IV.

Metal Mining

Metalliferous ores are mined either on the surface of the earth or underground. In opencut mining, the ore is removed from deposits that crop out at the surface, lie on a hillside, or are covered by a shallow overburden that is stripped before or simultaneously with the removal of the ore. In open-pit mining, benches are terraced into the earth or rock along the hillside or in the pit. The ore is usually loosened by blasting and loaded into trucks or rail cars by mechanical loaders, or shovels. As the pit increases in depth, the cost of mining by this method also increases, primarily because of the need to remove ever-increasing volumes of waste rock around the ore body to ensure a safe slope for the pit. The change from open-pit to underground mining occurs at the depth at which the costs of mining by the two methods are equal.

The selection of an underground method of mining depends on a number of conditions, primarily the grade, size, shape, and attitude of the ore body and the strength of the ore and wall rock. Generally, a system is used in which the force of gravity helps in the removal of ore.

The mine development work consists of driving a system of crosscuts that connect the shaft with the ore body at a number of levels, a suitable vertical distance apart. Vertical openings, called raises, are made to connect the various levels. The ore body is thus divided into blocks, which are bounded by the levels and the raises, and is ready for extraction. The ore may be removed from the bottom of the block upwards, the process being known as overhand stoping, or, more rarely, from the top of the block downward (underhand stoping). A stope is the chamber in which the ore is broken and mined. The stopes, on completion of mining, may be allowed to remain empty, if adequately supported, or may be filled with material, usually waste rock, brought down from the surface to support the exhausted stope and ensure safety of mining operations in adjacent stopes.

The specific method or methods used in removing the ore usually depends on local geometry and the physical characteristics of the ore and wall rock. Thick-bedded and massive deposits are usually mined by the so-called caving method. In block caving, the development work is confined below the lowest boundary of the block and consists of a network of operating tunnels, called drifts, and slanting raises, called finger-raises, which emanate from the drifts and terminate at the bottom surface of the block. The finger-raises are widened to funnel-shaped openings under the block. The block is then undercut as the rock supporting it is removed. This causes the collapse of the unsupported ore, which falls by gravity through the finger-raises into the drifts, from which it is scraped into mine cars. Under ideal conditions, no primary blasting is necessary, and in general, this is the cheapest underground mining method for handling large low-grade deposits. If the ore body does not disintegrate readily when the support is removed, blasting is necessary; this method is called forced block caving. The gradual extraction of the ore and the resulting fracturing of the rock around the mine workings cause subsidence at the ground surface, which may be counteracted by filling the resultant surface depressions with waste material from the ore-processing mill.

Placer mining is a special opencut method for deposits of sand, gravel, or other alluvium containing workable amounts of valuable minerals. Native gold is the most important placer mineral, but platinum and tin are also found in gravels. Other minerals found are zircon, diamond, ruby, and other gems, and monazite, ilmenite, and ores of columbium and tantalum.

Large placer operations involve excavation by power shovels, bucket-wheel excavators, or dragline conveyors, which deliver the sand and gravel to a system of screens, jigs, and sluices used to recover the ore mineral. Occasionally, the gravel bank is broken down by a high-pressure stream of water delivered through a large nozzle, called a hydraulic giant; this method is known as hydraulicking. More often, the placer is flooded, and the digging and processing equipment is mounted on a dredge. The mechanical excavator is usually of the chain-bucket type, and this method is known as dredging.