The thorium dioxide compound, ThO 2 , is known industrially as thoria, and it is used in most of the thorium applications, characterized by being the chemical compound with the highest melting point (3,300 ºC).
Thorium was discovered in 1828 by Mortar Thrane Esmark, who found a black mineral on the Norwegian island of Løvøya. Esmark gave the mineral to Jöns Jacob Berzelius, who analyzed it, finding an unknown metal in it.
Thorium is a radioactive, shiny, moderately hard, silvery-white, ductile, and malleable metal that tarnishes very slowly in air, turning gray and later black. It belongs to the group of actinides, identifying itself with atomic number 90 and an atomic weight of 232 g / mol.
Thorium-232 ( 232 Th 90 ) constitutes more than 99% of the total element thorium present in the earth’s crust. It can be considered to be a stable isotope, despite being radioactive, since its half-life is 1,405 x 10 10 years. It radioactively decays through the emission of α and β particles, and γ radiation.
Thorium-232 is transformed into radium-268 ( 268 Ra 88 ) by the emission of an alpha particle, made up of two protons and two neutrons. Thorium can undergo a series of radioactive decays until it becomes a stable element: lead-208.
Thorium-232 is capable of trapping neutrons to transform into the radioactive element uranium-233, emitting β-type radiation. Uranium, on the other hand, is used in nuclear reactors for energy production.
Thorium is a highly reactive and electropositive metal. It oxidizes very slowly in air, although corrosion can occur after several months. When heated in air it ignites, emitting a brilliant white light as the production of thorium dioxide, ThO 2 , proceeds .
It also dissolves in concentrated nitric acid with a small amount of catalytic fluoride or fluorosilicate. Thorium is a pyrophoric metal: when it turns into powder it is capable of igniting spontaneously in the air.
Thorium atoms form a face-centered cubic (fcc) crystal at room temperature. When heated above 1360 ° C, the crystal undergoes a transition to the lower- density body-centered cubic (bcc) phase . Meanwhile, thorium under high pressure (100 GPa or more), acquires a dense tetragonal body-centered structure (bct).
The abbreviated electron configuration for thorium is as follows:
[Rn] 6d 2 7s 2
Losing its four valence electrons, it becomes the Th 4+ cation . Note that despite being an actinide, it lacks electrons in its 5f orbitals, in contrast to the other actinides.
The main mineral used commercially to obtain thorium is monazite. The initial step is its separation from its primary deposit: the pegmatite. Alkaline earth metal carbonates are removed from pegmatite by reacting their fragments with hydrogen chloride.
The resulting fragments are calcined and filtered, then subjected to magnetic separation. Thus a sandy material of monazite is obtained. This sand is digested with 93% sulfuric acid, at a temperature of 210 to 230 ºC, and for several hours. The acidic solution formed is then diluted with water ten times its volume .
The monazite remains sink to the bottom, while the thorium and the other rare earth elements float in the acidic preparation. The pH is adjusted to 1.3, which results in the precipitation of thorium as phosphate, while the rest of the rare earths in suspension remain in solution.
Currently the separation and purification are carried out using liquid solvents, for example, tributyl phosphate in kerosene.
Thorium metal can be produced in commercial quantities by the metallothermic reduction of thorium tetrafluoride (ThF 4 ) and thorium dioxide (ThO 2 ), or by electrolysis of thorium tetrachloride (ThCl 4 ).
Thorium has had many applications, many of which have been discarded since the 1950s, because its radioactive nature posed a health risk.
Thorium has been alloyed with tungsten as an electrode in TIG (tungsten inert gas) welding, constituting 2% of the alloy.
In small amounts, thorium has been added to tungsten filaments to reduce their crystallization, thus allowing the emission of electrons at lower temperatures. Tungsten-thorium wires have been used in electronic tubes and in the electrodes of X-ray tubes and rectifiers.
Thorium dioxide has been used in tungsten arc welding, as it increases the resistance of tungsten to the high temperatures of metal electrodes. However, it has been superseded in this application by the oxides of zirconium, cerium or lanthanum.
Thorium tetrafluoride, on the other hand, has been used as a material to reduce reflections in multilayer optical coatings, which are transparent to light with a wavelength between 0.350 to 1.2 µm. However, the thorium salt has been replaced in this use by lanthanum tetrafluoride.
Thorium dioxide has been used in light blanket illumination, as it emits a bright light corresponding to visible light. Although thorium is still used in this application, thorium has been partially replaced by yttrium.
Thorium has also been used in the production of refractory materials for the metallurgical industry and in ceramic crucibles for teaching and research laboratories.
Thorium-232 is used in nuclear reactors to trap slow-moving neutrons, as it transforms into uranium-233 in doing so. This radioactive element is fissile and is used for energy production.
The development of nuclear reactors based on thorium-32 has been slow, and the first reactor with this characteristic was created at the Indian Point Energy Center, located in Buchanan USA, in 1962. Thorium-232 nuclear reactors do not emit plutonium, which makes them less polluting.