Tantalum preparation

Trisubstituted allylic alcohols. A low-valent tantalum prepared by reduction of TaCIs with Zn in DME/benzene adds to alkynes to form a complex that reacts with aldehydes to form (E)-allylic alcohols. The regioselectivity is determined by the bulkiness of the groups on the alkyne and of the R group of the aldehyde. 

F ure 329 AES depth profile of a 30-nm-thick tantalum oxide fikn on tantalum, prepared by anodization (adapted from ref. [8]). 

Eighteen isotopes of niobium are known. The metal can be isolated from tantalum, and prepared in several ways. 

Europium is now prepared by mixing EU2O3 with a 10%-excess of lanthanum metal and heating the mixture in a tantalum crucible under high vacuum. The element is collected as a silvery-white metallic deposit on the walls of the crucible. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Tantalum Compounds. Potassium heptafluorotantalate [16924-00-8] K TaF, is the most important tantalum compound produced at plant scale. This compound is used in large quantities for tantalum metal production. The fluorotantalate is prepared by adding potassium salts such as KCl and KF to the hot aqueous tantalum solution produced by the solvent extraction process. The mixture is then allowed to cool under strictiy controlled conditions to get a crystalline mass having a reproducible particle size distribution. To prevent the formation of oxyfluorides, it is necessary to start with reaction mixtures having an excess of about 5% HF on a wt/wt basis. The acid is added directiy to the reaction mixture or together with the aqueous solution of the potassium compound. Potassium heptafluorotantalate is produced either in a batch process where the quantity of output is about 300—500 kg K TaFy, or by a continuously operated process (28). 

Tantalum pentoxide [1314-61 -0] Ta20, is prepared by calcining tantaUc acid or hydrated tantalum oxide [75397-94-3] Ta20 at... 

Tantalum(II) oxide [12035-90-4] TaO, is the only other oxide the existence of which has been confirmed. It can be prepared from Ta20 by reduction with carbon at 1900°C or with H2 at 1100°C. 

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

Tantalum carbide is produced by carburization of the element or the oxide with carbon, ia a manner similar to the preparation of WC or TiC. Final carburization in a vacuum gives a golden yellow carbide, free of oxygen and nitrogen, that contains 6.1—6.3 wt % C and 0—0.2 wt % graphite. 

One was Ekeberg s tantalum and the other he called niobium (Niobe was the daughter of Tantalus). Despite the chronological precedence of the name columbium, lUPAC adopted niobium in 1950, though columbium is still sometimes used in US industry. Impure niobium metal was first isolated by C. W. Blomstrand in 1866 by the reduction of the chloride with hydrogen, but the first pure samples of metallic niobium and tantalum were not prepared until 1907 when W. von Bolton reduced the fluorometallates with sodium. 

Tantalum and tantalum alloys react with hydrogen, nitrogen and oxygen at temperatures above 300°C. Hydrogen is dissolved in the metallic matrix above 350°C and evolved at higher temperatures of about 800°C . The dissolved hydrogen embrittles the tantalum and its alloys. This effect can be used to prepare tantalum powder. 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

Using metallic precursors, HF solutions with higher concentrations of tantalum or niobium can be achieved. It is possible to prepare solutions that have maximum concentrations of about 1000 g/1 tantalum oxide and about 600 g/1 niobium oxide (Me205). 

Synthesis of the compounds from such HF solutions is performed by adding soluble fluoride compounds to the tantalum or niobium solution or by recrystallization of prepared fluoride compounds from water or HF solutions of different concentrations. In the first case, the composition of the compounds obtained depends on the ratio between Ta/Nb and the added metal and on the initial concentration of the HF used, whereas in the second case, it depends only on the HF concentration. 

Table 6 summarizes the main compounds that can be prepared by adding alkali metal fluorides to fluorine solutions that contain niobium or tantalum. 

The stoichiometry of the prepared compounds depends not only on the composition of the initial mixture, but also on the initial oxide s fluorination activity. Unlike tantalum oxide, fluorination of niobium oxide by an ammonium hydrofluoride melt results in the formation of oxyfluoroniobates, but not of fluoroniobates. During the first step of Nb205 fluorination, (NH4)3NbOF6 is formed according to the following interaction [51, 52, 105, 111, 121, 122] ... 

Preparation of the solutions was similar to that of niobium-containing solutions, i.e. by dissolving tantalum metal powder in hydrofluoric acid, HF, at a concentration of about 40% weight. 

The tantalum dissolution process takes longer compared to the preparation of the corresponding niobium solution, therefore the solution is heated and a small amount of nitric acid is added. A grey precipitate indicates saturation of the solution. The prepared solution is separated from the precipitate by filtration and used as the initial solution. 

An irreversible extinction of the SHG signal at 150-200°C is observed for a number of other fluoride and oxyfluoride compounds of tantalum and niobium that crystallize in centrosymmetric space groups. This phenomenon is especially typical for the compounds prepared by precipitation from solutions [206]. The appearance of the weak SHG signal for such compounds is related to imperfections in their crystal structure and the creation of dipoles. Nevertheless, appropriate thermal treatment improves the structure and leads to the disappearance of dipoles and to the irreversible disappearance of the corresponding SHG signal. 

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