THORIUM ORE

Listing description:

Thorium (pronounced /ˈθɔəriəm/ THOHR-ee-əm) is a chemical element with the symbol Th and atomic number 90.
Thorium is a naturally occurring, slightly radioactive metal. It is estimated to be about three to four times more abundant than uranium in the Earth's crust. It has been considered a waste product in mining rare earths, so its abundance is high and cost low.

Detailed description:

Thorium was successfully used as a breeding (fertile) source for nuclear fuel – uranium (233) in the molten-salt reactor experiment (MSR) from 1964 to 1969 (producing thermal energy for heat exchange to air or liquids), as well as in several light water reactors using solid fuel composed of a mixture of ²³²Th and ²³³U, including the Shippingport Atomic Power Station (operation commenced 1957, decommissioned in 1982), but a thorium-uranium mix was only used at end of life to demonstrate Th-to-U breeding. Currently, the Japanese Fuji project and officials in India are advocating a thorium-based nuclear program, and a seed-and-blanket fuel utilizing thorium is undergoing irradiation testing at the Kurchatov Institute in Moscow. Advocates of the use of thorium as the fuel source for nuclear reactors state that they can be built to operate significantly cleaner than uranium-based power plants as the waste products are much easier to handle.

When used in molten-salt reactors, thorium bred to ²³³U removes weaponization dangers, because no uranium exists in solid form and the reactor runs continuously, with no shutdown for refuelling—all thorium and fissile uranium is consumed and any undesired gasses and uranium/plutonium isotopes are flushed out as gasses (e.g., as uranium hexafluoride) as the hot, liquid salt is pumped around the reactor/exchanger system.

Characteristics

Physical properties

Pure thorium is a silvery-white metal which 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. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400 °C from a face-centered cubic to a body-centered cubic structure. Powdered thorium metal is often pyrophoric and requires careful handling. When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Thorium has the largest liquid range of any element: 2946 °C between the melting point and boiling point.

Chemical properties

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric acid. It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride ion.

Compounds

Thorium compounds are stable in the +4 oxidation state.

Thorium dioxide has the highest melting point (3300 °C) of all oxides.

Thorium(IV) nitrate and thorium(IV) fluoride are known in their hydrated forms: Th(NO₃)₄·4H₂O and ThF₄·4H₂O, respectively. The thorium center has square planar geometry. Thorium(IV) carbonate, Th(CO₃)₂, is also known.

When treated with potassium fluoride and hydrofluoric acid, Th⁴⁺ forms the complex anion ThF2−6, which precipitates as an insoluble salt, K₂ThF₆.

Thorium(IV) hydroxide, Th(OH)₄, is highly insoluble in water, and is not amphoteric. The peroxide of thorium is rare in being an insoluble solid.

Applications

Thorium

Thorium is part of a certain magnesium alloys called Mag-Thor, which are used in aircraft engines, imparting high strength and creep resistance at elevated temperatures. Thoriated magnesium was used to build the CIM-10 Bomarc missile, although concerns about radioactivity have resulted in several missiles being removed from public display.

Thorium is also used as an alloying agent in gas tungsten arc welding (GTAW) to increase the melting temperature of tungsten electrodes and improve arc stability. The electrodes labeled EWTH-1 contain 1% thorium, while the EWTH-2 contain 2%. In electronic equipment, thorium coating of tungsten wire improves the electron emission of heated cathodes.

Thorium is a very effective radiation shield, although it has not been used for this purpose as much as lead or depleted uranium. Thorium is a fertile material for producing nuclear fuel in a breeder reactor. Uranium-thorium age dating has been used to date hominid fossils.

Thorium compounds

Thorium dioxide is a material for heat-resistant ceramics, e.g., for high-temperature laboratory crucibles. When added to glass, it helps increase refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments.

Thorium dioxide (ThO₂) and thorium nitrate (Th(NO₃)₄) were used in mantles of portable gas lights, including natural gas lamps, oil lamps and camping lights. These mantles glow with an intense white light (unrelated to radioactivity) when heated in a gas flame, and its color could be shifted to yellow by addition of cerium.

Thorium dioxide was used to control the grain size of tungsten metal used for spirals of electric lamps. Thoriated tungsten elements were also found in the negative poles of magnetron tubes. Thorium was added because of its ability to emit electrons at relatively low temperatures when heated in vacuum. Those tubes were emitting electrons at microwave frequencies and were applied in microwave ovens and radars.

Thorium as a nuclear fuel

Thorium, as well as uranium and plutonium, can be used as fuel in a nuclear reactor. A thorium fuel cycle offers several potential advantages over a uranium fuel cycle, including greater abundance on Earth, superior physical and nuclear properties of fuel, enhanced proliferation resistance, and reduced nuclear waste production.

Although not fissile itself, ²³²Th will absorb slow neutrons to produce ²³³U, which is fissile. Hence, like ²³⁸U, it is fertile. It is at least 4-5 times more abundant in Earth's crust than all isotopes of uranium combined and is fairly evenly spread around Earth^{[citation needed]}, with many countries having large supplies of it. Also, preparation of thorium fuel does not require isotopic separation.

History

M. T. Esmark found a black mineral on Løvøy Island, Norway and gave a sample to Professor Jens Esmark, a noted mineralogist who was not able to identify it, so he sent a sample to the Swedish chemist Jöns Jakob Berzelius for examination in 1828. Berzelius analyzed it and named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the gas mantle in 1885.

Thorium was first observed to be radioactive in 1898, independently, by Polish-French physicist Marie Curie and English chemist Gerhard Carl Schmidt.^[34][35][36] 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.^[37]

Occurrence

Thorium is found in small amounts in most rocks and soils, where it is about four times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 12 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. Thorium-containing monazite(Ce) occurs in some quantities^{[clarification needed]} on all continents.^[5][39]

²³²Th decays very slowly (its half-life is comparable to the age of the Universe) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than ²³²Th, though on a mass basis they are negligible.

Thorium extraction

Main article: Monazite

Thorium has been extracted chiefly from monazite through a complex multi-stage process. The monazite sand is dissolved in hot concentrated sulfuric acid (H₂SO₄). Thorium is extracted as an insoluble residue into an organic phase containing an amine. Next it is separated or "stripped" using an ion such as nitrate, chloride, hydroxide, or carbonate, returning the thorium to an aqueous phase. Finally, the thorium is precipitated and collected.^[40]

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.

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