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(last updated 20 Nov 2010)
In the case of in-situ leaching (ISL) - also called in-situ recovery (ISR), or solution mining - the uranium-bearing ore is not removed from its geological deposit, but a leaching liquid is injected through wells into the ore
deposit, and the uranium-bearing liquid is pumped to the surface from other wells.
In-situ leaching gains importance for the exploitation of low grade ore deposits, for its low production cost. Many new projects for uranium in-situ leaching are being planned at present.
The USA produced 1684 t U from in-situ leaching in
1996, this corresponds to 93% of all uranium produced in that
year. The ISL operations are mainly located in Wyoming, Texas
For current U.S. ISL operations, see Operating Status of Nonconventional Uranium Plants and U.S. Uranium Mine Production (US DOE).
New ISL projects are being proposed for Texas, Wyoming, and New Mexico.
In Eastern Germany, an underground mine converted to an in-situ leaching facility was in operation at Königstein near Dresden until the end of 1990. It produced a total of 18,000 t U, 30% of which were from ISL with sulfuric acid.
In the Czech Republic, in-situ leaching with sulfuric acid was used on a large scale at Stráz pod Ralskem in North Bohemia: The ore deposit is located in Cretaceous sandstones with grades of 0.08 - 0.15% uranium. In an area of 5.6 km2, 9340 wells were drilled from the surface into the deposit. The total production to 1994 was 13,835 t U.
In Bulgaria, in-situ leaching was in use at many locations. The first uranium mines in Bulgaria were underground mines. From 1979, in-situ leaching was also applied, using wells, drilled from the surface. The leaching agent used in most cases was sulfuric acid. From 1981, in-situ leaching was also used to increase the yield from mined out conventional underground mines [Tabakov1993]. From 1981, 23 ore deposits were mined by conventional underground mining techniques, 17 by in- situ leaching from the surface, and 11 by in-situ leaching in combination with conventional mining techniques. In 1990, 70% of the uranium produced was from in-situ leaching of ore deposits with very low grades of 0.02 - 0.07% of uranium [Kuzmanov1993]. In the years 1991 - 1992, 14,000 wells in 15 in-situ leaching fields were in operation [OECD1994]. The total area used for in- situ leaching comprised 6 km2 [Vapirev1996]. The total production from in-situ leaching to 1994 was 5,175 t U [OECD1996].
In Ukraine, ISL has been used at the Devladove, Bratske, and Safonovskoye sites from 1966 - 1983.
In Russia, a new ISL project is being proposed for Dalmatovkoye in Western Siberia.
In Kazakhstan, in-situ leaching is being used at the Kandjugan, Uvanas, Mynkuduk, Karamurun sites. In 1994, the production from ISL was 1580 t U, a 70% share in the country's uranium production; the total production from ISL to 1994 was 19,961 t U [OECD1996]. The new projects of Muyunkum and Inkay are also planned for exploitation by ISL.
In Uzbekistan, in-situ leaching (with sulfuric acid) is being used at the Uchkuduk, Zarafabad, and Nurabad deposits, covering a total area of 13 km2. Since 1995, all production is from ISL (3050 t U annually) [OECD1996].
In China, ISL is being used at Tengchong and Yining.
In Australia, new ISL projects are being proposed for Beverley and Honeymoon in South Australia.
> View images: ISL general arrangement (19k) · ISL detail (7k) (WMA/Cogema)
The advantages of this technology are:
The disadvantages of the in-situ leaching technology are:
> View Typical failure modes during ISL operation (animation - 58k)
In the case of Königstein (Germany), a total of 100,000 tonnes of sulfuric acid was injected with the leaching liquid into the ore deposit. At present, 1.9 million m3 of leaching liquid are still locked in the pores of the rock leached so far; a further 0.85 million m3 are circulating between the leaching zone and the recovery plant. The liquid contains high contaminant concentrations, for example, expressed as multiples of the drinking water standards: cadmium 400x, arsenic 280x, nickel 130x, uranium 83x, etc. This liquid presents a hazard to an aquifer that is of importance for the drinking water supply of the region.
Groundwater impact is much larger at the Czech in-situ leaching
site of Stráz pod Ralskem: 28.7 million m3 of
contaminated liquid is contained in the leaching zone, covering
an area of 5.74 km2. This zone contains a total of
1.5 million tonnes of sulphate, 37,500 tonnes of ammonium, and
others. In addition to the chemicals needed for the leaching
operation (including 3.7 million tonnes of sulfuric acid, among
others), 100,000 tonnes of ammonium were injected; they were a
waste product resulting from the recovery of uranium from the
Moreover, the contaminated liquid has spread out beyond the leaching zone horizontally and vertically, thus contaminating another area of 28 km2 and a further 235 million m3 of groundwater. To the southwest, the groundwater contamination has already reached the second zone of groundwater protection of the potable water supply of the town of Mimon. In southeastern direction, the contaminated groundwater is still at a distance of 1.2 - 1.5 km from the second zone of groundwater protection of the Dolánky potable water wells, which supply 200 l/s for the city of Liberec [Andel1996]. The migration of the contaminated liquids in an easterly direction towards the Hamr I underground mine is at present intercepted by a hydraulic barrier: decontaminated water is injected into a chain of wells to prevent further migration of the contaminated groundwater.
In Bulgaria, a total of 2.5 million tonnes of sulfuric acid was
injected into the ore deposits exploited by in-situ leaching. It
is estimated that about 10% of the surface area used for ISL
could be contaminated from solution spills. This is of concern,
since the area is to be returned to its previous owners for
After termination of the ISL operations, the contaminated groundwater spreads offsite. Some in-situ leaching facilities (for example Bolyarovo, Tenevo/Okop) are located close to drinking water wells. [Vapirev1996]
The impacts of ISL on surface and groundwater are catastrophic:
"Very high concentrations of sulfate ions are measured in surface water and even in wells of private owners as a result of accidental spilling of solutions in sites of in-situ leaching. At the site "Cheshmata" (Haskovo), in the valley downstream from the sorption station, the measured content of sulfates is 1400 mg/l, free H2SO4 is 392 mg/l and pH is 2.2 (5.5 - 8.5 for 3-rd category water). A similar case has been recorded in Navusen where in a valley the sulfate concentration is 13362 mg/l and almost 5 g/l H2SO4, which means that actually the water is leaching solution.
In the underground water of such sites the salt content is >20 g/l, from which the sulfates are 12-15 g/l." [Dimitrov1996]
The Devladovo site in Ukraine was leached with sulfuric and nitric acid. The surface of the site was heavily contaminated from spills of leaching solutions. Groundwater contamination is spreading downstream from the site at a speed of 53 m/year. It has traveled a distance of 1.7 km already and will reach the village of Devladovo after 24.5 years. [Molchanov1995]
Typical examples for the incidents occuring during business as usual at in-situ operations, including surface spills and underground solution excursions, can be found here: Christensen Ranch (Wyoming), Highland (Wyoming), Smith Ranch (Wyoming), Crow Butte (Nebraska), Kingsville Dome (Texas), Rosita (Texas), Bruni (Texas), Beverley (South Australia)
The best results have been obtained with the following treatment scheme, consisting of a series of different steps [Schmidt1989], [Catchpole1995]:
But, even with this treatment scheme, various problems remain unresolved:
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 porevolume 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 [Nigbor1982] [Engelmann1982].
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." [Otton 2009] (see details)
For the Königstein (Germany) in-situ leaching mine, there
are still no large-scale proven methods to remove the remaining
leaching liquid from the deposit and to inhibit continued
leaching of uranium and other contaminants. The impact is rather
severe, as the mining activities damaged an aquifer used for the
drinking water supply in the Dresden area.
At present, it is planned to flood the Königstein mine (which is an underground mine converted to in-situ leaching in some areas), up to a certain groundwater level, to wash the leaching blocs. The flooding should be halted and the flooding waters be contained and treated, until their contaminant concentrations would only be marginal. It must be anticipated, though, that this procedure might take long periods of time, as the leaching zone is no longer washed under pressure, unlike during the leaching action.
The situation is even more difficult in the Czech in-situ
leaching facility of Stráz pod Ralskem: the goal of
restoring groundwater quality in the leaching zone to background
has been abandoned as unrealistic.
The restoration goal for the upper aquifer above the leaching zone (used for potable water supply), however, is the drinking water standard, to be achieved by pumping of contaminated waters. The goal seems to be attainable for this aquifer, although some contaminants, as aluminium, exceed the standard up to 1000-fold.
But, for the leaching zone and its surroundings, the goal of reaching the potable water standard is regarded as absolutely unrealistic. For this aquifer, the goal is defined that anticipated contaminant migration to the upper aquifer shall not worsen the water quality in this aquifer beyond potable water standards. But it is still unclear, which contaminant level in the lower aquifer is sufficient to achieve this goal. According to modeling results, a level of total dissolved solids of 10 g/l will be reached in the year 2014, and a level of 1 g/l in 2032, after continuous pumping.
> View details on Stráz groundwater restoration project.
In Bulgaria, a restoration attempt using recirculation of the solution without addition of acid failed: the tubes and filters of the sorption columns plugged, and all restoration attempts were stopped [Vapirev1996]. In some cases, heavy metals and rare earth elements (V, W, Mo, La) were detected in groundwater due to the recycling of solution [Dimitrov1996]. At present, the installations at the surface of the ISL sites are being decommissioned, and all pipes are being removed. But, there is no groundwater restoration: the ISL wells are being plugged; and the groundwater is submitted to "natural restoration".
The restoration of the Devladovo ISL site in Ukraine was limited to soil cleanup at the surface. Some heavily contaminated soil was replaced, while deep ploughing was the only remedy used at the major part of the site. Cleanup was finished in 1975. Subsequently, the site has been used for agriculture. Surveys performed in 1991 have shown that the radionuclide concentrations in the soil had not decreased at all, and that the anticipated self-cleaning of the soil had not taken place. Effective dose equivalents of up to 0.2 mSv/year were calculated for the members of the local population consuming the wheat grown on this soil. [Molchanov1995]
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