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Uranium Production
How Uranium is Produced
and Used
General Information
Uranium (U) – a weakly-radioactive metallic chemical element in the actinide series of the periodic table. Uranium atom consists of 92 protons, 92 electrons (including 6 valence ones) and between 141 and 146 neutrons, thus having six isotopes, the most common of which are U-238 and U-235 (99.27% and 0.72% of natural uranium).
Characteristics
• Atomic weight. Uranium is the heaviest naturally occurring element, also having high density - it is 18.7 times as dense as water.
• Fissionability. Uranium atom can be split into two lighter elements, releasing neutrons and nuclear binding energy. U-235 is the only naturally occurring fissile (easily splitting) isotope of uranium. Another fissile isotope, U-233, can be produced from natural thorium. U-238 is only fissionable with fast neutrons , but can be transmuted to fissile plutonium-239 in a nuclear reactor. A nuclear chain reaction can be induced with slow neutrons and maintained only when U-235 or U-233 are present among U-238 in sufficient concentrations (i.e. when U-238 is enriched).
• Weak radioactivity. All uranium isotopes are unstable and decay slowly by emitting an alpha-particle. Long half-life (U-238 – 4.5 billion years) makes uranium useful in dating the age of Earth and other objects.
Contemporary uses of uranium exploit its unique properties.
Induced nuclear fission of U-235
Uranium is covered with black oxide coating in the air.
Discovery and Studies
I century AD -
Uranium in its natural oxide form had been used as a
yellow, red and orange colorant in glassmakin
g and pottery.
1789 - the German chemist Martin
Klaproth obtained uranium
oxide from pitchblende, supposed it was an unknown element
and named it.
1841 - Eugène-Melchior Péligot isolated metallic uranium.
1896 - Antoine
Becquerel discovered radioactivi-ty by using uranium.
1898 - Pierre and
Marie Curie found
radioactive radium and polonium in
uranium ore.
Industrial mining of uranium
was started to extract radium.
1934-1939 - Research of radioactive elements
resulted in the discovery
of nuclear fission.
Importance of uranium as
a weapon material and
energy source had been widely acknowled-
ged.
1942 – Under the Manhattan
Project, Enrico Fermi
initiated the first
self-sustained nuclear chain
reaction.
1945, August 6th
– Uranium-
based “Little Boy”
bomb was detonated
over Hiroshima
1954, June 27th – the
first commercial
-scale nuclear power station began
operation in Obninsk,
USSR.
Martin Henrich Klaproth
One of the Manhattan Project’s calutrons in Oak Ridge, Tennessee
Military Application
Fissile explosive material for nuclear weapons. Early nuclear bombs used U-235 or plutonium-239 derived of U-238. More recent and complicated devices are based on uranium-cased plutonium causing hydrogen isotopes to undergo fusion.
Armour-piercing ammunition – due to high density.
Counterweights and balances in military equipment.
Anti-radioactive shielding – due to low radioactivity and high density (more effective than lead).
Nuclear explosion
Bullet shell made of depleted uranium
Civilian Application
Fuel for nuclear power plants. Controlled fission chain reaction creates heat by splitting U-235
atoms, making water steam spin a turbine to drive a generator,
producing electricity. One kilogram of uranium-235 can theoretically produce about 80 terajoules of
energy or as much as 3000 tonnes of coal. Nuclear plants typically use
fuel enriched to 3% of U-235. By now over 440 nuclear reactors
operate all over the world.
Making of high-energy X-rays
Radiometric dating
Formerly used in glass-making, pottery, photography
Uranium Minerals
The primary uranium ore mineral is uraninite (UO2) or pitchblende (UO3, U2O5), commonly collectively referred to as U3O8(the most stable form).
A large variety of secondary uranium minerals are known, many of which are brilliantly coloured and fluorescent. The most common are gummite, autunite (with calcium), saleeite (magnesium) and torbernite (with copper); and hydrated uranium silicates such as coffinite and uranophane (with calcium).
Uraninite or Pitchblende Autunite crystals under the microscope
Torbernite, an important secondary uranium mineral
Cuprosklodowskite - CuUO2SiO2OH2•6H2O.
Uranium MineralsPrimary uranium minerals
Name Chemical Formulauraninite UO2
pitchblende U3O8, rare U3O7
coffinite U(SiO4)1–x(OH)4x
brannerite UTi2O6
davidite (REE)(Y,U)(Ti,Fe3+)20O38
Secondary uranium mineralsName Chemical Formulaautunite Ca(UO2)2(PO4)2 x 8-12 H2O
carnotite K2(UO2)2(VO4)2 x 1–3 H2O
gummite gum like amorphous mixture of various uranium minerals
seleeite Mg(UO2)2(PO4)2 x 10 H2O
torbernite Cu(UO2)2(PO4)2 x 12 H2O
tyuyamunite Ca(UO2)2(VO4)2 x 5-8 H2O
uranocircite Ba(UO2)2(PO4)2 x 8-10 H2O
uranophane Ca(UO2)2(HSiO4)2 x 5 H2O
zeunerite Cu(UO2)2(AsO4)2 x 8-10 H2O
Uranium Ore DepositsUranium on Earth is 40 times more common than silver and 500 times more common than gold, and its tiny concentrations can be found almost everywhere, but the challenge is to find areas with adequate concentrations for economically viable extraction.
IAEA classification (1996) lists 15 main deposit types, of which the most economically significant are unconformity-related and sandstone deposits. The latter are common in the Southern Kazakhstan. Uranium reserves distribution
Uranium Mining• The contemporary economically viable
methods of uranium extraction include: Open pit mining Underground mining In-situ leaching Uranium recovery from seawater has been also
tried and found uneconomical.
Uranium mining distribution in 2007
The worldwide production of uranium in 2009 amounted to 50 572 tons, of which 27% was mined in Kazakhstan. Uranium mines operate in some 20 countries, 58% of world production comes from 10 mines in six countries, these six accounting for 85% of the world’s mined uranium. The diagram to the right shows distribution of uranium mining volumes by countries in 2007. Though the early uranium boom of 1950s made Moab, Utah “the world’s uranium capital” and large deposits were discovered in Siberia around that time, nowadays the “Big Three” of uranium market are Canada, Australia and Kazakhstan.
Open-Pit and Underground Mining
These mining methods are no different from mining other metals or minerals, and gold, silver or copper are often mined in association with uranium.
• If orebodies lie close to the surface, they can be accessed by open cut mining. Overlying rock is removed by blasting and excavation, and ore is extracted from a large pit. Disadvantages of the method include much waste material, high airborne dust levels (suppressed by water) and significant alteration of landscape, e.g. damage to fertile soils.
• Underground mining is employed when deposits occur at larger depths. A shaft is sunk in the vicinity of ore veins, and crosscuts are driven horizontally from the shaft to the veins at different levels. Ultimately the shaft and tunnels form a structure of the mine with several stopes, from which the ore is extracted. Though less waste material is removed from underground mines, underground mining of uranium is more dangerous for workers due to radiation exposure and release of Radon-222 gas causing cancer.
Uranium mine near Moab, Utah
In Situ Leach Mining
Some orebodies lie in groundwater in porous unconsolidated material (such as gravel or sand) and may be accessed simply by oxygenating the groundwater and pumping it out with dissolved uranium – this is in-situ leach (ISL) mining. ISL mining means that removal of the uranium minerals is accomplished without any major ground disturbance. Weakly acidified groundwater (or alkaline groundwater where the ground contains a lot of limestone with a lot of oxygen in it) is circulated through an enclosed underground aquifer. The leaching solution dissolves the uranium before being pumped to the surface treatment plant where the uranium is recovered as a precipitate. Most US and Kazakh uranium is produced by this method.
The general scheme of an ISL wellfield
In Situ Leach Mining: Leaching
The uranium ions are selectively transferred into the solution in the result of chemical processes involving decomposition of uranium-bearing substances by leachants and ion exchange. The resulting “pregnant” solution proceeds through several stages, circulating in a closed technological cycle.
In Situ Leach Mining: Processing Unit
As the process is complete, the uranium bearing solution is supplied into sorption columns. The ion-exchange resin intakes the uranium ions from the pregnant solution. Saturated resin is supplied to desorption columns. At this stage the uranium is stripped off the resin by a nitrate solution, producing eluate, a solution of high uranium content. The denitrated resin returns back to the sorption columns. For uranium precipitation from the eluate Kazatomprom uses caustic soda. Obtained slurry is filtered and dried into (“torrefaction”) yellow cake. In the refinery the yellow cake is dissolved for organic extraction, a process allowing for additional purification by organic substances. Following precipitation, drying and thermal decomposition operations the refinery produces U3O8.
In Situ Leach Mining: Recirculation
The solution resulted after sorption collected in settling ponds to reduce remaining solid impurities. The refined solutions after adding sulfuric acid are returned to the well house. From the well house solutions go via a distribution unit to the wellfield under the pressure of 5-6 atmospheres. The leaching solution penetrates through filters of the injection wells to the underground productive horizon where the uranium transfers into the solution. With the use of wellfield pipeline system including the submersible pumps or an airlift or their combination the uranium-bearing solutions are supplied to the surface through the productive wells. The uranium bearing solutions are accumulated from a number of productive wells in accumulating tanks. From the collecting tanks the solutions proceed to sorption columns.
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