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Tantalum (Ta)

Name

  Tantalum

Symbol

 Ta
Atomic Number  73
Atomic Weight  180.9479(1)
Color  Grey Blue

General Information

Tantalum is a grayish silver, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which can be used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below 150°C, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulphur trioxide. The element has a melting point exceeded only by tungsten and rhenium.

ISOLATION

Here is a brief summary of the isolation of tantalum. Isolation of tantalum appears to be complicated. Tantalum minerals usually contain both niobium and tantalum. Since they are so similar chemically, it is difficult to separate them. Tantalum can be extracted from the ores by first fusing the ore with alkali, and then extracting the resultant mixture into hydrofluoric acid, HF. Current methodology involves the separation of tantalum from these acid solutions using a liquid-liquid extraction technique. In this process tantalum salts are extracted into the ketone MIBK (methyl isobutyl ketone, 4-methyl pentan-2-one). The niobium remains in the HF solution.

After conversion to the oxide, metallic tantalum can be made by reduction with sodium or carbon. Electrolysis of molten fluorides is also used.

Tantalum — Raw Materials and Processing

Until recently the majority of the world’s production of tantalum was from the discard slags of tin smelters. The tin mineral cassiterite is frequently associated with columbite/    tantalite ore, especially in Thailand, Australia, Brazil and Central Africa, and to a smaller extent in Malaysia. When the tin concentrates are smelted, the tin is reduced to metal, but the tantalum reports unreduced in the slag, from which it can be recovered by electric smelting and/or chemical extraction. Tin slags, particularly those from the Thaisarco smelter in Phuket, Thailand, used to be an important supplier of tantalum, but the decline of the tin industry in South East Asia since 1985 has meant the replacement of much of that source of tantalum by primary mining of tantalite.  

There are a number of mines now operating, two of the largest are open-cut operations in Western Australia (Green bushes and Wodgina). A minor source resulted from a boom in tantalum prices in 1979/80 which caused the excavation of very large tonnages of old tantalum-bearing tin slags in South East Asia, where they had been used as ground fill since early this century. Some of these are still supplying the processors’ needs but it is generally considered that stocks of them are now small. There is also a sizeable amount (about 25%) of recycling of scrap metal and of compounds of tantalum.  

Tantalum and niobium are extracted from their ores, after concentration, by chemical means rather than by smelting. The concentrates are attacked by HF/H2SO4 which brings the tantalum and niobium compounds into solution. The acid solution is mixed thoroughly with MIBK (methyl-iso-butyl ketone) which dissolves the tantalum and niobium compounds into the ketone while leaving impurities in the aqueous solution. The organic and inorganic solutions form separate layers and the organic (ketone) solution can be separated from the aqueous layer (liquid-liquid separation). The niobium is then stripped with dilute acid, and the tantalum subsequently extracted by acid ammonium fluoride. For tantalum, the metal is usually produced in powder form by sodium reduction of the fluoride. It can then be compacted (as it is for capacitors) to final shape, or may be melted (and refined) in an electron beam furnace.

Tantalum and Niobium — Dissimilar Twins?

In 1801 an American chemist named Hatchett was testing a heavy black mineral from Connecticut and discovered that it contained a new element which he named ‘Columbium’. A year later Eckberg in Sweden discovered two minerals each containing an oxide of an unknown element. This proved very difficult to dissolve in acids and frustrating to work with, so he named it ‘Tantalum’ after the Greek god Tantalus (who could not reach the water to drink or the apple to eat).

In 1844 the chemist Rose showed that another element was present in the Swedish mineral, and he named it ‘Niobium’ after Niobe, the daughter of Tantalus. Only in 1866 did Marignac develop a method of separating the two elements chemically by taking advantage of the difference in solubility of the two potassium double fluorides (a procedure used until quite recently in the manufacture of the metals). The European ‘Niobium’ was soon shown to be identical to the American ‘Columbium’ and for nearly a century arguments raged over which name had priority. Finally in 1950 the international chemical body, by a majority decision, settled for niobium but the old name columbium is still in common use in the Americas, complicated by the fact that one of the two most common minerals of niobium is universally known as ‘Columbite’. Of the two elements, niobium is far more abundant in the earth’s crust than tantalum; nevertheless they almost always occur together. This results from the great chemical similarity of their oxides (which gave those early chemists so much trouble), and from their very similar atomic radii, so that they freely replace each other in minerals. The columbite mentioned above is an iron or manganese niobate and there is an iron manganese tantalate known as tantalite. A full range of mixtures between the extremes exists, all naturally occurring.  

When the two elements were finally separated, and the metals produced, it was obvious that the similarity did not extend to all their physical properties. Niobium metal is very similar in density to iron, but tantalum is nearly twice as heavy. As a result of all these factors, and the relative abundance (and cheapness) of niobium, they have very different applications, but in some uses, in particular of the high purity metal and its alloys, there is some overlap. Dissimilar twins indeed!  

Applications of tantalum and niobium are based on their ability to form a non-conducting layer of oxide on the surface of the metal, a dense, stable and adhesive layer of pent oxide.

APPLICATION

For a brief period early this century, tantalum had a use in wire form as lamp filament, but the development of cheap tungsten wire put paid to this.  

The biggest single use now of tantalum (accounting for 50% of the total) is as the powdered metal, mostly for the production of capacitors. These depend on the dielectric properties of a tantalum oxide film on a tantalum compact, but the amount of tantalum required for a particular service is reduced as the metal purity is raised. Technical advances in tantalum manufacture and increasing knowledge of the effect of other parameters such as particle size and shape have enabled capacitor makers to make large increases in their production of units with little significant increase in the weight of tantalum consumed. Also important as applications of tantalum are tantalum carbide used in cutting tools (with titanium and tungsten carbides), and pure or alloyed tantalum metal, much of it for corrosion or heat resistant chemical plant equipment, or in super alloys for jet engines.

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