Uranium is a naturally occurring element that is obtained as its oxides from ores in a wide variety of deposits in Earth’s crust. These ores primarily consist of uraninite (UO2) and pitchblende (U2O5 • UO3). The generic uranium oxide that is ultimately used as the feedstock for uranium enrichment (vide infra) is U3O8, often referred to as UOX or yellowcake. Pure uranium is hard, silvery, extremely dense (1.6 times denser than lead) and, of course, radioactive. The two naturally occurring isotopes—U-235
and U-238—are both radioactive, but only U-235 is fissile. The concentration of U-235 in naturally occurring uranium is only 0.7%, the remainder being U-238.
A majority (roughly 64%) of the world’s uranium production is from just three countries—Kazakhstan, Canada, and Australia (World Nuclear Association 2013b). Extraction of uranium-containing ores from Earth’s crust is also energy intensive and environmentally disruptive. About 20% of uranium ore is mined from open pits and roughly 30% from underground excavations where the ore is crushed and the uranium oxides leached out by treatment with sulfuric acid (Equations 9.11a and 9.11b).
UO3 (s) + 2H+ (aq) ^ UO2+ (aq) + H2O
UO2+ (aq) + 3SO4- (aq) ^ UO2(SO4)4- (aq)
A process that has grown steadily in the last few decades is in situ leach mining or in situ recovery (ISL or ISR) and, in fact, this method is now the most commonly used for uranium recovery. In ISL, the uranium oxide is dissolved directly from ore in situ using either an acid (typically sulfuric acid) or, when limestone or other carbonate minerals are present that would also dissolve in the acid, an alkali leaching solution. The uranium-laden solution is then pumped to the surface. This method, of course, requires careful monitoring to ensure that groundwater supplies are not contaminated (World Nuclear Association 2012). If the leachate is acidic, the dissolved UO2 (SO4)4- can be extracted into an organic solvent by treatment with a lipophilic quaternary amine (R3NH+) that coordinates with the trisulfate, extracting it into an organic solvent for subsequent concentration and conversion to the oxide.