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Thorium

History

Morten Esmark found a black mineral on Løvøya island, Norway and gave a sample to his father Jens Esmark, a noted mineralogist. The elder Esmark was not able to identify it and sent a sample to Swedish chemist Jöns Jakob Berzelius for examination in 1828. In 1829 Berzelius determined that it contained a new element, which he named thorium after Thor, the Norse god of thunder. The metal had no practical uses until 1885 when Carl Auer von Welsbach invented the gas mantle. Thorium was first observed to be radioactive in 1898, independently, by Polish-French physicist Marie Curie and German chemist Gerhard Carl Schmidt. Between 1900 and 1903, Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half-life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity. The crystal bar process (or "iodide process") was discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925 to produce high-purity metallic thorium. Because of health concerns, the thorium in classic lantern mantles has been replaced by rare-earth elements that also produce intense light without the radioactivity.

Isotopes

Twenty-seven thorium radioisotopes have been characterized, with a range in atomic weight from 210 to 236. All are unstable with the most stable being ²³²Th with a half-life of 14.05 billion years. Thorium-232 represents all but a trace of naturally occurring thorium. It is an alpha emitter and 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. Other isotopes of thorium are short-lived intermediates in the decay chains of higher elements, and only found in trace amounts. The longer-lived of these trace isotopes include: ²³⁰Th with a half-life of 75,380 years which is a daughter product of ²³⁸U decay; ²²⁹Th with a half-life of 7340 years and ²²⁸Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives less than ten minutes. Much of the internal heat the earth produces has been attributed to thorium and uranium.

Sources

Thorium-232 is a primordial nuclide, having existed in its current form for over 4.5 billion years, a half-life is comparable to the age of the Universe and thus predating the formation of the Earth. Thorium was forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovas. Thorium is found in small amounts in most rocks and soils. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO₄), thorianite (ThO₂ + UO₂) and monazite. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. Monazite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium.^[66] Thorium-containing minerals occur on all continents. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO₂ along with rare-earth minerals.

Properties

When pure, thorium is a silvery-white metal that is air-stable and retains 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 specimens often contain several tenths of a percent of the oxide. High-purity thorium has been made. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400°C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point of 3300°C, 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 handled carefully. When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Uses

The principal historic use of thorium has been in the preparation of the Welsbach mantle, used for portable gaslights. 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 high 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. Thorium 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.

Production

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.

Hazards

Powdered thorium metal is pyrophoric and will often ignite spontaneously in air. Thorium disintegrates with the production of "thoron" (²²⁰Rn), which is an alpha emitter and presents a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential. Secondary decay products of thorium include radium and actinium. Because of this, there are concerns about the safety of thorium mantles and some nuclear safety agencies state precautions about their use.

Further reading

• Mathias S. Wickleder, Blandine Fourest, and Peter K. Dorhout, (2006) Thorium, Chapter 3 in The Chemistry of the Actinide and Transactinide  Elements, Third Edition, L. R. Morss, J. Fuger, and N. M. Edelstein, Eds., Springer Publishers.
• Wikipedia - Thorium

Updated by Dr. David Hobart July 2013.