Open-pit mining is a surface-mining operation, in which rocks or minerals are extracted from the Earth by creating an open hole, or pit, in the Earth’s surface. These pits are generally 1km deep beneath the Earth’s surface, and are used to exploit low-grade ore bodies. This type of mining is used only when the minerals to be extracted are near the land surface, or when tunneling is unsuitable for mineral extraction.
Open-pit mines, which produce dimension stone and building materials, are known as quarries. This mining process involves digging vertical levels of the hole, known as benches. Each bench is dug on 4 to 60m intervals, based on the size of the machinery used. The excavation is usually performed by hydraulic shovels, or rope. Drill and blast methods are also commonly used. The walls of the pit are stepped, to prevent the rocks from falling down the wall surface. A haul road formed at the side of the pit provides a ramp up, through which the trucks carrying ore and waste rock can be transported.
Open-pit mining results in two types of waste products: waste rock - which lacks sufficient quantities of minerals - and tailings - formed as a result of a mineral separation process. Waste rock is usually piled up at the surface near the edge of the open pit, referred to as the waste dump. Tailings are pumped to a settling pond, or tailings dam, where the water evaporates.
Both high and low-grade zones of the mineral ore bodies can be mined through open-pit mining. The mining rate is usually greater than 20,000 tons per day (tpd). Some pits have a mining rate of more than 100,000 tpd.
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