Most uranium mining in USA is now by in situ leach (ISL) methods. In USA
it is seen as the most cost effective and environmentally acceptable
method of mining. There are two proposals well advanced for ISL mining of
uranium in Australia.
Conventional mining involves removing rock from the ground, breaking it up
and treating it to remove the minerals being sought.
What is ISL and How long has it been used?
In situ leaching (ISL), also known as solution mining, involves leaving
the ore where it is in the ground, and using liquids which are pumped
through it to recover the minerals out of the ore by leaching.
Consequently there is little surface disturbance and no tailings or waste
rock generated. However, the orebody needs to be permeable to the liquids
used, and located so that they do not contaminate ground water away from
ISL mining was first tried on an experimental basis in Wyoming during the
early 1960s. The first commercial mine began operating in 1974. Today
about a dozen projects are licensed to operate in the USA, (in Wyoming,
Nebraska and Texas) and most of the operating mines are less than 10 years
old. Most are small, by Australian standards but they supply some 85% of
the US uranium production. About 13% of world uranium production is by ISL
(including all Kazakhstan and Uzbekistan output).
ISL can also be applied to other minerals such as copper and gold.
ISL Uranium Deposit Characteristics
Uranium deposits suitable for ISL occur in permeable sand or sandstones,
confined above and below by impermeable strata and below the water table.
They may either be flat, or "roll front",- in cross section, C-shaped
deposits within a permeable sedimentary layer.
They were formed by the lateral movement of groundwater bearing oxidised
uranium minerals through the aquifer, with precipitation of the minerals
occurring when the oxygen content decreased, along extensive
oxidation-reduction interfaces. The uranium minerals are usually uraninite
(oxide) or coffinite (silicate) coatings on individual sand grains. The
ISL process essentially reverses this ore genesis, in a much shorter time
Techniques for ISL have evolved to the point where it is a controllable,
safe, and environmentally benign method of mining which can operate under
strict environmental controls and which often has cost advantages.
ISL Wellfield -an overview
Wells are cased to ensure that liquors only flow to and from the ore zone.
Submersible electric pumps draw from near the bottom of the production
A wellfield design is typically a grid with alternating production and
injection wells. The spacing between them usually ranges from 15 to 30
metres. A series of monitor wells are situated around the whole wellfield
to ensure that contaminated water does not move outside the mining area.
Oxygen is injected into the leach liquors as they are returned to the
injection wells and drawn into the ore zone to oxidise the uranium
minerals. They are then removed as the liquors are pumped out.
While uranium production in Australia uses acid leaching of the crushed
ore, ISL overseas normally uses alkaline leaching agents such as a
combination of sodium bicarbonate and carbon dioxide. At Honeymoon in SA,
the process will be acid leaching with weak sulfuric acid plus oxygen. The
leaching solution is at a pH of 2.8 - 2.0, about the same as vinegar.
In USA the production life of an individual ISL wellfield is usually less
than 3 years, typically 6-10 months. Most of the uranium is recovered
during the first 6 months of the wellfield's operation. The most
successful operations have achieved a total overall recovery of about 80%
of the ore. Over time, production flows decrease as clay and silt become
trapped in the permeable sediments. These can be dislodged to some extent
by using higher-pressure injection or by reversing the flow between
injection and production wells.
The pregnant solution from the production wells is pumped to the treatment
plant where the uranium is removed from it in an ion exchange system; or
by solvent extraction. The uranium is then stripped from the ion exchange
resin, and precipitated chemically.
Most of the solution is returned to the injection wells, but a little is
bled off and treated as wastewater. It contains various dissolved minerals
such as radium, arsenic and iron. Barium chloride is added to precipitate
the radium. This bleed of process solution ensures that there is a steady
flow into the wellfield from the surrounding aquifer, rather than having
any leach liquor move in the other direction. Before the process solution
is reinjected, it is oxygenated and if necessary recharged with sodium
bicarbonate or acid.
ISL in Australia
Two projects are currently proposed for ISL mining operations in
Australia; Honeymoon and Beverley, both in the Lake Frome area of South
Australia between Broken Hill and the northern Flinders Ranges.
At Honeymoon the uranium deposit occurs in porous sandstone at a depth of
100 to 120 metres and extending over about 24 hectares of a buried
Honeymoon was discovered in 1972, about 75 kilometres north west of Broken
Hill. Early in 1997 Sedimentary Holdings NL reached agreement with MIM
Holdings Ltd to acquire the Honeymoon and two adjacent uranium deposits
next to its own Chatfield (East Kalkaroo) deposits.
This brings together uranium resources of about 6800 tonnes U3O8 averaging
0.15% and amenable to ISL. The purchase was funded by Southern Cross
Resources Inc. of Toronto which raised funds in Canada for the development
of the SA uranium properties. Sedimentary Resources (an Australian
company) holds 35% of Southern Cross.
Plans had been developed in the 1970s to extract the uranium oxide by ISL,
and some $12 million was spent in preparation then. Draft and Final
Environmental Impact Statements were produced, and both South Australian
and Commonwealth environmental approval was subsequently obtained in 1981
for production to 450 t/yr. Field tests of the in situ leaching process
were carried out and a $3.5 million, 110 t/yr pilot plant was built and
remains on site, but the project was abandoned in 1983*.
Southern Cross now plans to bring Honeymoon and the associated deposits
into production in 1998, building up to 450 tonnes per year by 2000.
The Beverley deposit, 520 kilometres north of Adelaide is northwest of
Honeymoon and geologically similar to it. Plans to mine it by ISL were
abandoned in 1983 when the South Australian Government refused to grant
permission for development to proceed*. The deposit was sold to Heathgate
Resources Pty Ltd, an affiliate of General Atomics of USA, in 1990.
Three ore lenses lie at a depth of 110-140 metres, over some 4 km. Their 6
million tonnes of 0.27% ore contains 16 200 tonnes of uranium oxide. An
estimated 11 500 tonnes of this was considered recoverable by ISL at 900
tonnes per year, making it the largest Australian deposit of its kind.
Hydrogeological tests and operation of a continuous field leach trial are
being undertaken in 1997-98, along with preparation of a new Environmental
Impact Statement. Heathgate Resources is working towards commencing
production before 2000.
Environment & Health
At established operations overseas, after ISL mining is completed, the
quality of the remaining groundwater must be restored to a baseline
standard determined before the start of the operation, so that any prior
use can be resumed. Contaminated water drawn from the aquifer is either
evaporated or treated before reinjection.
In contrast to the main US operations, the water quality at the Australian
sites is very low to start with, and it is quite unusable. At Beverley the
groundwater in the orebody is very saline and orders of magnitude too high
in radionuclides for any permitted use. At Honeymoon the water is
similarly very saline, and high in sulfates and radium. When acid leaching
is discontinued, the water quality reverts to its original condition.
Upon decommissioning, wells are sealed or capped, process facilities
removed, any evaporation pond revegetated, and the land can readily revert
to its previous uses.
The usual radiation safeguards are applied at an ISL mining operation,
despite the fact that most of the orebody's radioactivity remains well
underground and there is hence no 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.
due to the ALP "3 mines" policy
Brunt, D, 1997; Proposed in situ leach mining at Beverley, SA; Uranium '97
conference, Darwin. Dobrzinski, I, 1997, Beverley and Honeymoon Deposits,
MESA Journal 5, April 1997. Szymanski, W N. 1993, Energy
Information Administration, Uranium Industry Annual. Uranium Institute,
1996, Global Nuclear Fuel Market,supply & demand 1995-2015.