If the concentration of uranium in the ore from underground mines or open pit mines is too low to be processed in the mill, the heaps of ore are subjected to leaching liquids such as acids, alkaline chemicals or peroxide solutions. The liquid flows down through the heap and dissolves uranium minerals. The liquid runs down a layer of plastic under the heap and collects in pools.
If the concentration of uranium in the ore from underground mines or open pit mines is high enough it is crushed to separate the uranium minerals from the rock matrix and to remove impurities. Then it is subjected to chemical processing similar to the leaching described above to produce a liquid rich in uranium..
In-situ leaching processes which rely on ground water and leaching chemicals pumped down into the orebody produce enriched liquid like the results of heap leaching and ore processing.
In these processes of ore processing, there can be problems with radioactive dust from ore heaps, release of radon gas from the heaps, and pollution of surface and ground water from leaching liquids.
The liquid produced from these processes also contains other elements like molybdenum, vanadium, selenium, iron, lead and arsenic. These are removed with organic solutions or ion exchange resins. Finally the liquid is filtered and the uranium compounds are precipitated and dried to produce a yellow powder called yellowcake which contains about 80% of triuranium octoxide (U3O8). It also contains other uranium oxides such as uranium dioxide (UO2) and uranium trioxide (UO3). Yellowcake is sent to enrichment facilities to produce different forms of uranium oxide and uranium metal for various applications.
Yellowcake can be smelted to produce purified uranium dioxide. Such natural unenriched uranium is used in pressurized heavy-water reactors and some other nuclear systems.
The natural ratio of the two primary uranium isotopes is 99.27 percent U-238 to .75% percent U-235. U-235 is the only naturally occurring element isotope that can be made to fission by the absorption of slow moving neutrons. This capability of U-235 is referred to as being fissile and is the key to creating certain types of nuclear reactors and creating nuclear weapons. In order to be utilized for these purposes, the uranium metal must have the percentage of U-235 increased above the natural ratio.
In order to produce enriched uranium metal, the isotopes of U-238 and U-235 must be separated and then recombined to yield increased percentages of U-235. In gas separation, yellowcake is processed in combination with fluoride to produce uranium hexafluoride gas (UF6). Next, isotopes of uranium are separated by gas diffusion or gas centrifuge. There are other aerodynamic processes that use special nozzles or vortex tubes to separate the different isotopes in gaseous form. There are laser separation techniques that rely on the ability of lasers to be tuned to just excite the U-235 isotope. Electromagnetic separation vaporizes the uranium metal and then uses magnetic fields to accelerate and deflect the different isotopes. Chemical methods have been developed as well as plasma separation techniques that utilize superconducting magnets and plasma physics, .to increase the percentage of the U-235 isotope.
When the percentage of U-235 is less than 20%, the resulting metal is called low-enriched uranium and it is used large civilian reactors. Above 20% percentage of U-235, the resulting metal is called highly-enriched uranium. This form is used in compact nuclear reactors in naval warships and submarines. Further processing that takes the U-235 percentage above 90% yields uranium suitable for nuclear weapons.
Range mine and mill in Australia:
