Separation of Niobium and Tantalum

Marignac s fractional crystallization is slow. Solvent-extraction methods are faster and much more effective. Concentrates containing niobium and tantalum are dissolved in a mixture of hydrofluoric and sulfuric acids. Extraction with isobutyl ketone takes both metals to the organic phase. Niobium and tantalum can be extracted selectively from the ketone phase, niobium with sulfuric add, tantalum with ammonium fluoride solution [22.2]. 

Surprisingly, niobium can be obtained using carbon reduction [22.3]. In a first reaction between niobium oxide and carbon, niobium carbide NbC is formed. This is ground together with a carefully calculated amount of Nb20j. The mixture is com- 

Other industrial methods are reduction of NbClj or K NbF with sodium. 

Ferroniobium is manufactured using the aluminothermic method described for ferrovanadium. The alloy is used as a furnace addition to give steel a specified niobium content. Brazil is the world s biggest producer of ferroniobium. An aluminothermic charge can be composed of 18 tonnes pyrochlore, 4 tonnes hematite, 6 tonnes aluminum powder and 0.5-0.75 tonnes each of lime and fluorspar. From the reaction an 11-tonnes ingot with 65-70% Nb is obtained. 

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Marignac 3 also showed that previous methods for the separation of niobium and tantalum were far from perfect, and for the first time he succeeded in preparing pure niobium and tantalum compounds. His methods are still in use to some extent, and his analyses provided the first reliable values for the atomic weights of these elements. It should be stated, too, that Rose s earlier researches, which extended over a period of nearly twenty years, have provided a valuable source of information for the chemistry of niobium and tantalum.4 His calculations and formulae were revised by Rammelsberg 5 in the light of subsequent discoveries. 

The use of solvating extractants in the recovery of gold and platinum-group metals (PGM) was described in the previous section. These extractants have also found some specialized applications in the extractive metallurgy of base metals. For example, they have been used in the recovery of uranium, the separation of zirconium and hafnium, the separation of niobium and tantalum, the removal of iron from solutions of cobalt and nickel chlorides, and in the separation of the rare-earth metals from one another. 

The separation of niobium and tantalum by solvent extraction has been carried out on a commercial scale for about 30 years, and has totally superseded the previously used Marignac process based on the fractional crystallization of K2TaF7 and KiNbOFvHiO from dilute hydrofluoric acid.317-318... 

Several techniques have been suggested for the separation of niobium and tantalum by ion-exchange, but it does not seem possible for any of them to... 

The separation of niobium from tantalum tluough the gaseous chlorides is carried out at higher temperature, about 900 K, and it is therefore to be expected, as is the case, that the thermodynamic data will provide a useful guide. These metals form a number of chlorides, mainly the ui- tetra- and pentachlorides. These latter are much more volatile than the tetrahalides, and the exchange reaction at 900 K... 

Precipitation of niobium and tantalum compounds in forms that are suitable for separation from the aqueous media (liquid-solid interaction) ... 

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. 

Separation of niobium from tantalum and impurity metals is the most important step in its extraction from the ore. It may be achieved by several methods that include solvent extraction, ion exchange, fractional crystallization, fractional sublimation, and other techniques. Solvent extraction is apphed mostly in several large-scale commercial processes. Although the classical fractional crystalhzation method forms effective separation, it is a tedious... 

Ruff and Schiller 8 determined the solubilities of the double fluorides of niobium and tantalum, K2NbF7 and KaTaF7, in varying quantities of hydrofluoric add and potassium fluoride, and based a method of fractional separation on the results, which showed that the solubility of both fluorides diminishes with increasing concentration of potassium fluoride and decreasing concentration of hydrofluoric acid the solubility increases rapidly with rising temperature, and is always... 

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... 

Gravimetric Methods.—The mixture of pentoxides of niobium and tantalum is redissolved in concentrated hydrofluoric acid and separated by Marignac s process (see p. 128) the potassium tantalum fluoride and potassium niobium oxyfluoride are then separately converted into the pentoxides as described above, and -weighed.1 This method has several disadvantages (1) The ratio of the solubilities of the two compounds on which the separation is based is only approximately 10 1, and the process is, therefore, necessarily inaccurate even when the recrystaflisation is repeated to a tedious extent the error approaches 1 per cent. (2) The concentration of the hydrofluoric add and of the potassium fluoride must be carefully controlled if the acidity is too low, an oxyfluoride of tantalum is precipitated if the acidity is too great, a normal fluoride of niobium is obtained. (3) Several platinum dishes are necessary. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

Production of niobium and tantalum is on a smaller scale and the processes involved are varied and complicated. Alkali fusion, or digestion of the ore with acids can be used to solubilize the metals, which can then be separated from each other. The process originally developed by... 

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