Open-pit mining, open-cut mining or opencast mining is a surface mining technique of extracting
rock or minerals from the earth by their removal from an open pit or borrow.
This form of mining differs from extractive methods that require tunneling into the earth
such as long wall mining. Open-pit mines are used when deposits of commercially useful
minerals or rock are found near the surface; that is, where the overburden is relatively
thin or the material of interest is structurally unsuitable for tunneling. For minerals that
occur deep below the surface - where the overburden is thick or the mineral occurs as veins in
hard rock - underground mining methods extract the valued material.
Open-pit mines that produce building materials and dimension stone are commonly referred
to as quarries. Open-pit mines are typically enlarged until
either the mineral resource is exhausted, or an increasing ratio of overburden to ore
makes further mining uneconomic. When this occurs, the exhausted mines are sometimes
converted to landfills for disposal of solid wastes. However, some form of water control
is usually required to keep the mine pit from becoming a lake, if the mine is situated in
a climate of considerable precipitation or if any layers of the pit forming the mine
border productive aquifers.
Extraction Open-cast mines are dug on benches, which
describe vertical levels of the hole. These benches are usually on four meter to sixty
meter intervals, depending on the size of the machinery that is being used. Many quarries
do not use benches, as they are usually shallow. Most walls of the pit are generally dug on
an angle less than vertical, to prevent and minimize damage and danger from rock falls.
This depends on how weathered the rocks are, and the type of rock, and also how many structural
weaknesses occur within the rocks, such as a fault, shears, joints or foliations.
The walls are stepped. The inclined section of the wall is known as the batter, and the
flat part of the step is known as the bench or berm. The steps in the walls help prevent
rock falls continuing down the entire face of the wall. In some instances additional
ground support is required and rock bolts, cable bolts and shotcrete are used. De-watering
bores may be used to relieve water pressure by drilling horizontally into the wall, which
is often enough to cause failures in the wall by itself.
A haul road is usually situated at the side of the pit, forming a ramp up which trucks
can drive, carrying ore and waste rock. Waste rock is piled up at the surface, near
the edge of the open pit. This is known as the waste dump. The waste dump is also tiered
and stepped, to minimize degradation. Ore which has been processed is known as tailings,
and is generally a slurry. This is pumped to a tailings dam or settling pond, where
the water evaporates. Tailings dams can often be toxic due to the presence of unextracted
sulfide minerals, some forms of toxic minerals in the gangue, and often cyanide which is
used to treat gold ore via the cyanide leach process. This toxicity can harm the surrounding
environment. Rehabilitation
After mining finishes, the mine area must undergo rehabilitation. Waste dumps are contoured
to flatten them out, to further stabilise them. If the ore contains sulfides it is usually
covered with a layer of clay to prevent access of rain and oxygen from the air, which can
oxidise the sulfides to produce sulfuric acid, a phenomenon known as acid mine drainage.
This is then generally covered with soil, and vegetation is planted to help consolidate
the material. Eventually this layer will erode, but it is generally hoped that the rate of
leaching or acid will be slowed by the cover such that the environment can handle the load
of acid and associated heavy metals. There are no long term studies on the success of
these covers due to the relatively short time in which large scale open pit mining has existed.
It may take hundreds to thousands of years for some waste dumps to become "acid neutral"
and stop leaching to the environment. The dumps are usually fenced off to prevent livestock
denuding them of vegetation. The open pit is then surrounded with a fence, to prevent
access, and it generally eventually fills up with ground water. In arid areas it may
not fill due to deep groundwater levels.
Typical open cut grades Gold is generally extracted in open-pit mines
at 1 to 2 ppm but in certain cases, 0.75 ppm gold is economical. This was achieved by bulk
heap leaching at Alkane Minerals Ltd. Peak Hill mine in western New South Wales, near
Dubbo, Australia. Nickel, generally as laterite, is extracted
via open-pit down to 0.2%. Copper is extracted at grades as low as 0.15% to 0.2%, generally
in massive open-pit mines in Chile, where the size of the resources and favorable metallurgy
allows economies of scale. Materials typically extracted from open-pit
mines include: Bitumen
Clay Coal
Copper Coquina
Diamonds Gravel and stone
Granite Gritstone
Gypsum Limestone
Marble Metal ores, such as Copper, Iron, Gold, Silver
and Molybdenum Uranium
Phosphate See also
List of open-pit mines Closure problem
Cut-and-cover Cut
Trench References
External links The biggest, deepest and deadliest mines in
the world Basics of an open pit mine
Large Surface Miners Deepest open pit mines