…
Uranium (U), although very
dense (19.1 grams per cubic centimeter), is a relatively weak, nonrefractory metal. Indeed, the metallic properties of uranium appear to be intermediate between those of silver and other true metals and those of the nonmetallic elements, so that it is not valued for structural applications. The principal value of uranium is in the radioactive and fissionable properties of its isotopes. In nature, almost all (99.27 percent) of the metal consists of
uranium-238; the remainder consists of
uranium-235 (0.72 percent) and uranium-234 (0.006 percent). Of these naturally occurring isotopes,
only uranium-235 is directly fissionable by neutron irradiation. However, uranium-238, upon absorbing a neutron, forms
uranium-239, and this latter isotope eventually decays into
plutonium-239—a
fissile material of great importance in
nuclear power and nuclear weapons. Another fissile isotope,
uranium-233, can be formed by neutron irradiation of thorium-232.
Extraction and refining
…
Leaching
…
Acid leaching is commonly performed by agitating an ore-leach mixture for 4 to as long as 48 hours at ambient temperature. Except in special circumstances,
sulfuric acid is the leachant used; it is supplied in amounts sufficient to obtain a final leach liquor at about pH 1.5. Sulfuric acid leaching circuits commonly employ either manganese dioxide or chlorate ion to oxidize the tetravalent uranium ion (U⁴⁺
) to the hexavalent uranyl ion (UO₂²⁺
). Typically, about 5 kilograms (11 pounds) of manganese dioxide or 1.5 kilograms of sodium chlorate per ton suffice to oxidize tetravalent uranium. In any case, the oxidized uranium reacts with the sulfuric acid to form a uranyl sulfate complex anion, [UO₂
(SO)₃
]⁴⁻.
Uranium ores that contain significant amounts of basic minerals such as calcite or dolomite are leached with 0.5 to 1 molar sodium carbonate solutions. Although a variety of reagents has been studied and tested, oxygen is the uranium oxidant of choice. Typically, candidate ores are leached in air at atmospheric pressure and at 75 to 80 °C (167 to 175 °F) for periods that vary with the particular ore. The alkaline leachant reacts with uranium to form a readily soluble uranyl carbonate complex ion, [UO₂
(CO₃
)₃
]⁴⁺.
Prior to further processing, solutions resulting from either acidic or carbonate leaching must be clarified. Large-scale separation of clays and other ore slimes is accomplished through the use of effective flocculants, including polyacrylamides, guar gum, and animal glue.
…
= new reply since forum marked as read