Thorium

History

Occurence

• Thorium occurs in thorite and in thorianite.
• Large deposits of thorium minerals have been reported in New England and elsewhere, but these have not yet been exploited.
• Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum.
• The metal is a source of nuclear power.
• There is probably more energy available for use from thorium in the minerals of the earth's crust than from both uranium and fossil fuels.
• Any sizable demand from thorium as a nuclear fuel is still several years in the future.
• Work has been done in developing thorium cycle converter-reactor systems.
• Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated.
• While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties.
• Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals.
• Much of the internal heat the earth produces has been attributed to thorium and uranium.
• Several methods are available for producing thorium metal; it can be obtained by reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal.
• Thorium was originally assigned a position in Group IV of the periodic table.
• Because of its atomic weight, valence, etc., it is now considered to be the second member of the actinide series of elements.

Properties

• When pure, thorium is a silvery-white metal which is air-stable and reatins its luster for several months.
• When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black.
• The physical properties of thorium are greatly influenced by the degree of contamination with the oxide.
• The purest spcimens often contain several tenths of a percent of the oxide.
• High-purity thorium has been made.
• Pur thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn.
• Thorium is dimorphic, changing at 1400C from a cubic to a body-centered cubic structure.
• Thorium oxide has a melting point of 3300C, which is the highest of all oxides.
• Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points.
• Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.
• Powdered thorium metal is often pyrophoric and should be carefully handled.
• When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Uses

• The principal use of thorium has been in the preparation of the Welsbach mantle, used for portable gas lights.
• These mantles, consisting of thorium oxide with about 1% cerium oxide and other ingredients, glow with a dazzling light when heated in a gas flame.
• Thorium is an important alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures.
• Because thorium has a low work-function and igh electron emission, it is used to coat tungsten wire used in electronic equipment.
• The oxide is also used to control the grain size of tungsten used for electric lamps; it is also used for high-temperature laboratory crucibles.
• Glasses containing thorium oxide have a high refractive index and low dispersion.
• Consequently, they find application in high quality lenses for cameras and scientific instruments.
• Thorium oxide has also found use as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid.

Isotopes

• Twenty five isotopes of thorium are known with atomic masses ranging from 212 to 236.
• All are unstable.
• 232Th occurs naturally and has a half-life of 1.4 x 10^10 years.
• It is an alpha emitter.
• 232Th goes through six alpha and four beta decay steps before becoming the stable isotope 208Pb.
• 232Th is sufficiently radioactive to expose a photographic plate in a few hours.
• Thorium disintegrates with the production of "thoron" (220Rn), which is an alpha emitter and presents a radiation hazard.

Costs & Handling