Thorium minerals

This is a radioactive element. It occurs in minute traces in barium and thorium minerals, but it can be produced by irradiation of bismuth in a nuclear reactor. (The study of its chemistry presents great difficulty because of its intense a radiation). 

Thorium occurs in thorite and in thorianite. 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. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% Th02 along with rare-earth minerals. 

The examination and analysis of minerals have provided x-ray emission spectrography with a challenge and an opportunity. This situation has arisen because of a great growth of interest in uranium and thorium minerals in the rare-earth oxides and in metals such as tantalum and niobium, or hafnium and zirconium. On the whole, x-ray emission spectrography has met the challenge successfully, and the investigations that prove this also demonstrate the versatility and the value of the method.70"72... 

Polonium, completing the elements of Group 16, is radioactive and one of the rarest naturally occurring elements (about 3 x 10 " % of the Earth s crust). The main natural source of polonium is uranium ores, which contain about lO g of Po per ton. The isotope 210-Po, occurring in uranium (and also thorium) minerals as an intermediate in the radioactive decay series, was discovered by M. S. Curie in 1898. 

The element exists as an intermediate in uranium and thorium minerals through their decay. There is no stable isotope. The longest-living isotope has a half-life of 8.3 hours. In the crust of the Earth, the total steady-state mass is estimated at a few tens of grams. Thus astatine is the rarest element (record ). A few atoms of this relative of iodine can be found in all uranium ore. It exhibits certain metallic properties. 

Lung levels of thorium (230 and 232) in workers occupationally exposed to thorium (miners and millers) are significantly higher than those not occupationally exposed (Gilbert et al. 1985 Singh et al. 1981 Vocaturo et al. 1983 Wrenn et al. 1985). In a review of the epidemiological evidence,... 

RADIOACTIVITY. The spontaneous disintegration of the nucleus of an atom with the emission of radiation. This phenomenon was discovered by Becquerel in 1896 by the exposure-producing effect on a photographic plate by pitchblende (uranium-containing mineral) while wrapped m black paper in the dark, Soon after this, it was found that uranium minerals and uranium chemicals showed more radioactivity than could be accounted for by the uranium content. About the same dmc. radioactivity of thorium minerals and thorium chemicals was also discovered. 

The principal resource minerals for thorium are Monazite, a rare earth phosphate, Thorite, ThSi04, and Thorianite or Uranothorianite, (U, Th)02, of which Monazite is the most important. Thorium minerals are mainly concentrated in sedimentary ores derived from the heavy refractory... 

Singh, H. and Gupta, C. K. (2000) Solvent extraction in production and processing of uranium and thorium, Miner. Process. Extr. Metall. Rev., 21(1-5), 307-349. 

Radioactive substances are widely distributed on the earth. Some are found in the atmosphere, but the major part is present in the lithosphere. The most important ones are the ores of uranium and thorium, and potassium salts, including the radioactive decay products of uranium and thorium. Uranium and thorium are common elements in nature. Their concentrations in granite are about 4 and 13mg/kg, respectively, and the concentration of uranium in seawater is about 3 pg/l. Some uranium and thorium minerals are listed in Table 1.1. The most unportant uranium mineral is pitchblende (UsOs). Uranium is also found in mica. The most important thorium mineral is monazite, which contains between about 0.1 and 15% Th. 

There are locations of locally elevated terrestrial radiation mainly due to uranium and thorium mineralization. Examples of locally elevated dose rates are ... 

It was known [38] before 1910 that the longest-lived (still radioactive) descendant of Rn ( radium D ), °Pb(ti/2 = 21 years), cannot be chemically separated from nonradioactive lead, but it was first shown [44,45] in 1914 that lead isolated from uranium minerals can be mainly Pb and lead from thorium minerals can be predominantly ° Pb, compared to the usual isotope mixture with 207.2 amu. Less than ten years later, it was established that isotopy is not restricted to + values above 205 but that the majority of elements show two or more stable isotopes in mass-spectra. 

Although anhydrous thorium sulfate dissolves in water at 0°C to the extent of 20 w/o, the solution is metastable and deposits hydrates on standing. Stable solutions at higher temperature require the presence of free sulfuric acid, as in solutions used to leach thorium minerals. Sec. 

The U.S. Geological Survey was scheduled to publish a revised study of U.S. thorium resources in August 1979. Partial results of this study, which cover most of these resources but do not include the beach placers of Florida, Georgia, and the Carolinas, were presented orally by Staatz [S5] of the U.S. Geological Survey in 1978. Table 6.14 lists the types of deposit, the principal districts in which potentially economic thorium-bearing deposits have been found, the principal thorium minerals, and estimates of thorium reserves and resources. Thorium from the vein deposits, the first type, could be produced for less than 30/lb. Thorium is the principal salable product in these deposits. Thorium could be coproduced with other elements from disseminated deposits, massive carbonatites, and placers the amount of thorium that might be produced from them, and its cost, depends on the marketability of the other minerals that occur with the thorium. 

Table 6.15 summarizes two sources of information on the annual rate of thorium production, by country. The first three columns give the production rate of monazite concentrates for the more recent years of 1976, 1977, and 1978 [El]. We have estimated total thorium production from a typical monazite thorium content of 6 weight percent (w/o). These columns do not include monazite production in the United States or Soviet Union, nor the small production of other thorium minerals. The last two columns give the U.S. Bureau of Mines figures [Ul] for total thorium production in 1973 and an estimate of total thorium production capacity in 1980, if demand were such as to support it. 

Type of deposit District Principal thorium minerals Other elements Reserves Probable potential resources... 

The principal steps in producing refined thorium compounds from thorium-bearing ores are concentration of thorium minerals, extraction of thorium from concentrates, purification or refining of thorium, and conversion to metal or the thorium compound finally wanted. This section describes the concentration of monazite, the principal source of thorium in the past the extraction of thorium from monazite and the recovery of thorium from leach liquors by solvent extraction. Purification of thorium is described in Sec. 9 and conversion in Sec. 10. 

Astatine is the heaviest member of group 17 and is known only in the form of radioactive isotopes, all of which have short half-lives. The longest lived isotope is At (fi = 8.1 h). Several isotopes are present naturally as transient products of the decay of uranium and thorium minerals At is formed from the 3-decay of Po, but the path competes with decay to Pb (the dominant decay, see Figure 2.3). Other isotopes are artificially prepared, e.g. "At (an a-... 

The first product of disintegration is mesothorium I, discovered by Hahn in 1907. It is isotopic with radium and is used as a substitute for certain radium preparations. As large quantities of thorium minerals are now worked up in connection with the gas-mantle industry, and mesothorium is a by-product, it has assumed commercial importance. It is separated from thorium in monazite being precipitated along with barium as sulphate. Thorium Xy discovered by Rutherford and Soddy in 1902, is another isotope of radium. Radiothorium, RdTh, is an active isotope of thorium and cannot be separated from it directly it has to be obtained from mesothorium I by disintegration if required free from isotopes. 

If the lead present in a pure uranium mineral has resulted from the disintegration of uranium atoms only, its atomic weight should approximate to that of uranium less 8 a-particles, that is, to 238 14 — 8 X 4 002, or 206 12. Actinium lead will be the same. But lead from a thorium mineral should have an atomic weight equal to that of thorium less 6 a-particles, that is 232 12 — 6 X 4-002, or 208 1T. Hence the atomic weight of lead may be expected to vary with its source. 

Frondel C, Riska DD, Frondel JW (1956) X-ray powder data for uranium and thorium minerals. U S Geol SurvEullE 1036-G 91-153... 

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