Niobium and Tantalum
This article describes in brief the history of niobium and tantalum from mining to metal, the minerals concentration process, and separation and purification of niobium and tantalum from the mineral concentrates by important solvent extraction processes practiced in industries using different extractants.
Niobium is an important microalloying element in steel. NIobium is used in nickel-, cobalt-, and iron-base superalloys which have been used in jet engines components, rocket sub-assemblies, and heat resisting and combustion equipments.
Tantalum is highly corrosive resistant and refractory metal like niobium and also finds use as a minor component in alloys. Being chemically inert to a great extent it is considered to be a substitute for platinum. Its main use is in capacitors..
These interesting elements are chemically similar and are associated with each other in nature. It is, therefore, very interesting to trace the history of these elements. Being chemically similar it is also interesting to know how these elements are separated from each other. This amusing story of these elements has been narrated in this article in brief.
Niobium was discovered by Charles Hatchett, an English chemist (1765-1847), in 1801. .Charles Hatchett analyzed the specimen of hitherto unknown mineral from the collection of the British Museum in London. This mineral was obtained from the collection of the first governor .of Connecticut, John Winthrop. John Winthrop was an alchemist, physician, and rock collector. This mineral was found near New London, Connecticut. Later this mineral was called columbite. Charles Hatchett, based on the analysis, concluded that this mineral contained a new element and he termed that element as Columbium, as the source of this mineral at that time was America.
Tantalum was discovered by Anders Gustaf Ekeberg (1767-1813) in 1802. He investigated two minerals, one from Ytterby, Sweden and the other from Kimito, Finland. The mineral from Sweden became known as yttrotantalite. He discovered that both these minerals were containing the same but hitherto .unknown element. He named this new element Tantalum after Tantalus, the son of Jupiter. The separation of this new element from the mineral was a tantalizing experience. Hence, the name Tantalum is a befitting name for this new element.
William Hyde Wollaston, a British chemist, in 1809 analysed columbite and tantalite minerals and concluded that both the columbium and tantalum are the same element. However, in 1844 Heinrich Rose could establish that these are two different elements. He distinguished these two elements by their differences in valence states. Columbium exhibited trivalent and pentavalent states while tantalum exhibited only pentavalent state. He changed the name of columbium as niobium after Niobe, the daughter of Tantalus.. .
Separaton of niobium from tantalum was very difficult due to the chemical similarities of their oxides. Ultimately after 65 years since the discovery of niobium Marignac in 1866 developed a process for the separation of niobium from tantalum. In 1864 Blomstrand prepared the niobium metal by the reduction of its salt.
The names columbium and niobium,were being used for the same element for about a century. In 1949 the International Union of Pure and Applied Chemistry (IUPAC) officially adopted the name niobium for this element. Some metallurgists still use the name columbium. Old habits just continue
Niobium is found in niobite (columbite), niobite-tantalite, pyrochlore, and euxenite. Large deposits of niobium have been found associated with carbonatites (carbon-silicate rocks) as a constituent of pyrochlore. Extensive ore reserves are found in Canada, Brazil, Nigeria, Zaire, and in Russia.
Tantalum ores are found primarily in Australia, Canada, Brazil, and central Africa, with some additional quantities originating in Southeast Asia and in China. There is also interest in exploration of this element in various regions of the world, such as Canada, Egypt and Saudi Arabia.
Most of the niobium and tantalum deposits are in specific belts in Africa and South America .
Pegmatite, a very coarse grained ( grain size 20 mm or more) igneous hard rock formed by cooling of magma (molten rock), is the host of tantalite minerals. In fact pegmatite contains more than 550 different minerals. It is a dominant source of tantalum In pegmatite tantalum content is in the range of 0.02 to 0.1%.and niobium around 0.01 %. Tantalum minerals of various chemical compositions are known, It is pertinent to note that, in general, any tantalum mineral concentrate is termed as tantalite.Tantalum mineral concentrate may contain sometimes even more than five different tantalum bearing minerals. In some deposits tantalum is a main product of tantalum mining but many a times tantalum is a by-product of niobium mining.
Minerals (Pyrochlore, Columbite-Tantalite) Concentration Process
Pyrochlore mineral, the main source of niobium, and columbite-tantalite mineral, the main source of tantalum are separated from the naturally occurring ore deposits to produce mineral concentrates of pyrochlore and columbite-tantalite respectively by physical beneficiation and separation techniques..Physical beneficiation is based on the differences in physical properties of different minerals to achieve the separation of desired minerals from the rest of the minerals. The common steps involved in the separation of pyrochlore and columbite-tantalite minerals from the ore deposits are the mining (open pit or underground), crushing and grinding. In the case of pyrochlore further steps involved after grinding are sizing and desliming, flotation, and removal of impurities to obtain pyrochlore concentrate. In the case of columbite-tantalite, after grinding coarse particles containing columbite are separated from fine particles containing tantalite, Further by gravity separation columbite mineral concentrate is obtained from coarse particles, and tantalite mineral concentrate is obtained.from fine particles.The concentrates of minerals thus produced at or near the mine site are transported to the processors' works for further chemical processing.
The single largest source of tantalum mineral concentrates is the production by Sons of Gwalia Ltd. from its Greenbushes and Wodgina mines in Western Australia. Overview of this plant at Wodgina is depicted below.. These two mines combined produce over 50% of global demand.
The main source of niobium was columbite-tantalite in the sixties. However, now pyrochlore is the major source of niobium. Blomstrand prepared the metal niobium for the first time in 1864 by reducing the niobium chloride by heating in a hydrogen atmosphere. Marignac process developed in 1866 is considered to be the first industrial process fo separate niobium from tantalum..This was based on the difference in solubility of potassium double fluoride salts of tantalum and niobium. The potassium niobium oxyfluoride, K2NbOF5 has very high solubility in comparison to the potassium tantalum fluoride, K2TaF7 . K2NbF7 is not formed in this process as it is stable only in strong hydrofluoric acid..The purity of tantalum produced by this route of crystallization was very satisfactory. However, niobium produced by this route was not high due to the presence of titanium in the mineral concentrate..
Scrap recycling generated within the various segments of the tantalum industry accounts for about 20 to 25% of the total input each year.
In 1950 U.S. Bureau of Mines and Ames Laboratories of Iowa State University developed the solvent extraction (SX) process using the extractant methyl iso-butyl ketone (MIBK) for the separation of niobium and tantalum. Marignac process was then abandoned in favor of SX process as SX process could produce high purity niobium and tantalum.
Solvent Extraction Process (MIBK - HF / H2SO4 System) for the Extraction and Separation of Niobium and Tantalum
Raw material concentrate (columbite - tantalite, pyrochlore, tin slag with high tantalum content) is finely ground and dissolved in hydrofluoric acid (HF) and then adjusted the acidity to greater than 8M with H2SO4. Iron, manganese, titanium get dissolved along with niobium and tantalum, Other impurities in the concentrate such as silica. calcium, rare earths, aluminum etc. remain insoluble and are removed by filtration. The solution containing tantalum and niobium along with some impurities is subjected to SX treatment using the extractant MIBK. Both niobium and tantalum extract at high concentration of H2SO4 (>8N), but only tantalum extracts at lower acidity (3N-8N). Initially niobium and tantalum are extracted together in the organic phase (MIBK) at .>8N H2SO4. Under these conditions most of the impurities such as iron, manganese , titanium, etc. remain in the aqueous phase. Organic phase (MIBK) containing niobium and tantalum is then brought into contact with fresh aqueous phase containing less <8N (preferably around 3N) H2SO4. Under this condition only niobium is back extracted in the aqueous phase keeping tantalum in the organic phase. The back extracted aqueous niobium is again re-extracted with MIBK to remove traces of tantalum (i.e., to re-extract traces of tantalum from niobium). Then ammonia is added to the aqueous solution containing pure niobium to precipitate niobium oxide hydrate. Oxide hydrate of niobium is then separated by filtration, dried and calcined in heated chambers or rotary furnaces.. Niobium oxide thus obtained is of high purity. This SX process is a counter-current process Now-a-days SX process is carried out using mixer-settlers.
Solvent Extraction Processes (Cyclohexanone / Tri-n-Butyl Phosphate - HF - H2SO4 Systems)
Cyclohexanone and tri-n-butyl phosphate (TBP) are the other extractants that have been used in industries for the extraction and separation of niobium and tantalum. The basic system of extraction is the same as that of the extractant MIBK.. Niobium and tantalum are extracted from the aqueous solution containing HF and H2SO4. Only the parameters such as concentrations of HF and H2SO4., and aqueous to organic volume ratios are slightly different from that of MIBK system. However, MIBK system is most popular in industries in various countries though in Russia sometimes cyclohexanone is used and in India TBP is used for the extraction and separation of these elements. MIBK and cyclohexanone are cheap extractants compared to TBP. However, the solubility of TBP in aqueous (0.5 vol%) is less than MIBK (2 vol%) and cyclohexanone (16 vol%). Similarly the boiling point of TBP (1780 C) is higher than cyclohexanone (1550 C) and MIBK (1160 C). These favorable properties of TBP outweighs the cost of TBP. But the most important and vital factor is that the purity of niobium and tantalum obtained using MIBK is far greater than that obtained using TBP and cyclohexanone. Hence, MIBK is preferred to TBP and cyclohexanone.
It is pertinenet to note that in all these systems (MIBK, TBP, and cyclohexanone) the required concentrations of HF(>2N) and H2SO4.(>8N) for the extraction of niobium and tantalum are very high. Extractants are also amenable to degradation due to high concentrations of acids, in particular HF. These are some of the drawbacks of these processes.
Novel Solvent Extraction Process for the Extraction of Niobium and Tantalum
Author of this knol has standardized a process for the industrial use for the extraction and purification of niobium and tantalum. This method requires much less concentrations of HF and H2SO4.. Moreover the purity of noibium and tantalum produced by this method is far greater than even MIBK The extractant used in this process is not degraded as in the case of TBP and other extractants. The losses of the extractant during the process are expected to be very low compared to MIBK, TBP, and cyclohexanone due to various reasons. . This process is much less hazardous too. In this process the drawbacks of the MIBK, TBP and cyclohexanone processes have been minimized. Interested readers may contact the author.
Following are the few pictures of pure niobium and tantalum metals.
Niobium and tantalum are like twins and it was an herculean task to separate them from each other. This is due to their nearly identical atomic radii, and hence, are virtually identical from a separation point of view. Scientists took 65 years to separate them from each other by fractional crystallization (Marignac process) and that too separation was not perfect. Then after about 85 years niobium and tantalum could be perfectly separated from each other (high purity) by (SX) process (U.S. Bureau of Mines and Ames Laboratories of Iowa State University). There are certain pairs of elements like cobalt-nickel, zirconium-hafnium, holmium-erbium, and praseodymium-neodymium which are difficult to separate from each other. SX and Ion Exchange processes could only separate them with a high degree of purity.
- Nicolas Dekun, economic Geology; May 1962; v. 57; no. 3; p. 377-404; DOI: 10.2113/gsecongeo.57.3.377.
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