In situ leach mining (ISL), also known as in-situ mining or solution mining, was first used as a means to extract low grades of uranium from ore in underground mines. First used in Wyoming in the 1950s, originally as a low production experiment at the Lucky June mine, it became a high-production, low cost method of fulfilling Atomic Energy Commission uranium requirements at Utah Construction Company's Shirley Basin mining operations in the 1960s. Pioneered through the efforts of Charles Don Snow, a uranium mining and exploration geologist employed by Utah, many of his developments are still used today in ISL mining.
What is ISL mining? According to the Wyoming Mining Association website, ISL mining is explained in the following manner. (We choose Wyoming because it is the birthplace of "solution mining" as it was originally called.)
"In-situ mining is a noninvasive, environmentally friendly mining process involving minimal surface disturbance which extracts uranium from porous sandstone aquifers by reversing the natural processes which deposited the uranium.
To be mined in situ, the uranium deposit must occur in permeable sandstone aquifers. These sandstone aquifers provide the "plumbing system" for both the original emplacement and the recovery of the uranium. The uranium was emplaced by weakly oxidizing ground water which moved through the plumbing systems of the geologic formation. To effectively extract uranium deposited from ground water, a company must first thoroughly define this plumbing system and then designs well fields that best fit the natural hydro-geological conditions.
Detailed mapping techniques, using geophysical data from standard logging tools, have been developed by uranium companies. These innovative mapping methods define the geologic controls of the original solutions, so that these same routes can be retraced for effective in situ leaching of the ore. Once the geometry of the ore bodies is known, the locations of injection and recovery wells are planned to effectively contact the uranium. This technique has been used in several thousand wells covering hundreds of acres.
Following the installation of the well field, a leaching solution (or lixiviant), consisting of native ground water containing dissolved oxygen and carbon dioxide, is delivered to the uranium-bearing strata through the injection wells. Once in contact with the mineralization, the lixiviant oxidizes the uranium minerals, which allows the uranium to dissolve in the ground water. Production wells, located between the injection wells, intercept the pregnant lixiviant and pump it to the surface. A centralized ion-exchange facility extracts the uranium from the barren lixiviant, stripped of uranium, is regenerated with oxygen and carbon dioxide and recirculated for continued leaching. The ion exchange resin, which becomes 'loaded' with uranium, it is stripped or eluted. Once eluted, the ion exchange resin is returned to the well field facility.
During the mining process, slightly more water is produced from the ore-bearing formation than is reinjected. This net withdrawal, or 'bleed,' produces a cone of depression in the mining area, controlling fluid flow and confining it to the mining zone. The mined aquifer is surrounded, both laterally and above and below, by monitor wells which are frequently sampled to ensure that all mining fluids are retained within the mining zone. The 'bleed' also provides a chemical bleed on the aquifer to limit the buildup of species like sulfate and chloride which are affected by the leaching process. The 'bleed' water is treated for removal of uranium and radium. This treated water is then disposed of through waste water land application, or irrigation. A very small volume of radioactive sludge results; this sludge is disposed of at an NRC licensed uranium tailings facility.
The ion exchange resin is stripped of its uranium, and the resulting rich eluate is precipitated to produce a yellow cake slurry. This slurry is dewatered and dried to a final drummed uranium concentrate.
At the conclusion of the leaching process in a well field area, the same injection and production wells and surface facilities are used for restoration of the affected ground water. Ground water restoration is accomplished in three ways. First, the water in the leach zone is removed by "ground water sweep", and native ground water flows in to replace the removed contaminated water. The water which is removed is again treated to remove radionuclides and disposed of in irrigation. Second, the water which is removed is processed to purify it, typically with reverse osmosis, and the pure water is injected into the affected aquifer. This reinjection of very pure water results in a large increment of water quality improvement in a short time period. Third, the soluble metal ions which resulted from the oxidation of the ore zone are chemically immobilized by injecting a reducing chemical into the ore zone, immobilizing these constituents in situ. Ground water restoration is continued until the affected water is suit
able for its pre-mining use.
Throughout the leaching and restoration processes, a company ensures the isolation of the leach zone by careful well placement and construction. The well fields are extensively monitored to prevent the contamination of other aquifers.
Once mining is complete, the aquifer is restored by pumping fresh water through the aquifer until the ground water meets the pre-mining use.
In situ mining has several advantages over conventional mining. First, the environmental impact is minimal, as the affected water is restored at the conclusion of mining. Second, it is lower cost, allowing Wyoming's low grade deposits to compete globally with the very high grade deposits of Canada. Finally the method is safe and proven, resulting in minimal employee exposure to health risks."
ISL mining may be the wave of the future of U.S. uranium mining, or it may become an interim mining measure, in areas where the geology is appropriate for IS. Until sufficient quantities of uranium are required by U.S. utilities to fuel the country's demand for nuclear energy, ISL mining may remain the leading uranium mining method in the United States. At some point, an overwhelming need for uranium for the nuclear fuel cycle may again put ISL mining in the backseat, and uranium miners may return to conventional mining methods, such as open pit mining.
Source: http://ezinearticles.com/?What-Is-ISL-Uranium-Mining&id=183880
