The process of extracting minerals from the earth from a pit which is open for the entire duration of the operation of the mine. Open pit mines are also known as strip mining or opencast mining or open cut mining. The open pit mines are always on the open and never extend underground. Unlike underground mining, the minerals are found closer to the earth’s surface making open pit mines commercially viable. Open pit mines are operated and expanded until the minerals are exhausted or if the open pit is structurally weak for further mining.
Open pit mines are characterized by the digging in steps or benches. Based on the type of mining machinery and transport used, the steps can be found in four to sixty meter intervals. To minimize the danger of falling rocks compromising the structural integrity of the pit, the walls of the pit are dug on an angle that is less than the vertical. The walls bear the steps. The flat bottom of the step is known as the berm or bench, while the inclination of the wall is known as the batter. The angle of inclination varies with the type and hardness the rock. The steps prevent and slow down the rocks from falling down, destroying the mine. Usage of concrete, rock bolts and cable bolts to prevent the falling of rocks is common. In addition to rocks, the damage to an open pit mine could also be caused by the walls weakened by the holes drilled to ease water pressure.
A service or haul road is usually constructed like a ramp at the side of the pit to transport minerals, rocks and construction materials in and out of the pit. The rock dug up while mining is piled up at the top and near the edge of the open cut. This arrangement of waste rocks is also in the form of steps to prevent degradation. The mineral ore dug up from the mine after undergoing appropriate processing is known as tailing. Tailing is usually damp and slurry. The slurry is diverted and stored in a settling pond to allow the water to evaporate. The presence of processing chemicals like unextracted sulphide deposits and even cyanide if the ore is Gold making the slurry a toxic substance.
When the mining is stopped in a open pit, it is subjected to rehabilitation. Rehabilitation is the process of making the open pit and the associated areas like the settling pond structurally sound and less toxic to the environment. The waste rocks at the top are used for strengthening the open pit structure from collapsing. The settling pond with the toxic slurry will then be covered with clay to prevent the formation of sulphuric acid when rain water and air come in contact with the slurry. Trees are planted over this layer of clay to further minimize the effect of the toxic materials. Some of the minerals mined in open pits are Gold, Coal, Diamonds, Granite, Marble, Limestone, Gypsum, Copper and Iron.