Page 119 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 105

its hexavalent state and dissolving it as an anionic complex (likely as a carbonate,
UO2(CO3)n 2- 2n, or as a sulphate complex, UO2(SO4)n 2- 2n), often at elevated temperatures.
As the water and the dissolved uranium migrated downstream, regions of reducing material
were encountered, either inorganic (e.g. pyrite) or organic (e.g. humic) matter, which
causexl reduction to U(IV). Since most U(IV) compounds are insoluble, the uranium
precipitated, possibly as the sulphide or, more likely, as the hydroxide. Many of these
original uranium precipitates were later covered by sedimentary material.
In most minerals uranium is in the tetravalent state. The most important one is uraninite
(UO2+ x, x = 0.01 to 0.25), in which the uranium concentration is 50 - 90%; it is found
in Western Europe, Central Africa (e.g. Katanga, Gabon) and Canada (e.g. Cigar Lake)
and Australia (e.g. Koongara). In the USA and Russia camotite (a K + U vanadate) is the
most important mineral and contains 54 % uranium. In the high grade ores the mineral is
mixed with other minerals so the average uranium concentration in the crushed ore is much
less: e.g. < 0.5 % on the Colorado Plateau. Uranium is often found in lower concentration,
of the order of 0.01 - 0.03 %, in association with other valuable minerals such as apatite,
shale, peat, etc.
At the 1999 price of --$30 per kg U, the known (total of measured, indicated and
inferred) world reserves of uranium which could be recovered economically was about 3.3
Mton U30 8. This corresponds to -40 years consumption assuming LWR:s and present
nuclear capacity to increase by - 1% a year. As energy cost increases, it is estimated that
about 20 Mton can be recovered at a higher cost. Sea water contains some 4 500 Mtons U
but it is uncertain if U can be recovered economically from this huge resource.
The uranium production in 1999 was -37 kton of U30 8. The ten largest producers, in
decreasing order, were Canada, Australia, Niger, Namibia, Uzbekistan, Russia, USA,
Kazakhstan, South Africa and the Czech Republic. The Cigar Lake mine in Canada is
interesting for its location (430 m below a lake), size (- 130 kton U), and high U-content
(up to 19%) which poses a difficult working environment (Rn plus quite high levels of
y-radiation); the mine is expected to open in 2005. The past over capacity in U-production
has caused large U stockpiles and a small interest in prospecting and opening of new mines.

5.5.3. Production techniques

Uranium ores differ widely in composition, containing a variety of other elements which
must be removed. As a result the production methods differ considerably depending on the
particular ore to be processed although in every case very selective processes must be used.
The following is a common scheme.
The ore is mined in open pits or underground. The ore is crushed and concentrated
through flotation. If the uranium is in the tetravalent state it is oxidized in piles by air,
sometimes with the aid of bacteria. The material is subsequently treated with sulfuric acid
which dissolves the uranium as the sulphate complex, UO2(SO4)22- (the feed). This complex
can be selectively removed from the aqueous solution by means of anion exchange resins
or, more commonly, by extraction into an organic solvent (solvent extraction). In the latter
case an extractant (i.e. an organic compound with ability to form a U-organic complex
soluble in organic solvents) dissolved in kerosene is used; depending on the aqueous feed
composition, various extractants have been applied. A typical flow-sheet is illustrated in
Figure 5.2, and the basic chemical principles in Table 5.3. The final product is commonly