Tantalum oxide fluorides

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]).

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. 

Kobayashi et al. [508] developed an effective method to control particle size and fluoride content in granular tantalum oxide and niobium oxide. The resultant powders are suitable for application in the manufacturing of ceramics, single crystals, optical glass, etc. 

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. 

Only two processes of tantalum metal production are of worldwide commercial significance. These are the electrolysis of fluoride-chloride melts containing potassium heptafluorotantalate, K TaF , and tantalum oxide, Ta20s, and the reduction with sodium of K-salt or K—salt that is dissolved in potassium fluoride-chloride melts. 

Anode processes yield gaseous chlorine, fluorine, carbon chloride or fluoride. In the case of melts containing dissolved tantalum oxide, carbon oxides (mostly carbon dioxide) are formed on the graphite anode [28,37]. 

Simple Binary and Related Compounds.—Reviews have been published which describe crystal structures of, and chemical bonding in, the Ta-O system. Structural aspects of niobium and tantalum oxides and oxide fluorides have... 

In a rather eccentric process, given the propensity of such systems for halide exchange, the reaction between tantalum(V) oxide and hydrogen fluoride at 90 C in an autoclave, followed by treatment of the mixture (now a hydrated fluorotantalic acid) with phosgene at 140 "C, has been used for the synthesis of anhydrous tantalum(V) fluoride [1106] ... 

KCl-NaCl-K2TaF7 melts On addition of potassium heptafluotantalate to the NaCl-KCl melt a peak of tantalum reduction to metal (Rl) from fluoro-chloro complex (as will be shown below) was observed on voltammograms (Fig. 1). On the anodic part of the curve several peaks of the deposited tantalum oxidation correspond to it. The anodic peaks OJ andOJ" should be referred to tantalum dissolution respectively in the form of Ta(lV) and Ta(V) chloro complexes, formed in the absence of excess fluoride ions. Recording of the cyclic voltammetric curve with a potential reverse at more positive potential -0.48 or -0.6 V (Fig.l), when the amount of tantalum deposited at the electrode is small and there is not enough time for diffusion of a considerable quantity of the liberated fluorine into the melt bulk, shows no such peaks, which corroborates our assumption. Peaks Oi and OJ were caused by tantalum disso-... 

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