Uranium Mining, and Processing

Primary Uranium Minerals
Uraninite	UO2.x
Pitchblende	UO2.x (x = 0.2-0.6)
Coffinite	U(SiO4)1-x(OH)4x
Brannerite	(U, CA, Y, CE)(Ti, Fe)2O6
Davidite	(REE)(Y, U)(Ti, Fe3+)20O38
Thucholite	Thorium- and uranium-bearing organic material

Secondary Uranium Minerals
Autunite	Ca(UO2)2(PO4)2·8-12 H2O
Carnotite	K2(UO2)2(VO4)2·1-3 H2O
Gummite	A mixture of uraninite and secondary uranium minerals of variable composition
Seleeite	Mg(UO2)2(PO4)2·10 H2O
Torbernite	Cu(UO2)2(PO4)2·12 H2O
Tyuyamunite	Ca(UO2)2(VO4)2·5-8 H2O
Uranocircite	Ba(UO2)2(PO4)2·8-10 H2O
Uranophane	Ca(UO2)2(HSiO4)2·5 H2O
Zeunerite	Cu(UO2)2(AsO4)2·8-10 H2O

Conventional Agitation Leach

Uranium is highly soluble as a sulfate in sulfuric acid, and as a carbonate in alkaline solution in the U6+ valence state. If it occurs in the U4+ state it must oxidized before becoming soluble; this is a two-step reaction, with a chemical oxidant first used to oxidize iron, for example, from the ferrous Fe2+to the ferric Fe3+ state, and in turn the oxidized iron causes oxidation of the uranium from U4+ to U6+

The first step in the agitated leaching process is to finely grind the ore (typically to about 300- to 500-micron size) in a water–slurry mixture. The ore slurry is thickened to a higher density (about 50 percent solids), and then forwarded to a series of stirred tanks where the leaching takes place. Acid and oxidants are added—for acid leaching, temperatures of 50°C to 60°C are used, whereas alkaline leaching requires a higher temperature of 90°C to 95°C. The tanks can be at normal atmospheric pressure or pressurized. Acid and a suitable oxidant (e.g., oxygen, hydrogen peroxide, sodium chlorate, or manganese dioxide) is added to oxidize U4+ to U6+. The acid is the lixiviant—or liquid solution—that dissolves the metal in the U6+ sulfate form. 

Alternatively, a mixture of sodium carbonate and sodium bicarbonate can be used if the ore gangue has a high acid consumption. The choice of a carbonate or acid leaching route is based on the consumption of each chemical by the ore matrix or host rock, reagent availability, and environmental and economic considerations. The choice of oxidant is based on many of the same considerations as the choice of lixiviant.

In either acid or alkaline leaching, the ore slurry—with the uranium in solution—requires the separation of the solids from the uranium-containing liquid. This is commonly performed using filters (horizontal belt, pressure, or drum filters) or a series of thickeners or decanters. In both cases, the slurry is washed with acidified water for the acid leach process, or water only in the case of the alkaline leach option, in what is termed countercurrent decantation. The washed solids, now referred to as tailings, are generally neutralized with lime or other alkaline material if acid leaching of the ore was employed to extract the uranium. The tailings are then forwarded to a tailings impoundment facility for storage.

The clear liquid containing the uranium in solution is further purified using a solvent extraction or ion exchange technology. After uranium removal, the solution—known as “raffinate” or “barren solution”—is recycled back to the filters or decantation process. The concentrated, purified uranium solution (referred to as “pregnant solution” or “eluate”) is advanced to a precipitation stage using hydrogen peroxide, magnesium oxide, or sodium hydroxide. The resultant uranium precipitate is then filtered or centrifuged, dried or calcined, and packaged into suitable drums for shipping

Uranium Enrichment

Uranium when mined is in the form of a stable oxide (U3O8) or peroxide. After initial purification to remove impurities, which is done by heating strongly and subsequent agglomeration and crushing, the purified uranium oxide is then reduced in a kiln by hydrogen to form uranium dioxide by the following reaction.

U3O8 + 2H2 → 3UO2 + 2H2O

The reduced oxide of uranium is then reacted with hydrogen fluoride to form uranium tetrafluoride which is further reacted with gaseous fluorine to produce uranium hexafluoride which is then used as a starting input material in an enrichment plant. The reactions by which the above mentioned conversion takes place are given below.

UO2 + 4HF → UF4 + 2H2O

UF4 + F2 → UF6

Enrichment Methods

Gaseous diffusion: Uranium hexafluoride is first heated and converted from a solid to a gas. It is then made to pass through a series of semi-permeable membranes which are a part of compressors and converters. Since the U-235 isotope molecules are lighter than the U-238 isotope molecules, they tend to diffuse faster through the pores of the membranes and hence can be separated. The two streams of gas produced have different U-235 concentrations and hence are separated. Several repetitions of this process are required to obtain concentrations of U-235 isotope in the gas are high enough to the extent that it can be used in a nuclear reactor. [3] The enriched UF6 gas is then allowed to liquefy and subsequently solidify before being transported to a nuclear power plant. This technology contributes to about 27% of the world's enriched uranium.

Gas centrifuge: This method of uranium enrichment also utilizes the difference in mass of the uranium isotopes are an aid in their separation. A gas centrifuge system consists of several cylinders which rotate at high speeds that create a strong centrifugal force within. The U-238 molecules being heavier tend to line the outer wall of the cylinder while the lighter U-235 molecules stay more towards the center. The two streams are separated and the depleted stream is pumped back for further U-235 extraction. This method of enrichment proves to be more energy efficient as compared to the diffusion method by a factor of 50:1 and hence is currently used to produce 50% of the world enriched uranium.

Grades of Enriched Uranium

Different grades of uranium can be produced through enrichment which then has different applications. These are broadly classified under the following categories.

Slightly Enriched Uranium (SEU): This has a concentration of 0.9% to 2% of U-235 isotope and is most commonly used as a substitute to natural uranium in heavy water reactors. One benefit of using this instead of natural uranium is that lesser quantity of fuel is required to produce the same amount of energy and as a result, the amount of nuclear waste to manage at the end of the production is significantly reduced.

Low Enriched Uranium (LEU): This has a concentration lower than 20% of U-235 isotope. It is commonly used in light water reactors and research reactors.

Highly Enriched Uranium (HEU): This has a concentration higher than 20% of U-235 isotope. When the concentration is over 80%, it is termed as weapons grade uranium which is necessary for a nuclear chain reaction to occur. Highly Enriched Uranium with concentrations greater than 40% is used in naval reactors in powering submarines and in fast neutron reactors.

Nuclear power reactors

The vast majority of all nuclear power reactors require 'enriched' uranium fuel in which the proportion of the uranium-235 isotope has been raised from the natural level of 0.7% to about 3.5% to 5%.  The enrichment process needs to have the uranium in gaseous form, so on the way from the mine it goes through a conversion plant which turns the uranium oxide into uranium hexafluoride.

About 27 tonnes of fresh fuel is required each year by a 1000 MWe nuclear reactor.

Uranium Oxide.

 The most common forms of uranium oxide are U3O8 and UO2. Both oxide forms are solids that have a low solubility in water and are relatively stable over a wide range of environmental conditions. U3O8 is the most stable form of uranium and is the form found in nature. The most common form of U3O8 is “yellow cake,” a solid named for its characteristic color that is produced during the uranium mining and milling process. UO2 is a solid ceramic material and is the form in which uranium is most commonly used as a nuclear reactor fuel.  At ambient temperatures, UO2 will gradually convert to U3O8.  Uranium oxides are extremely stable in the environment and are thus generally considered the preferred chemical form for storage or disposal.

 Isotope Separation

 Natural uranium is a mixture of 0.711% 235U and 92.89% of 238U. The enrichment process enriches the 235U content in natural uranium to the desired percentage. Low-enriched uranium which is typically used in nuclear reactors has 3-4% percent of 235U, while the highly enriched uranium has more than 50% of 235U and is typically used in nuclear weapons.

 Uranium is made from uraninite, which is a mixture of UO2, UO3, oxides of lead, thorium and rare earth elements. Uraninite is calcined to evaporate some impurities, then agglomerated and crushed. U3O8 is then put in a kiln with hydrogen:

 U3O8 + 2H2 =⇒ 3UO2 + 2H2O,                  Heat = −109kJ/mole       (1)

UO3 + H2 =⇒ UO2 + H2O                              Heat = −109kJ/mole       (2)

 The uranium dioxide is then treated with hydrogen fluoride in another kiln.

 UO2 + 4HF =⇒ UF4 + 2H2O,                        Heat = −176kJ/mole       (3)

 Finally the tetrafluoride is fed into a fluidized bed reactor with fluorine to produce the uranium hexafluoride (238UF6, 235 UF6) that is used in the separation process.

 UF4 + F2 =⇒ UF6                                                                                             (4)

 After the separation of molecules the UF6 is vaporized in autoclaves with steam and reacted with hydrogen at 700°C:

UF6 + 2H2O + H2 =⇒ U3O8 + 6HF                                                              (5)

 The final product is the so-called yellow cake, which is the basic raw material for nuclear fuel fabrication.

 Uranium hexafluoride is used because it has great storage properties. It can be used as a solid, liquid or gas, with minimum variations in pressure or temperature. It is usually stored as a solid, when in use it can be turned into liquid which is ideal for pumping. For the actual separation process it is used as a gas.

Uranium Hexafluoride.

 UF6  is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt.  UF6 does not react with oxygen, nitrogen, carbon dioxide, or dry air, but it does react with water or water vapor (including humidity in the air). When UF6 comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive hydrogen fluoride (HF) and a uranium-fluoride compound called uranyl fluoride (UO2F2). For this reason, UF6 is always handled in leak-tight containers and processing equipment. Although very convenient for processing, UF6 is not considered a preferred form for long-term storage or disposal because of its relative instability.

Enrichment processes require uranium to be in a gaseous form at relatively low temperature, hence uranium oxide from the mine is converted to uranium hexafluoride in a preliminary process, at a separate conversion plant.

Enriched UF₆ is transported to a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then pressed to form small fuel pellets, which are then heated to make a hard ceramic material. The pellets are then inserted into thin tubes to form fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are several meters long. 

The number of fuel rods used to make each fuel assembly depends on the type of reactor. A pressurized water reactor may use between 121-193 fuel assemblies, each consisting of between 179-264 fuel rods.

There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation), which consumes only 2% to 2.5%^[9] as much energy as gaseous diffusion, with centrifuges being at least a "factor of 20" more efficient.

The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong centripetal force so that the heavier gas molecules containing ²³⁸U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in ²³⁵U collect closer to the center.

Today, 5% U-235 is the maximum level of enrichment for fuel used in normal power reactors. 

Uranium Metal.

 Uranium metal is heavy, silvery white, malleable, ductile, and softer than steel. It is one of the densest materials known (19 g/cm3), being 1.6 times more dense than lead. Uranium metal is not as stable as U3O8 or UO2 because it is subject to surface oxidation. It tarnishes in air, with the oxide film preventing further oxidation of bulk metal at room temperature. Water attacks uranium metal slowly at room temperature and rapidly at higher temperatures. Uranium metal powder or chips will ignite spontaneously in air at ambient temperature.