Tantalum fluorine, fluorides

Boro4(III) fluoride, gold(III) fluoride, tantalum(V) fluoride and platinum(IV) fluorine are further examples of weak acids in fluorosulphuric acid ... 

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 is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Tantalum has excellent resistance to virtually all salts including chlorides (especially cupric and ferric chloride), sulphates, nitrates and salts of organic acids, provided (a) they do not contain fluorides, fluorine and free sulphur trioxide, or (b) hydrolyse to produce strong alkalis. 

The discoveiy of the process for the separation of tantalum and niobium using fluorination marked, in fact, the beginning of the development of the chemistry and technology of tantalum and niobium in general, and initiated the development of complex fluoride compound chemistry in particular. 

The second method of tantalum and niobium production is related historically to Marignac s process of tantalum and niobium separation, in the form of complex fluoride compounds, and is based on the fluorination of raw material. The modem production process consists of slightly different steps, as described below. 

Nevertheless, tantalum and niobium refining technology was, and remains, a part of fluorine chemistry, since its main processes are related to the chemistry of tantalum and niobium fluorides in solid, dissolved and molten states. 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

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

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

Modem processing of tantalum and niobium metals and their compounds is related to the treatment of fluoride compounds. Hence, successful technological improvements, the development of novel methods and the manufacturing of high-grade products depend on the application of technological achievements in the area of fluorine chemistry. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

The process of separating the intermediate products from the purified solutions, in the form of solid complex fluoride salts or hydroxides, is also related to the behavior of tantalum and niobium complexes in solutions of different compositions. The precipitation of complex fluoride compounds must be performed under conditions that prevent hydrolysis, whereas the precipitation of hydroxides is intended to be performed along with hydrolysis in order to reduce contamination of the oxide material by fluorine. 

Hydrofluoric acid, at relatively high concentrations and at elevated temperatures, dissolves columbite-tantalite concentrates at a reasonable rate. The dissolution process is based on the fluorination of tantalum, niobium and other metal oxides and their conversion into soluble complex fluoride acids yielding complex fluoride ions. 

The optimal temperature range for the fluorination process was found to be about 230-290°C. The resulting cake was leached with water. The prepared solution was separated from the precipitate by regular filtration and the separated insoluble precipitate was identified as lithium fluoride, LiF. The solution contained up to 90 g/1 Ta205. Solution acidity was relatively low, with a typical pH = 3-4, and was suitable for the precipitation of potassium heptafluorotantalate, K2TaF7, tantalum hydroxide or further purification by liquid-liquid extraction after appropriate adjustment of the solution acidity [113]. 

In both cases, the fluorination of the complex oxides of tantalum and niobium leads to the formation of the water-soluble compounds (NH4)2TaF7 and (NH4)3NbOF6, the insoluble lithium fluoride and die gaseous components H20, NH3 and HF. 

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 ... 

Another way of applying the selective extraction method directly on the initial solution is to produce a solution of low acidity. This can be achieved by using the hydrofluoride method for fluorination and decomposition of raw material. As was discussed in Paragraph 8.2.2, the raw material is fluorinated by molten ammonium hydrofluoride yielding soluble complex fluorides of ammonium and tantalum or niobium. The cake obtained following fluorination is dissolved in water, leading to a solution of low initial acidity that is related for the most part to the partial hydrolysis of complex fluoride compounds. The acidity of the solution is first adjusted to ensure selective tantalum extraction. In the second step, the acidity of the raffinate is increased to provide the necessary conditions for niobium extraction. 

First, it is important to note that complete fluorination of the elements ensures an effective separation process. Particularly, Maiorov and Nikolaev [477] developed and reported on the conversion of tantalum, niobium and titanium sulfates and chlorides into their respective fluorides. It was shown that such conversion leads to significant improvement in/enhancement of the separation of the elements. 

Substitution of fluoride ions by other suitable ions in the complex acid so as to separate the fluorine ions from tantalum and niobium (liquid-liquid interaction) ... 

The opposite process, i.e. pouring the strip solution into the ammonia solution, significantly reduces the fluorine concentration in the hydroxides formed. Bludssus et al. [495] developed a process comprising the introduction of tantalum- or niobium-containing acid solution to an ammonia solution until achieving pH = 9. It is reported that this method enables the production of tantalum or niobium hydroxides with fluoride contents as low as 0.5% wt. with... 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

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]. 

S Fluorination of tantalum and niobium oxides by hydrofluorides of ammonium or alkali metals yields fluorotantalate or monooxy-fluoroniobate compounds. Fluorination of tantalum or niobium oxides in the presence of oxides of other metals yields complex fluoride compounds containing both tantalum or niobium and added metals. 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

This monograph compiles the latest research on the chemistry of complex fluorides and oxyfluorides of tantalum and niobium, and covers synthesis and fluorination processes, crystal structure peculiarities and crystal chemical classification, as well as the behavior of complex ions in fluorine solutions and melts. 

Conversion of the separated fluorides into the corresponding oxides is effected by boiling with concentrated sulphuric acid until free from fluorine, and then hydrolysing the product by boiling with water. Alternatively, the hydrated acids are precipitated by the addition of ammonia to the solutions of the double fluorides.4 Niobium pentoxide, Nbg05, or tantalum pentoxide, TaaOs, is obtained on ignition of the precipitated hydrate. 

Tantalum Pentafluoride, TaFs, is the only known fluoride of tantalum, and has been successfully isolated by methods that avoid hydrolysis (1) Tantalum and fluorine are brought into reaction exactly as in the preparation of niobium pentafluoride.1 (2) Tantalum penta-chloride is treated in the cold with dry hydrofluoric add the hydrochloric acid liberated and excess of hydrofluoric add are evaporated oft, and the resulting tantalum pentafluoride is purified by redistillation in a platinum crucible between 300° and 400° C.2 (3) The double barium tantalum fluoride, 3BaF2.2TaF5, is very strongly heated in a platinum tube, one end of which is kept cold. ... 

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