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.
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.
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.
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.
First aid
If this chemical gets into the eyes, remove any contact lenses at once and irrigate immediately for at least 15 minutes, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts the skin, remove contaminated clothing and wash immediately with soap and water. Seek medical attention immediately.If this chemical has been inhaled, remove from exposure, begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR if heart action has stopped. Transfer promptly to a medical facility. When this chemical has been swallowed, get medical attention. Give large quantities of water and induce vomiting. Do not make an unconscious person vomit.
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.
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.
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.
Waste Disposal
Recovery and recycling is in the preferred route.
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.
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.
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.
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.
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.
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.
