Uranium, extraction economics

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). 

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. 

By calcnlating the residence times of the various solids in the tank and relating them to their corresponding extraction curves, the total uranium extraction for the entire train of mixers was estimated. The cost of the various mixer options, the prodnction efficiency net resnlt, and the cost of the installation and tank design conld be combined to yield the economic optimum for the plant. 

Uranium deposits occur in many countries, but as shown in Table 12.4 the economically recoverable reserves, like oil and gas reserves, are primarily located outside of Europe. The OECD Nuclear Energy Agency and the International Atomic Energy Agency (IAEA, 2008) expects that it will be possible to double the global uranium extraction at cost below US 80/ kgU to approximately 120,000 tU per year in 2016. At a constant rate of extraction these resources would allow for uranium extraction well into the 2040s. 

As most of the excavated uranium is subsequently used as nuclear fuel, the price of conversion of the ore (yellow cake) to UFg, the price of enrichment (separative work units—SWU), the cost of deconversion of the enriched UF to uranium oxide (or other chemical forms), and the production of fuel elements determine economic factors. In addition, the cost of electric power production by nuclear power plants in comparison with other plants (gas, coal, and oil) and the overall cost of disposal of the waste from all these processes will influence the worthiness of uranium extraction. In view of the rapid changes in the prices of these processes, it is difficult to assess the threshold concentration of uranium in the ore that will make mining viable economically. Furthermore, nations or organizations that cannot purchase uranium... 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

In most uranium ores the element is present in several, usually many diverse minerals. Some of these dissolve in sulfuric acid solutions under mild conditions, while others may require more aggressive conditions. Thus, while it may be comfortable to recover 90-95% of the uranium present, it may be tough or impractical to win the balance amount of a few percent economically. Some of the most difficult uranium minerals to leach are those of the multiple oxide variety, most commonly brannerite and davidite. These usually have U(IV) as well as U(VI), together with a number of other elements such as titanium, iron, vanadium, thorium, and rare earths. To extract uranium from these sources is not as easy as other relatively simpler commonly occurring sources. 

Lanthanides are also found as minor components in other ores, particularly in association with uranium or in phosphate rock. These are often coextracted with the major product and can be economically recovered from the waste streams resulting from the uranium or phosphoric acid extraction. 

Molybdenum can also be recovered economically from some uranium leach liquors, particularly those of the USA. When uranium is stripped from amine extractants by solutions of sodium chloride, any molybdenum present remains in the organic phase, and can be subsequently recovered by being stripped into a solution of sodium carbonate. A process has been operated in which the strip liquor is acidified to a pH value of 4.5 and the molybdenum is reextracted into a solution of quaternary amine chloride in kerosene.218 The extracted metal is stripped into a solution containing sodium hydroxide and sodium chloride to produce liquors containing 30-40 g of molybdenum per litre, from which calcium molybdate can be precipitated by the addition of calcium chloride. 

Low enrichment of uranium concentration (D 10 see Table 50. An economically viable process for extracting uranium is achievable only with D > 10. ... 

The achievement of an economically viable process for the extraction of uranium fix>m seawater, however, could be achieved only through use of a sorbent with uranium concentrating factors greater than those provided by titanium oxides [181]. Polyacrylamidoxime sorbents, characterized by D value > 10 ( 10 ) (see Table 5) made an appearance at the end of the 1970s to remove this impediment to the economic recovery of uranium fix>m seawater. Their appearance reoriented research in the field of uranium recovery toward highly selective organic resins [188, 189]. 

Current estimates of the available reserves and further resources of uranium and thorium, and their global distribution, are shown in Figs. 5.44-5.50. The uraruum proven reserves indicated in Fig. 5.44 can be extracted at costs below 130 US /t, as can the probable additional reserves indicated in Fig. 5.45. Figure 5.46 shows new and unconventional resources that may later become reserves. They are inferred on the basis of geological modelling or other indirect information (OECD and IAEA, 1993 World Energy Council, 1995). The thorium resource estimates are from the US Geological Survey (Hedrick, 1998) and are similarly divided into reserves (Eig. 5.47), additional reserves (Fig. 5.48) and more speculative resources (Fig. 5.49). The thorium situation is less well explored than that of uranium the reserves cannot be said to be "economical", as they are presently mined for other purposes (rare earth metals), and thorium is only a byproduct with currently very limited areas of use. The "speculative" Th-resources may well have a similar status to some of the additional U-reserves. 

Most of the procedures for extracting trace metals from sea water have been developed with the objective to analytically determine these metals till now, no trace metal including uranium can be economically recovered from sea water. An enrichment of some trace metals in marine organisms will briefly be mentioned as well. 

Preliminary studies on potential methods for the extraction of uranium from sea water took into consideration not only the extraction by solid sorbents, but also by solvent extraction, ion flotation, coprecipitation, and electrolysis. However, for a large-scale uranium recovery only the sorptive accumulation by use of a suitable solid sorbent seems to be feasible with regard to economic reasons and environmental impacts n9). 

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