Medium rare-earth elements

The problem is no longer the validity of Mendeleev s system, but the best way to represent it. Should it be the original short-form table with 8 columns, the familiar medium-long form with 18 columns, or perhaps even a long-form table with 32 columns, which more naturally accommodates the rare earth elements Into the main body of the table Altanahvely, some favor pyramidal tables, while others advocate the left-step form proposed by diaries Janet in the 1920s. Theodor Benfey and rhilip Stewart have proposed continuous spiral models. Hundreds, possibly even thousands, of periodic systems have been proposed, and each has its ardent supporters. 

In each representation, the alkali metal is black, the rare-earth element is shaded medium grey, the main group element is dark grey, and selenium is light grey (a) NagEu2(Si2Se6)2,... 

The selectivity of most methods can be increased by proper selection of the pH of the analytical medium. For reagents of the R-OH type there exists a relationship between colour reactions and hydrolytic reactions of certain elements. In strongly acid solutions, colour reactions proceed with those elements that have easily hydrolyzable cations, such as Zr, Hf, Th, U(rV) and Ti. In moderately acidic solutions the reactions also proceed with Fe(IlI), Al, and U(IV) in weakly acid and neutral solutions, with rare-earth elements, Fe(II), Cu, Mn, and in alkaline solutions, with Ca, Sr, and Mg. Easily hydrolyzable species react in more acidic solutions, whereas other ions react only in less acidic ones. 

Rare-earth elements may be isolated by precipitation as their oxalates, fluorides, or hydroxides. When the oxalates are precipitated from a weakly acidic medium (pH 1-4), Ca is used as a collector [14,15],... 

The anionic scandium-sulphate complex is sorbed by the strongly basic anion-exchanger Dowex 1 [1]. Under the conditions employed, rare-earth elements, Al, Be, and some other elements are eluted, but Th, Zr, and U are retained along with scandium. Scandium can also be sorbed on anion-exchangers as the thiocyanate complex [2]. Scandium and rare-earth elements have been retained on a strongly basic anion-exchanger from a medium of 90% glacial acetic acid and 10% 3 M HCl. First, yttrium and the lanthanides are run off the column, then scandium is eluted [3]. 

Precipitation of scandium with ammonium tartrate in neutral medium separates it from the rare-earth elements [15,16]. Yttrium is employed as a collector for microgram quantities of Sc. Larger amounts of other elements (>20 mg of Al, 20 mg of Fe, 2 mg of Zr, or 2 mg of Th) prevent the quantitative separation of scandium as the tartrate. Scandium is separated from yttrium by extraction as the thiocyanate complex. 

With a few exceptions, the characteristic problem of cultivation of plant explants in in vitro cultures is a low production of secondary metabolites by these cultures. One of the methods which can achieve an increase in the production of natural substances in in vitro cultures, is elicitation of cell cultures with biotic elicitors. For example, hairy root cultures of Cassia obtusifolia L. clones transformed with Agrobacterium rhizogenes strain 9402 were established to investigate anthraquinone production. It was found that changes of the elements in the culture medium and the addition of rare earth element Eu3+ can greatly influence the contents of free anthraquinones in the hairy root [320],... 

Fig. 6.28 A van t Hoff diagram comparing dissociation pressures of the Ti-doped NaAlH4 and NasAiHe with those of other well-known hydrides (from Ref [133]). HT, MT and LT stand for high-, medium- and low-temperature hydrides, respectively. Mm in MmNisHe is misch metal, a low-cost mixture of rare-earth elements.

Rare earth elements have an essential role in permanent magnets, lamp phosphors, rechargeable NiMH batteries, catalysts and other applications (see Chap. 5). In 2010, the European Union published the report Critical Raw Materials for the EU , indicating that the rare earths had the highest supply risk of all raw materials, while their economic importance was considered medium (European Union 2010) (Fig. 7.1). 

Fig. 7.2 DOE medium-term (2015-2025) criticality matrix, showing the five most critical rare earth elements (Y, Nd, Eu, Tb, Dy), and the non-critical ones (Ce, Pr, Sm). Reproduced with permission of the U.S. Department of Energy (2011)...

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. 

An outline of the Rhone-Poulenc separation flowsheet to produce high purity rare earth oxides from various ores such as monazite, bastnasite and euxenite was published by Agpar and Poirier (1976). After the leaching step, non-rare earth elements and radioactive products are removed. The first solvent extraction stream in a chloride medium produces non-separated rare earth compounds, such as dehydrated rare earth chlorides used as the feedstock for the electrolysis to produce mischmetal. 

Skornik NA, Serebrennikov VV (1965) Dependence of solubility of citrates of some rare earth elements on the pH of the medium. Zhum Neorg Khim 10 407 09... 

The classic method for the isolation of the rare earth group which is used for both qualitative and quantitative determination involves three methods. In the first method, rare earths are precipitated as fluorides in acidic medium. The elements precipitated include Mg, Cu, Fe, rare earths Th, Ca and Sr. The second method consists of precipitation as hydroxides resulting in the removal of alkaline-earth elements like calcium from the mineral. In the third method rare earths are precipitated as oxalates from moderately acidic solutions and the elements Ca, Zn, Pb, Cu, Cd, and Ag may be coprecipitated. In early times the above methods were repeated several times to isolate, the rare earth group in a relatively pure form. 

---