IN SITU RECOVERY TECHNOLOGY
uranium deposits can be recovered commercially by modern, low
cost in situ recovery (ISR) technology. As
described below, this type of mineral recovery involves the
circulation of groundwater with bubbled oxygen and
club-soda-like mixture through a series of injection and
extraction wells until the uranium in the sand of the aquifer
has been depleted.
ISR involves the circulation of naturally occurring
and benign groundwater, fortified with oxygen, through a uranium
ore body. This natural water plus oxygen is pumped into
injection wells, through the uranium ore body, where the uranium
in the host sandstone is oxidized and solubilized - continuing
through the sandstone to the extraction wells, where the
uranium-bearing groundwater is pumped to the surface. This water
proceeds to a ion exchange unit (like a big water-softener) for
uranium removal then is pumped back to the wellfield for
re-fortification with oxygen, down into the injection wells, and
again re-circulated through the ore body. This recirculation of
the same groundwater continues over and over, until the uranium
in the sandstone is depleted.
conventional mining involves removing the ore from open pits or
underground chambers and tunnels, after which it is crushed and
ground in mills, with the subsequent leaching and concentration
of the uranium using harsh chemicals. Because conventional
mining excavates the entire ore horizon, conventional mining
generates millions of tons of chemically contaminated waste
tailings, compared to the ISR process, which
generates no tailings and very few waste solids.
found in and adjacent to uranium orebody is naturally
contaminated and unsuitable for drinking, even if it is
surrounded by a drinking water aquifer. Uranium decays into
radium and radon gas, and other “daughter” products. Over
hundreds of thousands, or millions, of years, these decay
products build up and contaminate the water in and near the
orebody. For example, the U.S. EPA has a proposed maximum level
for radon gas of 300 pCi/L in drinking water. In a uranium
orebody, it is common for radon gas to exceed 1,000,000 pCi/L,
which is 3,300 times the drinking water standard. Water from a
uranium orebody is naturally contaminated and cannot be used for
any activity (e.g., irrigation), let alone as a source of
added to the naturally conraminated groundwater at the ore body,
and that water is continuously re-circulated until most of the
uranium is recovered. The technology used to take the uranium
out of the water is the same as that used in home-based water
softeners. Waste in ISR is only a tiny fraction of
that from a conventional mine, so tailing piles are not needed
at the site, and the footprint of ISR facilities
is far smaller than that for a conventional mine operation.
is highly regulated at every phase of operation, and monitor
wells surrounding the mine site are required, ensuring
protection of the surrounding aquifer. Unlike conventional
mining, in ISR the aquifer must be restored to
ISR is not new; it has been safely used for more than
thirty years. Projects using ISR occur in
Nebraska, Texas, and Wyoming, and in other countries around the
world, including Australia.
source of uranium is igneous (volcanic) rock, which includes all
of the earth at one time or another and which makes uranium
ubiquitous. Uranium is commonly found in water because it
oxidized and its oxidized form is very water soluble.
Conversely, when the soluble uranium comes into contact with a
reducing environment (e.g., sulfides such as pyrite or hydrogen
sulfide, and organic material such as coal, oil or gas), it
falls from solution.
enters an aquifer when it becomes dissolved in
waters, such as rainwater. The oxygenated water percolates
downward from the surface, through uranium bearing sediments,
such as rock interbedded with volcanic ash. As the uranium is
dissolved into the water, it
is carried even further through the porous rock and deeper into
the aquifer. Commercial grade ore deposits accumulate over
millions of years, as huge amounts of weakly oxidized
groundwater containing the dissolved uranium pass a reducing
interface in the sandstone rock, where the uranium accumulates.
uranium ore deposits must meet certain criteria to allow
recovery with the ISR process:
Multiple uranium ore horizons, 30 to 150 feet wide, are
identified through drilling of exploration boreholes. The exact
location of the ore is then mapped, showing its length, width,
and depth below surface. Injection and extraction (production)
wells are drilled, cased, cemented, tested, and completed in the
ore zones. These wells are placed in patterns, coupling
injectors with extractors, and spanning the ore body. These
patterns cause the oxygenated water that is injected at the
surface to be pulled through and across the ore, taking the ore
with it when it is then pumped from the
wells back up to the surface. This technology was first
developed and refined by the oil industry, which uses the same
technique in water floods or other secondary and tertiary oil
The ore deposit must be located in a water-saturated zone.
The deposit must have adequate rock permeability.
The deposit must be easily solubilized with oxygen.
wells completely surround the wellfield patterns, and are
sampled regularly (sometimes as often as twice weekly) to ensure
that all recovery solutions remain contained within the
mineralized area. For this same reason, monitor wells are placed
in sands immediately above and below the ore zone, as required.
and Production of Uranium
uranium mills, which treat rock taken from open pit or
underground mines, mostly use sulfuric acid to liberate the
uranium from the crushed rock. That solution is called "lixiviant"
or "leachate". In early tests of ISR, sulfuric
acid solutions or ammonium carbonate-hydrogen peroxide solutions
were tried (especially the ammonium carbonate lixiviant). While
these leach solutions readily removed the uranium mineral from
the host sand, they caused problems in the post-production
groundwater restoration phase, and were quickly replaced with a
more neutral, environmentally friendly solution.
ISR operations in the U.S. use a sodium
system, a solution similar to baking soda or club soda. This
system closely resembles the chemistry of native
oxygen is added to the water to mobilize the uranium, which then
is “washed” from the sand grains by the carbonate present in the
basically mimicking (in reverse order) the natural process of
uranium emplacement. This water is then pumped from the ground
to the surface plant for removal of the uranium.
ISR process begins when groundwater is pumped from
the extraction/ production wells. The water flows through the
uranium extraction plant described below. After the groundwater
is circulated through the plant, oxygen is added to the water.
This "fortified" water is injected into the ore horizon through
injection wells. As the oxygenated water passes through the
porous sandstone, it oxidizes and solubilizes the uranium, which
generally occurs as a thin precipitate covering the sand grains
of sedimentary rock. This water is pulled to the extraction
wells and pumped to the surface plant for uranium removal. The
cycle is repeated, and the groundwater is recirculated,over
ISR is a closed water system, circulating the same
water over and over again, except that approximately 1% less
water is injected than was extracted. This creates a "pressure
sink" in the wellfields, causing fluids in the area to naturally
flow into the wellfield, thereby helping to contain the
Facilities: Ion Exchange, Elution, Precipitation, and Drying
EXCHANGE. The surface plant for an ISR
operation is essentially a large water softener, very similar to
those used in homes. "Hard water" in the home is caused by high
levels of the chemical ions of calcium and magnesium. A home
water softener contains plastic "resin" beads held in a tank
(itself generally plastic or fiberglass). As water flows
through the tank and across these resin beads, the chemical ions
of calcium and magnesium are "exchanged" out of the water and
onto the resin. The other half of the "exchange" is sodium ions
transferring off the resin and into the water. This is called
"softening" of water, and the term "ion exchange" is used for
this general process.
concentrated salt water (or brine) will cause this ion exchange
to reverse direction. During the "regeneration" cycle of the
home water softener, a highly concentrated salt (NaCl) solution
is pumped over the resin, causing the calcium and magnesium to
"exchange" off the resin and into the salt water (opposite of
what it did in the "softening" process), while sodium transfers
back onto the resin, thus "regenerating" the resin and preparing
it again for "softening" of water.
type of "ion exchange" will transfer uranium out of water and
onto resin beads. An ISR facility is composed
mostly of large tanks that hold "resin" and "regenerating"
water, and pumps to move the water. A home water softener
contains "cation" resin because it involves the ion exchange of
the positively charged ions (cations): calcium, magnesium, and
sodium. Since uranium in solution is a negatively charged ion
(called anion), the tanks at a recovery plant hold "anion" resin
this, the ion exchange process for uranium ISR is
exactly the same as for a home water softener. The water pumped
from the ground contains uranium in solution, passes over the
anion resin, and the uranium transfers onto the plastic resin
beads. At the same time, chloride or bicarbonate (both negative
ions) transfers off the resin and into the water. The ion
exchange is then complete, and the groundwater has very little
uranium left in it. Once the water leaves the resin tank, it is
re-fortified with oxygen and re-injected into the ground. The
process is repeated again and again, until the uranium level
drops too low to continue the cycle.
ION EXCHANGE (“RIX”) Smaller ore US deposits may not support
a large central plant and but rather would call for RIX. In an
RIX system, each orebody is still mined with its own native
groundwater, and the recovery process is the same, except that
the surface facility is off-site. The resin, once loaded with
uranium, is transferred out of the ion exchange column, drained,
and trucked via trailer to a central plant for removal of the
captured uranium. The clean ion exchange resin is then
transferred back to the RIX in the same way.
Each RIX is a self contained, stand-alone unit that recovers
uranium in pressurized down flow ion exchange columns. A
pressurized system keeps the uranium solution contained within a
closed loop that eliminates the potential for release of radon
gas to the atmosphere. (Radon gas is a natural byproduct of
uranium decay, and is always present along with the parent
uranium.) The entire unit is curbed to provide containment from
spills or leakage.
system allows development of only one central process plant
to be used for multiple orebodies. This lowers capital outlay
and allows recovery from smaller ore deposits.
The "regeneration" phase for ISR resin is called
"elution", and the salt water used to regenerate the resin is
called "eluant". Just as for the home water softener, highly
concentrated salt water brine is used for regeneration. In the
case of ISR however, some sodium
bicarbonate-carbonate solution (baking soda and club soda) is
mixed with the brine. This brine water is then pumped over the
resin, and a reverse ion exchange occurs, just as it does in a
home water softener during the regenerating phase. The elution
process causes uranium to be concentrated in the saltwater brine
or "eluant". For a home water softener, the brine is used only
once and then is sent down the drain, but in the ISR
process, the eluant is recycled and reused after the uranium is
precipitated from it.
PRECIPITATION. Precipitation of the uranium from the eluant,
or saltwater brine, is also a relatively simple and
uncomplicated process. At this point, the uranium is actually
combined with carbonate, known chemically as a "complex". A
small amount of hydrochloric acid, also known as muriatic acid
when used in home swimming pools, is added to the brine or
eluant. This causes the carbonate to break apart into carbon
dioxide and oxygen, breaking the uranium-carbonate "complex".
is now free and can be chemically "reduced" to cause
precipitation. Chemical reduction is simply repeating the
natural process that formed the ore body in the first place.
Alternatively, the uranium can be oxidized even further, which
forms a precipitate of crystals. Since "crystals" are relatively
easy to filter from water, oxidation is the preferred method of
precipitation. Hydrogen peroxide, just like you buy in the
drugstore, is used for this step. The brine is then refortified
with salt and carbonate in preparation for another cycle.
Impurities, such as sulfate, can build up in the brine and
decrease efficiency of the regeneration process, so some of the
brine is replaced with fresh solution. The used or "spent"
eluant is considered a waste product, and is discharged to a
DRYING AND PACKAGING. The precipitated and filtered uranium,
also called uranium "slurry", is washed with a small amount of
fresh water while in the filter press, and then dewatered. This
wash water, which contains some soluble uranium, is returned to
the elution and precipitation circuit. The resulting moist
uranium slurry (or "yellowcake") may be dried prior to packaging
for shipment. If the uranium is to be dried, it is pumped into a
zero-emission vacuum dryer. The water liberated as steam during
the drying process is condensed, and recycled to the
precipitation circuit or discharged to a waste pond. The dried
yellowcake is placed in 55-gallon steel drums for shipment.
production terminates, the water quality of the affected aquifer
must be restored to levels defined in regulatory standards. The
same injection, and production wells, and surface facilities are
used for groundwater restoration. Restoration reduces the
effects of ISR by removing injected chemicals or
immobilizing substances produced by the process.
restoration methods are currently used. The most direct and
widely practiced technique is that of groundwater sweeping,
which is simply pumping waters out of the well field area. As
pumping continues, groundwater beyond the mineralized area flows
in to replace the fluid that was pumped out. The extent of
lixiviant recovery and removal of undesired ions in the aquifer
improves with increased groundwater extraction, and the process
continues until predetermined concentrations of constituents are
restoration technique involves surface treatment of recycled
groundwater before it is reinjected into the wells. Recycling
uses some sort of filtration or distillation process such as
reverse osmosis concentration to remove the desired constituents
from water and consumes less groundwater than sweeping. Reverse
osmosis is a well established water treatment process whereby
the majority of dissolved "ions" are separated from the
wastewater, and concentrated into a smaller volume of briny
wastewater. After processing by reverse osmosis, pure water is
then returned to the affected aquifer through injection wells.
This reinjection of very pure water results in a large increment
of water quality improvement in a short time period.
surface reclamation, well bores are filled with cement to
prevent groundwater migration between water-bearing formations.
Casings are then cut off below the ground surface,
and the resulting excavation backfilled. Surface soil may
require decontamination to defined limits. Contaminated surface
soil is considered a by-product material of the ISR
process, and disposed of at a licensed site. Any structures
remaining after license termination must be decontaminated to
regulatory limits allowing unrestricted use. The ground surface
must also be re-vegetated. All reclamation work is performed
according to standards set by regulatory agencies, and is
overseen by those agencies.
operations produce small amounts of both solid wastes, and
waste is disposed of permanently at ISR locations.
Liquid wastes from the wellfield, process circuit, and aquifer
restoration are often injected into deep Waste Disposal Wells
(e.g., in Nebraska, Texas, Wyoming). ISR
disposal wells are not classified as “hazardous”; as
compared to the hundreds of “hazardous” disposal
used in the U.S. As with all phases of ISR, work
is overseen by regulatory agencies, and follows standards
established by them.