Thorium

HistoryThorium 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.

UsesThorium 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.

DescriptionDiscovered 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.

OccurrenceThorium 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.

HistoryDiscovered 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.

UsesThorium 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.

UsesThorium 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.

UsesAs 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.

General DescriptionSilver 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.

PreparationThorium 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 proportionsThe 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 choiceThere 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 belowFinely-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 reductionIn 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

DefinitionMetallic 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.

DefinitionA 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.

Definitionthorium: 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.