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| Fig. 1: Photo of underground uranium mining operation. (Source: Wikimedia Commons) |
Uranium is a radioactive chemical element of the actinoid series of the periodic table, with an atomic number of 92. It is an important nuclear fuel. [1] This report concerns the logistics of Uranium Mining. More specifically, it will establish a typology of the different types of Uranium Mining, give a brief background of the extraction and then get into the details of the Uranium Mill Chemistry; which is how the Uranium Ore is refined into something usable. This finished product is then used primarily in civilian nuclear power, naval energy applications, and medical devices. [2]
There are three primary types of mining processes. The first is open pit and underground mining, the second is in-situ leach (ISL) mining, and the third is heap-leaching mining. Where pockets of Uranium ore are close to the surface, the open pit mining usually accesses them. This involves a large pit and the removal of much "overburden", which is the overlying rock above the ore, as well as a lot of waste rock. When the rock is slightly further below the surface, underground mining is used (see Fig. 1), which involves the construction of access shafts that create less waste rock and environment impact. In situ leach (ISL) mining involves when uranium is in groundwater in porous unconsolidated material such as gravel or sand. In this state, uranium is accessed by dissolving it, and then pumping it out of the ground. The last type of uranium extraction is heap leaching. This type of extraction is generally utilized with very low-grad ore. The process involves the breaking down of the ore which is then stack about 5 to 30 meters high on an impermeable pad and irrigated with an acid or alkaline solution over many weeks. The resulting concoction is then collected and treated to recover uranium, as with ISL, usually by utilizing a process of ion exchange. [2] However, these different extraction processes are not the end. The ore still must be subjected to the "uranium mill chemistry process" to become usable. The process is described in the following.
The ore is first crushed and then mixed with water so it can move through the milling system in a more liquid form. The ore is then exposed to acid in order to separate the uranium from other minerals in the ore. The uranium is now part of a liquid solution, which is separated from the other materials that have remained solid. Now the solution must be further purified. This is done by a solvent extraction process followed by a solidification of the uranium through chemical precipitation. At the end of the process, this uranium is dried. The resulting substance is uranium oxide (U3O8) concentrate, and is colored yellow in a powdery form. This end substance is often referred to as "yellowcake". [3]
Delving into the chemistry in more details, the leaching process utilizes sulfuric acid. The formula for how the chemicals change is below, which details how the UO2 is oxidized into UO3
In some cases, ores are subjected to a carbonate leaching that is used to create a soluble uranyl tricarbonate ion: UO2(CO3)3. This process can be precipitated with an alkali. The uranium within this solution is then extracted using a resin/polymer ion exchange or a liquid ion exchange. The products of other extraction processes such as ISL or heap leaching are treated similarly. The solvent extraction is a continuous cycle of loading and stripping that uses an organic liquid to carry the extracting chemical that removes the uranium from the solution. The cycle involves the stripping of the uranium with either a strong acid or chlorine solution. This solution is then precipitated by the addition of ammonia, hydrogen peroxide, or another chemical with similar properties. These solvents are then treated to remove impurities, including the cations and anions with sulfuric acid and gaseous ammonia respectively. The solvents are then stripped using ammonium sulfate solution. The precipitation of ammonium diuranate is achieved by adding gaseous ammonia to neutralize the solution.
This product is then "dewatered and roasted" to yield U3O8. This final product is then marketed and exported. [2]
The extraction and refinement of uranium is an involved and complex process. There are multiple types of extraction, from underground mining, to pit mining, to ISL mining and heap leaching. The chemistry behind the milling process involves significant attention and is slightly varied depending on the extraction process. The product however, the powdery yellowcake substance that emerges from the extraction and chemistry procedures, is vitally important for our contemporary society.
© Connor Kennedy. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. L. Clark, "The Discovery of Plutonium Reorganized the Periodic Table and Aided the Discovery of New Elements,." Los Alamos National Laboratory, LA-UR-09-04783, 16 Sep 09.
[2] "Uranium Extraction Technology," International Acomic Energy Agency, Technial Report No. 359, 1993.
[3] J. S. Morrell and M. J. Jackson, Uranium Processing and Properties (Springer, 2013).
