History
Thorium is a silvery-white metal that is air-stable and retains its lustre for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. Thorium oxide (ThO2), one of thorium s compounds, has many uses. Thorium was discovered by Jöns Jacob Berzelius, a Swedish chemist, in 1828. He discovered it in a sample of a mineral that was given to him by the Reverend Has Morten Thrane Esmark, who suspected that it contained an unknown substance. Esmark s mineral is now known as thorite (ThSiO4). Thorium makes up about 0.0007% of the Earth s crust and is primarily obtained from thorite, thorianite (ThO2), and monazite ((Ce, La, Th, Nd, Y)PO4).Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Because of these properties, thorium has found applications in lightbulb elements, lantern mantles, arc-light lamps, welding electrodes, and heat-resistant ceramics. Glass containing thorium oxide has a high refractive index and dispersion and is used in high-quality lenses for cameras and scientific instruments.
Occurance
Thorium is a naturally occurring, radioactive substance. In the environment, thorium exists in combination with other minerals, such as silica. Small amounts of thorium are present in all rocks, soil, water, plants, and animals. Soil contains an average of about 6 parts of thorium per million parts of soil. More than 99% of natural thorium exists in the form of thorium-232 and later breaks down into two parts – a small part called ‘alpha’ radiation and a large part called the decay product. The decay product is also not stable and continues to break down through a series of decay products until a stable product is formed. During these decay processes, radioactive substances are produced. These include radium and radon. These substances give off radiation, including alpha and beta particles and gamma radiation. Rocks of certain underground mines 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. After most of the thorium is removed, the rocks are called ‘depleted’ ore or tailings. Soil commonly contains an average of around 6 ppm of thorium. Thorium is more abundant than uranium and is widely distributed in nature as an easily exploitable resource in many countries and has not been exploited fully and commercially. Thorium fuels, therefore, complement uranium fuels and ensure long-term sustainability of nuclear power. Thorium fuel cycle is an attractive way to produce long-term nuclear energy with low radiotoxicity waste. In addition, the transition to thorium could be done through the incineration of weapons-grade plutonium (WPu) or civilian plutonium.
Uses
Thorium has extensive societal applications: to make ceramics, gas lantern mantles, and metals used in the aerospace industry and in nuclear reactions. In India, there has always been a strong incentive for development of thorium fuels and fuel cycles because of large thorium deposits compared to the very modest uranium reserves. Thorium oxide is also used to make glass with a high index of refraction that is used to make high-quality camera lenses. Thorium oxide is used as a catalyst in the production of sulphuric acid (H2SO4), in the cracking of petroleum products, and in the conversion of ammonia (NH3) to nitric acid (HNO3). Thorium exists in nature in a single isotopic form – Th-232 – which decays very slowly (its half-life is about three times the age of the Earth). The decay chains of natural thorium and uranium give rise to minute traces of Th-228, Th-230, and Th-234, but the presence of these in mass terms is negligible. Also, ThO2 is relatively inert and does not oxidise unlike UO2, which oxidises easily to U3O8 and UO3. Hence, long-term interim storage and permanent disposal in repository of spent ThO2-based fuel are simpler without the problem of oxidation.
Reaction
Thorium combines with practically all nonmetallic elements except noble gases, forming binary compounds. The most stable oxidation state is +4. Heating the metal in air or oxygen forms the oxide, ThO2. Heating the metal in hydrogen at 600°C yields the dihydride ThH2. Also, higher halides of thorium are known. They are produced by heating the dihydride in hydrogen at 250°C. Thorium hydrides are pyrophoric.
Thorium combines with nitrogen at elevated temperatures to form nitrides ThN and Th2N3. Reaction with carbon at elevated temperatures forms the carbides ThC and ThC2.
Thorium reacts with all halogens forming tetrahalides. Thorium also forms inter-metallic compounds with iron, copper, aluminum, selenium, nickel, cobalt, manganese, bismuth, and many other metals at elevated temperatures.
Toxicity
All thorium isotopes are radioactive. Also all its intermediate decay products including radon-220 are radioactive and present radiation hazard. Exposure can cause cancer.
References
handbook-of-inorganic-chemicals
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/cm
3.
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.
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.
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.
Uses
Thorium has several commercial uses. For example, thorium oxide (ThO
2) 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 (H
2SO
4), in the cracking procedures inthe petroleum industry, and in the conversion of ammonia (NH
3) into nitric acid (HNO
3).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.
Uses
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.
Uses
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.
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, ThCl
4, 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-ThCl
4 or NaCl-KCl-KF-ThF
4 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.
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 × 10
10 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, ThO
2) isused on gas mantles and in specialrefractories. The element was discoveredby J. J. Berzelius in 1829.
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.
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.
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.
Synthesis
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.
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.
