As with other types of hard rock mining there are several methods of extraction. The main methods of mining are box cut mining, open pit mining and in situ leaching (ISL)
[edit] Open pit
In the early years up until the 1960's uranium was predominantly mined in open pit mines from ore deposits located near the surface. In open pit mining, overburden is removed by drilling and blasting to expose the ore body which is mined by blasting and excavation via loaders and dump trucks. Workers spend much time in enclosed cabins thus limiting exposure. Water is extensively used to suppress airborne dust levels.
[edit] Underground uranium mining
If the uranium is too far below the surface for open pit mining, an underground mine might be used with tunnels and shafts dug to access and remove uranium ore. There is less waste material removed from underground mines than open pit mines, however this type of mining exposes underground worker to the highest levels of radon gas.
Underground uranium mining is in principle no different to any other hard rock mining and other ores are often mined in association (eg copper, gold, silver). Once the ore body has been identified a shaft is sunk in the vicinity of the ore veins, and crosscuts are driven horizontally to the veins at various levels, usually every 100 to 150 metres. Similar tunnels, known as drifts, are driven along the ore veins from the crosscut. To win the ore, the next step is to drive tunnels, known as raises when driven upwards and winzes when driven downwards through the deposit from level to level. These raises are subsequently used to develop the stopes where the ore is mined in the veins.
The stope, which is the workshop of the mine, is the excavation from which the ore is being extracted. Two methods of stope mining are commonly used. In the “cut and fill” method and open stoping method, the space remaining following removal of ore after blasting is filled with waste rock and cement. In the “shrinkage” method just sufficient broken ore is removed via the chutes below to allow the miners to work from the top of the pile to drill and blast for the next layer to be broken off; eventually leaving a large hole. Another method, known as room and pillar, is used for thinner flatter ore bodies. In this method the ore body is first divided into blocks by intersecting drives, removing ore while so doing, and then systematically removing the blocks, leaving sufficient for roof support.
[edit] Heap leaching
Waste rock is produced during open pit mining when overburden is removed, and during underground mining when driving tunnels through non-ore zones.
Piles of these tailings often contain elevated concentrations of radioisotopes compared to normal rock. Other waste piles consist of ore with too low a grade for processing. The transition between waste rock and ore depends on technical and economic feasibility criteria. All these piles threaten people and the environment after shut down of the mine due to their release of radon gas and seepage water containing radioactive and toxic materials.
In some cases uranium has been removed from this low-grade ore by heap leaching. This may be done if the uranium contents is too low for the ore to be economically processed in a uranium mill. The leaching liquid (often sulfuric acid) is introduced on the top of the pile and percolates down until it reaches a liner below the pile, where it is caught and pumped to a processing plant. Due to the potential for extreme damage to the surrounding environment , this practice is no longer in use.
[edit] In situ leaching
In situ leaching, which is sometimes referred to as solution mining, is performed by pumping liquids (weak acid or weak alkaline depending on the density of calcium in the area) down through pipes placed on one side of the deposit of uranium, through the deposit, and up through pipes on the opposing side of the deposit - recovering ore by leaching. ISL is also used on other types of metal extraction such as copper. ISL is cost effective as there is less machinery cost involved. Transfer of excess rock is also an advantage of ISL as there is minimal movement. Less time is involved in set up and maintenance of ISL though it is not suitable to all deposits of uranium as the deposit area must be permeable to the liquids used (for instance in sandstone coverage). Environmental impact studies are performed when surveying exploitable areas mainly because ground water can be affected by ISL. It’s the only type of uranium mining currently being done in the United States (2006).
[edit] Recovery from seawater
The uranium concentration of sea water is low, approximately 3.3 mg per cubic meter of seawater (3.3 ppb) but the quantity of this resource is gigantic (4.5 billion tons), this resource is practically limitless with respect to world-wide demand. That is to say, if even a portion of the uranium in seawater could be used the entire world's nuclear power generation fuel could be provided over a long time period. Although research and development for recovery of this low-concentration element by inorganic adsorbents such as titanium oxide compounds, has occurred since the 1960s in the United Kingdom, France, Germany, and Japan, this research was halted due to low recovery efficiency.
At the Takazaki Radiation Chemistry Research Establishment of the Japan Atomic Energy Research Institute (JAERI Takazaki Research Establishment), research and development has continued culminating in the production of adsorbent by irradiation of polymer fiber. Adsorbents have been synthesized that have a functional group (amidoxime group) that selectively adsorbs heavy metals, and the performance of such adsorbents has been improved. Uranium adsorption capacity of the polymer fiber adsorbent is high, approximately ten fold greater in comparison to the conventional titanium oxide adsorbent. Commercial installations are planed for the near future.
[edit] Open pit
In the early years up until the 1960's uranium was predominantly mined in open pit mines from ore deposits located near the surface. In open pit mining, overburden is removed by drilling and blasting to expose the ore body which is mined by blasting and excavation via loaders and dump trucks. Workers spend much time in enclosed cabins thus limiting exposure. Water is extensively used to suppress airborne dust levels.
[edit] Underground uranium mining
If the uranium is too far below the surface for open pit mining, an underground mine might be used with tunnels and shafts dug to access and remove uranium ore. There is less waste material removed from underground mines than open pit mines, however this type of mining exposes underground worker to the highest levels of radon gas.
Underground uranium mining is in principle no different to any other hard rock mining and other ores are often mined in association (eg copper, gold, silver). Once the ore body has been identified a shaft is sunk in the vicinity of the ore veins, and crosscuts are driven horizontally to the veins at various levels, usually every 100 to 150 metres. Similar tunnels, known as drifts, are driven along the ore veins from the crosscut. To win the ore, the next step is to drive tunnels, known as raises when driven upwards and winzes when driven downwards through the deposit from level to level. These raises are subsequently used to develop the stopes where the ore is mined in the veins.
The stope, which is the workshop of the mine, is the excavation from which the ore is being extracted. Two methods of stope mining are commonly used. In the “cut and fill” method and open stoping method, the space remaining following removal of ore after blasting is filled with waste rock and cement. In the “shrinkage” method just sufficient broken ore is removed via the chutes below to allow the miners to work from the top of the pile to drill and blast for the next layer to be broken off; eventually leaving a large hole. Another method, known as room and pillar, is used for thinner flatter ore bodies. In this method the ore body is first divided into blocks by intersecting drives, removing ore while so doing, and then systematically removing the blocks, leaving sufficient for roof support.
[edit] Heap leaching
Waste rock is produced during open pit mining when overburden is removed, and during underground mining when driving tunnels through non-ore zones.
Piles of these tailings often contain elevated concentrations of radioisotopes compared to normal rock. Other waste piles consist of ore with too low a grade for processing. The transition between waste rock and ore depends on technical and economic feasibility criteria. All these piles threaten people and the environment after shut down of the mine due to their release of radon gas and seepage water containing radioactive and toxic materials.
In some cases uranium has been removed from this low-grade ore by heap leaching. This may be done if the uranium contents is too low for the ore to be economically processed in a uranium mill. The leaching liquid (often sulfuric acid) is introduced on the top of the pile and percolates down until it reaches a liner below the pile, where it is caught and pumped to a processing plant. Due to the potential for extreme damage to the surrounding environment , this practice is no longer in use.
[edit] In situ leaching
In situ leaching, which is sometimes referred to as solution mining, is performed by pumping liquids (weak acid or weak alkaline depending on the density of calcium in the area) down through pipes placed on one side of the deposit of uranium, through the deposit, and up through pipes on the opposing side of the deposit - recovering ore by leaching. ISL is also used on other types of metal extraction such as copper. ISL is cost effective as there is less machinery cost involved. Transfer of excess rock is also an advantage of ISL as there is minimal movement. Less time is involved in set up and maintenance of ISL though it is not suitable to all deposits of uranium as the deposit area must be permeable to the liquids used (for instance in sandstone coverage). Environmental impact studies are performed when surveying exploitable areas mainly because ground water can be affected by ISL. It’s the only type of uranium mining currently being done in the United States (2006).
[edit] Recovery from seawater
The uranium concentration of sea water is low, approximately 3.3 mg per cubic meter of seawater (3.3 ppb) but the quantity of this resource is gigantic (4.5 billion tons), this resource is practically limitless with respect to world-wide demand. That is to say, if even a portion of the uranium in seawater could be used the entire world's nuclear power generation fuel could be provided over a long time period. Although research and development for recovery of this low-concentration element by inorganic adsorbents such as titanium oxide compounds, has occurred since the 1960s in the United Kingdom, France, Germany, and Japan, this research was halted due to low recovery efficiency.
At the Takazaki Radiation Chemistry Research Establishment of the Japan Atomic Energy Research Institute (JAERI Takazaki Research Establishment), research and development has continued culminating in the production of adsorbent by irradiation of polymer fiber. Adsorbents have been synthesized that have a functional group (amidoxime group) that selectively adsorbs heavy metals, and the performance of such adsorbents has been improved. Uranium adsorption capacity of the polymer fiber adsorbent is high, approximately ten fold greater in comparison to the conventional titanium oxide adsorbent. Commercial installations are planed for the near future.
posted by Tristan Tzara at
3:08 AM
|
0 comments
