Pitchblende uranium content

Of much greater significance than this, however, was Mme. Curie s observation that the activity of the uranium mineral pitchblende is four or five times as great as one might expect it to be from its uranium content (24). She concluded that the ore must contain another radioactive element in addition to uranium, and that, since the composition of the ore was known, the active element must be present in extremely small amount and must therefore be very active indeed. Therefore it became necessary to work up large quantities of pitchblende and to make elaborate and tedious fractionations of this complex ore. 

In 1896, a young Polish chemist, Marie Curie, and her French husband, Pierre, decided to find out why a certain uranium ore called pitchblende gave off rays that were much stronger than the uranium content of the ore could explain. 

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. 

She found that all uranium salts were active, but so was aesc/i ife, which contained no uranium. Pitchblende produced emanations of greater intensity than could be attributed to its uranium content. 

The uranium content of natural waters is generally determined by the geological conditions. The most important uranium mineral is uraninite (pitchblende, XUO3. yU02), of which there are sizeable deposits in Zaire, Canada and in Czechoslovakia (Joachimsthal), but also in Colorado (USA), North Africa (phosphate companion). South Africa (gold companion), Australia... 

Uranium occurs in most phosphate rocks, but its concentration varies from deposit to deposit. Some sedimentary rocks show notably higher concentrations than most igneous rocks. The most uranium-rich rocks found to date are those of Florida (up to 300 ppmw U), Morocco (up to 230 ppmw U), and Jordan (up to 240 ppmw U) [32,33[. In comparison, conventional uranium deposits such as pitchblende typically have a uranium content of 1,000-3,000 ppmw. When the phosphate rock is acidulated, up to 80% or 90% of the ur ium passes into solution in the phosphoric add. The exact amount of PR acidulated depends on its characteristics and the parameters of the phosphoric acid process employed. Examples of the uranium contents of different phosphate rocks and of phosphoric acids produced from them are indicated in Table 11.29. 

Almost immediately after the discovery of radioactivity, Marie Sklodowska Curie and Pierre Curie began more detailed studies of the new phenomenon. Guided by their observation that some natural uranium ores, such as pitchblende, were more highly radioactive than corresponded to their uranium content (Sklodowska Curie 1898), they fractionated the ores chemically, using the intensity of radioactivity in the fractions as evidence for further radioactive substances. The result was the discovery, in June 1898, of a new radioactive element in the bismuth fraction (Curie and Curie, 1898) the Curies named it polonium in honor of Marie s homeland. A few months later, in December 1898, they were able to report the discovery of another radioactive element, this one in the barium fraction separated from pitchblende (Curie et al. 1898) they named it radium. The subsequent isolation of radium from barium was accomplished by fractional crystallization of barium chloride, with radium chloride always being enriched in the crystalline phase. It soon became possible to characterize radium spectroscopically by optical emission lines (Demar9ay 1898) and, thus, to confirm the discovery by an independent identification. By 1902, M. Curie had isolated 120 mg of pure... 

Uranium and thorium are widely distributed in the earth s crust, with average concentrations of 2 and 8 ppm, respectively (Grainger 1958 Boyle 1982 Frondel et al. 1967 Wills 1997). Over 100 different uranium-based minerals are known, but deposits of high-grade ore (hke uraninite and pitchblende, with as much as 70% uranium by weight) are rare. Most of the world s supply of recovered uranium has been extracted from ores with a uranium content of only about 0.2%. Conversely, the mineralogy of thorium is less varied, and deposits of its principal mineral, monazite, are fairly common. The thorium content of pure monazite is typically between 5% and 10%. 

Curie measured the intensity of the radiation from the mineral pitchblende. The principal metal in pitchblende is uranium, but smaller quantities of many other metals are present as well. Curie found that the radioactivity of pitchblende was considerably greater than could be explained on the basis of its uranium content, and she concluded that there was an unknown and highly radioactive element present. She dissolved some pitchblende in acid and precipitated each metal in turn and checked with the electrometer to see if the radioactivity had come out of solution. She found that a high radioactivity was associated with the bismuth fraction, and named the new element which was clearly present polonium in honour of her native country. However, the spectrum of the precipitate contained no new lines, so it appeared that polonium must be present in extremely small quantity. 

At Montmassacrot (Fig. 5) the mineralized formations trend WSW-ENE and dip 60-80° to the north. They consist of very brecciated seams of disseminated pitchblende and coffinite (average grade, 0.8%o). Radon clearly indicates the mineralization with anomalous values close to the ore deposit. There is a good correlation between these results and those of the dry residual uranium content. 

Dominant ore minerals in the reduced zone are pitchblende and coffinite and, in some deposits, associated primary vanadium oxides—for example, montroseite. In oxidized zones the important uranium minerals are the uranyl vanadates (carnotite, tyuyamunite or francevillite). Accessory elements include Mo, Se and Cu. Average uranium content ranges from 0.01 to 0.40 7o U3O8. 

The genetic model may have major significance for the identification of additional resources in these deposits. A very simple calculation shows that if pure pitchblende veinlets only 1 cm thick were repeated at 20-m intervals, the uranium content of the whole rock volume would average 1000 ppm, which is the grade of the oxidized ores presently mined. Therefore, the presently known resources could possibly be increased several times in depth. It therefore appears highly desirable to undertake major scientific research on this type of deposit. 

All isotopes of radium are radioactive and occur naturally in very small quantities. In their studies of radioactive elements, husband and wife Pierre (1859-1906) and Marie (1867-1934) Curie discovered that pitchblende—the mineral that was their source of uranium—was more radioactive than its uranium and thorium content alone could explain. Therefore, they suspected that a new, undiscovered element... 

If a mineral is sufficiently old and rich in Th and U, Pb concentrations can be high enough to be detectable using an electron microprobe. Analysis of all three elements then provides an estimate of age. This method has been widely used in uranium-ore studies, as pitchblende produces high Pb contents in relatively short times. Recently this technique has been applied with success to monazite (Suzuki and Adachi 1991, 1994 Montel et al. 1994, 1996 Williams and Jercinovic 2002), and, with more difficulty, to zircon, and xenotime (Suzuki and Adachi 1991, Geisler and Schleicher 2000)... 

But Marie had then the dogged determination that would dominate her whole life, so she observed that radiation from uranium did not seem to depend on temperature and continued. Marie decided to investigate all known elements to see if she could find others that were radioactive. She found another thorium. But then so did others who were investigating this same effect at this same time. Marie s inspiration came when she decided to look at minerals for new sources of radiation. She found that pitchblende, the commonest ore of uranium, was more radioactive than its uranium and thorium content could account for. There was something else in there, and Marie set out to find it. 

Two of the most well-known applications of the sodium carbonate leaching of ores are the extraction of uranium from pitchblende in Canada and extraction of both uranium and vanadium from camotite, which is mined in numerous localities of the Colorado Plateau area of the United States. The latter deposits were originally worked for their vanadium content but since the 1939-45 war the emphasis has been on uranium, with vanadium a subsidiary product. 

There is convincing evidence that some uranium ores have in past geological epochs sustained natural chain reactions. Some samples of pitchblende, UjOg, from the Oklo Mine in Gabon, Africa, have a U content distinctly lower than... 

Earth, the content of uranium minerals relative to was therefore higher. The age of the Oklo deposit has been established at 1.74 x 10 years. Calculation indicates that the content of the Oklo pitchblende 1.74 x 10 years ago was about 3 %. At this concentration of fissile material, water suffusing the ore deposit could have brought regions of the deposit to criticality, and a slow or intermittent chain reaction could have ensued. It is believed that other concentrated uranium ore deposits could have achieved supercriticality in the presence of water as a neutron moderator (2-3) x 10 years ago. Such chain reactions conceivably played an important part in early geological events. It is interesting to note that the fission products produced at Oklo over an estimated period of 10 years are still retained in the rock in which they were formed more than a billion years ago [63]. 

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