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Scandium

[skan-dee-uhm]

Scandium is a chemical element that has the symbol Sc and atomic number 21. A silvery white metal that is always present as compounds, scandium ores occur as rare minerals from Scandinavia and elsewhere, and it is sometimes considered along with yttrium, and the lanthanides and actinides, to be a rare earth element.

Notable characteristics

Scandium is a rare, hard, silvery, rough very dark metallic element that develops a slightly yellowish or pinkish cast when exposed to air. It is not resistant to weathering when pure and is destroyed on prolonged contact with most dilute acids. However, like some other reactive metals, this metal is not attacked by a 1:1 mixture of nitric acid (H N O₃) and hydrofluoric acid, H F.

The rarity of scandium is not an arbitrary fact. The thermonuclear reactions that produce the elements in this range of atomic numbers tend to produce much greater quantities of elements with an even atomic number. These elements were usually produced by the fusion of lighter elements with helium-4 nuclei, starting with carbon-12 (element six). Thus, the common elements in the range of scandium are atomic numbers 18 (argon), 20 (calcium), 22 (titanium), and 24 (chromium); with elements with odd atomic numbers 19 (potassium), 21 (scandium), and 23 (vanadium) being rarely produced, and thus much less common. The production of the odd-numbered elements in this range result from much less common thermonuclear reactions, as is explained elsewhere.

Applications

Since it is not a very common metal, scandium does not have many applications. If it were more common, it might be useful in the making of aircraft and spacecraft structures, probably alloyed with other metals.

It is used in lacrosse sticks; a light yet strong metal is needed for precise accuracy and speed. Backcountry tent manufacturers sometimes use scandium alloys in tent poles. U.S. gunmaker Smith & Wesson produces a few variations including a large, medium, and small lightweight revolver with a frame composed of scandium alloy and a titanium cylinder.

Approximately 20 kg (as Sc₂O₃) of scandium is used annually in the United States to make high-intensity lights. Scandium iodide added to mercury-vapor lamps produces an efficient artificial light source that resembles sunlight, and which allows good color-reproduction with TV cameras. About 80 kg of scandium is used in light bulbs globally per year. The radioactive isotope Sc-46 is used in oil refineries as a tracing agent.

The main application of scandium by weight is in aluminium-scandium alloys for minor aerospace industry components, and for unusual designs sports equipment (bikes, golf clubs, baseball bats, firearms, etc.) which rely on high performance materials. However, titanium, being much more common, and similar in lightness and strength, is much more widely used, with tons found in some aircraft, especially military ones.

When added to aluminium, scandium substantially lowers the rate of recrystallization and associated grain-growth in weld heat-affected zones. Aluminium, being a face-centred-cubic metal, is not particularly subject to the strengthening effects of the decrease in grain diameter. However, the presence of fine dispersions of Al₃Sc does increase strength by a small measure, much as any other precipitate system in aluminium alloys. It is added to aluminium alloys primarily to control that otherwise excessive grain growth in the heat-affected zone of weldable structural aluminium alloys, which gives two knock-on effects; greater strengthening via finer precipitation of other alloying elements and by reducing the precipitate-free zones that normally exist at the grain boundaries of age-hardening aluminium alloys.

The original use of scandium-aluminium alloys was in the nose cones of some USSR submarine-launched ballistic missiles (SLBMs). The strength of the resulting nose cone was enough to enable it to pierce the ice-cap without damage, and so, enabling a missile launch while still submerged under the Arctic ice cap.

Scandium triflate is a catalytic Lewis acid used in organic chemistry.

History

Dmitri Mendeleev used his periodic law, in 1869, to predict the existence of, and some properties of, three unknown elements, including one he called ekaboron .

Lars Fredrik Nilson and his team, apparently unaware of that prediction in the spring of 1879, were looking for rare earth metals. By using spectral analysis, they found a new element within the minerals euxenite and gadolinite. They named it scandium, from the Latin Scandia meaning "Scandinavia", and in the process of isolating the scandium, they processed 10 kilograms of euxenite, producing about 2.0 grams of a very pure scandium oxide (Sc₂O₃).

Per Teodor Cleve of Sweden concluded that scandium corresponded well to the hoped-for ekaboron, and he notified Mendeleev of this in August.

Fischer, Brunger, and Grieneisen prepared metallic scandium for the first time in 1937, by electrolysis of a eutectic melt of potassium, lithium, and scandium chlorides at a temperature of 700 to 800°C. Tungsten wires in a pool of liquid zinc were the electrodes in a graphite crucible. The first pound of 99% pure scandium metal was not produced until 1960.

Occurrence

Scandium is distributed sparsely on earth, occurring only as trace quantities in many minerals. Rare minerals from Scandinavia and Madagascar, such as thortveitite, euxenite, and gadolinite are the only known concentrated sources of this element (which is never found as a free metal). It is also found in residues that remain after tungsten is extracted from wolframite, and from ores after uranium and thorium have been extracted.

Scandium is more common in the sun and certain stars than on Earth. Scandium is only the 50th most common element on earth (35th most abundant in the Earth's crust), but it is the 23rd most common element in the sun.

The blue color of the aquamarine variety of beryl is thought to be caused by scandium impurities in it.

Isolation

Thortveitite and kolbeckite are the primary mineral sources of scandium. Uranium-mill tailings by-products also are an important source. Pure scandium is commercially produced by reducing scandium fluoride with metallic calcium.

Scandium market

World production of scandium is in the order of 2,000 kg per year, generally as a by-product of uranium and nickel-cobalt-copper or platinum group element mining. Consumption is in the order of 5,000 kg, and typically is consumed in bicycle frames in Sc-Al alloys. The present main source of scandium metal to meet this shortfall is from the military stockpiles of the former Soviet Union (mainly in the country of Ukraine), which were extracted from uranium tailings. There is no primary production in the Americas, Europe, or Australia, although gigantic scandium deposits are associated with uranium, nickel-copper-cobalt laterite deposits, and ultramafic rocks worldwide.

Scandium can also be extracted from tantalum residues, tungsten processing wastes, tin slags, and a variety of other such industrial waste streams, and it is sometimes recovered from rare earth ores, particularly the rare earth oxide deposits of Bayan Obo, China.

The strength and commerciality of the scandium market is yet to be demonstrated as it is a specialty metal and a single producer could corner the supply with minimal tonnage production. The price in 2006 of 99.0% scandium oxide is of order of US$700 per kilogram

Compounds

The most common oxidation state of scandium in is +3. Scandium chemically resembles yttrium and the rare earth metals more than it resembles aluminium or titanium. Thus scandium is sometimes seen as the scandium oxide, Sc₂O₃, and as scandium chloride, ScCl₃.

Isotopes

Naturally occurring scandium is composed of one stable isotope ⁴⁵Sc. 13 radioisotopes have been characterized with the most stable being ⁴⁶Sc with a half-life of 83.8 days, ⁴⁷Sc with a half-life of 3.35 days, and ⁴⁸Sc with a half-life of 43.7 hours. All of the remaining radioactive isotopes have half lives that are less than 4 hours, and the majority of these have half-lives that are less than 2 minutes. This element also has 5 meta states with the most stable being ^44mSc (t_½ 58.6 h).

The isotopes of scandium range in atomic weight from 40 u (⁴⁰Sc) to 54 u (⁵⁴Sc). The primary decay mode at masses lower than the only stable isotope, ⁴⁵Sc, is electron capture, and the primary mode at masses above it is beta emission. The primary decay products at atomic weights below ⁴⁵Sc are calcium isotopes and the primary products from higher atomic weights are titanium isotopes.