Separation of Individual Rare Earth Elements

Methods of Separation of Individual Rare Earth Elements 

At the end of this chapter we will treat the working up of monazite as a representative for the mode of recovering rare earths from these minerals, but at first we will consider the different techniques for the separation of the individual components from a rare earth concentrate. 

The long story of the methods for the separation of the individual rare earths may broadly be divided into two main parts a) classical methods b) modem methods. Old-fashioned classical techniques like fractional crystallization, fractional precipitation and fractional thermal decomposition were not only used by the early workers in the past, but still remain as very important methods for economical production of rare earths on commercial scales. Modem methods like solvent (liquid-liquid) extraction, ion exchange or chromatographic (paper, thin layer and gas) techniques have both advantages and limitations. 

Eu and Gd can be concentrated by crystallization through double magnesium nitrates, followed by crystallization of bismuth magnesium nitrates [30. Sm and Eu are then removed by a proceedure based on valence change, and Gd is recovered by bromate crystallizations. Yttrium group earths may be conveniently separated by bromate crystallization. 

Brunisholz et al. [33—36] have very carefully and extensively studied the rare earth-EDTA systems following the suggestion of Marsh [37] and Moeller [35], and they were able to fractionate various rare earth mixture [39—42.  

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Chapter 2. Methods of Separation of Individual Rare Earth Elements 10... 

The separation of the rare earths as a group and the separation of individual rare earths from each other by solvent extraction procedures is possible and is the subject of another chapter. Only those extraction methods of particular importance in the determination of the rare earths are discussed in this section. The separation of the individual rare earths by solvent extraction involves a multistage process and is too cumbersome to be of practical importance for the determination of the rare earths. Solvent extraction procedures are extremely useful, however, in the separation of Th, Zr, Hf, U and other elements which are difficult to separate from the rare earths by other procedures. Ce(IV) can also be readily separated from the trivalent rare earths by solvent extraction. 

The industrial applications of the rare earths can be divided into two categories— uses that involve the mixed rare earths in proportion to their occurrence in their ores or in concentrates (not exceeding 90% of any one rare earth element), and uses that involve the separated individual rare earth elements (> 90% pure). Of the total volume of rare earths consumed about 95% is in the form of mixed rare earths or concentrates, but in monetary terms the contribution by both categories is about equal. 

Samarium is one of the rare earth elements found in Row 6 of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The rare earth metals are not really very rare in Earth s surface. The name comes from the fact that these elements were once very difficult to separate from each other. For a long time, chemists knew very little about the individual elements. A more correct name for these rare earth elements is the lanthanoid series. It is named after the element lanthanum, the first element in the series. 

This homologous placement of the rare-earth elements shows to what extend Mendeleev s viewpoints had changed by the end of the first half of 1870. What catches the eye is that Dmitrii Ivanovich had switched from placing the rare-earth elements as a group in the periodic system to an individual placement of each element separately (Fig. 11.6). That is, by breaking up the natural group of rare earths, Mendeleev ended up with a set of distinct elements which he set out to accommodate on an individual basis in dijferent groups of the system. 

Yttrium is found together with other rare earth oxides inmonazite sands (Ce, La, etc.) PO4] and in bastnasite [(Ce, La, etc.)(C03)F] (see Section 1.7.1). Yttrium is extracted together with other rare earth elements in a concentrated solution of sodium hydroxide at 140-150 °C after cooling, the hydroxides of the rare earth elements are separated by filtration. Alternatively, bastnasite may be calcined to drive off CO2 and fluorine, and then leached with hydrochloric acid to dissolve the trivalent rare earth elements. The rare earth hydroxides and chlorides obtained in this way are further processed to produce individual rare earth metal compounds... 

At some level, it is necessary in all of these scientific and technological applications to quantify the lanthanides present. Choice of an analytical technique is dictated by the type of information required and by the nature of the sample(s) being analyzed. Separation chemistry is central to many of the most successfirl analytical methods. For the rare earths, two distinct separations are important (1) separation of the rare earths as a group from the matrix elements, and (2) separation of the individual members of the series. Due to the chemical similarities of the rare earths and the existence of these metal ions in essentially one oxidation state, the latter is one of the greatest challenges in the separation of metal ions. 

The analytical separation of the individual rare-earth elements utilizing chromatographic techniques is one of the heroic accomplishments of 20th century chemistry. This... 

Rare earth abundances in natural materials have become an important geo-chemieal tool. The rare earth elements comprise a uniquely coherent group wherever one rare earth appears, all others are present as well. The group is coherent because under most natural conditions ail members share a common (3+) oxidation state, with anomalous behavior occurring under some conditions for Ce (4+) and Eu (2+). Natural materials differ substantially from each other in concentrations of the rare earths as a group and in abundances of individual rare earths relative to each other. In most natural situations, chemical separations within the rare earth group occur as a smooth function of atomic number. This makes it possible to find genetic relationships among diverse natural materials and to determine by what processes some natural materials formed. 

The history of rare earth separations dates back to the discovery of yttria in the year 1794, and the isolation of ceria in 1803 after which a total of 17 rare earth elements. Sc, Y, La, Ac and the lanthanide elements (except Pm), were isolated laboriously (in varying degrees of purity) by relatively inefficient fractional precipitation methods, prior to 1947. Such methods have (for the most part) been outmoded by the development of more elegant counter-current techniques during the last 30 years. While the purpose of this chapter is to summarize and comment upon recent progress in means of isolating individual lanthanides and yttrium, some mention of well-developed processes for the preliminary treatment of rare earth mixtures must be made, to place the subject of component resolution in proper context. 

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. 

About 25 000 tons of RE Metals - calculated as oxide - are currently consumed in the world per year. This quantity is divided among a dazzling variety of applications. In order to bring a certain systemization into this variety, these applications and possible applications have been reviewed from 3 different aspects from a historic development, from the special properties of the rare earths and from the degree of separation of the individual elements or grcfup of elements of the rare earth metal series. 

Radiochemical group separation has a major advantage over individual element separation in that it is far less time consuming, yet it can permit suflBcient separation to allow precise analysis. For example, by simply separating the rare earth elements (REE) as a group after neutron activation, it is possible to measure most of the rare earth spectra by direct counting and thus determine their distribution. 

The rare earth elements are very similar to each other. Separating them is a difficult task. The ores are first treated with sulfuric acid (H2SO4). The materials produced are then passed through a series of steps and the individual elements separated ftom each other. 

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