Opencast mining & Types

Opencast mining or quarrying of minerals is easier than mining by underground methods. During quarrying the alluvium and rocks below which the mineral lies, are removed and dumped, in the initial stages, in a place which is not required in future for quarrying, residential or other purposes. The mineral exposed is completely extracted. Opencast mining is also known as open-pit mining, open-cut mining, surface mining and also as strip mining, the latter term being commonly used in the U.S.A. for opencast mining of coal. The overburden and the mineral, coal, are excavated in long strips of a few metres thickness and hence the operations are termed strip mining. The operation of removing overburden and extracting mineral is done by one of the following methods.

(1)  Manual quarrying: In this case manual labour is employed. Small drilling machines, drilling 1.2 m to 1.8 m deep holes, 37 mm diameter, are used and the holes are blasted with gun-powder or other explosives. The overburden and mineral are manually loaded into tubs which are hauled by rope haulages or locomotives. Tipping trucks are also sometimes employed and manually loaded.

(2)  Mechanised opencast working: In this method heavy earth moving   machinery   like   draglines,   power shovels,   rear-dumping trucks (common type being Haulpaks), well-hole drills etc. are used. The blast holes are 6 m to 18 m deep, and 125 mm to 250 mm diameter. The rock is blasted by liquid oxygen, open cast gelignite or other high explosives. Mineral and overburden are transported by locomotives, belt conveyors or large trucks known as dumpers. Bucket wheel excavators are used in some mines for soft rocks e.g. at Neyveli lignite project.

A method of surface mining known as placer mining involves mining and washing together of generally unconsolidated or semi-consolidated rock near the ground surface and the method is normally not treated as opencast mining but a variation of it.

Glory hole mining is a method where the mineral is excavated in small open pits but is transported to the surface through underground excavations and transport system.

Nearly 70% of the mineral production in the world comes by opencast mining and in India this method of mining accounts for nearly 75% of our mineral output.