Mining Nature's Way
In Situ Leaching
Until the mid 1970s, uranium was produced either by open pit mining, which peels off layers of surface rock, or by shaft mining, which is entirely done underground.
A new approach was undertaken back then which is being used more and more. It is a technique called in-situ leaching. In situ or "in place" leaching is
very similar to the waterflood process commonly employed in the oil business to increase petroleum recovery. Treated water is circulated through an underground
ore body to dissolve and wash out the tiny uranium particular which coat the sand grains in sandstone formations. Then the dissolved uranium is pumped to the surface using
submersible pumps commonly found in most ranch and farm water wells.
Advantages of In situ leaching
Before starting commerical leaching, extensive pilot tests are made to ensure the process will work in the area and can be adapted to local conditions. It also has to be
determined if the ore can be economically produced. If there are any archaeologicalruns in the vicinity, archaeologists are brought in to determine how the project should
proceed.
When all these conditions can be met, in situ leaching can be a very advantageous technique. There is almost no disturbance of the ground surface. The local water table remains intact,
since relatively small amounts of water are used. The leaching operation also gives a strong boost to the community tax base without straining
local resources, for a typical commerical facility has only about 70 workers, plus another 40 in support activities.
The Leaching Process
Uranium is believed to have orginated millions of years ago in volcanic ash. Eventually the uranium in the ash was dissolved by rainfall and carried into underground formations,
where a chemical reaction caused it to concentrate and solidify as an ore body. Leaching merely reverses this process; it redissolves the uranium and then pumps it up through production wells.
Production is typically carried out within a wellfield that may have various patterns that conform to the shape of the ore body.
Typically a 5-spot pattern - a production well surrounded on 4 sides by injection wells - is commonly used in expansive areas. A water-based solution containing non-toxic chemicals in dilute form
is pumped down the injection wells and drawn up from the producer. By operating the producing well at slightly higher pumping rates than the injection wells, a pressure sink that forces the
solution to flow toward the producer is created.
Monitor wells surrounding the ore zone are sampled regularly to make sure no solution escapes. In the unlikely event this ever happened, it would not endanger natural groundwater, since only
a small area is leached at one time, and the chemicals used are dilute and non-toxic. Any seepage can be corrected almost immediately by increasing the pumping rates
in the production wells, drawing the solution back towards the wells.
The Recovery Plant
Once the uranium solution reaches the surface, it is sent to a recovery plant where the uranium is separated from the leach solution.
A portion of the plant works like an ordinary water purification unit, except that after subsequent processing, the end result is not water,
but "yellowcake" - the bright yellow slurry or powder which is sold to utilities for power generation after conversion into nuclear power plant fuel.
Solution mining is, in effect, a closed water circulation system since up to 98 per-cent of the water and chemicals employed are used over and over again.
The remainder is disposed of in an environmentally safe manner - for example, through solar evaporation or by pumping it into deep wells in zones where
water quality to too poor for domestic or agriculture use. This is done according to strict government standards and under regulatory agency permits.
Restoration
When leaching is completed, the water in the mining zone is restored to a quality consistent with its original condition under regulatory
agency requirements. The mining zone is flushed clean by natural groundwater which is pumped from the surrounding areas through wells which were previously
used for production and/or injection. This solution can be disposed of in a manner described previously, or a portion of the water
can be reinjected after being purified with reverse osmosis treatment at the surface. After restoration all wells are plugged with cement and the surface re-seeded,
if necessary. While one location is being restored, leaching operations move to another spot.