Niobium purification

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Once purification of the niobium has been effected, the niobium can be reduced to the metallic form. The double fluoride salt with potassium, K2NbFy, can be reduced using sodium metal. The reaction is carried out in a cylindrical iron vessel filled with alternating layers of K NbF and oxygen-free sodium ... 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

Two main methods exist for the production of tantalum and niobium from the mineral raw material. The first method is based on the chlorination of raw material, followed by separation and purification by distillation of tantalum and niobium in the form of pentachlorides, TaCl5 and NbCl5 [24, 29]. Boiling points of tantalum and niobium pentachlorides (236°C and 248°C, respectively) are relatively low and are far enough apart to enable separation by distillation. 

The next technological step is the purification and separation of tantalum and niobium in the form of complex fluoride compounds of tantalum and niobium. This process is performed using liquid-liquid extraction, using appropriate organic solvents. This technological step requires knowledge on... 

Solution purification and tantalum - niobium separation by liquid - liquid extraction... 

The filtrated solution can be successfully used for the precipitation of potassium heptafluoroniobate, K2NbF7 or niobium hydroxide. In addition, the solution is suitable for further purification by liquid-liquid extraction after adjustment of its acidity [118, 122]. 

Two main schemes exist for the separation and purification of tantalum and niobium using liquid-liquid extraction. The first is based on the collective extraction of tantalum and niobium from an initial solution into an organic phase so as to separate them from impurities that remain in the aqueous media, the raffinate. The separation of tantalum and niobium is subsequently performed by fractional stripping into two different aqueous solutions. In this case, stripping of niobium is performed using relatively weak acids prior to the stripping of tantalum. Fig. 125 presents a flow chart of the process. 

The second process of selective extraction is more effective and leads to better separation of tantalum and niobium and to more effective purification. Its performance, however, requires the initial solution to be of relatively low acidity. 

It is recommended that the concentration of sulfuric acid in the initial solution be kept at 2-4 mol per liter for the extraction of tantalum, whereas for the extraction of niobium, the concentration of sulfuric acid must be increased to a minimum of 6 mol per liter [458,481]. In some cases, the presence, in the initial solution, of titanium in the form of fluorotitanic acid ensures the successful and selective extraction and purification of tantalum and niobium with no addition of any other mineral acid [482]. 

Optimal parameters for the extraction, washing and stripping of niobium were determined to be number of stages for all three processes - 4, volumetric ratios Vorg Vaqu are 1 1, 20 1 and 8 1, respectively. Additional fine purification of the extractant was recommended by stripping of tantalum and niobium remainders using a 0.5% wt. ammonia solution. This additional stripping leads to final concentrations of both tantalum and niobium in the extractant that are < 0.001 g/1. Table 62 shows the purity of niobium oxide prepared by the described method. 

The precipitated precursor can be dissolved and re-crystallized from fluorine-free solutions. This provides excellent conditions for deep purification of the material and reduction of problematic impurities such as titanium, fluorine, etc. Peroxometalates decompose at relatively low temperatures forming tantalum or niobium oxides containing small amount of absorbed water. The absorbed water separation is achieved by further thermal treatment - drying and calcination - of the product ... 

Rare earth metals, as well as alkali earth metals, can be used as oxygen getters in the purification of tantalum powder. Osaku and Komukai [608] developed a method for the production of tantalum and niobium metal powder by a two-step reduction of their oxides. The second step was aimed at reducing the oxygen content and was performed by thermal treatment with the addition of rare metals. The powder obtained by the described method is uniform, had a low oxygen level and was suitable for application in the manufacturing of tantalum capacitors. 

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. 

There are several processes for extracting and refining niobium from its ores. (Payton, P.H. 1981. Niobium and Niobium Compounds. In Kirk-Othmer Encyclopedia of Chemical Technology, 3 . ed., Vol, 15, pp. 820-827. New York Wiley Interscience). The process of choice depends on nature of the ore and end use intended for the metal. Some common steps in these recovery processes involve ore preconcentration, breaking or opening the ore, obtaining pure niobium compounds, reduction of niobium compounds to niobium metal, purification or refining metal and fabrication. If niobium is extracted from a niobium-tantalum ore, the most important step is separation of niobium from tantalum, both of which are chemically very similar. 

Preparation of Niobium (Tantalum) Chloride and Purification from Iron Impurities. Assemble an apparatus as shown in Fig. 124. Place activated carbon preliminarily dried in vacuum during two hours at 200-250 Cinto section III of the reactor. Mix 2 g of niobium (tantalum) and 0.5 g of iron in a porcelain boat and place them in section I. Fill columns 5 and 6 with a mixture of glass wool and phosphoric anhydride, and fill wash bottle 7 with sulphuric acid. 

The method has also been examined for the production of other nonvolatile compounds. In these cases, the purification process involves subliming excess starting materials away from the product. For example, greater than 95 % yields of tungsten(IV) chloride from tungsten(VI) chloride and 85% yields of niobium-(IV) chloride from niobium(V) chloride can be obtained by using a 500-watt light bulb at 150°C. and a reaction period of 3 days. 

Niobium pentachloride decomposes readily in the presence of moisture thus it is very important in this synthesis that the acetonitrile be rigorously dried. The method described by Coetzee et al.1 is suitable for the purification of this solvent. The niobium pentachloride starting material should also be freed of any oxy species. This can be effected at about 100° by sublimation of the pentachloride away from the less volatile oxychlorides using a standard vacuum sublimer with a water-cooled probe. Appearance of commercial samples is a poor guide to their purity, and this purification step should not be omitted. The potassium thiocyanate should be purified by recrystallization and then thoroughly dried. 

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