Fractional crystallization, lanthanides

Fra.ctiona.1 Crystallization. Fractional crystallization, used until the early part of the twentieth century, is uneconomical for processing large quantities of lanthanides. Many recrystallization steps are required to recover high purity products. Several salts and double salts have been used ... 

RE(N0 )2 NH NO 4H20 for light lanthanide separation (La, Nd, Pr) 2RE(N02)3 3Mg(N03)2 24H20 for middle lanthanide separation (Sm, Eu, Gd). Bromates and ethylsulfates have been found useful. Fractional crystallization is particularly slow and tedious for the medium and heavy rare earths. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

Sulfates and nitrates are known and in all cases they decompose to the oxides on heating. Double sulfates of the type M (S04)3.3Na2S04.12H20 can be prepared, and La (unlike Sc and Y) forms a double nitrate, La(N03)3,2NH4N03.4H20, which is of the type once used extensively in fractional crystallization procedures for separating individual lanthanides. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

The last of the lanthanides, this metal is also the hardest and the densest of them. It is a component of cerium mischmetal. Lutetium has some applications in optoelectronics. Shows great similarities to ytterbium. Its discoverer, Georges Urbain, carried out 15 000 fractional crystallizations to isolate pure lutetium (record ). The element has special catalytic properties (oil industry). 176Lu is generated artificially and is a good beta emitter (research purposes). 177Lu has a half-life of six days and is used in nuclear medicine. 

The early separation of the lanthanides was beset by difficulties as a result of the similarity in size and charge of the lanthanide ions. The separations were generally based on slight differences in solubility, which were exploited through schemes of fractional crystallization. The differences in behavior resulting from a decrease in ion... 

Classical methods of separation [7] are (1) fractional crystallization, (2) precipitation and (3) thermal reactions. Fractional crystallization is an effective method for lanthanides at the lower end of the series, which differ in cation radius to a large extent. The separation of lanthanum as a double nitrate, La(N03)3-2NH4N03-4H20, from praseodymium and other trivalent lanthanide with prior removal of cerium as Ce4+ is quite a rapid process and is of commercial significance. Other examples are separation of yttrium earths as bromates, RE(Br03>9H20 and use of simple nitrates, sulfates and double sulfate and alkali metal rare earth ethylenediamine tetraacetate complex salts in fractional crystallization separation. 

These exhibit high coordination numbers. Double nitrates Mg3Ln2(N03)i2.24H20, which contain [Ln(N03)6] ions, are important historically as they were once used for the separation of the lanthanides by fractional crystallization. Use of counter-ions like Ph4P+ and Me4N+ allow isolation of the 10-coordinate [Ln(N03)5] ions. 

Nitrate complexes are of historical importance for their role in separation of the early lanthanides by fractional crystallization, when magnesium double nitrates, Mg3Ln2(N03)i2 -24H20, were used. These complexes contain 12-coordinate [Ln(N03)6] groups, also found in some crown ether complexes. Other nitrate complexes characterized include 10-coordinate [Ln(N03)5] , as salts with counterions... 

The separation of yttrium from the lanthanides is performed by selective oxidation, reduction, fractionated crystallization, or precipitation, ion-exchange and liquid-liquid extraction. Methods for determination include arc spectrography, flame photometry and atomic absorption spectrometry with the nitrous oxide acetylene flame. The latter method improved the detection limits of yttrium in the air, rocks and other components of the natural environment (Deuber and Heim 1991 Welz and Sperling 1999).Other analytical methods useful for sensitive monitoring of trace amounts of yttrium are X-ray emission spectroscopy, mass spectrometry and neutron activation analysis (NAA) the latter method utilizes the large thermal neutron cross-section of yttrium. For high-sensitivity analysis of yttrium, inductively coupled plasma atomic emission spectroscopy (ICP-AES) is especially recommended for solid samples, and inductively coupled plasma mass spectroscopy (ICP-MS) for liquid samples (Reiman and Caritat 1998). 

The optically active electrons involve the 4f shell, buried deep within the Xe-core of electrons. Because of the shielding effect of the outer closed shells (5s25p6), the chemistry of each lanthanide is quite similar to the others. Thus, the separation of each rare earth, free from the others, has proven to be very difficult. The most stable valence state is Ln3+, where three (3) electrons (4fi and Gs ) have been removed. Most separations were accomplished in the 1800 s by repeated fractional crystallization. Sometimes, literally thousands of recrystallization steps were required before a reasonably pure product could be Isolated and demonstrated to be pure. A number of false starts have been recorded where the investigator thought he had a pure product, only to be shown that he did not. The first rare earth to be documented, gadoliniiun - Gd, was actually named around 1805. But, the product was later shown to be a mixture of rare earths. Gd was finally isolated in a pure form some 75 years later. It was named after the Swedish investigator, J. GadoUn (1794) who characterized the first rare earth ore. 

Fractional crystallization Fractional crystallization can be used to successively crystallize Ln ions, whereby the solubility difference for the lanthanide elements can be exploited. In this way you can crystallize a particular fraction of the solution that contains the specific lanthanide element that you seek. 

Of typical separations methods, those based on volatility (like distillation) are of little importance in lanthanide/actinide separations. Precipitation methods and other biphasic separations processes are by far the most useful. For the separation of macroscopic amounts of the individual lanthanides, fractional crystallization was the principal technique in the early days of the investigation of the lanthanides. The small solubility differences required hundreds or even thousands of repetitions to achieve useful separation of the elements (Moeller 1963). 

The only practical chemical methods for the determination of any of the individual rare earths from each other and, especially, adjacent lanthanides are based on the oxidation of cerium(III) to cerium(lV) and the determination of Ce(IV) by gravimetric or volumetric methods as discussed in previous sections. Fractional crystallization methods can be used to separate the rare earths into subgroups, but clean separations of adjacent rare earths are not feasible. As mentioned previously, Eu, Sm, and Yb can be reduced to the plus two oxidation state, but only Eu(II) can really be satisfactorily determined by chemical methods. 

Fig. 21.6. Comparison diagram for volcanic rocks from Reunion island (Zielinski, 1975). The rock with lowest lanthanide concentrations is olivine-rich alkali basalt. Rocks with higher lanthanide concentrations are increasingly acidic in nature. The sequence is believed to be successive products of fractional crystallization.

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