FACTSHEET: Uranium mining and milling By Lizzy Bloem
Uranium mineral can be found everywhere on earth, but only mineral of economic interest is called ore. Often extraction is too costly, for uranium can occur in very complex combinations with other elements. Usually the quantity of uranium in rocks is too low to make extraction worthwhile.
Even the uranium ore usually mined contains a very small percentage of uranium (0.12 %). Since 1999, Canadian ore deposits have been explored containing as much as 13% uranium. Ores called pitchblende, uraninite and carnotite are mostly mined.
All ore contains radioactive uranium, lead, polonium, bismuth, thorium, radium, protactinium and radon gas. These are radioactive decay products from uranium. Also other toxic elements like nickel, cadmium, molybdenum, vanadium, arsenic and mercury may be present in the minerals. Radon gas is a particular danger. The radioactive gas easily escapes from the rock and always surrounds a rock or mine site.
2. Mining technologies
Different techniques are used to obtain uranium from the rock. Uranium is extracted from ore at uranium mills and at in situ leach (ISL) facilities. Uranium is also obtained as by-product from non-conventional uranium plants, like the phosphate industry. Mining and milling generate waste rock, sludge, tailings (dried sludge), wastewater and leached areas.
Currently, over half of the world's uranium comes from underground mines, about 27% from open pits and 19% from ISL. In the US, about 16% of the domestically produced uranium comes from the phosphate industry.
2.1.1. Conventional uranium mines
Traditionally, uranium ore was dug from open pits. In open pit mines, the ore deposits are located near the surface. To excavate the rocks, the traditional mining methods of drilling and blasting are used. These pits often have enormous dimensions.
In the 1970?s shaft mines opened to employ deep underground ore bodies. Underground mining involves construction of access tunnels and shafts.
Because ore contains such low concentrations of uranium, huge amounts of rock must be mined.
To keep groundwater out of the mine during operations, large amounts of contaminated water are pumped out and mostly released into rivers and lakes. When the pumps are shut down after the closure of the mine, there is a great risk of groundwater contamination from the rising water level in the shafts.
Mine waste, or spoil, is generated during open pit mining when overlying or surrounding strata are removed, and during underground mining when tunnels through non-ore zones are dug or when ore of a grade too low for processing is mined. Waste rock is stored in huge waste stockpiles, and often used after mining has finished to cover more dangerous waste buried in the original pits. In Germany millions of tonnes of radioactive and toxic waste rock have been processed into gravel or cement and used for road and railroad construction.
Truck hauling waste rock, SaskatchewanRadon gas and also the inevitable dust are a health hazard for the workers. Usually there is little protection against the wind blowing away radon gas and dust. Under most conditions these piles also contaminate the environment.
After the shutdown of uranium mines, hundreds of millions of tonnes of radioactive waste rock will be left over. The waste rock releases radon gas. Radon gas and dust is blown away by the wind and seepage water containing radioactive and toxic materials continually contaminates the groundwater and the environment.
2.1.2. Uranium mills
After transport from the mine to the mill, the ore is crushed and ground into fragments from about 2 cm or less. Then the leaching process begins. The ore particles are suspended in huge amounts of water. The leaching liquid, mostly sulphuric acid and thickener, is added to the sludge (slurry).
During leaching, insoluble uranium oxides bond with sulphuric acid to form soluble uranium sulphate. Then the uranium is extracted from the sludge through a series of stages. A last phase, including drying, produces a powder known as 'yellowcake'. It is packed in casks and transported.
Compared with uranium in ore, yellowcake is thousands of times more concentrated. Yellowcake consists of about 90-95% uranium oxide (U3O8) and various impurities.
The escape of radon gas during the milling process poses a great risk to the workers.
For the greater part the ore that is mined becomes waste, after the uranium-rich solution is separated. The waste of the leaching process is called sludge when still wet and, after drying out, tailings. The tailings are made up of hazardous radioactive and toxic by-products. The hazard per gram of mill tailings is perhaps low in comparison with other radioactive waste, but the large volume and lack of regulations for their containment have resulted in widespread contamination.
Often the sludge is dumped into huge reservoirs, often lakes within manmade dams. Frequently these tailing ponds are abandoned. The poisonous material is now exposed to the biosphere, free to erode, seep or disperse. Dam failures with huge spilling of contaminated sludge and water have occurred again and again. Sometimes the tailings are brought back to where the ore has been mined. Groundwater contamination is a common result of this operation.
Tailings are even more dangerous than waste rock. Uranium and other substances, which were previously held in the rock, have been forced into solution. Consequently, these chemicals are more mobile and chemically reactive than in the initial ore. They are now able to get into the environment by seepage, leaching and blown dust. Tailings also still contain 85% of the radioactivity of the original ore. The radioactive constituents have long radioactive half-lives, so the deposits are radioactive for millions of years.
2.2. In situ leaching (ISL)
Since a decrease in price of uranium in the 1980s the alternative technique of in situ leaching (ISL) is often used. In conventional mines the ore is excavated, crushed and chemically treated in large surface processing plants. In the process of ISL, the chemicals are injected underground directly into the underground ore. Uranium is extracted directly out of the ground. As such, ISL has in general lower production costs than conventional mining.
At the site, an extensive series of holes are drilled into underground uranium deposits. Millions of litres of strong acid solution, alkaline solution or oxygenated groundwater are then injected into the ore body. Uranium is directly dissolved from the rock. The uranium bearing liquid is then continuously pumped up from below and transferred to a processing plant where the uranium is recovered.
There are important criteria for ISL. The deposit must be confined above and below by low permeability materials, such as clays or shales. The deposit itself must occur within permeable materials, such as sands or sandstones. The deposit must be saturated within an ?aquifer? or groundwater system.
The most critical part of ISL is the control of the movement of the chemical solutions within the aquifer. Escape of the solution outside the ore zone leads to contamination of surrounding groundwater. Some of the most common causes of escape is through old exploration holes that were not adequately plugged.
Although in situ leaching (ISL) does not produce waste rock and tailings above ground, it contaminates the leached area undergound and surrounding environment irreversibly. The liquid left below ground creates a great deal of radioactive and chemical pollution within the groundwater system which may spread contamination downstream of the mine sites. ISL also produces extremely large quantities of wastewater and sludge, with high radioactivity and high concentrations of heavy metals. Most of the time these liquids are mixed and re-injected into the same deposit as that being mined.
2.3. Heap leaching
Waste rock is usually left in piles near the mine. Sometimes it is economically viable to leach these piles to extract uranium, a process called heap leaching. In this technology the leaching liquid percolates from the top to the bottom of a pile, where the liquid is caught and pumped to a processing plant.
2.4. Non-conventional uranium plants
IMC Agrico phosphate processing plant, FloridaDeposits mined in other industries often also contain uranium. The concentration of uranium in these deposits usually is even lower than the ores mined for their uranium content, but is nevertheless economically viable because very large quantities are mined.
Uranium can occur in association with other minerals such as gold and copper and is often mined as a by-product of these materials.
During the cold war it was kept secret in the US that the phosphate industry also produced uranium. Even the workers did not know about this. The phosphate industry has produced and sold sizeable quantities of uranium. In 1997, this amounted to roughly 16% of the domestically produced uranium in the US. This industry is responsible for highly toxic wastewater ponds.
3. Uranium production
Uranium production figures are still not freely available in all countries. The statistics continue to show major gaps, in particular those for the former Soviet Union and China. Since the rapid decline of uranium prices in the early 1980?s and the political changes in the late eighties, the situation has changed a great deal. Uranium is now no longer produced for political reasons without concern for cost, and only the most cost-effective production centres survive.
About ten years ago one third of the world's total uranium production was in Europe. Since then, uranium production has been shut down or greatly reduced in most European countries because of the high production cost. Nowadays, the Saskatchewan mines in Canada produce roughly one-third of the total global production.
Among the largest uranium producers of the world in total are Canada, the US, Germany, South Africa, Czechoslovakia, Kazakhstan and France. Also Niger, Russia, Uzbekistan and Australia have begun expanding their production.