Uranium concentrate

Seawater. The world s oceans contain ca 4 X 10 t of uranium (32). Because the uranium concentration is very low, approximately 3.34 ppm, vast amounts of water would be required to recover significant amount of uranium metal, ie, 10 m of seawater for each metdc ton of U. Significant engineering development and associated environmental concerns have limited the development of an economic means of uranium extraction from seawater (32) (see Ocean RAWMATORiALs). 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Concerns that diagenetic alteration could affect measured °Th ages of corals have existed since the early applications using alpha-counting methods. Thurber et al. (1965) offered the following list for reliable coral °Th ages a) no evidence of recrystallization, b) uranium concentration of 3 ppm, c) p°Th/ Th] of > 20, d) of 1.15 ... 

Initial U concentration. If uranium concentration has changed as a result of diagenetic reactions, one may, in principle detect this by comparison between uranium concentrations in modem corals and their fossil counterparts. Early work documenting and studying uranium concentrations in corals is extensive (e g., Barnes et al. 1956 Tatsumoto and Goldberg 1959 Veeh and Turekian 1968 Schroeder et al. 1970 Thompson and Livingston 1970 Gvirtzman et al 1973 Amiel et al. 1973 Swart 1980 Swart and Hubbard 1982 Cross and Cross 1983). This broad body of data shows that primary surface coral uranium concentrations lie between 1.5 and 4 ppm (see Fig. 1). Concentrations appear to be species dependent (Cross and Cross 1983). Furthermore, uranium concentrations vary within individual coral skeletons (Schroeder et al. 1970 Shen and Dunbar 1995 Min et al. 1995). 

Thompson G, Livingston HD (1970) Strontium and uranium concentrations in aragonite precipitated by some modem corals. Earth Planet Sci Lett 8 439-442 ThurberDL (1962) Anomalous in Nature. J Geophys Res 67 4518-4520... 

Veeh HH, Turekian KK (1968) Cobalt, silver and uranium concentrations of reef building corals in the Pacific Ocean. Limol Oceanogr 13 304-308... 

Figure 4. The uranium concentration in unfiltered water, 0.2 gm and 3 kD filtered water in river water from the Kalix River mouth and samples from the low salinity estuarine zone (0-3). Data plotted against conductivity (although the salinity scale is not defined below 2, a tentative scale is indicated). The lines represent the best fit for each fraction in the estuary. The data from the Kalix river mouth represent the river water component, which show <10% aimual variation in concentration. The analytical errors are smaller than the symbols. Data from Andersson et al. (2001). Copyright 2001 Elsevier Science.

Figure 5. The in 0.2pm and 3 kD filtered water and colloids phase (3kD - 0.2pm) and particles (>0.2 pm) as well as material from sediment traps plotted versus conductivity in the low salinity zone (0-3) of the Kalix River estuary. The stippled area marks the reported annual range in at the Kalix river mouth, which show a substantial variation compared to the uranium concentration. Data from Andersson et al. (2001). Copyright 2001 Elsevier Science.

The D-A model predicts the distribution of uranium and U-series isotopes across a bone section (Figs. 3 and 4). Under constant conditions Uranium is diffusing from the inner and outer surfaces of the bone, giving a u-shaped Uranium concentration profile that gradually flattens with time to a uniform uranium distribution when the bone reaches equilibrium with the uranium in the groundwater. Because the uranium is equilibrating with the outer portions of the bone section first, closed system U-series dates approach the true age of the bone towards the surfaces, but are underestimated towards the centre. Further details of the D-A model are given in the Appendix. 

The track density can be easily determined on a newly cut, polished, and etched surface by counting, under an optical microscope, the number of etched tracks in a measured area of the solid. The uranium concentration can be determined by a number of analytical techniques. Following these... 

The catalyst activity is so high that uranium concentration lower than 0.1 millimoles per liter allows a complete conversion of butadiene to be obtained in a few hours, at 20°C, The transfer reaction of uranium based catalyst is similar to that of conventional 3d-block elements (titanium, cobalt, nickel) so that the molecular weight of the polymer is affected by polymerization temperature, polymerization time and monomer concentration in the customary way. This is in contrast, as we shall see later on, to some catalysts based on 4 f-block elements. Uranium based catalysts are able to polymerize isoprene and other dienes to high cis polymers the cis content of polyisoprene is 94%, somewhat inferior to titanium based catalysts. In contrast, with 3d-block elements an "all cis", random butadiene-isoprene... 

The same is true of the geographical distribution of the uranium concentration in glacial till, the most common soil type in Finland (Geological Survey of Finland, 1985). The sometimes very large effect of ground permeability is the main reason that the details of the radon concentration distribution differ from the distribution of other radiation parameters. 

Leung et al. [ 104] and Kim and Zeitlin [105] described a method for the separation and determination of uranium in seawater. Thoric hydroxide (Th(OH)4) was used as a collector. The final uranium concentration was measured via the fluorescence (at 575 nm) of its Rhodamine B complex. The detection limit was about 200 jLg/l. 

The conclusions of Hurt s study of year-by-year oxygen isotope ratios in 72 years of S. gigantea are thus supportive of the conclusions of the CIAP study [49] that solar variations influence the abundances of many kinds of chemical species in the stratosphere, and therefore influence the.amount of solar energy they absorb and re-radiate to earth, and therefore influence the surface temperature of the earth and especially the surface temperatures of the oceans. It is the surface temperature of the oceans which produces the phenomena we have discussed the isotope ratio variations in rain and hence in tree rings, the isotope ratio variations in the Greenland ice cap, in the organic carbon and uranium concentrations in sea cores, and furthermore variations of the sea surface temperature produces variations in the carbon-14 to carbon-12 ratio fractionation at the sea air interface and hence in the carbon-14 content of atmospheric carbon dioxide and hence in the carbon-14 content of tree rings. 

Fig. 2. Uranium concentrations (ppm) in C-horizon soil samples, Nova Scotia. ...

Another potentially vast resource is seawater. Uranium resources associated with the oceans are estimated at around 4000 million tonnes however, the uranium concentration in seawater is only around 0.003 ppm. The recovery of uranium from seawater is still subject to basic research. Considerable technological developments as well as significant improvements of economics (or drastic increases in uranium prices) are crucial for the commercial use of this resource, which is unlikely in the foreseeable future. As the energy demand for uranium extraction increases with lower concentrations, the net energy balance of the entire fuel cycle is also critical. 

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