Sodium tantalum fluorides

Neither calcium nor magnesium is used as the reducing agent in the production of tantalum by fluoride reduction. The intermediate used is potassium tantalum fluoride (K2TaF7), and the reducing agent is sodium. The choice of sodium is primarily guided by the need to produce tantalum in the powder form. The pertinent reaction is ... 

The best described case is the one of potassium tantalum fluoride reduction. This salt is currently obtained in the Marignac separation, from the potassium columbium oxyfluoride, by crystallization. The fully dried salt is mixed with clean sodium chips, pressed lightly in a steel bomb which is sealed and heated externally with gas. After the flash the cooled bomb is drilled out and the cake is treated with alcohol, water, and various acids. The powder obtained is processed to compact metal by pressing in bars and high vacuum sintering. 

Fio. 9.11, Extraction of tantalum (fluoride dissolution, re rystallization of double fluoride, sodium reduction process). 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

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. 

The sodium reduction process was first implemented in Germany by Siemens and Halske A.G. and it remained a principally European method, while the American tantalum industry was founded exclusively on the electrolysis of molten fluorides [28, 576]. The production of tantalum by sodium reduction has gained worldwide acceptance and currently, nearly 90% of the world s tantalum is produced by the sodium reduction method [538]. 

L.A. Kamenskaya Phys.-Chem. investigations of interactions between niobium and tantalum complex fluorides and oxyfluorides and potassium and sodium fluorides and chlorides in molten conditions. - Abstracts of Dissertation, Kiev, 1976 (in Russian). 

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. 

Fluorides are nonhygroscopic, and their melting points are higher than those of the corresponding chlorides. Besides, the fluoride reduction reactions are considerably more exothermic. The prime examples of the use of fluorides as intermediates are the reduction of uranium tetrafluoride by calcium or magnesium the reduction of rare earth fluorides by calcium, reduction of beryllium fluoride by magnesium and the reduction of potassium tantalum double fluoride by sodium. 

Silver difluoride, 0014 Silver fluoride, 0013 Sodium chloride, 4036 Sodium iodide, 4623 Tantalum pentachloride, 4185 Tellurium tetrabromide, 0296 Thallium, 4922 Tin(II) chloride, 4116 Tin(IV) chloride, 4174 Tin(II) fluoride, 4331 Titanium(II) chloride, 4117 Titanium dibromide, 0284 Titanium diiodide, 4630 Titanium tetrachloride, 4176 Titanium tetraiodide, 4638 Titanium trichloride, 4158... 

Bulk holmium metal is prepared by reduction of holmium chloride or fluoride by sodium, calcium, or magnesium in a tantalum crucible under argon atmosphere ... 

Tantalum can be separated from niobium by recrystallization of the double potassium fluorides. In the commercial process the ore is fused with caustic soda. The insoluble sodium niobate, sodium tantalate, and... 

Another example is the silicidizing of tantalum, basically an oxidation— reduction reaction. The packing is sodium fluoride and silicon. After deposition, the coating diffuses continuously into the substrate, according to the following reactions ... 

A method of separation which avoids the preparation of the double fluorides consists in fusing the mixed niobic and t an tali c acids with sodium carbonate and nitrate, the product is digested with warm water and a current of carbon dioxide is passed through the solution. It is claimed that only tantalic acid is precipitated.5 This process has, however, been the subject of adverse criticism.6 Partial separation of niobium from tantalum can be effected by warming the mixed, freshly precipitated, hydrated oxides with a mixture of hydrogen peroxide and hydrochloric acid the niobium dissolves readily, while the tantalum dissolves only sparingly.7... 

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