THORIUM

Description

Discovered in 1828 by Berzelius, thorium is a naturally occurring radioactive metal with no stable isotopes, which is named for the Norse god Thor. It is about as abundant as lead. Soil commonly contains an average of about six parts of thorium per million parts (ppm) of soil. Thorium occurs in the minerals thorite, thorianite, orangite, and yttrocrasite, and in monazite sand. Rocks in some underground mines may also contain thorium in a more concentrated form. After these rocks are mined, thorium is usually concentrated and changed into thorium dioxide or other chemical forms. Thorium-bearing rock that has had most of the thoriumremoved from it is called ‘depleted’ ore or tailings.

Chemical properties

Soft metal with bright silvery luster when freshly cut, similar to lead in hardness when pure. Can be cold-rolled, extruded, drawn, and welded. Soluble in acids; insoluble in alkalies and water. Some alloys may ignite spontaneously, the metal in massive form is not flammable.

Chemical properties

Thorium is a silvery-white, soft, ductile metal which is a natural radioactive element.

Physical properties

Thorium is a radioactive, silvery-white metal when freshly cut. It takes a month or morefor it to tarnish in air, at which point it forms a coating of black oxide. Although it is heavy,it is also a soft and malleable actinide metal. The metal has a rather low melting point, but itsoxide has a very high melting point of about 3,300°C. Thorium reacts slowly with water butreacts more vigorously with hydrochloric acid (HCl).
Thorium’s melting point is 1,750°C, its boiling point is 4,788°C, and its density is 11.79g/cm³.

Isotopes

There are 30 radioisotopes of thorium. One isotope in particular, thorium-232,although a weak source of radiation, has such a long half-life (1.405×10⁺¹⁰ years, orabout 14 billion years) that it still exists in nature and is considered stable.

Origin of Name

Thorium was named for Thor, the Scandinavian (Norse) god of “thunder.”

Occurrence

Thorium is the 37th most abundant element found on Earth, and it makes up about0.0007% of the Earth’s crust. It is mostly found in the ores of thorite, thorianite (the oxide ofthorium), and monazite sand. It is about as abundant as lead in the Earth’s crust. As a potentialfuel for nuclear reactors, thorium has more energy potential than the entire Earth’s supply ofuranium, coal, and gas combined.

History

Discovered by Berzelius in 1828. Thorium occurs in thorite (ThSiO4) and in thorianite (ThO2 + UO2). 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 for 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. When 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. High-purity thorium has been made. Pure thorium is soft, very ductile, and can be coldrolled, 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 carefully handled. When heated in air, thorium turnings ignite and burn brilliantly with a white light. 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 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 has not found many uses due to its radioactive nature and its handling and disposal problems. Thirty isotopes of thorium are known with atomic masses ranging from 210 to 237. All are unstable. 232Th occurs naturally and has a half-life of 1.4 × 1010 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. Good ventilation of areas where thorium is stored or handled is therefore essential. Thorium metal (99.8%) costs about $25/g.

Characteristics

Thorium is chemically similar to hafnium (₇₂Hf ) and zirconium (₄₀Zr), located just above itin group 4 (IVB). Thorium-232 is found in nature in rather large quantities and goes througha complicated decay process called the thorium decay series. This series involves both alphaand beta emissions, as follows: Th-232 →Ra-228→Ac-228→Th-228→Ra-224→Rn-220→Po-216→Po-212→Pb-212→Bi-212→Ti-208→Pb-208. Thorium-232 can also be convertedinto thorium-233 or uranium-233 by bombarding it with neutrons. This results in Th-232adding a neutron to its nucleus, thus increasing its atomic weight. It then decays into uranium-233. This makes it potentially useful as an experimental new type of fissionable materialfor use in nuclear reactors designed to produce electricity.

The Uses of THORIUM

Thorium has several commercial uses. For example, thorium oxide (ThO₂) has several uses,including in the Welsbach lantern mantle that glows with a bright flame when heated by agas burner. Because of the oxide’s high melting point, it is used to make high-temperaturecrucibles, as well as glass with a high index of refraction in optical instruments. It is alsoused as a catalyst in the production of sulfuric acid (H₂SO₄), in the cracking procedures inthe petroleum industry, and in the conversion of ammonia (NH₃) into nitric acid (HNO₃).Thorium is used as a “jacket” around the core of nuclear reactors, where it becomes fissionableuranium-233 that is then used for the nuclear reaction to produce energy. Additionally,it is used in photoelectric cells and X-ray tubes and as a coating on the tungsten used to makefilaments for light bulbs. It has great potential to supplant all other nonrenewable energysources (i.e., coal, gas, and atomic energy). Thorium-232 can be converted into uranium-233,a fissionable fuel available in much greater quantities than other forms of fissionable materialsused in nuclear reactors.

The Uses of THORIUM

As fuel in nuclear reactors, as source of fissionable 233U. In manufacture of incandescent gas-light mantles, welding electrodes, ceramics. As hardener in Mg alloys; for filament coatings in incandescent lamps and vacuum tubes; as chemical catalyst.

The Uses of THORIUM

Thorium is present in nuclear reactor fuels, and is used in the manufacture of incandescent gas-light mantles, welding electrodes, and ceramics, as a hardener in magnesium alloys, and as a chemical catalyst. In addition, it is used in sun lamps, in photoelectric cells, and in target materials for X-ray tubes. Thorium is present in fires and explosions caused by thorium metal powder and has been recovered as a by-product of uranium production. The use of thorium in nuclear reactors is currently being explored, with India’s program being the largest and most well known.

Definition

Metallic element of atomic number 90, a member of the actinide series (group IIIB of periodic table), aw 232.0381, valence of 4; radioactive, no stable isotopes.

Definition

A toxic radioactive element of the actinoid series that is a soft ductile silvery metal. It has several long-lived radioisotopes found in a variety of minerals including monazite. Thorium is used in magnesium alloys, incandescent gas mantles, and nuclear fuel elements. Symbol: Th; m.p. 1780°C; b.p. 4790°C (approx.); r.d. 11.72 (20°C); p.n. 90; r.a.m. 232.0381.

Definition

thorium: Symbol Th. A grey radioactivemetallic element belonging tothe actinoids; a.n. 90; r.a.m.232.038; r.d. 11.5–11.9 (17°C); m.p.1740–1760°C; b.p. 4780–4800°C. It occursin monazite sand in Brazil,India, and USA. The isotopes of thoriumhave mass numbers from 223to 234 inclusive; the most stable isotope,thorium–232, has a half-life of1.39 × 10¹⁰ years. It has an oxidationstate of (+4) and its chemistry resemblesthat of the other actinoids. It canbe used as a nuclear fuel for breederreactors as thorium–232 capturesslow neutrons to breed uranium–233.Thorium dioxide (thoria, ThO₂) isused on gas mantles and in specialrefractories. The element was discoveredby J. J. Berzelius in 1829.

Production Methods

Thorium is extracted from monazite sand concentrates for metallurgical and other purposes by digestion with either hot, fuming sulfuric acid or caustic soda. The resultant mass is diluted with water that dissolves thorium, uranium, and rare earth metals, leaving unreacted monazite, silica, rutile (TiO2), and zircon (ZrSiO4). Neutralization of the liquor precipitates thorium phosphate, leaving behind uranium and most of the rare earth metals.
In 1974, U.S. domestic use of thorium was about 80 tons, about one-half of which was employed to produce nuclear fuels and for nuclear research. Principal nonenergy applications applications were in the production of Welsbach incandescent gaslight mantles, as a hardener in Th–Mg alloys, in thoriated tungsten electrodes, and for chemical catalytic uses. Overall, the consumption of thorium in the United States has decreased significantly over the past several decades as nonradioactive substances have replaced thorium in many applications.

Preparation

Thorium is recovered mostly from monazite, which is a phosphate mineral of the light-weight rare earths. Monazite occurs as sand associated with silica and a few other minerals in smaller proportions
The first step in the recovery process involves breaking down or opening upthe ore. This usually is done by one of two methods: (1) digesting with hot concentrated sulfuric acid or (2) treatment with hot concentrated sodium hydroxide. In the acid digestion process, finely-ground monazite is treated with hot sulfuric acid. Thorium and rare earths dissolve in the acid. Phosphoric acid is released from monazite (a phosphate mineral) by reacting phosphates with sulfuric acid. Insoluble residues are removed by filtration. In the caustic digestion process, monazite, on heating with a concentrated solution of sodium hydroxide, breaks down to form soluble trisodium phosphate and an insoluble residue containing hydrated oxides of thorium and rare earths. Thus, in the caustic process, trisodium phosphate is recovered as a by-product. The hydrated oxides are dissolved in sulfuric acid.
Thorium sulfate, being less soluble than rare earth metals’ sulfates, can be separated by fractional crystallization. Usually, solvent extraction methods are applied to obtain high purity thorium and for separation from rare earths. In many solvent extraction processes, an aqueous solution of tributyl phosphate is the extraction solvent of choice
There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metallic thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below
Finely-ground monazite is treated with a 45% NaOH solution and heated at 138°C to open the ore. This converts thorium, uranium, and the rare earths to their water-insoluble oxides. The insoluble residues are filtered, dissolved in 37% HCl, and heated at 80°C. The oxides are converted into their soluble chlorides. The pH of the solution is adjusted to 5.8 with NaOH. Thorium and uranium are precipitated along with small quantities of rare earths. The precipitate is washed and dissolved in concentrated nitric acid. Thorium and uranium are separated from the rare earths by solvent extraction using an aqueous solution of tributyl phosphate. The two metals are separated from the organic phase by fractional crystallization or reduction
In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metallic cerium and removed by filtration. Thorium then is recovered from solution.
Thorium metal may be produced from its salts—usually the oxide or a halide—by several methods that include electrolysis and reduction with calcium. In the calcium reduction process, thorium oxide is heated in a closed vessel at 950°C. The product is cooled and leached with water and dilute acid and then washed and vacuum-dried to form a free-flowing powder
Thorium metal also can be prepared by thermal reduction of its halides with calcium, magnesium, sodium, or potassium at elevated temperatures (950°C), first in an inert atmosphere and then in vacuum. Fluoride and chloride thorium salts are commonly employed. Berzelius first prepared thorium by heating tetrachloride, ThCl₄, with potassium. Magnesium and calcium are the most common reductant. These metals are added to thorium halides in excess to ensure complete reduction. Excess magnesium or calcium is removed by heating at elevated temperatures in vacuum. One such thermal reduction of halides produces thorium sponge, which can be converted into the massive metal by melting in an electron beam or arc furnace
Thorium can be obtained from its halides by electrolysis. A fused salt bath of NaCl-KCl-ThCl₄ or NaCl-KCl-KF-ThF₄ or similar eutectic mixtures is employed in electrolysis. The electrolysis may be carried out in a graphite crucible, and thorium is deposited as a coarse powder on the electrode, which is made of molybdenum or other suitable material.

General Description

Silver to grayish radioactive metal. Twice as dense as lead. Radioactive materials emit ionizing radiation, detectable only using special instruments. Exposure to intense levels of radiation or prolonged exposure to low levels can be harmful. Film is also damaged by radiation.

Air & Water Reactions

Pyrophoric material, spontaneously ignites in air.

Reactivity Profile

THORIUM when heated with chlorine (or sulfur), reacts vigorously with incandescence [Mellor 7:208 1946-47]. When thorium is heated with phosphorus, they unite with incandescence [Svenska Akad. 1829 p.1].

Hazard

Flammable and explosive in powder form. Dusts of thorium have very low ignition points and may ignite at room temperature. Radioactive decay isotopes are dangerous when ingested.

Hazard

As thorium undergoes natural radioactive decay, a number of products, including gases,are emitted. These decay products are extremely dangerous radioactive poisons if inhaled oringested.

Health Hazard

Radiation presents minimal risk to transport workers, emergency response personnel and the public during transportation accidents. Packaging durability increases as potential hazard of radioactive content increases. Undamaged packages are safe. Contents of damaged packages may cause higher external radiation exposure, or both external and internal radiation exposure if contents are released. Low radiation hazard when material is inside container. If material is released from package or bulk container, hazard will vary from low to moderate. Level of hazard will depend on the type and amount of radioactivity, the kind of material it is in, and/or the surfaces it is on. Some material may be released from packages during accidents of moderate severity but risks to people are not great. Released radioactive materials or contaminated objects usually will be visible if packaging fails. Some exclusive use shipments of bulk and packaged materials will not have "RADIOACTIVE" labels. Placards, markings and shipping papers provide identification. Some packages may have a "RADIOACTIVE" label and a second hazard label. The second hazard is usually greater than the radiation hazard; so follow this GUIDE as well as the response GUIDE for the second hazard class label. Some radioactive materials cannot be detected by commonly available instruments. Runoff from control of cargo fire may cause low-level pollution.

Safety Profile

Suspected carcinogen. Taken internally as Th02, it has proven to be carcinogenic due to its radioactivity. On an acute basis it has caused dermatitis. Flammable in the form of dust when exposed to heat or flame, or by chemical reaction with oxidizers. The powder may ignite spontaneously in air. Potentially hazardous reactions with chlorine, fluorine, bromine, oxygen, phosphorus, silver, sulfur, air, nitryl fluoride, peroxyformic acid.

Potential Exposure

Metallic thorium is used in nuclear reactors to produce nuclear fuel; in the manufacture of incandescent mantles; as an alloying material, especially with some of the lighter metals, for example, magnesium as a reducing agent in metallurgy; for filament coatings in incandescent lamps and vacuum tubes; as a catalyst in organic synthesis; in ceramics; and in welding electrodes. Exposure may occur during production and use of thorium-containing materials, in the casting and machining of alloy parts; and from the fume produced during welding with thorium electrodes. Thorium nitrate is an oxidizer. Contact with combustibles, and reducing agents will cause violent combustion or ignition.

Environmental Fate

Thorium’s usage may result in release of thorium compounds to the environment through various waste streams. As noted above, thorium is also found naturally, particularly in monazite sand. Thorium compounds are expected to exist in the particulate phase if released to the atmosphere based on their low vapor pressures and may be removed from the air by wet and dry depositions. Th and ThO2 have low mobility in soils. In aquatic releases, adsorption is expected to be the primary means of removal from the system.

Shipping

UN2975 Thorium metal, pyrophoric, Hazard class: 7; Labels: 7-Radioactive material, 4.2-Spontaneously combustible material. Note: UN/NA 2975 doesn’t appear in the 49 CFR Hazmat Table.

Toxicity evaluation

Thorium’s mechanism of toxicity is via binding with bone and other glucoproteins and, in some cases, an interaction with zinc. Thorium oxide is radioactive. As noted above, thorium accumulates in the liver, spleen, lymph nodes, and bone marrow, leading to long-term exposure with a diversity of cells. Almost all absorbed thorium stays in human systems after exposure.

Incompatibilities

The powder may ignite spontaneously in air. Heating may cause violent combustion or explosion. May explosively decompose from shock, friction, or concussion. Incompatible with strong oxidizers (chlorates, nitrates, peroxides, permanganates, perchlorates, chlorine, bromine, fluorine, etc.); contact may cause violent fires or explosions. Keep away from alkaline materials, strong bases, strong acids, oxoacids, epoxides, nitryl fluoride; peroxyformic acid; silver, sulfur.

Waste Disposal

Recovery and recycling is in the preferred route.