Uranium is a dense, radioactive element that is primarily extracted and refined as a fuel source for nuclear energy. In its metal form, uranium is used as a radioactive shield as well as in military armor and ammunition.
Properties
- Atomic Symbol: U
- Atomic Number: 92
- Atomic Mass: 238.03g/mol 1
- Element Category: Actinide
- Density: 18.95g/ cm3 at 20°C
- Melting Point: 1405°F (1132°C)
- Boiling Point: 6904°F (3818°C)
- Moh's Hardness: 6
Characteristics
Uranium is a radioactive, paramagnetic metal that is both malleable and ductile. Being nearly twice as dense as lead, it is one of the heaviest naturally occurring elements. The silvery metal form of uranium is, itself, very reactive, rapidly oxidizing in air and water.
For us, uranium's most important characteristic is that its atoms can be split - or fission - to release heat. More specifically, uranium-235, an isotope of uranium that makes up only about 0.7% of naturally occurring uranium is the fission-able material. Uranium-238, which makes up most of the remaining 99.3% of uranium available, does not have the same atomic properties.
History
In ancient times uranium oxide was used to produce yellow and green colored ceramics. Italian glass artifacts decorated using small amounts of uranium oxide have been dated back to 79 A.D.
Uranium was discovered in 1789 by German chemist, Martin Heinrich Klaproth as he studied the mineral pitchblende, which was thought to be a compound of zinc and iron. Although, at the time, Kalproth was unaware, he had only produced an oxide of uranium and not the pure element.
Not until 50 years later, in 1841, did the French chemist Eugene-Melchoir Peligot produce pure metallic uranium by reducing uranium tetrachloride (UCl4) with potassium.
In 1896 Henri Becquerel detected the radioactive nature of uranium, leading to studies on radium, nuclear chemistry and, ultimately, nuclear energy. This discovery gave rise to the atomic age and innovations like the atomic bomb and nuclear power reactors.
Production
The mineral ores with the highest concentration of uranium are pitchblende, uraninite, carnotite and brannerite. Uranium mining consists of both traditional underground and open-pit methods, which account for about 53% of global production, as well as in-situ leaching, which accounts for another 41%. Another 5% of uranium production is extracted as by-product from other mineral ores like gold and copper.
Uranium mining techniques are often determined by the grade of ore, which also reflects its geological classification. Low-grade ores, which contain less than 0.5% uranium, are usually treated with more cost-efficient heap leaching methods. Heap leaching require that the crushed ores are stacked 5 to 30 meters deep on a pad and irrigated with a leaching solution (often sulfuric acid). Over many weeks, the solution extracts uranium as it seeps through the ore and then is collected in pools. The solution, once precipitated, produces 50-80% uranium concentrate, which can then be further refined.
Higher-grade uranium ore, like that found in Canada's Athabasca basin, must be milled into a powder before being treated with sulfuric acid. The uranium containing acid can then be precipitated and treated with solvent extraction or ion exchange separation techniques to produce uranium oxide concentrate (U3O8), or yellowcake, which is has a uranium content of more than 80%.
In-situ mining methods have been developed to extract the uranium from sandstone bedded infiltration-type deposits, like those common in Kazakhstan. As opposed to conventional mining methods, this process allows the ore to stay underground. Instead, wells are dug into the body of the ore for injecting a sulfuric solution. The leaching process pulls uranium from the ore into the leaching solution that rises to the surface and can be collected and processed with ion exchange and precipitation techniques to form yellowcake.
Uranium metal is produced from depleted uranium (or DU), which is the uranium material that remains after yellowcake has been enriched to make fuel for nuclear reactors. Enriched uranium contains a higher content of U-235, while DU contains a lower content.
Depleted uranium is most often stored as the crystalline solid uranium hexafluoride. Uranium hexafluoride can be chemically reduced with hydrogen and exothermically reacted with magnesium to produce uranium metal. It can also be made through electrolysis is a alkali solution.
53,663 tonnes of uranium were produced worldwide in 2010. Around twenty countries worldwide have uranium mines, although Kazakhstan, Canada, and Australia account for over half of production, supplying 17,803, 9,783 and 5,900 tonnes, respectively. The largest producers of refined uranium are Cameco, Areva Resources, Kazatomprom and Rio Tinto.
Approximately, 5000 tonnes of uranium per year - or about 10% of global production - comes from recycling spent nuclear fuel rods.
Applications
Without question, the majority of uranium produced is used in the power generation sector, although a small amount is used for medical and research purposes, as well as military applications.
Before the advent of nuclear energy, uranium was very limited in its applications, but its key role in thermonuclear chain reactions have led to its use in energy generation, both for civilian and military purposes. Now, the global demand for uranium is estimated to be about 68,500 tonnes per year.
Uranium-235, which is separated from U-238 during the fuel enrichment process, is the required raw material for most nuclear reactors because of its unique ability to undergo nuclear fission. As a source of energy, uranium fission is highly efficient. 1kg (2.2lbs) of uranium-235 can produce as much energy as 1,500 tonnes of coal while producing far lower carbon emissions.
Uranium metal is used for its density and, in military applications, for its pyrophoric nature.
Uranium metal has been used as trim- or counterweights in aircraft and it continues to be used in radium shielding and in packaging containers for radioactive materials.
In military applications, DU metal is used to strengthen tank armor and in armor piercing munitions, Plated between steel, depleted uranium, is used as armor on some Abrams tanks. Bullets made with depleted uranium are not only heavy and able to pierce armor, but are more destructive than other metals because of its incendiary nature.
References
Holden, A.N. Physical Metallurgy of Uranium. Addison-Wesley Publishing Company, Inc.: 1958.
The World Nuclear Association. www.world-nuclear.org
