Environmental Engineering Reference
In-Depth Information
method, the space remaining following removal of ore after blasting is filled with waste rock and
cement. In the shrinkage method, just enough broken ore is removed via chutes below to allow
miners to work from the top of the pile while drilling and blasting for the next layer to be broken
off, eventually leaving a large hole.
As with coal, the environmental costs of deep mining uranium include impacts near tunnel
openings, ventilation shafts, and spoil piles; cross connections between aquifers, allowing con-
tamination of previously separate water supplies; and surface subsidence as a result of collapse
of cavity or tunnel roofs. Because uranium ore emits radon gases and other sources of radiation,
it can be more dangerous than other underground mining unless adequate ventilation systems are
installed.
In open pit or surface uranium mining, overburden is removed by drilling, blasting, and ripping
to expose an ore body, which is mined by blasting and excavation via loaders and dump trucks,
similar to surface coal mining as described in
Chapter 2.
Uranium ore is then broken up, loaded,
and hauled to a milling site. Unlike surface coal mining, there are no national requirements for
reclamation of uranium surface mines in the United States, which are regulated only by state laws.
Historically, uranium mines have a poor record of preserving topsoil for later distribution during
reclamation. Overburden material is usually hauled to a disposal site. Uranium ore is excavated,
broken up, and hauled to milling facilities before transport to fabrication facilities. Overburden
or spoil is sometimes used to refill a mined area, but often not. After spoil has been graded and
recontoured by bulldozers, topsoil may be redistributed and vegetation planted to hold surface soil
together and reduce erosion of spoil. In surface uranium mining operations, workers spend much
time in climate-controlled, enclosed cabins to limit their exposure to dust and radiation. Water is
used to suppress airborne dust.
In situ leaching, sometimes referred to as in situ recovery or solution mining, is performed by
pumping liquids (weak acid or weak alkaline depending on the calcium concentration in the ore)
down through injection wells placed on one side of a deposit of uranium. The liquid flows through
the deposit and is pumped up through recovery wells on the opposing side of the deposit, recover-
ing uranium by leaching. In situ leaching is often cost-effective because it avoids excavation costs
and may be implemented more quickly than conventional mining. However, it is not suitable for
all uranium deposits because the host rock must be permeable to liquids (as is often the case in
sandstone), in which case it may be impossible to contain leaching chemicals and prevent them
from contaminating nearby groundwater aquifers. Environmental impact studies and monitoring
must be performed continuously when evaluating in situ leaching.
Waste rock is produced during open pit mining when overburden is removed and during under-
ground mining when driving tunnels through non-ore zones. Piles of these tailings often contain
elevated concentrations of radioisotopes compared to normal rock. Other waste piles consist of
ore of too low a grade for processing. The difference between waste rock and ore depends on
technical and economic feasibility criteria, especially the market price of U
3
O
8
. All these piles
Search WWH ::
Custom Search