical and chemical production processes. For example, Ga is obtained during the production of alumina, A1₂O₃, and In is extracted from the tailings of Zn and Pb production.

Rare-earth metals have highly similar chemical properties. They are found together in ores and separation is extremely complex. Extraction with organic solvents and ion-exchange processes are used for this purpose.

The group of radioactive metals includes metals occurring in nature (Fr, Ra, Po, Ac, Th, Pa, U) and those produced artificially (Tc, Np, Pu). Uranium and plutonium are the most valuable of this group because of their use in the production of nuclear energy.

Rare metals are usually present in small concentrations in ores, and the ores are often complex. Therefore, ore dressing and chemical processes of isolation, separation, and purification of rare-metal compounds are highly important in the technology of rare-metal extraction. As a rule, rare metals are not directly smelted from ore concentrates but are reduced by various methods from purified chemical compounds. The diverse methods widely used in the metallurgy of rare metals include reduction of oxides and salts by gases, carbon, or metals, thermal dissociation of compounds, electrolysis in aqueous and molten media, and vacuum, arc, electron-beam, and zone melting. In addition, the methods of powder metallurgy are now widely used for refractory metals.