Rare earth elements fractional crystallization

The last years in the period of discovery and the some 30 years following it saw several developments in separation techniques for example, the reduction process was introduced for certain rare earths. The variety of chemical methods developed was then applied on an ever increasing scale, sometimes to many kilograms of starting materials especially large amounts were used in the search for the elusive element 61. As examples of the compounds used in the large-scale separation of the rare earths by fractional crystallization, ammonium nitrates and ferrocyanides may be mentioned. Spectroscopic methods were applied to evaluate the success of the separation. A review of the often very laborious separation procedures has been presented by Prandtl (1938). 

For small extents of crystallization, the maximum change, and thereby the most valuable information on F, will be obtained from elements with high Dt (compatible elements) such as Ni in basaltic olivine. Elements with ), 1 (incompatible elements), such as Th, Ba or rare-earth elements in basaltic systems, will provide basically no clue to F variations. In addition, information carried by incompatible elements, which do not fractionate with respect to each other, is entirely redundant. This is better shown by taking the relative change in the ratio of two elements il and i2 per increment of crystallization... 

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. 

Prior to the proposal of the Periodic Table, there was no information available on how many chemical elements could possibly exist. Even after the appearance of the numerous periodic tables of chemical elements, the rare earth elements were an especially difficult case because they could not be properly arranged into any of the Tables. Until the twentieth century, fractional crystallization was the only method of purification of elements. In most cases, this required thousands of recrystallizations involving months of work. As a result, there is a long list of various false claims among the rare earth elements, some of which are detailed below. 

Because, of the veiy heavy ionic, weight (250) of the perrhenate ion, it is one of the heaviest simple anions obtainable in readily soluble salts. It has found use as a precipitant for potassium and some other heavy univalent ions also as a precipitant for such complex ions as Co(NH )( "+. and for the separation of alkaloids and organic bases. Perrhenate also is used in the fractional crystallization ot the rare-earth elements. 

Fractional crystallization (or differential crystallization) is a process whereby two chemically compounds that form crystals with slightly different solubilities in some solvent (e.g., water) can be separated by a "tree-like" process. One should remember the herculean work by Marie Curie3, who by fractional crystallization isolated 0.1 g of intensely radioactive RaCl2 from 1 ton of pitchblende (a black mixture of many other salts, mainly oxides of uranium, lead, thorium, and rare earth elements). 

Diagrammatic representation of the system of fractional crystallization used to separate salts of the rare-earth elements (reproduced with permission from D.M. Yost, H. Russell and C.S. Garner, The Rare Earth Elements and their Compounds, John Wiley, 1947.)... 

Figure 4 Chondrite-normalized rare earth element (REE) abundances in a suite of cogenetic iavas from the Eastern Gaiapagos Spreading Center (aiso shown in Figures 3 and 6). increasing abundances of REE and fhe size of the negative europium anomaiy from MORE fo andesite are consistent with evoiution of fhe suife primariiy by fractional crystallization. Concave-down patterns are an indication of their normal depleted chemical character (N-MORB).

In 1912 Shibata moved to Paris to study under Georges Urbain (1872-1938). He intended to study the rare earth elements, but Urbain advised him not to do so because such study required tedious fractional crystallization, which was not suitable for a foreign chemist with only limited time to spend. Instead, Urbain suggested that Shibata carry out absorption spectrographic studies of cobalt complexes. Fortunately, Shibata was able to use the newly obtained medium-sized quartz spectrograph of Adam Hilger, type E2, and he carried out absorption measurements of cobalt-ammine complexes (5). In Urbains s laboratory Shibata also learned from Jacques Bardet the technique of emission spectrographic analysis, which Shibata later used to analyze the rare earth minerals found in Japan. 

Urbain, Georses (1872-1938) French chemist who specialized in the study of the rare earth elements. After enormous labor involving hundreds of thousands of fractional crystallizations, he discovered samarium, europium, gadolinium, terbium, holmium, lutetium, and hafnium. 

The 35 years after 1840 were uneventful regarding the discovery of rare earth elements. Berzelius had died in 1848, Mosander in 1858. Inorganic chemistry was under pressure from the growing organic chemistry. The introduction of a new analytical technique, spectroscopy, however, aroused new interest and opened new approaches. The gradual separation of one element from another could be followed. This was very valuable in the many and time-consuming fractional crystallizations. 

Charles James, Figure 17.12, was bom in 1880 at Earls Barton near Northampton, England and died in 1928 in New Hampshire, USA. He studied at University College, London, under Sir William Ramsay and graduated from the Institute of Chemistry in 1904. He moved to USA and the University of New Hampshire in 1906. There he developed new fractional crystallization techniques for separating rare earth elements. He was the first to separate large amounts of lutetium from ytterbium. 

Ever since his arrival in Florence, Rolla conducted research on the group of rare earths with the specific intention of isolating the element possessing atomic number 61. He smdied new methods of fractional crystallization, the method of choice for separating rare earth elements. He worked first with small, then with immense quantities of material so that in the 1930s the Instimte possessed, as its Director declared with great pride, the richest and purest collection of cerium oxides in the world [147]. ... 

Gladyshevsky, Bodak and Pecharsky have collected together summaries of the literature of the last quarter century on phase equilibria and crystal chemistry of ternary metallic systems containing a rare earth element with a metallic member from the group III, Illb, or IV and a transition metal. This literature contains studies of a surprising fraction of the enormous number of compounds possible (i.e. crystal structure data for over 1600 ternary intermetallic compounds are presented and the phase relationships of nearly 200 ternary systems are illustrated, generally as isothermal sections). The authors have been major contributors to these investigations. 

The early history of the rare earth elements is primarily dominated by the attempts to separate and purify the individual elements using the classical techniques of fractional crystallization and precipitation (Vickery, 1953). These procedures generally involved aqueous solutions which contained the hydrated ion and in this sense could be considered as the earliest examples of studies of the complexing properties of the rare earths. From a practical standpoint, however, the existence of the complexed ions was only incidental and was probably not even considered by the early workers. Early reference works of the 1920 s which summarize the extant information discuss only some double salts and adducts of the rare earths and do not consider them in terms of coordination compounds (Moeller, 1967). The first chelates prepared were probably the acetylacetonates used by Urbain (1896) in a separation procedure. 

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