Lanthanides- process

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

The lanthanides readily form double salts, eg, Lri2(SO NajS04 2H2O Lu2(SO MgSO 24H20, which were used historically in the fractionation process. 

Apatite and other phosphorites constitute a substantial resource of rare earths. The REO content is highly variable and ranges from trace amounts to over 1%. Apatite- [1306-05-4] rich tailings of the iron ore at Mineville, New York, have been considered a potential source of yttrium and lanthanides. Rare-earth-rich apatites are found at the Kola Peninsula, Russia, and the Phalaborwa complex in South Africa. In spite of low REO content apatites could become an important source of rare earths because these are processed in large quantities for the manufacturing of fertilisers (qv). 

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 ... 

There are a number of minerals in which thorium is found. Thus a number of basic process flow sheets exist for the recovery of thorium from ores (10). The extraction of mona ite from sands is accompHshed via the digestion of sand using hot base, which converts the oxide to the hydroxide form. The hydroxide is then dissolved in hydrochloric acid and the pH adjusted to between 5 and 6, affording the separation of thorium from the less acidic lanthanides. Thorium hydroxide is dissolved in nitric acid and extracted using methyl isobutyl ketone or tributyl phosphate in kerosene to yield Th(N02)4,... 

There are few principal lanthanide deposits, and there are no minerals that are sources for cerium alone. All the lighter lanthanides occur together in any potential deposit, and processes separating the lanthanides are necessary to obtain pure cerium products. 

Phosphate rock, mined widely throughout the world for its fertilizer value (see Fertilizers), in certain regions contains a few percent of lanthanides. For example, the apatite deposits in the Kola peninsula on the Russian/Finnish border. The Ln content is recoverable from the various processing residues, and because other Ln-containing minerals, such as loparite [12173-83-0], are also found there, the location suppHes a significant part of the demand in Eastern Europe. 

The production of cerium derivatives begins with ore beneficiation and production of a mineral concentrate. Attack on that concentrate to create a suitable mixed lanthanide precursor for later separation processes follows. Then, depending on the relative market demand for different products, there is either direct production of a cerium-rich material, or separation of the mixed lanthanide precursor into individual pure lanthanide compounds including compounds of pure cerium, or both. The starting mineral determines how the suitable mixed lanthanide precursor is formed. In contrast the separation... 

An alternative process for opening bastnasite is used ia Chiaa high temperature roastiag with sulfuric acid followed by an aqueous leach produces a solution containing the Ln elements. Ln is then precipitated by addition of sodium chloride as a mixed sulfate. Controlled precipitation of hydroxide can remove impurities and the Ln content is eventually taken up ia HCl. The initial cerium-containing product, oace the heavy metals Sm and beyond have been removed, is a light lanthanide (La, Ce, Pr, and Nd) rare-earth chloride. 

Historically the use of mona2ite, a thorium-containing mineral, as the principal lanthanide resource led to confusion regarding the relation between radioactivity and the lanthanides. Inadequate separations produced Th-contaminated Ln products. Modem processing technology results in products that meet all regulatory requirements. 

The role of cerium in these lighting phosphors is not as the emitting atom but rather as the sensitizer. The initial step in the lighting process is the efficient absorption of the 254 nm emission Ce ", with broad absorption bands in the uv, is very suitable. This absorbed energy is then transferred to the sublattice within the crystalline phosphor eventually the activator ion is fed and emission results. Cerium, as a sensitizer ion, is compatible in crystal lattices with other lanthanide ions, such as Eu and Tb, the usual activator atoms. 

There s it is required to keep unchangeable chemical forms of material components, as well as lanthanide concentration ratio in different degree of oxidation. Therefore, the main conception of this work is to combine process of the sample decomposition and analytical reaction of the determined chemical form. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

The bulk of both monazite and bastnaesite is made up of Ce, La, Nd and Pr (in that order) but, whereas monazite typically contains around 5-10% Th02 and 3% yttrium earths, these and the heavy lanthanides are virtually absent in bastnaesite. Although thorium is only weakly radioactive it is contaminated with daughter elements such as Ra which are more active and therefore require careful handling during the processing of monazite. This is a complication not encountered in the processing of bastnaesite. 

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

Because of the technical importance of solvent extraction, ion-exchange and precipitation processes for the actinides, a major part of their coordination chemistry has been concerned with aqueous solutions, particularly that involving uranium. It is, however, evident that the actinides as a whole have a much stronger tendency to form complexes than the lanthanides and, as a result of the wider range of available oxidation states, their coordination chemistry is more varied. 

When this analysis was first attempted [9-11] very few values of 1 had been obtained from series limits in the third spectra of the lanthanides, and the first comprehensive sets were calculated from Born-Haber cycles [9]. Subsequent spectroscopic values [12] confirmed the early work and are plotted in Eig. 1.1. In all cases they refer to the ionization process... 

The second class of reaction is that of processes in which the 4f electrons are conserved. The obvious examples are the complexing reactions of tripositive lanthanide ions. Here the irregularities due to changes in inter-electronic repulsion almost entirely disappear. We then get the slight smooth energy change whose consequences were so familiar to 19 century chemists, who struggled vdth the separation problem. 

The principle just outhned has two parts. The first part deals with redox processes and was developed here by examining the relative stabihties of the -i-2 and -i-3 oxidation states of the lanthanides. It can be extended in a variety of ways. Thus if the f variation is shifted one element to the right, it tells us the nature of the f variations, and accounts for the distribution of the -i-4 oxidation states of the lanthanides [2, 10, 15]. Their stability shows maxima at cerium(IV) and terbium(IV), decreasing rapidly as one moves from these elements across the series. 

Very often, the tetrad effect is not clearly discernible in the energies of processes in which 4f electrons are conserved. It may, for example, be obscured by irregularities caused by structural variations in either reactants or products. This is especially likely given the willingness of lanthanide ions to adopt a variety of coordination geometries. There is, however, no doubt that tetrad-like patterns are often observed. But does Table 1.2 provide a convincing explanation of what is seen ... 

In the lanthanide series, the equivalent values are much reduced by the retreat of the 4f electrons into the xenon core. This is so whether we consider processes that involve the condensation of gaseous ions, or conventional reactions. Table 1.3 includes data for the change... 

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