The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 561

Uranium Leaching
In situ leaching (ISL) of uranium ore has been carried out since the early 1960s, with early
production from deposits in Russia, Ukraine and Uzbekistan. After considerable research
during the 1960s, commercial production of ISL uranium in the US commenced in the
1970s and has continued ever since, resulting by 2001, in a combined production of 43,000
tonnes of uranium from 28 ISL operations. Other ISL projects have been developed in
China, Kazakhstan, Germany and Australia. Most ISL projects have been applied to 'roll-
front' type sandstone uranium deposits which are 'uniquely amenable to ISL exploitation
since ISL mining relies on physical and chemical processes similar to those that originally
deposited the uranium ore bodies' (Underhill 1992).
In situ leaching is usually used for relatively low grade uranium deposits which can not
be economically mined and processed by open pit or underground means. Uranium is
extracted from these low grade ore bodies with either acid-leach using sulphuric acid, or
alkaline-leach using sodium bicarbonate or ammonium bicarbonate. For both these proc-
esses, the uranium is subsequently recovered from the pregnant solution using either ion
exchange or solvent extraction methods. Selection of the leach process is inl uenced by:
●

In situ leaching is usually
used for relatively low grade
uranium deposits which can
not be economically mined
and processed by open pit or
underground means.

Regulatory requirements;

●

Composition and therefore relative solubility of ore and host rock;

●

Reagent consumption rates and costs;

●

Leaching rates and uranium recovery; and

●

Environmental factors including aquifer quality, groundwater gradients and l ow rates.

The salient features of acid and alkaline leaching are compared in Table 13.2 .
In many cases the choice of leaching process is dictated by regulatory requirements.
Groundwater in contact with uranium ore is inevitably radioactive and contaminated with
dissolved uranium and its decay products, including radon and thorium. Accordingly, it is
unsuitable for any form of domestic, agricultural or industrial use. US regulations require
that aquifer quality after solution mining be returned to a condition approaching potabil-
ity. This is more difi cult to achieve in the case of acid leaching, and, as a result, alkaline
leaching has been used in most uranium ISL projects in the US. Regulations in other juris-
dictions have been more pragmatic, in recognition of the fact that the groundwater had
no benei cial use before mining and therefore did not warrant expensive treatment after
mining. In most of these countries, acid leaching has been selected, as it generally involves
higher leaching rates, higher overall recoveries and consequently better economic returns.
Ores containing high concentrations of calcite, however, still require alkaline leaching.
A typical alkaline leach process is depicted in Figures 13.19 and 13.20 . The acid leach
process is similar except that sulphuric acid is the lixiviant. The effective location, spacing
and design of injection and recovery wells requires a thorough understanding of the hydr-
ogeology of the ore body and its surrounds. Detailed hydrogeological studies using drilling
and pump testing are used to evaluate aquifer conditions. Laboratory testing is used to
assess leaching rates. With these results as inputs, numerical modelling can then be used
to evaluate the optimum layout of bores, injection and extraction rates, and waste (barren
solution) re-injection scenarios.

Groundwater in contact with
uranium ore is inevitably
radioactive and contaminated
with dissolved uranium and its
decay products, including radon
and thorium.

Impacts
Compared to alternatives involving excavation, processing and disposal of waste rock and
tailings, ISL involves much less surface disturbance and accordingly much smaller impacts
on the landscape, soils, l ora, fauna, land use and visual amenity. Capital and operating
costs are generally also lower. On the other hand, the recoveries achieved are signii cantly
lower than can generally be achieved by conventional mining and processing.