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Uranium is a relatively common metal, found in rocks and seawater. Over half of the world's production of uranium from mines is in Canada and Australia.
Uranium ore is recovered by excavation or by in situ leaching techniques. Excavation may be underground or open pit mining. After the uranium has been mined it is milled. the uranium ore is crushed to a fine slurry, which is then leached with sulfuric acid to produce concentrated U3O8, commonly known as 'yellowcake'.
In situ leaching involves pumping a liquid into the ground to dissolve the uranium and then pumping that liquid back to the surface.
Mining and Milling
There are two main types of mining. In general, open pit mining is used where deposits are close to the surface and underground mining is used for deep deposits, typically greater than 120 m deep.
Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine.
In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect against airborne radiation exposure.
After the uranium ore has been mined it is milled. Milling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines
At the mill the ore is crushed and ground to a fine slurry. Sulfuric acid or a strong alkaline solution is used to dissolve the uranium to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U3O8) concentrate. This is sometimes referred to as 'yellowcake' and generally contains more than 80% uranium. The original ore may contain as little as 0.1% uranium.
After drying and usually heating it is packed in 200-litre drums as a concentrate.
The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are placed in engineered facilities near the mine (often in mined out pits). Tailings contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived. These materials need to be isolated from the environment.
In situ leaching
An increasing proportion of the world's uranium now comes from in situ leaching (ISL), where oxygenated groundwater is circulated through the uranium deposit underground to dissolve the uranium and bring it to the surface.
The decision as to which form of uranium recovery is used for a particular deposit is governed by the nature of the orebody, safety and economic considerations.
Further information:World Uranium MiningEnvironmental Aspects of Uranium MiningIn Situ Leach (ISL) Mining of UraniumUranium production figures, 1997-2005
Uranium Mining Education Paper
External Links:Australia's Uranium Mines + DepositsCanadian U mines: Environmental protection