In-situ leach

In-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining, is a mining process used to recover minerals such as copper and uranium through boreholes drilled into a deposit, in situ.

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 ore body without the need for conventional mining involving drill-and-blast, open-cut or underground mining.

Process

In-situ leach mining involves pumping of a lixiviant into the ore body via a borehole, which circulates through the porous rock dissolving the ore and is extracted via a second borehole.

The lixiviant 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 solubility of the ore minerals within the solution. For uranium ores, the lixiviant may be acid or sodium bicarbonate.

Soluble salts

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 Colorado to extract nahcolite (sodium bicarbonate).^[1] In-situ leaching is often used when the deposits are too deep, or the beds too thin for conventional underground mining.

Uranium

Solutions used to dissolve uranium ore are either acid (sulfuric acid or 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 States and the Soviet Union in the early 1960s. The first uranium ISL in the US was in the Shirley Basin in the state of Wyoming, which operated from 1961-1970 using sulfuric acid. Since 1970, all commercial-scale ISL mines in the US have used carbonate solutions.^[2]

In-situ recovery involves the extraction of uranium-bearing water (grading as low as .05% U₃O₈). The extracted uranium solution is then filtered through resin beads. Through an ion exchange process, the resin beads attract uranium from the solution. Uranium loaded resins are then transported to a processing plant, where U₃O₈ is separated from the resin beads and yellowcake is produced. The resin beads can then be returned to the ion exchange facility where they are reused.

At the end of 2008 there were four^[3] in-situ leaching uranium mines operating in the United States, operated by Cameco, Mestena and Uranium Resources, Inc., all using sodium bicarbonate. ISL produces 90% of the uranium mined in the US. In 2010, Uranium Energy Corporation began in-situ leach operations at their Palangana project in Duval County, Texas. In July 2012 Cameco delayed development of their Kintyre project, due to challenging project economics based on $45.00 U₃O₈. One ISR reclamation project was also in operation as of 2009.^[4]

Significant ISL mines are operating in Kazakhstan and Australia. The Beverley uranium mine in Australia uses in-situ leaching. ISL mining accounted for 41% of the world's uranium production in 2010.^[5]

Examples of in-situ uranium mines include:

• The Beverley Uranium Mine, South Australia, is an operating ISL uranium mine and Australia's first such mine.
• The 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 (operating), Kingsville Dome (operating), Rosita (standby) and Vasquez (restoration) are ISL uranium operations in the United States.
• In 2010 Uranium Energy Corp. began an ISL mining operation in the Palangana deposit in Duval County, Texas. The ion exchange facility at Palangana trucks uranium loaded resin beads to the company's Hobson processing plant, where yellowcake is produced. Uranium Energy Corp. has three additional South Texas deposits permitted or in development.^[6]

Copper

In-situ leaching of copper was done by the Chinese by 977 AD, and perhaps as early as 177 BC.^[2] Copper is usually leached using acid (sulfuric acid or hydrochloric acid), then recovered from solution by solvent extraction electrowinning (SX-EW) or by chemical precipitation.

Ores most amenable to leaching include the copper carbonates malachite and azurite, the oxide tenorite, and the silicate chrysocolla. Other copper minerals, such as the oxide cuprite and the sulfide chalcocite may require addition of oxidizing agents such as ferric sulfate and oxygen to the leachate before the minerals are dissolved. The ores with the highest sulfide contents, such as bornite and chalcopyrite will 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.^[7]

Gold

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 chloride and iodide solution. After obtaining poor results, perhaps because of the complex telluride ore, the test was halted.^[8]

Environmental Concerns: ISL Mining of Uranium

According to the World Nuclear Organization:

In the USA legislation requires that the water quality in the affected aquifer be restored so as to enable its pre-mining use. Usually this is potable water or stock water (usually less than 500 ppm total dissolved solids), and while not all chemical characteristics can be returned to those pre-mining, the water must be usable for the same purposes as before. Often it need to be treated by reverse osmosis, giving rise to a problem in disposing of the concentrated brine stream from this.

The usual radiation safeguards are applied at an ISL Uranium mining operation, despite the fact that most of the orebody's radioactivity remains well underground and there is hence minimal increase in radon release and no ore dust. Employees are monitored for alpha radiation contamination and personal dosimeters are worn to measure exposure to gamma radiation. Routine monitoring of air, dust and surface contamination are undertaken.

^[9]

Dr. Gavin Mudd: The leaching liquid used for in-situ leaching contains the leaching agent ammonium carbonate for example, or - particularly in Europe - sulfuric acid. This method can only be applied if the uranium deposit is located in porous rock, confined in impermeable rock layers.

The advantages of this technology are:

• Reduced hazards for the employees from accidents, dust, and radiation,

• Low cost, no need for large uranium mill tailings deposits.

The disadvantages of the in-situ leaching technology are:

• Risk of spreading of leaching liquid outside of the uranium deposit, involving subsequent groundwater contamination

• Unpredictable impact of the leaching liquid on the rock of the deposit,

and the impossibility of restoring natural groundwater conditions after completion of the leaching operations.

Moreover, in-situ leaching releases considerable amounts of radon, and produces certain amounts of waste slurries and waste water during recovery of the uranium from the liquid.

After termination of an in-situ leaching operation, the waste slurries produced must be safely disposed, and the aquifer, contaminated from the leaching activities, must be restored. Groundwater restoration is a very tedious process that is not yet fully understood. So far, it is not possible to restore groundwater quality to previous conditions.

The best results have been obtained with the following treatment scheme, consisting of a series of different steps: ^{[10] [11]}

• Phase 1: Pumping of contaminated water: the injection of the leaching solution is stopped and the contaminated liquid is pumped from the leaching zone. Subsequently, clean groundwater flows in from outside of the leaching zone.
• Phase 2: as 1, but with treatment of the pumped liquid (by reverse osmosis) and re-injection into the former leaching zone. This scheme results in circulation of the liquid.
• Phase 3: as 2, with the addition of a reducing chemical (for example hydrogen sulfide (H₂S) or sodium sulfide (Na₂S). This causes the chemical precipitation and thus immobilization of major contaminants.
• Phase 4: Circulation of the liquid by pumping and re- injection, to obtain uniform conditions in the whole former leaching zone.

But, even with this treatment scheme, various problems remain unresolved:

• Contaminants, that are mobile under chemically reducing conditions, such as radium, cannot be controlled.
• If chemically reducing conditions are later disturbed for any reasons, the precipitated contaminants are re-mobilized.
• The restoration process takes very long periods of time, not all parameters can be lowered appropriately.

Most restoration experiments reported refer to the alkaline leaching scheme, since this scheme is the only one used in Western world commercial in-situ operations. Therefore, nearly no experience exists with groundwater restoration after acid in- situ leaching, the scheme that was applied in most instances in Eastern Europe. The only Western in-situ leaching site restored after sulfuric acid leaching so far, is the small pilot scale facility Nine Mile Lake near Casper, Wyoming (USA). The results can therefore not simply be transferred to production scale facilities. The restoration scheme applied included the first two steps mentioned above. It turned out that a water volume of more than 20 times the pore volume of the leaching zone had to be pumped, and still several parameters did not reach background levels. Moreover, the restoration required about the same time as used for the leaching period. ^{[12] [13]}

In USA, the Pawnee, Lamprecht, and Zamzow ISL Sites in Texas were restored using steps 1 and 2 of the above listed treatment scheme.^[14] Relaxed groundwater restoration standards have been granted at these and other sites, since the restoration criteria could not be met.

A study published by the U.S. Geological Survey in 2009 found that "To date, no remediation of an ISR operation in the United States has successfully returned the aquifer to baseline conditions."^[15]

Baseline conditions include commercial quantities of radioactive U₃O₈. Efficient in-situ recovery reduces U₃O₈ values of the aquifer. Speaking at an EPA Region 8 workshop, on September 29, 2010, Ardyth Simmons, PhD, Los Alamos National Laboratory (Los Alamos, NM) on the subject "Establishing Baseline and Comparison to Restoration Values at Uranium In-Situ Recovery Sites" stated "These results indicated that it may be unrealistic for ISR operations to restore aquifers to the mean, because in some cases, this means that there would have to be less uranium present than there was pre-mining. Pursuing more conservative concentrations results in a considerable amount of water usage, and many of these aquifers were not suitable for drinking water before mining initiated."^[16]

The EPA is considering the need to update the environmental protection standards for uranium mining because current regulations, promulgated in response to the Uranium Mill Tailings Radiation Control Act of 1978, do not address the relatively recent process of in-situ leaching (ISL) of uranium from underground ore bodies. In a February, 2012 letter the EPA states, "Because the ISL process affects groundwater quality, the EPA’s Office of Radiation and Indoor Air requested advice from the Science Advisory Board (SAB) on issues related to design and implementation of groundwater monitoring at ISL mining sites."

The SAB makes recommendations concerning monitoring to characterize baseline groundwater quality prior to the start of mining operations, monitoring to detect any leachate excursions during mining, and monitoring to determine when groundwater quality has stabilized after mining operations have been completed. The SAB also reviews the advantages and disadvantages of alternative statistical techniques to determine whether post-operational groundwater quality has returned to near pre-mining conditions and whether mine operation can be predicted not to adversely impact groundwater quality after site closure acceptance. ^[17]

See also

• Mineral exploration
• Heap leaching

References

External links

• Heathgate Resources Acid Leach Infopage