The Rare Earths
The term rare earths denotes the group of 17 chemically similar metallic elements that includes scandium, yttrium, and the lanthanides (Spedding 1978; Connelly et al. 2005). The lanthanides are the series of elements with atomic numbers 57 to 71, all of which, except promethium, occur in nature. The rare-earth elements, being chemically similar to one another, invariably occur together in minerals and behave as a single chemical entity. Thus, the discovery of the rare earths themselves occurred over a period of nearly 160 years, from 1787 to 1941 (Szabadvary 1988; Weeks 1956). The problem of separating them from one another for scientific study or industrial use then followed. This has been one of the most challenging tasks of rare-earth technology. While the attempts to separate the rare earths began with the work of Mosander during 1839-1841, much of the effort directed to the separation of various rare earths occurred from 1891 to 1940. During this period, from the available mixed and separated compound intermediates, many rare-earth alloys and metals were prepared, and commercial applications were developed for mixed or roughly separated rare earths. The following two decades, 1940-1960, were the most productive in terms of effective process development. Most important were the development of modern separation methods, which resulted in the availability of sufficient quantities of pure individual rare-earth compounds (Powell and Spedding 1959b) for the investigation of reduction processes to prepare pure metals and alloys (Beaudry and Gschneidner 1978) and evaluation of their properties. Beginning in the 1960s, much progress was made in the large-scale production of purer rare earths, in the identification of newer properties, and in their use in a variety of important commercial applications. The usable forms of rare earths encompass naturally occurring oxide mixtures and products synthesized from them, high-purity individual metals, alloys, and compounds. The current annual demand for rare earths is in the range of 125,000 t calculated as rare-earth oxides. The world rare-earth reserves are large and sufficient to support the present level of consumption for many centuries to come.