Yellowcake

After mining, the uranium must be concentrated before further operations are carried out. This is done in the mills, which are located near the mines. Here the uranium ore content is increased from a few tenths of a percent (in the ore) to 85-95% (in a semirefined concentrate known as yellowcake ), while eliminating other elements that are present (the tailings ). 

Following solvent extraction, uranium is precipitated from the solution by the addition of gaseous ammonia with the yellowcake product (Fig. 16.4) being collected, packaged in 55-gal drums, and shipped to a refinery for further purification and conversion to UF6 for enrichment. 

Alkaline leach mills will use NaOH for final product recovery (in the form of sodium uranate, Na2U207). Often these uranium ore concentrates (yellowcake) are transformed into U3Os by drying at 200°C to remove water and calcining, that is, heating until decomposition. 

Figure 16.4 Ammonium diuranate (yellowcake) after solvent extraction. (Photo courtesy of UIC.) (Figure also appears in color figure section.)...

Following milling, the yellowcake is shipped for refining and conversion. In most refineries, the uranium ore concentrates are purified by solvent extraction and then converted to UF6 for enrichment. (A schematic diagram for this process is shown in Fig. 16.5.)... 

Define or describe the following terms or concepts (a) yellowcake, (b) orange oxide, and (c) green salt. 

Ammonia is used to precipitate the final uranium concentrate salt (yellowcake) prior to drying or calcination. It is generally economically preferred over sodium hydroxide or hydrogen peroxide57. 

Nitric acid is used to digest crude uranium concentrates (UjOg, yellowcake) in some uranium purification processes. About 1,000 tonnes of nitric acid may be used to process yellowcake in the United States. 

In TBP extraction, the yellowcake is dissolved in nitric acid and extracted with tributyl phosphate in a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N03)2(TBP)2 which is extracted into the diluent. The purified uranium is then back-extracted into nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N03)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated during a pyrolysis step to form uranium trioxide [1344-58-7], U03, as shown in equation 10. The pyrolysis is most often carried out in either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The U03 is reduced with hydrogen to uranium dioxide [1344-57-6], U02 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF4, with HF at elevated temperatures (eq. 12). The UF4 can be either reduced to uranium metal or fluorinated to uranium hexafluoride [7783-81-5], UF6, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to U03, U02, and ultimately UF4 have been discussed in detail (40). 

Damon EG, Eidson AF, Hahn FF, et al. 1984. Comparison of early lung clearance of yellowcake aerosols in rats with in vitro dissolution and IR analysis. Health Phys 46 859-866. 

Fort CW, Jr., Douglas RL, Moore WE. 1980. Radioactive emissions from yellowcake processing stacks at uranium mills. Report ISS ORP/LV-80-3. NTIS PB81-153389. 

Fig. 7.1. Schematic presentations of (a) the once-through cycle (b) the thermal reactor cycle and (c) the fast reactor cycle. UjO, = yellowcake, UF = uranium hexafluoride, MOX = mixed oxide fuel (uranium/...

Uranium ore is typically mined or leached in situ. For the mined ore, the raw material is milled to produce a high surface area slurry, which is then treated with H2SO4. The sulfuric acid oxidizes the uranium to the soluble hexavalent state. The addition of base to the solution precipitates an oxide of uranium known as yellowcake , UsOs. A similar process is used for the in situ leaching of uranium ore except that the initial treatment is performed on bores with a slightly acidic and high oxygenated aqueous solution that is raised to the surface and allowed to go through an extraction process to remove the uranium. 

---