All uranium fluorides are created using uranium tetrafluoride (UF4
itself is prepared by hydrofluorination of uranium dioxide. Reduction of UF4
with hydrogen at 1000o
C produces uranium trifluoride (UF3
). Under the right conditions of temperature and pressure, the reaction of solid UF4
with gaseous uranium hexafluoride (UF6
) can form the intermediate fluorides of U2
, and UF5
One method of preparing uranium tetrachloride (UCl4
) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of UCl4
by hydrogen produces uranium trichloride (UCl3
) while the higher chlorides of uranium are prepared by reaction with additional chlorine. All uranium chlorides react with water and air.
Bromides and iodides of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding UH3
to those element’s acids. Known examples include: UBr3
, and UI4
Occurrence and Production of Uranium
Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is also the highest-numbered element to be found naturally in significant quantities on earth and is always found combined with other elements. Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernova explosions. The decay of uranium, thorium and potassium-40 in the Earth’s mantle is thought to be the main source of heat that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.
Its average concentration in the Earth’s crust is (depending on the reference) 2 to 4 parts per million, or about 40 times as abundant as silver. The Earth’s crust from the surface to 25 km (15 mi) down is calculated to contain 1017 kg of uranium while the oceans may contain 1013 kg. The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizers), and 3 parts per billion of sea water is composed of the element.
It is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum. It is found in hundreds of minerals including uraninite (the most common uranium ore), autunite, uranophane, torbernite, and coffinite. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources with as little as 0.1% uranium).
Citrobacter species can have concentrations of uranium in their bodies 300 times higher than in the surrounding environment.Some microorganisms, such as the lichen Trapelia involuta or the bacterium Citrobacter, can absorb concentrations of uranium that are up to 300 times higher than their environment. Citrobactor species absorb uranyl ions when given glycerol phosphate (or other similar organic phosphates). After one day, one gram of bacteria will encrust themselves with nine grams of uranyl phosphate crystals; this creates the possibility that these organisms could be used to decontaminate uranium-polluted water.
Plants absorb some uranium from the soil they are rooted in. Dry weight concentrations of uranium in plants range from 5 to 60 parts per billion, and ash from burnt wood can have concentrations up to 4 parts per million. Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.
Production and Mining
Uranium ore is mined in several ways: by open pit, underground, or by in-situ leaching (see uranium mining). Low-grade uranium ore typically contains 0.1 to 0.25% of actual uranium oxides, so extensive measures must be employed to extract the metal from its ore. High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 70% uranium oxides, and therefore must be diluted with waste rock prior to milling. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides. Yellowcake is then calcined to remove impurities from the milling process prior to refining and conversion.
Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals. Uranium metal can also be made through electrolysis of KUF5 or UF4, dissolved in a molten calcium chloride (CaCl2) and sodium chloride (NaCl) solution. Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.
It is estimated that there is 4.7 million tonnes of uranium ore reserves (economically mineable) known to exist, while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction). An additional 4.6 billion tonnes of uranium are estimated to be in sea water (Japanese scientists in the 1980s proved that extraction of uranium from sea water using ion exchangers was feasible).
Exploration for uranium is continuing to increase with US$200 million being spent world wide in 2005, a 54% increase on the previous year.
Australia has 38% of the world’s uranium ore resources – the most of any country. In fact, the world’s largest single uranium deposit is located at the Olympic Dam Mine in South Australia. Almost all the uranium is exported, under strict International Atomic Energy Agency safeguards to satisfy the Australian people and government that none of the uranium is used in nuclear weapons. As of 2006, the Australian government was advocating an expansion of uranium mining, although issues with state governments and indigenous interests complicate the issue.
The largest single source of uranium ore in the United States was the Colorado Plateau located in Colorado, Utah, New Mexico, and Arizona. The U.S. federal government paid discovery bonuses and guaranteed purchase prices to anyone who found and delivered uranium ore, and was the sole legal purchaser of the uranium. The economic incentives resulted in a frenzy of exploration and mining activity throughout the Colorado Plateau from 1947 through 1959 that left thousands of miles of crudely graded roads spider-webbing the remote deserts of the Colorado Plateau, and thousands of abandoned uranium mines, exploratory shafts, and tailings piles. The frenzy ended as suddenly as it had begun, when the U.S. government stopped purchasing the uranium.
In 2005, seventeen countries produced concentrated uranium oxides, with Canada (27.9% of world production) and Australia (22.8%) being the largest producers and Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger (7.4%), Uzbekistan (5.5%), the United States (2.5%), Ukraine (1.9%) and China (1.7%) also producing significant amounts. The ultimate supply of uranium is believed to very large and sufficient for at least the next 85 years although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century. It is estimated that for a ten times increase in price, the supply of uranium that can be economically mined is increased 300 times.
Isotopes of Uranium
232U [140 neutrons]
Half life: 68.9 years
Decay Energy: 5.414 MeV
Decays to 228Th.
233U [141 neutrons]
Half life: 159200 years
Decay Energy: 4.909 MeV
Decays to 229
234U [142 neutrons]
Half life: 245500 years
Decay Energy: 4.859 MeV
Decays to 230
235U [143 neutrons]
Half life: 7.038 x 108
Decay Energy: 4.679 MeV
Decays to 231
U is unique in its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. In fact, U-235 is the only fissile isotope found in nature. It was discovered in 1935 by Arthur Jeffrey Dempster. A uranium nucleus that absorbs a neutron splits into two lighter nuclei; this is called nuclear fission. It releases either two or three neutrons which continue the reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.
236U [144 neutrons]
Half life: 2.342 x 107
Decay Energy: 4.572 MeV
Decays to 232
238U [146 neutrons]
Half life: 4.468 x 109
Decay Energy: 4.260 MeV
Decays to 234
Uranium-238 is important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which is fissile, that is, can be broken apart by thermal neutrons.