scandium

Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia.

Scandium is present in most of the deposits of rare-earth and uranium compounds, but it is extracted from these ores in only a few mines worldwide. Because of the low availability and difficulties in the preparation of metallic scandium, which was first done in 1937, applications for scandium were not developed until the 1970s, when the positive effects of scandium on aluminium alloys were discovered, and its use in such alloys remains its only major application. The global trade of scandium oxide is 15–20 tonnes per year.

The properties of scandium compounds are intermediate between those of aluminium and yttrium. A diagonal relationship exists between the behavior of magnesium and scandium, just as there is between beryllium and aluminium. In the chemical compounds of the elements in group 3, the predominant oxidation state is +3.

Properties

Chemical characteristics

Scandium is a soft metal with a silvery appearance. It develops a slightly yellowish or pinkish cast when oxidized by air. It is susceptible to weathering and dissolves slowly in most dilute acids. It does not react with a 1:1 mixture of nitric acid () and 48.0% hydrofluoric acid (), possibly due to the formation of an impermeable passive layer. Scandium turnings ignite in the air with a brilliant yellow flame to form scandium oxide.

Isotopes

In nature, scandium is found exclusively as the isotope ⁴⁵Sc, which has a nuclear spin of 7/2; this is its only stable isotope. Twenty-five radioisotopes have been characterized with the most stable being ⁴⁶Sc, which has a half-life of 83.8 days; ⁴⁷Sc, 3.35 days; the positron emitter ⁴⁴Sc, 4 hours; and ⁴⁸Sc, 43.7 hours. All of the remaining radioactive isotopes have half-lives less than 4 hours, and the majority of these have half-lives less than 2 minutes. This element also has five nuclear isomers, with the most stable being ^44m2Sc (''t''_1/2 = 58.6 h).

The known isotopes of scandium range from ³⁶Sc to ⁶⁰Sc. 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.

Occurrence

In Earth's crust, scandium is not rare. Estimates vary from 18 to 25 ppm, which is comparable to the abundance of cobalt (20–30 ppm). Scandium is only the 50th most common element on Earth (35th most abundant in the crust), but it is the 23rd most common element in the Sun. However, scandium is distributed sparsely and occurs in trace amounts in many minerals. Rare minerals from Scandinavia and Madagascar such as thortveitite, euxenite, and gadolinite are the only known concentrated sources of this element. Thortveitite can contain up to 45% of scandium in the form of scandium oxide.

The stable form of scandium is created in supernovas via the r-process. Also, scandium is created by cosmic ray spallation of the more abundant iron nuclei.
*²⁸Si + 17n → ⁴⁵Sc (r-process)
*⁵⁶Fe + p → ⁴⁵Sc + ¹¹C + n (cosmic ray spallation)

Production

The world production of scandium is in the order of 15–20 tonnes per year, in the form of scandium oxide. The demand is about 50% higher, and both the production and demand keep increasing. In 2003, only three mines produced scandium: 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; since then many other countries have built scandium-producing facilities, including 5 tonnes/year (7.5 tonnes/year ) by Nickel Asia Corporation and Sumitomo Metal Mining in the Philippines.
In the United States, NioCorp Development hopes to raise $1 billion toward opening a niobium mine at its Elk Creek site in southeast Nebraska which may be able to produce as much as 95 tonnes of scandium oxide annually. In each case, scandium is a byproduct of the extraction of other elements and is sold as scandium oxide.Scandium
USGS.

To produce metallic scandium, the oxide is converted to scandium fluoride and then reduced with metallic calcium.

Madagascar and the Iveland- Evje region in Norway have the only deposits of minerals with high scandium content, thortveitite ) but these are not being exploited. The mineral kolbeckite has a very high scandium content but is not a