The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 562

TABLE 13.2
Comparison of Acid and Alkaline Leaching

Acid Leaching Alkaline Leaching

Acid leaching achieves a high rate of uranium extraction - typically 70 to 90%. Extraction from alkaline leaching is relatively low - typically 60 to 70%.

Acid leaching yields faster dissolution of uranium - requiring 40 to 70 pore
volumes.

Slower kinetics of uranium dissolution. Alkaline dissolution typically requires
more pore volumes

Signifi cant increase in concentration of dissolved solids (TDS) in groundwater -
typically 10 to 25 g/L.

Insignifi cant increase in groundwater TDS.

High acid consumption for ores containing carbonate minerals. Potential to treat ores containing high carbonate content.

Requires use of corrosion resistant materials and equipment. Common materials and equipment can be used.

Addition of oxidant not always required. Addition of oxidant always required.

Possibility of recovering by-products. Leaching chemistry is highly selective for uranium.

Additional processing on surface may be required to produce contaminant-free
product.

Product solution from ion exchange should produce required product.

Risk of reducing permeability due to chemical and gaseous plugging. Precipitation of carbonate or sulphate minerals can also reduce permeability.

Restoration to pre-mining water quality requires an extended treatment period.
Such restoration has only been demonstrated at one pilot site 1 .

Restoration to pre-mining water quality has been demonstrated at several
sites.

Seepage beyond borefi eld is unlikely, due to formation of precipitates that
reduce porosity and permeability, and given natural attenuation due to reaction
of contaminants with adjacent barren rock and unaffected groundwater.

Potential for residual solutions to spread beyond the limits of the areas
being treated.

1 Note: For many acid ISL sites, restoration to pre-mining water quality has not been a requirement,
as the quality of pre-mining water was already poor.

Source:
Taylor et al. 2004

There have been examples of adverse environmental impacts as a result of ISL oper-
ations. These examples are from the former Soviet Union and resulted from the use of
improper practices. There are, of course, many more and far more serious examples of
environmental damage from conventional uranium mining operations in various parts of
the world, mostly from operations that predated the rise of environmental consciousness
that dates from about 1970.
Of primary concern are the impacts on groundwater quality. Although aquifers in con-
tact with uranium ore are naturally contaminated with radioactive elements and there-
fore unsuitable for consumption by humans, livestock or wildlife, the wastewater from the
process plant will contain other contaminants that could eventually migrate away from the
ore body area. In this respect the alkaline leach method is preferred by US regulators as it
facilitates restoration of groundwater quality to potable levels. However, in many situa-
tions such as remote, arid areas with low quality groundwater and no potential for signii -
cant groundwater interaction with the biosphere, there is no such requirement.

Closure
Given the small footprint associated with ISL operations, surface rehabilitation is rela-
tively straightforward. The main issue is usually groundwater restoration to ensure that
there are no long-term effects, particularly outside the area of the leach i eld. Given that