As a kind of surface mining, Open-pit mining or Opencast mining refers to a method of extracting rock or minerals from the earth by their removal from an open pit or borrow. Mining companies choose this way to get rocks and minerals out of the ground because it is the easiest and cheapest way to do it. Open-pit mining is only used if the rocks or minerals are close to the surface of the land or if a normal tunnel-type of mine isn’t possible.
At first, a geologist examines and tests the land to be mined. He will also check the site for what it will do to the environment. Even before they start to mine it, the scientists know the value of the rock/minerals below the surface and how much it will cost to dig them up. Mining companies make plans to get as much out as they can for their money. The plans and the amount of money it will cost to mine the area will change over the years because prices of rock and minerals change.
Once the plan is decided, the top layers of the land are removed. After that, layer after layer is taken away until the rock or mineral being crushed with stone crusher is visible. This is taken out, processed when it needs it, and sold. Taking away layer after layer of land creates a gigantic, open hole [or pit] that keeps getting deeper and deeper until there is nothing left to bring out. Explosives [like dynamite] are used when the miners want to get large blocks of materials out of the earth. The picture above shows how an open pit looks.
Most of the time, when the mining company is done with it, the pit is used as a landfill. A landfill is used as a dump to get rid of our garbage. When it is filled with garbage, it is covered over with layers of dirt. After the garbage rots a long time, the land can be used again. flotation cell:http://www.china-mills.com/p16.html
open-pit mining is used to differentiate this form of mining 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 (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would be the case for sand, cinder, and gravel). 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.
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