Tantalum oxide

Separation of tantalum from niobium requires several complicated steps. Several methods are used to commercially produce the element, including electrolysis of molten potassium fluorotantalate, reduction of potassium fluorotantalate with sodium, or reacting tantalum carbide with tantalum oxide. Twenty five isotopes of tantalum are known to exist. Natural tantalum contains two isotopes. 

A large deposit of loparite occurs ia the Kola Peninsula, Russia. The production of REO reaches 6500 t/yr. Loparite contains over 30% of rare-earth oxides from the cerium group. In addition, loparite contains up to 40% titanium oxide and up to 12% niobium and tantalum oxides. 

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

Tantalum Oxides. Tantalum pentoxide [1314-61 -0] Ta20, (mp = 1880°C, density = 8.73 g/cm ) is a white powder existing in two thermodynamically stable modifications. The orthorombic P-phase changes at 1360°C into the tetragonal a-modiftcation. The existence of an S-modiftcation has also been reported (70). Tantalum pentoxide reacts slowly with hot hydrofluoric acid but is insoluble in water and in most solutions of acids and alkalies. For analytical purposes, it can be dissolved by fusion with alkali hydroxides, alkali carbonates, and potassium pyrosulfate. 

During World War II, production of butadiene (qv) from ethanol was of great importance. About 60% of the butadiene produced in the United States during that time was obtained by a two-step process utilizing a 3 1 mixture of ethanol and acetaldehyde at atmospheric pressure and a catalyst of tantalum oxide and siHca gel at 325—350°C (393—397). Extensive catalytic studies were reported (398—401) including a fluidized process (402). However, because of later developments in the manufacture of butadiene by the dehydrogenation of butane and butenes, and by naphtha cracking, the use of ethanol as a raw material for this purpose has all but disappeared. 

Capacitance is related to the area of the plates (yi), the distance between the plates (d), and the dielectric constant (e) of the material between the plates (Figure 2, equation I). The dielectric constant or permittivity of a material is the increased capacitance observed compared to the condition if a vacuum was present between the plates. Common dielectric materials are polystyrene (e = 2.5), mylar (e = 3), mica (e = 6), aluminum oxide (e = 7), tantalum oxide (e = 25), and titania (e = 100). In the Leyden jar the dielectric is silica. 

The performance of different types of chlorination processes is discussed comprehensively in overview [31]. It should be mentioned that carbon tetrachloride can also be applied successfully in the chlorination of rare refractory metal oxides, including tantalum oxide. 

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). 

Hydrofluoride synthesis is based on the simultaneous fluorination by ammonium hydrofluoride of niobium or tantalum oxides with other metals compounds (oxides, halides, carbonates etc.) [105]. Table 13 presents some properties of ammonium hydrofluoride, NH4HF2 [51, 71]. Ammonium hydrofluoride is similar to anhydrous HF in its reactivity, but possesses some indisputable advantages. The cost of ammonium hydrofluoride is relatively low, it can be dried and handled easily, recycled from gaseous components, and its processing requires no special equipment. 

The fluorination of tantalum oxide, Ta205, is described by Rakov [51]. The interaction begins at a temperature of 156-190°C, yielding heptafluorotantalate of ammonium ... 

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] ... 

Analysis of isotherms of physicochemical properties can also clarify the interactions within molten systems that contain oxides. For example, the interaction between potassium fluorotantalate and tantalum oxide, Ta2Os, can be studied using density isotherms of molten mixtures. Fig. 68 demonstrates the dependence of the density on K2TaF7 concentration for mixtures K2TaF7 -KF - KC1 and K2TaF7 - KF - KC1 - Ta2Os. 

The proposed model of the structure of oxyfluoride melts corresponds with the conductivity results shown in Fig. 69. The specific conductivity of the melt drops abruptly and asymptotically approaches a constant value with the increase in tantalum oxide concentration. This can be regarded as an additional indication of the formation of oxyfluorotantale-associated polyanions, which leads to a decrease in the volume in which light ions, such as potassium and fluorine, can move. The formation of the polyanions can be presented as follows ... 

The second solution that results from the liquid-liquid extraction process is a high-purity niobium-containing solution. This solution is used in the preparation of niobium oxide, Nb205. The process is similar to the above-described process of tantalum oxide preparation and consists of the precipitation of niobium hydroxide and subsequent thermal treatment to obtain niobium oxide powder. 

Fig. 132. Distribution of tantalum oxide between cells of pilot extraction system, for organic and aqueous phases. Reproduced from [473], A. Agulyansky, L. Agulyansky, V. F. Travkin, Chemical Engineering and Processing 43 (2004) 1231, Copyright 2004, with permission of Elsevier.

Fig. 132 shows the content distribution of tantalum oxide among the cells, for the organic and aqueous phases. 

The tantalum strip solution was used for the preparation, by precipitation and thermal treatment, of tantalum oxide. The product was determined to be of high purity grade. Table 62 presents typical analysis results. 

In the case of tantalum-containing solutions, a sharp drop in Ta205 concentration was observed also at pH > 10. The precipitated material was identified as a pure amorphous powder, which after appropriate thermal treatment was converted into tantalum oxide. Fig. 137 presents isotherms (20°C) of Ta205 concentration versus pH for solutions with compositions close to those of industrial strip solutions. 

Fig. 137. Tantalum oxide content versus pH. Curve 1 - solution containing no ammonium fluoride. Curves 2-5 - solutions with increased initial ammonium fluoride content (after Agulyanskaya et al., [492, 493]).

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