- In-situ leach
In-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining, is a process of recovering minerals such as
copperand uraniumthrough boreholes drilled into the deposit. The process initially involves drilling of holes into the ore deposit. Explosive or hydraulic fracturing may be used to create open pathways in the deposit for solution to penetrate. Leaching solution is pumped into the deposit where it makes contact with the ore. The solution bearing the dissolved ore content is then pumped to the surface and processed. This process allows the extraction of metals and salts from an orebody without the need for conventional mininginvolving drill-and-blast, open-cut or underground mining.
In-situ leach mining involves pumping of a leachate solution into the ore body via a borehole, which circulates through the porous rock dissolving the ore and is extracted via a second
The leachate solution varies according to the ore deposit - for salt deposits the leachate can be fresh water into which salts can readily dissolve. For copper,
acids are generally needed to enhance solubilityof the ore minerals within the solution. For uranium ores, the leachate may be acid or sodium bicarbonate.
In-situ leach is widely used to extract deposits of water-soluble salts such as
sylvite( potash), halite(rock salt, sodium chloride), and sodium sulfate. It has been used in the US state of Coloradoto extract nahcolite( sodium bicarbonate). [http://www.agapito.com/SolutionMiningofNahcolite.pdf] In-situ leaching is often used when the deposits are too deep, or the beds too thin for conventional underground mining.
Solutions used to dissolve uranium are either acid (
sulfuric acidor less commonly nitric acid) or carbonate( sodium bicarbonate, ammonium carbonate, or dissolved carbon dioxide). Dissolved oxygen is sometimes added to the water to mobilize the uranium. ISL of uranium ores started in the United Statesand the Soviet Unionin the early 1960s. The first uranium ISL in the US was in the Shirley Basinin the state of Wyoming, which operated from 1961-1970 using sulfuric acid. Since 1970, all commercial-scale ISL mines in the US have used carbonatesolutions.http://www.sea-us.org.au/pdfs/tmw00/TMW00-Oz-USA.pdf Acid In Situ Leach Uranium Mining]
There are currently five in-situ leaching uranium mines operating in the
United States, operated by Cameco, Mestenaand Uranium Resources Company, all using sodium bicarbonate. ISL produces 90% of the uranium mined in the US. Two more ISL projects are in licensing and proposal stages in the US, and two in reclamation in 2006. [http://www.eia.doe.gov/cneaf/nuclear/dupr/uisl.html In-situ leach mines operating in the United States. EIA Domestic Uranium Production Report.]
Significant ISL mines are operating in
Kazakhstanand Australia. The Beverley uranium mine in Australia uses in-situ leaching. ISL mining produces around 21% of the world's uranium production. [http://www.uic.com.au/nip40.htm Overview of ISL uranium mines, 2006.]
Examples of in-situ uranium mines
Beverley Uranium Mine, South Australia, is an operating ISL uraniummine and Australia's first such mine.
Honeymoon Uranium Mine, South Australia, due 2008, will be Australia's second ISL uranium mine.
Crow Butte(operating), Smith Ranch-Highland(operating), Christensen Ranch (reclamation), Irigaray (reclamation), Churchrock (proposed), Crownpoint (proposed), Alta Mesa (operating), Hobson (standby), La Palangana (development), Kingsville Dome (operating), Rosita (standby) and Vasquez (operating) are ISL uranium operations in the United States. "See Uranium mining in the United States"
In-situ leaching of copper was done by the Chinese by 977 AD, and perhaps as early as 177 BC. Copper is usually leached using acid (
sulfuric acidor hydrochloric acid), then recovered from solution by solvent extractionelectrowinning ( SX-EW) or by chemical precipitation.
Ores most amenable to leaching include the copper carbonates
malachiteand azurite, the oxide tenorite, and the silicate chrysocolla. Other copper minerals, such as the oxide cupriteand the sulfide chalcocitemay require addition of oxidizing agents such as ferric sulfateand oxygento the leachate before the minerals are dissolved. The ores with the highest sulfide contents, such as borniteand chalcopyritewill require more oxidants and will dissolve more slowly. Sometimes oxidation is speeded by the bacteria " Thiobacillus ferrooxidans", which feeds on sulfide compounds.
Copper ISL is often done by "stope leaching", in which broken low-grade ore is leached in a current or former conventional underground mine. The leaching may take place in backfilled stopes or caved areas. In 1994, stope leaching of copper was reported at 16 mines in the US. At the San Manuel mine in the US state of
Arizona, ISL, underground mining, and open-pit mining were being done simultaneously in different parts of the same ore body. ["Extraction and beneficiation of ores and minerals", v. 4, (1994) US Environmental Protection Agency, EPA 530-R-94-031.]
In-situ leaching has not been used on a commercial scale for gold mining. A three-year pilot program was undertaken in the 1970s to in-situ leach gold ore at the Ajax mine in the Cripple Creek district in the US, using a
chlorideand iodidesolution. After obtaining poor results, perhaps because of the complex tellurideore, the test was halted. [Peter G. Chamberlain and Michael G. Pojar (1984) "Gold and silver leaching practices in the United States", US Bureau of Mines, Information Circular 8969, p.24.]
In-situ leach techniques are often controversial, sometimes because of
acidleachate solution. The concerns of environmental groups and landholders centre around;
* Acidification of
* Mobilisation of potentially hazardous
heavy metalsand, in the case of uranium, radioactive heavy metals. [http://www.heathgateresources.com.au/contentmine.jsp?xcid=117 Heathgate Resources Ltd. Concerns Surrounding Beverley Uranium Mine potential for contamination.]
* Disturbance of the groundwater table, mixing of groundwater
aquifers and general disturbance of the land atop the ore body
* Destruction of habitat for
stygofaunaand other rock-inhabiting organisms, bacteria, et cetera.
* Potential spills of acidic and metal-bearing or salt-bearing leachates upon the surface
Uranium ore deposits
* [http://www.heathgateresources.com.au/contentmine.jsp?xcid=113 Heathgate Resources Acid Leach Infopage]
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