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Titanium

A hard silvery-grey metal, titanium is found in nature as a compound within many minerals, but most notably in ilmenite (FeTiO3) and rutile (TiO2), more commonly found in sand or soil than hard rock. Other titanium-bearing minerals include perovskite, titanite, anatase and brookite. Major deposits of titanium minerals are found in Australia, Canada, India, Norway, South Africa, the Ukraine and the USA.

Titanium is particularly valued for its low density combined with high strength and excellent corrosion resistance. Pure titanium can reach an ultimate tensile strength of up to 740 N/sq mm, and an alloy like LT 33, containing aluminium, vanadium and tin, reaches 1,200 N/sq mm. The metal's coefficient of thermal expansion is around half that of stainless steel and copper, and one-third that of aluminium. Its density is around 60% of steel's, half that of copper's and 1.7 times aluminium's. Its modulus of elasticity is half that of stainless steel, making it durable and shock resistant.

The aerospace industry is still the largest single consumer of the metal. Titanium alloys capable of operating at temperatures from sub-zero to 600°C are used in aero­engines for discs, blades, shafts and casings. High-strength alloys are used in a wide variety of airframe applications -from small fasteners, weighing a few grammes, to landing gear trucks and large wing beams, weighing up to 1 tonne. Titanium can account for 10% of the unloaded weight of some commercial passenger aircraft.

Most titanium is actually consumed in the form of titanium dioxide - a non-toxic white pigment used in paints, paper, plastic and cosmetics.

Origins

Although the existence of titanium minerals has been known for over 200 years, commercial production of titanium metal and titanium dioxide pigment did not begin in earnest until the 1940s. W.J. Kroli patented a method for producing titanium metal via the carbo-chlorination of titanium dioxide in 1938. The element was originally named after the Titans of Greek mythology by German chemist M.H. Klaproth, who successfully separated titanium dioxide from rutile at the end of the eighteenth century.

The US Geological Survey (USGS) estimates that world mine production of ilmenite in 2004 totalled 4.8m tonnes, while world mine production of rutile totalled 400,000 tonnes. Ilmenite supplies about 90% of the world's demand for titanium minerals. World resources of anatase, rutile and ilmenite total more than 2bn tonnes by USGS estimates.

Production

The first step towards production of titanium metal is usually to make sponge by the chlorination of rutile ore (see flow diagram). Chlorine and coke are combined with rutile to produce titanium tetrachloride, which is then reacted with magnesium in a closed system to produce titanium sponge and magnesium chloride. The magnesium and magnesium chloride are removed for recycling by using a vacuum distillation process or the Kroll leach process. The USA, Russia, Kazakhstan, Ukraine, Japan and China are significant producers of titanium sponge.

A vacuum arc reduction (VAR) or electron beam cold hearth furnace is used to melt the sponge with scrap and/or alloying elements such as vanadium, aluminium, molybdenum, tin and zirconium, to produce remelt electrodes, which can be VAR melted to produce the strict specifications of aerospace and other high-technology applications, or directly cast into slabs.

VAR ingots are cylindrical and can weigh as much as 17,500 Ib (7.94 tonnes). They are forged to make slab or billet, or are used for investment casting stock. Roiling produces plate, sheet, bar, rod and wire. Tube and pipe are formed from strip cut from sheet.

Applications

In daily life titanium is most commonly associated with high-value products such as wristwatches, spectacle frames, sports goods and jewellery but, apart from its widespread use in aircraft, it has many other applications where its combination of physical properties and biocompatibility wins overbther materials. Depending on the exact nature of the application, titanium competes with nickel, stainless steel and zirconium alloys.

The automotive sector shows promising signs of growth. In suspension systems, for example, replacement of steel springs by titanium achieves a 60% weight advantage. Other applications include crankshafts, connecting rods and exhaust systems. Power plants and seawater desalination plants also continue to be important areas of growth for the metal while, for computers, titanium substrates for hard disk drives are under development.