Scandium production

High-purity scandium oxide (i.e., 99.0 to 99.99 wt.% Sc) is an initial raw material used to produce a metallic scandium. After fluorination of the oxide, pure scandium is then prepared by calciothermic reduction of scandium trifluoride (ScFj) with pure calcium metal. The metallic scandium obtained undergoes subsequent refining by vacuum distillation, which ensures a purity of metal at the level 99.99 to 99.999 wt.% Sc. Tentative annual demand for ultrapure metallic scandium for different fields of application is estimated for the near future at 800 to 1000 kg per year. Total annual world production in 2000 of scandium, excluding China, was about 30 kg. Union Carbide and Johnson Matthey, as well as the research company Boulder, are the main manufacturers of scandium products from thortveitite, wastes of uranium, and tungsten production. 

The production of the first pound of 99% pure scandium metal was announced in 1960. 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and hahdes. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

Benzyl and allyl alcohols which can generate stabilized caibocations give Friedel-Crafts alkylation products with mild Lewis acid catalysts such as scandium triflate. ... 

Skraup/Doebner-von Miller-type reactions with lanthanide catalysts under microwave radiation are efficient for a variety of different anilines. For example, cyclisation of aniline 44 with acetone in the presence of scandium triflate gave the desired product 45 in excellent yield. 

Fukuzawa et al. [99] found analogous scandium(III)triflate/ Pr-PyBOx complex as efficient catalyst for the asymmetric Diels-Alder reaction between cyclopentadiene or acyclic dienes and acyl-l,3-oxazohdin-2-ones with up to 90% ee. They latter described the same reaction in super critical CO2 in the presence of MSdA [ 100] that proceeded more rapidly than in CH2CI2 leading to the expected product with analogous selectivity. 

Scott Oakes et al. (1999a, b) have shown how adoption of SC conditions can lead to a dramatic pressure-dependent enhancement of diastereoselectivity. In the case of sulphoxidation of cysteine derivatives with rert-butyl hydroperoxide, with cationic ion-exchange resin Amberlyst-15 as a catalyst, 95% de was realized at 40 °C and with SC CO2. By contrast, with conventional solvents no distereoselectivity was observed. Another example is the Diels-Alder reaction of acrylates with cyclopentadiene in SC CO2 at 50 °C, with scandium tris (trifluoromethanesulphonate) as a Lewis acid catalyst. The endoiexo ratio of the product was as high as 24 1, while in a solvent like toluene it was only 10 1. 

Mausner, L. F. Kolsky, K. L. Joshi, V. Sweet, M. P. Meinken, G. E. Srivastava, S. C. In Scandium-47 A replacement for Cu-67 in nuclear medicine therapy with beta/gamma emitters, Isotope Production and Applications in the 21st Century, Proceedings of the International Conference on Isotopes, Vancouver, BC, Canada, 1999 Stevenson, N. R., Ed. World Scientific Publishing Singapore, 1999. 

Related to the nitrile oxide cycloadditions presented in Scheme 6.206 are 1,3-dipolar cycloaddition reactions of nitrones with alkenes leading to isoxazolidines. The group of Comes-Franchini has described cycloadditions of (Z)-a-phenyl-N-methylnitrone with allylic fluorides leading to enantiopure fluorine-containing isoxazolidines, and ultimately to amino polyols (Scheme 6.207) [374]. The reactions were carried out under solvent-free conditions in the presence of 5 mol% of either scandium(III) or indium(III) triflate. In the racemic series, an optimized 74% yield of an exo/endo mixture of cycloadducts was obtained within 15 min at 100 °C. In the case of the enantiopure allyl fluoride, a similar product distribution was achieved after 25 min at 100 °C. Reduction of the isoxazolidine cycloadducts with lithium aluminum hydride provided fluorinated enantiopure polyols of pharmaceutical interest possessing four stereocenters. 

Cyclopentadienylamine)scandium(2,3-dimethyl-l,3-butadiene) 7 was synthesized in good yield, as shown in Scheme 2. Complex 7 reacted with benzonitrile to form a /rz-imido complex 8, the structure of which was characterized by single crystal X-ray diffraction. This product 8 was proposed to be formed by nitrile insertion followed by an attack of another diene methylene group on the carbon atom of the imido intermediate.3 An unsaturated metal imido species was formed, which easily dimerized to produce 8. However, the yield of 8 was not reported. 

Undoubtedly, the best method for the production of pure anhydrous lanthanide trihalides involves direct reaction of the elements. However, suitable reaction vessels, of molybdenum, tungsten, or tantalum, have to be employed silica containers result in oxohalides (27). Trichlorides have been produced by reacting metal with chlorine (28), methyl chloride (28), or hydrogen chloride (28-31). Of the tribromides, only that of scandium has been prepared by direct reaction with bromine (32). The triiodides have been prepared by reacting the metal with iodine (27, 29, 31, 33-41) or with ammonium iodide (42). 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

The advantages of using ionic liquids as solvents for Diels-Alder reactions are exemplified by the scandium triflate catalysed reactions [14] in [bmim][PFg], [bmim][SbF6] and [bmim][OTf] for the reaction shown in Scheme 7.6. Whilst the nature of the anion seems to have little effect, all these solvents give rate enhancements for a range of Diels-Alder reactions compared to when the reactions are carried out in dichloromethane (DCM). Also, the selectivity towards the endo product is higher than in conventional solvents. As well as the enhanced rates and selectivities, the products can also be removed by extraction with diethyl ether and the ionic liquid and catalyst can immediately be reused. Experiments... 

Although the above demonstrated that product control could be achieved in scC02, the difference in selectivity was relatively small. However, later work using a Lewis acid catalyst, scandium triflate, on the Diels-Alder reaction of n-butyl acrylate and cyclopentadiene (Scheme 7.7) showed that the endo exo ratio was again found to rise to a maximum and then decrease again as the pressure, and hence density, was increased (Figure 7.3) [19]. 

A less specific type of adsorption can sometimes be used if the required product forms insoluble hydroxides but the target element does not. In this case, the solution is made alkaline, and the carrier-free radio-colloidal product is readily absorbed on to filter paper in good yield, when, after washing, it can subsequently be dissolved in acid. This has been used for the separation of magnesium from aluminium, scandium from calcium and for several other elements (17), (26), (42), (44), (66), (103), (104), (105), (106). 

Non-chlorinated Lewis acids, such as scandium triflate, were found to be good catalysts for Friedel-Crafts alkylation reactions (167). Although no aromatic hydrocarbon alkylation occurred in CH2CI2, [BMIMJPFg, Sc(OTf)3 catalyzed the alkylation of benzene with high yields of the monoalkylated product. The lower acidity of the ionic liquid led to fewer byproducts and therefore higher yields. The products were separated by simple decantation and the catalyst was reused. 

Scandium also is obtained as a by-product of processing uranium ores, although they contain only traces of the metal. 

In most recovery processes, scandium oxide is converted to its fluoride salt. The fluoride salt is the end product. The fluoride is converted to metallic scandium by heating with calcium in a tantalum crucible at elevated temperatures. A similar reduction is carried out with most rare earths. The metal is purified by distillation at 1,650 to 1,700°C under high vacuum in a tantalum crucible. 

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