Open-pit mining: Encyclopedia II - Open-pit mining - Extraction

Open-pit mining - Extraction

Open Cut mines are dug on benches, which describe horizontal levels of the mine. These benches are usually on 3 metre or 6 metre levels, depending on the size of the machinery being used. Quarries rarely use benches, as the majority of quarries are dug using (relatively) small-scale machinery.

The walls of the pit are generally dug on an angle less than vertical, to prevent and minimise 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 vertical section of the wall is known as the batter, and the flat part of the step is known as the bench. The steps in the walls help prevent rock falls continuing down the entire face of the wall. To support the wall, often rock bolts are used. Sometimes, de-watering bores are drilled horizontally into the wall to relieve water pressure, which is often enough to cause failures in the wall by itself.

A haul road is 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 surface near the edge of the open cut. This is known as the waste dump. The waste dump is also tiered and stepped, to minimise erosion.

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.

Adapted from the Wikipedia article "Extraction", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki