Uranium mining and milling is the starting process for all nuclear fuel cycles. In this process uranium ore is extracted from the Earth’s crust similarly as for mining of copper, zinc, and other metals. Uranium is often found with copper, phosphates, and other minerals; thus, it is often a co-product of other mining operations. The worldwide production of uranium in 2015 amounted to 60496 tonnes. Kazakhstan, Canada, and Australia are the top three producers and together account for 70% of world uranium production.
Uranium is commonly found at low levels (a few ppm – parts per million) in all rocks, soil, water, plants, and animals (including humans). Uranium occurs also in seawater, and can be recovered from the ocean water. But only a few of the uranium ores known contain sufficient uranium (greater than 0.1%) to extract commercially. Significant concentrations of uranium occur in some substances such as uraninite (the most common uranium ore), phosphate rock deposits, and other minerals.
Natural uranium refers to uranium with the same isotopic ratio as found in nature. It consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (238.03u). Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). The abundance of isotopes in the nature is caused by difference in the half-lifes. All three naturally-occurring isotopes of uranium (238U, 235U and 234U) are unstable. On the other hand these isotopes (except 234U) belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.5×109years for 238U).
Uranium Heap Leaching
As with other types of hard rock mining there are several methods of extraction. In case of uranium mining, it strongly depends on the concentration of uranium in the ore. For example, the ore extracted from the Australian Olympic Dam Mine has a concentration of 0.05 %. Most reserves have uranium with a concentration of between 0.1 bis 0.2 %. Noteworthy, in Canadian Saskatchewan ore is mined that contains more than 20 % uranium. There are three main methods of extracting uranium
Another method of uranium extraction is heap leaching. Heap leaching is an extraction process from ore which has been mined and placed in piles on the surface. Heap leaching is based on the use of a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials. In 2012, the percentage of the mined uranium produced by heap leaching was 1.7 percent. The remaining 7.3% was derived as a byproduct of mining for other minerals, and miscellaneous recovery.