Refractory Metals: Part Two

Abstract:

Tantalum is soluble in fused alkalis, insoluble in sulfuric, hydrochloric, and nitric acids, and soluble in hydrofluoric acid. It ignites in air to form tantalum pentoxide (sometimes referred to as tantalum (V) oxide or tantalic acid anhydride).
Tantalum belongs to the group of metals that includes vanadium and niobium. It occurs mainly in the mineral tantalite (see Columbite), FeTa2O6.

Alloys containing varying amounts of refractory metals are vital to virtually every major industry, including automotive, mining, aerospace, chemical and petroleum processing, electrical and electronics, medical electronics and prosthetics, metal processing, nuclear technology and ordnance. The principal areas of uses for the five refractory metals are shown in Table 1.

Applications W Mo Ta Nb Re
Electronics X X X X X
Alloying X X X X X
Nuclear Power X X   X  
Aerospace X X X X  
Chemicals/Catalysts X X X   X
Metal Cutting & Forming X X X    
Mechanical Parts X X      
Mining/Oil Drilling X        

Table 1: Major Uses of Refractory Metals

Tantalum, symbol Ta, is a white, ductile, malleable metallic element. Tantalum is one of the transition elements of the periodic table. The atomic number of tantalum is 73. Tantalum was first obtained in pure form in 1820 by the Swedish chemist Baron Jöns Jakob Berzelius, who heated potassium tantalifluoride, K2TaF7, with excess potassium.

Properties and Occurrence

Tantalum is soluble in fused alkalis, insoluble in sulfuric, hydrochloric, and nitric acids, and soluble in hydrofluoric acid. It ignites in air to form tantalum pentoxide (sometimes referred to as tantalum (V) oxide or tantalic acid anhydride), Ta2O5, a white infusible substance that combines with metallic oxides or hydroxides to form compounds called tantalites.

Tantalic acid (also know as tantalum (V) oxide hydrate), Ta2O5 . xH2O, is a gelatinous precipitate that has a varying number of water molecules associated with each tantalum pentoxide molecule. Tantalum melts at about 2996°C (about 5425°F), boils at about 5425°C (about 9797°F), and has a specific gravity of 16.6. The atomic weight of tantalum is 180.948.

Tantalum belongs to the group of metals that includes vanadium and niobium. It occurs mainly in the mineral tantalite (see Columbite), FeTa2O6. Tantalum ranks about 53rd among the elements in natural abundance in the earth's crust. Principal deposits of the metal occur in Australia and Scandinavia. In the United States, small deposits are found associated with the pegmatite veins of the Black Hills of South Dakota.

Most tantalum minerals contain some niobium metal, which is separated by solvent extraction or selective-crystallization procedures. Commercially, tantalum is prepared by the electrolysis of fused potassium tantalifluoride or of tantalum compounds dissolved in dilute sulfuric acid.

Titanium, symbol Ti, silver-white metallic element used principally to make light, strong alloys. Titanium is one of the transition elements of the periodic table. The atomic number of titanium is 22.

Titanium was discovered in 1791 in the mineral menachanite by the British clergyman William Gregor, who named the new element menachite. Four years later, the German chemist Martin Heinrich Klaproth rediscovered the element in the mineral rutile and named it titanium in allusion to the strength of the mythological Greek Titans. The metal was isolated in 1910.

Properties and Occurrence

Pure titanium is soluble in concentrated acids, such as sulfuric and hydrofluoric acids, and insoluble in water. The metal is extremely brittle when cold, but is readily malleable and ductile at a low red heat. Titanium melts at about 1660°C (about 3020°F), boils at about 3287°C (about 5949°F), and has a specific gravity of 4.5. The atomic weight of titanium is 47.88.

Titanium burns in oxygen at 610°C (1130°F) to form titanium dioxide, and it burns in nitrogen at 800°C (1472°F) to form titanium nitride, TiN. Titanium exhibits valences of 4, 3, and 2, and forms the salts titanium tetrachloride, TiCl4; titanium trichloride, TiCl3; and titanium dichloride, TiCl2. It ranks ninth in abundance among the elements in the crust of the earth but is never found in the pure state. It occurs as an oxide in the minerals ilmenite, FeTiO3; rutile, TiO2; and sphene, CaO • TiO2 • SiO2.

To obtain titanium oxide, the mineral is ground and mixed with potassium carbonate and aqueous hydrofluoric acid to yield potassium fluorotitanate, K2TiF6. The fluorotitanate is extracted with hot water and decomposed with ammonia. The resulting ammoniacal hydrated oxide, when ignited in a platinum vessel, yields titanium dioxide, TiO2. Titanium is obtained in the pure form by first treating the oxide with chlorine to form titanium tetrachloride, a volatile liquid, and then reducing the liquid with magnesium in a closed iron chamber to yield metallic titanium. The metal is then melted and cast into ingots.

Tungsten, symbol W (from the earlier name, wolfram), is a metallic element that has the highest melting point of any metal. Tungsten is one of the transition elements of the periodic table. The atomic number of tungsten is 74.

Some credit the Swedish chemist Carl Wilhelm Scheele with the discovery of tungsten in 1781, while others name the Spanish D'Elhuyar brothers Juan José and Fausto as its discoverers, in 1783. Pure tungsten is silver-white in color and is ductile; the more easily obtained impure form is steel-gray and is hard and brittle. Tungsten is insoluble in hot and cold water and in alcohol, slightly soluble in ammonia and nitric acid, and soluble in hot, concentrated potassium hydroxide. Tungsten melts at about 3410°C (6170°F), boils at about 5660°C (10,220°F), and has a specific gravity of 19.5. The atomic weight of tungsten is 183.85.

Tungsten ranks 57th in abundance among the elements in the earth’s crust. It is never found freely in nature, but occurs in combination with other metals, notably in the minerals scheelite and wolframite, which are the important tungsten ores. Mines in South Korea, Portugal, Austria, and Australia produce more than half of the world's supply of these ores.

To separate the element from its ore, the ore is first fused with sodium carbonate to yield sodium tungstate, Na2WO4. The soluble sodium tungstate is then extracted with hot water and treated with hydrochloric acid to yield tungstic acid, H2WO4. The latter compound is washed and dried to produce the oxide WO3, which is reduced by hydrogen in an electric furnace. The resulting fine powder is reheated in molds in an atmosphere of hydrogen and pressed into bars, which are hammered and rolled at high temperature to compact them and make them ductile.



Figure 1: Refractory metal products

Screw connections made of refractory metals and refractory metal fasteners are used in many applications because of their corrosion resistance as well as their high-temperature properties. Examples include high-temperature furnace construction, the chemical industry, medical technology and aerospace.



Figure 2: Fasteners from refractory metals

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