WikiPortallus: Scandium.html

Scandium is present in most of the rare earth element and uranium deposits, but it is extracted from these ores in only a few mines worldwide. Due to the low availability and the difficulties in the preparation of metallic scandium, which was first done in 1937, it took until the 1970s before applications for scandium were developed. The positive effects of scandium on aluminium alloys were discovered in the 1970s, and its use in such alloys remains the only major application of scandium.

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 from Scandinavia.³⁴ They named it scandium, from the Latin Scandia meaning "Scandinavia". The first attempt to isolate the scandium yielded only 250 mg of impure scandium oxide, but in the second attempt, they processed 10 kilograms of euxenite, producing about 2.0 grams of a very pure scandium oxide (Sc₂O₃).⁴

Per Teodor Cleve of Sweden processed 10 kilograms of gadolinite to search for the new element. He concluded that scandium corresponded well to the hoped-for ekaboron, and he notified Mendeleev of this in August.⁵ It took several years before William Crookes found a scandium concentration of 1% in the mineral Wiikite.⁶ 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.⁷ The first pound of 99% pure scandium metal was not produced until 1960. The use for aluminium alloys started after a patent was granted in 1971 in the United States. At the same time the Soviet Union also expanded the research on aluminium scandium alloys.⁸

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 dissolved on prolonged contact with most dilute acids. However, like some other reactive metals, this metal does not react with 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 are 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 of odd atomic numbers 19 (potassium), 21 (scandium), and 23 (vanadium) being rarely produced, and thus being much less common. The production of the odd-numbered elements in this range result from much less common thermonuclear reactions.⁹

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).¹⁰

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.¹⁰

Compounds

The most common oxidation state of scandium 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(III) oxide, Sc₂O₃, and as scandium(III) chloride, ScCl₃. Scandium chloride dissolves in a potassium chloride solution by forming K₃[ScCl₆. This shows that scandium prefers the coordination number six, similar to aluminium. In the compounds ScB and ScC, boron and carbon are incorporated non-stoichiometrically into the lattice of the scandium.¹¹ Alternative oxidation states of scandium are present in the hydrides scandium(II) hydride (ScH₂) and scandium(I) hydride (ScH). ScH₂ is stable at room temperature,² while ScH is only observable in high temperature gas phase spectroscopy.¹

Occurrence

Scandium is distributed sparsely on earth, occurring only as trace quantities in many minerals. The concentration in the upper continental crust is 7ppm and 25ppm in the lower continental crust.¹² Rare minerals from Scandinavia¹³ and Madagascar¹⁴ such as thortveitite, euxenite, and gadolinite are the only known concentrated sources of this element (which is never found as a free metal). The mineral thortveititecan contains 40 - 45 % of scandium(III) oxide.¹³

It is also found in residues that remain after tungsten is extracted from wolframite, and from ores after uranium and thorium have been extracted.¹⁵ 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.¹⁶

Production

World production of scandium is in the order of 2,000 kg per year as scandium oxide. The primary production is 400 kg while the rest is from stockpiles of Russia created during the Cold War. The production of metallic scandium is in the order of 10 kg per year.¹⁷¹⁸

In 2003 only three mines produced scandium oxide. The uranium and iron mines in Zhovti Vody in Ukraine, the rare earth mines in Bayan Obo, China and the apatite mines in the Kola peninsula, Russia, yield the scandium ore as a byproduct of mining activities for other minerals.¹⁸ The scandium from these mines is sold as scandium oxide, and only a small fraction is converted to scandium fluoride and reduced with metallic calcium.

Madagascar and Iveland-Evje Region in Norway are the only minable deposits of minerals with high scandium content, thortveitite (Y,Sc)₂(Si₂O₇) and kolbeckite ScPO₄·2H₂O.¹⁵ Other scandium deposits are associated with uranium, nickel-copper-cobalt laterite deposits, and ultramafic rocks worldwide. The nickel and cobalt mines at Syerston and Lake Innes, New South Wales, Australia, as well as several iron, tin, and tungsten deposits in China and several uranium deposits in Russia and Kazakhstan contain scandium. Up to 2003, none of these mines was extracting the scandium from the ore residues, but projects exist for several of the mines to start scandium extraction,¹⁸ so there is no primary production of scandium in the Americas, Europe, or Australia.

Applications

Since it is not a very common metal, scandium does not have many applications. The main application of scandium by weight is in aluminium-scandium alloys for minor aerospace industry components. The aluminium alloys contain between 0.1% and 0.5% of scandium.¹⁹ The Russian military aircrafts Mig 21²⁰ and Mig 29¹⁹ used aluminium scandium alloys. Some unusual designs for sports equipment (bikes, golf clubs, lacrosse shafts, baseball bats,²¹ firearms, etc.) which rely on high performance materials also use scandium to improve the properties of aluminium. 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 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.¹⁹

It is used in lacrosse sticks and baseball bats; 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.²²

Approximately 20 kg (as Sc₂O₃) of scandium is used annually in the United States to make high-intensity lights.²³ 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.²⁴ 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.²³

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 enable a missile launch while still submerged under the Arctic ice cap.^{citation needed}

Contrary to a misconception, the blue color of the aquamarine variety of beryl is not caused by scandium impurities.²⁶ Rather, the color centers of the aquamarine are due to iron(III) and iron(II) ions incorporated into the beryl structure.²⁷

Precautions

The pure metal is not considered toxic, but scandium compounds should be handled as if they are toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for scandium compounds.²⁸ The LD₅₀ levels for scandium(III) chloride for rats have been determined and were intraperitoneal 4 mg/kg and oral 755 mg/kg.²⁹

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