Thorium (Th)

Where Found

Thorium occurs in various minerals that contain uranium or rare earth elements. The most important source of thorium is monazite, which is usually found in sand. Sand containing monazite is found in India, Brazil, Australia, Madagascar, Sri Lanka, South Africa, and Canada. In the United States, thorium is found in Idaho, Florida, Michigan, California, Colorado, North Carolina, and South Carolina.

Primary Uses

Thorium is mostly used in the form of thorium 232. This can be used to produce uranium 233 for nuclear reactors.

Technical Definition

Thorium (abbreviated Th), atomic number 90, belongs to the actinide series of the periodic table of the elements and resembles uranium in its chemical and physical properties. All thorium isotopes are radioactive; thorium 232 dominates because it has a half-life of about fourteen billion years. Thorium has an atomic weight of 232.038. Pure thorium is a silver-white metal that turns gray or black when exposed to air. Its density is 11.7 grams per cubic centimeter; it has a melting point of about 1,700° Celsius and a boiling point of about 4,000° Celsius. (Exact figures cannot be given because these values are greatly changed by impurities.)

Description, Distribution, and Forms

Thorium is a fairly rare radioactive element resembling uranium. It is mostly obtained along with rare earth elements in the processing of monazite. Thorium serves as an indirect source of nuclear power because it can be changed into uranium.

History

Thorium was discovered in 1828 by the Swedish chemist Jöns Jacob Berzelius. Its radioactive nature was discovered in 1898. In the late nineteenth century and early twentieth century, thorium was mostly used in mantles for incandescent gaslights because it gave off a bright white light when heated.

Obtaining Thorium

Thorium is usually obtained from monazite. First the monazite is finely ground and mixed with hot sulfuric acid or hot sodium hydroxide to separate thorium and rare earth elements from the other substances found in monazite. Thorium compounds are then obtained from this mixture by a variety of chemical reactions. In general, these methods depend on the fact that certain thorium compounds have different solubilities from similar compounds of the rare earth elements in certain solvents.

Free thorium may be obtained by treating thorium oxide with calcium at about 950° Celsius. It may also be obtained by the electrolysis of thorium chloride. The thorium powder obtained by these methods may be transformed into thorium metal by compressing it and heating it in a vacuum.

Uses of Thorium

In a nuclear reactor thorium 232 can be transformed into uranium 233, which can undergo fission to release nuclear energy. Thorium is also used to strengthen magnesium alloys, to make photoelectric cells, as a catalyst, in welding electrodes, and in high-temperature ceramics.

Because thorium is radioactive, it poses a health hazard. Although thorium 232 is not particularly dangerous on its own, one of the substances it changes into as it decays, radon 220, is hazardous because it is a gas and may enter the lungs. Because of its radioactivity, the use of thorium products has decreased.

Bibliography

U.S. Geological Survey.

Mineral Information: Thorium Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/thorium/