Tantalum -containing solutions

Complexes in fluoride solutions - tantalum-containing solutions 

The first comprehensive investigation of the TaF5 - HF - H2O system was performed by Buslaev and Nikolaev [292]. Based on the analysis of solubility isotherms, and on conductometric and potentiometric titrations, the authors concluded that in this solution, tantalum forms oxyfluorotantalic acid, H2TaOF5, similar to the formation of H2NbOF5 in solutions containing NbF5. 

Tsikaeva et al. investigated the Raman and IR absorption spectra of solutions containing tantalum in hydrofluoric acid with no additional cations [290, 291], 

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. 

HF concentration, mol/1 Ta concentration, mol/1 NH4 +/Ta5+ Complexes found % TaF72 of total complexes 

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Table 4. Compounds precipitated from tantalum-containing solutions Reproducedfrom [61], D. V. Tsikaeva, A. /. Agulyansky, Y. I. Balabanov, V Y. Kuznetsov, V. T. Kalinnikov, Zh. Neorg. Khim. 34 (1989) 3046, Copyright 1989, with permission of Nauka (Russian Academy of Sciences) publishing.

Tantalum containing solution Niobium containing solution... 

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. 

Complexes in fluoride solutions — tantalum containing solutions... 

The products of the solvent extraction process are tantalum strip solution, niobium strip solution and raffinate - liquid wastes containing impurities and residual acids. 

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. 

Two kinds of tantalum-containing initial solutions were chosen according to their ionic complex structure. The first one contained mostly TaF6 ions (Ta F = 1 18) while the second was characterized predominantly by TaF72 ions (Ta F = 1 6.5). The ionic composition of the solutions was determined by Raman spectroscopy. 

Table 46. Composition of tantalum-containing complexes in fluoride solutions of different concentrations (after Keller and Chetham-Strode [155]).

In summary, it can be said that tantalum-containing fluoride solutions generally consist of two types of ions TaF72 and TaF6. Low acidity of the solution (i.e. low HF concentration) leads to the predominant formation of TaF72 complex ions, while higher concentrations of HF lead to the presence of TaF6" complex ions. 

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. 

As was discussed in Chapter 4, tantalum and niobium dissolve in fluorine-containing solutions in the form of complex fluoride ions of two types, namely TaF727TaF6" and NbOF527NbF6 [61, 155, 171, 291]. The equilibrium between the complexes depends on the acidity of the solution and can be represented schematically as shown in Equations (139) and (140) for tantalum and niobium, respectively ... 

The raffinate from the selective extraction process contains mostly niobium. The tantalum extract is treated by steam stripping to obtain a tantalum strip solution. The method results in the effective separation and relatively high concentration of tantalum and niobium in the respective strip solutions. 

The resulting product depends on the precipitation conditions, and in particular, on the over-saturation level of the solution. Formation of ammonium oxyfluorometalate crystalline compounds occurs at a relatively low pH of the solution. From the standpoint of the interactions described in Equation (143), this means that the interaction between NH4F and Me205 (denoted as interaction 1) is stronger than the interaction denoted as interaction 2. In this case, subsequent processes of the hydroxide treatment lead to some defluorination of the product, but the performance of such processes is usually very problematic. Precipitation at high pH values leads to a strong oversaturation of niobium- or tantalum-containing compounds, which in turn... 

Brown et al. [494] developed a method for the production of hydrated niobium or tantalum pentoxide from fluoride-containing solutions. The essence of the method is that the fluorotantalic or oxyfluoroniobic acid solution is mixed in stages with aqueous ammonia at controlled pH, temperature, and precipitation time. The above conditions enable to produce tantalum or niobium hydroxides with a narrow particle size distribution. The precipitated hydroxides are calcinated at temperatures above 790°C, yielding tantalum oxide powder that is characterized by a pack density of approximately 3 g/cm3. Niobium oxide is obtained by thermal treatment of niobium hydroxide at temperatures above 650°C. The product obtained has a pack density of approximately 1.8 g/cm3. The specific surface area of tantalum oxide and niobium oxide is nominally about 3 or 2 m2/g, respectively. 

Investigations of the plasma chemical decomposition of tantalum-containing fluoride solutions indicated no significant differences in the process and product parameters compared to the corresponding decomposition of niobium-containing fluoride solution [529, 532]. The particle diameter, shape and specific surface area of both niobium oxide and tantalum oxide powders attest to a gas-phase mechanism of the interaction, with sequential condensation and agglomeration of the oxides. 

Potassium heptafluorotantalate, K2TaF7, or as it is called by its commercial name K-salt, is a starting material for tantalum metal production. K-salt is produced by adding potassium fluoride, KF, or potassium chloride, KC1, to a tantalum strip solution that results from a liquid-liquid extraction process. In order to prevent hydrolysis and co-precipitation of potassium oxyfluoro-tantalate, a small excess of HF is added to the solution [24]. Another way to avoid the possible formation and co-precipitation of oxyfluoride phases is to use potassium hydrofluoride, KHF2, as a potassium-containing agent. The yield of the precipitation depends mostly on the concentration of the potassium-containing salt and is independent of the HF concentration [535]. 

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